U.S. patent application number 09/836928 was filed with the patent office on 2002-07-11 for impingement air dry process for making absorbent sheet.
Invention is credited to Edwards, Steven L., Watson, Gary M..
Application Number | 20020088577 09/836928 |
Document ID | / |
Family ID | 26894633 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020088577 |
Kind Code |
A1 |
Watson, Gary M. ; et
al. |
July 11, 2002 |
Impingement air dry process for making absorbent sheet
Abstract
A process for making absorbent sheet includes: (a) depositing an
aqueous furnish of cellulosic fiber on a forming fabric; (b)
dewatering the wet web to a consistency of from about 15 to about
40 percent; (c) transferring the dewatered web from the forming
fabric to another fabric traveling at a speed of from about 10 to
about 80 percent slower than the forming fabric; (d) wet-shaping
the web on an impression fabric whereby the web is macroscopically
rearranged to conform to the surface of the impression fabric; and
(e) impingement air drying the web. The process is particularly
suitable for making high bulk products form difficult to process
furnishes such as recycle furnishes and for making high basis
weight products without compressive dewatering with a papermaking
felt.
Inventors: |
Watson, Gary M.; (Vancouver,
WA) ; Edwards, Steven L.; (Fremont, WI) |
Correspondence
Address: |
Michael W. Ferrell, Esq.
Ferrell & Ferrell, L.L.P.
90 Crystal Run Road, Suite 401
Middletown
NY
10941
US
|
Family ID: |
26894633 |
Appl. No.: |
09/836928 |
Filed: |
April 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60199301 |
Apr 24, 2000 |
|
|
|
Current U.S.
Class: |
162/109 ;
162/141; 162/147; 162/207 |
Current CPC
Class: |
D21F 5/18 20130101; D21F
11/14 20130101; D21F 11/006 20130101; D21F 11/145 20130101 |
Class at
Publication: |
162/109 ;
162/207; 162/141; 162/147 |
International
Class: |
D21F 005/00 |
Claims
What is claimed is:
1. A method of making an absorbent sheet comprising: (a) depositing
an aqueous furnish comprising cellulosic fiber on a forming fabric;
(b) dewatering the wet web to a consistency of from about 15 to
about 40 percent; (c) transferring the dewatered web at said
consistency of from about 15 to about 40 percent to another fabric
traveling at a speed of from about 10 to about 80 percent slower
than the speed of the dewatered web prior to such transfer in order
to impart machine direction stretch into the absorbent sheet; (d)
macroscopically rearranging said web to conform to the surface of
an impression fabric; and (e) impingement air drying said web to
form said absorbent sheet.
2. The method according to claim 1, wherein the wet web is
dewatered to have a consistency of from about 20 to about 30
percent upon transfer in step (c).
3. The method according to claim 1, wherein said web is impingement
air dried at a water removal rate of from about 25 lbs/hr-ft.sup.2
to about 50 lbs/hr-ft.sup.2.
4. The method according to claim 3, wherein said web is impingement
air dried at a water removal rate of from about 30 lbs/hr-ft.sup.2
to about 40 lbs/hr-ft.sup.2.
5. The method according to claim 1, wherein said web is impingement
air dried over an impingement air drying length of from about 50 to
about 300 ft.
6. The method according to claim 5, wherein said web is impingement
air dried over an impingement air drying length of from about 75 to
about 200 ft.
7. The method according to claim 6, wherein said web is impingement
air dried over an impingement air drying length of from about 100
ft. to about 150 ft.
8. The method according to claim 1, wherein said step of
impingement air drying said web comprising drying said web with a
plurality of sequentially arranged impingement air dryers.
9. The method according to claim 8, wherein impingement exhaust air
from a downline dryer is cascaded backward to an upline impingement
air drier.
10. The method according to claim 1, wherein said absorbent sheet
has a basis weight of at least about 10 lbs/3000ft.sup.2.
11. The method according to claim 10, wherein said absorbent sheet
has a basis weight of at least about 15 lbs/3000ft.sup.2.
12. The method according to claim 11, wherein said absorbent sheet
has a basis weight of at least about 20 lbs/3000ft/.sup.2.
13. The method according to claim 1, wherein the cellulosic fiber
present in said furnish comprises recycle fiber.
14. The method according to claim 13, wherein the recycled fiber in
said aqueous furnish comprises at least about 50 percent by weight
of the fiber present.
15. The method according to claim 14, wherein the recycled fiber in
said aqueous furnish comprises at least about 75 percent by weight
of the fiber present.
16. The method according to claim 1, wherein said step of
impingement air drying said web comprises impingement air drying
said web on an impression fabric supported on a vacuum cylinder in
opposed facing relationship with an impingement air drying
hood.
17. The method according to claim 1, further comprising the steps
of: (f) adhering the impingement air dried web to a rotating
cylinder and (g) creping said web from said cylinder.
18. The method according to claim 17, wherein said rotating
cylinder is a heated rotating cylinder.
19. The method according to claim 17, wherein the impingement air
dried web is applied to said rotating cylinder with the aid of an
adhesive.
20. The method according to claim 1, wherein said web is
impingement air dried on said impression fabric to a consistency of
at least about 90%.
21. The method according to claim 20, wherein said web is
impingement air dried to a consistency of at least about 95%.
22. An absorbent sheet made by the method according to claim 1.
23. The method according to claim 1, wherein said aqueous furnish
comprises recycled fiber.
24. The method according to claim 23, wherein the recycled fiber in
said aqueous furnish comprises at least about 50 percent by weight
of the fiber present.
25. The method according to claim 24, wherein the recycled fiber
present in said aqueous furnish comprises at least about 75 percent
by weight of the fiber present.
26. The method according to claim 1, wherein at least about 10
percent of the fiber in said aqueous furnish has been subjected to
a curling process.
27. The method according to claim 26, wherein at least about 25
percent of the fiber present in said aqueous furnish has been
subjected to a curling process.
28. The method according to claim 27, wherein at least about 50
percent of the fiber in said aqueous furnish has been subjected to
a curling process.
29. The method according to claim 28, wherein at least about 75
percent of the fiber in said aqueous furnish has been subjected to
a curling process.
30. The method according to claim 29, wherein at least about 90
percent of the fiber in said aqueous furnish has been subjected to
a curling process.
31. The method according to claim 26, wherein said method of
curling said fiber comprises concurrently heat treating and
convolving said fiber at an elevated temperature.
32. The method according to claim 31, wherein said fiber is curled
in a disk refiner with saturated steam at a pressure of from about
5 to about 150 psig.
33. The method according to claim 1, wherein said step of
depositing said aqueous cellulosic furnish on said forming fabric
includes foam forming said furnish on said forming fabric.
34. The method according to claim 1, wherein said aqueous furnish
comprises a cationic debonding agent.
