Note: Descriptions are shown in the official language in which they were submitted.
CA 02206750 1997-OS-29
1
PAPERMAKING BELT HAVING SEMICONTINUOUS PATTERN
AND PAPER MADE THEREON
FIELD OF-THE INVENTION
The present invention relates to belts used for making cellulosic
fibrous structures, such as paper. Particularly this invention relates
to a belt used in a through-air drying process for making cellulosic
fibrous structures, and more particularly to a belt having a particular
pattern thereon which imparts properties to the paper in a like pattern.
BACKGROUND OF THE INVENTION
Cellulosic fibrous structures, such as paper, are well known in the
art. For example, cellulosic fibrous structures are a staple of every
day life and are found in facial tissues, toilet tissue, and paper
toweling.
One advancement in the art of cellulosic fibrous structures is
cellulosic fibrous structures having multiple regions. A cellulosic
fibrous structure is considered to have multiple regions when one region
of the cellulosic fibrous structure -differs in either basis weight,
density, or both from another region of the cellulosic fibrous structure.
Multiple regions within a cellulosic fibrous structure can provide
several advantages, such as economization of mater-ials, increasing
certain desirable properties and decreasing certain undesirable
properties. However, the apparatus used to manufacture the multiple
region cellulosic fibrous structure will greatly influence these
properties.
Specifically a secondary belt, or comparable other apparatus, can
affect the properties imparted to the cellulosic fibrous structure. As
used herein, a "secondary apparatus" or a "secondary belt" refers to an
apparatus or a belt, respectively, fiaving an embryonic web contacting
surface and which is used to carry or otherwise process an embryonic web
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of cellulosic fibers after initial formation in the wet end of the
papermaking machinery. A secondary belt may include, without limitation,
a belt used for molding an embryonic web of the cellulosic fibrous
structure, a through-air drying belt, a belt used to transfer the
embryonic web to another component in the papermaking machinery, or a
backing wire used in the wet end of the papermaking machinery (such as a
twin-wire former) for purposes other than initial formation. An
apparatus or belt according to the present invention does not include
embossing rolls, which deform dry fibers after fiber-to-fiber bonding. has
taken place. Of course, a cellulosic fibrous structure according to the
present invention may be later embossed, or may remain unembossed.
As an example of how a secondary belt may input specific properties
to a cellulosic fibrous structure, a wet molded and through-air dried
cellulosic fibrous structure made on a secondary belt according to Figure
4 of commonly assigned U.S. Patent 4,514,345 issued April 30, 1985 to
Johnson, et al. may experience less curling at the edges than a
cellulosic fibrous structure made on a secondary- belt according to
commonly assigned U.S. Patent 4,528,239 issued July 9, 1985 to Trokhan.
Conversely, a cellulosic fibrous structure made on a secondary belt
according to the aforementioned Trokhan patent may have a greater burst
strength than a cellulosic fibrous structure made on a secondary belt
according to Figure 4 of the aforementioned Johnson, et al. patent.
This difference in performance relative to properties such as
absorbency and burst strength may be attributed to the pattern of the
drying belt used in wet molding and the through-air drying process to
make the respective cellulosic fibrous structures. A cellulosic fibrous
structure made on a secondary belt according to Figure 4 of the afore-
mentioned Johnson, et al. patent will have discrete high density regions
and essentially continuous low density regions. Conversely, a cellulosic
fibrous structure made on a secondary belt according to the afore-
mentioned Trokhan patent will have continuous high density regions and
discrete low density regions. This difference in the pattern of the
regions influences other properties of the respective cellulosic fibrous
structures as well.
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For example, a cellulosic fibrous structure made on a belt according
to the aforementioned Trokhan patent may have a lower cross machine
direction modulus of elasticity and may have greater cross machine
direction extensibility than a cellulosic fibrous structure made on a
belt according to the aforementioned Johnson, et al. patent. However,
these properties are typically offset by less sheet shrinkage-and edge
curling in a cellulosic fibrous structure made on a belt according to the
aforementioned Johnson, et al. patent.
The caliper of certain cellulosic fibrous structures is closely
related to the crepe pattern caused by the impact angle of the doctor
blade. The doctor blade is used to remove the cellulosic fibrous
structure from the surface of a heated Yankee drying drum and to crepe
the cellulosic fibrous structure by foreshortening it in the machine
direction. However, maintaining-constant material properties (such as
machine direction extensibility), which properties are influenced try the
doctor blade is difficult. This difficulty is encountered because the
doctor blade wears over time. Such wear is rarely constant over time.,
due to the taper of the blade and the stiffness of the blade changing as
a third order power when wear occurs. Furthermore, the wear and changes
which occur on one papermaking machine utiliting a particular doctor
blade are often totally different than the wear and changes which occur
on another papermaking machine using an identical doctor blade.
As the doctor blade wears, and the impact angle between the doctor
blade and the Yankee drying drum becomes smaller, the cellulosic fibrous
structure typically becomes softer, but loses tensile strength. Also, as
the impact angle becomes smaller due to wear, the cellulosic fibrous
structure may have greater caliper. Conversely, as the impact angle
between the doctor blade and the surface of the Yankee drying drum
becomes greater, such as occurs when the bevel angle of the doctor blade
is increased, the doctor blade will typically wear at a faster rate.
