Low energy polymeric surfaces, such as polyethylene and fluorocarbons,
are notoriously difficult to bond with adhesives. However, their bonding mechanisms are well
established and represented by important theories of adhesion. In this editorial, we look into
why these substrates are difficult to join and offer insight into possible solutions.
The adsorption theory
of adhesion states that adhesion
results from molecular contact between two materials and the resulting surface
forces that develop. The process of establishing intimate contact between an
adhesive and the adherend is known as wetting. After contact is achieved
between adhesive and adherend through wetting, it is believed that adhesion
results primarily through forces of molecular attraction such as van der Walls
Figure 1 shows good and poor
of a liquid spreading over a surface.
Contact angle of an uncured epoxy adhesive on four surfaces of varying
critical surface tension
Good wetting occurs if the adhesive spreads out over the substrate in a uniform film and in doing so
makes a high degree contact angle between the substrate and the adhesives (e.g. epoxy adhesive on
metal substrate). Poor wetting occurs when the adhesive forms droplets on the surface making a low
degree contact angle (e.g., epoxy adhesive on fluoroethylene propylene substrate).
For an adhesive to wet a surface, the adhesive should
have a lower surface tension, , than the solids surface
energy (or critical surface tension), c.
Table 1 lists surface tensions of common adherends and adhesive liquids.
Critical Surface Tensions for Common Solids and Surface Tensions for Common
Thus, one of the rules relevant to bonding low energy
substrates is that:
For good wetting and resulting strong adhesion forces:
adhesive << c
For poor wetting and low adhesion forces:
adhesive >> c
A simple view of this relationship is provided in Figure1
where the contact angle of a drop of epoxy adhesive on a variety of surfaces is shown.
Some important concepts develop out of the concept
that for good wetting to occur adhesive
<< c substrate. You
would expect from this relationship that epoxies and similar adhesives would
bond very well to metal,
and other high-energy surface. But these same adhesives would bond poorly to
and low energy surfaces. This is fairly evident in every day situations.
However, you would also expect that from this relationship that
polyethylene and fluorocarbon polymers, if used as adhesives, would provide excellent adhesion to a
variety of surfaces including low surface energy polymers and metals. In fact, they do provide excellent
adhesion. However, commercial polyethylene generally has many low molecular weight constituents that
create a weak boundary layer, thus preventing practical adhesion, and fluorocarbons cannot be easily
melted or put into solution. Thus, fluorocarbons are difficult to get into a fluid state to wet the
surface and solidify without significant internal stresses. However, polyethylene does make an
excellent base for hot melt adhesive once the weak low molecular weight constituents are removed.
Researches are attempting to develop epoxy resins with fluorinated chains that can easily wet most
It is also important to note that with a low energy substrate and an
adhesive with relatively high surface tension, surface roughening as a pretreatment before bonding
generally does not always improve the resulting bond strength and, in fact, usually degrades bond
strength. This is because surface roughening does not normally change the surface energy, but the
many grooves and valleys that it creates on the substrate surface will not fill with adhesive before
cure due to lack of wetting and air remains entrapped between the substrate and the adhesive.
This reduces the effective bond area and creates stress risers at the interface. Thus, the best
surface treatment for low energy substrates is to actually raise the surface energy through chemical
or physical processes such as etching, plasma treating, flame treating, etc.
It is also easy to see why silicone and fluorocarbon coatings provide
good mold release surfaces. Most resins will not easily wet these surfaces. It is also easy to see
why mineral oil and other oils provide weak boundary layers. These contaminants will spread readily
on any substrate because of their low surface tension and most adhesives would not wet a surface
contaminated by these oils.
It is also interesting to note that by making a coating (or adhesive)
more likely to wet a substrate by lowering its surface tension you may inadvertently make it more
difficult for subsequent coatings or adhesives to bond to this material once it is cured. Graffiti
resistant paints work in this manner.
Should you have any comments or feedback,
please contact me.