Note: Descriptions are shown in the official language in which they were submitted.
1
FIBROUS-REINFORCED SHEET
Field of the Invention
The present invention relates to a highly filled,
fibrous-reinforced sheet and to a process for preparing the
sheet. More particularly, the present invention relates to a
fibrous-reinforced sheet that is highly filled with calcium
carbonate and to a process for preparing the,sheet.
Sheets which comprise a relatively small amount of
fibers and a relatively high amount of filler are well known.
An example of such a sheet comprises cellulosic fibers in an
amount of about 5 to about 25 wt.%, about 50 to about 85 wt%
of mineral filler such as talc, and about 2 to about 30 wt.%
of a binder. Such sheets can be used in a variety of
applications, for example, as backing for vinyl floor
covering, gasket paper, laminating bases for articles such
as, for example, apparel and bookcovers, and laminated and/or
compression molded thermoplastic composites, such as, for
example, the composite paper-reinforced thermoplastic article
described in U.S. Patent No. 5,188,888. Sheets manufactured
for use in such applications generally have a thickness of
about 10 to about 500 mils and widths of six to ten feet or
more.
The properties required to be present in such sheets
vary, depending on the particular application in which the
sheet is used. However, in general, the sheet should have
the following properties: sufficient tear resistance and
sufficient tensile strength to resist being snapped during
processing and handling; adequate cohesive strength to resist
delamination during processing; flexibility sufficient to
enable the sheet to be formed into a roll, and, in the case
2
of use in vinyl flooring, to be cooed; resistance to
indenting; and dimensional stability as changes in moisture
content and temperature are encountered in the environment in
which the sheet is used.
Properties such as tensile strength, tear resistance and
dimensional stability in the sheet are contributed in the
main by the type of fibers present in the sheet. Cohesive
properties are in the main contributed by the binder which
functions to adhere together the fibers and filler of the
sheet. Resistance to indenting is in the main the function
of the filler. And flexibility properties are contributed by
the fibers, filler, and binder.
The filler is a particularly important constituent of
the sheet simply because it comprises such a large proportion
of the sheet. To fulfill its function satisfactorily, it
must be relatively low in cost, impart desired properties to
the sheet, and not offset to any significant degree the
properties imparted to the sheet by other of its
constituents.
The method which is used most widely to prepare such
sheets is the basic method which is used to manufacture
paper, including the use of conventional paper-making
equipment such as, for example, a Fourdrinier machine. This
method includes forming a highly dilute aqueous slurry (for
example, 1 wt.% solids) of fibers, filler and other
constituents and continuously feeding a stream of such slurry
onto a moving horizontal porous conveyor belt. The method
includes the use of constituents which are effective in
causing the retention of the particulate solids comprising
the sheet on the porous belt as water in significant amount
drains through the porous belt; means for compacting the wet
sheet and removing additional water therefrom; and means for
evaporating residual water from the sheet to dry it.
One of the problems that has been encountered in
manufacturing a filled, fibrous-reinforced sheet by the
aforementioned type of "paper-making" process is the loss of
filler through the openings of the porous conveyor belt.
Typically, the size of the filler particles is smaller than
the openings of the porous belt. Although a certain amount
CA 02128072 2004-08-17
3
of filler is retained in the wet mass of solids deposited on
the belt by virtue of fibers blocking off openings in the
porous belt, a significant amount of filler can be carried
along by the water which flows through the porous belt. This
is, of course, undesirable for several reasons, including the
loss of filler from the product.
The present invention is directed to improved means
for improving the retention of filler in the sheet during
its manufacturing process.
Reported Developments
U.S. Patent No. 4,225,383 discloses a fibrous-reinforced
sheet which is. made by a "paper-making" type process and
which contains about 50 to about 80 wt.% of filler. This
patent discloses that such sheets can be made by including in
the aqueous slurry from which the sheet is formed about 2 to
about 30 wt.% of a binder which comprises a film-forming,
water-insoluble organic polymer in the form of an ionically
stabilized latex having no greater than 0.7 milliequivalent
of bound charge per gram of polymer in the latex. Such
.polymers include anionic polymers, that is, polymers which
carry a negative charge by virtue of their containing
ionizable groups such as carboxylate groups. This patent
discloses also the use in the slurry of a flocculating agent
which has a charge opposite that of the latex. Accordingly,
a cationic flocculant, that is, a flocculant bearing a
positive charge is suggested for use in combination with an
anionic polymer.
