Effect of PBAT on physical,morphological,and mechanical properties of PBS/PBAT foam

Abstract

This study examines the effect of poly(butylene adipate-co-terephthalate) (PBAT) content on the physical, morphological, and mechanical properties of poly(butylene succinate) (PBS)/PBAT foam. A compression molding technique was used to prepare the PBS/PBAT foam using the chemical blowing agent azodicarbonamide and the cross-linking agent dicumyl peroxide. The chemical structure and morphological properties of PBS/PBAT foam were examined via Fourier transform infrared and scanning electron microscopy techniques, respectively, whereas tensile and flexural properties were investigated using a universal testing machine. The results reveal that the incorporation of PBAT barely enhances the viscosity of the PBS/PBAT blend, producing only minor changes in the average cell size of PBS/PBAT foam. However, increasing the PBAT content contributes to a relatively significant improvement in the flexibility and toughness of PBS/PBAT foam, where a decrease in Young’s modulus and tensile strength of the PBS/PBAT foam is observed compared with those of the PBS foam. Similar behavior to the tensile results is noticed for the flexural properties of the neat and PBS/PBAT foams.

Keywords

Biodegradable polymer poly(butylene succinate)polymer blend foam morphology

Introduction

With growing concerns about the environment, the importance of bio-based and biodegradable polymers is rising due to plastic waste generation and disposal. Biodegradable polymers appear to provide a promising solution for such problems. Poly(butylene succinate) (PBS) produced from succinic acid and 1,4 butanediol^1

-4 is one of the most attractive commercial biopolymers owing to its excellent properties, such as biodegradability, thermal stability, thermal and chemical resistance, and good processing.⁵ However, PBS still has certain limitations, especially in industrial development, due to its high production cost. The foaming process appears to be feasible since significantly less polymeric material is used due to the gaseous voids in the polymer matrix.⁶ Polymeric foams have significant unique properties, such as being lightweight with greater thermal and sound insulation, and enhanced mechanical properties to weight ratio. Hence, they have vast application potential in several industries.

Azodicarbonamide (ADC) is a well-known blowing agent for producing foam. Numerous scientists are utilizing ADC for PBS.^7

-13 However, some researchers reveal that heat release from the exothermic reaction of ADC during the foam process could be an issue due to the low viscosity of linear PBS, cell collapse, and deteriorating mechanical or insulation properties of PBS foams. Yue et al.¹⁴ utilized endothermic expandable microspheres (EnEMs) to counteract the exothermic effect from ADC decomposition. With a decrease in the amount of ADC and an increase in EnEMs, the closed-cell fraction of PBS foams is improved from 21% to approximately 100%, the average cell size decreased from 450 µm to 30 µm, the flexural strength increased from 3 MPa to 21 MPa, and the modulus from 106 MPa to 268 MPa. Feng et al.⁸ disclosed the effect of ADC on PBS properties by examining different amounts of ADC (i.e. 2, 4, 6, and 8 phr). At the point when ADC content is 2 phr, PBS foams with regular and narrow cell size distribution are obtained, whereas with an ADC content of 4 and 6 phr, some cells rupture and coalesce, and tensile strength also decreases. Samples containing 8 phr of ADC are a complete failure, and all mechanical tests could not be performed. Accordingly, 2 phr of ADC was chosen for this study since this amount can generate adequate gas cells and does not cause cell damage from the exothermic reaction.

