Method for producing building materials

The invention relates to a process for producing high-strength construction and decoration materials which resemble natural stone, are in the form of panels and are used for cladding facades, walls and floors, both indoors and outdoors, from mixtures of broken glass, mineral components and finely divided additives, with a specified composition. A sintering aid, in the form of a solution of metal oxychlorides or metal alkoxides or tetrachlorides, preferably titanium tetrachloride, is added to the mixture, which is then introduced into a heat-resistant mould. In this mould, the mixture is subjected to a conventional sintering operation. The sintering aid which has been added makes it possible in particular to reduce the maximum sintering temperature and to improve the quality of the surface considerably.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a process for producing shaped sintered glass or
 sintered glass ceramic bodies and, more particularly, to a process for
 producing shaped sintered glass or sintered glass-ceramic bodies as
 high-strength construction and decoration materials simulating natural
 stone and used for cladding facades, walls and floors, both indoors and
 outdoors, that includes introducing a planar, uniformly thick layer of a
 mixture of broken glass, a mineral component and finely divided additives
 into a heat-resistant mold and drying it at a temperature of 60 to
 100.degree. C.; heating the layer at a rate of at least 0.5 to 3.0 K/min
 to a sintering temperature; maintaining the sintering temperature for at
 least 20 minutes and then cooling the layer at a rate of at least 0.5 to
 3.0 K/min to room temperature.
 2. Prior Art
 Numerous natural materials are employed for decorative applications both
 indoors and outdoors. Marble and granite are particularly widely used and
 are employed in large quantities for the internal and external cladding of
 prestige buildings. In addition to providing the decorative impression, it
 is also imperative that the corrosion resistance and economic production
 be assured. Naturally occurring materials do not always fulfil these
 demands, since it is often impossible to produce sufficiently large
 surfaces which have a uniform impression and since the corrosion
 resistance and strength are not always optimal, owing to the porosity of
 the natural material.
 It is therefore known to replace the natural materials with artificially
 produced, glass-based shaped bodies which resemble natural stone and are
 in the form of panels.
 For example, DE 41 25 698 C1 has disclosed a glass ceramic material having
 a composition (in % by weight, based on oxide) of

SUMMARY OF THE INVENTION
 The invention is based on this known method in accordance with DE 44 16 489
 C1.
 The invention is based on the object of producing sintered glass or
 sintered glass ceramic panels using this process which have a visually
 effective surface quality as can otherwise only be produced by expensive
 and technically complex grinding and polishing operations. A further
 object is to reduce the maximum temperature for the sintering operation.
 According to the invention, this object is achieved by the fact that,
 starting from the process described at the outset, a sintering aid, in the
 form of a solution of a metal tetrachloride compound, a metal oxychloride,
 a metal alkoxide or a hydrolysis product is added to the mixtures.
 The measure according to the invention resides in the surprising effect
 that the sintering temperature can be reduced by up to 130 K by using even
 small quantities of dissolved compounds mentioned above, advantageously
 leading to savings on energy costs and thus making the entire process more
 economical and technically simpler. The use properties, such as for
 example the strength, remain unchanged or may even be improved with regard
 to the surface quality and the possibility of deforming and bending the
 shaped bodies, which are in the form of panels. Thus it has been found,
 for example, that panels made from sintered glass ceramic which have been
 produced, for example, with the aid of a tetrachloride additive, in
 contrast to panels which have been produced without this additive, can be
 deformed, in particular bent, without problems at 700.degree. C., which is
 highly advantageous.
 The abovementioned additives increase the sintering activity of the glass
 granules used. An indicator of this is an improved smooth flow of the
 glass, as manifested by an increase in the gloss and improvement to the
 planarity of the panel surfaces produced. According to the tests which
 have been carried out so far, this even allows relatively coarse glass
 granules, with grain size upper limits of up to 10 mm (previous standard
 grains: &lt;5 m) to be used for production according to the process of the
 invention without adversely affecting the tensile bending strength.
 It has been found that various metal tetrachloride compounds or metal
 oxychlorides or metal alkoxides, or the hydrolysis products thereof, can
 be used. Particular importance is attached to compounds containing metals
 of subgroups 4 and 5 of the periodic system and tin, such as tetrachloride
 compounds or oxychlorides or alkoxides of tin, of vanadium or of zirconium
 or hydrolysis products thereof.
 Titanium tetrachloride (TiCl.sub.4) is preferably used, since tests have
 shown that this compound provides a particularly marked effect.
