Figure 5. TGA and DTA curves for the cellulose acetate sample (CA) (A) and for the sample of cellulose acetate nanofibers with annatto extract (CA@A) (B).
For the neat sample, an initial weight loss (3.8%) was seen before reaching 100 °C; this could be the result of an endothermic water loss process [38] in which water is physically adsorbed between the CA molecular backbones. This initial weight loss is also ascribed to the remains of solvents between the nanofibers [39]. For the CA@A sample a higher weight loss (5.3%) before 100 °C was observed. The presence of annatto most likely increased the amount of water between the polymer molecules (hydrophilicity). A second weight loss was observed between 245 ºC and 373 ºC for CA and between 237 ºC and 383 ºC for the CA@A samples. This second weight loss is attributed to thermal degradation of CA [26], deacetylation and pyrolytic decomposition of the polymer backbones (between 240 and 380 °C) [40]. The TGA results consequently suggest that CA was thermally stable during the preparation of the solution (25 °C) and electrospinning (25 °C), even when the annatto was present at 1% wt. Figure 5 (C) shows the weight loss curves for both the CA and CA@A samples; up to 275 ºC a slight increase in thermal stability can be perceived for CA@A. The presence of annatto extract molecules may hamper the reactions involved in thermal degradation of CA. At 368 ºC a crossover occurred, with inverted behavior above this temperature and the CA@A sample exhibiting a lower weight loss rate than the samples of neat CA. This phenomenon can be associated with the thermal degradation of bixin and norbixin molecules, which is reported to occur at 280–380 °C [41]. Another thermal event was observed around 533 °C for the CA sample and 566 °C for the CA@A sample, and can be attributed to carbonization of the remaining polymer [26]. The residual content at 900 ºC was around 2.2% for the CA sample and 3.9% for CA@A.