Note: Descriptions are shown in the official language in which they were submitted.
CA 02617812 2008-01-11
WET EMBOSSED PAPERBOARD AND METHOD AND APPARATUS FOR
MANUFACTURING SAME
FIELD OF THE INVENTION
The invention relates to embossing paperboards. More precisely, it relates to
a wet
embossed paperboard and a method and an apparatus for manufacturing same.
DESCRIPTION OF THE PRIOR ART
Embossing is the process of creating a three-dimensional image or design in
paper and
other ductile materials. It is typically accomplished with a combination of
heat and
pressure on the paper. This is achieved by using a metal die (female) usually
made of
brass or stainless steel and a counter die (male) that fit together and
actually squeeze
the fibers of the paper. This pressure and a combination of heat actuated
"irons" raise
the level of the image higher than the substrate and make it smooth. This can
be
performed on dry or wet papers. The process works because the paper is
malleable; it
will embrace and retain an image of whatever object is pressed against it.
A paperboard is a sheet of fibrous web material having a grammage higher than
125
grams per square meter, by comparison with papers which have a grammage below
125 grams per square meter . A paperboard is embossed to increase its volume
and,
simultaneously reduce the quantity of raw material necessary to manufacture
the
paperboard for a given thickness. It therefore increases the specific volume
(or bulk).
However, dry embossing crushes the fibers of the paperboard and therefore
weakens
substantially the resulting paperboard. Dry embossing delaminate boards made
of
multiple plies.
Peak to peak embossing perforates the pulp-based substrate and therefore
alters
substantially its mechanical properties.
Techniques other than embossing to increase the volume of the paperboard are
currently used but all yield unacceptable results with respect to volume of
the
paperboard, quantity of fibers used and strength of the resulting paperboard.
Such
techniques are, for example, reducing the wet pressing, reducing the refining,
adding
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sawdust in the wet mat, adding mechanical pulp and chemicals.
BRIEF SUMMARY OF THE INVENTION
It is therefore an aim of the present invention to address the above mentioned
issues.
According to a general aspect, there is provided a process for manufacturing
an
embossed paperboard. The process comprises the steps of: forming a wet mat
including more than 60 wt% of cellulose fibers; pressure molding, with at
least one
embossing roll, the wet mat having 20 to 70 wt% solid to create a nested
surface texture
thereon; and drying the embossed wet mat to obtain the embossed paperboard
with a
grammage ranging between 125 and 1500 grams per square meter.
The step of forming the wet mat can further comprise superposing 1 to 12 paper
plies
or, in an alternate embodiment, superposing 7 to 9 paper plies.
The pressure molding step can further comprise applying a pressure ranging
between
50 and 600 pounds per linear inch (PLI). The pressure molding step can be
carried out
with two embossing rolls having spaced-apart knobs in meshing engagement, the
two
embossing rolls being synchronously rotated.
In alternates embodiments, the solid content of the wet mat ranges between 35
and 55
wt% during the pressure molding step and/or the wet mat can comprise more than
80
wt% of cellulose fibers.
In alternates embodiments, the wet mat can comprise less than 30 wt% of
inorganic
fillers and/or the cellulose fibers of the wet mat comprises more than 60 wt%
of recycled
fibers.
The recycled fibers can comprise more than 40 wt% of old corrugated cardboard
(OCC)
fibers.
The embossed paperboard can have a specific volume density ranging between 1
and
6 cubic centimeter per gram, a tensile strength ranging between 100 and 700
Newtons
per inch, a thickness ranging between 250 and 5 000 micrometers, a moisture
content
below 15 wt%, and/or a grammage ranging between 250 and 900 grams per square
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meter.
In an embodiment, the process also includes the step of decelerating the wet
mat for
carrying the pressure molding step. It can also include the step of
accelerating the wet
mat for carrying the drying step. It can also include the step of withdrawing
excess water
while carrying the pressure molding step.
According to another general aspect, there is provided an embossed paperboard
comprising: a paper mat having a nested surface texture thereon created by
pressure
molding with at least one embossing roll when the paper mat contained between
20 to
70 wt% solid and then dried to contain less than 15 wt% of moisture content,
the paper
mat having more than 60 wt% of cellulose fibers and a grammage ranging between
125
and 1500 grams per square meter.
In this specification, the term "paperboard" is intended to mean paperboard,
cardboards
as well as boards including cellulose fibers and, more particularly,
paperboards and
boards thicker than 10 mils (0.01 inch). It includes medium and high weight
paper
substrates having a grammage higher than 125 grams per square meter. It
includes,
without limitation, virgin and recycled materials and single and multi-ply
materials.
The term "secondary paper" is intended to mean any recycled fibers, waste
papers, or
other sources of pulp and fiber that come from a previously created product or
process.
