Wettability of welded wood-joints investigated by the Wilhelmy method: part 1. Determination of apparent contact angles, swelling, and water sorption

2.1.1 Wettability

The welded specimens were prepared from clear pieces of Scots pine sapwood (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) with planed surfaces and dimensions of 20 mm × 20 mm × 230 mm (radial (R) × tangential (T) × longitudinal (L); Figure 1a). The specimens were conditioned for two weeks at a temperature of 20 °C and 65% relative humidity (RH) in an environmental chamber to 12% average moisture content (MC) and thereafter welded together in pairs to dimensions 20 mm × 40 mm × 230 mm (longitudinal welding of a tangential face to a radial face; Figure 1b). The linear vibration welding machine was a Branson, model M−624 (Branson Ultraschall, Dietzenbach, Germany) that was set to a frequency of 240 Hz and the most important parameter settings listed in Table 1 that could result in a maximum shear strength of the joints.

Figure 1:

Preparation of the welded specimens.

A Wilhelmy plate is a thin, generally rectangular plate which is vertically immersed in the test liquid along one of its larger dimensions. To make plates entirely of welded wood, the welded joints of the specimens should be opened to get access to their welded area. The welded joints of the specimens were opened with a steel chisel (Figure 1c), carefully cleaned with hexane before each use. Welded areas are generally very thin and brittle and cutting them to strips thinner than 0.5 mm in order to eliminate the non-welded wood adjacent the welded areas is difficult. The welded areas were band sawn to strips with a thickness of 0.5 mm (Figure 1d) and two chips 20 mm × 10 mm × 0.5 mm in size were cut from one of the strips (Figure 1e) with a scalpel carefully cleaned with hexane. The chips were glued together on their rear sides using a waterproofing adhesive (Contact A3, Bostik) to make plates (two-sided welded specimens) with dimensions of 20 mm × 10 mm × 1 mm (Figure 1f). The cross sections of the specimens were end-sealed with an alkyd/polyurethane varnish to prevent liquid sorption through these sides of the specimens. Chips with dimensions of 20 mm × 10 mm × 1 mm were cut from non-welded wood adjacent the welded areas to be used as control specimens (Figure 1c). For each group of welded and control specimens, three replicates were tested.

2.1.2 ATR-FTIR spectroscopy

The specimens were prepared from the welded interface of the split welded-joint in Figure 1c. Tiny splints were carefully cut from the welded areas with a scalpel, and similar splints were cut from adjacent non-welded wood to be used as control specimens. The scalpel was carefully cleaned with hexane before each use. For each group of welded and control specimens, three replicates were tested using a Perkin-Elmer frontier FTIR equipment (Waltham, MA, USA) with a frontier UATR ZnSe with reflection top plate and pressure arm (Spectrum 10^TM software) and four scans of a resolution of 4 cm−1 were collected for each replicate at room temperature.

2.1.3 Scanning electron microscopy

After the wettability test, the glued joints of the two-sided specimens (Figure 1f) were opened to examine both surfaces of the specimens. The welded chips were split into small pieces of 10 mm × 10 mm × 0.5 mm with a scalpel carefully cleaned with hexane before each use. The control specimens from the wettability test were also split to small pieces of the same dimensions. All specimens were mounted onto standard metal stubs using carbon paste and sputter coated with a gold layer (20 nm) in a Denton Desk II sputter unit before observation. The specimens were scanned in a JEOL 5200 scanning electron microscope with magnification of 300 000 at 10 kV of accelerating voltage.

2.2.1 Theoretical background

The basic equation of capillarity given by Laplace (1799) and Young (1805) is one of the fundamental equations used to describe surface wetting phenomena. The Young equation was modified later by Dupré and Dupré (1869) to

which is called the Young–Dupré equation or the work of adhesion equation, where YLG is the surface tension of the liquid and θ the apparent contact angle between the liquid and the substrate. The Young–Dupré equation provided a basis for advanced wetting models on rough and chemically heterogeneous surfaces (Oss 1994; Wenzel 1936). Wilhelmy (1863) first proposed indirect contact angle measurement by immersing a plate in a liquid and deriving the angle from the measured force by a simple equation originally developed for measuring the surface tension of a liquid that was later modified for the measurement of wetting forces on wood (Casilla et al. 1981; Gast and Adamson 1997; Neumann and Good 1979). The most updated model of Gast and Adamson (1997) is

where F(h,t) is the wetting force as a function of the depth of immersion h and time t, p the perimeter of the substrate, γl the surface tension of the liquid, θ the apparent contact angle between liquid and substrate, Fw(t) the force due to wicking (capillary action) and sorption of the liquid at time t, ρ the liquid density, A the cross-sectional area of the plate, and g the gravitational constant.

