LAMINATED BOARD AND ELECTRON BEAM CURABLE COMPOSITION USED IN MANUFACTURE THEREOF

Abstract: A composite board provided with a decorative or protective surface is manufactured by coating an electron beam (EB) curable pigmented composition onto a carrier sheet, curing the composition by EB irradiation and then bonding the cured coated carrier to the board. The carrier is preferably a paper selected for high opacity. The coating composition preferably comprises acrylate oligomers and one or more acrylate monomers having a viscosity desirably greater than 1000 cps at 20 DEG C applied to result in coating weights less than 15 gsm. A coating composition and a coated paper are also claimed.

Description: LAMINATED BOARD ANDJΞLECTRONJ3EAM CURABLE , COMPOSITION USED IN MANUFACTURE THEREOF Field of the Invention This invention relates to a method for manufacture of a composite board having a decorative or protective laminated surface and to laminates for use with the method. The invention is herein described with particular reference to laminates in which the composite board is particle board but it will be understood that the method of the invention can usefully be applied to other substrates/ for example chipboard, hardboard, reformed hardboard and the like. Background of the Invention Composite boards manufactured by bonding together particles or chips of wood are widely used in building and construction and for the manufacture of furniture. It is often desired to provide such board or other composites with a factory applied decorative or protective finish and that has hitherto been accomplished by laminating a thermosetting resin sheet, for example a melamine sheet, under pressure, to a surface of the board or by painting the board surface with a conventional air-dried or solvent based paint. When laminated particle board is intended for use in kitchens, bathrooms or the like the exposed surface is frequently subjected to harsh treatment including cleaning and scrubbing, often with abrasives and sometimes with solvent preparations. The decorative surface is thus required to have a high order of hardness, scuff resistance, water and solvent resistance. Melamine laminated surfaces provide an excellent hard, scuff-resistant decorative surface but stacked sheets of melamine are expensive to handle and the process of laminating individual stiff sheets to respective boards using heated presses is labour intensive and too costly for many end uses. Paint systems now in use for factory finishing hardboards and particle boards while less expensive to apply have less satisfactory properties than melamine laminates and tend to be more costly than is desired. The drying step of the painting process is especially costly in terms of space requirement. When applied to timber the paints tend to raise the grain requiring sanding between multiple coats. .The unevenness of hardboard surfaces also necessitates application of multiple coats or fillers, adding to materials cost. In an effort to more rapidly dry the paints, it has been proposed to cure paints applied to such substrates by UV radiation. The faster curing obtainable with UV curing reduces space requirement to some extent but UV-cureable formulations are limited in pigment loading capacity and are restricted in their ability to accept pigments of certain colours. An object of the present invention is to provide a composite board having a surface finish which has a satisfactory level of hardness, scuff resistance and solvent resistance and which can be produced more economically than conventional melamine laminates. A further objective is to provide a surface finish to hardboard which has a desired opacity (so as to be able to hide joins or defects in the substrate) and which provides the necessary gloss and chemical resistance. Disclosure of Invention According to one aspect, the invention consists in a method for manufacture of a ' composite board having a decorative or protective surface, comprising the steps of: a) coating an electron beam curable pigmented composition onto a carrier sheet; b) curing the composition by means including electron beam irradiation; and c) bonding the cured coated carrier sheet to a composite board. It has been found that compositions may be prepared which accept high loadings of pigment (more than 40% by weight) and which may nevertheless be cured by electron beam radiation at high speeds (e.g. 400 m/min.) after coating onto a relatively inexpensive carrier sheet such as paper. Surprisingly, it has been found that selected EB curable compositions are sufficiently flexible to be wound onto a roll at high EB curing speeds and yet when subsequently laminated to particle board, provide satisfactory hardness, water and solvent resistance. It is usual that cured polymer films which exhibit flexibility do so at the expense of hardness and solvent resistance. In highly preferred embodiments, the carrier sheet is a paper selected to have an intrinsic opacity Υ.b Yw>0.9 when measured on the MINOLTA chromometer and the cured coating is applied at less than 15 gsm. Desirably the coating composition has a viscosity greater than 1000 cps at 30°C and is applied at an elevated temperature, e.g. 40 - 50°C or up to 60°C, the application viscosity