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BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to an improved papermaking technique and to
the final product paper sheet material provided thereby.
2. Description of the Prior Art:
In conventional papermaking processes, raw paper pulp essentially
consisting of cellulosic fibers is formulated into a dilute aqueous
suspension which is introduced into the headbox of the papermaking machine
from whence it is distributed over a filtering surface, e.g., Fourdrinier
wire, on which the web of paper forms. The web is then drained and dried.
The quality and properties of the resulting paper are determined, notably,
by the operating conditions of the papermaking machine, the state of the
raw pulp, the various additives that are added to the suspension prior to
the formation of the web, as well as the materials that are deposited onto
the paper web downstream of the filter surface.
Among such additives that are formulated into the fiber suspension
introduced into the headbox, exemplary are mineral fillers, sizing agents,
agents for enhancing the mechanical characteristics of the paper, coloring
agents, and the like. The mineral fillers, such as CaCO.sub.3, TiO.sub.2,
etc., render the sheet of paper opaque, thus facilitating writing and
printing. Sizing agents, such as the alkylketene dimers, are added to
impart liquid resistance properties to the sheet, and to permit the sized
sheet of paper to be employed for writing and printing.
One of the qualities of paper, paperboard and other such products is the
regularity of its characteristics. Among these is the so-called
"look-thru" or "sheet formation" which is extremely important and is
representative of the general quality of the paper. The look-thru
effectively represents a greater or lesser degree of homogeneity of fiber
distribution over the surface and thickness of the sheet. The look-thru
quality will hence have considerable influence on the general quality of
the print in the case, for example, of paper used for printing and
writing, and on mechanical properties in the case, for example, of paper
used for packaging. In order to improve the look-thru, more and more
additives are being added to the suspension introduced into the headbox.
But, in order to meet the increasing strictness of pollution standards
requirements, papermaking plants are attempting to decrease their water
consumption, which results in an increase in the pollutant loading
associated with this type of industry (suspended matter, biological oxygen
requirement, chemical oxygen requirement, salinity, etc.). Other problems
are also encountered, such as the problem of pitches, rosins and the like,
and, more generally, pollutants associated with the presence of so-called
anionic trash. As the characteristics of the paper must, however, remain
identical and the problems associated with preserving the environment have
to be overcome, one of the solutions proposed in the prior art entails
increasing retention of the above elements within the web during formation
thereof on the filtering surface. This result is attained to a certain
degree, by adding retention agents to the fiber suspension introduced into
the headbox. Unfortunately, this technique is subject either to variations
in its effectiveness, or results in a significant deterioration in the
look-thru of the paper, depending on the products used.
Thus, serious need continues to exist in this art for an improved technique
for producing paper and the like, in which, in addition to the mineral
filler and the sizing agent, unique retention systems are admixed with the
fiber suspension or composition introduced into the headbox.
EP-A-348,366 describes a ternary such system comprising cationic starch, a
polyaluminum compound, and a silicic acid polymer. No anionic silica is
indicated and, in addition, the polysilicic acid must have a given
specific surface area (>1,050 M.sup.2 /g).
WO 88/6,659 describes a ternary system comprising a cationic polymer,
polyaluminum compound, and silica. The cationic polymer is preferably
polyacrylamide and cationic starch is again not indicated.
EP-A-285,486 describes a retention system based on cationic starch and
polyaluminum chloride. The mineral filler can, for example, be silica.
U.S. Pat. No. 4,643,801 describes a retention system based on cationic
starch, a high molecular weight anionic polymer and silica, in parts by
weight of starch/silica of 100/1 to 1/1 and of anionic polymer/silica from
20/1 to 1/10. An aluminum compound can also be added.
In "polyaluminum hydroxychloride application on neutral pH rosin sizing of
paper" by B. H. Wortley and J. C. Steelhammer, a retention system is
indicated comprising cationic starch, bentonite and an anionic polymer,
but not polyaluminum chloride (PAC).
SUMMARY OF THE INVENTION
Accordingly, a major object of the present invention is the provision of an
improved papermaking process, comprising incorporating into the aqueous
suspension of papermaking fibers introduced into the headbox of a
conventional papermaking machine, in addition to the inorganic filler
material and the sizing agent(s), a certain unique ternary retention
system.
Another object of this invention is the provision of high retention rates
in a papermaking process, as well as an improved dewatering of paper,
while maintaining the look-thru quality and other characteristics thereof.
The present invention thus permits a greater amount of filler to be
employed, notably fillers that are sensitive to acids, providing wastes
and effluents that are less polluting.
Briefly, the present invention features an improved method for
manufacturing paper which comprises incorporating into the fibrous
composition introduced into the headbox:
(a) an inorganic filler material,
(b) a sizing agent, and
(c) a retention system comprised of:
(c)(1) a cationic starch,
(c)(2) a polyaluminum chloride, and
(c)(3) a silica anionic in nature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
More particularly according to the present invention, the suspension
containing the cellulosic fibers can be crude or bleached pulp, bleached
chemical pulp of resinous, foliaceous, or annual vegetable matter, natural
unbleached chemical pulps of resinous or foliaceous origin, bleached or
unbleached mechanical pulps (SGW, TMP, CTMP, etc.), bleached or unbleached
de-inked pulps, pulps that already contain additives, and mixtures
thereof.
