Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-10-01
2003-12-09
Zalukaeva, Tatyana (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S083000, C526S180000, C526S220000, C526S264000, C526S271000, C526S303100, C526S317100, C526S318440, C526S329200, C526S330000
Reexamination Certificate
active
06660819
ABSTRACT:
The invention relates to a process for producing synthetic polymers based on acrylic acid and derivatives thereof, which polymers have an exceedingly low content of residual monomers. The polymers have high molecular weights and either are water-soluble or have high absorptive capacity for water, aqueous solutions and body fluids.
Various synthetic polymer products having absorptive capacity for water and body fluids have been described in numerous patents: e.g. crosslinked polymers and copolymers based on acrylic or methacrylic acid (U.S. Pat. No. 4,018,951, U.S. Pat. No. 4,066,583, U.S. Pat. No. 4,062,817, U.S. Pat. No. 4,066,583, DE-OS 26 13 135, DE 27 12 043, DE 28 13 634), or acrylamidopropanesulfonic acid copolymers (DE 31 24 008). These absorbents are virtually insoluble in water, absorbing a multiple of their weight of water, urine or other aqueous solutions at equilibrium. In addition to high absorptive capacity, other properties of synthetic absorbents such as low residual monomer content, low ratio of water solubles and high gel strength of the swollen polymer particles are also mentioned in some patent specifications.
In the production of high molecular weight water-soluble or water-swellable, i.e., partially crosslinked polymers and mixed polymers, complete conversion of the monomers—in particular, monomers based on acrylic acid has been found impossible. On an industrial scale, residual monomer levels of from 0.1 to 0.5 wt.-% normally are observed in the polymer product.
The toxicity of the monomers remaining in the polymer product is well-known and therefore, a process for producing polymers and mixed polymers virtually free of monomers would be highly convenient. Up to now, since it has not been possible as yet to conduct a polymerization process in such a way that virtually no residual monomers would remain, it has been attempted to remove the toxic residual monomers from the polymer product by converting them into harmless derivatives. Thus, DE-AS 1,070,377 and U.S. Pat. No. 2,960,486 use mixing the aqueous solutions of high molecular weight acrylamide polymers with sodium disulfite solution and drying at 80-120°. One precondition for these methods is processing the polymer product in a highly diluted (2-3%) polymer solution, which is uneconomic, and for this reason these methods have rarely been applied in practice.
The treatment of a polymer gel with an aqueous solution of sodium bisulfite or metabisulfite has been described in U.S. Pat. No. 3,755,280, and with a solid alkali sulfite in EP 175,554, where residual monomer contents of from 0.03 to 0.3 wt.-% have been obtained. Likewise, the JP-PS 56/103207 uses bisulfites, sulfites and pyrosulfites for the same purpose. U.S. Pat. No. 3,780,006 uses gaseous sulfur dioxide in order to decrease the acrylamide concentration in an emulsion polymer. EP 505,163 uses treatment of polymer products with a combination of metabisulfite and a surface-active agent (HLB from 3 to 40) subsequent to completed polymerization, thereby decreasing the residual monomer content down to 10 ppm. From 2 to 5 wt.-% of metabisulfite (relative to the polymer gel having 40% w.s., that is, from 5 to 12.5 wt.-% of metabisulfite relative to the dry polymer product) is necessary in such a secondary treatment of polymer gels in order to achieve the desired decrease in residual monomers. Such high quantities of added substances may have highly negative effects on the application-technical properties.
EP 303,518 A2 describes a process for producing absorbent polymer products based on acrylic acid/acrylamide, and according to this invention, a high degree of neutralization of from 70 to 100 mole-%, a high monomer concentration of at least 50%, and a combination of thermally decomposing azo and redox initiators are used. Owing to these conditions, the reaction proceeds in such a way that all of the water used in the batch undergoes vaporization even during the polymerization, so that subsequent drying can be omitted, and the residual monomer contents are said to drop below 500 ppm, preferably below 200 ppm. The residual monomer levels of the experimental examples have not been specified.
DE 37 24 709 A1 describes a process for producing polymer products having low residual monomer content, wherein the polymer products following production are reacted in a swollen form as a gel or in solution with compounds reacting with the double bonds in the residual monomers at 50-150° C.
In U.S. Pat. No. 4,766,173, the decrease of acrylic acid residual monomer in polymer products is effected by subsequent treatment of the polymer products using amino acids of the lysine and cysteine types at temperatures from 80° C. on.
WO 94/20547 describes additives such as bromate and chlorate in the polymerization solution and subsequent heating of the final polymer wherein, inter alia, a decrease of the residual monomers is effected by the additives. The bromates and chlorates may also be added subsequent to the polymerization. Despite these measures, the residual monomer content of the polymer products is between about 135 and 1100 ppm.
While the prior art processes described above permit a substantial decrease of residual monomers, they involve several drawbacks such as malodors as a result of liberated sulfur dioxide, corrosion of the plant by sulfur oxide resultant products (e.g. sulfurous acid and sulfuric acid as well as salts thereof in acidic medium). Above all, however, any subsequent treatment of the final polymer that has been produced represents an additional technological step necessitating additional apparatus and a quite considerable input of time, and also, a quite significant amount of additive remains in the final product and may adversely affect the application-technical properties.
It is therefore the object of the invention to produce synthetic polymer products of per se known chemical composition with high molecular weight or high retention capability for water, aqueous liquids and body fluids, using particular specific conditions so as to obtain final products having an extremely low residual monomer content and good application-technical properties, without necessitating a subsequent treatment of the produced polymers using additional chemical compounds.
Surprisingly, it has now been found that synthetic polymer products based on acrylic acid and having the desired properties and an extremely low content of residual monomers can be produced by free-radical polymerization in aqueous solution, provided a salt of at least one nitrogen compound is used during the preparation of the monomer mixture to be polymerized, and the polymer products are subjected to subsequent heating at 120 to 240° C., preferably 140 to 180° C. The polymer products according to the process of the invention have an extremely low content of residual monomers, preferably being below 50 ppm, particularly below 30 ppm. Of special importance is the fact that acrylamide, which is rated critical in toxicological terms, can be decreased to a residual content of below 10 ppm.
Predominantly, the salts of ammonia, hydroxylamine, of aliphatic mono- and polyamines, particularly aliphatic C
1
-C
10
amines, cycloaliphatic mono- and polyamines, particularly cycloaliphatic C
6
-C
12
amines, aromatic mono- and polyamines, particularly aromatic C
6
-C
12
amines, heterocyclic amines, and alkanolamines and mixtures thereof can be used as suitable salts of nitrogen compounds. In particular, the salts of ammonia, monoethanolamine and diethanolamine are preferably used.
Inorganic and organic acids are suitable as salt-forming acidic component of the amine salts to be used according to the invention. As inorganic acids, in particular, hydrofluoric acid, hydrochloric acid, hydrobromic acid and hydriodic acid, sulfuric acid, sulfurous acid, sulfoxylic acid, orthophosphoric acid, phosphorous acid, nitric acid, and nitrous acid form the counterion. Organic acids can be derived from aliphatic, aromatic and mixed aliphatic/aromatic compounds. They preferably contain a carboxyl group and/or acid functions
Chmelir Miroslav
Dahmen Kurt
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Stockhausen GmbH & Co. KG
Zalukaeva Tatyana
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