I've looked for literature to "explain" my results but i haven't found any similar behaviour.
The measurements were performed in nitrogen environment. The material is erythritol (sugar alcohol) and it usually has a one single step decomposition.
Also, between the onset point and the total decomposition there are only 15*C which is very low compared to literature (it usually lost its weight completely at 290-300*C)
Accepting that the sample in indeed pure erythritol, it seems that about 41% of its molecular weight was lost shortly before all the sample was completely vaporized, what might perhaps correspond to early contribution of volatile specie(s) accounting to the molecular weight of 0.41·122 = 50 g/mol. It is perhaps interesting to note that the combined molecular weight of methanol (32) and water (18) matches that figure. That might be completed by shortly later vaporized residue corresponding to 72 g/mol. The hypothetical lost of CH6O2 from the erythritol formula (C4H10O4) suggests that such residue might correspond to methylglyoxal or to acrylic acid, where the higher boiling point of the last (141 ºC) could perhaps better explain a slight delay for its complete vaporization, even if thermal decomposition occurs by a single step. Moreover, the well-known esterification reaction between methanol and acrylic acid to yield water and methyl acrylate ― which differs from erythritol by two water molecules ― suggests that acrylic acid may possibly be also involved, if methanol indeed occurs among decomposition products.
Of course, that would be just a first guess tentative explanation, that additional data, such as from DSC and from evolved gas analysis, might contribute to either confirm or refute.
Also to be noticed, is that a mixture of erythritol with its diastereomer threitol could perhaps alternatively explain a resembling TGA trace.
Onset about 170.36 C, it indicates its melting point. Rest is already discussed above.
Thank you very much Carlos Araújo Queiroz !!! I really appreciate your help.
However, there's still something that I haven't totally understood und that is the relationship between the esterification reaction between methanol and acrylic acid and the more likely possibility of acrylic acid to be the residue that remains after the small step at around 175°C instead of methylglyoxal.
The fact that acrylic acid has a higher boiling point (141 °C) is a good explanation about why this compound evaporates later though.
Thank you again for your answer
Paulina... In such cases, we watch the behavior of the sample in a melting poin capillary. The material may start decomposition and melts. The decomposition continues after melting... Again try to watch your sample in the melting oint apparatus specifically between 160 to 180 oC. ... It will be my pleasure to keep discussion if you feed me back... Regards
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