U.S. patent application number 10/778673 was filed with the patent office on 2005-08-18 for sodium sulfate treated pulp.
Invention is credited to Hajnal, Andre S., West, Hugh.
Application Number | 20050178518 10/778673 |
Document ID | / |
Family ID | 34838223 |
Filed Date | 2005-08-18 |
United States Patent
Application |
20050178518 |
Kind Code |
A1 |
West, Hugh ; et al. |
August 18, 2005 |
Sodium sulfate treated pulp
Abstract
Cellulose pulp sheets treated with sodium sulfate can be
fiberized to produce sodium sulfate treated fibers that exhibit
desirable densification properties. The sodium sulfate treated
fibers densify to a greater degree than fibers that have not been
treated with sodium sulfate.
Inventors: |
West, Hugh; (Seattle,
WA) ; Hajnal, Andre S.; (Anderson Island,
WA) |
Correspondence
Address: |
WEYERHAEUSER COMPANY
INTELLECTUAL PROPERTY DEPT., CH 1J27
P.O. BOX 9777
FEDERAL WAY
WA
98063
US
|
Family ID: |
34838223 |
Appl. No.: |
10/778673 |
Filed: |
February 13, 2004 |
Current U.S.
Class: |
162/181.2 ;
162/135; 162/158; 162/205 |
Current CPC
Class: |
D21C 9/004 20130101;
D21H 11/20 20130101 |
Class at
Publication: |
162/181.2 ;
162/158; 162/135; 162/205 |
International
Class: |
D21H 017/66 |
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A cellulose pulp sheet comprising: cellulose fibers; water; and
sodium sulfate applied to the pulp sheet in an amount ranging from
about 0.1 to 15 weight percent based on dry fiber weight.
2. The cellulose pulp sheet of claim 1, wherein the sodium sulfate
is applied to the pulp sheet in an amount ranging from about 1 to
10 weight percent based on dry fiber weight.
3. The cellulose pulp sheet of claim 1, wherein the cellulose
fibers are wood pulp fibers.
4. The cellulose pulp sheet of claim 2, wherein the sodium sulfate
is applied to the pulp sheet as an aqueous solution containing
about 5 to 33 weight percent sodium sulfate solids.
5. The cellulose pulp sheet of claim 4, wherein the sodium sulfate
is applied to the pulp sheet as an aqueous solution containing
about 5 to 30 weight percent sodium sulfate solids.
6. The cellulose pulp sheet of claim 5, wherein the sodium sulfate
is applied to the pulp sheet as an aqueous solution containing
about 5 to 27 weight percent sodium sulfate solids.
7. The cellulose pulp sheet of claim 1, wherein the water content
is less than about 20 weight percent based on total product
weight.
8. The cellulose pulp sheet of claim 7, wherein the water content
is less than about 15 weight percent based on total product
weight.
9. The cellulose pulp sheet of claim 1, further comprising an
oil.
10. Treated fibers comprising: cellulose fibers, the cellulose
fibers after application and release of a compression load being
densified to a first density; water; and sodium sulfate, the
cellulose fibers after treatment with the sodium sulfate densifying
to a second density after application and release of the
compression load, the sodium sulfate being present in an amount
effective to result in the second density being greater than the
first density.
11. The treated fibers of claim 10, wherein the second density is
at least 5 percent greater than the first density.
12. The treated fibers of claim 11, wherein the sodium sulfate is
present in an amount greater than about 1.0 wt. % based on dry
fiber weight.
13. The treated fibers of claim 10, further comprising an oil.
14. A method for producing a cellulose pulp sheet comprising
providing cellulose pulp; forming a cellulose pulp sheet from the
cellulose pulp; and applying sodium sulfate to the cellulose pulp
sheet.
15. The method of claim 14, wherein the cellulose pulp is wood
pulp.
16. The method of claim 14, wherein the sodium sulfate is applied
to the cellulose pulp sheet in an amount that results in a loading
of about 0.1 to 15 weight percent based on dry pulp.
17. The method of claim 14, wherein the sodium sulfate is applied
to the cellulose pulp sheet in an amount that results in a loading
of about 1.0 to 10 weight percent based on dry pulp.
18. The method of claim 14, wherein the sodium sulfate is applied
as an aqueous solution containing from about 5 to 33 weight percent
sodium sulfate solids.
19. The method of claim 14, wherein the cellulose pulp sheet
further comprises water and the water content of the cellulose pulp
sheet after application of the sodium sulfate is less than about 20
weight percent based on total product weight.
