Preparation And Foaming Properties Of Long Chain Branched Poly (Lactic Acid)

Poly(lactic acid)(PLA)foams is lightweight and has excellent mechanical,barrier and cushioning properties.The excellent biodegradability of PLA make it more promising than traditional foams in application areas such as packaging,medical materials and auto-material.However,the foaming process of PLA is limited because of it's low melt strength and low crystallization rate.Pyromellitic dianhydride(PMDA)and triglycidyl isocyanurate(TGIC)were added together as chain extenders(CE)to introduce long chain branched(LCB)structure into PLA and improve it's melt strength.Simultaneously,flexible polymers such as poly(butylene succinate)(PBS)and poly(butylene-adipate-co-terephtalate)(PBAT)were added into PLA to improve its toughness.In addition,talc was added as nucleating agent into PLA/PBS blends with LCB.Foams of LCB PLA/PBS and LCB PLA/PBAT blends were prepared through extrusion foaming.The rheological,crystallization properties and foamability of three blends above were studied by torque rheometer,rotational rheometer,DSC,POM and SEM.All results are described as follows:LCB structure is successfully obtained in PLA/PBS blends by adding PMDA and TGIC together with the ratio of 1:1.The melt strength,elasticity and elongational viscosity of extended PLA/PBS blends increases with the increase of CE concentration.The presence of LCB structure also promotes the crystallization behavior transform from cold crystallization to melt crystallization and enhances the crystallization temperature since branched chains can play a role of nucleating site.The crystal density increases while the crystal size decreases with the increasing of CE concentration,which promotes the bubble nucleation and improves the foamability of LCB PLA/PBS blends.The cell morphology becomes the best when the CE concentration is 0.8 wt%.LCB structure is also obtained in PLA/PBAT blends and the melt strength and elasticity increase with the increase of CE concentration.The LCB structure also promotes the appearance of melt crystallization and increases the crystallization temperature.Compared with LCB PLA/PBS blends,the degree of branching of LCB PLA/PBAT blends is higher and the increase of melt strength,elasticity and crystallization temperature were more remarkable.In addition,the cell density and expansion ratio of LCB PLA/PBAT foams are higher than LCB PLA/PBS foams.The PLA/PBAT foams with excellent cell structure is obtained when the CE concentration is 0.6 wt% and 0.8 wt%.Talc is an effective nucleating agent for PLA/PBS with more linear structure.The addition of talc increases the crystallization temperature and crystallization rate because of it's role of crystal nucleation.In PLA/PBS with more branched structure,talc slows down the crystallization rate instead of accelerates,suggesting that the branched structure dominates crystallization already regardless of the presence of talc.However,due to the role of crystal nucleation of talc,the crystal density increases and crystal size diminishes dramatically,which induces cell nucleation and thus enhances the cell density.The PLA/PBS foam with integrated and uniform cell structure is obtained when the CE concentration is 0.4 wt%.The cell density and mean diameter reach 6.5×108 cell/cm3 and 130 ?m,respectively.