Study of the Preparation and Properties of TPS/PBSA/PLA Biodegradable Composites

Abstract

Thermoplastic starch/butyl glycol ester copolymer/polylactic acid (TPS/PBSA/PLA) biodegradable composites were prepared by melt-mixing. The structure, microstructure, mechanical properties and heat resistance of the TPS/PBSA/PLA composites were studied by Fourier-transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), tensile test and thermogravimetry tests, respectively. The results showed that PBSA or PLA could bind to TPS by hydrogen bonding. SEM analysis showed that the composite represents an excellent dispersion and satisfied two-phase compatibility when the PLA, TPS and PBSA blended by a mass ration of 10, 30, and 60. The mechanical properties and the heat resistance of TPS/PBSA/PLA composite were improved by adding PLA with content less than 10%, according to the testing results.

1. Introduction

Starch, the second largest natural polymer on earth, is widely synthesized by plants. Due to the renewable, biodegradable resource and low price, starch is widely used in the manufacturing industry for food, medicine, plastic and chemical packaging [1,2,3,4,5,6]. At present, the production of starch degradable plastics accounts for more than 60% of the total production of degradable plastics [7,8,9,10,11]. However, natural starch is a polyhydroxy compound, and the adjacent molecules interact with each other by hydrogen bonds to form complete particles with a microcrystalline structure, which contributes to poor processing and mechanical properties [12,13,14,15,16,17]. Thermoplastic starch (TPS) is a mixture of starch and plasticizer, which acts thermal plasticity and shear effect at high-temperature. However, the products of TPS possess high hygroscopicity, awful heat tolerance, and poor dimensional stability, which restricts the use of TPS vastly.

In order to overcome these defects, biodegradable polyester materials often are mixed into TPS. Succinic acid butyl glycol ester adipic acid butyl glycol ester copolymer (PBSA) is one of the biodegradable polyester materials that reported widely at home and abroad in recent years. As a semi-crystalline polymer, PBSA has properties of excellent water resistance, well thermal plasticity and easy shaping and processing, etc. Meanwhile, polylactic acid (PLA) could increase the compatibility between PBSA and TPS and maintain the biodegradability of PBSA. In brief, the membrane-blown biodegradable composite with excellent comprehensive performance was prepared by blending TPS, PBSA and PLA [18].

2. Materials and Methods

2.1. Materials

The materials for the preparation of TPS/PBSA/PLA biodegradable composite are corn starch (the water content is 10% and particle size is 100 mesh, National Starch), glycerin from Tianjin Sitong Chemical Plant (analytically pure), PBSA with the melt flow rate (190 °C, 2.16 kg) of 2.4 g/10 min is purchased from Kingfa Sci. & Tech. Co., Ltd. (Guangzhou, China). PLA with the melt flow rate (190 °C, 2.16 kg) of 6.3 g/10 min is produced by NatureWorks.

2.2. Preparation of TPS

The corn starch was dried at 120 °C in the oven for 5 h to make the moisture content down to 1% and then mixed with glycerin in a high-speed mixer at the mass ratio of 100:20 for 10 min. The mixed materials were sent into the twin-screw extruder (d = 40 mm, L/D = 44:1), and the drawing rod was cooled and pelleted. The temperature of each section of the extruder barrel (from the feeding port to the extruder die) was set at 90 °C/100 °C/120 °C/120 °C/120 °C/130 °C, the screw speed was 200 r/min, and the feeding speed was 50 kg/h, the thermoplastic starch was prepared for use.

2.3. Preparation of TPS/PBSA

After drying at 80 °C for 5 h, the TPS and PBSA were mixed in a high-speed mixer at the mass ratio of 0:100, 10:90, 30:70 and 50:50. After being evenly mixed, the mixed materials were sent into a co-rotating twin-screw extruder for extrusion granulation at 130 °C–150 °C, the screw speed was 200 r/min, and the feeding speed was 50 kg/h, the TPS/PBSA blends with 0%, 10%, 30% and 50% thermoplastic starch were obtained.

