Page 318 - Handbook of Adhesives and Sealants

280 Chapter Eight

Many adhesives are made from organic polymers. There are also
adhesive systems with inorganic origin. The oldest polymers used for
adhesives were of natural origin. Often naturally occurring adhesives
are thought to be inferior to synthetic polymers because of their lower
strength and limited freedom in processing. However, in many appli-
cations, such as bonding of paper and wood where the emphasis may
be on the adhesive being biodegradable or repulpable, naturally oc-
curring adhesives find a strong market. Modern epoxies, urethanes,
acrylics, and other adhesive systems that are used in demanding
structural applications are made from synthetic polymers.
The classification of adhesives into synthetic and naturally occur-
ring categories is usually far too broad for many practical purposes.
The industry has settled on several common methods of classifying
adhesives that satisfy most purposes. These classifications are by:
Function
Chemical composition
Mode of application or reaction
Physical form
Cost
End-use

All of these classifications and distinctions overlap to some degree.

8.2.1 Function
The functional classification defines adhesives as being either struc-
tural or nonstructural. Structural adhesives are materials of high
strength and permanence. Generally, structural adhesives are defined
as those having shear strengths in excess of 1000 psi and resistance
to most common operating environments. Their primary function is to
hold structures together and be capable of resisting high loads without
deformation. Structural adhesives are generally presumed to survive
the life of the application.
Conversely, nonstructural adhesives are not required to support
substantial loads, but they merely hold lightweight materials in place.
Nonstructural adhesives creep under moderate load and are often de-
graded by long term environmental exposures. They are often used for
temporary or short term fastening. Nonstructural adhesives are some-
times referred to as holding adhesives. Certain pressure sensitive
adhesives, hot melt, and water emulsion adhesives are examples of
nonstructural adhesives because they have moderately low shear
strength, high creep, and poor resistance to temperature and chemi-