Page 234 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 203

example, low viscosity adhesives are more likely to penetrate a porous
oxide layer than high viscosity adhesives.
When working with metal adherends, one must recognize that the
nature of the surface can be significantly different for the same type
of metal. The surface characteristics will depend on how the metal
was processed and heat treated. They may also be determined by the
secondary finishing processes (machining, forming, etc.) and how the
material is handled and stored. The surface characteristics will also
depend on the type and conditions of any prebond surface preparation
process. For example with certain surface prebond treatments for alu-
minum, impurities in the rinsing water or etch solution and variations
in the treating temperature can critically change the nature of the
resulting surface.
The kind and degree of surface treatment required for optimal ad-
hesion will depend on many factors. It is sometimes desired, although
not always practical or wise, to have the pure, bulk adherend material
exposed directly to the adhesive, with no intervening layer of oxide
film, anodizing coating, or contaminant. However, with some sub-
strates, such as aluminum, the oxide layer is thin, dense, and strong
and will retard diffusion and further oxide growth. Certain oxide lay-
ers actually protect the aluminum surface from corrosion and trans-
formation during aging in service. Therefore, it may not be necessary
to remove a clean aluminum oxide layer. However, other oxide layers,
such as those formed on copper alloy surfaces, are cohesively weak
and should be removed before application of the adhesive or sealant.
Depending on the nature of the application and the substrate, at times
it may be best to completely remove the original (and unknown) oxide
layer that was delivered with the substrate, and ‘‘manufacture’’ a
known, protective oxide layer before bonding. Various chemical con-
versions and other prebond treatments have been developed to per-
form this task.

6.2.2 Polymeric surfaces
The situation with organic substrates, such as plastics or elastomers,
is even more complex than with metals. These materials have lower
surface energies and lower tensile strength than metals, and most
importantly, polymeric surfaces are more dynamic and likely to change
than metals. Thus, there is a greater probability of variation in the
surface. As shown in Fig. 6.2b, polymeric surfaces have the potential
for low molecular weight fragments, oxidation products, plasticizers,
processing aids, lubricants and slip aids, adsorbed water, and organic
contaminants along with various other surprises for the end-user.
These could all be present in the surface region. They will affect the