Page 68 - Handbook of Adhesives and Sealants

36 Chapter One

can also be incorrectly applied so that the flow of liquids in contact
with the sealant results in a residual buildup of extraneous matter at
the joint.

1.5.4 Chemical effects
Sealants can also have a chemical effect on the substrate. Chemical
incompatibility could cause the sealant or substrate to soften, harden,
crack, craze, inhibit cure, or cause other changes. An example of this
would be the use of an acid release sealant (such as a silicone sealant
that releases acidic acid on cure) on a surface like concrete, marble,
or limestone. On these surfaces, an acid/base reaction can cause the
formation of bond breaking salts at the bond-line.
Another example of chemical incompatibility is the bleed of plasti-
cizers or other low molecular weight volatiles through sealants, caus-
ing them to discolor after exposure to sunlight. This happens
frequently when sealants or coatings are applied over asphalt or or-
ganic rubber-based materials that are formulated with low molecular
weight plasticizers.

1.5.5 Production considerations
An important consideration for any sealing operation is the relative
ease of handling and applying the sealant. There are wide ranges of
sealants available with varying degrees of application difficulty. There
are single and two component sealants, primer and primerless sealant
systems, hot melt application systems, preformed sealant tapes and
sealants containing solvents. As with adhesives, the time required for
the sealant to harden from a liquid state into a semi-solid with some
degree of handling strength is very important.

1.6 Basic Mechanisms

1.6.1 General requirements for all
adhesives and sealants
If one looks at the adhesive bonding or sealing ‘‘process’’ as a complete
procedure, encompassing all aspects of material selection, joint design,
production, etc., then the basic requirements are the same no matter
what the application. These universal requirements for successful ap-
plication are:

1. Cleanliness of the substrate surface
2. Wetting of the substrate surface (intimate contact of the adhesive
or sealant on the substrate)
3. Solidification of the adhesive or sealant