Note: Descriptions are shown in the official language in which they were submitted.
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1
PAPER PRODUCTS HAVING WET STRENGTH
FROM ALDEHYDE-FUNCTIONALIZED CELLULOSIC FIBERS AND
POLYMERS
Field of the Invention
The present invention relates to paper products having a relatively high
initial wet strength.
More particularly, the invention relates to paper products comprising aldehyde-
functionali2ed
cellulosic fibers and polyhydroxy polymers that are reactive with the
cellufosic fibers.
Backsround of the Invention
Paper webs or sheets, sometimes called tissue or paper tissue webs or sheets,
find extensive
use in modern society. These include such staple items as paper towels, facial
tissues and sanitary
(or toilet) tissues. These paper products can have various desirable
properties, including wet and dry
tensile strength.
Wet strength is a desirable attribute of many disposable paper products that
come into
contact with water in use, such as napkins, paper towels, household tissues,
disposable hospital wear,
etc. In particular, it is often desirable that such paper products have
sufficient wet strength to enable
their use in the moistened or wet condition. For example, moistened tissue or
towel may be used for
body or other cleaning. Unfortunately, an untreated cellulose fiber assemblage
will typically lose
95% to 97% of its strength when saturated with water such that it cannot
usually be used in the
moistened or wet condition. Therefore, several approaches have been taken to
impart wet strength to
paper products.
Paper products develop dry strength in part due to interfiber hydrogen
bonding. When the
paper product is wetted, water disrupts the hydrogen bonds and, as a
consequence, lowers the
strength of the paper product. Historically, wet strength of paper products
has been increased
primarily by two approaches. One approach is to prevent water from reaching
and disrupting the
interfiber hydrogen bonds, for example, by coating the paper product. Another
approach is to
incorporate additives in the paper product which contribute toward the
formation of interfiber bonds
which are not broken or. for temporary wet strength, which resist being
broken, by water. The
second approach is commonly the technique of choice, especially for tissue
products. In this latter
approach, a water soluble wet strength resin may be added to the pulp,
generally before the paper
t,
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2
product is formed (wet-end addition). The resin generally contains cationic
functionalities so that
it can be easily retained by the cellulose fibers, which are naturally
anionic.
A number of resins have been used or disclosed as being particularly useful
for providing
wet strength to paper products. Certain of these wet strength additives have
resulted in paper
products with permanent wet strength, i.e., paper which when placed in an
aqueous medium
retains a substantial portion of its initial wet strength over time. Exemplary
resins of this type
include urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-
epichlorohydrin
resins. Such resins have limited wet strength decay.
Permanent wet strength in paper products is often an unnecessary and
undesirable
property. Paper products such as toilet tissues, etc., are generally disposed
of after brief periods of
use into septic systems and the like. Clogging of these systems can result if
the paper product
permanently retains its hydrolysis-resistant strength properties. Therefore,
manufacturers have
more recently added temporary wet strength additives to paper products for
which wet strength is
sufficient for the intended use, but which then decays upon soaking in water.
Decay of the wet
strength facilitates flow of the paper product through septic systems.
Numerous approaches for
providing paper products claimed as having good initial wet strength which
decays significantly
over time have been suggested.
For example, U.S. Pat. No. 3,556,932, Coscia et al., issued Jan. 19, 1971;
U.S. Pat. No.
3,740,391, Williams et at., issued June 19, 1973; U.S. Pat. No. 4,605,702,
Guerro et al., issued
August 12, 1986; and U.S. Pat. No. 3,096,228, Day et al., issued July 2, 1983,
describe additives
that are suggested for imparting temporary wet strength the paper. In
addition, modified starch
temporary wet strength agents are marketed by the National Starch and Chemical
Corporation
(Bloomfield, New Jersey). This type of wet strength agent can be made by
reacting
dimethoxyethyl-N-methylchloracetamide with cationic starch polymers. Modified
starch wet
strength agents are also described in U.S. Pat. No. 4,675,394, Solarek, et
al., issued June 23,
1987. Additional wet strength resins are disclosed in U.S. Pat. No. 3,410,828,
Kekish, issued
Nov. 12, 1968 and its parent, U.S. Pat. No. 3,317,370, Kekish, issued May 2,
1967.
Still other additives have been used in the papermaking process to impart a
level of dry
andlor wet strength to the paper product. One type of strength additive are
the galactomannan
gums such as guar gum and locust bean gum. These gums and their use in paper
are described in
more detail in Handbook of Pulp and Paper Technology, 2nd Ed., Britt, pp. 650-
654 (Van
Nostrand Reinhold Co. 1964). The galactomannan gums generally impart dry
strength to paper
products. Unfortunately, in addition to having dry strength, the paper
products incorporating such
gums tend to be harsh to the hand. Therefore, the galactomannan gums have
found utility in
printing and writing paper but generally have not been useful in paper
products where softness is
a desirable characteristic, such as toilet tissue and facial tissue.
It is also well known to those knowledgeable in pulp bleaching, that oxidative
bleaching
of
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3
cellulose fibers outside the optimum pH (10 or greater) and temperature
conditions can result in
formation of carbonyl groups in the fibers, in the form of ketones and/or
aldehydes. For example
hypochlorite bleaching in the neutral pH range, will produce such a result
(Cellulose Chemistry
and Its Applications, T.P. Nevell & S.H. Zeronian, Eds, pp. 258 - 260, Ellis
Harwood Ltd. Pub.,
West Sussex. England. 1985). Chlorine bleaching without free radical
scavengers, e.g. chlorine
dioxide, will also produce fibers with an elevated carbonyl content (The
Bleaching of Pulp 3rd
Ed.. R.P. Singh Ed., pp. 40 - 42 & 64 - 65, TAPPI Press, Atlanta. GA, 1979) as
will ozone
bleaching to the point of fiber degradation (M.P. Godsay & E.M. Pierce. AIChe
Symposium
Series, No. 246, Vol. 81, pp. 9 - 19). However, ketone groups do not provide
significant wet
strength properties to the paper product. Rather, the dry and the minimal wet
strength of the
untreated products are determined primarily by the existence of interfiber
hydrogen bonds.
