BACKGROUND OF THE INVENTION

[0001] The invention relates to resin systems for the manufacturing of foam core boards as well as wood based panels comprising a foamed core layer.

BACKGROUND OF THE INVENTION

[0002] It has been an issue in occupational medicine since decades to reduce the weight of manpower-moved goods in order to avoid or at least reduce the prevalence of dorsal pain and diseases of the musculoskeletal system. It is well known from occupational investigations, that workers in the indoor/outdoor building sector are endangered to suffer from that type of occupational disease.

[0003] One possibility is to reduce the weight of items which are mostly handled by manpower, namely boards for indoor and outdoor use, especially those which are used for furniture. Boards with less density than the usual approximately 650kg/m ³ can be easier handled.

[0004] For that reason Light Weight Boards, reducing the amount of manually moved weight, are one of those developments in board making, which have recently most attention in wood-based panel board industry. Light weight boards are applicable in many areas of board applications except flooring and weight-loaded construction issues.

FIELD OF THE INVENTION

[0005] Lightweight Boards usually require higher resin systemload than normal boards or a filler which substitutes partly the wooden fraction of the board. The latter approach to reduce the boards density is to substitute a part of the wood by lighter materials like polystyrene. One example of this is WO2008046892, which refers to a light wood-based material with a density of 200-600 kg/m ³ which contains 1-25 wt% of filler having a density of 10-150 kg per m ³ , selected from the group of plastic particles like expanded polystyrene. [0006] Other systems uses foaming particles or foaming resins to be mixed into another material to form the foamed core. The disadvantage is processing high-volume light-weight products before these materials perform their function in additional procedural steps. EP1561554 describes the use of polyurethane or polyethylene foam in a foam-core panel.

[0007] However, with these core materials, such panels have high flammability which would require strong flame-retardants, and the polyethylene will show very limited form stability because of its thermoplastic nature and relatively low melting point.

[0008] US5554429 also describes a wood board comprising a core layer with one or more strand boards formed by strips and by using a foamed binder. The binder comprises a substantial amount of wood chips, and a mixture of a foaming urethane resin and a nonfoaming other resin in a ratio of 4: 1 to 1 :4.

[0009] Publication DE 102007019416 describes the manufacturing of a basic material consisting of cellulosic and/or lignocellulosic material which is equipped with a foamable resin. Once mixed homogeneously with the material, the mixture is foamed by increasing volume that the mixture is at least partly duroplastic crosslinked. In this case the foaming is partly managed by mechanical devices like an extruder or shear- force mixers and partly by chemical ingredients which can expel gas under chemical acid or temperature regime. Additionally the foaming process may need the use of pressure higher than atmospheric pressure and before pressing the board the crosslinked material has to be crushed and sieved and added to 30% to a mat of wooden fibers to give a board after normal press.

[0010] A third technology may be the substitution of the middle layer by foamed core layers. The manufacturing of boards with in situ foam core material is a special embodiment. Tailored foamable resin system technology with controlled on-set and end-set of the foaming process has to be adjusted to the state-of-the-art of continuous working hot presses. In another potential process, multi-stack boards are formed. In both cases, the for boards with foam core material has to fit special properties, as there are a controlled foaming start and controlled foaming stop. The preferred process is a continuous belt process. WO2008071618 shows several options of such a manufacturing process for foam core board. DESCRIPTION OF THE INVENTION

[0011] One of the problems to be solved in foam resin systemmaking is the controlled foaming start and end process, which is necessary to obtain the ideal density of the foam harmonized with the industrial manufacturing of wood-based panels. The foam has to built up before and/or simultaneously to the curing process of the resin. That means: a) The increase of viscosity due to the proceeding curing (curing speed) has to be controlled in that way that the expanding process foams the growing resin network. b) The foaming process has to be controlled in that way that the foam is able to expand the middle layer of the board. That means that the voids have a closed-cell structure and the foaming forces are strong enough to overcome the weight of the upper layer(s), the pressing belt, and the rollers belt. c) The pressing/curing processes of the surfaces as well as the core have to be aligned regarding temperature, pressure and time to obtain proper cured and densifϊed surface layers, as well as cured and foamed core with sufficient mechanical stability. In WO2008071618 it is essential to first density the surface layers before starting with (substantial) expansion of the core resin.

[0012] The above problems are solved by the present invention providing an improved resin system for use in the process for making core foam laminates of wood panels, to such a process and to core foam laminates.

[0013] The present invention relates to a process of making wood panels from resinated wood chips, wherein a particulate foamable novolac resin system is used in the core of the panel. The wood panel comprises a laminate of a foamed core layer, comprising a foamed novolac resin, sandwiched between outer layers comprising resin bound wood chips.

