Page 354 - Handbook of Adhesives and Sealants

Adhesive Classifications 309

One part solution (liquid)
Solid (powder, tape, film, etc.)

Some specific types of adhesives, such as epoxy, can be available in
several forms through slight variation in the composition. The char-
acteristics and advantages of the various adhesive forms are sum-
marized in Table 8.9.
Often the desired form of the adhesive is dictated by the complexity
of the parts to be bonded and the availability of application equipment.
Two-part solvent systems would, for example, require metering and
mixing equipment, spray application equipment, and fixturing equip-
ment to hold the parts together until the adhesive dries and cures. On
the other extreme, a solid tape adhesive may have the catalysts al-
ready introduced into the formulation. Thus, there is no need to meter
or mix components, and application does not require any special equip-
ment. Furthermore, there is little waste with a tape adhesive, and
contact with liquid chemicals, a source for dermatitis and other health
problems, is eliminated. Solvent adhesives are generally used where
the adhesive can be uniformly sprayed on large areas. However, with
solvent based adhesives, provision must be made for evaporation of
the solvent with resulting environmental concerns.
Adhesives can be simply classified as being a liquid (solvent con-
taining or 100% solids) or a solid. Within each class there are impor-
tant types of adhesive systems. Table 8.10 outlines the various liquid
and solid adhesives that are in common use today.

TABLE 8.9 Adhesives Classified by Form
Type Remarks Advantages
Liquid ...... Most common form; practically Easy to apply. Viscosity often
every formulation available. under control of user. Major
Principally solvent-dispersed form for hand application
Paste....... Wide range of consistencies. Lends itself to high-
Limited formulations; production setups because of
principally 100% solids less time wait. High shear
modified epoxies and creep strengths
Powder ..... Requires mixing or heating to Longer shelf life; mixed in
activate curing quantities needed
Mastic ...... Applied with trowel Void-filling, nonflowing
Film, tape . . Limited to flat surfaces, wide Quick and easy application.
range of curing ease No waste or run-over;
uniform thickness
Other....... Rods, supported tapes, Ease of application and cure
precoated copper for printed for particular use
circuits, etc.