## 4 Conclusions  
The subject of the research constituted cotton/polyester blend textiles exposed to thermal radiation under controlled conditions that did not cause changes visible to the naked eye in these products. The purpose of the study was to show how thermal changes to this type of material can be revealed under laboratory conditions. In the case of the examination of changes to the single fibres, such an assessment was carried out using methods being applied in forensic laboratories. The performed investigations show that the changes occurring in the fibres due to the action of heat depend not only on the type of fibres but also on the structural parameters of the textile product. The most essential geometrical parameters of the textiles that determine their thermal resistance in the case of impact of heat radiation are porosity and the mutual positions of particular types of fibres in the three-dimensional structure of yarns and fabrics. The conducted research has shown that the greater porosity of the textile product, especially the loose, spatial structure of the surface layer, promotes the thermal destruction of the fibres under the influence of heat radiation. Among the tested textiles, the woven fabrics _Sample 3_ and _Sample 4_ (porosity values 74% and 76%, respectively) showed greater thermal resistance than the knitted fabrics _Sample 1_ and _Sample 2_ (porosity values 86% and 90%, respectively). Changes in the morphological structure due to heat appeared in polyester fibres before changes in cotton fibres occurred. Polyester fibres undergo permanent changes in terms of dimensions and shape (shrinkage, deformation, melting). In contrast, cotton fibres, under the influence of heat, changed their chemical structure earlier than PES fibres. This is evidenced by the results of FTIR analysis of cotton and PES fibres. The assumption that microscopic analysis of textile materials will make it possible to observe and identify some thermal changes in threads and fibres caused even during a short-term temperature increase has been positively verified. The changes observed in the structure of the fibres depended on the thermal properties of fibre polymer; cotton made of thermoreactive polymer (cellulose) underwent more or less extensive charring, while polyester fibres, made of thermoplastic polymer, tended to soften, melt and deform. However, in the examined samples, next to thermally changed fibres, i.e., charred (cotton) or deformed (polyester), fibres with no microscopically observable changes were detected. The studies also have shown that on the surface of some original cotton textiles containing small amounts of synthetic fibres, the presence of tiny deformed balls at the ends of the thermoplastic fibres can be expected. This is caused by a singeing process of the textile product during manufacture in order to make it smoother and lustrous. Out of the applied techniques, scanning electron microscopy showed the highest efficiency in tracking morphological changes in fibres occurring under the influence of heat radiation. Changes to the shape of whole yarns, particular fibres and their surface were observed. In turn, optical microscopic methods, despite their simplicity and ease of preparation of test samples, were characterised by relatively high usefulness in the identification of thermal changes in textile materials, especially in terms of the physical microstructure of PES fibres. Changes observed in the FTIR spectrum of cotton fibres derived from _Sample 2_ (exposure time: 40 s) were of a chemical nature, as evidenced by the emergence of a new band. No appearance of new bands or disappearance of the other ones were observed in FTIR spectra of the rest of the examined fibres. It can be concluded that neither of the instrumental methods used in this work, i.e., FTIR spectroscopy, optical and electron microscopy, was found useful for the complete evaluation of the thermal changes in the morphological and chemical structure of fibres if used on its own.