Opal glass composition with higher opacity

Abstract

The invention relates to a novel composition for producing opal glass, achieving varying degrees of opacity from light opalescence to dense white opal. This fluoride- free and environmentally friendly opal glass formulation prioritizes human health and ecological safety. By substituting fluoride with phosphorus pentoxide (P₂O₅) and zinc oxide (ZnO), the composition attains enhanced opacity levels exceeding 85%, coupled with a dense, milky-white appearance. The opal phase is characterized by spherically-shaped, glassy droplets within a composition that includes 58-74% SiO₂, 10-18.5% Na₂O, 5-10% BaO, 2-10% Al₂O₃, 3-10% B₂O₃, 2-10% CaO, 0.32-3% ZnO, and 2-7% P₂O₅ by weight. The resulting glass exhibits a softening point of 735-765°C and a density of at least 2.4 g / cm³, with phase separation driving its opacification. This innovative composition ensures safety and sustainability while maintaining the high-performance standards of opal glass.

Description

Opal Glass Composition with Higher OpacityFIELD OF THE INVENTION

[0001] The present invention relates to the field of glass production, specifically opal glass and a novel composition for opal glass production exhibiting a degree of opacity varying from a very light opalescence to a dense white opal.BACKGROUND

[0002] Opal glasses have been known to the glass art for many years. They are generally light-diffusing glasses ranging in appearance from near-transparent to opaque that can be blown or pressed into a wide variety of shapes. As recognized in the art, the opacity demonstrated by such glasses is generally the result of a phase separation wherein a light-diffusing phase is evenly separated throughout a transparent glassy matrix, the light-diffusing phase having an index of refraction differing from that of the matrix glass so as to cause light scattering and thus loss of transparency in the glass.

[0003] Opal glasses have commonly been classified into two broad categories, i.e., spontaneous opal glasses, and thermally opacifiable or reheatable opal glasses. Spontaneous opal glasses encompass those compositions which develop opacity as the molten batch is cooled to a glass article, this opacity resulting from the growth of crystals in the glass or from some other type of phase separation phenomenon taking place within the glass. Spontaneous opal glasses are characterized by the fact that the light-diffusing phase separates out ("strikes in") during the cooling and forming of the melt into a glass article. Thus, the opacifying agent strikes in during the shaping of the melt to a glass article utilizing such conventional glass forming techniques as blowing, casting, drawing, pressing, rolling, and spinning.

[0004] In the thermally opacifiable or reheatable opal glasses, little or no opacity is achieved when the molten batch is cooled. Opacity will develop, however, when the glass bodies are exposed to temperatures in the vicinity of or somewhat above the iannealing point of the glass. Here, again, crystal growth or some other phase separation phenomenon produces the desired opacity. The crystal content developed is generally quite small, most usually less than 10% by volume, so that the overall physical characteristics of the glass, other than optical transmission, are affected only slightly, if at all. In contrast, the rate of opal development, i.e., the rate at which the lightdiffusing phase separates out of the glassy matrix, is relatively slow in the thermally opacifiable glasses. Consequently, upon cooling and shaping the melt into a glass article, a substantially clear or only faintly opacified appearance is observed. The glass article must be reheated to temperatures in and / or above the transformation range of the glass to promote separation of the opacifying phase(s).

[0005] A dense, uniformly milky- white appearance has been deemed to constitute a most desirable attribute of opal glasses. Such glasses permit the manufacture of thinwalled and, therefore, lightweight articles, but which will still exhibit good opacity. However, such glasses have been subject to a serious problem termed "flask mark," when molded articles such as culinary and table ware have been produced therefrom. This defect is evidenced by an area of differential opacity near the bottom of such articles and is a frequent source of rejection for the ware. Also, because of their compositions and consequent physical properties, considerable difficulty has been experienced in devising applicable fired-on decorating materials.

