Changing Surface Properties for Better Adhesion

If it is challenging to adhere to a material due to its low surface energy, it is very critical to choose the correct adhesive to bond that material. There are various ways to increase the surface energy of a material, improving the ability to bond. Here are several different ways to modify surface energy.

Priming

The ability of a liquid to chemically interact at the surface can be improved with primers or adhesion promoters. Primers work by adding a monolayer or thin coating of a functional material to the surface. Primers are chemically designed so that one end of the molecules bond very well to low-surface-energy materials while the free end bonds very well to adhesives.
Plasma Treating

Plasma treatments expose the surface of a material to an ionized gas which changes the chemical reactivity of the surface atoms. There are several different varieties of plasma treating, and there many different companies specialising in plasma-treatment services.
Flame Treating

Flame treatment takes advantage of the combustion chemistry of a “flame” to chemically alter the surface atoms of a substrate. As with plasma treatments, there are a variety of companies specialising in flame treatment for different materials.

Surface Cleanliness

Although we just spent a great deal of time understanding the intricacies of surface energy of materials, in reality, a material to be bonded will arrive with a surface that has been contaminated. This can include dust, oil, grease, fingerprints or any other debris capable of winding up on a surface.

Contamination can result in:
- Reduced contact between adhesive and substrate, preventing a bond
- De-tackified adhesive, preventing effective wet out
- Reduced bond strength

Effective Wet Out

Clean surface allows for maximum wet out and surface contact potential
Ineffective Wet Out

A contaminated surface inhibits surface contact and wet out potential

It is very often necessary to thoroughly clean material prior to bonding to guarantee the adhesive is making proper contact. There are a few methods to do this:
- Use a mixture of 50/50 isopropyl alcohol (IPA)/water. This is generally adequate for plastics which don’t require oily coatings to protect their integrity.
- Clean with harsher solvents, including acetone or methyl-ethyl ketone (MEK). Many metals arrive with an oily coating that is present to prevent oxidation (rust) or as a result of processing (e.g. lubricants).
- Degrease with an industrial degreaser, followed by solvent cleaning. This is often reserved for metals that are not only coated with processing oil, but also very dirty.

Abrading – When Cleaning isn’t enough

There are times when cleaning alone is not capable of removing the contaminants on the surface of a material. Injection-moulded plastics may contain traces of mould release agents that are designed to resist many solvents. Oxide layers of metals (e.g. rusted iron) are “bonded” to the surface and cannot be removed by solvent cleaning alone. Some industrial oils are resistant to solvents and chemicals with the goal of keeping a metal surface safe from harm during transit.

Surface with no preparation
Surface after 1st cleaning

Surface Preparation Process Using Abrading

To prepare a surface adequately for bonding, it may be necessary to add an abrasion step to the process.

1. Clean first – a first round of cleaning can remove much of the contamination present and will prevent debris from smearing during abrasion.

2. Abrade – abrasion with standard abrasive paper or non-woven abrasive pads can remove the contaminated surface and provide roughness.

3. Clean – final cleaning removes any debris left over from abrasion and provides the new, clean surface for bonding.

Surface after abrasion
Surface after 2nd cleaning

The Science of Adhesion Educational Series

The Science of Adhesion Educational Series is designed to be a comprehensive introduction to Adhesion Science and the use of tapes and adhesives in design applications. Visit the Science of Adhesion page to view all articles or choose from the selected topics below.

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