Note: Descriptions are shown in the official language in which they were submitted.
CA 02423386 2003-02-10
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USE OF STARCH COMPOSITIONS IN PAPERMAKING
This application is being filed as a PCT application by CARGILL,
INCORPORATED., a United States national and resident, designating all
countries
except US.
FIELD OF THE INVENTION
The present disclosure is directed to improved starch compositions, and
methods of making and using the improved starch compositions. In particular,
the
disclosure is directed to starch compositions for use in papermaking
processes, and
to methods of preparing, manipulating and using the starch compositions during
manufacture of paper products.
BACKGROUND
Numerous paper products are manufactured from fibers. These products are
often manufactured from an aqueous slurry containing modified cellulose fibers
derived from various plant sources. The slurry is formed in the wet end of a
papennaking machine, where paper fiber is formed into a dilute water slurry
and
combined with a variety of materials before being distributed onto a paper
machine
wire. The water is subsequently removed from the slurry in a controlled manner
to
form a web, which is pressed and dried to create a finished paper product.
Additives can be incorporated into the slurry to enhance the papermaking
process and to improve the finished papers' aesthetic and functional
properties.
These additives can include starch compositions incorporated during the wet
end of
the papermaking process to improve drainage and retention, to add strength,
and to
improve formation properties of the paper. Starch compositions can increase
ink
penetration times, reduce lateral spread of printing inks, and improve imaging
and
contrast. Starch compositions can also increase the surface integrity of
papers,
thereby decreasing picking in uses such as printing and photocopying.
Other ingredients that can be,incorporated into paper are microparticles,
including specialty clays, silica, and other functional fine particles. These
microparticles are often added during the wet end of the papermaking machine.
Depending upon the type of paper being made, as well as the characteristics of
the
slurry, various different microparticles can be added. One of the challenges
of using
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microparticles during papermaking is that the microparticles are not all
retained on
the web as the paper is formed. The microparticles that are not retained often
end up
being discharged, which can be expensive because the particles are not used.
Therefore, it is desirable to enhance particle retention.
Drainage, or de-watering ability, is another important consideration in the
manufacture of paper because it is related to how fast a paper machine can
remove
water from the web. Typically, improved dewatering corresponds to higher
speeds
on paper machines and to higher production rates of paper. Papermakers often
seek
to retain all fiber and particulates on the wire at the greatest speed
economically
possible, without sacrificing product quality. However, papermakers often
experience drainage limitations while trying to maintain product quality, and
therefore it is desirable to have high drainage values such that the paper can
be made
at high speeds and high quality.
Although papermakers and suppliers of paper ingredients realize that high
retention and drainage are desirable, a considerable challenge in making
consistent,
high-quality paper has been that papermaking systems are not all alike and can
show
significant variation. This variation can be the result of changes in the
ingredients in
the paper furnish as well as variability in the papermaking equipment. These
variations can make it difficult to produce quality paper at high speeds due
to
changes in particle retention and drainage.
Presently, most ingredients added to the papermaking slurry are optimized
for use under specific conditions. This is true, for example, of starch
compositions
added to the wet end of the papermaking process. Unfortunately, conditions at
most
papermaking facilities vary over time as the ingredients and systems change.
Therefore, a need exists for improvements that allow for satisfactory drainage
and
particle retention over a range of papermaking conditions.
SUMMARY OF THE DISCLOSURE
The present disclosure relates to starches, for example cationic crosslinked
starches, and to the use of those starches in papermaking. More particularly,
the
present disclosure is directed to starch and its use in wet end processing of
a paper
machine. The practices of the disclosure are particularly adapted for
customization
of the starch properties for specific wet end systems, and allow for
modification of
the starch properties to correspond to variations in the wet end of the
papermaking
machine.
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The starch can also be modified during production by adjusting the starch
functionality in the papermaking process. By selectively changing the
crosslinking
level of the starch, the drainage and retention properties of the paper
furnish
containing the starch are altered, which permits the starch properties to be
tailored to
provide improved performance depending upon the characteristics of the paper
ftu-~lish in which it will be used.
The starch properties can further be adjusted immediately prior to use in the
wet end of the papermaking machine in order to tailor the starch to the
specific
conditions existing in the papermaking machine. In this manner, the starch can
be
tailored to improve drainage and retention. This customization occurs, for
example,
by modification of the temperature at which the starch composition is cooked
prior
to addition to the wet end, by changing the period of time for cooking the
starch, by
changing the pressure at which the starch is cooked, and/or by changing the
solids
content of the starch prior to cooking. By adjusting these parameters, either
1 S individually or in concert, the properties of the starch are altered and
can be
conformed to specific conditions of various papermaking processes. For
example,
by cooking at higher or lower temperatures the starch properties are altered,
and
these altered properties can be used to improve wet end performance.
