Olivier Arnould
email: olivier.arnould@univ-montp2.fr

Reinhard Stürzenbecher

Sandrine Bardet

Karin Hofstetter

Daniel Guibal

Nadine Amusant

Antonio Pizzi

Running title: Mechanical potential of eco-OSB Mechanical potential of eco-OSB produced from durable and nondurable species and natural resins

Keywords: mechanical properties, micromechanical modelling, natural resin, OSB, pine and cypress mixture

OSB panels were manufactured with different mixtures of pine and cypress heartwood and resins based on lignin or tannin in order to develop an eco-friendly wood composite with a natural durability against termite and fungi. Some physical properties and the major elastic moduli of bulk wood as well as of the manufactured panels were determined using different measurement techniques. In addition, a micromechanical model was adapted and validated with the experimental results. The good agreement obtained between the experimental data and model predictions indicates the proper assessment of the most influential parameters, such as raw material and adhesive properties, strand orientation, layer assembly, and density profile. A parameter study, enlightening the effect of strand orientation on several elastic constants, enlarges the scope of experiments. We conclude with an optimal combination of resin and wood species mixture resulting in the best performance from a biological and mechanical standpoint.

Introduction

Most of the wood-based composites are not naturally resistant to termite attack [START_REF] Muin | Termicidal performance of wood based composites treated with silafluofen using supercritical carbon dioxide[END_REF] because they are mainly manufactured from non durable wood species. Panels designed for end uses, in which decay or termite attack are potential hazards, often contain fungicides or insecticides. Leachability and toxicity are major problems for this type of products. Nowadays, the pressure to restrict the use of wood preservatives in wood products is increasing. Moreover, interactions between adhesives and preservatives damage the bond performance and ultimately reduce the physical properties of the panel [START_REF] Goroyias | The mechanical and physical properties of strand boards treated with preservatives at different stages of manufacture[END_REF]Hale 2004, Kirkpatrick and[START_REF] Kirkpatrick | Biocide treatments for wood composites -A review[END_REF].

Thus, alternative approaches are necessary to obtain good durability of environmentally friendly wood composites without loss of performance.

Modern product developments should consider both ecological and technical aspects. The resistance of wood products to biodegradation can be increased by using naturally durable wood species, especially in regions with low to moderate termite hazard [START_REF] Behr | Decay and termite resistance of medium density fiberboards made from wood residue[END_REF][START_REF] Yalinkilic | Biological, physical and mechanical properties of particleboards manufactured from waste leaves[END_REF][START_REF] Ecosb Layman ; Evans | Medium density fibreboard manufactured from blends of white cypress pine and non-durable wood species shows increased resistance to attack by subterranean C. lacteus[END_REF][START_REF] Kartal | Decay and termite resistance of medium density fibreboard (MDF) made from different wood species[END_REF][START_REF] Wan | Utilizing eastern white Cedar to improve the resistance of strand boards to mold and decay fungi[END_REF]. Another environmental concern is the control of volatile and semi-volatile compounds derived mainly from adhesives (resins).

Natural resins based on lignin [START_REF] Lei | Environment-friendly, mixed tannin/lignin wood resins[END_REF], Mansouri et al. 2007a) or tannin [START_REF] Garnier | Rheology of polyflavonoid tanninformaldehyde reactions before and after gelling -Part 2: Hardener influence and comparison of different tannins[END_REF][START_REF] Ballerini | Non-toxic, zero-emission tannin-glyoxal adhesives for wood panels[END_REF]) are options for environment-friendly products.

A political concern nowadays is on reducing the emission of climate gases (mainly CO 2 ) in production processes. Wood and wood products are a priori ecological materials, especially if productions processes are well optimized with reduced energy consumption [START_REF] Ecosb Layman ; Evans | Medium density fibreboard manufactured from blends of white cypress pine and non-durable wood species shows increased resistance to attack by subterranean C. lacteus[END_REF] and residues (by-products). Oriented strand board (OSB) panels are exemplary with this regard as their production permits the utilization of almost all the harvested trees including imperfect or young trees and fast growing species.

Results on the durability of ecological OSB products (shortly 'eco-OSB') have been published recently [START_REF] Amusant | Biological properties of an OSB eco-product manufactured from a mixture of durable and non durable species and natural resins[END_REF]. It has been shown that OSB made of a mixture of heartwood cypress (Cupressus sempervirens) and pine (Pinus sylvestris), with lignin (with paraformaldehyde and pMDI) or tannin (from pine with hexamine hardener)-based resin, show durability against termites and fungi.

The load bearing capacity of OSB panels in structural applications is essential. Thus, this paper focuses on the mechanical potential of these eco-OSBs. Firstly, mechanical properties of the raw material and of eco-OSB will be identified by several mechanical testing methods. Secondly, a micromechanical model will be applied, which provides a link between microstructural characteristics and the macroscopic mechanical behaviour. In particular, the overall elastic properties of the panels will be estimated considering the physical properties of bulk wood and resin as well as the morphological characteristics of the OSB such as strand orientation, density profile and layer assembly. The motivation for the modelling is to further explore the mechanical potential of the panels beyond the traditional experiences. The micromechanical model should serve as the basis for product development and optimisation. The expectation is that it allows identifying optimal panel designs in terms of microstructural characteristics and panel lay-ups.

Materials and methods

Characterization of the raw materials

OSB was produced of cypress heartwood, which is naturally durable against termites, and sap-and heart-wood of pine, which are both nondurable against termites. The different 60-year old trees were grown in the Grenouillet Arboretum (France), felled, and crosscut into 1 m long logs. Test specimens for determination of physical and mechanical properties were cut from the logs as depicted in Figure 1. All specimens, i.e., the raw material and the OSB panels, were conditioned and tested at a temperature of 20°C and a relative humidity (RH) of 65%. First, static compression tests on cubes, with a side length of 40 mm machined along the principal material directions (R, T and L) were performed on a universal electromechanical testing machine MTS 1/ME with a 5 kN load cell. Mean compression strain was assessed by using strain gages (from Kyowa and TML with 2 or 8 mm gage length depending on the annual ring thickness on the face considered) for calculating the elastic moduli E R , E T and E L . Moreover, transversally oriented gages were used to measure the transverse strains on each of the four accessible faces of the cubes for determination of the six Poisson's ratios ν RT , νΤ R , ν LT , ν TL , ν LR and ν RL . The maximum applied load corresponds to a mean compressive strain of around 0.2%. The test consists in three loading/unloading cycles at a strain rate of about 10 -4 s -1 . The elastic moduli are measured in the linear range of the unloading/reloading curves.

In addition, Bordonné's free vibration beam method (Bordonné 1989, Brancheriau and[START_REF] Brancheriau | Natural vibration analysis of clear wooden beams: a theoretical review[END_REF] was applied on samples sized 20×20×360 mm 3 (R-T-L). It allows measuring longitudinal bending elastic modulus, E L , and shear moduli, G TL or G LR depending on the sample rotation along the L-direction, at the natural frequency of the beam, which is approximately 700 Hz.

Furthermore, ultrasound measurements in the directions of the principal axes have been performed by means of Sofranel's 1 MHz longitudinal transducer on cubes with side lengths of 20 mm cut at the end of the free vibration beam (see Figure 1). Determining the ultrasound velocity V in the sample [START_REF] Bucur | Acoustics of wood[END_REF] and knowing the density ρ, it is possible to compute the elastic stiffness C ii of the sample that is linked to the modulus of elasticity E i and the Poisson's ratios ν ij (Guitard 1989):

( )

TR RL LT LR RT TL TL LT TR RT LR RL i ii jk kj 1 1 E C ν ν ν ν ν ν ν ν ν ν ν ν ν ν - - - + + = - (1)
where

2 ii i C V ρ
= and i,j,k = {R, T, L}, e.g., if i = R then 1-ν jk ν kj = 1-ν TL ν LT . Assuming a negligible effect of loading frequency on the Poisson's ratios, their values obtained with the compression tests are used to compute the elastic modulus E i from C ii .

