Page 198 - Handbook of Plastics Technologies

THERMOSETS

3.68 CHAPTER 3

3.1.9.3 Adhesives. Ideally, an adhesive should be a low-molecular-weight fluid for
easy and thorough application to a solid surface and then convert to a high-molecular-
weight solid for maximum mechanical strength. This is most easily achieved by apply-
ing a low-molecular-weight reactive oligomer and then cross-linking it up to a high-mo-
lecular-weight finished adhesive bond. Additionally, if the cure reaction can react with
the substrate as well, then the finished product is composed of primary covalent bonds
from adherend 1 to adhesive layer to adherend 2, which should give the maximum bond-
ing strength. Thus, it is not surprising that many of our best adhesives are thermosetting
resins.
Casein and starch adhesives are cross-linked by urea-formaldehyde, melamine-fomal-
dehyde, or resorcinol-fomaldehyde.
Many rubber adhesives are vulcanized to increase their strength: natural rubber, SBR,
neoprene, nitrile rubber, butyl rubber.
Polysulfide rubber is cured by peroxides.
Amino and phenolic resin adhesives for plywood, fiberglass insulation, laminates, fric-
tion products, and abrasive products are reactive oligomers that cure by self-condensation
of methylol groups.
Epoxy adhesives are cured by amines, amides, anhydrides, polysulfides, and amino and
phenolic resins.
Polyurethanes are cured by conventional polyol-isocyanate reactions.
Polyvinyl alcohol is copolymerized with cross-linkable comonomers; N-methylol
acrylamide is self-curing. Carboxylic acid comonomers are cured by epoxy, urea, or phe-
nolic resins.
Polyvinyl alcohol is cross-linked by glyoxal, epoxy, urea-formaldehyde, and phenol-
formaldehyde resins. It is also cross-linked by polyvalent metal salts.
Polyvinyl formal and butyral contain –OH groups that are cured by dialdehydes, diiso-
cyanates, anhydrides, epoxy, melamine, and phenolic resins.
Acrylics are copolymerized with a variety of functional groups that can either self-cure
or react with added curing agents: amide, carboxyl, hydroxyl, epoxy, alkoxymethyl.
Anaerobic adhesives are primarily glycol dimethacrylates, catalyzed by cumene hydro-
peroxide, and packaged under air to inhibit premature polymerization. When they are
pressed between two adherends, excluding air, they cure rapidly.
Silicone RTV sealants are HO-Silicone-OH oligomers + CH SiX cross-linkers, where
3
3
X is -OCH , -O CCH , or –N=C(CH ) .
3
3
2
3 2
Organosilane coupling agents are used to produce adhesion between a polymer ma-
trix and a reinforcing fiber or filler particle. They have a general formula (RO) SiRX,
3
where RO is CH O- or CH CO -, and RX is designed to copolymerize with the polymer
3
3
2
matrix during cure. For example, X would be –CH=CH for an unsaturated polyester
2
matrix, -NH or epoxy for epoxy, melamine, and phenolic matrixes.
2
Hopefully, this comparative review will provide stimulating cross-fertilization for fu-
ture developments in thermoset plastics and cross-linked polymers in general.
3.2 PROCESSES
There are a great variety of processes for manufacturing thermoset plastic products. Some
of them are modifications of conventional thermoplastic processes, but most of them are
uniquely designed for handling the simultaneous shaping and cross-linking that are in-
volved. They may be classified as molding processes, reinforced plastics processes, pour-
ing processes, and powder processes.

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