115 WOOD RESEARCH 55 (2): 2010 115-124 MICROWAVE DRYING OF WOOD-CEMENT COMPOSITES Ivan Makovíny Faculty of Environmental and Manufacturing Technology, Technical University, Zvolen, Slovak Republic ABSTRACT In the paper the microwave drying of wood-cement composite boards of the densities 400 kg.m -3 , 700 kg.m -3 and 1100 kg.m -3 was studied. e wood-cement boards were both without admixtures and with the admixture of carbon. High power density of the electro- magnetic field was used. e drying speed varied between 1.7 and 14 (%.min -1 ). No damages such as cracks, shape changes, or decrease of mechanical strength at the bend were observed. It may be stated that by means of microwave drying, it is possible to reduce the time of fi nalizing wood-cement boards without any noticeable changes in their quality. KEY WORDS: wood-cement composite, microwave drying, drying speed, carbon wood cement composites INTRODUCTION Wood-cement composites (WCC) are used mostly as a building material – as both exterior and interior wall claddings, as well as ceilings and floors claddings. eir desirable basic qualities are: heat-resistance, insect and fungi resistance, toughness, high absorption of sound, and low thermal conductivity. If appropriate electro-conductive admixtures are used, it is possible to increase the attenuation characteristics of WCC and to use them for shielding electromagnetic fields. e necessary element included in the technology process of the WCC production is water. It enables the hydration of mineral binder – cement. At the end of the technology process, a redundant water remains in WCC, which is removed by natural drying in the air, with simultaneous stiffening of the cement paste. is process is lengthy, it lasts approximately 28 days. e aim of the study was to test the possibility of fast drying of WCC using the microwave type of heating. e issue of microwave drying of wood has been treated with a considerable attention, see e. g. works of (Babiak et al. 2000, Cividini and Travan 2003, Perré and Turner 1999, Lee and Hong 1987, Zemiar et al. 2009). To a certain extent, it was also the case with e. g. heated gluing of particleboard materials and fibreboards. e drying of composite materials is not generally required in the technology process, with the exception of conventional heating during the process of pressing the mat of wood particles.
MICROWAVE DRYING OF WOOD-CEMENT COMPOSITES
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untitled115-124
Technical University, Zvolen, Slovak Republic
ABSTRACT
In the paper the microwave drying of wood-cement composite boards of the densities 400 kg.m-3, 700 kg.m-3 and 1100 kg.m-3 was studied. Th e wood-cement boards were both without admixtures and with the admixture of carbon. High power density of the electro- magnetic fi eld was used. Th e drying speed varied between 1.7 and 14 (%.min-1). No damages such as cracks, shape changes, or decrease of mechanical strength at the bend were observed. It may be stated that by means of microwave drying, it is possible to reduce the time of fi nalizing wood-cement boards without any noticeable changes in their quality.
KEY WORDS: wood-cement composite, microwave drying, drying speed, carbon wood cement composites
INTRODUCTION
Wood-cement composites (WCC) are used mostly as a building material – as both exterior and interior wall claddings, as well as ceilings and fl oors claddings. Th eir desirable basic qualities are: heat-resistance, insect and fungi resistance, toughness, high absorption of sound, and low thermal conductivity. If appropriate electro-conductive admixtures are used, it is possible to increase the attenuation characteristics of WCC and to use them for shielding electromagnetic fi elds.
Th e necessary element included in the technology process of the WCC production is water. It enables the hydration of mineral binder – cement. At the end of the technology process, a redundant water remains in WCC, which is removed by natural drying in the air, with simultaneous stiff ening of the cement paste. Th is process is lengthy, it lasts approximately 28 days. Th e aim of the study was to test the possibility of fast drying of WCC using the microwave type of heating.
Th e issue of microwave drying of wood has been treated with a considerable attention, see e. g. works of (Babiak et al. 2000, Cividini and Travan 2003, Perré and Turner 1999, Lee and Hong 1987, Zemiar et al. 2009). To a certain extent, it was also the case with e. g. heated gluing of particleboard materials and fi breboards. Th e drying of composite materials is not generally required in the technology process, with the exception of conventional heating during the process of pressing the mat of wood particles.
