Note: Descriptions are shown in the official language in which they were submitted.
CA 02417905 2002-12-23
COATED PAPER AND PROCESS FOR PRODUCING SAME
TECHNICAL FIELD
The present invention relates to a coated printing paper, made
predominantly from bleached thermomechanical pulp, that has high gloss,
brightness, and opacity that is comparable or superior to coated papers
that are produced primarily from chemical pulp, and a process for
manufacturing same. More particularly, the invention is concerned with the
provision of a paper furnish based on bleached thermomechanical pulp that
is used to produce a coated paper of good quality and at a reasonable cost.
BACKGROUND ART
Printing papers are produced in a number of grades, either coated or
uncoated, dictated by the end use for the paper. The important
appearance properties are opacity (see through), brightness (how reflective
the paper is at a particular wavelength, typically 457 nm), and gloss. As
opacity, brightness, and gloss increase, the quality (and price) of the paper
increases. Newsprint and directory papers typically have the lowest
brightness and negligible gloss whereas a Number 1 publication grade (for
say a glossy annual report) would have the highest brightness and gloss.
Table 1 herein below presents the range of values for some typical printing
grades. In the table %Mech and %Chem refer to the percentage of
mechanical pulp and chemical pulp, respectively, in the papermaking
furnish. The letters SC stand for supercalendered in grades C, B, or A, the
latter differing by the amount of filler present. C5 stands for coated paper
grade 5, etc., and LWC stands for lightweight coated. (The terminology, C1
- C5, is generally used in North America, whereas LWC is used
elsewhere). See John D. Peel, "Paper Science and Paper Manufacture".
Angus Wilde Publications Inc., Vancouver, 1999.
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CA 02417905 2002-12-23
Table 1. Some printing grades
Furnish FillerGrammage BrightnessOpacityGloss Smoothness
%Mech., % glm2 /S0 %TAPPI %HunterPPS=10S
%Chem Nm
Newsprint70-100, 0-16 40-48.8 57-60 90-94 -- 2.6-4.2
30-0
Catalogs
SCC 30-80, 0-7 45-90 65-85 85-92 25 1.7-2.6
70-20
SCB 30-80, 7-15 45-60 65-85 85-92 35 1.5-1.9
70-20
SCA 30-80, 16- 45-60 65-85 85-92 46- 1.1-1.4
70-20
30
Coated
Papers
C5(LWC)45-70, 4-15 42-80 68-75 85-92 50-58 0.9-1.9
55-30
C4 50, 50 4-12 50-70 72-78 90-94 60-65 1.3-1.6
Invention65-85, 10- 50-82 75-84 >90 60-75 0.9-1.5
15-35
20
C3 0-10, 10- 75-150 76-82 90-95 63-72 0.8-1.4
100-90
20
C2 0, 100 ~20 78-82 95-98 to 80 0.8-1.4
C1 0, 100 ~20 83-88 95-98 to 90 0.8-1.4
Brightness; as defined in the paper industry, is the reflectivity of the paper
at a particular wavelength, typically 457 nm. The reflectivity, in turn, is
the
ability of the paper to scatter light from the air-fiber or air-pigment
interfaces
within the paper web, usually defined in terms of a light scattering
coefficient. A "white" paper is one that would equally reflect or scatter all
the wavelengths in the visible spectrum, and not absorb any wavelengths.
Thus paper brightness is achieved by bleaching the pulp to remove
chromophores that may absorb light at some wavelength, adding more
fiber to create more air-fiber interfaces, and/or adding high brightness
fillers
that scatter the light at all wavelengths in the visible part of the spectrum.
Many people take brightness to be the "whiteness" of the paper, and while
this is not technically correct, higher brightness is generally perceived as
better quality.
Gloss is the ability to reflect light at a particular, specular angle and
higher gloss is generally related to higher quality. Gloss is thus an
indicator
of surface smoothness. Newsprint, which is not coated, may have a
brightness of less than 62° ISO and a gloss of less than 12% Hunter,
2
CA 02417905 2002-12-23
whereas a high quality coated printed grade (C1 ) will have a brightness
greater than 87° ISO and a gloss exceeding 75%.
Opacity is the property of not being able to see through the paper.
