# What is the difference between regular clear epoxy and "epoxy steel" that you can get?



## Jimothy (Oct 29, 2015)

I understand that they are both 2 part concontions, and that the latter has small metal particles in it. But why? When would standard epoxy be inferior to epoxy steel and vice versa? If you often glue wood to wood with a gap, metal to metal, wood to metal, etc. Whats the deal?


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## Novamr99 (Oct 9, 2020)

Do you want the glue that you just used to fix you piece of wood (that you now need to finish sanding) to be as hard as the wood, or as hard as a nail? You'll sand your chair away before you smooth out the joint. Think about your materials and your tools.

You are using epoxy to join two pieces together, sometimes Back-Together. You are trying to Unite them. Make them a Unit.

If you separate wood with metal, you are not creating the unity you seek.

It actually gets even funkier when you try to use wood adhesives on metal. DAMHIK

I know there will be those who have all the chemistry and stress test data to follow so let's all get Schooled.


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## CaptainKlutz (Apr 23, 2014)

Question is very broad, and without product PN's or names, plus substrates you want to bond; 
it is impossible give more than general answer?

Regardless:

- Polymer adhesives have temperature restrictions. Above glass transition of cured polymer, they lose physical strength, and lose adhesion. Epoxy polymer is an insulator, and the thicker the bond line; the harder it is for adhesive to dissipate heat during cure. Non filled (clear) epoxy has lowest operating temp range. Metal filled epoxy has highest working temp range, as the metal particles help dissipate heat and keep epoxy cooler. 
There are many different epoxy chemistries. But typically a room temp cure epoxy has lowest usable temp range. An oven cured chemistry will have much higher operating temp range. For example an oven cured phenolic epoxy can be used at 300°F easily, while some standard bisphenol based retail epoxy losses significant strength above 120°F.

- Metal filled epoxies behave like a composite of metal and epoxy. 
With high filler loading, they can be machined, filed, threaded, and used like metal in low stress applications. There are different metal fillers and each one has advantages/disadvantages. Aluminum fillers (like found in JB weld) are decent general purpose metal filled epoxy. But aluminum is not strong metal. So steel fillers (like found in Devcon Steel epoxy) will be stronger, but at same time dissipates heat slower. Metal filled epoxy PUTTY will be stronger than liquid epoxy as it contains more metal fillers per sq mm. Copper and metallic ceramic fillers are expensive and primarily used in thermal dissipation bonding applications.

- Adhesive bond line thickness is critical in many applications. 
Filled epoxies (including metal fillers) will increase the bond line thickness, as you need to have room for fillers. But they also allow a thicker bond line, thanks to adhesion between filler particles; before strength of joint is strictly based on bulk epoxy properties. So filled epoxy systems can be advantageous when bonded surfaces are rough, resulting in thicker bond line.

Hope this helps.


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## Jimothy (Oct 29, 2015)

thanks for the lengthy reply. So as a simple answer, either wood to wood or metal to metal with the same glueline thickness, the epoxy with metal particles will produce a stronger bond?


> Question is very broad, and without product PN s or names, plus substrates you want to bond;
> it is impossible give more than general answer?
> 
> Regardless:
> ...


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## CaptainKlutz (Apr 23, 2014)

> thanks for the lengthy reply. So as a simple answer, either wood to wood or metal to metal with the same glueline thickness, the epoxy with metal particles will produce a stronger bond? - Jimothy


Hmm,

Mismatch in terminology confuses me?
Whole idea to adhesive bonding is to use proper adhesive for material and joint.

Wood to wood, can use many different adhesives.

Epoxy adds no advantage to a wood to wood bond, compared to other adhesives. Regardless if filled with metal or not. Wood is weak. Even a lowest strength PVA (white glue) is stronger than the wood. So you don't need epoxy for wood gluing, unless you need epoxy for a different reason.

PVA adhesives (white/yellow wood glue) are single part adhesive intended for porous surfaces. They have more than enough strength to create a bond stronger than wood, as long as bond line is uniform and relatively small. There are simple PVA adhesives that are reversible with water, and modern water resistant blends that include secondary cure mechanism to reduce water permeation. All of these are stronger than wood.
PVA will not fill large gaps, as it is weak when used as a polymer filler. But when you fill the PVA with wood dust, it becomes a usable and strong putty.

Epoxy adhesives have higher bond strength than PVA, and do not depend on porous surfaces to remove water and enable hardening or curing. They are able to bond to many different base materials, and are great for bonding dissimilar materials. They are also more expensive (4x+ in volume) than PVA, and rarely used to bond wood to wood. 
Epoxy is sometimes preferred in wood bonding, when wood is very dense and oily; which reduces penetration of PVA adhesive. They are also popular by non-wood working types for repairing cracked items with larger bond line or where previous adhesive can not be adequately cleaned from surfaces to exposed wood pores.
Use of metal filled epoxy for wood is considered a waste of money. The improved properties created by metal have zero benefit when bonding a weak substrate like wood. Metal filled epoxies only help with dealing with stronger metal substrates.

Cheers!


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## MrUnix (May 18, 2012)

> So as a simple answer, either wood to wood or metal to metal with the same glueline thickness, the epoxy with metal particles will produce a stronger bond?


Simple answer, No.

Here is what West System says about using fillers:



> *Epoxy Fillers and Epoxy Strength*
> 
> While certain fillers will increase the density of epoxy, this does not correlate to higher physical properties or increased adhesion strength. *Epoxy is strongest without filler added to it*, but adhesive fillers are excellent for increasing the viscosity of the epoxy mixture, bridging gaps, preventing the epoxy from soaking too far into the wood, and preventing glue-starved joints. Fairing fillers turn the epoxy into an easily sanded fairing compound.
> 
> Source: https://www.westsystem.com/the-105-system/fillers/


Cheers,
Brad


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