[0006] A glass may be opacified by dispersing therein particles of such a nature that visible light is scattered or diffused, rather than directly transmitted. In general, the ability of a particle to scatter light, and thus impart opacity, depends on the degree of difference between the refractive indices of the particle and the glass, particle size, and particle concentration. Opal glasses wherein the opacity mechanism is the presence of alkali metal or alkaline earth metal fluoride crystals have been known to the art for many years. However, it has proven difficult to satisfactorily opacify such base glasses with conventional opacifying agents such as fluorides. The compositions containing fluorides were preferable for the commercial production of opal glasses despite anobjectionable tendency for the fluorides to volatilize during the glass melting process as well as during subsequent forming of ware from the melt. As a result, there is an undue loss of opacifying agent, pollution of the surrounding atmosphere with obnoxious vapors, chemical attack on the refractory of the melting unit, and a generally transparent or pale opal product, especially adjacent the surface of the molded article / ware. When such ware is tempered or strengthened in known manner by sudden controlled cooling from a given temperature, the losses due to breakage are too great for practical purposes on account of the unduly large number of stones which are eroded from the refractories into the glass and which cause severe non-uniform stresses in the ware during tempering. Many countries have implemented stringent regulations on the levels of fluorine content permitted in the air, in accordance with their Environmental Protection Laws. Furthermore, fluorine is a potentially toxic material which could contaminate food brought into contact therewith via diffusion from, or attrition of, the glass surface. This factor provides another reason for removing fluorine from the glass composition.

[0007] The currently used formulations of opal glass raise concerns about their environmental impact and potential risks to human health. Additionally, the opacity of the resulting product is both limited and inconsistent. Certain variations of these glasses even necessitate a secondary heat treatment, typically lasting around 4 to 8 hours. This not only results in significant energy consumption but also hampers streamlined and continuous production processes. While the above-mentioned opacifying agents have been employed by the prior art, their use has not been universal because the opal glass produced may be slightly colored or it may possess physical and chemical properties that make it unsuitable for manufacturing articles by known machine production techniques. Another disadvantage of known opal glass is that the degree of opacity is often difficult to control and nonuniformity in the finished article is frequently a problem. This non-uniformity is caused by improper mixing of the opacifying agent in the batch which results in localized uncontrolled development of the opalizing species.

[0008] Attempts have been made to remove Fluoride from the opal glass, such as in the prior art US349880, where Phosphate (usually Calcium Phosphate) has been used as an opacifying agent to produce opal glasses. The compositions described in US349880 are capable of exhibiting opacification, however, the opacification may further be improved by incorporating components of the glass composition which increase the refractive index of the glass.

[0009] Some of the other prior art introduces an emulsion opal glass that does not contain fluorine. However, this emulsion opal glass system requires an extra emulsification step to be conducted at temperatures ranging from 700 °C to 900 °C, with a heat treatment duration of 2 to 6 hours. This procedure results in substantial energy and resource consumption, making it unsuitable for efficient large-scale industrial production. Another prior art outlines a Na2O - CaO - SiCh emulsion opal glass, incorporating phosphorus compounds and zirconium oxide for opacity. However, due to its elevated content of alkali metal oxides, exceeding 16%, there exists a heightened risk of substantial volatilization of alkali phosphate compounds. Consequently, this has the potential to lead to environmental pollution and corrosion of the melting furnaces. Moreover, the prior glasses also contain lead oxide. Opal glasses containing substantial lead oxide have a relatively low viscosity at melting temperatures and hence are quite corrosive to refractories and are not suitable for use in feeders for automatic glass working machinery.

[0010] Other than Fluoride, prior art exists where Phosphate (usually Calcium Phosphate) has been used as an opacifying agent to produce opal glasses. While the Phosphate Opaque Glass requires lower glass melting temperatures than its Fluoride Opal Glass counterpart, consequently leading to a slight to moderate decrease in energy consumption for the melting process depending on the choice of composition, such phosphates opal glasses are pale compared to fluoride opals and do not become equally opaque even when the content of calcium phosphate is greatly increased. Instead, they become coarse grained or sandy in character and brittle.

