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Ind. Eng. Chem. Res. 2009, 48, 7594–7602
Properties of Poly(lactic acid)/Poly(butylene adipate-co-terephthalate)/ Nanoparticle Ternary Composites Long Jiang, Bo Liu, and Jinwen Zhang* Composite Materials and Engineering Center, Washington State UniVersity, Pullman, Washington 99164
Poly(lactic acid) (PLA) can be toughened by flexible poly(butylene adipate-co-terephthalate) (PBAT) at the cost of a certain degree of strength and modulus loss. In an attempt to achieve balanced overall properties, PLA ternary composites containing both PBAT and rigid nanoparticles, i.e., montmorillonite clay (MMT) or nanosized precipitated calcium carbonate (NPCC), were prepared by twin screw extrusion and subsequent injection molding. Mechanical testing demonstrated that the composites containing MMT exhibited higher tensile strength and modulus but lower elongation compared to the composites containing NPCC. Using maleic anhydride (MA) grafted PLA as a compatibilizer, the elongation of the ternary composites was substantially increased, possibly due to improved dispersion of the nanoparticles. The injection molded ternary composites were shown to have a skin-core layered structure. The skin and core layers were found to possess different microstructure, thermal behavior, and mechanical properties. The microstructure difference led to a sequential fracture behavior during tension testing: the fracture of the core layer was followed by the skin layer. The skin layer, with a higher degree of PLA chain alignment and conformational ordering than the core layer, exhibited a higher glass transition temperature, lower cold crystallization temperature, and a higher degree of perfection in crystalline structures. Introduction In recent years, polylactic acid (PLA), a biodegradable thermoplastic polyester derived from corn starch feedstock, has been extensively studied as an alternative to petrochemical plastics. Due to its high modulus and strength, PLA has been used to produce fibers, film, vehicle interiors, appliance components, food wares, food/beverage packaging, etc. However, unsatisfactory characteristics such as brittleness, low heat deflection temperature (HDT), and poor barrier properties have significantly restricted its expansion into new application areas. The development of methods for PLA toughening is a major topic which has attracted extensive research interest since the advent of PLA. Blending PLA with flexible polymers is a practical and economical way to obtain toughened PLA products. Polycaprolactone (PCL), a biodegradable polymer having a low Tg is probably the most widely studied PLA toughening agent. Studies have shown that blending PLA with PCL resulted in significant increases in PLA ductility and toughness but that the PLA strength and modulus were also substantially reduced.1,2 It has also been reported that the elongation-at-break of PLA/PCL blends was substantially improved when a PLA/PCL diblock copolymer was used as a compatibilizer.3 In situ compatibilization of PLA/PCL by reactive blending was also studied using different catalysts/ coupling agents.4,5 The results showed that under certain conditions significant toughening effects could be realized as the result of reactive compatibilization. PLA was also toughened by other biodegradable polymers such as poly(butylene adipateco-terephthalate) (PBAT),6-8 poly(propylene carbonate) (PPC),9 poly(para-dioxanone) (PPD),10 and poly(butylene succinate) (PBS).11 In general, these PLA blends displayed improved elongation-at-break but reduced strength and modulus due to the addition of a ductile phase. On the other hand, organically modified layered silicates (organoclay) have been shown to effectively improve HDT, * To whom all correspondence should be addressed. Tel.: 509-3358723. Fax: 509-335-5077. E-mail:
[email protected].
barrier properties, and dynamic mechanical and rheological properties of PLA.12-17 Our recent study showed that at low concentrations, e.g.
