Absorbent articles and methods for their preparation

Water swellable absorbent articles, made from solutions of carboxylic polyelectrolytes, together with methods for their preparation, and a composition useful to make said articles are disclosed. The articles are cured and/or crosslinked with polyamide-polyamine epichlorohydrin adducts by heating and/or removing subsantially all of the solvent from the precursor composition. The absorbent articles are useful as surgical sponges, diapers, tampons, meat trays, bath mats and the like.

BACKGROUND OF THE INVENTION 
This invention relates to water swellable absorbent articles made from 
crosslinked polyelectrolytes, methods for their preparation, and to a 
composition containing polyelectrolytes and 
polyamido-polyamine/epichlorohydrin adducts which is useful to make 
absorbent articles. 
It is known from U.S. Pat. Nos. 3,669,103 and 3,670,731 that cross-linked 
polymeric sorbents can be sandwiched between flexible supports to achieve 
disposable diapers or dressings. 
It is further known from U.S. Pat. Nos. 2,988,539; 3,393,168; 3,514,419 and 
3,357,067 that water swellable cross-linked carboxylic copolymers can be 
prepared. However, these prior art copolymers are all crosslinked during 
copolymerization or crosslinked after polymerization with subsequent 
neutralization of the carboxylic acid groups to form water swellable 
polyelectrolytes and hence these prior art polyelectrolytes cannot be 
crosslinked in situ as a coating on a substrate or as a flexible film 
thereof. 
The polyamido-polyamine epichlorohydrin adducts used herein have been used 
to cure water soluble polymers to water insoluble films as is shown by 
U.S. Pat. No. 3,224,986 to Butler, et al. However, this patent does not 
indicate how to make water swellable products. 
The foregoing adducts are believed to be water soluble resins having a 
plurality of 3-hydroxy azetidinium, epoxy propyl, and chlorohydrin groups 
as is shown by Carr et al., J. Applied Polymer Science 17:721-735 (1973). 
SUMMARY OF THE INVENTION 
The patent application by J. R. Gross, Ser. No. 468,794, filed May 9, 1974 
now U.S. Pat. No. 3,980,663, discloses methods of curing polyelectrolytes 
to water swellable articles wherein the curing or crosslinking agent is a 
polyhaloalkanol, a sulfonium zwitterion, a haloepoxyalkane or a 
polyglycidyl ether. 
It now has been discovered that faster cures, at lower temperatures, can be 
obtained by using polyamido-polyamine epichlorohydrin adducts. 
The present invention comprises a composition which is useful to form water 
swellable articles of a carboxylic type synthetic polyelectrolyte which 
consists of lower alcohols, water, or mixtures thereof, about 5 to about 
60 percent, preferably about 15 to about 40 percent by weight based on the 
solvent of a carboxylic polyelectrolyte, and an amount of a water soluble 
polyamido-polyamine/epichlorohydrin adduct sufficient to cure the 
polyelectrolyte into a water swellable article. 
The invention further comprises methods of making discrete films, absorbent 
articles, particulates, fibers, and the products of these processes 
wherein the above composition on various substrates, is dried to crosslink 
the polyelectrolyte. The use of elevated temperatures is advantageous to 
accelerate the crosslinking and drying of the polyelectrolyte. However, if 
desired, the use of heat can be eliminated. 
In order to obtain very high production rates of absorbent articles, it may 
be desirable to replace part or nearly all of the water in the 
polyelectrolyte solution with a lower alcohol such as methanol or ethanol. 
This substitution results in lower solution viscosities at a given percent 
solids and promotes rapid drying. 
The final products of the present invention are thus water swellable and 
are useful where ever aqueous solutions need to be absorbed. Examples of 
the diverse utilities are surgical sponges, catamenial tampons, diapers, 
meat trays, paper towels, disposable door mats, disposable bath mats and 
disposable litter mats for household pets. 
