A closer look at the “cleaner” plastic

Meghan Lena | July 08, 2013 | Newsletter

In terms of toxic content, polyethylene is generally thought of as relatively benign compared to other plastics.

Polyethylene materials require little maintenance, are resistant to moisture and insects, and can be quite durable.  Add to that the fact that these polymers have significantly higher recycling rates than other plastics, and you’ve got yourself a marketable material with which to make certain building products, right?

A couple months ago, a manufacturer submitted a product for assessment by the Pharos building materials evaluation system. The product is a solid surface used for applications like countertops. It consists of nearly 100-percent recycled polyethylene. Our research standards and commitment to transparency led us to take a closer look at this material to fully understand if recycled polyethylene is indeed a “cleaner” choice for building products.

To our surprise, we found the potential for persistent bioaccumulative toxicants to end up in recycled polyethylene, primarily from nonylphenol additives.

Polyethylene manufacturing process

The basic process for manufacturing polyethylene is to polymerize an ethylene monomer. Most ethylene is created by cracking ethane, propane and butane—compounds that are recovered from processing of natural gas and of heavier feedstocks such as naphtha.[1]  The toxicological profiles of these feedstocks are not as severe as those associated with other types of plastics, like polyurethane or polyvinyl chloride.

Manufacturers add a number of additives to polyethylene plastics in order to delay or prevent weathering, improve clarity, and reduce static and slipping during manufacturing or recycling. Although additives only account for a small percentage of polyethylene content, they do bring with them trouble and as a result, hesitancy in declaring the recycled plastic a clear, “cleaner” choice.

One class of additives in particular raises concern: antioxidants.[2]  Antioxidants terminate oxidation reactions that occur during weathering and reduce the degradation of organic materials. 

We found that some very commonly used antioxidants in polyethylene plastics contain persistent bioaccumulative toxicants (PBTs). The chemical hazards associated with PBTs have earned them the highest priority for elimination from building materials. 

Oh no: more nonylphenols

According to the Phosphite Manufacturers Consortium, the antioxidant tris(nonylphenyl1) phosphite (TNPP) is commonly used in polyethylene manufacture.  TNPP is a nonylphenol compound, part of a family of PBTs that have gained notoriety since the Pharos Signal first discussed them in our 2010 article, “No, no, nonylphenols.” 

Last month, the US EPA summarized its concerns with nonylphenols like TNPP:  “Nonylphenol is persistent in the aquatic environment, moderately bioaccumulative, and extremely toxic to aquatic organisms.  Nonylphenol has also been detected in human breast milk, blood, and urine.“   NP is also an endocrine disruptor.

The manufacturing of recycled polyethylene requires up to 25 times more antioxidants than are used in the production of virgin material.  Up to 5,000 parts per million (0.5% by weight) of the recycled polyethylene may contain TNPP or other antioxidant.  

A 1993 article from Plastics Technology explains, “Because virgin and regrind suffer from the same degradation mechanisms, they both benefit from the same stabilization products, notes GE Specialty Chemicals lead chemist Suzanne Dietz.  The required loadings can shift upwards in recycling, though.  For example, an antioxidant typically used at 220 ppm in a virgin polyolefin will often be added at 1000-5000 ppm to stabilize recycle, Dietz says."

Cleaning up polyethylene

On the bright side, several companies have developed nonylphenol-free polyethylene manufacturing processes. Total Petrochemicals advertises grades of polyethylene used in diapers [link to brochure] that are free of TNPP and a Kuwaiti petrochemical company takes it one step further by selling only TNPP-free polyethylene resins in “response to ecological concerns surrounding some chemistries with similarities to nonylphenol…” 

For Pharos users considering using polyethylene products in their construction projects, it’s best to assume that HDPE products could contain chemicals that are harmful for human health and the environment.  Ask manufacturers what antioxidants and other additives are in their product  – and please let us know what you find out. The answers could pave the way for a truly cleaner polyethylene.

Endnotes

[1] There is a rise in ethylene production from bio-ethanol. Read more about this in this recent Pharos Signal article.  

[2] In addition to antioxidants, manufacturers routinely add antiblock agents, slip agents, antistatic additive, and UV stabilizers, which have their own varying levels of hazard warnings.  These are all described in our new “common ingredients” record for polyethylene additives.

Meghan Lena provides administrative and research assistance to HBN and the Pharos Project team out of the Vermont office.

Follow Meghan on Twitter: @meghan_lena     

Senior Researcher Jim Vallette contributed to this article.