Page 542 - Handbook of Adhesives and Sealants

Sealant Classification and Composition 467

TABLE 12.8 Fillers Commonly Used in Sealant Formulations 2,4
Filler Benefit
Calcium carbonate Viscosity adjustment
Silica Common for silicone resins
Fumed silica Thixotropic agent
Titanium dioxide Used whenever white or light shades
are required
Carbon black Used when darker shades are required
Iron oxides Colorants in certain sealants
Aluminum and other metal stearates, Thixotropic agents
titanium esters
Glass and polymeric fibers Thixotropic agents

can be up to 90% in sealant putties and in oil and resin based caulks.
Fillers in silicone sealants, however, may constitute only 10–20% of
the total volume.

12.3.4 Plasticizers
Plasticizers are used to reduce the elastic modulus of some sealants.
A general approach is to use a high performance base polymer that
gives high modulus and high recovery and then incorporate a plasti-
cizer to reduce the modulus and increase flexibility. Plasticizers are
also used in chemically cured liquid polymers to assure softness and
the ability to relax stress.
The best results are achieved by using relatively nonvolatile plas-
ticizers. Volatile plasticizers are less expensive, but the beneficial
properties that they provide will dissipate with time. Common con-
struction sealant specifications will specify a maximum weight loss
(10–12%) after heat aging samples for 2 weeks at 158 F. This will
assure that the plasticizers in the sealant will not evaporate during
normal service aging. Satisfactory plasticizers include the less volatile
phthalates, adipates, sebacates, phosphates, and other ester types.
Most sealants, except for silicones, use plasticizers in their formula-
tions. Silicone sealants can be plasticized only by low molecular weight
silicone oils. 2

12.3.5 Additives to improve adhesion
Often the base polymer in the sealant formulation will provide ade-
quate adhesion to many substrates. However when necessary, adhe-
sion properties can be improved by using additives such as coupling
agents, primers, or a combination of the two. Phenol-formaldehyde