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Threshold cell diameter for high thermal insulation of water-blown rigid polyurethane foams
Journal of Industrial and Engineering Chemistry 73 (2019) 344–350
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Threshold cell diameter for high thermal insulation of water-blown rigid polyurethane foams Hyeon Choe, Yeongsu Choi, Jung Hyeun Kim* Department of Chemical Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2018 Received in revised form 1 February 2019 Accepted 8 February 2019 Available online 18 February 2019
Water-blown rigid polyurethane foams manufactured in eco-friendly manners attract great attentions for applications in various industrial products. Especially, the polyurethane foams are widely applicable as thermal insulation materials for LNG carrier, electronic appliances, pipes, and building. Heat transfer mechanisms in foamed materials have strong relationships with gaseous molecules in cells, solid parts, and cellular morphologies. In this study, thermal conductivity of the water-blown rigid polyurethane foams was investigated by controlling the cellular morphologies using different types of surfactant molecules and gelling catalysts. The cell sizes were controlled from 551 mm to 153 mm by varying surfactants and gelling catalysts. The small cell sizes showed low radiative thermal conductivity due to the high number of thick cell walls and struts to obstruct the photon transport process. More importantly, there was a clear cell threshold size (230 mm) in variations of the overall thermal conductivity value (about 24.3 mW/m K) of polyurethane foams, and the delayed slowly weaken thermal insulation property was also noted in the cell sizes less than 230 mm. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Polyurethane foam Thermal conductivity Radiation Cell morphology Insulation
Introduction The energy saving materials recently attract great attentions in various commercial applications such as electronic appliances, constructions, and marine ships due to the limited energy resources. In order to save energy efficiently in those applications, it is important to control heat transfer mechanisms (conduction, convection, and radiation) in those products. For example, the thermal insulation products used in LNG carrier, refrigerator, and pipes for material transport require delayed heat transfer and reduced heat loss [1,2]. For this purpose, various kinds of materials such as silica aerogel, mineral wool, expanded polystyrene, cellulose, cork, and polyurethane foams are widely used [3–5]. Among them, polyurethane foams have been recently applied for thermal insulation products [6–10] as well as absorbing [11–15], and cushioning components [16–20] due to their easy production, low density, and low thermal conductivity [21]. In applications of polyurethane foams for thermal insulation materials, gaseous components such as chloro-fluoro-carbons (CFCs) and hydro-chloro-fluoro-carbons (HCFCs) had been used in fabrications as physical blowing agents to obtain low thermal
* Corresponding author. E-mail address: [email protected] (J.H. Kim).
conductivity. However, those gas molecules have serious issues in uses because of the environmental ozone depletion and global warming. Therefore, enormous researches have been conducted recently to improve thermal insulation properties of polyurethane foams eco-friendly. First, the problematic ozone-depleting gases have been replaced by other physical blowing agents like hydrofluorocarbons (HFCs), hydrocarbons (HCs), and their derivatives, as well as by CO2 molecules (chemical blowing agent) generated from water-blown polyurethane foams [22–25]. Second, the effect of formulation ingredients in manufacturing polyurethane foams on thermal properties were reported [26]. In addition, various fillers such as cellulose nanocrystal [27], nanofillers [28,29] were also studied to improve the thermal insulation properties of polyurethane composite foams. All those above reported studies examined the overall thermal conductivities of polyurethane foams, but it could be much meaningful if the overall heat conductivity can be analyzed considering conductive mechanisms through gas/solid parts as well as radiative mechanism [30,31]. The relative contribution from radiative mechanism on overall heat conductivity is closely related to the amount of cell volume fraction in the PU foams, and therefore it would be useful to examine the overall heat transfer behavior with radiative thermal conductivity depending on the cell size of the PU foams. In this study, the water-blown rigid polyurethane foams were fabricated in order to examine the effect of morphology on thermal
https://doi.org/10.1016/j.jiec.2019.02.003 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
H. Choe et al. / Journal of Industrial and Engineering Chemistry 73 (2019) 344–350
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Table 1 Formulation of water-blown rigid polyurethane foams. Materials
Contents (pphp) based on polyol (100 g)
Formulation
L4 D0
Polyol mixture
Polyol Gelling catalyst Blowing catalyst Blowing agent Silicon surfactant
Isocyanate
GP-400 Dabco 33LV DBTDL Dabco BL11 Water L-3002 L-6900 M-200
100 g 1.6 – 0.5 4.0 4.0 – 156.8
L6 D0
L8 D0
L10D0
L10D1
L10D2
1.6 –
1.6 –
1.6 –
1.5 0.1
1.4 0.2
6.0 –
8.0 –
10.0 –
10.0 –
10.0 –
H0.5D0 100 g 1.6 – 0.5 4.0 – 0.5 156.8
H1 D0
H1.5D0
H2D0
H2D1
H2D2
1.6 –
1.6 –
1.6 –
1.5 0.1
1.4 0.2
– 1.0
– 1.5
– 2.0
– 2.0
– 2.0
Fig. 1. SEM images showing the variations of cavity sizes with various amounts of surfactant L-3002 and catalyst DBTDL: L4D0 (a), L6D0 (b), L8D0 (c), L10D0 (d), L10D1 (e), and L10D2 (f).
