Ask Dr. Dave
QUESTION: I have many questions about the basic chemistry of anaerobic adhesives, but the most important one is this: if these adhesives are really anaerobic, why don't they cure during their manufacture when a vacuum is pulled on them?
ANSWER: The term "anaerobic" was originally used back in the 1950s to signify that the adhesives would cure spontaneously when air was excluded. The original technology was based on generating peroxy compounds in acrylic monomers by bubbling air or oxygen through them. Unfortunately, air had to be bubbled through them continuously to maintain stability. These products took a leap forward from two major events: first, by the discovery that deliberately added hydroperoxides and stabilizers could give stable systems; and second, the development of the low-density polyethylene bottle, which had a very high oxygen permeability. With hindsight, we now know that these systems are not truly "anaerobic" systems but are free-radical redox systems where the hydroperoxide reacts with transition metals on the substrates - typically iron or copper - to generate free radicals, which then polymerise the monomers. It is possible to pull a vacuum on these systems, such as to remove air bubbles from high-viscosity products without affecting their stability. In this case, the highly activated resins are kept cool to minimize any possible pre-curing, and air is bubbled through them to maintain stability between vacuum cycles.
QUESTION: I would like to use a hot melt for bonding steel components together and require very high adhesive strength and good high-temperature resistance. What do you recommend for best performance and cost effectiveness?
ANSWER: First, a good ethylene vinyl acetate (EVA)-based hot melt would be the most cost effective and might work if formulated with a resin containing carboxylic acid functionality to enhance adhesion to the metal. Choice of appropriate resins and additives can also give good heat resistance. Secondly, try a hot-melt polyurethane. These adhesives fixture rapidly, bond well to metals and slowly crosslink with moisture from the atmosphere to give good high-temperature performance. You might also consider using a hot-melt polyamide. The downside to these systems is that the adhesive application temperatures tend to run very high (200
Q&A Exchange is written by Dr. Dave Dunn of F.L.D.Enterprises, a technical consultancy and full-service industrial market research firm specializing in the adhesives, sealants, specialty rubbers, and plastics fields. Dr. Dunn is a former Vice President and Director of Loctite Corp., and has spent many years troubleshooting adhesive and sealant problems. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; (330) 562-2930; fax (865) 251-9687; e-mail DrDave242@att.net ; or visit http://www.fldenterprises.com .