[0001] Generally, 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.
[0002] In the manufacture of papers, especially lightweight paper, a major concern is the
opacity of the paper. Generally, opacity is 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.
[0003] 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 properties are continually being sought. The problem is difficult to solve
because opacity and strength are generally inversely proportional characteristics.
The problem is particularly acute in lightweight papers where the amount of material
that can be employed to form the paper is minimal.
[0004] US-A-3.897,300 describes a paper filled with a blushed particulate polystyrene pigment
filler. This filler consists essentially of spherical particles having an air-containing
microporous structure. It is capable of increasing the opacity of a sheet of paper.
[0005] Further, it is known to add structured latex particles in a paper coating, which
are intended to be applied on the surface of a paper product.
[0006] US-A-4,134,872 relates to a paper product, which is coated with a layer comprising
structured latex particles as a binder. These coating layers provide coated paper
products having better gloss, ink receptivity, smoothness, wet binding strength, and
dry binding strength.
[0007] US-A-4,613,633 relates to a copolymer latex comprising heterogeneous polymer particles
having a hard and a soft polymer domain. These particles are used in paper coating
layers.
[0008] 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.
[0009] The present invention provides for a structured latex particle 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 T
g of 80°C or greater, and a shell portion having a T
g of 25°C 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.
[0010] The present invention further provides for a paper product prepared with the structured
latex particle as described above as a filler in the bulk. 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 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.
[0011] 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 (T
g) of 80°C or greater and the shell region has a T
g of 25°C 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 desired.
[0012] 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.
[0013] 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 accomplished by emulsion polymerizing
the desired shell portion monomers in the presence of 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 product.
[0014] Emulsion polymerization techniques such as 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.
[0015] The structured latex particle of the subject invention requires a core region which
has light scattering properties. Generally, this light scattering characteristic is
provided by a polymer or copolymer of ethylenically unsaturated monomers having a
T
g of 80°C or greater. The T
g 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 T
g would allow the core region to coalesce during paper making and result in a low light
scattering characteristic.
[0016] Difunctional monomers, typically useful as crosslinkers, can be employed in the core
region to increase the T
g of the core region. Examples of good crosslinkers would include allyl or crotyl acrylate
and methacrylates and divinyl benzene. The preferred polymer core composition is a
monovinyl aromatic polymer such as styrene and copolymers or derivatives thereof having
a T
g of 80°C or greater. More preferably, the core region is a homopolymer of styrene.
[0017] 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.
[0018] The particle size of the subject structured latex is important to the overall light
scattering properties and, therefore, a particle size of from 1,400 to 4,000 angstroms
(Å) (140 to 400 nm) is preferred. More preferably, the average particle diameter is
from 2,000 to 3,500 Å (200 to 350 nm).
[0019] The shell region of the subject structured latex particle is composed of a polymer
or copolymer of ethylenically unsaturated monomers having a T
g of 25°C 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 acids, vinyl or vinylidene halides or acrylates.
Optionally, reactive monomers such as N-methylolacrylamide and glycidylmethacrylate
can be included.
[0020] The term "monovinyl aromatic monomer", as used herein, is meant to include those
monomers with a radical of the formula
(wherein R is hydrogen or a lower alkyl such as an alkyl having from 1 to 4 carbon
atoms) attached directly to an aromatic nucleus containing from 6 to 10 carbon atoms,
including those wherein the aromatic nucleus is substituted with alkyl or halogen
substituents. The preferred monomer is styrene.
[0021] The term "aliphatic conjugated diene", as used herein, 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.
[0022] The term "monoethylenically unsaturated carboxylic acid monomer", as used herein,
is meant to include those monocarboxylic monomers such as 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.
[0023] Vinylidene halides and vinyl halides suitable for this invention include vinylidene
chloride and vinyl chloride, which are preferred. Vinylidene bromides and vinyl bromide
can also be employed.
[0024] The term "acrylate", as used herein, is meant to include monomers of the acrylate
or methacrylate type. Additionally, the acrylates can include acids, esters, amides
and substituted derivatives thereof. Generally, the preferred acrylates are C₁-C₈
alkyl acrylates or methacrylates. Examples of such acrylates include butyl acrylate,
4-biphenyl acrylate, hexyl acrylate, sec-butyl acrylate, tert-butyl acrylate, methylmethacrylate,
butylmethacrylate, lauryl methacrylate, hexylmethacrylate, isobutylmethacrylate, and
isopropylmethacrylate.
