GB2069994A

GB2069994A - Mouldable Fluoroaluminophosphate Glasses

Info

Publication number: GB2069994A
Application number: GB8105724A
Authority: GB
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1980-02-26
Filing date: 1981-02-24
Publication date: 1981-09-03
Also published as: DE3105664A1; JPS56149343A; FR2476631B1; GB2069994B; NL8100918A; DE3105664C2; FR2476631A1; JPS5933545B2

Abstract

A glass which exhibits a transition temperature of below 350 DEG C and which comprises, as analysed in weight percent on the oxide basis, from 30 to 75% P2O5, from 3 to 25% R2O, wherein R2O comprises from 0 to 20% Li2O, from 0 to 20% Na2O, from 0 to 10% Rb2O and from 0 to 10% Cs2O, from 0 to 20% K2O, from 0 to 40% RO, wherein RO comprises from 0 to 15% MgO, from 0 to 40% CaO, from 0 to 40% SrO and from 0 to 40% BaO, from 0 to 60% PbO, from 0 to 40% ZnO, from 0 to 30% CdO, from 0 to 60% RO+PbO+ZnO+CdO, from 3 to 20% Al2O3 and between 3 and 24% F, the atomic ratio F:Al being from 0.75:1 to 5:1 is disclosed.

Description

SPECIFICATION Mouldable Fluoroaluminophosphate Glasses This invention relates to mouldable fluoroaluminophosphate glasses.

United States Patent Application Serial No. 198,439 discloses a group of glasses having compositions within the alkali metal oxide-alkaline earth metal oxide-fluoride-phosphate field (R20- R0-F-P205) displaying low transition temperatures (Tg) and the capability for moulding and otherwise shaping glass bodies under pressure at low temperatures, viz. below 4500 C, without the need for hydration.Those glasses have base compositions consisting essentially, expressed in mole percent on the oxide basis as calculated from the batch, of from 1 5 to 45% R2O, consisting of from 0 to 45% Li20, from 0 to 20% Na2O and from 0 to 10% K2O, from 0 to 20% RO, consisting of from 0 to 10% MgO, from 0 to 15% Cao, from 0 to 20% SrO and from 0 to 20% BaO, from 25 to 55% P205 with from 0.3 to 3%, by weight, F, as analyzed in the final glass. Optional consistuents therefore include up to 20% PbO, up to 7% La203 and up to 12% ZnO. The essential absence of Awl203 is stated to be much preferred. When the P205 content is less than 30%, B203 may be incorporated in amounts of up to 25%.When the content of P205 is at least 45%, B203will be present at levels no greater than 5%. The sum of all optional ingredients other than RO and R20 will not exceed 25% and the total of all optional components exclusive of RO will be less than 35%.

It is explained therein that vitreous, phosphate-based compositions containing fluoride are well known in the glass art, but that such are commonly plagued by two seemingly intrinsic undesirable characteristics, viz. relatively poor chemical durability and a tendency to devitrify during forming and working. Many glasses demonstrated poor resistance to weathering, i.e. they were attacked upon exposure to the ambient environment.

A principai object of the present invention is to provide phosphate-based glasses containing fluoride demonstrating the capability of being moulded or otherwise shaped under pressure at temperatures below 3500C and which exhibit good chemical durability and resistance to weathering along with good glass stability, i.e. resistance to devitrification during forming and working.

A practical object is to design a preferred range of compositions of such glasses that will be expecially suitable for use in optical and ophthalmic applications.

It has been determined that these objects may be achieved with glass compositions consisting essentially, as analyzed in weight percent on the oxide basis, of from 30 to 75% P205, from 3 to 25% R20, consisting of from 0 to 20% Li20, from 0 to 20% Na2O, from 0 to 20% K20, from 0 to 10% Rb20 and from 0 to 10% Cs20, from 0 to 60% PbO, from 0 to 40% RO, consisting of from 0 to 15% MgO, from 0 to 40% CaO, from 0 to 40% SrO and from 0 to 40% BaO, from 0 to 40% ZnO, from 0 to 30% CdO, from 0 to 60% PbO+RO+ZnO+Cd0, from 3 to 20% Al203 and > 3% but < 24% F, the atomic ratio F:AI being at least 0.75:1, but no more than 5:1.

As was disclosed in the above-mentioned reference, the inclusion of Awl203 in the glasses cited therein increased the transition temperatures (tug) thereof to such an extent that its absence was directed. However, it has now been discovered that, when the glass compositions contain substantial amounts of fluoride, i.e. greater than 3% analyzed in the final glass, Al203 may be incorporated into the glass without raising the Tg thereof excessively. This incorporation of AI2O3 performs three vital functions: firstly, it increases the solubility of fluoride in the glass system; secondly, it inhibits liquid immiscibility which gives rise to phase separation in the glass; and, thirdly, it vastly improves the weathering resistance and chemical durability of the glass.

Fluorine is a necessary component of the present glasses since it fluxes the base phosphate glasses down to lower Tg temperatures for easier mouldability. The weathering resistance and chemical durability of the glasses sharply decrease, however, with increasing fluoride concentrations.

