Note: Descriptions are shown in the official language in which they were submitted.
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Title: KRAPT-SUB~ ~ BRISTOL PAPER PROD~CTS
FIELD OF THE INVENTION
The invention relates generally to kraft-
substitute bristol paper products comprising from 70 to 90
per cent mechanical pulp fibres and from 10 to 30 per cent
chemical pulp fibres and having a caliper of from 0.006 to
0.020 inches. The bristol paper products of the invention
have the necessary physical characteristics to substitute
for kraft products, including products such as file
folders, return postcards and index di~iders, and yet
weigh from 20 to 30 per cent less than the corresponding
conventional kraft paper product of equivalent caliper.
The invention also relates to a process for preparing
kraft-substitute bristol paper sheets by providing a
15 furnish of 70 to 95 per cent mechanical pulp and 5 to 30
per cent chemical pulp, depositing the furnish on the
wire of a paper machine to form a web and running the web
through the paper machine to form a paper sheet.
RA~ ~UNU QF T~E lNV~.~lON
Bristol paper products are paper and paperboard
products which are generally 0.006 inches or greater in
thickness and which are strong, stiff, smooth, tear
resistant and foldable. The physical properties of
bristol paper make it suitable for paper products
requiring strength and tear resistance, which will
maintain their shape and which can be folded without
tearing or splintering. Examples of products which are
manufactured from bristol paper includes for example, file
folders, return postcards, index dividers, sales tags,
garment tags and cards.
Bristol paper products are traditionally made
primarily from ch~ iral pulp, in particular kraft pulp.
Accordingly, commercial bristol paper products are also
frequently referred to as kraft products.
Chemical pulp is prepared by chemically treating
wood chips in an aqueous solution at ele~ated temperature
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and pressure to dissolve sufficient lignin in the cell
walls to allow the cellulose and hemicellulose fibres to
be separated with relatively little or no mechanical
action. The two major chemical pulping processes are
the kraft and sulfite processes.
In the sulfite process, wood chips are digested
with a mixture of sulfurous acid and bisulfite ions to
solubilize the lignin. The sulfites combine with the
lignin to form salts which dissolve in the cooking liquor.
The sulfite process produces somewhat brighter pulp, but
weaker paper sheets than the kraft process. The sulfite
process has been largely superseded by the kraft process,
which has become the dominant pulping process worldwide.
Rraft pulp for bristol paper products is
prepared with an alkaline pulping liquor cont~ining, for
example, sodium hydroxide and sodium sulfide. ~hen cooked
in this liquor the alkali results in fragmentation of the
lignin into smaller segments. The cellulosic fibres in
kraft pulp are long, flattened and deficient in lignin
compared to the fibres produced by the mechanical process
and yield a dense pulp.
The use of pulp cont~i~ing predo~in~ntly kraft
pulp fibres has been considered essential for bristol
paper products, where bending stiffness, folding
endurance, brightness, smoothness, tear strength and low
surface compressibility are required features of the paper
product. Although the use of pre~orinAntly kraft pulp
imparts strength and smoothness, kraft pulp is dense and
provides relatively heavy paper products which are
expensive to handle and ship.
The use of kraft pulp in the manufacture of
bristol paper products raises a number of environmental
concerns. Rraft pulping has a relatively low yield from
the wood or raw material of from between 40 and 50%.
Accordingly from 50 to 60 ~ of the original wood is
discarded as waste during chemical processing. Thus,
chemical pulping is an inefficient use of natural forest
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resources. ~raft pulp is dense and dark and usually
requires extensive bleaching and refining before it can be
used in the manufacture of kraft paper products.
Chemical pulping also generates large amounts of
environmental pollutants which may be troublesome and
expensive to dispose of. Volatile malodorous sulfur
compounds such as hydrogen sulfide, methyl mercaptan,
dimethyl sulfide and dimethyl disulfide are produced as
undesirable polluting by-products of the process for
preparing chemical pulps.
Particularly toxic pollutants are generated by
bleaching chemical pulp. The whiteness or brightness of
pulp varies over a wide range, depending on the type of
wood pulped and the pulping process itself. As noted
above, Kraft pulp tends to be quite dark, having a
brightness in the range of from 15 to 30, which is
unacceptable for most bristol paper products. Accordingly,
kraft pulps used in the manufacture of bristol paper
products are usually bleached by one or more of a number
of chemical treatments, such as chlorination, alkaline
extraction, chlorine dioxide, oxygen, hypochlorite,
peroxide or ozone.
