Heat treatment of a laser coating

COATING HEAT TREATMENT

BY LASER

The invention relates to the field of painting on a substrate and describes a method of drying and / or baking by laser, in particular suitable for paints or inks comprising an organic or aqueous-based solvent.

Different methods of applying paint or liquid or powder ink on flat substrates or slightly deformed compared to the plane (sin (angle / vertical)> 0.95) are available today, in particular the application by roller , by a curtain of paint, electro assisted spraying or not.

The paints are then dried and / or baked in an oven or oven. Three main techniques are currently available to ensure this drying and / or this cooking: drying in ambient air, drying / baking in the oven, UV crosslinking. The speed of travel of substrates coated with paint in an oven or an oven for drying or baking can range from a few 1 m / min for glass substrates to 1 km / min in the case of press presses for example.

The air drying technique is very slow (several hours of waiting are necessary) and is limited to the use of siccative drying paint (without cooking).

The technique of drying / baking in the oven is industrially the most widespread today. Based on ovens with short / medium infrared radiation, these installations require ovens several tens of meters long depending on the running speed of the substrate and the cooking time required.

Based on a technology using very little solvent, the so-called UV crosslinking firing is carried out purely photochemically by irradiating the paint with UV radiation resulting in crosslinking. This technique allows higher rates than drying / baking in the oven but generates environmental problems, in particular due to the significant generation of ozone, acrylates and free radicals in the production area. The present invention proposes to combine the power of intense laser type radiation (which of course covers the possibility of having several such radiations) with traditional paints or inks intended for an oven process. The invention is particularly suitable for the heat treatment of coated substrates having large surfaces, in particular ranging from 1 to 25 m².

The invention relates to a method of heating an organic coating applied to substrates, a laser radiation being applied to the organic coating while the substrates are continuously traveling.

The coating is organic insofar as it comprises at least one organic compound before the laser treatment according to the invention. For example, a paint commonly used to protect the back of mirrors is an organic coating because it contains an organic solvent or an organic resin. The coating may include an organic pigment. After treatment with the process according to the invention, the coating generally still contains an organic compound.

The invention is particularly suitable for drying or baking paints applied to a glass substrate as on the back of mirrors, in particular in the latter case in order to protect the silver layer from corrosion.

The laser treatment according to the invention also has the particularity, unlike annealing or quenching treatments, of not significantly heating the substrate. It is thus not necessary to carry out a slow and controlled cooling of the coated substrate before it is cut or stored. This method also makes it possible to integrate a heating device into existing continuous production lines, in particular a mirror production line, which may include a silver layer preheating zone to remove traces of moisture.

The substrate may in particular comprise or be a sheet of glass, of glass ceramic, or of an organic polymer. For the mirror application, it is preferably transparent. It can be colorless (it is then a clear or extra-clear glass) or colored, for example in blue, green, gray or bronze. The glass is preferably of the soda-lime-silica type, but it can also be made of borosilicate or alumino-borosilicate type glass. The preferred organic polymers are polycarbonate or polymethyl methacrylate or else polyethylene terephthalate (PET). The substrate can have at least one dimension greater than or equal to 1 m, even 2 m and even 3 m. The thickness of the substrate generally ranges from 0.5 mm to 20 mm, in particular for the mirror application from 0.7 to 9 mm, in particular from 2 to 8 mm, or even from 4 to 6 mm. The substrate can be flat or curved. It can be rigid or flexible.

The glass substrate is generally of the float type, that is to say capable of having been obtained by a process consisting in pouring the molten glass onto a bath of molten tin (“float” bath). In this case, the layer to be treated can be deposited on the “tin” side as well as on the “atmosphere” side of the substrate. The term “atmosphere” and “tin” faces is understood to mean the faces of the substrate having been respectively in contact with the atmosphere prevailing in the float bath and in contact with the molten tin. The tin face contains a small surface quantity of tin having diffused in the structure of the glass. The glass substrate can also be obtained by rolling between two rollers, a technique which makes it possible in particular to print patterns on the surface of the glass.

According to the invention, the substrate can in particular be of the glass type, coated with ink (comprising at least one pigment, in particular in the form of nanoparticles or comprising at least one organic dye) or of paint with organic solvent, hydro-diluted or even water-soluble. The invention is particularly suitable for inks and paints of the Alkyd, Acrylic and Polyurethane type, but not exclusively. The temperature ranges accessible by the technique according to the invention are particularly suitable for technologies based on crosslinking mechanisms of the urea / formaldehyde, epoxide or isocyanate type, but not exclusively.

The heat treatment is carried out using at least laser radiation. The surface power of the laser radiation at the level of the coating is preferably greater than or equal to 20 and even greater than or equal to 30 kW / cm². This very high energy density makes it possible to reach the desired temperature at the level of the coating very quickly (generally in a time less than or equal to 1 second) and therefore to limit the duration of the treatment accordingly, the heat generated n then having no time to diffuse within the substrate.

Thanks to the very high heat exchange coefficient associated with the process according to the invention, even the part of the substrate (especially glass) located 0.5 mm from the coating generally does not undergo temperatures above 100 ° C. The substrate therefore generally does not undergo a temperature above 100 ° C. at a depth of 0.5 mm from the substrate / coating interface.

