THE POTENTIAL USE OF ORGANOSILANE WATER SOLUBLE NANOMATERIALS AS WATER VAPOR DIFFUSION RETARDERS FOR WOOD

e retarding e ect of organosilane water soluble nanomaterials (nano-zycosil and nano-zyco l) on water vapor di usion through poplar wood (P.nigra) was evaluated in comparison with that of clear coatings (sealer and nitrocellulose lacquer and polyester lacquer) using cup and sorption methods. Two drying methods were applied to dry the nanomaterials -coated substrates: oven drying at temperature of 103 ± 2 °C for 24 h. and climatically drying at temperature of 25 °C and relative humidity of 65% for 20 minutes. e results showed that both coating materials decreased the water vapor di usion rate through the wood. e sealer and nitrocellulose lacquer coating represented a stronger e ect on the di usion coe cient of wood compared to the other coatings. In this case, the nano-zycosil represented a better performance compared to the nano-zyco l. Furthermore, the drying method of the nanomaterials -coated substrates can impact the nanomaterials performance. e water vapor di usion through the oven-dried substrates was faster than that through the climatically dried ones.


INTRODUCTION
e study of moisture di usion through wood is of great importance for many wood processing applications, such as drying, preservation, impregnation, moisture transfer through building wall systems, and packaging.Wood, as a hygroscopic material containing hydroxyl groups, is sensitive to humidity and temperature uctuations in terms of moisture sorption.e surface checks develop due to moisture sorption stresses can enhance the weathering process of wood.Moisture di usion into and out of wood can occur in response to a moisture gradient.In addition to the moisture sorption, water vapor di usivity through wood increases its weathering process.e mass di usivity refers to the ability of a porous medium to allow di usion ow under a concentration gradient.Considerable research has been conducted relative to the moisture di usion in porous media, proving the importance of this type of mass transfer (Nefzi and Jouini 2004, Zhelezny and Shapiro 2006, Nakashima and Kamiya 2010).Moisture di usion in wood is governed by the vapor di usion through the cell lumens and bound water di usion through the cell wall.Many research e orts have been conducted to measure the di usivity properties of wood by two main methods: the steady-state cup method and unsteady-state sorption method (Fick 1855, Martley 1926, Ping and Lianbai 2003, Burch et al. 1992, Absetz et al. 1993, Pang 1997, Mouchot and Zoulalian 2002).
Wood modi cation can be applied to alter its mass di usivity properties.e modi cation can involve thermal modi cation (Rousset et al. 2004), surface coating (Cerny et al. 1996)

Maderas.
modi cation (Giorgi et al. 2009).Among the modi cation methods, surface coating is widely used to reduce the mass di usivity through wood (Gholamiyan et al. 2010).e surface coating is basically used to improve the ultra violet (UV) stability of wood (Pai et al. 2008, Yu et al. 2007), to reduce water uptake (Grelier et al. 2007, Van et al. 2007), and to increase its attractive appearance.e measuring of di usivity properties of a coated substrate can be useful to evaluate its water repellency.Gholamiyan et al. (2011) evaluated the water-proo ng character of di erent coating materials by various methods.
Nanotechnology was one of the major elds of discovery of the twentieth century.Interest in nanotechnology and nanomaterials is stimulated by the fact that they demonstrate unique properties because of their small size and high surface to volume ratio.e potential use of nanomaterials for products based on natural materials, such as wood, has been investigated.e properties of wood and wood-based materials can be modi ed by the aid of nanotechnology (Lowry et al. 2008, Kaygin and Akgun 2009, Ashori and Nourbakhsh 2009, Lei et al. 2008).For example, the water absorption of wood can be modi ed through reinforcing of surface coating materials with nanoparticles (Gholamiyan et al. 2011). Giorgi et al. (2009) pointed out that the steaming of wood surface with nano-NAOH particles results in a substantial reduction in its gas and vapor di usivity. is study is aimed at investigating the potential use of organosilane water soluble nanomaterials (nano-zycosil and nano-zyco l) as water vapor di usion retarders for wood.

Sampling
Poplar wood (Populus nigra) from a forest near Taleghan in Iran was selected for the study.A tree with approximately 30 years of growth was felled.Several boards measuring 70 by 200 by 50 mm (T, R and L respectively) were cut.e average initial moisture content of the boards was 120%.e boards were conventionally dried in a laboratory kiln at a constant dry-bulb T of 60 o C and RH of 40% to the nal moisture content of about 12%.en, cylindrical specimens with 18 mm in diameter were drilled in longitudinal direction.e thickness of specimens was 7 and 3 mm in longitudinal direction for the cup and sorption experiments, respectively.e diagram of sample preparation is illustrated in Fig. 1.

