Thermal conductivity of foamed

Thermal conductivity of foamed plastics has been shown to vary with thickness (197). This has been attributed to the boundary effects of the radiant contribution to heat-transfer. 

The mechanical properties at low strain rates, dynamic mechanical properties, creep-recovery behaviour, thermal expansion and thermal conductivity of foams manufactured from blends of LDPE with an EVA and with an isoprene-styrene block copolymer were studied as a function of the LDPE content in the blends. The experimental results demonstrated important aspects related to the modification of the foam properties by blending. 16 refs. 

There are various models available to compute the thermal conductivity of foamed or filled plastics [39,47, 51]. A rule of mixtures, suggested by Knappe [39], commonly used... 

The apparent thermal conductivity of foams is dependent on the bulk density of the foamed material, the gas used... 

The experimental study of the thermal conductivity of foams has shown [78] that at bubble diameters larger than 3-4 mm, a significant role in thermal conductivity plays the convection inside the bubble. These data are described by the relation... 

Figure 23. Thermal conductivity of foam in panels (foam blown with CFC-11) (212).

Thermal Properties. Themud Conductivity. As stated above, the thermal conductivities of phenolic foams vary remarkable depending on whether they are closed cell or open cell. Generally, the thermal conductivities of foams with 90% or more closed cells are within the range of 0.015 kcal/mh°C but if they have open cells, the thermal conductivities increase to 0.030 to 0.035 kcal/mh C. If the foams have 50 to 80% closed cells their thermal conductivities will be an intermediate value between the above two figures. Meantime, the thermal conductivities of foams with 50% or less closed cells will be almost the same as that of open-cell foams. 

THERMAL CONDUCTIVITY OF FOAMED POLYSTYRENE IN A REGION OF THE SECOND ORDER TRANSITION POINT. 

The thermal conductivity of foams expanded with CO2 changes rapidly owing to diffusion of gases into, and CO2 out of, the cells. The permeability of PU to various gases is shown in Table I. Expansion of the foam with a fluorocarbon, such as trichlorofluoromethane (CCI3F) or dichlorodifluoromethane (CCI2F2)> lowers the thermal conductivity of the foam. The fluorocarbon also... 

Fig. 4.38. Thermal conductivity of commercially available graphite foam (Poco foam) compared with the thermal conductivity of foam originally produced for this project (nonirradiated samples OP-1, OP-4, and OP-10).

Foams constitute an especially important class of multiphase polymer systems, and their thermal conductivity behavior is both interesting and complex. As seen in Tables 1 and 6, the thermal conductivity of foams can be very low, in some cases considerably below that of air. 

To compare the thermal conductivity of foam samples, a simple way is to compare the time necessary for a given temperature increase. In our experiment, a hot plate was kqrt at 54°C, in touch the bottom of a foam sample. The initial temperature of the top of the foam sample was at room temperature. Two thermal couples... 

For the thermal conductivity measurement, sample C and pine PS foam sample were measured by the method described above. It was found that it took 33 minutes for a temperature increase from 33.2 to 34.5°C for pure PS sanple and 40 minutes for sample C. This indicates that with the addition of activated carbon, the thermal conductivity of foam samples decreases. Further experiments by heat flow meter (FOX 200, LaserComp) will be conducted to investigate the effect of activated carbon and moisture. 

Siace the pores ia an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is iaefficient. Coupled with the fact that sohd conduction is suppressed due to the low density, a siUca aerogel has a typical thermal conductivity of 0.015 W/(m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chloro uorocarbon)-blown polyurethane foams (54). 

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. 

The thermal conductivity of most materials decreases with temperature. When the foam stmcture and gas composition are not influenced by temperature, the k of the cellular material decreases with decreasing temperature. When the composition of the gas phase may change (ie, condensation of a vapor), then the relationship of k to temperature is much more complex (143,191,198). 

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). 

J. Isberg, The Thermal Conductivity of Poljurethane Foams, Chalmers University of Technology, Gothenburg, Sweden, 1988. 

A 2.54-cm Styrofoam plastic foam with thermal conductivity of ca 0.03 W/ (m-K) (0.21 (Btu-in.)/(ft-b°F)) is equivalent to 61 cm of gravel. Any synthetic foam having compressive strength sufficiently high and thermal conductivity sufficiently low is effective. However, the resistance of PS-type foams to water, frost damage, and microorganisms in the sod makes them especially desirable. An interesting and important appHcation of this concept was the use of Styrofoam in the constmction of the Alaska pipeline. In this case, the foam was used to protect the permafrost. 

Except where the foam is surrounded by a skin of relatively impermeable material, it would be expected that the blowing gas would diffuse out and be replaced by air and that the thermal conductivities of the foams would increase until they approached that of expanded polystyrene of similar density. Whilst this... 

The other principal thermal properties of plastics which are relevant to design are thermal conductivity and coefficient of thermal expansion. Compared with most materials, plastics offer very low values of thermal conductivity, particularly if they are foamed. Fig. 1.10 shows comparisons between the thermal conductivity of a selection of metals, plastics and building materials. In contrast to their low conductivity, plastics have high coefficients of expansion when compared with metals. This is illustrated in Fig. 1.11 and Table 1.8 gives fuller information on the thermal properties of pl tics and metals. 

Wall sections in foam moulding are thicker than in solid material. Longer cycle times can therefore be expected due to both the wall thickness and the low thermal conductivity of the cellular material. In contrast, however, the injection pressures in foam moulding are low when compared with conventional injection moulding. This means that less clamping force is needed per unit area of moulding and mould costs are less because lower strength mould materials may be used. 

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

Journal of Cellular Plastics 37, No. 1, Jan. 2001,p.21-42 THERMAL CONDUCTIVITY OF A POLYETHYLENE FOAM BLOCK PRODUCED BY A COMPRESSION MOULDING PROCESS Martinez-Diez J A Rodriguez-Perez M A De Saja J A Arcos y Rabago L O Almanza O A... 

Low density PE foam sheets having a thickness of 10 mm were cut from a block produced by compression moulding and their thermal conductivities over the temperature range from 24 to 50C determined. The evolution of the properties along the block was analysed and the cell structure, apparent mean cell diameter, anisotropy, mean cell wall thickness and relative fraction of polymer determined using quantitative image analysis and a previously reported model utilised to predict the thermal conductivity of the foams. 30 refs. 

The thermal conductivity of a section of a commercially produced high density polyethylene foam channel was measured. The walls consisted of a 6.4 mm foam core with a skin of 1.6 mm thickness on either side. Sqnares were machined from the outer surface of the channel, so that heat flow throngh the entire thickness the core pins one skin layer and the complete section conld be... 

An investigation was carried out on concrete containing up to 30% of PE foam waste in order to optimise the thermal conductivity of the concrete. An amorphous aggregate of ash-slag waste was utilised to decrease the thermal conductivity of the concrete. 5 refs. 

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. 

THE THERMAL CONDUCTIVITY OF POLYETHYLENE FOAMS MANUFACTURED BY A NITROGEN SOLUTION PROCESS... 

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