Thanks to the very high homogeneity of the power of the laser line associated with the method according to the invention, said power not varying more than 5% on the line, or even not varying more than 1% on the line, the coating undergoes a uniform temperature which allows drying or curing of paints or inks without causing defects.

The process according to the invention is continuous: a relative movement is created between the coated substrate and the means of heating by laser beam so as to treat the desired surface, in general the entire surface.

The laser radiation preferably has a wavelength between 266 and 1000 nm, especially between 530 and 1200 nm. It is indeed in this wavelength range that the absorption of the coating (paint or ink) is maximum. Thus the radiation is specifically absorbed by the coating and little by the substrate, which allows the layer to be heated quickly without heating the substrate.

Preferably, the absorption by the coating before laser heat treatment according to the invention (ink or paint) at the wavelength of the laser radiation is greater than or equal to 20%, in particular 30% (absorption = 100% - transmission - reflection, the transmission and the reflection being measured on the layer / substrate assembly for example by an apparatus of the Iambda900 type) for a characteristic coating thickness of 10 μιτι in normal transmission (perpendicular to the coated substrate). On the contrary, glass, especially clear or extra-clear glass, absorbs very little in this wavelength range so that the radiation mainly heats the layer. The absorption is defined as being equal to the value of 100% from which the transmission and the reflection of the layer are subtracted.

Laser diodes are preferably used, emitting for example at a wavelength of the order of 808 nm, 880 nm, 940 nm, or even 980 nm or 1032 nm. In the form of diode systems, very high powers can be obtained, making it possible to achieve surface powers at the level of the layer to be treated greater than 20 kW / cm², or even greater than 30 kW / cm².

For increased simplicity of implementation, the lasers used in the context of the invention can be fiberized, which means that the laser radiation (any gain medium: gas, liquid, solid) is injected into an optical fiber and then delivered near the surface to be treated by a focusing head. In particular, the laser can also be fiber, in the sense that the amplification medium (that is to say gain medium) is itself an optical fiber, generally doped with rare earth ions ("fiber laser " in English)

The laser radiation can come from at least one laser beam forming a line (called “laser line” in the remainder of the text) which simultaneously irradiates the entire width of the substrates coated with the coating to be heated. This embodiment avoids the use of expensive displacement systems, generally bulky, and delicate maintenance. The online laser beam can in particular be obtained using high-power laser diode systems associated with focusing optics. The thickness of the line is preferably between 0.01 and 1 mm. The length of the line is adapted to the width of the substrate to be treated, it is typically between 5 mm and 4 m. The intensity profile of the line (across its width) can in particular be a Gauss curve or a square wave.

Generally, the laser radiation is applied along a line substantially transverse with respect to the direction of travel of the substrates.

The laser line simultaneously irradiating all or part of the width of the substrates can be composed of a single line (then irradiating the entire width of the substrate), or of several lines, possibly disjoint. When several lines are used, it is preferable that they are arranged so that the entire surface of the coating to be heated is treated. The laser line may be arranged obliquely to the direction of travel of the substrate, but is preferably arranged perpendicular to the direction of travel of the substrate. In the case of several laser lines, these can process the substrate simultaneously, or in a time-shifted manner. In practice, different laser beams are either physically focused in the same place to have a simultaneous processing of the substrate, or else they are offset in space to process one after the other a given width of the moving substrate. The important thing is that the entire surface to be treated is.

In order to treat the entire surface of the layer continuously, a relative displacement is implemented between on the one hand the substrate coated with the layer and the laser line. The substrate coated with the layer to be treated by laser can thus be set in displacement, in particular in translation travel opposite the fixed laser line, generally below, but possibly above the laser line. Preferably, the difference between the respective speeds of the substrate and the laser is greater than or equal to 1 meter per minute, or even 4 and even 6, 8, 10 or 20 meters per minute, in order to ensure a high processing speed. Generally, the substrates run at a speed of 1 to 20 meters per minute.

For the displacement of the substrate in translation, the movement can be carried out using any mechanical conveying means, for example using bands, rollers, trays in translation. The conveyor system makes it possible to control and regulate the speed of movement. If the substrate is made of flexible organic material, generally of the polymer type such as PVC or PTFE, the displacement can be carried out using a film advance system comprising a succession of rollers.

The laser can also be set in motion so as to adjust its distance from the substrate, which can be useful in particular when the substrate is curved, but not only. Indeed, it is preferable that the laser beam is focused on the coating to be treated so that the latter is located at a distance less than or equal to 1 mm from the focal plane. Ideally, the coating merges with the focal plane. If the substrate or laser displacement system is not sufficiently precise as to the distance between the substrate and the focal plane, it should preferably be possible to adjust the distance between the laser and the substrate. This adjustment can be automatic, in particular regulated by measuring the distance upstream from the treatment.

All the relative positions of the substrate and the laser are possible as long as the surface of the substrate is suitably irradiated. The substrate is most generally arranged horizontally, but it can also be arranged vertically, or at any possible inclination. When the substrate is arranged horizontally, the laser is generally arranged so as to irradiate the upper face of the substrate.

