Development of smart gypsum composites by incorporating thermoregulating microcapsules

Energy and Buildings 76 (2014) 631–639

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Development of smart gypsum composites by incorporating thermoregulating microcapsules Ana M. Borreguero a , Ignacio Garrido b , José L. Valverde a , Juan F. Rodríguez a , Manuel Carmona a,∗ a b

Department of Chemical Engineering, University of Castilla – La Mancha, Av. Camilo José Cela s/n, 13004 Ciudad Real, Spain Department of Mechanical Engineering, University of Castilla – La Mancha, Plaza Manuel Meca, 1, 13400 Almadén, Spain

a r t i c l e

i n f o

Article history: Received 11 December 2013 Received in revised form 28 February 2014 Accepted 5 March 2014 Keywords: Gypsum PCM Thermal properties Building materials Thermal energy storage capacity Filler

a b s t r a c t Smart gypsum composites were manufactured by adding different kinds of microcapsules containing phase change materials (PCMs) in order to develop building materials with high thermal energy storage (TES) capacity useful for being applied in high comfort constructive systems. The physical, thermal and mechanical properties of these composites such as density, porosity, thermal stability, thermal conductivity (k), equivalent heat capacity (cp ), the accumulated heat power (qacc ) and the maximum compressive strength were studied. Results showed that the higher the microcapsules content, the lower the density and k and the higher the cp and qacc , due to the PCM action. Besides, the addition of 15 wt% of microcapsules respect to the hemihydrate would allow to save 4.5 kWh of energy per operating cycle in a standard room covered with 1 m3 of this gypsum. This energy is equivalent to the energy spent by three incandescent light bulbs of 60-W kept on all the day and a reduction of 1.395 kg of CO2 emissions to the atmosphere. The addition of these thermoregulating materials to gypsum decreases their compressive strength but all the developed materials satisfied the Spanish mechanical regulations for gypsum as building material, being possible to increase the total amount of added microcapsules. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The social concern about the consequences of global warming is increasing the regulation requirements for the energy consumption in most of the developed countries, promoting the reduction of the fossil fuels use. It is known that buildings are responsible of close to 40% of the final energy consumption and also the 36% of CO2 emissions in the Europe Community in 2002 [1] and the EU directive 2010/31/UE [2] focus its environmental objectives in the reduction of the total emission of the greenhouse gases in at a least a 20% in 2020 with respect to that of the year 1990. A similar situation is found in the USA, since there buildings consume the 30% of the total energy consumption [3]. The energy consumption is well related with the CO2 emissions and according to the Energy National Center of Spain (CNE), 0.31 kg of CO2 are emitted to the atmosphere per kWh of produced energy [4].

∗ Corresponding author. Tel.: +34 902204100; fax: +34 926 295318. E-mail address: [email protected] (M. Carmona). http://dx.doi.org/10.1016/j.enbuild.2014.03.005 0378-7788/© 2014 Elsevier B.V. All rights reserved.

The fossil fuel consumption in buildings could be minimized by the application of the solar energy but this energy is intermittent and its exploitation requires the development of proper technologies to storage it. A noteworthy alternative for storage this energy in buildings is the use of thermoregulating materials, whose inversion is more than recovered considering their economic and environmental benefits. The final properties of the building envelopes containing these materials must be analyzed in order to evaluate the suitability and real gains of the constructive system. Several authors have proposed the incorporation of phase change materials (PCMs) in order to regulate the inside building temperatures by storing or releasing the solar energy proportional to their latent heat of fusion depending on the external temperature. This way, a PCM stores energy at temperatures higher than its melting point but when the temperature outside the building goes down to the PCM melting temperature, it releases the previously stored energy [5–7]. One of the building materials used for the PCM incorporation is gypsum; which is environmental friendly, fire resistant, esthetics, presents low prices and can be applied in situ or as precast slabs. Besides, this material can be used for interior wall, ceiling or exterior if proper hydrophobization treatment of the wall

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Table 1 Microcapsules data. Product

dpn0.5 (␮m)

dpv0.5 (␮m)

Tf (◦ C)

mSD-(LDPE·EVA-RT27) mSP-(PS-RT27) Micronal® DS 5001X mSD-CNFs

3.9 ± 0.11 116.8 ± 2.84 7.1 ± 2.32 –

10.0 ± 0.42 584.0 ± 17.05 77.2 ± 21.06 –

28.40 28.46 27.67 27.6

is accomplished [8]. From the chemical point of view, gypsum is calcium sulfate dihydrate obtained by adding water to the calcium sulfate hemihydrate (hemihydrate) in powder according to the stoichiometric reaction (Eq. (1)) [9]. CaSO4 ·(1/2)H2 O + (3/2)H2 O → CaSO4 ·2H2 O + heat