Accordingly, the present invention is based upon the use of a combination of Awl203 and F to endow the glasses with low transition temperatures coupled with good weather resistance and chemical durability. Moreover, not only must Al203 and fluoride be present in the glass composition in significant amounts, but the atomic ratio F:AI is of critical importance in achieving the desired balance between Tg and chemical resistance of the glass. For example, lower ratios of F:AI provide greater chemical resistances, but accompanied by higher transition temperatures. As the ratio of F:AI increases, the transition temperature and chemical resistance of the glass decrease such that, at a ratio of above 5:1, the resistance of the glass to chemical attack is quite poor.

Laboratory studies have indicated that constant F:AI ratios, in a particular family of glasses, represent isotherms of transition temperatures and isodurabilities. Such phenomena are not unknown in the glass art. One instance of such is the so-called "mixed alkali effect", wherein alkali metal oxides may be manipulated in order simultaneously to reduce glass viscosity and increase the chemical durability thereof. Analogously, the effects of Al203 on increasing glass durability and transition temperature are well known, as is the knowledge that fluoride produces opposite effects. However, the combination of those two components in the proper quantities produces behaviour similar to the mixed alkali effect.

When the atomic ratio F:AI of about 5:1 is exceeded, the durability of the glass appears to decrease exponentially. This circumstance is believed to be caused by liquid immiscibility. The solubility of fluoride into glass compositions free from Al2O3 is poor with the level of fluoride retained therein being conjectured as associated with modifying cations. It is theorized that the increased solubility of fluoride in Al2O3-containing glasses is due to the fact that aluminium may readily change from four-fold to six-fold coordination and behave as a modifying cation, thereby permitting an increased number of anions in the glass structure (two F- ions replacing one 02- ion).

Minor amounts of compatible metal oxides, such as La2O3, WO3 MoO3 and Nod203, may be included to modify the refractive index and other physical properties of the glass. The total of such additions, however, will not exceed 10%, by weight.

Laboratory experience has demonstrated that, for practical purposes, the present glasses may be categorized into two general groups, viz. glasses consisting essentially solely of compositions within the R2O-A12O3-F-P2O5 system and glasses wherein significant amounts, e.g. about 5% total, of at least one modifying oxide, such as RO, PbO, ZnO and CdO, are included in the compositions. Glasses within the simple quaternary typically exhibit refractive indices of from 1.45 to 1.5 and the optimum combination of good chemical durability, low T and good glass stability is found with Awl203 and F contents of from 15 to 20% and > 15 but < 24 o7o, respectively.In contrast, where substantial amounts of RO, PbO, ZnO and CdO are incorporated into the compositions, the optimum combination of properties is produced with Awl203 and F contents of from 3 to 15% and > 3 but < 15%, respectively, and the refractive indices of the glasses are commonly from 1.5 to 1.7.

A study of the prior art is hampered by the fact that the workers generally reported glass compositions merely in terms of batch materials. As was observed in the above-mentioned reference and as will be demonstrated hereinafter, volatilization of fluoride during melting may be as high as 90%. Accordingly, unless a chemical analysis of a glass is provided, the fluoride content retained therein is simply conjectured. The present invention is based on the discovery that the inclusion of at least 3%, by weight, Al2O3 in alkali metal phosphate base glass compositions enables fluoride retentions up to 50% of that batched, while concurrently yielding products of relatively good chemical durability. Thus, fluoride contents approaching 24%, by weight, have been analyzed in such glasses.

Because Awl203 raises the transition temperature of the glasses, the fluoride content must exceed 3%, by weight, in order to ensure transition temperature below 4000 C.

United States Patent No. 2,430,539 circumscribes an area of titanium fluophosphate glasses statedly useful in optical applications. TiO2 was included in the compositions to improve chemical durability. The glasses were expressed by the formula AFTiO2M(POV)z wherein AF represented an alkali metal fluoride and M(POy)z designated a phosphate of aluminium or beryllium, commonly a meta orortho-phosphate. No analyses of the glasses are provided. The working examples indicated from 55.5 to 77 mole percent. NaF or KF, up to 19.8 mole percent Tit2, from 15 to 25 mole percent Al(PO3)3 or from 40 to 44.5 mole percent ALP4. There is no mention of PbO, RO or ZnO.

United States Patent No. 2,496,824 delineates iron fluophosphate glasses which, because of the low colouration observed therein, were asserted to be suitable for optical applications. The compositions, in weight percent as calculated from the batches, consisted essentially of from 20 to 40% NaF, from 22 to 40% LiF, from 2 to 1 5% Fe203 and from 55 to 76% Al(PO3)3. No analyses of the glasses are provided, PbO, RO and/or ZnO are not required components and the total alkali metal content is far in excess of that which may be tolerated in accordance with the present invention.