[0014] The foamable particulate novolac resin system can be used both in continuous belt processing, but also for example in discontinuous multi-stack plate processing.

[0015] The preferred embodiment is a continuous process wherein a layer of classical UF- resinated wooden particles is put on a belt into a continuous press. Before the press entrance, -A-

the resin system to be foamed is applied in form of a powder, whilst a second layer of classical UF-resinated wooden particles is applied on the top of the powder resin.

[0016] The sandwich- like board with two surfaces and one core layer runs into the hot press at a constant speed. While hot pressing the resin system starts to foam and the foaming resin system has to be strong enough to lift the top pressing and roller belt against the top press platen. Thus, the thickness of the board is determined by the gap between the bottom and the top press platen.

[0017] The resinated wood particles can be pre-pressed and/or partly cured before applying the foamable core resin. In WO2008071618 several variants of such a manufacturing process for foam core boards are described, which reference is herewith incorporated by reference.

[0018] As wood chips, small particle board chips can be used, but also powder (for MDF) or larger wood chips for common wood-chips board. The size of the wood particles may range from 0.4-0.6 mm (for MDF) to 40-80 mm length, 4-10 mm width and less than 1 mm thickness (for OSB). Preferably, the length of the wood chips is about 3 to 15 mm, and the width about 10 mm or less; the thickness preferably is about 1 mm or less, preferably 0.1-0.3 mm. Preferred resin for resination is commonly used UF resin. In case a water proof system is necessary, preferably MF resin is applied in at least the outer layers.

[0019] The foaming novolac system is solid at room temperature (below 30 ⁰ C), so that it is easily possible to execute a continuous process, while resinated chips are used for the upper and lower surface and the board can be produced in one pressing and heating step. Because the novolac resin system is in particulate form, it is possible to first press the wood chips to substantial cure level, before - by melting of the novolac - the novolac starts to foam.

[0020] The particulate novolac system of the present invention is differentiated from novolac resins that are liquid below 30 ⁰ C. Such low temperature liquid novolac systems are problematic in a laminate wooden board forming process because they tend to penetrate into the porous wood chips of the outer wood layers prior to the curing reaction. Such low temperature liquid systems also typically include substantial amounts solvents, such as methanol, to dissolve the novolac. The use of solvents is disadvantageous from an environmental point of view, as well as posing a fire risk, and potentially weakens the novolac foam when solvent remains subsequent to the cure process. In the present invention, the foam core is preferably substantially free of volatile organic solvents, and more preferably substantially free of organic solvents in general.

[0021] Novolacs can be made as solid types of phenolic resins with different ranges of melting/dropping points according to the state of the art and the knowledge of skilled persons. Preferably, the novolac has a dropping point of 40 ⁰ C or higher, more preferable about 60 ⁰ C or higher. The dropping point will be maximum about 160 ⁰ C or lower, preferably 140 ⁰ C or lower. The dropping point can be measured in accordance with DIN ISO 2176 on a FP 83 Mettler Toledo in accordance with the user manual of Mettler Toledo FP900.

[0022] The novolac is a phenol- formaldehyde resin in which the molar ratio of phenol to formaldehyde is higher than 1 As phenol is the common compound for phenolic type resins also other phenolic compounds can be used, and are encompassed in the term novolac. Phenol may partly be replaced by cresol, resorcinol or another phenolic compound. Resins with up to 30 mol% of phenolic compounds (other than phenol) can enhance the mechanical properties like flexibility.

[0023] The novolac resin preferably is cured with an external hardener for curing, which distributes e.g. formaldehyde for this process. During curing of a novolac/hardenermixture gaseous products are set free because of the polycondensation process and therefore byproducts expel from the resin. This is a first effect to achieve foaming Hexamethylentetramine (hexamine) is the most common hardener, made from ammonia and formaldehyde, and hexamine can be used as external hardener.

[0024] It is preferred to use further foaming agents, such as for example inorganic carbonates or bicarbonates that are able to release CO2 at higher temperatures. Preferably, the resin further comprises an acid, as the carbonates release CO2 faster in the presence of acids. Suitable examples of carbonates are sodium(bi)carbonate, ammoniumcarbonate, and potassium(bi)carbonate . [0025] Cured novolacs can be rather brittle; hence it is preferred to use modifications of a novolac that cause a toughening effect. For foamed material the toughness of the resin system does not only give mechanical stability but unexpectedly, it also causes an improvement of the foam factor. As modifier, polyalcohols can be used, preferably glycerol can be used, but also other polyalcohols such as for example polyethyleneglycol, sorbitol, pentaeritritol, ethoxylated trimethylolpropane diethyleneglycol, dipropyleneglycol and the like. Some of these alcohols may tend to dissolve the novolac, which would cause the dropping point to decrease. Such alcohols are used in smaller amounts.