Intermediate oxidations that may result in the formation of aldehydes have
heretofore not been
desired, since the presence of aldehyde groups tends to cause yellowing of the
cellulosic fibers
over time.
It is also known that certain chemicals can by intent produce cellulose fibers
with an
elevated aldehyde content. Examples of these are sodium periodate, periodic
acid and sodium or
potassium dichromate at mildly acidic pH (Cellulose Chemistry and Its
Applications, T.P. Nevell
& S.H. Zeronian, Eds., pp. 249 - 253 & 260 - 261, Ellis Harwood Ltd. Pub.,
West Sussex,
England 1985).
While some of the problems of providing paper products having wet strength
have at
least been partially ameliorated by the art, none has solved the problems in
the manner or to the
extent of the present invention. It is therefore an object of an aspect of
this invention to provide
paper products, and particularly paper tissue products, that have an initial
wet strength that is
significantly higher than that of the corresponding paper product formed from
unmodified and
untreated cellulosic fibers, and which retains sufficient strength during the
period of intended use.
Yet another object of an aspect of the present invention is to provide paper
products having an
initial wet strength sufficient for use of the paper product for body cleaning
in the moistened
condition. It is a fiuther object of an aspect of the present invention to
provide tissue paper
products having an initial total wet tensile strength of at least about 80
g/inch, preferably at least
about 120 g/inch.
Another object of an aspect of this invention is to provide paper products
that have such
initial wet strengths, and which also have a rate of wet strength decay
sufficient for a flushable
product. It is a further object of an aspect of the present invention to
provide paper products
having such initial total wet tensile strengths and a 30 minute total wet
tensile strength of not
more than 40 g/inch.
The present invention relates to paper products having relatively high levels
of initial wet
strength. The invention is particularly adapted to disposable absorbent paper
products such as
those used for household, body, or other cleaning applications and those used
for the absorption
of body fluids such as urine and menses.
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The paper products of the present invention comprise cellulosic fibers having
free
aldehyde groups. The cellulosic fibers are combined with a water-soluble
polymer having
functional groups capable of reacting with the aldehyde groups. The aldehyde
groups on the
fibers are reacted with the functional groups to form bonds joining the fibers
(inter-fiber
bonds are formed).
According to an aspect of the present invention, there is provided a paper
product
having initial wet strength, comprising cellulosic fibers having free aldehyde
groups, wherein
the fibers are derived from cellulosic fibers comprising a polysaccharide in
which the
hydroxyl groups of at least a portion of the repeating units of the
polysaccharide are cis-
hydroxyl groups, the fibers being combined with a water-soluble polymer having
functional
groups that react with the aldehyde groups, the aldehyde groups being reacted
with the
functional groups to form chemical bonds joining the fibers.
In a preferred embodiment, the cellulosic fibers contain a polysaccharide in
the
hemicellulose fraction in which the hydroxyl groups of at least a portion of
the repeating units
of the polysaccharide are cis-hydroxyl groups, preferred repeating units being
mannose and
galactose. Similarly, the water-soluble polymer is preferably a polysaccharide
in which the
hydroxyl groups of at least a portion of the repeating units of the
polysaccharide are cis-
hydroxyl groups. Preferred polysaccharides are those in which the repeating
units are derived
from one or more sugars selected from mannose, galactose, allow, altrose,
gulose, talose,
ribose, and lyxose.
D~taj~ed Description of Preferred Embodimentlsl
As used herein, the terms "paper" and "paper products" include sheet-like
masses and
molded products containing the modified cellulosic fibers and certain polymers
that are
reactive therewith. The paper products of the present invention comprise
cellulosic fibers
having free aldehyde groups that are combined with a water-soluble polymer
having
functional groups capable of reacting with the aldehyde groups. The cellulosic
aldehyde
groups are reacted with the polymer functional groups to form inter-fiber
chemical bonds,
typically covalent bonds (i.e., bonds are fprmed between different fibers).
These bonds
provide a high initial wet strength to the paper product, as compared to a
corresponding paper
product formed without cellulosic fibers having free aldehyde groups.
The cellulosic fibers having free aldehyde groups can be derived from
cellulosic
fibers of diverse natural origin. Digested fibers from softwood (i.e., derived
from coniferous
trees), hardwood (i.e., derived from deciduous trees) or cotton linters are
preferably utilized.
Fibers from Esparto grass, bagasse, kemp, flax, and other lignaceous and
cellulosic fiber
sources may also be utilized as raw material in the invention. Also useful in
the present
invention are fibers derived from recycled paper, which can contain any or all
of the above
categories as well as other non-fibrous materials such as fillers and
adhesives used to facilitate
the original paper making.
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T'he paper products may also contain non-cellulosic fibrous polymeric material
characterized by having hydroxyl groups attached to the polymer backbone, for
example glass
fibers and synthetic fibers modified with hydroxyl groups. Other fibrous
material, e.g.,
synthetic fibers, such as rayon, polyethylene and polypropylene fibers, can
also be utilized in
combination with natural cellulosic fibers or other fibers containing hydroxyl
groups.
Mixtures of any of the foregoing fibers may be used. Since the strength of the
paper product
tends to increase with the number of hydroxyl groups in the fibers, it will
usually be preferred
to employ primarily, more preferably wholly, fibers having hydroxyl groups.
Cellulosic fibers
are economically preferred.
Cellulose containing fibers in the natural state are composed of primarily of
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and lignin. 'The carbohydrates are atmost wholly composed of long polymeric
chains of anhydro-
sugar units known as polysaccharides, primarily cellulose which is composed of
long linear
chains of (3-linked anhydroglucopyranose units. Other polysaccharides present
in cellulose are
commonly referred to as hemicelluloses which are derived at least in part from
the sugars D-
5 glucose, D-mannose. D-galactose, D-xylose, L-arabinose, ribose (as
determined by hydrolysis).