Chlorination and alkaline extraction have
traditionally been employed in the initial stages of kraft
pulp bleaching to delignify the pulp and remove the
chromophores. The chlorination of pulp forms a host of
chlorinated organic compounds, such as polychlorinated
dioxins and dibenzofurans which are extremely toxic.
Environmental and health concerns mandate that the
generation and release of these chemicals by pulp and
paper mills be reduced and that more environmentally sound
substitute kraft paper products be developed.
Mechanical pulp has not been used a~ a major
component of bristol paper products as the fibres produced
by the mechanical grinding process are short, and weak.
The fibres of mechanical pulp retain the original cell
wall lignin and, as a result, yield paper products having
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high bulk, opacity, compressibility and surface roughness
but relatively low mechanical strength and density. Paper
prepared from mechanical pulp is considered to be weak and
mechanical pulp has been used primarily to manufacture
lower quality paper grades, such as newsprint, where
strength is not required. Typically, paper machines for
manufacturing these low grade papers from mechanical pulp
are not equipped with size presses to apply surface
treatments to the paper, as the low quality rough grades,
such as newsprint and cardboard are not coated.
There is a need for bristol paper products
having the necessary minimum physical characteristics,
such as stiffness, foldability and strength, to substitute
for traditional kraft products, but which may be
manufactured from pulp which is less wasteful of wood
resources. Accordingly, there is also a need for a more
economical and less polluting process for preparing the~e
products.
S~MMARY OF ~rn~ lNv~.llQN
It is an ob~ect of the present invention to
provide a bristol paper product, having acceptable m i n i
strength characteristics to substitute for a corresponding
kraft product of equivalent caliper, but which is made
primarily from mechanical pulp. It is a further object of
the invention to provide such a bristol paper product
which is less dense and thus weighs less than the kraft
product of equivalent caliper for which it may be
substituted. It is yet a further object to provide a
bristol paper product which utilises wood resources more
efficiently and which may be manufactured by a process
which results in fewer toxic by-products than the chemi~Al
pulping processes, such as the kraft process.
It is also an ob~ect of the present invention to
provide a bristol paper product having a caliper of from
3~ 0.006 to 0.020 inches and having the requisite ~ini ~m
stiffness, tear strength, density, foldability for a
bristol product, while being less dense and thus lighter
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than the corresponding conventional kraft paper product of
equivalent caliper.
The present inventors have surprisingly
det~r~ined that a kraft-substitute bristol paper product,
S having the necessary i n; mll~ physical properties to
substitute for a conventional kraft paper product of
equivalent caliper can be prepared from a furnish
contAining from 70 to 95 per cent mechanical pulp and 5 to
30 per cent chemical pulp.
An aspect of the invention provides a kraft-
substitute bristol paper product comprising from 70 to 95
per cent mechanical pulp fibres and from 5 to 30 per cent
chemical pulp fibres, wherein the paper product has a
caliper of from 0.006 to 0.020 inches and weighs from 20
lS to 30 per cent less than the corresponding conventional
kraft paper product of equivaient caliper. In an
embodiment, the paper product comprises from 70 to 80 per
cent mechanical pulp fibres and from 20 to 30 per cent
high yield sulphite pulp fibres.
In a preferred embodiment, the paper product
comprises from 80 to 95 per cent mechanical pulp fibres
and from 5 to 20 per cent kraft pulp fibres. It is
contemplated that, in an embodiment, the mechanical pulp
fibres are unbleached and the bristol paper product has a
brightness of from 45 to 75. In a further embodiment, the
paper product has a density of from O.S to 0.75 g/cm3 and
a tear strength of from 700 to 2300 milliNewtons (mN).
In particular embodiments, the paper product may
be a file folder, return postcard or index divider. In
preferred embodiments, a file folder is provided having a
caliper of from 0.010 to 0.011 inches and weighing from 90
to 115 pounds per ream.
In further embodiments, the invention provides
a kraft-substitute bristol paper product comprising a
postcard having a machine direction/cross-machine
direction stiffness ratio of from 1.5 to 2.5 and a machine
direction/cross-machine direction tear strength ratio of
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from 0.3 to 0.7.
In yet a further embodiment, the invention
provides a file folder comprising 80 to 90 per cent,
preferably 84 to 86 per cent, unbleached groundwood pulp
fibres and from 10 to 20 per cent, preferably 14 to 16 per
cent, kraft pulp fibres, having a caliper of from 0.0100
to 0.0110, preferably 0.0104 to 0.0106 inches and a weight
of from 90 to 115, preferably 100 to 106 pounds, per ream
and a tear strength of from 1100 to 1500 mN.
