Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C21/00—Treatment of glass, not in the form of fibres or filaments, by diffusing ions or metals in the surface
  • C03C21/001—Treatment of glass, not in the form of fibres or filaments, by diffusing ions or metals in the surface in liquid phase, e.g. molten salts, solutions
  • C03C21/002—Treatment of glass, not in the form of fibres or filaments, by diffusing ions or metals in the surface in liquid phase, e.g. molten salts, solutions to perform ion-exchange between alkali ions

Definitions

• This invention relates to the strengthening of glass articles by creating a surface layer of compressive stress therein through an ion exchange process. More particularly, this invention relates to the strengthening of sodium aluminosilicate glasses wherein potassium ions are exchanged for sodium ions and contemplates heating such glasses in air prior to the ion exchange process.
• alkali silicate glass articles can be strengthened several fold by means of an ion exchange process.
• a glass article containing lithium or sodium ions is contacted with a source of alkali metal ions having a larger ionic radius than lithium or sodium at an elevated temperature but below the strain point of the glass for a period of time such that i the larger ions will migrate into the surface of the .glass article and replace the smaller alkali ions therein. Since this exchange is carried out at a temperature below the strain point of the glass, there can be no viscous flow thereof to relieve the compressive stresses set up by the crowding of the larger ions into the glass structure in place of the smaller ions. Thus, a surface compression layer is set up in the glass article which greatly enhances the mechanical strength thereof.
• the means for carrying out this large-ion-for-small-ion type of ion exchange has generally utilized two approaches.
• This second mode of operation has founded a new dimension in commercial glass manufacturing since it has permitted the production of glass articles exhibiting mechanical strengths heretofore not obtainable. And it is with this second method that the present invention is an improvement thereon.
• British Pat. No. 966,733 discusses the unusual effect the addition of substantial amounts, i.e., greater than 5% by weight, of A1 0 to alkali silicate glasses has upon the strength than can be imparted thereto through ion exchange of the large-ion-for-small-ion type.
• Specific examples are recited therein of the exchange of potassium ions for sodium ions in sodium aluminocilicate glasses and the strengths obtained thereby compared with other sodium silicate glasses wherein the A1 0 content is less than 5% by weight. That patent also observed that a true realistic appraisal of the strength of glass involves the measurement thereof after the surface of the article has been subjected to at least moderate abrasion.
• the pristine strength of newly-formed glass articles is very high but usually of very short duration, since normal handling of the articles causes surface damage which drastically reduces the initial strength.
• the mechanical strength of a glass article is generally of little significance unless measured after the surface has been subjected to abrasion to simulate conditions like that which the glass might see in service. Therefore, two techniques were devised for simulating the abrasion underice gone by glass articles in service. These techniques were described in that patent and are set out below since these tests were utilized in examining the mechanical strengths of the glasses developed by the instant invention.
• a test piece In the first type of abrasion, a test piece, generally a 4 inch by inch diameter glass cane, is mechanically mounted and rapidly rotated for about 30 seconds in contact with 150 grit silicon carbide paper under a small constant pressure to maintain um'form contact.
• the second type of abrasion comprises placing ten similar-sized glass rods ma number 0 ballmill jar, adding 200 cc. of 30 grit silicon ca'rlgide particles, and then rotating the jar at 100 r.p. rn. for 15 minutes.
• the invention comprising British Pat. No. 966,733 lay in'the discovery that the presence of substantial amounts of A1 0 i.e., at least 5% by weight, resulted in glasses exhibiting high abraded or practical strengths.
• the glasses would still exhibit etrengths several fold that of annealed glasses of the same composition. Comparisons of this property were made between the aluminosilicate glasses and glasses containing little or no A1 0 and, specifically, the common soda lime glasses of commerce.
• our invention comprises heating glass articles of certain compositions in air or other inert atmosphere at temperatures from about 25 C. below the transformation range to temperatures within the transformation range of the glass and then carrying out a potassium-for-sodium-ion exchange at temperatures below the strain point of the glass.
• glass-forming batches consisting essentially, in weight percent on the oxide basis, of about 50-65% SiO 10-25% A1 0 and 10-20% Na O were melted in open crucibles at temperatures between about l5001600 C. for a period of about-16 hours. In most instances, the melts were mechanically stirred to insure homogeneity. Cane about A" in diameter was drawn from each crucible and, frequently,'-%" thick sheet was rolled.
