Questions and Answers Exchange
Question: We have been using an anaerobic threadlocking adhesive for many years with no problems. Recently, we had a problem with one of our components rusting in use and began using stainless-steel fasteners and other components. Shortly afterward, we noticed the fasteners were not holding the required torque throughout our assembly process. Could the change in metal have affected the threadlocking adhesive? We have been very careful to assure all the components are clean, but the problem persists.
Answer: Anaerobic threadlocking adhesives generally cure by polymerizing in the absence of atmospheric oxygen, certainly an environment found between the threads of an assembled fastener. But the initiation of the reaction is also catalyzed by the presence of transition metal ions in a redox fashion. Transition metals with such activity include cobalt, copper and iron. Unfortunately, stainless steel contains a very high chromium component, which is not a transition metal, and experience has shown stainless steel to retard the cure of traditional anaerobic adhesives and sealants. Your threadlocker supplier should be able to provide you an alternative formulated adhesive or suggest a suitable primer that could be applied to the stainless surface prior to assembly.
Question: We are a small user of a precut epoxy film adhesive. Our problem is the wide variations in mechanical properties of the adhesive. What suggestions can you offer? Answer: Assuming that the adhesive is properly manufactured, the storage, handling and shelf-life are potential problem areas. The adhesive’s tensile strength, tackiness and mechanical flexibility will change upon aging and exposure to high humidity. Moisture absorbed into the product can easily change the mechanical properties of the adhesive. Shipping the adhesive in hot trucks may also present problems. Therefore, tight containers are necessary. In addition, make certain that you have refrigerated storage and that you have inventory-control procedures that guarantee the use of in-date material.
-ROGER J. LOHMAN
Question: Previously, your column mentioned that silicones could provide some high-temperature-stability characteristics. How stable are the silicones to ozone and other weathering problems?
Answer: Silicones have outstanding stability to ozone attack as compared to organic elastomerics. The reason is that these polymers do not have any double-bond unsaturation. Additionally, silicones offer good weathering characteristics because they do not absorb UV radiation. Therefore, the aging cracks and deterioration that may be evident in some other elastomers are generally not a problem. In reference to their thermal properties, some industrial-grade silicone products are able to withstand constant heat of 350ºF to 400ºF for a year, and show only minor changes in their physical properties. In fact, the long-term weathering resistance of silicones is why they often find use in various construction and highway expansion joints, as well as in structural glazing. On a personal note, I have used a silicone sealant on a concrete driveway expansion joint to prevent water from undermining the slab since it was on a slight hill. After 13 years, it is still holding up quite well.
-ROGER J. LOHMAN
Q&A Exchange draws on the collective expertise of the staff of The ChemQuest Group, Inc., a management consulting firm specializing in the adhesives, sealants and coatings industries. Dr. William E. Broxterman serves as president of this specialist consulting firm. Questions for publication should be directed to him at The ChemQuest Group, Inc., 8170 Corporate Park Dr., Suite 317, Cincinnati, OH 45242; 513-469-7555; fax: 513-469-7779.