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

Plastics are widely used in food and drug packaging industry due to their intrinsic good properties such as light weight,excellent chemical stability,convenient processing etc.Food outer packaging,food inner packaging films,beverage bottles,drug bottles and infusion bags are commonly used plastic materials.Virgin plastic granules should be chosen to be used to manufacture food and drug packaging in order to ensure the food and drug safety.Recycled plastics are strictly forbidden to product the food and drug packaging.For this reason,research on the effects on the structure and properties of food-grade reclaimed plastics has significance.In this thesis,food-grade plastics such as polyethylene(PE),polypropylene(PP),polystyrene(PS)and polycarbonate(PC)are used as raw materials.Multiple reprocessings were performed by using twin-screw extruder.These processes were used for simulating the processing history of food-grade recycled plastics.In addition,a series of recycled PE samples which adulterated a few reclaimed PE were prepared by high-speed commixture and melt extrusion.ATR-FTIR spectroscopy,melt flow index(MFI)analysis,gel permeation chromatography(GPC),thermal gravimetric analysis(TGA),differential scanning calorimeter(DSC)and mechanical properties measurements were used to characterize and analyze the effect on the structure and properties of virgin and recycled plastics.The discoveries according to the data from kinds of instrumental analysis are elaborated as follows:(1)Comparing with virgin PE,the molecular structure of recycled PE was no longer single.The molecular weight of recycled ones increased and the molecular weight distribution began to be widened.The thermal decomposition temperature and degree of crystallinity experienced a slight decrease.Meanwhile the tensile strength of recycled PE increased.Crosslinking reaction was the main reaction of processing degradation of PE.(2)For recycled PP,PS and PC,the molecular structure was more complicated than virgin ones.The molecular weight decreased and the molecular weight distribution was wider.Thermal properties and tensile properties of all the recycled PP,PS and PC had a decreasing trend.After the three virgin plastic materials had multiple extrusion process,the degradation reaction was chain scission reaction.(3)The ingredient of reclaimed PE was sophisticated.The simplicity of the molecular structure of PE materials which adulterated different proportion of reclaimed PE was poor.The MFI increased along with the increasing doping content,which illustrated the improved processing fluidity and the decreased molecular weight.Adulterating reclaimed PE had effects on the thermal stability and tensile properties of PE,which led in lower thermal decomposition temperature and tensile strength.Besides,after adulteration of reclaimed PE,the melting peak had a significant variation: recycled LLDPE appeared bimodal or widened melting peak.The melting temperature of recycled LDPE decreased with the increasing of the doping content of reclaimed LDPE.