Ionic liquids – a versatile tool for (bio-) polymer treatment

 Since the early 2000s ionic liquids have been investigated and utilized as direct and efficient solvents for biopolymers and polymers like cellulose, chitin, silk, wool, aramid fibers, polyester (PET), polyamide (PA), etc. [1].

Characteristics
Ionic liquids offer favorable physico-chemical properties like negligible or low boiling points, non-flammability, low toxicity, high dissolution capacity and high recycling rates. Biopolymers like cellulose are directly dissolved under rather mild conditions without the need to add chemicals or additives during processing.

Inorganic salts like sodium chloride (NaCl) have very high melting points due to the organization of the cations and anions in a fixed lattice structure. By contrast, ionic liquids have – per definition – melting points below 100 °C because the ions are typically of organic nature and larger than inorganic ions. Therefore, ionic liquids are often named “high ordered liquids” or “low ordered solids”. They behave like tiny steel balls which are kept together by an invisible glue on their surface. If a bucket full of these tiny steel balls is poured on the floor the individual steel balls do not bounce back and jump around. They stay together and flow, like a liquid. This behavior contributes to the exceptional physico-chemical properties of ionic liquids like non-flammability and a negligible vapor pressure. Ionic liquids do not burn because they have no boiling point and they do not release any gaseous compounds into the atmosphere, this is one main reason why many candidates are non-toxic. Ionic liquids show high thermal and electrochemical stability but for the utilization of ionic liquids as solvents their unusual solvent properties are most relevant.

Ionic liquids only attract hydrogen bonds in biopolymers but do not affect or deconstruct the biopolymers itself which leads to a very mild and gentle dissolution of cellulose, lignin, chitin, chitosan, silk, and other biopolymers [2].

Besides that, ionic liquids are applied for the dissolution and treatment of other polymers like aramid fibers, PA, PET, etc. Recycling of textiles, dying of virgin fibers as well as hydrophobization of fiber surfaces and flame retardant are the key applications of ionic liquids in that regard.

State-of-the-art
Theoretically, there are billions of possible combinations of cations and anions, but in practice approx. 100 different ionic liquid candidates are used on a larger scale. A classic ionic liquid is liquid at room temperature with colors varying from colorless to brown and has a viscosity roughly comparable to olive oil. In the early day’s halide containing ionic liquids like EMIM Cl were used as solvents. Although the dissolution capacity of these “first generation ionic liquids” is comparable to the state-of-the-art ionic liquids like coordinating ionic liquids (EMIM OAc), superbase ionic liquids (mTBDH OAc), or protic ionic liquids (BHIM HSO4) there are some obvious drawbacks of using these types of liquids like environmental sustainability due to the halides contained or the expected corrosion issues.

The future trend clearly goes into the direction of using bio-based ionic liquids where the precursors originate from natural resources and the developed ionic liquids are non-toxic and biodegradable. A lot of research is currently ongoing to improve the ionic liquids in that regard [3].

Raw materials
The main raw materials that are currently under investigation are of cellulosic or lingo-cellulosic origin including agricultural waste, switchgrass, sorghum, poplar, bamboo, paper pulp, or cotton. Other feedstock for ionic liquid-based processes to generate fibers are used textiles as well as shrimp shells. Beside the selection of the most suitable ionic liquid candidate for the corresponding feedstock the development of improved processes is of importance since the general aim is to dissolve as much feedstock in terms of concentration in the ionic liquid as possible in a short period of time. A common approach is the use of a co-solvent which often helps to reduce the viscosity in the ionic liquid-based solvent system. Other technologies to support the dissolution of the feedstock in the ionic liquid are for example microwave irradiation, ultrasound, oscillating flow reactors, or kneaders.

In many cases the proof of concept and the ionic liquid screening is done on a gram scale level. Various feedstock are tested in combination with different ionic liquids, co-solvents, and aiding technologies on the gram scale level. For promising combinations of raw materials and ionic liquids the parameters for the dissolution process like temperature, time, mixing, amount of co-solvent, acceptable raw material loading, etc. are optimized. To isolate the desired compounds from the ionic liquid solvent, most frequently an anti-solvent like water is added to the system that competes with the dissolved species for the hydrogen bonds in the system and forces the product (e.g. cellulose) to precipitate from the ionic liquid mixture. In that case the product is filtered, washed, and dried followed by determination of the product quality. Once the quality of the generated product is proven the ionic liquid is produced on a larger scale and the dissolution process as well as the product isolation is translated to the kg scale.

