Note: Descriptions are shown in the official language in which they were submitted.
WO 94/05857 _ 214 3 3 4 ~ PCT/US93/07852
1
PROCESS FOR APPLYING CHEMICAL PAPERMAKING
ADDITIVES FROM A THIN FILM TO TISSUE PAPER
10
TECHNICAL FIELD
This invention relates, in general, to a process for preparing
tissue paper; and more specifically, to a process for applying low levels
of chemical papermaking additives to the surface of tissue paper for
enhancing the properties of the paper. e.g., strength, softness,
absorbency, and/or aesthetics.
BACKGROUND OF THE INVENTION
Consumer products such as toilet tissue, toweling :and facial tissue
made from cellulosic webs are a pervasive part of modern society. In
general, these products need to possess certain key physical properties
to be considered acceptable to consumers. .While the exact mix of key
properties and the absolute value of the individual properties will vary
depending on the nature of the product, nonetheless, softness, wet and
dry strength, absorbency, and pleasing aesthetic nature are universally
desirable properties. Softness is that aspect of the fibrous web that
elicits a pleasing tactile response and insures that the product is not
harsh or abrasive when it contacts human skin or other fragile surfaces.
Strength is the ability of the structure to retain its physical integrity
during use. Absorbency is the property of the fibrous structure which
allows it to acquire and retain contacted fluids in an acceptable time.
Aesthetic nature refers to the psycho-visual response that occurs when
the consumer views the product either alone or in the context of the
product's surroundings.
The most common method for the manufacture of tissue products is the
wet laid papermaking process. In such a process, individual fibers are
first suspended in a dilute slurry with water. This slurry is then laid
on a foraminous screen to remove a large portion of the water and to form
WO 94/05857 PCT/US93/07852
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a thin, relatively uniform-weight embryonic web. This embryonic web is
then molded and/or dried in a variety of ways to form the final tissue
web. As part of this process the molded and/or dried web is usually
glued to a drying drum and subsequently creped from the surface of the
dryer to impart desirable properties.
Products made by many existing wet laid processes fall under the
above description. Examples of such webs that are soft, strong, and
absorbent and contain at least two micro regions of density can be found
in, U.S. Patents: 3,301,746 which issued January 31, 1967, to Lawrence
H~ Sanford and James B. Sisson; 3,974,025 which issued August 10, 1976,
to Peter G. Ayers; 3,994,771 which issued November 30, 1976, to George
Morgan, Jr. and Thomas F. Rich; 4,191,609 which issued March 4, 1980, to
Paul D. Trokhan; and 4,637,859 which issued January 20, 1987, to Paul D.
Trokhan. Each of these papers is characterized by a repeating pattern of
dense areas and less dense areas. The dense areas can be either discrete
or continuous. These dense areas result from localized compaction of the
web during papermaking by raised areas of an imprinting carrier fabric or
belt.
Other high-bulk, soft tissue papers are disclosed in U.S. Patent
4,300,981 which issued November 17, 1981, to Jerry E. Carstens; and
4,440,597 which issued April 3, 1984, to Edward R. Wells and Thomas A.
Hensler.
Additionally, achieving high-bulk, soft and absorbent tissue paper
through the avoidance of overall compaction prior to final drying is
disclosed in U.S. Patent 3,821,068 which issued June 28, 1974, to D. L.
Shaw; and avoidance of overall compaction in combination with the use of
debonders and elastomeric bonders in the papermaking furnish is disclosed
in U.S. Patent 3,812,000 which issued May 21, 1974, to J. L. Salvucci,
Jr.
Chemical debonders such as those contemplated by Salvucci, referred
to above, and their operative theory are disclosed in such representative
U.S. Patents as 3,755,220 which issued August 28, 1973, to Friemark et
al.; 3,844,880 which issued October 29, 1974, to Meisel et al.; and
4,158,594 which issued January 19, 1979, to Becker et al.
Tissue paper has also been treated with cationic surfactants, as
well as noncationic surfactants to enhance softness. See, for example,
U. S. Patent 4,959,125 which issued September 25, 1990, to Spendel; and
U. S. Patent 4,940,513 which issued July 10, 1990, to Spendel, that
_~.__.. _T. _. .. __... _.,
t
_2143340
WO 94/05857 PCT/US93/07852
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disclose processes for enhancing the softness of tissue paper by treating
it with noncationic, preferably nonionic, surfactants.
It has been found that the softness of tissue paper, in particular,
high-bulk pattern densified tissue papers, can be improved by treatment
with various agents such as vegetable, animal or synthetic oils, and
especially polysiloxane materials typically referred to as silicone oils.
See, for example, U. S. Patent 5,059,282 which issued October 22, 1991,
to Ampulski et al. The Ampulski patent discloses a process for adding a
polysiloxane compound to a wet tissue web (preferably at a fiber
consistency of between about 20X and about 35X). These polysiloxane
compounds impart a silky, soft feeling to the tissue paper.
While the processes described above generally make acceptable
product properties, the product properties can be further enhanced.
However, processes to make current products and potentially enhanced
Products suffer from several drawbacks. For example, the chemicals used
to strengthen tissue webs are often added to the dilute slurry of water
and fibers prior to the initial lay down on the forming screen. This is
a relatively convenient and cost effective way to introduce additives.
However, other chemicals to aid absorbency or to improve softness are
also commonly added to the so called wet end of the tissue making
process. Because of the complex nature of the individual chemicals used
to generate the key properties, they often interact with each other in an
adverse manner. They can compete with each other for the desired
retention on the cellulose fibers as well as destroy properties that are
inherent in the fibers. For example softening chemicals often reduce
the natural tendency of fibers to bond to other fibers and hence reduce
the functional strength of the resulting web. Both the process and the
product benefit if the chemical papermaking additives introduced in the
wet end are kept to a minimum.
As previously mentioned, the majority of the existing tissue
manufacturing processes glue the web to the surface of a drying drum and
subsequently crepe the web from the dryer surface. Creping generally
produces a web with improved softness and importantly improves the
extensibility of the web. For proper creping to occur, it is imperative
that the web be securely attached to the surface of the drum. Many of
the chemicals added to the wet end of the machine, to ostensibly improve
key properties, end up interfering with the adhesion of the web to the
drying drum and hence adversely affect the creping process and the
CA 02143340 2000-05-29
3 '
4
quality of the tissue produced. The creping operation runs optimally when the
adhesive used to adhere the web to the creping surface is free of interference
from non-creping related chemicals such as those added at the wet end of the
overall tissue making process.
Additives introduced in the wet end of the process must be retained by
the cellulose fibers if the chemicals are to be functional. This is generally
done
by using chemicals that possess an ionic charge; most preferably a positive
ionic
charge which is attracted to the inherent negative ionic charge of cellulose.
Many additives which could improve the properties of the web are not charged.
Introduction of such chemicals into the dilute fiber slurry at the wet end of
the
process results in poor retention and exacerbates the interference problems
described above.
Another drawback to adding any chemical to the wet end of the process
is that the chemical, if retained, is distributed throughout the web. In many
instances it is desirable to apply active ingredients) only to the surface of
the
web. This may, for instance, be desirable with lubricious softening materials.
Application only to the surface insures efficient use of the material since
consumers only tactically interact with the surface. Application to the
surface
also avoids interference with other materials, such as strength additives,
that
might best be included in the center of the sheet. The present invention
overcomes all of the above mentioned drawbacks and generates desirable
additional benefits.
It is therefore, an aspect of an object of this invention to provide an
improved process to incorporate chemical papermaking additives into the tissue
web that enhance softness, strength, absorbency, and aesthetics or
combinations of these properties.
It is a further aspect of an object of this invention to provide an improved
process to incorporate chemical papermaking additives into the tissue web that
enhance softness, strength, absorbency, and aesthetics, or combinations of
these properties, without interference with the creping operation or
disruption of
the delicate water system balance or loss of beneficial properties generated
by
other means.
It is a further aspect of an object of this invention to provide an improved
process to incorporate chemical papermaking additives into the tissue web that
are typically poorly retained when added at the wet end of the papermaking
process.
It is a further aspect of an object of this invention to provide a process for
CA 02143340 2000-05-29
adding chemical papermaking additives to the dry web at the calender stack.
It is a further aspect of an object of this invention to provide an improved
process to apply diluted chemical papermaking additives (diluted to insure
5 controlled application of small quantities of additive) to a heated transfer
surface,
to preferentially evaporate the solvent or carrier material while the mixture
is on
the transfer surface but prior to addition to the dry web and subsequently to
apply a more concentrated mixture of the additive and solvent to the surface
of
the tissue web than was initially applied to the transfer surface.
It is a further aspect of an object of this invention to provide an improved
process to apply chemical papermaking additives to the tissue web via the
process described above where the vapor pressure of the carrier or solvent
material is higher than that of the additive material such that the carrier is
preferentially depleted after application to the heated transfer surface.
Preferably this carrier depletion also occurs prior to application to the
tissue web.
These and other aspects of objects are obtained using the present
invention, as will be seen from the following more detailed disclosure.
