UHMPE

Wang, Q., Ait-kadi, A. and Kaliaguine, S. (1992a). Catalytic grafting a new technique for polymer/fiber composites. II. Plasma treated UHMPE fibers/polyelhylene composites. J. Appl. Polym. Sci. 45, 1023-1033. 

Ultra-high modulus fibers such as aramid and carbon fibers have been currently utilized for composite material fabrication. Ultra-high modulus polyethylene (UHMPE) fiber is also applicable for composite fabrication because of the light weight in addition to its high modulus, vibration damping, and resistance to chemicals. However, this fiber has drawbacks such as poor interfacial adhesion with the polymer matrix of the composite because of highly hydrophobic nature of the fiber surface. 

Recently, various techniques that produce highly oriented linear polyethylene with a ultra high modulus (hereafter, referred to as UHMPE) have been developed. In this section, we will examine the structure of the UHMPE that was prepared by highly drawing a dried gel [69]. Even if bulk polyethylene is uniaxially highly drawn by a normal method at a temperature between the Tg and Tm, the phase structure is essentially similar to the undrawn sample. That is, it involves three phases of the crystalline and two noncrystalline phases, although the mass fraction and detailed content of each phase are somewhat different. However, UHMPE samples may have a particular phase structure. 

The wear behaviour of polytetrafluoroethylene (PTFE), carbon-filled PTFE, high density polyethylene (HDPE), ultrahigh molecular weight polyethylene (UHMPE), low density polyethylene (LDPE) and polymethyl methacrylate (PMMA) was studied. To ensure consistent and controlled properties of the samples, many of the materials were processed in the authors laboratory. The details of sample preparation and processing techniques are reported elsewhere ( ). ... 

Experimental data presented here indicate that the wear behaviour of PTFE, as well as linear polythenes, is quite different from that expected on the basis of the simple thin film transfer mechanism described earlier. The results are unambiguous but at first sight difficult to explain. In the search for an explanation of the unusual wear behaviour shown by PTFE and linear polythenes, an analysis of the relative motions in "vertical" pin-to-disc configuration was carried out. There was a strong indication that this unusual wear behaviour of PTFE, HDPE and UHMPE arises from a pecularity in the pin-on-disc method (10). Since the fixed pin describes a circular path, the motion is complex. To a good approximation it may be considered to be comprised of a linear vector V, combined with a pin effectively rotating about its axis with an... 

Tilting disc Leaflet Delrin Pyrolytic carbon (carbon deposited on graphite substrate) ultra high molecular polyethylene (UHMPE)... 

TPR thermoplastic rubber UHMPE ultrahigh modulus polyethylene... 

LDPE, PB, and UHMPE should be classified as semirigid thermoplasts on the basis of their moduli of elasticity, but other commodity plastics should be classified as rigid. In all cases, the moduli of elasticity of the commercial polymers are much lower than the values theoretically expected for the fully oriented chain (see also Chapter 11). Commodity thermoplastics range between soft and hard in terms of hardness. 

UHMW-PE Ultrahigh molecular weight poly(ethylene) (also UHMPE) (molecular mass over 3.1 x 106 g/mol)... 

The extrusion of powder billets ensmes a considerable improvement of some physical and mechanical characteristics (5). It is used in the case of materials for which the solid plug method is not efficient enough, eg UHMPE and PTFE (9,63). For PTFE, the maximum value of the so attained flexural modulus at 24°C is 20 GPa at an EDR 40 (64). 

In a further study, Evans and Alderson [61-63] showed that in both PTFE and ultrahigh molecular weight polyethylene (UHMPE) an isotropic microstructure of nodules and fibrils, shown schematically in Figure 7.24 can give rise to a negative Poisson s ratio. Essentially this arises because when the materials are stretched the extension of the fibrils causes the nodules to move apart. 

E reduction at large for compositions UHMPE/Al is due to not molecular mechanisms, but macroscopic ones (interfacial boundaries polymer matrix-filler fi acture) [16]. The linear dependence K2(Xj jj) is obtained, which at = 1 is extrapolated to = 0. This means, that polymer Ifacture is impossible without chain preliminary drawing. In other words, this assumes definite nonzero failure strain for nonoriented polymers samples. As it has been shown in Ref [22], the greatest value X j ( ) can be determined... 

FIGURE 7.2 The dependence of local overloading coefficient k, calculated according to the Eq. (7.3), on molecular draw ratio l j for UHMPE (1) and UHMPE/Al (2). The shaded region indicates literary values range [10]. 

The situation changes in the case of solid-phase (semicrystalline) polymer uniaxial drawing. As the experimental estimations shown [5], the Poisson s ratio value for initial pol5aneric materials (componors UHMPE-Al and UHMPE-bauxite) v 0.36 and for these materials extmdates with draw ratio X > 3-v 0.43. From the Eq. (14.3) it follows that A 0.857. This means componors volume obligatory increase, expressed in cracks formation on interfacial boundaries pol5nner matrix-filler [3] ... 

FIGURE 14.1 The dependences of structure fractal dimension on extrusion draw ratio X, calculated according to the Eq. (1.9) (1,2) and the Eq. (1.12) (3) for UHMPE-Al (1, 3) and UHMPE-bauxite (2,3). The shaded region shows the X range, corresponding DS spontaneous change [11]. 

However, in Ref [36] although the good conformity of theory and experiment was obtained, but parameters A, and were not identified within the frameworks of polymers structure or properties. Therefore, the goals propounded above were solved on the example of 5deld process description of polymerization-filled compositions (componors) on the basis UHMPE, prepared by solid-phase extrusion method [2],... 

FIGURE 14.5 The comparison of experimental and calculated according to the Eq. (14.15) Oy yield stress values for UHMPE (1), UHMPE-Al (2) and UHMPE-bauxite (3) [31],... 

FIGURE 14.6 The dependences of elasticity modulus E ) and fracture stress (2) on extrusion draw ratio X for componors UHMPE - bauxite [44],... 

The authors number was offered earlier [43] the new method of polymerization-filled compositions on the basis of UHMPE processing - powder billet ram extrusion, allowing to prepare rod-like products with mechanical properties high level. The studies, carried out on systems UHMPE-kaolin, UHMPE-Al, shown [2], that at large draw ratios X a componors strengthening reduction occurs. The obtained result comparison with the data by ex-trudates UHMPE and its compositions with smaller kaolin contents fracture [43] allows to suppose, that fracture stress extreme change can be connected with the filler availability. The Ref [44] is devoted to this question clarification. 

As and in the case [45], the studied componors fracture stress <7 depending on X changes extremely and similarly o elasticity modulus E, reaching the greatest values in the region of X. 5 (Fig. 14.6) at bauxite content 40 mas. %. The maximum for componors UHMPE-Al with Al content 70 mas. % is disposed at smaller X values that at the same filler content 54 mas. % [45]. Hence, at filler contents increase strengthening reduction process is displayed earlier (at smaller X). The X increasing induces also extrudates density p monotonous reduction also (Fig. 14.7). 

In Fig. 14.8, the change of samples limiting draw ratio corresponding to their fracture in mechanical tests, as a function extrusion draw ratio X is shown. As one can seem the X,. increase at X growth is observed. Such behavior Xj is differed from traditionally observed one for oriented polymers, when X increasing decreases X,. owing to molecular chains mobility exhausting [19], Personally, it has been established, that for matrix UHMPE X enhancement from 3 up to 5 is accompanied by reduction from 1.18 uptol.l2[44]. 

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