COMPOSITE BOARDS AND PANELS

Abstract: A composite board or panel has a lightweight central lay­ er of for example cellulosic material such as paper "honey­ comb". In each face of this there is adhered a facing layer of paper or paperboard. Each face of each facing layer is coated with an inert material impervious to water or water-vapour such as polymers, bitumen or waxes. Outermost paper or foil layers can be provided on each face of the composite. The composite is particularly resistant to uptake of water and thus does not warp.

Description: Title: COMPOSITE BOARDS AND PANELS This invention relates to composite boards and panels. As used herein the term "board" relates to a composite layered material alone, of a size and shape as produced by the manufacturing process of the nature described in more detail below or by similar or related manufacturing processes. The term "panels" is used as meaning unit pieces of board provided with a surround and/or internal longitudinal support strips e.g. battens or rails, whether made up during manufacture of the layer of material or made from a cut and machined piece of board with, for example, its peripheral regions removed to accommodate the rails or battens. The rails or battens can be made of timber, strips of medium density fibre (MDF) board or wood particle board, or of metal. While boards or panels of the invention may be used in permanent constructions, or elements thereof, such as doors or suspended ceilings, the invention relates more specially to light-weight composite boards or panels suitable for example for use in construction of temporary structures such as exhibition stands or for the fabrication of display materials such as hanging signs in supermarkets or like premises. Composite boards and panels composed of or faced with cellulosic fibrous layers are already known. Typically, they consist of a relatively thick central layrer generally known as the "core" to provide the necessary bulk and thinner facing layers, one on each face of the central layer, to provide the desired appearance, rigidity and surface characteristics. The facing layers known in this technology can be of plywood, for example the 3-ply or 4-ply surface panels used in preparation of flush doors. They can also be manufactured wood fibre boards such as hardboard or medium density fibre (MDF) board. They can alternatively be made of wood particle board, high pressure laminates, and single thicknesses of wood-based laminating materials. Of recent years there have been proposals to provide boards and panels in which facing layers are of thick paper or paper board e.g. cardboard. In practice, however, such panels while commercially and structurally attactive because of the low weight per unit area are difficult to manufacture and use. Hitherto, useful boards or panels could only be obtained and successfully used if three conditions were met. Firstly, great care must be taken to bring the facing layers to the same relative moisture content as each other prior to assembly. Secondly, great care must be taken to ensure that the gauge and the hygroscopic properties of the two facing layers are the same on both faces of the composite material. Even then, variability from one batch of such material to the next (or even within the same batch) may be encountered. Thirdly, care must be taken in applying adhesives to such facing layers during assembly since the adhesives used are usually water-based and usually need to be spread on the inner surfaces of the facing layers which must be kept as low as possible and as uniform as possible over different parts of each sheet. When the boards or panels have been manufactured, using all the aforementioned precautions, they are still vulnerable to warping, bending, bowing or twisting when in use as cladding materials for exhibition stands, signs or for any other use. This is because none of the manufacturing precautions can be applied with absolute reliability and also because, in use, the boards or panels are subjected to widely varying environments of temperature, relative humidity and differential treatment and exposure on each face of the boards or panels. Since typically such boards or panels may be decorated with expensive artwork and used in a visually prominent position, such warping is totally unacceptable. Any such warping, bending, bowing or twisting of the boards and panels is mainly due to dimensional changes arising from the uptake or loss of moisture in the facing layers. Typically, these changes are non-uniform and vary as between different panel directions, that is to say, those directions corresponding to the "machine" or the "transverse" directions ofthe surface layer when originally produced. The present invention is based upon the realisation that these dimensional problems arising from moisture vapour or water uptake are predominantly a "skin" effect and can be controlled by preventing water, whether in liquid or vapour form and whether arising from the adhesive during manufacture or from atmospheric water vapour during use, from entering or leaving the suface