Page 174 - Handbook of Adhesives and Sealants

144 Chapter Four

all too many cases, the substrate is considered to be a constant
throughout the life of a production part. In practice, there are a num-
ber of common reasons for substrates to change.
Metallic substrates change primarily with regard to their surfaces.
Different oxide layers can be formed that are depending on the chem-
istry of the surface and the way it was treated during fabrication of
the part. The degree of surface cleaning or surface roughness may also
change throughout a production run due to differences in processing
chemicals, or procedures, or to contamination of the materials that are
used.
For organic substrates, the possible types of change are numerous
and potentially more significant. Polymeric substrates may change in
modulus due to aging, loss of plasticizer, or continued chemical reac-
tion. They may change because of slight formulation variations that
take place from lot to lot. These changes may not be sufficient to cause
a change in the bulk physical properties. Therefore, the change may
go unnoticed to the quality control department because they are look-
ing at only bulk properties and not at subtle formulation variations
that could affect adhesion properties. Such substrate modifications
could result in drastic changes in the surface properties of the mate-
rial.
A case in point is an elastomeric substrate, such as compounded
nitrile rubber. Generally, the user will specify the rubber part by com-
pressive strength, extension, hardness, or other standard bulk prop-
erty tests. The supplier can formulate virtually thousands of formu-
lations that will meet these specified properties. In doing so he may
use an incompatible plasticizer, or a low molecular weight extender,
or a new filler that is more hydroscopic. These small changes could
have a very large effect on the resulting joint strength. Therefore, any
change in substrate formulation or processing must be re-verified with
regard to its adhesion characteristics.
Test methods used to determine the uniformity of substrates are
numerous and vary with the type of material. They are generally the
same tests used to characterize the material or to determine its fun-
damental physical properties. Tests that are commonly employed are
hardness, tensile strength, modulus, and surface characteristics such
as roughness or contact angle measured with a standard liquid. Often
a test similar to the non-volatile test mentioned above is used to de-
termine if there are any compounds in the substrate that are capable
of outgasing on exposure to elevated temperatures. Internal moisture
content of certain polymers, such as nylon and polycarbonate, is also
known to affect adhesion.