New Test Method for Chemical Durability of Optical Glass

Abstract

Dimming and staining resistivity of optical glass have generally been expressed in term of water durability (Dw), and acid durability (Da), which are determined by the powder method, in accordance with the specification of the Japanese Optical Glass Industrial Standards (JOGIS).
However, based on our experiences, the Da does not agree well with the staining resistivity for several glasses. Futhermore, Dw and Da do not represent the solubility of optical glass in water and detergent which is related to the generation of latent scratch with the hardness of the glass.
The rate of staining formation in water and the solubility of optical glasses in water, NaOH solution, and Na₅P₃O₁₀ solution which is a common corrosive inorganic-builder of detergent, have been studied.
The experimental method is as follows; polished glassed were immersed in H₂O controlled pH at 7, 1/100 M NaOH, 1/100 M Na₅P₃O₁₀at 50°C. After a given period of time of corrosion the weight loss of a glass was measured and interference color of the surface, which exhibits the optical thickness of the staining layer, was observed.
It has been found that,
1) In the case of the glass having poorer staining resistivity, Da exhibits lower value than that expected from the behavior of actual staining formation, because of the fault of the powder method. The staining resistivity should be measured directly.
2) Many newer type glasses easily generate latent scratch, in spite of having high chemical durability against H₂O and NaOH solution. It can been explained by reason that the glasses easily attacked by Na₅P₃O₁₀contained usually within detergent. The solubility of the glasses both in Na₅P₃O₁₀solution and in H₂O should be measured in order to know the resistivity of the latent scratch generation.