L De Vetter 
  
J Van Den Bulcke 
email: jan.vandenbulcke@ugent.be
  
J Van Acker 
  
Envelope treatment of wood based materials with concentrated organosilicons

Keywords: Oberflächenbehandlung von Holzwerkstoffen mit konzentrierten Organosiloxanen Um die Nutzungsdauer von Holz und Holzwerkstoffen zu verlängern, ist es wichtig, der Aufnahme von Feuchtigkeit entgegenzuwirken. Untersucht wird die Verwendung von lösemittel-sowie wasserbasierten Organosiloxanen zur Oberflächenbehandlung dieser Materialien. Dazu wurden Organosiloxane auf vier verschiedene Werkstoffe, nämlich OSB und MDF sowie Massivholz und Sperrholz, aufgetragen einer Reihe von Wasserlagerungs-und Tauchversuchen und künstlicher Bewitterung unterzogen, um die hydrophobierende Wirkung zu bestimmen. Die Oberflächenbehandlung mit Organosiloxanen zeigte Potential, die wasserabstoßenden Eigenschaften von Massivholz und Sperrholz zu verbessern, 2 allerdings mit reduzierter Wirkung nach künstlicher Bewitterung

Counteracting moisture ingress in wood and wood based products is crucial for service life prolongation. The use of solvent-as well as water-based organosilicons as envelope treatments on such materials is investigated. Therefore, organosilicons were applied onto four different substrates like OSB and MDF as well as solid wood and plywood. Both treated as well as untreated materials were subjected to a series of floating and imbibition protocols in combination with artificial weathering in order to quantify their hydrophobing action. Envelope treatments of organosilicons show potential in improving water repellent characteristics of solid wood and plywood but have a reduced effectiveness after artificial weathering. The wood based panels OSB and MDF behave opposite. Further research is needed to validate hypotheses on this.

Introduction

Wood and wood products are susceptible to physical and biological degradation when exposed outdoors. Moisture plays an important role in this degradation process. Lowering the water and water vapour uptake is one method to protect the material. This can be accomplished by modifying the material, making it less hydrophilic or by creating a protective envelope around it, inhibiting moisture ingress.

Since organosilicons are hydrophobing agents, they are supposed to interact in the woodwater relationship, leading to lower moisture uptake by the wood products. This should be beneficial for the service life of the wooden commodity. [START_REF] Donath | Creating water repellent effects on wood by treatment with silanes[END_REF] reported that silanes have a positive effect on water repellence and Hager (1995) found a reduction of water uptake by 70 % for silicone treated spruce. Previously limited evidence was found that organosilicons applied at low concentrations lead to lower water sorption (both liquid and water vapour) of wood, especially when tested and evaluated under laboratory conditions (De Vetter 2009). Higher concentrations of organosilicons increase protection, but since De Vetter et al. (2009) have already indicated that these concentrations are no longer economically feasible for full wood matrix treatment, the application was limited to the envelope or the edges of the material.

Although moisture related properties of wood based products and the effects of envelope treatments on them have been investigated before, no literature references were found concerning the influence of organosilicons as envelope treatments on these building materials. It was considered valuable to investigate whether organosilicons, when applied at high concentrations, could induce a change in the wood-water relationship. Therefore, all treated and untreated specimens were subjected to a series of floating and imbibition tests in combination with artificial weathering. Solid wood is often applied under use class 3 conditions, whereas swelling of the edges of wood based panels may occur when applied as e.g. wood flooring.

Materials and methods

Wood materials and organosilicon products

In addition to solid Scots pine sapwood (Pinus sylvestris L.), four kinds of plywood and some wood based materials like medium density fiberboard (MDF) and oriented strand board (OSB) were included. While the MDF-HLS was glued with melamine urea formaldehyde resin (MUF), the OSB/3 was glued with polymeric diphenylmethane isocyanate (pMDI) (EN 13986, 2002). The plywoods were made of poplar (11 plies, 15 mm thick), spruce (5 plies, 15 mm thick), birch (11 plies, 15 mm thick) and maritime pine (7 plies, 17 mm thick). All plywoods were glued with phenol formaldehyde glue (PF), except for poplar plywood which was glued with MUF.

