Patent Publication Number: US-2015064390-A1

Title: Core, panel blank, floor panel and methods of their production

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/EP2012/058443, filed May 8, 2012, of which priority is claimed, the disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to building panels and in particular to floor panels. 
     More specifically, the present disclosure relates to a core for inclusion in a building panel, such as a floor panel; a blank for forming building panels and to methods for forming such a core, blank and building panel. 
     BACKGROUND 
     It is known to provide parquet floor panels comprising a core which is formed of a plurality of low-grade wood blocks (also termed “slats” or “lamellae”, which are arranged with their fiber direction perpendicular to a longitudinal direction of the floor panel. The core is sandwiched between a decorative layer, typically of an about 3 mm wood surface layer, and a balancing layer, typically of an about 2 mm wood veneer. The decorative layer and the balancing layer are glued to the core. 
     Floor panels as described above have been manufactured since the 1940&#39;s, and, apart from improvements in surface treatment, glue technology and the introduction of mechanical locking systems, have undergone relatively little change. 
     Examples of such parquet floor panels are disclosed in e.g. WO96/30177 A1 and US2005/0208255A1. 
     It is desirable to further reduce the cost of floor panels of the above referenced type. 
     Definitions 
     The term “panel blank” is understood as a piece of material which has substantially the same format as the end product, but which has not been completed with respect to the formation of e.g. a locking system at edges of the panel blank. 
     The term “blank”, is understood as a piece of material, which is intended to be divided into a plurality of panel blanks. 
     The “major surface” is typically the upwardly or downwardly surface of a core, blank, panel blank or building panel or floor panel. These surfaces are typically parallel with each other. 
     A “principal direction” of a core, blank, panel blank or building panel or floor panel may be a longitudinal direction in a case where the item is elongate (i.e. substantially rectangular and non-square). As an alternative, or complement, the principal direction may be a direction which is substantially parallel with a direction along which another layer than the core layer, e.g. a decorative layer and/or a balancing layer, forming part of the blank, panel blank or floor panel presents its major bending strength and/or major tensile strength, such as a principal fiber direction of the decorative layer and/or of the balancing layer. 
     A “principal fiber direction” is defined as the direction along which most of the fibers of the wood block or veneer extend. Typically, the principal fiber direction substantially coincides with a longitudinal direction of the wood block since lumber is normally sawn so as to have its fibers extend substantially longitudinally. 
     An “expanded polymer material” is understood as a polymer material which contains a resin and a substantial amount of gas-filled inclusions. 
     SUMMARY 
     The present invention aims generally at overcoming or alleviating some disadvantages of prior art floor panels, with a particular focus on reduction of cost. 
     The invention is defined by the appended independent claims, with embodiments being set forth in the dependent claims, in the following description and in the drawings. 
     According to a first aspect, there is provided a core for inclusion in a building panel. The core presents a pair of substantially parallel major surfaces, and comprises a plurality of first material blocks, extending between said major surfaces and arranged with their respective principal directions substantially parallel with said major surfaces and extending 90°±45° to a principal direction of the core; and a plurality of second material blocks, extending between said major surfaces and being arranged alternatingly with said first material blocks along the principal direction of the core. The first material blocks and said second material blocks differ substantially with respect to at least one material property. 
     Such a core provides an opportunity for improved control of properties associated with the core, such as strength and weight, and of the cost of the core. 
     The “principal direction of the core” may be understood as a longitudinal direction of the core, which is particularly applicable when the core is to be produced as an elongate or substantially continuous sheet-like product. 
     The first material blocks preferably extend 90°±30°, 90°±15°, 90°±5° or substantially perpendicular to the principal direction of the core. The second material blocks may typically extend substantially parallel to the first material blocks. 
     By “differ substantially” is meant that the difference is more than what would be incurred by the mere random variation which occurs between blocks of material which are of the same type and orientation, such as would be the case between two pieces of the same type of wood, or as would be the case due to manufacturing tolerances, etc. 
     The core may present a thickness between its major surfaces of about 5 mm to about 50 mm, preferably about 7 mm to about 15 mm. 
     A cross section of the core, taken along the principal direction of the core, perpendicular to the major surfaces and containing an equal number of whole first material blocks and whole second material blocks, may present less than 50% by area of said second material blocks. 
     Hence, the first material blocks may form more than 50% by volume of the core, not counting any edge reinforcing insets. 
     The core may comprise at least one web, which may be attached to and substantially cover at least one of the core&#39;s major surfaces. 
     The web may comprise at least one of a non-woven material, a woven material, a pulp-based material and a polymer film. 
     The web may comprise a material having adhesive properties with respect to said first blocks and/or said second material blocks. 
     The web may be flexible enough to allow the core to bend so as to allow an angle of more than 5°, preferably more than 10°, more than 15°, more than 20° or more than 25°, between major surface normals of a pair of adjacent first material blocks. 
