Note: Descriptions are shown in the official language in which they were submitted.
115~5~4
PAPERMAKING
The present invention relates generally to paper-
making processesand, more particularly, to the use
of a binder in a papermaking process, the binder com-
prising a complex of cationic starch and colloidal
silicic acid to produce a paper having increased strength
and other characteristics. Such a binder, in addition,
also effects highly improved levels of retention of
added mineral materials as well as papermaking fines.
At the present time, the papermaking industry
is plagued with a number of serious problems. First,
the price of cellulosic pulp has escalated materially
and hign quality pulp is in relatively short supply.
Second, various problems including the problems inherent
in the disposal of papermaking wastes and the ecological
requirements of various governmerltal bodies have marked-
ly increased the cost of papermaking. Finally, the
costof the energv required to make paper has increased
materially. As a result, the industry and its custo-
mers are faced with two choices: either pay the higher
costs or materially decrease the amounts and/or quality
of the cellulosic fibers with a consequential loss
of quality in the finished paper product.
The industry has made various attempts to reduce
the cost of the paper products. One approach that has
been employed involves the addition of clay and other
mineral fillers in the papermaking process to replace
fiber but such additions have been found to reduce
the strength and other characteristics of the resulting
paper to a degree which is unsatisfactory. Also, the
addition of such mineral filler results in poor reten-
tion of the filler material, e.g. they pass through
the wire to the extent that the level of filler materials
builds up in the white water with the result that the
clean up of white water and the disposal of the material
1~4S~
becomes a serious problem. Various binders have been
employed in an attempt to al]eviate the retention problem
but their use has not been entirely satisfactory.
Attempts have also been made to use types of pulp
which are less expensive and of lower quality, but
this, of course, results in a reduction in the charac-
teristics of the paper and often results in excessive
fines which are not retained in the papermakir,g process
with the consequent white water disposal problems.
In Canadian application Serial llo. 373,025 filed
in the Patent Office conte~poraneously
with this application and assigned to the assignee
of this invention,
there is disclosed a binder system and method which
produce improved properties in paper and which will
permit the use of minimum amounts of fiber to attain
strengths and other properties which are required.
In general, the system of that invention includes the
use of a binder complex which involves two components,
i.e. colloidal silicic acid and cationic starch. The
weight ratio between the cationic starch and the SiO2
in the colloidal silicic acid is greater than one and
less than about 25. The two components are provided
in the stock prior to formation of the paper product
on the papermaking machine. It has been found that,
after drying, the sheet has greatly enhanced strength
characteristics. Also, it has been found that when
mineral fillers such as clay, chalk and the like are
employed in the stock, these mineral fillers are effi-
ciently retained in the sheet and further do not havethe degree of deleterious effect upon the strength
of the sheet that will be observed when the binder
system is not employed.
While the mechanism that occurs in the stock and
during paper formation and drying in the presence of
the binder is not entirely understood, it is believed
.. .
~s~
that the cationic starch and the anionic colloida~
silicic acid form a complex agglomerate which is bound
t~gether by the anionic colloidal silicic acid, and
that the cationic starch becomes associated with the
surface of the mineral filler material whose surface
is either totally or partially anionic. The cationic
starch also becomes associated with the cellulosic
fiber and the fines, both of which are anionic. ~pon
drying, the association between the agglomerate and
the cellulosic fibers provides extensive hydrogen bond-
ing. This theory is supported in part by the fact that
as the Zeta potential in the anionic stock moves towards
zero when employing the binder complex of the invention
both the strength characteristics and the retention
improve.
The principal object of this invention is to further
enhance the effect of the binder complex disclosed
in application Serial No, 373,025 _ . Other objects
and advantages of the invention will become known by
reference to the following description and the appended
drawings in which:
Fig. 1 is a flow diagram of a papermaking process
embodying various of the features of the invention;
Fig. 2 is a chart showing a test run on a papermak-
ing machine, the process employed embodying variousof the features of the invention.
