Page 331 - Handbook of Adhesives and Sealants

Adhesive Classifications 291

are the oldest of the synthetic adhesives. It is usually necessary to
also apply pressure to overcome the deleterious effects of the volatile
by-products.
2. Those formed by addition polymerization without by-product
formation. Among this group are polyesters, epoxies, urethanes,
cyanoacrylates, anaerobics, and radiation curable polymers. Most
elastomers and acrylics are also in this category. Only contact pressure
is necessary for these adhesives, and so they are well suited for bond-
ing large articles.
In order to achieve cure with these adhesive systems, a curing agent
or catalyst is necessary. The curing reaction then proceeds at room
temperature or at elevated temperatures depending on the nature of
the reaction. In some cases, these adhesives will cure on activation
with radiation (visible light, UV, or other form of energy). Anaerobic
adhesives are catalyzed by metal ions on the surface of the substrate
and cure rapidly when air is excluded from the bondline. Some poly-
urethane and silicone adhesives are cured by reacting with the mois-
ture in the air.
The main advantage of adhesives that cure by chemical reaction is
that they can be used for relatively large areas. With room tempera-
ture curing formulations, an oven or press is not needed. Being ther-
mosetting in nature, these adhesives also provide a high degree of
cohesive strength, temperature resistance, and resistance to common
environments.
There are many adhesives that cure by chemical reaction. Chemi-
cally reactive adhesives can be further subdivided into two groups:

Single component systems—moisture, heat activated, and free rad-
ical cure
Multiple component systems—mix-in and no-mix 4
These adhesives are used in both structural and non-structural ap-
plications. The most widely used chemically reactive structural ad-
hesives are epoxy, polyurethane, modified acrylic, cyanoacrylate, and
anaerobic systems. Table 8.4 describes the advantages and limitations
of these five adhesives. Typical application and end-use properties are
also presented.
8.2.3.1.1 Multiple part adhesive systems. Multiple component systems
consist of two or more components that must be kept separate until
just before the bonding operation. These components must be metered
in the proper ratio, mixed, and then dispersed. Once mixed according
to the manufacturer’s instructions, the adhesive will have a finite