WO2009121857A1

WO2009121857A1 - Process for application of stain

Info

Publication number: WO2009121857A1
Application number: PCT/EP2009/053771
Authority: WO
Inventors: Niclas Nils Erik Andersson; Ola Magnus HÅKANSSON
Original assignee: Akzo Nobel Coatings International B.V.
Priority date: 2008-04-04
Filing date: 2009-03-31
Publication date: 2009-10-08

Abstract

The invention relates to a process for application of a stain on a wood based substrate using a first roller with a resilient smooth surface contacting the substrate and a second roller with an engraved hard surface contacting the surface of the first roller, and wherein the substrate is moved relative to the first roller, characterized in that stain is applied to both the first roller and the second roller.

Description

Process for Application of Stain

The invention relates to a process for application of a stain on a wood based substrate using a first roller with a resilient smooth surface contacting the substrate and a second roller with an engraved hard surface contacting the surface of the first roller, and wherein the substrate is moved relative to the first roller.

Such a process is known from International patent application WO 2004/054806 A. This document describes a rotary pad printing system comprising an engraved steel roller in contact with a pad printing cylinder with a silicone rubber surface. The system is provided with a closed ink system where ink is applied to the engraved roller. The printing cylinder translates the desired patterns to the print object. Examples of such objects are boards of MDF or wood. Doctor blades are provided for scraping off excess ink. European patent application EP 1598185 A relates to an apparatus and a process for printing wood or plastic substrates. The process comprises the steps of applying printing ink into the depressions of an engraved cylinder, transferring the printing ink to an off-set roller having a surface hardness of less than 20 Shore, and printing the printing ink from the off-set roller to the substrate. German Utility Model DE 20 2004 018710 U1 describes an apparatus similar to the apparatus used in the present invention. This document describes the application of a structured top layer with the apparatus.

A drawback of the known process is that an uneven amount is applied to the substrate when said substrate has a length which is greater than the circumference of the first roller. This leads to variations in colour depth on the stained substrate, which is undesirable. In order to avoid such colour depth variations in the known process, it is required to apply a relatively large amount of stain. This is undesirable in its turn, because a large amount of applied stain increases the drying time and leads to more grain raising in wooden substrates.

The present invention seeks to provide a process which alleviates the above- mentioned drawbacks.

Accordingly, the present invention provides a process for application of a stain on a wood based substrate using a first roller with a resilient smooth surface contacting the substrate and a second roller with an engraved metal surface contacting the first roller, and wherein the substrate is moved relative to the first roller, characterized in that stain is applied to both the first roller and the second roller. When stain is applied using the process of the invention, an even amount of stain is applied to the substrate, also in cases where the substrate has a length which is greater than the circumference of the first roller. Therefore, colour depth variations on the stained substrate are minimized. Furthermore, the process allows the application of a minimum amount of stain, which leads to a short drying time and to decreased grain raising on wooden substrates.

The invention will now be described with reference to Figure 1 , which illustrates a process according to the invention. A wood based substrate (1 ) is moved relative to roller (2) along moving direction (6) on a conveyor belt (9). The substrate (1 ) is in contact with roller (2) having a resilient smooth surface. Roller (2) transfers stain to the wood based substrate. Roller (2) is also in contact with a second roller (3) with an engraved hard surface. Roller (2) rotates around axis (7) in the direction of rotation (11 ). Roller (3) rotates in the opposite direction (12) around axis (8). Stain is supplied via stain feed lines (10) to the surface of roller (2) and to the surface of engraved roller (3). A doctor blade (4) scrapes off excess stain from the surface of roller (2). A second doctor blade (5) removes excess stain from the surface of the second roller (3), leaving stain in the engraved parts of the surface of the second roller (3). The wood based substrate can be genuine wood, including veneer. Alternatively, the substrate may be reconstituted wood, such as chipboard, high or medium density fiberboard, or plywood. The wood based substrate may be in the form of boards, for example boards used for flooring or furniture. It is also possible to use substrates intended for outdoor application, such as framework for windows.

The first roller has a resilient smooth surface. By smooth surface is meant that the surface is essentially even and free of indentations and protrusions. The surface of the first roller is solid, but it is made of resilient material. Examples of suitable materials for the surface of the first roller are natural or synthetic rubber, silicone, and silicone rubber. The surface of the first roller can also be made of mixed or composite materials. In one embodiment, the roller surface has a sandwich structure having two or three layers of resilient material of different Shore A hardness values. Generally, the surface of the first roller has a Shore A hardness value of at least 0, or at least 5, or at least 10. A suitable upper limit of the Shore A hardness value is 70, or 50, or 30. The flexibility of the surface of the first roller allows the roller surface to also contact the edges or even the sides of the substrate when the roller is pressed towards the surface of the substrate. During the process the first roller contacts the substrate and stain is transferred from the surface of the first roller to the substrate by movement of the substrate relative to the first roller. Generally, the movement direction of the substrate is essentially perpendicular to the axis of the first roller. Movement of the substrate relative to the first roller can suitably be effected by placing the substrate on a conveyor belt. In a typical embodiment, the conveyor belt is made of flexible material and moving the substrate, which is sandwiched between the conveyor belt and the first roller.

