## 3 Results and Discussion
The solubility parameter, \(R_{A}\), between the two materials is calculated using the equation based on the corresponding partial components of the solubility parameter (2):  
\[R_{A}=\sqrt{4\cdot([\delta_{d2}-\delta_{d1}])^{2}+\left(\delta_{p2}[-\delta_{ p1}]^{2}\right)+(\delta_{h2}-\delta_{h1})^{2}} \tag{2}\]  
where \(R_{A}\) is the distance from the center of solubility of the polymer to the coordinates of the solubility parameter of plasticizers; \(\delta_{d}\), \(\delta_{p}\) and \(\delta_{h}\) are the solubility parameters of the dispersion, polar interaction and interaction due to hydrogen bonds; and indices 1 and 2 relate to the polymer and the plasticizer, respectively. For high compatibility, it is necessary that \(R_{A}\) be less than \(R_{0}\). The \(R_{A}/R_{0}\) ratio was called the \"red\" number (3), reflecting the relative energy difference:  
\[RED=R_{A}/R_{0} \tag{3}\]  
Table 3 shows the calculated values for the solubility parameters, the total solubility of \(R_{A}\) and the amount of \(RED\) for all the obtained diesters. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Compound** & \(\delta_{d}\) **(MJ/m3)1/2** & \(\delta_{p}\) **(MJ/m3)1/2** & \(\delta_{h}\) **(MJ/m3)1/2** & \(\delta\) **(MJ/m3)1/2** & \(R_{A}\) & \(RED\) \\ \hline DBEA & 15.29 & 8.06 & 4.80 & 17.94 & 7.07 & 0.850 \\ DBPA & 15.29 & 8.30 & 4.80 & 17.94 & 7.07 & 0.850 \\ DPEA & 18.99 & 9.02 & 5.49 & 21.73 & 1.34 & 0.296 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of solubility: total solubility of \(R_{A}\) and \(RED\) numbers for acid diesters. A _RED_ number less than 1.0 indicates a high affinity of the plasticizer and polymer, and with higher numbers, compatibility decreases. The calculated values for individual solubility parameters for dibutoxyethyl adipate and for dibutoxypropyl adipate coincide and are slightly below the limit \(\delta_{d}=15.4\) for PVC. The minimum value of this indicator was obtained for diphenyloxyethyl adipate. However, in general, for all semi-finished compounds, the _RED_ number is less than 1, which implies their good compatibility with PVC. A practical way to assess the compatibility of a compound with PVC is to determine the critical dissolution temperature of the polymer. When processing plasticized PVC, it is known that good manufacturability is achieved at temperatures exceeding the critical dissolution temperature by 20-80 \({}^{\circ}\)C. If the critical temperature of the product is in the range of 90-135 \({}^{\circ}\)C, the plasticizer demonstrates excellent compatibility and is suitable for the production of products at normal pressure with heating up to 160 \({}^{\circ}\)C. In the case when the critical dissolution temperature of the polymer in the plasticizer is 136-160 \({}^{\circ}\)C, its solubility decreases, and it is usually used in a mixture with basic plasticizers. For a comparative assessment of the compatibility of synthesized compounds with the reference plasticizer dioctyl phthalate, parameter A was used, which was calculated using a special formula (4):  
\[A\left(\%\right)=100\;T_{DOP}/T_{cr} \tag{4}\]  
where \(T_{DOP}\) is the critical temperature of PVC dissolution in DOP and \(T_{cr}\) is the critical dissolution temperature of PVC in the compound under study. The results are presented in Table 4.  
The results indicate a strong correlation between the critical dissolution temperature and parameter \(A\) with the Hansen solubility parameters calculated previously. Among the esters examined, dibutoxypropyl adipate stands out due to its notably low critical dissolution temperature with PVC, suggesting enhanced compatibility with this polymer. This compatibility implies its potential as an effective primary plasticizer, enhancing the performance and processing of PVC materials. In summary, dibutoxypropyl adipate's properties make it a promising candidate for applications requiring improved flexibility and stability in PVC formulations.