DE1496470B2 - ALKALINE SILICATE SOLIDIFIED BY ION EXCHANGE, THE ENVIRONMENTAL SURFACE LAYER AND THE PROCESS FOR ITS MANUFACTURING ARE CONSTRUCTED BY THE GLASS SUBSTANCE WITH A CORE AND A PRESSURE - Google Patents

Description

The present invention relates to an ion exchange strengthened alkali silicate glass article having improved mechanical strength and methods of making such an article.

It is known to strengthen glass articles by ion exchange, which involves the exchange of relatively large ions in the glass for relatively small ions from an external source at temperatures above the lower cooling point of the glass (10 ^{14 lS} poise). This type of ion exchange is z. In U.S. Patent 2,779,136. This patent describes the treatment of an alkali silicate glass in which the alkali metal is sodium and / or potassium with a material containing lithium ions at a temperature between the stress point and the softening point of the glass. The lithium ions are introduced into the glass surface in exchange for sodium and / or potassium ions from the surface of the glass in order to produce a layer of new glass on the object, the coefficient of thermal expansion of which is lower than that of the original glass. As a result, when the object is cooled, compressive stresses are generated in the new glass surface.

A second known way of strengthening the glass by ion exchange involves exchange of relatively small ions in the glass versus relatively large ions from an external source at temperatures below the lower cooling point of the glass. This type of ion exchange is z. B. from the French Patents 1,329,124 to 1,329,126 are known.

With this type of ion exchange, compressive stresses tend to arise directly from the ion exchange even than through the formation of a new glass surface.

The improvement in strength obtained in this way can, however, be lost quite quickly if the article is subsequently subjected to a temperature of 300 ^° C. or more for an extended period of time with subsequent treatment. Furthermore, there is often a serious decrease in the chemical resistance of the glass surface due to the ion exchange. From a raw glass production standpoint, the available lithium-containing raw materials are relatively expensive, and the viscosity and devitrification properties of the lithium silicate glass often make it difficult to use conventional glassmaking processes. These various problems are now avoided by the reinforced glass article and method of consolidation of the present invention.

The object according to the invention is a glass object made of a solidified by ion exchange Alkali silicate glass core with a larger alkali ion than lithium and made of one under compressive stress Standing alkali silicate glass surface layer with a different structure than the core, but same molar composition, in which a lower molar content of the alkali ions contained in the core is balanced by a corresponding molar content of lithium ions, which is characterized by is that at least some of the lithium ions in the outer edge zone of the enveloping Surface layer through larger monovalent metal ions without changing the structure of the surface layer is replaced.

In the production of such a solidified glass object, the object can initially be made from an alkali metal silicate glass which has a relatively large ion as the primary alkali ion contains. This item is then replaced by a layer of a lithium silicate glass of lithium ions against the larger alkali metal ions in a surface layer or zone of the object enveloped. This surface zone is then subjected to a further ion exchange, the lithium ions of this zone being exchanged for larger alkali ions at a temperature below the temperature at which the tension in the glass rapidly decreases.

The larger ions that take part in this second ion exchange can e.g. B. from a molten salt originate, which is in contact with the surface of the object, or they can come from the Inside the glass object by counter-diffusion or penetration of alkali ions during the further Originate from heating the glass object. For example, larger alkali ions from the Stem glass inside the object migrate to the outside in order to keep up with the lithium ions from the newly created Glass on the surface of the object get exchanged, with the lithium ions migrate deeper into the object in exchange with the larger ions. In both cases the obtained article characterized by an improved strength, which during substantial periods of time is maintained at temperatures up to 600 ° C.

The object can be, for example, a flat sheet of glass or pane of glass, as is the case with building or Vehicle windows are used, or it can be a hollow body, such as. B. a container or Container that is used in the packaging, manufacture or serving of food and beverages can be. The retention of strength at high temperature characteristic of the new item is of particular importance in items used in cooking and similar items that are necessarily exposed to elevated temperatures for prolonged periods of time. For test and Experimental purposes, however, have short glass tubes or glass rods with a diameter of 6.3 mm proved to be particularly suitable.

The larger alkali ion contained in the stem glass is preferably an ion usually known as Flux is used in melting glass; d. H. Sodium, potassium or a mixture of these both elements. Other alkali metals such as B.

Rubidium and cesium can also be used, but are of lesser interest because of their greater cost and their lower mobility during ion exchange. The expression "alkali metal silicate" is understood in the conventional sense to denote a glass composition in which silica (SiO ₂ ) is the only or primary glass-forming oxide and one or more alkali metal oxides represent the main glass modifications.

In addition, the glass normally contains at least 5 percent by weight Al ₂ O ₃ and / or ZrO ₂ in order to improve the strength properties. Glasses suitable for the present invention,

can therefore consist essentially of 5 to 25 percent by weight Na ₂ O and / or K ₂ O, 5 to 25 percent by weight Al ₂ O ₃ and / or ZrO ₂ , 45 to 80 percent by weight SiO ₂ and 0 to 20 percent by weight of other conventional glassmaking materials.

