Ultra high molecular weight polyethylene, Polymerization

The flow of liquid hydrocarbons can be enhanced by introducing into the stream a nonagglomerating suspension of ultra-high molecular weight polyethylene [490,1647] in water with small amounts of surfactant. The finely divided polyethylene is prepared by polymerization and then cryogrinded below the glass-transition temperature. 

Polyolefins. Ultra-High-Molecular-Weight Polyethylene.TIus is easily made by conventional low-pressure coordination polymerization. Hercules Hi-fax 1900 has a molecular weight of 2.5-5.0 million. Its outstanding properties are low coefficient of friction (0.11) abrasion-resistance superior to nylon, polyurethane, and steel unbreakable in the Izod notched impact test and high resistance to most inorganic and many organic chemicals. 

When polytetrafluoroethylene (PTFE) was Introduced in the early 1960 s the life of total replacement hip joints was limited to about three years by the poor wear characteristics of the polymer. When ultra-high molecular weight polyethylene (UHMWPE) replaced PTFE, the rate of penetration of the metallic component into the polymeric component was reduced to such an extent that loosening emerged as a major aspect of prosthetic life. There is, nevertheless, a need to pursue studies of the wear of prosthetic materials to facilitate the development of satisfactory materials which can be used with confidence in long-life prostheses. 

The blood bag case study illustrates the use of polymeric film as a flexible container. It considers the permeability of polymers, plus processes for fabricating plastics film. Plasticised PVC has dominated the market for years, but there could be a changeover to flexible polyolefin films. The case study on replacement joints for implanting in the body illustrates wear and the effects of wear debris. Research continues on improving the wear resistance of the ultra high molecular weight polyethylene (UHMWPE) and mitigating the effects of sterilisation on the implant properties. 

At a conceptual level, polyethylene consists only of carbon and hydrogen, as was described in the previous chapter. However, if the discussion of polyethylene is to proceed from ideal abstractions to actual physical implants, three "real world" steps need to occur. First, the ultra-high molecular weight polyethylene (UHMWPE) must be polymerized from ethylene gas. Second, the polymerized UHMWPE, in the form of resin powder, needs to be consolidated into a sheet, rod, or near-net shaped implant (Figure 2.1). Finally, in most instances, tire UHMWPE implant needs to be machined into its final shape (Figure 2.1). A small subset of implants are consolidated into their final form directly, in a process known as direct compression molding (DCM), witiiout need of additional machining. 

For SSE, monolithic or powder billets are used. In the latter case, the polymer powder is compacted, heated to the temperature close to the melting temperature and then extruded. The extrudates of ultra high molecular weight polyethylene (UHMWPE) (8), polymerization-filled polyethylene composites (7,9)... 

P.J. Lemstra, C.W.M. Bastiaansen, S. Rastogi, Basic aspects of solution (Gel)-spinning and ultra-drawing of ultra-high molecular weight polyethylene. Structure Formation in Polymeric Fibers Salem, D. R., Ed. Hanser Munich, 2000, pp. 186-223. 

Weiser, M.S., Wesolek, M., and Mulhaupt, R. (2006b) The synthesis and X-ray structure of a phenoxyimine catalyst tailored for living olefin polymerization and the synthesis of ultra-high molecular weight polyethylene and atactic polypropylene. Journal of Organometallic Chemistry, 691,2945-2952. 

Three advanced polymeric materials were discussed ultra-high-molecular-weight polyethylene, liquid crystal polymers, and thermoplastic elastomers. These materials have unusual properties and are used in a host of high-technology applications. 

Figures VII.3 and VII.4 show the experimental values of the Young s modulus and the tensile strength, respectively, for thick films of undoped trans-polyacetylene as a function of draw ratio (all samples were derived from the same polymerization batch). Although there is some scatter in the data, the modulus and tenacity increase approximately linearly with the draw ratio, as is commonly observed for most polymers drawn to moderate draw ratios. The modulus and tensile strength of trans-polyacetylene films stretched up to 15 times are 50 GPa and 0.9 GPa, respectively. These values are essentially equivalent to those observed for ultra-high molecular weight polyethylene [83] drawn to the same draw ratio. Recently, Akagi et al.[78] reported remarkable mechanical properties for drawn polyacetylene films prepared by non-solvent polymerization (100 GPa and 0.9 GPa for the modulus and tensile strength, respectively). The origin of difference in the modulus (in the two studies) is unknown.

H. -J. Park, J. Kim, Y. Seo, J. Shim, M. -Y. Sung, and S. Kwak. Wear behavior of in situ polymerized carbon nanotube/ultra high molecular weight polyethylene composites. Macromol. Res. 21 (9), 965-970 (2013). 

Lemstra, P.J., Bastiaansen, C.W.M., and Rastogi, S., Basic Aspects of Solution(Gel)-Spinning and Ultra-Drawing of Ultra-High Molecular Weight Polyethylene , in Structure Formation in Polymeric Fibers, Editor Salem, D.R., Hanser Gardner Publications Inc, 2000. 

Aerospace composite structures can also employ high-performance polymeric fibres such as the aramids Kevlar (DuPont) or Twaron (Teijin Twaron), poly(p-phenyl-ene-2,6-benzobisoxazole) (PBO) (Toyobo) and high-modulus polyethylene (PE) (Dynema, Certran and Spectra). Generally ultra-high-molecular-weight PE (UHMWPE) can be considered to be inert to most environments except that the service temperature is limited to <130 °C. 

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