WO2004103921A1

WO2004103921A1 - Glass and glass-ceramic articles and process to prepare same

Info

Publication number: WO2004103921A1
Application number: PCT/BR2004/000079
Authority: WO
Inventors: Dutra Zanotto Edgar; Eduardo Bellini Ferreira; Cátia FREDERICCI; Miguel Oscar Prado
Original assignee: Universidade Federal De São Carlos-Ufscar
Priority date: 2003-05-26
Filing date: 2004-05-26
Publication date: 2004-12-02
Also published as: BR0301484A; BRPI0301484B1

Abstract

Glass and glass-ceramic articles are obtained using a process based on the soda-lime-silica (NCS) system, the process comprising preparing frits by melting suitable NCS compositions between 1300°C and 1600°C, milling to suitable particle size distribution, limited to particle sizes < 15mm, compacting and sintering between 720°C and 1100°C, so as to favor viscous flow before the limiting occurrence of surface crystallization, to obtain maximum densification or, alternatively, interrupting the sintering process to obtain porous articles; the process parameters being optimized with the aid of a mathematical model and algorithm that allow one to attain suitable porosity and crystallinity for the end product. The glass and glass-ceramic articles that can be obtained are also described.

Description

GLASS AND GLASS-CERAMIC ARTICLES AND PROCESS TO PREPARE SAME

FIELD OF THE INVENTION

The present invention refers to glass and glass- ceramic articles and to a process for obtaining same through melting, fritting, milling and particle size distribution adjustment of colorless and colored glasses of the soda-lime- silica Na₂0-CaO-Sι0₂ (NCS) system, and further mixture of the frits according to color or particle size distribution criteria, compaction in refractory molds, viscous flow sintering thermal treatment and, for glass-ceramics, simultaneous or subsequent crystallization.

Sintering treatments with or without concurrent crystallization can be designed and optimized using mathematical models and algorithms via computer simulations developed specifically for this purpose. Although the properties of the so- obtained products are similar to those of present glass-ceramics, the fact of being based on a different chemical system - soda- lime-silica having compositions very close to those of window or container glasses, renders the present product as good as, but cheaper than other glass-ceramics. The proposed process makes possible to obtain monolithic, sintered pieces of variable porosity, from very low (close to zero) up to the order of 40% by volume. The invention is therefore also directed to glass or glass-ceramic articles resulting from the inventive process.

BACKGROUND INFORMATION

One of the basic concerns during the manufacture of vitreous articles has always been to avoid devitrification or uncontrolled crystallization during the manufacturing process. On the other hand, research and development work led to the manufacture of the so-called glass-ceramics, those being polycrystallme, ceramic materials made by the controlled crystallization of glass articles.

Such materials can be obtained through a conventional, three-step method: melting, shaping and thermal treatments for crystal nucleation and growth in the volume of material. In this case the introduction of nucleating agents in the glass composition is generally required.

Alternatively, another well-known method comprises the sintering of powdered glasses, with simultaneous or 5 subsequent superficial crystallization of the glassy particles. One advantage of this second method is that it does not require nucleating agents in the glass composition, since the vitreous particle surfaces themselves work favorably towards this end.

Sintered glass-ceramics known as Neoparies , the

10 main crystalline phase of which is wollastonite (CaO.Si0₂), are manufactured by the Nippon El ectric Gla ss company, however, their high price impair their commercialization for several applications .

Sintering allows obtaining glass-ceramic pieces y \ 5 of complex shapes such as design articles, kitchen appliances, basin cubs, sanitary articles, among others, of wide size scale, besides products of simpler geometry, such as flat or curved plates (floor and wall tiles) .

On the other hand, contemporary architecture more

20 and more demands wall and floor tiles having a wide variety of technical as well as aesthetic requirements. Thus, the demand for large-dimension (larger than 0.5 x 0.5 m) , monolithic curved or flat pieces has largely increased.

The resulting products can be white or colored, 25 polished or rugged, are attractive - due to their similarity to natural stones - and are fairly resistant to chemicals as well as to failure and scratching. Such products can be used instead of the conventional ceramic wall and floor tiles or translucent natural stones such as marble and granite. 30 The patent literature is abundant in publications on glass and glass-ceramics for tiles.

One class of these glass-ceramics materials is based on β-wollastonite (CaO.Si0₂) . The resulting products are sold as Neoparies . 35 Thus, US patent 3,955,989 teaches a process and formulation for producing partially crystallized glass articles containing numerous β-wollastonite crystals that are oriented in several directions. The objective is to mimic natural translucent materials, such as marble. The properties of these glass-ceramics such as resistance to staining and abrasion, and mechanical strength are fairly good. The process involves placing glass 5 grains of diameters between 2 and 5 mm in refractory molds and heating them to about 1150°C for 5 minutes, at heating and cooling rates of 2°C/minute.

The same US patent cited hereinbefore teaches a method for coloring the obtained material by wetting the surface

10 before the thermal treatment with a 10% NiCl aqueous solution. Finally, it is taught that the surface of the material becomes shining and attractive after mechanical polishing. The composition of the employed frit was (wt. %) 19.1 CaO, 6.8 Al₂0₃, 59.1 Si0₂, 1.7 Na₂0, 0.6 B₂0₃, 6.8 ZnO and 4.3 BaO. In one of the examples, • 15 such frit is admixed to a black frit containing 5 wt . % Fe₂0₃, so as to obtain a black spot glass-ceramic after sintering.

US patent 3,964,917 broadens the formulation of US 3,955,989 into (wt.) 15-25% CaO, 3-13% A1₂0₃, 50-60% Si0₂, 2-10% ZnO, including further variable amounts of minor elements.