[0011] While the some prior art relating to production of opal glass with Phosphate opal glass exists, there are still problems related to the level of opacification produced by the Phosphate opal glass from the prior art. This invention is, therefore, directed towards providing an improved composition of Phosphate opal glass, which is relatively simple and is highly effective for its intended purpose of producing striking and attractive opacification in the opal glass.SUMMARY

[0012] Embodiment of the present disclosure present technological improvements as solutions to one or more of the above-mentioned technical problems recognized by the inventors in conventional systems. The inventors have come up with a novel composition of opal glass which is free from fluoride and non-hazardous to human health. By replacing the fluoride content with not only just Phosphorous Pentaoxide (P2O5), but also Zinc Oxide (ZnO), the inventors have found that higher opacity can be achieved in opal glass.

[0013] Another object of the present invention is to provide an opal glass composition demonstrating a very high degree of opacity suitable for use as dinnerware.

[0014] Still another object of the invention is to is to produce a spontaneous opal glass of dense opacity having a composition that demonstrates a softening point in the range of 725-750 °C.

[0015] This invention introduces a novel fluoride-free composition for producing opal glass, emphasizing safety and environmental sustainability. The formulation replaces fluoride with phosphorus pentoxide (P2O5) and zinc oxide (ZnO), achieving enhanced opacity exceeding 85% and a dense, milky-white appearance. The composition includes specific weight percentages of SiO2, Na2O, BaO, AI2O3, B2O3, CaO, ZnO, and P2O5, optimized for performance. The opal glass demonstrates a softening point between 735°C and 765°C, a density of at least 2.4 g / cm3, and opacification through phase separation involving spherically-shaped, glassy droplets. This innovativematerial eliminates hazardous components, making it safe for human health and environmentally friendly while delivering high-quality opal glass properties.

[0016] These and other features and advantages of the present invention will become apparent from the following description of the invention that is provided in connection with the accompanying results of the composition of opal glass. It will be appreciated that features of the present disclosure are susceptible to being combined in various combinations without departing from the scope of the present disclosure as defined by the below mentioned detailed description.DETAILED DESCRIPTION

[0017] The following detailed description illustrates embodiment of the present disclosure and manners by which they can be implemented. The words "comprising," "having," "containing," and "including," and other forms thereof, are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It must also be noted that as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0018] The person skilled in the art will recognize many variations, alternatives, and modifications of the embodiment of the present disclosure. It should be understood that this invention is not limited to the particular methodology, protocols, and the like, described herein and as such may vary. The terminology used herein is for the purpose of describing particular embodiment only and is not intended to limit the scope of the present invention, which is defined solely by the claims.

[0019] Additional features and advantages of the glass compositions and glass articles formed from the glass composition will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein.

[0020] The term opal glass as used herein denotes any glass which has a light-diffusing medium or phase therein which renders the glass essentially light diffusing and thus translucent or opaque. The term opalescent glass refers to those opal glasses which have a light-diffusing medium therein which renders the glass translucent. Another disadvantage of known opal glasses is that the degree of opacity is often difficult to control and non-uniformity in the finished article is frequently a problem. This nonuniformity is caused by improper mixing of the opacifying agent in the batch which results in localized uncontrolled development of the opalizing articles. In order to attain the aforementioned objective, the technical approach employed for producing opal glass encompasses the following steps:The batch ingredients were appropriately selected weighed according to the composition shown in the table. The mixed batch composition was put in a platinum crucible for melting wherein the platinum crucible was introduced into a furnace at a temperature of approximately 1400-1500°C. The melting process continued for 2 hours. Subsequently, the molten glass was cast onto a preheated stainless steel mold. This quenching from high temperature generates thermal stress in the glass. To release this stress, annealing process was carried out. The casted shape was transferred to an annealing chamber, where the temperature was set near the glass transition temperature of the material between 550° - 580°C and maintained for 2 hours to release the internal stress. Lastly, allowing the glass to gradually cool to room temperature results in the formation of the opal glass article. The crucible used is not limited to platinum. However, any other similar material crucibles like platinum-rhodium and the like can also be used for melting.The raw material composition, quantified as weight percentages, includes the following elements:58 - 72% SiO2, 3 - 8% B2O3, 3 - 8% A12O3, 10-17% alkali metal oxide (Na2O), 2 - 10% CaO, 0 - 5% BaO, 0.5 - 4% ZnO, 2 - 8% P2O5and 0-0.02% Sb2O3.Following are the raw materials used to synthesize the glass of the present invention from the glass composition mentioned above.• Silica Sand (SiC )• Borax (Na2B4O?.10H2O)• Soda Feldspar (Na2O.AhO3.6SiO2)• Limestone (CaCCh)• Barium Carbonate (BaCCL)• Soda Ash (Na2COs)• Zinc Oxide (ZnO)• Sodium Hexameta Phosphate (NaPO3)6• Aluminum Hydrate (Al(0H)3)

