Note: Descriptions are shown in the official language in which they were submitted.
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C A N A D A
APPLICANT: NATIONAL SILICATES LTD.
TITLE: PAPER STRENGTH ENHANCEMENT BY SILICATE/STARCH
TREATMENT
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Background of the Invention
This invention relates to compositions for the
improvement of strength of cast cellulosic mats. The
production of paper, paperboard and other cellulosic
fibre mats is usually accomplished via the formation of
cast cellulosic mats on a moving table or cylinder. In
such processes an aqueous slurry of cellulose fibres is
spread onto the table, also known as the "wire" or
rotating cylinder, and is transformed into a mat by
removal of much of the water from the fibres by suction.
In subsequent steps this mat is converted into paper or
board by pressing and drying of the fibre mat.
A priority in the manufacture of much paper and
paperboard is the strength of the product. While the
long fibres present in virgin fibre derived from wood
have adequate strength properties for most commercial
applications, paper manufactured from fibres which have
been previously used commonly exhibits lower strength due
to damage caused to the fibres during the recycling
process. The recent trends to employ increasingly large
quantities of recycled fibres in paper products has led
to a significant decline in the strength of many
different types of paper products. Although the loss of
strength of such papers can in some cases be countered by
the utilization of larger quantities of virgin fibre,
this is not always economically desirable, virgin fibres
being very much more expensive than those derived from
used paper such as old cardboard cartons, newspapers and
the like. The strength properties of paper and
paperboard of most concern traditionally have been burst,
tear, tensile strength and compressive strength.
An approach commonly employed is to improve the
strength of papers by the incorporation of various types
of chemicals. The literature contains many examples of
such additives, and their properties and utilization is
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well covered in standard monographs on the chemistry of
papermaking. A useful summary of some of the chemicals
which are used for this purpose is to be found in
"Chemical Additives for Improved Compression Strength of
Unbleached Board," by Douglas B. Smith in the proceedings
of the 1992 TAPPI Papermakers Conference 393-404. The
use of sodium silicate either alone or in combination
with starch for the enhancement of the strength of paper
products has already been taught, and both of these have
a long history as additives for the improvement of paper
properties [See for example James G. Vail, Soluble
Silicates in Industry 1928, PP. 287-94. Also E.D.
Kaufman U.S. Patent No. 3,819,555 (1974); E. Strazdins
U.S. Patent No. 3,840,489 (1974); J.A. Sedlak U.S. Patent
No. 3,874,994 (1975); E. Strazdins U.S. Patent No.
4,002,588 (1977); H.S. Killam U.S. Patent No. 4,167,439
(1979); K.M. Seymour and D.G. Seymour U.s. Patent No.
5,358,554 (1994)].
Although it has long been known that incorporation
of soluble alkali salts of silicic acid within the
cellulosic fibres improves the strength properties of
paper products, the actual method of incorporating this
silicate into the sheet in a sufficient amount to be
effective, has until now faced insurmountable barriers.
A description of the different options available for
the incorporations of additives into cellulosic sheets
will help in the explanation of this invention. For
chemical additives to paper to be effective it is
necessary that they be well dispersed throughout the
fibres of the cellulosic mat. This can be carried out in
one of three ways: addition in the "wet end," in a size
press or after the "dry end" by the use of some kind of
application technique. The addition of chemicals in the
wet end of a paper machine (i.e., by introduction at the
head box or into the white water) is a well known common
practice which is relatively easy to do with undissolved
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chemicals (such as filler clays), or with soluble
additives which react chemically with the cellulosic
fibres (such as internal sizing agents), but the
retention of these materials in the sheet becomes
increasingly difficult if the chemicals being applied
have low affinity for fibre. Such anionic additives
require large quantities of retention aids which
significantly add to the cost and chemical complexity of
the chemistry in the paper machine.
The anionic nature of alkali silicates is such that
they cannot be retained in paper without the use of
excessive quantities of retention aids, the amount being
so high as to make the process economically unattractive.
In addition to this a second problem arises from the fact
that these retention aids are only partially effective
even when used in high quantities. As a result of this,
not all the alkali silicate is retained, and significant
quantities travel through the sheet into the "white water
system" where their interaction with other chemicals
negatively affects the chemical balance of the paper
machine.
