Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation

Transcription

1 Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation Sara Mangs 1, Olle Ramnäs 1 and Ulf Jarfelt 2 1 Chemical Environmental Science 2 Building Technology, Chalmers University of Technology, SE Göteborg, Sweden Received: 27 April 2005 Accepted: 26 May 2005 ABSTRACT Polyethylene terephthalate (PET) foam is a possible replacement option for polyurethane (PUR) foam as insulation material in district heating pipes. In this study, the diffusion coeffi cients and activation energies of cell gases in carbon dioxide blown PET foam (densities kg m -3 ) were determined at temperatures between 23 ºC and 90 ºC. The foam thermal ageing due to the mass transport of air into and carbon dioxide out of the foam was about ten times slower in PET foam than in PUR foam. The thermal conductivities of the PET foam boards were determined in a heat flow meter apparatus. The contribution to the foam thermal conductivity due to conduction in the solid polymer and radiation within the cell voids was determined to 17 mw m -1 K-1 at 20 ºC. This is higher than the value estimated for PUR foam in district heating pipes, 12 mw m -1 K-1. This contribution can probably be reduced by developing low density PET foam and reducing the cell size. INTRODUCTION The use of polyurethane (PUR) foam is associated with issues of environmental concern, due to the fact that it is usually made from non-renewable resources and because of the toxicity of isocyanates, which constitute one of the main components used in PUR foam production. The thermal conductivity of an insulation foam increases over time due to diffusion of blowing agent out of and air into the foam. Thus, the heat losses in an insulation application increase over time. The heat produced to compensate for these losses gives rise to environmental impacts (e.g. carbon dioxide, nitrogen oxides and sulphur oxides emissions), which vary depending on how the heat is produced. For insulation purposes, such as district heating pipes, it is therefore desirable to find materials with as good as or better long-term thermal performance than the PUR foam used today. Polyethylene terephthalate (PET) foam, a new Cellular Polymers, Vol. 24, No. 3,

2 Sara Mangs, Olle Ramnäs and Ulf Jarfelt insulation and construction material, is one possible replacement alternative. Carbon dioxide is an environmentally friendly blowing agent and was used in the PET foam studied in this report. Several trends indicate that the amount of recycled PET in Europe will increase (1-5). In the future it will probably be possible to produce PET foam from recycled PET food packaging materials. This resource efficiency gives PET foam a possible environmental and economic advantage compared to PUR foam (6). Two materials are placed around the media pipe in district heating applications today: PUR foam for insulation and polyethylene (PE) for the outer protective pipe casing. Since PET foam is quite strong, it might be possible to use the same material throughout the whole pipe. This would simplify the production as well as the waste treatment of pipes taken out of use. EXPERIMENTAL Two different carbon dioxide blown PET foam boards, produced by BC Foam in Italy (foam A) and Fagerdala World Foams in Belgium (foam B), were used in the study. Foam A was 29 mm thick and had a density of 157 kg m -3, while foam B was 35 mm thick and had a density of 148 kg m -3. PET foam is produced by an extrusion process, developed by BC Foam. The polymer granules are dried and mixed with a nucleating agent (talcum 0.5% by mass) before entering the extruder. Blowing agent (carbon dioxide, 1% by mass) of high pressure (70-80 bar) is mixed into the polymer melt in the extruder and kept under pressure until the melt exits through the die where the extrusion pressure is about bar. In the atmospheric pressure outside of the die lips, the blowing agent is transformed from the liquid phase to the gaseous phase, creating the foam cells. Determination of Foam Characteristics According to the European standard for district heating pipes (EN 253:2003), the cell size of PUR foam insulation in a district heating pipe is determined by counting the number of cells over a length of 10 mm in three samples and calculating the average value (7). The samples should be taken radially with the centre of the sample in the centre of the insulation. For the PET foam boards, the cell size was determined by counting the number of cells in eight samples of foam A and foam B taken from two perpendicular directions. 116 Cellular Polymers, Vol. 24, No. 3, 2005

3 Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation Figure 1. Close up of a cross-section of PET foam (Foam A), 10x10 mm, taken with a digital camera The glass transition temperatures (T g ) of the PET foams were determined by means of differential scanning calorimetry (DSC), using the Perkin-Elmer power-compensated DSC7. The temperature was increased from 50 ºC to 300 ºC at a heating rate of 10 ºC/min and then cooled at the same rate. This heating/cooling rate has been reported to give repeatable results for polymer samples and is a compromise between a too fast rate that could cause nonuniform heating and a too slow rate that could cause ageing in the sample (8). The instrument was calibrated against indium prior to the measurements. Samples weighing 5-10 mg were cut from foams A and B. Three samples from foam A were analysed, namely the centre of the foam, an area close to the outer surface and a cross section of the whole board thickness. One sample of the cross section of board B was also analysed. The weight fraction crystallinity (χ) was calculated by (9) : χ= h H f (1) Cellular Polymers, Vol. 24, No. 3,

