Urethane Foam Variables

[Hudson Force]

I've had some great experience with successfully replacing some wet balsa core with some sheets of end grain balsa tiles on a cloth matrix. I will continue with my technique in dealing with some more soft spots; however, there are some spaces where I would choose not to cut away the full top laminate and decide to pour a two part high density foam into these places. When shopping for these polyurethane two part foams, I see and understand the density variable, but I have not been able to notice any other variables in the products on the market. What are some comments from those with more experience? Are there any other variables among the two part, mix and pour, urethane foams that would be of interest for those replacing some damaged coring?

[Hudson Force]

Maybe I need to clarify my question. Are there advantages to using some particular two part foams?

Mixing time, strength, adhesion, shaping (forming/grinding). Are some easier to work with for any reason?

[minaret]

Not really. Pour foam is open cell by nature and not appropriate for this use. If you must use it, search and find the 8 lb. instead of using two or four lb. if you've never done it before, prepare for a huge mess. I insert paper funnels in each hole to be poured before mixing. You have only seconds after mixing to pour it. Make sure each batches expansion is complete before pouring more foam, and don't pour foam into an area where pressure from expansion will be trapped, ie more than one hole in each area to be poured. You can very easily make a high in the deck or worse, I've seen serious structures blown apart by too much pour foam.

[minaret]

Much better to install zerk fittings and pump in epoxy with a grease gun. Still an amateur repair which will eventually fail and make a proper repair more difficult though.

[Hudson Force]

Originally Posted by minaret

................ Still an amateur repair which will eventually fail and make a proper repair more difficult though.

Please tell me more of your opinions about the "will eventually fail" prediction. As I said before, I've had very good results with my end core balsa placed between epoxy layers. These repairs total about 7 square feet. They are not visually apparent and remain firm with the oldest one being completed 18 years ago. I have some areas where I'd rather not disturb the deck surface. I could still work in the balsa over an epoxy base and then cover with epoxy, but it would be far easier to pour the foam over the epoxy base and then shoot in the epoxy over the foam. In my experience the potential for mess comes with poor preparation. I'm more concerned with the nature of your prediction that the repair would "fail". I would truly like to know what characteristic of the foam or procedure would cause this failure. It's difficult to hear that it will be amateurish, messy and fail without some explanation. Are you suggesting that the foam will compress or detriorate? ....it will not adhere to the epoxy? ......there are unforseen causes of voids in the work? I could easily be convinced not to use the foam, but I'd like to know why.

[model 10]

I don't understand why you compare your experience with end grain Balsa with 2x foam? The foam will break away from the load bearing surface leaving gaps and then go on to fail even more. If you had a surface with no point loading that caused movrment in that surface. foam would be great, really great.
I'm surprised you don't see honeycomb used more. It would be better than foam.

[Hudson Force]

Originally Posted by Guy

I don't understand why you compare your experience with end grain Balsa with 2x foam? The foam will break away from the load bearing surface leaving gaps and then go on to fail even more. If you had a surface with no point loading that caused movrment in that surface. foam would be great, really great.
I'm surprised you don't see honeycomb used more. It would be better than foam.

I'm not making a comparison between my experience with end grain balsa and the urethane foam,- I'm asking for a comparison. I have never used two part urethane foam; therefore, I want to know what to expect compared to my successes with the balsa. Your's is the first definitive information I've been able to get from anyone and it's marginal. You say that the foam will break away from the load bearing surface. This implies that the foam compresses under pressure and does not adhere to the epoxy. Compression and lack of adhesion are huge negatives. Is this what you have observed? Im in short supply of facts. What do you mean by "go on the fail even more". Are you saying that the foam shrinks, decomposes, disinegrates.....?

[model 10]

Sorry for the marginal info. It's all I got.
Look at a failed component that used a foam core. You may have a good bond at the glue line but the foam fails right next to that and under continued use the gap gets ever wider beccause it just keeps compressing and with the failed glue joint the whole thing turns into a mess. Unles I misread your OP, you seemed to be comparing the structural properties of foam to end grain Balsa. End grain is not that that compresable, if at all.

[colemj]

I wouldn't do this.

If you insist on going with poured foam, don't use anything below 8lb density. Your challenges have mostly been listed already: you have only seconds to do the pour, if you mess up the pour in these few seconds you are screwed, the foam at this density expands ~10X and will rip the skins apart if it isn't relieved, the expansion into a closed dimension will almost certainly leave voids unless relief cuts are provided - which makes as much mess and finish work as simply cutting the top or bottom skin and recoring with structural foam or balsa, and you are making a large assumption that the bonding sides of the intact skins are prepped sufficiently for complete bonding to the foam - if they are moldy, dirty or wet, you won't have a bond.