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insulation properties. The cellular morphology was varied during blowing and gelling processes by applying different molecular weight surfactants and different types of gelling catalysts leading to different reaction rates. The cellular morphology was analyzed by scanning electron microscope (SEM) for cell sizes of the polyurethane foams. The NCO conversions by Fourier transformed infrared spectroscopy (FTIR) and surface tension measurements by surface tensiometer were also examined to understand the effects of reaction rates and water molecular dispersions on final cellular morphologies. Additionally, the effect of average cell sizes on the radiative thermal conductivity of polyurethane foams was investigated with the extinction coefficient measurements by FTIR transmittance. The foam’s aging of thermal insulation properties was also monitored for 22 days to evaluate the possible long-term usability by exchanging CO2 molecules with atmospheric air molecules. Experimental Materials Polyether based polyol GP-400 (OH value: 400 15, Mw: 400 g/ mol, fav = 3) was provided by KPX chemical, and polymeric MDI (methylene diphenyl diisocyanate) COSMONATE M-200 (%NCO: 31 1, fav = 2.7) was provided by Kumho Mitsui Chemicals. Dabco 33LV (33% triethylenediamine (TEDA) and 67% dipropylene glycol, Air Products and Chemicals) and DBTDL (95% dibutyltin dilaurate, Sigma–Aldrich) were used as gelling catalysts. Dabco BL11 (70% 2dimethylaminoethyl ether and 30% dipropylene glycol, Air Products and Chemicals) was used as a blowing catalyst, and carbon dioxide was generated from the blowing reactions between isocyanate and water molecules. Two different silicon surfactants (L-3002: low Mw = 116 g/mol & v = 42 10 6 m2 s 1, L-6900: high Mw = 214 g/mol & v = 720 10 6 m2 s 1) were purchased from Momentive to stabilize the cell structure for uniform cell size distributions. Synthesis The rigid polyurethane foams were prepared by the one-shot method in the following order. First, the polyol mixtures listed in Table 1 were mixed in 400 mL paper cups by a mechanical stirrer at 1700 rpm for 10 min. Then, pre-weighed isocyanates following the formulations were added to the polyol mixture for further mixing at 6000 rpm for 8 s, and finally the full mixtures were poured into the aluminum mold (230 mm 230 mm 20 mm) for fabrications of polyurethane foams. During the polymerization reactions, the mold temperature was maintained at 60 1 C, and the mold was tightened not for leaking from the joints. After completing the polymerizations in the aluminum mold for 20 min, the polyurethane foams were taken out from the mold and stored in a room condition for further measurements of the cellular structures and thermal conductivities.
four times with a resolution of 4 cm1. Then, the NCO conversions were calculated using the following Eq. (1) after baseline correction: NCO conversion = 1
It/I0,
(1)
where I0 and It are normalized peak heights of the free NCO stretching band at the beginning of the reaction and time t. Maximum peak heights of the free NCO stretching band between 2270 cm 1–2250 cm 1 were normalized over the invariable internal standard peak (2970 cm 1, C H stretching band) during reaction [32–34]. FTIR transmittances of the foam samples with various thicknesses (0.2 mm–2.0 mm) were also measured for assessments of the extinction coefficient of samples which are closely related to the radiation thermal conductivity. Surface tensions of the polyol mixtures were measured using a single fiber tensiometer (K-100sf, KRUSS) after pre-mixing at 1700 rpm for 10 min according to the ASTM D 1331. The thermal conductivities of as-prepared samples (230 mm 230 mm 20 mm) were measured using a heat flowmeter (l-meter EP-500e, Lambda-Meßtechnik GmbH Dresden) for various cell sizes according to the ASTM C177. The thermal conductivities of samples were measured in 6 h after fabrication. To further examine the effect of aging time on the thermal conductivities, it was also measured as a function of aging time up to 22 days after storing at room condition. Results and discussion Cell morphology Cell structures of rigid polyurethane foams are generally governed not only by the distributions of blowing agent but also by the strength of polymer matrix [35]. The selection of surfactant molecules can be considered as a primary factor in fabrications of polyurethane foams to control these properties. Two different silicon surfactants were used in foam fabrications. Fig. 1 shows the variations of cell sizes of the foams with increasing the contents of surfactant L-3002 (low molecular weight). The cell size decreased with increasing L-3002 contents at 1.6 pphp Dabco 33LV only, and it further reduced by combinations with the DBTDL gelling catalyst. As increasing the contents of low molecular weight surfactant L3002 (Fig. 1a–d), the small dispersed H2O droplets in the reactant mixtures become more stable and remain as their initial states without coalescing. In addition, at the same surfactant content of 10 pphp, decreasing tendency of the cell sizes with DBTDL catalyst can be attributed to the increased matrix modulus during cell formation process due to the high gelling reaction. In order to examine the effect of DBTDL catalyst addition on the initial polymerization reaction, NCO conversion was assessed using FTIR analysis. Fig. 2 shows the initial NCO conversion as a function
Characterizations SEM (SNE-3000M, SEC Co., Ltd., at 15 kV) was used to measure cell sizes as functions of gelling catalysts and surfactants. The mean values of the cell sizes were determined from about 15 images for each sample using the Image Pro Plus (Media Cybernetic) software. In order to determine the conversion of NCO functionalities as a function of reaction time, FTIR (Frontier, PerkinElmer) equipped with attenuated total reflection (ATR) accessory was used to measure the peak intensities of the functional groups at 6 s intervals for the initial reaction (up to 150 s) utilizing the time base program. The basic scanning in FTIR measurements was performed
Fig. 2. Initial NCO conversion by adding DBTDL to the Dabco 33LV base gelling catalyst, at total 1.6 pphp catalyst content.
