Process for production of highly water-absorbent resin

In a process for producing a highly water-absorbent resin comprising subjecting a mixture of (meth)acrylic acid with a water-soluble salt thereof as a monomer to reversed phase suspension polymerization in a hydrophobic solvent in the presence of a dispersing agent, improvements that at least a portion of said monomer is fed sequentially with the progress of polymerization reaction and at least a portion of said dispersing agent is fed sequentially with the progress of polymerization. The resulting resin is useful as a highly water-soluble resin component of sanitary products, or as a humectant/desiccant, a rheology modifier and so on.

BACKGROUND OF THE INVENTION 
The present invention relates to improvements in the reversed phase 
suspension polymerization process for the production of (meth)acrylic acid 
type highly water-absorbent resins. 
While a variety of water-absorbent resins are known today, the 
(meth)acrylic acid type highly waterabsorbent resins obtainable by 
subjecting a partially neutralized salt of (meth)acrylic acid to reversed 
phase suspension polymerization are particularly important. 
In regard to the technology of reversed phase suspension polymerization of 
such a partially neutralized salt of (meth)acrylic acid, a large number of 
patent applications have been filed to this day as listed below. 
Japanese Patent Publication No. 30710/1979 (Laid-open KOKAI 53-46389): a 
dispersing agent defined. 
Japanese Laid-open Patent Application KOKAI 56-26909 (1981): polymerization 
under reduced pressure. 
Japanese Laid-open Patent Application KOKAI 56-93716 (1981): an aliphatic 
alcohol used concomitantly as a medium. 
Japanese Laid-open Patent Application KOKAI 56-131608 (1981): a dispersing 
agent specified. 
Japanese Laid-open Patent Application KOKAI 56-147806 (1981): an aliphatic 
ketone is used as a medium. 
Japanese Laid-open Patent Application KOKAI 56-161408 (1981): certain two 
kinds of catalysts are used. 
Japanese Laid-open Patent Application KOKAI 57-44627 (1982): crosslinking 
with a crosslinking agent. 
Japanese Laid-open Patent Application KOKAI 57-94011 (1982): a carboxy 
groups-containing polymer is used as a dispersing agent. 
Japanese Laid-open Patent Application KOKAI 57-98513 (1982): a basic 
nitrogen-containing polymer is used as a dispersing agent. 
Japanese Laid-open Patent Application KOKAI 57-128709 (1982): a certain 
anti-tack agent is used. 
Japanese Laid-open Patent Application KOKAI 57-158209 (1982) : a certain 
protective colloid is used. 
Japanese Laid-open Patent Application KOKAI 57-167302 (1982): a certain 
nonionic surfactant is used. 
Japanese Laid-open Patent Application KOKAI 57-192416 (1982): a certain 
anti-tack agent is used. 
Japanese Laid-open Patent Application KOKAI 57-198714 (1982): partial 
dehydration of a polymer slurry. 
Japanese Laid-open Patent Application KOKAI 58-32607 (1983): addition of a 
surfactant during or after polymerization. 
Japanese Laid-open Patent Application KOKAI 58-42602 (1983) reacting the 
polymer with a crosslinking agent. 
Japanese Laid-open Patent Application KOKAI 58-117222 (1983): reacting with 
a crosslinking agent. 
Japanese Laid-open Patent Application KOKAI 59-8711 (1984) addition of 
inorganic powders during or after polymerization. 
Japanese Laid-open Patent Application KOKAI 59-62665 (1984): crosslinking 
with a crosslinking agent. 
Japanese Laid-open Patent Application KOKAI 60-179410 (1985): a tertiary 
amino group-containing monomer is concomitantly used. 
Japanese Laid-open Patent Application KOKAI 60-186506 (1985): reaction with 
a crosslinking agent after polymerization. 
Japanese Laid-open Patent Application KOKAI 61-40309 (1986): a certain 
protective colloid is used 
Japanese Laid-open Patent Application KOKAI 61-43606 (1986): a sucrose 
fatty acid ester is used as a protective colloid 
Japanese Laid-open Patent Application KOKAI 61-53308 (1986): certain two 
kinds of protective colloids are used in combination. 
