Note: Descriptions are shown in the official language in which they were submitted.
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
THROUGH-AIR DRYING APPARATUS HAVING DECREASING WET FLOW RESISTANCE IN THE
MACHINE DIRECTION AND PROCESS OF DRYING A WEB THEREWITH
FIELD OF INVENTION
The subject invention relates to through air drying for tissue paper
papennaking, and
more particularly to through air drying usable with micropore drying media.
BACKGROUND OF THE INVENTION
Micropore drying media are known in the art. Micropore drying media include a
ply,
or a plurality of plies superimposed in face-to-face relationship. The plies
provide
restrictions in 'the flow path for air flow therethrough. The restrictions in
the flow path may
comprise pores smaller than many of the interstitial areas in tissue paper, as
well as other
generally planar materials dried, or otherwise made, thereon. The following
discussion is
directed to tissue paper, it being understood that the invention is not so
limited.
By providing pores smaller than the interstices of the tissue paper,
differences in flow
resistance through the tissue paper, etc., are negated due to the greater flow
resistance being
provided by the micropore drying medium. Such differences in flow resistance
may occur
due to differences in intensive properties, such as caliper, basis weight and
density.
Typically, such differences occur on a very small scale due to localized
differences in the
various regions of the tissue paper.
The prior art discloses micropore media suitable for drying tissue paper
thereon.
Improvements to the micropore media include micropore drying apparatus having
multiple
zones, high fatigue strength/low pressure drop micropore drying media and
micropore
media having preferentially reduced wet pressure drop. Such micropore media,
suitable for
adaptation to the present invention, are illustrated in commonly assigned U.S.
Pat. Nos.
5,274,930, issued Jan. 4, 1994 to Ensign et al.; 5,437,107, issued Aug. 1,
1995 to Ensign et
al.; 5,539,996, issued July 30, 1996 to Ensign et al.; 5,581,906, issued Dec.
10, 1996 to
Ensign et al.; 5,584,126, issued Dec. 17, 1996 to Ensign et al.; 5,584,128,
issued Dec. 17,
1
CA 02452853 2007-05-30
1996 to Ensign et al.; 5,625,961, issuod May 6, 1997 to Ensign et al.;
5,912,072, issued
June 15, 1999 to Trokhan et al.; 5,942,322, issued Aug. 24, 1999 to Bnsign et
aL;
6,021,583, issued Feb. 8, 2000 to Stelljes, Jr. et aL; and 6,105,276, issued
Aug. 22, 2000 to
Bnsign et al.
There remain other ways to optimize energy consumption when using micropore
drying techniques. For example, as water is removed from the tissae paper,
etc., to be dried
by air flow therethrough, subsequent flow restrictions in the micropore media
need not be as
gt+eat. Thus, in the machine direction, flow restrictions in the micropore
media may be
reduced while maintaining a pore sim smaller thaa many, preferably most, and
most
preferably all, of the interstices in the tissne paper.
Thus, flow mstriations tbrough the micropore media may be reduced as the
tissue
paper to be dried travels across the micropore dryin.g medium in the machine
direction.
This an-angement provides the benefit of decoupling mechanical dewatering of
the tissue
paper from through air drying of the tissae paper. During mechanical
dewatering, a smail
pore size is better to promote dewatering by capillary action. During tbrough
air drying,
pore sizes which are relatively larger, but still provide a limitiug orifice
for air flow through
the tissue paper, have less flow resistance and thereby save energy.
Reduced flow resistance tbrough the micropore media may be provided by having
pore sizes which successively increase in the machine direction. Altematively,
micropore
media having a higher density of pores, i.e., more pores per square
centimeter, in the
machine direction may be utilized. Finally, hybrid media having both of the
above features
may be utilized. Furthermore, the reduced flow resistance apparatus of the
present
invention may be used with through air drying tissue paper pape,rmaking
processes which
are not limited to mieropore drying media. The variable flow resistance
apparatus and
process according to the present invention mey be applied ta other through air
drying tissue
paper papermaking techniques as well. For example, the disclosed apparatus and
process
may be used with the predryers of a through air drying tissue paper
papernnaking machine.
