A study on fabrication of nanocomposite polyethylene foam through extrusion foaming procedure

Abstract

Foaming a polymer not only turns it into a lightweight material but also gives some special properties to it. However, the most important issue is controlling the foaming process to achieve a desirable structure with high cell density and low relative density. In the present study, the extrusion foaming process of polyethylene was studied through stepwise amendments. An innovative extrusion system was designed and implemented to produce extrusion foams under different material and process conditions using N₂ as blowing agent. In the first step, the final cooling condition was investigated. The air-cooling condition led to a higher cell density/lower cell size compared to the water-cooling condition although a higher relative density was obtained. In the second step, the effects of the addition of talc and the synergetic effect of talc/nanoclay at different contents were investigated in detail. The hybrid of talc/nanoclay had a noticeably improving effect on the cellular structure. In the third step, the effects of processing parameters including the die temperature and screw speed were studied on the foam properties. Finally, up to 49.4% decrease in the relative density of samples was observed, also cell densities up to 2.5 × 10⁴ cell/cm³ and cell sizes as small as 280 µm were achieved.

Keywords

Foam extrusion polyethylene density morphology nanocomposite

Introduction

Polymeric foams are porous materials widely used in different applications such as automotive, piezoelectric, tissue engineering, aircraft, food, and packaging industries due to their light weight, excellent thermal and sound insulation properties,^1,2 high strength to weight ratio,³ and high energy absorption.⁴ Polymeric foam is a solid polymer containing a gaseous phase as dispersed bubbles. Cell nucleation, cell growth, and cell stabilization are the three main steps in the foaming process. Polymeric foams are categorized into two types including chemical and physical from the blowing agents’ viewpoint. In the latter one, gaseous materials such as carbon dioxide and nitrogen are injected into the melted polymer. In the chemical process, solid materials such as azodicarbonamide and sodium bicarbonate are dispersed in the polymer and release gas at a specific temperature to form a porous structure.⁵ Extrusion foaming is one of the continuous production processes. In this method, a single-phase polymer/gas mixture is obtained by injecting a gas into the extruder followed by shear stress supplied by the screw. Afterward, the cells are nucleated by rapid pressure drop generated in the die. Extruded polymeric foams have a variety of functions especially in the electrical insulation industry.⁶

Nowadays, nanomaterials are used to improve some properties of polymers such as mechanical,^7,8 physical,⁹ and chemical.¹⁰ Nanomaterials not only improve the morphology and properties of solid polymers but also act as a nucleating agent in the foaming process of these polymers which leads to the decrease of cell size and increase of cell density.^11–15

The first development of microcellular foams was carried out in the early 1980s using the solid-state foam method.¹⁶ After the introduction of microcellular foams by Martini et al.¹⁷ in 1981, in 1993, Park¹⁸ researched microcellular foam production systems. The main idea of this research was aimed at providing conditions for widespread thermodynamic instability to enhance the nucleation of gas bubbles in the polymer matrix. Lee and Park¹⁹ produced high-density polyethylene (HDPE) foam using nitrogen gas. The experimental results revealed that the void fraction of high-density foams blown with N₂ was not affected by the die temperature, contrasting the situation in low-density foams. Park et al.²⁰ investigated the effect of extrusion nozzle temperature on the expansion ratio of polystyrene foam. Their results showed that the high nozzle temperature increases the gas escape in the foam while exiting the nozzle and therefore, the high temperature of the nozzle reduces the expansion ratio due to the decrease of melt strength and occurrence of the coalescence phenomenon. Park and Suh²¹ investigated the extrusion foaming of polypropylene (PP) and high impact polystyrene (HIPS). They concluded that the big bubbles of gas in the polymer melt get smaller and smaller by the shear tension of the screw rotation, forming a single-phase dissolution of gas and polymer. Park et al.²² studied the effect of pressure drop rate on the nucleation of the gas bubbles in the extrusion foaming process of HIPS. According to their results, the final cell density of the samples was increased by increasing the pressure drop rate. Behravesh et al.²³ investigated the effect of extrusion nozzle temperature on the cell growth behavior of the samples. They concluded that a uniform cooling procedure is necessary to prevent the excessive growth of the nucleated cells.

