WO2016102777A1

WO2016102777A1 - Treatment method for wood and wooden surface obtained by such method

Info

Publication number: WO2016102777A1
Application number: PCT/FI2015/050932
Authority: WO
Inventors: Monika ÖSTERBERG; Alina LOZHECHNIKOVA
Original assignee: Aalto University Foundation
Priority date: 2014-12-22
Filing date: 2015-12-22
Publication date: 2016-06-30
Also published as: EP3265284A4; EP3265284A1

Abstract

The object of the present invention is a method for treating of wood that is to be used for example in building and furniture construction. In the method dispersion of micro- and/or nanoparticles is applied on wooden surface to form a non-continuous surface layer which allows moisture vapor permeation thus retaining moisture buffer value (MBV) of non-treated wood or even enhancing it. Another object of the invention is a wooden surface having a non-continuous surface layer of micro- and/or nanoparticles obtained by such a method.

Description

TREATMENT METHOD FOR WOOD AND WOODEN SURFACE OBTAINED BY SUCH METHOD

Field of the invention

The invention relates to a method for treating of wood that is to be used for example in building and furniture construction and to a wooden surface obtained by such a method. Background art

In mankind history wood has always been of major importance. Renewability, strength, visual appearance and good thermal insulation properties made it the material of choice for centuries, especially for building and furniture construction. Dur- ing past decades, however, wood was gradually replaced by new materials, such as concrete, steel, and most recently, plastic. In some areas timber as a material became almost fully disregarded.

On the other hand, recent environmental and sustainability concerns has made in- dustry look for new substitutes for fossil materials and demand for green and renewable materials, including wood, is increasing. However, there are some material properties of timber that are less desirable, e.g. perceptivity to biological attack and susceptibility to weathering. Hygroscopicity or ability of timber to attract, hold and release water molecules is also considered to be negative and disadvantageous. This is probably due to the known "side effects" of wood exposed to wet environment, such are dimensional instability of material due to swelling and shrinkage of the cell wall and lumen. Additionally, wet conditions create favorable conditions for the growth of various wood degrading living organisms (e.g. diverse fungi, bacteria and insects).

To avoid problems of wood degradation and to enhance durability and easy maintenance, wood is often hydrophobized. Surface hydrophobization methods include, but are not limited to, treatments with silicon containing compounds, the deposition of metal oxide nanoparticles and surface impregnation with various waxes, oils, polyelectrolytes and other compounds. Many of these approaches have a negative environmental impact and cause damage to the ecosystem because of the possibility for biocidal chemicals to leak from the surface. Nevertheless, recent findings suggest that hygroscopicity of timber can be used to our advantage. The ability of wooden material to store and release moisture helps to regulate the indoor climate naturally, to dampen the humidity variations and to avoid extreme conditions. This phenomenon is known as moisture buffering and it is an energy-efficient way to passively moderate moisture levels in living space. The use of hygroscopic materials together with a well-controlled ventilation system may further reduce the energy consumed for heating and cooling and increase the overall energy-efficiency of a building. Maintaining certain levels of relative humidity (RH) will also increase the perceived air quality and influence the occupants' health and comfort. The increased interest in hygroscopicity of wood puts focus on finish- ing and modification techniques, which are designed to enhance the moisture buffering performance.

Especially during hot periods, the dynamic moisture storage in hygroscopic materials reduces the moisture in the air, leading to increased comfort and consequently a reduced need for cooling, resulting in indirect savings. However, it has been shown that since the indoor air humidity is reduced, the indoor air enthalpy is also reduced and consequently less energy is needed for cooling, which leads to direct energy savings. The storage of moisture inside a hygroscopic material such as wood also means thermal storage, which can lead to passive heating or cooling of the building during the adsorption and desorption of water, and increased thermal comfort. To maximize the effect of moisture buffering, the surface area of wooden materials in interiors should be increased. This could be achieved by introducing more wooden surfaces into a living space, e.g. wooden floors, walls, ceilings, furniture, etc. Timber used indoors is certainly less susceptible to UV- and biodegradation by living organisms compared to wood used outdoors, but certain modification might still be required to improve the material properties and increase the lifetime of the material. Studies conducted on various building materials, including wood, generally suggest that paints and coatings decrease the moisture buffering effectiveness of the treated surfaces. Description of the invention

The object of the present invention is to provide a treating method of wood that preserves natural ability of timber to buffer moisture vapor while increasing resistance to liquid water as well as such wooden surface.

