Evaluation of Functional Insoles for Protective Footwear Under Simulated Use Conditions

The insoles were made from a composite consisting of three layers that were spot-welded using ultrasound. The composite was optimized by spot-welding to obtain an appropriate bonding strength between the layers while ensuring sufficient elasticity as compared to adhesive-bonded composites [18]. The three layers of the tested insoles were as follows (Figure 1):

upper layer in contact with the foot made of a polyester knitted spacer fabric;

Figure 1

The study involved two composite variants: reference insoles (variant I) and modified insoles (variant II) with middle nonwoven layer containing SAP and a biocide. The insoles were tested in conjunction with knee-high all-rubber safety footwear and reference socks made of cotton and polyamide (70/30%) in simulation studies. Prior to tests, the insoles, socks, and footwear were conditioned at a temperature of 23 ± 2°C and a relative humidity of 50 ± 5% for 72 h.

Inserts of size 42 and 43 were used in the tests. The thickness and length of both variants are summarized in the Table 1. The thickness of the inserts did not depend on the size and was 3.53 mm on the insole surface and 0.9 mm at the weld point, respectively.

Prior to dynamic tests, the insole materials were characterized in terms of moisture vapor transmission and sorption dynamics using a SORP 3 sorption meter [19] (Table 2).

Subsequently, dynamic tests were performed to investigate water absorption and desorption pursuant to the requirements specified for protective footwear in the standard [20], with two modifications that were designed to simulate footwear use in real-life conditions: -

the application of a load (500 N) equivalent to the average weight of a person;

Samples measuring 110 mm × 50 mm were cut out from the front part of insoles and mounted in the apparatus shown in Figure 2.

Figure 2

The apparatus applied the set pressure to the sample on the platform and bent it in a way that is similar to insole behavior during protective footwear use. Perspiration was simulated by the supply of artificial sweat [21, 22]. The setup was placed in a climatic chamber (A) maintaining a temperature of 30°C and a relative humidity of 80% RH to reflect the microclimate inside safety footwear. Prior to tests, the sample was weighed with an accuracy of 1 mg (m_o). The sample was mounted in such a way so that the side of the insole normally facing the foot was in contact with the gauze (G) covering the surface of the platform (H). One end of the sample was attached to the platform, and the other one to the cylinder (C), which exerted the set pressure on the sample. During tests, artificial sweat (I), either acidic (pH 5.5) or alkaline (pH 8), was supplied onto the platform at a rate of 5 mL/h to imitate human perspiration under conditions of strenuous work. Each test lasted 8 h, which equals to a work shift. At the end of the test, the sample was weighed with an accuracy of 1 mg (m_F). Subsequently, the sample was dried under controlled conditions (23°C and 50% RH) for 24 h, and again weighed with an accuracy of 1 mg (m_R).

Absorption (W_A) was calculated in (mg/cm²) from the formula: WA=mF−mOA {W_A} = {{{m_F} - {m_O}} \over A} where m_O is —initial weight (mg), m_F— is final weight (mg), and A— is sample area (cm²).

Desorption (W_D) was calculated in (%) from the formula: WD=mF−mRmF−mO×100 {W_D} = {{{m_F} - {m_R}} \over {{m_F} - {m_O}}} \times 100 where m_O— is initial weight (mg), m_F— is final weight (mg), and m_R— is weight after drying (mg).

After the hygienic tests, the samples were examined for their mechanical properties, that is, tear strength [23] is applied to investigate the changes in insole structure. So, the individual layers of a 30 × 100 mm sample were mounted in the clamps of a universal testing machine (Instron, USA) after 24 h of drying under a controlled atmosphere and pulled at a rate of 100 ± 10 mm/min (Figure 3). Tear strength was calculated in N/mm from the formula: Tearstrength=Meantearingforce[N]Samplewidth[mm] {\rm{Tear}}\,{\rm{strength}} = {{{\rm{Mean}}\,{\rm{tearing}}\,{\rm{force}}\,[{\rm{N}}]} \over {{\rm{Sample}}\,{\rm{width}}\,[{\rm{mm}}]}}

Figure 3

Further, the microstructure of the middle layer made of a melt-blown nonwoven with functional modifiers (SAP and a biocide) was analyzed using a scanning electron microscope (FEI NOVA NanoSEM 230, The Netherlands). Samples of the middle layer were mounted on the microscope platform with a carbon tape. Images were acquired under low vacuum using secondary electron detection with an electron beam of 10 keV.

The microclimate inside protective footwear, i.e., the conditions in the space between the foot, insole, and footwear, was measured using an integrated temperature and relative humidity (T/RH) sensor (iButton, USA). First, an insole and a sock were inserted into the footwear, and an activated TR/H sensor was fitted in the metatarsal region. The tested system was placed in a climatic chamber which maintains temperature of 30°C and a relative humidity of 80%. Continuous measurements were conducted for 8 h with data points recorded every minute. The records were read at the end of the trial. It is assumed that the relative humidity inside the footwear below 80% ensures comfort for the user, while after exceeding 80%, the user begins to perceive discomfort.