35. The method according to claim 34, wherein said aqueous furnish
further comprises a non-ionic surfactant.
36. The method according to claim 1, wherein said web is
impingement air dried at an impingement air drying rate of at least
about 30 pounds of water removed per square foot of impingement air
drying surface per hour.
37. The method according to claim 36, wherein said web is
impingement air dried at an impingement air drying rate of at least
about 40 pounds of water removed per square foot of impingement air
drying area surface per hour.
38. A method of making an absorbent sheet comprising: (a)
depositing an aqueous furnish comprising cellulosic fiber on a
forming fabric; (b) dewatering the wet web to a consistency of from
about 15 to about 40 percent; (c) transferring the dewatered web
from the forming fabric to a transfer fabric traveling at a speed
of from about 10 to about 80 percent slower than the forming
fabric; (d) transferring the web to an impression fabric whereupon
the web is macroscopically rearranged to conform to the surface of
the impression fabric; and (e) impingement air drying the web.
39. The method according to claim 38, wherein the wet web is
dewatered to a consistency of from about 20 to about 30 percent in
Step (b).
40. The method according to claim 38, wherein the transfer fabric
is traveling at a speed of from about 15 to about 40 percent slower
than the forming fabric.
Description
CLAIM FOR PRIORITY
[0001] This non-provisional application claims the benefit of the
filing date of U.S. Provisional Patent Application Ser. No.
60/199,301, of the same title, filed Apr. 24, 2000.
TECHNICAL FIELD
[0002] The present invention relates to methods of making absorbent
cellulosic sheet in general, and more specifically to a process for
making a non-compressively dewatered, impingement air dried
absorbent sheet.
BACKGROUND
[0003] Methods of making paper tissue, towel, and the like are well
known. Typically, such processes include conventional wet pressing
and throughdry processes. Conventional wet pressing processes have
certain advantages over conventional through air drying processes
including: (1) lower energy costs associated with the mechanical
removal of water rather than transpiration drying with hot air; (2)
higher production speeds are more readily achieved with processes
which utilize wet pressing to form a web; and (3) the process is
relatively robust in that it does not require a highly permeable
substrate. On the other hand, throughair drying processes have
become the method of choice for new capital investment,
particularly for producing soft, bulky, premium quality tissue and
towel products.
[0004] One method of making throughdried products is disclosed in
U.S. Pat. No. 5,607,551 to Farrington, Jr. et al. wherein uncreped,
through dried products are described. According to the '551 patent,
a stream of an aqueous suspension of papermaking fibers is
deposited onto a forming fabric and partially dewatered to a
consistency of about 10 percent. The wet web is then transferred to
a transfer fabric travelling at a slower speed than the forming
fabric in order to impart increased stretch into the web. The web
is then transferred to a throughdrying fabric where it is dried to
a final consistency of about 95 percent or greater employing a
vacuum of from about 3 to about 15 inches of mercury.
[0005] There is disclosed in U.S. Pat. No. 5,510,002 to Hermans et
al. various throughdried, creped products. There is taught in
connection with FIG. 2, for example, a throughdried/wet-pressed
method of making crepe tissue wherein an aqueous suspension of
papermaking fibers is deposited on a forming fabric, dewatered in a
press nip between a pair of felts followed by wet straining the web
on a throughair drying fabric, and throughair drying. The
throughdried web is adhered to a Yankee dryer, further dried and
creped to yield the final product.
[0006] Throughdried, creped products are also disclosed in the
following patents: U.S. Pat. No. 3,994,771 to Morgan, Jr. et al.;
U.S. Pat. No. 4,102,737 to Morton; and U.S. Pat. No. 4,529,480 to
Trokhan. The processes described in these patents comprise, very
generally, forming a web on a foraminous support, thermally
pre-drying the web, applying the web to a Yankee dryer with a nip
defined, in part, by an impression fabric and creping the product
therefrom.
[0007] As noted in the above, throughdried products tend to exhibit
enhanced bulk and superior tactile properties; however,
conventional thermal dewatering with hot air tends to be energy
intensive and requires a relatively permeable substrate. Thus,
wet-press operations are preferable from an energy perspective and
are more readily applied to high basis weight products and products
made from furnishes containing recycle fiber which tends to form
webs with less permeability than virgin fiber. However, wet press
operations tend to utilize more fiber and thus are more costly on a
square foot basis.
[0008] The state of the art is perhaps further understood by way of
the following patents. It will be appreciated that high production
rates (sheet speeds) are exceedingly important to the viability of
any particular production process due to the large investment. In
connection with paper manufacture, it has been suggested, for
example, to employ an air foil to stabilize web transfer off of a
Yankee dryer in order to maintain suitable production rates.
[0009] There is disclosed in U.S. Pat. No. 5,851,353 to Fiscus et
al. a method for can drying wet webs for tissue products wherein a
partially dewatered wet web is restrained between a pair of molding
fabrics. The restrained wet web is processed over a plurality of
can dryers, for example, from a consistency of about 40 percent to
a consistency of at least about 70 percent. The sheet molding
fabrics protect the web from direct contact with the can dryers and
impart an impression on the web.
[0010] There is disclosed in U.S. Pat. No. 5,087,324 to Awofeso et
al. a delaminated stratified paper towel. The towel includes a
dense first layer of chemical fiber blend and a second layer of a
bulky anfractuous fiber blend unitary with the first layer. The
first and second layers enhance the rate of absorption and water
holding capacity of the paper towel. The method of forming a
delaminated stratified web of paper towel material includes
supplying a first furnish directly to a wire and supplying a second
furnish of a bulky anfractuous fiber blend directly on to the first
furnish disposed on the wire. Thereafter, a web of paper towel is
creped and embossed.
[0011] There is disclosed in U.S. Pat. No. 5,494,554 to Edwards et
al. the formation of wet press tissue webs used for facial tissue,
bath tissue, paper towels, or the like, produced by forming the wet
tissue in layers in which the second formed layer has a consistency
which is significantly less than the consistency of the first
formed layer. The resulting improvement in web formation enables
uniform debonding during dry creping which, in turn, provides a
significant improvement in softness and reduction in linting. Wet
pressed tissues made with the process according to the '554 patent
are internally debonded as measured by a high void volume
index.
[0012] As will be appreciated from the foregoing, processes for
making absorbent sheet generally incorporate two types of drying:
(1) can drying where high density, low permeability can be
tolerated and (2) throughdrying which requires a permeable
substrate. The present invention is directed to making high bulk
products wherein the permeability of the substrate is not
critical.