Paper that is not too thin appears opaque because, as noted above, light is
scattered at the air-fiber interfaces and all of the light cannot pass through
the sheet. In fact "printing opacity" is defined as the ratio of the
reflectance
for a given paper with a backing of zero reflectance to the reflectance of the
same paper if it were infinitely thick. Opacity can be achieved by using
mechanical pulps that contain a lot of "fines" or particulate material that
increase the surface area available for light scattering, by increasing the
amount of fiber, by minimizing refining of the chemical pulp(s), by adding
inorganic filler materials that will scatter light, and avoiding densification
of
the paper web by wet pressing or calendering during manufacture.
Depending on the grade quality, however, not all of the above ways to
control brightness, opacity, or gloss can be used: Often, an action made to
enhance one paper property is detrimental to one of the other sheet
properties. For example, calendering a paper made from chemical pulps to
enhance gloss may decrease opacity due to the densification of the paper
web. As with all engineered papers, there will be tradeoffs between raw
materials and paper machine operating variables to produce a product with
the desired convertibility or end use performance criteria. Occasionally, a
new approach is discovered that creates a step change that allows one to
produce a new superior product or the same quality product for much lower
cost.
In North America there are five coated paper grades that are
defined, with "coated #1", or C1, being the highest total quality and cost
(for
example, a glossy annual report) and C5 the lowest quality and cost (for
example, used in directories or catalogs). In this case, total quality
includes
the appearance properties (brightness, opacity, and gloss) as well as other
desired attributes such as printability, tearing strength, internal bond
strength, tensile strength or bending stiffness (see Table 1 ). Of the total
3
CA 02417905 2002-12-23
coated paper market, the two largest segments in volume are C3 (for
example, advertising inserts) at 31 % and C5 at 41 %. For comparison C1,
C2, and C4 grades comprise 5%, 10%, and 13% of the market
respectively. The difference in quality (and cost) for each of these grades
is achieved by the use of different fiber types, pulping andlor bleaching
methods, coating technology and type of calendering. For example, with
reference to Table 1, the top three grades (C1-C3) are generally made from
bleached chemical kraft pulps (less that 10% mechanical pulp) and are
coated once, twice or even three times on each surface. These are often
referred to as coated free (meaning no mechanical pulp is used) sheets,
CFS. Grades C4 and C5 are primarily made from approximately equal
blends of bleached groundwood and bleached chemical pulps and are
referred to collectively as coated groundwood, CGW.
Because they are used in products that often go through the mail,
the C4 and C5 grades are the lowest basis weight grades, and in this case
there can be significant challenges in meeting brightness and gloss targets
while maintaining opacity. Mechanical pulps are particularly suited for
these grades because the fine or particulate material created during fiber
separation and processing provides a large surface area and many air-fiber
interfaces to scatter light. Accordingly, there are a variety of fiber types
and
manufacturing methods employed within each of the grades to achieve the
desired appearance and other properties.
A layer of coating may be applied to the paper in a variety of ways.
Each controls the amount of coating formulation (coating color) that is
applied to the base paper to produce a uniform film, and the coater designs
are classified according to how they meter the coating onto the paper.
Transfer roll coaters use a sequence of rolls to produce a uniform film of
coating similar to a gate roll size press, but these are not often used for
pigmented coatings. Blade coafers, the most widely used, deposit an
excess of coating formulation on the substrate and the excess is metered
off with a trailing blade. There are a number of variations in blade coating
4
CA 02417905 2002-12-23
technology. Rod coaters are similar to blade coaters except that a rotating
wound wire or threaded rod is used to doctor the excess coating off the
surface. Airknife coaters are also similar in that an excess of coating is
applied but in this case the excess is doctored off by a jet of air leaving
the
desired amount of coating remaining on the paper. Airknife coaters are
quite different from the other coater types in that the coating layer tends to
uniform in thickness over the undulations in the surface rather than being
thinner at the high spots in the paper. Film presses are used with
pigmentized coatings to apply low coat weights (4-10 g/mZ per side) at high
l0 speeds. Spray coaters may also be used in applying a coating.
A supercalender is built of a vertical stack of alternating resilient and
non-resilient rolls with the paper passing sequentially through the nips
defined by adjacent rolls. The non-resilient rolls are typically highly
polished, smooth, steel rolls and the resilient rolls are typically cotton or
polymer filled. During operation, the steel rolls are heated to some desired
temperature and a load is applied to the top roll in the stack so that a
pressure and high temperature is applied to the paper as it passes through
each nip. In doing so, a gloss is imparted to the surface of the paper in
each nip that is in contact with the smooth non-resilient roll.