The cationic starch, indicated as component (c)(1), of the present
invention comprises one or more products marketed under the generic name
of cationic starch. The starch advantageously has an average molecular
weight of 2.10.sup.4 to 2.10.sup.5 preferably from 3.10.sup.4 to
12.10.sup.4. The cationic starches have an exemplary degree of
substitution (D.S.) ranging from 0.01 to 0.1. The cationic starches
include, for example, those described in Kirk-Othmen, Encyclopedia of
Chemical Technology, 3rd edition, volume 21, page 503. Preferably, they
constitute, by weight, from 0.2% to 1.7% of the weight of the fibrous
suspension introduced into the headbox.
The polyaluminum chloride (PAC), indicated as component (c)(2) according to
the present invention, comprehends products that are typically designated
aluminum polychloride, basic polychloride of aluminum, basic
polychlorosulfate of aluminum and preferably is one or more of the
following products:
1. A salt of the formula:
A1.sub.n (OH).sub.m C1.sub.3n-m (I)
in which m and n are positive integers and 3n-m is positive; such salt may
contain a polyvalent anion Y selected from among the anions of sulfuric,
phosphoric, polyphosphoric, silicic, chromic, carboxylic, and sulfonic
acids, the molar ratio Y/A1 ranging from 0.015 to 0.4;
2. A salt of the formula:
A1.sub.n (OH).sub.m C1.sub.3n-m-2k (SO.sub.4)k (II)
in which k, m and n are positive integers and 3n>m+2k, the basicity m/3n
ranges from 0.3 to 0.7 and k/n ranges from 0.01 to 0.03; such a salt can
be prepared according to the process described in U.S. Pat. No. 3,929,666;
3. A salt of the formula:
A1.sub.n (OH).sub.3n-m-2p C1.sub.m (SO.sub.4).sub.p !z (III)
in which (3n-m-2p)/3n ranges from 0.4 to 0.7; p ranges from 0.04 to 0.25n;
m/p ranges from 8 to 35, k, m, n, and p are integers and z is at least 1;
such a salt is described in GB-A-2,128,977;
4. A chlorosulfate of basic aluminum of the formula:
A1.sub.n OH.sub.m (SO.sub.4).sub.k C1.sub.3n-m-2k (IV)
in which the basicity (m/3n) 100 ranges from about 40% to about 65% and has
an A1 equivalent/C1 equivalent ratio ranging from 2.8 to 5, an apparent
molecular weight MA measured by conventional light diffusion and apparent
hydrodynamic diameters OZ and OW measured by quasi-elastic light diffusion
as follows:
MA=7,000-35,000
OZ(.ANG.)=350-2,500
OW(.ANG.)=200-1,200
this salt being described in FR-A-2,584,699.
Preferably, the PAC is present in the fibrous composition introduced into
the headbox in an amount of 0.01% to 0.3% by weight, based on the weight
of said fibrous composition, said percentage being expressed on the basis
of Al.sub.2 O.sub.3. Preferably, the PAC is WAC of formula IV.
The anionic silica, indicated as component (c)(3) according to the present
invention, is one or more silicas selected from among silica sol, silica
gel, microparticulate silica, silico-aluminate, bentone and bentonite.
Preferably, the silica is present in the fibrous composition in an amount
of 0.01% to 0.3% by weight, based on the weight of said fibrous
composition, the percentage being expressed as a percentage of active
constituent, in other words in SiO.sub.2. Such silica of anionic nature
preferably is in sol form, advantageously comprising 15% SiO.sub.2.
The inorganic or mineral filler material, the component (a) indicated
above, can be one or more of the conventionally employed fillers, such as
the following, provided by way of example: clay, CaCO.sub.3, hydrated
alumina, talc, TiO.sub.2, and the like. The inorganic filler represents
less than 40% by weight, preferably 10% to 25% by weight, based on the
weight of said fibrous composition. The preferred inorganic filler
material is CaCO.sub.3.
The sizing agent, the component (b) indicated above, can be one or more of
the following conventional fillers employed for a neutral medium: alkyl
ketene dimers (AKD), fluorinated phosphates, carboxylic acid anhydrides,
styrene/maleic anhydride copolymers, and derivatives thereof. The amount
of sizing agent employed is advantageously less than 10% by weight,
preferably ranging from 0.01% to 2% by weight of the commercial product
having 6% of active material, based on the weight of said composition. The
preferred sizing agent is AKD.
The components (a), (b), (c)(1), (c)(2) and (c)(3) indicated above can be
added separately, or in the form of a mixture of two or more thereof.