The surface of the first roller is in contact with the surface of a second roller. Generally, the axis of the first roller and the axis of the second roller are essentially parallel. The second roller has an engraved hard surface. The hard surface of the second roller generally is an engraved metal surface, for example an engraved copper or steel surface. In one embodiment, the engraved hard surface is made of an engraved copper surface subsequently covered with a chrome layer. Alternatively, the engraved hard surface may also be a ceramic surface. Generally, the surface of the second roller is engraved so as to form a regular pattern of cells over the entire surface of the second roller. The cells are capable of storing liquid stain. The engraving can be in the form of lines of varying width or other geometrical forms. The most suitable type of engraving depends on the viscosity of the stain to be applied and on other process parameters. The size, depth, geometry, and distribution of the engraving determines how much stain can be stored in the hard roller surface and also influences the transfer of stain from the second roller via the first roller to the substrate.

The first roller suitably is in contact with a first doctor blade. In operation, the doctor blade scrapes off excess stain from the surface of the first roller. Any surplus of stain and scraped off stain flows along the doctor blade to the ends of the roller, and from there to a receiving container.

In a preferred embodiment, the second roller is also in contact with a doctor blade. In operation, the doctor blade removes excess stain from the surface of the second roller, leaving stain in the engraved parts of the surface of the second roller. The surplus of stain flows into the receiving container in the same way as described for the first roller. The stain collected in the receiving container is suitably re-circulated into the process.

The stain applied in the process is a liquid composition. It is typically formulated to impart or transport the pigment into the pores of the surface rather than creating a film on top of the surface. Wood stain is generally designed to add color to the wood based substrate while leaving the substrate mostly visible. Application of a stain does not essentially alter the surface structure of a wood based substrate. In particular, it does not contribute to the formation of a structured surface layer. Transparent varnishes or surface films are applied afterwards. Suitable stains for staining wood based substrates are generally known to the skilled person. The stain generally comprises a liquid diluent. The liquid diluent may be an organic solvent or water, or mixtures thereof. Preferably, the stain is a water based stain. Such a water based stain generally comprises water and pigments, and optionally dyes. The stain may also comprise a polymer binder. The polymer binder may be a physically drying binder. Alternatively, the binder may be crosslinkable. In another embodiment, the stain may be a radiation-curable stain comprising a radiation-curable binder, optionally in combination with a physically drying binder. Curing of the radiation-curable stain is carried out by exposure of the applied stain to radiation, typically ultraviolet radiation. The radiation-curable stain suitably comprises a liquid diluent, which may be selected from water, a volatile organic solvent, or a radiation-curable monomer. The radiation-curable stain suitably comprises the following components: 5 to 90 % by weight of one or more binders, at least one of the binders being radiation-curable, 0.1 to 40 % by weight of pigments and/or dyes, 1 to 10 % by weight of one or more photoinitiators, optionally up to 5 % by weight of additives, such as wetting agents or defoamers, and optionally up to 90% by weight of a liquid diluent. The % by weight is based on the total weight of the stain composition. If not radiation-curable, a typical stain contains 0.1 to 50 % by weight of binder, such as a resin or organic polymer, 0.1 to 60 % by weight of pigments and/or dyes, 0 to 3 % by weight of additives, such as wetting additives or anti-foaming additives, and 30 to 96 % by weight of water. The % by weight is based on the total weight of the stain composition.

The stain used in the process of the invention suitably has a higher non-volatile content than conventional stains. Typically, the stain has a non-volatile content of at least 3 % by weight, or 10 % by weight, calculated on the total weight of the stain. The non-volatile content generally does not exceed 50 % by weight, although for radiation-curable stains the non-volatile content may be up to 99 % by weight. The term non-volatile content refers to the weight fraction of the coating composition that remains in the coating film after drying and curing. The initial viscosity of the stain generally is at least 10 seconds, or at least 15 seconds, or even 50 seconds, measured in a DIN 4 cup at 23°C. Lower viscosities can give rise to an uneven appearance of the dried stain on the substrate. Generally, the viscosity does not exceed 50 seconds, or 40 seconds, or even 35 seconds. Stain is suitably transported to the first roller and the second roller via stain feed lines. The stain feed lines are connected to a stain reservoir and have stain outlet openings close to the roller surfaces. The stain is suitably transported through the stain feed line by a pump which pumps the stain from the stain reservoir towards the stain outlet opening. Looking into the rotation direction of the respective roller, the stain outlet opening generally precedes the doctor blade. Preferably, the stain outlet opening is located close to the doctor blade.

