Insulation materials, thermal density effects

The heat conductivity in solids occurs via phonons. This conductivity is ideal in single crystals and is considerably reduced in porous solids, by one to two orders of magnitude. Therefore thermal insulation materials are built up of small particles which should touch each other at only a few points. This effect is of course enhanced by a low density of the material. 

AN INVESTIGATION OF THE EFFECT OF DENSITY AND WATER VAPOUR CONDENSATION ON THE THERMAL CONDUCTIVITY OF LOW TEMPERATURE INSULATING MATERIALS. PH.D. THESIS. 

Solid foams have important applications as useful materials. The presence of gas bubbles results in a substantial reduction in the average density of the material. It also reduces the thermal conductivity, making sohd foams very suitable for insulation materials. Solid foams find applications as lightweight material in mechanical components. In this case, the challenge is to achieve an effective compromise between weight and strength. In foods, such as bread, icecream, or marshmallows, solid foams help to give a pleasant texture. 

EPS is a very effective insulating material and it has positive ratio of price to quality with excellent thermal insulation characteristics (0.040 W/m.K at a density of 15 kg/m and 0.035 W/M.K at a density of 30 kg/m ). EPS is used for roof insulation, insulation of walls and heat pipes, and for floors. 

The thermal conductivity of diatomaceous and vermiculite heat insulation materials is similar. Diatomaceous materials have a very fine pore structure in such materials, the radiation effect on the thermal conductivity is low. An interesting dependence of thermal conductivity vs. temperature appears in Fig. 2.86—diatomaceous bricks with density 400, 500, and 600 kg/m have different values of thermal conductivity at 200 °C, but rather similar values at the temperature of service—400-600 °C. At such temperatures, the thermal conductivity of materials... 

There are many thermal benefits of treating textile structures with PCM microcapsules such as cooling, insulation and thermo regulating effect. Without phase change materials the thermal insulation capacity of clothing depends on the thickness and the density of the fabric (passive insulation). The application of PCM to a garment provides an active thermal... 

Recall that Table 7.1 provided a range of thermal effectiveness for the more common insulation materials. However, from the previous sections it should be evident that the most thermally resistant material is not always the most adequate for a given application. Cryogenic systems typically have constraints other than thermal-insulating capacity that determine the selection of an insulant, such as the desire for a low-density material for insulating a space rocket. Thus, Table 7.1 should not be used exclusively in selecting an insulation, but just to show the relative thermal resistances. Table 7.5 presents a more qualitative yet detailed analysis of the more commonly used cryogenic insulations. 

Foam Insulation Since foams are not homogeneous materials, their apparent thermal conductivity is dependent upon the bulk density of tne insulation, the gas used to foam the insulation, and the mean temperature of the insulation. Heat conduction through a foam is determined by convection and radiation within the cells and by conduction in the solid structure. Evacuation of a foam is effective in reducing its thermal conductivity, indicating a partially open cellular structure, but the resulting values are stiU considerably higher than either multilayer or evacuated powder insulations. 

In Sections 24.3 and 24.5 the flammability and fire resistance of individual fiber/fabric type are discussed. However, as also discussed before, the fire resistance of a fabric not only depends upon the nature of components and the FR treatments applied, but also on fabric area density, construction, air permeability, and moisture content. Nonwovens, for example, will have superior properties to woven or knitted structure, even if all other variables are kept the same.93 The air entrapped within the interstices of any fabric structure and between layers of fabrics within a garment assembly provides the real thermal insulation. For effective thermal and fire resistance in a fabric structure, these insulating air domains need to be maintained.22 In general, for protective clothing and fire-block materials, for best performance multilayered fabric structures are employed. The assembly structures can be engineered to maximize their performance. It is beyond the scope of this chapter to go into details of these composite structures hence the reader is referred to the literature on specified applications and products available. 

The main purpose of middle layers is to provide additional (thermal) insulation. Nowadays, these layers are often made of fleece materials with good air entrapment properties. Their thermal conductivity (typically 0.03-0.04 W/mK) is near from air (0.026 W/mK). The thermal resistance of such layers is directly correlated with their thickness, provided that no air movement occurs within the fabric. Thermal conductivity and air permeability also are generally dependent on the fabric density (Yip and Ng, 2008). Conduction has been shown to be the main heat transfer mechanism through textile layers as long as the fibre volume fraction is higher than 9% (Woo et al., 1994). However, materials with very low density (like spacer materials) allow radiant and convective heat transfer. This was demonstrated by Das et al. (2012) who compared a spacer fabric middle layer with two non-woven middle layers and showed that the contribution of this spacer fabric to the overall insulation was higher than the two other samples in a non-convective mode, while it was the lowest in a forced convective mode. The positive effect of metallised interlayers with low emissivity on the reduction of... 

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