Aqueous dispersions of multiphase polymer particles

Abstract

The present invention relates to a composition comprising an aqueous dispersion of porous core-shell polymer particles having a water-occlusive core containing a core polymer phase comprising structural units of a salt of a carboxylic acid monomer, structural units of n-butyl acrylate, 2-ethylhexyl acrylate, or a combination thereof, structural units of methyl methacrylate, and a shell comprising one or more acrylate monomers, and styrene. The porous core-shell polymer is useful as a component of a basecoat interlayer for thermal paper products.

Description

[Technical Field]

[0001] The present invention relates to compositions comprising aqueous dispersions of multiphase polymer particles useful as porous hollow sphere pigments for coating applications.

[0002] Thermal paper is a multilayer recording material comprising a paper substrate, an intermediate heat insulating layer containing a binder and hollow sphere pigment (HSP), and an image-forming layer (see U.S. Pat. No. 10,730,334 B1). Printing performance in direct thermal printing applications is highly dependent on maximizing the porosity of the HSP. The greater the porosity, the less printing energy is required to produce an image. While porosity alone cannot predict printing performance, higher porosity results in thinner particle shells and a higher likelihood of particle collapse, simultaneously reducing printing performance. Therefore, in the field of thermal printing, it would be advantageous to improve printing performance by providing HSPs with increased porosity and collapse resistance. Summary of the Invention

[0003] The present invention addresses a need in the art in one aspect by providing a composition, the composition being an aqueous dispersion of porous core-shell polymer particles, comprising: a) a water-occlusive core containing a core polymer phase comprising: 1) 30 to 55 weight percent structural units of a salt of a carboxylic acid monomer; 2) 4.5 to 55 weight percent structural units of n-butyl acrylate, or 2-ethylhexyl acrylate, or a combination thereof; and 3) 4.5 to 55 weight percent structural units of methyl methacrylate; b) a shell comprising: 1) 3.4 to 16 wt. % structural units of one or more acrylate monomers selected from the group consisting of ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate; and 2) 80 to 96.4 wt. % structural units of styrene; The porous core-shell polymer particles have a number average particle size in the range of 750 nm to 2 μm.

[0004] The compositions of the present invention are precursors to porous HSPs that have high porosity and resistance to collapse. [Brief explanation of the drawings]

[0005] [Figure 1] 1 is a scanning electron micrograph of an intact porous hollow sphere pigment. [Figure 2] 1 is a scanning electron micrograph of a collapsed non-porous hollow sphere pigment. DETAILED DESCRIPTION OF THE INVENTION

[0006] In one aspect, the present invention provides a composition comprising an aqueous dispersion of porous core-shell polymer particles, the aqueous dispersion of porous core-shell polymer particles comprising: a) a water-occlusive core containing a core polymer phase comprising: 1) 30 to 55 weight percent structural units of a salt of a carboxylic acid monomer; 2) 4.5 to 55 weight percent structural units of n-butyl acrylate, or 2-ethylhexyl acrylate, or a combination thereof; and 3) 4.5 to 55 weight percent structural units of methyl methacrylate; b) a shell comprising 1) 3.4 to 16 wt. % structural units of one or more acrylate monomers selected from the group consisting of ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate, and 2) 80 wt. % to 96.4 wt. % structural units of styrene; The porous core-shell polymer particles have a number average particle size in the range of 750 nm to 3 μm.

[0007] All weight ranges for the recited structural units of the monomers in the core polymer phase are based on the weight of the core polymer phase. Similarly, all weight ranges for the recited structural units of the monomers in the shell are based on the weight of the shell.

[0008] Aqueous dispersions of porous, water-occluded polymer particles are advantageously prepared in multiple steps as follows: Non-water-occluded core polymer particles are prepared by copolymerizing a monomer emulsion (ME1) containing a carboxylic acid monomer, an acrylate monomer, and methyl methacrylate under emulsion polymerization conditions. The core polymer particles can be prepared by polymerizing ME1 in contact with seed polymer particles, typically a copolymer of methyl methacrylate and methacrylic acid. The ratio of seed polymer particles to ME1 is typically in the w / w range of 1:99 to 50:50. The core polymer particles can be isolated or used directly in the production of core-shell polymer particles.