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Water is an essential element in the process of the WCC production, being necessary for hydrating cement. Th e portion of water to cement, i. e. water coeffi cient, is a signifi cant factor aff ecting the strength of WCC. Th e optimal portion necessary for hydrating cement is practically unachievable, due to several reasons. In the production of WCC, water is added alongside with other input raw materials, and water exists in wood wool, as well. With regards to the changes in the composition of raw materials that contain water, and sometimes also to the needs to increase the content of water because of the function of technological junctions and reology of the mat of wood wool, cement and admixtures on the production line, at the end of the WCC production process, they contain redundant water.
Th e quickest method of heating wood materials is the microwave heating. With a low diff usional resistance of the dried material, this way of heating enables even the highest speed of heating. Wood-cement composites represent polar dielectric materials. With respect to a good permeability for a liquid water and also for steam, the best conditions are achieved for the application of microwave method of drying WCC.
A demerit of the application of microwave heating of materials in ventricular resonators may be an irregularity of the heating, which is caused by non-homogenity of electro-magnetic fi elds.
Th e microwave system used for drying WCC in our experiments includes the resonator, in which the operational box containing material moves within the scale of half a wave length during heating. In this way, the irregularity of heating in the dried material, and consequently, also of non-homogeneous thermal fi eld, is reduced.
Th e intensity of absorption of dielectric heating in electromagnetic fi elds depends, with a given frequency, on the intensity of electric element of the fi eld, and on the loss number ε″ of dried material. Th e loss number of material is predominantly aff ected by moisture.
Specifi c volumetric power is expressed by the relation
(1)
where ω is an angular speed, ε0 is the permitivity of vacuum, ε ` is the loss number of material, E is the intensity of electric fi eld. Generally, for a considered volumetrical element of wood, there is an equation for thermal
conductance
(2)
where λ is the coeffi cient of thermal conductivity, ϑ – temperature, c – specifi c heat capacity, ρ - density, t – time.
Th e fi rst element of the equation represents the heat conducted away to the environment for a unit of time, the second element – the output of heat consumed to increase the temperature, heating a particular element of wood, and the right side represents the inner source of heat, the output of heat of the element.
2
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For the sake of practical usage, it is important to know the total time of drying. Th e time needed for heating of material may be calculated from the relationship:
(3)
where c is the specifi c heat capacity ϑ 1 – temperature at the beginning of heating ϑ 2 – temperature at the beginning of intensive evaporation of water m – weight of material V – the volume of heated material
Th e time needed for heating may be determined from the relationship:
(4)
where mH2O is the weight of evaporated water r – evaporation heat of water
From the equations (3) and (4) the total time of drying may be obtained, but only under generalized preconditions that the process is adiabatic and parameters of equations are constant. In fact, it is only approximately true, and thus these relations may be used only for an approximate estimation of the drying time. It is especially diffi cult to ascertain specifi c output of heat, as the absorption of electromagnetic energy by dried material depends on electrophysical properties of the material and, at the same time, there emerges also the refl ection of electromagnetic waves.
MATERIAL AND METHODS
Samples of wood-cement boards of various densities 400, 700 and 1100 kg.m-3, which we produced in the laboratory by pressing wood-cement matt under various pressures, were used for drying WCC. Measurements were taken from samples of boards of dimensions 150 x 300 mm. As we used three diff erent values of pressure, the samples of diff erent values of thickness were obtained. After pressing, they were 23 mm, 13 mm and 9 mm thick. Th e pressing of each sample lasted 24 hours.
Th ere realized 12 measurements with various concentrations of carbon: 0, 5, 10 and 25 (wt.%) and of three densities: 400, 700, 1100 kg.m-3.Th e each measurement was perforated on six samples.
Th e samples of boards were prepared out of spruce wood-wool consisting of the stripes of geometrical dimensions 500 x 5 x 0,3 (mm).
