Note: Descriptions are shown in the official language in which they were submitted.
12815~1
,
SHEETS HAVING IMPROVED STIFFNESS FRO~I
FIBER, LATEX AN~ COALESCING AGE~T
The present invention generally relates to
sheets prepared from fiber, latex and a coalescing
agent. Further, the present invention provides for a
sheet having improved stiffness by employing a hard
latex and a fugitive coalescing agent.
A wide variety of sheets made from fibrous
materials have been described in the art. Typical
examples of such sheets incLude fine printing papers,
cardboard papers, underlayment felt for vinyl floor
coverings, gasket papers, roofing papers, sound-
deadening papers, pipewrap, heat deflection papers and
board products.
In an effort to improve the properties of these
products, various latex compositions have been
employed. While such practices have greatly
contributed to the art, it is still desirable to have
further improvements in their physical properties such
as stiffness.
Stiffness is very important in printing grades
of paper, especially in the lightweight grades.
30,110A-F -1-
1281511
--2--
Methods to increase sheet stiffness are therefore
desirable. One potential method for increasing
stiffness would be to employ a latex having greater
hardness; however, use of such a latex would be limited
by its ability to coalesce or form a film under the
sheet prepar~-tion conditions. That is, a latex with a
minimum film-forming temperature greater than the
temperature a sheet reaches during preparation ~.~ould
not be a viable choice. Unfortunately, such latexes
could provide the desired stiffness, but increasing the
drying temperature would be harmful to the fiber and
other organic components of the sheet system.
Therefore, it would be desirable to be able to employ a
latex having minimum film-forming conditions greater
than the sheet preparation conditions in order to take
advantage of the latex's physical properties (i.e.,
hardness which could translate to increased stiffness
when used in a sheet system).
U.S. Patent No. 4.225,383 generally teaches
composite sheet preparation and suggests polymers which
are made film-forming by the use of plasticizers can bi~
used. This reference is, however, limited to composite
sheets contalning relatively high levels of filler or
pigment from 60 to 95 percent based on the dried weight
of the composite sheet. The subject invention,
instead, focuses on sheets with no filler or medium
filler levels
The present invention provides a process for
preparing a sheet with improved stiffness which
comprises: (a) mi~ing in an aqueous medium a mixture
comprising (i) a fiber, (ii) a latex having minimum
film-forming conditions greater than the conditions the
sheet reaches during drying, and (iii) a fugitive
i
30,11OA-F -2-
. ~ . .
lZ8~511
3--
coalescing asent (b) forming the mixture into a sheet,
and (c) drying said sheet under conditions less t'nan
the minimum film-forming conditions required by the
latex. Additionally, the process can include the step
of mixing the latex with the fugitive coalescing agent
prior to the addition of the fiber in the aqueous
medium.
Further, the present invention may include the
addition of a pisment or filler to the aqueous mixture
of the process of up to 60 weisht percent based on the
total dry weight of the sheet. Additionally, the latex
can be destabilized by combining a chemical flocculant
to form the sheet.
The present invention further provides for a
sheet prepared from the aforementioned process. An
advantage of the sheet is improved stiffness for
various grades of paper, especially fine printing
paper.
The process of the invention requires a fiber,
a latex and a coalescing agent. More particularly, a
fugitive coalescing agent is employed. The process can
further comprise the incorporation of fillers from low
to medium levels (0 to 60 weight percent based on the
total weight of the dried sheet) which are generally
known in the art of sheet preparatïon.
3 As a first component, the process of the
invention requires a fiber material. The fiber can be
a water-insoluble, natural or synthetic water-
dispersible fiber or blend thereof. Either long or
short fibers, or mixtures of both, are useful. The
most common natural fibers are those made from wood
.
30,110A-F -3-
~Z81~
--4-- . .
pulp, cotton, wool and jute to name only a few.