SUMMARY OF THE INVENTION
[0013] There is provided in one aspect of the present invention a
method of making absorbent sheet including the steps of: (a)
depositing an aqueous furnish comprising cellulosic fiber on a
foraminous support; (b) dewatering (preferably non-compressively
dewatering) the wet web to a consistency of from about 15 to about
40 percent; (c) transferring the dewatered web at the aforesaid
consistency to another fabric traveling at a speed of from about 10
to about 80 percent slower than the speed of the web prior to
transfer; (d) macroscopically rearranging the web to conform to the
shape of an impression fabric; and (e) impingement air drying the
web to form an absorbent sheet. Typically, the web is dewatered to
a consistency of from about 20 to about 30 percent prior to
transfer and impingement air dried at a rate of from about 25-50
lbs of water removed per hour per square foot of drying area.
Drying rates of from about 30-40 lbs/hr-ft.sup.2 are typical, over
drying lengths of from about 50 to 300 feet. Impingement air drying
lengths are typically from about 75 to about 200 feet, with from
about 100 to 150 feet being a preferred construction of a paper
machine to practice the present invention.
[0014] Most typically, the step of impingement air drying is
carried out over a plurality of impingement air dryers including
rotating cylinders and drying hoods sequentially arranged in a row
opposing a row of reversing vacuum cylinders over which the web
travels. In this arrangement, impingement exhaust air from a
downline dryer can be cascaded backward to an upline dryer
operating at higher humidity.
[0015] A product of any typical basis weight may be made by way of
the present invention, suitably having a weight of at least 10
lbs/3000 ft.sup.2. Higher basis weight products, having basis
weights of at least 15 lbs/3000 ft.sup.2 or at least 20 lbs/3000
ft.sup.2 may also be produced as will readily be appreciated from
the discussion which follows.
[0016] Typically, the web is impingement air dried to a consistency
of at least about 90% and in preferred embodiments to a consistency
of about 95 percent or so.
[0017] In another aspect of the present invention, there is
provided the additional steps of: adhering the impingement air
dried web to a rotating cylinder and creping the web from the
cylinder. A creping adhesive may be used, and the cylinder may be
heated if so desired.
[0018] There is provided in still yet another aspect of the present
invention a method of making an absorbent sheet including the steps
of: (a) depositing an aqueous furnish comprising cellulosic fiber
on a forming fabric; (b) dewatering the wet web to a consistency of
from about 15 to about 40%; (c) transferring the dewatered web from
the forming fabric to a transfer fabric traveling at a speed of
from about 10 to about 80% slower than the forming fabric; (d)
transferring the web to an impression fabric whereby the web is
macroscopically rearranged to conform to the surface of the
impression fabric; and (e) impingement air drying the web.
Typically, the wet web is dewatered to a consistency of from about
20 to about 30% in step (b). So also, the transfer fabric is
typically traveling at a speed of from about 15 to about 40% slower
than the forming fabric.
[0019] Any suitable aqueous furnish may be employed; in many
embodiments the furnish includes recycled fiber. Recycled fiber may
be present in any amount; particularly preferred embodiments
oftentimes include at least about 50 percent by weight recycled
fiber, based on the amount of fiber present. More than about 75
percent by weight of the fiber may be recycled fiber or the
cellulosic fiber in the furnish may consist entirely of recycled
fiber.
[0020] In order to achieve enhanced bulk and softness it may be
desirable in many embodiments to subject at least a portion of the
fiber to a curling process. For example, one may subject at least
about 10 percent of the fiber in the aqueous furnish to a curling
process or at least about 25 percent of the fiber in the furnish to
a curling process. Where particularly high bulk is desired, one may
subject 75%, 90% or even more of the fiber present in the aqueous
furnish to a curling process. While any suitable curling process
may be used to increase the curl inherent in the fiber, a
particularly preferred process includes concurrently heat treating
and convolving the fiber at an elevated temperature. Such processes
may be carried out in a disk refiner, for example, with saturated
steam at a pressure of from about 5 to about 150 psig. Another
method of increasing the bulk may include foam forming the furnish
on the forming fabric as is known in the art. See, for example,
U.S. Pat. No. 5,200,035, the disclosure of which is incorporated
herein by reference.
[0021] In a typical embodiment, the aqueous furnish will further
include a debonding agent, such as a cationic debonding agent. In
some embodiments, it may be preferred to include both a cationic
debonding agent and a non-ionic surfactant.
[0022] It is desirable to dry the web at the highest rate
achievable with the impingement air dryer. Preferably a drying rate
of at least about 30 pounds of water removed per square foot of
impingement air drying surface per hour is preferred. More
preferably, a drying rate of at least 40 pounds of water removed
per square foot of impingement air drying surface per hour is
attained.
[0023] The present invention further includes absorbent sheet made
by the aforesaid process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The present invention is described in detail below with
reference to the various figures. In the figures:
[0025] FIGS. 1(a) and 1(b) are plots showing drying time and air
permeability for a 9 lb/3000 ft.sup.2 basis weight absorbent
sheet;
[0026] FIGS. 2(a) and 2(b) are plots showing drying time and air
permeability for a 13 lb/3000 ft.sup.2 basis weight absorbent
sheet;
[0027] FIGS. 3(a) and 3(b) are plots showing drying time and air
permeability for a 14 lb/3000 ft.sup.2 basis weight absorbent
sheet;
[0028] FIGS. 4(a) and 4(b) are plots showing drying time and air
permeability for a 28 lb/3000 ft.sup.2 basis weight absorbent
sheet;
[0029] FIG. 5 is a schematic diagram of a papermaking machine
useful for practicing the process of the present invention;
[0030] FIG. 6 is a schematic diagram of another papermaking machine
useful for practicing the process of the present invention;
[0031] FIG. 7(a) is a schematic diagram illustrating details of an
impingement air dryer useful in connection with the present
invention;
[0032] FIG. 7(b) is a diagram illustrating the operation of the
impingement air drying apparatus of FIG. 7(a);
DETAILED DESCRIPTION
[0033] The present invention is described in detail below for
purposes of exemplification only. Various modifications within the
spirit and scope of the present invention, set forth in the
appended claims, will be readily apparent to those of skill in the
art. According to the present invention, an absorbent paper web can
be made by dispersing fibers into aqueous slurry and depositing the
aqueous slurry onto the forming wire of a papermaking machine. Any
art recognized forming technique might be used. For example, an
extensive but non-exhaustive list includes a crescent former, a
C-wrap twin wire former, an S-wrap twin wire former, a suction
breast roll former, or a Fourdrinier former. The particular forming
apparatus is not critical to the success of the present invention.
