Question: I make both thermoplastic and thermoset plastic parts, and I want to start using structural adhesives to bond pieces prior to supplying my customers. Should I be aware of any potential problems?

Answer: There has been a tremendous increase in recent years in the use of plastics and composites to replace metals in component design. Since plastics do not always lend themselves to traditional fastening methods, the demand for adhesive bonding of these materials has increased.

From the adhesives technologist’s perspective, plastics are something of an anomaly: on one hand, bonded joints can be made where the plastic falls before the adhesive; on the other hand, certain plastics are very difficult to bond. Structural adhesives for plastics typically include epoxies, cyanoacrylates, polyurethanes or reactive acrylics. Epoxies often work but are sometimes too rigid and brittle. Rubber-toughened systems have improved this problem to a certain extent, and many successful applications have been reported in bonding thermoset plastics and repairing fiberglass panels.

Two-component polyurethanes are extremely versatile in plastics bonding and are used widely for polyester sheet-molding compound (SMC) bonding, where their good adhesion and high flexibility are key properties. Cyanoacrylates provide excellent adhesion and are probably the most versatile adhesives for plastics. Limitations include gap filling to only 0.5 mm, temperature resistance to about 100˚C and poor impact resistance. Reactive acrylic adhesives possess excellent plastic-bonding abilities due in part to their solvency for many plastics. However, in some cases, this can cause stress-cracking of thermoplastics. The major limitation of this class of adhesive is limited gap-filling to about 1 mm, not to mention strong odor and the flammability of some formulations. Two-part mixable versions of these adhesives have greater gap-filling ability, which leads to even greater capabilities for structural bonding and repair of plastics and composites. Photo-initiated acrylics that can be cured in 1-30 seconds using UV or visible light are also available. These are used successfully on transparent substrates like PVC and polycarbonate.

My initial recommendation would be to try cyanoacrylates, if they are cost-effective for you, and also the reactive acrylics.