Note: Descriptions are shown in the official language in which they were submitted.
~ ~598~4
Background of the Invention
The present invention is directed to a method
for producing a highly absorbent, cellulosic sheet hav-
ing excellent quality and a high level of surface-per-
ceived softness.
In the production of standard grades of paper on
a conventional paper machine, cellulosic fibers and water
are combined in a headbox to form an aqueous fiber slurry
having a fiber consistency of from about 0.05~ to 0.2% by
weight, based on the total weight of that slurry. The wet
web from the headbox is formed on a papermaking wire at a
consistency of at least about 5% for purposes of mechanic-
ally removing water therefrom. This mechanical dewatering
step increases the consistency of the web up to a level of
about 45-50%. As the water is mechanically displaced, the
fibers are moved into close proximity one with the other,
and chemical bonds, generally described as "hydrogen bonds",
are formed between the adjacent cellulosic fiber surfaces.
The formation of these hydrogen bonds serves to strengthen
the web, as measured! for example, by a substantial increase
in physical properties such as an increase in the breaking
length of the web.
The uncontroled formation of substantial numbers
of hydrogen bonds between the surfaces of adjacent fibers
is detrimental to certain paper properties such as softness
and absorbency. Specifically, various types of paper
products such as certain grades of tissue and towel in which
the sheet must be absorbent and soft to the touch cannot
tolerate the unlimited formation of hydrogen bonds. These
high softness and absorbency properties are not present in
paper products made by the above described conventional
papermaking techni~ues.
~ 159694
In an attempt to produce a sheet having the above
described requisite degree of softness and absorbency, cer-
tain prior art methods incorporate a chemical "debonding
agent" into the aqueous fiber slurry to inhibit the forma-
tion of hydrogen bonds. The incorporation of the debonder
must, however, be accomplished without destroying the integ-
rity of the web since it is necessary to form a sheet having
the requisite strength properties for its intended use as a
finished product of commerce, i.e., to produce a sheet hav-
ing a high, over-all quality.
U. S. Patent 3,812,000 to Salvucci et al.; U. S.
3,844,880 to Meisel et al.; and U. S. 3,903,342 to Roberts
et al., respectively, describe the addition of materials
such as the above mentioned debonding agents to an aqueous
slurry of cellulosic fibers to minimize hydrogen bonding in
an attempt to increase softness. Typically, the above
chemical debonding agents comprise surfactants such as
those described in the Hervey et al. U. S. patents, Nos.
3,554,862; 3,554,863; and 3,395,708. The addition of a
chemical debonding agent to the fiber slurry promotes the
over-all treatment of all ~ibers without regard to whether
the treated fibers are located at the web surface, where
hand-feel properties are measured! or in the interstices
o~ the sheet. Therefore, certain patents such as U. S.
3,556,931 to Champaigne; U. S. 2,756~647 to Thompson; and
U. S. 4,158,594 to Becker et al., disclose adding a debond-
ing agent to a cellulosic web after web formation, but prior
to drying. These latter patents describe softening the web
to a certain extent, but do not provide a sheet having the
requisite quality, absorbency, and surface-perceived soft-
ness level, as hereinafter defined.
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1 ~59~94
Summary of the Invention
This invention is directed to a method for pro-
ducing a highly absorbent cellulosic sheet having excellent
over-all quality and a high level of surface-perceived soft-
ness f respectively, which is capable of being employed in
high softness grades of tissue and towel~
In order to overcome the previously described
problems associated with the prior art resulting from the
addition to the headbox of chemical debonding agents, the
subject invention contemplates treating the cellulose fibers
in the formed web with a chemical bonding inhibitor having
an acidic pH. More specifically, the cellulosic web immedi-
ately after formation preferably has an initial pH of not
less than about 6.0, and preferably not less than about 6.5,
and more preferably not less than about 7Ø At least one
surface of the web is treated with the chemical bonding
....
i inhibitor, and the pH of the treated web immediately after
; the treatment step is not more than about 5.0, and prefer-
ably not more than about 4.5, and more preferably not more
than about 4.0, and most preferably not more than about 3.0,
The result of treating the web with a chemical bonding
inhibitor under the pH conditions described above is an
unexpected, dramatic improvement in qualityf absorbency,
and surface-perceived softness of the sheet products. The
treatment of the cellulosic fibers with the acidic chemical
~- bonding inhibitors does not occur until the consistency of
the web, as measured immediately after the treatment step !
is at least about 5% by weight, up to about 50% by weight.
Detailed Descrip~ on of the Drawing
FIGURE 1 is a schematic representation of a pre-
ferred method within the scope of this invention.
.