( )
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WOOD RESEARCH
Fig. 1: Design of the resonator of a microwave system with the position of installed magnetrons (MG1
to MG10), and a dried sample of wood-cement composite, (WCC).
Fig. 2: View at a microwave system with the operational box and a wood-cement board
Drying was carried out in the operational box installed movably in the resonator of a microwave system in the shape of ashlars with the parameters in the Tab. 1 (Makovíny 2009).
Tab. 1: Basic technical parameters of microwave system
Th e achieved specifi c volumetric power was of the value p = 6 kW.m-3. Drying was taking place under atmospheric pressure and relative moisture of air 90 %. It was carried out immediately after pressing, i.e. 24 h later.
At certain intervals during the process of drying, we weighed the samples and also took the surface temperature. Th e surface temperature of samples was measured by pyrometer GIM 530 MS at fi ve places, to obtain the average temperature. Moisture was determined by means of gravimetric method.
Th e quality of drying was assessed macroscopically, evaluating the occurrence of boundary, internal and surface cracks, or, as the case may be, of a collapse.
Input power W 12000
Frequency GHz 2.45 Size of rezonator mm3 300x250x1500
Size of operational box mm3 190x230x1400 Weight kg 70
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RESULTS AND DISCUSSION
Th e results of the experiment are summarized in (Fig. l - 8, and in Tab. 2). Moisture curves representing the dependency of moisture of wood-cement composite samples of diff erent density and without the admixture of carbon on the time interval of drying are depicted in (Fig. 3a - Fig. 6a).
Th e initial moisture both with content of WCC, both with and without the admixture of carbon, was decreasing with increasing density of WCC.
Th e growth of density was achieved through increasing pressure with the same proportion of elements of composites, hence, in the case of the samples of the densities 700 and 1100 (kg.m-3), a part of water was pressed off the matt.
Since the moisture of WCC decreased approximately linearly with the time of drying, the speed of drying may be considered constant within the whole moisture content interval.
Th e wood-cement boards with the carbon admixture of the concentrations 5, 10 a 25 (%) behaved similarly to the carbon-less samples.
Th e speed of the drying of samples (Tab. 2) depended on the initial moisture but also on the density of the samples. Th e speed of drying is from 1.7 – 14 (% .min-1).
Tab. 2: The drying speed of WCC by different density ρ concentration of carbon C and initial moisture wi
Despite the high drying speed, there was no occurrence of internal or surface cracks observed when assessing the quality of drying.
Th e thermal characteristics (Fig. 3b-6b) correspond to the moisture characteristics (Fig. 3a-6a).
Fig. 3a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration 0 (wt.%)
C (%) 0 5 10 25
(kg.m-3) 400 700 1100 400 700 1100 400 700 1100 400 700 1100
wi (%) 94 45 20 78 41 38 76 40 34 75 49 26
v (%.min-1) 14 4.3 1.7 11.7 4.2 4.4 12 4.2 3.2 15 4.9 2.9
0,0
20,0
40,0
60,0
80,0
100,0
w (%)
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Fig. 3b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration (wt.%)
Fig. 4a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration 5 (wt.%)
Fig. 4b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration 5 (wt.%)
0
20
40
60
80
100
(°C)
0
20
40
60
80
100
120
140
C)
0,0
20,0
40,0
60,0
80,0
w (%)
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Fig. 5a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after one day pressing the boards at carbon concentration 10 (wt.%)
Fig. 5b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after one days pressing the boards at carbon concentration 10 (wt.%)
Fig. 6a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after one day pressing the boards at carbon concentration 25 (wt.%)
0
20
40
60
80
100
120
( C)
0,0
20,0
40,0
60,0
80,0
w (%)
w (%)
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Fig. 6b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after one days pressing the boards at carbon concentration 25 (wt.%)
Th e changes of temperature, except for the samples of WCC with the lowest density 400 kg.m-3 at 0 (wt.%) carbon concentration, did not have any signifi cant distilling horizontals, which appear during the dielectric heating of wood at the moisture over FSP, when the free water is evaporating. Th e increased temperature accelerates the course of chemical reactions of the hydratation of cement. Portland cement is not a pure material but rather a multiphasic material whose composition varies at a wide range. A series of chemical reactions proceeds in it. In general, conditions for hydrating may vary depending on admixtures, however, there was not determined any defi nite infl uence of the carbon concentration.
