Page 62 - Handbook of Adhesives and Sealants

30 Chapter One

Yet, one must be certain that these accelerated conditions do not cause
reactions that are normally not experienced in the application.

1.4.8 Production limitations
Slow and critical processing requirements of some adhesives can be a
major disadvantage particularly in high-volume production opera-
tions. Several production concerns must be considered when bonding
operations are first projected. All adhesives require clean surfaces to
obtain the best results. Depending on the type and condition of the
substrate and the bond strength desired, surface preparations ranging
from a simple solvent wipe to chemical etching are necessary. Adhe-
sives should never be applied onto other coatings or over other adhe-
sives unless the characteristics of these materials are accurately
known. The resulting bond strength will be no greater than the ‘‘weak-
est link in the chain’’.
If the adhesive has multiple components, the parts must be carefully
weighed and mixed. The setting operation often requires heat and
pressure. Lengthy set time makes jigs and fixtures necessary for as-
sembly. Rigid process controls are also necessary, because the adhesive
properties are dependent on curing parameters and surface prepara-
tions. The inspection of finished joints for quality control is very dif-
ficult. This also necessitates strict control over the entire bonding pro-
cess to ensure uniform quality. Non-destructive test techniques cannot
quantitatively predict joint strength.

1.4.9 Other limitations
Since the true ‘‘general-purpose’’ adhesive has not yet been developed,
the end-user must allow time to test candidate adhesives and bonding
processes. Everyone involved in the design, selection, testing, and
manufacture of adhesive bonded assemblies should be trained as to
the critical requirements and processes. Adhesives are sometimes
composed of material that may present personnel hazards, including
flammability and dermatitis, in which case the necessary precautions
must be considered. Workers must be trained how to handle these
materials safely.
The following items contribute to a ‘‘hidden cost’’ of using adhesives,
and they also could contribute to serious production difficulties:

The storage life of the adhesive may be unrealistically short; some
adhesives require refrigerated storage.
The adhesive may begin to solidify before the worker is ready.
The cost of surface preparation and primers, if necessary, must be
considered.