layer as such. In one aspect the invention provides a composite board or panel in which a lightweight central layer of void-containing material has adhered thereto on both faces a facing layer of paper or paper-board, each such facing layer being itself coated (a) on its outer face with a layer of material inert thereto and impervious to water in liquid or vapour form, and (b) on its inner face with a layer of the same or different material of like properties. In a variant form of the invention, one or both inert layers are covered at least partly with one or more relatively thin additional, usually decorative or decoratable, applied layer. The term "relatively thin" means that the or each layer does not impart warping characteristics to the board or panel and in practice usually means that the outermost applied layer or layers of this nature is or are jointly less than one tenth of the thickness of the facing layer as described above. This outermost layer may itself be paper or may be a metal or polymeric foil in which latter cases the thickness can if desired be somewhat greater than that of a paper layer. The use of aluminium foil as the outermost layer is particularly useful when the final panel or board is required to have surfaces of low spread of flame, for example, Class 1 in accordance with BS476: Part 7. Such foil outermost layers can, if desired, be coated with a non-inflammable lacquer, but this is not essential. Usually, however, the thin additional layer is a paper layer. It can be provided on both external faces of the coated cardboard facing layer, whereby in one instance it faces outwards to be the outermost layer of the board or panel, and in the other it faces inwards to be a contact layer to ensure good adhesion to the core. It can also be provided on one such external face only, to be the eventual outermost layer. In cases where there is no such inner paper layer (next to the core) a surface treatment of the inner inert layer, e.g. by flame oxidation or by corona discharge is advantageous to ensure good core adhesion. When a paper layer is present on an external face of the cladding layer it will be advantageous to provide a somewhat thicker inert layer than would be used for an uncovered inert layer. The central layer or core, can be of any light, and usually voidcontaining but coherent layer of material. Typical materials usually fall into one of three groups, examples of which can be used alone or in combination either with one another or as between different groups. The three groups are respectively based on cellulosic materials, synthetic polymeric materials, and inorganic mineral-based materials. Cellulosic materials are ultimately based either upon wood or upon non-wood fibrous vegetable waste or like material. Wood-based materials can include composites made of thin wooden layers such as plywood or blockwood. The wood can be more extensively subdivided, into chips or splinters to give chipboard, or wood particle boards. Particles may be of a finer state of subdivision to provide "hardboard" and/or may be of a fibrous nature. Such fibrous wood-based material can be made up as the relatively harder "fibreboard", or the rather softer material grades known as "insulation boards". It is also envisaged for the fibrous wood-based material to be laid, to form cardboard or paper layers and further to be used as, or made up into, the core layer. Thus, monolayer or multilayer solid cardboard cores can be used. More usually, configured cardboard or paper is used for example "corrugated cardboard" (e.g. structures with one or two facing layers on each side of a corrugated layer) such corrugated cardboards themselves being usable to form a core of one or more such corrugated layers. A particularly preferred core layer material is cardboard or paper "honeycomb" material. Cardboard or paper "honeycomb" is a succession of strips of cardboard or paper each strip being adhered to adjacent strips at a number of transverse lines across each of its surfaces. The lines of adhesion on one surface are staggered in relation to those on the other surface, whereby the structure, when opened out at right angles to the strip surfaces has a cellular honeycomb-like appearance. Non-wood-based materials can include pressed board-like structures made from vegetable waste in food or other industries, based for example on cane, husks or stem residues such as flax shive board. Synthetic polymeric materials can be foamed as a sheet, either single or multilayered. Alternatively, they can be pressed layers of material 5 made of solid or foamed particles, whether regular in shape (e.g. beads) or irregular (e.g. scrap particle material) or fibrous (e.g. synthetic textile waste) and whether originally produced as a primary product or as scrap, recycled or byproduct. Examples are expanded polystyrene beads, scrap or other particulate polyurethane (foamed or not) or particulate polyphenol resins. Mineral-based materials can be naturally -occuring fibrous materials compacted into sheets or slabs. Alternatively, they can be fibrous glasses whether produced