But, the situation is even more complicated than described above.
Not all secondary belts produce cellulosic fibrous structures which
respond alike to changes in the impact angle of the doctor blade. For
example, a cellulosic fibrous structure through air dried on a belt made
generally in accordance with the teachings of commonly assigned U.S.
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Patent 3,301,746 issued January 31, 1967 to Sanford, et al. shows an
increase in caliper as the doctor blade impact angle is decreased.
However, the caliper generated on a cellulosic fibrous structure made on
a secondary belt according to the aforementioned Sanford, et al. patent
is not as great as the caliper of a like celluiosi.c fibrous structure
made on a secondary belt according to the aforementioned Trokhan patent.
But a disadvantage to the aforementioned Trokhan patent is that a
cellulosic fibrous structure made thereon does not show a correlation to
the doctor blade impact angle. Thus, one skilled in the art is forced to
select between greater caliper generation and control of the caliper (and
other properties) by adjusting the doctor blade.
Furthermore, wear of the doctor blade and the associated changes in
impact angle cause different effects in cellulosic fibrous structures,
which effects depend upon the pattern of the protuberances in~ the
secondary belt. A cellulosic fibrous structure made on a belt having
discrete protuberances will increase in caliper as the doctor blade
wears, if the blade impact angle is got adjusted to compensate.
Conversely, a cellulosic fibrous structure made on a secondary belt
having a continuous pattern of protuberances is less sensitive to such
wear.
It is not surprising that considerable effort has been expended in
the prior art to achieve constant material properties by adjusting the
impact angle of the doctor blades. In one example, illustrated by
commonly assigned U.S. Patent 4,919,75fi issued April 24, 1990 to Sawdai,
the doctor blade is continually adjusted to minimize the effects of
doctor blade wear on the material properties of the cellulosic fibrous
structure.
However, adjusting the doctor blade requires more equipment,
associated maintenance, and set-up time for the papermaking machinery
than machinery which simply tolerates changes in the doctor blade impact
angle. While, of course, it is desirable to produce paper having certain
consumer desired properties, the art clearly shows a need for greater
flexibility in the manufacturing process, and particularly a way to
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achieve greater flexibility by not having to adjust the doctor blade
impact angle using complex machinery.
More importantly, the prior art shows a need for a secondary belt
which generates relatively high caliper yet responds to changes in the
impact angle of the doctor blade with like changes in the caliper of the
cellulosic fibrous structures dried thereon.
As noted above, one way to achieve greater caliper is by adjusting
the doctor blade. Another way to increase the caliper of a cellulosic
fibrous structure having multiple regions is to increase its basis
weight. However, this arrangement also increases the basis weight of
other regions in which it may not be desirable to do so, requires greater
utilization of fibers, and increases the cost to the consumer.
With the present invention, a way has been found to decouple the
relationship between the Z-direction extent of the protuberances and the
caliper of the cellulosic fibrous structure. Furthermore, other
properties of the cellulosic fibrous structure may benefit from having
been made on a secondary belt according to the present invention.
For example, another problem frequently encountered with cellulosic
fibrous structures which try to minimise fiber utilization and present
less expense to the consumer is pinholing. Pinholing occurs when regions
of the cellulosic fibrous structure are deflected into the deflection
conduits of the secondary belts and break through, so that an opening is
present and light passes through the opening. Pinholing and transmission
of light therethrough present a cellulosic fibrous structure having a
less durable and lower quality appearance to the consumer, and is
accordingly undesirable to the consumer.
One cause of pinholing in a cellulosic fibrous structure made on a
belt according to the aforementioned Trokhan patent is caliper generation
resulting from protuberances which are too great in the 1-direction. By
generating caliper in this manner, Z-direction deflection of the
cellulosic fibrous structure occurs to an extent that pinholing results.
Thus, one using the aforementioned Trokhan belt is forced to select
between caliper generation and reduced pinholing.
Other problems~found in cellulosic fibrous structures made on a belt
according to the aforementioned Trokhan belt of the prior art are cross
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machine direction shrinkage and curling of the edges of the
cellulosic fibrous structure. Such shrinkage and curling are
caused by structural movement during machine direction
tensioning, such as inevitably occurs during winding and
converting. Shrinkage requires a wider cellulosic fibrous
structure for manufacture. Edge curling may cause fold over,
leading to breakage of the web during manufacture. Both cause
greater expense in the manufacturing process.
Unfortunately, the amount of shrinkage is also closely
related to the amount of cross machine direction extensibility
the cellulosic fibrous structure will undergo before rupture.
While relatively greater cross machine direction extensibility
is highly desired, due to allowing the cellulosic fibrous
structure to elastically deform without tearing or shredding
in use, the penalty for such desired cross machine direction
extensibility is paid for at the time of manufacture by
encountering greater cross machine direction shrinkage and
curling.
Accordingly, it is an object of an aspect of this
invention to provide a secondary apparatus or belt which
reduces occurrences of pinholing and shrinkage and curling of
cellulosic fibrous structures during manufacture. It is an
object of an aspect of this invention to provide a secondary
apparatus or belt which reduces occurrences of pinholing
without requiring a corresponding reduction in the caliper of
the cellulosic fibrous structure manufactured thereon.
