Compounded PC + radiation = discoloration?

[prdave00]

I am looking at using polycarbonate (Bayer Makralon 2458) resin compounded to add colorant for some parts. Unfortunately I don't know the specifics about the colorant/additives. We traditionally EtO sterilize, but are now exploring radiation for speed. Any ideas on if I should expect discoloration issues even though colorant is added? Bayer's brochure suggests that discoloration is only an issue for transparent, colorless parts made from grades that are not stabilized for radiation. I also realize this may be a case by case basis since not all radiation cycles are the same and not all compounded polycarbonate material are created equal. Right now I'm fishing for practical advise.

Any other concerns I should have (or keep in mind) other than discoloration? Are color additives known to change in appearance after being exposed to radiation?

 

[Pud]

Gamma sterilization causes yellowing of the base polymer. PC grades for sterilizing usually have a blue tint to offset this effect. The clear grades end up with a neutral(ish) grey tint. (Yellow is considered objectionable, apparently...)

So really, your problem will be how stable the pigments are, and how the yellowing of the base polymer will affect the final desired colour. (e.g. I can envisage a bright white being difficult to achieve.)

Trials in order I guess.

H

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[prdave00]

Thanks Pud It never occurred to me that it was a color enhancement to offset the yellow color shift. I always figured it was some fancy additive or the resin was irradiated under special conditions to stabilize it. I wouldn't say the yellowish-green color is a problem from a cosmetic standpoint, but moreso because you lose some of the transparency.

I posed the same question as in my OP to Bayer, but it sounds like I should run it by the compounded and/or pigment providers. I think you are right that trials are ultimately in order.

 

[patprimmer]

If it is 2458 FBL, the FBL means it is blue tint clear, not natual which is straw tint.

By adding blue tint to the natural yellow tint you end up with a neutral grey tint.

To stabilise against extra yellowing above the natural level, you need more blue tint, but it may then appear blue until the resin yellows further.

Dark colours will cover it. A deliberate dark straw tint will cover it fairly well as a slight increase in colour strength is not so noticeable as a shift in hue.

A light fawn or solid green or blue will show as they are sensitive to even slight change in yellow.

Regards
Pat
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