Induced crystalline fiber-like structure as reinforcement in PLA products through applied shear in injection molding

Abstract

In recent years, there has been a growing interest in sustainable, biobased, and biodegradable polymers due to the rise of public awareness towards petroleum-based polymers, the requirement to comply with new environmental laws, and the considerations about the ecological impacts of the product over the entire life cycle. Polylactic acid (PLA) is a biopolymer produced from renewable resources, considered an alternative to commodity and engineering applications due to its high strength, stiffness, and production capacity. However, PLA suffers from high brittleness, low melt strength, and slow crystallization, hampering processing and applicability. To improve the mechanical and thermal properties of PLA, methods such as PLA-based nanocomposite development, fiber modification methods, fiber reinforcement, mixing with rigid polymers or plasticizers, and chain branching have been applied. Although these attempts could eliminate the disadvantages of PLA, the biodegradability and natural advantages of PLA should be preserved while also reducing the end-product cost. Furthermore, the additives or fibers must be compatible with the mechanical and thermal recycling processes applied to the PLA products after use. Self-reinforced PLA composites (SR-PLAs) have been introduced to provide high strength and stiffness without traditional reinforcements. Furthermore, the absence of foreign reinforcements such as glass or carbon fiber can contribute to the complete biodegradability and easy recyclability of the composite. Self-reinforced composites are comprised of identical or similar types of polymers in the matrix and reinforcement phases. This concept aims to obtain a solid and stable matrix-fiber interface using polymers with similar chemical structures in both phases. However, the matrix and the reinforcement phases must have a certain melting temperature difference to produce these composites. Thus, while the matrix will be completely molten, the structure of the reinforcing polymer will be preserved. This study aims to determine the manufacturability and mechanical and thermal properties of in-situ SR-PLAs prepared through injection molding. Different types of PLAs were used to provide the required melting temperature difference in the matrix and reinforcement phase. The matrix was formed by an amorphous PLA (aPLA). The reinforcement phase was created by three semi-crystalline PLA grades (cPLA1, cPLA2, and cPLA3) with different crystallizability and molecular weight. To induce isothermal cold crystallization, the cPLAs were initially annealed at 90 oC, between the glass transition and melting temperature of PLA. Then, the aPLA/cPLA blends with different cPLA types and compositions with a weight ratio of 95/5, 90/10, 85/15, and 80/20 were prepared using a dry mixer. Finally, the aPLA/cPLA blends were dehumidified at 50 oC overnight and injection molded. Before processing, an aPLA/cPLA3 blend with a weight ratio of 85/15 was injection molded in different barrel and mold temperatures to determine the process parameters. The differential scanning calorimetry (DSC) analysis revealed non-isothermal cold crystallization peaks for aPLA/cPLA3 samples processed at an average barrel temperature of 160 oC. This was mainly due to the stretching and unfolding of some cPLA crystals during processing. In addition, the tensile results showed slightly lower mechanical properties for aPLA/cPLA3 samples processed at a mold temperature of 20 oC. Therefore, the barrel and mold temperatures were determined as 150 and 40 oC. Next, the thermal and mechanical behavior of neat PLA and aPLA/cPLA blends were studied. The neat PLA did not exhibit crystallinity due to the absence of the cPLA phase. On the other hand, the aPLA/cPLA blends with cPLA contents below 5 wt% also did not exhibit crystallinity. The aPLA/cPLA blends with cPLA contents above 10 wt% showed crystallinities due to unmelted cPLA crystals during processing as the process was carried below the melting temperature of cPLA crystals. Moreover, the crystallinities of aPLA/cPLA3 blends were more pronounced than aPLA/cPLA1 and aPLA/cPLA2 blends due to the higher melting temperature and crystallizability of cPLA3. The single glass transition around 60 oC indicated miscibility between aPLA and cPLAs. The tensile test results revealed that the tensile strength and modulus values of the aPLA/cPLA samples were significantly greater than those of the neat aPLA. Moreover, this reinforcing effect became even more prominent when cPLA3 was used. These enhancements were due to the induced fiber-like structure obtained through the inherent high shear rate of the injection molding process. The strain at break values did not reveal substantial improvements, although aPLA/cPLA samples were slightly more ductile than neat PLA. Finally, the HDT and impact strength analysis of aPLA/cPLA were investigated to improve the processability and applicability of PLA. While the cPLA addition slightly increased the heat deflection temperature (HDT) to 60 oC, it unaltered the impact strength at 12 kJ/m2. Therefore, polybutylene adipate terephthalate (PBAT) was selected to improve the tensile toughness of SR-PLA blends. cPLA3 was chosen as the reinforcing phase in SR-PLAs due to its most prominent enhancement effect on thermal and mechanical properties. The SR-PLA/PBAT blends with a fixed 15 wt. % PBAT was prepared similarly to SR-PLA preparation methods. In addition, 85 wt. % SR-PLAs contained different cPLA3 compositions with a weight ratio of 90/10, 80/20, and 70/30 to observe the effects of cPLA content on the material properties of SR-PLA/PBAT blends. The thermal analysis revealed that the SR-PLA/PBAT had higher crystallization than SR-PLAs. Moreover, the crystallinity was further increased with increasing cPLA3 content. The melting temperature of SR-PLA/PBAT blends was lower than SR-PLAs. This was attributed to the fact that the PBAT droplets could have penetrated between the cPLA crystals, resulting in a less ordered structure. The SR-PLA(80/20)/PBAT and SR-PLA(70/30)/PBAT blends exhibited non-isothermal cold crystallization due to the higher crystallizability of cPLA3. Furthermore, the cold crystallization in PLA(70/30)/PBAT occurred earlier than in PLA(80/20)/PBAT as the higher cPLA3 contents further enhanced the orientation of fiber-like crystals and supported heterogeneous cold crystallization. Finally, the two distinct glass transitions around -35 and 60 oC indicated immiscibility between PLA and PBAT. The stress-strain behavior of SR-PLA/PBAT blends showed that the PBAT addition resulted in a transition from ductile to brittle behavior. The tensile strength and modulus of SR-PLA/PBAT blends were lower than neat PLA. The maximum losses in terms of percentage for tensile strength and modulus were 15 and 25%, respectively. On the other hand, the strain at the break of the SR-PLA/PBAT blends was significantly greater than neat PLA. Moreover, this reinforcing effect was even more significant in higher cPLA3 contents due to the formation of a more uniform blend morphology. As a result, the ductility of neat PLA was increased from 3.9 to 44.6% in SR-PLA(70/30)/PBAT. Furthermore, the impact strength of neat PLA was enhanced to 16 kJ/m2 in all SR-PLA/PBAT blends. Finally, the improvements in blend morphology were demonstrated by SEM images. The microstructure of SR-PLA/PBAT blends revealed that the PBAT droplets were finely dispersed within the SR-PLA. Moreover, higher cPLA3 contents improved the blend melt strength, promoting the breakup of PBAT droplets. Therefore, a more uniform blend morphology corresponded to enhancements in ductility and impact strength.