Appropriate lifetimes, fitting deaths

The principle “Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products”, that appears 10^th in the original list of “Principles of Green Chemistry” (Fig. 1), published by Anastas and Warner,¹ must remain part of the fundamental basis of green chemistry. Consider the definition of two key words: principle – “a primary assumption forming the basis of a chain of reasoning”² and chemistry – “the branch of science concerned with the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances”.³ Clearly any practitioner of chemistry that purports to be ‘green’, must consider the end fate of the substances that are created and endeavour to ensure that these ultimately “do no net harm” (to borrow from the Hippocratian tradition of modern medicine, as interpreted by Sokol⁴). This has frequently been interpreted to mean that all compounds must be biodegradable, but this oversimplification can lead to the poor design of chemical products with lifetimes ill-matched to their intended uses. For example, many biodegradable polymers have been posited, but a robust, reusable, or recyclable, polymer that is kept in the (re)manufacturing cycle for as long as possible, could lead to reduced waste overall and, by enabling a “Circular Economy”, to reduced resource and energy use. In this model of a circular economy (an alternative to the more traditional linear economy, which has often been summarized as “make, use, dispose”) resources are retained in use for as long as possible, allowing the extraction of maximum value whilst in use and are then subjected to reuse, remanufacture, or recovery of materials at the end of each service life.⁵ Focusing on short term biodegradability can frustrate this potential for recycling at the highest utility, i.e. with least degradation of value. This is the tension between designing robust materials, amenable to reuse/remanufacture, that remain in the technical nutrient cycle, and ensuring that, at the end-of-life, these return to the biological nutrient cycle. None of this is to suggest that the principle is unsound, or has been superseded, but simply that it must be intelligently applied. Defining appropriate lifetimes and designing molecules and materials accordingly, may be as important as ensuring that these break down to innocuous products once finally reaching the end of their useful lifetime, which should be as long as possible, to reduce reliance on and exploitation of primary resources.⁶ Such appropriate lifetimes can only be defined by careful consideration of all impacts during the production, use and disposal of a product, as determined by life cycle analysis.

Fig. 1 The 12 principles of green chemistry.

To illustrate the concept of appropriate lifetimes, coupled to end-of-function considerations, we consider two large classes of chemical substances that are in wide use, in a plethora of applications and products: surfactants and organic polymers (as used in ‘plastics’).

Surfactants

Surfactants are ubiquitous in a range of formulated products, used daily by consumers (e.g. household cleaners, personal care products, cosmetics, anti-fog agents, and pharmaceutical formulations); in various industries (e.g. paints & coatings, inks, and adhesives); and in very large-scale applications such as agrochemical formulations, drilling and industrial cleaning, as well as fire-fighting and pollution mitigation (e.g. oil spill cleanups). They act as agents to achieve: wetting, cleaning, emulsification, foam control (enhancement or minimisation), and dispersion. As formulated surfactant containing products are dispersed during use (indeed, are designed to be dispersed during use), recovery/reuse, or recycling, of surfactants is seldom an option. In the majority of applications the end fate of the surfactant is an aqueous ecosystem (usually, but not always, via a water treatment plant). Not surprisingly then, surfactants were probably one of the first classes of chemical substances to be (re)designed with end-of-life in mind. For example, poorly biodegradable tetra-propylene benzenesulfonates were replaced with the more readily biodegradable linear alkylbenzenesulfonates in the middle of the last century.⁷ More recently, alkenesulfonates and fatty alcohol sulfates, based on renewable vegetable oils, have gained prominence and the demand for ‘milder’ surfactants (to human skin and eyes) as well as ‘natural’ products have prompted producers to revisit amino acid and sugar based surfactants, many of which show good biodegradability; this illustrates that, even at a molecular level, biomimicry can be a sound design principle. Similarly, natural biosurfactants, such as glycolipids, are also gaining in popularity, at least for specialist applications.

While biodegradation of the surfactant itself is important, the principle requires innocuous degradation products and even before this principle appeared in print, it had been recognized that breakdown products from alkylphenol ethoxylates (APEs) were more toxic to aquatic organisms than the parent APEs – indeed, nonylphenol is ca. 10× more toxic than its ethoxylate precursor and is an oestrogen mimic.^8,9

Clearly, there remains scope for designing effective, yet more readily degraded, surfactants that do not result in deleterious breakdown products, but an alternative strategy, that can be effective, is to replace significant quantities of surfactants with other materials. For example, dispersed, nanofibrillar oxidized cellulose can be used as a rheology modifier to generate stable reduced surfactant personal care products and emulsions (creams and lotions) with significantly reduced surfactant content.^10,11 This example illustrates the potential for mitigation of harm, by considering the impact of the whole product rather than individual components.

Considering the life cycle of inherently single-use substances, such as surfactants, suggests that the most significant impacts are in the use phase.^12,13 Given that impacts can occur at all stages of the life cycle, a systems approach, that includes all stages, is necessary to minimize overall impact. For products containing surfactants, LCA studies reveal that it is not always the chemical constituents of the formulated product itself that have maximum impact. For example, a recent analysis of six products using data provided by three large manufacturers (Henkel, Procter & Gamble, and Unilever): hand dishwashing detergent, compact powder and tablet laundry detergent, window glass trigger spray, bathroom trigger spray, acid toilet cleaner, and bleach toilet cleaner revealed that, for a number of the product classes, packaging had at least as great as, if not greater, impact than surfactant choice and quantity. Thus, transforming the packaging used could provide immediate environmental benefits, which leads to a discussion of the second class of chemical substance chosen as an exemplar: organic polymers.

