An overview of polymer foam technologies with a focus on polylactic acid,polypropylene,and polyamide 6-based materials

Abstract

Polymer-based composite materials are widely used in a number of engineering applications owing to their light weight, mechanical strength, chemical resistance and versatility in design. Using these materials in foam form offers several additional advantages, including reduced density, improved heat and sound insulation, and increased impact resistance. In the contemporary era, intensive research is being conducted into the evaluation of petrochemical-based polymers, such as polypropylene (PP) and polyamide 6 (PA6), and biopolymers, particularly polylactic acid (PLA), in foamed composite structures. Furthermore, hybrid systems formed by the combination of these different polymer types offer great potential for balancing environmental sustainability and performance. This review discusses the general structure, foam formation mechanisms, production techniques and basic properties of polymer-based foam composite materials. Prominent polymer-based systems, such as PP, PA6 and PLA, were examined comparatively. Additionally, contemporary methodologies concerning hybrid foam systems formed by PLA with PP and PA6, as presented in the existing literature, were evaluated.

Keywords

polymeric foams hybrid foam systems foaming lightweight structures biocomposite foams

Introduction

In the contemporary context, the selection of materials for engineering applications is influenced by a multitude of factors, including not only high mechanical performance but also criteria such as lightness, versatility, energy efficiency and environmental sustainability. In this context, polymer-based composite materials are extensively utilised in numerous industrial sectors due to their low density, ease of processability, chemical resistance and configurable properties. These composite systems, which are obtained by combining the polymer matrix with various fillers, fibres or additives, provide high performance, functional solutions, particularly in the automotive, construction, packaging and electronics industries.

Compared to conventional composites, polymer-based foam composite materials have a lower density and enhanced specific properties. This is attributable to their cellular structures, which are formed by the controlled dispersion of the gas phase within the polymer matrix. These structures provide ideal solutions for a range of applications, including heat and sound insulation, energy absorption and impact resistance, while simultaneously reducing production costs. Progress in manufacturing methods has allowed the improvement of these structures, which are created using physical and chemical foaming techniques, in terms of foam cell size, distribution and structural similarity. In recent years, there has been an increased focus on sustainability and the use of environmentally friendly materials. Consequently, there has been a rise in research interest in biodegradable and renewable polymers. In this regard, foam composites made from biopolymers, such as PLA, have become increasingly prominent. However, conventional thermoplastics such as PP and PA6 are also processed in foam form and offer attractive alternatives in terms of lightness and strength. Integrating PLA-based foams with conventional polymers such as PP and PA6 has been shown to create novel materials with enhanced environmental and mechanical performance.

Despite numerous studies on individual polymer foam systems, a comprehensive comparison that systematically examines PP-, PA6- and PLA-based foams together, particularly in terms of their processing–structure–property relationships, is lacking. Furthermore, the literature does not adequately address how hybrid systems, such as PLA/PP and PLA/PA6 foams, overcome the inherent incompatibility between biobased and petroleum-based polymers. There is also a lack of information on how these interactions affect cell morphology, stability, and final performance. Therefore, a consolidated review focusing on these polymer families is needed to clarify current knowledge gaps and guide future material design strategies.

This review article first discusses the general properties, production methods and foam formation mechanisms of polymer-based foam composite materials, followed by a comparative analysis of foam systems based on certain polymer types, including polypropylene (PP), polyamide 6 (PA6) and polylactic acid (PLA). Furthermore, the structural advantages, processing challenges and potential applications of hybrid foam composites formed by PLA-based systems with PP and PA6 were examined.

Polymer based composite materials

Composite materials consist of a matrix and a reinforcement phase that are structurally insoluble in each other. They are unable to provide the desired properties alone, but they do not lose their own properties within the formed structure either. While particles or fibres can be used as a reinforcement phase, the predominant matrix phase around this material is used. The selection of the matrix and reinforcement phases used can be determined by factors such as lightness, strength, and cost. The objective of the research is to demonstrate that composite materials are competitive with traditional materials and possess superior properties.^1,2

Polymer-based composites are among the most widely utilized materials in recent years, with a wide range of applications. Polymer materials are organic in nature and consist of large molecules formed by the combination of different types and a certain number of small molecules formed by hydrogen, carbon and other non-metallic elements. Due to their high molecular weight, polymer materials have unique properties. The materials’ low density, corrosion resistance, ease of processing and non-conductivity for heat and electricity make these materials suitable for a wide range of manufacturing processes and applications.

Polymers can be classified into two basic groups: thermosets and thermoplastics. Thermoplastics are composed of linear chains of molecules. They can soften and melt when heated to specific temperatures, depending on their characteristic glass transition temperatures. This property also makes them suitable for recycling, as they can be reshaped under heat and pressure. Thermoplastics are a diverse group of materials with a wide range of properties. They can be as flexible as rubber, as hard as metal, or as transparent as glass. They are also highly tough and can be produced using traditional plastics manufacturing techniques, such as injection moulding and extrusion. When compared to other composites, this enables them to be a cost-effective and efficient choice for production at higher speeds.³ In contrast, thermosets are a group of polymers that are cured in the presence of a curing agent and heat. These polymers exhibit a covalently bonded, insoluble, three-dimensional network structure and demonstrate no signs of softening when heated. The irreversible curing process is characterized by the formation of highly stable bonds between molecules, resulting in a high degree of resistance to heat and a wide range of chemicals, including alcohol, ketone and hydrocarbon. Compared with thermoplastics, thermosets are harder and stronger.⁴ As polymer composites evolve in line with these structural and processing principles, converting them into foam-based architectures is an important way to extend their functionality and industrial relevance.

Polymer based foam composite materials

Since the beginning of the twentieth century, petroleum-based polymers have been extensively used in all manufacturing sectors and have become a significant field for conducting various studies in line with advancements in manufacturing technologies. The development of foaming technology, which began with the integration of chemical and engineering technologies during and following the Second World War, has led to the identification of potential applications for polymeric materials in Europe and North America. This technological advancement has emerged as a significant sub-sector within the polymer industry, providing opportunities for manufacturing through methodologies such as extrusion, injection, reactive foaming, and gelation. The production and application areas of polymer-based foam composites, whose production trials began in the 1930s, increased rapidly in the 1980s. The need for lightness, cost effectiveness and reduced raw material consumption resulted in a significant expansion of studies on foaming polymeric materials. Polymer-based foam materials, which can provide improved strength-to-weight ratios and superior thermal and sound insulation properties, have become widespread in various fields, including automotive, defense, transportation, aviation, construction, medical, military, petroleum, and marine industries.⁵ Table 1 provides a comprehensive overview of the historical development of polymer foams.

Table 1.

Historical development of polymer-based foam composites.⁵

Year	Foam	Reference
1931	Polystyrene foam	⁶
1937	Polyurethane foam	⁷
1941	Polyethylene foam	⁸
1967	ABS foam	⁹
1967	PVC foam	¹⁰
1971	PET foam	¹¹
1972	Polypropylene foam	¹²

Polymeric foams are materials that contain gas voids in the form of porous cellular structures, which are surrounded by a dense polymeric matrix. They are produced by the expansion of foaming agents, which can be either an inert gas, a liquid or a chemical mixture capable of volatilization. Foams usually consist of at least two phases.¹³ The initial phase is the solid polymer matrix, and the subsequent phase is the gas created by the foaming agent. The voids formed in the polymer reduce the density of the material, and consequently less raw material is required, resulting in a reduction in the price of the product. Moreover, the density of the polymer can be adjusted by controlling the void ratio in the material. This process enables the production of polymer foams with a range of properties, which can be utilized in various applications.^14–17 In comparison to conventional materials and non-cellular polymers, polymer foams have become a notable research topic, especially in the automotive sector. The properties of polymer foams that make them a valuable research subject include their lower density, material savings, affordability, superior strength-to-weight ratio, high thermal and acoustic insulation, impact resistance, toughness, long fatigue life, thermal stability, and low dielectric constant. The weight reduction offered by polymer foams, and the consequent reduction in fuel consumption, demonstrates their value in the automotive sector. The primary advantages and disadvantages of polymer-based foam composites were listed in Table 2.

Table 2.

Advantages and disadvantages of polymer-based foam composites.¹⁸

Advantages	Disadvantages
Light weight	Variable density
High thermal insulation	Weakening of mechanical properties
Improved strength to weight ratio	—
Ease of moulding	—
Impact resistance	—
Low dielectric constant	—

Polymeric foams can be classified into different groups based on polymer matrix material, cell morphology, glass transition temperature, expansion rate and foam cell size (Table 3). Based on the polymer matrix material, polymer foams are analysed in two groups, thermoplastics and thermoset foams, and classified as rigid, flexible or semi-rigid (semi-flexible) depending on the glass transition temperature, which varies according to the chemical composition, degree of crystallisation and degree of cross-linking of the foam. If the glass transition temperature is below room temperature, the foam is called flexible; if it is above room temperature, the foam is called rigid. Foams that fall between these two categories in terms of density and functionality are known as semi-rigid foams. According to the cell morphology, polymeric foams are divided into two as closed-cell or open-cell foams. Cell geometry, cell size and cell shape are important in this classification, and the formation of an open- or closed-cell structure can be controlled by the type of material being foamed and the appropriate foaming process. Polymer based foams can be classified as microporous and porous according to the cell sizes formed in the foam structure, and as low and high density according to the densities obtained based on the expansion ratio.^19,20 Furthermore, the operational criteria and their influence on foam morphology and performance for PP, PA6 and PLA were summarised in Table 4 to understand which operational parameters are critical for controlling morphology and mechanical properties for each polymer system.

Table 3.

Classification of polymer based foam composites.⁵

Classification type	Foam type of structure
Polymer type	Thermoplastic/thermoset
Hardness	Flexible/rigid
Cell structure	Open cell/closed cell
Cell size	Microporous/porous
Density	High density/low density

Table 4.

Operational criteria and their influence on foam morphology and performance for PP, PA6 and PLA.

Polymer	Key operational targets/Levers	Influence on cell morphology	Impact on properties	References
PP	- Moderate melt strength	- Higher cell density with nucleating agents	- Increased stiffness and impact strength	²¹
	- Nucleating agents	- Uniform cell size	- Reduced shrinkage
	- Temperature control	- Closed-cell fraction improved	- Reduced shrinkage
PA6	- High melt viscosity	- Small, uniform cells with low coalescence	- High modulus and strength	^22,23
	- Moisture control (drying required)	- Risk of collapse if moisture present	- Lower toughness if collapse occurs
	- Nucleation strategy (chemical or physical)	- Risk of collapse if moisture present	- Lower toughness if collapse occurs
PLA	- Low melt strength (need to consider chain extenders)	- Fine, uniform microcells	- High stiffness but low toughness	^24,25
	- Moisture sensitive (drying is essential)	- High risk of cell collapse without melt strength or nucleation aid	- Dimensional instability if collapse occurs
	- Nucleation via supercritical CO₂ or additives		- Dimensional instability if collapse occurs

As the structural diversity and functional capabilities of polymer-based foams continue to increase, scientific attention has shifted towards foam systems derived from renewable and biodegradable polymers due to growing environmental concerns and sustainability-driven material choices.

Biopolymer based foam composite materials

With the beginning of the twenty-first century, rapidly depleting natural resources, climate change, social and environmental awareness have led to awareness in the production and consumption sectors, and global warming, recyclability, energy efficiency and sustainability of resources have become increasingly important. While recycling activities started to become popular in the mid-1990s, on the other hand, petroleum prices, which had been stable for many years, started to rise extremely rapidly in the early 2000s, which was reflected in the prices of petroleum-based polymer raw materials. These developments have revealed the need to develop raw materials that can be used as an alternative to petroleum-based polymers. The emergence of biopolymers and the beginning of studies on their use as a substitute for traditional petroleum-based materials is one of the most important developments of the last decade. With the realization that the technology that facilitates our lives on the one hand, on the other hand, causes CO₂ emissions at a level that may cause global warming and that CO₂ emissions are inevitable, especially in industrialised countries’ production processes, studies on biopolymer-based composite materials have started to increase. Although the focus of the studies is based on the ‘carbon footprint’, especially the regulations expected to be implemented regarding CO₂ emissions in the automotive sector, it has been seen that the emerging biopolymer composite material class can exhibit very good properties and can be used in sectors such as automotive, health, electronics, and transportation. This new class of materials not only replaced traditional petroleum-based polymers, but also led to the emergence of a new class of biocomposites with different polymers and fillers, whose properties were further improved.^26,27

Biopolymers are commonly defined as either bio-based or biodegradable, or both. The classification of biopolymers is typically divided into three categories.²⁸

• Bio-based and biodegradable biopolymers produced from renewable resources: PLA, polyhydroxyalkanoates (PHA), starch, chitosan…

• Bio-based but non-biodegradable biopolymers produced from renewable resources: bio-based polyethylene, bio-based polypropylene, bio-based polyethylene terephthalate...

• Biopolymers produced from fossil fuels but biodegradable: polycaprolactone (PCL), polybutylene succinate (PBS)...

The increasing demand for lightweight materials with lower CO₂ emissions has led to the initiation of foaming studies for biopolymers, and in recent years, the foaming behaviour of PLA, which is the most widely used commercially, has been investigated. The low foaming tendency of PLA is mainly due to its low melt strength and slow crystallization rate, and during foaming processes, the low melt strength of the polymer can cause cell coalescence and the disappearance of cell walls. It has been demonstrated by studies on the subject that the process of foaming can be enhanced by increasing the crystallization rate of PLA during extrusion and injection. This leads to significant improvements in the final foaming behaviour, including cell density and expansion rate. The result is the production of closed-cell foams with uniform morphology and controlled expansion rate. Studies have also been conducted on the impact of foaming agents on crystallisation kinetics, as well as their effect on foam quality and the final properties of foam structures. These findings clearly demonstrate that the foaming performance of biopolymers such as PLA depends heavily on interfacial interactions and phase behaviour. This makes compatibilising agents a critical parameter when blending these systems with other polymers to enhance or balance their properties.

Importance of compatibilising agent in polymer blend based foam composite materials

The preparation of polymer-based composite materials is a suitable and frequently preferred method for the development of new polymeric materials in which the advanced properties of more than one polymer are combined. With this method, materials with desired properties can be obtained in a shorter time and more cost-effectively from polymers to be developed by using new monomers or polymerization techniques. Twin-screw extruders are generally used in the preparation of composite materials to be obtained for polymers, which can be used in suitable areas in accordance with their individual properties, together in order to increase their mechanical and thermal properties and to improve their properties such as hardness, toughness, impact resistance and wear resistance.

Polymer blends are generally classified as homogeneous (miscible at the molecular level) or heterogeneous (immiscible) blends.²⁹ In homogeneous blends, both polymers may lose some of their properties, while the final structure may carry the properties of both polymers equally. In heterogeneous blends, the properties of both polymers can be observed in the final structure and the weaknesses of one polymer can be balanced by the strengths of the other. The morphology of heterogeneous blends can generally depend on the blend ratios, viscosities and process conditions.³⁰

The miscibility and immiscibility of polymer blends are determined according to the free energy change values (ΔG_m) in equation (1), and the blends can be fully miscible, partially miscible or immiscible with each other depending on the ΔG_m values.²⁹ If the free energy change value ΔG_m<0, the structure is miscible, if ΔG_m>0, it is immiscible.³⁰

{Δ G}_{m} = Δ H_{m} - T Δ S_{m}

(1)

In the equation, ΔS_m is the entropy change and expresses the disorder and is always positive. For this reason, this value, which can be significant alone, especially for blends of low molecular weight polymers, can be negligible since polymer blends have monomers with high molecular weight, and in this case, the value of the enthalpy change of mixing (ΔH_m) is decisive. ΔH_m expresses the enthalpy change during the mixing of polymers. If the mixing process is exothermic, ΔH_m will be negative, ΔG_m will also be negative and the system will tend towards miscibility. ΔH_m is exothermic only when strong specific interactions occur between the blend components. The most common specific interactions found in polymer blends are hydrogen bonding, dipole-dipole, Van der Waals and ionic interactions.