However, Aitcin (2005) states that during the acceleration of the process of hardening the concrete by means of heat, the initial hardness of concrete increases, while the f inal strength of concrete decreases. We assume that due to a good access of water to cement, contrary to concrete, the most of cement grains hydrate at increased temperature in a short time interval, i. e. still during pressing-in within the f irst 24 hours, (Makovíny 2009, Lee and Hong 1987, Moslemi and Pfister 1987).
The inf luence of the concentration of carbon on thermal curves was ambiguous. There appeared problems at the edges of boards with the highest concentration (C = 25 %) and the highest density (ρ = 1100 kg.m-3) when the temperature started to rise to an excessive degree after a certain time of heating.
CONCLUSION
It was discovered that it is possible to dry with microwave heating the wood-cement composites of the densities 400, 700 and 1100 kg.m-3 without the admixture of carbon and with various concentrations of carbon with a high quality and a high speed of drying at the same time. Th e drying speed depended on initial values of moisture and density of WCC, varying within the interval of 1.7 – 14 %.min-1. Th e higher values of drying speed correspond to the higher values of initial moisture content WCC. Th e drying speed was practically constant during the process of drying. It is possible to reduce the time of fi nalizing wood- cement composites by means of microwave drying without any marked changes of their quality. Hence it is possible to consider the microwave way of drying wood-cement boards suitable for industrial usage to reduce the time of storing because of the maturing of cement,
0
20
40
60
80
100
( C)
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and to reduce the moisture. At present, the storing time of 4 weeks is a usual practice in the production of WCC.
ACKNOWLEDGEMENTS
Th is research was supported by Slovak Agency for support of the research and of the development under project APVV– 0229-06 “Applied research of wood-cement boards for screening of the electromagnetic fi elds and for increasing of the fi re resistance.”
REFERENCES
1. Aitcin, P.C., 2005: High performause concrete Expodata – Didot, Praha, 320 pp. 2. Antti, L., et al., 2004: The Effect of drying method on the mechanical properties of
wood: Comparisons between conventional kiln and microwave dried Scots pine, Luleå University of Technology, Skellefteå, Sweden. Pp. 1-9
3. Antti, A.L., Perré, P., 1999: A microwave applicator for on line wood drying: Temperature and moisture distribution in wood. Wood Sci. Technol. 33: 123-138
4. Babiak, M., et al., 2003: Drying of robinia wood by microwave heating. Proccedigs “Interaction of Wood with Various Forms of Energy”. Technical University in Zvolen. Pp. 25-28
5. Cividini, R., Travan, L., 2003: Microwave heating in vacuum – Press drying of Timber: Practical Investigation. 8th International IUFRO Wood Drying Conferences. Pp. 150-155
6. Lee, A.W.C., Hong, Z., 1987: Effect of cement/wood ratios and wood storage conditions on hydration temperature, Hydration time, and compressive strength of wood – cement mixtures. Wood and Fiver Science 19(3): 262-268
7. Makovíny, I., Kurjatko, S., 2003: The drying of thin parquet boards with microwave heating. Proceeding “Electric heat in wood industry”. Technical University in Zvolen. Pp. 18-21
8. Makoviny, I., 2009: Hydratation of cement and mixtures of cement with carbon and wood – particles. In: XIXth Internation Symposium Adhensives in Woodworking Industry Zvolen. Pp. 142-149
9. Moslemi, A.A., Pfister, S.C., 1987: The if luence of cement/wood ratio and cement type on bending strength and dimensional stability of wood-cement composite panels. Wood and Fiber Science 19(2): 165-175
10. Perré, P., Turner, W., 1999: The use of numerical simulation as a cognitive tool for studying the microwave drying of softwood in an over waveguide. Wood Sci. Technol. 33: 445-464
11. Trebula, P., Dekrét, A., Klement, I., 1997: Beech wood drying with the microwave heating, Acta Facultatis Xylologiae, Zvolen. Technical University in Zvolen. Pp. 87-93