20 US patent 4,054,435 teaches a method whereby by lying on a refractory mold a first layer of glass particles, followed by a refractory film, or by bubble-containing particles, and finally a top layer of a few millimeters of again glass particles, it is possible to obtain porous glass-ceramics and

25 therefore, lighter thermal insulators, where only the top layer is dense and impermeable. The compositions are further extended to

(wt.) 15-40% CaO, 3-13% A1₂0₃, and 40-75% Si0₂, with the remainder being completed with what is being designed as "compatible" components. Twenty compositions are listed, none of them having

30 less than 56.6 wt . % Si0₂.

Further publications are related to sintered glass-ceramics. Thus US patent 5,066,524 teaches how to obtain glass-ceramics of several colors. In this case the addition of a second "crystallizable" glass is suggested, containing only a

35 small amount (~0.1%) of transition element oxides. By way of examples of these colored frits are cited the formulations (wt. %) 15-25 CaO, 15-25 A1₂0₃, 40-59 Si0₂, 0-12 MgO, 0-12 ZnO, 2-10 B₂0₃, 4-13 Na₂0, 0-5 K₂0, 0-5 BaO, 0-1 As₂0₃, 0-1 Sb₂0₃ + transition oxides. The role of each oxide in the glass (these being well- known literature data) is explained, as well as the color of the resulting glass from each transition element added to the frit and 5 - after crystallization - to the glass-ceramics and finally, three examples of glass-ceramics compositions are provided. The exact amount of dye is not informed.

US patent 5,275,978 extends US patent 5,066,524 to several well-known inorganic pigments. This aims at

10 diversifying the color of the obtainable glass-ceramics.

US patent 5,403,664 suggests some modifications in the original compositions, which, according to the inventors, improve fusibility and control the devitrification of the frit. The final listed composition are (wt. %) 6-16.5 CaO, 1-15 A1₂0₃, ■-I5 50-75 Si0₂, 0-1.5 MgO, 2.5-12 ZnO, 0-1.5 B₂0₃, 0.1-15 Na₂0 + K₂0, 0- 12 BaO, 0-1 As₂0₃, 0-1 Sb₂0₃, 0.1-5 Li₂0, 0-1.5 SrO, 0-1 Ti0₂, 0-1.5 Zr02, 0-1.5 P₂0₅ and up to 10 of oxide dyes. Specific composition examples, thermal treatments as well as the resulting glass- ceramics properties are supplied.

20 Additional glass-ceramics can be obtained from diopside (CaO. gO.2Si0₂) , with potential resistance to chemical attack and mechanical strength higher than those of wollastonite, hence the interest in these materials.

Bulgarian patent BG50879 relates to a process

25 that is similar to those based on wollastonite, but uses a different chemical composition, leading to a glass-ceramics containing between 50-60% of diopside crystals instead of wollastonite. The inventors claim that glass grains of 1 to 10 mm compacted in a mold are heated at 10°C/min up to 1150°C where they

30 are kept for 30 minutes, and the so-produced sintered piece is cooled at 10°C/min. The resulting properties are similar to those of glass-ceramics based on wollastonite and better than those of marble and granite.

US patent 5,061,307 also teaches compositions

35 that have diopside as the main crystalline phase. Six examples of specific compositions, thermal treatments and resulting properties are provided. A further group of patents, some of which are summarized below, relate to the use of particles from recycled glasses of the soda-lime-silica system. However, due to the presence of impurities and of various different compositions for 5 each family of recycled glass, the use of such materials of distinct origins does not allow the easy control of the viscous flow sintering rate and its competition with crystallization - that prevents sintering.

JP 9086942 teaches a process similar to those

10 previously described using cullet from bottles to obtain monolithic glass blocks. In this case milled glass are used, having particle diameter from 1 to 50 mm, heated between 700 and 1200°C (according to the authors this would prevent crystallization) to partially melt the particles and agglomerate --15 them. The resulting vitreous blocks are designed for use as building materials. However, we know that heating at temperatures below 900°C makes it very difficult to avoid crystallization (that prevents sintering) , this rendering the proposed process of a rather difficult industrial execution. In addition, it should be

20 emphasized that at about 1030 °C the crystals melt and then the whole material can flow and sinter.

In spite of the fact that the present invention equally contemplates the use of recycled soda-lime-silica glass, in an amount of up to 100% of the whole composition, such cullet

25 should be re-melted, fritted and comminuted in a controlled way to get pristine particles, which are further agglomerated and treated according to specific thermal treatment projects to obtain an end product having the desired properties.

US patent 5,649,987 teaches a sintering method

30 for milled glasses that may originate from recycled television tubes, flat or container glasses, having compositions in the range (wt. %) 55-82 Si0₂, 1-4 A1₂0₃, 2-16 Na₂0, 0-10 K₂0, 0-5 MgO, 0-12 CaO, 0-3 PbO, 0-15 BaO, 0-15 B₂0₃, 0-11 SrO, 0-1 ZnO, 0-3 Zr0₂, 0-1 Ti0₂, 0-1 Ce0₂, 0-1 Sb₂0₃, 0-1 As₂0₃ and 0-1 F, of particle size

35 distribution from 0.2 to 3 mm, and in amounts of from 85 to 98% by weight in the mixture. To complete the mixture, they add from 0 to 14.7 wt % of a mineral component or a mixture of same is added, selected among of limestone, sand, recycled ceramic materials or slag, at particle sizes distributed under 0.4 mm, and of from 0 to 5% of fine additives (< 60 μm) , that can be Zr0₂, MgO, SnO, CaO, Ti0₂, kaolin and ZrSi0₄, or still, inorganic pigments, metal 5 oxides, colorless or colored glasses or powdered metals. Also added is from 1 to 3% water to implement the mixture and avoid segregation, such water being later on eliminated by drying the material in the molds at temperatures between 60 and 110°C before sintering. A layer made up of 100% milled glass, having from 3 to

10 7 mm thickness, can also be added to the surface of the material to be sintered. Pieces are heated at 0.5-0.3 K/min and finally sintered between 720 and 1100°C, for 20 to 120 minutes. Using this process the inventor states that it is possible to obtain materials that bear similarity with natural rocks, and that can be ^,JI5 used in design and for tiles at faςades, walls and floors.