being about 3 - 400 cps at the application temperature. In preferred embodiments, the method is economically competitive with other laminated composite boards produced by the prior art methods. Preferred embodiments of the invention Various embodiments of the invention will now be described by way of example only. In general, the first step in methods according to the invention is to coat a carrier, for example paper or foil, with an EB curable composition. For preference, the carrier is a paper having low oil absorption characteristics in order to minimize penetration of the curable composition during the dwell time between coating and curing. In order to achieve an opacity in the cured coated paper which will adequately hide the substrate, it is important to select a paper having an intrinsic opacity Y.b/Yw>0.9 (Brightness L, >85) . Opacity herein expressed as . /Y refers to measurements in the YXY system as determined with a MINOLTA chromometer. A paper of opacity Y./Y >0.9, when coated and cured according to the invention, will have an intrinsic opacity >0.95 (Brightness >90) and will hide the substrate at surprisingly low coating weights of less than 15 gsm and usually 4 - 10 gsm depending on pigment loading in the coating. It is also desirable to select a paper which has the ability to "hold out" the wet coating such that gloss is maximized and transparency is minimized. The best papers have been found to be 60 gsm approximately and (usually) clay coated both sides. One * Trade Mark of APPM suitable example is Austral Cote* 63gsm ex APPM with intrinsic opacity Y b /Y >0.9*1 (brightness 86). When coated with 10 gsm of example H, this opacity rises to 0.98 (brightness 92). In addition a gloss level of 55 - 60 is achievable using this paper. Another important factor in selection of paper for use in the invention is the presence of lignins and wood. Papers made from pulp which has not been chemically treated (e.g. mechanically ground wood pulp or semi-chemical pulp from which lignin has not been removed) are favoured. It is believed that the presence of lignins in the paper reduces the tendency for the paper to appear translucent when a clear coating is absorbed into the paper causing a refractive index reduction. That tendency may be minimized by selecting papers containing less then 20% of wood free - or chemical - pulp. However, in less preferred embodiments the coating may be applied to foils, cellulose fibre batts, webs, felts or other carriers which may be bonded to a composite board. The EB coating composition comprises one or more cross-linkable acrylate oligomers. For preference, the oligomer is selected from the group consisting of urethane acrylate, epoxy acrylate, polyester acrylate and silicone acrylate oligomers. These oligomers may be multifunctional, self-crosslinking oligomers or may be used in combination with crosslinking agents. Preferred crosslinking agents are acrylate monomers and more preferably ether acrylate monomers or alkoxylated acrylate monomers, e.g. tripropylene glycol diacrylate. The EB curable composition essentially includes one or more pigments and optionally may include filters, extenders, dyes, and other additives. The selection of oligomer influences flexibility, hardness, abrasion and solvent resistance of the end product and pigmentability. Table I shows the viscosity of various compositions identified by characters "A" to "J". Of these, compositions "D" to "J" inclusive are embodiments of the invention and compositions "A" to "C" are included for comparison only. With reference to formulations "F" and "G" of Table 1, EBECRYL* 1810 is a low viscosity polyester acrylate having an "as supplied" viscosity of 340 cps with no diluent added. This has been found to provide an excellent grinding medium for pigments and enables a pigment loading of up to 71% w/w without addition of diluent. After EB curing formulation "F" and "G", a hard and glossy film is obtained from each. In * Trade Mark of UCB ** Trade Mark of Polycure Pty. Ltd. comparison with formulation "D", it is difficult to achieve pigment loadings in excess of 60% in URACRIL** T (an aromatic trifunctional urethane acrylate) without addition of a diluent. The films produced on curing formulation "D" are duller, have lower solvent resistance and are more easily scuffed. Formulation "E" utilizes a low molecular weight, highly reactive aromatic urethane triacrylate (LOCRIL** 3134) in which pigment loadings of 60% may be satisfactorily achieved without the use of diluent and on curing hard, glossy and very solvent resistant coatings are obtained. When thinned with monomer (TPGDA) , the LOCRIL 3134 system has superior solvent resistance, hardness and water resistance at identical viscosity to the EBECRYL 1810 system and has very similar gloss. In practice, pigment loadings of 60% represent a practical maximum above which additional pigment does not substantially increase opacity. By utilizing high pigment loadings, the opacity of the film is such that surprisingly thin films may be utilized. For example, formulation "J" may be used in films as thin as 6 - 7 grams per square meter .