The chemical properties of glass determine its resistance to attack by water, moisture, acids and alkalis. Different optical glass manufacturers use different approaches. Glass containing larger amounts of substance such as silicon dioxide (SiO2), aluminum oxide (Al2O3), titanium oxide (TiO2) or oxides of the rare earths, are more resistant to be leached by aqueous and acidic solutions. They also are usually more resistant to local corrosion. If the glass contains large quantities of more readily soluble substances such as alkalis, then reactions of varying degree can be expected depending on the operating conditions. These reactions are sufficient for layer formation or removal of the glass surface. SCHOTT uses the following five test methods to assess the chemical behavior of polished glass surfaces: Climatic resistance, Stain resistance, Acid resistance, Alkali resistance and Phosphate resistance.
Climatic resistance describes the behavior of optical glasses at high relative humidity and high temperatures. In sensitive glasses a cloudy film can appear that generally cannot be wiped off.
An accelerated procedure is used to test the climatic resistance of the glasses, in which polished, uncoated glass plates are subjected to a water vapor saturated atmosphere, the temperature of which is alternated between 40°C and 50°C on an hourly basis. Since the temperature increase in the glass plates follows that of the atmosphere, water condenses on the glasses during the warming phase. In the cooling phase the temperature of the atmosphere initially falls faster than that of the glass plates causing a drying of the glass surface.
After an exposure time of 30 hours the glasses are removed from the climatic chamber. The difference in haze before and after testing is used as a measure of the resulting surface change. The measurements are conducted using a hazemeter. The classifications are done based on haze difference ΔH after a 30-hour test period.
The glasses in class CR 1 display no visible attack after being subjected to 30 hours of climatic change. In normal humidity conditions during the fabrication and storing of optical glasses in class CR 1, no surface attack should be expected. On the other hand, the fabrication and storing of optical glasses in class CR 4 should be done with caution because they are sensitive to climatic influences.
The test procedure for stain resistance gives information on possible changes in the glass surface (stain formation) under the influence of lightly acidic water (for example perspiration, acidic condensates) without vaporization.
The stain resistance class is determined according to the following procedure: The plane polished glass sample to be tested is pressed onto a test cuvette, which has a spherical depression of max. 0.25 mm depth containing a few drops of a test solution.
Test solution I: Sodium Acetate Buffer pH = 4.6
Test solution II: Sodium Acetate Buffer pH = 5.6
Interference color stains develop as a result of decomposition of the surface of the glass by the test solution. The measure for classifying the glasses is the time that elapses before the first brown-blue stain occurs at a temperature of 25°C. This change in color indicates a chemical change in the previously defined surface layer of 0.1 µm thickness insofar as the glass can form layers at all.
Stain resistance class FR 0 contains all glasses that exhibit virtually no interference colors, even after 100 hours of exposure to test solution I. Glasses in classification FR 5 must be handled with particular care during processing.
Acid resistance classifies the behavior of optical glasses that come in contact with large quantities of acidic solutions (from a practical standpoint for example, perspiration, laminating substances, carbonated water, etc.).
The time t required to dissolve a layer with a thickness of 0.1 µm serves as a measure of acid resistance. Two aggressive solutions are used in determining acid resistance. A strong acid (nitric acid, c = 0.5 mol/l, pH 0.3) at 25°C is used for the more resistant glass types. For glasses with less acid resistance, a weakly acidic solution with a pH value of 4.6 (sodium acetate buffer) is used, also at 25°C.
Class SR 5 forms the transition point between the two groups. Included in it are glasses for which the time for removal of a layer thickness of 0.1 µm at a pH value of 0.3 is less than 0.1 h and at a pH value of 4.6 is greater than 10 hours.
The alkali resistance indicates the sensitivity of optical glasses in contact with warm, alkaline liquids, such as cooling liquids in grinding and polishing processes. The phosphate resistance describes the behavior of optical glasses during cleaning with phosphate containing washing solutions (detergents). Both test methods serve to show the resistance to aqueous alkaline solution in excess and use the same classification scheme.
The alkali resistance class AR is based on the time required to remove a layer thickness of glass of 0.1 µm in an alkaline solution (sodium hydroxide, c = 0.01 mol/l, pH = 12) at a temperature of 50°C. The phosphate resistance class PR is based on the time required to remove a layer thickness of glass of 0.1 mm in an alkaline phosphate containing solution (pentasodium triphosphate Na5P3O10, c = 0.01 mol/l, pH = 10) at a temperature of 50°C. The layer thickness is calculated from the weight loss per surface area and the density of the glass.
Copyright © 2016 Precision Micro-Optics Inc. All Rights Reserved.