Page 351 - Handbook of Adhesives and Sealants

Adhesive Classifications 307

ness of the substrate surface. Once the hot melt adhesive is applied
and in the molten condition, the substrates must be joined immedi-
ately. The adhesive can be applied and then the adhesive coated sub-
strate is placed into storage for later activation. At a later date, the
coated substrates could then be removed from storage, reheated to
soften the adhesive, and joined under slight pressure.
When hardened, the hot melt adhesive can have various degrees of
tackiness depending on the formulation. A completely pressure sen-
sitive adhesive could result from a hot melt formulation. Some pres-
sure sensitive tapes and films are manufactured in this manner. This
method eliminates the expense and environmental hazards of using
solvent simply to reduce the viscosity of a pressure sensitive resin for
application to a carrier.
Materials that are primarily used as hot melt adhesives include eth-
ylene and vinyl acetate copolymers, polyvinyl acetates, polyethylene,
amorphous polypropylene, block copolymers (styrene butadiene
rubber), polyamides, and polyester. In general, hot melt adhesive for-
mulations are solids at temperatures below 175 F. Typical application
temperatures are 300–400 F. Typical properties of hot melt adhesives
are shown in Table 8.8.
Special dispensing equipment must be used to apply hot melts. At
one time hot melts were mainly applied from a molten bath that must
be kept under an inert gas to prevent oxidation of the adhesive. New
application methods and forms of hot melt adhesives now avoid the
need for such equipment. Cylinders or cords of hot melt can be applied
to the substrate though hand guns that have a heated nozzle. Hot melt
adhesive formulations must be chosen carefully based on the appli-
cation equipment available and the speed at which the joint can be
made. Open time of the hot melt is especially critical with substrates
that are thermally conductive such as metals.
Foamable hot melt adhesives have been available since the 1980s.
These adhesives use nitrogen or carbon dioxide gas to increase the
volume of the adhesive by 20–70% as it is applied to the substrate.
The foaming process increases open time and provides for good gap
filling properties. The elastic foam also tends to relieve stresses that
might develop internally within the joint.

8.2.4 Physical form
A widely used method of distinguishing between adhesives is by their
physical form. Adhesive systems are available in a number of forms.
The most common forms are:

Multiple part solventless (liquid or paste)
One part solventless (liquid or paste)