Yellowing of polycarbonate urethane (pcu)

[prdave00]

I've noticed done yellowing on some tubes extruded from 80A pcu I've been evaluating. These parts are being cyclically compressed (5-10% strain) in a saline solution maintained at 90-100 deg F. The cyclic loading is being performed in a room with fluorescent lighting. I don't see this yellowing on samples held at the same temperature in a saline solution. I haven't sacrificed anything to cross section it to see if the bulk material is yellowed or just the skin. The material supplier hasn't offered a reason, so I thought I'd solicit some speculation from the various polymer experts that visit these threads.

There is still done investigation to be done. The control samples may have not been soaking at the same temperature and the saline concentration may have been lower than the test parts.

 

[Demon3]

Are concerned about color or actual polymer degradation?

Yellowing is common and usually comes from reaction of the stabilizer. For example phenolic antioxidants change to quinones which are yellow.

The amount of reaction needed to create a visible yellow color is tiny so seeing yellow does not mean there is any polymer degradation or loss in mechanicals.

Ways to prevent color would be to change stabilization package or add an extra ingredient to minimize color (you can find examples of that in the patent literature). Or add some blue dye. It's normal to add blue dye too mask yellow in polyesters and polycarbonates.

If you see a loss in mechanicals then again you probably need to change stabilizer. Probably use something that is higher molecular weight and less water soluble (so it stays inside the polymer).


Chris DeArmitt - PhD FRSC CChem

 

[patprimmer]

Also in addition to the excellent advice from Chris, does the real world use involve exposure to fluorescent light as these lights in the test area provide relatively high levels of UV.

Regards
Pat
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[prdave00]

Chris & Pat: Obvious material degradation such as surface cracking hasn't been noted, but my recollection (and I'd have to check with my colleague) is that there was some permanent set after application of the loading cycles. The concern is that this yellowing is an indication that the material and mechanical properties is changing such that the stiffness will be altered over its life time. This material is being considered for a medical application and I assume it's "cleaner" than an industrial grade equivalent, but I don't know if that precludes them from adding stabilizers, antioxidants, etc. I'll have to ask the resin manufacturer.

Typically these will see very little exposure to florescent lighting both before and during use. Unloaded samples, kept in similar environments under the same lighting conditions (although temperature and the concentration of saline might have been slightly different), have not exhibited this discoloration. One thought is that heating within the material due to cyclic loading at 5 Hz has accelerated oxidation of the surface, but our supplier hasn't commented on this theory.

 

[patprimmer]

Load cycling certainly increases polymer temperature. An increase in temperature increases bot oxidation and UV degradation and I would think degradation of stabilisers. I think the rule of thumb is for every 10 deg C increase in temperature the rate of reaction doubles. Note this is a rule of thumb, not a reliable accurate estimate and is randomly plucked from vague old memories.

Permanent set is due to creep under load which is a characteristic of all thermoplastics to some degree and nothing to do with degradation.

In fact if the polymer is degraded by UV it tends to initially increase in tensile and flex mod, but decreases in elongation and impact.



Regards
Pat
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[prdave00]

Thanks for all your help.