A broad range of water-and solvent-based organosilicons was applied. An overview of their components and active ingredient concentration is given in Table 1. Micro-emulsion W4 was purchased from Wacker-Chemie GmbH (Germany) and all other organosilicons were obtained from Dow Corning Corporation (Seneffe, Belgium).

To obtain highly concentrated envelope treatments, the organosilicons were brush applied twice onto the wood substrates as supplied (without dilution). Weighing the specimens prior to and after brushing yields the product retention (Eq. 1).

( ) ( ) ( )

² ² cm area g m g m m g product n applicatio before n applicatio after ret - = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ (1)
Floating test -envelope experiment (focus on Scots pine sapwood)

The organosilicons W1, W2, W3, W4, W5, S1, and S2 were brushed twice onto Scots pine sapwood specimens measuring 40 × 15 × 320 mm³ (R× T × L). OSB specimens (18 mm thick) were treated with W2 or W4, while the plywood specimens were only treated with organosilicon W2. All boards and half of the Scots pine sapwood specimens were conditioned at 20 °C and 65 % RH, while the other half of the Scots pine sapwood specimens was conditioned until fiber saturation point (FSP) prior to treatment. Since [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF] proved that the liquid moisture coefficient of wood (eastern white pine) is temperature dependent all handling was performed in a conditioned room (20 °C, 65 % RH). Next, the specimens, sealed at all edges, were floated onto demineralised water. At fixed intervals up to four hours the increase in mass was recorded after blotting of the specimens on drying tissue. Finally, the specimens were removed from the water and put with their longest coated face onto a rack, allowing desorption to occur at 20 °C and 65% RH.

After one night of desorption, the mass of the specimens was determined again.

Subsequently, the specimens were weathered in a UV cabinet with spray option (ATLAS UVCON). The first weathering cycle was a so-called wet cycle A. During this cycle the specimens were illuminated (λ = 340 nm) for 23 hours in combination with 1 hour without illumination; this 24-hour cycle was then repeated. The spray nozzles were turned on and off according to the diagram shown in Figure 1. After 144 hours the specimens were subjected to a second floating test, followed by a dry artificial weathering cycle B and consecutively a third floating test. During the weathering cycle B the specimens were illuminated constantly while the spray nozzles followed the spraying regime as depicted in Figure 1. The floating test consisted of a 4 hour sorption period, followed by 20 hours of desorption under conditioned circumstances (20 °C, 65 % RH). The specimens were weighed after 5s, 5, 10 min, 1, 2, 4, 5, and 24 hours. This process of alternating floating and weathering was performed several times. At the end of the experiment, the specimens were oven dried (103°C), allowing to calculate the moisture content at each interval (Eq.2).

(

( ) ( ) ( ) 100 % 103 103 × - = °°g m g m g m MC C C t t (2) ) 
Imbibition test -concentrated edge experiment (focus on wood based panels) Supplementary to this transverse water sorption and desorption, an imbibition test was performed starting with concentrated edge treatments. In this test set-up small blocks measuring 15 mm wide and 50 mm long (Figure 2) were cut from at least three MDF boards (15 mm thick), three OSB boards (18 mm thick) and three boards of the four plywood types (15 or 17 mm thick). As reference, Scots pine sapwood specimens (15 × 15 × 50 mm³, R × T × L) were included. [START_REF] Lukowsky | Some aspects of testing water repellents[END_REF] proved that sealing the side edges of solid wood blocks or treating them with a water repellent prevented superficial creeping at the sides. Therefore, water penetrated the samples only via the unsealed areas in direct contact with water. Since it was the purpose to simulate full size boards and evaluate the protective effectiveness of the water repellents only on the exposed side, all surfaces except for the exposed side and its opposite side were sealed with two coats of a white two-component polyurethane edge sealant system. This opposite side was left unsealed, to avoid pressure build-up of entrapped air which might slow down the water absorption [START_REF] Candanedo | Numerical simulation of water absorption in softwood[END_REF] (Figure 2).