The aforementioned '383 patent discloses the use of a
variety of fillers in preparing the highly filled sheets
described therein, including, for example, magnesium
hydroxide, magnesium silicate, magnesium carbonate, talc,
zinc oxide, calcium sulfate, calcium carbonate and others.
Developmental work has shown that it is feasible to prepare,
according to the disclosure of the '383 patent, highly filled
sheets utilizing a filler such as raw talc (about 50 wt.%
magnesite and about 50% talc). (Raw talc is a filler which
is used widely in manufacturing sheets which are used, for
1~ ~ ~~~
4
example, as backing for vinyl floor covering.) However,
developmental work has shown also that it is not possible to
make effectively a highly filled sheet (for example, about 80
wt.% filler) when the filler is calcium carbonate.
Experience has shown that critical amounts of calcium
carbonate are not retained with the other ingredients which
make up the sheet and are carried away with the water
constituent of the slurry as it flows through the porous belt
on which the slurry is deposited. It is a significant
disadvantage that calcium carbonate cannot be used as the
filler in highly filled sheets because it is about one third
the cost of raw talc. The present invention relates to the
provision of a fibrous-reinforced, highly filled sheet which
includes a filler that consists essentially of calcium
carbonate.
Summary of the Invention
In accordance with the present invention, there is
provided, in a process for forming a highly filled,
fibrous-reinforced sheet in which the sheet is formed from
an aqueous dispersion of solids which include fibers, filler,
and binder and cationic flocculating agent by continuously
feeding a stream of such aqueous dispersion onto a moving
porous support surface, wherein the filler comprises at least
about 50 wt.% of the solids portion of the dispersion, the
improvement comprising the use in the aqueous dispersion of a
filler consisting essentially of calcium carbonate and a
cationic polymeric flocculant having a charge-density of at
least about 2 equivalents of cationic nitrogen per kilogram
of polymer.
In preferred form, the cationic flocculant is a
copolymer of acrylamide and cationic acrylate ester of
essentially linear polymer structure with a weight average
molecular weight of at least about 5 million.
Another aspect of the present invention encompasses the
provision of a fibrous-reinforced sheet comprising:
(A) about 5 to about 25 wt.% fibers;
(B) at least about 50 wt.% calcium carbonate filler;
5
(C) about 2 to about 30 wt.% of resin binder;
and
(D) about 0.01 to about 0.5 wt.% of a cationic
polymeric flocculant having a charge-density of at
least about 2, preferably at least about 3.5,
equivalents of cationic nitrogen per kilogram of
polymer.
In preferred form, the sheet comprises at least about 70 wt.%
calcium carbonate, most preferably about 75 to about 85 wt.%
calcium carbonate. Also in preferred form, the
aforementioned sheet has a tensile strength of at least about
pounds per inch width, more preferably at least about 40
pounds per inch width.
An additional aspect of this invention is related to a
15 method for imparting opacity to white paper or a white,
highly filled, fibrous-reinforced sheet that includes a
filler, for example, calcium carbonate, and that is prepared
from a dilute aqueous dispersion of fibers and filler
comprising including in the dispersion carbon black in an
20 amount sufficient to render the paper or sheet opaque and to
retain the white appearance of the sheet or paper. In
addition, the present invention includes within its scope the
provision of white paper or white, highly filled, fibrous-
reinforced sheet that tends to lack opacity in which there is
included in the sheet a sufficient amount of carbon black to
impart opacity to the white paper or.sheet and to retain the
white appearance of the sheet or paper.
There are numerous advantages which are provided by the
present invention. It permits a manufacturer to utilize, in
relatively large quantities, calcium carbonate, which is one
of the lower cost filler materials available to the industry.
