P. J. Hine

The hot compaction of high modulus melt-spun polyethylene fibres

The production of solid section highly oriented polyethylene by compaction of melt-spun polyethylene fibres is described. Differential scanning calorimetry, X-ray diffraction and electron microscopy have been used to determine the structure of the compacted polymer. The essential feature of the process is shown to be selective surface melting of the fibres to form a polyethylene/polyethylene composite of very high integrity, yet maintaining a very high proportion of the strength and stiffness of the fibres.

The hot compaction of polyethylene terephthalate

A process is described for the successful compaction of polyethylene terephthalate fibres. The measurement of mechanical properties shows that a very high proportion of the original fibre properties are retained and that the compacted samples have a good degree of coherence. Electron microscopy studies of suitably etched samples reveals the effect of the compaction temperature on the structure of the compacted samples.

The hot compaction of woven polypropylene tapes

In this paper we describe the hot compaction of woven polypropylene (PP) tapes. It is shown that under suitable conditions of temperature and pressure, successful compaction is achieved by selective melting of the PP tapes. Mechanical measurements, combined with morphological studies, show that good tape to tape bonding, and good interlayer bonding, are achieved using an optimum compaction temperature of around 182 °C, while retaining a significant proportion of the original PP structure. Differential scanning calorimetry studies have shown that the compaction temperatures employed to produce a homogeneous coherent material have a significant annealing affect on the crystalline structure of the original drawn tapes, with a large change in the crystal size and a small increase in overall crystallinity (accompanied by a small increase in sample density). The mechanical properties of the compacted PP sheets show a combination of low density and good stiffness and strength.

The hot compaction of polypropylene fibres

Investigation of the compaction of unidirectionally arranged high-tenacity polypropylene fibres is described. A combination of techniques, with the major emphasis being morphological studies, show that controllable “selective” surface melting is not achieved at a high enough proportion to give substantial fibre-to-fibre bonding, and hence good lateral strengths.

Compaction of high-modulus melt-spun polyethylene fibres 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 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.

A COMPARISON OF THE HOT-COMPACTION BEHAVIOR OF ORIENTED, HIGH-MODULUS, POLYETHYLENE FIBERS AND TAPES*

The purpose of this article is twofold. First, there is an account of the hot-compaction behavior of a new, highly oriented, high-modulus polyethylene (PE) tape with the trade name of Tensylon® (manufactured by Synthetic Industries, USA). This tape, produced by a melt spinning route, has mechanical properties comparable to those of commercially available gel-spun fibers. Unidirectional samples were produced for a range of compaction temperatures to determine the optimum compaction conditions to obtain the best mechanical properties of the resulting compacted sheets. Second, the mechanical properties of the best Tensylon sample, manufactured at a compaction temperature of 153°C, was compared with three other hot-compacted, highly oriented PE materials, based on Certran®, Dyneema®, and Spectra® commercial PE fibers. The results showed that the optimum compaction temperature was in most cases about 1°C below the point at which substantial crystalline melting occurred. At this optimum temperature, differential scanning calorimetry (DSC) melting studies showed that approximately 30% of the original oriented phase had been lost to bond the structure together. In the case of Dyneema, the properties of the fiber were not translated into the properties of a compacted sheet, and morphological studies showed that this was because melting did not occur on the fiber surfaces, but rather in the interior of the fiber due to a skin structure. The properties of the compacted Tensylon tapes were found to be exceptional, combining very high modulus and strength with interlayer bonding and good creep resistance. Moreover, the optimum temperature appeared to be about 2°C below the point at which complete melting occurred, giving a wider processing window for this material. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

The hot compaction of 2-dimensional woven melt spun high modulus polyethylene fibres

In this paper we describe the production, properties and morphology of hot compacted 2-dimensional woven high modulus polyethylene fibres. The aims of the work were to establish the optimum conditions for production of the compacted woven PE sheets using a combination of mechanical measurements at Leeds and morphological investigations at Reading. This joint approach had proved very successful in a previous study on the compaction of unidirectional arranged PE fibres, where the optimum compaction temperature was established as 138°C, where ∼10% of the original fibres were melted. Morphological studies clearly showed that the melted material had recrystallised, epitaxially, onto the original fibre backbones, forming a coherent network to bind fibres into a continuous structure. The current studies, using the woven PE material, showed that a higher temperature was needed to fill all the space between the woven polyethylene fibres, and so produce a coherent material. Peel tests, where two layers of cloth are compacted together and then pulled apart, were carried out over a range of compaction temperatures to measure the interlayer bond strength; this increased with increasing compaction temperature. Significantly, reasonable bond strengths were established at the optimum temperature established for the unidirectional samples (138°C measured on the mould or 136°C in the centre of the fibre assembly) which produces ∼10% melted and recrystallised material, although a higher interlayer strength was measured at higher temperatures where more of the melted phase was produced. Morphological investigations of woven samples with ∼10% melted material, showed that while the individual fibre bundles were well bonded, not all of the complicated junctions between the fibre bundles in the woven network were completely filled with melted and recrystallised material, and that a temperature 2°C higher than for 1D compactions was probably optimum. The optimum temperature was found to fall very close to the temperature at which complete melting of the fibre occurred.

Differential melting in compacted high-modulus melt-spun polyethylene fibres

The compaction of high-modulus melt-spun polyethylene fibres has been investigated for compaction temperatures above the optimum. After such treatment the specimens are liable to be non-uniform because of differential melting. Individual compacted fibres are observed to melt not only from the outside inwards, but also in certain internal regions, depending upon the availability of local free volume. The regions of different stability have been identified and inferences drawn concerning the structure of the initial fibres. It is suggested in particular that the longitudinal regions of deficit density (which exhibit cratering in transverse sections and melt before their surroundings) are a result of initial crystallization occurring within a rigid framework inside the fibre, possibly nucleated on a strained molecular network. The presence of banded recrystallization around residual fibres demonstrates that this phenomenon develops via interaction of neighbouring lamellae as they grow.

On morphologies developed during two-dimensional compaction of woven polypropylene tapes

The microstructure of compacted woven polypropylene cloths prepared at their optimum compaction temperature of 184°C has been examined. Details of transverse and longitudinal cross-sections have been revealed by permanganic etching and observed with scanning electron microscopy. The original cloth was found to contain perpendicular cracks and biconical defects reported previously in other systems. After compaction, the cloth bonded together to form a thick solid sheet, with a melting point raised for the residual material but reduced for the recrystallized component. The higher melting regions form a continuous three-dimensional network with linear traces in a longitudinal section, in agreement with recent observations of fiber structure. Recrystallization occurs both within and externally from tapes: where parallel tapes meet, transcrystalline layers emanate from tape surfaces, with a distinct line where the two growth fronts meet. In some more extensive recrystallized regions row structures are formed, probably indicating local flow during compaction.