CA1058940A - Photochromic sheet glass compositions and articles - Google Patents

Abstract

PHOTOCHROMIC SHEET GLASS
COMPOSITIONS AND ARTICLES
Abstract of the Disclosure Chemically-strengthenable silver halide-containing lithium boroaluminosilicate glass compositions having physical and chemical characteristics required for sheet drawing processes and exhibiting excellent photochromic properties after heat treatment, and also strengthened photochromic drawn sheet glass articles provided there-from, are described.

Description

Back round oE the_Invention ~ ,*, ~
` ~ 10 The present in~ention i.s in the field of ?hoto-chromic glass and specifically relates to phot~ehromie , _, .. .
_ glass compositions which are both chemically strength-enable and suitable for use in the manufacture of , --=== photochrom~c sheet glass by direet drawing fro~ a melt.
:
. ~ The basic patent in the field of photoehromie '. glass, United States Patent No. 3,208,860 to Armistead . ~ and Stookey, discloses a broad range of silieate glass ~ compositions exhibiting reversible phototropic propér~
: ties. These glasses are rendered reversibly phototropie .~. 20 through the inelusion of speeified quantities of silver i . ~ halide in the glass composition, and through appropriate heat treatment of the glass after forming to eause the .~. preeipitation and growth of silver halide erystallites ln the glass. These erystallites are small enough to .~ be invisible, yet are darkenahle under the aetion of light to reduee the light transmitting eapabilities of ; the glass. Upon shielding f.rom light, the crystallites fade to the eolorl.ess state, restoring the original . .

.

,`,',` : ' .
:.-, : :
:, :
," ,.: . .
'~' "", .

, .
~:, :~ ~
r ....

~ ` ;, ' .` " :...... ,,.. : .` .... ; ~ ,`; ':,.,':: :, ': ~ . :

lii ; light transmission characteristics of the glass. In .
a photochromic glasses these darkening and fading cycles may be repeated indefinitely without fatigue.
'- ~ ~ A major use for photochromic glasses exhibiting reversible darkening under the action of visible light has been in the manufacture oE ph`otochromic ophthalmic .~, , .
-~ ~ lenses. United States Patent No. 31197,296 to Eppler ~ and Stookey, for example, describes a family of refrac-., tive index-corrected silicate glasses comprising silver '-halides which are useful for prescription lenses.
These g~asses exhibit, in conventional 2 mm thicknesses, photochromic properties sufficiently developed for pre-; ~ scription ~ophthalmic applications, and have refractive '~ ~ index characteristics compatible with conventional lens '~ ~ g~inding procedures.
Ophthalmic lens manufacture normally comprises the - pressing of glass lens blanks of optical quality from a melt foll'owed by grinding and polishing of the lens ~" blanks to specified prescriptlons. It will be appre-' _ 20 ciated that the manufacture of non-prescription photo-~ ~ chromic glass lenses, e.g., sungl'ass lenses, in large ;`r ~ ~ ' quantity by processes requiring grinding and polishing '~ is not only expensive but also wasteful of material.
'~ ' For these reasons it would be desirable to provide : ~ .
''~ ~ less costly means for producing photochromic glass sheet for lenses or other applications. It would be ,:, .
particularly desirable to provide means for producing ' such sheet in large quan-tities and in a surface and , , .
bulk quality suffic'ient for optical use. The sheet could then be in'expensively sagged to curvatures ~'''' - required for lenses, windshie'lds, or other sheet glass ~'; configurations.

. . .
~ -2-'11 -:` ~

Minimum requirements of photochromic glass sheet for applications such as described would be high opti-cal quality, good chemical durability, and high strength ~~ - ~ and good photochromic darkenabili-ty even in moderate ~ .
thickness. If the sheet is to be suitable for light-weight sunglass lenses, it must also be chemically streng-thenable in order to meet FDA requirements for eyegla~s lens safety. Federal safety requirements can-,,--~ .
not be routinely met in lightweight glass sheet ofmoderate thickness (1.3-1.7 millimeters) in the absence of tempering, or by utilizing air tempering processes.
One method of improving the darkenabllity of photo-chromic glasses for thin sheet applications is to increase the silver halide content thereof in the manner dçscribed by Stookey in United States Patent No.
3,449,103. However, this expedient requires that the glass be rapidly quenched from the melt in order to : :;~= .
~ avoid haze in the glass due to macroscopic silver hal-.
~ ~ ide crystal formation. Pressing and rolling are pro-~ . . . . . . . .
cesses which provide~relatively rapid quenching of the glass below silver halide crystallization temperatures;
` ~ ~ however, these processes do not normally prbvide glass articles or glass sheet of optical surface quality because of surface marking by the mold or roller sur-"~;, !_~ ' 't,~ ~ faces utilized to quench the glass.
Drawing processes are known which comprise direct formation of glass sheet from a melt, and in most of - i these processes the glass sheet surfaces axe not con-tacted by mold or roller surfaces until after the glass has cooled sufficiently to resist surface marking. The widely known sheet draw processes include the ~olburn ' '' , ' .. . .

~'.

:

`
1~5~ 0 -process, the Fourcault process, and the Pittsburgh Plate or Pennvernon process. These processes utilize rollers to draw the sheet up from a glass melt; but can provide~', ' glass of near-optical quality and without surface marks in,thicknessas on the order of about l.5 millimeters.
The downdraw sheet-forming processes described in United States Patents Nos. 3,338,696 and 3,682,609 to Dockerty ~, are particularly suitable for providing thin, lightweight sheet of controlled uniform thickness and optical quality at relatively low viscosities~ German Patentschrift , ,~ s ~-. , ~.
', 2,125,232 describes attempts to provide photochromic ' ', ~' glass of conventional composition utilizing forming ' ', , .. : , . . :
,, methods of~this type.
~, Unfortunately, these known sheet drawing processes ' ~, ,~ dq not provide the rapid quenching action of conventional~
pressing procedures, presenting an uncertainty regarding - ,~
l ~ the feasib~ y o producing haze-free~highly-darkenahlè
',, ~ silver ha~ide photochromic glass therefrom. '~
Moreover, all of these processes involve holding ~; ~ 20 substantial volumes of glass at rather low temperatures ',~
to obtain useful sheet forming viscosities in the 1104 , 106 poise range. These volumes~of glass are also ;;~-~
required to be in prolonged contact with the refractory ' metals or ceramics utilized as the~means for form1ng ,~ ' '"', the drawn sheet. Such processes impose severe con-, , straints on glass composition because of the formidable ,? '-, liquidus and glass stability problems assooi~ted with '`' -,~, the handling and processing of glass at relativeIy low --',', temperatures and high viscosities. , '~ 30 These problems are further aggravated by the fact ~':",, ... , :
'- , that the attainment of the required degree o . . . ,~ .

