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Choosing the Appropriate Resin

Epoxy vs. Vinylester vs. Polyester Resins

Jon Soller

Soller Composites

Copyright 2004
Updated 2018

All Rights Reserved 

I.                   Introduction

There are three main types of Resins used today for use with Carbon Fiber, Fiberglass, and Aramid (Kevlar). These are Epoxy, Vinylester, Polyester Resins.  Each has different characteristics and associated costs.  Below we briefly discuss each of these resins. 

Please be aware that all resins & hardeners have their safety related issues.  Please research the products you intend to use and fully read the manufacturer’s safety information and follow their recommendations.

II.                Epoxy resins

These are usually the most expensive of the three resin types, but well worth the cost.

Epoxy resins are typically about three times stronger than the next strongest resin type.  Epoxy adheres to Carbon Fiber, Fiberglass, and Aramid (Kevlar), very well and forms a virtually leak- proof barrier.  Epoxy also adheres to older epoxy and most materials quite well.

Most epoxies do have a tendency of yellowing when exposed to UV.  When purchasing epoxy for applications that have extreme temperature changes or are exposed to water, make sure you purchase an “all weather” epoxy hardener.  Examples of such a hardener is West System’s 207 Hardener or any of our Adtech 820 hardeners (822, 823 & 824).

Note that most epoxies are slightly amber in color.  When you apply the typical amount of resin to a composite, that is just enough to wet-out the composite, the epoxy is clear. The exception to this is when you wet-out yellow Kevlar or white colored fiberglass.  In these cases you will see the yellow kevlar darken slightly and you will most likely see a very slight yellow tint to a white colored fiberglass.

You can buy perfectly clear epoxy resins. West System's 207 or our Adtech 824 hardener are quite clear and have UV protection. They are the only epoxy hardeners we are aware of that has both of these characteristics.

Don't confuse the "yellowing" of epoxies over time with the slight amber initial color of most epoxies.  There are few epoxies in the market that will not yellow over time.  The West System 207 and any of our Adtech hardeners (822-824) are amoung the best.  Even for indoor applications, UV will eventually yellow your epoxy.  So if you don't plan on painting your piece/application and you want your piece to look good and last as long as possible, plan either protecting it with UV coating (such as a UV urethane), and/or use the 207 or 822-824 hardeners.

Note that West System Epoxy starts to soften at approc 135F; our Adtech softens at about 210F. The Adtech is also a much stronger epoxy and costs less, but we recommend curing above 80F for the Adtech epoxy. We still recommend West System for low temp applications (such as basement wall in colder climates, otherwise we recommend the Adtech 820 base epoxies).

III.             Vinylester Resins

These resins can have less strength than epoxy resins. They can adhere poorly to Carbon Fiber and Aramid (Kevlar), but that depend on the finish or sizing on the composite fiber.  Vinylester resins can be used with composite fabrics to avoid potential delamination issues if a polyester resin is to be put on top of the epoxy. Note that epoxy does not have a porblem sticking to polyester resins, but polyester resin can have difficulty sticking to epoxy. For example, if you want to put a polyester clear coat on top of epoxy, you may want to consider using a Vinylester resin instead of the epoxy.

IV.              Polyester Resins

These are the cheapest of all the resins.  They have poor bonding capability and should never be used for any structural carbon or aramid work. They typically work well only on fiberglass.  One should generally never consider using this resin with structural applications with Carbon Fiber or Aramid.

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