Page 454 - Handbook of Adhesives and Sealants

392 Chapter Ten

tions. Modified acrylics have good peel, impact, and tensile lap shear
strengths between temperatures of 160 and 250 F. High bond
strengths are obtained on metals and plastics and on oily surfaces.
Bonds have good resistance to high humidity. Many acrylic resins have
a pungent, monomeric odor making them objectionable to certain
users. Recent advances have eliminated some of these problems.
Certain acrylic formulations are easily converted to UV or light cur-
ing formulations with the addition of photoinitiators to activate the
free radical crosslinking mechanism. These single component adhe-
sives are usually termed ‘‘aerobic’’ adhesives. They have diminished
sensitivity to air inhibition. Aerobic adhesives cure rapidly at room
temperature to form tough, durable bonds with structural strength.
In order for the UV or visible light to get through to the adhesive, the
substrates must be transparent to light or else the adhesive is used
as a coating/sealant such as in the application of wire bonding on
circuit boards. Generally, the adhesive must be in the direct path of
the light source. Several recent formulations have been developed
where the adhesive will cure in deep (1/4 in) sections. With these
systems indirect UV light is sufficient to cure the adhesive.
An acrylic-polyurethane adhesive has been introduced recently that
has a urethane backbone crosslinked with acrylic. 34 This type of ad-
hesive develops flexible bonds in plastic and non-plastic assemblies
such as office furniture and architectural laminate. It has good weath-
ering properties and low odor because of the incorporation of urethane.
Recently, a new thermosetting acrylic adhesive was also introduced
with good cold temperature resistance and improved peel adhesion to
metals. 35 With conventional toughened acrylic adhesives, cold temper-
ature resistance is limited to approximately 0 F.

10.4 Nonstructural Adhesives

Non-structural adhesives are characterized by relatively low strength
and poor creep resistance at slightly elevated temperatures. The most
common non-structural adhesives are based on elastomers and ther-
moplastics. Although these systems have low strength, they usually
are easy to use and fast setting. Because of these characteristics, most
non-structural adhesives are used in assembly-line fastening opera-
tions where the ultimate joint strength is not required and the envi-
ronmental service conditions are not severe. The common types of
non-structural adhesives are pressure-sensitive adhesives, contact ad-
hesives, mastics, hot melts, and thermoplastic emulsions.
Although they may be classified as ‘‘non-structural’’ for the purpose
of this discussion, many of these adhesives can be used in structural