The aforesaid objects of the invention are attained according to the method of the invention in which dispersion of micro- and/or nanoparticles is applied on wooden surface to form a non-continuous surface layer which allows moisture vapor permeation thus retaining moisture buffer value (MBV) of non-treated wood or even en- hancing it. Preferably the non-continuous surface layer additionally repels water by increasing water contact angle (CA) thus slowing down the penetration of water. The size distribution of micro- and/or nanoparticles ranges from nano to microscale, preferably from about 10 nm to about 10 μιτι, more preferably from 100 nm to 1 μιτι.

The aforesaid objects of the invention are also attained according to the wooden surface according to the invention that is having a non-continuous surface layer of micro- and/or nanoparticles that is obtained by the method of the invention. The size distribution of the micro- and/or nanoparticles ranges from nano to microscale, preferably from about 10 nm to about 10 μιτι, more preferably from 100 nm to 1 μιτι.

Preferably micro- and/or nanoparticles of wax are used as the dispersion. The dispersion of wax micro- and/or nanoparticles is preferably obtained by melting wax and dispersing it in water using homogenizer or ultrasonic processor and then removing particles bigger than about 12-15 μιτι, preferably bigger than 10 μιτι, more preferably bigger than 1 μιτι. Preferably, the wax used in the method is carnauba wax. Micro- and/or nanoparticles are preferably applied to the wooden surface by spraying or by impregnation with the dispersion.

Brief description of the drawings

Fig. 1 shows a schematic representation of continuous and non-continuous coating on wood; Fig. 2 shows a diagram of water contact angle (CA) over time for differently treated surfaces of radially cut spruce;

Fig. 3 shows a diagram of water CA over time for differently treated surfaces of tangentially cut spruce;

Fig. 4 shows a bar graph of Moisture Buffer Value (MBV) for surfaces with different treatments for a radial surface; and

Fig. 5 shows a bar graph of MBV for surfaces with different treatments for a tangential surface.

Detailed description of the invention

Developed treatment is fundamentally different to conventional wood modification techniques. It doesn't form a continuous film on the surface as many commercial paints and lacquers do, and therefore doesn't limit moisture buffering. Instead, the method is based on the effect of hydrophobic micro- and/or nanoparticles that form a non-continuous surface layer, which slows down the penetration of water, but allows moisture vapor permeation. The benefits of this approach are the retained moisture buffering capacity and possibility for passive climate control. The micro- and/or nanoparticles may also partially fill the pores and pits of wooden surface. The difference in surface structure between conventional continuous film and the surface with particles according to the invention is schematically shown on Figurel. Special focus is put on the sustainability of the approach. Only natural materials are used with no harmful chemicals whatsoever, thus making this approach green and final product nontoxic for humans or nature. Carnauba wax is a natural wax produced by the leaves of the Carnauba palm. It was chosen for wax particles production for performing experiments of the method of the invention, as it is the hardest of natural waxes. It is also hypoallergenic, chemically inert and is not a food source for humans, thus not raising ethical questions for its use in wood modification. In the following non-limiting experiments of the method of the invention refined carnauba wax in the form of pellets (Sigma- Aldrich) is used.

For the particle production carnauba wax is melted and dispersed in water using homogenizer or ultrasonic processor (e.g. Polytron homogenizer PT2000, Kinemati- ca AG or Digital ultrasonic processor S-450, Branson). Obtained wax particles are mostly spherical in shape and have size distribution from nano to micro scale (~10 nm to ~10 μιτι, preferable 100 nm to lpm). Particles, which are bigger than about 12-15 μιτι will be visible on the surface; therefore they are removed e.g. by filtration (e.g. qualitative grade paper filter). Preferably particles bigger than 10 μιτι, more preferably particles bigger than 1 pm are removed. The dispersion is applied on wooden surface and allowed to dry and then buffed with a cotton cloth to distribute the particles evenly.

The application of micro- and/or nanoparticles to the surface may be done by spray- ing or by impregnation with the dispersion.

Moisture buffer value (MBV, g/m²*% Relative_humidity) is the accepted parameter to characterize moisture buffering efficiency. The moisture buffering effect is considered to be negligible with MBV ranging from 0 to 0.2, to be limited from 0.2 to 0.5, moderate from 0.5 to 1.0, good with 1 < MBV < 2 and excellent with MBV above 2. In the experiments of the method of the invention moisture uptake was measured for periods of high and low relative humidity, imitating daily humidity changes in living spaces. The experiments were performed using standard

NORDTEST method with the exception of the air velocity (varied between 0,10 and 0, 06 m/s instead of 0,10±0,05 m/s) and the size of the samples (wood samples of kiln dried spruce board around 2 cm thick). In the experiments, moisture buffer values were calculated for different treatment techniques utilizing carnauba wax. A continuous carnauba wax film coating was obtained by dipping the wood sample into molten wax. A thicker film was achieved by increasing the number of immersions of the sample into the molten wax.