[0021] Maintaining the base glass composition within the above-cited ranges is vital to achieving the desired softening point and dense white opacity. Na2O, BaO, and P2O5 are self-evidently the critical components in the development of the crystalline opal phase. In an embodiment, an opal glass comprises a composition that is environmentally sustainable or that does not comprise non-environmentally sustainable constituents. A glass composition as described herein does not contain fluorine (F) or is fluorine-free. In another embodiment, a glass composition as described herein does not contain barium (Ba) or is barium-free.

[0022] Silicon Dioxide (SiCL) is a main component forming the glass, and is a component that improves dielectric properties. The SiCF content advantageously ranges between 58% and 74% and preferably still is in excess of 58% and advantageously less than 68%. On this side of the lower limits, the glass compositions according to the invention become very problematic to work and the shaping of articles becomes almost unrealizable. On the other hand, above the upper limits, the glass compositions become difficult to melt. With this composition, a region suitable for phase separation is obtained.

[0023] Aluminum oxide (AI2O3) is a component that improves the mechanical strength of the glass precursor and suppresses the enlargement of the phase separation structure and suppresses shrinkage during sintering. AI2O3 plays a vital role in achieving glasses exhibiting excellent resistance to acid and alkali attack as well as helps increase in viscosity. However, too low AI2O3 content impairs the crystal growth as well as the chemical resistance, whereas too high AI2O3 content results in an undesirably high viscosity and an impaired meltability of the glass. Hence, within the scope of this invention, the AI2O3 content is advantageously fine-tuned to fall between 2 - 10% by weight, and preferably below 8% and preferably still below 7.5%.

[0024] Boric Trioxide (B2O3) functions as a fluxing agent and apparently promotes the liquid-liquid phase separation. It also promotes melting of the glass raw material and lowers the viscosity of the molten glass at high temperature. It also encourages the constituents of the glass to form interspersed amorphous phases manifesting substantial differences in refractive index. These differences in refractive indices provide the very dense white opacity of the opals. In the present invention, the B2O3 content, is advantageously is in excess of 3% and preferably in excess of 5% and advantageously does not go beyond 10% and preferably is below 8%.

[0025] Sodium oxide (Na2O) is useful for promoting melting of glass raw materials, adjusting thermal expansion, viscosity, etc. and serves as a stabilizing agent and prevents rapid, uncontrolled opalization of glass. It's essential to be cautious not to introduce an excessive amount in order to maintain a relatively low thermal expansion coefficient. Levels of Na2O of about 10-15% are preferred to ensure a desirable working viscosity in the glasses at temperatures below 1300° C. and to maintain an emulsification temperature below 1300° C. Hence, in this current invention, the optimal range for the Na2O content falls within 10- 17% by weight, with the ideal range situated between 11 - 16%

[0026] Calcium Oxide (CaO) does not increase the devitrification temperature of glass, improves solubility and promotes phase separation. It helps in the fusion of othercomponents during the manufacturing process. In this current invention, the CaO content is fine-tuned to fall within a range of 2 - 10% by weight, with the optimal range being 2.5 - 8.5% by weight.

