Note: Descriptions are shown in the official language in which they were submitted.
WO 91/01409 PCT/US90/04138
2~~3b~'~~v t.
Description
CELL WALL LOADING OF NEVER-DRIED PULP FIBERS
Technical Field of the Invention
This invention relates to a filled paper
composition wherein the filler is an insoluble
precipitate predominantly located within the cell wall
of never-dried cellulosic pulp fibers. The location of
the filler within the cell walls determines the
resulting filled paper composition having increased
strength relative to a corresponding conventionally
filled paper containing the same amount of the same
filler.
The present invention also relates to a
process for producing a filled paper composition having
increased strength relative to a conventionally filled
paper having the same concentration of the same filler
material.
Background of the Invention
The increasing cost of virgin pulp and the
energy associated with its transfonaation are familiar
problems to most papermakers. The boom in hardwoods
utilization, the optimization of high-yield pulping
processes, and the ongoing conversion to alkaline sizing
are only a few examples of many attempts made in recent
years to address papermaking problems. The most
economically useful approach has been to replace pulp
fibers with cheaper filler materials. High-filler
papers are also called ultrahigh-ash paper when calcium
carbonate (CaC03) is the filler. However, the major
constraint of ultrahigh-ash paper is an impairment of
interfibrillar bonding. This results in decreased paper
strength.
WO 91/01409 PCT/US90/04138
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2
Papermaking processes often use fillers or
opaque pigments to confer some desirable characteristics
to the paper product and to provide a cost savings for
paper raw materials. Fillers can increase opacity,
brightness and printing properties. Fillers are cheaper
substitutes than cellulose fibers and can reduce the
total cost of the finished paper product. Moreover,
fillers can be dried easier than fibers and reduce
energy consumption during the papermaking process.
An essential property of paper for many end
uses is its opacity. It is particularly important for
printing papers, where it is desirable to have as little
as possible of the print on the reverse side of a
printed sheet or on a sheet below it be visible through
the paper. For printing and other applications, paper
must also have a certain degree of brightness, or
whiteness. For many paper products, acceptable levels
of optical properties can be achieved from the pulp
fibers alone. However, in other products, the inherent
light-reflective characteristics of the fibers are
insufficient to meet consumer demands. In such cases,
the papermaker adds a filler.
A filler consists of fine particles of an
insoluble solid, usually of a mineral origin, suspended
in a slurry. By virtue of the high ratio of surface
area to weight (and sometimes high refractive index),
the filler particles confer light-reflectance to the
paper and thereby increase both opacity and brightness.
Adding fillers to paper pulp produces an enhancement of
the optical properties of the paper and further produces
the advantages of improved smoothness and improved
printability. Further, replacing fiber with an
inexpensive filler can reduce the cost of the paper.
However, filler addition poses some additional problems.
One problem associated with filler addition is
that the mechanical strength of the paper is less than
could be expected from the ratio of load-bearing fiber
WO 91/01409 PCT/US90/04138
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3
to non-load-bearing filler. The mechanical strength of
paper can be expressed in terms of burst index, tear
index, and tensile index. The usual explanation for
this is that some of the filler particles become trapped
between fibers, thereby reducing the strength of the
fiber-to-fiber hydrogen bonding. The hydrogen bonding
is the primary source of paper strength.
There exists a practical limit to the amount
of filler which can be used. The paper mechanical
properties depend primarily upon hydrogen bonding
between fibrous elements. Filler accumulates on the
external surface of the fibers. Accumulated filler
weakens the paper strength. Further, one must use
increasing amounts of retention aids to avoid excessive
pigment losses through the paper-forming wire.
Accordingly, filler concentrations are often limited to
a maximum of about 10% ash content.
Several techniques have been used to try to
overcome the problems of decreased strength from
increasing filler content. Most approaches have
involved filler surface modification, using retention
additives, and using supplemental bonding agents. For
example, preflocculated fibers and fillers have been
used to increase filler retention and reduce loss of
paper strength. Coarser particles of pigment or filler,
caused by the preflocculation procedure, are retained
more efficiently than the finer particles of pigment.
Thus, there is less interference with inter-fiber
bonding. This helps improve paper strength. However,
paper opacity is reduced with increasing particle size.
Moreover, the cost savings associated with the
preflocculation technique are insignificant and are
offset by additional problems.
Craig, U.S.. Patent No. 2,583,548 ("Craig"),
describes a process forming a pigmented cellulosic pulp
by precipitating pigment "in and around" the fibers.
According to Craig, dry cellulosic fibers are added to a
WO 91/01409 PCT/US90/04138
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solution of one reactant, for example, calcium chloride,
and the suspension is mechanically worked so as to
effect a gelatinizing of the dry fibers. A second
reactant, for example, sodium carbonate, is added so as
to effect the precipitation of fine solid particles,
such as calcium carbonate. The fibers are then washed
to remove the soluble by-product (sodium chloride).
The Craig process has considerable
limitations. The presence of filler on fiber surfaces
and the gelatinizing effect on the fibers are
detrimental to paper strength. The gelatinized fibers
are so severely broken that both the filler precipitate
and the gelled fibers form a slurry. Thus, the Craig
process has not achieved commercial success despite its
disclosure about 39 years ago.