Supercalendering is performed to produce the desirable glossy surface on
printing papers. The ability to achieve gloss depends upon the nature of
the paper surface and base paper, the speed of operation of the
supercalender, the temperature of the steel rolls and the applied pressure
on the stack.
Supercalendering is normally carried out off line as a separate
operation. Since the end of the 1990's, however, there has been a
tendency to supercalender in-line whenever possible in order to lower
costs. If it is possible to match the supercalender operating speed with the
speed of the papermachine, while getting the same paper quality, one
could expect some efficiencies since the paper would not have to be dried,
wound up on a reel, and then moved to a different location in the mill for
5
CA 02417905 2002-12-23
superealendering. In addition, if the web needs to be re-moistened in some
fashion prior to supercalendering to facilitate gloss development, as is often
the case, there could be advantages by simply placing the supercalender
right at the end of the papermachine and coater and eliminate the
moistening step. Placing the supercalender in-line, however, will require
careful control of the papermaking, coating, and supercalendering
operations, because a problem in any one of these will stop the entire
production in the mill. Thus, sophisticated process control measures are
required to make this viable.
While the prior art generally discloses the production of C4 or C3
coated printing papers starting from a furnish having a fiber content of
about 50 weight percent TMP and about 50 weight percent chemical pulp
that also contains about 12 weight percent of inorganic filler, it does not
describe nor suggest the possibility of using bleached thermomechanical
pulp especially high levels thereof such as up to 80 weight percent TMP
(fiber content), in such grades so as to produce C4 or C3 coated printing
papers. it is speculated that such bleached thermomechanical pulps have
not been used in such grades because of poor bonding (strength)
properties, optical properties or, energy cost to meet quality requirements.
As used in the present specification and claims, the term bleached
thermomechanical pulp is intended to mean a pulp having properties as
defined in tables 2 and 3 hereinbelow. The thermomechanical pulp should
also have a freeness below 50 ml Csf and a brightness over 75° ISO.
It is an object of the present invention to provide a coated paper of
the same or superior quality and at a much lower cost than the usual
coated paper of the prior art:
It is another object of the invention to provide a step change in the
conventional process of producing a coated paper that allows to produce a
new superior product or of the same quality at much lower cost.
It is another object of the present invention to provide a paper of
similar quality in certain respects to typical coated papers that are made
6
CA 02417905 2002-12-23
from 100°/a bleached kraft pulp, by using a paper furnish containing a
bleached thermomechanical pulp.
DISCLOSURE OF INVENTION
The above and other objects of the invention may be achieved by
providing a process for producing a coated paper comprising the steps of:
a. providing a paper furnish;
b. forming the paper furnish into a consolidated web on a paper
machine;
c. partially drying the consolidated web under conditions to form a
base paper;
d. coating the base paper with using a film coating technology
equipment;
e. drying the coated base paper; and
f, supercalendering the coated base paper;
characterized in that the said paper furnish has a sheet fiber content made
of about 60 to 85 weight percent of a bleached thermomechanical pulp
(TMP), about 10 to 35 weight percent of a bleached chemical pulp and, 0 to
about 15 weight percent of a deinked pulp, said paper furnish also
comprising about 12 to 20 weight percent of an inorganic filler, said
bleached thermomechanical pulp having a freeness below 50 ml Csf and a
brightness over 75° ISO.
The invention also relates to a coated paper of basis weight 50 to
82 glm2 containing a mixture of bleached thermomechanical pulp, bleached
chemical pulp, optionally a deinked pulp, and an inorganic filler, in the
percent ranges mentioned above, and having a PPS10-S smoothness less
7
CA 02417905 2002-12-23
than 1.5 pm, preferably less than 1.3 Nm, a TAPPI opacity exceeding 90%,
a brightness exceeding 75° ISO and a Hunter gloss exceeding 65%,
preferably 70%.
MODES OF CARRYING OUT THE INVENTION
The preferred inorganic filler according to the invention may be
ground calcium carbonate, precipitated calcium carbonate, calcined clay or
the likes.
The bleached thermomechanical pulp preferably has a pulp
brightness of about 80° ISO, a TAPPI opacity exceeding 90%, a
scattering
coefficient exceeding 49 m2/kg and, a light absorption coefficient less than
about 1.2 m2/kg.
