Shaped glass article and method for producing such a shaped glass article

Abstract

A shaped glass article is provided that is ultrathin, has two surfaces and one or more edges joining the two surfaces, and a thickness between the two surfaces. The shaped ultrathin glass article has at least one curved area with a non-vanishing surface curvature with a minimal curvature radius R if no external forces are applied. A method for producing a shaped glass article is also provided that includes providing an ultrathin glass with two surfaces and one or more edges joining the two surfaces, having a thickness between the two surfaces and shaping the ultrathin glass to a shaped ultrathin glass article by forming at least one curved area having a non-vanishing surface curvature with a minimal curvature radius R if no external forces are applied to the shaped ultrathin glass article.

Claims

1. A shaped ultrathin glass article, comprising: two surfaces; one or more edges joining the two surfaces; a thickness between the two surfaces; and at least one curved area with a non-vanishing surface curvature with a minimal curvature radius (R) when no external forces are applied.

2. The shaped ultrathin glass article according to claim 1, wherein the at least one curved area has a curvature that is one-dimensional with a constant curvature in one surface direction.

3. The shaped ultrathin glass article according to claim 1, comprising an alkali containing glass composition.

4. The shaped ultrathin glass article according to claim 1, wherein the thickness is equal or less than 0.4 mm.

5. The shaped ultrathin glass article according to claim 1, wherein the at least one curved area extends over an entirety of the two surfaces.

6. The shaped ultrathin glass article according to claim 1, wherein the at least one curved area comprises a plurality of curved areas.

7. The shaped ultrathin glass article according to claim 1, wherein the minimal curvature radius (R) is equal or smaller than 5000 mm and equal or larger than 1 mm.

8. The shaped ultrathin glass article according to claim 1, wherein the at least one curved area is formed by a non-precision-forming process selected from the group consisting of thermal bending, thermal slumping, thermal molding, and unbalanced surface compressive stressing.

9. The shaped ultrathin glass article according to claim 1, wherein the at least one curved area is a result of unbalanced surface compressive stressing and comprises a surface compressive stress and/or a depth of layer in the at least one curved area on one of the two surfaces that is larger than the surface compressive stress and/or the depth of layer on the other of the two surfaces.

10. The shaped ultrathin glass article according to claim 9, comprising a surface compressive stress in the at least one curved area on one of the two surfaces that is larger than the surface compressive stress on the other of the two surfaces, wherein the larger surface compressive stress lies in a range from 10 MPa to 1200 MPa and the smaller surface compressive stress amounts to at most 90% of the larger surface compressive stress.

11. The shaped ultrathin glass article according to claim 9, wherein the unbalanced surface compressive stressing comprises unbalanced annealing of the two surfaces and/or unequal ion-exchanged surface layers of the two surfaces.

12. The shaped ultrathin glass article according to claim 9, where the unbalanced surface compressive stressing comprises an ion-exchanged surface layer having a depth of layer (DoL) in a range from 1 μm to 50 μm at the one of the two surfaces that has the larger surface compressive stress.

13. The shaped ultrathin glass article according to claim 1, wherein that the glass article is bendable out of shape to a target minimal curvature radius R′≠R in the curved area without breakage, resulting in a non-vanishing static surface tensile stress in the curved area so that a resulting static surface tensile stress is not greater than $1.15.Math.\mathrm{Min}\left({\stackrel{\_}{\sigma }}_a-{\Delta }_a.Math.0.4.Math.\left(1-\ln \left(\frac{A_{\mathrm{ref}}}{A_{\mathrm{App}}}.Math.\Phi \right)\right),{\stackrel{\_}{\sigma }}_e-{\Delta }_e.Math.0.4.Math.\left(1-\ln \left(\frac{L_{\mathrm{ref}}}{L_{\mathrm{App}}}.Math.\Phi \right)\right)\right),$ L.sub.ref is the stressed edge length and A.sub.ref is the stressed surface area of the side faces of ultra-thin glass samples, σ.sub.a being the median of the tensile stress of samples of the ultra-thin glass upon break of the samples, where the break occurs within a side face of the samples within the stressed surface area, σ.sub.e is the median of the tensile stress of samples of the ultra-thin glass upon break of the samples, where the break emanates from an edge of the samples within the stressed edge length, Δ.sub.e and Δ.sub.a being standard deviations of the tensile stress upon break of the samples at the edge or within a side face of the samples, respectively (i.e. the standard deviations of the median values σ.sub.e, σ.sub.a), A.sub.app being the surface area of one side face of the ultra-thin glass and L.sub.app being the cumulated edge length of the longitudinal edges of the ultra-thin glass, and Φ being a specified maximum rate of breakage within a time interval of at least half a year.

