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NANO-BASED COATINGS AND THE INFLUENCE ON THE HYGROSCOPIC PROPERTIES OF WOOD

Selamawit Mamo Fufa1*, Per Jostein Hovde1, Goce Talev1 and Bjørn Petter Jelle1,2

1Norwegian University of Science and Technology (NTNU), Department of Civil and Transport

Engineering, Høgskoleringen 7A, NO-7491 Trondheim, Norway. 2SINTEF Building and Infrastructure, Department of Materials and Structures, NO-7465

Trondheim, Norway *Corresponding author: Email: [email protected]

ABSTRACT: Wood as a natural material can be easily affected by different climatic conditions when it is used as a building material. Water in its liquid and vapour form may lead to deterioration of wood where hygroscopic properties of the wood play a major role in the determination of its durability. Exterior wood coatings provide a barrier against moisture but the combined action of ultraviolet radiation, oxygen and water causes coating degradation. However, nano-based coatings provide new opportunities towards modification of wood attributes. The very small size and high reactivity of nanoparticles integrated into the wood coating may improve the hygroscopic properties of a wooden cladding. The objective of this work is to present moisture resistance of nano-based coated wood by exposing it to transient climatic conditions. The measured material properties were used as input data for the numerical tool WUFI in order to compare the measured properties of nano-based coated and uncoated wood under transient conditions with those input values used in the simulation.

KEYWORDS: Wood, Nano-based coating, Nano-based paint, Hygroscopic properties, WUFI

1. Introduction

Wood which is one of the main important building materials has its drawbacks. Moisture is one of the major factors which affect the service life of wooden claddings. Moisture flow through wooden claddings can promote the growth of mould and fungi which may degrade the wood; destruct aesthetic value; affect the thermal comfort and health of the dwellers; and finally it can lead to deterioration of the building. It has been estimated that 80% of the failure of building components are moisture related [1]. Various surface treatments are a common way used to control the moisture transport through wooden claddings. Coating the surface is used to limit the amount of moisture penetration below the level which may lead to wood deterioration. Nano-based coatings have a number of potential towards reducing amount of moisture deposited on the surface of the wood [2-4]. However, moisture has also reduced the performance of the coating through time. In response to the problem of moisture in buildings, a considerable effort has been made by researchers to identify the moisture performance of wooden materials [5-7]. Many user friendly transient numerical models have also appeared in order to predict the moisture performance of building materials. Warme- und Feuchtetransport Instationar (WUFI) is one of the numerical simulation tools which provides reliable results for various hygrothermal simulations [8]. WUFI predicts the combined heat and moisture transfer in multilayered constructions under transient conditions.

The aim of this work is to evaluate the moisture transport in surface treated spruce exposed to transient climatic conditions. Adsorption isotherm and water vapour resistance (WVR) properties

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were measured to determine the measured moisture content (MC) compare with the numerical tool WUFI.

2. Experimental

2.1 Sample description

Spruce samples without knots were selected for the experiment. Specimens of (100.0 ± 0.5) mm x (100.0 ± 0.5) mm x (20.0 ± 0.5) mm size were prepared for the sorption and transient tests. In order to minimize shrinkage, swelling and possibility of cracking of specimens, small circular specimens with a diameter of (78.0 ± 0.5) mm and thickness of (5.0 ± 0.5) mm were used for the cup measurements. All specimens were preconditioned at a temperature of (23.0 ± 0.5) oC and a relative air humidity (RH) of (50 ± 3) % until equilibrium was reached. Two commercially available transparent nano coating types labelled as SPN and SPf were used to coat the specimens. Uncoated spruce (SU) was used as a control reference for the test. Based on the information obtained from suppliers, both the SPN and SPf coatings are hydrophobic and oil free, contains no toxic chemicals and are suitable for outdoor wooden claddings. The nano coatings were applied on the surface of the specimens with a brush even if the coatings had a very thin consistency. Wood is a material with large variation in its properties. As a hygroscopic material, it absorbs and desorbs moisture when used for outdoor applications. In order to assess the influence of moisture, it is important to know how the wood reacts with the variation in climatic conditions. The material properties of the wood are the main factors which determine its moisture storage capacity.

