Pyrolysis-gas chromatography/mass spectrometry study of chromophores in dissolving wood pulp.

Abstract

Dissolving wood pulp (DWP) is bleached wood pulp that contains high cellulose content, typically >95%. The bleaching is done to remove remnants of lignin and hemicelluloses that are not necessary for the final DWP. The pulp is chemically processed to make high-grade products such as speciality papers, viscose fabrics, and microcrystalline cellulose used in applications such as textile fibres, fillers in pharmaceutical tablets, or as a thickener in food additives, and manufacture of paints. Frequently, the fully bleached DWP exhibits an unwanted yellowish haze. This phenomenon results in loss of brightness in the pulps and is referred to as brightness reversion. Remnants of polysaccharides on DWP are believed to give rise to chromophores responsible for the brightness reversion. Such reversion in brightness is viewed as an early sign of ageing or deterioration of the pulp. The impact of chromophores on the quality of DWP is a pressing issue in the global pulp and paper industry. Thus, identification of chromophores in the pulps is an important aspect to solving this problem and efforts have been made to develop techniques for identification of the chromophores responsible for brightness reversion. For example, an elegant method, termed chromophore release and identification, has been developed. However, application of the methodology for analysis of chromophores in pulps is tedious and long. It takes, on average, seven days to generate results, and a large quantity of pulp is required for the analysis. Such a long processing time is not ideal for industrial applications where time is of the essence. Hence, in this study, a novel methodology that allows for rapid and accurate characterization of chromophores directly from pulp fibres has been developed. The method entails direct analysis of chromophores on pulps without pre-extraction of chromophores, and results are obtained in less than an hour. The methodology entails the use of analytical pyrolysis combined with gas chromatography/mass spectrometry (Py-GC/MS) for direct detection and identification of chromophores in DWP. To develop the method, selected fully bleached DWP samples were induced for brightness reversion and then analysed by Py-GC/MS and any chromophores present were identified by mass spectrometry. Probable compounds that could have contributed to the brightness reversion were also induced for brightness reversion and analysed to ascertain any similarities with the pyrograms of the brightness reversed DWP samples as well as mass spectral identities of the compounds. The results showed that the pulps that were induced for brightness reversion contained relatively higher amounts of chromophores than the original pulps, with ketones and furan-type compounds (originating from degraded cellulose and remnants of the hemicelluloses) being the major chromophoric groups. Analysis of data on pulps with varying brightness reversion values did not show correlation with the relative amounts of chromophores detected by Py-GC/MS. The results confirmed literature reports that brightness is not a good indicator of the concentration of chromophores in pulps. The developed Py-GC/MS method was then used to identify chromophores in DWP samples produced from the industry and the laboratory. Considering that the industry-produced DWP was from a wood furnish comprised of a mixture of wood species, it was essential to understand how the different clones/species in the wood furnish influenced the formation of chromophores. the results showed that different wood species contained the same type of chromophores, with furantype compounds and conjugated ketones being the major groups of compounds detected. Pulps were sampled from different unit operations in a mill to ascertain the profiles of chromophores along the mill process. Additionally to the industrial in-process pulps, a number of single clone acid bi-sulphite pulps were bleached in the laboratory to produce pulps (in-process) that were also analysed for chromophore content using Py-GC/MS. The main groups of chromophores identified in all the samples were still furans-type compounds and conjugated ketones. Wet chemical analysis of in-process pulps showed that the remnants of hemicelluloses and degraded cellulose in the pulps have a significant impact on the formation of the ketones and furan-type compounds. Finally, the developed Py-GC/MS method was tested on other pulps (besides DWP) that contained higher amounts of lignin (viz., the newsprint and kraft pulp). This was done to ascertain if the method would apply to other pulps besides DWP. As expected, the results showed that newsprint contained very high amounts of chromophores due to lignin whereas kraft pulps exhibited smaller amounts of lignin-type pyrolysis products. Thus, lignin was the major cause of brightness reversion in pulps that contain lignin and residual amounts of lignin. Overall, the major chromophoric compounds identified in DWP were conjugated ketones and furan-type compounds. These compounds originated from remnants of hemicelluloses and degraded cellulose in the pulps: this agrees with literature reports on the origin of chromophores in cellulosic materials. The Py-GC/MS is a novel tool for rapid and direct analysis of residual chromophores in fully bleached and in-process DWPs and can be utilised for rapidly identifying the presence and chemistry of residual chromophores in DWP. The technique is currently being used to monitor the quality of industrially-produced DWP from different pulp mills.