GB2141132A - High density information records made of conductive resin compositions comprising vinyl chloride resins free of dispersants - Google Patents

Abstract

A high density information record of the electrostatic capacitance type comprising a record substrate on which signal information is recorded as geometric variations. The record substrate is made of a conductive resin composition comprising a vinyl chloride resin and a conductive powder. The vinyl chloride resin is obtained by suspension polymerization of at least one starting vinyl chloride monomer in an aqueous solution of a dispersant and washing the resulting resin slurry with water until the dispersant is substantially removed from the resin product.

Description

SPECIFICATION High density information records made of conductive resin compositions comprising vinyl chloride resins free of dispersants This invention relates to the record art and more particularly, to high density information recdrds of a variable electrostatic capacitance form, such as video or audio discs, on vvhich signal information is recorded as g#eometric variations.

In certain information playback systems of the electrostatic capacitance type, signal information is recorded as geometric variations or pits in a spiral plane or groove or in concentric planes or grooves by compression molding a conductive resin composition between stampers having modulated ridges on the surfaces thereof, thereby obtaining a disc with an impression of the stampers therein as signal information.

The individual geometric variations or pits of the disc are very small and are arranged in a very high density. When the geometric variations are traced with a pickup stylus such as of diamond having an electrode thereon, capacitive variations are established between the stylus and the disc according to the geometric variations, thereby playing back pictures and sounds.

Typical of the playback system is a video disc system of the electrostatic capacitance type. The video disc system is now put into practice.

In the video disc system, it is very important that the disc be of high quality. The disc should have good characteristics or properties with regard to signal-to-noise ratio showing a reproduction level of signal, durability against repeated cycles of reproduction, mechanical strength, heat stability, humidity proof and the like, by which good performance of the disc is maintained over a long term. To satisfy the above requirements, various conductive resin compositions for the discs and molding techniques have been heretofore proposed. At present, it is generally accepted that resin compositions comprising vinyl chloride resins, carbon black with a large surface area and, if necessary, stabilizers, lubricants and the like ar most preferable from the practical standpoint. Molding techniques of making video or audio discs from these resin compositions have been established.

As mentioned before, signal information is impressed in the disc as fine geometric variations of very high density. For instance, 50 billions of signal pits each with a depth of 0.3 of 0.4 pm are impressed with a track pitch of 1.35 pm. The pits are concentrically or spirally arranged in a disc surface with the innermost track turn having a diameter of 9 cm and the outermost track turn having a diameter of 25 cm. Adjacent pits have intervals of about 0.6 pm at the innermost track turn and of about 1.7 pm at the outermost track turn. Thus, geometric variations are very fine. Moreover, tracking signal is also encoded around the boundary between adjacent tracks in the form of pits with a width of 0.6 to 0.7 pm and a depth of 0.05 pm.

For reading of such fine signals, a diamond stylus having an electrode is slidably contacted with the concentrically or spirally arranged signal pits while controlling with tracking signal as proposed by Victor Company of Japan, Limited or is slidably contacted with a spiral groove in which signal information is impressed at the bottom thereof as proposed by RCA Co., Ltd.

U.S.A. The geometrical variations of the pits are traced as capacitive variations and reproduced as images and sounds. For accurate reproduction, it is important that the signals be molded exactly according to modulated ridges of stampers. If the signals are not exactly molded in the disc, picture images and sounds of high quality cannot be obtained. This lowers the commerical value of the disc. Especially, when formation of signals is very poor, the signal-to-noise ratio may deteriorate and crosstalking of signals from adjacent signal pits may occur. In an extreme case, tracking signal may become unclear or the guide groove may not be kept in shape, so that correct tracking cannot be achieved.

In order to solve the above problem, formation of fine pit signals was studied from various angles including selection of starting materials such as vinyl chloride resins, stabilizers, lubricants and the like, kneading conditions, press molding conditions, and the like. As a result, satisfactory video or digital audio discs are made using selected starting materials and optimum kneading and press molding conditions.

In the industrial production, however, there arose another problem of molding failure of signal mainly due to contamination of stampers on the surfaces thereof. Especially, when the press molding operation is repeated many times, the stampers are contaminated and have an accumulated deposit on the surfaces thereof, causing the signal-to-noise ratio to deteriorate and crosstalking to take place.

