Pre-
cursor
to	Functional
Group	Moiety
G_d	FM	R₁	R₂	R₃

DC1	SO₃H	CH₃	H	NH₂
DC2	SO₃H	CH₃	Cl	NH₂
DC3	SO₃H	Cl	CH₃	NH₂
DC4	SO₃H	Cl	CO₂H	NH₂
DC5	SO₃H	Cl	CH₂CH₃	NH₂
DC6	SO₃H	Cl	Cl	NH₂
DC7	SO₃H	H	NH₂	H
DC8	SO₃H	H	NH₂	CH₃
DC9	SO₃H	NH₂	H	Cl
DC10	SO₃H	H	H	NH₂
DC11	SO₃H	H	NH₂	H
DC12	SO₃H	NO₂	NH₂	H

DC13		NH₂	CH₃	H

DC14	CO₂H	H	H	NH₂
DC15	Cl	H	H	NH₂
DC16	NH₂	CH₃	H	H
DC17	NH₂	H	CH₃	H

DC18		NH₂	CH₃	H

DC19		H	NH₂	H

DC20	NH₂	H	H	H

DC21

In an embodiment, the coupling group G_cof Formula (1) can include β-naphthol and derivatives of Formula (4), naphthalene sulfonic acid derivatives of Formulas (5) and (6), pyrazolone derivatives of Formula (7), acetoacetic arylide derivatives of Formula (8), and the like. In formulas (4)-(8), the asterisk “*” denotes the point of coupling or attachment to the monoazo (N═N) linkage.
where FM represents H, CO₂H, SO₃H, —C(═O)—NH-Aryl-SO₃⁻ where the aryl group can be unsubstituted or substituted with either halogens (such as Cl, Br, I, F) or alkyl groups having from about 1 to about 10 carbons (such as methyl, ethyl, propyl, butyl and the like) CO₂H, halogen (such as Cl, Br, I), NH₂, —C(═O)—NH₂, substituted benzamides such as:
wherein groups R₂′ R₃′, R₄′ and R₅′ can independently be H, alkyl groups having from about 1 to 10 carbons (such as methyl, ethyl, propyl, butyl, and the like), alkoxyl groups (such as OCH₃, OCH₂CH₃, and the like), hydroxyl or halogen (such as Cl, Br, I, F) or nitro NO₂; or benzimidazolone amides such as:
and the like.
where FM represents preferably SO₃H, but also can represent CO₂H, —C(═O)—NH-Aryl-SO₃⁻ where the aryl group can be unsubstituted or substituted with either halogens (such as Cl, Br, I, F) or alkyl groups having from about 1 to about 10 carbons (such as methyl, ethyl, propyl, butyl and the like) CO₂H, halogen (such as Cl, Br, I), NH₂, —C(═O)—NH₂groups R₃and R₄independently represent H, SO₃H, and the like.
where FM represents preferably SO₃H, but also can represent CO₂H, —C(═O)—NH-Aryl-SO₃⁻ where the aryl group can be unsubstituted or substituted with either halogens (such as Cl, Br, I, F) or alkyl groups having from about 1 to about 10 carbons (such as methyl, ethyl, propyl, butyl and the like) CO₂H, halogen (such as Cl, Br, I), NH₂, —C(═O)—NH₂; R₁, R₂, R₃and R₄independently represent H, SO₃H, —C(═O)—NH-Phenyl, and the like.
where G represents CO₂H, straight or branched alkyl such as having from 1 to about 10 carbons atoms (such as methyl, ethyl, propyl, butyl, or the like), and the like; and R₁′, R₂′, R₃′ and R₄′ independently represent H, halogen (such as Cl, Br, I), SO₃H, nitro (NO₂) or alkoxyl group such as OCH₃or OCH₂CH₃and the like.
where R₁′ represents a straight or branched alkyl group having, for example, from 1 to about 10 carbon atoms (such as methyl, ethyl, propyl, butyl, and the like); R₂′ represents a benzimidazolone group:
or a substituted aryl group
where each of R_a, R_b, and R_cindependently represents H, a straight or branched alkyl group having, for example, from 1 to about 10 carbon atoms (such as methyl, ethyl, propyl, butyl, and the like), alkoxyl groups such as OCH₃or OCH₂CH₃and the like, halogen (such as Cl, Br, I), nitro NO₂, and the like.
Representative examples of the nucleophilic coupling component as a precursor of lakes monoazo pigments which have the functional moiety that is capable of non-covalent bonding with a complementary functional group on the stabilizer, include (but are not limited to) the following structures shown in Tables 2-6 (with the functional moiety “FM” denoted, if applicable):
TABLE 2
Precursor
to Class of Functional
group Coupling Moiety
G_c Component FM
CC1 β-Naphthol H
CC2 β-oxynaphthoic acid CO₂H
(“BONA”)
CC3 Naphthol ASderivatives
CC6 Benzimidazolone
TABLE 3
Precursor
to Class of
group Coupling
G_c Component FM R₃ R₄
CC4a Naphthalene Sulfonic SO₃H H H
Acid derivatives
CC4b Naphthalene Sulfonic SO₃H SO₃H H
Acid derivatives
TABLE 4
Precursor
to Class of
group Coupling
G_c Component FM R₁ R₂ R₃ R₄
CC5 Naphthalene SulfonicAcid derivatives SO₃H
H H SO₃H
TABLE 5
Precursor
to Class of
group Coupling
G_c Component G R₁′ R₂′ R₃′ R₄′
CC7 Pyrazolone deriv. CO₂H H H SO₃H H
CC8 Pyrazolone deriv. CH₃ H H SO₃H H
CC9 Pyrazolone deriv. CH₃ H SO₃H H H
CC10 Pyrazolone deriv. CH₃ Cl H SO₃H Cl

TABLE 6

Precursor
to	Class of
group	Coupling
G_c	Component	R₁′	R₂′	R_a	R_b	R_c

CC11	Acetoacetic arylide	CH₃		H	H	H

CC12	Acetoacetic arylide	CH₃		CH₃	H	H

CC13	Acetoacetic arylide	CH₃		Cl	H	H

CC14	Acetoacetic arylide	CH₃		H	OCH₃	H

CC15	Acetoaceticbenzimidazolone	CH₃		—	—	—

The organic pigment, and in some embodiments, the organic pigment precursor, also generally includes a counterion as part of the overall structure. Such counterions can be, for example, any suitable counterion including those that are well known in the art. Such counterions can be, for example, cations or anions of either metals or non-metals that include N, P, S and the like, or carbon-based cations or anions. Examples of suitable cations include ions of Ba, Ca, Cu, Mg, Sr, Li, Na, K, Cs, Mn, Cu, Cr, Fe, Ti, Ni, Co, Zn, V, B, Al, Ga, and other metal ions, as well as ammonium and phosphonium cations, mono-, di-, tri-, and tetra-substituted ammonium and phosphonium cations, where the substitutents can be aliphatic alkyl groups, such as methyl, ethyl, butyl, stearyl and the like, as well as aryl groups such as phenyl or benzyl and the like.
Representative examples of monoazo laked pigments comprised from a selection of substituted aniline precursor (denoted DC) which can also include Tobias Acid, nucleophilic coupling component (denoted as CC) and metal salts (denoted as M) to provide the counter-cation Mⁿ⁺ of the formula (1) are listed in Table 7, and other laked pigment structures may arise from other combinations of DC and CC and metal cation salt (M) that are not shown in Table 7.

TABLE 7

Color	Color
Index	Index		G_d	G_c	Metal
# (C.I.)	(C.I.) Name	Laked Pigment Class	precursor	precursor	Salt M

