Note: Descriptions are shown in the official language in which they were submitted.
2145554
PATENT
SOFT LAYERED TISSUES HAVING HIGH WET STRENGTH
Background of the Invention
In the manufacture of tissue paper useful as facial tissue, bath
tissue, paper towels and the like, it is well known to use various
additives to enhance the properties of the product. Such additives
include wet strength agents, which can be permanent or temporary, and
debonding agents. As the name implies, wet strength agents impart
strength retention to the tissue sheet when it becomes wet by
creating or retaining certain fiber-to-fiber bonds that withstand the
presence of water or moisture, which is particularly useful for most
tissue applications. Temporary wet strength agents are especially
useful for bath tissue, where wet strength is needed while the tissue
is being used, but is undesirable after the tissue has been flushed
into the sewer system. Debonding agents are desirable for the
purpose of enhancing the softness of the tissue sheet by reducing the
number of papermaking bonds between fibers and enhancing the surface
feel of the tissue.
However, the action of debonding agents inherently is counter to
the objective of the wet strength agents, since the debonding agents
can prevent the wet strength agents from forming the desired bonds.
Hence there is a need for a means of combining wet strength agents
and debonders which minimizes the counterproductive interaction of
the two classes of chemicals and enables the manufacture of tissue
sheets having the beneficial properties of softness and wet strength.
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Summary of the Invention
It has now been discovered that debonding agents and wet strength agents can
be
added to a tissue sheet in a layer-wise fashion to maximize the effectiveness
of each
additive while minimizing the interaction of the additives with each other.
This enables the
papermaker to take full advantage of the properties of the additives and the
fibers within
the various layers of the tissue, resulting in a soft, yet strong, tissue.
In one aspect of the present invention, there is provided a method for making
a soft
tissue sheet comprising the steps of forming a layered wet web of papermaking
fibers
using a layered headbox, said layered wet web having two outer layers and at
least one
inner layer, wherein the two outer layers comprise predominantly hardwood
fibers and
said at least one inner layer comprises predominantly softwood fibers, said
two outer
layers containing a debonding agent, and said at least one inner layer
containing a wet
strength agent; and drying the web, with the proviso that the outer layers do
not comprise
a polyhydroxy compound.
In another aspect, the invention resides in a method for making a soft tissue
sheet
comprising (a) forming a layered wet web of papermaking fibers using a layered
headbox,
said layered wet web having a first outer layer, a second outer layer, and at
least one
inner layer, wherein the two outer layers comprise predominantly hardwood
fibers and
said at least one inner layer contains a wet strength agent and comprises
predominantly
softwood fibers, and wherein at least the first outer layer contains a
debonding agent; (b)
transferring the layered wet web to a throughdrying fabric wherein said second
outer layer
is in contact with the throughdrying fabric; and (c) throughdrying the web to
form a soft
tissue sheet. The resulting throughdried web can be creped or uncreped. In an
alternative
embodiment, the layered web of step (a) can be dried in accordance with
conventional
"wet-pressing" processes wherein the web is carried by a papermaking felt,
pressed
against the surface of a Yankee dryer, dried and creped to produce a soft
tissue sheet.
In an additional embodiment of the layered web of step (a) at least one inner
layer
can be two inner layers, where one of the inner layers substantially contains
secondary
fibers.
In another aspect, the invention resides in a soft layered tissue comprising
two outer
layers and at least one inner layer, wherein said two outer layers contain
predominantly
hardwood fibers and at least one of the two outer layers, such as the airside
layer
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(hereinafter described), contains a debonding agent, and wherein said at least
one inner
layer contains predominantly softwood fibers and a wet strength agent. The
tissue can be
throughdried or wet-pressed and can be creped or uncreped.
In another aspect of the present invention, there is provided a layered tissue
comprising two outer layers and at least one inner layer, wherein said two
outer layers
comprise predominantly hardwood fibers and a debonding agent, and said inner
layer
comprises predominantly softwood fibers and a wet strength agent, with the
proviso that
the outer layers do not comprise a polyhydroxy compound.