The present invention makes a significant advancement over the C4
and C3 grades shown in Table 1 in that it produces a sheet that is
comparable or superior to a C4 coated grade and comparable to a C3
coated grade in the lower basis weight range, but preferably uses a
significantly higher proportion of mechanical fiber in the form of a bleached
thermomechanical pulp (TMP) concomitantly with significantly lower
proportions of bleached chemical pulps: The preferred furnish composition
is more closely aligned with a typical SC furnish, a supercalendered but
uncoated grade. For example, for SC grades the furnish is typically
between 30 - 80% mechanical pulp and 70 - 20% chemical pulp. For C4
grades the mechanical pulp would typically be 50% of the fibrous part of
the furnish and the chemical pulp fraction 50%. For a higher quality C3
grade the ratio of mechanical to chemical pulp would typically be 10190 or
0/100. In the present invention, with quality factors comparable to C3
grades, the preferred ratio of mechanical pulp to chemical pulp is 80120.
That can offer a considerable advantage in cost savings.
The bleached TMP is normally obtained from northern spruce and fir
softwood material and optionally, from birch hardwood. The present
8
CA 02417905 2002-12-23
invention preferably uses a thermomechanical pulp made from a blend of
fir, spruce and potentially birch woods, all of which are readily reduced to
fibrous form during pressurized refining and have good color and good
strength. Average fiber length for either softwood species is around 3 mm
with approximately 4 million fibers per gram. The average fiber length for
birch would be around 1.5 - 1.8 mm with approximately 10 million fibers
per gram. The thermomechanical pulp is preferably bleached with
hydrogen peroxide to a brightnesss of t80° ISO, for example between
75°
and 85° ISO, preferably about 80° - 81 ° ISO. In
addition, to the elimination
of the chromophores that cause color, hydrogen peroxide bleaching of the
TMP also increases the number of carboxylic acid groups on the fiber
surface by up to 100%, from, say, 95 to 200 mmol/kg (compared to 10 - 25
mmol/kg for a bleached sulfate chemical pulp}. The increase in carboxylic
acid groups, especially on the surface of the fibrous material, is known to
enhance pulp strength properties (Zhang, Y. et al., J. Wood Chem. & Tech.
1994, 14(1 ); 83-102). Bleached thermomechanical pulp obtained by
treatment with hydrogen peroxide also enables to reduce the specific light
absorption coefficient to less than 1.2 m2/kg, and to increase the specific
tensile strength of the pulp by 9 to 15%. For example, the bleached
thermomechanical pulp may comprise about 60 to 75 weight percent
balsam or eastern fir (Abies balsama) and about 40 .to 25 weight percent
Canadian spruce (Picea glauca), eastern spruce (Picea rubens), a black
spruce (Picea mariana), any of which may be substituted with about 0 to 15
weight percent paper birch (Betula papyrifera) andlor yellow birch (Betula
alleghaniensis) or any other members of the Betulaceae family. The
bleached chemical pulp is preferably bleached chemical softwood.
9
CA 02417905 2002-12-23
According to the present invention, the preferred thermomechanical
pulp has the following Bauer McNett fiber distribution:
Mesh < 16 3 - 6
Mesh 16 - 30 20 - 24
Mesh 30 -100 25 -31
Mesh 100 - 200 8 -10
Mesh > 200 33 - 38
thereby providing both high opacity and low porosity as well as high internal
bonding to minimize blistering in the final coated.paper.
The furnish can be formed into a web using a dauble wire, such as a
SynFormer NP, forming device, or any other appropriate device.
Preferably, the web is partially dried to a moisture content of less than 3.0%
prior to pre-calendering andlor coating.
The consolidation of the base paper is preferably accomplished
according to normal practice using a double wire forming section. The
target basis weight in this case is around 34. to 54 g/m2. After drying the
web to the proper moisture content, the base paper is pre-calendered, if
necessary, using a hard nip calender with steel rolls to provide a uniform
thickness to the base paper as it passes into the in-line or off-line
coater(s).
The desired range of Bendtsen roughness is preferably from 200 - 250
mllmin.
Coating is normally carried out by spreading equally about 25 to 30
weight percent coating material over both sides of,the base paper by using
a single coating of about ~6.5 to 10.0 g/m2 on each side of the base paper.