Some other researches have been performed on material parameters affecting the foaming process. Naguib et al.²⁴ used talc as the nucleation agent in the extrusion foaming process of PP. A significant increase in cell density was observed. In semi-crystalline polymers such as PE or PP, one of the main obstacles in the foaming process is low melt strength. Using peroxides as crosslinking agents²⁵ and adding high-density polymers²⁶ are some of the solutions. Nazri et al.²⁶ investigated the extrusion foaming process of low-density PE (LDPE). They observed that by adding 10 wt% of HDPE, the cell density was increased significantly as a result of higher melt strength. Nofar²⁷ investigated the effect of nano and micro-sized additives on extrusion foaming of polylactic acid (PLA) behavior using supercritical CO₂. The results showed that increasing the die pressure leads to reduced crystallization rate and slow growth rate of a large number of crystals, which contributes to increasing final cell density and the structural uniformity of the foam. The use of nanoparticles in polymer foams not only strengthens the cell wall but also reduces the cell size. It also provides foams with superior mechanical properties. Okolieocha et al.²⁸ studied the mechanical properties and cell morphology of extruded foam by the addition of graphene. The results showed that the cell morphologies of PS/thermally reduced graphite oxide foams show enhanced cell homogeneity with a tremendous increase of the cell densities by more than one order of magnitude compared to neat PS and its counterparts. Chaudhary and Jayaraman²⁹ extruded linear PP-clay nanocomposite foams using a chemical blowing agent. The results of their work showed that clay dispersion played a significant role in strain-hardening of the melt state along with slower crystallization, which led to extruded PP nanocomposite foams with smaller cell sizes and greater cell density due to reduction of cell coalescence. Sahagún et al.³⁰ investigated the effect of using a compatilizing agent in the extrusion foaming process of PP/HDPE blend. The results indicated that using 10 wt% of Kraton D1102 led to an increase in cell density and a decrease in cell size due to providing a larger space for nucleating sites.

Different chemical and physical extrusion foaming processes were carried out in former researches. However, in the present study, a stepwise investigation is conducted on the physical extrusion foaming process of LDPE. Different parameters such as the nanoclay and talc content, die temperature, cooling process, and rotation speed of screw have been taken into account to achieve low-density LDPE foams. The cellular structures of produced foams are investigated using scanning electron microscopy and their cell size and cell density are studied. Also, their relative density as a parameter introducing the degree of foaming is assessed. The efficacious approaches to obtain a foam with a desirable cellular structure having low relative density are recognized.

Experimental

Materials and equipment

A single screw extrusion made by Technic Company with length to diameter ratio (L/D) of 20 mm and cylinder diameter of 40 mm was used as the base machine. The maximum rotation speed was 50 rpm. A 14 mm threaded hole was made on the last third zone of the cylinder to inject the gas into the polymer. A one-way control valve (Control KA-02) was mounted on the hole to prevent the gas and polymer melt to enter the gas pipe. The valve was equipped with a porous material to prevent the return of the polymer into the valve. N₂ with maximum pressure of 130 bar was used as the physical blowing agent. A pressure gauge system was mounted on the metal connector of the capsule to the valve. An evacuation valve was also considered on the pipe for the sake of safety. The length of the main screw part of the extrusion was 80 cm. A static mixer was designed and manufactured with a length of 60 cm using CM5 metal pipe with an inner diameter of 40 mm with a static screw inside. The static mixer was connected to the main part of extrusion using proper fixtures.

The temperature of the final zones of extrusion is an important factor in the cell growth stage. A cooling device was designed and manufactured with a length of 40 cm using CM5 metal pipe with an inner diameter of 40 mm and was connected to the end part of the static mixer using proper fixtures. This cooling zone was for decreasing the temperature of the melted polymer after the static mixer. A sealing gasket was used in the connection of the main part to the static mixer and also the connection of the static mixer to the cooling device to prevent polymer leakage.

The melt extruder had three temperature controllers. To ensure continuous temperature control, three new temperature control zones were added on the static mixer, two new temperature zones were added on the cooling part, and a final temperature control zone was added on the nozzle die. Autonics TZN4M thermostats were used to control the temperature by regulating the heaters. Figure 1 shows the schematic representation of the designed foaming system and the temperature profile of the different zones. The final cooling condition is after the output of the melted material from the die. Two different final cooling conditions including water and air were implemented in the experiments.