Most polymers are immiscible due to the positive ΔG_m value which causes phase separation, poor interfacial interactions and deterioration of final properties. Therefore, the compatibility of immiscible polymer pairs must be taken into account during the design of high-performance polymer blends. Compatibilization can be achieved by the addition of a pre-synthesized copolymer or by chemical reactions at the interface between the polymers during processing.³¹

Blends are often improved by adding suitable compatibilising agents to reduce interfacial tension, improve compatibility and achieve a more homogeneous dispersion with smaller particle size. The use of compatibilising agents in heterogeneous blends can improve properties, which in most cases are lower than those of the combined polymers.³²

During the preparation of blends, it is known that the blend morphology is affected by the polymer ratios in the blend, viscosities, type and amount of compatibilising agent and process conditions. It is stated that the dispersed phase structure formed in immiscible polymer blends is directly related to the interfacial tension and will decrease if the interfacial tension decreases. Compatibilising agents are materials that have a high interfacial activity and reduce the interfacial tension between the polymers in the blend and support homogeneity. The use of these materials significantly affects the morphology. It is stated that the cell size in the foam structure decreases with the use of a compatibilising agent and the foaming agent creates nucleation zones by promoting the formation of nuclei.³³ Since compatibilisation directly governs phase morphology, cell nucleation behaviour and overall foam stability in polymer blends, the improvements achieved through interfacial engineering result in enhanced functional performance, which is the basis for the broad industrial application of polymer-based foam composite materials.

Application areas of polymer based foam composite materials

Polymer-based foam composite materials, which have properties such as low density, heat and sound insulation, improved strength/weight performance, toughness, impact resistance, are extremely common and it is difficult to think that there can be a single industry branch where they do not have any application. As illustrated in Figure 1, these materials are widely used in many fields such as automotive, aviation, transportation, maritime, building, construction, defence, packaging, white goods, furniture, electrical electronics, sports and entertainment, have an application area at every point of daily life. The applications of foam structures depend on the properties of these materials. Flexible foams are used in automotive parts and packaging applications where impact damping properties are at the forefront, while rigid foams are used in construction, transport, aviation sectors and in the automotive sector where lightness and strength are important. Polymer-based foam materials are developed according to the properties required for their intended application, and studies are carried out to ensure they can be manufactured and used effectively.^31,34,35

Figure 1.

Scheme of application areas of polymer based foam composite materials.

In the automotive sector, which is one of the leading areas where polymer-based foam materials are widely used, these materials contribute to the reduction of fuel consumption while providing weight savings in vehicles, provided that foam structures can meet the expected requirements for safety specifications. Polyolefin based foam composite structures are preferred in safety parts where impact resistance is important. For this purpose, polypropylene is the most preferred polymer in foam composite structures in the automotive sector. PP-based foam materials, which have impact absorbing properties as well as lightness contribution to vehicles, have been increasingly used in vehicle bumpers, headliners, poles and support parts in recent years. It is stated that the use of PP-based foam structures in bumpers can reduce weight by up to 40%, while meeting the legal requirements for impact damping, which is critical for automotive manufacturers. It is reported that head injury criteria can be met when used in headliners, which is another critical safety area.^34,36 Polymer-based foam composite structures can also be used in sealing parts, support parts, ventilation ducts, door panels, dashboards, interior claddings, lower supports of floor slabs, vibration damping pads, etc. to provide insulation and comfort, provided that adequate safety criteria are fullfilled. Polymer-based foams, which are especially preferred in the building and construction sector for their sound and heat insulation, long life and high strength properties, can also be used as insulation materials in buildings, interior and exterior claddings, roofs, under floor blocks where load support is required, shower cabins and piping systems such as water channels. Flame retardant additives prepared in the appropriate flammability class can be preferred as construction material in accordance with building regulations. Polymeric foams, which are used in the aerospace and defence industries with their light weight and advanced mechanical properties, can be used in structural and support parts such as fuselage, tail, door, wing, propeller, seat, armour and helmets of passenger aircraft, combat aircraft and unmanned aerial vehicles. Polymer-based foam composite materials used in aviation are expected to have flame retardant properties, which are important for almost every part and are sharply defined by standards. The use of polymer-based foam materials in the marine industry is important due to their light weight, high mechanical values and improved corrosion resistance. The first use of composite materials in the marine sector started with the aim of finding solutions to the corrosion problems associated with steel and aluminium, and then the studies were expanded due to the weight savings they provide. Polymer-based composite materials can be used in the hulls of ships, boats and yachts, bearings, covers, floats and hoses. Due to their high strength and abrasion resistance, polymer-based foam composites are also used in the health sector as connection and filling materials in orthopaedics and as filling and implant materials in oral and dental health. Polymer-based composite materials, which have high strength, light weight, strength and low maintenance demand, are preferred in sports, games and entertainment areas and equipment, and are used in pool bodies, slides, amusement parks and toys in children’s parks. In addition, due to their design flexibility, equipment such as bicycles, canoes, kayaks, skateboards, skateboards, golf clubs, tennis rackets can be produced from polymer-based foam materials, while providing lightness and durability to users, life safety is also taken into consideration. Polymer-based foam composite materials, which are used in electrical panels, panels, cable boxes in the field of electricity and electronics due to their electrical and thermal insulation and thermal resistance, have also found a place in the energy field due to their light weight and high strength, and have become an advantageous material class for wind turbines and solar panels. In electrical and electronic applications requiring conductivity, studies are being carried out to develop composite materials with the use of carbon black and carbon nanotubes. In the packaging sector, foam structures produced with low density polymers are generally preferred in polymer-based foam materials, which are generally selected due to their lightness and low cost, but also benefit from their shock absorbing properties in some special packaging applications.^31,34–36 The broad and demanding range of applications for polymer-based foam composites highlights the need for precise control over their cellular structure. This makes a clear understanding of the fundamental foaming mechanisms essential for tailoring the performance of materials to specific end-use requirements.

Basic mechanisms of foam formation in polymer based foams

The foaming process of polymers with chemical blowing agents occurs in several distinct stages, as illustrated in Figure 2: Diffusion, nucleation, bubble growth and stabilisation. During the diffusion stage, the gas generated by the chemical blowing agent disperses uniformly throughout the polymer matrix. Nucleation starts with the formation of expandable bubbles in the polymer melt saturated with a foaming agent. If this foaming agent is a physical foaming agent such as nitrogen or carbon dioxide, the dissolution of the gas in the polymer will occur under high pressure. A chemical foaming agent such as azodicarbonamide will release gas at a certain degradation temperature and the gas will then dissolve into the polymer. After the nucleation stage, the growth of the pores will continue until the foaming agent stabilises or the formed pore breaks down.³⁴

Figure 2.

Foaming stages of polymers with chemical blowing agents.

Formation of polymer/gas solution

As the homogeneity of the polymer/gas mixture significantly affects the final cell morphology and mechanical properties, the first step in foaming is to obtain a homogeneous mixture of polymer and foaming agent. The diffusion of gas in the foam formation mechanism is dependent on temperature variation. In addition, gas injected into the polymer matrix in excessive amounts can remain undissolved. Therefore, the amount of foaming agent injected must be below the solubility limit, which is directly affected by the process pressure and temperature, to ensure that the gas is completely mixed and dissolved in the polymer. When the solubility limit is exceeded and foaming agent is added to the polymer matrix in such a way as to create excessive saturation, uncontrolled large voids will form in the structure, so it is extremely important to determine the appropriate ratio of foaming agent for porous structures to be formed.³⁷ The solubility limit of the gas in polymer-based foam systems can be determined according to Henry’s law (equation (2)).³⁸

c_{s} = k_{D} P s = \frac{1}{H} P s

(2)

In equation (2), c_s is the solubility of the gas in the polymer (cm³/g or g_gaz/g_polimer), k_D (1/H) is the Henry solubility coefficient (cm³/g-Pa) and Ps is the partial pressure of the gas (Pa). k_D solubility coefficient can be calculated as a function of temperature by the formula in equation (3).³⁸

k_{D} = k_{D_{0}} \exp (- \frac{Δ H_{s}}{R T})

(3)

In equation (3), R is the gas constant (J/K), T is the temperature (K), k_D0 is the solubility factor (cm³/g-Pa) and ΔH_s is the molar heat adsorption (J). ΔH_s can be negative or positive depending on the polymer-gas system. Equations (2) and (3) can be used to determine the solubility of the foaming agent in the polymer matrix at a given temperature and pressure value.

Gas diffusion in the polymer matrix is a fundamental function of temperature and can be determined by equation (4).³⁸

D = D_{0} \exp (- \frac{E_{d}}{R T})

(4)

D₀ in equation (4) is the diffusion coefficient constant (cm²/s) and E_d is the activation energy (J) required for diffusion. Accordingly, the diffusion rate can be increased by processing the polymer/gas mixture at higher temperatures.

Cell nucleation

In order to initiate the process of gas bubble formation in the foaming processes of polymers, it is necessary that the gas dissolved in the polymer matrix reaches a state of saturation. At this time, thermodynamic instability resulting from a rapid decrease in the solubility of the gas with the effect of temperature and pressure triggers cell nucleation. There are two types of nucleation mechanisms in the foam formation: homogeneous nucleation and heterogeneous nucleation. In homogeneous nucleation, cells nucleate randomly within the polymer matrix, requiring a higher nucleation energy. In heterogeneous nucleation, there are different agents or additives within the matrix, allowing nucleation to occur more easily at the boundaries of these phases and requiring a lower energy.³⁹

According to the classical nucleation theory,^26,40 cellular bubbles larger than the critical radius (R_cr) grow spontaneously, while those smaller than R_cr collapse. R_cr can be determined by equation (5).

R_{c r} = \frac{2 λ_{\lg}}{P_{b u b, c r} - P_{s y s}}

(5)

λ_lg is the interfacial tension at the liquid-gas interface, P_bub,cr is the internal pressure of the bubble with critical radius and P_sys is the system pressure. The expression P_bub,cr-P_sys indicates the degree of saturation. Assuming that the polymer/gas mixture is a weak solution, the P_bub,cr term can be expressed by Henry’s law, in which case equation (6) is obtained. Here H is the Henry coefficient and C is the concentration of the gas dissolved in the polymer.

R_{c r} = \frac{2 λ_{\lg}}{H C - P_{s y s}}

(6)

Cell growth

After cell nucleation, since the pressure inside the cells is greater than the external pressure, the cells start to grow in a way to reduce the difference between internal and external pressure. Cell growth mechanisms depend on viscosity, diffusion coefficient, gas concentration and number of nucleated cells.^41,42 Temperature, which is one of the important parameters in the foaming process, has a direct effect on diffusion and melt viscosity. When the temperature decreases, the diffusion of the gas will decrease and the viscosity of the polymer matrix will increase, as a result, the growth rate of the cells will decrease. Therefore, it is important to keep the gas close to the polymer matrix in the foaming process in order to achieve good cell growth and higher volumetric expansion. Since the size of the cells in microporous foams is extremely small and their density is high, the cell walls separating the cells are smaller and the growth rate of the cells is higher compared to conventional foam materials. However, this can also lead to a disruption in cell density. As the nucleated cells grow, neighbouring cells will come into contact with each other and the cells will tend to merge as the total free energy decreases with the reduction in surface area caused by the merging of the cells.⁴²

Cell stability

The formation and growth of a large number of cells during the foaming process continuously increases the surface area and volume of the foam system, resulting in a thinning of the cell wall. As the foam system becomes unstable, the cells are usually stabilised by cooling or by the addition of surfactants.⁴³ As these fundamental mechanisms determine how cells nucleate, grow and stabilise within the polymer matrix, selecting the appropriate production method is crucial for reliably achieving the desired foam morphology and performance.

Production methods of polymer based foam composite materials

In the production of plastic foam materials, physical foaming agents (PFA), chemical foaming agents (CFA) and expandable microspheres are used to form gas bubbles in the polymer-based matrix. Although both methods perform similar processes, the difference between them is the source of gas production. Chemical foaming agents, which produce gas during foaming, are usually reaction products formed by decomposition as a result of a reaction. Physical foaming agents are gases or liquids that gasify a significant portion of the polymer matrix under foaming conditions. The amount and type of foaming agent directly affects the density of the finished product and cell formation.^13,31 The process-structure-property map was given in Table 5.

Table 5.

Process-structure-property map for different foaming routes (PFA, CFA, microspheres).

Foaming route	Solubility and diffusivity	P-T-t window	Nucleation behaviour	Cell growth and stability	Risk of collapse	Typical structure	Resulting properties
PFA (CO2)	High solubility (PLA > PA6 > PP), high diffusivity (rapid desorption)	High P (10–20 MPa), moderate T (80–150°C), short t	Mostly heterogeneous, strong supersaturation, high cell density	Fast growth; stability depends on viscosity and crystallinity	Medium-high (CO₂ diffuses quickly)	Very fine cells, high density	High stiffness; moderate strength; risk of shrinkage
PFA (N2)	Lower solubility than CO₂; fast diffusivity	Higher P required; wider T range	Lower nucleation rate; larger cells	Slower growth; generally more stable than CO₂	Low–medium	Larger, more uniform cells	Lower density reduction; moderate mechanical strength
CFA	No gas dissolution step; gas generated thermally	Controlled by decomposition T and kinetics	Broad, less controlled nucleation	Growth limited by melt strength	Medium	Larger cells; variability high	Moderate stiffness; lower homogeneity
Expandable microspheres	Gas encapsulated (independent of polymer solubility)	Low P, moderate T; microspheres expand sharply	No molecular nucleation, expansion-driven	Very stable; shell prevents collapse	Very low	Highly uniform, closed-cell	Very low density; good insulation; lower elastic modulus

Although PFAs are more advantageous compared to CFAs in terms of volumetric expansion rates in foam processes is taken into consideration, the ease of application and the ability to form thinner cell structures with the release of inorganic gases make the use of CFAs more preferable. In addition, the need for equipment that can provide high pressure in order to deliver the gas to the system in the processes where PFAs are used is one of the disadvantages that increase the process cost. When environmental emission restrictions are considered, the use of CFAs can be considered more environmentally friendly than PFAs. On the other hand, compared to fillers such as talc and calcite used as nucleating agents, CFAs leave no residue and can be added as nucleating agents to systems using PFA as foaming agent. Despite all the advantages of CFAs, which are easy to process and adapt to the device used, the factor that limits their use is the lack of a specific foaming agent that can produce inorganic gas with suitable solubility especially for foaming thermoplastic polymers.⁵ A a decision tree guided by matrix, target cell morphology (open/closed), mechanical requirements, and equipment constraints was presented in Figure 3.

Figure 3.

Concise decision tree for foaming-agent selection.