     The first material blocks and second material blocks may be formed from materials, which differ with respect to at least one of material type, material composition, material density, material hardness, material tensile strength and material elastic modulus. 
     With respect to “material type”, the blocks may be formed of different materials, such as wood and expanded polymer material or different types of polymer materials or different kinds of wood. 
     With respect to “material composition”, the blocks may be formed of polymer materials having different material compositions and/or of polymer materials containing different types of fillers or additives. 
     With respect to material density, the first material blocks may have a higher density than the second material blocks. 
     With respect to “material hardness”, the first material blocks may be harder than the second material blocks. 
     With respect to “material orientation” the first material blocks and the second material blocks may have anisotropic properties, and may be oriented in different directions. For example, the first material blocks may be have fibers extending substantially perpendicular to the principal direction of the core, while the second material blocks extend substantially parallel to the principal direction of the core, or parallel to a thickness direction of the core. 
     With respect to “material tensile strength”, the first material blocks may have higher tensile strength than the second material blocks. 
     With respect to the “material elastic modulus”, the first and second materials may present different bending stiffness. Such different bending stiffness may typically be presented at least with respect to the longitudinal direction of the material blocks. 
     The principal direction of the first material blocks may be substantially parallel with a direction of major bending stiffness and/or of major tensile strength of the first material blocks. 
     In one set of embodiments, the first material blocks may be formed from a wood material. 
     The principal direction of the first material blocks may thus be substantially parallel with a principal fiber direction of the wood material. 
     The wood blocks may comprise wood blocks of a wood quality, which is lower than or equal to V according to the Swedish “Gröna boken” (“The Green book”) standard of 1960, preferably lower than or equal to VI or lower than or equal to VII. As an alternative, the wood blocks may be of a wood quality which is lower than or equal to B according to the Swedish “Bl{dot over (a)}a boken” (“The Blue book”) standard of 1994, preferably lower than or equal to C or lower than or equal to D. As yet another alternative, the wood blocks may comprise wood blocks of a quality which is lower than or equal to G4-2/G2-2 of the European EN 1611-1 standard of 2000, preferably lower than or equal to G4-3/G2-3 or lower than or equal to G4-4/G2-4. These standards are hereby incorporated by reference. 
     Preferably, the wood is of quality V or VI according to the “Gröna boken” standard, or of a corresponding quality according to any other standard. 
     The wood blocks may also, or as an alternative, be of a strength class which is lower than or equal to C18 of the EN 338 standard, preferably lower than or equal to C16 or lower than or equal to C14. 
     In another set of embodiments, the first material blocks may be formed of a composite material. 
     The composite material may comprise a polymer material matrix, and a filler material and/or a reinforcing material. 
     The filler material may comprise wood dust, wood fibers, wood particles, wood chips, a mineral-based filler or a polymer material. 
     The reinforcing material may be arranged to provide anisotropic strength properties of the composite material. 
     For example, the reinforcing material may comprise elongate fibers extending substantially parallel with the principal direction of the first material blocks. Hence, the first material blocks may be formed from an engineered wood material. 
     Non-limiting examples of engineered wood materials include fiberboards, chip boards, particle boards, and reconstituted wood. It is possible to form the first and second material blocks from different types of engineered wood materials. 
     The second material blocks may be formed from an expanded polymer material, an engineered wood material or a non-expanded polymer material. Expanded polymer material blocks may comprise at least one of expanded polyethylene, expanded polypropylene and expanded polystyrene extruded polystyrene foam. 
     An expanded polymer material may be non-filled or filled with e.g. fillers in the form of wood dust, wood fibers, wood particles, wood chips, a mineral-based filler and/or a polymer material. 
     Typically, the second material blocks would be formed of an engineered wood material having lower cost, strength and/or density than that of the first material blocks. 
     Typically, a non-expanded polymer material may comprise one or more polymer matrix materials and fillers in the form of wood dust, wood fibers, wood particles, wood chips, a mineral-based filler and/or a polymer material, having lower cost and/or strength than that of the first material blocks. The density may be lower or higher. A low density would provide weight advantages, while a higher density may provide desirable sound absorption or reduction properties. According to a second aspect, there is provided a blank for forming a building panel, comprising a core as described above, and at least one structural layer substantially covering least one of the core&#39;s major surfaces. 
     A building panel may, as non-limiting examples, be a floor panel, a wall panel, a ceiling panel or a roof panel. 
     The structural layer may comprise a decorative layer and/or a wear layer. 
     The structural layer may further comprise a reinforcing layer arranged between the core and the decorative layer. 
     In one set of embodiments, the decorative layer may comprise a laminate material. 
     In another set of embodiments, the decorative layer may comprise wood. 
