Functionalization of cellulosic fibers via adsorption of native and light-responsive polysaccharides

Pulp fibers are widely used in different types of everyday products. Even though the demand for newspaper and printing paper is declining, there is a clear growth in the consumption of such fiber-based products as specialty papers, packaging, tissue, and hygiene products. The growing concern about plastic pollution stimulates the development of bio-based technologies and biodegradable bio-based products. In light of the current situation, this work focused on the surface functionalization of pulp fibers to improve their properties as well as to introduce novel features. Such modification provides an opportunity to increase the value of the fibrous materials and broaden the application of pulp fibers.Adsorption of biopolymers and their derivatives was chosen as a modification method that preserves the bulk structure of the fibers and that can be realized using facilities of the existing fiberlines of a pulp and paper mill. Two types of polysaccharides were used as surface modifying agents: extracted xylans and specially designed light-responsive derivatives of cellulose.Xylans were extracted from birch wood and bleached birch kraft pulp using pressurized hot water and cold alkali extraction methods. The extracted xylans were considerably different from each other with respect to the appearance, chemical composition, and molar mass. The cold alkali extracted xylan (CAX) was much purer than the hot water extracted one (HWX). Both xylans were applied to pine kraft pulp fibers at dosages of 2 and 5% (pulp based) during an oxygen delignification stage. The addition of CAX had a positive effect on the selectivity of the process. Hand-sheets made of the oxygen delignified as well as further bleached and beaten CAX treated pulps had superior mechanical properties compared to the reference samples. In contrast, HWX, which contained higher amounts of the side groups and lignin impurities, showed worse performance than CAX. In particular, HWX had lower adsorption onto the pulp fibers and impeded the oxygen delignification process. Thus, these results demonstrated that CAX can be applied as a dry strengthening agent to improve mechanical properties of a final fiber product, and that the modification step of pulp can be executed in the oxygen delignification stage of the fiber production line.Novel light-responsive multifunctional cationic cellulose derivatives (MCCDs) soluble in water were adsorbed onto bleached eucalyptus kraft pulp in aqueous solutions to impart photoactive functionalities to the fibers. The studies covered the adsorption mechanism and the effect of the structure of MCCDs on the adsorption process. Applied coumarintype MCCDs, namely mixed 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy] acetic acid–(3-carboxypropyl)trimethylammonium chloride esters of cellulose, had an intrinsic ability to crosslink under UV light irradiation via cycloaddition reaction of the pendant photoactive moieties. The pulp fibers modified with this type MCCDs gained the photocrosslinking functionality of the derivatives. Hand-sheets made of such fibers and post-irradiated with UV light possessed a much stronger fiber network due to the formed covalent inter fiber bonds. In addition, stiffness of the individual functionalized fibers increased after the irradiation. Another group of MCCDs, namely mixed N-(3-propanoic acid)- and N-(4- butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride esters of cellulose, endowed the fibers with fluorescence in the visible part of the spectrum. Under UV light exposure, fluorescence of the pulp fibers made them visually distinguishable from the reference fibers. Therefore, the fluorescent fibers can serve as an authenticity indicator when incorporated into the fibrous material, such as inner side of the packaging. Both types of the prepared light-responsive pulp fibers can be potentially used to design smart bio-based materials, e.g. in packaging, to increase its mechanical performance and to validate the packaging authenticity.