The present invention also relates to a process
for preparing a kraft-substitute bristol paper sheet
having a density in the range of from 0.5 to 0.75 g/cm3
and a tear strength of from 700 to 2300 mN, comprising:
i ~; ng unbleached and unrefined mechanical pulp and
chemical pulp to obtain a furnish comprising from 70 to 90
per cent mechAnir~l pulp and from 10 to 30 per cent
chemical pulp; depositing a jet of the furnish on the
wire of the forming section of a paper machine to form a
web and rllnn i ng the web through the paper machine to
produce the paper sheet.
In an embodiment of the process the furnish has
a brightness of from 45 to 70. In a further embodiment of
the process the furnish is deposited on the screen at a
~et/wire ratio of approximately 1Ø
DE~Tr~n D~rRTPTION OF THE lNv~.llON
The present invention relates to a range of
kraft-substitute bristol paper products and to the process
for preparing these products including specialty products,
such as file folders, cards, postcards, return postcards,
and tags. In a preferred embodiment of the invention, the
kraft-substitute bristol paper product is a file folder.
As hereinbefore mentioned, the present invention
relates to a kraft-substitute bristol paper product
comprising from 70 to 95 per cent mechanical pulp fibres
and from 5 to 30 per cent chemical pulp fibres. The
kraft-substitute bristol paper product has a caliper of
from 0.006 to 0.020 inches and weighs from 20 to 30 per
-
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cent less than the corresponding conventional kraft
product of equivalent caliper.
The term bristol paper product as used herein
refers to paper or paperboard which is 0.006 inches or
greater in thickness and which is strong, tear resistant
and foldable. Conventional kraft bristol paper products
are traditionally made from close to 100% per cent kraft
pulp fibres. The physical properties of bristol make it
suitable for paper products requiring strength and tear
resistance and which will maintain their shape. Examples
of products which may be made from the kraft-substitute
bristol include file folders, return postcards, index
dividers, drawing pads, coating base papers, ticket base
papers, sales tags, garment tags and cards.
A kraft-substitute bristol paper product refers
to a bristol paper product which has the necessary minimum
requisite physical characteristics, such as stiffness and
foldability to be used as a substitute for a corresponding
conventional kraft paper product, but which has no more
than 30, preferably 20, most preferably 15 per cent kraft
pulp fibres, as described in more detail below.
Corresponding conventional kraft paper products
refers to bristol paper products which are made up
pre~o-;n~ntly of kraft pulp fibres. Conventional kraft
paper products generally have from 80 to lO0 per cent
kraft pulp fibres, depending upon the specific
application. Such conventional products are well known
and are commercially available.
Pulp fibres are cellulosic fibres from which
paper products are manufactured. These may be obtAine~
from a variety of raw materials, primarily wood or from
other natural sources, such as cereal straws, bamboo and
bagasse. In the paper-making process, the raw material is
broken up and prepared into a pulp, which consists of an
aqueous suspension of cellulosic pulp fibres.
Chemical pulp fibres refer to those pulp fibres
which are obt~ineA from wood or other raw materials
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principally using chemicals to dissolve away the lignin to
release the fibres. Methods for preparing chemical pulp
are well known in the art. The invention is by no r~An~
restricted to a particular type of chemical pulp, and it
is contemplated that kraft, sulfite and high yield sulfite
pulp fibres, most preferably kraft, may be useful in the
paper products of the invention.
Sulfite pulp fibres may be prepared by the
sulfite process, whereby wood chips are cooked in a
digester with a mixture of sulfurous acid and bisulfite
ions to solubilize the lignin. The sulfites combine with
the lignin to form salts which dissolve in the cooking
liquor. High yield sulfite pulp may be prepared following
known procedures employing less severe cooking conditions
combined with some mechanical fiberizing to separate the
pulp fibres. High yield sulfite pulp fibres retain a
higher proportion of lignin and hemicellulose than sulfite
pulp fibres.
Rraft pulp may be prepared by stAn~rd
procedures with an alkaline pulping liquor cont~ining, for
example, sodium hydroxide and sodium sulfide. In the
kraft process wood chips are cooked in white liquor
contAining the active chemicals and the residual black
liquor cont~ining the reaction products of the lignin
solubilization is ~el"oved for further processing. Under
these conditions the wood fibres are de-lignified and the
lignin is chemically fragmented and dissolved in the
cooking liquor. Kraft pulp fibres typically contain very
small amounts of lignin generally less than 5 per cent of
the total weight. Cellulose and hemicellulose are al80
lost during this process.