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of reaction time, and it clearly demonstrates the accelerated reaction as increasing the DBTDL contents in the gelling catalyst system. As reported by Gwon et al. [36], tin-based DBTDL catalyst has much higher reaction rate constant (144 L2/g mol h) than that (109 L2/g mol h) of amine-based Dabco 33LV, and furthermore the combinations of those two types of catalysts can enhance the gelling reaction rate synergistically [37]. As a result, the higher gelling reaction strengthens the matrix modulus during the cell formation, and it effectively keeps the initially formed cell sizes from coalescing. Fig. 3 also shows the variations of cell sizes of the foams with increasing the amounts of surfactant L-6900 (high molecular weight). Comparing with the cell size formed with L-3002
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surfactant, at all L-6900 contents, it revealed much smaller cell sizes than those with L-3002 case. It is possibly due to the low drainage flow of the polyurethane matrix by increased viscosity of high molecular weight surfactant. With increasing the contents of surfactant and by combinations of DBTDL catalyst, changes in cell sizes revealed the same trend with the L-3002 case (shown in Fig. 1). Therefore, the smallest cell size in our experimental investigations could be obtained by adopting the high molecular weight L-6900 surfactant in combined gelling catalyst system (Dabco 33LV/DBTDL: 1.4/0.2). These various cell sizes can have significant effect on the thermal conductivity of polyurethane foams because the cell size is strongly related to the number of cell walls and cell wall thickness which influence photon transfers.
Fig. 3. SEM images showing the variations of cavity sizes with various amounts of surfactant L-6900 and catalyst DBTDL: H0.5D0 (a), H1D0 (b), H1.5D0 (c), H2D0 (d), H2D1 (e), and H2D2 (f).
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formations by CO2 generations through decompositions of intermediate carbamic acids. Thermal conductivity
Fig. 4. Surface tension of polyol mixtures with silicon surfactants of L-3002 and L6900 as function of the contents.
Fig. 4 shows the results of surface tensions of polyol mixture systems as a function of high and low molecular weight surfactants. Generally, the surface tension of polyol mixture decreases with increasing the contents of surfactant molecules, and it can be attributed to the more homogeneous distributions of dispersed water phases in the polyol matrix at higher contents. For the lower surface tension systems, the energy required to generate water phases in the mixtures is lowered, and thus higher number of small dispersed water phases can be further used in fabrications of polyurethane foams from the lower surface tension polyol systems. As a result, the average cell sizes and their distributions become more homogeneous and smaller with increasing the contents of surfactant molecules regardless of the molecular sizes, shown in Figs. 1 & 3. Relative difference of cell sizes formed with low molecular weight L3002 and high molecular weight L-6900 is possibly due to the robustness of the micelle stability of water phases in polyol mixtures. Longer hydrophilic branches in surfactant molecules can provide more stable water dispersed phases than that from shorter branches cases, and so they result into the smaller and more uniform cell
The cellular morphology of porous materials plays a crucial role in overall heat transfer process because the conductive heat transfer is mainly accomplished through gases in cells and solid medium. Additionally, the radiative heat propagations through cell walls and struts have a strong influence on overall heat transfer in polyurethane foams. Amongst these heat transfer mechanisms by thermal conductions through gases and polymer medium and by radiation, radiative heat transfer is considered as most importantly in foamed materials. It is because radiative heat transfer mechanisms are strongly related to photons transports through the cell walls and struts between cells, which are determined by the cell morphology [38]. In order to examine the effect of radiative heat transfer on overall heat conductivity, the radiative heat conduction is firstly assessed using FTIR analysis by calculating extinction coefficients of foams. Fig. 5 shows a schematic illustration for obtaining radiative thermal conductivity using the extinction coefficients measured by FTIR transmittances of the foam samples. The radiative thermal conductivity is strongly related to the average extinction coefficient (Kavg) which is counting for the photon absorption and scattering in the foamed materials. Fig. 6a shows the results of radiative thermal conductivities of the foam samples manufactured with various sizes of cell diameters. For example, the cell walls of the foam samples with large cell diameters have relatively thinner wall thickness than the small cell diameter cases, and so these thinner cell walls experience low photon absorption and scattering phenomena. Fig. 6b reports the decreased tendency of cell wall thickness and the number of cell walls per unit length (mm) as cell diameters increased. The less cell walls, the less time for the photon to penetrate completely the polyurethane insulation foam. As a result, the thinner cell wall foams revealed lower extinction coefficient, as shown in Fig. 6a. Therefore, it resulted into the increased radiative thermal conductivity of the foam
Fig. 5. Procedure for obtaining radiative thermal conductivity (lr) of the foam samples by FTIR transmittance measurements.
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Fig. 8. Overall thermal conductivity (l) of polyurethane foams with various cell diameters as a function of aging time.
Aging behavior
Fig. 6. Radiative thermal conductivity (lr) and average extinction coefficient (Kavg) of the polyurethane foams (a) and cell wall thickness and number of cell walls per unit length (mm) as a function of cell diameter.
samples with increasing cell diameters. Whereas, the samples with small cell diameters absorb and scatter high numbers of photons under heat transfer condition, and thus they reveal weak radiative thermal conduction behavior. Fig. 7 shows the overall thermal conductivity of foam samples as a function of cell diameter. In addition to the radiative thermal conductivity, the solid parts and gaseous cells also contribute to the overall thermal conductivity of the foamed materials. As shown in Fig. 7, a clear threshold point of the overall thermal conductivity was observed at the cell diameter of about 230 mm. Below this critical cell diameter, the dependence of overall thermal conductivity on cell diameter is almost negligible. It can be due to the enhanced relative effect of thermal conductivity from the solid parts because the solid wall thickness becomes thicker as the cell size decreases under the same gas contents. Above this critical cell diameter, the relative contribution of radiative thermal transfer on the overall thermal conductivity becomes significant. Therefore, it is important to keep the cell diameter smaller than about 230 mm in order to minimize the heat transfer through polyurethane foams for applications in various insulation products.