The forming fabric can be any suitable foraminous member including
single layer fabrics, double layer fabrics, triple layer fabrics,
photopolymer fabrics, and the like. Non-exhaustive background art
in the forming fabric area include U.S. Pat. Nos. 4,157,276;
4,605,585; 4,161,195; 3,545,705; 3,549,742; 3,858,623; 4,041,989;
4,071,050; 4,112,982; 4,149,571; 4,182,381; 4,184,519; 4,314,589;
4,359,069; 4,376,455; 4,379,735; 4,453,573; 4,564,052; 4,592,395;
4,611,639; 4,640,741; 4,709,732; 4,759,391; 4,759,976; 4,942,077;
4,967,085; 4,998,568; 5,016,678; 5,054,525; 5,066,532; 5,098,519;
5,103,874; 5,114,777; 5,167,261; 5,199,261; 5,199,467; 5,211,815;
5,219,004; 5,245,025; 5,277,761; 5,328,565; and 5,379,808 all of
which are incorporated herein by reference in their entirety. The
particular forming fabric is not critical to the success of the
present invention. One forming fabric particularly useful with the
present invention is Voith Fabrics Forming Fabric 2184 made by
Voith Fabrics Corporation, Shreveport, La.
[0034] Any suitable transfer fabric may be used to transfer the web
between the forming fabric and the impression fabric in embodiments
of the invention wherein an intermediate transfer fabric is
utilized. In this respect, note U.S. Pat. No. 5,607,551 to
Farrington et al., the disclosure of which is hereby incorporated
by reference. The speed of the transfer fabric is substantially
slower than the speed of the forming fabric in order to impart
machine direction stretch into the web. Transfer fabrics include
single layer, multi-layer or composite permeable structures.
Preferred fabrics have at least one of the following
characteristics: (1) on the side of the transfer fabric that is in
contact with the wet web (the "top" side), the number of machine
direction (MD), strands per inch (mesh), is from about 10 to 200
(4-80 per cm) and the number per cm of cross direction (CD) strands
per inch (count) is also from about 10 to 200. The strand diameter
is typically smaller than 0.050 inch (1.3 mm); and (2) on the top
side the distance between the highest point of the MD knuckle and
the highest point on the CD knuckle is from about 0.001 to about
0.02 or 0.03 inch (0.025 to about 0.5 or 0.75 mm). In between these
two levels, there can be knuckles formed either by MD or CD strands
that give the topography a three dimensional characteristic.
Specific suitable transfer fabrics include, by way of example,
those made by Asten Forming Fabrics Inc., Appleton Wis., and
designated as numbers 934, 937, 939 and 959 and Albany 94M
manufactured by Albany International, Appleton Wire Division,
Appleton Wis.
[0035] The impression fabric is also suitably a coarse fabric such
that the wet web is supported in some areas and unsupported in
others in order to enable the web to flex and response to
differential air pressure or other deflection force applied to the
web. Such fabric suitable for purposes of this invention include,
without limitation, those papermaking fabric which exhibit
significant open area or three dimensional surface contour or
depression sufficient to impart substantial Z-directional
deflection of the web and one disclosed, for example, in U.S. Pat.
No. 5,411,636 to Hermans et al., the disclosure of which is hereby
incorporated by reference.
[0036] Suitable impression fabrics sometimes utilized as
throughdrying fabrics likewise include single layer, multi-layer,
or composite permeable structures. Characteristics are similar to
those of the intermediate transfer fabrics noted above. Preferred
fabrics thus have at least one of the following characteristics:
(1) on the side of the impression fabric that is in contact with
the wet web (the "top" side), the number of machine direction (MD)
strands per inch (mesh) is from 10 to 200 and the number of cross
direction (CD) strands per inch (count) is also from 10 to 200. The
strand diameter is typically smaller than 0.050 inch; (2) on the
top side, the distance between the highest point of the MD knuckle
and the highest point on the CD knuckle is from about 0.001 to
about 0.02 or 0.03 inch. In between these two levels there can be
knuckles formed either by MD or CD strands that give the topography
a three dimensional hill/valley appearance which is imparted to the
sheet during the wet molding step; (3) on the top side, the length
of the MD knuckles is equal to longer than the length of the CD
knuckles; (4) if the fabric is made in a multi-layer construction,
it is preferred that the bottom layer is of a finer mesh than the
top layer so as to control the depth of web penetration to maximize
fiber retention; and (5) the fabric may be made to show certain
geometric patterns that are pleasing to the eye, which is typically
repeated between every two to 50 warp yarns. Suitable commercially
available coarse fabrics include a number of fabrics made by Asten,
Forming Fabrics, Inc., including without limitation Asten 934, 920,
52B, and Velostar V800. In embodiments where both a coarse
intermediate transfer fabric and an impression fabric are used, the
geometry and orientation of the fabrics are orthogonally optimized
to provide the desired machine direction and cross-direction
stretch.
[0037] The consistency of the web when the differential pressure is
applied to conform the web to the shape of the forming fabric must
be high enough that the web has some integrity and that a
significant number of bonds have formed within the web, yet not so
high as to make the web unresponsive to the differential air
pressure or other pressure applied to force the web into the
impression fabric. At consistency approaching dryness, for example,
it is difficult to draw sufficient vacuum on the web because of its
porosity and lack of moisture. Preferably the consistency of the
web about its surface will be from about 30 to about 80 percent and
more preferably from about 40 to about 70 percent and still more
preferably from about 45 to about 60 percent. While the invention
as illustrated below in connection with vacuum molding, the means
for deflecting the wet web to create the increase in internal bulk
can be pneumatic means, such as positive and/or negative air
pressure or mechanical means such as a male engraved roll having
protrusions which match up with the depressions in the coarse
fabric. Deflection of the web is preferably achieved by
differential air pressure, which can be applied by drawing vacuum
through the supporting coarse fabric to pull the web into the
coarse fabric or by applying the positive pressure into the fabric
to push the web into the coarse fabric. A vacuum suction box is a
preferred vacuum source because it is common to use in papermaking
processes. However, air knives or air presses can also be used to
supply positive pressure, where vacuums cannot provide enough
pressure differential to create the desired effect. When using a
vacuum suction box the width of the vacuum slot can be from
approximately {fraction (1/16)} inch to whatever size is desired as
long as sufficient pump capacity exists to establish sufficient
vacuum time. It is common practice to use vacuum slot from 1/8 inch
to 1/2 inch.
[0038] The magnitude of the pressure differential and the duration
of the exposure of the web to the pressure differential can be
optimized depending on the composition of the furnish, the basis
weight of the web, the moisture content of the web, the design of
the supporting coarse fabric and the speed of the machine. Suitable
vacuum levels can be from about 10 inches of mercury to about 30
inches of mercury, preferably from about 15 to about 25 inches of
mercury and most preferably about 20 inches of mercury.
[0039] Papermaking fibers used to form the absorbent products of
the present invention include cellulosic fibers commonly referred
to as wood pulp fibers, liberated in the pulping process from
softwood gymnosperms or coniferous trees and hardwoods (angiosperms
or deciduous trees). Cellulosic fibers from diverse material
origins may also be used to form the web of the present invention.
