Which component of an epoxy mix "times" the cure time?

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Banzai88

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Epoxy has 2 components, Resin and Hardener. In any given 'brand', what makes it actually cure at 5 minutes, or 15, or whatever. Is it both components, or does one component have a slightly different chemical cocktail for each cure level? Like for 24 hour cure, does it have X amount of cancer causing unpronounceable stuff, and at 30 minutes cure it has X+5 amounts of cancer causing unpronounceable stuff, and at 5 minutes cure it has X+10 amounts of cancer causing unpronounceable stuff?

For example, is all Bob Smith clear resin the same, and it's the hardener that is different? Or maybe it's the other way around? And because of this can you mix the generic component with timing component from a different batch (assuming the same product family) and actually change the cure time?

Clear as mud, right? :grin:

What makes epoxy....epox at different times? :lol:

(PS, I know it polymerizes or some such) :p
 
Generally speaking, the hardener is the kicker component, however epoxies are designed usually with a specific ratio of hardener to resin, so you might not be able to use Bob Smith 5min hardener with 20 min resin as the chemical proportions might be out of wack and you end up with a semi-hardened gooey mess. An example of different ratios is US Composites 635 resin, it is sold with the option of a Fast, Slow, or Medium cure hardener, the amount of hardener used varies by its speed but the hardeners are not necessarily chemically identical, so that the more hardener added to make it set faster.
 
...and you guys wonder why people don't post here.

I'm an electrical engineer, not a chemical engineer. I figured that someone could give me an elevator speech understanding that made more sense, faster, than that stuff (Which I had already found, BTW).

rharshberger, thanks, you're a bright spot in an otherwise dark hole.
 
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...and you guys wonder why people don't post here.

I'm an electrical engineer, not a chemical engineer. I figured that someone could give me an elevator speech understanding that made more sense, faster, than that stuff (Which I had already found, BTW).

rharshberger, thanks, you're a bright spot in an otherwise dark hole.
I'm a physical chemist and sometimes it's hard to know the level of understanding a newer user has when they ask a general question.....but the answer to your initial question is reaction kinetics.

All two-part epoxies have a Part A "Resin" tka (technically know as) a "monomer" or a "prepolymer" and a Part B "Hardener" tka a "linker". When mixed together after a certain "curing time" they have reacted together to form final epoxy polymer. Think of it like the time constant in an electronic circuit.

The Part A monomer molecule has two reactive ends that will react with the Part B linker molecule. The Part B linker molecule has 2 or more reactive ends that will react with a Part A monomer molecule. If the Part B linker has only 2 reactive ends, then the cured epoxy will be a linear polymer that looks like A-B-(A-B)n-A which is somewhat flexible. If the Part B linker molecule has 3 or more reactive ends, then you will have a 3-D cross-linked structure that is extremely rigid.

The Part A may have more than 1 (type or size) monomer molecules which is why I also labeled Part A as a prepolymer to provide the desired cured epoxy properties. The Part B may also have more than 1 (type or size) linker molecule so having 2 reactive ends and others having 3 or more to adjust the degree of cross-linking for adjusting the rigidity. Furthermore both the prepolymer and the linker may themselves be the product of a polymerization of one or more molecules to obtain desired properties.

The reaction kinetics (working and curing times or time constant) can be altered by using different linker molecules or by changing the curing temperature. If you are joining only one part to another, then you probably want a fast cure, however if you are laying up a composite, you need a slower curing epoxy so it doesn't set up before you assemble the composite. Many professional laminating resins have a several day curing time at room temperature, but will cure in several hours or even less in an oven. Some thermoset epoxies will cure almost instantaneously at a high temperature and are well suited for rapid injection molded fabrications.

There are literally billions of epoxy combinations and that's the polymer chemistry that the consumer doesn't see. Further details beyond this very elementary explanation really does require a lot of reading........

Bob
 
I'm a physical chemist and sometimes it's hard to know the level of understanding a newer user has when they ask a general question.....but the answer to your initial question is reaction kinetics.

All two-part epoxies have a Part A "Resin" tka (technically know as) a "monomer" or a "prepolymer" and a Part B "Hardener" tka a "linker". When mixed together after a certain "curing time" they have reacted together to form final epoxy polymer. Think of it like the time constant in an electronic circuit.

The Part A monomer molecule has two reactive ends that will react with the Part B linker molecule. The Part B linker molecule has 2 or more reactive ends that will react with a Part A monomer molecule. If the Part B linker has only 2 reactive ends, then the cured epoxy will be a linear polymer that looks like A-B-(A-B)n-A which is somewhat flexible. If the Part B linker molecule has 3 or more reactive ends, then you will have a 3-D cross-linked structure that is extremely rigid.

The Part A may have more than 1 (type or size) monomer molecules which is why I also labeled Part A as a prepolymer to provide the desired cured epoxy properties. The Part B may also have more than 1 (type or size) linker molecule so having 2 reactive ends and others having 3 or more to adjust the degree of cross-linking for adjusting the rigidity. Furthermore both the prepolymer and the linker may themselves be the product of a polymerization of one or more molecules to obtain desired properties.

The reaction kinetics (working and curing times or time constant) can be altered by using different linker molecules or by changing the curing temperature. If you are joining only one part to another, then you probably want a fast cure, however if you are laying up a composite, you need a slower curing epoxy so it doesn't set up before you assemble the composite. Many professional laminating resins have a several day curing time at room temperature, but will cure in several hours or even less in an oven. Some thermoset epoxies will cure almost instantaneously at a high temperature and are well suited for rapid injection molded fabrications.

There are literally billions of epoxy combinations and that's the polymer chemistry that the consumer doesn't see. Further details beyond this very elementary explanation really does require a lot of reading........

Bob

Thanks. That makes plenty of sense.
 
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