Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-1- 1310~53
PAPER PRODUCT PREPARED WITH STRUCTURFD LATEX PARTICLES
HAVING REINFORCING AND OPACITY CHARACTERISTICS AND
PROCESS FOR PREPARING THE SAME
Generally9 this invention is directed toward a
method for preparing high quality lightweight paper
having increased opacity. The method employs a
--- structured latex having both reinforcing and opacifying
characteristics.
In the manufacture of papers, especially
lightweight paper, a major concern is the opacity of
the paper. Generally, opacity iq provided by
conventional fillers such as clay, calcium carbonate
and titanium dioxide. Unfortunately, these fillers can
adversely affect the mechanical properties of the paper
if too much filler is attempted to be incorporated to
obtain the desired opacity~
Attempts have been made to offset the reduction
in mechanical properties through the use of various
formulations of latexes having increased strength.
However, improved methods of obtaining good opacity or
light scattering characteristics without reducing
mechanical propertie3 are continually being sought.
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131()45)
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The problem is diffLcult to solve because opacity and
strength are generally inversely proportional
characteristic~. The problem is particularly acute in
lightweight papers where the amount of material that
can be employed to form the paper is minimal.
It has been discovered that by specifically
- engineering a structured latex particle having a
core/shell morphology that both characteristics of
opacity and mechanical strength can be satisfied. This
is accomplished by preparing the latex particle such
that it not only reinforces or binds the constituents
of the paper but also scatters light to provide its own
measure of opacity to the paper.
The present invention provides for a structured
latex part,icle suitable for use in the preparation of
~` latex-containing products to increase opacity and -
strength comprising a core portion having a light
scattering characteristic and a Tg of 80C or greater,
and a shell portion having a Tg of 25C or lower. The
structured latex particles have a particle size of
1,400 to 4,000 angstroms (140 to 400 nm). In one
embodiment the structured latex particle has a
homopolymer core portion of styrene and a shell region
of styrene/butadiene/acrylic acid.
The present invention further provides for a
paper product prepared with the structured latex
particle as described above. Most advantageously the
latex particle is employed in the preparation of light
weight paper where good opacity and mechanical strength
are desired. The subject structured latex particle
characteristically provides opacity and reinforcement
which allows for the reduction of fillers, such as
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_3_ 13~0453
TiO2~ in the article of manufacture. That is, the
structured latex particle allows for the reduction or
elimination of expensive opacifying fillers in products
without a corresponding loss of opacity.
Preparation of the products of the present
invention requires a starting latex comprising a
structured latex particle having a core/shell
morphology wherein the core region has a second order
transition temperature (Tg) of 80C or greater and the
shell region has a Tg of 25C or lower. These
structured latex particles are suitable for use in the
preparation of paper products having good opacity and
mechanical strength characteristics. Preferably and
most advantageously, the subject latex is employed in
the preparation of light weight papers where good
opacity and mechanical strength are de~ired.
The term "opaque" is meant to define that
quality of resisting the passage of light by being
neither transparent nor translucent. The opaque
characteristic is measurable by measuring the
brightness of reflected light, light scattering or
reflectance, and opacity (TAPPI standard T-425 om 81).
By "mechanical strength" is meant the quality of
tensile strength, modulus, tear strength, and other
physical properties generally recognized in the art as
contributing to mechanical strength.
The preparation of structured latexes are well
documented and do not form a part of this invention
except as modified by the compositional requirements of
the invention. Generally, particles of a starting
latex are encapsulated with additional monomers
polymerized therewith. This can be conveniently
32,944A-F -3-
~ 3 ~ 0453
--4--
accomplished by emulsion polymerizing the desired shell
portion monomers in the presence o~ an existing latex
which has the desired core composition. Thus, the
polymerization is a conventional emulsion
polymerization of a latex but for the polymerization of
the shell monomer portion being conducted in the
presence of a preexisting latex particle which
represents the core region of the final latex productO
Emulsion polymerization techniques such as
0 staged or continuous addition of monomer feeds are
typically employed. Examples of such techniques are
further described in U.S. Patent Nos. 4,156,669 and
4,017,442.
The structured latex particle of the subject
invention requires a core region which has light
~ scattering properties. Generally, this light
scattering characteristic iq provided by a polymer or
copolymer of ethylenically unsaturated monomers having
Tg of 80C or greater. The Tg of the core region is
important in maintaining the identity, i.e., size and
distribution, of the core which thereby contributes to
the light scattering characteristic of the latex.