These fibers include non-woody fibers liberated from sugar cane,
bagasse, sabai grass, rice straw, banana leaves, paper mulberry
(i.e., bast fiber), abaca leaves, pineapple leaves, esparto grass
leaves, and fibers from the genus hesperaloe in the family
Agavaceae. Also recycled fibers which may contain all of the above
fiber sources in different percentages, can be used in the present
invention. Suitable fibers are disclosed in U.S. Pat. Nos.,
5,320,710 and 3,620,911, both of which are incorporated herein by
reference.
[0040] Papermaking fibers can be liberated from their source
material by any one of a number of chemical pulping processes
familiar to one experienced in the art including sulfate, sulfite,
polysulfide, soda pulping, etc. The pulp can be bleached if desired
by chemical means including the use of chlorine, chlorine dioxide,
oxygen, etc. Furthermore, papermaking fibers can be liberated from
source material by any one of a number of mechanical/chemical
pulping processes familiar to anyone experienced in the art
including mechanical pulping, thermomechanical pulping, and
chemithermomechanical pulping. These mechanical pulps can be
bleached, if necessary, by a number of familiar bleaching schemes
including alkaline peroxide and ozone bleaching.
[0041] Furnishes utilized in connection with the present invention
may contain significant amounts of secondary fibers that possess
significant amounts of ash and fines. It is common in the industry
to hear the term ash associated with virgin fibers. This is defined
as the amount of ash that would be created if the fibers were
burned. Typically no more than about 0.1% to about 0.2% ash is
found in virgin fibers. Ash as used in the present invention
includes this "ash" associated with virgin fibers as well as
contaminants resulting from prior use of the fiber. Furnishes
utilized in connection with the present invention may include
excess of amounts of ash greater than about 1% or more. Ash
originates when fillers or coatings are needed to paper during
formation of a filled or coated paper product. Ash will typically
be a mixture containing titanium dioxide, kaolin clay, calcium
carbonate and/or silica. This excess ash or particulate matter is
what has traditionally interfered with processes using recycle
fibers, thus making the use of recycled fibers unattractive. In
general recycled paper containing high amounts of ash is priced
substantially lower than recycled papers with low or insignificant
ash contents. Thus, there will be a significant advantage to a
process for making a premium or near-premium product from recycled
paper containing excess amounts of ash.
[0042] Furnishes containing excess ash also typically contain
significant amount of fines. Ash and fines are most often
associated with secondary, recycled fibers, post-consumer paper and
converting broke from printing plants and the like. Secondary,
recycled fibers with excess amounts of ash and significant fines
are available on the market and are inexpensive because it is
generally accepted that only very thin, rough, economy towel and
tissue products can be made unless the furnish is processed to
remove the ash. The present invention makes it possible to achieve
a paper product with high void volume and premium or near-premium
qualities from secondary fibers having significant amounts of ash
and fines without any need to preprocess the fiber to remove fines
and ash. While the present invention contemplates the use of fiber
mixtures, including the use of virgin fibers, fiber in the products
according to the present invention may have greater than 0.75% ash,
and sometimes more than 1% ash. The fiber may have greater than 2%
ash and may even have as high as 30% ash or more.
[0043] As used herein, fines constitute material within the furnish
that will pass through a 100 mess screen. Ash and ash content is
defined as above and can be determined using TAPPI Standard Method
T211 OM93.
[0044] The suspension of fibers or furnish may contain chemical
additives to alter the physical properties of the paper produced.
These chemistries are well understood by the skilled artisan and
may be used in any known combination.
[0045] The pulp can be mixed with strength adjusting agents such as
wet strength agents, dry strength agents and debonders/softeners.
Suitable wet strength agents are known to the skilled artisan. A
comprehensive but non-exhaustive list of useful strength aids
include urea-formaldehyde resins, melamine formaldehyde resins,
glyoxylated polyacrylamide resins, polyamide-epichlorohydrin resins
and the like. Thermosetting polyacrylamides are produced by
reacting acrylamide with diallyl dimethyl ammonium chloride
(DADMAC) to produce a cationic polyacrylamide copolymer which is
ultimately reacted with glyoxal to produce a cationic cross-linking
wet strength resin, glyoxylated polyacrylamide. These materials are
generally described in U.S. Pat. Nos. 3,556,932 to Coscia et al.
and 3,556,933 to Williams et al., both of which are incorporated
herein by reference in their entirety. Resins of this type are
commercially available under the trade name of PAREZ 631 NC by
Cytec Industries. Different mole ratios of
acrylamide/DADMAC/glyoxal can be used to produce cross-linking
resins, which are useful as wet strength agents. Furthermore, other
dialdehydes can be substituted for glyoxal to produce thermosetting
wet strength characteristics. Of particular utility are the
polyamide-epichlorohydrin resins, an example of which is sold under
the trade names Kymene 557LX and Kymene 557H by Hercules
Incorporated of Wilmington, Del. and CASCAMID.RTM. from Borden
Chemical Inc. These resins and the process for making the resins
are described in U.S. Pat. Nos. 3,700,623 and 3,772,076 each of
which is incorporated herein by reference in its entirety. An
extensive description of polymeric-epihalohydrin resins is given in
Chapter 2: Alkaline-Curing Polymeric Amine-Epichlorohydrin by Espy
in Wet Strength Resins and Their Application (L. Chan, Editor,
1994), herein incorporated by reference in its entirety. A
reasonably comprehensive list of wet strength resins is described
by Westfelt in Cellulose Chemistry and Technology Volume 13, p.
813, 1979, which is incorporated herein by reference.
[0046] Suitable dry strength agents will be readily apparent to one
skilled in the art. A comprehensive but non-exhaustive list of
useful dry strength aids includes starch, guar gum,
polyacrylamides, carboxymethyl cellulose and the like. Of
particular utility is carboxymethyl cellulose, an example of which
is sold under the trade name Hercules CMC by Hercules Incorporated
of Wilmington, Del.
[0047] Suitable debonders are likewise known to the skilled
artisan. Debonders or softeners may also be incorporated into the
pulp or sprayed upon the web after its formation. The present
invention may also be used with softener materials within the class
of amido amine salts derived from partially acid neutralized
amines. Such materials are disclosed in U.S. Pat. No.4,720,383.
Evans, Chemistry and Industry, Jul. 5, 1969, pp. 893-903; Egan, J.
Am. Oil Chemist's Soc., Vol. 55 (1978), pp. 118-121; and Trivedi et
al., J. Am. Oil Chemist's Soc., June 1981, pp. 754-756,
incorporated by reference in their entirety, indicate that
softeners are often available commercially only as complex mixtures
rather than as single compounds. While the following discussion
will focus on the predominant species, it should be understood that
commercially available mixtures would generally be used in
practice.
