Field of Art The invention relates to methods for production of wood-based panels, e.g., fibreboards or particleboards, with an improved density profile.

Background Art In the production of wood-based panels, pressing is the key step of the process. The pressing process is particularly important for achieving the appropriate density profile of the panel. The density profile represents the distribution of density in the panel cross- section and directly affects the physical and mechanical properties of the panel, thus, it is considered to be an important factor of the quality of the panels. The pressing is usually done in a hot-press with flat pressing boards.

Due to increasing demand for biomass during the past years competition for wooden raw materials used for particleboards and fibreboards have been stronger. Stategies to confront resource shortage include improved resource efficiency, higher productivity, the development of light-weight materials, and more use of alternative wood species. The mechanical properties of wood-based panels, i.e. internal bonding (IB), modulus of rupture (MOR). and modulus of elasticity (MOE), are connected to panels intrinsic density structure. For particlebards and medium density boards IB is known to be especially linked to the density of the panels core layer, while bending properties are more associated with the density of the surface layers.

The aim of the present invention is to further improve the density profile of wood-based panels.

Disclosure of the Invention

The present invention provides a method for production of wood-based panels with an improved density profile, which comprises the steps of pre-pressing, hot-pressing and curing, wherein during the hot-pressing step, a geometric pattern is impressed into both surfaces of the wood-based panel wherein the geometric pattern on one surface is shifted relative to the geometric pattern on the opposite surface.

The wood-based panels are panels made of a mixture of wood particles or fibres with adhesives (binders) and/or fillers and/or hardeners, such as resins and nitrates. The wood-based panels preferably include particleboards, fiberboards, MDF boards, OSB boards. The thickness of the panels is typically at least 5 mm, preferably at least 10 mm.

The geometric pattern must be regular, i.e., having the same distribution over the whole surface of the panel, and continuous, and preferably the geometric pattern is selected from honeycomb pattern, oval or circular patterns, rectangular patterns. The geometric patterns on the two opposite surfaces of the wood-based panel are preferably shifted relative to each other by one half of the pattern unit in at least one direction. The depth of the impressed geometric pattern is preferably at least 1 mm, more preferably at least 3 mm, or 1 to 5 mm, or 3 to 5 mm.

The wood based panel i.e. particleboard, fiberboard or oriented strandboard may be produced using common processing technology and conditions. Firstly the formed mattress of wood particles or fibers can be pre-pressed using pressure from 0,1 to 1,5 MPa, then the pre-pressed mattress is moved to the press where the shaped pattern is already placed according to description. The processing conditions differ according to the adhesive used in the boards. However, typical preferred range of temperature is from 60 to 200 °C and range of pressure is from 1.5 to 4 MPa. Wooden strands, particles or fibres as well as agricultural resources such as straw, hemp or kenaf in form of particles, strand or fibers, may be used as raw materials for the panel. This material may be bonded by various resins (urea-formaldehyde, melamine- forrnaldehyde, phenol-formaldehyde) as is common in industrial production. The boards may alternatively be produced without any adhesive in the form of binderless panels.

Thus, the present invention provides a method of improving the density profile of wood-based panels by impressing the geometrical pattern. The advantage of this invention is that with this process the targeted panel thickness remains unaltered, but the physical and mechanical properties of the panel are significantly improved.

The impressing of the geometric pattern is preferably performed by employing two pressing surfaces (such as pressing boards or pressing cylinders) in the hot-press, which bear the pattern in the form of a protruding geometric pattern. One pressing board impresses the geometric pattern into the top surface of the wood-based panel, and the second pressing board impresses the geometric pattern into the bottom surface of the wood-based panel. The geometric patterns on the boards are shifted relative to each other.

The edges of the protruding geometric pattern of the pressing surfaces are preferably rounded, in order to avoid formation of small defects during the pressing which could result in splitting and break-ups of the material. Thus, the present invention also encompasses a device for production of wood-based panels with an improved density profile, wherein the pressing tools are on the side adjacent to the pressed panel provided a protruding geometric pattern composed of geometric pattern elements. The geometric pattern elements may be, e.g., rectangles (incl. squares), circles, ovals, honeycomb pattern. The geometric pattern on the surface of one pressing tool is shifted relative to the geometric pattern on the surface of the second pressing tool.

Brief description of Drawings Figure 1 shows the two pressing surfaces (boards) of the hot-press and the pressed wood-based panel.