     In another set of embodiments, the decorative layer may comprise (or consist essentially of) a plastic material, e.g. including polyvinyl chloride, acrylic, epoxy, polyurethane; rubber or rubber-like material; or linoleum. 
     In some embodiments, the decorative layer may be formed essentially of a solid wood material. 
     The blank may further comprise a balancing layer comprising a wood veneer, a polymer film, a woven web, a non-woven web and/or a pulp based material. 
     The principal direction of the core may be substantially parallel with a direction of major bending stiffness of at least one of the structural layers. 
     The principal direction of the core may be substantially parallel with a longitudinal direction of the blank. 
     According to a third aspect, there is provided a building panel formed from a blank as disclosed above. Such floor panel may be provided with a locking system along at least two of its opposing edge portions. 
     According to a fourth aspect, there is provided a method of making a core for a building panel, the core presenting a pair of substantially parallel major surfaces. The method comprises providing a plurality of first material blocks, arranging the plurality of first material blocks with their respective principal directions substantially parallel with the major surfaces and substantially perpendicular to a principal direction of the core, and providing second material blocks arranged alternatingly with said first material blocks in said principal direction of the core. 
     The step of providing said second material blocks may comprise arranging pre-formed second material blocks alternatingly with said first material blocks. 
     The step of providing said second material blocks may comprise providing a polymer material resin in a space providing a longitudinal separation of two of said first material blocks and subsequently causing the polymer material resin to expand. 
     The step of providing the second material block may comprise providing the second material block with a thickness which at least initially is greater than that of said first material blocks. 
     The method may further comprise compressing said second material block in connection with the application of a web or structural layer to the core. 
     The method may further comprise using said second material block to provide adhesion between the core and the layer. 
     According to a fifth aspect, there is provided a method of making a building panel, the method comprising providing a blank as described above, cutting a building panel blank from the blank along the principal direction of the core to a format suitable for providing a building panel, and forming mechanical connecting means along at least one edge of the floor panel blank. 
     The method may further comprise providing a surface treatment on at least part of the mechanical connecting means. 
     As an alternative or additional measure, the surface treatment may comprise applying a coating to an exposed part of said second material block. 
     As an alternative or additional measure, the surface treatment may comprise thermoforming an exposed part of said second material block. 
     According to a sixth aspect, there is provided a floor panel, comprising a decorative layer and a core layer. The core is arranged below the decorative layer when the floor panel is substantially horizontally oriented. The core layer comprises a plurality of wood blocks arranged with their respective principal fiber directions substantially parallel with a plane defined by a major surface of the floor panel and substantially perpendicular to a principal direction of the floor panel. The core layer comprises a plurality of second material blocks, which are made from a different material than said wood blocks, and along the principal direction of the floor panel, the core layer presents alternating portions formed of said wood blocks and of said second material blocks. 
     A “second material block” is understood as a block of material which is different from the material of the wood blocks. The second material blocks may e.g. be of generally lower quality (strength and/or stiffness) and/or cost than the wood blocks. Hence, the second material blocks mainly serve to fill the space between the wood blocks, and possibly to provide sound reduction. 
     A floor panel comprising a plurality of second material blocks may be produced at reduced cost compared with a floor panel comprising only wood blocks, since such second material may be less costly than even the lowest quality wood. 
     Moreover, the second material may reduce weight of the floor panel as compared to a floor panel comprising only wood, thus reducing cost of transportation and facilitating installation. 
     Along the principal direction of the floor panel, a total length of the second material blocks may be less than a total length of the wood blocks. 
     Along the principal direction of the floor panel, a total length of the second material blocks may be more than 5%, preferably more than 10%, 20%, 30% or 40% of the total length of the floor panel. Typically, the total length of the second material blocks is less than 50% of the total length of the floor panel. 
     The second material blocks may have lower density than said wood blocks and/or lower tensile strength in a direction parallel to said principal fiber directions than said wood blocks. 
     The second material blocks may comprise expanded polymer material blocks, such as at least one of expanded polyethylene, expanded polypropylene and expanded polystyrene extruded polystyrene foam. 
     As an alternative or complement, the second material blocks may comprise an engineered wood product. 
     As yet another alternative or complement, the second material blocks may comprise a non-expanded polymer material comprising filler material. 
     The wood blocks may comprise, or consist essentially of, wood blocks of a wood quality, which is lower than or equal to V according to the Swedish “Gröna boken” (“The Green book”) standard of 1960, preferably lower than or equal to VI or lower than or equal to VII. As an alternative, the wood blocks may be of a wood quality which is lower than or equal to B according to the Swedish “Bl{dot over (a)}a boken” (“The Blue book”) standard of 1994, preferably lower than or equal to C or lower than or equal to D. As yet another alternative, the wood blocks may comprise wood blocks of a quality which is lower than or equal to G4-2/G2-2 of the European EN 1611-1 standard of 2000, preferably lower than or equal to G4-3/G2-3 or lower than or equal to G4-4/G2-4. 