Another technique is described in U. S. Patent
No. 4,510,020. This process has been called the "lumen-
loading" process and it involves placing the filler
material directly within the lumens of soft wood pulp
fibers. "Lumen-loaded" pulp is prepared by vigorously
agitating a dry softwood pulp in a concentrated
suspension of filler. The action of the agitation
encourages the filler to move through transverse pit
apertures in the fiber cell walls and into the lumen,
where the filler material is adsorbed against the
surface of the lumen cavity. Subsequent washing of the
lumen-filled pulp fibers rapidly eliminates residual
filler from the external surfaces of the fibers but only
slowly from the lumen. The result is an increased
retention of filler within the lumen, while removing the
hindrance to inter-fiber bonding by removing the filler
outside of the fiber lumens. The result is increased
paper strength for the amount of filler present. The
lumen-loading technique works best with fibers that have
been dried.
The lumen-loading technique, however, has not
proved to be economically or commercially viable. The
WO 91/01409 PGT/US90/04138
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technique requires the manipulation of large volumes of
relatively concentrated filler suspensions agitated at
high revolutions for prolonged periods of time.
Further, the lumen-loading technique requires a
5 relatively small particle size filler, such as titanium
oxide, which is an expensive filler material. Moreover,
the lumen-loading technique will only work for dry
softwood fibers having a sufficient number of pit
apertures. As the lumens are open at the pits, filler
may be lost in the same way that it is introduced.
Further, the pores in the cell walls are not filled by
the lumen-loading technique.
Accordingly, there is a need in the art to be
able to produce economical paper of high opacity and
strength using as much filler material as possible, and
to be able to use cellulosic pulp fibers from any source
(e.g., softwoods, hardwoods and annual plants, such as
sugarcane).
Summary of the Invention
The present invention refers to a filled-paper
composition comprising intact, never-dried cellulose
fibers and filler, wherein at least 50% of the filler
content is located within the pores or cell wall of the
never-dried cellulose fibers. The filled paper
composition is characterized by having increased
strength compared to a corresponding conventionally
filled paper containing the same amount of the same
filler. The filler is formed in s'tu as an insoluble
precipitate in an aqueous system. The paper composition
may further comprise a coloring agent wherein the
coloring agent is a colored precipitate formed in situ
that functions as a filler material.
Examples of insoluble precipitates that
function as filler materials include, for example,
calcium carbonate, other precipitates listed in Table 1
herein, and combinations thereof. The paper composition
WO 91/01409 PCT/US90/04138
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is selected from the group consisting of unbleached
kraft paper, bleached kraft paper, sulfite pulp
(bleached and unbleached) fine printing paper, fine
writing paper, and lightweight newsprint paper.
The invention further describes a process for
the production of filled paper wherein the starting pulp
is a never-dried pulp. The inventive process comprises
dispersing the never-dried pulp in a first solution,
wherein the first solution comprises a salt or salts, to
form a first dispersion; filtering the pulp from the
first dispersion; and redispersing the filtered, never-
dried pulp in a second solution to form a second
dispersion, wherein the second solution comprises a salt
or salts different from the salt or salts of the first
solution and with the proviso that the interaction of
the salt or salts from the first solution and the salt
or salts from the second solution form an insoluble
precipitate that acts as a filler within the pores of
the cell wall of the never-dried pulp. This forms a
filled pulp fiber that can be filtered and dried or used
wet for papermaking.
The paper is made by further process steps
known to those of ordinary skill in the art. The pulp
can be used directly for papermaking without drying, or
dried as filled pulp fibers and later used for
papermaking.
The present invention includes a filled paper
product made from filled, never-dried cellulose pulp
fibers, wherein the filled paper is made directly from
the filled, never-dried pulp or the filled, never-dried
pulp is made, dried, and later used to make paper. The
essential steps of the inventive process are as follows:
1. Immersing (or dispersing) the never-dried
pulp in a first solution, wherein the first solution
comprises a soluble salt or salts;
2. Filtering the immersed, never-dried pulp
and then redispersing (or reimmersing) the filtered,
WO 91/01409 PCT/US90/04138
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never-dried pulp in a second solution, wherein the
second solution comprises a soluble salt or salts
different from the soluble salt or salts of the first
solution and with the proviso that the interaction of
the salt or salts from the first solution and the salt
or salts from the second solution form an insoluble
precipitate ~ situ that acts as a filler within the
cell wall or pores of the never-dried pulp: and
3. Filtering and washing the filled never-
dried pulp.
The paper can be made directly with the
filled, never-dried pulp fibers by conventional
procedures. Alternatively, the filled, never-dried pulp
can be dried and later used for papermaking.
In another embodiment, the filled, never-dried
pulp fibers are beaten after filling in the never-dried
state or after being once dried. If the unbeaten,
filled pulp is dried, the papermaker can control the
specifications of the beating process in the papermaking
operation.
The never-dried cellulose pulp can be derived
from hardwoods, softwoods, annual plants such as
sugarcane (bagasse), and combinations thereof.