As used herein, a "debonding agent" is an additive that enhances the softness
of
tissue paper. The debonding agents may accomplish this by a variety of means:
a) by
interfering with formation of hydrogen bonds, such as with fatty quaternary
ammonium
compounds (debonder); b) by increasing the lubricity of the fibers and
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2145554
increasing flexibility of the web, such as with fatty acid salts and
derivatives (e. g., fatty amine derivatives) and silicones;
c) reducing surface tension and thereby reducing Campbell's forces
during web formation, resulting in reduced bonded area, such as with
surfactants; or d) by other means or combinations of means. Suitable
debonding agents include, without limitation, alkyl trimethyl
quaternary ammonium compounds, dialkyl dimethyl quaternary ammonium
compounds, trialkyl methyl quaternary ammonium compounds, dialkoxy
alkyl quaternary ammonium compounds, dialkoxy alkyl quaternary
ammonium compounds, diamidoamine quaternary compounds, imidazolinium
quaternary ammonium compounds, fatty acid derivatives, nonionic
surfactants, ampholytic surfactants, silicones, methyl cellulose,
hydroxypropyl cellulose, methyl hydroxyethyl cellulose, methyl
hydroxypropyl cellulose, methyl hydroxybutyl cellulose,
carboxyethylmethyl cellulose, and mixtures thereof.
The debonding agents are preferably incorporated into the outer
layers) by addition into the layer furnish prior to formation of the-
web. However, the debonding agents can also be applied to the wet
web after formation by spraying the debonding agent onto the web,
before drying of the web. In such instances the consistency of the
wet web can be about 40 percent or less, more specifically about 30
percent or less, and still more specifically about 20 percent or
less. The amount of debonding agent applied to the outer layers)
can be from about 0.125 kg/tonne per layer (0.25 lb./ton per layer)
to 25 kg/tonne per layer (50 lb./ton per layer).
As used herein, a "wet strength agent" is an additive that
increases the strength of wet tissue paper. It can provide permanent
or temporary wet strength to the tissue. Suitable wet strength
agents include, without limitation, urea-formaldehyde resins,
melamine-formaldehyde resins, epoxidized polyamide resins, polyamine-
polyamide-epichlorohydrin resins, glyoxalated polyacrylamide resins,
polyethylenimine resins, temporary wet strength resins described in
U.S. Patent No. 4,981,577 to Bjorkquist issued January 1, 1991,
dialdehyde starch, cationic
aldehyde starch, cellulose xanthate, synthetic latexes, vegetable
gums, glyoxal, acrylic emulsions and amphoteric starch siioxanes.
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The amount of wet strength agent added to the inner layer of
softwood fibers can be from 0.25 kg/tonne/layer (0.5 lb./ton) to
25 kg/tonne/layer (50 lb./ton).
Brief Description of the Drawing
Figure 1 is a schematic flow diagram of a tissue making process
useful for purposes of this invention.
Detailed Description of the Drawing
Referring to Figure 1, a method of making multi-layered tissues
suitable for purposes of this invention will be described. (For
simplicity, the various tensioning rolls schematically used to define
the several fabric runs are shown but not numbered.) It will be
appreciated that variations from the apparatus and method illustrated
in Figure 1 can be made without departing from the scope of the
invention. Shown is a twin wire former having a layered papermaking
headbox 10 which injects or deposits an aqueous suspension of
papermaking fibers between outer forming fabric 11 and inner forming
fabric 12. Inner forming fabric 12 serves to support and carry the
newly-formed wet web 13 downstream in the process as the web is
partially dewatered to a consistency of about 10 dry weight percent.
Additional dewatering of the wet web can be carried out, such as by
vacuum suction 14, while the wet web is supported by the forming
fabric. As shown, the dryerside layer of the wet web, which is the
outer layer of the web that ultimately faces the dryer surface during
drying, is in contact with the forming fabric 12. The airside layer
is the outer layer on the opposite side of the web and faces away
from the dryer during drying.
The wet web is then transferred from the inner forming fabric
to a transfer fabric 17 traveling at a slower speed than the forming
fabric in order to impart increased stretch into the web. Transfer
is preferably carried out with the assistance of a vacuum shoe 18 and
a kiss transfer to avoid substantial compression of the wet web.
Optional vacuum box 19 can be used to further dewater the web and
spray applicator 20 can be used to provide controlled addition of
additives such as debonders.
