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, 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.
Figure 1. Silane Hydrolyses and Polymerization
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
Figure 2. Effectiveness of Silanes on Different Substrates
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.
Figure 3. Overview of the Reactivity of Onto Adhesion Promoters
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-surface-energy
Figure 4. Example of Onto Adhesion Promoter Modifying a Polypropylene Substrate.
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).
Figure 5. T-Peel Testing with Loctite 4105 Cyanoacrylate Resin with Onto Adhesive Promoters
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 100°C) 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).
Figure 6. Lap Shear Testing Optitec 5054 Epoxy Resin with Onto Adhesive Promoters
promotion is a powerful technique for increasing the strength of adhesive
bonds. Adhesion-promoting systems should be chosen based on the chemical nature
of the substrate. For example, for high-surface-energy metal/inorganic, choose
silanes, titanates or zirconates; for low-surface-energy organic substrates, an
Onto system is appropriate.
For more information, contact Dr. Jon-Paul Griffiths at +44
(0)1865-854807 or visit www.oxfordsurfaces.com.