Thus the present invention provides a papermaking
process in which an a~ueous papermaking stock containing
a cellulosic pulp is formed into a sheet and dried, the
sheet comprising over 50% cellulosic fiber. The stock
includes a binder comprising colloidal silicic acid having
an average particle size of less than 20nm, and cationic
starch having a degree of substitution of between 0.01
and 0.05. The weight ratio of cationic starch to SiO2 is
35 between 1:1 and 25:1, and the weight of solids in the
binder is between 0.1-15% by weight of the pulp. The
improvement comprises intermixing in the stock first a
.
5tj~
portlon of the colloidal silicic acid and then the ca-tionic
starch and after an agglomerate has formed, adding and
intermixing the remainder of the colloidal silicic acid
in the stock prior to the formation of the sheet, said
first portion of colloidal silicic acid comprising 20-90%
of the total colloidal silicic acid added.
In another aspect the invention provides such a
papermaking process wherein the stock includes a binder
comprising a colloidal silicic acid sol having silica
particles with a surface area of from about 50 to 1000 m2/g
and cationic starch having a degree of substitution of
between 0.02 and 0.04, the weight ratio of cationic starch
to SiO2 being between 1.5:1 and 10:1. The weight of
solids in the binder is between about 0.1 and 15% of the
weight of the pulp. The improvement comprises intermixing
in the stock first a portion of the colloidal silicic acid
and then the cationic starcn and, after an agglomerate
has formed, adding and intermixing the remainder of the
colloidal silicic acid in the stock prior to the formation
of the sheet, said first portion of colloidal silicic
acid comprising 20-90% of the total colloidal silicic
acid added.
In still another embodiment the invention provides
such a papermaking process wherein the stock includes a
binder comprising colloidal silicic acid sol with particles
having a surface area of from about 300 to 700 m2/g and
cationic starch having a degree of substitution of between
0.02 and 0.04, the weight ratio of cationic starch to ciO2
being between 1.5:1 and 10:1. The weight of solids in the
binder is between 1.0-15% of the weight of the pulp. The
improvement comprises intermixing in the stock first a portion
of the colloidal silicic acid and then the cationic starch and,
after an agglomerate has formed, adding and intermixing the
remainder of the colloidal silicic acid in the stock prior
to the formation of the sheet, said first portion of colloidal
silicic acid comprising 20-90% of the total colloidal silicic
acid added. In another aspect the invention provides such a
papermaking process in which the weight ratio of cationic
;4
3b
starch to SiO2 is between about 1.5:1 and 4.5:1.
In a preferred emhodiment the process as set out in
the above aspects is provided wherein between about 30 and
about 80 percent of the colloidal silicic acid is added to
the stock to form an agglomerate and the remainlng portion
of the colloidal silicic acid is added after the formation
of the agglomerate.
We have discovered that when a binder of the type
disclosed in copending application Serial No. 373,025
is employed, the effect of the binder system may be
enhanced by adding the colloidal silicic acid compo-
nent in several increments, i.e. a portion of the colloi-
dal silicic acid is admixed with the pulp and the mineral
filler when present, then the cationic starch is added
and thereafter when a complex agglomerate of pulp,
filler (if any), silicic acid and starch is formed
and before the stock is fed to the head box of the
papermaking machine the remaining portion of the colloi-
S~
dal sillcic acld is admixecl with t~le stock containin~the complex agglomerate. This proc~dure of supplyinc~
the colloidal silicic acid in two or more steps results
in certain improvements in stren~th and other charac-
teristics, but the most striking improvement is theincrease in retention of filler and papermaking fines.
The reason for these improvements is not entirely
understood but it is believed that they result from
the production of complex filler-fiber-binder agglo-
merates, which are more stable, i.e. that the lateraddition of the colloidal silicic acid causes the agglo-
merates initially formed to bond together to form even
more stable agglomerates which are less sensitive to
mechanical and other forces during the formation of
the paper.