As is well known in the art, video or digital audio discs are made as follows: a molten conductive resin composition is placed or injected between stampers having modulated ridges on the surfaces thereof and press molded, followed by cooling to obtain a disc. In the case, the melt is charged from the central portion toward the peripheral portion of the stampers, press molded and cooled to obtain a thin disc.

The stampers are fabricated by electromolding of nickel from mother molds. Because the fabrication is so controlled as to accurately form fine ridges corresponding to intended signal, discs obtained under good molding conditions bear accurate pit signals thereon when the stampers are still fresh. However, it was found that when a pair of stampers were repeatedly used over a long term, the signal-to-noise ratio of the resulting discs lowered. The lowering of the signal-to-noise ratio starts from the central portion of disc and extends toward the peripheral portion. This problem may be readily overcome by changing the stampers by fresh ones.

However, frequent changes are disadvantageous in that the production cost of the disc rises because not only expensive stampers are consumed, but also time for the change is lost.

Additionally, starting materials are also lost by amounts required to control for the change the molding machine under optimum conditions.

A further serious problem involved in the high density information record is attributed to blisters produced on the record surface. As is well known, during playback of recorded signal information, the relative velocity between the stylus and the record reaches as high as 10 m/sec. If the record has blisters thereon, the stylus will jump from the record surface at a portion where blistered, so that signal information cannot be accurately picked up at the portion, leading to a defect or loss of a picture or image to be reproduced. Further, the jumped stylus may scrape off part of the record body upon contact with the record surface after the jumping, and thus signal contained in the scraped-off portion is lost, thereby increasing dropouts or generating dust which is unfavourable for the record.

In general, blisters are unfavorably produced from a number of sources such as impurities contained in starting thermoplastic resins, conductive materials and other additives, foreign matters incorporated during mixing or molding of the conductive resin composition, and thermal decomposition products of the thermoplastic resin. These sources may be suitably removed by improvements in preparation of the starting materials, and by developments of suitable kneading and molding systems. By the improvements, records obtained immediately after fabrication have little or no blisters, thus not generating dropouts when played back.

However, it was found that when such records were allowed to stand, dropouts increased in number as a function of time. It was also found that the increase of dropouts was ascribed to the fact that when records were molded and then allowed to stand, moisture in air was absorbed in the record to form fine blisters on the surface thereof.

It is accordingly an object of the invention to provide high density information records which can overcome all the problems described above.

It is another object of the invention to provide high density information records obtained from conductive resin compositions which enable stampers to be used over a longer term than known conductive resin compositions whereby the records are obtained in high yield and at low cost.

It is a further object of the invention to provide high density information records which are stable and free of fine blisters, as will be formed as time passes, even when placed under relatively high humidity conditions.

The above objects can be achieved, according to the invention, by a high density information record of the electrostatic capacitance type which comprises a record substrate on which signal information is recorded as geometric variations and which is made of a conductive resin composition comprising a vinyl chloride resin and a conductive powder dispersed in the resin, the vinyl chloride resin being obtained by suspension polymerization of at least one starting monomer in an aqueous solution of at least one dispersant for the suspension polymerization and washing the resulting polymer slurry until the dispersant is substantially removed from the polymer. Preferably, sodium polyacrylates are used as the dispersant.

Figures 1 and 2 are graphical representations of the relation between Y-signal-to-noise ratio and the number of disc molding cycles for different resin compositions, respectively; Figures 3(a) and 3(b) are graphs showing the relation between the number of video discs and the number of dropouts with regard to video discs obtained in Example 6 immediately and 120 hours after molding, respectively; Figures 4(a) and 4(b) are graphs similar to Figs. 3(a) and 3(b) but with regard to video discs of Comparative Example 3 obtained immediately and 120 hours after molding, respectively; and Figures 5(a) and 5(b) are graphs similar to Figs. 3(a) and 3(b) but with regard to video discs of Comparative Example 4 obtained immediately and 120 hours after molding, respectively.

As described above, the present invention is characterized by the use, as a resin component of high density information records, of vinyl chloride resins obtained by suspension polymerization from which dispersants essentially used for the suspension polymerization are removed to a substantial extent by washing.

