Adhesives and sealants troubleshooting tips from Dr. Dave Dunn.

Question: We formulate ultraviolet (UV) adhesives using methacrylate monomers. Can you give us some tips on how to maximize the cure speed?

Answer: Curing can be sped up in several ways. Obviously, using high-energy UV lamps is one way. A judicious choice of photoinitiator and perhaps amine synergists is then essential.

While methacrylate monomers have traditionally been used in UV adhesives, the coatings industry has usually used acrylates. Most adhesive formulators chose methacrylates because they produce less brittle adhesives and give higher adhesive strengths, and the monomers are less irritating and toxic than acrylate monomers.

However, many of the newer acrylate monomers and oligomers are much safer to use. They can be mixed with methacrylates and can give dramatic increases in cure speed.

Question: Can we use anaerobic threadlockers for screws used in plastics?

Answer: These materials are not normally recommended for use in plastics substrates, particularly on thermoplastics. Although some plastics are resistant, many plastics are stress-cracked by the monomers used in these adhesives. This phenomenon is made worse the longer the liquid threadlocker is in contact with the plastic before it cures to a solid. Unfortunately, plastics are very inactive surfaces for curing threadlockers, which normally require metals to make them cure quickly. This can sometimes be mitigated by using an activator to cure the adhesive.

Many people use cyanoacrylate adhesives as threadlockers in plastics. An alternative is to use metal-threaded inserts in the plastics.