' CA 02717923 2010--08
SPECIFICATION
TISSUE PAPERS FOR HOUSEHOLD USE
TECHNICAL FIELD
The present invention relates to tissue papers for
household use such as tissue papers including toilet tissue
papers, facial tissues, etc., and paper towels and more
specifically it relates to soft and pleasant-to-touch
tissue papers for household use with high strength.
BACKGROUND ART
Slushed pulp in a slurry state as prepared from
chemical pulp obtained by cooking hardwood or softwood
chips, dry pulp obtained by dewatering and drying this
slushed pulp, or deinked recycled pulp obtained by deinking
waste paper has been conventionally used for so-called
tissue papers for household use such as toilet tissue
papers, tissue papers including facial tissues, paper
towels, etc., and these pulps are used as unbleached or
bleached pulps or unbeaten or beaten pulps alone or in
combination depending on the quality design.
Techniques for improving softness or hand feeling as
an important quality of tissue papers have been previously
studied, and various proposals have been made, including
e.g., layered tissue papers in which the types or the
proportions of the pulps layered and converted into paper
are controlled and processes for preparing them (patent
document 1, patent document 2); methods for improving hand
feeling and softness by appropriately selecting paper
machines such as Fourdrinier machines, paper machines
having a short forming section, twin-wire machines,
cylinder machines with a Yankee dryer, etc., or by adding
auxiliary chemicals to slushed pulp to improve the
lubricity of the slushed pulp per se, such as paper
softeners, e.g., fatty acid ester-based softeners (patent
document 3), quaternary ammonium salt-based cationic
surfactants (patent document 4), urethane alcohols or salts
or cationized products thereof (patent document 5),
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non-cationic surfactants (patent document 6, patent
document 7), polyphosphates (patent document 8),
polysiloxanes (patent document 9, patent document 10),
etc., or by once concentrating and then mechanically
kneading pulp to bend fibers (patent document 11, patent
document 12).
The use of the auxiliary chemicals sometimes produced
good softening effects, but had the disadvantage that their
high foamability might disturb the paper-making operation
per se and in some cases invite a decrease in paper
strength and water absorbency. The mechanical processes
for bending fibers were disadvantageous in energy
consumption because of additional steps of concentrating
raw materials.
In order to improve the wet strength of tissue
papers, wet paper strength agents such as polyamides,
polyamines and epoxy resins are mainly used, but they had
negative effects on softness and hand feeling because they
rigidify the tissue papers per se.
In order to improve surface smoothness, one or two
calender sets consisting of a pair of an upper and a lower
sufficiently polished chilled rolls and metal rolls have
been conventionally used downstream of tissue paper making
machines. However, this calender caused problems such as
decreased thickness, increased rigidity and hard texture
when the nip pressure was increased to improve smoothness.
References:
Patent document 1: JPA No. Sho 54-46914.
Patent document 2: JUA No. Hei 4-66992.
Patent document 3: US Patent No. 3,296,065.
Patent document 4: JPA No. Sho 48-22701.
Patent document 5: JPA No. Sho 60-139897.
Patent document 6: JPA No. Hei 2-99690.
Patent document 7: JPA No. Hei 2-99691.
Patent document 8: JPA No. Hei 2-36288.
Patent document 9: JPA No. Hei 2-224626.
Patent document 10: JPA No. Hei 3-900.
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Patent document 11: JPA No. Hei 5-23262.
Patent document 12: JPA No. Hei 6-14848.
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
The present invention aims to provide soft and pleasant-to-touch tissue papers
for
household use with high strength.
MEANS FOR SOLVING THE PROBLEMS
As a result of careful studies, we achieved the present invention on the basis
of
the finding that soft and pleasant-to-touch tissue papers for household use
with high
strength can be attained by including a pulp obtained by applying an impact
force
produced during the collapse of bubbles generated by cavitation to pulp
fibres.
Accordingly, in one aspect, the present invention resides in a tissue paper
for
household use, consisting of two or more paper layers, at least one paper
layer of which
contains a cavitation-treated pulp obtained by applying an impact force
produced during
the collapse of bubbles generated by cavitation to pulp fibers, wherein the
cavitation-
treated pulp has an assembly of microfibrils having a width of 3 p.m or more
on the
surface of the fiber, and wherein said at least one paper layer is dried using
a Yankee
dryer and said at least one paper layer is placed on the outside.
