Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Removing and recycling removed material from an ongoing...
Reexamination Certificate
2002-02-07
2003-06-10
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Removing and recycling removed material from an ongoing...
C526S089000, C526S072000, C526S319000, C526S320000, C526S330000
Reexamination Certificate
active
06576720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for preparing polyvinyl alcohol by means of catalytic transesterification of an alcoholic polyvinyl ester solution and subsequent isolation of the polyvinyl alcohol formed.
2. Background Art
The preparation of polyvinyl alcohol (PVAL) by transesterifying polyvinyl esters obtained by free-radical polymerization of vinyl esters has long been known. Vinyl esters that can be used include esters of aliphatic carboxylic acids, such as vinyl acetate or vinyl propionate, for example. The transesterification usually takes place in the presence of monohydric aliphatic alcohols such as methanol or ethanol. The reaction may take place with either basic or acidic catalysis. The polyvinyl alcohols obtained are described primarily by their degree of hydrolysis and their viscosity in a 4% by weight aqueous solution.
Another characteristic frequently encountered instead of the degree of hydrolysis is the saponification number, which indicates the amount of KOH in milligrams per gram of PVAL that is needed for complete cleavage of all of the remaining ester groups. The saponification number indicates only the mean value of the KOH consumption for complete ester cleavage. For identical measured saponification numbers, therefore, it is possible for distinctly different breadths of the saponification number distribution to occur. Both the breadth of the saponification number distribution and blockiness have a great influence on the quality of the PVAL, especially for its use as a protective colloid in emulsion and suspension polymerization.
For identical saponification number and viscosity, there are nevertheless distinct differences in the makeup of polyvinyl alcohols depending on the preparation process. As an initial consideration, the esters groups remaining may be distributed differently, for example concentrated in relatively large blocks or distributed randomly. When basic catalysts are used, the polyvinyl alcohols obtained tend to be blocky; when using acidic catalysts, polyvinyl alcohols with random distribution of remaining ester groups are more likely to be obtained. Additions of other solvents, such as water, also influence the distribution of the ester groups in the polyvinyl alcohol.
One established process for preparing polyvinyl alcohol is known as the belt process, and is described, for example, in U.S. Pat. No. 3,278,505 and DE-A 2251603. In this process, aqueous alkali is mixed rapidly and thoroughly with generally methanolic polyvinyl acetate solution and the mixture is applied to a continuous belt. The mixture solidifies to a gel, which at the end of the belt is fractionated and cut. The resulting granules are generally neutralized with acetic acid and washed with methanol. This continuous process is especially suitable for preparing polyvinyl alcohol in large amounts, an advantage being very economical preparation of large amounts of polyvinyl alcohol with uniform product quality. A disadvantage is that the granulated gel is difficult to neutralize; associated with this difficulty is an undesirable increase in the breadth of the saponification number distribution. The process is economical only for the production of large amounts of PVAL. It is therefore prohibitive to employ for the introduction of new and innovative products in quantities which, at commencement of production, are small. Further disadvantages include the high acquisition costs and the large amount of space occupied by the plant.
A process very similar to the belt process is the extruder process, which is described, for example, in EP-B 54716 and EP-A 942008, in which the belt is replaced by an appropriate extruder. In this process, the gel is comminuted in the extruder during the reaction. In comparison to the belt process, the extruder process can be used to process more highly concentrated polyvinyl acetate solutions. A disadvantage are the acquisition costs, which are even higher than those for the belt process, at identical capacities.
Another continuous process which has been described is the suspension transesterification of polyvinyl acetate, U.S. Pat. No. 3,487,060, for example. Here, the polyvinyl acetate solution and the catalyst solution are added continuously to an agitated alcoholysis mixture, and a slurry of polyvinyl alcohol in methanol and methyl acetate is removed continuously from this alcoholysis mixture. By means of an appropriate process regime, gel formation can be substantially prevented. In comparison to the other continuous processes, the acquisition costs are relatively low. However, it is difficult to maintain a specific degree of hydrolysis, and thus product uniformity suffers as a result.
Among the principal established noncontinuous processes is that of transesterification in a kneading apparatus, which is described, for example, in DE-A 3000750. A highly concentrated, generally methanolic, polyvinyl acetate solution is mixed with the transesterification catalyst. The resulting gel is progressively comminuted during the reaction. Following termination of the reaction with acid, methanol and methyl acetate formed in the kneading apparatus are separated by distillation. off. The kneading apparatus is very well suited to producing small quantities of polyvinyl alcohol specialties. Large amounts of a product are difficult to produce at favorable cost, however. Since the kneading apparatus is a poor mixer, both for the mixing in of the catalyst and for the addition of acid upon neutralization, the resultant polyvinyl alcohols have very broad saponification number distributions.
The transesterification of polyvinyl acetate to polyvinyl alcohol may also take place in a standard stirred tank, as is described, for example, in DE-A 2304684. Dilute, generally methanolic, polyvinyl acetate solutions are mixed with the catalyst. By progressive stirring, the gel is comminuted and a very fine suspension of polyvinyl alcohol in methanol/methyl acetate is obtained. As a result of the rapid distribution of the catalyst in the alcoholic solution and the uniform termination of the reaction with an acid in the fine suspension, it is possible to obtain polyvinyl alcohols of very good quality with a narrow saponification number distribution. Besides the solid polyvinyl alcohol obtained as a result of separating the solvent mixture, it is possible to distill off methyl acetate/methanol and at the same time to add water, in order to obtain an aqueous polyvinyl alcohol solution. A disadvantage is that owing to the poor filterability of the fine suspension, the preparation of solid polyvinyl alcohol by this process is not economical. Furthermore, large amounts of solvent must be distilled from the stirred tank and subsequently worked up by distillation. The twofold distillation represents a time-consuming and energy-intensive process. For separating methyl acetate and methanol, moreover, a plurality of distillation columns are required.
From DD-A 251683 it is known that the production of the aqueous solution may be accelerated by isolating the polyvinyl alcohol from the reaction mixture and introducing it, at a residual moisture content of more than 50% by weight, into a water-filled dissolution tank. If vacuum is applied, superheated steam introduced, and at the same time the dissolution tank is heated, methanol, methyl acetate, and water are distilled away and the polyvinyl alcohol goes into solution. This still leaves the problem of the poor filterability of the polyvinyl alcohol suspension. Another disadvantage of this process is that there are large quantities of filtrate to be reprocessed.
It is known that the filterability of the resulting polyvinyl alcohol suspension may be improved significantly by the addition of aliphatic or cycloaliphatic hydrocarbons. For example, DD-A 238054 describes producing a heterogenous methanolic polyvinyl acetate solution by adding 20-50% of hydrocarbons with a chain length of from 5 to 10 carbon atoms. Alkali-catalyzed transesterification conducted in this mixture leads to fine
Ball Peter
Bauer Werner
Tschirner Peter
Brooks & Kushman P.C.
Cheung William K
Wacker Polymer Systems GmbH & Co. KG
Wu David W.
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