as a primary product e.g. for glass fibre or as a by-product e.g. as a slag material expanded to fibrous form for producing a sheet product. Particulate mineral material of lighter weight such as vermiculite or perlite materials, can also constitute a suitable core for cladding. Cores in which the material only partly comprises one of the above classes, or is a hybrid between two or more such classes, can also be used, e.g. cellulosic fibrous material together with particulate polymer material and/or fibrous or particulate inorganic material, such as wood-fibre/polymer-particle; wood-fibre/gypsum (or cement) for aerated insulation sheets, or wood-fibre/inorganic-fibre. We have discovered that the expansion or contraction of the facing due to changes in moisture content of the atmosphere, or due to the use of water-based adhesives during the manufacture of composite boards or panels is greatly diminished by the use of coated facing layers as described above. It is a particular feature of the invention that by providing effectively inert and moisture-impervious surface layers, not sensitive to changes in relative humidity, the manufacturing process and the final use of the invention are both improved. It is additionally advantageous that materials of different types, or if of the same type of different gauges, can be used on each face of the central layer or core; hitherto the two faces have been balanced by using facing material of the same gauge, since even if the cellulosic material were previously equilibrated for moisture content the difference of gauge of material in the prior art would itself led to warping even when minor humidity changes took place. However,according to the present invention a relatively thicker gauge material can be applied to one surface, that is to say, the normally outer surface of a panel as used for example e in an exhibition stand or similar lightweight construction, and a thinner gauge material can be provided on an inner or protected surface of such panel. We have further discovered that the structure as defined above according to the invention possesses further advantages over known prior art structures. A known defect of composite panels, especially when only a thin cladding layer is used (e.g. of cardboard) is the so-called "core shadow" or "frame shadow". This is a faint mottling of the surface corresponding to the underlying core structure or frame elements and appears to be an effect related to differential stretching and contraction of fibres in the facing layers due to humidity changes. In the structure as described above and in accordance with the invention, such core shadow or frame shadow does not manifest itself since the facing layers are themselves protected on both surfaces by the inert layers. The inert layers in each case (which can be the same on both faces, or can differ) can be of any flexible material which can be adherently and permanently applied to the paper or paperbaord. Metal foils are possible, especially aluminium foil for flame resistance. However, it will often be found cheaper and more convenient to use synthetic polymeric layers. These can be applied by spraying,dipping, or painting e.g. by roller coating, but are most preferably applied by extrusion of a molten film from a slit die directly on to the surface without the use of any water-based solvent or adhesive. Alternatively, a film of material can be unwound from a roll and adherently pressed, possibly with application of heat or other suitable expedient, on to both surfaces of the layers to form a laminate. Suitable polymeric materials are polyolefinic in nature such as polyethylene, polypropylene, or ethylene-propylene copolymer. They can also be plasticised polyvinyl chloride, or can be polyvinylidene chloride since this has a specially high resistance to water vapour. Other possible materials from which the inert layers can be formed are bitumens and waxes. Preferred numerical limitations on the composite material can be broadly stated without limiting the broad scope of the invention as described above. The central or core layer, can be from 3 to75 for example 6 to 50 mm in thickness. The paper or paperboard can be for instance from 0.25 to 3 e.g. from 0.5 to 3 mm in thickness as a preferred range,e.g. 250 - 1500 gsm (grams per sq.metre). The inert barrier layer on either side of this surface can generally be applied in a coating weight of from 10 to 100 grams per square metre, depending on e.g. the polymer used and the degree of protection necessary. Thus, for low density polyethylene the minimum coating weight is preferably 20 to 25 gsm, and if one or both of the inert barrier layers is to be paper-covered the minimum is preferably increased to 30 to 40 gsm. If one or both of the inert barrier layers is to be covered with metal foil or some other impervious foil, the lower of these coating weigh minimum ranges will be found adequate. The adhesives used to bond the surface layers as described above on to the central or core layer of the composite boards or panels are usually water-based such as urea-formaldehyde