In applications of polyurethane foams for insulation products, it is important to understand the degradation behavior of thermal conductivity of materials over usage time. It also has a strong relationship with the cell sizes of the foams due to the significant contribution from the radiative thermal conduction. Fig. 8 shows the overall thermal conductivity of polyurethane foams as a function of aging time. It revealed an increasing tendency for all samples regardless of the cell diameter sizes with aging time. The high thermal insulation performance of polyurethane foams is primarily due to the CO2 gas molecules in cells. However, these CO2 molecules of high partial pressures is gradually replaced with air molecules from outside the foam samples [39,40]. Eventually, all cells are filled with air molecules, and thus the thermal conductivity of aged samples reaches to the limiting values. As shown in Fig. 8, the samples with average cell diameters larger than about 230 mm reached their limiting thermal conductivities after about 18 days with fast increments comparing to the samples with smaller diameter cases. The smaller cell samples revealed slow increments in the overall thermal conductivity, and it can be due to the time delay in replacing the CO2 gases trapped in the original cells with air molecules from atmosphere through many small cells. To exchange all CO2 gases for air molecules, all molecules should pass through cell walls in the PU foams. Therefore, the time delay for exchanging gas molecules in the PU foams with smaller cells and thicker cell walls is inevitable. Moreover, higher number of cell walls can also retard the gas molecule exchanges in the PU foam samples, as shown in Fig. 6b. In addition, the ultimate values of overall thermal conductivity for small cell cases showed much lower values than for large cell cases because the radiative contribution on overall thermal conductivity become more significant for larger cell foams. Therefore, it is highly recommended to keep the cell diameter of polyurethane foams less than 230 mm to guarantee their improved thermal aging behaviors in applications for insulation products. Conclusions
Fig. 7. Overall thermal conductivity (l) including solid, gas, and radiative contributions (b) as a function of cell diameter.
Water-blown rigid polyurethane foams were fabricated using two types of surfactants and gelling catalysts to investigate the effect of cellular morphology on the overall thermal conductivity of the foams. By applying surfactant molecules, the cell size decreased with increasing the surfactant contents because of the reduced surface tension of polyurethane matrix which is advantageous for even distributions of water molecules and generated CO2 gases. The high molecular weight surfactant and the fast gelling reaction rate by DBTDL catalyst resulted in evenly distributed small cells in strengthened matrix without post coalescing. The decreasing tendency of cell sizes in polyurethane foams is closely related to the radiative thermal conduction. The
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high number of evenly distributed small cells and thick cell walls in polyurethane foams obstructed the photon transfers, and also the radiative thermal conductivity was decreased with high extinction coefficient. The overall thermal conductivity of the polyurethane foams was maintained at low level for the cell sizes less than 230 mm, but it started to increase above this threshold cell size due to the increased importance of the solid part contribution. The cell sizes also influenced the aging of thermal insulation properties because cell wall thickness plays a strong role in exchanging the existing CO2 molecules with foreign air molecules. The ultimate overall thermal conductivity led to the lower values with the smaller cell size due to reduced radiative thermal conductivity. Therefore, evenly distributed small cells (specifically, smaller than 230 mm) in polyurethane foams are strongly recommended in applications for thermal insulation products. Acknowledgements This research was partially supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017H1D8A1030582). References [1] S. Choi, Polym. Sci. Technol. 10 (1999) 621. [2] U. Stirna, I. Beverte, V. Yakushin, U. Cabulis, J. Cell. Plast. 47 (2011) 337, doi: http://dx.doi.org/10.1177/0021955X11398381. [3] N. Nazeran, J. Moghaddas, J. Non-Cryst. Solids 461 (2017) 1, doi:http://dx.doi. org/10.1016/j.jnoncrysol.2017.01.037. [4] B.P. Jelle, Energy Build. 43 (2011) 2549, doi:http://dx.doi.org/10.1016/j. enbuild.2011.05.015. [5] A.M. Papadopoulos, Energy Build. 37 (2005) 77, doi:http://dx.doi.org/10.1016/j. enbuild.2004.05.006. [6] C. Kim, J.R. Youn, Polym. Plast. Technol. Eng. 39 (2000) 163, doi:http://dx.doi. org/10.1081/PPT-100100022. [7] T.J. Bunning, H.G. Jeon, A.K. Roy, K.M. Kearns, B.L. Farmer, W.W. Adams, J. Appl. Polym. Sci. 87 (2003) 2348, doi:http://dx.doi.org/10.1002/app.11918. [8] A.A. Aydın, H. Okutan, Energy Convers. Manage. 68 (2013) 74, doi:http://dx.doi. org/10.1016/j.enconman.2012.12.015. [9] N.V. Gama, B. Soares, C.S.R. Freire, R. Silva, C.P. Neto, A. Barros-Timmons, A. Ferreira, Mater. Des. 76 (2015) 77, doi:http://dx.doi.org/10.1016/j. matdes.2015.03.032. [10] X. Huang, C.F. De Hoop, J. Xie, Q. Wu, D. Boldor, J. Qi, Mater. Des. 138 (2018) 11, doi:http://dx.doi.org/10.1016/j.matdes.2017.10.058. [11] H. Choe, G. Sung, J.H. Kim, Compos. Sci. Technol. 156 (2017) 19, doi:http://dx. doi.org/10.1016/j.compscitech.2017.12.024. [12] L. Kong, Y. Li, F. Qiu, T. Zhang, Q. Guo, X. Zhang, D. Yang, J. Xu, M. Xue, J. Ind. Eng. Chem. 58 (2018) 369, doi:http://dx.doi.org/10.1016/j.jiec.2017.09.050.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Threshold cell diameter for high thermal insulation of water-blown rigid polyurethane foams Hyeon Choe, Yeongsu Choi, Jung Hyeun Kim* Department of Chemical Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2018 Received in revised form 1 February 2019 Accepted 8 February 2019 Available online 18 February 2019