     The wood blocks may also, or as an alternative, comprise, or consist essentially of, wood blocks of a strength class, which is lower than or equal to C18 of the EN 338 standard, preferably lower than or equal to C16 or lower than or equal to C14. 
     Hence, wood blocks, which are of low or very low quality, and hence cost, may be used to a great extent. 
     The core layer may further comprise a reinforcing inset portion, which may be provided at a short side edge portion of the floor panel. 
     Typically, the core layer of the floor panel may consist of about 80-99% by volume, possibly 90-95% by volume, of alternating wood and second material blocks. The remainder may consist of, inter alia, inset pieces, adapted to provide specific parts of the locking system. 
     The second material blocks may be provided with surface coating at an edge portion where said second material blocks are exposed. 
     Alternatively or as a complement, the second material blocks may present a portion of increased density at an edge portion where said second material blocks are exposed. 
     The decorative layer may comprise, or consist essentially of, natural wood. 
     The floor panel may comprise a balancing layer, and the core layer may be sandwiched between the balancing layer and the decorative layer 
     According to a seventh aspect, there is provided a method of making a floor panel, comprising arranging a plurality of elongate wood blocks side by side in a co-planar manner, with their respective longitudinal directions substantially parallel with each other to form a core layer. The method further comprises providing the core layer with at least a decorative layer to form a panel blank, and cutting the panel blank to form an elongate floor panel blank, such that the longitudinal directions of the wood blocks are substantially perpendicular to a principal direction of the floor panel blank. The method further comprises providing a plurality of second material blocks arranged alternatingly with the wood blocks along the principal direction of the floor panel blank. The second material blocks have lower density than said wood blocks and/or lower tensile strength in a direction parallel to said principal fiber directions than said wood blocks. 
     The method may further comprise forming locking means along said principal direction of the floor panel blank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic exploded view of a panel blank. 
         FIG. 2  is a schematic perspective view of a panel blank. 
         FIG. 3  is a schematic perspective view of a part of a floor panel. 
         FIG. 4  is a schematic top view of a floor panel. 
         FIG. 5  is a schematic cross sectional view of a floor panel. 
         FIGS. 6   a - 6   c  schematically illustrate a method of producing a blank or a panel blank. 
         FIGS. 7   a - 7   d  schematically illustrate a first alternative method of producing a blank or a panel blank. 
         FIGS. 8   a - 8   b  schematically illustrate a second alternative method of producing a blank or a panel blank. 
         FIGS. 9   a - 9   c  schematically illustrate a core. 
         FIG. 10  schematically illustrates a third alternative method of producing a blank or a panel blank. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a schematic exploded view of a floor panel blank  1 , comprising a decorative layer  13 , a core layer  10 ,  11  and a balancing layer  12 .  FIG. 2  shows a floor panel blank  1 ′ in an assembled state. The core layer  10 ,  11  is formed of wood blocks  10  and expanded polymer blocks  11 , which are arranged alternatingly along a longitudinal direction L of the floor panel blank. For example, this longitudinal direction of the floor panel blank may be substantially parallel with a principal fiber direction of a wood veneer forming a visually exposed upper surface of the decorative layer  13 . 
     The wood blocks  10  may be substantially elongate, with their respective principal fiber direction substantially coinciding with a respective longitudinal direction l of the respective block  10 . A cross section of the wood blocks  10  may present a thickness Tw and a width Ww. The width Ww may be greater than the thickness Tw. 
     The longitudinal direction l of the wood blocks  10  may extend 90°±45° relative to the longitudinal direction L of the floor panel blank. Preferably, the wood blocks extend 90°±30°, 90°±15°, 90°±5° or 90°±1° relative to the longitudinal direction of the panel blank. 
     The wood blocks may comprise blocks of any appearance, but for cost reasons, it may be preferred that the wood blocks comprise, or consist essentially of, (e.g. consist of at least 50%, at least 70%, at least 90% or at least 95%) a wood quality lower than or equal to V according to the Swedish “Gröna boken” (“The Green book”) standard of 1960, preferably lower than or equal to VI or lower than or equal to VII; lower than or equal to B according to the Swedish “Bl{dot over (a)}a boken” (“The Blue book”) standard of 1994, preferably lower than or equal to C or lower than or equal to D; or lower than or equal to G4-2/G2/2 of the European EN 1611-1 standard of 2000, preferably lower than or equal to G4-3/G2-3 or lower than or equal to G4-4/G2-4. 
     Moreover, the wood blocks may have a strength class lower than or equal to C18 of the EN 338 standard, preferably lower than or equal to C16 or lower than or equal to C14. 
