Effect of Plasticizers on the Structure and Properties of Polylactic Acid/Thermoplastic Starch Blends

Publication Date: 2014-05-20 Qing Song Zhang, Jing Jing Deng, Xiang Wei He, Zhi Hua Wu

(College of Polymer Science and Engineering, Sichuan University, Chengdu, Sichuan 610065)

Abstract: Citric acid tributyl ester (TBC) and polyethylene glycol (PEG) were used to plasticize the poly(lactic acid) (PLA) / thermoplastic starch (TPS) blend system. The rheological properties of PLA were adjusted to improve the compatibility between PLA and TPS, melt blending characteristics, and the microstructure and mechanical properties of the blend. The results show that TBC has a better modification effect than PEG; TBC can increase the uniform dispersion of TPS, and the phase dispersion size is significantly reduced; the tensile strength and elongation at break of TBC-modified PLA/TPS are significantly improved, with lower water absorption.

Keywords: Poly(lactic acid); Thermoplastic starch; Plasticizer; Citric acid tributyl ester; Polyethylene glycol; Structure; Properties

Chinese Library Classification: TQ322.6 Document Code: A Article Number: 1005-5770 (2009) 11-0051-04

Poly(lactic acid) (PLA) is an aliphatic polyester synthesized from renewable resources [1]. PLA possesses good mechanical properties, biocompatibility, and processing performance. Compared to existing petroleum-based polymers, PLA can be completely degraded into CO2 and H2O in the natural environment, making it a truly green and environmentally friendly material. PLA has been widely used in packaging, medical, and daily necessities fields [2]. However, pure PLA materials have defects such as brittleness and low mechanical strength. Furthermore, the relatively high price of PLA limits the widespread application of PLA resin. To overcome these shortcomings, existing polymers are blended and modified with PLA. The main existing method is blending starch and thermoplastic starch to modify PLA [3]. PLA/thermoplastic starch improves the brittleness of PLA/starch to a certain extent, but the toughness of the PLA/thermoplastic starch blend system still cannot meet the requirements for widespread application [4], which has become a major obstacle to its application in various fields. Adding plasticizers is an effective means to improve toughness and increase flexibility [5].

In this experiment, thermoplastic starch (TPS) was prepared by blending starch and glycerol. Citric acid tributyl ester (TBC) and polyethylene glycol (PEG) were used as plasticizers to modify the PLA/TPS (mass ratio 70/30) blend system. The changes in the system's phase morphology, phase transition characteristics, rheological properties, mechanical properties, water absorption, and plasticizer migration were studied.

1. Experimental Section

1.1 Raw Materials and Equipment

Polylactic acid: Industrial grade, Cargill-Dow Chemical Company, USA; Corn starch: Industrial grade, Luzhou Bio-technology (Shaanxi) Co., Ltd.; Glycerol: Chemically pure, Chengdu Kelong Chemical Reagent Factory; Polyethylene glycol 10000: Analytical pure, Chengdu Kelong Chemical Reagent Factory; Tributyl citrate (TBC): Analytical pure, Tianjin Chemical Reagent Research Institute. Torque rheometer: XSS-300, Shanghai Light Industry Mould Factory; Plate vulcanizer: YJ66, Chengdu Hangfa Hydraulic Co., Ltd.; Differential scanning calorimeter: DSC-204F, Netzsch, Germany; Capillary rheometer: CFT-500D, Shimadzu, Japan; Scanning electron microscope: JSM-5900LV, JEOL Ltd., Japan; Universal electronic material testing machine: Instron-4302, Instron, USA.

1.2 Sample Preparation

Preparation of TPS: Mix starch with additives such as glycerol thoroughly according to the formula and knead evenly. The mass ratio of starch to glycerol is 100/40.

Preparation of PLA/TPS blend: Dry PLA, plasticizer, and TPS were melt-kneaded in a torque rheometer kneading head according to the formula, and then pressed into sheets to make standard specimens. The mass ratio of PLA to TPS is 70/30.

1.3 Performance testing and analysis

Mechanical property testing: According to GB/T 1040—2006, the tensile speed is 50 mm/min.

SEM observation: Tensile specimens were fractured in liquid nitrogen along the tensile direction, vacuum-gold-plated on the fracture surface, and then observed using a scanning electron microscope to examine the fracture morphology.

