Лако-красочные материалы — производство

There may be a tendency, with time, for a pressure-sensitive adhesive tape to slowly curl away from the surface to which it is applied, usually owing to a prolonged slight stress coupled with stress relaxation within the adhesive. Conventional testing may not indicate this weakness. One way to study this is to use a modified form of the 90° inverted peel test shown in Fig. 8a. The surface under evaluation should be marked into equal units, as for example, six 1 in. (25 mm) divisions. The adhesive tape under evaluation can be applied along this marked area, as for a conventional adhesion test, together with a control product known to adhere satisfactorily. Then in place of the usual 200-g loading, a much lighter load is applied, sufficient to apply a very slight stress. This will be in the area of 10 or 20 g, depending on the adhesive. Any adhesive system that has a tendency to slowly release from a surface will gradually peel away, sometimes within minutes, while a satisfactory product will hold indefinitely.

An alternative method that has provided interesting data is to run a 90° peel test but begin this test at 0.1 in. (2.5 mm)/min, then stop the tensile tester under stress but allow the recorder to continue. It will be necessary to reduce the speed of the recorder, as the test may take minutes. Stress relaxation will be shown by the rate of drop of the peel value obtained when the tester was stopped. A useful value of stress relaxation time is that time taken when the applied stress has been reduced to 40% of its original value.

The degree of initial contact that can be obtained from a given pressure-sensitive adhesive-coated material depends not only on the surface to which it is applied, but also on the roughness of the adhesive surface itself, which may come from its prolonged contact with its own irregular backing, such as a glass cloth or embossed or heavily creped paper or plastic facing or carrier. One simple method to evaluate surface contact is to apply the adhesive-coated product to a piece of plate glass as for a standard peel test, then examine the degree of contact area as seen by the degree of light reflection difference at the adhesive/glass interface, observing from the reverse side of the glass. A further method is to use carbon paper. By applying the adhesive-coated product to the carbon paper with a standard roll-down, then carefully removing it, the degree of ink removed and the pattern seen is a measure of the potential contact. The carbon paper may need reinforcement to prevent tearing on removal of the tape.