[0025] The shell region is 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 with the preferred core composition of styrene. Generally,
the shell composition has the following styrene/butadiene/acrylic acid ratios of 49/49/2
to 47/47/6; 49/49/2 to 69/29/2; and 69/29/2 to 67/27/6, respectively.
[0026] 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 as a latex 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.
[0027] These and other advantages will be readily apparent in view of the following examples.
Example 1
[0028] A structured latex was prepared in the following manner. To 80 parts by weight per
100 parts comonomer charge of an aqueous medium containing .01 part by weight of a
pentasodium salt of diethylenetriaminepentacetic 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 stream containing 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 90°C. 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.
[0029] Upon completion, the resulting latex had a total solids content of about 43 percent,
a pH of 3, and the structured latex particles had an average particle diameter of
1550 Å (155 nm), and a polystyrene core (T
g approximately 100°C) to styrene/butadiene/acrylic acid shell (T
g lower than 25°C) ratio of 70/30.
Example 2
[0030] A structured latex particle was prepared as in Example 1 except that the core/shell
ratio was 50/50.
Example 3
[0031] Paper compositions employing the latexes prepared in Examples 1 and 2 as 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 aqueous 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.
[0032] The suspensions described above were used to form paper handsheets having a basis
weight of 40 lbs./3300 sq.ft. (59.2 g/m²)on a Noble and Wood paper machine. The structured
particle latex content of each paper was measured 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.
|
Latex |
|
None 1 |
Example 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 |
¹Not an example of the invention; conventional paper sheet |
Opacity is TAPPI standard T-425 om 81 |
Brightness is TAPPI standard T-452 om 83 |
⁴Scattering coefficients obtained from reflectance measurements (Kubelka-Munk equation). |
[0033] 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 is 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 comprise 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.
1. A lightweight paper product containing a structured latex particle comprising:
(1) a core portion having a light scattering characteristic and a Tg of 80°C or greater; and
(2) a shell portion having a Tg of 25°C or lower;
whereby the opacity and strength of said paper is increased over a paper product
prepared in the absence of said structured latex particle.
2. A paper product as claimed in Claim 1 wherein the structured latex particle comprises
from 1 to 20 percent by dry weight of said paper.
3. A paper product as claimed in Claim 1 wherein said core portion comprises a monovinyl
aromatic polymer, copolymer or derivative thereof.
4. A paper product as claimed in Claim 3 wherein said monovinyl aromatic polymer is styrene.
5. A paper product as claimed in Claim 1 wherein said structured particle latex has a
homopolymer core region of styrene.
6. A paper product as claimed in claim 3 wherein said core portion additionally contains
difunctional monomers which contribute to increasing the Tg of said core.
7. A paper product as claimed in claim 6 wherein said difunctional monomer comprises
allyl or crotyl acrylates, and/or methacrylates, or divinyl benzene.
8. A paper product of claim 1 wherein said structured particle latex has a shell region
comprising a polymer blend of styrene/butadiene/acrylic acid.
9. A paper product as claimed in claim 8 wherein the ratio of said styrene/butadiene/acrylic
acid is 49/49/2 to 47/47/6; 49/49/2 to 69/29/2; and 69/29/2 to 67/27/6, respectively.
10. A process for preparing a lightweight paper product containing as a filler in the
bulk a structured latex particle which comprises
(a) providing a structural latex particle comprising:
(1) a core portion having a light scattering characteristic and a Tg of 80°C or greater; and
(2) a shell portion having a Tg of 25°C or lower;
(b) admixing said structured latex particle with an aqueous paper composition; and
(c) forming the mixture into a paper product.
1. Leichtgewichtiges Papierprodukt, enthaltend als Füllstoff in der Masse ein strukturiertes
Latexteilchen, umfassend:
(1) einen Kernanteil mit einer Lichtstreueigenschaft und einem Tg von 80° C oder größer; und
(2) einen Mantelanteil mit einem Tg von 25° C oder niedriger;
wodurch die Opazität und Festigkeit dieses Papiers gegenüber einem bei Abwesenheit
dieses strukturierten Latexteilchens hergestellten Papierproduktes erhöht ist.
2. Papierprodukt nach Anspruch 1, worin das strukturierte Latexteilchen von 1 bis 20
% Trockengewicht dieses Papiers ausmacht.