Examples

The invention is further illustrated by the following examples which should not be construed as limiting.

Example 1 Figure 1 shows a hot-pressing device i for pressing the panels 4 of the invention. It consists of the upper pressing board 2 and the lower pressing board 3. The panel 4 to be pressed in positioned between the pressing boards 2 and 4· The pressing boards 2 and 4 are provided with a protruding geometric pattern 7. In this particular embodiment, the geometric pattern 7 is a honecomb pattern, but other patterns can be used as well.

The geometrical pattern 7 on one pressing board is shifted relative to the geometric pattern on the other pressing board, thereby pressing a geometric pattern 8 into one surface of the wood-based panel 4 which is shifted relative to the geometric pattern on the opposite surface of the wood-based panel 4. The shift in the depicted embodiment is by one half of the pattern unit (one hexagon) in one direction and by one third of the pattern unit in perpendicular direction. The edges of the protruding pattern are rounded with the radius of 3 mm, and the contact edges between the flat part of the pressing board and the pattern are rounded with the radius of 2 mm. Example 2

Particleboards were produced at laboratory scale, using urea-formaldehyde resin (66% solid content), and ammonium nitrate (60% solid content) as a hardener. Particles, adhesive and hardener were blended in a laboratory resinating drum for 10 minutes, and the mixture then placed into a forming box (250x500mm). After pre-pressing the furnish was hot-pressed at 200°C. Two-side impressed particleboards at densities of 500 kg m ^"3 and 700 kg-m ^"3 (indicated OL500-IMP, and OL700-IMP), and a control (OL500- CO; OL700-CO), all at 12 mm thickness, were prepared. Production parameters are listed in table 1. Top and bottom surface impressments were done using pressing boards described above. After pressing both panel surfaces were sanded with a 180-grid sandpaper, and conditioned at 20°C and 65% relative humidity.

Mechanical characterization was carried out according to EN310 and EN319 using the universal testing machine ZWICK ^® Z050. As flat sample surfaces are required the impressed surface patterns on the IB samples were completely removed by sanding. Data were recorded by the testXpert software and statistically evaluated, using STATISTICA L 10. Table 1. Production parameters for the production of single-layer particleboard with impressed surface patterns

Production parameters of panels Single-layer particleboard Resination ([g] solid to [g] dry wood particles) 8% Hardener ([g] solid to [g] solid resin) 1,5% Targeted moisture content of particles 11%

The impressment dramatically changed the mechanical property profiles of the tested boards. Bending property data are listed in table 2 for all tested panel types MOR of OL500-IMP, relative to the control (OL500-CO) decreased by 23%, the 700kg/m3 type was lowered by 30%. Bending stiffness ( OE) was greatly reduced (up to 60%) For the internal bonding the different panels with impressment show significant increases. IB of OL500-IMP compared to OL500-CO doubled (+105%.), while OL700-IMP compared to OL700-CO increased even by 75%. The dramatic loss of bending strength can be assigned to applied surface shape alterations, which plays a crucial role in the bending performance. As a measure to improve bending the FEM-assisted approach with veneer overlaying was done, with the veneers mitigating the low stiffness. Statistical significance was present for all findings. The ANOVA with post-hoc Duncan test provided evidence that every panel with impressed density was significantly different from the control.

Table 2

OL500a OL500b OL700a OL700b

MOR [Mpa]

Mean value 308 2 ^~ 47 12.26 8.69 min [Mpa] 1.47 2.17 1 1.41 8.24 max [Mpa] 4.53 2.69 13.62 9.56

S dev 1.56 0.27 1.19 0.75 MOE [Mpa]

Mean value 707.08 297.33 2408.83 857.61 min 426 230 2221.32 825.2

max 914.67 369 2580.18 887.71

St.dev 252.49 69.6 179.97 31.32

IB [Mpa]

Mean value 0.17 0.35 0.49 0.86

min [Mpa] 0.12 0.31 0.34 0.62

max [Mpa] 0.25 0.39 0.58 1.09

St.dev 0.05 0.03 0.09 0.16

DENSITY [kg m-3]

Mean value 468.99 669.14

min 405.00 605.22

max 524.00 715.63

St.dev 42.47 39.17

Industrial Applicability The present invention is suitable for wood-based panels that need to have a higher internal integrity of layers, such as panels used in floors, ceilings, wall panels, or panels used in layered„sandwich-type" materials.