As is well known, vinyl chloride resins used to make high density information records are obtained by suspension polymerization. In the suspension polymerization, a starting monomer or monomers are dispersed as droplets along with a polymerization initiator in an aqueous solution of a high molecular weight dispersant for suspension polymerization. The monomer or monomers are polymerized while violently agitating the reaction system.

It was found that when high density information records were molded from the polymer obtained in the same manner as described above, stampers of a molding machine were gradually contaminated on the surfaces thereof with unknown substances derived from the polymer. Contamination of stampers and occurrence of dropouts in relation to time were confirmed as followes. Three types of vinyl chloride resins were prepared using ordinary ingredients necessary for suspension polymerization. More particularly, vinyl chloride homopolymer, vinyl chloride-ethylene copolymer having an ethylene content of 2%, and ethylene-vinyl acetate-vinyl chloride grafted copolymer, each having a degree of polymerization-of 450, were prepared. Each polymer was pelletized andwsubjected to repeated molding of video discs to determine the contamination of the stampers.Moreover, the regulting discs were checked with respect to the number of dropouts immediately and 120 hours after the molding. As a result, it was found that the vinyl chloride homopolymer gave a less influence on the contamination than the other two copolymers. This was true of the dropouts. It was also found that the contamination and the number of dropouts did not depend on the type of resin, but would be ascribed to an amount of the dispersant used for suspension polymerization because preparation of the vinyl chloride homopolymer needed only much smaller amounts of the dispersant than in the case of the other copolymers.

With vinyl chloride resins having a degree of polymerization as low as 350 to 500, it is usual to polymerize vinyl chloride monomers at a temperature as high as about 60 to 80 C. When the polymerization is effected at such a high temperature as indicated above, a relatively great amount of a dispersant for suspension polymerization is necessary in order to prepare a vinyl chloride resin product having a size distribution suitable for uniform mixing with a conductive powder such as carbon black.

A number of dispersants are ordinarily used singly or in combination for suspension polymerization of vinyl chloride monomer and/or monomers copolymerizable therewith. In general, there are used, singly or in combination, water-soluble partially saponified polyvinyl acetates, cellulose derivatives, copolymers of vinyl chloride and maleic anhydride, polyvinylpyrrolidone, sodium polyacrylates and the like. A large number of partially saponified polyvinyl acetates are known having different molecular weights, degrees of saponification and combinations with third components. Also, a large number of cellulose derivatives used for these purposes are known which have different molecular weights and different types of substituents.

These water-soluble dispersants are properly used depending on the intended nature of vinyl chloride resin. As a matter of course, oil-soluble polymeric compounds may be used in combination provided that they dissolve in monomers used.

In general, vinyl chloride resins used as a starting material for video or digital audio discs are prepared by polymerization at temperatures as high as 60 to 80 C as mentioned before. This leads to the tendency that the resulting vinyl-chloride resin particles become shall in porosity with relatively poor gelling property and that removal of unreacted monomer becomes difficult.

To avoid this, it is preferred to use partially saponified polyvinyl acetates having a degree of saponification of from 60 to 80% and a low molecular weight, and cellulose derivatives such as hydroxypropyl cellulose or hydroxypropylmethyl cellulose with a thermal gelling temperature exceeding 70 C. These dispersants are not so reduced in dispersing strength even when used at high temperatures and have the effect of increasing the porosity. Most preferably, sodium polyacrylates are used because they have much greater dispersability and are able to produce vinyl chloride resins having a small-size distribution even when used in much smaller amounts than other dispersants.However, when sodium polyacrylates are used singly, the porosity of the resulting resin particles becomes relatively small, making it difficult to suitably remove unreacted monomer from the particles. The use of sodium polyacrylates results in vinyl chloride resins which are suitable as a starting material for video or digital audio discs but is rather unfavorable from the standpoint of resin preparation. To avoid this, sodium polyacrylates are used in combination with other water-soluble polymer compounds such as, for example, partially saponified polyvinyl acetates, cellulose derivatives and the like.

