Making it Stick! – How to select the right adhesive

When selecting the right self-adhesive product for graphic applications, there are number of things to consider. It is important to choose a good top sheet to ensure optimal print results and durability but it is just as critical to choose the right adhesive to match the substrate, application and  environmental conditions.  An adhesive plays a significant role in determining a graphic's performance over time. By having a good understanding of adhesives and factors that influence adhesion, you can ensure your application will be a success.

There are three main aspects of an adhesive to understand: Tack, adhesion and cohesion.

Tack refers to the initial tack or ‘grab’ of the adhesive when it first makes contact with the surface. Some adhesives can be low tack to allow for repositioning when applying the graphic. Others can be high tack for applications where the surface is irregular, difficult to apply to or an immediate bond is required. The initial tack of the adhesive does not necessarily relate to the stick of the adhesive during its performance life. For example, a low tack adhesive may increase over time and end up forming a very strong bond with the surface once it’s cured. As a general rule of thumb, adhesives take 24 to 48 hours to cure. This is why the ‘thumb test’ is really not going to tell you much more than how tacky the adhesive feels. It is certainly not an indicator of how well the product will adhere to your intended substrate, unless of course it’s a thumb!

 Once applied and cured the bond of the adhesive is referred to as adhesion.

A good way to test compatibility and adhesion to a substrate is simply to apply it, wait 48 hours, then peel off. If the film is not sticking well at this point, it hasn't formed a good bond and is unlikely to. While this doesn't account for all things that can compromise adhesion -  like the migration of anti adherent material from the substrate into the film - it still is a valuable test. So many application failures could have been avoided if a simple test had been carried out first.

Cohesion refers to the internal strength of an adhesive. Scientifically it refers to the attraction of the materials within the adhesive that hold the adhesive mass together. The higher the cohesion value of the adhesive the stronger it is.  Adhesives that are used to hold heavy objects in place, such as double-sided industrial tapes, need high cohesion. The cohesiveness of the adhesive will also determine the shrinkage level of the top sheet. All PVC top sheets are prone to shrinkage, some more than others, depending on the manufacturing process and the ingredients used. The adhesive can control the shrinkage to the degree that two identical top sheets can show different shrinkage levels simply based on the adhesive. Often film shrinkage is regarded as a by-product of the manufacturing process and type of film  (monomeric, polymeric, cast or calendered) but it is actually the film and adhesive combination that is the final determinant of shrinkage. These levels are published in the technical data sheets.

There are many types of adhesives but within the graphics industry there are two main types – solvent-based acrylic and water-based acrylic. The difference is the solution in which the acrylic polymers are suspended in (water or solvent). Water based adhesives are generally applied to economy films simply because it is more expensive to produce and work with solvent based adhesives. Most high grade films or films produced for demanding applications have a solvent based adhesive which is a good indicator of the superior performance these types of adhesives can have.  Solvent adhesives tend to be structurally stronger and perform better when exposed to chemicals, moisture, humidity and temperature extremes. As sustainability and ecological considerations become more important in product choices water based adhesives will continue to improve and take a bigger share of the market.

 An adhesive can be removable, semi permanent or permanent. Adhesives are generally classed into these categories, based on their adhesion values in laboratory testing.

Removable adhesives are used for temporary applications where easy removal without damage to the surface is a requisite of the application. The life of a removable adhesive varies between brands and products but some can be removed for up to two years after application. After the stated removability time period the adhesive bond tends to increase as opposed to decrease.  While removable adhesives have a low adhesion , it is not just this characteristic that makes the adhesive removable but also the construction of the adhesive. Cohesion is important here. The adhesive needs to stay together to be able to be removed cleanly and have a higher bond to the top sheet then the substrate in order to go with the film when taken off.

The other expectation with removable adhesive is that can they be removed cleanly with no to minimal adhesive residue. However, this is not always the case and it is an acceptable industry standard to say ones' adhesive is removable if it leaves less than 30% glue residue. Some products perform better than others. Where the product you choose falls on the 0% to 30% spectrum may not matter for small areas but if you are removing large graphics in a cherry picker, or changing out mobile fleet advertising that requires a fast turn around time, you will want to have the least amount of residue possible.

Permanent adhesives are used for applications where the graphic is expected to be as permanent as possible and for long term or demanding applications. Permanent adhesives can be removed but not as easily as removable. Some products, while being permanent, have a highly cohesive adhesive and strong bond to the top sheet. When removed, they can leave minimal to no glue compared with some products that can leave a large amount of their adhesive on the substrate, resulting in a time consuming and costly clean up.

Two significant application considerations when choosing the right adhesive are the nature of the surface the graphic is to be applied to, and the conditions under which this is done. There are so many variances in surfaces and surface treatments that either a good knowledge of the surface prior to application is needed; and if this is not known then testing is always advisable. You will get the most accurate results if the test sample is in the same format as the end application. If the graphic is going to be printed and laminated then apply a printed and laminated sample.  Printing, cutting and laminating all influence an adhesive's performance.

Environmental factors will influence application, in particular temperature. Adhesion relates to the ability of the adhesive to maintain an interaction with the substrate and this is best achieved with maximum adhesive contact to the surface. The fluidity of the adhesive, how it flows across and coats the surface is the foundation for successful adhesion. If the ambient or surface temperature is too cold, then adhesive flow is reduced and consequentially so is adhesion. Manufacturers specify in the technical data sheets what the minimum application temperature is for each product so it is important to observe these guidelines. Using heat guns to increase the flow of the adhesive and speed up the curing process post application are a must for more demanding applications requiring conforming or on textured surfaces. For interior applications look out for hidden influences on temperature such as air conditioning units.

Printing a film also impacts adhesion levels and if printing with solvent inks, nothing is more important than drying the print correctly. Applying a laminate or the film onto a surface before the solvents are fully evaporated can result in solvents remaining trapped and adversely affecting the adhesive.

With so many factors impacting adhesion it is impossible to find a ‘one film suits all’. Even with a film that has been designed for a specific application, the range of substrates, surfaces, paints and environmental conditions will determine the end result. Having a good knowledge and understanding of the type of adhesive you are using, the performance requirements and qualifying your product choice by testing,  will go a long way to ensuring optimal performance of your prints and ultimately your customer’s satisfaction.

Written by Denise Kirby

Denise has over 25 years experience in the sign industry as a supplier of self adhesive products. Denise is highly passionate about the industry and enjoys writing about different applications and opportunities with the hope of inspiring people to explore new creative and functional opportunities with film.