Typically, synthetic fibers useful in the subject
invention comprise rayon, nylon, glass fibers and
polyester. More examples of other suitable fiber
material are disclosed in U.S. Patent No. 4,225,383.
.
The fiber is present in a reinforcing amount.
That is, sufficient fiber ~ill be present to impart
additional physical strength to the sheet, compared to
a similar sheet prepared without a fiber. Generally,
the sheet will contain from 1 to 99, preferably from 10
to 80, more preferably from 15 to 70 weight percent
fiber based on the total weight of the dried sheet.
As a second component, the process of the
invention requires a latex. By the term "latex" is
meant a colloidally stable dispersion of discrete
polymer particles in an aqueous medium. The latex
employed in the subject invention is critical to
imparting the desired stif~ness to the sheet. There-
fore, the latexes are termed "hard latexes" which is
meant to describe a latex havLng a relatively high
minimum film-forming temperature. That is a higher
temperature than the sheet normally reaches at drying
or due to the higher minim-lm ~ilm-~orming temperature?
the latex does not have su~ficient residence time to
adequately coalesce under normal sheet preparation
conditions. More particularly, these so called "hard
latexes" are non-film-forming under sheet process
conditions. "Non-film-forming" is meant to indicate a
degree of latex coalescence. Sheet preparation
"conditions" is meant to indicate the various
parameters which effect the ability of a latex to
coalesce such as temperatu`re, residence time and
moisture level. Because the hard latex would not
I ,
30,110A-F -~-
128~511
generally form a film (i.e., coalesce) the third
component of the subject invention is required which is
a fugitive coalescing agent.
The latexes employed in the subject invention
are water-insoluble, natural or synthetLc and may be
homopolymers or copolymers of two or more ethylenically
unsaturated monomers or a mixture of such polymers.
Monovinylidene aromatic compounds such as styrene and
aliphatic conjugated dienes such as 1,3-butadiene are
preferred latexes. Generally, the more preferred
latexes comprise styrene/butadiene. Other suitable
latexes generally known in the art of sheet preparation
can also be advantageously employed as described
herein. Suitable latexes are disclosed in U.S. Patent
No. 4,225,383.
The latex will be present in an amount
sufficient to maintain the ~inished sheet in the form
of a generalLy unitary sheet having su~ficient body to
enable it to be handled without crumbli.ng. Preferably,
there is sufficient late~ pres,enk to al~ow the dried
sheet to be put on rolls, coated, cut and printed
without crumbllng or tearing. Gerlerally, the sheet
will have from 1 to 30, preferabLy ~rom 1.-5 to 10, more
preferably from 2 to 7, weight percent based on the
total weight of the dried sheet.
As a third component, the subject process
requires a coalescing agent. More particularly, the
coalescing agent must be a fugitive coalescing agent.
Therefore, unless otherwise indicated the term
"coalescing agent" is meant to indicate a fugitive
coalescing agent. By "fugitive" is meant that the
coalescing agent is sufficiently volatile such that
30,11OA-F -5-
~81Sl~
under the conditions employed to form and dry the
sheet, the coalescing agent will be substantially
removed from the sheet. Useful coalescing agents
include ~lycol ethers such as ethylene glycol
monomethyl ether, ethylene glycol monoethyl ether,
- ethylene ~lycol monobutyl ether, dietnylene glycol
monobutyl ether, diethylene glycol diethyl ether, and
propylene glycol phenol ether; glyo~l ether acetates
such as diethylene glycol monoethyl ether acetate and
ethylene glycol monomethyl ether acetate; and ketones
such as acetone, methyl ethyl ketone, methyl propyl
ketone, and diacetone alcohol.
The coalescing agent is present in an effective
amount. That is, sufficient coalescing agent will be
present to assist in the deformation of the latex
particles such that the finished sheet has greater
stiffness than a similar sheet made without the use of
a coalescing agent. Generally, the coalescing agent
will be present from 1 to 20, preferably from 2.5 to
15, more preferably from 5 to 10 weight percent based
on the total weight of the latex solids.
