Research Highlight 4 – From Surgical Mask to Carbon Fibers.

"Yesterday I was someone’s face mask, today I got upgraded into high value carbon fibers”.

The current plastic pollution is worsened by COVID-19 pandemic, it was estimated that ~3.4 billion single use face masks were disposed of daily worldwide in mid 2020. The mismanaged waste was found littering the streets/roads and beaches, which not only has adverse affect on human health but it is also a threat to marine life. These single use/surgical face masks were made from nonwoven polypropylene (PP) fibers and are not biodegradable, thus can persist in the open environment and in the oceans for many years. Over time, they can disintegrate to form microplastics which can be swallowed by marine organisms – ultimately negatively impacting human health through food consumption. Most of the collected waste is landfilled – which also imposes pressure on land due to the staggering amount of waste, and most importantly the economic value invested for its production is also lost (~ 1.6 million tons per day produced in 2020). The incineration of these mask waste can recover some value in the form of energy, however it contributes to greenhouse gas emissions [1-3]. Urgent solutions are needed to address these issues. Researchers from School of Polymer Science and Engineering, The University of Southern Mississippi, USA, Dr. Zhe Qiang and group have found a perfect solution to mitigate global mask pollution. They have recently upcycled waste surgical/face mask into high value carbon fibers – via simple steps including thermal stabilization and pyrolysis – which can fully retain the initial fiberous structure of polypropylene in the mask. The work is published in ‘ACS Omega’ [4].

Before upcyling, the authors have cut the surgical mask to remove the elastic bands, metal nosepiece and inner melt-spun mat, and the outer fabric layers of the mask were used to produce carbon fibers. Their strategy involves an initial sulfonation step by dipping the mask in sulfuric acid at 155 °C, which introduces sulfonic acid groups (SO3H) into the secondary or tertiary carbons in the polypropylene chain (see Scheme 1). With continuous heating, these can undergo desulfonation, which results in unsaturated bonds in the polymer backbone that can react with another polymer chain to form crosslinked structure. The second step involves carbonization process by heating thermally stabilized mask at elevated temperature (800-1400 °C), which removes all functional groups (SO2, H2O, CO, CO2, etc.,) to form porous carbonaceous material.

The authors have stated in the article that the initial thermal stabilization step (that is, stabilization of PP chain via sulfonation-crosslinking) is necessary for successful conversion of PP in mask waste into carbon fibers. Otherwise, instead of producing carbon residue, upon pyrolysis, PP chain will undergo chain scission to produce oligomers or short chain hydrocarbons, and ends up volatilizing at carbonization temperature [5]. The authors strategy resulted in 58% yield carbon residue at 800 °C (under nitrogen atmosphere) whereas waste mask prior to thermal stabilization showed 0% carbon residue. Their sulfonation-crosslinking step also introduced sulfur heteroatoms into the carbon framework (via direct bonding of sulfur atoms to carbon). The authors were able to confirm this using Energy-Dispersive (5.6 wt% sulfur atoms) and Photo-Electron (0.4 at. % sulfur) X-ray Spectroscopy analysis. They hope that these sulfur doped multifunctional carbon fibers produced from mask waste could be useful for many diverse applications such as energy storage, CO2 adsorption and catalysis.

In the article, the authors have demonstrated several practical applications of this waste-mask derived carbon fibers. A prominent example is Joule heating experiment – heating of carbon fibers by application of voltage. Due to its high electrical conductivity carbon fibers can generally produce heat at a wide temperature range depending upon the voltage applied. This Joule heating is highly relevant for aerospace industry in the production of composite parts, for example welding of carbon fibers or post-curing of thermosetting resin in the presence of heat generated from carbon fibers instead of oven heating [6,7]. In Joule heating experiment, the authors carbonized mask showed a maximum temperature of 248 °C at a very low voltage of 9 V. The authors have stated that the existing waste-derived carbon products such as those derived from 'coal tar' need much higher voltage of 60 V to reach similar temperature. Moreover, the heat reaches fast, and upon removal of voltage heat can dissipate quickly and return to room temperature in a matter of 10 seconds – making them also suitable for thermotherapy and thermochromics applications. The authors envisage it’s use as fillers in Joule heating composite products, since the direct use can be a bit challenging due to brittle nature of the carbonized mask in its present size and shape.