The chemical resistance of a glass surface can be determined in a number of ways, depending on the particular nature and degree of durability required for the particular application. For the purposes of the invention, the acid resistance, which is determined by the conventional hot acid tests, is of particular interest. In this test, a piece of the glassware is immersed in a 5% hydrochloric acid solution at 95 ° C. for 24 hours, the acid resistance being expressed by the weight loss of the glass in milligrams per square centimeter. In general, it is considered that a glass whose weight loss is greater than 0.5 mg / cm ^{2 should be} regarded as a glass of insufficient strength.

In order to produce a glass article which is able to meet these qualifications after chemical strengthening by an ion exchange process, glasses are used in the present invention which consist essentially of 45 to 75 percent by weight SiO ₂ , 2 to 20 percent by weight P ₂ O ₅ , 5 to 25 weight percent Al ₂ O ₃ and 8 to 25 weight percent Na ₂ O and preferably essentially 45 to 70 weight percent SiO ₂ , 10 to 22 weight percent Na ₂ O, 10 to 22 weight percent Al ₂ O ₃ and 4 to 15 weight percent P ₂ O ₅ exist.

Glasses that. composed solely of alkali metal oxide and silica, generally have poor strength properties and are not well suited for chemical strengthening, particularly deep strengthening to withstand wear. It has therefore been found that it is preferred to add either alumina or zirconia in order to stabilize the glass while giving it significantly improved strength. Alumina is generally preferred for fusing purposes and at least 5 weight percent is required for strength and durability improvement. The shelf life usually peaks in glasses containing about 15 to 18 weight percent alumina and then drops so that glasses containing more than about 20 weight percent Al ₂ O ₃ have inadequate shelf life, especially with higher soda content. With a soda content of 8 to 15 percent by weight, a slightly higher alumina content can therefore be used before the drop in shelf life significantly exceeds ^{0.5 mg / cm 2.}

Silica is the basic glass-forming oxide. At least 45 percent by weight is required to obtain a glass with sufficient durability and strength.

It has been found that the presence of P ₂ O ₅ in soda-alumina silicate glasses avoids the surface defects of the crystal structure, namely cracking and / or shrinking, which otherwise normally takes place during lithium ion exchange. Relatively small amounts can inhibit crystal formation in some glasses. Usually, however, at least 2 percent by weight P ₂ O _{5 is} required and a level of 4 percent by weight is the optimum. Suitable phosphate raw materials are generally more expensive than alumina silicate materials, such as. B. feldspars and the like. It is therefore desirable that the P ₂ O ₅ content does not exceed about 10 percent by weight. On the other hand, however, up to about 20 percent by weight can be used.

Sodium oxide (Na ₂ O) is the base glass modifier and also provides ions for exchange with lithium during the first exchange. At least 8 percent by weight is desirable for this purpose and 15 to 18 percent by weight is the optimal amount. Up to about 25 percent by weight may be present in certain circumstances, but such larger amounts tend to impart lower chemical resistance and higher liquefaction temperature. The latter makes melting and molding more difficult.
Other known glass-forming oxides, such as. B.

the other alkali metal oxides, the alkaline earth metal oxides, PbO, TiO ₂ , glass colorants and fining agents can each be contained in limited amounts of 1 to 5 percent by weight. Larger amounts generally tend to crystallize in the glass, to lose the solidification potential or to have other adverse effects.

The solidified silicate glass object of the type described above is obtained by first producing a glass object from an alkali silicate glass in which the alkali ion is predominantly an ion other than lithium The main glass of the object is formed, this surface glass being characterized in that it has a significantly higher content of lithium ions than the content of the main glass, and then the lithium ions from the glass surface layer are exchanged with exchangeable ions with a larger ionic radius without significant rearrangement of the glass structure, whereby a pressure-tensioned surface is created on the object. Preferably, the base glass of the object contains at least one oxide, such as. B. Na ₂ O or K ₂ O, and a lithium silicate glass surface layer is obtained in that the object is brought into contact at a temperature above the glass tension point with a substance containing lithium ions, whereby these lithium ions with the larger alkali metal ions of the main glass at least be partially exchanged, so that the surface layer has a different composition and structure.

By exchanging the lithium ion or another smaller ion for a larger alkali metal ion A thin layer of glass forms above the glass tension point, which is structurally different as well as in their composition from the original alkali metal glass from which the object is made was produced differs. The new glass is referred to as silicate glass in that it is the Stammglas corresponds, except that in the new glass the lithium content is much higher and the sodium ion content is substantially less, but the total molar alkali metal oxide content is substantially is the same in both glasses. In so far as the exchange takes place above the glass tension point, ord-

net the stem glass structure so that it differs in terms of the size of the alkali metal ions present, i.e., accommodates the exchange of sodium for lithium ions in the glass composition.

The following description relates to embodiments of the invention in which the primary alkali metal ion in the main glass is sodium and the enveloping layer is a corresponding lithium silicate glass is. The description refers to the drawings.