The invention permits this to be done in a way which utilizes
available equipment which can be operated according to
processing conditions which have come to be standards in the
industry. Thus, it is not necessary to invest in equipment
or make processing changes in adopting the use of the present
invention. In addition, the highly filled, calcium
carbonate-containing sheet of the present invention has
6
properties which meet accepted standards. The invention
provides also economic and effective means for imparting
opacity to paper or a highly filled, fibrous-reinforced sheet
in a manner such that the white color thereof is
substantially retained.
Detailed Description of the Invention
The classes of constituents comprising the sheet of the
present invention are known. They include fibers, filler,
binder and cationic polymeric flocculant having a charge-
density of at least about 2 equivalents of cationic nitrogen
per kilogram of polymer (also referred to herein as "high
charge-density flocculant").
The fibers impart strength and other properties to the
sheet. They are water-insoluble and are capable of being
dispersed in the aqueous medium which functions as the liquid
carrier of the various constituents from which the sheet is
formed. The fibers can be natural or synthetic materials.
Two or more different kinds of fiber can be used to prepare
the sheet. Examples of such fibers are cellulosic fibers,
glass fibers, including, for example, chopped glass, blown
glass, and spun glass fibers and rock wool fibers, ceramic
fibers, and synthetic resinous fibers, for example,
polyester, polyethylene, polypropylene, and nylon fibers.
The sheet of the present invention can be made effectively
without the use of asbestos fibers which are associated with
health hazards.
It is preferred to use a mixture of cellulosic and glass
fibers, the latter functioning to impart good dimensional
stability to the sheet. The cellulosic fibers can be, for
example, cellulosic pulp fibers, either virgin or secondary
(recycled), and either bleached or unbleached. Such
cellulosic fibers can be produced, for example, by any
suitable chemical or chemi-mechanical pulp-making process.
The glass fibers are preferably chopped glass fibers which
are sized for water dispersibility, for example, with a
polyester coating.
7
Any suitable form of calcium carbonate can be used as
filler for the purpose of imparting desired properties to the
sheet and reducing its cost. Various forms of calcium
carbonate are known, for example, as described in U.S. Patent
No. 4,596,661. It is believed that the most widely used
source of the calcium carbonate will be limestone rock which
is crushed and ground to suitable particle size, for example,
by roller milling. However, the source of calcium carbonate
can also be other naturally occurring calcium carbonate or
precipitated calcium carbonate.
The calcium carbonate can be of any suitable particle
size. In general, the particle size of the calcium carbonate
will fall-within the range of about 1 to about 30 microns in
equivalent spherical diameter. Ball-milling of limestone can
be used to produce calcium carbonate which has a relatively
high content of fine particles, that is, < 3 microns, which
is desirable for imparting opacity to the sheet. There has
been used to good advantage dry ground limestone having the
following particle size distribution: 0 wt.% - > 100 microns;
15 wt.% to 35 wt.% - <100 microns, but > 15 microns; 50
wt.% - <7 to 15 microns; and 15 to 35 wt.% - <3 microns.
Excellent results have been achieved utilizing calcium
carbonate as the only filler in the sheet. However, one or
more other fillers can be used in combination with calcium
carbonate. When another fillers) is used, it is recommended
that it comprise about 1 to about 10 wt.% of the solids
content of the aqueous dispersion. Typically, the filler is
a particulate material which is essentially water-insoluble.
Examples of other fillers that can be used are clay,
magnesium hydroxide, magnesium carbonate, titanium dioxide,
zinc oxide, barium sulfate, calcium sulfate, amorphous
silica, aluminum silicate, magnesium silicate, diatamaceous
earth, aluminum trihydrate, talc, and vermiculite.
The binder functions to adhere together the various
constituents of the sheet and to impart cohesive and other
desired properties thereto. The binder typically comprises
water-insoluble, solid resin particles which are dispersed in
the slurry and which are film-forming in nature, for example,
the types of resin solids used in water-based paints. The
8
most convenient source of the binder is a latex, that is, an
aqueous dispersion of resin solids having a size which is
colloidal in nature.
For use in the present invention, the resin should be
anionic, that is, carry a negative charge. Any available
means can be used to formulate a negatively charged resin or
to impart to the resin a negative charge. In preferred form,
the resin is prepared from a monomer or mixture of monomers,
at least one of which includes an ionizable group, for
example, a negatively charged carboxylate, sulfate or
sulfonate group. Exemplary of the nature of the charge
carried by the resin is about 0.03 to about 0.7
milliequivalents of charge per gram of resin in a latex
thereof.