The present invention is able to load a
precipitate-type filler material within the cell walls
or pores located within the cell walls of never-dried
pulp fibers by the internal in s'tu precipitation of
insoluble fillers and pigments. Never-dried pulp fibers
are unique in having relatively large-sized pores
located within the interior of the cell wall. These
pores collapse when the pulp fiber is dried and are not
fully restored by the rewetting of the dried fiber.
Therefore, one can optimally precipitate filler material
within the cell wall surrounding the lumen only before
the fiber is dried. Similarly, filled fibers, filled by
the inventive process and dried, cannot be refilled by
the inventive process.
WO 91/01409 PCT/US90/04138
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~063~6"~
Filler materials, such as pigments and opaque
precipitates, are loaded into the pores of the cell
walls of never-dried wood pulp fiber by precipitating
the filler material inside the pores. This replaces the
fluid content of the pore. Excess filler is washed away
from the external surface of the fiber and an
insignificant amount, if any, of filler material remains
within the lumen of the fiber. As never-dried pulp
fibers are hollow, tubular structures, the fibers
l0 develop an extremely large surface area after pulping
and retain that large surface area while remaining wet
(i.e., never-dried). The large surface area within the
never-dried fibers is generously available to soluble
salts that are precipitated as papermaking fillers.
This preserves the bonding ability of the external
cellulosic layers and does not affect the strength of
the resulting paper.
Beef Description of the Drawings
Figure la is a scanning electron micrograph at
2142X magnification showing the surface of filled,
never-dried pulp fiber filled according to the inventive
process with NiC03 insoluble precipitate filler. Figure
lb is an electron dispersion analysis (EDAX) of the
filled fiber showing Ni location and distribution in the
cell wall of the fiber.
Figures 2a, 2b, and 2c are EDAX graphs of Ni
(07 box) of a NiC03-filled, never-dried pulp fiber
showing the surface of the fiber, the cell wall of the
fiber, and the fiber lumen, respectively. Filler was
predominantly present in Figure 2b, indicating the
presence of nickel in the cell wall.
Figures 3 and 4 illustrate the tensile index
and burst index, respectively, of different filler
content papers made from never-dried western hemlock
pulp (a softwood). The filled circles represent paper
made from fibers filled by the inventive process, and
.,,..... ,
206 3567
the open circles or squares represent paper made from fibers filled by a
conventional process, as described in Example 1. The different symbols
represent different batches run on different dates.
Figures 5, 6 and 7 illustrate tear index, burst index, and
tensile index, respectively, for different filler content papers made from
red alder pulp (a hardwood). The open circles or squares represent the
inventive process with CaC03 as the filler precipitated in situ, wherein,
for the open squares, CaC 12 was the first salt and Na2C03 was the second
salt, and for the open circles, Na2C03 was the first salt and CaCl2 was
the second salt. The closed triangles are data from paper made from
mixtures of cell wall filled and unfilled fibers in ratios of 1:3, l:l, and
3:1, respectively. The open diamonds are red alder, never-dried pulp
fibers filled by the conventional techniques described in Example 1. The
"x" designation used once-dried red alder pulp, rewetted and filled by the
inventive process.
Figures 8, 9, and 10 illustrate the tear index, burst index,
and tensile index, respectively, for different filler content papers made
from spruce CTMP pulp (a softwood). The open squares represent in
situ precipitated, never-dried pulp fibers filled by the inventive process.
The open circles represent never-dried pulp fibers loaded in a filled paper
by a conventional process described in Example 1.
Figures 11, 12, and 13 illustrate the tear index, burst index,
and tensile index, respectively, for different filler content papers made
from bagasse pulps (sugarcane). The squares represent in situ
precipitated, never-dried pulp fibers filled by the inventive process, with
the filled squares being bleached pulp and the open squares being
unbleached pulp. The diamonds represent never-dried pulps filled by a
conventional process, as described in Example 1,
WO 91 /01409 PCT/US90/04138
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2 0 6 3' ~~'6'~ ~ ~ ~ .~_.
with the filled diamonds being bleached pulp and the
open diamonds being unbleached pulp. The triangles
represent once-dried, bleached pulp filled by the
inventive process. The poor results obtained with the
once-dried pulps indicate that pores of the never-dried
pulp fibers are necessary to be able to fill the cell
walls of fibers.
Figure 14 compares the relative decrease in
tensile strength as a function of filler content
comparing literature data of the lumen-loading technique
(triangles or "x" figures) to never-dried pulps filled
by the inventive process using red alder hardwood pulp
(open squaresj, bagasse sugarcane pulp (diamonds), and
spruce CTMP softwood pulp (closed circles).
Detailed Description of the Invention
Never-dried pulp is formed by removing the
lignin and hemicellulose from cellulose wood fibers
during pulping. The pulp obtained is a composite of
several hundred concentric lamellae of cellulose
microfibrils. Each lamella is separated from the others
by water-filled spaces (pores) which vary in width from
about 25 to about 300 angstroms. The larger spaces are
located nearer the periphery, with the narrower spaces
located toward the lumen (a central channel of about 10
to about 20 microns in width). The spacing of the pores
more or less corresponds to the thickness of the lignin
in the cellulose wood fiber. The pore size generally
has a normal log distribution. A surprising result of
the inventive process is that most of the first solution
leaves the fiber lumen when the fiber is filtered
between the addition of the first and second solutions.
