Clay-based heat insulator composites: Thermal and water retention properties

Applied Clay Science 37 (2007) 90 – 96 www.elsevier.com/locate/clay

Clay-based heat insulator composites: Thermal and water retention properties Kamal Al-Malah a,⁎, Basim Abu-Jdayil b b

a Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan Visiting Associate Professor at Department of Chemical Engineering, University of Arab Emirates, Al-Ain, United Arab Emirates

Received 25 August 2006; received in revised form 2 January 2007; accepted 8 January 2007 Available online 17 January 2007

Abstract The formulation of unsaturated polyester composite as an insulating material that gives the best in terms of thermal and water retention properties was investigated as a function of filler type and content. Different types of local fillers were used in the formulations. Bentonite-based unsaturated polyester composite which is denoted as BBUPEC was found to have stable and compatible thermal, physical, and chemical properties. BBUPEC thermal conductivity, k, values lie between 0.1 and 0.2 W/(m K). It was found that at 50 wt.% filler content and 40 wt.% polyester content, k of BBUPEC is minimum. Calcium carbonate-based composite also gave a similar value. However, in terms of citric acid impregnation, calcium carbonate-based composite was not stable and dissolution took place. In terms of water retention value, citric acid and NaOH impregnation values, one could say that bentonite3-based composite was the best among BBUPEC. Consequently, one would say that BBUPEC shows good characteristics in terms of thermal conductivity and physical and chemical stability and with such inexpensive and abundant fillers from natural resources, they pose a potential thermal insulating material. Sandwiching of BBUPEC in wall structures by one-third of the total thickness will significantly reduce the overall heat transfer coefficient in home and industrial applications by at least 50%. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermal insulator; Jordanian bentonite; Composite material; Unsaturated polyester; Clay; Water retention; Thermal conductivity

1. Introduction There is a necessity for finding alternative thermal insulating materials to preserve energy by minimizing energy losses. Jordan suffers from a cold and humid weather during winter season, which requires minimization of energy loss from the shelters to the environment. Home shelters are usually heated by a kerosenebased stove and to a lesser degree by central, oil-burned heaters. On the other hand, there is also a necessity for ⁎ Corresponding author. Tel.: +962 27201000. E-mail address: [email protected] (K. Al-Malah). 0169-1317/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2007.01.001

air-cooling as some temperature spikes occasionally occur during summer. Therefore, thermal insulation is becoming an essential element in home shelters and in commercial and governmental buildings, and is commonly used between wall cavities. Insulation materials can be made in different forms including loose-fill form, blanket batt or roll form, rigid form, foamed in place, or reflective form. The choice of the proper insulation materials type and form depends on the type of application as well as the desired materials physical, thermal and other properties (Al-Homoud, 2005). Some typical properties of insulating materials that are considered as a must in terms of mechanical,

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physical, and thermal properties are: low thermal conductivity, prevention of water leak, ease of handling and machining, durability and light weight, fire resistance, and safe/healthy use and installation. In addition, the thermal insulation of buildings must be cost effective; i.e., reduction in running cost due to energy saving should outweigh the increase in fixed cost within the lifetime of the building. Al-Sanea (2002) found that the inclusion of a 5-cm thick molded polystyrene layer reduced the roof heattransfer load to one-third of its value in an identical roof section without insulation, while using a polyurethane layer instead, reduced the load to less than one-quarter. Hernandez-Olivares et al. (1999) studied mechanical and thermal properties of a composite material that is made of cork and gypsum. They found that cork-gypsum composites are characterized both by low thermal conductivity and low density. On the other hand, the mechanical properties of cork–gypsum composite were poor. Such a composite material was suggested for use in building applications as partitions. Kumar (2003) examined hollow blocks made of fly ash–lime–phosphogypsum (FaL-G) composites, as potential thermal insulating materials. Fly ash is rich in silica and to a lesser degree in alumina; lime being rich in calcium oxide; and calcined phosphogypsum in calcium sulfate di-hydrate. It was found that water absorption of FaL-G hollow blocks was between 19.2% and 37.2 wt.%, with a minimum value at 30 wt.% of fly ash and a maximum value at 80 wt.% fly ash content. Marcovich et al. (2001) examined composites made from unsaturated polyester/styrene thermoset matrix and woodflour. Using scanning electron microscopy (SEM), they showed that at low filler loading of woodflour, the resin managed to fill in the gaps between fibers of filler, however at high filler loading, the resin is insufficient to wet completely the filler, hence empty capillaries were found. Bureau et al. (2001) examined the effect of styrene content on fragility of unsaturated polyester resins. When the contents of styrene increases, they found that the molar ratio of maleic anhydride/styrene controls the length of the styrene chain that will be connected to the unsaturated polyester. Using differential scanning calorimetry (DSC), they found that the increase of the glass transition temperature observed for their samples in the range of styrene content included between 25% and 50% (w/w) is due to an increase of the average length of the styrenic chain, hence, the fragility of these networks increases as the styrene content increase. In this research, focus is made on the formulation of polyester–clay composite as an insulating material that gives the best in terms of thermal and mechanical

