EP0034494A1

EP0034494A1 - Curvilinear type fluorescent lamp

Info

Publication number: EP0034494A1
Application number: EP19810300626
Authority: EP
Inventors: Atsushi Sato; Noriyoshi Kikuchi; Takashi Oomori; Masanori C/O Techn. Group Nakamura
Original assignee: Toshiba Corp; Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1980-02-15
Filing date: 1981-02-16
Publication date: 1981-08-26
Also published as: AU530404B2; CA1158704A; DE3161854D1; AU6720281A; EP0034494B1

Abstract

A curvilinear type fluorescent lamp comprises a curvilinear, light-transmitting sealed tube (34) made of soda-lime glass or low lead glass containing 12 percent by weight or less of lead oxide, whose softening point is 640 to 720°C and whose coefficient of linear thermal expansion is 92 to 105 x 10^-7 cm/cm/°C. On the inner surface of the tube (34) a phosphor layer (36) is formed. The phosphor forming the layer (36) contains a binder consisting chiefly of calcium borate.

Description

This invention relates to a curvilinear type fluorescent lamp comprising a curvilinear, light-transmitting sealed tube made of soda-lime glass or low lead glass.
Various curvilinear type fluorescent lamps are known. Of these lamps, a circular fluorescent lamp, for example, is manufactured in the following manner. First, the inner surface of a straight, light-transmitting glass tube is coated with a phosphor. Then, a stem holding an electrode coated with an electron-emitting substance is sealed to either end of the straight glass tube. The tube is then heated and softened in a furnace. It is bent about a drum, thus being shaped like a circle. Thereafter, the tube is cooled and subsequently evacuated. Finally, an inert gas and mercury are introduced into the tube.
The light-transmitting, sealed tube of the known curvilinear fluorescent lamp is made of lead glass which contains 24 to 29 percent by weight of lead oxide and which has a low softening point. The lead glass is used because a tube made of it can easily be bent to form a circle at low temperatures. The glass, however, is expensive because it contains a great quantity of expensive lead oxide. A curvilinear type fluorescent lamp will be costly if provided with a curvilinear tube made of lead glass. Further, lead is one of the prominent sources of environmental pollution. Used and broken fluorescent lamps made of lead glass can hardly be disposed of in such a way that the lead does not promote environmental pollution.
Instead of lead glass having the above-mentioned drawbacks, soda-lime glass containing no oxide of lead is used on trial basis as the material of a straight, light-transmitting sealed tube. A straight tube of soda-lime glass cannot be bent easily, however, unless it is heated to a higher temperature than a tube of lead glass. This is because soda-lime glass has a higher softenging point than lead glass. While a soda-lime glass tube is being heated to a high temperature and bent, the phosphor laid on the inner surface of the tube is moved into the glass layer. As the tube is cooled, it may be broken by tension generated due to the difference in thermal expansion coefficient between phosphor and soda-lime glass. If not broken at the end of cooling process, the tube has its strength reduced to an alarming degree. In addition, the radiant efficiency of the phosphor will be reduced, therefore the initial luminous flux of the lamp will be lowered.
Low lead glass which may be used instead of high lead glass or soda-lime glass is disclosed in Japanese Patent Publication No. 50-1580 and Japanese Patent Disclosure (Kokai) No. 54-60778. The low lead glass disclosed therein has a softening point lower than that of sode-lime glass. The low lead glass disclosed in Japanese Patent Publication No. 50-1580 contains 1.91 to 10 percent by weight of lead oxide, whereas the low lead glass disclosed in Japanese Patent Disclosure (Kokai) No. 54-60778 contains 4 to 19 percent by weight of lead oxide.
The low lead glass similar to that disclosed in Japanese Patent Publication No. 50-1580 has such an oxide composition (wt. %) as shown in Table 1. The oxide compositions of two examples A, B are shown in Table 2 together with those of the lead glass and soda-lime glass which are used as the materials of the known curvilinear and straight fluorescent tubes respectively. The coefficients of linear thermal expansion and softening points of the low lead glasses A and B, the lead glass and the soda-lime glass are shown also in Table 2.
The inventors thereof made a number of curvilinear type fluorescent lamps of the same dimensions, whose curvilinear tubes were made of different glasses, i.e. low lead glasses A and B, the conventional lead glass and the soda-lime glass. These lamps were tested to determine their characteristics. The test showed that the lamps made of glass A and the lamps made of glass B had an initial lumen comparable with that of the lamps made of the conventional lead glass and better than that of the lamps made of soda-lime glass. However, these lamps were disadvantageous in the following repsects:

(1) These lamps had not a sufficient strength.
(2) Their life performance of luminous flux was poorer than that of the lamps made of the lead glass.

