Prior to discussing potential solutions to contaminant buildup, we must first understand the fundamentals of adhesion between our process equipment and the “stickies” in our process. A common misconception about glues and gums is that they are chemically bonded to the surface of our equipment. For most process contaminants there is typically no chemical bond between the glue and the surface. Instead, there are a large number of tiny electrostatic attractions called Van Der Waals forces as shown in Figure 1 that act like millions of tiny magnets between the glue molecules and the molecules on the surface of the equipment.
The other mechanism by which adhesives stick to the surface is through mechanical bonds. If our surface is porous and full of holes, the glue molecules can seep into the holes and lock into them as shown in Figure 2 similar to how ice cubes lock into an ice cube tray. However, for either of these two bonding mechanisms to occur, the adhesive must “wet out” and cover the surface of our roller. The greater the surface area that is covered by the contaminant, the more difficult the electrostatic and mechanical bonds are to break, and the stronger the glue will stick to the surface.
How easily the adhesive will “wet out” on the surface is a function of the relative adhesive and cohesive forces intrinsic to the type of contaminant and the type of surface. Cohesive forces are defined as the attraction between molecules of the same type while adhesive forces are the attraction between different molecule types. Cohesive forces will try to collapse the glue or contaminant into the smallest possible surface area while adhesive forces will cause it to flatten out on the surface of the substrate.
To visualize this, think of a drop of water (our “glue”) placed on a car hood. If we wax the car hood, the cohesive forces between the water molecules are stronger than the adhesive forces between the water molecules and the wax molecules, and the water molecules will pull themselves together into a droplet. However, on an unwaxed hood, the paint molecules have a higher adhesion strength to the water molecules than the cohesive strength between water molecules and the water will therefore “wet out” and cover the surface of the paint. This is further illustrated in Figure 3.
So why does the wax behave differently than the paint in terms of preventing the water to “wet out”? The answer is due to an intrinsic property of every material called surface energy, which is measured by the strength of bonding energies at the surface of any material. A simplified explanation of surface energy is that “unhappy” molecules at the surface do not have a full complement of molecules around them to balance out their attractive forces. These surface molecules therefore exhibit increased adhesive affinity to different molecules that touch the surface. Surface energy can be directly measured for every substance as a function of force per unit length.
However, due to intrinsic molecular structure, some materials have less “unhappy” molecules at the surface than others. As can be seen in Figure 4, fluoropolymers like polytetrafluoroethylene (PTFE) have the lowest surface energies of all materials. Therefore, these are the preferred materials to construct heat shrinkable rollers covers, which lowers the surface energy of process rollers and prevents glues, gums, and other “stickies” from wetting out and forming electrostatic and mechanical bonds.