F i g. 1 is a schematic representation of a cylindrical glass rod in cross section according to FIG Embodiment of the present invention;

Figures 2 and 3 graphically illustrate the manner in which the sodium ion concentrations in an article the in F i g. 1 can be changed;

F i g. Figure 4 graphically illustrates the retention of strength of such an article;

F i g. 5 is a schematic illustration of a cylindrical glass rod in cross section according to FIG further embodiment of the present invention;

F i g. 6 explained together with FIG. 2 graphs the manner in which the sodium ion concentration in the case of an item of the type shown in FIG. 5 can be changed.

In the glass rod of FIG. 1, the core part 10 is surrounded by a layer 12 which surrounds it in a circular manner surrounded by a glass with a lithium silicate glass structure, d. H. a structure that differs from that of the Glass core 10 differs in that the number of lithium ion sites in the glass structure is essential is larger than in the glass core, but the total number of alkali metal ion sites (the molar Content of the alkali metal oxide) is the same. The glass of layer 12 is further characterized by that at least some of the lithium ion sites in the lithium silicate glass structure of the surface layer 12 by larger ions, such as B. sodium or potassium ions, is taken without the lithium silicate glass structure is changed. The presence of such larger ions on the smaller lithium ion sites leads to a compressive stress in the surface layer 12, which remains as long as the Glass structure is not rearranged to accommodate the larger ions. The surface layer 12 is normally quite thin, but preferably at least 0.0508 to 0.1016 mm thick prevent loss of strength in the product due to abrasion. .

The composition in the surface glass layer 12 can be very different. For identification and to distinguish it from the inner glass, however, the surface layer 12 is used as a unit considered.

The manner in which the surface layer composition can vary is illustrated in FIG. 2, which shows the sodium ion line of a typical sodium aluminosilicate glass article subjected to lithium ion exchange for 15 minutes under conditions where the mutual ion diffusion rate is 10 ~ ⁶ cm ² / sec. fraud.

In the illustration, the sodium ion ratio is -q

plotted on the ordinate, while the depth of penetration into the glass surface in thousandths of a centimeter is plotted on the abscissa. The sodium ion ratio is the ratio of the sodium ion content at each indicated point to the sodium ion content of the stem glass before each exchange. It it is obvious that the lithium ion line can also be obtained by using the sodium

ion line rotates 180 ° on the ~ - = 0.5 axis,

what is shown.

A particular advantage of the present invention is that sodium oxide or potassium oxide or a mixture of these oxides as the primary flux for silica in the glass that makes up the glass article prepared in accordance with the present invention can be used. During execution According to the present invention, a silicate glass composition containing such a flux is mixed and melted to give a molten glass from which an article of the desired shape molded using conventional glassmaking techniques.

The glass article obtained in this way is placed in a suitable molten lithium salt bath immersed at a temperature above the glass stress point for a sufficient time to to achieve a desired depth of ion exchange between the glass and the bath. The essential The salt bath's requirement is that it contain an exchangeable lithium ion that is contained in the glass introduced for final exchange by a larger ion in a subsequent ion exchange stage will. Thus, the liquid glass bath normally contains an ionizable lithium salt, either alone or in conjunction with another salt.

A mixed salt bath may be necessary to create a temperature sufficiently above the glass tension point. For example, mixtures were (Li ₂ SO _4), and 50 used to 5% of sodium sulfate (Na ₂ SO ₄₎ from 50 to 95% of lithium sulfate to temperatures of 600 to provide up to 86o C ^0. Chloride baths were also used, but are less suitable because of the chemical action on metal containers and plant components and because of the glass itself.

The terms "stress point" or "stress temperature" denote a temperature at which the glass has reached a certain state with regard to relaxation. In general, they indicate a maximum temperature at which relaxation does not yet take place under the given circumstances and corresponds to ^{a viscosity of about 10 14} · ^{5 poises.}

The time required for the initial lithium ion exchange varies with the exchange temperature. The optimal replacement time can be between 5 minutes at a temperature near the The softening or deformation point of the glass up to about 100 hours at a temperature straight lie above the stress point. Usually temperatures of about 50 to 100 ° C will be below of the glass softening point at times of a few minutes to an hour. If ejn other ion than the lithium ion is used, the lower mobility requires a correspondingly longer replacement time. In any case, it is necessary to create conditions under which a layer from a silicate glass for subsequent exchange

large versus small ions of the type described below.

The glass article thus obtained, which has a synthetic lithium silicate glass layer, can now brought into contact with an ion exchange material that supplies ions larger than lithium ions will. These can be the same ions as originally melted in the glass and passed through the initial ion exchange was removed. The surface contact is preferably made by means of a molten salt bath instead. For an object with a synthetic lithium silicate glass surface layer The bath can contain a sodium salt, potassium salt, silver salt or a mixture of such a salt a modifying salt at which a suitable bath temperature is obtained. Typically a sodium nitrate bath can be used to introduce sodium ions.

The critical factor at this stage of the process is to exchange a larger ion for to effect the smaller lithium ion of the glass at a sufficiently low temperature so that no substantial molecular rearrangement can take place within the glass, thus creating the larger Ion can customize. Then the glass article is usually at a temperature below or near of the glass tension point in order to shorten the replacement time.

