Exploratory study on the impregnation of Scots pine sapwood (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) with different hot melting waxes

Figures 1, 2, 4 and 6 present the results of lateral wax penetration in pine sapwood and beech. The impregnations presented in Fig. 1 and 2 were carried out under a vacuum-pressure process (8 bar) at 100°C for 47 min. European beech is difficult to impregnate with liquid wax via the lateral surfaces, in contrast to pine sapwood (Fig. 4). Within pine, significant differences between the radial and tangential directions are observed (Fig. 1, 4 and 6). The deeper penetration depth in the radial direction of pine can be explained by Buro and Buro (1959) and Scholz et al. (2010a). These authors observed wax deposits in the ray tracheids of pine. For this reason, the conductance of ray tracheids explains the better radial wax penetration into pine sapwood.

However, the radial and tangential penetrations in beech are quite similar to each other (Fig. 2 and 5) and show much lower penetration depth compared to that of pine. According to the cited authors, this could be explained by the absence of conductance for liquids via the parenchyma tissue (Buro and Buro 1959; Scholz et al. 2010a). On the other hand, it is more difficult for non-polar particles such as carbon (Murmanis and Chudnoff 1979) or wax to cross the hardwood pits due to their membrane structure (Scholz et al. 2010a). In Fig. 2, the penetration of waxes with different polarity and viscosity are demonstrated. Wax 4 has no polarity and a viscosity of 9.4 mPa s at 100°C. The acid number of wax 1 lies between 15 and 20 mg KOH g⁻¹, and it has a viscosity of 28 mPa s (q.v. Scholz et al. 2010a). The importance of the polarity is obvious for wax 3 and wax 2 (Fig. 2). Although it has a slightly lower viscosity and comparable molecule size (Table 3, Scholz et al. 2010a), wax 2 does not penetrate across the lateral surface like wax 3. This can possibly be explained by an influence of polar groups such as ester bonds or free carboxyl groups and their reactivity with each other and with the hydroxyl groups of the cell wall.

The results of the impregnations on the semi-industrial scale are presented in Fig. 3, 4, 5 and 6. Waxes penetrate wood longitudinally via tracheids, fibers and vessels like other fluids too. The effective capillary radius influences the penetrating fluid volume exponentially (exponent 4) according to Poiseuille’s equation. The longitudinal penetration of softwood from tracheid to tracheid across bordered pits is marginal compared to the radial direction (Siau 1984). The vessels of hardwood possess a boundless extension due to the perforated cross-walls (Suchsland 1958). The standard deviations as well as Fig. 4a and 4b indicate that the wax uptake in pine sapwood is more homogeneous in comparison with beech. The penetration depth increases with pressure and temperature. The longitudinal uptake of wax in pine ranges between 5 and 11% (24–57 kg m⁻³) (4–12 bar) at 100°C and between 6 and 16% (30–78 kg m⁻³) at 120°C. Fig. 5 demonstrates that the penetration depth of wax in beech within 47 min exceeds the one in pine only marginally despite the presence of much longer vessel strands with a length between 800 and 2,000 mm (Braun 1970; Grosser 1977; Wagenführ 1984). Macroscopic observations showed that only a few vessels were completely filled with wax over the total specimen length of 1,000 mm. That phenomenon could be explained by the low occurrence of long vessels in distinct diffuse-porous wood species (Skene and Balodis 1968; Middleton and Butterfield 1990). A pressure of 4 bar at 100°C causes an average penetration of ~32 mm in the radial direction.

The kinetic effect is shown under consideration of the wax uptake for pine sapwood. The radial wax uptake at 12 bar increases from 59% (359 kg m⁻³) at 100°C to 77% (435 kg m⁻³) at 120°C. The prolongation of the process duration from 47 min to 360 min causes an uptake of up to 91%. According to Scholz et al. (2010b), this is in the range of fully impregnated Scots pine sapwood.