Physical foaming method

PFAs, which change phase during the process to form the cellular structure, are low boiling liquids or compressed gases that can turn into volatile gas. Liquids that can turn into volatile gas in the polymer vaporise with the decrease in temperature and pressure. The most common physical foaming agents are hydrocarbons, which can be divided into three general classes: aliphatic hydrocarbons (linear chains), chlorinated hydrocarbons, and chlorine-fluorine hydrocarbons. Aliphatic hydrocarbons are gases such as butane, heptane, hexane, propane, etc., which are easily transportable as they are liquids at room temperatures and have largely no harmful environmental effects.^34,44,45

Chlorinated hydrocarbons such as methyl chloride and methylene chloride, which can turn into volatile gas, are not preferred in foaming processes because they are harmful, toxic and dangerous for human health. Another group is chlorine-fluorine-hydrocarbons (CFCs), which provide great advantages in foaming processes since they have properties such as low diffusion rate and high expansion rate. However, CFCs are environmentally harmful materials as well as their advantages. As a result of the release of these materials in the atmosphere, it has been determined that they increase global warming and damage the ozone layer, and as a result, their use was banned in 1987 by the ‘Montreal Protocol’ and in 1992 by the ‘Copenhagen Amendments’.^16,44,46,47

The leading inert gases used as PFA are carbon dioxide, air, nitrogen, oxygen and helium. These gases dissolve in the polymer and form foam structures by releasing the pressure. Gaseous inert gases such as carbon dioxide and nitrogen can be considered as environmentally friendly since they do not damage the ozone layer, but their use may be limited since high-pressure feed systems are required for extrusion and injection processes.^44,48,49 Regardless of whether it is liquid or gas, there are some common features that are important and should be considered when selecting a PFA. PFAs should not harm the environment, be flammable or toxic. They should have sufficient solubility but not reactive. Low thermal conductivity and diffusion rate are other features they should have, while cost-effectiveness is another important consideration.⁵⁰

In the extrusion or injection processes, the gases are pumped directly into the polymer melt under pressure. In the extrusion or injection processes, the gases are pumped directly into the polymer melt under pressure. Large bubbles are converted into small bubbles by screws designed for these systems, and a homogeneous polymer melt and gas mixture is formed before the material reaches the mould outlet. At the mould exit, the high pressure drop causes the bubbles to nucleate and grow. In the injection moulding process, when the cooling stage is started after the mould is filled, the pressure inside the bubbles is balanced with the external pressure, cell stability is ensured and foamy structure formation takes place. The PFA to be used in the foaming process must be injected into the system at high pressure when the polymer melts. Depending on the shear force generated by the rotation of the screw, the gas and the polymer melt mix homogeneously, and the foaming process takes place during the passage of this homogeneous mixture. As a result of the pressure drop at the mould exit, the solubility of the gas in the polymer melt decreases, and in this case, the dissolved gas is separated from the polymer in the form of bubbles. At this stage, cell nucleation takes place with the formation of core areas in the polymer matrix. The formed nuclei continue to grow until the gas present in the mixture is exhausted.^51–53

One of the important points in the use of PFAs in the extrusion process is the application of high temperature from the feed hopper to the gas supply unit to complete the melting of the polymer matrix, while a decreasing temperature programme is applied until the extruder head. Since the viscosity of the PFA decreases when it is injected into the system, a low barrel temperature is preferable.⁴⁴ In the foaming process with PFA in the injection process, similar to the extrusion process, an additional feeding unit must be integrated into the injection moulding machine in order to inject the PFA into the system at high pressure. In this method, in order for foaming to occur, it is important not to completely fill the mould volume volumetrically and to leave a volume gap for pore formation. The polymer expanding due to the PFA would fill the mould volume and thus porous, low-density parts would be produced.^54,55

The process of forming microporous structures by injection method has been developed by Trexel company as ‘MuCell’ technology. In this method, PFA is made into a solution in the polymer, the resulting single-phase mixture is injected into the mould and a homogeneous micro foam structure is formed as a result of expansion.⁵⁵ The most important difference of MuCell technology from the conventional method is the formation and maintenance of a single-phase polymer-gas solution in the barrel and the control of temperature and pressure changes during the moulding phase in order to form the micropore structure.^56,57 In the PFA process, nucleating agents can also be used for better distribution of the gas in the polymer matrix. These nucleating agents can be fine minerals such as talc, silica, calcium carbonate, sodium, calcium or magnesium based carbonate structures, organic acids such as citric acid, tartaric acid, inorganic phosphates such as boric acid, and acidic phosphates.⁴⁴

Chemical foaming method

Chemical foaming agents are additives that degrade with the increase in temperature during the process and gas is produced with the decomposition of these additives. The thermal degradation process of CFAs starts with the increase in process temperature and complete degradation is realised with the temperature applied during injection and extrusion. As a result of this degradation, gaseous by-products such as nitrogen and carbon dioxide are formed, which create foam bubbles. Solid by-products also form, which act as nucleating agents and provide heterogeneous nucleation in the polymer melt.^44,58,59

In the extrusion and injection processes where chemical foaming can occur, the polymer/CFA mixture prepared with CFA mixed and the polymer matrix in appropriate ratios is fed into the system through the feed hopper. Keeping the pressure high at the part where the mixture starts to move in the screw is an important point to keep the gas that starts to form dissolved in the polymer melt, and in case of pressure drop, undesirable pre-foaming may occur. In addition, it is important to optimise the process temperatures in such a way that the degradation temperature of the CFA is achieved at least once but does not allow polymer degradation. Since the use of CFA does not require any additional equipment or the integration of any feeding unit into the system, foaming can be carried out with almost all standard extrusion and injection devices. Unlike the extrusion method, it is also important to optimise the mould parameters in the chemical foaming process with the injection process.

The polymer/CFA mixture advancing in the screw at the appropriate temperature and pressure fills the mould at a lower volume to allow foam formation, with as high an injection rate as possible to prevent premature expansion of the gas. A back pressure should then be applied to prevent the foaming process from starting prematurely and the nozzle should be kept closed. A holding pressure as low as possible should be applied to allow the dissolved gas to nucleate and the resulting cells to expand, and the mould should be opened slightly before cooling starts to allow volume expansion.⁵⁹ The process results in sandwich-shaped parts consisting of a central region where foaming takes place and an outer shell.⁵⁴ It is possible to produce structures with more complex geometries in the foaming process with the injection process compared to the foaming process with the extrusion process. While an increase in the flow rate of the material is achieved with the use of CFA in the injection process, a decrease of approximately 25% in the cycle time occurs due to the elimination of the application of the holding/grinding pressure applied in the standard injection process, in which some more molten material is injected into the mould to prevent the collapse of the material as a result of the volume decrease that may occur when the melt material fills the mould and starts to cool, and the shortening of the cooling time.⁴⁴

Glass bubble method

In addition to physical and chemical foaming agents, glass bubbles, which are a new technology and accepted as the third type of foaming agent, are structures consisting of a 0.5–1.5 μm thick thermoplastic shell and hydrocarbon fillers such as propane, butane, pentane, isobutane, isopentane with diameters generally ranging from 15 to 65 μm. The temperature increase in the process causes the hydrocarbon to expand and the pressure inside the sphere to increase, as a result, the thermoplastic shell softens and the volume of the sphere begins to increase. As a result of this expansion, the density of the polymer can be reduced up to 30%. During the expansion, the diameter of the sphere can increase up to 3–4 times and the shell thickness can decrease to 0.1 μm.⁶⁰ The type of thermoplastic used for the sphere’s shell affects the foam structure’s mechanical properties. The advantage of the glass bubble method over PFA and CFA is the ability to produce foam structures with a closed cell structure with a uniform and stable cell size. The disadvantages of the method are the necessity to adhere to the optimum temperature, since the application of a lower temperature than the temperature determined as optimum during the process may cause a lower foaming rate, and the application of a higher temperature may cause the foam cells to collapse, and the high cost of glass bubbles.⁶¹ The efficiency and stability of foam production are strongly influenced by the source and mode of gas generation within the polymer matrix. Therefore, it is essential to examine the characteristics and decomposition behaviour of chemical foaming agents, which are a major class of foaming material, in greater detail.

Industrial viability considerations

Cycle time and throughput

Concerning the cycle time and throughput of these materials, it was reported that in the case of the injection foam molding the effects of injection speed and core-back distance on microstructure and mechanical properties of PP/CFA foams, a “critical core-back distance” beyond which cell collapse occurs, implying limits for cycle time and reproducibility at industrial throughput.⁶² In addition, in the case of batch (Supercritical CO₂) foaming, according to a recent review on biodegradable polymer foaming, batch foaming via supercritical CO₂ typically takes several hours for saturation and expansion, making it less attractive for high-throughput industrial applications.⁶³

Dimensional stability and shrinkage

In the study realised by Xu et al. (2023), compounding with mixed gases (CO₂ + N₂) improves the dimensional stability of foams by reducing shrinkage after foaming. Furthermore, the slow diffusion of dissolved gas in certain matrices can lead to density variation over time; for example, the PBAT foam studied in that work showed gradual density increase during maturation.⁶⁴

Minimum foamable thickness

For extrusion-based supercritical-CO₂ foaming, the minimum density achievable is limited. In fact, some industrial extruded foams have densities > 30 kg/m³, unlike lab-scale TIPS foams which can go down to ∼10 kg/m³, limiting how thin industrial parts can be made.⁶⁵

Scrap/Rework pathways and robustness to scale-up

The review of biodegradable foaming strategies highlights that scale-up of sc-CO₂ foaming is often done via extrusion. However, limitations remain: for example, producing very low-density foams is more challenging in continuous processes, and block sizes remain constrained.^63,65 In injection-molded polymer composite foams (e.g., PP with talc/EPDM), Yetgin & Ünal (2022) demonstrated that process optimization (injection speed, feedback pressure, molding temperature) critically affects cell uniformity and mechanical properties. Such sensitivity may pose challenges for consistent scale-up and low scrap rates.^62,66

Based on the literature, each foaming method has trade-offs in terms of industrial viability. Injection-molded CFA foams offer relatively short cycle times and good dimensional control but require precise process optimization to avoid scrap. On the other hand, sc-CO₂ foaming (especially batch) offers lower density and excellent control of microstructure, but suffers from long cycle times, limited throughput, and scale-up challenges.

Chemical foaming agents

Chemical foaming agents (CFAs) are chemicals that can be in powder or masterbatch form and provide foaming through the formation of one or more gases as a result of thermal decomposition reactions. Both organic and inorganic chemicals can be used as CFAs.³¹ It is important that the gas formation takes place in a temperature range close to the processing temperatures of the polymers and that the degradation products are compatible with the polymer. Unlike PFAs, CFAs can be mixed with polymers at room temperature and no special equipment is required. Although CFAs are stable under normal storage conditions, they start to nucleate in response to temperature during the process. If the CFA generates heat during the decomposition to produce gas, it is exothermic and the decomposition continues spontaneously until the CFA is exhausted. In endothermic foamers, which require additional heat during the reaction, the reaction can be controlled more, the decomposition takes longer and a smaller cell structure can be formed compared to exothermic CFAs. Recently, mixtures of exothermic and endothermic CFAs can also be used in foaming processes. The most commonly used CFAs in foaming processes were given in Table 6.

Table 6.

The most commonly used chemical foaming agents.³¹

Chemical nomenclature	Abbreviation	Reaction type	Decomposition temperature (°C)	Gas content
Azodicarbonamide	ADC	Exothermic	200–220	N₂, CO, CO₂, NH₃
Modified ADC	—	Exothermic	155–220	N₂, CO, CO₂, NH₃
4, 4'- oxybis (benzenesulfonylhydrazide)	OBSH	Exothermic	140–165	N₂, H₂O
5-phenyltetrazole	5-PT	Exothermic	240–250	N₂
p-Toluenesulfonyl-semicarbazide	TSS	Exothermic	215–235	N₂, CO₂
p-Toluenesulfonyl-hydrazide	TSH	Exothermic	110–140	N₂, H₂O
Sodium bicarbonate	Bicab	Endothermic	120–150	CO₂, H₂O
Citric acid	—	Endothermic	90–120	CO₂, H₂O

Exothermic agents

Exothermic CFAs are types of CFAs that generate heat during their decomposition reactions and forming high volumes of nitrogen and/or ammonia gas. The decomposition reactions of exothermic CFAs were given in equation (7).⁵⁹ The most widely used exothermic CFAs are azo and diazo compounds, N-Nitrozo compounds, hydrazides, tetrazoles and carbazides.^44,68

CFA (exothermic) \overset{+ Δ E_{1}}{\to} N_{2} + Δ E_{2} + organic by ‐ products

(7)

Azodicarbonamide, a yellow-orange coloured powder with the structural formula H₂N-OC-N=N-CO-N₂H (C₂H₄O₂N₄), is the most widely used CFA with a usage rate of approximately 90% among all CFAs and is commonly referred to as ADC, AZ, ACZ or ABFA. Azodicarbonamide, which decomposes in the range of 200–210°C and has a gas yield of approximately 220 cm³/g, is quite harmless and its products separated from the system in foam formation are less dangerous. As a result of the decomposition of azodicarbonamide, 62% nitrogen (N₂), 35% carbon monoxide (CO), 3% CO₂ and ammonia (NH₃) are formed by volume with solid wastes.⁴⁴ ADC can be used with many polymers such as PVC, PE, PP, ABS.^67–69

Other important exothermic CFAs are OBSH, 5-PT and TSS.^47,70 OBSH is one of the most preferred CFAs for low temperature applications and is widely used for foaming low density polyethylene (LDPE), ethylene vinyl acetate (EVA), polyvinyl chloride (PVC), epoxies, phenols and especially rubbers.^71,72

With the increasing use of engineering plastics in recent years, the need for foaming agents with higher degradation temperatures suitable for the process temperatures of these materials has also increased. 5-PT, a white crystalline powder with a degradation temperature of about 250°C, is used with PC, ABS, thermoplastic polyester and other high temperature resistant plastics with its high degradation temperature.⁶⁷

Endothermic agents

The most commonly used endothermic chemical foaming agents, which require heat during decomposition reactions (equation (8)), are sodium carbonates and mixtures of bicarbonates and citric acid.^68,73

CFA (endothermic) \overset{+ Δ E}{\to} {CO}_{2} + H_{2} O + inorganic by ‐ products

(8)

Sodium bicarbonate (NaHCO₃) begins to decompose at around 160°C, whereas citric acid decomposes at approximately 210°C. By mixing these two components in different ratios, CFAs operating in different temperature ranges can be obtained.⁷³ Since one of the degradation products of such CFAs is water, if the polymer to be foamed is water or moisture sensitive (polycarbonates or polyesters), it is necessary to use CFAs that do not produce water in the environment. Equations (9) and (10) show the degradation reactions of NaHCO₃ and citric acid, respectively.

2 {NaHCO}_{3} \to {Na}_{2} {CO}_{3} + H_{2} O + {CO}_{2}

(9)

C_{6} H_{8} O_{7} \to C_{5} H_{6} O_{4} + H_{2} O + {CO}_{2}

(10)

The efficiency of chemical foaming agents depends strongly on the thermal stability, polarity and degradation behaviour of the polymer matrix. Therefore, it is essential to evaluate these agents in the context of specific polymers, which stand out as one of the most widely investigated systems for producing foam composites.