In this case, too, the time required for optimal amplification essentially depends on the 30; ions, the glass composition and the exchange temperature. For a given glass an optimal exchange of sodium for lithium ions for reinforcement at temperatures of 50 to 100 ° C below the glass tension point require about 1 hour. The rule of thumb is that this time roughly doubles for every temperature drop from 50 to about 250 ° C. Where larger ions than Potassium or rubidium ions are used, a considerably longer exchange time is required, than would be expected based on the ion mobility.

F i g. 3 illustrates the manner in which the sodium ion concentration line the F i g. 2 is changed by this second ion exchange. In Fig. 3 the sodium ion ratio is ig-J, i.e. H. the Ratio of the existing sodium ion content to that of the original stem glass, again based on the The ordinate is plotted and the depth of penetration into the glass (in thousandths of a centimeter) is on the abscissa worn away. Each of the different curves shows the sodium ion line at time "i" in hours, where- * identified by the curve as the time of the second ion exchange. The curve dor F i g. 2 shows the Ion distribution at time 0 (i = 0). This line changes gradually over time and approaches infinity where the lithium ions are completely replaced, which is indicated by the straight line in FIG. 3 will be shown.

Another embodiment of the invention is illustrated in FIG. 5 of the drawing. The glass rod the F i g. 5 is shown as a glass rod composed of three zones for explanation. It contains a core or central zone 50, an outer or surface zone 52 and an intermediate zone 54. The central or core zone 50 represents the alkali metal silicate master glass from which the article is made will be produced. The surface zone 52 consists of a lithium silicate glass layer that passes through received a first exchange between lithium and sodium ions and modified by sodium ions which occupy part of the lithium sites. The intermediate zone 54 consists of the alkali metal silicate trunk glass, from which the sodium ions have migrated and into the lithium ions during a second heat treatment.

The strengths of these zones represent the depth of penetration into the glass and are exaggerated for explanation drawn. In particular, the surface zone 52 is normally much shallower than it in FIG Drawing was shown. Furthermore, the compositions within the zones are not constant, but more of an irregular nature, so that the zones merge into one another.

When producing an object according to FIG. 5 can be used in the same way as in the production of an object according to FIG. 1 until the time of the second ion exchange. It therefore becomes an alkali metal silicate glass article produced with a surface layer of lithium silicate glass according to the method described.

The glass object is now subjected to a further heat treatment at an elevated temperature below the Subject to deformation area. This temperature is such that the glass does not have such a low viscosity that a rearrangement of the glass structure can take place. Usually it is below the glass tension point. The heat treatment does not have to be in a specific medium or in a specific atmosphere be made. It is therefore advisable to carry out the treatment in normal or ambient air in an oven or another conventional method Device for heat treatment to make.

The heat treatment time depends mainly on the temperature at which the treatment is carried out and the degree of internal ion exchange and the resulting enhancement desired away. Every rewarming has a certain degree of internal ion exchange and one of it resulting gain. However, like an ion exchange with an external source, increases the effective degree of exchange for reinforcement to an optimum value in a relatively short time. Then further heat treatment is of little * or no value.

The rate of exchange increases with temperature, with temperatures well above 200 ° C are usually required. The heat treatment is preferably from 50 to 150 ° C. below Glass tension point. In this temperature range, times range from about 4 to about 16 hours im is generally used to achieve the maximum gain that can be achieved with a given lens.

During this second heating stage, the larger alkali metal ions are caused to be in the Stem glass, e.g. B. the exemplified sodium ions, from the stem glass in the middle area of the object to and into the glass of the surface zone in exchange with lithium ions from this Surface zone penetrate. Because of the low temperature at which the exchange takes place, is subject to however, none of the glasses of structural rearrangement. As a result, the sodium ions contained in reach the surface zone,, lithium ion sites in this glass. This creates compressive stresses

109531/279

developed within the surface caused by tension be balanced inside, so that greater mechanical strength in the glass object is obtained.

F i g. 6 illustrates the manner in which the sodium ion concentration line the F i g. 2 is changed by a more extensive exchange that takes place during the heat treatment takes place. In Fig. 6 is the sodium ion ratio, i.e. the ratio of the Sodium ion content after treatment to that in the original glass, again on the ordinate and the depth of penetration into the glass (in thousandths of a centimeter) is shown on the abscissa. The worn one Sodium ion content line is typical of a sodium aluminum silicate glass that is on for 4 hours 450 ° C after initial lithium ion exchange as in F * g. 2 has been heat treated.

The nature of the invention, its practice and the advantages arising therefrom will be discussed will now be described and illustrated with reference to specific examples.

In these examples, the term "strength" refers to the breaking strength of a material or object, which is determined as the modulus of rupture (BM) by placing a test piece over two spaced knife edges, a second pair of knife edges on the test piece evenly spaced between the first two knife edges and loaded the second pair until the glass breaks. The maximum breaking strength found on the lower surface of the test piece is then calculated as BM in kg / cm ^{2 of} the load, the size and shape of the sample and the test geometry.

