Note: Descriptions are shown in the official language in which they were submitted.
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HIGH STRENGTH, CELL~LOSIC-GEL CONTAXNING
~RAFT PAPER AND PROCESS FOR ~KING SAME
BACXGROUND AND GENERAL STATEMENT OF THE INVENTIO~
This invention pertains to high strength, cellulosic-
gel-containing kraft paper and to a process for making the
same.
In a typical procedure, cellulosic gels are prepared
by feeding a chemical papermaking pulp such as a Kraft
pulp to a conventional hydrapulper in which it is dis-
integrated into a fibrous slurry having a consistency of
from l to 10%, preferably from 6 to 8~.
The resulting slurry is fed t~ a battery of several
conventional pulp refiners arranged in series. The flow
through the sequence of refiners is throttled down by
appropriate valving to give a pressurized dwell or residence
time su~ficient to exhaustively hydrate the feed material
and produce a hyclrated cellulosic gel product.
The use of c:ellulosic gels prepared in this manner
as bindexs in the manufacture of wet process composition
boards of various categories is illustrated in the following
patents:
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Roberts U.S. 3379608-g Roberts 4173248
Roberts V.S. 4043862 ~eritage Canada 449167
Roberts U.S. 407783~
Roberts U.S. 4087317
However, cellulosic gels heretofore have not been
applied successfully as efficien~ binders in the manufacture
of ~raft papers since it appeared that the physical properties
of the gels would prohibit their successful application to
this end use.
Papermaking machines operate at very hi~h speeds, for
example, at speeds of as much as 3500 feet per minute. It is
necessary that the furnish supplied to them drain quickly
through the wire, i.e. in a matter of seco~ds.
Cellulosic gels ar~ thick, viscous, sticky liquids.
It would seem self-evident that to include a significant
proportion o~ them in a papermaking furnish, in order to
improve the strength of the paper product, would increase
the drain time of the furnish to such a level as to make it
unsuitable for use on high speed paper machines. Accordingly,
heretofore the desired high strength of the paper pxoducts
has been developed by refining the pulp to a high degree,
and by using long fibered stock which inherently imparts high
strength ts the paper. This is undesirable, since prolonged
refining requires expensive equipment the use of which is
attended by high power costs. Long fibered stocks are
comparatively expensive. Substantial economies oould be
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achieved by reducing the refining time and using short
fibered stoc~s, f~r example, short fibered hardwood stocks.
I haYe discovered that, surprisingiy, the inclusi~n
of a substantial proportion of cellulosic gel in Rraft
papermaking furnishes does s~ot materially increase their
drain times on the Fourdrinier wire. Additionally, their
inclusion materially increases the strength of the paper
and reduces significantly the refining time which must be
applied to the freshly cooked pulp.
~ccordingly it is the general object of my invention
to provide a process which will result in the production
of Kraft papers of improved quality at significantly lower
cost.
~ further object of my invention is the provision of
a process fox the.production of Xraft papers of improved
strength, and having smoother, harder surfaces.
Another object of my invention is the provision of a
process for making kraft papers which permits the inclusion
of a significant amount of low cost, short fibered pulp
such as hardwood pulp without adversely affecting the
strength of the paper products.
~enerally stated, my presently described process of
making high strength kraft papers-characterized by the above
noted advantages comprises first mechanically refining kraft
paperma~ing pulp to a freeness value of about CSF 5~0-800.
Next, 2 fluid mixture is made of ~rom about 90 to about 99
of the resultin~ refine~ pulp and from about l to a~out
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10% of hydrated cellulosic gel binder.
The kra'~t papermaking pulp which is a primary component
o~ ~he mixture comprises from about 40 to about 100~ ~y ~
weight of freshly prepared or "virgin~ kraft pulp and from
S 0 to 60~ by weight of pulp prepared from reclaimed or
"secondary" lignocellulosic fibers, which may include a
substantial proportion of short fibers.
The mixture is run into a felted sheet which is pressed
and dried to form tbe final kraft papex product.
DESCRIPTION OF PREE'ERRED EIIBODIMENTS
OE' THE INVENTION
As noted above, the new kraft paper of my invention
broadly comprises kraft papermaking pulp and cellulosic
gel.
