Page 302 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 267

as finishes. Certain finishes have been specially developed to match a
particular fiber with a resin matrix. Without adhesion promoters, the
interfacial resin-to-glass fiber adhesion is relatively weak, and water
can diffuse along the interface with catastrophic results to the end-
properties of the composite.

7.3.1 Silane adhesion promoters
Silanes are the most common commercial adhesion promoter. They are
commonly used to enhance adhesion between polymeric and inorganic
materials. 21,22 They usually have the form X Si-R, where X is typically
3
a chlorine or alkoxy group and R is the organofunctionality. The or-
ganofunctional portion bonds with the resin in the adhesive or the
organic medium, and the silane portion bonds to the inorganic or sub-
strate surface. Silane coupling agents are commonly used between the
adhesive and the adherend, between resin matrix and reinforcing fi-
bers in composites, and between resin matrix and mineral fillers in
plastic compounds. The resulting interface provides:

A chemical bridge between the surface and organic polymer or be-
tween organic polymers
A barrier to prevent moisture penetration to the interface
Transfer of stress from the resin to the substrate or inorganic filler
component thereby improving joint strength or bulk properties
Effective dispersion of fillers and reduction in the apparent viscosity
of the system

These chemicals are usually applied to fibrous reinforcements or to
the substrate surface as an aqueous solution. The solutions usually
are very dilute, only 0.01 to 2% by weight of silane to keep the highly
reactive hydrolyzed molecules from reacting with one another. The
bond strength enhancement increases with silane concentration up to
a maximum of about 2%, and then the enhancement falls-off with
additional concentration. Silane coupling agents react with water in
aqueous solutions to form hydrolyzed silanes, which react with the
surface of the inorganic substrate. The bound silanes then polymerize,
building up layers outward from the substrate with the organic func-
tionality oriented toward the adhesive. This process is shown in Fig.
7.3.
Silanes form strongly adsorbed polysiloxane films on ceramic and
metal surfaces. The chemical and mechanical integrity of these films
are highly dependent on application parameters such as solution con-
centration, solution pH, and drying time and temperature. The char-