DSC analysis: Sample weight is about 5 mg, heating range is 0-200℃, heating rate is 10 K/min, under nitrogen protection.

流变性能测试:在170℃下测定在不同剪切速率的的熔体表观黏度,口模长径比为10∶1。

Plasticizer Migration Test: Weigh the plasticized PLA/TPS (or MATPS) sample, record as H0, then place it in a vacuum oven at a constant temperature of 80°C and a vacuum of 0.04 MPa. Every 12 hours, take out the sample, wipe it with absorbent paper, and weigh it, recording as Hi (i=1, 2, 3, 4...). After 48 hours, take out the sample. The migration rate of the plasticizer is calculated using the following formula:

Water absorption test: In a room temperature environment, the sample is placed on a partition in a sealed container with a saturated brine solution at the bottom (generating a relative humidity of 75%). The sample is then taken out every 24 hours to be weighed, denoted as Gi (i=1, 2, 3, 4...). After 20 days, the sample is dried in an oven (70°C, 24 h), cooled to room temperature in a desiccator, and then weighed, denoted as G0. The water content in the sample is calculated using the following formula:

2 Results and Discussion

The amount of various plasticizers (PEG10000, TBC) mentioned in this article refers to the amount of plasticizer used to plasticize the PLA system.

2.1 Effect of Plasticizers on the Mechanical Properties of PLA/TPS

Figure 1 shows the experimental curves of the effect of different plasticizer amounts on the mechanical properties of PLA/TPS.

From Figure 1, it can be seen that both plasticizers increased the elongation at break of the material. The tensile strength of PEG-plasticized PLA/TPS gradually decreased with the increase of plasticizer content; the tensile strength of the TBC-plasticized blend system showed a trend of first increasing and then decreasing with the increase of plasticizer content, exhibiting a plasticizing effect. For the PEG-plasticized system, the addition of plasticizer weakened the interaction forces between polymer chains, making the polymer chains easier to move and enhancing their ability to change conformation. The plasticization process is a separation process of macromolecules. The plasticizer shields the interaction centers of some polymer chains, weakening the forces between adjacent polymer chains and causing these chain segments to separate, thereby imparting fluidity and flexibility to the polymer material, increasing its elongation at break, and decreasing its tensile strength. Among these two plasticized systems, TBC showed a better plasticizing effect, especially when the mass fraction of TBC was ≤10%, exhibiting a plasticizing effect. This may be because TBC is a small molecule plasticizer that easily penetrates PLA, making the PLA macromolecular chain segments easier to change conformation, arrange neatly, reduce internal stress and defects, thereby improving the tensile strength of PLA. PEG, on the other hand, is a macromolecular plasticizer that takes time to penetrate into PLA molecules. During the mixing process, PEG molecules may wrap around TPS molecules, leading to uneven dispersion of TPS. After PEG penetrates PLA, the viscosity of the plasticized PLA system is higher than that of the TBC-plasticized system, and the dispersion of TPS is poorer.

2.2 Effect of Plasticizers on the Rheological Properties of PLA/TPS

Figure 2 shows the capillary rheology curves of plasticized PLA/TPS. Table 1 shows the non-Newtonian index of plasticized PLA/TPS. It can be clearly seen from the figure that as the shear rate increases, the viscosity of plasticized PLA/TPS decreases faster than that of unplasticized PLA/TPS. This may be because in the plasticized PLA/TPS system, the plasticizer penetrates between the PLA macromolecules, causing the macromolecular chains to stretch and entangle, increasing the diameter of the flow units, enhancing the non-Newtonian nature of PLA, and increasing its sensitivity to shear. This effect is more pronounced in the TBC-plasticized system.

2.3 Scanning Electron Microscopy Analysis of Plasticized PLA/TPS

Figure 3 shows the SEM images of the brittle fracture surfaces of plasticized PLA/TPS samples.

As can be seen from Figure 3a: The TPS phase distribution in the PLA/TPS system is relatively uniform, with a phase size of 20-35 μm, and the TPS phase is separated from the PLA phase. It can be clearly seen from Figure 3b: In the 10% PEG10000 plasticized PLA/TPS, the TPS distribution is uneven, with varying sizes and increased phase size. In Figure 3c, the TPS phase size in the 10% TBC plasticized PLA/TPS system is significantly reduced to 7-16 μm and is uniformly distributed. It can be clearly seen from the figure: the interface between the TPS phase and the PLA phase is blurred, which may be due to the plasticizing effect of TBC on PLA.

