Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-04-16
2003-10-28
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S194000, C525S221000, C525S238000, C525S329200, C525S329300, C525S340000, C525S343000, C525S344000, C525S355000, C525S374000, C525S383000, C525S384000, C525S386000, C525S212000
Reexamination Certificate
active
06639022
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the preparation of superabsorbent polymers (SAP) by alkaline hydrolysis of polyacrylonitrile emulsions (PAN emulsion).
BACKGROUND OF THE INVENTION
Superabsorbent polymers are known and are used mainly in the manufacture of nappies and incontinence articles, but also as water-storage materials in agriculture and in the covering of electric cables. In general, those superabsorbent polymers are wide-mesh crosslinked, water-insoluble polymers or copolymers based on alkali metal salts of polyacrylic acid or copolymers of alkali metal salts of acrylic acid and acrylamide, which are obtained by the free-radical initiated copolymerization of acrylic acid and polyfunctional monomers, such as divinylbenzene, ethylene glycol dimethacrylate, ethylene glycol diallyl ether, butanediol diacrylate, hexanediol dimethacrylate, polyglycol diacrylate, trimethylolpropane diacrylate, allyl acrylate, diallyl acrylamide, trisallylamine, diallyl ether, methylenebisacrylamide and N-methylol-acrylamide. Owing to their structure, such polymers are capable, by swelling and the formation of hydrogels, of absorbing large quantities of water and aqueous solutions and of retaining them even under pressure.
It is known from DE-A 196 00 163 that SAP's can be prepared by hydrolysis of finely particulate PAN emulsions. It is described that superabsorbent polymers from PAN emulsions can be prepared in a continuous and reproducible manner from concentrated emulsions if self-cleaning reactors having a volume sufficient for a dwell time (residence time) of from 0.5 to 2 hours are used for the alkaline hydrolysis of the PAN emulsion.
It is also described that suitable devices for the preparation of superabsorbent polymers by hydrolysis of PAN emulsions are especially those that permit relatively long dwell times with sufficiently good mixing and good heat transfer with the simultaneous removal of the gaseous ammonia that forms during the hydrolysis.
Especially suitable are so-called “List reactors” (manufacturer: List AG, CH-4422 Arisdorf, Switzerland) having a free volume of up to 16.5 m
3
, which are constructed as either single-shaft or twin-shaft devices and have special mixing and scraping members both on the shaft and in the reactor housing. Such reactors may also be connected in series, as a result of which a high degree of flexibility in use can be achieved.
According to DE-A 196 00 163, when the hydrolysis is complete the reaction mixture may be continuously introduced directly from the List reactor into a precipitation reactor equipped with a high-speed stirrer and containing a low-boiling alcohol, for example methanol or ethanol. The superabsorbent polymer precipitates thereby in the form of a fine, readily filterable powder. Drying and grinding to the desired particle size yield the finished superabsorber.
It is also described that, in order to neutralise the unconsumed alkali metal hydroxide or to adjust the pH value of the end product, acid is added in order to obtain the pH value required for use in hygiene articles, for example babies' nappies or incontinence articles for adults, which value is to be from 5.5 to 6.5.
Since alcohols must be used in the known process to isolate the product, the alcohol/water/salt mixture formed after the neutralization must be disposed of, or the alcohol is returned to the production process after it has been separated off. For reasons of explosion protection, drying of the SAP containing water and alcohol is possible only with a great financial outlay. For economic and ecological reasons, use is often made of other processes.
According to DE-A 196 00 163, it is said to be possible, as an alternative to working up with alcohols, to dry the hydrolysis product in vacuo at temperatures up to 100° C. Without neutralization of the unconsumed alkali metal hydroxide, the products have a pH value, greater than 7.0, which is too high for use in hygiene articles, for example babies' nappies or incontinence articles for adults.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the object of the present invention is to provide an SAP having a pH value of less than or equal to 7 by a process entailing alkaline hydrolysis of PAN emulsions or PAN precipitation polymers, without the need to use alcohols or any other organic solvents for isolation.
The invention provides a process for the preparation of SAP's, which comprises the following subsidiary steps:
a) preparing a PAN emulsion (as described above according to DE-A 42 33 026 and its corresponding U.S. Pat. No. 5,356,985 that is incorporated by reference herein), preferably uncrosslinked or crosslinked homopolymers of acrylonitrile,
b) subjecting the emulsion obtained in a) to alkaline hydrolysis in self-cleaning reactors suitable for high-viscosity media to obtain a hydrolysate,
c) neutralizing the hydrolysate by:
1. adding acid to the hydrolysate, or
2. adding the hydrolysate to a water/acid mixture in order to adjust the pH value,
to obtain a water-containing, gel-like product, and
d) drying of the water-containing, gel-like product,
e) granulating the dry product obtained in d) to the desired particle size,
f) modifying the surface modification of the granular SAP obtained in e).
Reactors suitable for high viscosity medium are these that are known in the art for handling gel-like compounds.
It has been found that, in contrast to DE-A 196 00 163, an organic solvent is not necessary for isolation of the SAP in the process according to the invention. It is also important that acid is added (according to c1.)) to the hydrolysate in order to neutralise the alkali metal hydroxide and adjust the pH value. In contrast to DE-A 196 00 163, the salts that form thereby remain in the product after drying. It is surprising that outstanding properties are obtained even in the presence of the salt originating from the neutralization. Alternatively, it is also possible for the purposes of neutralization to add the resulting hydrolysate to a mixture consisting of water/acid (according to c2.)) and isolate it again some time later. The low molecular weight salts that form thereby are washed out or remain in the liquor. The properties of the products obtained by that process are likewise outstanding.
The PAN emulsions according to subsidiary step a) have a mean particle size, determined by laser correlation spectroscopy, of from 60 to 500 nm, preferably from 80 to 200 nm, and a solids content of from 10 to 55 wt. %. The molecular weights (weight-average) of the uncrosslinked PAN emulsions are from 5*10
5
to 1*10
7
g/mol. Preferred PAN emulsions are uncrosslinked or crosslinked homopolymers of acrylonitrile. Crosslinked PAN emulsions contain polyfunctional monomers incorporated by polymerization, as described in DE-A 42 33 026, of from 0.2 to 4.0 wt. %, based on the total amount of monomers.
According to subsidiary step b), the low-viscosity starting materials (PAN emulsion and aqueous alkali metal hydroxide solution) are mixed, and the hydrolysis is carried out in self-cleaning reactors suitable for high-viscosity media (HV reactors). In the hydrolysis, high-viscosity gels having a solids content of from 10 to 70 wt. % are formed. Suitable HV reactors have a good mixing and kneading action for the high-viscosity gel that forms, good heat transfer, and permit removal of the ammonia that forms in the hydrolysis. Especially suitable HV reactors are those from List AG, CH-4422 Arisdorf, Switzerland, which are constructed as single- or twin-shaft devices and have mixing and scraping members both on the shaft and in the reactor housing.
After the starting materials have been mixed, the hydrolysis may be carried out in a batch process, in a semi-continuous process by the metering in of one or both starting materials, or in a continuous process. In the case of a continuous process it is also possible to meter in the starting materials at the beginning of the process or in part later in the process. It is possible to use one or more HV reactors connected in series. Mixing
Gross Thomas
Michels Gisbert
Bayer Aktiengesellschaft
Franks James R.
Gil Joseph C.
Reddick Judy M.
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