12. Zemiar, J., et al., 2009: Temperature and moisture profites at microwave heating of wood. Ann. WULS- SGGW, Forestry and Wood Technology, No 67. Pp. 283-288
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Prof. Ing. Ivan Makovíny, CSc. Faculty of Environmental and Manufacturing Technology
Technical University in Zvolen T. G. Masaryka 24
960 53 Zvolen Slovak Republic
E-mail: [email protected]
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Technical University, Zvolen, Slovak Republic
ABSTRACT
In the paper the microwave drying of wood-cement composite boards of the densities 400 kg.m-3, 700 kg.m-3 and 1100 kg.m-3 was studied. Th e wood-cement boards were both without admixtures and with the admixture of carbon. High power density of the electro- magnetic fi eld was used. Th e drying speed varied between 1.7 and 14 (%.min-1). No damages such as cracks, shape changes, or decrease of mechanical strength at the bend were observed. It may be stated that by means of microwave drying, it is possible to reduce the time of fi nalizing wood-cement boards without any noticeable changes in their quality.
KEY WORDS: wood-cement composite, microwave drying, drying speed, carbon wood cement composites
INTRODUCTION
Wood-cement composites (WCC) are used mostly as a building material – as both exterior and interior wall claddings, as well as ceilings and fl oors claddings. Th eir desirable basic qualities are: heat-resistance, insect and fungi resistance, toughness, high absorption of sound, and low thermal conductivity. If appropriate electro-conductive admixtures are used, it is possible to increase the attenuation characteristics of WCC and to use them for shielding electromagnetic fi elds.
Th e necessary element included in the technology process of the WCC production is water. It enables the hydration of mineral binder – cement. At the end of the technology process, a redundant water remains in WCC, which is removed by natural drying in the air, with simultaneous stiff ening of the cement paste. Th is process is lengthy, it lasts approximately 28 days. Th e aim of the study was to test the possibility of fast drying of WCC using the microwave type of heating.
Th e issue of microwave drying of wood has been treated with a considerable attention, see e. g. works of (Babiak et al. 2000, Cividini and Travan 2003, Perré and Turner 1999, Lee and Hong 1987, Zemiar et al. 2009). To a certain extent, it was also the case with e. g. heated gluing of particleboard materials and fi breboards. Th e drying of composite materials is not generally required in the technology process, with the exception of conventional heating during the process of pressing the mat of wood particles.
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WOOD RESEARCH
Water is an essential element in the process of the WCC production, being necessary for hydrating cement. Th e portion of water to cement, i. e. water coeffi cient, is a signifi cant factor aff ecting the strength of WCC. Th e optimal portion necessary for hydrating cement is practically unachievable, due to several reasons. In the production of WCC, water is added alongside with other input raw materials, and water exists in wood wool, as well. With regards to the changes in the composition of raw materials that contain water, and sometimes also to the needs to increase the content of water because of the function of technological junctions and reology of the mat of wood wool, cement and admixtures on the production line, at the end of the WCC production process, they contain redundant water.
Th e quickest method of heating wood materials is the microwave heating. With a low diff usional resistance of the dried material, this way of heating enables even the highest speed of heating. Wood-cement composites represent polar dielectric materials. With respect to a good permeability for a liquid water and also for steam, the best conditions are achieved for the application of microwave method of drying WCC.
A demerit of the application of microwave heating of materials in ventricular resonators may be an irregularity of the heating, which is caused by non-homogenity of electro-magnetic fi elds.