In yet another aspect, the present invention resides in a process for making a
tissue paper, comprising: preparing at least one paper layer containing a
cavitation-treated
pulp obtained by applying an impact force produced during the collapse of
bubbles
generated by cavitation to pulp fibers; and drying said at least one paper
layer containing
the cavitation-treated pulp using a Yankee dryer; and placing the dried layer
on the
outside of piles of tissue paper, wherein the cavitation-treated pulp has an
assembly of
microfibrils having a width of 3 pm or more on the surface of the fiber.
ADVANTAGES OF THE INVENTION
The present invention makes it possible to attain soft and pleasant-to-touch
tissue
papers for household use with high strength.
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CA 02717923 2016-08-04
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic diagram showing the cavitation jet treatment system
used
in the examples.
EXPLANATION OF THE REFERENCE NUMERALS
1: sample tank
2: nozzle
3: cavitation jet cell
4: plunger pump
5: upstream pressure regulating valve
6: downstream pressure regulating valve
7: upstream pressure meter
8: downstream pressure meter
9: water feed valve
10: circulating valve
11: drain valve
12: temperature sensor
13: mixer.
THE MOST PREFERRED EMBODIMENTS OF THE INVENTION
The tissue papers for household use of the present
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= CA 02717923 2010-09-08
invention are characterized in that they contain a pulp
obtained by applying an impact force produced during the
collapse of bubbles generated by cavitation to pulp fibers.
The application of an impact force produced during the
collapse of bubbles generated by cavitation to pulp fibers
will be hereinafter referred to as cavitation treatment.
Pulps that can be used as targets for the cavitation
treatment of the present invention are not specifically
limited, but include chemical pulps obtained by cooking a
lignocelluloses material with an alkaline cooking liquor
(such as bleached kraft pulp (NBKP) or unbleached kraft
pulp (NUKP) of softwoods, bleached kraft pulp of hardwoods
(LBKP)), mechanical pulps (such as groundwood pulp (GP),
refiner groundwood pulp (RGP), thermomechanical pulp (TMP),
chemithermomechanical pulp (CTMP), etc.), deinked pulp
(DIP), etc. Chemical pulps that can be used include kraft
pulp, polysulfide pulp, soda pulp, alkaline sulfite pulp,
sodium carbonate pulp, and oxygen-soda pulp, etc.
The chemical pulps may also include those obtained by
adding a cyclic keto compound (e.g., anthraquinone, 1,4-
dihydro-9,10-diketoanthracene, etc.) to the cooking liquor.
Among the chemical pulps, kraft pulp is preferred for the
present invention, which may be obtained by the so-called
modified alkaline cooking process comprising adding the
cooking liquor in portions and concurrent cooking and
countercurrent cooking within a digester. These chemical
pulps can be used as unbleached or bleached pulps or
unbeaten or beaten pulps alone or in combination, as
appropriate.
The cavitation treatment is more specifically defined
as a process for promoting external fibrillation of pulp by
applying an impact force produced during the collapse of
bubbles generated by cavitation to pulp fibers while
suppressing internal fibrillation to adjust freeness. The
cavitation treatment is preferably a cavitation jet
treatment as described in W02005/012632.
It should be noted that pulp fibers can also be
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externally fibrillated by combining the cavitation
treatment with mechanical beating. The pulp suspension may
contain inorganic fine particles from fillers or pigments
contained in waste paper or broke in addition to pulp
fibers. The cavitation treatment affords a bulkier and
stronger pulp as compared with those conventionally
obtained by beating with a mechanical force using a refiner
or the like at the same freeness because external
fibrillation of pulp fibers is promoted while internal
fibrillation is suppressed. Tissue papers for household
use containing a pulp having external fibrils obtained by
this cavitation treatment are softer and stronger.
The Canadian Standard Freeness of the pulp prepared
by the cavitation treatment is preferably 50-650 ml in the
case of chemical pulps and 50-400 ml in the case of
mechanical pulps or recycled (deinked) pulp. In the case
of mixtures of these pulps, the total Canadian Standard
Freeness is preferably 100-550 ml.