Attributable to linear molecular chains and low molecular weight, PBS has relatively low viscosity, which is a limitation in the foaming process since viscosity is a primary parameter in the foam stabilization stage, that is, low viscosity causes cell collapse during the foaming process. Numerous investigations have been conducted to address the viscosity issue in PBS foaming,^{8,10

-13,15
-17} for example, applying chemical reagents (e.g. cross-linking agent and branching agent) to modify the chemical structure,¹⁸ applying radiation to cross-link PBS,¹⁷ and incorporating fillers to enhance the viscosity of the matrix.^10,12,15 Incorporating the cross-linking agents appears to be the most favorable method since it is more cost-effective than conventional foam equipment. Generally, PBS foaming using the cross-linking agent provides rigid PBS foam, resulting in limited end-use applications. To address this drawback, alternative solutions have been revealed, such as supercritical fluid foaming,¹⁹ incorporating EnEMs,²⁰ and the polymer-blending approach.²¹

Foaming polymer blends have a high potential for better control over properties, such as the size of the cellular structure. Yu et al.²² revealed the effect of PBS on poly(lactic acid) (PLA) foaming behavior using supercritical carbon dioxide (CO₂) as a blowing agent via batch foaming. The formation of the open cell structure of PLA-based PBS foam was investigated as to the PBS content and foaming temperature. It was found that PLA/PBS (80/20) foamed at 100°C provides the highest cell opening rate (96%). Ketkul et al.²³ disclosed a PLA-PBS-activated carbon (AC) composite foam by investigating various PLA/PBS ratios with fixed AC content (i.e. 5 phr). A comparison between the PLA-PBS blended foam and the PLA-PBS-AC composite foam was performed, and the results indicated that the AC could improve the modulus and tensile strength of composite foam only in the PBS-rich samples, whereas for the blended foams, greater tensile strength and modulus were observed with more PLA content. Oliviero et al.²⁴ revealed that biodegradable thermoplastic gelatin (TPG)/PBS foams were prepared via subsequent batch foaming using supercritical CO₂. The addition of PBS reduced the melt viscosity and increased both the CO₂ diffusivity and thermal stability of TPG. The TPG/PBS foams show smaller cell size and higher cell density than the neat TPG foam.

The number of biodegradable polymer blend foaming studies is still limited in comparison to those on biodegradable polymer blends. The PBS/poly(butylene adipate-co-terephthalate) (PBAT) blend appears to be attractive since previous research^21,25 reveals the improved rheological properties of the blends once PBAT is incorporated. Studies on the PBS/PBAT (60/40, 70/30, and 50/50 wt%) blend have reported good compatibility in lower amounts of PBS, as evidenced by a uniform dispersion of the PBS phase in the PBAT phase. Boonprasertpoh et al.²⁶ investigated the morphological, rheological, and mechanical properties of PBS/PBAT blends with an extensive ratio range from 0 wt% to 100 wt%. They disclosed that PBS/PBAT blends with a PBAT ratio of at least 70 wt% and at most 30 wt% exhibiting improved viscosity and relatively uniform dispersion of one polymer phase into another, especially for PBAT ratios of 80 and 20 wt%.

In the light of our previous study,²⁶ this research aims to develop PBS/PBAT foam using a blend with two PBAT ratios, that is 20 and 80 wt%, to observe the physical, morphological, and mechanical properties of PBS/PBAT foam obtained. In addition, relationships among these properties are discussed.

Materials and methods

Materials

The PBS GS PLA FZ91PD (T _m = 118°C) was supplied by Mitsubishi Chemical Corporation (Japan), while the PBAT Ecoflex FBX7011 (T _m = 110°C) was supplied by BASF (Ludwigshafen, Germany). ADC (yellow powder; gas volume = 235 ± 5 ml g⁻¹ at STP) as a blowing agent was supplied by A.F. Goodrich Chemicals Co., Ltd (Bangkok, Thailand). The dicumyl peroxide (DCP) was supplied by Kij Paiboon Chemical LP (Bangkok, Thailand) for use as a cross-linking agent. The PBS, PBAT, and ADC were dried in a vacuum oven at 70°C for 6 h before compounding.