Figure 1. e diagram of sample preparation (A:
A cross-cut board, B: cylindrical specimen for measuring di usion coe cient with cup method, C: cylindrical specimen for measuring di usion coe cient with sorption method) Coating materials Sealer and nitrocellulose lacquer and polyester lacquer, which are commonly used in the wood furniture industry, were used as clear coatings.ey were purchased from Dorsa Chemistry (Brilliant) Co. Nano-zycosil and nano-zyco l, which are organosilane water soluble nanomaterials, were purchased from Zydex Company.ese nanomaterials have been mainly developed for waterproo ng.For concrete, Nano-zyco lwith the size of 10-20 nm can enhance the waterproo ng property of nano-zycosil treated surfaces by lling microcrackes and nano pores.However, based on some pre-testes conducted for wood in the present study, no enhancing e ect was observed; thus, the mentioned nano-materials were applied, separately.e critical properties of nano-zycosil are summarized in table 1.

Coating methods
First, the surface of samples was coated by epoxy resin to con ne the water vapor di usivity through the longitudinal direction.en, one of the end sections was coated by the coating materials.A er that, some nanomaterials-coated samples were oven dried at temperature of 103 ± 2 °C for 24 hrs, and the others were dried into a conditioning room (T = 25 °C and RH=65% ) for about 20 minutes.All samples coated by the clear coatings were climatically dried inside the conditioning room.Table 2 shows the treatments.e sealer and nitrocellulose lacquer diluted by a lacquer thinner (1:2) were applied on the clear wood surfaces by brushing method.e clear polyester lacquer diluted by the lacquer thinner (1:2) and mixed by 10% catalyst (hardener) was brushed on the wood surfaces.Also, the nano-particles were applied on the surfaces by brushing.e mean coating weight for all treatments was 0.004 g/cm 2 .

Di usion Coe cient Measurements
Cup method e water vapor di usivity of wood samples was measured under the steady-state condition (Agoua et. al. 2001;Avramidis 2007).Several cups were lled with a NaCl solution, providing a RH of 75%.e cups with attached 7 mm-thick samples were then placed in a conditioning chamber (RH=60%, T=20°C).e cups were weighed every 24 h until a constant weight was reached.A er 21 days, when a steady-state condition was reached, the cup weight loss was plotted against the time.In fact, the steady-state condition was determined by the daily measurements of the cup weight.
e dimensionless di usion coe cient through the wood samples was calculated from equation 1 (Agoua et al. 2001) where q is water vapor ux through the wood sample (kg.s -1 m -2 ), ρ g is air density (kg.m -3 ), ρ v is water vapor density (kg.m -3 ), D e is the e ective di usion coe cient of wood (m 2 s-1 ).In order to be more explicit, a dimensionless coe cient f was introduced (Agoua et al.where Q is the measured mass ux (kg.s -1 ), A is the cross section of sample (m 2 ), M v is the molar weight of vapor (kg.mole -1 ), RH 1 is the relative humidity inside the climatic chamber, RH 2 is the relative humidity inside the cup, R is the constant of perfect gas, l is the sample thickness (m), P vs is the pressure of saturated water vapor in temperature of T(K), and D v is the water vapor di usion coe cient in air.

Sorption method
For unsteady-state measurement of di usion coe cient, using the sorption method, 3 mm-thick samples were rst equilibrated at an initial RH of 20% in a conditioning chamber.en, the relative humidity of the chamber was changed to the new RHs, 40, 60, and 80%, respectively.e temperature of conditioning chamber was kept at 30 ºC. e weight change of samples was monitored to the nearest 0.001 g at regular intervals.e dimensionless change of sample moisture content at equilibrium where X* is dimensionless moisture content, Xi is initial moisture content, Xe is nal moisture content, and X(t) is the wood moisture content at the time of t. is normalized curve slope that can be calculated from equation 5: en, the di usion coe cient can be calculated from equation 6: Where Dc is the di usion coe cient (m 2 .s - ) and l is sample thickness (m).Five replications were considered for each treatment to measure di usion coe cient.

Statistical analysis
e statistical analysis was conducted using SPSS so ware.Analysis of variance (ANOVA) was sued to test for signi cant di erences between means.Duncan's Test at the 95% con dence level was also applied to statically compare the mean values.
was plotted as a function of the square root of time.e di usion coe cient was calculated from the initial linear part of the sorption curve using the following equations (Avramidis 2007) (Fig. 2):

