EP0577465A1 - Engravable surface coatings, specially for printing rolls, coating process and pieces produced according to this process - Google Patents

Abstract

Engravable coating compositions, especially for printing rolls, which comprise at least one resin capable of coking and producing only a very small proportion of liquid products at the temperature at which it decomposes, the chief products of its decomposition being, at this temperature, solid or gaseous; - and generally a reinforcing particulate filler exhibiting a small proportion of ridges exhibiting acute projecting angles, the said filler generally representing a proportion of 5 to 75 % of the resin + filler volume. The process for coating the rolls comprises: - mixing the resin and the filler and degassing the mixture obtained; - actual coating and drying of the successive layer or layers deposited; and - baking of the coating. he

Description

The present invention relates to coating compositions, in particular for making etchable surfaces and more particularly for the manufacture of printing rolls etchable by direct LASER etching. It also relates to the manufacture of these rollers.

Cylinders used in gravure printing are generally used for printing packaging materials, wall and floor coverings, magazines, advertising inserts and mail-order catalogs, either for large print runs or for printing non-absorbent substrates.

A rotogravable cylinder is generally constituted as described below:

On a cylindrical steel structure, a layer of copper is deposited (by reaction of copper sulphate) with a nickel bonding flash. This layer of copper is rectified, then engraved with blind holes called alveoli whose distribution and dimensions define the printed surfaces and their tone. After engraving, the cylinder is chrome-plated to present an abrasion-resistant surface that can be wiped off with the doctor blade. The chrome cylinder is then polished to remove the roughness of the chrome.

• a) Encrage : Le cylindre tourne dans de l'encre liquide et une racle en acier essuie la surface pour ne laisser l'encre que dans les alvéoles.
• b) Impression : Le support d'impression entre en contact, sous pression, avec le cylindre. L'encre est transférée de l'alvéole du cylindre au support qui devient encré.
• c) Séchage : L'encre employée étant en général une solution d'une résine dans un solvant volatil tenant en suspension des pigments, le solvant est évaporé par action d'un courant d'air chaud et l'encre se solidifie sous la forme d'un film.

The cylinder is generally used on rotary as follows:

• a) Inking: The cylinder rotates in liquid ink and a steel scraper wipes the surface to leave the ink only in the cells.
• b) Printing: The printing medium comes into contact, under pressure, with the cylinder. The ink is transferred from the cylinder socket to the support which becomes inked.
• c) Drying: The ink used is generally a solution of a resin in a volatile solvent holding pigments in suspension, the solvent is evaporated by the action of a current of hot air and the ink solidifies in the form of a film.

The cycle described above is that of a cylinder printing a single color; this cylinder is placed in a printing unit. To obtain polychrome prints, you have to put several cylinders in line, up to twelve for the presses with twelve groups.

The main advantages of gravure printing are known.

Rotogravure can print all materials: paper, cardboard, plastic films, metals, etc., thanks to its drying system. It allows you to print continuous strips which are only interrupted when the machine stops. Indeed, at the end of a reel, you can connect the strip of the next reel without stopping the machine.

In addition, gravure printing is a multiformat printing process: in fact, it is enough to change the format (the diameter) of the cylinder without any other adaptation.

In addition, compared to other printing processes, gravure printing includes the minimum of moving mechanical parts. Apart from the movement of the doctor blade, all the movements are rotary.

In addition, the ability to transfer a lot of ink allows for deep solids. The transparency of the inks makes it possible to reproduce very numerous color tones.

Finally, from an economic point of view, the cost of the inks, the machine investment ratio and the productivity of the material make it possible to produce quality prints at very low cost prices.

However, the current technique of engraving copper-plated, nickel-plated and chrome-plated steel rollers is polluting because of the electrolysis baths, on the one hand, and because of the solvent inks which take well on the chrome-plated rollers, on the other hand. Water-based inks are potentially less polluting, but do not cure well on chrome rollers.

Furthermore, the steel printing rollers are between heavy and very heavy and their use requires significant handling means.

The ink is dried by evaporation of the solvents from the ink. From 1993, in certain countries including France, these solvents can no longer be evacuated to the atmosphere and must be either destroyed by incineration or absorbed on activated carbon and distilled, which will require additional investments and costs. exploitation.

One can think of replacing the solvents of the ink by water. However, the surface tension of the cylinder chromium is unfavorable for the use of water-based inks. In addition, a rotogravable cylinder is a precision tool both in its geometrical characteristics and in its surface condition, but especially in its printing surface composed of cells from 10 microns to 150 microns sometimes 10 microns apart. The hollow of these cells is also important for adjusting the ink flow rates. A tolerance of 2.5 microns currently accepted gives visible effects. This precision applies to parts from 370 to 3000 mm in length and from 60 to 500 mm in diameter.