Figure 1.

Schematic of designed foaming extrusion system and the applied temperature profile.

LDPE-0200 product of Bandar Emam petrochemical company with the melt flow index (MFI) of 2 g/10 min (190°C, 2.16 kg) and the density of 0.920 g/cm³ was used as the base material together with HDPE-54404 product of Lorestan petrochemical company with the MFI of 4 g/10 min (190°C, 2.16 kg) with density of 0.945 g/cm3. Montmorillonite nanoclay with the trade name of Cloisite 30B product of Southern Clay Products (Inc.) with density of 0.6 g/cm3 and average thickness and length sizes of respectively, 1.5 nm and 9 µm was used as reinforcement agent. Talc powder product of Merck Company with density of 2.7 g/cm³ and average size of 5 µm was used as nucleation agent. The design of the experiments is listed in Table 1. It is noteworthy that in the foaming process one of the main effective parameters is the gas content in the gas/polymer solution. However, due to experimental limitations, the gas dosage is not controlled in this study.

Table 1.

Design of experiments.

Step	Code	Material parameters			Process parameters
Step	Code	Base material	Nanoclay (wt%)	Talc (wt%)	Die temperature (°C)	Screw speed (rpm)	Final cooling condition
1	A1	HDPE	0	0	150	10	Water
	A2	HDPE	1	0	150	10	Water
	A3	HDPE	1	0	150	10	Air
2	B1	LDPE/HDPE (9:1)	0	1	140	10	Water
	B2	LDPE/HDPE (9:1)	1	1	140	10	Water
	B3	LDPE/HDPE (9:1)	2.5	1	140	10	Water
3	C1	LDPE/HDPE (9:1)	2.5	1	125	8	Water
	C2	LDPE/HDPE (9:1)	2.5	1	125	5	Water
	C3	LDPE/HDPE (9:1)	2.5	1	125	5	Air

Strategies for improving the cellular structure

The first implemented strategy is to increase the pressure drop rate of the polymer melt as it exits the extrusion nozzle. The higher pressure drop rate leads to larger thermodynamic instability and consequently better cell nucleation in the polymer melt.²² The pressure drop rate in the extrusion nozzle can be calculated using equation (1)²²:

\frac{d p}{d t} \approx \frac{Δ p}{Δ t} = - 2 m {(3 + \frac{1}{n})}^{n} {(\frac{Q}{π R^{3}})}^{n + 1}

(1)

where $\frac{d p}{d t}$ is the pressure drop rate, R is the nozzle radius, Q is the volumetric flow rate, the parameters m and n are related to the power-law viscosity model: consistency index and exponent, respectively.²²

Firstly, the samples were produced by a nozzle with 6 mm diameter. It was found that the desired foam samples were not obtained according to pre-tests done by the authors. Regarding equation (1), $\frac{d p}{d t}$ is inversely related to R. By reducing the nozzle radius, the pressure drop rate is increased. The higher the pressure drop rate is, the better the foam properties are.³¹ Therefore, a nozzle with a diameter of 4 mm was designed and produced by the authors.

The temperature of melt can also affect the pressure inside the extrusion cylinder and die, therefore it plays a role in pressure drop rate and nucleation stage. However, due to controlling the pressure inside the cylinder with rotational speed, the temperature variation effect on the nucleation stage is negligible but it strongly affects the cell growth stage of foaming.

The second strategy is controlling the temperature. High nozzle temperatures lead to high gas loss and also the occurrence of coalescence and collapse phenomena and are detrimental to cell nucleation. On the other hand, the low nozzle temperatures lead to very high melt strength which prevents the cells from proper growing. It is also noteworthy that the final cooling condition in the final section of the extrusion plays an important role in stabilizing the polymer-gas dissolution and forming of crystals in the polymer melt which are potential regions for nucleation.

