U.S. patent application number 14/612548 was filed with the patent office on 2015-06-11 for superabsorbent polymer and method for making same.
This patent application is currently assigned to EVONIK CORPORATION. The applicant listed for this patent is EVONIK CORPORATION. Invention is credited to Michael M. Azad, Geoffrey Wyatt Blake, Michael S. Jarman, Mark Joy, Scott J. Smith.
Application Number | 20150157759 14/612548 |
Document ID | / |
Family ID | 41572570 |
Filed Date | 2015-06-11 |
United States Patent
Application |
20150157759 |
Kind Code |
A1 |
Azad; Michael M. ; et
al. |
June 11, 2015 |
SUPERABSORBENT POLYMER AND METHOD FOR MAKING SAME
Abstract
A process for the preparation of superabsorbent polymer
containing clay, the process including the steps of (I)
polymerizing a polymerization mixture comprising: (a) one or more
ethylenically unsaturated carboxyl-containing monomers, (b) one or
more crosslinking agents, (c) optionally one or more comonomers
copolymerizable with the carboxyl-containing monomer, (d)
neutralizing agent to partially neutralize the polymer to from
about 50% to about 99%, by weight, and (e) a polymerization medium,
to form a crosslinked partially neutralized hydrogel, (II) admixing
a clay with the crosslinked partially neutralized hydrogel to form
partially neutralized superabsorbent polymer-clay hydrogel; (III)
drying the crosslinked partially neutralized hydrogel at a
temperature from about 190.degree. C. to about 210.degree. C. and
for a time period of from about 15 minutes to about 120 minutes,
and (IV) comminuting the dried partially neutralized superabsorbent
polymer-clay hydrogel to particles.
Inventors: |
Azad; Michael M.;
(Reidsville, NC) ; Smith; Scott J.; (Dusseldorf,
DE) ; Joy; Mark; (Greensboro, NC) ; Blake;
Geoffrey Wyatt; (Kernersville, NC) ; Jarman; Michael
S.; (High Point, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EVONIK CORPORATION |
Parsippany |
NJ |
US |
|
|
Assignee: |
EVONIK CORPORATION
|
Family ID: |
41572570 |
Appl. No.: |
14/612548 |
Filed: |
February 3, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13543047 |
Jul 6, 2012 |
8962910 |
|
|
14612548 |
|
|
|
|
12254434 |
Oct 20, 2008 |
8222477 |
|
|
13543047 |
|
|
|
|
Current U.S.
Class: |
523/102 ;
523/105 |
Current CPC
Class: |
Y10T 428/2982 20150115;
A61F 13/53 20130101; C08J 3/12 20130101; A61L 15/46 20130101; A61L
15/18 20130101; C08J 2300/14 20130101; A61L 15/24 20130101; A61L
15/60 20130101; C08J 3/128 20130101 |
International
Class: |
A61L 15/60 20060101
A61L015/60; A61L 15/18 20060101 A61L015/18; A61L 15/46 20060101
A61L015/46; A61L 15/24 20060101 A61L015/24 |
Claims
1-34. (canceled)
35. A superabsorbent polymer containing kaolin particulate
comprising: a) a superabsorbent polymer particulate comprising an
intimately admixed blend of from about 90 wt % to about 99.5 wt %
of a superabsorbent polymer hydrogel, wherein the superabsorbent
polymer hydrogel is neutralized to from about 50 to about 80 mole
%, and from about 0.5 wt % to about 10 wt % of a kaolin; wherein
the admixed blend is dried and comminuted into superabsorbent
polymer particulate; b) wherein the superabsorbent polymer
particulate of a) is surface treated with from about 0.001 wt % to
about 5 wt % of a surface crosslinking agent to form a surface
crosslinked superabsorbent polymer particulate; c) wherein the
surface crosslinked particulate composition of b) is surface
treated with from about 0.01 wt % to about 5 wt % of a multivalent
salt to form the superabsorbent polymer containing kaolin
particulate; d) wherein the superabsorbent polymer particulate
composition of b), c), or b) and c) is further surface treated with
an additive selected from an odor-binding compound, cyclodextrin,
zeolite, anti-caking additive, multivalent metal salt, or a
surfactant.
36. The superabsorbent polymer containing kaolin particulate of
claim 35 wherein at least about 40% by weight of the superabsorbent
polymer particulate have a particle size from about 300 .mu.m to
about 600 .mu.m as measured by screening through a U.S. standard 30
mesh screen and retained on a U.S. standard 50 mesh screen.
37. The superabsorbent polymer containing kaolin particulate of
claim 35 further comprising e) from about 0.01 wt % to about 0.5 wt
% of a thermoplastic polymer.
38. The superabsorbent polymer containing kaolin particulate of
claim 35 wherein the additive is an odor-binding substance.
39. The superabsorbent polymer containing kaolin particulate of
claim 35 wherein the additive is an odor-binding substance
comprising an organic salt.
40. The superabsorbent polymer containing kaolin particulate of
claim 35 having a centrifuge retention capacity of from about 25
g/g to about 40 g/g as measured by the Centrifuge Retention
Capacity Test.
41. The superabsorbent polymer containing kaolin particulate of
claim 35 having an absorption under load at 0.9 psi of from about
10 g/g to about 20 g/g as measured by the Absorbency Under Load
(AUL0.9 psi) Test.
42. The superabsorbent polymer containing kaolin particulate of
claim 35 having a free swell gel bed permeability of from about 10
Darcy to about 100 Darcy as measured by the Free Swell Gel Bed
Permeability Test.
43. The superabsorbent polymer containing kaolin particulate of
claim 35 having a vortex time of from about 30 seconds to about 70
seconds.
44. The superabsorbent polymer containing kaolin particulate of
claim 35 wherein the multivalent metal salt of d) comprises an
aluminum salt.
45. The superabsorbent polymer containing kaolin particulate of
claim 35 wherein said superabsorbent polymer containing kaolin
particulate has a water content of up to about 10% by weight.
46. A process for the preparation of superabsorbent polymer
containing kaolin, the process comprising the steps of: (I)
polymerizing a polymerization mixture comprising: (a) one or more
ethylenically unsaturated carboxyl-containing monomers, (b) one or
more crosslinking agents, (c) optionally one or more comonomers
copolymerizable with the carboxyl-containing monomer, (d)
neutralizing agent to partially neutralize the polymer to from
about 50 to about 80 mole %, and (e) a polymerization medium, to
form a crosslinked partially neutralized hydrogel, (II) admixing
kaolin with the crosslinked partially neutralized hydrogel to form
a partially neutralized superabsorbent polymer-kaolin hydrogel
comprising an intimately admixed blend of from about 90 wt % to
about 99.5 wt % of a superabsorbent polymer hydrogel and from about
0.5 wt % to about 10 wt % of a kaolin; (III) drying the crosslinked
partially neutralized hydrogel at a temperature from about
190.degree. C. to about 210.degree. C. and for a time period of
from about 15 minutes to about 120 minutes; and (IV) comminuting
the dried partially neutralized superabsorbent polymer kaolin
hydrogel into superabsorbent polymer containing kaolin particulate;
and (V) surface treating the superabsorbent polymer particulate
with from about 0.001 wt % to about 5 wt % of a surface
crosslinking agent to form a surface crosslinked superabsorbent
polymer containing kaolin particulate; (VI) wherein the surface
crosslinked particulate composition of step (V) is surface treated
with an additive selected from an odor-binding compound,
cyclodextrin, zeolite, anti-caking additive, multivalent metal
salt, or a surfactant; and wherein the superabsorbent polymer
containing kaolin particulate has a free swell gel bed permeability
of at least about 10 Darcy, as measured by the Free Swell Gel Bed
Permeability Test.
47. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein the superabsorbent polymer is
present in an amount of about 90% to about 98%, by weight, and the
kaolin is present in an amount from about 0.5% to about 10%, by
weight.
48. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46, further comprising the step of
grinding the superabsorbent polymer kaolin hydrogel into
superabsorbent polymer particulate having a particle size from
about 150 .mu.m to about 850 .mu.m as measured by screening through
a U.S. standard 20 mesh screen and retained on a U.S. standard 100
mesh screen.
49. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein at least about 40% by weight
of the superabsorbent polymer particulate has a particle size from
about 300 .mu.m to about 600 .mu.m as measured by screening through
a U.S. standard 30 mesh screen and retained on a U.S. standard 50
mesh screen.
50. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein the kaolin is a swelling
kaolin selected from the group consisting of montmorillonite,
saponite, nontronite, laponite, beidelite, hectorite, sauconite,
stevensite, vermiculite, volkonskoite, magadite, medmontite,
kenyaite, and mixtures thereof.
51. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein the kaolin is a nonswelling
kaolin selected from the group consisting of a kaolin mineral, a
serpentine mineral, a mica mineral, a chlorite mineral, sepolite,
palygorskite, bauxite, and mixtures thereof.
52. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein said monomer solution
includes water-soluble ethylenically unsaturated monomer mixtures
or salts thereof.
53. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein said polymer is a cross
linked polymer of polyacrylic acid, sodium polyacrylate or
copolymers thereof, cross linked with a polyvinyl monomer.
54. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein said polymerizing step
produces a water-swellable, aqueous fluid absorbent polymer or
copolymer gel.
55. The process for the preparation of superabsorbent polymer
containing kaolin of claim 46 wherein the multivalent metal salt of
(VI) comprises aluminum sulfate.
Description
[0001] This is a continuation application of application Ser. No.
12/254,434, filed on Oct. 20, 2008, currently pending, the
disclosure of which is expressly incorporated herein by
reference.
BACKGROUND
[0002] A superabsorbent material in general refers to a
water-swellable, water-insoluble, material capable of absorbing at
least about 10 times its weight, and up to about 30 times or more
its weight in an aqueous solution containing 0.9 weight percent
sodium chloride solution in water. The present invention relates to
superabsorbent polymer particles, which absorb water, aqueous
liquids, and blood, and a method to make the superabsorbent polymer
and particles. The acronym SAP, as used herein and as generally
used in the industry, is used in place of superabsorbent polymer,
superabsorbent polymer composition, superabsorbent polymer
particles, or variations thereof.
[0003] A superabsorbent polymer is a cross linked partially
neutralized polymer that is capable of absorbing large amounts of
aqueous liquids and body fluids, such as urine or blood, with
swelling and the formation of hydrogels, and of retaining them
under a certain pressure in accordance with the general definition
of superabsorbent material. A superabsorbent polymer composition is
a superabsorbent polymer that has been surface treated that may
include surface cross linking and/or other treatment of the surface
of the superabsorbent polymer.
[0004] Commercially available superabsorbent polymer compositions
include cross linked polyacrylic acids or cross linked
starch-acrylic acid graft polymers, in which some of the carboxyl
groups are neutralized with sodium hydroxide solution or potassium
hydroxide solution. Superabsorbent polymer composition particles
are particles of superabsorbent polymers or superabsorbent polymer
compositions, and generally have a particle size of from about 150
microns to about 850 microns. A comprehensive survey of
superabsorbent polymers, and their use and manufacture, is given in
F. L. Buchholz and A. T. Graham (editors) in "Modern Superabsorbent
Polymer Technology," Wiley-VCH, New York, 1998.
[0005] A primary use of SAP and SAP particles is in sanitary
articles, such as babies' diapers, incontinence products, or
sanitary towels. For fit, comfort, and aesthetic reasons, and from
environmental aspects, there is an increasing trend to make
sanitary articles smaller and thinner. This is being accomplished
by reducing the content of the high volume fluff fiber in these
articles. To ensure a constant total retention capacity of body
fluids in the sanitary articles, more SAP content is being used in
these sanitary articles.
[0006] Clays and other mineral products have been added to SAPs in
an attempt to improve SAP performance. For example, the addition of
finely divided amorphous silica, such as AEROSIL.RTM., available
from Evonik GmbH, Germany, or CAB-O-SIL.RTM., available from Cabot
Corporation, or a bentonite onto the surface of SAP powders or
granules is known. U.S. Pat. Nos. 5,140,076 and 4,734,478 disclose
the addition of silica during surface crosslinking of dry SAP
powders. U.S. Pat. No. 4,286,082 discloses mixtures of silica and
SAP for use in hygiene articles.
[0007] Generally, in mixtures of dry SAP particles with a silica
powder, the silica adheres to the SAP particle surfaces and alters
the surface properties of the SAP particles, but not their
intrinsic absorption properties. For example, the silica powder is
hydrophilic or hydrophobic, which primarily influences the rate at
which a fluid is absorbed by the SAP particles.
[0008] Other patents and applications disclosing SAP particles and
a clay include GB 2,082,614 disclosing a dry, solid,
water-swellable absorbent composition prepared by blending dry SAP
particles and 1% to 75%, by weight of the blend, of an extender
material selected from uncrosslinked cellulose derivatives, starch,
certain clays and minerals, and mixtures thereof.
[0009] U.S. Pat. No. 5,733,576 discloses a process of producing
absorbing agents containing (a) a water-swellable, synthetic
polymer or copolymer, and (b) a natural or synthetic polymeric
compound which at normal temperature is a pourable powder and is
partially soluble or insoluble in water. The absorbing agents may
contain clay as a neutral filling agent.
[0010] WO 01/68156 discloses a hydrophilic swellable
hydrogel-forming polymer containing alumosilicate and having
enhanced permeability and improved odor-control properties. The
alumosilicates can be added before, during, or after
polymerization.
[0011] U.S. Pat. No. 7,329,701 discloses superabsorbent polymer
particles containing a clay, wherein the clay is added to an SAP
hydrogel prior to SAP neutralization to provide particles having
improved fluid acquisition rates and an improved permeability of a
fluid through the swollen SAP-clay particles.
[0012] The present invention is directed to improving the
properties of SAP particles by introducing clay into the SAP
hydrogel under specific conditions. It has been found that the
addition of clay to a partially neutralized SAP hydrogel and drying
the hydrogel-clay at elevated temperatures can improve SAP
performance properties. Therefore, the present invention is
directed to improving SAP absorption rate and permeability
performance, without adversely affecting other fluid absorption and
retention properties of the SAP particles, by the addition of clay
during the manufacturing process.
SUMMARY
[0013] The present invention is directed to SAP particles and
methods of manufacturing superabsorbent polymer containing clay.
More particularly, the present invention is directed to SAP
containing clay comprising a water-absorbing resin and clay, and a
method of manufacturing such SAP-clay particles.
[0014] An embodiment of the present invention comprises a process
for the production of superabsorbent polymer containing clay and
particles thereof based on a process for the preparation of
superabsorbent polymer comprising the steps of:
(I) polymerizing a polymerization mixture comprising: (a) one or
more ethylenically unsaturated carboxyl-containing monomers, (b)
one or more crosslinking agents, (c) optionally one or more
comonomers copolymerizable with the carboxyl-containing monomer,
(d) neutralizing agent to partially neutralize the polymer to from
about 50% to about 99%, by weight, and (e) a polymerization medium,
to form a crosslinked partially neutralized hydrogel, (II) admixing
clay with the crosslinked partially neutralized hydrogel to form
partially neutralized superabsorbent polymer-clay hydrogel; (III)
drying the crosslinked partially neutralized hydrogel at a
temperature from about 190.degree. C. to about 210.degree. C. and
for a time period of from about 15 minutes to about 120 minutes,
and (IV) comminuting the dried partially neutralized superabsorbent
polymer-clay hydrogel to particles.
[0015] An embodiment of the present invention includes the
preparation of superabsorbent polymer containing clay wherein the
superabsorbent polymer is present in an amount of about 90% to
about 99.5%, by weight, and the clay is present in an amount of
about 10% to about 0.5%, by weight.
[0016] Another embodiment of the present invention further includes
grinding the superabsorbent polymer containing clay hydrogel into
superabsorbent polymer particulate having a particle size from
about 150 .mu.m to about 850 .mu.m as measured by screening through
a U.S. standard 20 mesh screen and retained on a U.S. standard 100
mesh screen.
[0017] An embodiment of the present invention further includes a
superabsorbent polymer particulate made by the foregoing process.
In addition, the present invention is directed to absorbent
compositions or sanitary articles such as diapers that may contain
superabsorbent polymer compositions of the present invention.
[0018] Numerous other features and advantages of the present
invention will appear from the following description. In the
description, reference is made to exemplary embodiments of the
invention. Such embodiments do not represent the full scope of the
invention. Reference should therefore be made to the claims herein
for interpreting the full scope of the invention. In the interest
of brevity and conciseness, any ranges of values set forth in this
specification contemplate all values within the range and are to be
construed as support for claims reciting any sub-ranges having
endpoints which are real number values within the specified range
in question. By way of a hypothetical illustrative example, a
disclosure in this specification of a range of from 1 to 5 shall be
considered to support claims to any of the following ranges: 1-5;
1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5.
FIGURES
[0019] The foregoing and other features, aspects, and advantages of
the present invention will become better understood with regard to
the following description, appended claims, and accompanying
drawings where:
[0020] FIG. 1 is a side view of the test apparatus employed for the
Free Swell Gel Bed Permeability Test;
[0021] FIG. 2 is a cross-sectional side view of a cylinder/cup
assembly employed in the Free Swell Gel Bed Permeability Test
apparatus shown in FIG. 1;
[0022] FIG. 3 is a top view of a plunger employed in the Free Swell
Gel Bed Permeability Test apparatus shown in FIG. 1; and
[0023] FIG. 4 is a side view of the test apparatus employed for the
Absorbency Under Load Test.
DEFINITIONS
[0024] It should be noted that, when employed in the present
disclosure, the terms "comprises," "comprising," and other
derivatives from the root term "comprise" are intended to be
open-ended terms that specify the presence of any stated features,
elements, integers, steps, or components, and are not intended to
preclude the presence or addition of one or more other features,
elements, integers, steps, components, or groups thereof.
[0025] The term "absorbent article" generally refers to devices
that can absorb and contain fluids. For example, personal care
absorbent articles refer to devices that are placed against or near
the skin to absorb and contain the various fluids discharged from
the body.
[0026] The term "cross linked" used in reference to the
superabsorbent polymer refers to any means for effectively
rendering normally water-soluble materials substantially
water-insoluble but swellable. Such a cross linking means can
include, for example, physical entanglement, crystalline domains,
covalent bonds, ionic complexes and associations, hydrophilic
associations such as hydrogen bonding, hydrophobic associations, or
Van der Waals forces.
[0027] The term "Darcy" is a CGS unit of permeability. One Darcy is
the permeability of a solid through which one cubic centimeter of
fluid, having a viscosity of one centipoise, will flow in one
second through a section one centimeter thick and one square
centimeter in cross-section, if the pressure difference between the
two sides of the solid is one atmosphere. It turns out that
permeability has the same units as area; since there is no SI unit
of permeability, square meters are used. One Darcy is equal to
about 0.98692.times.10.sup.-12 m.sup.2 or about
0.98692.times.10.sup.-8 cm.sup.2.
[0028] The term "disposable" is used herein to describe absorbent
articles that are not intended to be laundered or otherwise
restored or reused as an absorbent article after a single use.
Examples of such disposable absorbent articles include, but are not
limited to, personal care absorbent articles, health/medical
absorbent articles, and household/industrial absorbent
articles.
[0029] The term "dry superabsorbent polymer composition" generally
refers to the superabsorbent polymer composition having less than
about 10% moisture.
[0030] The term "mass median particle size" of a given sample of
particles of superabsorbent polymer composition is defined as the
particle size, which divides the sample in half on a mass basis,
i.e., half of the sample by weight has a particle size greater than
the mass median particle size, and half of the sample by mass has a
particle size less than the mass median particle size. Thus, for
example, the mass median particle size of a sample of
superabsorbent polymer composition particles is 2 microns if
one-half of the samples by weight are measured as more than 2
microns.
[0031] The terms "particle," "particulate," and the like, when used
with the term "superabsorbent polymer," refer to the form of
discrete units. The units can comprise flakes, fibers,
agglomerates, granules, powders, spheres, pulverized materials, or
the like, as well as combinations thereof. The particles can have
any desired shape: for example, cubic, rod-like, polyhedral,
spherical or semi-spherical, rounded or semi-rounded, angular,
irregular, et cetera. Shapes having a high aspect ratio, like
needles, flakes, and fibers, are also contemplated for inclusion
herein. The terms "particle" or "particulate" may also include an
agglomeration comprising more than one individual particle,
particulate, or the like. Additionally, a particle, particulate, or
any desired agglomeration thereof may be composed of more than one
type of material.
[0032] The term "polymer" includes, but is not limited to,
homopolymers, copolymers, for example, block, graft, random, and
alternating copolymers, terpolymers, etc., and blends and
modifications thereof. Furthermore, unless otherwise specifically
limited, the term "polymer" shall include all possible
configurational isomers of the material. These configurations
include, but are not limited to isotactic, syndiotactic, and
atactic symmetries.
[0033] The term "polyolefin" as used herein generally includes, but
is not limited to, materials such as polyethylene, polypropylene,
polyisobutylene, polystyrene, ethylene vinyl acetate copolymer, and
the like, the homopolymers, copolymers, terpolymers, etc., thereof,
and blends and modifications thereof. The term "polyolefin" shall
include all possible structures thereof, which include, but are not
limited to, isotatic, synodiotactic, and random symmetries.
Copolymers include atactic and block copolymers.
[0034] The term "superabsorbent materials" refers to
water-swellable, water-insoluble organic or inorganic materials
including superabsorbent polymers and superabsorbent polymer
compositions capable, under the most favorable conditions, of
absorbing at least about 10 times their weight, or at least about
15 times their weight, or at least about 25 times their weight in
an aqueous solution containing 0.9 weight percent sodium
chloride.
[0035] The term "superabsorbent polymer composition" refers to a
superabsorbent polymer comprising a surface additive in accordance
with the present invention.
[0036] The terms "superabsorbent polymer" and "superabsorbent
polymer preproduct" refer to a material that is produced by
conducting all of the steps for making a superabsorbent polymer as
described herein, up to and including drying the material, and
coarse grinding in a crusher.
[0037] The term "superabsorbent polymer containing clay" and
superabsorbent polymer-clay" and "SAP-clay" will be used throughout
to represent superabsorbent polymer containing clay as set forth in
the present application.
[0038] The term "surface cross linking" means that the level of
functional cross links in the vicinity of the surface of the
superabsorbent polymer particle generally is higher than the level
of functional cross links in the interior of the superabsorbent
polymer particle. As used herein, "surface" describes the
outer-facing boundaries of the particle. For porous superabsorbent
polymer particles, exposed internal surfaces also are included in
the definition of surface.
