Page 445 - Handbook of Adhesives and Sealants

Adhesives Families 385

tics. Since they are very flexible, polyurethane adhesives are often
used to bond films, foils, and elastomers. Urethane structural adhe-
sives are used extensively in the automotive industry because of good
adhesion properties to sheet molding component (SMC) and urethane
reaction injection molded (RIM) plastics. Structural polyurethane ad-
hesives are widely used in SMC assembly and other plastics bonding
in the automotive market. Polyurethanes also bond exceptionally well
to wooden substrates. The properties of several commercial polyure-
thane adhesives are presented on Table 10.15.
Recently reactive polyurethane hot-melt adhesives and sealants
have been introduced. 22,23 These hot melts develop 2,500–4,000 psi
shear strength and elongation of 500–700%. They are produced from
polyurethane polymers with terminal isocyanate groups. Initially, pol-
yurethane hot melts function similarly to conventional thermoplastic
hot melts and eliminate the need for extended fixturing of assemblies.
They are generally applied in molten form at temperatures as low as
215 F. However, once the polyurethane material has been applied to
the substrate, the terminal isocyanate groups react with moisture in
the substrate and surrounding air to form a thermoset material. Dur-
ing this cure cycle, the product evolves from a thermoplastic to a ther-
moset adhesive.
Once the hot melt polyurethane is applied, the parts should be
joined rapidly. Green strength develops in seconds, and full cure will
occur in one to three days. The bond will continually strengthen to far
surpass the strength and environmental durability of conventional hot
melts. In addition to the automotive industry, these adhesives are
widely used in the textile industry for laminating fabric, seaming and
stitch replacement, and producing foam and fabric laminates. 24

10.3.10 Anaerobic resins
Acrylate monomer-based adhesives are called ‘‘anaerobic’’ adhesives
because they cure when air is excluded from the resin. They are es-
sentially monomeric, thin liquids that polymerize to form a tough plas-
tic bond when confined between closely fitting metal joints. The term
anaerobic is derived from the science of biology and refers to life forms
that live in the absence of air. Polymerization is essentially a free
radical type addition polymerization as shown in Fig. 10.8.
These materials will bond all common metals, glass, ceramics, and
thermoset plastics to each other. Anaerobics can be applied to three
types of surfaces: active, inactive, and inhibiting. Active surfaces, such
as clean metals and thermoset plastics, will produce the fastest cures.
Metal surfaces accelerate the polymerization in the absence of air.
Primers or heat must be used to achieve curing on inactive surfaces