elevated melt temp causing pigment degradation? 5

[dgowans]

I have a situation where parts molded from a 10% GF, UV stabilized, FR polycarbonate are changing colors, sometimes drastically, after a period of time (<1.5 years) in the field.

Consultation with the material supplier ruled out UV exposure as the cause because the discoloration is not consistently on UV-exposed surfaces. Chemical attack was also deemed not likely because there are two parts molded from the same material in close proximity to one another in the installed product. I have observed instances where part 1 is completely discolored and part 2 looks as if it just came out of the box, and vice versa.

Both molds have hot runner systems; in a recent conversation with the molder I was informed that the parts are often difficult to fill and that the melt temps have been elevated in order to eliminate this. One possibility that has been raised is that the higher temperatures are causing the pigment to degrade. We have not seen a color shift in parts as they are received, only after field deployment.

My questions:

1) Is pigment degradation a likely cause for discoloration over time?

2) If so, is the material (or UV / FR packages) likely to be degraded as well?

I'll offer up as much information as I can to answer any questions but am limited in what I can disclose (no pictures, unfortunately).

 

[btrueblood]

What is the pigment?

 

[dgowans]

Pigments are TiO₂ plus a combination of iron oxides.

 

[patprimmer]

FR and UV can interact at times.

FR can be degraded at top end of moulding temperatures.

I would expect both to show up at time of moulding.

It sounds like they have changed pigments to one with lower light fastness. Pigments can fade when exposed to visible light as well as when exposed to UV light.

Humidity and other chemicals can impact on light fastness. Some can have a catalytic effect on fading. I would suspect FR or degradation products from FR to be most likely to have a catalytic effect on fading.

I would not expect iron oxides not TiO2 to be prone to fading or catalytic effect. Look for a change in pigment formulation.

If you have a moulding that is faded in one area but not another get a full spectrograph of the unfaded part vs a moulding that was exposed but did not fade. A different spectrograph indicates a different pigment formulation

Regards
Pat
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[Demon3]

TiO2 can strongly degrade plastics when exposed to UV light as it generates radicals. In fact, that effect is used to make self-cleaning surfaces where the TiO2 destroys the dirt.

Also, all TiO2 is not the same. Each type has a unique coating to protect the polymer from being attacked by the TiO2. Thus you have different thickness and chemistry of coating on the pigment particles and that determines how color fast your system will be. So, although you say that your color change is not following the areas that are UV exposed, could it be that some parts have different TiO2? People tend to swap out TiO2 grades like they are interchangeable and they are not.

Raising the melt temperature will not make TiO2 or iron oxide degrade but it will increase their likelihood of reacting with your polymer or FR additive.

Iron oxides can be very detrimental to the long term stability of plastics. Any transition metal, like iron, copper, chromium, vanadium can destroy polymers (they catalytically decompose hydroperoxides if you want to get the technical jargon). Again, slight differences in grade can make a big different in stability. It is not the iron content that matters but the chemical form and availability of the iron.

Chris DeArmitt

http://www.phantomplastics.com

Consulting to the plastics industry

 

[Pud]

I thought Tio2 also degrades the glass fibres?

Can't understand why some parts are ok and some not though.

H

http://www.tynevalleyplastics.co.uk

 

[dgowans]

Thanks for the responses - very informative as usual. I did get the process setup from the molder and will review it with respect to the recommendations from the material supplier, although further discussion with the molder make me think that an elevated melt temp isn't the culprit. I could be wrong.

The potential effect that TiO2 might be having on the polymer has me concerned. I'll raise that issue with the material supplier when I get back in the office.

One thing that was perfectly clear to me when I submitted my initial post but I obviously didn't make clear to any of you is that the color change isn't a simple fade - the parts are starting life gray in color and are changing to a greenish color after field deployment. I'm told that the as-molded color does have a bit of a green tint to it (damn color blindness gets me again!) but it's nothing like what we're seeing in the field.

Hopefully this bit of information will trigger an AHA!! moment in one of you. I'm quickly reaching my limits on this one...

 

[patprimmer]

AHA

That makes Chris's explanation much more likely than mine.

The grey will turn green as the polymer degrades and yellows.

My explanation was based on the assumption of fading rather than yellowing.

Harry, I think (I don't actually know as I am recalling from long ago anecdotal evidence) rutile type TiO2 has a very sharp crystal shape and that wears equipment and breaks glass fibres.

Regards
Pat
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[dgowans]

Update:

Pat's initial response was correct; the reaction of the flame retardant used in the material to UV is what we're now suspecting as the root cause of the color shift. We exposed samples of the material that exhibited the color shift as well as the same material with no FR package to UV. The FR parts discolored rapidly, as expected, while the non-FR parts did not. Additional discolored samples returned from the field were much more consistent with UV exposure than the first parts I saw.

Fortunately, the parts that discolored maintained their mechanical properties when tested per UL 746C so we appear to be dealing with a cosmetic issue alone.

Why were some parts discolored when other adjacent parts were not? We had a supplier playing games with the material. In some cases, the parts were molded from the material we had specified - these didn't discolor. Parts molded from an "equivalent" grade were the issue. Said supplier has had their hand slapped, to say the least...

 

[Demon3]

Thanks for taking the time to update as that gives us valuable insight and helps us learn for the future.

I would suspect that a halogenated flame retardant would give off e.g. HBr and that would give discoloration as it leaves behind conjugated double bonds. As well as that, acidic species like HBr attack some UV stabilizers such as hindered amines.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Pud]

Cheers for the update - glad it's resolved! (sort of)

We never substitute "EQUIVALENT" materials - invariably they never are - without customer approvals - something always comes back and bites you in the bum!

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Sometimes the same grade from the same supplier is not even equivalent.

I have seen problems when raw materials suppliers change source for whatever reason.

I have also seen where a lack of respect for potential reactions with different stabilisers has caused an extruder barrel to explode and kill 2 people.

Regards
Pat
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