This is because the lumen is more open to the external
environment than the pores in the cell wall. Thus,
little, if any, filler is precipitated in situ in the
lumen. The normal log distribution of pore size is a
plot of the logarithm of the pore size versus pore frequency.
The never-dried pulp fiber has a surface area of about 1,000
m2/g. Upon drying, the surface area reduces to about 1 m2/g. Even
though the lamellae swell upon rewetting, the rewetted pulp has a surface
area of only about 100 m2/g. Thus, upon drying, most of the pores of
the never-dried pulp irreversibly collapse.
The inventive composition and processes depend upon the
special properties of the never-dried pulp or its equivalents. The never-
dried pulp has a large internal surface area of about 1,000 m2/g as a
result of the corresponding internal cell wall pore volume of about 1.2
mL/g. The internal cell wall pores are substantially lost by collapse
during drying. Anything placed within the pores drying becomes trapped
in the pores, as the pores collapse during drying.
We have shown that if never-dried pulp is sequentially
treated with a first solution containing a soluble salt, such as calcium
chloride, and altered to remove the soluable salt from the exterior of the
fiber and the lumen, and then a second soluble salt, such as sodium
carbonate, is added, the filler, calcium carbonate, is created within these
pores but not within the lumen. This process is appropriate for other
filler materials when the filler is an insoluble precipitate formed from the
interaction of two or more soluble salts.
When the filler is located within the cell wall by the in situ
process, interference with the hydrogen bonding between fibers is
reduced. As a consequence, the strength of paper made from such in situ
precipitation cell wall-filled fibers is greater than the strength of paper
made from the usual (conventional) combination of fibers and the same
amount of filler particles added to the fibers. The conventional mixture
of filler and fibers
~~
-~ ,
12 206 3567
locates the filler between the fibers. Furthermore, if the filler is located
inside the cell wall of the fiber in the inventive process and compositions,
the abrasive filler will have less contact with the forming wire on the
paper machine. This will result in fewer wire changes being needed for
the paper machine in a given period of time. Moreover, there is a
reduced opportunity for filler to dust off from the paper sheet because the
filler is located predominantly within the cell wall of the fibers rather
than outside of the fibers.
Another advantage of the inventive process and compositions
is that larger amounts of filler are used to form paper and maintain the
strength of the resulting paper. The paper filler does not require
incorporating adhesive polymers to maintain paper strength. Thus, paper
made using the inventive process without adhesive polymers can have
larger amounts of filler than conventionally made paper, while retaining
equal or superior strength characteristics. Since filler is generally more
economical than pulp fibers, the inventive process provides an economic
benefit by a lower cost of goods for the finished paper composition.
Moreover, it is less energy intensive and more economical to dry filler
than to dry fiber. Thus, reduced energy costs for paper forming will be
achieved by reduced drying costs.
The inventive process takes never-dried pulp and precipitates
a filler material in situ. In one embodiment, never-dried pulps are filled
by consecutively soaking the never-dried pulp in solutions comprising a
soluble salt or salts. The never-dried pulps are first soaked in a first
solution for approximately five minutes or less. The first solution
comprises a soluble salt or salts and functions to replace the water within
the pores in the cell wall and in the lumen with a solution containing the
soluble salt
WO 91/01409 PCT/US90/04138
"~. 13 ~, Q
or salts of the first solution. The never-dried pulp
fibers are filtered and washed, which removes the salt
or salts from the first solution from the exterior and
the lumen of the fibers. A second solution containing a
different soluble salt or salts is added to the filtered
fibers. The interaction of the salt or salts from the
first solution within the pores of the cell wall of the
never-dried pulp fibers and the soluble salt or salts of
the second solution forms an insoluble precipitate that
falls out of solution within the pores of the cell wall
of the never-dried pulp fibers. The precipitate within
the cell wall of the never-dried pulp fibers acts as a
filler. When the fibers are dried or used to make paper
and later dried, the insoluble precipitate acts as paper
filler. The filled, never-dried pulp fibers are
subsequently filtered and washed and used to form paper.
Alternatively, the filled fibers may be dried and
shipped to a papermaking facility as dry lap.
Pulp fibers are often beaten to certain
specifications as part of the papermaking procedure.
The beating of the pulp fibers occurs before forming the
paper. The inventive process allows the beating to
occur either before or after filling the fibers.
Moreover, never-dried pulp fibers can be filled, dried
and then beaten before use to form paper.
The order of the soluble salts in the first or
the second solution is not important to the process.
What is important is that the salt or salts of the first
and second solution be different and that they form an
insoluble precipitate upon interaction. Examples of
white (opaque) and various colored precipitates are
listed in Table 1.
