Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-10-20
2001-01-09
Yoon, Tae (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C528S491000, C528S501000
Reexamination Certificate
active
06172158
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a vinylidene chloride-based latex capable of forming a coating film which not only has satisfactory adhesion and satisfactory printability but has excellent gas (especially oxygen) and water vapor barrier properties and excellent resistance to hot water treatment. This invention further relates to a process for producing the latex.
BACKGROUND ART
Vinylidene chloride-based latexes have an important use in that the latexes are applied to films of plastics, e.g., polypropylene, polyesters, and nylons, directly or after application of an anchor coat to thereby greatly improve the gas and water vapor barrier properties of the plastic films.
These latexes are required to form a coating film having satisfactory adhesion to base films. Although the coating film formed must be satisfactory in other properties, including printability and suitability for laminating to other films, the most strongly required property of the coating film formed is excellent gas and water vapor barrier properties.
Such a coating film is used mainly for food packaging. Depending on the kinds of foods, the packages should be immersed in hot water to sterilize the contents after packaging. In this case, the hot water treatment may impair the transparency of the coating film to make the same milky (hereinafter referred to as “boil blushing”) or reduce the gas barrier properties of the coating film. There is hence a desire for a latex capable of forming a coating film which suffers from neither boil blushing nor a decrease in gas barrier properties through hot water treatment and also has excellent hot-water resistance.
For example, an examined Japanese patent publication 48-10941 discloses a technique for obtaining a latex which forms a coating film excellent in both gas barrier properties after hot water treatment and also in other properties. The technique comprises dialyzing a vinylidene chloride-based latex with a diaphragm to reduce the total content of inorganic salts to 0.5% to 0.1% to thereby produce the desired latex.
However, the example given in the above reference is insufficient in improving coating films in boil blushing.
An important requirement for food-packaging materials is to maintain the aesthetic appearance of articles. In particular, since boil blushing of a coating film significantly impairs the commercial value of the article, it is essential in the field of food packaging to use a technique for preventing the blushing. In addition, the performance required in the food-packaging market has become increasingly high in recent years, and a further improvement is necessary for markedly improving resistance to hot water.
An object of the present invention is to provide a vinylidene chloride-based latex which gives a coating film having satisfactory resistance to hot water without the disadvantages of prior art latexes, and to provide a process for producing the vinylidene chloride-based latex. More particularly, the object of the present invention is to provide a vinylidene chloride-based latex capable of providing a coating film which undergoes neither boil blushing nor a decrease in gas barrier properties through hot water treatment, and to provide a simple and efficient process for industrially producing the latex.
DISCLOSURE OF THE INVENTION
With respect to the vinylidene chloride-based latex, the copolymer produced generates hydrochloric acid during the polymerization. The latex obtained usually contains chlorine ions in an amount of about 1,000 ppm of the copolymer.
The present inventors made intensive investigations in order to eliminate the problem of the prior art technique. As a result, it was surprisingly found that a coating film formed by applying the latex having significantly improved resistance to hot water, in particular resistance to boil blushing, can be obtained by dialyzing a latex to reduce the content of chlorine ions contained therein to or below a given amount. The present invention has been completed based on this finding.
The present invention provides a vinylidene chloride-based latex which has a chlorine ion content reduced to or below 500 ppm, preferably 200 ppm of all solid matter in the latex by weight, as well as a process for producing the latex.
The present invention will be explained below in detail.
The vinylidene chloride-based latex used for the present invention is a copolymer latex obtained by the emulsion polymerization of a monomer mixture containing vinylidene chloride as the main monomer. The copolymer has a vinylidene chloride content of 50% by weight or higher. For applications where an especially high degree of gas and water vapor barrier properties are required, the vinylidene chloride content in the latex is preferably from 86 to 94% by weight.
If the vinylidene chloride content in the latex is lower than 86% by weight, the copolymer has reduced crystallinity and the latex forms a coating film having impaired gas and water vapor barrier properties. If the content thereof exceeds 94% by weight, the copolymer crystallizes immediately after polymerization, and the latex particles become too rigid resulting in insufficient film-forming properties.
There are no particular limitations on the kinds of emulsifying agents, polymerization initiators, surfactants, and the like that can be used in producing the vinylidene chloride-based latex through emulsion polymerization.
Comonomer ingredients usable for producing the vinylidene chloride-based latex used for the present invention are monomers copolymerizable with vinylidene chloride. Examples thereof include:
(a) (meth)acrylic esters such as alkyl or cycloalkyl (meth)acrylates in which the alkyl or cycloalkyl has 1 to 18 carbon atoms (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, glycidyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, and stearyl (meth)acrylate) and alkoxyalkyl (meth)acrylates in which the alkoxyalkyl has 2 to 18 carbon atoms (e.g., methoxybutyl (meth)acrylate, methoxyethyl (meth)acrylate, and ethoxybutyl (meth)acrylate);
(b) ethylenic &agr;,&bgr;-unsaturated carboxylic acids having carboxyl group(s), such as (meth)acrylic acid, crotonic acid, itaconic acid, itaconic anhydride, maleic acid, maleic anhydride, fumaric acid, and citraconic acid, and salts of these acids;
(c) unsaturated amide compounds such as (meth)acrylamide and diacetone acrylamide;
(d) monomers containing a nitrile group, such as acrylonitrile and methacrylonitrile;
(e) hydroxyalkyl (meth)acrylates in which the hydroxyalkyl has 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate, polyoxyethylene monoacrylate, polyoxyethylene monomethacrylate, and hydroxyl-containing monomers such as N-methylol(meth)acrylamide and allyl alcohol; and
(f) other polymerizable unsaturated monomers such as styrene, &agr;-methylstyrene, vinyl chloride, butadiene, vinyltoluene, and vinyl acetate.
Methods for removing impurities including inorganic salts from such a vinylidene chloride-based latex include the adsorption method using an ion-exchange resin, the membrane filtration method using a microfiltration membrane or ultrafiltration membrane, and the dialysis method using a semipermeable membrane.
The method of adsorption with an ion-exchange resin was found to be difficult to use industrially because vinylidene chloride-based latex particles are adsorbed onto the anion-exchange resin due to the negative charge of the latex particles themselves, and because neutral molecules cannot be removed, and regeneration of the ion-exchange resin requires a long period of time.
The membrane filtration method was found to be extremely difficult to industrially apply to a vinylidene chloride-based latex because it has a disadvantage that the latex is apt to form a deposit layer on the membrane surface du
Sakai Hideki
Yamawaki Yukio
Yokota Masahisa
Asahi Kasei Kogyo Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Yoon Tae
LandOfFree
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