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properties. Different types of local fillers were used in the formulations. In this paper, thermal conductivity and water absorption capacity as a function of filler type and content will be addressed. There are valuable fillers available in Jordan, which can be used in this aspect; for example, limestone and clays (smectite, illite, and kaolinite). Clay materials are available in abundant amounts in Al-Azraq area with a projected amount of 17 million metric tons. 2. Experimental methods 2.1. Materials Polyester used in this study was obtained from the Intermediate Petrochemicals Co., Jordan. Unsaturated polyester was chosen because of their ease of handling, low water absorption values, low cost and its rapid curing with no gases evolved (Wordingham and Reboul, 1968; Billmeyer, 1984). The type of polyester used is HM190 with a styrene content of 33–35%, an acid content of 21–23% and a viscosity of 300– 400 mPa s. Three different bentonite samples obtained from different areas in Jordan and one feldspar sample was used in this study as fillers. The International Company for Ceramics (Mufraq– Jordan) supplied us with the feldspar sample. The chemical analysis of the fillers used is shown in Table 1. In addition, commercial calcium carbonate obtained from the Jordan Carbonate Company (Amman–Jordan) was used for the sake of comparison. Calcite is the natural form of calcium carbonate which is widely used in plastics as filler due to its high dispersability, low oil absorption characteristics, higher impact resistance, smooth surface finish, easy processing and excellent dimensional stability. Pure calcite is a relatively soft material (Moh hardness 3.0) with a specific gravity of 2.7.

3. Sample preparation The fillers were crushed and then screened to a grain size of 0.5 mm. To overcome the low degree of crosslinking in polyester, styrene (C6H5CHCH2) was added. Styrene was chosen because of its compatibility with the polyester. Its low viscosity makes the mixing process (unsaturated polyester/styrene with filler) easier. For the curing process, methyl ethyl ketone peroxide as an initiator was used because of its solubility in styrene and when mixed with the accelerator (cobalt–octoate) decomposition occurs at room temperature to give free radicals. Different filler contents (25–60 wt.%) at a constant styrene/polyester ratio (18 wt ratio) and different styrene contents (5–12 wt.%) with constant filler content of 50 wt.% composites were prepared. The composition of the prepared composite was chosen upon the results of our preliminary study (Abu-Jdayil et al., 2002). The

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Table 1 Chemical analysis of used fillers Filler type

SiO2 (wt.%)

Al2O3 (wt.%)

Fe2O3 (wt.%)

MgO (wt.%)

CaO (wt.%)

Na2O (wt.%)

K2O (wt.%)

H2O (wt.%)