The dispersants used for suspension polymerization may, more or less, depend on the type of vinyl chloride resin, agitation conditions of reactor, the degree of polymerization of a dispersant used, and the like. The amounts are usually in the range of from 0.02 to 0.8 wt% of vinyl chloride monomer for vinyl chloride homopolymer and from 0.05 to 2.0 wt% of the total amount of monomers for copolymers.

As described, sodium polyacrylates have much higher dispersability than other dispersants.

For the suspension polymeriztion, the degree of polymerization of the polyacrylate suitably ranges from 5,000 to 100,000. This is for the following reasons. In the production of vinyl chloride resins, it is general that dispersants are dissolved in water and filtered in order to remove foreign matters therefrom. The filtration operation becomes difficult when using sodium polyacrylates whose degree of polymerization exceeds 100,000 because of too high a viscosity of an aqueous solution thereof. On the other hand, sodium polyacrylates having a degree of polymerization below 5,000 are not suitable for use in suspension polymerization because of the development of emulsifying ability thereof.

The amount of sodium polyacrylates having a degree of polymerization of, for example, 50,000 ranges from 0.005 to 0.05 wt% of monomer for vinyl chloride homopolymer and from 0.01 to 0.1 wt% of the total amount of monomers for copolymers.

If sodium polyacrylates are used in combination with other type of dispersant, it is preferable that other dispersant should bear about 30 to 70% of a required suspending and dispersing capacity. The dispersing capacity of sodium polyacrylates is 7 to 15 times as large as the highest capacity-of partially saponified polyvinyl acetates or cellulose derivatives. More particularly, when sodium polyacrylates are used along with partially saponified polyvinyl acetates or cellulose derivatives, sodium polyacrylates are used in amounts ranging from 0.002 to 0.04 wt% and the acetates or derivatives are used in amounts ranging from 0.01 to 0.2 wt%, both based on monomer for vinyl chloride homopolymer. Such amounts range from 0.003 to 0.007 wt% of sodium polyacrylates and from 0.01 to 0.2 wt% of the acetates or derivatives, both based on the total amount of monomers for copolymers.

The vinyl chloride resins useful in the present invention should have a degree of polymerization of 350 to 600 and include, for example, vinyl chloride homopolymer, copolymers of vinyl chloride with vinyl acetate, ethylene, propylene, alpha-olefins such as 1-octene, 1-hexene and the like, acrylate monomers such as ethyl acrylate, 2-ethylhexyl acrylate, n-butyl acrylate, methyl acrylate and the like, and ethylene-vinyl acetate copolymers grafted with vinyl chloride, and the like. Where copolymers are used, the content of co-monomers should preferably be below 25 wt%.

In the practice of the invention, vinyl chloride monomer and/or other copolymerizable monomers are suspension polymerized as usual, whereupon the reactor and agitator should preferably be washed with water to remove foreign matters therefrom. Likewise, monomers, dispersants, and other additives should peferably be filtered after dissolution in water, if necessary, thereby removing foreign matters with a certain size therefrom. The resulting resin particles are sufficiently washed with water to remove water-soluble dispersants used therefrom.

By this, contamination of stampers in repeated use and occurrence of fine blisters on the disc surface can be suitably avoided. Preferably, after completion of the suspension polymerization, the polymer slurry is washed with water by ordinary techniques including filtration and/or dehydration with a centrifugal dehydrator and/or settling, and addition of water to the resulting cake to form a slurry. This procedure is repeated several times. Termination of the washing may be readily determined by analysis of one component in the waste water or washing, e.g. sodium ions for sodium polyacrylates. In general, the washing is continued until water-soluble dispersants are removed to a level below 400 ppm, preferably 200 ppm in washing in order to obtain video or digital audio discs free of fine blisters and to overcome the problem of stamper contamination.The level below 200 ppm may be attained when the resin particles and washed by the above procedure using 5 to 20 times by volume of water in total. For one washing operation, the resin particles should preferably be washed with two or more times by volume of water because too small an amount of washing water is less efficient. The washing procedure including filtration and/or dehydration or the like is favorably repeated several times. In the event complete washing is not accomplished, a water-soluble dispersant used may be plated out on the stamper surfaces when such a resin is molded, thereby contaminating the stampers therewith. Water used for the washing should preferably have a temperature of from 10 to 50 C in order to ensure efficient washing.

