Process for the preparation of hydrophilic hydrogels of high swelling capacity

The present invention relates to a process for the preparation of hydrophilic hydrogels of high swelling capacity by (co)polymerization of hydrophilic monomers in a fluidized bed apparatus.

The present invention relates to a process for the preparation of 
hydrophilic hydrogels of high swelling capacity by (co)polymerization of 
hydrophilic monomers in a fluidized bed apparatus. 
DESCRIPTION OF THE PRIOR ART 
Hydrophilic hydrogels of high swelling capacity are, for example, polymers 
of (co)polymerized hydrophilic monomers. Such hydrogels are used as 
products which absorb aqueous solutions for the production of diapers, 
tampons, sanitary towels and other hygiene articles, and also as 
water-retaining agents in agriculture and horticulture. 
Processes are known for the preparation of such hydrogels in which 
monomers, such as, for example, acrylic acid and methacrylic acid, are 
polymerized in aqueous solution, in acid form or neutralized to a certain 
percentage, with the addition of catalysts, initiators and crosslinking 
agents. The reaction solution is diluted with water here to the extent 
that a gel forms after the reaction. The dilution with water is carried 
out on the one hand to limit the maximum temperature which occurs due to 
heating as a result of the heat of polymerization, and on the other hand 
the processability of the gel to granules imposes limits on the solids 
content. The reaction is as a rule carried out with heating either in the 
static state (pot reactor or belt reactor) or in apparatuses with rotors, 
the reaction mass in each case being mixed and kneaded up to the gel 
phase. EP-B 223 063 discloses a process for the continuous preparation of 
crosslinked finely divided polymers in gel form in which pre-neutralized 
acrylic acid is reacted with comonomers in aqueous solution in a 
continuously operating single-shaft, cylindrical kneader at 45-80.degree. 
C. and lumps of gel with a residual moisture content of 30 to 70% are 
discharged at the end. A disadvantage of this process is further 
processing of the viscous gel with expensive drying, comminution and 
sieving to the desired particle size. Dust and undesirable fine contents 
must be discarded or employed elsewhere or worked up. Furthermore, at a 
relatively high solids content, the molecular chains are destroyed due to 
the intensive shearing stresses on the gel, and the desired properties of 
the product are thus adversely influenced. 
DE-A 3519013 describes the preparation of pulverulent, water-soluble 
polymers by polymerization of water-soluble, ethylenically unsaturated 
monomers in a powder bed, water-insoluble polymerization initiators being 
employed in the form of liquid, organic peroxides. In this reaction, the 
heat of polymerization is removed by evaporation of water, the circulated 
powder bed being retained. 
DE-A 3842184 describes a process in which water-soluble, monoethylenically 
unsaturated monomers are polymerized to pulverulent hydrophilic polymers 
in a powder bed. The particle size is controlled in this process by 
atomizing the solution of the monomers together with an inert auxiliary 
liquid with the aid of a multi-component nozzle immersed in the bed. The 
inert stream of atomizing gas regulates the particle size of the polymers 
by its variation in its amount. 
DE-A 3842185 describes a process for the preparation of pulverulent 
monomers from acrylic and methacrylic acid in a powder bed. The monomers 
are added in a mixture of water and alcohols with polymerization 
initiators and regulators, the powder state being maintained, and the 
reaction mass is circulated mechanically and the heat of reaction removed 
by distilling off the solvents. Thiocarboxylic acid and/or mercapto 
alcohol and propionic acid and/or formic acid are employed as regulators 
in a defined range of amounts in this process. 
EP-A 113048 also describes a process for the preparation of pulverulent 
polymers based on water-soluble ethylenically unsaturated monomers in an 
agitated powder bed, a tank, stirred autoclave or flow tube being possible 
as the device. In this process, the monomers are added to the powder bed 
in mixtures with water or water/isopropanol, this being maintained. The 
heat of polymerization is removed by distilling off the solvents. 40-95% 
of the acid groups of the monomers are neutralized here and the 
polymerization proceeds in the presence of thiocarboxylic acid and further 
substances as regulators. 
The processes according to DE-A 3519013, DE-A 3842185 and EP-A 113048 do 
not relate to water-swellable polymers and therefore have no relevance 
with respect to the present invention. 
The process according to EP-B 223 063 has the disadvantage that the process 
proceeds via the gel phase and relatively high solids contents during 
further processing of the gel lead to difficulties due to the high energy 
consumption of the apparatus and to a reduction in the properties of the 
products. Furthermore, the gel particles must be dried, ground and sieved. 