[0026] The novolac (phenol-formaldehyde polycondensation) preferably is made using an acidic catalyst. Typical acids which are used as catalysts for novolac synthesis are oxalic acid, sulfuric acid, phosphoric acid and several others or combinations of them according to the knowledge of skilled persons.

[0027] The solid novolac resin system preferably consists of a particulate mixture comprising a novolac with acidic pH (as it comes from the production process after condensation, removal of remaining free phenol and drying), modifier, hardener and an inorganic carbonate. This powder can be used as foamable core of a Lightweight board.

[0028] The whole mixture preferably is ground for application in the form of a powder. The powder particles preferably are about 0.01 - 1 mm, preferably 0.05-0.2 mm, as measured with sieving equipment.

[0029] Preferably, the amounts of the respective materials of the novolac core resin system are such, that - in free expanding form, in a test beaker - an expansion is achieved, while curing at 135 ⁰ C, of a factor of 4 or more (the foam factor is defined as the post-foaming volume divided by the pre-foaming volume). This means that the average density of the foamed product after foaming and curing is l/4th of the density of the resin system before foaming. Preferably, the expansion is a factor of 6 or more. Generally, a factor of 20 or lower will be used, preferably a factor of 15 or lower, because very high expansion rates may decrease the strength of the foam. The foam may be (largely) open cell foam or (largely) closed cell foam. A predominant closed cell foam is preferred because of improved insulation properties. [0030] The novolac (phenol- formaldehyde resin with acid catalyst) generally will be present in an amount of about 10 wt% or more of the foamable material, preferably, about 20 wt% or more, and most preferable about 30 wt% or more. Generally, the amount will be 80 wt% or less, preferably about 60 wt% or less.

[0031] The amount of acid in the novolac preferably is about 1 wt% or more, even more preferable about 3 wt% or more related to novolac. Generally, the amount of acid will be about 10 wt% or less, preferably about 7 wt% or less.

[0032] The toughening agent can be present in an amount of about 3 wt%, preferably about 10 wt% or more, and even more preferable about 15 wt% or more related to novolak.

[0033] Generally, the toughening agent will be present in an amount of 60 wt% or less, preferably about 30 wt% or less. Higher amounts may be suitable if the toughening agent is itself relatively high molecular weight and co-reactive with the novolac or with one of the other reactive components.

[0034] The external hardener preferably is present in an amount of 5 wt% or more, preferably about 10 wt% or more related to the (premixed ) novolac (containing acid and glycerol). Generally, the amount will be about 30 wt% or less, preferably about 20 wt% or less.

[0035] The additional blowing agent preferably is present in an amount of 2 wt% or more, more preferably about 5 wt% or more, related to the (premixed ) novolac (containing acid and glycerol). Generally, the amount will be about 30 wt% or less, preferably about 20 wt% or less.

[0036] The above wt-percentages are relative to the resin system as described. In addition to the components and additives, particulate fillers, fibre reinforcement, flame retardants and surface active agents and the like can be present in amounts up to 200 wt% of the resin. Preferably, the total amount of these auxiliary agents is about 100 wt% or less relative to the resin. [0037] The foamable novolac resin system as defined above can be used as such, or in combination with other resins. In a preferred embodiment, the foamable core consists essentially of the foamable novolac resin system.

[0038] The resin system can be cured as core layer between wood- flake panels in a mold or belt press at a temperature at least of the dropping point temperature to 150 ⁰ C or more. Generally, the temperature will be about 28O ⁰ C or less, preferably about 240 ⁰ C or less. Preferably, the temperature shows a gradient, with a higher initial temperature of about 220 ⁰ C or higher, like between 220-230 ⁰ C. The temperature may show a natural decrease till about 200 ⁰ C or lower, like 160-180 ⁰ C.

[0039] In an embodiment, the board can be cooled to a temperature of below 100 ⁰ C on a cooled belt, to preclude micro or macro steam-rupture of the board. In a preferred embodiment, it is not necessary to have a special cooling step, because the novolac is stable and the cooling step can be skipped.

[0040] The pressure in the mold or belt press preferably is in the initial phase about 2 N/mm ² or higher, and generally will be about 5 N/mm ² or lower. The pressure can be released - depending on the specific use - to about 0.5 N/mm ² (like for example between 0.2 N/mm ² to 1 N/mm ² ).

[0041] The boards can be post cured if necessary at e.g. 80-100 ⁰ C.

