EP0395496B1 - Sheet suitable for exposure to light, with an identification and security mark, and use of such a sheet - Google Patents

Abstract

The invention concerns a sheet intended for prolonged exposure to light. It carries one at least light-resistant imprint having a first colour in natural light with a wavelength between 400 and 700 nanometres and a second colour, different from the first, in an artificial light with a wavelength of less than 400 nanometres and greater than 700 nanometres. The imprint is produced by photogravure printing with liquid ink containing at least one luminescent pigment capable of withstanding temperatures above 100 DEG C for at least 10 minutes. Use especially as a decorative sheet for laminates.

Description

The invention relates to a decorative sheet usable for the manufacture of laminated panels, this sheet further comprising recognition and security means. The invention further relates to a composition for printing such a sheet.

We know that there are mainly two kinds of laminated panels: so-called high pressure panels and so-called low pressure panels.

For many years, laminates have been used as materials in homes and commercial and industrial premises. Typical applications of such laminates are the surface coating of walls, table tops, furniture and the like.

The so-called high pressure laminates are produced from a core consisting of sheets impregnated with resin. The sheets are generally sheets of kraft paper which has been impregnated with a thermosetting resin and more particularly with a phenolic resin.

When the kraft paper has been impregnated with resin, the sheets are dried, cut, then stacked on top of each other. The number of sheets in the stack depends on the applications and can vary between 3 and 9, but can be higher.

Then placed on the pile of sheets constituting the core, a decorative sheet which is generally a sheet of paper carrying a printed or light-colored pattern, and impregnated with a thermosetting resin which does not blacken with heat, for example melamine-formaldehyde resins, benzoguanamine-formaldehyde resins, unsaturated polyester resins. In general, a protective cover sheet, without a pattern and transparent in the final laminate, is placed above the decorative sheet. In terms of profession, this protective sheet is called "over-lay". The stack of impregnated sheets is then placed in a press fitted with a sheet giving the surface condition. The stack is then densified by heating and pressing to obtain a unitary structure.

During densification, the thermosetting resins are transformed into thermosetting form and an extremely hard material with a decorative effect is obtained.

So-called low pressure laminates are produced in a similar manner to that of high pressure laminates, but the decorative sheet is laminated directly on a particle board or any other basic support.

The decorative sheet is traditionally made in the following manner. A sheet of paper is made by draining on a flat canvas an aqueous suspension of cellulose fibers, long and / or short, bleached and / or unbleached, fillers, binders. The sheet thus formed is drained, pressed and dried to form a sheet of paper. This sheet is then printed. The impression usually made is a gravure printing, by means of engraved rollers. The engraving technique consists in making a drawing, for example reproducing a wood or a purely fancy artistic drawing. Then we take the photograph of this drawing and from the films obtained, we engrave one or more printing rollers. Each cylinder then passes through an ink bath, and the indentations or imprints engraved in the cylinder are filled with ink which is then transferred to the paper. Generally one to five separate and successive prints are made using one to five rolls.

We understand that the gravure printing process allows from a drawing to print a sheet of paper. However, this poses a problem for the printer of the decorative sheet. Indeed, if the drawing is an original drawing, created by the printer, any other printer, from a printed decorative sheet, can have photographs taken of this sheet and have cylinders engraved to obtain the same impression. Counterfeits can therefore be numerous and it is generally not possible to detect them or have intangible proof that they are counterfeits.

The invention therefore aims to solve this problem.

Another problem that arises is the identification of the laminate manufacturer. Indeed, the decorative sheet manufacturer can sell the same authentic sheet to several laminate manufacturers. Each laminate manufacturer generally bears his mark on the laminate, by means of a more or less removable label. Again, there is the problem of identifying the manufacturer of a particular laminate. The invention aims to solve this second problem.

Another problem which arises is that the sheets which are generally used for the manufacture of furniture are subjected for prolonged periods to natural or artificial light.

An object of the invention is therefore to provide a decorative sheet for laminate panels, comprising means for recognizing the origin of the laminates.

To this end, the invention relates to a sheet usable for the manufacture of laminated panels, intended to be subjected to the prolonged action of light, characterized in that it comprises an impression at least resistant to light, this impression having a first shade in natural light and a second shade, different from the first, in artificial light.

Preferably, the first design is invisible to natural light and the second design is visible to artificial light. By natural light is meant a light source whose wavelengths are between 400 and 700 nanometers and by artificial light, a light source whose wavelengths are less than 400 nanometers and greater than 700 nanometers.

We know that certain pigments and dyes are luminescent. These materials are commonly called "phosphors". These pigments can be fluorescent or phosphorescent. The color sensation produced by conventional pigments or dyes which are not luminescent results from the selective absorption of wavelengths in the visible part of the spectrum. Wavelengths that are not absorbed are reflected or transmitted, which gives the sensation of color. This feeling of color is absent when there is no visible light source, because there is no energy to reflect or transmit.