CA 02063567 2000-09-07
WO 91/01409 PCT/US90/04138
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TABLE 1
EXAMPLES OF PRECIPITATES USED AS FILLERS
Color Name Formula
White Calcium carbonate CaC03
Aluminum phosphate AIPOa
Zinc ammonium phosphate Zn(NH4)P04
Calcium phosphate CaHP04, Ca(H2P04)2
Magnesium ammonium phosphate Mg(NH~)P04
Calcium borate Ca(BOz)z
Bismuth phosphate BiP04
Magnesium carbonate MgC03
Zirconium hydrogen phosphate Zr(HP04)2
Zirconyl hydrogen phosphate Zr0(HZP04)z
Aluminum hydroxide Al(OH)3
Bismuth hydroxide Bi(OH)3
Zinc hydroxide Zn(OH)z
Titanium hydroxide Ti(OH)4
Zirconium hydroxide Zr(OH)4
Calcium silicate CaSi03
Barium Sulfate BaSOa
Barium silicofluoride BaSiF6
Barium hydroxide Ba(OH)z
Barium orthophosphate Ba3(P04)z
Barium pyrophosphate BazP407
Barium metasilicate BaSi03
Barium carbonate BaC03
Bismuth oxycarbonate BiO2C03
Cadmium carbonate CdC03
Calcium metaborate hexahydrate Ca(BOz)z~6H20
Calcium hydroxide Ca(OH)z
Calcium orthophosphate Ca3(P04)2
Calcium pyrophosphate pentahydrateCazP207~5H20
Calcium sulfate CaS04
Lead carbonate PbC03
Magnesium metaborate octahydrateMg(BOz)z~8H20
Magnesium hydroxide Mg(OH)z
CA 02063567 2000-09-07
WO 91/01409 PCTlUS90/04138
Magnesium orthophosphate Mg3(P04)z
Strontium carbonate SrC03
Strontium metasilicate SrSi03
Strontium orthosilicate SrSi04
5 Thorium hydroxide Th(OH)4
Zinc carbonate ZnC03
Zinc orthophosphate Zn3(PO4)z'4H2O
Zinc metasilicate ZnSlO3
Blue Ferric ferrocyanide (Prussian Fe4[Fe(CN)6]3
blue)
10 Ferrous ferricyanide
(Turnbull's blue) Fe3[Fe(CN)6]z
Cupric phosphate Cu3(P04)z
Copper hydroxide Cu(OH)z
Copper basic carbonate 2CuC03Cu(OH)z
15 Violet Chromium orthophosphate hexahydrateCrP04~6H20
Red Mercurous iodide HgzIz
Mercuric iodide HgIz
Silver chromate AgCr04
Bismuth iodide BiIz
BiI3
Cobalt carbonate CoC03
Cobalt orthophosphate octahydrateCo3(P04)z~8H20
Cobalt ferricyanide Co[Fe(CN)6]z
Copper ferrocyanide CuzFe(CN)6~2H20
Stannous iodide SnIz
Pink Cobalt phosphate C03(PO4)2
Manganese ammonium phosphate Mn(NH4)P04
Cobalt orthophosphate dehydrateCo3(P04)z~2H20
Manganese carbonate MnC03
Yellow Cadmium sulfate CdS
Cadmium molybdate CdMo04
Barium chromate BaCr04
Antimony sulfide SbzS3
Calcium chromate CaCr04~2H20
Copper ferricyanide Cu3[Fe(CN)6]z~
14H20
Lead chromate PbCr04
Lead iodide PbIz
WO 91/01409 PCT/US90/04138
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16
Mercurous carbonate Hg2C03
Molybdenum metaphosphate Mo(P03)6
Silver iodide AgI
Silver orthophosphate Ag3P04
Tin sulfide SnS2
Green Chromium pyrophosphate Cr4(P20~)3
Copper metaborate Cu(B02)2
Copper basic carbonate CuC03Cu(OH)2
Nickel orthophosphate octahydrate Ni(P04)28H20
Nickel carbonate NiC03
Chromic phosphate ' CrP04
B ack Copper sulfide CuS
One of ordinary skill in the art would know
which salts would fona each precipitate.
Preferred examples of soluble salts that form
an insoluble precipitate include CaCl2 and Na2Si03,
yielding the precipitate CaSi03 (an opaque white
filler); BaCl2 and Na2S04, yielding BaS04 (a white
opaque filler); and CaCl2 and Na2C03, forming CaC03
(opaque white filler). It should be noted that it is
possible to replace a sodium cation with a potassium
cation in any of the soluble salts. Examples of green
precipitate fillers are NiC03, formed by the combination
of the aqueous salts NiCl2 and Na2C03; copper carbonate
(CuC03), from cuprous chloride (Cu2C12) and sodium
carbonate; and chromic phosphate (CrP04), from chromic
chloride (CrCl3) and sodium phosphate (Na3P04). The
preferred precipitate filler material is calcium
carbonate (CaC03). Calcium carbonate can be formed, for
example, by having one solution of calcium chloride and
the other solution of sodium or potassium carbonate. In
all of the insoluble precipitates that are formed, the
order of use of the soluble salts is not important.
