Does your adhesive stick?

When choosing the right self-adhesive product for graphic applications, many aspects need to be considered. Typically, the focus tends to be on the material’s top sheet, aspects such as colour, print receptivity, durability, and conformability. Products are categorised by how the top sheet is made such as, cast, calendered polymeric or calendered monomeric. These categories heavily influence decisions around whether a film is right for the job. If you are doing a vehicle wrap for example, you would likely choose a cast, or for a short- term window decal, a monomeric calendered film.

While these are important indicators of performance, another that is often overlooked, is the adhesive. It’s almost assumed that if the top sheet is the right choice, it must have the right adhesive but just as top sheets need to be assessed based on the performance requirement of the application, so does the adhesive. Denise Kirby tells us more in this, the first in a two-part series.

The adhesive is a critical component of any self -adhesive application. If it is not right for the substrate and the application or environmental conditions, your graphic or print will end up looking terrible or it simply won’t stick, regardless of the quality or features of the top sheet.

Therefore, it is important to understand adhesives and to be aware of factors that influence adhesion when making purchasing decisions. Different types of adhesives can also benefit the installation process and make the application easier or faster. When time is money this important factor cannot be overlooked.

Firstly, there are three main facets 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 or difficult to apply to. High tack is also beneficial for keeping a graphic in place if it’s going to be exposed to extreme environmental conditions or “stress” before the adhesive has time to cure and realise its full adhesive power.

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 over time increase in strength and end up forming a very strong bond with the surface once it has cured. Cure time is generally 24 to 48 hours after application. The cure time can be accelerated by post heating with a heat gun, which is essential for vehicle wraps or other applications where the material has been stretched. After this an adhesive’s “stickability” is referred to 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. Therefore, a good way to test compatibility and “stick” of an adhesive to a surface you are unsure of is to apply a sample piece, wait 48 hours, then peel off. If the adhesion is poor at this stage, then the application will not be a success.

Cohesion refers to the internal strength of an adhesive, that is how well an adhesive bonds together with itself. The higher the cohesion value of the adhesive the stronger its internal structure. Adhesives that are used to hold heavy objects in place, such as industrial double-sided mounting films, need to be highly cohesive to stop the bonding objects from sliding out of position. The cohesion level of an adhesive also influences the degree of shrinkage. The nature of PVC films mean they are prone to shrink. The degree of shrinkage can be directly influenced by the adhesive formulation and not purely determined by which manufacturing process was used to make the top sheet, or calendered.

There are many types of adhesives. Within the sign and print industry, the most popular formulations are acrylic adhesives. They can be either solvent based acrylic or water- based acrylic.

Solvent Based vs
Water Based Adhesives

Short term and more economical films tend to be produced with water-based adhesives. The reason for this is mainly cost. With environmental and safety aspects becoming more of a consideration, the production of solvent adhesives requires more processes and handling care. Manufacturing with solvent adhesives is hazardous so production is highly regulated to ensure the safety of workers and to protect the environment.  This means higher production costs.

Solvent adhesives are ideal for durable outdoor applications and for applications where graphics will be subjected to temperature fluctuations and high moisture. They also have good chemical resistance. This is why long term and vehicle wrapping films are generally coated with a solvent-based adhesive rather than water based. It doesn’t mean that water-based adhesives should never be used. Their advantage beyond cost is that they are more environmentally friendly and for promotional graphics and indoor decorative applications, they are perfectly suitable. The main thing to remember is that water- based adhesives are affected by water so do not choose them for areas where the edges of the graphic may be exposed to excessive water over a long period of time and never use the wet method during application.

Removable adhesive vs permanent

Acrylic adhesives can be removable  or permanent.

Removable Adhesives are used for temporary applications where clean, adhesive free removal without damage to the surface is a requisite of the application. The life of a removable adhesive will vary between brands. It is a misconception that a graphic with a removable adhesive will suddenly fall off once its removable life is over. Generally, the adhesion increases over time and becomes more permanent. While removable adhesives have a lower tack and adhesion value compared to a permanent adhesive, it is not just this characteristic that makes the adhesive removable but rather the construction of the adhesive and the bond of the adhesive to the top sheet.  The accepted industry definition of removability is an adhesive that leaves less than 30% of its glue on removal. If you have covered a large surface area this is still a lot to clean off. Unless you find this process cathartic, choose a quality adhesive as some brands offer removable adhesives that leave little to no residue.

Due to their lower initial tack, avoid using water to apply removable adhesives. Water is used to make the application of a graphic easier by reducing the initial tack. Removable already has a lower tack. By adding water, you can reduce the tack to a point where the graphic will not adhere at all.

Permanent Adhesives are used for applications where the graphic is expected to be as permanent as possible. Permanent adhesives can still be removed but how easily and cleanly is brand dependent. Some permanent adhesives are highly cohesive adhesive or a specific treatment/structure that when removed will leave minimal glue compared to other products. This can be a huge advantage when it comes to removal, especially for applications like vehicle wraps.

Choosing the right product for your application requires so much more than choosing the right top sheet. By understanding adhesives, you can be confident in your choices and ensure a successful end result.

This article first appeared in the August issue of Digital Image Magazine. In the next issue we look at different kinds of adhesive structures and things that can affect adhesion.

Written by Denise Kirby

Denise Kirby has over 27 years experience as a supplier of self adhesive products to the sign and print industry. Initially starting out in the family business as a distributor of  consumables she went on to work for leading self adhesive manufacturers in marketing , business and product development roles across Australia and New Zealand.  She now has her own business, Kirbyco, which focuses on environmentally friendly, recyclable and sustainable print solutions.

Denise is highly passionate about the industry and enjoys writing about applications and opportunities in print in and signage with the goal of inspiring people to explore new creative and functional opportunities with film as well as educating the industry on products, trends and new innovations.