(gsm) . In flexibility, comparable coatings of the three oligomers ranked URACRIL T > EBECRYL 1810 > LOCRIL 3134. The oligomers rank in the reverse order in respect of hardness and solvent resistance, LOCRIL 3134 being the hardest and most solvent resistant. If molecular weight of the oligomer is too low, the composition tends to "soak in" excessively prior to curing, while if the molecular weight is too great, it is difficult to obtain satisfactory pigment loading. Oligomer molecular weights in the range of from 1000 to 4000 or more have been found to be particularly suitable. Although some oligomers such as EBECRYL 1810 may be cured by electron beam (EB) irradiation without additional crosslinking agents (see formulation "G"), compositions according to the invention desirably comprise a crosslinking agent which is preferably one or a blend .of monomers, and more preferably is a blend of multifunctional ether acrylate, or alkoxy acrylate, monomers. Particularly suitable acrylate monomers include: Tripropylene glycol Diacrylate - "TPGDA" Propoxylated Glycerol Triacrylate . - "PGTA" Dipropylene Glycol Diacrylate - "DPGDA" Hexanedioldiacrylate - "HDDA" These monomers are compatible with the oligomer system and are efficient viscosity reducers. They have low odour, high crosslinking efficiency (and thus provide high cure speed) and low DRAIZE value (skin irritancy) . In a preferred embodiment a blend of TPGDA:DPGDA:PGTA (3:5:2) is employed with URACRIL T as the oligomer. In another embodiment a 60:40 blend of TPGDA/PGTA is used with URACRIL T. Increasing the level of the trifunctional monomer substantially increases solvent resistance in the finished product. It has been found to be desirable to include a wetting agent such as NUOSPERSE 657. However, levels in excess of about 0.5% are likely to adversely affect solvent resistance. Water resistance is of particular importance for laminates for use e.g. inside kitchen cupboards. Water resistance can be measured by two different but related parameters, namely water absorption into the paper laminate, and water transmission into the laminate substrate. Water resistance is important to prevent delamination and substrate swelling. We have found that water resistance decreases in the order LOCRIL 3134 > URACRIL T > EBECRYL 1810 but is enhanced by heavier film weights and by inclusion of hydrophobic monomers e.g. HDDA, TMPTA. Water resistance is also improved by including a silicone diacrylate such as EB 350 (available from UCB) in the oligomer blend. In general, it is preferable to select those compositions which include oligomers having the highest crosslinking density and the greatest hydrophobicity e.g. urethane acrylates with alkyl backbones and with high functionality. The coating formulation may be applied to the paper by any suitable means for example by use of a roller coater at elevated temperature, for example 45°C - 50°C, by knife coaters or by other means. The applied coating is cured by an electron beam. Curing is typically conducted at speeds of 400 m/min. with a dose of 3 M rads in the presence of an inert (N_) atmosphere. Under these conditions (400 m/min) the dwell time of the composition between coating and curing is about 0.3 seconds. Cured coatings of 1 - 15 gsm give adequate opacity when paints of 40 - 70% pigment loading are used depending on choice of paper. The cured coated sheet or foil so produced is sufficiently flexible to be wound onto a roll in long lengths at 400 m/min and may be economically produced and stored on the roll. The cured coated sheet or foil is subsequently unwound from the roll and laminated onto a composite board, particularly hardboard, particle board or other substrate. The laminating process varies slightly depending on the adhesive but is essentially application of adhesive to the substrate by roller coater followed by IR heating and application of the laminating foil to the substrate ia a modified roller coater set up to apply the - In ¬ appropriate ' laminating pressure. Typical temperatures after IR heating are 80°C and typical operating speeds are 30 m/minute. Ease of application of adhesives is PVA>EVA>U-F (higher laminating temperatures are required in the same order) . If desired, offcuts of hardboard or particle board may be glued together in a continuous edge-to-edge assembly and the coated paper may be laminated to the surface, the opacity of the paper being sufficient to mask the underlying joints. The "coated and cured" paper does not have the hardness or abrasion resistance of a high pressure melamine resin lamination sheet but performs very well in comparison with prior art paints in respect of scuff resistance, abrasion resistance and resistance to common household solvents and water. On the other hand, laminated board made according to the invention is inexpensive to manufacture in comparison with prior art laminates utilizing melamine sheets. Advantageously, coated and cured paper as described herein is sufficiently flexible for handling in rolls, and thus is able to be laminated to a chip board substrate on a continuous basis and at comparatively low production costs. Electron beam dose, dose rate and curing speed may be varied to an extent which can be determined by routine experiment. Dual cure systems in which EB curing is combined with UV, or with vapour catalyzed or isocyanate curing systems or the