[0042] The wood panels of the present invention are laminates comprising layers of wood- foam-wood. The foamed core layer comprises a foamed novolac resin and is sandwiched between outer layers comprising resin bound wood chips.

[0043] The core layer is preferably substantially free of wood chip material. The wood layers are preferably substantially free of novolac resin material, although some minimal penetration of novolac into the wood may be expected at the border of the foamed novolac core and the outer layers. In any case, it is preferable that the outer wood layers comprise a lower wt% of novolac resin than the core layer. It is also preferable that the outer wood layers are substantially unfoamed, although some minor foaming or expansion may occur due to curing processes in the wood layers. Preferably the wood layer has a foam factor of less than 3, and more preferably less than 1.5.

[0044] In one embodiment of the invention, the foamed core layer preferably has substantially only novolac resin as a binder material.

[0045] Preferably the novolac resin system comprises cured phenol-formaldehyde, a toughening agent and a one or more base or acid compounds. Most preferably the foamed core layer consists essentially of a novolac resin system comprising cured phenol- formaldehyde, a toughening agent and a one or more base or acid compounds; and optionally particulate filler(s), fibre reinforcement, flame retardant(s) and surface active agent(s).

[0046] The foamed core layer has a density that is lower than the density of the outer wood layers. In this manner the foamed core lightens the wood panel. The novolac resin is preferably foamed to by a factor of at least 4, more preferably at least 6, and more preferably at least 10. The lower density is achieved by the foam nature of the core layer.

[0047] The wood panel of the present invention preferably has a thickness of from 8 to 40 mm.

[0048] The average density of the board is less than 650 kg/m ³ , preferably less than 600 kg/m ³ , preferably less than 550 kg/m ³ , more preferably less than 500 kg/m ³ , even more preferably less than 400 kg/m ³ , more preferably less than 300 kg/m ³ , and most preferably 200 kg/m ³ or less. Generally, the density will be more than 50 kg/m ³ , preferably 80 kg/m ³ or greater, more than 150 kg/m ³ , and may be more than 200 kg/m ³ . Preferably the average density of the board is from 50 to 400 kg/m ³ , more preferably 75 to 200 kg/m ³ , even more preferably 100-150 kg/m ³ . Thus in one aspect of the present invention the process of manufacturing a board includes the step of foaming a particulate novolac to give a board having a density within the aforementioned density ranges.

[0049] The thickness of the foam core board will be between about 8 to about 40 mm, although thicker or thinner boards can be made. The insulation value preferably is improved with respect to non-foam boards. For example, the heat transport is about 0.1 W/mK or less and the noise absorption is about 30 dB or better. [0050] The foam core board may be further processed with decorative overlay papers, but preferably, such finishing is also performed in-line. Suitable finishing may comprise: MF impregnated Decorative Overlay paper (Decor Overlay); MF overlay + MF Decor overlay; PF Kraft paper (several layers) + MF Decor Overlay, optionally with a further MF Overlay.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0051] The invention is elucidated with the following examples, without being limited thereto.

Example 1

[0052] lOOg Prefere 4118X (trade marked product from Dynea) containing 5wt% oxalic acid and 30wt% glycerol are ground together with 15wt% hexamethylenetetramine and 10% Na-carbonate. The amounts of acid and glycerol are related to the novolak, hardener and foaming agent are related to the (premixed ) novolac (containing acid and glycerol).

[0053] This mixture realizes a foaming factor of about lOfold after treating at 135°C.

[0054] Resinated wood chips were provided in a loosely poured layer having a thickness of 2 cm on a plate; 0.5 cm of particulate novolac resin system as described was applied on the wood chips, and a further layer of resinated wood chips having a thickness of 2 cm was applied. The three layer system was pressed at 2 N/mm ² , and the plates were heated to 23O ⁰ C. The pressure was gradually released to 0.5 N/mm ² , while having a distance between the plates of 3 cm. The laminate was cured for 30 min. During that time, the novolac resin system gave a foam, lowering the density by about 50% as compared to a comparable board

Example 2

[0055] The same resin system and board production was followed as in example 1. However, after pressing, the plate was moved to an infra-red / hot air oven, and the plate was heated to 135 ⁰ C, while the upper wood panel was allowed to move to a total thickness of 3 cm.

Example 3 [0056] The experiment of example 1 was followed, but on the plate, first MF impregnated overlay decor paper was laid and thereon three layers of PF impregnated Kraft paper, before providing the wood chips, and the same Kraft and overlay decor layers were put on the second layer. In this way, a finished wood panel product was made in one process.

[0057] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art.

[0058] Further modifications in addition to those described above may be made to the techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.