Why not slurries?

[Toot]

I searched around and found only 13 links on this site to the use of slurries in boat construction and a number of those were in reference to odd repairs using chopped roving to fill in gaps.

When I was in A&P school, we made wet-layup parts on a foam core by first applying a slurry made up of microballoons (silica bubbles) and epoxy. The reason for this was: 1. The epoxy needs to fill in the tiny bubbles in the foam in order to achieve a good bond and a slurry is lighter and uses less resin than using straight epoxy; 2. the slurry is stronger; and 3. the slurry will give a good bonding surface for the fiberglass; 4. without the weight of the fabric pulling at it, the slurry (applied and allowed to cure on its own before laying up the glass) fills the voids more completely and provides a better bond.


But apparently boat people aren't using slurry on their foam before performing wet layups. Why not?

[antonfourie]

Probably costs too much in labour ....

[frosh]

Some racing craft where light weight is extremely critical do the initial coating of foam with a microballoon slurry which saves some resin absorption.
I am not sure if the motivation is to provide a stronger bond. It is obviously a bit slower so commercially not so desirable. However I am about to complete a foam composite construction and will do exactly as you recommend as time is not of the essence.

[Toot]

The more I think about it, I am not sure the bond would be stronger- it would be roughly equal, I should think. However, I do believe it makes bonding to vertical surfaces easier because, once you have the slurry cured, you have a smoother surface which seems to translate to greater amounts of surface tension... kind of the difference between a wet tissue laying flat on plate glass versus laying flat on wood.

I was just surprised not to see it in use by more boat people.. especially if you're using $100+/gal resins. If time isn't of the essence, tripling the volume of the resin for ~$30 ought to make it rather popular wherever you can use it.

Right now I'm playing around with Kevlar pulp for fabricating hard points for installing studs into fiberglass components for later attachment to metal surfaces. Very neat stuff and quite strong! And not quite the inhalation hazard of microballoons. Breathing that stuff is very bad. It may look like a different substance, but it is certainly not to be snorted.

I am also considering (just thought of this tonight), for my prospective mini-boat, using tiny V-channels scraped into polystyrene in a hash pattern and filled with a slurry in order to increase the bonding surface of the core as, for a ~150# boat, the low impact resistance shouldn't be so much of a concern.

[waikikin]

I'll often resin prime & then squeegee a resin/colloidal-glue mix to ply-timber esp' end grain when epoxy sheathing then lay glass to it whilst wet or at least tacky- best to pre wet/impregnate fabric as glue can hold back resin wet out.This technique saves time for me- try to use a falling ambient temp to reduce bubbles. Regards from Jeff.

[wave1235]

Cabosil fillets then increasingly wider glass tapes each layer when still wet makes hard jobs easy and seals so much better. Mixing the Cabosil/epoxy to the consistancy of peanut butter is the trick.

[Toot]

waikikin... you need to be careful when laying up fiberglass over uncured slurry. I can't speak for all types of slurry, but I know that microballoons can work their way up into the fiberglass and weaken the structure by making the matrix surrounding the strands of glass less uniform in strength. I recommend letting it cure before laying up the glass and have been explicitly told not to do it any other way. But, otoh, again, that's with airplanes, not boats.

[Toot]

Originally Posted by wave1235

Cabosil fillets then increasingly wider glass tapes each layer when still wet makes hard jobs easy and seals so much better. Mixing the Cabosil/epoxy to the consistancy of peanut butter is the trick.

That's the cool thing about slurries. It's the reason I actually enjoy working with them... you can make them into any consistancy you want so they are very versatile. The only thing I don't like about them is the way they will stick to your hands while working, making it a little harder to get uniform shapes. But tongue depressors and other such things work great. I'm particularly a fan of a little tool you can buy at your arts and crafts store for working with clay. They come in very thin flexible metal or else rubber and are shaped a little bit like a jelly bean only instead of the concave side, you have an extra convex surface for shaping. It gives you an infinite number of radii for shaping and, on its side, can also be used as a straight edge of sorts.

[waikikin]

Toot, I agree on that with micro ballon/ lightweight fill- at least wait until its tacky & be aware of resin scavenging propertys of some substrates/cores giving a dry bondline esp' with light weight extenders- U can really come unstuck chasing KG's sometimes.The slurry I mentioned was for colloidal sillica/cabosil- we do try to conserve resin but not at the expense of durability & repairability & will always resin prime cores on installation.Regards from Jeff.

[Toot]

Originally Posted by waikikin

Toot, I agree on that with micro ballon/ lightweight fill- at least wait until its tacky & be aware of resin scavenging propertys of some substrates/cores giving a dry bondline esp' with light weight extenders- U can really come unstuck chasing KG's sometimes.The slurry I mentioned was for colloidal sillica/cabosil- we do try to conserve resin but not at the expense of durability & repairability & will always resin prime cores on installation.Regards from Jeff.

Did you edit your previous post? Or did I misread it? Or both? Anyway, I agree with what you're saying now.

That's an interesting idea- priming the wood with straight resin, squeegeeing it off before adding the fabric.

But did I read you right that you always use straight resin to prime the (foam) cores?

[waikikin]

Toot, I never edit my posts- just leave em as is warts & all, & quite happy to be wrong & corrected & therefore better educated for it. Some times I'll describe a process & then on demonstation of it be told of the details left out of the description- pictures & demos often do a better job than words alone- maybe we need some kind of live action forum with resin & fabric remnants flying about with helmet cams catching all the ooozzzing boatbuilding fun times- but then I'm not that IT savvy & can but slowly type. Resin priming- so far as I remember it's part of the recomended process of most core manufacturers, by preference I'll use a proprietry? pre blended core bonding putty for core bedding particularly with poly resins as Ive come across some nasty delam issues in repairs with "home blend" bedding that didn't include resin prime as part of the process. Regards from Jeff.

[JR-Shine]

I use a plastic spreader to apply some epoxy to the foam core before glass - It just helps fill in the little voids in the pVC foam. I don’t add any filler to make a slurry. On the high density foam, its not as necessary as with the lighter (say 4-5 lb) foam

Joel
boatbuildercentral.com