MBV values for radial and tangential surfaces are shown on Figure 4 and Figure 5 respectively. Unmodified wood and wood treated with wax particles show good level of moisture buffering with MBV « 1. Coating with wax particles can even slightly enhance MBV compared to the unmodified wood, increasing the MBV up to 1.08 on the radial and 1.09 on the tangential surfaces. Both film-forming treatments decreased the reference MBV of wood, possibly due to sealing of the pores and the isolation of hydrophilic groups. Buffering of thin layer of wax can be described as limited with MBV « 0.3. Moisture buffering effect of thick layer of wax is negligible, MBV ~ 0.1. Thus, it can be concluded that utilizing particles is more advantageous comparing to continuous films in terms of preserving natural moisture buffering capacity of solid wood. In the experiments the wax particles have been shown to increase the roughness of the surface and this may have a positive effect on the moisture buffering.

In the experiments of the method of the invention the water contact angle (CA) is determined using a contact angle meter (e.g. CAM 200, KSV Instruments Ltd., Helsinki, Finland) and the full Young-Laplace equation. In the experiments the CA in- creased after first minute from 0 to 80° for radially cut surface of kiln dried spruce board and from 90 to 120° for tangential surface (Figures 2 and 3). Interestingly, tangential surface treated with wax particles had higher CA than surface with continuous wax film, 120 and 100° respectively. This is probably due to more complex surface roughness of surface treated with particles and formation of air pockets with trapped air preventing water droplet from penetrating through surface. Combination of hierarchical surface roughness of wood (on micro and nano level) and low surface energy of wax particles makes treated surface more resistant to liquid water. Carnauba was the wax of choice for the experiments of the invention, but other waxes are expected to behave similarly and be effective as well due to their hydrophobic nature and hardness. The coating of the invention will allow greater use of wood in living spaces due to improved resistance to water and durability. Increased hydrophobicity of surface allows for its use in wet spaces and possibly also makes the wooden surfaces easy to clean and maintain. Storage of moisture inside the wood structure means also thermal storage, which can lead to passive heating or cooling of the building and increased thermal comfort. Therefore, with the use of wood treated with the method of the invention in the living space the reduction in energy consumption due to reduced need of heating and mechanical ventilation is possible. Overall energy efficiency of a building could be improved.

The example was shown using wax micro- and/or nanoparticles but we suggest that the general idea of using non-continuous layers of particles to modify wood surfaces can be applied for various applications including introducing easy maintenance, wa- ter resistance, fire retardant, antimicrobial/antifungal properties or even as an alternative to painting to introduce coloring. The idea is that using micro- and/or nanoparticles instead of continuous layer the moisture buffering ability of the wood is retained or even enhanced. The invention can be of interest for the manufacturers of paints and varnishes for wooden surfaces, as it represents a sustainable alternative for conventional film- forming coatings, while enhancing the moisture buffering property.

Claims

1. Method for treating of wood that is to be used for example in building and furniture construction, characterized in that dispersion of micro- and/or nanopartides is applied on wooden surface to form a non-continuous surface layer which allows moisture vapor permeation thus retaining moisture buffer value (MBV) of non- treated wood or even enhancing it.

2. Method according to claim 1, characterized in that the non-continuous surface layer additionally repels water by increasing water contact angle (CA) thus slowing down the penetration of water.

3. Method according to claim 1 or 2, characterized in that the size distribution of the micro- and/or nanopartides ranges from nano to micro scale, preferably from about 10 nm to about 10 μιτι, more preferably from 100 nm to 1 μιτι.

4. Method according to one of claims 1 to 3, characterized in that the micro- and/or nanopartides of wax is used as a dispersion.

5. Method according to claim 4, characterized in that the dispersion of wax micro- and/or nanopartides is obtained by melting wax and dispersing it in water using homogenizer or ultrasonic processor and then removing particles bigger than about 12-15 μιτι, preferably bigger than 10 μιτι, more preferably bigger than 1 pm..

6. Method according to claim 4 or 5, characterized in that the wax used is carnau- ba wax.

7. Method according to any one of claims 1 to 6, characterized in that the application of micro- and/or nanopartides to the wooden surface is done by spraying or by impregnation with the dispersion.

8. Wooden surface having a non-continuous surface layer of micro- and/or nanopartides obtained by a method of any of claims 1 to 7.

9. Wooden surface according to claim 8, wherein the size distribution of the micro- and/or nanopartides ranges from nano to micro scale, preferably from about 10 nm to about 10 μιτι, more preferably from 100 nm to 1 μιτι.

10. Wooden surface of claim 8 or 9, wherein the surface layer consists of micro- and/or nanopartides of wax.

11. Wooden surface of claim 10, wherein the wax is carnauba wax.