How High-Loft Textile Thermal Insulation Properties Depend on Compressibility

Repeated compression-recovery test was carried out by the device developed by the Technical University of Liberec [11], as shown in Figure 3. This simple device consists of a transparent perspex cylinder (base diameter is 14 cm) and a pressure plate for a set of required compression values. To ensure accuracy of measurements, five layers of materials were measured simultaneously and the image processing method NIS Element system was used for both before and after loading measurements.

Figure 2

Figure 3

Mutual combinations of two different settings of loading time (10 and 30 min) and two settings of relaxation time (15 and 40 min) were used for the testing. In total, five cycles were done for each type of the test and each sample. The pressure of static loading was 300 Pa. The measuring conditions were arranged according to the standard EN ISO 33886-1: Polymeric materials, cellular flexible – Determination of stress–strain characteristic in compression.

The compression C [%] and recovery R [%] were determined by means of equations (1) and (2). Generally, compression is reduction in volume when pressure was applied. In this case, the compression after relaxation (15 min or 40 min) was measured. (1)C=(h1−h2h1)×100[%]\matrix{{C = \left({{{{h_1} - {h_2}} \over {{h_1}}}} \right) \times 100} \hfill & {[\% ]} \hfill \cr} where h₁ is the original height of the samples and h₂ is the height of the samples after removal of load (after relaxation time).

Recovery is given by equation (2) as the degree to which a sample mass recovered to its original height upon unloading. (2)R=(h4−h3h1−h3)×100[%]\matrix{{R = \left({{{{h_4} - {h_3}} \over {{h_1} - {h_3}}}} \right) \times 100} \hfill & {[\% ]} \hfill \cr} where h₁ is the original height of the samples, h₃ is the height of the samples under load, and h₄ is the height of samples after removal of load.

To study the thickness variation of insulation fabrics under dynamic loading, the measurement device shown in Figure 4 was used. This instrument was developed at the Technical University of Liberec [12]. A pressure plate with a contact area of 78.5 cm² (diameter is 10 cm) moved vertically up and down with a frequency of 400 cycles per min, applying a dynamic load of 6 kPa on the samples. The applied pressure of static loading corresponds to the average loading by straps of a 10 kg backpack. This pressure was determined by the XSENSOR^® X3 pressure mapping system. Twenty-four thousand cycles were applied to each tested sample to simulate real conditions of backpack wearing. Number of applied cycles should be reflecting how many times wearer uses backpack (puts backpack on or off) during two seasons approximately.

Figure 4

The variation of thickness of the tested samples was measured by digital thickness gauge SDL M034A both before and after dynamic loading. Applied pressure (during all measurements of thickness) was set to 50 Pa because the other devices use a low pressure for the measurement of thermal properties, for example, for Togmeter SDL M 259 the pressure is 5 Pa and equipment Fox 314 according to ASTM D1518 measures under a pressure of 70 Pa. Moreover, the carried experiment for fixing thickness of sample by different pressures confirmed the abovementioned conclusion.

The compression C [%] was determined by means of equation (1). Parameter h₂ is the height of the samples, which was measured immediately after the removal of load on the contrary of static loading that is measured under load. This parameter should simulate the sample behavior of the sample after taking the backpack off.

Two times of loading (10 and 30 min) and subsequently two times of relaxation (15 and 40 min) were applied to the tested materials.

These results are in accordance with recent studies [2, 5], indicating that intensity of high-loft insulations compressibility is influenced by the loading time and the time of relaxation. Generally, longer relaxation time ensures decreasing of thickness compression and thereby the reducing heat losses over the original value of fillings. It is caused by reappearing of air gaps in the fibrous structure of fillings. Furthermore, the compression C [%] becomes smaller as the weight of filling increased, in particular by PrimaLoft^® Sport sample A. The values of recovery R [%] confirm these results. This may be because the sample A contains the hollow fibers of bigger diameters that ensure high elasticity.

As can be seen from Figure 9, the trend of compression C [%] after dynamic loading (24,000 cycles) following above mentioned, namely the ability to recover is growing with increasing material thickness.

Figure 9