DD209431A1 - METHOD FOR PRODUCING MICROGLASS BALLS WITH CRACKED VALUES OVER 2.2 - Google Patents

Abstract

The invention relates to a method for the production of glass microglasses with refractive powers above 2.2, which are used as starting material for the production of retroreflective sheeting. The aim of the invention is to specify the composition of a glass which is stable against reduction, discoloration and devitrification, which is free of PbO, CdO, Sb 2 O 3 and Bi 2 O 3 and therefore toxic substances and has a relatively low density, as well as a method for melting the glass and the production the glass microspheres. The invention has for its object to provide high-index glass microspheres of a 5-component glass of metal oxides without the use of other common glass components. According to the invention, the glass microspheres are composed of 40 to 60% TiO 2 .20 to 35% BaO, 2 to 15% ZrO 2 .3 to 16% ZnO and 1 to 5% WO 3. They are melted by Plasmatron at an axis temperature of about 5000 degrees K from the mixture directly and then annealed at 1000 degrees to 1400 degrees K to a higher oxidation state of the Ti and thus to achieve a high refractive index over 2.2.

Description

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Fields of application of the invention

The invention relates to a process for the production of glass microspheres with refractive powers of more than 2.2, which are used as starting material for the production of retro-reflective films. From such films mainly reflective traffic signs and signs are made.

Characteristic of the known technical solutions

The opaque glass microspheres act as optical lenses to concentrate light from a remote source at a point close to the back of the glass microspheres. In such a retro-reflecting system, the light is united by the ball at a focal point located at its rear, ie at the point of the ball closest to the light source. At this back there is at the same time a reflector, z. 3. An aluminum miniumfolie _4, which reflects the light, that is, this sends back by the ball in such a direction that is substantially parallel with the incident light.

In principle, two types of reflective films can be distinguished:

a) "Regular Transparencies": In these the medium is in front of the high

refracting glass spheres air, d. H.

b) "Flattop films": In these, the medium before the high

refractive glass spheres transparent plastic, d. H. n ^ = 1.5

In Fig. 1 is a film, as it should be constructed according to the optical laws, shown in cross section. The glass beads 2 are enclosed between a cover layer 1 and a spacer layer 3. On the back of the spacer layer is the reflective layer 4. Seen from the optical point of view, the reflective film consists of a plurality of three-lens systems, one of which is shown schematically in FIG. For near-axis rays, the following results from the theory of refracting spherical surfaces

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Relationship:

h = f. d (I)

h is the thickness of the Distanζlayer, d is the ball diameter and f is a proportionality factor.

The proportionality factor "f holds: _f _. 3 2 1

2n. (n ^ -Пр) + Ja (Xi ₁ -Ii ₉ ) (II)

Here, n represents the refractive index of the medium in front of the ball lens, Пр the refractive index of the ball lens and n ~ the refractive index of the medium behind the ball lens.

In order to achieve good reflection both with vertically incident light beam A and obliquely incident light beam B, it is necessary that the reflective layer concentrates the ball back sides concentrated in the optically effective range at a distance "h".

Reflex films in which the reflective layer is located directly on the back of the sphere are technically easier to produce, ie, h = 0. Purely as a result of equations I and II, the refractive index of the glass spheres must be twice that of the medium The sphere, pure plastic films (n. = 1), this condition is feasible, since glass balls with the refractive index n _? = * 2 can be produced. In practice, glass beads with a refractive index ρр = 1.9 are used for regular films in order to achieve favorable reflection from off-axis radiation.

Regular films are limited for the production of traffic signs used because they wetting their surface, z. As by rain, lose their reflective properties, since the refractive index n, the medium of the ball in a disturbing manner of 1.0 (air) to 1.33 (water) increases.

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In Flattop films, this defect does not occur, since in this Pail a 7 / water drop behaves because of its relative size, relative to the glass spheres, as an optically ineffective Planlinse. For this reason, Flattop films are almost exclusively produced today.

However, glass flakes having a refractive index of 2.3 to 3.0 would be required for the production of flattop films in which the reflective layer is directly on the backside of the sphere. Ба glass beads with such a high refractive index can not be produced until now, a spacer layer must be introduced between G-laskugelrückseite and reflective layer, so that glass beads of achievable refractive index, z. B. 2.0 to 2.5 can be used.

In the production of glass with a refractive index above 2.0 previously required masses with a high content of relatively low melting metal oxides. Frequently used constituents are lead oxide and bismuth oxide, which, however, are very easy to reduce and thereby deepen the inherent color of the glasses from yellowish to brown.

