Particulate absorbent material

The present invention features methods of making absorbent material which exhibit excellent absorption for saline and other liquids as well as being biodegradable. The methods of the invention form a particulate which can be stored in dry form and rehydrated at any time. The particulate can be used to replace the presently utilized polyacrylate superabsorbers. The base material used in the methods of the invention is a carboxylated cellulosic material such as carboxymethylcellulose, preferably, a carboxymethylcellulose having a DS, or Degree of Substitution, of 0.5 or greater. The carboxylated cellulose material is reacted with two distinct agents; a cross-linking agent and a hydrophobicity agent to make the final absorbent.

BACKGROUND OF THE INVENTION 
The present invention relates to absorbent particles which swell in the 
presence of aqueous solutions including saline and urine to many times 
their own weight. These absorbent materials are biodegradable and have 
absorptive properties equal to or higher than those of present 
superabsorbers. 
Biodegradability of disposable products is no longer a preferred option; it 
has become a necessity. As the number of disposables in society has grown, 
the land fills and others methods of treatment for these disposables have 
been strained to the limits. Plastics are just one form of the problem of 
disposables while absorbent materials such as polyacrylates which are 
commonly used, e.g., in disposable diapers, have degradation times in the 
thousands of years. While the superabsorber such as the polyacrylates have 
advantages because of their high absorptive capabilities so that less is 
needed to attract a large amount of liquid, the non-biodegradability of 
these products makes them unacceptable if alternatives can be achieved. 
Particulate superabsorbers can be used either in lose or packed form or 
else can be dispersed among fibers, e.g., cellulose fluff, to act as part 
of a liquid absorption system. However, if they are used in a packed form, 
the gel blocking problem must be ameliorated in order to provide optimum 
action. 
Accordingly, an object of the invention is to provide a biodegradable 
superabsorbent particle which is competitive with the polyacrylate 
superabsorbers in terms of uptake for aqueous solutions and saline. 
Another object of the invention is to provide methods of making 
biodegradable superabsorbers which use inexpensive material and are rapid. 
A further objection of the invention is to provide a superabsorber which 
can act as a delivery system for a variety of materials, e.g., enzymes. 
These and other objects and features of the invention will be apparent from 
the following description. 
SUMMARY OF THE INVENTION 
The present invention features methods of making absorbent material which 
exhibit excellent absorption for saline and other liquids as well as being 
biodegradable. The methods of the invention form a particulate which can 
be stored in dry form and rehydrated at any time. The particulate can be 
used to replace the presently utilized polyacrylate superabsorbers. 
The absorptive material made using the methods of the invention differs 
from acrylate absorbents in that it is biodegradable; and it is 
hydrophobic which assists in limiting gel blocking and reduces clumping. 
The hydrophobic aspect of the present particulates makes them distinct 
from earlier superabsorbers. 
The base material used in the methods of the invention is a carboxylated 
cellulosic material such as carboxymethylcellulose. However, any cellulose 
derivative which has substantial carboxylation can be used. The preferred 
carboxylated cellulose is a carboxymethylcellulose having a DS, or Degree 
of Substitution, of 0.5 or greater, most preferably greater than 0.7. 
Carboxymethylcellulose with this high Degree of Substitution can have such 
a large amount of cross-linking that it would form an unworkable, almost 
glue-like material without the hydrophobicity treatment of the present 
invention. 
The carboxylated cellulose material is reacted with two distinct agents; a 
cross-linking agent and a hydrophobicity agent. The order of reaction can 
change the properties of the final absorbent, with the reaction first with 
the cross-linking agent yielding more of a shell-like absorber and, 
consequently, a firmer particle, while their reaction with the 
hydrophobicity agent first will yield a particle having higher overall 
absorptive capabilities with absorption throughout the body of the 
particle. The preferred cross-linking agents are those which are metal 
containing and include a metal with an effective valence of at least three 
such as aluminum, chromium, or iron. The most preferred cross-linking 
agents are acetates, alkoxides such as ispropoxides and hydroxides, and 
chlorides. These include materials such as aluminum acetate, aluminum 
isopropoxide, aluminum hydroxide, ferric chloride, and mixtures thereof. 
