In recent decades, awareness of the environmental, social and economic crisis, both at the national and international levels, has prompted researchers to develop new, low-cost composites that are more environmentally friendly and safer for humans, using plant materials as reinforcing elements. Compared to conventional synthetic reinforcing materials, lignocellulosic fibers have many advantages. In addition to being renewable, inexpensive, widely available and harmless to health, plant fibers have relatively high specific mechanical properties combined with low density. However, there is a problem with the biostability of plant materials used in the production of composite materials, and to solve it, it was decided to pre-modify plant materials using the monoethanolamine (N-B) trihydroxyborate modifier. In order to find out the effect of the modifier on the plant material, the chemical composition of the plant material was determined before and after the modification. Determination of cellulose content was carried out by the nitrogen-alcohol method according to Kurshner and Hoffer; the amount of hemicelluloses was analyzed by treatment with 2% hydrochloric acid, followed by precipitation of furfural obtained by the bromide-bromate method; lignin content was determined by the Clason method using 72% sulfuric acid; and, finally, the content of extractive substances soluble in organic solvents was determined. It should be noted that, compared with the component composition of the raw plant material, there is a change, and this change is noticeable in the amount of lignin after the modification of the raw material and after extraction, which gives a decrease of almost 2.75 times. This is due to the formation of weak esters under the action of the modifier and, most strikingly, a strong increase in the amount of hemicellulose, which contributes to the depolymerization of cellulose macromolecules under the action of the alkaline modifier used. All this allows us to state that the modification of the crushed stems of Sosnowski's hogweed with monoethanolamine (N→B)-trihydroxyborate changes the composition of the components of the lignocarbohydrate complex of the substrate in the direction of reducing the degree of polymerization and the formation of esters.

Keywords: component composition, cellulose, hemicellulose, lignin, extractives, vegetable raw materials, ligno-carbohydrate complex