Even when water-insoluble but monomer-soluble dispersants such as ethyl cellulose, or low molecular weight surfactants such as polyethylene glycol are used in combination with watersoluble dispersants removal of the dispersants, both water-soluble and insoluble, may be accomplished in the same manner as described above.

As described above, dispersants for suspension polymerization are removed by adding water to a polymer slurry or a wet polymer product. Even though vinyl chloride resins which have been once dried are washed with water, satisfactory washing cannot be achieved because the surfaces of the particles of the dried resin are unlikely to wet. Furthermore, the additional step of drying the resin particles would increase the opportunity of unfavorably incorporating foreign matters thereinto and the rise of energy cost for the drying. Accordingly, it is convenient to wash the slurry or wet polymer after completion of the polymerization but prior to drying.

For the washing, there is c#onveniently used clean water, particularly clean ion-exchanged water, -from which foreign matters are removed. In practice, the polymer slurry is first placed in a dehydrator to obtain a cake, to which is added fresh water to obtain a slurry, followed by the above procedure until the dispersant in waste water reached not larger than about 400 ppm.

Alternatively, the slurry may be allowed to stand in a tank so as- to permit the resin particles to settle. After removal of the supernatant, water is added to the particles to make a slurry, followed by repeating the above procedure until the same level of dispersant as indicated above is reached.

For fabrication of high density information records according the invention, the thus obtained vinyl chloride resins are mixed with a conductive powder such as carbon black, fine metal powders, or the like in an amount from 5 to 30 wt% based on the resin, thereby-obtaining a conductive resin composition. The composition may further comprise additives ordinarily used for these purposes, such as lubricants, thermal stabilizers, and the like. Mixing, kneading, pelletizing and compression molding of the composition can be effected by any techniques known to the art and are not specifically described herein.

The present invention is described in more detail by way of preparatory examples, examples, and comparative examples.

Preparatory Example 1 Preparation of Vinyl Chloride Resins Used in the Present Invention Vinyl chloride was polymerized with or without copolymerizable monomers in a 600 liters autoclave equipped with a jacket and an agitator. Scales deposited on the inner surfaces of the autoclave were removed by a scraper and tightly sealed. Methylene chloride was filled in the autoclave and then heated to 40 C, followed by agitation for 1 hour. Subsequently, the methylene chloride was withdrawn and pressurized water with a pressure of 60 kg/cm2G was injected into the autoclave to completely remove the scales therefrom. Ion-exchanged water which passed through a filter having openings of 0.5 jum was charged into the autoclave for washing.

Ion-exchanged water used for the polymerization was filtered with the same type of filter as used above. Vinyl chloride monomer and copolymerizable monomers were each filtered with a filter having openings of 1 clam. A suspension stabilizer for the polymerization was dissolved in the ion-exchanged water to make a 2 wt% aqueous solution, followed by filtering with a filter having openings of 1 jum.

A polymerization initiator was passed through a filter with openings of 1 ,um as it is.

The polymerization reaction was carried out as follows. The ion-exchanged water was charged into the washed autoclave, into which were further charged predetermined amounts of the suspension stabilizer and the polymerization initiator while filtering. Thereafter, the autoclave was tightly sealed, after which the air in the system was evacuated to an internal pressure of 35 mmHg, followed by charging vinyl chloride monomer while agitating. The system was heated to a temperature of polymerization at which the exothermic polymerization reaction started.

Thereafter, the system was kept at a temperature indicated in Table 1, under which the reaction was caused to proceed. At the time when the rate of polymerization reached about 70%, the temperature of the content was lowered down to 65 C and remaining vinyl chloride monomer was recovered. The internal pressure was reduced to O Kg/cm2G, after which 2 m3/hour of nitrogen gas was introduced from the bottom of the autoclave for 2 hours while passing through a filter having openings of 0.5 ,um and discharged from the upper portion of the autoclave in order to eliminate the remaining vinyl chloride monomer from the system. Then, the internal pressure was raised to 1 Kg/cm2G by the use of pressurized nitrogen gas.