SUMMARY OF THE INVENTION
The present invention encompasses a process for making soft, strong,
absorbent, and aesthetically pleasing tissue paper. This process includes the
steps of providing a dry tissue paper web and then applying a sufficient
amount
of a chemical papermaking additive to the dry web. More specifically, the
softener application process includes the steps of diluting a chemical
papermaking additive compound with a suitable solvent to form a diluted
papermaking additive solution; applying the diluted papermaking additive
solution to a heated transfer surface by, for example, spraying; and
evaporating
a portion of the solvent from the heated transfer surface to form a film
containing
the papermaking additive. Next, at least one outwardly-facing surface of the
dry
tissue paper web is contacted with the heated transfer surface resulting in a
transfer of a sufficient amount of the papermaking additive such that between
0.004% and about 2% of the papermaking additive is retained by the tissue
paper. By solvent is meant a fluid that completely dissolves a chemical
papermaking additive, or a fluid that is used to emulsify a
WO 94/05857 PCT/US93/07852
2143340
6
chemical papermaking additive, or a fluid that is used to suspend a
chemical papermaking additive. The solvent may also be a carrier or
delivery vehicle that contains the chemical additive or aids in the
delivery of chemical papermaking additive. All references are meant to
be interchangeable and not limiting. The solution is the fluid
containing the chemical papermaking additive. By solution is meant a
true solution, an emulsion, and/or suspension. For purposes for this
invention, all terms are interchangeable and not limiting. If the
solvent is water then, preferably, the hot web is dried to a moisture
level below its equilibrium moisture content (at standard conditions)
before being contacted with the papermaking additive film, however this
process is also applicable to tissue paper at its equilibrium moisture as
well, if most of the water is evaporated from the transfer surface.
The amount of papermaking additive retained by the tissue paper is
Preferably, between O.O1X to about 1.0X, based on the dry fiber weight of
the tissue paper. The resulting tissue paper preferably has a basis
weight of from about 10 to about 80 g/m2 and a fiber density of less than
about 0.6 g/cc.
As mentioned above, the papermaking additive is applied to the web
Preferably, after the web has been dried and creped. By adding the
papermaking additive to the web after drying and creping, there is no
interference with the glue on the Yankee dryer, which can cause skip
crepe and/or loss in sheet control. Further, papermaking additives
applied by means of the process described in this invention do not
interfere with the papermaking water system, since they are not added in
the wet end of the paper machine. A further advantage of this process is
that the additives do not need to be substantive to the paper. That is,
they do not need to contain a cationic charge for bonding with the
anionic charge on the cellulosic papermaking fibers. Preferably, the
PaPermaking additive is applied to a hot, creped web after it leaves the
doctor blade and before it is wound on the parent roll.
Surprisingly, it has been found that significant tissue softening,
strength, absorbency, and/or aesthetic benefits can be achieved by low
levels of a chemical papermaking additive when the papermaking additive
is diluted with a solvent, applied to a heated transfer surface which
evaporates the carrier solvent and then transfers the papermaking
additive to a hot web before the converting operation. An advantage of
the process disclosed herein, is that the amount of residual solvent
21 403 4d
7
transferred to the paper web is sufficiently low that it does not degrade
other product properties. In addition, the quantity of papermaking
additive used is low enough to be economical. Also, tissue paper treated
with low levels of chemical softeners, such as polysiloxanes, retain a high
level of wettability, an important feature for a tissue product.
In accordance with one embodiment of the invention, an improved
process is provided for applying a polysiloxane compound to a dried and
creped tissue paper web, the process comprising the steps of:
a) providing a dried and creped tissue paper web;
b) diluting a polysiloxane compound with a suitable solvent to form
a dilute polysiloxane solution;
c) applying the dilute polysiloxane solution to a heated transfer
surface, wherein the heated transfer surface is a hot calendar roll;
d) evaporating at least a portion of the solvent from the hot
calender roll to form a film containing the polysiloxane compound; and
e) transferring the film from the hot calender roll to at least one
outwardly-facing surface of the dried and creped tissue web by contacting
the outwardly-facing web surface with the hot calender roll, and
transferring a sufficient amount of the polysiloxane compound,
such that from about 0.004% to about 0.74% of the polysiloxane
compound, based on the dry fiber weight of the tissue web, is retained by
the tissue web.
In accordance with a further embodiment of the present invention,
a process is provided for applying chemical papermaking additives to a
dried and creped tissue paper web, the process comprising the steps of:
a) providing a dried and creped tissue paper web;
b) diluting a chemical papermaking additive with a suitable solvent
to form a dilute chemical solution;
21 403 4~
7a
c) applying the dilute chemical solution to a heated transfer surface,
wherein the heated transfer surface is a hot calender roll;
d) evaporating at least a portion of the solvent from the hot
calender roll to form a film containing the chemical papermaking additive;
and
e) transferring the film from the hot calender roll to at least one
outwardly-facing surface of the dried and creped tissue web by contacting
the outwardly-facing web surface with the hot calender roll, thereby
transferring a sufficient amount of the chemical papermaking additive
such that from about 0.004% to about 2.0% of the chemical papermaking
additive, based on the dry fiber weight of the tissue web, is retained by the
tissue web;
wherein the chemical papermaking additive is selected from the
group consisting of strength additives, absorbancy additives, softener
additives, and mixtures thereof.
Preferred softener additives for use in the process of the present
invention include an amino-functional polydimethylpolysiloxane wherein
less than about 10 mole percent of the side chains on the polymer contain
an amino-functional group. In addition to such substitution with amino-
functional groups, effective substitution may be made with carboxyl,
hydroxyl, ether, polyether, aldehyde, ketone, amide, ester, and thiol
groups. Of these effective substituent groups, the family of groups
comprising amino, carboxyl, and hydroxyl groups are more preferred than
the others; and amino-functional groups are most preferred.,
Exemplary commercially available polysiloxanes include DOW 8075
and DOW 200 which are available from Dow Corning; and Silwet L720
and Ucarsil EPS which are available from Union Carbide.
Other preferred softener additives suitable for the present
invention include nonionic surfactants selected from sorbitan esters,
ethoxylated sorbitan esters, propoxylated sorbitan esters, mixed
ethoxylated/propoxylated sorbitan esters, and mixtures thereof.
21 40~ ~p
7b
The process for preparing tissue paper treated with a chemical
softener additive such as the polysiloxane and/or nonionic surfactants
discussed above may further comprise the step of adding an effective
amount of an absorbency additive to enhance the tactile perceivable
surface smoothness of the tissue paper and/or to at least partially offset
any reduction of wettability the tissue paper which would otherwise result
from the incorporation of the polysiloxane or other chemical softener. Of
course, the wettability of the paper without the chemical softener additive
can be enhanced with the addition of a suitable absorbency additive such
as a surfactant. The effective amount of surfactant is such that,
preferably, from about 0.01 to about 2 percent on a dry fiber weight of the
tissue paper; and, more preferably, from about 0.05 to about 1.0 percent is
retained by the tissue paper. Also, preferably, the surfactant is
noncationic; and is substantially nonmigratory in situ after the tissue
paper has been manufactured in order to substantially obviate post-
manufacturing changes in the tissue paper's properties which might
otherwise result from the inclusion of
r
WO 94/05857 PCT/US93/07852
X1433 ~0
8
surfactant. This may be achieved, for instance, through the use of
surfactants having melt temperatures greater than the temperatures
cortmonly encountered during storage, shipping, merchandising, and use of
tissue paper product embodiments of the invention: for example, melt
temperatures of about 50'C or higher.
Also, the process for preparing tissue paper in accordance with the
present invention may further comprise the step of adding an effective
amount of a strength additive such as a starch-based material to at least
partially offset any reduction of tensile strength and/or increase in
tinting propensity which would otherwise result from the incorporation of
the chemical softener additive and, if present, absorbency additive. The
effective amount of strength additive is such that, preferably, from
about 0.01 to about 2 percent on a dry fiber weight basis of the tissue
paper, is retained by the tissue paper.
All percentages, ratios and proportions herein are by weight, unless
otherwise specified.
BRIEF DESCRIPTION OF THE INYENT10N
Figure 1 is a schematic representation illustrating a preferred
embodiment of the process of the present invention of adding chemical
papermaking additive compounds to a tissue web.
The present invention is described in more detail below.
QETA1LED DESCRIPTION OF THE INVENTION
Briefly, the present invention provides tissue paper having enhanced
tactile perceivable softness through the addition of a chemical softener
additive, improved strength through the addition of a strength additive,
enhanced absorbency through the addition of an absorbency additive, and/
or enhanced aesthetics by incorporating an aesthetic additive such as
inks, dyes, and perfumes to a dry tissue web. These properties can be
enhanced by applying these and other chemical papermaking additives alone
or in combination to a dry tissue web. Preferably, the tissue web is
dried to a moisture content below its equilibrium moisture content before
the chemical papermaking additive is applied to the web.