2.2 Test methods and conditions

2.2.1 Sorption test method

Sorption (adsorption and desorption) tests are useful to determine the hygroscopic sorption properties of the specimens by measuring the MC of the specimens in equilibrium with air at a specific temperature and RH. Sorption tests have been run according to EN ISO 12571 (2000) [9]. The hygroscopic moisture storage is described by means of material specific sorption curves called sorption isotherms [5, 8]. Within the context of this study, the adsorption isotherm test was carried out for SU material. The specimens were first dried to a constant mass and then exposed to a series of test environments with RHs at increasing levels from (40 ± 3) %, (50 ± 3) % and (75 ± 3) % RH in the climate chamber and to (94 ± 2) % RH in a box using a KNO3 salt solution (Fig.1). After establishing the equilibrium moisture content at each RH level, the adsorption curve was obtained.

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Figure 1: Sorption tests in a climate chamber (left) and in a saturated salt solution (right).

2.2.2 Cup test method

Like the sorption isotherm, the WVR is another vital moisture transport property used for assessing the moisture management ability of building materials. The cup method is a well-accepted method used to determine the WVR under isothermal conditions [10], though the method has some limitations [11]. The experiments were carried out according to EN ISO 12572 (2001) [12].

Figure 2: Cup tests used for determining WVR.

In this study, both coated and uncoated specimens have been tested under different RH of (11/50 ± 3) % RH, (100/75 ± 3) % RH and (100/50 ± 3) % RH at (23.0 ± 0.5) oC and 3 m/s air velocity (Fig. 2). The test conditions used in this experiment is described in Table 1 below.

Table 1: Test conditions used in the cup test.

Test series

RH inside / outside the cup [%]

Temperature [oC]

Test condition Inside the cup Outside the cup

1 11 / 50 23 LiCl (86.2g / 100ml distilled water)

Climate chamber

2 100 / 50 23 Distilled water Climate chamber 3 100 / 75 23 Distilled water Climate chamber

Specimens were sealed into the mouth of aluminium cups containing a vapour pressure regulator which was either distilled water or lithium chloride (LiCl). The test cups were then placed in a climate chamber with (50 ± 3) % and/ or (75 ± 3) % RH in order to create a water vapour flow either into the cup or out of the cup. The WVR of the specimens was determined from the test results.

2.2.3 Transient test method

Transient tests, i.e. relative humidity step change tests, were used to investigate the variation in MC of the wood exposed to changes in climatic conditions. The edges and back side of the

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specimens were sealed with aluminium foil in order to obtain one-dimensional moisture transport through the wood surface (Fig.3).

Figure 3: Wood specimens under transient test conditions.

The experiments were conducted under step changes in RHs and at a constant temperature. Specimens were exposed to weekly cycles between (75 ± 3) % and (40 ± 3) % RH step changes for a total of four weeks. The weight changes of the test specimens were measured as a function of time and the average MC was determined.

3. WUFI simulation tool The moisture conditions of wooden claddings were also calculated with the WUFI simulation tool in order to validate the measured values. The required data for the calculation of moisture distributions include the material properties, surface transfer coefficients and climate files. All material properties, except moisture storage and WVR, were obtained from WUFI for the simulation. The databases from WUFI provided by Germany, Sweden and Norway were selected for validating the result. Table 2 presents material properties and surface transfer coefficient values from the WUFI database used as input during simulation. Table 2: Material properties of spruce selected from different sources in WUFI simulation tool.

Parameters

Data from different sources

Germany [SG]

Norway [SN]

Sweden [SS]

Material properties

Bulk density [ kg/m3] 455 390 430 Porosity [m3/m3] 0.73 0.75 0.73 Heat capacity [J/(kgK)] 1 500 1 600 1 600 Thermal conductivity [W/(mK)]

0.09 0.13 0.14

Surface transfer coefficient

Exterior surface heat resistance [m2K/W]

0.0588 0.0588 0.0588

Interior surface heat resistance [m2K/W]

0.125 0.125 0.125

Interior surface equivalent air layer thickness (sd) value [m]

