The Thermogravimetric,Microscopic and Mechanical Analyses of PBT and PET yarns(5)

3.2 Thermal Analysis

The thermal behaviors of PBT yarn as well as unmodified and modified PET yarns were investigated by TGA thermograms. The thermal properties of the samples were presented in Table 2.
It was observed that all the fibres had different weight-loss stages and onset temperatures. It is generally known that thermal decomposition of polyester usually starts with the scission of the polymer chain and polyester generates benzoic acid,carbon dioxide, carbon monoxide,benzoic acid derivatives, oligomers and polycyclic aromatic hydrocarbons upon high temperature decomposition (9). From the Table 2, it can be said that flame retardant PET undergoes single stage degradation with a single peak at around 306,80°C, whereas the degradation temperatures of other samples are in the range of 350-375°C. This fact is in accordance with the previous studies (9, 12).In Figure 3, the TGA graphs of the regular PET and flame retardant PET (the most distinctive difference observed ones) are given.

From the data in Table 2, the onset temperature of flame-retardant PET was found to be the lowest one in comparison with the others. In the previous study of Chang et al, the reason of this reduction of Tdi (initial degradation temperature) was attributed to the fact that the O=P–O bond was less stable than the common C–C bond. Furthermore, it was seen that the char yield increased with phosphorus content, meaning the flame retardancy was improved. The accession to high char yield was estimated to be through two ways.Firstly, flame retardant component interrupted the exothermic processes and suppressed combustion by capturing free radical or secondly, it increased the char yield by redirecting the chemical reactions involved in decomposition in favor of reactions yielding carbon rather than CO or CO2 and through the formation of a surface layer of protective char (13).

When the UV protective and antibacterial polyester yarns were considered, it was determined that the initial degradation temperature and char residue were slightly higher than the regular PET. For antibacterial polyester yarn, this increase was attributed to the development of intermolecular attraction among the polymer chains by the presence of silver nanoparticles. The improved thermal stability could also be explained through the reduced mobility of the polyester chains. For UV protective yarn, this increase was estimated to be due to the presence of TiO2 particles in the yarn remained at higher temperatures (14, 15). When the thermal decomposition behavior of PBT was compared to that of PET, since PBT had high number of methylene groups in the main chain, it was observed that PBT had a low initial degradation temperature (Tdi=351,74°C) and high char residue.These results were in accordance with the involved papers (14-16).

Writen by Buket ARIK1, Ebru BOZACI1, Asli DEMIR2, Esen ÖZDOGAN1