Solar Energy Materials 22 (1991) 231-242 231 North-Holland

Obtaining an energy storing building material by direct incorporation of an organic phase change material in gypsum wallboard

D. Feldman, D. Banu, D. Hawes and E. Ghanbari Centre for Building Studies, Concordia University, Montreal, Quebec, Canada H3G I M8

Received 17 December 1990

A laboratory scale energy storage gypsum wallboard was produced by the direct incorporation of 21%-22% commercial grade butyl stearate (BS) at the mixing stage of conventional gypsum board production. The incorporation of BS was strongly facilitated by the presence and type of small amounts of dispersing agents. The physico-mechanical properties of the laboratory-pro-

. duced thermal storage wallboard compare quite well with values obtained for standard gypsum board. The energy storing board has a ten-fold increase in capacity for the storage and discharge of heat when compared with gypsum wallboard alone.

1. Introduction

Therma l s torage is an impor t an t factor in the effective use of solar hea t ing and in the conserva t ion of energy. Unfor tuna te ly , prac t ica l app l i ca t ions of research in this a rea have lagged that of o ther advances in the field. To rect ify this s i tuat ion, the a p p r o a c h taken in the presen t work involved the i nco rpo ra t ion of phase change mate r ia l ( P C M ) in gypsum wa l lboa rd to p rov ide a cost effective thermal s torage of adequa t e capac i ty which is easy to install . The P C M ' s abso rb and release hea t while changing their s ta te f rom solid (S) to l iquid (L) and f rom L to S, respect ively.

If p rope r ly selected S - L P C M ' s can funct ion very well in po rous mater ia l s such as gypsum and concrete , because, even in its L state, the P C M will be re ta ined in the s t ruc ture of the host mate r ia l by vir tue of surface tension.

G y p s u m wa l lboa rd was chosen as an a p p r o p r i a t e con ta ine r for P C M because it is a c o m m o n bu i ld ing p roduc t which is easi ly a d a p t e d to energy s tor ing app l i ca t ions and which could be p roduced and marke t ed by exis t ing manufac tu r ing and sales facilities.

The incorpora t ion of P C M into bu i ld ing mater ia l s has been a subject of research in recent years. Me thods of i nco rpo ra t ion are c i ted compr i s ing the add i t i on of small se l f -conta ined capsules of P C M when b lend ing the c o m p o n e n t s of the bu i ld ing mate r ia l or i nco rpo ra t ing P C M with o ther compone n t s which make up the bu i ld ing mater ia l s [1-4] . A l though all these inves t igat ions presen t in teres t ing results, their app l i cab i l i ty to indus t r ia l p roduc t ion is not clear.

0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

232 D, I:ehh~um el al. ()rgantu P ( M m ,~;'p.~'um wal]hoard

In a previous paper [5] technical and economic evaluation of the PCM wallboard performance was carried out. PCM's were incorporated by the immersion of regular wallboard samples at room temperature into a container of PCM at 80 °C. Two mixtures, 49% butyl stearate with 48% butyl palmitate and 55% lauric acid with 45~7 capric acid diluted 10% with fire retardant, were diffused into regular wallboard. No exudation of liquid PCM occurred when the weight proportion of PCM remained below 25%. In the wallboard, initial PCM freezing points were 21 and 220 ( ̀ . respectively, with melting points of 17 and 18°C. For a 4 ° C temperature swing, thermal storage capacities up to 350 k J / m 2 are available. Preliminary tests showed little extra flame spread beyond that of conventional wallboard. The thermal conductivity of the wallboard increased from 0.19 to 0.22 W / ° C - m with the incorporation of liquid PCM.

In the present paper the direct incorporation of PCM in wallboard is discussed. Incorporation of PCM in wallboard by immersion is simple and permits the impregnation process to be carried out either at the end of the conventional production process or at any appropriate time and place thereafter. The direct incorporation method involves introduction of the PCM into the wallboard produc- tion line at the point where the other ingredients are mixed. Even though it appears to require a somewhat more sophisticated process control than the immersion process, it may prove to result in the lower production cost alternative.

