Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a method and
apparatus and more particularly, relates to a method and
apparatus to produce a relatively dense high quality paper.
In general, the manufacture of paper involves
allowing a dilute water suspension of wood pulp fibers to
flow onto a travelling open mesh wire screen through which a
large portion of the liquid passes. It is also known in the
art to flow the dilute water suspension of wood pulp fibers
between two such screens. Further moisture is often removed
through processing steps such as the application of vacuum
and/or by the application of pressure. Thus, one may pass
the wet web through nips formed by opposing rolls, i.e. wet
pressing. It is also known to further reduce the moisture
content of the paper web by passing the web over rotating
heated cylinders where, through evaporation, the moisture
content may be reduced to a figure of less than 15%.
In the paper-making process it is often desirable
or required to improve specific properties of the paper web.
Thus, a problem which has recently been encountered in the
art is that, with the introduction of high speed printing, a
higher quality paper is required. Specifically, a paper
having a higher density along with a relatively low moisture
content is a prerequisite for certain types of printing.
For example, when printing a sheet in multi-colour,
subsequent to the application of a first colour, the printed
sheet is passed through a drying step prior to the next
colour being applied thereto. Should the moisture of the
sheet entering the drying step be too high (e.g. above 8%),
the sheet will shrink during the drying step and will not be
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in registry for application of the next colour.
In other words, for certain printing
requirements, it is desirable to have a paper web with a high
density and low moisture content. In order to achieve the
higher density without crushing the paper web, it has been
proposed to increase the moisture when machine calendering so
as to facilitate a compacting step. However, this leads to
blackening/mottling of the paper and acts in opposition to
the desirability of having a low moisture content. In order
to achieve the desired results, one must operate a relatively
expensive process using a plurality of highly structured
steps.
It is an object of the present invention to
provide a method and apparatus for producing a paper web
which has a relatively high density and low moisture and
which paper web is suitable for multi-colour printing.
There is provided an improved method for
producing a high quality paper suitable for multi-colour
printing, which method includes the steps of preparing a pulp
furnish of fibers, forming a wet paper web from the furnish,
removing moisture, if necessary, from the wet paper web
soft calendering the web to increase substantially the web
density, and treating the web to reduce its moisture content.
There is also provided an improvement in a paper
making apparatus, which apparatus includes means for
preparing a pulp furnish of fibers, means for forming a wet
paper web from the furnish, means for removing moisture from
the wet paper web to a level of between about 55% to about
15%, the improvement comprising means for soft-calendering
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the paper web, while the paper web has a moisture content
above 15%, to increase substantially the density of the web.
As previously mentioned, in the paper-making
process it is often desirable to improve specific properties
of the paper web and thus, the paper web is often subjected
to a calendering operation wherein the web is passed through
successive nips formed between heavy rotating rolls in a
calender stack. In this respect, one may subject the dried
paper web to either a "hard" or "super" calendering step. A
hard calendering step operation comprises passing the paper
web between paired rolls, the surface of each being formed of
a hard non-resilient material. Super calendering (also
sometimes referred to as soft calendering), on the other
hand, takes pace between a pair of rolls wherein the surface
of one of the rolls is made of a hard non-resilient material
while the surface of the opposed roll is made of a firm
resilient material. The calendering operations can take
place "in line" with the other steps of the paper-making
process - in other words, along with the web forming,
pressing and drying steps and is thus referred to as on-
machine calendering. If the calendering is not done in line,it is referred to as off-machine calendering. As presently
practiced in the paper-making art, super calendering is
usually performed off-machine.
Other calendering and/or wet pressing operations
or similar type operations are also known in the art. Thus,
in situations where the wire or the dandy roll or the press
felt leaves marks on the wet web, it is sometimes desirable
to remove these marks while the web is highly plastic. In
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such an instance, a smoothing press is used immediately
before the drying section; the smoothing press consists of
two rolls similar to plain press rolls except that no felt is
used and the web is very lightly pressed between two very
hard surfaces. These surfaces are usually made from a
metallic or granite material and in some instances, a very
hard rubber material has been used. More recently especially
for groundwood paper a calendering operation within the drier
section itself is being used. The point at which the
calendering is done will depend on the paper properties
desired and this operation is referred to as breaker (stack)
calendering. This breaker calendering operation is performed
utilizing essentially the same equipment as that used for
machine calendering - i.e. hard calendering where the surface
of the nip rolls are both made of hard non-resilient
material. While the main function of breaker calendering is
to smooth out the sheet and level out any high spots, low nip
pressures have to be used to avoid sheet damage, mottling or
blackening, since the moisture content can be high and the
web weak.