By taking advantage of its hydrophobicity, the carbonized mask also found application in oil sorption and oil-water separation. The mask waste derived carbon fibers can uptake 14 gram of mineral oil per gram of carbon fiber. The carbonized mask can also adsorb various organic solvents such as chloroform, tetrahydrofuran, toluene, dichloromethane and hexanes with varying adsorption capacity ranging from 5-12 gram of solvent per gram of carbon fiber. The authors have demonstrated this oil or solvent sorption capability of the waste mask derived carbon fibers by placing dyed (pink or blue) solvent droplets in oil and it’s disappearance. From a circular economy point of view, it is important to stress that the carbonized mask can be re-used many times in applications where the sorbent removal is easy – which is demonstrated in the article up to 5 cycles using chloroform as the sorbent. Activation of carbonized mask with potassium hydroxide (KOH) at 700 °C further allowed it’s use in water purification applications such as waste water treatment. Because KOH activation enhanced the porosity of the carbonized mask (an increase of surface area from 295 to 600 m2/g) which facilitated sorption of pollutants, dyes, etc., from water. In a dye sorption study, the authors activated carbon mask showed competitive performance in comparison to commercially available powder-activated carbons (PAC, surface area: 712 m2/g).

Due to its low density, high electrical and thermal conductivity, high mechanical properties, and so on, carbon fibers can be useful for a wide range of applications. However, its high cost ($16-25/kg) restricting their use. Currently carbon fibers are mostly utilized as reinforcement in composite parts in aerospace applications – where cost is not a major concern. Recent studies suggest that it’s price should be less than $11-15.4/kg to be successfully employed in automotive industry [8,9]. The major cost contribution is from the starting precursor polyacrylonitrile (PAN) synthesis and (wet)spinning process – ~90% industrial production of carbon fiber is based on PAN. The industry and researchers are currently exploring cheap alternative precursors for carbon fiber production – such as polyethylene (PE), which only costs a total of $9.39/kg – an approximate of 71% cost reduction for its polymerization, 53% cost reduction for its melt spinning process and 30% cost reduction for its carbonization process in comparison to PAN based carbon fiber production. In PE based carbon fiber production, the major cost contribution coming from thermal/wet stabilization process (sulphonation, 30% increase, from $2.41/kg to $3.25/kg as compared to PAN) – which is similar to the authors work. The innovation applied by the authors is the use of ‘waste mask' as the starting precursor which can be directly converted into carbon fibers without disrupting it’s fibril structure, size and shape; thereby eliminating the need for initial two steps (polymerization + spinning). Their strategy has the potential to reduce the cost of carbon fibers up to $7.5/kg or less than that. This could broaden it’s current commercial applications. The whole process is simple, scalable, and can be instantly implemented utilizing current infrastructure, at the same time creating value from waste – it is a powerful remedy for global mask pollution! We need more innovation like Dr. Qiang’s that has practical solutions and can bring instant tangible results toward the common goal of achieving a sustainable society.

References:

[1]. Benson, N. U.; Bassey, D. E.; Palanisami, T. COVID pollution: Impact of COVID-19 pandemic on global plastic waste foot print. Heliyon 2021, 7,e06343.

[2]. Patricio Silva, A. L.; Prata, J. C.; Walker, T. K.; Duarte, A. C.; Ouyang, W.; Barcelo, D.; Rocha-Santos, T. Increased plastic pollution due to COVID-19 pandemic: Challenges and recommendations. Chem. Eng. J. 2021, 405, 126683.

[3]. Hantoko, D.; Li, X.; Pariatamby, A.; Yoshikawa, K.; Horttanainen, M.; Yan, M. Challenges and practices on waste management and disposal during COVID-19 pandemic. J. Environ. Manage. 2021, 286, 112140.

[4]. Robertson, M.; Obando, A. G.; Emery, J.; Qiang, Z. Multifunctional carbon fibers from chemical upcyling of mask waste. ACS Omega 2022, 7, 12278-12287.

[5]. Younker, J. M.; Saito, T.; Hunt, M. A.; Naskar, A. K.; Beste, A. Pyrolysis pathways of sulfonated polyethylene, an alternative carbon fiber precursor. J. Am. Chem. Soc. 2013, 135, 6130-6141.

[6]. Yao, Y.; Fu, K. K.; Zhu, S.; Dai, J.; Wang, Y.; Pastel, G.; Chen, Y.; Li, T.; Wang, C.; Li, T.; Hu, L. Carbon welding by ultrafast Joule heating. Nano Lett. 2016, 16, 7282-7289.

[7]. Prolongo, S. G.; Moriche, R.; Del Rosario, G.; Jimenez-Suarez, A.; Prolongo, M. G.; Urena, A. Joule effect self-heating of epoxy composites reinforced with graphitic nanofillers. J. Polym. Res. 2016, 23, 189.

[8]. Choi, D.; Kil, H.-S.; Lee, S. Fabrication of low-cost carbon fibers using economical precursors and advanced processing technologies. Carbon 2019, 142, 610-649.

[9]. Soulis, S.; Konstantopoulos, G.; Koumoulos, E. P.; Charitidis, C. A. Impact of alternative stabilization strategies for the production of PAN-based carbon fibers with high performance. Fibers 2020, 8, 33.

Created by: Sini Nalakathu Kolanadiyil