[0039] The term "thermoplastic" describes a material that softens
when exposed to heat and which substantially returns to a
non-softened condition when cooled to room temperature.
[0040] The term "% by weight" or "% wt" when used herein and
referring to components of the superabsorbent polymer composition,
is to be interpreted as based on the weight of the dry
superabsorbent polymer composition, unless otherwise specified
herein.
[0041] These terms may be defined with additional language in the
remaining portions of the specification.
DETAILED DESCRIPTION
[0042] An embodiment of the present invention comprises a process
for the production of superabsorbent polymer containing clay based
on a process for the preparation of superabsorbent polymer
containing clay comprising the steps of:
(I) polymerizing a polymerization mixture comprising: (a) one or
more ethylenically unsaturated carboxyl-containing monomers, (b)
one or more crosslinking agents, (c) optionally, one or more
comonomers copolymerizable with the carboxyl-containing monomer,
(d) neutralizing agent to partially neutralize the polymer to from
about 50% to about 99%, by weight, and (e) a polymerization medium,
to form a crosslinked partially neutralized hydrogel, (II) admixing
clay with the crosslinked partially neutralized hydrogel to form
partially neutralized superabsorbent polymer-clay hydrogel; (III)
drying the crosslinked partially neutralized hydrogel at a
temperature from about 190.degree. C. to about 210.degree. C. and
for a time period of from about 15 minutes to about 120 minutes,
and (IV) comminuting the dried partially neutralized superabsorbent
polymer-clay hydrogel to particles.
[0043] The SAP component of the SAP-clay particles is prepared by
well-known continuous and discontinuous processes. The monomers
comprising the SAP component of the SAP-clay particles typically
are polymerized in aqueous solution to form an SAP hydrogel.
However, the SAP component of the present particles may be prepared
by any other method known to persons skilled in the art, like
inverse suspension polymerization.
[0044] A superabsorbent polymer as set forth in the embodiments of
the present invention is obtained by the initial polymerization of
from about 55% to about 99.9% by weight of the superabsorbent
polymer of polymerizable unsaturated acid group containing monomer.
A suitable monomer includes any of those containing carboxyl
groups, such as acrylic acid, methacrylic acid, or
2-acrylamido-2-methylpropanesulfonic acid, or mixtures thereof. It
is desirable for at least about 50% by weight, and more desirable
for at least about 75% by weight of the acid groups to be carboxyl
groups.
[0045] The acid groups are neutralized to the extent of at least
about 25 mol %, that is, the acid groups are desirably present as
sodium, potassium, or ammonium salts. In some aspects, the degree
of neutralization may be at least about 50 mol %. In some aspects,
it is desirable to utilize polymers obtained by polymerization of
acrylic acid or methacrylic acid, the carboxyl groups of which are
neutralized to the extent of from about 50 mol % to about 80 mol %,
in the presence of internal cross linking agents.
[0046] In some aspects, the suitable monomer that can be
copolymerized with the ethylenically unsaturated monomer may
include, but is not limited to acrylamide, methacrylamide,
hydroxyethyl acrylate, dimethylaminoalkyl (meth)-acrylate,
ethoxylated (meth)-acrylates, dimethylaminopropylacrylamide, or
acrylamidopropyltrimethylammonium chloride. Such monomer may be
present in a range of from 0% to about 40% by weight of the
copolymerized monomer.
[0047] The superabsorbent polymer of the invention also includes
internal cross linking agents. The internal cross linking agent has
at least two ethylenically unsaturated double bonds, or one
ethylenically unsaturated double bond and one functional group that
is reactive toward acid groups of the polymerizable unsaturated
acid group containing monomer, or several functional groups that
are reactive towards acid groups can be used as the internal cross
linking component and is desirably present during the
polymerization of the polymerizable unsaturated acid group
containing a monomer.
[0048] Examples of internal cross linking agents include, but are
not limited to, aliphatic unsaturated amides, such as
methylenebisacryl- or -methacrylamide or ethylenebisacrylamide;
aliphatic esters of polyols or alkoxylated polyols with
ethylenically unsaturated acids, such as di(meth)acrylates or
tri(meth)acrylates of butanediol or ethylene glycol, polyglycols or
trimethylolpropane; di- and triacrylate esters of
trimethylolpropane which may be oxyalkylated, desirably
ethoxylated, with about 1 to about 30 moles of alkylene oxide;
acrylate and methacrylate esters of glycerol and pentaerythritol
and of glycerol and pentaerythritol oxyethylated with desirably
about 1 to about 30 mol of ethylene oxide; allyl compounds, such as
allyl (meth)acrylate, alkoxylated allyl (meth)acrylate reacted with
desirably about 1 to about 30 mol of ethylene oxide, triallyl
cyanurate, triallyl isocyanurate, maleic acid diallyl ester,
poly-allyl esters, tetraallyloxyethane, triallylamine,
tetraallylethylenediamine, diols, polyols, hydroxy allyl or
acrylate compounds and allyl esters of phosphoric acid or
phosphorous acid; and monomers that are capable of cross linking,
such as N-methylol compounds of unsaturated amides, such as of
methacrylamide or acrylamide, and the ethers derived there from.
Ionic cross linkers such as multivalent metal salts may also be
employed. Mixtures of the cross linking agents mentioned can also
be employed. The content of the internal cross linking agents is
from about 0.001% to about 5% by weight such as from about 0.2% to
about 3% by weight based on the total amount of the polymerizable
unsaturated acid group containing monomer.
[0049] In some aspects, initiators can be used for initiation of
the free-radical polymerization. Suitable initiators include, but
are not limited to, azo or peroxo compounds, redox systems or UV
initiators, sensitizers, and/or radiation.
[0050] As previously noted, the polymerization reaction proceeds
rapidly to yield a highly viscous hydrogel that is extruded, for
example, onto a flat surface such as a continuously moving conveyor
belt. The neutralized SAP hydrogel then is comminuted, and the clay
is added to, typically as aqueous clay slurry, and intimately
admixed with, the comminuted SAP hydrogel particles. The clay may
also be added as solid particles, or a powder. The SAP hydrogel and
clay components may then be intimately admixed, e.g., by extrusion,
to disperse the clay in and on the hydrogel particles. The
resulting neutralized SAP-clay mixture is then dried and sized, and
optionally surface crosslinked to provide neutralized SAP-clay
particles. Comminution of the SAP-clay hydrogel particles may be
performed simultaneously or sequentially.
[0051] After comminutation, the viscous SAP-clay hydrogel particles
are dehydrated (i.e., dried) to obtain SAP-clay particles in a
solid or powder form. The dehydration step may be performed, for
example, by heating the viscous SAP-clay hydrogel particles at a
temperature of from about 190.degree. C. to about 210.degree. C.
for about 15 minutes to about 120 minutes in a forced-air oven, or
a time period of from about 15 minutes to about 110 minutes or from
about 15 minutes to about 100 minutes, or from about 20 minutes to
about 100 minutes. The dried SAP-clay hydrogel may then be
subjected to further mechanical means for particle size reduction
and classification including chopping, grinding, and sieving.
[0052] Such SAP-clay compositions may include superabsorbent
polymer present in an amount of about 90% to about 99.5%, or from
about 91% to about 99%, or from about 92% to about 98% by weight,
and the clay is present in an amount of about 0.5% to about 10%, or
from about 1 to about 9 wt %, or from about 2 to about 8 wt % by
weight.
[0053] Clay Component
[0054] An embodiment of the present invention includes the
preparation of superabsorbent polymer wherein the superabsorbent
polymer is present in an amount of about 60% to about 90%, by
weight, and the clay is present in an amount of about 0.5% to about
10%, by weight, or from about 1 to about 9 wt %, or from about 2 to
about 8 wt %. Clay useful in the present SAP-clay particles can be
swelling or nonswelling clay.
[0055] Swelling clays have the ability to absorb water and are
swellable, layered organic materials. Suitable swelling clays
include, but are not limited to, montmorillonite, saponite,
nontronite, laponite, beidelite, hectorite, sauconite, stevensite,
vermiculite, volkonskoite, magadite, medmontite, kenyaite, and
mixtures thereof.
[0056] The swelling clay may be a smectite or vermiculite clay.
More preferably, the clay is a smectite clay. Examples of suitable
smectites include, but are not limited to, montmorillonite (often
referred to as bentonite), beidelite, nontronite, hectorite,
saponite, sauconite, and laponite. Bentonite is a naturally
occurring combination of clay particles, rich in montmorillonite
and also including other smectites, as well as nonclay mineral
constituents.
[0057] Suitable nonswelling clays include, without limitation,
kaolin minerals (including kaolinite, dickite, and nacrite),
serpentine minerals, mica minerals (including illite), chlorite
minerals, sepolite, palygorskite, bauxite, and mixtures
thereof.
[0058] The clay also can be an organophilic clay. As used here and
hereafter, the term "organophilic" is defined as the property of a
compound to absorb at least its own weight, and preferably many
times its own weight, of an organic, water-immiscible compound. An
organophilic compound optionally can absorb water or a
water-miscible compound.
[0059] The terms "organophilic clay" and "organoclay" are used
interchangeably herein to refer to various types of clay, e.g.,
smectites that have organoammonium ions substituted for metal
cations (e.g., sodium and/or potassium) present between the clay
layers. The term "organoammonium ion" refers to a substituted
ammonium ion wherein one or more hydrogen atoms are replaced by an
aliphatic or aromatic organic group. The organoclays, therefore,
are solid compounds that have an inorganic component and an organic
component.
[0060] Clay substrates of organophilic clay may include the
smectite-type clays, particularly smectite-type clays that have a
cation exchange capacity of at least 75 milliequivalents per 100
grams of clay. Useful clay substrates include, but are not limited
to, the naturally occurring Wyoming variety of bentonite and
similar clays, and hectorite, which is magnesium-lithium silicate
clay. The clays may first be converted to the sodium form if they
are not already in this form. This conversion can be effected by a
cation exchange reaction using a soluble sodium compound by methods
well known in the art. Smectite-type clays prepared synthetically
also can be utilized, for example, montmorillonite, bentonite,
beidelite, hectorite, saponite, and stevensite. Other useful clay
substrates include nontronite, illite, attapulgite, and a fuller's
earth.
[0061] Organoclays useful in the present invention also include
those set forth in U.S. Pat. No. 2,531,427, wherein the organoclays
cited in the '427 patent are incorporated herein by reference.
These organoclays are modified clays that exhibit, in an inorganic
liquid, some of the properties an untreated clay exhibits in water.
For example, the ability to swell in organic liquids and form
stable gels and colloidal dispersions is desirous.
[0062] Generally, the organoammonium ion substituted onto the clay
substrate has an organic group that ranges from an aliphatic
hydrocarbon moiety having 1 to 24 carbon atoms to an aromatic
organic moiety, such as a benzyl group that can have a variety of
groups substituted on the phenyl ring. The number of benzyl versus
aliphatic hydrocarbon moieties substituted on the ammonium ion can
vary from 3 to 0 aromatic moieties per aliphatic moiety (i.e.,
dimethyl dioctadecyl 0:2, methyl benzyl dioctadecyl 1:2, dibenzyl
dioctabenzyl 2:2, tribenzyl octadecyl 3:1, and methyl dibenzyl
octadecyl 2:1). The amount of organoammonium ion substituted onto
the clay substrate typically is about 0.5% to about 50%, by weight
of the organophilic clay.