United States Patent No. 2,481,700 discloses fluophosphate glasses purportedly operable in optical applications which are encompassed within the formula AF-MF2-R, wherein AF designates a fluoride selected from LiF, NaF and KF, MF2 represents a fluoride selected from MgF2, CaF2, SrF2, BaF2 and ZnF2, and R indicates an aluminium and/or a beryllium phosphate. AF constitutes from 7 to 54%, by weight of the composition, MF2 constitutes from 0 to 58%, by weight, and R makes up from 30 to 90%, by weight, the atomic proportion of fluorine to phosphorus in the glass batch being from 0.23:1 to 2.9:1. No analyses of the glasses are provided, RO, PbO and/or ZnO are not required constituents and there is no teaching regarding the need for maintaining a particular ratio between the aluminium and fluoride components.With respect to this latter circumstance, it must be noted that Alp03 was merely an optional ingredient.

United States Patent No. 3,281,254 describes glasses stated to be useful for optical applications having batch compositions, expressed in weight percent, of from 16 to 23.8% alkali metal metaphosphate, from 23.8 to 41% alkaline earth metal metaphosphate and from 1 to 21% of a fluoride selected from PbF2, LiF, KHF2, ZnSiF2, BaF2 and Mg F2. Yet again, no analyses indicating the fluoride content in the final glass are provided nor is there even a suggestion of a need to maintain a particular ratio between the aluminium and fluoride components.

United States Patent No. 3,656,976 discloses fluophosphate glasses reportedly suitable for optical purposes consisting essentially, as calculated from the batch in cationic percent, of from 15 to 40% PO2.5, 5, from 0.5 to 21% Bio15, the ratio B:P being less than 0.7:1, from 0.7 to 40% alkali metal fluoride, from 10 to 60% alkaline earth metal fluoride and from 10 to 25% AlF3. No fluoride analyses of the final glass are provided, B203 is a required rather than an optional ingredient and there is no requirement that a specific ratio be maintained between the fluoride and aluminium components.

United States Patent No. 3,954,484 discusses alkaline earth aluminofluorophosphate glasses having indices of refraction of greater than 1.57, an Abbe number of less than 70 and a relatively high positive anomalous partial disperson. No analyses of fluoride present in the final glass are reported, alkali metal oxides are implicitly absent from the compositions and there is no requirement that a critical atomic ratio be maintained between the fluoride and aluminium contents.

Table 1 below records a number of exemplary glass batches melted on a laboratory scale, reported in terms of parts, by weight, of the actual batch ingredients utilized, illustrating the parameters of the present invention. Reagent grade batch materials were employed in these laboratory melts to forestall any anomalous reaction or phenomenon due to impurities in the starting materials. Particularly desirable phosphate compounds for batch materials included the alkali metal and alkaline earth metal meta- and ortho-phosphates. Al(PO3)3, Pb(PO3)2 and Zn3(PO4)2 were also found to be helpful in achieving good glass quaiity. P205 may be utilized as a batch ingredient although it is hygroscopic, it cannot be ball-milled with any ease and it volatilizes rapidly during the initial stages of melting.

Ammonium phosphate was considered to be an unacceptable batch material because, while it may be ball-milled, it volatilizes very readily during the early stages of melting. Furthermore, it is a reducing agent and, as such, is incompatible with easily-reducible metal oxides, such as PbO.

The batch ingredients were compounded, tumble mixed together to assist in obtaining a homogeneous melt and then charged into 96% silica or alumina crucibles, the type of crucible utilized being reported in Table 1 as SiO2 or Al2O3. Although the batches recorded in Table 1 represent laboratory scale melts only, it must be appreciated that larger melts thereof may be made in pots or commercial continuous melting tanks. The crucibles were introduced into a furnace operating at about 9000C and the batches melted for about one-half hour. The melts were thereafter poured into a steel mould to produce a glass slab having dimensions of about 1 5x 1 5x 1.3 cms (6"x6"x") and the slab immediately transferred to an annealer operating at about 25 C above the Tg of the glass.

Table 1 1 2 3 4 5 6 7 8 9 NaPO3 71 35 - - - - - - LiPO3 61 61 61 61 61 31 - - Pb(PO3)2 188 188 188 94 - - - - - P205 74 99 124 161 198 223 245 199 245 BaF2 115 115 115 115 115 115 115 58 AIF3. H20 46 46 46 46 46 46 46 46 46 NaF - 15 30 30 30 30 30 30 30 PbF2 - - - 63 126 126 126 126 126 LiF - - - - - 9 18 18 18 Ba(PO3)2 - - - - - - - 97 194 Crucible sia, 2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 10 11 12 13 14 15 16 17 18 NaPO3 - - 42 - - - - - LiPO3 37 - 73 73 73 37 - - Pb(PO3)2 - - 226 226 - - - - - P205 268 239 119 149 238 268 294 239 294 BaF2 138 70 138 138 138 138 138 70 AlF3.H2O 55 55 55 55 55 55 55 55 55 NaF 36 36 18 36 36 36 36 36 36 PbF2 151 151 - - 151 151 151 151 151 LiF 11 22 - - - 11 22 22 22 Ba(PO3)2 - 116 - - - - - 116 233 Crucible SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 19 20 21 22 23 24 Ai(PO3)3 44.7 44.7 44.9 41.2 41.4 41.9 LiPF6 - 8.2 7.3 27.7 36.6 42.4 KPF6 29.0 37.9 25.0 - 11.3 - NaPF6 26.3 9.2 22.8 30.8 10.7 15.6 The glasses of Table 1 were chemically analyzed for fluoride and alumina employing techniques customary in the glass art. Those values are recorded in Table 2 below and, in the case of F, a comparison is drawn (% retention) between the amount of fluoride in the batch materials in weight percent and the level thereof as analysed in the final glass.