As may be seen from the above, hard calendering
may be utilized both within the drier section and subsequent
to the drying step. Generally, breaker calendering utilizes
only very low nip pressures while machine calendering (hard
calendering) is utilized with moistures below 15~, and low to
high nip pressures. Super calendering, on the other hand, is
generally only utilized in the low moisture range,
usually off-machine. Other types of calendering, such as
gloss calendering or similar "finishing" operations, in which
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a substantial reduction in thickness/caliper or substantial
increases in density are NOT effected (because of various
problems e.g. "glossy"/mottle surfaces, etc.) also employ
soft calendering techniques, at moisture above 15%.
Commercial application usually involves paperboard grades.
This is known, for example, in U. S. Patent No, 3,124,504.
In the disclosure of this patent, it is taught that a nip
involving a hard metal surface and a hard resilient surface
may be utilized to impart certain surface characteristics to
an uncoated web. They do not teach any desirability of
achieving a substantial reduction in the thickness or thereby
an increase in the density of the web which is the aim of
applicant. Applicant, on the other hand, achieves a
substantial reduction. For gloss or similar calendering
operations a decrease in thickness ( or an increase in
density) of over 5-10% is considered a substantial decrease
(or increase) depending on the grade of paper or paperboard.
Furthermore, Mahoney et al do not give data to
support their process and inquiries indicate that there has
been no commercial success with this process. This lack of
commercial success is supported by Mihelich in his U.S.
Patent No. 3,759,785 where his soft calendering process could
only be applied to an uncoated paper (newsprint) in a
moisture range of up to 12 to 15% (lines 23-25, col.8.)
In view of the above, it is therefore important
to note that the terms "compacting/calendering/soft-
calendering/calenders" as used by applicant in the present
invention excludes those pressing/calendering operations that
involve the following: wet pressing/wet presses;
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smoothing/smoothing presses/calenders; finishing/~loss
calendering/gloss calenders.
The present invention, as previously mentioned,
utilizes a wet paper web which preferably has a moisture
content of between about 55% to about 15%. This wet paper
web may be furnished by conventional means - as in
conventional paper-making operations, a dilute water
suspension of the pulp fibers may be caused to flow onto a
travelling open mesh wire screen to permit removal of a
substantial portion of the water through the screen. Further
water may be removed by conventional steps such as the
application of vacuum or the use of press rolls or other
steps to partially dry the web to the desired moisture
content. If desired, the web may be additionally treated to
increase its integrity.
The web, while at a moisture content of between
about 55% to about 15% is subject to an on-machine soft-
calendering operation. Thus, the paper web, at the desired
moisture content is passed between a pair of rolls, one of
which is made of a hard non-resilient material such as a
metallic material and an opposed roll made of a firm
resilient material. While the materials of which the rolls
may be formed are known in the art and the terms "soft"
and/or "super" calendering are known to those knowledgeable
in the art, more specific aspects are discussed below. For
the purposes of the present invention, applicant will use
the term soft-calendering with the term super-calendering
intended to mean that calendering done at a relatively low
moisture level typically as one of the final finishing
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l~Q(~4~2
operations and frequently off line.
As mentioned above, there are a number of
variables/parameters involved in calendering, many of which
interact in a highly complex way and on which subject much
has been written. However, one parameter is of interest
here, namely the hardness and nature of the material of which
the resilient roll surface is made. There are those
materials, which are commonly referred to as "cotton filled",
others are referred to an "elastomeric". A particular
elastomeric material or calender roll which has been found to
be useful for the present invention is that made by Edouard
Kusters (West Germany) and sold by that company under the
trade Name of "MAT-ON-LINE". In general, the term soft-
calendering involves a roll which has a resilient roll
surface typically formed of an inner portion of a hard
(metallic) material with a softer material on the surface.