It is believed that anionic resins that will be used
most widely as the binder in the present invention will
comprise a polymer (a homopolymer or higher polymer) prepared
from one or more monomers which include ethylenic
unsaturation. Examples of suitable polymers include styrene-
butadiene resins, acrylate resins, ethylene-vinyl acetate
resins, acrylonitrile resins, acrylonitrile-butadiene-styrene
resins, polyvinyl chloride) resins, and poly(vinylidene
chloride) resins. The preferred binder is a carboxylated
styrene-butadiene resin supplied in the form of a latex.
As required, there should be included in the slurry a
coagulant which functions to impart to the fibers and fillers
positive charges which anchor these materials to the
negatively charged sites of the anionic resin binder.
Examples of coagulants that can be used are water-soluble
inorganic salts such as aluminum sulfate (alum), calcium
chloride, and magnesium chloride and water soluble, cationic
resins, for example, low molecular weight quaternized amines,
low molecular weight polyacrylamides, and low molecular
weight polyethylene imine. The low molecular weight
polyacrylamide resin can function also to improve the wet-
strength properties of the sheet, as, for example, resins of
the type that impart permanent strength to a sheet. It is
preferred to use a mixture of coagulants comprising an
inorganic bi- or tri-valent metal salt, for example, alum,
9
and a polymeric coagulant, for example, a conventional
cationic polyacrylamide wet-strength resin.
A slurry which includes the aforementioned type
coagulant is not capable of being formed into the highly
filled sheet which is the subject of the present invention.
There is insufficient retention of the coagulated particles
on the porous support on which the slurry is deposited. In
addition, the nature of the coagulated particles that are
formed is such that they interfere with the drainage of the
water through the porous support. When this occurs, it is
necessary to slow down the speed of the moving support in
order to permit the drainage of additional water. This is
disadvantageous because it slows the rate of production.
The high charge-density cationic flocculant used in the
practice of the present invention functions to form the
coagulated particles of the slurry into soft flocs. ~In
certain embodiments within the scope of the present
invention, the flocs have been observed to be similar in
structure and in size to snowflakes. The flocs are in effect
discrete clumps which are of larger size than the coagulated
particles and sufficiently large to resist passing through
the openings of the porous support surface on which they are
retained. Aiso, the nature of the flocs is such that water
of the slurry drains freely and quickly through the openings
of the support surface and at a rate such that normal
operating speeds of the moving porous support surface can be
maintained.
As mentioned above, the cationic flocculant for use in
the practice of the present invention has a charge-density of
at least about 2 equivalents of cationic nitrogen per
kilogram of polymer. It is believed that the flocculants
used most widely will have a charge-density within the range
of about 2 to about 5 or 5.2 of the aforementioned
equivalents. In preferred form, the charge-density is at
least about 3.5 equivalents of cationic nitrogen per kilogram
of polymer. Cationic resins or polymers which have such
charge-densities are known, for example, as described in U.S.
Patent Nos. 5,098,520 and 5,178,730.
10
Such a polymer can be prepared, for example, by the
polymerization of an ethylenically unsaturated cationic
monomer or monomers, with or without another monomer or
monomers (typically nonionic) utilizing amounts of monomers
such that the resulting polymer has the desired charge-
density. Examples of cationic monomers that can be used to
prepare high charge-density cationic polymers are: dialkyl
amino alkyl acrylates or methacrylates or acrylamides or
methacrylamides in acid salt form, or preferably in the form
of a quaternary ammonium salt. An example of a monomer that
can be polymerized with the cationic monomer is an
acrylamide. Examples of cationic polymers include
quaternized and unquaternized copolymers of dimethylamino
ethyl acrylate or methacrylate and acrylamide, polyethylene
imines, polyamine epichlorohydrin polymers and homo- and co-
polymers (with acrylamide) of diallyldimethylammonium
chloride.