One implementation of the disclosure is a process for improving a
papermaking method. The process comprises providing a papermaking furnish
containing cellulosic fibers in an aqueous slurry to which is added a starch
composition. The starch composition is typically a crosslinked cationic
starch. The
starch is cooked prior to
addition to the papermaking furnish at a cooking temperature typically below
330
~F, and more typically from 180 to 250 ~F, and even more typically less than
220 ~F
or 230 ~F. Such cooking temperatures are typically average cooking
temperatures,
which corresponds to the average temperature measured from two or more
temperatures over time.
Microparticles, including nanoparticles, are also incorporated into the
papermaking furnish to enhance machine performance, such as drainage and
retention, and these microparticles typically have an average diameter of less
than
1.0 micron, and more typically less than 0.1 microns. Suitable microparticles
include, for example, various silica and clays.
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The cationic crosslinked starch of the disclosure is typically mixed as a wet
end additive into a paper furnish having a pH of from about 4.0 to about 9.0
in the
wet end. The general manufacturing process for paper, including the term "wet
end", is described generally in Pulp & Paper Manufacture, Vol. III,
Papermaking
and Paperboard Making, R. G. McDonald, editor, J. N. Franklin, tech. editor,
McGraw Hill Book Co., 1970.
In specific implementations, the starch and methods are used to improve
dewatering of papermaking furnishes. As the furnish is dewatered during the
papermaking process, the dewatering rate is evaluated. If this dewatering rate
is
unsatisfactory, then the cooking temperature of the starch is modified in
order to
alter the dewatering properties. The modification of the cooking temperature
should
be sufficient to produce a modification in the dewatering or first pass
retention of the
papermaking furnish. Typically, the amount of modification in the temperature
is
greater than 1 °F, and more typically at least 5 ~F. In specific
implementations, the
amount is from 5 to 10 ~F. In certain implementations the modification is at
least
about 10 ~F. The temperature is increased in certain implementations, and
decreased
in other implementations, depending upon the dewatering or drainage
performance
prior to modification of the cooking temperature.
It is sometimes necessary to determine the proper change in temperature
through iterative changes in temperature followed by evaluation of the paper
properties. Such iterative changes allow for step-wise evaluation and
adjustment of
the furnish properties. For example, when dewatering properties are
unsatisfactory
or show deterioration, the temperature can be initially lowered by a specific
temperature (for example, 5 ~F). If this lowering shows improvement in
dewatering,
then the temperature can be maintained at this new temperature. Alternatively,
the
temperature can be further lowered to seek even greater improvements in
dewatering
levels. If this lower temperature improves the dewatering properties, then the
temperature can be kept at this level (or lowered further to seek even greater
improvements). However, if this lower temperature does not improve the
dewatering properties, then the temperature can be raised back to the previous
level.
Alternatively, the temperature can be raised part way back to the previous
level.
If the initial lowering does not result in an improvement in the dewatering
properties, then the temperature should typically be raised above the initial
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temperature to determine if the dewatering properties improve. If the
dewatering
properties do not improve, then the temperature should be returned to the
initial
temperature or returned to a temperature intermediate the initial temperature
and the
raised temperature. If the dewatering properties do improve, then the
temperature
can be maintained at the heightened temperature or raised again to seek an
even
greater temperature.
In this manner and similar manners the temperature at which the starch is
cooked is
used to alter the properties of the starch produced, thereby tailoring those
properties
to the wet-end properties of a paper machine. In addition to adjusting the
retention
and drainage properties by adjusting the cooking temperature of the starch,
these
properties can be adjusted by modification of the pressure at which the starch
is
cooked and by changing the solids content of the starch prior to being cooked.
For
example, the starch is typically cooked in a jet cooker at a pressure of less
than 100
pounds per square inch; and the starch is typically added to the jet cooker at
a solids
1 S content of less than 10 percent. By altering the pressure or the solids
content, the
starch composition can be tailored to the specific properties of the wet end
furnish to
which they are added.
Not only can the temperature, pressure, and solids levels be independently
modified to improve the wet end performance, but they can be modified together
to
change the starch properties. For example, all three parameters can be
changed? the
temperature and pressure can be changed, the temperature and solids content
can be
changed, or the pressure and solids content can be changed. Also, besides
drainage
and retention, other improvements can be made in the wet end properties, such
as
improvements in line speed that are often observed along with improvements in
drainage and retention.
