Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02239927 1998-06-30
PATENT
METHOD OF APPLYING
CHEMICAL SIZING AGENTS FOR MAKING REDUCED WET-THROUGH TISSUE
Background of the Invention
The use of sizing agents in the manufacture of tissues, such as facial and bath
tissue, is not common practice in the industry. However, it has been demonstrated that
the addition of sizing agents similar to those commonly used in fine papers reduces the
ability of fluid to penetrate the tissue and thereby keep the users hands dry during most
5 uses. Such tissues also have adequate absorbency to be functionally useful. These
tissues typically contain a blend of relatively long fibers, which are usually softwood fibers,
and relatively short fibers, which are usually hardwood fibers. Preferably, the sizing agent
is added to the long fibers prior to forming the web since better chemical retention is
achieved than with addition to short fibers. The long fibers are also treated with
10 strengthening agents (wet and dry) and refining. Both refining and strengthening agents
are often used because excessive use of either treatment may have an adverse effect on
the tissue making process and/or the resulting tissue product.
However, addition of sizing agents to the fiber furnish can have some
disadvantages. In one case, sizing agents added to a furnish prior to forming cause
15 internal debonding which results in loss of tensile strength and increased dust when the
sheet contacts the creping doctor blade. The loss in tensile strength may require
excessive refining or strength additive which can have a detrimental effect on tissue
softness. Excessive dust may increase the frequency of sheet breaks at the reel, provide
a less than desirable environment for machine operators and is detrimental to the
20 consumer perception of the tissue.
Therefore there is a need for a more effficient method of utilizing sizing agents in
the manufacture of tissues.
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Summary of the Invention
It has now been discovered that soft creped tissue can be produced by the indirect
addition of chemical sizing agents to the tissue web by applying the chemical sizing
agents to the surface of the Yankee dryer, such as by spraying. More specifically, the
5 sizing agents can be included as part of the creping adhesive formulation, which is
sprayed onto the surface of the Yankee dryer between the creping blade and the pressure
roll. The sizing agents are subsequently transferred to the tissue sheet surface as the
sheet is pressed against the Yankee dryer.
The sizing agent remains predominantly on the surface of the sheet where it is
10 most beneficial, thereby resulting in a barrier to rapid absorption of fluid. Some of the
sizing agent passes through the sheet while in the pressure roll nip and is recirculated
back to the wet end of the tissue machine with the white water. As such, additional
amounts become incorporated into the tissue sheet at that point of the process, but these
amounts are more evenly distributed throughout the sheet. The net result is a tissue
15 sheet having a greater concentration of sizing agent on the surface of the sheet, thereby
reducing rapid absorption of fluid into the sheet relative to sheets made in a conventional
manner.
Hence in one aspect the invention resides in a method for making creped tissue
comprising: (a) forming a wet tissue web by depositing an aqueous papermaking furnish
onto a forming fabric; (b) partially dewatering the tissue web; (c) applying a creping
adhesive and one or more sizing agents to the surface of a Yankee dryer; (d) adhering the
- tissue web to the surface of the Yankee dryer such that the sizing agent is transferred to
the tissue web; and (e) creping the web.
In another aspect, the invention resides in a tissue product made by the above-
mentioned method.
As used herein, a Usizing agent" is any chemical that imparts water repellency to
cellulosic fibers. Suitable sizing agents are disclosed in a text entitled UPapermaking and
20 Paper Board Making," second edition, Volume lll, edited by R. G. MacDonald, and J. N.
Franklin .
Sizing agents are commonly added to control the penetration of aqueous liquids
into paper or other fibrous structures. In many cases, a certain resistance is required for
end use. When surface treatments are applied with conventional equipment often sizing
25 is required in the base sheet to control pickup of the aqueous solution. Paper grades
such as butcher's wrap, milk cart3n, linQrb3ard, bleached and unbleached bag, fine paper,
cylinder board, newsprint and corrugated medium are routinely sized.
Internal sizing agents, which are those applied to the fibers within the paper structure,
provide a reduced rate of penetration by retarding the rate of flow through the inter-fiber
.
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capillaries. When sizing is accomplished, the contact angle at the fiber surface is 90 degrees
or greater. Internal sizing agents function through the use of low surface energy, hydrophobic
chemicals which when attached to the surface of cellulose reduce the surface energy of the
fiber surface.
