Page 193 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.63

FIGURE 3.68 Peroxide cross-linking of polyethylene.

FIGURE 3.69 Silane cross-linking of polyethylene.

ene has been melt-processed into the final product, it is immersed in low-pressure steam to
hydrolyze the alkoxy groups to silanol groups, which condense instantly to form siloxane
cross-links.
Radiation cross-linking is often done with electron-beam equipment, using doses up to
60 Mrad. This splits off hydrogen atoms, leaving polymer radicals. The resulting radicals
are trapped in the solid phase; cross-linking can be hastened by annealing, giving the radi-
cals enough mobility to meet and combine.
Cross-linking increases melt viscosity, which improves the foaming process. It is used
to produce heat-shrink film and tubing. It increases environmental stress-crack resistance,
and it retains hot strength beyond the melting point of the crystalline phase. These im-
provements are particularly useful in wire and cable, pipe insulation, gasketing, and sports
and orthopedic equipment.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.