Hot Melt Adhesives Defined

Hot Melt Adhesives Defined

Hot melt adhesives are solvent-free adhesives, that are characteristically solid at temperatures
below 180 degrees F (°F), are low viscosity fluids above 180°F, and rapidly set upon cooling. The
development of hot melt adhesive technology stemmed from the previous use of molten wax for
bonding. When this method no longer satisfied performance needs, 100 percent thermoplastic
systems were introduced. Today, hot melt adhesives are used in a variety of manufacturing
processes, including bookbinding, product assembly, and box and carton heat sealing.

There are a number of hot melt adhesives in use, with the most common being those used for hot
melt pressure sensitive adhesive applications:

* ethylene vinyl acetate (EVA) copolymers, compatible with paraffin, the original hot melt;
* styrene-isoprene-styrene (SIS) copolymers;
* styrene-butadiene-styrene (SBS) copolymers;
* ethylene ethyl acrylate copolymers (EEA); and
* polyurethane reactive (PUR). (Refs. 6, 11).

Generally, these polymers do not exhibit the full range of performance characteristics required for
an end product by themselves. Thus, "a variety of tackifying resins, waxes, antioxidants,
plasticizers, and other materials are added to the adhesive formulation to enhance the polymer
performance." (Ref. 6)

The latest hot melt adhesive advance is the PUR adhesive, which is a 100 percent solid,
one-component urethane prepolymer that "behaves like a standard hot melt until it reacts with
moisture to crosslink or chain extend, forming a new polyurethane polymer." (Ref. 12) By curing
the polymer in this way, PURs have performance characteristics that are more enhanced than
those of standard hot melts. (Ref. 12) Unlike many of the other hot melts, which require a slot die
or roll coater (Ref. 6), PURs are applied to a substrate as a dot or a thin glue line, set in seconds,
and are structurally rigid in minutes, following a final set. These adhesives have been accepted in
many manufacturing industries, where they can be applied in small bond points to eliminate use of
mechanical fasteners, such as staples, screws, rivets, clips, snaps, nails or stitching. (Ref. 13)

Uses and Properties

Hot melt adhesives are used "primarily for packaging, textiles, labels, and other pressure sensitive
applications, disposable products, stamps and envelopes, and product assembly processes."
(Refs. 2, 3) Table 2 describes several industrial sectors and applications in which hot melt
adhesives are commonly used.

Table 2
Common Uses of Hot Melt Adhesives

Industry

Application
Construction         Manufacture of laminated wood panels; kitchen countertops.
Non-rigid Bonding         Bonding of woven and non-woven fabrics; manufacture of athletic shoes,
books, and sporting goods.
Packaging         Manufacture of cartons, boxes and corrugated boards; bags, envelopes, disposable
products (diapers, paper products); cigarettes; and labels, stamps.
Vehicles         Aircraft and aerospace structural assemblies; automotive, truck, boat, and bus
assembly; mobile home manufacturing.

Hot melt adhesives form a strong bond quickly simply by cooling (Refs. 2, 3), are compatible with
most materials, and are clean and easy to handle. (Ref. 3) In general, hot melt adhesives are less
water sensitive than other thermoplastic polymers, and are unaffected by water, moisture, or
humidity, although if applied to a damp or wet surface the bonding may be poor. Hot melts can be
formulated to increase their water sensitivity, as when used for stamps, envelopes and paper
products that are to be recycled. (Refs. 2, 3)

Hot melt adhesives have some limitations that must be recognized. Hot melts cannot be used with
heat sensitive substrates; the adhesive bonds lose strength at high temperatures; chemical
resistance may be lacking with some types of hot melts; and exposure to high temperature
environments can cause the adhesive to melt. Consequently, hot melt adhesives are inappropriate
in situations where these limitations cannot be avoided. For example, hot melts should not be
used on a substrate that would be near a heat source, such as a kitchen cabinet that would be
placed near an oven. (Ref. 3) However, innovations in hot melts are removing some of these
limitations: PURs are resistant to heat once they are cured, and could be used on substrates
subsequently exposed to heat. (Refs. 2, 3)