GLASSES FOR CONTAINERS; PROCESSES FOR THE PRODUCTION OF SAME; AND CONTAINERS COMPRISING SAID GLASSES

Abstract: A green glass for a container, for example, a beer bottle capable of protecting beer from sunflavour is provided which glass comprises 0.3 to 1.5 % by weight of chromium oxide in terms of Cr₂O₃ and at most 0.06 % by weight of Fe₂O₃ in a glass composition of SiO₂-Na₂O-CaO type and having C.I.E. colorimetric values for 10 mm thickness: a brightness (Y) of 10 to 25 %, a dominant wavelength (λd) of 556 to 564 nm and an excitation purity (Pe) of at least 90 %, which is capable of intercepting substantially light of 340 to 460 nm in a thickness of 4 mm.

Description: This invention relates to green glasses of soda-lime-silica type for containers, capable of absorbing or intercepting ultraviolet rays and near ultraviolet visible rays, and more particularly, it is concerned with green glass compositions for beer bottles, capable of protecting beer from so-called sunflavour. It is known that beer is deteriorated by the action of sunlight so that it gives out the so-called 'sunflavor'. In particular, light within a wavelength range of 400 to 460 nm reacts with bitter substances of hops so that the beer loses its intrinsic amber colour and becomes red-tinted, and gives out a shunky odour, namely sunflavour. A beer bottle formed of ordinary amber glass (thickness: about 4 mm, shown in Fig. 1 and Fig. 2, Curve C) intercepts light of 400 to 460 nm, but another beer bottle of ordinary green glass does not have properties comparable to those of the amber glass, as shown in Fig. 1, although various efforts have been made to intercept ultraviolet rays. Therefore, beer in a bottle of green glass must be stored with greater care than in the case of the amber glass. If this is not done there arises the problem that the beer soon deteriorates. An example of the ordinary green glass commonly used for beer bottles is a glass having a composition comprising 72.2 % SiO₂, 12.7 % Na₂O, 10.8 % CaO, 1.94 % Al₂O₃, 1.34 % K₂O, 0.13 % Cr₂O₃, 0.15 % Fe₂O₃ , the percentages being by weight, and unavoidable impurities, and having a brightness (Y) of 33.8 %, a dominant wavelength (λd) of 558.7 nm and an excitation purity (Pe) of 68.3 %. This ordinary green glass intercepts light of 400 nm, but transmits a substantial proportion of light of 400 to 460 nm, as is evident from the percent transmission shown in Fig. 1, Curve D. In the production of such an ultraviolet light intercepting green glass, furthermore, there are various problems due to the necessity of maintaining hexavalent chromium in the glass. That is, since an oxidant such as sodium nitrate is used in a proportion of more than 1 part by weight to 100 parts by weight of silica sand in the glass composition, (1) seeds or small bubbles tend to occur in the glass, (2) the production cost is increased because of the necessity to raise the temperature of the melting furnace or decrease the production quantity of the melting furnace so as to control the occurrence of seeds and (3) nitrogen oxides (NOx) are formed through decomposition of the sodium nitrate. Since the operation is carried out by feeding a larger amount of air to secure a higher degree of oxidation than in the case of the ordinary transparent or coloured glass, (1) the energy consumption is consequently increased and the production cost is increased and (2) the quantity of nitrogen oxides is increased, resulting in the problem of environmental pollution. It is an object of the present invention to provide an improved green glass of soda-lime-silica type, for a container, which gives adequate light protection. It is another object of the present invention to provide a green glass composition capable of intercepting or absorbing ultraviolet rays and near violet visible rays and suitable for the production of containers used for packaging food or beverages. It is a further object of the present invention to provide a beer bottle made from a green glass free from seeds or small bubbles and capable of protecting beer from sunflavour. It is a still further object of the present invention to provide a process for the production of an improved green glass of soda-lime-silica type, whereby the disadvantages of the prior art can be overcome. These objects can be attained by a green glass for a container containing 0.3 to 1.5 % by weight of chromium