Lead oxide also favors the darkening of such glasses when exposed to sunlight, i. h., by so-called solarization. These glasses are also generally less resistant ^ egen chemical agents such. B. against SO ^ -haltire industrial gases. If the melting and shaping process of the glass spheres takes place under conditions which are not sufficiently oxidized, there is a risk of clouding of the glass by a precipitate of metallic lead, which is particularly difficult to control in such extremely basic glasses. These glasses have a very high density, which ultimately results in that a relatively smaller amount of retroreflective sheeting can be produced with a certain amount of glass beads of the described glass.

In order to counter these disadvantages, a variety of glass compositions have been tested which contain titanium dioxide as an essential ingredient.

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From the BED-PS 976,231 glasses with high refractive power are known, which from 30 to 70 mol <S TiOp and 0 to 5 mol *> of the usual, experience, the refractive power lowering oxides, such as silica, boron oxide, phosphorus oxide, iTatrrumoxid and otherwise lead oxide and optionally other oxides such as bismuth oxide, tantalum oxide, barium oxide and antimony oxide.

From the USA PS. 2,939,797, titanate glasses are known which have 35.6 to 57 moles of ⁶ O TiO ₂ > 22.8 to 40 moles * BaO, 5.5 to 26.9 moles of BpO 2, and 0 to 20 6 ZnO.

In the ВЙБ-OS 1.A96.S30 glasses are given, which consist of 2 5 to 30 Io TiO ₂ , 4 ^ to 59 % BaO, 3 to 10 <$> B ₃ O ₃ and 13 to 20 fo AlpQ-j ,

Other known glasses (32D-A3 1.248.872) have BaO and Sb ₂ O ₃ and parts of at least one oxide or sulfide of the following elements ITa, K, Li, Ca, B, As, Bi, Zn, Pb, Ti as Hauptbestard parts , Sr, Cd, Si, Al, BHg, Sn, Ta, La, Se and Te. 3RD-AS 1,250,977 specifies glasses consisting of 10 to 50 barrels of titanium dioxide, 10 to 6 # barium oxide, 0.2 to 30 % silica, 0.5 to 14 # boric oxide and 0.1 to 1 % OO consist.

The glass specified in BHD-AS 1.259.028 contains 55 to 75 # TiO ₂ , 10 to 4 Ъ BaO, CaO, SrO and / or HgO, 0.2 to 8 6 SiO ₂ , Al ₃ O ₃ , B ₂ O ₃ , GeO ₂ and / or P ₀ O ₁ ^, 0 to 35 l ZnO, 0 to 35 % GdO, 0 to 35 5 Bi ₂ O ₃ and 0 to 35 3 PbO. From the US-PS 3. ^ 93.403 glasses are known which consist of at least 75 wt .- ³ S from above 1800 ⁰ C melting metal oxides and at most 1 ¹ ^ wt. Ί> of such metal oxides, which at a temperature to 1800 ° 0, with the proviso that as above 1800 ⁰ C melting metal oxides at least 60 Gew. - ¹ ^ be used in the mixture of titanium dioxide plus at least one of the following metal oxides:

CaO, SrO, BaO, Sc ₂ O ₃ , T ₃ O ₃ , La ₃ O ₃ , ZrO ₃ , HfO ₂ , wherein the contents of SCpO ₃ 2 ¹ ^ wt .- ^ and La ₃ O ₃ or HfO ₂ 40 wt. -Л of the batch not exceed TIOP and the content of the mixture is between ¹ and 90 ^ by weight ³ lie S # t ·

-3MRI1983 * O73l4tG

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The BUD-AS 1771079 includes glasses that from 30 to 37 BaO Io, 4 ^ to 54 ^ TiO _2, 2 to 9 *> ZrO _2, from 1 to 5 "> Ia ₃ O _3, 0 to 10" S ' TO ₃ , 0 to * *> Ta ₂ O ₅ and JIb ₂ O ₅ , 0 to 3 "S ZnO, 0 to 2,7 # SiO ₂ and B ₂ O ₃ , 0 to 1, я 3 GeO ₂ and 1 Ъ Al ₃ O-, where the sum SiO ₂ + ³ p ° 3 ^{+ A1} p ⁰ -? ^Does not exceed ^the ' ^{Ѵeгѣ 2} > 7.

In the 33D-0S 2.5 ^ 5.633 glasses are described, the from 10 to ^ O # TiOp, from 2 5 to 70 ⁰ S BaO and CdO, up to 35 ^ riasbildenden oxide, selected from SiO ₂ , B ₂ O ₅ ^{and ff} Q ° p ^{unri from} 1 to 20 <& ZnO exist.

