MXPA97007750A - Ac resistant glass - Google Patents

Abstract

An acid resistant glass article is provided comprising: about 25-45 wt% SiO2, about 20-35 wt% TiO2, less than about 5 wt% B2O3, about 15-40 wt% of a alkaline earth modifier selected from the group consisting of BaO, SrO, and mixtures thereof, and no more about 25% by weight of alkali metal oxide selected from the group consisting of Na2O, K2O, and mixtures thereof. The glass article is preferably formed in the form of a microsphere that is adapted for use as a retroreflective element in markings or markings for the paving

Description

ACID RESISTANT GLASS Background of the Invention Such pavement markings or signals typically include microspheres to provide retroreflective properties. However, glass microspheres embedded in the surface of painted lines and other pavement markings or signals that serve as retroreflective elements are subjected to highly destructive conditions. This can result from, for example, the weight of the vehicles. Such conditions tend to crush and / or eventually scrape the microspheres, reducing or removing thereby transmitting light which is necessary for retroreflection. Glass microspheres in pavement markings or signs are also subject to attack by corrosive agents sometimes present in the air, such as acids formed by various emissions from automobiles, furnaces, industrial plants, etc. Such an attack can cause the microspheres to become nebulous and lose transparency, which also reduces or removes their REF: 25686 reflectivity. Conventional glass microspheres that have desirable resistance to crushing or deformation, scraping and opacity can be difficult to manufacture consistently. This is because a number of the components are volatile and tend to be released from the melting of the glass, making it difficult to control the refractive index of the glass, for example. In addition, such compositions use components, for example, sodium fluoride, which create dangerous emissions from the melting of the glass, for example, HF and SiF4. Such components can also promote devitrification, which limits the size of the glass particles that can be produced. In this way, what is necessary are glass articles, particularly glass microspheres, which are acid-resistant, resistant to crushing or deformation, and which are easier and safer to manufacture.