[0063] Organoclays may comprise one or more of the following types
of organoammonium cation-modified montmorillonite clays or benzyl
organoclays, such as dimethyl benzyl (hydrogenated tallow) ammonium
bentonite; methyl benzyl di(hydrogenated tallow) ammonium
bentonite; and more generally organoammonium-cation modified
montmorillonite clays.
[0064] The montmorillonite clays that can be so modified are the
principal constituents of bentonite rock, and have the chemical
compositions and characteristics described, for example, in Berry
& Mason, "Mineralogy," pp. 508-509 (1959). Modified
montmorillonite clays of this type (i.e., organoclays) are
commercially available from Southern Clay Products, Inc., Gonzales,
Tex., under trade designations such as CLAYTONE.RTM. 34 and 40, and
from NL Industries, Inc., New York, N.Y., under trade designations
such as BENTONE.RTM.. 27, 34, and 38. Other organoclays useful in
the invention are the higher dialkyl dimethyl ammonium organoclays,
such as dimethyl di-(hydrogenated tallow) ammonium bentonite; the
benzyl ammonium organoclays, such as dimethyl benzyl (hydrogenated
tallow) ammonium bentonite; and ethylhydroxy ammonium organoclays,
such as methyl bis(2-hydroxyethyl) octadecyl ammonium bentonite.
Examples of nonswelling organophilic clays are bentonite clays
treated with an amine containing three to eight carbon atoms, e.g.,
propylamine, butylamine, or octylamine.
[0065] Other commercially available clays include ULTRAGLOSS.RTM.
clays (hydrous kaolin) from BASF Corporation, Florham Park, N.J.;
Purified Clay from Nanocor Technologies, Arlington Heights, Ill.;
and HYDROGLOSS.RTM. from Huber, Atlanta, Ga.
[0066] Another suitable clay component comprises the
aluminosilicates. Useful aluminosilicates are nonzeolite silicates
wherein a portion of the silicon atoms are replaced by aluminum
atoms. Because the aluminum atom has one positive nuclear charge
less than the silicon atom, every aluminum atom replacing a silicon
atom increases the negative charge of the lattice anion by one
unit. Additional cations, therefore, are needed to neutralize the
molecule. Consequently, in addition to aluminum atoms,
aluminosilicates can include additional metal atoms, e.g., alkali
and alkaline earth metal atoms, such as sodium, potassium,
magnesium, calcium, iron, and zinc. Useful aluminosilicates have a
layered structure. In accordance with the present invention, both
naturally occurring and synthetic aluminosilicates are useful.
[0067] Naturally occurring aluminosilicates include the micas.
Micas are infinite sheet silicates containing layers of
tetrahedral. An example of a useful sheet-like aluminosilicate for
the purposes of the invention is the synthetic aluminosilicate
saponite. Synthetic saponite (CAS No. 1319-41-1) is commercially
available as a white, odorless powder. Examples of useful naturally
occurring micas are muscowite, biotite, phlogopite, lepidolite,
zinnwaldite, paragonite, and montmorillonite.
[0068] A clay does not perform, like an SAP with respect to
absorbing and retaining large amounts of an aqueous fluid. A clay
typically is referred to, and considered, as a diluent for SAP
particles in an attempt to improve one or more properties of the
SAP. It also is expected that other SAP properties would be
adversely affected by diluting an SAP with the clay. However, as
demonstrated hereafter, after adding the clay to SAP particles, the
beneficial properties associated with an SAP are diminished to a
substantially lower degree than expected, while other beneficial
properties are improved.
[0069] Surface Treatment
[0070] In embodiments wherein a surface crosslinking agent is used,
the surface crosslinking agent is applied to the dried SAP
particles. After application of the surface crosslinking agent, the
SAP-clay particles are subjected to conditions wherein the surface
crosslinking agent reacts with a portion of the carboxyl or amino
groups of the SAP to crosslink the surfaces of the SAP particles.
In general, surface cross linking is a process that is believed to
increase the cross link density of the polymer matrix in the
vicinity of the superabsorbent polymer particle surface with
respect to the cross linking density of the particle interior.
[0071] In some particular aspects, desirable surface cross linking
agents include chemicals with one or more functional groups that
are reactive toward pendant groups of the polymer chains, typically
the acid groups. The surface cross linking agent may be present in
an amount of from about 0.001% to about 5% by weight of the dry
superabsorbent polymer composition, and such as from about 0.1% to
about 3% by weight, and such as from about 0.1% to about 1% by
weight, based on the weight of the dry superabsorbent polymer
composition. Applicants have found that a heat treatment step after
addition of the surface cross linking agent is desirable.
[0072] In one particular aspect, the particulate superabsorbent
polymer may be coated or surface-treated with an alkylene carbonate
followed by heating to affect surface cross linking, which can
improve the surface cross linking density and the gel strength
characteristics of the superabsorbent polymer composition particle.
More specifically, the surface cross linking agent is coated onto
the superabsorbent polymer particulate by mixing the polymer
particulate with an aqueous alcoholic solution of the alkylene
carbonate surface cross linking agent. The amount of alcohol is
determined by the solubility of the alkylene carbonate and is kept
as low as possible for various reasons. Suitable alcohols may
include methanol, isopropanol, ethanol, butanol, or butyl glycol,
as well as mixtures of these alcohols. In some aspects, the solvent
desirably is water, which typically is used in an amount of about
0.3% by weight to about 5.0% by weight, based on the weight of the
dry superabsorbent polymer composition. In other aspects, the
alkylene carbonate surface cross linking agent is dissolved in
water without any alcohol. In still other aspects, the alkylene
carbonate surface cross linking agent may be applied from a powder
mixture, for example, with an inorganic carrier material, such as
silicone dioxide (SiO.sub.2), or in a vapor state by sublimation of
the alkylene carbonate.
[0073] To achieve the desired surface cross linking properties, the
alkylene carbonate is distributed evenly on the particulate
superabsorbent polymer. For this purpose, mixing is effected in
suitable mixers known in the art, such as fluidized bed mixers,
paddle mixers, rotary drum mixers, or twin-worm mixers. It is also
possible to carry out the coating of the particulate superabsorbent
polymer during one of the process steps in the production of the
particulate superabsorbent polymer. In one particular aspect, a
suitable process for this purpose is the inverse suspension
polymerization process.
[0074] The heat treatment, that may follow the coating treatment,
may be carried out as follows. In general, the heat treatment is at
a temperature of from about 100.degree. C. to about 300.degree. C.
Lower temperatures are possible if highly reactive epoxide cross
linking agents are used. However, if alkylene carbonates are used,
then the thermal treatment is suitably at a temperature of from
about 150.degree. C. to about 250.degree. C. In this particular
aspect, the treatment temperature depends on the dwell time and the
kind of alkylene carbonate. For example, at a temperature of about
150.degree. C., the thermal treatment is carried out for one hour
or longer. In contrast, at a temperature of about 250.degree. C., a
few minutes (e.g., from about 0.5 minutes to about 5 minutes) are
sufficient to achieve the desired surface cross-linking properties.
The thermal treatment may be carried out in conventional dryers or
ovens known in the art.
[0075] In some aspects, the superabsorbent polymer composition of
the present invention may include from 0% to about 5% by weight, or
from about 0.001% to about 5% by weight, or from about 0.01% to
about 0.5% by weight of the dry superabsorbent polymer composition
of a polymeric coating, such as a thermoplastic coating, or a
cationic coating, or a combination of a thermoplastic coating and a
cationic coating. In some particular aspects, the polymeric coating
desirably is a polymer that may be in a solid, emulsion,
suspension, colloidal, or solubilized state, or combinations
thereof. Polymeric coatings suitable for this invention may
include, but are not limited to, a thermoplastic coating having a
thermoplastic melt temperature wherein the polymeric coating is
applied to the particle surface coincident with or followed by a
temperature of the treated superabsorbent polymer particle at about
the thermoplastic melt temperature.
[0076] Examples of thermoplastic polymers include, but are not
limited to, polyolefin, polyethylene, polyester, polyamide,
polyurethane, styrene polybutadiene, linear low density
polyethylene (LLDPE), ethylene acrylic acid copolymer (EAA),
ethylene alkyl methacrylate copolymer (EMA), polypropylene (PP),
maleated polypropylene, ethylene vinyl acetate copolymer (EVA),
polyester, polyamide, and blends of all families of polyolefins,
such as blends of PP, EVA, EMA, EEA, EBA, HDPE, MDPE, LDPE, LLDPE,
and/or VLDPE, may also be advantageously employed. The term
polyolefin as used herein is defined above. In particular aspects,
the Applicants have found that maleated polypropylene to be a
desirable thermoplastic polymer for use in the present invention. A
thermoplastic polymer may be functionalized to have additional
benefits such as water solubility or dispersability.
[0077] Polymeric coatings of this invention may also include a
cationic polymer. A cationic polymer as used herein refers to a
polymer or mixture of polymers comprising a functional group or
groups having a potential of becoming positively charged ions upon
ionization in an aqueous solution. Suitable functional groups for a
cationic polymer include, but are not limited to, primary,
secondary, or tertiary amino groups, imino groups, imido groups,
amido groups, and quaternary ammonium groups. Examples of synthetic
cationic polymers include, but are not limited to, the salts or
partial salts of poly(vinyl amines), poly(allylamines),
poly(ethylene imine), poly(amino propanol vinyl ethers),
poly(acrylamidopropyl trimethyl ammonium chloride),
poly(diallyldimethyl ammonium chloride). Poly(vinyl amines)
include, but are not limited to, LUPAMIN.RTM. 9095 available from
BASF Corporation, Mount Olive, N.J. Examples of natural-based
cationic polymers include, but are not limited to, partially
deacetylated chitin, chitosan, and chitosan salts. Synthetic
polypeptides such as polyasparagins, polylysines, polyglutamines,
and polyarginines are also suitable cationic polymers.
[0078] The absorbent polymers according to the invention can
comprise include from 0 to about 5 wt % of a multivalent metal
salt, based on the weight of the mixture, on the surface of the
polymer. The multivalent metal salt is preferably water soluble.
Examples of preferred metal cations include the cations of Al, Fe,
Zr, Mg and Zn. Preferably, the metal cation has a valence of at
least +3, with Al being most preferred. Examples of preferred
anions in the multivalent metal salt include halides,
chlorohydrates, sulfates, nitrates and acetates, with chlorides,
sulfates, chlorohydrates and acetates being preferred,
chlorohydrates and sulfates being more preferred and sulfates being
the most preferred. Aluminum sulfate is the most preferred
multivalent metal salt and is readily commercially available. The
preferred form of aluminum sulfate is hydrated aluminum sulfate,
preferably aluminum sulfate having from 12 to 14 waters of
hydration. Mixtures of multivalent metal salts can be employed.
[0079] The polymer and multivalent metal salt suitably are mixed by
dry blending, or preferably in solution, using means well known to
those skilled in the art. Aqueous solutions are preferred. With dry
blending, a binder may be employed in an amount which sufficient to
ensure that a substantially uniform mixture of the salt and the
superabsorbent polymer is maintained. The binder may be water or a
nonvolatile organic compound having a boiling point of at least
150.degree. C. Examples of binders include water, polyols such as
propylene glycol, glycerin and poly(ethylene glycol).