Polypropylene based foam composite materials

Polypropylene

Polypropylene is a thermoplastic material produced by polymerising propylene monomers to form very long polymer chains. Although there are several methods for the synthesis of PP from propylene monomers, the most widely used method is carried out with catalysts capable of producing crystallisable PP chains. The result is ‘isotactic’ semi-crystalline PP with enhanced physical, mechanical and thermal properties. Non-crystallisable ‘atactic’ PP, which is produced in smaller quantities as a by-product of semi-crystalline PP production and has extremely poor mechanical and thermal properties, is a soft and sticky material most commonly used in adhesives and sealing products.³¹

PP is a polymorphic material with at least three crystal forms, α, β, and γ. The commercially used PP grade crystallises primarily in the α-crystal form under normal thermal conditions. The β-crystal form, which has many properties such as improved impact strength compared to the α-crystal form, can be obtained by the use of nucleating agents.⁷⁴ Semi-crystalline PP is a thermoplastic product containing both crystalline and amorphous phases and the amounts of these phases depend on the structural and stereochemical properties of the polymer chains and the conditions of the processes such as injection, extrusion, thermoforming where the polymer matrix is transformed into the final product. PP is a polymer with highly developed physical, mechanical and thermal properties at room temperature and characterised by its low density, high hardness, high melting point and high impact strength. These general properties of PP can be modified by changing the chain regularity (tacticity), distribution or length, adding a co-monomer such as ethylene or an additive such as impact strength enhancer to the polymer chain.⁷⁴ Depending on the orientation of the methyl groups in the main chain, polypropylene can be isotactic, syndiotactic or atactic.⁷⁵ In isotactic polypropylene, which is the most widely used commercially, the methyl groups in the main chain are all in the same configuration and are located on the same side of the polymer chain, which provides PP with a high degree of crystallinity. In syndiotactic PP, the methyl groups are oppositely linked to the polymer chain, whereas in atactic PP they have a random arrangement. Most polymers are predominantly isotactic with a small amount of atactic content and the production of syndiotactic PP can be carried out commercially with metallocene catalysts.⁷⁶ Isotactic polypropylene (iPP) is the most widely used type of polypropylene (PP) in industrial applications due to the poor mechanical properties of atactic PP and the difficulties in producing syndiotactic PP.⁷⁷ Polypropylene, which contains only propylene monomer in semi-crystalline solid form and is largely isotactic in structure, is called homopolymer PP (HPP), while PP is called random copolymer (RCP) when PP chains contain a co-monomer such as ethylene in the range of approximately 1–8%. HPP containing a mixed RCP phase with 45–65% ethylene content is called impact copolymer (ICP). HPP is the most widely used type of PP commercially.⁷⁸

Considering the advantages and disadvantages of PP, which is one of the most widely used polymers in the polymer industry (Table 7), PP is used in many sectors such as automotive, packaging, construction, building, transportation, white goods, etc. because it is affordable and easy to process with many processes such as injection, extrusion, blow moulding and thermoforming.⁷⁶

Table 7.

Advantages and disadvantages of PP polymer.⁷⁹

Advantages	Disadvantages
Easy to process	UV radiation effects
Enhanced impact resistance	Flammability
High hardness	Difficulty of bonding
Suitability of homopolymer PP types for food contact	Poor impact strength at low temperature

Polypropylene based composite materials

Polypropylene is one of the most widely used commercial plastics in automotive, packaging and white goods applications with its low density, reasonable price, improved chemical and moisture resistance, physical, thermal, and mechanical properties.⁸⁰ However, poor impact toughness, especially at low temperature, limits the use of PP in applications where improved impact properties are required. For this purpose, the use of blends of PP with elastomers and olefinic copolymers has been one of the traditional solutions.⁸¹ The industrial production of composite materials using polymer blends is an important process for developing high-performance materials by altering the basic properties of existing polymers. In recent years, especially in the automotive sector, the use of PP has increased and it can be used in many parts such as bumper, dashboard, support parts, interior and exterior coatings. In order to further improve the properties of PP, the production of PP based composite materials and the investigation of their properties and uses has become an important field of study.⁷⁵ For this purpose, the blends of PP with thermoplastics, thermosets and elastomers have been an important subject that has been studied for years.

PP/Thermoplastic blends

PP is used with other thermoplastic materials to develop composite materials with the desired properties, by evaluating the advantages and advanced properties of the polymers in the blend. The literature contains examples of studies that have used different thermoplastic materials such as PP/PA, PP/PC, PP/PE, PP/polystyrene (PS), PP/poly(methyl methacrylate) (PMMA), PP/poly(ethylene terephthalate) (PET) in the literature.⁷⁵ In PP/PA blends, PP provides easy processability and moisture resistance to the composite material, while PA contributes to mechanical and thermal properties, improves paintability and barrier properties.⁸² In PP/PS blends, it was observed that the stiffness of PP increased and the toughness and chemical resistance of PS were improved.⁸³ In PP/PET blends, the lower gas permeability of PET and the lower water permeability of PP complement each other, and the combination of these two polymers provides resistance to solvents, chemicals and air, which is particularly important in packaging applications.⁸⁴

PP/Thermoset blends

In blends prepared with resins that can be used with PP, such as unsaturated polyester or epoxy, it has been found that crosslinking can occur in the molten PP phase during mixing by a dynamic vulcanisation process, and that these resins improve the properties of PP, especially when different compatibilisers are used.⁸⁵ Investigating the properties of blends of polypropylene (PP) and low-molecular-weight, crosslinkable, unsaturated polyester resin prepared with a peroxide-based radical initiator revealed that a homogeneous morphology containing crosslinked unsaturated polyester in a PP matrix with low viscosity was obtained. Since crosslinking in these blends affects the properties of the blend, process parameters such as mixing time and ratio have a direct effect on the efficiency of the process and the properties of the final blend.⁸⁵ In the study of PP/epoxy blends, it was found that blends containing cross-linked epoxy resin homogeneously dispersed in the PP matrix were obtained and a dynamic vulcanisation process was observed. The use of maleic anhydride grafted PP (PP-g-MA) as a compatibilizer in blends containing PP and epoxy resin was found to increase the compatibility between the two phases even further, and the particle distribution in the matrix was more homogeneous and had smaller areas.⁸⁶

PP/Thermoplastic elastomer blends

PP/thermoplastic elastomer (TPE) blends can be divided into two categories: thermoplastic olefins (TPO) and thermoplastic vulcanisates (TPV). While TPOs are generally non-vulcanisable materials, TPVs are materials in which the vulcanisation of the elastomer phase occurs in the melt phase during the mixing of plastic and rubber materials.⁷⁵ When EVA, ethylene propylene diene monomer (EPDM) and styrene butadiene styrene (SBS) elastomers were added to the PP matrix, it was observed that mechanical properties improved and impact strength values increased by 3–15%.⁸⁷ Compared to EPDM and SBS, EVA provides the mixture with a higher toughness value due to its superior interfacial compatibility with PP and greater flexibility. Improved properties were observed in mechanical analysis results such as maximum stress, elongation at break, modulus of elasticity and stress values in TPE composites prepared using compatibilizers such as sulphonated EPDM, maleicised EPDM, maleicised PP, acrylated PP in EPDM and PP blends.⁸⁸ Similar results were obtained for blends of chloroprene rubber (CR) with PP and polystyrene butadiene rubber (SBR), though it was found that the elongation at break values increased as the ratio of CR in the blend increased, and in SBR/PP blends, especially in the presence of maleicised PP, the impact strength and modulus of elasticity values increased significantly.^89,90

Polypropylene based foam composite materials

Thermoplastic-based foams have been widely used in many industrial sectors, such as automotive, transport, construction and furniture, due to their low density, improved strength-to-weight ratio, insulating properties and impact resistance. Consequently, studies on the foaming of polypropylene, a material that is also widely used in these sectors, have become an important area of research. PP based foams have higher strength than polyethylene based foams and better impact resistance than polystyrene based foams, while being more resistant to chemicals and solvents.^77,91 PP-based foam materials, which provide advantages in industrial applications due to their superior properties, have been a remarkable subject for the automotive industry, where studies on different material groups have gained importance, especially in terms of lightness, CO₂ emission limitations and fuel efficiency. However, as the linear hydrocarbon structure of PP results in low melt strength and low viscoelasticity, this structure is not very favourable for the production of a high performance foam material. During foaming, it is difficult to maintain the nucleated cells, and cell collapse and coalescence occur, resulting in foam structures with poor mechanical properties.^92–94 Many studies have been carried out on the development of composite structures incorporating different polymers and additives, with the aim of improving the mechanical properties of PP-based foams by optimising their cell structure.

One known method of increasing the melt strength of polypropylene (PP) is to increase its crosslinking tendency. It has been found that crosslinking PP resins increases volumetric expansion in foam structures, forming more regular void structures and significantly improving thermoformability.⁹⁵ When the foaming behaviour of the composite structure formed by cross-linking PP and trimethacrylate was studied, it was found that the density decreased, a uniform and homogeneous cell structure was obtained and elongation values increased.⁷⁷ However, since the cross-links formed prevent the final product from being recycled, the focus has been on the development of alternative composite structures.

Recently, significant progress has been made in the foaming of immiscible polyolefin blends. It has been found that the foamed composite material obtained by physically foaming the composite structure containing PP and high density polyethylene (HDPE) in the presence of CO₂ foams more easily than pure polymers, and this may be due to heterogeneous nucleation occurring at the interface of the two immiscible polymers.⁹⁵ While optical images showed that phase separation occurred in the blends due to the different crystal structures of pure HDPE and PP polymers, it was found that the low activation energies and weak interfacial interactions of HDPE/PP blends facilitated the initiation of nucleation and cell formation. As a result of optimising the process conditions, a temperature of 175°C and a time of 30 s were found to be the most suitable parameters. Blends with a high HDPE ratio were found to exhibit cell coalescence due to the softer matrix structure. In contrast, blends with 50:50 or 30:70 HDPE/PP ratios were found to produce more suitable foam structures due to the matrix’s increased hardness and viscosity. Impact performance is related to the cell size of the foam structure and the distribution of the foam, with higher impact performance being achieved at higher temperatures and times.⁹⁵ When the properties of the foam structure formed as a result of chemical foaming of PP/HDPE polymer blends in the presence of azodicarbonamide were studied, it was found that a homogeneously dispersed foam structure with extremely small dimensions was formed, which was related to the fact that a second polymer phase in the main matrix acted as a nucleation zone in the formation of the foam structure.⁹⁶ Density values decreased by 50%, tensile strength values decreased as the HDPE ratio in the blend increased, and tensile strength values of foam composite structures were found to be 59.7–67.1% lower compared to unfoamed blends. In general, the mechanical values of the foam structures of pure polymers were found to be higher, while the foam structures of polymer blends with different morphologies and densities exhibited more brittle behaviour, elongation at break and impact strength values decreased.⁹⁶

Examining the foaming behaviour of PP blends with TPOs revealed that adding TPO to the PP matrix did not significantly alter the morphological structure or interfacial tension. However, at high TPO ratios, slower cell growth rates and lower cell densities were observed due to the increased elasticity of the matrix. The highest polyolefin elastomer (POE) ratio at which cell integrity could be maintained was determined to be 30%. The mechanical behaviour of the foam structures was investigated, and it was found that the elastic modulus values of the foam structures prepared with the addition of POE to the PP matrix decreased significantly, while the elongation at break values increased significantly.⁷⁷

It has been reported that nanofoam structures of PP/polytetrafluoroethylene (PTFE) blends prepared by conventional injection and mould opening injection processes have significantly improved mechanical properties compared to foamed PP structures, and even strength and ductility values were found to be higher than unfoamed PP. The impact strength values of nanofoams with nanopore structure were found to be 700% higher than normal foam and 200% higher than unfoamed PP.⁹⁷

The chemical foaming agents sodium bicarbonate, citric acid and azodicarbonamide can be used to foam PP-based composite materials. The density values of foam composite materials obtained with different CBAs are close to each other, and it is stated that the rate of use of CBA and the cell structure formed are the determining parameters for density values and mechanical properties.^31,98,99

Polyamide 6 based foam composite materials

Polyamide 6

Polyamides (PA) are thermoplastics containing repeating amide groups in the main chain, widely used among engineering plastics with superior mechanical properties, thermal and chemical resistance.⁹⁹ PA is one of the first polycondensation polymers and can be produced by both condensation and ring-opening polymerisation. Polyamides, which can be made from monomers such as caprolactam, diamine, dicarboxylic acid, can have an aliphatic or aromatic structure. Aliphatic polyamides are generally hygroscopic and absorb moisture, and this property of PA can be reduced by adding an aromatic ring to the monomer structure. Water is an important impurity for PA polymers and problems are encountered in the production processes for PAs containing water in their structure. PA polymers are also known as nylon, the following number refers to the number of carbon atoms.¹⁰⁰ Polyamide 6 (PA6), also known as Nylon 6, is a type of polyamide containing six carbon atoms in the monomer structure, generally produced by ring-opening polymerisation from caprolactam.¹⁰¹

PA6, which has high impact resistance, hard and tough due to its molecular weight and structure, can be substituted for brass, bronze, steel with its robustness, advanced mechanical strength, fatigue and friction resistance, and can be preferred in many applications where high mechanical strength is required. At low and normal sliding speeds and in environments containing abrasives such as dust and sand, the working life of PA6 is 2-10 times higher than steel.¹⁰¹ The ambient temperature and humidity have a significant effect on the properties of PA6, therefore the mechanical properties of the final products are also affected by temperature and humidity parameters. PA6 exhibits a noticeable decrease in modulus of elasticity at higher temperatures. However, glass fibre-reinforced blends can improve its mechanical behaviour at these temperatures.¹⁰¹ PA6 has a high melting point compared to other polymers and shows melting behaviour at approximately 220–225°C and the glass transition temperature is known to be 57–60°C. With their high thermal resistance and thermal stability, they behave thermally resistant as well as mechanical properties. PA6, which has high resistance to alcohols, hydrocarbons, esters, ketones, oils and bases, has poor resistance to strong acids.¹⁰²

PA6, which can be both injection moulded and extruded, is widely used in many sectors such as automotive, textile, transportation, construction, white goods, electrical and electronics, where cost-effective, high-strength, rigid and stable properties are required and provides ease of processing. PA6, which can be both injection moulded and extruded, offers the opportunity to prepare products with excellent properties with its advantages when the moisture retention feature that may cause dimensional instability is taken into consideration and necessary precautions are taken.¹⁰² In the automotive industry, in moving parts such as gears, bushings, intake manifolds, cooling fans, turbo air ducts, oil pans, impact-absorbant engine covers and under-bonnet parts and door handles; in the white goods industry, in the control modules of products; photovoltaic connectors and circuit breaker pins in the electrical and electronics industry; in the production of many ropes, fibres and woven clothing in textile and medical textile applications, in surgical suture threads in medicine, in the production of strings for acoustic and traditional instruments such as guitar, violin, viola and cello in the music industry.^103–105

Polyamide 6 based composite materials

PA6 is widely used in various engineering applications due to its superior mechanical and thermal properties, high impact strength and machinability. However, its use is limited in certain areas due to its sensitivity to moisture and notch sensitivity, as well as its hygroscopicity.^30,106,107 In cases where the properties of polymers alone are not sufficient, the development of composite materials is a widely used method to improve the desired properties by producing polymer blends. In order to overcome the disadvantages of PA6, PA6-based composite materials prepared with other polymers were produced and their properties were investigated. For this purpose, blends with various polymers such as PP, PC, ABS, EPDM were prepared.^108,109 PA6 based composites can be analysed under three subheadings.