The term "abrasion resistance" refers to the breaking strength of a as determined above Body with a variety of abrasions, d. H. visible scratches or defects that are random on the Surface were generated. The nature and degree of abrasion on the glass surface depend on the Conditions of use. Standard abrasion tests have therefore been put in place on a valid basis to have for comparison and imitation of known conditions of use.

Two types of abrasion were used for the present purposes. For one type, a test piece, z. B. a glass tube with a diameter of about 6.3 mm and a length of about 10 mm, mechanically mounted and quickly in contact with 150 grit silicon carbide paper for about 30 seconds rotated under a low constant pressure so that uniform contact was maintained. A second type is called chafing. In this case, ten equal sized glass rods are used 200 ecm silicone carbide particles of 30 grit mixed and a scrubbing motion for 15 minutes in a No. 0 ball mill at 90 to 100 rpm. The first type of abrasion became surface imperfections imitated that arise during use - when they rub against hard materials, for example when glass objects rub against each other. Errors generated in this way mimic damage caused by a combination of abrasion from chafing and direct impact develop.

The strength of the glass with an undamaged fresh surface is very high, which is shown by measured strengths of several 100 kg / cm ² in freshly drawn glass threads and rods. In practice, however, the strength of normal, commercially available glass objects is between 350 and 1757 kg / cm ² , depending on the type of glass, the manufacturing process and the nature of the subsequent treatment.

Examples 1 to 8

The following compositions, given in parts by weight on an oxide basis, explain the glasses used in these examples.

Table I.

SiO ₂ ..
Al ₂ O ₃
Na ₂ O
Li ₂ O.
TiO ₂ .
As ₂ O ₃
CaO.
B ₂ O ₃ .
ZrO ₂ .

MgO.
Sb ₂ O ₃

1 2 3 4th 5 6th 7th 8th 62.1 64.4 65.4 64.5 59.2 62.8 55.2 64.7 24.1 26.0 23.6 15.0 24.0 24.8 19.4 - 7.2 6.4 9.0 20.0 9.5 9.9 16.3 19.9 3.6 1.9 1.5 - 1.8 1.0 - - - · 0.8 - - 5.0 1.0 4.6 - - 0.5 0.5 0.5 0.5 0.5 0.5 0.5 - - - - - - 2.0 - - - - - - - 2.0 - - - - - - - 14.9 0.1 - - - - - - 1.9 - - - - - - 1.1 - - - - - -

72.0

11.0
1.0

16.0

In general, these glasses have been melted from frits using conventional methods consisted of common raw materials. A 2.7 kg frit was in a crucible that lasted 16 hours Was heated in an electric furnace to about 1550 to 1600 ° C, melted and then into a tube drawn.

The conditions of the various specific examples and the measured modulus of rupture in kg / cm ² are given in the following table, the example number in the table below corresponding to the number of the above glass, so that a glass of composition 1 was used in example 1 below . In every example

the glass test samples are glass rods with a diameter of 6.3 mm and a length of 10 cm that have been cut from glass tubes or rods drawn from a melt of the specified glass / Glass samples shaped in this way are particularly suitable for strength measurements. Corresponding untreated samples, which were annealed and then subjected to abrasion with a silicon carbide paper of 150 grit, show breaking modulus values of approximately 350 to 492 kg cm ² . The temperature is in ⁰ C, the time is in minutes, unless otherwise noted, and the salt baths are given in percent by weight.

Table II

example Salt bath Time Tempe
rature
⁰ C BM
(kg / cm ² ) 1 80% Li ₂ SO ₄
20% LiCl 2 min. 750 85% NaNO ₃
15% Na ₂ SO ₄ 2 hours. 450 4710 2 95% Li ₂ SO ₄
5% Na ₂ SO ₄
NaNO ₃ 10 min.
5h 800
400 3867 3 50% Li ₂ SO ₄
50% Na ₂ SO ₄
NaNO ₃ IStd.
6h 650
400 4689 4th 60% Li ₂ SO ₄
40% Na ₂ SO ₄
NaNO ₃ 15 minutes.
4h 650
400 2623 5 80% Li ₂ SO ₄
20% Na ₂ SO ₄
NaNO ₃ 15 minutes.
4h 800
400 4099 6th like No. 5 3832 7th 60% Li ₂ SO ₄
40% Na ₂ SO ₄
NaNO ₃ . 15 minutes.
4h 650
400 2953 8th like No. 7 3698

If a glass object is above the glass tension point at a temperature in the deformation or the annealing area is heated, the glass structure changes quickly afterwards Any tension easing. The reinforcement caused by compressive stress in glasses by means of however, an ion exchange treatment in which large ions are exchanged for small ions can easily get lost by opening the object

ι ο significantly lower temperatures heated for a long time. It is believed that this is also the result of the relaxation of compressive stress, however, by ion migration or diffusion within the glass, instead of becoming viscous Glass.