An alternate method for the incorporation of
chemicals into paper involves a size press. With this
piece of equipment, usually located near the middle of
the drier section of a paper machine, liquid chemicals
can be well dispersed and retained within the paper.
This method is widely used for the addition of starch
sizing agents to paper, and sodium silicate has also been
added to paper at the size press. The main drawback with
size presses involved their efficiency. Relatively low
quantities of chemicals can be applied by this method,
and if the addition of larger amounts (more than 3-4~ by
weight) the paper machine has to be slowed down which
results in a loss of production output. The rate of
absorption of the chemicals by the paper is not
instantaneous. For these reasons incorporation of
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chemicals via the size press frequently leads to a
reduction in machine speed and output which significantly
reduces the cost advantages of using the chemicals. The
third method of incorporating chemicals into paper
involves soaking, dipping, wiping or mechanically
impregnating the paper after it has emerged from the
driers. Although such techniques are ~men~hle for use
with liquids such as sodium silicate, they all suffer
from the drawback that the production rate is limited by
the need to slow the process for the adsorption of the
chemicals, after which the treated paper has to be
redried. This results in processing costs, and the need
for space for the extra driers which may not always be
available.
It is therefore evident that if the problem of
retention of the alkali silicates could be solved, the
preferred method of incorporating alkali silicates into
paper would be at the wet end for the reasons given
above. Conceptually such additions could be made in the
head box, or on the moving wire of paper machine via some
kind of applicator weir, die or spray bar.
In the course of investigating many possible
solutions to this problem, it occurred to the present
inventors that efficient incorporation of solid forms of
silicate into paper at the wet end could be achieved if
the solubility characteristics of the silicate were
correctly designed. A brief description of the physical
properties of alkali silicates will help clarify the
description of the invention is in order.
Alkali silicates are typically manufactured in a
high temperature furnace in which sand and alkali source
are fused together. Various types of product are
possible depending on the alkali component, sodium and
potassium being the most common, and the weight ratio of
sand to alkali. The anhydrous product of this reaction,
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known as glass, finds commercial use either in the dry
form, for which applications it is usually ground to a
fine powder, or it is dissolved in water to produce an
aqueous solution. To date all attempts to incorporate
silicates into paper have involved various aqueous
solutions of this product, those of sodium silicate being
the most common for reasons of cost.
The complexity of the process has until now
precluded the possibility of utilizing alkali silicates
in the solid form for this application. In the course of
considering this problem it became clear that the desired
properties could be achieved if the solubility properties
of the silicate could be designed in such a manner that
the silicate could be added to the wet end of the paper
machine in a solid form which could be easily retained by
the sheet, and such that it would dissolve and disperse
in the sheet before the paper or board leaves the paper
machine.
It also became apparent that the desired silicate
solubility behaviour might be achieved making use of the
temperature differential between the wet end and the
drier section of the paper machine. While the
temperature of the white water is seldom more than 60-70
degrees centigrade, that within the driers exceeds 100
degrees. It was conjectured that the solution to the
problem lay in finding a dry form of silicate which was
essentially insoluble in the lower temperature conditions
of the wet end, but became fully soluble, and
dispersible, within the driers. In order to be
successful, it would be necessary for the product to be
sufficiently insoluble in luke warm water in advance of
the driers and be retained in the paper long enough for
the paper to pass from the paper machine wire, through
the press section until it entered the drying section.
It would however have to be sufficiently soluble in hot
water that the effect of steam in the drier would
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completely dissolve the particles. Both the solubility
rate and the size of the individual particles should be
such that both during the introduction and dissolution
the silicate would be well distributed through the paper
in order to realize high efficiency.
General Description of the Invention
The invention here described discloses the
development of a class of alkali silicates with precisely
defined solubility and particle size characteristics to
achieve the desired silicate solubility properties. We
have discovered that these results can be achieved by
achieving the correct balance between solubility and
particle size of sodium silicate in the solid form. The
preferred products are a sub-class of a types of
partially hydrated alkali silicate which are obtained by
spray drying aqueous solutions of sodium silicate which
are commercially available under the trade names Silicate
G~, GD~ and Britesil, all manufactured by the PQ
Corporation. The present invention is not intended to be
limited to this class of silicate, however, for the
approach could also be extended to other dry forms of
silicate, such as sodium metasilicate, both anhydrous and
pentahydrate, and to the ground glasses of sodium and
potassium silicate. The preferred solubility
characteristics are found to be obtained by modification
of the properties of a partially dried solution of a 3.2
silica to alkali ratio of sodium silicate, one form known
as Silicate G~.