The term "virgin fibers" refer to fibers that come directly from original
pulping processes.
The term "nested pattern" refer to a pattern wherein the depressions created
on a first
paperboard side are in register with the protuberances created on a second
paperboard
side, opposed to the first side, and vice-versa. Nested embossing pattern can
be
created with two embossment rolls, each having embossment knobs and the
embossment knobs of one roll mesh between the embossment knobs of the other
roll or
with two embossment rolls, only one roll having embossment knobs and the other
roll
having a substantially smooth outer surface, which can be deformable.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows an apparatus used to emboss a wet web in accordance with an
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embodiment;
Fig. 2 is a schematic cross-sectional view of embossment rolls of the
apparatus shown
in Fig. 1, wherein both rolls include embossment knobs;
Fig. 3 is a schematic cross-sectional view of alternate embossment rolls of
the
apparatus shown in Fig. 1, wherein only one roll includes embossment knobs;
Fig. 4 is a perspective view of a wet embossing apparatus in accordance with
an
embodiment;
Fig. 5 includes Fig. 5A, Fig. 5B and Fig. 5C, wherein Fig. 5A is a micrograph
of a
depressed surface of a nested embossed paperboard; Fig. 5B is a detailed view
of the
surface of Fig. 5A; and Fig. 5C is a micrograph of a transversal view of the
nested
embossed paperboard of Fig. 5A, with the depressed surface at the top and the
protruding surface at the bottom;
Fig. 6 is a photograph of an example of a nested embossing pattern;
Fig. 7 includes Fig. 7A and Fig. 7B, wherein Fig. 7A is a schematic cross-
section view of
a paperboard which is not embossed and has a first specific volume and Fig. 7B
is a
schematic cross-section view of the paperboard of Fig. 7A which has been
embossed
with the present embossing technique and now has a specific volume
substantially
double of that of the non-embossed paperboard of Fig. 7A;
Fig. 8 includes Fig. 8A and Fig. 8B, wherein Fig. 8A is a micrograph of the
bottom
surface of a wet embossed paperboard and Fig. 8B is a micrograph of the top
surface of
the wet embossed paperboard of Fig. 8A, with the depressed surface at the top
and the
protruding surface at the bottom;
Fig. 9 includes Fig. 9A and Fig. 9B, wherein Fig. 9A and Fig. 9B are
micrographs of
different cross-section views of a wet embossed paperboard, with the depressed
surface at the top and the protruding surface at the bottom;
Fig. 10 includes Fig. 10A and Fig. 10B, wherein Fig. 10A is a micrograph of
the bottom
surface of a dry embossed paperboard and Fig. 10B is a micrograph of the top
surface
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of the dry embossed paperboard of Fig. 10A, with the depressed surface at the
top and
the protruding surface at the bottom;
Fig. 11 includes Fig. 11A and Fig. 11B, wherein Fig. 11A and Fig. 11B are
micrographs
of cross-section views of different portions of a dry embossed paperboard,
with the
depressed surface at the top and the protruding surface at the bottom;
Fig. 12 is a photograph of an embossing pattern in accordance with a first
embodiment;
Fig. 13 is a photograph of an embossing pattern in accordance with a second
embodiment, wherein the embossing pattern has a 65 mil depth;
Fig. 14 is a photograph of an embossing pattern in accordance with a third
embodiment,
wherein the embossing pattern has a 60 mil depth;
Fig. 15 includes Fig. 15a and 15b, Figs. 15a and 15b are photographs of an
embossing
pattern in accordance with a fourth embodiment, wherein the embossing pattern
has a
135 mil depth and wherein the embossing pattern of Figs. 15a and 15b was
created
with a 25 mil and 50 mil spacing between the embossment rolls respectively;
Fig. 16 is a photograph of an embossing pattern in accordance with a fifth
embodiment,
wherein the embossing pattern has a 125 mil depth;
Fig. 17 includes Fig. 17a and 17b, Figs. 17a and 17b are photographs of an
embossing
pattern in accordance with a sixth embodiment, wherein the embossing pattern
has a
100 mil depth and wherein the embossing pattern of Figs. 17a and 17b was
created
with a 30 mil and 20 mil spacing between the embossment rolls respectively;
Fig. 18 is a photograph of an embossing pattern in accordance with a seventh
embodiment, wherein the embossing pattern has a 70 mil depth;
Fig. 19 is a photograph of an embossing pattern in accordance with a eighth
embodiment, wherein the embossing pattern has a 70 mil depth;
Fig. 20 is a photograph of an embossing pattern in accordance with a ninth
embodiment, wherein the embossing pattern has a 60 mil depth;
Fig. 21 is a photograph of an embossing pattern in accordance with a tenth
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embodiment, wherein the embossing pattern has a 60 mil depth; and
Fig. 22 is a photograph of an embossing pattern in accordance with a eleventh
embodiment, wherein the embossing pattern has a 35 mil depth.