Sample preparation

The PBS/PBAT blends and ADC and DCP were mixed using an internal mixer (Brabender WIN/B/S 350E, Brabender, Duisburg, Germany) at 120°C with 50 r min⁻¹ rotor speed and then pelletized (Retsch SM 100, Retsch, Haan, Germany). In reference to our previous work,⁷ 2 phr of DCP was applied during compound preparation. To avoid the cellular structure’s damage caused by ADC,¹⁴ 2 phr of ADC was chosen, as previously mentioned in the Introduction section. Next, foam fabrication was performed using a Collin P500PM (Collin, Munich, Germany) compression molding machine at 160°C for 10-min heating time and 10-min cooling time at a pressure of 1000 N cm⁻². The composition of PBS/PBAT blends prepared in this study is summarized in Table 1.

Table 1.

Composition of PBS/PBAT blends.

Sample	PBS	PBS80	PBS20	PBAT
PBS (wt%)	100	80	20	0
PBAT (wt%)	0	20	80	100

PBS: poly(butylene succinate); PBAT: poly(butylene adipate-co-terephthalate).

Morphological studies

The cell morphology of the fractured surface on PBS/PBAT foams was examined using scanning electron microscopy (SEM) Hitachi SU3500 (Hitachi Krefeld, Germany). The samples were immersed in liquid nitrogen for about 3 min and then broke off. The fractured surface was coated with a thin layer of platinum using a sputter coater (Quorum Technologies SC7620, Laughton, East Sussex). Micrographs at 30× magnification with an accelerating voltage of 8 kV were obtained. SemAfore software (version 5.21, JEOL CO., Helsinki, Finland) was used to examine cell size and cell distribution.

The cell density, defined as the number of cells per cubic centimeter relative to unfoamed material, was characterized from the SEM micrograph using the following equation²⁷

Cell density = {[{(n M)}^{2} / A]}^{3}^{/ 2} \times expansion ratio

where n is the number of cells in the micrograph, A and M are the area and magnification factor of the micrograph, respectively.

Rheological studies

The viscosity of all compounds, that is, the neat PBS, PBS/PBAT blends, and the neat PBAT, containing 2 phr of DCP were determined using a Malvern Bohlin Gemini HR rotational rheometer (UK) nano (on a parallel plate) gap 1 mm (at 1 Hz frequency) and 160°C (i.e. foam processing temperature).

Chemical structure studies

Chemical structure analysis was performed using Fourier transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet 6700 ATR-FTIR, Massachusetts, USA). All samples were performed under conditions of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. For each sample of the spectrum, 64 accumulated scans were produced and the absorbance was recorded as a function of wavenumbers.

Mechanical studies

Young’s modulus, tensile strength, and elongation at break values of samples were measured according to ASTM D638-15 using a universal testing machine (Instron 5569, Instron, Massachusetts, USA). The dumbbell-shaped samples were tested at 50 mm min⁻¹ crosshead speed with a 10-kN load at room temperature (about 23 ± 2°C). At least five specimens were tested for each sample to obtain an average value.

Flexural strength and modulus samples were measured according to ASTM D790-03 using a universal testing machine (Hounsfield H10KM, Hounsfield, Redhill, England). The samples were tested at a crosshead speed of 30 mm min⁻¹ with a 75 N load at room temperature (about 23 ± 2°C). At least five specimens were tested for each sample to obtain an average value.

Results and discussion

Physical and morphological properties of foam samples

Figure 1 exhibits SEM micrographs of the neat PBS, PBS80, PBS20, and PBAT foams with spherical and oval-shaped cells distributed in all foam samples. It is noticeable that the greater the PBAT content the smaller the cell dimension is observed. This may be caused by the viscosity of PBAT being higher than the PBS, resulting in cell growth hindrance.²⁶

Figure 1.

SEM micrographs at ×30 magnification of foam samples. SEM: scanning electron microscopy.

