TONY GOUGH
Director of Innovation For Sustainability, Innospec Ltd

Abstract

1,4-Dioxane is a trace impurity by-product which forms during the manufacture of alkyl ether sulfate surfactants and is therefore found at trace levels in numerous household cleaning and personal care cleansing products containing alkyl ether sulfates. Its presence in these products has been known since the 1970s and, although it is classed as a possible human carcinogen, at the low levels at which it can be present in these products it is thought not to be a risk to consumers during the use of these products. However, concerns over levels of 1,4-dioxane found in drinking water prompted a recent ruling in New York State imposing severe restrictions on the levels of 1,4-dioxane permitted in these products which will come into force within the next few years. In order to comply with these restrictions, products containing alkyl ether sulfates need to be adapted or reformulated and there are various options for formulators for achieving this.

WHAT IS 1,4-DIOXANE?

1,4-Dioxane is a synthetic organic compound with structure shown in Figure 1 and is completely miscible with water. It is a commercially available material on industrial scale with numerous uses (1) and is manufactured by several companies around the world by synthetic routes such as acid catalysed dehydration of diethylene glycol which is made from ethylene oxide.

However, 1,4-dioxane is also unintentionally formed as a trace by-product during the manufacture of certain ethoxylated surfactants and trace levels of 1,4-dioxane can therefore end up in household and personal care products formulated with them.

MECHANISM OF FORMATION OF 1,4-DIOXANE IN ETHOXYLATED SURFACTANTS
It is ethylene oxide (structure shown in Figure 2), the ‘ethoxylating’ reagent used to make ethoxylated surfactants, that is ultimately responsible for the formation of 1,4-dioxane by-product in certain surfactants. However, whilst ethylene oxide can dimerise under certain conditions to form 1,4-dioxane, this is not believed to be the main mechanism for its formation in the surfactants in which it is present. 1,4-Dioxane can also potentially be formed during the ethoxylation of alcohols to give ethoxylated surfactants, however, it is the additional process of sulfation (reaction of alkyl ethoxylates with sulfur trioxide (SO3) to form alkyl ether sulfates) which promotes the formation of 1,4-dioxane, giving detectable quantities of the 1,4-dioxane as a trace by-product (2).
The issue with 1,4-dioxide trace contamination is therefore mainly preponderant with alkyl ether sulfates such as sodium laureth sulfate (SLES). It is therefore important to note that that not all surfactants that use ethylene oxide during their production necessarily contain 1,4-dioxane as a detectable by-product. Indeed, the author’s Company has undertaken an extensive analytical study on numerous surfactants in its portfolio and found that the great majority of them had no detectable 1,4-dioxane, allowing the claim ‘1,4-dioxane-free’ to be made for them (3, 4). These ‘1,4-dioxane-free’ surfactants include alkyl ether carboxylates, acyl isethionates and acyl N-methyl taurates; all of which include use of ethylene oxide during their manufacture.

ISSUES WITH 1,4-DIOXANE AND IMPENDING NEW REGULATIONS

1,4-Dioxane is toxic and is classed as a possible human carcinogen by various authorities around the world. This classification is based on animal studies but there is insufficient evidence to show that it is carcinogenic to humans. 1,4-Dioxane as an impurity in cleaning products, especially personal cleansing products, has been under scrutiny since the late 1970s and numerous analyses have been done over the years on groups of personal care products to ascertain the levels of 1,4-dioxane present in these. Studies done in 1979 and the early 1980s on selected shampoo and bath gel products, etc showed 1,4-dioxane could be present in them up to levels of 279 mg/L (1, 5). However, over the years since then there have been advances in modifying the process conditions to reduce the amount of 1,4-dioxane produced during the sulfation of alcohol ethoxylates and also in the technology for stripping 1,4-dioxane out of alkyl ether ethoxylates after it has been formed (2).

