EP3767032A1

EP3767032A1 - Shape forming of delignified wood

Info

Publication number: EP3767032A1
Application number: EP19187447.8A
Authority: EP
Inventors: Marion Andrea FREY; Clemens Dransfeld; Meri Tuuli ZIRKELBACH; Tobias KEPLINGER; Etienne TRACHSEL; Mikael Hannus; Ingo BURGERT
Original assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Current assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Priority date: 2019-07-19
Filing date: 2019-07-19
Publication date: 2021-01-20

Abstract

The present invention relates to a method for producing densified wood (41). A wet delignified piece of wood (40) comprising cellulose fibers oriented as naturally grown is placed in a mold (10, 16), wherein the mold comprises a porous section that forms a molding surface (13, 14, 15) and/or a porous layer (50) is present in said mold (10, 16) contacting said delignified piece of wood (40). The delignified piece of wood (40) is placed in such a way that the cellulose fibers align predominantly in parallel to the molding surface (13, 14, 15). Subsequently, a pressure difference is applied to press the delignified piece of wood (40) against said molding surface (13, 14, 15) yielding a densified piece of wood (41). Furthermore, the densified piece of wood (41) is provided.

Description

Background

Delignified densified wood is a new promising lightweight, high-performance and bio-based material, which potentially could substitute natural fiber reinforced composites or glass fiber reinforced composites. The process is based on delignification of wood (removal of the matrix lignin), which results in a flexible cellulose scaffold followed by a subsequent densification step, leading to a material with high stiffness, strength and toughness. (Patent application EP17168238.8, 2017 ) (M.Frey et al., 2018).
The densification is currently conducted in closed molds by simple pressing. The material cannot be processed in fully wet state, as the free water creates a counter pressure, leading to a reduced densification, distorted fiber alignment and cracks (Figure 1).
Therefore, the material has to be conditioned before densification in order to remove the free water. This, however, leads to a reduced formability of the scaffold and a longer processing time. A process, which combines shaping and densification in one step, allowing for processing wet delignified wood fast and in a scalable manner, is therefore necessary.
The present invention is directed towards shape forming of delignified wood, which allows for combining shaping, densification and drying in a simple and scalable approach.

Description

A first aspect of the present invention relates to a method for producing densified delignified wood. The method comprises the steps of

providing at least one delignified piece of wood comprising cellulose fibers oriented as naturally grown, wherein the delignified piece of wood is wet,
placing said delignified piece of wood in a mold comprising a molding surface, wherein said mold comprises a porous section that forms the molding surface and/or a porous layer is present in said mold contacting said delignified piece of wood, wherein the delignified piece of wood is placed in such a way that the cellulose fibers align predominantly in parallel to the molding surface,
generating a pressure difference in such a way that the piece of wood is pressed against said molding surface and densified, yielding a densified piece of wood.

For example, wet delignified wood - such as veneers, layers, strips - is draped onto a mold in a predefined lay-up. Delignified wood may be obtained by standard delignification methods, wherein lignin is removed from wood. The delignified wood is densified and dried in a vacuum bag surrounding the mold. This leads to an increased fiber volume content, resulting in higher mechanical properties, and drying locks the structure in its shape. Water is either removed through pores in the mold, or through a porous layer (e.g. mesh grid, textile) placed on top or below the delignified wood (Fig. 2).
The delignified piece of wood may be of any shape. The delignified piece of wood comprises mainly parallel aligned cellulose fibers. The minimal volume can be in the µm³ range. More easily treatable units in terms of the densification treatments are characterized by a volume in the cm³ range. The maximum volume is defined by the size of the densification apparatus and the feasibility of the delignification since the delignification solution has to infiltrate the whole piece of wood in order to achieve an almost complete removal of lignin. According to the Fick's laws of diffusion, the temporal change of concentration dc(x,t) (c, concentration; x, diffusion distance, t, time), the distance x is proportional to t^0.5.
The delignified piece of wood is densified in the direction perpendicular to the extension of the cellulose fibers. When placed in the mold, the cellulose fibers are parallel to the molding surface. If the molding surface is curved, the cellulose fibers are bent accordingly.
The molding surface relates to that section of the surface of the mold that forms the negative shape, which corresponds to the desired positive shape, i.e. the shape of the densified piece of wood.
To allow formability of the piece of wood and to reduce the overall processing time, wet delignified wood is used. Delignified wood is referred to as being "wet" if it comprises water. Water molecules might be present in form of bound water or free water. Bound water relates to water molecules, which are bound to macromolecules of the delignified wood e.g. by adsorption. Free water relates to water molecules that are present in lumina within the delignified wood structure. When stored at 65 % RH, delignified wood has a water content ≥ 11 %. The water is mainly bound water. Usually, the fibers within the delignified wood are saturated with bound water at a water content of approx. 23 %. Thus, free water is also present if the water content is ≥ 23 %. Small variations may occur depending on the type of wood.
The water content relates to the mass of water in relation to the total mass of the delignified piece of wood.
In certain embodiments, the water content of the delignified piece of wood is ≥ 11 %, particularly between 11 % and 440 % (fully water-saturated state).
In certain embodiments, the water content of the delignified piece of wood is ≥ 23 % (storage at 95% RH), particularly between 23 % and 440 %.
Besides a certain water content, the removal of lignin is important for the formability of the piece of wood. Delignified wood is obtained by an almost complete removal of lignin. A suitable method for the delignification of wood is described below in more detail.
In general, lignin contributes to the brown color of lignocellulosic material, i.e. the characteristic color of wood. In a fully delignified state, wood consists mainly of cellulose fibers that are whitish. If the delignification is inhomogeneous, the delignified wood does not appear evenly whitish. Upon tensile loading such materials will crack or break in the brownish colored areas or at the interface to the whitish areas.
In certain embodiments, the lignin content of the delignified piece of wood is below 10 %.
In certain embodiments, the lignin content of the delignified piece of wood is below 3 %.
In certain embodiments, the lignin content of the delignified piece of wood is < 1%.
In certain embodiments, the delignified piece of wood is obtained from a veneer, layer or a strip of wood.
High densification, i.e. a high number of fibers/volume, can be achieved if the cellulose fibers align parallel to the molding surface/perpendicular to the direction of the pressure applied. A large number of parallel fibers in a piece of wood can be achieved if it is prepared by cutting techniques in longitudinal direction / growth direction such as rift cut, crown cut or rotary cut. In contrast to other densification methods, the orientation of the fibers is maintained.
In certain embodiments, the delignified piece of wood is obtained from a veneer, layer or a strip of wood obtained by a rift cut, crown cut or rotary cut.
In certain embodiments, the delignified piece of wood has a thickness between 0.5 mm and 10 mm.
In certain embodiments, the delignified piece of wood has a thickness between 1 mm and 3 mm.
As described above, shaping and densifying wood requires the removal of lignin while the structural integrity of the delignified and densified piece of wood should be maintained to achieve a high tensile strength. The piece of wood, for example softwood, contains fibers (precisely tracheids, in a biological context) which are approximately 3 mm long and 30 µm in diameter. The cell walls of the fibers consist of cellulose microfibrils. A microfibril is formed by several cellulose chains and possesses a length of approximately 15 nm length and a diameter of approximately 3 nm. Lignin and hemicellulose fill spaces between the cellulose fibrils. The microfibrils in normal wood are predominately aligned in longitudinal direction (small microfibril angle). This mainly parallel alignment is maintained upon removal of lignin, in particular when the delignification method described in the following is applied.
In certain embodiments, the delignified piece of wood is obtained by treating a piece of wood with