(1)

Different authors have studied the addition of PCMs directly into the building materials and they have observed that PCMs are easily adsorbed into porous concrete or in the polyurethane foams matrix, enhancing the thermal energy storage (TES) capacity of the facade and reducing the consumed energy [10–17]. Unfortunately, in the direct application the PCMs can interact with the rest of materials and leak when remaining melted [18]. These problems can be solved by the PCMs encapsulation, it is putting them into containers, before their incorporation in the construction materials [19,20]. Microencapsulation of PCMs with polymeric shells stands out as one of the best encapsulation options for this particular application [21]. Additionally, the microencapsulation allows to increase the heat-transfer area, control the variations of volume during the change of phase and avoid deleterious effect on the mechanical properties of building materials [22]. Therefore, the development of a gypsum block with high TES capacity by means of the addition of microencapsulated PCMs into the hydrating hemihydrates could lead to a reduction of the energy demand in the residential and tertiary sectors [23]. A commercial product with this technology has been manufactured and distributed by the chemical company BASF. This gypsum wallboard is named Knauf Gips KG’s PCM SmartBoard® and contains 3 kg of Micronal® PCM per square meter and 15 mm of thickness [24]. Toppi and Mazzarella [9] synthesized gypsum wallboards containing PCMs by using the commercial product of BASF DS 5000 which consists of a dispersion of Micronal® PCM having a solid content of 42 wt%. They found that even a very small amount of microcapsules presents a big effect on the gypsum viscosity and provides crystallization nuclei, promoting the gypsum solidification and modifying the composite material properties. The microcapsules addition increases the porosity and reduces the thermal conductivity. Nevertheless, the effects of thermoregulating microcapsules having different shell materials on the thermal and mechanical properties of the lightweight gypsums have not been previously reported in literature. In previous works, microencapsulated PCMs with a polystyrene shell were successfully synthesized by means of a suspension like polymerization (SP) technique and called mSP-(PS-RT27) [25,26,6]. Furthermore, the microcapsules mSD-(LDPE·EVA-RT27), with shell from low density polyethylene (LDPE) and Ethyl vinyl acetate (EVA) and with Rubitherm® RT27 as core material, were prepared by Spray drying (SD) technique following the process described in the Patent EP2119498 [27]. Both types of microcapsules containing Rubitherm® RT27 were incorporated into rigid polyurethane foams, finding that the presence of the microcapsules into the foam enhanced the TES capacity and the insulating effect of wallboards [28–30]. Thus, these microencapsulated PCMs can be used to increase the TES capacity of building materials while keeping the insulating properties [28–30]. The aim of this paper is to develop smart gypsum blocks by adding three different kinds of microcapsules containing PCMs at a

hf (J/g) ± ± ± ±

0.90 2.74 0.94 1.55

98.14 96.74 116.2 95.64

± ± ± ±

2.17 1.95 4.11 5.73

mass ratio of microcapsules/hemihydrate in percentage from 7.5 to 15.0%. The physical, thermal and mechanical properties such as porosity, density, thermal conductivity, the equivalent specific heat and the maximum compressive strength must be determined. Besides, in order to know the effect of increasing the thermal conductivity of microcapsules on the thermal properties of the gypsum block, mSD-(LDPE·EVA-RT27) containing 2 wt% of carbon nanofibers (CNFs) as filler were incorporated and named mSDCNFs. 2. Materials and methods 2.1. Materials Four types of microcapsules containing PCMs were used in this work: mSP-(PSt-RT27), mSD-(LDPE·EVA-RT27) with and without CNFs and Micronal® DS 5001X. mSP-(PSt-RT27) were synthesized with polystyrene shell and paraffin wax Rubitherm® RT27 by suspension polymerization technique in our lab facilities. mSD(LDPE·EVA-RT27) with and without CNFs are also made in our laboratory from a polymeric shell of low density polyethylene (LDPE) and Ethylvinylacetate (EVA) copolymer and paraffin wax Rubitherm® RT27 as core material, following the process described in the European Patent EP2119498 [31]. The incorporated CNFs were synthesized following the method developed by Jiménez et al. [32]. Finally, the commercial microcapsules Micronal® DS 5001X supplied by BASF were selected since they have exhibited a large TES capacity in concrete blocks [33]. Micronal® DS 5001X contains n-heptadecane as the core material whose melting point is 26 ◦ C and a shell from Polymethylmethacrylate (PMMA). The average particle size in number (dpn0.5 ) and in volume (dpv0.5 ) of the microcapsules were obtained by low angle laser light scattering (LALLS) and their melting point (Tf ) and latent heat of fusion (hf ) were determined by differential scanning calorimetry (DSC) analyses. These data of the four microcapsules types are gathered in Table 1. Black gypsum was supplied by Yesos Juarez S.A. (Spain) and demineralized water with a conductivity value lower than 5 ␮S/cm were used for the gypsum blocks. 2.2. Gypsum blocks manufacturing Gypsum blocks were synthesized by weighting and mixing the masses of calcium sulfate hemihydrate, water and microcapsules according to the proportions shown in Table 2. The hemihydrate and microcapsules were mixed first and then the mixture was added to the water mixing constantly under vigorous agitation