[0048] Quasoft 202-JR is a suitable softener material, which may be
derived by alkylating a condensation product of oleic acid and
diethylenetriamine. Synthesis conditions using a deficiency of
alkylation agent (e.g., diethyl sulfate) and only one alkylating
step, followed by pH adjustment to protonate the non-ethylated
species, result in a mixture consisting of cationic ethylated and
cationic non-ethylated species. A minor proportion (e.g., about
10%) of the resulting amido amine cyclize to imidazoline compounds.
Since only the imidazoline portions of these materials are
quaternary ammonium compounds, the compositions as a whole are
pH-sensitive. Therefore, in the practice of the present invention
with this class of chemicals, the pH in the head box should be
approximately 6 to 8, more preferably 6 to 7 and most preferably
6.5 to 7.
[0049] Quaternary ammonium compounds, such as dialkyl dimethyl
quaternary ammonium salts are suitable particularly when the alkyl
groups contain from about 14 to 20 carbon atoms. These compounds
have the advantage of being relatively insensitive to pH.
[0050] Biodegradable softeners can be utilized. Representative
biodegradable cationic softeners/debonders are disclosed in U.S.
Pat. Nos. 5,312,522; 5,415,737; 5,262,007; 5,264,082; and
5,223,096, all of which are incorporated herein by reference in
their entirety. The compounds are biodegradable diesters of
quaternary ammonia compounds, quaternized amine-esters, and
biodegradable vegetable oil based esters functional with quaternary
ammonium chloride and diester dierucyldimethyl ammonium chloride
and are representative biodegradable softeners.
[0051] In some embodiments, a particularly preferred debonder
composition includes a quaternary amine component as well as a
nonionic surfactant.
[0052] The quaternary ammonium component may include a quaternary
ammonium species selected from the group consisting of: an
alkyl(enyl)amidoethyl-a- lkyl(enyl)-imidazolinium,
dialkyldimethylammonium, or
bis-alkylamidoethyl-methylhydroxy-ethyl-ammonium salt; wherein the
alkyl groups are saturated, unsaturated, or mixtures thereof, and
the hydrocarbon chains have lengths of from ten to twenty-two
carbon atoms. The debonding composition may include a synergistic
combination of: (a) a quaternary ammonium surfactant component
comprising a surfactant compound selected from the group consisting
of a dialkyldimethyl-ammonium salts of the formula: 1
[0053] wherein each R may be the same or different and each R
indicates a hydrocarbon chain having a chain length of from about
twelve to about twenty-two carbon atoms and may be saturated or
unsaturated; and wherein said compounds are associated with a
suitable anion; and (b) a nonionic surfactant component.
Preferably, the ammonium salt is a dialkyl-imidazolinium compound
and the suitable anion is methylsulfate. The nonionic surfactant
component typically includes the reaction product of a fatty acid
or fatty alcohol with ethylene oxide such as a polyethylene glycol
diester of a fatty acid (PEG diols or PEG diesters).
[0054] A convenient way to enhance product bulk is to provide in
the furnish at the forming end of the process at least a modicum of
curled fiber. This may be accomplished by adding commercially
available high bulk additive ("HBA") available from Weyerhauser or
suitable virgin or secondary fibers may be provided with additional
curl as described in one or more of the following patents, the
disclosures of which are hereby incorporated by reference into this
patent as if set forth in their entirety: U.S. Pat. No. 2,516,384
to Hill et al.; U.S. Pat. No. 3,382,140 to Henderson et al.; U.S.
Pat. No. 4,036,679 to Bach et al.; U.S. Pat. No. 4,431,479 to Barbe
et al.; U.S. Pat. No. 5,384,012 to Hazard; U.S. Pat. Nos.
5,348,620; 5,501,768 to Hermans et al.; or U.S. Pat. No. 5,858,021
to Sun et al. The curled fiber is added in suitable amounts as
noted herein, or, one may utilize 100% curled fiber if so desired
provided the costs are not prohibitive.
[0055] In this latter respect, a particularly cost effective
procedure is simply to concurrently heat-treat and convolve the
fiber in a pressurized disk refiner at relatively high consistency
(20-60%) with saturated steam at a pressure of from about 5 to 150
psig. Preferably, the refiner is operated at low energy inputs,
less than about 2 hp-day/ton and at short residence times of the
fiber in the refiner. Suitable residence times may be less than
about 20 seconds and typically less than about 10 seconds. This
procedure produces fiber with remarkably durable curl as described
in co-pending U.S. patent application Ser. No. 09/793,863, filed
Feb. 27, 2001 (Attorney Docket No. 2247) entitled "Method of
Providing Papermaking Fibers with Durable Curl and Absorbent Sheet
Incorporating Same", assigned to the Assignee of the present
invention, the disclosure of which is hereby incorporated by
reference. If so desired, bleaching chemicals such as caustic and
hydrogen peroxide may be included to increase the brightness of the
product as noted in U.S. patent application Ser. No. 09/793,874,
filed Feb. 27, 2001 (Attorney Docket No. 2159) entitled "Method of
Bleaching and Providing Papermaking Fibers with Durable Curl", the
disclosure of which is also incorporated by reference.
[0056] Impingement air drying is known, for example, in connection
with drying hoods about Yankee dryers. See Convective Heat Transfer
Under Turbulent Impinging Slot Jet at Large Temperature
Differences; Voss et al. Department of Chemical Engineering, McGill
University, Pulp and Paper Research Institute of Canada, Montreal,
Quebec, (Kyoto Conf., 1985). It is distinguished from throughdrying
where all or at least most of the drying fluid actually passes
through the web. Impingement air drying has been utilized in
connection with coated papers. See for example, U.S. Pat. No.
5,865,955 of Ilvesp{umlaut over (aa)}t et al. as well as the
following U.S. patents: U.S. Pat. Nos. 5,968,590and 6,001,421 to
Ahonen et al. the disclosures of which are hereby incorporated by
reference. In connection with impingement air drying, little, if
any, of the drying air passes through the web. Unlike the use of
impingement air drying known in the art, the present invention is
directed to a process wherein absorbent sheet is impingement air
dried on an impression fabric. In preferred embodiments, the web is
non-compressively dewatered prior to being impingement air dried.
By non-compressively dewatering it is meant that the web is not
"squeezed" as in a nip press or as in a nip between a roll and a
papermaking felt, for example, as in a typical shoe press prior to
being impingement air dried.
[0057] The advantages of the present invention over throughdry
processes is appreciated by considering FIGS. 1 through 4.