at least one acid, in particular an inorganic or organic acid, more particularly acetic acid, sulfuric acid, chloric acid, peracetic acid, or
at least one oxidizing agent, in particular hydrogen peroxide, sodium chlorite, sodium sulfite, ozone, more particularly hydrogen peroxide, under alkaline or acidic conditions, in particular under acidic conditions or
at least one base, in particular sodium hydroxide.

In certain embodiments, the piece of wood is hardwood or softwood.
The method can be performed using any kind of hardwood or softwood.
In certain embodiments, the piece of wood is from Acer spp., Betula spp., Fagus sylvatica, Quercus spp. Fraxinus excelsior, Eucalyptus spp., Populus spp., Prunus avium, Tilia spp., Robinia pseudoacacia, Tectona grandis, Ulmus spp., Juglans regia, Carpinus betulus, Ochroma pyramidale, Pseudotsuga menziesii, Taxus baccata, Picea abies, Pinus sylvestris, Larix decidua, Thuja plicata, Abies alba, or Pinus strobus.
Lignin constitutes approximately 20 to 35 % of the dry mass of wood, for example the lignin content in conifers ranges from 27 % to 32 % and in deciduous trees (Betula, Fagus) from 19 % to 23 % (H. G. Hirschberg: Handbuch Verfahrenstechnik und Anlagenbau. Springer Verlag 1999: p. 436). Besides lignin, an additional partial removal of hemicellulose and amorphous cellulose occurs. The delignification may also be performed until the respective loss of weight is reached, in particular a loss of weight is reached, which corresponds at least to the amount of lignin of the starting material.
For example, delignification causes a weight loss of 20 % to 40 % of the dry mass of the piece of wood, particularly a weight loss of 28 to 40 % of the dry mass of the piece of wood (lignocellulosic material) from a conifer and a weight loss of 20 to 32 % of the dry mass of the piece of wood (lignocellulosic material) from a deciduous tree.
The delignification process can also be monitored by assessing the loss of color. Lignin contributes to the brown color of lignocellulosic material, i.e. the characteristic color of wood. In a fully delignified state, wood consists mainly of cellulose fibers that are whitish. Thus, depending on the thickness of the piece of wood, delignification has to be performed until the delignified wood appears white.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with at least one acid. The pH is adjusted between 1 and 6, particularly 1.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood at least with one inorganic or organic acid.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with acetic acid, sulfuric acid, chloric acid or peracetic acid.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with at least one oxidizing agent under alkaline or acidic conditions. Under alkaline conditions, the pH is adjusted between 8 and 14. Under acidic conditions, the pH is adjusted between 1 and 6.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with hydrogen peroxide, sodium chlorite, sodium sulfite or ozone under alkaline or acidic conditions.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with hydrogen peroxide under alkaline or acidic conditions.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with at least one oxidizing agent under acidic conditions.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with hydrogen peroxide, sodium chlorite, sodium sulfite or ozone under acidic conditions.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with hydrogen peroxide under acidic conditions. In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with acetic acid in combination with H₂O₂.
The use of acetic acid in combination with H₂O₂ is a less poisonous treatment, compared to other delignification treatments.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with at least one base. The pH is adjusted between 8 and 14, particularly 14.
In certain embodiments, the delignified piece of wood is obtained by treating the piece of wood with sodium hydroxide.
To obtain an even delignification, the delignification solution has to infiltrate the piece of wood completely.
In certain embodiments, the delignified piece of wood is obtained by an incubation of the infiltrated piece of wood at a temperature between 20 °C and 90 °C.
In certain embodiments, the delignified piece of wood is obtained by an incubation of the infiltrated piece of wood at a temperature between 60 °C and 90 °C.
In certain embodiments, the delignified piece of wood is obtained by an incubation of the infiltrated piece of wood at a temperature between 75 °C and 85 °C.
In certain embodiments, the delignified piece of wood is obtained by an incubation of the infiltrated piece of wood at 80 °C.
The densification is performed in a mold which comprises a molding surface. The molding surface forms the negative shape, which corresponds to the positive shape of the desired densified piece of wood.
When the delignified piece of wood is placed into the mold, the cellulose fibers align parallel to the molding surface/perpendicular to the direction of the pressure applied. If the molding surface is curved, the cellulose fibers are bent accordingly.
The densification may be performed in an open mold or in a closed mold.
In certain embodiments, the mold is an open mold or a closed mold.
In certain embodiments, the mold is an open mold.
Alternatively to open molds, closed porous molds or closed solid molds with the addition of porous/conductive layers in between the two molds (or upper and lower part of a mold) can be used. The advantage of such an approach is the increased surface quality of the final composite part.
In certain embodiments, the mold is a closed mold.
The closed mold may consist of two parts, e.g an upper and a lower part. The lower part may comprise a molding surface, which forms the negative shape that corresponds to one side of the positive shape of the desired densified piece of wood and the upper part may comprise a molding surface, which forms the negative shape that corresponds to the other side of the positive shape of the desired densified piece of wood. Particularly, the shape of the molding surface of the upper and the lower part are complementary to each other. The upper and the lower part may be separable or connected, e.g. by a hinge.