15500:1	Red 50:1	β-Naphthol Lakes	DC14	CC1	½ Ba
15510:1	Orange 17	β-Naphthol Lakes	DC7	CC1	Ba
15510:2	Orange 17:1	β-Naphthol Lakes	DC7	CC1	⅔ Al
15525	Red 68	β-Naphthol Lakes	DC4	CC1	2 Ca
15580	Red 51	β-Naphthol Lakes	DC8	CC1	Ba
15585	Red 53	β-Naphthol Lakes	DC3	CC1	2 Na
15585:1	Red 53:1	β-Naphthol Lakes	DC3	CC1	Ba
15585:3	Red 53:3	β-Naphthol Lakes	DC3	CC1	Sr
15602	Orange 46	β-Naphthol Lakes	DC5	CC1	Ba
15630	Red 49	β-Naphthol Lakes	DC21	CC1	2 Na
15630:1	Red 49:1	β-Naphthol Lakes	DC21	CC1	Ba
15630:2	Red 49:2	β-Naphthol Lakes	DC21	CC1	Ca
15630:3	Red 49:3	β-Naphthol Lakes	DC21	CC1	Sr
15800	Red 64	β-oxynaphthoic acid (BONA) Lakes	DC20	CC2	½ Ba
15800:1	Red 64:1	β-oxynaphthoic acid (BONA) Lakes	DC20	CC2	½ Ca
15800:2	Brown 5	β-oxynaphthoic acid (BONA) Lakes	DC20	CC2	½ Cu
15825:2	Red 58:2	β-oxynaphthoic acid (BONA) Lakes	DC9	CC2	Ca
15825:4	Red 58:4	β-oxynaphthoic acid (BONA) Lakes	DC9	CC2	Mn
15850:1	Red 57:1	β-oxynaphthoic acid (BONA) Lakes	DC1	CC2	Ca
15860:1	Red 52:1	β-oxynaphthoic acid (BONA) Lakes	DC3	CC2	Ca
15860:2	Red 52:2	β-oxynaphthoic acid (BONA) Lakes	DC3	CC2	Mn
15865:1	Red 48:1	β-oxynaphthoic acid (BONA) Lakes	DC2	CC2	Ba
15865:2	Red 48:2	β-oxynaphthoic acid (BONA) Lakes	DC2	CC2	Ca
15865:3	Red 48:3	β-oxynaphthoic acid (BONA) Lakes	DC2	CC2	Sr
15865:4	Red 48:4	β-oxynaphthoic acid (BONA) Lakes	DC2	CC2	Mn
15865:5	Red 48:5	β-oxynaphthoic acid (BONA) Lakes	DC2	CC2	Mg
15867	Red 200	β-oxynaphthoic acid (BONA) Lakes	DC5	CC2	Ca
15880:1	Red 63:1	β-oxynaphthoic acid (BONA) Lakes	DC21	CC2	Ca
15880:2	Red 63:2	β-oxynaphthoic acid (BONA) Lakes	DC21	CC2	Mn
15892	Red 151	Naphthol AS Lakes	DC10	CC3	Ba
				(R₂′ = H,
				R₄′ = SO₃H)
15910	Red 243	Naphthol AS Lakes	DC2	CC3	½ Ba
				(R₂′ = OCH₃,
				R₄′ = H)
15915	Red 247	Naphthol AS Lakes	DC13	CC3	Ca
				(R₂′ = H,
				R₄′ = OCH₃)
15985:1	Yellow 104	Naphthalene Sulfonic Acid Lakes	DC7	CC4a	⅔ Al
15990	Orange 19	Naphthalene Sulfonic Acid Lakes	DC15	CC4a	½ Ba
16105	Red 60	Naphthalene Sulfonic Acid Lakes	DC14	CC4b	3/2 Ba
18000:1	Red 66	Naphthalene Sulfonic Acid Lakes	DC16	CC5	½ Ba, Na

The complementary functional group of the stabilizer can be one or more of any suitable moiety capable of non-covalent bonding with the functional moiety of the stabilizer. Illustrative complementary functional groups on the stabilizer include the following: sulfonate/sulfonic acid, (thio)carboxylate/(thio)carboxylic acid, phosphonate/phosphonic acid, ammonium and substituted ammonium salts, phosphonium and substituted phosphonium salts, substituted carbonium salts, substituted arylium salts, alkyl/aryl (thio)carboxylate esters, thiol esters, primary or secondary amides, primary or secondary amines, hydroxyl, ketone, aldehyde, oxime, hydroxylamino, enamines (or Schiff base), porphyrins, (phthalo)cyanines, urethane or carbamate, substituted ureas, guanidines and guanidinium salts, pyridine and pyridinium salts, imidazolium and (benz)imidazolium salts, (benz)imidazolones, pyrrolo, pyrimidine and pyrimidinium salts, pyridinone, piperidine and piperidinium salts, piperazine and piperazinium salts, triazolo, tetraazolo, oxazole, oxazolines and oxazolinium salts, indoles, indenones, and the like.
The stabilizer can be any compound that has the function of limiting the extent of pigment particle or molecular self-assembly so as to produce predominantly nanoscale-sized pigment particles. The stabilizer compound should have a hydrocarbon moiety that provides sufficient steric bulk to enable the function of the stabilizer to regulate pigment particle size. The hydrocarbon moiety in embodiments is predominantly aliphatic, but in other embodiments can also incorporate aromatic groups, and generally contains at least 6 carbon atoms, such as at least 12 carbons or at least 16 carbons, and not more than about 100 carbons, but the actual number of carbons can be outside of these ranges. The hydrocarbon moiety can be either linear, cyclic or branched, and in embodiments is desirably branched, and may or may not contain cyclic moieties such as cycloalkyl rings or aromatic rings. The aliphatic branches are long with at least 2 carbons in each branch, such as at least 6 carbons in each branch, and not more than about 100 carbons.
It is understood that the term “steric bulk” is a relative term, based on comparison with the size of the pigment or pigment precursor to which it becomes non-covalently associated. In embodiments, the phrase “steric bulk” refers to the situation when the hydrocarbon moiety of the stabilizer compound that is coordinated to the pigment/precursor surface, occupies a 3-dimensional spatial volume that effectively prevents the approach or association of other chemical entities (e.g. colorant molecules, primary pigment particles or small pigment aggregate) toward the pigment/precursor surface. Thus, the stabilizer should have its hydrocarbon moiety large enough so that as several stabilizer molecules become non-covalently associated with the chemical entity (pigment or precursor), the stabilizer molecules act as surface barrier agents for the primary pigment particles and effectively encapsulates them, and thereby limits the growth of the pigment particles and affording only nanoparticles of the pigment. For example, for the pigment precursor Lithol Rubine and for the organic pigment Pigment Red 57:1, the following illustrative groups on a stabilizer are considered to have adequate “steric bulk” so as to enable the stabilizer to limit the extent of pigment self-assembly or aggregation and mainly produce pigment nano-sized particles:
Representative examples of stabilizer compounds that have both the functional group that non-covalently associates with the pigment and the sterically bulky hydrocarbon moiety, include (but are not limited to) the following compounds:
Z=H; Metal cations such as Na, K, Li, Ca, Ba, Sr, Mg, Mn, Al, Cu, B, and others;

- Organic cations such as NH₄⁺, NR₄⁺, PR₄⁺, and others

Z=H; Metal cations such as Na, K, Li, Ca, Ba, Sr, Mg, Mn, Al, Cu, B, and others;

- Organic cations such as NH₄⁺, NR₄⁺, PR₄⁺, and others
  - and methylene units (m+n)>1

- Organic cations such as NH₄⁺, NR₄⁺, PR₄⁺, and others
  - and methylene units (m+n)>1
    - per branch

- Organic cations such as NH₄⁺, NR₄⁺, PR₄⁺, and others
  - and methylene units m≧1
  - and for iso-stearic acid, n≦1
    wherein m and n denotes the number of repeated methylene units, and where m can range between 1 and 50, and n can range between 1 and 5, however the values can also be outside these ranges.