DETAILED DESCRIPTION 
Examples of carboxylic synthetic polyelectrolytes useful in this invention 
are the ammonium or alkali metal salts of homopolymers of acrylic or 
methacrylic acid and copolymers with one or more ethylenically unsaturated 
comonomers. The only limitation being that any copolymer, to be useful in 
preparing highly absorbent polymers according to this invention, must be 
essentially water soluble in the sale form. The alternating copolymers of 
maleic anhydride and the maleic and fumaric acids and esters are useful 
when rendered water soluble by an appropriate base. One skilled in the art 
of radical addition copolymerization could prepare any number of suitable 
heteropolymers containing sufficient carboxylate functionality to render 
them water soluble and thus be useful in this invention. 
A list of applicable carboxylic polymers which could be prepared from 
readily available monomers and converted into their salt form is as 
follows: 
acrylic acid -- acrylate copolymers 
acrylic acid -- acrylamide copolymers 
acrylic acid -- olefin copolymers 
polyacrylic acid 
acrylic acid -- vinyl aromatic copolymers 
acrylic acid -- styrene sulfonic acid copolymers 
acrylic acid -- vinyl ether copolymers 
acrylic acid -- vinyl acetate copolymers 
acrylic acid -- vinyl alcohol copolymers 
copolymers of methacrylic acid with all the above comonomers 
copolymers of maleic acid, fumaric acid and their esters with all the above 
comonomers 
copolymers of maleic anhydride with all the above comonomers. 
If desired, the foregoing polyelectrolytes can also be sulfonated by 
treatment with SO.sub.3, chlorosulfonic acid or fuming sulfuric acid in an 
inert organic solvent. 
Illustrative examples of the crosslinking agents useful in this invention 
are set forth in U.S. Pat. Nos. 2,926,154; 3,224,986; and 3,332,901. The 
disclosures of these references are incorporated herein by reference. 
In the preferred method of making water swellable films by the present 
invention, the above composition of the polyelectrolytes is spread on a 
flat plate or roller of metal, plastic, or other impervious substrate and 
dried to crosslink the polyelectrolyte and drive off the excess water 
and/or alcohol. The film is then peeled off the plate or roller by a 
scraper to recover the intact film for subsequent storage or use. 
It is sometimes desirable to add a small amount of a surfactant to the 
polyelectrolyte composition to aid in flowing on and removing the 
continuous film from the water impervious substrate. A secondary benefit 
of using a surfactant is to increase the wettability of the final dry 
absorbent film. Either anionic or nonionic surfactants may be used. 
Examples of the useful surfactants are the sodium alkyl sulfonates and the 
ethylene oxide derivatives of alkylated phenols and the like. 
Similarly, when an absorbent article is prepared, the article which is to 
be the substrate is coated with the composition of polyelectrolyte and 
then the coating is crosslinked. It is to be understood that for the 
purposes of this invention the coating step implies a complete coating or 
a discontinuous coating, thus when a fiberous substrate such as cellulose 
batting, paper, woven or non-woven cloth, polyurethane foam and the like 
are used as the substrate, the composition can be applied in a 
discontinuous manner, i.e., in a pattern of large dots, squares, or grid 
lines to retain the inherent flexibility of the fiberous substrate and at 
the same time vastly improve its water absorbency. Wood pulp can be coated 
by slurrying it in the polyelectrolyte composition followed by a fluffing 
operation. 
If desired, the water swellable film prepared as above can be used per se 
as the inner absorbent layer in baby diapers. It is sometimes advantageous 
that the film be disintegrated into flakes, strips or powders. This is 
accomplished by crushing or comminuting the film in a hammer mill, 
blenders, or the like. If long flat strips are desired, the film can be 
sliced widthwise with appropriate slicers. 
In some instances, water swellable fibers are desired. These can be 
prepared by extruding the above composition of the polyelectrolytes into a 
bath comprising lower alkyl ketones such as acetone, methyl ethyl ketone, 
diethyl ketone and the like. Alcoholic compositions may be extruded into a 
non-aqueous coagulant such as chlorinated hydrocarbons, i.e., methylene 
chloride, perchloroethylene and the like. The soft extruded fibers are 
then removed from the bath by any convenient means such as a three or five 
roll cluster and carried through a heated chamber at a temperature greater 
than about 30.degree. C and preferably in the range from about 70.degree. 
to about 150.degree. C to dry and to crosslink the polyelectrolyte fibers. 