The fu~itive coale~cin~ agent is preferentially
present in the latex particle as opposed to the aqueous
phase of the ~iYture. In the late~ particle the
coalescing agent assists the hard latex to deform or
coalesce on the fiber or pigments if present. In
effect, the hard lateY is, thus, made film-formin~
under drying conditions less than the minimum film-
forming conditions required by the hard latex.
Further, during the drying of the sheet, the fugitive
coalescing agent is substantially removed whereby the
hard latex reverts and contributes to the stiffness of
30,11OA-F -6-
1281511
the final sheet. Trace amounts of fugitive coalescing
agent may, however, remain in the final sheet.
The invention also includes "composite papers"
which are non-woven fabrics containing a binder, a
fiber~ and a nonbindlng filler. These fillers are
generally finely divided solids (i.e., powders) such as
clay, magnesium hydroxide, or calcium carbonate.
Examples of other suitable fillers are disclosed in
U.S. Patent No. 4,225?383.
For use in the subject invention, the filler
level is from 0 to 60 weight percent based on the
weight of the dried sheet. The preferred filler range
is from 25 to ~5 weight percent. Lower filler levels
can be advantageously employed due to the excellent
physical properties contributed by the hard latex.
Generally, the fiber, latex, coalesc;ng agent
and optional filler are combined in an aqueous medium
prior to formation o~ the sheet. ~hile not critical to
the practice of the invention, it i9 ~enerally
convenient to premix the latex and coalescing agent,
and add this mixture to a dispersion o~ the fiber and
filler in water. In preparing the latex-coalescing
agent mixture, it is desirable to provide a standing -
period so that the coalescing agent may have time to
diffuse into the latex particles. Normally, a period
of about one hour is sufficient. If the latex ar;d
coalescing agent are added separately to the fiber
slurry, an extensi~/e waiting period (perhaps several
days or more) may be needed unless very large amounts
of coalescing agent are used.
30,110A-F -7-
1;~81
--8 -
After the fiber, latex and coalescing agent
have been combined in the aqueous medium, the late~ is
destabilized so as to precipitate it onto the fibers.
This is conveniently accomplished by means of a floccu-
lating (coagulating) agent. As the flocculant,substances capable of destabilizing the latec
dispersion can be used. ~amples of suitable
flocculants include modified cationic polyacrylamide
and diallyldie~hylammonium chlorid~ for anionic lat~xes
and partially hydrolyzed ?olyacrylamide for cationic
latexes.
After flocculation is completed, the aqueous
slurry is formed into a sheet or web and dewatered.
This sheet forming and dewatering process may be accom-
plished by any conventional paper-making apparatus such
as a sheet mold, or a Fourdrinier or cylinder machine.
After the composite is formed into a dewatered
sheet, it may be desirable to densify the sheet by
pressing it with a ~lat press or by sending it through
calendering rolls. Dryin~ of the sheet may be either
by air drying at ambi0nt conditlons or by oven drying.
The drying temperature of a sheet is limited by
the temperature at which the fiber or other organic
component of the system can withstand ~ithout being
damaged. Thus, heretobefore, a latex selection was
generally limited by the minimum film-forming
temperature of the latex. Therefore, an advantage of
the present invention is that for a given drying
temperature, a harder latex may be used or for a given
latex, the composite may be dried at a-lower
temperature.
.
30,11OA-F -8-
1~8~511
Other general teachings of the use of latexes
in the formation of fibrous sheets may be found, for
example, in U.S. Patent Nos. 3,875,097 and 3,518,113,
and r~est German issued patent 1 446 609.