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~ ~989~
Detailed Description of the Invention
In accordance with the present invention, a
method is provided for producing a highly absorbent, cel-
lulosic sheet which exhibit~ excellent over-all quality and
"~ a high degree of suxface-perceived softness. A schematic
....
~ drawing depicting a process configuration is set forth in
,....
FIGURE 1.
In the method of the present invention, an aque-
;~ ous furnish including cellulose papermaking fibers is ini-
.i tially formed. The cellulosic fibers have undergone some
degree of lignin modification, such as at least partial
chemical treatment, to produce materials such as chemi-
, mechanical pulp, semichemical pulp, chemical pulp, or mix-
;
tures thereof. Suitable materials from which the above
cellulose fibers can be derived include the usual species
`~ of coniferous and deciduous pulpwood, the cellulosic com-
~:,
ponents being preferably produced from coniferous pulpwood
because of its greater fiber length.
The aqueous furnish is transported to a headbox 2
at a le~el sufficient to permit the formation of a substan-
tially dry sheet upon completion of the hereinafter described
dewatering and thermal drying steps, respectively, without
requiring further drying thereof subsequent to creping.
As a practical-matter! however, the consistency of the
aqueous furnish used in forming the subject ~et web is
desirably maintained at a level of from about 0.05% by
weight~ and more preferably from about 0.1% by weight, based
on the total weight of cellulosic fibers in the a~ueous
furnish~ up to a preferred consistency of about 1.0% by
weight~ and more preferably up to about 0.75% by weight.
. .
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~ 1~9694
.
;; A wet web l is then formed by deposition of the
; aqueous furnish onto a web forming means 3, typically a
~.~ conventional papermaking system including a foraminous con-
j` veying means 4 such as a Fourdrinier wire, Stevens former,
: or the like.
- Dewatering of the wet web is then provided prior
`:~ to the thermal drying operation typically employing a non-
:~. thermal dewatering means 5. The nonthermal dewatering step .
.~` is usually accomplished by various means for imparting mech-
anical compaction to the web l such as vacuum boxes, slot
` boxes, coacting press rolls, or combinations thereof. For
; purposes of lllustration of the method of this invention,
the wet web 1 is dewatered by subjecting same to a series
of vacuum boxes and/or slot boxes, as shown in FIGURE 1.
Thereafter, the web is further dewa-tered by subjectlng same
~:`
to the compressive forces exerted by nonthermal dewatering
. means such as, for example, a pair of rolls, followed by a
pressure roll coacting with a thermal drying means.
` The wet web 1 is carried by the foraminous con-
veying means 4 through the nonthermal dewatering means 5,
: where it is dewatered to a consistency of at least about 5~,
preferably at l~ast 10%, and more preferably at least 15~,
: up to a consistency of preferably about 50%, and more prefer-
ably up to about 45%, and most preferably up to about 35%.
The cellulosic web formed, as described above, preferably
has an initial pH of at least about 6 in order to minimize
corrosion problems which can occur with respect to the for-
aminous conveying means, as well as other portions of the
.~ papermaking equipment if the pH per se is too low. More
~:. 30 preferably, the initial pH of the web is maintained at a
level, as previously described. The wet web 1 prior to the
,
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: 1 ~5~4
thermal drying step is treated with an acidic chemical
' material 6, which inhibits the formation of papermaking
..:
.....
bonds between adjacent cellulosic fibers. By treating the
web 1 in this manner, a sheet 20 having the hereinafter
, . .
`~ defined, unexpected properties can be produced. The pH of
web immediately after the treatment step, denoted "la", is
not more than about 5.0, and is preferably not more than
; about 4.5, and is more preferably not more than about 4.0,
,
and is most preferably not more than about 3Ø
:,;
The consistency of the web immediately after the
treatm~nt step is preferably within the previously defined
consistency parameters. Preferably, this chemical bonding
; inhibitor per se is a "chemical debonding agent". These
materials are well-known in the prior art, and are prefer-
ably substantially cationic in nature. Examples of suitable
chemical bonding inhibitor materials include Quaker 2001*
(~uaker Chemical), Ceranine HCS* (Sandoz), Leomin KP*
-~ (Hoechst AG), and Amasoft-PM* (American Color and Chemica~.
The bonding inhibitor 6 employed for treatment of
` 20 the web is provided at a treatment level which is sufficient
to minimize the formation of the above described hydrogen
bonds, but less than an amount which would cause significant
runnability and sheet strength problems in the final commer-
cial product. The amount of acid chemical bonding inhibitor 6
employed~ on a 100% active basis, is preferably from about
0.5 pound per ton (1 kg/tonne~ of cellulose pulp, up to about
15 pounds per ton (30 kg/tonne) of cellulose pulp. However,
a more preferred addition of from about 1 pound (2 kg), up
;~ to about 10 pounds (20 kg), of chemical bonding inhibitor
per tonne of cellulose pulp can be employed.
