Liquid rubber polyurethanes


#1

Hi there,

For a while now I have been making parts to sell from PLA plastic via 3D printing.

What I really need, however, is a way to make these parts from a semi flexible, non-brittle rubber/silicone.

I have found items such as:


and it looks like it could be suitable (I could make or 3D print a mould so that’s not an issue)

However, what’s complicating matters for me are the warnings that these liquid polyurethane rubbers can irritate skin. My part would be worn touching the skin for long periods of time (consecutive days etc, maybe up to a week or even every day constantly) without much of a break. Due to this, I need to be sure that the product isn’t going to cause a negative reaction for someone who uses the product.

From what I have read, it seems that these polyurethane rubber warnings are more to do with the stage before it has been mixed and has had chance to set. It’s not an issue for me if it is irritating to skin during this stage…but how about after it has set? Does anyone know if skin irritation due to the chemical make-up of the rubber is possible? If so, how likely?

I appreciate that these are not easy questions to answer and I am not looking for any legal guarantee…just the opinion of someone who has done something similar and observed the results!

Any answers or help would be greatly appreciated!

Best wishes,

Dan.


#2

In Most cases once cured it is inert… Some people could have a reaction just like they have a reaction to latex.

Added
One thing with Flexible Urethane. Make sure you use a good silicone release/Barrier agent… That stuff bonds to everything. Preferably Silicone/RTV molds are the best.


#3

wow…that was quick!

Thanks for your response, really appreciate it :+1:

Definitely something to look into then…


#4

Whether a product like that is skin-safe when cured is often going to depend on how it’s processed, especially if the liquids are not safe. I would try to shop for a flexible urethane (or silicone!) that is advertised as skin safe/biocompatible/fda-approved-for-food-contact/medical-grade or something Iike that, and has very detail instructions about what you need to do to make sure it meets that standard when it is cured. Bonus points if there’s a test you can run on your part to make sure it’s properly cured.


#5

Ordinary compounded urethane elastomers are NOT suitable for prolonged skin contact.

anything short of a perfect proportion perfectly mixed is going to have uncompounded material that will sweat out and is toxic.

Thermoplastic Urethanes can be medical grade and safe- but they require injection molding. You might be able to make an injection mold to inject TPU parts on the form 2 using high temp resin.

However you CAN mix and pour certain platinum cure silicones that are safe for prolonged skin exposure.
Given that compounded urethanes can cause allergic bronchitis, and skin reactions, I would recommend you source a Food Grade platinum silicone with very low cure inhibition.


#6

Thanks for your responses guys :+1:

My issue is that I need something around shore a hardness 70 (not 100% sure yet), and the skin-safe or medical grade rubbers/silicones all seem to be in the 15-30 range…

Maybe I’ll have to reconsider the process I’m going to use,

Thanks for your help!

Dan


#7

Maybe not quite as hard as you would like but here is a platinum-cure silicone with Shore 30A:

Smooth-On makes great products. Maybe consider having an insert to stiffen it up if necessary.


#8

you can find silicones in shoreA- 60 to 70
Silpak makes some-


#9

Thanks both of you :+1:

I am a little confused about the differences (of if there are any at all) between rubber urethanes and silicones, in terms of safeness for prolonged skin contact?

@Sculptingman are these shore a60/70 silicones suitable for prolonged use on skin? If so, they could be perfect:+1:

Thanks again,

Dan


#10

So I am not sure what you are doing that has prolonged skin contact.

I am in medical devices and we are required to do biocompatibility studies if there is prolonged contact with the skin. Some manufacturers have a strong master file with biocompatibility testing and you can rely on that rather than running the testing yourself (which is usually ~$20k and takes ~3 months). So if you locate a material you are interested in you can contact the manufacturer and ask them if they have a master file with biocompatibility testing. I would strongly recommend finding a supplier with the right testing on file.


#11

Ordinary skin contact does not require a surgical grade silicone.
Most any platinum cured system would likely do- but to be safe- i would opt for a Food Grade material.
You simply ask the suppliers if they have a food grade silicone between shore 55 and 70.

Be aware, tho, that silicone does NOT ‘breathe’- prolonged contact with the skin will cut off the skin from oxygen and not allow sweat or oils to evaporate- this will at the very least cause the kind of issue you get from having a bandaid on for a really long time- that white, slimy, deadlooking patch of skin.
And Some folks will still have a reaction because of the sweat and skin oils accumulating and causing irritation. They are not reacting to the silicone- but to the fact that it traps their own exuded toxins against their skin.

this is why rubber insoles are made of foamed rubber and even then covered with a fabric- to allow some air circulation and the wicking of moisture away from the skin.