PA6/Polyoleolefin blends

PA6/polyolefin blends are prepared to reduce the moisture retention of PA6, reduce costs, and improve processability. However, there is a reduction in the blends’ mechanical properties because PA6 and polyolefins are incompatible polymers that do not mix with each other. To harmonise PA6 with polyolefins, minimise loss of mechanical properties and improve the desired properties, studies have been carried out on the use of appropriate types and ratios of compatibilising agents. Polyolefins blended with PA6 include PP, PE, HDPE, LDPE, and linear low density polyethylene (LLDPE).³²

In PA6/PP blends, maleic anhydride (MA)-grafted PP, styrene-ethylene-butylene-styrene (SEBS) and ethylene-propylene rubber (EPR) are commonly used as compatibilisers, with varying results depending on the type and rate of use of the compatibiliser. In general, the use of compatibilisers increased the elasticity modulus and tensile strength values, while notable improvements in impact strength values were also observed with rubber-based compatibilisers.^109–111 In PA6/PE blends, MA-grafted PE (PE-g-MA), acetic acid-grafted PE (PE-g-AcA), ethyl methacrylate-glycidyl methacrylate (EMA-GMA), MA-grafted EPDM (EPDM-g-MA), acetic acid-grafted LDPE (LDPE-g-AcA) are used as compatibilisers. For PA6/LDPE blends, the network structure formed by LDPE-g-AcA in the matrix provides higher mechanical properties, while for PA6/HDPE blends, PE-g-MA and EMA-GMA are considered the best compatibilisers with improved toughness properties.^112,113 The use of LLDPE-g-MA was also found to improve the impact and tensile strength properties of PA6/LLDPE blends.^114,115

PA6/Ethylene based thermoplastic and elastomer blends

Ethylene-based thermoplastic polymers that can be prepared by blending with PA6 include ethylene vinyl alcohol (EVOH) and ethylene acrylic acid (EAA), while ethylene-based elastomers include EVA, ethylene butylene acrylate (EBA) and ethylene propylene rubber (EPR). When analysing the properties of PA6/EVOH blends prepared in different ratios, it was found that the 50:50 blends had the highest tensile strength value, which was related to the effect of the interactions between the hydroxyl groups of both polymers on the mechanical properties.¹¹⁵ It has been stated that acetic acid groups act as a compatibilising agent in PA6/EAA blends, and mechanical values also improve with increasing EAA content.¹¹⁶ Studies were carried out with the addition of MA-grafted EVA (EVA-g-MA) due to the incompatibility between polymers in PA6/EVA blends. It was determined that the mechanical values improved with the use of MA in blends prepared with MA at weight ratios of 0–6%, and that the most suitable compatibilising agent ratio was 2 wt.%.¹¹⁷ It has also been reported that tensile strength and crack propagation resistance are improved by using MA grafted EBA (EBA-g-MA) in PA6/EBA blends.^118,119 As with other ethylene-based elastomers, the use of a compatibiliser has improved the mechanical properties of PA6/EPR blends.^120,121

PA6/Elastomer blends

Studies were carried out by preparing blends using SEBS, TPV and polysulfone (PSU) in order to contribute the high toughness and high elongation at break properties of elastomers to PA6 polymer and especially to improve low temperature toughness behaviour.³² It has been reported that the impact strength properties of PA6/SEBS blends are remarkably improved, especially with the use of a secondary SEBS-g-MA phase grafted with MA.¹²¹ Similarly, it was stated that improved toughness properties were obtained with the use of MA in PA6/PSU and PA6/TPV blends.^122,123

Polyamide 6 based foam composite materials

PA6 foams are an interesting class of materials that have been used in many different fields in recent years due to their superior properties, combining both the advanced properties of PA6 and the advantages of foam structures. By successfully foaming PA6 polymer, specific properties such as higher impact resistance, toughness, sound and thermal insulation, energy adsorption can be obtained while maintaining the mechanical properties of the material.^124–127 However, in the foaming process, the melt strength and viscosity of PA6 may remain low to maintain the cell morphologies formed, which may cause the cells to collapse or rupture. Therefore, obtaining and commercialising PA6 foams with high performance and ideal structure has become an issue that needs to be studied in detail. The most commonly used chemical blowing agents to obtain PA6-based foam structures are those containing azodicarbonamide, sodium bicarbonate and citric acid. To improve the foaming behaviour of PA6, blends with epoxy resins have been successfully used to improve both the mechanical properties and the cell structure. An increase in melt strength has been observed due to the formation of cross-links.¹²⁷

In the studies carried out to increase the melt strength and viscosity of PA6, it has been observed that PA6 modified with chain extenders also gains a certain branching ability and reduces the breakage and coalescence of the cells formed during the foaming process.¹²⁸ By investigating the foaming behaviour and properties of the composite structures prepared by using fillers together with PA6 in the preparation of PA6 based foams, it was found that the mechanical and thermal properties of the structure as well as the foaming behaviour of PA6 were improved by the use of fillers. The most commonly used fillers in PA6 based foam composite structures are glass fibre (GF) and organically modified montmorillonite (OMMT).¹²⁸ It was found that while a decrease of up to 15% in the density value was observed in the foam structures produced by adding glass fibres to the PA6 matrix and using glass beads, an increase of 10% in the modulus of elasticity and tensile strength values was obtained.^129–131 In foam structures containing OMMT, it was determined that there was an improvement in thermal properties as well as mechanical properties.¹³¹ As the filler used in the matrix plays an active role in nucleation, the formation of crystalline structures proceeds in a correspondingly regular manner and the crystalline structure of the foams increases. In the study it was observed that the crystallinity of the PA6 based foam structure containing 33 wt.% OMMT increased from 24% to 61.5%. In addition, the T₅ and T_max values for the maximum degradation temperature at 5% mass loss in the blend increased by 3.5°C and 57.2°C, respectively. As the OMMT content in the mixture increased above 50 wt.%, the Tmax value started to decrease and it was found that the reason for this behavior was related to the decrease in the matrix’s crystallinity when OMMT became the dominant phase.¹³¹

Polylactic acid based foam composite materials

Polylactic acid

PLA, also known as polylactic acid or polylactide, is a biodegradable and bio-based aliphatic polyester, a biopolymer produced from 100% renewable sources rich in starch such as corn sugar, potato and sugar cane.¹³² Initially, its use was limited to biomedical applications as a biopolymer. However, as environmental sensitivity has increased over time, along with the introduction of sanctions for producers and consumers, research into the use of PLA in different areas has expanded. As a result, PLA has begun to be used in various sectors, as a better understanding of its properties has emerged.^27,28 High molecular weight PLA is commonly produced by polycondensation and/or ring-opening polymerisation. NatureWorks, one of the largest suppliers of PLA, produces approximately 150,000 tonnes of PLA per year.¹³³ Lactic acid (LA), also known as 2-hydroxy propionic acid, is the starting monomer of PLA and has two stereoisomers, L-LA and D-LA. LA can be used to produce PLA structures with different molecular weights, but since only high-molecular-weight PLA molecules are suitable for industrial use, process studies have been developed to produce high-molecular-weight PLA for this purpose. Commercial PLA is a copolymer of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) rather than a linear homopolymer. Like other known petroleum-based polymers, PLA can be processed using extrusion, injection moulding, melt processing, compression moulding, blow moulding, thermoforming, and film production. The limitations of these different processing methods are the degradation of the PLA structure at high processing temperatures and the difficulty in obtaining a homogeneous structure at low processing temperatures. However, if the rheological and viscoelastic properties of PLA are well understood, the process parameters can be optimised accordingly.¹³⁴ Looking at the properties of PLA, it has advantageous mechanical and physical properties such as high modulus, high strength, improved gas permeability and easy processability. The tensile and flexural modulus values of PLA are higher than those of high density PE, PP and PS. Because of these properties, PLA is being considered as an alternative biopolymer to widely used petroleum-based polymers, and PLA is being considered as an alternative that can be substituted.^135,136 In addition to these advantages, PLA also has several disadvantages including low toughness, low impact strength, low crystallisation rate, low thermal resistance, which limit its use in different sectors.^137,138

Polylactic acid based composite materials

Research has been undertaken to address the limitations of PLA, which are due to its superior mechanical properties and its status as a bio-based, environmentally friendly biopolymer. In this regard, it has been observed that PLA can be further enhanced in its properties by blending with other bio-based biopolymers, petroleum-based polymers or elastomers, depending on the desired application.^133,139,140

In recent years, studies on PLA-based composite blends, PCL, starch, lignin, PHA, PBS, poly(3-hydroxybutyrate) (PHB), poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate-co-adipate) (PBSA), polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH), EVOH, poly(propylene carbonate) (PPC) and poly(ethylene succinate) (PES) were among the bio-based biodegradable biopolymers in the relevant studies.¹³⁵ Different biopolymers can impart different properties to the composite, such as improving toughness, tensile strength, elongation at break and crystallisation rate in the desired direction.

Even though most biopolymers are compatible with each other, various compatibilizers, catalysts and binders can be used in composite structures to increase interphase compatibility, improve dispersion, extend chain structures and increase strength. Literature studies have shown that both synthetic petroleum-based polymers and biopolymers can be used to improve the properties of PLA and overcome its shortcomings. To this respect, polymers and elastomers such as PP, PA, PE, PS, ABS, PU, PC, PMMA, PET, polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT) and rubber have been included in recent studies. As with biopolymers, it is possible to improve the properties of synthetic polymers by creating composites that match the desired properties. In synthetic polymers, as in biopolymers, phase compatibility in the composite structure can be improved by the use of additives such as compatibilisers and chain extenders, and more homogeneous and resistant structures can be obtained. A review of the literature shows that there are also studies in which PLA-based composites are reinforced with fillers that affect the properties of the structure, and that the effects of industrial and natural additives on the matrix phase at both the nano and micro scales are great. In such systems, it is seen that not only the final morphology of the blend, but also the polymer-polymer and polymer-filler/additive interface properties have a great influence on the final properties of the product, and the localisation of fillers or additives can be altered as a result of interactions at the interfaces.^141–143 Therefore, the type, form, utilisation rate, compatibility of the selected filler or additive material, how it will affect the properties of the composite system and the specific physico-chemical interactions that may occur should be well determined.^144,145 The main fillers used on an industrial scale are talc, calcium carbonate, glass fibre, carbon fibre, titanium dioxide, wollastonite, mica and kaolin.^145–149 These reinforcements can be used not only to reduce costs, but also to provide many important properties to the system to which they are added, depending on whether they are inactive or active. The main purposes of using fillers in composite structures can be listed as increasing heat resistance, increasing rigidity, improving impact resistance, improving dimensional stability, improving thermal conductivity, increasing electrical conductivity. In the selection of fillers, important issues such as particle size, dispersion, dispersion properties, chemical structure, surface structure and size, abrasion risk, density, surface hardness, and price should be considered.^150,151 In recent years, environmentalist approaches have led to the widespread use of natural fibres, and studies on this subject have gained weight in various sectors due to the fibres’ lightness, low cost, high strength and hardness, easy accessibility, and environmental friendliness.¹⁵⁰

Polylactic acid based foam composite materials

For the development of PLA-based foam structures, it was observed that the preparation of PLA-based blends with the use of chain extenders and compatibilisers can be important to improve the low melt strength, slow crystallisation rate, and associated poor foaming properties of PLA. On the other hand, the addition of inorganic fillers to the structure was found to improve the slow crystallisation kinetics, low melt strength and foaming behaviour of PLA.^152,153 The design of PLA foam systems has become an important issue, especially in the automotive sector, where lightweight structures are becoming increasingly important. To this end, studies have been conducted to produce foam systems from composite structures of PLA, incorporating both biopolymers and synthetic polymers. These studies aim to enhance the weak properties of PLA through composite structures, while also achieving lightness by preparing foam structures, reducing costs, and lowering CO₂ emissions by using fewer raw materials. For this purpose, the foaming behaviour and properties of foam structures made of starch, PBAT, PCL, PVOH, and polymers such as polyhydroxybutyrate-valerate (PHBV), PMMA, PC, PP, PE, and PS were investigated.¹³ When the foam structures of PLA/starch blends prepared by different methods were investigated, it was found that water and CO₂ were used as foaming agents, while an improvement in the tensile strength and ductility values of the blend was observed, low density PLA/starch foams with open cell structure were generally obtained, and it was found that interfacial modification of PLA was necessary to have less open cell content in order to further improve the properties.^154–159 In PLA/PBAT blends physically foamed with CO₂, N₂ and chemically foamed with azodyl carbonamide, it was found that the use of PBAT improved the low melt strength and low elasticity of PLA, cell nucleation was further accelerated by the use of additives such as talc and nanoclay in the blends, while it was observed that the cell density of the foamed structure increased when a compatibiliser was added to the structure. The cell density of PLA/PBAT blends containing the compatibiliser was 2 ay10⁷ cells/cm³, while this value was determined to be 2 de10⁶ cells/cm³ in PLA/PBAT blends without compatibiliser and 2 in10⁵ cells/cm³ in PLA polymer. On the other hand, in the differential scanning calorimetry (DSC) analyses of PLA/PBAT blends without the addition of the compatibiliser, two melting peaks were observed at 127°C and 168°C, while only one melting peak was observed at 151°C after the addition of the compatibiliser.^158,159 The study of the foaming behaviour and properties of PVOH/PHBV blends prepared with PLA showed that process parameters and mixing speed have an effect on the size of the dispersing phase in the matrix, with finer dispersing phase improving the foam morphology and increasing cell densities with increasing mixing speed.^160–162 According to the results of the thermal analyses of the foam structures obtained by physical foaming of PLA/PHBV blends with N₂, it was observed that there was no significant change in the thermal properties of the foamed and non-foamed blends, and a decrease in the glass transition temperature (Tg) value of the PLA polymer was observed with increasing PHBV content in both structures. In addition, the crystallinity of PHBV increased in blends with PHBV content above 45%, which was related to the behaviour of PLA in the blend as a nucleating agent in the PHBV phase. While pure PLA and PHBV polymers showed a single stage thermal degradation behaviour at 320°C and 260°C, PLA/PHBV blends showed a multiple degradation behaviour between these two temperatures, with a lower thermal stability than PLA but higher than PHBV. It was noticed that blends with a PHBV ratio above 30 wt.% were immiscible, whereas a more homogeneous structure could be achieved with lower ratios. As a result of this situation, it was observed that increasing the PHBV ratio caused a decrease in tensile strength values, and when the PHBV ratio increased from 5 wt.% to 45 wt.%, the tensile strength value decreased from 54.99 MPa to 42.23 MPa, and it was stated that this decrease was due to the weak interfacial interactions between PLA and PHBV polymers.¹⁶⁰ The ductility values of the structures obtained by physical foaming of PLA/PCL blends with CO₂ and N₂ have been observed to improve in general, PLA/PE, PLA/PS, PLA/PET blends prepared by using synthetic polymers together with compatibilizers, and foam structures prepared mainly by using physical foaming agents have improved the toughness and strength properties of PLA, while increasing the fatigue resistance.¹⁶¹ When the studies are examined in general it can be noted that there are foaming studies of PLA with both bio-based and synthetic polymers, but in these studies the foaming processes are mainly carried out with physical foaming agents and the compatibilising agents used to increase the interfacial interactions between polymers and to obtain more compatible structures have a positive effect on the foaming performance and improve the mechanical properties of the foam structures.³¹

Polylactic acid/Polypropylene based foam composite materials

The preparation of blends of PP, which has low density, high fatigue resistance, impact strength and improved chemical resistance, with high strength PLA has shown that immiscible structures are formed due to the high polarity difference between the polymers. This indicates the need for compatibilising agents to increase the interfacial interactions of the prepared blends and improve their mechanical properties.^163,164 For this purpose, compatibilizers such as PP-g-MA,¹⁶³ SEBS-g-MA,¹⁶⁴ EBA-GMA,¹⁶⁵ EMA-GMA¹⁶⁶ were used and it was found that the mechanical properties and impact strength of the blends were improved, the compatibilizer helped to form a continuous phase for the transfer of the applied stress by forming stress transfer bridges between the two phases and thus improved the mechanical properties by reducing the interfacial tension and increasing the interaction. The study, where 1 to 5 wt.% PP-gMA was added to 70:30, 80:20 and 90:10 PLA/PP blends, found that 90:10 polymer and 5 wt.% compatibilizer were the most suitable formulation proportions, and the highest tensile strength value of 50.06 MPa was obtained for these proportions by mechanical analysis.¹⁶³ In a comparative study between the use of SEBS-g-MA and PP-g-MA as a compatibiliser, it was found that the interaction between the polymer and the compatibiliser was stronger due to the fact that the degree of branching of PP-g-MA was 1.01% while the degree of branching of SEBS-g-MA was 1.60%. While the tensile strength values of PLA/PP blends using PP-g-MA were in the range of 18–20 MPa, this value could be increased to 32–35 MPa with the use of 2 wt.% SEBS-g-MA.¹⁶⁴ In the research carried out on the use of EBA-GMA in varying ratios in PLA/PP blends, it was observed that the tensile strength was the highest with a value of 51 MPa and the use of the compatibilising agent was determined as 2.5 wt.%. Furthermore, at higher ratios, the tensile strength values decreased to 40 MPa due to EBA-GMA which is a softer material compared to PLA and PP. When the impact strength values were examined, it was observed that while the value of PLA/PP blends without compatibilising agent was 4.3 kJ/m², this value could increase up to 4.3 kJ/m² with the use of EBA-GMA.¹⁶⁵ Elongation at break values were found to increase in the range of 200–300% with the use of 3% EMA-GMA in PLA/PP blends, and the transfer of applied stress is enhanced by increasing interphase compatibility. Impact strength values increased from a mean value of 4.1 kJ/m² to a mean value of 6.8 kJ/m² with the use of EMA-GMA.¹⁶⁶

