4.3.2 Phosphate Resistance (PR)

Glass samples with dimensions of 30 x 30 x 2 mm are prepared with all surfaces polished to given specifications. They are hung by platinum wire and placed in an aqueous solution containing 0.01 mol/l purified tripolyphosphate at 50°C for specified lengths of time (15 minutes, 1 hour, 4 hours, or 16 hours).

After this treatment, the loss of mass of the sample is determined using an analytical balance. Calculation of the time t_0.1 necessary to etch a surface layer to a depth of 0.1 µm is made using the same formula as described in the previous section for obtaining the acid resistance (SR). In this case, however, the time unit is minutes.

The calculation is carried out, as a rule, using the value of the loss of mass that is observed under the minimum test conditions (i. e., test solution and test time for obtaining a loss of mass greater than 1 mg/sample).

The phosphate resistance class PR is obtained by comparison of the time required for the attack to a depth of 0.1 µm (min). Time scales are given in table 8.

In addition, changes in the surface of the sample following this test procedure are qualitatively evaluated with the naked eye. Additional classification numbers are given in table 7 which can be found in the previous section. For example, it is indicated that the phosphate resistance class (PR) is PR 2.0 for optical glass that needs 120 minutes for attack to a depth of 0.1 µm, with no visible changes in the surface after the attack.