Epoxy strength

[sharonov]

This has been bothering me for awhile. I tend to automatically add filler when performing repairs with epoxy. Sometime ago I stumbled upon the following info on West System site:
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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 the fillers are necessary to:

• Increase the viscosity of the mix
• Bridge gaps
• Prevent the epoxy from soaking too far into the wood
• Prevent glue-starved joints

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While I understand that in many cases (e.g fillets, vertical surface repair, etc.) higher viscosity is very useful, it just became a habit to always add filler. The statement from the manufacturer seems to say I should not do that unless required. Opinions?

 

[Tin Kicker]

While what you copied is generally true, especially for our use on polyester resin boats, there are fillers which can increase tensile or shear strength (or perform other functions) but there is always a tradeoff somewhere.

 

[Christian Williams]

All I know is that epoxy with colloidal silica is brutally hard and tough, and with sawdust, as a fillet, sure seems as strong as the wood it connects.

Like everybody else, I have whacked, drilled and marveled at the stuff left in the mixing cup after it goes off, and been impressed.

 

[bigd14]

That does not sound right. Like Christian, I have used various fillers to make a much harder and more durable compound. Common "thickeners" e.g. colloidal silica or microballoons, do not seem to add very much strength compared to neat epoxy. But add in some finally chopped fiberglass and suddenly you have a bulletproof material. In fact I have made it a point to almost always add finely chopped fiberglass to my thickened epoxy for any repairs that might be structural.

 

[u079721]

I took the in house "Glue U" course at West Systems in Bay City, MI many years ago, and I remember something similar being mentioned. As I recall a workaround was to first wet out surfaces with un-filled epoxy, and then follow up with thickened epoxy to ensure the best bond.

 

[Kenneth K]

I read the West Sytems 150-ish page Repair and Maintenance Manual before rebedding my deck hardware. Seems like colloidal silica was their own recommendation for hardware mounting applications.

When talking about "strength" of straight vs filled epoxy, it probably matters whether you are talking about compressive strength (hardware mounting), vs tensile, shear, or elastic strength.

 

[tenders]

I suspect there is some distinction between filler which changes the properties of the epoxy, and the addition of a structural component like glass fibers which creates a composite material. The strength of neat epoxy is no good if it won’t stay in place and fully absorb into a substrate like wood while it’s curing, thus the use of thickening fillers to result in a better final assembly. The strength/hardness of neat epoxy may result in a patch which can’t be nicely sanded flush with a surrounding softer material like gelcoat, thus the use of fillers which reduce its hardness and presumably its strength somewhat, which is irrelevant due to the low strength of the surrounding material. I don’t know if epoxy+fumed silica is harder than pure epoxy, they are both pretty darn hard, but I do know that epoxy+sand is harder than pure epoxy, and epoxy+chopped fiber is less brittle and stronger than pure epoxy when you’re molding a shape out of it.

As a rule I use cotton fibers or sawdust as a thickener when using epoxy with wood, silica as a thickener when using it with fiberglass or fiberglass cloth, chopped fibers when trying to make crude standalone shapes (not too often with boatwork), and a special epoxy paste with premixed softening fillers when patching or fairing gelcoat.

Despite this apparent simplicity, my epoxy toolkit entourage has somehow expanded over the years into two bushel basket-sized containers, however....