Adhesives for Low Energy Surfaces

SpecialChem | Edward Petrie - Nov 11, 2002

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 forces.

Figure 1 shows good and poor wetting of a liquid spreading over a surface.

epoxy adhesive

Figure 1:
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 solids surface energy (or critical surface tension), c. Table 1 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

Table 1:
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:

For good wetting and resulting strong adhesion forces: adhesive << c substrate

For poor wetting and low adhesion forces: adhesive >> c substrate

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, glasses, and other high-energy surface. But these same adhesives would bond poorly to polyethylene, fluorocarbon, 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 surfaces.

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.

Edward Petrie.


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