Furthermore, it is an object of an aspect of the present
invention to provide greater control over the caliper of the
cellulosic fibrous structure with the impact angle of the
doctor blade.
BRIEF SUI~SARY OF THE INVENTION
The invention in one aspect thereof comprises an
apparatus for manufacturing a cellulosic fibrous structure.
The apparatus may comprise an endless belt having a
reinforcing structure and a framework of protuberances joined
thereto in a semicontinuous pattern. Between the
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6a
protuberances are deflection conduits through which air may pass. The
protuberances may be generally parallel, or may be arranged to provide
individual cells within the deflection conduits. In another embodiment, the
invention comprises the paper made on this secondary belt or apparatus.
In accordance with one embodiment of the invention, a macroscopically
monoplanar cellulosic fibrous structure has two mutually orthogonal principle
directions, a machine direction and a cross-machine direction, said cellulosic
fibrous structure further having a first plurality of unembossed regions
having a
first density, and a second plurality of unembossed regions having a second
density different from and less than said first density, wherein said first
plurality
of regions form a semicontinuous pattern of high density regions separated
from
each other by said second plurality of regions which form a semicontinuous
pattern of low density regions, said low density regions comprising fibers
molded generally perpendicular to said two mutually orthogonal principal
directions, each of said high density regions and said low density regions
having
a vector component extending substantially throughout one of said principle
directions of said cellulosic fibrous structure.
In accordance with a further embodiment of the invention, a
macroscopically monoplanar cellulosic fibrous structure has two mutually
orthogonal principal directions, a machine direction and a cross-machine
direction, said cellulosic fibrous structure further having three regions, a
first
plurality of semicontinuous regions having a first density, a second plurality
of
regions having a second density different from and less than said first
density,and a third plurality of regions having a third density, said third
density
being greater than said second density and less than said first density,
wherein
said regions having said second density are bounded by said regions having
said
third density, said regions having said second density and said third density
combining to form a semicontinuous pattern and wherein said second and third
regions combined have a vector component extending substantially throughout
one of said principal directions of said cellulosic fibrous structure.
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BRIEF DESCRIPTION OF THE DRAWINGS
While the Specification concludes with claims particularly pointing
out and distinctly claiming the present invention, it is believed the
same will be better understood by the following Specification taken in
conjunction with the associated drawings in which like components are
given the same reference numeral, and:
Figure 1 is a top plan view of a secondary belt according to the
present invention having parallel protuberances with parallel
deflection conduits therebetween, the protuberances and
deflection conduits being oriented at a diagonal relative to
the machine direction and the cross machine direction;
Figure 2 is a vertical sectional view taken along lines 2-2rof
Figure 1; and
Figure 3 is a top plan view of an alternative secondary belt
according to the present invention having protuberances. which
are not equidistantly spaced from the adjacent protuberances
and which form individual cells within the deflection conduits.
DETAILED DESCRIPTION OF THE INVENTION
The invention comprises an apparatus for manufacturing a cellulosic
fibrous structure.The apparatus according to the present invention may be
embodied in a variety of forms, such as stationary plates for making hand
sheets, rotating drums for continuous processing and preferably endless
belts 10 for ordinary papermaking machinery as illustrated in Figure 1.
Although these, and other, embodiments ~of the present invention are
suitable, except as noted below, the preferred embodiment of the endless
belt 10 is the embodiment discussed below with the understanding that
other embodiments may be readily carried out by one skilled in the art.
The preferred endless belt 10 embodiment of an apparatus according
to the present invention comprises two primary elements: a patterned
framework of protuberances 20 and a reinforcing structure 30. The
reinforcing structure 30 of the belt 10 has two opposed major surfaces.
One major surface is the paper contacting side 32 and from which the
protuberances 20 extend. The other major surface of the reinforcing
structure 30 of the papermaking belt 10 is the backside 34, which
CA 02206750 1997-OS-29
contacts the machinery employed in a typical papermaking operation. Machinery
employed in a typical papermaking operation include vacuum pickup shoes,
rollers,
etc., as are well known in the art and will not be further discussed herein.
Generally, for a belt 10 according to the present invention, the "machine
direction' of the belt 10 is the direction within the plane of the belt 10
parallel to the
principal direction of travel of the cellulosic fibrous structure during
manufacture.
1 o The machine direction is designated by arrows "MD" in Figures 1 and 3. The
cross
machine direction is generally orthogonal to the machine direction and also
lies
within the plane of the belt 10. The Z-direction is orthogonal to both the
machine
direction and cross machine direction and generally normal to the plane of the
belt
at any position in the papermaking process. The machine direction, cross
machine direction, and Z-direction form a Cartesian coordinate system.
The belt 10 according to the present invention is essentially macroscopically
monoplanar. As used herein a component is "macroscopically monoplanar" if such
component has two very large dimensions in comparison to a relatively small
third
dimension. The belt 10 is essentially macroscopically monoplanar in
recogrstion
2 o that deviations from absolute planarity are tolerable, but not preferred,
so long as
the deviations do not adversely affect the performance of the papermaking belt
10 in
making cellulosic fibrous structures thereon.