The resulting slurry of vinyl chloride polymer was discharged through a 42 mesh metal sieve under the pessure of the nitrogen in the autoclave into a 1000 liters container which had been washed with filtered ion-exchanged water and which had a discharge nozzle and an agitator.

Part of the slurry was dehydrated with a centrifugal dewatering machine installed in a room, which was pressurized with clean air passed through an HEPA filter, thereby obtaining a cake.

To the cake was added water in an amount three times as much as the amount of the cake to make a slurry. The slurry was agitated for 1 hour, followed by repeating the above procedure two more times. The cake obtained by the fourth dehydration was dried in a conical dryer in such a way that no dust was incorporated thereinto, thereby obtaining fine particles of vinyl chloride resins. The polymerization conditions and compositions of vinyl chloride resins prepared in the same manner as described above are shown in Table 1 below.

Table 1 Experiment 1 2 3 Composition vinyl vinyl vinyl chloride chloride chloride homo- homo- homo polymer polymer polymer Ion-exchanged water 290Kg 290Kg 290Kg Vinyl chloride monomer 200Kg 200Kg 190Kg Ethylene - - 1 OKg Polymerization initiator 1 60g 1 60g 1809 (Note 1) Partially saponified polyvinyl acetate 6009 11009 (Note 2) Hydroxypropyl methyl- - 8009 cellulose (Note 3) Polymerization 80 C 80 C 77 C temperature Polymerization time 8.5 hrs. 8.5 hrs. 9.0 hrs.

Degree of polymerization 480 480 490 Content of co-monomer - - 2.3% in the final product Size distribution 42 mesh pass (%) 99 99 99 100 ,, 67 58 85 200 ,, 3 5 18 Apparent density 0.55 0.53 0.48 (Note 1) Polymerization initiator: Kaya ester 0, available from Kayaku Noury Co., Ltd.

(Note 2) Partially saponified polyvinyl acetate: Gosenol KH-17 available from The Nippon Synthetic Chem. Ind. Co., Ltd.

(Note 3) Hydroxypropylmethyl cellulose: Metollose 90SH 100 avail able from Shinetsu Chem. Co., Ltd.

Preparatory Example 2 Preparation of Vinyl Chloride Homopolymers for Comparison The general procedure for Experiment Nos. 1 and 3 was repeated except that part of the slurry withdrawn from the reator was dried in the dewatering machine and the conical dryer without washing with water, thereby obtaining frine particles of the vinyl chloride resins (Experiment Nos. 4 and 5.

Preparatory Example 3 The general procedure of Preparatory Example 1 was repeated except that 0.1 wt% of sodium polyacrylate and 2 wt% of other dispersant dissolved in filtered ion-exchanged water were used, thereby obtaining a resin slurry. The slurry was subsequently dehydrated and washed with ionexchanged water in an amount of 3 times by volume of the produced vinyl chloride resin, followed by agitating for one hour. The above procedure was repeated two more times. The content of sodium polyacrylate in the waste water was analyzed and found to be below 12 ppm.

The cake obtained by the final dehydration was dried while taking care of not incorporating dust into the resin particles. The content of the sodium polyacrylate in the resin was found to be about 6 ppm.

In this manner, four vinyl chloride resins were prepared in total. The preparation conditions and compositions, and resin characteristics are shown in Table 2 below.

Table 2 Experiment 6 7 8 9 Composition vinyl vinyl vinyl vinyl chloride chloride chloride/ chloride/ homo- homo- ethylene ethylene polymer polymer copolymer copolymer lon-exchanged water 290Kg 290Kg 290Kg 290Kg Vinyl chloride monomer 200Kg 200Kg 190Kg 190Kg Ethylene - - 1 OKg 1 OKg Polymerization initiator 1 60g 1 60g 1 80g 1809 (Note 4) Partially saponified polyvinyl acetate 3009 - 6009 500g (Note 5) Hydroxypropylmethyl - 5009 cellulose (Note 6) Sodium polyacrylate (Note 7) 40g 60g 80g 100g Polymerization 80 C 80 C 77 C 77 C temperature Polymerization time 8.5 hrs. 8.5 hrs. 9.0 hrs. 9.0 hrs.