It will be noted that throughout the appended drawings, like features are
identified by
like reference numerals.
DETAILED DESCRIPTION
Now referring to the drawings and, more particularly referring to Fig. 1,
there is shown
an apparatus 10 used for transforming and, more particularly, wet embossing a
wet mat
12 into an embossed paperboard 14. Wet embossing allows a better pattern
definition
and keeps the paperboard cohesiveness.
The process for manufacturing the embossed paperboard 14 is designed for
embossing
the wet mat 12 which, prior to the embossing step, includes more than 60 wt%
of
cellulose fibers and has between 20 to 70 wt% solid. Following the process,
the
embossed paperboard 14 with a grammage ranging between 125 and 1500 grams per
square meter is obtained.
The process includes the steps of forming the wet mat 12; pressure molding
with at
least one embossing roll the wet mat to create a surface texture thereon; and
drying the
embossed wet mat to obtain the embossed paperboard 14.
Embossing is typically performed by one of two embossing roll arrangements,
knob-to-
knob embossing or nested embossing. Knob-to-knob embossing, also referred to
as
peak-to-peak embossing, consists of axially parallel rolls juxtaposed to form
a nip
between the knobs on opposing rolls. As mentioned above, nested embossing
patterns
can be obtained with two embossment rolls. In a first embodiment, shown in
Fig. 2, both
rolls 16, 18 include embossment knobs 20a, 20b and the embossment knobs 20a of
one
roll 16 mesh between the embossment knobs 20b of the other roll 18. In an
alternate
embodiment, shown in Fig. 3, only one roll 22 has embossment knobs 24 and the
other
roll 26 has a substantially smooth outer surface 28, which can be deformable.
Thus, the
depressions created on one side of the mat nest with the protrusions created
on the
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opposite mat side. In a third embodiment (not shown), only one roll 16 has
embossment
knobs 20 and the other roll 18 has depression created to receive the
embossment
knobs 20 of roll 16.
Referring back to Fig. 1, there is shown that the wet fibrous mat 12 is first
formed from a
supply of pulp fibers from an aqueous slurry in a well known manner. Most
fibers are
cellulose fibers, which can provide from secondary materials, virgin fibers,
or a
combination of both, as is well known in the art.
In an embodiment, the wet mat 12 includes more than 60 wt% of cellulose
fibers. In an
alternate embodiment, the wet mat 12 includes more than 80 wt% of cellulose
fibers.
Additives may be added in the pulp to modify the appearance and/or physical
characteristics of the paperboard produced. Many types of additives are well
known in
the art, examples of such well known additives are mineral fillers (or
inorganic fillers),
dry strength resins, retention and drainage aids (chemicals), sizing agents,
etc.
The wet mat 12 can have a plurality of plies of superposed pulp-based
material. In an
embodiment, the paperboard has between 1 and 12 plies of pulp-based material.
Light
paperboards typically have two plies and have a grammage between 125 and 300
grams per square meter. Paperboards with a greater number of plies or thicker
boards
have a grammage of about 250 to 1500 grams per square meter. In an embodiment,
the
wet embossing apparatus and process are used to emboss paperboards having a
grammage between 125 and 1500 grams per square meter. In an alternate
embodiment, the wet embossing apparatus and process are used to emboss
paperboards having a grammage between 275 and 900 grams per square meter, with
seven to nine plies, for instance.
It is appreciated that the composition of each ply can vary. For example, in
an
embodiment, the outer plies, also referred to as liners, can have a first
composition in
pulp fiber while the inner plies, also referred to as fillers, can have a
second pulp fiber
composition.
For example, the wet mat can have seven plies and the outer plies (or liners),
i.e. plies #
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1 and # 7, can be made from pulp including between 60 and 80 wt% old
corrugated
cardboard (OCC) pulp and between 20 and 40 wt% recycled kraft boards. The
inner
plies (or fillers), i.e. plies # 2 to # 6, can be made from 100 wt% OCC pulp.
It is
appreciated that in alternate embodiments, the outer and inner plies can have
the same
fiber content or that the ply fiber content can vary from the one described
above.