like may be employed. As will be apparent to those skilled in the art from the teaching hereof, the invention may be embodied in other forms and the coating compositions may be varied from those described to an extent which is determinable by routine experiment. Modifiers such as gloss or matt additives, viscosity control agents, extenders, fillers and catalysts and the like may be added without departing from the invention herein disclosed. TABLE I B H Uracril T (1) 100 - - 34 - - - 22 22 - Locril 31342 (2) - 100 34 - - - - 18 EB1810 (3) - - 100 - - 34 29 - Nuosperse 657(4) - - - 0.5 0.5 0.5 0.5 0.3 0.3 0.5 TPGDA (5) - - - 21 21 26 - 9 33 26 TIONA 464 (6) - - - 45 45 40 71 44 44 55 DPGDA (7) - - - - - - - 17 - PGTA (8) - - - - - - - 7 - Anti-settling(9) - - - . - - - - 0.7 0.7 - Viscosity cps (30°C) 13K 13K 340 1.4K I.IK 85 23K I.IK I.IK 0.9K Solvent D/R (lOgs ) 8 15 18 - 15 5 20 (1) Aromatic trifunctional urethane acrylate ex Polycure, containing 30% TPGDA, Double bond EW=532. (2) Aromatic trifunctional urethane acrylate ex Polycure, containing 30% TPGDA, Double bond EW=357. (3) Low viscosity Polyester acrylate ex UCB. (4) Wetting agent ex A.C. Hatricks. (5) Tripropylene glycol diacrylate ex UCB. (6) Rutile titanium dioxide ex SCM. (7) Dipropyleneglycoldiacrylate ex Degussa. (8) Propoxylated glyceroltriacrylate ex Synthese.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:- 1. A method for manufacture of a composite board having a decorative or protective surface, comprising the steps of: a) coating an electron beam curable pigmented composition onto a carrier sheet; b) curing the composition by means including electron beam irradiation; and c) bonding the cured coated carrier to a composite board. 2. A method according to Claim 1 further including the steps of: d) winding the carrier sheet obtained from the curing step onto a roll; and e) feeding the carrier from the roll prior to or during the bonding step. 3. A method according to Claim 1 or Claim 2 wherein the electron beam curable composition includes one or more oligomers selected from the group consisting of urethane acrylates, polyester acrylates, epoxy acrylates and silicone acrylates and at least one pigment. 4. A method according to Claim 3 wherein the electron beam curable composition includes at least one acrylic monomer. 5. A method according to Claim 4 wherein the acrylic monomer is selected from the group consisting of polyfunctional acrylate monomers. 6. A method according to Claim 5 wherein the selected monomer is a polyfunctional alkoxy - or ether - acrylate monomer. 7. A method according to any one of the preceding claims wherein the carrier sheet is paper. 8. " A method according to Claim 7 wherein the paper has an opacity when measured on a MINOLTA chromometer. 9. A method according to any one of the preceding claims wherein the electron beam curable coating composition is applied at from 1 - 15 gsm to the carrier sheet. 10. A method according to any one of the preceding claims wherein the selected oligomer is predominantly a urethane acrylate., 11. A method according to Claim 10 wherein the oligomer is an aromatic trifunctional urethane acrylate. 12. A method according to any one of the preceding claims further comprising the step of disposing a plurality of composite board pieces into an assembly in which the faces of the pieces are co-planar, and in which an edge of one piece abuts an edge of an a jacent piece, and bonding the carrier obtained from step (c) to the co-planar faces of the pieces in the assembly. 13. A laminate comprising a composite board, a lamina bonded on at least one side of the board, the lamina being a carrier sheet having less than 15 gsm of a pigmented, electron beam cured coating composition on its exposed surface. 14. A laminate according to Claim 12 wherein the carrier sheet is paper. 15. A laminate according to Claim 14 wherein the paper has an opacity Yb, /Yw>0.9 when measured on a MINOLTA chromometer. 16. An electron beam curable coating composition for use in manufacture of the laminate of Claim 13 comprising, by weight of the uncured composition: 18% to 34% of an acrylate oligomer 0% to 33% of an acrylate monomer or monomers 40% to 71% of a pigment. 17. A composition according to Claim 16 wherein the oligomer is an aromatic trifunctional urethane acrylate. 18. A coating composition according to Claim 17 wherein the acrylate monomer comprises tripropyl glycol diacrylate. 19. A composition substantially as herein described with reference to any of examples "D" -to "J" . 20. A method substantially as herein described with reference to any one of the examples. 21. A lamina for use in a method according to Claim 1 comprising a paper having an opacity Y fa Y >0.9 when measured with a MINOLTA chromometer, the paper being coated with a pigmented, electron beam cured composition and having a coated weight of less than 15 gsm. 22. A lamina according to Claim 21 having an opacity Y j /Y >0.95 in the cured coated product.

Inventors: BETT STEPHEN JAMES, GARNETT JOHN LYNDON, BETT, STEPHEN, JAMES, GARNETT, JOHN, LYNDON
Applicants: BETT STEPHEN JAMES, GARNETT JOHN LYNDON, BETT, STEPHEN, JAMES, GARNETT, JOHN, LYNDON
Application year: 1988
Main classification: B32B21
IPC codes: B27N7, C08F2, C09D4