Throughdry processes for making absorbent sheet require relatively
permeable webs which may or may not be readily formed at high basis
weights or with recycle fiber having a relatively high fines
content. In this respect, a series of 100% recycle absorbent sheet
products were tested suitably for throughdrying by wetting them
300% (consistency of 25%) and drying them with hot air in a
throughdry apparatus.
[0058] FIG. 1(a) is a plot of drying time in seconds versus
moisture content for a dry creped, 9 lb/3000 ft.sup.2 product made
with recycle furnish, wherein the drying temperature was
230.degree. C. and the pressure drop was about 250 mm of water
through the sheet. FIG. 1(b) is a plot of air speed through the
sheet utilized to generate the drying data of FIG. 1(a) at 0%
moisture versus pressure drop in mm of water.
[0059] FIG. 2(a) is a plot of drying time versus moisture ratio for
a wet-creped, 13 lb/3000 ft.sup.2 product made with recycle
furnish, wherein the drying temperature was 220.degree. C. and the
pressure drop was about 480 mm of water through the sheet. FIG.
2(b) is a plot of air speed through the sheet versus pressure drop
at various moisture levels for the sheet used to generate the
drying data of FIG. 2(a).
[0060] FIG. 3(a) is a plot of drying time versus moisture content
for a dry creped, 14 lb/3000 ft.sup.2 product made with recycle
furnish, wherein the drying temperature was 230.degree. C. and the
pressure drop was about 370 mm water through the sheet. FIG. 3(b)
is a plot of air speed through the sheet utilized to generate the
drying time data in FIG. 3(a) versus pressure drop at 0% moisture
content.
[0061] FIG. 4(a) is a plot of drying time versus moisture content
starting at various moisture levels at time=0 for a 28 lb/3000
ft.sup.2, wet creped product made with recycle furnish wherein the
drying temperature was about 220.degree. C. and the pressure drop
was about 480 mm of mercury through the sheet. FIG. 4(b) is a lot
of air speed through the sheet utilized to generate the data of
FIG. 4(a) versus pressure drop through the sheet.
[0062] The data of FIGS. 1(a) through 4(b) may be utilized to
compare a throughdry process with an impingement air dry process of
the present invention as shown in Table 1 below, wherein drying is
calculated beginning at 25% consistency and continuing to 95%
consistency.
1TABLE 1 Comparison of Throughdry Processing With Impingement Air
Drying Basis TAD Length Invention Drying Weight Drying Time Air (@
Length * (lbs/ (From 25% Flow Rate Commercial (@ 30/40 lbs/hr- 3000
ft.sup.2) Cons) (500 mm .DELTA.p) Speed) ft.sup.2) 9 0.5 sec`s
>10 m/sec 50 ft 106/80 ft (6000 fpm) (6000 fpm) 13 5.0 sec`s
0.25-2 m/sec 433 ft 133/100 ft (5200 fpm) (5200 fpm) 14 >1.0
sec`s .about.6 m/sec >83 ft 138/103 (5000 fpm) (5000 fpm) 28
19.5 sec`s 0.75 m/sec 1170 ft 165/124 (3000 fpm) (3000 fpm
[0063] *Basis: Begin drying at 25% consistency (3 lbs water/lb
fiber) and finish drying at 95% consistency.
[0064] Clearly, while through air dry lengths of 50-100 feet could
be considered practical in connection with 16-18 foot diameter
throughdryers with 270 degrees of wrap, lengths above this would
not be. Thus, for sheet with low permeability, throughdrying is
simply not practical. Further savings can be reached by cascading
upline the relatively low humidity heated air used in downline or
subsequent impingement air dryers when a plurality of dryers are
used. This latter feature of the present invention is better
appreciated in connection with FIGS. 5 and 6, further discussed
below.
[0065] There is shown in FIG. 5 a papermaking apparatus 10 useful
for practicing the present invention. Apparatus 10 includes a
forming section 12, an intermediate carrier section 14, a transfer
zone indicated at 16, a pre-dryer/imprinting section 18 and a
plurality of impingement air dryers 20, 22, 24 which include
rotating vacuum cylinders and impingement air hoods as described
below. Also optionally provided is a crepe section 26.
[0066] In section 12 there is provided a headbox indicated at 28,
as well as a forming fabric 30 looped about a breast suction roll
32. A vacuum box 34 non-compressively dewaters furnish deposited on
fabric 30 by way of headbox 28. Fabric 30 is also looped over rolls
36, 38, 40 and 42.
[0067] Intermediate carrier section 14 includes an intermediate
carrier fabric 44 which is supported on rolls 46-56. Fabric 44 also
passes over another vacuum box 58 which further serves to dewater a
nascent web W, traveling in the direction indicated by arrows
60-64. Fabric 44 also passes over an arcuate portion of roll 38, as
well as transfer head 66. Biasing means may be provided to obviate
slack in the various fabrics if so desired.
[0068] Transfer zone 16 includes fabric 44 as well as an impression
of fabric 68, traveling in direction 70. Fabric 68 is looped around
a plurality of support rolls 72-76 which may include biasing means
as noted hereinabove, and is further lopped about cylinders 78, 80
and 82 respectively of impingement air dryers 20, 22 and 24 of
apparatus 10. Further provided is a molding vacuum box 84 which
pulls a vacuum of from about 10 to 30 inches of mercury and is
operative to thus macroscopically rearrange web W to conform to the
shape of impression fabric 68, that is, to shape the wet web and
provide a structure to the product defined by fabric 68. The speeds
of fabric 68 and 44 are independently controlled, with fabric 68
traveling slower than fabric 44, thereby carrying out a so-called
"rush-transfer" during manufacture of a web of the present
invention. The transfer from fabric 44 to 68 is thus carried out as
described in U.S. Pat. No. 4,440,597 to Wells et al., the
disclosure of which is incorporated by reference.
[0069] Apparatus 10 further includes a plurality of vacuum
reversing cylinders 85, 86 arranged in a row parallel to the row
defined by cylinders 78, 80 and 82 as well as another transfer
fabric 88 and a heated rotating creping cylinder 90 provided with a
creping blade 92 in creping section 26.
[0070] In operation, web W is formed on fabric 30, transferred to
fabric 44 which travels at a velocity, VI. From fabric 44, web W is
transferred to fabric 68 at transfer section 18 wherein transfer is
aided by way of vacuum transfer head 66 as shown. Transfer fabric
68, which is a coarse impression fabric as noted above, travels at
a velocity, V2, which is characteristically in accordance with the
invention smaller than velocity VI of fabric 44.
[0071] After transfer, web W is macroscopically rearranged at
imprinting section 18 by vacuum box 84 before it is further
impingement air dried on impression fabric 68 by impingement air
dryers 20, 22 and 24 which are arranged as shown. Typically,
impingement air dryers utilized in accordance with the invention
may be impingement air dryers with two drying zones, such as zones
94, 96 in a hood 98 of dryer 20. Vacuum cylinders, such as
cylinders 78-82 may be 12 feet in diameter and reversing vacuum
rolls 85, 86 may be 6 feet in diameter.
