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.
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.
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.
Data sheets for PSA tapes contain a range of physical property measurements. Understanding what these tests mean is important in being able to determine if a particular tape product is right for your application. The most common performance measures are peel, tack and shear. While there are many variants on how these tests are performed, the most common methods are described below.
Peel is a measure of adhesion – how well the tape sticks to a specific surface. It is measured by bonding a tape strip to a surface and then recording the force required to pull it off in a controlled manner.
For QC testing purposes, the most common test surface is stainless steel, although other materials are also used and may be seen on data sheets and application specific tests
(such as glass, HDPE, PP, ABS and more). The tape is applied to the surface with a defined amount of pressure and then allowed to remain on the surface for a controlled amount of time (often 20 minutes but tests using 24 hours, or more are also performed).
After that time, the tape is peeled off using a machine capable of measuring the force needed. A controlled speed is used (typically 12 inch/minute) as is a controlled angle (with 180° being the most common, although 90° may be used for some very thick tapes). The peel force is reported in units of force per width, such as ozf/in or N/25mm.
Controlling the speed and dwell time are key to getting consistent results due to the viscoelastic behavior of PSAs.
When the test strip is peeled away from the surface it can behave in one of several ways. The tape can remove cleanly from the surface, which is defined as adhesive failure. Alternatively, the tape can remove leaving some of the adhesive behind on the surface and some still on the tape backing, which is called cohesive failure.
Application with heat during lamination helps with adhesive flow and surface wet out to improve the bond.
Lower temperatures might lead to insufficient wetting of the adhesive on the substrate. Extremely high temperatures might cause the tape to stretch when being applied, which could create additional stress in the final application. The recommended application temperature range is 50°F – 100°F.
Be sure the application surface is clean, dry and free of grease, dirt or other contaminants. Clean the surface with a lint-free cloth and any suitable cleaner (water, solvents, etc.) and allow it to thoroughly dry. Suitable solvents for use include toluene, alcohols, esters, and ketones. Always test the substrate to be sure it is able to withstand the cleaner prior to application. When using a solvent, follow the manufacturer’s recommendations and precautions.
Pressure sensitive tapes need surface contact pressure to achieve a strong bond between the tape and substrate. In some cases, lower pressure can be used to achieve 100% contact. Conversely, excessive pressure can result in tape compression and/or stretch.
A general guide would be, light pressure of a few psi. Use more pressure for a rough surface versus a smooth surface. Bond strength is dependent upon a number of factors including specific adhesion, the amount of pressure, surface contact, and residence time.
Pressure sensitive tapes must be properly stored to maintain bonding characteristics. Extreme high and low temperatures can lead to degradation of the adhesive or backing materials. Store rolls in the original packaging in a controlled environment (72°F / 22° C and 50% RH) out of direct of sunlight.
Only remove the liner when ready to make the final lamination. Do not allow adhesive to remain exposed to air for extended periods of time as dust, moisture, dry heat and other ambient conditions can reduce its effectiveness.
Do not put excessive pressure on the bond line until the part is properly placed. Apply adequate pressure to the entire bond line to ensure proper adhesion.
There are some substrates that are generally easier to bond to than others. These are the materials with higher surface energy. It is important to know that coatings on the surfaces or fillers in the substrates might influence adhesion characteristics. A painted aluminum or plasticized PVC (polyvinyl chloride) might display completely different bonding characteristics as compared to pure, unaltered materials.
Consider using a primer or adhesion promoter to alter the surface characteristics of substrates to enhance bonding. Consult the product’s usage recommendations to be sure that adhesive systems and primers are cooperative in nature. Corona or flame treatments can also be applied.