It is important to note that the miniscule levels at which 1,4-dioxane are present in shampoos and body washes containing alkyl ether sulfates pose no risk to consumers from the use of these products despite there being a lot of erroneous information on the internet, etc surrounding 1,4-dioxane in this matter. The key issues with 1,4-dioxane are that it is highly mobile, it is not readily biodegradable and cannot therefore be easily removed in water-treatment and sewage plants which means that it can ultimately end up at trace levels in drinking water and thus cause a potential risk to humans and other lifeforms through cumulative effects. It is also important to note that there are other sources of 1,4-dioxane besides household and personal care products (including I&I) which contribute to the current levels at which 1,4-dioxane can be found in groundwater/drinking water in various regions in the USA. These include former industrial and military facilities, but the contributions of 1,4-dioxane from each of these sources to the total are not known and can vary from location to location.

Whatever the case, concern over 1,4-dioxane levels in drinking water in Long Island, USA prompted the passing of New York State Senate Bill S4389B by Governor Cuomo in December 2019 restricting the levels of 1,4-dioxane to 2 ppm for household cleaning products and personal care products by 31st December, 2022 and then to 1 ppm in these products by 31st December 2023. Many States in the USA have indicated that they will follow suite and adopt these same restrictions. However, the position on 1,4-dioxane regulations in the rest of the world is much less draconian. For example, there is currently no regulation on the levels of 1,4-dioxane in household and personal care products in Europe but manufacturers of personal care products are generally working to the Scientific Committee on Consumer Safety (SCCS) opinion of 2015 that “a trace level of <10 ppm in cosmetic products is safe” (6). For the ASEAN countries, the presence of 1,4-dioxane in personal care products was regulated to <25 ppm from June 2020 with a further restriction of <10 ppm coming into force in June 2023.

OPTIONS FOR FORMULATORS FOR MEETING THE NEW YORK STATE RESTRICTIONS
The New York State restrictions have prompted suppliers of SLES to provide 70% active grades with a maximum of 5 ppm of 1,4-dioxane. Use of 70% active SLES with 5 ppm allows formulations with a maximum of 14% active SLES to be made which will meet the New York State regulation (2023). A level of 14% active SLES is well within the levels at which conventional cleansers are typically formulated which means it is likely that most, if not all, personal care cleansing formulations containing SLES will not necessarily need to be reformulated and merely changing the grade of SLES to one with lower 1,4-dioxane content would suffice. However, these formulations are likely to be somewhat more expensive due to the higher cost of the SLES with lower 1,4-dioxane since SLES manufacturers will probably charge a premium for their low 1,4-dioxane grades due to the extra costs incurred from stripping the 1,4-dioxane levels down to a maximum level of 5 ppm.

Unfortunately, concentrated laundry detergents such as laundry pods containing SLES won’t be amenable to simply switching to an SLES grade with the lower 1,4-dioxane levels since, even at a level of 1,4-dioxane as low as 5 ppm in the SLES, the concentrated formulations are likely to exceed the impending New York State restriction levels. This is unfair since these products have the same 1,4-dioxane levels in the wash water as conventional products and are designed to have sustainability attributes such as smaller packaging and reduction of CO2 emissions during transportation. Concentrated laundry detergents will therefore need to be reformulated with surfactants that don’t contain measurable 1,4-dioxane.

As already mentioned above, for formulators who wish to formulate away from alkyl ether sulfates altogether due to their negative association with 1,4-dioxane, there is a fair number of ‘1,4-dioxane free’ surfactants on the market which can be used to create high-performing home care and personal care cleansing products (examples of 1,4-dioxane free surfactants are included in ref. 4). Furthermore, companies such as the one the author is affiliated to have great expertise in formulating with these surfactants and can help their customers create ‘1,4-dioxane free’ formulations for both home care and personal care cleansers which suit customer needs.