Table 2 Weight of raw material for the studied gypsum blocks. Microcapsules/ hemihydrate (wt%)

0.0 7.5 15.0

Component

Water (g)

Hemihydrate (g)

Microcapsules (g)

110 110 115

231 231 231

0 17.3 34.7

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state. This equipment and its performance for the thermal characterization of materials containing PCMs were described and proved in previous works [23,34]. Tests were carried out fixing initially the thermostatic bath temperature at 18 ◦ C and, once the gypsum block reached the steady state, the set point of the thermostatic bath was changed to 42 ◦ C and then left again until the new steady state. Temperatures at different gypsum block locations and the inlet and outlet heat fluxes were constantly registered during the test. Thermocouples of K-type were used to measure temperatures: two were put in the external block surface (Tup ), other two were placed at the cell (Tdown ) and the last one to measure the environmental temperature (T∞ ). In all cases, thermocouples were put at the inlet and outlet borders of the block in liquid flow direction. The liquid flow was high enough (100 l/h) to ensure the absence of thermal profile in liquid direction. Four heat flow sensors PU22T were used to monitor on line the inlet and outlet heat fluxes and they were located as shown in Fig. 1b. All these signals were registered continuously using the NOKEVAL program and recorded by means of a computer. Using these signals is possible to quantify by means of Eqs. (2)–(4), the equivalent specific heat in a temperature range (cp ), the TES capacity per cubic meter and the effective thermal conductivity at the final steady state (k), respectively. cp =

qacc msample · (Tupf − Tupi )

(2)

qacc b · msample 3.6 × 106

(3)

TES = = Fig. 1. Experimental set up for the materials thermal behavior tests: (a) global experimental set up; (b) heat flux sensor position.

during at least 1 min. Finally, the mixture was poured into a mold of 3 cm × 6 cm × 10 cm and left to set at atmospheric conditions. 2.3. Sample characterization 2.3.1. SEM analysis Synthesized lightweight gypsum composites were depicted by means of scanning electron microscopy (SEM) by using a FEI QUANTA 200. 2.3.2. Thermal degradation of gypsum composites Thermogravimetric analysis (TGA) of gypsum composites was performed by thermogravimetric analysis by using a TA Instrument equipment model SDT Q600. The used conditions for the analyses were a heating rate of 10 ◦ C/min from room temperature to 700 ◦ C under nitrogen atmosphere. 2.3.3. Density The bulk density of the gypsum block, b , was determined by weighing and size measurements of the test prisms. The matrix density, , was determined by helium pycnometry (Micromeritics Accupyc 1330). Small helium atoms penetrate into the pores of the studied gypsum samples and ensure that the obtained density value by this method corresponds only with the solid material. Values of bulk and matrix densities were used to determine the sample porosity (ε). 2.3.4. Experimental equipment and procedure for the thermal behavior tests The thermal behavior of gypsum blocks doped with microcapsules has been studied using a homemade equipment (Fig. 1a) that registers different temperature profiles with time of blocks when they are subjected to a disturbance and starting from the steady

