Improvement of impact fracture properties of PLA/PCL polymer blend due to LTI addition

Recent years, considerable attention has been paid to biodegradable polymers, mainly owing to increasing interest for preservation of environment and substitution of petrochemical polymers. Poly (lactic acid) (PLA), one of typical biodegradable polymers, is being considered for use in a variety of industrial fields including car, computer and electric appliances. PLA is also utilized as a biomaterial in medical fields such as orthopedics and oral surgery [1, 2], due to bioabsorbability and biocompatibility. Fracture properties and fracture behavior of PLA have been studied, and it was found that crazes are formed in crack-tip region prior to crack initiation, that is similar to the fracture behavior of brittle polymers such as PP and PS [3–7]. Improvement of the fracture properties of such brittle polymers can generally be achieved by blending a ductile secondary phase to the base polymer. Poly (e-caprolacton) (PCL), a ductile biodegradable polymer, has been chosen as a blending partner for PLA [8–14], and the fracture properties of PLA/PCL was found to be greater than those of neat PLA [9]. However, it was also found that the immiscibility of PLA and PCL causes phase separation, and tends to lower the fracture properties especially when PCL content increases. In the present study, lysine triisocyanate (LTI) was used as an additive for PLA/PCL blend to improve such immiscibility. Impact fracture properties of PLA/PCL/LTI blend was evaluated and compared to those of PLA/PCL blend to assess the effectiveness of LTI addition. Effect of LTI addition on fracture micromechanism was also investigated by observing their fracture surfaces using a scanning electron microscope (SEM). PLA/PCL and PLA/PCL/LTI blends were fabricated from PLA pellets (Lacty #9030, Shimadzu Co. Ltd), PCL pellets (CelgreenH7, Daicel Chemistry Industries Co.) and LTI (Kyowa Co. Ltd) by melt-mixing in a conventional melt-mixer at 180 C for 20 min at a rotor speed of 50 rpm. The mixing ratio of PLA and PCL was fixed at 85:15 in weight fraction, and the LTI content was chosen to be 1 wt% in this study. Plates of 2-mm thick were then fabricated from the mixtures using a hot press attached with water-cooling system. These mixtures were melted at 180 C and pressed at 30 MPa, and then quenched to room temperature using the cooling system. Single-edge-notchbend (SENB) specimens were prepared from these PLA/ PCL and PLA/PCL/LTI plates. The J-integral at crack initiation, Jin, and the average fracture energy, Jf, were evaluated at an impact rate of 1.4 m/s using an instrumented drop weight impact testing system with dynamic displacement measuring apparatus [15, 16]. Jin and Jf were calculated using the following formulae: