Note: Descriptions are shown in the official language in which they were submitted.
I
MODIFIED STARCHES AS EXTENDERS
FOR ABSORBENT POLYMERS
,:
BACKGROUND OF THE INVENTION
yield of the Invention
Polymeric substances which have the ability to absorb
large amounts of aqueous fluids are well known in the prior
art and are typically referred to as "super absorbents".
lo Starch-based super absorbents are particularly useful, and
are easily prepared by first graft polymerizing either
acrylonitrile or mixtures of monomers that contain pro-
dominantly acrylonitrile onto either starch or flour and
then, in a second step, saponifying the polyacrylonitrile
15 moiety by treating the graft copolymer with hot alkali.
This process has been reviewed by Santa and Barley in the
` Encyclopedia of Polymer Science and Technology, Supplement
Vol. 2 HO Marc and No M. Decals, ens., John Wiley &
Sons, 1977, p. 665)~ There are a multitude of uses for
` 20 super absorbents. For example, they are used in agriculture
to increase the water-holding capacity of poor or marginal
soils, as seed coatings to enhance germination, and as root
dipping compositions to reduce or eliminate transplant shock.
Super absorbents find application in disposable soft goods,
US such as diapers and feminine napkins for enhancing the
absorptivity of these articles toward body fluids. Another
important use for these substances is as thickening agents
for aqueous systems. Medical applications include
incorporation into body powders and wound dressings,
30 particularly or the treatment of decubitus ulcers or
bed sores.
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, Although super absorbents enjoy a broad field of
application, their cost is often an inhibiting factor in
I/ their widespread commercial acceptance, particularly in
Jo agriculture. Diluting the absorbent with an inexpensive
extender, such as starch, has obvious marketing advantages.
However, it is at once apparent that addition of an inert
delineate will decrease the water absorbency of the resulting
blend in an amount proportional to the amount of delineate
added. This invention relates to diluting super absorbents
10 without a proportionate sacrifice of absorbency.
)
Description of the Prior Art
In US. Patent No. 3,935,099 and related Patents
3,981,000, 3,985,616, and 3,997,484, Weaver et at. teach
lo that absorbent polymers may be extended by mixing dispersions
of the saponified, gelatinized starchpolyacrylonitrile
; graft copolymers (GS-HPAN) with dispersions of inexpensive
natural polymers or their derivatives, and then drying
Jo the resulting mixtures. Exemplary extenders include flour,
I 20 guard gelatin, starch, and dextrin. In Example 20 and
accompanying Table 9, Weaver et at. illustrate the
expected delineate effect on absorbency of several of these
extenders.
I US SUMMARY OF THE INVENTION
¦ We have now surprisingly discovered that selected
modified starches, particularly those whose molecular weights
have been significantly reduced may be used to extend
super absorbents without a commensurate reduction in the water
absorbency of the resulting blended compositions This
behavior is totally unexpected in view of the drastic
reduction in absorbency observed upon extension with us-
modified starch or the acid-modified flour of Weaver et at.
326~
We have also discovered that the physical properties of
the water-swollen gels obtained from the instant blends
are superior in certain applications to those resulting
from non diluted absorbents.
In accordance with this discovery, it is an object of
the invention to provide more economical absorbent
compositions by diluting GO HAN polymers with inexpensive
extenders.
It is also an object of the invention to extend GS-HPAN
1 polymers by blending with dextrinized starches which
i synergistically enhance the absorbency of the polymers.
In the preferred embodiments, blends comprising up to 50%
` 15 of selected dextrins display water absorbencies comparable
I to the undiluted absorbent.
Another object of the invention is to provide extended
Jo absorbent compositions which yield soft, smooth gels having
! 20 application as high-quality thickening agents.
Jo Other objects and advantages of this invention will
I become readily apparent from the ensuing description.
it
DETAILED DESCRIPTION
It is well known that modification of starch by
Jo treatment with acid or enzyme causes a lowering in the
molecular weight of the polysaccharide and provides a
I` 30 starch composition which is more water soluble than its
unmodified precursor. Increases in acid concentration,
I'
I` reaction temperature, and reaction time will decrease the
I` polysaccharide molecular weight and increase its water
volubility.
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The modified starches for use in the invention are
comparatively low molecular weight dextrins resulting from
relatively extensive acid or enzyme treatment. Indicative of the
molecular weight is inherent viscosity as determined by the
standard procedure of Myers et at. in Methods in Carbohydrate
Chemistry (Vol. IV, R. L. Whistler, Ed., Academic Press, 1964,
pp. 124-127. Dextrins having inherent viscosities in the range
of about 0.05 to 0.60 dug at 0.5~ (w/v) in lo Noah are considered
to be within the scope of the invention, with those having vise
cosities in the rang of 0.05 to 0.25 dug being preferred.
By comparison, the inherent viscosity of unmodified starch is
approximately 2.3 dug
The specific source of the starch intended for dew-
transition is not especially critical and may include any of
the common cereal grains such as corn, wheat, and rice, or the
root crops as exemplified by potato and tapioca. It is important,
however, that the starch be separated from the nonamylaceous
components which would otherwise wend to interfere with the
absorbency of the blended compositions. Substantially pure starch
fractions obtained from conventional grain wet milling or root
crop processing operations are suitable.
The extenders contemplated herein may advantageously
combined with any of the previously discussed starch-based
super absorbents; namely, those which are prepared by graft posy
meriting acrylonitrile or mixtures of monomers thaw contain pro-
dominantly acrylonitrile onto either starch or flours, and then
saponifying the resultant product. In the course of the super-
absorbent preparation, it is critical that the extender be
incorporated into the aqueous dispersion of the starch-graft
copolymer prior to the final drying
Lo
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step. Typically, it will be added in the sequence step
either immediately preceding or immediately after saponify-
cation. At the time of addition, the extender may be in
¦ a dry powdered state or in the form of an aqueous dispersion.
Upon thorough mixing of the components, the saponified and neutralized blend is dried by any conventional means
to a moisture content in the range of about 1-15~ water by
weight.
The aforementioned modified starch extenders have little
or no absorbent capacity of their own generally less than
about 1 g. warily g. extender. However, when blended in
combination with super absorbents in amounts of up to about
2 parts extender: 1 part absorbent on a dry weight basis,
j 15 they exhibit a significant synergistic behavior. Blends
comprising highly modified starch extenders with inherent
viscosities in the preferred range of 0.05 to 0.25 dug
I` and having a ratio of extender: adsorbent of 1:2, and in
some cases as much as 1:1, are characterized by absorbencies
I closely approximating those of the undiluted absorbent.
j The water volubility of the instant blends will be a function
¦ of both the degree of extender modification and its level
of addition.
Gels prepared from the blended absorbents are
characterized by a soft, smooth texture. This quality
¦ renders the products of the invention particularly well
I` suited for use as thickening agents for aqueous systems,
I where a grainy, particulate character would be objectionable.
5` 30 The gels become softer and smoother as the degree of
modification of the delineate increases. This tailoring of
super absorbent quality ho simple dilution is a facile
alternative to the precipitation process of the prior art.
.
7 ~2~3~60
The following examples are intended only to further
illustrate the invention and are not intended to limit the
scope of the invention which is defined by the claims.
EXAMPLE 1
This example illustrates one method for the synthesis
of the saponified starch-polyacrylonitrile graft copolymer
(GS-HPAN) reaction mixture useful in the blends of the
instant invention.
A 10-cu.-ft. ribbon blender was charged with 33.25 lb.
of unmodified wheat starch and 400 lb. of water, and the
stirred slurry was heated for 30 min. at 86-90C. After
the mixture was cooled to 32C, 34.5 lb. of acrylonitrile
was added, hollowed after 2 min. by a solution of 354.2 g.
of eerie ammonium nitrate in 8 1. of water containing 42 ml.
, of concentrated nitric acid. After the mixture had stirred
at 32-45C. for 40 min., it was neutralized with 45%
; 20 potassium hydroxide solution, and the unrequited acrylonitrile
was removed by azeotropic distillation. Fifty pounds of
water and 47 lb. of 45% potassium hydroxide solution was
then added to the dispersion of starch-PAN graft copolymer,
and the saponification was carried out by heating the
I mixture for about 2.5 hr. at 90-95C., while allowing the
ammonia formed in the reaction to vent to the atmosphere.
The alkaline GS-HPAN reaction mixture was stored at 4C.
EXAMPLES AYE
To 50 g. of the GS-HPAN reaction mixture of Example 1
(19.1% solids, pi 9.6~ was added ill g. (dry basis) of either
a commercial corn starch or a commercial acid-modified
starch (dextrin). The mixture way kneaded to thoroughly
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blend the powdered starch or dextrin with the GS-HPAN
dough. Glacial acetic acid was then kneaded into the
blend to adjust the pi to 7.2-7.4. The resulting blends
were then dry dried on an 18 x 12 in. double drum drier
that was heated with 58 prig steam and was rotated
at 4 rum Absorbencies of drum-dried products were
obtained by allowing an accurately weight sample (2-10 my.)
to soak in distilled water for 30 min. The swollen gel
was separated from unabsorbed water by pouring the
dispersion through a tared 280 mesh sieve and allowing it
to drain for 20 min. Absorbency, in grams of water per
gram of polymer, was calculated from the weight of water-
swollen gel and the weight of sample used fox the test.
Solubilities of drum-dried products were obtained by
allowing a known weight of sample to stand overnight in a
known weight of water. After removal of the gel fraction
by filtration a known weight of the clear filtrate was
; freeze dried, and the volubility was calculated based upon
the weight of the solid residue.
Table I compares the absorbencies and water solubilities
of a number of drum dried GS-HPAN products extended with
acid-modified starches (white dextrins~ having a range of
inherent viscosities to drum-dried, non extended GS-HPAN
US and to the drum-dried GS-HPAN extended with the corn starch.
The expected absorbency of each of the extended products
based upon the percentage of GS-HPAN present was 215 go
representing a variation from the control of -51%. The
actual variation from the control for each of the dextrin-
extended products was significantly less than expected That for the starch-extended product was actually more than
expected. The product of Example YE gave a softer, smoother
gel than the others when hydrated or the absorbency test,
end consequently it was not as truffle drained on the
sieve. This factor may have contributed to the absorbency
value being higher than that of the control.
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; EXAMPLE 3
To 50-g. samples of the GS-HPAN reaction mixture of
Example 1 were added either 5 g., 10 g., or 20 g. (dry
5 basis) of the dextrin used in Example YE. Samples were
prepared, drum dried, and tested for absorbency as desk
cried in Example AYE. As noted for Example YE, samples
gave soft, smooth gels when placed in water, and complete
; separation from unabsorbed water was difficult. Results
10 are given ill Table II.
EXAMPLE 4
To 50-g. samples of the GS-HPAN reaction mixture of
15 Example 1 were added either 5 g., 10 g., or 20 g. (dry
basis) of the dextrin used in Example ED Samples were
prepared, drum dried, and tested for absorbency as described
in Examples AYE, and results are given in Table III.
EXAMPLE 5
To 50-g~ samples of the GS-HPAN reaction mixture of
Example 1 were added either 5 g., 10 g., or 20 go (dry
basis) of corn starch. Samples were prepared, drum dried,
I and tested for absorbency as described in Examples AYE,
an results are given in Table IV. As indicated by a
comparison of the actual and expected variations from the
control, the unmodified starch extender actually had a slight
negative effect van the GS-HPAN absorbency, beyond the effect
30 attributed to dilution
It is understood that the foregoing detailed descrip-
lion is given merely by way of illustration and that
modification and variations may be made therein without
¦ departing from the spirit and scope of the invention.
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' TABLE ITJo
Amount of Drum-drled product
Example dextrin e GS-HPAN, Absorbency~g./g. Variation from control,%
No. extender, g. % by wt. Actual Expected Actual Expected
I, anyone (control) 100 360 ... ... I--
Jo 3B 5 66 440 236+22 -34
3C 10 49 340 176-6 -51
:!: ED 20 32 200 116-44 -68
TABLE III
¦ 10 Amount of _ Drum-dried product
Example dextrin d GS-HPAN, Absorbency, go Variation from control,%
No. extender , % by wt. Actual Expected Actual Expected
- - '- g
anyone (control) 100 410 ... ... ...
1 4B 5 66 390 268 -5 -34
lo 4C 10 49 320 200 -22 -51
ED 20 32 260 159 -37 -68
TABLE IV
i
Jo Amount of Drum-dried product
Jo Example stanch GS-~IPAN, Absorbency, go Variation from control,%
No, extender g. JO by wt. Actual Expected Actual Expected
anyone (control) lo 370 ... ... ...
1 SUB 5 66 230 243 -38 -34
r 5C 10 49 140 181 -62 ~51
l ED 20 32 72 120 -81 -68
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