Adhesives for Low Energy Surfaces

TAGS:  Epoxy Adhesives    

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.

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 vander Walls forces.

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 a metal substrate). Poor wetting occurs when the adhesive forms droplets on the surface making a low degree contact angle (e.g., the epoxy adhesive on fluoroethylene propylene substrate).

Let’s understand good and poor wetting of a liquid spreading over a surface through the figure below:

The figure above shows the contact angle of a drop of epoxy adhesive on a variety of surfaces.

Key to Good Adhesion - Wetting

For an adhesive to wet a surface it should have a lower surface tension, λ than the solid’s surface energy (or critical surface tension), λ_c.

The table below lists surface tensions of common adherends and adhesive liquids:

Solid Materials	Critical Surface Tension, dynes/cm
Acetal	47
Acrylonitrile-butadiene-styrene	35
Cellulose	45
Epoxy	47
Fluoroethylene propylene	16
Polyamide	46
Polycarbonate	46
Polyethylene terephthalate	43
Polyethylene	31
Polymethylmethacrylate	39
Polystyrene	33
Polytetrafluoroethylene	18
Polyvinyl chloride	39
Silicone	24
Aluminum	~ 500
Copper	~ 1000
Liquid Materials	Surface Tension, dynes/cm
Epoxy resin	47
Fluorinated epoxy resin	33
Gylcerol	63
Petroleum lubricating oil	29
Silicone Oils	21
Water	73

Critical Surface Tensions for Common Solids and Surface Tensions for Common Liquids

Thus, one of the rules relevant to bonding low energy substrates is that:



Some important concepts develop out of the concept that for good wetting to occur λ_adhesive < λ_{c substrate}:

1. You would expect from this relationship that epoxies and similar adhesives would bond very well to metal, glasses, and other high-energy surfaces. But these same adhesives would bond poorly to polyethylene, fluorocarbon, and low energy surfaces. This is fairly evident in everyday situations.


2. 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. Researchers are attempting to develop epoxy resins with fluorinated chains that can easily wet most surfaces.

Surface Treatment for Low Energy Substrates

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.

Adhesive Professionals Stay Alert!

Avoid premature adhesion failure when bonding dissimilar materials. Join the course: Bonding Dissimilar Materials: How to Reduce Internal Stress Levels and learn to manage the problem of internal stress and cure shrinkage efficiently.