Flow Properties of Molten EVA Copolymer

Abstract

The flow properties of molten EVA copolymer in the region of compratively higher shear rate were measured in connection with investigation of the behaviors and forming mechanism of melt fracture by means of a capillary type rheometer. The relation between the actual shear rate and viscosity obtained by means of the differential method indicates that their flow pattern is non-Newtonian and viscosity decreases with increase of shear rate.They are described by two straight lines of different slope. The shear rate at their intersecting point corresponds to the critical shear rate where melt fracture forms.The EVA copolymer differs slightly in the flow curve from low density polyethylene; it seems, however, that their difference is due to the difference in molecular weight distribution rather than in the materials themselves.And also, the fluidity of the molten EVA copolymer having about 3 in the ratio of Mw/Mn may be equivalent to that of low density polyethylene having from 4 to 7 and, generally, EVA copolymer has higher fluidity than low density polyethylene does. It is expected that the fluidity will increase with incroporation of vinylacetate in the same molecular weight distribution and the same average molecular weight.The critical shear rate increases with the melt index and temperature. It can not be found that the materials themselves and the molecular weight distribution gives direct influence to the critical point of melt fracture forming as far as the melt index is defined as the parameter. The critical viscosity ηc where melt fracture forms decreases almost in a straight line with increase of melt index.It is found from the studies of end correction and behavior of formation of melt fracture that melt fracture occurs at inlet of the die, and it supposed that formation of melt fracture has been caused by the result of the elastic turbulence of flow pattern due to failure of recoverable shear strain at inlet of the die.