A further implementation includes a process for adjusting a papermaking
method. The process entails adjusting the temperature at which the starch
composition is cooked in order to obtain improved drainage or retention
properties
of the papermaking furnish. The process includes providing a papermaking
furnish
containing cellulosic fibers and microparticles in an aqueous slurry, and
providing a
starch composition formulated for addition to the papermaking furnish. A
portion of
the starch composition is cooked at an initial temperature and then added to
the
papermaking furnish. The furnish is subsequently dewatered to form a
cellulosic
fiber web. An assessment regarding the rate of dewatering or particle
retention of
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the aqueous slurry is made, and if the dewatering is at an unsatisfactory rate
then the
temperature at which the starch composition is cooked is changed to a
different
temperature in order to modify the rate of dewatering of the aqueous slurry.
The above summary of the present disclosure is not intended to describe each
embodiment of the present disclosure. This is the purpose of the figures and
the
detailed description which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects and advantages will become apparent upon reading the
following detailed description and upon reference to the drawings in which:
Figure 1 is a chart depicting particle size distribution of example wet end
starches, one of which has been crosslinked and one of which has not.
Figure 2 is a chart depicting the average particle size of a crosslinked
cationic starch cooked at various jet cooking temperatures.
Figure 3 is a chart depicting the particle size distribution of example wet
end
additives.
Figure 4 is a chart depicting drainage of a crosslinked cationic starch and a
non-crosslinked cationic starch that have been cooked at various temperatures.
Figure 5 is a chart depicting the viscosity of a crosslinked cationic starch
and
a non-crosslinked cationic starch that have been cooked at various
temperatures.
Figure 6 is a chart depicting the average particle size of a crosslinked
cationic starch and a non-crosslinked cationic starch that have been cooked at
various temperatures.
While the disclosure is susceptible to various modifications and alternative
forms, specifics are shown by way of example and are described in detail. It
should
be understood, however, that the intention is not to limit the disclosure to
the
particular embodiments described. On the contrary, the intention is to cover
all
modifications, equivalents, and alternatives falling within the spirit and
scope of the
disclosure as defined by the appended claims.
DETAILED DESCRIPTION
The present disclosure relates to starches, including cationic crosslinked
starches, and to the use of those starches in papermaking. More particularly,
the
present disclosure is directed to cationized crosslinked starch and to use of
the starch
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in the wet end system of a paper machine. The starch is adapted for
customization
to various wet end conditions, and allows for modification to correspond to
variations in the wet end of the papermaking machine.
According to an aspect of the disclosure, a cationic starch which has been
crosslinked after cationization is added to paper pulp or fiu-nish during
paper
manufacture. The starch is cooked prior to addition at the wet end of the
papermaking machine and the cooking parameters are adjusted in order to
improve
the properties of the wet end furnish, such as particle retention and drainage
of the
furnish. In this manner the properties of the starch are customized so as to
conform
to the specific conditions of the wet end of the paper machine.
The following detailed description includes specific starch compositions,
methods of adjusting the properties of the starch compositions to conform to
the
conditions of the papermaking process, and improvements in the papermaking
performance, including improved drainage and improved first pass retention and
microparticle retention.
A. Modifiable Starch Compositions
In a first aspect, the present disclosure is directed to starch compositions
suitable for use in the wet end stage of paper manufacturing. The starch
compositions of the disclosure possess properties permitting them to be
modified
during cooking to improve performance during the papermaking process.
The starch can be selected from a variety of starches, including corn (such as
waxy corn or dent corn), potato, sorghum, tapioca, wheat, rice, etc. The
starch is
preferably a corn starch, and typically a dent corn starch, and more typically
a
cationized dent corn starch. The starch should have hydroxyl groups or another
functional groups to permit crosslinking. Additional properties relating to
crosslinking levels, viscosity, substitution levels, and particle size are
described
below.
The starch is typically crosslinked with a crosslinker which is reactive with
the hydroxyl functionality of the starch. .The crosslinked starch permits a
greater
range in particle sizes compared to non-crosslinked starch. This range of
particle
sizes allows greater opportunity to improve wet-end performance. Without being
limited to a theory of use, it is believed that improved performance is
obtained when
starch particle size closely correlates to that of other particles in the
furnish.
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Although the starch particles can be smaller than the paper fibers and larger
then
microparticle additives, a relationship is believed to exist between the sizes
of the
various particles (fiber, starch, and microparticle).
In reference now to Figure 1, a graph is shown depicting the particle size
profiles of non-crosslinked cationic dent corn starch compared to crosslinked
cationic dent corn starch. The non-crosslinked cationic dent starch has a
narrow
particle size distribution, while the crosslinked cationic starch has a wide
particle
size distribution. This greater distribution is believed to allow for greater
probability
of particle-particle collisions to occur among the particulates of the wet end
furnish
and the starch, thus resulting in increased retention of the microparticles.