4 Sara Mangs, Olle Ramnäs and Ulf Jarfelt where h is the determined net enthalpy change [J g -1 ] around the melting temperature of the polymer sample, and H f is the enthalpy of fusion of PET crystals, estimated to 125 J g -1 as suggested by Wunderlich (10). Determination of Diffusion Coefficients and Activation Energies The diffusion coefficients of carbon dioxide, oxygen and nitrogen in PET foam were determined at different temperatures (23 ºC, 40 ºC, 60 ºC and 90 ºC). Cylinders of foam B (length: 55 mm, diameter 20.8 mm) were stored at these temperatures. In order to prevent unwanted longitudinal mass transport of cell gases, plates of aluminium were glued with epoxy onto the ends of the cylinders. The foam samples were analysed after different storage times at the respective temperatures. The partial pressures of carbon dioxide, oxygen and nitrogen were determined by cutting the foam cylinders to a length of about 45 mm and then crushing the foam, measuring the total volume of the released cell gases and analysing the cell gas composition by gas chromatography (11). The changes in cell gas pressure in the cylinders were assumed to follow Fick s law. The mean partial pressure for each gas in an insulation foam can be calculated according to equation 2 for cylinders with only radial diffusion. The effective diffusion coefficient was calculated by fitting the calculated curve describing the change in cell gas pressure over time with the corresponding measured values. ( ) p i (t) = p 0i 4 exp β 2 0j D eff,i t j=1 β 0j a ( ) 2 (2) where p i mean partial pressure of component i [kpa] p 0i initial partial pressure of component i [kpa] D eff,i effective diffusion coefficient of component i [m 2 s -1 ] β 0j a roots of the zero order Bessel function [-] a cylinder radius [m] t time [s] The activation energies for the effective diffusion coefficients were calculated using a least square approximation and with the assumption that the diffusion follows an Arrhenius type relationship: 118 Cellular Polymers, Vol. 24, No. 3, 2005

5 Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation D eff = D o exp E D R T (3) where D o pre-exponential factor [-] E D activation energy [J mole -1 ] R the gas constant [J K -1 mole-1 ] T temperature [K] Determination of Foam Thermal Conductivity The foam thermal conductivity is the sum of the thermal conductivities due to radiation within the cell void, conduction in the solid polymer and conduction in the cell gas. Both the radiation and the conduction in the cell gas are temperature dependent. By measuring the thermal conductivity of the foam at different temperatures and calculating the cell gas conductivity, the contributions of the different mechanisms in PET foam were determined. The foam thermal conductivities for two PET foam boards (foam A: 157 kg m -3, 29 mm thick; foam B: 148 kg m -3, 35 mm thick) were measured in a heat flow meter apparatus (Holometrics Rapid-k) in accordance with the standard ISO8301 method at average temperatures of about 10 ºC, 20 ºC, 25 ºC, 30 ºC and 40 ºC. Both boards had a size of 300x300 mm. The cell gas composition was determined by taking cylindrical samples of each board and analysing the gas by gas chromatography (11). Foam A was then 11 months old and contained 58% carbon dioxide, 26% nitrogen and 16% oxygen. Foam B was 7.5 months old and contained 64% carbon dioxide, 23% nitrogen and 14% oxygen. The thermal conductivity of the cell gas (λ gas ) was calculated at each temperature according to the Brokaw method (equation 3), with the Brokaw coefficient equal to 0.5, as suggested by Isberg (12). The thermal conductivities for each cell gas component used in the calculations are presented below (13). n λ gas = 0.5 y i λ i + i=1 1 n y i λ i=1 i (4) where λ gas thermal conductivity of the cell gas mixture [mw m -1 K-1 ] y i molar fraction of component i [-] λ i thermal conductivity of cell gas component i [mw m -1 K -1 ] Cellular Polymers, Vol. 24, No. 3,