Based upon the work that has been done to date,
the principles of this invention are believed applicable
in the manufacture of all grades and types of paper
products. For example, printing grades, including news-
print, tissue, paper board and the like.
It has been found that the greatest improvementsare observed when the binder is employed with chemical
pulps, e.g. sulfate and sulfite pulps from both hard
and soft wood. Lesscr but highly significant improvements
occur with thermo-mechanical and mecllanical pulps.
It has been noted that thc presencc of excessive amounts
of lignin in ground wood pulps seems to interfere with
the efficiency of the binder so that such pulps may
require either a greater proportion of binder or the
inclusion of a greater proportion of other pulp of
low lignin content to acllieve the desired result. (~s
used herein, the terms "cellulosic pulp" and "cellulosic
fiher" refer to chemical, thermo-mechanical and mecha-
nical or ground wood pulp and the fibers contained
therein.)
The presence of cellulosic fibers is essential
to ohtain certain of the improved results of the inven-
~ 15~S~j~
tion WhiCIl occur because of the in-teraction or asso-
ciation of ac3glomcrate and the cellulosic fibers. Pre-
ferably, the finished paper should contain over 506
cellulosic fiber but paper containing lesser amounts
of cellulosic fibers may be produced which have greatly
improved properties as compared to paper made from
similar stocks not employing the binder agglomerate
described herein.
Mineral filler material which can be employed
includes any of the common mineral fillers which have
a surface which is at least partially anionic in charac-
ter. Mineral fillers such as kaolin (china clay), ben-
tonite, titanium dioxide, chalk, and talc all may be
employed sa~isfactorily. (The term "mineral fillers"
as used herein includes in addition to the foregoing
materials, wollastonite and glass fibers.) When the
binder complex disclosed herein is employed, the mineral
fillers will be substantially retained in the finished
product and the paper produced will not have its strength
degraded to the degree observed when the binder is
not employed.
The mineral filler is normally added in the form
of an aqueous slurry in the usual concentrations employed
for such fillers.
As pointed out above, the binder comprises a combina-
tion of colloidal silicic acid and cationic starch.
The colloidal silicic acid may take various forms,
for cxamplc, it may be in the form of polysilicic acid
or colloidal silica sols although bcst results are
obtained through the use of colloidal silica sols.
Polysilicic acid can be madc by reacting watcr
glass with sulfuric acid by known procedures to provide
molecular weights (as SiO2) up to about 100,000. However,
the resulting polysilicic acid is unstable and difficult
to use and presents a problem in that the presence
of sodium sulfate causes corrosion and other problems
in paperrnaking and white water disposal. rrhe sodium
~s~5~j~
sulfate may be renloved by ion exchange througll the
use of known methods but the resulting polysilicic
acid is unstable and without stabilization will deterio-
rate on storage. Salt-free polysilicic aci.d may also
be produced by direct ion exchange of diluted water
glass.
While substantial improvements are observed in
both strength and retention with a binder containing
polysilicic acid and cationic starch, superior results
are obtained through the use with the cationic starch
of colloidal silica in the form of a sol containing
between about 2-60~ by weight of SiO2 and preferably
about 4-30% SiO2 by weight.
The colloidal silica in the sol should desirably
have a surface area of from about 50 to 1000 m2/g and
preferably a surface area from about 200 ~o 1000 m2/g
with best results being observed when the surface area
is between about 300 to 700 m2/g. The silica sol is
stabilized with an alkali having a molar ratio of SiO2
to M20 of from 10:1 to 300:1 and preferably a ratio
of from 15:1 to 100:1 (M is an ion selected from the
group consisting of Na, K, Li and NH4), It has been
determined that the size of the colloidal silica particles
should be under 20 nm and preferably should have an
average size ranging from about 10 down to 1 nm. (~
colloidal silica particle having a surface area of
about 500 m2/g involves an average particle size of
about 5.5 nm),
In essence, it is pr~ferably sought to employ
a silica sol having colloidal silica parti.cles which
have a maximum active surfacc and a well defilled
small size generally averaging 4-9 nm.