A micromechanical model by [START_REF] Hofstetter | Development and experimental verification of a continuum micromechanics model for wood[END_REF][START_REF] Hofstetter | The influence of the microfibril angle of wood stiffness: a continuum micromechanics approach[END_REF][START_REF] Hofstetter | Micromechanical modeling of solid-type and plate-type deformation patterns within softwood materials. A review and an improved approach[END_REF] was also applied because a complete and consistent set of all nine independent elastic constants of the bulk wood was not always available or reliable. This model allows the prediction of the elasticity tensor of various wood species from the elastic properties of the basic constituents of wood (cellulose, hemicelluloses, lignin and water) and from morphological parameters such as microfibril angle (MFA), cell arrangement and macroscopic density. In order to estimate the properties of the raw material, density was chosen in accordance with the mean density of the tested bulk wood samples.

The microstructural characteristics, MFA and the lignin content, were determined by adjusting the resulting model predictions of E L , E T and G TL to the corresponding experimental results from bending free vibration and compression tests. The model estimated stiffness tensor obtained for these microstructural characteristics was finally used as input for the panel model presented below.

Manufacturing OSB panels

Flakes with dimensions of 0.6×10×100 mm 3 (R-T-L) were manually trimmed in thin veneers and the flakes of each species were dried to about 6-7% moisture content (MC) before gluing. Mat formation and strand orientation were done by hand. The full set of panel manufacturing parameters is presented in Table 1. A total of 24 OSB panels was prepared, which corresponds to three panels for each combination of resin and species.

Characterization of OSB test specimens

From each panel, 18 squared test specimens with dimensions of 50×50×14 mm 3 and 2 beams (one sized 300×40×14 mm 3 , mainly oriented in the x-direction, and one sized 260×40×14 mm 3 , mainly oriented in the y-direction) were cut (Figure 2). The beams and part of the squared specimens were used for determination of the elastic properties of the panels, while the remaining squared specimens were employed for the durability measurements [START_REF] Amusant | Biological properties of an OSB eco-product manufactured from a mixture of durable and non durable species and natural resins[END_REF]).

The mean density was measured for each test specimen. The vertical density profile was determined by means of the densitometer DENSE-LABX (Electronic Wood System, Germany) at increments of 0.05 mm for ten randomly chosen specimens. The strand orientation distribution was determined manually using pictures of the outer surfaces of three different panels (Figure 2) and ImageJ, a public domain image processing software.

Classical static face down 4 point-bending test (outer span: 250 mm, inner span: 160 mm, loading point diameter: 20 mm) were first done on the beam-shaped sample using again the electromechanical testing machine MTS 1/ME equipped with a 5 kN load cell, at a loading speed of 10 µm s -1 in order to reach the ultimate loading force in 300±120 s following EN 789 European standard (2005). The tests were performed in the elastic range, and the bending strain was measured through the difference in deflection between three points by means of a micrometer mounted on a specific fitting. Accordingly, the static bending moduli of elasticity in the two main panel directions, E x and E y , were obtained. In addition, the same samples were tested in free vibration bending using Bordonné's principle (Bordonné 1989, Brancheriau and[START_REF] Brancheriau | Natural vibration analysis of clear wooden beams: a theoretical review[END_REF]. Face down measurements allow to determine the bending moduli of elasticity, E x and E y , at a frequency of around 500 Hz and edgewise measurements yield estimates of the shear elastic modulus G xy (Brancheriau 2006) on the two types of beams (x and y-direction). Finally, ultrasound measurements through the thickness of the squared specimens were performed in order to obtain the elastic stiffness C zz .

Modelling the elastic properties of the panels A multiscale model for strand-based engineered wood products developed by Stürzenbecher et al. (2010a, b) was applied and adapted to the specific characteristics of the present panels. This multiscale model is based on the continuum micromechanics and lamination theory and predicts the in-plane tension and bending stiffnesses as well as the in-plane shear stiffness of multi-layer strand boards. Thereby, the boards are idealized consisting of ellipsoidally shaped and perfectly bonded wood strands. The following parameters are considered: the elastic properties of the wood species, the slenderness ratio and orientation distribution of the strands, as well as the panel lay-up described in terms of density profile and layer assembly. Here only the specifications of the model for the present study are explained. For a detailed description of the model approach, see Stürzenbecher et al. (2010b). The high resin mass content of the produced boards (Table 1), which equals about 6%

(by volume) of the final boards, requires an adjustment of the original model. This model had been developed for strand boards with moderately low resin content, which did not necessitate consideration of the adhesive as a separate material phase. In order to account properly for the higher adhesive content in this application, strands were modelled with an adhesive layer, applying the Composite Cylinder Assemblage (CCA) model for estimating their elastic properties [START_REF] Hashin | Analysis of properties of fiber composites with anisotropic constituents[END_REF]Rosen 1964, Hashin 1979). The transverse shear modulus, which cannot be estimated by means of the CCA model, was predicted by a Generalised Self Consistent Scheme developed by [START_REF] Christensen | Solutions for effective shear properties in three phase sphere and cylinder models[END_REF]. Based on the estimated elastic properties of adhesive coated strands, the homogenization procedure of Stürzenbecher et al. (2010b) was applied, accounting for the compaction, the strand orientation distribution, the layer assembly and the density profile across the panel thickness. The elastic behaviour of the tannin and the lignin adhesives in their cured state was assumed to be isotropic with a Poisson's ratio of 0.3 and a modulus of 1.8 GPa [START_REF] Garcia | Polycondensation and autocondensation networks in polyflavonoid tannins. II. Polycondensation versus autocondensation[END_REF]Pizzi 1998, Osman andPizzi 2002) and 2.1 GPa (Mansouri et al. 2007b), respectively. Since the density profiles were not measured at every test specimen, one characteristic representative of all measured density profile was taken for modelling of all panels. This procedure was feasible, since the production process was the same for all panel types and only little variation was observed between the measured density profiles.

Extending the original model by Stürzenbecher et al. (2010b), the stiffness component zz C in the plate thickness direction was estimated from the respective values of the individual board layers with different densities. This was done using the rule of mixtures for serially arranged materials, reading mathematically as:

1 1 i zz N i i zz C f C = = ∑ (2)
where i f denotes the relative layer thickness and i zz C the stiffness tensor component of this layer i in the thickness direction of the panel.

Results and discussion

Density and mechanical properties of the raw materials

The data for density and elastic properties of the bulk wood are reported in other ones even if it corresponds in that case to bending loading. This may be due to the relatively good homogeneity of the material at the considered cross section scale.

Microstructural characteristics of bulk wood were back-calculated by the micromechanical model [START_REF] Hofstetter | Development and experimental verification of a continuum micromechanics model for wood[END_REF][START_REF] Hofstetter | The influence of the microfibril angle of wood stiffness: a continuum micromechanics approach[END_REF][START_REF] Hofstetter | Micromechanical modeling of solid-type and plate-type deformation patterns within softwood materials. A review and an improved approach[END_REF] based on the values of E L , E T and G TL measured with the bending free vibration technique. MFAs of 21° were obtained for pine and 22° for cypress, whereby the lignin content of the former was 20% and 26% of the latter, which is in the range of possible mean lignin contents for softwood from 25 to 34% after [START_REF] Petterson | The chemical composition of wood[END_REF] or from 20% to 27% after [START_REF] Faix | Chemie des Holzes[END_REF]. Accordingly, the micromechanical model provides a full set of elastic constants of the (orthotropic) raw material, which is in full agreement with those obtained from experiments (Table 2).

Structural characterization of the produced strand boards

The average density of all panels is about 656 kg m -3 with a standard deviation of 24 kg m -3 . Figure 3 shows the characteristic measured density profile, which was used for the evaluation of the model for eco-OSB panels. It exhibits a moderate U-shape, as all the measured density profiles. For modelling purpose, this profile was discretized: constant density values were determined for layer thicknesses between 0.5 mm close to the surfaces and 2.5 mm in the centre of the board (Figure 3).

The strand orientation distribution measured on the surfaces of three different panels is depicted in Figure 4. A classical spread of orientations is observed, and a normal distribution was adjusted by the least-square method. This yields a mean orientation close to 0° and a standard deviation around 5°, reflecting the careful panel production by hand, which achieves better alignment of strands than industrial processes.