Alternately, coating may achieved by spreading equally about 25 to 40
weight percent coating over each sides of the base paper by using multiple
coatings totaling about 6.5 to 17.0 glm2 on each side of the base paper.
CA 02417905 2002-12-23
Although any suitable coating procedure may be used, according to the
present invention, it is preferred use a film coater, such as a transfer roll
coater, a film press, a rod water, a spray coater or the likes.
Using an on-line or off-line film coater, a single film coating may be
simultaneously applied to both sides of the base sheet or, with two such
units providing two separate coating operations, two coatings may be
applied to each side of the base sheet. Target coat weight is usually 6-16
glm2.
The coating formulation used herein is engineered to be compatible
with the on-line or off-line calendering operation and can include any or all
of GCC, clay, hollow sphere plastic pigments, glossing clay or binder.
The coated base paper is usually dried to a predetermined and
controlled optimum moisture content to yield the optimal desired and end
use perto~mance. Preferably, the coated base paper is dried to a moisture
content of about 7.5 to 8.5% prior to supercalendering.
Supercalende~ing of the coated paper base may be carried out on-
line or off-line, and is normally achieved under conditions to produce a
coated paper having a grammage of about 50 to 82 glm2 and a Hunter 75°
gloss over 70%, an ISO brightness over 80° and a TAPPI opacity over
90%. The use of an on-line supercalender immediately following the
coating dryers, rather than supercalendering off machine in a separate
operation, is advantageous from both a time and efficiency standpoint as
noted earlier. It eliminates the handling of the paper web that would be
required in an off tine supercalender operation and also allows the coated
paper web moisture content to be controlled going into the supercalender,
potentially minimizing the creation of internal stresses that could cause
other problems in the paper during printing or end use. One such problem
that sometimes arises with the use of mechanical pulps (or coarse
chemical pulp fibers) in coated papers is the ability of a pulp fiber that has
been collapsed during supercalendering to snap back to its previously
11
CA 02417905 2002-12-23
unco0apsed state when exposed to moisture and heat, e.g. during heatset
offset printing. It is believed that mechanical pulp fibers in the coated base
paper that pass directly into the supercalender at the proper moisture
content will lose some or all of this propensity to regain their former
uncollapsed state.
In the . current invention, for manufacturing cost and quality
considerations, it is preferred to place the supercalender in-line with the
paper machine and coater, thus avoiding unnecessary handling of the
paper and giving the papermaker a considerable amount of flexibility as to
the moisture content of the paper as it enters the plurality of resilient and
non-resilient rolls of the supercalender. It is believed that entering the
nips
at the proper moisture will help to stabilize collapsed fibers so that they
are
not prone to "pop open", as is often the case, when they encounter a hot
and moist environment at some later time, e.g. in heatset offset printing.
Supercalendering preferably immediately follows the drying step with
roll temperatures of about 140 - 150°C under an applied load of about
200
to 400 kNlm, under conditions to achieve a gloss of about 60 to 75% and a
surtace smoothness less than 1.5 pm PPS10, preferably less than 1.3 Nm
PPS10. Usually an eight to ten roll supercalender is used for this operation.
By virtue of the pulping method according to the invention, bleached
thermomechanical provides a large amount of fines that can scatter light,
and thus assist in achieving opacity and brightness.
According to the present invention, the inorganic filler, such as
ground calcium carbonate, GCC, that is present in the papermaking furnish
helps provide brightness, opacity, and gloss in the final product. This filler
must have high brightness itself and be present in sufficient amounts to
yield the desired effects, without disrupting bonding of the fibers in the
base
sheet. The desired range of high brightness filler in the papermaking
furnish is 12 to 20% of the total furnish, as mentioned above.
The present invention uses high quality bleached thermomechanical
' pulps in the base paper, as opposed to the use of bleached chemical pulps,
12
CA 02417905 2002-12-23
which are normally used in large quantities in C4 and C3 printed. paper
grades.
EXAMPLES
The following is a detailed description of certain embodiments of the
invention, given in the form of examples, deemed by the inventors to be the
best manner of implementing the invention.
A major consideration in the implementation of the invention is the
use of high quality and low cost bleached mechanical pulps in the base
paper, as opposed to the use of bleached chemical pulps, which are
normally used in C4 and C3 printed paper grades. In the present
embodiment, a bleached thermomechanicat pulp was used.