Contrarily a lower Tg would allow the core region to
coalesce during paper making and result in a low light
scattering characteristic.
Difunctional monomers~ typically useful as
crosslinkers, can be employed in the core region to
increase the Tg of the core region. Examples of good
crosslinkers would include allyl or crotyl acryla~e and
methacrylates and divinyl benzene. The preferred
polymer core composition is a monovinyl aromatic
polymer such as styrene and copolymers or derivatives
32,944A-F -4-
_5_ 1310~53
thereof having a Tg of 80C or greater. More
preferably, the core region is a homopolymer of
styrene.
The core portion of the structured latex
particle comprises from 40 to 90 percent of the total
particle. Preferably, the core/shell polymer weight
ratio is from 50/50 to 90/10, respectively.
The particle size of the subject structured
latex is important to the overall light 3cattering
properties and, therefore, a particle size of ~rom
1,400 to 4,000 angstroms (A) (140 to 400 nm) is
preferred. More preferably, the average particle
diameter is from 2,000 to 3,500 A ~200 to 350 nm).
The shell region of the subject structured
_ latex particle is compo~ed of a polymer or copolymer of
ethylenically unsaturated monomers having a Tg of 25C
or lower. Generally, the shell composition may include
monomers employed in the core region. Thus, the shell
compositions advantageously employed are various blends
of polymerized monomers such as monovinyl aromatics,
aliphatic conjugated dienes, monoethylenically
unsaturated carboxylic acidq, vinyl or vinylidene
halides or acrylates. Optionally, reactive monomers
such as N-methylolacrylamide and glycidylmethacrylate
can be included.
The term "monovinyl aromatic monomer", as used
herein, is meant to include those monomers with a
radical of the formula
32,944A-F ~5-
13~0453
--6--
CH2=C-
(wherein R i~ hydrogen or a lower alkyl such as an
alkyl having from 1 to 4 carbon atom~) attached
- 5 directly to an aromatic nucleus containing from 6 to 10
carbon atoms, including those wherein the aromatic
nucleus i~ substituted with alkyl or halogen
substituents. The preferred monomer is styrene.
The term "aliphatic conjugated diene", as used
harein, is meant to include compounds such as 1,3-
butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl- 1~3-
butadiene, 2-neopentyl~1,3-butadiene, piperylene (1,3-
pentadiene), and other hydrocarbon analogs of 1,3-
butadiene.
_ The term "monoethylenically unsaturated
carboxylic acid monomer", as used herein, is meant to
include those monocarboxylic monomers such a~ acrylic
acid, and methacrylic acid; dicarboxylic monomers such
as itaconic acid, fumaric acid, maleic acid, and their
monoesters. The preferred acid monomer is acrylic
acid.
Vinylidene halides and vinyl halides suitable
for this invention include vinylidene chloride and
vinyl chloride, which are preferred. Vinylidene
bromides and vinyl bromide can al~o be employed.
3 The term "acrylate", as used herein, is meant
to include ~onomers o~ the acrylate or methacrylate
type. Additionally~ the acrylates can include acids,
esters, a~ides and substituted derivatires thereof.
Generally, the preferred acrylate~ are C1-Cg alkyl
acrylates or methacrylates. Examples of such acrylates
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include butyl acrylate, 4-biphenyl acrylate, hexyl
acrylate, sec-butyl acrylate, tert butyl acrylate,
methylmethacrylate, butylmethacrylate, lauryl
methacrylate, hexylmethacrylate, isobutylmethacrylate,
and isopropylmethacrylate.
The shell region i3 preferably composed of a
-~ polymer blend comprising styrene, butadiene and acrylic
acid. This particular blend has been found to yield
desirable physical properties and compatibility ~ith
the preferred core coMposition of ~tyrene. Generally,
the shell composition has the following ~tyrene/~
butadiene/acrylic acid ratios of 49/49/2 to 47/47/6;
49i49/2 to 69/29/2; and 69/29/2 to 67/27/6,
respectively.
