Obviousness

US Patent 8640498, titled "Fining of boroalumino silicate glasses," concerns alkali-free glass compositions and methods for producing glass sheets for flat panel display devices, particularly active matrix liquid crystal displays (AMLCDs). The invention focuses on achieving effective fining (removal of gaseous inclusions) without using substantial amounts of arsenic or antimony, by carefully controlling the glass composition and employing tin oxide (SnO2) as a fining agent.

The key features of independent Claim 1 are:

• An alkali-free glass comprising SiO2, Al2O3, B2O3, MgO, and CaO (and optionally SrO, BaO).
• A method involving selecting, melting, and fining batch materials, followed by producing glass sheets via a downdraw process.
• Specific compositional characteristics:
  • Σ[RO]/[Al2O3] ratio greater than or equal to 1.0, where Σ[RO] is the sum of mole percents of MgO, CaO, SrO, and BaO.
  • MgO content greater than or equal to 1.0 mole percent.
• Specific fining agent characteristics:
  • At most 0.005 mole percent As2O3.
  • At most 0.005 mole percent Sb2O3.
  • SnO2 is used in fining, with an SnO2 content of at least 0.01 mole percent.
• A resulting glass with a liquidus viscosity greater than or equal to 100,000 poise.

Obviousness Analysis under 35 U.S.C. § 103

A person having ordinary skill in the art (POSA) in glass manufacturing for flat panel displays, at the time of the invention (priority date June 28, 2005), would have been motivated to combine several pieces of existing knowledge to arrive at the claimed invention.

Combination of Prior Art References and Motivation:

1. Baseline Glass and Forming Process:

   • U.S. Pat. Nos. 5,374,595 (Dumbaugh et al.) and 6,319,867 (Chacon et al.): These patents disclose alkali-free boroalumino silicate glasses exhibiting desirable properties like high strain points (e.g., in excess of 650° C.), which were known to be suitable for active plates based on thin film transistors (TFTs) in display applications. These references would provide a POSA with suitable base glass compositions.
   • U.S. Pat. Nos. 3,338,696 (Dockerty) and 3,682,609 (Dockerty): These patents describe the downdraw sheet drawing processes, particularly the fusion process, as a highly desirable method for producing glass sheets for substrates without requiring costly post-forming finishing operations. A POSA would understand that the fusion process imposes severe restrictions on glass properties, notably requiring relatively high liquidus viscosities, preferably greater than 100,000 poises.

   Motivation: A POSA would be motivated to combine the advantageous alkali-free boroalumino silicate glass compositions (Dumbaugh/Chacon) with the high-quality downdraw (fusion) manufacturing process (Dockerty) to produce substrates for AMLCDs. This combination establishes the need for a glass composition compatible with the downdraw process, specifically requiring a liquidus viscosity greater than or equal to 100,000 poise, and comprising the basic elements SiO2, Al2O3, B2O3, MgO, and CaO.

2. Elimination of Hazardous Fining Agents:

   • U.S. Pat. Nos. 5,785,726 (Dorfeld et al.), 6,128,924 (Bange et al.), 5,824,127 (Bange et al.), and co-pending patent application Ser. No. 11/116,669: These references disclose the ongoing efforts in the art to produce "substantially arsenic free" glasses due to environmental and health issues associated with arsenic. The patent itself also notes similar environmental and health issues with antimony, and that antimony is a "less effective" fining agent compared to arsenic.

   Motivation: Driven by these well-known environmental and health concerns, a POSA would be strongly motivated to develop glass compositions and manufacturing methods that eliminate or substantially reduce arsenic (to at most 0.005 mole percent As2O3) and antimony (to at most 0.005 mole percent Sb2O3) as fining agents.

3. Use of Tin Oxide as an Alternative Fining Agent:

   • Corning Incorporated Code 7059, 1737, and EAGLE 2000 glasses (and general knowledge in the art): The patent explicitly states that SnO2 was "a component of AMLCD glasses through the use of tin oxide electrodes in the Joule melting of the batch materials for such glasses." It also highlights that SnO2 "is a ubiquitous material which has no known hazardous properties." However, it acknowledges that "tin fining (i.e., SnO2 fining) is less effective" than As2O3 and Sb2O3 fining.