A particularly useful and commercially signi-
ficant embodiment of the in/ention includes a composite
made with a latex, a high grade cellulose fiber, a
filler, and a fugitive coalescing agent, wherein the
finished composite is relatively thin (i.e., ~rom 0.002
to 0.01 inches (0.05 to 0.25 mm)). These composites
generally will contain desirably 3 to 99, preferably 25
to 90, most preferably 35 to 70, weight percent fiber
based on the total weight of the dried composite;
desirably O to 60, preferably from 25 to 45, weight
percent filler based on the weight of the dried
compos;te; and desirably from 0.5 to 30, preferably
from o.8 to 10, more preferably ~rom 1 to 5, weight
percent (based on solids) of the late~, based on the
total weight of the dried composite. Such composites
are useful as printing papers, and have unexpectedly
superior ~tiff`ness compared to similar papers made
without the fugitive coalescin3 a~ent.
The invention is further illustrated by the
following examples. In the e~amples all parts and
percentages are by weight unless otherwise specified.
30 Example 1 - Fine Printin~ Paper
A. Pulp Preparation
A mixture of 50 parts kraft hardwood and 50
parts kraft softwood was dispersed in water at 1.35
percent consistency and mechanically refined into pulp
having 392 CSF (Canadian Standard Freeness). The pulp
30,11OA-F -9-
'. :
'
12815~1
--10--
was then diluted to 0.51 percent consistency. On the
following da-y, the pulp wa3 adjusted to a pH of about
4.1 with 0.1 N HCl.
B.Latex ~reDaration
A relatively hard latex wa3 prepared from 80
parts styrene, 17 parts butadiene and 2 parts acrylic
acid using co~ventional emuIsion polymerization tech-
niques. The resultin~ latex was filtered to remo~/e anycoagulum, and had 48 percent solids. To 100 wet grams
of latex (45 percent solids), 4.5 g of propylene glycol
phenyl ether were added, and mixed for at least 1 hour.
C.Sheet Preparation
In a large beaker, 444 wet grams (0.5 percent
solids) of pulp were thoroughly mixed with 1.3 g of
kaolin filler clay and 0.18 g of the above-prepared
latex were added. A~ter stirring was complete (3
minutes), cationic polyacrylamide (flocculating agent)
was added unti] water phase clarity occurred, and then
an additional 0.5 pounds/ton (0.025 percent) of
Betz 1260 thigh molecular weight polyacrylamide) was
added. The dispersicn was then immediately poured into
a Noble and Wood sheet mold (8 in ~ 8 in
(203 mm x 203 mm)) having 5 liters of pH 4.0 water
therein, and then additional water was added to bring
` the total volume in the mold to 10.5 liters. The mold
was then drained, and the sheet pressed and removed
from the screen. The sheet was drum dried at 160F
(71C) for 330 seconds. The moisture content of the
dried sheet was 4 percent.
30,110A-F -10-
~ .
~Z8~5~1
D. Evaluations
After aging for 1 day at 50 percent RH and 73F
(22.8C) (TAPPI method T-402-05-70), the sheets were
evaluated for riOidity on a Licence ~odak P~the tester
using 0.5 in (12.7 mm) wide test strips. The results
are shown in the Table which follows.
E.Compa~i 3 ons - - ~
10 For comparative purposes, two other papers,
Samples 2 and 3, were made. Sample 2 was identical to
the above-described paper, except that it was made
without the propylene glycol ?henyl ether coalescing
agent. Sample 3 did not contain a coalescing agent and
was made from a softer latex which was a blend of 40
percent of an 81 percent styrene, 17 percent 1,3-
butadiene, 2 percent acrylic acid latex, and 60 percent
of a latex comprising 49 percent styrene, 50 percent
1,3-butadiene, and 1 percent itaconic acid. The
results are also ~hown below.
FINE ~R N~ ~NG PAPER
Sample Late.~ Coalescin~ A~ent Ri~itv_
1 hard yes 35 . 7
2* hard no 28.6
303* Soft/hard no 27.7
blend
*Not an example of the invention
.
30, 110A-F -11-