,, ";. , ,, , ,; . . . . . . . .

strengthenability in silicate glasses of -the normal type require the inclusion in the glass of some minimum quantities of exchangeable alkali ions. In the case of the photochromic glasses herein described, the presence of lithium oxide is required. Unfortunately, the pres-ence of lithium has a detrimental effect on low temper-ature glass stability, particularly in the presence of refractory ceramic surfaces.
.; ~ .
Thereforel in order to be useful for the manufac-ture of lightweight photochromic glass sheet of optical quality.~tilizing conventional drawing processes, a . glass composition must exhibit specified chemical and ,~,*t~ , physical p~operties in addition to the aforementioned properties of optical clarity, chemical durability, chemical strengthenability and good photochromic darken-lng in low or moderate thicknesses. In particular, the~
. glass must exhibit high viscosity at the liquidus and.
~i excellent stability a~ainst devitrification at forming , :~ viscosities despite prolonged contact with refractory metal and ceramic materials presently available for use as forming means in sheet draw processes.

~:~ In terms of specific constraints relating to glass ~ .. ; ~ . - . .
~; properties for these processesj a glass composition must . ... .
~ exhibit a viscosity at its liquidus temperature of at , ~ ~
least about 104 and preferably about 105 poises, and it . . . must exhibit excellent long term stability against devitrification and interfacial crystallization in con-: . .

. tact with refractory metals and ceramics such as plati-. . . .
.~ num, mullite, sillimanite, and high density alumina-; ~ 30 containing refractories used to contain or form molten ::: . glass. This stability must be maintained down to , . .
:.:
, ~

.
. "
:::, , : . . .

~11 : s `
~58~

.. . .
~. temperatures in the range corresponding -to glass vis-~ cosities of 104~106 poises, which are the viscosities ~ at which the glass is normally formed.
~:
~ ~~ In addi-tion, the glass provided must be one which .; - is chemically strengthenable to an unabraded modulus of :` rupture strength of at least 45,000 psi and a depth of . ~ compression layer oE at least 3.5 mils utilizing known - ion-exchange strengthening processes This combination .. ;~ , of properties is required to enable the glass to rou-` ~ ~ 10 tinely meet Federal safety requirements. Finally, the -~ -.~ ` photochromic properties of the glass must be such that i.~ the glass exhibits, in thickness not exceeding about ~ . 1.7 millimeters, a luminous transmlttance in the dark-:. ~ ened or activated state (darkened luminous transmittance) ~:. o~ not more than about 25%, and a fading rate such that - ~ :. the glass exhibits a faded luminous transmittance at . least.l.5 times the darkened luminous transmittance in ~, . ~ ~ .
a five-minute fading interval.
. . In addition, it is required for most sunglass appli-cations that these darkening and fading characteristics be obtained in glasses exhibiting a luminous transm~t-: . , . .~ ,. . . . .
~ tance in the clear or unactivated~state (clear luminous ': transmittance) of at least about.60%, typicall.y in the ,. range of about 60-9~%.

; ~ . Summar~ of the Invention~ ~
'"-- ' ' ' ,.
. We have now discovered a regi.on of glass composi-tions which can be used in processes for forming drawn . ;~ .
. sheet, and which yet provides haze-free optical quality, . good chemical durability, chemical strengthenability, ~:~ ; 30 and photochromic properties which are sufficiently ,. developed ~o be useful in thin sheet form.
, .,; , , : ~ ~ -6-", ~
,: . , , ,, ' : , ' . ' ' ' ~LOS~3~4~
Broadly, glass compositions in accordance with the present invention consist essentially, in weight percent on the oxide basis as calculated from t~e batch, of about 54-66~ SiO2, 7 15% A1203, 10-25~ B203, 0.5-4.0%
Li20, 3.5-15~ Na20, 0-10~ K20, 6-16~ total of Li20 +
Na20 + K20, 0-3.0% PbO, 0.1-1.0% Ag, 0.1-1.0% Cl, 0-3.0%
Br, 0.008-0.16% CuO, and 0-2~5~ F. The glass may optionally additionally contain colorant oxides selected - in the indicated proportions from the group consisting of 0-1% total of transition metal oxide colorants and 0-5% total of rare earth oxide colorants.
~ .
;~ Glasses provided from the above-described composi-.. ...
` tions exhibit viscosities of at least about 104 poises ~; at the liquidus temperature thereof, a liquidus~-viscosity ~', r~lationship which permits forming by direct sheet draw-ing from the melt.
Glasses of these compositions also exhibit long~
~ - term stability against devitrification in contact with ~ -~
`~ platinum at temperatures corresponding to glass viscos- `;~
ities in the range of 104-106 po:ises, and can thus be drawn from the melt at those viscosities using platinum or platinum-clad drawbars, downdraw pipes, or other `~
sheet forming means to provide glass sheet of optical - quality. By long term stability against devitrification `
is meant good resistance to surface crystal growth in ;~
~ . :
contact with platinum at temperatures corresponding to glass viscosi~ies in the 104-106 poise range. The growth of a crystalline layer not exceeding 10 microns in thickness at a glass-platinum interface over a thirty-day interval at these viscosities is deemed good resistance to crystal growth.