Qin · x Tdown − Tupf

(4)

where qacc is the amount of accumulated heat in the sample during the experiment (J), msample is the gypsum block mass (kg) and Tupf and Tupi are the Tup at the final and initial conditions, TES in kWh/m3 , b is the bulk density of the gypsum block (kg/m3 ), Qin is the inlet heat flux at the final steady state condition (W/m2 ) and x (m) is the gypsum thickness. The qacc is calculated by subtracting the outlet heat fluxes from the inlet one. 2.3.5. Compression test The mechanical properties of the new developed composites were analyzed by using a MTS 370.02. The compressive strength was measured by using the blocks made for thermal studies which were cut having faces of 40 mm × 40 mm. A load rate of 1 N/mm2 ·s was applied over the entire load application until fracture. The compressive strength was calculated according to standard No EN 13279-2:2004 for gypsum binder and gypsum plasters [35]. The physical and thermal characterization tests were performed three times and the results are given in the tables with 95% confidence interval. 3. Results 3.1. SEM analysis It is well known that morphology of calcium sulfate dihydrate crystal depends on the formation conditions and the presence and type of chemical additive [36]. The structure of lightweight gypsum composites synthesized with a mass ratio microcapsules/hemihydrate of 7.5% by weight and without microcapsules is shown in Fig. 2. Fig. 2a shows that needle shaped gypsum crystals with a high degree of interlocking are formed during the hydration of hemihydrates. These needles presented a typical size in ␮m thus, it is expected that particles of similar of higher size present a big effect

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Fig. 2. SEM images of calcium sulfate dehydrate, (a) without microcapsules; synthesized with a mass ratio microcapsules/hemihydrate of 7% (b) mSD-(LDPE·EVA-RT27); (c) mSP-(PS-RT27); (d) Micronal® DS 5001X.

on the gypsum properties. Fig. 2b indicates that mSD-(LDPE·EVART27) which have a particle size of 3 ␮m can be located on the needle gypsum occupying the pores formed by them and hence, changing the gypsum capillary porosity. It is observed in Fig. 2c that the higher particle size of the mSP-(PS-RT27) which have a dpn0.5 of 116.8 ␮m avoid the gypsum growing and they are found in an agglomerated form, being the gypsum found on the surface of the microcapsules. Finally, it is observed in Fig. 2d that single particles of Micronal® DS 5001X can be found on the needle gypsum and also as an agglomerated material. Thus, according to these results, microcapsules of a small particle size must be used in order to enhance the continuous growth of the gypsum. It is not observed changes in the needle form by adding these different types of microcapsules. Fig. 3 shows the SEM images of composites synthesized with a mass ratio microcapsules/hemihydrate of 15%. It is observed a high microcapsules agglomeration, forming clusters into the gypsum. The presence of clusters in this lightweight gypsum could be deleterious for the mechanical properties. 3.2. Thermal degradation of gypsum composites Thermogravimetric analyses are shown in Fig. 4. This figure shows that the calcium sulfate dihydrate presents three regions of weight loss in the thermal curve. The first region is observed between 80 and 140 ◦ C attributed to absorbed water (hygroscopic water) and the chemically bound water of the hydrated salt, the second region that took place mainly at 525 ◦ C refers to the water bound to hydraulic compounds and the last one corresponds to the carbon dioxide developed during the decomposition of carbonates [37,38]. As can be seen in this figure, independently on the microcapsules type, TGA curves present two steps of weight

loss; the first one between 150 and 250 ◦ C, corresponding to the core material evaporation and the second step due to the polymer degradation, between 370 and 480 ◦ C, that depends on the polymer network. These analyses confirm the temperature of degradability of the polymers as PMM < PS < LDPE·EVA according to the values found in literature [39,40]. No separate steps were observed for the EVA and LDPE [40]. The TGA of the lightweight gypsum composites indicates that independent of the microcapsule type the evaporation of the core material goes forward with respect to the boiling point for pure material. The same effect was only observed in the case of the PS shell, whereas the thermal decomposition for the other two shells started at a slightly higher temperature. The faster evaporation of the PCMs into gypsum blocks could be explained by the high rate of water evaporation that takes place during the dehydration of the calcium sulfate dihydrate and the higher thermal conductivity of gypsum respect to the PCMs and polymers which improves the heat transfer. The thermal conductivity of gypsum is ranged between 0.19 and 0.76 W/m K [9,11,41], whereas the values for PCMs and polymers shown in Table 3 are lower than 0.335 W/m K. In the case of the shells, this different behavior could be attributed to the particle size of the microcapsules. The dpn0.5 of mSP-(PS-RT27) is more Table 3 Thermal conductivity of microcapsules components. Material

k (W/m K)

LDPE EVA PS Rubitherm® RT27 PMMA CNFs

0.15–0.335 [42–44] 0.13 [45] 0.14–0.18 [46] 0.2 [47] 0.12 [39,48] 1000–2000 [49,50]

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635

Fig. 3. SEM images of composites synthesized with a mass ratio microcapsules/hemihydrate of 15% (a) mSD-(LDPE·EVA-RT27); (b) mSP-(PS-RT27); (c) Micronal® DS 5001X.