Now it will be discussed why the lamps made of glasses A and B were disadvantageous despite their good initial lumen. Table 3 shows the initial luminous flux, luminous flux at 3000 Hr and luminous flux at 5000 Hr of the curvilinear type fluorescent lamps made by the inventors and subsequently tested. "Initial luminous flux" was detected at the 100th hour of use, in accordance with the regulation JISC7601. All the lamps were circular fluorescent lamps of 100V, 30W. The numerical data given in Table 3 were the average of 50 lamps of every type tested.
Fig. 1 shows the typical outer appearance of a circular fluorescent lamp made of the conventional low lead glass, upon lapse of 3000 hours of continuous use. A portion 24 of the circular tube 22 made of the low lead glass, which contained 12 percent by weight or less of lead oxide (PbO), was yellow brown. There was ascertained the trend that this color change occurred at that portion 24 of the tube 22 which was heated to the highest temperature during the bending process, i.e. that portion 24 which is positioned at the highest place in the furnace.
It may be well supposed that this color change was caused partly by sodium amalgam formed by reaction between Na⁺ diffused from the glass and mercury vapor and partly by so-called dangling bond, i.e. bonding of Hg⁺ and radical of the glass, which were generated by the phosphor moved into the glass. As shown in Table 2, Na⁺ was diffused in a small amount from the conventionally used lead glass because the glass contained a small amount of sodium oxide (Na₂0). The tubes of the lead glass were bent in a desired fashion at a relatively low temperature, and the phosphor moved into the glass but in a small amount and no prominent dangling bond tended to occur.
The above-mentioned test further revealed that, the more phosphor moved into glass, the weaker became the tubes made of the glass. The reason why so will be discussed.
Binders generally contained in the phosphor for fluorescent lamps contains boron oxide (B₂0₃). The boron oxide therefore is partly in contact with sode-lime glass or low lead glass. A portion of the boron oxide therefore melts when the tubes of either soda-lime glass or low lead glass are heated and bent in the furnace. The molten borate is fused with the glass, thus forming a B₂0₃-Si0₂-Na ₂0 glass. The viscosity of the glass thus formed sharply decreases when the glass is heated to about 800°C. When the glass tubes are heated to about 800°C, the inner surfaces as well as their outer surfaces are softened, whereby the phospher moves into them. As a result, when the tubes bent are cooled, the inner surface portion of each tube has fine cracks due to the difference in thermal expansion coefficient between the phosphor and the glass. The fine cracks seem to weaken the tubes.
Indeed a binder will more strongly bind phosphor particles if it contains barium oxide (BaO). But barium oxide will lower the melting point of the binder. And the binder may be possibly melt before the glass softens. If this happens, more phosphor particles will move into the glass layer of a tube while the tube is being heated, softened and bent. With reference to Figs. 2(a), 2(b) and 2(c), it will be described how phosphor particles containing the binder consisting barium calcium borate move into the glass layer of the tube.
Fig. 2(a) shows the particles 28 of phosphor coated on the inner surface of the glass tube 22 and subsequently baked. Fig. 2(b) illustrates the phosphor particles 28, some of which lie in the glass layer of the tube 22 which has been already heated, softened and bent. In the recesses 30 between the glass layer and each phosphor particle 28 that lies partly or wholly within the glass layer a dangling bond will be formed. Fig. 2(c) illustrates a part 26 of the portion 24 of the tube 22 which has been used for 3000 hours (see Fig. 1). Since mercury vapor has flowed into the recesses 30 through the spaces among the phosphor particles 28, the portion 24 of the tube is turned into yellow brown by reaction between dangling bond of glass and this mercury.