[0080] The superabsorbent polymer compositions according to the
invention may include from about 0.01% to about 2% by weight or
from about 0.01% to about 1% by weight based on the dry
superabsorbent polymer composition of a water-insoluble inorganic
metal compound. The water-insoluble inorganic metal compound may
include, but are not limited to, a cation selected from aluminum,
titanium, calcium, or iron and an anion selected from phosphate,
borate, or sulfate. Examples of water-insoluble inorganic metal
compounds include aluminum phosphate and an insoluble metal borate.
The inorganic metal compound may have a mass median particle size
of less than about 2 .mu.m, and may have a mass median particle
size of less than about 1 .mu.m.
[0081] The inorganic metal compound can be applied in the dry
physical form to the surface of the superabsorbent polymer
particles. For this, the superabsorbent polymer particles can be
intimately mixed with the finely divided inorganic metal compound.
The finely divided inorganic metal compound is usually added at
about room temperature to the superabsorbent polymer particles and
mixed in until a homogeneous mixture is present. For this purpose,
mixing is effected in suitable mixers known in the art, such as
fluidized bed mixers, paddle mixers, rotary drum mixers, or
twin-worm mixers. The mixing of the SAP particles with the finely
divided water-insoluble inorganic metal compound may take place
before or after any surface cross linking, for example during the
application of the surface cross linking agent.
[0082] Alternatively, a suspension of a finely divided
water-insoluble inorganic metal compounds can be prepared and
applied to a particulate SAP. The suspension is applied, for
example, by spraying. Useful dispersion media for preparing the
suspension include water, organic solvents such as alcohols, for
example methanol, ethanol, isopropanol, ketones, for example
acetone, methyl ethyl ketone, or mixtures of water with the
aforementioned organic solvents. Other useful dispersion media
include dispersion aids, surfactants, protective colloidal,
viscosity modifiers, and other auxiliaries to assist in the
preparation of the suspension. The suspension can be applied in
conventional reaction mixers or mixing and drying systems as
described above at a temperature in the range from room temperature
to less than the boiling point of the dispersion medium, preferably
at about room temperature. It is appropriate to combine the
application of the suspension with a surface cross linking step by
dispersing the finely divided water-insoluble metal salt in the
solution of the surface cross linking agent. Alternatively, the
suspension can also be applied before or after the surface cross
linking step. The application of the slurry may be followed by a
drying step.
[0083] In some aspects, the superabsorbent polymer compositions
according to the invention can include from 0% to about 5%, or from
about 0.01% to about 3%, by weight of the dry superabsorbent
polymer composition of silica. Examples of silica include fumed
silica, precipitated silica, silicon dioxide, silicic acid, and
silicates. In some particular aspects, microscopic noncrystalline
silicon dioxide is desirable. Products include SIPERNAT.RTM. 22S
and AEROSIL.RTM. 200 available from Evonik Corporation, Parsippany,
N.J. In some aspects, the particle diameter of the inorganic powder
can be 1,000 .mu.m or smaller, such as 100 .mu.m or smaller.
[0084] In some aspects, the superabsorbent polymer compositions may
also include from 0% to about 30% by weight of the dry
superabsorbent polymer composition, such as from about 0.1% to
about 5% by weight, of water-soluble polymers based by weight of
the dry superabsorbent polymer composition, of partly or completely
hydrolyzed polyvinyl acetate, polyvinylpyrrolidone, starch or
starch derivatives, polyglycols, polyethylene oxides, polypropylene
oxides, or polyacrylic acids.
[0085] In some aspects, additional surface additives may optionally
be employed with the superabsorbent polymer particles, such as
odor-binding substances, such as cyclodextrins, zeolites, inorganic
or organic salts, and similar materials; anti-caking additives,
flow modification agents, surfactants, viscosity modifiers, and the
like. In addition, surface additives may be employed that perform
several roles during surface modifications. For example, a single
additive may be a surfactant, viscosity modifier, and may react to
cross link polymer chains.
[0086] In some aspects, the superabsorbent polymer compositions of
the present invention may be, after a heat treatment step, treated
with water so that the superabsorbent polymer composition has water
content of up to about 10% by weight of the superabsorbent polymer
composition. This water may be added with one or more of the
surface additives from above added to the superabsorbent
polymer.
[0087] The superabsorbent polymer compositions according to the
invention may be prepared either continuously or discontinuously in
a large-scale industrial manner, the after-cross linking according
to the invention being carried out accordingly. The partially
neutralized monomer, such as acrylic acid, is converted into a gel
by free-radical polymerization in aqueous solution in the presence
of cross linking agents and any further components, and the gel is
comminuted, dried, ground, and sieved off to the desired particle
size. The superabsorbent polymer composition particles of the
present invention generally include particle sizes ranging from
about 150 to about 850 microns. The present invention may include
at least about 40 wt % of the particles having a particle size from
about 300 .mu.m to about 600 .mu.m, or at least about 50 wt % of
the particles having a particle size from about 300 .mu.m to about
600 .mu.m, or at least about 60 wt % of the particles having a
particle size from about 300 .mu.m to about 600 .mu.m as measured
by screening through a U.S. standard 30 mesh screen and retained on
a U.S. standard 50 mesh screen. In addition, the size distribution
of the SAP particles of the present invention may include less than
about 30% by weight of SAP particles having a size greater than
about 600 microns, and less than about 30% by weight of SAP
particles having a size of less than about 300 microns as measured
using for example a RO-TAP.RTM. Mechanical Sieve Shaker Model B
available from W. S. Tyler, Inc., Mentor Ohio.
[0088] It is well known to those skilled in the art that particle
size distribution of the SAP particles resembles a normal
distribution or a bell shaped curve. It is also known that for
various reasons, the normal distribution of the particle size
distribution may be skewed in either direction.
[0089] Surprisingly, it has been found that the absorption and
retention properties of SAP particles may be improved by the
addition of clay to a partially neutralized SAP hydrogel and drying
the hydrogel-clay at elevated temperatures while maintaining the
absorption under load at 0.9 psi properties. In particular,
incorporating the resulting SAP-clay particles into a diaper core
provides cores having improved fluid acquisition rates.
[0090] The superabsorbent polymer containing clay of the present
invention exhibits certain characteristics, or properties, as
measured by Free Swell Gel Bed Permeability (GBP), Centrifuge
Retention Capacity (CRC), and absorbency under load at about 0.9
psi (AUL(0.9 psi)), and vortex time. The Free Swell Gel Bed
Permeability (GBP) Test is a measurement of the permeability of a
swollen bed of superabsorbent material in Darcy (e.g., separate
from the absorbent structure) under a confining pressure after what
is commonly referred to as "free swell" conditions. In this
context, the term "free swell" means that the superabsorbent
material is allowed to swell without a swell restraining load upon
absorbing test solution as will be described. The Centrifuge
Retention Capacity (CRC) Test measures the ability of the
superabsorbent polymer composition to retain liquid therein after
being saturated and subjected to centrifugation under controlled
conditions. The resultant retention capacity is stated as grams of
liquid retained per gram weight of the sample (g/g). The vortex
time is a measure of the free swell absorbing rate of the
polymer.
[0091] A superabsorbent polymer containing clay particulate made by
a process of present invention may have a centrifuge retention
capacity of from about 25 g/g to about 40 g/g, or from about 26 to
about 30 g/g; and may have an absorption under load at 0.9 psi of
from about 10 g/g to about 20 g/g, or from about 13 g/g to about 18
g/g, and a free swell gel bed permeability of from about 5 to about
100 Darcy, and a vortex time of about 70 seconds or less, or from
about 30 to about 70 seconds.
[0092] The superabsorbent polymer containing clay according to the
present invention can be employed in many products including
sanitary towels, diapers, or wound coverings, and they have the
property that they rapidly absorb large amounts of menstrual blood,
urine, or other body fluids. Since the agents according to the
invention retain the absorbed liquids even under pressure and are
also capable of distributing further liquid within the construction
in the swollen state, they are more desirably employed in higher
concentrations, with respect to the hydrophilic fiber material,
such as fluff, when compared to conventional current superabsorbent
compositions. They are also suitable for use as a homogeneous
superabsorbent layer without fluff content within the diaper
construction, as a result of which particularly thin articles are
possible. The polymers are furthermore suitable for use in hygiene
articles (incontinence products) for adults.
[0093] The above test results show that the absorbent SAP-clay
particles of the present invention can be used to absorb aqueous
fluids. The fluid can be a body fluid, an industrial waste, or any
other fluid that one desires to absorb. The absorbed fluid can be
any water-containing fluid, and typically contains electrolytes,
for example, urine, blood, saline, menses, and similar liquids.
[0094] The present SAP-clay particles, therefore, are useful in
personal hygiene articles comprising: (A) a fluid-pervious top
sheet; (B) a fluid-impervious back sheet; (C) a core positioned
between (A) and (B), said core comprising: (C1) about 10 to 100% by
weight of the SAP-clay particles of the present invention, and (C2)
0 to about 90% by weight of a fiber material; (D) optionally one or
more tissue layers positioned directly above and/or below said core
(C); and (E) optionally an acquisition layer positioned between (A)
and (C).
[0095] The fluid-pervious top sheet (A) is the layer which is in
direct contact with the skin of the wearer. Top sheet (A) generally
comprises synthetic or cellulosic fibers or films, i.e.,
polyesters, polyolefins, rayon, or natural fibers, such as cotton.
In the case of nonwoven materials, the fibers generally are joined
together by binders such as a polyacrylate. Preferred materials are
polyesters, rayon and blends thereof, polyethylene, and
polypropylene. The fluid-impervious layer (B) is generally a sheet
of polyethylene or polypropylene.
[0096] The core (C) includes SAP-clay particles (C1) of the present
invention, and also can include a fiber material (C2). Fiber
material (C2) typically is hydrophilic, i.e., aqueous fluids are
rapidly distributed across the fibers. The fiber material typically
is cellulose, modified cellulose, rayon, or polyester, such as
polyethylene terephthalate. Preferred fibers are cellulose fibers,
such as pulp. The fibers may have a diameter of about 1 to about
200 .mu.m, or about 10 to about 100 .mu.m, and a minimum length of
about 1 mm.
[0097] The amount of fiber material (C2) based on the total weight
of the core is typically about 20% to about 80% by weight, or about
40% to about 70% by total weight of C(1) and C(2). Core (C)
typically also can be a heavily loaded core (e.g., 60-95 wt %
SAP-clay particles/5-40 wt % fluff).
[0098] The SAP-clay particles often are present in core (C) as a
pressed sheet containing the particles, and optionally fluff and/or
nonwoven fibers. A single absorbent layer or sheet containing
SAP-clay particles of the present invention can be used as the
absorbent component of a core (C). A plurality of absorbent layers
or sheets may be used in the core (C), together with a wicking
layer (e.g., a tissue layer) between absorbent layers or sheets to
provide improved wicking of a fluid between and through the
absorbent sheets. In other embodiments, at least one of the
absorbent layers or sheets in a core (C) contains nonwoven fibers
to improve wet strength of the absorbent core and assist in
wicking.
[0099] A core (C) may contain two to five absorbent layers or
sheets. By utilizing a laminate of thin absorbent layers or sheets,
as opposed to a single, thicker absorbent layer or sheet,
horizontal expansion of the core is decreased, and vertical
expansion is promoted. This feature provides a good fluid transport
through the core, provides a better fitting diaper after an initial
insult, and avoids leaking when the diaper is subsequently rewet by
a second and additional insult. In more preferred embodiments, core
(C) contains a laminate of two or more absorbent layers or sheets
of SAP-clay particles wherein a wicking layer is positioned between
each absorbent sheet layer or sheet, and on top and at the bottom
of the laminate.
