Page 329 - Handbook of Adhesives and Sealants

Adhesive Classifications 289

characteristics of the adhesive in the particular application. For ex-
ample, if the processing requirements dictate that only a room tem-
perature curing adhesive system can be used, then high temperature
resistance will generally be sacrificed. There are very few room tem-
perature curing adhesives that exhibit good elevated temperature re-
sistance because of the lack of extensive crosslinking and the nature
of the polymers formed with room temperature curing formulations.
The assembly engineer is often constrained in selection of an adhesive
by available plant facilities, production speed requirements, cost, and
other important business parameters. Cure conditions required for
major adhesives and sealants are shown in Table 8.3.
If grouped by method of reaction, adhesives can be further classified
by type or how they are used. For example, within the group that cures
by chemical reaction there are systems that cure by reacting with a
hardener, by reacting with moisture in the air, by reacting with a
source of radiation, or by reacting with the surface ions on the sub-
strate. This hierarchy can be summarized as follows:

1. Adhesives that harden by chemical reaction:
a. Two part systems
b. Single part, cured via catalyst or hardener
c. Moisture curing adhesives
d. Radiation (light, UV, electron beam, etc.) curing adhesives
e. Adhesives catalyzed by the substrate
f. Adhesives in solid form (tape, film, powder, etc.)
2. Adhesives that harden by solvent or water loss:
a. Contact adhesives
b. Pressure sensitive adhesives
c. Reactivatable adhesives
d. Resinous adhesives
3. Adhesives that harden by cooling from the melt:
a. Hot melt adhesives
b. Hot melt applied pressure sensitive and thermosetting adhe-
sives

The following sections will describe the adhesive systems that reside
within each of these types.

8.2.3.1 Chemical reaction. Most thermosetting adhesives crosslink
and cure by two primary chemical reactions. 3
1. Those formed by condensation reaction, usually with water as a
by-product. This group includes the phenolic and amino resins, which