Next, the cavitation treatment is explained in
detail. When cavitation bubbles collapse, a high impact
force reaching several Giga Pascal is produced in a local
region on the order of several micrometers and the
temperature microscopically rises to several thousand
degrees Celsius due to adiabatic compression during the
collapse of bubbles, as described in a book of Katoh (New
Edition Cavitation: Basics and Recent Advance, Written and
Edited by Yoji Katoh, Published by Makishoten, 1999). As a
result, the temperature rises when cavitation occurs. For
these reasons, cavitation has harmful influences on fluid
machinery such as damage, oscillation and performance loss
and it has been a technical challenge to control
cavitation. Recently, rapid advances in studies on
cavitation made it possible to precisely control the region
in which cavitation occurs and even the impact force by
using hydrodynamic parameters of cavitation jet as
operation factors. This led to expectations that the
strong energy of bubbles could be effectively utilized by
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controlling the impact force induced by collapse of
bubbles. Thus, it became possible to precisely control
cavitation by operation/adjustment based on hydrodynamic
parameters. This shows that the stability of technical
effects can be maintained, and the present invention is
characterized in that bubbles generated by controlled
cavitation are actively introduced into a pulp suspension
to effectively utilize their energy rather than the
conventional uncontrollably harmful cavitation
spontaneously occurring in fluid machinery.
Means for generating cavitation in the present
invention include, but not limited to, a liquid jet, an
ultrasonic transducer, a combination of an ultrasonic
transducer and a horn amplifier, and laser irradiation. It
is preferable to use a liquid jet, which is more effective
for pulp fibers because it efficiently generates cavitation
bubbles and forms cavitation bubble clouds having a
stronger impact force of collapse. The cavitation
generated by the means described above is clearly different
from the conventional uncontrollably harmful cavitation
spontaneously occurring in fluid machinery.
As noted above, the cavitation treatment is
preferably a cavitation jet treatment using a liquid jet as
described in W02005/012632 and as will be explained in
detail below.
The liquid jet refers to a jet of a liquid or a fluid
containing solid particles or a gas dispersed or mixed in a
liquid, including a liquid jet containing a slurry of pulp
or inorganic particles and bubbles. The gas here may
include bubbles generated by cavitation.
Flow rates and pressures are especially important for
cavitation because it occurs when a liquid is accelerated
and a local pressure decreases below the vapor pressure of
the liquid. Therefore, the cavitation number 0, which is a
basic dimensionless number expressing a cavitation state.
is defined as follows (New Edition Cavitation: Basics and
Recent Advance, Written and Edited by Yoji Katoh, Published
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by Makishoten, 1999).
P. Pp
a = -
1
¨ pU.
2
where p-: local pressure (absolute pressure), U-:
characteristic flow velocity, pv: vapor pressure of fluid
(absolute pressure), p: density.
If the cavitation number here is high, it means that
the flow site is in a state where cavitation occurs hard.
Especially when cavitation is generated through a nozzle or
an orifice tube as in the case of a cavitation jet, the
cavitation number o can be rewritten by equation (2) below
where RI: nozzle upstream pressure (absolute pressure), 132:
nozzle downstream pressure (absolute pressure), pv: vapor
pressure of fluid (absolute pressure), and the cavitation
number o can be approximated as follows in the case of a
cavitation jet because the pressure difference between pi,
P2 andPv is significant so that p1 p2 p, (H. Soyama, J.
Soc. Mat. Sci. Japan, 47 (4), 381 1998).
P2-Pv P2
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191 P2 191
In this manner, the cavitation number cr is expressed
by two values, i.e., the pressures upstream and downstream
of the nozzle. It should be noted that all of the
pressures measured in the examples herein are gauge
pressures, and the cavitation number a in the present
invention is expressed by equation (3) below:
0 - P4/113
where p3: nozzle upstream pressure (gauge pressure),
p4: nozzle downstream pressure (gauge pressure).
Cavitation conditions in the present invention are as
follow: the cavitation number o defined above is desirably
0.001 or more and 0.5 or less, preferably 0.003 or more and
0.2 or less, especially 0.01 or more and 0.1 or less. If
the cavitation number o is less than 0.001, little benefit
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is attained because the pressure difference against the
surroundings is small when cavitation bubbles collapse, but
if it is greater than 0.5, the pressure difference in the
flow is too small to generate cavitation.
When a jetting liquid is emitted through a nozzle or
an orifice tube to generate cavitation, the pressure of the
jetting liquid (nozzle upstream pressure) is desirably
0.01 MPa (gauge pressure) or more and 60 MPa (gauge
pressure) or less, preferably 0.7 MPa (gauge pressure) or
more and 30 MPa (gauge pressure) or less, especially 2 MPa
(gauge pressure) or more and 15 MPa (gauge pressure) or
less. If the nozzle upstream pressure is less than
0.01 MPa (gauge pressure), little benefit is attained
because a pressure difference is less likely occur against
the nozzle downstream pressure. If the nozzle upstream
pressure is greater than 60 MPa (gauge pressure), special
pumps and pressure vessels are required and energy
consumption increases, leading to cost disadvantages, but
also pulp fibers are excessively damaged so that they
become unsuitable for use as raw materials for papermaking.