[0100] An absorbent layer or sheet containing SAP-clay particles of
the present invention, or a laminate comprising such layers or
sheets, is present in an absorbent core to provide a desired basis
weight (i.e., weight of SAP in the core) of about 50 to about 800
gsm (grams/square meter), or from about 150 to about 600 gsm. To
achieve the full advantage of the present invention, the basis
weight is from about 300 to about 550 gsm. The desired basis weight
of the core is related to the end use of the core. For example,
diapers for newborns have a low basis weight, as opposed to a
medium basis weight for toddlers, and a high basis weight for
overnight diapers.
[0101] In another embodiment, a present diaper core may include a
topsheet (A), a core (C), and a backsheet (B), i.e., an acquisition
layer is not present. An example of a topsheet (A) is staple length
polypropylene fibers having a denier of about 1.5, such as
Hercules-type 151 polypropylene marketed by Hercules, Inc.,
Wilmington, Del. As used herein, the term "staple length fibers"
refers to having a length of at least about 15.9 mm (0.62 inches).
The back sheet (B) is impervious to liquids, and typically is
manufactured from a thin plastic film, although other flexible
liquid impervious materials also can be used. The back sheet
prevents exudates absorbed and contained in the absorbent core (C)
from wetting articles, such as bed sheets and undergarments, that
contact the diaper.
[0102] For an absorbent article having a core (C) containing a
"fluff" component, the "fluff" comprises a fibrous material in the
form of a web or matrix. Fibers include naturally occurring fibers
(modified or unmodified). Examples of suitable unmodified/modified
naturally occurring fibers include cotton, Esparto grass, bagasse,
kemp, flax, silk, wool, wood pulp, chemically modified wood pulp,
and jute.
[0103] The cores also can include an optional nonwoven fiber, for
example, polypropylene, polyethylene, polyethylene terephthalate,
viscose, and mixtures thereof. Also, an open fiber mesh of nonwoven
fibers can be used, for example, cellulose acetate fiber. Nonwoven
fibers can be made by drylaid thermobonded, carded air-through
bonded, spunbond, or spun-meltblown-spun processes. Nonwoven fibers
impart additional wet strength to an absorbent layer or sheet when
used in an amount of about 10 to about 20 grams per square meter
(gsm) of sheet material.
[0104] Suitable fibers, and fiber meshes, can be made from
polyvinyl chloride, polyvinyl fluoride, polytetrafluoroethylene,
polyvinylidene chloride, polyacrylics such as ORLON.RTM., polyvinyl
acetate, polyethylvinyl acetate, nonsoluble or soluble polyvinyl
alcohol, polyolefins such as polyethylene (e.g., PULPEX.RTM.) and
polypropylene, polyamides (e.g., nylon), polyesters (e.g.,
DACRON.RTM. or KODEL.RTM.), polyurethanes, polystyrenes, and the
like.
[0105] Hydrophilic fibers are preferred, and include rayon,
polyester fibers, such as polyethylene terephthalate (e.g.,
DACRON.RTM.), hydrophilic nylon (e.g., HYDROFIL.RTM.), and the
like. Suitable hydrophilic fibers can also be obtained by
hydrophilizing hydrophobic fibers, such as surfactant-treated or
silica-treated thermoplastic fibers derived from, for example,
polyolefins, such as polyethylene or polypropylene, polyacrylics,
polyamides, polystyrenes, polyurethanes, and the like.
[0106] The improved results demonstrated by a core containing
SAP-clay particles of the present invention permit the thickness of
the core to be reduced. Typically, cores contain 50% or more fluff
or pulp to achieve rapid liquid absorption while avoiding problems
like gel blocking. The present cores, which contain SAP-clay
particles acquire liquids sufficiently fast to avoid problems, like
gel blocking, and, therefore, the amount of fluff or pulp in the
core can be reduced, or eliminated. A reduction in the amount of
the low-density fluff results in a thinner core, and, accordingly,
a thinner diaper. Therefore, a core of the present invention can
contain at least 50% SAP-clay particles, preferably at least 60%,
and up to 80% of the SAP-clay particles. In various embodiments,
the presence of a fluff is no longer necessary, or desired.
[0107] The superabsorbent polymer containing clay according to the
invention may also be employed in absorbent articles that are
suitable for further uses. In particular, the superabsorbent
polymer compositions of this invention can be used in absorbent
compositions for absorbents for water or aqueous liquids, desirably
in constructions for absorption of body fluids, in foamed and
non-foamed sheet-like structures, in packaging materials, in
constructions for plant growing, as soil improvement agents, or as
active compound carriers. For this, they are processed into a web
by mixing with paper or fluff or synthetic fibers or by
distributing the superabsorbent polymer composition particles
between substrates of paper, fluff, or non-woven textiles, or by
processing into carrier materials. They are further suited for use
in absorbent compositions such as wound dressings, packaging,
agricultural absorbents, food trays and pads, and the like.
[0108] The present invention may be better understood with
reference to the following examples.
Test Procedures
[0109] Centrifuge Retention Capacity Test
[0110] The Centrifuge Retention Capacity (CRC) Test measures the
ability of the superabsorbent polymer to retain liquid therein
after being saturated and subjected to centrifugation under
controlled conditions. The resultant retention capacity is stated
as grams of liquid retained per gram weight of the sample (g/g).
The sample to be tested is prepared from particles that are
pre-screened through a U.S. standard 30-mesh screen and retained on
a U.S. standard 50-mesh screen. As a result, the superabsorbent
polymer sample comprises particles sized in the range of about 300
to about 600 microns. The particles can be pre-screened by hand or
automatically.
[0111] The retention capacity is measured by placing about 0.2
grams of the pre-screened superabsorbent polymer sample into a
water-permeable bag that will contain the sample while allowing a
test solution (0.9 weight percent sodium chloride in distilled
water) to be freely absorbed by the sample. A heat-sealable tea bag
material, such as that available from Dexter Corporation (having a
place of business in Windsor Locks, Conn., U.S.A.) as model
designation 1234T heat sealable filter paper works well for most
applications. The bag is formed by folding a 5-inch by 3-inch
sample of the bag material in half and heat-sealing two of the open
edges to form a 2.5-inch by 3-inch rectangular pouch. The heat
seals are about 0.25 inches inside the edge of the material. After
the sample is placed in the pouch, the remaining open edge of the
pouch is also heat-sealed. Empty bags are also made to serve as
controls. Three samples are prepared for each superabsorbent
polymer composition to be tested.
[0112] The sealed bags are submerged in a pan containing the test
solution at about 23.degree. C., making sure that the bags are held
down until they are completely wetted. After wetting, the samples
remain in the solution for about 30 minutes, at which time they are
removed from the solution and temporarily laid on a non-absorbent
flat surface.
[0113] The wet bags are then placed into the basket wherein the wet
bags are separated from each other and are placed at the outer
circumferential edge of the basket, wherein the basket is of a
suitable centrifuge capable of subjecting the samples to a g-force
of about 350. One suitable centrifuge is a CLAY ADAMS DYNAC II,
model #0103, having a water collection basket, a digital rpm gauge,
and a machined drainage basket adapted to hold and drain the flat
bag samples. Where multiple samples are centrifuged, the samples
are placed in opposing positions within the centrifuge to balance
the basket when spinning. The bags (including the wet, empty bags)
are centrifuged at about 1,600 rpm (e.g., to achieve a target
g-force of about 290 g force with a variance from about 280 to
about 300 g force), for 3 minutes. G force is defined as a unit of
inertial force on a body that is subjected to rapid acceleration or
gravity, equal to 32 ft/sec.sup.2 at sea level. The bags are
removed and weighed, with the empty bags (controls) being weighed
first, followed by the bags containing the superabsorbent polymer
composition samples. The amount of solution retained by the
superabsorbent polymer sample, taking into account the solution
retained by the bag itself, is the centrifuge retention capacity
(CRC) of the superabsorbent polymer, expressed as grams of fluid
per gram of superabsorbent polymer. More particularly, the
retention capacity is determined by the following equation:
sample / bag after centrifuge - empty bag after centrifuge - dry
sample weight dry sample weight ##EQU00001##
[0114] The three samples are tested, and the results are averaged
to determine the Centrifuge Retention Capacity (CRC) of the
superabsorbent polymer composition.
[0115] Free-Swell Gel Bed Permeability Test (FSGBP)
[0116] As used herein, the Free-Swell Gel Bed Permeability Test,
also referred to as the Gel Bed Permeability (GBP) Under 0 psi
Swell Pressure Test, determines the permeability of a swollen bed
of gel particles (e.g., such as the surface treated absorbent
material or the superabsorbent material prior to being surface
treated), under what is commonly referred to as "free swell"
conditions. The term "free swell" means that the gel particles are
allowed to swell without a restraining load upon absorbing test
solution as will be described. A suitable apparatus for conducting
the Gel Bed Permeability Test is shown in FIGS. 1, 2 and 3 and
indicated generally as 500. The test apparatus assembly 528
comprises a sample container, generally indicated at 530, and a
plunger, generally indicated at 536. The plunger comprises a shaft
538 having a cylinder hole bored down the longitudinal axis and a
head 550 positioned at the bottom of the shaft. The shaft hole 562
has a diameter of about 16 mm. The plunger head is attached to the
shaft, such as by adhesion. Twelve holes 544 are bored into the
radial axis of the shaft, three positioned at every 90 degrees
having diameters of about 6.4 mm. The shaft 538 is machined from a
LEXAN rod or equivalent material and has an outer diameter of about
2.2 cm and an inner diameter of about 16 mm.
[0117] The plunger head 550 has a concentric inner ring of seven
holes 560 and an outer ring of 14 holes 554, all holes having a
diameter of about 8.8 millimeters as well as a hole of about 16 mm
aligned with the shaft. The plunger head 550 is machined from a
LEXAN rod or equivalent material and has a height of approximately
16 mm and a diameter sized such that it fits within the cylinder
534 with minimum wall clearance but still slides freely. The total
length of the plunger head 550 and shaft 538 is about 8.25 cm, but
can be machined at the top of the shaft to obtain the desired mass
of the plunger 536. The plunger 536 comprises a 100 mesh stainless
steel cloth screen 564 that is biaxially stretched to tautness and
attached to the lower end of the plunger 536. The screen is
attached to the plunger head 550 using an appropriate solvent that
causes the screen to be securely adhered to the plunger head 550.
Care must be taken to avoid excess solvent migrating into the open
portions of the screen and reducing the open area for liquid flow.
Acrylic solvent Weld-on 4 from IPS Corporation (having a place of
business in Gardena, Calif., USA) is a suitable solvent.
[0118] The sample container 530 comprises a cylinder 534 and a 400
mesh stainless steel cloth screen 566 that is biaxially stretched
to tautness and attached to the lower end of the cylinder 534. The
screen is attached to the cylinder using an appropriate solvent
that causes the screen to be securely adhered to the cylinder. Care
must be taken to avoid excess solvent migrating into the open
portions of the screen and reducing the open area for liquid flow.