2
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
SUMMARY OF THE INVENTION
The invention comprises a micropore drying apparatus having a machine
direction
and a Z-direction orthogonal thereto. The micropore drying apparatus is
permeable to air
flow therethrough. The micropore drying apparatus has a wet flow resistance to
air flow
therethrough, which wet flow resistance to air flow decreases in the machine
direction of
the micropore drying apparatus.
The micropore drying apparatus has a grid of pores which provide the air flow
therethrough. The wet flow resistance may decrease in either a step wise
fashion or in a
gradient. The decrease may occur within sections of, or entirely throughout,
the drying
apparatus. The decreased flow resistance may be achieved by increasing the
size and/or
number of pores. In yet another embodiment, the decreasing pore resistance may
be
provided by coating the micropore drying apparatus to reduce the surface
energy, or
changing the flow path through the pores to be less tortuous and provide a
lesser flow
resistance in the Z-direction. In yet another embodiment, the hydraulic radius
of the pores
may be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic vertical side elevational view of an apparatus according
to the
present invention having a generally vertically oriented major axis and a
stationary
micropore drying medium.
Fig. 2 is a schematic top plan view of a micropore drying medium according to
the
present invention.
Fig. 3 is a schematic side elevational view of a micropore drying apparatus
according
to the present invention having first and second discrete units, the first
discrete unit being
somewhat larger than the second. An optional through air drying belt carries
the web to be
dried to the first unit.
3
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
DETAILED DESCRIPTION OF THE INVENTION
Referring to Fig. 1, the micropore drying apparatus 10 according to the
present
invention comprises at least one, and typically a plurality of, micropore
drying media 15.
The apparatus 10 is used for drying a web thereon. Each of the at least one
micropore
drying media 15 preferably comprises one, and preferably a plurality of, plies
22,24,26,28,30,32 superimposed in face-to-face relationship. Such a micropore
drying
medium 15 is generally planar, and has a Z-direction oriented orthogonal to
the plane. The
micropore drying apparatus 10 may be executed in a flat geometry or,
preferably, is
disposed in a curvilinear geometry and adapted to be used in a roll. The
apparatus 10 has a
machine direction. The web moves in the machine direction relative to the
apparatus 10.
The micropore drying apparatus 10, and particularly the micropore drying
medium 15, have
a flow resistance therethrough. The flow resistance varies in a decreasing
fashion in the
machine direction.
Referring to Fig. 2, as used herein, a micropore drying apparatus 10 is any
apparatus
which introduces a micropore drying medium 15 in the flow path of the through
air
drying process, which micropore drying medium 15 has a field of pores 40
disposed in a
grid. A plurality of the pores 40 are smaller than the interstitials of the
web to be dried in
the through air drying process. A suitable micropore drying apparatus 10
includes a
laminate of one or more woven mesh screens, wherein at least one of the woven
screens has
openings, or pores 40, therethrough which are smaller than the interstitials
of the web to be
dried thereon.
Referring back to Fig. 1, the micropore drying apparatus 10 according to the
present
invention may be used to dry any web comprising a generally planar sheet
material. Webs
usable with the micropore drying apparatus 10 include tissue paper 5,
synthetic nonwovens,
hard grades of paper, cloth, etc. The following description will be directed
to a web of
tissue paper 5, it being understood that the invention is not so limited.
As illustrated in Fig. 2, the micropore drying apparatus 10 according to the
present
invention may comprise a single integral unit. By integral unit, it is meant
that tissue paper
5 disposed on the micropore drying medium 15 of such an apparatus 10 is, or
may be,
4
CA 02452853 2007-05-30
subject to the mimpore drying process without sabstantial inteszvption dnring
the entire
time period the tissue paper 5 is on the micropore drying medium 15.
As illustrated in Fig. 2, the first ply 22 of the plurality of plies
22,24,26,28,30,32
oontacts a web of tissue paper 5 disposed thereon. TU .first ply 22 has pores
40
therethrough, which provide a pore 40 size smaller than at least some of, and
preferably
smaller than many o~ the interstioes of the tissue paper 5 disposed tbereon.
