Thermal insulation through polymeric coatings

Heat transfer can take place by means of conduction, convection or radiation across a barrier from the hot side to the cooler side. In conduction, heat passes from a hotter to a colder region along the static material. Convection is the transfer of heat by a flow of gases or liquids of different temperatures. In radiation, thermal energy is transmitted as electromagnetic waves. 

PVC and PU are some of the widely used thermal insulators. They are invariably used in their foamed, expanded forms. The reason for this is the contribution of entrapped air or other gases to the total thermal insulation.  

The total heat conduction Ap of a polymeric material is the sum of components Aq, Ar, As and A, where G = gas in the cell, R = radiation across the cell of the foam, S = conduction through the wall and C = convection of the gas. 

The highest resistance to heat flow comes from the air entrapped in the cell. Still air has one of the lowest thermal insulations, 25mW/mK (Table 4.4). Therefore, polymeric materials in the form of foam have more entrapped air and show improved thermal insulation, the As factor, which depends on the type of polymer itself and contributes approximately 30% of the total resistance to heat flow. Therefore, the type of polymer used in thermal insulators is the next major factor to be considered while designing a thermal insulator. The other two contributions, i.e. Aj, due to the radiation across the cell of the foam and due to the convection of the gas, is kept to optimum levels by proper design of the cell structure of the foam coating. Requirements for extreme cold weather are  

To avoid the condensation of perspiration in a garment, breathable fabrics are required. The garment can also become damp in rain or snowfall or in sportswear. The moisture diffuses through the clothing and can reach the human skin. As the moisture evaporates, the wearer feels cold and uncomfortable. 

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