In a rotating drum embodiment of the present invention (not shown), the
reinforcing structure 30 may comprise a generally cylindrical shell having a
2 5 plurality of holes therethrough. In a papermaking belt 10 embodiment, the
reinforcing structure 30 comprises a series of filaments, preferably woven in
a
rectangular pattern to define interstices therebetween. The interstices allow
fluids,
such as drying air, to pass through the belt 10 according to the present
invention.
The interstices form one of the groups of openings in the papermaking belt 10
3 0 according to the present invention, which openings are preferably smaller
than
those defined by the pattern of the framework.
If desired, the reinforcing structure 30 may have vertically stacked machine
direction filaments to provide increased stability and load bearing
capability.
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By vertically stacking the machine direction filaments of the reinforcing
structure 30,
the overall durability and performance of a belt 10 according to the present
invention is enhanced.
The reinforcing structure 30 should not present significant obstruction to the
flow of fluids, such as drying air therethrough and, therefore, should be
highly
permeable. The permeability of the reinforcing structure 30 may be measured by
l0 the airflow therethrough at a differential pressure of about 1.3
centimeters of water
(0.5 inches of water). A preferred reinforcing structure 30 having no
framework of
protuberances 20 attached thereto should have a permeability at this
differential
pressure of about 240 to 490 standard cubic meters per minute per square meter
of
belt 10 area (800 to 1,600 standard cubic feet per minute per square foot). Of
course,
it will be apparent that the permeability of the belt 10 will be reduced when
the
framework of protuberances 20 is attached to the reinforcing structure 30. A
belt 10
having a framework of protuberances 20 preferably has an air permeability of
about
90 to 180 standard cubic meters per minute per square meter (300 to 600
standard
cubic feed per minute per square foot).
2 0 ~ In an alternative embodiment, the reinforcing structure 30 of a belt 10
according to the present invention may have a textured backside 34. The
textured
backside 34 has a surface topography with asperities to prevent the buildup of
papermaking fibers on the backside 34 of the belt 10, reduces the differential
pressure across the belt 10 as a vacuum is applied thereto during the
papermaking
2 5 process, and increases the rise time of the differential pressure prior to
the
maximum differential pressure occurring.
A particularly preferred reinforcing structure 30 for use with the present
invention may be made in accordance with the teachings of commonly assigned
U.S.
Patent 5,098,522 issued March 24,1992 to Smurkoski, et al. which patent is
3 o referenced herein for its showing of how to make a particularly preferred
reinforcing structure 30 suitable for use with a papermaking belt 10 in
accordance
with the present invention and showing a process for making cellulosic fibrous
structures using such a papermaking belt 10.
The other primary component of the papermaking belt 10 according to the
3 5 present invention is the patterned framework of protuberances 20.
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The protuberances 20 define deflection conduits 40 therebetween. The
deflection conduits 40 allow water to be removed from the cellulosic
fibrous structure by the application of differential fluid pressure, by
evaporative mechanisms, or both when drying air passes through the
cellulosic fibrous structure while on the papermaking belt 10 or a vacuum
is applied through the.belt 10. The deflection conduits 40 allow the
cellulosic fibrous structure to deflect in the Z-direction and generate
the caliper of and aesthetic patterns on the resulting cellulosic fibrous
structure.
The protuberances 20 are arranged in a semicontinuous pattern. As
used herein, a pattern of protuberances 20 is considered to be
"semicontinuous" if a plurality of the protuberances 20 extends
substantially throughout one dimension of the apparatus, and each
protuberance 20 in the plurality is spaced apart from adjacent
protuberances 20.
The protuberances 20 in the semicontinuous pattern may be generally
parallel as illustrated in Figure 1, rnay form a wave pattern as
illustrated in Figure 3, and/or may form a pattern in which adjacent
protuberances 20 are offset from one another with respect to the phase of
the pattern as ill-ustrated in Figure 3. The semicontinuous protuberances
may be aligned in any direction within the plane of the papermaking
belt 10.
Thus, the protuberances 20 may span the entire cross machine
direction of the belt 10, may endlessly encircle the belt 10 in the
machine direction, or may run diagonally relative to the machine and
cross machine direct ions. Of course, the directions of the protuberance
20 alignments (machine direction, cross machine direction, or diagonal}
discussed above refer to the principal alignment of the protuberances 20.
Within each al ignment, the protuberance 20 may have segments al igned at
other directions, but aggregate to yield the particular alignment of the
entire protuberance 20.
Protuberances ZO arranged in a framework having a semicontinuous
pattern are to be 'distinguished from a pattern of discrete protuberances
20, in which any one protuberance 20 does not extend substantially
throughout a principal direction of the papermaking belt 10. An example
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of discrete protuberances 20 is found at Figure 4 of commonly assigned U.S.
Patent
4,514,345 issued April 30, 1985 to Johnson, et al.
Similarly, a pattern of semicontinuous protuberances 20 is to be distinguished
from protuberances 20 forming an essentially continuous pattern. An
essentially
continuous pattern extends substantially throughout both the machine direction
and
cross machine direction of the papermaking belt 10, although not necessarily
in a
straight line fashion. Alternatively, a pattern may be continuous because the
l0 framework forms at least one essentially unbroken net-like pattern.
