CA2366870A1 - Aqueous binders for ink compositions - Google Patents

Abstract

A composition comprising (A) a carboxylated styrene/butadiene latex having a monomodal particle size distribution; and (B) a dispersed alkali-soluble polymer having from 35 to 50% of polymerized residual acid monomer units, based on the total weight of monomers in the alkali-soluble polymer.

Description

k ' 6086 AQUEOUS BINDERS FOR INK COMPOSITIONS
Background of the Invention The present invention relates to latex compositions that are especially useful as binders for ink compositions.
The ink industry historically has used solvent based ink formulations. There is a trend toward using water-based ink formulations in view of the current regulatory environment and the disadvantages associated with the use of solvents in inks.
Many water-based ink formulations require a water-based binder, such as a latex. Blends of alkali-soluble polymers with alkali-insoluble polymers have been utilized as the binder in water-based inks, as well as in, paints, adhesives, and architectural and pharmaceutical coatings to provide performance improvements to these systems. An example of the first generation of such polymers is illustrated in U.S. Pat. No. 4,151,143, which discloses the preparation of a surfactant-free emulsion polymer coating composition containing an alkali soluble polymer prepared by solution polymerization. The method includes ( 1 ) a first stage wherein a carboxyl group containing polymer is prepared by a conventional solution or bulk polymerization technique and then is water-dispersed or solubilized by partial or full neutralization with an organic amine or base and high shear agitation, and (2) a second stage polymerization wherein a mixture of polymerizable monomers and polymerization catalyst is added to the first-stage emulsion at an elevated temperature to affect polymerization of the stage two monomers resulting in the formation of an emulsion coating composition.
Second generation alkali-soluble and alkali-insoluble polymers were prepared using "core-shell technology." So-called "core-shell" polymers are formed when the alkali-soluble polymers form a "shell" or coating around a discreet domain or "core" of the alkali-insoluble polymer. Examples of such core-shell polymers are disclosed in U.S.
Pat. No. 4,916,171. Further improvements were made by chemically grafting the alkali-soluble shell and alkali-insoluble polymer core using methods such as those described in U.S. Patents 4,876,313 and 5,403,894.
"Inverse core-shell" emulsion polymers have also been disclosed. Inverse core-shell polymers are those where the second-stage polymer becomes the core and is surrounded by the shell which, in the inverse process, is the first stage polymer. These " t CA 02366870 2002-O1-07 608'!6 inverse compositions can be formed when the first stage polymer is more hydrophilic than the second stage monomer; see, e.g., U.S. Patent 4,894,397.
U.S. Patent 5,073,591 describes a method for producing highly acidic polymeric emulsions by a two-stage method wherein the first stage contains a high level of acid monomer and crosslinker, and the second stage contains an amino-containing monomer.
EP 0 587 333 A2 describes water-resistant multi-stage polymers having an alkali-insoluble polymer and an alkali-soluble polymer that are prepared by sequential emulsion polymerization of a monomer mixture having acid functionality in the alkali-soluble stage and, optionally, a polyfunctional compound; and an alkali-insoluble polymer having, optionally, an amine functionality.
U.S. Patent 3,037,952 discloses a clear, aqueous coating composition that is a dispersion of a water-insoluble polymer and certain salts of a linear copolymer of acrylic or methacrylic acid with methyl methacrylate. The patent teaches that it is essential that the copolymer does not contain more than 30 weight percent of acrylic or methacrylic acid salt units.
WO 96!19536 discloses an emulsion of an organic polymeric material comprising a hydrophobic polymer part that has a polymodal particle size distribution, and an oligomeric part bearing acid groups which is capable of imparting reversibility to the polymeric material.
U.S. Patent 4,644,031 discloses a composition for coating pharnnaceutical dosage forms that are resistant to gastric juice and that release their active ingredient rapidly at a predetermined pH value in the range from pH 5 to pH 8. The coating composition contains a polymer containing carboxyl groups that is water soluble between pH
5 and pH 8, and a water insoluble film forming polymer based on an acrylate, such as ethyl acrylate or methyl methacrylate.
U.S. Patent 5,804,632 discloses a process for preparing aqueous emulsions of polymer systems comprising an acid-functional polymer and a hydrophobic polymer. The patent teaches that this polymer may be used in coating applications and graphic arts applications such as inks and overprint lacquers. However, the disclosed process requires a transition metal chelate complex, particularly a cobalt chelate complex, for the purpose of controlling molecular weight.