[0009] The monomers used to form the shell may be polymerized in one or two steps. A one-step polymerization can be carried out as follows: After the addition of ME1 to the reactor is complete, a second monomer emulsion (ME2) containing 3.4 to 16 wt. % acrylate monomer and 80 to 96.4 wt. % styrene, based on the weight of the monomers in the second monomer emulsion, is added to the core polymer dispersion in the reactor under emulsion polymerization conditions. The weight-to-weight ratio of shell to core polymer phase is preferably in the range of 3:1 to 7:1. When the addition of ME2 is complete, hot deionized water and a neutralizing amount of base, such as NH4OH or an alkali metal hydroxide, such as NaOH, are added to the mixture. This neutralization step causes swelling of the particles with simultaneous occlusion of water in the core.

[0010] Two-stage polymerization can be accomplished as follows: After the reaction of ME1 and ME2 is complete and a suitable hold time (approximately 15 minutes), a third monomer emulsion (ME3) containing styrene is fed to the reactor in the presence of a radical inhibitor, such as 4-hydroxyTEMPO. After the addition of ME3 is complete, hot deionized water and a neutralizing amount of base are added to the mixture. The dispersion is advantageously chased with t-butyl hydroperoxide (t-BHP) and isoascorbic acid (IAA), and the contents are filtered to remove coagulum. The weight-to-weight ratio of ME3 to ME2 typically ranges from 0.08:1, or 0.1:1, to 0.5:1, 0.3:1, or 0.2:1. The water occlusive core contains from 30% by weight, preferably from 35% by weight, more preferably from 38% by weight, to 55% by weight, preferably to 45% by weight, more preferably to 42% by weight of a core polymer phase comprising structural units of a salt of a carboxylic acid monomer, preferably a salt of acrylic acid or methacrylic acid, more preferably a salt of methacrylic acid, most preferably sodium methacrylate or ammonium methacrylate. As used herein, the term "structural units" refers to the remainder of the listed monomers after polymerization. For example, M + is a counterion, preferably a lithium, sodium, potassium, or ammonium counterion, the structural unit of a salt of methacrylic acid is as shown below:

[0011] [ka]

[0012] The total contribution of butyl acrylate and 2-ethylhexyl acrylate structural units in the core polymer phase ranges from 4.5 to 55 wt%. Preferably, the core polymer phase comprises from 9 wt%, or from 12 wt%, to 55 wt%, to 50 wt%, or to 45 wt% of acrylate, preferably n-butyl acrylate, structural units. The core polymer phase further comprises from 9 wt%, or from 13 wt%, or from 18 wt%, to 55 wt%, to 50 wt%, or to 45 wt% of methyl methacrylate structural units. Preferably, at least 90 wt%, more preferably at least 95 wt%, and most preferably at least 99 wt% of the core polymer phase comprises structural units of methyl methacrylate, acrylate monomers, and salts of methacrylic acid.

[0013] The shell comprises from 3.4% by weight, preferably from 4% by weight, more preferably from 5% by weight, to 16% by weight, preferably to 15% by weight, more preferably to 13% by weight of structural units of an acrylate monomer, preferably n-butyl acrylate, and from 80% by weight, preferably from 84% by weight, more preferably from 85% by weight, most preferably from 87% by weight, to 96.4% by weight, preferably to 94% by weight, most preferably to 93% by weight of structural units of styrene.

[0014] The shell may further comprise structural units of acid monomer salts, such as ethylenically unsaturated carboxylic acid salts, including lithium, sodium, potassium, and ammonium salts of acrylic acid and methacrylic acid; structural units of ethylenically unsaturated sulfonates, such as sodium 4-vinylbenzenesulfonate (sodium styrenesulfonate); and structural units of phosphorus acid salts, such as phosphoethyl methacrylate (PEM). The concentration of the acid monomer salt structural units in the shell is preferably in the range of from 0.1% by weight, 0.5% by weight, 1.0% by weight, or 1.5% by weight to 5% by weight, 3.5% by weight, or 3.0% by weight. The shell preferably comprises structural units of a salt of methacrylic acid, more preferably a salt of sodium methacrylate or ammonium methacrylate.

[0015] The shell may further comprise structural units of a polyethylenically unsaturated monomer, such as allyl methacrylate (ALMA) or divinylbenzene. The concentration of the structural units of the polyethylenically unsaturated monomer in the shell preferably ranges from 0.05% by weight, more preferably from 0.1% by weight to 1% by weight, more preferably to 0.5% by weight.

[0016] The weight to weight ratio of shell to core polymer phase preferably ranges from 2.5:1, from 3.0:1, from 3.5:1, or from 3.8:1, to 7:1, to 6.5:1, to 6.0:1, to 5.0:1, to 4.5:1, or to 4.2:1.