100

90 85

40 80 20

75 70 0

100

200

300

400

500

600

100 mSP-(PS-RT27)

95

Weight blocks (%)

95

100

Microcapsules content in gypsum blocks (wt%) 0.0 7.5 15.0

90

60

85 40 80 20

75 70

0 700

80

0

100

200

300

400

500

600

Weight microcapsules (%)

mSD-(LDPE·EVA-RT27) microcapsules content in gypsum blocks 80 (wt%) 0.0 7.5 60 15.0

Weight microcapsules (%)

Weight blocks (%)

100

0 700

Temperature (ºC)

Temperature (ºC)

a)

b) 100

100 ®

Microcapsules content in gypsum blocks (wt%) 0.0 7.5 15.0

90 85

80

60

40 80 20

75 70 0

100

200

300

400

500

600

Weight microcapsules (%)

Weight blocks (%)

Micronal DS 5001X 95

0 700

Temperature (ºC)

c) Fig. 4. Weight loss with temperature for the gypsum blocks containing 0, 7.5, 15.0 and 100% of: (a) mSD-(LDPE·EVA-RT27); (b) mSP-(PS-RT27); (c) Micronal® DS 5001X.

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Table 4 Densities and porosity of the composite blocks. Block

Microcapsules/ hemihydrate (wt%)

b (kg/m3 )

Gypsum Gypsum with mSD-(LDPE·EVA-RT27)

0 7.5 15 7.5 15 7.5 15 7.5

1285.65 1242.24 1177.25 1282.92 1123.29 1182.41 1104.97 1288.88

Gypsum with Micronal® DS 5001X Gypsum with mSD-CNFs

than fifteen times higher than those of mSD-(LDPE·EVA-RT27) and Micronal® DS 5001X. Hence, the small sample of lightweight gypsum with mSP-(PS-RT27) contains mainly broken microcapsules whereas for the other two microcapsules, the gypsum provided good protection, preventing their evaporation. 3.3. Density The porosity of the lightweight gypsum composites (Eq. (5)) were estimated from the bulk density, the matrix density () and assuming that the gypsum pores are filled of air which remains trapped once the gypsum has solidified. For the value of air density air was assumed that is the one at normal conditions (air = 1.186 kg/m3 ). ε=

 − b  − air

1.05 2.50 1.41 0.77 7.13 3.24 2.76 7.43

2451.4 2191.0 2043.0 2261.9 2167.2 2291.3 2057.9 2211.6

± ± ± ± ± ± ± ±

ε 2.81 5.59 4.52 3.19 19.4 3.70 3.87 6.19

0.476 0.433 0.424 0.433 0.482 0.484 0.463 0.417

± ± ± ± ± ± ± ±

0.0013 0.0030 0.0025 0.0016 0.0105 0.0023 0.0025 0.0045

Fig. 7 shows the accumulated heat powers and, as expected; the higher the microcapsule content, the higher the accumulated heat power. Heat capacities, thermal energy storage capacities per cubic meter and thermal conductivities determined from thermal analyses are gathered in Table 5. The cp values of lightweight gypsum composites increase with the increase of the content and heat of fusion (Hf ) of added microcapsules. As can be seen, this increase is directly proportional to the microcapsule content, passing from a 50 to a 100% when the mass ratio of mSD-(LDPE·EVA-RT27) or Micronal® DS 5001X respect to the hemihydrate is increased from 7.5 to 15%. Nevertheless, mSP(PS-RT-27) present the lowest increase in the cp value for a content into the gypsum of 15%. This difference respect to the other microcapsules could be related with the lowest thermal conductivity of PS respect to the PMMA or LDPE-EVA copolymer. Fixed values of cp

(5)

Table 4 shows both densities and the porosity of each one of the studied lightweight gypsum composites. As expected, the higher the microcapsules content, the lower the bulk density due to the lowest density of the microcapsules materials compare to the one of the gypsum. There is an exception in the case of the mSD-CNFs addition because this lightweight gypsum presents a lower porosity than that of pure gypsum. It is also observed a decrease in the matrix density with the microcapsules content, but increasing this density slightly with the particle size. These results also show that the higher the particle size, the higher the porosity of the gypsum block. Besides, the composites bulk densities are higher than 600 kg/m3 , satisfying the mechanical European regulation for gypsum (EN 13279-2) [35].