The chemistry of cellulose, in regard to wood cellulose, is described in more
detail in Handbook
of Pulp and Paper Technology, 2nd Ed., Britt, pp. 3-12 (Van Nostrand Reinhold
Co. 1964).
Cellulosic fibers that are preferred for use herein are those that comprise a
hemicellulose in which
the hydroxyl groups of at least a portion of the repeating units of the
hemicellulose are cis-
hydroxyl groups. Without intending to be bound by theory, it is believed that
hemicelluloses
having monosaccharides with a cis-hydroxyl group stereochemistry, e.g. mannose
and galactose
are more rapidly oxidized than polysaccharides having sugars with trans-
hydroxyl groups, e.g.
cellulose, and thereby enable a more rapid process for the development of high
wet strength
paper. Thus, preferred cellulosic fibers are those that comprise a
hemicellulose derived from one
or more sugars selected from mannose and galactose. Typically, the
hemicellulose is derived
from mannose, galactose or both. The composition of a given cellulose can be
determined by
hydrolysis of the cellulose material to the constituent sugars by known
methods with subsequent
qualitative and quantitative analysis of the hydrolyzate by separation
techniques such as paper,
thin-layer, or gas-liquid chromatography.
The optimum cellulosic fiber source utilized in conjunction with this
invention will
depend upon the particular end use contemplated. Generally wood pulps will be
utilized.
Applicable wood pulps include chemical pulps, such as Kraft (i.e., sulfate)
and sulfite pulps, as
well as mechanical pulps including, for example, groundwood, thermomechanical
pulp (i.e.,
TMP) and chemi-thermomechanical pulp (i.e., CTMP).
Fibers prepared by a mechanical pulp process tend to provide the highest
initial total wet
tensile strengths and may be preferred for this reason. Without intending to
be bound by theory, it
is believed that mechanical pulp processes tend to preserve the hernicellulose
content and the
lignin content of the cellulosic fibers. Thus, such fibers tend to have a
higher content of lignin and
of polysaccharides having cis-hydroxyl groups in the repeating units of the
polysaccharide.
Chemical pulping processes tend to remove hemicelluloses and lignin such that
potential initial
wet strengths may not be achieved with such pulps. However, chemical pulps are
often preferred
since they impart a superior tactile sense of softness to paper products made
therefrom. Paper
products made with aldehyde-functionalized chemical pulp fibers in accordance
with the present
invention may therefore provide a particularly suitable balance of softness
and initial wet
strength. In further regard to chemical pulps, kraft fibers are preferred to
sulfite fibers since less
hemicellulose tends to be removed in the kraft process leading to higher
levels of initial wet
tensile strength.
Both softwood fibers and hardwood kraft fibers contain cellulose and
hemicelluloses.
However, the types of hemicelluloses in softwood fibers are different from
those in hardwood
fibers
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and contain a higher weight % of galactose and mannose. For example, The
Handbook ojPulp and
Paper Technology, 2nd Ed.. Britt, p. 6 (Van Nostrand Reinhold Co. 1964),
describes the mannan
content of softwoods in the range of from 10 to 15%, while hardwoods have a
mannan content of
usually only 2 to 3%. The types of lignin in softwood also differs from that
of hardwood.
In a preferred embodiment, the cellulosic fibers are softwood fibers. !t has
been found that
the softwood fibers having aldehyde groups in accordance with the present
invention produce paper
structures having unexpectedly higher levels of initial wet tensile than paper
structures formed from
hardwood fibers having aldehyde groups. While it is well known in the
papermaking art that
softwood fibers are longer than hardwood fibers and that softwood fibers
produce paper having a
higher strength than paper formed from hardwood fibers, it is believed that
softwood fibers, having a
higher weight percentage of hemicelluiose galactose andlor mannose than
hardwood fibers, form a
greater number of aldehyde groups on the cellulosic polymer chain. This
greater number of aldehyde
groups tends to increase the difference in wet strength typically observed
between softwood and
hardwood fibers.
Completely bleached, partially bleached and unbleached fibers are applicable.
It may
frequently be desired to utilize bleached pulp for its superior brightness and
consumer appeal.
The cellulosic fibers employed in the paper products of the present invention
are modified
to contain free aidehyde groups. By "free" aldehyde groups, it is meant that
the aldehyde groups are
capable of reacting with the water-soluble polymer material having functional
groups. The cellulosic
fibers can be modified to contain the aldehyde groups by converting at least a
portion of the
cellulosic hydroxyl groups to intrafiber and/or interfiber aldehyde groups,
typically both intrafiber
and inte~ber aldehyde groups. Alternatively, this can be accomplished by graft
copolymerization of
aldehyde functionalities, for example as described by T.G. Gafurov et al. in
Strukt. Modif. Khlop.
Tsellyul., Vol. 3, pp. 131-135 (1966), which describes the application of
acrolein to cellulose. In a
preferred embodiment, the cellulosic fibers are modified by converting
cellulosic hydroxyl groups to
intrafiber and/or interfiber aldehyde groups (more preferably both intrafiber
and interfiber aldehyde
groups). The hydroxyl groups can be convened to aldehyde groups by treating
the fibers with an
oxidizing agent under conditions that cause the formation of such groups. In
general, the
modification by way of oxidation is performed by dispersing the fibers in an
aqueous liquid medium,
contacting the fibers with an oxidizing agent, and reacting hydroxyl groups in
the cellulosic fibers
with the oxidizing agent to form aldehyde groups. The oxidizing agent may
react with hydroxyl
groups in any of the components of the fibers, including the cellulose,
hemicellulose, and/or lignin.
The aqueous liquid medium enables the dispersion of the fibers such that
intimate and
uniform contact can be achieved between the fibers and the oxidizing agent to
thereby provide a
higher and more uniform yield. In addition to water, the liquid medium may
comprise one or more
materials which in combination will effect such dispersion but which will not
dissolve the
unmodified fibers or the oxidized fibers. Water is an economically preferred
liquid medium.
