Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-04-16
2003-06-03
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S490000, C528S491000, C521S142000, C523S105000, C523S173000, C525S329100
Reexamination Certificate
active
06573358
ABSTRACT:
The invention relates to a process for the preparation of superabsorbent polymers (SAP) by alkaline hydrolysis of polyacrylonitrile precipitation polymers (PAN precipitation polymer).
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-methylolacrylamide. 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.
Processes for the preparation of SAPs are known from DE-A 44 06 951, DE-A 44 29 318, DE-A 196 00 163, DE-A 196 46 856 and EP-A 936 223; a common feature of all those specifications is that superabsorbent polymers are formed by alkaline hydrolysis of finely particulate polyacrylonitrile emulsions (PAN emulsions).
The above-mentioned processes use PAN emulsions which can be prepared only by using a particular emulsifier, which is described in DE-A 42 33 026; without the use of that emulsifier, it has hitherto scarcely been possible to prepare PAN emulsions economically. However, the use of that particular emulsifier is disadvantageous for economic and qualitative reasons. The quality of the superabsorbers is impaired because the emulsifier is water-soluble or water-dispersible.
The obvious solution, namely to use PAN precipitation polymers as the raw material, is not possible, as DE-A 44 06 951 and DE-A 44 29 318 teach:
“The acid or alkaline hydrolysis of polyacrylonitrile also yields polymers having carboxyl or carboxylate groups. Those polymers are generally water-soluble (and therefore not swellable), because the polyacrylonitrile used as starting material and obtained by precipitation polymerisation has a too low molecular weight.”
Accordingly, the object of the invention was to find a raw material based on polyacrylonitrile instead of on finely particulate PAN emulsions, which raw material can be prepared without the use of the above-mentioned particular emulsifiers according to DE-A 42 33 026 and which, after alkaline hydrolysis and, optionally, further steps, has outstanding SAP properties.
The invention provides a process for the preparation of SAPs, which process comprises the following subsidiary steps:
a) preparation of the PAN precipitation polymers, preferably uncrosslinked or crosslinked homopolymers of acrylonitrile,
b) alkaline hydrolysis of the PAN precipitation polymers, preferably in self-cleaning reactors suitable for high-viscosity media, to obtain a hydrolysate (=hydrolysis product)
c) optionally, neutralisation of the hydrolysate if necessary and desired by:
1. addition of acid to the hydrolysis product, or
2. addition of the hydrolysis product to a water/acid mixture, or
3. addition of the hydrolysis product to an alcohol/water/acid
d) drying of the water-containing, gel-like product,
e) grinding the dried product obtained in d) to the desired particle size,
f) surface modification of the granular SAP.
The purpose of the optional neutralization step C) is the adjustment of the pH value of the water-containing, gel-like product to a level smaller than 7.
Accordingly, in the present invention, high molecular weight PAN precipitation polymers are used instead of PAN emulsions.
It is already known from DE-A 196 00 163 that SAPs can be prepared by hydrolysis of PAN emulsions of fine particle size. 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 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 residence 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 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 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 neutralize the unconsumed alkali metal hydroxide or to adjust the pH value of the end product, acid must be added in order to maintain the pH value of ≦7 required for use in hygiene articles, for example babies' nappies or incontinence articles for adults.
However, it is in principle also possible, as described in DE-A 19 646 856, to carry out the hydrolysis in a water/alcohol mixture in stirred containers.
The present invention also provides superabsorbers prepared by the process according to the invention.
The invention relates also to the use of the superabsorbers according to the invention in the manufacture of nappies, incontinence articles, water-storage materials and electric cable coverings.
The invention further provides nappies, incontinence articles, water-storage materials and electric cable coverings manufactured by the process according to the invention.
Suitable high molecular weight PAN precipitation polymers according to subsidiary step a) may be prepared by methods known in the art; see in this connection, for example, Houben-Weyl, Methoden der organischen Chemie, E20, p. 1208-1216, Georg Thieme Verlag, Stuttgart. The molecular weights (weight-average) of the uncrosslinked PAN precipitation polymers are from 5*10
5
to 1*10
7
g/mol, and the Staudinger indices, measured in dimethylformamide with the addition of 1 g/liter of LiBr, are greater than 2 dl/g, preferably greater than 4 dl/g. Preferred PAN precipitation polymers are polyacrylonitriles; both uncrosslinked and crosslinked homopolymers of acrylonitrile are preferred. Crosslinked PAN precipitation polymers contain, incorporated by polymerization, polyfunctional monomers, as described, for example, in DE-A 42 33 026 (U.S. Pat. No. 5,356,985 incorporated herein by reference), or divinyl ethers, such as, for example, diethylene glycol divinyl ether, ethylene glycol divinyl ether, butanediol divinyl ether, which list does not constitute a limitation, of from 0.01 to 4.0 wt. %, based on the total amount of monomers.
According to subsidiary step b), the liquid starting materials (PAN precipitation polymers dispersed in water, 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 tha
Michels Gisbert
Sackmann Günter
Struss Klaus
Preis Aron
Yoon Tae H.
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