Properties of ionic liquids for spinning of cellulose and recycling via freeze crystallization

The overall hypotheses underlying this PhD work were that: a) It is possible to produce conductive carbon nano-tube-cellulose based fibers with improved strength using ionic liquids (ILs) and enzymes in the spinning process. b) It is possible to predict the cellulose solubilisation properties of ILs via thermodynamic modeling of the cellulose and IL properties using COSMO-RS. c) Freeze crystallization of water is an energy saving way to separate IL+H2O mixtures for ILs recycling in cellulose spinning processes as compared to separation by evaporation.

In turn, the main work objectives were: (1) Identify the best ILs for dissolving cellulose. (2) Assess energy-use via freeze crystallization for recycling of the ILs to reduce the cost. (3) Prepare conductive fibers based on spinning of cellulose and carbon nano-tubes using ILs for dissolution and assess enzymatic improvement of fiber tensile strength by use of laccase.

Due to strong the hydrogen bonds (H-bonds) in its supra-molecular structure, cellulose is insoluble in water and in most organic solvents. Traditional cellulose spinning processes (e.g. for textile manufacture) therefore often involve use of strong acid or alkali, which in turn poses an environmental burden.

As outlined above, this study was based on that ionic liquids might be useful for cellulose dissolution. For this, the following research parts were completed: First, 375 ILs candidates were screened by COSMO-RS modelling for cellulose dissolution properties. As a part of this, a comparison of cellulose models in COSMORS was performed, identifying the mid-monomer in cellotriose as best, and enthalpy calculations of cellulose dissolution in 7 relevant, selected ILs was accomplished, and the cellulose dissolution experimentally verified.

COSMO-RS excess enthalpy predictions indicated that the cellulose dissolution process is mostly anion dependent and the main forces in the cellulose dissolution in ILs are H-bonds: Ac−, Cl−, DEP−, and Br−were the most promising anions for cellulose dissolution. Cations with ethyl, allyl, 2-hydroxylethyl, 2-methoxyethyl and acryl-oyloxypropyl functional groups exhibited particularly good properties for cellulose dissolution.

Second, recycling ILs from water mixtures by freeze crystallization was conducted. Six ILs, EmimAc, EmimDep, AmimCl, HOEtpyBr, HOEtmimBr, and EtOMmimCl, which had been found to have high cellulose solubility in phase one, were selected for freezing point measurements alone and in ILs+H2O mixtures. The results illustrated that the ILs exhibit similar features as inorganic salts of MgCl2 and MgSO4 in depressing the freezing point of water.

Ice starts forming when the IL aqueous solutions are cooled to their freezing points in the dilute region. At higher IL concentrations, the solid phases formed were presumed to be solid IL or hydrates of the form IL·nH2O. The HOEtpyBr+H2O and HOEtmimBr+H2O systems formed simple eutectic systems and the freeing points of these two system were higher than those of the other four ILs.

This result indicated that these two bromide ILs could be separated from water directly by freeze crystallization of the water. From the experimental results it was moreover determined that the freezing points of IL+H2O systems were affected by the nature of both the cations and the anions of the ILs.

Third, separation of IL+H2O by freeze crystallization was applied to recycling EmimAc and EmimDep, respectively, in the cellulose spinning process, and the methodology was named as the freeze crystallization method. The energy requirement for EmimAc and EmimDep recycling after cellulose spinning by the freeze crystallization method followed by evaporation was calculated and compared to the energy use in a traditional (patented) evaporation method.

It was found that to fabricate 1 kg dry cellulose fiber using freeze crystallization+evaporation rather than evaporation, 21.5 MJ can be saved for EmimAc and 37.3 MJ for EmimDep recycling. It was also shown that significantly less H2O is required in the cellulose spinning process with ILs than with N-methylmorpholine oxide.

Finally, the production of multiwall carbon nanotubes (MWCNTs)-cellulose conductive fibers using a selected IL with good cellulose dissolution properties, namely EmimDep, was studied: The material used was cotton pulp and MWCNTs. The work verified that EmimDep can dissolve cellulose and disperse MWCNTs.

The fiber electrical conductivity increased with increasing mass ratio of MWCNTs relative to the cellulose. The highest electrical conductivity measured was 760 S/m at a spinning extrusion flow of 1 mL/min, spinneret diameter of 0.46 mm and a ratio of 5:1 of MWCNTs versus cellulose. Thermogravimetric analysis and surface area results gave evidence that the fiber was more stable at the spinneret diameter of 0.46 mm than at 0.98 mm.

The surface area of the conductive fiber reached 188 m2/g. The SEM data also revealed pore structures in the fiber surface and hinted that the alignment of MWCNTs in the fiber decreases the fiber electrical conductivity. Tensile strength measurements showed that the fiber tensile strength could be increased by laccase enzyme treatment, provided that phenols were added to the spun cotton pulp-CNTs fiber bath solution.

The data obtained verified the hypotheses formulated for the PhD work and provided new knowledge of: ILs cellulose solubility properties, COSMO-RS modelling of cellulose, freezing points and freeze thermodynamics properties of ILs and ILs+H2O mixtures, energy saving for ILs recycling by freeze crystallization, and finally indicated the promising potential of using ILs and enzymes in the production of conductive CNT-cellulose based fibers.