Cross-References to Related Applications

None

Field of the invention

This invention relates to the field of inorganic paints with a formulation that match the Coefficient of Thermal Expansion (CTE) of the glass substrate where the paint is applied to.

Background of the invention

In response to the government regulatory requirements for increased automotive fuel economy, as well as the growing public awareness and demand for energy efficient environmentally friendly products, automotive original equipment manufacturers, around the world, have been working to improve the energy efficiency of their vehicles.

One of the key elements of this strategy to improve efficiency has been the concept of light weighting. Often, more traditional, less expensive, conventional materials and processes are being replaced by innovative new materials and processes which while sometimes being more expensive, still have higher utility than the materials and processes being replaced due to their lower weight and the corresponding increase in fuel efficiency. Sometimes, the new materials and processes bring with them added functionality as well as lighter weight. Vehicle glazing has been no exception.

By reducing the weight of the vehicle, not only substantial improvements can be made in energy consumption, but additional challenges emerge in the mechanical properties point of view. This is especially important for electric vehicles where the reduction in weight directly translates into an increase in the range of the vehicle which is a key consumer concern. However, when reducing weight in the windshield or roof glasses, mechanical and acoustic properties substantially decrease. For instance, the glass may break easily, or it would be difficult to reduce the noise outside the car. Glass manufacturers are trying to increase the mechanical conditions of the layers of laminate glasses, for instance, by using Borosilicate and/or Boroaluminosilicates and/or a family of aluminosilicates glasses instead of Soda Lime glasses in automotive windshields. For example, borosilicate glasses can weight be 60 to 70% less compared to a standard Soda Lime glass with the same mechanical conditions. However, the problem nowadays is the painting process during the fabrication of an automotive laminated glasses, since the glass has a Coefficient of Thermal Expansion (CTE) lower than the paint that is currently available in the market. Typically, the edges of automotive glasses are painted with glass frits or inorganic paints often referred as the black band, which plays both a functional and an aesthetic role. The substantially opaque black print on the glass serves to protect the poly-urethane adhesive used to bond the glass to the vehicle from ultra-violet light and the degradation that it can cause. It also serves to hide the adhesive from the exterior view of the vehicle.

Frits or paints available on the market has a high CTE, similar to soda lime glass or aluminosilicate glass. The problem appears when those paints are applied on glasses with a low CTE as the Borofloat glasses. When the glass is painted and then vitrified, due to the mismatch of CTE of the frit and the glass, both the glass and the frit are potentially weakened with different tension forces along the surface of the glass or frit, which then may generate micro fractures or cracks in the laminates. The vitrification process consists of having a glass, painting the desired area with a frit, and then elevating the glass and paint temperature for a certain amount of time in order to obtain viscous flow and good adhesion of the frit to the glass.

One approach of the prior art to solve this problem is to take ground glass of the same type of the glass that is going to be laminated, break that portion of glass, grind it and then form a composite paint. The ground glass in the frit may have a similar CTE of the glass substrate where the frit is going to be painted, however, the problem of doing this is that the temperature of vitrification of the frit will increase and will not allow the glass frit to flow, leaving a rough surface behind. Also, if the vitrification process gets too hot or too long, it will not yield the desired results and in fact most likely will generate defects and poor adhesion. A wide variety of soda lime based frits, with different properties, are available for the various applications, however, there is no available specialt glasses based frit compositions for low CTE glass substrates. This is especially true for thin (2 mm or less) specialty glass substrates.

Accordingly, there is a need in the industry to develop a formulation of inorganic paints that have a lower CTE than existent frits that during the process create no surface tensions and breakage of the glass substrates where the paint is used. Specifically, there is the need to provide inorganic paints for glasses with low CTE.

Brief summary of the invention

The present invention relates to inorganic paint compositions that overcome the Coefficient of Thermal Expansion (CTE) mismatch issue between the frit and the glass substrate. Specifically, the inorganic paint of the present invention comprises a CTE lower or equal to the glasses where the inorganic paint is being applied. The present invention also comprises a laminated glazing including the inorganic paint.