Japanese Utility Model Disclosure (Kokai) No. 53-92976 discloses the technique of providing a metal oxide layer between the inner surface of a curvilinear glass tube and a phosphor layer thereby to reinforce the glass tube. This technique may indeed be effective if the tube is made of glass having a softening point of about 600°C. But the technique does not seem to work if the tube is made of soda-lime glass or low lead glass which has a relatively high softening point ranging from 640°C to 720°C. Japanese Patent Disclosure (Kokai) No. 53-92976 further teaches that on the metal oxide layer there is formed a layer of phosphor containing 0.1 to 4 percent by weight of a B₂0₃-containing binder and that the binder may be a barium calcium borate binder. A barium calcium borate binder, however, melts before soda-lime glass or low lead glass softens which has a high softening point. If the tube according to Disclosure (Kokai) No. 53-92976 is made of soda-lime glass or low lead glass, the binder will help many phosphor particles move into the glass layer of the tube and hence will create many dangling bond when the tube is heated, softened and bent. The dangling bond thus formed in great numbers will eventually reduce the performance of luminous flux of the resultant curvilinear fluorescent lamp.
Japanese Patent Disclosure (Kokai) No. 49-20972 discloses a method of improving the life performance of luminous flux of a curvilinear fluorescent lamp. More specifically, a titanium dioxide film 0.02 to 0.2 micron thick is laid on the inner surface of a glass tube. Patent Disclosure -(Kokai) No. 49-20972 teaches that, unless the titanium dioxide film is provided, the sodium ions liberated from the glass reacts with mercury vapor to form amalgam upon lapse of a predetermined burning time of the lamp and that the amalgam, if formed, causes fluorescent paint (e.g. Ca₅(P0₄)₃X; Sb, Mn, where X = F, Cl) to turn gray. The above-mentioned titanium dioxide film is too thin to prevent the aforementioned yellow brown change of color taking place in a curvilinear fluorescent lamp which is made of soda-lime glass or low lead glass.
Japanese Patent Publication No. 35-12085 discloses a binder consisting of CaO and B ₂0₃ in weight ratio of 3:1 to 1:2. The binder is mixed with phosphor, and the mixture is coated on the inner surface of a glass tube and subsequently baked, thereby forming calcium borate. This technique aims to enhance the strength of the phosphor layer without reducing the luminous efficacy of a fluorescent lamp comprising the glass tube. In contrast, the binder used in the present invention consists chiefly of calcium borate. It is used in order to prevent phosphor particles from entering a layer of soda-lime glass or low lead glass having a high softening point, while a tube made of such glass is being heated, softened and bent, thereby to improve the strength of the glass tube. It is used also in order to suppress diffusion of sodium ions into the glass layer, thereby to enhance the luminous efficacy of the lamp.
It is an object of.this invention to provide a curvilinear type fluorescent lamp which comprises a curvilinear, light-transmitting sealed tube made of soda-lime glass or low lead glass and which.has an improved strength and 'its improved life performance of luminous flux.
According to this invention there is provided a fluorescent lamp comprising a curvilinear, light-transmitting sealed tube having an electrode attached to either end and coated with electron-emitting substance, said tube being made of soda-lime glass or low lead glass containing 12 percent by weight or less of lead oxide, whose softening point is 640 to 720°C and whose coefficient of linear thermal expansion is 92 to 105 x 10^-7 cm/cm/°C; and a phosphor layer laid on the inner surface of the tube and containing binder consisting chiefly of calcium borate.
The lamp of the above-mentioned structure has initial luminous flux comparable with that of a curvilinear type fluorescent lamp made of the conventionally used high lead glass. The binder melts at the same time that a straight tube made of said glass is bent, thus suppressing movement of phosphor particles into the glass layer of the tube. Further it is possible with the lamp to prevent sodium ions from difussing from the glass layer while the lamp is being used.