Water-blown rigid polyurethane foams manufactured in eco-friendly manners attract great attentions for applications in various industrial products. Especially, the polyurethane foams are widely applicable as thermal insulation materials for LNG carrier, electronic appliances, pipes, and building. Heat transfer mechanisms in foamed materials have strong relationships with gaseous molecules in cells, solid parts, and cellular morphologies. In this study, thermal conductivity of the water-blown rigid polyurethane foams was investigated by controlling the cellular morphologies using different types of surfactant molecules and gelling catalysts. The cell sizes were controlled from 551 mm to 153 mm by varying surfactants and gelling catalysts. The small cell sizes showed low radiative thermal conductivity due to the high number of thick cell walls and struts to obstruct the photon transport process. More importantly, there was a clear cell threshold size (230 mm) in variations of the overall thermal conductivity value (about 24.3 mW/m K) of polyurethane foams, and the delayed slowly weaken thermal insulation property was also noted in the cell sizes less than 230 mm. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Polyurethane foam Thermal conductivity Radiation Cell morphology Insulation
Introduction The energy saving materials recently attract great attentions in various commercial applications such as electronic appliances, constructions, and marine ships due to the limited energy resources. In order to save energy efficiently in those applications, it is important to control heat transfer mechanisms (conduction, convection, and radiation) in those products. For example, the thermal insulation products used in LNG carrier, refrigerator, and pipes for material transport require delayed heat transfer and reduced heat loss [1,2]. For this purpose, various kinds of materials such as silica aerogel, mineral wool, expanded polystyrene, cellulose, cork, and polyurethane foams are widely used [3–5]. Among them, polyurethane foams have been recently applied for thermal insulation products [6–10] as well as absorbing [11–15], and cushioning components [16–20] due to their easy production, low density, and low thermal conductivity [21]. In applications of polyurethane foams for thermal insulation materials, gaseous components such as chloro-fluoro-carbons (CFCs) and hydro-chloro-fluoro-carbons (HCFCs) had been used in fabrications as physical blowing agents to obtain low thermal
* Corresponding author. E-mail address: [email protected] (J.H. Kim).
conductivity. However, those gas molecules have serious issues in uses because of the environmental ozone depletion and global warming. Therefore, enormous researches have been conducted recently to improve thermal insulation properties of polyurethane foams eco-friendly. First, the problematic ozone-depleting gases have been replaced by other physical blowing agents like hydrofluorocarbons (HFCs), hydrocarbons (HCs), and their derivatives, as well as by CO2 molecules (chemical blowing agent) generated from water-blown polyurethane foams [22–25]. Second, the effect of formulation ingredients in manufacturing polyurethane foams on thermal properties were reported [26]. In addition, various fillers such as cellulose nanocrystal [27], nanofillers [28,29] were also studied to improve the thermal insulation properties of polyurethane composite foams. All those above reported studies examined the overall thermal conductivities of polyurethane foams, but it could be much meaningful if the overall heat conductivity can be analyzed considering conductive mechanisms through gas/solid parts as well as radiative mechanism [30,31]. The relative contribution from radiative mechanism on overall heat conductivity is closely related to the amount of cell volume fraction in the PU foams, and therefore it would be useful to examine the overall heat transfer behavior with radiative thermal conductivity depending on the cell size of the PU foams. In this study, the water-blown rigid polyurethane foams were fabricated in order to examine the effect of morphology on thermal
https://doi.org/10.1016/j.jiec.2019.02.003 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
H. Choe et al. / Journal of Industrial and Engineering Chemistry 73 (2019) 344–350
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Table 1 Formulation of water-blown rigid polyurethane foams. Materials
Contents (pphp) based on polyol (100 g)
Formulation
L4 D0
Polyol mixture
Polyol Gelling catalyst Blowing catalyst Blowing agent Silicon surfactant
Isocyanate
GP-400 Dabco 33LV DBTDL Dabco BL11 Water L-3002 L-6900 M-200
100 g 1.6 – 0.5 4.0 4.0 – 156.8
L6 D0
L8 D0
L10D0
L10D1
L10D2
1.6 –
1.6 –
1.6 –
1.5 0.1
1.4 0.2
6.0 –
8.0 –
10.0 –
10.0 –
10.0 –
H0.5D0 100 g 1.6 – 0.5 4.0 – 0.5 156.8
H1 D0
H1.5D0
H2D0
H2D1
H2D2
1.6 –
1.6 –
1.6 –
1.5 0.1
1.4 0.2
– 1.0
– 1.5
– 2.0
– 2.0
– 2.0
Fig. 1. SEM images showing the variations of cavity sizes with various amounts of surfactant L-3002 and catalyst DBTDL: L4D0 (a), L6D0 (b), L8D0 (c), L10D0 (d), L10D1 (e), and L10D2 (f).