The
atraagement having -the re,lafively smaller pwe 40 sizes in the first ply 22
provides a
limiting orifice for air flow through the fust ply 22 and any tissu paper 5
placed thereom.
Air may first pass tbrough the tissue paper 5, then through the micropore
drying medium 15,
vice versa, or a combination thereof as the tissue paper 5 traverses
sequentially spaced
portions of the micropore drying medium 15. Alternatively, the micropore
drying apparatus
according to the present invention may comprise a plurality of micropore
drying media
15.
Subjacent the fnst ply 22 is preferably a plurality of plies 22,24,26,28,30,32
of
increasing pore 40 size. In a preferred embodiment, five or six plies
22,24,26,28,30,32 of
increasing pore 40 size may be utilized to form a unitary laminate comprising
the mimpore
drying medium 15. Each snccessive ply below the first ply 22 povides less flow
resistance
and increased strength for the laminate comprisin4$ the mierapore drying
medium 15.
The pore 40 size under consideration is the finest pore 40 size in the
micropore drying
medium 15, as this provides the maximum resistance to air flow therethrougb
and controls
the flow of air through the micropore drying medium 15 and any tissue paper 5
disposed
thereon. The pore 40 sizes of the sub,jaaent plies 24,26,28,30,32 may be
constant in the
machine direction or, preferably, are variable in the machine direction as
desern'bed
hereinbelow.
Pore 40 size may be measured using a bubble point test method according to SAB
Standard ARP 901. If the micropore drying medium 15
comprises a laminate of plural plies 22,24,26,28,30,32, the micropore drying
medium 15 is
measured as a unitary laminate. If the micropore drying medium 15 is held
stationasy, and
the web moved relative to the micropore drying medium 15, there are
prophetically less
fatigue stresses enconntered by the micropore drying medium 15.
5
CA 02452853 2007-05-30
Flow resistance is measured according to the teehnique discussed in commonly
assigned U.S. Pat. No. 6,021,583, issaed Feb. 8, 2000 to Stelljes, Jr. et al.
SpeciScally, as used herein, flow resistance measures the wet pressure
drop tluvugh the micropore drying medium 15. A saitably-sized sample of the
miaropore
drying medium 15 is provided so that a round, 4 inch (10.2 cm) diatneter
portion of the
miaopore drying medium.15 may be exposed to flow therethrougIL A test fixture
is also
provided. The test fixture comprises pipe having a length of saven inches
(17.8 cm) and a 2
inch (5.1 cm) nominal diameter. The pipe is joined to a rodum. The redaoer has
a length
of 16 inches (40.6 ean) and has a two inch (5.1 cm) nominal inside diameter.
The inside
diameter of the reducer tapers at a 711 included angle over a 16 inch (40.6
om) length to a
four inch (10.2 cm) nominal inside diameter.
The sample of the micropore drying medium 15 is disposed at the four inch
(10.2 cm)
nominal inside diameter portion of the test fixture. The micropore drying
medium 15 is
oriented so that the firat ply 22 faces the high-pressure (upstream) side of
the air flow. The
test fixtm is symmetrical about the sample of the micropore drying medium 15.
Downstream of the sample of the mieropore drying medium 15, the test fixture
again tapers
through a reducer at an incbded angle of 7 from a four inch (10.2 cm) nominal
inside
diameter to a two-inch (5.1 cm) nominal inside diameter. This reducer is also.
joined to a
pipe. Such pipe has a length of sevep inches (17.8 cm), is shaight, and has a
two inch (5.1
cm) nominal inside diameter.
800 SCFM per square foot of air flow (377.61iters per second) per square foot
(929
square centimeters) is applied through the mieropore drying medium 15 for a
total of about
70 SCFM (33.04 standard liters per second) per 0.087 square feet (80.8 squsre
centimeters)
for the sample desen'bed herein. The air flow is sapplied at 75 2 F. (23.9
1 C.).
The static pressure drop across the micropore drying medium 15 is measured by
a
manometer, a pair of pressure traasduoers, or other suitable.mcans known in
the art: The
static pressure differential is the drypressure drop for that micropore drying
medium 15.
