## 2 Materials and Methods
### Methods
The choice to use the studied temperature range between +5 \({}^{\circ}\)C and +25 \({}^{\circ}\)C is explained as follows. The upper temperature +25 \({}^{\circ}\)C is the typical indoor temperature at which the clothing is stored and accumulates heat between working shifts. The temperature +5 \({}^{\circ}\)C is the typical temperature in the off-season, which is taken as the boundary between the cold and normal conditions and allows for the initial human cooling [32]. The samples have a square shape. The length of the side of the square is 250 mm. The measurement of the sample sizes corresponds to [33]. The sample is dried to a constant weight in a drying oven. The heat flow in the experiment is considered to be stationary if the thermal conductivity values, as measured at 5 consecutive time points with an interval of 300 s, differ from each other by less than 1% with constant signals from the measuring devices. Measurement errors in the present experiment:  
* that of temperature sensor signals: 0.5% max. ;
* that of the heat flux density meter signal: 0.6% max. ;  
Figure 2: Schema of the apparatus for measuring the thermal conductivity of test samples: 1, sample of the test material; 2, heater; 3, insulation block; 4, housing; 5, built-in heat meter, including heat flux density meter; 6, temperature sensors; 7, refrigeration unit; 8, lower back panel (switch, fuse, power cord outlet, and heater connector); 9–14, control keyboard (9, start and end of work; 10, access to the menu and the control to save measurement results; 11, work with the function menu, 12, reset data; 13 and 14, control of measurement conditions); 15, height adjustment platform for measuring elements (fixing the thickness of material samples). * that of the sample thickness measurements: over 0.5% (less than 0.1 mm) as the arithmetic mean of four point measurements of the sample;
* that of the pressure 0.2 kPa [34]: 1.5% max. ;
* the deviation of the front faces of a hard sample specimen from parallel alignment: 0.5 mm max. ;
* the loss of its mass after drying: 0.1% max. The study of the fibrous microstructure of the bonded Termofiber (No. 1 in accordance with Table 2) canvas allowed us to perform a quantitative and qualitative assessment of the 2D and 3D digital parameters of the studied fiber structure (Figure 3). Termofiber has large, internal free cavities between fibers. This allows for the design of the interface of such a structure with parts of the microencapsulated HPM. The analysis of the Outlast textile (No. 2 in accordance with Table 2) showed that it has a three-layer structure. Each layer was examined separately from different surfaces. A cross-section was used to examine the inner layer (Figure 4). Based on the research, we established the structural parameters of the Termofiber fibrous canvas:  
- fiber diameter: 27 to 30 \(\mu\)m;  
- cavity depth: 310 to 545 \(\mu\)m;  
- average cavity diameter: 80 to 120 \(\mu\)m.  
For the Outlast textile heat-preserving material, we have established the structural parameters as presented in Table 4.  
Figure 4: The microstructure of fibrous, non-woven heat-preserving Outlast textile material: (**a**) the microstructure for a surface with Outlast (500\(\times\)); (**b**) the microstructure for the cross-section (100\(\times\)). The Termofiber and Outlast fiber size ratios, including the sizes of the heat-preserving capsules, have a common averaged range of values. However, an uneven concentration of the encapsulated bonding substance in the Outlast volume does not allow us to consider it for use in a complex insulating material as elements with sizes that are similar to Termofiber fibers. Given that a relative structural regularity of the encapsulated fabric is established for sections over 2000 \(\upmu\)m, it is advisable to consider the complex structure of the insulant as a combination of layers of the main Termofiber and Outlast textile in common multi-layer insulating materials [35].