From various known solid forms of sodium silicate, a
limited number of species have the correct properties to
obtain the desired properties for the process described.
For the process to be efficient the particles must be of
such a size that they are able to penetrate within, but
not all the way through, the cellulosic mat. Thus a
correct balance must be found between retention and
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dispersion: particles which are too large might be
retained on the very surface of the mat, and would form a
surface layer with adverse properties when the sheet
passed through the driers. Particles which are too
small, however, might pass through the sheet and not be
retained. Particles of a size range preferably but not
limited to between about 20 and 60 microns have been
found to have these properties. The particles must also
retain their insolubility in cool or lukewarm water so
that they can be retained within the sheet until the
paper reaches the drier section.
Another important feature of the present invention
arises from the well known fact that the solubility rate
of particles is also a function of size. A solution to
the problem of finding the correct balance between these
properties was obtained when we determined that the rate
of dissolution of dried silicate such as Silicate G~, was
also dependent upon the residual moisture content of the
product.
As will be illustrated in the Examples below, the
speed at which Silicate G~ has almost the ideal
combination of properties required for this application,
and that only minor modifications in the process used to
manufactured Silicate G~ are required to make it
perfectly acceptable for this application. Thus Silicate
G~ containing moisture in the range of about 10 to 20~ by
weight has been found to be suitably insoluble in
lukewarm water, and rapidly soluble in hot water to
achieve the performance required.
It will therefore be apparent to those skilled in
the art that the properties of the dried silicate such as
those described, can be manipulated by modification of
both particle size and moisture content in order to
achieve the desired properties for the conditions of the
paper machine under consideration. It is also recognized
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that other factors present during the manufacture of
dried silicates, such as the dwell time and temperature
in the drier affect the properties of the product.
Accordingly, this invention is not limited to the
specific methods of manufacture of these products which
are its currently preferred embodiments.
A third aspect of this invention stems from the
discovery that dry forms of sodium silicate can be
combined with starch in the applications described, the
use of starch as an additive for the strength improvement
of paper being well known and of wide spread application.
It was our surprise to discover that the use of starch in
combination with the dried silicates here described led
under some conditions to a synergistic improvement in the
strength properties of the paper. Good results were
obtained with unmodified starches derived from various
plant products such as corn, potato, rice, etc., as well
as starches which have been modified by various chemical
processes, such as cationization as is a common
application of starch in the pulp and paper.
The importance of this synergism will be understood
when the complexity of the different types of strength
tests which are carried out on paper is taken into
account. Of the various methods used to evaluate the
strength of paper, those which are increasingly
considered desirable include taber stiffness, edge crush
(also known as ECT); ring crush; short span compression
strength (also known as STFI); corrugated medium test
(also known as CMT); and corrugated flute crush (also
known as CFC). It is well known that starch and silicate
do not improve the strength of paper in the same way.
The incorporation of starch into a paper sheet is
particularly good, for example, for burst and tensile
strength, while silicate is most valuable for this
improvement of the stiffness and ring crush of paper.
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As will be illustrated in the Examples below, when
starch and silicate are applied to the paper according to
the technique here described, the increase in strength of
the paper observed is unexpectedly in excess of that
which might have been predicted if either of the
components was used alone.
Detailed Description of the Invention
As noted above the invention concerns the
improvement in the strength properties of cellulosic
products such as paper or board by incorporation of
sodium silicate in particulate form either alone, or in
combination with certain other particular materials such
as starch. In a process which is an embodiment the
chemicals are applied to the surface of the moving wet
web on a fourdrinier forming table, although this process
is not limited to fourdrinier machines but can also be
applied to other paper making processes such as cylinder
machines. Addition of these chemicals can be
accomplished by a number of application techniques well
known in the art and include spray showers, overflow
weirs, or dies.
It is an important feature of the process of the
invention that all the chemicals employed be in
particulate form, be sparingly soluble in cool or
lukewarm water, but rapidly soluble under the high
temperature conditions encountered in the drier section
of the paper machine. The particulate nature of the
additives causes them to be retained on the sheet without
the use of expensive chemical retention aids, while their
high solubility at elevated temperatures permits rapid
distribution of the material throughout the sheet as the
water is removed in the drier section of the machine,
good distribution of product being well known to be of
importance for such applications.