The yield of pulp from chemical pulping, such as
the kraft process is generally between from 40 to 60 per
cent of the weight of the original wood and an improved
yield of up to from 70 to 75 per cent may be achieved
using the high yield sulfite process. Mechanical pulping
is much more efficient and has about a 95 per cent yield
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from the wood.
Suitable equipment for digesting the wood i8
well known and includes stationary horizontal and
spherical digesters and rotating vertical digesters.
Continuous batch digesters, such as Messing and Durkee,
~A ~ or IMPCO digesters may be used.
Chemical pulps, especially kraft pulpq, are
generally used wherever strength is an important feature
of the paper product or is required for applying a finish
to the paper during processing. More particularly, kraft
pulps are used where bending stiffness, folding endurance,
brightness, smoothness and low surface compressibility are
required features of the paper product.
Mechanical pulp fibres may be prepared by
mechanical pulping processes, whereby the wood is pulped
primarily using mechanical energy, with resulting high
yields of pulp. Mechanical pulp typically contains from
5 to 35 per cent lignin based on the weight of the pulp.
Methods for preparing mechanical pulp are well
known in the art. Stone groundwood pulp is prepared by
pressing a block of wood lengthwise against a wet
grindstone revolving at high speeds. Other suitable
methods for preparing mechanical pulp are well known in
the art and include pressurized groundwood PGW, refiner
mechanical pulp RMP, thermo-refiner mechanical pulp TRMP,
pressure refined mechanical pulp PRMP, chemi-mechanical
CMP, thermo-mechanical pulp, pressure/pressure thermo-
mechanical pulp PPTMP, chemi-refiner mechanical pulp CRMP,
chemi-the ~-mechanical pulp CTMP, th~ - chemi-mechanical
pulp TCMP and th~rmo-mechanical-chemi pulp TMCP.
The mechAnic~l and chemical pulps, once
prepared, may be further treated to screen out large
particles and the pulps may be stored prior to further
processing.
The kraft-substitute bristol paper products of
the invention may comprise from 70 to 95 per cent
mechanical fibres and from 5 to 30 per cent chemical pul~
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-- 10 --
fibres. Preferably, the paper products of the invention
have from 80 to 95, more preferably 84 to 86, per cent
mechanical pulp, preferably stone groundwood pulp fibre~,
most preferably unbleached and unrefined stone groundwood
S pulp fibres; and from 5 to 20, more preferably 14 to 16
per cent respectively kraft pulp fibres.
Where high yield sulfite pulp fi~res are used as
the chemical pulp fibres in the paper products of the
invention, the appropriate percentage of chemical pulp
selected should be higher, preferably from 20 to 30 per
cent. As a rough guide, about twice as much high yield
sulfite fibres are required to duplicate the results
achieved with kraft pulp fibres.
As hereinbefore mentioned, the kraft-substitute
bristol paper products of the invention have a caliper of
from 0.006 to 0.020 inches, preferably from 0.008 to
0.016, more preferably form 0.010 to 0.011, most
preferably from 0.0104 to 0.0106 inches. The appropriate
caliper may be selected based on the desired
specifications of a particular paper product.
The kraft-substitute bristol paper products of
the invention have the requisite strength and
characteristics to substitute for conventional kraft paper
products and yet weigh from 20 to 30, per cent less than
the corresponding conventional kraft products of
equivalent caliper. Accordingly, the kraft-substitute
bristol paper products are more economical to manufacture
and to transport. Thus, not only are the paper products
of the invention made from pulp which has a higher yield
from the wood, but the paper products themselves require
less weight of pulp to achieve the same caliper as the
corresponding conventional kraft product. Accordingly,
the paper products of the invention are less expensive to
~nllfacture. The paper products of the invention may have
a density of from 0.5 to 0.75 g/cm3.
In the competitive environment of the commercial
paper industry, the weight of paper products may be a
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crucial factor for the distributor. For exampl~, pap~r
products are generally purchased by the distributor based
on a per weight basis, such as tons of paper and the
distributor pays a certain amount per ton in shipping
costs. However, frequently, the distributor sells the
paper product at a set price based on the number of
units, such as reams of paper or number of file folders.
