Method of treating a substrate comprising cellulose fibres, and a product manufactured by the method

Description

Background of the invention

[0001] The invention relates to a method of treating a substrate, such as paper or paper coated with plastic, comprising cellulose fibres, and a product manufactured by the method.

[0002] A known method is to print paper webs for instance by means of flexographic or gravure printing, using water-dilutable inks and varnishes. A problem herein is that the method does not enable surfaces that are sufficiently water-resistant to be achieved for demanding uses, such as for use in furniture, packages to be used in humid environments, building papers or films, or outdoor advertising structures. The ink or varnish may start sagging anew, resulting in flaws or damaged surfaces.

[0003] The aforementioned problems may be eliminated by employing a second known method, namely that of printing with solvent-based inks and varnishes but, as demands for eco-friendliness are on the increase, this is not a good solution.

[0004] A third known method is to use solvent-free, UV curing inks and varnishes for printing. These are widely used in label printing, i.e. on small objects and narrow webs. However, in these objects, the consumption of ink and varnish is high, and the method cannot be advantageously employed in connection with wide webs since changing the ink/varnish from a solvent-free ink/varnish to a water-dilutable ink/varnish normally being used takes up expensive machine time as well.

[0005] A fourth known method is to use for printing a water dispersion containing ink or varnish and including cross-linking agents for curing a surface treatment while at the same time water is being dried in a drying process. A problem with this method is that the shelf life of the ink or varnish becomes shorter, i.e. the quantity of ink/varnish to be disposed of increases. Another problem with this method is that the composition of the ink/varnish changes as a function of time, which may lead to variations in quality of the end product and cause additional work in cleaning the printing machine.

Brief description of the invention

[0006] For the purpose of describing the invention, this application employs the following terms: A surface treatment layer refers to a water-containing layer which is printed onto a surface of a substrate. A surface treatment refers to a complete surface treatment being formed onto the surface of the substrate, and dried up and cured. Further, this application employs the term "water dispersion". A water dispersion refers to a dispersion comprising water and at least one polymer. Such a water dispersion may include at least one colour pigment, in which case the term "ink" or "printing ink" is used. If the water dispersion contains no colour pigment, the term "varnish" is used.

[0007] An object of the invention is to provide a method to enable the problems mentioned in the background of the invention to be solved. The object of the invention is achieved by a method which is characterised by what is stated in the independent method claim. The method is used for manufacturing a product as claimed in the independent product claim. Preferred embodiments of the invention are disclosed in the dependent claims.

[0008] The method according to the invention has several advantages. It enables waterproof surfaces to be achieved for demanding uses. The method employs no harmful solvents, but the printing ink/varnish is water-dilutable. The consumption of printing ink/varnish is at the same level as that of other water-dilutable inks/varnishes. It takes no extra time to change the ink/varnish at the printing machine. No problem exists with the shelf time or stability of composition of the printing ink/varnish since it is only exposure to radiation that causes curing. In connection with water dispersions to be cured by exposure to radiation, only little or no harmful monomers occur. When using the method according to the invention, it is also possible to achieve extremely thin but usable surface treatments. In addition, some ink/varnish compositions enable extremely wear-resistant surfaces to be achieved. Further, the method enables water and grease resistance as well as washability, i.e. stains being able to be removed from a surface by washing, to be achieved.

[0009] The method is based on applying onto a surface of the substrate a water dispersion to form a surface treatment layer which is dried up and cured into a surface treatment by exposure to radiation. A particularly advantageous result is achieved when first is formed a surface treatment layer which is dried up and, on top of this, a new surface treatment layer is formed which is then dried. The procedure may be repeated until a desired number of superimposed layers is achieved. This is followed by a curing phase. Generally, the number of superimposed surface treatment layers formed is 2 to 3. Even if the number of surface treatment layers is greater, the grammage of the surface treatment is generally less than 5 g/m².

[0010] The method according to the invention comprises treating a substrate comprising cellulose fibres. The substrate may be paper or paper covered with plastic, or nonwoven fabric. The grammage of paper is generally 40 to 200 g/m², preferably 40 to 120 g/m², and its fibre contents consist of mechanical and/or chemical pulp. Paper may contain a filler, or it may be filler-free. The surface of paper is sealed by gluing or coating, for instance by using a kaolin or talc coating. Gluing or coating may be provided on both sides or on one side only. If the paper is paper coated with plastic, the grammage of the coating is generally 5 to 20 g/m². The coating plastic may be polyolefin, such as polypropylene or polyethene. Preferably, the substrate is in a web form such that the width of the web is generally at least one metre, and the treatment may be performed as a reel-to-reel process. The treatment is performed as a reel-to-reel process such that the web is being treated continuously on the same production line. A surface treatment layer is formed by printing the water dispersion onto the surface of the web by employing a known printing method, such as flexographic printing, gravure printing or inverse gravure printing. Different layers may be formed by using different printing methods; for instance, a first layer by flexographic printing and layers thereupon by inverse gravure printing. The surface treatment layer/s is/are dried up and cured into a surface treatment by exposure to radiation. Preferably, the curing takes place by exposure to ultraviolet radiation (UV radiation). Alternatively, the curing may be performed by electron beam curing (EB radiation).

[0011] The surface treatment layer is usually formed from a water dispersion comprising 20 to 25 percent by weight of acrylate oligomers and/or acryl polymers, 55 to 60 percent by weight of water and, in addition, fillers and auxiliaries as well as photoinitiators. The amount of solids is generally 2 to 10 g/m².