Silica sols meeting the above specifications are
commercially available from various sources includ-
ing Nalco Chemical Company, Du Pont & de Nemours Corpo-
ration and the assignee of thi.s invention.
The cationic starch which is employed in the binder
may be made from starches derived from any of the common
Stj~
starch producing mater1als, e.g. corn starch, wheat
starch, potato starch, rice starch, etc. ~s is ~Jell
known, a starch is made cationic by ammonium group
substitution by known procedures. Best results have
been obtained when the degree of substitution (d.s~)
is between about 0.01 and 0.05 and preferably between
about 0.02 and 0.04, and most preferably over about
0.025 and less than about 0.04. While a wide variety
of ammonium compounds, preferably quaternary, are employ-
ed in making cationized starches for use in our binder,we prefer to employ a cationized starch which was pre-
pared by treating the base starch with either 3-chloro-2-
-hydroxypropyl-trimethyl ammonium chloride or 2,3-epoxy-
propyl-trimethyl ammonium chloride to obtain a cationized
starch having 0.02-0.04 d.s.
In the papermaking process the binder is added
to the papermaking stock prior to the time that the
paper product is formed on tlle papermaking machine.
In the initial addition, a portion of the colloidal
silicic acid component and the cationic starch may
be mixed together to form an aqueous slurry of the
silica-cationic starch binder complex which then can
be added to and thoroughly mixed with the papermaking
stock. However~ this procedure does not provide maximized
results. It is preferable that the initial silica-cationic
starch complex is formed in situ in the papermaking
stock. This can be accomplished by adding the initial
portion of the colloidal silicic acid componcnt in
the form oE an aqueous sol and the cationic starch
in the form of an aqueous solution separatcly to the
stock in a mixing tank or at a point in the system
where there is adequate agitation so that the two com-
ponents are dispersed with the papermaking components
so that they interact witll each other, and with the
papermaking components at the samc time.
Even better results are obtained if the initial
portion of the colloidal silicic acid component is
~s4'j~
added to a portLon of the stock and thorouyhly mixed
therewit}l after wlllch -the make-up of the stock is complet-
ed and the cationic starch component is added and -tho-
roughly mixed with the stock prior to the formation
of the paper product.
In the event that a mi,neral filler is to be added
to the stock it has been found preferable to slurry
the mineral filler in water with the initial portion
of the colloidal silicic acid component and then to
introduce the filler-colloidal silicic acid component
slurry into a mixing device where it is incorporated
into the stock along with the pulp and cationic starch.
Thereafter, the final portion or portions of the
colloidal silicic acid component are thoroughly mixed
with the stock after the initial agglomerate is formed
and prior to or at the time the stock is conducted
into the head box. The initial addition of the colloidal
silicic acid should comprise about 20 to 90 percent
of the total amount to be added and then, after the
initial agglomerate is formed, the remainder should
be added before the sheet is formed. Preferably, the
initial addition should comprise 30-80% of the colloidal
silicic acid component.
It has been found that in a papermaking process
employing the binder complex described herein, the
p~l of the stock is not unduly critical and may range
from a p~l of from 4 to 9. I-lowever, pll ranges higher
than 9 and lower than 4 are undesirablc. Also, other
paper chemicals such as si~ing agents, alum, and the
li)ce may be employed but care should be taken that
the level of thcse agents is not great cnough to interferc
with the formation of the silica-cationic starch agglo-
merate and that the level of the agent in recirculating
white water does not become excessive so as to inter-
fere with the formation of the binder agglomerate.Therefore, it is usually preferred to add the agent
at a point in the system after the agglomerate is for-
med.
~5~LS~i~
~s pointed out ahove, the ratio of cationlc stal-ch
to the total colloidal silicic acid componerlt should
be between 1:1 and 25:1 by weight. Preferably, tlle
ratio is between 1.5:1 and 10:1 and most preferably
between 1.5:1 and 4.5:1.