As mentioned above, the wet mat fibers can include secondary fibers as well as
virgin
fibers. In an embodiment, the wet mat 12 can include between 50 and 100 wt%
secondary fibers. The secondary fibers can include low grade fibers such as
OCC, old
newpapers (ONP), old magazines (OMG), and mixed office paper, for instance. It
can
also include high grade fibers such as computer print-out (CPO), white ledges
(offece
paper) and colored ledger (office paper), for instance. The secondary fibers
can also
include, without being limitative, residential mixed paper, soft and hard
mixed papers,
boxboard cuttings, mill wrappers, news (de-ink quality or not, special, over-
issue, etc.),
double-sorted corrugated, new double-lined kraft corrugated cuttings, fiber
cores, used
brown kraft, mixed kraft cuttings, carrier stock, new colored kraft, grocery
bag scrap,
kraft multi-wall bag scrap, new brown kraft envelope curttings, mixed
groundwood
shavings, telephone directories, white blank news, groundwood computer
printout,
publication blanks, flyleaf shavings, coated soft white shavings, hard white
shavings,
hard white envelope cuttings, new colored envelope cuttings, semi bleached
cuttings,
sorted office paper, manifold colored or white ledger, sorted white ledger,
coated book
stock, coated groundwood sections, printed bleached board cuttings, misprinted
bleached board, unprinted bleached board, bleached cup stock, printed bleached
cup
stock, unprinted and printed bleached plate stock, and the like. It is
appreciated that this
enumeration is not limitative and that other secondary fibers can be used.
In an embodiment, the wet mat should contain long and strong fibers such as
OCC and
recycled kraft board fibers to reduce fiber breakage during the embossing
process.
Long and strong fibers typically have a length longer than 1 millimeter.
The wet mat is then drained to allow water to drain by means of a force such
as gravity
or a pressure difference.
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The wet mat 12 is further partially dewatered in a press unit 29, using press
rolls 30,
where the wet mat 12 is squeezed, to obtain a wet mat 12 having between about
20 wt% to about 70 wt% solids with an acceptable thickness and smoothness, as
is
known in the art. In an embodiment, the thickness of the wet mat 12, when
measured
wet, can vary between 250 and 5000 micrometers. In an alternate embodiment,
the wet
mat 12 has 40 to 60 wt% solid at the entry of the wet embossing process step.
In the embodiment shown in Fig. 1, a double felt press, with one felt 32 on
each side of
the web 12, is used. However, it is appreciated that in alternate embodiments,
other
presses such as, for instance, smoothness presses and shoe presses can be
used.
The wet mat 12 is then pressure molded in an embossing unit 33, with two
embossing
rolls 34, 36, each rotatable on an axis, the axes being parallel to one
another. In an
embodiment, the embossing roll 34 is a male roll since it includes a plurality
of
embossing knobs, or protrusions, on it surface. The other embossing roll 36 is
a
deformable rubber roll, having a substantially smooth outer surface, to create
a nested
surface texture thereon.
In an alternate embossing process, the second roll 36 includes depressions
which
corresponds to the embossing knobs extending outwardly from the male embossing
roll
34. The protrusions and the depressions are disposed in a non-random pattern
where
the respective non-random patterns are coordinated with each other. The
embossing
rolls are axially synchronously rotated with the protrusions and the
depressions being in
register to create nested protrusions and depressions in the wet mat 12.
In another alternate embodiment, both embossing rolls 34, 36 include
protrusions and
the wet mat 12 is embossed on both sides, i.e. protrusions and depressions are
provided on both sides of the resulting paperboard. The embossing rolls 34, 36
can also
include depressions which are in register with the protrusions of the opposed
roll or the
outer surface material of the rolls 34, 36 can be deformable. Thus, the two
rolls 34, 36
are aligned such that the respective coordinated non-random pattern of
protrusions and
nest together such that the protrusions of the two rolls 34, 36 mesh each
other.
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All alternate embossing apparatuses produce a pattern of protrusions and
depressions
in the cellulose fibrous structure of the wet mat 12, thereby increasing the
wet mat
specific volume. If only one male embossing roll 34, i.e. including
protrusions, is used,
the paperboard 14 is only embossed on one side, the other side of the
paperboard 14
having corresponding depressions. On the opposite, if two embossing rolls 34,
36 are
used, depressions and protrusions are provided on both sides of the paperboard
14.
In an embodiment, the wet mat 12 is carried between two embossing rolls 34, 36
which
are not heated.
Usually, when manufacturing a paperboard web, the paperboard speed along the
manufacturing apparatus is continually increased. Thus, from the press unit 29
towards
the drying unit 38, the paperboard web accelerates. The paperboard web, which
is a
viscoelastic material, slightly stretches in each unit.
On the opposite, in the wet embossing unit 33, the wet mat 12 decelerates. The
wet mat
12 is carried at a slower speed in the embossing unit 33 than in the press
unit 29. The
wet mat 12 slowly accelerates in the drying unit 38.
In an embodiment, if the drying unit 38 includes several drying rolls 40, the
wet mat 12
can still decelerates in the first drying rolls 40 and accelerate thereafter.