Bentonite 1 Bentonite 2 Bentonite 3 Feldspar

51.64 66.03 54.26 74.00

16.10 13.23 25.48 13.00

11.58 6.26 5.46 1.47

2.65 2.32 0.66 0.40

1.98 0.36 0.67 0.81

4.58 2.22 0.73 4.58

1.79 2.60 1.52 3.61

5.16 5.72 0.88 0.34

composites were prepared at room temperature using a high viscosity mixer. Then the mixture was poured in the suitable mold prepared from stainless steel under mechanical vibration to get rid of the air bubbles that may occur while pouring. Different types of molds, with different shapes and sizes, were fabricated to meet the requirements of the tests that were performed on the prepared composites. The interior surface of the molds was coated with paraffin wax and poly vinyl acetate to prevent sticking of the sample with the mold. 3.1. Thermal conductivity test The Hilton B480 Thermal Conductivity of Buildings & Insulating Materials Unit was used to measure the thermal conductivity of the prepared composites. The apparatus is available at the University of Applied Science/Jordan. The measurement conditions followed the standard methods reported by ISO 8301. The steady state method was used in these measurements, where the thermal conductivity was determined from measurements of the temperature gradient in the composite material and the heat input. According to the ISO 8301, the dimensions of the samples were 300 mm × 300 mm × 10 mm. The measurements were performed in duplicates and the average value was reported. It should be pointed out that in all experiments carried out the standard error of measurement is, on the average, less than 5% of the measured value. 3.2. Water retention This test was performed according to the standard test ASTM D-570-81. Distilled and tap waters were used in this test. The test specimens were of the form of a bar 42 mm long by 35 mm wide by 20 mm thickness. The specimen was placed in a container of water at room temperature, and rested at its edge and entirely immersed. At the end of 24 h the sample was removed from water, wiped free of surface moisture with a dry cloth, weighed to the nearest 0.001 g immediately, and then replaced in water. The weighing was repeated at the end of the first week and every week thereafter and for 4 weeks. This time was enough to reach the saturation

(equilibrium) condition, where no change in the sample weight was noticed. During this period, the specimens' weight difference was recorded at different times. The percentage water retention (WR%) for unsaturated polyester composites was calculated using WR% ¼

weight of equilibrated sample−weight of dry sample weight of dry sample  100%

ð1Þ It is a measure for percent relative increase in weight due to water imbibement or retention within the solid matrix. 3.3. Resistance to chemical reagents This test was performed according to the test ASTM D-543-84. Two chemical reagents were used, namely; citric acid and sodium hydroxide solutions. 104 g of citric acid crystals were dissolved in 935 mL of water to produce a solution with 1% concentration. On the other hand, 107 g of NaOH in 964 mL of distilled water to produce a 10% NaOH solution. Then the same procedure of water retention was followed. The percentage acid or base impregnation for unsaturated polyester composites was calculated using acid ðbaseÞ impregnation % ¼

weight of equilibrated sample−weight of dry sample weight of dry sample 100% ð2Þ

It is a measure for percent relative increase in weight of the solid matrix due to acid or base impregnation. It should be pointed out that in all experiments carried out the standard error of measurement is, on the average, less than 5% of the measured value. 4. Results and discussion The thermal conductivity coefficient of un-saturated polyester was experimentally found to be 0.1 W/(m K).

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Applying Eq. (3) to, for example, bentonite 1-based unsaturated polyester will result in: 1 k

j

BBUPEC

¼

j

j

0:50 0:40 þ 3:5 filler 0:1 unsatPE 0:10 þ ¼ 4:8 0:15 styrene

ð5aÞ

j

kBBUPEC ¼ 0:2 W=ðm KÞ

Fig. 1. Effect of filler content on the thermal conductivity of unsaturated-polyester composite materials.