US patent 5,536,345 teaches a process similar to those previously described but using complex thermal treatments and three layers: a sand layer at the bottom of the refractory mold, another one of crushed glass of several origins such as

20 windows, bottles, television sets, metallurgical slag, etc and another one of colored frit at the surface, all being sintered together.

The sintering of pulverized recycled glass does not allow one to directly obtain materials with nearly zero

25 porosity. Recycled glass fragments from glass packaging (or any other source) always carry hard-to-eliminate dirt resulting from the discarding and collection process. Besides, the inherent mixture of glasses from various origins results in particles of distinct chemical compositions, even if only slightly different,

30 and consequently, of different viscous flow sintering rates and crystallization kinetics.

The presence of dirt and different glasses in the mixture limits the control of the process for obtaining materials that would be free from porosity or having a controlled porosity.

35 Besides, other materials, such as sand and slags, are also used and undergo complex thermal transformations, without any relationship with the concept of the present invention (that uses pure, home made customized glass) .

International published application WO02/26650 teaches one to obtain materials for tiles by sintering vitreous silicate frits. The chemical composition, the thermal treatments and the particle size distribution are referred to only in general terms, so as to refer this process to already known patents and articles. This publication claims the process for obtaining such materials by the piling of several layers of frits of particular features, aiming at obtaining different aesthetic effects in the end product .

Contrary to publications that suggest the use of particles from milled recycled glass to the same end, the present application presents the manufacture of frits (that is, a glass of controlled composition, that melts at pre-determined temperatures and times, sufficient for the complete reaction between the raw materials and homogenization, and is quickly cooled and milled at previously designed particle size distributions) especially for sintering and simultaneous or subsequent crystallization, in this way avoiding problems with dirt and heterogeneity.

Thus, according to the present invention, the process for obtaining glasses or glass-ceramics from colorless and colored glasses of the soda-lime-silica Na₂0-Ca0-Si0₂ (NCS) system is rigorously designed, with strict control of the sintering and crystallization degrees, by obtaining thermal treatment parameters through computer simulations, using an algorithm specially developed by the Applicant for this purpose. Detailed descriptions of such algorithm can be found in several publications by the same authors of the present application. One of these publications is the article by E.D. Zanotto and M.O Prado, "Isothermal sintering with concurrent crystallization of monodisperse and polydispersed glass particles". Part 1 in Physics and Chemistry of Glasses , vol 42, n " 3, June 2001 , pp . 191 -198. A further publication is the article by M. 0 Prado, C. Fredericci , E. D. Zanotto - "Glass sintering with concurrent crystallization Part II. Non-isothermal sintering of jagged polydispersed particles" in Physics and Chemistry of Glasses, vol . 43, n . 5, October 2002, pp. 215-223. The proposed process can still contemplate the use of a certain amount of recycled glass in the composition of the frits, with this amount attaining up to 100%, provided those are re-melted, fritted and comminuted in a controlled way. Thus, it is observed that in spite of the technological progress in the processes for manufacturing glass and glass-ceramic materials, the technique still needs a process based on the NCS system where the frit is fabricated to have a strictly controlled composition and particle size distribution and the thermal treatment is rigorously designed, with a strict control of the sintering and the crystallization degrees through computer numerical simulations, so as to make possible to obtain monolithic sintered pieces having: i) specific crystalline phases formed after crystallization; and ii) variable porosity, from a very low (close to zero) up to the order of 40% of the volume, such process being described and claimed in the present application.

SUMMARY OF THE INVENTION

Broadly, the present invention relates to a process for obtaining glass and glass-ceramic articles by the mixture of the raw materials, melting, fritting, milling and particle size distribution adjustment of colorless and colored glasses of the soda-lime-silica (NCS) - Na₂0-Ca0-Si0₂ - system and then, mixture of the frits according to coloring or particle size distribution criteria, compaction in refractory molds, sintering treatment between 720°C and 1100°C, so as to favor viscous flow before the limiting occurrence of the superficial crystallization, for obtaining maximum densification or alternatively interrupting sintering by temperature reduction to obtain highly porous materials and, in case of the glass-ceramics, simultaneous or subsequent crystallization, with the aid of a mathematical model and algorithm for computer simulation in order to obtain the desired porosity and crystallinity .

Thus, the present invention provides a process for obtaining glass or glass-ceramic articles based on the soda- lime-silica system by mixing the raw materials, melting, fritting, milling and particle size distribution adjustment of colorless and colored glasses and further mixture of the frits, compaction and sintering thermal treatment with the aid of a mathematical model and computer simulation algorithm, the sintering occurring with or without crystallization. The invention provides still a process for obtaining glass or glass-ceramic articles based on the NCS system where the thermal treatment process is rigorously designed, having a strict control of the sintering and crystallization degree, through computer simulations using an algorithm developed by the Applicant.

The invention provides further a process for obtaining glass or glass-ceramic articles based on the NCS system that makes possible to obtain sintered monolithic pieces of variable porosity, from very low (close to zero) and up to the order of 40% of the volume.

The invention provides still a process based on the above-mentioned numerical simulation for obtaining glass or glass-ceramic materials based on the NCS system that favors the viscous flow sintering before the limiting occurrence of the superficial crystallization.

The invention provides also glass or glass- ceramic articles based on the NCS system having typical features of 3 point bend strength of from 40 to 100 MPa, Vickers Microhardness 500 to 600 Pa, and chemical durability (% by weight loss) in the presence of 0.01 M HC1 (95°C/1 h) 0.1 to 0.3 and in the presence of 0.01 M NaOH (95°C/1 h) , 0.2 to 0.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 attached is a flowsheet that illustrates the process of the invention. FIGURE 2 attached is a flowsheet that illustrates the algorithm developed by the Applicant and used in the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODES According to the invention, the expressions article and ceramic material are used having a very close meaning.

