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KStock (Materials) (OP)
25 Sep 10 12:20
I am in the running athletic footwear industry and have designed an oval shaped part positioned in the midfoot for the pupose of high energy return after deflecting 50-70% when loaded.  Pebax 7233 is performing perfectly due to its high flex modulous, energy return, and ability to withstand cyclical fatigue (does not seem to take a set), low crack propagation, etc.  The only problem... it's cost prohibitive.  
   What do you think is the best substitute (Nylon 66, 6, 12, 6/12,  Hytrel, TPU, etc.) and will it perform close to the Pebax 7233 for less cost?   Thanks!     
Demon3 (Materials)
25 Sep 10 16:17
I'd search for a polymer with properties like Pebax 7233 which are listed here:

Places to search free include and the Campus Database.

Chris DeArmitt PhD FRSC CChem
Consultant to the plastics industry

patprimmer (Publican)
25 Sep 10 19:56
Polyester block co-polymer elastomer's Like Hytrel (DuPont) Arnitel (DSM) Riteflex (Ticona) and one I cant remember by LG are the polymers with most similarity to PEBAX.

TPU will be to soft unless it has some fillers.

Nylons without an elastomer block co-polymer component cannot get the combined flexibility and resiliency of PEBAX

Closest will be type 12 with some plasticiser, but it will still fall well short.

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KStock (Materials) (OP)
26 Sep 10 13:56
Thanks Chris and Pat!
   Pat (and Chris if you'd like), if I understand you correctly,  a polyester block co-polymer elastomer (Hytrel) or similar brand is by far my best choice for acheiving required flexibility while maintaining a degree of stiffness and energy return, correct?  
   I was thinking of trying a nylon with some glass content for needed stiffness... but since it needs to deflect 50-70% (imagine a hollowed out oval shaped donut hole being compressed)...I suppose the glass would diminish my energy return as well as make it succeptible to cracking and cyclical fatigue (donut becomes a pancake over time)?  Going the other way and adding a plasticiser would diminish the stiffness that I need to support the body when running... would you agree?
    Any idea if how the polyester block co-polymer elastomer class of material compares with Pebax in terms of cost?  No problem if not, I will contact a Sales rep and go from there.  Thanks for all your help!
patprimmer (Publican)
26 Sep 10 17:57
Hytrel types are expensive. PEBAX is VERY expensive.

I am somewhat perplexed by your requirements that it be both stiff and flexible.

Glass filled nylon, while very strong and rigid is not at all flexible and has very low elongation at break. It has very good recovery from deflection for a thermoplastic, however that deflection will be very small. If this sounds confusing, yes I am confused.

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Pud (Mechanical)
26 Sep 10 18:57
So what's wrong with NBR (Natural Rubber!)
Why is there no synthetic elastomer used for car tyres?
Ok of course if black is what you want...

You could investigate TPV's - e.g.

I have no idea on relative costs apart from medical grades are $$$$$! Shoe stuff may be more competitive.


patprimmer (Publican)
26 Sep 10 19:54
Maybe I miss read the application.

I was thinking the sole of the shoe as I know both Hytrel and PEBAX are used for soles and studs on top end football boots where flexibility in the arch area and good abrasion resistance is required in the one moulding.

If you can design the part around the flex mod of the material and it can be hard rather than an elastomer, ie it is a spring, not bladder, acetal might be good. Acetal is used extensively as key return springs on computer keyboards and some mobile phone and calculator keypads.  

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KStock (Materials) (OP)
26 Sep 10 22:30
Hi Pat,
Sorry if I confused you.  I'm attaching a photo of the side profile of the shoe with the oval shaped injected molded plastic part circled (located right under the arch).  This is a photo taken of the shoe just before impact.  When the shoe is loaded, the oval compresses 50% or more... and needs to return this kenetic energy as much as possible.  As you can imagine, cyclical fatigue is a main concern... and the part must have the proper stiffness acheived through geometry and material (must also have an element of flexibility).  Too stiff and it feels like a rock under your foot.   Too flexible and it bottoms out and is not resilient.  Hope that helps.

patprimmer (Publican)
27 Sep 10 5:11
I will backtrack and suggest Hytrel or equivalent with a shore D of about 70.

How is it retained in place. Will adhesion or lack thereof  be an issue.  

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KStock (Materials) (OP)
27 Sep 10 14:21
Hi Pat,
It will be cemented (contact system using solvent based primer and cement) to the sole (bottom) and upper (top).   I know that Hytrel is often used in plates in Basketball shoes... I suppose for the same reasons Pebax is used (flexbility and energy return with good durability).  I don't know why I didn't think of it until you suggested it.  You have have been a tremendous help!  Thanks.

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