The amount of binder to be employed varies ~ith
the effect desired and the characteristics of the par-
ticular components which are selected in making up
the binder. For example, if the ~inder includes poly-
silicic acid as the colloidal silicic acid component,
more binder will be required than if the colloidal
silicic acid component is colloidal silica sol having
a surface area of 300 to 700 m2/g. Similarly, if the
cationic starch, for example, has a d.s. of 0.025 as
compared to a d.s. of 0.030, more binder will be re-
quired assuming the colloidal silicic acid component
is unchanged.
In general, when the stock does not contain a
mineral filler the l~vel of binder may range from 0.1
to lS~ by weight and preferably from 1 to 15% by weight
based upon the weight of the cellulosic fiber. ~s point-
ed out above, the effectiveness of the binder is greater
with chemical pulps so that less bindcr will be required
with these pulps to obtain a given e~ffect than other
types. In the event that a mineral filler is employed
the amount of binder may be based on the weight of
the filler material and may range from 0.5 to 25~ by
weight and usually betwcen 2.5 to 15~ by weight of
the filler.
Tlle following are specific examples of a process
embodying features of the invention.
EXAMPLE 1
A commercial trial run was made making a coated,
off-set, super calendered printing paper having a gram-
mage of 85 g/m2. The machine employed was a twin wire
Beloit "Bel-Baie" machine having a capacity of about
10,000 kg/hour at a speed of about 600 m/min. The coat-
1 ll54S~i~
1 0
ing was accomplislled "on-line" with lO g/m2 of calcium
carbonate applied to each side of the sheet. The cellu-
losic fiber comprised 70% sulfate hardwood ancl 30~
sul ate soft~ood pulp both of which were fully bleached.
The ~1~ of the white water was about ~.5.
In the operation of the machine which was employed,
the ~uality requirements for the paper produced by
it were very rigid. As a result, in normal operation,
a high proportion of the finished coated paper, about
25%, is classified as "broke". Broke, is unsatisfactory
paper which is recycled into the stock and is reformed
into a paper web. As a result, the stock to the machine
head box contains a large proportion of filler in the
form of reslurried coating from the hroke. The propor-
tion of the broke is often as high as 50% of the solidsin the total stock.
The presence of the additional filler from the
broke constitutes a serious problem in normal operation
of the machine since its retention on the papermaking
wire is extremely poor and most of it finds its way
into the white water and eventually into the sewer.
Also, since the arnount of broke always varies, the
filler content in the base sheet varies causing uneven
sheet properties with t'ne result that there are numerous
breaks in the paper wcb during production with attendant
loss of production.
Fig. l is a flow diagram indicating the general
operation which was employed in the run of this example
employing various of the teachings of the invention.
In Mixing Tank No. l, the two types of bleached
pulp which were typically uscd in the plant, i.e. the
70% sulfatehardwood and 30% sulfate softwood ?ulp, both
fully bleached, were mixed together with the slurried
broke. In order to compensate for varying amounts of
filler in the stock caused by differing amounts of
broke, arrangements were rnade to add a desired amount
of extra filler (calcium carbonate). At this point,
1 1
the amoullt o~ extra filler addecl was depenclent ~Lpon
the ash con-tent which was measured continuoucly on
line in the base sheet and enouyh calcium carbonate
fil]er was added to mcLintain the level of ash in the
finished paper base sheet at 15% by weight of dry paper.
In addition, in Mixing Tank No, 1, there was added
in the form of an aqueous solution of colloidal silica
containing 15~ by weight SiO2, in an amount equivalent
to 1.7 kg of SiO2 per metric ton of dry base sheet
(prior to coating)~ The colloidal silica sol was stabi-
lized with alkali with a molar ratio of SiO2:Na20 of
45:1. The silica had a particlc si~e in the range of
from about 5-7 nm and a surface area of approximately
500 m2/g.
