The invention relates to a digitally printed product, a method of manufacturing the product and the use of the product. In particular, the invention concerns relatively large-area plate-like products, which are suitable for wall elements, billboards or the like, or to the coating of such walls and the like, the products preferably being sufficiently weather-resistant to be suited to outdoor use.

A general advantage of digital printing is the possibility to quickly change the printing (of text and/or figures) without interrupting the process, which is enabled by electric control and which results in short differentiated production runs and advantageous costs.

Present billboards and posters that are mounted on outdoor poster stands or frames are printed on paper or board and, in many cases, provided with a transparent protective glass or plastic. However, even when protected, they do not re- sist the effects of weather for long periods; therefore, they are intended to be short-lived and to be changed frequently. As far as the applicant knows, a printed, plate-like product, which mainly consists of a natural fibre material and which as such would endure outdoors for several months or even years without a separate protective cover without essentially changing its appearance, has not existed so far.

Digital printing of outdoor advertisements on plastic material (channel plastic) is well-known. Such advertisements resist solar UV radiation for a few months.

Fibre-based plates, such as plywood, chipboard, fibre board, hardboard or OSB board, which are used as construction materials, are protected for outdoor use by coating them with a film impregnated with resin. A conventional procedure is to absorb phenol-formaldehyde resin or amino resin into kraft paper, after which the paper is attached to a base board at a raised pressure and temperature/ the attachment being based on the hot curing of resin. In applications, where the boards are subjected to mechanical stress, one or more thermoplastic layers are also possibly included in the protective coating. Generally, prints are included in the coatings by printing the paper by flexographic printing before impregnating it with the resin. The patent application Fl 20050172 discloses a coating, which is intended for wood panels, such as plywood or chipboard and which is based on kraft paper that is impregnated with a resin mixture. The patent specification mentions the advantages and disadvantages related to the weather resistance of various types of res- ins; phenol-formaldehyde resins do not crack but they change their colour under the effect of sunlight and UV light, whereas amino resins have a good weather resistance but they are more susceptible to cracking. The purpose of the specification is to provide a wood panel coating that has an even, light colour shade, whereby the darkening that the phenol-formaldehyde resins undergo, in particular, is considered a problem. As a solution, the specification discloses the pigmenting of the resin mixture and printing the kraft paper with a desired printing ink. The purpose of the colour printing is thus to provide a coating of an even colour and an even quality, which has good coverage.

The application Fl 20050172 mentions no digitally printed products or any prod- ucts that would contain information produced by printing. The only goal of the colour printing disclosed is to provide the uniform and even, preferably light colour shade of the product. In its description of the prior art, the specification mentions the use of coloured decorative paper in the coating, but merely as an alternative to brown kraft paper, i.e., the goal also in this case is the desired colour shade of the paper, not information or a decorative patterning of the board product.

The problem that the invention seeks a solution to is thus to provide a digitally printed plate-like product, where the printing consists of image or text information or decorative patterning and which resists weather, i.e., sunlight, UV radiation and varying humidity, rain and changing temperatures well enough to be suitable for outdoor use. The product according to the invention is characterized in that it comprises a plate-like base and a fibre-based film, which constitutes its coating and is impregnated with softened amino resin, and that the coated side of the product is provided with digital printing.

The invention is based on an observation, according to which the paper, which is impregnated with colourless amino resin and attached to the plate-like base by means of heat and pressure, can be digitally printed and, thereafter, subjected to a long-term effect of weather without the quality of the printing essentially weakening or with relatively minor changes in the quality of printing. An advantage of amino resins compared with phenol-formaldehyde resins is their good resistance to UV radiation, i.e., that they do not darken under the effect of visible or UV light. The mechanical strength of the resin is a less critical factor, e.g., in billboards and, according to the invention, the mechanical strength of amino resin is improved by mixing a softener with it. Suitable well-known softeners include caprolactam, or- tho/para toluene sulfonamide (Ketjenflex), acetoguanamine, monoethylene glycol (MEG), diethylene glycol (DEG), polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyacrylate, acrylic resin, phenol-formaldehyde resin and the like. A small amount of phenol-formaldehyde resin as softener does not yet cause a significant darkening of the product.

Suitable amino resins include melamine-formaldehyde resin, urea-formaldehyde resin or a mixture thereof. Melamine-formaldehyde resin is preferable in outdoor use and urea-formaldehyde resin in indoor use. The share of amino resin in the mixture is preferably about 60-95 weight-% and that of softener is preferably about 5-40 weight-%. To enhance its chemical strength, other additives, such as stabilizers, UV absorbents, pigments, surfactants, moisteners, anti-foam agents and diluents can be added to the mixture in a total amount of about 0.1-10% of the weight of the mixture.

The product according to the invention can be coated in the manner described above and digitally printed either on one side only or both sides thereof. The films that serve as coating on different sides of the product are most preferably similar to each other, and the prints of the different sides of the product can be mutually similar or different.