Still according to the invention, the glass- ceramic materials resulting from the proposed process are called NCS-based glass-ceramics, in an abbreviated form, glass- ceramics/NCS or GC/NSC

Also, the term "frits" encompasses as synonyms, "beads" and "pellets". The soda-lime-silica (NCS) system is technologically very important since conventional packaging and window glasses and others commodity glasses having diverse applications are derived from said system. The wide application of this system is due to the combination of excellent properties such as chemical stability, transparency (even if the glasses can also be colored and even opaque) , abrasion resistance and thermal insulation .

In spite of the fact that the three main components, Na₂0, CaO and Si0₂ convey the name to this class of materials, other components are added to the composition in order to adjust the required properties to the manufacture and further use. Thus, an example of typical average composition in this system is (wt. %) 72 Si0₂, 14 Na₂0, 11 CaO, 1 MgO and 2 A1₂0₃. In practical terms, a small amount (0.1 wt . %) of sulfates is also added to help removing bubbles.

The combination of Na₂0, CaO, A1₂0₃ and Si0₂ is used to suppress the liquid-liquid immiscibility occurring in binary systems of alkaline and alkaline-earth silicates and thus prevent the liquid-liquid phase separation that degrades the hydrolytic resistance of these materials, besides reducing the tendency to devitrification. In this system, crystallization mainly produces the crystalline phases crystoballite (Si0₂) and devitrite (Na₂0.3Ca0.6Si0₂) , that convey to the material improved hardness and abrasion strength. The satisfactory manufacture of these materials by sintering at temperatures below 1100°C has been made possible only after the progress of the knowledge and control of the viscous flow sintering and concurrent crystallization kinetics.

The process for producing sintered glass and glass-ceramic materials from glasses of the soda-lime-silica (NCS) according to the invention comprises the following steps: i) Providing raw materials for the manufacture of controlled composition frits, mixing the raw materials, melting at controlled temperatures between 1300°C and 1600°C, for predetermined periods of time, homogenizing, refining and quickly cooling (fritting) in water or in rolls (or any other process for the manufacture of particulate glasses) so as to obtain frits, beads or pellets; colored frits can be obtained based on the composition of colorless frit, by adding small percentages of transition elements: Fe, Cr, Co, Ni, Ti, V, Mn, Cu; or rare-earths (Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) ; ii) Milling at previously designed particle sizes, smaller than 15 mm, and effect the particle size distribution adjustment of the particulate material according to the aesthetic design and porosity of the final article. For example, particle size distributions with average size around ~ 4 mm can lead to granite-like glass-ceramics; iii) Admixing the colorless and colored glass particles according to the design of the final article; iv) Settling the particulate material using gravity or vibration in refractory molds; of from 1 to 5% by weight of conventional lubricants and water being added to the mixture in order to maximize the particle packing and avoid segregation; v) Thermally treating the so settled material to sinter by viscous flow with or without simultaneous crystallization between 720°C and 1100°C, densification and partially or completely crystallizing the materials, so as to obtain glass or glass-ceramic monoliths; vi) Optionally, effect rectification and polishing of the materials obtained after sintering,

Wherein the selection of the chemical composition of the frits, the particle size distribution adjustment and the thermal treatment are optimized with the aid of a mathematical model and an algorithm that predict the features resulting from the sintered material as a function of the treating parameters and the raw material features. The above steps will now be described in more detail .

Manufacture of Frits

The manufacturing process of glass and glass- ceramic objects through sintering starts with the preparation of particulate glasses (that can be crystallized in the process, in the case of the glass-ceramics) especially produced to this end.

Such glasses are manufactured based on commercial raw materials (sand, sodium, calcium and magnesium carbonates, alumina, etc) that, after melting, yield frits having components in the composition intervals listed in Table 1. A specific example is the above-cited composition (weight %) 72 Si0₂, 14 Na₂0, 11 CaO, 1 MgO and 2 Al₂0₃. In practice, a small amount (from 0.1 to 2% by weight) of sulfates or Sb₂0₃ is also added to help removing bubbles.

Impurities the content of which is lower than 0.01% by weight are not listed in this Table and are a function only of the purity of the raw materials employed.

The manufacture of frits through selection, mixture, controlled melting of the raw materials and further cooling allow a more precise control of the composition and impurities than the use of cullet, those being from different origins and carrying high impurity degrees.

The cullet produced at some industry and not packed off as an end product, but often recycled into same, is also suitable for milling and preparing particles for sintering; skipping the step of frit preparation.

One kind of glass useful for making frits for the sintering process according to the invention is flat soda-lime- silica glass.

The addition of Sb₂0₃, for example, can aid in eliminating pore-generating dissolved gases. A further possibility that results in a relevant effect is to include certain amounts of the components A1₂0 and MgO into the frit composition, in order to minimize its tendency to crystallization during sintering - again aiming at minimizing pores. These two oxides also improve the chemical durability. Please note that the compositions suggested in the present specification are based on conventional average compositions of NCS glasses, since the concept of the invention is not directed to a new composition, but instead to the application of known compositions to the described process.

The patentably distinguishing aspect of the invention in the frit manufacturing step is that the glass is of a controlled composition, being melted at pre-determined temperatures and periods of time, quickly cooled, milled and assembled at previously designed particle size distributions, especially for sintering with or without concurrent crystallization.

As cited hereinbefore, the glass can be partially or completely recycled, provided it obeys the composition and melting criteria of the described process.

TABLE 1

Components Compos ition (wt % )

Si0₂ 50 - 75

B₂0₃ 0 - 10

PbO 0 - 49

Na₂0 0 - 17

Cao 5 - 20

A1₂0₃ 1 - 15

MgO 0 - 5

K₂0 0 - 16

BaO 0 - 11

S0₃ Up to 1

ZnO 0 - 12

Sb₂0₃ 0 . 1 - 0 . 5

In order to obtain the frits, the raw materials are mixed and melted between 1300°C and 1600°C, with the melt being quickly cooled for the manufacture of granulated glasses (frits) .

