Authors: (Long Jiang, Jinwen Zhang, Wood Materials and Engineering Laboratory, Washington State University, Pullman WA)
Abstract: Polylactic acid (PLA) is a cornstarch-based biodegradable polyester. Commercial PLAs exhibit much higher Young’s modulus and tensile strength than mainstream petroleum-based polymers such as polyethylene (PE) and polypropylene (PP). However, brittleness is a major problem preventing PLA from widespread applications. In this chapter, three PLA toughening strategies by physical blending are presented, including soft polymer inclusion toughening, rigid nanoparticle toughening and hybrid multicomponent toughening. Poly(butylene adipate-co-terephthalate) (PBAT) is a flexible biodegradable polyester with low strength and modulus. It was blended with PLA using a twin-screw extruder. It was found that at only 5 wt% PBAT content, the blend demonstrated a 10-fold increase in tensile elongation. However, this toughening effect is achieved by sacrificing strength and modulus of the blend. Nanoparticle (e.g., montmorillonite and nanosized calcium carbonate) reinforced polymer composites often exhibits superior mechanical properties because of MMT’s unique layered platelet nanostructure. At 2.5 wt% MMT content, elongation of the PLA/MMT blend tripled and most importantly, strength and modulus of the blend increased as well. However, Both PBAT and MMT toughening had their limitations, i.e., lack of the flexibility in the design of product properties. Ternary blends of PLA/PBAT/MMT provided an ideal platform to tailor mechanical properties of PLA to meet specific requirements of different applications. Ternary system brought more flexibility in controlling the properties of the system and provided a much larger property envelope for PLA blends. PBAT was the main controlling factor of specimen elongation while MMT were primarily used to boost modulus and strength of the blend. By properly adjusting their contents in the blend, desired properties could be realized.