20. The method of claim 14, wherein the temperature of the
cellulose pulp sheet when the sodium sulfate is applied is at least
25 degrees Celsius.
21. A method for producing a densified web of cellulose fibers
comprising: providing cellulose fibers treated with sodium sulfate;
fiberizing the cellulose fibers treated with sodium sulfate;
forming the fiberized cellulose fibers treated with sodium sulfate
into a web; and compressing the web.
22. The method claim 21, wherein the cellulose fibers are wood pulp
fibers.
23. The method of claim 21, wherein the sodium sulfate is present
on the cellulose fibers in an amount ranging from about 0.1 to 15
weight percent based on dry fibers.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to cellulose pulp that has
been treated with sodium sulfate and to methods for applying sodium
sulfate to cellulose pulp.
BACKGROUND OF THE INVENTION
[0002] Cellulose fibers have found widespread application in
absorbent articles, such as diapers and feminine hygiene products.
The cellulose fibers are generally used as an absorbent medium to
acquire, transport, and hold fluids. While cellulose fibers are
effective at acquiring, transporting, and holding fluids, many
improvements to cellulose fibers have been made over the past
decades to improve the performance properties of cellulose fibers
in absorbent products. For example, U.S. Pat. Nos. 6,340,411 and
5,547,541 describe that webs of cellulose fibers treated with
certain polymeric and nonpolymeric materials require less pressure
to densify a web of the fibers to a given density as compared to
the pressure needed to densify a similar web of fibers without the
polymeric or nonpolymeric material present.
[0003] The cellulose fibers treated with the compositions described
in U.S. Pat. No. 5,547,541 are manufactured by applying the desired
compositions to a wet laid web of cellulose fibers which has been
produced, for example, using a Fourdrinier machine. The treated wet
laid web of cellulose fibers is generally formed into a roll for
bulk delivery to an absorbent product manufacturer. The absorbent
product manufacturer typically unrolls the roll and processes the
web in a fiberization unit that individualizes the fibers and
prepares them for further processing.
[0004] Absorbent products including an absorbent core of
superabsorbent material and cellulose fibers are typically
manufactured by a process that combines cellulose fibers and
superabsorbent material. In such a process, rolls or bales of
cellulose fibers without superabsorbent material are fiberized by a
fiberizing apparatus such as a hammermill. These fiberized
cellulose fibers are entrained in air and superabsorbent material
is introduced to the air entrained fibers. The air entrained
combination of cellulose fibers and superabsorbent material is
delivered to an air lay device such as a pad former, which draws
the fibers and superabsorbent material onto a screen and forms the
fibers and superabsorbent material into a particular shape. These
formed pads are then removed from the pad former for further
processing, including subjecting the formed pads to compression in
order to densify the pad by decreasing its thickness.
[0005] Reducing the thickness of the formed pads which are used in
diapers is important to diaper manufacturers so that they can
reduce the size of packaging which allows them to ship more diapers
per volume and to display a larger number of diapers in a limited
amount of shelf space. In addition, consumers find thinner diapers
more desirable.
[0006] With this background, the present inventors have worked to
address the challenges above and have developed compositions that
can be compressed to achieve articles of desirable densities and
methods of providing and utilizing such compositions.
SUMMARY OF THE INVENTION
[0007] The present invention provides cellulose pulp sheets and
cellulose fibers treated with sodium sulfate that are useful in
absorbent cores formed from the treated pulp or fibers. The
compositions of the present invention can be formed into absorbent
articles for absorbing fluids such as aqueous fluids like urine or
blood. The compositions are useful in absorbent articles such as
diapers, incontinent devices and feminine hygiene products. The
compositions of the present invention can be compressed to
densities that manufacturers of absorbent articles should find
desirable.
[0008] In one aspect, the present invention relates to a cellulose
pulp sheet that includes cellulose fibers, water and sodium sulfate
applied to the cellulose fibers in an amount ranging from about 0.1
to 15 weight percent based on dry fiber weight. The cellulose pulp
sheet can be fiberized into individualized fibers, laid into a pad,
and then compressed. The invention also relates to a method for
producing a cellulose pulp sheet which includes the steps of
providing cellulose pulp, forming a cellulose pulp sheet from the
cellulose pulp, and applying sodium sulfate to the cellulose pulp
sheet.