" ' CA 02366870 2002-O1-07 U.S. Patent 5,739,196 discloses latex compositions having desirable wet adhesion properties.
It would be desirable to have an improved water-based binder for water-based ink formulations that would provide improved rub resistance and water resistance.
Summary of the Invention The present invention provides such a binder composition that comprises:
(A) a carboxylated styrene/butadiene latex having a monomodal particle size distribution;
(B) a dispersed alkali-soluble polymer having from 35-50% of polymerized acid monomer units, based on the total weight of monomers in the alkali-soluble polymer;
the (B) polymer being prepared without using a transition metal complex catalytic chain transfer agent, with the proviso that the composition is substantially free of wet adhesion promoters.
The latex compositions of the present invention provide desirable Theological properties for ink formulations into which they are incorporated. The compositions of the invention also provide surprisingly improved rub resistance and/or water resistance.
Detailed Description of the Invention The composition of the invention comprises an alkali-soluble polymer (the first polymer) and a carboxlyated styrene/butadiene latex (the second polymer).
The first polymer is preferably water insoluble below pH 8 but becomes water soluble above pH 8. The first polymer is described as 'water soluble' if a 5% solids dispersion of the first polymer in water turns from a white milky color to a translucent or clear solution at a pH above 8. Polymerization of the first polymer is typically carried out by emulsion polymerization of a mixture of monomers comprising at least one vinyl acid or anhydride functional monomer or monomer that imparts alkaline solubility to the first polymer, and a comonomer such as an alkyl (meth)acrylate, styrene or a substituted styrene, acrylamide, or a methacrylate or hydroxyalkyl ester of a carboxylic acid.
Suitable acid functional monomers for preparation of the first polymer include acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, itaconic acid, malefic acid, fumaric acid, and the like including mixtures thereof. In one embodiment of the "' CA 02366870 2002-O1-07 ' 60876 present invention, the preferred acid monomer is selected from the group consisting of acrylic acid and methacrylic acid.
The first polymer suitably contains an amount of carboxyl functionality such that the first polymer will be soluble at a pH above about 8. A minimum amount of carboxyl functionality is needed to solubilize the polymer depending on the hydrophobicity of the comonomers, molecular weight of the polymer, chemical nature of the carboxyl monomer and sequence distribution of monomers in the polymers. The amount of carboxyl functionality influences the rheology of both the latex and inks prepared from the latex and influences how the first polymer interacts with pigments. When used in excess, carboxyl groups make ink water sensitive. Based on the above criteria, the preferred total weight of acid functional monomer present in the first polymer is preferably from about 35 percent to about 50 percent; and more preferably is from about 40 percent to 45 percent, based on the total weight of monomers charged. In a preferred embodiment, from 0 to about 15 weight percent of acrylic acid and from about 35 to about 50 weight percent of methacrylic acid are employed, based on the total weight of monomers charged.
Examples of comonomers useful in polymerization of the first polymer include: vinyl aromatic monomers, such as styrene, and substituted styrenes such as alpha methyl styrene; acrylates and methacrylates, including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, and other C1-C12 alkyl or hydroxy alkyl acrylates and methacrylates; acrylonitrile, vinyl acetate, and the like, and/or mixtures thereof. Styrene is the preferred comonomer in the first polymer, and preferably is employed in an amount ranging from about 50 to about 65 weight percent, based on the total weight of monomers charged.
It is desirable that the first polymer have a low to moderate number average molecular weight, preferably less than about 20,000, more preferably less than about 10,000, as determined by gel permeation chromatography. The molecular weight is controlled by the addition of a suitable chain transfer agent as is known in the art.
Preferably, from about 5 to about 15 weight parts of chain transfer agent are employed per 100 weight parts of monomer. Examples of suitable chain transfer agents include, for example, alkyl mercaptans such as octyl mercaptan and decyl mercaptan, esters of mercaptoacetic acid, such as an ethyl ester of mercaptoacetic acid and 2-ethylhexyl ester of mercaptoacetic acid, and esters of mercapto- propionic acid, such as isooctyl ester of mercaptopropionic acid, and " CA 02366870 2002-O1-07 ' 60876 compounds containing mufti mercapto functional groups, such as trimethylolpropane tri mercapto propionate. The preferred chain transfer agent is t-dodecylmercaptan.
In carrying out the emulsion polymerization to form the first polymer, at least .
one initiator or catalyst is used at a concentration sufficient to initiate or catalyze the polymerization reaction. The particular concentration used in any instance will depend upon the specific monomer mixture undergoing reaction and the specific initiator employed, as is well known to those skilled in the art, and is frequently from about 0.01 to 3 weight percent;
more preferably from about 0.05 to 2 weight percent based on the weight of monomers charged. Examples of suitable initiators include hydrogen peroxide, peracetic acid, t-butyl hydroperoxide, ammonium persulfate, potassium persulfate, and sodium persulfate, as well as any of the other known initiators. Also useful are redox catalyst systems such as sodium persulfate-sodium formaldehyde sulfoxylate, cumene hydroperoxide-sodium metabisulfite, hydrogen peroxide-ascorbic acid, and the other known redox systems.
The polymerizable feed compositions can also contain any of the other known additives conventionally used in emulsion polymerization processes in the usual known quantities, such as crosslinkers, surfactants, dispersion aids, emulsifiers, photosensitizers, colorants, bactericides, fungicides, etc.
The first polymer preferably has a pH of from about 1 to about 4, and an average particle diameter of from about 100 to 200 nanometers as measured by light scattering techniques. The solids content of the first polymer in emulsion form can vary considerably and can be adjusted to desired levels using techniques well known in the art, but preferably is from about 40 to about 55 weight percent solids, and more preferably is from about 45 to about 50 weight percent solids.
Monomers useful for polymerization in the second polymer include any monomers that have acrylic, styrenic or vinyl functionality. Examples of such monomers include conjugated dienes, acrylate and methacrylate esters, styrene, alkyl styrenes, vinyl toluene, vinyl acetate, vinyl alcohol, acrylonitrile, vinylidene chloride, and vinyl ketones.