Th e microwave system used for drying WCC in our experiments includes the resonator, in which the operational box containing material moves within the scale of half a wave length during heating. In this way, the irregularity of heating in the dried material, and consequently, also of non-homogeneous thermal fi eld, is reduced.
Th e intensity of absorption of dielectric heating in electromagnetic fi elds depends, with a given frequency, on the intensity of electric element of the fi eld, and on the loss number ε″ of dried material. Th e loss number of material is predominantly aff ected by moisture.
Specifi c volumetric power is expressed by the relation
(1)
where ω is an angular speed, ε0 is the permitivity of vacuum, ε ` is the loss number of material, E is the intensity of electric fi eld. Generally, for a considered volumetrical element of wood, there is an equation for thermal
conductance
(2)
where λ is the coeffi cient of thermal conductivity, ϑ – temperature, c – specifi c heat capacity, ρ - density, t – time.
Th e fi rst element of the equation represents the heat conducted away to the environment for a unit of time, the second element – the output of heat consumed to increase the temperature, heating a particular element of wood, and the right side represents the inner source of heat, the output of heat of the element.
2
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Vol. 55 (2): 2010
For the sake of practical usage, it is important to know the total time of drying. Th e time needed for heating of material may be calculated from the relationship:
(3)
where c is the specifi c heat capacity ϑ 1 – temperature at the beginning of heating ϑ 2 – temperature at the beginning of intensive evaporation of water m – weight of material V – the volume of heated material
Th e time needed for heating may be determined from the relationship:
(4)
where mH2O is the weight of evaporated water r – evaporation heat of water
From the equations (3) and (4) the total time of drying may be obtained, but only under generalized preconditions that the process is adiabatic and parameters of equations are constant. In fact, it is only approximately true, and thus these relations may be used only for an approximate estimation of the drying time. It is especially diffi cult to ascertain specifi c output of heat, as the absorption of electromagnetic energy by dried material depends on electrophysical properties of the material and, at the same time, there emerges also the refl ection of electromagnetic waves.
MATERIAL AND METHODS
Samples of wood-cement boards of various densities 400, 700 and 1100 kg.m-3, which we produced in the laboratory by pressing wood-cement matt under various pressures, were used for drying WCC. Measurements were taken from samples of boards of dimensions 150 x 300 mm. As we used three diff erent values of pressure, the samples of diff erent values of thickness were obtained. After pressing, they were 23 mm, 13 mm and 9 mm thick. Th e pressing of each sample lasted 24 hours.
Th ere realized 12 measurements with various concentrations of carbon: 0, 5, 10 and 25 (wt.%) and of three densities: 400, 700, 1100 kg.m-3.Th e each measurement was perforated on six samples.
Th e samples of boards were prepared out of spruce wood-wool consisting of the stripes of geometrical dimensions 500 x 5 x 0,3 (mm).
( )
118
WOOD RESEARCH
Fig. 1: Design of the resonator of a microwave system with the position of installed magnetrons (MG1
to MG10), and a dried sample of wood-cement composite, (WCC).
Fig. 2: View at a microwave system with the operational box and a wood-cement board
Drying was carried out in the operational box installed movably in the resonator of a microwave system in the shape of ashlars with the parameters in the Tab. 1 (Makovíny 2009).
Tab. 1: Basic technical parameters of microwave system
Th e achieved specifi c volumetric power was of the value p = 6 kW.m-3. Drying was taking place under atmospheric pressure and relative moisture of air 90 %. It was carried out immediately after pressing, i.e. 24 h later.
At certain intervals during the process of drying, we weighed the samples and also took the surface temperature. Th e surface temperature of samples was measured by pyrometer GIM 530 MS at fi ve places, to obtain the average temperature. Moisture was determined by means of gravimetric method.
Th e quality of drying was assessed macroscopically, evaluating the occurrence of boundary, internal and surface cracks, or, as the case may be, of a collapse.