[0072] Optionally, a downstream dryer hood, such as the hood 100 of
dryer 24 is coupled to an upstream hood such as hood 98 by way of a
conduit 102. In this way, exhaust air from impingement dryer hood
100, operating at relatively low humidity, can be cascaded upline
to hood 98 in order to conserve energy, that is, to reduce the
energy needed by gas-fired dryers to pre-heat the drying air.
[0073] Generally, drying air temperatures may be from about
125.degree. C. to about 175.degree. C. in the hoods with about
150.degree. C. being typical. In general, the consistency (solids
content) of the web is from about 30-70 percent prior to being
impingement air dried and is preferably dried to a consistency of
at least about 90 percent solids, more preferably web W is dried to
a solids content of at least about 95 percent by dryers 20-24.
[0074] After impingement air drying, web W may be calendared and
wound or optionally transferred to fabric 88 which may be a coarse
impression fabric as described above. The web is then knuckled onto
a creping cylinder by way of roll 104 to selectively density the
web and creped to provide further machine direction stretch to the
product as described in U.S. Pat. No. 3,301,746 to Sanford et al.,
and U.S. Pat. No. 4,529,480 to Trokhan et al., the disclosures of
which are hereby incorporated by reference.
[0075] Typical impingement air drying lengths in accordance with
the invention may be between about 100 and 150 feet with drying
rates of from about 30-40 lbs/ft.sup.2-hr. Drying lengths are
calculated for each dryer shown as degrees of wrap about the dryer
cylinder divided by 360.degree. times .pi. times the cylinder
diameter in feet whereas the impingement air drying area per dryer
is the drying length per cylinder times the (axial) length of the
drying cylinder of the dryer.
[0076] Another papermaking machine 110 suitable for producing
uncreped, impingement air dried products in accordance with the
present invention is shown in FIG. 6. Machine 110 includes
generally a twin wire forming section 112, an intermediate transfer
section 114 and an impingement air drying section 116 shown
schematically in FIG. 6. Section 112 includes a headbox 118 which
may be a layered or unlayered headbox which deposits a cellulosic
papermaking furnish on a forming wire 120 which is supported by a
plurality of rolls 122, 124, 126, 128 including a vacuum roll 130.
Forming wire 132 is provided to assist in forming the nascent web
W, and is supported by a plurality of cylindrical rolls such as
roll 134. The respective forming wire 120, 132 travel in the
direction 136, 138 as shown on FIG. 6 and web W may be dewatered by
a vacuum box before being conveyed to transfer section 114 as shown
in FIG. 6.
[0077] Transfer section 114 includes a transfer fabric 140 which
may be an impression fabric provided with substantial texture
orthogonal to the machine direction supported about a plurality of
rolls 142-146 including roll 148. Also provided is a transfer head
150 which provides vacuum assist for the transfer of web W from
wire 120 to fabric 140. Fabric 140 typically moves at a speed which
is less than the speed of fabric 120 in order to provide
microcontractions to web W as noted, for example, in U.S. Pat. No.
5,607,551, the disclosure of which is incorporated herein by
reference, as well as has been noted in connection with FIG. 5
above.
[0078] Web W is transferred to another impression fabric 152 which
is looped about a plurality of rolls 154-158 as well as about
cylinders 160-164 of impingement air dryers 166-170 shown in FIG.
6. Impingement air dryers 166-170 are equipped with dual zone
impingement air hoods 172-176 as described in connection with FIG.
5 and further described in connection with FIGS. 7(a) and 7(b)
below.
[0079] Transfer of the web to fabric 152 is assisted by a vacuum
head 178. Fabric 152 may be traveling at a velocity lower than
fabric 140 to impart further machine direction stretch to web W.
There is provided adjacent fabric 152 a vacuum box 180 for molding
web W into fabric 152, generally by applying a vacuum of from about
10 to about 30 inches of mercury to web W which may have a
consistency of about 50 percent which vacuum is operative to
macroscopically rearrange the web and conform it to the shape of
fabric 152.
[0080] After molding, the web is conveyed to dryers 166-170 and
impingement air dried typically to a consistency of at least about
90 percent prior to being removed from fabric 152 at vacuum roll
182 and calendared by rolls 184, 186. Following calendaring, the
web may be further processed in the direction 188 indicating, for
example, the absorbent sheet might be embossed prior to being wound
up.
[0081] The air flow in the impingement air dryer hoods is
illustrated in FIGS. 7(a) and 7(b). FIGS. 7(a) and 7(b) are
schematic illustrations of the construction of the surface of the
impingement drying device utilized in connection with the present
invention and described herein. In the impingement blowing device,
blow holes are denoted by reference N2 and direct air flow P.sub.N2
toward the web and exhaust air pipes are denoted by reference N1
and remove an air flow P.sub.N1 from the vicinity of the web. The
diameter of each exhaust air pipe N1 is about 50 mm to about 100
mm, preferably about 75 mm and the diameter of each blow hole is
about 3 mm to about 8 mm, most commonly about 5 mm. The paper web W
runs at a distance of from about 10 mm to about 150 mm, preferably
about 25 mm, from the face of the nozzle plate and the nozzle
chamber of the hood is denoted by reference letter N. The vacuum
cylinder against which the impingement air drying device is
arranged is denoted by reference letter C in FIG. 7(b), it being
understood that this is the arrangement of the various elements of
FIGS. 5 and 6. The open area of the blow holes and the nozzle plate
in the area of web W is from about 1 percent to about 5 percent and
most commonly about 1.5 percent. The velocity of air in the blow
holes is about 40 meters per second to about 150 meters per second,
preferably about 100 mps. The heated air impinges upon fabric W
which is on an impression fabric, further shaping the web. The air
quantity that is blown is from about 0.5 to about 2.5 cubic meters
per second per square meter which is calculated for the effective
area of the drying unit. Most commonly an air quantity of from
about 1 to about 1.5 cubic meter per second per square meter is
used. The open area of the exhaust air pipes is from about 5
percent to about 15 percent, most commonly about 10 percent. In
addition to the nozzle face illustrated in FIG. 7(a) it is possible
to use a slot nozzle construction, fluid nozzle construction, foil
nozzle construction or a direct blow nozzle construction as well
as, for example, infra dryers. As can be seen, both the impinging
air and the exhaust thereof is on the same side of web W.
[0082] While the invention has been described and illustrated in
connection with numerous embodiments, modifications within the
spirit and scope of the present invention, set forth in the
appended claims, will be readily apparent to those of skill in the
art.
* * * * *