The Preparation And Mechanical Properties Of Cassava Starch-Vinyl Acetate Grafted Copolymers

In this paper, with cassava starch as graft framework, VAc as graft monomer, grafted starch was prepared through the choices of initiating system and preparing conditions. The effects of PVA content, kinds and content of cross-linking agent, cross-linking time and cross-linking temperature on mechanical properties and degradation property of grafted starch materials were investigated, and the grafted starch materials with good mechanical and degrading properties were prepared. The structure and properties of grafted starch copolymer were determined with FTIR,TG/DTG,SEM,DSC,XRD and DMA.The results show that initiating system has an effect on grafting copolymerization ofcassava starch and VAc monomer. In the initiating systems studied, (NH₄)₂S₂O₈ has the best initiating efficiency , and Fe²⁺-H₂O₂ second; (NH₄)₂Ce(NO₃)₆ last.Higher monomer conversion, grafted ratio and efficiency could be obtained with initiating system of (NH₄)₂S₂O₈.The optimum preparing condition is concentration of starch 5%, mass ratio of monomer and starch 2.0, concentration of initiator 18.0 mmol/L, reaction temperature 60℃and reaction time 3h. The product with above preparing technology is grafted copolymer of Cassava Starch and Vinyl Acetate. The copolymer is amorphous structure. The degrading course of cassava starch by heat is one step, whereas the degrading course of copolymer three and provided with better heating stability.Mechanical properties of cassava starch-VAc grafted copolymer is improved with PVA and cross-linking agent added. PVA content, Kinds and content of cross-linking agent and cross-linking treatment have relatively big influence on mechanical properties of copolymer. As for cross-linking agent, epoxypropane has the best effect , formaldehyde second, and hexametaphosphate third.The excellent mechanical properties of cassava starch-VAc grafted copolymer could be prepared with PVA content 1%, dosage of epoxypropane 1%, cross-linking temperature 60℃and cross-linking time 30min.Content of graft monomer has an effect on mechanical properties of copolymers. With content of monomer increasing, ultimate tensile strength of copolymer decreases, elongation at break increases, water absorpton ratio of copolymer decreases at first then decreased.Mass ratio of VAc and starch has an effect on dynamic mechanical properties of grafted starch.DMA study shows that E ' of copolymer increases with increasing monomer content as temperature less than -50℃, decreases in -50℃to 50℃,and remains the same value more than 120℃. There appear two peaks of internal friction as the ratio of monomer and starch is 1:1,and three peaks 2:1 and 3:1. The peak area of internal friction increases with increasing content of monomer.The grafted copolymers show a good biological degrading property. Bacillus andα-amylase have excellent degrading effect on grafted copolymer, the biological degrading rate of copolymers is the fastest in 5 days. Burying with clay also speeds up the degradation of copolymer. Degrading rate increases with increasing starch content of copolymer. The branch structure of copolymer appears biological degradation besides starch.