Page 192 - Handbook of Plastics Technologies

THERMOSETS

3.62 CHAPTER 3

phthalonitriles at 250 to 350°C produces highly cross-linked thermoset heterocyclic poly-
mers. These have high T s (e.g., 450°C), resist temperatures of 200 to 550°C, and have
g
low water absorption.
Aside from simply varying the mid-section of the bis-phthalonitrile, researchers have
copolymerized them with aromatic diamines. They have also inserted different metal cat-
ions into the center of the phthalocyanine ring. For example, germanium and tin give semi-
conductivity, and doping with iodine brings electrical conductivity up to 0.01 Ω-cm. Put-
ting a silicon atom into the center of the ring permits them to stack layers of phthalocya-
nine rings around a siloxane center. This family of polymers offers a number of
possibilities in terms of adhesion, heat stability, conductivity, colorability, and chemical
resistance.

3.1.8 Cross-Linking of Thermoplastics
Thermoplastics are generally stable linear molecules, but we do occasionally cross-link
them to improve specific processes or properties. Some of them contain reactive groups
that can be cross-linked directly. Others we modify so as to make them accessible to cross-
linking. Quite a number of typical improvements can be made by cross-linking:
• Foam processing
• Modulus
• Strength
• Creep-resistance
• Adhesion
• Abrasion-resistance
• Dimensional stability
• Heat deflection temperature
• Heat-shrink film and tubing
• Hot strength
• Dimensional stability
• Flame retardance
• Solvent resistance
• Water resistance
• Gelation
• Age resistance (Cage effect)

3.1.8.1 Polyethylene. Polyethylene is not very reactive, but it can be cross-linked
lightly, either by chemical reaction or by high-energy radiation. Chemical cross-linking is
most often accomplished by adding a high-temperature peroxide, such as dicumyl perox-
ide or 2,5-dimethyl-2,5-di-t-butylperoxy hexane, melt processing below the decomposi-
tion temperature of the peroxide, and then increasing the temperature to decompose the
peroxide and initiate the cross-linking process (Fig. 3.68). Addition of carbon black often
improves both process and properties.
A newer chemical method is to melt-blend polyethylene with vinyl trialkoxy silane and
peroxide, grafting the silane as side-groups onto the polyethylene chain (Fig. 3.69). As
long as it is kept very dry, the alkoxysilane groups are stable. After the grafted polyethyl-

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