I would like to understand how the polyester resin or epoxy catalyst works and to what degree it must be mixed with the base material in order to be effective.

Typical product instructions advise to thoroughly mix the catalyst for a specified period of time.

How thorough does this really have to be in order to have the material cure properly? Is the mixing done just to initiate a thermosetting behavior and does that just continue to propagate heat to the rest of the material to complete the curing of the entire mass? If this is the case, wouldn't the base material cure if it were heated sufficiently without the presence of a catalyst?

Or is something else happening when the catalyst begins the cure? And how does mixing effect the quality of the cure?

  • 1
    $\begingroup$ Are you sure that epoxy relies on a catalyst? I thought that at least sometimes, the materials mixed are two solutions of two different monomers which can undergo a condensation polymerization with each other but not with themselves. $\endgroup$
    – Curt F.
    Jun 18, 2015 at 22:34
  • $\begingroup$ If you look at the two tubes you will probably see that one component is a diamine and the other is a di- or tri-terminated oxirane / epoxide. Both are required for polymerisation as the manufacturers intended. Water may in theory react with the epoxy but it might be a lot slower and not give you a hardened adhesive. $\endgroup$
    – Beerhunter
    Jun 18, 2015 at 22:54

1 Answer 1


The degree of mixing is very important in "two part" epoxy binder systems for coatings and adhesives.

In most cases the two liquids are quite viscous and the liquids themselves are better described as suspensions or colloidal dispersions rather than solutions. Unlike say two simple aqueous solutions when combined our two prepolymer liquids will not readily homogenize in a reasonable amount of time unless they are mechanically mixed in some way. Even if the liquids do combine very slowly they will often gel or vitrify by the molecules at the interfaces reacting before the liquids homogenize.

In many epoxy systems we have, as Beerhunter stated above, some oligomer or polymer with epoxy functionality in one part and some liquid with amine (or sometimes hydroxyl) functionality in the other. Every amine needs to "find" an oxirane ring to attack or the curing process simply can't occur. The curing process works by nucleiphillic attack from amines to open epoxied rings.

The presence of leftover unreacted amine in cured epoxy materials often results in a phenomenon known as amine blushing or amine blooming. leftover amines or polyamides migrate to the surface and may in some cases react with air to form carbamates. This can be disastrous for desired materials properties like tack, adhesion (for the epoxy as a substrate), or cohesive strength of the bulk material.

In summary you can't build a robust three dimensional polymer network without thoroughly mixing because all the reactants can't react. Always mix as thoroughly as possible to get complete conversion.

My experience with polyesters is limited so I will not comment on that.

  • $\begingroup$ Excellent response, for a chemist kind of mind, a layman need to have a little interpretation .. what he basically said id that the hardener (part b) affects the resin (part a) chemically. the manufacture has simplified the mixing by changing the basic concentrations so that ...generally part A & B are the same amounts. $\endgroup$
    – SkipBerne
    Nov 16, 2015 at 18:22
  • $\begingroup$ I have been using epoxies for decades and I can tell you mixing is the most important step. I use ziplock bags to mix the parts in . one corner a the other b. and you can knead the two together in the bag then icing $\endgroup$
    – SkipBerne
    Nov 16, 2015 at 18:29

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