Accordingly, if the distributor buys one ton of paper
product that is 30% lighter than average, the distributor
can sell the paper product at a competitive reduced price,
for example 20% less, and still have increased profits of
10%.
Where tear strength is a necessary feature of
the paper product, both the chemical and mechanical pulps
may preferably be unrefined. Where the paper product is
to be folded, the present inventors have determined that
the chemical pulp fibres should preferably be kraft pulp
fibres in order to prevent splintering at the fold.
In a preferred embodiment, the invention relates
to a file folder made up of 80 to 90, preferably 84 to 86,
most preferably 85 per cent unbleached groundwood pulp
flbres and from lO to 20, preferably 14 to 16, most
preferably about lS per cent kraft pulp fibres. The file
folders of the invention have a caliper of from O.OlO0 to
O.OllO, preferably from 0.0104 to 0.0106, most preferably
about O.OlO5 inches and a weight of from 90 to 115,
preferably lO0 to 106 pounds per ream and a brightness of
from 45 to 75 preferably from 60 to 65.
The file folders of the invention have an
equivalent caliper to commercial file folders, such as
Grand & Toy Permax file folders, but weigh approximately
25 per cent less. The present inventors have determined
that the use of the small amounts of kraft pulp fibres
detailed above, permit the file folders to hold a clean
fold, without splintering. The file folders may be
efficiently and economically prepared by the process
describêd in more detail below, with a high yield from the
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wood, without the need for extensive bleaching or refining
and with the generation of minimal toxic by products.
Accordingly, the file folders may be produced and shipped
more cheaply than conventional kraft file folders and are
less harmful to the environment.
The invention still further relates to cards,
such as tag cards, postcards and return postcards. The
return postcards of the invention are made from stock
having a basis weight of from 90 to llOg/m2 and a caliper
of at least 0.007 inches, preferably from 0.007 to 0.008
inches. This thickness helps to provide sufficient
stiffness to enable the return postcards to be processed
by post office sorting equipment without being damaged.
Index dividers are contemplated as paper products of the
invention. The index dividers may preferably have from 70
per cent groundwood pulp fibres and 30 per cent high yield
sulfite pulp fibres and have a thickness of about 165 um
at a basis weight of about 103 g/m2.
As hereinbefore mentioned, the present invention
also relates to a process for preparing a kraft-substitute
bristol paper sheet having a density in the range of 0.5
to 0.75 g/cm3 and a tear strength of from 700 to 2300,
preferably 1100 to 1500 mN, comprising: mixing unbleached
and unrefined mechanical pulp and chemical pulp to obtain
a furnish comprising from 70 to 95 per cent mechanical
pulp and from 5 to 30 per cent chemical pulp; depositing
a jet of the furnish on the wire of the forming section
of a paper machine to form a web and running the web
through the paper machine to produce the paper sheet.
Preferably, the furnish has a brightness of from 45 to 70.
Suitable pulps for use in the process of the
invention are described elsewhere herein. The mechanical
pulp and preferably also the chemical pulp may be
unrefined. Refining refers to the mechanical treatment of
pulp, usually carried out in conical or disc type
refiners, where the pulp fibres are modified by flowing
parallel to bar crossings.
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The mechanical and chemical pulps are mixed or
blended together, resulting in a furnish having from 70 to
95, per cent mechanical pulp and 5 to 30, cent chemical
pulp. Blen~i~g of the pulp furnish may be carried out in
a furnish beater or large chest with a pump having
sufficient agitation to ensure a high turnover rate and
adequate iYing. The resulting furnish has 15 to 30 per
cent ligneous material.
The furnish may be prepared as described above
and fed into a headbox on a paper making machine, such as
a fourdrinier, double wire or cylinder machine.
In a fourdrinier machine, for example the
furnish is fed from the head box onto the continuously
moving fourdrinier wire in the forming section of the
machine. As the slurry advances on the wire down the
forming section of the machine an initial amount of the
water in the pulp furnish drains through the wire into
drainage units to form a web of pulp product supported on
the wire. Close to the downstream end of the wire
additional amounts of water are forcibly le.ovad from the
web by means of suction boxes in contact with the lower
surface of the wire.
Once the web of pulp product reaches the end of
the wire it is passed over a suction couch roll which
extracts further water from the web of pulp product which
emerges from the couch roll as a self supporting web of
pulp product which can be peeled off the wire and
subjected to further processing steps, such as pressing,
drying and calendering to ~ ve water and improve the
surface and finish of the product.