Mechanical properties of the produced strand boards

The elastic properties of the final OSBs, measured with different techniques, are presented in Table 3 and grouped according to the mixture of wood species and the resin types. Here, the medians and the ranges are given, showing the difference between the maxima and the minima of the three replicates of each setting.

The values obtained in static bending are in good agreement with those obtained in free vibration despite the difference in the loading frequency. Free vibration yielded quite similar results

for the in-plane shear-modulus obtained for the beams oriented in x and y-direction. The order of the values of the measured moduli is as expected, i.e., E x > E y > G xy , because E x and E y are mainly linked to E L and E T of the bulk wood, respectively, as the outer layers contribute dominantly to the overall bending stiffness of the panels. Remarkably, the results for the elastic moduli do not correlate with the amount of cypress in the mixture except for the stiffness C zz . The latter decreases when the amount of cypress is reduced irrespective of the resin. This is in line with the slightly higher moduli E R measured on the bulk wood samples of cypress than on those of pine. For the bending moduli E x and E y measured on the panel, the effect of cypress content is not obvious, probably because the two moduli of the bulk material controlling the panel bending stiffness, namely E T and E L , are close to each other for the two species, as can be seen in Table 2. The variability of the out-of-plane modulus rather results from variations of the wood and resin properties in individual panels than from different extents of bonding defects. On the other hand, the variability of the bending properties of the panels is -amongst others -a consequence of varying bonding quality between strands. Altogether, the mechanical properties were comparable to that of conventional, industrially produced boards, highlighting the potential of the investigated biocomposite. In this study, panels made with lignin-based resin give the best results in terms of elastic properties. This is all the more interesting as lignin-based resin yields the best durability too [START_REF] Amusant | Biological properties of an OSB eco-product manufactured from a mixture of durable and non durable species and natural resins[END_REF]).

Comparison of model predictions and experimental results

The suitability of the micromechanical model was validated experimentally. For this purpose, the model is evaluated with the specifications of the produced boards, including the elastic properties of the raw material and resin, the characteristic density profile adjusted to the mean final density, the strand orientation distribution and the layer assembly. Thereupon, a one-to-one comparison is made between the model estimates and the corresponding results of bending free vibration tests (E x , E y and G xy ) and ultrasonic experiments (C zz ), respectively (Figure 5). Both MOE, E x , and E y , estimated by the model show on average good agreement with the experimental results obtained from bending free vibration tests. Natural fluctuations of elastic properties of the raw material and variations in the production process were not considered in the model, so that the considerable variations of the experimental results were not reproduced by the model. The mean prediction error of the MOE E x amounts to 12.1% with a standard deviation of 17.1%, while it is 3.8% with a standard deviation of 21.4% for E y . The in-plane shear modulus G xy is overestimated by the model by 24.6% with a standard deviation of prediction errors of 46.5%. Particularly, experimental shear moduli below 1 GPa are not well predicted by the model. Further, the model overestimated the transverse stiffness component C zz by about 24.6%, with a standard deviation of 35.2%.

Model parameter studies on the effect of strand orientation

The experimentally validated model was extended to the experimental investigations of the mechanical behaviour of eco-OSB to non-tested configurations. Particular emphasis is placed on examining the effect of strand orientation distribution on the elastic properties of the final panels.

The strand orientation of industrially produced boards is expected to be not as strictly oriented as currently observed in the hand-made panels. Taking this into consideration, the model allows estimating elastic properties of panels from a commercial production line. The parameter study is performed for pine wood as raw material, lignin adhesive, and a mean board density of 650 kg m -³.

Adhesive content, density profile, and the ratio of strand mass in the face and core layers respectively, are the same as in the actually produced boards considered in the model validation.

The distribution of strand orientation is described by a normal distribution with a mean orientation of 0° (coinciding with the x-axis) and a variable standard deviation. Increasing the standard deviation finally leads to a random strand orientation distribution. Figure 6 shows the pronounced effect of less tight strand orientation, modelled by increasing the standard deviation of the assumed normal distribution, on the mechanical properties of the panel. The MOE in the principal direction of the panel decreases dramatically when the strands are less aligned with the principal panel direction, whereas the MOE perpendicular to this direction increases only slightly.

The in-plane shear modulus G xy rises with increasing standard deviation of the strand orientation distribution from about 1.4 GPa to about 2.5 GPa. This means that higher deviations of strand orientations from the main panel direction in commercial production, improves the performance of the panel for shear stiffening applications, but degrades it for bending applications with a single pronounced load bearing direction.

Conclusion

Characterization of wood species as raw materials for OSB production with various methods (static vs. dynamic and compression vs. bending) led to very similar and satisfactory results. This good agreement is due to the low viscosity of dry wood and the relatively high homogeneity of the sample in the scales of L-direction and cross section (i.e., relatively small annual ring width compared to the cross section characteristic length). Additionally, a micromechanics model was applied delivering all stiffness components of the input wood and, thus, completing the characterization. The mechanical behaviour of the laboratory-made panels was also determined by dynamic and static measurement techniques. The best quality (with highest stiffness) has been obtained for the panels glued with lignin-based resin. As this type of panels show the best durability too, they might be suitable for developing eco-OSB panels at the industrial scale. Further, a multiscale model has been developed and applied in order to explore and to quantify the influences of the microstructural characteristics on the mechanical behaviour of the boards for non-tested configurations. The established model for eco-OSB is able to reflect suitably the microstructural characteristics of raw material and adhesive properties, strand orientation, density profile and layer assembly. It delivers reasonably accurate predictions for the mean elastic properties, e.g., both the in-plane bending moduli and the in-plane shear modulus as well as the out-of-plane or transverse stiffness tensor component. Employing the validated model for parameter studies gives insight into the (micro)mechanical behaviour of strand boards. In an exemplary manner, the effect of strand orientation distribution on bending and shear stiffness was demonstrated to be able to estimate the influence of the production process on the mechanical properties of the panels. The combination of the theoretical model, capable to describe the underlying mechanics, and complementary experiments, affording direct insight into the mechanical performance, seems to be a fruitful and efficient approach. This combination permits the further development of products.

Figures' legend Figure 1 .

 legend1 Figures' legend

Figure 2 .

 2 Figure 2. Face view of a manufactured OSB (50% cypress-50% pine with the lignin based resin)

Figure 4 .

 4 Figure 4. Strand orientations measured on the surfaces of three produced panels and normal

Figure 5 .

 5 Figure 5. Comparison of experimental values from bending free vibration tests and corresponding

Figure 6 .

 6 Figure 6. Effect of strand orientation on the elastic bending constants.

Figure 1 .

 1 Figure 1. Cutting plan for specimens for measurements on the raw material.

Figure 2 .

 2 Figure 2. Face view of a manufactured OSB (50% cypress-50% pine with the lignin based resin)

Figure 3 .

 3 Figure 3. Characteristic measured density profile, DP, in thickness and layer-wise average for

Figure 4 .

 4 Figure 4. Strand orientations measured on the surfaces of three produced panels and normal

Figure 5 .

 5 Figure 5. Comparison of experimental values from bending free vibration tests and corresponding

Figure 6 .

 6 Figure 6. Effect of strand orientation on the elastic bending constants. 40

Table 2

 2 LR is missing because of experimental difficulties. The measurement of Poisson's ratio ν LT is difficult as well, leading to too high values on one side of the sample, of only limited reliability, that leads to the high standard deviation reported. The values for ν LT have been checked by measuring ν TL as well, but the measurement results were not better in this case due to the small absolute values of these ratios. The results for the longitudinal elastic moduli, E L , of the raw material measured by different techniques are in reasonably good agreement with each other. Similarly good agreement is obtained for the elastic moduli E T and E R determined by ultrasound measurements and static compression tests. The beam free vibration measurements delivered in addition to the E L both shear moduli G TL and G LR . Values obtained with this last method for E L are in good agreement with the

. For the static compression tests, only one sample per species was tested several times. This may explain the very low standard deviation of the respective results. For the compression tests on pine, Poisson's ratio ν

Table 1 .

