In recent years, the increasing desire to reduce toxic pollutants in work environments has prompted adhesives manufacturers to seek elimination of volatile solvents from adhesive compositions. Moreover, elimination of volatile solvents is desirable to reduce energy consumption, since considerable heat energy is required to evaporate volatile solvents when curing the composition.
Radiation curable adhesive compositions have been developed which contain no volatile solvents, but rather contain substituents which react upon exposure to radiation to become incorporated into the cured adhesive.
It is important that the cured adhesive composition has properties which are tailored to its intended end use. The pressure sensitive adhesives area is particularly demanding in terms of the properties required for commercial utility. In pressure sensitive adhesives applications (e.g. pressure sensitive adhesive tapes) it is frequently required that the adhesive composition have a good balance of peel strength and shear strength. Peel strength is a measure of the adhesivity of the composition, that is, how strongly it bonds to other materials. Shear strength is a measure of cohesivity of the composition, that is, its internal strength and resistance to plastic deformation.
A pressure sensitive adhesive may have high peel strength but poor shear strength, in which case it will tend to creep under shear loading. Another problem may exist wherein a pressure sensitive adhesive has a high degree of internal cohesion but is too hard and inflexible and not sufficiently adherent to be useful in many pressure sensitive adhesive applications. In such a case, a pressure sensitive adhesive tape may undergo an erratic type of failure in which the adhesive tape, upon being pulled away from the applied surface, releases in a series of jerks, rather than peeling away from the surface in a smooth, continuous motion. This type of failure is commonly called "zip" failure. Another problem associated with a poor balance of adhesivity and cohesivity is "adhesive transfer," which is the tendency of an adhesive on an adhesive tape to transfer from its backing material to the applied surface when the tape is peeled away from the applied surface.
While a pressure sensitive adhesive having high peel strength alone or high sheer strength alone may have utility in certain specific end uses, a pressure sensitive adhesive having a good balance of peel strength and shear strength generally has a much broader scope of utility.
Obtaining a desirable balance of properties in radiation curable pressure sensitive adhesive compositions has been a problem in the prior art. It has been found that some inherently tacky polymers with a low degree of cohesion can be crosslinked by exposure to actinic radiation, thus improving cohesion. However, we have observed that it is difficult or impossible to obtain an adhesive having a good balance of properties in this manner. Moreover, the tacky polymer usually has a viscosity such that it cannot be applied as a thin film at room temperature without the addition of solvents.
In our copending application, Ser. No. 673,801, we described a radiation curable adhesive composition comprising, as the major substituent, poly(vinyl alkyl ether) having admixed therewith from 3 to 40 weight percent of a multifunctional acrylate compound, and, optionally, up to 25 weight percent of a monofunctional acrylate compound and a photoinitiator. When cured by exposure to radiation, these compositions exhibited an excellent balance of adhesive properties. It was found that the multifunctional acrylate, which is highly reactive in the presence of actinic radiation and a photoinitiator, reacted to impart crosslink density, and thereby good cohesion, to the radiation cured composition. This increased cohesion was achieved without a concomitant loss of adhesion such as had been observed upon irradiation of poly(vinyl alkyl ether) alone in a conventional solvent medium.
In addition to the above mentioned properties desired in a pressure sensitive, it is usually also desirable that the adhesive exhibit a reasonable degree of "quick stick," that is, the property of adhering on contact without the external application of pressure.
Yet another desirable property in a pressure sensitive adhesive is that the adhesive, in the uncured state, have a viscosity such that it can easily be applied by conventional coating techniques.
While the radiation curable compositions of our previously mentioned copending application exhibit a good balance of pressure sensitive adhesive properties when cured, it is always desirable to develop new radiation curable pressure sensitive adhesive compositions having a good balance of adhesive properties.