Yellowing: bromine fire retardants, ABS, and PBT

[ohaimark]

I posted something similar in the Great/Interesting Finds thread. I think this should have its own topic, though.

After realizing I parroted the "bromine causes yellowing" opinion I decided to do some (cursory) research. The results are below.

"Tetrabromobisphenol A is also used as an additive in acrylonitrile butadiene styrene (ABS)."

Source: https://en.wikipedia.org/wiki/Brominate ... _retardant

So we know it's in there.

The industry *knows* there are issues with UV stability in brominated plastics. Below is a document from ICL Industrial products. Admittedly, this is a PBT datasheet.

"FR-1410 (decabromodiphenyl ethane - 82% bromine) is cost efficient in
PBT but has limited UV stability and is not melt blendable during injection
molding."

Source: http://icl-ip.com/wp-content/uploads/20 ... 130729.pdf

The datasheet and article lead me to believe that less bromine strongly correlates with greater UV stability and weathering endurance, though the plastic itself can also be a large factor (perhaps the primary factor).

Source for the next paragraphs: Article where a guy actually talked with an expert -- http://www.vintagecomputing.com/index.php/archives/189

"The plastics most commonly used to make the structural cases for electronic equipment are polypropylene, impact styrene, and ABS," replied Deanin. "These all tend to discolor and embrittle gradually when exposed to UV and/or heat. They become oxidized and develop conjugated unsaturation, which produces color. They crosslink or degrade, which causes brittleness."

SNES Systems, according to a Nintendo reply, yellowed due to fire retardants.

"Deanin weighed in on the flame retardant issue only by saying that most retardants are harmless, but certain "aliphatic bromine compounds" are unstable to heat and UV, and should be avoided. Flame retardants unstable to heat! How ironic."

PBT can have bromines added to it -- the explanation for the long term color difference likely lies in the different manufacturing processes or chemical makeups of the plastics. ABS is known for being "weak" to UV light, meaning it could break down and release bromine compounds for reaction much faster than PBT.

My conclusion is that ABS plastic, when coupled with bromine, is the perfect storm for yellowing. Oxidation is likely the primary yellowing factor, but it seems that bromine fire retardants act as an accelerant in the process. I don't know if the yellow color of bromine is part of the extreme yellowing shown in ABS, but I think it could be a contributing factor due to the breakdown of the plastic and the reactivity of bromine (which is off the charts).

Any chemistry or thermoplastics experts here who would like to weigh in?

[andrewjoy]

I know that the better quality of ABS the like likely it is to yellow. The caps on my beamspring are ABS and they are still a loverly bright white after more than 30 years. Yet some cheap china crap you get can go yellow in less than 2 years .

[Chyros]

I made two or so extensive posts in an older topic about it where I explained the chemistry in detail. I'm on my phone right now so I can't link it but it should be easy to find. One basic thing is forget the goddamn bromine . I don't know why people keep bringing up the goddamn bromine xD .

[Halvar]

Chyros' post:

keyboards-f2/retrobrite-the-yellow-stri ... ml#p228424

There's also a discussion following that.

[ohaimark]

I made two or so extensive posts in an older topic about it where I explained the chemistry in detail. I'm on my phone right now so I can't link it but it should be easy to find. One basic thing is forget the goddamn bromine . I don't know why people keep bringing up the goddamn bromine xD .

So it isn't even an accelerant for the process?

[Chyros]

I made two or so extensive posts in an older topic about it where I explained the chemistry in detail. I'm on my phone right now so I can't link it but it should be easy to find. One basic thing is forget the goddamn bromine . I don't know why people keep bringing up the goddamn bromine xD .

So it isn't even an accelerant for the process?

MANUFACTURERS DON'T ADD BROMINE TO KEYBOARDS .

TBBPA is one of many chemicals present in industrial ABS which includes stabilisers, fire retardants, plasticisers, leftover catalyst, etc. Several of these compounds have been shown to accelerate the UV-mediated breakdown of ABS concomitant with the formation of carbonyls which are responsible for the colouring. Once degraded the materials can be temporarily whitened with retrobright but the material will still yellow again over time even in the absence of UV as decomposition occurs through reaction with oxygen.