The compound viscosity, foam density, average cell size, and cell density are listed in Table 2. Compared with the neat polymers, a decrease in compound viscosity for both PBS80 and PBS20 is observed. This may be caused by phase separation in an immiscible blend, providing weak interaction between PBS and PBAT phases, as discussed by Boonprasertpoh et al.²⁶ Numerous researchers^21,25,28 also report the same phenomenon, in that an immiscible blend could lead to decreased viscosity even when incorporating compatibilizers into the system. Kowalczyk et al.²⁰ studied the PLA/PBS blend system incorporating a compatibilizer, that is, glycidyl methacrylate, and DCP and revealed that the phase separation of an immiscible blend still remained.

Table 2.

Viscosity of foam compound, foam density, average cell size, and cell density.

Sample	Compound viscosity at 160°C 1 Hz (Pa·s)	Foam density (g cm⁻³)	Average cell size (mm)	Cell density (105 cells cm⁻³)
PBS	13,713.33 ± 105.99	0.64 ± 0.01	0.45 ± 0.16	1.71 ± 0.38
PBS80	12,746.67 ± 101.16	0.61 ± 0.01	0.53 ± 0.27	1.54 ± 0.29
PBS20	14,133.33 ± 49.33	0.64 ± 0.01	0.30 ± 0.13	4.59 ±0.57
PBAT	24,233.33 ± 755.27	0.66 ± 0.01	0.22 ± 0.80	19.69 ± 0.48

PBS: poly(butylene succinate); PBAT: poly(butylene adipate-co-terephthalate).

An insignificant change in foam density can also be observed in Table 2. The average cell size of the foam sample seems to decline as the compound viscosity increases, that is, greater viscosity provides a smaller cell size. This is because the viscosity has a pronounced effect on cell growth. A similar analysis was reported by Bradley and Phillips,²⁹ whereby the viscosity appears to be one of the parameters controlling cell collapse. As expected, the cell density shows an opposite trend in compound viscosity, conforming to the SEM micrographs in Figure 1.

Additionally, a graph showing the cell size distribution of all foam samples obtained from the SemAfore software is shown in Figure 2. Again, the cell size distribution may relate to the compound viscosity, that is, greater viscosity provides more uniform cell size distribution, as seen by the narrower curve base.

Figure 2.

Cell size distribution graphs of foam samples.

Chemical structure studies

To identify the chemical interaction between polymers, FTIR analysis was performed. The FTIR spectra of the neat polymer foams and their blends are shown in Figure 3.

Figure 3.

FTIR spectrum of foam samples. FTIR: Fourier transform infrared.

The asymmetric stretching vibration of –CH₂ in the PBS chain was observed at 1327 cm⁻¹. The peak decreases with a decreased amount of PBS. A sharp peak at 720 cm⁻¹ represents four or more adjacent methylene (–CH₂–) groups. The peak decreases with an increased amount of PBS, while the ester linkage vibration in PBAT is shown at 1263 cm⁻¹. The peak also decreases with a decreased amount of PBAT. Muthuraj et al. revealed that a transesterification reaction might occur in the PBS and PBAT blend reflecting a peak shift in the carbonyl group toward higher wavenumbers (from 1705 cm⁻¹ to about 1722 cm⁻¹).²¹ The occurrence of a transesterification reaction would enhance the tensile strength of the blend. As seen from the peaks in Figure 3, it can be concluded that no transesterification reaction^21,25 between PBS and PBAT occurs since no peak shift in the carbonyl group is observed.

In comparison to our previous work,²⁶ blending neat PBS and neat PBAT results in a transesterification reaction, increasing the viscosity compared to their matrix. However, in this research, DCP was incorporated into the foaming process. Maybe the transesterification reaction was disturbed by the DCP, resulting in no increase in the viscosity and mechanical properties, as confirmed by the results.

Mechanical properties

Tensile properties

Table 3 presents the tensile properties of the neat polymer and PBS/PBAT foams. It is noticeable that Young’s modulus and the tensile strength at break exhibit the same trend, that is, these values decrease with an increase in PBAT content. Conversely, the elongation at break increases with an increase in PBAT content, probably due to the high flexibility and toughness properties of PBAT.