The BIId-OS 2,824,797 finally does a glass consisting of 40 to ^ h <*> TiO _2, 24 to 44 Ъ BaO, ^ to 15 <$> ZnO, 3 to 13 ί ZrO _2, 0.2 to 3 ^ GaO, 0.2 to 3 # MfeO, 0.2 to 2 ^ SiO ₂ , 0.1 to 0.5 ^ ITa ₅ O and / or K ₂ O.

For the production of high refractive index glass beads, it is known to use processes in which either the glass melted in a shark or in an electrically or gas heated melt basin is fritted, ground and shaped in a known manner by means of a gas / air mixture burner with possibly the use of oxygen to form micro glass particles or the glass stream leaving the port furnace or the melting tank is atomized directly into glass microspheres using a burner or a combination is used in the process.

Object of the invention

The aim of the invention is to specify the composition of a glass which is stable against reduction and discoloration for glass microbeads with refractive powers above 2.2, which is free of PbO, GdO, Sb ₂ O- and 3i- _o- i- which have high toxicity and thus pose a risk to the workplace and the environment, have a relatively low density, and specify a process for the melt of the glass and the molding of the glass microspheres.

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Presentation of the 'yesens of the invention

The invention has for its object to provide high-index glass microspheres of a 5-component glass of metal oxides, without the use of other conventional G-lasbestandteile,

Features of the invention

According to the invention, glass microbeads having a refractive index greater than 2.2 are melted from a G-emenge, which is composed of the light metal oxide TiO ₂ , the alkaline earth metal oxide 3a0 and the Schwerniet al! Oxides ZnO, ZrO ₂ and Ж) _ and their composition in mass percent as follows:

40 to 60 # TiO ₂

2 0 to 35 # BaO

2 to 1Я fo ZrO ₂

3 to 16 <$ ZnO

1 to 5 # WO ₃

Characteristic of the glass according to the invention is the absence of any "strong glass former" such as SiO ₂ , B ₃ O and P ₂ O ₁ -, with simultaneous lack of alkali oxides and the stabilization of the glassy state at high temperatures by low levels of tungsten oxide, without thereby To have to accept lowering of the refractive index. Glass microspheres from the glass according to the invention can not be readily prepared by the usual methods of both the direct and the indirect method, since on the one hand the melting point of the glass is very high and on the other hand TiO ₂ at such high percentages in the conventional melting of the glass and the Microglass bead molding technology tends to devitrify and stain by low oxidation state.

Accordingly, the melting of the glass from the batch and the formation of the microspheres is undertaken in one step by achieving the transformation of the raw material into a homogeneous and amorphous state in a rapid and instantaneous manner by passing the specially prepared raw material through a zone of extreme high temperature is passed.

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The correspondingly high temperatures can be z. 3. be generated by means of LPG oxygen, acetylene-oxygen or preferably by a high-temperature plasma.

To generate a plasma flame while a plasma torch is used with non-transmitted arc. The basis is a high-current arc which burns between an acute tungsten cathode and a water-cooled nozzle made of plucked anode. From the anode nozzle, the arc plasma is blown out by a vigorous gas flow in the form of a bright, slender flame, referred to as a plasma jet. The plasma jet contains a non-current-carrying plasma, which is used to transfer heat to the homogeneously mixed raw material without any repercussions on the discharge mechanism. The cooling of the arc by the gas flow and its constriction in the nozzle lead due to the thermal Pincheffektes to a high temperature of the beam. The axis temperature and the shape and length of the plasma jet are dependent on the current intensity of the discharge, the gas flow rate and the nozzle shape and the gas type.

The working gases used are gases which do not chemically attack the cathode material. It comes here argon, nitrogen, hydrogen and mixtures of these gases into consideration. Particularly cost-effective is the use of compressed air. In this case, axis temperatures of approx. 50OQ ⁰ K are reached.

However, the exact value of the axis temperature of the jet does not matter crucially. Much more important is its enthalpy. It indicates the 'Värrae content of the arc gas as a function of temperature and is thus decisive for the amount of heat that is transmitted from the beam. Molecular gases such. B. compressed air have for the intended use here significant advantages over monatomic gases. With the onset of dissociation, the dissociation energy must additionally be supplied to the molecular gas during a temperature increase. As a result, its content above the dissociation temperature greatly exceeds the yar content of a monatomic gas of the same temperature.