resin solution (applied e.g. at 150 - 300 g.s.m. of 50 -80% solid content solution) or the polyvinyl acetate - based emulsion adhesives. The invention further extends to a method of manufacturing composite boards or panels as described above in which paper or paperboard facing sheets clad with the inert barrier layer are arranged in face-to-face pairs and provided with adhesive layers on each free outer face and; the pairs of facing sheets are stacked alternately with layers of lightweight void-containing material to provide a supply of wet composite boards or panels. After this, the wet boards and panels can be pressed as a stack or individual boards and panels may be hot pressed in suitable platten presses. In either case, the object is to ensure that each central or core layer is permanently adhered to a facing layer on each face. Facing sheets may be selected with one clad surface itself provided with a paper or foil lamination, and the other surface otherwise treated to facilitate adhesion. In such cases the method is carried out by placing the facing sheets in pairs with paper or foil surfaces in contact. If different thicknesses of facing sheets are used it is preferred to place them in pairs with one of each thickness,whereby the eventual composite board has one surface of different strength from the other. In another method for the production of composite boards and panels two facing sheets are glued at a time and placed one each side of a single central core layer. The wet composite is then mechanically moved forward into a single-opening hot press with a very short press cycle time. On opening the press, the pressed board or panel is automatically ejected and the press refilled with another wet composite. Composite boards with cardboard facing sheets may also be produced by continuous production processes. Typically, the cardboard facing materials are on large reels, and can therefore be continuously unwound, coated with adhesive and continuously pressed on to the central or core layer of the boards supplied as a roll, as from a folded "magazine" or as a sequence of abutting sheets. This gives a continuous band of composite board, which is cut to the required length by a traversing saw of cutter. The invention will be further described with reference to the accompanying drawings, in which: Figures 1 to 4 are diagrammatic fragmentary cross-sections of boards according to the invention, Figure 5 is a partial diagrammatic cross-section of a panel according to the invention, and Figure 6 is a graph showing the dimensional changes in an untreated cardboard material against changes of water content. Figure 1 shows inert thin layers 1 of polymeric material covering both surfaces of a cardboard material 2 to form a laminate adhered by adhesive layer 3 to either face of a paper honeycomb core 4. The dimensions shown are in any case not to scale, but since the core thickness 4 is so much greater than the facing layer thickness (about 3 to 20 times) It is shown discontinued in the drawing. Example 1 gives details of materials and dimensions of a typical such product. Figure 2 is generally similar but possesses thicker polymer layers 6 to which outermost paper layers 5 are laminated without the use of adhesive. The paper layers form a suitable basis for adhesion (at 3) or for external decoration. Example 2 gives details of materials and dimensions. Figure 3 shows a composite board in which the inner surface of the cardboard layer 2 possesses a thin polymeric layer 1 while the outer surface possesses the two layers 5 and 6 as in Figure 2. Example 3 describes a typical such material in more detail. Figure 4 shows a composite board identical at one facing layer to that board in Figure 1. The other facing layer, however, is based upon thinner cardboard 7, having the same thin polymer coating 1 . Example 4 describes this material in more detail. Figure 5 shows assembly of a board as shown in Figure 1 (the reference numerals being therefore identical) to a peripheral wooden batten 8 as part of a panel. Figure 5 also shows in more detail a cross-section of the paper honeycomb, with a double thickness of paper at 9 adhered to layer 10. Figure 6 is a graph of moisture content of cardboard material per se in weight percent plotted against percentage dimensional change. It is apparent that transverse dimensional change as shown by the full line (in relation to the machine direction of original manufacture) is much more marked than longitudinal dimensional change as shown by the dotted line. Readings were taken at 31%, 50%, 60% and 85% relative humidity of the environment, with the 50% value (in equilibrium at approximately 8% water content) taken as the datum line. Surfacing the cardboard with an inert polymer as a barrier layer decreased the slope of both curves, and was especially valuable in controlling the troublesome transverse expansion tendency. The invention will be further described with reference to the following non-limiting Examples. Example 1 Cardboard 1 mm in thickness was coated with molten low density polyethylene (LDPE) on