Composite fiberglass/foam panel build question

#32
I wanted to know more about that topic and called our plastics engineer in Germany. He told me that they do very expensive tests in a lab to simulate years of usage on the road. Standard hardware store foams failed! They simply fractured or turned into dust. After all they are designed to be used in buildings that don't move.
The XPS foam used for panel manufacturing appears to be the same as the hardware store material, only with an added planing process to acheive better tolerance (I can attest to the thicknesses of hardware foam being approximate and variable) and a better gluing surface since the cells on the outer surface are cut. If the thickness variability doesn't bother you, you can acheive a similar gluing surface by sanding. IMO the dog brush is much better than sanding as a surface prep, but you could do both easily enough.

OC panel foam: http://commercial.owenscorning.com/assets/0/144/172/174/3ebc8d51-67ac-4c49-a251-7db4d8f20fc9.pdf
OC hardware foam: http://www.foamular.com/assets/0/144/172/174/98cf58e1-c3d2-4b6c-beb5-2063215bea18.pdf

Boat builders don't use XPS because it is too weak for optimized strength/weight hulls. They use foams >5 lb/ft^3, while XPS ranges from 1.15 lb/ft^3 for 15 psi and 2.2 lb/ft^3 for 60 psi. But panels made for vehicles do not need to withstand repeated pounding of waves.

In my testing the foam core is definitely the weak link in flexural failure, but I don't see that as an issue. Skins need to be "overbuilt" (much stronger than needed for optimal flexural strength/weight) to resist impacts and denting, and panel thickness is primarily determined by the degree of insulation desired. I can jump up and down (hard) on a 6"x36" 2" thick wall sample (supported at the ends) without it breaking.
 
#34
The XPS foam used for panel manufacturing appears to be the same as the hardware store material, only with an added planing process to acheive better tolerance (I can attest to the thicknesses of hardware foam being approximate and variable) and a better gluing surface since the cells on the outer surface are cut. If the thickness variability doesn't bother you, you can acheive a similar gluing surface by sanding. IMO the dog brush is much better than sanding as a surface prep, but you could do both easily enough.

OC panel foam: http://commercial.owenscorning.com/assets/0/144/172/174/3ebc8d51-67ac-4c49-a251-7db4d8f20fc9.pdf
OC hardware foam: http://www.foamular.com/assets/0/144/172/174/98cf58e1-c3d2-4b6c-beb5-2063215bea18.pdf

Boat builders don't use XPS because it is too weak for optimized strength/weight hulls. They use foams >5 lb/ft^3, while XPS ranges from 1.15 lb/ft^3 for 15 psi and 2.2 lb/ft^3 for 60 psi. But panels made for vehicles do not need to withstand repeated pounding of waves.

In my testing the foam core is definitely the weak link in flexural failure, but I don't see that as an issue. Skins need to be "overbuilt" (much stronger than needed for optimal flexural strength/weight) to resist impacts and denting, and panel thickness is primarily determined by the degree of insulation desired. I can jump up and down (hard) on a 6"x36" 2" thick wall sample (supported at the ends) without it breaking.
I used OC 1" plus Filon on either side with 2 part polyurethane adhesive, no sanding/scratching and my panels were very stiff and took quite a bit of abuse during destructive testing. I'll be using DOW when I do my panels though.

 
#37
I used OC 1" plus Filon on either side with 2 part polyurethane adhesive, no sanding/scratching and my panels were very stiff and took quite a bit of abuse during destructive testing. I'll be using DOW when I do my panels though.
Cool!
How much does that adhesive cost?
Does it work better than epoxy? The stuff I've been using is Ebond brand and seems to work well. <$30/gal, about $40/gal including 2k mile shipping. You aren't using as much though, so price isn't such a big deal.

A potential issue with moisture cure is that there really isn't any way for moisture to get in. I used PL Premium on foam/plywood and it was fine, but you are gluing two waterproof materials.

Maybe ask the foam manufacturers what they suggest?

For adhesion tests I'd suggest making narrow samples and peeling them apart. I'm pretty sure a properly prepped foam surface will stick a lot better than "raw".
 
#38
For bonding any skins to a foam core, Id recommend a good marine grade epoxy.

Polyurethane adhesives have their place, but for the DIYer, the epoxy will be much more forgiving.

When using poly, you need considerably better/more clamp load to keep things in check, as it expands. Epoxy does not.
 
#39
I agree with what IdaSho is saying, epoxy is very easy and available, but the other thing to consider is the time required for epoxy vs polys. 1-2 hr clamp time for poly vs 12-24 for epoxy. The easiest set up for small shop is vacuum bagging for either adhesive. The invest can be rather small even for large panels. Pumps are often available used for reasonable, and you just need a large flat table and plastic sheeting. Especially if your only doing 6 panels for one box, durability isnt as important. Wood working and boat forums are full of advice, some of it even good!

My best answer is contact some of the larger suppliers for adhesives and talk to their tech. Most chemical companies have so many types its easy to get lost and they dont advertise all their offerings. Tell them the type of panels and the foam and they can make a recommendation. FRP and XPS is a very common lay up even in the US so there are plenty of options. The only problem is they are accustomed to dealing with production volumes using 450G totes so it might take a little talking and waiting to get smaller amounts. Henkel/Loctite is a great place to start, HB Fuller is another. Your regional distributors are another great resource.
 
#41
How much does that adhesive cost? Does it work better than epoxy? For adhesion tests I'd suggest making narrow samples and peeling them apart. I'm pretty sure a properly prepped foam surface will stick a lot better than "raw".
This stuff is about $130/gal. Pretty expensive but it works quite well.

Polyurethane adhesives have their place, but for the DIYer, the epoxy will be much more forgiving.

When using poly, you need considerably better/more clamp load to keep things in check, as it expands. Epoxy does not.
This poly was very forgiving and easy to work with. I plan on building a vacuum table for lamination.
 
#42
So what PU adhesive, specifically, are you using?

All single component are moisture cure, making and type of vacuum table lamination much more tricky.
 
#44
You are using a potting urethane?

The shore hardness is pretty low compared to an actual bonding adhesive or epoxy.

Have you done any actual bonding/adhesion tests yet?
 
#45
You are using a potting urethane?

The shore hardness is pretty low compared to an actual bonding adhesive or epoxy.

Have you done any actual bonding/adhesion tests yet?
I called Epic Resins and told them what I was doing and that is what they sent me. I've done quite a bit of testing and adhesion is great. Destructive testing showed the foam failing and no urethane failure. The test panels were incredibly rigid and took a lot of abuse.

Keep in mind the samples below had zero surface prep on the foam.