oxide in terms of Cr₂O₃ and at most 0.06 % by weight of Fe₂O₃ in a glass composition of SiO₂-Na₂O-CaO type and having C.I.E. colorimetric values for 10 mm thickness: a brightness (Y) of 1.02 % to 25 %, a dominant wavelength (λd) of 556 to 564 nm and an excitation purity (Pe) of at least 90 %, which is capable of intercepting substantially a light of 340 to 460 nm in a thickness of 5 mm. The accompanying drawings illustrate the principle and merits of the present invention in greater detail. Fig. 1 and Fig. 2 are respectively graphs showing the percent transmissions of light in the ultraviolet region and near ultraviolet visible region as to green glasses of Example 1 to Example 3 according to the present invention, and amber glass and green glass of the prior art, each having a thickness of 4 mm, except Curve F (1 mm) (Fig. 1) and 2 mm (Fig. 2). The inventors have made various efforts to develop a green glass for a container capable of preventing photochemical deterioration of the contents, in particular, protecting beer from so-called sunflavour and have succeeded in obtaining such a green glass by the use of a silica sand for transparent glasses, which has a smaller Fe₂O₃ content. According to the present invention there is provided a green glass for a container containing 0.3 to 1.5 % by weight, preferably 0.35 to 1.5 % by weight, in particular, 0.35 to 0.5 % by weight of chromium oxide in terms of Cr₂O₃ and at most 0.06 % by weight of Fe₂O₃ in a glass composition of SiO₂-Na₂O-CaO type, having C.I.E. colorimetric values for 10 mm thickness: a brightness (Y) of 1.02 % to 25 %, a dominant wavelength (λd) of 556 to 564 nm and an excitation purity (Pe) of at least 90 % and being capable of substantially intercepting light of 340 to 460 nm in a thickness of 4 mm, and a process for the production of the above described green glass, which process comprises preparing and melting a batch composition comprising 100 parts by weight of silica sand, 24 to 40 parts by weight of soda ash, 20 to 30 parts by weight of limestone, 0.5 to 3.0 parts by weight of sodium sulfate, 0.8 to 4.0 parts by weight of a bichromate, 0 to 3 parts by weight, preferably 0 to 1 part by weight of a nitrate and unavoidable impurities. Generally, the glass composition of SiO₂-Na₂O-CaO type used in the green glass of the present invention comprises 60 to 75 % by weight of SiO₂, 10 to 16 % by weight of Na₂O, 8 to 12 % by weight of CaO, 0.5 to 6 % by weight of Al₂O₃, 0 to 5 % by weight of K₂O and unavoidable impurities. In the green glass composition of the present invention, chromium oxide is added in a proportion of 0.3 to 1.5 % by weight, preferably 0.35 to 1.5 % by weight in terms of Cr₂O₃, for example, by the use of a bichromate such as potassium bichromate, and the iron content is adjusted to 0.06 % or less as Fe₂O₃. The thus-added chromium is preferably retained in the hexavalent chromium state as much as possible. The iron components unavoidably come from the silica sand and limestone and are added to the glass composition. When using the ordinary silica sand and limestone for coloured glasses, the content of Fe₂O₃ is about 0.15 to 0.2 % by weight, whereas in the present invention, a silica sand for transparent glasses, containing a smaller amount of Fe₂O₃ is particularly used to adjust the iron content to that described above. In the green glass of the present invention, no colouring constituents other than chromium oxides, for example, metal oxides such as CuO, Co₃O₄, NiO, As₂O₅, Fe₂O₃, MnO, CeO₂ and Sb₂O₃ are added. When these metal oxides are added, transmission of heat rays is lowered, resulting in deterioration of the melting and forming properties, and Cr⁶⁺ is converted into Cr³⁺ so that interception of light near 450 nm is insufficient. The amount of an oxidant such as sodium nitrate is generally 0, but, if necessary, the oxidant can be added in a proportion of less than 1 part by weight to 100 parts by weight of silica sand. Even if the operation is carried out under the ordinary conditions except using the oxidant in such a smaller proportion, including 0, a glass capable of substantially intercepting light of 340 to 460 nm can be obtained. In the glass compositions of the present invention, chromium oxides are contained in a proportion of 0.3 to 1.5 % by weight, preferably 0.35 to 1.5 % by weight, because the green colour is enhanced in order to