Method of bending dissimilar glass compositions

Abstract

Glass laminates, comprising more than one glass composition, are becoming increasingly common as the industry moves towards lighter and stronger glazing. Bending dissimilar glass compositions can present problems. A mismatch in the glass viscosity curves, especially in the viscoelastic region of the compositions can result in one layer becoming softer than one of the other layers during the thermal bending process. As a result, economical processes, such as gravity or press bending in which multiple glass layers are simultaneously bent, may not be practical to use forcing the use of more expensive single glass layer bending processes. By thermal treatment processes the fictive temperature of at least one of the glass compositions prior to bending can be shifted to better match the other compositions allowing the glass layers to be simultaneously bent.

Claims

1. A bent laminated glass, comprising: a first glass layer comprised of a first glass composition having a fictive temperature; and a second glass layer comprised of a second glass composition having a fictive temperature T2; wherein the first glass composition is different from the second glass composition; and the difference between the fictive temperature of the first glass layer and the fictive temperature T2 is no more than 30° C.

2. The bent laminated glass of claim 1, wherein in the second glass layer, the fictive temperature T2 is reached by shifting an original fictive temperature of the second glass composition to the fictive temperature T2.

3. The bent laminated glass of claim 2, wherein the fictive temperature of the first glass layer is greater than the fictive temperature T2, and the fictive temperature T2 is greater than an original fictive temperature of the second glass layer.

4. The bent laminated glass of claim 1, wherein the fictive temperature of the first glass layer is the fictive temperature T2.

5. The bent laminated glass of claim 1, wherein the difference between the fictive temperature of the first glass layer and the fictive temperature T2 is not more than 20° C.

6. The bent laminated glass of claim 1, wherein the fictive temperature of the first glass layer is greater than the fictive temperature T2.

7. The bent laminated glass of claim 1, wherein the fictive temperature T2 is greater than the fictive temperature of the first glass layer.

8. The bent laminated glass of claim 1, wherein the first glass layer is an aluminosilicate glass, and the second glass layer is a soda-lime glass.

9. The bent laminated glass of claim 1, wherein the first glass layer is an aluminosilicate glass or soda-lime, and the second glass layer is a borosilicate glass.

10. The bent laminated glass of claim 1, wherein there is no more than a 2 mm gap between the mating surfaces of the first and second glass layers.

11. A method for producing a bent laminated glass having a first glass layer comprised of a first glass composition and a second glass layer comprised of a second glass composition, the first glass composition being different from the second glass composition, the method comprising the steps of: treating the first glass layer by a thermal process, such that the fictive temperature of the first glass composition is shifted from T1 towards the fictive temperature T2, wherein the fictive temperature T2 is the fictive temperature of the second glass composition and the difference between the shifted fictive temperature of the first glass layer and the fictive temperature T2 is no more than 30° C.; bending said at least two glass layers simultaneously by a bending process; and cooling said at least two glass layers.

12. The method for producing a bent laminated glass of claim 11, further comprising the step of: treating the second glass layer with a thermal process, such that the fictive temperature T2 is reached by shifting its fictive temperature from T3 to T2.

13. The method for producing a bent laminated glass of claim 11, wherein the step of treating the first glass layer by a thermal process comprises the steps of: heating the first glass layer to a temperature over the fictive temperature T2; and cooling the first glass layer at a cooling rate selected from the group consisting of a fast cooling rate and a slow cooling rate.

14. The method for producing a bent laminated glass of claim 13, wherein the fast cooling rate ranges from 20° C./min to 100° C./min.

15. The method for producing a bent laminated glass of claim 13, wherein the slow cooling rate ranges from 0.1° C./min to 20° C./min.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1A shows a cross section: typical laminated glazing.

(2) FIG. 1B shows a cross section: typical laminated glazing with coating and performance film.

(3) FIG. 2 shows dilatometry results on a glass indicating the change in fictive temperature due to a glass pre-bending heat treatment.

REFERENCE NUMERALS OF DRAWINGS

(4) 2 Glass 4 Plastic Interlayer 6 Obscuration 12 Film 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

(5) The term “glass” can be applied to many organic and inorganic materials, including many that are not transparent. For this document we will only be referring to nonorganic transparent glass.

