Crosslink , ing

De Gennes [41] predicted that percolation theory should hold for crosslink-ing of small molecule precursors. However, he argued that for vulcanizing polymers (high Mw), only a very narrow regime near the gel point exists for which percolation is valid, i.e. these polymers should exhibit more mean fieldlike behavior. 

Fusite. [H.B. Fuller] Two-part crosslink-ing urethane adhesive. 

Fibersil. [Goldschmidt] Self-crosslink-ing silicone finish to synthetic fill fi-bm. 

Fig. 4. Double logarithmic plot of the rdax-ation shear modulus vs time for a crosslink-ing system curve a - before the gel point curve fc - at the gel point curve c - after the gel point...

Polyurethane paints - even at relatively low crosslinking density - exhibit high resistance to water, solvents, and chemicals compared to other paints. Increasing the crosslinking density increases chemical resistance. This can be achieved both by using high-functional polyols as well as by an NCO/OH ratio > 1 (over-crosslink-ing). Often both measures are combined. 

The higher the average valence of a system, the greater its density of crosslink-ing. The description and the characterization of polymer networks will be described in Section 3.5. 

During the 1980s, two other noteworthy developments occurred relative to polyethylene in joint replacements. In the early 1980s, Char F. Thackray, Ltd. of Leeds began development on an injection molded HOPE that could be crosslinked by silane chemistry (the same type of crosslink-ing used in silicone polymers) [5, 6]. Only 22 of these implants were produced and implanted by Dr. Wroblewski starting in 1986 [7]. After an initial bedding in period, these crosslinked HOPE components have been found to exhibit very low clinical wear rates. 

There are two methods of incorporating vitamin E into UHMWPE. One is to blend vitamin E with UHMWPE powder prior to consolidation. When consolidated, the blend can be crosslinked with the use of ionizing radiation. However, the presence of vitamin E in UHMWPE during irradiation reduces the efficiency of crosslinking [3-5]. The properties of vitamin-E-blended UHMWPE are covered in Chapter 16. An alternative method is the diffusion of vitamin E into UHMWPE following radiation crosslink-ing [1, 6]. The crosslinking efficiency of UHMWPE is not adversely affected in this method because vitamin E is not present during irradiation. 

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