The effects of insoles on the relative humidity and temperature in protective footwear were evaluated on a thermal foot model (ATT Electro, Poland). Experiments were conducted at 33°C for 8 h at a sweating rate of 10 g/h, which is characteristic of medium used for hard physical exertion. The objective was to determine relative humidity in the space between the plantar foot region and insole inside the footwear [24].

This part of the study involved five volunteers who were professionally active firefighters. Footwear microclimate tests were conducted on a treadmill set to a speed of 5 km/h (Figure 4). Relative humidity and temperature in the plantar foot region were measured with an accuracy of ±1.5% using an integrated T/RH sensor positioned between the insole and sock, under the longitudinal arch of the foot. Temperature and relative humidity were measured on continuously, with readings taken at every 60 s. The tests were conducted in a laboratory at constant climatic conditions: an air temperature of 23 ± 2°C, a relative humidity of 50 ± 5%, and an air flow of 0.10 m/s [25, 26]. This test consisted of three phases: phase I – 10 min of rest; phase II – walking on the treadmill for 60 min; and phase III – 30 min of rest.

Figure 4

Statistical analysis was performed using Statistica ver. 13.1 software. Statistical significance was evaluated using oneway analysis of variance (ANOVA). Differences between insole variants were assessed by means of Tukey's test at a significance level of p ≤ 0.05. Results were given as means ± standard deviation (SD).

The results of insole tests for liquid absorption and desorption under dynamic conditions are given in Table 3.

Both insole variants met the standard requirements, that is, absorption of 70 mg/cm² and 80% desorption. Variant II samples exhibited higher absorption of alkaline sweat characteristic under strenuous exertion (134.62–155.22 mg/cm² at pH 8) as compared to acidic sweat reflecting skin pH (133.22–152.13 mg/cm² at pH 5). The absorption values obtained for variant I samples were lower by approximately 55–64% for alkaline sweat and by 30–33% for acidic sweat. But, desorption was close to 100% irrespective of insole variant and sweat pH.

Statistical analysis of absorption results revealed significant differences between insole variants, but not between different sweat pH values. No statistically significant differences were observed in the case of desorption results.

Further, scanning electron microscopy was employed to qualitatively evaluate morphological changes in the microstructure of insole samples after the liquid absorption and desorption studies. Figure 5 presents examples of SEM images of samples before and after tests at a magnification of 10,000x, which makes it possible to observe SAP and biocide particles deposited on the surface of polypropylene fibers. As it is seen, modifier particles became redistributed or coagulated (especially in the case of the biocide) in some areas, and their overall quantity slightly decreased. However, it should be noted that no strictly quantitative analysis was possible but the recorded SEM images testify the stability of the bonding of additives to the nonwoven structure, which is important for the development of an innovative product. Importantly, the nonwoven structure (e.g., fiber diameter and orientation) did also not reveal any major alterations, which is crucial in terms of the practical use of hygienic footwear insoles.

Figure 5

Table 4 presents the results of tear strength tests that reflect the bonding strength between the three ultrasonically spot-welded layers. This parameter is important as it indicates the durability of insoles under practical conditions (Table 4).

As it is observed from Table 4, the value 0.4 N/mm does not exceed the threshold value specified in the standard in the case of both variants.

Microclimate test results (temperature and relative humidity) for insole variant I containing an unmodified polypropylene melt-blown nonwoven and variant II with a modified nonwoven were compared and presented in Figures 6 and 7.

Figure 6

Figure 7

The insoles did not increase temperature inside the footwear, which reached 29°C for both variants that is well below the discomfort threshold of 35°C reported in the literature [27].

Higher relative humidity inside footwear (76–78%) was recorded for insole variant I without modifiers as compared to variant II containing a modified melt-blown nonwoven (74–76%). This indicates that the addition of SAP helped in absorbing moisture leading to drier conditions in the footwear. It should be noted that neither insole variant exceeded the discomfort threshold of 80% RH defined in the literature [27].

The effects of insoles on relative humidity and temperature in protective footwear were also investigated using a thermal foot model. It was observed that the type of insole affected the speed of reaching the discomfort level. Footwear with insole variants I was found to exceed the 80% RH threshold after 3 h of use under the conditions of strenuous physical exercise (Figures 8 and 9), thus compromising user comfort. But, in variant II the level of discomfort was achieved only after 5 h. It should be noted that the presented experiments reflected the continuous use of footwear with insoles irrespective of any rest intervals that are normally required during actual hard physical work.

Figure 8

Figure 9

The results obtained from tests on human subjects show that the discomfort threshold for temperature (35°C) was not exceeded in most cases irrespective of insole variant, indicating comfortable conditions during strenuous physical work (Figure 10). However, the application of SAP in variant II decreased temperature by 2–3°C, which suggests superior ventilation in that insole variant. In the case of two out of five volunteers, relative humidity also remained below the discomfort threshold of 80% (Figure 11). The relative humidity recorded for the remaining users was 85–95%, which may be attributable to individual physiological traits. Notwithstanding the above, the insoles with an unmodified middle nonwoven layer did not ensure sufficient sweat absorption from the skin, resulting in 82–100% RH for all participants.

Figure 10

Figure 11