Few studies have been carried out in the literature to investigate the properties of foam structures of PLA/PP and PLA/PP blends whose properties are further improved with a compatibiliser. In one such study, blends with PP-g-MA were prepared to improve the foaming behaviour of PLA, which is weak due to its low melt strength. The morphology of the foam structures obtained using CO₂ as a physical foaming agent was then studied. It was found that the compatible interface between PP-g-MA and PLA increased the nucleation tendency, resulting in a higher cell density.¹⁶⁷ The study revealed that the thermal properties of the PLA polymer are affected when it interacts with MA, and this situation is effective on the foaming process. In general, it is known that the crystallisation behaviour has an effect on the expansion rate and the preservation of the cell integrity in the foaming process. In particular, in the physical CO₂ foaming process, PCA will significantly increase the crystallisation rate of PLA, and since the foaming temperature is generally close to the crystallisation temperature, changes in crystallisation behaviour will directly affect the viscosity of PLA during the foaming process and, as a result, the foaming behaviour of PLA will be affected. The results of the DSC analysis demonstrate that the Tg value of the PLA polymer (60°C) is not evidently influenced by the incorporation of MA. The crystallisation temperature (Tc) value of pure PLA, which is 125°C, decreased to 107°C with the addition of MA. It is observed that the enthalpy of crystallisation value increases with the addition of 1 wt.% MA, but it begins to decrease at higher MA ratios. It has been posited that this phenomenon is associated with the process of branched polymer formation. The existence of branching points has been demonstrated to result in a reduction in the activation energy required for crystal nucleation. Consequently, cold crystallisation occurs at comparatively low temperatures. While pure PLA produced a single melting peak at around 152°C, the addition of MA resulted in two melting peaks: one at a higher temperature (154–155°C) and one at a lower temperature (149–150°C). Furthermore, an observation was made that the melting point at higher temperatures tended to increase gradually with increasing MA content. The results of X-ray diffraction (XRD) analyses, which were conducted to investigate the melting behaviour that gave double melting point, indicated a correlation with the crystal structure of PLA. It has been hypothesized that the two-point melting behaviour of PLA is attributable to the fact that smaller and more irregular crystals are not afforded sufficient time to form crystals of higher structural perfection within the heating and recrystallisation mechanism. Consequently, the crystals form successively and exhibit re-melting behaviour at relatively lower temperatures. In addition to the crystallization temperature, the degree of crystallinity of the MA-added PLA blends was also reported to have a significant effect in controlling the cellular morphology of the foams. A higher crystallization rate and a higher degree of crystallinity will facilitate the rapid solidification of cells during foaming processes. The addition of MA to the structure is known to affect the degree of crystallinity of PLA. Indeed, it was observed that the general degree of crystallinity increased from 5.05% to 6.89% as the amount of MA added to the PLA structure increased. Consequently, it was asserted that the foaming capacity of MA-added PLA blends exhibited an enhancement.¹⁶⁷ In a separate study, n-pentane was utilised as a physical foaming agent, with the foaming behaviour of PLA/PP foam structures obtained by autoclave process being the focus of investigation.¹⁶⁸ The study observed that foam structures with high cell density and regular cell structures were obtained by physical foaming, and the high gas solubility of PLA due to its low crystallinity increased the foaming performance of PLA/PP blends. In the present study, the addition of 1 wt.% talc was utilised as a nucleating agent to the PLA/PP blends, which were prepared using various proportions of PLA, ranging from 5 wt.% to 30 wt.%. An investigation was conducted into the impact of foaming on the melt behaviour of PP and PP/PLA blends. The findings revealed that the melting temperature of pure PP polymer remained largely unaltered following the foaming process. However, a modest increase in the melting enthalpy was observed, with a rise from 67.5 J/g to 70.5 J/g. The addition of PLA to PP polymer was found to result in a decrease in melting enthalpy values, attributable to the reduced crystallisation rate of PLA. A study of the crystallisation enthalpies revealed that the value of the enthalpy of crystallisation of PLA increased from 4.4 J/g to 27.5 J/g. This finding suggests that the foaming process facilitated the development of crystallinity in PLA.¹⁶⁸ A further study investigated, the biodegradability of the blends was investigated by preparing foam structures of PLA and PP and PP blends with high melt strength using CO₂ as a physical foaming agent.¹⁶⁹ Thermal gravimetric analyses (TGA) of the mixtures prepared using PLA at ratios of 10, 12, 30 and 50 wt.% demonstrated that the degradation temperatures remained largely unaltered with varying PLA ratios. In a study in which blends obtained by using PLA and PP were foamed by using a chemical foaming agent, PLA/PP blends with different ratios were studied, but it was determined that a more effective foaming was obtained in the PLA (30 wt.%)/PP (70 wt.%) mixture in which PP was used at a higher ratio and the most suitable CFA ratio was 1.5 wt.%.¹⁷⁰ In order to enhance the compatibility of polymers in foam structures with identical proportions, compatibilisation with PLA-g-MA was conducted, and it was ascertained that the incorporation of a 1 wt.% compatibilising agent resulted in an improvement of the properties.¹⁷¹

Polylactic acid/Polyamide 6 based foam composite materials

PLA/PA6 blends are of interest due to their high tensile strength, impact strength, stiffness, and gas barrier properties. They are more compatible than PLA/PP blends due to hydrogen bonding interactions between the polymers. Polymer blends prepared with only PLA and PA6 polymers exhibit high ductility and strength as a result of high interfacial interactions.¹⁷² The utilisation of chain extenders and compatibilisers has been demonstrated to enhance the properties of blends, thereby fortifying the blend’s structure and optimising its characteristics, including elongation at break, impact, and tensile strength.^173,174 Multifunctional epoxide (MCE) and polycarbodimide (PCD) are the most commonly used chain extenders, while polyethylene-octene elastomer (POE), maleic anhydride grafted POE (POE-g-MA) and thermoplastic polyurethane (TPU) are the preferred compatibilising agents to further increase interfacial compatibility.^173,175,176

In a study, PLA/PA6 blends were physically foamed with CO₂ foaming agent, with microfibril structures prepared using PA6 at ratios of 3, 7, 15 and 25 wt.%. It was observed that the addition of PA6 had a positive effect on the foaming behaviour of the PLA polymer in the PLA matrix. With low PA6 content, the collapse of the cells decreased and the formation of foams with uniform structures could be achieved.¹⁷⁷ Although the crystallization kinetics of PLA were found to improve with the addition of PA6 polymer, the crystallinity values for these blends were generally lower in comparison to those of pure PLA. It has been demonstrated that the crystallinity of PLA diminished from 34.8% to 28.2–24.9% with the incorporation of PA6. It has been hypothesized that this phenomenon is attributable to the restricted mobility of PLA molecules, consequent to the formation of a substantial number of crystals within the structure. The observation that pure PLA exhibits a lower propensity for crystal formation, attributable to the absence of a heterogeneous nucleation effect, is posited as a contributing factor to this process, as it engenders a reduced number of physical obstacles that impede crystal growth.¹⁷⁷ In a separate study, PLA/PA6 blends were foamed using a chemical foaming agent, and it was determined that more uniform foam structures were obtained in the PLA (30 wt.%)/PA6 (70 wt.%) mixture. The most suitable CFA ratio was determined to be 1.5 wt.%.¹⁷⁰ To improve the compatibility between polymers, different ratios of PLA-g-MA were added to the foam structures at the same ratios and it was determined that the use of 3 wt.% compatibilising agent improved the properties in the most optimum way.¹⁷¹

Discussion

A comparative evaluation of PP-, PA6- and PLA-based foam composite systems shows that the behaviour of foaming is governed by the complex interplay of melt rheology, crystallinity, gas diffusion kinetics and interfacial interactions. While all three polymers can be processed into cellular structures, they each have different advantages and limitations that directly affect the resulting morphology and performance. The relative density, cell size, cell density, open/closed cell fraction, and specific mechanical properties for the studied materials under PFA (CO₂/N₂), CFA, and expandable microsphere foaming were compiled in Table 8.

Table 8.

Summary of reported ranges for cell morphology and mechanical properties of PP, PA6, PLA and their blends under different foaming methods.

Material/Blend	Foaming method	(ρ/ρ₀)	Cell size (µm)	Cell ρ (cells/cm³)	Open/Closed cell fraction (%)	Elastic modulus E (MPa)	Yield strength σ_y (MPa)	Impact strength (kJ/m²)	Reference
PP	PFA CO₂	0.04–0.86	1–50	10⁶–10⁸	∼100% closed	50–500	1–20	0.5–10	¹⁷⁸
PP	CFA	0.05–0.8	2–60	10⁶–10⁸	90–100 closed	40–400	1–18	0.5–9	^179,180
PLA	PFA CO₂	0.1–0.5	5–100	10⁴–10⁷	60–80 open	50–300	2–15	1–8	^24,25
PLA	Expandable microspheres	0.15–0.6	10–120	10⁴–10⁶	60–80 open	60–250	3–12	1–7	^181,182
PA6	Core-back injection	—	∼83–60	1.1 .110⁵–5.9 .910⁵	—	—	—	—	^22,23
PLA/PP	PFA CO₂	0.1–0.6	10–80	10⁴–10⁷	50–80 open/closed	40–250	2–12	1–6	^183,184

PP foams benefit from favourable melt elasticity and moderate crystallinity, which promote stable cell nucleation during physical and chemical foaming processes. However, their relatively low melt strength can lead to cell coalescence at high expansion ratios. PA6, on the other hand, has superior thermal stability and mechanical strength; however, its hygroscopic nature and narrow processing window often make it difficult to control cell growth and uniformity. PLA is an attractive sustainable alternative as it is biodegradable and renewable; however, its inherently low melt strength and brittle nature often limit foam expansion and structural homogeneity. These observations highlight the need for modifications, such as the use of chain extenders, branching agents or plasticisers, to improve PLA’s foaming capability.

The immiscible nature of hybrid foams based on PLA/PP and PLA/PA6 introduces further complexity. The interfacial tension between the polymer phases directly affects gas distribution and cell stabilisation during foaming. While previous studies have demonstrated that hybridisation can produce synergistic effects, combining the rigidity of PLA with the toughness of PP or PA6, the ultimate performance depends heavily on compatibilisation efficiency. Using maleic anhydride-based compatibilisers, reactive blending or nanofiller-assisted interfacial engineering has been reported to improve morphology and mechanical integrity. However, there have been few systematic studies connecting compatibilisation strategies with foam morphology. In particular, the influence of interfacial adhesion on cell nucleation density, gas diffusion pathways and collapse resistance has not yet been addressed comprehensively.

A gap that has been identified in the literature is the scarcity of standardised foaming methodologies that can be applied to both biopolymers and conventional thermoplastics. Differences in rheological behaviour, crystallisation kinetics and thermal degradation profiles often necessitate polymer-specific foaming parameters, complicating direct comparisons and industrial scalability. The growing demand for lightweight, sustainable materials necessitates the development of unified processing frameworks incorporating experimental data, kinetic modelling, and machine learning-assisted optimisation.

The collective findings from the literature indicate that polymer-based foam composite systems have significant potential for advanced engineering applications. However, realising this potential will require a deeper understanding of the underlying mechanisms and improved interfacial design. Hybrid foams, in particular, are a promising way to achieve sustainability and performance, but their behaviour is not fully understood. Closing these knowledge gaps will support material innovation and accelerate the transition towards environmentally responsible polymer technologies.

Conclusion

Polymer-based foam composite materials continue to attract increasing attention due to their advantageous properties, including low density, high specific strength and excellent thermal and acoustic insulation capabilities. This review comparatively examined the structural characteristics, processing routes and foam performance of petrochemical-based polymers (polypropylene (PP) and polyamide 6 (PA6)) and biodegradable polymers (polylactic acid (PLA)), as well as hybrid systems formed by PLA/PP and PLA/PA6 blends. PLA emerges as a sustainable alternative thanks to its bio-based origin and minimal environmental impact, while PP and PA6 offer significant advantages in terms of mechanical robustness and thermal resistance. Hybrid foam composites that combine these polymer families have the potential to offer balanced performance, integrating the environmental benefits of PLA with the processing stability and durability of conventional thermoplastics. Despite these promising outcomes, several critical challenges remain unresolved. Notably, limited interfacial compatibility in hybrid foam systems, inadequate melt strength in PLA during foaming, and an incomplete understanding of gas diffusion, nucleation, and cell stabilisation mechanisms continue to hinder performance optimisation. Additionally, the absence of standardised foaming protocols and the scarcity of multiscale structure–property correlations restrict the broader industrial implementation of these materials.

Future studies should therefore prioritise the following:

• the development of advanced compatibilisation strategies for PLA/PP and PLA/PA6 foams;

• in situ characterisation methods and multiscale modelling to monitor cell formation and growth;

• the design of bio-based additives to enhance melt strength and foam stability;

• the establishment of optimised processing windows for consistent, large-scale production.

Addressing these research gaps is essential to unlocking the full potential of polymer-based foam composites for use in lightweight, energy-efficient, environmentally sustainable engineering applications.

Footnotes

ORCID iD

Meral Akkoyun Kurtlu

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References

. An introduction to composite materials. Cambridge University Press, 2017. DOI: 10.1007/978-981-10-5696-3_1. Epub ahead of print 2017.

Fan

Njuguna

. An introduction to lightweight composite materials and their use in transport structures. In: Lightweight composite structures in transport: design, manufacturing, analysis and performance. Elsevier, 2016, pp. 3–34.

Hsissou

Seghiri

Benzekri

, et al. Polymer composite materials: a comprehensive review. Compos Struct 2021; 262: 113640.

Ben Ameur

El Mahi

Rebiere

, et al. Damping analysis of unidirectional carbon/flax fiber hybrid composites. Int J Appl Mech 2018; 10: 1850050.

Lee

S-T

Scholz

DPK

. Polymeric foams: technology and developments in regulation, process, and products. CRC Press, 2009.

Gum

Riese

. Reaction polymers: polyurethanes, epoxies, unsaturated polyesters, phenolics, special monomers, and additives: chemistry, technology, applications, markets. Hanser Publishers, 1992.

Beltrán

Boyacá

. Production of rigid polyurethane foams from soy-based polyols. Lat Am Appl Res 2011; 41: 75–80.

Grünbauer

HJM

Bicerano

Clavel

, et al. Rigid polyurethane foams. Polymer Foams. CRC Press, 2004, pp. 253–310.

Park

Cheung

. A study of cell nucleation in the extrusion of polypropylene foams. Polym Eng Sci 1997; 37: 1–10.

10.

Dämmstoffe im Hochbau (Insulation Materials in Buildings), Information for builders, architects and engineers, wirschaftsministerium Baden-Würtenberg. 1999.

11.

Steildachdämmung mit Polyurethan-Hartschaumstoff (Pitched Roof Insulation with Rigid Polyurethane Foam), IVPU. 1995.

12.

European thermal insulation markets: the current and future prospects for polyurethane, UTECH 2000 (urethanes technology), Crain Communications Ltd.

2000.

13.