Brief Description of the Invention The present invention provides glassware, particularly microspheres, which combine high durability, a desirable index of refraction (eg, about 1.6-1.9 and preferably about 1.75-1.85), and good resistance to chemical, acidic agents. The term "microsphere" or "spherical" is used in the present for the unitary, rounded glass elements used for retroreflection. In any case, the elements are perfect spheres. One aspect of the increased durability of glassware, e.g., microspheres, is improved crush resistance, i.e., crush resistance. As measured by a test described in the working examples, the microspheres of the invention have a crush resistance of at least about 3500 kg / cm2, and preferably at least 5000 kg / cm2. Another aspect of the increased durability is improved acid resistance. As measured by a test described in the working examples, no more than about 20% of a set of microspheres have reduced transparency as a result of acid exposure. That is, no more than about 20% (in number) shows any visible haze, frost or opacity as a result of 120 hours in a 1% by volume solution of concentrated sulfuric acid. Thus, as used herein, an "acid-resistant" glass composition is one that can be exposed to 1% by volume of H2SO4 for 120 hours with no more than about 20% of a sample of microspheres having the claimed glass composition exhibiting a reduced transparency. Preferably, the glass articles are visibly transparent (i.e., they transmit a sufficient amount of light such that they are sufficiently retroreflective for marking or pavement marking applications) and contain no more than about 5% by weight of glass. scattered crystallinity. Glassware can be of any size and shape and can be used in a variety of applications. For microspheres that are suitable for use in markings or signals, these are preferably less than about 2 mm in diameter. The glass microspheres of the present invention can be used in ribbons for structured markings or pavement markings. However, they are preferably used in ribbons for flat markings or pavement markings. The components of the glassware of the present invention are as follows: approximately 25-45% by weight of Si02; about 20-35% by weight of Ti02; less than about 5% by weight of B203; about 15-40% by weight of an alkaline earth modifier selected from the group consisting of BaO, SrO and mixtures thereof; and not more than about 25% by weight of an alkali metal oxide selected from the group consisting of Na0, K20, and mixtures thereof. The ratio of the amount of SiO2 to the total amount of the alkaline earth modifier plus the alkali metal oxide is greater than about 0.7. Such compositions provide acid-resistant glass. For significant acid resistance, ease of melting and optimum refractive index (i.e., about 1.75-1.85), the glass compositions of the present invention include about 30-35 wt.% SiO2, about 20-35 wt.% of Ti02, less than about 1% by weight of B203, about 25-35% by weight of BaO and / or SrO, and about 4-8% by weight of Na20 and / or K20. As is common in the glass technique, the components are described as oxides, which is the form in which these are presumed to exist in the completed glass articles of the invention, and which is correctly considered for the chemical elements and its proportions in the composition. The initial materials used to make the glass may be some chemical component other than an oxide, such as a barium carbonate, but the composition becomes modified to the oxide form during the melting of the ingredients. In this way, the compositions of the glassware of the present invention are discussed in terms of a theoretical oxide base. The novel glass compositions of the invention are described herein in percent by weight (ie, "% by weight") of the respective components. Glass compositions are also sometimes described in mol percent (ie "% mol"). Glass compositions in mole percent can be converted to weight percent as follows (1) converting the mole percent of each oxide to mass by multiplying the molecular weight of each oxide by the respective mole percent; (2) calculate the total mass for the composition; and (3) calculate the weight percent of each oxide by dividing the mass in (1) by the total mass in (2). For example, consider a glass having the following composition: 55 mol% Si02; 30% in mol of BaO; and 15% mol of Ti02. The molecular weights for Si02, BaO, and Ti02 are 60.1 g / mol, 153.3 g / mol, and 79.9 g / mol, respectively. The mass of each oxide in this formulation is as follows: Si02 is 0.55 X 60.1 = 33.1 g; BaO is 0.30 X 153.3 = 46.0 g; and Ti02 is 0.15 X 79.9 = 12 g. In this way, the glass is shown to comprise, in percent by weight, the following: 36.3% by weight of SiO2, 50.5% by weight of BaO, and 13.2% by weight of TiO2.

Detailed description The glass components in the compositions each generally contribute different properties or degrees of a property in proportion to their amount in the composition, and combine with each other to achieve the unique properties of the glassware of the present invention. For at least the components present in large quantities, usually there are no severe changes in the properties with small changes in proportions, and the numerical ranges set forth herein for such components must be understood in that context. The components and amounts of each are selected to provide compositions having good acid resistance, good crush or deformation resistance, melt flow fluidity, and a reasonable melting temperature (i.e., less than about 1450 ° C) . Preferably, the components. and quantities of each are also selected to provide compositions with low volatility at operating temperatures (ie, less than about 0.5% by weight less at vaporization at operating temperatures) and low toxicity. Preferably, the glass compositions of the present invention do not require fluidizing agents to improve melt flow during the manufacturing process. However, if fluidizing agents are used, they should not be volatile. In addition, they should not create dangerous compounds. In this manner, metal fluorides are not used in the compositions of the present invention.