[0009] In another aspect, the present invention relates to a method
for producing a densified web of cellulose fibers that includes the
step of providing cellulose fibers treated with sodium sulfate. The
treated cellulose fibers are fiberized and formed into a web. The
web is compressed to form a densified web.
[0010] In yet another aspect, the present invention relates to
fibers that have been treated with sodium sulfate. The cellulose
fiber before treatment with sodium sulfate exhibit a first density
after application and release of a compression load. The treated
fibers include water and sodium sulfate. The cellulose fibers after
treatment with sodium sulfate densify to a second density after
application and release of the compression load. The sodium sulfate
is present in an amount so that the second density is at least 5
percent greater than the first density.
[0011] The sulfate treated fibers of the present invention may be
further treated with an oil in order to provide fibers that retain
materials such as superabsorbent materials within a web of the
fibers.
[0012] Manufacturers of absorbent articles will find the sodium
sulfate treated pulp and fibers of the present invention useful in
their absorbent products due to the densification properties of
cellulose pulp fibers treated in accordance with the present
invention. The methods of the present invention provide suitable
means for producing the treated cellulose pulp fibers that exhibit
densification properties that absorbent article manufacturers
should find desirable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0014] FIG. 1 is a graph illustrating the results of compression
testing to determine the densification properties of absorbent
structures containing cellulose fibers treated in accordance with
the present invention;
[0015] FIG. 2 is a graph illustrating the results of compression
testing to determine the densification properties of structures
containing cellulose fibers treated in accordance with the present
invention; and
[0016] FIG. 3 is a schematic illustration of a wet laid web
manufacturing line illustrating the application of sodium sulfate
to a wet laid web of cellulose fibers in accordance with the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] As used herein, the term "fiber" refers to natural or
synthetic fibers. Such fibers may be physically pretreated, e.g.,
by subjecting the fibers to steam, or chemically treated, e.g., by
crosslinking the fibers. The fibers may also be twisted or crimped
as desired.
[0018] A particular type of fiber are cellulose fibers. A
particular example of a cellulose fiber is wood pulp fiber. Wood
pulp fibers can be hardwood pulp fibers or softwood pulp fibers.
The cellulose pulp fibers may be chemical, thermomechanical,
chemithermomechanical or combinations thereof. Such wood pulp
fibers can be obtained from well known chemical processes such as
the kraft or sulfite processes. Other cellulose fibers include
lyocell, linen, chopped silk fibers, bagasse, hemp, jute, rice,
wheat, bamboo, corn, sisal, cotton, flax, kenaf, peat moss, and
mixtures thereof. When the fibers are cellulose fibers, they may be
pretreated with chemicals to result in lignin or cellulose-rich
fiber surfaces. In addition, the fibers may be bleached.
[0019] Examples of synthetic fibers include acrylic, polyester,
carboxylated polyolefin, and polyamine fibers.
[0020] Sodium sulfate (Na.sub.2SO.sub.4) is available in the form
of white crystals or powder from numerous commercial sources.
[0021] As used herein, the term "superabsorbent material" refers to
polymers that swell on exposure to water and form a hydrated gel
(hydrogel) by absorbing large amounts of water. Superabsorbent
materials exhibit the ability to absorb large quantities of liquid,
i.e., in excess of 10 to 15 parts of liquid per part thereof. These
superabsorbent materials generally fall into three classes, namely
starch graft copolymers, crosslinked carboxymethylcellulose
derivatives and modified hydrophilic polyacrylates. Examples of
such absorbent polymers are hydrolyzed starch-acrylonitrile graft
copolymer, a neutralized starch-acrylic acid graft copolymer, a
saponified acrylic acid ester-vinyl acetate copolymer, a hydrolyzed
acrylonitrile copolymer or acrylamide copolymer, a modified
crosslinked polyvinyl alcohol, a neutralized self-crosslinking
polyacrylic acid, a crosslinked polyacrylate salt, carboxylated
cellulose, and a neutralized crosslinked isobutylene-maleic
anhydride copolymer.
[0022] Superabsorbent particles are available commercially, for
example starch graft polyacrylate hydrogel fines (IM 1000F) from
Hoechst-Celanese of Portsmouth, Va., or larger particles such as
granules. Other superabsorbent particles are marketed under the
trademarks SANWET (supplied by Sanyo Kasei Kogyo Kabushiki Kaisha),
SUMIKA GEL (supplied by Sumitomo Kagaku Kabushiki Kaisha and which
is emulsion polymerized and spherical as opposed to solution
polymerized ground particles), FAVOR (supplied by Stockhausen of
Greensboro, N.C.), and NORSOCRYL (supplied by Atochem).
