Highly transparent impact resistant glass ceramic

202080/3 nwaa 'Jal iw nnw rna'pw byi 'n'lin-'ni'p

Highly transparent impact resistant glass ceramic

Schott AG C. 196881 202080/4

HIGHLY TRANSPARENT IMPACT RESISTANT GLASS CERAMIC

Field of the Invention

The present invention describes a highly transparent impact resistant glass ceramic.

Background of the Invention US 2007/0232476 A1 relates to a transparent glass ceramic plate with high-quartz mixed crystals as the prevailing crystal phase, which is exposed to operationally high thermal stresses and which has an opaque, colored, high temperature-stable bottom coating over the entire surface or over part of the surface. The glass ceramic plate has a content of Nd2O3 of 40 to 4000 ppm, a Yellowness Index of less than 10% with a 4 mm thickness and a variegation of colors of the glass ceramic in the CIELAB color system of C* of less than 5. US 6 846 760 B2 relates to a flat float glass that can be prestressed or transformed into a glass ceramic with high quartz mixed crystals or keatite mixed crystals. To eliminate undesirable surface defects during floating and to achieve superior characteristics of the glass or of he glass ceramic, in particular with regard to a low coefficient of thermal expansion and high light transmittance, the glass has a concentration of less than 300 ppb Pt, less than 30 ppb Rh, less than 1.5 wt. % ZnO and less than 1 wt. % SnO2 and is refined during melting without the use of the conventional fining agents arsenic oxide and/or antimony oxide. US 6 673 729 relates to a glass and a glass-ceramic comprising beta-quartz and/or keatite solid solutions, and to a process for their production, and to their use as substrate material for coating. Glass-ceramic comprising beta-quartz and/or keatite solid solutions with a surface roughness without polishing of Ra<50 nm, a thermal expansion in the temperature range between 20°C and 300° C of <1.2Ί0-6/ K and a transmission in the near infrared region at 1050 nm of >85% for a 4 mm thickness. CA 2 299 292 A1 discloses a translucent or opaque glass-ceramic containing β-quartz solid solution as the predominant crystal phase, having a composition (in % by weight) of Li2O 3-5, Na2O 0-1, K2O 0-1, Na2O + K2O 0.2-2, MgO 0-1.8, BaO 0-3.5, SrO 0-1, CaO 0-1, BaO + SrO + CaO 0.2-4, ZnO 0-2.8, AI2O3 17-26, SiO2 62-72, TiO2 0-2.5, ZrO2 0-3, TiO2+ ZrO2 1- <3.5, Sb2O3 0-2, As2O3 0-2, SnO 0- <1, and P2O5 0-8, an average coefficient of linear thermal expansion a2O-7oo°c of < 0.5-10'6/K, an average crystal size of the β-quartz solid solution of > 80 nm, and a transmission (sample thickness 4 mm) x380-780nm of< 30%. The glass-ceramic may be preferably used as a heatable plate for cooking and grilling, as a cooking utensil, as a stove window, and/or as a base plate for microwave ovens. 1 202080/4 EP 1 837 312 A1 relates to a lithium aluminosilicate glass which can be converted, with ceramicization times of less than 2.5 hours, into a transparent glass-ceramic comprising beta-quartz solid solutions as the predominant crystal phase, having a haze of less than 1% according to ASTM D 1003 and having a neutral color. WO 2003/068501 A1 discloses a transportation anti bullet glazing concept construction with controlled injury free splinter projection based on inorganic rearmost ply materials with scratch and abrasion resistance and chemical durability comparable to soda-lime glass with the central tension of the last ply facing the environment tailored to release injury free splinters without generating perforations on the aluminum witness foil of the respective standard(s) ballistic testes) for a given ballistic level and an induced residual field of vision after impact and said construction having an optimized cross section design in terms of weight (mass per unit area) and thickness reduction upon a ballistic level specific requirement.