Other patents which refer to cationic polymers having a
high charge-density include: U.S. Patent Nos. 3,658,474;
3,962,332; 4,174,279; 4,396,513; and 4,711,727. Various of
these patents disclose different bases for the charge-
densities of the cationic polymers described therein. For
example, aforementioned Patent No. 4,396,513 discloses a
charge-density based on the molar ratio of the cationic
monomer used to prepare the polymer. For example, a polymer
made from 80 to 90 mole% of a cationic monomer is considered
to have a charge-density of 80 to 90%, which is referred to
as a very high charge-density. Cationic polymers disclosed
in this patent include those prepared by polymerizing a
quaternary ammonium salt of 2-(meth)acryloyloxyethyl-N, N-
dimethylamine with acrylamide.
Examples of commercially available high charge-density
cationic flocculants that can be used in the practice of the
present invention are described hereafter. WT-24761, which
is sold by Calgon Corporation, is a high molecular weight
cationic polyelectrolyte in the form of a latex of cationic
polymer that has a charge-density of about 2 to about 5.2
equivalents of cationic nitrogen per kilogram of polymer.
The_latex is an opaque white liquid having a density of 8.6
11
lbs./gallon and a viscosity of about 1500-3500 cps. This
cationic polymer is described by its manufacturer as being
effective for use in various liquid/solids separating
processes such as sludge thickening, clarification,
floatation, vacuum filtration and centrifugation. (It is
believed that this polymer is the equivalent of Calgon's
Hydraid CMP-452 which is described as being effective for use
in liquid/solids separations involving paper machine
retention, clarification, paper machine drainage, and save-
all applications.) Optimer 7194 and Optimer 7195, which are
sold by Nalco Chemical Company, are described as high charge
and high molecular weight cationic flocculants which are in
latex form. These cationic flocculants are described as
being effective for use in.various types of dewatering
processes. They have a charge density of about 2 to about 5
equivalents of cationic nitrogen per kilogram of polymer.
Although various of the high charge-density cationic
flocculants are available commercially in the form of latexes
(colloidal dispersions of resin solids), the flocculants can
be treated according to known procedures to solubilize them,
as recommended by their suppliers. Thus, the flocculants can
be added to the slurry in solution form.
The art recognizes the use in aqueous slurries of the
type involved herein of other ingredients which are typically
used in small amounts and which can be considered. as
additives or optional ingredients. Examples of such
materials include antioxidants, anionic or cationic
mildewcides, and defoaming agents.
The aqueous slurry used in making the sheet of the
present invention can be formulated from ingredients that are
used in amounts which are known in the art. The aqueous
slurry contains a very high proportion of water and a
relatively small amount of materials which are dissolved
therein or which are dispersed therein as solid particles
(both dissolved materials and solid particles are encompassed
by the use herein of the term "solids content"). Typically,
the solids content of the slurry will comprise about
0.5 to about 15 wt.% of the slurry, based on the total weight
of the water and the solids content of the slurry. For the
12
purpose of manufacturing a sheet for use as a backing for
vinyl floor covering, it is recommended that it be formed
from a slurry that has a solids content of about 3 to about
6 wt.%.
The proportion of constituents comprising the solids
content of the slurry will generally comprise: (A) about 5
to about 25 wt.% of fibers; (B) at least about 50 wt.% of
calcium carbonate; (C) about 2 to about 30 wt.% of binder;
(D) 0 to about 2 wt.% of a coagulant; and (E) about 0.01 to
about 0.5 wt.% of a high charge-density cationic flocculant.
When present, an additive will generally comprise about 0.001
to about 1 wt.% of the slurry. In the manufacture of a sheet
for use as a backing for vinyl floor covering, it is
preferred that the solids content of the slurry comprise:
(A) about 5 to about 15 wt.% of fibers; (B) at least about 70
wt.%, preferably about 75 to about 85 wt.%, of calcium
carbonate; (C) about 5 to about 15 wt.% of binder; (D) about
0.1 to about 2 wt.% of a coagulant; and (E) about 0.01 to
about 0.1 wt.% of a high charge-density cationic flocculant.
In preferred form, the slurry will include also about 0.05 to
about 0.5 wt.% of an antioxidant.
