Adhesion and Silicone Adhesives
The term “adhesion” refers to the state in which two substrate surfaces are held together by interfacial
forces, or bonds 1 . Adhesion can come about in different ways and last for varying amounts of time
depending on such factors as substrate surface preparation, adhesive used, and compatibility of the
substrates being held together by the adhesive. In some cases, failure of the adhesive bond results in
catastrophic device failure; in other circumstances, a strong adhesive bond is not desirable. Generally, the
stronger the bond between substrate and adhesive, the longer adhesion will last. Tacky gels and pressure-
sensitive adhesives have been used for a variety of applications in which temporary bonds are needed, such
as in transdermal patches and processing aids 2 .
Most silicone adhesives are made of silica-filled silicone polymers that cure in place by crosslinking. The
overall mechanical strength is known as cohesive strength. The strongest bonds are those which occur
between atoms from chemical bonding; however, adhesion is not limited to chemical bonding. Conversely,
materials can have the ability to form a strong “bond” when brought into contact with another surface for a
short time (seconds) under light pressure. This short-term, pressure-induced attachment is known as tack 2 .
Tack and adhesion are influenced by how a force (and how much of it) is applied to the bond, as well as the
bond thickness (thinner can be better) 1 .
Basic Adhesion Mechanisms
Many factors affecting adhesion can be manipulated according to an adhesive’s intended use, but the
following compilation entails fundamental concepts essential to how silicone adhesives function from a
chemical and mechanical perspective:
• Chemisorption - Covalent bonding
– Bond formed between adhesive and substrate when their molecules share electrons
– Strongest type of bonding
• Adsorption - Hydrogen bonding, Van der Waals, polar interactions
– Bonding between hydrogen and unpaired electrons
– Not as strong as covalent
• Mechanical interlocking of pieces
– Dependent on strength of materials, not chemistry at the surface
– “Lock and Key”
– Occurs when materials are soluble in each other
– Occurs with polymers and alloys
– Similar to mechanical interlocking on molecular level
As regards evaluating an adhesive, failure modes can be categorized in two different ways: adhesive failure
and cohesive failure. Adhesive failure occurs when the bond fails between adhesive and substrate.
Cohesive failure occurs when the bond fails through the adhesive/silicone, leaving a coating of silicone on
both of the substrates (test panels). This means the adhesive bond is stronger than the silicone. (For
information on adhesion test methods, see Appendix.)
Figure 1: Adhesive and cohesive failure.
Adhesive performance is largely dependant upon the substrate to which it is applied 1 . How receptive a
substrate is to an adhesive is generally related to wettability, which refers broadly to substrate acceptance of
a coating. Poor wetting is indicated by beading, which means the coating is not flowing, or “wetting out” 3
Beading is illustrated in Figure 2 below.
Figure 2: Comparison of poor surface wettability and good surface wettability.
Poor wetting will result in poor adhesion, and good wetting may result in good adhesion. If a substrate has
low surface energy, or more of a resistance to an adhesive, one should take steps to make the surface more
reactive before applying the adhesive. The following factors 4 influence how “welcoming” a substrate will
be to an adhesive:
• Rough surface creates more area for chemical and mechanical bonding
• It is more difficult to bond to polished and smooth surfaces.
• Eliminates surface contaminants (cutting oils, etc.)
• Removes moisture
• Molecular compatibility with substrate—wettability
• Chemical reactivity with adhesive
Methods of Improving Adhesion
Choosing an adhesive should be based on its molecular compatibility with the substrate to which it is being
applied. Improving adhesion to a substrate can be accomplished by modifying the surface of the substrate
to increase the reactivity and compatibility of the substrate, and/or by adding adhesion promoters to the
silicone formulation itself. Each of these methods has trade offs but can dramatically increase the bond
strength if engineered well. Surface preparation of the substrate is often crucial. Methods of surface
modification include the following 3 :
• Applied to surface as a preparatory step
• Ideally forms monolayer on substrate
• Modifies substrate surface for better wetting of silicone
• Covalent bonds across joint when cure complete
2.) Flame/plasma/corona treatment
• Cleaves bonds to expose reactive groups
• Surface is more reactive with adhesive.
Silane primers are used to promote adhesion between two non-bonding surfaces. Prior to applying the
adhesive, a monolayer of primer can be applied to a substrate as surface preparation for the adhesive. The
silane in primer is a coupling agent containing two reactive groups, one which reacts to the substrate and
another which reacts to the adhesive. By increasing the covalent bonds between the adhesive and substrate,
the amount of force needed to break adhesion is increased and higher lap shear values are achieved.
Primers also greatly increase the silicone adhesive’s ability to wet out the substrates. (See “Choosing a
Silicone Primer Adhesive System” and “Application and Storage Recommendations for NuSil's Primers”
for more information on primers.) Figures 3 and 4 depict the process of interaction between silanols in the
primer and the substrate to which the primer is being applied. Figure 3 is the first step of the primer
reaction, during which the silane undergoes hydrolysis into the more reactive silane.
R - S i( O R ') 3 + 3 H
R - S i( O H ) 3 + 3 R - S i( O R ') 3 + 3 H
R - S i( O H ) 3 + 3 H O R '
Figure 3: Step 1. In the ideal silane primer reaction, the silane is hydrolyzed. (NuSil Recommends 30
minutes minimum at 30-70% RH).
Figure 4 is the second step, during which the reactive silanols react onto alcohol functional molecules that
will form covalent bonds with the primer and the substrate.
Figure 4: Step 2. After silane is hydrolyzed, silanols react with the substrate.
Figure 5: Lap shear results using R31-2186 adhesive on a primed (with SP-270), an unprimed, and
treated and primed (with SP-270) substrates.
Applying flame, plasma or corona to substrates before applying a silicone adhesive, makes the substrate
surface more reactive to the silicone 5 . These treatments may accomplish the following:
Oxidize surface which increases reactivity to adhesive and/or primer
Increase surface energy
Hydrophobic => Hydrophilic
Flame and corona treatments each generate radical oxygen atoms that oxidize the substrate’s surface. From
gases such as oxygen, plasma treatment creates high energy molecules that form oxygen-containing
reactive groups on the substrate, initiating reaction with the adhesive. The best method of treatment
depends on the substrate composition, topography and surface area. Because substrate response generated
by flame, plasma and corona decreases over time, it is recommended to apply the adhesive within one week
of treatment for the best performance.