Water is pressed out of the wet delignified wood during densification. During densification, the free water is removed completely. Also the bound water is removed partially. The final humidity content depends on the temperature and the time of the densification process, e.g. by applying vacuum. To avoid a counter pressure of the water, the water drains off through open pores that are present in the mold. Therefore, the mold may comprise a porous section that forms the molding surface. The pores of this section form a network of channels and interstices so that the water can flow out of the mold. For example, the whole mold may be made of a porous material or a part of the mold, which is connected to a drain, is made of a porous material.
In certain embodiments, the mold comprises a porous section, which forms the molding surface and at least one section of the outer surface of the mold or the mold comprises a porous section, which forms the molding surface and is connected to a drain.
In certain embodiments, the mold comprises a porous section, which forms the molding surface and at least one section of the outer surface of the mold.
In certain embodiments, the mold comprises a porous section, which forms the molding surface and is connected to a drain.
In certain embodiments, the mold is porous.
In certain embodiments, the open mold is porous.
In certain embodiments, the closed mold is porous.
In certain embodiments, the closed mold comprises at least two parts, in particular an upper and a lower part.
In certain embodiments, both the upper and the lower part are porous or one of the upper and the lower part is porous and the other part is solid.
In certain embodiments, the upper and the lower part are porous.
Suitable molds withstand the pressure that is applied to generate the pressure difference. For example, a mold has to withstand at least a pressure of 1 atm (101325 Pa) in the vacuum process.
Furthermore, suitable molds should be heat-resistant if the densification is performed at elevated temperatures such as 65 °C.
The pores of the mold comprise open pores which form a network of cavities, channels and/or interstices. This network allows water, which is pressed out of the delignified wood, to flow from the molding surface towards the outer surface of the mold. Finally, the water exits the mold.
The water may also be removed via a porous layer. Such layer may additionally be used in a porous mold as described above or in a solid mold. The porous layer may consist of one layer, may comprise sublayers or may be composed of several porous layers. For example, the porous layer may comprise a flow mesh and a textile layer.
Further porous layers may be present in the mold assembly. For example, an open porous mold may be covered by a textile layer to allow a smooth surface finish of the delignified wood and below the mold may be a flow mesh connected to a vacuum tube to facilitate the removal of water. Alternatively, the flow mesh may be placed on top of the delignified piece of wood.
In certain embodiments, the porous layer consists of at least one layer.
In certain embodiments, the porous layer comprises one or more layers.
In certain embodiments, the porous layer comprises one or more layers made of different materials.
In certain embodiments, the porous layer is placed below and/or above the delignified piece of wood.
If an open mold is used, the porous layer may be placed below or above the delignified piece of wood.
If a closed mold is used, one or more porous layers may be used.
In certain embodiments, one or more, particularly two, porous layers are present in the mold.
In certain embodiments, one or more, particularly two, porous layers are present in the closed mold.
In certain embodiments, the closed solid mold comprises at least two parts, in particular an upper and a lower part. The upper and the lower part each comprise a molding surface which comprises a negative shape, which corresponds to the upper and the lower side of the desired densified piece of wood (positive), respectively. Before densification, the porous layer is placed on the molding surface of the lower part and the delignified piece of wood is placed on the (lower) porous layer. Optionally, a further (upper) porous layer is placed on the piece of wood before the upper part is placed on the delignified piece of wood/the upper porous layer.
If a solid mold is used, the size and shape of the porous layer matches the size and shape of the molding surface. Furthermore, the porous layer either contacts a drainage system within the mold and/or the porous layer extends to the outer surface of the mold so that the water can flow out. Suitable porous layers withstand the pressure that is applied to generate the pressure difference, i.e. the pores are still water-permeable when pressure is applied. For example, a porous layer has to withstand at least a pressure of 1 atm (101325 Pa) in the vacuum process. Furthermore, suitable porous layers should be heat-resistant if the densification is performed at elevated temperatures such as 65 °C.
In certain embodiments, the porous layer is a continuous-fiber fabric, a polymer mesh or a flow grid.
In certain embodiments, the porous layer is placed on top and/or below the delignified piece of wood.
In certain embodiments, the porous layer is placed on top and below the delignified piece of wood.
In certain embodiments, the porous layer is placed on top or below the delignified piece of wood.
If the pores of the mold and the porous layer are too big, the negative of the porous structure may be visible on the densified piece of wood. Thus, the size of the pores should be small to obtain a smooth surface of the densified piece of wood. On the other hand, the pores have to be big enough to provide sufficient water flow.