As aforementioned, the paper web may be subjected
to the soft-calendering step while preferably having a
moisture content of between about 55% to about 15%. While
applicant has found that this invention works within the
moisture range of about 55% to about 15%, the low moisture
limit is really that limit beyond which the prior art itself
found it could not work effectively without sheet blackening
taking place. The higher moisture level, on the other hand,
was found to be that level where no further moisture could be
extracted by the usual wet pressing operations; furthermore,
depending on prior compacting/drying steps, it was found to
be the point where the integrity/strength of the web was
sufficient for it to withstand a soft-calendering
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operation/step. This moisture level was also found to be
largely dependent on the nature of the web furnish. While
web adhesion to the rolls can be severe at these high
moistures, release agents/surfaces can be used effectively to
counteract this, otherwise it, too, can determine the high
moisture limit.
It is preferred however that the soft-calendering
be done while the moisture content of the paper web is
between about 20% to about 45%. The optimum specific
moisture content for any particular paper web will, however,
depend on many variables or parameters. Thus, one must taken
into account factors such as the machine speed, the nip
pressure load, the roll diameter, the number of nips through
which the web is passed, the calendering temperature, the
type of furnish used for the web itself, the nature and
hardness of the surface of the resilient roll, etc.
The temperature of the web entering the nip will
also depend on other factors. Thus, one could utilize steam
showers to increase the moisture and temperature of the web
up to near the boiling point of water and by utilizing a
compartmental steam box and varying the steam at various
locations, one could control the nip pressure profile of the
soft-calendering operation. However, one limiting factor for
the calendering temperature would be the type of material
utilized to create the firm resilient roll surface for the
soft-calendering step. When elastomeric materials are used,
too high a temperature would deleteriously affect the roll
surface/bonding between the elastomer and the metallic core.
Also, too high a temperature might tend to dry out the sheet
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while it is being processed. Nevertheless, by controlling
both the web temperature and the temperature of the surface
of each of the mating rolls, applicant found he could control
the overall soft calendering temperature. Thus, within the
temperature limitation of the resilient roll surface,
applicant found that by using higher calendering
temperatures, he was able to decrease the nip pressure
intensity/charge for a given compacting action or conversely
obtain greater compacting for a given nip charge, all in the
interest of a higher quality sheet.
Another embodiment that applicant found
advantageous, was to divide the soft calendering operation at
the higher moistures into two or more stages, with or without
inter-stage drying. For example, a light soft calendering
stage in the 45 to 55% moisture range where the density would
be increased by 10 to 20%, this would be followed by drying
the web to moisture within the 25 to 35% range, to be follow-
ed by a heavier soft calendering stage/step where the density
would be further increased. While infra-red drying can be
used to good effect in this embodiment, it is preferable to
use a drying technique where the web is held under pressure
while it dries, e.g. a dryer felt and cylinder combination.
Embodiments involving a combination of applicant's novel soft
calendering step with other prior art calendering steps are
disclosed below with the accompanying data.
Having thus generally described the invention,
references will be made to the accompanying drawings
illustrating the practice of the invention, and in which:-
Figures 1, 2 and 3 are graphs showing the
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relationship between accumulated applied nip pressure chargeversus sheet density according to various embodiments.
In a first test embodiment, a commercially
produced newsprint web was subjected to a super-calendering
step on pilot plant machinery. The web had an initial
moisture content of 9%; the paper web was subjected to a
plurality of passes through nips with varying nip pressures
as are set forth in Table 1 and plotted in Figure 1 (curve
A). A sample of the paper web was tested for various
properties as set forth hereinbelow.
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Susequently, a web of newsprint was removed from
a commercial paper machine at the breaker stack location at a
moisture of approximately 31~. By the time the web was
prepared for further processing on the pilot plant machinery,
the moisture had dropped to a range of 25-30~. This moist
paper web was then subjected to a soft calendering step
similar to the previous embodiment with the parameters and
results being set forth below in Table 2, and plotted in
Figure 1 (curve B).
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In the next test embodiment, the moist paper webwas again taken from a commercial paper machine at the
breaker stack location with the same moisture content as the
sample for Table 2. The sample was subjected to a soft
calendering operation at a moisture content of between 22 -
23%, air dried to approximately 9 - 10% moisture and then
subjected to a further soft calendering operation. The
results are given below in Table 3 and plotted in Figure 2.