According to this invention, another curvilinear type fluorescent lamp is provided, which comprises a curvilinear, light-transmitting sealed tube having an electrode attached to either end and coated with electron-emitting substance, said tube being made of soda-lime glass or low lead glass containing 12 percent by weight or less of lead oxide, whose sofening point is 640 to 720°C. and whose coefficient of linear thermal expansion is 92 to 105 x 10^-7 cm/cm/°C; a metal oxide layer coated on the inner surface of the tube in an amount of 0.6 to 6.2 mg per square centimeter and made of at least one of the group consisting of aluminum oxide, magnesium oxide, silicon oxide and titanium oxide; and a phosphor layer laid on the metal oxide layer and containing binder consisting chiefly of calcium borate.
The curvilinear sealed tube of the latter-mentioned lamp is made stronger than that of the first-mentioned lamp, owning to the use of the metal oxide layer. Needless to say, the metal oxide layer effectively works to suppress movement of phosphor particles into the glass layer and thus suppress creating of radical of the glass. Moreover, it prevents the radical (dangling bond), if any, from trapping Hg+, thereby improving the life performance of luminous flux of the lamp. Still further, the metal oxide layer suppresses reaction between Hg and sodium ions diffused from the glass layer.
If calcium borate is used in an amount of 0.2 to 2.0 percent by weight on the basis of the amount of phosphor used, the phosphor layer becomes strong and the initial luminous flux of the lamp is improved. Further, if the phosphor containing the binder is coated on the inner surface of the glass tube or on the metal oxide layer coated on the inner surface of the tube in an amount of 2.9 to 3.9 mg per square centimeter, the characteristics of the lamp will be improved.
All the effects mentioned above will be promoted if the above-mentioned glass contains 12 to 17 percent by weight of sodium oxide (Na₂0). In addition, the characteristics of the lamp will be further improved if the above-mentioned glass contains 3 to 8 percent by weight of calcium oxide, which effectively suppresses diffusion of sodium ions.
Moreover, both the initial luminous flux and life performance of luminous flux of the lamp will be further enhanced if the calcium borate contained in the binder is obtained from a mixture of CaO and B ₂0₃, the mol ratio of CaO to B ₂0₃ being 1/2 to 3/2 and is deposited on the inner surface of the glass tube or on the metal oxide layer.
The lamp of either structure mentioned above can be manufactured at a reduced cost. Further, since the glass used contains a small amount of lead, the lamp of this invention is preferred in view of environmental pollution.
Other objects and advantages of the invention will become apparent during the following discussion of the accompanying drawings.
This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 shows a known circular type fluorescent lamp which has been continuously used for 3000 hours;
Fig. 2(a) is an enlarged sectional view of the layer of a known glass tube and a phosphor layer coated on the glass layer;
Fig. 2(b) is an enlarged sectional view of the layer of the known glass tube now bent in the form of a circle and the phosphor layer coated on the glass layer;
Fig. 2(c) is an enlarged sectional view of a portion 26 of the circular type fluorescent lamp shown in Fig. 1, taken along line II-II' in Fig. 1;
_Fig. 3 shows a circular type fluorescent lamp according to this invention;
_Fig. 4 is a cross sectional view of the lamp shown in Fig. 3, taken along line IV-IV' in Fig. 3;
_Fig. 5 is a graph showing the relationship between the strength of a phosphor layer and the mixing ratio of a binder consisting chiefly of calcium borate;
_Fig. 6 is a graph illustrating the relationship between total luminous flux and the mixing ratio of a binder consisting chiefly of calcium borate;
Fig. 7 is a graph showing the relationship between the quantity of phosphor coated, on one hand, and initial luminous flux and ratio of generated blackening, on the other;
_Fig. 8 shows another circular type fluorescent lamp according to this invention;
Fig. 9(a) is an enlarged sectional view of the layer of a glass tube of this invention and a phosphor layer coated on the glass layer;
Fig. 9(b) is an enlarged sectional view of the glass tube of this invention, now bent in the form of a circle, and the phosphor layer coated on the glass layer; and
Fig. 9(c) is an enlarged sectional view of a portion of the lamp shown in Fig. 8 which has been continuously used for 3000 hours.