Japanese Laid-open Patent Application KOKAI 61-69812 (1986): reaction with 
a monoglycidyl compound. 
Among the above literature, 
Japanese Laid-open Patent Publication KOKAI 57-158209 (1982), ditto KOKAI 
58-32607 (1983), ditto KOKAI 60-179410 (1985) and ditto KOKAI 60-186506 
(1985) give examples in which the monomer is fed by dripping to a 
hydrophobic medium. 
Among the above literature, Japanese Laid-open Patent Publication KOKAI 
58-32607 (1983) describes that the addition of a water-soluble or 
water-dispersible surfactant during or after polymerization results in an 
improvement in water absorption characteristics inclusive of initial water 
absorbency. In this prior art, an oil-soluble cellulose ester or cellulose 
ether is essentially used as a protective colloid and no surfactant is 
used as a protective colloid. 
It is stated therein that the surfactant is added either "during" or 
"after" polymerization and that generally the addition thereof to a 
polymer slurry after polymerization reaction is preferred. 
As to why the addition of a surfactant results in an improved 
water-absorbing characteristic, the inventor concerned postulates that the 
surfactant breaks the protective colloid covering polymer particles and 
imparts penetrability to the whole water-absorbent polymer. 
As examples of said water-soluble or water-dispersible surfactant, the same 
patent literature lists nonionic surfactants with HLB values not less than 
7, such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol 
ethers, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene fatty 
acid esters, polyoxy- ethylene-oxypropylene block polymers, etc.; anionic 
surfactants such as salts of fatty acids, alkylnaphthalenesulfonic acid, 
dialkyl sulfosuccinates, alkyl sulfate ester salts, higher alcohol sulfate 
ester salts, etc., and cationic surfactants such as alkylamine salts, 
alkyl quaternary ammonium salts and so on. 
Any highly water-absorbent resin is required to satisfy the following 
requirements. 
(a) It does not undergo uneven gelation on contact with water or body 
fluid. 
(b) When brought into contact with water or body fluid, it shows a high 
absorption rate and a large absorption capacity. 
(c) Even under stress loading, it retains a high absorption rate and a 
large absorption capacity for water and body fluid. 
(d) It shows a high gel strength when swollen on absorption of water or 
body fluid. 
(e) In production, it can be obtained in an appropriate particle size range 
(100-300 .mu.m) in good yield. 
However, the (meth)acrylic acid type highly absorbent resin prepared by the 
usual reversed phase suspension polymerization process fails to well 
satisfy the requirements (b) and (e) although it meets the other 
requirements. 
The present inventors also had a preconception, prior to this invention, 
that once charged in a single dose, a dispersing agent remains at 
substantially the same concentration level throughout polymerization 
reaction and accordingly made trials and errors on the supposition that a 
homogeneous polymer would be forthcoming coming if an ingenuity be 
excercised in the manner of monomer charging. The outcome was, however, a 
virtual failure. 
Then, the inventors suspected that even if the dispersing agent fed in a 
single dose may not be destroyed or otherwise eliminated during 
polymerization, its effective concentration may undergo change in the 
course of polymerization reaction. The present invention is based on 
results of research endeavors made along the above way of thinking. 
Thus, through an ingenuity exercised in respect to the conditions of 
reversed phase suspension polymerization, the present invention has 
successfully made it possible to produce a (meth)acrylic acid type highly 
water-absorbent resin meeting all the aforementioned requirements (a) 
though (e). 
SUMMARY OF THE INVENTION 
The present invention is concerned with a method of producing a 
(meth)acrylic acid type highly waterabsorbing resin which comprises 
subjecting a monomer comprising a mixture of (meth)acrylic acid with a 
water-soluble salt of (meth)acrylic acid to reversed phased suspension 
polymerization in a hydrophobic medium by feeding at least a portion of 
the monomer sequentially according to the progress of polymerization and 
feeding at least a portion of a dispersing agent sequentially according to 
the progress of polymerization reaction. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is carried into practice using as a monomer a mixture 
of (meth)acrylic acid with a water-soluble salt of (meth)acrylic acid. The 
watersoluble salt may be the corresponding sodium salt, potassium salt, 
ammonium salt or the like. 