For this study butyl stearate (BS) was chosen. It is a commercial product (Emery 2325) and consists of a mixture of 49% butyl stearate and 48% butyl palmitate. It represents more than half of the stearic acid esters production [6].

The fatty acid esters are already produced in large amounts for plastic, cosmetic textile and lubricant industries and many of them are obtained from renewable sources such as oil seeds and tallow [7].

The BS was chosen because it meets the thermodynamic and kinetic criteria for low temperature heat storage. It has a relatively low melting point and a high latent heat of transition per unit mass. It exhibits a small volume change during the phase transition and does not supercool during freezing. It possesses good chemical stability, low vapour pressure at room temperature, high flash point and high autoignition temperature: moreover it is non-toxic, non-corrosive and odourless [8].

The objective of this study is to examine the feasibility of incorporating a typical PCM in the early stage of gypsum board production and to characterize the obtained energy storing wallboard obtained thereby.

2. Experimental

2.1. Materials

(a) PCM - Butyl stearate (Emery 2325) commercial mixture of 49% butyl stearate and 48% butyl palmitate. Its thermal characteristics are (average values for three samples tested): freezing point 21°C, melting point 17°C, latent heat of freezing 141 J / g and latent heat of melting 138 J /g .

D. Feldman et al. / Organic P C M in g~'psum wallboard 233

(b) Stucco - (Plaster of Paris) and additives (accelerator, binder, wetting agent, foaming agent). All these materials were obtained from Westroc Industries, Ste. Catherine, Quebec. (c) Paperboard - Commercial product obtained from Westroc Industries. (d) Dispersing agents - Three different materials were finally used as dispersing agents:

- Westroc foaming agent Westroc Industries; - Poly(vinyl alcohol) A relatively low molecular weight product, partly

saponified; produced by Hoechst Canada; - Sodium salt of Kraft Lignin Produced by Westwaco Co. USA.

2.2. Procedure

The industrial continuous manufacture of gypsum wallboard is achieved by mixing a slurry of calcinated gypsum, CaSO 4- ½H20 (Stucco), with water and various other additives to form a gypsum paste which is then poured between two layers of paperboard. After setting, the board is cut to size and passes through a dryer to remove the excess water.

The additives mentioned above include an accelerator to speed the setting time so that the board can be handled shortly after pouring, a binder to assure adherence of the paperboard backing to the core, a wetting agent and a pregenerated foam to produce the desired density of the product.

This foam is normally generated by incorporating air in an aqueous solution of a foaming agent in a foam generator.

Whereas it is not possible to reproduce typical production facilities in the laboratory, a serious attempt was made to arrive at the same results manually. This was done by giving due attention to the quantities and condition of the ingredients as well as to the timing and order of their introduction to the mix.

The work began with the manufacture of plain wallboard in the laboratory which was achieved as follows. The CaSO 4 • ½ H20 and solid additives were weighed with a precision of +0.1 g and were then added to a specified volume of water. These materials were first mixed by hand for 15 s to ensure complete blending and then mixed for a further 20 s at 800 rmp with a Heidolf RZR-50 laboratory mixer with a 5 cm helical impeller. At this point, a quantity of foam equal to 5% of the weight of the paste was introduced and mixed for about 20 s at 500 rpm. This foam had been prepared simultaneously by mixing 120 ml of a 1% solution of foaming agent in a Waring model 33BL 73 blender for 2 min at high speed. Then, about 75 s after blending, the required quantity of foam was poured into the paste and mixed as previously mentioned. The density of this foam was 0.15-0.25 g / c m 3.

The resulting mixture was immediately poured into a 20.3 x 20.3 x 1.17 cm mold with a sheet of wet paperboard on the bottom to form the back covering for the wallboard. As the mixture stiffened, the excess slurry was removed and a wet sheet of facing paperboard was laid on top and rolled gently in with a rubber roller. After 15 min the specimen was removed from the mold and after a further 15 rain it was placed in a forced circulation oven and dried for 2 h at 105 o C.