intercept light of 400 to 460 nm by increasing the content thereof to more than that of the prior art green glass. If less than 0.3 % by weight, in particular, 0.35 % by weight, of chromium oxides is included the interception of light of 400 to 460 nm is sufficient, as shown by the percent transmission curve of Comparative Example 1 in Fig. 1, while if more than 1.5 % by weight of chromium oxides is included the brightness of the glass is excessively lowered. When 1.5 % by weight of chromium oxides are contained in the glass composition, a light protection effect can be given even in the case of a thickness of 1 mm, which effect is substantially similar to the case of adding 0.35 % by weight of Cr₂O₃ for a thickness of 4 mm. If the amount of iron oxides exceeds 0.06 % by weight as Fe₂O₃, Cr⁶⁺ is converted into Cr³⁺ depending on the amount of the excess and the interception of light near 450 nm will probably be insufficient. Since it is assumed that hexavalent chromium has a large influence upon the interception of light of near 450 nm, as described above, Cr⁶⁺ is fed in the form of a bichromate and the content of Fe₂O₃ is controlled so as to be reduced and metal oxides such as CuO are not added to secure the hexavalent chromium state. Furthermore, the melting and forming properties of the glass are deteriorated depending on the increment of Fe₂O₃. On the other hand, the amount of an oxidant can largely be decreased by decreasing the content of Fe₂O₃ to a greater extent as compared with the coloured glass of the prior art, namely from 0.15-0.2 % by weight to less than 0.06 % by weight, thereby obtaining a melt which is excellent in fining and a seed-free glass. According to the present invention, the following advantages can be obtained.: (1) The content of Cr₂O₃ is increased to thicken the green colour thereby completely intercepting near ultraviolet visible rays of near 400 to 460 nm and preventing occurrence of the so-called sunflavour. (2) The amount of oxidant can be decreased, resulting in a melt excellent in fining, and a green glass free from seeds can thus be produced by largely decreasing the content of Fe₂O₃. (3) Production of a green glass can be carried out without lowering the percent transmission of heat rays, deteriorating the melting and molding properties of the glass and lowering the production speed by decreasing the content of Fe₂O₃ and excluding addition of metal oxides such as copper oxide. (4) In the case of a thickness of 2 mm, the percent transmission of the glass according to the present invention in the wavelength region of ultraviolet to 460 nm is smaller than that of amber glasses of the prior art. Therefore, the glass of the present invention can be widely applied to containers or bottles capable of protecting the contents such as food and beverages. (5) In a situation requiring lightweight containers, the glass composition of the present invention will be more favourably applied. Even in the case of a thickness of 1 mm, increase of the quantity of Cr₂O₃ to 1.5 % by weight results in benefits comparable to those in the case of a thickness of 4 mm (Cr₂O₃ 0.35%). (6) Since the use of excessive air and sodium nitrate is controlled according to the present invention and the quantity of NOx generated in the process can be largely decreased, the problem of environmental pollution can be alleviated. The following examples are given in order to illustrate the present invention in detail, and in which parts and percents are to be taken as those by weight unless otherwise indicated. Example 1 To 100 parts of silica sand containing 0.04 % of Fe₂O₃ were added 28.8. parts of soda ash, 24 parts of limestone containing 0.025 % of Fe₂O₃, 1.0 part of sodium sulfate and 1.2 parts of potassium bichromate and the resulting mixture was melted to prepare a green glass having a composition of 72.1 % SiO₂, 13.6 % Na₂O, 10.2 % CaO, 1.8 % Al₂O₃, 1.5 % K₂O, 0.46 % Cr₂O₃, 0.04 % Fe₂O₃ and trace unavoidable impurities. This glass had C.I.E. colorimetric values consisting of the following for a typical 10 mm specimen: a brightness (Y) of 16.6 %, dominant wavelength (λd) of 560.4 nm and excitation purity (Pe) of 97.8 % and had a percent photo-transmission as shown in Fig. 1 in the case of a thickness of 4 mm (Curve A). It will clearly be understood from the results that the above described green glass is more suitable for a beer bottle