What is the best Rubber for Watchbands? 2

[gciriani]

I need a tip from a real elastomer experts!
I'm looking for an improved NBR (Acrylo-Nitrile Butadiene) for watchbands, that lasts longer than what is usually found in the market. I'm actually not sure if NBR is the best rubber around for this job, or if there are FKM-, EPDM-based compounds that do a better job. I've perused without reaching any conclusion, datasheets available on the internet, and all threads in ENg-Tips regarding nitrile.

I understand that weathering of rubber is mainly caused by the polymer chains breaking down; this is accelerated by water, ozone, sun light (UV); please correct me if I'm wrong. What is the best elastomer mix to enhance this resistance?

Standard watchbands used in my current production have a specific weight of 1.21 g/cm3, and 70 Shore-A (I'm actually not sure if it rather is IRHD). I would like to have also a mild-to-reasonable chemical resistance to alcohol and acetone, since these are household items. Absence of odor is also important. Some customers of mine also request FDA compliance, in the belief that something that can be used in food processing is for sure non-allergenic to the skin. In future production I may be required to offer, besides the basic black color, also bright colors, like red green, yellow, blue.

Any good suggestions, or compound mixes?

 

[berti]

Hi all

silicone (VMQ) could be a good choice, maybe with addition cure. It is- provided correct processing including postcure- very skin-indifferent and is available in all bright colors.

I remember that WACKER as one of the big producers of the polymer had a wrist-watch as a giveaway.

HTH

Berti

 

[gciriani]

Thanks Berti! VMQ seems a good choice; is it also weather resistant? Do you think that FVMQ is good too?

An injection molding supplier said that NBR, start breaking down, even after 1 hour in salt water, and suggested EPDM. What would be the pros and cons of VMQ compared to EPDM?

 

[btrueblood]

EPDM will break down in the presence of any hydrocarbon oils or greases. Even most animal/vegetable oils/fats will cause it trouble over the long run. But it's excellent for water-based compounds.

Silicones are nice, but have poor tensile/tear strength.

 

[gciriani]

Thanks Btrueblood! Would you say that skin grease is enough to break down EPDM over the long run? And would the break down turn eventually into cracks?

 

[berti]

Hi all again,

FVMQ is maybe even better then VMQ but it has a very bad property: it is 10 times more expensive.

EPDM is probably not bad, one weak point is skin fat resistance, second is: transparent and bright-colored materials are difficult to make. On the other hand it is much cheaper even then VMQ so it is worth a try.


HTH Berti

 

[btrueblood]

Don't know how human skin oils/fats would affect EPDM. It is rated as a "2" out of 4 in the Parker O-ring Handbook for sealing against tallow and lard (1 is best, 4 is worst). Would expect some swelling and softening over time.

 

[gciriani]

Thanks both Berti and BTrueBlood for your tips. After all input, I'm leaning toward VMQ (especially because of the easier colorabilty).

However, the vendor assures me that there are scubadiving goggles made of NBR, and that any compuond can be tweaked to improve resistance in certain environments; it seems also that FDA compatibility can be met by any compound after some adjustment.

Other potential suppliers say that goggles are made of silicone, i.e. VMQ, to testify to the better properties of VMQ vs NBR or vs EPDM. As said I'm orienting myself toward VMQ, but I've ordered sample of all to test them against artificial sweat, salt fog, UV, and repeated pliability.

Thanks again, your help has been enormous in shaping my opinion. Should anything else come to mind, please give more feedback, either here or at Ciriani@Promotion-Spa.Com