The starch is formulated with a crosslinker, which can be a polyfunctional
organic or inorganic compound wherein functional groups, such as epoxides or
anhydrides, on the crosslinker are reactive with hydroxyl groups on the
starch. The
starch can be crosslinked with polyepoxide compounds such as a
polyaminepolyepoxide resin, phosphorousoxychloride, 1,4 butanediol diglycidyl
ether, dianhydrides, acetals, and polyfunctional silanes. The crosslinker can
also be
sodium trimetaphosphate. These and other suitable crosslinkers, and
crosslinking
methods, are described in U.S. Pat. Nos. 3,790,829; 3,391,018; 3,361,590, and
5,122,231, incorporated herein by reference.
Starch Viscosity
Typically, the level of crosslinking relates substantially to the starch
viscosity. Thus, changes in viscosity can be implemented in part by altering
the
level of crosslinking. The amount of crosslinking is a function of the time
and kind
of crosslinker, as well as reaction conditions, all of which are chosen to
provide a
viscosity in a specified range.
The cationic crosslinked starch is typically crosslinked to a hot paste
viscosity in the range of about 10 cps to about 3000 cps, typically from about
SO cps
to 3000 cps, preferably from about 200 cps to about 3000 cps as measured on a
Brookfield viscometer at 2.0 percent starch solids at 95~ C, at 20 rpm, using
a
number 21 spindle in accordance with the method taught in U.S. Patent No.
5,122,231, incorporated herein by reference.
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Alternatively, the starch viscosity can be measured using breakdown
viscosity in accordance with the methodology disclosed in U.S. Patent No.
5,368,690, even though this methodology can be less precise for measurements
of
high viscosity starches. The percent breakdown viscosity is typically greater
than 85
percent, and more typically greater than 90 percent. In a preferred
implementation
the breakdown viscosity is greater than 95 percent.
Starch Substitution Levels
Suitable starch compositions are desirably cationic starches that retain a
positive charge when dissolved in water. The starch preferably contains a
quaternary ammonium ion, which gives enhanced flexibility in pH. Frequently,
such quaternary ammonium-containing starch is derivatized by etherification of
hydroxyl groups with an appropriate etherifying agent. The etherifying agent
has a
cationic character such as (3-chloro-2 hydroxypropyl) trimethyl ammonium
chloride, the methyl chloride quaternary salt of N-(2,3-epoxypropyl)
dimethylamine
or N-(2,3-epoxypropyl) dibutylamine or N-(2,3-epoxypropyl)methylaniline.
As used herein, the degree of substitution (DS) is defined as the average
number of hydroxyl groups on each anhydroglucose unit which are derivatized
with
substituent groups. The DS serves as a measure of the charge on the cationized
and
crosslinked starch and is related to the average number of monovalent cations
on the
hydroxyl groups on each anhydroglucose unit. Degree of substitution is
described
generally in STARCH: Chemistry and Technology, second edition, R. L. Whister,
J.
N. Bemiller, and E. F. Paschall, editors, Academic Press, Inc., 1984. The
starch is
typically canonized to a degree of substitution (DS) of greater than 0.005,
but not
greater than 0.100, more typically to a DS of about 0.030 to about 0.070. The
starch preferably has a DS of from 0.030 and 0.040.
The starch can be cationized by any known method, such as by reacting it in
an alkaline medium with tertiary or quaternary amines followed by
neutralization,
and washing and drying as desired. Known methods for cationizing starch are
described in U.S. Pat. Nos. 4,146,515 to Buikema et al. and 4,840,705 to Ikeda
et al,
incorporated herein by reference. In one aspect of the disclosure, corn starch
is
cationized by reaction of the starch with (3-chloro-2-hydroxypropyl) trimethyl
ammonium chloride in an alkaline medium provided by sodium hydroxide to form
the cationic (2-hydroxypropyl) trimethyl ammonium chloride starch ether with a
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molar degree of substitution (DS) of the ether on the starch in the range of
from
0.030 to 0.040.
Starch Particle Size
The starch compositions of the present disclosure are advantageous in that
they permit modification of the particle size based upon the nature of the
starch
composition and the manner in which it is cooked, including the temperature,
pressure, and solids level. Thus, the particle sizes can be changed in order
to
provide the most advantageous properties in the wet end finish. The starch
particle
size can be determined using a particle size distribution analysis, such as
using Mie
scattering theory incorporated into size distribution analyzers made by
Horiba, Inc.,
including LA910 size distribution analyzer. Mie scattering theory does not
provide
a direct measurement of diameter, but indicates at least the relative particle
size.
