Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Absorbent disposable tissue and towel products
have recently become increasingly softer and smoother, while
retaining or increasing their absorbent capacity. Consumers
have become increasingly more conscious about the tactile
properties of such products, which include facial and
bathroom tissue, and household and industrial towels and
wipers. Therefore, a number of processes have been
proposed to produce products having increased softness
and smoothness coupled with adequate absorbency
characteristics.
For example, U.S. Patent Nos. 4,158,594 and
4,208,459, Becker et al, disclose a process ~and a
product produced thereby) for forming a soft fibrous
sheet material wherein a bonding material is applied
to one surface of a web in a quantity of at least
1.4~ nonvolatile constituents in a "fine pattern",
with the web being adhered to a creping surface and
then differentially creped. The bonding material performs
the dual function of adding strength to the debonded,
creped sheet as well as serving as a creping adhesive.
The patterns disclosed may be in the form of a
reticular pattern or a pattern of dots. The base sneet
which is adhered to the creping cylinder in the fine
pattern may have been previously either throughdried
without compression or creped from a first creping
cylinder. However, the cost of applying adhesive in a
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quantity of 1.4~ based upon the dry web weight, when
such adhesives cost on the order of $28.00 per ton of product,
are enormous. Likewise, the costs involved in building,
installing and operatlng a throughdrying apparatus are
substantially greater than those required to operate
a conventional Yankee dryer.
A somewhat different approach is taken in
U.S. Patent No. 4,225,382 Kearney et al, wherein
separate headboxes and forming zones are utilized to
form separate layers of fibers, which are juxtaposed
after dewatering the layers differentially. After
the combined web is throughdried to at least 85~
consistency, it i9 creped from a creping cylinder to
produce crepe folds which are equal in frequency and
simultaneous in phase in each layer of the stratified
web. Alternatively, a single headbox capable of
forming distinct layers in a single operation may be
utilized to produce the stratified web having
differential consistency in the various layers upon
juxtaposition.
Still another method of making an improved
fibrous web is disclosed in U.S. Patent No. 4,125,659,
Klowak et al, wherein a fibrous web is uniformly adhered
to a creping cylinder without substantial compression
and is thereafter differentially adhered to the cylinder
with the use of either a patterned pressure roll or an
endless fabric. The pattern of the pressure roll or
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fabric results in corresponding areas of the web
becoming more firmly adhered to the creping surface than
adjacent uncompressed areas of the web, such that
the more firmly adhered areas are finely creped and
the less firmly adhered areas are coarsely creped.
Lastly, U.S. Patent No. 4,166,001, Dunning
et al discloses a method of forming a multiple layer
web having outer layers of strongly bonded fibers
separated by an intermediate layer of weakly bonded
fibers, which, upon being creped and recreped, exhibit
ply separation, with each layer shearing away from
the other. The resulting product simulates a two-ply
tissue in bulk a~d softness while being formed as a
single ply.
While the act of creping a wet laid tissue
web produces beneficial effects upon its tactile and
absorbent properties, the web properties may be further
, enhanced by creping a second time, as noted in a number
of the patents above. However, Applicant has found
that, without the addition of significant quantities
of costly adhesives, a second full crepe reduces
the tensile strength of a tissue sheet below
acceptable levels. For instance, a tissue
web may have a machine direction (md) tensile strength
of 3500 grams, which is reduced to 1500 grams after
a first overall crepe, which is above the expected
target range of 1200 grams for commercially available
tissues. However, after a second overall crepe
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the md tensile strength will be reduced below this target
figure. With the addition of signi~icant quantities of
adhesive this strength may be increased into the
acceptable range. While the purpose of the second
crepe is to break or disrupt a greater number of
hydrogen bonds than is possible with a single crepe,
thereby increasing consumer perception of softness
and s~oothness, this heretofore could only be accomplished
at the expense of md tensile strength or increased
adhesive costs to build up strength lost by reason of
the second crepe.
An alternative method o~ producing a
tissue web which has increased bulk, absorbency and
softness is by "throughdrying" the sheet as first
proposed in U.S. Patent No. 3,301,746, Sanford et al.
In this and a number of subsequent patents issued to
the same assignee (the Procter & Gamble Co.), a
process is disclosed whereby a tissue web is dried
; 20 without compaction, with a relatively dry sheet
being adhered to a Yankee dryer at discrete knuckle
points corresponding to the knuckles of an imprinting
fabric. However, while the web produced by such a
throughdrying process has qualities deemed desirable
by consumers, the capital investment involved in
building and maintaining such a throughdrying system
are enormous.