Unlike conventional dyes or pigments, luminescent pigments do not reflect light, but they are themselves sources of light. They have the property of absorbing certain wavelengths, generally wavelengths less than 400 nanometers and invisible to the eye, and of transforming these wavelengths into visible wavelengths. The wavelengths thus emitted produce a sensation of color. When the emission of light continues for a non-negligible period of time, after the excitation energy source has been switched off, the material is said to be phosphorescent. Pigments based on zinc sulfide have been described in the Kirk-Othmer documenet: Encyclopedia of Chemical Technology, 3rd ed, volume 17. These pigments are described as chemically stable.

Many publications have described the synthesis of such luminescent materials.

The following publications will be cited as references: EP-A-15382, EP-A-21342, EP-A-123947.

We also know that it is known to introduce pigments inside the paper, in particular by the publications FR-A-2 478 695 and EP-A-226367. These pigments are dispersed in the paper and the latter is used as security paper for checks, banknotes, etc.

Document EP-A-147252 describes a mixture of a luminescent substance and an anti-storage substance. Such a mixture can be used for marking documents to prevent counterfeiting, for example bank notes, fiduciary papers. The compositions can be introduced onto paper and can be introduced into printing inks. These inks are fatty inks, that is to say containing an oil and the printing is done by offset or intaglio. So far, the problem of marking heat-resistant sheets that can be used to decorate laminates has never been posed.

Document AU-A-518,156 describes a sheet of paper comprising an at least light-resistant printing having a first light shade of wavelength between 400 and 700 nm and a second shade different from the first at the wavelength less than 400 nm, the impression being a heliographic impression. However, it is not a paper usable for the manufacture of laminates.

Document DE-A-2 910 176 describes a method for characterizing disc packaging using fluorescent printing.

GB-2,147,542 describes an article printed using fluorescent or synthetic material.

EP 0253 543 describes a printing ink comprising a metallic compound, a luminescent compound and one or more inert compounds. Such ink is used for credit cards.

The document "Research Disclosure" n ° 160, August 1977, HAVANT GB, page 60 years, describes a document printed using a fluorescent ink and a non-fluorescent ink.

None of these documents describes a paper printed using at least one fluorescent ink, usable for the manufacture of laminated panels obtained by low or high pressure and subjected to high temperatures during lamination.

However, an essential technical problem to be solved is the fact that the sheets are intended to be subjected to heat, for a period of several tens of minutes, or even several hours. Indeed, when it comes to laminate sheets, during densification, the sheet is subjected to temperatures of more than 100 ° C., for 20 minutes. Similarly, when manufacturing papers covered with metal or metallized, these papers are subjected to high temperatures, especially when hot stamping is carried out.

Marking sheets, for example for laminates, also poses different and additional problems of marking paper for banknotes and other security documents.

In fact, the decorative sheets for laminates are necessarily impregnated with a thermosetting resin. When densification is carried out, the impregnated sheet is placed on a particle board or a stack of impregnated kraft paper sheets. Then we press the assembly against a sheet. However, a technical problem posed is that the prints must not transfer from the decorative sheet onto the sheet which must remain clean.

A second technical problem to be solved is that during the impregnation, the decorative sheet passes

in an impregnation bath. The impressions must therefore not migrate into the impregnation bath and must not pollute it. In fact, if the impregnation resin is polluted by printing, during the subsequent stratification, the thermosetting resin creeps under the action of heat and pressure and if it contains luminescent printing substances, these these flow together and cause smudging of the final impression.

Another technical problem to be solved is that when the printed sheet is rubbed, the luminescent substance must not come off. The friction resistance must therefore be high. In addition, the impregnability of the decorative sheet must be preserved.

Finally, it is essential that the luminescent substances used have a very good resistance to light. Indeed, these sheets can be used for panels used as materials in homes and furniture tops and they are therefore intended to be subjected to the action of light for extended periods of time.

The Applicant has tried many luminescent substances sold on the market and has been able to find that those which are well suited are mineral pigments. Indeed, organic molecules are very sensitive to external conditions and degrade easily in light, as well as in heat.

The Applicant has been able to use mineral pigments having a very good resistance to light and resist high temperatures for long periods of time, namely temperatures above 100 ° C., preferably between 140 and 200 ° C., by example 140-150 ° C for at least 10 min to 2 hours, for example 20 to 30 minutes.

It has surprisingly been found that luminescent substances used according to the invention such as zinc sulfides or their mixture with other metallic sulfides are resistant to heat, for periods of time of more than 10 min.

More preferably, the luminescent substances are pigments based on zinc sulfide in admixture with cadmium sulfide. These pigments can have a particle size of less than 40 micrometers.

Pigments of zinc sulfide or zinc sulfide and cadmium which are activated by copper can be used. Their preparation method is known and is for example described in H.W. Leverenz, An introduction to luminescence of solids: S. John, Wiley & Sons, Inc. New York, Chapman & Hall Ltd London, 1950.

