Choosing the correct self-adhesive product for graphic applications requires you to consider numerous issues around the job.

Typically, signage and print professionals will focus on the materials top sheet, along with aspects such as colour, print receptivity, durability, and conformability.

However, another consideration often overlooked, involves the performance aspects of the adhesive. With any digital print it is important to choose a good top sheet to ensure optimal print results and performance but if the adhesive you choose is not right for the substrate, application or environmental conditions, then the risk of failure is inevitable.

Because the adhesive plays an equally important role in determining a graphics performance and appearance as the top sheet, it helps to have a decent understanding of factors that influence adhesion when making purchasing decisions.

Start with tack

Firstly, we need to consider 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.

The tack can vary, depending on the job. For example, some adhesives need to have a low tack to allow for repositioning when applying the graphic. Others can have a high tack for applications where an irregular surface presents challenges or looks difficult to apply to.

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 is cured. As a general rule of thumb, adhesives take 24 to 48 hours to cure. After this, we refer to an adhesive’s stick-ability as adhesion.

Adhesion refers to the final adhesion value of the adhesive once it has set. If your graphic has not adhered well to the surface after the 24-48 hour period, then it never will.

A good way to test compatibility, and the stick of an adhesive to a surface you are unsure of, is to apply a sample piece to it, wait 48 hours, then peel off. If you are not happy with the adhesion at this point, then it is not the right choice as it is unlikely to improve.

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. So adhesives that you use to hold heavy objects in place, such as double sided industrial tapes, need to be highly cohesive.

The cohesiveness of the adhesive will also determine the shrinkage level of the top sheet. All PVC top sheets have a tendency to shrinkage and some more than others, depending on the manufacturing process and the ingredients used.

The adhesive can control the shrinkage to the point that two identical top sheets can show different shrinkage levels simply based on the adhesive.

Often, companies make film choices, in regards to shrinkage concerns, focused on the top sheet construction (monomeric, polymeric, cast or calendered). However, it is actually the film and adhesive combination that is the final determinant of shrinkage levels. You can see these levels published in the technical data sheets.

Solvent and water

There are many types of adhesives but, within the graphics industry, we concern ourselves with two main kinds: solvent based acrylic and water based acrylic. The difference between them involves the solution in which the acrylic polymers are suspended in, either water or solvent.

We generally see water-based adhesives 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 of these types of adhesives. Water-based adhesives have their place but solvent adhesives tend to perform better when exposed to solvents, moisture, humidity and temperature extremes.

Environmental concerns

Two significant application considerations when choosing the right adhesive include the nature of the surface the graphic to which you will apply your graphic and the conditions under which you will complete the application.

We know that, with so many variances in surfaces and surface treatments, we really need a thorough knowledge of the surface, prior to application: this is critical.

If you don’t have this knowledge, then you need to consider testing. You will gain greater accuracy 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, laminating, and cutting can affect various aspects of a films performance.

Environmental factors play a crucial role in successful application. Of these factors, temperature plays a significant role.

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.

So, if the ambient or surface temperature is too cold, then adhesive flow is reduced and consequently, so is adhesion.

Manufacturers specify, in their technical data sheets, the minimum application temperature for each product. So we need to observe these guidelines.

Using heat guns increases the flow of the adhesive and speeds up the curing process post application; a must for more demanding applications requiring conforming or on textured surfaces. For interior applications, you need to look out for hidden influences on temperature, for example, air conditioning units.

Printing a film also influences adhesion levels and, when 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 influencing 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 can impact on the result.

Possessing a thorough knowledge and understanding of the type of adhesive you use, and knowing the performance aspects required will go a long way to ensuring optimal performance of your prints and ultimately your customer’s satisfaction with the end result.

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