Acrylic solvent Weld-on 4 from IPS Corporation (having a place of
business in Gardena, Calif., USA) is a suitable solvent. A gel
particle sample, indicated as 568 in FIG. 2, is supported on the
screen 566 within the cylinder 534 during testing.
[0119] The cylinder 534 may be bored from a transparent LEXAN rod
or equivalent material, or it may be cut from a LEXAN tubing or
equivalent material, and has an inner diameter of about 6 cm (e.g.,
a cross-sectional area of about 28.27 cm.sup.2), a wall thickness
of about 0.5 cm and a height of approximately 7.95 cm. A step is
machined into the outer diameter of the cylinder 534 such that a
region 534a with an outer diameter of 66 mm exists for the bottom
31 mm of the cylinder 534. An o-ring 540 which fits the diameter of
region 534a may be placed at the top of the step.
[0120] The annular weight 548 has a counter-bored hole about 2.2 cm
in diameter and 1.3 cm deep so that it slips freely onto the shaft
538. The annular weight also has a thru-bore 548a of about 16 mm.
The annular weight 548 can be made from stainless steel or from
other suitable materials resistant to corrosion in the presence of
the test solution, which is 0.9 weight percent sodium chloride
solution in distilled water. The combined weight of the plunger 536
and annular weight 548 equals approximately 596 grams (g), which
corresponds to a pressure applied to the sample 568 of about 0.3
pounds per square inch (psi), or about 20.7 dynes/cm.sup.2 (2.07
kPa), over a sample area of about 28.27 cm.sup.2.
[0121] When the test solution flows through the test apparatus
during testing as described below, the sample container 530
generally rests on a weir 600. The purpose of the weir is to divert
liquid that overflows the top of the sample container 530 and
diverts the overflow liquid to a separate collection device 601.
The weir can be positioned above a scale 602 with a beaker 603
resting on it to collect saline solution passing through the
swollen sample 568.
[0122] To conduct the Gel Bed Permeability Test under "free swell"
conditions, the plunger 536, with the weight 548 seated thereon, is
placed in an empty sample container 530 and the height from the top
of the weight 548 to the bottom of the sample container 530 is
measured using a suitable gauge accurate to 0.01 mm. The force the
thickness gauge applies during measurement should be as low as
possible, preferably less than about 0.74 Newtons. It is important
to measure the height of each empty sample container 530, plunger
536, and weight 548 combination and to keep track of which plunger
536 and weight 548 is used when using multiple test apparatus. The
same plunger 536 and weight 548 should be used for measurement when
the sample 568 is later swollen following saturation. It is also
desirable that the base that the sample cup 530 is resting on is
level, and the top surface of the weight 548 is parallel to the
bottom surface of the sample cup 530.
[0123] The sample to be tested is prepared from superabsorbent
polymer composition particles which are prescreened through a U.S.
standard 30 mesh screen and retained on a U.S. standard 50 mesh
screen. As a result, the test sample comprises particles sized in
the range of about 300 to about 600 microns. The superabsorbent
polymer particles can be pre-screened with, for example, a RO-TAP
Mechanical Sieve Shaker Model B available from W. S. Tyler, Inc.,
Mentor Ohio. Sieving is conducted for 10 minutes. Approximately 2.0
grams of the sample is placed in the sample container 530 and
spread out evenly on the bottom of the sample container. The
container, with 2.0 grams of sample in it, without the plunger 536
and weight 548 therein, is then submerged in the 0.9% saline
solution for a time period of about 60 minutes to saturate the
sample and allow the sample to swell free of any restraining load.
During saturation, the sample cup 530 is set on a mesh located in
the liquid reservoir so that the sample cup 530 is raised slightly
above the bottom of the liquid reservoir. The mesh does not inhibit
the flow of saline solution into the sample cup 530. A suitable
mesh can be obtained as part number 7308 from Eagle Supply and
Plastic, having a place of business in Appleton, Wis., U.S.A.
Saline does not fully cover the superabsorbent polymer composition
particles, as would be evidenced by a perfectly flat saline surface
in the test cell. Also, saline depth is not allowed to fall so low
that the surface within the cell is defined solely by swollen
superabsorbent, rather than saline.
[0124] At the end of this period, the plunger 536 and weight 548
assembly is placed on the saturated sample 568 in the sample
container 530 and then the sample container 530, plunger 536,
weight 548, and sample 568 are removed from the solution. After
removal and before being measured, the sample container 530,
plunger 536, weight 548, and sample 568 are to remain at rest for
about 30 seconds on a suitable flat, large grid non-deformable
plate of uniform thickness. The thickness of the saturated sample
568 is determined by again measuring the height from the top of the
weight 548 to the bottom of the sample container 530, using the
same thickness gauge used previously provided that the zero point
is unchanged from the initial height measurement. The sample
container 530, plunger 536, weight 548, and sample 568 may be
placed on a flat, large grid non-deformable plate of uniform
thickness that will prevent liquid in the sample container from
being released onto a flat surface due to surface tension. The
plate has an overall dimension of 7.6 cm by 7.6 cm, and each grid
has a cell size dimension of 1.59 cm long by 1.59 cm wide by 1.12
cm deep. A suitable flat, large grid non-deformable plate material
is a parabolic diffuser panel, catalogue number 1624K27, available
from McMaster Carr Supply Company, having a place of business in
Chicago, Ill., U.S.A., which can then be cut to the proper
dimensions. This flat, large mesh non-deformable plate must also be
present when measuring the height of the initial empty assembly.
The height measurement should be made as soon as practicable after
the thickness gauge is engaged. The height measurement obtained
from measuring the empty sample container 530, plunger 536, and
weight 548 is subtracted from the height measurement obtained after
saturating the sample 568. The resulting value is the thickness, or
height "H" of the swollen sample.
[0125] The permeability measurement is initiated by delivering a
flow of the 0.9% saline solution into the sample container 530 with
the saturated sample 568, plunger 536, and weight 548 inside. The
flow rate of test solution into the container is adjusted to cause
saline solution to overflow the top of the cylinder 534 thereby
resulting in a consistent head pressure equal to the height of the
sample container 530. The test solution may be added by any
suitable means that is sufficient to ensure a small, but consistent
amount of overflow from the top of the cylinder, such as with a
metering pump 604. The overflow liquid is diverted into a separate
collection device 601. The quantity of solution passing through the
sample 568 versus time is measured gravimetrically using the scale
602 and beaker 603. Data points from the scale 602 are collected
every second for at least sixty seconds once the overflow has
begun. Data collection may be taken manually or with data
collection software. The flow rate, Q, through the swollen sample
568 is determined in units of grams/second (g/s) by a linear
least-square fit of fluid passing through the sample 568 (in grams)
versus time (in seconds).
[0126] Permeability in cm.sup.2 is obtained by the following
equation: K=[Q*H*.mu.]/[A*.rho.*P], where K=Permeability
(cm.sup.2), Q=flow rate (g/sec), H=height of swollen sample (cm),
.mu.=liquid viscosity (poise) (approximately one centipoise for the
test solution used with this Test), A=cross-sectional area for
liquid flow (28.27 cm.sup.2 for the sample container used with this
Test), p=liquid density (g/cm.sup.3) (approximately one g/cm.sup.3,
for the test solution used with this Test) and P=hydrostatic
pressure (dynes/cm.sup.2) (normally approximately 7,797
dynes/cm.sup.2). The hydrostatic pressure is calculated from
P=.rho.*g*h, where .rho.=liquid density (g/cm.sup.3),
g=gravitational acceleration, nominally 981 cm/sec.sup.2, and
h=fluid height, e.g., 7.95 cm for the Gel Bed Permeability Test
described herein.
[0127] A minimum of two samples is tested and the results are
averaged to determine the gel bed permeability of the sample.
[0128] Absorbency Under Load (AUL0.9 psi) Test
[0129] The Absorbency Under Load (AUL) Test measures the ability of
the superabsorbent polymer composition particles to absorb a 0.9
weight percent solution of sodium chloride in distilled water at
room temperature (test solution) while the material is under a load
of 0.9 psi. The apparatus for testing AUL consists of: [0130] An
AUL assembly including a cylinder, a 4.4 g piston, and a standard
317 gm weight. The components of this assembly are described in
additional detail below. [0131] A flat-bottomed square plastic tray
that is sufficiently broad to allow the glass frits to lay on the
bottom without contact with the tray walls. A plastic tray that is
9'' by 9''(22.9 cm.times.22.9 cm), with a depth of 0.5 to 1''(1.3
cm to 2.5 cm) is commonly used for this test method. [0132] A 12.5
cm diameter sintered glass frit with a `C` porosity (25-50
microns). This frit is prepared in advance through equilibration in
saline (0.9% sodium chloride in distilled water, by weight). In
addition to being washed with at least two portions of fresh
saline, the frit must be immersed in saline for at least 12 hours
prior to AUL measurements. [0133] Whatman Grade 1, 12.5 cm diameter
filter paper circles. [0134] A supply of saline (0.9% sodium
chloride in distilled water, by weight).
[0135] Referring to FIG. 4, the cylinder 412 of the AUL assembly
400 used to contain the superabsorbent polymer composition
particles 410 is made from one-inch (2.54 cm) inside diameter
thermoplastic tubing machined-out slightly to be sure of
concentricity. After machining, a 400 mesh stainless steel wire
cloth 414 is attached to the bottom of the cylinder 412 by heating
the steel wire cloth 414 in a flame until red hot, after which the
cylinder 412 is held onto the steel wire cloth until cooled. A
soldering iron can be utilized to touch up the seal if unsuccessful
or if it breaks. Care must be taken to maintain a flat smooth
bottom and not distort the inside of the cylinder 412.
[0136] The 4.4 g piston (416) is made from one-inch diameter solid
material (e.g., PLEXIGLAS.RTM.) and is machined to closely fit
without binding in the cylinder 412.
[0137] A standard 317 gm weight 418 is used to provide a 62,053
dyne/cm.sup.2 (about 0.9 psi) restraining load. The weight is a
cylindrical, 1 inch (2.5 cm) diameter, stainless steel weight that
is machined to closely fit without binding in the cylinder.
[0138] Unless specified otherwise, a sample 410 corresponding to a
layer of at least about 300 gsm. (0.16 g) of superabsorbent polymer
composition particles is utilized for testing the AUL. The sample
410 is taken from superabsorbent polymer composition particles that
are pre-screened through U.S. standard #30 mesh and retained on
U.S. std. #50 mesh. The superabsorbent polymer composition
particles can be pre-screened with, for example, a RO-TAP.RTM.
Mechanical Sieve Shaker Model B available from W. S. Tyler, Inc.,
Mentor Ohio. Sieving is conducted for about 10 minutes.
[0139] The inside of the cylinder 412 is wiped with an antistatic
cloth prior to placing the superabsorbent polymer composition
particles 410 into the cylinder 412.
[0140] The desired amount of the sample of sieved superabsorbent
polymer composition particles 410 (about 0.16 g) is weighed out on
a weigh paper and evenly distributed on the wire cloth 414 at the
bottom of the cylinder 412. The weight of the superabsorbent
polymer composition particles in the bottom of the cylinder is
recorded as `SA,` for use in the AUL calculation described below.
Care is taken to be sure no superabsorbent polymer particles cling
to the wall of the cylinder. After carefully placing the 4.4 g
piston 412 and 317 g weight 418 on the superabsorbent polymer
composition particles 410 in the cylinder 412, the AUL assembly 400
including the cylinder, piston, weight, and superabsorbent polymer
composition particles is weighed, and the weight is recorded as
weight `A`.
