The first model was calculated based on the collected data to determine the optimal region. Statistically insignificant variables were identified on the Pareto chart and excluded from the model. Based on the Shapiro-Wilk test, the normal distribution of residuals was confirmed, which allows the use of the RSM method. Because of the lack of repetitions, the model could not be tested statistically. The main purpose of this model is to make fast screening of studies process and propose, not to determine, optimal conditions. As the optimal region, COOH conversion was assumed to be 70-73% because these values are close enough to the gell point (75%) to ensure the possibility of both ending the reaction before gelling and rapidly reaching the gelling point during thermal cross-linking. Due to that, a heat map (Figure ) was created to image the optimal region (green). Due to the apparatus used, the temperature of 145℃ was favourable because water collection starts before reducing pressure, and after reducing pressure, there was no violent boiling. Three experiments at 145℃ and 105, 110, and 115 min were performed to estimate optimal conditions. These experiments and previous data were used to calculate the final model (Table ). Like before, statistically insignificant variables were excluded from the model by using Pareto chart. The normal distribution of residuals was confirmed by the Shapiro-Wilk test. The last three experiments could be used as repetitions, so the lack of fit could be tested this time. The calculated model fit the obtained results very well, which was confirmed statistically (lack of fit insignificant). Based on the model, optimal conditions were calculated. The final optimal experiment was performed to verify the validity of optimization. Results are shown in Table . The model and response surface are presented in the figure Figure .