After conditioning at 20 °C and 65 % RH, one of the two unsealed edges of each block was brushed twice with one of the pure products W2, W3 or W5. Consequently the specimens were subjected to an imbibition experiment simulating the water absorption caused by wind-driven rain on the façade or exterior cladding of a building envelope [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF]. Therefore, the specimens were put with their treated edge onto a stainless steel grid in contact with a shallow layer of demineralised water, this way avoiding internal pressure [START_REF] Lukowsky | Some aspects of testing water repellents[END_REF][START_REF] Candanedo | Numerical simulation of water absorption in softwood[END_REF]. The mass and width of each specimen was determined prior to water contact and after 5 s, 5, 10 min, 1, 2 and 4 h. The specimens were removed from the water after 4 hours of sorption and allowed to dry under constant temperature (20 °C) and humidity conditions (65 % RH). They were reweighed and measured after 1, 20 and 44 h desorption. All these values were then used to calculate the MC (Eq. 2), the water repellent effectiveness WRE based on relative water uptake (RWU) (Eqs. 3 and 4) and the swelling in the middle of each specimen (Eq. 5) at different times t.

( )

100 % × - = untreated treated untreated RWU RWU RWU WRE (3) ( ) ( ) ( ) ( ) 100 % × - = g m g m g m RWU untreated dry dry wet (4) ( ) ( ) ( ) ( ) mm width mm width mm width swelling t t t t 0 0 % - = (5) 
During sorption water penetrates the wood block by different forms of diffusion and capillary forces (de Meijer and Militz 2000; [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF]. The moisture moves up in the block and finally reaches the opposite side. From that moment the wood block is capillary saturated and moisture absorption is the result of diffusion, dissolution and removal of air inclusions in the water [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF][START_REF] Roels | Interlaboratory comparison of hygric properties of porous building materials[END_REF][START_REF] Hens | Building physics -Heat, air and moisture. Fundamentals and engineering methods with examples and exercises[END_REF]). These two phases can clearly be distinguished from each other when the water influx per square meter contact surface is plotted versus the square root of time.

Both phases have a linear relationship being typical for hygroscopic porous materials, but with a distinctly different slope [START_REF] Krus | Porosity and liquid absorption of cement paste[END_REF]. The intersection defines the capillary saturated volumetric moisture content w c (kg/m 3 ) and is a net material property. The slope of the curve in the first phase is called the water absorption

coefficient A w [kg/(m² s 1/2 )]
and is a composite property indicating how easily a material absorbs water. When in direct contact with liquid water, this water absorption coefficient determines, together with the moisture-liquid diffusivity D w (m²/s), the moisture transport and the redistribution of the moisture into the wood [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF]. When assuming constant diffusivity and taking the shape of the advancing moisture front into consideration these two parameters can be linked to each other as given in Equation 6( [START_REF] Kumaran | Moisture diffusivity of building materials from water absorption measurements[END_REF][START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF][START_REF] Candanedo | Numerical simulation of water absorption in softwood[END_REF]. Although liquid diffusivity D w is assumed to be constant, [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF] proved that D w of untreated eastern white pine increased with increasing temperature. That is why this test set-up was performed under constant climatic conditions (20 °C and 65 % RH).

Since A w might be influenced by the superficial treatment with organosilicons it was calculated according to Equation 7 [START_REF] Kumaran | Moisture diffusivity of building materials from water absorption measurements[END_REF][START_REF] Candanedo | Numerical simulation of water absorption in softwood[END_REF].

( )

2 2 1 ³ ² 4 ² ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = m kg w s m kg A s m D c w w π (6) ( ) ( ) ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × - = ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ 2 1 2 1 ² ² s m time area contact kg m kg m s m kg A initial time w (7)

Results

Floating test -envelope experiment with Scots pine sapwood

The product retentions of the envelope treatment of Scots pine sapwood are given in Table 2. The specimens were floated onto demineralised water prior to and after weathering for one week (144 h) in an ATLAS UVCON. Figure 3 shows that prior to weathering all organosilicons were able to lower the MC significantly, while this was only valid for some of the treatments after weathering.

In the next experiment pure products were applied onto fibre saturated wood. Figure 4 shows the resulting MC over five floating tests alternated with artificial weathering.

Water has accumulated in the wood during this process of artificial weathering and floating. Just like in the first floating test prior to weathering all products show their ability to lower the MC of the wood by up to 60 %. Afterwards half of the products are losing their effectiveness and the MC is gradually approaching that of untreated Scots pine sapwood. The other products continue to lower the MC by 30 to 70 % even after more than 696 hours of alternating floating and weathering. The solvent-based products S1 and S2 perform best, followed by the emulsions W4 and W5.

