January 2007

Question: We want to look at UV adhesives for bonding clear plastic and glass samples but are wary about having high-intensity UV lamps in our plant. Are our concerns valid?

Answer: Fifteen or 20 years ago, some end users became disillusioned with UV systems because of several observed limitations: excessive heat buildup during cure; inability to cure through colored substrates; surface tackiness of the adhesives; and the necessary use of short-wavelength, high-intensity UV lamps for curing. This short-wavelength radiation made it mandatory to have considerable shielding around the equipment, plus skin and eye protection for the operators. Fortunately, several developments over the years have made UV curing much more useful and considerably safer to handle. By using a combination of new monomers, better photoinitiators and new lamp technology, it is now possible to cure these adhesives in seconds using visible light rather than UV. Some of the latest systems even use high-intensity LEDs to cure the adhesives rather than conventional UV-type bulbs. These systems are much safer than the “dinosaur” systems of 20 years ago, they do not generate heat and they cure well through most colored plastics. Although it is still recommended to use eye and skin protection when operating these systems, the possibilities of severe skin or eye damage are minimal when the systems are handled with care.

Question: In an earlier column you mentioned alternatives to fumed silica as thixotropes for epoxy adhesives. Specifications, however, dictate that we continue to use fumed silica even though we find it difficult to handle and it generates huge clouds of irritating dust when poured from its bags. Is there a better way to handle the material?

Answer: There are several ways of handling fumed silica that virtually eliminate dust. Some very large users, such as those making silicone sealants, use completely closed mixing systems, and they can use vacuums to transfer the fumed silicone into tanks. For more common mixing systems using open tanks, fumed silica suppliers offer handling systems that are usually based on using either vacuum or diaphragm pumps to transfer the material directly from large bulk containers, thus avoiding the use of simple paper sacks. Some powder-handling companies also offer specialized systems for fumed silica.