The first roller transfers the stain from the second roller to the substrate. More precisely, the first roller transfers the stain from the surface of the second roller to its own surface and subsequently to the substrate. The stain filled cells of the surface of the second roller are at least partly emptied during this transfer process. Application of stain to both the surface of first roller and the surface of the second roller ensures that there is a continuous wetting of the surface of the first roller. This eliminates variations in colour strength within the stained substrate. If stain is applied to the surface of the second roller only, the wetting of the surface of the first roller is supplied by the second roller only. In such a case, the amount of stain applied to a substrate decreases with each full of rotation of the first roller, until after a certain number of rotations, for example 10 rotations, an equilibrium is reached. This leads to the above-mentioned variations in colour strength on the stained substrate. This effect will be particularly visible when the length of the substrate is greater than the circumference of the first roller and when there are interruptions in the substrate feed, i.e. when the substrate is fed discontinuously. Under practical circumstances, the length of the substrate is generally greater than the circumference of the roller. Also, a fully continuous feed can hardly be reached. The problem of colour strength variation arising under these conditions is eliminated when stain is applied to both the second roller and the first roller.

The invention also relates to the use of an apparatus having a) a first roller with a resilient smooth surface, b) a second roller with an engraved hard surface contacting the surface of the first roller, c) a first coating agent feed line connected to a coating agent reservoir for supply of coating agent to the surface of the first roller, and d) a second coating agent feed line connected to the coating agent reservoir for supply of coating agent to the surface of the second roller, for application of stain to a wood based substrate.

Claims

1. A process for application of a stain on a wood based substrate (1 ) using a first roller (2) with a resilient smooth surface contacting the substrate (1 ) and a second roller (3) with an engraved hard surface contacting the surface of the first roller (2), and wherein the substrate (1 ) is moved relative to the first roller (2), characterized in that stain is applied to both the first roller (2) and the second roller (3).

2. A process according to claim 1 , wherein the first roller (2) is in contact with a first doctor blade (4).

3. A process according to claim 1 or 2, wherein a second doctor (5) blade is in contact with the second roller (3) and wherein the second doctor blade (5) removes excess stain from the surface of the second roller (3), leaving stain in the engraved parts of the surface of the second roller (3).

4. A process according to any one of the preceding claims, wherein the substrate (1 ) is genuine wood.

5. A process according to any one of claims 1 to 3, wherein the substrate (1 ) is reconstituted wood.

6. A process according to any one of the preceding claims, wherein the surface of the first roller (2) consists of a material selected from the group consisting of natural rubber, synthetic rubber, and silicone rubber.

7. A process according to any one of the preceding claims, wherein the surface of the first roller (2) has a sandwich structure having at least two layers of resilient material of different Shore A hardness values.

8. A process according to any one of the preceding claims, wherein the surface of the second roller (3) consists of metal.

9. A process according to any one of the preceding claims, wherein the moving direction (6) of the substrate (1 ) is essentially perpendicular to the axis (7) of the first roller (2).

10. A process according to any one of the preceding claims, wherein the axes (7, 8) of the first roller and the second roller are essentially parallel.

11. A process according to any one of the preceding claims, wherein the substrate (1 ) is moved relative to the first roller (2) by a conveyor belt (9).

12. A process according to any one of the preceding claims, wherein the length of the substrate (1 ) is greater than the circumference of the first roller (2).

13. A process according to any one of the preceding claims, wherein the stain is a water based stain.

14. A process according to claim 13, wherein the stain comprises water and pigments and/or dyes.

15. A process according to claim 14, wherein the stain additionally comprises a crosslinkable or physically drying polymer binder.

16. A process according to any one of the preceding claims, wherein the stain has a non-volatile content of at least 3 % by weight, calculated on the total weight of the stain.

17. A process according to any one of the preceding claims, wherein the viscosity of the stain, measured at a temperature of 23°C in a DIN 4 cup, is in the range of 15 to 40 seconds.

18. A process according to any one of the preceding claims, wherein the stain is a radiation-curable stain.

19. A process according to any one of the preceding claims, wherein the surface of the first roller (2) has a Shore A hardness value in the range of 5 to 70.

20. Use of an apparatus having a) a first roller (2) with a resilient smooth surface, b) a second roller (3) with an engraved hard surface contacting the surface of the first roller (2), c) a first coating agent feed line (10) connected to a coating agent reservoir for supply of coating agent to the surface of the first roller (2), and d) a second coating agent feed line (10) connected to the coating agent reservoir for supply of coating agent to the surface of the second roller (3), for application of stain to a wood based substrate.