Question: Hot melt adhesives seem almost perfect. They don’t need any mixing. They have no solvents. They do require some heated dispensing equipment, but it seems pretty simple. Why haven’t hot melt adhesives become the dominant type of adhesive in the world?
Answer: Hot melt adhesives are exactly as you state. They are simple, require no mixing and are among the most environmentally acceptable adhesives currently known. However, until recently, hot melt adhesives were thermoplastic materials that would melt and re-melt. This meant that a hot melt adhesive could not be used on an application that might subsequently see temperatures near the adhesive’s melting point. Despite this weakness, hot melt adhesives are among the largest volume adhesive type sold in the world today, and are also among the fastest growing.
Recently, newer hot melt technology has been introduced that allows for subsequent crosslinking. The most notable are moisture-cure polyurethane materials that apply and result in instant fixture, but then subsequently cure by reacting with atmospheric moisture. After a slow start, these new hot melt adhesives are gaining acceptance rapidly. There are also some new plural-component curable hot melt technologies just being introduced that may also expand the market for hot melt adhesives.
Question: We have used thread-locking adhesives for many years and have noticed that sometimes they work very quickly and other times they seem to take forever, or just don’t lock the threads at all. Are we misusing these adhesives? We always have a fear the assembly might not be properly engaged.
Answer: Anaerobic adhesives, which comprise most thread-locking adhesives on the market, were developed in the early 1950s and have quickly become a standard tool for maintenance. They have been proven to be very reliable, but you have noted a potential weakness of these almost magical adhesives.
To understand why you see these inconsistencies, an understanding of the basic chemistry of these systems is required. These adhesives are called anaerobic because they crosslink and cure only in the absence of oxygen. As you know, about 20% of the atmosphere we breathe is composed of oxygen. There is plenty around and more than enough to keep these adhesives liquid in their containers. Ideally, once confined between the threads of a fastener or in a bearing race, for example, these adhesives will quickly cure and lock the assembly together.
However, there is other chemistry that comes into play as well. The cure systems of these acrylic-based adhesives are what polymer chemists classify as free-radical polymerization. As in most such reactions, oxygen inhibition is a factor, but the reactions are also accelerated by the presence of certain metal ions, such as iron or copper. An anaerobic adhesive used on a copper or brass fitting will set very rapidly indeed, but when used on stainless materials, can cure very slowly. The cure on nonmetallic materials can also be very slow or almost negligible. Some suppliers offer primers or specialized formulations for such difficult materials.
Another common problem with anaerobics – in fact with almost all adhesives – is cleanliness. If you try to use these adhesives on very dirty or oily surfaces, you can expect erratic results. -Bob Smith