14. The shaped ultrathin glass article according to claim 13, where the maximum rate of breakage Φ is less than 0.1.

15. The shaped ultrathin glass article according to claim 13, further comprising a deviation of the target curvature radius R′ from the curvature radius R of up to 50%.

16. The shaped ultrathin glass article according to claim 13, further comprising a ratio of the resulting static surface tensile stress in the curved area and an average breakage stress of the ultrathin glass article that is equal or lower than 20%.

17. The shaped ultrathin glass article according to claim 13, wherein the resulting static surface tensile stress in the at least one curved area is less than 75 MPa.

18. The shaped ultrathin glass article according to claim 1, further comprising a target surface having a target curvature radius R′≠R in the at least one curved area, the target surface being laminated onto one of the two surfaces.

19. A method for producing a shaped ultrathin glass article, comprising: providing an ultrathin glass with two surfaces, one or more edges joining the two surfaces, and a thickness between the two surfaces; and shaping the ultrathin glass to a shaped ultrathin glass article by forming at least one curved area having a non-vanishing surface curvature with a minimal curvature radius (R) if no external forces are applied, wherein the at least one curved area has an one-dimensional curvature with an constant curvature in one surface direction.

20. The method according to claim 19, wherein the step of shaping comprises applying a non-precision-forming process selected from the group consisting of thermal bending, thermal slumping, thermal molding, and unbalanced surface stressing.

21. The method according to claim 19, where the step of shaping comprises unbalanced surface stressing comprising unbalanced annealing of the two surfaces and/or unequal ion-exchange on the two surfaces.

22. The method according to claim 21, wherein the unequal ion-exchange comprises applying alkaline metal salts to the ultrathin glass, the alkaline metal salts being selected from the group consisting of NaNO.sub.3, Na.sub.2CO.sub.3, NaOH, Na.sub.2SO.sub.4, NaF, Na.sub.3PO.sub.4, Na.sub.2SiO.sub.3, Na.sub.2Cr.sub.2O.sub.7, NaCl, NaBF.sub.4, Na.sub.2HPO.sub.4, K.sub.2CO.sub.3, KOH, KNO.sub.3, K.sub.2SO.sub.4, KF, K.sub.3PO.sub.4, K.sub.2SiO.sub.3, K.sub.2Cr.sub.2O.sub.7, KCl, KBF.sub.4, K.sub.2HPO.sub.4, CsNO.sub.3, CsSO.sub.4, and CsCl.

23. The method according to claim 21, wherein the unbalanced surface stressing comprises masking at least one of the two surfaces in the at least one area to be curved, the masking fully or partially preventing surface stressing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] The exemplary figures used for illustration of the invention show schematically:

[0090] FIG. 1 is a Weibull diagram for a soda-lime glass as well as glasses A and C according to Table 2 at different thicknesses and different stressing status;

[0091] FIG. 2 is a log-linear plot of fatigue lifetime vs. ratio of static stress and breakage stress for glass samples of glass A with a thickness of 0.07 mm;

[0092] FIG. 3 is a shaped ultrathin glass sheet according to the invention;

[0093] FIG. 4a-4d are several different embodiments of an ultrathin glass article with unbalanced ion-exchanged surface layers in a flat configuration;

[0094] FIG. 5a is an ultrathin glass article with stripe patterned ion-exchange surface layers on both surfaces;

[0095] FIG. 5b is the shaped ultrathin glass article according to the invention resulting from the ion-exchanged surface layers according to FIG. 5a;

[0096] FIG. 6 is the lamination of the shaped ultrathin glass article with constant curvature radius to a target surface of a device with different curvature radius;

[0097] FIG. 7 is a thermal slumping and thermal bending setup for shaping the shaped ultrathin glass article.

[0098] The dimensions and aspect ratios in the figures are not to scale and have been oversized in places for better visualization. Corresponding elements in the figures are generally referred to by the same reference numerals.