2.4. Preparation of TPS/PLA

The TPS prepared as in 2.2 and PLA were placed in the oven and dried at 80 °C for 5 h to make the moisture content less than 1000 ppm. Composites of TPS/PLA were prepared by melt-mixing. First, TPS and PLA were mixed in a high-speed mixer. Then, the mixture was sent to the co-rotating twin-screw extruder (d = 40 mm, L/D = 44:1). The drawing rod was cooled and pelleted. The temperature of each section of the extruder barrel (from the feeding port to the extruder die) was set at 130 °C /140 °C /140 °C /150 °C /150 °C /140 °C /130 °C, the screw speed was 200 r/min, and the feeding speed was 50 kg/h. The proportion for the composites of TPS/PLA was described in

Table 1. Formula of thermoplastic starch/polylactic acid (TPS/PLA blends) (PHR).

Sample	TPS	PLA
a	0	100
b	20	80
c	40	60
d	60	40

.

2.5. Preparation of TPS/PBSA/PLA

Before the preparation of TPS/PBSA/PLA composite, PBSA and PLA were dried for 5 h at 80 °C. Then the mixture of TPS/PBSA/PLA was feed into a co-rotating twin-screw extruder, as is mentioned in 2.2, after being pre-mixed by a high-speed mixer. The temperature for each section (from the feeding port to extrusion die) of the extruder barrel was set at 130 °C/140 °C/140 °C/150 °C/150 °C/140 °C/130 °C. The screw speed was 200 r/min, and the feeding speed was 50 kg/h. Then the TPS/PBSA/PLA composites were prepared. The formula for the composites of TPS/PBSA/PLA is shown in

Table 2. Formula of TPS/PBSA/PLA blends (PHR).

Sample	TPS	PBSA	PLA
a	30	70	0
b	30	65	5
c	30	60	10
d	30	55	15
e	30	50	20

.

2.6. Characterization and Testing

All the composites were dried after prepared and before the test, and the water absorptions were controlled down to 1%.

The chemical composition of the TPS/PBSA/PLA composites was detected by using Fourier-transform infrared spectroscopy (BRUKER VECTOR22). The microstructure of TPS/PBSA/PLA blends was characterized by a scanning electron microscope (XL-30).

The mechanical properties of the blends were tested using an extension test under the universal tester (WDW) produced by Tianjin Sansi Test Instrument Manufacturing Co. Ltd. (Tianjin, China). The specifics testing method was as per the GB/T 1040-2006. The blends were injected into dumbbell samples in the injection molding machine at 190 °C after drying for 4 h at 80 °C. The stretching velocity of the extension test was 20 mm/min. The fracture surface of the tensile specimen was characterized by SEM. The anti-heat property of the TPS/PBSA/PLA composites was tested with a thermal gravimetric analyzer (TGA) in a nitrogen protection environment, with the temperature up to 700 °C at a speed of 20 °C/min.

3. Results and Discussion

3.1. Characterization of the TPS/PBSA Blend

3.1.1. Infrared Analysis of TPS/PBSA Blend

The structure of TPS/PBSA composites was characterized by FT-IR. The spectra of pure PBSA and TPS/PBSA composites are shown in Figure 1. As presented in the spectra of TPS/PBSA composites, several absorption peaks raised at 500–800 cm⁻¹ and became stronger with the increase of the TPS content, which indicated that PBSA broke the symmetry of single rings in starch. Hydroxide radical (OH) in the PBSA and TPS acts out a binding effect by forming a hydrogen bond. As shown in the spectra of TPS/PBSA blends, a wide peak occurred at 3000–3700 cm⁻¹, which was contributed by the stretching vibration of OH. With the increase of the TPS content, the peak located at 3000–3700 cm⁻¹ became more strength, which is caused by the associating OH in TPS. The peaks at 2930, 2910, 1760, 1260~1470, 1160~940 cm⁻¹ were aroused by the vibration of -CH3, CH2, carbonyl group in PBSA, -CH2, the asymmetric stretching vibration of C-O-C and the stretching and skeleton vibration absorption of C-O, respectively.

3.1.2. Properties of the TPS/PBSA blend

Figure 2 shows the elongation and tensile properties of the TPS/PBSA blend. When the content of thermoplastic starch was 10%, the elongation at break and tensile strength of the TPS/PBSA blend were reduced to 478% and 18.1 MPa, respectively. Moreover, the amount of decline was lower than 10%. However, with the continuous increase of the TPS content, the elongation at break and tensile strength of blends diminished rapidly, and when the content of TPS upped to 50%, the elongation at break and tensile strength decreased to 278% and 10.5 MPa. Tests for mechanical properties showed that the elongation at break and tensile strength of the TPS/PBSA blend reduced inordinately. When the content of thermoplastic starch was low, the TPS dispersed evenly in the PBSA matrix, and the OH on the surface of TPS interacted with PBSA. Thus, the elongation at break and tensile strength are not significantly reduced.