Examples of
protuberances 20 forming an essentially continuous pattern is illustrated by
Figures
2-3 of the aforementioned U.S. Patent 4,514,345 issued to Johnson, et al or by
the
aforementioned U.S. Patent 4,528,239 issued to Trokhan.
As illustrated in Figure 2, the framework of semicontinuous protuberances 20
according to the present invention is joined to the reinforcing structure 30
and
extends outwardly from the paper contacting side 32 thereof in the Z-
direction. The
prohiberances 20 may have straight sidewalk, tapered sidewalls, and be made of
any material suitable to withstand the temperatures, pressures, and
deformations
which occur during the papermaking process. Particularly preferred
protuberances
2 0 20 are made of photosensitive resins.
The photosensitive resin, or other material used to form the pattern of
semicontinuous protuberances 20, may be applied and joined to the reinforcing
structure 30 in any suitable manner. A particularly preferred manner of
attachment
and joining is applying liquid photosensitive resin to surround and envelop
the
2 5 reinforcing structure 30, cure the portions of the liquid photosensitive
resin which
are to form the semicontinuous pattern of the protuberances 20, and wash away
the
balance of the resin in an uncured state. Suitable processes for manufacturing
a
papermaking belt 10 in accordance with the present invention are disclosed in
the
aforementioned U.S. Patent 4,514,345 issued to Johnson, et al., commonly
assigned
3 o U.S. Patent 4,528,239 issued July 9, 1985 to Trokhan, and the
aforementioned U.S.
Patent 5,098,522 issued to Smurkoski, et al., which patents are referenced
herein for
their showing a particularly preferred manner
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12
of forming the protuberances 20 and joining the protuberances 20 to the
reinforcing structure 30.
As is evident from a reading of any of the three aforementioned
patents incorporated by reference, the pattern of the protuberances 20 is
determined by transparencies in a, mask through which an activating wave
length of light is passed. The activating light cures portions of the
photosensitive resin opposite the transparencies. Conversely, the
portions of the photosensitive resin opposite the opaque regions of the
mask are washed away, leaving the paper contacting side 32 of the
reinforcing surface exposed in such areas.
Thus, to form a particularly preferred embodiment of a papermaking
belt 10 according to the present invention, the mask must be formulated
with transparent regions having a semicontinuous pattern as described
above. Such a mask will form a like pattern of protuberances 20 on the
papermaking belt 10.
For the embodiments described herein, protuberances 20 forming a
semicontinuous pattern should have characteristics which produce desired
properties of the cellulbsic fibrous structures. The geometry of the
protuberances 20 significantly influences the properties of the resulting
cellulosic fibrous structure made on the secondary belt 10. For example,
the protuberances 20 may produce hinge lines in the cellulosic fibrous
structure, which hinge lines impart softness or burst strength thereto.
Furthermore, the semicontinuous pattern of protuberances ZO will
yield a like semicontinuous pattern of high and low density regions in
the cellulosic fibrous structure made on this belt 10. Such a pattern in
the resulting cellulosic fibrous structure occurs for two reasons.
First, the regions of the cellulosic fibrous structure coincident the
semicontinuous deflection conduits 40 will be dedensified by the air flow
therethrough or will be dedensified by the application of a vacuum to the
deflection conduits 40. Preferably, the regions of the cellulosic
fibrous structure coincident the protuberances 20 will be densified by
the transfer of the cellulosic fibrous structure to a rigid backing
surface, such as a Yankee drying drum.
The geometry of the protuberances 20 may be considered in a single
direction, or may be considered in two dimensions, and may be considered
CA 02206750 1997-OS-29
13
as either lying within or normal to the plane of the secondary belt 10
according to the present invention.
Particularly, the Z-direction extent of the protuberances 20 in a
single direction normal to the plane of the belt 10 determines the height
of the protuberances 20 above the paper contacting surface of the
reinforcing structure 30. If the height of the protuberances 20 is too
great, pinholing and apparent transparencies or light transmissipn
through the cellulosic fibrous structure will occur. Conversely, if the
Z-direction dimension of the protuberances 20 is smaller, the resulting
cellulosic fibrous structure will have less caliper. As noted above,
both pinholing and low caliper are undesirable because they present an
apparently lower quality cellulosic fibrous structure to the consumer.
For the embodiments described herein, the protuberances 20
preferably have a height between 0.05 and 0.64 millimeters (0.002 and
0.025 inches), preferably between 0.13 and 0.38 millimeters (0.005 and
0.015 inches), and more preferably between 0.20 and 0.26 millimeters
(0.008 and O.OiO inches). -
Referring back to Figure ~ and continuing the single direction
analysis, the spacing between inwardly facing edges of adjacent
protuberances 20 must be considered. If, within limits, the spacing is
too great for a given Z-direction extent, pinholing is more likely to
occur. Also, if the spacing between the inwardly facing edges of
adjacen t protuberances 20 is too great, another undesired resultant
phenomenon may be that fibers will not span the distal ends 46 of
adjacent protuberances 20, resulting in a cellulosic fibrous structure
having lesser strength than can be obtained if indivi-dual fibers span
adjacent protuberances 20. Conversely, if the spacing between the
inwardly facing edges of adjacent protuberances 20 is too small, the
cellulosic fibers will bridge adjacent protuberances 20, and in an
extreme case little caliper generation will result. Therefore, the
spacing between the inwardly facing surfaces of adjacent protuberances 20
must be optimized to allow sufficient caliper generation to occur and
minimize pinhol~ing.