Summary of the Invention

Until now, the inherent color of transparent glass ceramics has been too strong. The reasons for the inherent color of transparent glass ceramics can vary. The constituents of the raw material mixtures for the melts contain the coloring element Fe as an impurity. The use of the refining agents Sb2O3 and CeO2 also results in a slight inherent color. The described brownish-yellow intrinsic color of the transparent glass ceramics is based substantially on electronic transitions occurring on colored complexes which absorb in the region of visible light and in which the component required for nucleation, namely the Ti ion, takes part. The most frequent absorbing color complex stems from the formation of adjacent Fe and Ti ions between which electronic charge transfers take place. Sn-Ti complexes also impart an inherent color. The Fe/Ti color complexes lead to a red-brown discoloration and the Sn/Ti color complexes to a yellow-brown one. The formation of these adjacent color complexes takes place already during the cooling of the parent glass and particularly during the subsequent ceramization of the glass ceramic. In the melt, the ions are still uniformly distributed, but during cooling at high temperatures and during ceramization they preferably bind to each other. As a result, during the ceramization of the transparent glass ceramics, the inherent color intensifies very markedly compared to that of the parent glass. By absorption in the short-wave region of the visible spectrum, transparent flat glasses and: particularly the glass ceramics produced therefrom assume a pronounced inherent color which increases considerably with thickness.

It is known that the inherent color of glass ceramics can be reduced by overcoloring. The principle of overcoloring an undesirable color tinge naturally leads to stronger light absorption thus reducing the overall transmission, because the absorptions taking place are neutralized by the absorptions of complementary light portions by the overcoloring agent.

Glass ceramic plates find use in, among other applications, bullet-proof glass plates. In the production of such glass plates, several different glass or glass ceramic layers and plastic sheets are 1a 202080/4 linked. The temperature- and pressure-controlled process of linking the individual layers and plastic materials to each other, in particular, Is time-consuming and cost-intensive. The many 1b interfacial transitions between glass plates and plastic materials result in poor transmission characteristics which may lead to the formation of interference fringe patterns in the form of Newton fringes. Also, the large amount of glass, namely the high number of glass plates in the known bullet-proof plates results in their exhibiting a very high weight per unit area. The high weight per unit 5 area leads to a pronounced constructive cost of installation and vitrification.

Hence, one object of the present invention is to provide plates of glass ceramic free of the disadvantages shown by the prior-art plates and which are suitable for producing armored glass. 10 Another object of the present invention is to provide glass ceramic plates exhibiting high overall transmission for visible light. In particular, these glass ceramic plates show a high overall transmission for visible light which manifests itself in a high brightness value for transmitted standard light A under 2° observation (Ya/2-)· 15 Another object of the present invention is to provide plate laminates, comprising at least one glass ceramic plate, exhibiting high overall transmission for visible light and/or a high hardness. In particular, these plate laminates show a high overall transmission for visible light and their light brightness value Y for transmitted standard light A under 2° observation is Ya/2°> 50. 2 0 Another object of the present invention is to provide plate laminates, comprising at least one glass ceramic plate, said laminates exhibiting better resistance to dynamic stresses compared to plate laminates with conventional glass ceramic plates and the same weight per unit area.

Another object of the present invention is to provide plate laminates, comprising at least one glass 25 ceramic plate, said laminates exhibiting improved transparency thus ensuring bullet impact resistance meeting the requirements of NATO Standardization Agreement, STANAG 4569, Level 2 and 3. The plate laminates also ensure bullet impact resistance against armor-piercing projectiles, namely against armor-breaking projectiles. 3 0 The afore-indicated objectives are reached by providing a transparent plate made of lithium alumi nosilicate glass ceramic containing the following constituents in weight %, based on the overall composition: 35 2 10

Li2O AI2O3

SiO2

TiO2

SnO2

ZrO2 ΣΤϊΟ2+ΖγΟ2

MgO

Fe2O3

As2O3 3.0-4.5 18.0-24.0 55.0 - 70.0 0-2.3 0-0.2 0-2.0 0.5 -4.3 0-0.8 40 - 200 ppm and 0.3 - 0.9 wt. % as chemical refining agent.

The plate preferably contains from 40 to 130 ppm of Fe2O3, and the TiO2 content is preferably higher than 0.01 wt. % and particularly higher than 0.5 wt. %. 15 The reduction in Fe content is economically feasible only to a certain degree. A certain amount of Fe or Fe2O3 always enters the mixture with the industrially used raw materials for the production of the glass and with the abraded material from the units for the production, homogenization and transportation of the mixture. Because of the elevated cost of high-purity raw materials or of the special measures applied to industrial units, it is economically no longer feasible to drop the Fe2O3 2 0 content of industrially produced transparent glass ceramics below about 40 ppm.

