Method of pretreating glass preform with oxygen plasma

According to the present process for producing a molded glass article, the surface of a lead oxide-containing glass material to be press molded is subjected in advance to an oxidation treatment with activated oxygen ion; thereby, there takes place no reduction reaction of lead oxide at the glass surface, at the time of press molding of the glass material, and there can be obtained a molded glass article with a clear surface. Further, there occurs no adhesion of lead to the surface of the mold used, and consequently the mold life is extended, which is very economical.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a process for producing a molded glass 
article and more particularly to a process for producing a molded glass 
article from a glass material containing lead oxide. 
(2) Description of the Prior Art 
In recent years, many attempts have been made to press mold an optical lens 
or the like at a high precision without conducting post-treatments such as 
polishing and the like. In one of such molding processes, a glass material 
placed between an upper mold and a lower mold both made of a ceramic or 
the like is heated to a temperature close to the softening point of said 
glass and then subjected to press molding. This press molding is effected 
generally in a non-oxidizing or reducing gas atmosphere such as nitrogen 
gas or the like. The reason is that the use of a non-oxidizing gas 
atmosphere can prevent not only the oxidation of the material (e.g. 
stainless steel) constituting the pressing machine used but also the 
oxidation of the mold surface (having a mirror surface) which is in 
contact with the glass material (the mold surface, when the mold is 
repeatedly used in an oxidizing atmosphere, gets rough and loses the 
mirror surface smoothness). 
However, when a glass material containing, for example, about 20% by weight 
or more of lead oxide is subjected to press molding in a non-oxidizing 
atmosphere containing oxygen in a very small amount of 100 ppm or below, 
the lead oxide (PbO) at the glass surface is chemically reduced to become 
other lead compound or metallic lead (Pb), and the surface of the molded 
glass article gets cloudy. When the mold is used repeatedly under such a 
condition, the mold surface reacts with the lead component of the glass 
surface to form a reaction product and thereby to lose the mirror surface 
smoothness. Presently, there has been found no effective method for 
dissolving the reaction product in order to restore the original mirror 
surface. Moreover, the molded glass article obtained with the mold whose 
surface has lost the mirror surface smoothness, has an uneven surface. 
In order to solve the above problem encountered when a lead 
oxide-containing glass is subjected to press molding, Japanese Patent 
Application Kokai (Laid-Open) No. 162629/1987 discloses a process wherein 
press molding is effected by changing the oxygen concentration in 
non-oxidizing atmosphere after every given times of molding. 
However, when a lead oxide-containing glass material is subjected to press 
molding by the process described in the above patent document, neither the 
cloudiness at the molded glass article surface nor the adhesion of lead to 
the mold surface can be prevented completely. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a process for producing a 
molded glass article by press molding a glass material containing lead 
oxide, which process has such advantages that the molded glass article has 
no surface cloudiness and there occurs no lead adhesion to the surface of 
the mold used. 
The above object of the present invention has been achieved by, when a 
molded glass article is produced by press molding a glass material 
containing lead oxide, subjecting, prior to the press molding, the surface 
of the glass material to an oxidation treatment with activated oxygen ion. 
According to the present invention, there is provided a process for 
producing a molded glass article by press molding a glass material 
containing lead oxide, which process comprises subjecting, prior to the 
press molding, the surface of the glass material to an oxidation treatment 
with activated oxygen ion.

DETAILED DESCRIPTION OF THE INVENTION 
As a typical example of the oxidation treament with activated oxygen ion, 
there can be mentioned oxygen plasma ashing. In the present invention, the 
oxidation treatment can be effected by any method as long as the surface 
of a glass material to be press molded can be subjected to an oxidation 
treatment with activated oxygen ion. It is also possible to give rise to 
oxygen plasma ashing by the use of, for example, an ion-bombarding or 
ion-showering apparatus. A glass material containing lead oxide, when its 
surface is subjected to an oxidation treatment with activated oxygen ion 
prior to press molding, not only causes no reduction reaction of lead 
oxide at the glass surface even when press molding is effected in a 
reducing atmosphere at temperatures of the glass softening point or above, 
but also gives a molded glass article free from surface cloudiness. 