[0023] The term oil as used generally applies to a wide range of
substances. Oils may be derived from animals or from plant seeds or
nuts, and these types of oils tend to be chemically identical with
fats, with the only difference being one of consistency at room
temperature. Animal and plant oils are composed largely of
triglycerides of the fatty acids, oleic, palmitic, stearic, and
linolenic acid. Oils may also be derived from petroleum sources.
Petroleum-based oils generally include a mixture of hydrocarbons.
As used herein, the term "oil" refers to oils that have melting
points below the temperature at which the oil is applied to the
fibers as described below in more detail. Such temperature will
generally be below 25.degree. C., but could be higher. If the
melting point of the oil is greater than the ambient temperature at
which the oil is applied to the fibers, the oil can be heated to
liquefy it. This ensures that the oils remain liquid during their
application to the fibers. Oils useful in the present invention
should also have a vapor pressure sufficiently low to prevent
evaporation either during their application or during use.
[0024] The oil should not penetrate the walls of the fibers so
rapidly that it becomes unavailable to retain the superabsorbent
material when superabsorbent material is contacted with the oil
treated fibers. The oil preferably resides on the surface of the
fibers during the useful life of the absorbent article made from
the fibers. To that end, oils of higher molecular weight penetrate
the fiber wall more slowly than oils of a lower molecular
weight.
[0025] Examples of "oils" as that term is used herein include fats
and their component fatty acids. As described above, fats are
naturally occurring esters of long chain carboxylic acids and the
triol glycerol. These esters are also referred to as triglycerides.
The hydrolysis of fats yields glycerol and three component
carboxylic acids. These straight chain carboxylic acids which may
be obtained from the hydrolysis of fats are called fatty acids and
include one carboxylic acid group. Fatty acids may be saturated or
unsaturated. The most common saturated fatty acids are lauric acid,
myristic acid, palmitic acid, and stearic acid. Other fatty acids
include oleic acid, linoleic acid, and linolenic acid. Generally,
the melting point of a fat depends on the amount of unsaturation in
the fatty acids. Fats with a preponderance of unsaturated fatty
acids generally have melting points below about 25.degree. C.
Specific examples of oils as that term is used herein include
soybean oil, cottonseed oil, linseed oil, tung oil, castor oil,
coconut oil, olive oil, canola oil, safflower oil, corn oil or
jojoba oil. Jojoba oil is a light yellow liquid at room temperature
that is not technically an oil or fat, but rather is a wax. A wax
is an ester of fatty acids with long chain monohydric alcohols. The
term oil as used herein is intended to include jojoba oil and other
waxes that are liquid at temperatures that they are applied to
fibers. It should be understood that the foregoing is a list of
exemplary oils and that oils useful with the sodium sulfate treated
fibers of the present invention are not necessarily limited to the
foregoing oils. It should be understood that use of the term "oil"
in this application refers not only to the oil itself comprising a
mixture of various fat and fatty acid components, but also includes
the individual isolated fats, and the isolated fatty acids that
result when the fats are hydrolyzed. For example, the term "oil" as
used herein also refers to the fatty acids oleic, palmitic,
stearic, and linolenic, that form the most common triglycerides in
many oils derived from animals and plants and would be useful to
retain superabsorbent material in an absorbent structure comprising
oil-treated fibers and superabsorbent material.
[0026] The term "oil" as used herein also refers to unsubstituted
alkanes, alkenes, alkynes, cycloalkanes, cycloalkenes,
cycloalkynes, aromatics, and mixtures thereof derived from
petroleum or animal sources that have melting points below the
temperature at which the oil is applied to the fibers, e.g., about
25.degree. C. Such oils are generally derived from petroleum
sources, but may also be derived from animal sources. Oils of this
type should have vapor pressure sufficiently low to prevent
evaporation of the oil during application or use. Specific examples
of these types of oils include mineral oil, paraffin oil,
hexadecane, squalane, and squalene.
[0027] As used herein, mineral oil is an example of a highly
refined liquid petroleum derivative. Mineral oil is light, clear,
colorless, and odorless and is also referred to as medicinal oil.
Mineral oil is used medicinally as an internal lubricant and for
the manufacture of salves and ointments.