The atomic ratio F:AI is also tabulated. Also, various physical properties are recorded as measured in accordance with conventional methods. Thus, refractive index (nD) was determined by the Becke line technique, transition temperature was derived from differential scanning calorimetry and density was measured utilizing a modification of the Westphal balance. The chemical durability of the glasses was investigated via an immersion of 10 minutes into an aqueous 10%, by weight, HCI solution at room temperature and an immersion of one hour in boiling water. The loss of weight manifested by the glasses is reported in terms of mg/cm2. N.C. means essentially no change observed.

Table 2 1 2 3 4 5 6 7 8 9 FBatched 9.46 10.6 11.7 13.3 14.85 15.9 17.07 15.04 11.25 FAnalysed 3177 3.91 4.94 5.4 5.36 4.93 5.64 5.32 2.11 %FRetention 39.95 36.88 42.2 40.6 36.1 31.00 33.1 35.37 18.75 Al2O3Analysed 5.77 5.84 5.90 6.08 5.34 5.97 6.44 6.39 5.27 F:AI 1.74 1.79 2.24 2.45 2.69 2.21 2.34 2.23 1.07 Density 3.673 3.653 3.660 3.725 3.556 3.750 3.804 3.768 3.379 T@ 335 C 330 C 325 C 320 C 300 C 330 C 330 C 335 C 365 C Boiling H20 N.C. N.C. N.C. N.C. N.C. N.C. N.C. N.C. N.C.

10% HCI 1.13 1.01 1.62 1.92 2.75 1.67 2.25 1.78 0.83 nod 1.602 1.596 1.594 1.580 1.604 1.604 1.604 1.604 1.586 10 11 12 13 14 15 16 17 18 FBatched 15.9 15.04 10.6 11.7 14.85 15.9 17.07 15.04 11.25 FAnalysed 3.71 1.37 6.62 7.14 7.83 8.07 10.7 3.16 4.33 %F Retention 23.33 9.11 62.45 61.02 52.72 50.75 62.68 21.00 38.48 Al203 Analysed 5.60 4.10 6.16 6.75 7.10 7.25 7.32 6.08 6.18 F:AI 1.77 0.895 2.88 2.83 2.95 2.98 3.92 1.39 1.88 Density 3.694 3.307 3.635 3.682 3.744 3.760 3.839 3.533 3.572 T@ 337 C 355 C 300 C 310 C 315 C 318 C 295 C 350 C 330 C Boiling H20 N.C. N.C. 0.0099 0.0247 0.0127 0.0193 0.0048 N.C. N.C.

10% HCI 0.99 0.32 1.67 1.48 1.84 1.82 9.55 0.16 0.38 nod 1.604 1.578 1.580 1.586 1.586 1.588 1.580 1.594 1.592 19 20 21 22 23 24 FBatched 36.0 36.0 36.0 41.0 41.0 41.0 FAnalysed 18.1 18.4 18.7 20.4 19.5 20.8 %F Retention 50.3 51.1 51.9 49.8 47.6 50.7 Al203Analysed 16.8 16.8 17.4 18.5 19.5 18.3 F:AI 2.9 2.9 2.9 3.0 2.9 2.5 Density - 2.616 2.636 2.656 2.613 2.561 Tg 290 300 290 290 283 313 Boiling H20 0.001 0.0005 0.0005 0.0035 0.0018 0.0017 10% HCI 0.35 0.24 0.07 0.44 1.52 0.34 nD 1.452 - - 1.464 - - In general, the batches melted in Al2O3 crucibles retained more fluoride in the final glass than those melted in 96% SiO2 crucibles.It is conjectured that the molten batch reacts with the SiO2 of the crucible to form the highly volatile SiF4. The batches melted in Al2O3 crucibles picked up from 1 to 2% Al203 therefrom when the amount of batched AI2O3 is compared with the analysed value therefore. An unexpected effect upon fluoride retention was found in batches melted in uncovered crucibles, viz.

those glasses melted in uncovered crucibles showed a higher retention of fluoride than those melted in covered crucibles. It is theorized that covering the crucibles causes more water to be retained in the melt which, in turn, leads to the forming of the highly volatile HF. However, P205 and other relatively more volatile compounds are lost more readily from the molten glass when melted in open crucibles. The fluoride retention levels listed in Table 2 were calculated from a comparison of the analysed fluoride concentrations with the batched values. That calculation is not fully accurate, however, since it does not take into account other volatile products. Total accuracy would require complete chemical analyses of all exemplary compositions.

Melting the glasses in alumina crucibles with all or virtually all of the P2O5 batched from P205 anhydride and alkali metal metaphosphates is recommended inasmuch as the batches melt down rapidly and exhibit high fluoride retentions. The use of lead and barium metaphosphates as batch materials causes slow melting with considerable foaming.

