An adhesion promoter is a bi-functional compound that can chemically
react with both the substrate and the adhesive. Known for increasing an
adhesive’s bond strength, it can be applied in two ways by being mixed
with the adhesive or applied directly to the substrate. Unlike priming
systems,
silane adhesion promoters
are generally applied at thinner film thicknesses. An adhesion
promoter’s effectiveness depends on both the substrate and the adhesive
being used. Surface pretreatments, such as solvent cleaning or
mechanical etching, can be used with adhesion promoters as part of a
pretreatment method.
ADHESION PROMOTERS FOR METAL AND HIGH-SURFACE-ENERGY INORGANIC SUBSTRATES
The most common commercial adhesion promoter is based around
silane coupling agents. Silanes are most often used to promote the adhesion between polymeric systems and inorganic substrates.
Silane promoters typically comprise a tetravalent Si core (which has an
organo functional tail) and some form of hydrolyzable group, such as a
chloro or alkoxy attached. When applied to the substrate surface, the
silane is hydrolyzed to form a silanol, which condenses and polymerizes
with itself to form an extended network. If the silanol is on a
substrate with sufficient oxide functionality, cross coupling can take
place, anchoring the polymerized silanol to the surface (see Figure 1).
The choice of organofunctional tail on the silane is dictated by the
adhesive class that is being used (e.g., for an epoxy adhesive system, a
tail containing an amino or epoxy moiety would be suitable).
The effectiveness of silanes depends on the substrate being used;
smooth, high-surface-energy substrates are better than
low-surface-energy or discontinuous substrates (see Figure 2).
Titanate and zirconate coupling agents are growing in popularity. They
are predominately used to improve filler polymer adhesion in composites.
Both titanates and zirconates react similarly to silane coupling agents
by way of condensation to surface hydroxyl groups; however, unlike
silanes, there is not condensation polymerization to produce a network
at the interface.
ADHESION PROMOTERS FOR ORGANIC AND LOW-SURFACE-ENERGY INORGANIC SUBSTRATES
Low-surface-energy and solvent and chemical inertness all make organic
materials difficult to bond. The lack of “surface chemistry”(such as
hydroxyl) on most organic substrates renders
silane adhesion promoters ineffective.
Recently, Oxford Advanced Surfaces developed Onto®, a novel class of
adhesion promoters for use on organic and low-surfaceenergy inorganic
substrates.
Adhesion promoters conceptually resemble those based around a silane — a
functional tail covalently linked to a reactive head. The reactive head
in the Onto adhesion promoter is based around a latent reactive
intermediate, a class of organic functionality, which, upon application
of an external stimulus, converts from a stable state to a highly
reactive radical intermediate. This radical intermediate is capable of
reacting with C-H, O-H and N-H bonds, as well as C C and C C bonds (see
Figure 3). This range of reactivity allows the adhesion promoter to
react with nearly all organic substrates — from polyolefins to
polyimides, as well as polyesters and inorganic materials such as carbon
black and diamond (see Figure 4).
Onto adhesive promoters are applied as solventborne formulations in MEK
or toluene by way of appropriate coating techniques (such as spray,
dip, spin or roll-to-roll) then cured by activated heat (approximately
100oC) or UV light (254 nm).
These adhesive promoters have been demonstrated on both Melinex-OD
polyester film and Kapton-HN polyimide film, and have been shown to
increase the T-peel and lap sheer forces when used in conjunction with
cyanoacrylate (see Figure 5) or epoxy adhesives (Figure 6)
标签： silane adhesion promoters, Silane coupling agents