[0028] Paraffin oil is an example of an oil that is either pressed
or dry distilled from paraffin distillate obtained from the
distillation of petroleum.
[0029] Squalane is an example of an alkane derived from animal
sources, such as the sebum. Squalene is an example of an alkene;
more specifically, a terpene derived from animal sources, such as
the human sebum or shark liver oil. Squalene may also be isolated
from oils derived from plants, such as olive oil, wheat germ oil,
rice bran oil, and yeast.
[0030] In accordance with the present invention, the amount of
sodium sulfate added to the cellulose pulp sheet can vary over a
wide range. Amounts of sodium sulfate solids in the treated
cellulose sheets can range from about 0.1 wt. % to 15 wt. % based
on the dry fiber. weight. A narrower range of amounts is about 1.0
to 10.0 wt. % based on dry fiber weight, and an even narrower range
is about 1 to 7 wt %. These amounts of sodium sulfate can be
provided in the treated cellulose sheets by applying an aqueous
solution of sodium sulfate. The amount of sodium sulfate in the
aqueous solution can vary over a wide range. Preferably, the amount
of sodium sulfate in the aqueous solution that is applied to the
cellulose pulp sheet is chosen such that a desired level of loading
of sodium sulfate solids onto the cellulose pulp sheet is achieved
without the water content of the cellulose fiber sheet rising above
about 15 to 20 wt. % based on the total product weight. If the
water content of the pulp sheet is excessive, the pulp sheet is
difficult to fiberize and may be susceptible to premature
degradation, such as from mold growth or rotting. In certain
embodiments, when the aqueous solution of sodium sulfate is applied
to the pulp sheet before the pulp sheet is dried, larger amounts of
water can be introduced into the pulp sheet provided that the
subsequent drying steps reduce the water content of the pulp sheet
to a level below about 20 wt. % based on the total product weight.
Sufficient amounts of sodium sulfate solids should be added to the
cellulose pulp sheet so that when the cellulose pulp sheet is
fiberized the resulting fibers exhibit densification properties
that are superior to the densification properties of fibers that
have not been treated with sodium sulfate.
[0031] Aqueous solutions containing from 5 to 33 wt. % sodium
sulfate are useful in accordance with the present invention.
Aqueous solutions containing from about 5 to 30 wt. % or about 5 to
27 wt. % sodium sulfate are preferred because it has been observed
that the percent increase in the density of the fibers when the
aqueous solution contains about 25 wt. % sodium sulfate is greater
than when the aqueous solution contains about 33 wt. % sodium
sulfate.
[0032] The sodium sulfate solution can be applied to the cellulose
pulp sheet in a number of different ways. The present invention is
not limited to any particular application technique. Examples of
suitable application techniques include spraying, rolling, dipping,
and the like. The solution can be applied to one or both sides of
the cellulose pulp sheet. Alternatively, the solution can be
applied to fibers that are not in sheet form, e.g., individualized
fibers. The solution can be heated prior to its application,
although this is not required. Alternatively, the cellulose pulp
sheet can be at a temperature above room temperature when the
sodium sulfate solution is applied. In view of the decreasing
solubility of sodium sulfate in water as the temperature of the
solution decreases, in certain embodiments, particularly those
where the concentration of the sodium sulfate in the solution is
near its solubility limit, it is advantageous to preheat the
solution or the pulp sheet in order to reduce crystallization of
sodium sulfate from the solution during or right after its
application. Heating the aqueous solution of sodium sulfate or
heating the cellulose pulp sheet prior to application of the sodium
sulfate solution is one means for introducing more sodium sulfate
into the web.
[0033] As described above, when the sodium sulfate treated fibers
are subjected to and released from a compression load, they densify
to a density that is higher than the density that is achieved when
fibers that have not been treated with sodium sulfate are subjected
to the same compression loading and releasing. In some instances,
the density is increased 10% or more.