Surprisingly, it has been found that very low levels of chemical
additives, e.g. polysiloxane softeners provide a significant tissue
softening effect when applied to dry tissue webs in accordance with the
present invention. Importantly. it hac hppn found that the levels of
2~~3~40
WO 94/05857 _ PCT/US93/07852
9
softener additives used to soften the tissue paper are low enough that
the tissue paper retains high wettability. Furthermore, because the
tissue web is preferably overdried and at an elevated temperature when
the papermaking additive is applied and because carrier water is depleted
on the hot transfer surface, further drying is not required.
As used herein, hot tissue web refers to a tissue web which is at an
elevated temperature that is higher than room temperature. Preferably
the elevated temperature of the web is at least 43'C, and more preferably
at least 65'C.
The moisture content of a tissue web is related to the temperature
of the web and the relative humidity of the environment in which the web
is placed. As used herein, the term ~overdried tissue web' refers to a
tissue web that is dried to a moisture content below its equilibrium
moisture content at standard test conditions of 23'C and 50% relative
humidity. The equilibrium moisture content of a tissue web placed in
standard testing conditions of 23'C and 50% relative humidity is
approximately 7%. The tissue web in the present invention can be
overdried by raising it to a elevated temperature through use of
conventional drying means such as a Yankee dryer. Preferably, an
overdried tissue web will have a moisture content of less than 7%, more
preferably from about 0 to about 6%, and most preferably, a moisture
content of from about 0 to about 3X, by weight.
Paper exposed to the normal environment typically has an equilibrium
moisture content in the range of 5 to 8~G. 4lhen paper is dried and creped
the moisture content in the sheet is generally less than 3X. After
manufacturing, the paper absorbs water from the atmosphere. In the
preferred process of the present invention, advantage is taken of the low
moisture content in the paper as it leaves the doctor blade. By applying
a chemical papermaking additive solution on the paper while it is
overdried, any residual water that is added to the paper is less than
what would normally be taken up from the atmosphere. Thus, no further
drying is required, and no tensile loss is observed other than that which
would normally occur if the paper were absorbing moisture from the air.
The present invention is applicable to tissue paper in general,
including but not limited to conventionally felt-pressed tissue paper;
pattern densified tissue paper such as exemplified by Sanford-Sisson and
its progeny; and high-bulk, uncompacted tissue paper such as exemplified
by Salvucci. The tissue paper may be of a homogenous or multilay~red
WO 94/05857
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construction; and tissue paper products made therefrom may be of a
single-ply or mufti-ply construction. The tissue paper preferably has a
basis weight of between 10 g/m2 and about 80 g/m2, and density of about
0.60 g/cc or less. Preferably, basis weight will be below about 35 g/m2
5 or less; and density will be about 0.30 g/cc or less. Most preferably,
density will be between 0.04 g/cc and about 0.20 g/cc.
Conventionally pressed tissue paper and methods for making such
paper are known in the art. Such paper is typically made by depositing
papermaking furnish on a foraminous forming wire. This forming wire is
10 often referred to in the art as a Fourdrinier wire. Once the furnish is
depos i ted on the formi ng wi re, i t i s referred to as a web. The web 1 s
dewatered by pressing the web and drying at elevated temperature. The
particular techniques and typical equipment for making webs according to
the process just described are well known to those skilled 1n the art.
In a typical process, a low consistency pulp furnish is provided in a
pressurized headbox. The headbox has an opening for delivering a thin
deposit of pulp furnish onto the Fourdrinier wire to form a wet web. The
web is then typically dewatered to a fiber consistency of between about
7X and about 45X (total web weight basis) by vacuum dewatering and
further dried by pressing operations wherein the web is subjected to
pressure developed by opposing mechanical members, for example,
cylindrical rolls. The dewatered web is then further pressed and dried
by a stream drum apparatus known in the art as a Yankee dryer. Pressure
can be developed at the Yankee dryer by mechanical means such as an
opposing cylindrical drum pressing against the web. Multiple Yankee
dryer drums may be employed, whereby additional pressing is optionally
incurred between the drums. The tissue paper structures which are formed
are referred to hereinafter as conventional, pressed, tissue paper
structures. Such sheets are considered to be compacted, since the web is
subjected to substantial overall mechanical compressional forces while
the fibers are moist and are then dried while in a compressed state.
Pattern densified tissue paper is characterized by having a
relatively high-bulk field of relatively low fiber density and an array
of densified zones of relatively high fiber density. The high-bulk field
is alternatively characterized as a field of pillow regions. The
densified zones are alternatively referred to as knuckle regions. The
densified zones may be discretely spaced within the high-bulk field or
may be interconnected, either fully or partially, within the high-bulk
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CA 02143340 2000-05-29
11
field. Preferred processes for making pattern densified tissue webs are
disclosed in U.S. Patent No. 3,301,746, issued to Sanford and Sisson on
January 31, 1967, U.S. Patent No. 3,974,025, issued to Peter G. Ayers on
August 10, 1976, and U.S. Patent No. 4,191,609, issued to Paul D. Trokhan on
March 4, 1980, and U.S. Patent 4,637,859, issued to Paul D. Trokhan on
January 20, 1987.
In general, pattern densified webs are preferably prepared by depositing
a papermaking furnish on a foraminous forming wire such as a Fourdrinier wire
to form a wet web and then juxtaposing the web against an array of supports.
The web is pressed against the array of supports, thereby resulting in
densified
zones in the web at the locations geographically corresponding to the points
of
contact between the array of supports and the wet web. The remainder of the
web not compressed during this operation is referred to as the high-bulk
field.
This high-bulk field can be further dedensified by application of fluid
pressure,
such as with a vacuum type device or a blow-through dryer, or by mechanically
pressing the web against the array of supports. The web is dewatered, and
optionally predried, in such a manner so as to substantially avoid compression
of
the high-bulk field. This is preferably accomplished by fluid pressure, such
as
with a vacuum type device or blow-through dryer, or alternately by
mechanically
pressing the web against an array of supports wherein the high-bulk field is
not
compressed. The operations of dewatering, optional predrying and formation of
the densified zones may be integrated or partially integrated to reduce the
total
number of processing steps performed. Subsequent to formation of the densified
zones, dewatering, and optional predrying, the web is dried to completion,
preferably still avoiding mechanical pressing. Preferably, from about 8% to
about 65% of the tissue paper surface comprises densified knuckles having a
relative density of at least 125% of the density of the high-bulk field.
The array of supports is preferably an imprinting carrier fabric having a
patterned displacement of knuckles which operate as the array of supports
which facilitate the formation of the densified zones upon application of
pressure. The pattern of knuckles constitutes the array of supports previously
referred to. Imprinting carrier fabrics are disclosed in U.S. Patent No.
3,301,746,
Sanford and Sisson, issued January 31, 1967, U.S. Patent No. 3,821,068,
Salvucci, Jr. et al., issued May 21, 1974, U.S. Patent No. 3,974,025, Ayers,
issued August 10, 1976, U.S. Patent No. 3,573,164, Friedberg et al., issued
March 30, 1971, U.S. Patent No. 3,473,576, Amneus, issued October 21, 1969,
U.S. Patent No. 4,239,065, Trokhan, issued December 16, 1980, and U.S.
CA 02143340 2000-05-29
12
Patent No. 4,528,239, Trokhan, issued July 9, 1985.
Preferably, the furnish is first formed into a wet web on a foraminous
forming carrier, such as a Fourdrinier wire. The web is dewatered and
transferred to an imprinting fabric. The furnish may alternately be initially
deposited on a foraminous supporting carrier which also operates as an
imprinting fabric. Once formed, the wet web is dewatered and, preferably,
thermally predried to a selected fiber consistency of between about 40% and
about 80%. Dewatering is preferably performed with suction boxes or other
vacuum devices or with blow-through dryers. The knuckle imprint of the
imprinting fabric is impressed in the web as discussed above, prior to drying
the
web to completion. One method for accomplishing this is through application of
mechanical pressure. This can be done, for example, by pressing a nip roll
which supports the imprinting fabric against the face of a drying drum, such
as a
Yankee dryer, wherein the web is disposed between the nip roll and drying
drum. Also, preferably, the web is molded against the imprinting fabric prior
to
completion of drying by application of fluid pressure with a vacuum device
such
as a suction box, or with a blow-through dryer. Fluid pressure may be applied
to
induce impression of densifled zones during initial dewatering, in a separate,
subsequent process stage, or a combination thereof.
Uncompacted, nonpattern-denslfied tissue paper structures are described
in U.S. Patent No. 3,812,000 issued to Joseph L. Salvucci, Jr. and Peter N.
Yiannos on May 21, 1974, and U.S. Patent No. 4,208,459, issued to Henry E.
Becker, Albert L. McConnell, and Richard Schutte on June 17, 1980. In general,
uncompacted, nonpattern-densified tissue paper structures are prepared by
depositing a papermaking furnish on a foraminous forming wire such as a
Fourdrinier wire to form a wet web, draining the web and removing additional
water without mechanical compression until the web has a fiber consistency of
at least 80%, and creping the web. Water is removed from the web by vacuum
dewatering and thermal drying. The resulting structure is a soft but weak high-
bulk sheet of relatively
WO 94/05857 _ 214 3 3 4 d PCT/US93/07852
13
uncompacted fibers. Bonding material is preferably applied to portions
of the web prior to creping.