The fibrous suspension can also contain conventional additives such as
coloring agents, optical brighteners, mechanical strengthening agents,
anti-foaming agents, anti-slime agents, as well as products selected from
among the polyacrylamides, polyethylene imines, carboxymethyl cellulose,
urea/formol resins, melamine/formol resins, aminopolyamide/epichlorhydrin
resins, polyamide/epichlorhydrin.
Preferably, the components added are:
(a) from 5% to 30% of CaCO.sub.3,
(b) from 0.01% to 1% of AKD, and
(c) (1) from 0.2% to 1.7% of a cationic starch of molecular weight
2.10.sup.4 to 12.10.sup.5 and a degree of substitution of from 0.01 to
0.1;
(c) (2) from 0.1% to 3% of WAC as 10% Al.sub.2 O.sub.3 commercial product;
and (c)(3) from 0.01% to 0.3% of silica sol, expressed as % of SiO.sub.2.
In order to further illustrate the present invention and the advantages
thereof, the following specific examples are given, it being understood
that same are intended only as illustrative and in nowise limitative.
In said examples to follow, WAC was a product marketed by Atochem and was a
polyaluminum chloride of formula IV with 10% of Al.sub.2 O.sub.3. The
silica was in the form of a silica sol and is available commercially under
the trademark CECASOL from Ceca. The cationic starch was manufactured by
Roquette and marketed under the trademark HICAT 142. The AKD sizing agent
was manufactured by Hercules and marketed under the trademark AQUAPEL.
EXAMPLE 1:
The paper manufactured was paper for printing or writing, suitable for
carbonless copying paper.
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(i) Machine employed flat bed fitted with a
Bel Bond former,
(ii) Speed 590 m/min,
(iii) Width 3 m,
(iv) Weight 50 g/m.sup.2
Fiber composition:
(1) Resinous bleached kraft pulp
45%
(2) Foliaceous bleached kraft pulp
55%
(3) Filler CaCO.sub.3
(4) Amount of filler in finished paper
14%
(5) Sizing (AKD) 0.15%
(6) Cationic starch 0.6%
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The tests carried out on the product obtained, under the conditions
reported in the Table I below, provided the results indicated in said
Table:
TABLE I
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Total
WAC Silica (1) Retention
Look-thru (2)
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Test 1 0 0.5% 58% 104
Test 2 1.65% 0 57% 70
Test 3 1.65% 0.15% 67% 72
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(1) In commercial product, 15% active material.
(2) The lookthru was continuously measured using apparatus from the
"Centre technique du papier" of Grenoble (France).
Comparing Test 1 and Test 2, the very significant improvement in look-thru
obtained by the use of WAC instead of silica was apparent, contrary to
that which would have been expected by one skilled in this art.
Test 3 illustrates the synergy provided by the association of small amounts
of silica in the presence of WAC. For equivalent look-thru, retention was
increased by 10 points, equivalent to an increase of 15% to 20%. This
results in reduced water pollution from the circuits and greater ease of
treating waste water at a reduced cost.
EXAMPLE 2
The paper manufactured was a paper for printing and writing. The machine
was identical to that of Example 1.
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(i) Machine speed 540 m/min,
(ii) Paper Weight 60 g/m.sup.2
Fiber composition:
(1) Resinous bleached kraft pulp
40%
(2) Foliaceous bleached kraft pulp
60%
(3) Filler CaCO.sub.3
(4) Amount of filler in finished
11%
paper
(5) Sizing (AKD) 0.15%
(in commercial product)
(6) Cationic starch 0.5%
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The tests carried out on the product obtained, under the conditions
reported in the Table II below, provided the results indicated in said
Table:
TABLE II
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Total
WAC Silica Retention Look-thru
HBC (1)
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Test 1
1.6% 0 60% 76 8.4 g/l
Test 2
1.5% 0.15% 73% 75 6.5 g/l
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(1) HBC = headbox concentration
EXAMPLE 3:
The same machine was used as in Example 2.
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(i) Machine speed 520 m/min,
(ii) Paper Weight 70 g/m.sup.2
Fiber composition:
(1) Resinous bleached kraft pulp
35%
(2) Foliaceous bleached kraft pulp
65%
(3) Filler CaCO.sub.3
(4) Amount of filler in finished paper
14%
(5) Sizing (AKD) 0.15%
(in commercial product)
(6) Cationic starch 0.6%
______________________________________
The tests carried out on the product obtained, under the conditions
reported in Table III below, provided the results indicated in said Table:
TABLE III
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Total
WAC Silica Retention Look-thru
HBC
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Test 1
1.5% 0 67% 75 8.7 g/l
Test 2
1.5% 0.25% 78% 76 7.2 g/l
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While the invention has been described in terms of various preferred
embodiments, the skilled artisan will appreciate that various
modifications, substitutions, omissions, and changes may be made without
departing from the spirit thereof. Accordingly, it is intended that the
scope of the present invention be limited solely by the scope of the
following claims, including equivalents thereof.
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