[0034] As described above, oil can be applied to the sodium sulfate
treated fiber. The particular way that oil is applied to the fibers
is not critical. Examples of techniques for applying oil to the
fibers include the use of a gravure-type roll coater to coat a web
of the fibers. Alternatively, oil can be sprayed onto a web of the
fibers or the fibers can be immersed in a bath of oil. The oil may
also be added to the fibers as a web of the fibers is being broken
up, such as in a hammermill. The amount of oil applied to the
fibers should be sufficient to achieve retention of superabsorbent
material, but not so much as to have a significant adverse affect
on the fluid absorption properties of the fibers, such as the fluid
acquisition rate or the amount of fluid absorbed by a web of the
fibers. Manufacturers of absorbent articles that include absorbent
structures containing oil-treated fibers desire that the fluid
absorption properties of such structures be similar to or superior
to the fluid absorption properties of the absorbent structures that
the manufacturer is considering replacing. Ideally, the absorbent
structures would exhibit fluid acquisition properties that are at
least as desirable as the fluid acquisition properties of similar
absorbent structures manufactured from fibers that have not been
treated with oil. The amount of oil applied to the fibers should
also not be so great that it adversely impacts the fiberization of
the web of oil-treated fibers. Suitable amounts of oil applied to
the fibers include about 0.5 wt. % to about 20 wt. % oil based on
the weight of oven dried fibers. A narrower range is 1.0 wt. % to
about 15 wt. % oil based on the weight of oven dried fibers and an
even narrower range is 1.0 wt. % to about 10 wt. % oil based on the
weight of oven dried fibers.
[0035] The form of the fibers to which the oil is applied can vary.
If a roll coater is used, the fibers can be in the form of a sheet
of fibers. For example, the oil can be applied to a wet laid sheet
of fibers having a basis weight of at least 350 grams per
meter.sup.2 and a density of at least about 400 kg/meter.sup.3.
[0036] The oil may be added neat, or it may be diluted with solvent
that evaporates after application of the oil to the fibers. The
solvent should not adversely affect the attachment of
superabsorbent material to the fibers or the fluid acquisition and
fluid retention properties of an absorbent article that contains
the oil treated fibers.
[0037] FIG. 3 illustrates a wet laid sheet manufacturing line such
as a wood cellulose pulp sheet manufacturing line 10. In this
manufacturing line, a pulp slurry 12 is delivered from a headbox 14
through a slice 16 and onto a Fourdrinier wire 18. The pulp slurry
12 typically includes wood pulp fibers and may also include
synthetic or other non-cellulose fibers as part of the slurry.
Water is drawn from the pulp deposited on wire 18 by a conventional
vacuum system, not shown, leaving a deposited pulp sheet 20 which
is carried through a dewatering station 22, illustrated in this
case as two sets of calendar rolls 24, 26 each defining a
respective nip through which the pulp sheet or mat 20 passes. From
the dewatering station, the pulp sheet 20 enters a drying section
30. In a conventional pulp sheet manufacturing line, drying section
30 may include multiple canister dryers with the pulp mat 20
following a serpentine path around the respective canister dryers
and emerging as a dried sheet or mat 32 from the outlet of the
drying section 30. Other alternate drying mechanisms, alone or in
addition to canister dryers, may be included in the drying stage
30. The dried pulp sheet 32 has a maximum moisture content pursuant
to the manufacturer's specifications. Typically, the maximum
moisture content is no more than 10% by weight of the fibers and
most preferably no more than about 6% to 8% by weight. Unless
overly damp fibers are immediately used these fibers are subject to
degradation by, for example, mold or the like. The dried sheet 32
is taken up on a roll 40 for transportation to a remote location,
that is, one separate from the pulp sheet manufacturing line, such
as at a user's plant for use in manufacturing products. The dried
pulp sheets have a basis weight of about 200 g/m.sup.2 to about
1000 g/m.sup.2 or more and a density on the order of at least about
0.5 g/cm.sup.3 to about 1.2 g/cm.sup.3. Dried pulp sheets having
the foregoing basis weights are structurally distinct form lighter
basis weight sheets of wet laid or airlaid wood pulp fibers such as
tissue paper, paper towels, or other types of paper-like wet laid
or airlaid webs of cellulose fibers. Alternatively, the dried sheet
32 is collected in a baling apparatus 42 from which bales of the
pulp 44 are obtained for transport to a remote location.
[0038] The sodium sulfate solution can be applied to the pulp sheet
from one or more applying devices, one of which is indicated at 50
in FIG. 3. Any applying device may be used, such as streamers,
sprayers, roll coaters, curtain coaters, immersion applicators, or
the like. Sprayers are typically easier to utilize and incorporate
into a pulp-sheet manufacturing line. As indicated by the arrows
52, 54, and 56, the sodium sulfate may be applied at various
locations or at multiple locations on the pulp sheet manufacturing
line, such as ahead of the drying stage 30 (indicated by line 52),
intermediate the drying stage 30 (as indicated by line 54), or
downstream from the drying stage 30 (as indicated by the line 56).
