Can anyone explain, in layman’s terms, the chemical reaction that makes resin harden? Both polyester and epoxy.
clusters of molecules line up and join hands to form a chain. The hand holding is typically a covalescent bond but ionic bonding also happens for example when a monomer flips its oxygen.
In the case of polyester I believe styrene being evaporated has something to do with it as well.
surferdave has put it well, but until the real resin chemists get on here, I’ll try to add a little…
In poly, the catalyst by definition does not participate in the reaction, it just initiates. In the pre catalyzed liquid state, the polymers
are just floating around in the resin and styrene mixture. The catalyst makes them form polymer chains, the longer the better. Over
catalyzing makes them hurry too much, resulting in shorter chains (weaker). UV initiation makes them all jump into chains in a quick
but very orderly fashion. Once the chains are locked together you’ve gone from a liquid to a solid.
Epoxy is a little different in that the hardener is not a catalyst, it does participate in the reaction. That’s why it’s so important to mix epoxy
thoroughly and correctly. The rest of the reaction is similar to poly, but I’d better let one of the experts explain the slight differences.
…yes, and the catalyst in reality is not a catalyst
cause by definition a cat. is obtained unmodified after the reaction
Polyester is initiated by free radicals. MEKP is a free radical. Free radicals are very unstable compounds and need to be treated with respect. The more free radicals introduced the faster the reaction.
UV initiators use UV to cleve the free radical from the initiator and then the reaction takes place.
Epoxies are an addition reaction. That means you are adding a molecule that reacts with the resin forming the chains. The hardener molecule actually forms a part of the chain. The addition reaction in epoxies forms longer chains than other thermosets and the connections are at or near the ends. This gives better physicals and better bonding charateristics. As Mike said, this is why it’s important to mix accurately and thouroughly because you don’t want unreacted material to be floating around in your finished polymer.
Quote:
Polyester is initiated by free radicals. MEKP is a free radical. Free radicals are very unstable compounds and need to be treated with respect. The more free radicals introduced the faster the reaction.
So, what does the MEKP do to the polymers (or are they just ‘mers’ prior to the reaction) to make them form the long chains and harden?
Also, what function does styrene play?
I’ll think you’ll find some more answers in this thread
http://www.swaylocks.com/forum/gforum.cgi?post=352287;guest=18298537#352287
It just initiates the reaction. Styrene is used to lower viscosity but is reactive in the mixture. Lots of styrene is lost during the reaction which is why emissions are so high. The vapor pressure in a standard polyester surfboard resin is 5 while epoxy is .1. With epoxy you use less than 1/2 the resin that’s used to glass a polyester board so your emissions with polyester are 100 times as great. Not good stuff to breath, that styrene, which is why the EPA is on the stuff. There will soon be free radical initiated high solids resins for finishing. But it’s probably hard or perhaps impossible to match epoxy’s physicals in a laminate not to mention ease and safety of use.
My personal vision is that epoxy will be used for laminating while other high solid resins will be used for hot coat and finishing. Styrene based polyesters will fade into the past and everyone, workers, shop owners, consumers, and the environment will benefit. Workers will have a better and safer environment to work in, shop owners will be able to reduce or eliminate compliance issues and lower costs with more efficent material use and faster turn around times, the environment will benefit from lower emissions and lower material useage and consumers will get better boards. Sorry to get a little off the subject here … it just came out.
The resin chemist calvary has arrived!!!
I’m outta here…