Increasing the filler content, in general, causes an increase in thermal conductivity coefficient, see Fig. 1. This may be due to the higher conductivity of the filler. The thermal conductivity of most mineral fillers is about one order of magnitude higher than that of thermoplastics and their incorporation considerably increases the conductivity of a composite (Rothon, 1999). For example, pure, dense MgO and pure, dense Al2O3 do have thermal conductivity values about 38 W/(m K) at room temperature, while for fused silica it is about 2.7 W/(m K) (Rothon, 1999). Bentonite and feldspar used in our study are rich in silica and alumina, see Table 1. If the heat transfer resistance is defined as the reciprocal value of the thermal conductivity of a substance, then the overall resistance of a composite material can be calculated as the sum of the individual resistance for each constituent while weighted by their volume (or weight) fraction. 1 koverall

¼

N X wi w1 w2 w2 wN ¼ þ þ þ N þ ki k1 k2 k3 kN i¼1

ð3Þ

ð5bÞ

Thus, the thermal conductivity value of BBUPEC is around 0.2. Such a value is quite comparable with the range of empirical values reported in Fig. 2, for instance. The experimental value at the same composition is about 0.13 W/(m K). The presence of voids (or, porosity) is expected to be behind an experimental value that is less than that predicted by Eq. (3). Compared with building bricks (or cement plaster) which have thermal conductivity of 0.72 W/(m K), with concrete (stone) which has a thermal conductivity of 0.93 W/(m K), and with reinforced concrete with a thermal conductivity of 1.73 W/(m K), BBUPEC's have thermal conductivity values between 0.1 and 0.2, at 50 wt.% filler content and about 40 wt.% polyester. If an average resistance, using Eq. (3), is taken to the aforementioned materials without incorporating BBUPEC, then 1 kaverage

N X wi 0:333 0:333 0:333 þ þ ¼ 1:73 0:93 0:72 ki i¼1 ¼ 1:01

¼

ð6Þ

A kaverage of 1.0 W/(m K) is obtained. On the other hand, if a wall is built with such building materials (building blocks, concrete and reinforced concrete) while this time BBUPEC comprises one-third of the

where wi represents the weight fraction of species i and ki is the thermal conductivity, summed over all entering constituents of a composite. Eq. (3) will be applied, first, to the filler itself, and second, to the Bentonite-Based Unsaturated PolyEster Composites (BBUPEC). For the filler itself, it will be divided into three major constituents: alkali and alkali earth metal oxides, silica, and water. Thus, the average thermal conductivity for typical filler, like bentonite 1, will be: 1 k

j

bentonite1

¼

j

0:43 0:52 þ 38 alkalimetaloxide 2:7 0:05 þ ¼ 0:29 0:61 water

j

kbentonite1 ¼ 3:5 W=ðm KÞ

j

silica

ð4aÞ

ð4bÞ

Fig. 2. Effect of polyester content on the thermal conductivity of unsaturated-polyester composite materials.

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Fig. 3. Effect of filler content on the distilled water retention of the composite materials.

Fig. 5. Effect of polyester content on the distilled water retention of the composite materials.

wall thickness while the rest of the wall is made of the previous materials, then

range. At low polyester content (i.e., 37 wt.% or less) it is more likely that the amount of resin is not sufficient to fill in the gaps within the filler, in the presence of a relatively high filler load, which will result in void formation in specimens and therefore the thermal conductivity coefficient is reduced. Increasing the polyester content above 40%, the composites show an increase in the thermal conductivity. This increase is more likely due to the co-polymerized polyester/styrene products, which act as strong binders and hold the grains of bentonite together into one solid mass. Fig. 3 gives the distilled water retention percent for different composites as a function of filler content., In general, WR% value of composite is low and it increases with increasing bentonite content for bentonite 2 and 3. On the other hand, bentonite 1 shows a reverse behavior where the water retention decreases with increasing filler content. This behavior of bentonite 1 was also noticed when the distilled water was replaced

1 kð1=3Þinsulation

¼

N X wi 0:333 0:67 þ ¼ 2:344 ¼ 0:2 0:987 ki i¼1

ð7Þ

which means that the new value of kaverage will be 0.427 W/(m K). If 0.1 W/(m K) is used instead of 0.2 for BBUPEC, then kaverage will be 0.25 W/(m K). Consequently, constructing a wall made of BBUPEC that comprises one-third a wall thickness, the minimum percent relative reduction in overall thermal conductivity will be about 57% and the maximum will be about 75%. Fig. 2 shows the effect of polyester content on the thermal conductivity coefficient for the specimen containing 50 wt.% filler content. It should be noted that increasing the polyester content will be at the expense of styrene content. In the case of calcium carbonate, feldspar and bentonite 3 increasing the polyester content from 37.5 to 40.0 wt.% causes a decrease in the thermal conductivity coefficient of the composite to reach its minimum within the examined

Fig. 4. Effect of filler content on the tap water retention of the composite materials.