Silica (Si02) promotes glass formation and provides significant improvement in the acid resistance characteristic of the compositions of the present invention. For the best acid resistance, at least about 25% by weight of Si02 is included. However, much silica tends to make the glass viscous and difficult to form the molten glass directly into the spheres without the formation of irregularly shaped, excessive particles and fibers. In this way, the glass compositions of the present invention include no more than about 45% by weight of SiO2. For particularly advantageous results, the silica level should preferably be about 30-35% by weight. The refractive index of the glass elements of the invention in general increases with the increase in the proportions of Ti02. At least about 20% by weight, preferably about 20-35% by weight of TiO2 should be included to achieve the desired refractive indices, for example, about 1.6-1.9 and preferably about 1.75-1.85. In amounts above about 35% by weight, the refractive index is too high. In amounts below about 20% by weight, the refractive index is too low and the fusibility is poor. For particularly advantageous results, the level of Ti02 should preferably be about 25-35% by weight. An alkaline earth modifier, such as BaO or SrO (as a total or partial substitute for BaO), is included to improve the flowability of the molten glass during glass formation and to increase the refractive index of the glass. At least about 15% by weight, and preferably not more than about 40% by weight of BaO and / or SrO, is used in the compositions of the present invention. In the amounts below about 15% by weight, the composition is difficult to melt. In amounts above about 40% by weight of BaO, the acid resistance and the crushing or deformation resistance of the microspheres tend to fall below the desired levels. Thus, about 15-40% by weight is preferably used in the compositions of the present invention, more preferably about 20-35% by weight, and much more preferably about 25-35% by weight. An alkali metal oxide, such as Na20 or F or (as a total or partial substitute for Na20), also promotes the formation of glass; however, too much causes the glass compositions to have poor acid resistance. Greater than about 25% by weight causes the glassware of the present invention to be undesirable for pavement markings or markings. Very little alkali metal oxide results in poor melt behavior. Thus, the alkali metal oxide content is preferably about 4-25% by weight, and more preferably about 4-8% by weight. In preferred embodiments, the alkali metal oxide is sodium oxide, preferably the potassium oxide, because the potassium oxide is more volatile than the sodium oxide during typical glass melting operations. Although glass compositions containing Si02, Ti02 / Na20 and / or K20, and BaO and / or SrO are known, not all are resistant to acid. In addition, not all compositions having the amounts described herein are acid resistant.

In this way, the glass articles of the present invention include Si02, Na20 and / or K20, and BaO and / or SrO in amounts such that the ratio of the amount of SiO2 to the total amount of alkaline earth oxides (BaO and / or SrO) plus alkali metal oxides (Na20 and / or K20) is greater than about 0.7, preferably greater than about 0.8. Glass compositions containing these component ratios have good acid resistance. Boria (B203) provides improvement in the level of resistance to crushing or deformation and promotes the formation of glass; however, 5% by weight or more of B203 is undesirable. This is because too much B203 can cause manufacturing problems, such as poor melt behavior and phase separation. Preferably, not more than 1% by weight of B203 is included in the glass compositions of the present invention. In particularly preferred embodiments, the glass compositions are essentially free of B203. Another desirable component of the glass compositions of the present invention is a rare earth metal oxide, such as La203. Lanthanum oxide (La3O3), for example, promotes the formation of glass, aids in melting, and helps to raise the refractive index while not perniciously affecting acid resistance or resistance to crushing or deformation. If used, the compositions of the present invention do not include more than about 10% by weight of La203 and preferably not more than about 5% by weight of La203. If La203 is present in the compositions of the present invention, preferably no more than about 1% by weight of B203 is used. More preferably, if La203 is present, the glass compositions are essentially free of B203. As stated above, the glass compositions of the present invention are more particularly advantageous because they do not require the use of fluidizing agents. The commonly used fluidizing agents are metal fluorides (eg, NaF, LiF, BaF2, KF), which can create dangerous emissions from melting glass in the form of HF and volatile SiF4. The presence of fluoro, an anion without connection, also promotes devitrification which limits the size of the glass articles that can be prepared from the compositions of the present invention. In this way, the glass articles, for example microspheres, of the present invention are advantageous because they are resistant to acid, they can be made in a wide variety of sizes, and these are made without flux metal fluxing agents . These are also advantageous because they do not include toxic materials such as PbO. Many other components can be included, either to contribute some specific property or to take the place of a portion of one of the other components. In general, these other components do not total more than about 10 mol%, preferably no more than about 5% by weight, and most preferably no more than about 1% by weight of the composition (theoretical oxide base) . ZnO is an example of a possible additional component that can be used to provide the metability and flowability to molten glass; however, it also appears to reduce resistance to crushing or deformation and resistance to acid. Thus, if the ZnO is present, it is present in an amount of not more than about 10% by weight, and preferably not more than about 5% by weight. A1203 is also useful, in general in an amount of 1-5% by weight, to contribute to the strength. Another useful component, although not desirable due to its toxicity, is As203, which can be added to make the glass elements colorless. Another component that can be included in the compositions, but is not desired, is CaO; Nevertheless, this is present preferably in no more than about 5% by weight. The dyes may also be included in the composition of the present invention. Such dyes include, for example, Ce02, Fe203, CoO, Cr203, NiO, CuO, Mn02, and the like. Typically, the glass compositions of the present invention include not more than about 5% by weight, preferably not more than 1% by weight of colorant, based on the total weight of the composition (theoretical, oxide base). Also, rare earth elements, such as europium, can be included for fluorescence. The glass compositions of the present invention also include oxides of elements that exist as impurities in certain starting materials. For example, Ti02 can include impurities containing Al, Si and K that are incorporated in the glass. Typically the glass compositions include no more than a total of about 5% by weight of such impurities based on the total weight of the composition (theoretical oxide base). The glassware of the invention can be prepared by conventional processes. In a process useful for making microspheres, the starting materials are measured in particulate form, each starting material is preferably about 0.01 and 50 micrometers in size and intimately mixed together. These are then melted in a gas or electric fire oven, until all the starting materials are in liquid form. The liquid is then rapidly cooled in water, dried and crushed to a desired size for the final microspheres. The microspheres can be sifted to ensure that they are in the proper range of sizes. The crushed or deformed microspheres are then passed through a flame having a temperature in general between about 1100 ° C and 1450 ° C to spheroidize the particles.