Due to the promising results obtained for protecting solid wood, some organosilicons were applied onto the envelope of plywood and OSB. In Figures 5 and6 the MC during floating prior to and after one cycle of artificial weathering of untreated and organosilicon treated plywood and OSB is illustrated. The plywood specimens were brushed twice with product W2, while OSB was treated with either W2 or W4. Figure 5 shows that for OSB only slight improvements (15-30 %) can be observed, while for plywood (Figure 6) even lower impacts must be noted (< 20 %). However, it must be stressed that for Scots pine sapwood better results were obtained with other organosilicons, indicating that better performance might also be achieved on plywood and OSB when those other organosilicons would have been applied.

Imbibition test -concentrated edge experiment (focus on wood based panels)

Following the superficial application of organosilicons at high concentrations onto solid wood and wood based panels, a concentrated edge experiment was set up. Table 3 gives the obtained product retentions and shows that for Scots pine sapwood this retention does not differ greatly among the organosilicons, while it does for poplar and birch plywood.

These differences are however small compared to the large organosilicon dependent product retentions noted for the wood based materials OSB and especially MDF. For plywood the graphs are not univocal (Figures 8a and8b). While the MC is lowered when organosilicons were applied, the swelling is only reduced for the products W3 or W5. Application of W2 leads to swelling which is at least as high as the one of untreated plywood. As expected the WRE values for MDF and OSB are negative, while WRE values of 50 to 65 % are observed for Scots pine sapwood after 4 hours sorption. For plywood WRE-values range from 10 to 70 %, depending both on the organosilicon applied and on the kind of plywood (data not presented).

Table 4 lists the resulting water absorption coefficients (A w ) for untreated and organosilicon treated Scots pine sapwood, plywood, OSB, and MDF. As expected the saturated capillary moisture content (w c ) of Scots pine sapwood was not reached within 4 hours of sorption. This means that the sorption was still in the first phase and all data points of cumulative inflow versus square root of time were used in the linear regression as the basis for the determination of the water absorption capacity. This was also valid for untreated OSB and MDF. On the other hand, the untreated plywoods reached the saturated capillary moisture content (w c ) already after circa 10 to 115 min, depending on the type of plywood. Therefore, a distinction had to be made between the first and second phase of sorption based on visual inspection. Only those points belonging to the first phase were used in the regression analysis for calculation of the water absorption coefficient.

Table 4 shows that all organosilicons lower A w considerably for solid Scots pine sapwood. This is to a lesser extent the case for all plywoods, except for poplar plywood treated with W3. For the three other types of plywood best performance was achieved with product W5, followed by W3 and W2. The reduction of the water absorption coefficient for all three plywoods treated with W5 and maritime pine and spruce plywood treated with W3 was such that w c was not reached within 4 hours of sorption.

In contrast, the wood based materials OSB and MDF behaved opposite. While their A w values were the lowest of all untreated specimens, these values increased after application of organosilicons. This increase was so dramatic for OSB treated with W3 and for MDF treated with all organosilicons that they reached the saturated capillary moisture content already after 10 to 70 min.

Discussion

The results of the envelope treatment confirm the statements for solid wood (De Vetter 2009): (1) organosilicons are able to lower the moisture uptake of treated wood, (2) but the effectiveness of most organosilicons is reduced significantly by weathering.

Extrapolation towards wood based materials is however not straightforward. [START_REF] Nofal | Behaviour of engineered wood materials under the effect of wetting and drying cycles[END_REF] examined the behaviour of OSB in a cyclic wetting and drying experiment. They found that sealing the edges of panels reduced the MC by 5 % and slowed down the board thickness increase. This is important since thickness increase affects the volume of air cavity when the boards are used in e.g. a wall assembly and may subsequently affect its functionality.