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The amount of the aqueous liquid medium and the fibers in the dispersion may
vary over a
wide range. Typically, the dispersion comprises from about 0.1 to about 50
weight % of the fibers
and from 99.9 to about 50 weight % of the liquid medium. Thus, the dispersion
can be low
consistency (e.g., about 3% fiber/97% aqueous liquid medium, medium
consistency (e.g.. about 8-
16% fiber/92-84% aqueous liquid medium), or high consistency (e.g., about 20 -
50% fiber/SO - 80%
aqueous liquid medium).
The fibers can be dispersed in the medium by any suitable method such as are
known in the
art. Conventional agitation equipment, e.g., mechanical stirrers, are
typically used for low
consistency oxidation. Dispersion is typically achieved after agitating for a
period of about 30 - 60
minutes.
The fibers in the dispersion are then contacted with an oxidizing agent under
conditions to
cause oxidation of cellulosic hydroxyl groups to form aldehyde groups on the
fibers (i.e., infra-fiber
aldehyde groups are formed). Contact of the fibers and the oxidizing agent is
preferably assisted by
the use of an agitation means, e.g., a mechanical stirrer.
IS Suitable oxidizing agents are those compounds that will react with the
hydroxyl groups of
the cellulosic fibers to increase the carbonyl content of the fibers. Suitable
oxidizing agents include
oxidizing agents that are believed to function through free radical oxidation,
such as hypochlorous
acid, hypobromous acid, hypoiodous acid, persulfates, peroxides, perborates,
perphosphates, and any
of the known free radical polymerization initiators. Other oxidizing agents
suitable for use herein
include ozone, chromic acid, nitrogen dioxide, and periodates. In general, the
oxidizing agent of
choice is dissolved in water and mixed with the fibers to be oxidized. During
the oxidation step, at
least a portion of the hydroxyl groups of the cellulosic fibers are converted
to aldehyde groups.
Without intending to be bound by theory, it is believed that at least a
portion of the aldehydes are
present on the fiber surface to facilitate interfiber bonding during the
papermaking process. The
formation and quantification of aldehyde groups can be detected by known
analytical techniques
such as infrared analysis. Alternatively, the presence of aldehyde groups is
evidenced by an increase
in the wet strength of a paper product formed from the oxidized fibers,
relative to a corresponding
paper product formed from non-oxidized fibers. In general, for a given fiber
weight, oxidizing agent
concentration, and set of reaction conditions, oxidation increases with
increasing time of exposure to
the oxidizing agent. Thus, the degree of oxidation can be readily optimized
for a given fber weight
by quantifying the aldehyde content as a function of time by any of the
foregoing methods. It will
be desired to avoid the over oxidation of the fibers to cause significant
formation of carboxylic acid
groups, which can be detected and quantified using similar techniques.
Ozone oxidation can be accomplished by introducing ozone into the dispersion.
The ozone
may be introduced by injecting the gas under pressure into the dispersion. The
pH of the dispersion
is preferably adjusted to an initial pH of from about 4 to about 8, more
preferably from about 7 to
about 8. Although the flow rate and pressure of the ozone may vary over a wide
range, exemplary
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conditions include a flow rate of about 8.0 litersiminute and a flow pressure
of about 8 psig. The
dispersion is preferably cooled, e.g., to temperatures of about 0°C or
less, to maximize the solubility
of the ozone in the dispersion. Antifoaming agents such as are known in the
art may be added to the
mixture to minimize foaming. The oxidation reaction is typically completed by
introducing the
ozone under the foregoing conditions for a period ranging from about 30 to
about 60 minutes.
Oxidation by other oxidizing agents is further described in the Examples.
The oxidized fibers are then combined with a water-soluble polymer that
contains functional
groups capable of chemically reacting with the aldehyde groups. As used
herein, "water soluble"
includes the ability of a material to be dissolved, dispersed, swollen,
hydrated or similarly admixed in
water. Similarly, as used herein "substantially dissolving" includes
dissolution, dispersion,
hydration, swelling, and the like admixture with a liquid medium. Typically
the mixture forms a
generally uniform liquid mixture having, to the naked eye, one physical phase.
Suitable water soluble polymers contain functional groups selected from
hydroxyl groups
and amide groups, including polysaccharides, polyvinyl alcohol, and
polyacrylamides. Since the wet
strength decay rate tends to increase with polymers having amide groups,
polymers containing such
groups are not preferred for where paper having temporary wet strength is
desired. In a preferred
embodiment, the polymer is a water-soluble polysaccharide in which at least a
portion of the
hydroxyl groups in at least a portion of the repeating units of the
polysaccharide are cis-hydroxyl
groups. For example, the polysaccharide is suitably derived from one or more
sugars selected from
the group consisting of mannose, galactose, allow, altrose, gulose, taiose,
ribose, and lyxose.
Economically preferred polysaccharides are derived from mannose, galactose or
both. Such
preferred polysaccharides therefore include guar gum and locust bean gum. The
polysaccharides
may contain sugars other than those specifically mentioned. The sugar content
of the polysaccharide
can be determined by hydrolysis of the polysaccharide to the constituent
sugars by known methods
with subsequent qualitative and quantitative analysis of the hydrolyzate by
separation techniques
such as paper, thin-layer, or gas-liquid chromatography.
The polymer may be a neutral polymer, an electronically charged polymer, or a
charge
balanced mixture of polymers. Electronically charged derivatives, e.g., ionic
derivative, include
cationic and anionic derivatives. By "charge balanced mixture" of polymers, it
is meant that the total
amounts of each of the electronically charged polymers are selected so as to
provide an essentially
neutral polymer mixture. However, the polymer should be selected such that it
will not result in
excessive electrostatic repulsion between the fibers and the polymer. Since
the fibers are typically
negatively charged (anionic), neutral polymers or cationic polymers will
typically be preferred.