Alternatively, once the batch has been heated to the point where all the starting materials are liquid, the liquid batch can be emptied in a high speed air jet. Glass microspheres of the desired size are formed directly in the resulting stream. The air velocity is adjusted in this method to cause a proportion of the microspheres formed to have the desired dimensions. The above teachings about the spheres have observed that for the best retroreflective microspheres they should have good clarity, for example, they should contain no more than 5% by weight of dispersed crystallinity and should be at least 95% by weight free of bubbles. However, while such a degree of clarity is preferred, it is not essential for the satisfactory use of microspheres in markings or pavement markings. The microspheres of the invention can be incorporated into the coating compositions (see, for example, the North American patent No. 3,410,185 (Harrington); the american patent No. 2,963,378 (Palmquist et al.); and U.S. Patent No. 3,228,897 (Nelleseen), which generally comprise a film-forming binding material in which the microspheres are dispersed. Alternatively, the microspheres can be used in drop applications for painted lines or incorporated into the preformed retroreflective sheet or tape. As taught, for example, in U.S. Patent No. 2,354,018 (Heltzer et al.) Or U.S. Patent No. 3,915,771 (Gatzke et al.) The useful tape for pavement markings or signals in general comprises a backing, a layer of binder material, and a layer of microspheres partially embedded in the layer of the binder material. The backing can be made of various materials, for example polymeric films, metal sheets, and sheets based on fiber. The glass microspheres of the present invention particularly useful in the pavement marking sheet material as described in U.S. Patent No. 4,248,932 (Tung et al.), And other retroreflective assemblies, such as those described in U.S. Pat. Nos. 5,268,789 (Bradshaw), 5,310,278 (Kaczmarcik et al.), 5,286,682 (Jacobs et al.) And 5,227,221 (Hedblom).

Preferably, preferably large microspheres, for example, microspheres in excess of 250 microns in diameter, are used in markings or pavement markings for "exposed lenses" (i.e., with microspheres particularly embedded in, and partially protruding from and exposed above, the upper layer of the mark or signal) to provide the best retroreflective properties during precipitation and to minimize the effects of impurity collection. However, the microspheres of the invention can be made and used in various sizes, although 200-600 microns is typically desired. It is difficult to deliberately form microspheres smaller than 10 micrometers in diameter, although a fraction of microspheres below 2 or 3 micrometers in diameter is sometimes formed as a byproduct of the manufacture of larger microspheres. In general, uses for glass microspheres called for the microspheres to be less than about 2 millimeters in diameter, and more often less than about 1 millimeter in diameter.