(6) From a scientific standpoint, glass is defined as a solid material comprising a non-crystalline amorphous structure that lacks the ordered molecular structure of polycrystalline and crystalline materials and possess a glass transition region. Glasses have the mechanical rigidity of crystals with the random structure of liquids.

(7) Glass is formed by mixing various substances together and then heating to a temperature where they melt and fully dissolve in each other, forming a viscous homogeneous liquid.

(8) When heated or cooled sufficiently glass undergoes a glass transition. While most materials go through the phase change, the change in state is abrupt and occurs as a precise temperature as the molecules go from moving about freely to being locked in place and vice versa. This is because of all the bonds between the molecules are identical and break at the same temperature.

(9) In a glass, due to the random order of the molecules, the bonds are all different. The bond strength is a function of the stress on the bonds and temperature. In a glass, as the material is heated, it reaches a point where the bonds just begin to break, and the glass starts to soften. As the temperature is increased, more of the bonds continue to break and the glass becomes softer until the glass reaches its melting point where is it considered a liquid. This range of temperatures where the glass transitions from a liquid to a solid is known as the glass transition range. The center of this glass transition range is the glass fictive temperature, T.sub.f. It can also be described as the temperature at which the enthalpy curves of the solid and the “liquid” glass cross-over.

(10) The glass transition region is primarily a function of the composition of the glass but is also a function of the temperature profile experienced during cooling from the liquid state to the solid state. This is because the rate of cooling influences the ordering of the molecules in the solid and the residual stress on the bonds. Each cooling treatment generates a new glass enthalpy vs. temperature curve and therefore the glass assumes a different volume and consequently a different density.

(11) Glass if rapidly cooled will tend to have higher residual stress, higher volumes, and higher fictive temperatures than glass that has been slowly cooled. Therefore, the fictive temperature of glass characterizes the glass structure and its thermal history.

(12) We can re-fictivate the glass by heating the it to its glass transition range, holding at that temperature for some period of time and then slowly/or rapidly cooling the glass back down to below the glass transition range.

(13) We can take advantage of this phenomena to shift the bending viscosities up or down, to allow for curving simultaneously dissimilar glass compositions made by dissimilar processes.

(14) Most of the worlds' flat glass is produced by the float glass process, first commercialized in the 1950s. In the float glass process, the raw ingredients are melted in a large refractory vessel and then the molten glass is extruded from the vessel onto a bath of molten tin where the glass floats. The thickness of the glass is controlled by the speed at which the molten glass is drawn from the vessel. As the glass cools and hardens, the glass ribbon transfers to rollers. Float glass thickness can typically vary by +/50 μm over a short distance due to what is known as draw line distortion. This is caused by the mechanical means used to draw the molten glass extruded from the vessel into a thin ribbon on the flat glass float line.

(15) We distinguish between the “air” side of the float glass and the “tin” side. A thin coating of tin remains on the glass which will fluoresce and thus can be detected with ultra-violet light.

(16) The fusion or overflow downdraw method is another technique for producing flat glass. The method has the advantage in that the glass surfaces never come in contact with other materials such as the molten tin as in the float glass process, leaving a surface with significantly less defects and other impurities when compared to float glass.

(17) The fusion method was originally developed in the 1960s as a low-cost method for manufacturing optically superior glass for automotive windshields but was replaced by the float glass method. Previously windshield had been made from plate glass which required grinding and polishing to improve the optical quality of the glass. The fusion technology was reintroduced to produce very thin glass for the flat screen display market. A sheet of glass is formed when molten glass overflows from a supply trough, flows down both sides, and rejoins (fuses) at the tapered bottom, where it is drawn away in sheet form. Glass produced by the fusion method tends to have higher residual stress than float glass, and much higher fictive temperatures than a similar glass composition formed via the float method.

(18) The following terminology is used to describe the laminated glazing of the invention.

(19) Typical automotive laminated glazing cross sections are illustrated in FIGS. 1A and 1B. A laminate is comprised of two layers of glass, the exterior or outer, 201 and interior or inner, 202 that are permanently bonded together by a plastic layer 4 (interlayer). In a laminate, 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 glass 2 surface that is on 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. An obscuration 6 may be also applied to the glass. 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 have a coating 18 on one or more of the surfaces. The laminate may also comprise a film 12 laminated between at least two plastic layers 4.