For the floating test with edge sealed specimens a reduction in MC of up to 15 % was recorded when the boards were brushed with an organosilicon. However, comparing Figure 5 with Figures 3 and4 shows that OSB boards in direct contact with water during the floating test have a moisture uptake which is several times higher than for solid Scots pine sapwood. In contrast this is not the case when the edges of untreated OSB and MDF are considered and compared to the transverse surface of untreated Scots pine sapwood (Figure 7). Although it is already known that the thickness swell of OSB is higher than the swelling of solid wood due to release of compaction stress created during pressing, this should be seen in its proper context. At the beginning of the edge swelling test, Scots pine sapwood swells fast, but also reaches its maximum soon (Figure 8). OSB and MDF in contrast have a continuous increase in swelling during the whole sorption period and continue to swell at the beginning of the desorption stage. Initially OSB and MDF are less swollen than Scots pine sapwood, but after approximately 2 and 3.5 hours of sorption, respectively, this is reversed. Taking the higher moisture uptake and ultimate increased swelling into consideration, it is not surprising that negative WRE values were recorded for OSB and MDF (data not presented).

It has been proven before that the moisture content of wood based panels exposed to direct wetting is influenced by wetting time and by panel characteristics that affect capillarity such as wood species and wax additives of OSB. Figure 6 shows that the organosilicon W2 (40 % DMS/n-OTES) succeeds in a slight reduction (< 20 %) of the MC of each type of plywood except for birch plywood. Focussing on the edges of the boards, reductions of MC between 5 and 50 % can be achieved, depending on the plywood type and organosilicon applied (Figure 8). The order of magnitude of MC and swelling of plywood during the envelope and edge swelling tests are comparable to that of Scots pine sapwood. In contrast to OSB not all organosilicons succeed in reducing the swelling of plywood. Between the different plywood types differences in MC and swelling are seen, which cannot directly be linked to panel characteristics like wood species, thickness and number of plies, glue type, etc.

As stated above the water absorption coefficient A w governs the liquid moisture movement into a material. For wood used in use class 3 conditions, e.g. cladding as part of a building envelope, it is of utmost importance to determine this water absorption coefficient as it is one of the most important hygrothermal properties necessary to deal with the overall moisture management of a building envelope [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF]. Up to now A w was determined for coatings as such or coatings applied on wood [START_REF] Hora | The dynamic contact angle. A characteristic to predict the lifetime of a wood topcoat[END_REF][START_REF] Ahola | Water protection of wooden window joinery with low organic solvent content paints with known composition[END_REF][START_REF] De Meijer | Moisture transport in coated wood. Part 1: Analysis of sorption rates and moisture content profiles in spruce during liquid water uptake[END_REF] and for untreated softwood [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF][START_REF] Candanedo | Numerical simulation of water absorption in softwood[END_REF][START_REF] Miniotaite | Hygric properties of building materials[END_REF] but not yet for wood treated with organosilicons.

The second sorption stage was not reached for Scots pine sapwood in this edge imbibition test. This means that the moisture content was still below the capillary saturated moisture content. [START_REF] Mukhopadhyaya | Effect of surface temperature on water absorption coefficient of building materials[END_REF] found a water absorption coefficient at a water bath temperature of 21 °C of on average 112.0 . 10 -4 kg/(m 2 s 1/2 ). [START_REF] Miniotaite | Hygric properties of building materials[END_REF] and [START_REF] Kumaran | Moisture diffusivity of building materials from water absorption measurements[END_REF] both reported an A w value for spruce equal to 96.0 . 10 -4 kg/(m 2 s 1/2 ). The water bath temperature was 22 °C in the first report. [START_REF] Candanedo | Numerical simulation of water absorption in softwood[END_REF] found A w values between 96.6 . 10 -4 and 156 . 10 -4 kg/(m 2 s 1/2 ) for undefined longitudinally sawn softwood. Logically they found lower values for radially [19.4 . 10 -4 kg/(m 2 s 1/2 )] or tangentially [26.5 . 10 -4 -28.6 . 10 -4 kg/(m 2 s 1/2 )] sawn specimens.