In contrast to this, a special characteristic or a special property is one according to the invention reinforced object to maintain its strength at high temperature. This is based on the ability to maintain the increased strength under the subsequent heat treatment at relatively maintain high temperatures below the glow range. The nature of this extraordinarily beneficial Property is explained in the following comparative example.

Example 10 Example 9

A series of five glass test rods which had been produced from a melt of composition 9 were immersed in a molten salt bath consisting of 80 parts by weight Li ₂ SO ₄ and 20 _{parts by weight Na 2} SO ₄ and held there at 720 ^° C. for 15 minutes. After cooling and cleaning, the series was then placed in a molten sodium nitrate bath for 4 hours at a temperature of 400 ° C. The calculated average modulus of rupture for this series of samples after the indicated treatment was 3213 kg / cm ² .

For comparison purposes, a similar series of five glass rods were treated with the indicated lithium sulfate bath and then tested. The calculated average modulus of rupture for this series of samples was 984 kg / cm ² .

To complete the comparison, a third series of glass test samples were treated in the sodium nitrate bath for only 4 hours. The calculated average modulus of rupture was 1055 kg / cm ² . Glass tube samples of the ax described in the preceding examples are made from a glass having the following approximate composition, expressed in percent by weight, on an oxide basis: 50% SiO ₂ , 23% Al ₂ O ₃ , 7% P ₂ O ₅ , 18% Na ₂ O, 1% Li ₂ O and 1% K ₂ O. The glass has a stress point of about 580 ^° C.

A number of the pipe samples, hereinafter referred to as Series A, were subjected to a one-step large versus small ion exchange enhancement treatment. This treatment was that one for 16 hours immersing the samples in a series Kaliumnitratschmelzbad with a temperature of 450 ⁰ C (KNO _3). The treatment caused the potassium ions from the bath to be exchanged for the sodium and lithium ions from the glass.

A second series of pipe samples, hereinafter referred to as series B, was treated according to the invention in such a way that a double ion exchange took place. This treatment consisted in that the set of samples was immersed for 10 minutes in a bath, was consisting of 75% Li ₂ SO ₄ and 25% Na ₂ SO _4, wherein the bath temperature at 750 ⁰ C was. This led to an introduction of lithium ions into the glass in exchange for sodium and potassium ions from the glass. The tube samples were then purified and brought into a molten bath of 450 ⁰ C, which consisted of 85% _NaNO3 and 15% Na ₂ S0 ₄ and left for 2 hours. This second bath treatment served to introduce sodium ions into the glass in exchange for some of the lithium ions introduced by previous ion exchange. The two large sets of samples were then divided into smaller groups, and all samples were again heated in air to 500 ⁰ C, with a small group of each sample type was removed periodically for strength measurements. The strength was measured on each individual tube after abrasion of the type described above.

The average modulus of rupture for Group A and for Group B was calculated for the various rewarming times and these values are shown in FIG. 4 of the drawing. This figure is a graph with reheating time on the abscissa and average modulus of rupture (in thousands of kg / cm ² ) on the ordinate. The two curves embodying the foregoing data are designated A and B , as previously described, to correspond to the names of the Examples and the ion exchange treatments therein which are compared.

It should be noted that although the single ion exchange treatment plotted as curve A has the higher initial strength, this strength quickly drops on rewarming and appears to be completely lost after about 250 hours. In contrast, the inventive reinforced samples, plotted as curve B, experience a small initial drop, then recover easily and remain relatively constant even after 1000 hours of rewarming. Finally, as can be seen from the graph, there is a loss of strength. Glass objects that have been reinforced by the double replacement process retain the essential part of their increased strength for a sufficient period of use at 500 ^° C.

Example 11

Rods 6.3 mm in diameter were drawn from a large crucible melt of a glass which had the following composition, expressed in percent by weight, calculated on the oxide basis: 50% SiO ₂ , 20% Al ₂ Q ₁ , 10% P ₂ O ₅ , 18 % Na ₂ O, 1% K ₂ O and 1% Li ₂ O. This glass was melted from conventional raw material mass in 16 hours at 1550 ⁰ C and had a measured voltage point of 576 ⁰ C and a softening point of 86o ⁰ C. The drawn rod was cut into pieces 10 cm in length for comparative ion exchange treatments and experiments.

All rod pieces were then immersed for 10 minutes in a molten salt bath consisting of 75 mol percent Li ₂ SO ₄ and 25 mol percent Na ₂ SO ₄ . was, the bath was held at 75O ⁰ C. The test tubes were then removed and cleaned prior to further treatment. The various test tubes were now divided into four groups, and each group was placed in an oven to be heated to different temperatures in air, all temperatures being below the glass tension point. During the heat treatment of each group, individual groups of six samples were taken so that the effect of the different heat treatment times could be determined.

After the hot air treatment, each bar sample from each group was subjected to abrasion, after which a modulus of rupture for the individual bar and an average value for each group would be calculated.

The following table shows the temperatures in ° C and the hot air treatment times in hours for the individual sample groups. The table shows the average breaking modulus values (in kg / cm ² ) for each sample group.

