Page 455 - Handbook of Adhesives and Sealants

Adhesives Families 393

applications where the substrate, environmental conditions, and
strength characteristics are suitable. For example, water emulsions,
such as polyvinyl acetate, are used to bond wood in the furniture in-
dustry. They are used in ‘‘structural’’ applications but the porous sub-
strate and generally mild service conditions do not put a severe burden
on the adhesive. Non-structural adhesives may receive little notice
because of their pedestrian applications. However, they enjoy by far
the major portion of the overall adhesive market.

10.4.1 Elastomeric resins
Natural- or synthetic-rubber-based adhesives usually have excellent
peel strength but low shear strength. Their resiliency provides good
fatigue and impact properties. Temperature resistance is generally
limited to 150–200 F, and creep under load occurs at room tempera-
ture. The basic types of rubber-based adhesives used for nonstructural
applications are shown in Table 10.18.
These systems are generally supplied as solvent solutions, latex ce-
ments, and pressure-sensitive tapes. The first two forms require driv-
ing the solvent or water vehicle from the adhesive before bonding. This
is accomplished by either simple evaporation or forced heating. Some
of the stronger and more environmental resistant rubber-based ad-
hesives require an elevated-temperature cure. Once the solvent is re-
moved, these adhesives are used as either contact or pressure sensi-
tive adhesives. The ease of application and the many different
properties that can be obtained from elastomeric adhesives account
for their wide use.
A contact adhesive is one that is apparently dry (after solvent evap-
oration) to the touch. Many formulations, however, will adhere to itself
instantaneously upon contact with very slight pressure. Other for-
mulations require the application of pressure or heat and pressure to
create a bond. Contact adhesives form relatively strong bonds with
high shear and peel strengths. They are often used for metal to metal
bonding applications and to bond decorative sheet to wood cores. Al-
though they will creep at elevated temperatures, depending on the
nature of the base resin, contact adhesives are used in many struc-
tural applications.
Pressure-sensitive adhesives are permanently tacky and flow under
pressure to provide intimate contact with the adherend surface. Pres-
sure-sensitive tapes are made by placing these adhesives on a backing
material such as rubber, film, canvas, or cotton cloth. After pressure
is applied, the adhesive tightly grips the part being mounted as well
as the surface to which it is affixed. Pressure-sensitive adhesives are