• the glass articles were then placed in an electricallyheated kiln and the temperature raised therein to slightly below or within the transformation range of the particular glass.
• the transformation range of a glass is that 'temperature area within which a liquid melt is deemed 'to I have become an amorphous solid. This temperature range has been considered as lying between the strain pointcand the annealing point of a glass.
• the strain points of the glasses of this invention range between about 550-600 C. and the annealing points between about 600 6507 C.
• the glass articles were maintained within the kiln operating at temperatures slightly below or within the transformation range of the individual glasses for at-least four hours.
• the glass articles were im mersed in a bath of molten KNO operating at an elevated temperature but below the strain point of the g'lass involved. We have found that the highest strengthslare obtained where the ion exchange is carried out at temperatures between about 50150 C. below the strain point of the glasses.
• the glass articles were then removed from the salt bath, the adhering salt removed with water, and thereafter abraded according to one of the abovedescribed procedures preparatory to testing for modulus of rupture.
• Table I records various glass compositions, expressed in weight percent, on the oxide basisascalcuIated from the batch, which were treated in accordance with this invention.
• the batches may be composed of any materials, either oxides or other compounds, which, on being melted together, are converted to the desired oxide compositions in the proper proportions.
• a conventional fining agent such as AS203 may be included in the batch.
• AS203 a conventional fining agent
• about 0.5-1.0% by weight is added but, since the quantity remaining in the glass after the batch has been melted is too small to have any significant effect on the fundamental properties of the glass, this constituent, if employed, was not included in the table.
• Table II reports various heat treatments and salt bath treatments applied to Example 1 along with modulus of rupture (MOR) measurements made after the tumble abrasion test.
• the strain point of this glass is 581 C. and the annealing point is 631 C.
• a bath of molten KNO was utilized in each instance although other potassium salts which are molten at the required temperature range can obviously be employed, as can mixtures of potassium salts be utilized.
• Table II also supplies a comparison in mechanical strength obtained employing the preliminary heat treatment of the present invention with that attained where the preliminary heat treatment is omitted.
• Each MOR figure represens an average of measurements made on six samples and is believed to be within an error of :3000 p.s.i.
• Table II clearly demonstrates the effectiveness of the preliminary heat treatment in improving the mechanical strength of Example 1, the preferred glass for this invention, based upon such practical considerations as meltability and formability as well as the capability of exhibiting very high mechanical strengths.
• the preferred strengthening technique involves heating the glass article in air for about 24 hours. Such procedure generally assures a tumble abraded strength of about 90,000 p.s.i. (r3000 p.s.i.). Where the glass is abraded with silicon carbide paper rather than being tumble abraded, this preferred strengthening technique has yielded modulus of rupture measurements of about 110,000 p.s.i. as compared with about 72,000 p.s.i. wher no preliminary heat treatment in air is employed.
• Table III records the effect of a preliminary heat treatment in enhancing the mechanical strengths of the other examples reported in Table I.
• the glasses were immersed in a bath of molten KNO for 24 hours at 500 C. to accomplish the exchange of potassium for sodium ions.
• a method for strengthening a sodium aluminosilicate glass article by replacing the sodium ions in a surface of the glass article with potassium ions by contacting a surface of the glass article with a source of potassium ions at an elevated temperature but below the strain point of the glass to place a compression layer in the surface of the article, the improvement which comprises forming the glass article to be strengthened from a sodium aluminosilicate glass consisting essentially, by weight on the oxide basis, of about 1020% Na O' 10- 25% A1 0 and 50-65% SiO heating said glass article to a temperature from about 25 C.

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• Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Geochemistry & Mineralogy (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Surface Treatment Of Glass (AREA)
• Glass Compositions (AREA)

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Cited By (30)

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Citations (3)

• 1967
  • 1967-06-01 US US642686A patent/US3524737A/en not_active Expired - Lifetime
• 1968
  • 1968-05-21 JP JP3381168A patent/JPS4610430B1/ja active Pending
  • 1968-05-22 DE DE19681771427 patent/DE1771427A1/de active Pending
  • 1968-05-23 GB GB24590/68A patent/GB1212123A/en not_active Expired
  • 1968-05-25 ES ES354347A patent/ES354347A1/es not_active Expired
  • 1968-05-31 BE BE716046D patent/BE716046A/xx unknown
  • 1968-05-31 FR FR1570118D patent/FR1570118A/fr not_active Expired

Patent Citations (3)

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