Example 1. The wood species utilized and the preparation of the pulps is
described here. Two separate blends were studied: a blend of 75%
Balsam fir and 25% White spruce (75/25) and a blend of 70% Balsam fir,
20% White spruce and 10% White birch. For either blend the wood chips
were preconditioned in water followed by three stages of pressurized
refining using typical processing conditions and followed by screening,
cleaning and rejects treatment. The pulps were then bleached using two-
stage peroxide bleaching at typical conditions. Handsheets were prepared
according to TAPPI standards using recirculated white water. Table 2
presents a summary of the pulp properties and Table 3 presents the paper
properties for the two blends.
13
CA 02417905 2002-12-23
Table 2: Final pulp properties
Unbl eachedulps Ble achedpulps
p
JVood species : Balsam Fir l White7012011075/25 75/25 7012011075/2575125
Spruce l White Birch
Screening I Cleaning S S S & S S S & C
C
Pulp properties
Units
Energy consumption kWhIBDMT 3963 3869 4005 3963 38694005
Freeness CSF MI 32 28 15 26 26 12
Shives, Somerville 0,15 mm % 0.03 0.03 0.02 0.02 0.010.01
Shives , PQM 1000 !0 0.01 0.08 0 0.01 0.020.00
Fiber length, PQM 1000 mm 1.23 1.27 1.2 1.21 1.251.22
l
Bauer McNett, mesh
>16 ~6 4.6 5.6 5.9 3.8 3.7 6
16 - 30 % 20 21.3 22 21.5 23.521.4
30 - 10Q % 31 28.6 25.4 30.1 30.325.9
100 - 200 % 8.4 8.6 9.6 8.8 9.1 9.4
< 200 % 36 35.9 37.1 35.9 33.437.4
ISO Brightness 8457 55.5 56.1 55.3 75.4 79.779.0
14
CA 02417905 2002-12-23
Table 3: Paper properties for final pulp
Unbleached ulpsBl eached pulps
p
good species : Balsam Fir I White70/2011075!2575/2570/2011075/2575!25
Spruce I White Birch
creeping J Cleaning S S S S S S & C
&
C
aper properties
Units
ensity klmg 534 562 615 612 637 684
gar index mNmZlg 5.42 5.234.665.04 4.504.60
urst index kPamslg 2.84 3.273.553.07 3.323.60
ensile index Nm/g 52.6 56.558.754.5 63.866.3
oughness mUmin 85 51 26 49 54 28
orosity mllmin 20 14 8 12 52 5
colt bond JIm2 249 264 335 278 194 285
pacity (TAPPI) % 99.1 98.898.890.7 90.789.4
pec. light scattering coeff. m2lkgfi3.660.961.852.3 51.649.0
)
pec. light abs. coeff. (K) m2lkg 7.6 6.7 7 1.2 1.2 1.0
Referring to Table 2, we note that the fiber lengths for either blend
are very similar, ranging from 1.2 to 1:3 mm for _ both the unbleached and
bleached pulps. Table 2 also illustrates the effectiveness of the two-stage
peroxide bleaching. The 70120110 blend brightness increases from 55.5 to
75.4, almost 20 points, while the- 75/25 blend goes from 56.1 to 79.7 or
23.6 points. The screened and cleaned (S&C) 75/25 blend increased 23.7
points. Table 3 shows the impact of bleaching on tensile strength on the
pulps. The tensile index of the 70120110, 75125, and 75/25 S&C blends
increase by 3.6%, 12.9%, and 12.9% respectively.
CA 02417905 2002-12-23
This is indicative of the carboxylic acid groups created during peroxide
bleaching as noted earlier. The peroxide bleaching not only removes the
light absorbing chromophores (as evidenced by the specific light absorption
coefficient, K, or the significant increase in brightness) but also
significantly
increases the tensile strength ofthe pulp. In fact, the tensile strength of
the
TMP described here is higher than most other mechanical pulps, including
aspen pressure groundwood. This is a reason that less bleached chemical
pulp will be required in the final base paper blend.