[0017] The resulting porous core-shell polymer particles have a number average particle size ranging from 750 nm, 900 nm, or 1.1 μm, to 2 μm, to 1.8 μm, or to 1.5 μm, as measured by scanning electron microscopy. The solids content of the aqueous dispersion of the core-shell porous polymer particles is preferably in the range of 10 to 20 wt. %.

[0018] Surprisingly, it was discovered that the inclusion of acrylate monomer structural units in the core polymer phase and shell is essential for preparing porous HSPs with high porosity and collapse resistance. The inclusion of acrylate monomer structural units increases the T of the polymer phase. g The T value is reduced, thereby providing a method for preparing a water-occluded HSP precursor. However, the shell (ME3) plasticized by the presence of unreacted styrene does not yield an HSP with comparable porosity and collapse resistance. g The decrease in β alone cannot explain the improved efficiency of HSP.

[0019] The composition of the present invention is useful as a component of a basecoat intermediate layer for a thermal recording material. The basecoat formulation is prepared by blending an aqueous dispersion of porous, water-occlusive core-shell polymer particles with one or more binders, such as styrene butadiene latex, styrene acrylic latex, or polyvinyl alcohol. The basecoat formulation optionally contains an additional pigment, such as calcined clay. The basecoat formulation is then coated onto the surface of a sheet of paper, followed by drying and conditioning by means well known in the art. A thermal layer is then coated onto the basecoat and further dried. Therefore, another aspect of the present invention is a thermal recording material comprising a paper substrate, a basecoat layer having a thickness of 1 μm to 20 μm or 10 μm, comprising the composition of the present invention and a binder, and a thermal recording layer having a thickness of 1 μm to 30 μm, the basecoat layer being disposed between the thermal recording layer and the paper substrate.

[0020] The dried basecoat comprises a porous HSP having a porosity ranging from 50%, preferably from 55%, more preferably from 60%, more preferably from 65%, and most preferably from 70%, up to 80%, or up to 75%. As used herein, "porous" refers to one or more channels extending from the surface of the HSP to the void portion. The HSP comprises, based on the total weight of the core polymer phase and the shell, from 60 wt.%, from 64 wt.%, from 66 wt.%, from 80 wt.%, from 78 wt.%, or from 76 wt.% styrene structural units; from 1 wt.%, from 3.5 wt.%, from 7 wt.%, or from 9 wt.%, from 20 wt.%, or from 18 wt.% acrylate monomer, preferably n-butyl acrylate structural units; from 0.5 wt.%, from 2 wt.%, from 4 wt.%, from 15 wt.%, or from 10 wt.% methyl methacrylate structural units; and from 5 wt.%, or from 6 wt.%, from 20 wt.%, from 15 wt.%, or from 10 wt.% methacrylic acid or a salt thereof, preferably methacrylic acid or sodium methacrylate structural units. [Example]

[0021] Preparation of seed polymer dispersion Deionized water (1948.40 g) and sodium dodecylbenzenesulfonate (NaDDBS, 1.78 g, 22.5% in water) were charged to a 5 L four-neck round-bottom flask and heated to 85 °C under N2. In a separate container, a monomer emulsion containing deionized water (715.00 g), NaDDBS (20.00 g, 22.5% in water), methyl methacrylate (MMA, 780.00 g), and methacrylic acid (MAA, 10.00 g) was prepared. A portion of the monomer emulsion (10.8%, 165.00 g) was charged to the reactor and rinsed with deionized water (35 g). After removing this portion of the monomer emulsion, additional deionized water (50 g), NaDDBS (14.80 g), and MAA (510.00 g) were added to the monomer emulsion. A solution of sodium persulfate (5.50 g) in deionized water (30 g) was charged to the reactor. An exotherm was observed and the peak temperature was held for 15 minutes. The remainder of the ME was fed to the reactor over 120 minutes with the temperature set at 85°C. Upon completion of the feeds, the addition vessel was rinsed with deionized water (50 g) and the reaction was held at 85°C for 20 minutes. The reactor contents were then cooled to room temperature and filtered to remove coagulum. The resulting dispersion had a solids content of 31.2% and a particle size of 187 nm.