In reference now to Figure 2, an example particle size distribution of a
starch
composition made in accordance with the disclosure is shown. Depending upon
the
cooking temperature, the particle size distribution changes significantly for
the
crosslinked starch. In the example depicted, the average particle size changes
as
cooking temperature changes from about 190 to 265 ~F. The maximum size is
obtained at low cooking temperatures. By changing the cooking temperature, the
average particle size also changes, allowing for close tailoring of the
particle size to
the specific wet end furnish to which the starch is added.
B. Microparticle Ingredients
Microparticles are also incorporated into the papermaking furnish. The
microparticles typically aid in drainage, and can function as a flocculent.
Suitable
microparticles include silica and clays. The concentration of microparticles
added to
the wet end furnish in accordance with the disclosure will vary depending upon
the
desired properties of the finished paper product, along with retention levels
obtained.
Microparticles are typically added at a concentration of less than 5.0 pounds
per ton
of fiber, and more typically less than 2.0 pounds per ton of fiber.
The microparticles typically have an average diameter of less than 1.0
micron, and more typically less than 0.5 microns. Drainage aids such as
colloidal
silica often have an average diameter of about 0.1 ~,m, fillers are typically
1 to 50
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Vim, latex conglomerates are from 10 to 100 Vim, and fiber is often 200 p,m or
greater.
In reference now to Figure 3, a particle size distribution for typical wet end
ingredients is shown, including silica, titanium dioxide, precipitated calcium
carbonate (PCC), and white water. As indicated, the particle sizes vary
depending
upon the type of particle. Silica has a particle size of approximately 0.10
pm,
titanium dioxide has an average particle size of approximately 0.8 ~m and a
range of
0.5 to 1.0 pm. PCC ranges from approximately 1.0 pm to over 10 Vim. White
water
ranges broadly from less than 1.0 ~m to over 10 Vim. Depending upon the amount
of microparticles that are added, the starch composition should be adjusted to
optimize drainage and particle retention. It is often necessary to adjust the
starch
particle size to correspond to the various microparticle sizes. This is true
when the
microparticle sizes vary during paper manufacture, either as an intentional
change in
particle size or concentration, or as an inadvertent result of changes in the
papermaking furnish.
C. Starch Preparation Conditions
The present disclosure allows papermakers to cook the starch compositions
in a manner such that the starch properties, including particle size and
particle size
distribution, are optimized to coincide with the papermaker's general wet end
properties, including particle size and particle size distribution of the
particulates in
the wet end. Without being limited by theory, it is believed that this
variation in
particle size of the starch compositions is correlated to changes in drainage
and
retention. When the particle sizes of the starch properly correlate to the
particle
sizes of the inorganic and organic (e.g. fiber, latex) particles added to the
starch, the
furnish achieves enhanced drainage and retention properties, among other
properties.
The broad starch particle size range can be manipulated with the papermaker's
starch
cooker. The ability to change the particle size and particle size population
to
coincide with the papermakers' wet end allows superior retention and drainage
performance.
D. Methods of adjusting the properties of the starch composition
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Modification of cooking teiriperature
In a first implementation, the starch properties are modified by altering the
starch cooking temperature, in particular the temperature at which the starch
is jet
cooked. In specific implementations, the improved starch and cooking
temperature
adjustment methods are used to improve dewatering of papermaking furnishes
and/or the retention of microparticles. As the furnish is dewatered during the
papermaking process, the dewatering rate is evaluated. If this dewatering rate
is
unsatisfactory, then the cooking temperature of the starch is modified in
order to
alter the dewatering properties.
The modification of the cooking temperature should be sufficient to produce
a modification in the dewatering or first pass retention of the papermaking
furnish.
Thus, temperature changes should be of great enough magnitude to impact the
papermaking furnish properties. Typically, the amount of modification in the
temperature is greater than 1 °F, and more typically greater than 5 ~F.
In specific
1 S implementations, the amount is from 5 to 10 °F. In certain
implementations the
modification is at least about 10 ~F. The temperature is increased in certain
implementations, and decreased in other implementations, depending upon the
dewatering or drainage performance prior to modification of the cooking
temperature.
In a particular implementation, the papermaking process includes the steps of
cooking a starch component, dewatering a paper furnish, and then adjusting the
dewatering rate by changing the cooking temperature of the starch component.
