Лако-красочные материалы — производство

Because of the unique properties of pressure-sensitive adhesives, special tests not applicable to other types have been developed. While standard physical tests such as nonvolatile content, viscosity, and specific gravity are performed to ensure consistency of application, these tests do not predict adhesive performance. For pressure-sensitive adhesives, three critical performance characteristics are usually measured: tack, peel, and shear strength.

A. Tack

The classic test for tack of a pressure-sensitive adhesive film is the rolling ball tack test. Here a ball is rolled down an inclined plane onto a film of the adhesive. The length the ball travels across the film before stopping is a measure of the tack of the film. This test gives a good indication of tack with elastomer adhesives but is unreliable with water-based systems.

A more universal test is the probe test, in which the end of a cylinder of standard diameter is brought lightly into contact with the film for a very short time and the force required to separate it from the surface is measured. Similar in principle is the loop tack test, in which a loop of coated film is lowered onto a steel plate, making contact under its own weight, and the force required to withdraw the plate is then measured. All of these tests are markedly affected by the cleanliness of the ball, probe, or plate. Figure 4 illustrates the loop tack test.

1 I Adhesive

B. Peel

Peel strength is usually tested by laminating a coated film either to itself or to a specified substrate. The film is then peeled off the substrate at a steady speed at 90 or 180 degrees to the bond axis, and the force required for removal is measured. The result is always quoted as the force per unit width of film at a given rate of peel. Figure 5 shows the geometry of a 90- or 180-degree peel test.

C. Shear Strength

This test is a measure of the ability of a pressure-sensitive adhesive to withstand creep. A standard area of coated film is bonded to a steel plate and a weight suspended from it. The assembly is placed in an oven. In some shear tests the time for the assembly to delaminate at a fixed temperature is recorded, while in other tests the temperature at which failure occurs when the oven temperature is increased at a certain rate is the shear value quoted.