For the embodiments described herein, the inwardly facing surfaces
of adjacen t protuberances 20 may be spaced about 0.64 to about 1.40
CA 02206750 1997-OS-29
14
millimeters apart (0.025 to 0.055 inches) in a direction generally
orthogonal to such surfaces. This spacing will result in a cellulosic
fibrous structure which generates maximum caliper when made of
conventional cellulosic fibers, such as Northern softwood kraft or
eucalyptus.
A further single dimension analysis relates to th.e
width across the distal edge of the protuberance 20. The width is
measured generally normal to the principal dimension of the protuberance
20 within the plane of the belt 10 at a given location. If the
protuberance 20 is not wide enough, the protuberance 20 will not
withstand. the pressures and temperature differentials encountered during
and incidental to the papermaking process. Accordingly, such a
papermaking belt 10 will have a relatively short life and have to be
frequently replaced. If the protuberances 20 are too wide, a .more
one-sided texture will again result and the cell size, discussed below,
must be increased to compensate.
Of course, it is to be recognized that the protuberances 20 are
typically tapered and may occupy a greater projected surface area at the
proximal edge of the protuberance 20. For the embodiments described
herein, typically the proximal area of the protuberances 20 is about 25
to 75 percent of the belt 10 surface area and the distal area of the
protuberances 20 is about 15 to 65 percent of the belt 10 surface area.
Generally, for the embodiments described herein, protuberances 20
having a width at the proximal ends of about 0.3 to 1.3 millimeters
(0.011 to 0.050 inches) are suitable. The protuberances 20 may have a
width at the distal ends 46 of about 0.13 to 0.64 millimeters (0.005 to
0.025 inches), and preferably may have a width at the distal ends 46 of
about 0.20 to 0.46 millimeters (0.008 to 0.018 inches).
Examining the pattern of semicontinuous protuberances 20 in two
dimensions, particularly the machine and cross machine directions, it is
apparent that two different types of protuberances 20 may be utilized in
accordance with the present invention. All of the protuberances 20 are
generally nonintersecting. The first type of protuberance 20,
illustrated in Figure 1, utilizes generally parallel (although not
necessarily straight) protuberances 20. These protuberances 20 have
CA 02206750 1997-OS-29
generally equal spacings in the deflection conduits 40 therebetween, so
that individual cells 42 are not formed.
Conversely, as illustrated in Figure 3, the secondary belt 10 may
have noncontacting protuberances 20 which are not equidistantly spaced
from the adjacent protuberances 20 and which may define individual cells
42 within the deflect ion conduits 40. The protuberances 20 of such a
bel t 10 may not be paral l el . Furthermore, the protuberances 20 may not
be of constant width. Either arrangement may yield deflection conduits
40 having fiber bridging of adjacent protuberances 20 in certain areas
and fiber deflection into the deflection conduits 40 in other areas.
This arrangement provides the advantage that a cellulosic fibrous
structure having a semicontinuous pattern and three mutually different
densities may be formed. The three densities occur due to: 1) low
density fibers spanning adjacent__protuberances 20 and which deflect in
the Z-direction from the distal end 46 of the protuberances 20 an amount
at least about the thickness of the high density regions of the
cellulosic fibrous structure; 2) intermediate density fibers which bridge
adjacent protuberances 20 and deflect in the Z-direction an amount less
than about 50 percent of the Z-direction deflection found in the low
density fibers of the cellulosic fibrous structure; and 3) high density
densified fibers coincident the distal ends 46 of the protuberances 20.
A semicontinuous pattern three density cellulosic fibrous structure
such as this provides the benefits of more isotropic flexibility, better
softness, and a more pleasing texture than a like cellulosic fibrous
structure made on a secondary belt 10 having parallel protuberances 20.
The three densities may be arranged in cells 42 of low density regions
bounded by regions of intermediate and high density.
Cells 42 are defined as the discrete low density regions in the
cellulosic fibrous structures that occur between and are bounded by the
semicontinuous high density regions and the discrete intermediate density
regions in a cellulosic fibrous structure containing at least three
different densities, or are defined as the corresponding regions of the
secondary belt l0~producing such a cellulosic fibrous structure.
If the individual cells 42 in deflection conduits 40 between the
protuberances 20 are too large, the caliper generated during the drying
CA 02206750 1997-OS-29
16
process may not withstand subsequent calendering or other converting
operations, particularly for relatively low basis weight cellulosic
fibrous structures. Thus, a relatively lower caliper (and apparently
lower quality) product will be presented to the consumer - despite
adequate caliper generation occurring during manufacture. Also, large
cells may increase the one-sidedness of the texture.
Conversely, if the individual cells 42 in the deflection conduits 40
between adjacent protuberances 20 are too small, low caliper generation
may result, as noted above relative to the one-dimensional spacing
between adjacent protuberances Z0. Furthermore, if the individual cells
42 are too smal l , the width of the distal edges of the cel l s may be too
small for a given cell size and poor belt 10 life will again result.