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The precise location of the application point on the
paper machine is also of importance in this process.
Since the consistency of fibres on a paper machine
increases from about 1~ near the head box, to about 15
immediately prior to the press section after which the
paper enters the driers. In order to maximize the
retention and dispersion of the chemicals within the
sheet, it is preferable to add the chemicals to the
moving web of the paper machine at a position where the
wet mat has a solids content between about 2 and 10~.
Application of the products much before this point
results in poor retention, while addition of chemicals to
a mat of much higher than about 8~ consistency results in
accumulation of the chemicals on the surface. This is to
be avoided because of various deleterious consequences
such as the transfer of the chemicals to machinery during
drying, as well as later processing.
The treatment process can also be applied on those
paper machines which manufacture multiply paper in which
the wet mats from two or more head boxes are brought
together usually immediately prior to the press section,
during the manufacture of multiply paper or laminates.
The excellent retention of the particles between the
plies which is achieved by application at the point of
convergence of the mats as described in this process,
eliminates concerns about transfer of the product to
downstream processes such as drying and printing.
Example 1
Solubility rates of Silicate G~ of differing moisture
content at various temperatures
In order to study the effect of moisture content of
Silicate G~ on the rate of dissolution, samples of the
commercial grade of the product (containing 19.3~
moisture) were dried at 60~C in a laboratory oven for
between 0.5 and 2 hours after which the moisture content
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of the material was determined by weight loss at 525~C
(LOI). The rate of dissolution of these various
materials in water was then determined at three different
temperatures: 15~, 40~ and 60~C, after 20 and 60
minutes. The experiment was carried out by stirring the
10~ by weight slurry of Silicate G~ in distilled water,
and filtering and weighing the undissolved product.
Table 1 demonstrates very clearly the relationship
between water content and dissolution rate.
Table 1: Di5~b ~ '- of Silicate G~ under d;fl~.. ' ec ' ' 5
Percent Dissolved at:
Drying time Moisture 15~C 15~C 40~C 40~C 60~C 60~C
(hrs~60~C) (% LOI) 20 min 60 min 20 min 60 min 20 min 60 min
0 19.33 20.4 71.5 92.9 94.4 97.5 97.7
0.5 17.72 23.4 55.2 88.9 91.7 96.7 97.1
1 14.75 10.6 13.1 57.5 71.7 91.6 92.3
2 12.70 6.3 0.0 27.3 29.5 89.6 91.2
Example 2
Use of sodium silicate to increase the strength of paper
This example illustrates the increase in the
strength of paper which can be realized by incorporation
of alkali silicate. The alkali silicate used here was a
solution of the sodium salt of silicic acid in which the
ratio of silica to soda was 3.2:1 (sold by the PQ
corporation under the commercial name Silicate N~).
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It was found necessary in these experiments to
include certain retention aids, because of the anionic
charge of sodium silicate which renders it otherwise
difficult to retain the silicate on the fibres. A wide
number of such retention and drainage aids are
commercially available and known to those skilled in the
art. Because of the highly anionic nature and dispersive
properties of sodium silicate, those polymers which have
some degree of cationicity are preferred. In the example
below two such polymers: a coagulant Diaflocc 4439 (sold
by Diachem), and a flocculent Diafloc 7794, were
employed.
In the experiments the silicate was mixed into a
dispersion of unbleached kraft fibres at the
concentrations shown, the consistency of the fibres being
1~ by weight of water. After twenty seconds the
coagulant was added at the level of 1~ by weight of the
fibres, followed by the flocculent at a level of 0.1~ by
weight of fibres. After another twenty seconds the
handsheet was prepared for testing under standard
conditions, the amount of silicate retained in the
treated sheets being determined by analysis. The Mullen
burst and Short Span Compression Strength (also known as
STFI) strength of the treated samples and controls were
determined at 50~ humidity, 23~C. To correct for the
variation in the individual sheet weights due to addition
of the chemicals, the results shown in Table 2, have been
normalized to 205 g/m2. (Note that the most important
independent variable is the ~ silica retained. The
variation between this number and the amount of Silicate
N~ added is a reflection only of the efficiency of the
retention aids used in this experiment, and does not
reflect on the strength enhancing efficiency of the
silicate.)
