Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MAKING A CELLULOSIC WEB WITH IMPROVED WET-TENSILE STRENGTH
Technical Field
This invention relates generally to an absorbent web and a method for
preparing the absorbent web. More particularly, the invention relates to a
cellulosic
web having improved wet-tensile characteristics and methods for making the
same.
Background
Cellulosic web-based products, such as hand towels, wipers, and the like, are
widely manufactured in the paper making industry. Each product has unique
characteristics requiring appropriate attributes. The proper attributes ensure
that the
product is suitable for its intended purpose. For many products, these
attributes
include high tensile strength and high water absorbency.
Tensile strength of cellulosic webs is often measured under dry conditions to
determine "dry-tensile strength", and under wet conditions to determine "wet-
tensile
strength". Wet-tensile strength and dry-tensile strength are often closely
related.
Products having high dry-tensile strength frequently have high wet-tensile
strength.
The relationship between dry-tensile strength and wet-tensile strength occurs,
in
part, because certain manufacturing processes enhance both properties. For
example, pressing a cellulosic web under high pressure often imparts increased
dry-
tensile and wet-tensile strengths.
A cellulosic web must have adequate dry-tensile strength and adequate wet-
tensile strength. However, since cellulosic webs often deteriorate and lose
strength
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when wet, wet-tensile strength is normally significantly lower than dry-
tensile
strength. Thus, many cellulosic webs have inadequate wet-tensile strength,
even
though they have adequate dry-tensile strength.
While a hand towel must have some dry-tensile strength, high dry-tensile
strength can be disadvantageous because it correlates closely with stiffness
and stiff
webs. Hand towels manufactured from stiff webs are received with disfavor by
consumers, since the towels do not have the soft feel of traditional cotton
hand
towels. In contrast, consumers desire high wet-tensile strength, because it
allows
more vigorous use of the hand towel to pick up spills and scrub wet soiled
surfaces.
Traditionally, the wet-tensile strength of hand towels is improved by using
increased
pressure or a wet strength resin. While these methods increase the wet
strength of
the hand towels, they usually also increase the dry strength. For example, the
addition of CMC (carboxymethyl cellulose) causes significant increases in both
wet
tensile and dry tensile strengths.
The relationship between wet-tensile and dry-tensile is often expressed as a
tensile strength ratio in which wet-tensile strength is divided by dry-tensile
strength
to get the wet/dry tensile ratio (wet-tensile value in numerator, dry-tensile
value in
denominator). The wet/dry tensile ratio is normally less than 0.3, since the
wet-
tensile strength is usually significantly less than the dry-tensile strength.
The ratio is
preferably as close to 1 as possible, since this indicates a high wet-tensile
strength
relative to dry-tensile strength. Cellulosic webs with high wet/dry tensile
ratios are
soft, yet strong.
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Therefore, a need exists for an improved cellulosic web that has high wet-
tensile strength and a high wetldry tensile ratio.
Summary of the Invention
The present invention provides an improved method for making a cellulosic
web, as well as a web made using the improved method. The method includes
addition of surfactant and wet strength resin to a paperrnaking furnish. The
resulting
papermaking furnish is formed into a cellulosic web that is dried in
accordance with
conventional paper making methods. The finished web has improved strength
properties. In a certain implementation, the finished web has improved wet-
tensile
strength and an improved wet/dry tensile ratio.
A specific implementation of the method of the invention includes providing
a papermaking furnish containing a cellulosic material and water. Surfactant
is
added to the papermaking furnish. A wet strength resin is added to the
papermaking
furnish after addition of the surfactant. In a specific implementation, the
surfactant
includes a sulfonate, such as the surfactant Witco 5175-26A, sold by the Witco
Chemical Co. Following addition of the surfactant, the cellulosic material is
removed from the papermaking furnish to form a cellulosic web. A high
percentage
of the wet strength resin that was added to the papermaking furnish is
retained by the
cellulosic web. Finally, the cellulosic web is dried and formed into finished
products, such as hand towels. The resulting web has improved wet-tensile
strength
as well as an improved the wet/dry tensile ratio.
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The above summary of the present invention is not intended to describe each
discussed embodiment of the present invention. While the invention is
susceptible
to various modifications and alternative forms, specifics thereof will be
described in
detail. It should be understood, however, that the intention is not to limit
the
invention to particular embodiments described. On the contrary, the intention
is to
cover modifications, equivalents, and alternatives falling within the spirit
and scope
of the invention as defined by the appended claims.
