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Polycarboxylic acids (PCAs) aroused interest as crosslinking agents for natural polymers, such as cellulose, starch, chitosan, and hemicelluloses (HC), for improving their water resistance and imparting them new properties. Here, PCAs were studied for the crosslinking of HC films. HCs were isolated from gray alder (Alnus incana L.) wood as a water-soluble fraction by sawdust extraction with 7% potassium hydroxide. Citric acid (CA), maleic acid (MA), and butane tetracarboxylic acid (BTCA) served as crosslinking agents in the presence of sodium hypophosphite as a catalyst. The crosslinking reaction was followed by the analysis of free and esterified COOH groups in HC films. At temperatures below 100–110°C, the HC esterification process with BTCA proceeded very slowly. At 140°C, most of the acids’ COOH groups reacted with HC during 1–1.5 h. BTCA had a higher esterification activity than CA and MA. The obtained films retained their form on immersing in water and subsequent drying in the open air. The water retention value of films followed the reaction kinetics, that is, decreased with increasing reaction time. Water-resistant films were obtained with no <10–11% of BTCA and 20% of CA based on the weight of HC. The water vapor sorption capacity of the films after modification with 10% BTCA decreased approximately twice in the whole range of the relative humidity of air compared with the case of unmodified HC films.

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For reinforcing of paper, nanoparticle gels from black alder, birch and pine bark were obtained. Non-extracted bark and that extracted in biorefinery were used. For producing nanoparticles, the materials were destructed using the thermocatalytic destruction method and then dispersed in water medium in a ball mill. At a sufficient concentration, gel-like dispersions were obtained, which contained nanoparticles with the size ~300 nm. The effect of nanoparticle gels on the properties of paper sheets was investigated by introducing the dissolved gels in paper furnish and by covering both sides of paper sheets with nanoparticle gel coatings. It has been established that the nanoparticle fillers increase the tensile and burst strength. The nanoparticle fillers from extracted bark increase the mechanical indices to a higher extent. The coatings from nanoparticle gels considerably improve the Gurley air resistance of paper and increase the mechanical indices of paper sheets, especially burst strength. The effect of nanoparticle gel coatings is dependent on the coating thickness and gel concentration. The coatings decrease the tensile strength in a wet state.

Recycling of paper materials or other type of cellulose fibres is important regarding saving nature resources and environmental protection. Cellulose dissolution and regenerating from various materials is one of recycling possibilities. Cellulose dissolution by NaOH/urea system has a great potential due to the use of harmless reagents, and at the same time is challenging due to the forming of gel-like substances, when cellulose has degree of polymerization more than 1000. It was found that microcrystalline cellulose and cotton cellulose were dissolved in NaOH/urea, but tissue paper, filter paper and waste paper were not dissolved completely. Additives of Kraft pulp and pulp waste dissolved and regenerated from NaOH/urea and cupriethylendiamine (CuETD) in amount 0-30% from paper composition increased mechanical properties of paper sheet. CuETD additive was more effective than NaOH/urea additive, however latter has potential for further research as environmentally friendlier.