In order to measure wet pressure drop, the apparatus 10 and sample described
above
are provided. Additionally, a spray nozzle is provided and mounted upstream of
the sample
of the micropore drying medium 15. The spray nozzle is a Spraying System
(Wheaton,
6
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
Illinois) Type TG full cone spray nozzle (1/4 TTG 0.3) with a 0.020 inch (0.05
centimeters)
orifice and 100 mesh screen or equivalent. The nozzle is mounted at a distance
of 5 inches
(12.7 centimeters) upstream of the sample of the micropore drying medium 15.
The nozzle
supplies 0.06 gallons per minute (227 cubic centimeters per minute) of water
at 40 psi
(2810 grams per square centimeter) at a 58 full cone spray angle. The water
is sprayed at a
teinperature of 72 2 F. (22.2 (D 1 C.). The spray completely covers the
sample of the
micropore drying medium 15 and increases the pressure drop therethrough. Wet
pressure
drop is measured at various flow rates. For purposes of determining flow
resistance in
accordance with the present invention, wet pressure drop is measured at 40 and
80 scfin
(18.88 and 37.76 liters per second) per 0.087 square feet (80.8 square
centimeters). If the
wet flow resistance at either flow rate is less at one point in the machine
direction of the
micropore drying apparatus 10 than at a preceding section, the wet flow
resistance is judged
to be less for purposes of the present invention. The wet flow resistance is
judged to be less
at any point in the machine direction of the micropore drying apparatus 10 if
it decreases by
at least 5% preferably at least 10% and more preferably at least 20% as
measured at any two
points spaced apart in the machine direction.
The micropore drying medium 15 of the micropore drying apparatus 10 according
to
the present invention may be stationary, and arranged in a configuration which
allows a
papermaking belt 7 and web disposed thereon to be moved relative to the
stationary
micropore drying medium 15. Suitable stationary configurations for micropore
drying
media 15 include generally cylindrical geometries and geometries having
unequal major and
minor axes. If the latter arrangement is selected, preferably the major axis
MA-MA is
greater than the minor axis MI-MI and disposed in a generally vertical
orientation.
If a stationary micropore drying medium 15 is utilized, the tissue paper 5 may
be
carried on a through air drying belt 7. In such an arrangement, the tissue
paper 5 is
interposed between a movable through air drying belt 7 and a stationary
micropore drying
medium 15.
Suitable papermaking belts 7 include through air drying belts 7 as are well
known in
the art. Preferred through air drying belts 7 are described in commonly
assigned U.S. Pat.
Nos. 3,301,746, issued Jan. 31, 1967 to Sanford et al.; 3,905,863, issued
Sept. 16, 1975 to
7
CA 02452853 2007-05-30
Ayars; 3,974,025, issued Aug. 10, 1976 to Ayers; 4,191,609, issued March 4,
1980 to
Trokhan; 4,239,065, issued Dec. 16, 1980 to Trokhan; 5,366,785 issued Nov. 22,
1994 to
Sawdai; and 5,520,778, issued May 28,1996'to Sawdai; 4,514,345, issued
Apri130, 1985
to Johnson et al.; 4,528,239, issned July 9, 1985 to Trokhan; 5,098,522,
issued March 24,
1992; 5,260,171, issued Nov. 9, 1993 to Smurkosld et al.; 5,275,700, issued
Jan. 4, 1994 to
Trokhan; 5,328,565, issued July 1Z,1994 to Rasch at al.; 5,334,289, issued
Aug. 2,1994 to
Trokhan et al.; 5,431,786, issued July 11, 1995 to Rasch et al.; 5,496,624,
issued March 5,
1996 to Stelljes, Jr. et aL; 5,500,277, issued March 19, 1996 to Trokhan et
al.; 5,514,523,
issued May 7, 1996 to Trokhan et al.; 5,554,467, issued Sept. 10, 1996, to
Trokhan et al.;
5,566,724, issued Oet. 22, 1996 to Trokhan et al.; 5,624,790, issaed April 29,
1997 to
Trokhan et al.; 5,628,876 issued May 13, 1997 to Ayers at al.; 5,679,222
issued Oct. 21,
1997 to Raseh et al.; and 5,714,041 issned Feb. 3, 1998 to Ayers et al.