(20) FIG. 2 is an example of adjustment of the viscous-elastic region of the viscosity curve of a glass composition by subjecting the glass to a heat treatment. This Figure shows dilatometry results obtained on a borosilicate glass composition comparing three glass conditions, as-received glass samples and samples that were heat treated at two different rates. The later samples were firstly cut to the standard dilatometry size and heated to slightly above their T.sub.g temperature (around 550° C.). They were subsequently cooled at slow and fast controlled rates of 1° C./min and 50° C./min, respectively down to room temperature. Two samples of each type of glass were measured in a dilatometer at a heating measuring rate of 10° C./min in static air. FIG. 2 curves show the dilatometry results obtained for each of the glass types where ΔL is the difference between the initial and measured sample length (μm) and the temperature (° C.) is the sample temperature. The as-received dilatometry curve (curve named BG_AR) indicates a measured T.sub.f of 535° C. The slow cooling samples, or dead annealed samples (curve named BG_DA) show a shift in fictive temperature to T.sub.f of 517° C. T.sub.f was obtained using a dilatometry software by measuring the intersection of the tangent curves on the lower and the upper portions of the measured sample curves. It was possible to lower the T.sub.f and consequently the T.sub.g of the samples by submitting the glass samples to a heating treatment. It is important to note here that T.sub.g and T.sub.f are very close temperatures. This results clearly demonstrate that it is possible to re-engineer the visco-elastic region of the viscosity curve of a glass to allow for dissimilar composition glass bending.

(21) In several embodiments, the outer glass layer is a soda-lime glass or a borosilicate glass, and the inner glass layer is an aluminosilicate glass or soda-lime. In some embodiments, the fast cooling rate ranges from 20° C./min to 100° C./min, and the slow cooling rate ranges from 0.1° C./min to 20° C./min. In addition, in some embodiments, the outer glass layer is 2.1 mm or 2.3 mm thick, and the inner glass layer is 0.7 mm thick.

Example 1

(22) In the Example 1, a laminated glazing is comprised of an outer glass layer and an inner glass layer. The fictive temperature (T.sub.f) of the outer glass layer as received from the glass supplier is 570° C. and the inner glass layer as received from the glass supplier is 600° C. The T.sub.f of inner glass layer is shifted, prior to bending by heating the glass to a temperature over 600° C. and then cooling down at a slow cooling rate of 5° C./min. By doing so the T.sub.f may be shifted by up to 30° C. to about 570° C. The outer glass layer is not treated to shift its fictive temperature. This way, both glass compositions, outer and inner glass layers, may be bent together to its final shape. The gap between the mating surfaces of the first and second glass layers reaches no more than 2 mm, achieving a desirable surface match. The laminate may then be assembled and subjected to a lamination process.

Example 2

(23) In the example 2, a laminated glazing is comprised of an outer glass layer and an inner glass layer. The fictive temperature (T.sub.f) of the outer glass layer, as received from the glass supplier, is 570° C. and inner glass layer as received from the supplier is 600° C. The T.sub.f of outer glass layer is shifted prior to bending by heating the glass to a temperature over 570° C. and then cooling it down at a fast cooling rate of 50° C./min. By doing so the T.sub.f is shifted by up to 30° C. to about 600° C. The inner glass layer is not treated to shift its fictive temperature. Both glass layers, outer and inner, are bent together to its final shape. The gap between the mating surfaces of the first and second glass layers is about 1 mm. The laminate is assembled and subjected to a lamination process.

Example 3

(24) In the example 3, a laminated glazing is comprised of an outer glass layer and an inner glass layer. The fictive temperature (T.sub.f) of the outer glass layer, as received from the glass supplier, is 570° C. and inner glass layer as received from the supplier is 600° C.

(25) The T.sub.f of outer glass layer is shifted, prior to bending by heating the glass to a temperature over 570° C. and then cooling down to a slow cooling rate of 15° C./min. By doing so the T.sub.f is shifted by up to 15° C. to about 585° C. The T.sub.f of the inner glass layer is shifted by heating the glass to a temperature over 600° C. and then cooling down to a fast cooling rate of 30° C./min. By doing so the T.sub.f is shifted by up 15° C. to about 585° C.

(26) Both glass, outer and inner glass layers, are bent together to its final shape. The laminate is assembled and subjected to a lamination process.

(27) The forms of the invention shown and described in this specification represent illustrative preferred embodiments and it is understood that various changes may be made without departing from the spirit of the invention as defined in the following claimed subject matter.