As may be seen via a study of Table 2, the atomic ratio F:AI dictates the transition temperature exhibited by a glass. Hence, generally, the lower the ratio, the higher the transition temperature. The concentration of fluoride in the glass is also of vital importance in this regard. Thus, Examples 9 and 11 of Table 1, containing 2.1 1% and 1.37% fluoride, respectively, as chemically analysed, demonstrate Tg values in excess of 3500C.

Table 3 below reports the results of parallel plate viscosity measurements conducted over the 105101 poise range on five glasses. The difference between the softening point (Tsp) and the transition range, the analysed weight percent Al203, the analysed weight percent F and the atomic ratio F:AI are provided for each glass.

Table 3 Glass TspTg %at203 %F F.AI A 85 5.34 5.36 2.69 B 98 5.97 4.93 2.21 C 84 2.34 1.6 1.83 D 90 5.77 3.77 1.74 E 88 5.27 2.11 1.07 A large difference in temperature between the softening point and transition temperature indicates a shallow viscosity curve, while a small difference in temperature means a steep curve. The steepness of the curve in this viscosity range does not appear to be directly related to the fluoride concentration or to the atomic F:Al ratio. Nevertheless, it seems to follow the Al2O3 level. i.e. the lower the Al2O3 value, the steeper the curve. it is postulated that this circumstance is due to the capability of the A13+ cation to tie up one alkali metal cation, thereby stiffening the glass in this region of viscoelastic flow.

Two test methods were employed for rating the weathering resistance of the present glasses as reported in Table 4 below. The first of these is referred to in the optical glass industry as the Schott test. The second technique was more recently developed by Corning Glass Works, Corning, New York, U.S.A., and is beiieved to be more descriptive and informative. In both tests the specimens consist of polished squares about 5x5x0.6 cms (2"x21,x1/4'1).

In the Schott test the polished squares are exposed to a water vapour-saturated atmosphere with air temperature changes between 400C and 500C at an hourly rhythm. Condensation (with no run-off) occurs on the test specimen surface for about 1 5 minutes of each hourly cycle. After exposure times of 30, 100 and 1 80 hours, the specimens are removed from the test chamber and classified by conducting diffused light measurements; i.e. the fraction of transmitted visible lights that is diffusedly scattered by the weathered surface is utilized as a measure of the degree of weathering.The glasses are rated according to the following categories: Class 1-negligible attack, if any, after 7 hours Class 4-marked attack after 30 hours Classes 2 and 3-grades of sensitivity between Classes 1 and 4, but not further specified.

The Corning weathering test exposes the polished plates to a static humidity atmosphere of 98% relative humidity and 50bC. Three identical samples are employed with one being visually evaluated, washed and re-evaluated every three or four days; another evaluated on the same schedule, but washed only biweekly; and the remaining sample periodically examined, but not washed until the termination of the test after one month, at which time the sample is washed.The glasses are then visually classified as follow: Class A-no visible spots or haze when examined under an intense parallel beam light source Class B--a few spots or a slight haze visible only with lighting conditions as in A Class C-many spots or much haze visible with lighting conditions as in A Class D-spots or haze visible under ordinary ambient lighting Class E-excessive accumulation of weathering products (test terminated).

In table 4 the Corning weathering test was conducted for the periods shown and the glasses were not washed off between inspections. Classes A to E designate the same visual appearances as set forth above.

Table 4 Corning Test Analysed Analysed Glass Schott Test 1 Day 4 Days 7 Days Tg % Al203 %F F:AI 4 3 C C D 320 6.08 5.4 2.45 3 3 B D E 325 5.9 4.94 2.24 8 3 B D D 335 6.39 5.32 2.23 9 2 B B C 365 5.27 2.11 1.07 5 4 D E E 300 3.78 3.34 2.36 10 3 B D E 337 5.60 3.71 1.77 11 2 B C C 355 4.10 1.37 0.895 As a comparison, the most preferred glass of the first-mentioned reference consisted essentially, as analysed in weight percent on the oxide basis of, 48.0 P2O5, 2.09 Li2O, 4.08 Na 23.8 PbO, 21.43 BaO and 0.56 F.

A chemical analysis of the glass indicated the retained fluoride content to be about 0.56%. That glass was destroyed after a hour-hour exposure to the Corning weathering test.

A study of Table 4 indicates that weathering resistance does not strictly follow the trend of transition temperature or atomic F:AI ratio. This factor is due to the unequal comparison of glasses containing higher levels of Al2O3, since, as was noted previousiy, A13+ cations have the ability to tie up alkali metal cations, thereby improving the chemical durability of the glass.

A study was undertaken to evaluate the effect of diva lent cations upon the properties of the present fluorophosphate glasses. The properties of immediate concern were Tg, chemical durability, refractive index and fluoride retention. The cations chosen for the investigation were the alkaline earth metals Mg, Ca, Sr and Ba, along with Cd, Pb and Zn. Three glass compositions were selected to serve as general bases for the study, the compositions thereof being recited below in terms of parts, by weight, on the oxide basis as calculated from the batch. Also included are the F:AI ratios as calculated from the batches and the F:AI ratios as determined from analysed values of those elements.