The pH of the slurry should be such that the calcium
carbonate is not degraded. For this purpose, the pH should
be at least about 7.0, and is typically in the range of about
7.5 to about 8.5.
By virtue of the excellent retention of solid-contents
of the slurry on the porous support surface that is achieved
by the use of the present invention, the proportion of
ingredients comprising the highly filled sheet will
correspond closely to the proportion of solid contents of the
slurry. Thus, the sheet will generally comprise: (A) about
5 to about 25 wt.% of fibers; (B) at least about 50 wt.% of
calcium carbonate; (C) about 2 to about 30 wt.% of binder;
(D) 0 to about 2 wt.% of coagulant; and (E) about 0.01 to
about 0.5 wt.% of a high charge-density cationic flocculant.
For a sheet that is used as a backing for vinyl floor
covering, it is preferred that it comprise (A) about 5 to
about 15 wt.% of fibers; (B) at least about 70 wt.%,
preferably about 75 to about 85 wt.%, of calcium carbonate;
13
(C) about 5 to about 15 wt.% of binder; (D) about 0.1 to
about 1 wt.% of coagulant; and (E)~about O.Oi to about 0.1
wt.% of a high charge-density cationic flocculant.
Sheet parameters such as density and thickness will tend
to vary within relatively broad ranges depending on the
particular application in which the sheet is used. For
example, the density and thickness of the sheet can be
respectively about 30 to about 120 lbs/cu ft, and about
to about 500 mils. For use as backing for vinyl floor
10 covering, it is preferred that the density be about 50 to
about 90 lbs/cu ft and that the thickness be about 15 to
about 40 mils.
One of the features of the present invention is the
provision of a highly-filled calcium carbonate sheet that has
a combination of excellent properties. For example, the
present invention can be used to form sheets that have a
tensile strength of at least about 20 lbs (1"-wide strip,
evaluated at 74°F), and at least about 5 lbs (1"-wide strip,
measured at 350°F). Sheets with such strength
characteristics are particularly useful as backings for vinyl
floor covering.
As descr-ibed above, the sheet of the present invention
can be made by any suitable process which involves forming
the sheet~from a highly dilute aqueous slurry of constituents
comprising the sheet. Examples of suitable processes are
referred to in aforementioned U.S. Patent No. 4,225,383.
As mentioned above, one of the advantages of the present
invention is that standard processing steps do not have to be
changed in an adverse way to accommodate the use of the
invention. In this regard, it is noted that line speeds of
the type generally used in the manufacturing process can be
maintained or even increased. For example, in the
manufacture of a backing for vinyl floor covering, the porous
support surface can be operated at a speed of at least about
100 feet/minute, and preferably is operated at a speed of at
least about 200 feet per minute.
An exemplary sequence of process steps for forming the
aqueous slurry is described hereafter. Calcium carbonate is
added with agitation to water which preferably has been
14
heated to a temperature of about 90 to about 110°F.
Thereafter, there can be added, in sequence, coagulant,
fibers, and optionally, a polyacrylamide resin or equivalent
material which functions to improve the wet strength of the
sheet and as a coagulant, and an antioxidant, when used.
Additional water is added with agitation to the. slurry to
reduce the solids content thereof to, for example, about 3 to
about 7 wt.~ Thereafter, the binder, preferably in the form
of a latex, is added to the dilute slurry. The dilution
facilitates a thorough blending of the binder with the
fiber/filler constituents. There is then added to the
slurry, with agitation, the high charge-density cationic
flocculant.
Examples
Examples below are illustrative of the practice of the
present invention. Comparative examples are set forth also.
Table 1 below includes a description of four aqueous
slurries, one of which can be used to form a highly filled
sheet which is an exemplary embodiment of the present
invention (Example 1) and the others of which are comparative
in that they include talc as the filler instead of calcium
carbonate (Examples C-2 and C-3) or they include a prior art
cationic flocculant (Examples C-1, C-2 and C-3).
15
Table 1
Amount of Ingredients,
wt.%,
except water, as indicated
Ingredients Ex. Ex. C-3 Ex.
C-1 1
Ex.