The
first step, cooking the starch component, includes cooking at a first average
cooking
temperature below 330 ~F for a first period of time a cationized crosslinked
starch
having a hot paste viscosity in the range of from about 50 cps to about 3000
cps as
measured in a Brookfield viscometer in at 2.0 percent starch solids at 95~ C,
at 20
rpm, using a number 21 spindle. The furnish includes cellulosic fibers in an
aqueous
slurry, inorganic particles comprising at least 50 percent by weight particles
having
an average particle size of no greater than 1 micron, and the cooked starch
component. The rate of dewatering is adjusted by cooking the starch
composition at
a second temperature at least 10 of different than the first average cooking
temperature. In specific implementations, the second average cooking
temperature
is from 200 to 250 ~F, and in other'implementations the second average cooking
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temperature is less than 230 ~F. The microparticles can include silica, clay,
and
combinations thereof.
Additional steps can include determination of particle retention and
modification of temperature to adjust retention. The starch composition can be
cooked in a jet cooker, and at a pressure of less than 150 pounds per square
inch. In
specific implementations the starch is added to the jet cooker at a solids
content of
from 1 to 10 percent.
Due to the complexity of the furnish properties, it is sometimes necessary to
determine the proper change in temperature through iterative changes in
temperature
followed by evaluation of the paper properties. These changes involve making
adjustments from the initial cooking temperature to determine if the changes
will
improve retention, drainage, or other paper products. Such changes seek to
optimize
starch properties by adjusting the cooking temperature until the properties
approach
preferred ranges.
For example, when dewatering properties are unsatisfactory or show
deterioration, the temperature can be initially lowered by a specific
temperature (for
example, 5 nF). If this lowering shows improvement in dewatering, then the
temperature can be maintained at this new temperature. Alternatively, the
temperature can be further lowered to seek even greater improvements in
dewatering
levels. If this lower temperature improves the dewatering properties, then the
temperature can be kept at this level (or lowered further to seek even greater
improvements). However, if this lower temperature does not improve the
dewatering properties, then the temperature can be raised back to the previous
temperature.
Alternatively, the temperature can be raised to part way back to the previous
temperature. If the initial lowering does not result in an improvement in the
dewatering properties, then the temperature should typically be raised above
the
initial temperature to determine if the dewatering properties improve. If the
dewatering properties do not improve, then the temperature should be returned
to the
initial temperature or returned to a temperature intermediate the initial
temperature
and the raised temperature. If the dewatering properties do improve, then the
temperature can be maintained at the heightened temperature or raised again.
In this
manner the temperature at which the starch is cooked is used to alter the
properties
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of the starch produced, thereby tailoring those properties to the wet-end
properties of
a paper machine.
Modification of cooking pressures
In another implementation, the starch properties are modified by altering the
starch cooking pressures, in particular the pressures at which the starch is
jet cooked.
As the furnish is dewatered during the papermaking process, the dewatering or
retention rate is evaluated. If the dewatering or retention rates are
unsatisfactory,
then the cooking pressure of the starch is modified in order to alter the
dewatering or
retention properties. The modification of the cooking pressure should be
sufficient
to produce a modification in the dewatering or first pass retention of the
papermaking furnish.
Typically, the amount of modification in the cooking pressure is greater than
at least 1 psi, and more typically greater than 5 psi. In specific
implementations, the
amount is from 10 to 60 psi. In certain implementations the modification is at
least
about 20 psi. The pressure is increased in certain implementations, and
decreased in
other implementations, depending upon the dewatering or drainage performance
prior to modification of the cooking pressure.
In a particular implementation, the papermaking process includes the steps of
cooking a starch component, dewatering a paper furnish, and then adjusting the
dewatering rate by changing the cooking pressure of the starch component. The
first
step, cooking the starch component, includes cooking a cationized crosslinked
starch
having a hot paste viscosity in the range of from about 10 to 3000 cps, more
typically 50 to 3000 cps, and preferably 200 cps to about 3000 cps as measured
in a
Brookfield viscometer at about 2.0 percent starch solids and about 95 ~C using
a No.
21 spindle.
The furnish includes cellulosic fibers in an aqueous slurry, inorganic
particles comprising at least 50 percent by weight particles having an average
particle size of no greater than 1 micron, and the cooked starch component.
The rate
of dewatering is adjusted by cooking the starch composition at a second
pressure at
least 10 psi different than the first average cooking pressure.
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Additional steps can include determination of particle retention and
modification of pressure to adjust retention. In specific implementations the
starch
is added to the jet cooker at a solids content of from 1 to 10 percent.