 In principle, the solution may be an aqueous solution. However, it has been
 found that the abovementioned additives, in particular titanium
 tetrachloride, are particularly effective as sintering aids if they are
 added when dissolved in ethanol or other alcohols.
 In this context, the word solution is also taken to mean a suspension and a
 colloidal system.
 The metal alkoxides may be added as such in solution. They may also be
 formed only in the alcoholic solution by the metal reacting therein.
 The additives make up from 80 to 600 ppm of the total batch. By way of
 example, a significant reduction in the maximum sintering temperature
 required can be observed even with a low level of added tetrachloride
 within the range from 80-250 ppm, preferably in the vicinity of 160 ppm.
 The sintering temperature for large-area panels can be reduced from
 990.degree. C. to 900.degree. C. without having any adverse effect on the
 strength of the material.
 The gloss and planarity of the surface of the panels produced in this way
 approach the quality of polished surfaces.
 A visible effect occurs even with a TiCl.sub.4 addition of 160 ppm, the
 TiCl.sub.4 advantageously being introduced in dissolved form.
 Additives, in particular tetrachloride additives &gt;400 ppm, make it possible
 to produce panels with a completely planar yet obscured surface, which
 otherwise can only be achieved by means of a further process step, such as
 sand-blasting or grinding.
 The abovementioned smooth flow of sintered glass materials is produced by
 the corresponding dioxides of the abovementioned additives, which dioxides
 is [sic] formed by decomposition of the added compounds, in particular of
 the tetrachlorides, in some cases at the sintering temperature. Clearly,
 this is only dependent on the dioxide formed being distributed in a
 sufficiently finely divided and uniform manner on the surface of the glass
 grains, and this is achieved via the route of dissolving the additives in
 a suitable solvent. The effect is particularly pronounced in the case of
 TiO.sub.2 which is formed from components which have been dissolved in any
 way whatsoever. If the TiCl.sub.4 is dissolved, for example, in water,
 TiO.sub.2 is formed and at the same time finely distributed. If it is
 dissolved in alcohol, it is finely distributed and is only formed as a
 result of sintering.
 The tetrachlorides listed above, namely of tin, vanadium, zirconium and
 titanium, in principle cannot be dissolved in water, since they are
 immediately hydrolyzed. This hydrolysis also takes place in an
 alcohol/water mixture. By way of example, when TiCl.sub.4 is hydrolyzed, a
 water-containing compound TiO.sub.2.H.sub.2 O (previously also known as
 metatitanic acid, H.sub.2 TiO.sub.3) is formed. As far as it has been
 possible to establish given current knowledge, partially or completely
 hydrolyzed reaction products formed from the reaction of the halogen
 compounds of the additives generally represent the substance which is
 actually active in the effect described. For this reason, it is also
 possible to use other compounds which hydrolyze in a similar manner to
 alcohol or water, e.g. titanium oxysulphate TiO (SO.sub.4), previously
 also known as titanyl sulphate. Therefore, solutions which are formed from
 the reaction of water with organic titanium compounds of the general
 formula Ti(OR).sub.4 are also suitable. These titanium alkoxides which
 contain alkyl radicals of up to about four carbon atoms also hydrolyze
 more or less rapidly, initially forming oligomers, to give the end product
 TiO.sub.2, which, as described, is ascribed particular importance. Such
 titanium alkoxides or suitably halogenated titanium alkoxides of the
 general formula Ti(OR).sub.n X.sub.m, where X=halogen and m.ltoreq.n, are
 therefore also suitable.
 Although the abovementioned DE 44 16 489 C also describes SnO or TiO.sub.2
 as finely divided additives, these additives are not added in dissolved,
 i.e. impregnatable, form and therefore cannot act as sintering aids in the
 sense of the invention.
 The effect of the additive according to the invention consists in a very
 considerably improved planarity of the panel surface, combined with a
 significantly improved gloss. Consequently, in principle it is possible to
 use glass granules with grain size upper limits of up to at least 10 mm,
 which would otherwise lead to surfaces with excessive unevenness and hence
 to problems in grinding and polishing.
 With a high level of added tetrachloride (&gt;400 ppm) it is evident that too
 many oxide particles will be deposited in the surface of the grains, which
 particles again impair the sintering activity and hence the gloss. The
 panel surface becomes rougher, but its improved planarity is retained. The
 panel surfaces then have the attractive appearance of a ground or
 sandblasted material.
