Injection Molding And Properties Of Thermal Expandable Microspheres Foamed Polypropylene Composites

Polypropylene has excellent comprehensive properties due to its molecular structure characteristics.It is an ideal resin matrix for preparing high-performance foamed composites.At present,the research of foamed polypropylene mainly focuses on supercritical CO2 foaming.This method has some problems,such as high requirements on melt strength of polypropylene resin matrix,uneven distribution of cell size and high preparation cost.Thermal expandable microspheres foaming is a new plastic foaming method,but there is little systematic research on the preparation of carbon fiber reinforced thermal expandable microspheres foaming polypropylene at present.In this paper,carbon fiber reinforced foamed polypropylene materials were injection molded by using thermal expandable microspheres as foaming agent,and the following three aspects were mainly carried out:(1)The composition,free expansion/collapse dynamic process at different temperatures,micro-morphology of four kinds of thermal expandable microspheres(TEMs),and the rheological properties of microspheres/PP system were systematically studied by FT-IR,GCMS,TGA,TMA,SEM,torque rheometer and rotary rheometer.The results show that the chemical composition of the shells of four kinds of thermal expandable microspheres is basically the same,which is formed by copolymerization of methacrylamide,methyl methacrylate and acrylonitrile,and the inner core of the microspheres is composed of isooctanol and C8 alkane.Among them,the initial foaming temperature of F-230D microspheres and F260D microspheres was higher,reaching 186℃ and 200℃,respectively.And the cell structure was stable.When the temperature exceeded 200℃,and the temperature droped to room temperature,the expansion volume of the two microspheres remained basically unchanged.The addition of thermal expandable microspheres has an important influence on the rheological properties of the melt,which can greatly improve the complex viscosity,storage modulus and loss modulus of PP.Therefore,unlike chemical foaming and supercritical CO2 foaming,the thermal expandable microspheres foaming method does not require the use of high melt strength PP resin.(2)The injection molding of thermal expandable microspheres foamed polypropylene was studied.The effects of the amount of thermal expandable microspheres and its injection progress conditions on the micro-morphology,density and mechanical properties of the materials were investigated.The results showed that the parameters that have the greatest influence on the material properties are:the content of microspheres,injection temperature and cooling time.A certain minimum critical injection pressure and injection speed were required for the melt to reach the flow length of the cavity.After reaching the critical value,the injection pressure and injection speed had little effect on the density and mechanical properties of the material,and the system had wide range of processing adaptability.The appropriate processing conditions are as follows:the amount of thermal expandable microspheres:6wt%,injection temperature:210℃,cooling time:5 s,mould temperature:75℃,injection pressure:15-80 MPa,injection speed:19-74 cm3/s,injection volume:11-14 cm3.Under these conditions,the density of polypropylene foamed material was 0.67 g/cm3,tensile strength was 25.4 MPa,bending strength was 32.2 MPa,bending modulus was 725 MPa and notched impact strength was 2.275 kJ/m2.Clear sea-island structure is the structural requirement of microspheres/PP material to obtain high mechanical properties.Among various influencing factors,the addition amount of microspheres had the greatest influence.Such structure can be obtained with the addition amount of 1-6%.But beyond this range,the sea-island structure would be destroyed and the mechanical properties were poor.(3)The injection molding of carbon fiber reinforced thermal expandable microspheres foamed polypropylene was studied.The rheological properties of melt,micro-morphology,mechanical properties and resistivity of injection samples were systematically studied.The results showed that the complex viscosity,storage modulus and loss modulus of microspheres/PP foamed system increased.with the addition of carbon fiber.The effects of injection parameters on the cell structure,density and mechanical properties of carbon fiber/microspheres/PP composites were similar to the microspheres/PP composites.However,with the addition of carbon fiber,it was necessary to adopt higher injection temperature,greater minimum injection speed and injection pressure to meet the flow length requirement of full mold.The appropriate injection molding conditions of carbon fiber reinforced microsphere foamed polypropylene composites were as follows:30%3 mm short carbon fiber,6%thermal expandable microspheres,injection temperature:230℃,mould temperature:75℃,injection speed:46-74 cm3/s,injection pressure:30-80 MPa,cooling time:8 s,and injection volume:14cm3.Under these conditions,the density of carbon fiber reinforced foamed polypropylene composites was 0.75 g/cm3,and the tensile strength and bending strength were 55.6 MPa and 78.5 MPa,respectively.Compared with unfoamed pure polypropylene,the density decreased by 17.4%,and the tensile strength and bending strength increased by 85%and 124%respectively,which realized the light weight and high strength of polypropylene composites.The thermal expandable microspheres can promote the carbon fibers in PP matrix to connect with each other to form three-dimensional network,which was beneficial to the formation of conductive channels and reduced the resistivity of composite materials.Especially in the case of low carbon fiber addition,the effect was more remarkable.For the system with only 5%carbon fiber and 6%thermal expandable microspheres,the volume specific resistance and surface specific resistance of the material were both 2 orders of magnitude lower than that of the system without thermal expandable microspheres,reaching the standard of antistatic material.