Page 541 - Handbook of Adhesives and Sealants

466 Chapter Twelve

12.3.2 Solvents
Solvents are needed in large amounts to help liquefy the base resin
so that the other additives may be easily incorporated into the for-
mulation and to lower the viscosity of the system to make it easier to
apply. In small amounts, solvents are used to improve the extrudabil-
ity of the sealant.
Usually, the solvent type and concentration can be spotted quickly
from the manufacturers’ data sheet, the information on the sealant
container, or the information on the Material Safety Data Sheet
(MSDS). Total solvent concentration is usually quantitatively repre-
sented as 1.00 solids content in weight percentage.
Normal solvent contents for typical sealants are shown in Table
12.7. It is possible to use too much solvent in a formulation. This may
result in excessive shrinkage when the sealant cures. Volume shrink-
age will always be greater than the weight percent of solvent because
the density of the solvent is much lower than the other components
in the sealant. Generally, for every 1% weight loss there is about 2%
volume shrinkage.
The common solvents used in sealant formulations are toluene, xy-
lene, petroleum spirits, water, and others. Solvent mixtures are care-
fully chosen so that rapid evaporation of one component does not cre-
ate a problem. Care must be used in selecting the solvent mixture so
that the sealant’s skin does not dry so quickly that it prevents the
escape of solvent from the body of the sealant. The solvent system is
also an important determinant for the viscosity and application char-
acteristic of the sealant. Often the solvent in which the resin is the
most soluble is also the most volatile, and application problems are
common with solvents having excessive volatility.

12.3.3 Fillers
Fillers are generally used in sealant formulations as additives to in-
crease the viscosity of the sealant and provide better gap filling prop-
erties and to lower the cost of the formulation. For sealants, fillers do
not give much reinforcement or added strength. However, fillers can
significantly affect other performance factors such as water resistance
and hardness.
The most common filler is calcium carbonate, because it is readily
available and comes in various sizes. Other common fillers include
various clays, silica, colorants (such as titanium dioxide), zinc sulfide,
zinc oxide, carbon black, and various iron oxides. Table 12.8 provides
examples of the fillers that are commonly used in sealant formulations
and the beneficial effects that are derived from each.
Filler content can vary significantly depending on the base polymer
and the requirements of the formulation. The volume percent of fillers