Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-08-06
2001-02-06
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
active
06184300
ABSTRACT:
TECHNICAL FIELD
This invention relates to bowl-shaped polymer particles; an aqueous dispersion of polymer particles capable of, when it is dried, forming the bowl-shaped polymer particles; and processes for producing the bowl-shaped polymer particles and the aqueous polymer particle dispersion.
The bowl-shaped polymer particles of the present invention are used as a heat-insulating material, an organic pigment and an opacifying agent, and as ingredients to be incorporated in a material for forming a heat-insulating intermediate layer of a thermal recording material, an aqueous coating composition, a coating color for paper, and others.
BACKGROUND ART
It has been hitherto known that hollow polymer particles cause irregular reflection of irradiated light due to the presence of a void in the interior thereof, and thus, when the hollow polymer particles are coated on a material, whiteness of the material is enhanced and the material is made opaque.
It is considered that, as the diameter of the hollow polymer particles becomes large, the opacifying effect becomes prominent.
Polymer particles having a special shape, other than the hollow polymer particles, also are known as exhibiting an opacifying effect. For example, in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) No. H5-222108, polymer particles having at least one through-hole extending from the particle surface to the interior thereof, which is formed when the particles are dried, and a process for producing the polymers are described. It is described therein that the polymer particles are usable as a coating composition and an organic pigment for paper coating color. However, the particle size of the polymer particles, produced by this process, are small, i.e., at most about 0.2 &mgr;m, and thus, the opacifying effect of the pigment is poor.
A thermal recording material has been proposed in JP-A H2-164580, which has a heat-insulating intermediate layer between a support and a thermal recording layer, wherein the intermediate layer contains porous and non-spherical hollow polymer particles having undulations on the surface which are formed by allowing hollow polymer particles to shrink. These non-spherical hollow polymer particles exhibit good thermal response, but have poor adhesion between the thermal recording layer and the support.
Another thermal recording material has been proposed in JP-A H5-573, which has a heat-insulating intermediate layer between a support and a thermal recording layer, wherein the intermediate layer contains spherical or ellipsoidal plastic hollow particles. The plastic hollow particles are fine foamed plastic hollow particles having an average particle diameter of 2.0 to 20 &mgr;m, preferably 3 to 10 &mgr;m. It is however difficult to prepare fine foamed plastic hollow particles which are uniform and have a narrow particle size distribution.
A further thermal recording material has been proposed in JP-A H2-57382, which has a heat-insulating intermediate layer between a support and a thermal recording layer, wherein the intermediate layer contains fine hollow polymer particles having a particle diameter of not larger than 5 &mgr;m, preferably 0.1 to 3 &mgr;m. These hollow polymer particles exhibit a poor effect for enhancement of thermal response, and further have problems of causing sticking and deposition of tailings on a thermal head.
Emulsified vinyl polymer particles have been proposed in JP-A-H2-14222, which have a flat shape, and at least one major surface of which is concave. It is described therein that these polymer particles are used as an additive for a paint, a paper coating color, or a coating color for information recording paper and other materials. It is however indispensable for the preparation of the polymer particles to conduct emulsion polymerization using a non-aqueous organic solvent such as an aliphatic hydrocarbon, and, after the completion of polymerization, to remove the organic solvent from the reaction system. Due to the flat shape, the polymer particles exhibit poor opacifying effect, and, where they are used for a thermal recording paper, they exhibit a poor heat-insulating effect.
DISCLOSURE OF INVENTION
An object of the present invention is to provide polymer particles having a special shape, which exhibit excellent heat-insulating effect, whiteness-enhancing effect, opacifying effect, and other effects.
Another object of the present invention is to provide a process for stably producing the polymer particles of a special shape.
Still another object of the present invention is to provide a thermal recording material, which exhibits excellent thermal response, high dynamic sensitivity, and good reproducibility of printed letter images, i.e., dots, and in which problems of causing sticking and deposition of tailings on a thermal head are minimized.
In one aspect of the present invention, there is provided bowl-shaped polymer particles characterized by having a bowl-like shape as is formed by partly cutting along a plane each spherical hollow polymer particle having a void in the inside, and in which the cross-section perpendicular to the cut plane and passing through the center of the particle has a shape as is formed by partly cutting a double circle along a straight line, and the lengths H of the perpendiculars from the outer arc of the double circle in the cross-section to the straight line have a maximum value H
max
equal to or larger than the radius of the outer circle of the double circle.
In another aspect of the present invention, there is provided an aqueous dispersion of polymer particles, which is capable of, when the aqueous dispersion is dried, producing the above-mentioned bowl-shaped polymer particles.
In still another aspect of the present invention, there is provided a process for producing bowl-shaped polymer particles, characterized by comprising the steps of:
(1) copolymerizing a monomer mixture comprising 30 to 65% by weight of an acidic group-containing monomer and 70 to 35% by weight of a monomer copolymerizable therewith to prepare a nucleus polymer particle;
(2) copolymerizing a monomer mixture comprising 10 to 35% by weight of an acidic group-containing monomer and 90 to 65% by weight of a monomer copolymerizable therewith in the presence of the nuclear polymer particle to prepare a core polymer particle substantially surrounding the nucleus polymer particle, wherein the content of the acidic group-containing monomer units in the core polymer particle is equal to or smaller than the content of the acid group-containing monomer units in the nucleus polymer particle;
(3) copolymerizing a monomer mixture comprising 1 to 12% by weight of an acidic group-containing monomer and 99 to 88% by weight of a monomer copolymerizable therewith in the presence of the nucleus/core polymer particle to form at least one intermediate polymer layer which substantially surrounds the nucleus/core polymer particle;
(4) polymerizing an aromatic vinyl monomer alone or copolymerizing a monomer mixture comprising up to 0.5% by weight of an acidic group-containing monomer and at least 99.5% by weight of an aromatic vinyl monomer copolymerizable therewith in the presence of the polymer particle which is prepared in step (3) and comprising the intermediate polymer layer substantially surrounding the nucleus/core polymer particle to prepare an outer polymer layer which substantially surrounds the polymer particle prepared in step (3);
(5) adding a base to an aqueous dispersion of the polymer particle which is prepared in step (4) and has a multi-layer structure comprising at least four layers, to adjust the pH value of the aqueous polymer dispersion to at least 7;
(6) adding an aromatic vinyl monomer or a monomer mixture comprising at least 90% by weight of an aromatic vinyl monomer and not larger than 10% by weight of a monomer copolymerizable therewith, to polymerize the aromatic vinyl monomer or copolymerize the monomer mixture in the presence of the polymer particle with the multi-layer structure, prepared in step (5), to form an outermos
Ishizu Osamu
Shimokawa Yoshiharu
Armstrong, Westerman Hattori, McLeland & Naughton
Boykin Terressa M.
Nippon Zeon Co. Ltd.
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