Why Plastics Yellow - Part 5: Does Bromine Cause Yellowing?

I decided to dedicate a whole post on this because bromine gets so much of the yellowing blame. It gets a lot of blame for this because of its yellow color. So does it cause it or not? There are arguments that go both ways. The word that summarizes my conclusion is: possibly.

Why not a hard no?

Most of the articles online simplify the process of adding "just bromine" to plastic as a fire retardant. While it's true that bromine is added, it's not elemental bromine and often in a very complicated compound. Simplifying it to bromine oversimplifies it. There are numerous types of Bromine Fire Retarders (BFRs), and it's never simply a matter of adding in “a little bromine” because polymer science is incredibly complicated. Some of these BFRs include: 

 

 

It’s important to note that some of these may not be yellow in color at all. Some may be white and often are in a crystal form. So, BFRs come in many different type and they may be yellow may not be. How do we know if they influence the plastic or not? Let's look at some experiments that capture controlled experiments. The next three sections are taken from the Handbook of Polymer Degradation (a great resource to understand this stuff by the way). They provide some interesting conclusions.

  •   "A yellow coloration was formed when irradiation was carried out with wavelengths less than 320 nm, for all additives [BFRs] containing HDPE (high density polyethylene) samples. No coloration was observed for the sames without the additive.” So, for the same type of plastic (HDPE), yellowing occurred with BFRs and no yellowing occurred without BFRs. So, it is reasonable to conclude that BFRs (the additive referenced) could have something to do with yellowing. Though it is not conclusive, it does lead one to speculate that neglecting bromine all together is not the best assumption.
  •   “The deepest coloration was observed on the irradiating additive-containing HDPE samples at 300nm. Little coloration was found for LDPE (low density polyethylene) containing the additives, except for the samples containing TBA [a BFR] as an additive when irradiated at 300nm.” So, it is reasonable to conclude that for various BFRs used, yellowing could depend on the type of plastic utilized. Again, not conclusive, but a very good data point.
  •   "Although the photogegradation of PS [polystyrene] estimated by these three methods showed some irregularities in the order of efficiency of flame retardants, the most efficient additive for the acceleration of the degradation of PS was TBA. On the contrary, ETBP or TBNPP can protect against the photdegrdation of PS. The photostability of PS containting ETBP or TBNPP was excellent in these experiment." So, it is reasonable to conclude that various BFRs can have different levels of effects on  yellowing and degradation. Again, not conclusive, but again, a very good data point.

So, with these three pieces of evidence, I think it's fair to say that BFRs are not off the hook, so to speak, in the plastic yellowing debate. Depending on the BFR selected and the type of plastic they are applied to, there may be numerous interactions which cannot be "hand-waved" away with a simple yes/no answer. So the truth is we don’t necessarily know if these BFRs do or do not contribute to yellowing without knowing more information about the specific applications. I would be skeptical of anyone who says they know for sure unless they have done controlled experiments with repeated results.

A special note about ABS & electronics: I think the same thing can be said for electronics. While I do believe you can make a large assumption that ABS is or isn't impacted by BFRs, I haven't been able to find as good of source as the Handbook on Polymer Degradation which discusses ABS yellowing of electronics. (If you know please let me know). The main reason why is that while ABS may remain the same from electronic part to electronic part, the type of BFR used may or may not contribute to yellowing. That's why I've wondered many times as old electronic components that have seemingly the same plastic and are exposed to the same environmental conditions have one part yellowed and one part that isn't on the same piece of hardware.

So there you have it! This research is why the answer to the question Does Bromine Cause Yellowing is "possibly". This isn’t immensely satisfying, but I think it is a mistake to simply write off bromine (BFRs) or blame it. Polymer sciences is incredible complicated and I think its appropriate to have some honesty and realize the enormity of this question. To truly answer this question we need to look at the specific plastic formulation, the type of BFR used, the wavelengths exposed, and then we may start to glean some some understanding. That may just have to be a subject for a later post, perhaps Part 6... but for now, I'm calling it.

I hope that your enjoyed reading this series of posts. As an engineer, it is fun digging into questions like this, doing the research, asking questions, and finding answers. If you have questions, comment, or feedback on this post, please email us at support@www.retro-brite.com. We'd love to hear from you!

Part 1: Why Plastics Yellow: Intro/Summary

Part 2: Why Plastics Yellow: Chromophores

Part 3: Why Plastics Yellow: Mechanisms of Degradation

Part 4: Why Plastics Yellow: What Happens In The Plastic?

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