*3.1. Synthesis of Maleinized Hemp Seed Oil and Brazil Nut Oil*

The evolution of the AV values along the three temperature stages of 180 ◦C, 200 ◦C and 220 ◦C used throughout the maleinization process of BNO and HO has been plotted in Figure 2. Initially, the AV of BNO is 0.20 mg KOH g−<sup>1</sup> and 8 mg KOH g−<sup>1</sup> for the HO. In both cases, it can be observed how these values increase significantly and analogously after the first hour at 180 ◦C to approximately 47 mg KOH g−<sup>1</sup> and 50 mg KOH g−<sup>1</sup> for the BNO and HO, respectively, indicating that maleinization is occurring. After two hours, at the second temperature of 200 ◦C, another significant increment can be observed up to values close to 90 mg KOH g−<sup>1</sup> in the case of BNO and up to 80 mg KOH g−<sup>1</sup> in the

case of HO. Finally, at 220 ◦C, after three hours, there is a drastic increase in the AV up to 105 mg KOH g−<sup>1</sup> in HO and even up to 130 mg KOH g−<sup>1</sup> in BNO. In the two oils, the AV follows a very similar trend in which a clear increase is observed as the temperature increases. This is due to the fact that at temperatures close to 200 ◦C, the "ene" reaction is favored, in which maleic anhydride can easily bind to an allylic position of the unsaturated fatty acid [35]. As the temperature of 220 ◦C is maintained over time, the acidity index tends to stabilize, in the case of HO at 105 mg KOH g−<sup>1</sup> and at 130 mg KOH g−<sup>1</sup> for BNO. Both values are in complete agreement with the AV of commercial MLO reported by Quiles-Carrillo et al. [36], which indicates that it ranges between 105 and 130 mg KOH g−<sup>1</sup> , suggesting that both the MBNO and MHO developed have similar characteristics to one of the few maleinized oils available on the market today. HO. Finally, at 220 °C, after three hours, there is a drastic increase in the AV up to 105 mg KOH g−1 in HO and even up to 130 mg KOH g−1 in BNO. In the two oils, the AV follows a very similar trend in which a clear increase is observed as the temperature increases. This is due to the fact that at temperatures close to 200 °C, the "ene" reaction is favored, in which maleic anhydride can easily bind to an allylic position of the unsaturated fatty acid [35]. As the temperature of 220 °C is maintained over time, the acidity index tends to stabilize, in the case of HO at 105 mg KOH g−1 and at 130 mg KOH g−1 for BNO. Both values are in complete agreement with the AV of commercial MLO reported by Quiles-Carrillo et al. [36], which indicates that it ranges between 105 and 130 mg KOH g−1, suggesting that both the MBNO and MHO developed have similar characteristics to one of the few maleinized oils available on the market today. HO. Finally, at 220 °C, after three hours, there is a drastic increase in the AV up to 105 mg KOH g−1 in HO and even up to 130 mg KOH g−1 in BNO. In the two oils, the AV follows a very similar trend in which a clear increase is observed as the temperature increases. This is due to the fact that at temperatures close to 200 °C, the "ene" reaction is favored, in which maleic anhydride can easily bind to an allylic position of the unsaturated fatty acid [35]. As the temperature of 220 °C is maintained over time, the acidity index tends to stabilize, in the case of HO at 105 mg KOH g−1 and at 130 mg KOH g−1 for BNO. Both values are in complete agreement with the AV of commercial MLO reported by Quiles-Carrillo et al. [36], which indicates that it ranges between 105 and 130 mg KOH g−1, suggesting that both the MBNO and MHO developed have similar characteristics to one of the few maleinized oils available on the market today.

in Figure 2. Initially, the AV of BNO is 0.20 mg KOH g−1 and 8 mg KOH g−1 for the HO. In both cases, it can be observed how these values increase significantly and analogously after the first hour at 180 °C to approximately 47 mg KOH g−1 and 50 mg KOH g−1 for the BNO and HO, respectively, indicating that maleinization is occurring. After two hours, at the second temperature of 200 °C, another significant increment can be observed up to values close to 90 mg KOH g−1 in the case of BNO and up to 80 mg KOH g−1 in the case of

in Figure 2. Initially, the AV of BNO is 0.20 mg KOH g−1 and 8 mg KOH g−1 for the HO. In both cases, it can be observed how these values increase significantly and analogously after the first hour at 180 °C to approximately 47 mg KOH g−1 and 50 mg KOH g−1 for the BNO and HO, respectively, indicating that maleinization is occurring. After two hours, at the second temperature of 200 °C, another significant increment can be observed up to values close to 90 mg KOH g−1 in the case of BNO and up to 80 mg KOH g−1 in the case of

*Polymers* **2021**, *13*, x FOR PEER REVIEW 6 of 18

*Polymers* **2021**, *13*, x FOR PEER REVIEW 6 of 18

**Figure 2.** Effect of temperature and time on the efficiency of the maleinization of HO and BNO with maleic anhydride. **Figure 2.** Effect of temperature and time on the efficiency of the maleinization of HO and BNO with maleic anhydride. **Figure 2.** Effect of temperature and time on the efficiency of the maleinization of HO and BNO with maleic anhydride.

Another consequence observed as a result of the maleinization process is an important change in the color of the oils. Figure 3 shows a progressive color change of BNO, in which, at 180 °C, the color is light yellow, similar to the color of the original oil, and changes to a reddish color as maleinization occurs. The same occurs with HO. Ernzen et al. [37] reported a similar change during the maleinization process of soybean oil, in which the oil changed from a yellow to an orange–reddish color. Another consequence observed as a result of the maleinization process is an important change in the color of the oils. Figure 3 shows a progressive color change of BNO, in which, at 180 ◦C, the color is light yellow, similar to the color of the original oil, and changes to a reddish color as maleinization occurs. The same occurs with HO. Ernzen et al. [37] reported a similar change during the maleinization process of soybean oil, in which the oil changed from a yellow to an orange–reddish color. Another consequence observed as a result of the maleinization process is an important change in the color of the oils. Figure 3 shows a progressive color change of BNO, in which, at 180 °C, the color is light yellow, similar to the color of the original oil, and changes to a reddish color as maleinization occurs. The same occurs with HO. Ernzen et al. [37] reported a similar change during the maleinization process of soybean oil, in which the oil changed from a yellow to an orange–reddish color.

**Figure 3.** Influence of the reaction temperature and time on the color during maleinization of BNO, (**a**) 180 ◦C–0 min, (**b**) 200 ◦C–120 min and (**c**) 220 ◦C–180 min.