A B C K2O 8.6 9.0 9.0 P205 52.2 54.4 54.3 Li2O 2.8 2.9 2.8 Al2O3 8.2 4.3 4.6 BaO 28.2 29.3 29.2 F 10.5 13.4 20.5 Batched F:AI 3.65 9.04 13.58 Analysed F:AI 1.75 4.1 5.9 In each of the compositions examined, BaO was replaced in the base compositions by the substituting divalent metal cation with the other components of the batch being maintained relatively constant on a molar basis. The majority of the melts was conducted in platinum crucibles equipped with tight fitting platinum covers. Porcelain, alumina, glassy carbon and "VYCOR" brand crucibles were employed with the glasses containing lead and cadmium. Those crucibles were also covered, but with "VYCOR" brand or porcelain lids.

Table 5 below reports the three groups of exemplary glass batches in terms of parts, by weight, on the oxide basis as calculated from the batch. Because the sum of the ingredients totals or approximately totals 100, for all practical purposes, the values recorded may be deemed to reflect weight percent. Since it is not known with which cation(s) the fluoride is combined, it is simply recited as the fluoride. With the exception of cadmium and zinc which were batched as ortho-phosphates, the diva lent cations were added as metaphosphates.

The batch ingredients were compounded, tumble mixed together to aid in securing a homogeneous melt and thereafter charged into crucibles, the type of crucible employed being reported in the Table. The crucibles were introduced into a furnace operating at 1 000C and maintained therewith for about 10 minutes. The melts were quenched into glass slabs by pouring onto graphite blocks and the slabs annealed by holding for two hours in a furnace operating at from 15 to 400C above the Tg of the glass and then allowed to cool at furnace rate. In Table 5 the group A glasses reflect those composition based upon Glass A, supra, and the Group B and Group C glasses are founded upon Glass B and Glass C, respectively, reported above.

Table 5 Group A 25 26 27 28 29 30 31 K2O 8.1 8.9 10.1 7.2 8.6 7.2 9.8 Li2O 2.6 2.8 3.2 2.3 2.8 2.3 3.1 Al2O3 7.7 8.4 9.6 6.9 8.2 6.9 9.3 P205 49.1 53.6 61.0 43.8 52.1 43.8 58.9 BaO 26.4 - - 25.5 - - - SrO - 19.6 - - - - - MgO - - 8.6 - - - - PbO - - - - - 34.4 - CdO - - - - 21.8 - - CaO - - - - - 1 11.6 F 10.5 11.5 13.0 20.2 11.2 9.4 12.6 Crucible Pt Pt Pt Porcelain Pt Pt Pt Table 5 (cont.).

Group B 32 33 34 35 36 37 38 K2O 8.3 9.1 9.9 10.3 7.5 8.5 7.5 Li2O 2.5 2.7 3.0 3.1 2.2 2.6 2.2 Al2O3 4.0 4.4 4.8 4.9 3.6 4.1 3.6 P2O5 48.0 52.3 57.3 59.1 43.0 49.3 43.0 BaO 25.5 - - - - - SrO - 18.8 - - - - - CaO - - 11.1 - - - - MgO - - - 8.2 - - - PbO - - - - 33.2 - 33.2 CdO - - - - - 23.5 F 20.2 22.0 24.1 24.9 18.1 20.7 18.1 Crucible Pt Pt Pt Pt Porcelain Pt Pt Group C 39 40 41 42 43 44 45 46 K2O 8.3 9.1 10.0 10.4 7.4 8.3 7.8 9.6 Li2O 2.6 2.9 3.2 3.3 2.4 2.6 2.5 3.1 Al2O3 4.0 4.4 4.8 5.0 3.6 4.0 3.7 4.6 P205 50.2 55.0 60.6 62.7 44.7 50.2 47.0 58.0 CaO - - 12.2 - - - - BaO 27.1 - - - - 27.1 31.7 15.7 SrO - 20.1 - - - - - - MgO - - - 8.8 - - - PbO - - - - 35.1 - - F 13.5 14.7 16.2 16.8 12.0 13.5 12.6 15.6 Crucible Pt Pt Pt Pt Porcelain Pt Pt Pt 47 48 49 50 51 52 53 54 K2O 10.4 9.1 8.7 9.5 9.1 8.7 8.2 9.5 Li2O 3.3 2.9 2.8 3.0 2.9 2.8 2.6 3.0 Al2O3 5.0 4.4 4.2 4.6 4.4 4.2 4.0 4.6 P205 63.0 55.0 52.3 57.5 55.3 52.5 49.6 57.2 BaO 8.5 - - - - - - - SrO - 21.0 24.0 - - - - - ZnO - - - 16.4 19.7 - - CdO - - - - - 23.7 28.0 CaO - - - - - - - 16.9 F 16.9 14.7 14.0 15.4 14.8 14.1 13.3 15.3 Crucible Pt Pt Pt Pt Pt Al203 Al2O3 Pt 55 56 57 58 59 60 61 62 K2O 10.3 9.9 10.9 8.8 10.4 8.7 9.9 6.3 Li2O 3.3 3.2 3.5 2.8 3.3 2.8 3.1 2.0 Al2O3 5.0 4.8 5.2 4.2 5.0 4.2 4.7 3.0 P2O5 62.5 60.1 65.6 53.2 62.7 52.5 59.6 38.0 CaO 9.2 - - - - - - - MgO - 12.7 4.6 - - - - ZnO - - - 22.8 8.9 - - CdO - - - - - 23.7 13.4 PbO - - - - - - - 44.9 F 15.3 16.7 16.1 17.6 14.2 16.8 14.1 16.0 Crucible Pt Pt Pt Pt Pt C C Al2O3 63 64 65 66 67 K2O 10.0 6.3 9.0 10.0 7.4 Li2O 3.2 2.0 2.9 3.2 2.4 Al2O3 4.8 3.0 4.3 4.8 3.6 P2O5 60.6 38.0 54.2 60.6 44.7 PbO 11.9 44.9 21.3 11.9 35.1 F 16.2 10.2 14.5 16.2 12.0 Crucible Al2O3 VYCOR VYCOR VYCOR Pt The fluoride content in the glass was determined via conventional chemical analysis practice and the percent retention between the amount of fluoride in the batch materials and the level thereof as analysed in the final glass is reported in Table 6 below. The atomic ratio F:AI is also recorded along with the refractive index (nD), as determined via the Becke line technique, and the transition temperature (Tg), as measured by differential scanning calorimetry.

