Note: Descriptions are shown in the official language in which they were submitted.
CA 0223~0l3 l998-04-l6
WO97/14847 PCT~S96/16606
ACID RESISTANT CALCIUM CARBONATE FILLER
R~CKGROUND OF THE INVENTION
This invention relates generally to calcium
carbonate for use in papermaking, and related industries,
and more particularly to a calcium carbonate having acid
resistant properties.
Titanium dioxide and calcined clay have
traditionally been utilized as ~iller materials in the
preparation o~ neutral to weakly acidic paper in order to
improve the optical properties, especially the
brightness, o~ the resultant product. These materials,
however, especially titanium dioxide, have the
disadvantage o~ being very expensive, resulting in higher
manufacturing costs and an uncompetitively priced paper
product.
Calcium carbonate, particularly precipitated calcium
carbonate, has been used as a filler material in the
making o~ alkaline paper. Such usage results in a paper
with enhanced optical properties, without the expense
incurred in using titanium oxide ~illers, resulting in a
much less expensive product. Calcium carbonate, however,
cannot generally be used as a ~iller in acidic paper
because it decomposes in an acidic environment.
Consequently, there has long been a need to develop a
calcium carbonate composition which is acid stabilized
and-resistant to decomposition at low pH, so that it can
be utilized as a ~iller material in the manufacture o~
acidic paper, such as groundwood paper, where the use o~
an alkaline ~iller would have a negative impact on the
~inal paper properties.
Paper made ~rom mechanical pulps has been
traditionally produced under acidic papermaking
conditions because o~ "~iber alkaline darkening" that
occurs as pH rises. This means that there is a reduction
in brightness o~ the paper (brightness reversion) when
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WO97/14847 PCT~S96/16606
the pH is raised ~rom acid to alkaline in wood-containing
systems. Alkaline darkening will occur to some degree in
any wood pulps with significant lignin content. The
degree o~ darkening depends on the particular pulps, Ph,
and water quality. In general, ground calcium carbonate
and precipitated calcium carbonate fillers buffer wet end
in the 7.5-8.2 pH range. Acid-resistant calcium
carbonate compositions thus provide a means for reducing
the degree o~ fiber alkaline darkening and brightness
reversion due to their ability to maintain a stabilized
pH.
U. S. Patent 5,043,017 discloses and claims an acid-
stable calcium carbonate resistant to degradation in a
mildly acidic environment which comprises a mixture of a
calcium-chelating agent or a conjugate base, and a weak
acid, such that calcium carbonate is coated by, and is in
equilibrium with, the calcium-chelating agent or
conjugate base and the weak acid. Pre~erred calcium
carbonate compositions contain sodium hexametaphosphate
and phosphoric acid.
ORJF~TS OF TH~ JNVF~TION
It is an object of the present invention to provide
stabilized a~d acid resistant calcium carbonate
compositlons especially suitable for use in papermaking
applications.
It is a further object of the present invention to
provide a process for the preparation o~ the aforesaid
calcium carbonate compositions.
A still further object of the present invention is
to provide a paper having enhanced optical qualities
prepared using the calcium carbonate compositions of the
present invention.
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WO97/14847 PCT~S96/16606
SUMMARY OF T~ T~VENTION
The present invention relates to an improved form of
calcium carbonate which is stabilized and thus, acid
resistant, to enable its use as a filler material in the
making of neutral to weakly acid paper, and a process for
producing this acid resistant calcium carbonate. More
particularly, this invention concerns an acid resistant
calcium carbonate consisting essentially of at least
about O.l percent, based on the dry weight of the calcium
carbonate, o~ a mixture of two or more weak acids, in
admixture with the calcium carbonate. It has
surprisingly been found that the inclusion of the mixture
of two or more weak acids confers a higher degree of
stability and acid resistance for calcium carbonate in
the presence o~ ~iber slurry, and a longer term of pH
stability than the prior art acid-stabilized calcium
carbonate compositions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE l is a graph comparing the 24 hours ageing of
scalenohedral precipitated calcium carbonate compositions
of the present invention containing l, 2 or 6%
polyacrylate acid, and various concentrations of
phosphoric acid.
FIGURE 2 is a graph comparing the 24 hours ageing of
rhombic precipitated calcium carbonate compositions of
the present invention containing 1% or 2% polyacrylate
acid, and various concentrations of phosphoric acid
FIGURE 3 is a graph comparing the 24 hours ageing of
ground precipitated calcium carbonate compositions of the
present invention containing 1% or 2% polyacrylate acid,
and various concentrations of phosphoric acid
FIGURE 4 is a graph showing the pH of a
scalenohedral precipitated calcium carbonate composition
of the present invention containing 3~ polyacrylate acid,
and 5% phosphoric acid.