     The wood blocks may be formed of elongate wood beams, and/or of shorter pieces of wood which have been joined together (e.g. using finger joints, dovetail joints and/or glue). 
     The expanded polymer blocks  11  may be so-called “rigid polymer foams”, which may be defined as foams that have a modulus of elasticity (E) of about 0.02-0.4 GPa, preferably of about 0.1-0.4 GPa. The density may be in the area of 20-400 kg/m3, preferably of about 10-100 kg/m3, most preferably of about 20-50 kg/m3. 
     As an alternative, the expanded polymer blocks  11  may be so-called “flexible polymer foams”, which may be defined as foams that have a modulus of elasticity (E) of about 0.0003-0.0015 GPa. The density may be in the area of 5-200 kg/m3. 
     The expanded polymer blocks  11  may be so-called closed cell expanded polymer or open cell expanded polymer. 
     Examples of materials include polystyrene, polyurethane, polyolefin (e.g. polyethylene, polypropylene), thermosets (phenolic, melamine, urea or combinations thereof) and polyvinyl chloride (PVC). 
     The expanded polymer material blocks may comprise additives, such as wood chips, wood particles, wood dust (saw dust, wood flour) or wood fibers; mineral fillers (calcium, dolomite, etc); polymeric fillers (e.g. recycled polymers, rubbers or rubber-like materials). The expanded polymer blocks  11  are illustrated as being substantially elongate, with their respective longitudinal direction parallel with the longitudinal directions l of the wood blocks  10 . A cross section of the expanded polymer blocks  11  may present a thickness Tp and a width Wp. The width Wp may be greater than the thickness Tp. 
     The thickness Tw of the wood blocks may typically be on the order of about 5-50 mm, more commonly about 5-25 mm or about 7-15 mm, and most typically about 10 mm. 
     Typically, the thickness Tw of the wood blocks  10  may be substantially the same, or exactly the same, as the thickness Tp of the expanded polymer blocks  11 . 
     The width Ww of the wood blocks  10  may be greater than the width Wp of the expanded polymer blocks  11 . As non-limiting examples, the width Wp of the expanded polymer blocks  11  may be less than about 90%, less than about 80%, less than about 70% or less than about 60% of the width Ww of the wood blocks  10 . 
     The floor panel  1 ″ may thus have a core formed of alternating wood and expanded polymer blocks along substantially its entire length, optionally with the exception of one or both short side edges, which may be provided with a reinforcing inset, which may be formed of wood of a higher quality than that of the wood blocks  10 , a different type of wood, and/or of wood having a different fiber direction, e.g. perpendicular to, that of the wood blocks  10 . Alternatively, the insets may be formed of another material, such as MDF, HDF, polymer or polymer composite (e.g. fiber reinforced polymer). Each of the insets may extend over less than 15% of the floor panel&#39;s  1 ″ length, preferably less than 10%, less than 5% or less than 3% of the floor panel&#39;s  1 ″ length. 
     Such insets may, additionally, or as a complement, be provided for one or both long side edges, in which case the insets would extend along all or part of the long side edge(s), and perpendicular to the principal direction of the floor panel. 
     Referring to  FIGS. 3 and 4 , the floor panel  1 ″ may be provided with a locking system  30 ,  31 ,  32 ,  33  adapted for interconnection of the floor panel  1 ″ with similar or identical floor panels. Such a locking system may be arranged to provide mechanical or form-fit locking horizontally and/or vertically, i.e. the floor panels may be locked together wholly or partially without the use of glue. 
     In one embodiment, the long side locking system  30 ,  31  may be identical with the short side locking system  32 ,  33 . In another embodiment, the long side locking system  30 ,  31  may be different from that of the short side locking system in terms of e.g. geometry and/or mechanical properties of the locking system. 
     A plurality of different systems for mechanical interconnection of floorboards are known as such, and so are the methods for producing them. Non-limiting examples of such systems are shown in WO94/26999A1 U.S. Pat. No. 4,426,820, DE1211175, DE19851200C1, FR2675174, WO99/66151, WO97/47834, JP3169967, WO96/27721, GB1430423, WO02/55809, U.S. Pat. No. 4,819,932, EP1863984A1 and EP1802827A1, the entire contents of which documents are hereby incorporated herein by reference. 
     Hence, further description of the locking system is deemed unnecessary. 
     Referring to  FIGS. 6   a - 6   c,  a method of producing a floor panel  1 ″ is disclosed. 
     Pre-cut blocks  10  of wood as well as pre-formed (e.g. cut or moulded) blocks  11  of expanded polymer may be arranged on a first sheet, which may be the balancing layer  12  or the decorative layer  13 . The first sheet  12 ,  13  and/or the side of the blocks  10 ,  11  facing the first sheet may be provided with glue prior to the joining of the blocks  10 ,  11  to the first sheet  12 . Moreover, a second sheet  13  is provided and its side facing the blocks  10 ,  11  and/or the side of the blocks  10 ,  11  facing the second sheet are provided with glue, after which the second sheet  13  is joined with the blocks. 