The absorbency of the crosslinked polyelectrolytes (grams solution gelled 
per gram of polyelectrolyte) is determined in the following manner using 
synthetic urine (0.27 N sodium chloride solution). 
A 0.5 gram sample of a crosslinked polyelectrolyte is weighed into a 250 
ml. beaker, a 0.27 N sodium chloride solution (150 ml.) is poured into the 
beaker and allowed to soak for 15 minutes at room temperature, with 
occasional stirring. The swelled polyelectrolyte is then collected by 
filtration and the gel capacity is reported as grams of solution gelled 
per gram of polymer salt. 
For the purposes of this invention, a moisture or water absorbent or water 
swellable polyelectrolyte is defined as one which absorbs greater than 
about 20 times its weight of synthetic or natural urine. Preferably, the 
absorbency should be in the range from about 30-60 grams of water per gram 
of resin or polyelectrolyte. As can be seen from Table III hereinafter, 
the level of crosslinking agent (Kymene 557) used is a variable factor 
which is dependent upon the particular polyelectrolyte used and the 
molecular weight of the polyelectrolyte. In general, the amount used 
varies from the 0.5 to 5.0 percent based on the weight of the 
polyelectrolyte. However, this range is varied for each polyelectrolyte in 
order to adjust the absorbency of the final crosslinked gel so that it is 
at least 20 and preferably in the range from about 30 to about 60 grams of 
water per gram of resin.

EXAMPLES 1 - 5 
Three mixtures were made up having the following compositions. 
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Part A Part B Part C 
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600 g. deionized 
437.5 g ethyl 175 g deionized 
s,54 water acrylate water 
0.75 g Triton GR-5* 
77.2 g methacrylic 
2.0 g sodium 
acid bisulfite 
1.75 g sodium 
persulfate 
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*dioctylsodium sulfosuccinate 
Part A was charged to a 2 liter reactor and brought to 40.degree. C while 
under vigorous nitrogen purge. Eighteen milliliters of Part B was added to 
the reactor followed by all of Part C. The remainder of Part B was added 
over the next 2.5 hours while the temperature was held at 
39.degree.-41.degree. C. The latex was then digested at 60.degree. C for 
1.5 hours, cooled to 30.degree. C and bottled. The latex contained 40.6% 
non-volatiles. 
1125 g. of the above latex was added in a small stream over a period of 25 
minutes to a slowly stirred solution of 187.16 g. 50% NaOH in 547.9 g. 
deionized water. After the polymer has all dissolved, the viscous solution 
was heated at 50.degree. C for 22 hours to complete the saponification. 
The resulting solution (25.4% solids) had a Brookfield viscosity of 16,200 
cps. at 25.degree. C (No. 5 spindle, 10 rpm). The polymer is 50% 
ethylacrylate by moles with the remainder being sodium acrylate and 
methacrylate. 
Samples of the above solution were blended with Kymene 557.RTM. (a liquid 
adduct of epichlorohydrin and a polyamide having about 12.5% solids, a pH 
from 4.6-4.9, and a nitrogen content of about 12.8%) and cast on polished 
chromium plate with a 25 mil draw bar. After air drying, the films were 
lifted from the plate and placed in a 95.degree. C oven for various times. 
The absorbency (gal capacity) of the various films in 0.27 N. NaCl is set 
forth in Table I. 
TABLE I 
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Wt.% Curing Agent* 
Based on dry Wt. Absorbency 
Ex. of Polymer Time in Oven 
(gms H.sub.2 O/gm film) 
______________________________________ 
1 0.847 10 min. 38 
2 0.847 25 min. 37 
3 0.6 10 min. 51 
4 0.6 25 min. 50 
5 0.5 10 min. 54 
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*Kymene 557 (Hercules, Inc.) 
