FR2736348A1 - Enamel coating of glass or glass ceramic substrate - involves addition of graphite to enamel composition - Google Patents

Abstract

A substrate is coated with an enamel containing graphite.

Description

PROCESS FOR ENAMELING GLASS SUBSTRATES,
ENAMEL COMPOSITION USED AND PRODUCTS OBTAINED
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 capable of being used for the manufacture of a layer on a glass substrate and the enameled products obtained.

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

 The term “glass substrate” is understood to mean, according to the invention, all glass-based substrates, in particular glass substrates proper (for example glazing) but also substrates made of partially crystalline glass materials such as glass-ceramic substrates ( for example hobs).

 Enamels are well known in the state of the art and used in particular on actual glass substrates such as vehicle glazing or building glazing or even mirrors. Enamels are thus used to form borders, decorative layers, protective layers (masks) against ultraviolet radiation, in particular for the adhesive layers used for mounting the glazing in the bodywork bay when it comes, by example, windshield or rear window or for mounting accessories, for example mirror supports, or even as a protective layer for header strips of heating networks, etc. We also use enamels for the coating of glass ceramic substrates such as hobs.

 The enamel used for these applications is generally formed from a powder comprising a glass frit (glassy matrix) and pigments used as dyes in particular, the frit and the pigments being based on metal oxides, and a medium also called vehicle allowing the application of the enamel composition on the substrate and its adhesion with it at the time of deposition.

 The application of the enamel on the substrate can be done by different processes 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 substrate and general resistance so that , if necessary, the enamelled substrate can be handled without 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 high temperature heat treatment in order to vitrify the enamel and obtain the final coating.

 The enameled layers can however reduce certain mechanical properties at the level of the enameled substrates obtained compared to the same substrates not coated with enamel. It is nevertheless desirable to avoid such a reduction for glass substrates which have to satisfy certain safety requirements for applications in the building industry or in the automotive industry, in particular in the case of glass substrates used for the manufacture of automobile roofs, these substrates having to have properties of mechanical resistance, in particular resistance to mechanical shock (test of the ball drop according to the ANS Z 26.1 standard) particularly high.

 A related problem related to the enamel layers is that to fulfill its UV protection functions, an enameled layer must have a determined opacity. The opacity is obviously linked to the thickness of the enamel layer. Thus, when it is an enameled layer intended to protect a bonding bead, the layer must generally have a thickness greater than approximately 30 μm in the wet state (that is to say after application of the layer on the substrate and before drying and firing), which corresponds approximately to a thickness of 20 μm for the layer after firing and is considered to be important for an enamel layer. This therefore requires the use of a large amount of enamel and the cost is high.

 Furthermore, when it is a question of manufacturing a glazing comprising enameled zones of different densities, for example when it is a glazing for a car roof which has to present an enameled central zone with a fine screened pattern, and a strip enameled device of high opacity, it is generally necessary to use at least two passes (that is to say two screen printing operations) for the production of the enamel layer. In a first pass, the enamel layer is deposited at least in the central zone in the form of a thin layer having a very fine weft to obtain a good definition of the screened pattern and, in a second pass, we carry out the deposition of the peripheral strip using a second screen fabric to obtain a thicker layer having the required opacity. This process is long and costly because it requires two screen stations and two dryers.

 It is possible to increase the content of pigments in the enamels used to coat the glass in order to improve the opacity of the enameled layers obtained while reducing the thickness of these layers. However, the increase in the pigment content in the enamel layers also contributes to the lowering of the mechanical properties at the level of the enameled substrates obtained.

 Another problem linked to the deposition of enamel on a glass substrate arises when the enamelled substrates are subjected to bending operations, the bending and firing of the enamel generally taking place jointly. During bending operations, it often happens, in fact, that the enamel coating the glass substrate intended to be bent adheres to the elements brought into contact with it, in particular to the bending dies or to other substrates bent simultaneously, this leading, among other things, to a deterioration of the enamel layer.

 Certain enamels which do not adhere or adhere little to the elements coming into contact with them during bending exist and are qualified as enamels having non-stick properties or non-stick enamels. These are mainly enamels comprising oxidizable metals or enamels having components forming crystalline phases; however, such enamels cause, on the glass substrates which are coated therewith, an embrittlement often greater than the embrittlement caused by traditional enamels without anti-adhesive properties and further contribute to the lowering of the mechanical properties of the enameled substrates obtained. .