Organic polymers

Materials prepared from organic polymers (so-called ‘plastics’) are ubiquitous in modern society, where they are widely used in applications ranging from packaging to building materials – in developed regions of the world these applications each account for one third of the total quantity of plastics used, with packaging rising to >40% in developing regions!¹⁴ Negative perceptions of plastics may arise from their association with excessive packaging, as major contributors to street litter and consumption of finite resources (fossil oil), but many also contribute to enhanced public health. For example, light, impermeable, but easily shaped, plastic pipes are an excellent way to deliver clean water.

Feedstocks for the production of polymers can be fossil-, bio- or waste-based (via bio-refining), and the products may be biodegradable, or persistent in the environment. Currently, the five most widely used commodity plastics – poly(propylene), poly(ethylene), poly(vinyl chloride), poly(styrene), and poly(ethylene terephthalate) – are largely produced from fossil oil and are not biodegradable. It is tempting to suggest that polymers for single use applications (e.g. food packaging) should be bio-based/made from bio-refined waste products, and be compostable, while selection of the base polymer for products designed for lengthy or multiple uses must be made on the basis of the longest possible life… but even this is too simplistic an approach!

What is needed is a life cycle approach to plastic use, to determine not only what is most technically appropriate, but also how the product is actually used and disposed of. For example, a plastic car bumper needs to be UV, moisture, and heat and cold resistant, coupled with strength and durability. Such an item has the potential to be reused in the designed form, rather than needing to be recycled to the base polymer and reformed, or bio-degraded. Clearly, in this case, the maximum value is returned when the bumper is taken from one vehicle and refitted to another (this assumes that car bumper design has not altered in the interim period) and this fits well within the context of the circular economy.

Packaging for items that require a level of security have a different function. Their function is completed when the items are removed, and indeed the packaging is no longer able to perform its function. In this case, there is value in being able to recycle the polymer, such that it can be reformed into new packaging. This is still within the ‘circular economy’, but has a higher associated energy cost and also assumes that the waste streams can be efficiently segregated and treated.

Consider the example of plastic bags. These items have been the subject of considerable attention over the recent years, with heated discussions ranging from whether the bags should be made of a non-biodegradable, durable plastic allowing multiple reuse of the bag, or biodegradable, single-use, thus ensuring that these eventually disappear when discarded. In isolation, a good technical case could be made for either of these options, but the best outcome depends on how the users of the bags actually behave. A consumer who disposes of a heavy duty plastic bag into general waste after just one use, is creating a waste that will remain intact in a landfill site for many years, and consuming more resources. A single-use biodegradable, or compostable, lightweight bag may use less resources and often has a second life as a waste bin bag before disposal in landfill. The drive to move away from landfilling towards anaerobic digestion of household waste also has influence here: heavy-duty, durable, multi-use bags cannot be composted, but could act as a feedstock for waste-to-energy conversion, while single-use, compostable bags can be digested, but have lower value as energy sources (as these are usually prepared from oxygenated polymers, with lower calorific value than hydrocarbons). Thus, waste policy influences the selection of the most appropriate type of bag for consumers to use, and should be determined by evaluating the costs and benefits in terms of resources consumed, as well as final end-of-life options.

This latter example illustrates the need for a holistic life cycle approach, which is inherently complicated. The complexity of analysing impacts without consideration of the systems within which these impacts occur, or those due to a single chemical component in a complex product, has led some to suggest that a cradle to gate approach is justified, but such reductionism must be avoided if truly greener chemical substances are to be designed. Meta-analyses, similar to those conducted in the medical sciences, could provide broad guidelines for whole classes of products.

Thus, the principle remains a valid guide, but cannot be used in isolation and certainly should not lead to the oversimplified conclusion that only biodegradable materials are ‘green’. A life cycle perspective is required to define appropriate lifetimes and fitting deaths. One answer to the question “what relevance does this principle have for today's (global) challenges?” is “it's a ‘wicked problem’¹⁵” and there is a need to also consider policy and regulation, which can rapidly shift the goalposts with regards to which substance offers the least impact over its entire life cycle.

References

1. P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, 1998.
2. “principle, n.” OED Online, Oxford University Press, September 2016. Web. 10 October 2016.
3. “chemistry, n.” OED Online, Oxford University Press, September 2016. Web. 10 October 2016.
4. D. K. Sokol, Br. Med. J., 2013, 347, f6426.
5. Growth within: a circular economy vision for a competitive Europe, report by the Ellen MacArthur Foundation, the McKinsey Centre for Business and Environment and the Stiftungsfonds für Umweltökonomie und Nachhaltigkeit (SUN), June 2015. https://www.ellenmacarthurfoundation.org/assets/downloads/publications/EllenMacArthurFoundation_Growth-Within_July15.pdf.
6. Closing the loop – An EU action plan for the Circular Economy, Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions, 2015.
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11. J. L. Scott, C. Smith and G. F. Unali, US2014148407, 2014.
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14. A. L. Andrady and M. A. Neal, Philos. Trans. R. Soc., B, 2009, 364, 1977–1984.
15. A. W. Churchman, Manage. Sci., 1967, 14, B-141–B-146.

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