Types and use of epoxy

JohnCoker

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Here is a short primer on the types and usages of epoxy for hobby rocket purposes.

Types of Epoxy

In rocketry, we most commonly use epoxy two purposes: bonding and laminating. For bonding a thicker epoxy is desirable for its gap-filling ability and because the fillers add strength (particularly chopped fiberglass). Epoxy bonds via a mechanical process, which means that the surfaces to be bonded must have some roughness for the epoxy molecules to lock into. Always rough up both surfaces to be bonded with 100grit sandpaper and wipe clean with alcohol first.

Laminating epoxy is used most commonly to wet out a fabric to build up a composite structure or to reinforce a component (wood, paper, phenolic). For laminating, a thin epoxy is desirable because we want the fabric to be completely saturated (no dry spots) and we want to be able to remove the excess epoxy. (Epoxy by itself adds no strength to the part, only weight.)

Epoxy Manufacturers

Here is a quick overview of different commonly-used epoxies, with the focus on the ones I have most experience with.

Bob Smith

The most commonly used epoxy in hobby rocketry is "hobby shop" epoxy, usually made by Bob Smith Enterprises. Bob Smith epoxy comes in 5m, 15m and 30m bonding epoxies and a laminating epoxy (20m). Bob Smith bonding epoxy is mixed in a 1:1 volume ratio (fillers are included in the hardener).
https://www.bsi-inc.com/Pages/hobby/epoxies.html

There are several "epoxy systems" that are commonly used as well in the hobby: West Systems, Raka, System 3 and Aeropoxy. I have extensive experience with West System and Aeropoxy so I will discuss them at more length. Others have had good experiences with Raka and System 3 as well.

West System

This is a good product, easily available locally. While designed for use in boats, it's a good choice for hobby rocketry and I've built may rockets with it. West System has several products, but I'll concentrate on the most useful line for us: the "105 System".
https://www.westsystem.com/ss/the-105-system/

The 105 system is at base a laminating epoxy and is mixed 5:1 (5 parts resin to 1 part hardener). For bonding, one mixes a batch of the epoxy as normal, then thickens it with an appropriate filler. This makes the system extremely versatile because the filler can be tuned for the desired purpose:
  • 404 High-Density filler is good for general bonding especially of non-porous surfaces
  • 403 Microfibers is good for bonding of porous surfaces
  • 410 Microlight is good for filleting and other cosmetic purposes and is easily sandable

West System can easily be found at West Marine stores. That and its versatility make it a very popular choice.

Aeropoxy

The system I currently use is Aeropoxy. (This system is more expensive and less flexible than West System, but has superior mechanical properties.)
https://www.ptm-w.com/index.asp?pgid=15

Unlike the West 105 system, which has a single epoxy and a set of thickeners, Aeropoxy has several sets of products, and one uses different epoxies for bonding and laminating. I use these two pairs of products:
  • ES6209 A/B for bonding (1:1 mix)
  • PR2032 / PH3660 for laminating (3:1 mix by volume)

Notes

Safety

Epoxy products can cause allergic reactions over time. It's best to avoid contact with skin and highly recommended to wear gloves when working with it.
https://en.wikipedia.org/wiki/Allergy

Latex gloves are cheap, but latex itself is an allergen for many people so Nitrile gloves are a better choice.

General clean up of epoxy should be done with alcohol (common 70% isopropyl is fine).

Strength

I've done some tests comparing various epoxy systems for bonding strength:
https://jcrocket.com/adhesives.shtml

More important than that though is the preparation. To properly prepare surfaces for bonding, non-porous surfaces should be sanded with 100grit sandpaper and both surfaces should be wiped clean with alcohol.
 
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CarVac

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Latex gloves are really not effective protection against epoxy. Nitrile gloves are capable of preventing epoxy from diffusing through for about an hour, but it only takes a few minutes for latex.
 

4kids49

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Thanks for the primer John. I read your nice original report a few years ago.


Chris
 

Area66

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Latex gloves are really not effective protection against epoxy. Nitrile gloves are capable of preventing epoxy from diffusing through for about an hour, but it only takes a few minutes for latex.

I ordered Butyl gloves for when I'm go do laminating, but for quick job I will use Latex as I have them for free and I have only few risk to have epoxy on the gloves anyway .
 

fyrwrxz

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Thanks, John! It's always good to have a reminder we need a review of the basics from time to time, esp. when there is such a diverse number of readers with different levels of experience and needs.
 
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