Pigments based on zinc sulfide emitting in various wavelengths have been described in patent applications EP-A-34059, EP-A-78538, EP-A-91184 filed by the Japanese company KASEI OPTONIX. Pigments which are very suitable are also pigments based on zinc sulphide, manufactured by the German company RIEDEL DE HAEN.

Of course, according to the invention, mixtures of luminescent substances can be used, provided that they are resistant to light, heat, pressure, as mentioned above.

The following description, with reference to the examples, will make it possible to understand how the invention can be put into practice.

EXAMPLE 1

18.9 parts of pigment sold by RIEDEL DE HAEN under the reference Lumilux CD 704 are mixed successively.

zinc sulfide.

9 parts of water

3 parts alcohol, preferably ethanol

8.8 parts of varnish or binder for ink 992209 sold by SICPA 26.9 parts of varnish 991129 sold by SICPA 33.5 parts of an ethanol (2/3) water (1/3) mixture.

This mixture is applied to a sheet of white paper of 90 g / m2 by gravure printing.

The cylinder was etched by electronic etching, the cells having a depth of 37 micrometers, the height of the frame being 190 micrometers, and the width of the frame being 210 micrometers.

The size of the pigment particles is about 0.1 to 40 micrometers, which allows printing with such cells. We obtain a paper which is white in daylight, but when it is illuminated by ultraviolet rays, a blue impression appears while the white paper becomes brown because it does not reflect and does not transmit light.

This sheet is subjected to a temperature of 160 ° C. by placing it between two hot plates for 15 minutes. Then we illuminate the paper with UV rays. The blue print appears again.

COMPARATIVE EXAMPLE

The same mixture is made as in Example 1, but the mineral pigment is replaced by an organic pigment which is an N-substituted anthranilic acid of the type described in document DE-A-3225966. When the paper is subjected to the temperature of 200 ° C for 0.5 seconds, the fluorescent print no longer appears.

EXAMPLES 2 to 9

The same mixture is made as in Example 1, with pigments sold by the company OPTONIX, under different numbers.

The resistance to daylight is 3 to 6 measured according to the Wool scale test and the resistance to ultraviolet rays is 5 to 6.

The Wool scale test corresponds to standards NF T 51056 and NF T 51058. It consists in illuminating a sample comprising an impression with a Xenon lamp for 90 hours or 300 hours and in comparing the sample obtained with strands of wool with shades ranging from 1 to 8, the number 1 corresponding to color degradation, the number 8 corresponding to non-degradation.

EXAMPLE 10

Paper is printed by heliography, using one to five color prints in the traditional way. This paper is then decorated and has this decorative aspect and visible by lighting in natural light. A heliography printing is then carried out by carrying out one to three prints using the ink mixtures of Examples 1 to 9. The initial design is not modified in natural light. When we illuminate the paper in ultra-violet light, the initial drawing disappears and it is the superimposed drawing that becomes visible.

The invention relates not only to a means of marking and recognizing sheets subjected to temperature, and / or to pressure and to light, for long periods of time, but also relates to a new means of decorating these sheets. Polyvinyl chloride sheets can also be printed which are laminated to particle board by bonding and pressure. These plates used for furnishing must be resistant to light. It is also possible to print sheets of polyethylene, polypropylene, optionally coated and possibly impregnated with a resin.

Claims (8)

1. Sheet usable for the manufacture of laminated panels, and intended to be subjected to the prolonged action of light, comprising print at least resistant to light, presenting a first tone in light of wavelength included between 400 and 700 nanometers and a second tone different from the first, in light of wavelength less than 400 nanometers and higher than 700 nanometers, chacacterized in that the print is a heliographic print with the aid of liquid ink, the ink comprising at least one luminescent pigment resistant to temperatures higher than 100°C for at least 10 minutes.

2. Sheet according to Claim 1, characterized in that the pigment is resistant to temperatures of 140 to 200°C for 20 to 30 minutes.

3. Sheet according to one of Claims 1 and 2, characterized in that the pigment is a zinc sulfide.

4. Sheet according to one of Claims 1 to 3, characterized in that the pigment contains at least one zinc sulfide, a second metal sulfide and possibly at least one activation agent.

5. Sheet according to one of Claims 1 to 4, characterized in that the pigment is a mixture of zinc sulfide and of cadmium sulfide activated by copper.

6. Sheet according to one of Claims 1 to 5, characterized in that the pigment has a resistance to ultra-violet rays greater than 5, preferably 6.

7. Sheet according to one of Claims 1 to 6, characterized in that the pigment has a particle size smaller than 40 micrometers.

8. Sheet according to one of Claims 1 to 7, characterized in that it is constituted by paper or polyvinyl chloride or polyethylene or polypropylene, said sheet possibly being coated and possibly impregnated with a thermosettable resin.