Abstract

A cold rolling process was performed with extruded polylactic acid (PLA) as the crystalline polymer at different rolling ratios. The variations of the crystalline morphology, crystallinity, density, molecular orientation, and the microhardness were investigated during the process. Moreover, the dynamic mechanical properties and mechanical properties were evaluated by dynamic mechanical analysis and tensile test, respectively. The results showed that plastic deformation more easily occurred on the surface than on the interior. In addition, with the increase of the rolling ratio, the crystallinity decreased; however, the molecular orientation increased. Dynamic mechanical analysis revealed that the rolled PLA displayed an anisotropy during the process. Moreover, the tensile strength increased from 51.1 MPa to 86.0 MPa, and the fracture strain increased from 5.3% to 103.1%, in the case of a 60% rolling ratio along the rolling direction, indicating that the PLA was homogenized during the rolling process. This certified that an appropriate rolling process (i.e., the 60% rolling ratio) was conducive for improving the comprehensive mechanical properties of PLA in the rolling direction.

Introduction

In the field of materials, recycling is one of the most important technologies to conserve resources and to reduce waste. Biological degradation is widely considered as an effective recycling technology. In recent years, many biologically degradable polymers, e.g., polybutylene succinate (PBS), polycaprolactone (PCL) and polylactic acid (PLA) have been developed. Out of these polymers, PLA is considered as one of the most promising biodegradable, compostable, thermoplastic, and crystalline polymers. Additionally, PLA is a sustainable alternative to petrochemical-derived products and can be derived from renewable resources (Jonoobi et al., 2010). PLA has many advantages, such as high-strength, high-modulus (Garlotta, 2001) and good stiffness (Jonoobi et al., 2010). However, because PLA belongs to the group of brittle materials, the fracture strain is very low (approximately 5%) (Garlotta, 2001). Hence, this disadvantage may limit its applications in industry