In an alternate
embodiment, the paperboard web accelerates as soon as it enters the drying
unit 38.
Thus, in the embossing unit 33, the wet mat 12 retracts instead of stretching.
In an
embodiment, both embossing rolls 34, 36 have a 12 inch diameter. Moreover, the
solid
content of the wet mat increases in the embossing unit since water is released
during
embossing. It is appreciated that in alternate embodiments, the embossing
rolls 34, 36
can have a different diameter and their diameter can range between 10 and 60
inches.
In the embossing unit 33, the embossing rolls 34, 36 apply a pressure ranging
between
50 and 600 pounds per linear inch (PLI). In an alternate embodiment, the
pressure
applied to the wet mat 12 can range between 250 and 400 PLI. The pressure can
be
controlled by adjusting the spacing between both rolls 34, 36 and is selected
in
accordance with the wet mat thickness. Less pressure is applied to the wet mat
12 if the
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spacing is wider while, on the opposite, an increased pressure is applied to
the wet mat
12 if the spacing is narrower. In an embodiment, the spacing between the
embossing
rolls 34, 36 can range between 1 and 100 milli-inch (mils). The spacing
between the
embossing rolls 34, 36 is measured peak-to-peak if both embossing rolls 34, 36
have
embossing knobs or between the peak of an embossing knob and the substantially
smooth outer surface of the opposite embossing roll.
In an embodiment, the wet embossed mat 12 can be sprayed with an anti-adhesive
product before being inserted or while being carried between the embossing
rolls 34,
36. The anti-adhesive product, such as vegetal oil, for instance, greases the
embossing
rolls 34, 36 and prevents the wet embossed mat from entirely or partially
adhering to the
embossing rolls 34, 36.
Referring to Fig. 4, there is shown an embodiment of a wet embossing apparatus
33
having two embossing rolls 34, 36, with parallel rotation axis, and a nip
therebetween in
which the wet mat is inserted. The embossing apparatus can include, for
instance,
suction boxes 43 to adequately remove excess water and prevent web crushing,
anti-
adhesive applicators 45, and air jet cleaning apparatuses 47 mounted proximate
to the
embossing rolls 34, 36.
Finally, referring back to Fig. 1, the wet embossed mat 12 is then dried in a
drying unit
38 having multiple drying rolls 40 to obtain the embossed paperboard 14. The
drying
rolls 40 can be heated and the wet mat 12 is dried through contact with the
rolls 40 or
the dryer 38 can have blowers (not shown) which generate warm air currents
within the
dryer 38. For instance, without being limitative, other drying systems can be
used to dry
the wet embossed mat 12 such as drum dryers, filled with steam, infra red
dryers, air
dryers, evaporation tables, ovens (forced convection drying), dryer felts,
etc.
The embossed paperboard 14, once dried, has a thickness ranging between 0,01
and
0,2 inch and a grammage above 125 and below 1500 grams per square meter. This
grammage is measured in the dried finished product but depends on the
dewatering
and wet mat formation process.
It should be noted that drying, with drying rolls, a wet embossed mat 12 is
more difficult
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than drying a non-embossed mat because once embossed the mat has less surface
in
contact with the drying rolls. However, embossing allows to reduce the
quantity of fibers
used and there will be thus less fibers to dry.
Using the wet embossing technique described above, embossed paperboards having
a
specific volume density ranging between 1 and 6 square centimeters per gram, a
tensile
strength ranging between 100 and 700 newtons per inch, a thickness ranging
between
500 and 2,500 micrometers and a grammage ranging between 125 and 2,500 grams
per square meter can be obtained. The embossed paperboard is produced with a
moisture content below 15 wt%. In an alternate embodiment, the embossed
paperboard
is produced with a moisture content below 10 wt%.
The properties of the embossed paperboard vary in accordance with the feed
material
content (% of fibers, fiber nature, % inorganic filler, inorganic nature,
etc.), the
embossing process operating parameters, the embossing pattern, the embossing
unit
(one or two male embossing rolls), amongst others. The wet nested embossed
paperboard has a specific volume gain while reducing mechanical property
losses
comparatively to dry embossing. More particularly, the specific volume gain is
more
important than with prior art dry embossing technique.
Fig. 5A shows a surface of the embossed paperboard made using the present wet
nested embossing technique. The surface of the paperboard shown is the surface
which
was depressed using the protrusions on the male embossing roll 34, the
opposite roll 36
having a substantially smooth outer surface. Each dot is a depression caused
by a
protrusion on the male embossing roll 34. This creates a corresponding
protrusion on
the other surface of the paperboard (not shown). The other surface is
therefore the
surface having a raised volume. Depending on the proximity of the protrusions
on the
male embossing roll 34, the resulting raised volume on the other surface of
the
paperboard can appear to be raised continuously along a line or raised with a
dotted
pattern along a line.