     The blocks  10 ,  11  may be arranged so as to abut each other along their longitudinal direction l. Alternatively, a small space in the longitudinal direction L of the floor panel may be provided between the members of each pair of adjacent blocks. The space may be in the order of 0.1-5 mm, preferably 1-3 mm. 
     After the sheets  12 ,  13  and blocks  10 ,  11  have been joined, they may be subjected to pressure, either batchwise or continuously, e.g. by rollers or a belt press. The pressure may be complemented with addition of heat or other means for activating the glue, such as ultrasound, microwaves, etc. 
     Typically, these steps are performed to provide a blank, which may have a size of on the order of about e.g. 1.2 m×2.4 m. This blank may then be cut into floor panel blanks  1 ′, i.e. blanks having the general shape of the finished floor panel  1 ″, but without any locking system  30 ,  31 ,  32 ,  33  formed along its edges. The floor panel blanks  1 ′ may then be subjected to milling and optionally addition of separate elements (locking tongues, locking elements, locking strips, etc.) to provide the desired locking system  30 ,  31 ,  32 ,  33 . 
     The decorative layer  13  may be in the form of a solid wood sheet, which typically has a thickness of about 2-5 mm, most often about 3 mm. The wood sheet may be composed of a single or few wood blocks, or of a plurality of lamellae to form a block pattern on the surface. Typically, most, or at least some, of these lamellae are elongate and extend substantially parallel to the longitudinal direction L of the floor panel. Alternatively, some of the lamellae extend perpendicular to the longitudinal direction L of the floor panel, as would be the case with floor panels for providing a so-called Dutch floor pattern. 
     Alternatively, the decorative layer may be formed of a plurality of layers, such as e.g. a reinforcement layer combined with a thin surface layer, which is laminated or glued to the reinforcement layer. The surface layer may be formed of e.g. a thin (0.5-2 mm) wood veneer, or a laminate, i.e. an artificially patterned (e.g. printed) layer, which may be laminated to, or integrated with, a wear layer, which may comprise a polymer material with fillers and additives for providing a hard surface. The reinforcement layer may be formed of a plywood, MDF, HDF or other sheet material. 
     A decorative layer  13  of wood or wood veneer may be natural, i.e. uncoated, or coated/treated with e.g. lacquer, wax, oil, varnish or any other type of surface finish, such as acrylic or urethane based floor finishes. 
     The balancing layer  12  may be formed of a thin (0.5-2 mm) sheet of wood or plywood. Alternatively, the balancing layer may be formed of a polymer film, a woven or non-woven sheet, a pulp-based material (such as paper) or other wood-polymer composite material. 
     The floor panel typically forms part of a set of substantially identical floor panels, which are to be joined together to provide a flooring. 
     Referring to  FIGS. 7   a - 7   d,  a first alternative method of forming a blank will now be described. A first structural layer, such as a balancing layer  12  (or a decorative layer  13 ) is provided ( FIG. 7   a ). A number of wood blocks  10  are provided and arranged on the first structural layer. Optionally, the wood blocks  10  may be attached (provisionally or permanently) to the structural layer. The wood blocks  10  are arranged in a spaced manner along the principal direction L of the blank. 
     A polymer resin  9  adapted for being expanded is arranged in the spaces between the wood blocks ( FIG. 7   b ). The polymer resin may e.g. contain one or more expanding agents. 
     A second structural layer, such as a decorative layer  13  (or balancing layer  12 ) is arranged so as to sandwich the wood blocks  10  between the first and second structural layers ( FIG. 7   c ). The second structural layer may be attached to the wood blocks (provisionally or permanently). 
     The expanding agent(s) present in the polymer resin  9  is then activated so as to expand to substantially fill up the space between the wood blocks  10 . As the polymer resin sets, hardens or cools, the expanded polymer material blocks  11  will be provided. 
     The activation of the expanding agent(s) may be achieved by provision of heat, microwaves, ultrasound, vacuum, UV light, etc. 
     The activation of the expanding agent may also cause an activation of a glue or other attachment means for attaching the structural layers to the wood blocks. 
     In a variant of this method, the wood blocks  10  may be sandwiched between the structural layers  12 ,  13  prior to the introduction of the resin  9 . In this case, the resin may be subsequently injected into the spaces formed between the wood blocks and structural layers. In such case, the resin may, but need not, be activated immediately on introduction, e.g. upon contact with the surrounding atmosphere. 
     It is also possible to inject a non-expanding, and possibly filled, polymer resin into the spaces formed between the wood blocks (or other material blocks). 
