Canadian Plastics

Anti-oxidants let resins age gracefully

By Jim Anderton,technical editor   



Getting a little sunshine and fresh air is so synonymous with healthy living that it's hard to get some people to wear sunscreen, despite the possibility of skin cancer in this ozone-depleted age. If ...

Getting a little sunshine and fresh air is so synonymous with healthy living that it’s hard to get some people to wear sunscreen, despite the possibility of skin cancer in this ozone-depleted age. If you’re developing or processing resins for outdoor applications, however, you’re likely keenly aware of what the sun can do to your products. It’s called photodegradation, most often photo-oxidative degradation, and how it operates can give you a sense of how to keep it from ruining your garden furniture without busting the bottom line.

If you’ve been following this column recently, you’ll recall that polymers are long chains of carbon atoms with various attached groups of atoms that give each resin its characteristic properties. Size, in this case molecular weight (which is really just the length of the chains), matters and anything that breaks a link in the chain is trouble. For general-purpose polymers exposed to sunlight and the oxygen in air, the ultraviolet and infrared components of the light have enough energy to occasionally knock a few of the hydrogen atoms that cling to the chain’s carbon backbone clean off the molecule. That wouldn’t be so bad, except that the oxygen attacks the now exposed area of the chain where the hydrogen used to be, like dryer lint on Velcro, and that new combination promptly rips another hydrogen off another molecule, creating the dreaded free radical chain reaction. Peroxides are formed in the process, which have an additional damaging effect by creating unwanted branching in the polymer main chains when heat is added.

How fast can polymers degrade? Unprotected PE films can turn completely brittle after as little as a month in the sun. Even worse, the thin skin of embrittled material at the surface of the product is highly susceptible to cracking at low stresses; the cracks can then propagate into the unaffected material underneath.

Want more bad news? The process can be accelerated by the presence of trace metal impurities, which act as a catalyst. “There’s no tramp metal in my press” you say, but for catalysis the small amounts left over from polymerization at the resin supplier, or at the surface of molded inserts or wire will do.

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What’s the answer? You have two choices: Change your material to something naturally more resistant (PVC is an example), use additives, or both.

UV-inhibiting additives work by one or more of several strategies. In the case of photo-oxidative degradation, a simple and cost effective strategy is to use a colorant that absorbs the damaging light wavelengths, which is one of the reasons why carbon black is popular in PE agricultural films.

Another additive technique is to attack the free radicals, stopping the chain reaction. Antioxidant additives are often classed as “primary” and “secondary”. Two types of common primary antioxidants are “hindered” phenolics and “hindered” secondary aryl amines (sometimes referred to as hindered amine light stabilizers). “Hindered” means their molecules are restricted and strained like a loaded elevator. They operate by slightly different methods; both operate by breaking up the free radical chain reaction. The amines give generally better protection than the phenols, but they can discolor or stain some products. Amines are popular in elastomers, where photodegradation can be a serous problem. Aromatic secondary antioxidants don’t work well alone, but are synergistic with primary types. Phosphorous and sulfur compounds are common varieties. What about the trace metal catalysts? Metal deactivators trap the metal ions and have some secondary antioxidant properties. And these choices are just the tip of the proverbial iceberg. Stay tuned for more on additive technology.

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