When the content of TPS was too much, the interaction of TPS with the PBSA matrix was enhanced, which limited the movement of PBSA segments. Moreover, with the increase of TPS content, the TPS tended to agglomerate and separate from PBSA, which lead to the performance degradation of the composites.

Figure 3 was the TGA curve of the TPS/PBSA blend in N2. For PBSA, there were two obvious steps in the TGA spectrum, which represented the two-degradation process of PBSA during heating. The first degradation process occurred around 400 °C, which was the pyrolysis process of the PBSA molecular chain. The second degradation process occurred higher than 500 °C, which was the carbonization process of the PBSA molecule. For TPS/PBSA, there were four obvious steps in the TGA spectrum. The first degradation process related to the loss of glycerol and water in TPS came up at a temperature lower than 200 °C. The second degradation process took placed at a temperature of about 300 °C, which was resulted from the thermal cracking of starch molecules. The third degradation process occurred around 400 °C, which was caused by the pyrolysis of the PBSA molecule. The fourth degradation process happened after 500 °C, which was induced by the carbonization of PBSA and starch. As a result, the thermal degradation temperature of TPS/PBSA blends decreased with the increment of the TPS content.

3.1.3. SEM Results of TPS/PBSA Blends

Figure 4 shows the SEM images of TPS/PBSA blends. As a comparison, the SEM for pure PBSA was also presented. As shown in Figure 4, it can be seen that the TPS is distributed in the PBSA matrix. When the amount of thermoplastic starch was low (10%), the distribution of particle TPS was homogeneous. However, when the amount of thermoplastic starch was up to 50%, an obvious agglomeration phenomenon appears. The TPS particles and PBSA matrix were not closely bonded, and a clear interface appeared between the two phases. The surface of pits after the starch particles fall off was smooth, which indicated that the starch particles had a good dispersion in the PBSA matrix, but their binding force was very weak due to the polarity difference between them.

3.2. Characterization of TPS/PLA Blend

3.2.1. Infrared Analysis of TPS/PLA Blend

The structure of the TPS/PLA blend was characterized by FT-IR shown in Figure 5. Compared with the spectra of PLA blends, a long and wide stretching vibration peak appeared at 3000~3700 cm⁻¹ of spectra of TPS/PLA blends, which was the characteristic of OH in starch. The absorption peak shifted to the low wavenumber directly with the increase of TPS content, which indicated that the OH group in starch and OH group in PLA exist intermolecular hydrogen bond. The results showed that there were several absorption peaks with different strength at 500–800 cm⁻¹, and the absorption peaks became stronger with the increase of TPS content. The result further indicated that PLA broke the starch monocyclic symmetry after blending with thermoplastic starch and then caused the change of the position of a heterocyclic hydroxyl group, which meant PLA combined with TPS through hydrogen bond association.

3.2.2. Properties of TPS/PLA Blend

The mechanical properties data of the TPS/PLA blend are shown in

Table 3. Mechanical properties of TPS/PLA blend.

Sample	a	b	c	d
Tensile strength/MPa	72.5 ± 2.6	56.1 ± 2.3	42.7 ± 3.1	24.8 ± 2.1
Elongation/%	5 ± 0.3	26 ± 0.9	57 ± 2.4	7 ± 0.4
Notch impact strength/KJ·m2	3.2 ± 0.2	5.3 ± 0.1	7.8 ± 0.2	3.9 ± 0.1
Bending strength/MPa	110.8 ± 9.6	92.5 ± 9.6	68.6 ± 7.2	27.9 ± 5.8
Bending modulus/MPa	3805 ± 132	4010 ± 212	2516 ± 264	1222 ± 109
MI/g·10 min (190 °C, 2.16 kg)	5.1	9.8	22.6	32.5
Density/g·cm3	1.227	1.266	1.304	1.329