[0027] Barium Oxide (BaO) is unique among the alkaline earth metal oxides in not only enhancing opacity but also exerting a positive effect upon the acid and alkaline durability of the glasses as well as contribute essentially to the formation of microcrystals. It also enhances the glass's refractive index, density, glossiness, and chemical stability. Additionally, a small quantity of BaO acts as a catalyst to expedite the fusion process. However, excessive BaO content can pose challenges by making refining more difficult. More particularly, CaO essentially affects the nature of the crystals and BaO essentially affects their size. BaO values of about 5-10% are preferred to provide minimum microwave susceptibility consistent with use as culinary ware or food service ware in a microwave oven and to hold the density of the glasses below 2.7 g / cc Thus, the present invention incorporates BaO within the range of 1 and 4% and preferably is in excess of 3.5% and below 5%.

[0028] Zinc Oxide (ZnO) functions as an intermediary in the glass network and its introduction can enhance the glass's chemical stability, heat resistance, and reduce its thermal expansion coefficient. Moreover, ZnO can also simultaneously improve the refractive index of glass and thereby increase its opacity. The present invention incorporates ZnO within the range of 0.5 - 3% in weight.

[0029] In the present invention the phosphates along with zinc oxide are used as opacifying agents, and they have this especial advantage, that glasses which contain phosphates have no problems of atmospheric pollution, whereas the opal glasses which use fluorine as an opalizing agent are troubled with this problem. Traditionally, fluorine (e.g., as CaF2 or NaF) has been used to create microcrystals like fluorspar (CaF2), which scatter light. However, P2O5 can achieve a similar effect without fluorine by forming alternative crystal phases, such as Calcium phosphate or other phosphates depending on the glass composition. Phosphorus pentoxide (P2O5) promotes phase separation. Itcontributes to the glass's opalescent or milky appearance by scattering and diffusing light within the glass matrix. When the content of P2O5 is 3% or less, brick erosion and volatilization during glass melting can be suppressed in a mass production furnace, and excessive increase in the coefficient of thermal expansion can be suppressed to suppress glass breakage during acid treatment. Where the level of P2O5 is less than 1%, the glasses become translucent, losing the desired dense opacity. The content of P2 O5 will be maintained less than 9% to keep the liquidus below 1300° C., to reduce pollution of the environment through volatilization from the melt, to lessen attack on molds and other glass forming means, and to improve the melting behavior of the glasses. P2O5 at the 6-7% level is generally preferred to develop the crystalline opal phase for dense, milky-white opacity. To achieve specific characteristics desired for the final glass product P2O5 is included within a weight range of 2 - 7%. In this way, a very homogeneous glass, susceptible of being shaped and worked under classic conditions is formed. The addition of P2O5 provides an alternative mechanism for light scattering by creating dispersed crystalline or phase-separated regions within the glass matrix. Addition of zinc oxide between (0.5 - 3%) further enhances the glossiness and shine of the opal ware by its ability to increase the refractive index. This approach is environmentally friendly, versatile, and effective in producing high-quality opal glass.

[0030] Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples. The following compositions, expressed in percent by weight as calculated from their batches, are examples of opal glasses which illustrate but do not limit the invention. An empirical visual estimation of the density of opacification is also recorded. The term "dense" signifies that the sample exhibited no translucency in the pressed ware or annealed slabs. The opacity of the glass is measured by the Diffuse Reflectance Meter device already known in the art. However, any other measurement device known in the art may be used to measure the opacity of the glass.

[0031] Embodiments of the glass compositions described herein will be further clarified by the following examples. A plurality of exemplary glass compositions were prepared according to the batch compositions (by wt%) listed in Table 1 below.Table 15

[0032] Whereas the following description reflects laboratory and pilot plant scalemelting, it will be understood that the recited compositions would also be operable in large scale commercial melting units.

