How does a pressure-sensitive adhesive affect initial tack?

The effect of pressure-sensitive adhesives on initial tack.

　 　I. Intrinsic Viscosity

　　In addition to the critical surface tension, the bulk viscosity of a pressure-sensitive adhesive is another important factor determining its initial adhesion measured by the touch-test method. The effect of bulk viscosity on initial adhesion via the touch-test method is complex: as the bulk viscosity decreases, the effective contact area increases, leading to a rise in initial adhesion values; however, a decrease in bulk viscosity also reduces the stress required for the pressure-sensitive adhesive to deform, thereby causing the initial adhesion values to decline. Experimental evidence shows that significant initial adhesion can only be achieved when the bulk viscosity of the pressure-sensitive adhesive falls within a specific range—between 100,000 and 10,000,000 Pa·s.

　　II. Basic Mechanical Properties

　　Only when the dynamic storage modulus of a pressure-sensitive adhesive is on the order of 100,000 Pa can it achieve good initial adhesion. Further research indicates that the initial adhesion measured by the touch-test method at room temperature for a series of experimental acrylic pressure-sensitive adhesives exhibits an inverse proportional relationship with the logarithm of their dynamic storage modulus at 22°C. Thus, within a certain range, the smaller the dynamic storage modulus of a pressure-sensitive adhesive, the greater its initial adhesion measured by the touch-test method.

　　III. Glass Transition Temperature

　　The glass transition temperature of pressure-sensitive adhesives is positively proportional to the logarithm of their dynamic storage modulus. In other words, the lower the glass transition temperature of a pressure-sensitive adhesive, the smaller its dynamic storage modulus will be. Within a certain range, the lower the glass transition temperature of a pressure-sensitive adhesive, the better its initial tack will be. In fact, the relationship between the initial tack performance of pressure-sensitive adhesives and their glass transition temperatures is far more complex than this simple correlation suggests—lower glass transition temperatures do not necessarily lead to better initial tack performance.