Degree of polymerization 480 480 490 490 Content of co-monomer in the final product - - 2.3% 2.3% Size distribution 42 mesh pass (%) 99 99 99 99 100 67 58 85 75 200 , 3 5 .18 18 Apparent density 0.55 0.53 0.48 0.50 (Note 4) Polymerization initiator: Kaya ester 0, available from Kayaku Noury Co., Ltd.

(Note 5) Partially saponified polyvinyl acetate: Gosenol KH-17 available from The Nippon Synthetic Chem. Ind. Co., Ltd.

(Note 6) Hydroxypropylmethyl cellulose: Metollose 90SH 100 available from Shinetsu Chem. Co., Ltd.

(Note 7) Sodium polyacrylate: Aron A-20P Toa Synthetic Chem. Ind. Co., Ltd.

Preparatory Example 4 Preparation of Vinyl Chloride Resins for Comparison The general procedure of Experiment No. 3 was repeated except that part of the slurry withdrawn from the reactor was dehydrated and dried in the dewatering machine and the conical dryer without washing with water, thereby obtaining fine particles of the vinyl chloride resin (Experiment No. 10). Moreover, the procedure of Experiment No. 8 was repeated except that no sodium polyacrylate was used but 11009 of the partially saponified polyvinyl acetate was used, thereby obtaining a vinyl chloride resin (Experiment No. 11).

Examples 1-3 One hundred parts by weight of each of the resins obtained in Experiment Nos. 1 through 3 in which the dispersants used for the suspension polymerization were substantially removed by washing with water, 5 parts by weight of dibutyl tin mercapto ester stabilizer (STANN JF-95, made by Snakyo Organic Chemicals Co., Ltd.), 2.0 parts by weight of a fatty acid-glycerine ester lubricant (RES-210, made by Riken Vitamin Co., Ltd.), 0.5 part by weight of an alkyl ester of fatty acid (RES-310, made by Kao Soap Co., Ltd.), and 1.0 part by weight of dimethylpolysiloxane (RES-421, made by Sinetsu Chem.Co., Ltd.) were mixed in a 20 liter Henschel mixer to a temperature of 11 0,, followed by operating the mixer at a low speed so that the temperature was lowered down to 70 C. To the mixture was added 20 parts by weight of conductive carbon black (CSX-1 50A, made by Cabot Co., Ltd., U.S.A.), followed by heating again to 110 C and agitating at a high speed for 15 minutes. Thereafter, the mixture was cooled down to room temperature.

The mixture was pelletized by means of a kneader, PR-46 made by Buss Co., Ltd., Switzerland. The resulting pellets were subjected to a metal separator to remove metals or metal-containing pellets from the pellets and pressed by a video disc press machine to obtain video discs.

Comparative Examples 1 and 2 The general procedure of Examples 1 through 3 was repeated using the resins obtained in Experiment Nos. 4 and 5, thereby obtaining video discs.

In the above examples and comparative examples, about 300 video discs for each conductive resin composition were press molded in a continuous manner using stampers on which reference signal for signal measurement was recorded.

The video discs obtained in these examples and comparative examples were each subjected to measurement of the signal recorded therein to determine a degree of deterioration of Y-signal-tonoise ratio. The results are shown in Figs. 1 and 2.

As will be clear from the figures, little contamination of the stampers occur when the vinyl chloride resins used are substantially free of the suspension dispersants. Thus, the video discs of the invention have little singal defects and good playback characteristics. On the other hand, Fig. 2 reveals that use of the vinyl chloride resins from which the dispersants are not removed by washing but contained therein as they are results in contamination of the stampers when the molding is repeated. The molding with the contaminated stampers leads to video discs having signal defects and poor playback characteristics.

Observation, through a scanning electron microscope, of the stampers after completion of the 300 molding cycles demonstrated that no foreign matters were deposited on the stamper surface.

Examples 4-7 The general procedure of Examples 1 through 3 was repeated using 100 parts by weight of the vinyl chloride resins of Experiment Nos. 6 through 9, thereby obtaining video discs.

Comparative Examples 3 and 4 The general procedure of Examples 1 through 3 was repeated using 100 parts by weight of the vinyl chloride resins of Experiment Nos. 10 and 11, thereby obtaining video discs.

