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The rheological behavior of ethylene vinyl acetate copolymers (EVA) with different vinyl acetate (VA) contents during the extrusion process was investigated by means of viscosity and positron lifetime measurements. The viscosity of EVAs in the liquid state under shearing stress initially decreased with processing time before passing througha minimum at the transition point. The variation of processing time at this transition point with extrusion temperature showed that higher extrusion temperature leads to lower viscosity and faster degradation. The relationship between the inverse of the processing time for the transition point and the correspondingprocess temperature suggested that the VA content is a key factor for the degradation process of EVA. The positron results indicated that the free-volume size exhibits a corresponding variation to the rheological behavior.

Correlation study between the rheological property and the free volume for ethylene vinyl acetate copolymer under melt extrusion