 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, this process making it possible, in particular, to obtain enamelled substrates having mechanical properties improved.

 The invention consists in using graphite in enamel compositions intended to coat glass substrates, this graphite making it possible, in particular, to improve the mechanical properties of the enameled substrates obtained.

 The invention therefore proposes a new method for the enamelling of a glass substrate, in particular a glazing, in which an enamel composition containing graphite in view is deposited on at least part of the surface of the substrate, in particular , to improve the mechanical properties of enameled substrates, the layer formed is optionally dried and, subsequently, the enamel layer is melted or vitrified.

 The invention also relates to an enamel composition for a glass substrate, this composition comprising a powder formed from a glass frit and pigments, a medium allowing the desired viscosity to be applied for application to the substrate and allowing bonding. with the substrate, and graphite with a view, in particular, to improving the mechanical properties of enamelled substrates. The composition advantageously comprises at least 0.2% by weight of graphite relative to the total weight of the enamel composition in order to obtain an improvement in the mechanical properties at the level of the enameled substrates obtained and, preferably, at least 0.5 % by weight of graphite relative to the total weight of the composition.

 The use of graphite in the enamel compositions and in the previously mentioned process has several advantages. In particular, the loss of mechanical resistance at the level of enamelled glass substrates compared to the same glass substrates not coated with enamel is much lower for the substrates coated with an enamel containing graphite according to the invention than for the coated substrates of the same enamel without graphite.

 Furthermore, the presence of graphite in the enamel compositions also has the effect of imparting non-stick properties to many enamels according to the invention and thus of preventing their sticking during bending operations which can occur in the process of enamelling. Graphite can therefore also be used in enamel compositions intended to coat glass substrates, with a view to rendering said compositions non-stick.

 Since graphite makes it possible to increase the mechanical properties of enamelled substrates, it is also easier to make use of enamel compositions with a high rate of pigments (for example of compositions comprising more than 30-35% by weight of pigments), presence of graphite compensating for the loss of mechanical properties of enamelled substrates due to the presence of a high level of pigments.

The use of graphite and / or the increase in the level of pigments in the enamel makes it possible to improve the opacity of the enameled layers obtained and this improvement allows the production of layers of smaller thickness.

 This very good opacity to layer thickness ratio made possible by the use of the composition according to the invention makes it possible, for certain glazed glazing applications, to produce glazed layers in a single pass when it was necessary to use at least two passes with known enamel compositions. For example, in the case of glazed glazing for a car roof which must have a central zone formed by a thin layer having a fine weft and a peripheral zone of high opacity, it is possible with the enamel composition according to the invention, depending on the level of pigments and / or graphite in the composition, of operating the deposition of the enamel layer in a single pass corresponding to the deposition of a thin layer, this single pass nevertheless making it possible to obtain opacity that meets the acceptance criteria for the peripheral zone.

 The possibility of using an enamel composition made more opaque by the presence of a high level of pigments and / or graphite also has other advantages, such as the removal of the coating of the substrate by a black primer in certain applications. , or the improvement of the surface appearance, etc ...

 Another advantage linked to the use of graphite in enamel compositions is that graphite does not slightly modify the rheology of the enamel paste during its application to the substrate by screen printing (which would not be the case, for example , carbon black). In addition, graphite called pyrolitic carbon has very good resistance to the high temperatures used for the vitrification of the enamel. At these temperatures, of the order of 550-650 "C for enamels intended to coat glass substrates proper or of the order of 900" C for enamels intended to coat glass-ceramic substrates, it does not decompose not or very little and remains in the enamel (while carbon black for example would decompose into carbon dioxide before reaching these temperatures and would lead to obtaining a porous enamel).

 As indicated above, graphite is used in an amount, preferably, of at least 0.2% by weight of the total weight of the enamel composition. The effectiveness of graphite in terms of improving the mechanical properties of enameled substrates is all the greater the greater its quantity in the composition of enamel.

However, above 20% by weight of the total weight of the enamel, graphite can possibly make the cooking of the enamel more difficult.

 The powder present in the enamel composition according to the invention and formed of a glass frit and of pigments represents, generally, 60 to 90% by weight approximately relative to the total weight of the enamel composition.

 The glass frit can be any known glass frit based on oxides chosen, for example, from oxides of silicon, lead, zinc, bismuth, titanium, zirconium, sodium, boron, lithium , potassium, calcium, aluminum, tin, vanadium, molybdenum, magnesium, etc.

