I'm going to attempt to balance really complicated organic chemistry (with all types of variables depending on the plastic, mechanism, impurities, and other factors) with a framework on how to understand what occurs in the plastic during the yellowing process. A good portion of this article is taken from 1) Photodegradation and Photostabilization of Polymers, Especially Polystyrene and 2) The Handbook of Polymer Degradation. Both great reads if you have time!
When plastics yellow and degrade, they undergo three steps 1) initiation, 2) propagation, and 3) termination. Within these steps inactive products are formed, chromophores are altered, and the polymers are broken. Thus the plastic can become brittle, yellowed, or experience further negative effects. This process varies in nearly ever type of plastic, but if you have a great desire to read more about it, check out the #1 reference above.
So what generally happens during each phase?
- Initiation: The plastic is exposed to heat, light, etc., (those things mentioned in Post 3 in this series) and radicals are formed
- Propagation: The radical reacts with other compounds within the plastic.
- Termination: The radical fully reacts with other plastic and the plastic becomes stable again with a potentially altered chromophore (and hence yellowed color) or other undesirable effect.
Specifically looking at polystyrene (PS), when PS undergoes photoirradiation, yellowing is "due to the formation of conjugated double bonds or other types of chromophoric groups." So during the propagation phase chromophore bonds are created (an increase in pi-bonds) and the object goes through a bathochromic or hypsochromic shift.
A couple other interesting points relating to what happens in plastic include first, as I mentioned in Part 3 of Why Plastics Yellow, it's likely a surface effect, but propagates through the plastic over time. And second, "the existence of chromophoric groups in the macromolecules is a prerequisite for the initiation of any photochemical reaction" (Schnabel 1981). What that says is that chromophores must already be present within the plastic before any reaction with light can take place. If chromophores are not already in the base plastic material, there may be impurities which may be unintentionally incorporated to the plastic. These include internal impurities and external impurities which may serve as catalysts for yellowing.
Internal impurities which may contain chromophoric groups:
- Hydroperoxide.
- Carbonyl.
- Unsaturated bonds (C=C)
- Catalyst residue.
- Charge–transfer (CT) complexes with oxygen.
External impurities which may contain chromophoric groups:
- Traces of solvents, catalyst, etc.
- Compounds from a polluted urban atmosphere and smog, (polynuclear hydrocarbons such as naphthalene and polybutadiene)
- Additives (pigments, dyes, thermal stabilizers, photostabilizers, fire retardants)
- Traces of metals and metal oxides from processing equipment and containers, such as Fe, Ni or Cr.
I hope this post was enlightening to you. I tried to balance describe a framework which can be applied to most any plastic with solid technical details. Every plastic is unique, and there is nearly an infinite number of combinations of chemical reactions that can occur which are best taken on a case-by-case basis. I hope that this series of posts so far has provided you with a framework for understanding how plastics yellow!
And, if you've read till now, you are probably wondering "What about the Bromine?!?!". I've written up a post solely dedicated to that topic in Part 5.
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
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