The glassware of the invention can be used in other forms besides the microspheres and for other purposes besides retroreflection. For example, these can be used as fibers or flakes, and their high resistance to crushing or deformation and resistance to abrasion adapt them to be used for blasting with shot or as means for forming platelets, mechanical. The invention is further illustrated by the following examples, but the particular materials and amounts thereof cited in these examples, as well as other conditions and details, should not be constructed to unduly limit this invention.

EXAMPLES Acid Resistance The resistance to attack by acidic agents can be indicated by immersing a sample of glass microspheres in an excess of 1% by volume of concentrated sulfuric acid for 120 hours; then wash and dry the microspheres and then visibly determine the percentage of the microspheres that have been reduced in transparency. Different degrees of opacification are possible, from turvidity, to frosting, to the separation of a thin outer layer of the microsphere from the rest of the microsphere. The microspheres that have been reduced in transparency stand out among the unaffected microspheres in a sample. The test is preferably severe, but in a microsphere bath of the present invention no more than about 20%, preferably no more than about 15%, and most preferably no more than about 5%, have reduced the transparency.

Resistance to Crushing or Deformation The glass microspheres were formed and sieved at sizes of approximately 100-200 microns. The crushing or deformation resistance of the microspheres was measured in a bead or bead crushing machine, the main feature of which is two parallel plates made of very hard non-deforming material (e.g., a ceramic, such as polycrystalline aluminum oxide). An individual microsphere of known diameter was placed on the lower plate, which was raised until the microsphere fails. The resistance to crushing or deformation is the force exerted on the microsphere in the fault divided by the cross-sectional area of the microsphere (pr2). Ten microspheres of a given composition were tested and the average result was reported as crush resistance or deformation for the composition.

Refractive Index The Refractive Index was measured by Becke's method, which is described in F. Donald Bloss, "An Introduction to the Methods of Optical Crystallography"; Holt, Rinehart and Winston; New York; 47-55 (1961), the description of which is incorporated herein by reference.

Example 1 1.8 Index of White Beads or Pearls A batch of glass was made by mixing the following powders: 32 parts of SiO2, 31 parts of Ti02, 36.04 parts of BaC03, 10.96 parts of NaN03, 1.78 parts of H3B03 and 4 parts of La203. This mixture will produce a glass composed of 32% by weight of SiO2, 31% by weight of TiO2, 28% by weight of BaO, 4% by weight of Na20, 1% by weight of B203 and 4% by weight of La203. The raw material was placed in a platinum crucible and subsequently placed in an oven that has been preheated to 1400 ° ± 50 ° C. The materials were observed to melt in less than about 7 minutes. The crucible was allowed to sit for a total of fifteen minutes instead of the furnace, at which point the crucible was removed and the beads or glass beads were formed directly from the melt by emptying the liquid glass directly in a high air stream. speed (2,808 kg / cm2 (40 psi)). The glass microspheres were formed and sieved to a size between 100 micrometers and 250 micrometers. These microspheres were subsequently tested for acid resistance as follows: 0.3 g of the beads or beads sample (approximately 20,000 beads or beads or so) were placed into a glass vial to which approximately 20-30 ml of 1% by volume of concentrated H2SO4. The mixture was allowed to settle for five days at room temperature at a point where the acid was decanted and beads or beads were washed several times with deionized water and then allowed to dry, at which point they were observed under a microscope. For each sample, various fields of view were examined, each containing several hundred microspheres. After this test, beads or beads showed no signs of degradation or frosting (ie less than 5% exhibited any frosting, haze or turbidity). The crush or deformation resistance was measured as described above and found to be 5065 kg / cm2. The refractive index was measured as described above which is 1.82.