Fig. 3 shows the outer appearance of a circular type fluorescent lamp 32 according to this invention. _Fig. 4 is a cross sectional view of the lamp 32, taken along line IV-IV' in Fig. 3. The lamp 32 comprises a ring-shaped sealed glass tube 34 and a phosphor layer 36 coated on the inner surface of the glass tube 34. The tube 32 is made of low lead glass containing 12 percent by weight or less of lead oxide (PbO) and having a softening point of 640 to 720°C and a coefficient of linear thermal expansion of 92 to 105 x 10^-7 cm/cm/°C. The layer 36 is made of calcium halophosphate phosphor activated by manganese and antimony (Ca₅(PO₄)₃X; Sb, Mn, where X = F, Cl) containing a binder consisting of calcium borate.
Fifty circular fluorescent lamps of the structure shown in Figs. 3 and 4 were made which were provided with phosphor layers of the same thickness. These were 100 V - 30 W fluorescent lamps. They were continuously used for 3000 hours. Upon lapse of these hours all the fifty lamps were found to have performance of luminous flux ranging from 92 to 94 %. Further, their sealed glass tubes 34 had almost never turned yellow brown, unlike the conventionally used glass tubes inner surfaces of which are coated with phosphor containing barium-calcium borate binder.
It was found that calcium borate should be used in mixing ratio of 0.2 to 2.0 percent by weight with respect to the phosphor. If calcium borate is used in an mixing ratio outside this specific range, the phosphor layer will not be sufficiently strong and the initial luminous flux of the lamp will not be sufficiently high.
Fig. 5 illustrates the relationship between the strength of phosphor layer and the mixing ratio of a binder. Curve a indicates this relationship observed in the above-mentioned another fifty fluorescent lamps according to this invention. Curve shows this relationship observed in fifty circular fluorescent lamps of the same size and structure but using a known binder consisting of barium calcium borate. Curve y indicates said relationship observed in fifty circular fluorescent lamps of the same size and structure but using a known binder consisting of calcium phosphate and thus having an extremely high melting point. The binders used in the lamps of the three categories have such melting point as given in Table 4. This melting point was obtained by detecting the maximum heat-absorption temperature of the respective binders, using differential thermal analysis.
The phosphor layers of all the fluorescent lamps of the three categories were formed by coating the phosphors on the inner surfaces of the respective glass tubes and subsequently baking the phosphor thus coated. The strength of each of the phosphor layer was detected by blowing air at the rate of 300 Torr ℓ/sec against the layer through a nozzle spaced from the layer by 10 mm. The strength of the layer is the reciprocal of the ratio of the diameter of the largest flake of phosphor exfoliated from the inner surface of the glass tube to the diameter of the largest flake of known phosphor exfoliated from the inner surface of a tube made of the conventionally used lead glass, said known phosphor containing a binder consisting of barium calcium borate and calcium phosphate in weight mixing ratio of 2:1. In _Fig. 6 said reciprocal is given in percentage.
Fig. 6 illustrates the relationship between the total luminous flux of each fluorescent lamp tested and the mixing ratio of a binder used. Curve a' indicates this relationship observed in the fifty fluorescent lamps according to this invention. Curve S' shows the relationship observed in fifty fluorescent lamps using the known binder consisting of barium calcium borate. Curve y' shows the relationship observed in the fifty fluorescent lamps using the known binder consisting of calcium phosphate.
As evident from Figs. 5 and 6, the circular fluorescent lamps using calcium botate in a mixing ratio of 0.2 to 2.0 % exhibited performance of luminous flux better than that of the fluorescent lamps made of the conventionally used lead glass and exhibited a total luminious flux greater than the fluorescent lamps using the known binder consisting of barium calcium borate or calcium phosphate. The fluorescent lamps using calcium borate in a mixing ratio of more than 2.0 % exhibited a poor performance of luminous flux, and the phosphor containing calcium borate in a mixing ratio of less than 0.2 % was found likely to exfoliate from the inner surface of the glass tubes.
As Table 4 shows, barium calcium borate has a melting point 44°C to 77°C lower than the softening points of the soda-lime glass A and glass B all given in Table 2. Barium calcium borate, if used as a binder, will melt before any of these glasses softens. While the sealed glass tube is being heated, softened and bent, Na⁺ diffused from the glass used will enter the molten barium calcium borate. This may be why the lamps using barium calcium borate as a binder exhibited a total luminous flux smaller than the lamps using calcium borate as a binder.
As Table 4 further shows, calcium phosphate has an extremely high melting point, 1670°C. For this reason, the layer of the phosphor containing calcium phosphate did not adhere to the glass layer so strongly as did the layer of the phosphor containing calcium borate and the layer of the phosphor containing barium calcium borate.