In additional embodiments, other stabilizer compounds having different structures than those described previously may be used in addition to sterically bulky stabilizer compounds, to function as surface active agents (or surfactants) that either prevent or limit the degree of pigment particle aggregation. Representative examples of such surface active agents include, but are not limited to, rosin natural products such as abietic acid, dehydroabietic acid, pimaric acid, rosin soaps (such as the sodium salt of the rosin acids), hydrogenated derivatives of rosins and their alkyl ester derivatives made from glycerol or pentaerythritol or other such hydrocarbon alcohols, acrylic-based polymers such as poly(acrylic acid), poly(methyl methacrylate), styrene-based copolymers such as poly(styrene sodio-sulfonate) and poly(styrene)-co-poly(alkyl (meth)acrylate), copolymers of α-olefins such as 1-hexadecene, 1-octadecene, 1-eicosene, 1-triacontene and the like, copolymers of 4-vinyl pyridine, vinyl imidazole, and vinyl pyrrolidinone, polyester copolymers, polyamide copolymers, copolymers of acetals and acetates, such as the copolymer poly(vinylbutyral)-co-(vinyl alcohol)-co-(vinyl acetate).
The types of non-covalent chemical bonding that can occur between the functional moiety of the precursor/pigment and the complementary functional group of the stabilizer are, for example, van der Waals' forces, ionic or coordination bonding, hydrogen bonding, and/or aromatic pi-stacking bonding. In embodiments, the non-covalent bonding is predominately ionic bonding, but can include hydrogen bonding and aromatic pi-stacking bonding as additional or alternative types of non-covalent bonding between the functional moieties of the stabilizer compounds and the precursor/pigment.
The “average” pigment particle size, which is typically represented by Z-average, which is defined as an intensity mean which is derived from the cumulants analysis of an intensity signal obtained from dynamic light scattering method, and also by d₅₀, which is defined as the median particle size value at the 50th percentile of the particle size distribution, wherein 50% of the particles in the distribution are greater than the d₅₀particle size value and the other 50% of the particles in the distribution are less than the d₅₀value. Average particle size can be measured by methods that use light scattering technology to infer particle size, such as by dynamic light scattering (DLS). The term “particle diameter” as used herein refers to the length of the pigment particle at the longest dimension (in the case of acicular shaped particles) as derived from images of the particles generated by Transmission Electron Microscopy (TEM). The term “nano-sized”, “nanoscale”, “nanoscopic”, or “nano-sized pigment particles” refers to for instance, an average particle size, d₅₀, or Z-average, or an average particle diameter of less than about 150 nm, such as of about 1 nm to about 100 nm, or about 10 nm to about 80 nm. Typically, the distribution of a population of particle sizes is expressed by a width parameter or the polydispersity index (PDI), such as can be derived by DLS technique, and also by geometric standard deviation (GSD) which is a dimensionless number that typically estimates a population's dispersion of a given attribute (for instance, particle size) about the median value of the population and is derived from the exponentiated value of the standard deviation of the log-transformed values. If the geometric mean (or median) of a set of numbers {A₁, A₂, . . . , A_n} is denoted as μ_g, then the geometric standard deviation is calculated as:
$\begin{matrix} σ_{g} = \exp \sqrt{\frac{\sum_{i = 1}^{n} {(\ln A_{i} - \ln μ_{g})}^{2}}{n}} \end{matrix}$
The method of making nano-sized particles of the monoazo laked pigments such as those listed in Table 7 is a process that involves at least one or more reaction steps. A diazotization reaction is a key reaction step for synthesis of the monoazo laked pigment, whereby a suitably substituted aniline precursor (denoted as diazo component DC) such as those listed in Table 1, and Formulas (2) and (3), is either directly or indirectly converted first to a diazonium salt using standard procedures, such as that which includes treatment with an effective diazotizing agent such as nitrous acid HNO₂(which is generated in situ by mixing sodium nitrite with dilute protic acid solution such as hydrochloric acid), or nitrosyl sulfuric acid (NSA), which is commercially available or can be prepared by mixing sodium nitrite in concentrated sulfuric acid. Initially, it may be necessary to first dissolve the precursor substituted aniline in alkaline solution (such as aqueous potassium hydroxide solution, or ammonia water) followed by treatment with the diazotizing agent and acid solution, so as to generate the diazonium salt. The diazotization procedure is typically carried out at cold temperatures so as to keep the diazonium salt stable, and the resulting reaction mixture will comprise mainly the diazonium salt either dissolved or suspended as a precipitate in acidic medium. If desired and effective, an aqueous solution of the metal salt (Mⁿ⁺) can be optionally added that will define the specific composition of the desired monoazo laked pigment product, such as those listed in Table 7. A second solution or suspension is prepared by dissolving or suspending the nucleophilic coupling component (denoted as CC, such as those shown in Tables 2-6, and Formulas (4)-(8)) mainly into water, which may optionally contain another liquid such as an organic solvent (for example, iso-propanol, tetrahydrofuran, methanol, or other), and either acids or bases to render the coupling component into solution or a fine suspension and aid reaction with the diazonium salt solution, and additionally any buffers or surface active agents including the sterically bulky stabilizer compounds such as those described previously.
The reaction mixture containing the dissolved or suspended diazonium salt is then transferred into the solution or suspension of the desired nucleophilic coupling component, and the temperature of the mixture can range from about 10° C. to about 75° C., in order to produce a solid colorant material suspended as a precipitate in an aqueous slurry.
The solid colorant material may be the desired monoazo laked pigment product formed as nano-sized particles, or it may be an advanced synthetic intermediate for making the monoazo laked pigment product. In the case of the latter, a two-step process is required for preparing the nano-sized particles of monoazo laked pigment, whereby the second step involves rendering the advanced synthetic intermediate of the first step above (the pigment precursor) into homogeneous liquid solution by treatment with either strong acid or alkaline base, then treating this solution with one or more surface active agents in addition to the sterically bulky stabilizer compounds, as described previously, followed lastly by treatment with the required metal salt solution to provide the desired laked monoazopigment composition as a solid precipitate, said metal salt solution effectively functioning as a pigment precipitating agent. There are several chemical as well as physical processing factors can affect the final particle size and distribution of the monoazo laked pigment nanoparticles, including stoichiometries of the DC and CC starting reactants, metal salt, surface active agents, and stabilizer compounds, the concentrations of chemical species in the liquid medium, pH of liquid medium, temperature, addition rate, order of addition, agitation rate, post-reaction treatments such as heating, isolation and washing of particles, and drying conditions.
In embodiments is disclosed a two-step method of making nano-sized monoazo laked red pigments, for example Pigment Red 57:1, wherein the advanced pigment precursor Lithol Rubine is first synthesized as a potassium salt and is a water-soluble orange dye. The first step involves the diazotization of 2-amino-5-methyl-benzenesulfonic acid (DC1 in Table 1) by first dissolving the DC in dilute aqueous potassium hydroxide solution (0.5 mol/L) and cooling to a temperature anywhere in the range of about −5° C. to about 5° C., and then treating the solution with an aqueous solution of sodium nitrite (20 wt %), following with slow addition of concentrated hydrochloric acid at a rate that maintains the internal reaction temperature between −5° C. and +5° C. The resulting suspension that forms is stirred for additional time so as to ensure completeness of diazotization, and then the suspension is carefully transferred to a second solution containing 3-hydroxy-2-naphthoic acid dissolved in dilute alkaline solution (0.5 mol/L potassium hydroxide), using vigorous agitation as the colorant product is produced in the aqueous slurry. After stirring for additional time of at least 1 hour at room temperature, the colorant product (Lithol Rubine-potassium salt) is isolated by filtration as an orange dyestuff and washed with deionized water to remove excess salt by-products.
The second step of the process involves redispersing the orange Lithol Rubine-potassium salt dyestuff in deionized water to a concentration that can range from about 0.5 wt % to about 20 wt %, such as from about 1.5 wt % to about 10 wt % or from about 3.5 wt % to about 8 wt %, but the concentrations can also be outside of these ranges. The colorant solids in the slurry is then dissolved completely into liquid solution by treatment with aqueous alkaline base, such as sodium hydroxide or potassium hydroxide or ammonium hydroxide solution, until the pH level is high, such as above pH 8.0 or above pH 9.0 or above pH 10.0. To this alkaline solution of dissolved Lithol Rubine colorant can be optionally added a surface active agent as described previously, in particular embodiments surface active agent such as rosin soaps, delivered as an aqueous solution in the amount ranging from 0.1 wt % to 20 wt % based on colorant solids, such as in an amount ranging from 0.5 wt % to about 10 wt %, or in an amount ranging from 1.0 wt % to about 8.0 wt % based on colorant solids, but the amount used can also be outside of these ranges.
In embodiments, the preparation of ultrafine and nanosized particles of the monoazo laked Pigment Red 57:1 was only enabled by the additional use of a sterically bulky stabilizer compound having a functional group that could non-covalently bond to the complementary functional moiety of the pigment as well as branched aliphatic functional groups that could provide steric bulk to the pigment particle surface. In embodiments, particularly suitable sterically bulky stabilizer compounds are branched hydrocarbons with either carboxylate or sulfonate functional groups, compounds such as di[2-ethylhexyl]-3-sulfosuccinate sodium or sodium 2-hexyldecanoate, and the like. The stabilizer compound is introduced as a solution or suspension in a liquid that is predominantly aqueous but may optionally contain a water-miscible organic solvent such as THF, iso-propanol, NMP, Dowanol and the like, to aid dissolution of the stabilizer compound, in an amount relative to colorant moles ranging from about 5 mole-percent to about 100 mole-percent, such as from about 20 mole-percent to about 80 mole-percent, or from about 30 mole-percent to about 70 mole-percent, but the concentrations used can also be outside these ranges and in large excess relative to moles of colorant.
Lastly, the metal cation salt is added to transform the pigment precursor (Lithol Rubine-potassium salt in embodiments) into the desired monoazo laked pigment (Pigment Red 57:1 in embodiments), precipitated as nano-sized pigment particles. The aqueous solution of metal salt (calcium chloride in embodiments) with concentration ranging anywhere from 0.1 mol/L to about 2 mol/L, is slowly added dropwise in nearly stoichiometric quantities such as amounts ranging from 1.0 molar equivalents relative to about 2.0 molar equivalents, or from 1.1 to about 1.5 molar equivalents, or from 1.2 to about 1.4 molar equivalents relative to moles of colorant, however the amounts used can also be outside of these ranges and in large excess.
The type of metal salt can have an impact on the extent of forming nano-sized pigment particles of monoazo laked pigments, in particular the type of ligand that is coordinated to the metal cation and the relative ease with which it is displaced by a competing ligand from either the pigment functional moiety or the complementary functional moiety of the stabilizer compound, or both. In embodiments for monoazo laked Pigment Red 57:1, the nano-sized particles are formed using calcium (II) salts with ligands such as chloride, sulfate, acetate, and hydroxide; however a particularly desirable metal salt is calcium chloride for fastest reactivity.
The rates of addition of metal salt solution can also vary. For example, the slower the rate of addition, the more controlled is the rate of pigment crystal formation and particle aggregation, and therefore the smaller in size the pigment particles become.
Also important is the agitation rate and mixing pattern as the pigment formation/precipitation step is occurring. The higher the agitation rate and the more dynamic or complex is the mixing pattern (i.e. with baffles to prevent dead mixing zones), the smaller is the average particle diameter and the more narrow is the particle size distribution, as observable by Transmission Electron Microscopy (TEM) imaging. Agitation can be made more effective by using high-shear mixers such as homogenizers, attritors, our even the use of ultrasonic probes.
