FR2707624A1 - Process for enamelling a glass substrate and enamel composition employed - Google Patents

Abstract

Process for the manufacture of a layer of enamel on a glass substrate. According to the process at least a part of the drying of the layer of enamel deposited on the substrate is performed by microwave energy, the enamel composition containing graphite as doping agent.

Description

PROCESS FOR ENAMELLING A GLASS SUBSTRATE AND
EMAIL COMPOSITION USED
The present invention relates to the deposition of enamel on a glass substrate, in particular on a glazing. It relates in particular to a new process for the manufacture of an enameled layer on a glass substrate. It also relates to an enamel composition suitable for use in the manufacture of a layer on a glass substrate.

 By enamel is meant according to the invention both the composition or paste of enamel, used for deposition, as the layer of enamel formed at different stages of the manufacturing process as described below.

 Enamels are well known in the state of the art and used in particular on glass substrates such as vehicle glazing or building glazing or even mirrors. Enamels are thus used to form borders, decorative layers, protective layers in particular for the adhesive layers used for mounting the glazing in the bodywork bay when it is for example a windshield or rear window. , or for mounting accessories, for example mirror supports, or even as a protective layer for header strips for heating networks, etc.

 The enamel used for these applications is generally formed of a powder comprising a glass frit and pigments used as dyes, based on metal oxides, and of a medium also called vehicle generally comprising a binder allowing the adhesion of the enamel with the glass at the time of deposition, and of an organic solvent or diluent allowing the desired viscosity to be set for the application of the composition to the substrate.

 This application can be done by different methods such as spraying, screen printing, roller coating, etc.

 Screen printing is preferably used, which makes it possible to obtain very varied and reproducible shapes and layer structures.

 To manufacture the enameled layer, it is therefore preferably carried out by depositing the enamel on the substrate by screen printing, the wet layer formed is dried until this layer has sufficient adhesion to the glass and general resistance so that where appropriate, the glazing can be handled without any traces appearing on the formed layer, other layers are possibly deposited which are dried, and the enameled layer (s) is (are) finally treated by a treatment. high temperature thermal in order to vitrify the enamel and obtain the final coating.

 This high temperature vitrification treatment is generally linked to the heat treatment used for the transformation of the glass sheet, for example for the bending of the glass sheet.

 The drying of the enamel corresponds either to the removal of the solvent or diluent from the medium and for this purpose, infrared (IR) heating is generally used, or to curing by polymerization by Uv radiation, depending on the composition of enamel used. These types of drying have drawbacks.

 Thus drying by IR is energy consuming.

It also requires an extended drying line. It also poses the problem of evaporation of organic solvents and protection of the environment.

 UV drying can only be applied to an enamel containing organic components polymerizing under the action of W and in particular acrylic compounds. This also poses the problem of the harmfulness of these products. In addition, recently, it has been shown that the use of W leads to ozone production.

In addition, the polymerization by W can only be applied to the manufacture of enamel layers of very small thicknesses, generally too thin and therefore not sufficiently opaque for an enameled border of an automobile glazing for example.

 The invention overcomes the disadvantages mentioned. It proposes a new process for the manufacture of at least one layer of enamel on at least part of a glass substrate, in particular a glazing, using microwave drying, rapid, economical in energy and reducing the risks. for the environment, this process provides an opaque and non-porous enamel layer.

 By glass substrate is meant according to the invention both a bare glass substrate and a glass substrate already coated with one or more layers of enamel.

 The invention therefore provides a new method for the enamelling of a glass substrate, in particular a glazing, in which an enamel composition containing graphite as a coating is deposited on at least part of the surface of the glass. microwave doping agent, the layer formed is dried at least partially by microwave heating, and subsequently, the enamel layer is fused or vitrified.

 According to the invention, a microwave doping agent is an agent capable of absorbing microwave energy, and which consequently makes it possible to speed up the drying of the enamel by using this microwave energy.

 In addition to the advantages already mentioned, microwave energy, by being absorbed preferentially by the enamel composition containing the doping agent and little by the glass substrate, not only is energy efficient, but also due to the low temperature rise of the substrate which results therefrom, makes it possible to eliminate or at least reduce the subsequent stage of cooling of the substrate often necessary during IR drying. Furthermore, this low temperature rise of the substrate does not cause no condensation, which could eventually lead to appearance defects.