DETAILED DESCRIPTION

[0099] FIG. 1 shows a double logarithmic plot for the cumulative probability in % of the breakage strengths of different glass types at different thicknesses and stressing status (Weibull diagram). The breakage strength of 0.5 mm soda-lime glass indicates the breakage strength of a comparison sample representing a typical inorganic glass of similar thickness. The plot for 0.05 mm glass A (see Table 2 below) without ion-exchange represents the typical strength of inorganic glass thinner than 0.1 mm. The plot for 0.05 mm toughened glass A (Table 2) with ion-exchange shows the strength that can be achieved by unbalanced ion-exchanging glass A in a K-ion salt bath. The plot for 0.07 mm glass C (Table 2) shows the achievable strength after ion-exchange of an aluminosilicate glass in a K-ion salt bath.

[0100] FIG. 2 shows a log-linear plot of the fatigue lifetime of ultrathin glass samples made from glass A (Table 2) at a thickness of 0.07 mm. On the one hand, the plot shows that the logarithm of the fatigue lifetime (in seconds) is linear to the ratio between static stress and breakage stress of the glass. The plot can be used for extrapolation of the fatigue lifetime. A reduction of the static stress to 1/10 of the original stress can increase the fatigue lifetime of the glass sheet about 1,000 times. On the other hand, the force needed to deform the glass sample to a certain deflection is proportional to the cube of the glass thickness. Hence, ultrathin glasses can be bent at significantly lower forces as compared to thicker glasses, i.e. at lower static stresses if e.g. laminated to a target surface, with fatigue lifetimes that still exceed the usual lifetime of the corresponding application.

[0101] FIG. 3 shows a shaped ultrathin glass article 1 (hereinafter referred to “glass article 1”) according to the invention as it can be obtained from a rectangular flat glass sheet with a length L, a width W and a thickness t. The glass article 1 can also have other shapes as e.g. a circular shape or any other shape as required by the desired application.

[0102] The glass article 1 has a first surface 2 and an opposing second surface 3 which are joined by four edges 4. The glass article 1 has a one-dimensional curvature with an essentially constant radius R in a direction perpendicular to an axis A and no curvature along A. The shape of the glass article 1 therefore corresponds to a section of the mantle surface of a virtual regular circular cylinder with cylinder axis A and thus, in the terminology used herein, has one curved area extending over the whole glass article 1. According to the invention, the curvature of the glass article 1 can be achieved by a non-precision-forming process such as thermal bending, thermal slumping, thermal molding and/or unbalanced surface compressive stressing, the latter including unbalanced annealing and/or unbalanced ion-exchanging on the two surfaces.

[0103] FIGS. 4a to 4d show partial sectional views of exemplary embodiments of the glass article 1 in a flattened state with ion-exchanged surface layers. The figures exemplarily depict the depth of layer DoL for the different embodiments. The diagrams above and/or below the surfaces 2 and 3 show the qualitative profile of associated surface compressive stresses CS along a surface coordinate x. It is to be understood that the corresponding profiles of CS and DoL can also derive from an annealing treatment and the basic principle is not limited to ion-exchanged surface layers. Furthermore, the present surface layers are to be understood to vary only in the surface direction x whereas they are constant in a perpendicular surface direction (one-dimensional variation).

[0104] FIG. 4a shows an embodiment of the glass article 1 having an ion-exchanged surface layer 5 with a constant DoL on the surface 2 whereas the surface 3 has no ion-exchanged layer. This configuration results in a constant surface compressive stress CS.sub.2 on surface 2 and a surface compressive stress CS.sub.3=0 on surface 3. The glass article 1 results in a shaped glass article 1 as shown in FIG. 3 due to the unbalanced surface compressive stresses CS.sub.2 and CS.sub.3, where the surface 2 is the convex surface.

[0105] FIG. 4b shows an embodiment of the glass article 1 having a surface layer 5 with a constant DoL.sub.2 on surface 2 and a surface layer 6 with a constant DoL.sub.3 on surface 3, where DoL.sub.2 >DoL.sub.3≠0. As a result, the surface compressive stresses CS.sub.2 and CS.sub.3 are also different and non-vanishing. Dependent on the difference ΔCS=|CS.sub.2-CS.sub.3|, the glass article 1 receives a curved shape, where in this case, the surface 3 becomes the convex surface.

