Why Do Adhesives Stick?
There are several theories to describe why adhesives stick. Usually the bond is achieved through a combination of different mechanisms describe by the theories. A few of the more common theories are highlighted below:
Wettability & Surface Energy
Wettability and surface energy are very important to pressure sensitive adhesive bonding. If an adhesive cannot wet or spread out over a surface, its ability to bond to the surface will be greatly diminished. Surface energy is basically a measure of the attractive forces holding the surface molecules or particles on its surface. The theory states if the surface energy of the liquid (in this case our PSA) is less than the surface energy of the substrate then the liquid or adhesive will tend to wet out or spread over the surface. The adhesive will flow and generate more intimate contact with the surface. If the surface energy of the liquid is higher than the surface energy of the substrate, then the liquid will not spread. The liquid will bead up. Visualize how rain or water beads up on a freshly waxed car. The water has a higher surface energy than the wax so the water beads up. The viscous or liquid properties of pressure sensitive adhesives will work through the same bonding mechanism. If the pressure sensitive adhesive can wet and spread over the surface, it will bond better.
Your more difficult to bond to materials tend to be lower in surface energy. Typical low energy surfaces are polyethylene, polypropylene, and polystyrene. Higher energy surface such as metals, glass, ABS, polycarbonate, Nylon, and rigid PVC are easier to bond with PSAs.
Diffusion and Solubility
Diffusion and solubility are very important aspects in regards to how PSAs bond to various materials. In chemistry it is often referred to as "like likes like". If materials are similar and compatible with each other they will tend to mix and stay together. The solution will look uniform. If the materials you are mixing together don't like each other, they will tend to separate over time. Visualize how oil and water mix together. The oil and water will separate once you stop mixing. With diffusion and the proper solubility, a pressure sensitive adhesive may penetrate and "mix" with the surface you are bonding to depending on how similar the adhesive is to the surface.
Summary
Pressure Sensitive Adhesives (PSAs) are a unique adhesive technology that offers companies convenience, higher productivity, and lower total costs when compared to other bonding methods. PSAs are flexible and can be easily tailored to your specific application or processing issue. Nova Films & Foils Inc. can help you solve your bonding or processing problems. Contact us today, we'll be glad to help.