 1 Parameters of panel manufacturing

	Panel dimensions	350×350×14 mm	3
	Three layers panel construction	Core perpendicular to face flakes
	Mass distribution (side/core/side)	20% / 60% / 20%
	Wood species	Pine, cypress
	Target mat moisture content	6-7%
	Resin mass content	13% side and 11% core
	Blender type for mixing strands with resin Dakota
	Blender rotation speed	900 rpm
	Pressing cycle for gluing	90 s 35 bar, 120 s 16 bar, 150 s 8 bar
	Press temperature	175°C (plate surface)
	Total press time	6 min
	Replicate	3

Table 2 .

 2 Mean values of measured wood bulk properties obtained by different measurement methods and by micromechanical model predictions for the nine independent elastic constants of bulk wood.

	Wood	Properties	Beam free vibration (~700 Hz) Sample: 20 × 20 × 360 mm³	Ultrasound (1 MHz)	Static compression test Sample: 40 × 40× 40 mm³	Computed
		ρ (kg m -3 )	579±4	569±8	580
		E R (GPa)		1.99±0.09	1.75±0.03	1.21
		E T (GPa)		1.44±0.08	1.16±0.04	0.86
		E L (GPa)	13.17±0.97	12.55±0.96	11.21±1.79	13.03
	Cypress (Cupressus sempervirens)	ν RT ν LR			0.63±0.05 0.36±0.03	0.49 0.32
		ν LT			0.71±0.22	0.37
		G TL (GPa)	1.00±0.01			1.00
		G LR (GPa)	1.12±0.06			1.02
		G RT (GPa)				0.12
		ρ (kg m -3 )	547±26	537±21	535	550
		E R (GPa)		1.86±0.12	1.79±0.01	1.13
		E T (GPa)		0.73±0.19	0.91±0.01	0.80
		E L (GPa)	14.39±1.73	13.99±1.12	15.85±0.25	13.84
	Pine	ν RT			0.58±0.14	0.52
	(Pinus sylvestris)	ν LR			--	0.32
		ν LT			0.61±0.29	0.36
		G TL (GPa)	1.02±0.09			0.98
		G LR (GPa)	1.37±0.10			1.00
		G RT (GPa)				0.10

Table 3 .

 3 Elastic properties of the manufactured panels: median values and range (in parenthesis). Density values for the edgeways free vibration 5 bending are the same as the face down bending in the same direction. 6

		Cypress		E X (GPa) Face down bending		E Y (GPa) Face down bending	G XY (GPa) Edgeways free vibration		C ZZ (GPa)
	Resin	content	Density		Free vibration	Density		Free vibration	Bending (~1.5 kHz)	Density	Ultrasound
	base	(% wt.)	(kg m	-3 )	Static	(~500 Hz)	(kg m	-3 )	static	(~500 Hz)	x sample	y sample	(kg m	-3 )	(100 kHz)
		100	(35) 10.9 (0.3)	9.6 (2.7)	643 (23)	4 (0.1)	4.1 (0.6)	1.35 (0.4)	1.8 (1.9)	649 (40) 0.65 (0.25)
	Tannin	75 50	(10) (15) 13.5 (1.4) 8 (2.2)	7.7 (1) 11.9 (3.1) 657 (15) 667 (32)	4.9 (4) 5.1 (1)	5.1 (1.5) 4.7 (1.8)	1.3 (1.4) 1.1 (0.7)	0.9 (0.2) 1 (0.5)	705 (26) 0.61 (0.19) 661 (61) 0.55 (0.18)
		0	(28) 5.4 (2.3)	7.8 (1.6)	667 (53)	3 (0.3)	3.3 (0.5)	0.7 (1.2)	2.1 (4.4)	690 (58) 0.36 (0.08)
		100	(27) 10 (1.9)	9.5 (0.6)	661 (68) 4.5 (1.5)	4.4 (1.2)	1.5 (1)	1.3 (1.4) 654 (153) 0.85 (0.38)
	Lignin	75 50	(90) 12.9 (2.4) (37) 10.7 (2.3)	11.2 (2.7) 628 (41) 4.9 (0.2) 11.1 (0.8) 643 (46) 4.4 (4.4)	4.9 (0.7) 4 (0.7)	1.6 (1) 1.4 (0.4)	1.3 (0.7) 636 (150) 0.67 (0.25) 2.1 (1) 647 (174) 0.57 (0.39)
		0	(80) 12.1 (1)	11.3 (0.9) 652 (29)	3.4 (4)	5.9 (2.2)	1.1 (0.8)	1.1 (0.4) 643 (142) 0.52 (0.28)

Acknowledgements

The authors gratefully acknowledge Remy Marchal and Jean Claude Buteaud (from the Labomap laboratory, Ecole Nationale Supérieure d'Arts et Métiers, Cluny, France) for the peeling of the strands.


Aida Se Golpayegani
email: aida.golpayegani@gmail.com

Iris Brémaud

Joseph Gril

Marie-France Thevenon

Olivier Arnould

Kambiz Pourtahmasi

Effect of extractions on dynamic mechanical properties of white mulberry (Morus alba L.)

Keywords: anisotropy, damping coefficient, DMA, extractives, vibrational properties, white mulberry wood

Vibrational properties of wood are affected by several parameters, of which extractives can be one of the most important ones. Wood for European musical instruments has been often studied, but traditional Middle Eastern ones had been left unnoticed. In this study white mulberry (Morus alba L.), the main material for long-necked lutes in Iran, was extracted by five solvents of various polarities (water included). Free-free bar forced vibrations were used to measure longitudinal (L) loss tangent (tanδ), storage (elastic) modulus (E′) and specific modulus (E′/γ) in the acoustic range. Their anisotropy between the 3 axes of orthotropy was determined by DMA (dynamic mechanical analysis). Native wood had a quite low E L ′/γ but its tanδ was smaller than expected, and the anisotropy of tanδ and E′/γ was very low. Removal of extractives caused tanδ to increase and moduli to decrease. Acetone, the most effective solvent on damping despite a moderate extraction yield, increased tanδ L by at least 20% but did not modify E′/γ as much. When used successively, its effects masked those of solvents used afterwards. Anisotropy of E′/γ was nearly unchanged after extraction in methanol or hot water, while tanδ was much more increased in R than in T direction. Results suggest that in white mulberry, damping is governed more by nature and localization of extractives rather than by their crud abundance.

Introduction:

Wood has always been considered as a multifunctional material. Each of its versatile properties makes it appropriate for a particular final use. Wood historical use in musical instruments is in relation with its dynamic mechanical, or vibrational, properties, including mechanical damping (loss tangent) tanδ, storage (elastic) modulus E′ and specific modulus E′/γ. As a quasi-orthotropic material at the macroscopic scale, wood has different properties in longitudinal (L), radial (R) and tangential (T) directions (Backman and Lindberg 1 ; Nairn 2 ; Brémaud et al [START_REF] Brémaud | Anisotropy of wood vibrational properties: dependence on grain angle and review of literature data[END_REF] ). In the case of wood used for Western classical string instruments, it is generally accepted that a low tanδ and high E′/γ in axial direction, and a high anisotropy, are convenient for obtaining a soundboard of good quality. However, few studies have addressed the vibrational properties of wood used in Middle-East instruments. String instruments in Iran can be traced back to 970 B.C. Long-necked lutes Tar, Setar and Kamancheh are specifically made from white mulberry (Morus alba L.), which is also used for the Japanese short-necked lute Biwa (Yoshikawa 4 ). Sources of variations in dynamic mechanical properties of wood include cell arrangement and grain angle, microfibril angle within the cell wall, proportions of constitutive polymers, as well as the presence of additional compounds (Ono and Norimoto [START_REF] Ono | Study on Young's modulus and internal friction of wood in relation to the evaluation of wood for musical instruments[END_REF][START_REF] Ono | On physical criteria for the selection of wood for soundboards of musical instruments[END_REF] ; Obataya et al [START_REF] Obataya | Vibrational properties of wood along the grain[END_REF] ; Noda et al [START_REF] Noda | Physical and chemical characteristics of the blackened portion of Japanese persimmon (Diospyros kaki)[END_REF] ; Bucur 9 ). Although in lower quantities than cell wall constitutive polymers, extractives can have a considerable effect on mechanical and damping properties of wood. Exceptionally low damping of some musically important woods was somehow correlated with their extractives.