The materials were thoroughly mixed and were con-
ducted to Mixing Tank No. 2 where cationic starch was
added to the stock, in an amount equal to 10.2 kg of cat-
ionic starch ~er metric ton of dr~ base sheet.TIle cationic
starch was nre?ared by treating otato starch with
3-chloro-2-hydroxypropyl-trimethyl-ammonium chloride
to provide a degree of substitution (d.s.) in the starch
of 0.03. It was dispersed in cold water at a concen-
tration of about 4~ by weight, heated for 30 minutes
at about 90C, diluted with cold water to a concentra-
tion of about 2~ by weight and then added to MixingTank No. 2.
~ fter the cationic starch was thoroughly intermixed
the stock was conclucted to Mixing Tank No. 3 whercin
a second increment of colloidal silica sol, of the
type described above, was added to the stock in an
amount cqual to 2.1 kg per metric ton of dry base sheet.
From Mixing Tank No. 3 the stock was fed into
the head box of thc paper machine which was operated
at normal speeds to form the base sheet which was sub-
sequently dried, coated with a coating slip containingcalcium carbonate and calendered in the same manner
as before.
~-~54~
12
Fig. 2 ~3raphically illustrates the effect of the
addition of the colloidal silica ancl cationic starch,
as set forth ahove. The Jeft hand side of the chart
shows the condition of the stock of the whi-te water
in the commercial run prior to the addition oE the
colloidal silica and the cationic starch as outlined
above. ~s will be noted, the total solids in the stock
at the former or head box is approximately 15.5 g/l,
of which approximately 8.5 g/l is fiber and 7 g/l is
ash. The base sheet produced from this stock contain-
ed approximately 3 percent ash.
As appears from Fig. 2, the white water in the
commercial run before the addition of the colloidal
silica and cationic starch, contained approximately
10.5 g/l of solids; 6.0 g/l ash; and 4.5 g/l fiber.
The dramatic effect of the addition of the colloidal
silica and cationic starch as ou~lined above, is shown
on the right hand side of FIGURE 2 where thc total
solids in the head box decreased to approximately 6 g/l;
slightly less than 5 g/1 fiber; and about 1.5 g/1 ash.
The total solids in the white water dropped to about
1 g/l; about 0.5 g/1 fiber; and about 0.5 g/l ash. The
base sheet contained approximately 15 percent ash and,
the machine breaks during operation were substantially
less than in the commercial operation where the sheet
contained only 3 percent ash.
Test results showed that even -though the finished
base sheet made, as outlined abov~, had an increased
amount of filler, i.c. from abollt 3 pcrcent to about
15 percent which normally degrades the properties of
the sheet, the additional filler did not materially
decrease the strcngth propcrtics or printing properties
of the paper. To the contrary, certain properties were
increased markedly. For example, Z-strength or Inter-
nal bond strength measured by the Scott-bond method
increased by 85 percent at the 15 percent filler level
as compared to the 3 percent filler level in the commer-
S~L
13
cial run~ c IGT (Instituut Voor Grafische rl'ecllniek,
.~msterda~,l) surface pickincJ resis-tance increased by
40 yercent and the bursting strell~th increased by 40
percerlt .
During the trial, which extended over a several
week period, it was found that it was possible to add
much more broke to the stock than before. At one period
extending for about 16 hours, the entire stock was
broke. Further, with the addition of additional filler
material it was found that it was possible to maintain
15 percent filler in the base sheet over a two-week
period and that the resulting even level of ash per-
mitted an increase in the productivity of the paper
machine due to fewer breaks and a saving of fiber.
It was also found that the coupling of increased
retention and decreased head box consistency resulted
in a marked improvement in the drainage rate of the
stock on the wire. This, of course, means that an in-
crease in the machine speed is made possible which
will even further enhance the productivity.
The retention of fibers and fines on the wire
in the papermaking machine was also greatly improved.
Retention percentagc is determined by dividing the
difference betwcen the concentration of total solids
in the head box and the concentration of total solids
in the white water by the concentration of total solids
in the head box and multiplying by 100. Thus, on the
commercial run preceding the addition of the silica
sol and cationic starch as outlined above, the percentage
30 of retention was 15-~510-5 x 100 or 32~. As a result
of the use of our process the percentage of retention
increased to about 83~(6 0_o'-0- x 100). This high level
of retention simplified white water clean up and dispo-
sal.
