All-cellulose composite production using an ionic liquid : dissolution of wood derived high purity cellulose with BmimCI

Abstract

The feasibility of producing all-cellulose composites (ACCs) films from dissolving wood pulp by partial dissolution of the cellulose was investigated. It was thought that the undissolved fraction of the cellulose fibres would have a reinforcing effect on the final product. The ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) was used to dissolve sheets of the dissolving wood pulp on a laboratory scale. Before the cellulose sheets could be produced, the recovery of BmimCl was considered. Reuse of the solvent was important for the process to be cost effective and environmentally benign. Since little published information existed on the rinsing process to recover the solvent, a rinsing method was developed. The rinsing was performed using sequential rinsing stages with demineralised water. Increasing the temperature of the rinsing water was found to improve the efficiency of the rinsing stage and to accelerate the recovery of BmimCl. When the rinsing water was heated to 80 °C, approximately 30 % better recovery of BmimCl during the first rinsing stage was achieved compared to rinsing at 25 °C. When the BmimCl recovery for cellulose samples which were dissolved to different degrees was compared, it was found that a high degree of dissolution leads to slower recovery. Although the recovery was slower for the cellulose samples that were highly dissolved, the total recovery after four rinsing stages was 97.6 % (+/- 2.7 %) independent of the degree of dissolution. The concentration profiles for the duration of each rinsing stage was determined. The concentration profile of the first rinsing stage showed that more than 65 % of the BmimCl used could be recovered in 10 min when only 2.4 ml rinsing water per gram of BmimCl was used. For the 0.16 mm thick starting material films used in this study, four rinsing stages of 10 min duration each, using rinsing water at 80 °C was found to be sufficient for BmimCl recovery, independent of the dissolution conditions. Partial dissolution was achieved for few of the dissolution conditions investigated. Complete dissolution occurred for the majority of dissolution conditions and the produced films did not qualify as ACCs. Some of the dissolution conditions resulted not only in complete dissolution of the cellulose, but also in possible degradation and weakening of the cellulose. Especially when high concentrations of BmimCl was used, the regenerated cellulose structure had more defects and was weakened. It is recommended that shorter dissolution times and lower BmimCl concentrations be investigated. For the samples that did show some remnants of the native cellulose fibres, the undissolved fractions did not have a reinforcing effect on the regenerated cellulose matrix. For this reason, dissolving wood pulp was not suitable for the production of ACCs by partial dissolution. It is recommended that the starting material sheets be prepared as a mixture of the dissolving wood pulp and a different, stronger, high purity cellulose source, such as cotton. If the starting material contains stronger cellulose fibres, partial dissolution might result in the desired strengthening effect. Most of the sheets showed a large amount of variation which was suspected to be due to uneven dispersion and removal of the BmimCl through the film, the defects discussed or internal stresses resulting from the drying process. It is recommended that the process be refined to eliminate the large variation observed within each sample. The presence of BmimCl was found to have a plasticising effect on the produced cellulose film. The BmimCl inhibited the formation of crystals, probably resulting in the plasticising effect. It was also determined that crystals did not form as soon as cellulose precipitated from solution, due to the presence of BmimCl. Only after BmimCl was removed from the cellulose structure did crystals form in the solid cellulose structure. The crystal formation in the solid phase continued as the cellulose films were allowed to age. The long term effect of BmimCl was to reduce this crystal formation in the regenerated phase over time, but an increase in crystallinity was still observed independent of the presence of BmimCl in the cellulose. The temperature at which regeneration of the cellulose and rinsing of the regenerated material was performed visually affected the produced material. Regeneration and rinsing at 25 °C resulted in more transparent films with reduced crystallinity when compared to the films that were regenerated and rinsed at 80 °C. The films were more plastic due to the reduced crystallinity resulting from the lower temperature.