Textiles True Colours: How Sustainable are they?

By Brian Tsui, Website Coordinator for the GCI

The rise of fast-fashion trends in our global economy has led to a booming textiles manufacturing industry. According to the Ellen MacArthur Foundation, globally the industry consumes over 98 million tonnes of non-renewable resources per year.1 The majority of these items have seen less utilization compared with early 2000s. With an increasing consumer awareness of eco-friendly and sustainable practices in manufacturing, pressures at the consumer level have led to changes to these labour-intensive processes. Of note is the dyeing process, which historically has been a wet process. The dyeing, printing, and washing steps consume large amounts of energy and generate large volumes of wastewater, which contain the excess dyes, surfactants, and salts. In 2014 alone, coloration of textiles generated over 1.5 billion tons of wastewater.1 Much of this wastewater is neither environmentally benign nor biodegradable and requires further energy investment during the remediation step. Smaller eco-conscious manufacturers such as Ikeuchi Organic, a company located in Japan, have been tackling these issues by utilizing 100% wind power for their operations.2 The smaller scale allows for easier implementation of wastewater treatment and a final product which passes Class 1 certification as set by OEKO-TEX STANDARD 100, a global leader in textiles regulations.3 The underutilization of sustainable textiles dyeing processes on a large scale, however, continues to be a problem. The search for highly competitive, as well as energy and cost-efficient processes along with bridging the gap between laboratory and industrial scales to deliver the benefits of eco-friendly manufacturing is an ongoing goal in the textile research community.

Figure 1

Figure 1. Growth of clothing sales and decline in clothing utilization since 2000.1

Traditionally, the dyeing process involves an interaction between a dye molecule and a fibre (adsorption). This is accompanied by the movement of the dye to the interior of the fibre (diffusion). Consider cotton, which comprises an estimated 21% of global fibre usage.4 Cellulose is the primary component of cotton fibers, totalling up to 95% by mass. The most common method for dyeing cotton involves reactive dyes. As their name suggests, a reactive dye is a high energy molecule which can react with natural cellulose and form a covalent bond. The reactive molecule is low cost, easily tuned to a variety of vibrant colors, and simple to apply.

Figure 2

Figure 2. Types of reactions involving reactive dye and cellulose. Top is substitution, bottom is addition.

At each step of the process, there are several factors that determine the quality of the final product.5 Temperature has a pronounced effect on increasing dye penetration and more rapid diffusion, but at the cost of increased dye hydrolysis. Tight cotton fibres require higher temperature for effective permeation of the dye into the fibre, while loose cotton fibres require lower temperatures for the same effect. The alkalinity of the solution also plays an important role, as the concentration of the cellulose anion is proportional to the pH of the solution. During the dyeing process, some alkali is consumed, and therefore the pH of the final dye solution is lower than the initial pH. Electrolytes are also important to the dyeing solution, acting to overcome the electrostatic repulsions between anionic cellulose and the anionic dye linkers. Finally, the concentration of these additives also impacts the dyeing process. There is a fine line between rate of dyeing and negative impacts on production costs and the environment. These are just some examples of challenges textiles research scientists face when developing a new dye process.

Kim et al highlighted the use of nanofibrillated cellulose (NFC) as a supplement to normal cotton cellulose.6 The large surface area of NFC resulted in an abundance of surface cellulose hydroxyls. Consequently, the salt, alkali, and water amounts could be reduced by an order of magnitude, generating significantly less wastewater. Most importantly, the dyeing process was not significantly impacted by these changes and the resulting fabric is largely indistinguishable from traditional dyeing methods. A life-cycle assessment based on the experimental data suggested that the NFC-based process would result in a significant reduction on wastewater load.

Figure 3

Figure 3. Comparison of various dyeing methods. a) conventional dyeing method; b) conventional dyeing method with decreased salt and alkali concentration; c) NFC-based dyeing with decreased salt and alkali concentration.