. Due to the glycerin in thermoplastic starch permeated into PLA, which destroyed the hydrogen bond in PLA and reduced the intermolecular force of PLA, the tensile strength decreased with the increase of thermoplastic starch content. The notched impact strength and elongation at break of TPS/PLA blend increased first and then decreased with the increase of thermoplastic starch content, which due to the interaction between hydroxyl groups on the surface of starch particles and PLA matrix. When the thermoplastic starch content was low and well dispersed, the notched impact strength and elongation at break were increased. When the amount of starch was too much, on one hand, the interaction with the PLA matrix was strengthened, which limited the movement of PLA chain segment. On the other hand, with the increase of starch content, the phase separation trend in the blend increased, which led to the decline of its properties. The toughness of the blends increased with the increase of thermoplastic starch content in the composite. The addition of glycerol reduces the crystallinity and intermolecular force of starch, soften the starch, improved the mobility of starch segments and the whole macromolecules, increased the compatibility of starch and polylactic acid, and improved the toughness of the TPS/PLA blends. In addition, the melt flow rate and the density of thermoplastic starch/PLA blends increased with the increase of thermoplastic starch content.

The TGA curve of TPS/PLA blends in nitrogen atmosphere presented in Figure 6. TGA results showed that there were two obvious steps in the degradation process of PLA. The first degradation process occurred before 500 °C, and the maximum decomposition temperature was around 400 °C, which is the thermal cracking process of the PLA molecular chain; the second degradation process occurred after 500 °C, which was the carbonization process of PLA molecule. The degradation process of TPS/PLA blend has four obvious steps, and the first degradation process was lower than that of TPS/PLA blend 200 °C was related to the volatilization of glycerol and water in TPS. The second degradation process occurs between 225 °C and 375 °C, and the maximum decomposition temperature was about 300 °C, which was related to the thermal cracking of starch molecules. The third degradation process occurred between 375 °C and 475 °C, and the maximum decomposition temperature was around 400 °C, which was caused by the thermal cracking of PLA molecules. The fourth degradation process occurred after 500 °C, which was the carbonization of starch and PLA. The thermal degradation temperature of the TPS/PLA blend decreases with the increase of thermoplastic starch content. PLA could significantly increase the heat resistance temperature of TPS.

3.2.3. SEM Results of TPS/PLA Blend

Figure 7 shows the SEM result of the TPS/PLA blend. As a dispersion phase, TPS is distributed in the matrix of continuous phase PLA. When the amount of TPS was reduced (20%), the granular TPS was evenly distributed. With the amount of TPS increased to 60%, the agglomeration phenomenon appeared. The granular TPS was not closely bound to the PLA matrix, there was a clear interface between the two phases, and the surface of the pit was smooth after starch granules fall off. Although the good dispersion of starch particles in PLA matrix, the polarity difference between TPS and PLA, their binding force was weak, and the compatibility between the two phases was poor.

3.3. Characterization of the TPS/PBSA/PLA Composites

3.3.1. Infrared Analysis of the TPS/PBSA/PLA Composites

Figure 8 was the FT-IR spectrum of the TPS/PBSA/PLA composites. As shown in the spectra, the TPS/PBSA/PLA blends had a long and wide infrared absorption characteristic peak at the 3000~3700 cm⁻¹, which was the stretching vibration of O-H. Compared with the FT-IR spectra of PLA and PBSA, the peak at 3000~3700 cm⁻¹ of the TPS/PBSA/PLA composites was contributed by the stretching vibration of -OH from starch. However, the absorption peak shifted to lower wavenumber, which may be the existence of hydrogen bonds between the molecules of O-H in starch and PLA/PBSA. There were multiple absorption peaks with varying strength at 500–800 cm⁻¹, which further indicated that PBSA or PLA broke the single ring symmetry of TPS after blending with TPS, thus causing the change of the position of the starch heterocyclic hydroxyl group.

3.3.2. Properties of the TPS/PBSA/PLA Composites

Table 4. Mechanical properties of the TPS/PBSA/PLA composites.

Sample	a	b	c	d	e
Tensile strength/MPa	13.2 ± 0.9	14.3 ± 1.0	16.1 ± 1.0	15.9 ± 0.4	12.9 ± 0.9
Elongation/%	451 ± 12	340 ± 10	360 ± 9	317 ± 12	240 ± 13
Bending strength/MPa	6.5 ± 0.2	8.0 ± 0.1	10.5 ± 0.8	13.9 ± 0.6	11.9 ± 0.6
Bending modulus/MPa	141 ± 8	199 ± 10	261 ± 8	390 ± 10	370 ± 12
MI/g·10 min (190 °C, 2.16 kg)	2.1	1.9	1.5	1.1	0.9
Density/g·cm3	1.227	1.266	1.224	1.228	1.226