The concentration of salt or salts in the
aqueous solution can vary from about 1% to about 40%,
depending upon the solubility of the salt in an aqueous
WO 91/01409 PCT/US90/04138
l~ .2 ~:~3'~~'~ ~
system, the temperature of the process, and the amount
of filler desired. Preferably, the concentration of
salt or salts in the aqueous solution should be as
saturated as the solubility characteristics and the
temperature of the process permit so as to maximize the
filler content of the resulting filled, never-dried pulp
fibers. When using colored or pigmented filler
precipitates, it is desirable not to maximize the amount
of filler in the cell wall of the never-dried fibers.
The inventive process allows for the improved
retention of mechanical properties of never-dried pulp
when the cell wall is loaded with a precipitated filler
~n_ situ. When never-dried pulp was filled with NiCO3,
formed from the soluble salts NiCl2 and NaC03, the
nickel precipitate can be visualized by electron
dispersion analysis (EDAX).
Loaded, never-dried pulps were washed on a
wire screen (mesh #100) with tap water. Microscopic
observation of the washed, never-dried pulp indicated
that this procedure was not efficient enough to
completely remove excess filler material from around
internally filled, never-dried fibers. Handsheet
formation, drying, and conditioning were done in
accordance with TAPPI standards. See TAPPI Official
Test Method T 205 om-81 from the American National
Standard, April 1982.
Figure la shows the location of nickel, and
Figure lb shows the nickel distribution. The white dots
in Figure lb represent nickel, and the higher density of
the white dots enables the fiber cell wall to be
visualized. Figures 2a, 2b and 2c show different
aspects of a cross section of a never-dried pulp fiber
loaded with nickel carbonate filler material by a
process described herein. Figure 2a shows the surface
of the filled, never-dried pulp fihers with essentially
zero nickel present in the third box from the right.
Figure 2b shows a high nickel level strongly above
WO 91 /01409 PCT/US90/04138
r.y~=~t~~~,~ 1$
background in a peak in the third box from the right for
the cell wall areas of the fibers. Figure 2c shows the
nickel concentration in the lumen of the filled, never-
dried pulp fiber with very little nickel present.
Paper made from never-dried fibers that have
been loaded in the cell wall pores with precipitate-type
filler material can be used for a wide variety of
applications. The following are some of the widest
categories, bearing in mind there are also many
specialty products which are produced in smaller
quantities.
Fine papers are a broad class of papers used
for printing and writing. Generally, fine papers
contain fillers. One advantage of feeding the filled,
never-dried pulp fibers, filled within their cell wall
to a paper machine used in making fine paper, rather
than the usual mixture of separate fiber and filler, is
a greater retention of the filler material within the
fibers. This leads to better control of properties and
cleaner machine operation. In addition to the paper
being stronger than a corresponding paper conventionally
filled with the same concentration of filler material,
the paper made from cell wall filled, never-dried pulp
exhibits less "two-sidedness." Two-sidedness is due to
an unequal distribution of filler across the thickness
of the sheet. Further, there is less tendency for the
filler to "dust off" from the sheet during the
converting processes of wetting and slitting.
Unbleached kraft pulp is used for paper
products such as paper bags and wrapping papers because
of its high strength. However, it has a low brightness,
thus making it both unattractive and a poor substrate
for printing paper. Never-dried, unbleached kraft pulp
fibers with filled cell walls improve the brightness of
the paper produced and less strength is lost from filler
loading than with conventional loading techniques and
dried pulp fibers.
WO 91/01409 PCT/US90/04138
19
.~,.
Most newsprint is currently made from a
mixture of mechanical and chemical pulp without filler.
There is a demand for such products of lower basis
weight (pulp weight per unit area). One of the barriers
to achieving substantial decreases in basis weight is
that such changes reduce the opacity of the sheet.
Filler is not currently added to offset the loss in
opacity for various reasons, including the loss of
strength it causes in the sheet and the "messiness" it
imparts to the papermaking operation. Using cell wall
filled, never-dried pulp fibers, the newsprint problems
are reduced and newsprint can be made with improved
levels of opacity.
The following examples are set forth to
illustrate the inventive method and compositions
produced by the inventive method and not to limit the
scope of the invention.
EXAMPLE 1
This example illustrates a comparison using
softwood never-dried pulp from western hemlock,
comparing the properties of the paper made from the
inventive process and a conventional process. In each
case, the pulp was beaten to 400 CSF before treatment.
For the inventive process, a sample of never-dried pulp
(l0 g) was dispersed in a 5%, 10%, 20%, or 35% solution
of CaCl2 in 500 mL of water. After 30 minutes, the
CaCl2-impregnated fibers were collected by filtration
under reduced pressure and redispersed in a saturated
Na2C03 solution (1,000 mL). After one hour, the
dispersion was filtered into a 200 mesh wire screen and
then washed with water until the filtrate was clear.
The never-dried pulps used for the preparation
of conventionally loaded papers were also washed over a
200 mesh wire screen five times at 0.5% consistency.
The conventionally filled pulp had its pH value of 8.0
adjusted using NaOH. A retention aid (Reten 210,
2 ~6 356 7
Hercules Corp.) was added at various rates (0.5-1.5 lb/ton of pulp) to
achieve the appropriate retention of the commercial CaC03 slurry. The
time of agitation was one minute.
