An investigation was made of tube grades of high-density polyethylene with a unimodal and a bimodal molecular weight distribution during shear and longitudinal deformation under constant force conditions. It was shown that these polymers have similar melt flow indices and flow curves but differ considerably in other characteristics during shear deformation and in their behaviour during longitudinal deformation.
Melts of bimodal (tube) polyethylenes relax considerably more rapidly than normal monomodal specimens. At exit from an l/d = 20 capillary, a melt of bimodal polyethylene behaves like an unrelaxed melt, whereas normal polyethylene does not manage to relax entirely even at exit from an l/d = 30 capillary. The obtained results point to one of the reasons for the high durability of polyethylene tubes with a bimodal molecular weight distribution.
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