Extractives impact can be studied either through correlations between their amounts and vibrational properties variation resulting by their removal, or by the re-injection of extracts into a "neutral" wood. Methanol extraction increased the tanδ of heartwood by 15 to 37% in red cedar (Thuja plicata) and up to 60% in African Padauk (Pterocarpus soyauxii), while E′/γ slightly decreased (Yano 10 ; Brémaud et al. [START_REF] Brémaud | Effect of extractives on vibrational properties of African Padauk (Pterocarpus soyauxii Taub.)[END_REF] ). Impregnation of water-soluble extractives of pernambuco (Caesalpinia echinata) into spruce decreased tanδ by a half (Matsunaga et al. [START_REF] Matsunaga | Vibrational property changes of spruce wood by impregnation with water-soluble extractives of pernambuco (Guilandina echinata Spreng.)[END_REF] ).

The formation of hydrogen bonds between extractives and cell wall matrix was proposed as a possible cause of tanδ reduction (Matsunaga et al. [START_REF] Matsunaga | Working mechanism of adsorbed water on the vibrational properties of wood impregnated with extractives of pernambuco (Guilandina echinata Spreng.)[END_REF] ). Impregnation of Sitka spruce with isolated key chemical compounds from muirapiranga (Brosimum rubescens), a Moraceae as is mulberry, decreased tanδ by up to 25%, both in axial and radial directions. This was explained by motion restriction of matrix macromolecules due to extractives entering (Minato et al. [START_REF] Minato | Extractives of muirapiranga (Brosimun sp.) and its effects on the vibrational properties of wood[END_REF] ).

Contrary to these studies, water-soluble extractives of Reed (Arundo dunax L.) increased both tanδ and E′ (Obataya and Norimoto 15 ; Obataya et al. [START_REF] Obataya | The effects of watersoluble extractives on the acoustic properties of reed (Arundo donax L.)[END_REF] ). Extractives can also change wood equilibrium moisture content (EMC) (Hernandez 17 ). Higher EMC induces higher tanδ and lower E′/γ (Akitsu et al. [START_REF] Akitsu | Effect of humidity on vibrational properties of chemically modified wood[END_REF] ; Obataya et al. [START_REF] Obataya | The effects of adsorbed water on dynamic mechanical properties of wood[END_REF] ). The potential effect of extractives on vibrational properties could come from their chemical structure, cellular location and relation with basic polymers (Matsunaga et al. [START_REF] Matsunaga | Working mechanism of adsorbed water on the vibrational properties of wood impregnated with extractives of pernambuco (Guilandina echinata Spreng.)[END_REF] ; Brémaud et al. [START_REF] Brémaud | Effect of extractives on vibrational properties of African Padauk (Pterocarpus soyauxii Taub.)[END_REF] ). Removing extractives using solvents of different polarities could give a first idea about their polarity and possible cellular location.

Our work aims at evaluating the dynamic mechanical properties of white mulberry and understanding the possible contribution of extractives. Basic properties of samples from different regions of Iran were compared in a previous study (Se Golpayegani 20 ; Pourtahmasi and Se Golpayegani [START_REF] Pourtahmasi | Introducing Mulberry's wood (Morus alba L.) used in bowl shaped musical instruments of Iran[END_REF] ). In the present article, mulberry wood was extracted in five solvents of different polarities. In order to separate the overlapping effect of various solvents, they were used both as independent extracting agents, and in a successive order on the same specimens, to determine changes in acoustical properties along the grain. Furthermore, the effects on the anisotropy of tanδ, E′ and E′/γ were also studied after extraction by the two solvents with highest yields.

Material and Methods

Material

Mulberry trees were cut in Iran and kept for one month in outdoor conditions. Wood was selected there by a professional instrument maker. Several 500×50×50 mm (L×R×T) rods were cut from the middle of the heartwood and sent to France, in three batches within one year. The 1 st batch (used for powder samples and independent extractions on vibrational specimens) and 2 nd batch (successive extractions on vibrational specimens) came from the same tree; the 3 rd (specimens for DMA) came from another tree.

A portion of each rod was ground, the fraction passing through 40 mesh and retained on 60 mesh sieves was kept for powder extractions (TAPPI standard method T207_cm [START_REF]Water solubility of wood and pulp[END_REF] ). For vibrational tests, the rods were cut into approx. 200 axial specimens of dimensions 150×15×2 mm 3 (L×R×T), from which 77 specimens were kept for extractions and controls (Table 1). For DMA (Dynamic Mechanical Analysis), specimens were cut to a cross section of 4×3 mm 2 and a length of 42-48 mm (Fig. 1). 14 specimens were prepared in each direction of orthotropy leading to a total number of 42.

Conditioning and physical measurements

The specimens were oven dried at 60˚C for 48 h. This procedure ensured that the equilibrium was reached in adsorption. Residual moisture content, as compared with oven drying at 103°C, was around 1.36±0.49%. Specimens were then kept for three weeks in controlled conditions of 20±2˚C and 65±5% RH. This procedure was repeated before any mechanical measurement. Specific gravity γ (both oven dried and air dried) and EMC were recorded. EMC of native wood was of 7.9±0.8%.

Although mechanical measurements could not be run in a climatic chamber, mass change was small: -0.3±0.4% for native state specimens, both during vibrational and DMA tests. For treated specimens, mass change was negligible (-0.1±0.1%) during vibrational tests, and less than during DMA tests (-0.6±0.7%).

Measurement of dynamic mechanical properties

Axial dynamic mechanical properties were measured both by vibrational tests and by DMA, while anisotropy was studied only by DMA.

Non-contact forced bending vibrations of free-free bars

A frequency scan was imposed through an electric magnet facing a tiny steel plate (15-20 mg, a negligible additional weight) glued at one end of each specimen. A laser triangulation sensor measured the displacement. A program developed with LabView® software (Brémaud 23 ) monitored vibration emission and detection. Specific elastic (storage) modulus (E L ′/γ) was calculated from the first resonance frequency by the Euler-Bernoulli formula and elastic modulus (E L ′) was obtained by multiplying E L ′/γ by specific gravity (γ). Damping coefficient was measured using both bandwidth at half power in the frequency domain (or quality factor) and logarithmic decrement of amplitude in the time domain after stopping the vibration. Both measurements shall be equivalent to loss tangent tanδ, if tanδ << 0.1, which is the case for air-dry wood in this temperature/frequency range. Three repetitions were made for each specimen. Resonance frequencies were in the range of 200-400 Hz.

DMA (Dynamic mechanical analyzer)

The viscoelastic behavior of specimens was measured using a BOSE® ELF3230 DMA equipped with tension/compression fatigue grips, a 22 N (±0.17% maximum error) load cell and a high-resolution displacement sensor (1 mm range with ±0.26% maximum error). The specimens were fixed between two clamps with a working distance of 35 mm and tested in tension along their longest direction. The tests were done in purely alternative tension/compression loading controlled in displacement with an amplitude of ±0.0175 mm corresponding to a maximum tension/compression strain of ±0.02% (i.e., within the linear viscoelastic region, see Sun et al. [START_REF] Sun | Dynamic mechanical analysis of dry wood: Linear viscoelastic response region and effects of minor moisture changes[END_REF] ). Using BOSE WinTest® analysis software, E* (norm of the complex modulus), E′ (storage modulus), E′′ (loss modulus) and tanδ (loss tangent) were calculated. Both moduli and tanδ were corrected for the stiffness of the whole apparatus (around 1785 N/mm with a negligible viscous contribution), measured using a stiff steel specimen. Frequency sweeps from 0.1 to 10 Hz were run in triplicate for each specimen, native and then treated.

Extraction procedures

Extractions used solvents of increasing polarity: hexane (HX), dichloromethane (DM), acetone (AC), methanol (ME) and hot water (HW). Most used Soxhlet extractors. Groups of specimens, with similar ranges in properties, were defined after vibrational tests in native state and subjected to different treatments (Table 1).

