Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-05-27
2001-07-31
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S062000, C526S204000, C526S220000, C526S344200
Reexamination Certificate
active
06268441
ABSTRACT:
POLYMERIZATION OF VINYL MONOMERS
The present invention relates to the use of N-oxyl compounds for preventing wall deposits during the addition polymerization of vinyl monomers in aqueous phase.
In the course of polymerization reactions it is common for polymer deposits to appear on the reactor walls. These polymer deposits cause processing difficulties since they have to be removed at regular intervals, generally after each polymerization reaction, often entailing considerable expense. Moreover the deposits, which generally consist of polymers that are atypical in terms, for example, of their molecular weight distribution, can in some cases become detached and so lead to inhomogeneities in the polymers.
Various measures have been developed to remedy the formation of deposits. Since the extent of formation of deposits depends heavily on the surface structure of the reactor walls—their roughness, for example—the measures are aimed primarily at smoothing the surfaces. In addition to mechanical measures attempts have also been made to maintain the quality of the surface by adding corrosion inhibitors to the polymerization (see, for example, G. W. Becker, D. Braun (eds.), Kunststoff Handbuch, Hanser Verlag, Munich, Vienna 1986, Volume 2/1, p. 153). These measures, however, are normally unable to adequately prevent the formation of deposits.
The earlier German Patent Application 19 609 312.0 describes a process for preventing the premature polymerization of certain vinyl monomers in the course, for example, of their distillation, purification, storage and transportation, by adding N-oxyl compounds of secondary amines. The use of N-oxyl compounds has also been described for stabilizing styrene and other vinylaromatic compounds in the course of distillation (U.S. Pat. No. 5,254,760). It is also known, however, that even traces of such nitroxyl compounds disrupt the subsequent polymerization process; they cause retarded polymerization and uncontrolled chain termination, resulting in polymers deficient in reproducibility and of short chain length. These deleterious effects are described by Mardare et al. in Polym. Prep. (Am. Chem. Soc., Div. Polym. Sci.) 35 (1), 778 (1994).
It is an object of the present invention to find a vinyl monomer polymerization process in which the formation of wall deposits is substantially eliminated without perceptibly affecting the polymerization process.
We have found that this object is achieved by the use of N-oxyl compounds of secondary amines which carry no hydrogens on the a carbons to prevent deposits on reactor surfaces during the polymerization of vinyl monomers in aqueous phase.
By vinyl monomers are meant all addition-polymerizable monomers which carry a terminal olefinic double bond. Examples that may be mentioned are acrylic and methacrylic acid and their derivatives such as nitrites and esters, especially methyl, ethyl, propyl and butyl acrylate and methacrylate, and also vinyl esters of C
2
-C
5
carboxylic acids, especially vinyl acetate, vinyl propionate, vinyl butanoate, aromatic vinyl compounds, especially styrene, dienes, especially butadiene, vinyl halides, especially vinyl chloride, vinyl ethers, such as methyl, ethyl or butyl vinyl ether, vinyl thioethers, vinylcarbazoles, vinylpyrrolidones, vinylphthalimides, vinyl isocyanates, vinylcaprolactams, vinylimidazoles, vinylformamide, vinylsulfonic acid, and vinyl silanes such as vinyltriacetoxysilane, vinyltrichlorosilane or vinyltrimethoxysilane.
&agr;-Olefins are also suitable, such as ethylene, propylene, 1-butene, 1-hexene and 1-octene, especially together with other comonomers in copolymerization processes.
The process of the invention can be employed to particular advantage in the case of such copolymerization processes. Examples that may be mentioned of suitable copolymers which can be prepared by the processes are styrene-butadiene, synthetic rubberlike block polymers, such as styrene-butadiene-styrene, styrene-butyl acrylate, butadiene-acrylonitrile, acrylonitrile-butadiene-styrene, and ethylene-vinyl chloride.
The use according to the invention is particularly suitable in the polymerization of styrene, alone or together with further comonomers, and, if desired, in the presence of volatile blowing agents, such as pentane, and for the polymerization of vinyl chloride.
The process of the invention relates to polymerizations in aqueous phase. The term aqueous phase is intended to denote solutions, emulsions and suspensions of the corresponding monomers or polymers in water and in solvent mixtures comprising a substantial proportion , i.e., at least 20% by weight, of water. All customary emulsion polymerizations and suspension polymerizations, therefore, can be conducted in accordance with the process of the invention. The process conditions which can be chosen are those which are customary for such polymerization processes and are described, for example, in Houben Weyl, Methoden der organischen Chemie, Vol. E20, p. 218 ff and in Ullmanns Encyklopäldie der technischen Chemie, 4th edition, Vol. 19, p. 132 ff. and the literature cited therein.
In accordance with the invention, N-oxyl compounds of secondary amines are present during the polymerization process. The application of these N-oxyl compounds can be made in various ways.
It has been found advantageous to add the N-oxyl compound to the polymerization mixture. Alternatively, the N-oxyl compound may already be present in the monomer in order to protect it against premature polymerization.
The concentration of the N-oxyl compound in the polymerization mixture should be such that it has little effect on the rate of polymerization. The sensitivity of the polymerization to the amount of N-oxyl compound depends on a variety of parameters: in particular, on the nature of the monomer, the structure of the N-oxyl compound, the temperature, and other free-radical initiators and free-radical scavengers which may be present in the reaction mixture. The optimum concentration, however, can be determined in a few preliminary experiments under given process parameters.
A concentration range which has been found advantageous for the N-oxyl compound(s) in the polymerization mixture is that from 0.5 to 50 ppm, in particular from 1 to 20 ppm and, especially, from 2 to 10 ppm (1 ppm=10
−6
parts by weight of N-oxyl compound based on the overall weight of the monomers). At these concentrations, the formation of deposits is generally inhibited completely without any notable effect on the kinetics of the reaction.
In addition to adding the N-oxyl compound to the polymerization mixture, a process variant which has proven particularly appropriate is that in which the reactor surface is wetted with a solution of the N-oxyl compound before the reactor is filled with the polymerization mixture.
Wetting can be carried out simply and effectively by spraying the reactor walls and other components within the reactor, such as the stirrer, with a solution of the N-oxyl compound. The N-oxyl compounds are in many cases of poor solubility in water but dissolve for the most part in organic solvents such as methanol, ethanol, propanol, acetone, ethyl acetate, dimethylformamide, etc. A particularly appropriate solvent for many N-oxyl compounds is methanol. The concentration of the N-oxyl compound in the spray solution is not critical and is advantageously set at between 0.01 and 1% by weight, based on the overall mass of the spray solution.
Alternatively, the reactor surface can be wetted by filling it with a solution similar to the spray solution and then draining off that solution.
Wetting of the reactor surface with the N-oxyl compound has the advantage over its addition to the polymerization mixture that the effect on the rate of polymerization is extremely small and, in addition, that the region above the liquid level, especially the region just above the liquid level, in which deposits are common, is also protected against the formation of deposits.
It is also advantageous to combine both forms of application, in which case very small amounts are suff
Aumuller Alexander
Lynch John
Sutoris Heinz Friedrich
Zubiller Jürgen
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Teskin Fred
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