 The pigments used according to the invention can be chosen from the compounds containing metal oxides such as chromium oxides, copper oxides, iron oxides, cobalt oxides, nickel oxides, or can be chosen from copper chromates, cobalt chromates, etc.

 Due to the presence of graphite compensating for the losses in mechanical properties, the pigment content can be much higher than the usual contents, as explained above, and can range in particular up to 70% by weight of pigments relative to the sintered mixture plus pigments. , the usual contents generally not exceeding 30-35% by weight relative to the sintered mixture plus pigments. Preferably, in order to obtain a composition having a usual melting temperature, a glass frit is used having a melting point which is lower the higher the level of pigments and / or graphite.

 The medium present in the enamel composition according to the invention can be any medium usually used in known enamel compositions. It can include solvents, diluents, oils such as pine oils and other vegetable oils, resins such as acrylic resins, petroleum fractions, film-forming materials, etc.

 The powder is suspended in this medium so as to obtain a paste capable of being printed on a glass substrate.

 The enamel composition according to the invention can advantageously be prepared from existing enamel compositions or from components of the enamel compositions, such as glass frits. The preparation can be done by simply adding graphite to the preceding compositions or components and does not necessarily imply a complete reformulation of the enamel compositions used, just as it does not involve modifications of the devices for carrying out the processes traditional enamelling.

 The glass substrate on which the enamel composition is deposited can be both a bare glass substrate and a glass substrate already coated with one or more layers of enamel. This substrate may consist of one or more sheets of glass and may be toughened so as to have improved mechanical and thermal resistance properties. The enamel-coated substrate according to the invention thus comprises at least one glass sheet coated on at least part of one of its faces with a composition according to the invention.

 In the process of enamelling glass substrates, in particular glazings according to the invention, the enamel composition is preferably deposited by screen printing on the substrates, then baked, the baking taking place if necessary during the heat treatment linked to the bending and / or tempering of the substrates, the firing temperatures generally being of the order of 600-700 "C for the glass substrates themselves or of the order of 900-1000 C for the glass ceramic substrates.

 The deposition of the enamel composition can also be done by spraying, by coating with a curtain or with a roller, etc.

The deposition is generally followed by drying to allow sufficient adhesion and resistance of the layer on the substrate, for example drying by infrared, ultraviolet radiation, hot air or microwave, graphite also having the advantage in this last case to have a microwave doping effect. In
in the case where several layers are deposited on the substrate, each layer is preferably dried before the deposition of the next, the cooking taking place on
all of the layers.

 The bending and possible tempering of the glass substrates are carried out according to known methods. In particular, the bending can be carried out by gravity (bending in particular of glass substrates in pairs with a view to producing laminated glasses) or using dies and, when the substrates are curved and toughened, the toughening can take place. operate after bending the enamelled substrates, possibly within the same device.

 In the case where glass substrates must be curved simultaneously in order to produce laminated glazings, the enamel is deposited on at least one of the substrates and the substrates are assembled and then curved, the glass substrates then being separated to insert at least an interlayer film of different material, then the laminated glazing is obtained by assembling the assembly hot and under pressure.

Other advantages and characteristics of the invention will appear in the following examples:
WITNESS EXAMPLE 1
An enamel composition is prepared by mixing 100 parts by weight of a powder comprising approximately 16% by weight of SiO2, 50% by weight of PbO and 34% by weight of other intermediate or modifying oxides among which pigments with based on iron oxides, chromium oxides and cobalt oxides (accounting for approximately 30% by weight of the powder), with 25 parts by weight of a medium based on pine oil comprising 3% by weight of acrylic resin.

 This composition is applied by screen printing on a glass sheet of format 305 x 305 mm then dried and baked around 640 "C before determining the impact resistance of the sheet thus enamelled under the conditions defined by the ANS Z standard. 26.1 (ball drop test NO 6) This resistance is evaluated by measuring the height from which a ball of approximately 227 g dropped from this height and falling on the enamelled substrate causes the breakage of the latter. the greater, the greater the mechanical resistance of the enameled substrate.

 As an indication, this height must be at least 3.05 m for enamelled glazing used for the manufacture of automobile roofs.

 The drop height necessary to cause the breakage of the enamel-coated substrate according to this example is approximately 1.80 m.

