Adhesive coverage

The paster is a nonheated operation. The most common paster adhesive formulation consists of poly(vinyl alcohol)—clay—starch blends (10). A 100% area adhesive coverage is used. The rate of bond strength development of the adhesive is an important commercial concern and rapid bond formation rates are desirable. 

Insufficient or nonuniform adhesive coverage (generally, a minimum thickness of 2 mils is required). 

Of these failure mechanisms, the major ones associated with delamination include contaminated surfaces, inadequate adhesive coverage, stresses, voids, and moisture absorption. Many of these mechanisms are also responsible for other failure modes besides delamination. 

Too much material Correct amount Too little material Figure 6.2 Accept/reject criteria for adhesive coverage. 

Visual inspection of adhesives is performed on both uncured and cured samples. Uncured materials are inspected for uniformity of filler distribution, consistency, and adhesive coverage. Inspection methods are defined in paragraphs 3.4.1 and 3.8.3 of Method 5011.4 of MIL-STD-883F. ... 

Even polished, lapped, metal surfaces have irregularities of about 10 m and such pits can trap air. If no air is trapped in the pits of the metal then one can see that the surface wetting and adhesive coverage would be complete. Any entrapment of air will lead to incomplete adhesive/metal coverage and much will depend upon the contact angle of the adhesive and capillary attraction. The critical angle for perfect contact between adhesive and substrate approaches zero. 

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