Nylon 6 and Nylon 6,6

5 06 2012

Nylon is a synthetic polymer called a polyamide  because of the characteristic monomers of amides in the backbone chain.  Polyamides are also naturally occurring – proteins such as wool and silk are also polyamides.

We commonly see two basic types of nylon used in fabrics: nylon 6 and nylon 6,6:

  • Nylon 6,6:  Two different molecules (adipic acid and hexamethylene diamine)  are combined to create repeat units of 6 carbon atoms, thus the name nylon 6,6.
  • Nylon 6:  Only one type of molecule is used in the formation of nylon 6, which also has 6 carbon atoms.  The repeat unit for type 6 nylon is made from caprolactam (also called ε-caprolactam).

Remember polyester is also a polymer (as are lots of naturally occurring things).  And like polyester, the nylon polymers are theoretically unreactive and not particularly harmful, but that’s not true of the monomers:

  • A small % of the monomers escape during production (off gassing or into water), which have environmental consequences.
  • With production expected to be over  4.4 million pounds/year by 2020, burden on water treatment facilities is immense.
  • Monomers are precipitated out during treatment, so they are present in the sludge.

The manufacture of both nylon 6,6 and nylon 6 uses cyclohexane as a precursor [1] – and cyclohexane is made from benzene, “one of the most challenging processes in the chemical industry”.[2]  Benzene is listed as a human carcinogen by the US Department of Health and Human Services.  It is associated with acute myeloid leukemia (AML), aplastic anemia, myleodysplastic syndrome (MDS), acute lymphoblastic leukemia (ALL), and chronic myeloid leukemia (CML)[3]  The American Petroleum Institute (API) stated in 1948 that “it is generally considered that the only absolutely safe concentration for benzene is zero.” [[4]

But the real culprits are the generation of unwanted by-products of nylon manufacture:  ammonium sulfate [5] in the case of nylon 6 and nitrous oxide in the case of nylon 6,6.

For nylon 6, the conventional synthesis route to caprolactam uses toxic hydroxylamine (NH2OH) and, in the last two steps, concentrated sulfuric acid. Every metric ton of caprolactam produces up to 4.5 tons of ammonium sulfate as a by-product [6].  As with many chemicals now in use, there is no data to evaluate ammonium sulfate as to toxicity to humans, though it has been shown to affect development, growth and mortality in amphibians, crustaceans, fish, insects, mollusks, and other organisms.[7]

In addition, waste water generated during production of nylon-6 contains the unreacted monomer, caprolactam. Owing to the polluting and toxic nature of ε-caprolactam, “its removal from waste streams is necessary”[8]

In evaluating the chief components of nylon 6,6  (hexamethlylenediamine and adipic acid), we find a darker situation.   Hexamethlylenediamine is a  petroleum derivative,  with the usual consequences of petroleum processing. It is considered “mildly toxic”[9] (though in one study, ten administrations of 700 mg/kg to mice killed 3 of 20[10]).   But the production of the other monomer,  adipic acid,  requires the oxidation of cyclohexanol or cyclohexanone by nitric acid, a process which produces nitrous oxide (N2O) –  a greenhouse gas 300 times more potent than CO2.[11]  A study published in 1991 credits the production of nylon – and the concurrent by-product of nitrous oxide – as contributing as much as 10% to the increased observance of atmospheric N2O.[12]  And this is a great concern, so much so that there is increased talk of our “nitrogen footprint”.

Nitrogen is one of the 5 elements (the others are carbon, hydrogen, oxygen, and phosphorus) that make life possible. It is essential for the creation of DNA, amino acids and proteins. 79% of the earth’s atmosphere is made up of nitrogen, but living things can’t use it in this form called dinitrogen (N2).  So in the nitrogen cycle, lightning  converts N2 into nitrate, which is carried to Earth by rain, where it enters the food chain.  When organisms die, bacteria recycles the nitrogen in them and it returns to the atmosphere.  Pretty elegant, isn’t it?

From: Nitrous Oxide Focus Group

But we have disrupted this nitrogen cycle.  A study by University of Virginia environmental scientist James Galloway and colleagues reported that from 1970 to 2008, world population increased by 78% and reactive nitrogen creation grew 120%.[13] The turning point, according to the International Nitrogen Initiative, came in 1909 when humans figured out how to combine hydrogen with N2 to create ammonia – which was used to produce fertilizer. Humans have introduced additional reactive nitrogen into the environment by expanding the production of soybeans, peanuts and alfalfa, (leguminous) crops which host nitrogen-fixing bacteria that convert N2 into reactive nitrogen. We use ammonia to manufacture nylon, plastics, resins, animal and fish feed supplements, and explosives. Fossil fuel burning industries and vehicles produce nitrogen emissions, and nitrogen is a component of the electronics, steel, drug, missile and refrigerant industries.