Compacted non-pattern-densified tissue structures are commonly known
in the art as conventional tissue structures. In general, compacted,
non-pattern-densified tissue paper structures are prepared by depositing
a papermaking furnish on a foraminous wire such as a Fourdrinier wire to
form a wet web, draining the web and removing additional water with the
aid of a uniform mechanical compaction (pressing) until the web has a
consistency of 25-50X, transferring the web to a thermal dryer such as a
Yankee and creping the web. Overall, water 1s removed from the web by
vacuum, mechanical pressing and thermal means. The resulting structure
is strong and generally of singular density, but very low in bulk,
absorbency and in softness.
The papermaking fibers utilized for the present invention will
normally include fibers derived from wood pulp. Other cellulosic fibrous
' pulp fibers, such as cotton linters, bagasse, etc., can be utilized and
are intended to be within the scope of this invention. Synthetic fibers,
such as rayon, polyethylene and polypropylene fibers, may also be
utilized in combination with natural cellulosic fibers. One exemplary
Polyethylene fiber which may be utilized is PulpexTM, available from
Hercules, Inc. (Wilmington, Delaware).
Applicable wood pulps include chemical pulps, such as Kraft,
sulfite, and sulfate pulps, as well as mechanical pulps including, for
example, groundwood, thermomechanical pulp and chemically modified
thermomechanical pulp. Chemical pulps, however, are preferred since they
impart a superior tactile sense of softness to tissue sheets made
therefrom. Pulps derived from both deciduous trees (hereinafter, also
referred to as 'hardwood') and coniferous trees (hereinafter, also
referred to as 'softwood") may be utilized. Also applicable to the
Present invention are fibers derived from recycled paper, which may
contain any or all of the above categories as well as other non-fibrous
materials such as fillers and adhesives used to facilitate the original
pap:ermaking.
In addition to papermaking fibers, the papermaking furnish used to
make tissue paper structures may have other components or materials added
thereto as may be or later become known in the art. The types of
additives desirable will be dependent upon the particular end use of the
tissue sheet contemplated. For example, in products such as toilet
CA 02143340 2000-05-29
14
paper, paper towels, facial tissues and other similar products, high wet
strength
is a desirable attribute. Thus, it is often desirable to add to the
papermaking
furnish chemical substances known in the art as "wet strength" resins.
A general dissertation on the types of wet strength resins utilized in the
paper art can be found in TAPPI monograph series No. 29, Wet Strength in
Paper and Paperboard, Technical Association of the Pulp and Paper Industry
(New York, 1965). The most useful wet strength resins have generally been
cationic in character. Polyamide-epichlorohydrin resins are cationic wet
strength
resins which have been found to be of particular utility. Suitable types of
such
resins are described in U.S. Patent Nos. 3,700,623, issued on October 24,
1972,
and 3,772,076, issued on November 13, 1973, both issued to Keim. One
commercial source of a useful polyamide-epichlorohydrin resins is Hercules,
Inc.
of Wilmington, Delaware, which markets such resin under the mark KymemeT""
557H:
Polyacrylamide resins have also been found to be of utility as wet
strength resins. These resins are described in U.S. Patent Nos. 3,556,932,
issued on January 19, 1971, to Coscia, et al. and 3,556,933, issued on January
19, 1971, to Williams et al. One commercial source of polyacrylamide resins is
American Cyanamid Co. of Stanford, Connecticut, which markets one such resin
under the mark ParezT"" 631 NC.
Still other water-soluble cationic resins finding utility in this invention
are
urea formaldehyde and melamine formaldehyde resins. The more common
functional groups of these polyfunctional resins are nitrogen containing
groups
such as amino groups and methylol groups attached to nitrogen.
Polyethylenimine type resins may also find utility in the present invention.
In
addition, temporary wet strength resins such as Caldas (manufactured by Japan
Carlit) and CoBond 1000 (manufactured by National Starch and Chemical
Company) may be used in the present invention. It is to be understood that the
addition of chemical compounds such as the wet strength and temporary wet
strength resins discussed above to the pulp furnish is optional and is not
necessary for the practice of the present development.
In the process of the current invention the chemical papermaking
additives are applied after the tissue web has been dried and creped, and
preferably is still at an elevated temperature. It has been found that
~I43340
WO 94/05857 ' PCT/US93/07852
addition of some chemical papermaking additives to the tissue web before
the web is dried and creped can result in interference with the coating
on the dryer (i.e., glue coating on Yankee dryer), and also cause skip
crepe and a loss in sheet control. These problems are eliminated by the
5 process of the present invention wherein the chemical papermaking
additives are applied to the web after the web has been dried and creped.
Preferably, the chemical papermaking additives are applied to the dried
and creped tissue web before the web is wound onto the parent roll.
Thus, in a preferred embodiment of the present invention the chemical
10 PaPermaking additives are applied to a hot, overdried tissue web after
the web has been creped, but before the web passes through the calender
rolls.
The chemical papermaking additives are preferably applied to the hot
transfer surface from an aqueous solution, emulsion, or suspension. The
15 chemical papermaking additives can also be applied in a solution
containing a suitable, nonaqueous solvent, in which the chemical
papermaking additive dissolves or with which the chemical papermaking
additive is miscible: for example, hexane. The chemical papermaking
additive may be supplied in neat form or, more preferably, emulsified
with a suitable surfactant emulsifier. Emulsified chemical papermaking
additives are preferable for ease of application since a neat chemical
papermaking additive aqueous solution must be agitated to inhibit
separation into water and chemical papermaking additive phases.
The chemical papermaking additive should be applied to the heated
transfer surface in a macroscopically uniform fashion for subsequent
transfer to the tissue paper web so that substantially the entire sheet
benefits from the effect of the chemical papermaking additive. Following
application to the heated transfer surface, the solvent preferably
evaporates leaving a thin film containing the chemical papermaking
additive. By thin film is meant any thin coating, hale or mist on the
transfer surface. This thin film can be microscopically continuous.
discrete or patterned, but should be macroscopically uniform. On
microscopic scale the chemical papermaking additive may be distributed in
a uniform, random, discrete, patterned, continuous, or discontinuous
fashion. Applying the chemical papermaking additive to the tissue paper
web in continuous and patterned distributions are both within the scope
of the invention and meet the above criteria. Likewise, the chemical
WO 94/05857 PCT/US93/07852
,.
,. 16
papermaking additive can be added to either side of the tissue web
singularly, or to both sides.
Methods of macroscopically uniformly applying the chemical
papermaking additive to the hot transfer surface include spraying and
gravure printing. Spraying has been found to be economical, and
susceptible to accurate control over quantity and distribution of the
chemical papermaking additive, so it is most preferred. Preferably, an
aqueous mixture containing an emulsified chemical papermaking additive is
applied from the transfer surface onto the dried, creped tissue web after
the Yankee dryer and before the parent roll.
Figure 1 illustrates a preferred method of applying the chemical
papermaking additive to the tissue web. Referring to Figure 1, a wet
tissue web 1 is on carrier~fabric I4 past turning roll 2 and transferred
to Yankee dryer 5 by the action of pressure roll 3 while carrier fabric
14 travels past turning roll 16. The paper web is adhesively secured to
the cylindrical surface of Yankee dryer S by adhesive applied by spray
applicator 4. Drying is completed by steam-heated Yankee dryer 5 and by
hot air which is heated and circulated through drying hood 6 by means not
shown. The web is then dry creped from the Yankee dryer S by doctor
blade 7, after which it is designated creped paper sheet 15. An aqueous
mixture containing an emulsified chemical papermaking additive compound
is sprayed onto an upper heated transfer surface designated as upper
calender roll 10 and/or a lower heated transfer surface designated as
lower calender roll 11, by spray applicators 8 and 9 depending on whether
the chemical papermaking additive is to be applied to both sides of the
tissue web or dust to one side. The paper sheet 15 then contacts heated
transfer surfaces IO and 11 after a portion of the solvent has been
evaporated. The treated web then travels over a circumferential portion
of reel 12, and then is wound onto parent roll 13. Equipment suitable
for spraying chemical papermaking additive-containing liquids onto hot
transfer surfaces include external mix, air atomizing nozzles, such as
the 2 mm nozzle available -from V.I.B. Systems, Inc., Tucker, Georgia.
Equipment suitable for printing chemical papermaking additive-containing
liquids onto hot transfer surfaces include rotogravure or flexographic
Printers.
While not wishing to be bound by theory or to otherwise limit the
present invention, the following description of typical process
conditions encountered during the papermaking operation and their impact
_ r
_2143340
WO 94/05857 PCT/US93/07852
17
on the process described in this invention is provided. The Yankee dryer
raises the temperature of the tissue sheet and removes the moisture. The
steam pressure in the Yankee is on the order of 110 PSI (750kPa). This
pressure is sufficient to increase the temperature of the cylinder to
about 173'C. The temperature of the paper on the cylinder is raised as
the water in the sheet is removed. The temperature of the sheet as it
leaves the doctor blade can be in excess of 120'C. The sheet travels
through space to the calender and the reel and loses some of this heat.