Nofar

Park

. Poly (lactic acid) foaming. Prog Polym Sci 2014; 39: 1721–1741.

14.

Yetgin

Ünal

. Polimer esaslı köpük malzemeler. Dumlupınar Üniversitesi Fen Bilim Enstitüsü Derg 2008; 17: 117–128.

15.

Saçak

. Polimer Kimyası, Ankara üniversitesi, Fen Fakültesi, Gazi Kitabevi, 7.

16.

Shen

Han

Zeng

, et al. Polymer nanocomposite foams. Compos Sci Technol 2005; 65: 2344–2363.

17.

Dias

de Andrade e Silva

. Polyethylene foams cross-linked by electron beam. Radiat Phys Chem 2007; 76: 1696–1697.

18.

Ebnesajjad

. Fluoroplastics: the definitive user’s guide and databook. Melt processible fluoroplastics. Plastics Design Library, 2003.

19.

Sauceau

Fages

Common

, et al. New challenges in polymer foaming: a review of extrusion processes assisted by supercritical carbon dioxide. Prog Polym Sci 2011; 36: 749–766.

20.

Pop-Iliev

Processing of fine-cell polypropylene foams in compounding-based rotational foam molding. MSc thesis, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada, 1999.

21.

Z-M

Jiang

X-L

Liu

, et al. Foaming of polypropylene with supercritical carbon dioxide. J Supercrit Fluids 2007; 41: 299–310.

22.

Jiang

Zeng

, et al. The effect of α-olefin–maleic anhydride copolymer on the rheological and crystalline properties and microcellular foaming behavior of polyamide 6. Polymers (Basel) 2023; 15: 2056.

23.

Zhao

Zheng

Guo

, et al. Study on chain extension blending modification and foaming behavior of thermoplastic polyamide elastomer. ACS Omega 2023; 8: 9832–9842.

24.

Milovanovic

Lukic

Horvat

, et al. Green processing of neat poly (lactic acid) using carbon dioxide under elevated pressure for preparation of advanced materials: a review (2012–2022). Polymers (Basel) 2023; 15: 860.

25.

Villamil Jiménez

Le Moigne

Bénézet

J-C

, et al. Foaming of PLA composites by supercritical fluid-assisted processes: a review. Molecules 2020; 25: 3408.

26.

Ravenstijn

JTJ

. The state-of-the-art on bioplastics: products, markets, trends, and technologies. Ravenstijn, 2010.

27.

Loi

Delgado

, et al. Reactive compatibilization of polylactide/polypropylene blends. Ind Eng Chem Res 2015; 54: 6108–6114.

28.

Niaounakis

. Biopolymers: processing and products. William Andrew, 2014.

29.

Thomas

Grohens

Jyotishkumar

. Characterization of polymer blends: miscibility, morphology and interfaces. John Wiley & Sons, 2015.

30.

Koning

Van Duin

Pagnoulle

. Strategies for compatibilization of polymer blends. Prog Polym Sci 1998; 23: 707–757.

31.

Wypych

. Handbook of foaming and blowing agents, second edition. Elsevier, 2022.

32.

Aparna

Purnima

Adusumalli

. Review on various compatibilizers and its effect on mechanical properties of compatibilized nylon blends. Polym - Plast Technol Eng 2017; 56: 617–634.

33.

Bärwinkel

Bahrami

Löbling

, et al. Polymer foams made of immiscible polymer blends compatibilized by janus particles - effect of compatibilization on foam morphology. Adv Eng Mater 2016; 18: 814–825.

34.

Mills

. Handbook of polymeric foams and foam technology. Hanser Munich, 1993.

35.

Gnanavel

Kanagamadhuran

Prabhu

, et al. Review on manufacturing of cellular polymers and its applications. J Polym Text Eng 2019; 6: 9–22.

36.

Eaves

. Polymer foams trends in use and technology - rapra industry analysis report series. Rapra Technology Limited Shawbury, 2001.

37.

Nofar

Park

. Introduction to plastic foams and their foaming. Polylactide Foams. 2018, pp. 1–16.

38.

Crank

Park

. Diffusion in polymers. Academic Press, 1968, pp. 3–9.

39.

Colton

Suh

. The nucleation of microcellular thermoplastic foam with additives: part II: experimental results and discussion. Polym Eng Sci 1987; 27: 493–499.

40.

Leung

Wong

Guo

, et al. Change in the critical nucleation radius and its impact on cell stability during polymeric foaming processes. Chem Eng Sci 2009; 64: 4899–4907.

41.

Amon

Denson

. A study of the dynamics of foam growth: analysis of the growth of closely spaced spherical bubbles. Polym Eng Sci 1984; 24: 1026–1034.

42.

Leung

Park

, et al. Computer simulation of bubble-growth phenomena in foaming. Ind Eng Chem Res 2006; 45: 7823–7831.

43.

Jin

Zhao

Park

, et al. Recent trends of foaming in polymer processing: a review. Polymers (Basel) 2019; 11: 953.

44.

Yetgin

. Otomotiv sektörü için polimer köpük malzeme üretimi ve karakterizasyonu.

45.

Siripurapu

Gay

Royer

, et al. Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process. Polymer (Guildf) 2002; 43: 5511–5520.

46.

Park

. Effects of pressure and dissolved carbon dioxide on the rheological properties of molten polymers.

47.

Suryanarayanan

. Development of low-density structural foams of nylon 6 for injection molding: influence of blowing agents, nucleating agents and processing variables. University of Massachusetts, 1997.

48.

Harper

. Modern plastics handbook. McGraw Hill Professional, 2000. https://accessengineeringlibrary.com/browse/modern-plastics-handbook#fullDetails

49.

Landrock

. Handbook of plastic foams: types, properties, manufacture and applications. Elsevier, 1995. https://www-sciencedirect-com-443.web.bisu.edu.cn/science/article/pii/B9780815513575500048

50.

Han

Koelling

Tomasko

, et al. Effect of die temperature on the morphology of microcellular foams. Polym Eng Sci 2003; 43: 1206–1220.

51.

Yeung

AK-F

. Development of microcellular foam sheet extrusion systems. Canadian theses = Thèses canadiennes, 1998. https://hdl.handle.net/1807/15655

52.

Lee

Jeong

, et al. Growth of gas bubbles in the foam extrusion process. Adv Polym Technol 2000; 19: 97–112.

53.

Han

Kennedy

Zheng

, et al. Numerical analysis of microcellular injection molding. J Cell Plast 2003; 39: 475–485.

54.

Spörrer

ANJ

Altstädt

. Controlling morphology of injection molded structural foams by mold design and processing parameters. J Cell Plast 2007; 43: 313–330.

55.

Clariant , 2026. Available from: https://www.clariant.com/en/Company/Clariant-at-a-glance/Locations#/Turkey.

56.

Chen

Yang

Hwang

, et al. Effects of process conditions on the mechanical properties of microcellular injection molded polycarbonate parts. J Reinf Plast Compos 2008; 27: 153–165.

57.

Bledzki

Faruk

. Effects of the chemical foaming agents, injection parameters, and melt-flow index on the microstructure and mechanical properties of microcellular injection-molded wood-fiber/polypropylene composites. J Appl Polym Sci 2005; 97: 1090–1096.

58.

Trainer

. A study of the expansion of thermoplastic microballoons in an extruded syntactic foam. University of Massachusetts, 1989.

59.

Clariant HYDROCEROL® CFA Masterbatches Cut Plastics Content in Packaging by 10%. Clariant, 2019. Available from: https://www.clariant.com/en/Corporate/News/2019/09/Clariant-HYDROCEROLreg-CFA-Masterbatches-Cut-Plastics-Content-in-Packaging-by-10.

60.

Yalcin

Amos

Urquiola

, et al.Effect of processing conditions on the extent of glass bubble survival during twin screw compounding. 3M Technical Paper, 2016. Available from: https://multimedia.3m.com/mws/media/800424O/3m-glass-bubbles-survivability-technical-paper.pdf

61.

Ries

Spoerrer

Altstaedt

. Foam injection molding of thermoplastic elastomers: blowing agents, foaming process and characterization of structural foams. In: AIP conference proceedings. American Institute of Physics, 2014, pp. 401–410.

62.

Akkoyun

Badem

Öztoksoy

, et al. Polypropylene/chemical blowing agent foams: effect of the injection speed and core back distance on microstructure and mechanical properties. Int J Eng Res Dev 2020; 12: 638–647.

63.

Gonçalves

LFFF

Reis

Fernandes

. Forefront research of foaming strategies on biodegradable polymers and their composites by thermal or melt-based processing technologies: advances and perspectives. Polymers (Basel) 2024; 16: 1286.

64.

Long

Shaoyu

, et al. High-strength bio-degradable polymer foams with stable high volume-expansion ratio using chain extension and green supercritical mixed-gas foaming. Polymers (Basel) 2023; 15: 895.

65.

Jacobs

LJM

. Carbon dioxide as a sustainable means to control polymer foam morphology.

66.

Yetgin

Ünal

. Talk/EPDM/Polipropilen polimer kompozit köpük üretimi ve üretim şartlarının optimize edilmesi. Gümüşhane Üniversitesi Fen Bilim Derg 2022; 12: 864–876.

67.

Çebişli

. Modifiye Edilmiş Pet Köpük Üretimi Ve Karakterizasyonu.

68.

Liu

Processing of polyethylene and polypropylene foams in rotational molding. MSc thesis. Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada, 1998.

69.

Nadeau

. A study of direct gas injection foam extrusion of polyolefins for a wire coating application. University of Massachusetts, 2006.

70.

Marchant

Jayaraman

. Strategies for optimizing polypropylene-clay nanocomposite structure. Ind Eng Chem Res 2002; 41: 6402–6408.

71.

Khare

. The relationship between processing conditions and properties in chemically blown injection molded nylon foams. University of Massachusetts, 1996.

72.

Dong

. Study of fundamental foaming mechanisms in chemical-blowing-agent based foaming process. National Library of Canada= Bibliothèque nationale du Canada, Ottawa, 2004.

73.

Bradley

Phillips

. Novel foamable polypropylene polymers. SPE ANTEC Tech Pap 1990; 36: 717–720.

74.

Moore

. Polypropylene handbook: polymerization, characterization, properties, processing, applications. Hanser Publishers, 1996.

75.

Karger-Kocsis

Bárány

. Polypropylene handbook. Choice Rev Online 2006; 43: 43-2825-43–2825.

76.

Maier

Calafut

. Polypropylene: the definitive user’s guide and databook. William Andrew, 1998.

77.

McCallum

TJ.

Properties and foaming behaviour of thermoplastic olefin blends based on linear and branched polypropylene. MSc thesis, Department of Chemical Engineering, Queen’s University, Kingston, Canada, 2007.

78.

Karian

. Handbook of polypropylene and polypropylene composites, revised and expanded. CRC Press, 2003.

79.

Maddah

. Polypropylene as a promising plastic: a review. Am J Polym Sci 2916; 6: 1–11.

80.

Zhu

Luo

Bai

, et al. Synergistic effects of β-modification and impact polypropylene copolymer on brittle-ductile transition of polypropylene random copolymer. J Appl Polym Sci 2013; 129: 3613–3622.

81.

Zhang

Wang

Chen

, et al. Toughened polypropylene with balanced rigidity (II): morphology, melt flow rate and melting point of toughening master batch. Polym Adv Technol 2000; 11: 342–348.

82.

Liu

. Compatibilizing effect of maleated polypropylene on the mechanical properties of injection molded polypropylene/polyamide 6/functionalized-TiO2 nanocomposites. Compos Sci Technol 2009; 69: 421–426.

83.

Zhang

Wang

, et al. Preparation and investigation of the β‐nucleated polypropylene/polystyrene blends. J Appl Polym Sci 2013; 127: 1114–1121.

84.

Gartner

Suárez

López

. Grafting of maleic anhydride on polypropylene and its effect on blending with poly(ethylene terephthalate). Polym Eng Sci 2008; 48: 1910–1916.

85.

Wan

Patel

Xanthos

. Reactive melt modification of polypropylene with a crosslinkable polyester. Polym Eng Sci 2003; 43: 1276–1288.

86.

Jiang

Huang

Zhang

, et al. Dynamically cured polypropylene/epoxy blends. J Appl Polym Sci 2004; 92: 1437–1448.

87.

Öksüz

Eroǧlu

. Effect of the elastomer type on the microstructure and mechanical properties of polypropylene. J Appl Polym Sci 2005; 98: 1445–1450.

88.

Chakraborty

Ganguly

Mitra

, et al. Effect of liquid additives on morphology and properties of thermoplastic elastomers prepared from phase-modified EPDM elastomer and isotactic polypropylene blends. J Mater Sci 2008; 43: 6167–6176.

89.

Hristov

Lach

Krumova

, et al. Fracture toughness of modified polypropylene/poly(styrene-ran-butadiene) blends. Polym Int 2005; 54: 1632–1640.

90.

Salmah

Azra

Yusrina

, et al. A comparative study of polypropylene/(chloroprene rubber) and (recycled polypropylene)/(chloroprene rubber) blends. J Vinyl Addit Technol 2015; 21: 122–127.

91.

Jancar

Dibenedetto

. Effect of morphology on the behaviour of ternary composites of polypropylene with inorganic fillers and elastomer inclusions - part I tensile yield strength. J Mater Sci 1995; 30: 1601–1608.

92.

Zhao

Wang

Zhang

, et al. Lightweight and strong fibrillary PTFE reinforced polypropylene composite foams fabricated by foam injection molding. Eur Polym J 2019; 119: 22–31.

93.

Bao

Nyantakyi Junior

Weng

, et al. Tensile and impact properties of microcellular isotactic polypropylene (PP) foams obtained by supercritical carbon dioxide. J Supercrit Fluids 2016; 111: 63–73.

94.

Lee

Y-D

Wang

L-F

. Properties of polypropylene structural foam crosslinked by vinyltrimethoxy silane. J Appl Polym Sci 1986; 32: 4639–4647.

95.

Rachtanapun

Selke

SEM

Matuana

. Relationship between cell morphology and impact strength of microcellular foamed high-density polyethylene/polypropylene blends. Polym Eng Sci 2004; 44: 1551–1560.

96.

Abreu

FOMS

De Forte

MMC

Liberman

. Morphology and mechanical properties of polypropylenes/TPEs blends. Polimeros 2006; 16: 71–78.

97.

Wang

Zhao

Zhang

, et al. Lightweight and tough nanocellular PP/PTFE nanocomposite foams with defect-free surfaces obtained using in situ nanofibrillation and nanocellular injection molding. Chem Eng J 2018; 350: 1–11.

98.

Garbacz

Palutkiewicz

. Effectiveness of blowing agents in the cellular injection molding process. Cell Polym 2015; 34: 189–214.

99.

Liu

Chen

, et al. Novel polyamide 6 composites based on schiff-base containing phosphonate oligomer: high flame retardancy, great processability and mechanical property. Compos Part A Appl Sci Manuf 2021; 146: 106423.

100.

Saldívar-Guerra

Vivaldo-Lima

. Handbook of polymer synthesis, characterization, and processing. John Wiley & Sons, 2013.

101.

Deopura

Alagirusamy

Joshi

, et al. Polyesters and polyamides. Elsevier, 2008. DOI: 10.1533/9781845694609.

102.

Saylan

. PA 6 kompozitlerinin termal, mekanik ve tribolojik özelliklerinin incelenmesi.

103.

Mallick

. Fiber-reinforced composites: materials, manufacturing, and design. CRC Press, 2007.

104.

Xiao

Xiong

, et al. Preparation and tribological properties of C fibre reinforced C/SiC dual matrix composites fabrication by liquid silicon infiltration. Solid State Sci 2013; 16: 6–12.

105.