A technique for measuring wetting properties of fibers based on the Wilhelmy principle was reported by Young (1976). The Wilhelmy slide method is based on the principle that a thin plate will support a liquid meniscus whose weight depends upon the surface tension of the liquid and the perimeter of the plate.

2.2.2 The experiment

This study was performed based on the Wilhelmy method described by Casilla Steiner (1981) using a Sigma 70 tensiometer from KSV Instruments with water and heptane as a non-swelling liquid (Mantanis et al. 1994). Since accurate measurement of the perimeter of a porous material like wood is difficult, and the perimeter of the specimen changes during water immersion due to wicking and sorption, it is necessary to also use a non-swelling liquid. Drying the specimens before water immersion to zero MC could lead to opening of the glued joint. Therefore, the specimens were dried at room temperature, 22.5 ± 0.5 °C and 35 ± 5% RH, to 7% MC and weighed immediately before the wettability measurement. Each specimen was mounted in the tensiometer and partially immersed in ultra-pure water at a speed of 12 mm⋅min−1 to minimize wicking effects (Gardner et al. 1991), so that the Fw(t)-term in Eq. (2) could be neglected. After 50 s (corresponding to a depth of immersion of 10 mm), the specimen was withdrawn at the same speed, until the water was detached from the wet solid surface (5 mm above the surface), and thereafter, with no pause, immersed again. Changes in the force as a function of depth of immersion (h=0) were recorded graphically for 20 cycles in water (Figure 2). Thereafter the specimen was immersed and withdrawn for two cycles in a non-swelling liquid (heptane) to determine its perimeter after water absorption. The specimen was again dried at room temperature to the same weight as before the measurement and immersed for another two cycles in heptane to measure its original perimeter. Measuring the original perimeter before water immersion could lead to interference of the heptane in the measurement of contact angle in water. The perimeter of the specimen can be estimated by a linear regression of the receding curve or second advancing curve, on condition that the contact angle of the heptane is zero. If the surface tension of the liquid is known, then the perimeter of the specimen can be estimated by Eq. (2). The contact angles can then be calculated. Vice versa, if the perimeter is known the liquid surface tension can be estimated (Wålinder and Johansson 2001).

Figure 2:

Typical Wilhelmy curves showing force normalized with the sample perimeter (F/P) versus immersion depth (h). The dotted circles show the initial uptake (non-linearity at the beginning of the first cycle), the dashed squares show a drop in the advancing curves, and the rectangles show a dip at the right-hand end of the cycles.

The liquids used were distilled water (surface tension and density of 72.0 ± 0.2 mN⋅m−1 and 0.997 g⋅cm−3, at 25 ∘C) and heptane (surface tension and density of 21.3 mN⋅m−1 and 0.397 g⋅cm−3, at 25 ∘C). The measurements were performed at about 22 °C and 50% RH. Three independent replicates were tested, and fresh liquid substrate was used for each specimen.

The dynamic force measurements were made as described in the literature (Gardner et al. 1991; Mantanis and Young 1997; Wålinder and Johansson 2001) and are shown in Figure 2k. The advancing force FA and receding force FR were measures by linear regression of the advancing and the receding curves, respectively (Figure 2k). Ff is the final force, i. e. equal to the amount of sorbed liquid during a test cycle or Fw(t)-term in Eq. (2) at the time after the rupture of the meniscus (h=0). Fw(t) is assumed to be zero when the specimen touches the liquid in the first test cycle (at h=0).

The advancing contact angle θA and the receding contact angle θR were determined by linear extrapolation of FA, FR, and Ff at zero depth (h=0) in water, respectively

Dynamic sorption by the wood specimens was determined by extrapolation of the final forces Ff to zero depth in both water and heptane curves (Figure 2k), i. e., the force that remains after each cycle due to liquid sorption and wicking (Wålinder and Johansson, 2001). The liquid uptake by the specimens after cycle n was calculated as follows:

where wn (g) is the specimen weight after cycle n∈{0,1,…,N}, w0 (g) the initial weight of the specimen before testing, Ff,n the final force of cycle n (gms−2), and g the gravitational constant (ms−2). By definition, the final force of the zeroth cycle, Ff,0, is zero. The initial and final perimeters of the specimen (after wetting) were directly determined by immersion in heptane, but the specimen perimeter at intermediate cycles was estimated based on a linear combination of the measured force and final change in specimen perimeter (Sedighi Moghaddam et al. 2013):

where Pn and Pn−1 are the perimeters of the specimen after n and n−1 cycles, respectively; Ff,n and Ff,n−1 the final forces of cycle n and cycle n−1, respectively; ΔP the perimeter difference between the initial perimeter P0 (after drying) and the perimeter after the final cycle, PN; and ΔFf the final force difference between the initial and final cycles (ΔFf=Ff,N−Ff,1).