Modern photogravures process computer graphics images with a precision of 1 micron by manipulating 16 million colors but transfer the image to the cylinder, either with photoresistors or by mechanical engraving and thus lose computer precision.

A LASER engraving allows this computer precision to be transmitted and the process to be made more reliable.

When using techniques which allow for etching cells or "bridges" (distance between two cells) of 10 microns, it seems absurd to make 8 micron chrome coatings that change the size of the cells and bridges.

An object as demanding as a rotogravable cylinder implements expensive techniques, much more expensive than those necessary for flexographic, serigraphic or offset printing forms.

Despite computer graphics equipment, the cost of which is the same regardless of the process, a gravure cylinder is ten times more expensive than an offset plate.

To reduce the cost of the cylinder cycle to that of its main competitor the offset, it would be advisable to change the current technologies which date for some 150 years: the technologies of copper plating and chromium plating have hardly evolved.

Electrolytic deposition of heavy metals generates gaseous and wastewater discharges which must be treated before discharging them.

The presence of volumes of baths proportional to the size of the rooms to be treated presents industrial risks which position this activity among the classified industries.

The abandonment of metallic coatings and their replacement by a single synthetic coating would avoid copper plating and chrome plating and thus eliminate the treatment of wastewater which currently represents a significant operating cost.

In general, the present invention aims to preserve the advantages of gravure printing while eliminating the drawbacks, by using a non-metallic coating layer, which can be etched directly by LASER, and which takes the inks well. 'water.

For this purpose, it has already been proposed to use compositions based on resins which are easy to destroy thermally, such as epoxy resins, more or less charged with pigments to absorb the LASER radiation well, and / or charges causing the formation of gas, intended to expel the material burned by the LASER impact. However, these compositions are not satisfactory, because it forms at the opening of the dug hole a cone of several microns in height of partially decomposed material, capable of partially filling the dug hole during a subsequent polishing operation or under the action of the doctor blade.

The compositions of the present invention do not have the drawbacks mentioned above.

Based on resins with a high thermal resistance, they coke instead of thinning, and give, under the LASER impact, holes with clear edges, which, moreover, take ink well with water.

In general, the compositions of the invention can be defined as comprising a resin capable of coking and producing only a very small proportion of liquid products at the temperature at which it decomposes, the main products of its decomposition being, at this temperature, solid or gaseous, and optionally a filler consisting of particles of high hardness, this filler being in the form of particles of very small particle size, and preferably having a small proportion of sharp salient angles.

Among the resins capable of coking under the action of very high temperatures, there will be mentioned mainly the resins resulting from the condensation of formalin on furfuryl alcohol, furfural, melamine, a phenol or urea (generically called formalin resins ), or a resin resulting from the reaction of furfuryl alcohol or furfural with formalin, urea, phenol or itself (generically called furan resins). Note that the resins resulting from the action of formalin on furfuryl alcohol or furfural belong to two groups at the same time.

More particularly, the resin can be chosen from melamine formaldehyde resins, phenol-formaldehyde resins, such as resols, as well as furan resins.

The reinforcing filler brought into play in the coating compositions according to the invention in order to improve the resistance to wear can more particularly consist of glass micro-particles, silica in microfine flour, zirconium oxide, carborundum, alumina, or carbon such as fullerenes.

The particles constituting the charge may preferably have dimensions which are smaller by an order of magnitude than those of the holes which it is desired to make later by LASER impact, and more particularly less than ten microns.

The proportion of particulate fillers involved is generally from 5 to 75% by volume relative to the total volume of resin + fillers.

As will be described later, the coating may comprise a first coating consisting of one of the cured filled resins described above and a thin layer of cured uncharged resin, mainly consisting of melamine-formaldehyde resin.

The process for coating a cylindrical surface, in particular a printing roller engraved by LASER, is described below using a coating composition according to the invention.

• (a) une étape de préparation de la composition d'enduction incluant une opération de mélange et une opération de dégazage du mélange obtenu ;
• (b) l'étape d'enduction proprement dite ; et
• (c) une étape de cuisson du revêtement.

In general, this process includes:

• (a) a step of preparing the coating composition including a mixing operation and a degassing operation of the mixture obtained;
• (b) the actual coating step; and
• (c) a step of baking the coating.

In the step of preparing the coating composition, a mixture is made between at least one resin and at least one particulate filler, as defined above, in appropriate proportions. Thus, the particulate filler can be brought into play in a proportion ranging from 5 to 75% of the total volume of resin + filler, the resin being counted only for its dry extract when it is used in the form of a solution.