For purposes of the present invention, by a glass ceramic is meant an inorganic, nonporous material with a crystalline phase and a glass phase in which, as a rule, the matrix, namely the continuous phase, is a glass phase. The combination of a crystalline and a glass phase imparts to a 2 5 glass ceramic its special properties.

For purposes of the present invention, visible light is light with a wavelength from 380 to 780 nm.

The brightness value Y of the CIE-xyY color-measuring system is always reported for transmitted 30 standard light A and for an observation angle of 2° and can be determined from wavelength-resolved transmission spectra with the aid of the CIE-defined eye-sensitivity curves χ(λ), y(A) and ζ(λ) (tristimulus curves) (International Illumination Commission Proceedings, 1931, Cambridge University Press, Cambridge, or DIN 5031): 35 40 3 «9 Ο # β λ: wavelength of monochromatic light; Ι(λ): intensity of monochromatic light [1(A) = τ(λ)χίΑ/2°(λ); wavelength-solved transmission of the sample τ(λ) multiplied by the wavelength-solved intensity 15 factor for standard light A observed under a 2° angle fA/2°]·

The factor coordinates can be calculated from:

X x =---

20 X + Y + Z

Y y =-----

X + Y + Z 25

Z z =----

X + Y + Z 3 0 The achromatic point for standard light A is defined by: xu,A = 0.4476 YUiA = 0.4074 35 The abbreviation CTE refers to the linear thermal expansion coefficient of a material that can be indicated for different temperature ranges. CTE (ΔΙ/ΙοΔΤ) in ppm/K is determined by dilatometric measurements with a Thermal Dilatometric Analyzer, Harrop model TD 710, in accordance with DIN ISO 7991. 4 0 Preferably, the transparent plate contains the following additional constituents, in wt. % based on the overall composition: 4

BaO 0 - 3

Na2O 0-1.5

ZnO 0 - 2.5 5 ZrO2 0.5 - 2

The oxides Li2O, AI2O3 and SiO2 are required constituents of the glass ceramic and are present within the preferred limits cited in the claims. In general, a minimum amount of 3 wt. % of Li2O is needed, but Li2O contents exceeding 4.5 wt. % often cause undesirable devitrification during the manufacturing process. An Li2O content of 3.2 to 4.3 wt. % gives particularly good results. 10

To prevent excessively high glass viscosities and to suppress the tendency toward undesirable devitrification during shaping, the AI2O3 content is limited to a preferred minimum of 18 wt. % to a preferred maximum of 24 wt. %. When the AI2O3 content is below 19 wt. %, the transparency of the glass ceramic is reduced. The SiO2 content should preferably amount to a maximum of 70 wt. 15 %, particularly 68 wt. %, because this component causes a marked increase in the viscosity of the glass. Hence, higher SiO2 contents are deleterious for the melting of the glasses because of the temperature stress exerted during shaping. The minimum SiO2 content should preferably amount to 55 wt. % and particularly 60 wt. %.

2 0 MgO and ZnO can be incorporated into the crystalline phase as additional components. Because of the problem of undesirable formation of crystal phases such as Zn spinel during ceramization, the ZnO content is limited to a maximum of 2.5 wt. % and preferably to a maximum of 2 wt. %. The MgO content is limited to a maximum of 0.8 wt. % and preferably to a maximum of 0.65 wt. %, because otherwise the expansion coefficient of the glass ceramic is increased inadmissibly. A

25 low MgO content is also advantageous in suppressing the inherent color of the glass ceramic. As a rule, a minimum MgO content of 0.3 wt. % is required to prevent the thermal expansion of the glass ceramic in the temperature range from 30 °C to 700 °C from decreasing to negative values below-0.3x106/K 30 The glass ceramic contains as nucleating agents TiO2 and ZrO2 and preferably only TiO2. SnO2 can also serve as nucleating agent. The amount of TiO2 used is preferably in the range between 1.8 and 2.3 wt. % and most preferably between 2.0 and 2.3 wt. %. The amount of ZrO2 is preferably in the range between 0.5 and 2 wt. % and most preferably between 1.5 and 2 wt. %. If the two nucleating agents are used at the same time, the sum of TiO2 + ZrO2 should not exceed 4.3 35 wt. % and preferably not 2.3 wt. %.