On the other hand, the pressure in the vessel (nozzle
downstream pressure) is preferably 0.05 MPa (gauge
pressure) or more and 2.6 MPa (gauge pressure) or less
expressed in static pressure. A pressure is also applied
on the downstream side to increase the pressure in the
region where cavitation bubbles collapse by pressurizing
the vessel containing a target liquid (pulp suspension),
resulting in an increase in the pressure difference between
bubbles and the surroundings, whereby bubbles more
vigorously collapse to produce a stronger impact force.
However, cavitation per se hardly occurs if the pressure in
the vessel excessively increases. The ratio between the
pressure in the vessel and the pressure of the jetting
liquid ((gauge pressure)/ (gauge pressure)) is preferably
in the range of 0.001-0.5.
The jet flow rate of the jetting liquid is desirably
in the range of 1 m/sec or more and 200 m/sec or less,
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preferably in the range of 20 m/sec or more and 100 m/sec
or less. If the jet flow rate is less than 1 m/sec, little
benefit is attained because the pressure drop is too small
to generate cavitation. If it is greater than 200 m/sec,
however, cost disadvantages occur because high pressure is
required and therefore, a special equipment is required.
The cavitation treatment in the present invention
takes place at a site that can be selected from, but not
limited to, the inside of any vessel such as a tank or the
inside of a pipe. The treatment can be performed in one
pass, but a greater effect can be obtained by repeating a
necessary number of cycles. The treatment can be performed
in parallel or in series using multiple generating means.
A jet for generating cavitation may be injected into
a vessel open to the atmosphere such as a pulper, but
preferably within a pressure vessel to control cavitation.
In the method for generating cavitation by a liquid
jet in the present invention, liquids that can be jetted to
a pulp suspension include, but not limited to, e.g.,
distilled water, tap water, industrial water, recycled
water recovered from papermaking processes, pulp drain
water, white water, pulp suspensions, alcohols, etc.
Preferably, a pulp suspension per se is jetted to provide a
greater benefit because not only cavitation is generated
around the jet but also a hydrodynamic shear force is
obtained when the jet is emitted from an orifice at a high
pressure. When a pulp suspension is used as a jet liquid,
the total amount to be treated can be circulated.
When a pulp suspension is treated by cavitation
generated by a liquid jet, the solids content of the
suspension is preferably 5 % by weight or less, more
preferably 3 % by weight or less, still more preferably
0.1-1.5 % by weight in terms of the bubble generating
efficiency..
The pH of the pulp suspension is preferably pH 1-13,
more preferably pH 3-12, still more preferably pH 4-11. If
the pH is less than 1, problems such as corrosion of
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equipments occur, which are disadvantageous in terms of
materials and maintenance or the like. If the pH exceeds
13, however, alkaline discoloration of pulp fibers occurs
to unfavorably lower brightness. Alkaline conditions are
more desirable because pulp fibers are highly swollen and
more OH active radicals are produced.
According to the present invention, the flow rate of
the jetting liquid increases by increasing the jetting
pressure of the liquid to generate stronger cavitation.
Moreover, the vessel containing a target liquid is
pressurized to increase the pressure in the region where
cavitation bubbles collapse, resulting in an increase in
the pressure difference between bubbles and the
surroundings, whereby bubbles vigorously collapse to
produce a stronger impact force. Here, the jetting liquid
refers to a liquid emitted from an orifice at a high
pressure, and the target liquid refers to a liquid exposed
to a jet within a vessel or a pipe. Cavitation is
influenced by the amount of gas in the liquid, and if the
gas is excessive, bubbles collide with each other and join
together to create a cushioning effect so that the impact
force of collapse is absorbed by other bubbles and the
impact force decreases. Thus, the process temperature must
be the melting point or more and the boiling point or less
in view of the influence of dissolved gas and vapor
pressure. When the liquid medium is water, significant
effects can be obtained at a temperature of preferably
0-80 C, more preferably 10 C - 60 C. Considering that the
impact force is normally maximal at the midpoint between
the melting point and the boiling point, temperatures
around 50 C are most preferred in the case of aqueous
solutions, though significant effects can be obtained even
at lower temperatures within the range defined above
because there is no influence of vapor pressure.
Temperatures exceeding 80 C are unsuitable because the
pressure resistance of the pressure vessel for generating
cavitation considerably decreases so that the vessel may be
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liable to damages.