Brief description of drawings Figure 1 shows a cross section of a typical automotive laminate with black frit obscurations (black inorganic paint) on surfaces number two and four.

Figure 2 shows an automotive laminate including a black inorganic paint composition. Reference numerals

2 Glass

4 Plastic Bonding Interlayer

6 Black inorganic paint (frit obscuration)

16 Plastic Interlayer

18 Coating

101 Surface one 102 Surface two

103 Surface three

104 Surface four

201 Outer glass layer

202 Inner glass layer

Detailed description of the invention

The present invention is directed towards to an inorganic paint that has a Coefficient of Thermal Expansion (CTE) that matches the CTE of the glass substrate where the paint is being employed or is lower than the glass substrate.

Generally, the inorganic paints or frits are comprised by a plurality of organic materials, such as pigments, carriers, binders and finely ground glass and fluxes. Other materials are also sometimes added to enhance certain properties: the firing temperate, anti-stick, chemical resistance, etc. Each one of these materials add a correspondent multiplier to the CTE of the final paint, lowering or increasing the CTE of the inorganic paint.

The terminology used to describe the inorganic paint or frits composition will be exemplified using laminated glasses as an example. However, it should be noted that the following description can be also applied to any kind of glass on which an inorganic paint can be applied, as follows.

Black paints or frit obscurations are commonly printed on laminated and/or tempered automotive glazing where they serve to hide the adhesive used to bond the glazing to the body and to protect the adhesive from exterior degradation generated, for instance, from ultra-violet light. However, frit has a number of drawbacks. The frit weakens the surface of the glass. It can create distortion in the glass due to thermal gradients. Printed frit obscurations are incompatible and cannot be used with many types of coatings. Standard frit blocks ion exchange and so cannot be used with the chemical tempering process. The black paint or frit obscuration is applied to the glass using a silk screen or ink jet printing process prior to the heating and bending of the glass. As the flat glass is heated during the bending process, the powdered glass in the frit softens and melts, fusing to the surface of the glass. The frit becomes a permanent part of the glass. The frit is said to be“fired” when this takes place. This is a vitrification process which is very similar to the process used to apply enamel finishes on bathroom fixtures, pottery and appliances.

The paint or frit obscuration interferes with the annealing process. Laminated glass, as required for windshields, must be annealed to meet safety requirements. Glass is annealed by heating the glass into the glass transition range to soften it, letting it soak for some period to allow any stress to be relieved, and then slowly cooling back down through the glass transition range until the glass freezes. The ideal would be to have inorganic paints matching the CTE of the glass where it is going to be used. Other way, as stated before, the mismatch between the CTE of the laminated glass and the paint or frit obscuration may result in residual stress in the laminate when the surfaces of the laminated glass are forced together during lamination.

Regarding the glass used for automotive applications, it should be noted that historically windows have been made from standard soda lime glass, since this is the most common and economical formulation and accounts for a large percentage of the world’s glass productions. Soda-lime glass is made from sodium carbonate (soda), calcium carbonate (lime), silicon dioxide (silica), aluminum oxide (alumina), and small quantities of other substances added to alter the color and other properties. Other types of glass are known but have primarily been reserved for specialty applications. There they are used to provide for thinner and lighter glazing. While many types of paints are available for printing on soda lime glass, to the best of our knowledge there is no inorganic paints for non-soda lime glasses in the market today.