The third strategy is increasing the cell nucleation by adding nucleation agents to the polymer melt. Particles can be used as nucleation agents, especially with smaller sizes leading to higher specific surface areas. These particles are potential nucleating regions. The presence of these particles in the polymer melt also results in the formation of local tensions which is desirable for decreasing the needed energy for nucleation.^31–33

Foam production

According to the weight percentages of additives mentioned in Table 1, LDPE pellets with various amounts of talc, nanoclay, and HDPE were physically mixed and then fed into the barrel through the hopper and were completely melted by the screw motion and heaters. Then, the blowing agent was injected into the melted polymer. Then, the gas is dissolved in the polymer melt with the rotation of the screw in the barrel. The polymer-gas mixture passes through the static mixer to obtain the desired mixing. Then, the polymer-gas mixture enters the cooling zone. The cooling zone has a lower temperature than the mixer zone. The polymer-gas mixture reaches a stable state in this stage. The stabilized polymer-gas mixture enters the output mold. Mold diameter is an important parameter that influences the pressure drop rate (see equation (1)). In this region, the nucleation stage occurs. By the exit of materials from the mold, the temperature and pressure are decreased rapidly and the foaming process is completed. Rapid cooling of the shell at ambient temperature prevents gas to escape and allows a high expansion ratio to be achieved.

Foam characterization

The Archimedes density method was used to measure the density of the samples. Relative density (RD) is the density ratio of the foamed sample ( $ρ_{f}$ ) to the solid sample ( $ρ_{s}$ ) as equation (2):

R D = \frac{ρ_{f}}{ρ_{s}}

(2)

SHS-FX300GD balance with accuracy of 0.001 g and maximum capacity of 320 g was used for measuring the density of the samples.

The cell density is calculated with respect to the unfoamed polymer volume.

Samples were dipped in liquid nitrogen and then fractured to expose the cellular morphology. The fractured surface was coated with gold and the microstructure was characterized using Mira 3 Tescan scanning electron microscope (SEM). The cell density of the foamed samples was calculated with respect to the solid polymer using equation (3):

N = {(\frac{n}{A})}^{\frac{3}{2}} \times \frac{1}{R D}

(3)

where N is cell density, n is the number of cells in the area of A.

Also, SEM pictures were used to derive the cell size of the samples, the diameter of the observable cell inside the SEM pictures was measured and the average amount is reported.

Result and discussion

Step 1

Table 2 indicates the results of foamed samples produced in step 1. In the first step, HDPE was used to obtain the appropriate process conditions. Sample A1 was produced but the results showed that no foaming phenomenon takes place. Although the relative density was slightly decreased, no cells were observed in SEM pictures (see Figure 2(a)). To achieve an appropriate cellular structure, 1 wt% nanoclay was added to HDPE, and A2 sample was produced. The results demonstrated that a cellular structure was obtained which is a witness of taking place the foaming procedure. The cellular structure is observable in Figure 2(b). The relative density was improved by 13.7% (from 0.998 to 0.861) compared to A1. This foam structure is due to the role of the nucleating of nanoclays. Nanoclay can cause local tensile stresses and these stresses increase the nucleation in the foaming process by decreasing the critical radius and making the cells more likely to survive.³² A cell density of 1028 cells/cm³ and a cell size of 553 µm were obtained for A2. To investigate the effect of final cooling conditions, sample A3 was produced with the same process and materials conditions of sample A2. The only difference was in the final cooling condition. Sample A2 was cooled in the water while sample A3 was cooled in the air. Figure 2(c) shows the cellular structure of sample A3. The results revealed that the final cooling condition has a significant impact on the foaming behavior of the samples.³⁴ Sample A3 has a higher cell density/lower cell size and higher relative density than sample A2. Cooling in the water has a higher cooling rate compared to cooling in the air. With a higher cooling rate of the sample, thicker solid skin is formed around the sample. Figure 3 shows the solid skin of samples A2 and A3. As can be seen, a narrower solid skin was formed in the samples cooled in the air i.e. A3. Cells are formed near the sample skin due to the low cooling rate in the sample produced under air-cooling conditions. Therefore, the possibility of gas escape is higher in the samples cooled in the air. This gas escape leads to an increase of the foam density and consequently, the relative density. The higher the cooling rate, the faster the outer layer of the sample is frozen, which prevents the gas from escaping the material and causes a higher expansion ratio.³⁴ Due to the lower cooling rate in the air, a higher cell density/lower cell size was obtained for sample A3. As mentioned in the experimental section, the uncontrolled gas dosage parameter can also be effective on the results, especially on the foam density and cell growth process.