The afore-indicated amounts of nucleating agents allow ceramization to be carried out in a short time, preferably within a period of 1 to 2 hours. Because of the low amount of TiO2, the strength 5 of the inherent color is also reduced.

ZnO, MgO and BaO, are added to the composition to improve the melt properties of the glass ceramic and to stabilize the glass phase. ZnO also makes it possible to influence the thermal ex- 5 pansion coefficient (CTE) while MgO and BaO cause this coefficient to increase. Also, higher contents of the afore-said metals can affect the crystallization behavior during the conversion of the glass into the glass ceramic and they exert a deleterious effect on the thermal stress resistance of the glass ceramic. Preferably, the glass ceramic of the present invention contains 1 to 2.6 wt. % of ZnO, 0.3 to 0.8 wt. % of MgO and 0 to 3 wt. % of BaO. Most preferably, the glass ce- 10 ramie of the present invention contains 1 to 2 ZnO, 0.3 to 0.65 MgO and 0 to 2 BaO, each expressed in wt. %.

The glass ceramic of the present invention can also contain 0 to 1.5 wt. % and preferably 0 to 1 wt. % of Na2O. After ceramization, the alkali metal ions, for example the sodium ions, remain in 15 the residual glass phase. They increase the thermal expansion coefficient and can therefore be used when the thermal expansion coefficient values are too negative. If, however, they are used in excessive amounts, the thermal expansion becomes too high, and the nucleation is difficult to control. 2 0 The glass ceramics according to the invention are refined by use of arsenic oxide, the refining agent commonly used in the LiO2-AI2O3-SiO2 system. Alternatively or in combination, SnO2 can also be used in amounts of up to 0.2 wt. %, especially for high-temperature refining at > 1700 °C. Other refining agents, for example, Sb2O3, CeO2, sulfate compounds, chloride compounds or fluoride compounds, can also be added to the glass melt. The total amount of refining agents and 25 additives should not exceed 1.2 wt. %. Preferably, As2O3 is used as the only refining agent.

At low contents of As2O3, Sb2O3 or SnO2 refining agents, it may be necessary to combine the chemical refining with high-temperature refining above 1700 °C if good bubble quality with bubble numbers below 5 bubbles/kg of glass (based on bubble sizes >0.1 mm) is desired. It is parti- 30 culariy advantageous, if inherent color is to be avoided, to refine the glass ceramic exclusively with As2O3 as the refining agent and not to use antimony oxide and tin oxide as refining agents. Other refining agents, such as sulfate, chloride or fluoride compounds, may also be added in a total amount of up to 1 wt. %. 35 Preferably, the thickness of the transparent glass ceramic plate is in the range from 2 and 20 mm, particularly in the range from 4 and 15 mm and especially in the range from 6 and 12 mm. 6

The transparent plate according to the invention, at a thickness of 4 mm, preferably shows a brightness value Ya/2° of > 80, preferably > 85 and most preferably > 89.

The thermal expansion coefficient (CTE) between 30 and 700 °C is preferably in the range be- 5 tween -0.15 to +0.15x10'6/K and more preferably in the range between -0.05 to 0.1 x10’6/K.

The Knoop hardness of a glass ceramic according to the invention is determined in accordance with DIN ISO 9385, edition of 1991-01. The Knoop diamond is impressed under a load of 0.1 N for 20 seconds. The Knoop hardness of the glass ceramic plate of the invention is HK0.1/20 > 500, 10 preferably HKo.1/2o 550 and most preferably HK0.1/20 > 580.

Another object of the invention is a process for producing the glass ceramic of the invention.

It is generally known that glasses of the Li2O-Ai2O3-SiO2 system can be converted into glass ce- 15 ramies in which high-quartz mixed crystals are the main crystal phase. To this end, the following procedure is used: A suitable composition of suitable raw materials is melted, refined, homogenized and then, while hot, shaped to form a glass blank or green body, for example by rolling, casting, pressing or, recently, floating. By "green body" of a glass ceramic is meant a glassy body obtained from a melt of suitable composition which by treatment according to a suitable tem- 2 0 perature program can be converted into a glass ceramic.