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Therefore, a process is needed whereby the
desirable product attributes of throughdried sheets
are obtained without the capital investment or
adhesive costs necessary with prior art processes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a first
embodiment of the present invention;
FIG. 2 is a schematic representation of a
second embodiment of the present invention;
FIG. 3 is a schematic representation of a
third embodiment of the present invention;
FIG. 4 is a schematic representation of a
fourth embodiment of the present invention;
FIG. 5 iS a schematic representation of a layered
headbox used with the present invention;
FIG. 6 is a schematic representation of an
alternative method of recreping;
FIG. 7 iS an isometric view of the process of
the present invention;
FIG. 8 is a representation of a theoretical
single creped web;
FIG. 9 is a representation of a theoretical web
recreped according to the present invention;
FIG. 10 is a graph representing the relation-
ship of tensile strength to softness/smoothness of a
creped product;
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FIG. 11 is a photomicrograph of a single
creped tissue web; and,
FIG. 12 is a photomicrograph of a web
recreped according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The process of the present invention may be
carried out on apparatus as shown in FIG. 1. A slurry
of papermaking fibers is deposited from headbox 20
onto forming wire 22. The web 24 formed tkereby is
partially dewatered by conventional means, such as
vacuum boxes 26 prior to being affixed to the Yankee
dryer 28. The web 24 may be affixed to the first
Yankee dryer 28 by means of a pressure roll 30, which
compresses the web against the Yankee with a force of
approximately 400 pli. The web when affixed to the
first dryer 28 has a fiber consistency of approximately
38~ which is increased to between about 65 to 95%
upon being removed from the dryer. The web is conventionally
creped, as by doctor blade 32, resulting in a creped web
34 as has been produced for many years in the paper
industry. Depending upon the consistency of the web
at the point of application to the Yankee dryer 28,
it may be necessary to apply adhesive (such as animal
glue) to insure proper adhesion between the web and
the dryer. It is to be understood that the papermaking
machine thus far described is in all respects well known
in the art and has been used by Applicant to produce
.
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"creped wadding" for at least 50 years.
However, the product thus produced, while
acceptable for many wiping purposes in the ~ast, now
suffers in comparison to products made in accordance
with many of the current papermaking technologies which
produce tissues having a significantly increased
consumer perception of softness and smoothness. There-
fore, the creped web 34 is conveyed to a second Yankee
dryer 36 and is affixed thereto at a pressure nip
formed between pressure roll 38 and Yankee dryer surface
36. An imprinting fabric 40 is passed around pressure
roll 38 so that the creped web 34 is impressed against
Yankee dryer 36 in a pattern corresponding to the
raised knuckle areas of imprinting fabric 40. In
order to assure that proper adhesion is effected between
the knuckled areas of fabric 40 and Yankee dryer 36,
the fabric 40 may be "wrapped" about a portion of
the dryer 36 between pressure roll 38 and roll 50.
The recreped tissue web 52 is creped from Yankee 36
by doctor blade 54 and rolled into a roll of finished
product 56.
It has been found th~at imprinting fabrics
in the range of from about lO meshes/in2 to about 150
meshes/in2 are acceptable for this purpose, with a
preferred range of 25-lO0 mesh. The finer mesh fabrics
produce a greater degree of adhesion of the creped
sheet to the second Yankee and therefore produce a
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a greater number of "second crepes" per unit than
would a coarse mesh fabric. Depending upon the product
characteristics desired, a fabric of almost any mesh
could be used-- an extremely fine mesh fabric
producing a large number of very fine crepes (although
also producing a greater degree of debulking) and a
coarse fabric leaving most of the bulk in but producing
a fewer number of relatively coarse crepes.
Adhesives may be required to adhere the web
34 to cylinder 36. An overall adhesive spray 42 may be
applied either to the web 34 or cylinder 36, or the
adhesive may be applied by a conventional rotogravure
roll (not shown). Adhesives suitable for such use may
be polyvinyl acetate, polyvinyl alcohol, starch or
animal glue.
Additionally, while the present invention
finds particular application to a process whereby a
web is produced of cellulose fibers, such a web could
be produced with a furnish comprising a combination
of cellulosic and noncellulosic textile fibers.