According to the present invention, the energy
required for generating cavitation can be reduced by adding
a material that lowers the surface tension of a liquid,
such as a surfactant. Materials that are added include,
but not limited to, known or novel surfactants, e.g.,
nonionic surfactants, anionic surfactants, cationic
surfactants and ampholytic surfactants such as fatty acid
salts, higher alkyl sulfates, alkyl benzene sulfonates,
higher alcohols, alkyl phenols, alkylene oxide adducts of
fatty acids or the like, or organic solvents, etc. These
may be added as single components or mixtures of two or
more components. They may be added in any amount necessary
for lowering the surface tension of the jetting liquid
and/or target liquid. They may be added at any site in a
process preceding the site where cavitation is generated,
and when the liquid is circulated, they may be added even
after the site where cavitation is generated.
According to the present invention, the proportion of
the cavitation-treated pulp to the total amount of pulp is
not specifically limited, but stronger and softer tissue
papers for household use can be obtained as the proportion
increases. From this point of view, the proportion is
preferably 5 % by weight or more, more preferably 30 to
100 % by weight, still more preferably 60 to 100 t by
weight based on the bone dry weight of the total pulp. If
the proportion is less than 5 % by weight, the softness and
hand feeling remain unchanged and the strength cannot be
improved.
The tissue papers for household use consist of a
single layer or multiple layers, and the single layer or
multiple layers may be prepared from a single cavitation-
treated pulp or a mixture of two or more cavitation-treated
pulps or a mixture of a cavitation-treated pulp and a
conventional slushed pulp, dry pulp or deinked pulp (DIP).
When two plies of a tissue paper for household use
consisting of multiple layers are layered, hand feeling is
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further improved if a layer containing a cavitation-treated
pulp faces outward so that the layer containing a
cavitation-treated pulp contacts hands. Hand feeling is
further improved by drying the layer containing a
cavitation-treated pulp against a Yankee dryer and placing
this face on the outside of two plies of a tissue paper for
household use.
In addition to the cavitation-treated pulp, chemical
pulps (bleached kraft pulp (NBKP) or unbleached kraft pulp
(NUKP) of softwoods, bleached kraft pulp (LBKP) or
unbleached kraft pulp (LUKP) of hardwoods, etc.),
mechanical pulps (groundwood pulp (GP), refiner groundwood
pulp (RGP), thermomechanical pulp (TMP),
chemithermomechanical pulp (CTMP), etc.), and deinked pulp
(DIP) may be used as a mixture at any ratio.
The cavitation-treated pulp shows improved external
fibrillation, and sometimes has scale-like external fibrils,
as explained below.
According to a book of Isogai et al. (Akira Isogai:
"Materials Chemistry of Cellulose", The University of Tokyo
Press, p. 68, 2001), beating of pulp refers to a process in
which a mechanical shear stress is applied to hydrated pulp
fibers to form gaps between microfibrils within the pulp
fibers (internal fibrillation) and to raise fibrils on the
outer sides of the pulp fibers (external fibrillation),
thereby increasing the specific surface area to improve
swelling of the pulp fibers with water, and at the same
time, partially cutting the fibers and generating fine
fibers flaked off the outer peripheral faces of the fibers.
The beating process of pulp increases the bonding
area between fibers formed during papermaking, thereby
causing changes in various mechanical properties, optical
properties and liquid absorption. However, when pulp
fibers are observed at the molecular level, the molecular
weight of cellulose decreases only slightly and the
crystallinity is almost unchanged during the beating
process. This is attributed to the fact that amorphous and
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hydrophilic hemicellulose moieties serve as a cushion to
absorb mechanical energy.
According to a book of Shimaji et al. (Ken Shimaji et
al.: "Wood Tissue", Morikita Publishing, p. 55, 1976),
external fibrils seen in wood pulp beaten by conventional
methods refer to filamentous structures having a width of
about 0.4 - 1 pm observable by light microscopy, while
microfibrils are elemental structural units present in cell
walls as an assembly of cellulose molecules having a width
of about 9 - 37 nm. In the pulps having scale-like
external fibrils used in the present invention, the
characteristic scale-like external fibrils refer to flakes
or hairs on the surface of a fiber having a width of 3 pm
or more, preferably similar to the width of the pulp fiber
and consisting of a wide layer formed of an assembly of the
microfibrils aligned side by side, i.e., the microfibrils
on the surface of the fiber wall are flaked while retaining
a layer structure. They are also characterized by a
thickness ranging from 90 nm to 2 pm. When a fiber is
observed by electron microscopy, it is desirably observed
in the dry state eliminating hydrogen bonding, but it is
difficult to observe external fibrils with high precision
because such fibrils would be attracted to the surface of
the fiber by capillarity so that they would be difficult to
discern if the fiber was simply dried.
