Elastomer discoloration

[Sprocket59]

Hi All,

We use small suction cups on the equipment we sell. The material is a clear polyurethane. The cups we pull from stock (in clear plastic containers) often have discolored to yellow/amber. Aesthetics aside, I'm concerned about a loss of flexibility. Will the durometer be affected over time? Should we store the cups in airtight, light-proof containers?

Thanks for any input

 

[Demon3]

No harm in storing them in the dark and keeping them cool but a discoloration is usually not a sign that the material has degraded. Often the additives in it are discoloring. For example some antioxidants become yellow over time when in the dark, once exposed to sunlight it goes away.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

Polyurethane will gradually degrade and cross link when exposed to ultra violet light.

If it was originally clear or light coloured, yellowing will be apparent as this degradation proceeds.

It can be avoided by storing in a container that is opaque to ultra violet light. This is often also, but not necessarily opaque to visible light, and being opaque to visible light is not necessarily opaque to ultra violet light.

The degradation of polyurethane in ultra violet light will reduce flexibility and impact strength as well as causing discolouration.

The air will not do much damage, it is the ultra violet light and possibly elevated temperature that does the damage.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Compositepro]

Actualy air does slowly cause the degradation of many polymers. Various contaminants and impurities can accelerate this a lot. Free radicals is the mechanism. Many urethanes are also particularly succeptible to hydrolyisis. I've even seen where the polymerization catalysts in packaging materials cause problems in the packaged product.

 

[Demon3]

Pat's right about UV light. Are you storing these articles outdoors?

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[Sprocket59]

Sorry for the late response. My post was sent late Friday.

The parts are kept indoors in our stockroom at room temperature. There are overhead fluorescent lights that are turned off at night and weekends. The parts are shaded by the metal shelves above and the cardboard bins in which they're kept, but the worst discoloration is in parts that are at the end of a shelf where light can come in from the side. Does UV penetrate cardboard? What material would be a good UV-proof container?

To add a twist to this, I've had one of these parts on my desk for several years, unprotected, under fluorescent lighting, and it still looks good. The ones in stock are about a year old.

Thanks for the replys

 

[patprimmer]

I have seen cases where a chemical that leaches out of some cardboard has caused yellowing. I forget the details.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Sprocket59]

Thanks Pat,

Actually, the parts don't contact the cardboard bin. They have their own plastic containers as received from the vendor. These containers are then placed into the cardboard bins. I'm beginning to suspect a problem with the material itself. The clarity/discoloration differs with the batch numbers printed on the containers. I'm checking with the vendor to see if it's a quality issue.

 

[Demon3]

"Often the additives in it are discoloring. For example some antioxidants become yellow over time when in the dark, once exposed to sunlight it goes away."

So far that seems to fit well with what you're seeing. Try stretching them. If they fail at less elongation than the non coloured ones then it's degradation. If the elongation to failure is the same then it's just discoloration. Test more than one of each type to be sure.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[tom1953]

Besides antioxidants, there are UV stabilizers that can protect against yellowing upon UV exposure, some being better than others for non-yellowing. Also, if it's a soft urethane, there may be a plasticizer in the formulation that may be contributing to the yellowing.

As you suspect, and I do as well, there's some inconsistency in the different lots of material, whether absence of or incorrect antioxidant or UV stabilizer, or (if present) a variation in the plasticizer used, or something else.

The basic materials used in making polyurethanes have to be tightly controlled when making clear polymers. We fight with suppliers all the time to control the color of the polyols that we use for making clear (millable, rubber) urethanes.


Tom Jablonowski, TSE Industries, Inc.

http://www.tse-industries.com

 

[Sprocket59]

I've just spoken with the vendor, and they confirm a problem in manufacturing that has since been corrected. Our parts will be replaced.

Thanks for the education everyone.