In order to gain a good understanding of this issue, we first need to consider the two ways of looking at the meaning of ‘water resistance’. Each relates to a specific aspect and has an impact on the final result of the product:
The ‘whitening’ procedure, which is caused by the incorporation of water molecules into the adhesive film (see Fig. 1 with a visible white color in a display window). This phenomenon represents a visual impairment and is of particular importance in visible applications such as the ‘no label look’ or graphic protection films or lamination films.
Water resistance depends on the bond strength of bonded adhesive tape when exposed to water or moisture. In many technical applications, it does not matter whether an adhesive film discolors. The bond strength between the bonded substrates is the decisive factor.
From the start we can be clear that the nature of the substrate to which the self-adhesive film is applied plays a decisive role. The substrate’s physical properties, its surface roughness, porosity or polarity are significant factors affecting bond strength. When immersed in water it is possible for a polymer film to absorb up 30% of its weight in water. For example, a top-coat applied to a film to make it receptive to inkjet printing readily absorbs humidity and water can easily penetrate the boundary layer between the self-adhesive film and the substrate. Notwithstanding, the challenge of ‘water-resistance’ as it relates to the bond strength of the adhesive is essentially the same, whether the adhesive system is water-based or solvent-based.
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ATP old generation
ATP new generation
Separation force on a polyamide film after ageing in water (24h)
However, coming back to the issue of whitening, it is important first of all to understand the difference between a water-based adhesive system and one which is based on solvents. In both cases the adhesive consists of acrylate polymers or acrylate co-polymers produced with the monomers normally used in pressure-sensitive adhesives, e.g. butyl acrylate or 2-ethyl hexyl acrylate. Accordingly, the two types of adhesive system are not different in this respect. It is argued, therefore, that the difference lies in the additives or, more precisely, the surfactants that are essential for the production of water-based dispersions.
These surfactants are auxiliary substances that help to blend and stabilise two components such as water and oil that are not miscible. However, having investigated this issue over the last two years, we have concluded that the quality of the film and the coating process have a much bigger influence on the ‘whitening’ behaviour of a pressure sensitive film than do surfactants. So at ATP we have leveraged this understanding and worked with our raw material suppliers to develop pressure-sensitive films that can match the performance of solvent-based adhesive systems in terms of water whitening.
Indeed this development of high bond strength water-based adhesive systems with excellent whitening behaviour continues apace. We now have water-based adhesive systems, which even after being immersed in water for several days exhibit no visual impairment or any loss of adhesive force. These systems have been applied to products for the graphics industry such as overlaminates requiring water-resistance, particularly when used for outdoor applications.
The pictures show our adhesive systems after being fully immersed in water for a total of 24 hours. Whilst the two adhesives on the left show both whitening and a loss of adhesion, our new adhesive development on the right exhibits neither whitening nor loss of adhesive force.
New adhesive System (NEW adhesive) in comparance to old adhesive Systems. The Picture Shows a water ageing complete immersed under water after 24hrs.
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