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The web is then transferred from the transfer fabric 17 to the
throughdrying fabric 25 with the aid of a vacuum transfer roll 26.
The throughdrying fabric can be traveling at about the same speed or
a different speed relative to the transfer fabric. If desired, the
throughdrying fabric can be run at a slower speed to further enhance
stretch. Transfer is preferably carried out with vacuum assistance
to ensure deformation of the sheet to conform to the throughdrying
fabric, thus yielding desired bulk and appearance. Optional vacuum
box 27 and spray applicator 28 can be used as described above.
The level of vacuum used for the web transfers can be from about
3 to about 15 inches of mercury (75 to 380 millimeters of mercury),
preferably about 5 inches (125 millimeters) of mercury. The vacuum
shoe (negative pressure) can be supplemented or replaced by the use
of positive pressure from the opposite side of the web to blow the
web onto the next fabric in addition to or as a replacement for
sucking it onto the next fabric with vacuum. Also, a vacuum roll or
rolls can be used to replace the vacuum shoe(s).
While supported by the throughdrying fabric 25, the web is dried
to a consistency of about 94 percent or greater by the throughdryers
30 and 31. The dried basesheet 35 is transferred to carrier fabric
36 with the aid of vacuum roll 37 and transported to the reel 38
using carrier fabric 36 and an optional additional carrier fabric 39.
An optional pressurized turning roll 40 can be used to facilitate
removal of the web from the carrier fabric 36. Suitable carrier
fabrics for this purpose are Albany International 84M or 94M and
Asteri 959 or 937. Reel calender 45 or subsequent off-line
calendering can be used to improve the smoothness and softness of the
basesheet, if desired.
Examgles
Exampl a 1.
A pilot scale twin wire papermaking machine, as described in
Figure 1, was used to produce tissues in accordance with this
invention. More specifically, the papermaking machine had a layered
headbox with a top chamber, two central chambers, and a bottom
chamber. A first fibrous slurry composed primarily of short
papermaking fibers, namely eucalyptus hardwood kraft (EHWK), was
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pumped through the top and bottom headbox chambers and,
simultaneously, a second fibrous slurry composed primarily of long
papermaking fibers, namely northern softwood kraft (NSWK), was pumped
through the central headbox chambers and delivered in superposed
relation between the inner and outer forming fabrics to form thereon
a three-layered embryonic (wet) web. The inner and outer forming
fabrics were Asten 866 fabrics. The dry weight ratio of the three
layers (outer layer/inner layer/outer layer), referred to as the
layer split, was 37.5%/25%/37.5%.
The EHWK fibers of the first slurry had been previously
processed in a Maule shaft disperser with a power input of 80kW at a
consistency of about 34% and at a temperature of about 184°F. The
resulting EHWK fibers were treated with Beroce11~596 debonder in the
machine chest at a rate of 5 kg/tonne. Berocell 596 is a dimethyl
dialkyl ammonium chloride debonder supplied by Eka-Nobel. Fiber
consistency of the first slurry was about 0.12%.
The NSWK fibers of the second slurry were treated with
Parei 631 NC temporary wet strength resin at a rate of 5.45 kg/tonne.
(Parez 631 NC is a glyoxalated cationic polyacrylamide resin supplied
by Cytec.) The second fibrous slurry was also mechanically refined
to maintain target tensile strengths. Fiber consistency of the
second slurry was about 0.04%.
Partial dewatering of the embryonic web through the forming
fabric was assisted by vacuum boxes. The embryonic web was
transferred from the inner forming fabric to a Lindsay 3080-CCW
transfer fabric with the assistance of a vacuum transfer shoe at a
consistency of about 29%. The speed of the forming fabric was about
2285 feet/minute and the speed of the transfer fabric was about 1800
feet/minute, yielding a negative draw (rush transfer) of 27%.
The web was then transferred from the transfer fabric to the
throughdryer fabric (Asten Uelostar 800) at a consistency of about
29%. The web was dried by the throughdriers to a consistency of
about 94%. The dried web was transferred to the reel between two
transfer fabrics (Asten 866 and Lindsay~3070) and wound into a roll
on the reel.
The resulting tissue paper had a basis weight of 29 9/m2,
geometric mean tensile of 710 g/3 inches, wet CD tensile of 123 g/3
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inches (7.62 centimeters), wet/dry tensile ratio of 20.3% and a bulk
of 12.5 cc/g. The tissue paper had high tactile softness as
determined by panel evaluation.