The temperature of the paper wound in the reel is measured to be on the
order of 65'C. Eventually the sheet of paper cools to room temperature.
This can take anywhere from hours to days depending on the size of the
paper roll. As the paper cools it also absorbs moisture from the
atmosphere. As previously mentioned, the moisture content in the sheet
is related to the sheet temperature and the relative humidity of the
environment in which the paper is placed. For example the equilibrium
moisture content of a sheet placed in standard testing conditions of 23'C
and 50% RH is approximately 7%. Increasing the moisture content of the
sheet above 7% can have a deleterious effect on the tensile strength of
the paper. For example, a moisture increase to 9X can cause the tensile
strength of the paper to decrease by as much as 15X.
One very surprising attribute of chemical softeners, such as
polysiloxane, is their ability to improve softness at very low levels on
the surface of the paper. The chemical softener, however needs to be
fairly uniformly distributed on the paper surface in order for the
consumer to recognize the improved softness. From a process standpoint,
there was previously no satisfactory method of uniformly applying low
quantities of a chemical softener to a paper web traveling at a high rate
of speed. Belt speeds of 700 to 1000 meters/minute (25 to 40 miles/hour)
are typical in modern high speed paper machines. Webs traveling at these
rates of speed generally have an air boundary layer on their surface.
One method for applying low quantities of liquids is to use a spray
system and adjust the air and/or liquid pressures. For example, one
could go to low flow rates by using high air pressures. This generally
produces extremely small particles. It is difficult to impart sufficient
momentum into these small particles so they can penetrate the air
boundary layer traveling on the surface of the fast moving paper web.
Moreover, if one increases the particle size of the spray fluid so it can
WO 94/05857 PCT/US93/07852
penetrate the air boundary layer at low flow rates the surface coverage
becomes nonuniform.
One commonly used method for applying low levels of an active
material is to first dilute the material with a solvent. The spray
systems can then be adjusted to deliver larger particle sizes at high
flow rates. The larger particles can penetrate the air boundary layer.
However one is now faced with the problem of having to remove the solvent
from the paper. Generally volatile organic solvents are not used in
papermaking, since they can be fire or environmental hazards. Water can
be used as a solvent for water soluble papermaking additives. Water can
al so be used as a sol vent, or more appropri ately as a di 1 uent, for the
non-water soluble papermaking additives, such as organic oils, polymers,
and polysiloxanes, if the non-water soluble papermaking additive, such as
a polysiloxane is first emulsified with a suitable surfactant system.
While water does not pose the same process risks as an organic solvent,
. water can degrade the product, causing a loss in crepe and/or tensile
strength. Further the water needs to be removed from the paper.
One remedy to the water problem is to apply a dilute chemical
papermaking additive to the paper while it is overdried. The water added
to the paper with the chemical papermaking additive by this method is
usually less than the paper would normally take up from the atmosphere
upon cooling to room temperature. Thus, no further drying is required,
and no loss in tensile strength occurs from addition of the water.
However, the water solution is capable of penetrating the entire sheet
causing the active material to spread to the inside of the sheet rather
than staying on the surface of the paper where it is most effective.
Further, this process is limited to an overdried sheet, making
application to the paper during a converting process (an off paper
machine process) difficult without adding an additional drying step to
the process. A further limitation to this process is the limited
dilution range and application range of the chemical papermaking additive
emulsion imposed by the emulsion properties, (i.e., high concentrations
tend. to have high viscosities, whereas low concentrations increase the
amount of water sprayed on the sheet).
The present invention solves the above described problems by first
spraying a dilute water soluble chemical papermaking additive or
emulsified non-water soluble chemical papermaking additive solution onto
__....
2143340
WO 94/05857 - PCT/US93/07852
19
a hot transfer surface and evaporating the solvent from the chemical
papermaking additive solution before transferring it to the dry web.
For exemplary purposes, a typical commercially available silicone
emulsion chemical softener is Dow Corning~ Q2-7224 Conditioning Agent
marketed by the Dow Corning Corporation. This material generally
contains about 35X by weight of an amino-functional polysiloxane
emulsified in water. This silicone receipt emulsion is diluted with
water to less than about 20X concentration, by weight, before being
applied to the heated transfer surface. More preferably, chemical
paPermaking additive emulsions used in the present invention are first
diluted with water to less than about 15X concentration by weight before
being applied to the transfer surface.
Exemplary materials suitable for the heated transfer surfaces
include metal (e. g., steel, stainless steel, and chrome), non-metal
(e'gw suitable polymers, ceramic, glass), and rubber.
When a diluted silicone emulsion of the type described above was
sprayed on the hot transfer surface, in this case a steel calender roll,
it was most surprising to discover that little or no water was
transferred to the paper web by this process. In fact, under one set of
Process conditions, it was expected that the sheet moisture content would
increase from a base of 4X to 5X after spraying. However, it was found
that the moisture content did not increase at all, while the silicone
content in the web did increase to its expected concentration. It was a
further surprise to find that an attempt to increase the sheet moisture
by 3.5X (i.e., raising the sheet moisture from 4 to 7.5X) only resulted
in a moisture increase of 0.7X, that is the measured moisture content was
only 4.7X.
This is most surprising since the roll temperature is on the order
of 80'C (20'C below the boiling point of water) and the time between the
Point of application and point of transfer is on the order of 0.1 sec.
It was surprising to discover that greater than 50X of the water had
evaporated from the roll under these conditions, leaving behind a thin
film of polysiloxane emulsion. This thin film was calculated to be on
the order of 0.25 microns thick (I micron ~ 10-6 meters). The films of
the present invention are preferably less than about 10 microns in
thickness, and more preferably, less than about one micron in thickness.
In the process of the present invention it is preferred that at
least about 50y., more preferably at least about 80X, of the water is
ff ~ ~ ~ ~PCT/US93/07852
evaporated from the dilute chemical papermaking additive solution which
1s applied to the heated transfer surface before transferring it to the
dry tissue web. This leaves a film with a calculated thickness of about
0.075 microns. Most preferably greater than about 95X of the water is
5 evaporated from the solution on the heated transfer surface, leaving a
calculated film thickness of about 0.05 microns for transfer to the paper
web.
The temperature of the heated transfer surface is preferably below
the boiling point of the solvent. Thus, if the solvent is water, the
10 temperature of the heated transfer surface should be below 100'C.
Preferably the temperature is between 50 and 90'C, more preferably
between 70 and 90'C when water is used as the solvent.
The heat on the transfer surface can also cause a lowering of the
viscosity of the chemical papermaking additive, thus increasing its
15 ability to spread into a thin film on the transfer surface. This film is
then transferred to the paper web surface by contacting the web with the
transfer surface. Surprisingly, it has been found that the chemical
papermaking additive transfer efficiency to the web is quite high.
Efficiencies on the order of 40 to 80X are typical, based on the flow out
20 of the spray nozzles to the transfer surface and the quantity measured on
the paper web. Moreover, this process is not limited to overdried paper.
Depending on the amount of water removed from the spray mixture by the
hot transfer surface, the process described herein is capable of
delivering chemical papermaking additives to equilibrated dry paper as
well. However application to a hot overdried web is preferred, to insure
that any residual water in the film does not interfere with any paper
properties.
An additional benefit in applying the chemical papermaking additive
solution to a hot overdried web is that the decreased viscosity of the
solution aids in insuring that the solution is uniformly applied across
the surface of the web. (It is believed that the low viscosity solution
is more mobile).
CHEMICAL PAPERMAKING ADDITIVES
The chemical papermaking additives for use in the improved process
of the present invention are preferably selected from the group
consisting of strength additives, absorbency additives, softener
_.
CA 02143340 2000-05-29
21
additives, aesthetic additives, and mixtures thereof. Each of these types of
additives will be discussed below.
~ Strenqth Additives
The strength additive is selected from the group consisting of permanent
wet strength resins, temporary wet strength resins, dry strength additives,
and
mixtures thereof.
If permanent wet strength is desired, the chemical papermaking additive
can be chosen form the following group of chemicals: polyamid-epichlorohydrin,
polyacrylamides, styrene-butadiene latexes; insolubilized polyvinyl alcohol;
urea
formaldehyde; polyethyleneimine; and chitosan polymers. Polyamide
epichlorohydrin resins are cationic wet strength resins which have been found
to
be of particular utility. Suitable types of such resins are described in U.S.
Patent
Nos. 3,700,623, issued on October 24, 1972, and 3,772,076, issued on
November 13, 1973, both issued to Keim. One commercial source of a useful
polyamide-epichlorohydrin resins is Hercules, Inc. of ~Imington, Delaware,
which markets such resin under the mark KymemeT"" 557H.
Polyacrylamide resins have also been found to be of utility as wet
strength resins. These resins are described in U.S. Patent Nos. 3,556,932,
issued on January 19, 1971, to Coscia, et al. and 3,556,933, issued on January
19, 1971, to Williams et al. One commercial source of polyacrylamide resins is
American Cyanamid Co. of Stanford, Connecticut, which markets one such resin
under the mark ParezT"" 631 NC.