The initial wet tensile strength tends to increase with the molecular weight
of the polymer.
Therefore, for high initial wet strength, it is generally preferred to use
polymers having a relatively
high molecular weight. Electronically charged polymers tend to have lower
molecular weights than
the corresponding neutral polymer from which they are made, such that the
neutral polymers may
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provide higher initial wet tensile strengths, if each polymer has comparable
retention.
Polysaccharides that are suitable for use herein are commercially available
guar gums from
Hercules Chemical Co. of Passaic, New Jersey under the trade names Galactosol
and Supercol. and
anionic or cationic derivatives.
The aldehyde-functionalized cellulosic fibers and the water-soluble polymer
are combined
in a manner which allows the polymers to form a bonded fiber mass, generally
in the form of a sheet
containing the fibers (i.e., a paper product is formed). The bonded fiber mass
has a dry strength and
an initial wet strength that is higher than a comparable fiber mass with non-
aldehyde functionalized
fibers and the polymer. The polymer may be combined with the celiulosic fibers
in the wet-end of a
papermaking process such as are known in the art. Alternatively, the polymer
may be combined with
the cellulosic fibers by spraying or printing the polymer, typically in the
form of an aqueous solution
or dispersion, onto the fibers after they are set in the paper making process.
In forming paper generally in the form of sheets, the polymer is preferably
combined with
the cellulosic fibers in the wet-end of a wet-laid paper-making process such
as are known in the art.
IS Wet laid paper making processes typically include the steps of providing a
slurry containing the
cellulosic fibers {the slurry is alternatively referred to herein as a paper
making furnish), depositing
the slurry of fibers on a substrate such as a foraminous forming wire (e.g., a
Fourdrinier wire), and
setting the fibers into a sheeted form while the fbers are in a substantially
unflocculated condition.
The step of setting the fibers into sheeted form may be performed by allowing
the fluid to drain and
pressing the fibers against the foraminous wire (dewatering), for example,
with a screened roll, such
as a cylindrical Dandy Roll. Once set, the fibrous sheet may then be dried and
optionally compacted
as desired.
Thus, in a wet-laid paper making process, the polymer is preferably combined
with the
cellulosic fibers by adding the polymer to the paper making furnish, generally
an aqueous paper
making furnish comprising water and the cellulosic fibers. In a preferred
embodiment, the polymer
is added to the furnish after substantially dissolving the polymer in a
suitable liquid medium. Where
the polymer is hydrated by the medium, for example, in the case of guar gum,
it is preferred to bring
the polymer to its equilibrium swell. In an alternative embodiment, the
polymer is added directly to
the furnish. The furnish is adjusted, if necessary, to a pH of about 7 or
less, preferably from about 4
to about 7.
The polymer must remain in contact with the cellulosic fibers, prior to
setting the fibers, for
a period sufficient to allow adsorption of the polymer by the fibers and
bonding between the polymer
and the cellulosic fibers. Otherwise the polymer may be lost during the
setting step such that the wet
strength improvements are not obtained. A sufficient period is typically
achieved by leaving the
polymer in contact with the cellulosic fibers for a period of from a few
seconds to about 60 minutes
prior to setting the fibers, more typically on the order of a few to 60
seconds. Bonding may involve
ionic bonding and/or covalent bonding.
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The temperature of the furnish will generally be between greater than
0°C and less than
100°C and is more typically at about room temperature (20 -
25°C). The paper making process is
generally conducted in air at atmospheric pressure, although other
environments and pressures may
be used.
Once the furnish is prepared, it is convened into final web or sheet form by
any suitable wet
laying method, including a method previously described as involving deposition
of the furnish,
setting of the fibers, drying and optionally compacting.
The amount of polymer that is combined with the aldehyde-functionalized
cellulosic fibers
is generally selected to provide a balance of initial wet tensile strength,
wet tensile decay and
10 optionally other properties, including dry strength, consistent with the
objects of the invention. In
general, an increase in the amount of the polymer tends to result in an
increase in dry strength and
initial wet tensile strength (particularly in dry strength) and a decrease in
softness. The paper
products will typically contain from about 0.01 to about 5 weight % of the
polymer, based on the
weight of the aldehyde-functionalized cellulosic fibers. Preferably, the paper
products will contain
from about 0.01 to about 3 weight % of the polymer, based on the weight of the
cellulosic fibers.
For example, a particularly suitable paper product contains about 2 weight %
of the polymer, based
on the weight of the aldehyde-functionalized cellulosic fibers.
The wet strength develops through the formation of fiber-fiber bonds and/or
fiber-polymer
bonds. Without intending to be limited by theory, fiber-fiber bonding is
believed to occur through
the reaction of the aldehyde groups with hydroxyl groups on proximate fibers
to form hemi-acetal
bonds. The polymer provides additional sites for the aldehyde groups to react
to form bonds, with
inter-fiber bonding then occurring through bonding of a fiber to the polymer
to another fiber. More
specifically, the polymer is believed to react with the cellulosic fibers to
fornt hemiacetal or N-
acylhemiaminal groups through reaction of at least a portion of the aldehyde
groups on the cellulosic
fibers and at least a portion of the functional groups of the polymer
(hydroxyl groups or amide
groups, respectively) as the paper product dries. The resultant network tends
to have a higher
flexibility and a relatively high initial wet tensile strength compared to
interfiber bonded networks
formed from non-oxidized fibers or from oxidized fibers in the absence of the
polymer. The
hemiacetal and N-acylhemiaminal linkages are reversible in water, slowly
reverting to the original
aldehyde-functionalized fibers and polymer materials. This reversibility
confers temporary wet
strength to the paper product. The N-acylhemiaminal groups revert more slowly
than the hemiacetal
groups such that the wet strength of paper products comprising such groups has
a more permanent
nature.