Yet other papermaldng belts 7 are diselosed in
U.S. Pat. Nos. 5,429,686 issued July 4y 1995 to Clriu at al. and 5,672,248
issued Sept. 30,
1997 to Wendt et al.
The micropore drying medium 15 according to the present invention may provide
for
a residence time thereon of at least 1, preferably at least 25, and more
preferably at least 250
milliseconds, but not more tban 10,000, preferably not more than 5,000, and
more
preferably not more than 1,000 milliseconds. If desired, the micropore drying
medium 15
may comprise multiple zones of differing pressnre,s, as described in the
aforementioned
patents incorporated herein by reference. The micropore drying medium 15
according to
the preswt invention may have a length ranging from 5 nvllimeters to 50
meters, with a
pre&rred length of about 4 to about 30 meters, in order to provid.e adequate
residence time.
The plies 22,24,26,28,30,32 of the micropore drying medium 15 may be joined
together to form aimitary support for the tissue paper 5 as follows. The fnst
ply 22 is
optionally calendered and the subjacent plies 24,26,28,30,32 are preferably
individually
ealendered. The ealendering must be sufficient to provide adequate laiuckle
area for the
sintering operation descnbed below. The calendering must not unduly reduce the
open area
of the pores 40. The calendering may reduee the thiclmess of each ply 22, 24,
26, 28, 30 to
8
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
approximately 65-85% of its original thickness. A considerable range of
calendering levels
may be utilized to provide the desired knuckle area.
The plies 22, 24, 26, 28, 30, 32 are then superimposed upon each other in the
desired
arrangement. As noted above, preferably but not necessarily, the plies 22, 24,
26, 28, 30, 32
are monotonically arranged in order from the smallest pore 40 size to the
largest pore 40
size to form a laminate. Table I below shows a preferred six-ply arrangement.
This
arrangement illustrates a preferred embodiment of one laminated micropore
drying medium
15 suitable for use as the first micropore drying medium 15 which the tissue
paper 5 to be
dried encounters during the drying process.
TABLE I
Ply Warps/Shutes per Warp/Shute diameter (cm)
2.54 cm for plies 1-5 for plies 1-5.
Perf Plate/Hole Size/Pitch Perf Plate Thickness Weave
for P1y 6 for Ply 6.
1 165 x 1400 0.0071/0.0041 Dutch Twill
2 150 x 150 0.0066 Square
3 60 x 60 0.0191 Square
4 30 x 30 0.0406 Square
16 x 16 0.0711 Square
6 1.65 mm diameter holes 24 gauge ss None
on a 2.77 min pitch
In a preferred embodiment, the micropore drying apparatus 10 according to the
present invention may have three sections, each of decreasing flow resistance.
Successive
sections may be provided with a relatively coarser first ply 22. The second
through sixth
ply 32 may be the same in all three sections of mutually differing flow
resistances. Table
IIA below illustrates three different suitable embodiments of the first ply
22. The
successive numbers below indicate the successive positions in the machine
direction in
which the micropore media 15 of the micropore drying apparatus 10 having the
specified
9
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
first ply 22 may be disposed. Position 1 in the Table below precedes Position
2 which
precedes Position 3 as the positions are taken in the inachine direction.
Thus, the tissue
paper 5 will encounter positions 1, 2, 3, respectively, in that order.
TABLE IIA
Position of Warps/Sliutes Warp/Shute Weave Pore Density Pore Size
First Ply per 2.54 cm in diameter (cm) (pores per square (microns)
the First Ply centimeter x 10-5)
1 325 x 2300 0.0038/0.0025 Dutch Twill 2.9 7-8
2 165 x 1400 0.0071/0.0041 Dutch Twill 0.89 15-18
3 80 x 700 0.0102/0.0076 Dutch Twill 0.22 34-36
Specific examples of weaves usable in accordance with the first ply 22 of the
present
invention are shown in Table IIB below. Each of the first ply 22 shown in
Table IIB is
made with a Dutch Twill weave.