Hydrophobicity agents useful in the methods of the invention include 
monobasic and polybasic carboxylic acids or their salts, chlorides or 
anhydrides, most preferably those with 2-16 carbon atoms. Examples of 
useful hydrophobicity agents include acetic acid, proprionic acid, butyric 
acid, isobutyric acid, acetyl chloride, sodium acetate, sodium 
proprionate, proprionyl chloride, sodium butyrate, acetic anhydride, 
proprionyl anhydride, succinic acid, adipic acid, phthalic acid, citric 
acid, and mixtures thereof. 
The reactions between the carboxylated cellulosic material and the 
cross-linking and/or hydrophobicity agent can be carried out in either 
aqueous or organic solutions. If an aqueous solvent is used, a moderate 
concentration saline, e.g., 0.9%, is preferred to get a better charge 
separation in the interior of the particle while organic solvent such as 
neutral petroleum spirits may be use if the reactants chosen are not 
readily soluble in aqueous solutions. In one most preferred embodiment to 
the invention, carboxymethylcellulose is first pre-swollen in an aqueous 
solution which allows better access of the cross-linking and 
hydrophobicity agents to the interior of the particle. In addition, 
pre-treatment of the carboxymethylcellulose with a small quantity of an 
alcohol such as isopropyl alcohol may improve wetting and, accordingly, 
the reactions. 
The following description will further explain the methods of the 
invention. 
DESCRIPTION OF THE INVENTION 
The preferred absorbers of the present invention use a 
carboxymethylcellulose having a DS of 0.7 or above in order to provide 
sufficient cross-linking while allowing the hydrophobicity agent to 
eliminate the glue-like clumping problem. Although it is not necessary for 
understanding the invention, it is theorized that the metal ion, e.g., 
aluminum, reacts with the carboxyl groups on the adjacent chains of 
carboxymethylcellulose, forming an ionic cross-link between the chains. 
The aluminum has a third group thereon, most normally a hydroxide group 
although it could be an isopropyl or acetate group. The hydrophobic group, 
which most preferably is a small group such as acetate or proprionate is 
linked to a carboxymethylcellulose residue by aluminum ions as already 
described for cross-linking. The reason that the shorter chain carboxylic 
acids are preferred, e.g., acetate or proprionate is that with the same 
Degree of Substitution using larger molecules such as benxoic or palmitic 
acid, the hydrophobicity is so great that the water is not as easily 
accessible to the interior of the molecule. To use the longer chain 
molecules the degree of hydrophobic substitution must be much lower. 
The reason why the particulates of the present invention work so well as 
superabsorbers is not completely understood but is theorized that the 
Donnan effect may be involved. The Donnan effect relates to a charge 
separation whereby having a high concentration of net negative charge in 
the interior of the particle will cause flow of saline. This type of 
effect is expected since the absorption for the particulates made using 
the present methods is improved for saline as compared with a salt free 
aqueous solution. 
The following Examples will further illustrate the methods of the 
invention.

EXAMPLE 1 
In this Example, an organic method of making the particulate is described. 
In this, and all the following Examples, similar carboxymethylcellulose 
(CMC) was used. The carboxymethylcellulose had a DS of about 0.7 and was 
first sieved to remove any particles smaller than 500 microns. The 
particular CMC used was CMC 7H obtained from Aqualon. 
Five g of CMC was dissolved in 4 ml of petroleum ether having a boiling 
point of 70-90.degree. C. The petroleum ether contained 1 g of aluminum 
isopropoxide (Manalox 130, Manchem, Princeton, N.J.). The resulting slurry 
was stirred at 45-55.degree. C. in a closed vessel. After 2-4 hours 
(determined by no further release of isoproponal), 0.4-0.8 g of anhydrous 
glacial acetic acid is added and stirring is continued for another 1-2 
hours. After completion of the reaction, the solvent is removed from the 
slurry by filtration, the slurry is washed with other solvents such as 
petroleum ether and/or anhydride isopropl alcohol and air dried. 
The resulting product was tested by uptake in capillary action with 0.15 N 
NaC1 under applied load of 0.22 lbs./in.sup.2 for sixty minutes at room 
temperature. The amount of fluid uptake was then measured gravimetrically. 
The superabsorber absorbed 15 ml saline/g superabsorber. 