Detailed Description
The present invention is directed to an improved method for making a
cellulosic web, as well as a web made using the improved method. The method
includes addition of surfactant and wet strength resin to a papermaking
furnish. The
resulting papermaking furnish is formed into a cellulosic web. This web is
subsequently dried in accordance with conventional paper making methods. The
finished web has improved strength properties, for example, improved wet-
tensile
strength and an improved wet-tensile strength to dry-tensile strength ratio
(wet/dry
tensile ratio).
The method includes providing a papermaking furnish containing a cellulosic
material and water. The cellulosic material is, for example, Northern Soft
Wood
Kraft (NSWK), chemi-thermo-mechanical pulp (CTMP), natural redwood (RW),
Mobile Pine (MP), or a recycled wood fiber, such as bleached recycled office
waste
fiber.
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Surfactant is added to the papermaking furnish and a wet strength resin is
added after the addition of the surfactant. When an appropriate surfactant is
added
to the papermaking furnish, the quantity of wet-strength resin added to the
furnish
may be greatly reduced without sacrificing wet-tensile strength. When even
very
low levels of surfactant are added, marked enhancements to the wet-tensile
strength
of the resulting web are observed. In some such implementations, the wet-
tensile
strength increases significantly while the dry-tensile strength decreases.
This
resulting cellulosic web shows improved softness and wet-strength properties.
While the present invention is not limited to a specific surfactant, one
appropriate surfactant is Witco 5175-26A, manufactured by the Witco Chemical
Co.
Witco 5175-26A includes a significant sulfonate component. Preferred
surfactants
include an anionic alkyl sulfate or anionic alkyl sulfonate component.
Acceptable
surfactants include Witco EP-5327-97 and EP-5293-150, also manufactured by the
Witco Chemical Co. In addition to the anionic alkyl sulfate or anionic alkyl
sulfonate component, the surfactant may include other wetting agents to
improve
water compatibility and absorbency on the fiber.
The quantity of surfactant added to the papermaking furnish is between 0.1
and 10.0 pounds per ton of cellulosic material. In specific implementations,
the
quantity is from 0.5 to 5.0 pounds per ton. In yet another implementation, the
quantity is between 0.5 and 2.5 pounds per ton of cellulosic material. The
quantity
is approximately 1.0 pounds per ton in specific implementations in which the
cellulosic material is primarily NSWK fibers.
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Following addition of the surfactant, a wet strength resin is added to the
papermaking furnish. The wet-strength resin provides a binding force securing
the
cellulosic web when wet. The wet-strength resin is preferably water soluble.
Suitable resins include polyamide-epichlorohydrin resins such as those sold by
Hercules Incorporated of Wilmington, Delaware, under the brand name Kymene
557H and Kymene 2064. These resins include low molecular weight polymers
having reactive functional groups, including epoxy and amino groups.
The wet-strength resin is added at a rate of between 5.0 and 30.0 pounds per
ton of cellulosic material. While the complete mechanism of the present
invention is
not fully understood, without limiting the invention, it is believed that the
addition
of the surfactant to the papermaking formulation provides greater absorbency
or
functionality of the wet-strength resin. The improved absorbency or
functionality of
the wet-strength resin allows for use of reduced wet-strength resin.
After addition of the surfactant and wet-strength resin, the cellulosic
material
is removed from the papermaking filrnish to form a cellulosic web. A high
percentage of the wet strength resin added to the papermaking fiunish is
retained by
the cellulosic material as the cellulosic web is formed. In specific
implementations,
at least 80 percent of the wet strength resin is secured to the cellulosic
web. The
cellulosic web is then dried and formed into finished products, such as hand
towels.
The resulting cellulosic web has improved wet-tensile strength as well as an
improved ratio of wet-tensile strength to dry-tensile strength . Referring now
to the
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tables produced below, example formulations of cellulosic webs constructed in
accordance with the present invention are shown.
Each table shows five different formulations of cellulosic webs. The
summary data shown reflects the averaged values for eight different samples of
each
formulation. In producing each formulation, a cellulosic fiber feed stock was
added
and mixed with water to form a papermaking formulation. A surfactant and wet
strength resin were sequentially added (in the orders, as indicated). A
cellulosic web
formed from the papermaking formulation was dried, and the resulting dried web
was tested for wet-tensile strength and dry-tensile strength according to
paper testing
methods on Instron and Twing-Albert tensile testing instruments.
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Table lA: Addition of Surfactant followed by Wet Strength
Resin to NSWK feed stock.