The proportions of (meth)acrylic acid and said water-soluble salt of 
(meth)acrylic acid in said mixture are preferably selected from the range 
of 10/90 through 50/50 in molar ratio. Outside this range, the water 
absorption characteristic of the resulting resin will not be as 
satisfactory as desired. 
The mixture of (meth)acrylic acid with a water-soluble salt thereof can be 
generally obtained by partial neutralization of (meth)acrylic acid with an 
alkali such as sodium hydroxide, potassium hydroxide, ammonia or the like. 
The above monomer may be used in combination with small proportions of 
other vinyl monomers such as (meth)acrylic esters, unsaturated sulfonic 
acids or salts thereof, (meth)acrylamide, (meth)acrylonitrile, vinyl 
esters, vinyl ethers and so on. 
As the hydrophobic medium, there may be employed C.sub.6-10 hydrocarbons 
and halogenated aromatic hydrocarbons such as hexane, heptane, octane, 
cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, 
cyclooctane, decalin, benzene, toluene, xylene, chlorobenzene, 
dichlorobenzene, ethylbenzene and so on. In consideration of boiling point 
and commercial availability, cyclohexane and n-hexane are particularly 
preferred. 
As the dispersing agent, any dispersing agent capable of dispersing an 
aqueous solution of the monomer into said hydrophobic medium can be 
employed, irrespective of whether it is a surfactant or a polymeric 
dispersing agent. However, nonionic surfactants with HLB values in the 
range of 3 to 12 and particularly those sorbitan fatty acid esters and 
sucrose fatty acid esters which have HLB values in the range of 3 to 12 
are most suitable for the purposes of the invention. 
The polymerization reaction is carried out by dispersing an aqueous 
solution of the monomer in a hydrophobic medium with the aid of a 
dispersing agent. 
This is in contrast with the usual suspension polymerization which is 
conducted by dispersing the very monomer in an aqueous medium and is 
therefore called reversed phase suspension polymerization. 
The polymerization catalyst is added to the aqueous monomer solution. The 
use of a chain transfer agent is not required but if used, it is added to 
the aqueous monomer solution. 
The preferred polymerization temperature is in the range of about 
50.degree. C. to the reflux temperature of the reaction system. 
In the practice of the invention, at least a portion of the monomer is fed 
sequentially according to the progress of polymerization reaction and at 
the same time, at least a portion of the dispersing agent is also fed 
sequentially according to the progress of reaction. That is to say, there 
is no initial charge of the monomer or the dispersing agent or their 
initial charge is limited to a given quantity and the remainder is fed in 
small portions (especially dropwise). Such additional feeds of the monomer 
and dispersing agent may be simultaneous or independent. 
The preferred process parameter settings are as follows. 
Mi/Ma=0/100-50/50 
Di/Da=0/100-60/40 
D/M=1/100-6/100 
Di/S=0/100-0.30/100 
where M is the total monomer charge, Mi is the initial monomer charge, Ma 
is the additional monomer charge, D is the total charge of the dispersing 
agent, Di is the initial charge of the dispersing agent, Da is the 
additional charge of the dispersing agent, S is the charge of the 
hydrophobic medium (all by weight; the subscripts i and a stand for 
initial and additional, respectively). It is especially desirable to set 
Mi/Ma in the range of 0/100 to 20/80 and Di/Da in the range of 0/100 to 
40/60. It is more desirable to set Mi/Ma in the range of 0/100 to 10/90 
and Di/Da in the range of 1/99 to 30/70. 
During or after polymerization, there may be added, as required, various 
additives such as a crosslinking agent, an inorganic particulate substance 
and so on. 
The highly water-absorbent resin obtainable by the process of this 
invention finds application as, for example, a body fluid absorbent in 
sanitary products, a soil improving agent, a seed coating agent, a water 
sealant, a thickner or rheology modifier, anti-condensation agent, a 
sludge flocking agent, a desiccant, a humectant or the like. 
