Research On The Foaming Process Of Semi-crystalline Thermoplastic Composites

A polymer matrix composite,which is easy to process and has good performance,is becoming a substitutes for structural parts of metal and ceramic materials.In order to further meet the market demand for lightweight,the processing of foaming products has become a hot topic in recent years.Compared with traditional solid products,it not only reduces the use of raw materials and costs,but also has the advantages of light weight,heat insulation,energy absorption,high specific strength and impact resistance,and thus is widely used in buffer packaging,building insulation,household products,automobile industry,biological tissue engineering and other fields.However,the forming process of foaming products is repeatable and unreliable,and the control mechanism of the cell shape,size and density need further study.In this paper,the influence of molding technology on "material-morphology-property" is analyzed by means of experiment and numerical simulation.Taking biodegradable plastic polylactic acid(PLA)and general plastic polypropylene(PP)as the matrix materials,the relationship between cell morphology,process conditions and material properties was studied by supercritical injection foaming and batch foaming,and the formation mechanism and growth conditions of cell nuclei were discussed.For PLA,the composite material was prepared by combining it with spent coffee grounds(SCG),and the feasibility of the PLA/SCG injection foaming process and the properties of the product were further studied.The main work and conclusions are as follows.(1)The effects of SCG content,pretreatment,particle size and compatibilizer on the thermal stability and mechanical properties of PLA/SCG composites were studied.The results showed that the extraction efficiency of n-hexane with Soxhlet method was the best,reaching more than 15.0%.The mechanical properties(tensile and impact)and thermal stability of PLA/SCG composites decreased with the increase of the content of SCG(5,10,15 wt.%).After oil removal,the tensile strength of the composites(SCG proportion is 15 wt.%)increased by 137.0% compared with untreated.In the range of micron size,the particle size of spent coffee grounds had little effect on the properties of composites.When 5 wt.% methylenediphenyl diisocyanate(MDI)compatibilizing agent was added,the best mechanical properties were obtained,and the tensile strength and elongation at break increased by 48.6% and 123.7%,respectively.(2)PLA/SCG foaming products were prepared by means of supercritical nitrogen microporous injection molding,and the foaming behavior and mechanical properties of PLA/SCG composites were discussed.The results showed that the PLA/SCG foaming material presented a "skin layer-shear layer-core layer" cell distribution in the direction of product thickness.As the content of spent coffee grounds increased,the mechanical properties decreased.After the introduction of MDI,it can enlarge the chain of PLA and increase the viscosity,so that the diameter of the cell decreases and the diameter distribution becomes narrow.Besides,the specific tensile strength and elongation at break were improved by 122.5% and 113.4%,respectively.The numerical simulation(Moldflow Insight 2018)software was used to analyze the microporous foaming process of composites.According to the simulated results of velocity field and temperature field,the formation mechanism of cell was revealed,and the size and distribution of cell were obtained.By comparing the results of simulation and experiment,it is found that the two trends are consistent,which can provide reference for actual production.For PP,the effect of crystallization on the foaming behavior was studied.By setting different intermittent foaming processes,the thermal history of the material before unloading foaming was changed,and the relationship between crystallization and foaming was discussed.The intermittent foaming behavior of PP/GF composites was further studied by introducing GF enhancement.Through experiments,it is found that:(1)The crystal can act as the heterogeneous nucleation point of the cell.The foaming pressure is fixed at 12 MPa,and under certain foaming temperature and time,the bimodal cell size and the "stamen-like" structure are formed respectively.(2)For PP/GF composites,the existence of glass fiber restricts the molecular chain movement,significantly improves melt strength and reduces expansion rate.