234 D. f 'eldman et all Organic P ( ' M in gypsum wallboard

Wallboard specimens produced in this manner were found to have a density of about 0.68-0.69 g / c m 3 which closely approximates the density of standard wall- board, 0.62-0.66 g / c m ~.

It should be noted that the laboratory procedure differs from the production process, particularly in respect to its discontinuity so, consequently, a longer time is required to achieve the same result. It follows that the setting time had to be correspondingly adjusted so the accelerator quantities were reduced to less than half of the proportions which would be used in normal production.

Once the foregoing laboratory procedure was established for the production of plain wallboard, a number of experiments were conducted to investigate the process of incorporating PCM into the paste. To begin with, BS, in the proportion of 21%-22% by weight of dry product, was introduced to the paste by mixing it into the homogenous mixture for 25 s at 1300 rpm. However, when the foam was added, the PCM showed a tendency to migrate to the surface of the paste and render the foam unstable. The resulting product density was 0.90-0.95 g / c m 3 and a consider- able degree of PCM migration was observed.

The next step considered was the reduction of PCM migration. To accomplish this, a dispersing agent (DA) was added to the water used for paste preparation to ensure good dispersion of PCM in the mix. For this purpose about 15% of the water was replaced with a like quantity of 1%-2.5% solution of a dispersing agent, i.e., solutions of Westroc foaming agent (WFA), poly(vinyl alcohol), (PVA) or sodium salt of Kraft Lignin (Na-L).

The presence of the DA prevented both the migration of BS from the gypsum paste and instability of the foam. The resulting mixture had similar characteristics to plain gypsum paste. It could be moulded, demoulded and dried without an apparent BS migration. The resulting product density was about 0.77-0.79 g / c m 3.

It appears likely that the mixing of an oleous PCM in an aqueous gypsum slurry in order to produce an energy storing wallboard is feasible. Results indicate that the laboratory procedure can easily be adapted to the manufacturing scale.

2. 3. Testing

Various performance tests were conducted on the energy storage wallboard in order to establish a comparison between its characteristics and those of a plain commercial wallboard.

The testing procedure can be classified as follows: - analysis of gypsum paste;

analysis of energy storing wallboard. In addition, BS stability in wallboard at long and short term was verified by tests

and analysis described below.

2.3.1. Analysis of gypsum paste Three basic characteristics of gypsum paste and BS-gypsum paste were de-

termined: (i) compressive strength, (ii) density, (iii) time of setting. The specimen preparation and the testing procedure were in accordance to ASTM C-472.

D. Feldman et al. / Organic P C M in ~,psum wallboard 235

Table 1 Characteristics of different types of gypsum paste determined according to ASTM C-472 (content of BS 21%-22%)

Type of specimen Compressive Density a) Setting strength a) ( g / c m 3) time b~ ( N / c m 2 ) (rain)

Plain gypsum paste 190.7 BS-gypsum paste; W F A as DA 103.3 BS-gypsum paste; PVA as DA 129.0 BS-gypsum paste; Na-L as DA 157.7

0.677 17.6 0.740 21.3 0.779 22.6 0.778 19.6

~) Test results represent an average of 5 determinations. b} Test results represent an average of 3 determinations.

To find out the effect of the addition of BS and various DA's on the strengths and density of the gypsum paste, 4 sets of cubes with dimensions of 5.0 x 5.0 x 5.0 cm were prepared. The samples were conditioned at a relative humidity of 52% + 2% for 24 h and dried at 3 8 - 4 2 ° C in an air circulating oven for a week, until a constant weight was attained. The specimens were then tested in compression using an lnstron Universal testing machine model 1125. The cross head speed was 0.5 m m / m i n . The results of these tests for various samples are shown in table 1. The densities were determined prior the compressive strength and the results are also illustrated in table 1.

The time of setting is the period required for setting gypsum to attain its highest temperature during rehydration. This temperature was determined by placing a thermal sensor approximately 30% of the distance from the bot tom of each 200 g specimen that had been poured into an insulated cup. The elapsed time was measured from the time when the stucco was first added to the water to the time of maximum temperature rise. The setting time data are also shown in table 1.