The scale-like external fibrils in the present
invention are characterized in that they are stained by a
high molecular dye having a molecular weight of 10,000 or
more. Thus, the external fibrils refer to an assembly of
microfibrils to which a high molecular dye having a
molecular weight of 10,000 or more can be adsorbed. Dyes
having a molecular weight of 10,000 or more include orange
dyes such as CI Constitution nos. 40000 to 40006 including
Direct Orange 15 (old Color Index (CI) no. 621, or CI
Constitution no. 40002/3) as described in a literature of
Simon et al. (F.L. Simons, Tappi Journal, 33 (7), 312
(1950)) and a literature of Xiaochun et al. (Y. Xiaochun et
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al., Tappi Journal, 78 (6), 175 (1995)), but they are not
specifically limited so far as they can stain cellulose-
based fibers.
According to the literature of Xiaochun et al., the
dyes having a molecular weight of 10,000 or more described
above are molecules having a hydrodynamic size of 5 nm or
more as measured by light scattering and cannot permeate
into pores of less than 5 nm present on the surfaces of
pulp fibers. However, the dyes having a molecular weight
of 10,000 or more described above can readily access and
selectively stain fibrillated regions by adsorbing to them
because fibrils consisting of an assembly of microfibrils
on the surfaces of pulp fibers are exposed outside the pulp
fibers.
In order to optically highlight fibrillated regions,
they can be observed with enhanced contrast by staining the
entire fiber using a low molecular dye such as Direct Blue
1 (old Color Index (CI) no. 518, or CI Constitution no.
24410) or Direct Blue 4, Direct Blue 15, Direct Blue 22,
Direct Blue 151 or the like as described in the literatures
above. The low molecular dye is adsorbed to the entire
fiber, but displaced by a high molecular dye having a
higher bonding force. As a result, the fibrillated regions
to which the high molecular dye (orange dye) can be
adsorbed can be stained in orange while fiber pore regions
to which the high molecular dye cannot be adsorbed can be
stained with the low molecular dye (blue dye), whereby the
fibrillated regions can be highlighted. Suitable
low-molecular dyes contain 51 % or more of molecules having
a molecular weight of less than 10,000, preferably less
than 2000, more preferably 300-1500.
In a single unit of the pulps having scale-like
external fibrils of the present invention, the area ratio
of the externally fibrillated part expressed by equation 4
below is preferably 20 % or more and the peripheral length
index of the externally fibrillated part expressed by
equation 5 below is 1.5 or more. In the pulps of the
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present invention, these values increase because the
scale-like external fibrils have a greater surface area as
compared with conventional fibrils.
Area ratio of externally fibrillated part (%) = [(area
of externally fibrillated part) / (area of externally
fibrillated part + total surface area of pulp fiber)] x 100
(equation 4).
Peripheral length index of externally fibrillated part
= (peripheral length of externally fibrillated part +
entire peripheral length of fiber) / (entire peripheral
length of pulp fiber) (equation 5)
MECHANISM
The reason why tissue papers containing cavitation-
treated pulp show good hand feeling and strength is assumed
as follows.
Generally, tissue papers having a high bulk (low
density) and a smooth surface show improved hand feeling.
As noted above, cavitation-treated pulp has been
specifically promoted in external fibrillation. Thus,
external fibrillation has been promoted while retaining
fiber rigidity as described in W02006/085598, so that the
strength increases at a similar bulk or the bulk increases
when the pulp is prepared at a similar strength as compared
with conventional mechanical treatments such as double disc
refiners.
Moreover, papers prepared from cavitation-treated
pulp tend to be smoother because smooth surfaces such as
metal rolls are readily transferred.
For these reasons, tissue papers containing
cavitation-treated pulp are assumed to tend to show good
hand feeling and strength at the same time.
EXAMPLES
The following examples and comparative examples
further illustrate the present invention without, however,
limiting the invention thereto. Unless otherwise
specified, t in the examples and comparative examples
refers to % by weight. The pulps prepared in the following
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examples and comparative examples were converted into
papers in a twin-wire triple layer paper machine with a
Yankee dryer. The web was dry-creped by driving the dryer
and the winder reel at different speeds. Unless otherwise
specified, two plies of this raw tissue paper were layered
in such a manner that the face having contacted the Yankee
dryer (YD face) might form the outside (i.e., the side
contacting users' hands) and the face dried against the
Yankee dryer was soft-calendered. The pulps used in all of
the examples and comparative examples were bleached
hardwood pulps prepared by the kraft process from hardwood
chips produced in Japan and bleached to a Hunter brightness
of 84 %.