An alternative configuration is shown in
F~G. 2, wherein a headbox 58 lays a web 60 of paper-
making fibers onto forming wire 62, which is partially
dewatered thereon as by vacuum boxes 64. The
relatively wet paper web 60 then passes through a
standard belted press roll stack 66 which further dewaters
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il~7f~i886
the web to a fiber consistency of approximately 25 to
30%. The web 60 is fed into a second press roll stack
68 and adheres to one of the rolls 70 after being
further dewatered to about 35%. The web 72 is there-
after creped from roll 70 by a standard doctor blade 74
and conveyed to the Yankee dryer 76. The creped web
72 is affixed to the Yankee 76 at discrete points
corresponding to the knuckles of fabric 78 which
passes through a pressure nip formed between pressure
roll 80 and Yankee dryer 76. The recreped web 90
is removed from Yankee dryer 76 by doctor blade 92
and reeled into a roll of finished product 94 for
conversion into absorbent tissue products.
Both of the processes described above
relating to FIGS. 1 and 2 may be performed on
conventional papermaking machines to substantially
increase the desirable product attributes with minimal
capital investment. Depending upon the drying load
expected of Yankee dryers 36 and 76, these may be
provided as relatively small creping cylinders and
may simply be added onto the end of a conventional
creped wadding machine. Thé current invention may
also produce significant benefits when utilized with
a "throughdrying" machine as shown in FIG. 3. A
machine of this type may include, for example, headbox
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96 which lays a web 98 of papermaking fibers on
forming wire 100. The web 98 is transferred to a
throughdrying fabric 110 which carries the web around
a through - air drying cylinder 112, with the web 98
being thermally predried to a consistency of
approximately 90% B.D. The thermally predried web is
thereafter transferred to Yankee dryer 114 and adhered
thereto with the addition of creping adhesives 116.
Preferably, the web is differentially adhered to the
Yankee 114 by impressing the web 98 against the Yankee
surface 114 with the raised knuckle areas of an
imprinting fabric 118 at the location of a pressure
roll 120. ~he creped web 122 is creped from Yankee 114
by doctor blade 124 and carrled to a second creping
cylinder 126 and again differentially adhered thereto
between imprinting fabric 128 and the surface of the
cylinder 126. The web is affixed to the cylinder 126
at discrete points corresponding to the knuckles of
fabric 128 between pressure roll 140 and cyllnder 126
by adhesive 130 applied to cylinder 126 or web 122
immediately prior to pressure roll 140. The recreped
thermally predried web 142 is creped from cylinder
126 by doctor blade 144 and reeled into a roll of
finished product 146.
When utilizing a pressure roll 38, 80 or 140
as shown in FIGS. 1, 2 and 3, pressures in the range
of from about 50 pli to about 100 pli may
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sufficiently adhere the web to the cylinder at the
knuckle areas.
FIGS. 1 - 3 depict a process whereby the same
side of the tissue sheet is recreped. Applicant has
found that, when utilizing lower basis weight base webs,
such as 7-1/2 pound per 2880 square feet, such a
process produces maximum bulk and tactile qualities
with a minimum of investment. Such basis weights are
typically utilized as one ply of two-ply facial or
bathroom tissue products. However, it is applicant's
experience that when recreping a heavier basis weight
web, such as a web suitable for one-ply tissue products
(15 pounds basis weight) or for disposable towels (on
the order of 25 to 30 pounds per 2880 square feet),
the web is too thick and heavy for a second crepe on
the same side of the web to optimize product quality.
In this case, the web must be "turned over" and the
opposite side creped in order to obtain the ~aximum
benefit of a second crepe. It i5 hypothesized that
due to the high basis weight of the sheets, the second
crepe cannot "strike through" the entire web thickness
as it can in the lighter basis weight sheets and soften
the side not creped, resulting in a very one-sided
sheet having a soft side (the side creped) and a harsh
side(not creped).