If a non-soda lime glass is used, there is the need to find a compatible paint or frit that matches the CTE of the non-soda lime glass. Commonly soda lime glasses use different methods for painting any of the layers of the laminate, for instance, a painted or frit obscuration may be printed and fired if needed in any face of the layers of the laminate. However, this cannot be done when the CTE of the paint or frit does not match the CTE of the glazing. In response to those needs, the present invention discloses inorganic paint compositions comprising the following formulation:

The inorganic paint composition of the present invention may have additional metallic materials commonly found in inorganic paints. In embodiments where the composition is used as a paint over the surface of a laminated glass, the combination of the foregoing composition may result in a black band composition having a CTE lower or equal than the CTE of the laminated glass where the composition is applied. The formulation of the inorganic paint of the present invention achieve an adequate CTE (i.e. lower or equal to than the glass substrate or glaze) while maintains appropriate melting point during the vitrification process. Using boron oxide in the composition, is advantageous since it has a negative CTE. That is, boron oxide contracts when heated, therefore helping the composition of the present invention to achieve a lower CTE than the glass substrate. The inorganic paint of the present invention may also comprise of colorants in different ratios, for instance, the composition may include around 2-6% of a black colorant.

The resulting inorganic paint has a CTE between 30-70 xlO ⁷ /°K, and a viscosity less or equal to 10 ⁵ Poise at temperatures between 500 °C to 750 °C. In preferred embodiments, the resulting inorganic paint has a CTE between 30-55 xlO ⁷ /°K. In more preferred embodiments, the resulting inorganic paint has a CTE between 30-45 xlO ⁷ /°K.

Paints or frit obscurations have historically been black. One of the reasons for this comes from the limitations imposed by ceramic frits. It is difficult to produce frits, in colors other than black, that have the durability needed and for which the color can be reliably replicated from run to run. The other reason is aesthetics and it is commonly called the“black band.” However, in the present invention as will be described, it should be understood that the inorganic paint can be of any color.

In a first embodiment, an inorganic paint comprises from about 8 % to about 50% silicon oxides, from about 5% to about 15% boron oxides, no more than about 45% bismuth oxides, no more than about 20 % zinc oxides and no more than about 10% sodium oxides in percent by weight achieving a translucid color and a CTE no more than about 70 x 10 ⁷ / ⁰ K.

In a second embodiment, an inorganic paint comprises the same as the first embodiment and further comprises copper oxides no more than about 25% and iron oxides no more than about 5 % and chrome oxides from about 10% to about 40% in percent by weight achieving an opaque color (below L 25) and a CTE no more than about 70 xlO ⁷ /°K.

In a third embodiment, an inorganic paint comprises the same as the second embodiment and further comprises molybdenum oxides no more than about 1%, aluminum oxides no more than about 5%, tin oxides no more than about 5% and zirconium oxides no more than about 3% in percent by weight achieving an opaque color (below L 25), a CTE no more than about 70 xlO ⁷ /°K and an improvement in durability.

The terminology used to describe the inorganic paint or frits composition will be exemplified using laminated glasses as an example wherein at least one of the outer or inner glass layers have a low CTE from about 30 xlO ⁷ / °K to about 70 xlO ⁷ / °K (e.g. Aluminosilicate, Alkali aluminosilicate, Borosilicate, Boroaluminosilicate or Alkali Boroaluminosilicate) and the inorganic paint is applied in at least one of the glass layers which have a low CTE.

A typical automotive laminate cross section is illustrated in Figure 1. The laminate is comprised of two layers of glass, the exterior or outer layer, 201 and interior or inner layer, 202 that are permanently bonded together by a plastic layer 4 (interlayer). The glass surface that is on the exterior of the vehicle is referred to as surface one 101 or the number one surface. The opposite face of the exterior glass layer 201 is surface two 102 or the number two surface. The surface of glass 2 that is in the interior of the vehicle is referred to as surface four 104 or the number four surface. The opposite face of the interior layer of glass 202 is surface three 103 or the number three surface. Surfaces two 102 and three 103 are bonded together by the plastic layer 4. Black inorganic paint (frit obscuration) 6 may be also applied to the glass, as shown in Figure 1 and 2. Obscurations are commonly comprised of black enamel frit printed on either the number two 102 or number four surface 104 or on both. The laminate may also comprise a coating 18 on one or more of the surfaces.