Due to the complexity of the furnish properties, it is sometimes necessary to
determine the proper change in pressure through iterative changes in pressure
followed by evaluation of the paper properties. These changes involve making
adjustments from the initial cooking pressure to determine if the changes will
improve retention, drainage, or other paper products. Such changes seek to
optimize
starch properties by adjusting the cooking pressure until the properties
approach
preferred ranges.
For example, when dewatering or retention properties are unsatisfactory or
show deterioration, the pressure can be initially lowered by a specific amount
(for
example, 10 psi). If this lowering shows improvement in dewatering or
retention,
then the pressure can be maintained at this new pressure. Alternatively, the
pressure
can be further lowered to seelc even greater improvements in dewatering or
retention
levels. If this lower pressure improves the dewatering properties, then the
pressure
can be kept at this level (or lowered further to seek even greater
improvements).
However, if this lower pressure does not improve the dewatering properties,
then the
pressure can be raised back to the previous pressure. Alternatively, the
pressure can
be raised to part way back to the previous pressure.
If the initial lowering does not result in an improvement in the dewatering or
retention properties, then the pressure should typically be raised above the
initial psi
to determine if the dewatering or retention properties improve. If the
properties do
not improve, then the pressure should be returned to the initial pressure or
returned
to a pressure intermediate the initial pressure and the raised pressure. If
the
properties do improve, then the pressure can be maintained at the heightened
pressure or raised again to seek an even greater temperature. In this manner
and
similar manners the pressure at which the starch is cooked is used to alter
the
properties of the starch produced, thereby tailoring those properties to the
wet-end
properties of a paper machine.
Modification of solids levels
In a further implementation, the starch properties are modified by altering
the
starch solids levels at which the starch is cooked, and in particular the
solids levels a
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which the starch is j et cooked. In specific implementations, the improved
starch and
methods are used to improve dewatering of papermaking furnishes and/or the
retention of microparticles. As the furnish is dewatered during the
papermaking
process, the dewatering rate is evaluated. If this dewatering rate is
unsatisfactory,
then the solids levels of the starch is modified in order to alter the
dewatering
properties.
The modification of the solids levels should be sufficient to produce a
modification in the dewatering or first pass retention of the papermaking
furnish.
Typically, the amount of modification in the solids levels is greater than 1
percent,
and more typically greater than 2 percent. In specific implementations, the
amount
is from 3 to 10 percent. In certain implementations the modification is at
least
about 5 percent. The solids levels is increased in certain implementations,
and
decreased in other implementations, depending upon the dewatering or drainage
performance prior to modification of the solids levels.
In a particular implementation, the papermaking process includes the steps of
cooking a starch component, dewatering a paper furnish, and then adjusting the
dewatering rate by changing the solids levels of the starch component prior to
cooking in a jet cooker. The first step, cooking the starch component,
includes
cooking a cationized crosslinked starch in a jet cooker. The starch component
prior
to cooking has a hot paste viscosity in the range of from about 10 cps to 3000
cps,
more typically SO cps to 3000 cps, preferably 200 cps to about 3000 cps as
measured
in a Small Sample Brookfield Viscometer System (SSB) at 2.0 percent starch
solids
at 95~ C, at 20 rpm, using a number 21 spindle as measured after 10 minutes.
The
furnish includes cellulosic fibers in an aqueous slurry, inorganic particles
comprising
at least 50 percent by weight particles having an average particle size of no
greater
than 1 micron, and the cooked starch component. The rate of dewatering is
adjusted
by cooking the starch composition at a second solids levels at least 1 percent
different than the first average solids levels.
In specific implementations, the second average solids levels is from 5 to 6
percent, and in other implementations the second average solids levels is less
than 5
percent. The microparticles can include silica, clay, and combinations
thereof.
Additional steps can include determination of particle retention and
modification of solids levels to adjust retention. The starch composition can
be
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cooked in a jet cooker, and at a pressure of from 10 to 30 pounds per square
inch. In
specific implementations the starch is cooked at a temperature from 200 to 300
F.
Due to the complexity of the furnish properties, it is sometimes necessary to
determine the proper change in solids levels through iterative changes in
solids
S levels followed by evaluation of the paper properties. These changes involve
making adjustments from the initial solids level to determine if the changes
will
improve retention, drainage, or other paper products. Such changes seek to
optimize
starch properties by adjusting the solids level until the properties approach
preferred
ranges.
For example, when dewatering properties are unsatisfactory or show
deterioration, the solids levels can be initially lowered by a specific
amount. If this
lowering shows improvement in dewatering, then the solids levels can be
maintained
at this new solids levels. Alternatively, the solids levels can be further
lowered to
seek even greater improvements in dewatering levels. If this lower solids
levels
improves the dewatering properties, then the solids levels can be kept at this
level
(or lowered further to seek even greater improvements). However, if this lower
solids levels does not improve the dewatering properties, then the solids
levels can
be raised back to the previous solids levels. Alternatively, the solids levels
can be
raised to part way back to the previous solids levels.
