Ask Dr. Dave / Columns

Ask Dr. Dave

February 2007

QUESTION: Some time ago, you wrote about using organosilanes as adhesion promoters for glass surfaces. Can I use them with my adhesives for bonding metals?

ANSWER: Organosilanes have been used for many years to enhance the durability of adhesive bonds and to improve the interfacial bonding between adhesives and glass fillers. It is important to note that the term “adhesion promoter” is often a misnomer with respect to organosilanes; in many cases, the organosilanes don’t necessarily improve the initial adhesion, but prevent hydrolysis of the adhesive at the adhesive/glass interface. Organosilanes act as bridging molecules between the inorganic glass surface and the organic adhesive.  Most organosilanes consist of a silicon atom with three alkoxy groups and a fourth group (such as vinyl, methacryloxy, or glycidoxy) chosen to be compatible with the adhesive and (preferably) to be able to copolymerize into the adhesive. A range of organosilanes is available these days with different functional groups, and the answer to your question is that the correct choice of silane may help you on metals, particularly in terms of bond durability. But you must be careful because there is some evidence in coatings literature that suggests certain organosilanes actually reduce the hydrolysis resistance of organic coatings on steel. Work closely with your supplier of organosilanes, and thoroughly test your system with and without organosilanes.

QUESTION: How do the new one-component polyurethane adhesives being sold in consumer markets compare with products like superglues and epoxies?

ANSWER: The real value of cyanoacrylates (superglues) in the home is the fact that they are one-component and have the ability to cure in seconds. However, cyanoacrylates have always had limitations in terms of ability to fill gaps and bond certain surfaces. Although manufacturers have produced special versions to overcome most of these limitations, it is still mind-boggling to look at retail shelves and try to decide which adhesive to use. Epoxies are probably the most versatile high-strength and gap-filling adhesives, but they always involve the mixing of two components, though twin-syringes and static mixers have helped to overcome this limitation. Polyurethane adhesives, on the other hand, are single-component adhesives that are sold in a simple bottle and touted as being all-purpose. Although they normally don’t give strong bonds for several hours, they are thick adhesives that will hold surfaces together during curing. Some repair jobs require care in applying the right amount of adhesive and making sure the surfaces are clamped together in some way. I have done several successful repair jobs recently with these adhesives where I needed to fill some large gaps and other adhesives had failed.


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