Sanding of composites before bonding

[prfesser]

Hi fellow rocketnutz,
Something I've been doing for years...I heard it on Rocketry Online or some such...is to make a mix of epoxy and hardener, don gloves, and sand the parts to be bonded with sandpaper wetted with the mix. I don't see any downside. Sandpaper grit is either aluminum oxide or silicon carbide; loose material from wood is wood flour, and from composites it's either tiny bits of glass or of polymer. All of those are fillers used for epoxy anyway. Sanding while wetted with the mix pretty much guarantees that no significant amount of moisture can get to the bonded area before bonding.

Anyone else does this? Can anyone speak authoritatively? Pedantically? Any comments? Physical threats?

Best -- Terry

 

[heada]

That method would help ensure that the epoxy matrix was fully embedded into the rough surface rather that just sitting on top of it which would give a stronger bond. I assume you have a very lose fit before you start this as if it were too tight you might not be able to slide parts past each other such as on couplers.

 

[prfesser]

My main thought is that it would virtually completely exclude oxygen and water from the bond area.

 

[BDB]

Are you saying to wet the sand paper with laminating epoxy, or something thin like West System? This sounds really interesting, but I can’t imagine it working with Rocketpoxy or Proline.

 

[prfesser]

Yes, wet the paper with mixed epoxy and hardener. Laminating should be best as it's thin, but any epoxy that isn't a paste and doesn't set in ten minutes should work. Mind, it would be the final sanding before assembly. I doubt that it's something that could be done in production; the epoxy would constantly harden and the paper would have to be replaced. But for one-offs like we usually do...

Ordinarily I use very slow epoxies, so time is not an issue.

Best -- Terry

 

[BDB]

That sounds like an interesting idea. I usually use thick epoxy, like Rocketpoxy or Proline, for construction, but I would be game for trying to sand with some laminating epoxy on my second MD project. [I just need to fly (and lose) my first MD project first.]

 

[dhbarr]

I know for some of the toolmaking epoxies they recommend a thin layer of laminating epoxy before putting on the thick stuff.

 

[pcalvin]

In my short 14 years of composites for flight (satellite and launch vehicles), we've never abraded into fibers. Just break the resin glaze with scotch bright or 220. But we tend to over fixture (tighter bondline control) and use longer pot life resins.

 

[6PTsocket]

Epoxies like JB Weld or Marinetex are already filled. I think JB Weld uses aluminum dust. Epoxy cures faster when it is thicker. Roughening the surface and cleaning,with a solvent that gets rid of any dirt and grease and does not damage the substrate is what is important. Micro balloons are sometimes added to reduce weight. Thixotropic powder is sometimes added to make it more viscous so it does not run off vertical surfaces before it cures. I recently discovered JB Plastic Bonder (in tan or black) That is phenominal for most plastics, even for bonding two different types. Works well in a lot of other materials, too.

 

[prfesser]

Epoxies like JB Weld or Marinetex are already filled. I think JB Weld uses aluminum dust.

Yes; my point was that the sanded material that remains in the epoxy after sanding is unlikely to have any adverse effect, since those materials---aluminum oxide, silicon carbide, bits of cured epoxy resin, wood dust---are used as fillers in filled epoxy anyway.

 

[Bat-mite]

I sand dry and wash with acetone before bonding. Works for me. If it ain't broke, don't fix it, yadda yadda.

 

[prfesser]

In my short 14 years of composites for flight (satellite and launch vehicles), we've never abraded into fibers. Just break the resin glaze with scotch bright or 220. But we tend to over fixture (tighter bondline control) and use longer pot life resins.

Yes; my point was that any claim that "sanding that way will leave behind dust and other stuff that would contaminate the epoxy" is probably invalid. Even if one does happen to abrade into the fibers, the bits of glass/kevlar/carbon fiber that remain in the resin after sanding are unlikely to have any adverse effect, since those materials are used as fillers for epoxies anyway.

 

[prfesser]

I sand dry and wash with acetone before bonding. Works for me. If it ain't broke, don't fix it, yadda yadda.

Agreed. One point to be made is that sanding with epoxy-wetted sandpaper should be at least as effective (if not more so) as removing moisture from the joint, than solvent cleaning after sanding. The only moisture that can remain behind with epoxy wet-sanding should be any moisture that is already present in the epoxy mixture.

When I was in grad school a few centuries ago, my surface chem prof (James Wightman) said that nine out of ten times, adhesive joint failure is due to inadequate surface preparation---in particular, water that's left behind after cleaning. I still have my notes from that class; they say that a surface that has been adequately cleaned while in vacuum takes approximately one second to collect a monolayer of contamination (mostly in the form of moisture) when exposed to ordinary room air.

Anyone have a setup for measuring strength of an aluminum-to-aluminum epoxied joint? Aluminum is notorious for being difficult to epoxy. I'd like to make a number of joints (no, not THAT kind) and compare tensile strength of abraded-wetted-with-epoxy joints to those prepared following dry abrasion and solvent-cleaning, and to those prepared using a commercial aluminum prep such as the ones described in Lee and Neville's "Epoxy Resins" book. Without numbers, all I've got is a hypothesis...

Best -- Terry
"If it can't be expressed in figures, it is not science; it is opinion." -- another Heinlein quote, but a good 'un