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The addition of stabilizers is essential for maintaining the value and properties of plastics after they are recycled. However, the kind and quantity of stabilizers that are needed can vary according to the resin involved and the purpose for which it will be used.

Interrupting degradation

“The reason for stabilizers is, of course, they interrupt the natural degradation process for plastics through oxidation,” says Rudolf Pfaendner, division director for plastics at the Fraunhofer Institute for Structural Durability and System Reliability, based in Darmstadt, Germany. “By doing this, you get a material which can be processed thermally at high temperatures and which will last for many years.”

Processing stabilizers, long-term heat stabilizers and UV and light stabilizers are the most commonly added. While stabilizers sometimes are added when virgin plastic is produced and compounded, recyclers cannot assume the protection still is effective because it degrades over time, especially when plastics are reprocessed. The resulting plastic would exhibit inferior properties. For example, plastic that is used outdoors without the additional protection of a stabilizer can develop cracks and fade.

“That’s the risk you face if you don’t add the right amount of stabilizers for the applications you are going for,” says Pfaendner.

He says another challenge is that in most cases, recyclers don’t know the quantity of stabilizers added to plastic during its previous life. Additional stabilizers might be needed if the purpose of a polymer is changing from a short-term use to a long-term use. For example, a recycler might be taking a plastic used for a food container, where it was not exposed to much sunlight, and reusing it in an outdoor setting where a UV stabilizer would be necessary.

“The reason for stabilizers is, of course, they interrupt the natural degradation process for plastics through oxidation. By doing this, you get a material which can be processed thermally at high temperatures and which will last for many years.” -- Rudolf Pfaendner, Fraunhofer Institute

Songwon, a Korea-based plastic additives company with U.S. operations headquartered in Friendswood, Texas, cites a case in which a polymer that was used in a packaging application with a short lifetime requirement is recycled and returned to the processing stream for automotive applications. In that case, Songwon says, it would recommend a customer add its Songnox 3001-2, a long-term stabilizer.

Preserving molecular weight

Repeated recycling also can damage polymers through molecular weight degradation. Certain stabilizers can help preserve molecular weight, which is essential for mechanical characteristics. For example, Songwon says its Songnox 21B stabilizer protects the molecular weight of polymers, including polypropylene (PP) and polyethylene (PE), during processing.

In partnership with chemicals manufacturer Sabo SpA, Levate, Italy, the company also distributes Sabo Stab UV 228 50 PP, which improves the light stability of PP.

Recycling combinations

Within the past couple of years, Pfaendner says he has seen an increase in the availability of new stabilizers designed specifically for recyclers.

“There are more and more companies like Baerlocher, PolyAd Services [headquartered in Bensheim, Germany] and Mitsui [Chemicals] [headquartered in Tokyo] that offer products specially designed for recycled materials,” Pfaendner says. The companies put together specific stabilizer combinations that typically are needed for recycling, making it easier for recyclers than buying stabilizers separately. The prepackaged combinations also are designed to ensure proper dosing.

Baerlocher, with global headquarters in Unterschleissheim, Germany, and U.S. locations in Cincinnati and Dover, Ohio, offers custom Baeropol blends with stabilizers including UV, light and color, with resin-stabilization technology (RST). In October, it introduced Baeropol RST and Baeropol T-Blends, a line of preformulated polymer stabilization “one packs” that blend Baeropol RST with traditional stabilizers. An antioxidant that acts as a heat and long-term stabilizer, Baeropol RST can replace or reduce the amount of antioxidant phosphites recyclers normally add, according to the company. Baeropol RST and T-Blends allow customers to take advantage of the phosphites that already are contained in the resin.

While stabilizers sometimes are added when virgin plastic is produced and compounded, recyclers cannot assume the protection still is effective because it degrades over time, especially when plastics are reprocessed.

Baerlocher says its Baeropol RST also can reduce the need for traditional antacids. Many recycling streams are contaminated with polyvinyl chloride, as well as with organic byproducts, which can be acidic and can degrade polymer properties.

Baeropol T-Blends combine processing and long-term thermal stabilizers with RST “to provide economical, high-performance stabilization for the technically demanding and price-sensitive reprocessing markets,” according to the company.

Concerning costs

Some recyclers are reluctant to add stabilizers because of cost, but Pfaendner says it isn’t always necessary to add a lot of them.

“In any case, whenever you recycle and process, the addition of a small amount of stabilizers is always beneficial,” he says. “It’s often sufficient to add only 0.1 percent or 0.2 percent. We’re talking about relatively small amounts of additives. There is some additional cost, but sometimes if they realize they can sell a product with a higher value, it is no longer a discussion.”

The author is a senior staff reporter for Plastics Machinery Magazine and can be contacted at bgeiselman@plasticsmachinerymagazine.com.

For more information: Baerlocher USA, 513-482-6300, www.baerlocherusa.com; Fraunhofer Institute for Structural Durability and System Reliability LBF, 49-6151-705-0, www.lbf.fraunhofer.de/en.html; Songwon, 281-648-1585, www.songwon.com