As can be seen from table 1, for the same content of BS in the gypsum paste (21%-22%) the compressive strength of 5 x 5 x 5 cm cubes is strongly influenced by the type of DA employed for dispersing BS in the paste. The compressive strength diminishes by only 17% when Na-L was utilized as the DA and by 32% and 46%, respectively, when PVA and WFA were used. Since the rest of the characteristics, i.e., density and setting time, do not differ too much as a function of the dispersants, PVA and Na-L were selected for further tests.

The setting time data indicate that neither BS nor DA interfere with the hydration process of gypsum.

2.3.2. Analysis of energy storing wallboard The final evaluation of energy storing wallboard was carried out on laboratory

scale samples having a BS content of 21%-22%. PVA and Na-L were selected as the DA in accordance with the results from paragraph 2.3.1.

For the evaluation of energy storing wallboard the following tests were done: - Calorimetric tests to establish latent heats as well as melting and freezing points. - Thermal conductivity.

236 D. I-~,Abnan et al.

Table 2 Results of the calorimetric tests

O r g a m c P ( ' M m g y p s u m ~ta l lh ,ard

Type of specimen BS Melting Freezing

wt~ ) Mehing Latent Freezing Latent

point heat point heat

I °C) ( J /g) I °C) I J / g )

BS-gypsum board - PVA as DA: 21.5 17.8 30.1 20.9 29,9

BS-gypsum board - PVA as

DA; cycled sample 22.4 17.6 30.8 20.9 31,2

BS-gypsum board - Na-L as DA: 21.0 17.6 29.7 20.6 29.4

BS-gypsum board

Na-L as DA: cycled sample 20.5 17.7 29.5 20.6 30.3

Test results represent an average of at least 3 scans.

Accelerated aging test by exposure to the effect of temperature cycling. - Bending (flexural) test.

- Compatibili ty with selected paints and wallpapers. - Water absorption. - Preliminary tests for flame spread and fire resistance.

2.3.2.1. Calorimetric tests. Calorimetric tests of energy storing wallboard were conducted by differential scanning calorimetry (DSC) by using a DuPont 910 Differential Calorimeter connected with a DuPont 1090 Thermal Analyzer. A 2° C / m i n heating and cooling rate was considered optimal, taking into account the relatively broad peaks of the melting and freezing characteristics of the fatty ester mixtures.

The instrument was calibrated by scanning pure Indium samples (melting point 156.6°C, latent heat of melting 28.4 J /g) . Samples of 15-25 mg were weighed in aluminum pans (with an accuracy of +0.001 mg) and then scanned in a nitrogen atmosphere.

The average calorimetric data (results of at least 3 DSC scans) for 21%-22% BS in wallboard with both PVA and Na-L as DA are shown in table 2.

As can be seen from these data, the melting and freezing points of BS do not shift when incorporated in wallboard and the absorption and release of heat during the phase transition of the incorporated BS are not affected by its presence in gypsum,

2.3.2.2. Thermal conductivity. Thermal conductivities were measured simulta- neously for two 20 x 20 cm 2 samples in a Dynatech Model TC F G M - N 4 Guarded Hot Plate Apparatus.

The tests on various specimens performed according to ASTM C-177 were used to compare the effects of incorporated BS on the conductivity of energy storage wallboard with that of plain gypsum board.

D. Feldman et al. / Organic PCM in gypsum wallboard

Table 3 Thermal conductivities determined according to ASTM C-177

237

Type of specimen BS Thickness Average K factor (wt%) (ram) temp. ( ° C) (W/o C. m)

Conventional gypsum 13.0 22.4 0.190 wallboard tile 13.0

BS-gypsum wallboard 21.3 12.9 22.2 0.158 tile: PVA as DA 21.6 12.9

BS-gypsum wallboard tile; 21.3 13.0 22.2 0.175 Na-L as DA 21.9 12.9

Results of the tests are presented in table 3. These results demonstrate that the thermal conductivity ( K factor) of the BS incorporated tile is reduced form 0.190 W / ° C • m (plain gypsum board) to 0.158 W / ° C • m when PVA was used as DA and to 0.175 W / o C- m when the DA was Na-L. The small changes in conductivity with addition of PCM showed, as expected, that the majority of the heat flow is through the gypsum matrix rather than the PCM. It is, incidentally, this flow of heat through the matrix which allows the wallboard to serve not only as a thermal store but as a heat exchanger as well.