-7- ~ -: . , .. : .
.. ::. ~ ~ ' . . :

-l ~ -~
ilj ,These glasses also have excellent chemical dura- ~, , bilityr meaning for the purpose of the present descrip-tion tha-k the glass exhibits no visible surface film ~~~ ~ formation following a 10-minute exposure to 10% aqueous HCl.
Glasses within the above-described composition , unabrad~d "
region are chemically strengthenable to an a~a~e~ modu-~, ~ lus of rupture strength of about 45,000 psi with a de,pth ' of ion-exchanged layer of at least 3.5 mils as determined by conventional stress layer examination techniques util-' ~ 'izing, for example,'a polarizing microscope with a Babinet compensator. These strength and compression layer characteristics are- obtainable through conventional sodium-for-lithium salt bath ion-exchange processes at ' nQrmal ion-exchange temperatures (300-450C.), the sur-_ 'face compression being generated by the substitution into the glass of larger sodium ions for smaller lithium =cq . , - ions. ~he recited physical properties permit glass ~ sheet of 1.3-1.7,millimeters thickness to pass Government '~ 20 'impact tests for eyeglass lens safety.
~ Finallyl glasses within the above-described co~mpo-Y~ ~ sition region typically provide an excellent combination of photochromic properties following heat treatment in accordance with conventional practice. These properties , include,'in glass sheet not exceeding about 1.7 milli-, meters in, thickness, a darkened luminous transmittance not exceeding 25% and a fading rate such that the glass ~' , exhibits a faded luminous transmittance at least 1.5 times the darkened luminous transmittance after a five~
minute fading interval from the darkened state. For the , purpose of the present description the luminous ,' ' -8- ' ~' ' .,11 1~5~394~

transmittance of the glass is the value Y defined in terms of the 1931 C.I.E. trichromatic colorimetric system, utilizing the light source Illuminant C. This-~
--- colorimetric system and light source are described by A. C. Hardy in the Handbook of Colorimetry, Technology Press, M.I.T.~ Cambridge, Mass. (1936). Also for the purpose of this description, the darkened state may be obtained by exposure of the glass to direct, normally incident sunlight for 30 minutes, and the clear or un-activated state may be obtained by overnight (8 hour) fading of the glass in the absence of light.
Preferably, glass to be utllized for sunglass lens Y~p~ applications will additionally have a clear luminous transmittance of at least about 60g6, and this value can r~adily be achieved in combination with the other required photochromic properties in the composition r region hereinabove described. However, darker glasses ~ , .
having unactivated luminous transmittances of less than - 60% can also be routinely obtalned within the described s~ 20 composition region if maximum or near maximum concentra-~ ~ tions of the prescribed colorants are employed.
_ ~ - Glasses of the above-described composition are util-ized in accordance with the method of the invention which comprises an improved process for the manufacture of drawn photochromic glass sheet, wherein glass of the aforesaid composition is melted, adjusted in temperature to a viscosity of 104-106 poises, and then drawn past refractory forming means at viscosities in the 104-105 poise range to form potentially photochromic glass sheet.
-By potentially photochromic glass sheet is meant glass sheet comprising silver halides and activators such as _9_ [
i..~ l:
;ll ~L~5894~

copper oxlde which can be rendered photochromic through an appropriate heat treatment subsequent to forming.
The reEractory forming means may be a submerged drawbar~
~_. an overflow downdraw forming trough, termed a fusion pipe, or other known forming means utilized for forming glass sheet by updraw or downdraw processes from molten glass.
Observance of the above-described composition and process limitations, together with the use of supple-10 mental heat treatments and strengthening processes util-J izing conuentional time-temperature schedules, permits the production in accordance with the invention of chem-'7~ ' ically strengthened photochromic drawn~sheet glass articles hàving the strength characteristics and photo-chromic properties above set :Eorth. These properties are particularly useful, for example, in the production of thin, lightweight, photochromic eyeglass or sunglass __,;w ' lenses- or similar high strength articles for optical applications.

Detailed Description ."~,,~ I ' .
S-ince the photochromic, chemical and physical proper-ties provided in drawn sheet glass articles provided in - accordance with the invention are complex functions of the oxide constituents of the glass composition, observ-ance of the composition limitations above set forth is critical in order to obtain the combination of properties , required.
As previously noted, the presence of controlled ~ quantities of lithium oxide in the glass is required for _ 30 achieving the specified modulus of rupture strength of -~` :
lOS8~40 45,000 psi and depth of compression layer of 3.5 mils.
The use of less than 0.5% of Li20 by weight produces glass which cannot be consistently strengthened to r these'values, while the use of more than 4~ Li20 has detrimental effects on glass stability against platinum ¦ at 104-106 poise viscosities, and can produce haze in _~ .
the'glass. The required strength and depth of layer values are not obtainable in the absence of lithia using, for example, a potassium-for-sodium ion exchange ~ 10 treatment to strengthen the~glass.
,~' ' Control over the concentration of the other alkali :, ` . . ' metals isirequired because of the effect thereof on glass strengthening and photochromic properties. If less than the specified levels of soda and potash are ;' used, 'strengthenability and photochromic darkenability ''' are reduced. 'Excess total alkali content reduces the ~-~ ' fading rate of the glass below the required levels, ' whereas excess potash detracts from the chemical .
' ~ strengthenability of the glass.
The presence of A1203 and B203 in the glass counters ~the effect of alkali on the fading rate; thus glass~s containing less than the specified quantities of these -' ' constituents will exhibit inferior photochromic proper-tiesO However, the use of more than about 25~ B203 by ~ weight'reduces the chemical durability of the glass below _ ' the required levels. The use of more than 15~ alumina '`~''by weight substantially decreases glass stabllity against ;' ' ' devitrification, with the excess alumina tending to com-bine with the lithium in the composition to produce ~z ~. 30 spodumene (Li20 A1203 4SiO2) crystals.