6 Sara Mangs, Olle Ramnäs and Ulf Jarfelt CO 2 N 2 O 2 10 ºC ºC ºC ºC ºC RESULTS AND DISCUSSION Foam Characteristics The average cell size including both the cell void and walls was determined to 0.4 mm for foam A and 0.5 mm for foam B. In the case of foam A, no significant difference in the cell structure could be seen in any direction, whereas with foam B, the cell walls were thicker in one direction, which is probably a result of the production process. Reported cell size values of PUR foam are generally lower than those of PET. The cell size, determined by Biedermann et al. with a standard method (ASTM D ) for closed cell polyurethane foam (densities kg m -3 ) from district heating pipes, was mm (14). The glass transition temperature was determined to 78 ºC in foam A and 80 ºC in foam B. One explanation of the difference could be the degree of crystallinity: 35% in foam A and 37% in foam B. The glass transition temperature in purely amorphous PET is normally 67 ºC and increases with the degree of crystallinity (15). The thermal history can also affect T g. A faster cooling rate produces a material with lower T g. Due to the differences of foam structure, lower thickness and higher density, foam A is expected to have cooled down faster than foam B during the foam production. In foam A, a variation across the foam was observed. For the sample close to the surface, the glass transition temperature was 74 ºC, which probably is due to the faster cooling of the surface compared to the centre of the foam board. Effective Diffusion Coefficients and Activation Energies The determined effective diffusion coefficients of carbon dioxide, oxygen and nitrogen in PET foam are presented in Table 1. When calculating the diffusion coefficients, the average cell size (0.4 mm) of the damaged outer layer was subtracted from the diameter of the PET cylinder sample. Values 120 Cellular Polymers, Vol. 24, No. 3, 2005

7 Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation Table 1 Effective diffusion coefficients (Deff) and activation energies (ED) for carbon dioxide, oxygen and nitrogen in PET and PUR foam at different temperatures D eff E D 10 3 J mole m 2 s-1 23 ºC 40 ºC 60 ºC 90 ºC (calculated between ºC and 60 ºC) PET foam CO (δ=157 kg m -3 ) O N PET foam (6) O (δ=120 kg m -3 ) N PUR foam (16) CO 2 500* (δ=61-71 kg m -3 ) O 2 150* N 2 25* Recalculated values from a previous study. Foam blown with HFC-152a (6) * Determined at 20 ºC from previous studies of HFC-152a blown PET foam (6) and carbon dioxide/ cyclopentane blown PUR foam (16) are also included in the table. These values were determined with the same experimental methods as in this study. The diffusion coefficients of the HFC-152a blown foam were recalculated taking account of the damaged outer layer (determined to 0.4 mm). This resulted in diffusion coefficients that were approximately 10% lower than those reported previously. The diffusion coefficients of oxygen and nitrogen are about twice as high as those of the carbon dioxide blown foam. This is probably a result of the lower density of the HFC-152a blown foam (120 kg m -3 ) in relation to the carbon dioxide blown foam (157 kg m -3 ). The effective diffusion coefficients of the carbon dioxide blown PET foam as a function of the inverse temperature are shown in Figure 2. All curves follow the Arrhenius relationship between 23 ºC and 60 ºC. The diffusion coefficients from the study on HFC-152a blown PET, also shown in the figure, were not taken into account when calculating the curves. The diffusion coefficient at 90 ºC deviates from the Arrhenius plot. The increased mobility of the polymer segment chains above the glass transition temperature can result in increased mass transport of gases (17,18). Cellular Polymers, Vol. 24, No. 3,

8 Sara Mangs, Olle Ramnäs and Ulf Jarfelt Figure 2. Effective diffusion coefficients of cell gases in carbon dioxide blown PET foam (foam A) determined at different temperatures. The unfilled symbols represent diffusion coefficients from a previous study on HFC-152a blown PET (6). The lines were calculated assuming an Arrhenius relationship All effective diffusion coefficients are times lower in the carbon dioxide blown PET foam than those presented for PUR foam (Table 1). The resulting slower ageing of PET foam is illustrated in Figure 3. Here the calculated change of cell gas thermal conductivity over time, at 10 ºC in 35 mm thick homogeneous insulating boards stored at 23 ºC, 40 ºC and 60 ºC, is shown. The figure demonstrates that the decrease in insulating capacity due to the diffusion of cell gases is about ten times slower for PET foam than for PUR foam. It is well known that crystallinity generally enhances the barrier properties of gases through polymers (19). This has been confirmed for PET by, for example, Sekelik et al. (20) and Lin et al. (21). The crystallinity variation over PET foam boards affects the overall diffusion through the boards. If the crystallinity can be controlled during the production process, the long-term thermal performance of PET foam could be improved. The amount of talcum also has an effect on the diffusion of gases. Sekelik et al. have shown that talcum additive can significantly increase the barrier properties towards oxygen in PET (20). Thermal Conductivities of PET Foam Boards The measured thermal conductivities of PET foam between 10 ºC and 40 ºC are shown in Figure 4 (upper line). The temperature dependence for each of 122 Cellular Polymers, Vol. 24, No. 3, 2005

9 Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation Figure 3. Calculated change of thermal conductivity of the cell gas mixture in PET and PUR foam over time at 10 ºC for 35 mm thick carbon dioxide blown boards stored at different temperatures. The initial partial pressures were 80 kpa for carbon dioxide and 2.5 kpa for oxygen and nitrogen respectively Figure 4. Thermal conductivities of PET foam (boards measured at 10 ºC, 20 ºC, 25 ºC, 30 ºC and 40 ºC, upper line) and the contributions due to conduction in the cell gas (calculated, lower line) Cellular Polymers, Vol. 24, No. 3,