The types of glass 2 that may be used in either the exterior or interior glass layers include but are not limited to: the common soda lime variety typical of automotive glazing as well as aluminosilicates, lithium aluminosilicates, borosilicates, glass- ceramics, and the various other inorganic solid amorphous compositions which undergo a glass transition and are classified as glass included those that are not 100% transparent. These are generic families which can overlap, and which can be applied to a broad range of glass formulations. As an example, there is not a single specific formulation that comprises of aluminosilicate glass. Rather there are a broad range of glass formulation that can be classified as an aluminosilicate based upon containing aluminum compounds. Typical thickness used for the exterior and interior glass layers may range from 1 mm to 8 mm for the exterior glass layers and from 0.5 mm to 3 mm for the interior glass layers. However, it should be noted that the composition of the present invention may be used over even lower ranges of glass thicknesses.

Usually in automotive glasses, annealed glass is used. Annealing is a process where the glass has been slowly cooled from the bending temperature down through the glass transition range. This process relieves any stress left in the glass from the bending process in a few minutes. Annealed glass breaks into large shards with sharp edges. When laminated glass breaks, the shards of broken glass are held together, much like the pieces of a jigsaw puzzle, by the plastic layer helping to maintain the structural integrity of the glass. A vehicle with a broken windshield can still be operated. The plastic layer 4 also helps to prevent penetration by objects striking the laminate from the exterior and in the event of a crash occupant retention is improved.

The glass layers are formed using gravity bending, press bending, cold bending or any other conventional means known in the art. Gravity and press bending methods for forming glass are well known in the art and will not be discussed in the present disclosure.

Cold bending is a relatively new technology. As the name suggest, the glass is bent, while cold to its final shape, without the use of heat. On parts with minimal curvature a flat sheet of glass can be bent cold to the contour of the part. This is possible because as the thickness of glass decreases, the sheets become increasingly more flexible and can be bent without inducing stress levels high enough to significantly increase the long term probability of breakage. Thin sheets of annealed glass, in thicknesses of about 1 mm, can be bent to large radius cylindrical shapes (greater than 6 m). When the glass is chemically, or heat strengthened the glass can endure much higher levels of stress and can be bent along both major axis. The process is primarily used to bend chemically tempered thin glass sheets (<l mm) to shape. Cylindrical shapes can be formed with a radius in one direction of less than 4 meters. Shapes with compound bend, that is curvature in the direction of both principle axis can be formed with a radius of curvature in each direction as small as approximately 8 meters. Of course, much of this depends upon the surface area of the parts and the types and thicknesses of the substrates.

The cold bent glass will remain in tension and tend to distort the shape of the bent layer that it is bonded to. Therefore, the bent layer must be compensated to offset the tension. For more complex shapes with a high level of curvature, the flat glass may need to be partially thermally bent prior to cold bending.

The glass to be cold bent is placed with a bent to shape layer and with a bonding layer placed between the glass to be cold bent and the bent glass layer. The assembly is placed in what is known as a vacuum bag or also on a ring that serves as suction hose. The vacuum bag is an airtight set of plastic sheets, enclosing the assembly and bonded together it the edges, which allows for the air to be evacuated from the assembly and which also applies pressure on the assembly forcing the layers into contact with each other. The assembly, in the evacuated vacuum bag, is then heated to seal the assembly. The assembly is next placed into an autoclave which applies heat and high pressure on the assembly. This completes the cold bending process as the flat glass at this point has conformed to the shape of the bent layer and is permanently affixed. The cold bending process is very similar to a standard vacuum bag/autoclave process, well known in the art, except for having an unbent glass layer added to the stack of glass.

Heat strengthened, full temper soda lime float glass, with a compressive strength in the range of at least 70 MPa, can be used in all vehicle positions other than the windshield. Heat strengthened (tempered) glass has a layer of high compression on the outside surfaces of the glass, balanced by tension on the inside of the glass which is produced by the rapid cooling of the hot softened glass. When tempered glass breaks, the tension and compression are no longer in balance and the glass breaks into small beads with dull edges. Tempered glass is much stronger than annealed laminated glass. The thickness limits of the typical automotive heat strengthening process is about 3 mm. This is due to the rapid heat transfer that is required. It is not possible to achieve the high surface compression needed with thinner glass using the typical blower type low pressure air quenching systems.