According to observations, the adherence of printing ink to the surface of the product and the visual quality of the product depend, first of all, on the quality of the colour and the suitably even absorbance of the surface. The adherence can be influenced by UV-hardening the ink. The adherence can be sufficiently strong as such, but in some cases it is weaker, even so that the printing can be scratched off by a fingernail. However, the problem can be eliminated by providing the digitally printed coated side of the product with a protective lacquering. A colour pigment that dyes or shades the printing surface can be incorporated into the protective lacquer, naturally, so that the pigment does not cover the printing, however. The protective lacquer also provides the printing surface with a good wear resistance.

In comparative tests, it was observed that even an uncoated wooden surface or plywood can be printed digitally and lacquered to attach the printing and to protect the product against scratching. However, the non-homogeneous matter of the wood or plywood absorbs printing ink unevenly and the printing shows in different ways when viewed at different angles. The printing surface also darkens considerably under the effect of UV light. These problems are avoided in the digitally printed product according to the invention that is coated with the impregnated film. The weather resistance of the unlacquered or lacquered wood or plywood cannot be otherwise compared to the product according to the invention, which is coated with the film that is impregnated with the amino resin mixture.

The base, which the film impregnated with the amino resin mixture is attached to, can in the product according to the invention comprise cardboard, corrugated board or a wood panel, such as plywood, block board, chipboard, fibre board, hardboard or OSB board (Oriented Strand Board). The film coating of such boards is conventional technology as such. Furthermore, according to the invention, the base can comprise glued laminated timber or machined solid wood, whereby the film impregnated with the amino resin mixture adheres to the wood as such. As far as the applicant knows, wood that is coated with a film impregnated with amino resin mixture (to be distinguished from wood panels made of pulp), which is included in the present invention, is not known from before.

According to the invention, the fibre-based film to be impregnated preferably comprises paper that mostly consists of chemical pulp. Regarding the visibility of the printing and the possibilities to shade the product, bleached paper is preferable. As a coating of a wooden base, white paper that is impregnated with resin leaves the grains of the wood partially visible. If the product is lacquered, it can be shaded by pigmenting the lacquer, so that the grains of wood are still visible under the transparent film (the impregnated white paper). The level of the film coverage can be adjusted with pigments that are added to the resin. When using brown kraft pa- per, white pigment can be included in the impregnation resin to whiten the product.

Instead of paper, the film can comprise non-woven material. Wood fibres can partially or fully be replaced with plastic or glass fibres.

The method according to the invention for manufacturing a digitally printed product described above is characterized in that (i) a fibre-based film is impregnated with softened amino resin, (ii) the impregnated film is attached to a plate-like base by hot curing to constitute the coating, and (iii) the coated side of the product is digitally printed.

If UV stabilizers are incorporated into the film, UV-hardening printing inks can be used. The printing method can be, e.g., the ink-jet printing known as such. The invention comprises use of a digitally printed product described above or manufactured by the said method, when subjected to weather in outdoor locations, in particular. The product can serve as a separate decorative panel, billboard or signboard that is attached, e.g., to a stand intended for the same, or the product can comprise an element of a building wall, fence, frame or scaffolding or the like. Naturally, the products according to the invention are also suitable for indoor use, e.g., on furniture and wall surfaces, wherein exposure to sunlight is possible and protection against light and UV light is thus desirable. Other suitable applications include external wall elements, interior walls, ceilings and floors of buildings, out- door advertisements on roadsides, bus stops, poster stands, lamp posts, in sports fields, in front of shops, noise barriers, garbage shelters, construction site fences, playground fittings, lorry platforms and other vehicles and means of transport.

Examples

In the following test examples and related figures, digitally printed plywood boards according to the invention, which were coated with a protective film, and reference boards, which had no protective films, were subjected to climate testing. The testing arrangement and performance were as follows.

The thicknesses of the reference plywood boards were 4 mm and 9 mm. The plywood boards were coated with commercial impregnate films 60/174 Novox NW and 60/174 Novox OW (Laminating Papers Oy), which comprised kraft paper that was impregnated with a resin mixture, or they were left uncoated (reference boards). In Novox NW, 4.6% of Tiθ ₂ pigment that reflects UV radiation was added to the softened amino resin. The impregnation resin in Novox OW was otherwise the same, except that there was only 0.7% of Tiθ ₂ pigment. Coloured digital ink-jet printing was carried out on all boards, both protected at the edges and without protection. After printing, part of the boards were lacquered with gloss lacquer or dull lacquer, part was not lacquered.