10 Sara Mangs, Olle Ramnäs and Ulf Jarfelt the foams follows the relations: λ foam, A = 0.14 T(ºC) λ foam, B = 0.15 T(ºC) The lower line represents the calculated contribution to foam thermal conductivity due to the conduction in the cell gas mixture. The slightly higher values for foam A are due to the lower amount of blowing agent (58% carbon dioxide) compared to foam B (64% carbon dioxide). Both foams had very similar contributions to the foam thermal conductivity due to radiation and conduction (λ rad+cond ) in the solid polymer a. This contribution is expected to increase with foam density, but the differences in cell structure between foam A and foam B probably counteract this effect. At 20 ºC λ rad+cond was 17.2 mw m -1 K -1 for foam A and 17.3 mw m -1 K -1 for foam B. In a previous study on HCFC-142b/HCFC-22 blown PET foam of a lower density (91 kg m 3 ), it was determined to 15 mw m -1 K -1. The corresponding average value for typical PUR foam used in district heating pipes (density: kg m -3 ) from previous measurements is mw m -1 K -1. CONCLUSIONS The thermal ageing of carbon dioxide blown PET foam due to mass transport of air into and blowing agent out of the foam was about ten times slower than in carbon dioxide blown PUR foam. Today, cyclopentane blown PUR foam is commonly used for insulation applications. In PUR foam, cyclopentane diffuses significantly more slowly than carbon dioxide. It is likely that other blowing agents with a slower diffusion rate than carbon dioxide can also be developed for the production of PET foam. In order to obtain an insulation material that can compete with PUR foam in the future, the contribution to foam thermal conductivity due to conduction through the solid polymer and radiation within the cell voids in PET foam must be reduced. This can be achieved by reducing the foam density and the size of the cells. a The contribution to the foam thermal conductivities due to radiation and conduction in the solid polymer: λ rad+cond, A = T ( C) λ rad+cond, B = T ( C) Cellular Polymers, Vol. 24, No. 3, 2005

11 Mass Transport of Cell Gases in Carbon Dioxide Blown PET (Polyethylene Terephthalate) Foam Insulation ACKNOWLEDGEMENTS The financial support from the Swedish District Heating Association is greatly appreciated. Special thanks to the PET foam producers BC Foam (Volpiano, Italy) and Fagerdala World Foams (Gamle Fredrikstad, Norway), who provided the material for the project, and to Prof. Thomas Hjertberg at Polymer Technology and Dr. Thomas Schuman at Material Technology at Chalmers University of Technology for assistance with DSC measurements and interpretation of the results of the experiments. REFERENCES 1. Plastics - An Analysis of Plastics Consumption and Recovery in Europe 1999, Association of Plastics Manufacturers in Europe (2001). 2. Plastics - An Analysis of Plastics Consumption and Recovery in Europe 2000, Association of Plastics Manufacturers in Europe (2002). 3. Plastics - Insight into Consumption and Recovery in Europe, Association of Plastics Manufacturers in Europe (2001). 4. Plastics- An Analysis of Plastics Consumption and Recovery in Europe 2001 & 2002, Association of Plastics Manufacturers in Europe (2003). 5. Council Directive 94/62/EC of 15 December 1994 on Packaging and Packaging Waste, European Council (1994). 6. Mangs, S., Ramnäs, O. and Jarfelt, U., The 9th International Symposium on District Heating and Cooling, Helsinki University of Technology, Energy Engineering and Environmental Protection Publications (2004). 7. EN 253:2003, District heating pipes - Preinsulated bonded pipe systems for directly burled hot water networks - Pipe assembly of steel service pipe, polyurethane thermal insulation and outer casing of polyethylene, European Committee for Standardization (2003). 8. Zaroulis, J.S. and Boyce, M.C., Polymer, 38 (1997) Blundell, D.J., Beckett, D.R. and Willcocks, P.H., Polymer, 22 (1981) Wunderlich, B., Macromolecular Physics Vol. 1 Crystal Structure, Morphology, Defects, New York, USA (1973) Svanström, M. and Ramnäs, O., Cellular Plastics, 31 (1995) Isberg, J., The thermal conductivity of polyurethane foam, Doctoral thesis, Chalmers University of Technology, Göteborg, Sweden (1988). 13. Encyclopédie des gaz = Gas encyclopaedia [réd. sous l égide de la Division scientifi que de] l Air liquide, Amsterdam, The Netherlands (1976). Cellular Polymers, Vol. 24, No. 3,

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