[0033] Trials 1-6 are mere examples. A person skilled in the art will recognize many variations, alternatives, and modifications of the embodiment of the present disclosure.10

[0034] The present invention has both technical as well as economic significance with respect to the conventional glass composition or the like. The compositions in the present invention yield spontaneous opal glasses of dense opacity. This opal glass may be used for dinnerware, tableware , lampshades, architectural lighting, display covers, optical diffusers, semiconductors and many more applications.15

[0035] While a particular embodiment of the invention has been illustrated and described, modifications thereof will readily occur to those skilled in the art. It is understood that the various embodiment, details and composition of the opal glass and their features described above and illustrated in the attached table may be interchangedamong the various embodiment while remaining within the scope of the invention. Additionally, it is understood that various modifications could be made to any of the glass composition and / or elements described herein above while remaining within the scope of the invention.

Claims

We claim:

1. An environmentally friendly spontaneous opal glass demonstrating a dense, milky- white appearance having a level of opacity in excess of 85% , a softening point less than 765° C but in excess of 735° C, wherein spherically-shaped, glassy droplets constitute the opal phase containing the following composition essentially in terms of weight percent on the oxide basis as calculated from the batch of, 58-74% SiO2, 10-18.5 % Na2 O, 5- 10% BaO, 2-10% Ah O3, 3-10% B2O3, 2- 10% CaO, 0.32-3% ZnO , and 2-7% P2Os, wherein, the total of the weight quantities of the ingredients amounting to 100%, having a density of at least 2.4 g / cm3and is opacified as a result of phase separation, and having said that the opal glass composition is free from elements which are hazardous to environment as well as human health.

2. An environmentally friendly spontaneous opal glass as set forth in claim 1 , characterized in that it has the following composition in terms of weight percent on the oxide basis, 58-69% SiO2, 11-16.9 % Na2O, 3-6.9% BaO, 2.7-7.5% Ah O3, 3-8% B2O3, 2.5- 8.5% CaO, 0.5-3% ZnO , and 2.8-6.1% P2O5, wherein, the total of the weight quantities of the ingredients amounting to 100%.

3. An environmentally friendly spontaneous opal glass as claimed in claim 1, wherein the glass composition is free from Fluorine (F) and compounds containing (F).

4. An environmentally friendly spontaneous opal glass as claimed in claim 1, wherein the level of opacity is greater than 85%.

5. An environmentally friendly spontaneous opal glass as claimed in claim 1, wherein the level of opacity greater than 85% is achieved with minimum amount of Phosphorous Pentoxide (P2Os).

6. An environmentally friendly spontaneous opal glass as claimed in claim 2, wherein the softening point is less than 760° C but in excess of 740° C7. An environmentally friendly spontaneous opal glass demonstrating a dense, milky-white appearance having a level of opacity in excess of 84% , a softening point less than 765° C but in excess of 735° C, wherein spherically-shaped, glassy droplets constitute the opal phase containing the following composition essentially in terms of weight percent on the oxide basis as calculated from the batch of, 60-71% SiO2, 11-17 % Na2 O, 0- 4.3% BaO, 2.8-8% Ah O3, 1.8-5.2% B2O3, 2.5- 9.5% CaO, 0.31-1.5% ZnO , 3.8-6.6% P2Os, and 0-0.02% Sb2O3wherein, the total of the weight quantities of the ingredients amounting to 100%, having said that the opal glass is opalized as a result of phase separation during formation, and a density of at least 2.4 g / cm3•8. An environmentally friendly spontaneous opal glass as set forth in claim 1, characterized in that it has the following composition in terms of weight percent on the oxide basis, 59-68% SiO2, 12-17 % Na2O, 0-3% BaO, 2.4- 7.7% Ah O3, 3-7.7% B2O3, 2.3- 7.5% CaO, 0.5-2.5% ZnO , 3.8-6.2% P2O5, and 0-0.02% Sb2O3wherein, the total of the weight quantities of the ingredients amounting to 100%.

9. An environmentally friendly spontaneous opal glass as claimed in claim 8, wherein the level of opacity is greater than 85%.

10. An environmentally friendly spontaneous opal glass as claimed in claim 1, wherein the thermal expansion coefficient of opal glass makes it moderately resistant to thermal stress and can be used for dinnerware,tableware, lampshades, architectural lighting, display covers, semiconductors, optical diffusers and many more applications.

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