Expert: Dr. Jeffery Raymond - 11/7/2011

Question
So when you use gorilla glue, moisture has to be applied so that it foams and "fills cracks", thus making it a more effective adhesive. I was wondering why this is so? I found that it is a "polyurethane glue" but what is the specific chemical reaction that is occurring?  Thank you!

Answer
The atmospheric or liquid moisture promotes the decarboxylation (CO2) component of the urethane polymerization.  Initially the glue has long-ish chains that give it some viscoscity thicker than water or oil.  The exposure to moisture allows the ends of these chains to come together forming a network that gives the glue structure and makes it a solid.  The process of this "cross-linking" that occurs generates one molecule of carbon dioxide gas for each cross-link formed.  The structural/engineering function of wanting the foaming action is three fold: 1- it generates a mobile polymer which can get into microdefects in the surfaces, forming actual mechanical entanglement. 2- The air pockets function as "shock absorbers" that allow the pure polyurethane (which is innately a bit brittle) to have partially flexible regions that prevent the hard, but brittle, polymer from cracking.  3.  The air pockets do not seriously detract from the mechanical properties but keep the material light and less dense and means you need less actual glue to bind/fill a given volume.  The same reaction that makes the ends of the initial chains to link together is also what occurs between the glue network and the surfaces that it binds to.

I hope this helps.

Take care.