Conventional (left) and ‘welded’ (right) silk yarns – NFW’s Clarus technologies enable new all-natural composite structures that enables new performance (source: NFW) [4]

A very good example for the beneficial use of ionic liquids is the Clarus technology by Natural Fiber Welding Inc. (NFW), Peoria, IL/USA, an ionic liquid-based process to reshape and finish cellulosic biopolymers by ionic liquid welding. The raw materials are not completely dissolved but only on the surface to allow fibers to selectively fuse, leading to high quality yarns.

Conventional and ‘welded’ silk yarns are shown in Fig. 1. NFW’s Clarus technologies enable new all-natural composite structures that were not possible before and enable new performance [4]. Fibers produced by applying the Clarus technology exhibit an improved tear resistance and the number of process steps during knitting/weaving is significantly reduced. NFW is planning and building out tens of thousands of tons of production with strong demand from brands such as Ralph Lauren.

Another focus of NFW is the recycling and the reuse of used textiles utilizing the ionic liquid-based process. Besides the production of new fibers from old textiles other innovative approaches are developed like furniture and similar items like the tabletop made from original Denim shown in Fig. 2.

Tabletop made of pure Denim by NFW (source: turnstone)

Another process highlighting the benefits of imidazolium-based ionic liquids was developed at the German Institutes of Textile and Fiber Research Denkendorf (DITF), the HighPerCell technology [5-9]. The IL-based spinning process allows the production of endless multi-filament fibers with properties exceeding those of commercial tire cord fibers regarding the obtained Young´s modulus by a factor of 2. The developed IL-technology does not rely on chemical pre-treatment/conversion or chemical stabilization of the spinning solution to dissolve and process the cellulose.

HighPerCell and HighPerCellCarbon fibers (source: DITF)

The fibers produced by an air gap spinning process have a dense structure and a smooth surface. Due to these properties, they are predestined for technical applications. Possible applications of HighPerCell fibers range from reinforcing materials in composites to the production of carbon fibers (HighPerCell_Carbon) with very interesting material properties. Carbon fibers made from cellulose are a cost-effective and environmentally friendly alternative to non-sustainable petroleum-based carbon fibers [10].

The HighPerCell technology can be also applied for cellulose/chitin blend fibers consisting up to 90 wt.% of chitin. The process scheme is shown in Fig. 4 [11-12].

Production of cellulose/chitin fiber blends (source: DITF)

As mentioned, for each raw material the corresponding ionic liquid needs to be selected and the dissolution process as well as fiber coagulation needs to be optimized. Critical for the economic success of an ionic liquid-based process is the recycling of the ionic liquid-based solvent after the dissolution process. After removal of the desired products, contaminants and the excess of anti-solvent must be removed from the ionic liquid to an extent that allows efficient dissolution as well as constant product quality during the subsequent dissolution cycles. Recycling rates higher 99 % are realizable considering different classes of ionic liquids and feedstock/product combinations.

The Austrian company proionic GmbH has a strong background and focus on the production and sales of ionic liquids (ILs) with more than 20 years of experience in developing processes based on ILs and the implementation of the developed processes together with customers. ILs are produced according to the patented and registered CBILS process delivering halide-free ionic liquids with exceptional high purities. The process is robust and easily transferrable to the multi-ton scale without generating waste.

References
[1]     Swatloski, R.P. et al.: J. Am. Chem. Soc. 124 (2002) 4974
[2]     Zhang, S. et al. (eds.): Structures and Interactions of Ionic Liquids, Springer-Verlag Berlin Heidelberg 2014
[3]     Sun, N. et al.: Green Chem. 16 (2014) 2546
[4]     Haverhals, L.M. et al.: Chem. Commun. 48 (2012) 6417
[5]     Ingildeev, D. et al.: J. Appl. Polym. Sci. 128 (2012) 4141
[6]     Spörl, J.M. et al.: Mater. Today Commun. 7 (2016) 1
[7]     Hermanutz, F. et al.: Macromol. Mater. Eng. 304 (2019) 1800450
[8]     Hermanutz, F. et al. (eds.): Commercial Applications of Ionic Liquids, Springer International Publishing, Cham (2020) 227-259
[9]     Vocht, M.P. et al.: Cellulose 28 (2021) 3055
[10]     Spörl, J.M. et al.: Macromol. Mater. Eng. (2017) 1700195
[11]     Mundsinger, K. et al.: Carbohydr. Polym. 131 (2015) 34
[12]     Ota, A. et al.: Polym. Adv. Techn. 32 (2021) 335

Clarus = trademark
turnstone, CBILS, HighPerCell, HighPerCell_Carbon = registered trademarks