The present invention is particularly suitable for paper products which are to
be disposed
into sewer systems, such as toilet tissue. However, it is to be understood
that the present invention is
applicable to a variety of other paper products including disposable paper
products including writing
paper and more absorbent products such as those used for household, body, or
other cleaning
~M
CA 02250175 2003-03-13
11
applications and those used for the absorption of body fluids such as urine
and menses.
Exemplary paper products thus include writing paper, tissue paper including
toilet tissue and
facial tissue, paper towels, absorbent materials for diapers, feminine hygiene
articles including
sanitary napkins, pantiliners and tampons, adult incontinent articles and the
like.
Tissue paper of the present invention can be homogeneous or mufti-layered
construction;
and tissue paper products made thereftom can be of a single-ply or mufti-ply
construction. The
tissue paper preferably has a basis weight of between about 10 g/m2 and about
65 g/m2, and
density of about 0.6 g/cm' or less. More preferably, the basis weight will be
about 40 g/mz or less
and the density will be about 0.3 g/cm3 or less. Most preferably, the density
will be between about
0.04 g/cm3 and about 0.2 g/cm3. See Column 13, lines 61-67, of U.S. Patent
5,059,282 (Ampulski
et al), issued October 22, 1991, which describes how the density of tissue
paper is measured.
(Unless otherwise specified, all amounts and weights relative to the paper are
on a dry basis.) The
tissue paper may be conventionally pressed tissue paper, pattern densified
tissue paper, and
uncompacted, nonpattern-densified tissue paper. These types of tissue paper
and methods for
making such paper are well known in the art and are described, for example, in
U.S. Patent
5,334,286, issued on August 2, 1994 in the names of Dean V. Phan and Paul D.
Trokhan.
The following illustrates the preparation and testing of exemplary paper
products
according to the present invention. Abbreviations appearing in the Examples
have the following
meaning:
NSK - Northern softwood kraft
E - eucalyptus
sulfite - acid sulfite pulp fibers
CTMP - chemical thermomechanical pulp fibers
"OX" - oxidized fibers
Handsheet preparation
Handsheets are made essentially according to TAPPI standard T205 with the
following
modifications:
(1) tap water, adjusted to a desired pH, generally between 4.0 and 4.5, with
HZS04 and/or
NaOH is used;
(2) the sheet is formed on a polyester wire and dewatered by suction instead
of pressing;
(3) the embryonic web is transferred by vacuum to a polyester papermaking
fabric;
(4) the sheet is then dried by steam on a rotary drum drier.
In accordance with the present invention, in some examples a water-soluble
polymer is
combined with the aldehyde-fiinctionalized cellulosic fibers. After dispersing
the fibers in tap
water, the polymer is added to the disintegrated pulp and the slurry is
agitated for a fixed period
of time ranging from 1 to 60 minutes.
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12
Streneth Tests -
The paper products are aged prior to tensile testing a minimum of 24 hours in
a conditioned
room where the temperature is 73 °F + 4 °F {22.g °C + ~.2
°C) and the relative humidity is 50% +
I 0%.
I . Total Drv Tensile Strength {"TDT")
This test is performed on one inch by five inch (about 2.5 cm X 12.7 cm)
strips of paper
(including handsheets as described above, as well as other paper sheets) in a
conditioned room where
the temperature is 73°F + 4°F (about 28°C + 2.2°C)
and the relative humidity is 50% _+ 10%. An
electronic tensile tester (Model 1122, Instron Corp., Canton, Mass.) is used
and operated at a
crosshead speed of 2.0 inches per minute (about 5.2 cm per min.) and a gauge
length of 4.0 inches
(about 10.2 cm). Reference to a machine direction means that the sample being
tested is prepared
such that the S" dimension corresponds to that direction. Thus, for a machine
direction (MD) TDT,
the strips are cut such that the 5" dimension is parallel to the machine
direction of manufacture of the
paper product. For a cross machine direction (CD) TDT, the strips are cut such
that the 5" dimension
is parallel to the cross-machine direction of manufacture of the paper
product. Machine-direction
and cross-machine directions of manufacture are well known terms in the art of
paper-making.
The MD and CD tensile strengths are determined using the above equipment and
calculations in the conventional manner. The reported value is the arithmetic
average of at least six
strips tested for each directional strength. The TDT is the arithmetic total
of the MD and CD tensile
strengths.
Z. Wet Tensile
An electronic tensile tester (Model I t22. Instron Corp.) is used and operated
at a crosshead
speed of I.0 inch (about 1.3 cm) per minute and a gauge length of 1.0 inch
(about 2.5 cm), using the
same size strips as for TDT. The two ends of the strip are placed in the upper
jaws of the machine,
and the center of the strip is placed around a stainless steel peg. The strip
is soaked in distilled water
at about 20°C for the desired soak time, and then measured for tensile
strength. As in the case of the
TDT, reference to a machine direction means that the sample being tested is
prepared such that the 5"
dimension corresponds to that direction.
The MD and CD wet tensile strengths are determined using the above equipment
and
calculations in the conventional manner. The reported value is the arithmetic
average of at least six
strips tested for each directional strength. The total wet tensile strength
for a given soak time is the
arithmetic total of the MD and CD tensile strengths for that soak time.
Initial total wet tensile
strength ("ITWT") is measured when the paper has been saturated for 5 ~ 0.5
seconds. 30 minute
total wet tensile ("30 MTWT") is measured when the paper has been saturated
for 30 ~ 0.5 minutes.
The following non limiting examples are provided to illustrate the present
invention. The
scope of the invention is to be determined by the claims which follow.
Preparation of Oxidized Cellulosic Fibers
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E3
Example 1
The following examples illustrate the effect of fiber type on the attainment
of wet strength.
Handsheets are prepared from various types of fibers and ozone oxidized fibers
of the same type.
