Organic compounds -- part of the class 532-570 series – Organic compounds – Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
2002-05-23
2003-07-22
Kifle, Bruck (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
active
06596862
ABSTRACT:
The present invention relates to a process for purifying N-vinyl-&egr;-caprolactum.
N-Vinyl-&egr;-caprolactam is prepared on an industrial scale by vinylation of &egr;-caprolactam by means of acetylene under superatmospheric pressure in the presence of basic catalysts such as hydroxides and alkoxides (cf., for example, U.S. Pat. No. 2,327,804 and W. Reppe et al., Liebigs Ann. Chem. 601, 1956, pp. 81-137). Other preparative methods are the elimination of alcohols from N-(&agr;-alkoxyalkyl)caprolactams and the transvinylation of &egr;-caprolactam with vinyl ethers (U.S. Pat. No. 2,891,058). The isolation of the N-vinylcaprolactam from the reaction mixture customarily comprises a single or multiple distillation of the crude product obtained in the reaction. The product obtained after distillation generally contains from 98 to 99.95% of the desired product, with the main impurity being &egr;-caprolactam. The N-vinyl-&egr;-caprolactam which has been prepared in this way generally has a color number in the range from 20 to 200 (APHA color numbers in accordance with DIN EN 1557). On storage, further discoloration tends to occur, i.e. the color number increases. Such discoloration occurs particularly when the N-vinyl-&egr;-caprolactam, which is customarily stored as a solid, is melted.
Homopolymers and copolymers of N-vinyl-&egr;-caprolactam are employed in many fields. N-Vinyl-&egr;-caprolactam is frequently used for preparing polymers for cosmetic products, for the pharmaceutical sector and for food technology. In addition, N-vinyl-&egr;-caprolactam and its polymers are used for coating optical fibers. In all these fields, the purity of the N-vinyl-&egr;-caprolactam has to meet strict requirements. Particularly for the last-mentioned application, the abovementioned discoloration of the N-vinyl-&egr;-caprolactam is problematical. An important quality criterion for the N-vinyl-&egr;-caprolactam is therefore low discoloration. The degree of discoloration is customarily measured by means of the APHA color number which is determined by a method such as that described in DIN EN 1557.
U.S. Pat. No. 2,891,058 reports the analytical crystallization of N-vinyl-&egr;-caprolactam from diethyl ether. The product has a purity of 95%.
It is an object of the present invention to provide a process for purifying N-vinyl-&egr;-caprolactam which gives an N-vinyl-&egr;-caprolactam which displays little if any susceptibility to discoloration, i.e. has a higher color number stability than does customary N-vinyl-&egr;-caprolactam obtained by distillation.
We have found that this object is achieved by converting an N-vinyl-&egr;-caprolactam-containing product which consists predominantly, generally at least 95% by weight, of N-vinyl-&egr;-caprolactam into a melt, partially crystallizing this and separating the crystals from the mother liquor in which the impurities are present to give the purified N-vinyl-&egr;-caprolactam.
The present invention accordingly provides a process for purifying N-vinyl-&egr;-caprolactam, in which an N-vinyl-&egr;-caprolactam having a purity of at least 95% by weight is converted into a melt, this melt is partially crystallized and the crystals are separated from the mother liquor. The crystallized material obtained in this way is an N-vinylcaprolactam having a low tendency to discoloration and thus an increased color number stability compared to the at least 95% by weight pure N-vinylcaprolactam used in the form of the melt. The invention accordingly provides, in particular, a method of removing color-imparting impurities from N-vinyl-&egr;-caprolactam and of reducing the tendency to discoloration or improving the color number stability of N-vinyl-&egr;-caprolactam.
In the process of the present invention, the pure N-vinyl-&egr;-caprolactam is obtained from the crystals crystallized from the N-vinyl-&egr;-caprolactam-containing melt. Crystallization of the N-vinyl-&egr;-caprolactam gives a crystalline phase depleted in impurities and a liquid phase which has a higher content of impurities (mother liquor).
The crude N-vinylcaprolactam product used for the purification preferably contains at least 95% by weight and up to 99.95% by weight, in particular from 96% by weight to 99.95% by weight and particularly preferably from 97 to 99.95% by weight, of N-vinyl-&egr;-caprolactam.
The crystallization of the melt is usually carried out to a degree of crystallization of at least 5% and up to 95%, preferably to a degree of crystallization in the range from 10 to 90%, in particular in the range from 20 to 80% and particularly preferably in the range from 25 to 75%. The degree of crystallization referred to here is the proportion by weight of the crystals, based on the N-vinyl-&egr;-caprolactam present in the melt.
In a preferred embodiment of the invention, use is made of an N-vinylcaprolactam which has been obtained by purification by distillation of an N-vinyl-&egr;-caprolactam-containing crude product from the production process, preferably the above-mentioned vinylation of &egr;-caprolactam with acetylene. For the purposes of the present invention, “purification by distillation of the crude product” means that the reaction product obtained in the production process is worked up in a customary manner and the crude product obtained in this way is subjected to at least one distillation step, preferably a fractional distillation. The distillation is preferably carried out under reduced pressure, in particular from 5 to 200 mbar corresponding to a temperature at the top in the range from 100 to 150° C.
The upper limit to the temperature at which the crystallization is carried out is naturally the temperature at which the N-vinyl-&egr;-caprolactam which has already crystallized is in equilibrium with the N-vinyl-&egr;-caprolactam present in the mother liquor (equilibrium temperature). This temperature can, depending on the type and amount of impurities, be in the range from 32 to +34.9° C. and can be determined by a person skilled in the art using customary methods. Of course, the melt can also be crystallized by cooling to temperatures below the equilibrium temperature, e.g. to temperatures which are up to 35 K and preferably from 0.1 K to 20 K below the equilibrium temperature of the melt. However, highly super-cooled melts are preferably avoided, so that the temperature employed for the crystallization of the melt is in particular no more than 10 K and particularly preferably no more than 5 K below the equilibrium temperature. The crystallization is therefore preferably carried out at temperatures of the melt to be purified in the range from 25 to 34.9° C., in particular in the range from 30 to 34.9° C.
The actual crystallization of the N-vinyl-&egr;-caprolactam-containing melt can be carried out by methods analogous to known crystallization processes of the prior art. Suitable crystallization processes are known from, for example, U.S. Pat. No. 5,329,021, DE-A 26 06 364, DE-A 17 69 123 and EP-A 475 893.
To carry out the crystallization, it is usual to introduce the crude product to be purified in the form of its melt into a crystallizer and to crystallize out part of the N-vinyl-&egr;-caprolactam by cooling the crude product. The N-vinyl-&egr;-caprolactam-containing mother liquor obtained is separated off.
The crystallizer used in the process of the present invention is subjected to no particular restriction. Crystallizers which have been found to be particularly useful are ones whose function is based on the formation of crystals on cooled surfaces. Such crystallization processes are also referred to as layer crystallization. Suitable apparatuses are described in DE-A 17 69 123, DE-A 26 06 364, EP-A 218 545, EP-A 323 377, CH 645278, FR 2668946 and U.S. Pat. No. 3,597,164.
To carry out layer crystallization, the N-vinyl-&egr;-caprolactam-containing melt is brought into contact with the cooled surfaces of the heat exchanger. The temperature of these heat exchanger surfaces can be up to 40 K below the abovementioned equilibrium temperature and is preferably from 35 to 10 K, in partic
Becker Heike
Böttcher Arnd
Lorenz Rudolf Erich
Pinkos Rolf
BASF - Aktiengesellschaft
Keil & Weinkauf
Kifle Bruck
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