Study On Effect Of Processing History On Structure And Properties Of Food-grade Plastics

Plastic packaging is widely used in food and drug packaging industry due to their excellent intrinsic properties such as light weight,low cost and convenient processing et al.Polyethylene(PE),polypropylene(PP),polyvinyl chloride(PVC),polycarbonate(PC)and polystyrene(PS),et al.are common food packaging materials.“Hygienic management methods for plastic products and raw materials for food” enacted in 1993 in our country states that,all materials meant for processing plastic tableware,containers and food packaging materials,shall not be made of recycled plastic.But the chaotic phenomena of adding recycled materials into plastic product for food packaging is still prevalent despite repeated prohibitions,which will post a serious treat to public food safety and affects the health of human beings.Therefore,the identification of recycling materials doping in food grade plastic has important significance.In this project,food-grade plastics based on PE,PC,PP,PS as raw materials,are obtained through multiple extruded process by using twin-screw extruder.This process was used to simulate the processing history of food-grade recycled plastics.In addition,the new materials,six recycled materials and recovered materials from market were doped with different proportions by high-speed mixing machine,and the samples were obtained by melt extrusion.The melt flow index(MFI)analysis,gel permeation chromatography(GPC)analysis,thermal gravimetric analysis(TGA),differential scanning calorimeter(DSC)and universal testing machine were used to characterize and analyze the effect of processing history on various plastic structures and properties systematically.The results from numerous analysis are shown as follows,(1)for PE with repeating melt extrusion and doping in recycled materials,the melt flow rate(MFR)and the oxidation induction time(OIT)obviously decreased,molecular weight increased,and the molecular weight distribution began to widen out.Moreover,the tensile strength increased compared with new materials,but the elongation at break decreased.The thermal decomposition temperature had increased and the crystallinity reduced.There would be two melting peaks when the doping contents reached a certain level.(2)After repeated melt extrusion of PC,the elongation at break,impact strength,thermal decomposition temperature,glass transition temperature and molecular weight are all decreased,and the molecular weight distribution was widened.There would be two Tg appearing when the doping contents reached a certain level.(3)For PP and PS,the MFR increased with increasing of recycled numbers.The crystallinity and OIT of PP would reduce.The molecular weight and Tg of PS decreased with increasing of recycled numbers.According to the TGA analysis,we found that there were some materials which were not easily thermal degraded.Because of the new material doped with recycled materials and the phenomenon current market packaging material doped with recycling materials is very similar.The system detection method used in this paper provides testing basis to identify whether the addition of recycled materials in food plastic packaging materials.Therefore this topic has a very strong practical application value.