D. Improving Papermaking Performance
Under some conditions improved performance of the papermaking machine
is obtained, such as by reducing the number of runability upsets, and will
allow
paper makers increased production throughput. This enhanced throughput can be
the result of reducing the amount of paper that fails to conform to
performance
specifications, to improving drainage of the paper slurry, and increasing
machine
speed.
The starch is cooked and added to the wet end furnish, which contains
cellulosic fibers. The furnish can include hardwood, softwood or a
hardwood/softwood fiber blend. Addition of the cationic crosslinked starch can
occur at various points in the papermaking process; including prior to
conversion of
the wet pulp into a dry web or sheet. Thus, for example, it can be added to
the fiber
while the latter is in the headbox, beater, hydropulper, or stock chest. The
furnish
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can include additives, dyes, and/or fillers such as clays, CaC03, alum and the
like.
The disclosure advantageously permits the use of higher levels of starch and
fillers
in lieu of more expensive cellulosic fiber, the result being paper with
enhanced
strength made with less expensive raw materials in shorter process times with
higher
retention of fines and fillers.
E. Example
Paper stock was prepared to compare the effect of changes in retention,
drainage, and viscosity using crosslinked and non-crosslinked cationic dent
corn
starches based upon changes in cooking properties. For each type of starch,
thirty
pounds of starch were added per dry ton of wood fiber. The starches were
cooked at
temperatures from 192 to 265 °F, and solids levels were maintained from
1.28 to
1.39 percent. Average particle size of the starch particles was measured using
a
model LA910 Horiba Particle Size Distribution analyzer, and drainage was
measured using a Dynamic Drainage Jar procedure. Preparation details are
summarized in Table 1 and Table 2, below.
TABLE 1- Crosslinked Cationic Starch
Sample Cooking ViscosityDiluted Average Drainage Retention
Temp (CPS)* Solids Particle -
(F) Percent Size (~.m)(ml f 20
sec)
A 196 250 1.28 118 41.6 60.77
B 217 205 1.39 114 54.7 62.25
C 233 150 1.37 102 58.6 63.44
D 245 115 1.34 76 42.1 64.46
E 265 95 1.28 61 38.5 61.91
* Brookfield viscosity of the cooked starch as measured at 150 °F, at
the solids percent
shown in Table 1, using a #21 spindle.
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TABLE 2 - Non-Crosslinked Cationic Starch
Sample Cooking Viscos Diluted Average Drainage Retention
Temp *ty Solids Particle(ml l 20
(F) (CPS) Percent Size sec)
(pm)
F 192 40 1.66 34.1 33.8 61.71
G 218 30 1.43 8.4 31.4 60.78
H 240 20 1.23 10.1 33.2 64.12
I 265 20 1.38 5.3 32.9 62.84
Brookfield viscosity of the cooked starch as measured at 150 °F, at the
solids percent
shown in Table 1, using a #21 spindle.
Differences in papermaking properties of the two types of starch are depicted
in Figures 4, 5, and 6, which show the drainage, viscosity, and particle size
distribution for crosslinked cationic dent corn starch and non-crosslinked
cationic
dent corn starch at various jet cooking temperatures. As indicated in Figure
4, the
crosslinked cationic starch demonstrated dynamic drainage from approximately
200
to 260 °F, with a peak at approximately 230 °F. In contrast, the
non-crosslinked
cationic starch demonstrated relatively flat drainage (at a level below that
of the
crosslinked cationic starch) in this example.
Furthermore, as indicated in Figure 5, crosslinked cationic starch shows a
change in viscosity over a broad temperature range. In the example shown in
Figure
5 the crosslinked cationic starch had highest viscosity at low temperatures
and
lowest viscosity at elevated temperatures. The temperature range extended from
about 200 °F up to 265 °F. Over the same temperature range the
non-crosslinked
cationic starch did not show significant variations in viscosity.
Figure 6 demonstrates changes in particle size distribution over an extended
jet cooking range when using a crosslinked cationic starch compared to a non-
crosslinked cationic starch. The particle size distribution was greatest at
low jet
cooking temperatures and decreased as the temperature increased. Generally,
the
non-crosslinked starch showed a substantially lower change in particle size
distribution over the same temperature range.
These examples demonstrate the variation in starch properties over a range of
cooking temperatures. In particular, they demonstrate changes in drainage and
retention based upon changes in the cooking temperature.
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