The evaluation methods used in the examples and
comparative examples are as follows.
<Hand feeling>
Feel against hands and skin was evaluated by ten
panelists. The results were expressed as follows. C): very
good, 10: good, A: fair, X: poor.
<Determination method of (longitudinal) tensile strength>
Tensile strength: Samples of 15 mm in MD and CD
directions were cut and the tensile strength of a single
ply in each direction was measured to calculate the total
tensile strength by the equation below.
Tensile strength (g) = (tensile strength in MD x
tensile strength in CD)112.
<Determination method of bulk>
Bulk was expressed as the thickness (mm) of 10 plies
of each sample.
<Determination of basis weight>
Basis weight was determined according to JIS P 8124:
1998 (ISO 536: 1995).
<Preparation of cavitation-treated pulp>
Cavitation treatment was performed in the cavitation
jet treatment system shown in Figure 1. In Figure 1, a
pulp suspension (consistency 1.1 %) not shown is contained
in a sample tank 1, and a temperature sensor 12 and a mixer
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CA 02717923 200--09-08
13 are inserted into the sample tank 1. The pulp
suspension in the sample tank 1 is introduced as a jetting
liquid into a cavitation jet cell 3 via a given line
provided with a plunger pump 4. A nozzle 2 is provided at
the bottom of the cavitation jet cell 3, and more
specifically, the pulp suspension in the sample tank 1 is
injected from the nozzle 2 into the jet cell 3. On a line
extending from the periphery of the sample tank 1 to the
jet cell 3 are provided a water feed valve 9 and a
circulating valve 10, through which the pulp suspension in
the sample tank 1 is supplied as a target liquid into the
jet cell 3. On another line extending from the periphery
of the sample tank 1 to the nozzle 2 is provided an
upstream pressure regulating valve 5. On another line
extending from the top of the jet cell 3 to the sample tank
1 is provided a downstream pressure regulating valve 6 so
that the jetting pressure of the pulp suspension into the
nozzle 2 can be controlled by adjusting each valve 5, 6.
An upstream pressure meter 7 is provided at the inlet of
the nozzle 2, and a downstream pressure meter 8 is provided
at the top of the jet cell 3. A drain valve 11 is provided
at the bottom of the jet cell 3.
EXAMPLE 1
The raw material for the top and bottom layers of
triple layers was prepared as follows. A pulp sheet of a
bleached hardwood kraft pulp was disintegrated in a
low-consistency pulper and adjusted to a desired
consistency, and then treated in one pass by using the
cavitation jet treatment system (nozzle diameter 1.5 mm)
shown in Figure 1 at a jetting liquid pressure (nozzle
upstream pressure) of 8 MPa (gauge pressure, jet flow rate
80 m/sec.) and a pressure in the target vessel (nozzle
downstream pressure) of 0.4 MPa (gauge pressure). A pulp
suspension having a consistency of 3 % by weight was used
as a jetting liquid to treat the pulp suspension
(consistency 3 % by weight) in the vessel by cavitation,
thus giving raw material A having a Canadian Standard
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CA 02717923 2010-09-08
Freeness of 435 mL. A facial tissue was prepared from raw
material A and raw material B for the middle layer
(prepared by disintegrating a pulp sheet of a bleached
hardwood kraft pulp in a low-consistency pulper and having
a Canadian Standard Freeness of 500 mL). The basis weight
of the resulting facial tissue was adjusted to 16.6 g/m2.
COMPARATIVE EXAMPLE 1
The raw material for the top and bottom layers was
prepared by beating in a double disc refiner instead of the
cavitation treatment to give raw material C having a
Canadian Standard Freeness of 470 mL. A facial tissue was
prepared from raw material C and raw material B for the
middle layer. The basis weight of the resulting facial
tissue was adjusted to 16.6 g/m2.
Table 1
Example 1 Comparative example 1
Hand feeling A A
Bulk (mm/10 plies) 0.88 0.91
Tensile strength (g) 263 214
As shown in Table 1, Example 1 and Comparative
example 1 were comparable in hand feeling, but the tensile
strength of Example 1 was higher by about 20 % than that of
Comparative example 1. Example 1 remarkably increased in
strength over Comparative example 1 despite of a slight
decrease in bulk.
EXAMPLE 2
The raw material for the top and bottom layers was
prepared by the same treatment as in Example 1 to give raw
material D having a Canadian Standard Freeness of 420 mL.
