Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in reactor of specified material – or in reactor...
Reexamination Certificate
2001-10-17
2004-04-06
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in reactor of specified material, or in reactor...
C526S065000, C526S067000, C526S071000, C526S087000, C526S219600, C526S331000, C422S138000
Reexamination Certificate
active
06716934
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing an ethylene-vinyl acetate copolymer (EVA), which product provides a saponified ethylene-vinyl acetate copolymer (EVOH) having excellent melt-extrusion stability and melt-moldability and that undergoes very little discoloration or gelation.
2. Related Background Art
Conventionally, EVOH has been produced industrially by preparing EVA by copolymerizing ethylene and vinyl acetate using a polymerization initiator and saponifying the EVA. When produced using conventional methods, however, load fluctuation tends to occur when EVOH is supplied to an extruder and subjected to melt extrusion, thereby not necessarily providing sufficient melt-extrusion stability. Drawdown and neck-in fluctuations tend to occur when EVOH is subjected to melt extrusion, so reducing melt-moldability as well. Furthermore, when produced by conventional methods, molded EVOH tends to undergo discoloration and gelation.
To solve the foregoing problems, various methods have been proposed, for instance, in JP49(1964)-134763A, JP62(1987)-143954A, and JP1(1989)-135852A. However, considering the levels of performance that recent molding materials must provide, further improvements are required.
SUMMARY OF THE INVENTION
Specifically, it is desirable to achieve at least one of the following properties: (i) improved melt-extrusion stability, (ii) improved melt-moldability, (iii) reduced discoloration, and (iv) reduced gelation, in comparison with EVOH that is obtained by saponifying EVA produced in accordance with conventional methods.
Viewed from one aspect, the invention thus provides a method for producing an ethylene-vinyl acetate copolymer containing from 5 mole percent (mol %) to 60 mol % ethylene, said method comprising adding a vinyl acetate-containing material, ethylene, a polymerization initiator, a polymerization solvent comprising an aliphatic alcohol having not more than four carbon atoms and a saturated acetic ester to a polymerization vessel and copolymerizing said vinyl acetate and said ethylene at a temperature of from 30° C. to 150° C., wherein the concentration of acetaldehyde added to the polymerization vessel is not more than 200 ppm with respect to the vinyl acetate and the concentration of said saturated acetic ester added to the polymerization vessel is from 10 ppm to 1500 ppm with respect to the vinyl acetate.
According to the present invention, an ethylene-vinyl acetate copolymer containing 5 mol % to 60 mol % of ethylene is produced by copolymerizing ethylene with vinyl acetate in a polymerization solvent. The copolymerization utilizes a polymerization initiator. Herein, an aliphatic alcohol having not more than four carbon atoms is used as the polymerization solvent. In a vinyl acetate-containing material for supplying the vinyl acetate, a content of acetaldehyde with respect to vinyl acetate is preferably not more than 200 ppm, and a content of saturated acetic ester with respect to vinyl acetate is 10 ppm to 1500 ppm. It should be noted, however, that the source of the trace component such as saturated acetic ester is not limited to vinyl acetate. Preferably, the present invention achieves at least one of the foregoing (i), (ii), (iii), and (iv), optimally all of (i) to (iv).
Viewed from a further aspect, the present invention provides EVOH obtained by saponifying EVA obtained by the method herein described. Molded products containing the EVOH are also considered to form a part of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The following description will depict a preferred embodiment of the present invention.
In the present invention, an ethylene content in EVA is 5 mol % to 60 mol %. The ethylene content in EVA is preferably not less than 7 mol %, more preferably not less than 10 mol %, and particularly preferably not less than 20 mol %. Further, it is preferably not more than 58 mol %, more preferably not more than 55 mol %, and further preferably not more than 53 mol %. In the case where the ethylene content is lower than 5 mol %, the effect of reducing discoloration and gelation of EVOH obtained by saponification of the resulting EVA decreases. In the case where the ethylene content exceeds 60 mol %, the melt-moldability of EVOH obtained by saponification is impaired.
In the present invention, an aliphatic alcohol having not more than four carbon atoms is used as a polymerization solvent. If an aliphatic alcohol having five or more carbon atoms or an aromatic alcohol is used, the desired effects of the invention are reduced. Examples of aliphatic alcohols with not more than four carbon atoms include methanol, ethanol, propanol, and butanol. To achieve the effects of the present invention to a satisfactory extent, an aliphatic alcohol with not more than three carbon atoms is preferable, and an aliphatic alcohol with one carbon atom, that is, methanol, is more preferable.
Acetaldehyde is present as an impurity in the vinyl acetate that is supplied to the polymerization reaction. In the present invention, in the vinyl acetate-containing material, the content of acetaldehyde with respect to vinyl acetate is set to be not more than 200 ppm. The content of acetaldehyde is preferably not more than 150 ppm, more preferably not more than 100 ppm, and further preferably not more than 50 ppm. If the content of acetaldehyde with respect to vinyl acetate exceeds 200 ppm, the melt-extrusion stability and the melt-moldability of the obtained EVOH are impaired, and significant discoloration and gelation occur when molded products are produced with the obtained EVOH. It should be noted that vinyl acetate itself or a solution containing vinyl acetate may be used as the vinyl acetate-containing material. As the vinyl acetate-containing solution, an aliphatic alcohol solution in which the alcohol has not more than four carbon atoms is preferable. An alcohol solution of the same type as the polymerization solvent, particularly a methanol solution, is more preferable.
The mechanisms causing adverse effects due to the presence of acetaldehyde have not yet been proved. At the present time, it is considered that acetaldehyde functions as a chain transfer agent during polymerization, and affects the degree of polymerization, molecular weight distribution, branching, etc. of the EVA obtained by copolymerization, thereby adversely affecting the melt-extrusion stability and the melt-moldability of the EVOH. The discoloration and the gelation are considered to stem from condensation of acetaldehyde during polymerization of ethylene and vinyl acetate. The condensation product of acetaldehyde tends to cause discoloration and gelation and cannot be removed even in a later polymer-purifying process.
Recently, it was discovered that an excellent effect was achieved by reducing the content of acetaldehyde present as impurities in the vinyl acetate when ethylene and vinyl acetate were polymerized. Acetaldehyde was reduced by high precision distillation or with an ion-exchange resin. This, however, did not suffice to sufficiently improve the desired properties of the saponified product mentioned above. The inventor of the present invention further studied and consequently discovered that a conventional polymerizing system inherently tends to produce acetaldehyde.
It is deemed that vinyl acetate causes transesterification with a lower aliphatic alcohol that is used as a polymerization solvent, thereby producing acetaldehyde and an acetic ester as expressed by a reaction formula 1 below:
where R represents a lower alkyl group.
In other words, it is presumed that even if acetaldehyde in vinyl acetate is reduced before polymerization, a certain amount of heat could cause the foregoing transesterification, which produces acetaldehyde anew. Since this reaction is an equilibrium reaction, the addition of an acetic ester or the removal of lower aliphatic alcohol serves to prevent the production of acetaldehyde to some extent.
The inventor of the present invention studied both of these possibilities, and discover
Kuraray Co. Ltd.
Mulcahy Peter D.
Sastri Satya
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