Still other water-soluble cationic resins finding utility in this invention
are
urea formaldehyde and melamine formaldehyde resins. The more common
functional groups of these polyfunctional resins are nitrogen containing
groups
such as amino groups and methylol groups attached to nitrogen.
Polyethylenimine type resins may also find utility in the present invention.
If temporary wet strength is desired, the chemical papermaking additive
can be chosen from the following group of chemicals. Cationic dialdehyde
starch-based resin (such as Caldas produced by Japan Carlet or Cobond 1000
produced by National Starch); dialdehyde starch; and/or the resin described in
U.S. Patent No. 4,981,557 issued on January 1, 1991, to Bjorkquist.
If dry strength is desired, the chemical papermaking additive can be
chosen from the following group of chemicals. Polyacrylamide (such as
combinations of CyproT"" 514 and AccostrengthT"" 711 produced by American
Cyanamid of Wayne, N.J.); starch (such as corn starch or potato starch);
CA 02143340 2000-05-29
22
polyvinyl alcohol (such as AirvoITM 540 produced by Air Products Inc of
Allentown, PA); guar or locust bean gums; polyacrylate latexes; and/or
carboxymethyl cellulose (such as Aqualon CMC-TT"" from Aqualon Co.,
Wilmington, DE). In general, suitable starch for practicing the present
invention
is characterized by water solubility, and hydrophilicity. Exemplary starch
materials include corn starch and potato starch, albeit it is not intended to
thereby limit the scope of suitable starch materials; and waxy corn starch
that is
known industrially as amioca starch is particularly preferred. Amioca starch
differs from common corn starch in that it is entirely amylopectin, whereas
common corn starch contains both amplopectin and amylose. Various unique
characteristics of amioca starch are further described in "Amioca - The Starch
From Waxy Corn", H. H. Schopmeyer, Food Industries, December 1945, pp.
106-108 (Vol. pp. 1476-1478). The starch can be in granular or dispersed form
albeit granular form is preferred. The starch is preferably sufficiently
cooked to
induce swelling of the granules. More preferably, the starch granules are
swollen, as by cooking, to a point just prior to dispersion of the starch
granule.
Such highly swollen starch granules shall be referred to as being "fully
cooked".
The conditions for dispersion in general can vary depending upon the size of
the
starch granules, the degree of crystallinity of the granules, and the amount
of
amylose present. Fully cooked amioca starch, for example, can be prepared by
heating an aqueous slurry of about 4% consistency of starch granules at about
190°F (about 88°C) for between about 30 and about 40 minutes.
Other
exemplary starch materials which may be used include modified cationic
starches such as those modified to have nitrogen containing groups such as
amino groups and methylol groups attached to nitrogen, available from National
Starch and Chemical Company, (Bridgewater, New Jersey). Such modified
starch materials have heretofore been used primarily as a pulp furnish
additive
to increase wet and/or dry strength. However, when applied in accordance with
this invention by application to an overdried tissue paper web they may have
reduced effect on wet strength relative to wet-end addition of the same
modified
starch materials. Considering that such modified starch materials are more
WO 94/05857 _ ~ ~ ~ 3 3 ~ p PCT/US93/07852
23
expensive than unmodified starches, the latter have generally been
preferred. These wet and dry strength resins may be added to the pulp
furnish in addition to being added by the process described in this
Invention. It is to be understood that the addition of chemical compounds
such as the wet strength and temporary wet strength resins discussed
above to the pulp furnish is optional and is not necessary for the
practice of the present development.
For purposes of this invention, the strength additive is preferably
applied to the heated transfer roll in an aqueous solution. Methods of
aPPlication include, the same previously described with reference to
application of other chemical additives preferably by spraying; and, less
preferably, by printing. The strength additive may be applied to the
tissue paper web alone, simultaneously with, prior to, or subsequent to
the addition of softener, absorbency, and/or aesthetic additives. At
least an effective amount of a strength additive, preferably starch, to
provide lint control and concomitant strength increase upon drying
relative to a non-binder treated but otherwise identical sheet is
preferably applied to the sheet. Preferably, between about 0.01% and
about 2. OX of a strength additive is retained in the dried sheet,
calculated on a dry fiber weight basis; and, more preferably, between
about 0.1% and about I.Oy. of a strength additive material, preferably
starch-based, is retained.
Softener Additives
The chemical softener additives are selected from the group
consisting of lubricants, plasticizers, cationic debonders, noncationic
debonders and mixtures thereof. Debonders which are preferred for use in
the present invention are noncationic; and, more preferably, are nonionic
surfactants. However, cationic surfactants may be used. Noncationic
surfactants include anionic, nonionic, amphoteric, and zwitterionic
surfactants. Preferably, the surfactant is substantially nonmigratory in
situ after the tissue paper has been manufactured in order to
substantially obviate post-manufacturing changes in the tissue paper's
properties which might otherwise result from the inclusion of surfactant.
This may be achieved, for instance, through the use of surfactants having
melt temperatures greater than the temperatures commonly encountered
during storage, shipping, merchandising, and use of tissue paper product
embodiments of the invention: for example, melt temperatures of about
WO 94/05857 PCT/US93/07852
~~4334~
24
...
50'C or higher. Also, the surfactant is preferably water-soluble when
applied to the wet web.
The level of noncationic surfactant applied to tissue paper webs to
provide the aforementioned softness/tensile benefit ranges from the
minimum effective level needed for imparting such benefit, on a constant
tensile basis for the end product, to about 2x: preferably between about
0.01% and about 2f. noncationic surfactant is retained by the web; more
preferably, between about 0.059: and about 1.09:; and, most preferably,
between about 0.059: and about 0.3%. The surfactants preferably have
alkyl chains with eight or more carbon atoms. Exemplary anionic
surfactants are linear alkyl sulfonates, and alkylbenzene sulfonates.
Exemplary nonionic surfactants are alkylglycosides including
alkylglycoside esters such as CrodestaTM SL-40 which is available from
Croda, Inc. (New York, NY); alkylglycoside ethers as described in U.S.
Patent 4,011,389, issued to 41. K. Langdon, et al. on March 8, 1977;
alkylpolyethoxylated esters such as PegosperseTM 200 ML available from
Glyco Chemicals, Inc. (Greenwich, CT); alkylpolyethoxylated ethers and
esters such as Neodole 25-12 available from Shell Chemical Co; sorbitan
esters such as Span 60 from ICI America, Inc, ethoxylated sorbitan
esters, propoxylated sorbitan esters, mixed ethoxylated/propoxylated
sorbitan esters, and polyethoxylated sorbitan alcohols such as Tween 60
also from ICI America, Inc. Alkylpolyglycosides are particularly
preferred for use in the present invention. The above listings of
exemplary surfactants are intended to be merely exemplary in nature, and
are not meant to limit the scope of the invention.
The surfactant may be applied to the hot transfer surface by
spraying, gravure printing, or flexographic printing. Any surfactant
other than the chemical papermaking additive emulsifying surfactant
material, is hereinafter referred to as "surfactant," and any surfactant
Present as the emulsifying component of emulsified chemical papermaking
additives is hereinafter referred to as 'emulsifying agent". The
surfactant may be applied to the tissue paper alone or simultaneously
with, after, or before other chemical papermaking additives. In a
typical process, if another additive is present, the surfactant is
aPPlied to an overdried web simultaneously with the other additive(s).
It may also be desirable to treat a debonder containing tissue paper with
a relatively low level of a binder for lint control and/or to increase
tensile strength. As used herein the term "binder" refers to the various
CA 02143340 2000-05-29
wet and dry strength additives known in the art. The binder may be applied to
the tissue paper simultaneously with, after or before the debonder and an
absorbency aid, if used. Preferably, binders are added to the overdried tissue
5 webs simultaneously with the debonder (i.e., the binder is included in the
dilute
debonder solution applied to the heated transfer surface).
If a chemical softener that functions primarily by imparting a lubricous feel
is desired, it can be chosen from the following group of chemicals. Organic
materials (such as mineral oil or waxes such as paraffin or carnuba, or
lanolin);
10 and polysiloxanes (such as the compounds described in U.S. Patent No.
5,059,282 issued to Ampulski).
It has been found, surprisingly, that low levels of polysiloxane applied to
hot, overdried tissue paper webs can provide a softened, silky, flannel-like,
nongreasy tactile sense of feel to the tissue paper without the aid of
additional
15 materials such as oils or lotions. Importantly, these benefits can be
obtained for
many of the embodiments of the present invention in combination with high
wettability within the ranges desirable for toilet paper application.
Preferably,
tissue paper treated with polysiloxane in accordance with the present
invention
comprises about 0.75% or less polysiloxane. It is an unexpected benefit of
this
20 invention that tissue paper treated with about 0.75% or less polysiloxane
can
have imparted thereto substantial softness and silkiness benefits by such a
low
level of polysiloxane. In general, tissue paper having less than about 0.75%
polysiloxane, preferably less than about 0.5%, can provide substantial
increases
in softness and silkiness and flannel-like quality yet remain sufficiently
wettable
25 for use as toilet paper without requiring the addition of surfactant to
offset any
negative impact on wettability which results from the polysiloxane.