Other illustrative monomers useful in this invention include, for example, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, n-amyl methacrylate, sec-amyl methacrylate, hexyl methacrylate, lauryl methacrylate, stearyl methacrylate, ethyl hexyl methacrylate, crotyl methacrylate, cinnamyl methacrylate, oleyl methacrylate, hydroxy ethyl methacrylate, hydroxy propyl methacrylate, vinyl propionate, vinyl butyrate, vinyl tert-"' CA 02366870 2002-O1-07 butyrate, vinyl caprate, vinyl stearate, vinyl laurate, vinyl oleate, vinyl methyl ether, vinyl ethyl ether, vinyl n-propyl ether, vinyl isopropyl ether, vinyl n-butyl ether, vinyl iso-butyl ether, vinyl isooctyl ether, vinyl phenyl ether, alpha -chlorovinyl phenyl ether, vinyl naphthyl ether, methacrylonitrile, acrylamide, methacrylamide, N-alkyl acrylamides, N-aryl acrylamides, N-vinyl pyrrolidone, N-vinyl-3-morpholinones, N-vinyl-oxazolidone, N-vinyl-imidazole and the like including mixtures thereof. Preferably, the second polymer is a carboxlyated styrene/butadiene latex having from about 40 to 100 percent styrene and from about 0 to about 60 percent butadiene based on the total weight of styrene and butadiene.
The preferred second polymer further comprises from about 0 to about 15 weight parts of a comonomer selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, acrylamide, methacrylamide, vinylidene chloride, hydroxyethylacrylate, acrylonitrile, and combinations thereof. Many suitable second polymer latexes are commercially avialable.
Optional ingredients which may be included in polymerization of the second polymers include small amounts of other mufti-functional monomers such as crosslinkers, chain transfer agents, and other materials, as is known in the art.
Various methods of preparing the polymer compositions of this invention may be used. In a preferred embodiment, the first polymer in latex form is blended with the second polymer in latex form. The ratio for blending the first and second polymers of the present invention covers a wide range depending on the desired properties of the final latex product and its intended uses. For example, the first polymer suitably is from about 1 to about 30 weight percent , more preferably from about 5 to about 20 weight percent, and the second polymer is from about 70 to about 99, more preferably from about 80 to about 95 weight percent of the composition of the invention based on the total weight the first and second polymers.
Specific Embodiments of the Invention The following examples are given to illustrate the invention and should not be construed as limiting its scope. All parts and percentages are by weight unless otherwise indicated.
Example 1- Preparation of a First Polymer (Latex 1 ) Into a stainless steel jacketed reactor was charged the following initial reaction mixture: 89.3 partsl100 parts monomer of deionized water, 1.08 parts/100 parts "' CA 02366870 2002-O1-07 ' 60876 monomer of a 40% solids polystyrene polymer seed having an average diameter of 22 nm, and 0.01 parts/100 parts monomer of a 1% solution of VERSENOL 120, a chelating agent available from The Dow Chemical Company, Midland, MI, U.S.A., which is a 1%
solution of the trisodium salt of N-(carboxymethyl)-N'-(2-hydroxyethyl)-N,N'-ethylenediglycine.
VERSENOL is a trademark of The Dow Chemical Company. The reactor was purged with nitrogen and heated under agitation to 90°C. A monomer mixture of 43.3 parts methacrylic acid, 56.7 parts styrene, and 13 parts/100 parts monomer of trimethylol propane tri mercapto propionate was continuously added to the reactor in 240 minutes. Also, 50.0 parts/100 parts deionized water, 2 parts/100 parts ammonium persulfate, 1.80 parts/100 parts of a 45%
solution of DOWFAX 2A1 were continuously added to the reactor in 300 minutes.
A 45%
solution of DOWFAX 2A1, the sodium salt of dodecylated sulonated phenyl ether is available from The Dow Chemical Company, Midland MI, U.S.A. DOWFAX is a trademark of The Dow Chemical Company. The reaction vessel was maintained at 90°C
during the additions and for 60 minutes thereafter. The resulting latex is designated Latex l, and comprises a first polymer as that term is used herein.
Example 2 - Preparation of a Second Polymer (Latex 2) Into a stainless steel jacketed reactor was charged the following initial reaction mixture: 71.9 parts/100 parts monomer of deionized water, 2.28 parts/100 parts monomer of a 40% solids polystyrene polymer seed having an average diameter of 22.5 nm, and 0.02 parts/100 parts monomer of a 1% solution of VERSENOL 120. The reactor was purged with nitrogen and heated under agitation to 98°C. A monomer mixture of 38.5 parts butadiene, 53.5 parts styrene and 8 parts acrylic acid, and 1.8 parts/100 parts monomer of t-dodecyl mercaptan was continuously added to the reactor, with 8% of the monomers being added in 30 minutes, and the remaining 92% being added in an additional 175 minutes.
Also, 27.7 parts/100 parts deionized water, 1.1 parts/100 parts sodium persulfate, 0.05 parts/100 parts of a 28% solution of ammonium hydroxide were continuously added to the reactor in 225 minutes. The reaction vessel was maintained at 98°C
during the additions and for 20 minutes thereafter. The resulting latex is designated Latex 2, and comprises a second polymer as that term is used herein.
_7-' 60876 Example 3 2 with agitation. The blend ratio is based on the weight of solids in the latexes. The pH of the blend is adjusted to a value of 7.5 using ammonium hydroxide.
Example 4 Example 3 is repeated except that the blend ratio is 20/80.
Example 5 A water-based flexo carton ink is prepared using the following formulation:
A 10/90 blend of Latex 1 and Latex 2 is prepared by adding Latex 1 to Latex DB 153-1738 cyan pigment dispersion) 37.14 JONCRYL 60 (styrene/acrylic solution resin) 13.97 Latex of Example 3 48.39 DEHYDRAN 4100° (polysiloxane defoamer) 0.50 100.0 a Alper Dispersions b SC Johnson Polymer Cognis The ink is prepared by first preparing a vehicle (all components except pigment dispersion). The vehicle is added to the pigment dispersion with mixing. The percent solids of the ink is 44.4 %, and the pH of the ink is in the range of 8.5 to 8.75.
Example 6 Example 5 is repeated except that 11.03 parts of JONCRYL 60, 5.30 parts of water, and 46.03 parts of the latex of example 4 are employed. The percent solids of the ink is 42%.
Comparative Example 1 (Not an embodiment of the present invention) Example 5 is repeated except that the latex of example 4 is replaced with ECO 2177, a latex that is commercially available from S.C. Johnson Polymer.
_g_ Example 7 The inks of Examples 5 and 6 and Comparative Example 1 are proofed on the backside of Laneta 3NT-3 chart. Gloss, optical density, Sutherland rub testing, and water spot testing are performed and the results are summarized as follows.
Property Example 5 Example 6 Comparative Exam 1e 1 Print Densi 1.59 1.59 1.57 Glosse 38.5 39.4 38.4 Rub Resistance 30 20 10 Water Resistanceg_~3p-. I 30 ~ 10 -..