Further in this case, there occurrs no adhesion of lead to the surface of 
the mold used. 
The present process for producing a molded glass article is hereinafter 
described in detail by way of Examples. 
FIG. 1 is a schematic drawing of an apparatus (DES112-304AV manufactured by 
Plasma System Co.) for effecting oxygen plasma ashing. In FIG. 1, the 
numeral 1 is an oxidation treatment chamber; the numeral 2 is a rotary 
pump for reducing the inside pressure of the chamber 1 to a desired 
vacuum; the numeral 3 is an oxygen inlet for introducing oxygen into the 
chamber 1 after the inside pressure of the chamber 1 has been reduced to a 
desired vacuum by the rotary pump. The numerals 4 and 5 are high frequency 
electrodes, and a high frequency electric power is generated between these 
two electrodes. The numeral 6 is a quartz-made inner chamber, in which a 
glass sample 8 is mounted on a quartz-made holder 7. The numeral 9 is an 
infrared lamp for heating the glass sample 8 and said infrared lamp is 
arranged outside the quartz-made inner chamber 6. 
As the glass sample 8 whose surface was to be subjected to an oxygen plasma 
ashing treatment with the apparatus shown in FIG. 1, there were used three 
kinds of glass materials which were different in lead oxide content and 
which had been thoroughly cleaned by ultrasonic cleaning, i.e. glass 
sample No. 1 (PbO=66% by weight, softening point=470.degree. C.), No. 2 
(PbO=40.5% by weight, softening point=465.degree. C.) and No. 3 (PbO=23% 
by weight, softening point=480.degree. C.). 
Each of these glass materials as the glass sample 8 was mounted on the 
quartz-made holder 7; the inside pressure of the chamber 1 was reduced to 
0.03 Torr by the rotary pump 2; then, oxygen was introduced into the 
chamber 1 through the oxygen inlet 3 and thereby the vacuum inside the 
chamber 1 was adjusted to 0.8 Torr. 
A given high frequency electric power was generated between the high 
frequency electrodes 4 and 5 while the glass sample 8 was heated and kept 
at a given temperature by the infrared lamp 9, to effect an oxygen plasma 
ashing treatment. 
This oxygen plasma ashing was effected under the following two treatment 
conditions (A) and (B) which were different in high frequency output and 
treatment temperature. 
______________________________________ 
Treatment conditions 
(A) (B) 
______________________________________ 
High frequency output (W) 
600 1200 
Treatment temperature (.degree.C.) 
50 130 
Treatment time (min) 
2.5, 5 and 10 
2.5, 5 and 10 
______________________________________ 
The inside of the chamber 1 is kept always clean. Therefore, the glass 
sample 8 after oxygen plasma ashing could be used for press molding as it 
was, without being subjected to recleaning. 
Next, mention is made on press molding of the glass sample subjected to the 
plasma ashing treatment. 
FIG. 2(a) is a drawing showing the "right before glass press molding" 
condition in a pressing machine. In FIG. 2(a), the numeral 10 is an upper 
mold; the numeral 11 is a lower mold; the numeral 12 is a sleeve; the 
numeral 13 is fixtures for holding the upper and lower molds; the numeral 
14 is press heads; and a glass sample 8 to be press molded is placed in a 
space formed between the lower mold 11 and the sleeve 12. FIG. 2(b) is a 
drawing showing the "right after glass press molding"condition in the same 
pressing machine. In FIG. 2(b), an intended molded glass article 15 has 
been obtained by lowering the upper mold 10 to effect pressing. 
Using the pressing machine shown in FIG. 2, the above mentioned three lead 
oxide-containing glasses subjected to oxygen plasma ashing were press 
molded at 570.degree. C. in a nitrogen atmosphere to obtain molded glass 
articles. For comparison, the same three lead oxide-containing glasses not 
subjected to any oxygen plasma ashing were press molded under the same 
conditions to obtain molded glass articles. 