At location 52, the water remaining in the sheet or mat 20 at this
stage tends to interfere with the penetration of the materials into
the sheet. Consequently, application of the sodium sulfate solution
after some drying has taken place, for example at location 54, is
preferable. If the sodium sulfate solution is applied at location
56 in an amount which would cause the moisture content of the sheet
to exceed the desired maximum level, an additional drying stage
(not shown) may be included in the pulp manufacturing line to bring
the moisture content down to the desired level.
[0039] The oil can be applied to the pulp sheet from the same types
of devices and locations as described above with respect to the
sodium sulfate solution.
[0040] The rolls 40 or bales 44 of the treated wet laid web of
fibers may be transported to a remote location for use by a user.
These rolls or bales are then refiberized by a fiberizing device,
such as a hammermill which may be used alone or in conjunction with
other devices such as picker rolls or the like for breaking up the
sheet 32 or bales 42 into individual fibers. Depending on the end
use, the individualized fibers may be combined with particulate
material, such as superabsorbent particles, and/or airlaid into a
web and densified.
[0041] With this approach, the end user of the treated fibers may
readily select particles to be combined with the fibers. The user
has flexibility in air laying or otherwise processing the treated
fibers of the present invention into a finished product.
[0042] The treated fibers and superabsorbent material can be
combined and then formed into an absorbent structure in the
following manner. Rolls or bales of treated fibers, without
particles, are fiberized by a fiberizing device such as a
hammermill. The individualized fibers are air entrained during
which time the superabsorbent material can be added thereto. The
air entrained fibers and superabsorbent material are then delivered
to an air laying device, such as a pocket former, and formed into a
desired shape. The formed pad is removed from the air laying device
for further processing, including subjecting the pad to a
compression load to reduce the thickness of the pad and increase
its density. The formed pads are in the form of a web or mass of
fibers used as absorbent structures in absorbent articles such as
the ones discussed above.
[0043] It should be understood that in an alternative embodiment,
the sodium sulfate solution and oil can be applied to the fibers
while they are air entrained.
[0044] As illustrated in the examples that follow, fibers treated
with a sodium sulfate solution in accordance with the present
invention exhibit desirable densification properties.
[0045] The following examples are intended to illustrate certain
embodiments of the present invention and are not intended to limit
the scope of the present invention.
EXAMPLE 1
Preparation of Sodium Sulfate Treated Cellulose Pulp
[0046] Southern Pine wood cellulose pulp sheet available from
Weyerhaeuser Company under the designation NB 416 from New Bern,
N.C. with a starting moisture content of 6% by weight (based on
total sheet weight) was brought to a temperature of 120-140.degree.
F. by storing in a zippered plastic bag in an oven. The pulp sheet
was then quickly removed from the bag and coated in a Black
Brothers gravure-type roll coater with a solution of sodium
sulfate. The gravure coater results in the application of a uniform
coating of the sodium sulfate solution over one entire surface of
the pulp sheet from where it is rapidly soaked up by the sheet. The
sodium sulfate was obtained from Sigma-Aldrich (CAS number
7757-82-6 anhydrous sodium sulfate, 99% reagent grade). The sodium
sulfate solution had a solids content of 24.8% with the balance
being water. The sodium sulfate is dissolved in water at 33.degree.
C. This 24.8 wt. % solution was applied to the wood pulp sheet at a
rate of 10.5 parts by weight solution to 100 parts by weight of
pulp sheet, resulting in a loading of active (dry basis) sodium
sulfate solids of .about.2.8 wt. % based on the dry fiber content
of the pulp sheet. The final total moisture content of the wood
pulp cellulose sheet treated with sodium sulfate is .about.12.6 wt.
% based on the total final product weight.
[0047] The treated sheet was stored in a plastic zippered bag for
24 hours at room temperature to allow the added moisture to migrate
and reach equilibrium within the whole sheet. The sheet was then
fiberized in a laboratory hammermill and the resultant fluff was
fed to a rotary pocket former and resulted in fluff pads measuring
.about.12".times.5" with a basis weight of .about.300 grams per
square meter (gsm). The pads were placed in zippered plastic bags
to preserve moisture until used in the densification test
below.