Particularly suitable sizing agents are acid or alkaline sized such as acid rosin,
alkenyl ketene dimers, alkenyl succinic anhydride, alkyl ketone dimers and alkenol ketene
dimers of the formula:
R, - CH = C - CH - R2
O-C=O
wherein R, and R2 are based on C16 - C,8, aliphatic carbon chains, which can be the
same or different. Exemplary commercially available sizing agents of this type are Hercon
15 79 and Precis 3000 from Hercules, Inc., Wilmington, Delaware. The amount of the sizing
agent to the fibers can be from 1 to about 10 pounds per ton of fiber, more specifically
from about 1.5 to about 3 pounds per ton of fiber, and still more specifically, from about 2
to about 2.5 pounds per ton of fiber.
As used herein, the "Absorbency Rate" is a measure of the water repellency
20 imparted to the tissue by the sizing agent. The Absorbency Rate is the time it takes for a
product to be thoroughly saturated in distilled water. To measure the Absorbency Rate,
samples are prepared as 2 1/2 inch squares composed of 20 finished product sheets using
a die press (e.g. TMI DGD from Testing Machines Incorporated Inc., Amityville, N. Y.
11701). The ply of a finished product dictates the number of individual sheets: 1-ply: 20
25 individual sheets; 2-ply: 40 individual sheets; 3-ply: 60 individual sheets. When testing
soft rolls (single ply of tissue coming off the tissue machine before plying at the rewinder),
40 individual softroll sheets are used per sample (if the intended finished product is 2-ply.)
The samples are stapled in all four corners using Swingline S.F 4 inch speedpoint
staples. Samples are tested in a constant temperature water bath at a depth of at least 4
30 inches (maintained though out testing) containing distilled water at 30 +/- 1~ Celsius. The
sample is held approximately one inch above the water surface (staple points in the down
position) and then dropped flat on the water surface. A stopwatch (readable to 0.1
seconds) is started when the sample hits the water. When the sample is completely
saturated the stop watch is stopped and the Absorbency Rate is recorded. A minimum of
35 five samples are tested and the test results are averaged. All tests are conducted in a
laboratory atmosphere of 23 +/- 1~ Celsius and 50 +/- 2% RH. All samples are stored
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under these conditions for at least 4 hours before testing. (Sizing agents distribute
themselves and react faster at higher temperatures.)
The tissues of this invention have an Absorbency Rate, for tissue naturally aged5 days, of about 10 seconds or greater, more specifically about 100 seconds or greater,
still more specifically about 200 seconds or greater, still more specifically about 300
seconds or greater, and still more specifically from about 100 to about 400 seconds.
Brief DescriPtion of the Drawing
Figure 1 is a schematic flow diagram of a wet-pressed tissue making process,
illustrating the addition of sizing agents to the surface of the Yankee dryer. Also shown is
the white water recycle flow.
Detailed DescriPtion of the Drawing
Figure 1 is a schematic flow diagram of a conventional wet-pressed tissue makingprocess useful in the practice of this invention, although other tissue making processes
can also benefit from the stock prep method of this invention, such as throughdrying or
other non-compressive tissue making processes. The specific formation mode illustrated
in Figure 1 is commonly referred to as a crescent former, although many other formers
well known in the papermaking art can also be used. Shown is a headbox 21, a forming
fabric 22, a forming roll 23, a paper making felt 24, a press roll 25, a spray boom 26,
Yankee dryer 27, and a creping blade 28. Also shown, but not numbered, are various
idler or tension rolls used for defining the fabric runs in the schematic diagram, which may
differ in practice. As shown, the headbox 21 continuously deposits a stock jet 30 between
the forming fabric 22 and felt 24, which is partially wrapped around the forming roll 23.
Water is removed from the aqueous stock suspension through the forming fabric bycentrifugal force as the newly-formed web traverses the arc of the forming roll. As the
forming fabric and felt separate, the wet web 31 stays with the felt and is transported to
- the Yankee dryer 27.
At the Yankee dryer, the creping chemicals are continuously applied in the form of
an aqueous solution to the surface of the Yankee dryer on top of the residual adhesive
remaining after creping. In accordance with this invention, the creping chemicals can
include one or more sizing agents. The solution is applied by any conventional means,
preferably using a spray boom 26 which evenly sprays the surface of the dryer with the
creping adhesive solution. The point of application on the surface of the dryer is
immediately following the creping doctor blade 28, permitting suffficient time for the
spreading and drying of the film of fresh adhesive before contacting the web in the press
roll nip.
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The wet web 31 is applied to the surface of the dryer by means of the press roll 25
with an application force typically of about 200 pounds per square inch (psi). The
incoming web is nominally at about 10% consistency (range from about 8 to about 20%)
at the time it reaches the press roll. Following the pressing and dewatering step, the
5 consistency of the web is at or above about 40%. Sufficient Yankee dryer steam power
and hood drying capability are applied to this web to reach a final moisture content of
about 2.5% or less.