EXA~lPLE II
. . _ .
To further demonstrate the advantages of the two-
step operation, extended runs were made under various
5tj~
14
conditions on the commcrci,-ll machine described ln
Example I. The results of these runs are set forth in
tabular form in Table I.
Run 1 reflects the avera~e o~eration of the machine
of Example I in making coated, supercalendcred printing
paper over an extended period of time. The celluloslc
fibercom~rised 70% sulfate hardwood and 30% sulfate soft-
wood, both fully bleached. Normal amounts of broke
were recycled. The base sheet was coated with 10 g/m2
of calcium carbonate per side.
Run 2 reflects the average operation of the machine
of ~xample I over an extended period in maklng coated,
supercalendered printing paper in which the same fiber
was employed and normal amounts of hroke were recycled
in which the colloidal silicic acid employed was a
15'~ aqueous sol having the specifications set forth
in Example I. It was added to Mixing Tank No. 1 at
a level of 3.8 kg of SiO2 per metric ton of dry base
sheet. Cationic starch was added in Mixing Tank No.
2 at a level of 11.8 kg of cationic starch per metric
ton of dry base sheet, the cationic starch having the
specification as set forth in Example I and the method
of addition was as set forth in Example I. No additions
were made in Mixing Tank No. 3. The base sheet after
drying was coated on cacll side with 10 g/m2 of calcium
carbonate.
Run 3 followed the procedure of Run 2 except that
the addition of the silica sol was added in two increments.
Tllere was added in Mixing 'l'an~ No. 1, 2.9 kg of SiO2
per metric ton of dry base she~et. In Mixing Tank No.
2 the cationic starch was addcd at a level of 13.7 kg
of cationic starch per metric ton of dry base sheet.
In Mixin~ Tank No. 3 a second addition of the silica
sol was added at a level of 1.5 ]cg of SiO2 ~er metric ton
of dry base sheet.
The results are tabulated below:
1~5~5~
TABL~,
RUN 1 RUN 2 RUN 3
_ . .
Grarnmage g/m2 85 85 85
Ash content ~ 17 28 24
Tensile index
machine direction
Nm/g 66.2 64.2 64.5
cross direction
Nm/g 21.7 22.5 26.8
Burst Strength
kPa 214 294 310
Surface picking
resistance JGT
top side 73.4 92 112
~ire side 68.7 83 112
Internal bond
strength 2
Scott bond J/m 225 506 525
Concentration
at head box
g/l solids 15.5 10.1 ~.3
White water
concentration
g/l solids 10.5 5.2 1.2
Retention ~ 32.3 48.5 81.0
~.~54~fi9L
16
i~s will be SC'ell from the forec30ing, the use of
a colloidal silicic acid-cationic starch binder complex
in W}liCh t}-e colloidal silicic acid componen-t is added
incrementally, a portion being added after the initial
agglomerate is formed, makes possible substantial econo-
mics in the papermaking process as well as an improved
paper product. Also, a mineral filler may be employed
in much larger proportions than heretofore used while
maintaining or even improving the characteristics and
properties of the sheet. Some of the properties of
a sheet containing filler are enhanced.
In addition, the use of the binder system results
in increased retention of both minerals and fines so
that white water problems are minimized.
Further, because of the ability to reduce the
basis weight of a sheet or to increase the mineral
content, it is possible to reduc~ the energy required
to dry the paper and to pulp the wood fibers since
less fiber can be employed. ~lso, the increased rate
Of drainage and the higher retention on the wire make
possible higher machine speeds.
While a preferred embodiment has been shown and
described it will be understood that there is no intent
to limit the invention by such disclosure, but rather
it is intended to cover all modifications and alternate
constructions falling within the spirit and scope of
the invention as defincd in the appended claims.