In Examples 4 through 7 and Comparative Examples 3 and 4, 20 video discs were molded from each composition using stampers on which reference signal for signal measurement was recorded. Immediately and 120 hours after the molding, these video discs were each subjected to a dropout counter to count out the number of dropouts which were not good from the standpoint of the image quality.

The video discs obtained in Examples 4 through 7 involved no dropouts, whereas some video discs for comparison produced dropout noises, e.g. the video discs of Comparative Example 3 involved 0.75 dropout on average and the video discs of Comparative Example 4 had 11.9 dropouts on average. From the above, it will be seen that the video discs of the present invention involving no dropouts evaluated as NG are very excellent.

The video discs of these examples and comparative examples were further allowed to stand under conditions of a temperature of 40"C and a relative humidity of 90% for 96 hours and then under normal temperature and normal humidity conditions for 24 hour. Subsequently, each disc was subjected to the dropout counter. As a result, it was found that the discs of the present invention did not lower in image quality even when placed under severe conditions but the discs for comparison considerably deteriorated.

The results are summarized in Figs. 3(a) and 3(b) through 5(a) and 5(b) in which the results of the discs obtained immediately after the molding are shown in Figs. 3(a), 4(a) and 5(a) and the results of the discs obtained 120 hours after the molding are shown in figures. (b). It will be noted that the video discs obtained in-Examples 4 through 7 according to the invention are similar with respect to the dropout characteristic and the results of the discs of Example 4 are representatively shown in Figs. 3(a) and 3(b).

Claims (13)

1. A high density information record of the electrostatic capacitance type which comprises a record substrate on which signal information is recorded as geometric variations and which is made of a conductive resin composition comprising a vinyl chloride resin and a conductive powder, the vinyl chloride resin being obtained by suspension polymerization of at least one monomer in an aqueous solution of at least one dispersant and washing the resulting resin slurry until the at least one dispersant is substantially removed from a final resin product.

2. The high density information record according to Claim 1, wherein said vinyl chloride resin has a degree of polymerization of 350 to 600 and is vinyl chloride homopolymer.

3. The high density information record according to Claim 1, wherein said vinyl chloride resin is a copolymer of vinyl chloride and vinyl acetate, ethylene, propylene, alpha-olefin or acrylate monomer.

4. The high density information record according to Claim 1, wherein said vinyl chloride resin is a copolymer of ethylene and vinyl acetate grafted with vinyl chloride.

5. The high density information record according to Claim 1, wherein said at least one dispersant is a water-soluble polymer.

6. The high density information record according to Claim 5, wherein said water-soluble polymer is a member selected from the group consisting of partially saponified polyvinyl acetates having a degree of saponification of from 60 to 80%, cellulose derivatives and mixtures thereof, and is used in an amount of from 0.002 to 0.8 wt% of the at least one monomer when said at least one monomer is vinyl chloride monomer.

7. The high density information record according to Claim 6, wherein said water-soluble polymer is used in an amount of from 0.05 to 2.0 wt% of the at least one monomer where said at least one monomer is a mixture of vinyl chloride monomer and a monomer copolymerizable therewith.

8. The high density information record according to Claim 5, wherein said water-soluble dispersant is a sodium polyacylate material having a degree of polymerization ranging from 5,000 to 100,000 and is used in an amount ranging from 0.005 to 0.05 wt% of the at least one monomer when said at least one monomer is vinyl chloride monomer.

9. The high density information record according to Claim 8, wherein said sodium polyacrylate material is used in an amount of from 0.01 to 0.1 wt% of the at least one monomer when said at least one monomer is a mixture of vinyl chloride monomer and a monomer copolymerizable therewith.

10. The high density information record according to Claim 8 or 9, further comprising another type of water-soluble dispersant selected from the group consisting of partially saponified polyvinyl acetates, cellulose derivatives and mixtures thereof.

11. The high density information record according to Claim 1, further comprising a waterinsoluble dispersant which is soluble in the at least one monomer.

12. The high density information record according to Claim 1, wherein the at least one dispersant is removed to a level below 400 ppm in the washing.

13. The high density information record according to Claim 12, wherein the level is below 200 ppm.