Page 360 - Handbook of Adhesives and Sealants

Adhesive Classifications 315

8.2.4.4.3 Hot melts forms. Solid hot melt adhesives are formulated
with thermoplastic resins. They can be applied in several ways. The
adhesive can be applied to the surface in molten form using a heated
tank and heated hoses. In this case, the substrate is generally pre-
heated so that the adhesive will not gel immediately on contact with
the cooler substrate. The surfaces are then quickly mated together and
held under contact pressure until the adhesive solidifies. The sub-
strate can also be precoated with the adhesive prior to bonding.
The hot melt adhesive can also be applied to the joint as a preformed
thermoplastic film or mat. The joint is assembled, and heat and pres-
sure are applied. The joint is then cooled to solidify the adhesive and
achieve a bond.
Hot melt products are available in several shapes and forms. Sticks
and cord that can be fed through heated extrusion guns avoid the
necessity of having a heated bath of molten hot melt and heated hoses
to pump the hot melt from the bath to the joint area.

8.2.5 Cost
Cost is usually not used as a method of classifying adhesives; however,
it is an important factor in the selection of a specific adhesive and a
factor in determining whether adhesives should be used at all. Thus,
cost becomes a means of classification and selection, if not directly, at
least indirectly. The raw materials cost or ‘‘first cost’’ of an adhesive
can vary significantly.
Adhesive price is dependent on development costs and volume re-
quirements. Adhesives that have been specifically developed to be re-
sistant to adverse environments are usually more expensive than gen-
eral-purpose adhesives. Adhesive prices range from pennies a pound
for inorganic and animal-based systems to several hundred dollars per
pound for some heat-resistant synthetic types. Adhesives in film or
powder form require more processing than liquid or paste types and
are usually more expensive.
When estimating the cost of using adhesives, one must not only
consider the price of the adhesive but also the cost of everything re-
quired to obtain a reliable, complete joint. Therefore, the cost of fas-
tening with adhesives must also include the cost of labor, the cost of
equipment, the time required to cure the adhesive, and the economical
loss due to rejects of defective joints. The following parameters may
be important in analyzing the real cost of an adhesive system:

Efficiency of coverage in relation to bonding area or number of com-
ponents