Method for manufacturing a printed overlaying material

Field of the invention

The invention relates to a method for manufacturing a printed overlaying paper using electromagnetic radiation as the means to remove excess solvent, usually water, from the impregnated or lacquered printed paper. Subsequently the printed paper is used in the manufacture of a decorative laminate. The invention therefore also relates to laminate products manufactured using printed decor paper prepared by the method according to the invention.

Background of the invention

For the manufacture of laminates such as laminate flooring a decorative base paper, usually with a printed design, and an overlay paper are impregnated with a resin in which these overlaying materials serve as a carrier for the resin material. For laminate flooring a balancing paper is placed on the non-decorative side of the laminated substrate. To improve the abrasion and scratch resistance of the final laminate, hard particles such as corundum are incorporated in the surface layer above the printed design. These paper carrier materials also impart an internal stability to the final cured resin through their fibrous structure. In the manufacture of low pressure melamine flooring (LPM-flooring), also known as direct pressure melamine (DPM) and thermo- fused melamine (TFM), the overlaying paper or papers, impregnated with resin starting material and partially cured, are pressed onto a substrate in a hot press where the resin flows and bonds the carrier materials to the substrate and is cured to its final state to form the laminate.

Laminates can also be manufactured using so-called finish foils whether based on pre- impregnated or post-impregnated papers. Pre-impregnated papers are partially impregnated with a resin during the papermaking process. The pre-impregnated paper may subsequently be printed and a lacquer applied to protect the print and to provide the technical parameters of the decorative surface. By contrast post-impregnated papers are impregnated after the papermaking process and where applicable after a

print has been applied. Post-impregnated papers may also have a lacquered surface. Both pre-impregnated and post-impregnated finish foils are glued to the substrate to form the laminate.

In the manufacture of high pressure laminates (HPL) several resin impregnated kraft papers are assembled to form the substrate or core material and a resin impregnated decor paper is placed above the kraft papers. In some cases, for instance when a printed decor paper is used, a resin impregnated clear overlay paper is placed above the decor paper to provide wear protection for the print. Furthermore, in some cases the decorative paper is not impregnated and the clear overlay provides sufficient resin and the high pressure press sufficient pressure to form a uniformly impregnated HPL, this process being known as dry pressing. Hard particles such as corundum, also known as fused alumina, may be included in or on the clear overlay paper, or directly on the impregnated printed paper.

In another process known as continuous pressure laminate (CPL), the impregnated decorative paper and impregnated kraft paper or papers, and optionally an impregnated clear overlay paper, are fed from rolls into a double band hot press. The pressure and heat bonds the impregnated papers into a continuous laminate. Alternatively, the impregnated papers may be applied directly and simultaneously onto a wood based panel substrate within the continuous press. Hard particles such as corundum may be included in or on the decor paper or overlay paper.

The prior art for application of the print to the surface of the unicolour decor paper or pre-impregnated paper is at this time predominately by the rotogravure method and the inks are predominately water based. The printed decor papers are later impregnated, or impregnated and coated with resin, usually an amino resin such as urea formaldehyde or melamine formaldehyde or combinations of these. Printed pre-impregnated papers are usually lacquered. Post impregnated papers may also be lacquered.

Impregnating resins are usually water based resins with a water content of approximately 35% to 55%. For impregnated papers the industrial equipment used for removing the excess water and advancing the resin condition to the so-called b-stage,

with a volatile content of approximately 6%, is predominately hot air ovens. The applicant has, in international patent publication WO 2007/065222, disclosed the use of electromagnetic radiation, preferably by way of near infrared (NIR) emitters, for the removal of the excess water. However, it has been found that in some cases where the NIR radiation is applied to printed papers, localised blistering of the resin occurs. The effect of the blistering is to create areas of over-cured resin which may break away and cause dusting problems, and which can cause variations in the optical refractive index that lead to unsightly blemishes in the finished laminate. Moreover, the blisters are voids that cause variations in the quantity of the resin. The differences in flow characteristics between over-cured resin and ideal b-stage resin can cause the resin in the final laminate to have irregular surface properties.