     The resin  9  may, in addition to providing the expanded polymer material blocks  11 , also provide attachment of blocks to the structural layers, both of the expanded polymer material blocks and of the wood blocks. For example, the wood blocks may be designed or arranged so as to allow for a sufficient amount of resin to enter between the wood blocks and the structural layers, thereby providing adhesion. 
     Referring to  FIGS. 8   a - 8   b,  another alternative method of producing the blank will be described. 
     Expanded polymer material blocks  11  are provided with a thickness Tp which exceeds the thickness Tw of the wood blocks  10  ( FIG. 8   a ). The blocks may be provided as pre-shaped blocks or by activation of the expanding agent prior to application of the second structural layer. 
     The second structural layer  13  may then be applied, upon which the expanded polymer material blocks  11  are compressed ( FIG. 8   b ). The compression may be provided in the pressing step described with respect to  FIGS. 6   a - 6   c.    
     Referring to  FIGS. 9   a - 9   c,  a method of producing a core (without structural layers) will be described. 
     In a first embodiment ( FIG. 9   a ), the core may be produced by connecting alternating wood blocks  10  and expanded polymer material blocks  11  through e.g. adhesion (surface to surface) to form a continuous core without any structural layers. 
     If the expanded polymer material blocks  11  are relatively flexible, the core may be bent and possibly even wound as a roll. 
     In a second embodiment ( FIG. 9   b ), the core may be provided with a web  20 , to which the blocks  10 ,  11  will be attached in to a major surface of the core in a manner similar to what was disclosed with respect to the first structural layer  12 . 
     The web  20  may be a polymer film, a non-woven web, a woven web, a pulp-based (paper or paper-like) web, a rubber film, etc. Hence, the web may be flexible enough to allow the web to bend and to be wound as a roll. The web may have a thickness in the area of about 0.1-3 mm and preferably of about 0.1-1 mm. 
     In the third embodiment ( FIG. 9   c ), a second web  21  may be provided at the second face Sc 1  of the core. The second web may be identical to the first web, or it may be a different type of material. 
     It is possible to form the web of a material which, e.g. on activation will operate as a binder, adhesive or glue relative to the core and/or to the structural layers. It is also possible to apply a coating to, or impregnate the web with, a material which e.g. on activation will operate as a binder, adhesive or glue relative to the core and/or to the structural layers. In the event that such material is tacky or pressure activated, it is possible to provide also a protective film to one or both exposed faces of the webs  20 ,  21 . Such a protective film may thus be removed when the core provided with the web is to be joined to e.g. a structural layer. 
     It is noted that the methods of forming the panel described with respect to  FIGS. 6   a - 6   d,    7   a - 7   d  and  8   a - 8   b  may be applied to the forming of the core as described with respect to  FIGS. 9   a - 9   c,  provided that the possibly outwardly tackiness or stickiness is managed through e.g. a protective film and provided that the pressure is applied correctly. 
     Hence, it is possible to provide a core, which can be wound onto a roll, as schematically illustrated in  FIG. 10 . The core may then be unwound from the roll and any type of structural layers may be laminated to it under pressure Fp to produce a substantially continuous blank, such as wood or laminate decorative layers or backing layers. 
     It is noted that the descriptions above on production of a blank may be applied to a panel blank as well, in the event that it is desirable to produce the panel blank directly, rather than forming it by dividing a blank into several smaller pieces. 
     The combination of wood blocks and expanded polymer material blocks, is to be understood as an example of an implementation of the present disclosure. More generally, the wood blocks may be seen as a first material block and the expanded polymer material blocks may be seen as a second material block. 
     The first material blocks may thus be provided from another material than wood, such as e.g. a polymer/plastic material. The blocks may be formed from a composite material comprising a binder, such as a polymer matrix, one or more fillers and/or one or more reinforcing fibers. 
     Examples of fillers include one or more of wood chips, wood particles, wood dust (saw dust, wood flour) or wood fibers; mineral fillers (calcium, dolomite, etc); polymeric fillers (e.g. chunks or particles of recycled polymer, rubbers or rubber-like materials). 
     Examples of reinforcing fibers include one or more of glass fibers, carbon fibers, plant fibers (e.g. hemp, grass, bamboo or wood fibers or wood strands), or polymer fibers. 
     Hence, the first material blocks may be formed of so-called “engineered wood” or “reconstituted wood”, which as such is known from e.g. WO 2009/140136A2 and WO2010/082137A1, the entire contents of which is hereby incorporated herein by reference. 
     Examples of polymer matrices include one or more of thermoplastic material (polyethylene, polypropylene, etc.), epoxy, polyurethane, thermosets (phenol, melamine, urea or combinations thereof), acrylic and polyester. 
