Obviousness

Obviousness Analysis of US Patent 8642491 Under 35 U.S.C. § 103

This analysis identifies combinations of prior art references, as disclosed within US Patent 8642491, that would render the independent claims 1 and 13 obvious to a person having ordinary skill in the art (PHOSITA). The primary motivation for combining these references stems from well-known industry challenges and scientific principles detailed within the patent itself.

Independent Claims Overview

• Claim 1 (Glass Composition): Describes an alkali-free boroalumino silicate glass with specific oxide ranges (SiO2: 64.0-71.0; Al2O3: 9.0-12.0; B2O3: 7.0-12.0; MgO: 1.0-3.0; CaO: 6.0-11.5; SrO: 0-2.0; BaO: 0-0.1). Critical features include a Σ[RO]/[Al2O3] ratio ≥ 1.00 (where Σ[RO] is the sum of MgO, CaO, SrO, and BaO), low concentrations of As2O3 (≤ 0.05 mol %) and Sb2O3 (≤ 0.05 mol %), at least 0.01 mol % SnO2, and a liquidus viscosity ≥ 100,000 poise for downdraw processing.
• Claim 13 (Glass Sheet): Describes an alkali-free glass sheet produced by a downdraw process, comprising a glass with SiO2, Al2O3, B2O3, MgO, and CaO. Similar to Claim 1, it requires a Σ[RO]/[Al2O3] ratio ≥ 1.00 and MgO content ≥ 1.0 mol %. It specifies even lower As2O3 (≤ 0.005 mol %) and Sb2O3 (≤ 0.005 mol %), with at least 0.01 mol % SnO2, and a liquidus viscosity ≥ 100,000 poise.

Motivation for a Person Having Ordinary Skill in the Art (PHOSITA)

The patent explicitly outlines several driving forces in the art at the time of the invention:

1. Environmental and Health Concerns: There was a "continuing effort in the art to produce glasses with lower arsenic levels and, preferably, glasses which are substantially arsenic free." Similar concerns existed for antimony, which also presents "environmental and health issues." This provides a clear and strong motivation to eliminate or substantially reduce arsenic and antimony as fining agents.
2. Known Alternative Fining Agent: Tin oxide (SnO2) was already a "ubiquitous material which has no known hazardous properties." Furthermore, SnO2 "has been a component of AMLCD glasses through the use of tin oxide electrodes in the Joule melting of the batch materials for such glasses (e.g., the melting of Corning Incorporated Code 7059, 1737, and EAGLE 2000 glasses)." This indicates that a PHOSITA would readily consider SnO2 as a replacement fining agent, despite its acknowledged lower effectiveness compared to arsenic or antimony.
3. Need for High-Performance AMLCD Glass: The background details the stringent requirements for AMLCD glass substrates, including tight dimensional control, suitability for downdraw processes (requiring high liquidus viscosities, preferably >100,000 poises), desirable coefficient of thermal expansion (CTE) (28-34 × 10−7 /° C.), high strain point (> 650° C.), chemical durability, and low density (≤ 2.45 grams/cm3). These are all known objectives for a PHOSITA.
4. Achieving Low Gaseous Inclusions: The patent stresses the need for extremely low gaseous inclusion levels (≤ 0.10 inclusions/cm3, preferably ≤ 0.05 inclusions/cm3 for sheets ≥ 500 cm3 volume) consistently over sequential sheets.

Obviousness Argument: Combination of Prior Art References

A PHOSITA, aiming to develop an improved alkali-free boroalumino silicate glass for AMLCDs, particularly one that uses environmentally friendly fining agents while maintaining properties suitable for downdraw processing, would have been motivated to combine the following teachings:

1. Primary Reference: A Known Alkali-Free Boroalumino Silicate Glass Composition for AMLCDs.
A PHOSITA would start with a base glass composition for AMLCDs known in the art (e.g., Corning's own commercial glasses like Code 7059, 1737, or EAGLE 2000), which would provide a general framework for the required components (SiO2, Al2O3, B2O3, alkaline earth oxides) and target properties (e.g., high strain point from Dumbaugh et al. or Chacon et al., and liquidus viscosity for downdraw processes from Dockerty patents). These existing glasses were also known to contain SnO2 as a component.