Granulated glasses are generally manufactured as frits, by the forced cooling of the molten glass poured into water

(this being the origin of the word frit) or through squeezing between metal rolls. Other manufacturing processes, such as granulation with water jet, can equally be used.

In the case of frit manufacture involving water cooling, a drying step should be introduced into the process. Comminution and particle size distribution adjustment

After the selection of the materials that are to be part of the compact (using the criterion of color, for example) , the next step for manufacturing glasses or glass- ceramics by sintering corresponds to the particle size distribution adjustment.

The control of the particle size distribution of the particulate material is paramount to obtain sintered materials of minimum porosity, or having the suitable porosity for the desired use. Porosity is determining of the mechanical strength, of the staining resistance and of the visual aspect of the materials used for tiles and floors.

For each desired end material, the particle size distribution will be duly selected with the aid of the algorithm described in the scientific articles cited hereinbefore.

In some cases, however, the porosity does not definitely intervene in the utilization of the material. For dark or texture colored glasses and glass-ceramics, low porosity (< 5%) , at such a level so as not to weaken the mechanical strength, does not impair the visual aspect of the piece, as for instance occurs with porous granites of disseminated use, since eventual stains do not contrast easily, as would be for predominantly white materials . A similar situation is that of thermal insulating materials, where a high residual closed porosity is desirable. Or still, in the manufacture of filter materials where the presence of interconnected, specific dimension pores is of paramount importance. It is also noticed that the presence of a controlled porosity in the volume of the piece can diminish the total density, resulting in lighter materials for building.

Besides porosity, in the particle size distribution project, by the inclusion of large particles in the mixture (particles of the order of millimeters) , natural rock textures (such as marbles and granites) can be promoted in the sintered material, this having a strong aesthetic and commercial appeal . The frits directed to the manufacture of glass or glass-ceramic articles resulting from the inventive process should contain particles below 15 mm, in order to obtain different textures in the end product, as well as for the porosity control of the piece.

Whenever required, the frits can be comminuted in crushers and mills, separated by sieves into different particle size ranges and again mixed according to a suitable project of particle size distribution. Alternatively, the material can be comminuted and separated between the limits of a certain particle size distribution, or below a maximum size, using the total of the so- collected material, which will possess a nearly continuous distribution of particle sizes between the chosen limits. This kind of frit processing is of an easier industrial implementation, since it simplifies the sieving and particle size distribution separation process, while eliminating the disposal of particles having not designed sizes and need of a further mixing step of the selected particle size distributions. However, the control of the article end properties is somehow limited.

It should be noticed that even the distribution being between certain limits of particle size, it is possible to control, through the method used and the milling time, the features of particle size distribution, as wider, narrower, bimodal, etc., aiming for example to obtain maximum "green" density of the compact.

The separation process and particle size distribution adjustment can yield materials having particle sizes that are not within the desired particle size distribution, but that can be recycled and re-melt during the process of frit manufacture .

Mixture, Compaction, Shaping

The mixture of the particulate material is a step that depends on the desired properties and aesthetic features in the obtained pieces, such as for example, the combination of colors and texture. A further expected effect is to harmonize in the product an attractive visual aspect with a suitable mechanical performance, such as fracture strength.

The mixture of the particulate frit(s) can be effected in industrial, conventional mixers. Mixtures of different colors or particle size distributions can be placed in the molds in superimposed layers to yield certain effects in the end product. For example, the addition to the compact surface of a particle layer with sizes of the order of 5 to 10 mm, and after sintering, carrying out grinding and polishing at a depth that is half the final surface thickness can minimize the porosity of this area, as will be explained below.

The shaping process for sintered glasses and glass-ceramics is much simpler than the pressing in molds of the particulate material, with addition of binders and further lubricant, this being the process generally used in the manufacture of ceramic tiles. In the present case, the particulate glass is placed in refractory molds, previously coated with a finely particulate material, such as an alumina or kaolin aqueous suspension, which is dried before the frits placement, in order to avoid adherence of the glass to the mold.

Compaction of the pieces can be effected just by gravity action, aided by the forced vibration of the particles in the refractory trays. In order to avoid segregation during the molding, up to 5 wt % water can be added to the glass mixture and, through capillarity, aid in the particle packing. In this case, before sintering, the material should be dried between 60 and 110°C, as taught in US patent 5,649,987. Refractory molds should be dimensioned so as to compensate for the piece retraction, up to 40% by volume, caused by sintering. The retraction is not isotropic, but mainly at the height of the piece, this being caused by gravity action on the fluency of the glass material. Sintering, Crystallization and Surface and Dimensional Finishing

The sintering thermal treatment is conveniently planned with the aid of the above-cited algorithm. The sintering thermal treatment is carried out between 720°C and 1100°C, depending on the specific glass composition, on the particle size distribution and on the final desired porosity.

The materials manufactured according to the present process have lower sintering/crystallization temperatures

(<1100°C) , as compared to materials the main crystalline phases of which are wollastonite (CaO.Si0₂) and diopside (CaO.MgO.2Si0₂) , which are of the order of 1150°C.

In order to obtain materials of uniform dimensions, the temperature should be homogenous throughout the piece .

The treating time at some temperature also varies according to the specific chemical composition or the parent glass, the frit size distribution and the desired properties of the end product, and can vary from a few minutes to hours.

The heating can be isothermal by instantaneously inserting the compact in a furnace previously set to the desired sintering temperature and holding the material for half to several hours . Alternatively the sintering step can be non- isothermal, i.e. if the heating rate of the compact is between 1 and 20°C per minute.

