Foam Cores PVC vs PET ??

Discussion in 'Fiberglass and Composite Boat Building' started by Mark C. Schreiter, Jan 19, 2021.

  1. Mark C. Schreiter
    Joined: Nov 2020
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    Location: Tampa, Fl

    Mark C. Schreiter Junior Member

    I recently started a PVC foam core build but along the way I stumbled on PET foam. PET foam is (at least from the company I looked) much cheaper per sheet. Everything "non" scientific I read about PET foam it seemed like a no brainer to use for a core build, however, i contacted the company and they almost insisted that I use their PVC foam. I've attempted to attach the spec sheets. the PVC foam is 4 lbs and the PET is 5.
    [​IMG]
    [​IMG]
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Last edited: Jan 19, 2021
  3. Alik
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    Alik Senior Member

    Look at shear strength. For same density, PET will have much lower shear strength compared to PVC.
    (Compare marketing values with care: I would take values from certificate rather than form brochure).
    Shear strength is major factor affecting structural calcs of sandwich...
     
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  4. Mark C. Schreiter
    Joined: Nov 2020
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    Mark C. Schreiter Junior Member


    Makes sense, what about for non structural components? how much does shear strength matter? say I build the center console out of PET and the structure of the boat out of PVC. does Pet work well with epoxy, same as pvc? is the decking in small boats considered structural?

    just curious.
     
  5. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    It is all that matters in the initial design as Alik has said. I tabulated it with other foam according to strength vs weight. It has low specific strength, again as Alik has said.

    To expound further, It will be heavier than other foam for the same strength you are looking for. It is in the same category as rigid PU foam, EPS, Styrofoam, and Polystyrene. If I am looking for a foam with a shear strength of 1.0 N/mm2, candidates are T500, A500, H80, all in the lower range of 100 kg/m3 or less than T90-150 PET which is in the 150 kg/m3.
     
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  6. fallguy
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    fallguy Senior Member

    As much as I like to be named in the company of rxcomposite; that would be unfair to him.

    there are really three foams afaic
    Pet
    Pvc
    San

    The engineering qualities vary. I decided for my build that the core price was not a significant amount of the total build cost. I spent about 10% of the build costs on cores and that was buying Corecell M, which is not the cheapest and has pretty great engineering attributes. I had a hard time rationalizing using a cheaper core. If the core I purchased was 80% of the cost of the M, it was a savings of 2% of the build. And I could not justify using a poorer core to save 2%. The original core order with M foam perfed for vac was about $12000 for the Skoota. Other cores get me to the 10%. If I had skimped on core to save 2 grand; it would have been foolish.

    If the core were going to cost a LOT more; at some point, it would become more relevant.

    For me, I really only compared the M foam and the Gurit pvc.

    I ended up with mostly corecell M. Some pvc.

    However, I did use the following:
    Plascore-for the panel size
    Aquaplas-for 20+# density transom
    Corelite-ditto aquaplas
    Coosa-for some backers and places screws will bear.
    Marine plywood. I regret this decision. I used marine ply for bench tops and one of them filled with water during a heavy rain period here before I had drains installed. The bench filled clear to the top and the ply got wet and swelled and cracked the finishes and the grains came through the paint finishes from swelling. Do again, coosa; despite it costing 5x marine plywood. I still have marine ply in the cabin out of the rain.
     
    Last edited: Jan 21, 2021
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  7. KeithO
    Joined: Jul 2019
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    Location: Michigan

    KeithO Senior Member

    Is adhesion to PET not another weak point ? Obviously any foam has a texture that would provide a certain amount of mechanical keying, but in my experience polyethylene as a base material is very slick and resists most adhesives. I think it is commonly used as abrasion shields on the bottom of boats and as wear plates on snowmobiles because of their anti adhesion properties. Teflon for example is great for a lot of things, but lord help you if you need to bond it to something. I think there might be an acid etch process used to prepare the surface prior to bonding but this sounds impractical for core materials.
     
  8. fallguy
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    fallguy Senior Member

    Airex makes no such recommendation.

    https://www.3accorematerials.com/uploads/documents/AIREX-Processing-Guidelines_09.2020.pdf

    However, Airex does not have an equivalent shear across similar densities afaik. One would anticipate a lower shear easier to delam is all; not fail to bond.

    I am awfully curious and will try to compare shear between Airex and Corecell M.
     
  9. fallguy
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    fallguy Senior Member

    Done under different standards.

    Airex T10
    Density 100kg/m^3
    shear len iso 1922 0.95 mPa min
    shear cross iso 1922 0.75 min

    Gurit M100
    Density 107.5 kg/m^3
    Shear strength astm c273 1.45

    Roughly 1.5 times better rating for the M, or almost 2 one way, but I am not qualified for a complete interpretation.

    Gotta clean some tools before they cook up!
     

  10. rxcomposite
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    rxcomposite Senior Member

    I never worked with PET foam on a commercial basis. Maybe I did as a sample test as we were always given sample products.

    We did however used Rohacell foam as a substitute for HT divinycell. Roha has a smooth surface and core bonding is an issue as we were using prepreg and adhesive film to bond the prepreg to the core. Film has limited thickness.

    We were told to abrade the surface of the foam by sandpaper or prick it with a small size nail. Later we were able to obtain prick roller. Pricking was better than sandpaper and the roller showed better consistency.

    In house drum roll test proved sufficient and local testing laboratory validated test result. Pricking provides good mechanical bond.

    I would never try that to a Teflon or polyolefin based material as it was not designed to bond at all.
     
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