Table 2.

Results of the foamed samples of step 1.

Sample code	Relative density	Cell density (cells/cm3)	Cell size (µm)	Skin layer thickness (µm)
A1	0.998	—	—	—
A2	0.861	1028	553 ± 80	210
A3	0.933	2626	514 ± 90	260

Figure 2.

The SEM picture of samples’ section produced in step 1; (a) A1, (b) A2, and (c) A3.

Figure 3.

Closer look on SEM picture of samples produced in step 1; (a) A2 and (b) A3.

Step 2

Table 3 indicates the results of foamed samples produced in step 2. In the second step, LDPE/HDPE mixture with a ratio of 9:1 was used as the matrix. Therefore, the die temperature was set at a lower temperature compared to step 1 due to the lower processing temperature of LDPE. The first sample of step 2 i.e. B1 was produced to investigate the effect of the addition of talc as a potential nucleating agent. The results showed that no foam structure was obtained in this condition. Figure 4(a) shows that sample B1 does not have any cellular structure. Also, the relative density was 1. It shows that foaming does not take place. Sample B2 was produced for investigating the synergetic effect of nanoclays and talc with a ratio of 1:1 on the cellular structure of foamed samples. Figure 4(b) illuminated that a considerable improvement was observed in the cellular structure. The cell density of 1930 cells/cm³ and the cell size of 354 µm were obtained. The relative density was reduced to 0.991. Sample B3 was produced under the synergetic effect of nanoclay and talc with a ratio of 2.5:1. Figure 4(c) shows the cellular structure of sample B3. According to the results, sample B3 presented improved cellular structure which is a consequence of the higher content of nanoclay. The cell density was enhanced by almost 13 times (from 1930 cells/cm³ to 25938 cells/cm³) and the cell size was improved by 20.9% (from 354 µm to 280 µm). A significant reduction was also observed in the relative density by 10.2% (from 0.991 to 0.890). Both samples of B2 and B3 were cooled in the water and these foams had a solid skin without any nuclei as can be seen in Figure 5. It is notable that including the gas content parameter, which was experimentally unavailable, would make these conclusions more accurate.

Table 3.

Results of the foamed samples of step 2.

Sample code	Relative density	Cell density (cells/cm³)	Cell size (µm)	Skin layer thickness (µm)
B1	1	—	—	—
B2	0.991	1930	354 ± 45	1000
B3	0.890	25938	280 ± 50	1000

Figure 4.

The SEM picture of samples’ section produced in step 2; (a) B1, (b) B2, and (c) B3.

Figure 5.

Closer look on SEM picture of samples produced in step 2; (a) B2 and (b) B3.

Step 3

The results of previous steps showed that an appropriate cellular structure is achieved using the synergetic effect of nanoclays and talc with a ratio of 2.5:1. This synergetic effect was confirmed in the literature with other additives.³⁵ Therefore, the additive content was fixed at this ratio and the effects of the processing parameters were studied in step 3. As mentioned above the gas dosage parameter was not measured, therefore it is not considered in the analysis of the results. Nevertheless, Table 4 indicates the results of foamed samples produced in step 2. Sample C1 was produced with a lower die temperature and screw speed compared to B3. Figure 6(a) shows the cellular structure of sample C1. The results revealed that the cell density was considerably decreased but the relative density was reduced by 5.6% at almost the same cell size. The die temperature was fixed at 125°C and the screw speed was reduced to 5 rpm in sample C2 resulting in a cellular structure as shown in Figure 6(b). The results indicated that the relative density was reduced significantly by 48.3% from (0.840 to 0.434) at an almost same cell density although the cell size was increased from 284 µm to 360 µm. Decreasing the screw speed provides more time for gas diffusion into the polymer and a lower relative density is achieved.³⁶ To check the effect of the final cooling condition on this sample, sample C3 was produced at the same processing and material conditions as sample C2. The only difference was in the final cooling condition. Sample C3 was cooled in the air. Figure 6(c) shows the cellular structure of sample C3. A similar trend with previous results was obtained (see Table 2).

Table 4.

Results of the foamed samples of step 3.

Sample code	Relative density	Cell density (cells/cm³)	Cell size (µm)	Skin layer thickness (µm)
C1	0.840	5359	284 ± 50	500
C2	0.434	5590	360 ± 50	160
C3	0.506	20404	340 ± 50	110

Figure 6.

