Delayed Action PSAs

An interesting type of pressure sensitive adhesive (PSA) is one that only becomes pressure sensitive when exposed to an exterior element such as heat, moisture, or solvent. These adhesives are beginning to become known by the term "delayed action adhesives" (DAA). The advantage, of course, is that the adhesive is non-blocking until pressure sensitivity is desired. This provides for the possibility of linerless tapes and labels and a number of other innovative products and processes.

While linerless tapes and labels reduce cost by eliminating the need for the liner, they also address environmental concerns in that they also offer savings in the process waste. The technology also gives the supplier more products per roll, thereby improving the productivity of the end-user as well.

The main disadvantage of conventional DAA technology is that DAA adhesives remain pressure sensitive for periods of only a few months to a few years at most after activation. After this time interval, they lose their pressure sensitive character and adhesive failure can be induced.

Solvent and water activated PSAs have been around for some time. Back in the 60s, I vividly recall a problem that I was asked to look into within a consumer product company. The nametags were coated with a solvent activated PSA and these were falling off the client's product in astounding numbers. The source of the problem was that the worker applying the nametags believed that the solvent was an adhesive, and the more that was applied the better the adhesion. Another problem with moisture activated PSAs is that they lose their non-blocking characteristics in high humidity environments. This can be a problem in shipping or storage.

Today, fortunately, DAAs have progressed and heat activated systems have gained commercial interest. These adhesives are not pressure sensitive as they come from the converter. Just prior to final application the adhesive is momentarily heated and becomes pressure sensitive and tacky to the touch. There are several ways to produce such a product.

Tackiness can be achieved because the DAA coating consists of a discrete mixture of at least two different solid plasticizers held to the substrate with a binder vehicle. When the adhesive is heated, the plasticizer forms a eutectic melt, which has a melting point lower than their individual plasticizer melting points. A typical DAA adhesive formulation of this type is shown in Table 1.

* Vinylpyrrolidone / ethyl acrylate ratio is 60 / 40 by weight

A second step to the "linerless" vision of the future may be possible due to pressure sensitive hot melt adhesives based on styrene block copolymers. Although these adhesives lose their pressure sensitive characteristics when cooled from the melt, they represent another attempt at eliminating the release liner.

Table 2 provides a solvent-based DAA type adhesive formulated from styrene butadiene styrene copolymer. The adhesive can be coated on two substrates, dried, and then activated by heating both surfaces to 75?. Bonding is achieved by pressing both substrates together while the adhesive is in the molten state. The adhesive is pressure sensitive at elevated temperature and nonblocking at room temperature. This type of adhesive system could be used in a hot roll laminating process.

Should you have any comments or feedback, please contact me.

Edward M Petrie