For extractions on powder and on solid specimens for vibrational tests, extractions were run both in an independent (or parallel) and in a successive (or serial) way. Independent extraction means that a sample is submitted to one extraction in a given solvent, and its properties are measured before and after this single treatment. In this case, hot water extraction from vibrational specimens was run (for 8h) at 70°C (not using Soxhlet). Successive extractions means that a given group of specimens is submitted to extraction, first by the less polar solvent (HX), then oven-dried (brief process: weighted, air-dry stabilized, weighted, mechanically tested, and dried again for solid wood specimens), then extracted by the next solvent (DM), and so on. In this case, a "standard" Soxhlet extraction (<95°C) was run for water (complete description of conditions can be found in table 1). Specimens were dried before being extracted, so that less polar solvents could not enter cell walls. Control specimens underwent only physical steps (drying and stabilization) and were measured in the same time and condition as extracted ones. For determination of extractive content, 3 g of powder were put in a cellulose cartridge, extracted for 8 h, and their oven-dry (48 h at 60°C) weight loss was measured. For solid wood, as color hardly changed during extraction, duration of 12 h was chosen. Based on the yields of extractions in vibrational specimens, ME and HW were applied as independent solvents on DMA specimens. For each anisotropic direction, 4 specimens served as controls, 5 were extracted in ME and 5 in HW (70°C, without Soxhlet). Although hemicelluloses may be partially depolymerized at moderately high temperatures for water-saturated wood (Placet et al [START_REF] Placet | Viscoelastic properties of wood across the grain measured under water-saturated conditions up to 135 °C: evidence of thermal degradation[END_REF] ; Assor et al [START_REF] Assor | Concomitant Changes in Viscoelastic Properties and Amorphous Polymers during the Hydrothermal Treatment of Hardwood and Softwood[END_REF] ), treatment temperatures and duration were sufficiently low to neglect such effect.

Results and Discussion

Properties of native mulberry wood

The specific gravity γ of the mulberry wood under study ranged from 0.45 to 0.61 (Fig. 2). This was significantly lighter than wood from different regions in a previous study (Se Golpayegani 20 ). The 1 st batch (used for independent extractions) was significantly denser than the 2 nd one (used for successive extractions), although they came from the same tree. The 3 rd batch (used for DMA), which came from a different tree, covered a broad range, but its average γ was not significantly different from the 2 nd batch. Differences in density were not clearly related to those in vibrational properties (Fig. 3).

The specific dynamic modulus in L direction of all studied batches of wood was rather low (Fig. 3) and variable (11-18 GPa). Although dispersion was quite large, tanδ L was negatively related to E' L /γ. However, all batches generally had a lower tanδ L than the "standard trend" from Ono and Norimoto [START_REF] Ono | Study on Young's modulus and internal friction of wood in relation to the evaluation of wood for musical instruments[END_REF][START_REF] Ono | On physical criteria for the selection of wood for soundboards of musical instruments[END_REF] , a statistical relationship obtained on 20 softwoods and 30 hardwoods that can be considered as a reference. In the 1 st and 3 rd batches tanδ L was in average 6% and 11% lower than "standard", but with a higher dispersion than in 2 nd batch and in wood from a previous study (Se Golpayegani [START_REF] Golpayegani | Evaluation of mulberry's wood (Morus alba L.), considering its use in making Iranian musical instruments[END_REF] ), for which tanδ was always within the lower range (21% and 23% lower than the standard). tanδ L measured with DMA method was higher than that measured with free-free vibration.

However, it exhibited a decreasing trend against frequency, so that the difference could be attributed to the higher frequency of the free-free vibration. This decrease of tanδ with frequency was also observed in other directions, as well as a slight increase of elastic moduli.

However, the anisotropic ratios remained constant in the observed frequency range. In the following analysis, only the values measured at 10 Hz will be used. Ordering of the different tanδ in the 3 principal directions was the same as previously reported, i.e., tanδ T > tanδ R > tanδ L (Ono and Norimoto 27 ). However, damping anisotropies between the three main directions were small: R/L≈ 1.03, T/L≈ 1.34, and T/R≈ 1.30. These values were lower than those collected from several studies by Brémaud et al. [START_REF] Brémaud | Anisotropy of wood vibrational properties: dependence on grain angle and review of literature data[END_REF] in which the average ratios were of 2.7 (R/L), 2.9 (T/L) and 1.14 (T/R) for hardwoods. Similarly, the anisotropy of E′ agrees with well-known relationships, i.e., E′ L >> E′ R ≥ E′ T , but actual values of ratios (L/R ≈ 4, L/T ≈ 8 and R/T ≈ 1.9) were in the lower range of anisotropy compared with literature reviews (e.g., Guitard and El Amri 28 ,Nairn 2 ). It can be noted that mulberry has a low longitudinal E′ /γ and a very reduced anisotropy when compared with resonance spruce that is used for top plates of Western string instruments, and is closer to maple (used for back and sides) in this respect.

Yield of extraction

In wood powder (Fig. 4a), approximately the same cumulated amount of extractives was removed when applying solvents independently or successively. In solid wood (Fig. 4b), on the contrary, cumulated weight losses differed between these two extraction procedures, suggesting some structural effect on the accessibility to various solvents. This was supported by different extraction yields from axial, radial and tangential DMA specimens: 9, 14 and 18% respectively for ME.

Less polar solvent HX and DM, which should reach only the lumen, removed small and comparable amounts (1.6% and 1.7%) when used independently on powder. Their yields were much smaller in solid wood. When DM was used after HX, its yield was very low, suggesting that HX had already removed most of extractives accessible to apolar solvents. Similarly, AC had a smaller yield when used after HX and DM, suggesting that when used independently, it also removed compounds from lumens. Although AC and ME are thought to be able to solubilize similar types of compounds, ME caused the most exhaustive extraction from solid wood: 6.2% when used independently. Its efficiency was even increased when used after AC (7.14% from solid wood). On the contrary, in powder, HW had the highest yield (8.31%) when used independently, while it had a much smaller yield on solid. This suggests that in solid wood, polar extractives are more easily removed by ME than by HW, as the final cumulated yields were however similar between powder and solid.

Extraction effects on E L ′/γ

As extractions change the mass and specific gravity (Table 2, Table 3) of wood specimens, E L ′/γ values should be corrected for the contribution of extractives to γ:

' ' ap L L ef c a p E E                   , (1)
where (E L ′/γ) ap and γ ap are the apparent measured values, and (E L ′/γ) c is the corrected value taking into account the specific gravity γ ef after the most effective extraction (ME). (E L ′/γ) c should be more representative of an "extractives-free cell wall" modulus, and appeared 0.01 to 1.2 GPa higher than the apparent one (Table 2, Table 3).

Corrected E L ′/γ gradually decreased due to extractions. HX, DM, AC, ME and HW modified E L ′/γ by 0.2%, -0.6%, -5%, -11% and -1.8% respectively after independent extractions (Fig. 5a). Cumulated changes (i.e. relative to native state) after successive extractions followed a similar trend but with a bigger amplitude (Fig. 5b). E L ′ shows a similar gradual decrease.

Decrease in E L ′/γ appears nearly proportional to weight loss for different solvents (Fig. 5b), which suggests that extractives removed by AC, ME and HW bulked the cell wall matrix, and affected elasticity independently of compounds removed by these various solvents.

Changes in tanδ L related to weight loss

Figure 6 shows relative changes in tanδ L caused by each solvent either used individually or after others (successively). Apolar solvents (HX and DM) removed small amounts (approx.

1%) of extractives, probably from the lumens, resulting in small changes in tanδ L . In successive extraction, however, DM increased tanδ L by 14%, which might be linked to previous exposition to HX. On the other hand, in another species (Padauk), DM was able to slightly attain cell wall and increase tanδ L of approx. 20% for approx. 3% extracts removed (Brémaud et al. 11 ).

More polar solvents (AC, ME and HW) are expected to remove higher proportions of cell wall extractives, more susceptible to alter tanδ L . However, the highest change in tanδ L resulted from AC extraction (≈+20% independently or used after DM, ≈+38% for cumulated successive effect), although its weight loss was moderate. ME and HW, despite their higher yields, had smaller effects on tanδ L both in independent and in successive-cumulated order.