[0106] FIG. 4c shows an embodiment of the glass article 1 with a surface layer 5 on surface 2 with a variable depth, i.e. DoL=DoL(x), whereas the surface 3 has no ion-exchanged surface layer. DoL(x) has a maximum DoL.sub.max at a location x.sub.max in the center of the glass article 1. As a result, the surface compressive stress CS.sub.2 is also a function of coordinate x with a maximum at x.sub.max. The glass article 1 in this case receives a variable curvature R=R(x) with a minimum curvature radius R.sub.min at x.sub.max, where the curvature radius R(x) increases with increasing distance from x.sub.max. Dependent on the function of DoL(x) or CS(x), the resulting shape of the glass article can be e.g. cylindrical parabolic or hyperbolic.

[0107] FIG. 4d shows another embodiment of the glass article 1 with a surface layer 5 on surface 2 with a variable depth, i.e. DoL.sub.2=DoL.sub.2(x), whereas the surface 3 has a surface layer 6 with a corresponding but in x-direction inverted profile for DoL.sub.3(x). DoL.sub.2(x) has a minimum at the smallest x and a maximum at the largest x. In this case, the curvature radius R=R(x) is largest at the edges of the glass article 1 whereas there is no curvature at x.sub.eq where DoL.sub.2(x.sub.eq)=DoL.sub.3(x.sub.eq). The glass article 1 according to FIG. 4d therefore has two curved regions with alternating curvature, separated at x.sub.eq.

[0108] FIG. 5a shows another embodiment of the glass article 1 with a stripe patterned surface layer 5 on surface 2 and a stripe patterned surface layer 6 on surface 3. DoL.sub.2(x) and DoL.sub.3(x) are described by a periodic rectangle function. DoL.sub.2 is shifted in x-direction by half a period length with respect to DoL.sub.3 such that each area with a DoL.sub.max is opposed on the other surface by DoL=0. The alternating surface stresses CS.sub.2 and CS.sub.3 are therefore unbalanced with respect to a surface parallel center-plane of the glass article 1. Such a configuration results in a corrugated or wave shaped glass article 1 as shown in FIG. 5b with 4 curved regions with alternating curvatures.

[0109] FIG. 6 shows the lamination of the glass article 1 with a constant curvature radius R to a target lamination surface 7 of a device 8 with a constant target curvature radius R′. The target curvature radius R′ in this embodiment is larger than the curvature radius R of the glass article 1. During the lamination, the glass article 1 is forced out of its shape in order to fit the larger target radius R′ by lamination forces F. The forces F induce a surface tensile stress TS on the surface of the glass article 1. Further tensile stresses are also induced at the edges of the glass article 1. These static stresses need to be maintained in a laminated configuration 1′ in order to keep the glass article 1 out-of-shape. The lamination force can be provided e.g. by an Optical Clear Adhesive (OCA) between the target surface 7 and the corresponding surface of the glass article 1.

[0110] According to the invention, the shaped ultrathin glass article 1 can be laminated out-of-shape with very limited lamination forces resulting in very limited static stresses. As such, the shaped ultrathin glass article 1 can be produced at comparatively low precision since it can be adapted to its final high-precision shape in the laminated configuration 1′ without running the risk of breakage during lamination or due to static fatigue.

[0111] FIG. 7 shows a configuration/method for shaping the ultrathin glass article 1 e.g. thermal slumping. A graphite mold 10 which has a molding surface 11 is used for shaping the glass article 1. The mold 10 can be produced at low precision and as such at low cost. The mold 10 is located underneath an ultrathin glass sheet 12 which is to be formed into the shaped glass article 1. The mold 10 and the ultrathin glass sheet 12 are heated by e.g. infrared radiation IR to temperatures of about 20-30° C. above the glass transition temperature T.sub.g. The shaping time needed is approx. 2-5 min. for ultrathin glasses. Under the influence of gravity G, with assistance of a vacuum V applied at the mold surface 11 if necessary, the glass sheet 12 deforms and adapts to the molding surface 11. This process can be performed at low precision such that the curvature R of the finished shaped glass article 1 can deviate from the curvature of the molding surface 12.