2.3.2.3. Accelerated aging test. Accelerated aging tests are experiments in which one or more of the stress variables typically encountered in service are elevated to higher than normal levels, thus increasing the rate of the degradation of the materials under investigation [9]. In the case of energy storing wallboard, the main stress variable is the PCM melting-freezing cycle which is associated with small changes in its volume. On the other hand, an essential characteristic of a PCM is that it should maintain its properties after continual freeze-thaw cycles.

Plain and BS-gypsum board specimens having 20.3 × 20.3 × 1.27 cm were pre- pared for the laboratory scale cycling test. In addition to the samples mentioned above, specimens were painted with two different types of paints (general applica- tion grade alkyd and water base paint). Wallpaper was also applied to some samples. The application of paint and wallpaper to the surface of the samples was done to verify their compatibili ty with PCM wallboard in the freeze-thaw cycling test.

The freeze-thaw cycling tests were carried out in a controlled temperature chamber which permitted a cycling operation of 4 cycles per day between a maximum temperature of + 35°C and a minimum of - 2 6 ° C. The samples were maintained in the chamber for a period of 100 cycles. Periodic inspection of the samples was performed during the test. After 100 cycles no apparent physical changes were observed such as deterioration of the matrix, cracking, blistering or spotting of the paint and peeling of the paperboard.

After the cycling test, the BS-gypsum board specimens were analyzed by DSC for melting and freezing points and latent heats of transition. The calorimetric tests were conducted under conditions similar to those specified in paragraph 2.3.2.1. The

238 D. t-~,ldtnan et aL Organtc P ( ' M in Kvpsum wallt,oard

Table 4 Flexural strength determined according to AS]M C-473 (content of BS 21% 22% I

Type of specimen AV breaking load/deflection (kg/tuna

Parallel ~' Perpendicular ~''

Conventional gypsum board tile Westroc gypsum board tile Canadian Gypsum gypsum board tile BS-gypsum wallboard tile - PVA as DA BS-gypsum wallboard tile - Na-L as DA

21.5/0.77 57./2.03 28.0/1.10 59.8/2.47 19.8/0.87 64.0/2.37 18.0/0.80 46.0/ 1 / 77 18.2/'0.67 50.3/,' 1.83

Test results represent an average of at least 3 determinations. ~' Direction of paper fibre to the long specimen axis.

results are indicated in table 2 and demonstrate that f reeze-thaw cycling does not alter the thermal properties of PCM incorporated in wallboard.

2.3.2.4. Bending (flexural) tests. Mechanical flexural tests were performed on an Instron Model 1125 Universal Tester. The procedure was according to ASTM C475 using 203 x 152 x 12.7 mm samples. Two sets of samples were prepared for these tests, one with the predominant paper fibre direction parallel to the long axis of the samples and another with the paper fibres perpendicular to the long axis.

Prior to the bending test, the specimens were conditioned at 23 _+ 2 ° C and 50% + 2% RH until all had reached a constant weight. The results of the tests carried out on laboratory and commercial products of the same size are indicated in table 4. The specimen size used in the ASTM C-473 flexural strength test is 305 x 405 x 12.7 mm. Since it was not practical to produce specimens of standard size in the laboratory, an alternative size of 203 × 152 × 127 mm was used. Al- though the thickness-to-length ratio of the specimens used is not the same as the standard size, the purpose of the test was to obtain comparative values between the strength of plain gypsum board and those of BS-gypsum board.

As can be seen from the data presented in table 4, the addition of 21%-22% BS had little effect on the flexural strength of the laboratory scale samples of gypsum wallboard. However, in comparison with products made by Westroc Industries and Canadian Gypsum Co., the breaking strength of specimens tested perpendicular to the long sample axis showed a slight decrease, while those tested parallel to the edges were comparable.