The undesirable fine particle content must be recycled or discarded, which 
is undesirable from an economic and ecological aspect. 
DE-A 3842184 also relates to the preparation of crosslinked polymers 
containing acrylic acid. However, no fluidized bed polymerization process 
is described here, but a process of fixed bed polymerization, i. e. the 
reaction solution is sprayed onto a mechanically agitated or stirred fixed 
bed of polymerized particles. An inert diluent, by evaporation of which 
the heat of polymerization can be removed, must be added here. A 
temperature of the fixed bed is established according to the evaporation 
temperature of the inert solvent, i. e. the polymerization temperatures 
rises to values which are prohibitive for achieving certain properties of 
the products. 
Devices which are mentioned for this process are tanks, stirred autoclaves, 
combinations of a stirred tank and flow tube or stirred tank cascades, 
that is to say devices which produce a mechanically agitated powder bed. 
In contrast, in a fluidized bed, the heat is removed from the polymerized 
particles dispersed in the fluidizing gas to the gas easily and according 
to the temperature and amount of fluidizing gas fed in. In this case, the 
bed temperature can be kept in the range which is more advantageous for 
the polymerization and the resulting properties via the intake parameters. 
Furthermore, by dispersing the particles, their sticking together is 
counteracted far better than in a powder bed densely packed with 
particles. 
The object of the present invention is to provide a process for the 
preparation of hydrophilic hydrogels of high swelling capacity which does 
not have the abovementioned disadvantages and in which as far as possible 
no dust and no fine contents are obtained. 
SUMMARY OF THE INVENTION 
The present invention therefore relates to a process for the preparation of 
hydrophilic hydrogels of high swelling capacity by polymerization of 
hydrophilic (co)monomers, wherein the polymerization is carried out in a 
fluidized bed apparatus. 
The present invention preferably relates to a process for the preparation 
of hydrophilic hydrogels of high swelling capacity by polymerization of 
hydrophilic (co)monomers in the presence of water, aqueous alkali and a 
crosslinking agent, which comprises carrying out the polymerization in a 
fluidized bed apparatus, wherein 
the (co)monomers, water, aqueous alkali and crosslinking agent are sprayed 
into a fluidized bed through multi-component nozzles which are charged 
with pressurized gas and sprayed from the bottom upward, 
the multi-component nozzles are immersed in the fluidized bed, 
the fluidized bed is generated by a hot stream of inert gas which is passed 
through an inflow tray and granules on the top from the bottom upward, 
the particles in the fluidized bed are heated by the hot stream of inert 
gas and as a result the reaction of the (co)monomers and crosslinking 
agent sprayed onto this is initiated and controlled, and 
drying of these particles takes place in the fluidized bed, utilizing the 
heat of polymerization and/or neutralization. 
The process according to the invention can be operated both discontinuously 
and continuously.

DETAILED DESCRIPTION 
If desired, one or more polymerization initiators can additionally be 
employed to initiate the polymerization reaction. 
The comonomers can be premixed with the aqueous alkali, in particular 
sodium hydroxide solution or potassium hydroxide solution in a 
concentration of 30-50%, in a static or dynamic mixer, for example 
Supraton, directly before the nozzle discharge. However, it is also 
possible for the two components to be fed separately to the nozzle tip, so 
that the neutralization takes place only after the nozzle. 
If an initiator is employed, this can be fed to the pressurized gas at the 
nozzle discharge with a metering pump which generates a higher pressure 
than the inert gas pressure. 
The solids content in the reaction mixture can be between 30 and 90% by 
weight, advantageously between 50 and 70% by weight. Solids contents in 
aqueous reaction mixtures at which the heat of neutralization and 
polymerization liberated meets the heat requirement for the noticeable 
heat and the evaporation enthalpy of the water are particularly favorable. 
For example, in the case of polymerization of acrylic acid, this range is 
a solids content of 60-65% by weight. 
The degree of neutralization depends on the desired use profile of the 
granules to be prepared. Customary pH values of the granules are 5.5-6.5. 
The process according to the invention can proceed at temperatures in the 
fluidized bed of 60 to 120.degree. C., preferably between 80 and 
105.degree. C. The intake temperature of the fluidizing gas in the bed is 
essentially determined by the solids content and the heat of 
neutralization and polymerization, and can be up to 30.degree. C. below 
and up to 80.degree. C. above the fluidized bed temperature. 