Since, in accordance with the invention, at least a portion of the monomer 
is sequentially fed to the reaction system according to the progress of 
polymerization reaction, the monomer charge is kept in balance with the 
polymer production so that the concentration of the unpolymerized monomer 
in the hydrophobic medium is maintained in a definite range. 
In addition, because at least a portion of the dispersing agent is 
sequentially fed to the system according to the progress of polymerization 
reaction, the effective concentration of the dispersing agent in the 
hydrophobic medium is also maintained in a definite range. 
The reason why the effective concentration of the dispersing agent fed 
initially varies in the course of polymerization is not clear but it is 
suspected that the dispersing agent loses its monomer-dispersing power 
with the progress of polymerization reaction or is adsorbed on the polymer 
formed to undergo attrition of its functionality as a dispersing agent. 
Thus, although it is not conceivable that the added dispersing agent as 
such is destroyed or lost, it appears that its monomer-dispersing ability 
is gradually lost with the progress of polymerization to cause a decrease 
in the "effective concentration of the dispersing agent". 
Since, in accordance with the invention, both the concentration of the 
monomer and the effective concentration of the dispersing agent in the 
hydrophobic medium are each maintained in a definite range, the 
environment surrounding the suspended monomer during polymerization is 
kept constant, thus assuring the formation of a highly water-absorbent 
polymer meeting all the aforesaid requirements (a) through (e), which are 
reiterated here as follows. 
(a) It does not undergo uneven gelation on contact with water or body 
fluid. 
(b) When brought into contact with water or body fluid, it shows a high 
absorption rate and a large absorption capacity 
(c) Even under stress loading, it retains a high absorption rate and a 
large absorption capacity for water and body fluid. 
(d) It shows a high gel strength when swollen on absorption of water or 
body fluid. 
(e) In production, it can be obtained in an appropriate particle size range 
(100-300 .mu.m) in good yield.

EXAMPLES p The following examples are further illustrative of the 
invention. In these examples, all parts and % are by weight. 
EXAMPLE 1 
A highly water-absorbent resin was prepared by the following procedure. 
1. A two-liter round-bottomed separable flask (A) fitted with a stirrer, 
reflux condenser and nitrogen gas inlet line was charged with 1000 ml (780 
g) of cyclohexane as a hydrophobic medium and 0.2 g of sorbitan 
monostearate (HLB=4.7) as a dispersing agent. Then, nitrogen gas was 
bubbled into the mixture for 30 minutes to drive out the dissolved air and 
plenum air and the mixture was maintained at 70.degree. C. 
2. Another separable flask (B) was charged with 140 g of acrylic acid 
(reagent special grade) and, then, a solution of 62 g of sodium hydroxide 
(purity 95%) in 185 g of water was gradually added dropwise thereto under 
stirring and cooling for neutralization. Then, nitrogen gas was bubbled 
into the mixture under stirring to drive out the dissolved air. 
Thereafter, 4.2 ml of a 2% aqueous solution of ammonium persulfate and 1.4 
ml of a 1% aqueous solution of N,N'-methylenebisacrylamide, which had been 
stripped with nitrogen gas, were added into the flask and the mixture was 
stirred thoroughly. The mixture was then transferred to a drip funnel (C) 
which had been purged with nitrogen gas. 
3. A drip funnel (D) was charged with a solution of 4 g of sorbitan 
monostearate in 100 ml (78 g) of cyclohexane, and nitrogen gas was bubbled 
into the solution. 
4. While the mixture in flask (A) was maintained at 0.degree. C. with 
stirring, the solution in drip funnel (C) was added, by dripping, to flask 
(A) over a period of 60 minutes. At the same time, the solution in drip 
funnel (D) was also dripped into the flask (A) over 60 minutes. After 
completion of dropwise addition, the reaction was further conducted at 
70.degree.-75.degree. C. for 60 minutes, with the byproduct water being 
distilled off azeotropically with cyclohexane. 
5. After completion of the reaction, the reaction system was cooled and the 
product polymer was recovered by filtration through a 325-mesh wire 
filter, washed with warm cyclohexane and dried in vacuo at 80.degree. C. 
The parameter settings used in the above operation were as follows. 