The SEM picture of samples’ section produced in step 3; (a) C1, (b) C2, and (c) C3.

Conclusions

In this study, polyethylene was selected as the base material and talc powder and nanoclay were utilized as the additives. The specimens were manufactured under a variety of process and materials parameters using an extrusion machine equipped with a gas injection and mixing system. The effects of different weight percentages of additives and their synergetic effect were investigated in detail. The effects of the final cooling condition, the die temperature, and screw speed were studied as the processing parameters. The results indicated that adding nanoclay led to significant improvement of the cellular structure due to the nucleation effect of nanoclays and up to 14% decrease occurred in the relative density of samples. Regarding cooling conditions, it markedly affected the final cellular structure. The higher the cooling rate was, the faster the solidification of the skin was, which led to prevent gas loss and reduce the relative density. On the other hand, higher cell density/lower cell size was obtained at a lower cooling rate. The synergetic effect of talc/nanoclay significantly improved the cellular structure and this improving behavior was more considerable at higher contents. Finally, the relative density was markedly improved by reducing the screw speed due to the sufficient time for dissolution of gas inside the polymer which led to an increase of the cell density up to 2.5 × 104 cell/cm3. The cell size of samples was also decreased to 280 µm.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Taher Azdast

References

Azdast

Hasanzadeh

. Increasing cell density/decreasing cell size to produce microcellular and nanocellular thermoplastic foams: a review. J Cell Plast 2020; DOI: 10.1177%2F0021955X20959301.

Hasanzadeh

Azdast

Doniavi

, et al. Multi-objective optimization of heat transfer mechanisms of microcellular polymeric foams from thermal-insulation point of view. Therm Sci Eng Prog 2019; 9: 21–29.

Azdast

Lee

Hasanzadeh

, et al. Investigation of mechanical and morphological properties of acrylonitrile butadiene styrene nanocomposite foams from analytical hierarchy process point of view. Polym Bull 2019; 76(5): 2579–2599.

Cai

Huang

Chen

, et al. Thermo-expandable microspheres strengthened polydimethylsiloxane foam with unique softening behavior and high-efficient energy absorption. Appl Surf Sci 2021; 540: 148364.

Okolieocha

Raps

Subramaniam

, et al. Microcellular to nanocellular polymer foams: progress (2004–2015) and future directions—a review. Eur Polym J 2015; 73: 500–519.

Moradian

Azdast

Doniavi

. Investigating the effect of foam properties on the attenuation of coaxial cables with foamed polyethylene dielectric. Polym Adv Technol 2020; 31(12): 3328–3340.

Abedini

Asiyabi

Campbell

, et al. On fabrication and characteristics of injection molded ABS/Al₂O₃ nanocomposites. Int J Adv Manuf Technol 2019; 102(5): 1747–1758.

Azdast

Hasanzadeh

Moradian

. Improving impact strength in FSW of polymeric nanocomposites using stepwise tool design. Mater Manuf Process 2018; 33(3): 343–349.

Hasanzadeh

Azdast

Doniavi

. Thermal conductivity of low-density polyethylene foams part II: deep investigation using response surface methodology. J Thermal Sci 2020; 29(1): 159–168.

10.

Carrino

Durante

Franchitti

, et al. Mechanical performance analysis of hybrid metal-foam/composite samples. Int J Adv Manuf Technol 2012; 60(1): 181–190.

11.

Daryadel

Azdast

Hasanzadeh

, et al. Simultaneous decision analysis on the structural and mechanical properties of polymeric microcellular nanocomposites foamed using CO2. J Appl Polym Sci 2018; 135(14): 46098.

12.

Leung

Wong

Park

, et al. Ideal surface geometries of nucleating agents to enhance cell nucleation in polymeric foaming processes. J Appl Polym Sci 2008; 108(6): 3997–4003.

13.

Hasanzadeh

Azdast

Doniavi

, et al. A prediction model using response surface methodology based on cell size and foam density to predict thermal conductivity of polystyrene foams. Heat Mass Transf 2019; 55(10): 2845–2855.

14.