Table 3.

Tensile properties of neat polymers and PBS/PBAT foams.

Sample	Young’s modulus (MPa)	Tensile strength at break (MPa)	Elongation at break (%)
PBS	290.32 ± 16.65	8.16 ± 0.36	3.84 ± 0.25
PBS80	164.45 ± 4.46	5.59 ± 0.22	4.90 ± 0.31
PBS20	46.07 ± 0.67	3.49 ± 0.03	32.81 ± 2.87
PBAT	34.27 ± 5.44	3.74 ± 0.43	86.78 ± 5.10

PBS: poly(butylene succinate); PBAT: poly(butylene adipate-co-terephthalate).

Compared with the neat PBS foam, a dramatic decline in Young’s modulus and tensile strength values of the PBS/PBAT foams is observed. This may be caused by a heterogeneous blend in the system,²⁶ the nature of PBAT (i.e. high flexibility), and also the nonoccurrence of a transesterification product in the blend system as supported by the FITR results.

John et al.¹ also reported a similar phenomenon for the PBS/PBAT blend system, where the tensile strength of the PBS/PBAT blends decreases with increasing PBAT content. As anticipated, the elongation at break of the foam sample increases with increasing PBAT content due to the nature of PBAT²⁶ properties. John et al.¹ also found the same phenomenon, that is, the elongation at break of the blends increases with increasing PBAT content.

Flexural properties

In Table 4, similar behavior to the tensile results is detected for the flexural properties of the neat polymer and the PBS/PBAT foams, whereby the neat PBS foam exhibits higher flexural modulus and flexural strength than the neat PBAT foam because of the nature of PBAT properties (i.e. high flexibility and toughness). As anticipated for the blend system, both flexural modulus and flexural strength decrease with increasing PBAT content. In this study, it can be presumed that the reduction of flexural modulus and flexural strength of the blends is simply attributable to the heterogeneous blend in the system²⁶ and the nature of PBAT since no transesterification product is detected in the blend system as shown by the FTIR results.

Table 4.

Flexural properties of neat polymers and PBS/PBAT foams.

Sample	Flexural modulus (N m⁻² )	Flexural strength (N m⁻² )
PBS	580.00 ± 41.08	20.78 ± 1.17
PBS80	437.80 ± 43.46	17.10 ± 2.10
PBS20	99.90 ± 3.08	4.67 ± 0.12
PBAT	74.19 ± 7.92	2.89 ± 0.48

PBS: poly(butylene succinate); PBAT: poly(butylene adipate-co-terephthalate).

Finally, it is noticeable that the gas phase shows a relatively inadequate effect on the mechanical properties in this study since there is no obvious variation in foam density, indicating an equivalent ratio between the polymer and gas phase for all samples.

Conclusion

The morphological studies reveal that the incorporation of PBAT barely enhances the viscosity of the PBS/PBAT blend resulting in a minor change of PBS/PBAT foam density. As expected, the cell density is in reversal to the average cell size. However, increasing the PBAT content assists in a relatively significant improvement of flexibility and toughness in PBS/PBAT foam, where a decrease in Young’s modulus and tensile strength of the PBS/PBAT foams is observed compared to those of PBS foam. Similar behavior to the tensile results is noticed for the flexural properties of the neat and PBS/PBAT foams. Finally, it can be summarized that the effect on the mechanical properties of this foam system is caused by the matrix phase because of no obvious variation of foam density revealed here.

Footnotes

Acknowledgements

The authors also gratefully acknowledge the Department of Materials Science, Faculty of Science, Chulalongkorn University, A.F. Goodrich Chemicals Co., Ltd, and PTT Research and Technology Institute for their support.

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was funded by Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok, Thailand.

ORCID iD

Aekartit Boonprasertpoh

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