In certain embodiments, the size of the pores of the mold and/or the porous layer is between 0.1 mm and 2 mm, particularly between 0.3 mm and 0.5 mm.
In certain embodiments, the size of the pores of the mold is between 0.1 mm and 2 mm, particularly between 0.3 mm and 0.5 mm.
In certain embodiments, the size of the pores of the porous layer is between 0.1 mm and 2 mm, particularly between 0.3 mm and 0.5 mm.
In certain embodiments, the size of the pores of the mold and/or the porous layer is between 0.1 mm and 2 mm, particularly between 0.3 mm and 0.5 mm, at the molding surface.
To allow the removal of water, the mold or the porous layer is made of a material with a high number of pores per volume. For example, suitable materials are clay or wood. Also 3D printed porous molds or porous layers made of ABS (acrylonitrile butadiene styrene), PVA (polyvinyl alcohol) or PLA (polylactic acid) may be used.
The pores of the mold may vary in diameter and form a gradient, for example from the molding surface towards the outer surface of the mold. For example, the pore size increases from the molding surface towards the outer surface. The same may apply to the porous layer. As described above, the pore structure may result in an uneven surface of the densified piece of wood if the pores are too big. Thus, smaller pore size towards the surface leads to a smoother surface finish. The drying speed can be increased by using a mold or a porous layer with increasing pore size towards the outer surface.
In certain embodiments, the porosity of the mold and/or the porous layer is homogeneous or gradual.
In certain embodiments, the porosity of the mold is homogeneous or gradual.
In certain embodiments, the porosity of the mold is homogeneous.
In certain embodiments, the porosity of the mold is gradual.
In certain embodiments, the porosity of the porous layer is homogeneous or gradual.
In certain embodiments, the porosity of the porous layer is gradual.
In certain embodiments, the porosity of the porous layer is homogeneous.
To increase the surface quality or to protect the mold from contamination, the molding surface may be covered by a (porous) textile layer.
In certain embodiments, the molding surface is covered by a textile layer.
The use of wet delignified wood allows various shape forming. Thus, densified wood with a flat or a curved surface may be obtained.
In certain embodiments, the molding surface is characterized by one or more radii of curvatures between 200 µm to infinite (flat).
In certain embodiments, the molding surface is flat.
The inventive method provides particularly densified pieces of wood having at least one section with a curved surface. The surface might be curved everywhere or the surface may comprise curved and flat sections.
In certain embodiments, the molding surface is characterized by at least one curvature having a radius of curvature ≥ 200 µm.
In certain embodiments, the molding surface is characterized by at least one curvature having a radius of curvature ≥ 1 mm, particularly ≥ 1 cm.
In certain embodiments, the molding surface is characterized by at least one radius of curvature between 200 µm and 10 m, particularly 200 µm and 1 m.
In certain embodiments, the molding surface is characterized by at least one radius of curvature between 1 mm and 10 m, particularly 1mm and 1 m.
Curved surfaces may be required for example in the automotive industry or in the field of aviation. Formed parts such as door panels, covers or door handles comprise curved surfaces having curvatures in the cm to m range.
In certain embodiments, the molding surface is characterized by at least one curvature having a radius of curvature ≥ 1 cm.
In certain embodiments, the molding surface is characterized by at least one radius of curvature between 1 cm and 100 m, particularly between 1 cm and 10 m.
For densification, a pressure difference is applied. The pressure difference may be generated by using a positive pressure, e.g. pressing the upper part of the mold in the direction of the lower part of the mold in combination with vacuum or by using a negative pressure only (vacuum). For example, negative pressure may be applied by using a vacuum bag. Vacuum may be applied from above the densified piece of wood or from below the mold. Also a combination of using a positive pressure and negative pressure is possible. For example, pressing the upper part of the mold on the lower part of the mold may be further supported by applying a vacuum. Intracellular water is removed during densification. Hereby, water molecules cross the cell walls which comprise mainly cellulose fibrils after delignification. By applying a vacuum or vacuum in combination with a positive pressure, the integrity and orientation of the cellulose fibrils is maintained. The maximum pressure applied depends on the materials and devices used. For example, autoclave processing allows applying an external pressure of up to 10 bar.
In certain embodiments, a negative pressure or a negative pressure in combination with a positive pressure is applied, wherein the pressure difference to atmospheric pressure is ≥ 1 bar, particularly ≥ 2 bar, when a positive pressure is applied, or the pressure difference is ≥ 0.9 bar, particularly ≥ 0.99 bar, when a negative pressure is applied. For example, a positive pressure of 3 bar (pressure difference to 1 bar atmospheric pressure = 2 bar) may be combined with a negative pressure (vacuum) of 10^-2 bar. Alternatively, only a negative pressure may be applied.
In certain embodiments, a negative pressure is applied.
In certain embodiments, the pressure difference is ≥ 1 bar, particularly ≥ 2 bar, when a positive pressure is applied, or the pressure difference is ≥ 0.9 bar, particularly ≥ 0.99 bar, when a negative pressure is applied.
In certain embodiments, the pressure difference is ≥ 1 bar, particularly ≥ 2 bar, when a positive pressure is applied. The pressure difference relates to the pressure applied and the atmospheric pressure (approx. 1 bar). For instance, if a pressure of 3 bar is applied, the pressure difference to the atmospheric pressure is 2 bar.
