Heat transfer in microcellular polystyrene/multi-walled carbon nanotube nanocomposite foams
Introduction
Polymeric foams with thermal insulation are expected to make great contribution to addressing urgent economic and environmental concerns by preserving energy resources, reducing greenhouse gas emissions, and helping to alleviate climate change. For example, thermal-insulation polymer foams with low thermal conductivity are widely used to save energy in the heating and cooling of buildings and factories.
Numerous studies have been carried out on the thermal conduction (i.e., heat transfer) properties of polymeric foams. It is considered that gas conduction, solid conduction and radiation jointly influence the heat-transfer properties of polymeric foams [1]. The heat-transfer properties can be effectively decreased by using an insulation gas to decrease gas conduction [2], by applying a large expansion ratio to decrease solid conduction [3], and/or by mixing with carbonaceous materials to decrease radiation [4]. The thermal conductivity of an insulation gas, such as fluorocarbons, is on the order of ∼12 mW/m K, which is much lower than that of air (i.e., 26 mW/m K), but the insulation gas either imposes environmental problems or is cost-prohibitive to the manufacturers [2]. The insulation gas would also diffuse out of the foams and deteriorate the long-term thermal insulation performance.
Another efficient method to lower the gas conductivity is to make use of the Knudsen effect via small cells [5]. Aerogel, a nanoporous material, has superior thermal insulation performance due to the Knudsen effect (low gas conductivity) and large porosity (low solid conductivity) [6]. Unfortunately, because of the time-consuming processing method, the large amount of an organic solvent involved in the fabrication, and the low cost-efficiency, the aerogel usage is still limited in industry. Supercritical CO2 (scCO2) foaming is a promising method to produce eco-friendly thermal insulation materials with smaller cells compared to the conventional foams. Especially nanocells have been developed successfully [7]. Although the smallest cell size using scCO2 foaming is 10–40 nm, the expansion ratio that has been achieved so far is less than 1.5 [8], [9], [10]. As a consequence, the thermal conductivity of nanofoam with 1.5-fold expansion is typically 80 mW/m K at least [5], which is much higher than the required thermal conductivity of an insulation material. Recently, Costeux et al. modified the foaming process and produced 5-fold expanded nanocellular foams (cell size < 80 nm) [11]. Our analysis to be shown below predicts that the thermal conductivity of these materials should be at least 45 mW/m K. To achieve superthermal insulation, both a small cell size and a large expansion ratio are the prerequisites, but it is extremely difficult to produce nanocellular foams with a large expansion ratio of over 10 with the current state-of-the-art foam technology due to the high chance of cell opening through the thin cell walls [7], [9], [11], [12]. Once cell opening is active, the gas contained within the cells can easily escape to the environment, and therefore, the available gas to foam expansion becomes limited, resulting in a small expansion ratio. If cell opening occurs at the last minute of expansion, we may be able to maintain the large expansion ratio that was achieved, by quickly freezing the foam structure [13]. But if cell opening occurs before major expansion takes place because of too thin a wall thickness between nano cells, then there will be less amount of gas available for expansion, and thereby, expansion will be suppressed. So until the limiting factors of these existing technologies are overcome, we will not be able to fabricate nanocellular foams with low thermal conductivity.
Herein, we prepared the polystyrene (PS)/multi-walled carbon nanotube (MWCNT) foams by scCO2 foaming for thermal insulation and studied the fundamentals of heat transfer in PS/MWCNT foams. To the best of our knowledge, these PS/MWCNT foams have the smallest cell size of 5–6 μm at such a high expansion ratio of 18-fold. Although the cell size was increased, compared to the nanofoams, to achieve large expansion, the Knudsen effect in the 5 μm cells still attributes to decrease the gas conductivity significantly by 1.2 mW/m K. In addition, we discovered for the first time that the radiation with wavelength less than the cell size was intensively attenuated by reflection at the interfaces between cell walls and air, while the radiation with wavelength larger than the cell size was strongly attenuated by absorption via MWCNTs. The radiative thermal conductivity contributes up to 22% of the total thermal conductivity in PS foam. Dispersed MWCNTs in PS foams are quite efficient to block the long wavelength radiation. In order to understand the specific effects of MWCNTs on the heat transfer in PS/MWCNT foams, we made an in-depth study using the gas conduction model, the Glicksman model, and the Rosseland model. According to those models, we found that MWCNTs decrease the radiative thermal conductivity but increase the solid conductivity. The minimal thermal conductivity was 32.8 mW/m K without using any insulation gas when a 1 wt% of MWCNTs were added to the PS foam with 18-fold expansion. In the future, if an improved scCO2 foaming technology enables us to fabricate 18-fold PS/MWCNT foam with a reduced cell size on the order of 100 nm or smaller, the thermal conductivity will go below 16.4 mW/m K according to our models because the Knudsen effect will decrease the gas conductivity to 7.1 mW/m K. The superthermal insulation PS/MWCNT foams will then greatly help to save energy.
Section snippets
Theoretical fundamentals of heat transfer in MWCNT nanocomposite foams
Since their discovery by Iijima in 1991, carbon nanotubes (CNTs) with unique thermal, electrical, and mechanical properties have been widely studied [14], [15], [16], [17]. The superior properties of CNTs makes them potential design candidates for novel materials [18], [19]. In this work, CNTs were used to reduce thermal conductivity of foams. Thus, a clear understanding of how and to what degree the addition of CNTs relates to the thermal conductivity of polymer/CNT foams needs to be explored.
Materials
General purpose Polystyrene, GPPS 3100, was purchased at Styrolution (melt flow rate at 200 °C/5 kg = 10 g/10 min, density = 1.04 g/cm3) and used as received. MWCNTs (average outer diameter: 10 nm, average length: 1.5 μm, surface area: 250–300 m2/g, carbon purity 90%, density = 1.44 g/cm3) were supplied by Nanocyl™, Belgium, and were produced by Catalytic Carbon Vapor Deposition (CCVD). Carbon dioxide (Linde Gas, purity over 99%) and pentane (Caledon Laboratories Ltd., water content less than 0.02%) were used
Structure characterization of PS/MWCNT nanocomposites and foams
Synthesized MWCNTs are formed as entangled curved agglomerates, and intermolecular Van Der Waals interactions have a tendency to aggregate the MWCNTs in melt polymers. Therefore, to minimize the aggregation, a large shear force is necessary to disentangle them [54]. The prolonged mixing time can enhance the polymer chains’ diffusion between the matrix and the MWCNT aggregates of the masterbatch, but because of strong shear that occurs during that extended mixing period, the MWCNT’s lengths will
Conclusion
In this study, we applied scCO2 foaming to create PS/MWCNT foams with large expansion (18-fold) and a small cell size (5 μm). The gas conduction model, Glicksman model and Rosseland model were applied to investigate in depth the three following modes of heat transfer in the PS/MWCNT foams: the gas conductivity, the solid conductivity, and the radiative thermal conductivity. The gas conductivity of the PS/MWCNT foams was reduced 1.2 mW/m K. This was because the Knudsen effect induces a temperature
Acknowledgment
The authors would like to thank the Korea Institute of Construction Technology (KICT) for their financial support of this project.
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