Many different factors can have an effect on how well a tape will bond to a surface. Understanding these factors and how to control them can help ensure that the tape will perform as expected in your application. In particular, it is important to consider surface energy, surface contaminants, roughness, overall shape, temperature and time. All of these play a role in how well and how quickly the tape will bond.
The surface energy of a substrate (and an adhesive) will determine how easily the adhesive will wet out on the surface and how quickly an optimal bond will form. The term wet out refers to the flow of the adhesive across the substrate, forming close contact and therefore a strong bond. For a tape to wet out a substrate quickly, the surface energy of the adhesive must be lower than the surface energy of the substrate. This is common when bonding to metals and many engineered plastics, and so good bonds form readily with many different adhesives.
When bonding to low surface energy materials (such as polyethylene, polypropylene and many powder coated metals) the adhesive could have a higher surface energy than the substrate and so not wet out rapidly. Rubber adhesives are generally lower surface energy than acrylics, and so will often bond more quickly. Also, additional treatments can be applied to the substrate (such as corona or plasma treatment) in order to temporarily raise its surface energy and therefore promote good bonding.
Contaminants – Cleanliness
When a tape bonds to a surface, it forms an intimate contact between the adhesive and the surface. If there are contaminants on the bonding surface (such as dust, oil, rust etc.) the adhesive will contact those materials, and not the surface underneath.
In this case, the amount of adhesive contacting the substrate is reduced, as so, the strength of the adhesive bond is also reduced, and the tape may fail in the application.
Having a clean, dry surface is important to forming a good bond.
Sometimes the surface that the tape will stick to is not smooth – for example, wood, foams, textiles, or a plastic surface with an embossed texture. In these cases, the adhesive contact will initially be strongest only at the high points of the surface. Over time, or with pressure, the adhesive will flow and start to fill out the texture of the surface. Of course, it becomes important to use a thick enough tape to allow the adhesive to fill the depth of the texture. Thinner tapes may not be able to fill all of the surface voids, and may struggle to provide a strong enough bond.
Some adhesives will flow more than others, and more easily fill voids on the surface of the substrate. If the texture is extreme, then it may become necessary to choose a very aggressive adhesive in order to maximize the bond strength in the areas where contact occurs.
When bonding to a surface that is not flat, some additional factors can affect bonding. The curved, or angled surface will induce forces on the edges or bend points in the tape, which can result in a debonding peel force and cause the tape to pull away from the surface. These forces are indicated by the arrows in the diagrams below. More flexible tapes will obviously conform better to the surface.
In these cases, it becomes important to ensure that the tape selected has a strong enough adhesion to overcome these forces. This may require selection of a quite different tape than would be used to bond to the same material on a flat surface.
Time & Temperature
The way that a PSA tape performs is related to both time and temperature. A tape that bonds very well at room temperature, may not perform as well if you try to form a bond at much lower temperatures. That’s why our data sheets provide a minimum application temperature. The adhesive will become “glassy” as it gets cold and appear less tacky. The tack will recover though as the tape warms up again.
Time is also a very important factor to consider when using a tape. The adhesive on a tape needs time to flow and wet out the surface fully. As such, the bond strength will increase over time on most tapes, reaching its maximum strength a day or more after the bond is made. This is important to consider for applications which need to bear a load soon after bonding – a faster wetting, more aggressive product may be needed. At higher temperatures, wet out will be faster and bond strength will increase more quickly. This process will be slower at lower temperatures.
All of these points demonstrate the importance of testing a tapes performance in your exact application – which can differ substantially from the “ideal” situation represented on a product data sheet. At CCT we are happy to help translate your product needs into the ideal tape for your application.