Cellular polymers thermal conductivity

Thermal Conductivity. More information is available relating thermal conductivity to stmctural variables of cellular polymers than for any other property. Several papers have discussed the relation of the thermal conductivity of heterogeneous materials in general (187,188) and of plastic foams in particular (132,143,151,189—191) with the characteristic stmctural variables of the systems. 

As a good first approximation (187), the heat conduction of low density foams through the soHd and gas phases can be expressed as the product of the thermal conductivity of each phase times its volume fraction. Most rigid polymers have thermal conductivities of 0.07-0.28 W/(m-K) and the corresponding conduction through the soHd phase of a 32 kg/m (2 lbs/fT) foam (3 vol %) ranges 0.003-0.009 W/(m-K). In most cellular polymers this value is deterrnined primarily by the density of the foam and the polymer-phase composition. Smaller variations can result from changes in cell stmcture. 

Table 5. Thermal Conductivity at 20°C of Gases Used in Cellular Polymers ...

The variation in total thermal conductivity with density has the same general nature for ah. cellular polymers (143,189). The increase in at low densities is owing to an increased radiant heat transfer the rise at high densities to an increasing contribution of k. ... 

The thermal conductivity of a cellular polymer can change upon aging under ambient conditions if the gas composition is influenced by such aging. Such a case is evidenced when oxygen or nitrogen diffuses into polyurethane foams that initially have only a fluorocarbon blowing agent in the cells (32,130,143,190,191,198-201). 

A form of cellular rubber in which the cells are non-intercommunicating, self-contained units. It has low thermal conductivity. Expanded rubber is buoyant and does not absorb water and was therefore initially used in both the soft rubber and ebonite forms in the construction of lifebuoys and other marine buoyancy equipment. The most commonly used polymer is now polyurethane for both flexible and rigid systems. 

A series of low density polyolefin foams were manufactured and studied in terms of their thermal conductivity, cellular structure and polymer matrix morphology. In order to predict the thermal conductivity of a specified material a mathematical equation is presented. 26 refs. 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

Thermal Properties. More information is available relating thermal conductivity to structural variables of cellular polymers than for any other properly. 

A low (<0.4 W / (m-K)) thermal conductivity polymer, fabricated into alow density foam consisting of a multitude of tiny closed cells, provides good thermal performance. Cellular plastic thermal insulation can be used in the 4—350 K temperature range. Cellular plastic materials have been developed in... 

The thermal conductivity of cellular polymers has been thoroughly studied in heterogeneous materials [33,34] and plastic foams [25,35-38]. 

Figure 10.3 Effect of density on thermal conductivity of rigid cellular polymers. A, polystyrene [25] B, polystyrene [37] C, polyurethane-air [37] D, polyurethane-CFC 11 (CCI3F) [70] E, polyurethane [37] F, phenol-formaldehyde [37] G, ebonite [37]. To convert kg/m3 to Ib/ft3, multiply by 0.0624. Reproduced from F. O. Guenther, SPE Transactions, 2, 243 (1962), with permission from the society of Plastics Engineers, Brookfield, Connecticut, USA...

Thermal insulation is the second largest application of cellular polymers, and the largest application for the rigid materials, because of their thermal conductivity, ease of application, cost, moisture absorption, and water vapor transmission (or permeance). 

Cellular polymers, especially polystyrene and polyurethane, are also widely used for pipe and vessel insulation. The use of cellular rubber and cellular poly(vinyl chloride) in insulation for small pipes is attributed to their ease of application, combustion properties, and low thermal conductivity. 

With almost all cellular polymers there is a trade-off between acceptable foam physical properties and the unit mass of the cellular material. Usually the physical property under consideration shows a smooth curve relationship with the density of the product. This is not always the case, however, and care should always be exercised when adopting this rule. The relationship between the thermal conductivity of rigid foams and density is an example of where this rule can break down (Fig. 8). 

The thermal conductivity of a cellular polymer varies with temperature and, amongst other things, the density of the foam [2,33], The nature of the specimen surfaces is also important. An example of how the thermal conductivity of rigid EPS foam [34] changes with respect to these variables is shown in Fig. 8. 

The study of the thermal insulation behavior of cellular polymers is complex, and further reading of the cited references is recommended. There are essentially two ways in which the thermal conductivity can be determined, both ways being described in BS 4370, Part 2, Method 7A 7B, 1993. 

To reduce the heat transfer through the solid it is therefore necessary to increase the cellular anisotropy and to lower the thermal conductivity of the polymer structure. 

Cellular vinyls n. Vinyls containing occulted gas in bubbles or cells. Cellular vinyls are used to form polymers with very low densities, down to below O.lgcm , due to the high gas volume fraction, and hence can have exceptionally low thermal conductivities. 

Packaging. Because of the extremely broad demands on the mechanical properties of packaging materials, the entire range of cellular polymers from rigid to flexible is used in this application. The most important considerations are mechanical properties, cost, ease of application or fabrication, moisture susceptibility, thermal conductivity, and aesthetic appeal. 

Other polymers for which thermal conductivity data are available include polyamide 6,6, polypropylene, polymethyl methacrylate, rigid polyvinyl chloride, cellular polyethylene [58], polyvinylidene fluoride [59], polyvinylidene difluoride-ceramic composite [60], polyethylene [61], and polyamide films [61]. 

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