Page 197 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.67

fully suggest further development of cross-linking processes in thermoset plastics as
well.
3.1.9.1 Elastomers. Rubber is based on long, flexible molecules. These must be bonded
to each other to increase strength, elastic recovery, creep resistance, heat resistance, and
chemical resistance. In thermoplastic elastomers, the bonding is based on secondary at-
tractions such as polarity, hydrogen-bonding, and crystallinity, gathered into nano-size do-
mains dispersed in the rubber matrix. The majority of the rubber industry, however, uses
primary covalent cross-linking (vulcanization) to ensure intermolecular bonding.
Most rubber is based on polymers of isoprene or butadiene and contains many reac-
tive C=C double bonds available for cross-linking. It is cross-linked by sulfur, aided by
metal oxides and organic catalysts, producing sulfide cross-links between the polymer
chains. Ethylene-propylene rubber is mostly made with several percent of diene ter-
monomer to introduce C=C double bonds, which can then be vulcanized in the same
way. Similarly, butyl rubber is made with a few percent of isoprene comonomer to intro-
duce C=C double bonds and permit sulfur vulcanization. Even saturated elastomers are
sometimes cured by sulfur, using peroxides and catalysts to activate C-H bonds, and
metal oxides to attack C-Cl bonds.
Saturated elastomers are often cured by peroxide, often aided by catalysts. These in-
clude chlorinated polyethylene, fluorocarbon, acrylic ester, epichlorohydrin, polysulfide,
polyurethane, and silicone. The peroxide radical abstracts an unstable hydrogen from the
polymer, leaving a polymer radical, and then polymer radicals couple to produce C-C
cross-links.
Halogenated elastomers are often cured by metal oxides, in combination with other in-
gredients. These include chlorosulfonated polyethylene, chloro- and bromo-butyl, and
neoprene.
Some of the more unusual curing agents include phenolic resin, quinone dioxime, ma-
leimide, diamine, diisocyanate, tetraethyl silicate, and triallyl isocyanurate. These are lim-
ited to very specific polymer systems.

3.1.9.2 Coatings. Although thermoplastic polymers may make good coatings, cross-
linking is generally preferred to achieve maximum performance.
Alkyd resins are made from vegetable oils containing many C=C double bonds. Atmo-
spheric oxygen attacks these bonds, causing addition polymerization and cross-linking.
Polyesters are usually produced with terminal –OH groups. These are then cured by re-
action with methylol melamine or isocyanate.
Acrylic polymers are made with some hydroxyalkyl acrylate comonomer, and the –OH
groups are then cured by methylol melamine or epoxy resin.
Urea-formaldehyde and melamine-formaldehyde are used to cross-link acrylic, alkyd,
epoxy, and polyester coatings.
Epoxy resin coatings are cured by copolymerization with acrylic, polysulfide, polyure-
thane, polyamine, polyamide, amino, and phenolic oligomers.
Polyurethane coatings are cured by reaction with isocyanate or copolymerized with
alkyds and then cured by atmospheric oxygen.
Phenolic resins can be cured by simple homopolymerization or by copolymerization
with alkyd or epoxy resins.
Silicones are cured by hydrolysis of CH SiCl or (C H O) Si to form silanols, which
2 5
4
3
3
condense with the –OH end-groups of the silicone oligomers. CH Si(OH) condenses to
3
3
CH SiO 1.5 “glass resin;” baking this at high temperatures burns out the CH and produces
3
3
SiO ceramic coating. Silicones are also often copolymerized with alkyd, acrylic, epoxy,
2
and polyester to upgrade coatings for resistance to heat, moisture, and weather.
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