According to this invention, the calcium halophosphate phosphor containing calcium borate in a mixing ratio of 0.2 to 2.0 % is coated on the inner surface of the glass tube in an amount of, preferably, 2.9 to 3.9 mg per square centimeter. Curve 6 in Fig. 7 shows the relationship between the initial luminous flux of the lamps using calcium borate in a mixing ratio in said range and the amount of the phosphor used (mg/cm²). And curve e in Fig. 7 illustrates the relationship between the amount of phosphor used (mg/cm²) and the ratio of generated blackening of the glass tubes. As curve 6 clearly shows, the lamps using the phospher in an amount of less than 2.9 mg per square centimeter exhibited an initial luminous flux smaller than that of a circular fluorescent lamp made of the conventionally used lead glass. As curve e clearly shows, the lamps using the phosphor in an amount of more than 3.9 mg per square centimeter encountered the blackening of the glass tubes.
According to this invention it is preferred that the binder be prepared by calcium oxide (CaO) and boric anhydride (B₂0₃) in mol ratio of 1:2 to 3:2. A number of 100 V - 30 W white circular fluorescent lamps were made which comprised a circular sealed tube made of glass A and phospher layer coated on the inner surface of the tube and made of the above-mentioned calcium halophosphate phosphor containing a binder prepared by calcium oxide and boric anhydride in mol ratio (CaO/B₂0₃) ranging from 0.28 to 1.82. These fluorescent lamps were put to an impact strength test. More precisely, a steel ball having a diameter of 10 mm and weighing 3.5 g was dropped from different levels onto that portion of each fluorescent lamp which was about 100 mm from the exhaust tube of the lamp. The results of the test were as shown in Table 5. The impact strength of the circular fluorescent lamps made of the conventionally used lead glass is approximately 50 g-cm or more. If the fluorescent lamps of this invention are to have comparable impact strength, the mol ratio of calcium oxide to boric anhydride has to be 0.5 or more as well understood from Table 5. But, when the mol ratio exeeded 3/2 as in tests Nos. 9 and 10, the phospor layer exfoliated from the inner surface of the glass tube as the steel ball hit the tube, though the tube had a great impact strength.
Fig. 8 shows a cross sectional view of another circular type fluorescent lamp according to this invention. As shown in Fig. 8, this lamp comprises a ring-shaped sealed glass tube 34, metal oxide layer 38 formed on the inner surface of the tube 34 and a phosphor layer 36 formed on the metal oxide layer 38. The tube 34 is made of the same low lead glass as the tube of the lamp shown in Figs. 3 and 4. The phosphor layer 36 is made of the same phosphor as the phosphor layer of the lamp shown in Figs. 3 and 4. The metal oxide layer 38 is made of y-alumina and 0.1 to 1.0 micron thick. The thickness of the layer 38 may be detected by a scanning electron microscope.
Fifty circular fluorescent lamps of the structure shown in Fig. 8 were made which were provided with phosphor layer of the same thickness within said range and of different mixing ratios of calcium borate contained in the phosphor. These were 100 V - 30 W fluorescent lamps. They were continuously used for 3000 hours. Upon lapse of these hours it was found that their sealed glass tubes 34 has turned far lighter yellow brown than those of the lamps shown in Figs. 3 and 4. Further it was found that less phosphor particles were partly embedded in the glass layer while the glass tube was being heated, softened and bent.
Fig. 9(a) schematically shows metal oxide layer 38 formed on the inner surface of a straight glass tube and phosphor layer 36 formed on the metal oxide layer 38. Fig. 9(b) schematically shows the metal oxide layer 38 and the phosphor layer 36 of the glass tube now bent in the form of circle. Fig. 9(c) schematically illustrates the metal oxide layer 38 and the phosphor film 36 of the lamp which has been continuously used for 3000 hours. As evident from Fig. 9(b), far less phosphor particles were partly embedded in the glass layer while the glass tube was being heated, softened and bent. Further, as Fig. 9(c) shows, no mercury did not enter the recess 30 between the phosphor particles and the metal oxide layer 38. This is perhaps because the creation of dangling bond was effectively suppressed as is proved by the fact that the glass tube did not turn yellow brown.
Needless to say, the metal oxide layer 38, i.e. y-alumina layer successfully prevented the diffusion of Na⁺ from the glass tube 22. As mentioned above, the metal oxide layer 38 is 0.1 to 1.0 micron thick according to this invention. This is because it was found that the layer 38 failed to effectively suppress the creating of dangling bond and failed to prevent Na⁺ diffusion when it was less than 0.1 micron thick and that the phosphor layer 36 exfoliated from it when it was more than 1.0 micron thick. The metal oxide layer 38 may be formed 0.1 to 1.0 micron thick if y-alumina is used in an amount of 0.6 to 6.2 mg per square centimeter.
As mentioned above, it is preferred that calcium borate be used in an amount of 0.2 to 2.0 percent of the phosphor used. In case the metal oxide layer 38 is 0.5 micron thick, the phosphor layer 36 will exfoliate from the layer 38 if it is made of phoshor containing less than 0.2 percent by weight of calcium borate. If the phosphor layer 36 is made of phosphor containing more than 2.0 percent by weight of calcium borate, boron oxide will increase and thus promote Na⁺ diffusion despite the metal oxide layer 38 and will wrap up the phosphor particles, whereby the radiant efficiency of the phosphor is inevitably degraded.
The above-described impact strength test was conducted on 100 V - 30 W circular fluorescent lamps of the following five categories:

_I: Lamps each comprising a curvilinear tube of the conventionally used lead glass and phosphor layer coated on the inner surface of the tube and made of phosphor containing a binder consisting of barium calcium borate alone.
II: Lamps each comprising a circular tube of the conventionally used lead glass, an alumina film formed on the inner surface of the tube and a phosphor layer coated on the alumina film and made of phosphor containing a binder consisting of barium calcium borate alone.
III: Lamps each comprising a circular tube of soda-lime or low lead glass and a phosphor layer coated on the inner surface of the tube and made of phosphor containing a binder consisting of barium calcium borate alone.
IV. Lamps each comprising a circular tube of soda-lime or low lead glass, an alumina layer coated on the inner surface of the tube, and a phosphor layer coated on the alumina layer and made of phosphor containing binder consisting of barium calcium borate alone.
V. Lamps each comprising a circular tube of soda-lime or low lead glass, an alumina layer coated on the inner surface of the tube and a phosphor layer coated on the alumina film and made of phosphor containing a binder consisting of calcium borate alone.

In the lamps of categories II and IV the alumina layer was 0.5 micron thick. In the lamps of category of V the phosphor contained 1.0 percent by weight of calcium borate. The results of the impact strength test were as shown in Table 6.
As Table 6 shows, the lamps of category IV, which were made of soda-lime or low lead glass and provided with an alumina layer, proved stronger than the lamps of category III which were made of soda-lime glass or low lead glass but not provided with an alumina layer. Surprizingly, the lamps of categories IV and V, the lamps made of soda-lime glass or low lead glass and provided with an alumina layer, proved far stronger than those made of the conventionally used lead glass. Further, as will be evident by comparing the lamps of categories IV and V, lamps made of soda-lime glass or low lead glass will become stronger if their phosphor layer contains a binder consisting of calcium borate.
The metal oxide layer used in the second embodiment of this invention is not limited to an alumina layer. Use may be made of other metal oxide such as magnesium oxide, silicon oxide, titanium oxide and the mixture of them to bring forth the same effect as does alumina.
Both embodiments thus far described use soda-lime glass or low lead glass containing 12 percent by weight or less of lead oxide. If the tube is made of soda-lime glass which contains 12 to 17 percent by weight of _Na₂0 or low lead glass which contains the same amount of Na ₂0, the effect of the present invention will become apparent. The more the content of Na ₂0 in the glass is, the more the diffusion of Na⁺ is. Soda-lime glass containing 12 to 17 percent by weight of Na ₂0 and low lead glass containing the same amount of Na ₂0 in addition to 12 percent by weight or less of lead will more effectively prevent diffusion of Na⁺ if they contain further 3 to 8 percent by weight of calcium oxide (CaO).
As mentioned above, the phosphor used in this invention contains binder consisting of calcium borate alone. In view of this it is desired that the phosphor be antimony-manganese activated calcium halophosphate.
The basic idea of this invention may be applied to a U-shaped fluorescent lamp and a W-shaped fluorescent lamp, besides a circular fluorescent lamp.