since the percent transmission in the case of a thickness of 4 mm corresponding to the thickness of a beer bottle is zero for light of 400 and 460 nm which is strongly related to sunflavour in beer, in comparison with other comparative glasses, Curves D and E in Fig. 1. A glass bottle with a volume of 350 ml, weight of 240 g and side wall thickness of 2.5 mm was made from the above described green glass composition and an emerald green bottle with the same dimensions corresponding to an ultra-violet ray intercepting green glass shown in Fig. 1, Curve D, was used for comparison. Both the bottles were filled with beer and then subjected to a sensing test in which periods of time were measured until their sunflavours reached a same level. The green glass bottle of the present invention showed a period of time of 3.9 times as long as the comparative bottle. Example 2 The mixing and melting procedure of Example 1 was repeated except that the quantity of the potassium bichromate was decreased to 0.9 part, thus obtaining a green glass with a composition comprising 72.1 % SiO₂, 13.6 % Na₂O, 10.2 % CaO, 1.8 % Al₂O₃, 1.5 % K₂O, 0.35 % Cr₂O₃, 0.06 % Fe₂O₃ and trace unavoidable impurities. The thus resulting glass had C.I.E. colorimetric values as follows for a typical 10 mm specimen: a brightness (Y) of 23.9 %, a dominant wavelength (λd) of 561.7 nm, and an excitation purity (Pe) of 96.2 % and a percent photo-transmission as shown in Fig. 1 in the case of a thickness of 4 mm (Curve B). Similarly, this green glass is suitable for a beer bottle. The green glasses of Examples 1 and 2 each having a thickness of 2 mm showed percent photo-transmissions shown in Fig. 2, Curves A and B, which were favourably compared with an amber glass, Curve C, and the emerald green glass, Curve D, for light of 400 to 460 nm. Comparative Example 1 The procedure of Example 1 was repeated except using 0.6 part of potassium bichromate and additionally using 3.0 parts of sodium nitrate, thus obtaining a glass having a composition as shown in Table 1. The percent transmission of the glass of Comparative Example 1 is shown in Fig. 1, Curve E. Example 3 The procedure of Example 1 was repeated except for increasing the quantity of the potassium bichromate in the raw materials used in Example 1 to 3.9 parts, thus obtaining a glass with a composition of 72.1 % SiO₂, 13.6 % Na₂O, 10.2 % CaO, 1.8 % Al₂O₃, 1.5 % K₂O, 1.5 % Cr₂O₃, 0.04 % Fe₂O₃ and trace unavoidable impurities. This glass had C.I.E. colorimetric values as follows for a 1 mm specimen: a brightness (Y) of 49.1 % (corresponding to a calculated value of 1.02 % for a sample thickness of 10 mm), dominant wavelength (λd) of 565.0 nm and excitation purity (Pe) of 88.0 %, and had a percent transmission at a thickness of 1 mm, as shown in Fig. 1, Curve F.

Claims: A green glass for a container containing 0.3 to 1.5 % by weight of chromium oxide in terms of Cr₂O₃ and at most 0.06 % by weight of Fe₂O₃ in a glass composition of SiO-Na₂O-CaO type and having C.I.E. colorimetric values for 10 mm thickness: a brightness (Y) of 1.02 % to 25 %, a dominant wavelength (λd) of 556 to 564 nm and an excitation purity (Pe) of at least 90 %, which is capable of intercepting substantially light of 340 to 460 nm in a thickness of 4 mm. A process for the production of the green glass as claimed in Claim 1, which process comprises preparing and melting a batch composition comprising 100 parts by weight of silica sand, 24 to 40 parts by weight of soda ash, 20 to 30 parts by weight of limestone, 0.5 to 3 parts by weight of sodium sulfate, 0.8 to 4 parts by weight of a bichromate, 0 to 3 parts by weight of a nitrate and unavoidable impurities. A process as claimed in Claim 2, wherein the silica sand is one for transparent glasses, containing at most 0.06 % by weight of Fe₂O₃. A process as claimed in Claim 2 or Claim 3, wherein the batch composition does not contain metal oxides other than chromium oxides. A process as claimed in Claim 4, wherein the metal oxides are CuO, Co₃O₄, NiO, As₂O₅, Fe₂O₃ , MnO, CeO₂ and Sb₂O₃. A container such as a beer bottle whenever comprising a green glass as claimed in Claim 1.

Inventors: ABE, YOSHIHIRO, KITAYAMA, MASAO, MINOURA, NAOYA/MESON MIYAMA 2-13, NOGUCHI, AKIRA
Applicants: KIRIN BEER KABUSHIKI KAISHA, YAMAMURA GLASS CO. LTD.
Application year: 1987
Main classification: C03C4
IPC codes: C03C3, C03C4, C03C6