Therefore, as shown in FIG. 3, a web 148 is
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laid down from headbox 150 onto foxming wire 152,
partially dewatered, as by vacuum boxes 154 and
carried to Yankee dryer 156 where it is adhered with
compression onto the Yankee by pressure roll 158. A
first side of the web 160 is creped from the Yankee
dryer 156 by doctor blade 162, and the web 160 is
thereafter conveyed to a second creping cylinder 164
either above or beneath the first Yankee 156. A
second side of web 160 is adhered to the creping
10 cylinder 164 between impression fabric 166 and the
surface of cylinder 164 at the location of pressure
roll 168. If necessary, adhesive 170 may be utilized
to adhere the discrete points of web 160 corresponding
to knuckles of fabric 166 to the cylinder 164. The
recreped web 180 is removed from cylinder 164 by
doctor blade 182 and rolled into a roll of finished
product 184.
As disclosed in U.S. Patent ~o. 4,166,001,
jointly owned by the assignee of the present application,
a multiple layer web having layers of fibers with
different bonding characteristics may produce a
finished product having desirable characteristics upon
creping. Therefore, a headbox as shown in FIG. 5,
generally designated 185, and having individual fiber
stock inlets 186 A, B, and C, may be provided with
any of the papermaking machines of FIGS. 1 through 4.
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As described in the '001 patent, a weakly bonded
fiber stock supplied through inlet 186B, sandwiched
between more highly bonded fiber stocks of inlets 186A
and C may produce a ply separable web upon creping.
The headbox of FIG. 5 would find particular application
with the heavier basis weights envisioned in the machine
of FIG. 4, wherein the opposing sides of the web thus
formed are each creped, thereby increasing the likelihood
of the outer plies shearing away from the inner ply
formed by headbox 185.
While the once-creped sheet of FIGS. 1 through 4
is adhered to the second creping cylinder by an impression
fabric which at least partially wraps the creping cylinder,
the creped web 188 of FIG. 6 may be affixed to the creping
cylinder 190 in the nip formed between pressure roll 192,
impression fabric 194 and cylinder 190. In this case,
pressure roll 192 may be provided with a pressure of
approximately 150 pli. As in FIGS. 1, 3 and 4, the
relatively dry creped web 188 may be affixed to cylinder
190 by suitable creping adhesive applied at 196.
As depicted schematically in FIG. 7, the
relatively dry creped web Wc is adhered to the creping
cylinder or Yankee dryer Y, as by use of a creping
adhesive A applied as an overall spray, and compressed
against the cylinder Y by impression fabric F. As
depicted in FIG. 7, fabric F partially wraps the
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cylinder Y as it moves about pressure rolls R and R ' .
The creped web Wc is adhered to the surface of the
cylinder Y only at the discrete locations Wk corresponding
to the knuckles of fabric F, with a substantial portion
of web Wc remaining uncompressed by fabric F and not
adhered to cylinder Y.
A schematic representation of a tissue web
200 resulting from a conventional single creping process
is depicted figuratively in FIG. 8. Of course, a creped
web would not exhibit the regularity of crepe folds
shown in FIG. 8, but this figure is intended for
illustrative purposes only and should not be taken as
an accurate depiction of an actual web.
The undulating sinusoidal nature of the creped
web results from the scraping action of the doctor blade
against the creping cylinder which compresses the web
adhered to the cylinder and creates crepe "folds" upon
; the surface of the web. A typical wet-pressed web
exhibiting high levels of interfiber hydrogen bonding is
debonded as the doctor blade scapes the web from the
creping cylinder, with these debonded areas typically
alternating with non-debonded areas where the web has
"popped" off the creping cylinder, creating the crepe
fold. (see Holger Hollmark, Study of the Crepe Process
in an Experimental Paper Machine, Swedish Forest Products
Research Lab Report No. 144, Series B) Those portions
of the web 202 which were adhered to the cylinder surface
and which were scraped off are finely creped, producing
li'76~386
a high level of debonding in those localized areas. This
action results in relatively large areas 206 between
the debonded areas 202 which have not been
debonded and exhibit relatively coarse, stiff tactile
properties.
The present invention involves utilizing the
web of FIG. 8 and recreping it at discrete points over
its entire surface. When the web of FIG. 8 is
adhered to a creping cylinder overall, as with a plain
pressure roll, the applicant has found that the web
loses sufficient tensile strength that, unless
the process is extremely closely monitored, in many
instances the web will be too weak for use by consumers.
It is believed that this loss of strength results
because the web is debonded a second time in essentially the
same locations 202 as indicated from the first crepe in
FIG. 8, thereby locally debonding the web to too great
an extent in these locations and potentially weakening
the sheet.
