Page 245 - Handbook of Adhesives and Sealants

214 Chapter Six

On many nonporous surfaces a useful and quick method for testing
the effectiveness of the surface preparation is the ‘‘water-break test.’’
If distilled water beads when sprayed on the surface and does not wet
the substrate, the surface-preparation steps should be repeated. A
break in the water film may signal a soiled or contaminated area. If
the water wets the surface in a uniform film, an effective surface op-
eration may be assumed. However, the water-break test only provides
a rough approximation of the surface condition. For quantitative in-
terpretation of the cleanliness of a substrate, one could also measure
contact angle directly on the treated substrate with a drop of reference
liquid. The reference liquid could be distilled water or a liquid having
a surface tension similar to the adhesive or sealant that will be used.
Another test to determine cleanliness of the substrate involves wip-
ing a clean white towel across the prepared surface to see if any gross
contaminants are present. This technique is often used to check flat
surfaces and surfaces that are not easily cleaned because of the part
geometry. Similarly, a small strip of common office tape could be ap-
plied to the surface, peeled off and then examined on a white back-
ground for evidence of contamination. Of course, any residue left by
the tape will then need to be cleaned from the substrate. Certain forms
of contamination, notably oils, can also be more easily seen under ul-
traviolet (UV) light. An ultraviolet detection process has been sug-
gested which requires soiling the substrate with a fluorescent oil, nor-
mal cleaning, and then inspecting the surface under ultraviolet light. 13
The degree of cleanliness can be quantified through photoelectron
emission or reflectance measurements. The higher the reflectance, the
cleaner the surface is. Other advanced analytical methods for detect-
ing surface contamination are described in Chapter 5.
The objective of treating an adherend prior to bonding is to obtain
a joint where the weakest link is the adhesive layer and not the in-
terface. Thus, destructively tested joints should be examined for the
mode of failure. If failure is in the cohesive-mode (within the adhesive
layer or adherend), the surface treatment may be considered to be
optimum for that particular combination of adherend, adhesive, and
testing conditions. If an adhesion-mode of failure appears to be at the
interface, one may assume that additional surface optimization is nec-
essary if higher bond strengths are required. It must also be realized
that specimens may exhibit cohesive failure initially and then inter-
facial failure after aging for a period of time. In these cases, a devel-
oping weak boundary layer can be considered a possible culprit. Both
adhesive and surface preparations need to be tested with respect to
initial bond strength and permanence in the intended service environ-
ment.