The mixing can be carried out in a diluent or a solvent such as for example water or an aliphatic alcohol, in particular ethanol.

The advantage of dilution is that it allows a high filler content in the finished product while allowing easy degassing. It may in some cases require an evaporation phase during application to the cylinder.

The mixture thus formed is then placed under reduced pressure, so as to at least partially eliminate the gases contained in the charge, part of the gases dissolved in the resin and most of the air introduced during mixing.

The gases contained in the mixture are then expanded and a foam is formed, the volume of which can reach more than 10 times the initial volume of the mixture. This foam "breaks" with release of the gases thus released and the operation can be repeated one or more times until no more foam is formed, but there is a gentle boiling of the solvent or thinner. It can then be considered that there is practically no more gas dissolved in the composition and that the degassing is sufficient.

In step (b) of coating the roll, one can proceed as follows:

The degassed coating composition is placed as prepared during step (a) described above, and optionally added with a catalyst to subsequently obtain crosslinking of the resin, in a "gutter" placed under the roller to be coated. . The cylinder rotates around its axis, placed horizontally, and "licks" the surface of the liquid so as to filling said cylinder with a thin layer of the coating composition. The gutter is then moved so as to interrupt licking and a stream of warm air is then passed over the cylinder intended to facilitate the evaporation of the diluent contained in the layer deposited on the roller. The temperature of this warm air stream can for example be around 90 ° C. when the diluent of the composition is water. Heating is then continued until the deposited layer becomes viscous enough that it is possible to draw a wire from it. It often happens that at this stage of the operation, the resin becomes transparent because the refractive indices of the filler and of the diluted resin arrive in agreement. In this case, visual monitoring of the operation becomes particularly easy.

A second layer of composition, then a third and so on can optionally be added to the first thus deposited by alternating the licking and drying phases, ensuring that the duration of the licking is short enough to obtain the deposit of a new layer, but does not allow the dilution of the first layer by the bath of diluted composition. The unitary licking and evaporation operations can be repeated several times so as to deposit enough thin unitary layers to obtain the desired coating thickness. Thus, each unitary layer can have a thickness of 30 to 100 microns, and the thickness of the final coating can be for example from 100 microns to 1 millimeter.

According to a variant of the coating step (b), provision can be made, in the case of a steel roll of sufficient thickness, to preheat said roll in an oven at 60-70 ° C. and to use the calories stored to allow the diluent to evaporate in a stream of air at room temperature. In any case, it is important to keep the roller in rotation during this drying operation in order to avoid any sagging.

In step (c) of baking the coating, the coated roll is generally subjected to a heat treatment, in an oven or in an autoclave, so that the resin, optionally added with a hardener and / or a catalyst , is hardened by crosslinking. The temperature conditions, pressure and duration of this operation are well known in general to those skilled in the art, but it is particularly important in the case of the compositions according to the invention intended for printing rolls engraved by LASER, to use a temperature-pressure couple such that the by-products of the polycondensation reaction, on the one hand, and the diluents or solvents, or finally the azeotropes which can form in the resin during crosslinking, remain in the dissolved liquid state in the resin and do not boil to form micro-bubbles within the composition.

Advantageously, the cooking temperature will be approximately 10 ° C. lower than the boiling temperature of the most volatile product present.

Since the polycondensation reaction generally provides water as a by-product, the cooking will advantageously be carried out at a temperature slightly below 90 ° C. if the diluent used is also water, and at a temperature slightly below 65 ° C. if the diluent used is ethyl alcohol, operating at normal atmospheric pressure. This temperature can be increased by operating in an autoclave.

After deposition and hardening of the coating, the latter is generally subjected to a correction and / or a polishing.

By the method as described above, it is possible to deposit on printing rollers, loaded resin coatings having hardness characteristics suitable for use in direct LASER etching. The resins preferably used to produce these coatings are resols.

Furthermore, in certain cases, it may be advantageous to deposit on a coating formed as described above from any resin composition, loaded with reinforcing particles (such as a loaded resol composition), a thin layer of melamine-formaldehyde resin having a very satisfactory hardness, even in the absence of filler. In such a case, the requirement noted above when the shape of the charge particles is no longer as imperative as in the cases described above, since these no longer flush on the surface of the coating. Any roughness is leveled by the layer of melamine-formaldehyde resin deposited on top.

The following example illustrates the invention.

