Hand Tools Archive

how it cures
Response To:
Re: epoxy ***Links*** ()

Bill Tindall, E.Tn.
"Curing" is a chemical reaction A+B to yield cured epoxy. The most useful thing learned in a chem class is that a whole A molecule reacts with a whole B molecule. It doesn't happen that 2/3 of a molecule can do anything. If you have 3 A molecules and 2 B molecules and they are allowed to react there will be
2 A-B epoxy molecules formed and one A molecule left unreacted.

The curing chemistry is like building a long train (polymer) from individual cars with one exception. Coal cars will only couple to tanker cars. Coal cars can not couple to themselves. So, as long as you have an equal number of coal cars and tanker cars you can build a train as long as you want if you have the time. But, if there are more coal cars than tanker cars when all the tanker cars are hooked up tanker- to- coal, there will be coal cars left over.

Long molecules (the train/polymer) get tangled and become strong. Small molecules can't tangle and they are not strong.

Left over hardener molecules weaken the cured structure and can make it rubbery.

The polymer we call cellulose acetate is hard and brittle. Dissolve 25% of a small molecule into it and it becomes flexible. This mixture is what we know as the flexible backing of Scotch tape.

In the case of epoxy somebody knows how many coal cars and tanker cars respectively are in a squirt each of resin and hardener. Following directions results in them being equal in number.

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