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insulation properties. The cellular morphology was varied during blowing and gelling processes by applying different molecular weight surfactants and different types of gelling catalysts leading to different reaction rates. The cellular morphology was analyzed by scanning electron microscope (SEM) for cell sizes of the polyurethane foams. The NCO conversions by Fourier transformed infrared spectroscopy (FTIR) and surface tension measurements by surface tensiometer were also examined to understand the effects of reaction rates and water molecular dispersions on final cellular morphologies. Additionally, the effect of average cell sizes on the radiative thermal conductivity of polyurethane foams was investigated with the extinction coefficient measurements by FTIR transmittance. The foam’s aging of thermal insulation properties was also monitored for 22 days to evaluate the possible long-term usability by exchanging CO2 molecules with atmospheric air molecules. Experimental Materials Polyether based polyol GP-400 (OH value: 400 15, Mw: 400 g/ mol, fav = 3) was provided by KPX chemical, and polymeric MDI (methylene diphenyl diisocyanate) COSMONATE M-200 (%NCO: 31 1, fav = 2.7) was provided by Kumho Mitsui Chemicals. Dabco 33LV (33% triethylenediamine (TEDA) and 67% dipropylene glycol, Air Products and Chemicals) and DBTDL (95% dibutyltin dilaurate, Sigma–Aldrich) were used as gelling catalysts. Dabco BL11 (70% 2dimethylaminoethyl ether and 30% dipropylene glycol, Air Products and Chemicals) was used as a blowing catalyst, and carbon dioxide was generated from the blowing reactions between isocyanate and water molecules. Two different silicon surfactants (L-3002: low Mw = 116 g/mol & v = 42 10 6 m2 s 1, L-6900: high Mw = 214 g/mol & v = 720 10 6 m2 s 1) were purchased from Momentive to stabilize the cell structure for uniform cell size distributions. Synthesis The rigid polyurethane foams were prepared by the one-shot method in the following order. First, the polyol mixtures listed in Table 1 were mixed in 400 mL paper cups by a mechanical stirrer at 1700 rpm for 10 min. Then, pre-weighed isocyanates following the formulations were added to the polyol mixture for further mixing at 6000 rpm for 8 s, and finally the full mixtures were poured into the aluminum mold (230 mm 230 mm 20 mm) for fabrications of polyurethane foams. During the polymerization reactions, the mold temperature was maintained at 60 1 C, and the mold was tightened not for leaking from the joints. After completing the polymerizations in the aluminum mold for 20 min, the polyurethane foams were taken out from the mold and stored in a room condition for further measurements of the cellular structures and thermal conductivities.
four times with a resolution of 4 cm1. Then, the NCO conversions were calculated using the following Eq. (1) after baseline correction: NCO conversion = 1
It/I0,
(1)
where I0 and It are normalized peak heights of the free NCO stretching band at the beginning of the reaction and time t. Maximum peak heights of the free NCO stretching band between 2270 cm 1–2250 cm 1 were normalized over the invariable internal standard peak (2970 cm 1, C H stretching band) during reaction [32–34]. FTIR transmittances of the foam samples with various thicknesses (0.2 mm–2.0 mm) were also measured for assessments of the extinction coefficient of samples which are closely related to the radiation thermal conductivity. Surface tensions of the polyol mixtures were measured using a single fiber tensiometer (K-100sf, KRUSS) after pre-mixing at 1700 rpm for 10 min according to the ASTM D 1331. The thermal conductivities of as-prepared samples (230 mm 230 mm 20 mm) were measured using a heat flowmeter (l-meter EP-500e, Lambda-Meßtechnik GmbH Dresden) for various cell sizes according to the ASTM C177. The thermal conductivities of samples were measured in 6 h after fabrication. To further examine the effect of aging time on the thermal conductivities, it was also measured as a function of aging time up to 22 days after storing at room condition. Results and discussion Cell morphology Cell structures of rigid polyurethane foams are generally governed not only by the distributions of blowing agent but also by the strength of polymer matrix [35]. The selection of surfactant molecules can be considered as a primary factor in fabrications of polyurethane foams to control these properties. Two different silicon surfactants were used in foam fabrications. Fig. 1 shows the variations of cell sizes of the foams with increasing the contents of surfactant L-3002 (low molecular weight). The cell size decreased with increasing L-3002 contents at 1.6 pphp Dabco 33LV only, and it further reduced by combinations with the DBTDL gelling catalyst. As increasing the contents of low molecular weight surfactant L3002 (Fig. 1a–d), the small dispersed H2O droplets in the reactant mixtures become more stable and remain as their initial states without coalescing. In addition, at the same surfactant content of 10 pphp, decreasing tendency of the cell sizes with DBTDL catalyst can be attributed to the increased matrix modulus during cell formation process due to the high gelling reaction. In order to examine the effect of DBTDL catalyst addition on the initial polymerization reaction, NCO conversion was assessed using FTIR analysis. Fig. 2 shows the initial NCO conversion as a function
Characterizations SEM (SNE-3000M, SEC Co., Ltd., at 15 kV) was used to measure cell sizes as functions of gelling catalysts and surfactants. The mean values of the cell sizes were determined from about 15 images for each sample using the Image Pro Plus (Media Cybernetic) software. In order to determine the conversion of NCO functionalities as a function of reaction time, FTIR (Frontier, PerkinElmer) equipped with attenuated total reflection (ATR) accessory was used to measure the peak intensities of the functional groups at 6 s intervals for the initial reaction (up to 150 s) utilizing the time base program. The basic scanning in FTIR measurements was performed
Fig. 2. Initial NCO conversion by adding DBTDL to the Dabco 33LV base gelling catalyst, at total 1.6 pphp catalyst content.