Xia et al demonstrated the use of a co-solvent, ethanol, effective for eliminating the need for electrolytes.7 The ethanol was found to impact the dyeing process in four main ways: improving the solubility of the reactive dyes, decreasing the dielectric constant and electrostatic repulsion of the solution, increased aggregation of dyes leading to enhanced dyeing kinetics, and decreasing the surface tension improving dyeing efficiency. These efforts, too, improved the clarity of the resulting waste solution compared to commercial wastewater without loss of fabric colour.

Figure 4

Figure 4. Comparison of fabric wastewater with ethanol-water salt-free method and traditional electrolyte-added dyeing method.

Ding et al developed a method for dyeing without the use of reactive organic dyes, instead opting for base metal nanoparticles.8 Carbon black nanoparticles are first functionalized onto cotton fibres via radiation grafting. Subsequent immobilization of iron oxide red, cobalt green, or cobalt blue for red, green, and blue colors respectively, give rise to colorized cotton fabric. Various approximate colors can be created by varying the concentration of each nanoparticle, and the colour is retained even after 20 wash cycles. Most importantly, the lack of reactive dyes results in pronounced clarity of the resulting dye solution compared to real wastewater.

Figure 5

Figure 5. Images of wastewater samples. W0 is real wastewater from a dyeing factory. WCB1–WCB5, WCoG1–WCoG5, WCoG1–WCoG5, WFeR1–WFeR5 are samples from various concentrations of nanoparticle dyes.

Textiles and clothing play an important part of everyday life. The current clothing manufacture system is wasteful and polluting to the environment. With material and process scientists at the forefront, a new textiles economy built from sustainable methodologies will be vital as we continue to deplete our non-renewable resources.

 

References

  1. Ellen MacArthur Foundation, A new textiles economy: Redesigning fashion’s future, (2017, http://www.ellenmacarthurfoundation.org/publications).
  2. Ikeuchi Organic – Our Philosophy. https://www.ikeuchi.org/about-us/en/concept/ (accessed Mar 15, 2020)
  3. STANDARD 100 by OEKO-TEX. https://www.oeko-tex.com/en/our-standards/standard-100-by-oeko-tex (accessed Mar 15, 2020)
  4. Yang, Q. M. Global Fibres Overview, Synthetic Fibres Raw Materials Committee Meeting at APIC, Pattaya, 16 May 2014.
  5. Shang, S. M. Process control in dyeing of textiles. Process Control in Textile Manufacturing, 300–338. (2013) doi:10.1533/9780857095633.3.300
  6. Kim, Y.; McCoy, L. T.; Lee, E.; Lee, H.; Saremi, R.; Feit, C.; Hardin, I. R.; Sharma, S.; Mani, S.; Minko, S. Green Chem. 2017, 19, 4031.
  7. Xia, L.; Wang, A.; Zhang, C.; Liu, Y.; Guo, H.; Ding, C.; Wang, Y.; Xu, W. Green Chem. 2018, 20, 4473.
  8. Ding, X.; Yu, M.; Wang, Z.; Zhang, B.; Li, L.; Li, J. Green Chem. 2019, 21, 6611

A Greener Future for Chemicals

By Jose Jimenez Santiago, member-at-large for the GCI

Over the last four decades, the development of the chemical industry has transformed the global economy and provided a better quality of life for societies worldwide. However, the mass production and disposal of persistent chemicals has been a threat for the environment and the human health. Understanding the effect of chemicals in the environment is necessary in order to replace current harmful molecules with more benign ones1. Fortunately, chemist have learned from negative past experiences and the next generation of chemicals are expected to cause less toxicity and be less environmentally persistent.

Some chemicals have had a negative impact on wildlife populations, some of those effects appear after several years of exposure and bioaccumulation of the toxic molecules in the environment (Figure 1). The first step to prevent further tragedies in wildlife populations is to know the environmental impact of every chemical that is released into the market. However, in places where this data is available, like the United States and Europe, the percentage of chemicals with known environmental information is relatively small. There are around 75, 000 to140, 000 chemicals on the market, out of those, empirical data on persistence is available for only 0.2%, bioconcentration data for only 1% and aquatic toxicity for 11%2,3. Without this information, how can we make an accurate risk assessment of those chemicals? Moreover, molecules with high persistence in the environment may show a negative impact after many years of accumulation. It is important to keep in mind that that for most molecules on the market, we do not know the extent of their negative effects on the environment in the short term and long term2,4.