The cooling and annealing of the molten green body is followed by a heat treatment whereby the glass is converted into a glass ceramic by controlled bulk crystallization. In the course of this heat treatment, in a first conversion step ("nucleation agent separation") crystallization nuclei of the 25 same or different kind are formed in the glass. By crystallization nuclei or crystal nuclei are meant submicroscopic crystalline aggregates of a characteristic size. In a second conversion step ("crystallization"), possibly at a slightly higher temperature, crystals or crystallites grow on the crystal nuclei. The glass ceramic of the invention is preferably produced in accordance with the following ceramization program: 30 heating to a temperature of 750 °C ± 20 °C and holding this temperature for 20 minutes ±15 minutes; further heating, for purposes of ceramization, to a temperature of 900 °C ± 20 °C and holding this temperature for 20 minutes ±15 minutes, then cooling to room temperature. 35

The cooling to room temperature requires no particular temperature program. As a rule, it is carried out by exposing the hot plates to room air. The cooling time usually ranges from 10 K/min to < -100 K/min [sic]. 7

Another object of the present invention is a transparent plate of lithium aluminosilicate glass ceramic which contains the following constituents, expressed in wt. % based on the total composition:

Li2O 5 AI2O3

SiO2

TiO2

SnO2

ZrO2 10 ZZrO2+TiO2 MgO Fe2O3 AS2O3 3.0-4.5 18.0-24.0 55.0 - 70.0 0-2.3 0-0.2 0-2.0 0.5 -4.3 0-0.8 40 - 200 ppm and 0.3 - 0.9 wt. % as chemical refining agent. 15 and is prepared in accordance with the following ceramization program: heating for the purpose of nucleation to a temperature of 750 °C ± 20 °C and holding this temperature for 20 minutes ±15 minutes; further heating for the purpose of ceramization to a temperature of 900 °C ± 20 °C and 2 0 holding this temperature for 20 minutes ±15 minutes, then cooling to room temperature.

The afore-described transparent glass ceramic plate is appropriate for the production of plate laminates with a high bullet penetration resistance and high transparency for visible and infrared light in the wavelength range between 380 and 1100 nm. Moreover, the thermomechanical pro- 2 5 perties of the glass ceramic according to the invention make it possible to use it in fire safety glaz ing, as fireplace sight glass, ceramic hob, substrate for semiconductor materials or substrate for magnetic storage plates.

Another object of the present invention is a transparent plate laminate which, in particular, shows 30 a high resistance to bullet penetration and a high optical transmission.

The plate laminate according to the invention comprises at least one transparent glass ceramic plate (a), as described in the foregoing, optionally at least one plate (b) selected from the group consisting of borosilicate glass, soda lime glass and aluminosilicate glass which can also be che- 35 mically or thermally prestressed, and at least one plate (c) selected from the group consisting of polycarbonate, polyacrylate, particularly poly(methyl methacrylate), cellulose acetate butyrate, nylon, polyolefin, polyester, polyurethane and a mixture thereof. 8

According to German industrial standard DIN 52290, part 3 (06/1984), and/or according to German industrial standard DIN 52290, part 2 (11/1988), the plate laminate of the invention exerts breakthrough-inhibiting properties. The plate laminate according to the invention also ensures bullet penetration resistance that meets the requirements of the NATO Standardization Agree- 5 ment, STANAG 4569, Levels 2 and 3.

Preferably, plate (c) consists of transparent polycarbonate (PC), poly(methyl methacrylate) (PMMA) or polyurethane (PU). 10 The number of plates in the transparent plate laminate is limited only by the requirements for high transmission it must meet The number of plates (a), (b) and (c) preferably amounts to 2 to 10, more preferably to 3 to 9 and most preferably to 4 to 8. A plate (a) is preferably oriented outward or is furthest away from the object to be protected. For armored glass, for example, this means that plate (a) is the first plate that must withstand external influences. Preferably, plate (c) is ori- 15 ented closest to the object to be protected. In the case of armored glass, this means that plate (c) is the plate that lines the internal space of a motor vehicle.