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of reaction time, and it clearly demonstrates the accelerated reaction as increasing the DBTDL contents in the gelling catalyst system. As reported by Gwon et al. [36], tin-based DBTDL catalyst has much higher reaction rate constant (144 L2/g mol h) than that (109 L2/g mol h) of amine-based Dabco 33LV, and furthermore the combinations of those two types of catalysts can enhance the gelling reaction rate synergistically [37]. As a result, the higher gelling reaction strengthens the matrix modulus during the cell formation, and it effectively keeps the initially formed cell sizes from coalescing. Fig. 3 also shows the variations of cell sizes of the foams with increasing the amounts of surfactant L-6900 (high molecular weight). Comparing with the cell size formed with L-3002
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surfactant, at all L-6900 contents, it revealed much smaller cell sizes than those with L-3002 case. It is possibly due to the low drainage flow of the polyurethane matrix by increased viscosity of high molecular weight surfactant. With increasing the contents of surfactant and by combinations of DBTDL catalyst, changes in cell sizes revealed the same trend with the L-3002 case (shown in Fig. 1). Therefore, the smallest cell size in our experimental investigations could be obtained by adopting the high molecular weight L-6900 surfactant in combined gelling catalyst system (Dabco 33LV/DBTDL: 1.4/0.2). These various cell sizes can have significant effect on the thermal conductivity of polyurethane foams because the cell size is strongly related to the number of cell walls and cell wall thickness which influence photon transfers.
Fig. 3. SEM images showing the variations of cavity sizes with various amounts of surfactant L-6900 and catalyst DBTDL: H0.5D0 (a), H1D0 (b), H1.5D0 (c), H2D0 (d), H2D1 (e), and H2D2 (f).
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formations by CO2 generations through decompositions of intermediate carbamic acids. Thermal conductivity
Fig. 4. Surface tension of polyol mixtures with silicon surfactants of L-3002 and L6900 as function of the contents.
Fig. 4 shows the results of surface tensions of polyol mixture systems as a function of high and low molecular weight surfactants. Generally, the surface tension of polyol mixture decreases with increasing the contents of surfactant molecules, and it can be attributed to the more homogeneous distributions of dispersed water phases in the polyol matrix at higher contents. For the lower surface tension systems, the energy required to generate water phases in the mixtures is lowered, and thus higher number of small dispersed water phases can be further used in fabrications of polyurethane foams from the lower surface tension polyol systems. As a result, the average cell sizes and their distributions become more homogeneous and smaller with increasing the contents of surfactant molecules regardless of the molecular sizes, shown in Figs. 1 & 3. Relative difference of cell sizes formed with low molecular weight L3002 and high molecular weight L-6900 is possibly due to the robustness of the micelle stability of water phases in polyol mixtures. Longer hydrophilic branches in surfactant molecules can provide more stable water dispersed phases than that from shorter branches cases, and so they result into the smaller and more uniform cell
The cellular morphology of porous materials plays a crucial role in overall heat transfer process because the conductive heat transfer is mainly accomplished through gases in cells and solid medium. Additionally, the radiative heat propagations through cell walls and struts have a strong influence on overall heat transfer in polyurethane foams. Amongst these heat transfer mechanisms by thermal conductions through gases and polymer medium and by radiation, radiative heat transfer is considered as most importantly in foamed materials. It is because radiative heat transfer mechanisms are strongly related to photons transports through the cell walls and struts between cells, which are determined by the cell morphology [38]. In order to examine the effect of radiative heat transfer on overall heat conductivity, the radiative heat conduction is firstly assessed using FTIR analysis by calculating extinction coefficients of foams. Fig. 5 shows a schematic illustration for obtaining radiative thermal conductivity using the extinction coefficients measured by FTIR transmittances of the foam samples. The radiative thermal conductivity is strongly related to the average extinction coefficient (Kavg) which is counting for the photon absorption and scattering in the foamed materials. Fig. 6a shows the results of radiative thermal conductivities of the foam samples manufactured with various sizes of cell diameters. For example, the cell walls of the foam samples with large cell diameters have relatively thinner wall thickness than the small cell diameter cases, and so these thinner cell walls experience low photon absorption and scattering phenomena. Fig. 6b reports the decreased tendency of cell wall thickness and the number of cell walls per unit length (mm) as cell diameters increased. The less cell walls, the less time for the photon to penetrate completely the polyurethane insulation foam. As a result, the thinner cell wall foams revealed lower extinction coefficient, as shown in Fig. 6a. Therefore, it resulted into the increased radiative thermal conductivity of the foam
Fig. 5. Procedure for obtaining radiative thermal conductivity (lr) of the foam samples by FTIR transmittance measurements.
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Fig. 8. Overall thermal conductivity (l) of polyurethane foams with various cell diameters as a function of aging time.
Aging behavior
Fig. 6. Radiative thermal conductivity (lr) and average extinction coefficient (Kavg) of the polyurethane foams (a) and cell wall thickness and number of cell walls per unit length (mm) as a function of cell diameter.
samples with increasing cell diameters. Whereas, the samples with small cell diameters absorb and scatter high numbers of photons under heat transfer condition, and thus they reveal weak radiative thermal conduction behavior. Fig. 7 shows the overall thermal conductivity of foam samples as a function of cell diameter. In addition to the radiative thermal conductivity, the solid parts and gaseous cells also contribute to the overall thermal conductivity of the foamed materials. As shown in Fig. 7, a clear threshold point of the overall thermal conductivity was observed at the cell diameter of about 230 mm. Below this critical cell diameter, the dependence of overall thermal conductivity on cell diameter is almost negligible. It can be due to the enhanced relative effect of thermal conductivity from the solid parts because the solid wall thickness becomes thicker as the cell size decreases under the same gas contents. Above this critical cell diameter, the relative contribution of radiative thermal transfer on the overall thermal conductivity becomes significant. Therefore, it is important to keep the cell diameter smaller than about 230 mm in order to minimize the heat transfer through polyurethane foams for applications in various insulation products.