Jose blog picture

Figure 1. Examples of wildlife scenarios where chemicals have had or are having population effects 1,6,7,8.

 

 

It is hard to shift to a greener chemical industry that prioritizes the environment due to the cost involved in testing the toxicity of molecules. However, there have been advances from the government and chemistry community towards that goal:

  1. More regulation of chemicals in the environment.

Previously, chemical regulations were targeting the emission of a limited number of pollutants into the environment. New regulations, such as Registration, Evaluation, Authorization and Restriction of Chemicals (REACH2006) in the European Union, are looking to ensure that new chemicals entering the market will conform to minimum human safety and environmental standards5. Thus, manufacturers are responsible for evaluating the impact of new compounds being introduced into the market.

  1. Analytical development and computer models

Many analytical methods have achieved low limits of detection (LOD). It is now possible to identify all the molecules present in a sample. For example, these methods have been used to analyze urban runoff water and identify unusual pollutants4. With the help of these new, sensitive methods approach it is possible to investigate pollution incidents and identify the industrial location responsible for those incidents.

One major ethical concern when running toxicity tests is the large number of animals needed2. In addition, the many thousands of chemicals yet to be tested make this approach unreasonable. Recently, computer models were used to predict which chemicals will be of greatest concern; namely chemicals which are persistent, bioaccumulative and toxic (PBT). In this survey, out of 95,000 chemicals, only 3-5% were likely to be PBT4. These tolls can make feasible the lab test of the most hazardous chemicals and will reduce the ethical concerns.

  1. Better wastewater treatment

The incorporation of a secondary biological treatment into wastewater has considerable benefits for the water quality and chemicals reduction. The Activated Sludge Process is the most common method and has been applied in cities all around the world. For example, in China, 81% of the water distributed to the urban population undergoes the Activated Sludge Process9. This shows how we can find cost effective ways to remove chemicals that have accumulated in the environment for many years.

The amount of chemicals used worldwide, their production, diversity, and incorporation has never been greater1. As chemists, it is important to understand the environmental challenges that we face. In addition, chemists should be aware that chemical problems require chemical solutions. We can be pessimistic about the current status of pollutants, but there are tangible reasons to be optimistic about solutions and methods to reduce the negative impact of our current chemical production. Learning from the past is the starting point to ensure that the next generation of chemicals will have less negative impact on the environment.

 

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The Great Step Backwards: Polymer to Monomer

By Hyungjun Cho, member-at-large for the GCI

There is a movement to develop a new type of product life system called ‘the circular economy’ [3]. Part of this movement aims to manufacture products from recycled or raw materials, and after its useful lifetime, re-introduce the product (now considered waste) as recycled material. The motivation for the introduction of the circular economy is to minimize the need for virgin raw material, especially when it originates from non-renewable resources. This effort is being spearheaded by the Ellen MacArthur Foundation with major industry partners like Google, Unilever, Solvay, and Philips, among others [3]. A critical component to the function of the circular economy is developing the capability to turn waste into a desirable product.

There are several methods of recycling all the different types of materials we use in every day life. This blog will discuss a niche in the ‘plastic to monomer’ field. Evidently, in April of 2019, IUPAC named ‘plastic to monomer’ as part of the 10 chemical innovations that could have high impact in society [6]. Before discussing ‘plastic to monomer’, I must clarify the term ‘plastic’. Generally, plastic is made up of many polymer chains that are physically entangled with one another. A macroscopic analogy is when many electrical wires (think of Christmas tree lights) become entangled: the wires are stuck to each other and the rigidity of the ball of wire is greater than the rigidity of a single wire.