A weathering test was conducted in a humidity cabinet wherein the temperature and relative humidity were maintained constant at 50 C and 98%, respectively. Small pieces were broken off the glass slabs and placed upon a nichrome wire screen. After a soak period of eight days in the humidity cabinet, the specimens were removed and visually graded as to surface change. The rankings ranged from no change, A, to severe attack, F.

To investigate the water durability of the glasses, sections were cut from the slabs and each side ground with 320 grit silicon carbide paper such that all surfaces were uniformly abraded. The sections were then weighed and the external dimensions thereof carefully measured. The specimens were placed upon nichrome wire screens, the screens introduced into individual beakers, 100 ml of deionized water at 750C added thereto and the beakers inserted into a constant temperature water bath operating at 95 C. After about 15 minutes the contents of the beakers were brought to a constant temperature (N900C). The beakers were held within the bath for another hour, the specimens removed from the beakers, washed and air dried overnight. After weighing, the samples were evaluated in terms of weight loss per unit surface area (mg/cm2) and those results are listed in Table 6.

Table 6 Example Cation %FRetention Weathering mg/cm2 nD Tg 25 Ba 61.9 B 0.081 1.534 367 C 26 Sr 69.5 A 0.150 1.522 3700C 27 Mg 58.7 A - 1.512 4170C 28 Ba 27.7 B 0.186 1.618 3500C 29 Cd 52.7 A 2.775 1.534 3590C 30 Pb 38.4 C 0.051 1.608 3270C 31 Ca 68.3 A 0.259 1.508 3390C 32 Ba 51.7 D 0.818 1.506 2750C 33 Sr 63.0 B 1.467 1.488 2680C 34 Ca 62.4 F 72.203 1.470 2350C 35 Mg 51.1 A 1.134 1.486 3430C 36 Pb 18.0 B 0.026 1.624 3260C 37 Cd 42.3 A 0.577 1.528 3020C 38 Pb 37.1 D 0.435 1.584 2450C 39 Ba 56.3 C 0.484 1.518 2970C 40 SR 68.7 C 1.266 1.504 2870C 41 Ca 58.6 E 2.648 1.490 2930C 42 Mg 47.6 A 0.744 1.504 3640C 43 Pb 19.2 A 0.065 1.626 3050C 44 Ba 47.4 E 0.587 1.520 2940C 45 Ba 57.9 C 0.504 1.526 284 C 46 Ba 37.2 F 11.546 1.508 2980C 47 Ba 26.6 C 0.364 1.506 3010C 48 Sr 43.5 B 0.410 1.512 321 OC 49 Sr 63.0 B 1.467 1.488 2680C 50 Zn 40.3 E 7.792 1.510 2820C 51 Zn 42.6 E 1.599 1.516 2870C 52 Cd - A 0.323 1.544 3290C 53 Cd - A 0.540 1.522 3350C 54 Ca 57.5 A 0.588 1.510 3300C 55 Ca 38.3 B 1.019 1.508 3330C 56 Mg 57.1 A 0.749 1.494 3660C 57 Mg 33.0 B 0.580 1.498 344 C 58 Zn 46.5 D 0.902 1.520 2880C 59 Zn 30.4 E 5.063 1.506 2860C 60 Cd 40.4 A 1.611 1.542 3190C 61 Cd 30.6 D 0.874 1.522 3120C 62 Pb 36.9 E 0.185 1.652 3210C 63 Pb 23.5 F 0.429 1.532 2920C 64 Pb 30.4 B 0.094 1.656 2920C 65 Pb 31.0 F 3.907 1.546 2690C 66 Pb 25.0 E 1.576 1.524 2790C 67 Pb 36.7 C 29.261 1.602 2620C Several pertinent observations may be drawn from a reading of Table 5 in conjunction with Table 6. The Group A glasses, having the lowest batched F:AI ratio, exhibited higher fluorine retentions than the glasses of the other two groups with the SrO-modified composition demonstrating the highest retention of those glasses analysed. The Group A glasses manifested the best durability, but disadvantageously showed high transition temperatures.The Group B glasses displayed the lowest fluorine retention and the lowest transition temperatures of the three series.