Temperature during the pigment precipitation step using the metal salt solution is also important. In embodiments, lower temperatures are desired, such as from about 10° C. to about 50° C., or from about 15° C. to about 30° C., but the temperature can also be outside of these ranges.
In embodiments, the slurry of pigment nanoparticles is not treated nor processed any further, such as performing additional heating which is often practiced by pigment manufacturers, but instead is isolated by vacuum filtration or centrifugal separation processes. The pigment solids can be washed copiously with deionized water to remove excess salts or additives that are not tightly associated or bonded with the pigment particle surface. The pigment solids can be dried by freeze-drying under high vacuum, or alternatively, they can be pre-rinsed with a water-miscible solvent such as isopropanol or acetonitrile to remove excess water and then vacuum-oven dried. The resulting nano-size pigment particles are generally non-aggregated and of high quality, which when imaged by TEM (Transmission Electron Microscopy), exhibit primary pigment particles and small aggregates ranging in diameters from about 10 nm to about 300 nm, and predominantly from about 50 nm to about 150 nm. (Here, it is noted that average particle size d₅₀or Z-average, and GSD particle size distributions are measured by Dynamic Light Scattering, an optical measurement technique that estimates the hydrodynamic radius of non-spherical pigment particles gyrating and translating in a liquid dispersion via Brownian motion, by measuring the intensity of the incident light scattered from the moving particles. As such, the d₅₀or the Z-average particle size metric obtained by DLS technique is always a larger number than the actual particle diameters observed by TEM imaging.)
Characterization of the chemical composition of washed and dried nano-sized pigment particles are performed by NMR spectroscopy and elemental analysis. In embodiments, the composition of the monoazo laked pigment Red 57:1 indicated that the nano-sized particles prepared by the methods described above, particularly when using di[2-ethylhexyl]-3-sulfosuccinate sodium as the sterically bulky stabilizer, retained at least 80% of the sterically bulky stabilizer that was loaded into the process of making the nanoparticles, even after copious washing with deionized water to remove excess salts. Solid state¹H- and¹³C-NMR spectroscopic analyses indicated that the steric stabilizer compound was associated non-covalently with the pigment as a calcium salt, and the chemical structure of the pigment adopted the hydrazone tautomer form, as shown in Figure below.
Pigment particles of monoazo laked pigments such as PR 57:1 that have smaller particle sizes could also be prepared by the above two-step method in the absence of using sterically bulky stabilizers and with the use of surface active agents alone (for example, only rosin-type surface agents), depending on the concentrations and process conditions employed, but the pigment product did not predominantly exhibit nano-sized particles nor did the particles exhibit regular morphologies. In the absence of using the sterically bulky stabilizer compound, the two-step method described above typically produced rod-like particle aggregates, ranging in average particle diameter from 200-700 nm and with wide particle distribution, and such particles were difficult to disperse into a polymer coating matrix and generally gave poor coloristic properties. In embodiments, the combined use of a suitable sterically bulky stabilizer compound, such as branched alkanesulfonates or alkylcarboxylates, with a minor amount of suitable surface active agent such as derivatives of rosin-type surfactants, using either of the synthesis methods described previously would afford the smallest fine pigment particles having nanometer-scale diameters, more narrow particle size distribution, and low aspect ratio. Various combinations of these compounds, in addition to variations with process parameters such as stoichiometry of reactants, concentration, addition rate, temperature, agitation rate, reaction time, and post-reaction product recovery processes, enables the formation of pigment particles with tunable average particle size (d₅₀or Z-average) from nanoscale sizes (about 1 to about 100 nm) to mesoscale sizes (about 100 to about 500 nm) or larger.
The advantages of this process include the ability to tune particle size and composition for the intended end-use application of the monoazo laked pigment, such as for use in toners and inks and coatings, which for example include phase-change, gel-based and radiation-curable inks, solid and non-polar liquid inks, solvent-based inks and aqueous inks and ink dispersions. For the end-use application in piezoelectric inkjet printing, nano-sized particles are advantageous to ensure reliable inkjet printing and prevent blockage of jets due to pigment particle agglomeration. In addition, nano-sized pigment particles are advantageous for offering enhanced color properties in printed images, since in embodiments the color properties of nano-sized particles of monoazo laked pigment Red 57:1 were tunable with particle size. It was observed that as average particle size was decreased to the nanometer-scale, the hue angles were shifted from yellowish-red hues to bluish-red hues by an amount ranging from about 5 to about 35° in the color gamut space.
In embodiments, the nanosized pigment particles that were obtained for monoazo laked pigments can range in average particle size, d₅₀or Z-average, or the average particle diameter, from about 10 nm to about 250 nm or about 500 nm, such as from about 25 nm to about 175 nm, or from about 50 nm to about 150 nm, as measured by either dynamic light scattering method or from TEM images. The pigment average particle size can accordingly be tuned by the above method, to provide compositions with desired average particle sizes. For example, within the broad suitable size range of about 1 nm to about 500 nm or larger, the pigment composition can be provided to have an average particle size that is generally in the nanoscale size range (about 1 to about 100 nm) or generally in the mesoscale size range (about 100 to about 500 nm) or larger. In embodiments, the pigment composition can be provided to have an average particle size in the ranges of from about 10 nm to about 50 nm, about 50 nm to about 100 nm, about 100 nm to about 150 nm, or about 150 nm to about 300 nm. For example, in embodiments, the pigment composition can be provided to have an average particle size of about 50 nm to about 12550 nm. In embodiments, the particle size distributions can range such that the geometric standard deviation can range from about 1.1 to about 1.9, or from about 1.2 to about 1.7, as measured by dynamic light scattering method. The shape of the nanosized pigment particles can be one or more of several morphologies, including rods, platelets, needles, prisms or nearly spherical, and the aspect ratio of the nanosize pigment particles can range from about 1:1 to about 10:1, such as having aspect ratio between about 1:1 and about 5:1; however the actual metric can lie outside of these ranges.
In addition, nanosized pigment particles are advantageous for offering enhanced color properties in printed images, since in embodiments the color properties of nanosized particles of monoazo laked pigment Red 57:1 were tunable with particle size. In embodiments is disclosed the coloristic properties (hue angle, a*, b*, and NLSI as measure of specular reflectivity) of nanosized pigment particles, particularly of monoazo laked red pigment, that are directly correlated and tunable with the average pigment particle size, measured by either Dynamic Light Scattering or electron microscopy imaging techniques, as well as pigment composition with the non-covalently associated stabilizer, the latter which enables the control of particle size during pigment synthesis, and also enables enhanced dispersability within certain polymer binders for coating or other applications. For example, as average particle size was decreased to nanometer-scale, the hue angles were shifted from yellowish-red hues to bluish-red hues by an amount ranging from about 5 to about 35° in the color gamut space. The color of the nanosized pigment particles have the same general hue as is found with larger pigment particles. However, in embodiments, is disclosed coloristic properties of thin coatings of the nano-sized pigment particles of red monoazo laked pigments dispersed in a polymer binder (such as of poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate)), that exhibited a significant shift to lower hue angle and lower b* values that revealed more bluish magenta hues, and having either no change or a small enhancement of a* value. In embodiments, any suitable polymer can be used to effect both good pigment dispersion and to act as a pigment binder to allow a good film forming quality about the substrate. In other embodiments, it is advantageous when assessing coloristic properties of pigments, as they are dispersed in coatings, for example, that the polymer used to disperse a pigment is transparent to visible light, either chemically or by way of not scattering light owing to the polymer being more amorphous than crystalline, not be prone to fluorescence, and does not discolor during a dispersion making process or during the drying process after it has been coated on a substrate.
In embodiments, the hue angles of the coatings dispersed with the nanosized particles of monoazo laked pigment such as Pigment Red 57:1 measured in the range from about 345° to about 5° on the 2-dimensional b* a* color gamut space, as compared with hue angles ranging from about 0° to about 20° for similarly prepared polymer coatings dispersed with conventional larger sized particles of Pigment Red 57:1. The coatings dispersed with the nanosized particles of monoazo laked pigment can accordingly exhibit hue angles of from about 345° to about 5°, such as from about 345° to about 0°, or from about 345° to about 350° or to about 355°, on the 2-dimensional b* a* color gamut space.
Additionally, the specular reflectivity of the coatings of the nanosized monoazo laked red pigment was significantly enhanced from coatings produced with conventional larger sized pigment particles, which is an indicator of having very small particles being well-dispersed within the coating. Specular reflectivity was quantified as the degree of light scattering for the pigmented coating, a property that is dependent on the size and shape distributions of the pigment particles and their relative dispersability within the coating binder. The Normalized Light Scatter Index (NLSI) was quantified by measuring the spectral absorbance of the coating, using a Shimadzu UV-160 spectrophotometer, in a region where there is no absorbance from the chromogen of the monoazo laked pigment, but only absorbance due to light scattered from large aggregates and/or agglomerated pigment particles dispersed in the coating binder. The light scattering absorbance data is then normalized to a lambda-max optical density of 1.5, resulting in the NLSI value, in order to directly compare the light scattering indices of several pigmented coatings. The lower is the NLSI value, the smaller is the pigment particle size within the dispersed coating matrix. In embodiments, the NLSI value of the nanosized monoazo laked red pigments can range from about 0.1 to about 3.0, such as from about 0.1 to about 1.0. In comparison, the NLSI values observed with similarly prepared coatings containing conventional larger sized monoazo laked red pigments range anywhere from about 3.0 to about 75 (which indicates a very poorly dispersed coating).
The formed nanoscale pigment particle compositions can be used, for example, as coloring agents in a variety of compositions, such as in liquid (aqueous or non-aqueous) ink vehicles, including inks used in conventional pens, markers, and the like, liquid ink jet ink compositions, solid or phase change ink compositions, and the like. For example, the colored nanoparticles can be formulated into a variety of ink vehicles, including “low energy” solid inks with melt temperatures of about 60 to about 130° C., and solvent-based liquid inks or radiation-curable such as UV-curable liquid inks comprised of alkyloxylated monomers, and even aqueous inks.
In addition to ink compositions, the nanoscale-sized pigment composition can be used in a variety of other applications, where it is desired to provide a specific color to the composition. For example, the nanoscale-sized pigment composition can also be used in the same manner as conventional pigments in such uses as colorants for paints, resins, lenses, filters, printing inks, and the like according to applications thereof. By way of example only, the nanoscale-sized pigment composition of embodiments can be used for toner compositions, which include polymer particles and nanoscale pigment particles, along with other optional additives, that are formed into toner particles and optionally treated with internal or external additives such as flow aids, charge control agents, charge-enhancing agents, filler particles, radiation-curable agents or particles, surface release agents, and the like.
An example is set forth herein below and is illustrative of different compositions and conditions that can be utilized in practicing the disclosure. All proportions are by weight unless otherwise indicated. It will be apparent, however, that the disclosure can be practiced with many types of compositions and can have many different uses in accordance with the disclosure above and as pointed out hereinafter.