 The invention also relates to an enamel composition comprising a powder formed of a frit and of pigments in an amount of approximately 60 to 90% by weight relative to the total weight of the enamel composition, of a medium formed of '' a binder allowing the connection with glass and a diluent and / or solvent for the desired viscosity, and graphite as microwave doping agent, at a rate of at least 1% by weight for the graphite, preferably at least 3% by weight, generally 3 to 20% and preferably 3 to 15%.

 The use of graphite as a doping agent has several advantages compared to the use of other doping agents such as carbon black. If the absorption capacities of the microwave energy by graphite are equivalent to those of carbon black, on the other hand its use does not modify the rheology of the enamel paste during its application on the substrate by screen printing. which is not the case of carbon black which when used generally requires a significant modification of the sintered / medium ratio. In addition, graphite called "pyrolitic carbon" exhibits very good resistance to the high temperatures used for the vitrification of the enamel. At these temperatures of the order of 550-650 C it does not decompose or very little and remains in the enamel while carbon black which is an amorphous product decomposes into carbon dioxide before reaching these temperatures. It follows in the latter case obtaining a porous enamel.

 Graphite can be used alone or in combination with one or more other doping agents.

 These other doping agents can be chosen from ferrites, silicon carbides such as Carborundum (R), chalcopyrite.

 As indicated above, graphite is used in an amount preferably of at least 3% by weight of the total weight of the enamel composition. The effectiveness of graphite in terms of speed of drying of the enamel composition is all the greater when its quantity is important in the enamel composition. However, beyond 15% to 20% by weight of the total weight of the enamel, no improvement is generally observed.

 The graphite will therefore be used in proportions preferably ranging between 3 and 15% of the total weight of the enamel composition.

 The medium generally consists of a binder which can be based on alkyd resins, a diluent which can be based on pine oils and optionally an organic solvent. It can also contain water.

 The amount of medium used, that is to say the sintered / medium ratio can vary in particular according to the amount of graphite contained in the enamel composition. The purpose of this variation or adaptation is in particular to conserve the rheology of the enamel composition suitable for application by screen printing, this rheology being moreover little modified by the addition of graphite.

 Microwave drying in the presence of graphite in the enamel according to the invention can advantageously be combined with drying by blowing hot air. In these combinations, of course, the advantage of microwave drying is retained, in particular a length of drying line less than that of a whole line I.R.

 The particularly advantageous association of microwave drying with drying by blowing hot air can be achieved by alternating sequences of microwave drying and drying by blowing hot air.

The first sequence is preferably a microwave drying sequence. Advantageously, these two types of drying are combined to act simultaneously. In this case, the drying efficiency is further improved by using combined drying sequences (action) - nothing (non-action of microwaves and blowing) - combined drying - nothing, etc ...

 The temperature of the hot air is advantageously quite low so as not to bring the substrate to an equally high temperature. An air temperature is generally used between 30 and 900C and preferably between 50 and 800C.

 The drying time can, according to the invention, be reduced up to more than 25% compared to an all IR drying. This gnin in drying time, with equivalent energy used, between an IR drying and a drying according to the invention may also depend on other components, than graphite, on the enamel composition used, in particular on the choice of medium. Thus adding water increases the absorption coefficient of microwave energy.

 The energy consumed by microwave drying is much lower, on the order of two times less than the energy consumed by I.R.

 The drying method according to the invention can be implemented using the device described in the publication of European patent 446,114 from which the skilled person can draw the necessary instruction, suitable for drying the enamel on a glass substrate. When it is desired to dry the enamel layer by a combination of microwave drying and drying by blowing hot air, use is preferably made of an introduction of microwaves into the waveguide cavity applicator. parallelepiped, laterally with respect to the device axis, and perpendicular to the introduction of air into said applicator, this hot air then being introduced perpendicular to the face of the enameled layer to be dried. The device can be equipped with several applicators arranged one after the other, being able to be separated by more or less large intervals, for example intervals of 10 to 30 cm.

Other advantages and characteristics of the invention will appear in the following examples
EXAMPLE 1
An enamel composition is prepared by mixing 80 parts by weight of a powder of the type containing at least part of the following main components (1) and commonly used in the enamel industry for substrate glass, 20 parts by weight of a medium consisting of an organic binder based on an alkyd resin and an organic diluent based on pine oils, 5 parts by weight of artificial graphite or pyrolitic graphite for example that marketed under the name artificial graphite 1476 by the company Carbone-Lorraine (I) sio2
A1203 Fe2O3
MnO
MgO
CaO B203
TiO2
KzO Na, O
PbO
CuO Cr0
The number of parts of SiO2 added to the parts of Au203 being in this example greater than 50 parts per 100 parts in total.