EXAMPLE 2 
In this Example, the same carboxymethylcellulose was formed into a particle 
using an aqueous procedure. First, approximately 1 g of the CMC was 
pre-wet with 0.4 g of an alcohol such as isopropyl alcohol. The alcohol 
was removed and then 10 g of saline containing aluminum acetate (20 mg/g 
CMC) and 40 mg glacial acetic acid was added. The reaction was allowed to 
proceed for 4 hours at 50.degree. C. The swollen particles were then 
removed and dried under a hot air flow. 
Using the same test of described in Example 1, the saline uptake under load 
was approximately 18 ml/g of superabsorber. 
EXAMPLE 3 
In the Example, the carboxymethylcellulose was first allowed to swell for 
several hours in normal saline (0.9%) before the reactions described in 
Example 2 were carried out. By allowing the particles to swell to five 
times their initial weight before reaction, a value of approximately 17 
ml/g was obtained using the weight test. By allowing the particles to 
swell to approximately ten times their initial volume, a value of 19 ml/g 
was obtained. The pre-swelled described herein was "free" swelling, e.g., 
swelling without any applied load. 
There are several factors which can be modified in order to obtain optimum 
performance for a particular task. First, a more highly cross-linked 
absorber will exhibit a slower rate of saline uptake that will be able to 
hold more total saline. Further, if the reaction between the hydrophobic 
agent and the carboxylated cellulosic material is carried out before the 
cross-linking, a more absorbent material is formed. In any case, not all 
of the carboxyl groups were involved in the cross-linking. The highly 
cross-linked materials have 15-20 mole percent cross-linked while in some 
materials, such as that described in Example 2, only about 1 mole percent 
cross-linking is used. 
EXAMPLE 4 
In this Example, a low molecular weight carboxymethylcellulose (CMC) was 
used as a basis of the particulate superabsorber. Two distinct 
hydrophobicity agents, a monobasic acid and a bibasic acid, were used to 
modulate the cross-linking so as to obtain a product having improved 
properties. Without the use of both hydrophobicity agents, the material 
becomes too heavily cross-linked to use. 
One gram of low molecular weight CMC (Akzo PL820) was mixed with 0.5 g of 
isopropanol. Thirty grams of a cross-linking/hydrophobicity agent solution 
was then added. This solution was made with 90 mg NaC1, 20 mg aluminum 
acetate/borate, 40 mg glacial acetic acid, and 50 mg succinic acid in 28 
ml water. The resulting solution is stirred continuously until a 
homogeneous gel is formed. The gel is then transferred into a syringe and 
injected into 150 g of an isopropyl alcohol solution through a 16 gauge 
needle. A white precipitate in the form of small fibers appears in the 
alcohol solution. The alcohol is removed by filtration and evaporation and 
the resulting fibers are air dried. 
Using the same test as described in Example 1, the material showed an 
absorption of 15 ml saline/g superabsorber under load. The particulate 
superabsorber also showed a free-swell (swelling without any applied load) 
of 48 ml saline/g superabsorber. 
A further test was conducted with this material by placing the free-swollen 
superabsorber in a "tea-bag" of a non-woven fabric with a 150 mm mesh. The 
tea-bags were transferred to 50 ml centrifuge tubes and centrifuged to 
3000 X g for 30 minutes. At the end of 30 minutes centrifugation, the free 
fluid was collected. The amount of liquid released was determined 
gravimetrically. The results show a liquid retention after centrifugation 
of approximately 22 ml saline/g superabsorber. 
In addition to those described above, other means of making a particulate 
could be used. For example, a gel could be formed which is then dehydrated 
and ground to particulate form. Although this procedure can be useful, see 
Example 1 of the previously cited U.S. Pat. Application Ser. No. 320,944, 
is unlikely to yield particles of a single size without significant waste 
of material. Further, tests of particulate made using the grinding 
procedure without adding additional cellulose (such as described in the 
aforementioned Example) yielded an absorption under load of 10-18 ml/mg. 
The values using the methods described herein are much more consistent and 
simpler without yielding problems of gel blocking. 
Those skilled in the art will be able to determine other modifications of 
the Example procedures and materials set forth herein. Such other 
modifications are within the scope of the following claims.