Bermdez

Carrin-Vilches

Martnez-Mateo

, et al. Comparative study of the tribological properties of polyamide 6 filled with molybdenum disulfide and liquid crystalline additives. J Appl Polym Sci 2001; 81: 2426–2432.

106.

Campbell

. Phase diagrams: understanding the basics. ASM international, 2012.

107.

Litmanovich

Platé

Kudryavtsev

. Reactions in polymer blends: interchain effects and theoretical problems. Prog Polym Sci 2002; 27: 915–970.

108.

Krause

. Polymer–polymer compatibility. In: Polymer blends. Elsevier, 1978, pp. 15–113.

109.

Ide

Hasegawa

. Studies on polymer blend of nylon 6 and polypropylene or nylon 6 and polystyrene using the reaction of polymer. J Appl Polym Sci 1974; 18: 963–974.

110.

Huber

Misra

Mohanty

. Mechanical properties of compatibilized nylon 6/polypropylene blends; studies of the interfacial behavior through an emulsion model. J Appl Polym Sci 2014; 131: 9455–9462.

111.

Kuang

Chen

, et al. Fabrication of high strength PA6/PP blends with pressure-induced-flow processing. Mater Chem Phys 2015; 164: 1–5.

112.

Agrawal

Araújo

Mélo

TJA

. Effect of the processing method on the mechanical properties and morphology of compatibilized PA6/LDPE blends. J Mater Sci 2008; 43: 4443–4449.

113.

Agrawal

Rodrigues

AWB

Araújo

, et al. Influence of reactive compatibilizers on the rheometrical and mechanical properties of PA6/LDPE and PA6/HDPE blends. J Mater Sci 2010; 45: 496–502.

114.

Kudva

Keskkula

Paul

. Morphology and mechanical properties of compatibilized nylon 6/polyethylene blends. Polymer (Guildf) 1999; 40: 6003–6021.

115.

Földes

Pukánszky

. Miscibility-structure-property correlation in blends of ethylene vinyl alcohol copolymer and polyamide 6/66. J Colloid Interface Sci 2005; 283: 79–86.

116.

Valenza

Visco

Acierno

. Characterization of blends with polyamide 6 and ethylene acrylic acid copolymers at different acrylic acid content. Polym Test 2002; 21: 101–109.

117.

Bhattacharyya

Ghosh

Misra

. Ionomer compatibilised PA6/EVA blends: mechanical properties and morphological characterisation. Polymer (Guildf) 2003; 44: 1725–1732.

118.

Balamurugan

Maiti

. Influence of microstructure and deformation behavior on toughening of reactively compatibilized polyamide 6 and poly(ethylene-co-butyl acrylate) blends. Eur Polym J 2007; 43: 1786–1805.

119.

Prasath

Maiti

. The influence of reactive compatibilization on uniaxial large strain deformation and fracture behavior of polyamide 6 and poly (ethylene-co-butyl acrylate) blends. Polym Test 2008; 27: 752–764.

120.

Okada

Keskkula

Paul

. Mechanical properties of blends of maleated ethylene-propylene rubber and nylon 6. Polymer (Guildf) 2001; 42: 8715–8725.

121.

Oshinski

Keskkula

Paul

. Rubber toughening of polyamides with functionalized block copolymers: 2. Nylon-6,6. Polymer (Guildf) 1992; 33: 284–293.

122.

Tang

Yang

Shan

, et al. Double yielding in PA6/TPV-MAH blends: effect of dispersed phase with different content, modulus. Polymer (Guildf) 2007; 48: 7404–7413.

123.

Charoensirisomboon

Chiba

Torikai

, et al. Morphology-interface-toughness relationship in polyamide/polysulfone blends by reactive processing. Polymer (Guildf) 1999; 40: 6965–6975.

124.

Han

Jiang

, et al. Influence of crystallization on microcellular foaming behavior of polyamide 6 in a supercritical CO2-assisted route. J Appl Polym Sci 2020; 137: 49183.

125.

Song

Zhou

Wang

, et al. Role of chain extension in the rheological properties, crystallization behaviors, and microcellular foaming performances of poly (butylene adipate-co-terephthalate). J Appl Polym Sci 2019; 136: 47322.

126.

Zhao

Wang

, et al. Fabrication of high-expansion microcellular PLA foams based on pre-isothermal cold crystallization and supercritical CO2 foaming. Polym Degrad Stab 2018; 156: 75–88.

127.

Zhijuan

Lin

Luo

, et al. The application of a tri-functional epoxy resin as a crosslinking agent in extruded polyamide-6 foam. Polym Sci - Ser B 2019; 61: 574–581.

128.

Jiang

Gong

, et al. Preparation methods, performance improvement strategies, and typical applications of polyamide foams. Ind Eng Chem Res 2021; 60: 17365–17378.

129.

Yáñez-Macías

Rivera-Salinas

Solís-Rosales

, et al. Mechanical behavior of glass fiber-reinforced nylon-6 syntactic foams and its young’s modulus numerical study. J Appl Polym Sci 2021; 138: 50648.

130.

Wang

Xie

, et al. Microstructure and properties of glass fiber-reinforced polyamide/nylon microcellular foamed composites. Polymers (Basel) 2020; 12: 1–11.

131.

Wang

Zhao

, et al. Super-high fraction of organic montmorillonite filled polyamide 6 composite foam: morphologies, thermal and mechanical properties. Polym Adv Technol 2021; 32: 544–552.

132.

Castro-Aguirre

Iniguez-Franco

Samsudin

, et al. Poly (lactic acid)—mass production, processing, industrial applications, and end of life. Adv Drug Deliv Rev 2016; 107: 333–366.

133.

Theryo

Jing

Pitet

, et al. Tough polylactide graft copolymers. Macromolecules 2010; 43: 7394–7397.

134.

Auras

Lim

L-T

Selke

SEM

, et al. Poly (lactic acid): synthesis, structures, properties, processing, and applications. John Wiley & Sons, 2011.

135.

Nofar

Sacligil

Carreau

, et al. Poly (lactic acid) blends: processing, properties and applications. Int J Biol Macromol 2019; 125: 307–360.

136.

Saeidlou

Huneault

, et al. Poly (lactic acid) crystallization. Prog Polym Sci 2012; 37: 1657–1677.

137.

Dorgan

Williams

Lewis

. Melt rheology of poly(lactic acid): entanglement and chain architecture effects. J Rheol 1999; 43: 1141–1155.

138.

Grijpma

Pennings

. (Co) polymers of l‐lactide, 2. Mechanical properties. Macromol Chem Phys 1994; 195: 1649–1663.

139.

Rasal

Janorkar

Hirt

. Poly (lactic acid) modifications. Prog Polym Sci 2010; 35: 338–356.

140.

Cabedo

Feijoo

Villanueva

, et al. Optimization of biodegradable nanocomposites based on aPLA/PCL blends for food packaging applications. In: Macromolecular symposia. Wiley Online Library, 2006, pp. 191–197.

141.

Zhang

, et al. Selective localization of multiwalled carbon nanotubes in poly(ε-caprolactone)/polylactide blend. Biomacromolecules 2009; 10: 417–424.

142.

Shentu

Weng

. Preparation and properties of polylactide/poly(ethylene-co-octene)/nano- SiO 2 ternary composites. J Macromol Sci, Part B: Phys 2012; 51: 1766–1775.

143.

Lapčík

Maňas

Lapčíková

, et al. Effect of filler particle shape on plastic-elastic mechanical behavior of high density poly(ethylene)/mica and poly(ethylene)/wollastonite composites. Compos Part B Eng 2018; 141: 92–99.

144.

Liang

Yang

. Effects of carbon fiber content and size on electric conductive properties of reinforced high density polyethylene composites. Compos Part B Eng 2017; 114: 457–466.

145.

Savas

Tayfun

Dogan

. The use of polyethylene copolymers as compatibilizers in carbon fiber reinforced high density polyethylene composites. Compos Part B Eng 2016; 99: 188–195.

146.

Lapčík

Maňas

Vašina

, et al. High density poly(ethylene)/CaCO3 hollow spheres composites for technical applications. Compos Part B Eng 2017; 113: 218–224.

147.

Krásný

Lapčík

Lapčíková

, et al. The effect of low temperature air plasma treatment on physico-chemical properties of kaolinite/polyethylene composites. Compos Part B Eng 2014; 59: 293–299.

148.

Dasari

Rohrmann

Misra

RDK

. On the scratch deformation of micrometric wollastonite reinforced polypropylene composites. Mater Sci Eng A 2004; 364: 357–369.

149.

Malik

Farooqi

Vachet

. Mechanical and rheological properties of reinforced polyethylene. Polym Compos 1992; 13: 174–178.

150.

Murariu

Dubois

. PLA composites: from production to properties. Adv Drug Deliv Rev 2016; 107: 17–46.

151.

Ameli

Nofar

Jahani

, et al. Development of high void fraction polylactide composite foams using injection molding: crystallization and foaming behaviors. Chem Eng J 2015; 262: 78–87.

152.

Najafi

Heuzey

Carreau

, et al. Mechanical and morphological properties of injection molded linear and branched-polylactide (PLA) nanocomposite foams. Eur Polym J 2015; 73: 455–465.

153.

Preechawong

Peesan

Supaphol

, et al. Preparation and characterization of starch/poly(l-lactic acid) hybrid foams. Carbohydr Polym 2005; 59: 329–337.

154.

Hao

Geng

, et al. Study of different effects on foaming process of biodegradable PLA/starch composites in supercritical/compressed carbon dioxide. J Appl Polym Sci 2008; 109: 2679–2686.

155.

Mihai

Huneault

Favis

, et al. Extrusion foaming of semi-crystalline PLA and PLA/thermoplastic starch blends. Macromol Biosci 2007; 7: 907–920.

156.

Zhang

Sun

. Biodegradable foams of foly(lactic acid)/starch. I. Extrusion condition and cellular size distribution. J Appl Polym Sci 2007; 106: 857–862.

157.

Zhang

Sun

. Biodegradable foams of folydactic acid/starch. II. Cellular structure and water resistance. J Appl Polym Sci 2007; 106: 3058–3062.

158.

Pilla

Kim

Auer

, et al. Microcellular extrusion foaming of poly(lactide)/poly(butylene adipate-co-terephthalate) blends. Mater Sci Eng C 2010; 30: 255–262.

159.

Kramschuster

Turng

. An injection molding process for manufacturing highly porous and interconnected biodegradable polymer matrices for use as tissue engineering scaffolds. J Biomed Mater Res - Part B Appl Biomater 2010; 92: 366–376.

160.

Zhao

Cui

Sun

, et al. Morphology and properties of injection molded solid and microcellular polylactic acid/polyhydroxybutyrate-valerate (PLA/PHBV) blends. Ind Eng Chem Res 2013; 52: 2569–2581.

161.

Hamad

Kaseem

Ayyoob

, et al. Polylactic acid blends: the future of green, light and tough. Prog Polym Sci 2018; 85: 83–127.

162.

Choudhary

Mohanty

Nayak

, et al. Poly (L‐lactide)/polypropylene blends: evaluation of mechanical, thermal, and morphological characteristics. J Appl Polym Sci 2011; 121: 3223–3237.

163.

Hafidzah

Bijarimi

Alhadadi

, et al. Statistical study on the interaction factors of polypropylene-graft-maleic anhydride (PP-G-MA) with graphene nanoplatelet (GNP) at various poly(lactic acid)/polypropylene (PLA/PP) blends ratio. Indones J Chem 2021; 21: 234–242.

164.

Nuñez

Rosales

Perera

, et al. Nanocomposites of PLA/PP blends based on sepiolite. Polym Bull 2011; 67: 1991–2016.

165.

Kang

. Properties of immiscible and ethylene-butyl acrylate-glycidyl methacrylate terpolymer compatibilized poly (lactic acid) and polypropylene blends. Polym Test 2015; 43: 173–181.

166.

Sui

Jing

Zhao

, et al. A comparison study of high shear force and compatibilizer on the phase morphologies and properties of polypropylene/polylactide (PP/PLA) blends. Polymer (Guildf) 2018; 154: 119–127.

167.

Wang

Liu

, et al. Role of maleic- anhydride-grafted- polypropylene in supercritical CO2 foaming of poly (lactic acid) and its effect on cellular morphology. J Cell Plast 2016; 52: 37–56.

168.

Tang

Zhai

Zheng

. Autoclave preparation of expanded polypropylene/poly(lactic acid) blend bead foams with a batch foaming process. J Cell Plast 2011; 47: 429–446.

169.

Cardoso

ECL

Scagliusi

Lugão

. Gamma-radiation effect on biodegradability of synthetic PLA structural foams PP/HMSPP based. In: Minerals, metals and materials series. Springer, 2017, pp. 111–119.

170.

Tuna

Akkoyun Kurtlu

. Chemical foaming of polylactic acid/polypropylene and polylactic acid/polyamide 6: evaluation of changes in their properties. J Thermoplast Compos Mater 2024; 37: 842–863.

171.

Tuna

Akkoyun

. Effect of coupling agent on polylactic acid/polypropylene and polylactic acid/polyamide 6 foam composites. J Appl Polym Sci 2024; 141: e54849.

172.

Feng

. Structure and property of polylactide/polyamide blends. J Macromol Sci, Part B: Phys 2010; 49: 1117–1127.

173.

Khankrua

Pivsa-Art

Hiroyuki

, et al. Effect of chain extenders on thermal and mechanical properties of poly(lactic acid) at high processing temperatures: potential application in PLA/polyamide 6 blend. Polym Degrad Stab 2014; 108: 232–240.

174.

Hung

Wang

Chen

. Enhanced the thermal stability and crystallinity of polylactic acid (PLA) by incorporated reactive PS-b-PMMA-b-PGMA and PS-b-PGMA block copolymers as chain extenders. Polymer (Guildf) 2013; 54: 1860–1866.

175.

Wang

, et al. Polyamide-6/poly(lactic acid) blends compatibilized by the maleic anhydride grafted polyethylene-octene elastomer. Polym - Plast Technol Eng 2010; 49: 1241–1246.

176.

Chen

Zeng

, et al. Preparation and properties research of PA6/PLA blends toughening modified by TPU. In: Applied mechanics and materials. Trans Tech Publ, 2012, pp. 278–281.

177.

Kakroodi

Kazemi

Ding

, et al. Poly(lactic acid)-based in situ microfibrillar composites with enhanced crystallization kinetics, mechanical properties, rheological behavior, and foaming ability. Biomacromolecules 2015; 16: 3925–3935.

178.

Yang

Wang

Xing

, et al. A new promising nucleating agent for polymer foaming: effects of hollow molecular-sieve particles on polypropylene supercritical CO 2 microcellular foaming. RSC Adv 2018; 8: 20061–20067.

179.

Muñoz-Pascual

Saiz-Arroyo

Vuluga

, et al. Foams with enhanced ductility and impact behavior based on polypropylene composites. Polymers (Basel) 2020; 12: 943.

180.

Gong

Zhang

, et al. Study on foaming quality and impact property of foamed polypropylene composites. Polymers (Basel) 2018; 10: 1375.

181.

Mort

Peters

Curtzwiler

, et al. Biofillers improved compression modulus of extruded PLA foams. Sustainability 2022; 14: 5521.

182.

Oluwabunmi

D’Souza

Zhao

, et al. Compostable, fully biobased foams using PLA and micro cellulose for zero energy buildings. Sci Rep 2020; 10: 17771.

183.

Huang

Zhao

, et al. Lightweight polypropylene/polylactic acid composite foams with controllable hollow radially gradient porous structures for oil/water separation. Ind Eng Chem Res 2022; 61: 10982–10989.

184.

Peng

Mubarak

Diao

, et al. Progress in the preparation, properties, and applications of PLA and its composite microporous materials by supercritical CO2: a review from 2020 to 2022. Polymers (Basel) 2022; 14: 4320.