Page 543 - Handbook of Adhesives and Sealants

468 Chapter Twelve

resins and mercaptan terminated silane coupling agents are probably
the most widely used adhesion additives for sealants.
Integral coupling agents may greatly improve the durability of the
joint by providing additional resistance to the penetration of water
(see Chapter 7). It is assumed that the coupling agent will migrate
through the sealant and preferentially adsorb at the adherend inter-
face. This can only be true if the coupling agent is not already ad-
sorbed at the far greater competing interface that exists between the
base resin and filler materials. Thus, it may be necessary to add the
coupling material as a primer directly to the substrate rather than as
an integral ingredient in the sealant formulation.
Silane coupling agents are used both as additives and as primers
for sealant systems. They are organofunctional monomers that possess
dual reactivity. This characteristic enables groups at one end of the
molecule to react with active hydroxy sites on glass and metals and
even masonry while the other end reacts with the organic base resin.
This chemical bond is more lasting against the water and UV. Mars-
5
den describes the numerous silanes available and provides recom-
mendations for a wide range of applications. Silanes are used in most
sealants including latex caulks. Only a very small amount of silane
additive is necessary to achieve improved adhesion. Usually less than
several percent by weight is sufficient. Commercial sealants, with sil-
ane additives to improve adhesion to a variety of substrates, are some-
times marketed as ‘‘siliconized’’ sealants.
Primers are needed when the standard sealant formulation does not
adhere to a surface. Conventional primers for sealants either are a
monomolecular film or film formers. The monomolecular film is gen-
erally a dilute solution of an appropriate silane at about 5% solids
content in anhydrous solvent such as toluene or xylene. The primer is
wiped on the surface and dries in minutes. The sealant should be
applied within 15 min of primer application; otherwise, the primer will
react with moisture in the air. Silane primers should always be clear
water white. Any milky appearance indicates that the silane has lost
its potency. These primers may be used on steels, various forms of
anodized aluminum, baked finishes, fluorocarbon surfaces, and any
unusual surface in question.
Film forming primers are applied by brush or spray and actually
form a film across the surface, sealing most porosity. These film for-
mers are especially recommended on masonry and other porous sur-
faces. Film forming primers are often combinations of phenolic resins,
epoxy resins, and polyester resins with silanes. They are formulated
with solvent to a final product that is approximately 25% solids con-
tent. These primers generally dry within 15 min.