By spraying the reaction solution onto particles present at a defined 
temperature, the solution applied in a very thin layer is reacted very 
rapidly and granules with an onion-like shell build-up are formed. 
The advantage of this procedure is that the tacky and very viscoelastic gel 
phase, which causes enormous difficulties during processing, especially in 
the case of gels with a relatively high solids concentration, is avoided 
here, since drying in the thin layer also takes place simultaneously 
during the reaction. The reaction in a fluidized bed, i. e. in a stream of 
gas, in which the particles are dispersed also prevents the particles from 
being able to stick to the walls. By the formation of an onion-like shell 
build-up during the reaction, there is furthermore the possibility of 
producing different properties in the granules by varying the (co)monomers 
at varying times. 
The stream of inert gas for generating the fluidized bed is heated up to 
the required temperature before entry into the apparatus and adjusted to a 
throughput of 2,000 to 8,000 m.sup.3 /m.sup.2 h, preferably 5,000 to 7,000 
m.sup.3 /m.sup.2 h. The stream of inert gas which leaves the fluidized bed 
and is loaded with water vapor is freed from entrained fine contents, 
which in turn fall back into the fluidized bed, in a cyclone or a filter 
cleaned by pressurized gas. 
In the case of monocomponent granules, the process according to the 
invention can be employed particularly readily in a continuous procedure. 
Requirements for defined particle distributions of the granules can be met 
here in a simple manner. In the process according to the invention, the 
continuous procedure is preferred, which comprises a process in which 
a) a solution of the reaction components is sprayed into a fluidized bed 
through multi-component nozzles which are charged with pressurized gas and 
sprayed from the bottom upward, an initiator, which is employed if 
appropriate, being fed separately to the nozzle tip and being mixed with 
the solution in the atomizing cone, this fluidized bed being generated by 
a hot stream of inert gas which is passed through an inflow tray and 
granules on top from the bottom upward, and the multi-component nozzles 
being immersed in the fluidized bed, and 
b) the reaction of the reaction components sprayed onto the surface of the 
particles takes place in the fluidized bed in a controlled manner due to 
the temperature of the stream of inert gas entering, and drying of these 
particles takes place here in the fluidized bed, utilizing the heat of 
polymerization and/or neutralization, and 
c) a stream of granules is removed continuously from the fluidized bed and 
passed over a sieve, the oversize particles are comminuted on a mill and 
recycled to the fluidized bed together with the undersized particles 
separated off at the sieve, correctly sized particles being added to this 
material to be recycled in a proportion which exceeds the amount of solid 
fed to the fluidized bed apparatus in the form of the aqueous reaction 
mixture, and 
d) the solid is removed via a pipe located centrally or off-center at the 
base, at the end of which a fluidized bed is generated by a small stream 
of inert gas fed via an inflow tray from the bottom upward, so that the 
same circumstances as in the fluidized bed apparatus are established in 
the discharge pipe and uniform removal via a cellular wheel sluice located 
on the side of the inflow tray is ensured. 
The stream of inert gas loaded with water vapor leaves the fluidized bed 
apparatus via a cyclone or, more advantageously, via a filter cleaned by 
pressurized gas. This stream of gas can be recirculated and fed back to 
the fluidized bed. In this case, the entire stream is fed over a washer 
condenser, operated under neutral or alkaline conditions, with a 
subsequent aerosol separator. The amounts of this purified inert gas 
stream required for the discharge and the compression of the pressurized 
gas are removed and the remainder is fed, heated to the intake 
temperature, into the fluidized bed. 
If there are strict requirements of the classes of particle size of the 
granules to be prepared, sieving is carried out with several decks in the 
sieve, which have mesh widths of the sieve screen corresponding to the 
desired particle size classes. In this case, the upper deck advantageously 
determines the upper limit and the lower deck the lower limit of the 
particle size distribution. The mesh width of the middle decks is then 
chosen such that a specified particle size class is achieved in the 
properly sized particles by sluicing out an appropriate amount for each 
deck fraction. Using metering screws, the particular weight content is 
removed from the stream of granules leaving the decks and the individual 
streams are combined to the product stream. The excess properly sized 
particle classes sieved off is combined and recycled into the fluidized 
bed apparatus. 
The ratio of recirculated amount of material to amount of properly sized 
particles removed is preferably between 1 and 10. 