2. Secondary Reference: Arsenic/Antimony-Free Fining Processes (Dorfeld et al., Bange et al., and general knowledge of SnO2).
Motivated by the well-established desire to eliminate toxic fining agents, as taught by Dorfeld et al., Bange et al. (for arsenic-free processes) and the general industry trend, the PHOSITA would deliberately remove As2O3 and Sb2O3 from the batch. Given that SnO2 was already a known component in some AMLCD glasses and had no known hazardous properties, the PHOSITA would naturally elect to use SnO2 as the primary fining agent.

3. Secondary Reference: Principles of Glass Chemistry and Phase Diagrams (Taylor et al., McMillan et al., Navrotsky et al., Geisinger et al., and Levin et al.).
To compensate for SnO2's acknowledged lower effectiveness as a fining agent, the PHOSITA would seek to optimize the glass composition to improve fining efficiency or reduce initial gas solubility. The patent itself extensively details the principles of glass chemistry related to gas solubility: "The solubility of gases is comparatively high in aluminum-rich glasses and falls steeply as the Σ[RO]/[Al2O3] ratio increases beyond 1.00." This explicit teaching would motivate a PHOSITA to adjust the glass composition to ensure the Σ[RO]/[Al2O3] ratio is greater than or equal to 1.00, thereby predictably reducing initial gas solubility and making the fining process "easier to perform and more effective" with SnO2.

Furthermore, regarding MgO, while the patent notes a "prior belief" that MgO concentrations ≥ 1.0 mole percent "raised liquidus temperatures (lowered liquidus viscosities), thereby compromising high viscosity forming processes, such as, downdraw processes", the patent also provides counter-instruction rooted in fundamental glass chemistry. It references the MgO—Al2O3—SiO2 phase diagram from Levin et al., highlighting its uniqueness in having an Σ[RO]/[Al2O3] ratio slightly greater than 1 at a eutectic composition. More importantly, the patent explains that "the addition of MgO not only forces all initial melting to the RO-rich side of the phase diagram, but also produces local minima in liquidus temperatures below those that might be expected from either the MgO— or CaO-based systems alone." This is attributed to "freezing point depression," where "a suitably small amount of any oxide will initially reduce liquidus temperatures."

This detailed explanation, drawing on established phase diagrams and glass chemistry principles, would provide a PHOSITA with a clear motivation to explore MgO concentrations within the 1.0-3.0 mole percent range. Despite the general "prior belief," the scientific understanding presented in the patent (and implicitly available in the cited phase diagrams) would teach that, under careful control of the overall composition (including the SiO2 concentration and Σ[RO]/[Al2O3] ratio), MgO could reduce liquidus temperatures (and thus increase liquidus viscosity) while also contributing desirable properties like lower density and CTE, and higher strain point and modulus.

Conclusion of Obviousness:

Given the strong motivation to use environmentally friendly fining agents and achieve optimal properties for AMLCDs via downdraw processes, a PHOSITA would have been motivated to:

1. Eliminate arsenic and antimony as fining agents.
2. Utilize SnO2 as the primary fining agent, a known component in commercial AMLCD glasses.
3. Adjust the base glass composition to achieve a Σ[RO]/[Al2O3] ratio ≥ 1.00 to predictably improve fining by reducing gas solubility, a known effect in glass chemistry.
4. Incorporate MgO within the disclosed range (1.0-3.0 mol %) to achieve desired physical properties. Despite a general "prior belief" about MgO's negative impact on liquidus viscosity, the fundamental principles of glass chemistry, particularly concerning phase equilibria (Levin et al.) and freezing point depression as detailed within the patent itself, would have provided a PHOSITA with a reasonable expectation that optimizing MgO within a carefully controlled overall composition could indeed lead to the desired high liquidus viscosity necessary for downdraw processes.

The specific compositional ranges for other oxides (SiO2, Al2O3, B2O3, CaO, SrO, BaO) are well-known parameters to adjust for achieving the full suite of desired AMLCD glass properties (CTE, density, strain point) within boroalumino silicate systems. The combination of these steps, guided by the explicit motivations and scientific principles discussed within the patent itself, would have rendered the claimed alkali-free glass compositions and methods of producing glass sheets obvious to a PHOSITA.