Page 37 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 7

TABLE 1.1 Factors Influencing Selection of an Adhesive or Sealant 1
Stress
Tension.............. Forces acting perpendicular to the plane of the adhesive.
Not commonly encountered in bonding thin plastic or metal
sheets, leather, cork compositions, etc.
Shear ............... Forces acting in the plane of the adhesive. Pure shear is
seldom encountered in adhesive assemblies; substantial
tension components are usually found
Impact .............. Minimum force required to cause the adhesive to fail in a
single blow. May be determined in tension or shear.
Measures brittleness
Peel ................. Stripping of a flexible member fastened with adhesive to
another flexible or rigid member. Stress is applied at a line;
test loads are expressed in pounds per inch width.
Commonly used angles of peel in tests are 90 for relatively
stiff and 180 for flexible members
Cleavage............. Forces applied at one end of a rigid bonded assembly which
tend to split the bonded members apart. Can be considered
as ‘‘peel’’ of two rigid members
Fatigue .............. Dynamic—alternate loading in shear or tension-
compression. Static—maximum load sustained for long
periods of time in tension or shear; tests are also used to
determine creep
Chemical Factors
External............. Effect of chemical agents such as water, salt water,
gasoline, by hydraulic fluid, acids, alkalies, etc.
Internal ............. Effect of adherend on adhesive (i.e., exuded plasticizers in
certain plastics and rubber); effect of adhesive on the
adherend (crazing, staining, etc.)
Exposure
Weathering .......... Combined effect of rainfall, sunlight, temperature changes,
type of atmosphere
Light ................ Important only with translucent adherends. Effect of
artificial or natural light, or ultraviolet
Oxidation............ Usually tested by exposure to ozone with the joint either
unstressed or stressed, in which case deterioration is faster
Moisture............. Either adhesive or adherend may be affected by high
humidity or wet conditions. Cyclic testing with alternate
moist and dry conditions can be valuable. May cause
dimensional changes
Salt spray ........... Important only in coastal or marine atmospheres. Possible
corrosion of adherend should also be considered
Temperature
High ................ Normal atmospheric variations may be encountered, or
exceptional conditions. Bond strength may be affected by
reactions in adhesive or adherend; decomposition or
changes in physical properties of adhesive are important