EXAMPLE

• une résine de type résol, commercialisée par PERSTORP, en solution aqueuse à 70% en poids d'extrait sec ;
• un durciseur consistant en un mélange d'acides sulfoniques en solution aqueuse, également commercialisé par PERSTORP ;
• une charge constituée de particules de silice ayant la granulométrie indiquée ci-dessous.

taille > > > > > > > (microns) 1 1 1 8 6 4 2 6 2 0 % - 2 2 2 4 1 2 7 cumulés , , , , 3 8 1 0 0 2 2 , , , 0 8 2 In this example, we used:

• a resol type resin, sold by PERSTORP, in an aqueous solution at 70% by weight of dry extract;
• a hardener consisting of a mixture of sulfonic acids in aqueous solution, also marketed by PERSTORP;
• a filler consisting of silica particles having the particle size indicated below.

cut > > > > > > > (microns) 1 1 1 8 6 4 2 6 2 0 % - 2 2 2 4 1 2 7 cumulative , , , , 3 8 1 0 0 2 2 , , , 0 8 2

On the micrograph presented in the appendix, we see that the particles are small, of approximately spherical or oblong shape, and have a small proportion of sharp salient angles compared to the large number of obtuse salient angles.

25 g of resol was mixed with the spatula to 25 g of powdered silica to give a firm paste which was not suitable for degassing under vacuum.

To thin the mixture, 20 g of demineralized water were added.

The assembly was placed under reduced pressure in a desiccator for a few minutes to remove the air included. Part of the water evaporated during this treatment. First, a foam with a volume of about ten times the initial volume was formed, this foam "broke" in a few seconds, then the liquid began to boil gently and regularly at room temperature.

Once the product had been degassed, 1.2 g of hardener (about 5% by weight of the crude resin) were added, taking all precautions to avoid the introduction of air into the mixture, then poured the product into a gutter which it was brought under the roller to be filled, the latter being placed in slow rotation (about 5 revolutions per minute) with a horizontal axis, so that the roller licks the surface of the product which is deposited therein in a thin layer during two towers. We then stop licking down the gutter and bring a hot air heater (Leister) set to 100 ° C to facilitate the evaporation of water from the first layer. After about 5 minutes, the mixture, which was pink, becomes transparent (the refractive index of the solution becoming equal to that of the filler) while the product is highly viscous (a wire can be drawn from it). At this time, the current of hot air is temporarily stopped and the licking operation is repeated for two turns to deposit an additional layer of fresh product, then the drying is resumed until transparent, and this three times in succession to thus deposit four thin unit layers and obtain a sufficient thickness of coating.

After evaporation of the last layer, the whole was wrapped in a sheet of Terphane under slight tension (manual application) and placed in an oven at 60 ° C for two hours, the temperature then being brought to 80 ° C for two hours additional.

The finished product has an opaque pink appearance (like a lacquer) and has crimps due to the irregularity of the Terphane tension.

It is then rectified and polished to give a suitable sample, which can be engraved directly by LASER.

Claims (11)

Method for etching a surface for a printing press, characterized in that a coating resulting from the hardening of a composition comprising at least one resin capable of coking and producing only a very high degree of etching by means of LASER radiation low proportion of liquid products at the temperature at which it decomposes, the main products of its decomposition being, at this temperature, solid or gaseous, and optionally a charge consisting of particles of high hardness, this charge being in the form of particles of very small particle size. Process according to Claim 1, characterized in that the said resin is chosen from the resins resulting from the condensation of formalin on furfuryl alcohol, furfural, melamine, a phenol or urea (generically called formalin resins), or a resin resulting from the reaction of furfuryl alcohol or furfural with formalin, urea, phenol or itself (generically called furan resins). Process according to Claim 2, characterized in that the said resin is chosen from melamine-forrnol resins, phenol-formaldehyde resins and furan resins. Method according to one of claims 1 to 3, characterized in that said composition comprises a reinforcing filler consisting of micro-particles of glass, silica, zirconium oxide, carborundum, alumina, or carbon, having a small proportion of sharp salient angles. Method according to one of claims 1 to 4, characterized in that in said composition the particles of the filler have a size less than 16 microns. Method according to one of claims 1 to 5, characterized in that in said composition the proportion by volume of the filler is from 5 to 75% of the total volume: resin + filler. Method according to one of Claims 1 to 6, characterized in that, in the said composition has been hardened by heating to a temperature 10 ° C below the boiling temperature of the most volatile product present therein. Method according to one of claims 1 to 7, characterized in that said coating to be etched has a thickness of 100 microns to 1 millimeter. Coating method according to one of claims 1 to 8, characterized in that a thin layer of uncharged melamine-formaldehyde resin has been deposited on the coating to be etched. Etched coating produced by a process according to one of claims 1 to 9. Printing roller comprising an engraved coating according to claim 10.

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