was the result for the mechanical properties of the TPS/PBSA/PLA composites. As shown in Table 4, the tensile strength, bending strength and bending modulus increased with the increase of PLA content at first, and when the content of PLA was 5~15%, the value of the tensile strength, bending strength and bending modulus was increased significantly. The increased strength was due to the hardness of PLA. However, the increased PLA decreased the breaking elongation. When the PLA content was 20%, the elongation at break of the blend decreased significantly, mainly because of the fragility of PLA, and the addition of PLA into the system reduced the toughness of the blend, leading to a reduction in the breaking elongation.

From the above results, it can be concluded that the addition of PLA could improve the properties of the blends. As a reinforcement phase in the blend, hydroxy and carboxy groups in the PLA built a good interfacial binding effect with TPS through intermolecular hydrogen bond, which made for the disperse of TPS into PBSA. When the content of PLA is 10%, PLA had the best compatibility with PBSA, which contributed the best properties for the blends. However, when the content for PLA was elevated, the compatibility between PLA and PBSA was deteriorated, resulting in the degradation of system performance. Melt flow rate (MFR) for the TPS/PBSA/PLA composites was decreased with the increment of PLA, which could provide acceptable mobility for film blowing of the blends.

Figure 9 was the TGA spectrum for the TPS/PBSA/PLA composites in N2. As displayed in the TGA spectrum, there were four steps in the spectrum of the TPS/PBSA/PLA composites, and steps in the spectrum meant the degradation of composites. Based on the thermal decompositions of TPS/PBSA and TPS/PLA shown in Figure 3 and Figure 6, the first degradation process of the TPS/PBSA/PLA composites was lower than 200 °C, which was related to the volatilization of glycerin and water in TPS. The second degradation process occurred between 225 °C and 375 °C, and the maximum decomposition temperature was about 300 °C, which was related to the thermal decomposition of TPS molecules. The third degradation process occurred between 375 °C and 475 °C, and the maximum decomposition temperature was about 400 °C, which was caused by thermal cracking of PBSA and PLA molecules. The fourth degradation process occurred after 500 °C and was the carbonization of PBSA, PLA and TPS. In conclusion, PBSA and PLA can obviously improve the heat-resistant temperature of TPS, especially when the content of PLA was low, but the heat-resistant performance decreased when the content of PLA was high.

3.3.3. SEM Results of the TPS/PBSA/PLA Composites

Figure 10 was the SEM images of the TPS/PBSA/PLA composites. As a comparison, the SEM of pure starch was also presented in Figure 10a. Pure starch is a spheroidal particle with a size of about 10 μm. As shown in Figure 10b, TPS was distributed in the matrix of continuous phase PBSA as the dispersed phase. When the content of PLA was 10%, PLA and PBSA were mutually soluble to form a continuous phase, and the TPS particles dispersed more evenly in the PLA/PBSA phase, as shown in Figure 10c. When the content of PLA is up to 15%, TPS tended to reunite to large particles and precipitates on the surface of the matrix, as shown in Figure 10d. From the results of SEM, it can be concluded that PLA could improve the compatibility of TPS and PBSA. However, the high content PLA would weak the compatibility of TPS and PBSA.

4. Conclusions

Corn starch was plasticized with glycerin to make it for thermoplastic processing. Then the thermoplastic starch (TPS) was blended with PBSA and PLA, and a series of the TPS/PBSA/PLA composites were prepared. FT-IR was used to characterize the structure of the TPS/PBSA/PLA composites, and the results showed that PBSA or PLA combined with TPS through hydrogen bonding. The mechanical properties of the TPS/PBSA/PLA composites were improved. TGA analysis showed that PLA could significantly improve the heat resistance of TPS/PBSA/PLA. However, the heat resistance of TPS/PBSA/PLA decreased when the PLA content was up to 50%. SEM showed that the TPS was well dispersed in PBSA substrate when PLA content was low, while TPS and PBSA presented weak compatibility and poor adhesion with a high content of PLA. The optimum content of PLA addition was the amount of 10%. In conclusion, the best ratio for the new type of filler-modified TPS/PBSA/PLA composites is 30:60:10. The starch had an excellent dispersion performance and good mechanical properties during the film-forming process. The starch membrane material can be completely used as biodegradable plastic.