[0141] A sintered glass frit 424 (described above) is placed in the
plastic tray 420, with saline 422 added to a level equal to that of
the upper surface of the glass frit 424. A single circle of filter
paper 426 is placed gently on the glass frit 424, and the AUL
assembly 400 with the superabsorbent polymer composition particles
410 is then placed on top of the filter paper 426. The AUL assembly
400 is then allowed to remain on top of the filter paper 426 for a
test period of one hour, with attention paid to keeping the saline
level in the tray constant. At the end of the one hour test period,
the AUL apparatus is then weighed, with this value recorded as
weight `B.`
[0142] The AUL(0.9 psi) is calculated as follows:
AUL(0.9 psi)=(B-A)/SA
[0143] wherein
[0144] A=Weight of AUL Unit with dry SAP
[0145] B=Weight of AUL Unit with SAP after 60 minutes
absorption
[0146] SA=Actual SAP weight
[0147] A minimum of two tests is performed and the results are
averaged to determine the AUL value under 0.9 psi load. The samples
are tested at about 23.degree. C. and about 50% relative
humidity.
[0148] Vortex Time Test
The vortex test measures the amount of time in seconds required for
2 grams of a superabsorbent material to close a vortex created by
stirring 50 milliliters of saline solution at 600 revolutions per
minute on a magnetic stir plate. The time it takes for the vortex
to close is an indication of the free swell absorbing rate of the
superabsorbent material.
Equipment and Materials
[0149] 1. Schott Duran 100 ml Beaker and 50 ml graduated cylinder.
2. Programmable magnetic stir plate, capable of providing 600
revolutions per minute (such as that commercially available from
PMC Industries, under the trade designation Dataplate.RTM. Model
#721). 3. Magnetic stir bar without rings, 7.9 millimeters.times.32
millimeters, Teflon.RTM. covered (such as that commercially
available from Baxter Diagnostics, under the trade designation
S/PRIM. brand single pack round stirring bars with removable pivot
ring).
4. Stopwatch
[0150] 5. Balance, accurate to +/-0.01 g 6. Saline solution, 0.87
w/w % Blood Bank Saline available from Baxter Diagnostics
(considered, for the purposes of this application to be the
equivalent of 0.9 wt. % saline 7. Weighing paper 8. Room with
standard condition atmosphere: Temp=23.degree. C.+/-1.degree. C.
and Relative Humidity=50%+/-2%.
Test Procedure
[0151] 1. Measure 50 ml+/-0.01 ml of saline solution into the 100
ml beaker. 2. Place the magnetic stir bar into the beaker. 3.
Program the magnetic stir plate to 600 revolutions per minute. 4.
Place the beaker on the center of the magnetic stir plate such that
the magnetic stir bar is activated. The bottom of the vortex should
be near the top of the stir bar. 5. Weigh out 2 g+/-0.01 g of the
superabsorbent material to be tested on weighing paper. NOTE: The
superabsorbent material is tested as received (i.e. as it would go
into an absorbent composite such as those described herein). No
screening to a specific particle size is done, though the particle
size is known to have an effect on this test. 6. While the saline
solution is being stirred, quickly pour the superabsorbent material
to be tested into the saline solution and start the stopwatch. The
superabsorbent material to be tested should be added to the saline
solution between the center of the vortex and the side of the
beaker. 7. Stop the stopwatch when the surface of the saline
solution becomes flat and record the time. 8. The time, recorded in
seconds, is reported as the Vortex Time.
Examples
[0152] The following examples and are provided to illustrate the
invention and do not limit the scope of the claims. Unless
otherwise stated all parts, and percentages are by weight.
Preproduct [A Typical Preparative Procedure]
[0153] Into a polyethylene vessel equipped with an agitator and
cooling coils was added, 25.0 kg of 50% NaOH to 37 kg of distilled
water and cooled to 20.degree. C. 9.6 kg of glacial acrylic acid
was then added to the caustic solution and the solution again
cooled to 20.degree. C. 47.8 g of polyethylene glycol
monoallylether acrylate, 47.8 g of ethoxylated trimethylol propane
triacrylate SARTOMER.RTM. 454 product, and 19.2 kg of glacial
acrylic acid were added to the first solution, followed by cooling
to 4-6.degree. C. Nitrogen was bubbled through the monomer solution
for about 10 minutes. The monomer solution was then discharged in
7.7 kg batches into rectangular trays. To each batch 80 g of 1% by
weight of H.sub.2O.sub.2 aqueous solution, 120 g of 2 wt % aqueous
sodium persulfate solution, and 72 g of 0.5 wt % aqueous sodium
erythorbate solution was added homogeneously into the monomer
solution stream by injection of the sodium erythorbate solution
into the stream of the monomer solution being conveyed from the
monomer tank into a tray. The initiated monomer was allowed to
polymerize for 20 minutes. The resulting hydrogel was chopped and
extruded with a Hobart 4M6 commercial extruder, followed by drying
in a Procter & Schwartz Model 062 forced air oven at
195.degree. C. for 12 minutes with up flow and 6 minutes with down
flow air on a 20 in.times.40 in perforated metal tray to a final
product moisture level of less than 5 wt %. The dried material was
coarse-ground in a Prodeva Model 315-S crusher, milled in an MPI
666-F three-stage roller mill and sieved with a Minox MTS 600DS3V
to remove particles greater than 850 .mu.m and smaller than 150
.mu.m.
Comparative Examples 1-6
[0154] For the Comparative Examples 1-6 set forth herein, the
Kaolin clay was added to the extruded gel by the aid of a nozzle
and kneaded well before drying.
TABLE-US-00001 TABLE 1 Base Polymer Dried at 185.degree. C. for 18
minutes Vortex Kaolin Clay % CRC Sec Comp Ex 1 0 32.2 40 Comp Ex 2
2 30.05 35 Comp Ex 3 3 29.8 36 Comp Ex 4 4 29.1 34 Comp Ex 5 5 28.4
33 Comp Ex 6 10 26.5 30
Comparative Examples 7-12
[0155] The Preproduct was coated in an Anvil MIX9180 mixer with 1%
ethylene carbonate, 4% water, and 350 ppm Chemcor 43G40SP
(available from Chemcor Corporation, Chester, N.Y.) maleated
polypropylene based on the dry superabsorbent polymer composition
weight. The coated superabsorbent polymer was heat treated to about
195.degree. C. for about 40 minutes residence time in order to
effectuate the surface crosslinking of the polymer particles.
TABLE-US-00002 TABLE 2 Examples from Table 1 after Surface
Crosslinking CRC Vortex AUL (0.9 psi) Kaolin Clay % g/g Sec g/g
Comp Ex 7 0 27.5 85 14.5 Comp Ex 8 2 27.1 36 15.1 Comp Ex 9 3 26.45
31 14.85 Comp Ex 10 4 25.13 32 16.6 Comp Ex 11 5 24.1 33 14.85 Comp
Ex 12 10 23.3 31 14.85
Comparative Examples 13 & 14
Examples 1-10
TABLE-US-00003 [0156] TABLE 3 Base Polymer Dried at 195.degree. C.
for 18 minutes Vortex Kaolin Clay % CRC Sec Comp Ex 13 0 31.5 35 1
2 31.4 28 2 3 31 31 3 4 30.9 32 4 5 30.3 34 5 10 28.8 28
TABLE-US-00004 TABLE 4 Examples from Table 3 after Surface
Crosslinking CRC Vortex AUL (0.9 psi) Kaolin Clay % g/g Sec g/g
Comp Ex 14 0 27.9 94 14.2 6 2 27.7 39 14.8 7 3 27.4 40 14.5 8 4
26.6 40 17.3 9 5 27.2 33 13 10 10 26.2 33 15.6
Comparative Example 15 and Example 11
[0157] In Example 11, clay was added to the superabsorbent polymer
hydrogel of the commercial product SXM9200 (SXM9200 is commercially
available from Evonik Stockhausen, Greensboro N.C.) and dried at
195.degree. C. for 18 minutes. Table 5 shows a comparison of
SXM9200 without and with Kaolin clay. The properties of polymer
stayed intact after .about.4 wt % clay was added to the
polymer.
Also, as shown in Table 5, the results of the Vortex test improves
with the addition of clay to the SXM9200 hydrogel (i.e., the
polymer becomes faster under the Vortex test) and the permeability
of the SAP--clay polymer, as measured by Free Swell Gel Bed
Permeability, increases.
TABLE-US-00005 TABLE 5 SXM9200 with clay in hydrogel CRC AUL (0.9
psi) Free Swell Vortex Bulk Density g/g g/g GBP (sec) Comp Ex 15
0.58 30.43 12.62 5 89 SXM9200 Ex 11 0.60 29.96 14.49 29.6 32
Comparative Example 16 and Example 12
Diapers
[0158] Diapers were constructed using 45% SAP structure wherein the
SAP was one of 1) SXM9200, 2) SXM9200 including Kaolin clay in the
polymer. SXM9200 is commercially available from Evonik Stockhausen,
Inc., Greensboro, N.C. Construction for all diaper cores were a 600
GSM core and density of 0.13 with a homogenous blend of SAP and
pulp with a light pulp dusting layer. Target weight specifications
were achieved and maintained within normal production variability.
Each core was assembled using a poly back sheet, 18GSM hydrophilic
top sheet and 30GSM SBPP ADL.
[0159] A weighted (3.6 Kg) 4''.times.4'' block is placed on the
flat diaper core 2.5 cm forward of the diaper core centerline;
resulting in a test pressure of 0.5 PSI. The article was insulted
with 80 ml of saline and allowed to penetrate into the diaper core.
Record acquisition time. The weighted block was allowed to remain
on the diaper core for a period of 5 minutes. After 5 minutes
period, the block is removed. Rewet is measured by placing a
pre-weighed stack of 9.0 cm filter paper over the insult point and
applying a weight of 2.2 kg. After 2 minutes remove the weight,
weigh filter paper and record the rewet values. The test was
repeated for a total of 3 acquisitions. If the diaper core at any
time fails to acquire the liquid volume within the 5 minutes
allowed, the test is stopped and recorded as such.
TABLE-US-00006 TABLE 6 Diaper Testing Results Acquisition Time
(Sec) Rewet (grams) Insult Insult Insult Insult Insult Insult # 1 #
2 # 3 # 1 # 2 # 3 Comp Ex 16 98 208 248 0.1 5.4 12.6 (Without Clay)
Example 12 81 153 186 0.1 5 10.4 (With Clay)
[0160] As shown in Table 6, the addition of the clay to the
hydrogel according to the conditions of the invention provides
adequate time of fluid in the diaper.
[0161] Other than in the operating examples, or where otherwise
indicated, all numbers expressing quantities of ingredients,
reaction conditions, and so forth used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the following specification
and attached claims are approximations that may vary depending upon
the desired properties sought to be obtained by the present
invention. At the very least, and not as an attempt to limit the
application of the doctrine of equivalents to the scope of the
claims, each numerical parameter should be construed in light of
the number of significant digits and ordinary rounding
approaches.
[0162] Notwithstanding that the numerical ranges and parameters
setting forth the broad scope of the invention are approximations,
the numerical values set forth in the specific examples are
reported as precisely as possible. Any numerical value, however,
inherently contain certain errors necessarily resulting from the
standard deviation found in their respective testing
measurements.
* * * * *