Disclosed in international patent publication WO 2004/106027 is the use of NIR for postforming laminates of resin-impregnated components. A significant proportion of laminates that are postformed are printed decors. As with the impregnated printed decor paper, localised blistering and the aforementioned associated problems have been found to occur.

The problems just discussed have also been observed where, as divulged in International patent publication WO 2008/134823, NIR radiation is employed to process to b-stage decor papers impregnated with a urea based solution, eg water based urea formaldehyde resin. The thus treated papers are subsequently pressed onto a substrate eg particleboard or fibreboard. The pressed panel with a urea formaldehyde surface is subsequently coated to provide a chemical resistant, water resistant and non- porous surface. The coating may be one or several coatings and may contain additives to provide additional benefits such as but not limited to abrasion and scratch resistance. The coating may be cured by electron beam or by UV light, or may be a heat curing coating.

As described in the applicant's pending international patent publication WO 2007/079547, NIR irradiation may be used to fully cure a coating after pressing. The aforementioned problems are on occasion again observed.

These issues can also arise where impregnated papers or laminates being treated with NIR radiation contain pigments in the body of the paper, whether colouring pigments or otherwise.

It is an object of the invention to at least in part alleviate this problem that arises with the use of NIR radiation to dry resin-impregnated printed papers, to dry the lacquer applied to printed laminated panels, to dry the lacquer applied to pre-impregnated or post impregnated printed decorative finish foils, and to treat printed laminates that will be postformed.

Summary of the invention

This invention entails the realization that the aforedescribed problem with NIR drying of papers or laminates containing papers arises from differential absorption of the NIR radiation pigments in the print or in the papers. The differences in NIR absorption characteristics of various pigments are most noticeable after water has been substantially removed from the resin medium.

In a first aspect, the invention provides a method of manufacturing a resin-impregnated printed paper including applying printed indicia to a paper substrate, impregnating the printed paper with resin and thereafter treating the resin-impregnated printed paper to reduce its water content and to at least partially cure the impregnating resin, whereby to produce a further cured resin-impregnated printed paper, wherein said reduction of water is effected by irradiation with near infrared (NIR) radiation, and wherein said application of printed indicia utilises one or more pigments selected for their relatively low absorption of NIR radiation and/or a plurality of pigments selected for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering of the resin arising from only NIR absorption by the pigment(s).

The invention further provides, in its first aspect, a method of manufacturing a resin- impregnated printed paper including applying printed indicia to a paper substrate, impregnating the printed paper with resin and thereafter treating the resin-impregnated printed paper to reduce its water content and to at least partially cure the impregnating

resin, whereby to produce a further cured resin-impregnated printed paper, wherein said reduction of water is effected by irradiation with near infrared (NIR) radiation, and wherein said paper and printed indicia contain a plurality of pigments selected for their relatively low absorption of NIR radiation and/or for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering of the resin arising from any NIR absorption by the pigments..

In a second aspect, the invention provides a method of manufacturing a pre- impregnated printed paper including applying printed indicia to the pre-impregnated paper substrate, coating the printed pre-impregnated paper with a heat curable lacquer and thereafter treating the lacquered printed pre-impregnated paper to reduce its water content and to at least partially cure the lacquer, whereby to produce a further cured lacquered printed pre-impregnated paper, wherein said reduction of water is effected by irradiation with near infrared (NIR) radiation, and wherein said application of printed indicia utilises one or more pigments selected for their relatively low absorption of NIR radiation and/or a plurality of pigments selected for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering of the lacquer arising from any NIR absorption by the pigment(s).

The invention also provides, in its second aspect, a method of manufacturing a pre- impregnated printed paper including applying printed indicia to the pre-impregnated paper substrate, coating the printed pre-impregnated paper with a heat curable lacquer and thereafter treating the lacquered printed pre-impregnated paper to reduce its water content and to at least partially cure the lacquer, whereby to produce a further cured lacquered printed pre-impregnated paper, wherein said reduction of water is effected by irradiation with near infrared (NIR) radiation, and wherein said paper and printed indicia contain a plurality of pigments selected for their relatively low absorption of NIR radiation and/or for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering of the lacquer arising from any NIR absorption by the pigment(s).