Preferred is a plate arrangement that shows the following plate sequence, in all cases from the outside (furthest away from the object to be protected) toward the inside (closest to the object to 20 be protected): 8-plate laminate (a) -(a)-(a)-(a)-(a)-(a)-(a)-(c) (b) -(b)-(a)-(a)-(b)-(b)-(a)-(c) 25 (a)-(a)-(a)-(b)-(b)-(b)-(b)-(c) (a)-(a)-(b)-(b)-(a)-(a)-(a)-(c) (a) -(a)-(b)-(b)-(a)-(a)-(b)-(c) 7-plate laminate 30 (a)-(a)-(a)-(b)-(b)-(b)-(c) (b) -(b)-(b)-(a)-(a)-(b)-(c) (a)-(a)-(b)-(a)-(b)-(b)-(c) 5-plate laminate [sic] 35 (a)-(a)-(b)-(b)-(a)-(c) (a)-(a)-(a)-(b)-(b)-(c) (a)-(b)-(a)-(b)-(b)-(c) 9

Plate (b) preferably has a thicknes ranging from 3 to 20 mm, particularly from 5 to 15 mm and most preferably from 5 to 10 mm. The thickness of plate (c) is preferably in the range from 3 to 15 mm, particularly from 5 to 15 mm and most preferably from 8 to 13 mm. The thickness of plate (a) in the plate laminate is preferably in the range from 3 to 20 mm, particularly in the range from 4 to 5 15 mm and most preferably in the range from 6 to 12 mm.

The thickness of the plate laminate is preferably in the range from 30 to 100 mm, particularly in the range from 40 to 80 mm and most preferably in the range from 60 to 80 mm. 10 Preferably, plates (a)-(a), (a)-(b) and (b)-(b), independently of each other, are attached to each other with a bonding agent selected from the group consisting of casting resins or reactive resins based on polyurethanes, polyvinylbutyral (PVB), cross-linked polyurethanes, partly cross-linked polyurethanes, polyureas, epoxides, unsaturated or saturated polyesters, polybutylene tere-phthalates (PBT), poly(meth)acrylates, silicones or silicone resin polymers, or from the group of 15 hot-melt adhesives, coatings or sealants selected from the group consisting of hot-melt adhesives based on polyethylene or copolymers thereof, particularly ethylene vinylacetate (EVA) or polyvinyl acetate or mixtures thereof. More preferably plates (a)-(a), (a)-(b) and (b)-(b), independently of each other, are connected to each other with a bonding agent selected from the group consisting of casting resins or reactive resins based on polyurethanes, polyvinylbutyral and hot-melt adhe- 20 sives based on ethylene vinylacetate. Most preferably, plates (a)-(a), (a)-(b) and (b)-(b) are connected with polyvinylbutyral or polyurethane in the form of a film.

Plates (a)-(c), (b)-(c) and (c)-(c), independently of each other, are attached to each other with a bonding agent selected from the group consisting of casting resins or reactive resins based on po- 25 lyurethanes, polyvinylbutyral (PVB), cross-linked polyurethanes, partly cross-linked polyurethanes, polyureas, epoxides, unsaturated or saturated polyesters, polyethylene terephthalates (PET), polybutylene terephthalates (PBT), poly(meth)acrylates, silicones and silicone resin polymers, or from the group of hot-melt adhesives, coatings or sealants selected from the group consisting of hot-melt adhesives based on polyethylene or copolymers thereof, particularly ethylene vinylace- 30 tate (EVA), or polyvinyl acetate or mixture thereof. Most preferably, the (a)-(c), (b)-(c) and (c)-(c) plates, independently of each other, are connected by means of a bonding agent selected from the group consisting of casting resins or reactive resins based on polyurethanes, polyvinylbutyral and hot-melt adhesives based on ethylene vinylacetate. Most preferably, the (a)-(c), (b)-(c) and (c)-(c) plates are connected by means of polyvinylbutyral in the form of a film. 35 A transparent plate laminate according to the present invention that uses the transparent glass ceramic plates of the invention shows a lower weight per unit area than do corresponding transparent plate laminates containing conventional glasses or glass ceramics. Preferably, the weight 10 per unit area of the plate laminate is in the range between 50 and 150 kg/m2, more preferably in the range between 50 and 120 kg/m2 and most preferably in the range between 50 and 100 kg/m2 and it meets the requirement of "NATO AEP-55 STANAG 4569 - Level II or 111" by stopping, for example, hard-core bullets of the 7.62x39 API BZ type striking at 695 m/s, and bullets with a 5 tungsten carbide core of the 7.62 x 51 AP type striking at 930 m/s or hard-core bullets of the 7.62 x 54R B32 API type striking at 854 m/s.