When used after AC, ME even reduced tanδ L lower than the value for the previous state (ACextracted, Fig. 6b). On the contrary, in other species, methanol extractives often have a high "anti-damping" effect in wood (Yano HW in successive order could partly reflect drying history of specimens. However, a smaller effect on tanδ L of ME and HW than that of AC was also observed for independent extractions, without cumulated drying cycles. This brings us to the second hypothesis ; (ii) Two kinds of extractives compounds would co-exist in cell walls, some decreasing, and some increasing, tanδ L in native wood. AC would have removed the first ones, so that extraction of remaining, "tanδ L raising", compounds by ME and HW would decrease again the tanδ L of solid wood.

In various woods, methanol is reported to take out non-structural carbohydrates and phenolics (e.g. Rowe and Conner 29 ). White mulberry has quite important content of phenolic extractives (De Rosso et al. [START_REF] De Rosso | Chemical compounds released from five different woods used to make barrels for aging wines and spirits: volatile compounds and polyphenols[END_REF] ). In some woods, polyphenols can form part of the fiber cell wall matrix (Kleist and Bauch 31 ), while in some other woods, they may be either preferentially in vessels, parenchyma walls (Dünisch et al. [START_REF] Dünisch | Wood properties of juvenile and mature heartwood in Robinia pseudoacacia L[END_REF] ), or too much condensed to enter the cell wall substance (Koch and Kleist 33 ). In the later hypothesis, ME might have removed some lumen deposits, resulting in a lower ratio between changes in tanδ L and extraction yield. However, decreases in E′ L /γ after ME or HW suggested cell wall location. Thus, ME could have removed compounds such as sugars or simple phenols, which presence will increase tanδ L (Obayata et al. [START_REF] Obataya | Acoustic properties of a reed (Arundo donax L.) used for the vibrating plate of a clarinet[END_REF] ; Sakai et al. [START_REF] Sakai | Effects of impregnation of simple phenolics and natural polycyclic compounds on physical properties of wood[END_REF] ). A decrease in tanδ L is also observed after ME extraction of Thuja plicata sapwoodwhich should contain non-structural carbohydrateswhereas a completely opposite effect is observed in the heartwood of the same species (Yano 10 ). In mulberry, extraction in HW, following ME, did not bring additional changes in tanδ L , although it further removed ≈4% extractives. In Caesalpinia echinata, water soluble extractives decrease tanδ L in wood, which was first ascribed to their ability to form hydrogen bonds with cell walls components (Matsunaga et al. [START_REF] Matsunaga | Vibrational property changes of spruce wood by impregnation with water-soluble extractives of pernambuco (Guilandina echinata Spreng.)[END_REF] ). But other extractives, without hydroxyl groups and/or insoluble in water, also reduce tanδ L (Minato et al. [START_REF] Minato | Extractives of muirapiranga (Brosimun sp.) and its effects on the vibrational properties of wood[END_REF] ; Brémaud et al. [START_REF] Brémaud | Effect of extractives on vibrational properties of African Padauk (Pterocarpus soyauxii Taub.)[END_REF] ). Finally, differences in tanδ L might be related to changes in wood moisture content (Dunlop 35 ; Obayata et al. [START_REF] Obataya | The effects of adsorbed water on dynamic mechanical properties of wood[END_REF] ;

Inokuchi et al. [START_REF] Inokuchi | Effects of water extractives on the moisture-content dependence of vibrational properties of Bamboo[END_REF] ). However, changes in EMC due to independent extractions were small (Table 2). In successive extractions (Table 3), with higher cumulated yields, EMC was more significantly increased after ME and HW. This, however, should lead to an increase in tanδ L , instead of the observed decrease for successive use of ME and HW.

Therefore, the present results suggest the co-existence of some "tanδlowering" and of some "tanδ -raising" compounds in the heartwood of white mulberry. The first ones are extractable by ME but also with AC and HW, consequently independent extraction with those solvents had always resulted in an increase in tanδ L . The latter compounds, being probably hydrophilic, are not extractable by HX, DM or AC. Thus a successive extraction using ME of variations. Backman and Lindberg 1 stated that, for softwood, in tensile test latewood contributes mostly to the response in tangential direction while most of the strain occurs in early wood in radial direction. In mulberry, a ring-porous hardwood, the response in radial tension tests would more involve vessels and rays, whose cell walls are thinner, but are also susceptible to contain higher proportions of extractives (Koch 38 ; Kleist and Bauch 31 ; Dünisch et al. [START_REF] Dünisch | Wood properties of juvenile and mature heartwood in Robinia pseudoacacia L[END_REF] ). Thus, the very high change in tanδ R after ME extraction (≈ 50%) could express a bigger modification of rays and vessels, than of fibers.

Conclusion

Dynamic mechanical properties E′/γ, tanδ, and their anisotropic ratios in the 3 principal axes were measured on white mulberry wood (Morus alba L.), a representative raw material for musical instruments in Iran. They were compared before and after extractions by different solvents in order to investigate extractives effects. Results could be synthesized as follows:

-Mulberry wood has a quite low E' L /γ but its damping factor (tanδ L ) is lower than expected. The anisotropy of these two properties is also much lower than average.

-Combining two methods of extractions -using each solvent as an individual agent and using solvents in a successive order on a single sampling-could suggest the importance of extractives nature compared to their amount.

-The highest changes in tanδ L were not due to the most polar solvents, with highest extraction yields, but to acetone. Nevertheless, acetone did not alter E′ L /γ with the same intensity, suggesting that the extracted amounts or compounds were not essential for wood stiffness.

and when used successively after acetone, suggest the existence of two types of tanδ altering compounds in white mulberry. Some would be able to increase, some other would be able to reduce the damping, and both types would be accessible by acetone and methanol independently.

-Changes in E′/γ were nearly isotropic between the 3 principal axes of orthotropy, may be due to the very low anisotropy of mulberry in native state. On the contrary, tanδ was much more modified in radial than in tangential direction, suggesting different responses to extractions of rays and vessels walls as compared to fibers ones.

It should be noted that even though extractives are found to definitely affect vibrational properties of Morus alba L., it would be enlightening to identify the most important compounds present in those extractives and how they cause changes in vibrational properties. "Apparent" stands for measured values;; "Corrected" take into account the contribution of extractives to weight (for EMC) and specific gravity (for E'/γ).

Tables

(a, b, c): homogenous subsets in one-way ANOVA at a level α:0.05 (=comparison between groups in a given column, based on measured values).

(ns, *, **): differences between untreated and treated properties of a given group (based on corrected values whenever applicable) in t-test for paired samples; **: significant at α: 0.01, *: α: 0.05, ns: not significant. d) EMC relative to 103°C oven-drying. "Apparent" are measured values;; "Corrected" is calculated by taking into account the total extractive content (cumulated % on powder) from 1 st batch of wood (see Hernandez 2007).

e) "Treated" state for controls stands for values measured after one cycle of oven-drying followed by air-dry restabilization.

f) tanδ L are raw values, whereas tanδ L variations in Fig. 6 are corrected for controls.

Fig. 1 .

 1 Fig. 1. Cutting plan for vibrational and DMA specimens. Variations in length of DMA specimens were due to limitations in raw material.

Fig. 2 .

 2 Fig. 2. Box plot of specimens specific gravity (γ) for the three batches of wood under study, and for previous data on wood from different origins (Se Golpayegani 20 ). (a, b, c) significantly different groups (one-way

Fig. 3 .

 3 Fig.3. Relationship between E′ L /γ and tanδ L for 4 batches of native mulberry wood (N specimens: see Fig.2).

Fig. 4 :

 4 Fig. 4: Comparison of weight loss due to independent and successive extraction in (a) powder, and (b) solid (vibrational) specimens. "Successive" weight losses are relative to the previous step;; "independent" and "successive-cumulated" are relative to native oven-dry weight.

Fig. 5 .

 5 Fig. 5. Relationship between weight loss (%) and relative change of E′ L /γ (%) after (a): independent extractions and (b): successive extractions where changes are relative to initial state of native specimens (i.e. cumulated changes). E′ L /γ was corrected for contribution of extractives to γ.