The climate testing comprised a 1000-h testing of test samples in a climate chamber, where the following cycle was repeated:

(i) 102 min of heating by a lamp that simulated the spectrum of sunlight, its power being 0.40 W/m ² , the surface temperature of the samples under the effect of radiation was 63 C°, the temperature of air was 50 C°, the relative humidity was 50%; (ii) 18 min of spray irrigation with cold tap water, air blowing at a temperature of 50 C°; (iii) return to stage (i).

The test samples were photographed at the beginning of the test (series of images a and c) and after 1000 h from the beginning of the test (series of images b and d). The images are attached to the present application.

It is assessed that the 1000-h test that was carried out corresponds to an actual exposure of about 5 years to weather, except for periods of frost.

Example 1; 9 mm plywood + 60/174 Novox NW + printing + gloss lacquer

In Fig. 1a, a test sample without edge protection at the beginning of the test; in Fig. 1b, the test sample without edge protection after 1000 h; in Fig. 1c, a test sample with edge protection at the beginning of the test and in Fig. 1d, the test sample with edge protection after 1000 h.

Observations: No differences could be observed between the plywood boards without edge protection and those with edge protection. In the climate testing carried out according to standards, the lacquer sticks firmly to the coating and the ink. The lacquer on top of the printing ink ensures that the ink does not detach under abrasion or scratching (e.g., playground fittings). The Novox NW coating film improves the durability of the colour shades compared with the uncoated printed wood panel.

Example 2; 4-mm plywood + 60/174 Novox NW + printing

In Fig. 2a, a test sample without edge protection at the beginning of the test; in Fig. 2b, the test sample without edge protection after 1000 h; in Fig. 2c, a test sample with edge protection at the beginning of the test and in Fig. 2d, the test sample with edge protection after 1000 h.

Observations: The printing ink sticks firmly in climate testing (the edge protection has no observable effect). It is observed that the printing ink can be scratched by a similar force as soft plastics. The adherence of printing ink to several applications is sufficient without lacquering. The lacquer provides an improved abrasion resis- tance. The UV resistance of the ink on top of the coating is good (NB. the reference image and the test images are of different colours). The surface does not crack, there is no so-called rippling phenomenon and the surface is smooth. Example 3; 9 mm plywood + 60/174 Novox OW + printing + dull lacquer

In Fig. 3a, a test sample without edge protection at the beginning of the test; in Fig. 3b, the test sample without edge protection after 1000 h; in Fig. 3c, a test sample with edge protection at the beginning of the test and in Fig. 3d, the test sample with edge protection after 1000 h.

Observations: The Novox OW film gives the plywood a weaker UV protection than the Novox NW film. The film as such does not crack and the surface is smooth, but the OW film transmits UV radiation more easily than the Novox NW film, whereby the plywood darkens. However, the OW film protects the plywood significantly against darkening and cracking compared with a case, where the plywood is merely lacquered. No rippling phenomenon occurs.

Example 4; 9 mm plywood + 60/174 Novox OW + printing

In Fig. 4a, a test sample without edge protection at the beginning of the test; in Fig. 4b, the test sample without edge protection after 1000 h; in Fig. 4c, a test sample with edge protection at the beginning of the test and in Fig. 4d, the test sample with edge protection after 1000 h.

Observations: A very transparent coating similar to Example 3, wherein the UV protection is weaker than the Novox NW films, the coating remains smooth and does not crack and no rippling phenomenon occurs. The coating protects well the surface of the plywood but is poorer in preventing the effect of UV radiation that darkens the plywood. There is no great difference between the boards with or without edge protection. The UV resistance of the ink on top of the coating is good.

Reference example 5; 9 mm uncoated plywood + printing + gloss lacquer

In Fig. 5a, a test sample without edge protection at the beginning of the test; in Fig. 5b, the test sample without edge protection after 1000 h; in Fig. 5c, a test sample with edge protection at the beginning of the test and in Fig. 5d, the test sample with edge protection after 1000 h.

Observations: It was necessary to use 3 layers of lacquer to provide a sufficient gloss (in the other samples, there were 2 layers of lacquer). However, the lac- quered surface cracks and rippling occurs. The lacquer does not protect the plywood against darkening. The plywood surface that was merely lacquered was not as smooth as the surface that was coated with the Novox film, whether lacquered or unlacquered.

Reference example 6; 9 mm uncoated plywood + printing + dull lacquer

In Fig. 6a, a test sample without edge protection at the beginning of the test; in Fig. 6b, the test sample without edge protection after 1000 h; in Fig. 6c, a test sample with edge protection at the beginning of the test and in Fig. 6d, the test sample with edge protection after 1000 h.

Observations: With merely the lacquer, the plywood surface is not nearly of as good quality as the coated plywood (the appearance), and cracking and rippling occurs on the surface. The printing ink sticks firmly to the wood. However, the lacquer does not protect the wood against darkening (the same lacquer as in the coated samples). By selecting the best UV-resistant lacquer on the market, it was still not possible to protect the plywood so as to be suitable for outdoor use.