These two materials are employed in the following
general and preferred proportions, given in percent by weight,
dry sollds basis:
General Preferred
Kraft papermakinq pulp 90-99 94-98
~ Cellulosic gel 1-10 2-6
The kraft papermaking pulp may be derived from the
usual.sources and is prepared by the.sulfate pulping of the
woods of various species of trees Dr other lignocellulosic
material.
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After the pulp has been r2moved from ths digester,
it is washed and screened and then subjec~ed to ~ re~tively
brief treatment in a con~entional refiner, i.e. a double
d~sc refiner such as a Sprout-Waldron or Beloit Jones re-
finer. The refining need be carri~d ou~ only to a limiteddegree, i.e. until ~he Canadian Standard Freeness has reachea
a value of from about 500 to about 820J preferably from about
600 to about 750, with consequent saYing of energy.
The kraft papermaking pulp component may comprise
1~ 100% virgin pulpt prepared as outlined above, or, it may
include a proportion, i.e. from 0 to 60% of its weigbt,
dry solids basis, of pulps other than virgin kraft pulp.
These secondary fibers may include fibers derived ~rom
cl.eaned papermaking sludge, short fibers derived from the
"save all" of the paper mill, from waste paper, from waste
cartons and bags, and even from sawdust. The weakening
e~fect of these relatively short, lower qiality fibers
is overcome by the reinforcing action of the added cellulosic
gel.
Where the secondary fibers are derived from waste paper
or waste cartons, they are converted to a pulp of papermaking
consistency by processing them in a hydrapulper followed by
suitable refining i~ a conventional deflaker or refiner to
break up ~iber clumps and bundles. This refined stock then
is combined with ~he virgin pulp stock.
~ he gel component of the.papermaking mixture comprises
the exhaustively hydrated cellulosic gel described, for
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example, in my U.S. patents 4043862, 40~7833, and 4173248.
In its manufacture, che~ical cellulose pulp such as
kraft cellulose pulp is refined and hydrated exhaustively to
a,high degree at which fiber structure is almost completely
destroyed. SpeciXically, this is accomplished by breaking
down cellulose pulp sheets to their component individual
fibers and fiber clumps, preferably by adding the dry sheets
and water to a conventional hydrapulper and hydrapulping at
a stocX consistency of from 1 to 10%, preferably from 6 to
8%. This requires about 30 minutes.
The resulting pulp then is fed in controlled flow to a
selec~ed disc-type or conical-type primary refiner. There
preferably are three such refiners arranged in series with
a flow-restricting valve downstream from the last refiner
to insure an adequate pressurized refiner dwell time. These
abrade the pulp and hydrate it to a high degree, thereby
forcing water through the cellulosic structure.
The resulting par~ially refined and hydrated pulp is
then fed to a secondary refiner of the same general class
as the primary refiners~ but which is effective to complete
the hydration~ The pulp is reduced in size to values impart-
ing to it TAPPI standard drain times of at least 350 seconds,
preferably above 900 seconds. This is accomplished by a
shearing action which almost completely destroys the fi~er-
structure o~ the pulp and hydrates i~ exhaustively. Thissupple~ental and exhaustive refining greatly improves the
gualities of the pulp as a ~ind2r, dispe~sing agen~ and re
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tention agent when used in the manufacture of the herein
described high strength paper.
In particular, it makes of the gel an ~ixreversible~
binder. As a result, paper made with it is highly water
resistant.
A method other than the TAPPI drain time test for
characterizing a gel suitable for the present purpose is to
determine the shrinkage upon drying of the handsheet
produced by the TAPPI drain time test. A suitably hydrated
gel will form a hand sheet which shri~ks upon drying to a
diameter which is at least 25~ smal~er than its original
diameter.
In another method, the hand sheet is dried and a small
fl'ame applied to its underside. If the cellulose is suffi-
lS ciently hydrated, this instantaneously will produce a blisteri~ the sheet.
In still a further test procedure, 250 milleliters
of the gel slurry is dried into a solld ball. If the gel
is sufficiently hydrated, the ball will sink when dropped
into water and thereafter will remain hard without swelling
for an indefinite period of submergence.