Example 2 1.8 Index of beads or yellow beads The glass batch described in Example 1 was prepared with the addition of 5 parts of Ce02. The glass was formed into beads or beads as described above and beads or glass beads were observed to retro-reflect yellow. Beads or beads were tested for acid resistance as described above and no frosting or degradation was observed after the test.

Example 3-15 and Comparative Examples A & B The additional glass compositions were tested according to the method described in Example 1. The following table describes these compositions and their physical properties.

Example Si02 / Si02 Ti02 BaO Na 0 K20 B203 La203 ZnO Al203 SrO Resistance Index resistance (Alkaline acid crushing &or deformation Alkaline earth (% Attacked) (kg / cm) 3 0.74 25 34 30 1 2 4 0 0 0 < 5% 1.84-1.85 4 0.74 25 32 30 0 0 4 5 0 0 < 5% 5925 1.84-1.85 0.74 28 31 30 0 0 3 0 0 0 < 5% 5155 1.80-1.81 6 1.29 45 20 15 20 0 0 0 0 0 0 < 5% 6462 1.64-1.65 7 1.00 40 20 30 10 0 0 0 0 0 0 0 < 5% 7458 1.69-1.70 8 1.05 40 20 30 8 0 0 2 0 0 0 < 5% 7618 1.80-1.81 9 0.75 30 30 37 0 0 0 0 0 0 15% 6395 1.80-1.81 1.00 32 31 0 1 4 0 0 28 < 5% 6280 1.80-1.81 11 1.00 32 31 10 0 1 4 0 0 18 40% 7466 1.78-1.79 12 0.69 25 30 30 0 0 4 0 5 0 < 5% 7262 1.81-1.82 13 1.00 40 20 20 20 0 0 < 5% 6701 1.65-1.66 14 1.14 32 31 28 10% 6701 1.78-1.79 15 0.82 30 33 30 6.5 0.5 0 < 5% 6863 0.81-1.82 Comparative Examples A 0.60 25 30 30 12 < 50í B 0.60 25 33 25 17 0 0 0 > fifty% 3 Full descriptions of all patents, patent documents, and publications are incorporated herein by reference as if incorporated individually. It will be appreciated by those skilled in the art that various modifications may be made to the embodiments described above of the invention without departing from the essential nature thereof. The invention is proposed to cover all modifications of this kind within the scope of the appended claims.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, the content of the following claims is claimed as property.

Claims (22)

1. An acid resistant glass article, characterized in that it comprises, on a theoretical oxide basis: (a) approximately 25-45% by weight of SiO2; (b) about 20-35% by weight of Ti02; (c) less than about 5% by weight of B203; (d) about 15-40% by weight of an alkaline earth modifier selected from the group consisting of BaO, SrO, and mixtures thereof; and (e) not more than about 25% by weight of an alkali metal oxide selected from the group consisting of Na20, K20, and mixtures thereof; wherein the ratio of the amount of SiO2 to the total amount of the alkaline earth modifier plus the alkali metal oxide is greater than about 0.7.

2. The acid-resistant glass article according to claim 1, characterized in that it also includes not more than about 10% by weight of La203.

3. The acid resistant glass article according to claim 2, characterized in that it is essentially free of B2034.

The acid-resistant glass article according to claim 1, characterized in that it has a refractive index of about 1.6-1.9.

5. The acid-resistant glass article according to claim 4, characterized in that it has a refractive index of about 1.75-1.85.

6. The acid-resistant glass article according to claim 1, characterized in that it has a crush or deformation resistance of at least about 3500 kilograms per square centimeter.

7. The acid-resistant glass article according to claim 1, characterized in that it comprises: (a) approximately 30-35% by weight of SiO2; (b) about 25-35% by weight of Ti02; And (c) about 25-35% by weight of the alkaline earth modifier.

8. The acid-resistant glass article according to claim 1 in the form of a microsphere, characterized in that it has diameters of less than about 2 millimeters.