Di usion Coe cient Measured by Cup Method
e di usion coe cients obtained for control specimens were greater than those obtained for all coated specimens, i.e., the clear coating and nanoparticles had a positive e ect to decrease the water vapor di usivity (Table 3).e di usion coe cient of specimens ranged from 66.13×10 -9 m 2 s -1 for sealer and nitrocellulose lacquer-coated to 93.71×10 -9 m 2 s for uncoated (control) specimen.e D Uncoated sample / D Coated sample ratio ranged from 1.02 for sample coated by nanozyco l and oven dried (CZ dry) to 1.42 for sealer and nitrocellulose lacquer-coated sample (CSC).
e sealer and nitrocellulose lacquer coating represented a stronger e ect on the di usion coe cient of wood compared to the other coatings and nano-particles.Nevertheless, the mean coating weight is the same for the other coating and nano particles.is may be due to rigid lm formation on the wood surface (Gholamiyan et al. 2011). Cerny et al. (1996) also found di erent degrees of waterproo ng for various coating materials.Based on di usion modeling from simulated porous structures, Laudone et al. (2008) pointed out that the di usion rate is small for tiny porous media.e nanozycosil-coated specimen had a higher resistance to water vapor di usion than the nanozyco lcoated one, resulting from special properties of each nano-particle.e smaller size of nano-zycosil particles (4-6 nm) compared to nano-zyco l (10-20 nm) particles may play an important role in better reaction of the particles with OH groups of wood.Di usion through the oven-dried specimens was faster than that through the climatically dried ones. is is probably attributed to crack formation in the coating nanomaterials (Oosterbroek et al. 1991).

Di usion Coe cient Measured by Sorption Method
Figure 3 shows dimensionless weight change of samples versus the square root of time.e same trend of mass variation against RH-steps was observed for all coated and uncoated specimens.e water vapor di usivities calculated from absorption cycle are presented in table 4. Similar to what was obtained from the cup method, the di usion coe cient for all coated-specimens were lower than that for uncoated specimens.e sealer and nitrocellulose lacquer coating had a dominant e ect on the mass di usivity behavior of wood.e measured di usion coe cient was in the range of 0.9×10 -9 to 3.05×10 -9 m 2 s -1 .In the RH-step of 20-40%, the D Uncoated sample / D Coated sample ratio ranged The potential use of... Gholamiyan et al. from 1.12 for the sample coated by nano-zyco l and oven dried (CZ dry) to 3.35 for the sealer and nitrocellulose lacquer-coated sample (CSC).For the RH-step of 40-60%, the ratio was in the range of 0.97-2.84for the mentioned coated samples, respectively and similarly, for the RH-step of 60-80%, it ranged from 0.89 to 2.31.Indeed, the D Uncoated sample / D Coated sample ratio was lower when the sorption measurements made with a higher RH-step.As expected, the mass di usivities measured by the two techniques for the same specimen did not coincide numerically.It can be interesting that there is a signi cant di erence between the di usion coe cients obtained by the cup and sorption methods.In addition, the D Uncoated sample / D Coatedsample ratio obtained in steady-state condition and transient sorption was di erent.Overall, a higher ratio was observed for sorption measurements.Furthermore, the sorption measurements made with the higher RH-steps resulted in lower di usion coe cient.Houngan et al. (2006) also reported the same result for longitudinal and tangential di usion coecient of beech (Fagus sylvatica) wood calculated by the sorption method.

CONCLUSION
e present study evaluated the potential use of nanotechnology in addition to clear coatings to modify the mass di usivity properties of wood.e surface coating of wood by both clear coatings (sealer and nitrocellulose lacquers and polyester lacquer) and nanoparticles (nano-zycosil and nanozyco l) can reduce the rate of water vapor di usivity through the wood.Among all coating materials and nanoparticles used in this study, sealer and mitrocellulose lacquer had a pronounced e ect on the mass di usion coe cient of wood.In this case, the nano-zycosil represented a better performance compared to the nano-zyco l.Although both nano-zycosil and nano-zyco l particles were found to be e ective nano-particles in decreasing water vapor di usivity through wood, attention should be focused on the coating condition and drying method of coated substrate.Our study showed that the drying method of the nanoparticles-coated substrate can impact the nanoparticles performance.
e potential e ect of other nano-particles on the moisture di usivity of the surface coated wood is recommended for further work.
2001), which represents the ratio of mass di usivity in the porous medium over what would have been obtained in a sample of air at rest.f is de ned by the relation D e where D v is the binary di usion coe cient of vapor in air.D v is calculated from equation 2: D v = 2.26x10 -5 T / 273 1.81 Patm / P Where T is temperature (K) and P is pressure.For a very di usive material such as glass bers of very low density, f is close to the unit whereas the f value of an impervious material equals zero.e dimensionless di usivity f is calculated by the formula 3 (Agoua et al. 2001):

Figure 2 .
Figure 2. A sorption curve (moisture content vs. square root of time)

Fig. 3 .
Fig. 3. Dimensionless weight change of samples versus the square root of time in sorption method

Table 1 .
Critical information of the employed nano-zycosil

Table 2 .
Guide for treatmentsThe potential use of...Gholamiyan et al.

Table 4 .
Di usion coe cients measured by sorption method