Table 3
lO SAMPLE 00 0 2 3 4 5
SB Nips x pli 0 lx850 lx8502x850 lx850 2x850
(per nip)
S~l Nips x pli 0 2x8502x850 4x850 5x850
(per nip)
Acc. Nip 0 850 2550 34004250 5100
Charge (pli)
Caliper 108 71.5 64.1 62.061.4 62.0
(um)
Density 452 683 761 787 795 787
(grm/cm
PPS Felt (um) 2.76 2.26 2.48 2.59
PPS Wire (um) 2.90 2.39 2.46 2.56
SB = Soft calendering at a moisture above 15% (e.g. at the
Breaker Stack)
SM = Soft calendering at a moisture below 15~ (e.g. at the
Machine Stack~
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A further test embodiment was run similar to that
of Examples 2 and 3 again utilizing a web of newsprint from a
commercial paper machine at the breaker stack location. In
this embodiment, the paper web was subjected to an initial
soft calendering operation at a moisture content of 22 - 23%,
air dried to around 9 - 10% and then subjected to a final
hard calendering at the reduced moisture level. The result~
are shown in Table 4 below and plotted in Figure 3.
Table 4
SANPLE 00 0 1 2 3 4
SB Nips x pli 0 2x350 2x3502x350 2x500 2x500
(per nip)
HM Nips x pli 0 3x50 5x50 3x50 5x50
(per nip)
Acc Nip 0 700 850 9501150 1250
Charge (pli)
Caliper 108 77.8 73.1 72.0 70.6 69.9
(um)
Density 452 627 668 678691 698
~grm/cm
PPS Felt (um) 3.20 3.06
PPS Wire (um) 3.52 3.22
SB = Soft calendering at a moisture above 15% (e.g. at the
Breaker Stack)
HM = Hard calendering at a moisture below 15% (e.g. at the
Machine Stack)
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Referring to Figure 1, curve A indicates how
the density of the web increased as the web is subjected to
prior process of off-machine super calendering at a low
moisture content - approximately 9%. Curve B, on the other
hand, shows the increasing density of the web as it is
subjected to the process of the present invention - on-
machine soft calendering at a higher moisture content (25-
30%). In comparing curves A and B, it is evident that one
can obtain a higher density web more readily with the same
"accumulated nip charge" using the practice of the present
invention.
Thus, referring back to Table 1 (curve A) the
density for sample 11 represents a "standard news" sheet and
not that of a higher quality roto-gravure sheet which is
achieved in samples 5 to 9 in Table 2.
Figure 2 and Table 3 illustrates that one is
able to obtain a sheet with super quality roto grade
equivalent through the practice of the present invention
combined with super-calendering subsequent to the steps of
breaker stack soft-calendering and air drying. In this
process the density was increased about 80%.
Figure 3 and Table 4 illustrates that one can
obtain a good quality roto sheet with the combination of
soft-calendering at a higher moisture, air drying, and then
hard calendering with several light nips. In general, the
use of the soft-calendering at higher moistures reduces
substantially the number of nips/the magnitude of nip
pressure that is required for the final calendering of the
sheet. Since calendering, especially hard calendering, can
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be a damaging and expensive operation, this is extremely
useful.
Referring back to Tables 3 and 4, comparing the
results of super calendering versus hard calendering (both at
low moistures) after soft calendering at higher moistures by
applying an ink film with a special draw bar to the samples,
it was found that the hard calendering still had a greater
mottling propensity than that for the soft calendering which
showed no mottling whatsoever. Thus, while prior art
processes could produce a high density paper, mottling or
"galvanizing" still plagued these attempts. The present
invention, it is clear, eliminated this mottling effect which
made high quality printing very difficult if not impossible.
In addition, the density profile is more uniform and the
surface more "flat".
The apparatus for soft calendering is well
known in the art and thus, reference may be made to U.S.
Patents 3,365,774 and 4,256,034 to Kusters and 3,124,504 to
Mahoney et al as examples of a suitable type of apparatus.
While the above examples have dealt with newsprint grades of
paper, other grades of paper and paperboard, covering a wide
caliper or thickness range, could equally well be used, when
a higher density is required together with acceptable
printing characteristics.
As will be seen from the above, in the practice
of the present invention, a substantial increase in the
density is achieved using a soft-calendering operation. The
web "remembers" the previous calendering operation to which
it has been subjected and it has been found that it is the
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total nip pressure to which the web has been subjected
throughout the soft-calendering which is extremely important.
It will also be apparent that other changes and
modifications may be made to the above described specific
embodiments without departing from the spirit and scope of
the invention.
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