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
TABLE IIB
Mesh Count Pore Density Pore Size (microns)
(warps/shutes per 2.54 cm) (pores per square centimeter x 10-5
510 x 3600 7.1 4.5
325 x 2300 2.9 7.5
260 x 2000 2.0 9.3
165 x 1400 0.89 15
130 x 900 0.45 23
80 x 700 0.22 35
24 x 300 0.03 117
The micropore drying apparatus 10 according to the present invention may have
a
pore 40 size which is variable in, and which preferably increases in, the
machine direction.
The increasing pore 40 size may be provided by having different first ply 22
joined to each
other in abutting relationship. The plies 22,24,26,28,30,32 may be sequenced
without
interruption, except for the means used to sequentially join each first ply 22
to the
succeeding plies 24,26,28,30,32. The plies 22,24,26,28,30,32 may be joined
together using
any known means, including a full penetration tungsten weld or panels bolted
into place.
Alternatively, the first ply 22 may be abutted in end-to-end relationship,
with each first ply
22 being joined to subjacent plies 24,26,28,30,32 as described above. The
subjacent plies
24,26,28,30,32 may be joined together using welding or other means known in
the art. If the
micropore drying medium 15 is held stationary, by abutting adjacent first ply
22 together
with a joining technique which does not involve the first ply 22, interruption
in the air flow
therethrough, and hence adverse effects on the drying rate can be minimized.
Prophetically
a bolted construction can be used instead of a welded construction if the
micropore media
15 is held stationary.
In another execution, the micropore drying medium 15 may be movable in the
machine direction. In such an embodiment, preferably the micropore drying
medium 15 is
disposed as or on the cover of an axially rotatable roll, as is known and
illustrated in the art.
11
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
The axially rotatable roll carries the tissue paper 5 thereon. Optionally, a
through air drying
belt 7 thereon. The tissue paper 5 and/or through air drying belt 7 may be
utilized. The
tissue paper 5 and/or through air drying belt 7 do not move relative to the
cover of the
axially rotatable roll while it is rotating to minimize tearing of the tissue
papers 5.
Referring to Fig. 3, the micropore drying apparatus 10 may comprise two or
more
discrete units instead of a single integral unit. By discrete units, it is
meant that each unit is,
of itself, an integral unit. However, the two discrete units are mutually
separate and spaced
apart in the machine direction. At the space between the discrete units, the
tissue paper 5 is
not subjectable to the micropore drying process.
If a plurality of discrete units are selected for the micropore drying
apparatus 10
according to the present invention, first and second discrete units, or any
number of
successive discrete units, may be provided. Each discrete unit is spaced apart
in the
machine direction from the preceding discrete unit. Each successive discrete
unit preferably
has larger pores 40 than that of the preceding discrete unit.
The execution of Fig. 3 illustrates a movable micropore drying medium 15. The
movable micropore drying medium 15 is in the form of an endless belt
comprising a closed
loop. Two discrete units are provided. The first discrete unit illustrates the
optional
through air drying belt 7. The through air drying belt 7 and the first unit of
the micropore
drying medium 15 are juxtaposed such that the tissue paper 5 to be dried is
interposed
therebetween.
One of ordinary skill will recognize that the through air drying belt 7 may
further
transport the tissue paper 5 to be dried closer to the second discrete unit
prior to transfer.
Alternatively, and perhaps preferably, the through air drying belt 7 may carry
the tissue
paper 5 to be dried entirely throughout the second discrete unit of the
micropore drying
apparatus 10.
One of ordinary skill will recognize that the execution of Fig. 3, having
first and
second discrete units, may comprise two axially rotatable rolls. The first
axially rotatable
roll is usable as the first discrete unit, while the second axially rotatable
roll is usable as the
second discrete unit. Using axially rotatable rolls as the micropore drying
apparatus 10 of
12
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
the present invention provides the benefit of ease of construction and a
micropore drying
medium 15 which moves in tandem with the tissue paper 5 to be dried.
