Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-06-05
2004-01-27
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S331000, C526S348800, C525S330600
Reexamination Certificate
active
06683148
ABSTRACT:
FIELD OF THE INENTION
The present invention relates to methods for producing an ethylene-vinyl acetate copolymer and a saponified product thereof.
BACKGROUND OF THE INVENTION
Saponified ethylene-vinyl acetate copolymers (hereinafter referred to as “EVOH”) are excellent in melt moldability, gas barrier properties, water resistance, oil resistance, antistatic property and mechanical strength, and are used as various types of packages in the form of a film, a sheet, a container or the like. In these packages, visible imperfections generated at the time of molding, e.g. coloration, fish eye, rough surface, etc., are significant problems that need to be addressed. Therefore, several improvements have been proposed in the process of producing EVOH (e.g. JP 43(1968)-14958B, JP 47(1972)-38558B, JP 61(1986)-197603A, JP 9(1997)-71620A and JP 11(1999)-140136A).
On the other hand, to reduce production cost, it is desirable to recover and reuse unreacted materials and solvents used in the process of producing EVOH. Particularly, in the production of EVOH on a commercial scale, it is highly desirable to reuse alcohol-based solvents such as methanol. However, recovered solvents contain impurities formed as by-products in the steps of copolymerization, saponification and purification. Thus, when copolymerization is carried out using recovered solvents, the above-mentioned imperfections are generated at the time of molding the EVOH, reducing the commercial value of the molded product as a packaging material.
DISCLOSURE OF THE INVENTION
Thus, there is a need for alternative methods for producing an ethylene-vinyl acetate copolymer and a saponified product thereof in which recovered solvents can be used without generating visible imperfections at the time of molding.
Surprisingly, the inventor has found that the presence of an aldehyde compound in the recovered solvents induces the above-mentioned imperfections.
Accordingly, viewed from one aspect the invention provides a method for producing an ethylene-vinyl acetate copolymer which comprises copolymerizing ethylene and vinyl acetate in an alcohol-based solvent, wherein at least a portion of the alcohol-based solvent is prior treated whereby to reduce the amount of any aldehyde compound.
In a further aspect the present invention provides a method for producing an ethylene-vinyl acetate copolymer, which includes copolymerizing ethylene and vinyl acetate in an alcohol-based solvent, wherein at least a portion of the alcohol-based solvent is treated in advance by making contact with an acid, preferably a solid acid. According to this method, the concentration of an aldehyde compound in the solvent can be decreased by making contact with the acid.
In the method of the invention, it is preferable that the alcohol-based solvent to be treated is recovered by separating a solvent used in a copolymerization of ethylene and vinyl acetate from products and unreacted raw materials. Thus, according to the present invention, a recovered solvent can be reused while preventing a reduction in the qualities of the final EVOH molded product.
Furthermore, in the method of the present invention, for example, an alcohol-based solvent having a concentration of an aldehyde compound of at least 1 ppm but not more than 500 ppm (ppm is based on weight throughout, unless otherwise stated) is treated with an acid. It is preferable that the alcohol-based solvent after treatment with an acid has a concentration of an aldehyde compound of not more than 100 ppm. To sufficiently inhibit generation of visible imperfections at the time of molding, it is preferable that the concentration of an aldehyde compound in the treated solvent is not more than 30 ppm, more preferably not more than 5 ppm.
In a yet further aspect the present invention provides a method for producing an ethylene-vinyl acetate copolymer, which includes copolymerizing ethylene and vinyl acetate in an alcohol-based solvent, wherein the alcohol-based solvent contains an acetal compound. In this method, a solvent in which an aldehyde compound is acetalized is used.
In this method, it is preferable that the alcohol-based solvent contains the acetal compound in a concentration of at least 0.1 ppm but not more than 10,000 ppm.
The present invention also provides a method for producing EVOH. This method includes saponifying an ethylene-vinyl acetate copolymer obtained by any of the above-described methods.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a preferred embodiment of the present invention, an aldehyde compound contained in a recovered alcohol-based solvent is acetalized by making contact with a solid acid.
As the solid acid, any acid which is substantially insoluble in the alcohol-based solvent and which has an acid center on the surface and can act as an acetalization catalyst may be used without particular limitation. As the solid acid, for example, metal oxides (e.g. Al
2
O
3
, V
2
O
5
), sulfates (e.g. NiSO
4
, CuSO
4
), phosphates (e.g. AlPO
4
), chlorides (e.g. AlCl
3
, CuCl
3
) and the like may be used. Typical solid acid catalysts such as zeolite catalyst, silica-alumina catalyst and the like also may be used. Minerals such as montmorillonite also may be used. However, cation exchange resins are particularly suitable for carrying out the present invention on commercial scale.
Cation exchange resins function as insoluble solid acids, and various products suitable for treating a large amount of liquid and repeated use are commercially available. As the cation exchange resins, weak acid cation exchange resins may be used, but strong acid cation exchange resins are preferred.
The concentration of an aldehyde compound in an alcohol-based solvent used for copolymerization of ethylene and vinyl acetate has a great influence on the appearance characteristics of an EVOH molded product. When the concentration of an aldehyde compound present in the solvent is not more than 100 ppm, the appearance characteristics of the molded product is greatly improved.
Although the concentration of an aldehyde compound in the recovered alcohol-based solvent depends on the recovery method and other conditions for the recovery, it is approximately in the range of 150 to 500 ppm. By treating the recovered alcohol-based solvent in advance to such an extent that the concentration of an aldehyde compound is not more than 100 ppm, preferably not more than 30 ppm, more preferably not more than 5 ppm, copolymerization may be carried out using only the recovered solvent. Furthermore, when the concentration of an aldehyde compound in the recovered alcohol-based solvent is from 1 to 150 ppm, it is also preferable that the alcohol-based solvent is treated in advance such that the concentration of an aldehyde compound is decreased further. As described later, in the process of producing EVOH, an alcohol-based solvent such as methanol is also used to recover unreacted vinyl acetate. When this alcohol-based solvent for the recovery of vinyl acetate moreover is recovered together with the alcohol-based solvent from the copolymerization, and the entire recovered solvents are treated to decrease the content of aldehyde, solvents used at all stages of the copolymerization process (in copolymerization and separation stages) may be supplemented by the recovered and treated alcohol-based solvent.
As described above, in one embodiment of the present invention, acetalization with a solid acid is carried out as a treatment for decreasing the content of an aldehyde compound. However, the method of the present invention can also be effected by acetalizing an aldehyde compound using any means other than contacting with a solid acid. As a method for acetalization, for example, contacting with a liquid acid, e.g. an aqueous solution of an inorganic acid such as sulfuric acid, nitric acid or phosphoric acid; a carboxylic acid such as formic acid, acetic acid or oxalic acid; or a Lewis acid such as zinc chloride or aluminum chloride, may be employed.
An example of an aldehyde that is a problem in the production of an ethylene-vinyl acetate copolymer is acetalde
Kuraray Co. Ltd.
Oblon, Spivak, McClellamd, Maier & Neustadt, P.C.
Pezzuto Helen L.
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