EXAMPLE 6 
Following the procedures of the above examples, a film of the above polymer 
solution containing 0.5% of Kymene 557 was cast on an aluminum plate 
heated to 150.degree. C and then the plate was placed in a 150.degree. C 
oven for 2 minutes. The crosslinked polymer was scraped off the plate with 
a razor blade and the absorbency was 50 gms H.sub.2 o/gms polymer. 
EXAMPLES 7-8-9 
Films prepared in the manner of examples 1-5 were allowed to cure at 
25.degree. C for 48 hours in the laboratory. The atmosphere absorbency was 
then determined with allowance made for moisture absorbed from the air. 
The results are set forth in Table II. 
TABLE II 
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Wt. % Absorbency 
Example Curing Agent (gms H.sub.2 O/gm film) 
______________________________________ 
7 0.847 46 
8 0.6 54 
9 0.5 50 
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COMATIVE EXPERIMENTS 
In order to illustrate the improvement in curing rates over the above 
mentioned Ser. No. 468,794, the following experiment was performed. 
A diglycidyl ether cross-linking agent (DER 736) was used at a recommended 
level of 0.3% by weight polyelectrolyte. The oven was adjusted to 
110.degree. C. It is known that many chemical reaction rates double for 
every 10.degree. increase in temperature. A film was prepared as in the 
above examples and placed in the 110.degree. oven for 5 minutes to 30 
minutes. Even after 20 minutes, no gel was obtained after exposing the 
film to 0.27N, NaCl solution. After 30 minutes a very slimy, under 
crosslinked gel of 89 g/g absorbency was obtained. Allowing the same film 
to cure at room temperature for 2 days produced only a somewhat thickened 
solution when the film was placed in water. That the level of diglycidyl 
ether was sufficient to cause cross-linking is evidenced by the fact that 
leaving the film at 110.degree. C. for 16 hours did produce a swellable 
film with a 45 absorbency. 
EXAMPLES 10-11-12 
A sample of polyacrylic acid (Acrysol A-5 from Rohm and Haas) was brought 
to pH 9.2 with sodium hydroxide to give a solution of polysodium acrylate 
of 24.6% solids content. This was then diluted with water to a 12.3% 
solids content, mixed with various amounts of Kymene 557, and the mixture 
was used to cast a 15 mil film on glass plates. These were then dried in 
an air circulating oven at 105.degree. C. for 30 minutes. The absorbency 
results are shown in Table III. 
TABLE III 
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Level of Kymene (% 
based on solids con- 
Absorbency 
Example tent of polyacrylic acid) 
(0.27 N-NaC1) 
______________________________________ 
Control 1 20 11.2 
10 5 25 
11 4 31 
12 3 34 
Control 2 0.5 soluble 
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These data show that for this particular polyelectrolyte, amounts of Kymene 
557 at 0.5% or less do not give a crosslinked gel but a solution. On the 
other hand, amounts of the crosslinker greater than about 5%, give a gel 
which is too crosslinked to be useful, i.e., the absorbency is not 
sufficient to be of a practical value. 
EXAMPLE 13 
The crosslinkable solution prepared in Example 1 is coated on a laboratory 
paper towel, a polystyrene meat tray, a thin polyurethane foam sheet and 
cured at room temperature. Increased absorption is observed due to the 
water swellable polymer coated thereon. 
EXAMPLE 14 
The polyelectrolyte prepared in Example 1 is mixed with 2.0 grams of Kymene 
557 and the mixed solution diluted with water to a solution having 22% 
total solids with 0.5% of the solids being Kymene 557. 
The solution is then cast on two glass plates coated with Panshield .RTM. 
(for quick release) with a spreader to achieve 15 mil films. One plate was 
air dried for 48 hours in the laboratory air at 25.degree. C and at 40-50% 
relative humidity. The other plate was heated in an air circulating oven 
for 10 minutes at 107.degree. C. The first plate had an absorbency of 54 
gms H.sub.2 O/gm of resin and the second had an absorbency of 56 gms 
H.sub.2 O/gm of resin.