[0022] Intermediate Example 1 - Preparation of a Dispersion of Core Polymer Particles Deionized water (1301.00 g) and glacial acetic acid (0.51 g) were charged to a 5 L four-neck round-bottom flask and heated to 91°C under N2. In a separate container, a monomer emulsion was prepared containing deionized water (827.34 g), Disponil FES-993 surfactant (31% active, 13.87 g), MMA (770.00 g), butyl acrylate (BA, 140.00 g), and MAA (490.00 g). An initiator solution of sodium persulfate (3.50 g) in deionized water (150 g) was also prepared. A solution of sodium persulfate (3.50 g) in deionized water (45 g) was charged to the reactor and rinsed with deionized water (10 g). A portion of the seed polymer dispersion (157.05 g, 31.2% solids, 187 nm) was charged to the reactor and rinsed with deionized water (30 g). The monomer emulsion and initiator solution were then fed into the reactor over 120 minutes, with the monomer emulsion fed at a 50% rate for the first 20 minutes, while the reaction was maintained at 83°C. After the feeds were completed, the vessel was rinsed with deionized water (60 g), and the reaction was held at 83°C for 30 minutes. The reactor contents were then cooled to room temperature and filtered to remove coagulum. The resulting core polymer dispersion had a solids content of 36.4% and a particle size of 552 nm.

[0023] The core polymer dispersions of Intermediate Examples 2-5, and Comparative Intermediate Examples 1 and 2 were prepared according to the procedure of Intermediate Example 1. The weights of BA and MMA used to prepare the cores from the seed polymer dispersions, as well as the solids content and particle size, are shown in Table 1. The particle size was the z-average particle size measured by dynamic light scattering.

[0024] [Table 1]

[0025] Example 1: Preparation of water-occluded core-shell polymer particles, 6% BA in the shell Deionized water (2650.00 g) and glacial acetic acid (0.20 g) were charged to a 5 L four-neck round-bottom flask and heated to 96 °C under N2. In a separate container, a monomer emulsion was prepared containing deionized water (148.89 g), NaDDBS (1.71 g, 22.5% in water), styrene (STY, 367.20 g), BA (24.00 g), allyl methacrylate (ALMA, 0.80 g), and MAA (8.00 g). An initiator solution of sodium persulfate (1.51 g) in deionized water (76.80 g) was also prepared. A solution of sodium persulfate (0.76 g) in deionized water (10.40 g) was charged to the reactor and rinsed with deionized water (4 g). A portion of the core polymer dispersion from Intermediate Example 1 (273.45 g, 36.6% solids, 541 nm) was charged to the reactor and rinsed with deionized water (32 g). The monomer emulsion and initiator solution were then fed into the reactor over 120 minutes while the reaction was maintained at 90° C. After the feeds were completed, the vessel was rinsed with deionized water (28 g total), and the reaction was held at 90° C. for 15 minutes. A solution of iron sulfate heptahydrate (12.12 g of a 0.15% solution) and VERSENE™ chelating agent (trademark of The Dow Chemical Company or its affiliates, 1.0% solution, 1.90 g) was added to the reactor. A solution of t-butyl hydroperoxide (t-BHP, 70% solution, 2.40 g in 20 g deionized water) was added to the reactor, followed by the gradual addition of isoascorbic acid (0.66 g IAA in 38.40 g deionized water) over 15 minutes. A neutralizer solution prepared from deionized water (120 g), NaDDBS (10.68 g, 22.5% in water), and ammonium hydroxide (30%, 35.56 g) was gradually added to the reactor over 15 minutes. The vessel containing the neutralizer solution was rinsed with deionized water (16 g) and added to the reactor. The reaction temperature was maintained at 90°C for 60 minutes, after which the reactor contents were cooled to room temperature and filtered to remove coagulum. The resulting dispersion had a solids content of 13.0%, a porosity of 70.4%, and a particle size of 1.35 μm.

[0026] Examples 2-4 and Comparative Examples 1 and 2 were prepared essentially as described in Example 1, except that the core polymer dispersions of the corresponding intermediate examples and comparative examples were used. For each example, the shell was prepared using BA (6 wt%), styrene (91.8 wt%), acrylic acid (2.0 wt%), and ALMA (0.8 wt%). The concentration of ammonium methacrylate structural units was held constant at 39.2 wt% based on the weight of the core. The void fraction (VF%), optical evaluation (OR), and optical density at 0.25 mJ / dot (OD) were measured for each example by the following procedures.