The cellulosic fibers are ozone oxidized in the following manner. A mixture of
fibers and
tap water (0.9 - 1.3 wt% fibers) in a suitable container is stirred at room
temperature until the fibers
are well dispersed. The pH of the mixture is then adjusted to about 8 with an
inorganic acid or base
and the fibers are ozone oxidized. The fibers are oxidized with ozone by
bubbling ozone through the
mixture with vigorous stirring (mechanical stirrer) for a period of 30 - 35
minutes. The ozone is
introduced into the mixture using a Polymetrics model T-816 ozone generator,
run on oxygen feed at
a gauge pressure of 8 psig, a flow rate of 8.0 liters/minute, and a voltage of
115 volts. The mixture is
generator cooled to a temperature of from I SoC to - 20°C during the
oxidation process.
After the oxidation, the water is drained from the fibers on a Buchner funnel
and the fibers
are further dewatered by centrifugation.
Handsheets ( 18 1b/3000 ft2 or 26 1b/3000 ft2) are prepared as previously
described and
ITWT, TDT and 30MTWT are determined. The handsheets provide tensile values as
shown in Table
TahlP 1
Fiber identitybasis wt. InitialFinal ITWT TDT 30 MTWT
(Ib/3000ft2)pH pH (gm/in)(gm/in)(gm/in)
NSK 18 7.92 - 15 1165 -
NSK-OX 18 6.44 - 186 2418 75
NSK 26 7.8 - 42 3732 -
NSK-OX 26 7.8 6.18 348 3896 191
E 18 7.84 - < 10 85 -
E-OX 18 5.02 - 66 673 12
EMSK 18 7.88 - 8 464 -
(80/20
by wt)
E/NSK-OX 18 4.96 - 108 1520 52
(80/20
by wt)
sulfite 18 7.88 _ 8 128 -
sulfite-ox18 7.88 4.45 30 202 -
CTMP 18 7.99 - 38 1461 .
CTMP-OX 18 7.99 5.28 184 1608 184
CTMP 26 8 - 60 2627 -
CTMP-OX 26 8 6.05 450 3075 2I4
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14
CTMP/NSK 36 - - - 70 3476 ?6
(60/40
by wt)
CTMP/NSK-26 - - 384 3678 214
OX (60!40
by
wt )
CTMP/NSK 26 7.92 - 66 3509 -
(60/40
by wt)
CTMPMSK- 26 7.92 6.10 430 4111 l gp
OX (60/40
by
wt)
1 able 1 shows that oxidized kraft (softwood) fibers provide over three times
the ITWT that
oxidized eucalyptus (hardwood) fibers produce. This is believed to be due to
the much higher
weight percentage of wood polysaccharides (hemicelluloses) having the cis-
hydroxyl group
stereochemistry in the monomer units in softwood fibers than is present in
hardwood fibers.
Table I further shows that oxidized CTMP fibers provide even higher ITWT than
kraft
fibers. In the mechanical pulp most of the hemicellulose and lignin that is
present in the natural
wood is retained, which is believed to contribute to the higher ITWT. The
presence of the oxidized
lignin appears to contribute a more permanent type of wet tensile strength
which may be particularly
useful for paper towel applications.
The presence of hemicelluloses containing galactose and/or mannose tend to
contribute to
the attainment of ITWT and TDT. Therefore, fibers that have a greater
percentage of these sugars in
the native fiber tend to provide higher ITWT. In addition, pulping processes
that tend to preserve the
hemicellulose content tend to provide higher ITWT. Therefore, softwood fibers
will provide higher
ITWT than hardwood fibers, and mechanical pulps tend to provide higher ITWT
than chemical
pulps. Moreover, chemical pulp processes that tend to preserve the
hemicellulose content will tend
to provide higher ITWT. An acid sulfite process tends to efficiently remove
the galactose and
mannose containing hemicelluioses with a resultant low ITWT relative to !craft
fibers. Modifications
of conventional kraft process (e.g., lcraft/oxygen, polysulfide pulping, high
sulfidity digesting, etc.)
that preserve a higher weight percent of hemicellulose in the fibers would
tend to provide higher
ITWT in ozone oxidized fbers prepared by such processes, than the ITWT
provided by ozone
oxidized fibers produced through conventional kraft pulping.
Example 2
The following example illustrates the effect of oxidation pH on the initial
total wet tensile
strength of handsheets prepared from the ozone oxidized cellulosic fibers.
The cellulosic fibers are ozone oxidized in the following manner. A mixture of
Eucalyptus
fibers, NSK fibers (80/20 by wt) and tap water (about 0.9 wt % fibers) is
stirred at room temperature
until the fibers are well dispersed. The pH of the mixtures is then adjusted
with inorganic acid or
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base to an initial value and the fibers are ozone oxidized as described in
Example 1 for a period of 30
minutes. The final pH of the mixture is measured after the 30 minute oxidation
period. Handsheets
( 18 Ib/3000 ft2) are prepared as previously described and ITWT and TWT
determined. The
handsheets provide tensile values as shown in Table 2. Where more than one
value is shown, this
5 reflects that multiple of samples tested.
Table 2
Initial Final pH ITWT ITWT average
pH
2 2.04 53.3 _
3 2.93 58.8 _
4 3 .91 /3 .7 48.1 /61 54.6
5 4.26/4.13 42.4/76.4 59.4
6 4.39 gg_3
7 4.61 /4.43 61.4/78.5 70
8 5.11 I 06.4 _
9 6.32/5.92 60.9/97.4 79?
10 7.31 87.2 _
1 7.64 64.2 _
I2 11.63/11.49 26.8/29.8 2g,3
Table 2 shows that the maximum initial wet tensile strength is produced with
an initial
oxidation pH of about 8. Highly alkaline pH (> about 1 I ) is detrimental to
the development of wet
tensile with ozone oxidized fibers.
10 Example 3
The following example illustrates the effect of fiber oxidation time on the
development of
wet strength in paper tissue comprised of one cellulosic fiber mixture.
A mixture of 54 gm Eucalyptus fibers and 36 gm Northern Softwood Kraft fibers
(60/40
E/NSK) is slurried in 3.0 liter of tap water. The fibers are ozone oxidized as
described in Example 1
15 for a period of 40 - 75 minutes in 5 minute increments. Handsheets ( 18
1b/3000 ft2) are prepared as
previously described and ITWT and TDT determined. The handsheets provide
tensile values as
shown in Table 3.