[0012] Products manufactured by the method according to the invention are used for instance as an outer surface of different laminated materials; it may, for example, be laminated onto a surface of chipboard, MDF plate, HDF plate or honeycomb plate so as to form a barrier layer against humidity, grease or dirt. The product according to the invention also has a good tear strength. The main use for the product according to the invention is in imitations used in furniture industry, such as wood imitations.

Detailed description of the invention

[0013] In the following, the invention will be described by means of examples.

Example 1

[0014] As a printing substrate, a 135 g/m² Kemi Art Graph Plus liner by Metsabotnia was used. This was first printed using a flexographic printing method, most of the water was dried up by blowing hot-air onto the path, and subsequently coated from two units by using an inverse gravure printing method. A similar water-removing drying procedure was carried out between inverse gravure printing layers. Final drying with hot-air in a through drying machine, where the temperature of air was at 135°C. Next, the dried up varnish layer was cured further through exposure to UV radiation by using a mercury discharge lamp, radiation dose 50 mJ/cm².

[0015] For coating, a UV dispersion varnish WH/69298-20 by Becker Acroma was used in all units.

[0016] Eventually, a total amount of the varnish applied was 4 g/m² of dry varnish.

[0017] Properties achieved:

• water resistance R4 = 16 h water on top, after drying up no marks visible
• grease resistance R7 = 6 h paraffin oil on top, wiped and after 24 h washed with soap solution, and after drying up no grease strike-throughs

[0018] End use:

• furniture industry: surfaces in accordance with R7 requirements, such as back plates for bookcases.

Example 2

[0019] As a printing substrate, a 185 g/m² Kemi Art Graph Plus liner by Metsabotnia was used. A dispersion coating was carried out from two varnishing units by employing an inverse gravure printing method. Between the units, prior to a second application, water was dried from the dispersion by a hot-air blower blowing onto the surface of the dispersion. Final drying by a hot-air through drying machine at 135°C. This was followed by UV radiation drying by a mercury discharge lamp 50 mJ/cm².

[0020] For coating, a UV dispersion varnish WH/69298-20 by Becker Acroma was used in all units.

[0021] Eventually, a total amount of the varnish applied was 3 g/m² of dry varnish.

[0022] Properties achieved:

• water resistance R4 = 16 h water on top, after drying up no marks visible
• grease resistance R7 = 6 h paraffin oil on top, wiped and after 24 h washed with soap solution, and after drying up no grease strike-throughs

[0023] End use:

• furniture industry: surfaces in accordance with R7 requirements, such as back plates for bookcases.

Example 3

[0024] As a printing substrate, a 80 g/m² Modo Laser paper, coated with 12 g/m² LDPE plastic, was used.

[0025] The plastic surface is corona-treated 48 dyn.

[0026] A dispersion coating was carried out from two varnishing units by employing an inverse gravure printing method. Between the units, prior to a second application, water was dried from the dispersion by a hot-air blower blowing onto the surface of the dispersion. Final drying by a hot-air through drying machine at 135°C. This was followed by UV radiation drying by a mercury discharge lamp 50 mJ/cm².

[0027] For coating, a UV dispersion varnish WH/69298-20 by Becker Acroma was used in all units.

[0028] Eventually, a total amount of the varnish applied was 3 g/m² of dry varnish.

[0029] Properties achieved:

• water resistance R4 = 16 h water on top, after drying up no marks visible
• grease resistance R4 = 24 h paraffin oil on top, wiped and after 24 h washed with soap solution, and after drying up no grease strike-throughs
• coffee resistance R4 = 1 h coffee on top. Washed off with soap solution, and dried up. No discoloration whatsoever is accepted.

[0030] End use:

• furniture industry: surfaces in accordance with R4 requirements, such as side plates for bookcases and cabinets or cupboards, lower surfaces of shelves.

[0031] It is obvious to a person skilled in the art that, as technology advances, the basic idea of the invention may be implemented in many different ways. The invention and its embodiments are thus not restricted to the examples described above but may vary within the scope of the claims.

Claims

1. A method of treating a substrate comprising cellulose fibres, c h a r a c t e r i z e d by applying onto a surface of the substrate a water dispersion to form a surface treatment layer which is dried up and cured into a surface treatment by exposure to radiation.

2. A method as claimed in claim 1, characterized by printing the surface treatment layer onto the surface of the substrate.

3. A method as claimed in claim 2, characterized by printing the surface treatment layer by employing flexographic printing, gravure printing or inverse gravure printing.

4. A method as claimed in claim 2 or 3, characterized by printing onto the surface of the substrate a surface treatment layer which is dried up, and, subsequently, printing a new surface treatment layer on top of the dried-up surface treatment layer.

5. A method as claimed in any one of the preceding claims, characterized by curing the surface treatment layer by ultraviolet radiation or by electron beam curing.

6. A method as claimed in any one of the preceding claims, chara c t e r i z e d by printing continuously onto the substrate a surface treatment layer which is cured on the same production line.

7. A method as claimed in any one of the preceding claims, chara c t e r i z e d by the surface treatment layer comprising acrylate oligomers and/or acryl polymers.

8. A method as claimed in any one of the preceding claims, chara c t e r i z e d by the substrate comprising cellulose fibres being made of pigment-covered paper, plastic-covered paper or nonwoven fabric.

9. A product manufactured by a method as claimed in any one of the preceding claims.