I don't understand how you plan to remove the rotten balsa while leaving both skins intact AND provide a prepped surface for the core bond.

BTW, pour foam is closed cell, so I wouldn't worry too much about the water absorption. That is your only bright spot down this path IMO...

Mark

[Hudson Force]

Of course the producers of the two-part foam deny that their high density foam will compress. Once again, I'm not comparing the foam to the balsa,- I'm asking for the comparison and thanks for your observation that the foam is more compressable than the balsa. Was this a high density foam sandwiched between epoxy? Your speaking of experience with a material glued to foam and "glue joints". This might be an application far differrent than a layer of 3/8" foam laminated between two 1/4 inch layers of expoxy saturated cloth. My balsa repairs have no "glue lines". I'm not sure what your application was that showed your compression and separation.

Don't get me wrong! I'm ready to discard the 2 part urethane foam application, but I'm not talking about gluing formica to foam.

[Hudson Force]

Thanks Guy & Colmj, I need to be more specific with my project. I have areas of my deck that I have replaced the core with end grain balsa by cutting away the top layer; removing all the old damp core; laying the new balsa on a yet uncured wet epoxy layer overr the bottom; grinding down the balsa to an even layer beneath the top deck; rebuilding the top layer with fiberglass cloth layers and a final coat painted with epoxy paint with quartz sand to match my non-skid. I have areas of deck without non skid that I'd rater not cut away for cosmetic reasons, but I will still have adjacent cut-away portions where I will use the balsa. These sections are no less than four inches wide and sometimes as much as twelve inches. I will have large areas of open deck on two sides of these spaces for full release of foam pressure under expansion. Certainly, I can work under these bridges of deck and insert balsa, but I will not have the same access to easily fill the space remaining with the epoxy. I also have a void under a cockpit coaming where the fiberglass is a full half inch thick and structurally sound with no "soft spots", but the space is inaccessable to fill without major cutting away. I can leave these places void, but I was considering a low density foam here as insulation only. So my application would be to use the tw part urethane foam as an added support for under strips of deck that are not non-skid and spanning 4" to 12" areas.

So, quit on the foam idea or use it in this application?

[model 10]

The foam makers are perhaps talking apples and I may be talking oranges. Over a wide area the foam has the properties they tout. If your laminate is thin or flexable enough and wack with a ball peen hammer you will have a void at the point of impact. The foam fails at that point, where as with end grain Balsa it would not. The glue joint would still be fine in either case but with foam it has compressed and sheared from the glue. What you have left is just a very thin layer of foam and glue attached to the fiberglass then an air gap or void
I think we are exceding my ablity to exlain any more. I am a mechanic not an engineer, good luck.

[Hudson Force]

Originally Posted by Guy

...............I think we are exceding my ablity to exlain any more. I am a mechanic not an engineer, good luck.

All your information has been valuable to me. I'm just pushing to get all I can! Thanks!

[hellosailor]

Force, I wouldn't use foam for this. The deck skin will flex and every time it flexes, the foam will crumble a bit in response. Eventually you've got all crumbled dust in there and no more "foam" and you're back to square one.

AFAIK pourable foams are not necessarily open cell, the bubbles can be isolated and that leaves a closed cell foam. The fact that they are used for waterproof vaportight home insulation should be the clue that they have to be closed cell in order to prevent water and air from migrating through them. I wouldn't doubt if the real truth is somewhere in between and varies with the exact materials. But I still wouldn't use it for anything but a stopgap "get us home" repair.

Something like a filled epoxy (microballoons or vermiculite dust) that actually creates a rigid block of "plastic" with enough strength not to break down and go crumbly, if you really don't want to cut up the deck skin. The folks at West Systems (Goudgeon Bros.) are the usual culprits to get top-quality tech advice on that front. Always worth a free phone call for truly expert advice.

[Captain Bill]

PPourable foams are almost all urethane based. Urethane foams make great flotation and insulation, but lousy structure. Urethane foam is weak in shear and is brittle. Every time the deck flexes the foam will break free from the skins and impacts will cause it to fracture. Foam used in hulls and decks for structure is most often based on PVC. PVC foams flex and stretch a bit so they work well in a dynamically loaded structure. The problem is that they require pretty specific conditions to foam and they are not easily replicated so I am not aware of any pourable version of these foams. I think the idea of using Epoxy with a high percentage of microballons has some merit, but stay away from the pourable urethanes.