O-Chem in Real Life: Epoxy Glues

As mentioned in other "Real Life" pages, we don't talk much about polymers in this class, but the chemistry of epoxides gives us an excuse to do so again. Most glues are polymers of one form or another, epoxy glues are one of the strongest of the commercial glues.

Shown below is a mechanistic view of how the glue curing process works. You will all be aware that epoxy glues are contained in two tubes, which when mixed cure to form the glue. In one of the tubes is a structure that contains at least two epoxide functional groups. In the other is the "hardener", which often contains multiple amine functional groups.

Amines are moderate nuclophiles/Lewis bases that can undergo a standard Lewis acid/base with an epoxide, as shown above. Note that as expected, the amine attacks the least substituted carbon of the epoxide, the epoxide is not protonated. After proton transfer, the product of this reaction has a new N-C bond and an alcohol functional group (highlighted in green). Another amine can attack the other epoxide, and then one of the "attached" amines can attack another molecule that contains an epoxide, resulting on the formation of a POLYMER. The resulting polymer can be very hard, and the -OH groups help it to adhere to other structures via hydrogen-bonding.

This is a pretty good introduction of how epoxy glues work, but there are many more details. The exact structure of the molecule that contains the epoxide groups can vary, and especially the multifunctional hardener molecule can be very different from that shown and may contain different nucleophilic groups instead of or in addition to amines, depending upon the required properties of the glue. how quickly it sets, how waterproof it needs to be etc.