The average A w value found in this research [210.5 . 10 -4 kg/(m 2 s 1/2 )] for radial/tangential Scots pine sapwood was in the same order of magnitude, but higher than the previously mentioned values. Application of organosilicons lowers this water absorption coefficient three to six times, but still remains higher than literature values reported for transverse sections of softwood. [START_REF] Ahola | Water protection of wooden window joinery with low organic solvent content paints with known composition[END_REF] determined water absorption coefficients for coatings on spruce and pine and reported values between circa 50 . 10 -4 and 40 . 10 -4 kg/(m 2 h 1/2 ), much lower than the obtained values in this research for organosilicon treated wood. This means that coatings are better in reducing the water absorption coefficient of Scots pine than organosilicons. It is suggested that moisture movement through a coating is controlled by diffusion and not by capillarity [START_REF] Ahola | Water protection of wooden window joinery with low organic solvent content paints with known composition[END_REF]) while capillary liquid flow is important in uncoated wood after longer exposure times [START_REF] De Meijer | Moisture transport in coated wood. Part 1: Analysis of sorption rates and moisture content profiles in spruce during liquid water uptake[END_REF].

The mode of action of the organosilicons may be similar when applied onto earth substrates. As such the silicone can adsorb on the substrate capillaries to form a thin hydrophobic polysiloxane film which can significantly increase the contact angle of water with the substrate capillary wall and reduce the water absorption of the substrate through capillary depression [START_REF] Noll | Chemistry and technology of silicones[END_REF][START_REF] Ren | Study of water repellent effect of earth substrates impregnated with water-based silicones[END_REF]. Since positive correlations between contact angles of water on coated wood and water absorption rates have been found before, this seems a plausible explanation [START_REF] Hora | The dynamic contact angle. A characteristic to predict the lifetime of a wood topcoat[END_REF][START_REF] De Meijer | Moisture transport in coated wood. Part 1: Analysis of sorption rates and moisture content profiles in spruce during liquid water uptake[END_REF]. It was stated before that water repellents, meant to lower the velocity of water uptake and not the maximum water uptake, can cause a reduction of the capillary tension of wood and even a capillary depression [START_REF] Lukowsky | Water-based silicones on wood[END_REF]. [START_REF] Vetter | Detection and distribution analysis of organosilicon compounds in wood by means of SEM-EDX and micro-CT[END_REF] found a discontinuous micro-porous network on the cell walls and in the lumens of tracheids and wood rays of Scots pine sapwood after dipping in a low concentrated organosilicon. It can therefore be assumed that highly concentrated organosilicon at the surface could lead to a thin film on the surface, leading to increased contact angle and lower water absorption coefficient. Since the formation of a polysiloxane film could be crucial in explaining the behaviour of the treated material, further investigation should be performed. This can be done by doping the organosilicon prior to application and then combining image analysis (SEM or µ-CT) with elementary compositional analysis (EDX) as described in [START_REF] Vetter | Detection and distribution analysis of organosilicon compounds in wood by means of SEM-EDX and micro-CT[END_REF]. While also wavelength-dispersive spectrometry [START_REF] Niu | Microstructure and properties characterization of silicon coatings prepared by vacuum plasma spraying technology[END_REF] 

Figures

  Figures 7a and 7bshow the moisture contents and swelling of (un)treated Scots pine sapwood, OSB and MDF. These figures indicate a reduction of moisture content and swelling during sorption and desorption for organosilicon treated Scots pine sapwood, while increased moisture content and swelling is seen for organosilicon treated OSB and MDF compared to their untreated counterparts. Although a fast reduction in moisture content is noted once the specimens were removed from the water, the swelling of most specimens continued for at least another hour.

Figure 1 :Figure 2 :Figure 4 :Figure 5 :Figure 6 :

 12456 Figure 1: Spray nozzle cycles during 24 hours weathering in an ATLAS UVCON (minutes) Figure 2: Picture of an edge experiment specimen Figure 3: Moisture content during sorption (S) and desorption (D) prior to and after artificial weathering (W) of organosilicon treated Scots pine sapwood preconditioned at 20 °C and 65 % RH Figure 4: Moisture content during sorption (S), desorption (D) and artificial weathering (W) of organosilicon treated Scots pine sapwood pre-conditioned at fiber saturation point. Weathering consisted of alternating a wetter (Ww) and dryer cycle (Wd) Figure 5: Moisture content of untreated and with organosilicon W2 or W4 treated OSB during 4 hours sorption (S) and 19 hours desorption (D) prior to and after 144 h artificial weathering (W) Figure 6: Moisture content of untreated (continuous line) and with organosilicon W2 treated plywood (dashed line) 4h sorption (S) and 19h desorption (D) prior to and after 144h artificial weathering (W) Figure 7: (a) Moisture content and (b) edge swelling during sorption (S) and desorption (D) of MDF (M), OSB (O) and Scots pine sapwood (SP), either left untreated or brushed with organosilicons W2, W3 or W5 Figure 8: (a) Moisture content and (b) edge swelling during sorption (S) and desorption (D) of poplar plywood (P P), spruce plywood (P S), Maritime pine plywood (P MP) and birch plywood (P B). The specimens were either left untreated or treated with organosilicons W2, W3 or W5