Formulation 1 2 3 4 5
Feed Stock NSWK NSWK NSWK NSWK NSWK
First Additive WSR Witco Witco Witco Witco
First Additive 10 1 3 5 IO
Dose (lbJton)
Second Additive --- WSR WSR WSR WSR
Second Additive ---- 10 10 10 10
Dose (Ib./ton)
Dry-tensile 18.65 17.67 17.42 18.69 14.74
Change Dry-tensile---- - 5.3 - 6.6 + 0.2 - 2l
.0
Wet-tensile 1.85 3.29 3.05 2.86 1.52
Change Wet-tensile-- + 77.8 + 64.9+ 54.6 - 17.8
Wet-tensile / 0.10 0.19 0.19 0.15 0.10
Dry-tensile
Ratio
Improvement in - 90 90 50 0
Wet-tensile
/
Dry-tensile Ratio
(percent)
NSWK = Northern Softwood Kraft
WSR = Wet strength resin
Witco = Surfactant containing sulfonate manufactured by Witco Chemical Co.,
sold
under the name Witco 5175-26A
First Additive Dose and Second Additive Dose are measured in pounds of
additive
to pound of feed stock.
Table 1 A shows that addition of a surfactant prior to addition of the wet-
strength resin results in an increase in wet-tensile strength and an increase
in the
wet/dry tensile ratio when applied to NSWK feed stock. Formulation 1, which is
the
control in which no surfactant was added, has a significantly lower wet/dry
tensile
ratio and lower wet-tensile strength than formulations 2, 3, 4, and 5, in
which the
surfactant Witco 5175-26A was added first.
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Table 1B: Addition of Wet Strength Resin followed
by Surfactant to NSWK Feedstock
Formulation 1 2 3 4 ~~S
Feed Stock NSWK NSWK NSWK NSWK NSWIC
First Additive WSR WSR WSR WSR WSR
First Additive IU - 10 10 !0 10
Dose (lb./ton)
Second Additive----- Witco Witco Witco Witco
Second Additive--- 1 3 5 10
Dosee (Ib./ton)
Dry-tensile 18.65 16.4 15.20 14.72 13.45
Change Dry-tensile---- - 12.1 - 18.5 - 21.1 - 27.9
Wet-tensile 1.85 1.52 1.32 1.27 1.24
Change Wet-tensile---- - 17.8 - 28.6 - 3l - 33.0
.4
Wet-tensile 0.10 0.09 0.08 0.09 0.09
/ Dry-tensile
Ratio
Improvement ---- -l0 -20 - 10 - 10
in Wet-tensile
/
Dry-tensile
Ratio (percent)
NSWK = Northern Softwood Kraft
WSR = Wet strength resin
Witco = Surfactant containing sulfonate manufactured by Witco Chemical Co.,
sold
under the name Witco 5175-26A
First Additive Dose and Second Additive Dose are measured in pounds of
additive
to pound of feed stock.
Table 1 B, when compared with Table 1 A, demonstrates the importance of
adding surfactant prior to adding the wet strength resin to NSWK feed stock.
In
Table 1B, the formulations have a first additive of the wet strength resin and
a
second additive of the Witco 5175-26A surfactant. Compared to formulations
from
Table 1 A using the same dosage and different order of addition, the
formulations in
Table 1 B have significantly lower wet-tensile strength and wet/dry tensile
ratios.
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Table 2A
Formula8on 1 2 3 4 5
Feed Stock NSWK NSWK NSWK NSWK NSWK
First Additive WSR Witco Witco Witco Witco
First Additive 20 I 3 5 10
Dose (IbJton)
Second Additive -- WSR WSR WSR WSR
Second Additive - 20 20 20 20
Dose (Ib./ton)
Dry-tensile 13.13 13.08 12.80 12.97 12.87
Change Dry-tensile---- - 0.4 - 2.5 - I - 2.0
.2
Wet-tensile 2.65 3.22 2.79 3.63 3.01
Change Wet-tensile--- 21.5 5.3 37.0 13.6
Wet-tensile / .20 .25 .28 .28 .23
Dry-tensile
Ratio
Improvement in ---- 25 40 40 I S
Wet-tensile
/
Dry-tensile Ratio
(percent)
NSWK =Northern Softwood Kraft
WSR = Wet strength resin
Witco = Surfactant containing sulfonate manufactured by Witco Chemical Co.,
sold
under the name Witco 5175-26A
First Additive Dose and Second Additive Dose are measured in pounds of
additive
to pound of feed stock.
