UHMWPE fibres

Fig. 10 Sherby-Dorn plots of creep rate versus strain showing the different creep phases a U PVC pipes for constant hoop stresses of 37 to 42 MPa at a temperature of 20 °C (60 K below Tg) (using data of Castiglione et al. [42]) b Highly oriented UHMWPE fibres at room temperature (using data of Berger et al. [43])...

The unidirectional (UD) construction was first used in soft body armour for the chest and helmet by air crewmen in the Second World War and was reintroduced by AlliedSignal with the UHMWPE fibre Spectra in the mid-1980s. The unidirectional technology is based on the idea of combining the cross-plied filaments with an elastomeric matrix in a laminated systan. Fig. 6.7 shows a four-plied unidirectional system laminated by two films. Other companies, such as DSM and Park Technologies, also provide similar fabrics for personnel protection. 

Lee S-G, Kang T-J, Yoon T-H. Enhanced interfacial adhesion of UHMWPE fibres by oxygen plasma treatment. J Adhes Sci... 

Ghand N, Kreuzberger S and Hinrichsen G (1994) Influence of processing conditions on the tensile properties of unidirectional UHMWPE fibre-LDPE composites, Compos 9 878-880. 

As described earlier, the main prerequisite for the creation of high performance self-reinforced polymer composites is a thermoplastic polymer fibre with the necessary mechanical properties to make the creation of the final composites attractive. Ultrahigh molecular weight polyethylene (UHMWPE) fibres with veiy high mechanical properties, produced by gel spinning, became commercially... 

Devaux and Caze reported the characterisation of model composites comprising single UHMWPE fibres embedded in LDPE [47,48], It was shown that an oxidising chemical treatment of the UHMWPE fibre prior to embedding in the LDPE matrix improved the interface between the UHMWPE fibre and the LDPE matrix [48]. Ogawa et al. also reported the creation of unidirectimial UHMWPE fibre-reinforced PE composites [49], The authors report the use of commercial UHMWPE fibre to produce composites by the combination of these fibres with films of HDPE or LDPE to yield composites with >70% fibre weight fraction. 

V aisman et al. [52] reported the effect of bromination of the surface of commercial UHMWPE fibres in order to polarise the surface of the fibres. This bromination process was shown to result in an increase in the degree of order of the transcrystalline zone when these fibres were combined with HDPE to produce a self-reinforced polymer model composite. While these pubUcations report the use of different types of PE to create self-reinforced polymer composites, UHMWPE fibres have also been combined with ethylene-based copolymers. Kazanci et al. [53, 54] reported the creation of commercial UHMWPE fibre-reinforced ethylene-butene copolymers. Filament wormd structiues were produced, with fibre volume fractions of 65%, with the suggestirm of a potential application for these materials in unspecified medical devices. 

A method for achieving matrix impregnation of UHMWPE fibres by a UHMWPE matrix by a solution impregnation route has been presented by Cohen et al. [86]. A heated solution of UHMWPE in tetralin was used to coat commercial UHMWPE yams. In this way, the subsequent removal of the tetralin solvent from the coated yam results in a UHMWPE yams coated in the UHMWPE solute. Cohen et al. reported the subsequent thermal processing of these coated yams into unidirectional laminates with fibre volume fractions of up to 85%. Unlike many of the other technologies presented so far, the solution coating process presented by Cohen et al. yields a tmer self-reinforced polymer composite in the sense that the... 

Maity et al. reported the creation of commercial UHMWPE short (3-5 mm) fibre-reinforced LDPE composites [90, 91]. The composites were reported to have been created by first dissolving the LDPE matrix phase in heated toluene. Short chopped UHMWPE fibres were then added to this solution and stirred to achieve fibre distribution. Eollowing the extraction of the solvent, compression moulding was performed to yield a composite laminate, albeit with fibre volume fractions of typically only 15%. 

While early studies by the group at the University of Leeds focused on melt-spun UHMWPE fibres, the hot compaction process was also subsequently applied to commercial UHMWPE gel-spun fibres [124,125]. It was reported that for some PE fibres, the optimum hot compaction temperature was only 1-2 C less than the temperature at which substantial crystalline melting occurred [117]. At these temperatures, approximately 30% of the oriented fibre content could be lost to melting, and so would be expected to strongly affect the mechanical properties of the final composite. A comparison of the hot compaction of different commercial UHMWPE assemblies, such as fabrics and felts, with commercial UHMWPE laminates was later presented by Morye et al. [126] and Puente Orench et al. [127]. 

Ratner et al. [132] reported the creatirai of UHMWPE composites formed via a similar hot compaction route, and also describe the affect of a chemical crosslinking pretreatment on commercial UHMWPE fibres. This crossUnking treatment was shown to increase the mechanical properties of the final composite constructs and may also enhance interfibrillar bonding. The use of crossUnking to enhance the mechanical properties of hot compacted PE fibres was also reported by Ward and Hine, although in this case, crosslinking was achieved by gamma irradiation in an atmosphere of acetylene [33]. 

A different method of creating UHMWPE fibre-reinforced UHMWPE composites was presented by Mosleh et al. [179]. In this method, dry UHMWPE powder was mechanically oscillated through a funnel onto subsequent layers of short (25 mm) chopped UHMWPE fibres, or pieces of continuous UHMWPE fabric. By repeating this process with many layers of short chopped UHMWPE fibres or UHMWPE fabric, a layered stmcture was reported to have been created. These fibre assemblies were then heated under pressure to consolidate the structures. The short UHMWPE fibre-reinforced composites had a fibre volume fraction between 25 and 75%, while the continuous UHMWPE fabric-reinforced composites had a fibre volume fraction of 60%. Investigations into the potential application of these homocomposites in an articulation surface for a knee joint prosthesis were also described, as were the challenges associated with measuring the tribological performance of such fibre-reinforced materials [180]. 

As described earher, PE is inherently more suitable for high-performance fibre production than PP. While the PE molecule has a planar zigzag conformation, the presence of the methyl side group of PP causes steric hindrance and means that the PP molecule is helical in conformation. This helical conformation of the PP molecule means that the theoretical maximum attainable stiffness of a PP fibre is much less than the theoretical maximum attainable stiffness of a UHMWPE fibre. 

While self-reinforced polymer composites based on PE and PP have received most attention in recent years, there is also great scope for the use of PET in self-reinforced polymer composites, since many high performance PET fibres are commercially available. Although not comparable to commercial UHMWPE fibres in terms of mechanical properties, the mass productimi of PET fibres and... 

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