The results of press molding of the three lead oxide-containing glasses 
subjected to oxygen plasma ashing are shown in Table 1, together with the 
results of press molding of the three lead oxide-containing glasses not 
subjected to oxygen plasma ashing. These results are summarized as 
follows. 
(1) When there was effected no oxygen plasma ashing, all of the molded 
glass articles obtaiend from the glass sample Nos. 1, 2 and 3 had 
cloudiness. (In each test, 50 test pieces were used for each sample.) 
Cloudiness was particularly severe in the molded articles obtained from 
the glass sample Nos. 1 and 2. 
(2) When there was effected oxygen plasma ashing, good results were 
obtained even though the high frequency output and treatment temperature 
were varied over a wide range. That is, at a high frequency output of 600 
W and a treatment temperature of 50.degree. C., all the molded articles 
obtained from the test pieces of the glass sample No. 1 had no cloudiness 
when the ashing time was 5 min or longer. In all the molded articles 
obtained from the glass sample Nos. 2 and 3, there was no cloudiness when 
the ashing time was 2.5 min or longer. 
At a high frequency output of 1200 W and a treatment temperature of 
130.degree. C., all the molded articles obtained from the glass sample 
Nos. 1, 2 and 3 had no cloudiness when the ashing time was 2.5 min or 
longer. 
It is appreciated from the above results that oxygen plasma ashing is a 
very effective means for prevention of surface cloudiness in molded glass 
articles obtained by press molding of lead oxide-containing glass 
materials. 
The press molding of the glass sample Nos. 1, 2 and 3 subjected to 
oxidizing treatments under the oxygen plasma ashing conditions (A) and (B) 
was repeated using the same mold. In each case, there was no adhesion of 
lead to the mold surface even after 8,000-10,000 times repetition of press 
molding. In contrast, when no oxygen plasma ashing was effected, there was 
seen adhesion of lead to the mold surface after about 500-800 times 
repetition of press molding. 
TABLE 1 
__________________________________________________________________________ 
Oxygen plasma ashing conditions 
Glass sample (A) (B) 
PbO content 
Softening 
Oxygen plasma 
High frequency output: 600 W 
High frequency output: 1200 
W 
in glass point 
ashing: Not 
Treatment temperature: 50.degree. C. 
Treatment temperature: 
130.degree. C. 
No. 
(wt. %) 
(.degree.C.) 
effected 
2.5 min 5 min 10 min 
2.5 min 
5 min 
10 
__________________________________________________________________________ 
min 
1 66.0 470 Inspection 1 
Inspection 1 
Inspections 1, 2 
Same as 
Same as 
Same 
Same as 
Cloudiness in 
Very slight 
No cloudiness in 
left left left left 
all pieces 
cloudiness in 
any pieces 
several pieces 
(In both inspec- 
Inspection 2 
tions) 
Slight cloudi- 
ness in all pieces 
2 40.5 465 Inspection 1 
Inspections 1, 2 
Same as left 
Same as 
Same as 
Same 
Same as 
Cloudiness in 
No cloudiness in left left left left 
all pieces 
any pieces (In 
both inspections) 
3 23.0 480 Inspection 1 
Inspections 1, 2 
Same as left 
Same as 
Same as 
Same 
Same as 
Slight cloudi- 
No cloudiness in left left left left 
ness in all 
any pieces (In 
pieces both inspections) 
__________________________________________________________________________ 
Number of test pieces: 50 for each test 
Inspection method: In Inspection 1, each molded glass article was 
inspected visually. 
Thus, according to the present process for producing a molded glass 
article, the surface of a lead oxide-containing glass material to be press 
molded is subjected in advance to an oxidation treatment with activated 
oxygen ion; thereby, there takes place no reduction reaction of lead oxide 
at the glass surface, at the time of press molding of the glass material 
and there can be obtained a molded glass article with a clear surface. 
Further, there occurs no adhesion of lead to the surface of the mold used, 
and consequently the mold life is extended, which is very economical.