Other shapes and sizes of protrusions can be used to create corresponding
shapes of
depressions and protrusions on the surface of the paperboard. For example, a
star-
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headed protrusion can be provided on the embossing roll to create star-shaped
depressions and protrusions in the embossed paperboard.
Different sizes of protrusions on the male embossing roll 34 can also be
provided to
create interesting patterns on the paperboard, as it will be described in more
details
below in reference to Figs. 12 to 22. It should be noted that any embossing
pattern
respecting the required physical characteristics of the embossed paperboard
can be
produced by the present wet embossing technique and that the pattern shown is
only
one example of an embossing pattern. Moreover, the embossing pattern can be
created
by a combination of knobs provided on both embossing rolls.
Fig. 5B shows a detail of the surface of Fig. 5A. The fibers are apparent and
it can be
noted that some fibers were broken by the protrusions of the embossing roll
34. Fig. 5C
is a transversal view of the paperboard of Fig. 5A. The top surface is the
surface shown
in Fig. 5A and the bottom surface is the other surface of the paperboard, the
depressed
surface is therefore at the top and the protruding surface at the bottom. As
is apparent
on Fig. 5C, the paperboard is made of a plurality of plies. The top plies have
suffered
the most damage from the embossing technique with some delaminated plies while
the
bottom plies have simply curved under the embossing roll pressure.
Fig. 6 is an example of a nested embossing pattern that can be created using
the
present technique and is also an example of an embossed paperboard produced
with
the present technique.
Fig. 7 includes Fig. 7A and Fig. 7B, wherein Fig. 7A shows a schematic
transversal view
of a paperboard 114 which is not embossed having a top surface 140, an opposed
bottom surface 142, and a first specific volume and Fig. 7B shows a
representation of a
transversal view of the paperboard 214 of Fig. 7A which has been embossed with
the
present technique and now has a specific volume substantially double of that
of the
paperboard of Fig. 7A. The protrusions on the male embossing roll have
contacted the
top surface 140 of the paperboard 114 of Fig. 7A and have created the
depressions 236
in the top surface 240 of the paperboard 214 and the corresponding protrusions
238 on
the bottom surface 242 of the paperboard 214 as shown in Fig. 7B. The
resulting
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thickness of the paperboard 214 is substantially greater than the thickness of
the
original non-embossed paperboard 114 with the same amount of fibers used.
Fig. 8 includes Fig. 8A and Fig. 8B, wherein Fig. 8A shows the bottom surface
of a wet
embossed paperboard and Fig. 8B the top surface of the wet embossed paperboard
of
Fig. 8A, with the depressed surface at the top and the protruding surface at
the bottom.
Fig. 9 includes Fig. 9A and Fig. 9B, wherein Fig. 9A and Fig. 9B show
transversal views
of different portions of a wet embossed paperboard, with the depressed surface
at the
top and the protruding surface at the bottom. Some delamination 90 of the
plies of the
paperboard can be noticed but it is relatively minor.
Fig. 10 includes Fig. 10A and Fig. 10B, wherein Fig. 10A shows the bottom
surface of a
dry embossed paperboard and Fig. 10B the top surface of the dry embossed
paperboard of Fig. 10A with the depressed surface at the top and the
protruding surface
at the bottom. The dry embossed paperboard of Fig. 10 is embossed using prior
art
techniques.
When compared to the wet embossed paperboard of Fig. 8, one can note that when
the
protrusion contacted the surface of the paperboard in the dry embossing
technique, it
created a fracture in the bottom surface of the paperboard (see Fig. 10A). It
resulted in
an embossed paperboard with inferior mechanical properties than a paperboard
embossed when still having a moisture content higher than 30 wt%.
Fig. 11 includes Fig. 11A and Fig. 11B, wherein Fig. 11A and Fig. 11B show
transversal
views of different portions of a dry embossed paperboard, with the depressed
surface at
the top and the protruding surface at the bottom.
When compared with the views of Fig. 9, the dry embossing technique was more
destructive and created fractures 92 in the paperboard in addition to
delamination 90.
As mentioned above, the mechanical properties of a dry embossed board were
inferior
to the mechanical properties of a wet embossed board, particularly for
stiffness. Dry
embossing reduced the external as well as the internal mechanical properties
of the
embossed paperboard.
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Table 1 gives an example of the impact of dry and wet embossing on the
mechanical
properties of paperboards. The embossing was carried out with two embossing
rolls.
The first embossing roll had embossing knobs on its outer surface while the
second
embossing roll had a substantially smooth and deformable outer surface.
The mechanical properties were measured in accordance with the industry
standards.
