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There are some glasses that lack durability d ue to the chemical
behavior of the constituents utilized in the composition. These glasses
are influenced by water vapor, acid, gasses, etc., as well as ions in the
polishing slurry. Consequently, dimming and staining will occur on the
surfaces of these glasses during processing and storage. Since such
phenomena have to do with surface conditions and environment, no single
test can be accepted as a criterion of durability under all conditions. We
listed resistance to water and acid by the powder test method and
resistance to weather by the surface test method. We have also listed
resistance to acid and phosphate, following the test method of ISO8424 and
9689.
4.1 Water Resistance [RW(p)] and Acid Resistance
[RA(p)] (Powder Method)
The
glass to be tested is crushed to 425μm ~ 600μm
grains. A sample of this powder
equivalent to the specific gravity in grams is placed on a platinum
basket. This is put in a flask of silica glass containing the
reagent and boiled for 60 minutes. The sample is then carefully
dried and re-weighed to determine the loss of weight (percent) and
classified as per Tables 1 and 2. The reagent used for the water
resistance test is distilled water (pH 6.5 ~ 7.5). 1/100N nitric
acid is used for the acid resistance test.
Table1
Water Resistance
| Class |
1 |
2 |
3 |
4 |
5 |
6 |
| Loss of wt% |
<0.05 |
>0.05
<0.10 |
>0.01
<0.25 |
>0.25
<0.60 |
>0.60
<1.10 |
>1.10 |
Table2
Acid Resistance
| Class |
1 |
2 |
3 |
4 |
5 |
6 |
| Loss of wt% |
<0.20 |
>0.20
<0.35 |
>0.35
<0.65 |
0.65>
1.2< |
1.20>
2.20< |
2.20> |
4.2 Weathering Resistance [W(s)] (Surface
Method)
This test is carried out by putting freshly polished
glass plates in a chamber at +50°C, 85% humidity for 24 hours. If
the glass surface is severely attacked, another 6 hour test is
carried out with new pieces. The classification into four groups is
then obtained by inspecting the treated surface through a 50x
microscope as per Table 3.
Table
3
| Class |
Classification |
| 1 |
When there is no fading on the glass exposed in
the chamber for 24 hours and observed at 6000
luxes. |
| 2 |
When there is no fading observed on the glass
exposed in the chamber for 24 hours at 1500 luxes but fading
is observed at 6000 luxes. |
| 3 |
When fading is observed on the glass exposed in
the chamber for 24 hours when inspected at 1500
luxes. |
| 4 |
When fading is observed on the glass exposed in
the chamber for 6 hours when inspected at 1500
luxes. |
4.3 lSO
Method
4.3.1 Acid Resistance (SR)
Glass samples with dimensions of 30 x 30 x 2 mm are
prepared. The surface of these samples are polished to the specified
polishing conditions. They are hung by platinum wire into nitric
acid solution (pH 0.3) or acetic acid buffer solution (pH 4.6) at
25degC for the length of times specified (10 minutes, 100 minutes,
16 hours or 100 hours). After this treatment, the loss of mass of
the sample is determined using an analytical balance. Calculation of
the time t0.1 in hours, necessary to etch a surface layer
to a depth of 0.1μm is done using the following
formula:
t 0.1=
(tedS) /
{(m1-m2)100}
0.1 : the time (h) necessary
to etch a surface layer to a depth of0.1μm.
te :the time (h) for attack in the
experiment
the specific gravity
of the sample
S :the surface area （cm2 ） of the
sample
m1 : the mass (mg) of the sample before the
test
m2 : the mass (mg) of the sample after the
test
The calculation is
carried out by use of the value of the loss of mass which is
ob-served by the minimum test condition (i.e., test solution and
test time) for obtaining a loss of mass greater than 1 mg / sample.
If the loss of mass is less than 1mg / sample after 100 hours
exposure to pH 0.3, this value shall be accepted. The acid
resistance class SR is obtained by comparison of the pH of the test
solution and the time required for the attack to a depth of 0.1μm
(h) with time scales given in the classification Table
4.
Table
4
Acid resistance class
SR |
1 |
2 |
3 |
4 |
5 |
51 |
52 |
53 |
| pH of the attacking solution |
0.3 |
0.3 |
0.3 |
0.3 |
0.3 |
4.6 |
4.6 |
4.6 |
4.6 |
| Time t0.1 needed to etch
to a depth of 0.1μm （h) |
>100 |
100 ~10 |
10 ~1 |
1 ~0.1 |
<0.1 |
>10 |
10 ~1 |
1 ~0.1 |
<0.1 |
In addition, changes in the surface of the sample
following the treatment are qualitatively evaluated with the naked
eye. Additional classification numbers are given ac-cording to Table
5.
Table
5
| Additional Number |
Changes in the Surface |
| .0 |
No visible changes |
| .1 |
Clear, but irregular surface (wavy,
pockmarked) |
| .2 |
Interference colors (slight selective
leaching) |
| .3 |
Tenacious thin whitish layer (stronger
selective |
| .4 |
Loosely adhering thick layer (Surface
crust) |
4.3.2 Phosphate
Resistance (PR)
Glass
samples with dimensions of 30 x 30 x 2 mm are prepared and all
surfaces are polished to given specifications. They are hung by
platinum wire into aqueous solution containing 0.01 mol / l purified
tripolyphosphate at 50degC 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 t0.1 necessary to etch a
surface layer to a depth of 0.1μm is made using the same formula
which is used for obtaining the acid resistance (SR) in the previous
section. In this case, however, the time units are minutes. The
calculation is carried out, as a rule, using the value of the loss
of mass which 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
0.1μm (min) with time scales given in classification Table 6.
Table 6
| Phosphate resistance class PR |
1 |
2 |
3 |
4 |
| Time t0.1 needed to etch to a
depth of 0.1μm/min) |
>240 |
240 ～60 |
60 ～15 |
<15 |
Next, changes in the surface of the sample following
the treatment are qualitatively evaluated with the naked eye.
Additional classification numbers are given in addition to the class
number according to Table 5used for obtaining the acid resistance
(SR) in the previous section. For example, it is indicated that the
phosphate resistance class is PR 2.0 for optical glass which needs
120 minutes for attack to a depth of 0.1μm
, with no visible changes in the surface
after the attack.
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