Example 2.
A three-layer tissue paper sheet was produced in accordance with
Example 1, except that the first short fiber slurry did not contain a
debonding agent. Instead the outer EHWK layers of the undried web
were sprayed with a solution of Berocell 596 debonding agent. The
debonding agent solution was applied to the outer layers using spray
applicators 20 and 28 and corresponding vacuum boxes 19 and 27 as
shown in Figure 1. The debonding agent was applied to the outer
layers at a rate of 5 kg. debonding agent/tonne of EHWK fiber. Fiber
consistency of the web at the point of spray addition was about 29%.
The resulting tissue paper had a basis weight of 28.6 g/mZ, geometric
mean tensile of 723 g/3 inches, wet CD tensile of 113 g/3 inches
(7.62 centimeters), wet/dry tensile ratio of 19.3.% and a bulk of 12.2.
cc/g. The tissue paper had high tactile softness as determined by
panel evaluation.
Example 3.
A three-layer tissue paper sheet was produced in accordance with
Example 1, except: 1) the first short fiber slurry (EHWK) was treated
with 7.5 kg/tonne of Berocell 584 debonding agent (Berocell 584 is a
nonionic, cationic surfactant system supplied by Eka-Nobel); 2) the
second long fiber slurry (NSWK) was treated with 6.36 kg/tonne of
Parez 631 NC temporary wet strength agent; 3) the negative draw
between the forming fabric and transfer fabric was 29%; 4) the layer
split was 40/20%/40%; 5) the inner and outer forming fabrics were
Lindsay 2164 fabrics, the wet end transfer fabric was an Albany 94-
MSS, the TAD fabric was a Lindsay T216-3, and the dry end transfer
fabrics were an Albany 94-M and a Lindsay 3070; and 6) the transfer
from the inner forming fabric to the transfer fabric occurred at a
consistency of about 26% and the transfer to the TAD fabric occurred
at a consistency of about 27%.
The resulting tissue paper had a basis weight of 27.8 g/m2,
geometric mean tensile of 696 g/3 inches, wet CD tensile of 102 g/3
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inches (7.62 centimeters), wet/dry tensile ratio of 18.1% and a bulk
of 11.31 cc/g. The tissue paper had high tactile softness as
determined by panel evaluation.
Example 4.
A three-layer tissue paper sheet was produced in accordance with
Example 3, except: 1) the first short fiber slurry was composed of
southern hardwood kraft fibers (SHWK); 2) the second long fiber
slurry (NSWK) had been treated with 9.66 kg/tonne of Parez 631 NC
temporary wet strength agent; 3) the layer split was 40%/20%/40%; and
4) the transfer from the inner forming fabric to the transfer fabric
occurred at a consistency of about 28% and the transfer to the TAD
fabric occurred at a consistency of about 29%.
The resulting tissue paper had a basis weight of 29.4 g/mZ,
geometric mean tensile of 726 g/3 inches, wet CD tensile of 107 g/3
inches (7.62 centimeters), wet/dry tensile ratio of 18.1% and a bulk
of 9.95 cc/g. The tissue paper had high tactile softness as
determined by panel evaluation.
Exampl a 5.
A three-layer tissue paper sheet was produced in accordance with
Example 2, except: 1) the outer layers of the undried web were
sprayed with Ucarsil HCP textile softener (debonding agent) at a rate
of 10 kg Ucarsil HCP/tonne EHWK (Ucarsil HCP is an organomodified
silicone softener obtained from Union Carbide); Z) the second long
fiber slurry (NSWK) was treated with 4.33 kg/tonne of Parez 631 NC
temporary wet strength agent; 3) the negative draw between the
forming fabric and transfer fabric was 30%; 4) the layer split was
35%/30%/35%; 5) the wet end transfer fabric was an Albany 94-M, the
TAD fabric was a Lindsay T216-4, and the dry end transfer fabrics
were both Lindsay 3070 fabrics; and 6) the dried web was calendered
using a reel calender consisting of a 20-inch steel roll and a
20.5-inch rubber roll (110 P&J hardness, 0.75 inch cover thickness)
engaged to a nip width of about 32 millimeters.