[0112] In an alternative method for shaping the ultrathin glass article 1 (indicated in FIG. 7 by dashed lines), i.e. thermal bending, the ultrathin glass sheet 12 is sandwiched between the molding surfaces 11 and 13 of the mold 10 and an inversely shaped second mold part 14 which are forced against each other by a force S in order to bend the ultrathin glass sheet 12 into the desired shape. The parts 10 and 14 of the mold are heated similar to the above described thermal slumping.

Exemplary Embodiments

[0113] The glass compositions A, B and C as listed in the below Table 2 are used for the exemplary embodiments 1-6 as described below:

TABLE-US-00017 TABLE 2 Several exemplary glass compositions Composition weight-% Glass A SiO.sub.2 64.0 B.sub.2O.sub.3 8.3 Al.sub.2O.sub.3 4.0 Na.sub.2O 6.5 K.sub.2O 7.0 ZnO 5.5 TiO.sub.2 4.0 Sb.sub.2O.sub.3 0.6 Cl.sup.− 0.1 Glass B SiO.sub.2 61 B.sub.2O.sub.3 10 Al.sub.2O.sub.3 18 MgO 2.8 CaO 4.8 BaO 3.3 Glass C SiO.sub.2 62 Al.sub.2O.sub.3 17 Na.sub.2O 13 K.sub.2O 3.5 MgO 3.5 CaO 0.3 SnO.sub.2 0.1 TiO.sub.2 0.6
Glasses A to C have the following selected properties:

TABLE-US-00018 TABLE 3 Parameters of glasses A to C according to Table 2 Parameter Glass A Glass B Glass C CTE (20-300° C.) [10.sup.−6/K] 7.2 3.2 8.3 T.sub.g [° C.] 557 717 623 Density [g/cm.sup.3] 2.5 2.43 2.4
CTE in Table 3 refers to the coefficient of thermal expansion and T.sub.g refers to the glass transition temperature.

EXAMPLE 1

[0114] A sheet of 100 mm×60 mm was cut from glass A (see Table 2) with a thickness of 0.05 mm. The glass sheet was pasted with an ink mixed with KNO.sub.3 powder by a screen printing method fully covering one of its surfaces. Subsequently, the sheet was dried at 180° C. during 1 hour to remove the ink. After drying, the sheet was annealed at 330° C. for 2 hours to drive an ion-exchange process. As a result, the ultrathin glass sheet experienced a bending into a widely cylindrical curved shape with a curvature radius of 52 mm.

EXAMPLE 2

[0115] A sheet of 100 mm×60 mm was cut from glass A (Table 2) with a thickness of 0.05 mm. The sheet was coated with an Indium Tin Oxide (ITO) film on one of its surfaces in order to prevent ion-exchange and was subsequently submersed into a KNO.sub.3 salt bath. The ultrathin glass sheet was toughened at a temperature of 400° C. for 1 hour. The CS is approximately 270 MPa and the DoL is approximately 7 μm. As a result, the ultrathin glass sheet experienced a bending into a widely cylindrical curved shape with a curvature radius of 48 mm.

EXAMPLE 3

[0116] A sheet of 100 mm×60 mm was cut from glass A (Table 2) with a thickness of 0.1 mm. The sheet was masked according to a regular stripe pattern. The sheet was then coated with an ITO-film, resulting in coated areas in order to prevent ion-exchange in the coated areas. After removing the masking, the ultrathin glass sheet was submersed into a KNO.sub.3 salt bath and toughened at a temperature of 400° C. for 1 hour. This resulted in an ion-exchange in the uncoated areas and in no ion-exchange in the ITO-coated areas. The CS is approximately 270 MPa and the DoL is approximately 7 μm. As a result, the ultrathin glass sheet experienced an alternating bending with several curved areas into a wave shape.

EXAMPLE 4

[0117] Samples of glass A were drawn directly from the glass melting furnace with unbalanced cooling to cause unbalanced annealing of the glass surfaces. Due to the resulting unbalanced surface compressive stresses, the ultrathin glass bends into a curved shape. The surface with the larger compressive stress thereby forms the convex side whereas the surface with the smaller surface compressive stress forms the concave side. An excess surface compressive stress of 60 MPa and a depth of annealed layer of 8 μm on a 50 μm thick ultrathin glass lead to a curvature radius of 190 mm. It is to be noted that the glass shaped by unbalanced annealing has different CS and depth of layer profiles as ion-exchanged glasses. The depth of the annealed layer is generally about ⅙ of the thickness of the glass. When fitting the bent glass to a curved surface with 150 mm curvature radius, the static stress is calculated to be 2.6 MPa. Over 99% of the shaped ultrathin glass articles produced by this process were laminated without breakage.