The invention will be more fully understood by
reference to the following examples. However, these
examples are merely intended to illustrate embodiments of
the invention and are not to be construed to limit the
scope of the invention.
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EXAMPL~S
EXAMPLB 1
File ~older
File folders were prepared from groundwood pulp
as follows. A groundwood pulp furnish, cont~ning 70 to
85 per cent stone groundwood pulp and 15 to 30% per cent
northern softwood kraft pulp or high yield sulphite, was
used. The northern softwood kraft pulp was used without
refining, so as to maximize the tear strength. The stone
groundwood was used at a Canadian stAn~Ard freeness of
about lOOmL. No chemicals were added to the pulp.
A fourdrinier paper machine was used, running at
a speed of about 550 feet per minute. Trial runs were
performed using a range of basis weights from about 75 to
110 lbs/3000ft2 and various amounts of calendering. When
the file folder stock was calendered by a stack of six
calender rolls to a constant caliper of 0.0105 inches and
the basis weight was about 103 pounds per 3000 ft2, the
smoothness achieved was found to match that of competitive
conventional kraft file folders.
The paper was converted into file folders using
scoring dies. As the paper was less dense than
conventional kraft file folders, increased pressures were
used to obtain a good quality score line in the paper and
thus a good quality folder. Paper stock contAining 85%
stone groundwood and 15 % kraft was generally found to
give the best results. Paper stock contAin;ng 70~ stone
groundwood and 30 ~ high yield sulphite pulp gave poorer
quality folds with stiff high yield sulphite fibres
sticking out from the fold.
The file folders prepared from paper stock
cont~ining 85% stone groundwood and 15 % kraft as
described above were 0.0105 inches in caliper but weighed r
only 103 pounds per 3000 ft2 ream. This is in contrast to
conventional kraft file folders, such as commercially
available Grand & Toy Permax file folders, which are
0.0105 inches in caliper and weigh 137 pounds per ream.
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- 15 -
Accordingly, the novel file folders of the invention weigh
approximately 25% less than conventional kraft file
folders.
r EXA~PL~ 2
5 ReLuln Postc_rds
Return postcards were prepared from groundwood
pulp as follow~. A groundwood pulp stock, contAining 70
per cent groundwood pulp and 30 per cent high yield
sulphite pulp, was used. Return postcard stock was made
with a basis weight of 106.5 g/m2 and a thickness of 0.0076
inches. Kraft return post card stock is generally made at
a mini~lm of 110 g/m2 with a thickness of 0.0070 inches.
Return postcards are enclosed for example with
magazines and are designed to be completed and returned by
mail. Accordingly, the post cards must have sufficient
stiffness as well as tear resistance in order that they
can be processed in post office sorting equipment. The
increased thickness of the return postcards prepared from
groundwood pulp provides added stiffness for this purpose.
The cards are always processed in "landscape mode~, but
depending on how they are cut, they can be grain long" or
grain short", i.e. the machine direction of the paper
m2chine is either the longer or the shorter dimension of
the rectangular postcard. Thus, the stiffness and tear
strength need to be sufficient in both the machine
direction and the cross-machine direction of the paper
machine. This was achieved by making a sheet with a low
machine-direction/cross-machine direction (MD/CD) fibre
orientation ratio.
The low MD/CD fibre orientation ratio was
achieved by obtAining the same speed at the ~et of stock
exiting the he~hox slice as the fourdrinier section wire.
This is in contrast to conventional operation of the paper
machine where the wire is run at a higher speed than the
~et. This tends to orient a ma~ority of the fibres in the
machine direction and gives a typical MD/CD stiffness
ratio of 2.S to 5.0 and a MD/CD tear strength ratio of
=
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- 16 -
0.25 to 0.4. Having the jet/wire speed ratio at close to
1.0 allowed the MD/CD stiffness ratio to be reduced to 1.5
to 2.5 and the MD/CD the tear strength ratio to 0.3 to
0.7. ,~
S ~XAMPL~ 3
Tndex dividers
Index dividers were prepared from groundwood pulp as
follows. A groundwood pulp stock, contAining 70 per cent
groundwood pulp and 30 per cent high yield sulfite pulp,
was used. A stack of six calender rolls was used to give
a smooth surface and to control the thickness to 165 ~m.
The index dividers were prepared with a
thickness of 165 um at a basis weight of 103 g/m2. In
contrast, conventional index dividers made from 100 per
cent kraft pulp have a basis weight of 120 g/m2 at the same
thickness.