[0048] After removal from the bags, the rectangular 12".times.5"
pads were cut into smaller square pads measuring 10 cm.times.10 cm
using a die. The 10.times.10 cm pads were densified in a hydraulic
flat press under loads of either 50 psi, 100 psi, and 150 psi. The
pressure was only held momentarily and then released. Different
pads were used for each of the successively higher loads. Caliper
(thickness) of the pads was determined using a caliper gauge with a
wide "foot" designed to apply only moderate pressure to the pad of
.about.0.2 psi (i.e., it does not materially densify the pad in the
act of determining caliper). The densities of the pads were
calculated from the caliper and basis weight measurements.
[0049] Results of the density measurements versus applied pressure
are shown in FIG. 1 and show that the sodium sulfate treated pulp
(2.8% loading from 24.8 wt. % solution) attains a higher density
(up to about 14%) for a given pressure compared to the untreated
NB416 pulp having a moisture content of 6 wt. % which was
incorporated as a control.
EXAMPLE 2
Preparation of Sodium Sulfate Treated Cellulose Pulp
[0050] Example 2 was identical to Example 1 in every respect except
that the 24.8 wt. % solution of sodium sulfate was applied to the
wood pulp sheet at a rate of 21.6 parts by weight solution to 100
parts by weight of pulp sheet, resulting in a loading of active on
a (dry basis) sodium sulfate solids of .about.5.7 wt. % based on
the dry fiber content of the pulp sheet. The final total moisture
content of the wood pulp cellulose sheet treated with sodium
sulfate is .about.18.3 wt. % based on the total final product
weight.
[0051] Again, results of the density measurements versus applied
pressure are shown in FIG. 1 and show that the sodium sulfate
treated pulp (5.7% loading from 24.8 wt. % solution) attains a
higher density (up to about 25.7%) for a given pressure compared to
the untreated NB416 pulp having a moisture content of 6 wt. % which
was incorporated as a control. The data also shows that the higher
loading of sodium sulfate at higher final moisture content is
advantageous.
EXAMPLE 3
Preparation of Sodium Sulfate Treated Cellulose Pulp
[0052] Several air dry (.about.6% moisture content) 4" width strips
of NB416 pulp sheet weighing about 40 g were brought to a
temperature of 120-140.degree. F. by placing in zippered plastic
bags in an oven. The strips were quickly removed from the oven and,
whilst hot, were treated with 5.7 g of a 33 wt. % solution of
sodium sulfate solution that had been preheated to 33.degree. C.
Application of the solution was made to one side of the wood pulp
sheet using a syringe. Liquid was applied in lines along the full
length of the machine (long) direction of the paper and were spaced
apart by approx. 1/4". This resulted in a loading of active (on a
dry basis) sodium sulfate solids of .about.5.0 wt. % based on the
dry fiber content of the pulp sheet. The final total moisture
content of the wood pulp cellulose sheet treated with sodium
sulfate is .about.13.61 wt. % based on the total final product
weight. Control strips of NB416 having a moisture content of 6 wt.
% (no sodium sulfate addition) were also included in the
fiberization and densification procedures that follow below.
[0053] The pulp strips thus treated are placed in zippered plastic
bags for 24 hours and then removed and fiberized using a laboratory
Fitz hammermill. Resultant fluff was stored for about 16 hours in a
room at 50% RH (in bags with the tops open) and then is formed
(using a laboratory pad former) into 6" diameter round pads of
.about.300 gsm basis weight from which 10 cm.times.10 cm square
pads are cut and subject to the densification procedure as
described in Example 1. Density of the resultant pads is shown in
FIG. 2.
[0054] Results illustrated in FIG. 2 show that the sodium sulfate
treated pulp (5.0% loading from 33 wt % solution) attains a higher
density for a given pressure compared to the untreated NB416 pulp
which was incorporated as a control. However, in comparison with
FIG. 1 it is clear that the level of density increase over the
control is reduced when using a 33% sodium sulfate solution versus
the 24.8% solution. In the case of the 33% solution (at 5% chemical
add-on) the increase at 150 psi is .about.4% higher than the
control (0.17 g/cc vs. 0.163 g/cc) whereas in the case of the 24.8%
solution of Example 2 (at a similar 5.7% chemical add-on) the
density at 150 psi is 25.7% higher than the control (0.176 g/cc vs.
0.14 g/cc). It should be pointed out that in these types of
experiments the density that the control pulp attains varies
considerably with the type of fiberizer, pad former and ambient
humidity conditions. Given this, each experiment had its own
internally consistent control as the basis for judging the
performance of treated pulps.
* * * * *