36

32

Temperature (ºC)

Gypsum with mSP-(PS-RT27)

± ± ± ± ± ± ± ±

 (kg/m3 )

28

mSD-(LDPE·EVA-RT27) Content (wt%) 0.0 7.5 15.0

24

20

16 0

5000

15000

20000

25000

Time (s)

3.4. Thermal behavior of the composite gypsum blocks

Fig. 5. Temperature profiles of the external surface of the composite blocks containing 0.0, 7.5 and 15.0 wt% of mSD-(LDPE·EVA-RT27).

36

32

Temperature (ºC)

As example of the thermal behavior of the composite gypsum blocks, Fig. 5 shows the temperature profiles of the composites containing mSD-(LDPE·EVA-RT27) when they are subjected to the heating process. This figure shows that the higher the microcapsules content the lower the slope of the temperature profiles when the temperature reaches the region in which the PCM starts to melt. The decrease in the temperature change indicates that the heat is being used for the melting process instead of for the temperature increase. This decrease in the temperature curve slope when the composite reached the paraffin wax melting point was also observed for the rest of materials (Fig. 6). Hence, it can be expected that the gypsum blocks containing the maximum amount of microcapsules will exhibit a smoother temperature profile and close to the PCM melting point when they are subjected to a temperature change. This way, by means of their latent heat, the microcapsules are storing the thermal energy that reaches the gypsum blocks. On the other hand, the higher the microcapsules content, the lower the final temperature in the steady state (Fig. 5), what seems to indicate that the microcapsules addition reduce the composite thermal conductivity.

10000

28

Microcapsule type mSD-(LDPE·EVA-RT27) mPS-(PS-RT27) Micronal®DS 5001X mSD-CNFs

24

20

16 0

5000

10000

15000

20000

25000

Time (s) Fig. 6. Temperature profiles of the external surface of the composite blocks containing 7.5 wt% of the different studied microcapsules.

Accumulated Heat Power (W)

66.29 61.73 −4.10

1.5

mSD- (LDPE·EVA-RT27) Content (wt%) 0.0 7.5 15.0

1.2 0.9 0.6 0.3 0.0 -0.3

10000

15000

20000

61.07 30.42 10.16 73.60 47.64 17.68 51.64 38.64 8.87 101.74 63.88 15.88 51.76 34.39 6.02

0.26 7021.8 ± 6.91 18.89 30.78 2315.6 8.87 0.239 ± 0.002 0.23 7401.6 ± 13.89 18.82 31.63 2495.9 10.04 0.219 ± 0.005 0.26 6879.3 ± 5.89 18.89 31.03 2180.1 9.43 0.242 ± 0.002 0.25 8520.9 ± 8.08 18.79 30.37 2900.4 11.15 0.223 ± 0.005 0.26 6886.9 ± 10.03 18.94 30.91 2181.8 9.14 0.250 ± 0.009

Fig. 7. Heat fluxes stored by the gypsum blocks containing 0.0, 7.5 and 15.0 wt% of mSD-(LDPE·EVA-RT27).

cp increase (%) TES increase (%) kreduction (%)

0.27 5141.9 ± 6.81 18.80 31.92 1437.7 6.80 0.266 ± 0.001 Weight (kg) qacc (J) Tupi Tupf cp (J/kg ◦ C) TES (kWh/m3 ) k (W/m ◦ C)

7.5% 7.5% 7.5% 0%

15%

mSP-(PS-RT27) mSD-(LDPE·EVA-RT27) PCMs Block property

5000

Time (s)

15%

Micronal® DS 5001X

0

Table 5 Thermal properties of gypsum composites.

