Question: We are looking to formulate some acrylic adhesives, including ultraviolet (UV) adhesives. Should we be looking at acrylate or methacrylate monomers?
Answer: Historically, most adhesive and coatings formulators chose methacrylates over acrylates. Methacrylate polymers tend to have higher glass-transition (Tg) temperatures and less brittleness, and exhibit better weathering than their acrylate analogs. In addition, acrylates were perceived to have a poorer toxicity profile, particularly from a skin irritation or allergy point of view.
A key feature of acrylate monomers is their very fast curing. Chemically speaking, this is because the secondary free radical in acrylates is more reactive than the tertiary radical in methacrylates. In recent years, acrylates and acrylate oligomers have become very prevalent, particularly for fast-curing UV systems. Modern monomers have low skin irritation and are non-allergenic. I would encourage you to look at both types of monomers and mixtures of the two to optimize properties in your adhesives.
Question: How do we increase the thermal resistance of anaerobic adhesives?
Answer: You should look at using multifunctional monomers to get more crosslinking, as well as those with an aromatic backbone. A favorite monomer has been ethoxylated bisphenol dimethacrylate.
In addition, there are additives that can impart heat resistance. Check the patent literature; for example, there are expired patents that teach the use of materials like bismaleimides, which only crosslink at high temperatures. Also note that two methods are used to evaluate thermal resistance, namely hot strength (i.e., the strength when tested at high temperatures) and the long-term heat resistance of bonds aged at temperature for a period of time but tested at ambient temperature. ASI
Dr. Dave is a former vice president and director of Loctite Corp. and has spent many years in troubleshooting adhesive and sealant problems in the adhesives, sealants, specialty rubbers and plastics fields. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; phone (440) 477-5164; email DrDave242@windstream.net; or visit www.fldenterprises.com.
Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.