3.1.1 Water uptake

Figure 3 shows the water uptake as functions of the number of cycles. The sorption of water by welded pine and beech wsa respectively 2.5 and 6 times less than that by their controls, in line with previously published statements by Kollmann and Schneider (1963) and by Skaar (1988) that the water uptake in thermally modified wood is lower than that in non-welded wood.

Figure 3:

Average liquid uptake in water and heptane expressed in % of the specimen’s weight vs. cycle number. PW denotes non-welded (control) pine in water, PH non-welded pine in heptane, BW non-welded (control) beech in water, BH non-welded beech in heptane, WP4W welded pine for 4 s in water, WP4H welded pine for 4 s in heptane, WP5W welded pine for 5 s in water, WP5H welded pine for 5 s in heptane, WBW welded beech in water, and WBH welded beech in heptane.

The water sorption of non-welded specimens was initially very high, but it decreased rapidly after some cycles. The average water sorption was however significantly higher than that of the welded specimens. The average water sorption was significantly lower in the welded beech than that in the welded pines (WP4W and WP5W).

The sorption of heptane was also lower in welded than in non-welded specimens and pine specimens welded for 4 and 5 s showed similar heptane sorption (Figure 3, WP4H and WP5H). However, a two-cycle test was not sufficient to distinguish all the wetting kinetic regimes of heptane uptake. Similar heptane sorption indicates a similar micro-morphology for the welded pine specimens.

3.1.2 Contact angle

The multicycle Wilhelmy method was used to investigate dynamic contact angle changes of the welded and non-welded wood specimens after 20 cycles. Figure 4 shows the advancing contact angle, θA, and the receding contact angle, θR, where the terms “advancing” and “receding” refer to the movement of the liquid in relation to the specimen, advancing over a surface to be wetted (immersion), and receding from the same surface after being wetted (emersion; Casilla et al., 1981).

Figure 4:

Average dynamic contact angles of welded and non-welded pine and beech specimens for 20 cycles. P denotes non-welded (control) pine, WP4 welded pine for 4 s, WP5 welded pinefor 5 s, B non-welded (control) beech, and welded beech.

The contact angle of water on the welded specimens did not reach zero until 7, 10, and 20 cycles respectively for pine welded for 4 s (WP4), pine welded for 5 s (WP5), and welded beech (WB), implying that the wood surface did not become completely wet until these cycles. The multiple range test with the 95% Least Significant Difference procedure indicated that the contact angles on the welded specimens were significantly greater than that on the respective non-welded specimens. Welded beech specimens showed a significantly higher contact angle than welded pine specimens (Figure 4). The welded pine specimens welded for a longer time (5 s) showed a significantly higher contact angle than welded pine for 4 s.

The initial value of the contact angle shows the affinity of the wood surface to water and is mainly influenced by the surface structure of the wood species. The dynamic contact angle is additionally directed by the extractives, the capillary and the topographic structure, and the density of the wood (Boehme and Hora 1996).

The magnitude of contact angle hysteresis (difference between advancing and receding contact angles) is dependent on roughness, topography, morphology, and chemical homogeneity of the solid surface. Good (1979) suggested that the advancing contact angle represents hydrophobic areas on the surface, while the receding contact angle characterise hydrophilic areas.

3.1.3 Perimeter change

The relative change in the perimeter of the specimens after each of the 20 cycles of water immersion is presented in Figure 5. The swelling of the welded specimens was significantly lower than that of the non-welded specimens. Non-welded pine and non-welded beech differed in their wetting properties.

Figure 5:

Average perimeter change as a function of time (cycles) for pine and beech specimens. P denotes non-welded (control) pine, B non-welded (control) beech, WP4 welded pine for 4 s, WP5 welded pine for 5 s, and welded beech.

The total amounts of absorbed water were almost the same, but the swelling of the beech was significantly greater than that of the pine. Beech has higher density than pine, which probably explains the higher swelling (Stamm 1935). Welding reduced the water uptake of beech, but the swelling of welded beech was significantly greater than that of welded pine. The swelling of the pine welded for 4 s (WP4) was greater than that of the welded beech, but the swelling of the pine decreased when the welding time was extended to 5 s (WP5). Unexpectedly, welding pine for 4 s did not decrease the swelling compared to non-welded pine, as shown in Figure 5 for cycle six and later.