Fig. 6. Effect of filler content on the citric acid impregnation for the composite materials.

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Fig. 7. Effect of polyester content on the citric acid impregnation for the composite materials.

Fig. 9. Effect of polyester content on the NaOH impregnation for the composite materials.

by tap water, see Fig. 4. The comparison between Figs. 3 and 4 reveals that the type of water does not play a significant role in the amount of water retained. The greatest difference between the distilled and tap water retention was recorded for bentonite 2. From Fig. 5, it is noticed that increasing the polyester content does not cause a pronounced change in the water retention percentage. It is expected that in the examined polyester content range, the cross-linking process is very efficient, which reduces voids needed for water retention (Abu-Jdayil et al., 2002). The results of water retention are comparable with those reported by Ismail et al. (1999). Fig. 6 illustrates variation of citric acid impregnation capacity of the composite specimens as a function of filler content. It is clear that increasing the filler content decreases the citric acid absorption to reach a minimum at the filler content of 50 wt.%. It is believed that at this filler content the composites possess the best network structure. However, the calcium carbonate-based composites show a negative absorption percentage, which

means that the citric dissolves parts of the composite. It should be mentioned that the percentage of citric acid absorption is comparable with that of water. Increasing the polyester content decreases generally the citric acid impregnation, as shown in Fig. 7. As shown in Fig. 8, the NaOH impregnation decreases with increasing filler content to reach a minimum value at a filler content that lies between 40 and 50 wt.%. On the other hand, the prepared composites show a greater resistance against NaOH than citric acid. For example, in the case of a bentonite 2 based composite with a filler content of 50 wt.%, the citric acid impregnation is 2.9% while the NaOH impregnation is 1.0%. Bentonite 3 shows the best resistance against the NaOH. Except for bentonite 1 composite, the NaOH impregnation percent is independent of polyester content in the range examined, see Fig. 9. It should be mentioned here that it is really hard for us to compare our results for water, acid, and base retention values with those of other investigators, simply, because, the operating conditions in terms of immersion or equilibration time, type of composite, and temperature are different. 5. Conclusions

Fig. 8. Effect of filler content on the NaOH impregnation for the composite materials.

Natural clay (bentonite) can be utilized to manufacture a stable and compatible composite material, which was denoted as BBUPEC. In general, with 50 wt.% filler content and 40 wt.% polyester content, the thermal conductivity of un-saturated polyester composite showed a minimum thermal conductivity, k. At the afore-mentioned condition, calcium carbonate-based composite showed the minimum value of k which is 0.1 W/(m K), followed by bentonite-1-based and bentonite-2-based composites, followed by bentonite-3-based, and finally followed by feldspar-based composites. However, in

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terms of citric acid impregnation, calcium carbonatebased composite was not stable and dissolution took place. In terms of water retention value, citric acid and NaOH impregnation values, one could say that bentonite-3-based composite was the best among BBUPEC. Consequently, one would say that bentonite-based composites show good characteristics in terms of thermal conductivity and physical and chemical stability and with such cheap and abundant fillers from natural resources, they show a promising thermal insulating material both for domestic and industrial applications. Nomenclature BBUPEC Bentonite-Based Unsaturated Poly-Ester Composite Acknowledgment This research was funded by the Higher Council for Science and Technology, Amman–Jordan. References Abu-Jdayil, B., Al-Malah, K., Sawlaha, R., 2002. Study on bentoniteunsaturated polyester composite materials. Journal of Reinforced Plastics and Composites 21, 1597–1607.

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