Table 2
Sample % Silica Mullen % Scott % STFI % Ring %
in sheet (PSI) incr. Bond incr. (lb.ft/in) incr. Crush incr.
Series I
1 Control* 0.00 46.8 - 50 - 15.5 - 58.8
2 0.00 46.8 - 50 - 15.5 - 58.8
Control*
7 1% Silicate 0.28 51.6 10.2 78 56.0 17.35 12.0 65.6 11.6 D
N3**
8 1% Silicate N3 0.42 51.6 10.2 78 56.0 17.35 12.0 65.6 11.6 r
6 1% Silicate N$ 0.54 49.6 6.0 74 48.0 18 16.1 64.8 10.2
5 2% Silicate N~ 0.61 49.6 6.0 74 48.0 18 16.1 64.8 10.2
Series 2 ~,
26 Control* 0.02 44.1 - n/a 1 13.6 - 50.6
27 1% Silicate N3 0.40 45.2 2.5 n/a - 15.46 13.7 61.5 21.5
28 2% Silicate N3 0.62 47.0 6.6 n/a - 15.42 13.4 62.9 24.3
29 3% Silicate N3 0.81 50.0 13.4 n/a - 15.74 15.7 66.7 31.8
*0.1 % Diaflocc 7794 added to the control
**pH adjusted to 6.7
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Example 3
Use of Silicate G~ to increase the strength of paper under
laboratory conditions designed to mimic a paper machine
A fresh sample of linerboard pulp was dispersed in
water to a consistency of 0.83~. Silicate G~ was added to
362 ml of this furnish in sufficient quantities to prepare
handsheets weighing on average 3 gms, and containing 5, 10
and 20~ Silicate G~ respectively. In order to simulate the
conditions of a paper machine drier, the handsheets were
covered with a metal template and subjected to a
temperature of 93~C at a pressure of 20 psi for 11 minutes.
The results are shown in Table 3.
Table3
Sample STFI % Increase
(kNm/kg)
Control 16.45
5% Silicate G~ 17.94 9.10
10% Silicate G~ 20.27 23.22
20% Silicate G~ 23.89 45.22
Example 4
Various combinations of Silicate N~ and starch to increase
the strength of paper
This series of experiments involved impregnation of
commercial samples of linerboard and corrugating medium
with Silicate N~ and starch. The paper was first cut into
sheets 20x30 cm in size. The starch solution was then
prepared by heating with stirring a 7.5~ w/w dispersion of
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hydroxyethylated starch (Casco Inc.) in water for 10
minutes. The required amount of Silicate N~ was then
stirred into this solution, after which the test sheets
were immediately immersed into this mixture (still at
82~C). After 1 minute the sheets were removed from the
solution, blotted and dried in an oven at to 82~C for 3
minutes. The following mixtures of starch and Silicate N~
were prepared: 10:1, 10:2; 10:5; 10:10, all ratios being
calculated on a dry solids basis. "Blank" represents the
results obtained with untreated paper, while "control
refers to sheets which were treated with water alone before
testing. STFI was determined at 50~ relative humidity,
23~C. The results, shown in Table 4, include the raw data
and STFI values normalized to a basis weight of 125 g/m2.
Table4
Sample Treatment Basis STFI (kNm/kg) %
Incr. over
wt (g/m2) raw normal Control
Linerboard Blank 125.80 20.38 20.25
Linerboard Control 129.47 23.34 22.53
Linerboard Starch/Silicate N~ 10:1 144.63 27.49 23.78 5.50
Lhlell)odld Starch/Silicate N~ 10:2 138.11 34.20 20.95 37.37Li,l.,ll,oa.. l Starch/Silicate N~ 10:5 155.53 36.16 29.06 28.98
Lhlclbod.d St~rch/Silicate N~ 10:10 152.16 37.38 30.71 36.30Medium Blank 123.49 20.il 20.35
Medium Control 134.23 25.69 23.92
Medium Starch/Silicate N~ 10:1140.59 29.47 26.20 28.74
Medium Starch/Silicate N~ 10:2141.77 30.75 27.11 33.22
Medium Starch/Silicate N~ 10:5147.46 33.00 27.97 37.44
Medium Starch/Silicate N~ 10:10 147.39 33.87 28.72 41.13
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Example 5
Demonstration of the improvement in the strength of paper
which can be realized as a result of various combinations
of Silicate G~ and starch being retained on the moving web
of a pilot paper machine
This experiment was carried out on a pilot paper
machine, the paper being made from unbleached recycled
stock. Four different ratios of starch to Silicate G~ were
applied to the sheet halfway between the head box and the
press section, and the total amount of additives retained
on the sheet was 4~ by dry weight. The starch was
unmodified pearl starch (Casco Inc. 3005), while the
Silicate G~ contained 19~ moisture. Control samples were
taken at the beginning and end of the run. Strength
properties on the finished sheet determined both in the
machine direction (MD) and cross direction (CD) under the
conditions described above. The results presented in Table
5 show the percentage increase in strength properties over
the control. As in the previous examples, the results have
been normalized to the weight of the control (150 g/m2) in
order to correct for the increase in the basis weight of
the sheet caused by the additives. The results are
summarized in Table 5.