Also illustrated in Figure 1 is the white water recycle system. At the press roll nip,
white water effluent 35 expressed from the wet web is collected in catch pan 36. The
collected white water 37 drains into wire pit 38. Thick stock 40 having a consistency of
about 3 percent is diluted with white water at the fan pump 39 to a consistency of about
0.1 percent. The diluted stock 41 is subsequently injected into the headbox 21 to form the
wet web.
Examples
Example 1. (Control)
A soft tissue product was made using the overall process of Figure 1. More
specifically, a papermaking furnish was prepared consisting of 35% northern softwood
20 kraft (NSWK), 15% High Maple Quinnesec and 50% Eucalyptus fibers. The Quinnesec
and eucalyptus pulps were beaten together at about 3.5% consistency. The NSWK was
beaten separately at about 3.5% consistency. The two pulp streams were blended
together before dilution.
The blended furnish was then further diluted to about 0.1 weight percent based on
25 dry fiber, fed to a headbox and deposited from the headbox onto a multi-layer polyester
forming fabric to form the tissue web. The web was then transferred from the forming
fabric to a conventional wet-pressed carrier felt. The water content of the sheet on the felt
just prior to transfer to the Yankee dryer was about 88 percent. The sheet was
transferred to the Yankee dryer with a vacuum pressure roll. Nip pressure was about 230
30 pounds per square inch. Sheet moisture after the pressure roll was about 42 percent.
The adhesive mixture sprayed onto the Yankee surface just before the pressure roll
consisted of 28 percent polyvinyl alcohol, 58 percent polyamide resin (KymeneLX) and 14
percent release agent (Quaker 2008) . The spray application rate was about 4.2 pounds
of dry chemical per metric ts),n 3f fiber. A natural gas heated hood partially enclosing the
35 Yankee had a supply air temperature of 626 degrees Fahrenheit to assist in drying. Sheet
moisture after the creping blade was about 2 0 percent. Machine speed was about 4100
feet per minute. The crepe ratio was 1.30, or 30 percent. The resulting tissue was plied
*denotes trademark
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together and calendered with two steel rolls at about 70 pounds per lineal inch. The two-
ply product had the dryer side plied to the outside. When converted, the finished basis
weight of the two-ply facial tissue at TAPPI standard temperature and humidity was about
17 pounds per 2880 square feet.
Examl~le 2. (Invention)
A soft tissue product with reduced moisture penetration can be made in
accordance with this invention using the overall process of Figure 1. More specifically, a
papermaking furnish is prepared consisting of 35% northern softwood kraft (NSWK), 15%
High Maple Quinnesec and 50% Eucalyptus fibers. The Quinnesec and eucalyptus pulps
are beaten together at about 3.5% consistency. The NSWK is beaten separately at about
3.5% consistency. The two pulp streams are blended together before dilution.
The blended furnish can be further diluted to about 0.1 weight percent based on dry fiber,
fed to a headbox and deposited from the headbox onto a multi-layer polyester forming
fabric to form the tissue web. The web is then transferred from the forming fabric to a
conventional wet-pressed carrier felt. The water content of the sheet on the felt just prior
to transfer to the Yankee dryer can be about 88 percent. The sheet is transferred to the
Yankee dryer with a vacuum pressure roll. Nip pressure can be about 230 pounds per
square inch. Sheet moisture after the pressure roll can be about 42 percent. Theadhesive mixture sprayed onto the Yankee surface just before the pressure roll can
consist of about 20 percent polyvinyl alcohol, 35 percent polyamide resin (KymeneLX) and
45 percent sizing agent (PTD-M-1332 or PTD-M-1331 available from Hercules Inc.). The
spray application rate can be about 8 pounds of dry chemical per metric ton of fiber. A
natural gas heated hood partially enclosing the Yankee can have a supply air temperature
of 626 degrees Fahrenheit to assist in drying. Sheet moisture after the creping blade can
be about 2.0 percent. Machine speed can be about 4100 feet per minute. The creperatio can be about 1.27, or 27 percent. The resulting tissue is plied together and
calendered with two steel rolls at about 70 pounds per lineal inch. The two-ply product
can be plied with the dryer side to the outside. When converted, the finished basis weight
of the two-ply facial tissue at TAPPI standard temperature and humidity can be about 17
pounds per 2880 square feet. The resulting facial tissue can resist the penetration of
moisture thus providing the benefit of keeping the user's hands dry during normal use.
This tissue can have an Absorbency Rate of from 10 to 400 seconds compared to 3
seconds for the control (Example 1).
It will be appreciated that the foregoing examples, given for purposes of
illustration, are not to be considered as limiting the scope of this invention, which is
defined by the following claims and all equivalents thereto.
