Creating An Optimum Glue Surface.
Giving the glue a helping hand!
Are you using industrial glue? If so, the question ‘what is surface wetting’ may at first sound like a very technical question requiring a very abstract, perhaps technical answer. But for users of our glues, mastic sealants or tapes, it’s understanding is a critical factor in the success of any gluing job. Let me explain further.
For optimum adhesion, an adhesive must thoroughly “wet out” the surface to be bonded. When we say “wet out” we refer to the property of the adhesive to flow and completely cover the gluing surface. In so doing, the glue will maximize it’s contact area with the bonding surface and hopefully lead to a strong, indestructible joint.
For an adhesive, and indeed any liquid to effectively wet out a surface, the surface energy of the adhesive must be as low or lower than the surface energy of the substrate to be bonded.
Many glues have the ability to wet out and bond high surface energy surfaces such as many metal’s and some plastic, but fail to bond to low surface energy surfaces, such as most polyolefin plastics such as polypropylene and polyethylene. Why is this?
Surface wetting can be measured by the contact angle of a bead of liquid on the surface. If we look at water drops upon a leaf, we see that the water drop forms a perfect circle. The angle between the bottom of the drop and the surface of the leaf, is referred to as the contact angle.
By measuring the contact angle of a droplet of water on the surface of a leaf, we can easily determine the degree of surface wetting. The smaller the contact angle indicates a surface that is more wettable and therefore would allow a more effective bond to be achieved.
But in practice, what does this mean? If we apply a film of adhesive onto a surface, and the adhesive film contracts, this is typically a sign that the adhesive is not wetting the surface onto which it has been applied. Likewise, if the adhesive separates into individual drops, again, we have a serious surface wetting problem on our hands.
If this happens, how do we overcome it? How can we increase the basic bondability of a surface? How do we improve the glues wettability and surface flow characteristics?
The most immediate way is to convert the surface. As an example of this, priming the surface with the correct primer, will convert the surface from a low energy surface to a high surface energy surface.
Still again, we have other surface treatments, such as plasma treatment and corona treatment, which will give us a high surface energy surfaces. For any gluing challenge, understanding the wettability of the surface is often the first aspect to consider. This is a very complex subject, however, we have decades of expertise in all areas of adhesive bonding. Count on us.