Of course, it will be recognized by one of ordinary skill that a second
micropore
drying medium 15 may be disposed in face-to-face relationship with the
backside of the
through air drying belt 7. This arrangement provides the benefit that the
through air drying
belt 7 may be separately dewatered, preventing rewet of the tissue paper 5.
In a hybrid arrangement, the pore 40 size may be variable within one or more
discrete
units of the micropore drying apparatus 10. The largest pore 40 size of a
first discrete unit
may be matched to, larger than or smaller than the smallest pore 40 size of a
second or
succeeding discrete unit, and so on. Preferably, the largest pores 40 of the
first discrete unit
are slightly smaller than or the same size as the smallest pores 40 of the
second discrete unit
in order to efficiently remove mechanically bound water. It is to be
recognized that
variations in both residence time and pore 40 sizes may be utilized with any
of the
foregoing arrangements.
If desired, at least one of the discrete units of the micropore drying
apparatus 10 may
comprise pores 40 which are smaller than the interstices of the tissue paper
5. Further, such
pores 40 may have a vacuum applied thereto, which vacuum is provided at a
pressure less
than the breakthrough pressure of the pores 40. Such a discrete unit may be
made according
to the teachings of commonly assigned U.S. 4,556,450, issued. 1985 to Chuang
et al., or
5,584,126, issued Dec. 17, 1996 to Ensign et al., and incorporated herein by
reference.
For the embodiments described and claimed herein, the smallest pores 40 of the
micropore drying apparatus 10, whether comprised of a single integral unit or
a plurality of
discrete units, may range from a lower limit of at least 1, and preferably at
least 5 microns
to an upper limit of 20, and preferably an upper limit of 10 microns. The
largest pores 40 of
the micropore drying apparatus 10 according to the present invention again,
whether
comprised of a single integral unit or a plurality of discrete units, may
range from a lower
limit of at least 20, and preferably at least 30 microns to an upper limit of
not more than
120, and preferably not more than 40 microns.
Additionally, the decrease in flow resistance of the micropore drying
apparatus 10
which occurs in the machine direction may be provided by increasing the
density of the
13
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
pores 40 in one or more micropore drying media 15. By density or pore 40
density, it is
meant the number of pores 40 through the first, or most restrictive, ply of
the micropore
drying medium 15 per unit area. As the pore 40 density, or number of pores 40
per unit area
of micropore drying medium 15, increases at constant pore size, greater air
flow will occur
for a given area of the micropore drying medium 15 and wet flow resistance
will be
decreased. For the variable pore 40 size embodiments described herein, the
first ply 22 of
the micropore drying media 15 may have the pore 40 density and pore 40 size
listed in
Table III for the first, second and third positions of the first ply 22 of the
micropore drying
medium 15.
TABLE III
Position of Pore Density Pore Size
First Ply (pores per square (microns)
centimeter x 10-5)
1 2.0-7.1 4.5-9.3
2 0.45-2.0 9.3-23
3 0.03-0.45 23-117
If desired, the mesh count and wire size of such a micropore drying medium 15
may
be adjusted to achieve constant pore 40 size.
Of course, while embodiments having three successive positions in the machine
direction are described, it is to be realized that embodiments having any
number of
positions may be utilized for the micropore drying apparatus 10 of the present
invention.
The positions may be contiguous or may be spaced apart in the machine
direction.
Preferably, the wet flow resistance monotonically decreases in each successive
section,
however, it is possible that in a less preferred embodiment, certain sections
may be of
increasing wet flow resistance.
14
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
If a tissue paper 5 has low and high density regions, as described above, the
sizes of
the pores 40 may be optimized relative to the sizes of the interstitials of
the low and high
density regions of the tissue paper 5. Typically, the low density regions have
larger sized
interstitials than the high density regions. The sizes of the interstitials
are distributed in a
normal distribution, commonly measured as a pore volume distribution. Pore
volume
distribution is measured by a Pore Volume Distribution Analyzer, made by TRI
of
Princeton, New Jersey.
