Page 211 - Handbook of Adhesives and Sealants

182 Chapter Five

datory. Checks should be made to determine if cloths or solvent con-
tainers may have become contaminated.
The surface preparation process can be checked for effectiveness by
the water-break free test. After the final treating step, the substrate
surface is checked for a continuous film of water that should form
when deionized water droplets are placed on the surface. A similar
test for treatment of polymeric fabric is shown in Fig. 5.2. A surface
which is uniformly wet by distilled water will likely also be wet by the
adhesive since the specific surface energy of water is 72 dynes/cm and
most organic adhesives is 30–50 dynes/cm. However, this test tells
little about weak boundary layers or other contaminants that may be
present on the substrate’s surface but still be capable of wetting with
water.
After the adequacy of the surface treatment has been determined,
precautions must be taken to assure that the substrates are kept clean
and dry until bonding. The adhesive or primer should be applied to
the treated surface as quickly as possible.
5.2.4 Quality control of the bonding
process
All parts should be fitted together first without adhesive or sealant to
minimize production problems due to fit. The suitability of fit is either
established by visual inspection or direct measurement with a gauge
or shim. It is desirable that the extremes in mechanical tolerances
also be noted and that test specimens be made with the worst possible
fit to assure that the bonding process will always provide reliable
joints.
The adhesive metering and mixing operation should be monitored
by periodically sampling the mixed adhesive and testing it for adhe-
sive properties. Simple viscosity measurements, flow tests, or visual
inspection of consistency are the best methods of monitoring confor-
mance. A visual inspection can also be made for air entrapment. The
quality-control engineer should be sure that the oldest adhesive is
used first and that the specified shelf life has not been exceeded.
During the actual assembly operation, the cleanliness of the shop
and tools should be verified. The shop atmosphere should be controlled
as closely as possible. Temperature in the range of 65 to 90 F and
relative humidity from 20 to 65% is best for almost all bonding oper-
ations.
The amount of applied adhesive and the final bond-line thickness
must also be monitored because they may have an effect on joint
strength. Curing conditions should be monitored for pressure, heat-up
rate, maximum and minimum temperature during cure, time at the
required temperature, and cool-down rate.