10 000 10 000 10 000

Initial condition RH [%] 50 50 50 Temperature [oC] 23 23 23

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Weekly step cycles of (75 ± 3) % and (40 ± 3) % RH for a total of 4 weeks were taken as indoor and outdoor climate files. The building component was assumed as one wood material layer with 0.021 m thickness where the simulation was run for four weeks assuming weekly step changes between the two RH. The results of four weeks exposure values were compared with numerical simulation results. 4. Results and discussion 4.1 Sorption and cup tests

Figure 4 shows adsorption curves for SU obtained from this experimental work and three other values available in literature [6, 7, 13]. All wood test specimens were obtained from different trees and tested at different times. The main reason for the deviation of the results may be due to the variation in the moisture storage capacity of wood. As wood is a material with very complex composition, its moisture storage capacity can vary from place to place and from time to time, i.e the spruce wood samples from this work and the literature are not exactly the same wood materials.

Figure 4: Adsorption isotherm data for SU obtained from this work compared with different litrature values [6, 7, 13]. The measurement error might also be another parameter for the discrepancy. The accuracy of the salt solution, the accuracy of the balance and the accuracy of the temperature and RH in the climate chamber may explain parts of the discrepancies in the results. Similarly, the WVR values from the cup test for both coated and uncoated wood are depicted in Fig. 5. The result reveals that the WVR of spruce coated with SPf (S+SPf) and SPN (S+SPN) was slightly higher than that of SU. At 11/50 % RH, the WVR value of S+SPN and S+SPf was found to be around 1.1 times higher than SU value while it was about 1.2 times higher than SU value at 100/75 % RH. On the other hand, both coated and uncoated specimens show significant WVR reduction with increasing RH. Although the resistance of the nano coating was not as significant as expected, the SU was found to be relatively vapour open.

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Figure 5: Measured values of WVR plotted as a function of RH for SU , (S+SPN) and (S+SPf) for all cup test results (left) also depicting a close-up of the high RH test results (right).

The measured WVR of SU was compared with three other literature values from other researchers as shown in Fig. 6 [6, 7, 13].

Figure 6: WVR comparison of SU in this work with literature values [6, 7, 13]. For all tests the WVR value decreased with increasing RH and this might be due to the additional moisture transport governed by the liquid water transport occurring at higher RH. The WVR comparison shows significant difference between the studies. This may partly be due to the uncertainty in the cup measurements including: the quality of the salt solution, the accuracy of the climate room and the tightness of sealing agent. Furthermore, the variation in the wood properties (e.g. moisture storage capacity) may also lead to differences in the WVR value. 4.2 Transient tests

The result of the measured MC versus time from the weekly step change in RH for all specimens (SU, S+SPf and S+SPN) are presented in Fig.7. Although the WVR of specimens coated with SPN and SPf is somewhat larger than SU, their MC is almost of the same order of magnitude. During adsorption, there is about 3 % and 6 % increase in MC of specimens coated with SPf and SPN compared with SU, respectively.

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Figure 7: Measured MC values of SU, (S+SPN) and (S+SPf).

The type of surface treatment seems to be a very important aspect for increasing the hydrophobic property of the coated wood surface. The quality and the amount of surface treatment applied can determine the efficiency of the coated specimens. As the two nano treatments are very thin, it was rather difficult to determine whether the amount of the treatment applied was sufficient or not. Reliability of results also depends on a number of other factors including the type, particle size and the rate of dispersion of the nanomaterials used in the paint. In addition, the measurement errors while running the experiment, including the accuracy of the RH and temperature in the climate chamber may be the causing variations in the results. Furthermore, variation in the wood material samples may also cause variations in the results. 4.3 Numerical simulations

The measured MC of SU was compared with numerical results in WUFI and are plotted as moisture content versus time in Fig. 8 below. The weekly step change values of the SU found from experimental work are in good agreement with the simulation results. The simulation results from WUFI databases overlap showing the same MC. The discrepancy of the test values from the simulation may be due to the measurement errors during the adsorption and desorption process. Although the influence of material properties, except the adsorption curve and WVR, are neglected and used directly from WUFI, they might contribute to the discrepancies in the results. In addition, the assumption of constant surface transfer coefficients in the simulation, especially heat resistance factors, might also have an impact.

Figure 8: MC comparison of transient test results of SU with WUFI simulation by inserting measured material properties of SU.

