- August 01, 2003, David J. Bentley Jr., RBS Technologies
An intriguing aspect of pressure-sensitive adhesives is the wide variety available. They come in different forms for application as solutions in solvent, as dispersions or emulsions in water, and as 100% solids materials such as hot melt adhesives. They can use various polymers such as rubbers, acrylates, etc., to give them different performance characteristics. They might have high tack or low tack. They can provide high cohesive strength with low adhesive strength or low cohesive strength with high adhesive strength.
Another interesting characteristic is they can be permanent or removable. This is perhaps a feature of pressure-sensitive adhesives that not only contributes to their versatility but also complicates their use. A permanent pressure-sensitive adhesive destroys the substrate with attempts to remove it. A typical application is a label indicating the selling price for an item. A removable pressure-sensitive adhesive removes cleanly from the substrate. An example is a name tag applied at a party for identification.
Most people in the converting industry know suppliers offer both types of products. The manufacturer selects different ingredients to make the adhesive aggressive or non-aggressive.
Another factor that is less appreciated but equally important in the manufacture of a product using a pressure-sensitive adhesive is the substrate being used.
A permanent pressure-sensitive adhesive relies on poor internal strength of the substrate to which it is applied. As expected, a removable pressure-sensitive adhesive requires good internal strength of the substrate.
Hidden between the two previous statements is the fact that a product sold as a permanent pressure-sensitive actually could be a removable product depending on the substrate being used and vice versa.
Consider an example: A converter purchases a product labeled as a permanent pressure-sensitive adhesive and coats it onto papers for use as labels. When the labels are adhered to something, the aggressive nature of the adhesive is stronger than the internal strength of the paper. Attempts at removal therefore result in tearing of the paper. By definition, this is a permanent pressure-sensitive adhesive.
Suppose the same pressure-sensitive adhesive was coated onto a rather thick polyester film and made into labels. After application of these labels onto something, a person probably could remove them because of the high internal strength of the polyester film. The so-called permanent adhesive now is removable.
Likewise, a removable adhesive on high-internal-strength paper will deliver a product more removable than the same adhesive applied on a low-internal-strength paper. In other words, relying on the manufacturer's definition that pressure-sensitive adhesives are removable or permanent can be somewhat misleading. No pressure-sensitive adhesive truly is removable or permanent in its own right. Rather, a finished product using a pressure-sensitive adhesive applied onto a substrate can be removable or permanent.
The difference is the degree of permanence or ability for removal depend on two factors: the aggressiveness of the adhesive and the internal strength of the substrate to which it is applied. The adhesive manufacturer controls the first factor, but the converter making the label or other pressure-sensitive product controls the second.
Knowing both factors are important components of a finished pressure-sensitive product is necessary to ensure any product labeled permanent or removable will perform as advertised.
David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at firstname.lastname@example.org.