[0118] In comparison, the same unbalanced annealing causes an excess surface compressive stress of 90 MPa and a depth of layer of 83 μm for 0.5 mm thick glass, resulting in a bending radius of 1220 mm. Lamination attempts to a target curvature radius of 150 mm resulted in breakage of all samples.

EXAMPLE 5

[0119] Samples of glass B (Table 2) with a thickness of 0.1 mm and 100 mm×100 mm surface area were thermally bent in an IR furnace at 780° C. for 15 min on a graphite mold with a 52 mm curvature. After bending, the curvatures of the samples were 52 mm. These shaped ultrathin glasses could almost all (99%) successfully be laminated to a curved target surface with a curvature radius of 50 mm without breakage. The static stress is calculated as 2.9 MPa, resulting in an extrapolated fatigue lifetime of more than 1 year. In a further example, a low precision tungsten carbide mold is used at the same thermal treatment condition to get a radius curvature of 50.5 mm, which is still 1% deviation from the target radius. When laminating to the 50 mm target, the resultant static stress is 0.7 MPa and the expected life time is over 5 years. The yield during laminating was 100% i.e. all samples could successfully be laminated to the target surface.

[0120] In comparison, samples of 0.7 mm thick glass B (Table 2) resulted in a curvature radius of 58 mm under the same forming conditions. The lamination to 50 mm radius curvature surface leads to a static force of 70 MPa. Lamination could not be achieved due to the high de-lamination force.

[0121] In a further comparison, laminating of flat, i.e. having vanishing curvature radius, ultrathin glass samples with a thickness of 0.1 mm to a 75 mm target curvature radius leads to a yield of about 90% due to breakage of some samples. The static stress is calculated to about 50 MPa. Furthermore, over 80% of these samples break after 1 day which is presumably due to the high static stress that amounts to approx. 30.2% of the average breakage strength.

EXAMPLE 6

[0122] Samples of glass C (Table 2) with a thickness of 0.1 mm and 100 mm×30 mm surface area were coated with a Si02 coating on one surface and then submersed in a molten KNO.sub.3 salt bath at 390° C. for 30 min. The resulting glass article receives a curvature radius of 36 mm due to unbalanced ion-exchange since the SiO.sub.2 coating slows down the ion-exchange rate. The uncoated surface becomes the convex side and has a surface compressive stress (CS) of 730 MPa and depth of ion-exchanged layer (DoL) of 11 μm, whereas the coated surface has a lower CS of 500 MPa and DoL of 6 μm. This shaped ultrathin glass article has a high bending strength due to the ion-exchanged surface layers. When laminating to a curved display with a target curvature radius of 35 mm, the yield is 100%. The calculated fatigue lifetime is higher than 10 years due to a low static stress of 3.6 MPa and extremely high breakage strength of 1131 MPa, as shown in FIG. 1. The static stress is only 0.3% of the breakage strength.

[0123] Table 4 below summarizes several parameters of the embodiments according to Examples 4 to 6, and three comparative examples:

TABLE-US-00019 TABLE 4 Summary of parameters Static Curvature Target Average stress Static Surface radius radius Breakage Young's if stress Glass Thickness area R R′ stress modulus laminated Lamination vs. average Fatigue type (mm) (mm × mm) (mm) (mm) (MPa) (MPa) yield breakage lifetime Comparative Soda-lime 0.5 100 × 100 flat 100 124 73 182.5 0 >100%   0 samples Glass B 0.05 100 × 100 flat 50 173 74.8 37.4 9% 21.6%  <10 days.sup.  Glass B 0.1 100 × 100 flat 75 165 74.8 49.9 90% 30.2%  <1 day  Example 4 Glass A 0.05 100 × 100 190 150 170 72.9 2.6 99% 1.5% >1 year Example 5 Glass B 0.1 100 × 100 52 50 165 74.8 2.9 99% 1.7% >1 year Glass B 0.1 100 × 100 50.5 50 165 74.8 0.7 100% 0.4% .sup. >5 years Example 6 Glass C 0.1 100 × 30  36 35 1131 74 3.6 100% 0.3% >10 years.sup.  indicates data missing or illegible when filed