637

1.8

106.11 55.46 25.84

0.27 8295.3 ± 7.96 18.66 31.74 2390.7 11.00 0.277 ± 0.003 0.24 8267.5 ± 12.37 18.88 30.72 2963.3 10.57 0.197 ± 0.005

7.5% 15%

mSD-CNFs

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of 1300 and 1466 J/kg K for PS and PMMA are reported whereas the cp for LDPE-EVA is within 1800 and 3400 J/kg K [42,46,48]. Besides, as commented above, the cp increase agrees with the Hf of the microcapsules which is 116.2, 98.14, 96.74 J/g for the Micronal® DS 5001X, mSD-(LDPE·EVA-RT27) and mSP-(PS-RT27), respectively. As expected, TES values of smart gypsum composites increase with the microcapsules content. The addition of 7.5 or 15% microcapsules respect to the hemihydrate would rise the TES of the gypsum from 2.25 to 4.5 kWh/m3 , doing a theoretical calculation based on the Hf of the microcapsules, respectively. It is observed that only the lightweight gypsum composites containing microcapsules from LDPE·EVA as shell material showed a similar increase, being this increase ranged between 30 and 60% from the TES of the gypsum block without microcapsules which is only due to sensible heat change. These TES values indicate that for the studied experimental conditions the total amount of core material inside the mSP-(PS-RT27) and Micronal® DS 5001X were not completely melt at the final steady state and mainly for the gypsum composites having high amount of microcapsules. This effect could be due to the low thermal conductivity of these shell materials. Hence, the application of gypsum boards with 3 cm of thickness containing 15% of microcapsules respect to the hemihydrate in the interior walls of a standard room of dimensions 3 m × 4 m × 2.5 m requires 1 m3 of gypsum board approximately. This means that 4.5 kWh could be saved in this room, avoiding an emission of 1.395 kg of CO2 when the material works catching the energy from the sun during the day and releasing it to the room during the night. However if a conditioner air is sequentially turned on and turned off following working cycles would be possible to save more energy and money and reduce the emissions of CO2 because the microcapsules will absorb and release the energy several times with each temperature change. It is important to point out that this saving energy is three times higher than the energy spent by an incandescent 60-W light bulb kept on all the day (1.44 kWh). The addition of CNFs to the microcapsules confirms the effect of the low thermal conductivity of the microcapsules on the TES because they allowed to improve the TES even at values higher than the expected ones. This is related to the higher final temperature of the gypsum block as shown in the external temperature profile (Fig. 6). Thermal conductivity of CNFs favors the heat transfer and thus the paraffin melting, reaching at steady state a final temperature higher than that of the composite without CNFs. This higher temperature allows to explain the higher cp exhibited by this gypsum board. The thermal conductivity obtained for the pure gypsum is 0.266 W/m K, which is in the range from 0.19 to 0.76 W/m K, values found in literature for different gypsum mixtures

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Maximum Compressive Strength (MPa)

15

Microcapsules by weight (%) 0 7.5 15.0

12

9

6

3

0 Gypsum

mSD-(LDPE-EVA-RT27)mSP-(PS-RT27)

Micronal®DS 5001X

mSD-CNFs

Lightweight Gypsum Composites

Fig. 8. Maximum compressive strength of lightweight gypsum composites.

[9,11,41]. Besides, according to the results showed in Table 5, the following trend can be established in the thermal conductivity: mSD-CNFs > gypsum > 7.5%mSD-(LDPE·EVA-RT27) > 7.5%mSP-(PS-RT27) > 7.5%Micronal® DS 5001X > 15%mSD(LDPE·EVA-RT27) > 15%mSP-(PS-RT27) > 15%Micronal® DS 5001X. Therefore, it can be said that the higher the microcapsules content, the lower the thermal conductivity, except for the case of the mSD-CNFs. This trend is explained considering the thermal conductivity values of the paraffin wax, the shell polymers and the CNFs reported in Table 3. The reduction in the thermal conductivity could be an additional advantage, because the microcapsules do not only improve the TES capacity, but also enhance the insulating properties of the gypsum.

3.5. Compression test Fig. 8 shows the maximum compression strength of developed smart gypsum composites as function of microcapsules type and their composition. It is observed that independently on the microcapsule type the addition of microcapsules produces a decrease in the maximum compression strength. Lightweight gypsum composites developed by the addition of mSP-(PSt-RT27) becomes harder and stronger, despite of their larger average particle size. On the other hand, the decrease of the mechanical resistance observed with the increase of the microcapsules content could be attributed to the greater agglomeration of the capsules, similar to that observed in polyurethane composite foams doped with these kinds of microcapsules [30]. This figure also shows that lightweight gypsum composites containing mSD-(LDPE·EVA-RT27) or Micronal® DS 5001X present similar mechanical resistances to the compression and thus, microcapsules from LDPE·EVA or PMMA and having particle size within 4–7 ␮m exhibit the same mechanical effect. Finally, the addition of microcapsules containing CNFs to the smart gypsum composites not only increases the thermal conductivity, but also the mechanical resistance. This is possible because it was reported in a previous paper that the force required to produce the same microcapsule deformation is approximately 183% higher when a 2 wt% of CNFs is added in the microcapsule recipe [31]. It is also important to point out that all the developed smart gypsum composites satisfied the mechanical European regulation (EN 13279-2) which imposes that the mechanical resistance of gypsum composites for building construction be higher than 2 MPa [35]. Hence, there is a considerable margin for the increase of the amount of microcapsules into the gypsum while satisfying the European regulation.

