Preparation Of Microcellular Polylactide/Wood Flour Composite Foams And The Tailoring Of Cell Structure

Polylactide(PLA)is an environmental-friendly,biocompatible,biodegradable and renewable thermoplastic polyester,and PLA exhibits superior mechanical strength:high strength and high modulus,all of these features make PLA an ideal alternative to the traditional petroleum-based polymers.Supercritical fluid microcellular foaming technology could improve the toughness,realize the light weight,and reduce the cost by producing microcellular structure inside the PLA matrix.Wood flour is a natural biomass material,which could be combined with PLA to prepare new classes of fully biodegradable and bio-renewable composite foams.In addition,wood flour could improve the thermostability,the mechanical properties of PLA,and promote the cell nucleation of the composite foams.However,PLA is a semi-crystalline polymer,with slow crystallization rate and low melt strength.The continuous phase structure of the PLA matrix was destroyed in the high wood flour loadings.These factors are not conducive to the foaming processing of the polylactide/wood flour composites.In this paper,the suitable foaming technology of neat PLA was explored.Then,the maximum loadings of the wood flour were determined via the cell nucleation efficiency of the obtained composite foams.In addition,the cell distribution characteristics around the wood flour were studied.Finally,the desirable microcellular polylactide/wood flour composite foams were fabricated based on the chain branching of the PLA matrix,blending with the flexible polymer respectively.The main contents and results were summarized as follows:The saturation above the melting temperature for 20 min could promote the dissolution and the difussion of supercritical CO2,and the saturation under the crystallization temperature for another 20 min could improve the melt strength during the foaming.Finally,the cell structure was formed by the way of instantaneous depressurization,this two-step foaming technology could improve the formation of the homogeneous phases,the cell growth,and cell stabilization.With the increasing foaming temperature,the expansion ratio improved first and then reduced,the preferable cell structure was obtained at 100 ?.By increasing the saturation pressure,the expansion ratio increased first and then reduced as well,and the preferable cell structure was obtained at 16 MPa.The incorporation of wood flour improved the complex viscosity and storage modulus of the PLA melt.The viscoelastic response of the composites changed from a pseudo plastic-like to a pseudo solid-like behavior in the high wood flour loadings.Compared to the neat PLA foams,the PLA/wood flour composite foams had a smaller cell size,and a higher cell density,in spite of the lower expansion ratio.The cell size presented a gradient distribution around the wood flour.When the wood flour loadings were 20 wt%,the cell density reached 7.2x108cells/cm3,with a uniform cell size distribution and the average cell diameter was 20?m,whereas the open-cell ratio increased.Further increasing the wood flour loadings,the cell structure was destroyed.The branched structure was introduced via the reaction extrusion.Therefore,the viscoelasticity of the composites was improved,indicating the improved melt strength.The modified the PLA/wood flour composite foams had a better cell morphology,and more closed-cell structure.The incorporation of 2.5 wt%proved effective in optimizing the cell structure of the composite foams,the expansion ratio was up to 18 times,the cell density was 1.2x108 cells/cm3,and the average cell diameter was 25 ?m.The viscoelasticity of the composites was improved by the melt blending with the flexible degradable poly(butylene adipate-co-terephthalate)(PBAT).The double melting peaks moved to the lower temperature,the crystallization temperature decreased.PBAT obviously promoted the crystallization rate and the crystallinity.In addition,PBAT could decrease the glass transition temperature,storage modlus,and Tan?,thus improving the foamability of PLA/wood flour composites.There was another interesting phenomenon that the PBAT improves the non-continuous phase structure which was attributed to the introduction of wood flour.When the mass ratio of PLA and PBAT was 85:15,the cell structure and the expansion ratio were significantly improved compared to the PLA/wood flour system without PBAT.