Diffusion of Gases in Rigid Polyurethane Foam at Different Temperatures

Abstract

If the diffusion behaviour in a polyurethane (PUR) foam is known for all cell gases, it is possible to predict the change in gas composition and consequently the change in thermal conductivity over time for any configuration or dimension of insulating foam [1,2]. Carbon dioxide, which is always present, diffuses much faster out of the foam than e.g. cyclopentane. A simultaneous inward diffusion of air reduces the insulating capacity of the foam [3,4]. Calculation of heat loss presupposes knowledge of the initial partial pressures of the cell gases in the foam and their diffusion coefficients at operational temperatures. One application of PUR foam is in district heating pipes, transmitting hot water (80–100°C) for space heating purposes. The pipes consist of a PUR insulated steel pipe with a polyethylene casing. The temperature gradient over the foam cross-section influences the cell gas pressure and thus the diffusion properties. Use of cyclopentane as blowing agent in rigid PUR foam insulation has increased during the last decade, especially in Europe and Japan. In the foam, cyclopentane is present as a gas in the cells and dissolved in the polymer matrix. Due to concentration and temperature, cyclopentane may also be present as a condensed liquid in the cells. This reservoir of cyclopentane can compensate for losses of the gas due to diffusion [5,6]. Thus it keeps up the concentration of cyclopentane in the gas phase in the cells of the foam, which is beneficial for the long-term thermal performance The diffusion coefficients of oxygen, nitrogen, carbon dioxide and cyclopentane in PUR foam were determined in the temperature range 20–60°C. Samples were taken from district heating pipes: carbon dioxide-blown foam (density 71 kg·m−3) and cyclopentane blown foam (density 61 kg·mm−3). The cell gas concentrations were determined after grinding the foam sample and analysis of the released cell gases by gas chromatography [7]. The effective diffusion coefficients in the PUR foam were evaluated by a curve fitting procedure from the change of the partial pressures of the cell gases over time [1,6]. See Table 1. Very different activation energies of the cell gases, as in our study (See Table 1.), means that the insulating performance determined at a certain time and temperature cannot easily be extrapolated. Thus, the diffusion rates of these gases must be affected by temperature to a different extent, which is in agreement with die effective diffusion showed in Table 1. Table 1 Effective diffusion coefficients and activation energies for samples of PUR foam taken from district heating pipes (densities 61–71 kgm−3). Gas Effective diffusion coefficient Deff(·10−13m2·s−1) Activation energy ED (J-mole−1) 20°C 40°C 60°C Nitrogen 25 220 44–103 Oxygen 150 650 30–103 Carbon dioxide 500 1300 19–103 Cyclopentane 0.6* 4 7 59.103 * Determined at 23° 610PUR foams exposed to elevated temperatures get darker and become more brittle. This is probably due to oxidation of the polymer. In an unpublished study we observed that the amount of oxygen in PUR foams stored at 100°C was less than in foams stored at 80°C. Hence, oxygen must have been consumed in an oxidation process. This means that oxygen diffusion coefficients based on studies of cell gas concentrations at high temperature may be underestimated.