 The sintered glass or sintered glass ceramic panels according to the
 invention are produced in accordance with the process steps described in
 DE 44 16 489 and the examples described therein.
 After the homogenized and compacted mixtures have been placed in the
 heat-resistant moulds with the addition, according to the invention, of an
 ethanolic tetrachloride solution as sintering aid, the heat treatment
 begins with drying in the mould at temperatures of between 60 and
 110.degree. C. This is followed by the conventional sintering process
 steps which are known per se in the glass and ceramic industry, are
 explained in the abovementioned DE document in process steps 1.3-1.5 and
 are also incorporated in the present application. However, the measure
 according to the invention makes it possible to reduce the maximum
 sintering temperature of 990.degree. K. by up to 130 K, resulting in the
 advantages described at the outset.
 If the abovementioned DE document makes statements relating to the
 composition of glass materials, the grain size of the glass granules used,
 the added mineral components and the finely divided additives, this
 information is also incorporated in the disclosure of the present
 application.
 The results of laboratory samples given in Tables 1 and 2 are intended to
 illustrate on a comparative basis certain properties of the sintered
 panels produced using the process according to the invention.
 Table 1 summarizes, on a comparative basis, the results of tests relating
 to the optimization of the surfaces of the sintered panels produced using
 the process in accordance with the embodiment of the invention where the
 sintering aid employed is the titanium tetrachloride solution. In this
 table, the quantities of titanium dioxide formed from titanium
 tetrachloride during sintering, which quantities are equivalent to the
 titanium tetrachloride added, are shown for three maximum sintering
 temperatures Tmax.
 The table shows that even at the relatively low maximum sintering
 temperature of 860.degree. C., a titanium dioxide content of 35-110 ppm
 makes it possible to produce a smooth, planar panel with a satin-glazed
 sheen, while at the higher sintering temperature of 900.degree. C. the
 surface has a satin-glazed gloss. A titanium dioxide content of 70 ppm
 corresponds to a titanium tetrachloride content of 160 ppm.
 Table 2 shows a comparison between the tensile bending strength of a
 sintered panel produced using the abovementioned process according to the
 invention with and without an addition of 160 ppm of titanium
 tetrachloride, or 70 ppm of titanium dioxide.
 The sintered panel tested was produced in a hearth car furnace at a maximum
 sintering temperature of 900.degree. C., with a holding time of 60 minutes
 at this temperature. Compared to a panel produced under otherwise
 identical parameters but at Tmax=990.degree. C. with a holding time of 120
 minutes, the planarity and gloss of the surface was significantly
 improved. Comparing the Weibull distribution analysis for the tensile
 bending strength in the context of the measurement accuracy reveals no
 change from the reference sample, as shown by the comparison presented in
 Table 2, which moreover contains information for two different grain sizes
 and different sizes of the panel.
 The abovementioned Table 2 in particular shows that the process according
 to the invention maintains the mechanical property values even with a
 reduced maximum sintering temperature and a holding time which has been
 reduced by half.
 TABLE 1
 T
 max TiO.sub.2 content [ppm]
 [.degree. C.] 0 35 70 110 140 180
 210
 860 Surface: -- smooth, smooth, smooth, slightly slightly slightly
 planar planar planar rough, rough,
 rough,
 planar planar
 planar
 Gloss: -- satin satin satin matt/satin matt/satin
 matt/satin
 glaze glaze glaze glaze glaze
 glaze
 880 Surface: smooth, smooth, slightly smooth, smooth, smooth,
 slightly
 corrugated planar rough, planar planar planar
 rough,
 planar
 planar
 Gloss: satin satin satin satin satin satin
 satin
 glaze glaze glaze glaze glaze glaze
 glaze
 900 Surface: smooth, smooth, smooth, smooth, smooth, smooth,
 smooth,
 corrugated planar planar planar planar planar
 planar
 Gloss: satin glossy/ glossy glossy/ glossy/ glossy/
 glossy/
 glaze satin glaze satin satin glaze satin
 glaze satin glaze
 glaze
 TABLE 2
 Panel containing 70 ppm Panel containing 70 ppm
 TiO.sub.2 TiO.sub.2
 Grain size &lt; 5 mm Grain size &lt; 10 mm
 Format 1200 .times. 900 mm Format 300 .times. 300 mm
 Panel without TiO.sub.2
 Charact. value [MPa] 38 40 41
 Weibull's modulus 13 12 5
 5% quantile [MPa] 30 31 30
 Sample number 6 8 10