4. Conclusions Smart gypsum composites containing up to a 15% of three different thermoregulating microcapsules respect to the initial hemihydrate were produced. It was found that microcapsules of small particle size can be located on the needle gypsum occupying the pores and hence, changing its capillary porosity. Microcapsules of higher particle size avoid the gypsum growing and they are found in an agglomerated form, being the gypsum found on the surface of the microcapsules. The addition of microcapsules produces a decrease of the bulk density and modifies the gypsum porosity. It was observed a strong thermal stability of lightweight gypsum composites. It was clearly seen that the higher the microcapsules content the lower the slope of the temperature profiles. The cp values of lightweight gypsum composites increase with the increase of the microcapsules content and their heat of fusion (Hf ). Lightweight gypsum composites containing microcapsules from LDPE·EVA as shell material showed an increase in the TES similar to those theoretical values based on the Hf of the microcapsules. mSP-(PS-RT27) and Micronal® DS 5001X were not completely melt at the final steady state given lower values of TES mainly for the gypsum composites having high amount of microcapsules. As lower is the microcapsules content, greater is the thermal conductivity. The addition of CNFs enhanced the thermal conductivity of the material and promoted the melting of the PCM. The addition of 15% of microcapsules respect to the hemihydrate would allow to save 4.5 kWh of energy in a standard room covered with 1 m3 of panels from these smart gypsum composites and reduce the amount of CO2 (1.395 kg) emitted to the atmosphere per operating cycle. Finally, the addition of microcapsules produced a small decrease of the maximum compression strength, so small that the material assayed satisfied the mechanical European regulation EN 13279-2. Acknowledgements Financial support from European Commission through the NANOPCM Project (NMP4-SL-2010-260056) and the fellowship and grant from the Spanish Ministry of Science and Innovation (AP2007-02712) are gratefully acknowledged. References [1] EU Directive 2002/91/EC, European Parliament, Brussels, 2003. [2] EU Directive 2010/31/UE, European Parliament, Strasburg, 2010. [3] X. Wang, J. Niu, Performance of cooled-ceiling operating with MPCM slurry, Energy Conversion and Management 50 (2009) 583–591. [4] http://www.cne.es/cne/doc/publicaciones/Memoria Garantias y Etiquetado 2010.pdf (accessed on 10.12.13). [5] D.A. Neeper, Thermal dynamics of wallboard with latent heat storage, Solar Energy 68 (2000) 393–403. [6] A.M. Borreguero, M. Carmona, M.L. Sánchez, J.L. Valverde, J.F. Rodríguez, Improvement of the thermal behaviour of gypsum blocks by the incorporation of microcapsules containing PCMs obtained by suspension polymerization with an optimal core/coating mass ratio, Applied Thermal Engineering 30 (2010) 1164–1169. [7] L.F. Cabeza, A. Castell, C. Barreneche, A. de Gracia, A.I. Fernández, Materials used as PCM in thermal energy storage in buildings: a review, Renewable and Sustainable Energy Reviews 15 (2011) 1675–1695. [8] Q. Yu, H. Brouwers, Development of self-compacting gypsum-based lightweight composite, Cement and Concrete Composites 34 (2012) 1033–1043. [9] T. Toppi, L. Mazzarella, Gypsum based composite materials with microencapsulated PCM: experimental correlations for thermal properties estimation on the basis of the composition, Energy and Building 57 (2013) 227–236. [10] D. Feldman, M.A. Kahn, D. Banu, Energy storage composite with an organic PCM, Solar Energy Materials 18 (1989) 333–341. [11] D. Feldman, D. Banu, D. Hawes, E. Gahnbari, Obtaining energy storing building material by direct incorporation of an organic phase change material in gypsum wallboard, Solar Energy Materials 22 (1991) 231–242. [12] D.W. Hawes, D. Banu, D. Feldman, Latent heat storage in concrete. II, Solar Energy Materials 21 (1990) 61–80.

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