Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2000-07-26
2002-05-21
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06392011
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for producing granules of a polyvinyl chloride resin for preparation of paste. More particularly, the present invention relates to a process for producing granules of a polyvinyl chloride resin for preparation of paste which are produced from an aqueous dispersion of a vinyl chloride polymer obtained in accordance with emulsion polymerization or micro suspension polymerization, do not cause significant attachment of powder and exhibit excellent powder fluidity and excellent dispersion into plasticizers in preparation of paste (plastisol).
BACKGROUND ART
To prepare a polyvinyl chloride resin for preparation of paste, in general, an aqueous dispersion of a vinyl chloride. polymer which is obtained by emulsion polymerization or micro suspension polymerization of vinyl chloride or a mixture of vinyl chloride as the main component and a comonomer copolymerizable with vinyl chloride and contains primary particles having diameters of 0.05 to 5 &mgr;m as the main components and aggregates of the primary particle is used. Where necessary, surfactants and particles of a vinyl chloride polymer having diameters of primary particles of 0.2 to 2 &mgr;m are added to the aqueous dispersion in an amount of 5 to 30% by weight. The prepared mixture is spray-dried and a granular powder containing resin granules which are aggregates of polymer particles formed from individual droplets of the spray as the main components and aggregates of such resin granules is obtained.
Heretofore, the obtained granular powder is pulverized to prepare a fine polyvinyl chloride resin which is used for preparation of paste. Paste (generally referred to as plastisol or sol) is prepared by adding plasticizers and compounding ingredients such as heat stabilizers and fillers to the polyvinyl chloride resin and mixing the components.
The polyvinyl chloride resin for preparation of paste which has heretofore been used have some drawbacks due to fineness of the particles. For example, when a bag packed with the resin is opened for preparation of paste, dust of the resin powder is formed and the working environment is deteriorated. Powder fluidity of the resin is poor and automatic measurement and pneumatic transportation of the resin powder require sophisticated processes and large scale dust collectors. In Japanese Patent Application Laid-Open No. Heisei 2(1990)-133410, spherical granules of a polyvinyl chloride resin are proposed to improve these drawbacks. In Japanese Patent Application Laid-Open No. Heisei 2(1990)-225529, it is shown that resin granules which can be dispersed in a plasticizer without pulverization after drying are obtained by spray-drying using heated air having a specific humidity at a temperature lower than 100° C. However, the above processes have problems in that productivity of the drying apparatus is markedly low due to the spray-drying at a low temperature and that aggregates having large sizes which are formed by cleavage of resin layers attached to the inner face of the drying apparatus during the operation for a long time are mixed into the product and dispersion of the resin in the formed sol becomes poor.
In Japanese Patent Application Laid-Open No. Heisei 5(1993)-117404, it is disclosed that, even when the spray-drying is not conducted at a low temperature, a uniform polyvinyl chloride resin for preparation of paste can be obtained smoothly by removing grains having large sizes from a granular powder obtained after spray-drying using a pneumatic sieve of a fixed screen type. Although the granular powder obtained in accordance with this process exhibits excellent dispersion in plasticizers, the granular powder is polished by contacting the pneumatic sieve and fine particles having the size of the primary particles are formed. Therefore, the powder fluidity and the attachment of the powder to the inside of pipes for air transfer are not sufficiently improved.
In the production of granules of a polyvinyl chloride resin for preparation of paste, it has long been practiced that a screen having a specific opening or a plate having many holes of a specific size is fixed to a frame, the combination of the frame and the screen or the plate having many holes is vibrated and grains larger than the opening of the screen or the size of the holes are left on the screen or the plate. As the size of the powder decreases, the specific surface area of the powder increases and the relative surface energy becomes relatively greater. Therefore, in accordance with this method of classification, aggregation of particles and attachment of particles to the screen increase and efficiency of the classification markedly decreases as the size of the powder decreases. When a sieve allowing passage of powder by the weight of the powder itself is used, continuous classification of a powder having a diameter of particles of 200 &mgr;m or smaller is difficult. In the blower shifter process, which is a process for stable classification using a sieve having a relatively small opening such as that described above, powder is brought against the face of a screen at a high speed and passed through the screen by the kinetic energy of the powder. However, this process has drawbacks such as break down of the powder by the crushing against wires of the screen and attachment of powder having a low melting point by softening. Moreover, because a great amount of the air is required, an apparatus for separation of the powder which has passed the screen from the air stream becomes more expensive.
As the process for classification of fine powder which can overcome the drawbacks of the blower shifter process, a process using an ultrasonic vibration sieve has been developed. In the ultrasonic vibration sieve, an ultrasonically vibrated screen is attached to a frame vibrated at a lower frequency. The ultrasonic vibration sieve has been attracting attention as a remarkable sieve which can classify fine powder having diameters of 10 to 100 &mgr;m by a screen having an opening of about 30 to 200 &mgr;m. However, since the ultrasonic vibration is transferred from an oscillator to the screen directly or via a resonance plate, the area of the screen per one oscillator is naturally limited. To achieve uniform distribution of the ultrasonic vibration on the surface of the screen, a circular vibration sieve having a circular screen and a circular frame is used as the ultrasonic vibration sieve. The upper limit of the diameter of the screen per one oscillator is about 2,000 mm&phgr;. When the efficiency of the treatment per area of the screen is considered, an ultrasonic vibration sieve having a diameter of the screen of 1,000 mm&phgr; or smaller exhibits a great efficiency. This means that a great number of ultrasonic vibration sieves are required for classification of a great amount of powder. When powder for the treatment is distributed to a plurality of sieves and classified, each sieve must be equipped with an apparatus for quantitative supply of the powder because, when the speed of the classification is smaller than the speed of supplying the powder, a portion of the powder which should have passed the sieve overflows the sieve as a component above the screen. When fluctuations in the speed of the classification is taken into consideration, it is also necessary that the amount of supply of the powder be limited to an amount smaller than the maximum amount which the sieve can handle. As the result, the required number of the ultrasonic vibration sieves increases and cost of the apparatuses increases.
When the classification of powder is conducted continuously, fine particles in the powder for the classification are attached to the wire of the screen and the attached powder grows. The speed of the classification decreases with time and performing the continuous classification becomes difficult. In general, fine particles having diameters of 10 &mgr;m or smaller exhibit strong attachment (stickiness). In the classification by the ultrasonic vibration sieve which is particularly advantageo
Kose Tomoyuki
Makino Manabu
Nagatomo Yoshimitu
Nakamura Eitaro
Takizawa Tadashi
Armstrong Westerman & Hattori, LLP
Boykin Terressa M.
Shin Dai-ichi Vinyl Corporation
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