A facial tissue was prepared from raw material D and raw
material B for the middle layer by adding 0.1 % (based on
the bone dry weight of the pulp) of a wet paper strength
agent. The basis weight of the resulting facial tissue was
adjusted to 13.0 g/m2.
COMPARATIVE EXAMPLE 2
The raw material for the top and bottom layers was
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CA 02717923 2010-09-08
prepared by the same treatment as in Comparative Example 1
to give raw material E having a Canadian Standard Freeness
of 410 mL. A facial tissue was prepared from raw material
D and raw material B for the middle layer by adding 0.1 %
(based on the bone dry weight of the pulp) of a wet paper
strength agent. The basis weight of the resulting facial
tissue was adjusted to 13.0 g/m2.
Table 2
Example 2 Comparative example 2
Hand feeling 0 0
Bulk (mm/10 plies) 0.78 0.73
Tensile strength (g) 155 160
As shown in Table 2, Example 2 showed better hand
feeling than that of Comparative example 2 and a nearly
comparable tensile strength. Moreover, Example 2 improved
in bulk by about 7 % over Comparative example 2.
EXAMPLE 3
The raw material for the top and bottom layers was
prepared by the same treatment as in Example 1 except that
2 pass treatment was performed by using the cavitation jet
treatment system (nozzle diameter 1.5 mm) at a jetting
liquid pressure (nozzle upstream pressure) of 8 MPa (gauge
pressure, jet flow rate 80 m/sec.) and a pressure in the
target vessel (nozzle downstream pressure) of 0.4 MPa
(gauge pressure) to give raw material F having a Canadian
Standard Freeness of 390 mL. A facial tissue was prepared
from raw material F and raw material B for the middle layer
by adding 0.1 % (based on the bone dry weight of the pulp)
of a wet paper strength agent. The basis weight of the
resulting facial tissue was adjusted to 15.1 g/m2.
COMPARATIVE EXAMPLE 3
The raw material for the top and bottom layers was
prepared by the same treatment as in Comparative Example 1
to give raw material G having a Canadian Standard Freeness
of 470 mL. A facial tissue was prepared from raw material
G and raw material B for the middle layer by adding 0.1 %
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CA 02717923 2010-09-08
(based on the bone dry weight of the pulp) of a wet paper
strength agent. The basis weight of the resulting facial
tissue was adjusted to 15.1 g/m2.
Table 3
Example 3 Comparative example 3
Hand feeling 0 0
Bulk (mm/10 plies) 0.85 0.87
Tensile strength (g) 205 174
As shown in Table 3, Example 3 showed better hand
feeling and a tensile strength higher by about 20 % as
compared with Comparative example 3. Example 3 remarkably
increased in strength over Comparative example 3 despite of
a slight decrease in bulk.
EXAMPLE 4
The raw material for the top and bottom layers was
prepared by the same treatment as in Example 1 except that
a bleached hardwood kraft pulp sheet tending to favor
strength but compromise hand feeling (a pulp sheet having a
moisture content of about 50 % by weight (based on the bone
dry weight of the pulp)) was used to give raw material H
having a Canadian Standard Freeness of 440 mL. A facial
tissue was prepared from raw material H and raw material B
for the middle layer by adding 0.1 % (based on the bone dry
weight of the pulp) of a wet paper strength agent. The
basis weight of the resulting facial tissue was adjusted to
15.1 g/m2.
COMPARATIVE EXAMPLE 4
The raw material for the top and bottom layers was
prepared by the same treatment as in Comparative Example 1
except that a bleached hardwood kraft pulp sheet tending to
favor hand feeling (a pulp sheet having a moisture content
of about 10 % by weight (based on the bone dry weight of
the pulp)) was used to give raw material I having a
Canadian Standard Freeness of 480 mL. A facial tissue was
prepared from raw material I and raw material B for the
middle layer by adding 0.1 % (based on the bone dry weight
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= CA 02717923 2010-09-08
of the pulp) of a wet paper strength agent. The basis
weight of the resulting facial tissue was adjusted to
15.1 g/m2.
Table 4
Example 4 Comparative example 4
Hand feeling 0 0
Bulk (mm/10 plies) 0.79 0.84
Tensile strength (g) 247 177
As shown in Table 4, Example 4 using a bleached
hardwood kraft pulp tending to compromise hand feeling was
comparable in hand feeling as compared with even
Comparative example 4 using a bleached hardwood kraft pulp
sheet tending to favor hand feeling. Moreover, Example 4
improved in tensile strength by about 40 % over Comparative
example 4.
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