3. Papierprodukt nach Anspruch 1, worin dieser Kernanteil ein monovinylaromatisches Polymeres,
Copolymeres oder Derivat hiervon umfaßt.
4. Papierprodukt nach Anspruch 1, worin dieses monovinylaromatische Polymere Styrol ist.
5. Papierprodukt nach Anspruch 1, worin dieses strukturierte Latexteilchen einen Homopolymer-Kernbereich
aus Styrol hat.
6. Papierprodukt nach Anspruch 3, worin dieser Kernanteil zusätzlich difunktionelle Monomere,
welche zur Erhöhung des Tg dieses Kernes beitragen, enthält.
7. Papierprodukt nach Anspruch 6, worin dieses difunktionelle Monomere Allyl- oder Crotylacrylate
und /oder -methacrylate oder Divinylbenzol umfaßt.
8. Papierprodukt nach Anspruch 1, worin dieser strukturierte Teilchenlatex einen Mantelbereich
hat, der eine Polymermischung aus Styrol/Butadien/Acrylsäure umfaßt.
9. Papierprodukt nach Anspruch 8, worin das Verhältnis von diesen Styrol/Butadien/Acrylsäure
49/49/2 bis 47/47/6; 49/49/2 bis 69/29/2 bzw. 69/29/2 bis 67/27/6 beträgt.
10. Verfahren zur Herstellung eines leichtgewichtigen, als Füllstoff in der Masse ein
strukturiertes Latexteilchen enthaltenden Papierproduktes, welches umfaßt:
(a) Bereitstellung eines Strukturlatexteilchens, umfassend:
(1) einen Kernanteil mit einer Lichtstreueigenschaft und einem Tg von 80° C oder größer; und
(2) einen Mantelanteil mit einem Tg von 25° C oder niedriger;
(b) Zusammenmischen dieses strukturierten Latexteilchens mit einer wässrigen Papiermasse;
und
(c) Umwandeln der Mischung in ein Papierprodukt.
1. Produit à base de papier, de faible poids, contenant, comme charge dans la masse,
des particules de latex structurées, comprenant :
(1) une partie coeur ayant des propriétés de diffusion de la lumière et une Tg supérieure ou égale à 80°C, et
(2) une partie enveloppe ayant une Tg inférieure ou égale à 25°C,
grâce à quoi l'opacité et la résistance dudit papier sont augmentées par rapport
à celles d'un produit à base de papier, préparé en l'absence desdites particules de
latex structurées.
2. Produit à base de papier selon la revendication 1, dans lequel les particules de latex
structurées représentent de 1 à 20 % en poids à l'état sec dudit papier.
3. Produit à base de papier selon la revendication 1, pour lequel ladite partie coeur
comprend un polymère aromatique monovinylique, un copolymère ou un dérivé des précédents.
4. Produit à base de papier selon la revendication 3, pour lequel ledit polymère aromatique
monovinylique est un polymère de styrène.
5. Produit à base de papier selon la revendication 1, dans lequel lesdites particules
de latex structurées présentent une région coeur, constituée d'un homopolymère de
styrène.
6. Produit à base de papier selon la revendication 3, pour lequel ladite partie coeur
renferme en plus des monomères bifonctionnels qui contribuent à l'augmentation de
la Tg dudit coeur.
7. Produit à base de papier selon la revendication 6, pour lequel ledit monomère bifonctionnel
comprend les acrylates et/ou méthacrylates d'allyle ou de crotyle, ou le divinylbenzène.
8. Produit à base de papier selon la revendication 1, dans lequel lesdites particules
de latex structurées présentent une région enveloppe comprenant un mélange polymère
de styrène/butadiène/acide acrylique.
9. Produit à base de papier selon la revendication 8, pour lequel le rapport styrène/butadiène/acide
acrylique dudit mélange polymère est 49/49/2 à 47/47/6, 49/49/2 à 69/29/2, et 69/29/2
à 67/27/6, respectivement.
10. Procédé de préparation d'un produit à base de papier, de faible poids, contenant,
comme charge dans la masse, des particules de latex structurées, qui comprend :
(a) la fourniture de particules de latex structurées, comprenant :
(1) une partie coeur ayant des propriétés de diffusion de la lumière et une Tg supérieure ou égale à 80°C, et
(2) une partie enveloppe ayant une Tg inférieure ou égale à 25°C,
(b) le mélange desdites particules de latex structurées avec une composition aqueuse
de papier, et
(c) la mise du mélange sous la forme d'un papier.