, --11--.Ij ~L~5~3940 The presence of lead oxide in some quantity is helpful to provide the required combination of photo-chromic properties in 'che glass.
- , The addition of minor quantities of unspecified constituents to the glass is permissible but is gener-ally avoided because of the possibility of interfering with the required combination of properties. Thus ~ -while alkaline earth, bivalent and other multivalent _, :
metal oxides, such as BaO, CaO, SrO, MgO and ZnO can be ~ present in very minor quantities, they offer no proper-ties advantages and generally increase the liquidus and reduce the long-term stability of the glass.
-- ! - , ~ , Minor amounts of the alkali metal oxides Cs20 and Rb20 may be pre;sent but tend to detract from the strengthen-; ~ ~ ~ ab,ility of the composition.
Zirconia and titania should be specifically avoided due to their known tendenc~ to nucleate spodumene crys-C3~ ' , . . ' tallization.~ As little as 0.8% zirconia will promote ; zircon crystalliæation at temperatures in the forming range.
Polyvalent cations such as Sn, Sb and As may be ~x~ ~ added to the composition, for example, as minor quanti-ties of SnO~2, Sb203, and As203. Such additions may be useful in modifying the characteristics of the glass ~ - . .
melt, particularly in regard to oxidation state.
Glass compositions within the scope of the present invention may be compounded of conventional glass batch constituents in proportions which will yield the speci-fied oxide constituénts in the required amounts at tem-peratures utiliæed for melting the glass. Melting may - be carried out in accordance with conventional optial ~ -12-,._, .,. ~11 3L~589~0 glass melting prac-tice ln pots, tanks, crucibles, or _ other mel-ting units at temperatures in the 1200-]550C.
~ range.
- - Molten glass of the composition set forth may be formed by any conventional means such as pressing, roll-ing, blowing, spinning or the like. However, the glass is sufficiently stable that it may also be formed into sheet by direct dIawing from the melt, at Ieast where ~p~ plàtinum or other refractory metal-based drawbars, down-draw troughs, or other forming means are utilized.
Glass sheet or other articles formed as described may then be heat treated in accordance with thermal schedules conventional for photochromic glasses in order - to develop the specified photochromic properties therein.
S,ul*able heat treatments comprise exposure of the glass sheet to temperatures in the range of 580-750C. for `~; times ranging from a -few seconds to 2 hours. To main-G~ . ' : ' tain the required optical surface quality, the glass rnay be supported in a manner effective to prevent surface _ 20 ~marking, e.g., by edge support means.
Finally, the photochromic glass articles may be ~m~ subjected to conventional chemical strengthening treat-ments in, for example, a molten sodium saIt bath such as NaNO3 or a mixed NaN03/KNO3 bath containing 30~ or more NaNO3. The required strength and depth of compression layer may be obtained with these glasses by immersion in molten salt, for exarnple, at temperatures of about 300-450C. for times of about 4-24 hours.
The best cornbination of photochromic and physical ~ 30 properties utilizing conventional heat treating and - strengthening processes as above described are found .

~ -13-~ ~a i j~.

~[)513~

Within a preferred group of glasses having compositions consisting essentially, in weight percent cn the oxide basis as calculated from the batch, of about 57.1-65.3%~
SiO2, 9.6-13.9% A1203, 12.0-22.0% B203, 1.0-3.5% Li20, 3.7-i2.0% Na20, 0-5.8% K20, 6-15% total of Li20 -I Na20 +
K20, a ratio of Li20 content to Na20 + K20 content not exceeding about 2:3, 0.7-3.0% PbO, 0.1-1.0% Ag, 0.15-1.0%
Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.12% CuO, 0-1.0% tota,l of transition metal oxides selected in the indicated propor-tions from the group consisting of O-0.5% CoOI 0-1.0%
NiO, and 0-1.0% Cr2b3, and 0-5.0% total of rare earth , metal oxides selected from the group consisting of Er203, Pr203, Ho203~ and Nd23 ; ~ Specific examples of preferred glass compositions meeting the above limitations are set forth in Table I

~ below. The concentrations o~ constituents are set forth =~GS in parts by weight on the oxide basis as calculated from ~ the glass batch, except that the halogens and silver are ,, .
~ ~ ~ reported on an elemental basis in accordance with conven-. -_ 20 tional practice.
All of the preferred glasses above described,linclud-ing the examples of Table I below, have viscosities at the liquidus of at least 105 poises, as well as excellent chemical durability characterized by stability in aqueous ~ acidic solutions as hereinabove described. These glasses _ ~ : also exhibit long term stability against devitrification characterized by resistance to crystallization in contact with platinum at viscosities in the 104-106 poise range, as above defined. When heat treated according to conven-tional schedules to develop photochromic properties, all glasses have darkened luminous transmittances not exceeding , . , , ~

.
1~51~9~0 about 25% and fading characteristics such that the glasses exhibit a faded luminous transmittance at least 1.5 times the darkened luminous transmittance within a five-minute fading interval. These glasses also exhibit clear luminous transmittances greater than 60~.
All of the preferred glasses are ion-exchange strengthenable to a modulus of rupture strength of at ` ~ least 45,000 psi and a depth of compression layer at ' least 3.5 mils utilizing conventional ion-exchange strengthening treatments.
Table I includes photochromic property data where reported on individual samples, including the clear luminous transmittance of the glass (YO), the darkened laminous transmittance of the glass (YDj, and the lum-in~us transmittance of the darkened glass after a five-mlnute fading interval (YF5). For purposes of repro-'~ ducibility in measurement, the darkened glasses were all provided by exposure of the glass to two 15 watt black-light blue fluorescent UV lamp bulbs at a distance ~20 of 3-3/4 inches for a twentv-minute interval, to approx-imate the darkening produced bv exposure to direct sun-llght as hereinabove described.

-, ~

.,, ' ' ~

h ~

_ !~; . , ~IE
11 .

TABLE I
.
~ ~ .
1 2 ~ 4 5 6 SiO2 59.5 59.5 59.5 58.1 60.9 57.1 B203 17-3 17.0 17.7 17.3 15.8 17.0 A123 11-6 11.6 11.6 13.4 11.9 13.1 _ Na20 5.8 7.6 5.8 5.6 5.9 7.6 1~ . ` .
Li20 2.0 2.8 3.0 2.9 3.0 2.8 ~ .
PbO 2.2 2.1 2.2 2.2 2.3 2.1 F .27 .27 .27 .27 _ .27 ~ A~ .25 .22 .22 .22 .22 .22 i~ Cl .30 .30 .48 .47 .49 .30 ~ q~
Br .25 .20 .15 .48 .11 .35 ~ ~
CuO .018 .018.018 .018 .026 .018 ~ -CoO - .14 ~ ~ NiO .13 - - - - - -= YO 74.7 91.1 90.8 - 91.891.6 ' YD 22.0 21.0 22.0 - 22.721.9 YF5 42.1 - 38.840.0 ~: .' '' ' ~ -., ' .
. .", ~, .. .
~' ,' ' , ~ .
, .
, ~ .' ' ' ' : . ' ' .j , .
~_ ' ' ' , . , .