Claims

1. A curvilinear type fluorescent lamp comprising: a curvilinear, light-transmitting sealed tube having an electrode attached to either end and coated with an electron-emitting substance, said tube being made of soda-lime glass or low lead glass containing 12 percent by weight or less of lead oxide, whose softening point is 640 to 720°C and whose coefficient of linear thermal expansion is 92 to 105 x 10-⁷ cm/cm/°C; and a phosphor layer laid on the inner surface of the tube and containing binder consisting chiefly of calcium borate.

2. A curvilinear type fluorescent lamp according to claim 1 wherein said phosphor layer contains 0.2 to 2.0 percent by weight of calcium borate.

3. A curvilinear type fluorescent lamp according to claim 2, wherein said phosphor layer is coated on the inner surface of said tube in an amount of 2.9 to 3.9 mg per square centimeters.

4. A curvilinear type fluorescent lamp comprising:

a curvilinear, light-transmitting sealed tube having an electrode attached to either end and coated with an electron-emitting substance, said tube being made of soda-lime glass or low lead glass containing 12 percent by weight or less of lead oxide, whose softening point is 640 to 720°C and whose coefficient of linear thermal expansion is 92 to 105 x 10^-7 cm/cm/°C;

a metal oxide layer coated on the inner surface of the tube in an amount of 0.6 to 6.2 mg per square centimeters and made of at least one of the group consisting of aluminum oxide, magnesium oxide, silicon oxide and titanium oxide; and

a phosphor layer laid on the metal oxide layer and containing binder consisting chiefly of calcium borate.

5. A curvilinear type fluorescent lamp according to claim 4, wherein said phosphor layer contains 0.2 to 2.0 percent by weight of calcium borate.

6. A curvilinear type fluorescent lamp according to claim 5, wherein said phosphor layer is coated on the inner surface of said tube in an amount of 2.9 to 3.9 mg per square centimeters.

7. A curvilinear type fluorescent lamp according to any one of claims 1 to 6, wherein said soda-lime glass or low lead glass contains 12 to 17 percent by weight of sodium oxide (Na₂0).

8. A curvilinear type fluorescent lamp according to claim 7, wherein said soda-lime or low lead glass further contains 3 to 8 percent by weight of calcium oxide (CaO).

9. A curvilinear type fluorescent lamp according to any one of claims 1 to 6, wherein said low lead glass consists of the following components in the following ratio in weight percentage:

10. A curvilinear type fluorescent lamp according to any one of claims 1 to 6 and 8, wherein the calcium borate is obtained from a mixture of calcium oxide (CaO) and boron oxide (B₂0₃), the mol ratio of calcium oxide (CaO) to boron oxide (B₂0₃) being 1/2 to 3/2.

11. A curvilinear type fluorescent lamp according to claim 7, wherein the calcium borate is obtained from a mixture of calcium oxide (CaO) and boron oxide (B₂0₃), the mixing ratio of calcium oxide (CaO) to boron oxide (B₂0₃) being 1/2 to 3/2.

12. A curvilinear type fluorescent lamp according to claim 9, wherein the calcium borate is obtained from a mixture of calcium oxide (CaO) and boron oxide (B₂0₃), the mixing ratio of calcium oxide (CaO) to boron oxide (B₂0₃) being 1/2 to 3/2.

13. A curvilinear type fluorescent lamp according to any one of claims 1 to 6, 8, 11 and 12, wherein said binder contains no barium oxide (BaO).

14. A curvilinear type fluorescent lamp according to claim 7, wherein said binder contains no barium oxide (BaO).

15. A curvilinear type fluorescent lamp according to claim 9, wherein said binder contains no barium oxide (BaO).

16. A curvilinear type fluorescent lamp according to claim 10. wherein said binder contains no barium oxide (BaO).