Question: What tips can you provide for bonding plastics and composites with adhesives?
Answer: Adhesives are often a better way to assemble plastics than by using mechanical assembly with fasteners. For example, fiberglass composites are routinely bonded with epoxy or reactive acrylic adhesives in the automotive and marine industries because they give high strength and durable bonds. However, caution must be exercised in some cases. The inherent flexibility of plastics can induce peel and cleavage stresses in bonds. In addition, the high thermal expansion coefficients of plastics can also be an issue. Although it is not usually a problem in bonding plastics to plastics because plastics and adhesives often have similar thermal expansion coefficients, it can be an issue when bonding plastics to metals. Also, be aware that some acrylic adhesives can cause stress cracking in thermoplastics—there are warnings from manufacturers not to use anaerobic adhesives on several thermoplastics.
Several low-energy plastics can cause problems with adhesives. For example, polyolefins such as polyethylene and polypropylene or thermoplastic polyolefins (TPOs) must be treated with special primers or with corona or plasma treatment to achieve optimum bond strengths. These treatments clean the surface of mold release agents, for example, and also oxidize the surfaces to produce more polar surfaces, which are much more compatible with polar adhesives. Open-air plasma has become a very common technique in the automotive industry, where the low-polarity polyolefins are used for cost and performance reasons. ASI
Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.