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Correspondingly, the heat release during cooling drops below the dissociation temperature. The high heat content of the plasmas produced from molecular gases leads to the formation of relatively long plasma jets of up to 50 cm in length. The supply of the homogeneously mixed raw material is effected by an additional air flow, the necessary uniform supply the raw material is taken over by a pneumatic conveying. Bas cold gas flows past the cathode in the space filled by the bow.

Sin part of the gas enters the inner area of the arc, where it is heated to its axis temperature and forms the central part of the plasma jet. It is surrounded by a jacket of cooler and slower flowing gas, which is mainly heated by heat conduction from the central part.

Purely the conversion of homogeneously mixed raw material in microglass spheres of homogeneous amorphous state, a slow, flowing, but long plasma jet is used in which the particles have enough time to be heated and durchrollolzen.

The raw materials selected for the glass mass are ground into particles of colloidal size and intimately mixed together with the addition of water. During the homogeneous mixing ns, a binder such. For example, alkali metal silicate solution or organic binder may be added in an amount which is sufficient to bind the raw materials, but which is without significant lasting influence on the glass composition. The ground material is then either dried to a cake in an oven or converted into dried particles in a spray dryer. The preferred drying temperatures are in the conventional drying at 120 - 14O ⁰ C, while in the spray drying Я00 be set to GOO ⁰ G.

The resulting cake or the resulting particles are then subjected to ceramic sintering at temperatures of 1000 to 1600 ⁰ C.

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This sintering serves to firmly bind the particles and to fix the homogeneity, so that the cake or particles can be crushed or ground into grains of the size desired for the microglaskules.

Advantageously, the grains brought to the required size are fed to the plasmatron by means of a pneumatic conveying system and converted by the plasmatron into a spherical glassy state.

Under the glass microspheres is a part that is noticeable by gray or SchwärζVerfärbung. This is due to the reduction of a part of titanium to a lower oxidation state.

stage of Ti, z. B. to Ti, due. In order to correct this discoloration, an additional heat treatment, which lies below the transformation range for the glass, is used so that ultimately all the glass microspheres are in a light, translucent and clearly transparent form after the heat treatment has been completed. As a result of this heat treatment, the refractive index of the microglass spheres increases by 0.05 to 0.2 units to the desired final value of at least 2.2.

The raw materials used for the glass according to the invention are further explained in more detail below.

Titanium dioxide is an essential ingredient in the manufacture of high refractive index glass because it has the highest refractive index of the known glass additives. Another major advantage of titanium dioxide z. B. lead oxide, Gadmiumoxid or bismut oxide is the much lower density. This is especially important for the manufacturer of retro-reflective sheeting, because the lower the density of the glass, the greater the area covered for a given mass of the glass microspheres used. In addition, titanium dioxide in glasses is colorless, unless they contain certain oxides of the transition elements. Due to the easy accessibility of titanium dioxide, this is very priced.

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Another advantage over heavy metal oxides is the low toxicity.

Barium oxide is an important next to titanium dioxide component to obtain a glass with high refractive index and acts on ^a TiO ₂ ls flux for eutectic melting temperature lowering.

Zinc oxide also contributes significantly to increasing the refractive index of the glass and has a relatively low density. Ss lowers the viscosity and contributes to devitrification stability. Although ZnO has a high vapor pressure such that parts thereof can volatilize upon melting, the refractive index of ZnO is so close to the refractive index desired for the glass microglass that a loss of ZnO during melting does not significantly alter the refractive index of the glass.

Zirconia also significantly increases the refractive index of the glass, contributes to the lack of glass, improves chemical resistance and enhances glass stability. Another advantage is the relatively low density.

The tungsten oxide is mainly used to improve the glass stability and is an indispensable component of the proposed glass composition.

Overall, it was found that the proposed composition has the lowest tendency to crystallize when securing the high refractive index of> 2.20 and thus enables the stable production of glassy spherical particles with the proposed method, without a usual problematic in glasses such high refraction problematic melt in elaborate melting plants is required with high Sdelmetalleinsatz.

With respect to the raw materials for the glass of the present invention, it is desirable to use those raw materials normally used for optical glass or raw materials having a similar purity.

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The micro glass beads produced according to the invention have some decisive advantages over the micro glass beads produced by the previous methods. Due to the possible glass composition obtained glass microspheres that are extremely resistant to solarization and chemical agents such. B. against SOp-containing industrial waste gases are. It follows that the retro-reflective film made therefrom has a 5 to 7 times longer life than the film made with conventional glass microspheres. Due to the low intrinsic color of the invention produced according to Mikroglaskugelη, the resulting reflective films are characterized by an exceptional "white" hue (without paint), so that these films already in daylight еілеп cause very high attention compulsion.