Table III

Time (hours) 525 ° C 450 ° C 0 1898 1898 Va 2180 - 1 2672 2461 4 " 3586 3305 16 3656 3797 64 4148 4078 240 3586 4148

Similar modulus of rupture data not given in the Understanding have been given for corresponding ones Groups of pipe samples identified that were not subjected to abrasion testing. In general do these values closely agree with the preceding values, with the difference that they are approximately 10 to 20% higher.

B e i s ρ i e I 12

A series of glass rods, generally corresponding to those of Example 11, were made from a glass melt having the following calculated composition in percent by weight on an oxide basis: 72% SiO ₂ , 16% ZrO ₂ , 11% Na ₂ O and 1% Li ₂ O.

Various rows of such rods were immersed in a molten salt bath, which consisted of 80 mol percent Li ₂ SO ₄ and 20 mol percent Na ₂ SO ₄ , and held in this bath at a constant bath temperature of 720 ^° C. for 15 minutes. After cooling and cleaning, the rod sample sets were divided into two groups. One group was then heated in air to 450 ° C, while the other group was heated in a similar manner to 550 ⁰ C. A number of samples were periodically removed to allow comparison when the heat treatment was changed. Each individual rod sample was then rubbed and broken when bent.

The following table shows the average breaking modulus values (in 1000 kg / cm ² ) for each group of samples, the temperature and time of the hot air treatment being given in ⁰ C or hours for each group of samples.

Table IV

Time (hours) 550 ° C 450 ° C 0 984 984 2 1617 1547 4th 1617 1898 8th 1547 2109

Examples 13-23

The following table gives the composition of a number of glasses in percent by weight on an oxide basis, which were obtained from the corresponding glass compositions. The table also sets forth certain pertinent measurements of properties on the glasses as described below. · '■ ":' ■■ ■■■■. ■

Table V

13th 14th 15th 16 17th 18th 19th 45
15th
20th SiO ₂ 60
18th
5 60
16
7th 59
20th
4th 65
15th
5 60
20th
5 55
15th
10 18th Ai-, α. 15th 15th 15th 13th 13th 18th 1
1 P ₇ Os 1
1
2953 1
1
3305 1
1
2672 1
1
2812 1
1
2461 1
1
3023 2673 Na ₂ O 4219
0.05
clear 3797
clear 3867
0.33
easy 3305
0.03
clear 4078
0.28
easy 4008
0.08
easy 3234 Li-, 0 clouded clouded opal easy K ₇ O opal Modulus of rupture
(1) hot air (2) DoD gel salt Weight loss (mg / cm ² ) ..
Appearance

A glass mass corresponding to each of the above compositions was mixed, melted, and the glass was drawn into rods 6.3 mm in diameter. These wands were Cut into 10 cm lengths and divided into three groups for further treatment and examination.

Each set of samples was weighed, immersed in a 5% solution of hydrochloric acid at 95 ° C. for 24 hours, and then weighed again to measure the glass loss in mg / cm ^{2 of} the surface of the object. The resistance property that is important relates to the surface after ion exchange. However, in the successive exchanges that were carried out with these glasses, surfaces were created which essentially corresponded to the original and had corresponding resistance values within the test accuracy. The chemical resistance measurements were therefore expediently carried out on the trunk glass.

The remaining two sets of pipe samples were immersed in a molten salt bath consisting of 75 percent by weight lithium sulfate and 25 percent sodium sulfate. The bath temperature was kept constant at 750 ⁰ C. Each series of samples was immersed for 10 minutes at this temperature in order to cause an exchange of lithium for sodium ions in the surface layer of the glass tube. After this ion exchange treatment, the pipe samples were examined by visual inspection of their surface.

One row was then immersed in a second molten salt bath, which consisted of 85% sodium sulfate and 15% sodium chloride, and was kept there at a constant temperature of 450 ^° C. for 2 hours. The other row was placed in an electrically heated oven held at 450 ° G and remained there for 16 hours. After this sequential ion exchange treatment, each sample was scrubbed after which modulus of rupture measurements were made.

The fracture moduli obtained in this way are given in Table V in 1000 kg / cm ² under the designation “BM x 10 ³ kg / cm ² ”. The data for these series of samples, which were subjected to successive ion exchange enhancement treatment by immersion in two successive salt baths, are identified by the designation "double salt". The data for the samples that were subjected to a lithium ion exchange and then a heat treatment in air are identified by the designation »hot air«. The data for the chemical resistance with regard to the various glasses are identified by the designation »weight loss (mg / cm ² )«.

In addition to these data, the appearance of the samples is described as either "clear" or "slightly cloudy". The former designation denotes a glass surface that is essentially free of visible crystallization the latter indicates that slight formation of crystals was noted on the glass surface could be.

In the case of Examples 18 and 19, the glass surface was clear, but opal glass formation could be observed in the body of the glass. Such a separation of an opaque crystal phase is characteristic of glasses with a relatively high P ₂ O ₅ content. It may or may not be acceptable, depending on the demands placed on the transparency of a particular object.