With respect to the effect exerted by the individual divalent cations upon fluorine retention, Tables 5 and 6 illustrate that the alkaline earth metals foster greater retentions than do Cd, Zn and Pb.

Sr provides the highest retention, followed by Ca, Ba and Mg in that order. Of the other three divalent metal ions investigated, Cd appeared to cause the greatest fluorine retention with Pb exhibiting the least. Upon an increase in the batched F:Al ratio, the retention of fluorine decreases. Likewise, an increase in the molar content of the divalent metal cation effects an increase in the fluorine retention.

Those batches melted in platinum crucibles seemingly retained more fluorine than when the melting was conducted in crucibles fashioned from other materials.

An examination of Tables 5 and 6 illustrates that, generally speaking, as the concentration of the divalent metal is increased, the amount of fluorine retained in the glass also increases. There is some scatter in the durability data, but there appears to be a higher weight loss in the middle of the ranges studied than at both extremes thereof.

As might be expected, the results of the weathering tests generally compare with the water durability test with the best performances being evident with the lower F:AI ratios. However, the MgO containing glasses appeared to be less affected than those containing Pb.

Accordingly, to achieve practical durability and weathering resistance in the present glasses, the F:AI ratio will be held to below 5:1.

In summary, while the chemical durability and weathering resistance of the present glasses do not approach that of soda-lime or borosilicate glasses, they are quite comparable to many optical glasses having transition temperatures greater than 6000C. Moreover, as observed in the firstmentioned reference, anti-reflective coatings, such as Mg F2, are commonly applied to glasses designed for use in optical applications. Such coatings serve to protect the surface of the glass from ambient moisture.

To secure the best optical quality products coupled with good weathering resistance and excellent mouldability, i.e. having a Tg of less than about 3250C, the glass compositions will consist essentially, as analysed in weight percent on the oxide basis, of from 30 to 65 P205, from 0 to 50 PbO, from 0 to 30 BaO, from 0 to 30 ZnO, from 0 to 50 PbO+BaO+ZnO, from 3 to 20 Al2O3,from 0 to 10 Li2O, from 0 to 20 Na2O, from 0 to 20 K20, from 3 to 20 Li2O+Na2O+K20 and > 3 but < 24 F.

Claims

1. A glass which exhibits a transition temperature of below 3500C and which comprises, as analysed in weight percent on the oxide basis, from 30 to 75% P205, from 3 to 25% R2O, wherein R2O comprises from 0 to 20% Li2O, from 0 to 20% Na20, from 0 to 10% Rb2O and from 0 to 10% Cs2O, from 0 to 20% K2O, from 0 to 40% RO, wherein RO comprises from 0 to 15% MgO, from 0 to 40% CaO, from 0 to 40% SrO and from 0 to 40% BaO, from 0 to 60% PbO, from 0 to 40% ZnO, from 0 to 30% CdO, from 0 to 60% RO+PbO+ZnO+CdO, from 3 to 20% Awl203 and between 3 and 24% F, the atomic ratio F:AI being from 0.75:1 to 5:1.

2. A glass as claimed in claim 1 having a transition temperature of below 3250C and comprising, as analysed in weight percent on the oxide basis, from 30 to 65% P205, from 0 to 5% PbO, from 0 to 30% BaO, from 0 to 30% ZnO, from 0 to 50% PbO+BaO+ZnO, from 3 to 20% Al203, from 0 to 10% Li2O, from0to20%Na2O, from 0 two 20% K20, from 3 to 20% Li2O+Na2O+K20 and between 3 and 24% F.

3. A glass as claimed in claim 1 or claim 2 comprising up to 10% total of La2O3, and/or W03 and/or MoO3 and/or CdO and/or Nd203.

4. A glass as claimed in any of claims 1 to 3 having a refractive index of from 1.45 to 1.5 and comprising R2O, Al2O3, F and P2O5, the Awl205 content being from 1 5 to 20% and the F content being between 1 5 and 24%.

5. A glass as claimed in any of claims 1 to 3 having a refractive index of from 1.5 to 1.7 and comprising at least 5% total of at least one of RO, PbO, ZnO and CdO, the Awl203 content being from 3 to 15% and the F content being between 3 and 15%.

6. A glass as claimed in claim 1 substantially as herein described.

7. A process for the production of a glass as claimed in claim 1 which comprises compounding and melting appropriate components.

8. A process as claimed in claim 7 substantially as herein described.

9. A glass as claimed in claim 1 when produced by a process as claimed in claim 7 or claim 8.

10. An optical instrument or system which comprises an element comprising a glass as claimed in any of claims 1 to 6 or 9.