The in-line laser can be integrated into a production line for lacquered glass or mirrors, in particular solar mirrors.

In the case of the mirror application, the line laser is located in the production process after the silvering steps, for example as a preheating element for the glass before deposition of a layer of paint or just after the deposition of this layer. . The coated substrate can thus be treated in line after the deposition of the layer to be treated (ink or paint), at the outlet of the deposition installation and before the optical control devices, or after the optical control devices and before the devices for stacking substrates.

A laser line as for example described in FIG. 1 allows a coating (ink or paint) of thickness generally between 1 μιτι and 200 μιτι to be heated extremely quickly before the laser treatment (that is to say the heating) according to the invention. The inks and paints used for baking are naturally very absorbent in the infrared, a laser emitting typically in a wavelength band ranging from 266 nm to 11000 nm thus allows an optimal transfer of energy between the radiation source and the paint layer.

The laser heating method according to the invention can in particular be used according to four main modes: drying, rapid temperature rise, cooking or powder painting:

- drying mode: in this case, laser irradiation makes it possible to very quickly transfer energy corresponding to the latent heat of vaporization (L) of the solvent to be volatilized; in this case, a strong air flow ensures the extraction of solvent vapors; - rapid rise in temperature: after drying, the coating (paint or lacquer or ink) retains its absorbent properties in the infrared; the laser treatment then allows an extremely rapid rise of the dry coating with a view to its subsequent baking in a baking oven; the drying itself can be carried out in an oven or by the treatment according to the invention, the drying being followed by a heat treatment by laser according to the invention;

- baking: here it is a question of keeping the coating above the baking temperature for a sufficient time which can generally range from a few seconds to a few minutes; in particular, two processing possibilities are then possible:

-> successive use of several laser lines so as to maintain the layer temperature above the firing threshold for sufficient time;

-> laser scanning of the surface to be treated;

- powder paint: the application of a powder paint makes it possible to use a single treatment with a laser ramp to melt the powder and then harden it.

The laser treatment according to the invention mainly heats the coating by heating the substrate to a minimum. This therefore makes it possible to reduce the total energy necessary for the treatment of the coating and / or to increase the treatment rates.

In particular, the method according to the invention can be used for drying or baking paints for indoor or solar mirrors, and also for finishing the paint of a lacquered glass. The method according to the invention can be used to shorten the lengths of the drying or baking ovens.

For the case where the laser treatment according to the invention provides the elimination of a combustible organic material (a solvent for example) from the coating, it is possible to ensure sufficient dilution and convection using a gas such as air above the coated substrate to limit the risk of ignition or even explosion.

For the implementation of the method according to the invention, the following parameters are generally to be taken into consideration: P [W / m²]: power density of laser radiation;

I [m]: width of the laser beam (ie thickness of the laser line);

L [m]: length of the beam or of all the laser beams;

e: coating thickness before laser treatment;

p: density of the wet or dry coating layer respectively depending on whether one dries (solvent evaporation) or baked the coating (no solvent evaporation)

τ: solvent content in the coating before laser treatment;

a: absorption coefficient of the coating before laser treatment;

Cp [J / Kg / K]: heat capacity of the coating before laser treatment;

Lv: latent heat of vaporization of the organic matter to be eliminated

(solvent) during laser treatment;

V: running speed of the substrate;

Table 1 has collected the values ​​between which these parameters can generally be situated, limits included.

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Table 1

Then, the quantity of heat transferred per unit of area is estimated by:

Q [J / m²] = Pl / V,

and the temperature reached is estimated by

ΔΤ = ———

Cp.Vep ΔΤ representing the difference between the temperature reached and the ambient temperature.

FIG. 1 represents the method according to the invention. Substrates 1, coated with a coating to be dried or baked, pass one behind the other continuously in a direction represented by the arrow, being conveyed by a bed of rollers (not shown). They pass under a laser source 2 which delivers a laser line 3 focused on the surface of the moving substrates and over their entire width. The laser line produces heating to dry or bake the coating.

Example 1

On a production line for moving mirrors at a speed of 5 m / min, drying is carried out according to the invention of a layer of paint deposited on the back of the mirror as a protective coating. The coating before drying had a thickness of 50 μιτι, a density of 2 T / m³, a heat capacity of 0.7kJ / Kg / K, an absorbance a of 1. The solvent level (xylene: Lv = 300kJ / Kg) τ was 30% by weight (ie 0.3 in the above formula). A power of 330 kW / m² agrees. Once the paint is dry, the density of the coating is 1, 3 T / m³ and each KW / m² leads to an increase in the temperature of the paint by 4 Kelvins. The laser radiation essentially heats the coating, the glass being heated only by conduction from the coating and in a very short time (&lt;1 s) limiting the increase in its average temperature to less than 1 K over its total thickness. Example 2

Baking is carried out of a paint of the polyurethane type with isocyanate blocked of the industrial type requiring a temperature of 180 ° C. for unblocking and crosslinking of the layer. By the method according to the invention, a power of 40 kW / m² agrees.