The individual cells 42 may be arranged in any desired matrix. The
individual cells 42 may be aligned in either or both the machine
direction and/or cross machine direction. The individual cells 42 may be
staggered in either the machine direction, the cross machine direction,
or, alternatively, preferably the individual cells 42 are bilaterally
staggered. For the embodiments,destribed herein, protuberances 20 having
approximately 16 to 109 cells 42 per square centimeter (100 to 700 cells
42 per square inch), and preferably approximately 31 to approximately 78
individual cells 42 per square centimeter (200 to 500 individual cells 42
per square inch) and more preferably about 62 cells per square centimeter
(400 cells per square inch) are judged suitable.
In an alternative embodiment of the invention, the belt 10 having a
semicontinuous pattern of protuberances 20 and semicontinuous pattern of
defl ecti on condui is 40 may be used as a formi ng wi re i n the wet end of
the papermaking machine. When such a belt 10 is used as a forming wire
in the papermaking machine, a cellulosic fibrous structure having regions
of at Teast two mutually different basis weights will result. The at
least two mutually different basis weights in the cellulosic fibrous
structure may be aligned in either the machine direction, the cross
machine direction, or diagonally thereto.
This cellulosic fibrous structure provides the advantage, for
example,~that if the semicontinuous pattern of mutually different basis
weights is aligned in the cross machine direction and the cellulosic
CA 02206750 1997-OS-29
17
fibrous structure is to be utilized as a core-wound paper
product (such as toilet tissue or paper toweling) the low
basis weight regions provide a tear line. This tear line is
useful when the free end of the core-wound paper product is
pulled in tension, such as occurs when the user desires a
finite length of product for household tasks. The cellulosic
fibrous structure will usually tear at the line formed through
the low basis weight region. This arrangement provides the
advantage that the perforating operation may be eliminated
during paper converting and the further advantage that the
consumer may select sheets of almost any different size, as
may be needed for the task, rather than being limited by the
spacing between the perforations provided by the converting
operation.
EXAMPLES
Comparative examples of cellulosic fibrous structures
were made on a secondary belt 10 having a continuous pattern
according to the aforementioned Trokhan patent. a secondary
belt 10 having a discrete pattern according to Figure 8 of
commonly assigned U.S. Patent 4,239,065 issued December 16,
1980 to Trokhan, and a secondary belt 10 having a
semicontinuous pattern according to the present invention were
constructed.
The semicontinuous pattern belt 10 has a large sized
pattern of roses superimposed on the semicontinuous
protuberance 20. This rose pattern is illustrated in commonly
assigned U.S. Patent No. 5,328,565 issued July 12, 1994, Rasch
et al., corresponding to Canadian Patent Application
2,069,193, published December 20, 1992. The protuberances 20
were 0.33 millimeters (0.013 inches) in thickness, as
designated in Figure 3 by dimension T._ The protuberances 20
formed generally rectangularly shaped cells 42 having a major
dimension of 1.22 millimeters (0.048 inches), as designated by
dimension A and a minor dimension of 0.69 millimeters (0.027
inches), as designated by dimension N. Each protuberance 20
CA 02206750 1997-OS-29
17a
was most closely separated from the adjacent protuberance 20
by a distance of 0.23 millimeters (0.009 inches), as indicated
by dimension C.
The continuous pattern belt and semicontinuous pattern
belt 10 each had 62 cells 42 per square centimeter (400 cells
42 per square inch). The discrete pattern belt had a mesh
count of 23 x 17 filaments per
CA 02206750 1997-OS-29
18
square centimeter (59 x 44 filaments per square inch), yielding approximately
67
cells per square centimeter (433 cells per square inch). A cell was determined
to be
either a individual polygonal deflection conduit in the continuous pattern
belt made
according to the aforementioned Trokhan patent, a unit formed by six filament
knuckles in the discrete pattern belt made according to the aforementioned
Trokhan
'065 patent, or a unit cell 42 within a deflection conduit 40 as previously
defined in
the belt 10 according to the present invention.
l0 The continuous pattern and semicontinuous pattern secondary belts 10 each
had a Z-direction protuberance 20 extent of about 0.23 millimeters (0.009
inches).
The apparent protuberance 20 height for the belt 10 made according to the
aforementioned Trokhan'065 patent was measured by the pattern of the weave.
Particularly, the apparent protuberance 20 height was taken as the caliper of
the
secondary belt, less the shute filament diameter. To maintain approximately
equal
cell 42 counts and an appropriate diameter of the filaments forming the
reinforcing
structure 30 in the discrete pattern belt 10, the aforementioned 0.23
millimeters
(0.009 inches) protuberance 20 height could not be maintained for the discrete
pattern belt 10. Instead the apparent protuberance 20 height was 0.32
millimeters
2 0 (0.013 inches).
This example illustrates the choice that must be made between cell size and
protuberance 20 height when using a discrete pattern belt 10 made according to
the
aforementioned Trokhan '065 patent. However, given the great commercial
success
of cellulosic fibrous structures made on belts 10 according to the
aforementioned
Trokhan'065 patent, it was judged to be a suitable standard against which to
compare cellulosic fibrous structures made on a semicontinuous pattern belt 10
according to the present invention.