Since the heating periods in industrial furnaces are relatively long and sintering occurs simultaneously to the limiting glass particle superficial crystallization, a further parameter of paramount importance for the control of the process is the heating rate. See at this respect the article by M. 0. Prado, C. Fredericci, and E. D. Zanotto - "Glass sintering with concurrent crystallization Part II. Non-isothermal sintering of jagged polydispersed particles" in Physics and Chemis try of Glasses , vol . 43, n . 5, October 2002, pp . 215-223. Period of time, temperature and heating rate are interacting factors in thermal treatments and should be selected together.

The choice of temperatures and times for thermal treatments is also very meaningful for determining the process energy consumption and thus the cost of the end product.

It should be noticed that the material is not promptly sintered at 100% of the theoretical density, or up to a minimum porosity, without a reasonable glass formulation strategy, care in the manufacture of dense frits, a suitable project of particle size distribution in the compact and planning of the thermal treatments, so as to privilege the viscous flow sintering before the limiting occurrence of the superficial crystallization. This applies if maximum densif.ication is desired, but if the desired aim is a high porosity, at a certain moment it is possible to wish to interrupt sintering or accomplish it at a suitable rate .

According to the invention and as cited hereinbefore, such parameters are optimized with the aid of the mathematical model developed by the Applicant and detailed in the scientific articles cited above.

The suitable balance of all these parameters aiming at obtaining glass and glass-ceramics products having adequate properties for the intended use are part of the concept of the invention, such as described and claimed in the present application.

Rectifying and polishing are usual steps at the end of the manufacture of ceramic tiles of the porcelanatto kind, glass-ceramics or even in the superficial finishing of natural rocks. Polishing is a highly-sought feature by consumers, since besides the highly aesthetic gloss, cleaning is also made easier.

One way of minimizing the superficial porosity of the manufactured pieces is by adding, at the compact surface, a layer of particles having sizes of the order of 5 to 10 mm and, after sintering, further grinding and polishing at a depth of the order of half the thickness of the resulting layer, formed by the outer particles. This therefore occurs at the layer that contacts the surface and nearly all the porosity is eliminated by the path opened to the atmosphere. Pores entrapped due to the fast viscous flow (if the effect of surface crystallization is delayed in this step) are located only below this layer, in the final densification steps. If the outer layer has a sufficient thickness, allowing grinding and polishing at a depth approximately half this thickness (that is why particles of the order of 5 to 10 mm are used) , it is possible to obtain a smooth surface, free from porosity, this being highly desirable in the case of wall and floor tiles, chiefly of lighter colors.

Polishing can be accompanied by rectification for dimensional adjustment.

Glass-ceramics are manufactured by the superficial crystallization of the glass particles, obtained by keeping the piece in the furnace, after sintering and initial densification, at a sufficiently high temperature so as to allow crystal growing at the desired rate and during a sufficient period of time to attain the crystallized fraction designed for the specific material. The flowsheet of Figure 1 illustrates the several process steps of the invention, that is, at 1, the mixture of raw materials, 2, melting, homogenization and refining, 3, fritting or granulation, 4, comminuting, 5, separation and particle size distribution adjustment, 6, mixture, 7, shaping in refractory molds, 8, sintering/crystallization, that leads to the natural end product 9 or to a polishing/rectification step 10 and to the polished end product 11.

One important aspect of the invention is the use of a mathematical model or algorithm that eliminates for the most part the empirical approach used in state-of-the-art processes for obtaining sintered glass and glass-ceramics articles. The mathematical model, by considering the concurrent crystallization of the surface (and volume, if it is the case) during sintering allows to calculate the suitable thermal treatment in terms of time and temperature in order to obtain the desired porosity and crystallinity for the compact body, as detailed in the above-cited articles . Based on the glass intrinsic features for a certain chemical composition, such as viscosity, surface tension and crystal growth rate, as well as the features determined by the manufacturing process - such as particle size distribution, number of superficial nuclei, green density of the compact and heating rate, densification curves can be calculated as a function of time for different temperatures with the aid of known equations. Such curves take into consideration the effect of the superficial crystallization on sintering. Further, the model allows one to draw time versus temperature curves that lead to working windows (time, temperature or heating rates), which allow the establishment of desired conditions of density and crystallinity .

Thus, viscosity, surface tension, number of nucleation sites per unit area, crystal growth rate, particle size distribution, green density, time, temperature and heating rate are entry parameters for the algorithm response, which are density or porosity, and crystallinity.

It is also possible to test the resulting density or porosity by varying one of these entry parameters and observing the generated response.

Provided the glass intrinsic properties are known (viscosity, surface tension and crystal growth rate) the algorithm can indicate the best or the most promising conditions for sintering, such as dwell time, temperature and particle size distribution, for obtaining articles having the desired properties. Thus it is possible to assess for which values of these variables the obtained material will attain the desired values for density, porosity and crystallinity. More details can be obtained in the previously cited articles and in a further article by M. 0. Prado & E D Zanotto - "Glass sintering with concurrent crystallization" in Comptes Rendus Chimie 5 -(2002) pp. 773-786.

In short, the algorithm used in the invention and described in detail in the above-cited articles comprises the following steps: i) entry of the physical chemical parameters referring to the frit composition, that is, viscosity as a function of temperature (η (T) ) and glass surface tension (γ) that can be measured or calculated, besides the crystal growing rate of each of the crystalline phases found in the material (u_x(T)), that shall be measured; ii) entry of the parameters referring to the manufacture process steps, that is, the particle size distribution v(r), the number of superficial nuclei for crystallization of each different crystalline phase found in the material (Ns and the green density of the compact (p_v) , that should be measured, besides the thermal treatment parameters time (t) , temperature (T) and heating/cooling rate (q) ; iii) processing of the algorithm with the entry data, yielding as response the density or porosity, as well as the compact crystallized superficial fraction after thermally treating under the specified conditions; iv) alternatively, the algorithm can be processed leaving one or more entry parameters as variable, thus obtaining as response the variation of density (or porosity) and of the crystallized fraction as a function of this or these variables; v) alternatively, the density (or porosity) can be used as entry parameter, and the algorithm is processed so as to seek for which glass parameters or process conditions the desired response can be attained.