* trademark
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:, . ~ . .
6 9 ~
Treatment of the wet web with the bonding inhibi-
tor material can be accomplished by various means. Eor
instance, the treatment step can comprise spraying, applying
with a direct contact applicator means or by employing an
~' applicator felt. However, the preferred method of applica-
tion is by spraying the web such as by employing spray
header 6a! at various points prior to thermal drying means 8
~see FIGURE 1). The adjustment of the pH of the web is
localized at a giYen point of treatment ! as opposed to hav-
ing the web adjusted to the desired acidic pH level in the
headbox. In a preferred method, the pH of the chemical
bonding inhibitor 6 is adjusted to a pH level, prior to
treatment therewith, which will facilitate the requisite mini-
mum pH of the treated web at a level within the previously
; set forth pH limits, In a further preferred method, the pH
of the web is first adjusted to a prescribed level and
thereafter the sheet is treated with the chemical bonding
inhibitor~
The acidity level of the bonding inhibitor material
6 can be adjusted employing numerous materials capable of
performing that function. However, organic acids such as
formic acid, acetic acid, propionic acid and benzoic acid,
and inorganic acids such as hydrochloric acid~ sulfuric
acid, phosphoric acid and nitric acid! or salts thereof, are
preferred. Of the above acidic materials, however, sulfuric
acid is the most preferred.
The surface treated web la is then applied to the
surface 7 of the thermal drying means 8, preferably a ther-
mal drying cylinder such as a Yankee drying cylinder,
employing preferably an adhesive to supplement the adhesion
process. Examples of typical adhesive compounds which may be
l;
*` l 15969~
used include carboxymethyl cellulose, polyvinyl alcohol,
. .
~, anionic starch, various soluble natural polymers such as
; gums and the like, and synthetic resins such as polyamide
:,
;; resins, and the like. Adhesion of the treated web la to
; the cylinder surface 7 is preferably facilitated by the
'~' mechanical compressive action exerted thereon, generally
~, using one or more press rolls 9, which form a nip in combi-
,~, nation with thermal drying means 8 and which brings the web
`'l into more uniform contact with the thermal drying surface 7.
; 10 The ,web is then dried on the thermal drying sur-
face preferably to a consistency of at least about 92%, and
'~'' more preferably to a consistency of at least about 97~.
:~;
The creping means 11 then removes the dried,
creped sheet 20 from the thermal drying surface, the creping
action disrupting bonds between respective fibers and causing
~` a softening effect to be imparted to the sheet. In general,
~; the creping means is a doctor blade which crepes and removes -
the sheet from the thermal drying surface.
Sheet 20 has a high degree of bulk softness.
, 20 "Bulk softness" is measured by conducting a Handle-O-Meter
test (,HOM~ according to TAPPI T-498. The bulk softness
~reciprocal of stiffness) of a given sheet is then calculated
~- by dividing the HOM value by the square o~ the caliper of a
: given single sheet being tested, the quotient thereof being
multiplied by 105. For example, in single-ply tissue and
, ; towel applications~ depending on the type of furnish
, employed~ bulk softness, expressed as HOM/(Caliper)~ ~ 105
', is pre~erably not-more than about 1.0, and more preferably
; not more than about 0.8, and most preferably not more than
' 30 about 0.6.
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I ~g694
An important aspect of this invention is the
ability of the subject method to produce sheets in an
, extremely broad basis weight range. The basis weight of a
: given sheet is determined according to TAPPI test number
T-410. The basis weight of the sheet produced by the sub-
ject method can, in general, range from about 5 pounds per
` 3,000 square feet (118.8 kg/3000 m2), and preferably from
about 8 pounds per 3,000 square feet (190.1 kg/3000 m2), up
to about 50 pounds per 3,000 square feet (1180 kg/3000 m2),
and preferably up to about 40 pounds per 3,000 square feet
(950.4 kg/3000 m2).
Each sheet must have enough structural integrity
so that it will be capable of being manufactured without
being damaged. A measure of the structural integrity of a
sheet is breaking length. This procedure is run according
to TAPPI T-222, except that instead of a 15-millimeter-wide
- sample strip, a one-inch (25.4 mm) wide strip is used.