Verdu

Zoller

Marcilla

Plastisol foaming process. Decomposition of the foaming agent, polymer behavior in the corresponding temperature range and resulting foam properties. Polym Eng Sci 2013; 53(8): 1712–1718.

15.

Hopmann

Hendriks

Spicker

, et al. Surface roughness and foam morphology of cellulose acetate sheets foamed with 1,3,3,3-tetrafluoropropene. Polym Eng Sci 2017; 57(4): 441–449.

16.

Lee

Park

Ramesh

. Polymeric foams: science and technology. Boca Raton, FL: CRC Press, 2006.

17.

Martini

Ellen

Suh

. The production and analysis of microcellular foam. Master’s Thesis, Mechanical Engineering, MIT, 1981.

18.

Park

. The role of polymer/gas solutions in continuous processing of microcellular polymers. Doctoral dissertation, Massachusetts Institute of Technology, 1993.

19.

Lee

Park

. Use of nitrogen as a blowing agent for the production of fine-celled high-density polyethylene foams. Macromol Mater Eng 2006; 291(10): 1233–1244.

20.

Park

Behravesh

Venter

. Low density microcellular foam processing in extrusion using CO₂. Polym Eng Sci 1998; 38(11): 1812–1823.

21.

Park

Suh

. Filamentary extrusion of microcellular polymers using a rapid decompressive element. Polym Eng Sci 1996; 36(1): 34–48.

22.

Park

Baldwin

Suh

. Effect of the pressure drop rate on cell nucleation in continuous processing of microcellular polymers. Polym Eng Sci 1995; 35(5): 432–440.

23.

Behravesh

Park

Venter

. Challenge to the production of low-density, fine-cell HDPE foams using CO2. Cell Polym 1998; 17(5): 309–326.

24.

Naguib

Park

Lee

. Effect of talc content on the volume expansion ratio of extruded PP foams. J Cell Plast 2003; 39(6): 499–511.

25.

Rodríguez-Pérez

. Crosslinked polyolefin foams: production, structure, properties, and applications. In: Abe

Dušek

Kobayashi

(eds) Crosslinking in Materials Science. Berlin: Springer, 2005, pp. 97–126.

26.

Nazri

Behravesh

Shahi

. Investigation on microstructure and production of coaxial cable foam insulation using butane gas. ADMT J 2008; 2(1): 37–42.

27.

Nofar

. Effects of nano-/micro-sized additives and the corresponding induced crystallinity on the extrusion foaming behavior of PLA using supercritical CO₂. Mater Des 2016; 101: 24–34.

28.

Okolieocha

Köppl

Kerling

, et al. Influence of graphene on the cell morphology and mechanical properties of extruded polystyrene foam. J Cell Plast 2015; 51(4): 413–426.

29.

Chaudhary

Jayaraman

. Extrusion of linear polypropylene–clay nanocomposite foams. Polym Eng Sci 2011; 51(9): 1749–1756.

30.

Sahagún

González-Núñez

Rodrigue

. Morphology of extruded PP/HDPE foam blends. J Cell Plast 2006; 42(6): 469–485.

31.

Shaayegan

Wang

Costa

, et al. Effect of the melt compressibility and the pressure drop rate on the cell-nucleation behavior in foam injection molding with mold opening. Eur Polym J 2017; 92: 314–325.

32.

Chauvet

Sauceau

Fages

. Extrusion assisted by supercritical CO₂: a review on its application to biopolymers. J Supercrit Fluids 2017; 120: 408–420.

33.

Mohebbi

Mighri

Ajji

, et al. Current issues and challenges in polypropylene foaming: a review. Cell Polym 2015; 34(6): 299–338.

34.

Zhang

Zhou

. Effects of process variables on microcellular structure and crystallization of polypropylene foams with supercritical CO₂ as the foaming agent—a study of microcellular foaming of polypropylene. Polymer-Plast Technol Eng 2007; 46(9): 885–891.

35.

Hasanzadeh

Darvishi

Azdast

. Synergetic effect of MWCNT/nanoclays on microcellular polystyrene hybrid nanocomposite foams. Carbon Lett 2020; 30: 367–371.

36.

Larsen

Neldin

. Physical extruder foaming of poly (lactic acid)—processing and foam properties. Polym Eng Sci 2013; 53(5): 941–949.