In addition to the cellulosic gel prepared as described
abov~, varîous additives may also be included in the paper-
making ~urnish as desirable or necessary. These may include
small proportions of dyes, rot-proofing agents, wet strength
agents, wax, and the lik~.
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The pulp may be bleached if desired, in which case it
usually is preferred to use a bleached gel as well.
To prepare the furnish, all the foregoing materials.
a~e added to a suitable mixer such as a Waring blender and
mixed to uniformity. The resulting mixture then is added
to the headbox of the paper machine, and run into a felted
sheet, which is pressed and dried to ~orm the final paper
product.
E~PLES
The process of my inve~tion is illustrated in the
following examples wherein the amounts of the constituents
are given in percent by weight, dry weight basis.
A conventional kraft bag-making stock refined to a
freeness of CSF S50 was mixed with about 24~ by weight
of pulp having a freeness of about CS~ 550 and containing
secondary fibers derived from hydxapulping principally
corrugate~ kraft ~oxes and passing the resulting pulp through
a deflaker for preliminary refining in order to break down
fiber bundles and clumps. The resulting pulp mixture had
a consistency of about 0.3~. It was mixed with exhaustively
hydrated cellulosic gel prepared as outlined above and
having a TAPPI drain time of 550 seconds and a shrinkage
of 27~ when subjected to the handsheet drying test.
~he pulp and gel were formulated into 6 test mixtures.
containing respectively 0, 2, 4, 6, 8 and 10~ gel.
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The mixtures were run into standard test hand sheets
having a weight of 200 grams per square meter. These then
were ;tested by the Standard TAæPI Test for measuring stiff-
ness, CSF freeness, burst, tear and tensile. The results
are given in conventional TAPPI test units in the table
below.
% GEL STIF~NESS FREENESS BURST TEl~RTENSII;E
0 55 720 16~ 320 17,1
2 50 700 172 342 17,6
4 80 69D 198 ~76 19,3
6 75 670 190 392 .25,1
8 75 670 187 408 22,7
63~ 161 32~ 19.8
Inspecti~n of the above test data illustrates clearly
tha~ inclusion of as much as 8% gel reduces the CS~ freeness
but a small amount, i.e. from 720 to 670. This is not a
sufficient reduction to materially increase the drain time
of the furnish on the wire.
The results inclicate further a significant increase in
strength, for example, an increase in tensile strength of
from 17.1 to 22.7.
If.the gel binder were to be omitted, refining of the
original virgin stock to a freeness of CSF 400 to 45~ w~uld
be reguired to attain equivalent sheet strength, with
at~endant high ref ining costs.
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Other benefits accrue from the use of cellulosic gel
in the manufacture of kraft paper.
Its use permits the addition of lower cost fibers
t~ the paper sheet without significant reduction o~ strength
properties.
The qel can be made from a variety of fiber raw materials
as well as from conventional kraft and sulfite pulp.
~ t 6% gel usage, with con~entional pressing, it ma~es
a thicker sheet at the same welght as the regular pulp.
~ccordingly a lighter wei~ht sheet having acceptable physical
properties can be made at present thicknesses.
The gel usage gives a paper sheet with a harder,
smoother surface.
The reinfDroing effect of the gel shows on papers made
from hardwoods, so~t woods, recycled paper and thermo
mechanical pulp.
Use of gel in corrugating medium gives a substa~tial
increase in compres~ion resistance. This pexsists under
conditions of very high humidity.
Gel is an excellent fiber retention aid. It holds onto
most of the very fine fibers which normally are lost to the
formi~g wire of the paperma~ing machineO
If the papçr machine is improved so it will ru~ faster,
as by installing an ~extended nip~ or ot~er thermal or
mechanical impro~ements, the use of gel will permit the
additional pulp ~eeded to come from the lower cost secondary
fiber sources listed above.
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In sum, the use of cellulosic gel in kraft paper
improves all impo~tant sheet physical properti~ and ~nakes
possible the production of higher quality paper ~t lower
Gost.
S Having thus described my inventiorl in spscific embodi-
ments, I claim as new and desire to protect by Letters Pa~ent:
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