In certain embodiments, the pressure difference is ≥ 0.9 bar, particularly ≥ 0.99 bar, when a negative pressure is applied. The pressure difference relates to the negative pressure applied and the atmospheric pressure (approx. 1 bar). If a negative pressure of 10^-2 bar is applied, the difference to the atmospheric pressure is 0.99 bar.
The fiber alignment of shaped densified parts is retained by drying. To prevent deformations of the shaped densified piece of wood, the densification is performed until at least the free water is completely removed.
In certain embodiments, the delignified piece of wood is simultaneously densified and dried.
The densification may be performed at ambient temperature or elevated temperature. At higher temperatures, the drying/densification time can be reduced.
In certain embodiments, the densification is performed at a temperature between 18°C and 180 °C.
In certain embodiments, the densification is performed at a temperature between 18°C and 30 °C.
In certain embodiments, the densification is performed at a temperature between 30°C and 180 °C.
In certain embodiments, the densification is performed at a temperature between 30°C and 70 °C.
In certain embodiments, the densification is performed at a temperature between 40°C and 65 °C.
In certain embodiments, the densification is performed at 65 °C.
After simultaneous shaping, and removal of at least the free water by vacuum, which can lead to and/or be used for densification, the piece of wood may be further processed by applying an additional positive pressure and/or an additional drying step. The additional positive pressure may be applied in the same set up used for simultaneous shaping, densification and removal of at least the free water or in another device. Further densification is achieved by applying an additional positive pressure. Also the additional drying step may be performed in the set up used in the steps before or in another device or drying chamber. Thus, the delignified piece of wood may be processed in one process or machine at one station or in several processes/machines at several stations. Furthermore, shaping may be finished by dye-cutting the edges of the densified piece of wood.
In certain embodiments, the densified piece of wood is further dried in an additional drying step.
In certain embodiments, the densified piece of wood is further shaped by cutting the edges, in particular by dye-cutting the edges.
Upon densification, the cellulose fibers of the delignified wood are interconnected. In wood as naturally grown, the fibers are oriented in growth direction. To further enhance the tensile strength of the densified wood, several pieces of densified wood may be combined upon densification.
In certain embodiments, one or more delignified pieces of wood form one or more layers.
In certain embodiments, the cellulose fibers of one or more delignified pieces of wood are oriented in the same direction within one layer.
In certain embodiments, the cellulose fibers of one layer are oriented in a different direction with regard to the direction of the cellulose fibers of an adjacent layer. For example, the cellulose fibers in one layer extend perpendicular to the fibers in an adjacent layer (0°/90°). Also quasi-isotropic arrangements are possible.
To further enhance the tensile strength of the densified wood, adhesives may be applied. In contrast to other processes, there is no need to dry the delignified wood in order to get a good interfacial adhesion with the adhesive, e.g. a polymer matrix. In conventional methods, the fiber volume content is controlled by the fiber:matrix ratio and not by densification through the applied vacuum.
In certain embodiments, an adhesive is applied between the layers or infiltrated into the delignified piece of wood before the pressure difference is generated.
In certain embodiments, the adhesive is selected from natural glues and/or synthetic adhesives.
In certain embodiments, the natural glue is selected from starch, tannins, microfibrillated cellulose (MFC), nanocrystalline cellulose (NCC), methylcellulose and the synthetic adhesive is selected from thermoplastic polymers, in particular polyethylene polymers (PE), polypropylen polymers (PP), polymethylmethacrylat polymers (PMMA), polylactic acid polymers (PLA), or duroplastic polymers, in particular epoxy polymers, melamine urea formaldehyde polymers (MUF), urea formaldehyde polymers (UF).
A second aspect of the invention relates to a densified wood comprising one or more layers of delignified wood, wherein the cellulose fibers within one layer are oriented in the same direction and/or wherein the cellulose fibers of one layer are oriented in a different direction with regard to the direction of the cellulose fibers of an adjacent layer, characterized in that fiber density is between 0.3 gcm^-3 and 1.5 gcm^-3 in particular between 1 gcm^-3 and 1.5 gcm^-3.
In certain embodiments, the densified wood comprises one layer of delignified wood, wherein the cellulose fibers within the layer are oriented in the same direction, characterized in that fiber density is between 0.3 gcm^-3 and 1.5 gcm^-3 in particular between 1 gcm^-3 and 1.5 gcm^-3.
In certain embodiments, the densified wood comprises two or more layers of delignified wood, wherein the cellulose fibers within one layer are oriented in the same direction and wherein the cellulose fibers of one layer are oriented in a different direction with regard to the direction of the cellulose fibers of an adjacent layer, characterized in that fiber density is between 0.3 gcm^-3 and 1.5 gcm^-3 in particular between 1 gcm^-3 and 1.5 gcm^-3.
In certain embodiments, the densified wood is obtained by the method according to the first aspect of the invention.
In certain embodiments, the densified wood is characterized by one or more radii of curvatures between 200 µm to infinite (flat).
Reference is made to the embodiments of the first aspect of the invention.

