Polymerist Basics: Epoxy Resins
A continuing education on polymer basics
Welcome to another edition of Polymerist Basics where I attempt to explain a particular polymer without going to in-depth into the chemistry. This is meant to be a 1-pager explanation so brevity is part of the goal, but if you are a professional chemist or someone with deep knowledge in a particular topic and feel that I am missing something essential let me know. If you are just a regular person looking to learn more about chemistry and how it plays a pivotal role in your life then you are at the right place.
If you have ever wanted to do your own polymerizations at home you can with a five minute epoxy resin kit from the hardware store. Epoxy resins differ significantly from the majority of plastics in that their polymerizations typically occur during their deployment in the end application. This means that polymerizations are typically occurring at the site of the application either as an adhesive, composite matrix, or a coating. The next time you mix together the two parts of an epoxy resin, take note of how hot it gets after mixing and then how long it takes to harden or how quickly it becomes “unworkable” after mixing because you have just performed a polymerization.
Epoxy resins derive their ability to polymerize at room temperature from the epoxide ring, which is very unstable under the right conditions, typically in the presence of something that is capable of opening the ring. Chemicals such as thiols and amines are what might be considered to be electron rich with hydrogens that are easily moved and the epoxy ring opening occurs when an electron rich group with labile hydrogens gets close enough to spur a ring opening and a transfer of a hydrogen. Chemical bonds are broken and reformed during epoxy polymerizations and the overall number of free molecules decreases as they are joined together and this is the source of the heat that I mentioned earlier.
When the mixture of the two components becomes difficult to move around it is due to the polymer approaching what a polymer chemist might refer to as a “gel point” or the point at which a three dimensional network has formed. Another way to look at this is that the epoxy resin polymer has reached a molecular weight that approaches infinity as the ocean of molecules that it had started out with previously are now for the most part completely linked together. The three dimensional network is driven primarily by most epoxy resins having a functionality or an ability to react twice while the hardener component typically is able to react at least three times. This high degree of functionality and reactivity is what leads to this class of polymer being able to crosslink. As the epoxy resin fully polymerizes or crosslinks it becomes very resistant to heat, chemicals, and water, which is why they are often used to protect critical infrastructure such as metal bridges, adhere car frames together, or keep the handle of your favorite coffee mug attached to the cup.
For the most part epoxy resins are almost always crosslinked when applied so this means that once the crosslinking has occurred the polymer will never melt or flow if heated and the only way to remove it is either by physically removing it from your substrate or heating it up hot enough to burn it off. The next time you redo your bathroom and you want to utilize an epoxy paint to go over your tile remember that it will not be as easy to remove as it was to apply. One of the main benefits of the epoxy resin and similar systems is that it is a liquid when applied, which means it can very easily conform to the substrate that it is being applied to and once it hardens it becomes very difficult to remove.
The next time you ride or walk over a bridge supported by metal and marvel at how it’s stayed standing and not broken down due to rusting just think about the coatings based on epoxy resins that are working hard to keep water from touching the metal.