EXAMPLESExamples of Monoazo Laked Red Pigment Compositions and Methods of Making in Various Particle SizesComparative Example 1Synthesis of Pigment Red 57:1 Particles Using a Two-Step Method

Step 1: Diazotization and Coupling: Into a 500 mL round bottom flask equipped with a mechanical stirrer, thermometer, and addition funnel was dissolved 2-amino-5-methylbenzenesulfonic acid (8.82 g) into 0.5M KOH aqueous solution (97.0 mL). The resulting brown solution was cooled to 0° C. A 20 wt % aqueous solution of sodium nitrite (NaNO₂; 3.28 g dissolved into 25 mL water) was added slowly to the first solution while maintaining the temperature below 3° C. To the red-brown homogeneous mixture was added dropwise concentrated HCl (10M, 14.15 mL) over 1 hour, maintaining the internal temperature below 2° C. The mixture formed a pale brown suspension, and following complete addition of conc. HCl, the suspension was stirred an additional 30 min.

In a separate 2-L resin kettle was dissolved 3-hydroxy-2-naphthoic acid (8.86 g) into an aqueous solution of KOH (8.72 g) in water (100 mL). An additional 250 mL of water was added, and the light-brown solution was then cooled to 15° C. while stirring vigorously. The cold suspension of the diazonium salt suspension was then added slowly to the coupling solution while mixing vigorously. The color changed initially to a dark red solution, and ultimately to a yellowish-red (orange) slurry of precipitated dyestuff. The mixture was stirred for 2 hours while warming up to room temp, then filtered and diluted with about 500 mL of deionized water to produce an orange aqueous slurry of Lithol Rubine-Potassium salt dye (a synthetic precursor of Pigment Red 57:1) having solids content of about 1.6 wt %.

Step 2: Laking Step to Produce Pigment Red 57:1 Particles

Into a 500 mL round bottom flask equipped with mechanical stirrer and condenser was charged 126 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor from above having about 1.6% wt solids content. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 13 mL) was added dropwise to the slurry while stirring vigorously. A red precipitate formed immediately, and after addition was completed, the slurry was stirred for an additional 1 hour. The red slurry was then heated to about 75° C. for 20 min, then cooled to room temp. The slurry was filtered under high vacuum through a 1.2 μm acrylic polymer membrane, then reslurried twice with 200 mL portions of deionized water. The pH and conductivity of the filtrates after each filtration were measured and recorded, with the final wash filtrate having nearly neutral pH of 6.2 and conductivity of about 13.5 μS/cm, indicating low residual salts. The red pigment filtercake was reslurried into about 200 mL of DIW and freeze-dried for 48 hours, to afford a red colored powder (1.95 grams). TEM microscopy revealed long rod-like particles and aggregates, with particle diameters ranging from about 200 nm to about 700 nm, and large aspect ratios ranging from about 4:1 to about 10:1.

Example 1Synthesis of Nano-Sized Particles of Pigment Red 57:1 by a Two-Step Method

In a separate 2-L resin kettle was dissolved 3-hydroxy-2-naphthoic acid (8.86 g) into an aqueous solution of KOH (8.72 g) in water (100 mL). An additional 250 mL of water was added, and the light-brown solution was then cooled to 15° C. while stirring vigorously. The cold suspension of the diazonium salt suspension was then added slowly to the coupling solution while mixing vigorously. The color changed immediately to a dark red solution, and ultimately to a yellowish-red (orange) slurry of precipitated dyestuff. The mixture was stirred for 2 hours while warming up to room temp, then filtered and reslurried with about 500 mL of deionized water to produce an orange aqueous slurry of Lithol Rubine-Potassium salt dye having solids content of about 1.6 wt %.

Step 2: Laking Step to Produce Pigment Red 57:1 Particles:

Into a 500 mL round bottom flask equipped with mechanical stirrer and condenser was charged 126 g of aqueous slurry of Lithol Rubine-Potassium salt dye from above (Example 1, Step 1) having about 1.6% wt solids content. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. Anaqueous solution 5 wt % Dresinate X (4.0 mL) was added, followed by a solution containing sodium dioctyl sulfosuccinate (0.96 g) dissolved in 100 mL of 90:10 deionized water/THF mixture. No visible change was observed. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 13 mL) was added dropwise to the slurry while stirring vigorously. A red precipitate formed immediately, and after complete addition of the calcium chloride solution, the slurry was stirred for an additional 1 hour. The red slurry was then heated to about 75° C. for 20 min, then cooled to room temp. The slurry was filtered under high vacuum through a 0.45 μm Nylon membrane cloth, then reslurried twice with 75 mL portions of DIW. The pH and conductivity of the final wash filtrate was 7.4 and about 110 μS/cm, respectively, indicating that residual acids and salt by-products were removed. The red pigment filtercake was reslurried in about 250 mL of DIW and freeze-dried for 48 hours to afford a dark red colored powder (2.65 grams). Transmission electron microscopy images of the powder revealed platelet-like particles with particle diameters ranging from 30-150 nm, and aspect ratios that were less than 3:1.¹H-NMR spectroscopy analysis (300 MHz, DMSO-d₆) of the pigment indicated that the pigment adopted the hydrazone tautomer form, and that the dioctyl sulfosuccinate stabilizer compound was present at approximately 40 mol % (representing about 80% remaining of actual loading) and was associated with a calcium cation (determined by ICP spectroscopy).

Example 2Synthesis of Nano-Sized Particles of Pigment Red 57:1 by a Two-Step Method

The procedure ofStep 1 of Example 1 above was reproduced.