Sheets were made with both the conventional pulp and filler
mixes and cell wall loaded, never-dried pulp by using TAPPI standard
sheetmaking conditions. The filler (CaC03) content of the sheets was
calculated by the ash content, as determined by the standard TAPPI
procedure, except that the temperature of the furnace was 575°C.
In Figures 3 and 4, the papers made from the cell wall
loaded, never-dried pulp are shown by the closed circles. The papers
made by conventional techniques are shown by the open points.
Figure 3 shows the effect of filler level on the tensile index
for conventional and cell wall loaded, never-dried pulp. These data
indicate that at equal CaC03 filler concentrations, the sheets made with
fibers filled by the inventive process have tensile properties superior. to
those made by a conventional process.
Similar comparative data are obtained in Figure 4, where the
burst strength of the papers is measured. Figure 4 is a plot of the burst
index versus filler concentration in the paper for both types of filled
papers. These data demonstrate the superior burst strength values
obtained using fibers filled by the inventive process.
These data indicate that the inventive process allows more
filler to be added at the same paper strength or it provides for a higher
level of strength at the same concentration of filler. Filled paper sells for
approximately $1,000/ton or $0.50/lb when pulp costs $500/ton and filler
costs $200/ton. Thus, every additional percent of filler that can be
placed in a sheet instead of fiber represents a significant
WO 91/01409 PCT/US90/04138
2ofi3~G7
manufacturing cost savings of about $3-$4/ton to the
papermaker. Moreover, the inventive process does not
require a retention aid and thus the formation of the
paper can be improved. Thus, when using a softwood
kraft pulp, the inventive process improves the strength
properties of the resulting paper.
EXAMPLE 2
This example illustrates a comparison of
various mechanical properties of paper made with never
dried, cell wall loaded pulps from red alder versus
never-dried red alder pulps combined with filler by
conventional means versus once-dried red alder pulp
fibers filled by the inventive process. In each
instance, the never-dried pulps were initially beaten to
400 mL CSF prior to filler loading by either technique.
The methods used for filling red alder pulps by the
inventive process or combining by the conventional
techniques are described in Example 1. Calcium
carbonate was provided as a slurry for the conventional
technique or precipitated ~ situ according to the
inventive process. The concentration of filler was
determined from the ash content.
Figures 5, 6, and 7 compare the tear index,
burst index, and tensile index, respectively, comparing
red alder never-dried pulps filled by the inventive
process or by the conventional technique. In each
illustration, the ash content indicates the percent of
filler in the paper. Therefore, in each figure it is
possible to compare the tear index, burst index, and
tensile index of paper made from each type of filled
fiber at equivalent filler concentrations.
In Figures 5, 6, and 7, the upper line with
the higher tear burst or tensile indices is for papers
made with fibers filled by the inventive process. The
squares represent never-dried pulp fibers filled wherein
the sequence of solution addition is first calcium
WO 91/01409 PCT/US90/04138
22 -
~~~~~~?
chloride followed by sodium carbonate and the circles
have the reverse sequence of sodium carbonate followed
by calcium chloride. The lower line with the X-shaped
points represents once-dried pulp fibers filled by the
inventive process. The lower line with the diamond
points represents conventionally loaded, never-dried
pulps.
In each instance, the strength of the
resulting paper, as measured by tear index, burst index,
and tensile index, was higher for the inventive process
using never-dried pulp fibers. Further, the order of
addition of the two solutions is not important.
EXAMPLE 3
This example illustrates a comparison of
spruce CTMP (chemithermomechanical pulp) never-dried
pulp fibers filled by the inventive process or by
conventional techniques. The never-dried fibers were
initially beaten to 400 mL CSF. The inventive process
and the conventional process used to fill the fibers are
described in Example 1. Figures 8, 9, and l0 illustrate
the tear index, burst index, and tensile index,
respectively, of papers made from spruce CTMP never-
dried pulp fibers filled by the inventive process and by
the conventional technique. In each of the three
figures, the inventive process is illustrated by squares
and the conventional admixture process by circles.
A characteristic of spruce CTMP pulp is that
the tensile, burst, and tear indices decrease faster
3o with increasing ash contents (i.e., increasing filler
contents). For each strength parameter, the paper made
from never-dried pulp fibers filled by the inventive
process demonstrated increased strength as compared with
paper whose fibers were filled by conventional
techniques.
WO 91/01409 PCT/US90/04138
23
EXAMPLE 4
This example compares bagasse pulps derived
from sugarcane fibers comparing bleached and unbleached,
never-dried pulps filled by the inventive method to
bleached pulps that were once dried and filled by the
inventive method to conventionally loaded bleached
pulps. The processes used to make each paper and to
combine the fibers and the filler are described in
Example 1.
Figures 11, 12, and 13 illustrate the tear
index, burst index, and tensile index, respectively, of
each of the three types of paper. The squares
illustrate the inventive process, wherein the data from
paper made from bleached, never-dried pulp fibers are
indicated by filled-in squares and unbleached, never-
dried pulp fibers by open squares. The data from paper
made from never-dried bagasse fibers loaded by the
conventional process is illustrated by the triangles.