It shall be noticed that the values of the entry parameters of the algorithm can, for known systems, also be obtained from the scientific literature.

The present invention will now be illustrated by the following Examples, which should not be construed as limiting same .

EXAMPLES

Table 2 below shows an example of a glass composition leading to a glass-ceramics of good mechanical and physical chemical performance.

In order to obtain such material, particulate glass - having the particle size distribution listed in Table 3 below - is sintered at 720°C for 2 hours without controlled cooling as refers to densification and crystallization, but sufficiently slow (l-5°C/min) to avoid cracking of the piece by thermal shock and to release tensions in the residual glass phase.

The properties of the resulting material are shown in Table 4, as compared with the properties of a commercial material (Neoparies) - based on wollastonite - and those typical of natural marble and granite. Similar results are obtained for several combinations of colored glasses, of the same basic chemical composition and containing small amounts of dyes, provided they have a similar particle size distribution.

Low porosity articles can also be obtained from different particle size distribution combinations, by adjusting sintering times and temperatures.

TABLE 2

Components Average Composition (wt %)

Si0₂ 72.8

Na₂0 13.2

Cao 11.2

A1₂0₃ 2.13

MgO 0.16

Fe₂0₃ 0.04

K₂0 0.09

BaO 0.04

S0₃ 0.20

TABLE 3

Signs - and + mean respectively that the particles have completely trespassed the sieve of which the sieve opening dimension appears in the first place and were retained by the immediately following sieve.

TABLE 4

An additional example showing a reasonable mechanical and physical chemical performance is summarized in, Table 5 and the properties of this material can be found in Table 6. The mold is filled up with frits of a particle size distribution as stated in Table 5 and covered with a layer of colorless frit having particle size distribution (-2+1 mm) . Pieces are treated at 1000°C during 40 minutes.

TABLE 5

TABLE 6

Table 6 lists the mechanical and physical chemical properties of the sintered glass the composition of which is presented in Table 2 above.

Other pieces of circa 60 x 90 mm and weighting approximately 85 g, are manufactured from glass particles having the composition of Table 2, according to particle size distribution conditions and thermal treatments shown from Table 7 to Table 10.

Data obtained from visual inspection of the pieces are shown in the same Tables. Marks in Tables 7 to 10 mean T t = heating rate;

T = thermal treatment temperature; t = dwell period in the temperature level; T ^ = cooling rate; and "isc isothermal treatment, that is, the sample is placed inside the preheated furnace straight in the thermal treatment temperature.

Table 7 lists data for a sample having particle size distribution between 1 and 2 mm, continuous in this interval, obtained by milling in a mortar and separation in laboratory sieves .

TABLE 7

From the data shown in Tables 7 to 10, it is observed that the frits of the cited composition and particle size distribution sinter satisfactorily above 900°C and crystallize intensively at 1000°C, but at around 1030°C the crystals melt already and the sample remains mainly vitreous with the cooling to ambient temperature, even at rates as low as l°C/min. Polishing at a depth close to half the size of the particles sintered at the surface of the piece allows to obtain pore-free surfaces, even if the inner material porosity is considerable. This is one of the patentably distinguishing aspects of the invention. When polishing and manufacture of pieces the surface of which is pore-free are aimed, in order to make the process easier and to assure the desired results, it is recommended to apply a layer of large diameter particles (of the order of 5 mm or more) on the surface of the material and thoroughly control the polishing at the indicated thickness, that is, close to half the size of the superficial particles.

Table 8 below lists samples of particle size distribution between 2 and 4 mm, continuous in this interval, obtained by milling in a mortar and separation with laboratory sieves . In Table 8, the expression "stretch" means that the surface has turned into a single, smooth and non-segmented film, no longer keeping the features of the first topography of the compact particles, before sintering.

TABLE 8

And Table 9 below lists the properties of a sample with a mixture of particle size distributions in the amount of 71 wt % between 2 and 4 mm and 29 wt % smaller than # 100 (that is, smaller than 150 μm) and continuous in the intervals, obtained by milling in a mortar, separation with the aid of laboratory sieves and further mixture.

TABLE 9

And Table 10 lists samples having a continuous particle size distribution < 0.85 mm, (that is, below sieve # 20), obtained by milling and industrial separation, 60 minutes treatments at T.

TABLE 10

Tables 11 and 12 below illustrate the porosity that can be obtained from the materials used in the invention.

Table 11 lists frit densities and porosity having particle size distribution between 0.5 and 1 mm, isothermically treated for sintering at 700°C and 750°C at the indicated times.

TABLE 11

Table 12 lists further examples of sintered frits that reached 100% densification that is approximately zero porosity at the surface, at a depth of the order of the particle size used.

TABLE 12

Porosity is assessed using Methods ASTM C 373 -88 "Standard Test Method for Water Adsorption, Bulk Density, Apparent Porosity, and Apparent Specific Gravity of Fired Whiteware Products", and ASTM 693-84 , "Standard Test Method for Density of Glass by Buoyancy".

The more immediate use of the partially crystallized glasses and glass-ceramics obtained by sintering is in wall tiles. The highly crystallized glass-ceramics, which are therefore harder, can also be used as floor tiles, in relatively large blocks, of the order of 1 m², or smaller. Applications in residence and public, commercial and eventually industrial building floors and facades, in laboratories provided with sanitary control, are foreseen. Basins and flat stoppers for kitchens and bathrooms can also be manufactured. The inventive process can also be directed to obtain curved pieces.

The materials produced through the inventive process can be adapted to the manufacture of toilets, lavatory basins, kitchen utensils, filters, provided they are combined to suitable parameters of for example binding additives and lubricants, shaping methods, burning with dimensional control, etc.