2.3.2.5. Compatibility with selected paints and wallpapers. Since the paint or adhesives found on wallpaper could be affected by the presence of PCM, it is important to establish thecompat ib i l i ty between these materials. These tests are only intended to demonstrate the feasibility of such combinations and cannot be considered a comprehensive compatibility test. Alkyd- and water-based paints were applied to the laboratory prepared samples of directly incorporated BS wallboard to assess their compatibility. Representative paints were applied as follows: (a) one undercoat of alkyd-based paint, followed by a finishing coat of alkyd based paint; (b) two coats of water-based paint.

D. Feldman et al. / Organic P C M in gypsum wallboard 239

The following compatibili ty test results were obtained with respect to paint covering: (i) Examination of the painted specimens in ambient conditions for three months demonstrated apparent compatibili ty between paint and coverpaper. (ii) The compatibili ty test performed in the cycling chamber for 100 cycles. Section 2.3.2.3 indicated that there was no apparent change in paint condition (e.g., cracking, peeling, blistering and or flaking). (iii) The heating test at 50 °C for 168 h in an air circulating oven did not show any apparent change. (iv) Face-down exudation tests were also performed. These did not give rise to any exudation of the BS, nor was any peeling and blistering of the paint observed.

The following two types of normal, prepasted wallpaper were used for this compatibili ty test:

- Light, pulp based wallpaper; - Heavy, fibre based, knitted cross fibrous wallpaper. In both instances, as in the test for paint compatibility, test points (i) to (iv) were

performed for wallpaper as well. The results indicate that there were no signs of apparent physical change in the condition of the wallpaper.

2.3.2.6. Water absorption test. In consideration of the fact that wallboard is often used in places where there is a high degree of moisture, it was considered essential to verify the degree of water absorbency of the PCM board. To test this property, samples of 20.3 x 20.3 x 1.27 cm of both plain and BS incorporated boards were weighed at 23°C and 52% RH. They were then placed in a bath of clean tap water (20 o C) for a period of 2 h. At the end of the immersion time, the samples were drained for 1 rain and wiped dry using a paper towel and immediately weighed to determine the percentage of water absorption.

The plain wallboard which had been immersed showed a weight increase of 52% due to absorbed moisture. The BS incorporated wallboard with PVA and Na-L as DA showed increases in weight of 16.7% and 12.3% respectively due to absorbed moisture.

The fatty esters in BS-incorporated wallboard fill up a certain percentage of voids and are, moreover, highly water repellent organic compounds which do not allow easy passage of water molecules into the voids. These two factors clearly account for the less hygroscopic nature of the BS incorporated board. As has been seen from the test results, the moisture absorption property of the impregnated board is less than 1 /3 that of standard wallboard, which indicates that BS incorporated board may be used in those areas which have higher moisture contents.

2.3.2.7. Preliminary tests for flame spread and fire resistance. Flame spread tests were done with a propane torch according to ASTM D3806 for fire retardant paints, in a 60 cm tunnel.

Three samples of 20.3 x 20.3 x 1.27 cm were butted together tightly in the above mentioned apparatus at an angle of approximately 30 ° from the horizontal. A flame of approximately 700 ° C was directed vertically upward at a distance of 5 cm from the lower end of the test specimen to strike the sample. Flame travel was observed for 10 min against the scale and its extent was recorded.

24~J l). f~,ldtmtn et aL O r g a n w PC.~,I tn Kvp~um wal lboard

For the unimpregnated plain board the maximum flame spread was of 15 cm. For the 21%-22% BS incorporated wallboard the maximum flame spread was 23 cm for both types of DA (PVA and Na-k).

For the fire resistance tests, specimens were subjected to a direct perpendicular flame striking at distance of 5 cm and a temperature of 700 °C for 10 rain. In these tests, no evidence of burn-through on the opposite side of the samples was observed for the BS-incorporated board specimens. However, in the case where the plain board was tested, a slight discoloration of the opposite facing paperboard was noticed. It is evident that the fire resistance of wallboard appears to be slightly enhanced by the presence of the PCM, possibly by its absorption of heat during evaporation. Smoke evolution was also observed in the BS incorporated samples which were exposed to the flame for 10 rain.