The resulting tissue paper had a basis weight of 30.1 gm/mZ,
geometric mean tensile of 679 g/3 inches, wet CD tensile of 100g/3
inches (7.62 centimeters), wet/dry tensile ratio of 18.1% and a bulk
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of 8.35 cc/g. The tissue paper had high tactile softness as
determined by panel evaluation.
Exampl a ,6 .
A pilot scale twin wire through air dried papermaking machine,
similar to that described in Figure 1 but having a Yankee dryer
between roll 40 and the reel 38, was used to produce a creped,
throughdried tissue in accordance with this invention.
The paper machine had a layered headbox with a top chamber, two
central chambers, and a bottom chamber. A first fibrous slurry
composed of southern hardwood kraft (SHWK) fibers was pumped through
the top and bottom headbox chambers, and, simultaneously, a second
fibrous slurry composed of northern hardwood kraft fibers (NHWK) was
pumped through the central headbox chambers and delivered in
superposed relation onto the forming fabric to form thereon a three-
layer embryonic web. The layer split was 33.3%/33.3%/33.3%.
The SHWK fibers pumped through the top chamber of the headbox
were treated with 2.5 kg/tonne of Berocell 596 debonding agent. The
NSWK fibers of the second long fiber slurry were treated with Parez
631 NC temporary wet strength resin at a rate of 8.15kg/tonne. The
second fibrous slurry was also treated with sufficient starch to
maintain target tensile strengths. The SHWK fibers pumped through
the bottom chamber of the headbox were untreated.
Dewatering of the embryonic web occurred through the forming
fabric and was assisted by vacuum boxes. The embryonic web was
transferred from the forming fabric to a transfer fabric with the
assistance of a vacuum transfer roll at a consistency of about 29%.
The web was then transferred from the transfer fabric to the
throughdryer fabric at a consistency of about 29%. The web was dried
by the throughdriers to a consistency of about 94%, adhered to a
Yankee dryer, creped off the Yankee with a doctor blade, and wound
into a roll on the reel.
The resulting tissue paper had a basis weight of 27.1g/m2,
geometric mean tensile of 768g/3 inches, wet CD tensile of 107 g/3
inches {7.62 centimeters), wet/dry tensile ratio of 20.0% and a bulk
of 7.88cc/g. The tissue paper had high tactile softness as
determined by panel evaluation.
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Exampl a 7 .
A pilot scale crescent former wet-pressed papermaking machine
was used to produce a creped tissue in accordance with this
invention.
The paper machine had a layered headbox with a top chamber, a
center chamber, and a bottom chamber. A first fibrous slurry
composed of EHWK fibers was pumped through the top and bottom headbox
chambers, and, simultaneously, a second fibrous slurry composed of
NSWK fibers was pumped through the center headbox chamber and
delivered in superposed relation onto the felt to form thereon a
three-layer embryonic web. The layer split of the embryonic web was
30%/40%/30%.
The EHWK fibers of the first slurry were processed with
1 kg/tonne of Berocell 584 debonder in a Maule shaft disperser with a
power input of 55 kW at a consistency of about 34% and at a
temperature of about 178°F.
The NSWK fibers of the second slurry were treated with
Kymene 557 LX permanent wet strength resin at a rate of
2.27 kg/tonne. (Kymene 557 LX is a cationic polyamide-epichlorohydrin
resin supplied by Hercules, Incorporated.) Forty percent of the
second fibrous slurry was mechanically refined to maintain target
tensile strengths.
The web was carried by the felt to the Yankee dryer, where the
web was adhered to the dryer and then creped off the Yankee with a
doctor blade and was wound into a roll on the reel.
The resulting tissue paper had a basis weight of 18.1
1b/2880 ftZ, geometric mean tensile of 1091 g/3 inches (7.62
centimeters), wet CD tensile of 109 g/3", wet/dry tensile ratio of
13% and a bulk of 6.4 cc/g. The tissue paper had high tactile
softness as determined by panel evaluation.
All of the foregoing examples illustrate that a soft tissue
having good wet strength can be made by the method of this invention.
It will be appreciated that the foregoing examples, given for
purposes of illustration, are not to be construed as limiting the
scope of this invention, which is defined by the following claims.
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