2.4 DSC Analysis of Plasticized PLA/TPS

Figure 4 shows the DSC curves of PLA/TPS and plasticized PLA/TPS.

It can be seen from the figure that the transition temperature from the glassy state to the highly elastic state of PEG-plasticized PLA/TPS is lowered, and the transition from the glassy state to the highly elastic state of the TBC-plasticized PLA/TPS system basically disappears. The addition of small molecule plasticizers enhances the mobility of polymer macromolecules and lowers the glass transition temperature. The chain segment mobility of TBC small molecule plasticization is greater than that of PEG plasticization, and the glassy state basically disappears. The cold crystallization temperature of PLA in PEG10000 plasticized PLA/TPS becomes wider, but the cold crystallization enthalpy becomes smaller. This may be due to the larger and uneven TPS phase size in the PEG plasticized system, which hinders the crystallization of PLA. PEG-plasticized PLA/TPS has only one melting peak, which may be because the melting point of PEG10000-plasticized PLA is lowered and coincides with the melting point of TPS, causing the two melting peaks to overlap and shift towards lower temperatures. In TBC-plasticized PLA/TPS, the glass transition and cold crystallization peaks of PLA disappear, and there is only one melting peak, which is consistent with the reason for the PEG plasticized system. The disappearance of the cold crystallization peak and glass transition temperature indicates that the comprehensive properties of the blend will be more excellent. This may be because the addition of TBC small molecules makes the mobility of polymer macromolecules very active, thereby preventing the crystallization of PLA.

2.5 Migration of plasticizer in plasticized PLA/TPS

Figure 5 shows the migration rate curve of plasticizer in the PLA/TPS system.

The migration of plasticizer is mainly composed of two parts: one is the migration of glycerol in TPS, and the other is the migration of plasticizer in PLA. It can be seen from the figure that in the system containing 10% plasticizer, the migration rate of the TBC plasticizing system is much lower than that of the PEG plasticizing system. This may be due to two reasons: first, the compatibility of the plasticizing system. Better compatibility leads to less migration; second, the change in PLA crystallization. PEG is a crystalline polymer, and PEG crystallization accelerates the phase separation of PEG and PLA, increasing the migration of plasticizer in the system. TBC is a liquid substance, has good compatibility with PLA, and has a compatibilizing effect. Its cold crystallization peak for plasticized PLA is very weak, so the migration of plasticizer in the TBC plasticizing system is smaller.

2.6 Effect of plasticizer on water absorption of PLA/TPS

It can be seen from the figure that the water content at equilibrium of PLA/TPS plasticized with 10% TBC is the lowest, and the water content at equilibrium of PLA/TPS plasticized with 10% PEG10000 is the highest. The water content at equilibrium of PLA/TPS is 12.23%. The main factors affecting the water absorption of plasticized PLA/TPS system are the migration of plasticizer and the morphology structure of TPS in the system. When the phase size of TPS in the system is small and the distribution is uniform, TPS is better wrapped by PLA, blocking its contact with moisture and reducing its water absorption.

Figure 6 shows the effect of 10% plasticizer on the water absorption performance of PLA/TPS specimens.

3 Conclusion

1) SEM observation shows that PEG increases the phase dispersion size of PLA/TPS blends, and reduces the glass transition temperature and cold crystallization temperature. The dispersion phase size of the TBC plasticized PLA/TPS system is significantly smaller, with a uniform distribution and blurred phase interface, and the glass transition temperature and cold crystallization temperature disappear. This is especially true for the 10% TBC plasticized PLA/TPS system.

2) Both plasticizers PEG10000 and TBC can significantly alter the tensile properties of the PLA/TPS system. When the mass fraction of TBC is less than 10%, the plasticized system exhibits a plasticizing effect. Among them, 10% TBC plasticized PLA/TPS shows the best effect, with the maximum tensile strength of 35.33 MPa. The elongation at break of the system increases monotonically, from 4.41% to 15.62%.

3) The plasticizer migration rate and water absorption rate of TBC plasticized PLA/TPS materials are much lower than those of the PEG plasticized system.