Page 305 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 269

acter of the substrate may also influence the polysiloxane film struc-
ture. 24 Silanes are applied as primers to the surface of the substrate
by wiping, spraying, brushing, or dipping. The film thickness is gen-
erally less than 0.1 mil. The solvent in the silane system is removed
by drying at 122–140 F for 10 min. The advantage of the primer ap-
plication method is that it efficiently utilizes the silane material, and
there are minimal stability problems. The disadvantages are that it
is a two step process, and it is difficult to see the clear silane coating
unless it is pigmented. Therefore, complete surface coverage may be
in doubt.
Tests have shown that silanes arrange themselves in layers with a
high degree of order, influenced to a great extent by the surface of the
substrate. The molecules order themselves virtually perpendicular to
the surface to which they attach, and subsequent layers arrange them-
selves step-like in a head-to-head fashion. A rough surface can
break-up the first ordered layer, preventing the formation of the sec-
ond. The thickness of the silane interphase has important effects on
mechanical performance. Thin, but continuous, layers seem to provide
stronger and more durable adhesive bonds.
Three primary mechanisms have been suggested for enhanced ad-
hesion via silane coupling agents. 25 The classical explanation is that
the functional group on the silane molecule reacts with the adhesive
resin. Another possibility is that the polysiloxane surface layer has an
open porous structure and the liquid adhesive penetrates this and
then hardens to form an interpenetrating interphase region. The third
mechanism applies only to polymeric adherends. It is possible that the
solvent used to dilute and apply the silane adhesion promoter opens
the molecular structure on the substrate surface, allowing the silane
to penetrate and diffuse into the adherend.
The coupling agent interphase may be hard or soft and could affect
mechanical properties of the interphase region. A soft interphase, for
example, can significantly improve fatigue and other properties. Soft
interlayers reduce stress concentrations in the adhesive or in the resin
matrix if the system is a composite. In composites, a rigid interlayer
improves stress transfer of resin to the fiber and improves interfacial
shear strength. Coupling agents generally increase adhesion between
the resin matrix and substrate, thus raising the fracture energy re-
quired to initiate a crack. However, the same bond prevents the resin
matrix from stretching. Because of this reduction in tearing capacity,
the interface is less able to resist crack propagation once a crack is
initiated.
There are a number of silane adhesion promoters available, and
they differ from each other in the degree of their reactivity. Silanes
may be produced with amine, epoxy, mercaptan, and other function-
alities. Some examples are given in Table 7.5.