Preparation And Study Of High Density Polyethylene (HDPE) / Ultra High Molecular Weight Polyethylene (UHMWPE) Composite Foams

HDPE foams have attracted increasing interest because of their excellent toughness,chemical stability,abrasion resistance and low cost.However,it is rather difficult to find an appropriate foaming system by compression molding under high temperature for HDPE because of the higher processing temperature compared with low density polyethylene(LDPE)and thus narrower foaming window.Furthermore,the low melt strength of HDPE leads to the unsatisfactory foaming performances.Therefore,it is necessary to adjust the foaming window and melt strength.UHMWPE has been widely used to improve the melt strength,thermostability and mechanical properties of polyethylene(PE)due to its supernal viscosity,high melting temperature and outstanding mechanical properties.Herein,firstly,HDPE/UHMWPE blends were prepared by melt blending and the effects of UHMWPE on rheological properties,thermal properties and mechanical properties of HDPE were studied.Then,HDPE foams and HDPE/UHMWPE composite foams were successfully prepared by the single-stage compression molding and the effects of foaming temperature,UHMWPE,dicumyl peroxide(DCP)and azodicarbonamide(ADC)were also optimized to match the crosslinking reaction with the decomposition of foaming agent under high temperature.HDPE foams have potential fire hazards,so it's imperative to optimize the flame retardant properties of HDPE foams.As the intumescent flame retardant has high flame retardancy,no droplet,low smoke and non-corrosive gas,ammonium polyphosphate(APP)/melamine(MEL)as the common intumescent flame retardant was used in this paper and the effects of APP/MEL on the cell structure,foaming properties,mechanical properties and flame retardancy of HDPE/UHMWPE composite foams were studied.In addition,there is more and more fly ash discharged in China,causing serious soil pollution,water pollution and air pollution.Fly ash possesses the porous structure,low density,high surface activity and large specific surface area,and is easily dispersed in the polymers.Therefore,in this paper,fly ash was added to HDPE/UHMWPE blends,and HDPE/UHMWPE/fly ash composite foams were prepared.The effects of fly ash on the cell structure,foaming properties,mechanical properties and flame retardancy were studied.The results were as follows:(1)the addition of UHMWPE can improve the melt strength,thermal stability and mechanical properties of HDPE matrix,and UHMWPE also can enhance the foaming ability of HDPE matrix,stabilize the cell morphology and improve the mechanical properties of HDPE foams.And,when the foaming temperature is 210°C,the contents of UHMWPE and DCP are 10 wt%and 2 wt%,respectively,HDPE/UHMWPE composite foams possess the most uniform and stable cell morphology and prominent mechanical strength.In addition,HDPE/UHMWPE composite foams with the foaming density as low as0.039 g/cm~3 can be prepared just by adjusting the content of ADC,which largely broadens the applications of HDPE foams.(2)Furthermore,when APP/MEL content is 40 wt%,the limiting oxygen index of the composite foams increases to 24.8%,reaching HBF horizontal burning rating.(3)HDPE/UHMWPE/fly ash composite foams with fly ash content up to 35wt%can be prepared and have excellent expansion ratio.Finally,adding10 wt%APP/MEL to HDPE/UHMWPE/fly ash composite foams with 30wt%fly ash can reach HBF horizontal burning rating.