Surface tension of kraft lignin (KL) and KL/poly(N,Ndimethylaminoethyl methacrylate) mixtures, with different composition at the air-water interface depending on pH and temperature, was measured. It was found that the surface activity of both KL and the reaction mixtures grows with decreasing pH and increasing temperature. At the same time, the surface activity of the KL/poly(N,Ndimethylaminoethyl methacrylate) reaction mixture, irrespective of its composition under the same environment conditions, considerably exceeds the surface activity of KL, indicating the synergetic effect governed by the peculiarities of the polymeric structure of the formed soluble particles of the polyelectrolyte complex.

A lignin-containing adhesion enhancer, a water-soluble polyelectrolyte complex (LPEC) composed of soda hardwood lignin (HASL) and polyethylenimine (PEI), and the effect of the treatment of hydrolyzed and ammoxidized aspen sawdust with the LPEC nanoparticles were studied relative to the properties of wood-plastic composites (WPCs) based on recycled polypropylene (rPP). The presence of the excess of free amine groups and salt bonds between PEI and soda lignin, forming hydrophobic sites in the LPEC structure, caused the enhanced surface activity of the LPEC. The treatment with the LPEC nanoparticles increased the content of the fixed nitrogen in the modified sawdust samples and was accompanied by decreasing their water sorption and increasing contact angles that favored the decrease in the polar part of their surface free energy. The decreasing wetting ability enhanced the mechanical and water sorption properties of the obtained WPC samples. The improvement of the interfacial adhesion between the nitrogen-containing groups of the treated sawdust and the oxygen-containing groups of rPP was explained by the formation of both covalent and physicochemical bonds.

The aim of this work was to obtain eco-friendly constituents for making wood-polymer composites (WPCs) by rational utilisation of hardwood residue with its mechanical processing and purposeful chemical modification. To reduce the energy input for residue milling for obtaining a lignocellulosic filler as well as to activate its surface for the further modification, the optimal parameters of low temperature acid hydrolysis of the hardwood residue under mild conditions were found. To enhance the wetting of the lignocellulosic filler with respect to recycled polypropylene, the milled hydrolysed wood particles were modified by ammoxidation. For obtaining a lignin-containing surface tension regulator (compatibiliser) that is able to reinforce the interaction between the constituents at the interface, soda lignin was obtained via delignification of the hydrolysed hardwood residue under laboratory conditions. The developed compatibiliser represented nanoparticles of a non-stoichiometric polyelectrolyte complex, incorporating soda lignin and synthetic polyamine - polyethylenimine. By using appropriate laboratory equipment and the determined technological regime, aspen wood-recycled polypropylene composites were made and tested.

The effect of softwood sulphate lignin modified by its oxidation in alkaline medium on its behaviour at the air-water and oil-water interface, where rapeseed oil and technical fish fat oil served as the oil phase, was investigated. It has been found that, irrespective of the рН values of the water solution, the ability of the modified lignin to be adsorbed at the interfaces exceeds that of unmodified lignin, which is obviously caused by the pronounced differentiation of the hydrophilic and hydrophobic parts of the molecular structure in the oxidised lignin and the decrease in the content of the high-molecular fraction (Mw is greater than 1000) in it. At low concentrations (≤ 0.01 g/dl), the stabilising action of the modified lignin in the rapeseed oil-water emulsion is the same as that of sodium dodecyl sulphate. The enhanced surface activity of the oxidised sulphate lignin enables it not only to substitute the 30% of the mass of the commercial surfactant in the detergent composition for washing stainless steel plates, used during fish smoking, but also to upgrade its properties.

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