Example 2. A first trial was conducted to demonstrate that the proposed
concept was valid, namely that a high content bleached thermomechanical
pulp could replace bleached chemical pulp in C4 and C3 grades, and that
the coating and supercalendering operations could be carried out in-line
with the papermachine. Two different papermaking furnishes were utilized
in these trials. The first furnish, A, consisted of 59.0% bleached TMP,
26.6% bleached kraft (chemical) pulp and 14.4% GCC. The second
furnish, B, consisted of 64.5% bleached TMP, 16.1% bleached kraft pulp
and 19.4% GCC. In either case, the wood used for the bleached TMP was
a blend of 75% Balsam fir and 25% White spruce.
The TMP was processed from newsprint cull rolls and the kraft
purchased in the marketplace. The TMP pulp was separated and
processed in four batches. The cull rolls were slushed and the pulp first
refined at high consistency in an atmospheric Sunds Defibrator RG32136
with plates 9811 B to reduce CSF and chive content and increase pulp
strength. The pulp was then screened in a 2-stage MUST screen with
#0.10 mm slot baskets. The screen accepts were thickened to about 3%
consistency and bleached using sodium hydrosulfite. Rejects were refined
at high consistency in two stages, thickened and bleached as for the
accepts. The reject line accepts and main line accepts were then
combined. Three of the final TMP pulps had a CSF (freeness) of 35-36 ml,
low shive content, length weighted fiber length L(1) of 1.3 mm, and tensile
16
CA 02417905 2002-12-23
index, tear index, and iS0 brightness of 55 Nm/g, 5.2-5.3 mNm2lg, and 77-
78%, respectively. One batch of TMP was slightly lower in all properties
having L(I), tensile index, tear index, and ISO brightness of 1.15mm,
53Nm/g, 4.9mNm2/g, and 75°, respectively.
The bleached kraft chemical pulp was refined to a CSF of 320 (from
670) and had a tensile index and tear index of 95-101 Nmlg and 11.9-11.5
mNm2lg, respectively. The TMP and BKP were then blended into batches
having TMPIBKP ratios of 70/30 or 80120.
The filler was then added to the pulp batches in the percentages
specified in the first paragraph of this example. The filler was Omya GCC
with 95°!° 1S0 brightness and a Percol 47 retention aid system
was used.
The base paper was produced at 800 m/min using a SymFlo
headbox with short vanes and .a SymFormer MB. The press section
consisted of two straight through nips, -the first a double felted shoe press
and the second nip a single felted roll press with a transfer belt on the
topside. The target basis weight was 43.611 g/m2 (BD). In this pilot trial
the rolls were shipped to another location to be dried.
Pre-calendering of the base paper was carried out with an OptiHard
hard nip calender at load of 60 kNlm and 70° C. The smoother topside of
the paper was calendered against the hot roll. The target Bendtsen
roughness level was 200 - 250 mllmin, which was achieved with the
specified loading. The caliper and ISO brightness of the "A" paper was 78
wm and 76.5°, respectively, and the "B" paper caliper and ISO
brightness
was 77 ~m and 73.7°, respectively.
The two base papers were coated using an OptiSizer station,
treating both sides of the paper simultaneously with 10 g/m2. The top rod
diameter was 12 mm with a hardness of 40P&G and the lower rod diameter
was 15 mm with a hardness of 32 P&G. The nip pressure was 20 kNlm.
The coating formulation is presented in Table 4.
17
CA 02417905 2002-12-23
Table 4. Coating forr»ulation for the simulated an-line trials
Material Trade Name Parts
CaC03 HC90 60
Kaolin clay Hydragloss 40
SB-latex DL966 12
CMC FF10 0.3
PVA Moviol 6-98 0.5
Stearate Nopcote C 0.8
OBA Blankophor P 0.8
To achieve the trial target coat weight of 10 glm2 per side, the solids
content was adjusted to 66% for the bottom beam and 65% for the top
beam.
Supercalendering was done with an OptiLoad-8 at speeds of 1000 -
1200 m/min. Following a calibration procedure, the "A" coated paper was
calendered at 145°C (temperature of first 3 thermo rolls) with a linear
load
of 300 kNlm at 1000 mlmin while the "B" paper was calendered at
140/145°C with a load of 250 kN/m and speeds of either 1000 mlmin or
1200 m/min. The first trial was conducted to demonstrate that it was
possible to use a high thermomechanical pulp content, a concomitant low
kraft content, a much higher filler content in the base sheet and to run the
paper through a film coater. Table 5 presents a summary the results.