The data presented in Table V illustrate the ability to minimize crystallization and other surface defects during sequential ion exchange for reinforcement of glass articles, as evidenced by rupture modulus values after abrasion of 1752 to 4219 kg cm ² and a correspondingly excellent acid resistance.

Another group of explanatory glass compositions, as well as data for the characteristic ones Properties that were determined for the corresponding glasses are shown in Table VI below, specified. The data were obtained in the same manner as in Table V in all cases and specified.

109 53.1 / 279

Table VI

20th 21

23

SiO ₂

Al ₂ O ₃

P ₂ O ₅

Na ₂ O

Li ₂ O

Rupture modulus x 10 ~ ³ kg / cm ²

(1) hot air

(2) double salt

Weight loss (mg / cm ² ) ..
Appearance

50 25

5 18

3305 4780

15.2 slightly cloudy 55
25th

5
13th

1687
3023

33.8
jumped

63
18th

2
15th

1687.
1967
0.07

heavy
Crystal formation

50 30

18 1 1

Specimen breakage

Examples 20 and 21 of Table VI illustrate that reinforced glass articles with marginal surface appearance are obtained with a maximum content of alumina. However, as stated, the acid resistance of such glasses is very poor. Example 22 illustrates the tendency for surface crystallization with relatively small amounts of P ₂ O ₅ in the glass and a somewhat lower level of reinforcement compared to glass 13 of Table V. Example 23 illustrates the extreme problems that can arise with a glass, the one Contains excessive amount of clay and no P ₂ O _5. In general, these data indicate that at least 2% P ₂ O _5, and preferably 5%, of this oxide should be present in a glass for the present purposes. They also show that the combined content of Al ₂ O ₃ + R ₂ O should not exceed about 38% for reasons of acid resistance.

Claims (11)

Claims: 40 1. Glass object strengthened by ion exchange made of an alkali silicate glass core with a larger alkali ion than lithium and from an alkali silicate glass surface layer under compressive stress with a different structure than the core, but the same molar composition, in which a lower molar content of the alkali ions contained in the core due to a corresponding molar content of lithium ions is balanced, characterized in that at least part of the Lithium Ioneri in the outer edge zone of the enveloping surface layer by larger monovalent metal ions is replaced without changing the structure of the surface layer. 2. Glass object strengthened by ion exchange according to claim 1, characterized in that that there is an intermediate glass layer between the surface layer and the core, which the The structure of the alkali silicate glass has in the core, however it differs from this in the composition differs in that it has a lower content of alkali ions and contained in the core has a correspondingly higher content of the lithium ion of the surface layer, although the molar amount of total alkali ions remains the same. 3. Glass object strengthened by ion exchange according to claim 1, characterized in that that the relatively large alkali ions are the same as the alkali ions contained in the core. 4. Glass object strengthened by ion exchange according to one of the preceding claims, characterized in that the larger alkali ions contained in the core are sodium, potassium or a mixture of these substances. 5. Glass object strengthened by ion exchange according to one of the preceding claims, characterized in that the alkali silicate glass contains 5 to 25 percent by weight Al ₂ O ₃ or ZrO ₂ and mixtures of Al ₂ O ₃ and ZrO ₂ in the core. 6. Solidified by ion exchange glass object according to one of the preceding claims, characterized in that the core glass 8 to 25 percent by weight Na ₂ O and / or K ₂ O, 5 to 25 percent by weight Al ₂ O ₃ and / or ZrO ₂ , 45 to 75 percent by weight SiO ₂ and no more than 20 percent by weight of any other ingredients used in glass manufacture. 7. A glass object strengthened by ion exchange according to claim 6, characterized in that the core glass contains 5 to 25 percent by weight Al ₂ O ₃ and 2 to 20 percent by weight P ₂ O ₅ . 8. A method for producing an ion-exchange solidified article according to a of the preceding claims, characterized in that first an object from a Alkali silicate glass core is made with a larger alkali ion than lithium, lithium ions against some of the alkali ions that were originally in the outer zone of the glass object found to be exchanged at a temperature above the stress point of the glass, whereby a surface glass layer is formed on the article which is in the composition and glass structure differs from the core glass, and then at a temperature below of the lower cooling point some of the lithium ions from the glass of the surface layer, which are on this Way was obtained by ions of larger ion diameter without rearranging the glass structure is replaced in the surface. 9. The method according to claim 8, characterized in that the lithium ion consists of a salt Molten bath consisting of 50 to 90 mole percent lithium sulfate and 10 to 50 mole percent sodium sulfate
exists, at a temperature between 650 and
850 ⁰ C is exchanged. 10. The method according to claim 8 or 9, characterized
characterized in that the second ion exchange
is achieved in that the glass object with
a molten salt bath of salt with the larger one
Exchangeion is brought into contact. 11. The method according to claim 8 or 9, characterized
characterized in that the second ion exchange IO by heating the glass object obtained by the first ion exchange is performed at a temperature less than that used in the first ion exchange Temperature so that the alkali ions from the inner part of the object are exchanged with lithium ions from the surface zone of the object without any substantial Regrouping the glass structure and relaxing during this second exchange takes place. 1 sheet of drawings