:

S~

We haye previously described the diEficulties with respect to glass stability presented by the use of llthium oxide in the present glasses. Nevertheless,l -we have found that the presence of lithium oxide is required to achieve the required glass strengths and depths of compression layer, since a sodium-for-lithium ion exchange process must be utilized to achieve these properties. Table II below illustrates glass composi,--tions outside the scope of the present invention and shows the difficulties encountered when strengthening ' by a potassium-for-sodium ion exchange process is attempted. Compositions are set forth in parts by weight on the oxide basis as calculated from the batch, except for the halogens and silver which are reported on, the`conventional elemental basis.
~ ..
The ion-exchange strengthening processes employed .~ comprised immersion of the samples in molten KNO3 salt ~;
baths at temperatures in the range of 300-450C. for . .
times in the range of 2-24 ho~rs in accordance with known procedures. It will be observed that none of the ~ compositions shown in Table II was strengthenable t~
W~F~ provide the combination of 45,000 psi strength and 3.5 mil depth of layer which is required-to enable 1.3-1.7 millimeter glass sheet to routinely pass eyeglass impact safety requirements.

.. . . .
~ , ' /
- -17- ~

~ID5~

.
~ , .
~ .
,, O CS~ N ~) NLll('I~`1~) ~1 0 , ~1. . . . I. . . . . .
Co CO ~ O ~ ~ ~ o o o O
. -n ~ ~ n ~r ~ o ~ ~ ~ ~ O O ~
~ ~41 ~ ~ I I . . .
I~lli o > ~ o ~1 ~1 0 o o o O ~`I
~ ; , ~ ' O
_ ,~
~ , . ' ~j3 - g ~ ~ ~ ~ ~ ~ ~ o o : ; ~
o. ~ o ~ ~ o o oo o .`.,. ~ ; ~ ~ ,, ` o . . ~ . o Cl~lr) d' ~ ~t ~I N ~ ~10 1-1 i H, : -:
~ 1O I ~n o a) ~ 1 0 u~
~ a~ t5~ o GO ~ ~DO O O O O ~r) ~ ~ ,~ o ~; .:, ~ ~ ~ ~O ~
00 N ~ ~ O O OO O r-l : . - In N o N
~ I~ c~ ~ o o O oO ~-1 ., . ~ ~ O
O
. :

~ a) û, .C ~1 0 ,, ~ (~ ~ O O O
5,~,~ o o t`J N N o h Q~
'.~ tn m~ ~ æ ~ K Q ~
o ,1 " .

,If ~: .
~0 589 4!~

Although lithium oxide in some minimum concentra-tion is required to obtain sheet glass with useful chemical strengthenability characteristics/ the herein--_ , described maximum concentration limitations for this constituent must also be observed in order to obtain the glass stability required for sheet drawing applica-tions. Moreover, relative freedom from alkaline earth metal oxides and other non-essential oxides such as ZnO, ~p~ TiO2 and ZrO2 is desirable in order to preserve the stability and resistance to devitrification of the glass In fact, glasses which are essentially free of CaO, BaO, SrO, MgO, ZnO, TiO2~ and ZrO2 are preferred for use in the invention.
Table III below sets forth further glass composi-tions outside the scope of the present invention whlch ~d confirm the effects bf excess;ve non-essential oxide concentrations or excessive lithium oxide concentrations - on the physical-chemical properties of the resulting glass. The common feature of all of these glasses is insufficient low temperature glass stability, character-ized in each case by a liquidus temperature substantially above the 10~ poise viscosity temperature of the glass, , .
in most cases above the 104 poise viscosity temperature r .
of the glass. Also, all of these glasses exhibit poor long-term stability in contact with alumina-containing refractories at temperatures in the forming range.

Compositions are set forth in parts by weight on the oxide basis as calculated from the batch, except for silver and the halogens which are reported on an ele-mental basis. These compositions would not be utilized for continuous sheet drawing processes because the ..

D58~

relative instability of the glass would, under normal - conditions, eventually lead to devitri~ication and the ~ formation of crystalline defects or stones in the ~~~~ ~ drawn glass.

_ ~ ~!
.. ., ~
'~ .
. ',' , : "' . ' ~ , r",~ ~ ' ' ' - ' , , ' -' , ' ~
., .

; . .
.~",; , ~ .
'' ' '' ~ '.

.

~.' ' ' ~'.

., .
~ -20~

~58g'~

Lr ~
o~r ~c~ ~ ~ ~ ~ ~ ,~ o oo ~,-1 In ~ ~ ~ o o o o ~ o ~ ~ ~ ~ ~ ~ o 01 ~` ~ ~ I I I I -cn ~ ~ L~ `3 0 0 ~O o ~:, ~ ~ ~1 ~L~7 ) o o o o :i Ul ~,1 , :~ ~ o ~ t~ I~ ~ ~ ~ ~ ,~ o ~ :
- ~ O O O O

~: ~ . H ¦, ` - '.
~ . ~
I Lr, ,1 ~ CO ~9 o ~ ~ ~ ~ ~ ~ o Is~ D ~ O O O O

:

oo ~ CO O ~) O t~l O O O O

~ ' .
~ , r~r~l r~) I ~ o ~ , ~ ~D , ) ~) O ~ O O O O
. , ' ' ~''.
r~
H ¦ ~~ ' ~1 0 , ~
~ co r a~ ~ ~
~.` ' , .. .
r~ o ~ a~r~1 r~l r~l ~ ~ o ~ r~ r s> ~1 :

o o o r~l o o ~ r~l ~ o o o o o o O
~ u~ m ~ Z ,~ K`~ Q ~ m ~ , h - g o ,: " . , ., ~ :
.