Examples The tests performed were carried out taking into account the following criteria to select the materials of the inorganic paint: i) that the paint has a CTE lower or equal than the CTE of the glass where the paint is going to be used; and ii) that the paint frits has a lower melting point than the glass substrate. It should be noted that the state of the art provides paintings with low melting point but this at the expense of having a frit with high CTE. The present invention discloses an inorganic paint that does not raises the CTE of the glass frit.

1. Example 1. Referring to the composition shown in Figure 1, a first example was carried out on a laminate windshield comprising a standard Borosilicate glass, 1.1 mm thick exterior glass layer 201 and a 0.7 mm thick Aluminosilicate interior glass layer 202. A sheet of 0,76 mm PVB interlayer 4 is placed between the exterior and interior glass layers. A black frit is printed on the number four surface 104 of the interior glass layer 202, comprising the following composition: 14% silicon dioxide, 10% boron oxide, 40% bismuth (III) oxide, 5% zinc (I) oxide, 14% chrome (III) oxide, 8% copper (II) oxide, 2% iron (III) oxide, 1% molybdenum (VI) oxide, 1% aluminum oxide, 4% tin oxide and 1% sodium oxide.

The tension stress was measured in fourteen different points along the surface of a curved laminated glaze were the frit was printed and vitrified. The results are shown in the table below, and are below the standard (< 8 MPa) for windshields:

The color properties of the frit (lightness value L) were also measured using the CLI Lab color measurement standard. The measured lightness of the glass frit was below 14 L. Example 2. A second example consisted of a laminate windshield comprising a standard Borosilicate glass, 1.1 mm thick exterior glass layer 201 and a 0.5 mm thick Aluminosilicate interior glass layer 202. A sheet of 0.76 mm PVB interlayer 4 is placed between the exterior and interior glass layers. A black frit is printed on the number four surface 104 of the interior glass layer 202, comprising of the following composition: 8% silicon dioxide, 10% boron oxide, 17% zinc (I) oxide, 34% chrome (III) oxide, 20% copper (II) oxide, 1% molybdenum (VI) oxide, 1% aluminum oxide, 3% zirconium (II) oxide, and 6% sodium oxide.

The tension stress was measured in fourteen different points along the surface of a curved laminated glaze were the frit was printed and vitrified. The results are shown in the table below, and were below the standard (< 8 MPa) for windshields:

The measured lightness of the glass frit of this example was also measured and was below 14 L.

3. Example 3. A third example consisted in a laminate windshield comprising a standard Borosilicate glass, 1.1 mm thick exterior glass layer 201 and a 0.5 mm thick Aluminosilicate interior glass layer 202. A sheet of 0,76 mm PVB interlayer 4 is placed between the exterior and interior glass layers. A black frit is printed on the number four surface 104 of the interior glass layer 202, comprising the following composition: 36% silicon dioxide, 10% boron oxide, 7% bismuth (III) oxide, 28% chrome (III) oxide, 16% copper (II) oxide, and 3% aluminum oxide.

The tension stress was measured in fourteen different points along the surface of a curved laminated glaze were the frit was printed and vitrified. The results are shown in the table below, and were below the standard (< 8 MPa) for windshields:

The measured lightness of the glass frit of this example was also measured and was below 25 L. It should be stated that the maximum edge tension measured as acceptable should be 12 MPa (or a stress of +12 MPa, i.e. +13 MPa is not acceptable) and the minimum edge compression measured as acceptable should be 5 MPa (or a stress of -5 MPa, i.e. - 3 MPa, is not acceptable). It must be understood that this invention is not limited to the embodiments described and illustrated above. A person skilled in the art will understand that numerous variations and/or modifications can be carried out that do not depart from the spirit of the invention, which is only defined by the following claims.