5
Table 2A shows significant increase in the wet/dry tensile ratios in
formulations 2, 3, 4, and 5 compared to formulation 1. The difference in the
formulations is that a surfactant was not used in formulation 1, but was used
in
formulations 2, 3, 4, and S.
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Table 2B
Formulation 1 2 3 4 5
Feed Stock NSWK NSWK NSWK NSWK NSWK
First Additive WSR WSR WSR WSR WSR
First Additive 20 20 20 20 20
Dose (lb./ton)
Second Additive ---- Witco Witco Witco Witco
Second Additive -- 1 3 5 10
Dose (lb.lton)
Dry-tensile 13.13 13.67 12.71 13.13 11.42
Change Dry-tensile---- 4.1 - 3.2 0 - 13.0
Wet-tensile 2.65 3.02 2.84 3.33 2.40
Change Wet-tensile---- 44.2 7.2 25.7 - 9.4
Wet-tensile / .20 .22 .22 .25 .21
Dry-tensile Ratio
Improvement in --- 10 10 25 5
Wet-tensile /
Dry-tensile Ratio
(percent)
NSWK = Northern Softwood Kraft
WSR = Wet strength resin
Witco = Surfactant containing sulfonate manufactured by Witco Chemical Co.,
sold
under the name Witco 5175-26A
First Additive Dose and Second Additive Dose are measured in pounds of
additive
to pound of feed stock.
The formulations of Table 2B, compared to the formulations of Table 2A,
show that order of addition of the surfactant containing a sulfonate
influences the
wet/dry tensile ratios, and that the ratio is improved by adding the wet-
strength resin
after addition of the surfactant.
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Table 3A: Addition of Surfactant followed by Wet Strength
Resin to Bleached Recycled Office Waste Fiber Feed stock
Formulation I 2 3 4 5
Feed Stock RO RO RO RO RO
First Additive WSR Witco - WitccoWitco Witco
First Additive 20 1 3 5 10
Dose (Ib.hon)
Second Additive --- WSR WSR WSR WSR
Second Additive ---- 20 20 20 20
Dose (Ib./ton)
Dry-tensile 11.04 11.2 11.21 12.50 1 I
.49
Change Dry-tensile---- + 1.4 + 1.5 + ( + 4.1
3.2
Wet-tensile 1.20 1.74 1.15 1.39 I .40
Change Wet-tensile---- + 45 - 4.2 + ( + 16.7
5.8
Wet-tensile / 0.11 0.16 0.10 0.1 0.12
Dry-tensile I
Ratio
Improvement in -- 45 - 9 0 9
Wet-tensile
/
Dry-tensile Ratio
(Percent)
RO = Bleached recycled office waste fiber
WSR = Wet strength resin
Witco = Surfactant containing sulfonate manufactured by Witco Chemical Co.,
sold
under the name Witco 5175-26A
First Additive Dose and Second Additive Dose are measured in pounds of
additive
to pound of feed stock.
Tables 3A and 3B show that even very low levels of surfactant can
dramatically increase the wet-tensile strength of recycled office waste fiber
feed
stock.
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Table 3B: Addition of Wet Strength Resin followed by
Surfactant to Bleached Recycled Office Waste Fiber Feed Stock
Formulation 1 2 3 4 5
Feed Stock RO RO RO RO RO
First Additive WSR WSR WSR WSR WSR
First Additive 20 20 20 20 20
Dose {Ib./ton)
Second Additive ---- Witco Witco Witco Witco
Sccond Additive -- 1 3 5 10
Dose (Ib./ton)
Dry-tensile 11.04 11.75 11.55 11.95 11.28
Change Dry-tensile---- + 6.4 + 4.6 + 8.2 + 2.2
W et-tensile 1.20 1.78 1.16 1.35 1.14
Change Wet-tensile--- 48.8 - 3.3 12.5 - 5.0
Wet-tensile / 0.11 0.15 0.10 0.11 0.10
Dry-tensile
Ratio
Improvement in ---- 36 - 9 1 - 9
Wet-tensile
/
Dry-tensile Ratio
(Percent)
RO = bleached recycled office waste fiber
WSR = Wet strength resin
Witco = Surfactant containing sulfonate manufactured by Witco Chemical Co.,
sold
under the name Witco 5175-26A
First Additive Dose and Second Additive Dose are measured in pounds of
additive
to pound of feed stock.
While the present invention has been described with reference to several
particular implementations, those skilled in the art will recognize that many
changes
may be made hereto without departing form the spirit and scope of the present
invention.