The opacity of the enamel is also determined. This opacity can be expressed in terms of optical density. As an indication, when it is an enamelled layer intended to protect a bonding bead, the desired opacity corresponds to an optical density at least equal to 3. The optical density is measured with a
R densitometer, for example the GRETAG D 200 device. The optical density of the enamel according to this example is approximately 3.2.

EXAMPLE 1
The procedure is as in Example 1, adding to the enamel composition of Example 1, 15 parts by weight of artificial graphite or pyrolitic graphite, for example synthetic graphite powder 1-2, im sold under the designation 28.286 / 3 by the company ALDRICH.

 The drop height necessary to cause the breakage of the substrate coated with the enamel according to this example (enamel according to the invention) is approximately 4.4 m. There is therefore a significant improvement in the mechanical properties of the enameled substrate due to the presence of graphite in the enamel.

 The optical density of the enamel according to the present example is approximately 4.2. There is also, therefore, an improvement in opacity.

 It is also observed that the enamel according to the present example has non-stick properties and that the deposition of this enamel on a glazing having enameled zones of different characteristics can be done in one screen printing pass instead of two.

WITNESS EXAMPLE 2
In this example, an enamel composition is prepared by mixing 100 parts by weight of a powder comprising approximately 25% by weight of SiO2, 38% by weight of PbO, 7% of B203 and 30% of other intermediate oxides or modifiers including pigments based on iron oxides, chromium oxides and cobalt oxides (accounting for approximately 20 to 25% by weight of the powder) with 25 parts by weight of a medium based pine oil comprising 3% by weight of acrylic resin. Then proceed as in Example 1 to determine the mechanical properties of a glass substrate coated with enamel according to the present example.

 The drop height necessary to cause the breakage of the enamel-coated substrate according to the present example is approximately 3.2 m.

EXAMPLE WITNESS 3
The procedure is as in Example 2, adding 10 parts by weight of pigments such as copper chromates in spinel form to the enamel composition of Example 2.

 The drop height necessary to cause the breakage of the enamel-coated substrate according to the present example is approximately 2.5 m.

EXAMPLES 2, 3 AND 4
The procedure is as in control example 3, adding, to the enamel composition of control example 3, respectively 0.5 (example 2), 1 (example 3) and 5 (example 4) parts by weight of graphite such as that used in Example 1.

The drop height required to cause the substrate coated with
The enamel according to the present example is approximately 3.1 m (example 2), 3.3 m (example 3) and 3.8 m (example 4).

 We note, therefore, that the addition of a very small amount of graphite in the enamel makes it possible to compensate for the loss of mechanical properties of the enamelled substrates due to the addition of pigments in this same enamel and that the improvement of the Mechanical properties of enameled substrates is all the more important as the quantity of graphite is important in the composition of enamel.

 The enamels according to the invention are mainly used for coating actual glass substrates, such as glazing, for the automotive or building industry, or glass-ceramic substrates such as cooking plates.

Claims (8)

 1. A method for enamelling a glass or glass-ceramic substrate, in particular a glazing, in which an enamel composition is deposited on at least part of the surface of the substrate, the layer formed is optionally dried, and subsequently, the enamel layer is melted or vitrified, the enamel composition comprising graphite in order to improve the mechanical properties of the enameled substrate and / or to impart non-stick properties to the composition.  2. Method according to claim 1, characterized in that the composition is deposited in a single screen printing operation.  3. Enamel composition used in the method according to one of claims 1 or 2, comprising a powder formed from a glass frit and pigments, a medium and graphite in order to improve the mechanical properties of the enamelled substrate and / or impart non-stick properties to the composition.  4. Composition according to claim 3, characterized in that it comprises at least 0.2% by weight of graphite relative to the total weight of the composition.  5. Composition according to one of claims 3 or 4, characterized in that it comprises at least 30% by weight of pigments relative to the weight of the sintered mixture plus pigments.  6. Glazing comprising at least one glass sheet coated, on at least part of one of its faces, with a composition according to one of claims 3 to 5.  7. Glass-ceramic plate comprising at least one glass-ceramic sheet coated, on at least part of one of its faces, with an enamel composition according to one of claims 3 to 5.  8. Use of graphite, in an enamel composition intended to coat a glass or glass-ceramic substrate, to improve the mechanical properties of the enameled substrate and / or to impart non-stick properties to the composition.