Terms and definitions

In the context of the present invention, the term "delignification" relates to the removal of lignin from lignocellulosic materials such as wood. Lignin is a branched polymer located between the cellulose microfibrils in the cell walls of lignified parts of a plant, in particular wood. The lignin polymer contains several functional groups such as ether linkages, phenolic hydroxyl groups, aliphatic hydroxyl groups, unsubstituted or methyl-substituted C2, C3, C5 or C6 moieties, unsaturated moieties and ester groups that may react during delignification. For example, ethers may be cleaved by nucleophilic attacks, carbonyl and aldehyde groups react with nucleophiles, hydroxyl groups may be ionized or -O-methyl groups demethylated to allow a nucleophilic attack of the oxygen ion. Furthermore, conjugate addition, formaldehyde addition, epoxide addition and aldol condensation reactions might contribute to the depolymerisation of lignin. These chemical reactions result in the depolymerization of the lignin polymer into smaller moieties that diffuse from the lignocellulosic material into the surrounding solution and/or that are removed by several washing steps.
In the context of the present invention, the term "delignified piece of wood" relates to a piece of wood that is obtained by delignification of said piece of wood. Naturally grown wood comprises cellulose, which forms fibers that extend in growth direction. The delignified wood also comprises cellulose, wherein the cellulose forms fibers that are arranged in a mainly parallel orientation. Thus, the structural integrity of the fibers is maintained.
In the context of the present invention, the term "structural integrity" relates to the spatial arrangement of fibers in wood, delignified wood and densified wood in longitudinal direction, i.e. in growth direction. Fibers in wood are arranged in a mainly parallel orientation. This parallel alignment is maintained during delignification and densification. If a mold having a curved molding surface is used during densification, the fibers are bent according to the curvature of the molding surface.
In the context of the present invention, the term "densification" relates to the compression of delignified wood. By applying a vertical pressure force, particularly in radial or tangential direction, on delignified wood, the volume is reduced. The volume may be reduced by the loss of water without further compression of the cellulose fibers. Further compression relates to the reduction of the distance between the cellulose fibers. A pressure is particularly applied until a predefined thickness of the delignified wood or the maximum compression is achieved.
In the context of the present invention, the term "densified piece of wood" relates to a wood that is obtained by densification of delignified wood. The densified wood comprises cellulose, wherein the cellulose forms fibers that are arranged in a mainly parallel orientation. If a mold having a curved molding surface is used during densification, the fibers are bent according to the curvature of the molding surface. The density of densified wood ranges from more than 100 kg/m³ to 1500 kg/m³.
In the context of the present invention, the term "hardwood" relates to wood of deciduous trees. The deciduous trees may be non-modified or genetically modified. Non-limiting examples for deciduous trees are Acer spp., Betula spp., Fagus sylvatica, Quercus spp. Fraxinus excelsior, Populus spp., Prunus avium, Tilia spp., Robinia pseudoacacia, Tectona grandis, Ulmus spp., Juglans regia, Carpinus betulus, Eucalyptus spp., Ochroma pyramidale.
In the context of the present invention, the term "softwood" relates to wood of conifers. The conifers may be non-modified or genetically modified. Non-limiting examples for conifers are Pseudotsuga menziesii, Taxus baccata, Picea abies, Pinus sylvestris, Larix decidua, Thuja plicata, Abies alba, Pinus strobus.
In the context of the present invention, the term "oxidizing agent" relates to agents that oxidize lignin. Such oxidizing treatments are achieved by treatments comprising enzymes such as laccase, fungi or chemical substances such as Cl₂ in water, HCIO, peracetic acid (PAA), NaOH, NaClO₂, Na₂S₂O₄, ClO₂, HAc/H₂O₂ or O₃, ionic liquids or treatments which use catalysts, e.g. manganese salts.
In the context of the present invention, the term "radial direction" relates to a direction that is perpendicular to the longitudinal direction of the cellulose fibers and crosses the annual rings of a piece of wood.
In the context of the present invention, the term "tangential direction" relates to a direction that is perpendicular to the longitudinal direction of the cellulose fibers and aligns to a tangent of an annual ring of a piece of wood.
In the context of the present invention, the term "elastic modulus" relates to the slope of a stress-strain curve in the elastic deformation region and is a measure of the elastic deformation of the material. Stress is the force per cross section in a tensile test that can cause deformation of a material and strain is the elongation of the material related to its original length. The SI unit for the elastic modulus is pascal (Pa) or N/m², the practical unit is gigapascal (GPa). In the conduced tensile test, the loading direction was in the longitudinal direction, which means in the direction of the cellulosic fibers.
In the context of the present invention, the term "tensile strength" relates to the maximum tensile stress the cellulosic material can withstand. The loading direction was in the longitudinal direction, which means in the direction of the cellulosic fibers. The SI unit for tensile strength is pascal (Pa) or N/m², the practical unit is megapascal (MPa) or N/mm².
In the context of the present invention, the term "mold" relates to at least one block of material having a molding surface. The molding surface relates to that section of the surface of the mold that forms the negative shape, which corresponds to the desired positive shape, i.e. the shape of the densified piece of wood. The mold can be made of a solid or porous material or a mix thereof. The mold may consist of two parts, e.g. an upper and a lower part, wherein the delignified piece of wood may be placed between the upper and the lower part and subsequently densified by pressing the upper and the lower part together in combination with vacuum. If the mold is porous, the pores may be of irregular shape, i.e. variations in length, diameter and degree of branching occur. It is also possible, that the mold comprises pores of the same shape, for example open-ended through-pores (channels) having the same diameter. The mold can also be a grid if the grid possesses mechanical stability when shaped and the pore/opening range is in the proper range, i.e. the pores/openings should be small enough that the surface of the delignified piece of wood remains smooth upon densification and the pores/openings should be bid enough that water can flow through.
In the context of the present invention, the term "pore" or "porous" is understood according to the IUPAC definition (Pure & Appl. Chem. 1994, Vol. 66, No. 8, pp. 1739-1758, particularly the paragraph "Porous solid" on page 1742 and the section "Qualitative description of a porous solid" on pages 1742 to 1743).
A pore is a cavity, channel or interstice of a solid. The cavity, channel or interstice is deeper than wide. In contrast to this, a rough surface is not porous unless it has irregularities that are deeper than they are wide.
Pores may be classified according to their availability to an external fluid. Pores that are totally isolated from their neighbors are related to as closed pores. They influence such macroscopic properties as bulk density, mechanical strength and thermal conductivity, but are inactive in such processes as fluid flow and adsorption of gases. Closed pores may occur in porous molds or porous layers according to the invention. However, as they lack any continuous channel to the external surface, they do not contribute to draining off water.
Pores which have a continuous channel of communication with the external surface of the body, such as the molding surface, the outer surface of the mold or the surface of the porous layer, are related to as open pores. Some may be open only at one end; they are then described as blind (i.e. dead-end, or saccate) pores. Others may be open at two ends (through pores). Pores can also form a branched network of cavities, channels and/or interstices. The porous mold or porous layer according to the invention comprises open pores, which allow draining off the water during densification.
In the context of the present invention the term "positive pressure" relates to a pressure above the atmospheric pressure (> 1 bar). The term "negative pressure" relates to a pressure below the atmospheric pressure (<1 bar). 1 bar corresponds to 100000 Pascal (Pa). The atmospheric pressure is 1 bar. A pressure difference Δ(p) is calculated as follows: $Δ (p_{pos}) = positive pressure - atmospheric pressure$
$Δ (p_{neg}) = atmospheric pressure - negative pressure$