C-2
water (ml) 5000 5000 5000 5000
talc (filler) - 79.7 80.04 -
CaC03 (filler) 79.73 - - 80.04
alum (coagulant) 0.5 0.5 0.1 0.1
cellulosic fibers 7.4 7.4 7.4 7.4
glass fibers 1.00 1.00 1.00 1.00
polyacrylamide
(coagulant) 0.19 0.19 0.19 0.19
antioxidant 0.15 0.15 0.15 0.15
styrene-butadiene
resin (binder) 10.99 11.00 11.00 11.00
prior art flocculant 0.05 0.13 0.13 -
high charge-density
cationic flocculant - - - .08
The talc used in the slurries of Examples C-1 and C-3 was
sold by Cypress Industrial Minerals as Vertal 8. The calcium
carbonate used in the slurries of Examples C-1 and 1 was
roller-milled limestone. The cellulosic fiber consisted of
northern bleached softwood kraft. The polyacrylamide
coagulant is sold as Kymene 557H by Hercules Incorporated.
The glass fibers had a diameter of 7.5~, and length of 1/8".
The anti-oxidant is a 50% solids aqueous emulsion of 50% of a
hindered bis-phenol and 50% of high molecular weight
thioester. The styrene-butadiene resin is carboxylated and
was added in the form of a latex sold by Dow Chemical
Company. The prior art flocculant consisted of a medium
molecular weight cationic polyacrylamide emulsion flocculant
sold by Calgon Corporation as Hydraid TRP948. The high
charge-density cationic flocculant was Hydraid CMP-452 sold
by Calgon Corporation.
16
Each of the aqueous slurries described in Table 1 above
was formed into a sheet by adding the required aliquot of
slurry to a Williams Standard Sheet Mold, pressing the formed
sheet between standard blotting paper using a Williams Oil
Hydraulic Press, and drying against a heated drum dryer.
Thereafter, various properties of the sheets and process
conditions associated with the making of the sheets were
evaluated. Table 2 below includes the results of the
evaluations.
Table 2
Values of
Properties/ Properties/Processing ons
Conditi
Processing of Sheets formed of
from
Slurries
Conditions Ex. C-1 Ex. C-3 Ex.C-4
Ex. C-2
sample gauge .024" .024" .0235" .0235"
ream weight
(480 sq ft basis) 65.17 64.69 66.24 66.24
density (#/cf) 69.25 68.87 70.42 70.42
tensile, 1"-wide
strip ~ 74F (#) 27 24 27 29
elongation
at 74'F (%) 4.8 4.8 5 4
tensile, 1"-wide
strip ~ 350F (#) 10 10 12 11
elongation
at 350F (%) 3 4 3 3
plasticized .
tensile (#) 11 8 9 12
wet tensile,
1"-wide strip,
30 seconds (#) 10 8 11 13
mullen (psi) 48 43 49 59
elmendorf tear
(grams-force) 118 150 160 160
stiffness 48 52 60 52
plasticized
stiffness 16 20 16 16
Williams freeness,
80 mesh screen
(wt.% loss) 2.4 1.06 1.89 1.45
Williams freeness
(seconds) 11 9 11 10
17
Attention is directed to Table 2 above in respect of the
wt.% loss values recorded for non-aqueous constituents of the
slurry passing through the 80 mesh screen. The value of 2.4
wt.% recorded for the slurry of Example C-1 is considered to
be unsatisfactory for the effective manufacture of sheet on
an industrial scale. However, the 1.45 wt.% loss value for
the slurry of Example 1 is considered to be satisfactory,
and, as can be seen from other values in Table 2, the slurry
of Example 1 is capable of being formed into a sheet which
has comparable or better properties than sheets formed from
the comparative slurries.
The effectiveness of the present invention has been
demonstrated also in other ways. For example, sample stock,
which normally would be used in the Williams Sheet Mold, is
exposed to a very high level of shear in a jar with a 1300
rpm agitator for 60 seconds. This duplicates the high shear
which actually occurs as slurry is pumped onto the -moving
porous support surface. With limestone and Hydraid TRP-948
flocculant (prior art), freeness (drainage) increases from
about 10 seconds to about 35 seconds (unacceptably slow
drainage) and % weight loss increases from about 1.5% to
about 15 % (totally unacceptable). Using the improved
flocculant of this invention, freeness (drainage) increases
from 10 seconds (unsheared) to only 15-25 seconds
(acceptable) and % weight loss increases from about 1.5% to
5-9% (acceptable). Therefore, the improved flocculants are
much more resistant to shear (of the slurry) than the prior
art flocculants.
