# High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers
## 1 Introduction  
Striving for a clean world that is free from polymeric waste, including microplastics, stimulates the development of polymeric materials based on renewable feedstock. The use of products obtained from vegetable sources allows for a solution to this problem because they are renewable and, as a rule, biodegradable. Polyurethanes (PU) are some of the most commonly used industrial polymers from medicine to construction and even in the space industry. The broad range of PU applications leads to a wide range of requirements of their material properties. This problem can potentially be addressed by variation of the components used at PU synthesis. The world flora provides a number of chemicals which can be applied at PU production. Castor oil, obtained from castor beans as the vegetable source, is one of the most used polyols for PU synthesis [1]. However, the mechanical performance and protective qualities of PU materials based on castor oil are far from perfect [2]. One of the ways to improve the performance of castor oil-based PU is the chemical modification of castor oil itself to increase its reactivity and functionality [3]. Pathways of the chemical modification of castor oil that have been used include epoxy ring construction on the double C=C bonds of castor oil [4; 5; 6; 7], ester exchange using ester fragments of castor oil [8; 9], the hydroformylation of double C=C bonds [10] and its oxzonolysis [11; 12]. Other possible modifications of C=C double bonds include a metathesis reaction [13] or being subjected to a click reaction using a thiol-olefin reaction [14; 15; 16; 17]. However, these methods are very laborious and their transfer to industry is highly problematic. The synthesis of composite materials based on the PU matrix is one of the many possible ways to overcome these disadvantages [18], and the developments in this field have recently been reviewed [19]. Bio-based fillers are used for the preparation of composites [20]. Recommendations concerning the choice of fillers in bio-based composites are given in review [21]. The forest is an inexhaustible source of fillers based on wood. The human population has used wood since prehistoric times for products with different properties, shapes, and sizes. Wood is used for the production of paper, household products, construction, furniture, and in industry, including aerospace [22]. Wood-polymer composites are also well studied, and their development has a long history [23]. The intensive exploitation of forests, primarily tropical, leads to a decrease in their area, causing irreparable damage to the environment, and resources of valuable wood are constantly being depleted. In Russia, this applies to conifers that substitute \"weed\" trees such as the birch tree. Wood fibers from birch trees are the shortest in comparison with fibers obtained from conifers, resulting in a negative effect on the properties of polymer composites with thermoplastics [24]. Birch is a fast-growing tree, making this resource quickly renewable. This has led to the idea of introducing wood fibers of birch origin to the production of polymer composites. One of the ways to improve the mechanical performance of polymer composites filled with birch wood fibers is the formation of chemical bonds between these fibers and the polymer matrix [25]. Cellulose has free hydroxyl groups in its polymer chains that can be used in the process of polymer composite preparation [26]. This approach was used in the preparation of polymer composites filled with different vegetable fibers [27]. However, the number of such studies is limited. The economy requires lower production costs to ensure sustainable development. This stimulates the search for inexpensive starting chemicals while maintaining high consumer characteristics of the final products. Birch flour matches the necessary environmental and economic conditions.