Claims

1. GLASS AND GLASS-CERAMIC ARTICLES AND PROCESS TO PREPARE SAME , a process for obtaining glass and glass-ceramic articles based on glasses of the soda-lime-silica system (NCS) , wherein said process characterized for comprises the following steps : a) Providing raw materials based on soda-lime- silica compositions for the manufacture of controlled-composition frits; b) From the composition of a) , obtaining glass frits by mixing raw materials melting between 1300°C to 1600°C, homogenizing, fining and quickly cooling in air or water to obtain easily ground glass pieces (fritting); c) Milling the frits obtained in b) and effecting the particle size distribution adjustment of the particulate material at particle sizes smaller than 15 mm according to the desired end product; d) Compacting the particulate material of c) in refractory molds; e) Thermally treating for viscous flow sintering between 720°C and 1100°C, the compacted material of d) , whereby the desired partial or total densification and crystallization of the materials is attained; and f) Recovering glass or glass-ceramic monoliths; wherein the selection of the frit chemical composition, the particle size distribution adjustment and the thermal treatment are optimized with the aid of a mathematical model and algorithm that predict the features resulting from the sintered material as a function of the treatment parameters and of the physico-chemical parameters of the parent glass.

2. A process according to claim 1, characterized by the fact of alternatively cullet is used for milling and preparing particles for sintering, skipping the preparation of frits .

3. A process according to claim 1, characterized by the fact of viscous flow sintering is effected with concurrent crystallization.

4. A process according to claim 1, characterized by the fact of viscous flow sintering is free from concurrent crystallization.

5. A process according to claim 1, characterized by the fact of the glass composition comprises:

6. A process according to claim 1, characterized by the fact of the frits comprise a controlled composition that is melted at pre-determined temperatures and times and quickly cooled.

7. A process according to claim 1, characterized by the fact of the frits are obtained by pouring the molten composition into water or between metallic rolls.

8. A process according to claim 1, characterized by the fact of the frits are obtained from any industrial process used for obtaining particulate glasses.

9. A process according to claim 1, characterized by the fact of the soda-lime-silica composition contains of from 0 to 100% of recycled material.

10. A process according to claim 1, characterized by the fact of the recycled material is re-melt, made into frits, and comminuted in a controlled way to obtain the final material having the desired properties.

11. A process according to claim 1, characterized by the fact of colored frits are obtained by adding small amounts of transition elements: Fe, Cr, Co, Ni, Ti, V, Mn, Cu; or rare- earths such as Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb .

12. A process according to claim 1, characterized by the fact of in the compaction step of from 1 wt % to 5 wt % of conventional lubricants and water are added to the glass particles (frit) in order to maximize particle packing and prevent segregation.

13. A process according to claim 1, characterized by the fact of in the sintering e) step the heating is isothermal by inserting the compact in a furnace previously set to the desired sintering temperature and holding the material for half to several hours.

14. A process according to claim 1, characterized by the fact of alternatively the sintering e) step is non- isothermal, i.e the heating of the compact is between 1 and 20°C per minute.

15. A process according to claim 1, characterized by the fact of additionally after sintering the glass or glass- ceramic materials undergo rectification and polishing.

16. A process according to claim 1, characterized by the fact of the particle size distribution comprises 90 wt % of frits below 0.074 mm, 3.5 wt % between 1 and 2 mm, 1.5 wt % between 2 and 3 mm and 5.0 wt % between 3 and 4 mm.

17. A process according to claim 15, characterized by the fact of the particle size distribution is between 1 and 2 mm.

18. A process according to claim 1, characterized by the fact of the porosity by volume of the resulting glass and glass-ceramic products is close to zero.

19. A process according to claim 1, characterized by the fact of the porosity by volume of the resulting glass and glass-ceramic products is up to 40%.

20. A process according to claim 1, characterized by the fact of the superficial porosity of the manufactured articles is minimized by the addition, at the compact surface, of a layer of particles of sizes of the order of 5 to 10 mm and polishing at a thickness of the order of half the thickness of said layer, or without any polishing.

21. A process according to claim 1, characterized by the fact of the glass-ceramics are manufactured by the surface crystallization of the vitreous particles.

22. A process according to claim 20, characterized by the fact of the surface and volumetric crystallization of the glass particles is obtained by keeping the piece in the furnace, after initial sintering and densification, at a sufficiently high temperature to allow crystal growth for a time sufficient to attain the crystallized fraction designed for the material.

23. Glass and glass-ceramic articles, characterized by the fact of said articles are obtained according to a process comprising the following steps: a) Providing raw materials based on soda-lime- silica (NCS) for the manufacture of frits of controlled- composition; b) From the composition of a) , obtaining glass frits by admixing raw materials, melting between 1300°C to 1600°C, homogenizing, refining and quickly cooling with granulation (fritting) ; c) Milling the frits obtained in b) and effecting the particle size distribution adjustment of the particulate material at particle sizes smaller than 15 mm according to the desired end product; d) Compacting the particulate material of c) in refractory molds; e) Thermally treating for viscous flow sintering between 720°C and 1100°C, the compacted material of d) , whereby is attained the desired partial or total densification and crystallization of the materials; and f) Recovering glass or glass-ceramic monoliths, wherein the selection of the frit chemical composition, the particle size distribution adjustment and the thermal treatment are optimized with the aid of a mathematical model and algorithm that predict the features resulting from the sintered material as a function of the treatment parameters and of the raw material features.

24. Glass and glass-ceramic articles according to claim 22, characterized by the fact of the physical chemical features of said articles comprise 3 point bend strength of from 40-100 MPa, Vickers Microhardness 500-600 MPa and chemical durability in terms of % loss in the presence of 0.01 Molar HC1 (950°C/1 h) 0.1-0.3 g and in the presence of 0.01 Molar NaOH (950°C/1 h) 0.2-0.6 g.

25. Glass and glass-ceramic articles according to claim 22, characterized by the fact of the porosity of same is nearly zero by volume.

26. Glass and glass-ceramic articles according to claim 22, characterized by the fact of the porosity of same is up to 40% by volume.