     The second material blocks may differ from the first material blocks with respect to at least one of material type (i.e. it may be formed from a completely different material), material composition (i.e. it may be formed from the same materials as the first material blocks, but with different proportions), material density (i.e. it may be formed (e.g. expanded) to have a different density than the first material blocks), material hardness (i.e. it may be treated so as to achieve a higher or lower hardness) and material elastic modulus. Typically, the first material blocks may have a higher elastic modulus along a direction l which is perpendicular or otherwise transverse to the principal direction L of the core than the second material blocks, such that the blocks as such are stiffer. This may be achieved by e.g. arranging elongate fibers to extend substantially perpendicular to the principal direction L of the core. 
     The second material blocks may have a substantially lower density than the first material blocks, e.g. due to being expanded. 
     In the alternative, the second material blocks may have a substantially higher density than the second material blocks, e.g. due to comprising a relatively heavy filler. This may be desirable in order to reduce sound. 
     While the first material blocks may be selected to provide the necessary bending strength and joining strength, the second material blocks may be selected to provide another property, such as weight reduction, cost reduction and/or sound reduction. 
     The second material blocks referred to above may be formed of any material, which may be less costly, have lower density and/or lower tensile strength in the direction which is parallel with the principal direction of the wood fibers of the wood blocks. 
     Hence, the second material blocks may comprise an engineered wood product. 
     A first example of such a material is a so-called LDF (low density fiberboard) or VLDF (very low density fiberboard), which may be a board which is formed through moulding or compression moulding of wood chips, particles and/or dust. It is possible to mould such material during wet or moist condition, in which no or very little additional binder need to be added. 
     For example, an LDF board may be used having a density of 160-500 kg/m3 may be used. 
     In the alternative, a VLDF board may be used having a density of 200-300 kg/m3. LDF and VLDF boards are known from Rowell, R. et al: “Dimensionally Stabilized Very Low Density Fiberboard”, Wood and Fiber Science, 27 (4). pp. 428-436 (1995), the entire content of which is hereby incorporated herein by reference. 
     In yet another alternative, a ULDF (ultra low density fiber) board may be used having a density of 50-200 kg/m3. Such fiber boards are known through Xie, Y. et al: “Manufacture and Properties of Ultra-Low Density foamed Fibreboard from Wood Fibre”, BioResources 6 (4), 4055-4066 (2011) the entire contents of which is hereby incorporated herein by reference. 
     Hence, fiber boards having low, very low or ultra low density may be provided substantially without binder (wet/moist compression moulding), with binder and/or with additional additives. 
     It is also possible to replace or complement the expanded polymer material blocks or second material blocks with non-expanded composite polymer material blocks comprising a polymer matrix and fillers, such as e.g. wood chips, wood particles, wood dust (saw dust, wood flour) or wood fibers, mineral fillers (calcium, dolomite, etc), polymeric fillers (e.g. recycled polymers, rubbers or rubber-like materials). 
     Exposed portions of second material blocks of wood fiber (e.g. LDF, VLDF, ULDF) or filled polymer material may also be provided with surface treatment in order to improve e.g. strength, friction (reduce) or wear properties. Such treatment may include the application of penetrating or coating forming matter, such as lacquer and/or thermosetting polymers. 
     At least one of the material block types may be flexible enough to allow the core to bend so as to provide an angle of more than 5°, preferably more than 10°, more than 15°, more than 20° or more than 25° between major surface normals of a pair of adjacent (but with at least one other material block in between) material blocks of the same type. 
     The bending may also be rendered possible, or facilitated, by a longitudinal space between immediately juxtaposed first and second material blocks. Such a space may be on the order of 0.1-5 mm, preferably 0.5-3 mm, most preferably 1-3 mm. 
     Also, where relevant, the web  20 ,  21  may be flexible enough to allow such bending as described above. 
     It is understood that the core may comprise two or more types of material blocks, which may be arranged alternatingly in the principal direction L of the core and with their principal directions I substantially perpendicular to the principal direction L of the core. 
     It is also understood that while the above blanks, panel blanks and floor panels have been disclosed as a three layer structure (decorative layer, core layer and balancing layer), it is possible to use the core of the present disclosure also for two-layer panels, i.e. panels comprising a decorative layer and a core layer. Such panels may be installed and glued to a subfloor. They may be flexible to such an extent as to absorb, or conform to, height variations of the subfloor. 
     In specific embodiments, the following core types may be mentioned: 
     a combination of wood blocks (first material) and filled or non-filled expanded polymer material blocks (second material); 
     a combination of wood blocks (first material) and low density fiber board (ULDF, VLDF, LDF) blocks (second material); and 
     a combination of wood blocks (first material) and filled non-expanded polymer material blocks (second material). 
     Such cores may be used for floor panels having any type of decorative layer and balancing layer disclosed herein.