A single nitrogen molecule can cascade through the environment affecting air and water quality, human health and global warming in numerous ways(click here for a summary):

  • Runoff from agriculture—from fertilized crops fed to animals, from manure, and from biofuel and crops—enters rivers and streams and can contaminate groundwater. When nitrogen-loaded runoff makes its way to the ocean, it can result in eutrophication, where algae bloom, then die, depleting the oxygen and suffocating plants and animals. Runoff from urban areas, sewage treatment plants, and industrial wastewater also contribute to eutrophication.
  • Nitrogen is also a component of acid rain, which can acidify soils, lakes and streams. While some trees may utilize the extra nitrogen to grow, others experience foliage damage and have reduced tolerance for stress.
  • Our air quality is affected by nitrogen emissions from vehicles, fossil fuel burning industries (like coal), and the ammonia from agriculture, which cause ground-level ozone. High concentrations of ozone affect human respiratory and cardiovascular health and disrupt photosynthesis in plants.
  • Climate change is both influenced by and exacerbated by nitrogen. For example, nitrogen may stimulate plant growth, resulting in more carbon dioxide uptake in some forests.

Scientists have stressed the need to reduce fossil fuel emissions, improve wastewater treatment, restore natural nitrogen sinks in wetlands, and both reduce the use and increase the efficiency of nitrogen fertilizers. Galloway’s study also underscores the importance of better management of animal waste from the concentrated animal feeding operations that produce most of our meat today.

Another concern of using nylon is that all nylons break down in fire and form hazardous smoke.  Also smoke from burning nylon at a landfill emits the same chemicals,  typically containing  hydrogen cyanide, nitrous oxide (N2O) and dioxins[14].

Because nylon 6,6 is made from two different molecules, it is very difficult to recycle and/or repurpose.  Trying to separate and re-use them is like “trying to unbake a cake”.  However, nylon 6, because it is made from only one molecule, can easily be re-polymerized, and therin lies it’s claims to environmental superiority.  But  nylon production uses a lot of energy – about double that of polyester.  If recycling it uses about half the energy as is needed to produce virgin nylon, then recycled nylon and virgin polyester use about the same amount of energy.

Nylon 6 is becoming the new green darling of designers – but unless the recyling process captures all emissions, treats wastewater and sludge and also recaptures the energy used, the claim is tepid at best.  And nylon, unlike polyester, does degrade,  but slowly[15], giving it plenty of time to release its chemical load into our groundwater

I couldn’t find any data on the toxicity of nylon as fabric, but the government of Canada has evaluated nylon 6,6 because it is also used in cosmetics, and classified it as a “medium human health priority”; it is also on the Environment Canada Domestic Substance List.[16]  Another study found that some of the chemicals in nylon kitchen utensils migrated into food.[17]

[1] The remaining less than five percent of installed caprolactam capacity is via the cyclohexane photonitrozation process of Toray, which goes directly from cyclohexane to the oxime, or the SNIA Viscosa process, which utilizes toluene as feedstock and proceeds via oxidation-hydrogenation-nitrozation.  http://www.chemsystems.com/about/cs/news/items/PERP%200910_1_Caprolactam.cfm

[2] Villaluenga, J.P. Garcia, Tabe-Mohammadi, A., “A review on the separation of benzene/cyclohexane mixtures by pervaporation processes, Journal of Membrane Science, Vol 169, issue 2, pp. 159-174, May 2000.

[3] Smith, Martyn T. (2010). “Advances in understanding benzene health effects and susceptibility”. Ann Rev Pub Health 31: 133–48. DOI:10.1146/annurev.publhealth.012809.103646.

[4] American Petroleum Institute, API Toxicological Review, Benzene, September 1948, Agency for Toxic Substances and Disease Registry, Department of Health and Human Services

[6] Hoelderich, Wolfgang and Dahlhoff, Gerd, “The Greening of Nylon”, Chemical Innovation, February 2001, Vol 31, ppg. 29-40 and Weston, Charles et al, “Ammonium Compounds”, Encyclopedia of Chemical Technology, June 20, 2003, http://onlinelibrary.wiley.com/doi/10.1002/0471238961.0113131523051920.a01.pub2/abstract

[8] Kulkarni, Rahul and Kanekar, Pradnya, “Bioremediation of e-Caprolactum from Nylon 6 waste water…” MICROBIOLOGY, Vol 37, Number 3 1997

[10] “Handbook of Toxic Properties of Monomers and Additives”, Victor O. Sheffel, CRC Press, Inc., 1995

[11] 2007 IPCC Fourth Assessment Report (AR4) by Working Group 1 (WG1), Chapter 2 “Changes in Atmospheric Constituents and in Radiative Forcing” which contains information on global warming potential (GWP) of greenhouse gases

[12] Thiemens, Mark and Trogler, William, “Nylon Production: An unknown source of atmospheric nitrous oxide”, Science, February 1991, vol 251, pp 932-934

[13] Galloway, JN, and Gruber,  “An Earth-system perspective of the global nitrogen cycle.” Nature 451, 2008, 293-296.

[15] For nylon fabric, current estimates are 30 – 40 years.