Description of the figures

Fig. 1: shows delignified wood veneers after wet processing in a non-porous closed mold by simple pressing.
Fig. 2: shows embodiments of the inventive method, (a) The densification is performed in a mold that may be a closed mold 10 comprising a lower part of the mold 11 and an upper part of the mold 12. The lower part of the mold 11 comprises a lower molding surface 13 and the upper part of the mold 12 comprises an upper molding surface 14, wherein the lower molding surface 13 is complementary to the upper molding surface 14. The upper and the lower part of the mold 11, 12 comprise pores 20. (b) A wet delignified piece of wood 40 is placed at the lower molding surface 13 of the lower part of a closed mold 11 or at the molding surface 15 of an open mold 16. (c) The wet delignified piece of wood is shaped, densified and dried in one step by pressing the upper part of the mold 12 on the lower part of the mold 11 (not shown) or by applying a negative pressure (block arrows) by using a vacuum bag 30 surrounding the open mold 16 and the delignified piece of wood 40. The water (thin arrows) drains off through the pores 20. (d) The densified piece of wood 41 (final composite) is demolded and the mold can be reused. The shape of the final composite corresponds to the shape of the upper and lower molding surface 13, 14 of the closed mold 10 (not shown) or to the molding surface 15 of the open mold 16. (e) The densification is performed in a solid mold that may be a closed mold 10 comprising a lower part of the mold 11 and an upper part of the mold 12. The lower part of the mold 11 comprises a lower molding surface 13 and the upper part of the mold 12 comprises an upper molding surface 14, wherein the lower molding surface 13 is complementary to the upper molding surface 14. (f) A wet delignified piece of wood 40 is placed at the lower molding surface 13 of the lower part a closed mold 11 or at the molding surface 15 of an open mold 16. (g) A porous layer 50, e.g. a flow mesh 32, is placed on top of the delignified piece of wood 40. The wet delignified piece of wood is shaped, densified and dried in one step by pressing the upper part of the mold 12 on the lower part of the mold 11 (not shown) or by applying a negative pressure (block arrows) by using a vacuum bag 30 surrounding the open mold 16, the delignified piece of wood 40 and the porous layer 50. The water (thin arrows) drains off through pores of the porous layer 50. A further textile layer 51 may be present between the molding surface 13, 14, 15 and the delignified piece of wood 40 (not shown). (h) The densified piece of wood 41 (final composite) is demolded and the mold can be reused. The shape of the final composite corresponds to the shape of the upper and lower molding surface 13, 14 of the closed mold 10 (not shown) or to the molding surface 15 of the open mold 16.
Fig. 3: shows a vacuum setup for densification of veneers. (a) An open mold 16 is placed on a flow mesh 32, which connects the mold 16 and the vacuum tubing 33. The mold is a porous 3D printed mold. The molding surface is covered by a textile layer 51 to obtain a nice surface finish. The delignified pieces of wood 40 are placed on the textile layer 51. Alternatively, the flow mesh 32, which supports the removal of water, may be placed on top of the delignified piece of wood 40 (not shown). (b) The setup from (a) is covered by an additional breather 31 for a better air flow and a vacuum bag 30.
Fig. 4: shows that vacuum-densified veneers (thickness = 3mm) reveal a good sample integrity. Samples (1 layer) were densified on top of a flat porous mold by applying a vacuum of approximately 5^∗10^-2 bar for 3 hours.
Fig. 5: shows that the mechanical properties increase with increasing delignification degree up to 4 hours delignification. (a) Elastic modulus (E-Modul) in relation to the delignification time. (b) Ultimate tensile strength in relation to the delignification time. Circle: open-mold process of wet delignified rotary cut spruce veneers (fully water-saturated (440 wt.%)); square: closed-mold process of wet delignified rotary cut spruce veneers (fully water-saturated (440 wt.%)); triangle: reference (dried at 65 % RH).
Fig. 6: shows the handling of delignified veneers. (a) Delignified veneers 40 are draped in wet state by using a metal grid 60 as support. The veneers are draped in a porous open mold 16. (b) Delignified veneer 40 (t=1.5 mm) draped on top of porous mold 16.
Fig. 7: shows manufacturing of 8-ply [0°/90°] lay-up on a flat porous mold. (a) Application of starch (glue) 70 in between layers 42; (b) Draping of next layer of delignified wood 42; (c) Densified and dried composite part 43.
Fig. 8: shows the production of a car door handle. (a) vacuum bag set-up. (b) finished starch-cellulose composite part.
Fig. 9: shows a lay-up of delignified wood veneers. Glue is applied between the layers (arrows).
Fig. 10: shows an example for an application in the automotive industry: covering of a tachometer out of delignified wood produced in the open-molding process.

Examples

Example 1: Mechanical properties of single veneer-layer

Rotary cut spruce veneers were cut to the dimensions 150 x 30 x 3 mm³ (longitudinal x tangential x radial) and were then delignified in a 1:1 H₂O₂ and HAc solution followed by heating-up to 80°C. Delignification times were set to 80, 160, 240 and 360 minutes. After delignification, samples were washed with water until a pH value of minimum 4.5 was reached.
Wet delignified (or partially delignified) veneers were then densified and dried by applying a vacuum densification approach (see Figure 3) using an oil vacuum pump to obtain a vacuum in the range of 10^-2 bar. Samples were densified and dried in the vacuum bag for approximately 3 hours.
Samples fully densified by the vacuum densification approach reveal a good sample integrity (Fig. 4), in contrast to the currently conducted simple closed-mold densification without applying vacuum, in which the material cannot be processed in fully wet state (see Figure 1 for comparison).
Tensile testing was conducted at 65% RH (relative humidity) to analyze the mechanical properties. A clear increase in the elastic modulus and strength is observed for densified (open-mold and closed-mold) samples compared to the reference due to a higher fiber volume ratio achieved by the densification process. A lower lignin content eases densification and thus results in lower thickness, a higher density and improved mechanical performance. A similar trend was obtained for the tensile strength values (Figure 5).

Example 2: Draping of curved layers

For the manufacturing of curved parts, only fully delignified veneers were considered due to their ability to be shaped into various forms. Open-porous molds or non-porous molds with a porous/conductive layer (e.g. continuous-fiber fabric, polymer mesh, flow grid...) were used for vacuum processing. Optionally, a textile layer can be placed between mold surface and delignified wood to increase the surface quality or to protect the mold from contamination. The mold is then covered by a vacuum-bag setup as shown in Figure 6.