d determined with an X-Rite densitometer a 600 f # of rubs needed to obtain deep scratching; Sutherland rub tester, slow speed, 4# weight g a water droplet was placed on the print and allowed to sit for various time intervals, then wiped with a tissue. The values noted above is the time in seconds needed to damage the print.
_g_

Claims (4)

1. A composition comprising (A) a carboxylated styrene/butadiene latex having a monomodal particle size distribution;
(B) a dispersed alkali-soluble polymer having from 35-50% of polymerized residual acid monomer units, based on the total weight of monomers in the alkali-soluble polymer;
the (B) polymer being prepared without using a transition metal complex catalytic chain transfer agent, with the proviso that the composition is substantially free of wet adhesion promoters.

2. A composition comprising (A) a carboxylated styrene/butadiene latex having a monomodal particle size distribution;
(B) a dispersed alkali-soluble polymer having from 35 to 50% of polymerized residual acid monomer units, based on the total weight of monomers in the alkali-soluble polymer;
the (B) polymer being substantially free of transition metal complex catalytic chain transfer agent, with the proviso that the composition is substantially free of wet adhesion promoters.

3. A composition comprising (A) a carboxylated styrene/butadiene latex having a monomodal particle size distribution;
(B) a dispersed alkali-soluble polymer having from 35 to 50% of polymerized residual acid monomer units, based on the total weight of monomers in the alkali-soluble polymer.

4. An ink composition comprising the composition of Claim 3.