COMPACTION OF HIGH-MODULUS MELT-SPUN POLYETHYLENE FIBERS AT TEMPERATURES ABOVE AND BELOW THE OPTIMUM

In the process of hot compaction developed at the University of Leeds, high-modulus fibres are compacted to form coherent thick-section prod ucts with stiffnesses unobtainable by current processing techniques. U sing high-modulus polyethylene fibres (trade name TENFOR) produced by the melt-spinning/hot-drawing route as the starting material, it was d iscovered that under optimum conditions of pressure and temperature it is possible controllably to melt a small proportion of each fibre. On cooling, this molten material recrystallizes to bind the structure to gether and fill all the interstitial voids in the sample, leading to a substantial retention of the original fibre properties. For a hexagon al close-packed array of cylinders, only 10% of melted material is nee ded for this purpose. If the compaction temperature is too low, there is insufficient melt to fill the interstices, the fibres deform into p olygonal shapes, and insufficient transverse strength is developed. Ab ove the optimum temperature, the proportion of melt increases, causing the stiffness of the composite to be reduced. The recrystallization o f the melt is nucleated on the oriented fibres, giving similarly orien ted cylindrulitic growth. Where the regions of melt are large enough, and cooling sufficiently rapid, development away from the nucleus is a ccompanied by a cooperative rotation in chain orientation, analogous t o banding in spherulites.