Example 3: Multiple layers - lay-up

8 ply-composite with an initial thickness of 8x1.5 mm in native state and a final thickness of 2.5 mm, which corresponds to a densification down to approximately 1/4^th of the initial thickness of dry delignified wood, when taking into account the layer shrinkage upon delignification and drying (Fig. 7). Vacuum applied: in the range of 10^-2 bar.
Exactly the same approach as for the flat 8-ply part was used for manufacturing e.g. curved automotive parts. Depending on the targeted thickness and bending radii of the mold, the amount of layers needs to be reduced and the fiber directionality/orientation of the layers is adjusted. (Fig. 8)

Example 4: Layup of wet delignified veneers and "glue"

The delignified wood veneers are draped to the mold in the desired layup (unidirectional (UD), 0°/90°, quasi-isotropic...) to optimize for loading-conditions in in the final composite. The layers are glued by using natural glues (starch, tannins, MFC, NCC, methylcellulose...) or synthetic glues (thermoplastic or duroplastic polymers).

Example 5: Application of densified wood

The inventive process will lead to eased and fast production of large-scale densified cellulose fiber composites, e.g in the automotive industry (door panels, floor, dashboard ...), where the material could replace metals or fiber reinforced composites in order to reduce the weight for better fuel efficiency and to improve recyclability (Fig. 10).

List of references

10: closed mold
11: lower part of the mold
12: upper part of the mold
13: lower molding surface
14: upper molding surface
15: molding surface
16: open mold
20: pore
30: vacuum bag
31: breather
32: flow mesh
33: vacuum tubing
40: delignified piece of wood
41: densified piece of wood
42: layer of delignified wood
43: densified composite
50: porous layer
51: textile layer
60: metal grid
70: glue

Claims

A method for producing densified wood (41) comprising the steps of
- providing at least one delignified piece of wood (40) comprising cellulose fibers oriented as naturally grown, wherein the delignified piece of wood (40) is wet,

- placing said delignified piece of wood (40) in a mold (10, 16) comprising a molding surface (13, 14, 15), wherein said mold comprises a porous section that forms the molding surface and/or a porous layer (50) is present in said mold (10, 16) contacting said delignified piece of wood (40), wherein the delignified piece of wood (40) is placed in such a way that the cellulose fibers align predominantly in parallel to the molding surface (13, 14, 15),

- generating a pressure difference in such a way that the delignified piece of wood (40) is pressed against said molding surface (13, 14, 15) and densified yielding a densified piece of wood (41).
The method according to claim 1, wherein the water content of the delignified piece of wood (40) is ≥ 11 %, particularly ≥ 23 %.
The method according to any one of the preceding claims, wherein the lignin content of the delignified piece of wood (40) is below 10 %, particularly below 3 %.
The method according to any one of the preceding claims, wherein the delignified piece of wood (40) is obtained from a veneer, layer or a strip of wood, particularly from a veneer, layer or a strip of wood obtained by a rift cut, crown cut or rotary cut.
The method according to any one of the preceding claims, wherein the delignified piece of wood (40) has a thickness between 0.5 mm and 10 mm, particularly between 1 mm and 3 mm.
The method according to any one of the preceding claims, wherein the delignified piece of wood (40) is obtained by treating a piece of wood with
- at least one acid, in particular an inorganic or organic acid, more particularly acetic acid, sulfuric acid, chloric acid, peracetic acid, or

- at least one oxidizing agent, in particular hydrogen peroxide, sodium chlorite, sodium sulfite, ozone, more particularly hydrogen peroxide, under alkaline or acidic conditions, in particular under acidic conditions or

- at least one base, in particular sodium hydroxide.
The method according to any one of the preceding claims, wherein the size of the pores (20) of the mold (10, 16) and/or the porous layer (50) is between 0.1 mm and 2 mm, particularly between 0.3 mm and 0.5 mm, and/or wherein the porosity of the mold (10, 16) and/or the porous layer (50) is homogeneous or gradual.
The method according to any one of the preceding claims, wherein the porous layer (50) is placed on top and/or below the delignified piece of wood (40).
The method according to any one of the preceding claims, wherein the molding surface (13, 14, 15) is characterized by one or more radii of curvatures between 200 µm to infinite (flat).
The method according to any one of the preceding claims, wherein a negative pressure or a negative pressure in combination with a positive pressure is applied, wherein the pressure difference to atmospheric pressure is ≥ 1 bar, particularly ≥ 2 bar, when a positive pressure is applied, or the pressure difference is ≥ 0.9 bar, particularly ≥ 0.99 bar, when a negative pressure is applied.
The method according to any one of the preceding claims, wherein one or more delignified pieces of wood (40) form one or more layers and/or wherein the cellulose fibers of one or more delignified pieces of wood (40) are oriented in the same direction within one layer and/or wherein the cellulose fibers of one layer are oriented in a different direction with regard to the direction of the cellulose fibers of an adjacent layer.
The method according to claim 11, wherein an adhesive is applied between the layers or infiltrated into the delignified piece of wood (40) before the pressure difference is generated.
The method according to claim 12, wherein the adhesive is selected from natural glues and/or synthetic adhesives, wherein in particular the natural glue is selected from starch, tannins, microfibrillated cellulose (MFC), nanocrystalline cellulose (NCC), methylcellulose and the synthetic adhesive is selected from thermoplastic polymers, in particular polyethylene polymers (PE), polypropylen polymers (PP), polymethylmethacrylat polymers (PMMA), polylactic acid polymers (PLA), or duroplastic polymers, in particular epoxy polymers, melamine urea formaldehyde polymers (MUF), urea formaldehyde polymers (UF).
A densified wood (41), in particular obtained by the method according to claims 1 to 13, comprising one or more layers of delignified wood (40), wherein the cellulose fibers within one layer are oriented in the same direction and/or wherein the cellulose fibers of one layer are oriented in a different direction with regard to the direction of the cellulose fibers of an adjacent layer, characterized in that the fiber density is between 0.3 gcm^-3 and 1.5g cm^-3 in particular between 1 gcm^-3 and 1.5 gcm^-3.
The densified wood (41) according to claim 14, wherein the densified wood (41) is characterized by one or more radii of curvatures between 200 µm to infinite (flat).