Compaction of high-modulus melt-spun polyethylene fibres at temperatures above and below the optimum

Abstract

In the process of hot compaction developed at the University of Leeds, high-modulus fibres are compacted to form coherent thick-section products with stiffnesses unobtainable by current processing techniques. Using high-modulus polyethylene fibres (trade name TENFOR) produced by the melt-spinning/hot-drawing route as the starting material, it was discovered 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 together and fill all the interstitial voids in the sample, leading to a substantial retention of the original fibre properties. For a hexagonal close-packed array of cylinders, only 10% of melted material is needed for this purpose. If the compaction temperature is too low, there is insufficient melt to fill the interstices, the fibres deform into polygonal shapes, and insufficient transverse strength is developed. Above the optimum temperature, the proportion of melt increases, causing the stiffness of the composite to be reduced. The recrystallization of the melt is nucleated on the oriented fibres, giving similarly oriented cylindrulitic growth. Where the regions of melt are large enough, and cooling sufficiently rapid, development away from the nucleus is accompanied by a cooperative rotation in chain orientation, analogous to banding in spherulites.