The thickness of the bonding agent layers preferably ranges from 0.05 to 2 mm, more preferably from 0.1 to 1 mm and most preferably from 0.2 to 0.8 mm. 10

Another object of the present invention is a process for producing the plate laminate that comprises the following steps: cutting out plates (a), (b) and (c), cleaning plates (a), (b) and (c), 15 - stacking up the plates and introducing a bonding agent into the spaces between the indi vidual plates (a), (b) and (c), possibly exposing the stack to heat treatment to activate the bonding agent, possibly under vacuum or by exerting high pressure on the stack, cooling the laminate. 20 A similar process for producing a laminate is described, for example, in US 2008-187721 AA (SO-CLIMA GmbH) or EP 0 331 648.

By the invention-provided possibility to use only few and possibly thin glass ceramic plates, the 25 transmission properties of the plate laminate of the invention are much improved. Also, the plate laminates according to the invention possess outstanding color neutrality because of the use of glass ceramic plates which can be thinner compared to those of the prior art.

Another object of the present invention is the use of a glass ceramic plate according to the in-30 vention or the use of a plate laminate according to the invention, as described in the foregoing, as part of an armored glass or as part of a bullet-proof vest.

The armored glass according to the invention ensures maximum energy absorption when a bullet strikes. This splinter-proof design preferably serves to protect the persons present in a motor ve- 35 hide or building provided with such an armored glass from being hit by flying glass splinters stemming from a glass pane hit by a bullet. 11

Description of the Figures

Fig, 1 Description of a plate laminate of the invention 5 G1 to G8 represent plates made of glass, glass ceramic or plastic material. K1 to K7 represent layers of bonding agents based on a plastic material, particularly films of poly-vinylbutyral or polyurethane that as a rule are currently used for the production of laminated glass. 10 Examples

Four different glass ceramics of different composition were prepared (data are given in wt. % based on oxides). The brightness value Ya/2- for a glass thickness of 4 mm and the linear thermal expansion coefficient (CTE) are given for the range between 30 and 70 °C. 15

To prepare the glass ceramics, first a green glass of the same composition was prepared in the usual manner. From this green glass , glass plates of the desired thickness were then made. The glass plates were then ceramized in the known manner to form glass ceramic plates, the nucleating agents having been formed in the glass at a temperature of 750 °C and a residence time of 20 20 minutes. The glass was then heated to the crystallization temperature of 830 °C. The crys tallization was allowed to occur at a temperature ranging from 830 °C to 900 °C, the temperature being raised from 830 °C to 900 °C at a rate of 10 K per minute with a 10-minute residence time at 900 °C.

25 To determine the brightness value Ya/2°, first the wavelength-dependent transmission τ(λ) of a 4-mm-thick sample of the glass ceramic was measured and standardized to normal light A Ι(λ) = τ(λ) x fA (λ) (λ = 350....800 nm) 30

Then Y was then calculated with the aid of the CIE-defined eye sensitivity curves yavg(A) (International Illumination Commission Proceedings, 1931, Cambridge University Press, Cambridge, or DIN 5031): 35

f- f/(A)XA)rfA 12

The linear thermal expansion coefficient CTE was determined in the 30...700 °C temperature range in accordance with DIN ISO 7991 by use of a Thermal Dilatometric Analyzer, Harrop model TD710. 5 The compositions, brightness values and expansion coefficients are collected in Table 1. 10 15 20 25 1 2 3 4 AtiOi 21,96 21,70 21,50 21,67 0,36 0,31 4 tia 0,81 4 «4 6,88 Bay FejO3 ΜχΟ 0,0050. 3,78 ' ijw 0,6110 3,76 0,0120 ' 3,66 0,0180 3,72' Μ0Θ 0,88 0,58 0,58 Na2o 0,55 0,63 0,52 0,52 §102 65,04 85,60 65,80 64,96 TI02 2,27 . 2,11 2,20.:. . 0®· ZnO 1,70 1,00 1,84 1,87 Ife.' :· i ·... ··:· 1,73 1,80 1,76 1,77 SnO·. <0,01 <0,01 <0,01 <0,01 . YtAflPh 4mtn 80,7 69,5 89,3 86,7 GTE (30...700¾) -0,08 4,11 4,1 4,1

The glass ceramic of the invention according to Example 4 was incorporated into a plate laminate. The plate laminate was prepared in an autoclave by common lamination methods using different glass and glass ceramic plates by interposing in each case one PVB film in accordance with the 30 process parameters recommended by the manufacturer of the PVB film (Butacite® Clear, manufactured by DuPont).