Fig. 6 .

 6 Fig. 6. Relative changes (%) in tanδ L plotted against weight loss (%) for (a) independent and (b) successive extractions. Changes are relative to initial state of native specimens (independent and successive-cumulated).Arrows: order of successive extractions. Values of tanδ L in independent extractions are corrected for controls.

FiguresFig. 1

 1 Figures

  Fig. 1.

29 Fig. 2 .

 292 Fig. 2.

Fig. 3 . 31 Fig. 4 .

 3314 Fig. 3.

Fig. 5 . 33 Fig. 6 . 34 Fig. 7 .

 5336347 Fig. 5.

Table 1 :

 1 Specimens number and treatments for both independent and successive methods of extraction

	Number of specimens	Solvent	Drying and	Number of cycles of
			stabilization condition	drying and stabilization
	12	Hexane (HX)		2
	12	Dichloromethane (DM)		2
	12	Acetone (AC)		2
	12	Methanol (ME)		2
	5	Hot Water (HW, 70°C)	60°C for 48 h	2
	12	HX/DM/AC/ME/HW (100°C)	followed by	6
	12	Controls	20±2˚C & 65±5% RH for 3weeks	6
	15 DMA(5L, 5R, 5T)	ME		2
	15 DMA(5L, 5R, 5T)	HW (70°C)		2
	12 DMA(4L, 4R, 4T)	Controls		2

Table 2 :

 2 Basic statistics for differences in properties between groups submitted to independent extractions and to one cycle of drying/re-stabilization (=control).

	Treatment	EMC (%) d	Specific gravity γ	E′ L /γ Corrected (GPa)	tanδ L (10 -3 )
		Corrected			E′ L /γ Apparent (GPa)
		Apparent
		Native	Treated	Native	Treated	Native	Treated	Native	Treated f
	Control e	9.2±0.5	9.2±0.2 ns	0.556±0.027	0.553±0.030 ns	15.0±2.2	15.2±2.4 ns	8.6±0.3	7.9±0.5 **
		8.2±0.4	8.2±0.2	(a)	(ab)	14.0±1.6	14.2±1.6	(a)	(a)
		(a)	(ab)			(b)	(a)
	Extracted
	HX	9.1±0.2	8.9±0.4 ns	0.570±0.031	0.566±0.030 **	13.5±2.0	13.7±2.0 **	10.2±0.6	9.6±0.6 **
		8.1±0.2	7.9±0.4	(a)	(b)	12.3±1.4	12.5±1.4	(b)	(bc)
		(a)	(a)			(a)	(a)
	DM	9.0±0.3	9.6±0.4 **	0.560±0.031	0.557±0.031 **	14.3±2.2	14.3±2.2 ns	9.6±1.0	9.3±0.8 *
		8.0±0.3	8.6±0.3	(a)	(b)	13.2±1.6	13.3±1.6	(b)	(bc)
		(a)	(c)			(ab)	(a)
	AC	9.1±0.4	9.4±0.4 *	0.567±0.026	0.550±0.025 **	14.6±1.6	13.9±1.6 **	8.7±0.4	9.8±0.6 **
		8.1±0.3	8.6±0.3	(a)	(ab)	13.3±1.1	13.2±1.1	(a)	(c)
		(a)	(c)			(ab)	(a)
	ME	9.2±0.4	8.7±0.2 **	0.554±0.026	0.520±0.025 **	14.4±2.2	12.9±2.0**	8.9±0.5	9.5±0.5 **
		8.2±0.4	8.3±0.3	(a)	(a)	13.5±1.4	12.9±1.4	(a)	(bc)
		(a)	(bc)			(ab)	(a)
	HW	9.2±0.3	9.0±0.2 ns	0.550±0.015	0.530±0.017 **	14.1±1.7	14.0±1.9 ns	8.5±0.5	8.6±0.6 ns
		8.2±0.3	8.3±0.1	(a)	(ab)	13.3±1.3	13.7±1.7	(a)	(ab)
		(a)	(bc)			(ab)	(a)

Table 3 :

 3 Basic statistics for the successive ranges in properties for the group of specimens submitted to successive extractions."Apparent" stand for measured values;; "Corrected" take into account the contribution of extractives to weight (for EMC) and specific gravity (for E′ /γ). a) EMC relative to 103°C oven-drying. "Apparent" are measured values;; "Corrected" is calculated by taking into account the total extractive content (cumulated % on solid specimens) from 2 nd batch of wood (seeHernandez 2007).

	Treatment EMC (%) a	Specific gravity γ E′ L /γ Corrected (GPa)	tanδ L (10 -3 )
		Corrected		E′ L /γ Apparent (GPa)
		Apparent
	Untreated	7.0± 0.3	0.515± 0.011	13.72± 1.47	8.2± 0.5
		6.1± 0.3		12.99± 1.25
	HX	7.1± 0.3 ns	0.512± 0.016 ns	13.46± 1.68 ns	7.9± 0.4 **
		6.2± 0.2		12.82± 1.35
	DM	9.2± 0.2 **	0.515± 0.012 ns	13.51± 1.26 ns	9.3± 0.6 **
		8.0± 0.2		12.78± 1.07
	AC	9.0± 0.4 ns	0.515± 0.016 ns	13.53± 1.64 ns	11.3± 0.8 **
		8.0± 0.3		12.79± 1.30
	ME	8.6± 0.1 **	0.505± 0.014 **	12.69± 1.38 **	10.6± 0.6 **
		8.2± 0.1		12.24±1.13
	HW	8.8± 0.1 **	0.488± 0.011 **	11.85± 1.23 **	10.2± 0.5 *
		8.8± 0.1		11.84± 1.05

(ns, *, **): differences between each successive extracted state in relation to the previous phase of ttreatment (based on corrected values whenever applicable) in t-test for paired samples; **: significant at α: 0.01, *: α: 0.05, ns: not significant.

Fig. 7. Variations in mechanical properties after extractions measured by DMA at 10 Hz. Values for controls were not deduced from treated ones.

Acknowledgments Authors are grateful to SCAC (Service de Coopération et d'Action Culturelle) of French embassy in Tehran and ISMO (Iranian Center for international research and collaboration) for supporting this project.

and HW had reduced tanδ L , as the "tanδ -lowering" had been already removed by previous solvents and there were only the "tanδ -raising" components left. The methodology using both independent and successive series of extraction might reveal similar trends in other species.

Changes in anisotropic properties due to extractions

Figure 7 shows the variations in E′/γ and tanδ in the three principal directions of orthotropy after extractions with HW and ME or 2 drying cycles (controls). Variations in E′/γ after ME extraction were similar in all three directions: -18% to -20%, although weight losses were different (9%, 14% and 18% for L, R and T specimens respectively). Hot water caused both lower weight losses (6-8% in L, R and T), and smaller decrease in E′/γ along L and R directions, while it was similar to ME in T direction.

Concerning tanδ, uncertainty was high in L direction, as clamping of specimens may crush wood in the softer transverse direction. Even controls showed important variations, which cannot be ascribed to physico-chemical changes. However, after ME extraction, change in radial tanδ was at least twice more important than in L (DMA estimations and changes in vibrational tests) and T direction (Fig. 7).

The quasi-isotropic effect of extractions on E′/γ is quite surprising. In previous works, extractives stiffened the transverse moduli (Yano et al. [START_REF] Yano | Acoustic properties of Brazilian rosewood used for guitar back plate[END_REF] ; Minato et al. [START_REF] Minato | Extractives of muirapiranga (Brosimun sp.) and its effects on the vibrational properties of wood[END_REF] ), but not the axial one, in which the influence of the microfibrillar reinforcement predominates (Ono and Norimoto 5, 6 ; Obataya et al. [START_REF] Obataya | Vibrational properties of wood along the grain[END_REF] ). However, mulberry wood has a low longitudinal E′/γ, suggesting high microfibril angle, and is characterized by a very low anisotropy in native state. Effects of extractions on tanδ were clearly different in radial and tangential directions. This is interesting, as there are relatively few works on the tanδ R /tanδ T anisotropy and sources