Sticky Business
By Denise Nathan
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. Another consideration, one that is often overlooked, is the performance of the adhesive.
If the adhesive you choose is not right for the substrate, application and environmental conditions, then your print will end up looking terrible or it simply won’t stick regardless of the quality or features of the top sheet.
Because the adhesive plays an equally important role in determining a graphic’s performance and appearance as the top sheet, it is important to understand adhesives and to be aware of factors that influence adhesion when making purchasing decisions.
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 and difficult to apply to, or the graphic can’t be squeegeed down; or the graphic will be exposed to extreme conditions very soon after application. High tack is also beneficial if the graphic will be exposed to “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 tack and end up forming a very strong bond with the surface once it has cured and for the duration of its life. Generally adhesives take about 24 to 48 hours to cure. 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 object from sliding out of position. The cohesion level of an adhesive also influences the degree of shrinkage. While all PVCs are prone to shrinkage, how much and when can be directly influenced by the adhesive formulation and not purely determined by which manufacturing process was used to make the top sheet, i.e. cast or calendered.
There are many types of adhesives. Within the sign industry the choices are narrower and the most popular are Removable, Permanent and Bubble Free.
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 from brand to brand but some products can be removed for up to 2 years after application. After this time period the adhesive tends to set more as opposed to weaken. It is a misconception that a graphic with a removable adhesive will suddenly fall off once its removable life is over. While removable adhesives tend to have a slightly lower initial tack than some permanent adhesives it is not just this characteristic that makes the adhesive removable but rather the construction of the adhesive. Removable adhesives still have a high enough adhesion to be used for demanding applications such as fleet markings (flat surfaces). Their lower initial tack however, is an important consideration if you use water to apply the graphic. Water is used to make the application of a graphic easier for the applicator and it does this by reducing the initial tack. If the product you are using already has a lower initial tack i.e. is removable, you will end up reducing this further to a point where there is little adhesion and the graphic will start to lift. Therefore removable adhesives should always be applied dry.
Permanent Adhesives are used for applications where the graphic is expected to be as permanent as possible and the removability requirement is secondary, or for applications requiring a higher level of adhesion. Of course permanent adhesives can be removed, easily and cleanly but this will be very dependant on the brand. Some products while being permanent have a highly cohesive adhesive or a specific treatment/structure that when removed will leave minimal glue up to 5 years compared to other some products that leave almost all their adhesive on the substrate resulting in a time consuming clean up.
Bubble Free Adhesives have a special structure with channels that allow air to escape during application. This enables fast and easy application as the applicator can easy eliminate trapped air for a smooth finish. The goal of bubble free adhesives is to save labour time. Unfortunately in some applications, especially vehicle graphics they are being used to make up for skill. While they are much easier to apply then full adhesive systems, the application know how when it comes to stretching the film and post application stabilisation are as much of a requirement, if not more so with bubble free adhesives. Due to the structure of the adhesive there are less contact points for the adhesive to make with the substrate therefore the overall adhesion to a substrate can be slightly weaker. This is an important consideration when applying into very deep recesses and indentations.
There are two types of adhesive formulations- Solvent Based and Water Based.
Shorter term and more economical films tend to be produced with water based adhesives. The reason for this is simply cost. It is becoming increasingly more expensive to product solvent based adhesives. With environmental and safety aspects the production of solvent adhesives require more processes and handling care, which means higher costs. Generally solvent adhesives are superior in performance. It doesn’t mean that water-based adhesives should never be used but they are a reflection of quality. It is very rare to find a high or premium grade film with a water based adhesive for this reason. Two vinyls can have the exact same top sheet but be coated with different adhesives and as a result have different performance aspects.
When choosing the right product for your application it is important to look beyond the top sheet. With adhesives being equally influential on the performance of a graphic, assessing the suitability of the adhesive prior to application is just as critical. Understanding the way adhesives perform and their characteristics will ensure a successful end result and prevent costly reapplications.
Denise Nathan is Marketing Manager - MACtac in Oceania
denise@mactac.co.nz
www.mactac-europe.com