[ohaimark]

So... I'm confused by what you mean. Not trying to be difficult. It sounds sort of like we're agreeing about the effects of bromine based fire retardants, but disagreeing that they're in keyboards.

Keyboards aren't considered industrial ABS?

Or TBBPA isn't considered a bromine based fire retardant?

Or you're certain that keyboards didn't have TBBPA or other bromine based fire retardants added to them?

[Chyros]

TBBPA is a bromine-based compound, but it is not bromine, which is Br2, which is what most people appear to think is sloshing around in keyboards. Bromine is a very strongly coloured compound so people think it colours their keyboards yellow. It doesn't.

Keyboards, or anything made out of ABS, is made out of industrial ABS. It is possible to make pure ABS, but this has no applications for anything as far as I'm aware (the material isn't very good). As such, the effects of additives is a more or less moot point because they're always in ABS materials - any ABS you'll get your hands on have this in it, and will yellow. Additives, for all intents and purposes, are inherent to the material, and in fact the literature usually treats it as such.

TBBPA is also not the only material responsible for the yellowing from what I've read, so I'm not sure why people are so fascinated by it xD .

tl;dr: ABS yellows. Forget about the bromine :p .

[ohaimark]

Understood. I didn't really specify whether I thought bromine or bromine based compounds were sloshing around, as it were, in keyboards. I don't think they dump elemental bromine in their plastic mixes. (Hopefully that clears things up.)

Now I'm going to be the devil's advocate.

I don't think it's a point that isn't arguable -- moot actually means: "subject to debate; arguable or unsettled." (Forgive me. English major nitpicking.)

So, even if an additive such as a brominated compound can accelerate UV damage (as mentioned in the PDF AND industry literature), it should be completely disregarded as a factor because everything has additives? What about the amount or composition of the additives? Could those things influence yellowing speed or UV stability?

I don't want to ignore something that may influence yellowing, even if it is not the primary factor. To give an extreme example... That's like saying "this man died of heart failure" while ignoring the fact that someone stabbed him in the heart, speeding up the process.

Would you agree if I said: "UV damage unrelated to bromine compounds is the primary source of yellowing, though bromine compounds can accelerate the reaction?"

[Chyros]

Fair enough . The following:

"I don't know if the yellow color of bromine is part of the extreme yellowing shown in ABS, but I think it could be a contributing factor due to the breakdown of the plastic and the reactivity of bromine (which is off the charts)."

made me suspect you thought it was elemental bromine, because TBBPA is neither yellow nor very reactive, whereas Br2 is very yellow and very reactive. You wouldn't have been the first though xD .

Probably a more accurate as well as more useful statement would be "UV light causes ABS to yellow, a process that's accelerated by additives and impurities in the material."

A technical explanation would be that "the ABS, specifically the polybutadiene part of it, is radicalised by UV, which leaves it vulnerable to oxidation by molecular oxygen. The subsequently formed species include esters, hydroperoxides, and unsaturated carbonyls. The latter are responsible for the yellowing, and are also capable of engaging in cross-linking reactions with the rubber component of the material, which results in destruction of the elastomeric properties of the material and a concomitant embrittlement and loss of impact strength. Some impurities and additives in the plastic are even more susceptible to radicalisation than the ABS itself, accelerating the process."

As for Retrobright, which usually goes hand in hand with discussions on this process, the best statement would be "Retrobright can temporarily reverse the yellowed appearance, but can't prevent the process from reoccurring, even in the absence of further UV light."

[ohaimark]

I assumed TBBPA was yellow. My bad!

Want to co-write a wiki page titled "Yellowing?"

[Chyros]

Haha, I would if I had time and experience with web design of ANY kind xD .

[ohaimark]

How about I write it, based on your previous posts and what we discussed, then you review it for accuracy?

[Chyros]

Yeah sure, can do .