51~94~
The photochromic prope.rties of the drawn sheet are of course affected within limits by -the thermal treatment utilized to develop these properties in the -~~ ! glass. However, these properties are also critically dependent on the composition of the glass. Composi-tional variations affecting these properties include not only variations in the concentrations of the photo-- chromic constituents silver, the halogens, and copper, oxide~ but also variations in alkali metal oxide, silica, and B293, and PbO concentrations.
-~ Table IV below sets forth further examples o glass compositions outside the scope of the present invention w~hich exhibit insufficient photochromic dark-ening or poor fading characteristics attributable to cqmpositional variations beyond the limitations herein described. The concentrations of the glass constitu-ents are set forth in parts by we.ight on the oxide ~ .
: basis as calculated from the batch for each glass, . Pxcept for silver and the halogens which are reported ~;
on an elemental basis. The observed insufficiencies in photochromic behavior noted for each glass are allso reported, and are with reference to the specific dark-: -~
ening and fading requirements hereinabove set forth.
~' ', " ' ' '' ' . -22-:"~ , ' ' , . . , ~; , ,:

'11 `

~11958~

~ O O ~ ~ ~a ~ o o ~ ~ In ~ ~ ~ ,~ o ~1 ........... 4~
- r~ O ~ O
Ln ~ r~
o o o In ~ ~
O ~r O N r-l ~) N N ~r-l 0 ~1 b ~, j ~ oo ~ ~) ~ ~ rc~
v ~,.,* (~ 1 r-l :,1~` . ' ' S-J

,. ~, .
~ ` ~ ~:
H ~ ~ ~ a ) N NtYl -I ~ ' ~ .
~ . O O O u~ ~ ~a ~ I t~ ~ cr~ ~1 0 N N ~ ~-1 0 (a o .
. . ~ ,, .
....

~,r~ O O ~r~ ~
o~l . . . . . . . ~a -U~ r-J r~

Q~ , , . .
. j , ~ a) O rl , )-I ~

O 0~ ~ ~ ~ o u, m ~ Z ~ ~ ~ ~ ~) m r ~ .

~2 3 :~ . . : .

1~5~

The optional addition of the above-described trans-ition and rare earth metal oxide colorants to the glass compositions of -the present invention is useful to pro-3 vide some light attenuation and coloration in the faded state, usually for cosmetic purposes, and also to pro-vide some coloration and attenuation in the darkened state. However, care must be exercised in the selection of colorants to be incorporated into photochromie glasses because the effectiveness of some multivalent colorants depends strongly upon the oxidation state of the glass, and because some colorants absorb in the ultraviolet and .` ,~,7. ~ .
' ~; thus reduce the darkening capabilities of the glass.
For these reasons, the foregoing transition metal and rare earth metal oxide colorants are preferred. How-everl minor quantities of additional colloidal or ionic colorants such as uranium, cadmium sulfide, cadmium selenide, metallic gold or the like can be added pro-~ .
vided such additions do not detrimentally affect the ~
..
photochromic properties of the glass Table V below sets forth examples of tinted glass compositions provided in accordance with the invent~ionl - - illustrating the use of some of the preferred eolorants and the colors indueed thereby in the Gompositions of the inventionO Only the concentrations of the color~
ants, in parts by weight, are set forth, -the remainder of each glass consisting of 59.2 parts SiO2, 17.2 parts B203l 11.6 parts A1203, 8.7 parts Na20, 1.5 parts Li20,

2.3 parts PbO and minor concentrations of photochromic constituents by weight.
,': ''.
q~_ .

- : . . . ~

~51~4(~

1 1 1 1 o b~
. ,,,_ _. !

a) a) '.
I tr) .
~a .- O
' ~

. ' ~ I I ' I I O I ~ ' : ~?.
, ~ ,,~ ~1 . . . ; - , .

. ~ a) ` O , ' ~1 ~ ~ . .
o 3 ,~, . .

,, 1 ~ , , , , -O O Q
. . .
. - .

~ ~
_~ . ~1 o ,~
.. . O O a~

, O O Q

1--1 o o I I I I
O O ' ~
. , Q
~L L~l ~, 3~ o o 01 ~ 1 N
t~ ~ h .. . . . . . . .. .
.. . .

1~5~9~

The highest quality drawn glass sheet produced from glass compositions provid~d in accordance with the present invention is obtained utilizing downdraw sheet forming apparatus such as described in United States Patent No. 3,338,696 to Dockerty. The use of apparatus of this type, particularly in combination with the sheet thickness control means and methods described in s ~ United States Patent No. 3,682,609 to Dockerty, permi,ts the production of glass sheet of optical quality and uniform thic]~ness which is especially suited to the pro-duction of photochromic drawn sheet glass articles, such ;
. : : ,. -.;
as sunglass lenses, having the properties hereinabove described.
However, these processes also place the ~ighest damands on the physical-chemical properties of the glass because of the extreme glass stability required for 1,,,p, ~ . success~ul drawing in accordance with these mèthods.
~ . , .
Stability is particularly critical where the downdraw forming member, referred to as a fusion pipe or trough, is composed of an alumina-containing refractory ceramic such as sillimanite or dense alumina.
~e~ Glasses within the preferred compositional limita-.
tions hereinabove set forth, such as the glass composi-tions shown in Table I, exhibit long-term stability against platinum but in some cases are not sufficiently _ stable for use in combination with alumina or alumina-containing refractories. Hence glass compositions which are par~icularly preferred in accordance with the pres-ent invention are those exhibiting long-term stability, as hereinabove defined, even in contact with alumina-containing refractories at glass viscosities in the lO4-lO6 poise range. Such glass compositions '11 ~L~S1394~

