Preparation And Characterization Of High Melt Strength Polylactic Acid By Reactive Extrusion

Polylactic acid(PLA)is a high strength,high modulus,biodegradable thermoplastic resin with broad application potential.However,Due to the shortcomings of low melt strength,the strain hardening effect of PLA is not obvious,which limits the application of PLA in foaming and blown film.In view of the defect of low melt strength of PLA,In this paper,high melt strength PLA with branched structure is prepared by reactive extrusion method;the brittleness of PLA is toughened and modified.The main content of the experiment includes the following three parts:The 1,6-hexanediol diacrylate(HDDA)of high boiling point is selected as the grafting monomer by investigating the influence of different polyfunctional monomers on the melt strength,The grafted product(HDDA-g-PLA)is prepared by reactive extrusion.The melt flow rate and the sag height of the product decrease as the increasing of the graft ratio,and the melt strength is improved.The rheological properties of the product show that the chain relaxation time and viscoelastic properties of the product are obvious.This qualitatively indicates that the main chain has a long-chain structure,and the increase of the notched impact strength of the product also indicates that it is related to the branch of the product.Based on the experimental results and theoretical basis,a reasonable reaction mechanism is proposed,The carbonyl radical is formed by the chain cleavage reaction of the tertiary carbon radical.On the one hand,the carbonyl radical reacts with the chain of PLA to form a comb-type long-chain branch molecule;on the other hand,it reacts with HDDA to form a chain radical and continue to react with the backbone.The melt strength of the PLA is improved by the long-chain structure.The crystallization properties of high melt strength PLA are investigated.The results show that the crystallinity of the product is improved with the increase of grafting rate.When HDDA of 3 wt%(graft ratio:1.0%)is added,the crystallinity of the product is 40%,the spherulite density increase remarkably and the spherulite size become small.It is indicated that the branched structure produces by grafting functions as a nucleation,the crystal structure of the product is still an alpha crystal form.PLA is toughened by adding ethylene-vinyl acetate copolymer(EVA).The effect of the content of ethylene-vinyl acetate(EVA)on toughness is investigated by one-step reaction extrusion in PLA/EVA/HDDA/DCP system.With the increase of the content of EVA,the notched impact strength of the material first rises and then decreases.When the content of EVA is 8wt%,the impact notched strength is 7.5kJ/m~2,the toughness is increased by 97%.