Eleven plate laminates of different structures were prepared and are summarized in Table 2. 35 In Table 2, G1 to G8 indicate the bullet-proof layers and K1 to K7 the layers of the lamination film. The constituents of the individual layers are numbered, and when necessary their thickness is indicated in mm. The numbering of the individual layers has the following meaning: 13 1: Glass ceramic of the invention as per Example 4, Table 1. 2: Floated borosilicate glass with an expansion coefficient CTE30...30D of 3.3x10'6K'1 (Borofloat® 33, Schott AG, composition in wt. % about 81 SiO2,13 B2O3, 4 Na2O + K2O, 2 AI2O3). 5: Prior-art glass ceramic (EP 1,837,312, Example 3, rounded off composition in wt. %: 65.3 5 SiO2, 21.8 AI2O3, 3.7 Li2O, 2.3 TiO2, 2 BaO, 1.7 ZnO, 1.8 ZrO2, 0.6 MgO, 0.5 Na2O, 0.3

As2O3, 0.1 Nd2O3). 6: Commercial impact-resistant poly(methyl methacrylate) (Plexi-glas Resist®, Evonic Industries, US 5,726,245 A). 7.: Polyvinylbutyrate film, thickness 0.38 mm (Butacite® Clear, DuPont). 10

To determine the weight per unit area, kg x m'2, a 50 x 50 cm2 plate was weighed and the result was converted into 1 x m2. The antiballistic limit m x s'1 was determined with a 7.62-mm caliber bullet weighing 10 g and containing steel core. Different bullet speeds were produced by means of different propelling charges. To determine the antiballistic limit, the test laminates were shot at 15 with projectiles of different speed, and the antiballistic limit was then determined on the basis of the impact pattern for the different speeds.

Table 2 clearly indicates that plate laminates of the glass ceramic of the invention show a lower weight per unit area than do plate laminates of a different composition but with a comparable 2 0 ballistic limit. 14

Ex ample G1 K1 G2 K2 G3 K3 G4 K4 G5 K5 2 2 2 2 2 6b 7 7 7 7 7 4 mm 10 mm 10 mm 10 mm 10 mm 10 mm 2 2 1 1 2 6a 7 7 7 7 7 4 mm 10 mm 8 mm 8 mm 10 mm 2 1 1 1 1 6 7 7 7 7 7 4 mm 8 mm 8 mm 8 mm 8 mm 1 1 1 1 1 5 7 7 7 7 7 3 mm 8 mm 8 mm 8 mm 8 mm 1 1 1 2 2 4a 7 7 7 7 7 3 mm 8 mm 8 mm 10 mm 10 mm 1 1 1 1 1 4 7 7 7 7 7 3 mm 8 mm 8 mm 8 mm 8 mm 1 1 1 2 2 3a 7 7 7 7 7 3 mm 8 mm 8 mm 10 mm 10 mm 1 1 1 1 1 3 7 7 7 7 7 3 mm 8 mm 8 mm 8 mm 8 mm 1 1 1 2 6 2a 7 7 7 7 3 mm 8 mm 8 mm 10 mm 12 mm 1 1 2 1 6 2 7 7 7 7 3 mm 8 mm 10 mm 8 mm 12 mm 5 5 5 5 6 1 7 7 7 7 8 mm 8 mm 8 mm 8 mm 12 mm

Table 2 G6 K6 G7 K7 G8 Thick ness mm, Wt./unity area, kg/m2 (A/2°) Antiballistic limit 2 2 6 7 7 78.66 158 52.35 820 10 mm 12 mm 1 1 6 7 7 70.66 151.5 51.2 830 8 mm 8 mm 12 mm 1 1 6 7 7 66.66 144 50.72 850 8 mm 8 mm 12 mm 1 1 6 7 7 65.66 147 49.84 860 8 mm 8 mm 12 mm 2 6 7 7 63.28 131 53.9 760 10 mm 12 mm 1 6 7 57.28 126 53.11 780 8 mm 12 mm 6 7 52.9 109 57.17 620 12 mm 6 46.9 105.5 56.6 630 12 mm 44.52 88 60.96 520 40.52 85 60.33 520 45.52 97.5 49.4 600

I