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
2001-06-28
2003-11-25
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C210S348000, C210S360100, C210S780000, C210S781000, C525S057000, C525S060000, C525S062000, C525S330600
Reexamination Certificate
active
06653400
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing a saponified ethylene-vinyl acetate copolymer (hereinafter referred to as “EVOH”).
BACKGROUND OF THE INVENTION
An EVOH is 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. fish eyes, are significant problems that need to be addressed. Thus, traditionally, to inhibit the above-mentioned visible imperfections, various improvements in the process of producing an EVOH have been proposed.
For example, JP 1(1989)-52404B discloses a method including a step of copolymerizing ethylene and vinyl acetate, in which the ratio of the average residence time within a polymerization vessel with respect to the half value period of a polymerization initiator is not less than a fixed coefficient, which is determined from the polymerization degree of vinyl acetate. This method can be realized by using a polymerization initiator having a short half-life.
Furthermore, for example, JP 9(1997)-71620A discloses a method in which a conjugated polyene compound is added after copolymerizing ethylene and vinyl acetate, and further unreacted materials are removed. Also, JP 10(1998)-158325A discloses a method in which a thiol compound is added after copolymerizing ethylene and vinyl acetate, and further unreacted materials are removed.
DISCLOSURE OF THE INVENTION
It is therefore desirable to add further improvements to the methods for producing EVOH, thereby inhibiting visible imperfections in molded EVOH products more effectively.
The inventors have found that appearances of an EVOH molded product can be improved by preparing a mixture solution (mixed solvent) of methanol and water after saponification, and removing insoluble materials from this solution.
That is, the present invention provides a method for producing an EVOH, which method includes: copolymerizing ethylene and vinyl acetate in a methanol solution to form an ethylene-vinyl acetate copolymer; saponifying the copolymer in a methanol solution; adding water to the methanol solution containing the copolymer saponified, thereby to prepare a solution; removing insoluble materials from the solution; and obtaining the EVOH from the mixture solution from which the insoluble materials have been removed.
In the method of the present invention, foreign materials included, and insoluble components formed as by-products in various steps such as copolymerizing ethylene and vinyl acetate, removing (recovering) unreacted materials, saponifying the copolymer and the like are removed after saponification. Thus, the concentration of agents causing visible imperfections in a molded product can be reduced, regardless of which steps caused their introduction.
In the method of the present invention, it is preferable that the insoluble materials are removed from the mixture solution in which the ratio of methanol and water is in the range of 5:5 to 9:1 by weight. This makes it easy to remove the insoluble materials that will cause defective appearances while the EVOH is dissolved.
The method of the present invention is suitable for obtaining an EVOH having an ethylene content of at least 20 mol % but not more than 70 mol % and a saponification degree of at least 95 mol %.
In the method of the present invention, it is preferable that the insoluble materials are removed from the mixture solution in which the concentration of the EVOH is at least 30 wt % but not more than 60 wt %. When the concentration is too low, it may be disadvantageous in terms of cost. Conversely, when the concentration is too high, the viscosity of the mixture solution may increase, resulting in difficult filtration.
Furthermore, it is preferable that the insoluble materials are removed from the mixture solution in which the temperature of the mixture solution is at least 55° C. but not higher than 80° C. When the temperature is too low, the viscosity of the mixture solution may increase, resulting in difficult filtration. Conversely, when the temperature is too high, the mixture solution may foam or discolor.
In the above method, it is preferable that at least a part of the insoluble materials is a polyvinyl alcohol having a saponification degree of at least 95 mol % and/or an EVOH having an ethylene content of not more than 10 mol % and a saponification degree of at least 95 mol %. A polyvinyl alcohol (PVA) as a homopolymer of vinyl acetate, or an EVOH containing ethylene but having a low ethylene content such as the above and having a high saponification degree (hereinafter may be referred to as “a low ethylene EVOH”) causes a molded product to form as a gel. Thus, by removing PVA and/or a low ethylene EVOH from the mixture solution, the appearance characteristics of an intended EVOH molded product can be improved.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following, a preferred embodiment of the present invention is described.
In the present invention, a mixture solution is prepared by adding water to a methanol solution of an EVOH obtained by saponifying an ethylene-vinyl acetate copolymer. The form of the water added is not critical, and for example, it may be added in the form of water vapor. The insoluble materials in the mixture solution thus prepared are removed by solid-liquid separation such as filtration or centrifugal separation.
Particularly, PVA and/or low ethylene EVOH contained in the methanol solution after saponification should be removed to prevent deterioration of the appearance of an EVOH molded product. Therefore, the mixture ratio of water to methanol in the mixture solution may be in a range where the solubility of the intended EVOH is high and the solubility of PVA or a low ethylene EVOH is low.
The above-mentioned range of ratios depends on the ethylene content of the intended EVOH, copolymer concentration, solution temperature, etc. However, when producing an EVOH having an ethylene content of 20 to 70 mol % and a saponification degree of at least 95 mol %, approximately it is preferable that the weight ratio of methanol to water is from 5:5 to 9:1. When the amount of water added is too large, PVA and/or low ethylene EVOH are dissolved, and cannot be removed sufficiently as insoluble materials.
It is preferable that as the ethylene content in the EVOH increases, the content of methanol in the mixture solution also increases a little. For example, when the saponification degree is at least 95 mol %, the weight ratio of methanol to water when producing an EVOH having an ethylene content of 20 to 30 mol % is preferably from 5:5 to 7:3; the weight ratio of methanol to water when producing an EVOH having an ethylene content of 30 to 55 mol % is preferably from 6:4 to 8:2; and the weight ratio of methanol to water when producing an EVOH having an ethylene content of 55 to 70 mol % is preferably from 7:3 to 9:1.
It is generally preferable that the concentration of EVOH in the mixture solution prepared to remove insoluble materials is at least 30 wt % but not more than 60 wt %. Furthermore, it is generally preferable that the temperature of this mixture solution is at least 55° C. but not more than 80° C. There may be more preferable ranges for these concentration and temperature depending on the ethylene content in the intended EVOH etc.
The solid-liquid separation when removing insoluble materials from the mixture solution may be carried out using various types of filter or centrifugal separator. Insoluble materials can be removed efficiently, for example, when using at least one solid-liquid separation method selected from filtration with a plate filter, filtration with a bucket filter, filtration with a candle filter, and separation with a centrifugal separator. But when solid-liquid separation is carried out for a mixture solution containing an EVOH, filtration methods in which even a solution with high viscosity can be separated
Kawahara Takaharu
Tuboi Toshio
Kuraray Co. Ltd.
Reddick Judy M.
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