Input power W 12000
Frequency GHz 2.45 Size of rezonator mm3 300x250x1500
Size of operational box mm3 190x230x1400 Weight kg 70
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RESULTS AND DISCUSSION
Th e results of the experiment are summarized in (Fig. l - 8, and in Tab. 2). Moisture curves representing the dependency of moisture of wood-cement composite samples of diff erent density and without the admixture of carbon on the time interval of drying are depicted in (Fig. 3a - Fig. 6a).
Th e initial moisture both with content of WCC, both with and without the admixture of carbon, was decreasing with increasing density of WCC.
Th e growth of density was achieved through increasing pressure with the same proportion of elements of composites, hence, in the case of the samples of the densities 700 and 1100 (kg.m-3), a part of water was pressed off the matt.
Since the moisture of WCC decreased approximately linearly with the time of drying, the speed of drying may be considered constant within the whole moisture content interval.
Th e wood-cement boards with the carbon admixture of the concentrations 5, 10 a 25 (%) behaved similarly to the carbon-less samples.
Th e speed of the drying of samples (Tab. 2) depended on the initial moisture but also on the density of the samples. Th e speed of drying is from 1.7 – 14 (% .min-1).
Tab. 2: The drying speed of WCC by different density ρ concentration of carbon C and initial moisture wi
Despite the high drying speed, there was no occurrence of internal or surface cracks observed when assessing the quality of drying.
Th e thermal characteristics (Fig. 3b-6b) correspond to the moisture characteristics (Fig. 3a-6a).
Fig. 3a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration 0 (wt.%)
C (%) 0 5 10 25
(kg.m-3) 400 700 1100 400 700 1100 400 700 1100 400 700 1100
wi (%) 94 45 20 78 41 38 76 40 34 75 49 26
v (%.min-1) 14 4.3 1.7 11.7 4.2 4.4 12 4.2 3.2 15 4.9 2.9
0,0
20,0
40,0
60,0
80,0
100,0
w (%)
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WOOD RESEARCH
Fig. 3b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration (wt.%)
Fig. 4a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration 5 (wt.%)
Fig. 4b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after 24 h pressing the boards at carbon concentration 5 (wt.%)
0
20
40
60
80
100
(°C)
0
20
40
60
80
100
120
140
C)
0,0
20,0
40,0
60,0
80,0
w (%)
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Vol. 55 (2): 2010
Fig. 5a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after one day pressing the boards at carbon concentration 10 (wt.%)
Fig. 5b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after one days pressing the boards at carbon concentration 10 (wt.%)
Fig. 6a: Dependence of moisture content in WCC, from the time when microwave drying three boards
density 400, 700, 1100 kg.m-3 after one day pressing the boards at carbon concentration 25 (wt.%)
0
20
40
60
80
100
120
( C)
0,0
20,0
40,0
60,0
80,0
w (%)
w (%)
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WOOD RESEARCH
Fig. 6b: Temperature dependence WCC, from the time when microwave drying three boards density
400, 700, 1100 kg.m-3 after one days pressing the boards at carbon concentration 25 (wt.%)
Th e changes of temperature, except for the samples of WCC with the lowest density 400 kg.m-3 at 0 (wt.%) carbon concentration, did not have any signifi cant distilling horizontals, which appear during the dielectric heating of wood at the moisture over FSP, when the free water is evaporating. Th e increased temperature accelerates the course of chemical reactions of the hydratation of cement. Portland cement is not a pure material but rather a multiphasic material whose composition varies at a wide range. A series of chemical reactions proceeds in it. In general, conditions for hydrating may vary depending on admixtures, however, there was not determined any defi nite infl uence of the carbon concentration.
However, Aitcin (2005) states that during the acceleration of the process of hardening the concrete by means of heat, the initial hardness of concrete increases, while the f inal strength of concrete decreases. We assume that due to a good access of water to cement, contrary to concrete, the most of cement grains hydrate at increased temperature in a short time interval, i. e. still during pressing-in within the f irst 24 hours, (Makovíny 2009, Lee and Hong 1987, Moslemi and Pfister 1987).