As mentioned hereinabove, another aspect of the present
invention relates to the use of carbon black to impart
opacity to either paper or to a highly-filled, fibrous-
reinforced sheet that is white in color, but tends to lack
opacity, that is, for example, translucent. Opacity in paper
or the aforementioned type sheets is a desired characteristic
for many applications in which the paper or sheet is used.
For example, with paper, opacity is a desired property when
the application involves printing the paper on both sides.
In the use of fibrous reinforced sheet as backing for vinyl
18
floor covering, it is not aesthetically desirable to have the
backing appear transparent; opacity is desired.
Developmental work has revealed that the use of carbon
black in relatively small quantities provides significant
increases in opacity with but minor degradation in the white
appearance of white paper or white fibrous-reinforced sheets.
Both paper and sheets of the aforementioned type have in
common. their being comprised of fibers and filler.
Typically, the filler component of paper comprises about 5 to
about 35 wt.% of the paper. The filler component of highly-
filled fibrous reinforced sheets, on the other hand,
comprises a higher proportion of the sheet, for example, at
least about 50 wt.%. The use of carbon black is effective
for the purpose described herein in connection with both
paper and sheet-type products, and also with products that
comprise a filler other than calcium carbonate.
The amount of carbon black used for its "opacity"
application will tend to vary depending on various parameters
including, for example, the thickness of the paper or sheet,
the proportion of ingredients comprising the paper or sheet,
and~the whiteness of the paper or sheet. It is recommended
that there be used about 0.0001 to about 0.05 wt.% of the
carbon black in the paper or sheet, based on the total weight
of the ingredients comprising the paper or sheet, and that
adjustments in amounts be made as needed for the specific
application. In the manufacture of sheet of the type used as
backing for vinyl floor covering, it is recommended that the
carbon black comprise about 0.0001 to about 0.05 wt.%, more
preferably about 0.001 to about 0.01 wt.% of the sheet. The
retention characteristics of the carbon black are excellent
in that it is~well retained with other ingredients comprising
the sheet or paper on the moving porous support surface
during manufacture of the product. Accordingly, the
proportion of carbon black comprising the solids content of
the aqueous dispersion used to make the paper or sheet can
correspond to the proportion of carbon black comprising the
finished paper or sheet.
There is set forth in Table 3 below the identity of
aqueous dispersions which include carbon black therein
19
(Examples 2 and 3) and a comparative dispersion which does
not include carbon black (Example C-4). The dispersions
described in Table 3 were formed into white sheets in the
basic manner described hereinabove in connection with
previous examples.
Table 3
Amount of Ingredients, wt.%,
except water, as indicated
Ingredients Ex. C-4 Ex. 2 Ex. 3
water (ml) 5000 5000 5000
talc (filler) 80.04 - -
CaC03 (filler) - 80 80
alum (coagulant) 0.1 0.15 0.15
cellulosic fibers 7.4 7.4 7.4
glass fibers 1 1 1
polyacrylamide
(coagulant) 0.19 0.19 0.19
carbon black - 0.005 0.0025
antioxidant 0.15 0.15 0.15
styrene-butadiene
resin (binder) 11 11 11
prior art flocculant 0.14 - -
high charge-density
cationic flocculant - 0.14 0.14
The opacities of samples of sheetsformed from the
aqueous dispersions described in Table 3 were evaluated by
placing a hand between the back ide the sheet and a
s of
ceiling light, and examining the front side of the sheet with
the eye. The sheet made from the dispe rsion of Example C-4
was translucent in that there could distinctly seen
be
through the sheet the shadow of he' d and fingers thereof.
t han
In contrast, nothing could be see n through the sheet made
from the dispersion of Example and he shadow of the hand
2, t
could be barely seen through the sheet made from the
dispersion of Example 3.
20
In summary, it can be said that the present invention
provides practical means for improving in significant ways
the manufacture of products of the type described above.