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FIGURE 5 is a graph showing the pH of a
scalenohedral precipitated calcium carbonate composition
of the present invention containing 1% polymaleic acid,
and 4% phosphoric acid.
FIGURE 6 is a graph showing the pH of a
scalenohedral precipitated calcium carbonate composition
of the present invention containing 6~ polymaleic acid
and 1% phosphoric acid.
DET~TTFD DESCRIPTION OF T~ INVFNTION
The improved form of calcium carbonate prepared by
the instant invention is stabilized, and thus, acid
resistant, to enable its use as a filler material in the
making of neutral to weakly acid paper. While not
wishing to be bound by any particular theory as to the
operability of the present invention, it is believed that
the acid resistance conferred upon the calcium carbonate
compositions of the present invention is a result of the
inactivation of the surface of the calcium carbonate by
the addition of the mixture o~ the~two weak acids.
In the practice of the present invention, the
calcium carbonate compositions are rendered acid
resistant by the inclusion of at least about 0.1 percent,
based on the dry weight of the calcium carbonate, o~ a
mixture of two or more weak acids. Especially preferred
as one component of the mixture of weak acids is an
organic, polymeric weak acid, such as polyacrylate or
polymaleic acid.
While not wishing to be bound by any theory, it is
believed that the capability of the acid-stabilized
calcium carbonate o~ the present invention to resist
dissociation in an acidic environment is due to the
polymer adsorption on the surface of the calcium
carbonate, absorption of polymer functional groups on the
calcium carbonate surface, and the formation of a
bu~fering system between anionic functional groups of the
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W097/14847 PCT~S96/16606
polymer and a weak acid. This mechanism of polymer
adsorption is distinct from absorption or reaction of the
prior art sodium hexametaphosphate on the surface of
calcium carbonate. Polymer adsorption can provide a
barrier coating on the surface of calcium carbonate which
reduces the dissolution reaction of calcium carbonate.
On the other hand, the absorption or reaction of sodium
hexametaphosphate is limited to the surface of the
calcium carbonate.
The weak acids utilized in the compositions of the
present invention are preferably weak acids selected from
the group consisting of phosphoric acid, metaphosphoric
acid, hexametaphosphoric acid, ethylenediaminetetraacetic
acid (EDTA), citric acid, sulfurous acid, boric acid,
15 acetic acid, weak acids derived from organic polymeric
acids, such as polyacrylate acid, polymaleic acid and
polycarboxylic acid, and mixtures thereof. As noted
hereinbefore, the mixture of weak acids preferably
contains at least one weak acid which is derived from an
20 organic, polymeric acid. These organic polymeric acids
are typically an organic polymer having a weight average
molecular weight, ~w, in the range of 750-1,000,000,
consisting of regularly repeating units or chemically
similar units, connected by primary covalent bonds. The
25 total amount of the weak acids utilized is at least 0.1
percent, based on the dry weight of the calcium
carbonate, and is preferably about 1 to about 8 percent,
based on the dry weight of the calcium carbonate.
A preferred combination of weak acids for use in the
v 30 present invention is polyacrylate acid with phosphoric
acid.
The calcium carbonate utilized is preferably finely
divided and it can be either a precipitated calcium
carbonate or a natural ground limestone.
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The process-~or producing this acid resistant
calcium carbonate involves forming a mixture of calcium
carbonate with at least about O.l percent, based on the
dry weight o~ the calcium carbonate, o~ the mixture of
weak acids. The resultant mixture is blended ~or a
su~iciently long period o~ time to ensure uniform mixing
o~ the ingredients.
The calcium carbonate can be utilized in the above-
described process either as a dry powder or an aqueous
slurry with up to about 60 percent by weight solids
content.
The weak acids can be utilized in the process o~
preparation in either pure concentrated ~orms or as
aqueous solutions.
In a preferred embodiment o~ the instant process,
either the polymeric acid or the phosphoric acid can be
first added, followed by the addition of the second acid.
Alternately, the polymeric acid and phosphoric acid can
be added at the same time, or the polymeric acid and
phosphoric acid can be mixed together and then added to
the calcium carbonate.
The composition o~ the present invention can be
utilized to improve the optical properties of neutral to
weakly acidic paper by its addition to the paper during
standard manufacturing processes. Typically, the calcium
carbonate composition o~ the present invention is added
to a ~irst paper furnish containing components necessary
~or making acidic paper to thereby form a second paper
furnish.
The invention will be further illustrated by the
~ollowing Examples, which are to be considered
illustrative of the invention, and not limited to the
precise embodiments shown.
