Footwear for cold environments. Thermal properties, performance and testing

Present standard on safety footwear (EN 344) checks the insulation only at one point in the shoes by means of measuring the temperature change. A method that uses thermal foot model allows to measure footwear insulation simultaneously at various locations and for whole footwear as well. In the present work the method of heated foot model was developed further. It is possible to simulate sweating and evaluate reduction of insulation of footwear due to wetting and evaporative heat loss. The conditions with various sweat rates, wear length and foot motion were tested. Footwear with various insulation levels (from thin rubber boots to thick winter boots) was evaluated. Some footwear was manufactured both with and without steel toe cap and this allowed to study the thermal effect of steel toe cap in different conditions. Comparative studies between various methods (thermal foot model, humans, EN 344) for evaluating footwear thermal properties/insulation were carried out. Field studies were carried out for evaluation of footwear and feet conditions in real wear situation. The insulation of footwear can vary depending on region and insulation level of the footwear. Heavy winter boots had lowest insulation in toe zone and thin boots had heel zone as the coldest region. Sweat rates of 3 g/h can reduce footwear insulation considerably (9-19 % depending on initial dry insulation). At higher sweat rates (10 g/h) the reduction could be up to 36 %. Combined effects of sweating, walking and wind could reduce insulation by about 45 %. Reduction was bigger in warm winter boots. Only small amount of moisture evaporates from winter footwear during use. Insulation reduction levelled off over longer periods of use. The reduction can be calculated by simple equations. The thermal foot model gave similar insulation values as measured on human subjects in thermal comfort. The insulation values were used for validation of a mathematical model for foot skin temperature prediction. The results obtained with a thermal foot model give more useful information on footwear than does the present standard for footwear thermal testing. Thus, the thermal foot method is recommended for use as a standard. A steel toe cap in a footwear seems to have no influence on insulation, but may modify the heat losses from the foot. The influence could be related to the “after effect” that probably depends on the mass of steel toe cap and its thermal inertia. Some recommendations for use and choice of footwear are given.