Step 2: Laking

Into a 500 mL round bottom flask equipped with mechanical stirrer and condenser was charged 126 g of aqueous slurry of Lithol Rubine-Potassium salt dye from above (Example 1) having about 1.6% wt solids content. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. Anaqueous solution 5 wt % Dresinate X (4.0 mL) was added, followed by a solution containing sodium dioctyl sulfosuccinate (0.96 g) dissolved in 100 mL of 90:10 deionized water/THF mixture. No visible change was observed. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 13 mL) was added dropwise to the slurry while stirring vigorously. A red precipitate formed immediately, and after complete addition of the calcium chloride solution, the slurry was stirred for an additional 1 hour. The red slurry was then heated to about 75° C. for 20 min, then cooled to room temp. The slurry was filtered under high vacuum through a 0.45 μm Nylon membrane cloth, then reslurried twice with 75 mL portions of DIW. The pH and conductivity of the final wash filtrate was 7.15 and about 155 μS/cm, respectively. The red pigment filtercake was reslurried in about 250 mL of DIW and freeze-dried for 48 hours to afford a dark red-colored powder (2.62 grams). Transmission electron microscopy images of the powder revealed platelet-like particles with particle diameters ranging from 50-175 nm, and aspect ratios equal to or less than 3:1

Example 3Synthesis of Nano-Sized Particles of Pigment Red 57:1 by a Two-Step Method

Step 1: Diazotization and Coupling: Into a 500 mL round bottom flask equipped with a mechanical stirrer, thermometer, and addition funnel was dissolved 2-amino-5-methylbenzenesulfonic acid (12.15 g) into 0.5M KOH aqueous solution (135 mL). The resulting brown solution was cooled to 0° C. A 20 wt % aqueous solution of sodium nitrite (NaNO₂; 4.52 g dissolved into 30 mL water) was added slowly to the first solution while maintaining the temperature below −2° C. Concentrated HCl (10M, 19.5 mL) was then slowly added dropwise over 1 hour while maintaining the internal temperature below 0° C. The mixture formed a pale brown suspension and following complete addition of conc. HCl, the suspension was stirred an additional 30 min.

In a separate 2-L resin kettle was dissolved 3-hydroxy-2-naphthoic acid (12.2 g) into an aqueous solution of KOH (12.0 g) in water (130 mL). An additional 370 mL of water was added, and the pale brown solution was then cooled to about 15° C. while stirring. The cold suspension of the diazonium salt solution was then added slowly to the coupling solution while mixing vigorously. The color change was immediate to dark black-red solution, and ultimately to a yellowish-red (orange) slurry of precipitated dyestuff. The mixture was stirred for at least 2 hours while warming up to room temp, then filtered and reslurried with about 600 mL of deionized water to produce an orange-colored slurry of Lithol Rubine-Potassium salt dye having solids content of about 3.75%-wt.

Step 2: Laking Step to Produced Nano-Sized Particles of Pigment Red 57:1

Into a 1-L resin kettle equipped with mechanical stirrer and condenser was charged 265 g of aqueous slurry of Lithol Rubine-Potassium salt dye prepared fromStep 1 of Example 2 above, having approximately 3.75%-wt solids content). The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. Anaqueous solution 5 wt % Dresinate X (20.0 mL) was added while stirring, followed by a solution containing sodium dioctyl sulfosuccinate (4.8 g) dissolved in 220 mL of 90:10 deionized water/THF mixture was slowly added to the mixture with stirring. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 65 mL) was added dropwise to the slurry while stirring vigorously. A red precipitate formed immediately, and after complete addition of the calcium chloride solution, the slurry was stirred for an additional 1 hour. The red slurry was then heated to about 60° C. for 30 min, then cooled immediately in a cold water bath. The slurry was filtered under high vacuum through a 0.8 micron Versapor membrane cloth (obtained from PALL Corp.), then reslurried twice with about 750 mL portions of DIW, and filtered once more. The pH and conductivity of the final wash filtrate was 7.5 and about 208 μS/cm, respectively. The red pigment filtercake was reslurried in about 600 mL of deionized water and freeze-dried for 48 hours, to afford a dark red-colored powder (12.75 grams). Transmission electron microscopy images of the powder revealed predominantly platelet-like particles with particle diameters ranging from 50-150 nm, and aspect ratios that were equal to or less than about 3:1

Example 4Preparation of Nano-Sized Particles of Pigment Red 57:1 Using the Two-Step Method

Into a 250 mL round bottom flask equipped with mechanical stirrer and condenser was charged 10 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor prepared as inStep 1 of Example 3, except that the solids concentration in the aqueous slurry was about 10.0 wt %. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. Anaqueous solution 5 wt % Dresinate X (1.0 mL) was added, followed by a 0.05 mol/L solution (34.5 mL) containing sodium dioctyl sulfosuccinate dissolved in 90:10 deionized water/THF. No visible change was observed. An aqueous solution of calcium chloride dihydrate (1.0 M solution, 2.15 mL) was added dropwise by syringe pump to the slurry while stirring vigorously. A red precipitate formed immediately, and then the slurry was stirred at room temperature for an additional 30 min. The red slurry was then filtered under high vacuum through a 0.8 μm Versapor membrane cloth (obtained from PALL Corp.), then reslurried twice with 50 mL portions of deionized water and filtered each time after reslurrying. The pH and conductivity of the final wash filtrate was 7.5 and about 135 μS/cm, respectively, indicating that residual acids and salt by-products were removed. The red pigment filtercake was reslurried in about 30 mL of deionized water and freeze-dried for 48 hours to afford a dark red colored powder (1.32 grams). Transmission electron microscopy images of the powder revealed very small platelet-like particles with particle diameters ranging from 50 to 175 nm, and aspect ratios were equal to or less than about 3:1. Dynamic Light Scattering analysis measured an average particle size, d₅₀, of 189 nm and GSD of 1.54 (Z-average particle size of 176 nm; polydispersity index, PDI, of 0.143).¹H-NMR spectroscopy analysis (300 MHz, DMSO-d₆) of the material indicated that the pigment adopted the hydrazone tautomer form, and that the dioctyl sulfosuccinate stabilizer compound was present at a level ranging from approximately 50-75 mol %.

Example 5Preparation of Small Particles of Pigment Red 57:1 Using the Two-Step Method

Into a 500 mL round bottom flask equipped with mechanical stirrer and condenser was charged 126 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor (prepared as inStep 1 of Example 1) having about 1.6% wt solids content. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. Anaqueous solution 5 wt % Dresinate X (4.0 mL) was added, followed by a solution containing sodium dioctyl sulfosuccinate (1.92 g) dissolved in 100 mL of 90:10 deionized water/THF mixture. No visible change was observed. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 13 mL) was added dropwise to the slurry while stirring vigorously. A red precipitate formed immediately, and after complete addition of the calcium chloride solution, the slurry was stirred for an additional 1 hour. The red slurry was then heated to about 75° C. for 20 min, then cooled to room temp. The slurry was filtered under high vacuum through a 0.45 μm Nylon membrane cloth, then reslurried twice with 75 mL portions of DIW. The pH and conductivity of the final wash filtrate was 7.75 and conductivity of about 500 μS/cm. The red pigment filtercake was reslurried in about 250 mL of DIW and freeze-dried for 48 hours to afford a dark red-colored powder (2.73 grams). Transmission electron microscopy images of the powder showed a distribution of particle sizes, with diameter ranging from 50 to 400 nm and having particle morphologies that were predominantly platelets.

Example 6Preparation of Small Particles of Pigment Red 57:1 Using a Two-Step Method

The sterically bulky stabilizer compound used was potassium salt of 2-hexyldecanoic acid, prepared by treatment of 2-hexyldecanoic acid with potassium hydroxide dissolved in THF, after which the THF solvent was removed. Into a 500-mL round-bottom flask equipped with condenser and mechanical stirrer was charged 126 g of aqueous slurry of Lithol Rubine-Potassium salt (prepared as inStep 1 of Example 1) having about 1.6% wt solids content. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. Anaqueous solution 5 wt % Dresinate X (4.0 mL) was added, followed by a solution containing potassium 2-hexyldecanoate (1.28 g) dissolved in 100 mL of 80:20 deionized water/THF mixture, added dropwise while stirring vigorously. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 13 mL) was added to the slurry while stirring vigorously causing a bluish-red pigment precipitate to form. The slurry was stirred for 1 hour, heated to about 75° C. for 20 min, then cooled to room temperature. The slurry was filtered under high vacuum through a 0.8 μm Nylon membrane cloth, then reslurried once with 150 mL of DIW and filtered again. The pH and conductivity of the final wash filtrate was pH 8.38 and conductivity of about 63 μS/cm. The red pigment 57:1 filtercake was reslurried into about 150 mL of DIW and freeze-dried for 48 hours to afford a red powder (2.95 grams). TEM micrograph images showed a distribution of particle sizes, with diameters ranging from 50 to about 400 nm and having particle morphologies that included platelets as well as rods.