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~X~MPT,~ 1
Sc~lenohedral Precipitated Calcium Carbonate
St~hilized with Polyacrylate Acid and Phosphoric Acid
Acid stabilized scalenohedral precipitated calcium
carbonate slurry can be obtained by the addition of a
polymeric acid such as polyacrylate acid, and a weak acid
such as phosphoric acid. Initially, 1%, 3%, and 6%
polyacrylate acid, based on the dry weight of calcium
carbonate, was added into a 19.7% solids slurry of
scalenohedral precipitated calcium carbonate. The pH of
the untreated scalenohedral precipitated calcium
carbonate slurry was 9.05. After mixing, 1-6% phosphoric
acid, based on the dry weight of calcium carbonate, was
added. A plot of the pH was measured for each sample
after 24-hours ageing as shown in Figure 1. A
composition containing 1% polyacrylate acid, based on the
dry weight of calcium carbonate, and 3% phosphoric acid,
based on the dry weight of calcium carbonate was found to
have an initial pH of 5.28, which rose to 5.44 after 24
hours ageing.
EX~PT~ 2
Rhombic Precipitated Calciu~ Carbonate
Stabilized with Polyacrylate Acid and Phosphoric Acid
The initial pH of rhombic precipitated calcium
carbonate was 8.29. First, 1% or 2% polyacrylate acid,
based on the dry weight of calcium carbonate, was added
into a 17.3% solids slurry of rhombic precipitated
calcium carbonate. After blending, 1-6% phosphoric acid,
based on the dry weight of calcium carbonate, was added
as shown in Figure 2. The samples containing 2%
polyacrylate acid have lower pHs than the 1-6% phosphoric
acid and 1% polyacrylate acid, after 24 hours ageing.
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WO97/14847 PCT~S96/16606
~MPTF 3
Ground C~lcium Carbonate
Stab;l-zed with Pslyacrylate Acid ~nd Phosphoric Acid
The initial pH of ground calcium carbonate was 8.0l.
Initially, 1% and 2% polyacrylate acid, based on the dry
weight of calcium carbonate, was added into 20~ solids
slurry of ground calcium carbonate. After blending, 1-6%
phosphoric acid, based on the dry weight of calcium
carbonate, was added as shown graphically in Figure 3.
The samples with the addition of 2% polyacrylate acid had
a lower pH at the addition of 3-6~ phosphoric acid
compared to 1% polyacrylate acid after 24 hours ageing.
EXP~PT~ 4
Tong Term pH Stability of Scalenohe~ral Precipitated
C~lclum Carbonate Slurry
A long-term low pH stabilized scalenohedral
precipitated calcium carbonate slurry can be obtained by
the combination of polyacrylate acid and phosphoric acid.
The initial pH of scalenohedral precipitated calcium
carbonate slurry was 9.05. At the beginning, 3%
polyacrylate acid, based on the dry weight of calcium
carbonate, was added to l9.8% solids scalenohedral
precipitated calcium carbonate slurry, followed by the
addition of 5% phosphoric acid, based on the dry weight
of calcium carbonates, as shown in Figure 4. The pH of
scalenohedral precipitated calcium carbonate slurry were
measured and found to be 6.32 after two weeks ageing.
PTIF~ 5
Sc~lenohe~r~l Prec~p;tated Calcium Carbonate
St~hill~ed with Polymaleic Acid and Phosphoric ~cid
Acid stabilized scalenohedral precipitated calcium
carbonate slurry can be obtained by the addition of a
polymeric acid such as polymaleic acid, and a weak acid
such as phosphoric acid. Initially, 1% polymaleic acid,
based on the dry weight of calcium carbonate, was added
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into 19.7% solids slurry of scalenohedral precipitated
calcium carbonate. The pH of untreated scalenohedral
precipitated calcium carbonate slurry was 9.05. After
mixing, 4% phosphoric acid, based on the dry weight of
calcium carbonate, was added. A plot of the pH was
measured a~ter 47 hours ageing as shown in Figure 5. The
initlal pH of the slurry was measured and found to be
5.38, which rose to 6.80 after 47 hours ageing.
FXA~PT~ 6
Sc~lenohedral Precipitated Calcium Carbonate
Stabili7ed with Polymaleic Acid and Phosphoric Acid
Acid stabilized scalenohedral precipitated calcium
carbonate slurry can be obtained by the addition of a
polymeric acid such as polymeric acid, and a weak acid
such as phosphoric acid. Initially, 6% polymaleic acid,
based on the dry weight of calcium carbonate, was added
into 19.7% solids slurry of scalenohedral precipitated
calcium carbonate. The pH of untreated scalenohedral
precipitated calcium carbonate slurry was 9.05. After
mixing, 1~ phosphorlc acid, based on the dry weight o~
calcium carbonate, was added. A plot Q~ the pH was
measured after 118 hours ageing as shown in Figure 6.
The initial pH of the slurry was measured and ~ound to
be 5.70. After 118 hours ageing, the pH was again
measured and found to be 6.46.
