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Part III

Thermal Garments:

 

The effective way for protection against cold is to optimize the clothing insulation, so we can avoid heat loss due to heat exchange. Protection from cold also depends on the body metabolic rate; if it is less the required thermal resistance will be higher to maintain balance. In contrast to this at high metabolic rates the generated heat must be transferred to environment, this can be achieved by reducing the number of insulating layers. Insulation increases with increases in number of layers and thickness of clothing. Insulation is build up by air layers by trapping them inside the structure and minimizes internal convection. Better thermal insulation can be obtained by controlling thermal conductivity of the material. Textile materials are good thermal conductors. To design them to meet thermal insulation we have to trap the air inside the fabric, air is the good thermal insulator. In the cold, there is a steep in the temperature gradient from the skin across clothing layers to the ambient air. The dew point temperature and also the freezing temperatures may reach inside the clothing which causes condensation. This moisture build up leads to wetting of clothing layer, results in poor thermal insulation.

 

Basically textiles are good thermal conductor so by trapping air or giving some special coatings we can improve its insulation property. The nature of wool fiber such that due to its lofty and bulky nature it can trap more air inside its fiber arrangement and this entrapped air serves better role towards the thermal insulation. Apart from these wool fiber can generates heat if it absorbs moisture, which is called heat of sorption.

US Army report shows us the thermal insulation of various garment item, which states that thermal insulation is higher in case of fleece material and the table gives us the idea about the insulation behavior of various products.

 

The rate of decrement of insulation as the wind velocity increases. The main reason is that the structural distortion of the fiber or air pockets, which directly influences the decrease in thermal insulation. Research work4 shows us, the insulation achieved by the polished surface is 40% higher than the blackened surface.

 

Thermal insulation can also be provided by the reflecting layers such as aluminium foils. Aluminium can be coated into the structure or layered with the textile structure to provide required thermal insulation. Perfect insulation can be achieved by having low compression / high resiliency to maintain the entrapped air inside the structure.

 

Permeability mainly influenced by the structure of the textile materials. Irrespective of their type of structure each structure has their own permeability factors. Permeability also plays vital role towards comfort of garments.

If the air permeability is high during wind chill conditions the structure may not solve the purpose. Higher permeability results distortion of air pockets in the structure and the thermal insulation becomes less.

 

Our body continuously generates heat energy to facilitate comfort to with the environment. Performance of the clothing system at cold regions not only depends on the material thermal insulation characteristics, along with that the material should have moisture vapor transmission behavior. Decrease in environmental temperature results condensation in the structure. If the cloth becomes wet due to the vapor condensation or absorption, presence of moisture leads to the increased thermal conductivity. Which results sudden drop in the body temperature and in some cases results dangerous situation with respect to environment condition. Water vapor resistance of cloths increases in low temperatures, such as in mountains. Although in some cases the resistance of cloth decreases due to the reduction in pressure. Condensation is high at high altitude because of reduced vapor diffusion resistance and lower saturated vapor concentration at lower temperatures. Reduction in moisture vapor transmission enhances the condensation as well as discomfort. Apart from these there is a chance of hypothermia which can lead to fatal at high altitude10.

Water vapor transmission can be increased by increasing the pore size of the textile structure. The vapor transfer rate is determined by the concentration gradient, which is depends on the temperature. The research11 shows, the thermal insulation, moisture vapor transmission results for the different structures like polyester fleece, micro porous membrane, PVC layers. These layers were subjected to different sweat levels.

In the fleece structure vapor can pass through much better than the micro porous membrane and PVC. Higher air permeability of fleece structure facilitates water vapor transport to the next layers. Fleece structure can hold more water vapor in their inter spaces which results low thermal insulation behavior. Fleece structure shows higher temperature and lower vapor pressure than the micro porous membrane in the cold condition. The reason is higher thermal insulation property of the fleece structure. In case of micro porous membrane higher vapor pressure is due to the condensation which blocks the pores.

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