Mi/Ma: 0/100 
Di/Da: 4.8/95.2 
D/M: 3/100 
Di/S: 0.0233/100 
EXAMPLE 2 
Reversed phase suspension polymerization was conducted in the same manner 
as Example 1 except that the initial to additional charge ratio Di/Da of 
sorbitan monostearate (HLB=4.7) was adjusted to 20/80. The parameter 
settings were as follows. 
Mi/Ma: 0/100 
Di/Da: 20/80 
D/M: 3/100 
Di/S: 0.098/100 
EXAMPLE 
Reversed phase suspension polymerization was conducted in the same manner 
as Example 1 except that sorbitan monolaurate with an HLB value of 8.6 was 
used as the dispersing agent and that the following parameter settings 
were used. 
Mi/Ma: 0/100 
Di/Da: 10/90 
D/M: 5/100 
Di/S: 0.1/100 
EXAMPLE 4 
Reversed phase suspension polymerization was conducted in the same manner 
as Example 1 except that sucrose stearic acid ester with an HLB value of 
3.6 was used as the dispersing agent and that the following parameter 
settings were used. 
Mi/Ma: 10/90 
Di/Da: 30/70 
D/M: 4/100 
Di/S: 0.196/100 
EXAMPLE 5 
Reversed phase suspension polymerization was conducted in the same manner 
as Example 1 except that sucrose stearic acid ester with an HLB value of 
3.6 was used as the dispersing agent and that the following parameter 
settings were used. 
Mi/Ma: 0/100 
Di/Da: 20/80 
D/M 4/100 
Di/S: 0.13/100 
Reference Example 1 
Reversed phase suspension polymerization was conducted in the same manner 
as Example 1 except that the initial to additional charge ratio of the 
monomer 
was changed to Mi/Ma=30/70 and that the initial to additional charge ratio 
of sorbitan monostearate (HLB=4.7) as the dispersing agent was changed to 
Di/Da=30/70. The parameter settings were as follows. 
Mi/Ma: 30/70 
Di/Da: 30/70 
D/M: 3/100 
Di/S: 0.147/100 
Comparative Example 1 
In this comparative example, the whole amount of the aqueous monomer 
solution and of the dispersing agent were fed both in a single dose at 
commencement of polymerization reaction. 
A highly water-absorbent resin was prepared by the following procedure. 
1. The step 1 of Example 1 was repeated except that flask (A) was charged 
with 1100 ml (858 g) of cyclohexane and 4.2 g of sorbitan monostearate 
(HLB=4.7) as the dispersing agent. 
2. The step 2 of Example 1 was repeated except that the transfer from flask 
(B) to drip funnel (C) was omitted. Then, flask (B) was heated at an 
internal temperature of 70.degree. C. 
3. The step 3 of Example 1 was omitted. 
4. While the mixture in flask (A) was maintained at 70.degree. C. with 
constant stirring, the solution in flask (B) was transferred to flask (A) 
using care to avoid exposure of the solution to atmospheric air. After the 
transfer, the reaction was carried out at 70.degree.-75.degree. C. for 2 
hours, with the byproduct water being distilled off azeotropically with 
cyclohexane. 
5. The step 5 of Example 1 was repeated. 
The parameter settings in the above operation were as follows. 
Mi/Ma: 100/0 
Di/Da: 100/0 
D/M: 3/100 
Di/S: 0.49/100 
Comparative Example 2 
In this comparative example, the polymerization reaction was conducted in 
the same manner as Example 1 except that the whole amount of the 
dispersing agent was fed in a single dose at commencement of the reaction. 
A highly water-absorbent resin was prepared by the following procedure. 
1. The step 1 of Example 1 was repeated except that flask (A) was charged 
with 1100 ml (858 g) of cyclohexane and 4.2 g of sorbitan monostearate 
(HLB=4.7) as the dispersing agent. 
2. The step 2 of Example 1 was repeated. 
3. The step 3 of Example 1 was omitted. 
4. While the mixture in flask (A) was maintained at 70.degree. C. with 
constant stirring, the solution in drip funnel (C) was transferred 
dropwise to flask (A) over a period of 60 minutes. After completion of 
dropwise (reaction was further conducted at 70.degree.-75.degree. C. 
addition, the for 60 minutes, with the byproduct water being distilled off 
azeotropically with cyclohexane. 
