Chapter 3: Adhesive Application

3.1 Surface Preparation

There have been many publications on bonding that discuss the preparation of substrates. They all stress the fact that a clean surface is paramount to achieving a good bond. Laboratory experimentation shows that good primary adhesion may be obtained to rusty metal, however, when a part is subjected to an environment such as water or salt spray, the bond will fail in a short time. There are 3 basic metal preparations used today.

The first is mechanical preparation. Here the metal surface to be bonded is blasted with iron oxide grit that is angular as opposed to round shot. Blasting consists of impinging abrasive particles against the metal surface with an air stream. The duration of the blast, the shape and size of the blasting media, the particle velocity, and the hardness, porosity and other properties of the metal, determine the surface profile. Using grit is preferred over using shot because grit produces a rough, open surface, while shot peens the surface and sometimes causes occlusion with loose particles. The grit size most commonly used is G-40. The air used in blasting should be free of oil and water. Once blasted, the parts need to be degreased.

For many years, chlorinated solvents were used for vapour degreasing. However today, alkaline (hot detergent) baths are common. Some equipment is much like a dishwasher in that the parts to be degreased are washed with hot detergent followed by a hot rinse. The oils are then skimmed off the water...