Research On The Performance And Structure Of Poly(Lactic Acid)(PLA)Based Composites

Poly(lactic acid)(PLA) is one kind of biodegradable polymer with high strength which has been applied in the field of medicine, packaging and so on. However, the drawback of fragile, poor heat-resistance and impact strength, low crystallization rate and high cost limited its applications in other fields. Different fillers and modification agent were chosen to compound with PLA to improve its mechanical property and crystallization in the present study. The inherent relationship of structure and performance was also studied. The present study aimed to expand the application field of PLA, and provide fundamental data and reference frame for the manufacturing of PLA composites.Firstly, hemp fiber with high cost performance was chosen as the reinforcement, and a new fiber feeding stock---compounding fiber pellets obtained from the mix-spinning roving of hemp fibers and PLA fibers was used to prepare the PLA/hemp fiber composites via twin-screw extruding. Partial of the fiber were treated by silane coupling agent. The effects of fiber content, interface adhesion on the mechanical property, crystallization behavior and heat-resistance of composites were studied. Results showed that the addition of hemp fiber not only improved the mechanical property of composites, but also promoted the crystallization of PLA markedly without reducing the ductility. The strength and modulus can be further improved after the fibers were treated by silane, but the impact strength and heat-resistance of composites were reduced. The modulus of composites was decreased when the dosage of silane agent was higher than3wt%. The compound fiber pellets were considered as a convenient and effective feeding stock which can provide a new approach for the manufacturing of natural fiber reinforcing plastic composites.Then, microfibrillated cellulose extracted from natural cellulose fibers was chosen as additives to compound with PLA via a solvent extraction-hot pressing method. The tensile property, dynamic mechanical property, melting behavior and isothermal crystallization behavior of PLA/MFC composites were studied. And the heating and cooling process of samples were detected on line by an X-ray diffraction detector. Results showed that the MFC has great potentialities both on reinforcing and nucleating. It shortened the half time of crystallization of PLA. and improved the tensile strength and modulus drastically. Mechanical properties of the materials annealed at different temperature were different which was attributed to the different crystal style of PLA. Different PLA crystals were obtained in the pure PLA and PLA/MFC composites under the cooling process, which was attributed to the improving of crystallization temperature of PLA by addition of MFC. The storage modulus of composite with20wt%MFC loading can retain high modulus which is above1GPa at135℃。 This is significant for shortening the molding time and improving productive efficiency.Poly (ethylene-methyl acrylate-glycidyl methacrylate)(EGMA) was used compounding with PLA to improve its toughness via twin-screw extruding. The effects of EGMA content and dispersed phase size on the property of PLA/EGMA blend were studied by adjusting the content of EGMA and extruding parameters. Annealing was also carried out to observe the effect of PLA crystallization on the impact strength of blend. Results showed that a brittle-ductile transition occurred when the EGMA content was15wt%. And the impact strength became constant at about70kJ/m2when the content of EGMA was higher than30wt%. The impact strength of blend was affected by the temperature and extruding rate. When the content of EGMA was lower than30wt%, the impact strength increased drastically after annealing, but an inverse result was obtained when EGMA was higher than30wt%. It predicted that the toughening mechanism changed after annealing because of the improvement of crystallization of PLA.At last, a ternary blend system of PLA/EGMA/talcum was designed to improve the impact strength of PLA blend, meanwhile, retain its strength, modulus and heat-resistance. A two-step method was used to prepare PLA/EGMA/talcum blend---which the EGMA and talcum powder were melting blended firstly using a twin-screw extruder, then the obtained blend were cut to pellets and melting blended with PLA. The ternary blend has superior mechanical properties than binary blend of PLA/EGMA and PLA/talcum no matter the blend was annealed or not. Core-shell particle formed for the talcum/EGMA blend, which is a main reason for the outstanding properties. When PLA/EGMA/talcum=90/5/5(w/w/w), the impact strength was26.6kJ/m2and almost reach the value of PLA/EGMA(90/10) obtained in the former study. But the tensile strength and modulus were much higher than that of it. Comparing with the pure PLA, the impact strength was improved for3.5times, but only have10%and7%reduction on the strength and modulus respectively. Therefore, the preparing of this ternary blend including a reactive polymer and a rigid inorganic particle is considered as very effective method for toughening modification of PLA.