5. The step 5 of Example 1 was repeated. 
The parameter settings in the above operation were as follows. 
Mi/Ma: 0/100 
Di/Da: 100/0 
D/M: 3/100 
Di/S: 0.49/100 
The particle sizes and various properties of the polymers prepared in 
Examples 1 to 5, Reference Example 1 and Comparative Examples 1 and 2 
above are set forth in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Swelling 
Absorption Ion exchanged water 
speed under 
capacity (times) Incidence 
load Average particle 
Percentage of 
Ion exchanged Physiological 
Swelling 
of uneven 
(times/5) 
Gel size of polymer 
particles 
water saline time (sec) 
gelation 
min.) strength 
(.mu.m) .ltoreq.74 (wt. 
__________________________________________________________________________ 
%) 
Example 1 
350 60 120 None 45 .circleincircle. 
180 3 
Example 2 
300 55 300 None 35 .circle. 
150 8 
Example 3 
350 60 120 None 45 .circle. 
130 4 
Example 4 
300 58 180 None 40 .circle. 
150 7 
Example 5 
300 58 160 None 40 .circle. 
180 5 
Ref. 300 50 .gtoreq.600 
Occurred 
15 .DELTA. 
100 15 
Example 1 
Comp. 300 50 .gtoreq.600 
Occurred 
15 .DELTA. 
70 20 
Example 1 
Comp. 300 50 .gtoreq.600 
Occurred 
15 .DELTA. 
80 18 
Example 2 
__________________________________________________________________________ 
The conditions of determinations were as follows. 
Absorption capacity for ion exchanged water or physiological saline 
A 500-ml beaker was charged with 0.2 g (on a dry basis) of each particulate 
polymer, followed by addition of 200 g of ion exchanged water or 60 g of 
physiological saline (0.9% aqueous sodium chloride solution). The mixture 
was gently stirred with a glass rod and, then, allowed to stand at room 
temperature for 1 hour. The mixture was then filtered through a 325-mesh 
wire filter and the residual gel was weighed. The absorption capacity was 
calculated by means of the following equation. 
EQU Absorption capacity =(weight of filtration residue gel (g) -0.2)/0.2 
Ion exchanged water absorption rate and incidence of uneven gelation 
In a 100-ml measuring cylinder containing 100 ml of ion exchanged water was 
put 0.5 g (on a dry basis) of the test particulate polymer to cause 
swelling of the polymer particles and the incidence of uneven gelation was 
investigated by visual inspection. In addition, a stopwatch was started at 
addition of the particulate polymer and the time to complete coverage with 
gel was measured and regarded as swelling time. 
Swelling speed under load 
There was provided a funnel constructed so that its top is configured as a 
cylinder with an inside diameter of 30 mm and its bottom configured as a 
cone with a shank consisting in a small-diameter tube extending downwardly 
from the center of the conicalcylindrical portion. 
A glass filter (G-2) was installed at the depth of 25 mm from the top end 
of the funnel and 0.5 g of the test particulate polymer was uniformly 
placed on the glass filter. Then, a weight having a diameter substantially 
equal to the inner diameter of the cylindrical portion of the funnel was 
put on the particulate polymer so that the polymer particles would be 
subjected to a load of 15 g/cm.sup.2. 
The bottom end of a buret containing physiological saline (0.9% aqueous 
solution of NaCl) was connected to the lower end of the aforementioned 
shank of the funnel with a polyvinyl chloride tube and the cock of the 
buret was opened. The weight of the liquid in the buret was previously 
adjusted so that the top surface of the glass filter would be wetted with 
physiological saline. The amount of liquid in the buret was kept constant 
by adding physiological saline continuously. 
After 5 minutes, the swollen gel was taken out and weighed and the amount 
of physiological saline absorbed in 5 minutes was determined in the unit 
of times the dead weight of the polymer. 
Gel strength 
A 0.5 g portion of the filtration residue gel obtained in the determination 
of physiological saline absorption capacity was taken on the palm and 
rubbed against by the palms of both hands to assess the gel strength 
organoleptically.