consis-t essentially, in weight percent on the oxide

3 , basis as calculated from the batch, of about 57.1-65.3 .
SiO21 9~6-13~9~o A1203, 12.0-22-0% B203~ 1-0-3-5~ Li2~
''~ ! 3.7-12.0% Na20, 0-5.8% K20, 6-15% Li20 + Na20 ~ K20, a ratio of Li20 content to Na20 ~ K20 content not exceed-ing about 1:3, 0.7-3.0~ PbO, 0.1-1.0% Ag, O.lS-1.0% Cl, 0-3~0~ Br, 0-2.5% F, 0.008-0.12% CuO, 0-1.0% total of transition metal oxide colorants selected in the indi-cated proportion from the group consisting of 0-0.5%
CoO, 0-1.0% NiO, and 0-1.0% Cr203, and 0-5.0% total of ,~
'rare earth metal oxIde colorants selected from the group s sting of Er203~ Pr23~ ~23 and Nd23 Glasses '~
within the~broad scope of the present invention but out- -7,. side these defined limitations are not sufficiently s~able with time for use with alumina-containing over- -' 'flow downdraw forming members of the kind described ' , above.
Examples of glasses suitable for use in combination ~, , with alumina-containing refractory downdraw fusion pipes or troughs in the production of photochromic drawn sheet ~ glass articles in accordance with the invention are~ set _ forth in Table VI below. The compositions of the glasses s , are reported in parts by weight'on the oxide basis as calculated from the batch except for silver and the' halogens which are conventionally reported on an elemen-' tal basis,. All of the glasses set forth in Table VI
exhibit long-term stability against devitrification in ' , contact with alumina-containing refractories at glass viscosities in the 104-106 poise range, as evidenced ~ 30 in each case by a crystallization layer less than 10 ~~, ' microns in depth at a glass refractory interface even .

~-27-.

,Ij after 30 days at a temperature corresponding to the 105 . poise viscosity temperature of the glass.
Also shown in Table VI are the modulus of rupture ~~-~ ! strengths and compression layer depths observed after ion-exchange strengthening of the glasses when deter-mined Qn individual samples, these properties being : produced by a 16-hour treatment in a molten salt bath of 60~ NaN03 and 40% KN03 at 390C. , . ~
~ Also shown in Table VI are the photochromic proper- :
.. :,~'.~ .
~ 10 ties of the glasses, includi.ng the clear luminous trans-, - mittance (Y0), the darkened luminous transmittance (YD), and the faded luminous transmittance (YF5), obtained ~ according ~o the procedures utilized in generating the :~- data set forth in Table I
It~ ~ . . ~ .
~ : ~
~ " . ' ' ;
~ ~ .
. .
~ , .

~ : :
, -~' " , ' '' .

.

~' .
'' .:

, ,. ~ , . . .
.

~
~`

: .~q~
f,-- ~ O t~
:: C10 N ~ ~ ~ O ~ ~ 1 O O ~:1 t~l - O ~ ~ ~ ~D
> ls~ ~J~1 ~ 1'1 ~,.. ~`."~ a~ ~ O 'i' ~)~1~) ~ ~ ~ N O O r-l .
; O I .. . . . . . .
. f~ !. .'~ ' 1~1 CO . r-l ~ ~ ~ : , , OLr~ O~9r-lI--1 W 1-- Il') ~r)~It`~l ~ t~l O Q
' L~

~,. ' ' ' o ~' o ~
' . ', . 1~7 ~ ~ r`1~ ~ ~ N ~~1 O Or-l _- 'I ' . . . . .. .
.~
N~) ~In N ~~
If ) ~~ I~ O ~l ~ ~~1 0 00 r-In o ~,~
~n ~rf.` ~ O ~ I ~t~ O O~1 ~D .. . . , I ,. . . ..
0~ 1`~ ~ ~ ~.
, In ~~ :
. ~ . , ~ ~ ~ ~ o o o i ~ o o ~ ~ ~ o o o o o .~ ,1 (`1~1 (d rl ~ Q t~ o~1 ~n m ~ z ~ K P~ O m ~ f0 Z

~ O

.

1' ~58~

Ln O
o N O

~ ' _ I ~1 ~a I ~, o ~r o o ~) I . cn ~ ~ o ~ I . ~ -, :
I ~ In Ll~ o ' , ~, ~ o r o o o ~r ' I` ~1 ~ O ' ' ' ~ ~ .
~ .
U'~

O
) ~ I r~ ~ o I~ o ~1 ~ ....
~ . ~:
~ ` H 0~ . . . . . .
,~"y,.,v,~,~ . ~ r-l tY~ ~ ~ O ~ ' " ' N ') O
~1 , .
S~ ~
In ~ ' n o o ~
O In r , ~ ~

' ' '~ ~ ' .

8~ :
. i . , . _, ~, .
.

~o ~
, _30 ~S~4~

The invention may be further understood from the following illustrative example describing the produc~
tion of strengthened photochromic drawn sheet glass articles in accordance therewith.

Example ,', ' . - , . . .
A glass batch is compounded and melted in a batch ;~ ;
melter at a temperature on the order of about I400C
to provide a molten glass having a composition, in parts ;
by weight as calculated from the batch, of about 58.1 .: ~
parts SiO2, 16.9 parts B203, 11.3 parts A1203, 3.8 parts ; Na20, 5.8 parts K20, 1.8 parts Li20/ 0.24 parts Ag, 0.30 parts Cl, 0.26 parts Br, 0.27 parts F, and 0.018 parts CuO
by weight. ;
,, ~ . -The molten glass is fed into a refractory overflow ;;
~downdraw fusion pipe at a viscosity of about 104 poises and is delivered from the pipe as drawn glass-sheet about 1.5 millimeters in thickness. This drawn sheet is cooled below the glass softening temperature and , ~ . .. - ~ .
;~ separated~1nto~sections of glass sheet from which small 20~ ~ ;samples~o~ drawn sheet glass are cut.
The drawn sheet glass articles prepared as described are then subjected~to a heat treatment to develop the photochromic properties thereof. ~The heat treatmen~ com~
prises heating the articles in a lehr, supporting the article in a manner to prevent~surface damage, at a rate of about 600C. per hour to a temperature of 630C., hold~
ing the articIes at this temperature for 1j2 hour, cooling the articles at a rate of about 600C. to a temperature at Ieast below 350C. per hour, and finally removing the - 30 articles from the lehr. ~

., ~lj .