 The composition is applied by screen printing on a glass sheet of format 200 × 200 × 3 mm at room temperature at approximately 20 ° C. in order to form two enameled squares of 80 × 80 mm and 30 μm thick in the wet state. The glass sheet passes through a multi-mode microwave oven to be subjected to microwave energy of 1000 W for 2 min 30 s.

 When removed from the oven, the enamel layer is dry. The glass coated with the dry enamel layer is then subjected to a heat treatment of vitrification of the enamel by passing through an oven between 650-700 "C. After this firing, the enamel layer has good opacity and is non-opaque. porous.

WITNESS EXAMPLE 1
The procedure is as in Example 1, except that the enamel composition does not contain graphite. Drying must be applied for 3 min 25 s, to obtain a dry enamel layer.

 This example shows, compared to Example 1, that microwave drying applied to an enamel composition containing graphite makes it possible to reduce the duration of this operation.

WITNESS EXAMPLE 2
The procedure is as in Example 1, except that the graphite in the enamelled composition is replaced by 5% by weight of carbon black as a microwave dopant.

 Drying must be applied for 2 min 35 s to obtain a layer of dry enamel.

 Compared to Example 1, an equivalent drying time of the enamel is observed. But after cooking the enamel, a slightly porous layer is obtained.

EXAMPLE 2
The procedure is as in Example 1, except that 10 parts by weight of graphite are introduced into the enamel composition, the other components and the quantities being unchanged, and the drying time is thus reduced to 2 min 15 s.

 After firing, the enamel strip is opaque and non-porous.

EXAMPLE 3
The procedure is as in Example 1, except that 10 parts by weight of graphite and 10 parts by additional weight of medium are introduced, in order to facilitate the application of the enamel composition to the glass. The drying time is 2 min 15 s. If we compare with Example 1, a higher proportion of graphite makes it possible to further reduce the drying time.

 The invention applies in particular to the manufacture of enamel layers in the form of a strip, border, any other shape, and of any size, such as those known and used in glazing for motor vehicles or in glazing for the building or on mirrors.

 Another advantage of the invention resides in the possibility of manufacturing a thicker and more homogeneous layer of enamel than when drying by IR or UV. It is possible, for example, to produce a layer thicker than 25 μm which generally corresponds to the thickness limit for a layer dried by IR or W under industrial conditions. According to the invention, a layer can be manufactured industrially which may have a thickness of approximately 35 μm, knowing however that 30 μm already makes it possible to obtain, better than the opacity generally desired for a layer of enamel, for example intended to protect a collage against W.

Claims (9)

 1 Method for manufacturing an enamel layer on at least part of a glass substrate, in particular a glazing, in which an enamel composition is deposited on the substrate to form a layer, at least dried partially by microwaves the layer formed, and subsequently the enamel layer is melted, characterized in that the enamel composition contains graphite as a microwave doping agent.  2. Method according to claim 1, characterized in that the enamel composition contains at least 1% by weight and preferably at least 3% by weight of graphite.  3. Method according to claim 2, characterized in that the enamel composition comprises between 3 and 15% by weight of graphite.  4. Method according to claim 1 to 3, characterized in that the microwave drying is associated with drying by blowing hot air.  5. Method according to claim 4, characterized in that the microwave drying and the drying by blowing hot air are carried out simultaneously.  6. Method according to claim 5, characterized in that the microwave drying and the drying by blowing hot air simultaneously are carried out by sequences of drying action and non-action.  7. An enamel composition used for implementing the method according to one of claims 1 to 6, characterized in that it comprises - a frit at a rate of 60 to 90 t by weight, - a medium, - graphite to 3 to 20% by weight and preferably 3 to 15% by weight.  8. Glazing comprising at least one glass sheet comprising at least on a part of its surface an enamel layer obtained by the method according to one of claims 1 to 6.  9. Glazing according to claim 8 comprising at least one glass sheet comprising at least one layer of homogeneous non-porous enamel with a thickness greater than t5 w