By contrast, a web produced according to
applicant's invention, shown schematically in FIG. 9
is eminently suited for commercial sale and use. The
recreped web 208, being basically the same base web
200 as shown in FIG. 8, exhibits the same localized
areas of debonding 202 resulting from the first
crepe. However, because the impression fabric compresses
the creped web 208 only in localized areas corresponding
to the fabric knuckles, the web 208 will be recreped
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at regular points 210 on the web tthese points 210
appear random when seen in partial cross-section)
rather than in the same locations as the first crepe.
Therefore, there is a likelihood that a substantial
number of those uncreped areas of web 208 will be
contacted by a discrete knuckle of fabric F and be
compressed against cylinder Y and be creped therefrom.
Therefore, the likélihood is that the same areas 202
will not be creped again to the same extent as would
occur with a solid pressure roll, but additional
points of debonding 210 will be provided in previously
undebonded areas 206, which softens the sheet witnout
reducing its tensile strength below commercially
acceptable levels. Of course, random knuckle impressions
210 will occur in the areas previously creped, but the
level of additional debonding in these areas 202
will be significantly reduced.
FIG. 11 is a photomicrograph of a typical
creped tissue web shown in cross section. The sinusoidal
nature of the web 220 is apparent, although not as
regular as that depicted figuratively in FIG. 8. The
degree of debonding in web 220 is relatively low; by
Applicant's in-house subjective measurement, ~rom 35-50~
of the length of the web may exhibit detectable debonding.
FIG. 12, a photomicrograph of a base web similar to
that of FIG. 11 which has been recreped with a fabric
according to the present invention, also exhibits
the sinusoidal wave pattern, but the web 222 exhibits
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considerably higher levels of debonding, on the order
of 75% of its length. This increased level of debonding
results in a sheet having increased consumer perception
of softness and smoothness.
The graph of FIG. 10 illustrates the concept
of the present invention. Applicant has determined that
a tensile strength in the machine direction of at
least 1200 grams is necessary for conventionally formed
creped consumer tissue products. Applicant has likewise
developed an internal softness/smoothness scale of 1 to 10,
with acceptable product being rated at least a "6" softness/
smoothness, with exceptional product being somewhere
between "7" and "8". All creped tissue products exhibit
an inverse relationship between tensile strength and
softness/smoothness, in that as one increases the
strength of the web (thereby having less debonding~,
one decreases the softness. Likewise, in order to
increase softness/smoothness, one must sacrifice stength.
Heretofore, applicant had discovered that an exceptionally
so t product could be obtained (rating an "8U) by simply
recreping a previously creped sheet with a smooth
pressure roll. However, due to the severe debonding
noted above, such products fell below applicant's
target strength level of 1200 grams (FIG. 10). On
occassion, Applicant was successful in obtaining a
tissue sheet meeting the target strength level after
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recreping with a solid pressure roll, however this was
accomplished under conditions where all process variables
were relatively easily controlled. It would be
exceedingly difficult to obtain such process precision
on a commercial tissue machine 200 inches in width
running in excess of 5000 feet per minute. With the
use of a fabric to differentially adhere the previously
creped sheets to the creping cylinder, the sheet having
an "8" softness may be obtained while maintaining the
necessary minimum strength of 1200 grams (although
some strength is sacrificed to attain this softness/
smoothness level). The use of the fabric provides a
mechanism of controlling the crepe of the previously
creped sheet. The creping process need not be "fine
tuned" to the point noted above, which would be
necessary to produce the target strength on a commercial
machine. Because the sheet is creped only in discrete
locations, debonding of previously debonded areas will
not be as severe as with a smooth pressure roll,
- 20 resulting in a softer, smoother sheet requiring less
precision to make.
It will be apparent that the degree of
debonding from the second crepe will depend upon the
mesh of the imprinting fabric used. A relatively
coarse imprinting fabric in the range of 10-50 meshes/in.2
will cause the web to be adhered to the second creping
cylinder in a relatively coarse pattern, therefore
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resulting in a coarse crepe and relatively less
debonding. However, if a fabric mesh above 50, and
particularly 100 or above is used, the web will be
recreped in a very fine pattern resulting in a greater
percentage of the sheet being debonded. The mesh of
the imprinting fabric, therefore, may be utilized
as a process control to determine the product qualities
desired through the creping frequencies desired.
It will be appreciated that variations and
modifications of the disclosed processes may ~e
effected without departing from the spirit and scope
of the novel concepts of this invention.
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