The elimination of radiative heat transfer in fine celled PU rigid foams

Considerable research in the early 1980s led to the identification of radiative heat transfer as a major contributor to the overall thermal conductivity of PU rigid foam. Fundamental heat transfer analysis showed that for foam model matrices in which the primary variable was cell-size, an approximately linear relationship was obtained between the radiative heat transfer thermal conductivity contribution λ rad and cell-size. λ rad was shown to theoretically decrease to below 1 mW/mK when foam cell-sizes were reduced to about 50 μ. This decrease to below 1 mW/mK is considered in this presentation as an effective elimination of the radiative heat transfer mechanism. With the establishment of a phase-out timetable for R-11, considerable global research activity was focused on the development of fine celled technology capable of exploiting these predictions. In recent years technology for the production of closed cell foams with cell-sizes in the range 100-300 μ have been developed. More recently open celled foams have now been developed for vacuum panel applications with cell-sizes down to the 50 μ level. This series of foams provided a unique opportunity to determine directly the real level of radiative heat transfer that occurs and to therefore determine for the first time the real effectiveness of polyurethane foam matrices to attenuate radiative heat transfer. The theoretical framework for the investigation adopts what is known as a 3 flux approximation, which allows both absorption and scattering contributions to be taken into account. Spectral I-R directional-hemispherical transmission and reflection measurements were performed using an integrating sphere accessory on a BIORAD FTS-40 spectrometer equipped with a CsI beam splitter. The wavelength region of investigation covered 98% of the black body spectrum. From the experimental reflection and transmission spectra and the three flux approximation method, the spectral extinction coefficient was determined and then Rosseland averaged. From this Rosseland averaged extinction coefficient, the foam radiative thermal conductivity λ rad at a mean temperature of 283°C was calculated. Using a mean intercept length analysis of an SEM characterization of the foam both parallel and perpendicular to the symmetry axis, it was possible to determine the Z-average cell-size. Analysis of the results displayed a strong linear relationship of λ rad with the Z-average foam cell size. It is shown that for the very fine open celled foams designed for vacuum panel applications, λ rad values below 1 mW/mK could be obtained. This remarkable level of radiative attenuation for PU foam, in combination with the expected thermal conductivity contribution from solid conduction, is shown to strongly support their suitability as potential vacuum insulation matrices.