More particularly, the grammage, the thickness, the specific volume, the Z-
direction
tensile strength (ZDT), the breaking length, the stretch, the elasticity
modulus, and the
tensile energy absorption (TEA) were respectively measured in accordance with
the
standards TAPP! T410, TAPP! T411, Paptac D.4, and T494.
The wet nested embossed paperboard has a gain in specific volume of 68 % while
having a loss of 52 A) of Z-Directional Tensile tester (ZDT) and 45 % of
breaking length.
Therefore, the gain in specific volume is greater than the dry nested
embossing
technique while the loss in breaking length and ZDT is similar to that of dry
nested
embossing. Wet embossing does not break the surface and create fractures
comparatively to dry embossing.
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Mechanical Non-embossed Wet nested embossed Dry nested embossed
Properties paperboard paperboard paperboard
Result Result Difference Result
Difference
(%) (%)
Grammage (g/m2) 357 347 -3 358 0
Thickness (pm) 628 1027 +64' 988 +57
Specific volume 1.76 2.96 +68 2.76 +57
(cm3/g)
ZDT (psi) 61.1 29.6 -52 25.8 -58
Breaking length 4.05 2.21 -45 2.27 -44
(km)
Stretch (%) 2.62 2.30 -12 1.94 -26
Modulus of elasticity 1.55 0.49 -68 0.46 -70
(Gpa)
TEA (J/m2) 232 114 -51 90.1 -61
Table 1. Mechanical properties and differences between non-embossed
paperboards,
wet and dry nested embossed paperboards.
Table 2 shows the thickness variation for dry and wet embossed paperboards
following
the application of 180 psi load during 1 minute. Two tests were carried. The
first test
was carried with a relatively high embossing pressure while the second test
was carried
with a relatively low embossing pressure. The embossing pressure was adjusted
by
varying the spacing between the embossing rolls.
The thickness variation following compression of the embossed paperboards,
shown in
Table 2, was more important for dry embossed paperboards since more
delamination
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thus an inferior compression strength. Therefore, the thickness reduction
during winding
and reeling is less important for wet embossed paperboards than for dry
embossed
paperboards.
Embossing
Spacing Thickness prior Thickness
Thickness
Samples loading following
variation
pressure (mil)
(pm) loading (pm) (%)
High pressure Wet 25 2974 2962 -0.4
emboss.
Dry emboss. 25 2608 2222 -14.8
Low pressure Wet 50 2128 2099 -1.4
emboss.
Dry emboss. 40 2274 1304 -42.7
Table 2. Thickness variation following 180 psi load application during 1
minute.
For two different embossing patterns (Patterns A and B), the effect of the
embossing
pressure on the mechanical properties of the wet embossed paperboards was
evaluated. Pattern A is shown on Fig. 17 while pattern B is shown on Fig. 22.
Embossing pattern A had a 100 mil depth while embossing pattern 6 had a 35 mil
depth. The spacing between two consecutive embossing knobs on one embossing
roll
is 290 and 188 milli-inches for patterns A and B respectively. The mechanical
properties
obtained were compared to the mechanical properties of a non-embossed
paperboard
and are shown in Table 3 in percentages.
For embossing pattern A, the thickness gain was higher for high embossing
pressure
while the embossing pressure had no effect on the thickness gain for the
embossing
pattern B. A high embossing pressure lowered the stiffness of the resulting
wet
embossed paperboard.
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Emboss. Pattern A Pattern B
pattern
Embossing High Medium Low (30 High (25 Medium Low
(15
pressure (50 mil) (40 mil) mil) mil) (20 mil)
mil)
Grammage +6 +16 +15 -2 +2 0
gim2)
Thickness (pm) +122 +100 +93 +50 +50 +51
Specific volume +109 +73 +69 +54 +48 +50
(cm3/g)
ZDT (psi) -29
Stiffness(mN) -55 -51 -42 -52 -27
-27
Table 3. Embossing pressure effect on the wet embossed paperboard mechanical
properties.
In accordance with the embossing pattern, the wet mat thickness, which is
related to the
wet mat grammage, can or cannot influence the thickness of the resulting wet
embossed paperboard as shown in Table 4. The thickness of the sample (736 and
1067
pm) was measured on the dry non embossed paperboard. However, increased wet
mat
grammages provided stiffer wet embossed paperboards. Thus, the wet embossed
paperboard thickness should be controlled by the embossing pressure while the
stiffness should be controlled by the wet mat grammage.
Thickness (pm) Stiffness (mN)
Pattern A
Embossing Sample Sample Variation Sample Sample Variation
pressure 736 pm 1067 pm (cY0) 736 pm
1067 pm (%)
Medium 2367 2276 -4 518 809
+56
High 2616 2528 -3 508 743
+46
Pattern B
High 1451 1710 +18 523 800
+53
Table 4. Wet mat thickness effect on the wet embossed paperboard mechanical
properties.