Structured latex particles prepared in the
foregoing manner are advantageously employed as paper
coatings or fillers where good opacity and mechanical
properties are desirable. More particularly, paper
products prepared with the subject structured particle
latex will have increased opacity and strength over a
similar paper prepared in the absence of the structured
particle latex. While not limited to paper
applications, the subject latex can also be employed as
a filler or component in other areas where latexes may
be employed such as in protective or decorative
coatings, e.g., paints, etc. More advantageously, the
subject structured latexes are employed a~ a late~
3 binder or as a partial substitute for the latex binder
in the preparation of high quality lightweight or fine
papers. Lightweight papers are generally defined as
those papers having less than 34 pounds/3300 sq.ft. or
approximately 50 g/sq.meter.
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These and other advantages will be readily
apparent in view of the following examples.
Example 1
A structured latex was prepared in the
following manner. To 80 parts by weight per 100 parts
comonomer charge of an aqueous medium containing oO1
part by weight of a pen~asodium ~alt of diethylene-
triaminepentacetic acid and 0.52 part by weight of a
97/3 weight ratio styrene/acrylic acid copolymer seed
latex was continuously added 100 parts by weight of a
styrene monomer feed with stirring over a period of 2.8
hours. An aqueous ~tream oontaining 40 parts deionized
water, 1.0 part by weight of sodium dodecyldiphenyl
ether disulfonate, 0.5 part by weight of sodium
persulfate and 0.1 part by weight of sodium hydroxide
~`` was added with said styrene feed for a period of 4.-5
hours at 90C. Following the addition of the styrene
feed a 49/49/2 weight ratio of styrene/butadiene/-
acrylic acid comonomer feed was continuously added with
stirring over a 1~2 hour period. In addition, the
comonomer feed also contained 0~15 part by weight of
tertiary dodecylmercaptan.
Upon completion, the resulting latex had a
total solids content of about 43 percent, a pH of 3,
and the struotured latex particles had an average
particle diameter of 1550 ~ (155 nm), and a polystyrene
core (Tg approximately 100C) to
styrene/butadiene/acrylic acid shell (Tg lower than
25C) ratio of 70/30.
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_9_ 131~453
Example 2
A structured latex particle was prepared as in
Example 1 except that the core/shell ratio was 50/50.
Example 3
Paper compositions-employing the latexes
~~ prepared in Examples 1 and 2 a3 reinforcing opacifiers
were prepared. Seven and one-half parts (polymer
solids basis) of the latexes from Examples 1 and 2 were
admixed with individual samples of an aqueous paper
composition~ The paper composition consisted of 6,424
parts of an aqueou~ suspension containing 92~5 parts
(dry weight basis) of bleached kraft wood fibers (50/50
dry weight ratio of hardwood/softwood), 100 parts of a
2 percent by weight aqueous solution of aluminum
sulfate octadecahydrate, and 3.5 parts of 0.1 percent
by weight aqueous solution of a high molecular weight
polyacrylamide.
The suspensions described above were used to
form paper handsheets having a basis weight of
40 lbs./3300 sqOft. (59.2 g/m2)on a Noble and Wood
paper machine. The structured particle latex content
of each paper was mea~ured by pyrolysis-gas
chromatography. The opacity, brightness, tensile
strength and elongation of the handsheets were measured
and are shown below with a comparative paper example
prepared as above except in the absence of the
structured latex particles of this invention.
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Latex
None 1Example 1 Example 2
Core/Shell Ratio - 70/30 50/50
Percent Latex 0 3.9 4.87
Opacity2 73.4 76.6 74.3
Brightness3 77.4 79.9 78.4
-~ Tensile (psi) 3373 3566 3579
Elongation (%) 2.6 2.7 2.8
Scattering4 (cm/g) 302 1491 544
lNot an example of the invention; conventional paper
sheet
Opacity is TAPPI standard T-425 om 81
15 Brightness is TAPPI standard T-452 om 83
4Scattering coef~icients obtained from reflectance
measurements (Kubelka-Munk equation).
The results indicate that just a small addition
of the subject structured latex particle increased the
opacity without a reduction in strength, in fact
increasing the strength, of the paper sheet over the
comparative paper sheet containing no structured latex
particle. This result is significant inasmuch as
opacity and strength are generally inversely
proportional characteristics. It i~ indicated that
further improvement in properties would be observed at
higher levels of the subject latex inclusion; however,
this would tend to increase the overall cost of the
paper. Generally, the subject latex can oomprise from
1 to 20 percent by dry weight basis of the paper
composition. Preferably, the latex content of the
paper is from 2 to 10 percent by weight.
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