The minimum level of polysiloxane to be retained by the tissue paper is at
least an effective level for imparting a tactile difference in softness or
silkiness or
flannel-like quality to the paper. The minimum effective level may vary
depending upon the particular type of sheet, the method of application, the
particular type of polysiloxane, and whether the polysiloxane is supplemented
by
starch, surfactant, or other additives or treatments. Without limiting the
range of
applicable polysiloxane retention by the tissue paper, preferably at least
about
0.004%, more preferably at least about 0.01 %, and most preferably at least
about 0.05% polysiloxane is retained by the tissue paper.
WO 94/05857 PCT/US93/07852
26
Preferably, a sufficient amount of polysiloxane to impart a tactile sense
of softness is disposed uniformly on both surfaces of the tissue paper:
i.e., disposed on the outwardly facing surfaces of the surface-level
fibers. When polysiloxane is applied to one surface of the tissue paper,
some of it will, generally, at least partially penetrate to the tissue
paper interior. However, preferably, the polysiloxane is applied to both
sides of the tissue paper to ensure that both surfaces have:~imparted
thereto the benefits of the polysiloxane. In addition to treating tissue
paper with polysiloxane as described above, it has been found desirable
to also treat such tissue paper with an absorbency additive. This is in
addition to any surfactant material that may be present as an emulsifying
agent for the polysiloxane. In some cases it has also been found
- desirable to omit the polysiloxane from the additive solution and to
treat tissue paper with surfactant material alone to improve wetting
and/or softness. Tissue paper having in excess of about 0.3%
polysiloxane is preferably treated with surfactant when contemplated for
uses wherein high wettability is desired. Most preferably, a noncationlc
surfactant is applied to the hot, overdried tissue paper web, in order to
obtain an additional softness benefit, on a constant tensile basis, as
Previously discussed. The amount of surfactant required to increase
hydrophilicity to a desired level will depend upon the type and level of
polysiloxane and the type of surfactant. However, as a general
guideline, between about O.OIy. and about 2X surfactant retained by the
tissue paper, preferably between about 0.05X and about 1.0X, is believed
to be sufficient to provide sufficiently high wettability for most
applications, including toilet paper, for polysiloxane levels of about
0.75X or less.
If a chemical softener that functions primarily by plasticizing the
structure is desired, it can be chosen from the following group of
chemicals: polyethylene glycol (such as PEG 400); dimethylamine; and/or
glycerine.
If a cationic chemical softener that functions primarily by
debonding is desired, it can be chosen from the following group of
chemicals. Cationic quaternary compounds (such as dihydrogenated tallow
dimethyl ammonium methyl sulfate (DTDMAMS) or dihydrogenated tallow
dimethyl ammonium chloride (OTDMAC) both produced by Sherex Corporation
of Dudlin, OH; Berocel 579 (produced by Eka Nobel of Stennungsund,
Sweden); materials described in U.S. Patent No.'s 4,351,699 and 4,447,294
CA 02143340 2000-05-29
27
issued to Osborn and/or diester derivatives of DTDMAMS or DTDMAC).
C) Absorbency Additives
If an absorbency aid is desired that enhances the rate of absorbency it
can be chosen from the following group of chemicals: polyethoxylates (such as
PEG 400); alkyl ethoxylated esters (such as Pegosperse 200ML from Lonza
Inc.); alkyl ethoxylated alcohols (such as Neodol~); alkyl polyethoxylated
nonylphenols (such as Igepal CO produced by Rhone-Poulenc/GAF) and/or
materials described in U.S. Patent No.'s 4,959,125 and 4,940,513 issued to
Spendel.
In those instances where the surfactant debonder softener decreases
wetting, a wetting agent, e.g., a second surfactant, may be added to the
application solution. For_example, a sorbitan stearate ester can be mixed with
an alkyl polyethoxylated alcohol to produce a soft wettable paper.
If an absorbency aid is desired that decreases the rate of absorbency it
can be chosen from the following group of chemicals. Alkylketenedimers (such
as Aquapel~ 360XC Emulsion manufactured by Hercules Inc., Wilmington, DE.);
fluorocarbons (such as Scotch Guard by 3M of Minneapolis, MN).
The absorbency additive can be used alone or in combination with a
strength additive. Starch based strength additives have been found to be the
preferred binder for use in the present invention. Preferably, the tissue
paper is
treated with an aqueous solution of starch, and, as mentioned above, the sheet
is overdried at the time of application. In addition to reducing tinting of
the
finished tissue paper product, low levels of starch also imparts a modest
improvement in the tensile strength of tissue paper without imparting
boardiness
(i.e., stiffness) which would result from additions of high levels of starch.
Also,
this provides tissue paper having improved strengthlsoftness relationship
compared to tissue paper which has been strengthened by traditional methods
of increasing tensile strength: for example, sheets having increased tensile
strength due to increased refining of the pulp; or through the addition of
other
dry strength additives. This result is especially surprising since starch has
traditionally been used to build strength at the expense of softness in
applications wherein softness is not an important characteristic: for example,
paperboard. Additionally,
WO 94/05857 PCT/US93/07852
parenthetically, starch has been used as a filler for printing and
writing paper to improve surface printability.
Q~~ Aesthetic Additives
If an aesthetic additive is desired, it can be chosen from the
following group of chemicals: inks; dyes; perfumes; opacifiers (such as
Ti02 or calcium carbonate), optical brighteners, and mixtures thereof.
The aesthetics of the paper can also be improved utilizing the
process described in this invention. Inks, dyes, and/or perfumes are
Preferably added to the application fluid which is subsequently applied
to the hot transfer roll. The aesthetics additive may be applied alone
or in combination with the wetting, softening, and/or strength additives.
Analvtical Methods
Analysis of the amounts of treatment chemicals herein retained on
. tissue paper webs can be performed by any method accepted in the
applicable art. For example, the level of polysiloxane retained by the
tissue paper can be determined by solvent extraction of the polysiloxane
with an organic solvent followed by atomic absorption spectroscopy to
determine the level of silicon in the extract; the level of nonionic
surfactants, such as alkylglycosides, can be determined by extraction in
an organic solvent followed by gas chromatography to determine the level
of surfactant in the extract; the level of anionic surfactants, such as
linear alkyl sulfonates, can be determined by water extraction followed
by colorimetry analysis of the extract; the level of starch can be
determined by amylase digestion of the starch to glucose followed by
colorimetry analysis to determine glucose level. These methods are
exemplary, and are not meant to exclude other methods which may be useful
for determining levels of particular components retained by the tissue
Paper.
Hydrophilicity of tissue paper refers, in general, to the propensity
of the tissue paper to be wetted with water. Hydrophilicity of tissue
paper may be somewhat quantified by determining the period of time
required for dry tissue paper to become completely wetted with water.
This period of time is referred to as 'wetting time.' In order to
provide a consistent and repeatable test for wetting time, the following
procedure may be used for wetting time determinations: first, a
Conditioned sample unit sheet (the environmental conditions for testing
21433~Q
WO 94/05857 PCT/US93/07852
29
of paper samples are 23~1'C and 50~2% RH as specified 1n TAPPI Method T
402), approximately 4-3/8 inch x 4-3/4 inch (about 11.1 cm x 12 cm) of
tissue paper structure is provided; second, the sheet is folded into four
(4) juxtaposed quarters, and then crumpled into a ball approximately 0.75
inches (about 1.9 cm) to about 1 inch (about 2.5 cm) in diameter; third,
the balled sheet is placed on the surface of a body of distilled water at
23 ~ 1'C and a timer is simultaneously started; fourth, the timer is
stopped and read when wetting of the balled sheet is completed. Complete
wetting is observed visually.
The preferred hydrophilicity of tissue paper depends upon its
intended end use. It is desirable for tissue paper used in a variety of
applications, e.g., toilet paper, to completely wet in a relatively short
period of time to prevent clogging once the toilet is flushed.
Preferably, wetting time is 2 minutes or less. More preferably, wetting
time is 30 seconds or less. Most preferably, wetting time is 10 seconds
or less.
Hydrophilicity characters of tissue paper embodiments of the present
invention may, of course, be determined immediately after manufacture.
However, substantial increases in hydrophobicity may occur during the
first two weeks after the tissue paper is made: i.e., after the paper
has aged two (2) weeks following its manufacture. Thus, the above stated
wetting times are preferably measured at the end of such two week period.
Accordingly, wetting times measured at the end of a two week aging period
at room temperature are referred to as 'two week wetting times.'
The density of tissue paper, as that term is used herein, is the
average density calculated as the basis weight of that paper divided by
the caliper, with the appropriate unit conversions incorporated therein.
Caliper of the tissue paper, as used herein, is the thickness of the
paper when subjected to a compressive load of 95 g/in2 (15.5 g/cm2).
XAMP
The purpose of this example is to illustrate one method that can be
used to make soft tissue paper sheets treated with a softening additive
in accordance with the present invention.
A pilot scale Fourdrinier papermaking machine is used in the
practice of the present invention. The paper machine has a layered
headbox having a top chamber, a center chamber, and a bottom chamber.