The granules prepared by the process according to the invention as a rule 
have particle sizes of 100 .mu.m to 2 mm, preferably 200 to 800 .mu.m, 
particularly preferably between 300 and 600 .mu.m. 
Bi- or multi-component granules can also be prepared by the process 
according to the invention in a discontinuously proceeding operation, 
which comprises a process in which 
a) material, for example polymerized granules, natural substances, for 
example starch granules, or inert substances, is initially introduced into 
the fluidized bed and 
b) various reaction mixtures are sprayed on and polymerized successively 
with respect to time or at varying times. 
It is possible, by the process according to the invention, to modify the 
surface properties of the hydrophilic gels, for example to hydrophobize or 
hydrophilize them, or to modify the permeability, swelling properties or 
absorbency and the like by spraying on modifying agents in the fluidized 
bed after the polymerization reaction. 
The modifying agents are as a rule dissolved or emulsified in water or a 
solvent. The temperature in the fluidized bed is controlled here by the 
temperature of the stream of inert gas entering the fluidized bed such 
that the temperature conditions necessary for initiation and progress of 
the reaction and/or drying of the modifying agent are maintained. 
A requirement of the process according to the invention, both in the 
discontinuous and in the continuous procedure, is initial introduction of 
granules before the reaction mixture is sprayed in. The particle size 
distribution of this material is chosen according to application. As a 
rule, in the case of mono-component granules, a material of the same 
composition as the product which is to be prepared is initially 
introduced. 
Monomers which are particularly suitable for the process according to the 
invention are, for example, acrylic acid, methacrylic acid, 
acrylamidopropanesulfonic acid, vinylphosphonic acid, vinylsulfonic acid, 
styrenesulfonic acid, crotonic acid, maleic acid, maleic acid half-esters, 
maleic anhydride, maleic acid half-amides, acrylamides, methacrylamides, 
vinylpyrrolidones, vinylamides, such as N-vinyl-N-methylacetamide and 
N-vinylformamide, N-vinylcaprolactam, hydroxyalkyl esters of acrylic or 
methacrylic acid, vinylpyridines, N,N-dimethyl-diallylammonium chloride 
and aminoalkyl esters and aminoalkylamides of acrylic and methacrylic 
acid. 
These monomers are preferably employed in the form of aqueous solutions. If 
they are acids, they can be employed in the form of the free acids or in 
the form of the alkali metal, ammonium or amine salts and mixtures 
thereof. Mixtures of the monomers mentioned can also be employed in the 
process according to the invention. Particularly preferred monomers are 
acrylic acid and methacrylic acid. 
Further comonomer components, which can be employed in an amount of up to 
30% by weight of the total monomers, are, for example, (C.sub.1 -C.sub.22) 
-esters of acrylic, methacrylic or maleic acid and polyoxyalkylene esters 
of these acids, vinyl esters, such as vinyl acetate, or versatic acid, as 
well as polyoxyalkylene esters of these acids, vinyl esters, such as vinyl 
acetate or versatic acid vinyl ester, styrene, vinyltoluene and 
acrylonitrile. 
Crosslinking agents which can be employed in the process according to the 
invention are compounds which contain more than one olefinically 
unsaturated group in the molecule, for example acrylic and methacrylic 
acid esters of polyhydric alcohols, such as, for example, 
trimethylolpropane triacrylate and butanediol dimethacrylate, or allyl 
ethers of polyhydric alcohols, for example neopentylglycol diallyl ether, 
tetraallyloxyethane, allylamines, such as triallylamine or 
tetraallylammonium chloride, and divinylbenzene, divinylsulfone, divinyl 
adipate and N-methylenebisacrylamide. 
Polymerization initiators which can be employed in the process according to 
the invention, if appropriate, are the customary per-compounds, such as 
dibenzoyl peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, 
potassium peroxodisulfate, sodium peroxodisulfate and ammonium 
peroxodisulfate, and azo initiators, such as azoisobutyronitrile. Redox 
systems, such as, for example, hydrogen peroxide/ascorbic acid, 
peroxodisulfate/Na pyrosulfite and aldehydesulfoxylates/peroxides, are 
preferably employed. 