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Likewise, the moisture performance of the samples coated with the two types of nano paints predicted by WUFI is presented in Fig. 9. In this test the discrepancy between the experimental value and WUFI with measured material properties is about 20%.

Figure 9: MC comparison of WUFI simulation with experimental values of (S+SPf) (left) and (S+SPN) (right) exposed to transient test. All the barriers mentioned for SU versus simulation result can also be the cause for the variation in the result of coated specimens. Specially, the values of surface transfer coefficients used for the coated specimens may lead to large discrepancies. 5. Conclusion

Increasing concern on the durability of wooden buildings as well as environmental damage caused by wood treatments requires a better understanding of the moisture transport in exterior coated wooden claddings exposed to different climatic conditions. In this work the influence of transparent nano-based surface treatments on the moisture transport was presented. This preliminary study has demonstrated a rather poor performance of the surface treatments. However, further studies and experimental investigations are needed to modify coatings using appropriate and advantageous nanoparticles. Acknowledgements This work was carried out under the Sustainable Infrastructure Project in which nano-based wood modifications and their environmental impact were studied. The Research Council of Norway is greatly acknowledged for financial support. The authors would also like to express special thanks to Ole Aunrønning for his support during sample preparation and measurements. References

1. Galbraith, G.H., McLean, R.C., Guo, J., Kelly, D. (1998) A system of permeability specification for use in moisture simulation models. Journal of Drying Technology 7, 216-223.

2. Ritschkoff, A.-C., Mahlberg, R., Löija, M., Kallio, M., Mannila, J., Vesa, A. (2005) Sol-gel hybrid coatings for wood products with improved surface durability and repellence properties in nano and hybrid coatings, Manchester, UK.

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3. Tshabalala, M.A., Sung, L.-P. (2007) Wood surface modification by in-situ sol-gel deposition of hybrid inorganic-organic thin films. Journal of Coatings Technology Research 4, 483-490.

4. Mahltig, B., Swaboda, C., Roessler, A., Bottcher, H. (2008) Functionalising wood by nanosol application. Journal of Materials Chemistry 18, 3180-3192.

5. Kunzel, H.M. (1995) Simultaneous heat and moisture transport in building components. One-and two- dimensional calculation using simple parameters. In Fraunhofer Institute of Building Physics, Holzkirchen.

6. Time, B. (1998) Hygroscopic moisture transport in wood. Volume 1998:20, PhD thesis, NTNU, Trondheim.

7. Valen, M.S. (1998) Moisture transfer in organic coatings on porous materials- influence of varying environmental. Volume 1998:86, PhD thesis, NTNU, Trondheim.

8. WUFI 5 Pro (2009) Transient heat and moisture transport, Holzkrichen, Germany.

9. ISO12571 (2000) Hygrothermal performance of building materials and products- determination of hygroscopic sorption properties. European Committe for Standardization, Brussels, Belgium.

10. Söderström, O. (1998) On analysis of cup method measurements on hygroscopic materials with surface coatings. Nordic Journal of Building Physics.

11. Roles, S. (2009) How reliable are standard cup tests for the determination of the vapor permeability of the building materials? Proceedings of the fourth international building physics conference, Energy efficiency and new approaches, Istanbul, Turkey.

12. NS EN ISO12572 (2001) Hygrothermal performance of building materials and products - determination of water vapour transmission properties. European committee for standardization, Brussels, Belgium.

13. Tveit, A. (1966) Measurements of moisture sorption and moisture permeability of porous materials. Norwegian Building Research Institute, Oslo, Norway.

Selamawit M. Fufa, PhD studentSelamawit M. Fufa, PhD studentNorwegian University of Science and Technology (NTNU)Norwegian University of Science and Technology (NTNU)

NanoNano--based coatings and the influence based coatings and the influence on the hygroscopic properties of woodon the hygroscopic properties of wood

ContentContent

IntroductionObjective

Materials and methodSamples usedSorption, water vapor resistance and transient testsNumerical simulation

ResultsExperimental and simulation

Conclusion

Nano-based coatings and the influence on the hygroscopic properties of wood

WoodUntreated wooden cladding can be easily affected by climatic conditionMoisture is one of parameters that reduce service life of wooden buildings