Table 3
Oxidation time (minutes)ITWT (gmiin) TDT (gm/in)
40 81 1104
45 91 775
50 152 566
55 141 1083
60 ~ 166 ~ 1253
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16
65 159 1224
70 120 1126
75 117
1197
a~~~ ~ wows mac maximum initial total wet tensiles are provided with a fiber
oxidation
period of from SS minutes up to 65 minutes. Extended periods of oxidation may
result in the
oxidation of aldehyde groups, thereby causing a decrease in the initial total
wet strength.
Preparation of Paper Tissue From Oxidized Fibers and Poivhvdroxy Polymers
S Example 4
The following examples illustrate the preparation of paper tissue from ozone
oxidized
cellulosic fibers and several additives.
A mixture of S4 gm Eucalyptus fibers and 36 gm Northern Softwood Kraft fibers
(60/40
E/NSK) is scurried in 3.0 liter of water. The initial pH of the slurry is from
7 - 8. The fibers are
oxidized with ozone as described in Example I for a period of one hour.
Handsheets ( 18.5 Ib/3000
ft2 basis weight) are prepared as previously described with several additives
shown in Table 4. The
handsheets include 2% of the additive on fiber basis. The handsheets provide
ITWT and TDT as
shown in Table 4.
T.,1.1.. w
additive identity* ITWT ITWT TDT TDT
(gm/in) avg. (gtn/in)avg.
no additive 111/108/134c 18 IO8S/19871439
1244
Guar Gum (Sigma Chemical 274/289 282 2834/15582196
Co.)
Cationic Guar Gum (Hercules170 - 2733
Chemical
Co.)
Locust Bean Gum (Sigma 268 - 3132 -
Chemical Co.)
ozone oxidized guar gum 24S - 1666 -
(laboratory
preparation; to contain
aldehyde groups)
Cato - 31 Cationic Stareh193 - 3~ I -
(National
Starch & Chemical)
Redibond 5327 Cationic 1 S 1 - 2489 -
Starch
(National Starch & Chemical)
Oxidation Filtrate 106 - 1 I 13 -
Polyacrylamide 19,092-6 129 - 1713
(Aldrich
Chemical Co.)
Acco 711 (polyacrylamide;118 - 3181 -
Cytec
Industries)
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17
Polyvinyl Alcohol 36,306-583 - 1080 -
(Aldrich
Chemical Co.)
As shown in Table 4, several additives increase the initial total wet tensile
of the paper
product formed from the oxidized fibers. Exceptionally high levels of ITWT are
surprisingly
obtained when guar gum or locust bean gum, which contain monomers having the
cis-hydroxyl
group stereochemistry, are included in the handsheet.
Example 5
The following examples illustrate the preparation of paper tissue using
cellulosic fibers
oxidized with various oxidizing agents and treated with guar gum. 80%
eucalyptus/20% NSK fiber
is oxidized with the agents listed in Table 5.
Hypochlorous acid oxidation is performed as follows. An 80/20 by weight
mixture of
eucalyptus fibers and NSK fibers is slurried in water at 1% consistency. The
pH is adjusted to 3.5
4.0 with sulfuric acid. A 5% aqueous solution of sodium hypochiorite is then
added to the fiber
slurry to bring the slurry to a pH of 7.5-8Ø Sulfuric acid is then added to
bring the pH to 6Ø The
slurry is stirred overnight at room temperature. The pH is then adjusted to
4.0-4.5 and handsheets
( 18.5 1b/3000 ft2) are prepared with 2 weight % guar gum on a fiber basis, as
previously described.
Persulfate oxidation is performed as follows. An 80/20 by weight mixture of
eucalyptus
fibers and NSK fibers is slurried in water at 1% consistency. The pH is
adjusted to 7.0 with nitric
acid and 5 weight % sodium persuifate on a fiber basis is added. t weight %
cupric sulfate on a fiber
basis is added and the slurry is stirred for about 12 hours at room
temperature. The pH is then
adjusted to 4.0-4.5 with nitric acid and handsheets (18.5 Ib/3000 ft2) are
prepared with 2 weight
guar gum on a fiber basis, as previously described.
Hydrogen peroxide oxidation is performed as follows. An 80/20 by weight
mixture of
eucalyptus fibers and NSK fibers is slurried in water at 1% consistency. 5
weight % hydrogen
peroxide on a fiber basis is added. The pH is adjusted to 8.0 with sodium
hydroxide, and 0.5 weight
cupric sulfate on a fiber basis is added. The pH is readjusted to 8Ø The
slurry is stirred for about
12 hours at room temperature. The pH is then adjusted to 4.0-4.5 with sulfuric
acid and handsheets
(18.5 1b/3000 ft2) are prepared with 2 weight % guar gum (GG) on a fiber
basis, as previously
described. Handsheet ITWT and TDT are displayed in Table 5.
Table S
Oxidizing Agent/AdditiveITWT ~T
(Pin) (~~)
ozone/none 108 1987
ozone/GG 274 2834
hypochlorous acid/none186 1184
hypochlorous acid/GG493 2845
sodium persulfate/none155 ~ 1070
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18
sodium persulfate/GG 354 2812
hydrogen peroxide/none 88 566
hydrogen peroxide/GG 217 1625
Ozone, hypochlorous acid, sodium persulfate and hydrogen peroxide produce
oxidized
fibers which when combined with guar gum in papermaking have exceptional
levels of ITWT.
Fibers oxidized with hypochlorous acid provide a particularly high level of
ITWT.
While particular embodiments of the present invention have been illustrated
and
described, it would be obvious to those skilled in the art that various other
changes and
modifications can be made without departing from the spirit and scope of the
invention. It is
therefore intended to cover in the appended claims all such changes and
modifications that are
within the scope of this invention.