Example 7Preparation of Pigment Red 57:1 Particles Using the Two-Step Method

Into a 250 mL round bottom flask equipped with mechanical stirrer and condenser was charged 25 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor prepared as inStep 1 of Example 3, except that the solids concentration in the aqueous slurry was about 4.0 wt %. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. An aqueous solution of 5 wt % Dresinate X (1.0 mL) was added, followed by a 0.05 mol/L solution (11 mL) containing sodium dioctyl sulfosuccinate dissolved in 90:10 deionized water/THF. No visible change was observed. An aqueous solution of calcium chloride dihydrate (0.5 M solution, 6.5 mL) was added dropwise by syringe pump to the slurry while stirring vigorously. A red precipitate formed immediately, and then the slurry was stirred at room temperature for an additional 30 min. The red slurry was then filtered under high vacuum through a 0.8 μm Versapor membrane cloth (obtained from PALL Corp.), then reslurried twice with 50 mL portions of deionized water and filtered each time after reslurrying. The pH and conductivity of the final wash filtrate was 6.7 and about 22.5 μS/cm, respectively, indicating that residual acids and salt by-products were removed. The red pigment filtercake was reslurried in about 30 mL of deionized water and freeze-dried for 48 hours to afford a dark red colored powder (0.92 grams). Transmission electron microscopy images of the powder revealed irregular rod-like particles with particle diameters ranging from 100-600 nm with the majority of particles less than about 300 nm, and aspect ratios of greater than about 4:1. Dynamic Light Scattering analysis measured an average particle size, d₅₀, of 259 nm and GSD of 1.60 (Z-average particle size of 224 nm; polydispersity index, PDI, of 0.145).

Example 8Preparation of Pigment Red 57:1 Particles Using the Two-Step Method

Into a 250 mL round bottom flask equipped with mechanical stirrer and condenser was charged 25 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor prepared as inStep 1 of Example 3, except that the solids concentration in the aqueous slurry was about 4.0 wt %. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. An aqueous solution of 5 wt % Dresinate X (4.0 mL) was added, followed by a 0.25 mol/L solution (2 mL) containing sodium dioctyl sulfosuccinate dissolved in 90:10 deionized water/THF. No visible change was observed. An aqueous solution of calcium chloride dihydrate (1.0 M solution, 2 mL) was added dropwise by syringe pump to the slurry while stirring vigorously. A red precipitate formed immediately, and then the slurry was stirred at room temperature for an additional 30 min. The red slurry was then filtered under high vacuum through a 0.8 μm Versapor membrane cloth (obtained from PALL Corp.), then reslurried twice with 50 mL portions of deionized water and filtered each time after reslurrying. The pH and conductivity of the final wash filtrate was 6.9 and about 75.4 μS/cm, respectively, indicating that residual acids and salt by-products were removed. The red pigment filtercake was reslurried in about 30 mL of deionized water and freeze-dried for 48 hours to afford a dark red colored powder (0.70 grams). Transmission electron microscopy images of the powder revealed large rod-like particles with particle diameters ranging from 180-900 nm with the majority of particles less than about 400 nm, and aspect ratios of greater than about 4:1. Dynamic Light Scattering analysis measured an average particle size, d₅₀, of 269 nm and GSD of 1.64 (Z-average particle size of 252 nm; polydispersity index, PDI, of 0.185).

Example 9Preparation of Pigment Red 57:1 Particles Using the Two-Step Method

Into a 250 mL round bottom flask equipped with mechanical stirrer and condenser was charged 25 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor prepared as inStep 1 of Example 3, except that the solids concentration in the aqueous slurry was about 4.0 wt %. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. An aqueous solution of 5 wt % Dresinate X (4.0 mL) was added, followed by a 0.05 mol/L solution (34.5 mL) containing sodium dioctyl sulfosuccinate dissolved in 90:10 deionized water/THF. No visible change was observed. An aqueous solution of calcium chloride dihydrate (1.0 M solution, 2 mL) was added dropwise by syringe pump to the slurry while stirring vigorously. A red precipitate formed immediately, and then the slurry was stirred at room temperature for an additional 30 min. The red slurry was then filtered under high vacuum through a 0.8 μm Versapor membrane cloth (obtained from PALL Corp.), then reslurried twice with 50 mL portions of deionized water and filtered each time after reslurrying. The pH and conductivity of the final wash filtrate was 6.8 and about 77 μS/cm, respectively, indicating that residual acids and salt by-products were removed. The red pigment filtercake was reslurried in about 30 mL of deionized water and freeze-dried for 48 hours to afford a dark red colored powder (0.62 grams). Transmission electron microscopy images of the powder revealed platelets as well as rod-like particles with particle diameters ranging from 100-400 nm with the majority of particles less than about 250 nm, and aspect ratios of less than about 4:1. Dynamic Light Scattering analysis measured an average particle size, d₅₀, of 235 nm and GSD of 1.61 (Z-average particle size of 224 nm; polydispersity index, PDI, of 0.152).

Example 10Preparation of Small Particles of Pigment Red 57:1 Using the Two-Step Method

Into a 250 mL round bottom flask equipped with mechanical stirrer and condenser was charged 10 g of aqueous slurry of Lithol Rubine-Potassium salt dye precursor prepared as inStep 1 of Example 3, except that the solids concentration in the aqueous slurry was about 10.0 wt %. The pH of the slurry was adjusted to at least 9.0 or higher by addition of 0.5 M KOH solution, after which the dyestuff was fully dissolved. An aqueous solution of 5 wt % Dresinate X (4.0 mL) was added, followed by a 0.05 mol/L solution (11 mL) containing sodium dioctyl sulfosuccinate dissolved in 90:10 deionized water/THF. No visible change was observed. An aqueous solution of calcium chloride dihydrate (1.0 M solution, 3.25 mL) was added dropwise by syringe pump to the slurry while stirring vigorously. A red precipitate formed immediately, and then the slurry was stirred at room temperature for an additional 30 min. The red slurry was then filtered under high vacuum through a 0.8 μm Versapor membrane cloth (obtained from PALL Corp.), then reslurried twice with 50 mL portions of deionized water and filtered each time after reslurrying. The pH and conductivity of the final wash filtrate was 7.5 and about 42.7 μS/cm, respectively, indicating that residual acids and salt by-products were removed. The red pigment filtercake was reslurried in about 30 mL of deionized water and freeze-dried for 48 hours to afford a dark red colored powder (0.61 grams). Transmission electron microscopy images of the powder revealed platelets as well as rod-like particles with particle diameters ranging from 75 nm to about 300 nm with the majority of particles less than about 200 nm, and aspect ratios of less than about 3:1. Dynamic Light Scattering analysis measured an average particle size, d₅₀, of 209 nm and GSD of 1.57 (Z-average particle size of 193 nm; polydispersity index, PDI, of 0.148).

Examples of Liquid Dispersions Containing Monoazo Laked Pigment Particles (Including Nano-Sized Particles) and Their Coloristic PropertiesExample 11Method for Preparation of Liquid Dispersions and of Polymer Coatings

A series of liquid, non-aqueous dispersions were prepared using a polymeric dispersant and the nano-sized PR 57:1 pigments from Examples 1, 2, 3, 4, 5 and 6; the larger-sized pigment particles prepared as described in the Comparative Example 1; as well as two commercial sources of PR 57:1 obtained from Clariant (lot #L7B01) and Aakash. Coatings on clear Mylar film were prepared from these liquid dispersions, and evaluated in the following manner: Into a 30 mL amber bottle was added 0.22 g of pigment, 0.094 g polyvinylbutyral (B30HH obtained from Hoescht), 7.13 g n-butyl acetate (glass-distilled grade, obtained from Caledon Laboratories) and 70.0 g of ⅛′ stainless steel shot (Grade 25 440C obtained from Hoover Precision Products). The bottles were transferred to a jar mill and were allowed to gently mill for 4 days at 100 RPM. Two draw-down coatings were obtained for each dispersion using an 8-path gap on clear Mylar film such that the wet thicknesses for each coating comprised of PR 57:1 pigment sample were 0.5 and 1 mil. The air-dried coatings on clear Mylar film were then dried in a horizontal forced-air oven at 100° C. for 20 minutes.

Example 12Measurement of Pigment Particle Size by Dynamic Light Scattering

The measurements of various example PR57:1 pigments' particle sizes were performed on a Malvern ZetaSizer HT at 25° C. Each of the pigment dispersions prepared in Example 8 were diluted in a solution of polyvinylbutyral (B30HH obtained from Hoescht) in n-butyl acetate (glass-distilled grade, obtained from Caledon Laboratories) and sonicated at low power for 1 minute. For each determination of particle size and distribution, 3 replicates of 12 runs were performed wherein the repeatability of particle size data was realized for each sample. The particle size data for the diluted samples that were prepared in Example 8 can be found in Table 10.