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Table 5
% Change in strength l,.u~,~. lies with silicate retained in the sheet
Run # 1 2 3 4 5 6
Chernicals applied: (Control)
Starch (kg/T) - 40 36 32 28 20
Silicate G~ (kg/T): - 0 4 8 12 20
Silicate retained (kg/T) - - 3.7 5 7 9 5 18.2
Silicate retentiOD (%) - - 91.6 71.7 78.9 91.1
MD, Ring crush (Ib/6") - 6.2 17.9 20.0 20.9 18.5
CD, Ring crush (Ib/6") - 4.8 13.3 16.5 10.1 12.5
CMT (lbs) - 51.4 30.1 40.0 50.3 45.4
CFC (Ibs) - 17.6 19.6 23.7 27.6 28.2
Mullen - 14.1 18.7 23.6 12.1 15.4
MD, STFI (Ib/") - 5.1 13.9 10.7 8.4
CD, STFI (Ib/") - 5.5 3.9 7.0 11.3 9.6
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Example 6
Use of Silicate G~ and starch to increase paper strength on
an operating papermill
This trial was carried out at on the paper machine
which is used to produce 500 tons/day of 100~ recycled
linerboard at a basis weight of 220 g/m2 and a production
speed of 490 m/min. Unmodified (pearl) corn starch (CPC
3005) and Silicate G~ containing 12~ moisture, slurried
with water to a combined concentration of 7~, were fed onto
the papermachine by means of an overflow ("weir")
applicator. The weir was positioned about 1/3 of the way
between the headbox and the press section at which point
the consistency of the sheet was about 4~. The weir was 1
meter in length, or about 1/5 of the width of the paper
machine, a feature which allowed comparison of treated and
untreated sections from the same reel of paper. The
additive feed rate was adjusted to between 12 and 18 US
gal/min in order to achieve the retention shown below.
Starch alone was added at the start of the trial, after
which a mixture of silicate and starch was applied.
Although there was some variation over the course of the
trial, the average amount of material retained on the sheet
was about 3~ w/w, and the average starch:silicate ratio was
4:1 (dry basis). The results presented in Table 6 compare
the STFI values obtained on the treated edge of the paper
with that portion of the sheet which did not receive any
chemicals.
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Table 6
% Change in C~ e Strength of Treated vs U ~ ' a Board Samples
(treated back vs I " ' - ~ front sections on the same reel)
Time/Sample Silicate in Starch in MD STlil CD STFI
sheet % sheet % % increase% Increase
8:20 blank - - 0.19 -4.77
8:45 starch - - 1.42 5.19
9:40 starch - 510.19 14.29
11:10 blarlk - - 4.78 6.46
11:40 starch/silicate 0.62 1.41 4.53 7.34
12:15 starch/silicate 1.26 n/a 10.82 13.66
12:45 starch/silicate 0.92 2.53 11.04 20.89
13:15 starch/silicate 0.33 2.67 7.28 9.7
13:45 starch/silicate 1.09 n/a 10.17 13.81
14:10 starch/silicate 1.02 2.75 10.06 10.43
15:10 starch/silicate 0.97 n/a 9.72 14.89
15:40 starch/silicate 0.72 2.01 5.8 3.94
16: 15 starch/silicate 0.49 2.17 5.42 9.41
16:55 blarlk - - 3.32 -0.6
*3.2 ratio sodium silicate retained, dry basis