4lhere applicable as indicated in the following examples, the procedure
WO 94/05857 PCT/US93/07852
~,~ X33 ~~
described below also applies to such later examples. Briefly, first a
fibrous slurry comprised primarily of short papermaking fibers is pumped
through the top and bottom headbox chambers and, simultaneously, a second
fibrous slurry comprised primarily of long papermaking fibers is pumped
through the center headbox chamber and delivered in superposed relation
onto the Fourdrinier wire to form thereon a three-layer embryonic web.
The first slurry has a fiber consistency of about O.11X and its fibrous
content is Eucalyptus Hardwood Kraft. The second slurry has a fiber
consistency of about 0.15X and its fibrous content is Northern Softwood
10 Kraft. Dewatering occurs through the Fourdrinier wire and 1s assisted by
a deflector and vacuum boxes. The Fourdrinier wire is of a 5-shed, satin
weave configuration having 87 machine-direction and 76
cross-machine-direction monofilaments per inch, respectively. The
embryonic wet web is transferred from the Fourdrinier wire, at a fiber
15 consistency of about 22X at the point of transfer, to a carrier fabric
having a S-shed satin weave, 35 machine-direction and 33
cross-machine-direction monofilaments per inch, respectively. The web is
carried on the carrier fabric past the vacuum dewatering box, through the
blow-through predryers after which the web is transferred onto a Yankee
20 dryer. The fiber consistency is about 27X after the vacuum dewatering
box and, by the action of the predryers, about 65X prior to transfer onto
the Yankee dryer; creping adhesive comprising a 0.25X aqueous solution of
polyvinyl alcohol is spray applied by applicators; the fiber consistency
is increased to an estimated 99y. before dry creping the web with a doctor
25 blade. The doctor blade has a bevel angle of about 24 degrees and is
positioned with respect to the Yankee dryer to provide an impact angle of
about 83 degrees; the Yankee dryer is operated at about 350oF (177oC);
the Yankee dryer is operated at about 800 fpm (feet per minute) (about
244 meters per minute). The heated calender rolls are sprayed with a
30 chemical softener emulsion, further described below, using a 2 rtm spray
nozzle. The web is then passed between the two heated calender rolls.
The two calender rolls are biased together at roll weight and operated at
surface speeds of 660 fpm (about 201 meters per minute).
The spray solution is made by diluting Neodole 25-12 Shell Chemical
to SX by weight with water. The surfactant solution is then sprayed onto
a heated steel calender roll. The volumetric flow rate of the aqueous
solution through the nozzle is about 2 gal/hr cross-direction ft (about
25 liters/hr-meter).
~. __...__.____. i
_214334)
WO 94/05857 PCT/US93/07852
31
Greater than about 95x of the water is evaporated from the calender
rolls leaving a calculated chemical softener film thickness of less than
0.07 microns. The dry web, which has a moisture content of about 1X,
contacts the hot calender rolls. The chemical softener compound is
transferred to the dry web by direct pressure transfer. The transfer
efficiency of the chemical softener applied to the web, in general, is
about 45X.
The resulting tissue paper has a basis weight of 30g/m2, a density
of 0.10g/cc, and contains 0.17X by weight, of the alkylpolyethoxylated
alcohol compound and has an unequilibrated initial moisture content of
1.2X. Importantly the resulting tissue paper has an improved tactile
sense of softness relative to the untreated control.
RAMP
The purpose of this example is to illustrate one method that can be
used to make soft tissue paper sheets wherein the tissue paper is treated
with a softening additive and starch.
A 3-layer paper sheet is produced in accordance with the
hereinbefore described process of Example I. The tissue web is treated
with CrodestaTM SL-40 (an alkyl glycoside polyester nonionic surfactant
marketed by Croda Inc.) and with a fully cooked amioca starch prepared as
described in the specification. The surfactant and starch are applied
simultaneously on the heated transfer roll as part of the aqueous
solution sprayed through the paper machine spray nozzle. Concentration
of the CrodestaTM SL-40 nonionic surfactant in the aqueous solution is
adjusted so that the level of surfactant retained is about 0.15X, based
upon the weight of the dry fibers. Similarly, concentration of the
starch in the aqueous solution is adjusted so that the level of amioca
starch retained is about 0.2X, based upon the weight of the dry fibers.
The treating mixture is sprayed onto an upper and a lower heated
transfer roll. The water is evaporated from the rolls and the active
surfactant, and binder are transferred to both sides of the tissue web.
The volumetric flow rate through the upper and lower spray nozzle onto
the heated rolls is about 1 gal/hr cross-direction ft. The combined flow
rate through both nozzles is 2 gal/hr cross-direction ft.
The resulting tissue paper has a basis weight of 30g/m2, a density of
O.IOg/cc, and contains 0.15% by weight of CrodestaTM SL-40 nonionic
surfactant and 0.2X by weight of the cooked amioca starch. Importantly,
WO 94/05857 PCT/US93/07852
32
the resulting tissue paper has enhanced tactile softness and has higher
wettability and lower propensity for lint than untreated tissue paper.
EXAMPLE III
The purpose of this example is to illustrate one method that can be
used to make soft tissue paper sheets wherein the tissue paper is treated
in accordance with the present invention and converted into a two ply
product.
A 2-layer paper sheet is produced in accordance with the
hereinbefore described process of Example I with the following
exceptions. The volumetric flow rate through the nozzle is approximately
1.05 gal/hr cross-direction foot (about 13.3 liters/hr-meter). The film
thickness after 95X of the water is evaporated is calculated to about
0.035 microns. The resulting single ply tissue paper has a basis weight
of 16 g/m2.
Following papermaking, two sheets of treated paper are combined
together with the treated surfaces facing outward.
The resulting two-ply tissue paper product has a basis weight of 32
g/m2, a density of 0.10 g/cc,~and contains 0.17% by weight, of the
alkylpolyethoxylated alcohol.
Importantly, the resulting tissue paper has enhanced tactile
softness.
EXAMPLE IV
The purpose of this example is to illustrate one method that can be
used to make soft tissue paper sheets wherein the tissue paper is treated
with a mixed surfactant system containing a softener additive and an
absorbency enhancing agent. A 3-layer paper sheet is produced in
accordance with the hereinbefore described process of Example I. An
aqueous dispersion of softener is prepared from 11.9x GLYCOMUL-S CG (a
mixed sorbitan stearate ester surfactant made by Lonza, Inc.), 3.2X
Neodol~ 23-6.5T (an ethoxylated Clp-C13 linear alcohol dispersing
surfactant and wetting agent made by Shell Chemical Company), 0.8X DOW 65
Additive (a silicone polymer foam suppressant made by Dow Corning
Corporation), and 84.1x distilled water.
The treating mixture is sprayed onto a lower heated calender
(transfer) roll. The water is evaporated from the roll and the active
softener and absorbency enhancing agent are transferred to one side of
_ _ r_ _._ _ . _ ._ _ _..
~14334p
WO 94/05857 - PCT/US93/07852
33
the tissue web. The flow rate through the spray nozzles is adjusted such
that about 0.6y. softener (Glycomul-S CG) is retained by the sheet. The
resulting tissue paper has a basis weight of 30g/m2, a density of
O.lOg/cc, and contains about 0.6% by weight of the Glycomul-S CG
surfactant. Importantly, the resulting tissue paper has an enhanced
tactile softness and has high wettability.
XAMP
The purpose of this example is to illustrate one method that can be
used to make soft tissue paper sheets wherein the tissue paper is treated
with a biodegradable quaternized amine-ester softening compound. A
3-layer paper sheet is produced in accordance with the hereinbefore
described process of Example I. A 1% aqueous dispersion of softener is
prepared from a mixture of diester dehydrogenated tallow dimethyl
a~onium chloride (DEDTDMAC) (i.e., ADOGEN DDMC from the Sherex Chemical
Company) and a polyethylene glycol wetting agent (i.e., PEG-400 from the
Union Carbide Company). The solution is prepared according to the
following procedure: 1. An equivalent molar concentration of DEDTDMAC
and PEG-400 is weighed; 2. PEG is heated up to about 180oF; 3. DEDTDMAC
is dissolved into PEG to form a melted solution; 4. Shear stress is
applied to form a homogeneous mixture of DEDTDMAC in PEG; 5. The pH of
the dilution water is adjusted to about 3 by the addition of hydrochloric
acid; 6. The dilution water is then heated up to about 180'F; 7. The
melted mixture of DEDTDMAC/PEG400 is diluted to a 1% solution; 8. Shear
stress is applied to form an aqueous solution containing a vesicle
suspension of DEDTDMAC/PEG-400 mixture.
The treating mixture is sprayed onto a lower heated calender
(transfer) roll. The water is evaporated from the roll and the active
softening compound and absorbency agent are transferred to one side of
the tissue web. The flow rate through the spray nozzles is adjusted such
that about 0.05% softener (DEDTDMAC) is retained by the sheet. The
resulting tissue paper has a basis weight of 30g/m2, a density of
O.lOg/cc, and contains about 0.05% by weight of the DEDTDMAC softener.
Importantly, the resulting tissue paper has an enhanced tactile softness
and has high wettability.