Under the microscope with reflected light, the glass microbeads appear 100 % optically pure, the reflectance of the reflective foil is up to twice the reflectance of the film, which is made with conventional glass microspheres. Due to * the high refractive index over 2.2 spacer films of relatively low thickness can be used, which allow a high degree of deformation. Thus, the production of extremely wide-angle retro-reflective films is possible. Due to the opposite with TbO and ЗірО manufactured micro glass beads, the inventively fabricated glass microspheres have a much lower density, so that with the same mass of glass microbeads a much larger area of retro-reflective film can be produced. During the production of glass microspheres, there is a significant improvement in the working and environmental conditions due to the breakdown of such toxic substances as PbO and CdO. Overall, the method itself is more cost-effective than the conventional methods, since the glass is melted and the glass microglass is formed in one process step.

Embodiment:

The method according to the invention is explained in more detail below with reference to an exemplary embodiment:

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A mixture of 50.8 kg anatase, 51.3 kg barium nitrate, 10.2 kg zirconite, 8.1 kg zinc white and 1.5 kg folframoxid was ground for 24 hours in a drum mill with flint stones and then mixed with 7 / asser so that a thick porridge was made. This product was then dried for 4 hours at 130 ^° C. to form a cake and the cake was sintered for 30 minutes in an oven kept continuously at 1200 ^° C. The BEG sinter cake was then comminuted in a sieve drum mill, with the resulting particles passing through a 90 micron screen mesh screen spanned on the screen drum mill.

The obtained particles were passed through the flame of a plasmatron operated at 420kW with a power of 320kW to produce a temperature of about 5000 ^° C. Compressed air was used as the working gas and nitrogen as the protective gas for the cathode. The particles melted together sufficiently under the temperature conditions used in this treatment so that they could form into spheres by surface forces. They were collected in a refrigerated container filled with oil.

The glass microspheres had a gray color. The refractive index was η γ 2.15. They were then subjected to an annealing process under oxidizing conditions, where they were exposed to a temperature of 121O ⁰ C for 10 min. They were then converted to a virtually white color when viewed in solid form under daylight. Under retro-reflective conditions, they were virtually colorless and transparent. They had a refractive index that was 2.24.

Claims (3)

235552 8 PatentansOrüche Process for the production of glass microbeads with refractive indices above 2.2, characterized in that the glass is free of PbO, CdO, Sb ₂ O., and Bi _p 0 ~ and is composed of the light metal oxide TiOp, the alkaline earth oxide BaO and the heavy metal oxides ZnO, ZrOp and WOo composed without further additives, the percentages in percent by mass at TiO ₂ 40 to 60 #, at BaO 20 to 35 4 ₉ at ZrO ₂ 2 to 15 I ₉ at ZnO 3 to 16 "and at TTo 1 to 5 # be that without mixing a conventional glass melt by milling and mixing the raw materials, a mixture is formed, which is mixed with water or produced by Kaßmahlung, then dried, sintered and ground into particles that correspond to the size of the desired micro glass beads and these particles in one slowly flowing long plasma jet led there to be heated and melted through, so that due to the surface tension is a ball formation. Process according to item 1, characterized in that the proportion of ZrOp + ZnO does not exceed 25% by weight. A process according to pt. 1 and 2, characterized in that the glass microspheres are exposed after its formation a Temperprozoß in an oxidizing atmosphere, wherein the temperatures are depending on the glass-modifiers of between 1000 and UOO ⁰ C. Process according to the items 1 to 3, characterized in that the drying of the mixed raw materials to a cake at 120 to 14O ⁰ C is made. Method according to the Pkt. 1 to 3, characterized in that when using the spray drying of the mixed raw materials, the drying temperature between 400 and SOO ⁰ C. Method according to the pt. 1 to 5, characterized in that the sintering of the dried raw materials at 1000 to 1400 ⁰ C is made. 236652 8 7. "Process according to item 3, characterized in that for binding the raw materials alkali metal silicate or an organic binder with a mass fraction of * 0.5 fo based on the mass of the raw materials is used. S. The method according to the Pkt. 1 to 7, characterized in that the comminution and grinding of the sintered batch mixture is done in screen drum mills. 9. The method according to the Pkt. 1 to 8, characterized in that the plasmatron works with recirculated air as the working gas. 10. The method according to item 9, characterized in that nitrogen is used as protective gas. - For this 1 sheet drawings - -ЗМЯ7 іРЯЗ * П7Я I 4G