In a third aspect, the invention provides a method of producing a printed laminate product comprising postforming a printed laminate having resin-impregnated

components, by irradiation with NIR radiation, to form the printed laminate product, wherein the print on the printed laminate comprises one or more pigments selected for their relatively low absorption of NIR radiation and/or a plurality of pigments selected for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering of the resin arising from any NIR absorption by the pigment(s).

In this third aspect, the invention further provides a method of producing a printed laminate product comprising postforming a printed laminate having resin-impregnated components, by irradiation with NIR radiation, to form the printed laminate product, wherein the print on the printed laminate and the components of the laminate contain a plurality of pigments selected for their relatively low absorption of NIR radiation and/or for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering of the resin arising from any NIR absorption by the pigment(s).

In a fourth aspect, the invention provides a decor paper containing one or more pigments selected for relatively low absorption of NIR radiation and/or a plurality of pigments selected for their relatively low absorption of NIR radiation and/or for their mutual relatively low differences in absorption of NIR radiation whereby to minimize localised blistering, arising from any NIR absorption by the pigment(s) of resin that then impregnates the paper.

By "near-infrared" (NIR) is herein meant the wavelength range between the visible region and 2.5 μm, 700 to 2500nm.

It is thought that, where conventional more NIR absorbent pigments are employed and the water content is sufficiently reduced, the printing ink pigments as well as the paper pigments continue to absorb the NIR radiation and convert the radiation to heat which in turn heats the surrounding resin or lacquer. With the variety of pigment colours typically used to achieve the desired decorative effect it is often the case that a print can include black pigments in close vicinity to pigments with much lower NIR absorption characteristics. When the difference in NIR absorption creates sufficient difference in

the amount of heat generated, the result is an overheating of the black areas that causes blistering of the resin or lacquer in the vicinity of the black pigment while the rest of the surface resin is unaffected by blistering.

Preferred embodiments of the invention

In the first aspect, the curing of the impregnating resin may be partial only, whereby the paper produced is a b-stage resin-impregnated printed paper.

Preferably, the absorption of NIR radiation is less than 50%, more preferably less than 30%, most preferably less than 20%. The aforesaid differences in absorption of NIR radiation are preferably less than 50 percentage points, more preferably less than 30, most preferably less than 20.

Where the printing requires black colouring or colours utilising a black pigment, the selected pigment(s) may include, for example, a black pigment that is transparent to radiation in the NIR range, for example Lumogen ^® Black FK, or that is NIR reflecting, for example Sicopal ^® Black K. Both these exemplary products are available from BASF of Ludwigshafen, Germany. Lumogen Black FK4280, for example, relies in part on a perylene base structure and is reported by BASF to exhibit about 10% or less transmission over the range 300 to 700nm, and 80-90% over the range 800 to 1300nm. Lumogen Black FK4281 exhibits a transmission at 1200nm of greater than 85%. In general, pigments based on carbon black are preferably avoided.

With the desire of manufacturers to promote their decorative laminates as resistant to sunlight degradation it is practical, and preferred according to the invention, to substitute NIR absorbing pigments with pigments that are transparent to NIR radiation or have NIR reflecting attributes, both in the manufacture of the decor paper as well as in the printing inks.

In the decor paper of the fourth aspect of the invention, at least one of the pigments may be contained within the body of the paper, and/or at least one of the pigments may be printing pigment applied to the paper. The paper may be impregnated with resin.

It is believed that in addition to substituting the NIR absorbing pigments with NIR transparent or NIR reflecting pigments, it would be advantageous to avoid excessive resin on the surface of impregnated papers as is suggested in applicant's international patent publication WO 2007/065222. A method to avoid excess resin on the surface would be to apply the impregnation resin only from the non-printed side preferably by a pressure impregnation technique. Suitable such techniques include pulling the unprinted surface over a pre-wetting roll or over a slotted die resin applicator, such as in the ARP system from the firm VITS Langenfeld Germany. After the printed paper has passed over the slotted die and the desired amount of resin has soaked into the paper, the resin-soaked paper is passed through an NIR radiation unit and the NIR radiation preferably applied to the non-printed side to remove the solvent, usually water, to reach the desired b-stage. It may be desirable that the resin in the slotted die applicator is pigmented to increase the opacity of the decor paper.