Big air bubbles

Hi all
Was doing a hand layup with 2 layers 225 gms/sqmtr of CSM and one layer of carbon/ Kevlar and trialled using cork as reinforcement using in a few big flat areas and finally using VE resin The result surprised me , the areas of cork blew up with huge air bubbles, obviously gassing caused this result, has any one experienced anything like this and is there anything I can do to not have this happen ? .....

Thanks

 

First comes to mind of gassing with cork. Cork is very good insulation and the heat builds up. Start with warm materials and make sure the temperature is dropping some during the process.
BR Teddy

 

Second the temperature causing air in pores in the cork expanding and causing the problem. It happens when people use epoxy on timber in the sun, with varnish too. If you rub down and recoat with static or lightly falling temperature you should be OK.

 

Pre-coating the cork with resin, letting that set-up and then doing the actual laminating might help also.

 

When mixing synthetic and natural materials, all sorts of issues such as this are common.

As everyone else has said, it is because your cork off gassed due to increasing temperatures between the start of the project and the cure of your VR resin.

When using natural core such as balsa, cork, or even just sheathing plywood, it is essential the materials are properly low in moisture and if laminating outdoors, essential to start after lunch at the hotrest part of the day. Then, rather than expelling air and making bubbles, the natural, porous material will actually suck a little resin in, creating an even better bond.

 

To prevent outgassing, there's two approaches that actually work. The first and most common is the hot on hot method, where you heat the resin and the substrate, but permit the substrate to begin cooling, before it's applied. This way the pockets of air are contracting, when the goo goes on and it's drawn into the substrate. The method I now use is the mash and scrape technique. Outgas bubbles can only appear, if the goo is permitted to pool on the surface. This forces the bubbles to rise up through it. I apply the goo and mash it into the surface with a putty knife or plastic applicator, forcing it into the pores of the substrate. Next, after a short wait to let it soak in, I scrape the pooled goo to other areas so it can soak into unwet portions. This is a sealing treatment and only suitable on raw surfaces. By cleaning the surface of any pooled resin, there's nothing there for the bubbles to rise up through. This technique is easier and faster, leaving a dull satin surface, instead of the glossy one, if the resin has a film thickness on the surface. These both are extra steps, but eliminate outgassing in hand laminates.

 

I was going to use thin cork as padding on a trolley and used vinyl ester to laminate the frames. Very bad result, similar to what you describe. It's like the styrene attacked the cork and wrinkled it up, made a complete mess. I imagine its something to do with the glue holding the cork particles together

 

Have you tried de-gassing the material after you have mixed it?

We use a vacuum chamber type thing at work when using 2 part materials to ensure no bubbles are present once cured.

 

PAR,

In your Mash and Scrape technique is this done when applying epoxy on newly prepared wooden surfaces or when glassing a wood substrate?

I used a similar technique when glassing my stripper, only I scraped the excess epoxy off the cloth and collected it in a container, not reusing it. It had been sitting on the cloth weting it and didn't seem to be able to wet any additional cloth. The first coat had a satin sheen and subsequent coats were added to fill and bury the cloth. Turned out great.

phil w.

 

Phil, the technique I use is only necessary on raw wood or other porous surfaces, where outgassing might occur with the first sealing coat. Once the surface is sealed, it can't out gas, unless you uses thinned epoxy (not wise, unless you know the chemistry involved). The idea is to smash the goo into the pores, filling them, then scraping the surface, so no pools of goo remain. Without the pools or standing puddles of goo, there's nothing to have bubbles appear in and if they should show up, the just burst open on the surface, instead of being captured in a pool of goo.

As you're smashing and pushing goo around, excess can be moved from one area to the next, with little harm. If I push it off the edge of a piece, I catch it and reuse it someplace else. So log as the epoxy is still workable, you can use it.

Subsequent coatings don't need this technique, because the surface is sealed, so outgassing isn't possible. You can still get bubbles, but these are self generated and can be popped with a torch. Filling the weave of fabrics is best done with fairing compound, not straight resin. You can use straight resin, but it takes a lot more epoxy and often a few more coats, whereas a fairing compound is a one shot deal.

 

Bubbles

Par,

Thanks for the detailed description of the process. I now have a better understanding of it and see where it has lots of merit.
Have a good day!

pw

 

I didn't invent this Phil, but have blatantly stolen the process for my use. I can't remember where I originally saw it, but it instantly made sense, so I "adopted" it.