9. The acid-resistant glass article according to claim 8, characterized in that no more than about 15% of a sample of the microspheres have reduced transparency when exposed to 1% by volume of concentrated H2SO4 for 120 hours.

10. The acid-resistant glass article, characterized in that it comprises, in a theoretical oxide base: (a) approximately 30-35% by weight of SiO2; (b) about 25-35% by weight of Ti02; (c) less than about 1% by weight of B203; (d) about 25-35% by weight of an alkaline earth modifier selected from the group consisting of BaO, SrO, and mixtures thereof; and (e) about 4-25% by weight of an alkali metal oxide selected from the group consisting of Na20, K20, and mixtures thereof; wherein the ratio of the amount of SiO2 to the total amount of the alkaline earth modifier plus the alkali metal oxide is greater than about 0.7.

11. The acid-resistant glass article according to claim 10, characterized in that it includes not more than about 10% by weight of La203.

12. The acid-resistant glass article according to claim 11, characterized in that it is essentially free of B203.

13. The acid-resistant glass article according to claim 10, characterized in that it has a refractive index of about 1.75-1.85.

14. The acid-resistant glass article according to claim 10, characterized in that it comprises about 4-8% by weight of Na20.

15. The acid-resistant glass article according to claim 10 in the form of a microsphere, characterized in that it has a diameter of less than about 2 millimeters.

16. A tape for markings or signals for the pavement, characterized in that it comprises a backing and a layer of acid resistant microspheres carried on the backrest; wherein the acid-resistant microspheres comprise: (a) about 25-45% by weight of SiO2; (b) about 20-35% by weight of Ti02; (c) less than about 5% by weight of B203; (d) about 20-40% by weight of an alkaline earth modifier selected from the group consisting of BaO, SrO, and mixtures thereof; and (e) not more than about 25% by weight of an alkali metal oxide selected from the group consisting of Na20, K20, and mixtures thereof; where the ratio of the amount of SiO? to the total amount of the alkaline earth modifier plus the alkali metal oxide is greater than about 0.7.

17. The tape for pavement markings or signals according to claim 16, characterized in that no more than about 15% of a sample of the microspheres have reduced transparency when exposed to 1% by volume of concentrated H2SO4 for 120 hours.

18. The tape for markings or signals for the pavement according to claim 16, characterized in that the acid-resistant microspheres comprise: (a) approximately 30-35% by weight of SiO2; (b) about 25-35% by weight of Ti02; (c) less than about 1% by weight of B203; (d) about 25-35% by weight of the alkaline earth modifier; and (e) about 4-8% by weight of Na20.

19. The tape for markings or signals for the pavement according to claim 16, characterized in that the acid-resistant microspheres have a refractive index of about 1.6-1.9.

20. The tape for markings or signals for the pavement according to claim 19, characterized in that the acid-resistant microspheres have a refractive index of about 1.75-1.85.

21. An acid-resistant glass article, characterized in that it consists essentially of, in a theoretical oxide base: (a) approximately 25-45% by weight of SiO; (b) approximately 20-35% by weight of Ti02; (c) less than about 5% by weight of B: 0:; (d) 20-40% by weight of an alkaline earth modifier selected from the group consisting of BaO, SrO, and mixtures thereof; (e) 4-25% by weight of an alkali metal oxide selected from the group consisting of Na20, K20, and mixtures thereof; and (f) not more than 10% by weight of La3; wherein the ratio of the amount of SiO2 to the total amount of the alkaline earth modifier plus the alkali metal oxide is greater than about 0.7.

22. The acid-resistant glass according to claim 21, characterized in that it consists essentially of: (a) about 30-35% by weight of SiO2; (b) about 25-35% by weight of Ti02; (c) less than about 1% by weight of B203; (d) about 25-35% by weight of the alkaline earth modifier, (e) about 4-8% by weight of Na20; Y (f) not more than about 10% by weight of La203.