In summary, the simulated on-line trial showed that the use of
bleached TMP together with the in-line concept could be utilized to produce
a C4 grade with gloss, opacity, and ISO brightness exceeding 60°,
88°, and
78°, respectively. Final paper quality parameters were adjusted during
the
second set of trials by varying different settings.
Examine 3. Three separate trials were conducted to determine the paper
gloss potential between different coating formulations with and without
plastic pigments, and also to compare gloss potential with single and
double film coating operations. Each trial was carried out at a different
18
CA 02417905 2002-12-23
location. The base paper for trials one and two was a commercial SCA
paper very similar to the preferred base paper described in Example 1. It
had a high content of TMP, a high filler content, and a low kraft chemical
pulp content. The paper had a basis weight of 49.2 glm2 and an ISO
brightness of 73°. The SCA rolls were pre-calendered. The third trial
used
a commercial CGW base sheet that had a high content of bleached TMP
but typical levels of chemical pulp and filler. The CGW base paper had a
basis weight of 56:2 glm2 and an ISO brightness of 78°.
19
CA 02417905 2002-12-23
Table 5: Summary of the supercalendered paper trials
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The first trial used a Beloit PMSP film coater running at 1190 mlmin
with either a single coating on both sides (1 C2S) or two coatings on both
sides (2C2S). The coating formulation was typical but included Omya GCC
HC90 and Covercarb HP, tmerys Brazilian clays Capim DG, and a high
content of glossing clay. Some formulations included hollow sphere plastic
pigments HS3000NA.
The second trial utilized a Valmet OptiSizer MSP film coater at 1200
mlmin also in 1C2S and 2C2S, configurations. In this case the coating
formulation included Omya GCC HC90 and Covercarb HP, Huber clays
Hydragloss and Covergloss, and Dow latex. Again some formulations
included HS3000NA hollow sphere plastic pigments.
The third trial used a Jagenberg FifmPress coater at 1200 mlmin in
1 C2S and 2C2S configurations. The coating formulation based on best
CA 02417905 2002-12-23
current practices included the same GCC and clays utilized in trial one,
solid sphere plastic pigments, and BASF latex and additives.
The coated papers from the three coating trials were
supercalendered using an Optiload calender with a ten-roll configuration
running at 1200 m/min. The linear loading and temperature calibrations
determined in Example 2 with the same coating formulation were utilized.
For the SCA base paper the loading was 200 kNlm and for the CGW base
paper 250 kNlm was used. The coated papers were not remoistened and
no steam showers were employed.
Summaries of the rarige's of all of the results of the three trials are
presented in Table 6. For the 1 C2S samples the coat weights varied from
6.5 to 10 g/m2 per side with a final paper basis weight range from 50.3 to
66.6 glm2 and for the 2C2S samples the coat weights were 12 to 17 g/m2
per side with a final paper basis weight range from 66.6 to 81.4 g/m2.
These results demonstrate that by varying the appropriate parameters, the
desired final quality targets of the present invention can be reached.
Table 6: Coating trial results
Trial1 Trial2 Trial3
Base sheet brightness, 73.0 73.0 78.4
Brightness after coating, 79.0-80.6 79.8-81.5 86.1-87.8
Brightness after calendering,74.3-75.9 76.0-79.0 83.8-85.5
TAPPI opacity, % 93.7-95.1 93.8-95.4 91.2-91.9
Hunter 75 gloss, ! 66.6-77.6 60.2-70.6 53.5-65.7
PPS 10-S smoothness, p,m 0.86-1.00 0.92-1.22 1.42-1.78
,
Considering that the base papers were coated either 1 C2S or 2C2S
with differing coat weights, and the coating formulations were varied within
each trial, we can draw the following general conclusions.
21
CA 02417905 2002-12-23
~ PPS smoothness levels increase with higher chemical pulp content
and lower filler levels in the base paper.
~ PPS smoothness levels around 1.0 p,m can be achieved with a base
paper containing high levels of bleached TMP and high levels of
filler.
~ A 70% Hunter gloss can be achieved with a single coating on both
sides with about 5 parts of hollow sphere plastic pigments and high
glossing clay content.
~ A 75% Hunter gloss can be achieved with no or only a few parts of
hollow sphere plastic pigments and high glossing clay content with a
double film coating (2C2S).
It is understood that modifications are possible according to the
invention, provided they fall within the scope of the appended claims.
22