For example, a micropore drying apparatus 10 having three sections, each of
decreasing flow resistance, may be provided. The pore 40 size of the first ply
22 in the first
section may be less than the size of the midpoint of the pore volume
distribution of the
interstitials of the high density regions in the tissue paper 5. The pore 40
size of the first ply
22 in the second section may range from approximately the size of the midpoint
of the pore
volume distribution of the interstitials in the high density region to
approximately the
midpoint of the pore volume distribution of the interstitials of the low
density regions. The
pore 40 size of the first ply 22 in the third section may approximate the
midpoint of the size
of the interstitials in the low density region.
Alternatively, decreasing flow resistance in the machine direction may be
accomplished by providing successive micropore drying media 15 with an
intrinsically
lesser wet flow resistance. For example, successive micropore drying media 15,
or
successive portions of a single micropore drying medium 15, may be treated
with, or made
of, a material having an inherently lesser surface energy. For exainple, one
or more
micropore drying media 15, and particularly the surface of the pores 40 which
provide the
limiting flow path through the micropore drying media 15, may be treated with
low surface
energy extruded plastics such as polyesters or polypropylenes or the micropore
drying
medium 15 may be woven from such materials. Alternatively, the micropore
drying media
15 may be treated with a dry film uniform coating of silicone. Any means which
reduces
the flow resistance through the micropore drying medium 15 is considered to be
suitable.
Further information on such coatings and reducing the surface energy of the
micropore
drying media 15 may be found in commonly assigned U.S. Patent Nos. 5,912,072,
issued
CA 02452853 2007-05-30
June 15,1999 to Trokhan et al. and 6,021,583, issued Feb. 8, 2000 to Stelljes,
Jr. et al.
Additionally, one of ordittary siill will recognize there are other ways to
decresse the
flow resistance in various parts of the micropore drying medium 15 according
to the present
invention. For example, the first ply 22 of the micropore drying medium 15 may
be
provided with pores 40 therethrough having a less tortnous flow path.
Far.example, the Z-
direction dimension of the pores 40 could become etraigbter or shorter.
Altenuatively, the
flow resistance through the pores 40 of the firat ply 22 may be impacted by
the hydraulic
radius of the pores 40. As the hydraulic radius of the pores 40 increases, the
flow resistance
therethrough will likewise deaease.
Optionally, a stationary mieropore drying spparatus 10 may have a cover
therearound.
The cover rotates with, and preferably at the smne surface speed as, the
tissue paper 5 to be
dried thereon. If such a rotatable cover is used, preferably the cover has
pores 40 laW
than the interstitiabi in the tissue paper 5 to be dried, so that the flow
restrir,rim sti71 occurs
at the stationary micropore drying medium 15. This arrangement provides the
benefit that,
if desired, air flow may be drawn in through the web and into the interior of
the micaopore
drying apparatus 10. Alternatively, the micxopore drying apparatus 10 may blow
air out
through the micropore drying medium 15 and then through the web.
If the micropore drying medium 15 is deployed as a stationary cover, it may be
utilized on a mll having a noncircular paofile. The profile of the roll is
taken orthogonal to
the machine direction. The profile of the rnll may be flat, elliptioal as
shown, and have a
major axis MA-MA greater than the minor axis MI-NII. If the major axis MA-MA
is
generally vertically oriented, as illustrated, a smaller footprint will be
necessary to
accommodate a micropore drying medium 15 of increased residenae time.
Altematively,
the micropore drying medium 15 having a noncircular profile may be exocutod in
the form
of an endless loop and be movable in the machine direction.
If desired, a roll may be used to lightly press the web against the micropore
drying
medinm 15. A roll to lightiy press the web against the micropore drying medium
15 may be
disposed at the fcrst zone of the micropore drying apparatus 10, the second
zone of the
16
CA 02452853 2003-12-31
WO 03/016620 PCT/US02/25739
micropore drying apparatus 10, or both. Lightly pressing a web against a roll
is generally
described in U.S. Pat Nos. 5,598,643; 5,701,682; and 5,772,845.
In the description of the invention, various embodiments and/or individual
features
are disclosed. All combinations of such inventions and features are possible
and can result
in preferred executions of the invention.
17