Much like the type of wire influences the tangled ball it forms, the chemical structure of the polymer influences the material properties of the plastic. Examples of properties of plastics include rigidity, elasticity, malleability, gas permeability, friction to skin, transparency, and many others. The polymers that are used for commercial plastic products have been studied and developed for decades to be able perform a specific function. For example, polyvinyl chloride and polystyrene were initially discovered in the 1800s [2,8]. Thus, it would be ideal if the currently used polymers can be de-polymerized back into monomers for recycling purposes. This would be a major move by the plastics industry to become environmentally friendly.

The conventional method to turn polymers into monomer is thermal decomposition. Samples of polymer can be heated to high temperature (typically 220-500 °C) to break some of the bonds that hold the monomers together [10]. When this occurs, radicals can form at the site of the broken bond, which can lead to de-polymerization [10]. The required temperature and how much monomer is formed is dependent on the chemical structure of the monomers that are formed. Thermal decomposition to recover monomer is suitable only for a few types of polymers, such as poly(α-methylstyrene), which has ceiling temperature of 66 °C to propagate depolymerization; the monomer recovery after thermal decomposition of poly(α-methylstyrene) is excellent at 95% [11]. However, for polymers like polyethylene (PE, the most produced polymer) and polypropylene (PP, 2nd most produced polymer), the monomer recovery yield is poor (0.025-2%) [11]. In some cases such as polyvinylchloride (PVC, 3rd most produced polymer), thermal decomposition is even more problematic because PVC will release harmful hydrochloric acid and vinylenes upon heating [11]. Thus, the monomer recovery is poor (1 %) and the process is highly corrosive.

Therefore, one of the key challenges to address for ‘polymer to monomer’ is to perform de-polymerization at a low temperature. There are 4 recent publications that explore this challenge [5,7,9,12]. In general, the authors synthesized polymers using reversible-deactivation radical polymerization (RDRP) techniques and explored the de-polymerization reactions they encountered. Below is a brief highlight from the publications from the Haddleton group [9] and the Gramlich group [5].

Picture1

Scheme 1: De-polymerization of RAFT polymers with trithioester end-group [5]. Reproduced from ref. [5] with permission from The Royal Society of Chemistry.

Flanders et al. polymerized methacrylate monomers, including methylmethacrylate (MMA), using reversible addition-fragmentation chain-transfer (RAFT) polymerization with a trithioester chain-transfer agent (CTA) [5]. This type of polymerization places trithioester end-group at end of the polymer chain (Scheme 1). Typically, this end-group is used to re-start the polymerization at the trithioester end of the polymer. However, as we will see, it may have another function. The authors isolated the polymer, then re-dissolved the polymer in 1,4-dioxane at 70 °C (Scheme 1). This caused monomers to be released from the polymer chains at a temperature much less than the ceiling temperature of MMA, which is 227 °C [13]. Analysis of the polymer after partial de-polymerization demonstrated that the trithioester end-group was still attached to the polymer and the size dispersity (range of polymer ‘molecular weight’) was low, which suggested that the de-polymerization was moderated by the trithioester end-group. The authors observed 10-35% de-polymerization after heating at 70 °C for 12-60 hrs.

Picture2

Scheme 2: ATRP of NIPAM in carbonated water, followed by de-polymerization [9]. Reproduced from ref. [9] with permission from The Royal Society of Chemistry.

Lloyd et al. used an alkylbromide initiator, Cu-based catalyst system to polymerize N-isopropylacrylamide (NIPAM) in Highland Spring carbonated water at 0 °C (Scheme 2) [9]. This type of polymerization places a halide at the end of the polymer chain. The authors monitored the monomer conversion into polymer using 1H-NMR spectroscopy. They measured that ca. 99% of the monomer was converted into polymer chains within 10 min. Unexpectedly, in the next 50 min. the authors observed 50% de-polymerization. The authors attempted to optimize de-polymerization conditions by changing the pH, using dry ice in HPLC grade water instead of Highland Spring carbonated water, etc. which led to 34-71% de-polymerization after 0.5-24 hrs. Years later, the same group used a very similar polymerization condition to polymerize NIPAM [1]. This time, non-carbonated water was used as the solvent and they did not report any de-polymerization.

