Study On Rheological Properties Of High-expansion-ratio PET Resin And Regulating Mechanism Of Foam Structure

Polyethylene terephthalate(PET)is a kind of cost-effective engineering plastics.In order to optimize PET's relatively single product structure and make full use of PET,some studies have been done to develop the application in lightweight materials by industry and academia.PET foam not only has the advantage of light weight but also has excellent mechanical properties and physical properties.However,at present,commercial PET has a relatively low molecular weight and linear molecular structure,which lead to low melt viscosity and poor melt elasticity.This kind of PET cannot be used to obtain high-expansion-ratio PET foam.Besides,the high processing temperature of PET makes the solubility of foaming agent low,and PET has a slower crystallization rate,which makes it difficult to solidify the bubbles.All these factors are not helpful to prepare excellent microcellular materials.In this study,we focus on the preparation of high-expansion-ratio PET foam and the regulation of cell structure in the foaming process.Firstly,the high-melt-elasticity PET was prepared through the reactive extrusion process,and PET foams with an expansion ratio of up to 30 times(saturated pressure:15 MPa)were obtained.Secondly,by analyzing the relationships among molecular structure,rheological properties and foaming behavior,rheological standards were established,which can guide the preparation of foamable PET resin and predict the foaming ability of one PET.Finally,incompatible blends were used to control the bubble nucleation and growth during foaming.And PET foams with better cell structure were prepared.The key points are as follows:1.A commercial bottle-grade PET was modified by using reactive extrusion process with polyfunctional epoxy(ADR)as the chain extender.The modified PETs were characterized rheologically.Common chain extender pyromellitic anhydride(PMDA)was selected as a reference.The effects of chain extender content on the viscoelasticity of modified PETs were investigated.Results show that higher chain extender content can lead to better rheological properties.PET modified with ADR has a higher melt elasticity and a more pronounced strain hardening than that modified with PMDA.In order to improve the viscoelasticity of modified PET,response surface optimization method was used to study the effects of processing parameters during reactive extrusion.Besides,the plasticizing effect of CO2 during reactive process is utilized for further increasing the reactive efficiency between PET and chain extender.2.PETs with different molecular structure were prepared by using different chain extenders.These PETs include linear PET,long-chain branched PET,and partially cross-linked PET.Their rheological behavior and foaming properties were investigated systematically.It was found that partically cross-linked PET with high melt elasticity had better foaming properties,its foam has an expansion ratio of more than 30 times,a cell density of 108/cm3 and an average diameter of 60 ?m.The relationship among molecular structure-rheological properties-foaming behavior had been found,which indicated that rheological parameters can predict the foamability of one PET resin.3.Three polymers,high-density polyethylene(HDPE),low-density polyethylene(LDPE)and polydimethylsiloxane(PDMS),were blended with PET as a dispersed phase.Their effects on the foaming properties of PET were studied.Results showed that the incompatible interface can promote the bubble nucleation,and larger interfacial tension led to more significant effect on bubble nucleation.ADR was used to improve the compatibilization between PET and PA66 by the formed copolymer during reactive extrusion.The effects of ADR on the interface and the blend structure were investigated.The addition of ADR increased the viscoelasticity of each phase,and enhance the interfacial adhesion between the two phases.As the second phase,PA66 increased the cell density of blends,and it can crystallize rapidly during foaming process,which would limit the cell growth and lead to a smaller cell size.4.A single screw extruder was used to study the reactive extrusion-foaming process.By optimizing the foaming temperature,C02 content,head pressure and die structure,PET foams with average cell diameter of 20-120 ?m,cell density of 1×107-3×109/cm3,expansion ratio of 5-27 were obtained.The industrial-scale pilot production line for PET foam was developed.This line integrated the function of free drying,reactive extrusion,and foaming.PET foam board with a thickness of 1-10 mm and a foam density of 100-500 kg/m3 was produced.