both sides, using slit extrusion dies, cooled to solidify the polymer and treated with ultra-violet light and by a flame treatment or a corona discharge on each polymer surface prior to winding in reels. The coating weight of polymer on each side was 23gm/m2 (gsm), equivalent to a notional coating thickness of 0.025 mm, whereby each coated surface of the cardboard possesses a water vapour transmission of not greater 2 0 than 16 gm/m2for 24 hours at 38 C at a water vapour pressure gradient of 90% to a notional zero percentage within the cardboard, i.e. 0.0608/cm2abs. A paper honeycomb core material known per se, made of cardboard about 0.25 mm thick was cut into 20 mm wide strips which were expanded to give an expanded area of 15 mm hexagonal cell size, measured as the diameter of the inserted circle. From this area was cut a rectangle 2400 by 1200 mm, which if necessary was heat-treated to provide handling stability. The coated cardboard was cut into unit lengths of the same size which were assembled in face-to-face pairs and passed as such through the nip of a glueing roller to give on each outer face a loading of 220 gms of an aqueous solution of urea-formaldehyde resin of solids content 60%. A first double sheet of the adhesive-carrying coated cardboard was placed on a table, and a first sheet of the expanded paper honeycomb material was aligned therewith and superimposed thereon. Thereafter, and similarly, an alternation of double sheets of cardboard, and of paper honeycomb was built up to provide a suitable stack of wet composite boards for handling. The uppermost layer of paper honeycomb is conveniently covered with the lowest sheet of cardboard, when this is separated from its neighbour. The stack used was 24 - high, but could be from 8 to 48 - high depending on operating convenience, and to some extent the catalyst used in the adhesive. The stack was conveyed to an 8 - daylight press with heated platens where in each cycle eight composite boards, each consisting of a pauper honeycomb layer with adherent coated cardboard sheets on each face, were pressed individually between platens at 700 for 5 - 7 minutes at 14 p.s.i. (pounds per square inch), the temperature chosen being the maximum at which the polymer layer itself does not begin to soften, and the pressure being related to the cell and core thickness, usually within the range 5 to 30 psi. (One p.s.i. approximately equals 0.73 kg/cm2). The individual boards were then unloaded and trimmed, to give a board in which warping was minimal and subsequent warping in use was not significant. Example 2 Cardboard 1 mm, in thickness (about 650 g.s.m) was coated as before with LDPE but to a coating thickness on both sides of 34 g.s.m. and an additional thin paper layer of 39 g.s.m. was laminated to each polymer layer without use of adhesive. No corona discharge or like treatment was used. Subsequent procedure was as described in Example 1, to give a board which was essentially unwarped at the end of the manufacturing process and which resisted warping in use. The cardboard sheets were adhered to the paper honeycomb by urea-formaldehyde adhesive, and the outer paper surfaces of each composite board were readily decoratable even with water-based paints. The 39g.s.m. paper external surfaces did not cause the 650 g.s.m. cardboard facing layers to warp the composite. Example 3 Cardboard 650 g.s.m. in thickness was coated on one face with 34 g.s.m. LDPE and a 39 g.s.m. thin paper laminate superficial layer and on the other face with a 23 g.s.m. LDPE layer which was treated with a corona discharge and ultra-violet light as in Example 1. The coated cardboard was cut to unit sizes which were placed in face-to-face pairs having their paper laminate superficial layers adjacent. The glueing, stacking and pressing stages were carried out as before, to give a board unwarped by manufacture and resistant to warping in use.In the composite board the outer, thicker, polymer layer on each cardboard facing layer, which is paper-covered and decoratable resists water transmission since its 34 g.s.m. thickness is sufficient to separate any possible contact between the fibres protruding from the superficial paper and the cardboard. The inner 23 g.s.m. polymer layer is thinner since it has effective contact with cellulosic fibres from one face only and is in any case in a relatively protected position, next to the core, in relation to water or water vapour when in use. Example 4 The procedure of Example 1 was repeated except that two different grades of cardboard , one of 650 g.s.m. and the other of 350 g.s.m. were used. These were coated, treated with corona discharge, and arranged in face-to-face pairs containing one sheet of each grade separating the core layers. The boards produced had one face of 650 g.s.m. cardboard, for use as an outer face in a construction, and one of 350 g.s.m. cardboard as in inner face for a more protected location. However, since the cardboard layers were in each case protected by the inert layers of polymer there was no warping on manufacture and resistance to warping in use.