The reports on RDRP followed by de-polymerization highlighted here are not yet ready to make an impact to ‘plastic to monomer’. The authors admit that the mechanism of de-polymerization is unknown. However, these seem to be the first set of reports on de-polymerization occurring at low temperatures. Perhaps these publications could be the birth of the reversible-deactivation radical de-polymerization (RDRDe-P) field. This is especially intriguing because RDRP have already been studied for decades in academia and are being adopted by the polymer industry [4]. Companies like BASF, Solvay, DuPont, L’Oréal, Unilever, 3 M, Arkema, PPG Industries, etc. already claimed patents for technology and products based on RDRP [4]. Somewhat ironically, RDRP was also part of the IUPAC’s 10 chemical innovations for impact on society but not for its potential to recycle polymer [6].

The polymers of the future may not be made from monomers abundantly used today, but the polymers of the future may be degradable through a low energy process.

References

  1.  Alsubaie, F.; Liarou, E.; Nikolaou, V.; Wilson, P.; Haddleton, D. M. Thermoresponsive Viscosity of Polyacrylamide Block Copolymers Synthesised via Aqueous Cu-RDRP. European Polymer Journal 2019, 114, 326–331.
  2. Baumann, E. Ueber Einige Vinylverbindungen. Justus Liebigs Annalen der Chemie 1872, 163 (3), 308–322.
  3. Circular Economy – UK, USA, Europe, Asia & South America – The Ellen MacArthur Foundation https://www.ellenmacarthurfoundation.org/ (accessed Jan 5, 2020).
  4. Destarac, M. Industrial Development of Reversible-Deactivation Radical Polymerization: Is the Induction Period Over? Chem. 2018, 9 (40), 4947–4967.
  5. Flanders, M. J.; Gramlich, W. M. Reversible-Addition Fragmentation Chain Transfer (RAFT) Mediated Depolymerization of Brush Polymers. Chem. 2018, 9 (17), 2328–2335.
  6. Gomollón-Bel, F. Ten Chemical Innovations That Will Change Our World: IUPAC Identifies Emerging Technologies in Chemistry with Potential to Make Our Planet More Sustainable. Chemistry International 2019, 41 (2), 12–17.
  7. Li, L.; Shu, X.; Zhu, J. Low Temperature Depolymerization from a Copper-Based Aqueous Vinyl Polymerization System. Polymer 2012, 53 (22), 5010–5015.
  8. Liebig, J. Justus Liebig’s Annalen Der Chemie. Annalen der Chemie 1832, 1874-1978.
  9. Lloyd, D. J.; Nikolaou, V.; Collins, J.; Waldron, C.; Anastasaki, A.; Bassett, S. P.; Howdle, S. M.; Blanazs, A.; Wilson, P.; Kempe, K.; et al. Controlled Aqueous Polymerization of Acrylamides and Acrylates and “in Situ” Depolymerization in the Presence of Dissolved CO2. Commun. 2016, 52 (39), 6533–6536.
  10. Microwave-Assisted Polymer Synthesis. Springer eBooks 2016
  11. Moldoveanu, Șerban. Analytical Pyrolysis of Synthetic Organic Polymers; Techniques and instrumentation in analytical chemistry; Elsevier: Amsterdam ; Oxford, 2005.
  12. Sano, Y.; Konishi, T.; Sawamoto, M.; Ouchi, M. Controlled Radical Depolymerization of Chlorine-Capped PMMA via Reversible Activation of the Terminal Group by Ruthenium Catalyst. European Polymer Journal 2019, 120, 109181.
  13. SFPE Handbook of Fire Protection Engineering, 5th ed.; Hurley, M. J., Gottuk, D. T., Jr, J. R. H., Harada, K., Kuligowski, E. D., Puchovsky, M., Torero, J. L., Jr, J. M. W., Wieczorek, C. J., Eds.; Springer-Verlag: New York, 2016.