According to Fig. 5 and 6, increased temperature in the case of pine sapwood and especially prolonged process duration are more efficient for the tangential wax uptake (Fig. 4a). Furthermore, Fig. 4a demonstrates the blocking effect of heartwood tissue within pine, exemplified on the image with the tangential wax penetration. A prolongation of the process duration shows that the wax uptake is based on kinetic backgrounds especially for beech. An influence of effusing water steam on the wax uptake was excluded after 3 days of drying under 0.1 bar vacuum at 100°C and 120°C, respectively. After a regular impregnation process of about 47 min, beech shows an irregular wax penetration pattern (Fig. 3). This “zig-zag-pattern” is possibly caused by the latewood vessels, which possess scalariform perforation plates according to Wagenführ (2007). During a process of 360 min, the colored wax deposits indicate a more homogeneous pattern (Fig. 3). Furthermore, the penetration depth as well as the wax uptake increases from 4% (47 min) to 30% (360 min), which complies to 28 kg m⁻³ and 373 kg m⁻³, respectively. These results are in accordance with Hackbarth (1978), who pointed out the importance of the process duration in combination with the pressure level.

In accordance with Bellmann (1987), liquid wax disperses like water-based solutions across the bordered pits of the ray tracheids into adjacent radial and longitudinal tracheids (Scholz et al. 2010a). For the lateral wax uptake and dispersion into adjacent longitudinal tracheids, the squarrose ray tracheids arranged in 1–3 rows (Wagenführ 2007) are important. The tangential wax dispersion is also possible locally across damaged window-like pits (Liese 1957; Scholz et al. 2010a). The tangential wax migration is more impeded.

The results of the determination of wax viscosity and surface tension are shown in Table 3 and Fig. 7. The influence of wax viscosity is already in Fig. 5 and 6. But at this point, the influence of viscosity and surface tension shall be discussed theoretically in detail.

The extension properties of water-based solutions within wood are basically different from those of liquid waxes. The internal friction of waxes is much higher than between water molecules, interacting via hydrogen bonds, in consequence of the steric indenting of the aliphatic wax molecules. Atkins (1993) mentioned such indenting as a reason for the viscosity of heavy oil. In addition, dispersion forces of larger molecules also have an influence. Such dispersion forces are based on induced dipoles, which are caused by inner molecular electron motions. They were reduced disproportionately by an increasing molecule distance. As a matter of fact, unbranched wax molecules are closely packed according to Ivanovszky (1965). In accordance with Atkins (1993), dispersion forces probably influence the viscosity of waxes. The dynamic viscosity of wax 4 is 9.4 mPa s (100°C), which means more than nine times the viscosity of water (20°C). The viscosity of wax 1 is more than 28 times of that of water (see Table 3). In practice, the melting points and temperature-dependent viscosities are important. Fig. 7 and Table 3 depict distinct viscous properties of waxes at a defined temperature. The viscous melts of the exemplified polyolefin can be attributed to the long molecule chains (3,000–20,000 g mol⁻¹) as well as (atactic, syndiotactic, isotactic) side chains. Scholz et al. (2010a) did not measure any swelling of the cell wall after wood impregnation with hot melting waxes.

The results indicate an increased indenting but also higher processing temperatures in the range of 150°C causing the beginning of wood decomposition (Militz and Mai 2008) and effusing gases. Due to that fact, wax penetration within the wood will be impeded. In contrast to the mentioned polyolefin, wax 4 has a low viscosity due to the lack of reactive groups and short molecule chains (Table 3, Fig. 6). The glycerin ester wax (Fig. 7) with a comparable molecule size to that of wax 4 is more viscous (35 mPa s at 120°C). Because of an incomplete esterification of the polyvalent alcohol, free hydroxyl groups exist causing increased interaction with adjacent molecules and thus higher viscosity. Wax 3 is the heaviest of all waxes investigated but with a low melting point. According to Ivanovszky (1965), the low viscosity and the low melting point are caused by the low packing density of the branched wax molecule.