   Motivation: Given the strong motivation to eliminate arsenic and antimony (as discussed above), a POSA would naturally consider SnO2 as a primary alternative fining agent due to its non-hazardous nature and existing presence in AMLCD glass manufacturing. Despite its perceived lower effectiveness, the lack of toxicity would compel a POSA to explore its use, aiming for a concentration of at least 0.01 mole percent SnO2.

4. Optimizing Glass Composition for Enhanced Fining with SnO2:

   • Scientific Literature and General Knowledge of Glass Chemistry (e.g., M Taylor, G E Brown, P. McMillan, B. Piriou, A Navrotsky, K L Geisinger, G V Gibbs, and E M Levin, C R Robbins, H F McMurdie referenced in the patent): The patent itself explains that "the solubility of gases is comparatively high in aluminum-rich glasses and falls steeply as the Σ[RO]/[Al2O3] ratio increases beyond 1.00." It also notes that "In RO-rich systems, silica dissolution occurs at a comparatively low temperature, thus further inhibiting initial gas solubility." This fundamental understanding of glass chemistry would be known to a POSA.

   Motivation: Knowing that SnO2 is a "less effective" fining agent, a POSA would be motivated to optimize the base glass composition to improve its inherent fining capabilities, i.e., to reduce gas solubility. Drawing upon the known principles of glass chemistry, the POSA would understand that increasing the Σ[RO]/[Al2O3] ratio to be greater than or equal to 1.0 would lead to a steep fall in gas solubility, thereby making the fining process "easier to perform and more effective" even with less potent fining agents like SnO2.

   Motivation for MgO content: The patent states that MgO is "particularly important with regard to melting and fining" and provides benefits such as "lower density and CTE, and a higher chemical durability, strain point, and modulus." These are all desirable properties for AMLCD substrates. While the patent mentions a prior belief that MgO concentrations ≥ 1.0 mole percent "raised liquidus temperatures (lowered liquidus viscosities), thereby compromising high viscosity forming processes", a POSA would still be motivated to investigate MgO given its broad benefits. The patent's finding that "higher levels of MgO can be used, provided that simultaneously, the Σ[RO]/[Al2O3] ratio and SiO2 concentration are carefully controlled" suggests that overcoming the liquidus viscosity challenge at these MgO levels was achievable through routine optimization and careful control, which is within the skill set of a POSA, especially when seeking synergistic effects with the desired Σ[RO]/[Al2O3] ratio for improved fining.

Conclusion:
A POSA, motivated by the need for environmentally friendly fining in fusion-formable alkali-free boroalumino silicate glasses for AMLCDs, would combine:

1. Known alkali-free boroalumino silicate glass compositions suitable for AMLCDs (Dumbaugh/Chacon) and the downdraw process requiring high liquidus viscosity (Dockerty).
2. The clear motivation to eliminate arsenic and antimony fining agents (Dorfeld/Bange et al.).
3. The known non-hazardous nature and incidental presence of SnO2 in AMLCD glasses, leading to its selection as an alternative fining agent (Corning Code glasses).
4. The fundamental principles of glass chemistry regarding the relationship between Σ[RO]/[Al2O3] ratio and gas solubility (scientific literature/general knowledge), which would motivate increasing this ratio to ≥ 1.0 to enhance fining effectiveness.
5. The known benefits of MgO for various glass properties, leading to its inclusion at ≥ 1.0 mole percent, with the understanding that compatibility with downdraw processes would require careful compositional control and optimization.

This combination of known elements, driven by clear motivations to address known problems (hazardous fining, need for effective alternative fining, maintaining fusion compatibility), would render the method of Claim 1 obvious to a POSA. The resulting glass sheets would meet all the recited limitations, including being alkali-free, produced by a downdraw process, having the specified compositional ranges (SiO2, Al2O3, B2O3, MgO ≥ 1.0 mol%, CaO, Σ[RO]/[Al2O3] ≥ 1.0), containing at most 0.005 mol% As2O3 and Sb2O3, utilizing SnO2 (≥ 0.01 mol%) for fining, and possessing a liquidus viscosity ≥ 100,000 poise.