The data from papers made from bleached, never-dried
pulp fibers are shown by closed diamonds and unbleached,
never-dried pulp fibers by open diamonds. Paper made
from once-dried, bleached pulp and filled by the
inventive process is shown by the triangles.
As shown in Figures 11, 12, and 13, paper made
with never-dried bagasse pulp fibers filled by the
inventive process demonstrated superior strength
characteristics at each concentration of filler tested.
EXAMPLE 5
This example illustrates a comparison of paper
tensile strength characteristics when using fibers
filled by the inventive process with the lumen-loading
process as described in United States Patent No.
4,510,020, the disclosure of which is incorporated by
reference herein. Figure 14 illustrates the relative
decrease in tensile strength of paper expressed as a
percentage versus the filler content expressed as a
CA 02063567 2000-09-07
WO 91/01409 PCT/US90/04138
24
percentage with red alder never-dried pulps, bagasse never-dried pulps, and
spruce
CTMP never-dried pulps filled by the inventive process as compared with lumen-
loading techniques using softwoods, as derived from Miller et al. in
Proceedi~s 1983
TAPPI International Paper Physic Conference, Harwichport, p. 237 ("Miller et
al."),
and Green et al., Pulp & Paper Canada, 83:T203 (1982) ("Green et al.").
Larger amounts of filler were loaded within hardwood never-dried pulp
fibers using the inventive process when compared with Green et al.'s data for
softwoods
and similar amounts when compared with the Miller et al. softwoods. However,
it
should be noted that Miller et al. conducted their experiments with the
inclusion of 2%
PEI. PEI (polyethyleneimine) is a polycationic polymer which can form ionic
bonds
between the fibers in paper and acts to strengthen paper. PEI will function to
flocculate
the very fine filler particles within the lumen. The agglomeration of filler
particles into
larger masses improves the retention of filler inside the lumen, thus
minimizing
unloading mechanisms. We were able to achieve almost 40% filler loading with
bagasse never-dried pulps, but at the expense of mechanical properties. The
relative
decrease of tensile strength of the inventive process showed the same pattern
as the
Green et al. data with softwood fibers. Miller et al.'s attempt showed
encouraging
results, but the presence of 2% PEI may have added significantly to the
strength of the
resulting paper.
In Figure 14, the open squares indicate red alder never-dried pulps filled
by the inventive process, the open diamonds represent bagasse pulps filled by
the
inventive process, the filled circles represent spruce CTMP never-dried pulps
filled by
the inventive process, the closed triangles represent the data in Miller et
al., and the X
figures represent the data in Green et al.
WO 91/01409 PCT/US90/04138
., .,
EXAMPLE 6
This example illustrates how never-dried
eucalyptus pulp (a hardwood pulp) can be filled with
aluminum hydroxide ~ situ. Eucalyptus pulp was
dispersed in a first solution containing the soluble
salt aluminum sulfate. The first solution contained a
saturated concentration of aluminum sulfate at room
temperature. The first solution was removed after five
minutes by filtering the pulp. This also removes the
first solution from the pulp lumens.
A second solution containing 20% (w/v) sodium
hydroxide was used to disperse the pulp fibers. This
formed aluminum hydroxide precipitates' predominantly in
the cell wall of the fibers.
Paper was made from the fibers filled with
aluminum hydroxide filler. The amount of filler in the
paper was 9% as determined by ash content of A1203
(alumina).
EXAMPLE 7
This example illustrates the effect of beating
filled, never-dried fiber and the effect of different
beating conditions. Eucalyptus (hardwood) never-dried
pulp was filled with CaC03 by the inventive process as
described herein. The unbeaten, never-dried pulp had a
Canadian Standard Freeness (CSF) of 570 mL. A sample of
the filled, never-dried pulp fibers was first beaten for
10,000 revolutions in a PFI mill (beating apparatus).
The CSF value was 416 mL. The pulp was then formed into
a crude first sheet by filtration onto a wire screen.
The ash content of the first sheet was 43%. The pulp
was then redispersed in water and refiltered to form a
second sheet. The ash content of the second sheet was
38%. This process of redispersion and filtration was
repeated three more times. The ash contents of the
third, fourth, and fifth sheets were 34%, 36%, and 34%,
WO 91/01409 PCT/US90/04138
't ~ ~'~63 i6'~ 26
respectively. Thus, approximately only 7%-9% of the
filler was located outside the cell wall, even after
beating for 10,000 revolutions. That is, the filler
mainly stays in the cell wall during beating.
The entire procedure was repeated: except this
time the filled, never-dried pulp fibers were first
beaten for 20,000 revolutions, as described above. The
CSF value was 366 mL. The first filtered sheet had 46%
filler, the second sheet 41% filler, and the third sheet
38% filler. Thus, approximately only 8~ filler was
located outside of the cell wall even after beating for
20,000 revolutions.
Moreover, it is known that the pulp fibers
filled by the lumen-loading technique will lose most of
the filler upon beating. The inventive filling process,
by contrast, does not lose an excessive amount of the
filler upon beating.
From the foregoing, it will be appreciated
that, although specific embodiments of the invention
have been described herein for purposes of illustration,
various modification may be made without deviating from
the spirit and scope of the invention.
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