Does filler weaken epoxy?

Hi

I'm building a wood boat with marine plywood and epoxy resin and glass mat. If I need to glue more wood to the boat, for instance a rub rail, can I use epoxy thickened with micro ballons to make it easier to glue to vertical surfaces and fill small gaps? Do the microballons weaken the epoxy?

Thanks

 

Microballoons, in my opinion, weaken the bond strength. However, people use fillers all the time with great success.

I prefer aerosil or cabosil or fumed silica (all the same thing) to balloons. I find the balloons are difficult to hold shape for the same final volumes. This means more filler and less epoxy is used for the same viscosity. The balloon mix is also easy to sand whereas the aerosil mix is hard as nails.

I have not tested them against each other, but good bets on the silica as the better bonding agent.

Many wood boat s&g guys use wood flour as the thickener for bonding panels.

Most all your bonds will also be laminated with glass or glass tapes, right?

 

You would need to use a huge amount of microballoons to make it stiff enough to stick on a rubrail, thus diluting the epoxy, microballoons are used to make easily sanded filler.
For what you want, I use a mixture of microfibres and wood flour. It is argued they strengthen epoxy as they make it less brittle.

 

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. Epoxy Fillers - modify marine grade WEST SYSTEM Epoxy https://www.westsystem.com/the-105-system/fillers/

For bonding fasteners:
We typically use 800 psi as the ultimate shear stress when testing the filled epoxy mix. We base this conservative value on long-term fatigue data. At this stress level, the epoxy is the limiting component. The interface between the fastener threads and the epoxy is where a fatigue failure typically starts. This is good. The fatigue failure mechanism for epoxy—cracking and the gradual destruction of the matrix—provides you with some warning. High-Density Adhesive Filler - Marine Grade WEST SYSTEM Epoxy https://www.westsystem.com/404-high-density/

 

That is a bad choice. You should use woven or stitched fabric instead.

 

this is a good point

 

actually I got the terms wrong. I am using woven fiberglass. being new to this I got the terms wrong!
What I did is I glued some oak rub rails(it's what I had on hand, strip oak flooring) to the top outside edge of the boat. I don't plan on glassing it in place, the thinking being it is a wear item and can be replaced when needed.???

 

Yes, rub rails and rub strakes on the bottom are supposed to be replaceable. You can install them with bedding compound.

 

Cotton flox and add cabosil until it doesn't sag