2.3. 3. B S stability in energy storing board In addition to the calorimetric tests performed after the accelerated aging test

(section 2.3.2.3) the BS stability in gypsum wallboard was verified by infrared analysis (IR). The analysis was performed to verify the presence of any chemical interaction between BS and the other components of the gypsum board.

The spectra were investigated with an IR Spectrophotometer Beckman 4240. Solid samples were dispersed in KBr.

If any chemical reaction takes place, new products can be readily detected by comparing a sample spectrum with that from a known material and locating the presence of spurious bands in the former. Hence, if any chemical reaction occurs in the BS incorporated board, the characteristic IR absorption peaks should, at least be partially shifted from saturated aliphatic esters at 1740 c m - t or some other peaks, should appear.

IR absorption spectra of samples made of 21%-22% BS showed no characteristic signs indicative of ester hydrolysis. Furthermore, there were no signs of an 3 ' chemical reactions since no new peaks became evident.

Specimens were tested again after eight months storage at ambient temperature and humidity to determine if any slow reaction had occurred. Again, the IR spectra were unchanged when compared with BS spectra confirming the chemical stability of BS incorporated in gypsum wallboard.

3. Conclusion

The present work has demonstrated the feasibility of manufacturing an energy storing gypsum wallboard by direct incorporation of PCM at the mixing stage of conventional gypsum board manufacture.

This method of production would have the twin advantages of requiring very little modification to existing manufacturing facilities and offering the economy of continuous processing. Moreover, it is anticipated that this technique would be more efficient and more economical in producing the energy storing wallboard than the immersion process discussed elsewhere [5] since it would involve less labour and

D. Feldman et al. / Organic P C M in gypsum wallboard 241

would obviate the necessity for additional handling and immersing facilities at the end of a production line.

Laboratory scale energy storing gypsum wallboard was produced by the direct incorporation of 21%-22% commercial grade BS (the proportion of BS is related to the dry product).

The direct incorporation of BS was greatly facilitated by the presence and type of small amounts of dispersing agents. The amount of dispersing agent was of the same order of magnitudes as the amount of the other solid additives normally utilized in the gypsum wallboard manufacture.

The setting time of gypsum paste without an accelerator in the presence of BS shows a slight increase (2-4 min) but this is considered to be easily adjustable by choosing the appropriate amount of accelerator. DSC analysis carried out to evaluate the stability of BS in wallboard which had been subjected to freeze-thaw cycling, produced no evidence of any change in its thermal characteristics. IR analysis showed that no chemical reactions took place between BS and the gypsum wallboard components. Thermal conductivity data demonstrated that the thermal transmission characteristic of thermal storage wallboard are comparable to those of plain wallboard.

The flexural strength of laboratory produced thermal storage wallboard compares well with values obtained for conventional wallboard.

Durability of the samples and components of thermal storage board was evaluated by the use of accelerated freeze-thaw tests. The results obtained are completely satisfactory.

Compatibility tests with representative paints (water- and alkyd-based) show no sign of any peeling, blistering, or evidence of exudation of the BS one year after manufacture.

The water absorption test demonstrates that this product absorbs less than 1 /3 as much water as plain wallboard and is, therefore, a potential candidate for use in humid environments.

For a rise of about 4 ° C through the melting range of PCM, 1 m 2 of energy storing wallboard with 22% BS has a total thermal storage capacity of 370 kJ. This capacity was computed by adding the sensible heat capacity of gypsum alone (33 kJ) with the sensible heat capacity of BS (16 k J) and the latent heat capacity of BS (321 k J).

Consequently, energy storing wallboard has a ten-fold increase in capacity for storage and discharge of heat when compared with conventional gypsum wallboard.

Acknowledgements

Financial support for this research provided by the Natural Science and En- gineering Research Council of Canada is gratefully acknowledged. The authors also wish to thank Westroc Industries Co. Canada for providing us with materials for the laboratory scale gypsum wallboard production.

242 I). k~'hh~ltln et aL . Or~am¢ P ( ' M tn g y p s u m wal lboard

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