In applications of polyurethane foams for insulation products, it is important to understand the degradation behavior of thermal conductivity of materials over usage time. It also has a strong relationship with the cell sizes of the foams due to the significant contribution from the radiative thermal conduction. Fig. 8 shows the overall thermal conductivity of polyurethane foams as a function of aging time. It revealed an increasing tendency for all samples regardless of the cell diameter sizes with aging time. The high thermal insulation performance of polyurethane foams is primarily due to the CO2 gas molecules in cells. However, these CO2 molecules of high partial pressures is gradually replaced with air molecules from outside the foam samples [39,40]. Eventually, all cells are filled with air molecules, and thus the thermal conductivity of aged samples reaches to the limiting values. As shown in Fig. 8, the samples with average cell diameters larger than about 230 mm reached their limiting thermal conductivities after about 18 days with fast increments comparing to the samples with smaller diameter cases. The smaller cell samples revealed slow increments in the overall thermal conductivity, and it can be due to the time delay in replacing the CO2 gases trapped in the original cells with air molecules from atmosphere through many small cells. To exchange all CO2 gases for air molecules, all molecules should pass through cell walls in the PU foams. Therefore, the time delay for exchanging gas molecules in the PU foams with smaller cells and thicker cell walls is inevitable. Moreover, higher number of cell walls can also retard the gas molecule exchanges in the PU foam samples, as shown in Fig. 6b. In addition, the ultimate values of overall thermal conductivity for small cell cases showed much lower values than for large cell cases because the radiative contribution on overall thermal conductivity become more significant for larger cell foams. Therefore, it is highly recommended to keep the cell diameter of polyurethane foams less than 230 mm to guarantee their improved thermal aging behaviors in applications for insulation products. Conclusions
Fig. 7. Overall thermal conductivity (l) including solid, gas, and radiative contributions (b) as a function of cell diameter.
Water-blown rigid polyurethane foams were fabricated using two types of surfactants and gelling catalysts to investigate the effect of cellular morphology on the overall thermal conductivity of the foams. By applying surfactant molecules, the cell size decreased with increasing the surfactant contents because of the reduced surface tension of polyurethane matrix which is advantageous for even distributions of water molecules and generated CO2 gases. The high molecular weight surfactant and the fast gelling reaction rate by DBTDL catalyst resulted in evenly distributed small cells in strengthened matrix without post coalescing. The decreasing tendency of cell sizes in polyurethane foams is closely related to the radiative thermal conduction. The
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high number of evenly distributed small cells and thick cell walls in polyurethane foams obstructed the photon transfers, and also the radiative thermal conductivity was decreased with high extinction coefficient. The overall thermal conductivity of the polyurethane foams was maintained at low level for the cell sizes less than 230 mm, but it started to increase above this threshold cell size due to the increased importance of the solid part contribution. The cell sizes also influenced the aging of thermal insulation properties because cell wall thickness plays a strong role in exchanging the existing CO2 molecules with foreign air molecules. The ultimate overall thermal conductivity led to the lower values with the smaller cell size due to reduced radiative thermal conductivity. Therefore, evenly distributed small cells (specifically, smaller than 230 mm) in polyurethane foams are strongly recommended in applications for thermal insulation products. Acknowledgements This research was partially supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017H1D8A1030582). References [1] S. Choi, Polym. Sci. Technol. 10 (1999) 621. [2] U. Stirna, I. Beverte, V. Yakushin, U. Cabulis, J. Cell. Plast. 47 (2011) 337, doi: http://dx.doi.org/10.1177/0021955X11398381. [3] N. Nazeran, J. Moghaddas, J. Non-Cryst. Solids 461 (2017) 1, doi:http://dx.doi. org/10.1016/j.jnoncrysol.2017.01.037. [4] B.P. Jelle, Energy Build. 43 (2011) 2549, doi:http://dx.doi.org/10.1016/j. enbuild.2011.05.015. [5] A.M. Papadopoulos, Energy Build. 37 (2005) 77, doi:http://dx.doi.org/10.1016/j. enbuild.2004.05.006. [6] C. Kim, J.R. Youn, Polym. Plast. Technol. Eng. 39 (2000) 163, doi:http://dx.doi. org/10.1081/PPT-100100022. [7] T.J. Bunning, H.G. Jeon, A.K. Roy, K.M. Kearns, B.L. Farmer, W.W. Adams, J. Appl. Polym. Sci. 87 (2003) 2348, doi:http://dx.doi.org/10.1002/app.11918. [8] A.A. Aydın, H. Okutan, Energy Convers. Manage. 68 (2013) 74, doi:http://dx.doi. org/10.1016/j.enconman.2012.12.015. [9] N.V. Gama, B. Soares, C.S.R. Freire, R. Silva, C.P. Neto, A. Barros-Timmons, A. Ferreira, Mater. Des. 76 (2015) 77, doi:http://dx.doi.org/10.1016/j. matdes.2015.03.032. [10] X. Huang, C.F. De Hoop, J. Xie, Q. Wu, D. Boldor, J. Qi, Mater. Des. 138 (2018) 11, doi:http://dx.doi.org/10.1016/j.matdes.2017.10.058. [11] H. Choe, G. Sung, J.H. Kim, Compos. Sci. Technol. 156 (2017) 19, doi:http://dx. doi.org/10.1016/j.compscitech.2017.12.024. [12] L. Kong, Y. Li, F. Qiu, T. Zhang, Q. Guo, X. Zhang, D. Yang, J. Xu, M. Xue, J. Ind. Eng. Chem. 58 (2018) 369, doi:http://dx.doi.org/10.1016/j.jiec.2017.09.050.
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