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.