Example 13Evaluation of Coatings Prepared from Liquid Pigment Dispersions

The coatings on clear Mylar film prepared as described in Example 11 were assessed for coloristic and light scattering properties in the following manner: The UV/VIS/NIR transmittance spectra of each coating were obtained using a Shimadzu UV160 spectrophotometer, and the results showed dramatically reduced light scattering and remarkable specular reflectivity for the nano-sized PR57:1 pigment samples described herein, compared with the spectra of coatings prepared with commercial PR57:1 pigment samples obtained from Clariant and Aakash. The degree of light scattering in a coating is dependent on both the size and shape distributions of the pigment particles and their relative dispersability within the coating matrix, and the Normalized Light Scatter Index (NLSI) method was developed to be a measure of this characteristic for the pigmented coatings. NLSI is quantified by first measuring the spectral absorbance of the coating in a region where there is no absorbance from the chromogen of the monoazo laked pigment (for PR57:1, a suitable region is 700-900 nm), but only absorbance due to light scattered from large aggregates and/or agglomerated pigment particles dispersed in the coating binder. The Normalized Light Scatter Index (NLSI) is then obtained by normalizing each of the samples' light scattering indices (from 700 to 900 nm) to a lambda-max optical density=1.5. In this way, the degree of light scattering for each pigmented coating could be compared directly against each other. The lower the NLSI value, the smaller the inferred particle size of the dispersed pigment in the coating. A relationship between decreasing average particle size and decreasing NLSI value was found to exist with the coatings prepared from the example pigments shown in Table 8. In particular, the nano-sized monoazo laked pigment PR57:1 of Example 3 had by far the lowest degree of light scattering, with an NLSI value of 0.3. The coloristic properties of the Mylar coatings were determined using an X-RITE 938 spectrodensitometer. L* a* b* and optical density (O.D.) values were obtained for each of the samples, and the L* a* b* were normalized to an optical density of 1.5, and used to calculate the hue angle and chroma (C*), as listed in Table 8.

TABLE 8

Normalized Light Scatter Indices (NLSI), Coloristic properties normalized to O.D. = 1.5, Particle size ranges and
Dispersion stabilities of example PR57:1 pigments

Metric	PR57:1	PR57:1	Example 1	1	2	3	4	5	6	7	8	9	10

L*^a	47.9	48.0	44.8	50.8	50.6	51.7	53.0	49.9	49.6	50.1	49.0	50.8	52.0
a*^a	71.1	71.2	71.5	76.5	77.2	79.4	78.8	76.7	73.6	76.4	74.7	77.2	77.7
b*^a	8.7	17.5	34.8	−16.4	−17.4	−18.8	−15.0	−18.9	1.4	−2.4	6.1	−5.8	−12.5
Hue Angle (°)^a	6.6	13.8	28.1	347.9	347.1	346.6	349.2	346.1	0.9	358.3	4.6	355.8	350.9
C*^a	72.6	73.4	78.1	78.6	77.5	81.3	80.5	78.9	73.9	78.2	76.7	78.9	79.9
Particle Size	~50 to	~50 to	~200 to	~30 to	~50 to	~50 to	~50 to	~50 to	~50 to	~100 to	~180 to	~100 to	~75 to
Diameter Range^b	~400	~400	~700	~150	~175	~150	~175	~400	~400	~600;	~900;	~400;	~300;
(nm)										most	most	most	most
										<300	<400	<250	<200 nm
										nm	nm	nm
Normalized Light	5.5	9.9	74.1	0.3	1.3	1.0	0.7	0.9	4.8	1.8	2.2	1.5	0.4
Scatter Index^a
Dispersion	2 days	2 days	<1 day	6	3	3	3	6	2	3	3	3	3
Stability				months	months	months	months	months	months	months	months	months	months

In Table 8,
^adenotes that the various coloristic and light scatter index metrics were normalized to optical density of 1.5, and,
^bdenotes that the particle size diameter was determined by Transmission Electron Microscopy.

For the data in Table 9, the various coloristic properties, normalized light scatter indices, of the example pigments as they were dispersed and coated onto Clear Mylar® and the dispersion stabilities found in Table 8 have been organized into various ranges by way of the following:

a)¹L*, where A denotes 49≦L*≦54, B denotes L*>54, C denotes L*<49,
b)²a*, where A denotes a*≧78, B denotes 75≦a*<78, C denotes a*<75,
c)³b*, where A denotes b*≦−18, B denotes −10≦b*<−18, C denotes −2≦b*<−10, D denotes 6≦b*<−2, E denotes b*>6,
d)⁴NLSI, where A denotes NLSI≦1, B denotes 1<NLSI≦2, C denotes NLSI>2, and
e)⁵Dispersion Stability at Room Temperature, where A denotes stability ≧3 months, B denotes stability ≧1 week but <3 months, C denotes stability <1 week.

From table 9, it is clear that a variety of coloristic properties can be realized by adjusting the synthetic process of nano-sized PR57:1 to suit pigment compositions having relatively smaller particle sizes, relatively larger particle sizes or a combination of particle sizes to suit the needs of a particular application. For example, it may useful in a particular application, such as in ink formulations, to have the ability to tune the coloristic properties of an ink, a magenta ink, for example, without using excessive energy to grind down the pigment particles. The ability to control various properties of nano-sized PR57:1, such as hue angle, as determined from its synthesis and work-up history, is beneficial to the environment as lower energy consumption would be required to process a dispersion or ink for a given application, such as for coatings or ink jet inks, such as piezo inkjet inks.

TABLE 9

Ranged and Normalized Scatter Indices (NLSI), Coloristic and Dispersion
Stability Properties of example PR57:1 pigments, normalized to O.D. = 1.5

L*¹

a*²

b*³

Example 14b*a* Coloristic Properties of Coatings prepared from Liquid Pigment Dispersions

The graphs inFIGS. 1 and 2 visually illustrate the significant shifts in b* a* gamut observed with coatings prepared with the nano-sized PR57:1 pigments from Examples 1, 2, 3, 4 and 5, in addition to the extended C* chroma for the nano-sized pigment examples. Furthermore, the graph inFIG. 1 shows a clear blue-shifting of hue that directly corresponds to decreasing particle size/particle diameters of the example PR57:1 pigments, a relationship which is also inferred from the Normalized Light Scatter Index (NLSI) values of Table 8. (Note: For ease of generating the graph, the b* vertical axis shows “negative” hue angles, which represent the number of degrees <360 degrees.) The light scattering and coloristic data accumulated provide evidence for the ability to tune color properties and specular reflectivity of pigmented coatings with tunable particle size of surface-enhanced fine particles of monoazo laked red pigments, in particular Pigment Red 57:1. This is achieved by using the methods of making such nano-sized pigments of PR57:1 as described herein, in particular using the two-step process which uses sterically bulky stabilizer compounds to limit particle aggregation and thereby limit particle size as well as enhance dispersion and color characteristics of the nano-sized pigment particles. Furthermore, the ability to easily tune color properties of such monoazo laked pigments provides a means to control the color quality so that inexpensive azo laked pigments like PR57:1 can be used to obtain magenta color that are normally exhibited by higher cost red pigments, such as the quinacridone-type Pigment Red 122 and Pigment Red 202.

It can be seen from the data inFIG. 2 that there is a semi-logarithmic correlation between hue angle and Normalized Light Scatter Index (NLSI) of the example PR57:1 pigments, normalized to an optical density of 1.5. The relatively greater degree of blue-shifting of hue for some example pigments, especially those made in Examples 1, 2, 3, 4, and 5 occurred with correspondingly lower NLSI values. As well, red-shifting of hue for some example pigments occurred with correspondingly higher NLSI values. Furthermore, the direct correlation between hue angle of example pigments coated onto clear Mylar® (normalized to O.D.=1.5) and of particle size (prepared and measured as Z-average as disclosed in Example 9) as seen inFIG. 3 clearly indicate a range of hue angles (colors) suitable for various applications, such as inks or coatings. Thus various properties of example PR57:1 pigments can be easily, including pigment particle size and dispersed pigment hue, by adjusting certain aspects of the PR57:1 composition and method of making, including, for example, reactant loading and stoichiometry, reactants rate of addition and concentration, stirring speed, temperature and various pigment work up variances, including types and volumes of pigment washes.

FIG. 4 shows the nearly linear correlation between NLSI and Z-average particle size indicating that the particle size distributions in the dispersions made with example PR57:1 pigments were retained in the coatings, after evaporation of n-butyl acetate by oven drying, as inferred by the NLSI data.

It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.