Claims: CLAIMS 1. A composite board or panel in which a lightweight central layer or core of void-containing material has adhered thereto on both faces a facing layer of paper or paper-board, each such facing layer being itself coated (a) on its outer face with a layer of material inert thereto and impervious to water in liquid or vapour form, and (b) on its inner face with a layer of the same or different material of like properties. 2. A composite board or panel as claimed in claim 1 in which one or both inert layers are covered at least partly with one or more additional applied layers which does not impart warping characteristics. 3. A composite board or panel as claimed in claim 2 in which the additional applied layer is formed of paper a thickness less than one-tenth of the thickness of the facing layer, a metal foil, or a polymeric foil. 4. A composite board or panel as claimed in claim 3 in which the applied layer is aluminium foil of thickness from 0.005 to 0.1 mm. 5. A composite board or panel as claimed in claim 4 in which the aluminium foil layer is coated with a non-inflammable lacquer. 6. A composite board or panel as claimed in any of claims 1-5 in which the central core comprises cellulosic material. 7. A composite board or panel as claimed in claim 6 in which the cellulosic central core is a coherent layer composed of thin layers of wood,chips or splinters of wood,or wood subdivided into fibres. 8. A composite board or panel as claimed in claim 7 in which the wood fibres of the cellulosic central core layer are laid to form a paper or cardboard and this is used as one or more flat, corrugated or otherwise configured layer. 9. A composite board or panel as claimed in claim 8 in which the cellulosic central core layer is formed as a paper honeycomb, as herein defined. 10. A composite board or panel as claimed in claim 6 in which the cellulosic central core is a coherent layer composed of a compressed non-wood fibrous materials. 11. A composite board or panel as claimed in any one of claims 1 to 10 in which the facing layer on one face of the central core layer is thicker than the facing layer on the other core face. 12. A composite board or panel as claimed in any one of claims 1 to 11 in which the inert layers on the facing layer are composed of a synthetic polymer, of a bitument, or of a wax. 13. A composite board or panel as claimed in any one of claims 1 to 12 in which the central core layer is from 3 to 75 mm in thickness. 14. A composite board or panel as claimed in any of claims 1 to 13 in which the facing layer is from 0.15 to 3 mm in thickness. 15. A composite board or panel as claimed in any of claims 1 to 14 in which each inert layer on the facing layers has a thickness corresponding to a coating weight of 10 to 100 gm/m 2 16.A composite board or panel as claimed in any of claims 1 to 15 in which the inert layer is polymeric and coated with paper, and has a minimum thickness corresponding to a 20 gm/m2 coating weight. 17. A composite board or panel as claimed in any one of claims 1 to 15 in which the inert layer is polymeric and coated with metal foil, and has a minimum thickness corresponding to a 30 gm/m2 coating weight. 18. A composite board or panel as claimed in any one of claims 1 to 17 in which the facing layers are adhered to the central core layer by a urea-formaldehyde or polyvinvylacetate adhesive. 19. A method for the production of composite boards or panels as claimed in claim 1 in which paper or paperboard facing sheets clad with the inert barrier layer are arranged in face-to-face pairs and provided with adhesive layers on each free outer face; the pairs of facing sheets are stacked alternately with layers of lightweight void-containing material to provide a supply of wet composite boards or panels; and these wet composite boards or panels are thereafter either, (a) hot-pressed as a stack and thereafter separated or, (b) separated into individual boards or panels and individually hot-pressed. 20. A method for the production of composite boards or panels as claimed in claim l, in which paper or paperboard facing sheets clad with the inert barrier layer are arranged in face-to-face pairs and provided with adhesive layers on each free outer face; the pairs of facing sheets are separated and applied with their adhesive surfaces each to one face of a layer of lightweight void-containing material to provide a wet composite board: and this board is individually hot-pressed. 21. A method as claimed in claim 19 or 20 used when one inert layer on each facing sheet is clad with a paper or foil lamination and the other treated to facilitate adhesion; in which the facing sheets are adhesive coated in pairs with the paper of foil surface in contact. 22. A continuous method for the production of composite boards or panels as claimed in claim 1 in which: two reels of the facing sheet material clad with the inert barrier layer are continuously coated with adhesive; the adhesive surfaces are applied one to each side of a continuous supply of the central core layer from a reel, magazine or continuously abutting sequence of sheets: the wet continuous composite is cut to desired lengths; and the cut lengths are hot-pressed individually or in groups.

Inventors: KAINES, COLIN CHARLES
Applicants: QUINTON & KAINES (HOLDINGS) LIMITED
Application year: 1982
Main classification: B32B3
IPC codes: E04C2, B32B3