The cellulosic fibrous structure made on these three aforementioned belts 10
were layered in a trilaminate. The two outboard layers each comprised at least
forty
3 0 percent of the total furnish and were eucalyptus fiber. The,central layer
comprised
the balance of the furnish and was Northern softwood kraft (NSI~ fiber. The
layering process is described in more detail in commonly assigned U.S. Patent
3,994,771 issued November 30, 1976, to Morgan, Jr., et al., which patent is
referenced
herein for its showing how these layered cellulosic fibrous structures were
made for
3 5 this example.
CA 02206750 1997-OS-29
19
The cellulosic fibrous structures made for these examples had a consistency of
20 percent at the couch roll. The vacuum shoe used to transfer the embryonic
web
from the forming wire to the secondary belts had a vacuum of 31.8 centimeters
of
Mercury (12.5 inches of Mercury).
The resulting cellulosic fibrous structures were tested for basis weight as
measured according to ASTM Standard D585-74, tensile strength as measured on a
Thwing Albert tensile machine having a cross head separation rate of 10.2
l0 centimeters per minute (4 inches per minute), and a gage length of 5.08
centimeters
(2 inches). Caliper was measured under a confining pressure of 14.7 grams per
square centimeter (95 grams per square inch). The tensile strength varied
little from
sample to sample, when the effect of different percentages of Northern
softwood
kraft fibers is taken into account.
As can be seen from Table I, the basis weights of all three samples were
essentially constant. The cellulosic fibrous structure made on the discrete
pattern
belt 10 had considerably less caliper than the cellulosic fibrous structures
made on
the semicontinuous and continuous patterned belts 10.
The cellulosic fibrous structure made on the continuous pattern belt 10
2 o showed no correlation of doctor blade impact angle to caliper. The
cellulosic fibrous
structures made on the semicontinuous and discrete belts 10 showed a
monotorucally decreasing relationship in caliper as the impact angle of the
doctor
blade was increased. Thus, the only belt 10 to provide both relatively high
caliper
and a linear and monotonic correlation of doctor blade impact angle to such
caliper
2 5 is the belt 10 according to the present invention.
The caliper benefits shown in Table I were maintained throughout
subsequent converting operations.
CA 02206750
1997-OS-29
20
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CA 02206750 1997-OS-29
21
Additional testing was conducted to determine the effects of
- protuberance 20 pattern on sheet curl, shrinkage, and pinholing. For
these tests the doctor blade impact angle was held at a constant impact
angle of 81 degrees. A discrete pattern belt 10 made generally according
to Figure 4 of the aforementioned Johnson, et al..patent was substituted
for the discrete pattern belt 10 made according to the Trokhan '065
patent utilized in the prior Examples. The discrete pattern belt 10
utilized for this example had 62 cells per square centimeter (400 cells
per square inch) and a protuberance 20 height of 0.2 millimeters (0.009
inches). The protuberances 20 were generally rectangularly shaped with
rounded ends, had an aspect ratio of 3.375 and alternating protuberances
20 were oriented at 90 degree angles, as illustrated by the imprint
pattern of Figure 1 of the aforementioned Trokhan '065 patent.
The cel 1 ul os i c ~ f i brous structures made on these three bel is 10 had
approximately equal basis weights, to compare the effects of protuberanc a
20 pattern on sheet curling, shrinkage and pinholing. Pinholing was
measured by a Paperlab-1 Formation RoboTester supplied by Kajaani
Automation of Norcross, Georgia. _
Sheet curl and sheet shrinkage were ascertained by measuring the
sheet width just prior to the Yankee (PY), between the calender rolls and
the reel (BCR), and after cutting from the parent roll (AC). Sheet curl
is then given by the formula: (PY - BCR)/PY. Sheet shrinkage is given by
the formula: (PY - AC)/PY.
Table IIA illustrates three cellulosic fibrous structures made
according to the aforementioned belts 10 and having a total tensile
strength of approximately 400 grams per inch. Table II8 illustrates the
same cellulosic fibrous structures, except the total tensile strength is
about 500 grams per inch. In both Table IIA and Table IIB, softness
(which is strongly influenced by tensile strength) is corrected to the
appropriate tensile strength by O.1 PSU of softness per 25 grams per inch
of tensile strength.
CA 02206750 1997-OS-29
22
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CA 02206750 1997-OS-29
23
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CA 02206750 1997-OS-29
24
As can be seen from Tables IIA and IIB, the cellulosic fibrous
structure made on the belt 10 according to the present invention had
better sheet shrinkage and curl than the cellulosic fibrous structure
made on, the continuous pattern belt, but had shrinkage and curl
generally equivalent to that of the cellulosic fibrous structure made
on the discrete pattern belt. Also, the cellulosic fibrous structure
made on the belt 10 according to the present invention had a better
burst strength to tensile strength ratio than the cellulosic fibrous
structure made on a discrete pattern belt, however the burst strength
to tensile strength ratio was not as good as that of the cellulosic
fibrous structure made on the continuous pattern belt. Furthermore,
the cellulosic fibrous structure made on the belt 10 according to the
present invention--had better pinholing than the cellulosic fibrous
structure made on the continuous pattern belt, but had mixed results
relative to pinholing compared to the cellulosic fibrous structure made
on the discrete pattern belt.
It is recognized that many variations and combinations of
patterns, protuberance 20 sizes, and spacings may be made within the
scope of the present invention. All such variations are within the
scope of the following claims.
