# High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers
## 3 Results  
For the synthesis of PU-based coatings, we chose the trifunctional oligomer of methylene diphenyl diisocyanate because it is the cheapest commercial isocyanate, which, combined with the three hydroxyl functionalities of castor oil, offers the simplest combination of the PU polymer matrix. The problem of commercial castor oil is its relatively high content of water that is connected to its origin from the vegetable raw materials processed by pressing castor beans. The presence of water induces the formation of bubbles during PU synthesis [31], which negatively affects the mechanical performance (Figures 2 and 3). Hence, one of the most important tasks of this study is the search for methods and additives to castor oil that can help solve this problem. The tensile strength and elongation at break of PU prepared from the as-received castor oil and PM 200 are given in Table 1. As one can see, the obtained value of tensile strength is low. The presence of urea units in the synthesized polymer is due to the presence of water in castor oil. The reaction of water with isocyanate forms amine groups, further reacting with isocyanate with the formation of urea units [33]. Normally, the presence of urea groups in a PU structure leads to an improvement in the mechanical performance of the composition [34]. However, the presence of urea groups in synthesized polymer leads to the formation of two types of urea units (H-bonded (1664 cm\({}^{-1}\)) and free (1684 cm\({}^{-1}\))), and two types of urethane units, which are attributed to free carbonyls (i.e., non-H-bonded), and hydrogen bonded corresponding to the bands at carbonyl groups 1728 and 1702 cm\({}^{-1}\), respectively (Table 2). The presence of urea units causes an increase in the fraction of free carbonyl groups in the urethane fragments. Hence, the presence of urea units leads to the formation of \"loosened\" rigid domains in PU, contributing as an additional factor to the deterioration of the mechanical properties [35]. The band at 1742 cm\({}^{-1}\) is connected to the presence of ester bonds in castor oil [35]. The presence of wood fibers in the composites has practically no effect on the IR spectra (Figure 4). The band at 3050 cm\({}^{-1}\) is attributed to the valence vibration of aromatic C-H bonds presented in lignin contained in wood [36]. The analysis of the IR spectra (Table 2) shows that the introduction of birch flour leads to an increase in the fraction of H-bonded carbonyl groups in the urethane units. Therefore, the degree of phase separation (DPS) in the PU matrix grows, although not as significantly. Typically, the increase in DPS leads to an improvement in the mechanical performance [37]. The increase in DPS can be connected with the surface of the birch particle, which is covered by hydroxyl functionalities from cellulose and lignin macromolecules. The interactions of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Samples** & **Description** & **1664 cm\({}^{-1}\), \%** & **1684 cm\({}^{-1}\), \%** & **1702 cm\({}^{-1}\), \%** & **1728 cm\({}^{-1}\), \%** \\ \hline
1 & With neat castor oil & 10.9 & 3.3 & 42.4 & 43.4 \\
2 & Castor oil dried with zeolites & 0 & 0 & 50.3 & 49.7 \\
3 & With PDMS & 8.5 & 0 & 47.2 & 44.3 \\
5 & 10 wt.\% birch flour & 0 & 0 & 54.8 & 45.2 \\
6 & 20 wt.\% birch flour & 0 & 0 & 52.3 & 47.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Deconvolution of valence band of C=O in FTIR spectra of PU composites filled with birch flour. Figure 5: FTIR spectra in the carbonyl stretching region with deconvolution ((a) sample 1; (b) sample 6)). these functionalities with urethane units leads to the ordering of hard segments of the PU matrix near the surface of the birch fibers, which contributes to phase separation. This is one factor in the improvement of the mechanical performance of the PU composites after the addition of birch flour. To solve this problem, we used the most common antifoaming approaches: the drying of castor oil and the addition of polydimethylsiloxane (PDMS) [38].