Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2002-06-12
2003-02-18
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C430S138000, C525S288000, C525S902000
Reexamination Certificate
active
06521342
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns cationic core-shell particles having an acid-insoluble polymeric core and an acid-swellable polymeric shell component disposed generally about said core so that upon neutralizing said shell with acid, a portion of said shell remains attached or associated with said core. This invention further concerns polymer latex compositions wherein said polymeric shell component has been neutralized and substantially, but not totally, dissolved so as to form a blend of neutralized core-shell polymer and an aqueous solution of neutralized shell polymer. More particularly, the invention concerns the use of latex compositions produced from these cationic core-shell particles to formulate ink jet receptive coatings.
BACKGROUND OF THE INVENTION
Ink jet printing is widely used to print on a variety of substrates (including paper, textiles, and plastic films). These substrates are often coated with a material that enhances their receptivity for the ink jet ink. In the case of aqueous dye-based inks, which comprise the majority of inks currently used in ink jet printing, two properties are of paramount importance. The first is an affinity for water, as the coating must absorb a large amount of water from the ink in order to obtain a print that is dry to the touch in few seconds. Ink jet ink formulations often contain over 90% water. Furthermore, the coating must maintain its physical integrity while absorbing all of this water. In other words, the receptive coating must be hydrophilic enough to absorb a large quantity of water without actually being water-soluble.
The second important property is dye fixation. The majority of aqueous ink jet inks are based on dyes rather than pigments. To obtain sharp prints with high color density, the dye molecules must be immobilized almost immediately upon contact of the ink with the substrate. Penetration of the dyes into the substrate will result in reduced color density, while lateral migration of the dye molecules will cause indistinctness in the image formed.
In order to obtain high water absorption, ink jet receptive coatings have traditionally been formulated with both hydrophilic pigments (such as silica or alumina) and hydrophilic binders. While the most commonly used hydrophilic binders are polyvinyl alcohol (PVOH) and polyvinylpyrrolidinone (PVP), other suitable natural and synthetic polymers are known in the art (e.g., gelatin, starch, polyethylene oxide (PEO), hydroxyethylcellulose, carboxymethylcellulose, and the like). Those polymers that contain hydroxyl groups (such as starch, PVOH, and PEO) are often cross-linked with a compound such as glyoxal or glutaraldehyde to render them water-insoluble while maintaining their hydrophilicity. Fully hydrolyzed polyvinyl alcohol is particularly useful because it can be dissolved in hot water and remains in solution when cooled to room temperature. When the PVOH is coated on a substrate and then dried, it will not readily redissolve in room-temperature water. However, a problem exists with use of PVOH in that PVOH has no dye trapping properties.
The dyes that are commonly employed in aqueous ink jet inks are anionic, containing sulfonic acid groups. Thus dye fixation is usually accomplished by the employment of cationic polymers, although some highly polar nonionic polymers (such as PVP and polyethyloxazoline) have a limited amount of dye fixing capability. These latter polymers fix dyes by dipolar interactions between their amide groups and the sulfonic acid groups of the dye molecules, while the cationic polymers work by the more efficient mechanism of salt formation.
The most widely used cationic dye fixative in ink jet receptive coatings is poly(diallyldimethylammonium chloride), although other water-soluble cationic polymers are known in the art. For example, U.S. Pat. No. 6,010,790 teaches the use of poly(vinylbenzylquaternary ammonium salts). Other examples of water-soluble cationic polymers are cationic starch, cationic polyvinyl alcohol, guanidine-formaldehyde resins, epichlorohydrin-polyamine condensates, and water-soluble cationic acrylic resins.
However, a problem exists with the use of both soluble dipolar polymers and soluble cationic polymers in that, unless these polymers are cross-linked in some way, their presence has a detrimental effect on the water resistance of the ink jet prints.
As an alternative to water-soluble cationic resins, cationic acrylic or styrenic latices can be used as fixatives for anionic dyes in ink jet receptive coatings. These latices behave like plastic pigments. Attaching the positive charges to the surface of a pigment particle rather than to a soluble resin greatly enhances the water resistance of the ink jet prints. However, using substantial amounts of these cationic plastic pigments in a coating also results in rheological problems, which make the coatings difficult to employ on coating machinery. Furthermore, the level of cationic charge achievable with the cationic plastic pigment may not be high enough to achieve the desired level of dye fixing.
Anionic acrylic colloidal dispersion polymeric resins are widely used in the ink and coatings industries. Such polymers are manufactured by first making an acrylic latex that contains a substantial amount of an acidic monomer (such as methacrylic acid). Usually the amount of acidic monomer is sufficient to give the polymer an acid number of about 60 to about 120. The acidic functionality of the polymer is subsequently neutralized via the employment of a base (such as sodium hydroxide, potassium hydroxide, ammonia, or a water-soluble amine). This greatly enhances the hydrophilicity of the polymer, and the latex particles first swell and then lose their discreteness. While the resulting anionic acrylic colloidal dispersion polymeric resin product appears to be clear and homogeneous on a macroscopic basis, it is actually inhomogeneous on a molecular level, having some domains that are rich in hydrophobic moieties and other domains that are rich in hydrophilic ones. If a fugitive base is employed in the neutralization, these polymer dispersions dry to form water-insoluble films.
The production of anionic polymer latexes having alkali-insoluble cores and colloidal dispersion type shells are taught in U.S. Pat. No. 4,916,171. However, these anionic polymers are unsuitable for use in ink jet coating formulations as cationicity is required for proper dye fixation.
Therefore, an object of the present invention is to avoid the major rheological problems associated with traditional binders employed in ink jet coatings by disclosing a cationic core-shell polymer latexes having an acid-insoluble polymeric core and an acid-swellable polymeric shell component.
Another object of this invention is to disclose cationic core-shell particles.
A further object of this invention is to disclose core-shell particle latexes suitable for use in formulating ink jet receptive coatings.
SUMMARY OF THE INVENTION
The objects of this invention are met via the production of cationic core-shell particles having an acid-insoluble polymeric core and an acid-swellable polymeric shell component disposed generally about said core in such a manner so that, upon neutralizing the said shell with acid, the shell polymer remains attached or associated with the particle core and becomes a colloidal dispersion. Ink jet receptive coatings formulated with latex compositions produced from these cationic core-shell particles produce prints having enhanced definition and superior color density. These coatings also exhibit excellent rheological properties, which make them suitable for use with high-speed paper coating equipment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The cationic core-shell particles suitable for use in formulating ink jet receptive coatings comprise:
(A) a polymeric core component, said core component comprising the free radical polymerization reaction product of a mixture comprising:
(1) about 80.0% to about 99.5% by total weight of the mixture of a member selected from the group consisting of acry
Hutter G. Frederick
Stebbins Camille K.
Le H. Thi
McDaniel Terry B.
Reece IV Daniel B.
Schmalz Richard L.
Westvaco Corporation
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