1~5~

: ~.
The photochromic drawn sheet glass articles pre-pared as described are then subjected to an ion-exchange strengthening treatment comprising immersion in a molten - ~ , NaNO3 bath at a temperature of 390C. for 16 hours.
After treatment the samples are removed from the salt bath, cooled, washed to remove excess salt, and tested for strength and photochromic properties. The unabraded modulus of rupture stxengths of the articles averages about 50,200 psi with depths of surface compression :~ 10 being about 3.5 mils.
~! The faded luminous transmittance (Y) of a typical ; ~ 1.5 mm thick photochromic drawn sheet glass article pro-duced as above described is about 90.9%. When darkened by a 20-minute exposure to a pair of 15 watt black-li,ght blue fluorescent UV lamp bulbs spaced a distance of about 3 3/4 inches from the glass sheet, a darkened luminous transmittance of about 22.8% is obtained. The ~ . , darkened glass typically fades about 15.6 percentage ~ . , points to a luminous transmittance of about 38.4% in a ~, 20- five-minute fading interval.
From the foregoing example, which is merely illus-trative of the various eompositions and proeedures whieh may be employed in accordance with the present invention, - the effectiveness of our compositions in providing - strengthened photochromie drawn sheet glass artieles exhibitin~ the required properties for optieal and other applications is readily apparent.
' ' ~ -32-f ... . . . . ..

Claims (11)

We Claim:

1. A glass composition suitable for the production of chemically-strengthenable photochromic drawn sheet glass articles consisting essentially, in weight per-cent as calculated from the batch, of about 54-66% SiO2, 7-15% A1203, 10-25% B203, 0.5-4.0% Li20, 3.5-15% Na20, 0-10% K20, 6-16% total of Li20 + Na20 + K20, 0-3.0% PbO, 0.1-1.0% Ag, 0.1-1.0% Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.16% CuO, 0-1.0% total of transition metal oxide color-ants, and 0-5.0% total of rare earth metal oxide colorants.

2. A glass composition in accordance with claim 1 which consists essentially, in weight percent as calculated from the batch, of about 57.1-65.3% SiO2, 9. 6-13.9%
A1203, 12.0-22.0% B203, 1.0-3.5% Li20, 3.7-12.0% Na20, 0-5.8% K20, 6-15% total of Li20 + Na20 + K20, a ratio of Li20 content to Na20 + K20 content not exceeding about 2:3, 0.7-3.0% PbO, 0.1-1.0% Ag, 0.15-1.0% Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.12% CuO, 0-1.0% total of transition metal oxide colorants selected in the indicated propor-tion from the group consisting of 0-0.5% CoO, 0-1.0% NiO
and 0-1.0% Cr203, and 0-5.0% total of rare earth metal oxide colorants selected from the group consisting of Er203, Pr203, Ho203, and Nd203.

3. A glass composition in accordance with claim 2 wherein the ratio of Li20 content to Na20 + K20 con-tent does not exceed about 1-3.

4. A chemically-strengthened photochromic drawn sheet glass article formed of a glass having a composition consisting essentially, in weight percent as calculated from the batch, of about 54-66% Si02, 7-15% A1203, 10-25% B203, 0.5-4.0% Li20, 3.5-15% Na20, 0-10% K20, 6-16% total of Li20 + Na20 + K20, 0-3.0% PbO, 0.1-1.0%
Ag, 0.1-1.0% Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.16% Cu0, 0-1.0% total of transition metal oxide colorants, and 0-5.0% total of rare earth metal oxide colorants, said article having excellent chemical durability, an unabraded modulus of rupture strength of at least about 45,000 psi, and a compressively-stressed surface layer at least about 3.5 mils in thickness, and said glass having, in a thickness not exceeding about 1.7 milli-meters, a darkened luminous transmittance not exceeding about 25% and a fading rate such that the glass exhibits a faded luminous transmittance at least about 1.5 times the darkened luminous transmittance after a five-minute fading interval from the darkened state.

5. A photochromic drawn sheet glass article according to claim 4 having a thickness in the range of about 1.3-1.7 millimeters.

6. A photochromic drawn sheet glass article according to claim 5 having a clear luminous transmittance in the range of about 60-92%.

7. A photochromic drawn sheet glass article according to claim 6 which is an optical lens.

8. A photochromic drawn sheet glass article according to claim 4 having a composition consisting essentially, in weight percent as calculated from the batch, of about 57.1-65.3% SiO2, 9.6-13.9% A1203, 12.0-22.0%
B203, 1.0-3.5% Li20, 3.7-12.0% Na20, 0-5.8% K20, 6-15%
total of Li20 + Na20 + K20, a ratio of Li20 content to Na20 + K20 content not exceeding about 2:3, 0.7-3.0%
PbO, 0.1-1.0% Ag, 0.15-1.0% Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.12% CuO, 0-1.0% total of transition metal oxide colorants selected in the indicated proportion from the group consisting of 0-0.5% CoO, 0-1.0% NiO and 0-1.0%
Cr203, and 0 5.0% total of rare earth metal oxide color-ants selected from the group consisting of Er203, Pr203, Ho203, and Nd203.

9. A photochromic sheet glass article according to claim 8 wherein the ratio of the Li20 content to the Na20 + K20 content does not exceed about 1:3.

10. An improved process for the manufacture-of poten-tially photochromic drawn glass sheet comprising:
(a) melting a glass consisting essentially, in weight percent as calculated from the batch, of about 54-66% SiO2, 7-15% A1203, 10-25% B203, 0.5-4.0% Li20, 3.5-15% Na20, 0-10% K20, 6 16% total of Li20 + Na20 +
K20, 0-3.0% PbO, 0.1-1.0% Ag, 0.1-1.0% Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.16% CuO, 0-1.0% total of transition metal oxide colorants, and 0-5.0% total of rare earth metal oxide colorants;
(b) adjusting the temperature of at least a region of the glass melt to provide a glass viscosity in the range of about 104-106 poises, and (c) drawing the glass at a viscosity of 104-106 poises directly from the melt past refractory forming means to provide potentially photochromic drawn glass sheet.

11. An improved process according to claim 10 wherein the refractory forming means is an overflow downdraw forming trough.