; Average breaking length (BL vg) of a sheet is then calcu-
lated after the tensile strength of the sample in the mach-
ine direction (MD) and cross-machine direction (CMD),
respectively, is determined, using the following equation:
BLaVg - 685 (tensile MD + tensile CMD)
(2) (Basis Weight)
In order to insure runnability o~ a web on paper-
making equipment, the BLaVg of preferably at least about 150
meters, and more preferably at least about 250 meters is
provided. Furthermore, if the creped sheet has too high a
BLayg value, it will be too harsh to the feel and! therefore,
unacceptable to the consumer. Thus, it is preferred that the
' 30 BLaVg be not more than about 450 meters, and more preferably
not greater than about ~00 meters.
~t~
~ il
59~4
Another important sheet property is its ability
to absorb water. The water absorbency parameter is expressed
as the number of seconds it takes for a single sheet
(4.5 inches by 4.5 inches) (11.43 cm by 11.43 cm) to absorb
0.1 cc of water, the test being described in TAPPI T-432.
If the method of the present invention is employed
in a wet treatment step, as opposed to the prior art methods
in which chemical bonding inhibitor per se is employed, an
unexpected increase in the absorbency will result, Prefer-
ably, an increase in water absorbency of at least about 50%,
and more preferably at least 100%~ and most preferably at
least 150% can be provided.
EXAMPLE 1
A series of three experiments (A-C) was conducted,
employing a paper machine having a configuration set out in
FIGURE 1. In the first experiment~ 0.25% of a chemical
bonding inhibitor material, namely, Quaker 2001, a cationic
quaternary ammonium compound produced by Quaker Chemical
Company, at a pH of 5.48 was sprayed on the formed web at
a consistency of about 10%. In Experiments s and C, 25 and
50 ml of a 10% solution of sulfuric acid was added to 4 gal-
lons (15.2 liters) of bonding inhibitor solution prior to
spraying same on the wet web. This produced a pH of 2.78
i .
and 2.48, respectively, immediately after treatment of the
wet web in Experiments B and C. Sheets produced in Experi-
ments A-C were tested, and the results re water absorbency
and bulk softness are summarized in Table I.
TABLE I
A B C
Water Absorbency (Sec.) 37.64 11.60 14.10
HOM/(Caliper) x 10 1.02 0.55 0.53
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~ ""
; 1 159S94
~ The effect of employing an acidic bonding inhibitor
. ~
~ treatment solution is clearly demonstrated by the above
:,
~ experimental comparison. SpeciEically, the stiffness of
the subject sheets (B and C) have been substantially reduced
(softness increased) by employing the method of the subject
invention to about one-half that of the sheet (A) employing
a chemical bonding inhibitor per se. At the same time, the
; water absorbency of sheets B and C is dramatically increased,
as compared to its higher pH counterpart, so that the sheets
produced in Experiments B and C are 324% and 260~, respect-
ively, more absorbent than the sheet produced in Experiment A.
The over-all quality and surface-perceived soft-
ness, respectively, of a cellulose sheet are subjectively
determined to a great extent by the hand-feel as discerned
by the ultimate consumer. Objective testing of the sheets
in question are a measure of these properties and cannot
totally act as a substitute for such a subjective determina-
tion. Therefore, the over-all quality and surface-perceived
softness of sheets such as those produced by the method of
this invention can best be subjectively determined by pol-
-~ ling randomly selected respondents who have compared cellu-
~,
losic sheets prepared by various methods ! including the
subject method. More specifically, four sheets prepared in
`` substantially the same manner, except for pH of the chemical
bonding inhibitor employed, were compared. Each of the four
- sheets was then rated by a group of ten respondents with
respect to over-all quality and surface-perceived softness~
An average over-all ~uality and surface perceived softness
for each of the sheets tested was then determined by adding
the total points received for each of the sheets tested
from a given respondent and dividing that total by ten!
'
~ l 1$~9~
i.e., the total number of respondents. An over-all quality
factor and surface-perceived softness factor for sheets pro-
duced by the subject invention were then determined by
dividing the average over-all quality, or the average
surface-perceived softness, for the subject sheet by the com- --
parable average value of the sheet, using a chemical bonding
inhibitor per se. For example, a series of four experimental
sheets, including the three cellulosic sheet products de-
scribed in Example 1 (Nos. A-C), were tested, and an aYerage
over-all quality factor and an average surface-perceived
softness factor for each were determined.
TABLE II
A B C
Average Over-all
Quality Factor 1.0 3.6 3.4
Average Surface-
perceived Softness
Factor 1.0 3,4 3~5
Accordingly, the sheets produced by the method of
this invention have an over-all quality factor and a surface-
perceived softness factor of preferably at least about 1.5,and more preferably at least about 2.0, and most preferably
at least about 2.5.
. ~
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