The inf luence of the concentration of carbon on thermal curves was ambiguous. There appeared problems at the edges of boards with the highest concentration (C = 25 %) and the highest density (ρ = 1100 kg.m-3) when the temperature started to rise to an excessive degree after a certain time of heating.
CONCLUSION
It was discovered that it is possible to dry with microwave heating the wood-cement composites of the densities 400, 700 and 1100 kg.m-3 without the admixture of carbon and with various concentrations of carbon with a high quality and a high speed of drying at the same time. Th e drying speed depended on initial values of moisture and density of WCC, varying within the interval of 1.7 – 14 %.min-1. Th e higher values of drying speed correspond to the higher values of initial moisture content WCC. Th e drying speed was practically constant during the process of drying. It is possible to reduce the time of fi nalizing wood- cement composites by means of microwave drying without any marked changes of their quality. Hence it is possible to consider the microwave way of drying wood-cement boards suitable for industrial usage to reduce the time of storing because of the maturing of cement,
0
20
40
60
80
100
( C)
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Vol. 55 (2): 2010
and to reduce the moisture. At present, the storing time of 4 weeks is a usual practice in the production of WCC.
ACKNOWLEDGEMENTS
Th is research was supported by Slovak Agency for support of the research and of the development under project APVV– 0229-06 “Applied research of wood-cement boards for screening of the electromagnetic fi elds and for increasing of the fi re resistance.”
REFERENCES
1. Aitcin, P.C., 2005: High performause concrete Expodata – Didot, Praha, 320 pp. 2. Antti, L., et al., 2004: The Effect of drying method on the mechanical properties of
wood: Comparisons between conventional kiln and microwave dried Scots pine, Luleå University of Technology, Skellefteå, Sweden. Pp. 1-9
3. Antti, A.L., Perré, P., 1999: A microwave applicator for on line wood drying: Temperature and moisture distribution in wood. Wood Sci. Technol. 33: 123-138
4. Babiak, M., et al., 2003: Drying of robinia wood by microwave heating. Proccedigs “Interaction of Wood with Various Forms of Energy”. Technical University in Zvolen. Pp. 25-28
5. Cividini, R., Travan, L., 2003: Microwave heating in vacuum – Press drying of Timber: Practical Investigation. 8th International IUFRO Wood Drying Conferences. Pp. 150-155
6. Lee, A.W.C., Hong, Z., 1987: Effect of cement/wood ratios and wood storage conditions on hydration temperature, Hydration time, and compressive strength of wood – cement mixtures. Wood and Fiver Science 19(3): 262-268
7. Makovíny, I., Kurjatko, S., 2003: The drying of thin parquet boards with microwave heating. Proceeding “Electric heat in wood industry”. Technical University in Zvolen. Pp. 18-21
8. Makoviny, I., 2009: Hydratation of cement and mixtures of cement with carbon and wood – particles. In: XIXth Internation Symposium Adhensives in Woodworking Industry Zvolen. Pp. 142-149
9. Moslemi, A.A., Pfister, S.C., 1987: The if luence of cement/wood ratio and cement type on bending strength and dimensional stability of wood-cement composite panels. Wood and Fiber Science 19(2): 165-175
10. Perré, P., Turner, W., 1999: The use of numerical simulation as a cognitive tool for studying the microwave drying of softwood in an over waveguide. Wood Sci. Technol. 33: 445-464
11. Trebula, P., Dekrét, A., Klement, I., 1997: Beech wood drying with the microwave heating, Acta Facultatis Xylologiae, Zvolen. Technical University in Zvolen. Pp. 87-93
12. Zemiar, J., et al., 2009: Temperature and moisture profites at microwave heating of wood. Ann. WULS- SGGW, Forestry and Wood Technology, No 67. Pp. 283-288
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Prof. Ing. Ivan Makovíny, CSc. Faculty of Environmental and Manufacturing Technology
Technical University in Zvolen T. G. Masaryka 24
960 53 Zvolen Slovak Republic
E-mail: [email protected]
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