Monday, November 4, 2013

Green chemistry for more sustainable carpet

Although we don’t usually think about the chemistry behind carpet fiber, it’s a major component of carpet sustainability because almost all face fiber is produced from synthetic polymers. Synthetic polymers were discovered nearly 100 years ago but today, green chemistry is rapidly changing the way they are manufactured as well as the polymers themselves.

Joshua Drew Vaughn/Flickr

Green chemistry is a design philosophy that seeks to reduce the impact of chemical substances on humans, animals, plants, and the environment. There are two primary ways to achieve that goal. First, maximize efficiency and reduce or eliminate unnecessary chemicals and processing steps. Second, reduce or eliminate hazardous and toxic chemicals to reduce the risk if exposure occurs. Whether applied to the initial laboratory synthesis or volume manufacturing, green chemistry offers a way to reduce the impact, risk, and expense of chemical production. The core concepts of green chemistry are best demonstrated through the 12 Principles of Green Chemistry:
  1. It is better to prevent waste than to treat it afterward
  2. Design syntheses to maximize incorporation of all materials used into the final product
  3. Use methods that use and generate less hazardous chemicals
  4. Design products that maximize performance while minimizing toxicity
  5. Use safer solvents (or eliminate them completely)
  6. Increase energy efficiency
  7. Use renewable feedstocks
  8. Minimize derivatives
  9. Use catalysts to minimize waste and energy
  10. Design for degradation into the environment
  11. Use real-time analysis to minimize byproducts
  12. Minimize the potential for accidents
What does all that mean for carpet? Let’s take a look at Nylon 6,6.

The traditional synthesis of nylon 6,6 is a two-step process that first combines cyclohexene and nitric acid to produced adipic acid, which is then combined with hexamethylene diamine under high pressure to produce nylon. Cyclohexene is a reasonably safe chemical, but it is produced from benzene, which is obtained from crude oil and is a known carcinogen. Nitric acid also poses environmental risks and the results in the emission of nitrous oxide (N2O), a greenhouse gas.

How can green chemistry improve this synthesis? Well, instead of using nitric acid, we can use sodium tungstate, Na2WO4, as a catalyst for the first step (Principle #9). The reaction can instead be done under mild conditions in water (Principle #5) and the only byproduct is water (Principle #3). Although using sodium tungstate introduces a heavy metal to the process, because it acts as a catalyst, it is not used up during the reaction and can produce large amounts of adipic acid before needing replacement.

While this is a step in the right direction, there are potentially greener ways to produce adipic acid. Last year, cancer researchers at Duke University derived a new enzyme (the biological equivalent of a catalyst) that catalyzes the conversion of 2-oxoadipate to (R)-2-hydroxyadipate (Principle #9). This is a key step in engineering a biological system to convert sugars to adipic acid. Instead of relying on petroleum, we might one day be able to produce nylon from renewably sourced sugar (Principle #7).

Producing carpet completely from natural feedstocks instead of petroleum would greatly reduce the impact of carpet production. In fact, synthetic polymers for carpet fiber that contain renewable feedstocks are already available. Dupont uses 37% renewable resources in their Sorona polymer, which produces fibers that are both soft and stain resistant. The production of Sorona uses 30% less energy and 63% less greenhouse gases than an equal amount of nylon. Sorona is used in Mohawk’s SmartStrand carpet.

We look forward to seeing how more of the 12 principles become incorporated into carpet production. And until we can shift to rapidly renewable feedstocks, the recycle of post-consumer face fiber remains an important component of continuing to reduce the environmental footprint of this important product.

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