To evaluate the operational problems which could occur at the end of the
embossing
unit resulting from embossed mat strength losses, wet tensile tests have been
carried.
Wet embossed paperboard samples have been wet, sponged, to a solid content
ranging
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between 35 and 39 wt %, and then tested. The results were compared to two non-
embossed test webs. The results are shown in Table 9.
Embossing lowered the tensile strength of the embossed paperboard in
accordance
with the embossing pressure applied.
Thickness Embossing Tensile property or Tensile Tensile
TEA
(Pm) pressure Variation strength (N/m) stretch CYO
(J/m2)
957 2.37
12.6
Tensile property 398 --- --
-
736 pm Medium
Variation (%) -58
Hi Tensile property 231 9.80
11.3
gh
Variation (%) -76 +315
-10
Test web Tensile property 1024 2.56
12.1
Low Tensile property 591 7.12
17.6
Variation (%) -42 +178
+45
1067 pm Medium Tensile property 469 9.84
17.5
Variation (%) -54 +284
+45
Hi Tensile property 272 8.58
12.3
gh
Variation (%) -73 +235 +2
Table 5. Wet state tensile properties.
It is appreciated that the embossing pattern influences the mechanical
properties of the
resulting embossed paperboard. If the embossing pattern reproduced on both
surfaces
of the paperboard are symmetrical, better properties are observed and, more
particularly, adhesive application is facilitated.
To obtain symmetrical embossing patterns, two male embossing rolls, including
embossing knobs, are used in the embossing unit. The embossing rolls are
disposed in
a non-random manner where the respective non-random patterns are coordinated
with
each other. The embossing knobs on a first embossing roll are in register with
depressions provided on a second embossing roll. The embossing rolls are
axially
synchronously rotated. Protrusions and depressions are provided on both sides
of the
resulting paperboard. Specific volume gain up to 300 % can be obtained with
symmetrical embossing patterns.
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Thus, it has been observed that increasing the embossing pressure reduces the
strength of the embossed paperboard while increasing the specific volume gain,
the
paperboard shrinkage, and the dryness gain for paperboard having the same
thickness.
Even if increasing the embossing pressure reduces the strength of the embossed
paperboard, the strength of wet embossed paperboards is higher than the
strength of
dry embossed paperboards for the same embossing pressure.
To increase the embossed paperboard strength, the grammage can be increased.
Grammage increase also further increases the specific volume gain.
A specific volume gain is generally accompany with an increased shrinkage and
grammage.
The paperboard thickness variation can be controlled either by adjusting the
embossing
pressure or the grammage, depending on the embossing pattern. The embossing
pressure can be adjusted by varying the spacing between the embossing rolls.
As mentioned above, the manufacturing speed in the embossing unit is reduced.
This is
particularly important since the mat shrinks during the embossing process.
Now referring to Figs. 12 to 22, embodiments of embossing patterns are
described. It is
appreciated that these embossing patterns are exemplary only and other
embossing
patterns can be used. The depth of the embossing knob can vary between 30 and
150
mils. Moreover, the spacing between two consecutive embossing knobs can vary
between 40 to 1000 milli-inches.
Referring to Figs. 12 and 13, there is shown two embodiments of embossing
patterns
wherein all the protuberances are located on a same side of the embossed
paperboard.
On the opposite, the protuberances are located on both sides of the embossed
paperboards in the embodiments shown in Figs. 14 and 15.
In the embodiment, shown in Fig. 15, the embossing pattern was created with
two
embossing pressures. In the embodiment shown in Fig. 15a, the spacing between
both
embossing rolls was 25 mils while, in the embodiment shown in Fig. 15b, the
spacing
between both embossing rolls was 50 mils. Thus, the embossing pressure was
higher in
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the embodiment of Fig. 15a and the resulting embossing pattern is more
defined.
Figs. 16 to 22 show alternate embodiments of embossing patterns.
Similarly to Fig. 14, in Fig. 17, the embossing pattern was created with two
embossing
pressures. In the embodiment shown in Fig. 17a, the spacing between both
embossing
rolls was 30 mils while, in the embodiment shown in Fig. 15b, the spacing
between both
embossing rolls was 20 mils. Thus, the embossing pressure was higher in the
embodiment of Fig. 15b and the resulting embossing pattern is more defined.
The embodiments of the invention described above are intended to be exemplary
only.
It is appreciated that the wet embossing process described above can be
carried out not
only to increase the bulk of the paper web but also for aesthetic purposes.
The scope of the invention is therefore intended to be limited solely by the
scope of the
appended claims.
OR File No. 8244-71CA -21 -