Suitable modifying components for the process according to the invention 
are liquid substances, such as, for example, polyalkylene oxides, in 
particular polyethylene glycols, polypropylene glycols and polyglycols 
having a molecular weight of up to about 600, paraffins, polyamines, such 
as, for example, ethylenediamine and diethylenetriamine, 
polyethyleneimine, polyglycidyl compounds, such as, for example, ethylene 
glycol diglycidyl ether, propylene glycoldiglycidyl ether, polyethylene 
glycol diglycidyl ether and glycerol polyglycidyl ether, liquid polyhydric 
alcohols, such as, for example, glycerol, pentaerythritol, 
trimethylolpropane, neopentyl alcohol, sorbitol and polyvinyl alcohol, and 
solutions of poly(meth)acrylates, polyamidoamines, polyvinyl acetate and 
copolymers. 
Diglycidyl compounds, polyglycidyl compounds and polyamines, for example 
ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, 
diethylenetriamine and polyethyleneimine, are particularly preferred. 
In the process according to the invention, modifying components are bonded 
absorptively to the surface if they contain no groupings incorporated in 
the molecule which react chemically with the surface of the hydrogen 
particles. Modifying components which contain groupings which are capable 
of reacting chemically with the surface of the hydrogel particles can form 
covalent, ionic or complex bonds, such as, for example, polyglycidyl 
compounds, polyanions, polycations or polyvalent metal cations. 
Turning now to the Drawing, FIG. 1 illustrates substantially the complete 
apparatus 10 utilized in the process of this invention, including the 
fluidized bed reactor or zone 12 and the related feed, product discharge, 
metering, size selection (sieving), and recycling devices or conduits. The 
fluidized bed 14 is generated above the inflow tray (distribution plate or 
grid) 18 by introducing granules through a suitable feeding device 16, 
which is in communication with the region of fluidized bed zone 12 above 
the inflow tray 18, and by passing heated inert gas (introduced at 20) 
through the inflow tray 18 and the granules inside fluidized bed zone 12, 
thereby elevating the mass of granules to form the fluidized bed 14. 
A solution of reactants 22 is introduced my a metering pump 27 over the 
pressure provided by a pressurized gas and is sprayed onto the granules in 
fluidized bed 14 through multi-component nozzles, which are in the form of 
atomizer cones 24, cones 24 being fed by manifold 26. The initiator is 
introduced separately with the aid of metering pump 28 (and through its 
own separate manifold 30), but the initiator is also sprayed through the 
atomizer cones 24. 
The hot inert gas effluent (which includes water vapor from the spray 
emanating from atomizer cones 24) passes through a filter 34(cleaned by 
pressurized gas) and outlet 32 to a recovery system which includes a 
washer and condenser unit 36 and an aerosol separator 37. 
Solids are removed from the fluidized bed 14 via a pipe 38 at the base of 
zone 12 and fed to a second fluidized bed 114, similarly generated above a 
second inflow tray 118 with the aid of a relatively smaller inert gas flow 
passed upward through inflow tray 118. This use of a second fluidized bed 
114 insures trouble-free removal of granules through sluice 119. 
This stream of granules 121 is continuously removed from fluidized bed 14 
and is passed by a pneumatic conveyor 122 to a sieve 44 having a plurality 
of screens or decks 46, so that fines and oversize particles can be dealt 
with without detracting from the overall efficiency of the process. 
(Pneumatic conveyor pressure can be bled or released through filter 45.) 
Oversize particles are flowing to a mill 50, so that they can be reduced 
to the proper size and recycled via granule reservoir 52 and conduit 54 
through feeding device 16 to the fluidized bed 14, together with 
undersized particles flowing from sieve 44. Correctly-sized particles from 
the middle of sieve 44 are flowing to reservoir 55 from which the product 
is removed by a metering device 57. The proportion which exceeds the 
amount of solid fed to the fluidized bed zone 12 in the form of the 
aqueous reaction mixture, i.e. solution 22 is the overflow 56 of reservoir 
55 and comes back to the fluid bed reactor via 52, 54 and 16. 
In a preferred embodiment, sluice 119 is a cellular wheel sluice. 
FIG. 2 illustrates a typical form of modified granule 60 resulting from the 
spraying of solution 22 and initiator onto the surfaces of the granules 
fluidized in fluidized bed 14. Because of the temperature in fluidized bed 
14, at least one very thin layer of hydrophilic polymer of high swelling 
capacity forms very rapidly on granule 60 and is simultaneously dried in 
situ utilizing the heat of polymerization and/or neutralization. An 
onion-like shell 62 builds up on granule 60, and, if desired, the granular 
product 60 can be further modified with spray treatments after the 
polymerization reaction.