  Fig2

  

  

  

  

  

  

  

Table 1 :

 1 Overview of the products used with their main components and active ingredient

	concentrations		
	Product code	Main components	Concentration active ingredient (%)
	Water-based organosilicons	
	W1	n-OTES	40
	W2	DMS/n-OTES	40
	W3	PDMS	60
	W4	PDMS/TES	100
	W5	PDMS	60
	Solvent-based organosilicons	
	S1	n-OTES	100
	S2	MTM/n-OTES	100

Table 2 :

 2 Product retention (g/m 2 ) of Scots pine sapwood pre-conditioned under different circumstances and brushed twice with seven undiluted organosilicons

	Organosilicons	20 °C, 65 % RH	Fiber saturated
	W1	-	145 ± 21
	W2	148 ± 33	164 ± 51
	W3	215 ± 54	131 ± 38
	W4	136 ± 43	115 ± 41
	W5	167 ± 17	155 ± 16
	S1	179 ± 16	147 ± 32
	S2	158 ± 4	141 ± 34

Table 3 :

 3 Product retention (g/m 2 ) of wood based materials brushed twice with the undiluted organosilicons W2, W3 or W5

	Wood based material	W2	W3	W5
	Scots pine sapwood	175 ± 23	184 ± 31	134 ± 15
	Poplar plywood	295 ± 40	549 ± 60	289 ± 111
	Spruce plywood	298 ± 25	705 ± 52	1368 ± 149
	Maritime pine plywood	352 ± 67	630 ± 62	547 ± 90
	Birch plywood	315 ± 53	345 ± 63	296 ± 92
	OSB	209 ± 23	238 ± 45	213 ± 24
	MDF	229 ± 54	401 ± 51	329 ± 35

Table 4 :

 4 Water absorption coefficient A w [10 -4 kg/(m² s 1/2 )] for different wood materials either untreated or brush-treated with organosilicon W2, W3 or W5. Values given in italics did not reach the saturated capillary moisture content w c during the experiment

	Wood based	Control	W2	W3	W5
	material				
	Scots pine	210.5	73	115.8	83.1
	sapwood				
	Poplar plywood 533.4	344.9	619.4	385.9
	Spruce	436.6	307.4	82.8	71.7
	plywood				
	Maritime pine	142.1	130.0	50.5	36.7
	plywood				
	Birch plywood	750.4	505.9	380.2	189.9
	OSB	58.6	266.6	1202.2	189.3
	MDF	91.2	1524.2	3582.9	1636.1

Acknowledgements

Since the European Commission supported the research project HYDROPHOB (QLK5-CT-2002-01439), which was the framework for this study, the authors would like to thank the Commission for the financial support. The authors also wish to thank the Fund for Scientific Research-Flanders (FWO -Belgium) for the postdoctoral funding granted to the second author.

The authors declare that they have no conflict of interest.

Produktaufnahme (g/m²) von unterschiedlich klimatisiertem Kiefernsplintholz, das zweimal mit sieben unverdünnten Organosiloxanen bestrichen wurde. Tabelle 3 Produktaufnahme (g/m²) von Holzwerkstoffen, die zweimal mit unverdünntem Organosiloxan W2, W3 und W5 bestrichen wurden Tabelle 4 Wasseraufnahmekoeffizient A w [10 -4 kg/(m² s 1/2 )] von Kiefernsplintholz und verschiedenen Holzwerkstoffen, entweder unbehandelt oder mit den Organosiloxanen W2, W3 oder W5 bestrichen. Die Werte in Kursivschrift haben während des Versuchs den gesättigten Kapillarfeuchtegehalt w c nicht erreicht