NanotechnologyOne of the current promising area in the construction and building industriesWidely integrated towards wood modification

Improved wood properties

Nanoparticles Reference

Fire performance Clay,SiO2,TiO2 (Mahltig et al., 2008; Morgan, 2006; Sepeur, 2008)

Hydrophobic Clay,CeO2,TiO2 (Fauchadour et al., 2005; Mizutani et al., 2006; Turri et al., 2008)

UV-protection TiO2,ZnO,SiO2,Clay, CeO2 (Bauer and Mehnert, 2005; Fauchadour et al., 2005; Franziska et al., 2009; Landry et al., 2008; Sepeur, 2008)

Biocide Ag (Kumar et al., 2008)Scratch resistance SiO2 (Bauer et al., 2006)Photocatalytic TiO2 (Chen and Poon, 2009)

Table 1: Nanomaterials used in wood modification

IntroductionIntroduction


Evaluate the moisture transport in surface treated spruce exposed to transient climatic conditions

Measure material properties of the specimen

Compare experimentally obtained moisture content with numerical result

MotivationMotivation


Wood type

Spruce: widely used material in Norwegian buildingsSquare: length and width of (100±0.5)mm* & (20±0.5)mm thickness Circular: with (78±0.5)mm diameter* (5±0.5)mm thickness

Surface treatments

Commercially available two types of transparent coatings (SPf & SPN) Used for exterior wood claddings

Experimental: MaterialsExperimental: Materials


Experimental: Test methodsExperimental: Test methods

Preconditioning50±3 % RH & 23±0.5oC

SorptionClimate room (40, 50& 75)±3% RH

Water vapor resistanceCup method11±2% /50±3% RH, 100±2% /50±3% & 100±2% /75±3% RH inside/outside

the cup is used

Transient testWeekly step change within (40 & 75)±3% RHExposed to a total of 4 weeks


SimulationSimulation

WUFISoftware designed to perform hygrothermal analysis of building envelopeUsed to calculate heat and/or moisture transport in one dimensional

multilayered building components

Data from different sourcesParameters Germany [SG] Norway [SN] Sweden [SS]

Bulk density [ kg/m3] 455 390 430Porosity [m3/m3] 0.73 0.75 0.73Heat capacity [J/(kgK)] 1 500 1 600 1 600

Material properties

Thermal conductivity [W/(mK)] 0.09 0.13 0.14Exterior surface heat resistance [m2K/W]

0.0588 0.0588 0.0588

Interior surface heat resistance [m2K/W] 0.125 0.125 0.125Surface transfer

coefficient Interior surface equivalent air layer thickness (sd) value [m]

10 000 10 000 10 000

RH [%] 50 50 50Initial condition Temperature [oC] 23 23 23

Table 1: Material properties used for simulation


SimulationSimulation


Fig 4: WUFI simulation

Results: Material propertiesResults: Material properties

Moisture sorption increases with increase in RH

Fig 1: Sorption isotherm

WVR property of the wood reduced with increasing in RH

At higher RH, the moisture transport is by diffusion and liquid water transportWVR value of SPf and SPN are around 10-18% better than US

Fig 2.2: Comparison of measured & literature WVR values

Fig 2.1: Measured water vapor resistance (WVR)


Results: Transient testResults: Transient test

Fig 3: Measured moisture content

Varying the RH affects mainly the moisture content of the coated species


Results: simulationResults: simulation

Fig: 5.1 MC comparison of SU with WUFI simulation

Both experimental and simulation results follow the same trend in absorption and desorptionMajor difference can be seen in absorption

Fig: 5.2 MC comparison of S+SPf with WUFI simulation

Fig: 5.3 MC comparison of S+SPN with WUFI simulation


ConclusionConclusion

Measured material properties agree with literature values. The variation might be due to:The property of the woodApparatus used or measurement error

Nanopainted wood specimens show poor performance towards varying the climatic condition. The type, particle size and rate of dispersion of the nanoparticles used in the paint.Uncertainties in the application of the coating

There is some discrepancy between the measured and simulation results. Accuracy of the climate room (RH and temperature) and Measurement errorMaterial properties considered as constant


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Selamawit M. FufaSelamawit M. [email protected]


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