Many surfactants which are 1,4-dioxane free are also sulfate-free which is an ever growing trend for personal care cleansers and is also now an emerging trend for home care cleaning products. An extensive selection of examples of sulfate-free surfactants are included in ref. 4. It is commonly known that sulfate-free systems can be a challenge to thicken and often need inclusion of polymeric rheology modifiers to achieve desired viscosities. However, a popular sulfate-free (and 1,4-dioxane free) anionic surfactant is sodium lauroyl methyl isethionate (SLMI) (4) which, in combination with amphoteric surfactants such as cocamidopropyl betaine (CAPB), has excellent response to thickening with electrolytes such as sodium chloride (7). Another important tip here for formulators is that the viscosities attainable with SLMI/CAPB systems can be enhanced even further by using CAPB made from ‘topped’ or ‘stripped’ fatty acids (these are palm kernel oil or coconut oil derived fatty acid fractions which have had the majority of their C8 and C10 fatty acids removed by distillation) (7).

In summary, developers and manufacturers of household cleaning products containing alkyl ether sulfates face some challenges in adapting their formulations to meet the New York State regulations which come into force over the next few years, where they need to. However, the gearing up of surfactants manufacturers to either provide alkyl ether sulfates with sufficiently low levels of 1,4-dioxane and/or promote high performing surfactants in their ranges which are 1,4-dioxane free, along with providing formulating expertise assistance to customers requiring it, should lessen the burden on the customers of developing formulations which comply with the new regulations.

Figure 1. Structure of 1,4-dioxane depicted as its main conformer.

Figure 2. Structure of ethylene oxide.

REFERENCES AND NOTES

1. Interstate Technology and Regulatory Council (ITRC) document: 1,4-dioxane – History of Use and Potential Sources (2020) https://14d-1.itrcweb.org/history-of-use-and-potential-sources/ (2020), (Accessed 5th August, 2021), and references therein.
2. C. Bettenhausen, Chemical & Engineering News, 98(11), March, 2020.
3. 1,4-Dioxane content was analysed for with a level of detection of 1 ppm.
4. See the Innospec Product Guides for their full portfolio of surfactants and other products:
   - Personal Care Product Guide: https://innospec.com/wp-content/uploads/2020/09/EMEA-Product-Guide-3.pdf (Accessed 6th August, 2021).
   - Home Care Product Guides: https://innospec.com/wp-content/uploads/2021/07/IOSP-HC-Product-Guide.pdf and https://innospec.com/wp-content/uploads/2020/09/Dried-Surfactants-Guide-1.pdf (Accessed 6th August, 2021).
5. “1,4-Dioxane in Cosmetics: A Manufacturing Byproduct”, US Food and Drug Administration article, 29th January, 2019: www.fda.gov/cosmetics/potential-contaminants-cosmetics/14-dioxane-cosmetics-manufacturing-byproduct. (Accessed 12th August, 2021).
6. Scientific Opinion on The Report of the ICCR Working Group: Considerations on Acceptable Trace Level of 1,4-Dioxane in Cosmetic Products, 15th December 2015, SCCS/1570/15.
   (NB – According to EU Regulation 1223/2009 on cosmetic products (Annex II), 1,4-Dioxane is a prohibited substance (ie, cannot be intentionally added) but is permitted at technically unavoidable levels (Article 17)).
7. Presentation at In Cosmetics entitled “Advances in Formulating High Performance, Sulfate-Free Cleansing Products”, T. Gough, 2nd April, 2014. (Available on request).

ABOUT THE AUTHOR

Tony Gough has a PhD in organic chemistry and has worked in R&D/technical roles within the personal-care product industry for 33 years. He is Director-of-Innovation-for-Sustainability at Innospec UK and previously held management/senior-management roles at Unilever, Alberto-Culver, COSi, Surfachem and ISP/Ashland. Tony is also on the Praesidium of the IFSCC, becoming President of the IFSCC in 2022, and is a past-President of the SCS.