[0027] Measurement of particle size and determination of disintegration rating The size of HSPs was measured based on scanning electron micrographs (SEM). Two drops of emulsion were drop-cast onto conductive carbon tape on an aluminum SEM tab. After drying for 2 h at ambient temperature, the samples were coated with a thin layer of chromium in an EMS 150T ES metal coater using a sputtering current of 100 mA for 100 s. SEM images were acquired at 5 kV accelerating voltage from a Schottky field-emission electron source using an Everhart-Thornley secondary electron detector in a Thermo Fisher Nova NanoSEM 630 scanning electron microscope. All images were acquired at 20,000x magnification with an image size of 1024 × 884 pixels and a bit depth of 8 (grayscale ranged from 0 to 255, with 0 being the darkest and 255 being the brightest). All images had a horizontal field of view of 7.46 μm and a pixel size of 7.28 nm. Each image contained 40–60 HSPs. Two images for each sample were analyzed using ImageJ software (version 1.53c). Diameters were manually measured for all particles within the image, excluding particles at the edge of the image. A number-weighted average particle size was reported. The number of disintegrated particles was counted and compared to the total number of particles based on the percentage of disintegrated particles, and a disintegration rating was assigned as follows: Rating 1: Less than 10% of particles disintegrate Rating 2: 10% to 50% disintegrated particles Rating 3: Disintegrated particles > 50%

[0028] Porosity measurement Particle porosity was determined as follows: An aqueous dispersion of porous, water-occluded core-shell polymer particles (40 g) was added to a 50 mL polypropylene centrifuge. The tube was placed in the centrifuge and spun at 18,500 rpm for 120 minutes. The clear supernatant was decanted from the hard pack and weighed. From the latex mass, percent solids, and supernatant mass, the percent void fraction (VF%) was determined using the following formula:

[0029]

number

[0030] Preparation of basecoat and thermal recording formulation A portion of Example 2 (154.4 g), RHOPLEX™ P-308 styrene acrylic binder (7.5 g, 50 wt. % solids, trademark of The Dow Chemical Company or its affiliates), and polyvinyl alcohol (8.3 g, 15 wt. % in demineralized water, cat. #67710 Kremer Pigmente) were mixed in a container equipped with an overhead blade mixer and then diluted with deionized water (22.0 g) to a solids content of 13 wt. % to form a basecoat formulation. The basecoats of the other examples and Comparative Example 1 were made using equal dry weights of all components.

[0031] The thermosensitive recording layer was prepared from formulations and materials obtained from Nissho Kogyo Co., Ltd. Deionized water (51.6 g) was placed in an 8-ounce container, followed by the addition, in order, of Tunex-E precipitated calcium carbonate (4.4 g), P-603 Mizucasil silicon dioxide (3.7 g), PVA-203 buffer solution (1.0 g, Kuraray, 15 wt%), D-8 4-hydroxy-4'-isopropoxydiphenyl sulfone developer (8.8 g, Mitsubishi, 50 wt%), 2-benzyl-oxy-naphthalene sensitizer (4.0 g, 40 wt%), PVA-117 binder (15.8 g, Kuraray, 10 wt%), zinc stearate lubricant (3.1 g, 36 wt%), and PSD-290 2-anilino-6-(dibutylamino)-3-methylfluoran dye (5.7 g, Mitsubishi, 35 wt%), and mixing with an overhead blade mixer.

[0032] Preparation of thermal recording products NewPage free sheet paper (basis weight: 58 g / m 2 A sheet of paper (roughness: 4.00 μm, Gurley porosity: 24.6 s) was cut to 37.9 cm × 20.1 cm with the long dimension in the machine direction and then placed in a controlled temperature room (22°C, 50% humidity) for at least 2 hours. Using masking tape, the paper was taped to a piece of copy paper, which was then attached to a hand drawdown plate with the masking tape. A bead of base coat formulation was then pipetted onto the masking tape on the free sheet. A wire-wrapped rod was then manually lowered onto the strip of base coat formulation and moved across the paper to ensure the paper was evenly coated. The paper was then exposed to hot air for 45 seconds, after which the paper was transferred to an oven and dried at 80°C for an additional 45 seconds. After drying, the paper was conditioned in a controlled temperature room (22°C and 50% humidity) for 2 hours. This paper with the base coat was then coated with a thermosensitive layer using the same procedure used to apply the base coat and dried at 80°C for 1 minute.

[0033] Optical density measurements The fully coated paper was cut in the machine direction into two 2.5 inch (1 cm) wide strips. The two strips were taped end to end and printed using an Atlantek Paper Tester Model 200 under the following conditions: a) Sequence dot pulse duration = 0.8ms b) Full cycle time (T cycle )=5.0ms c) Print head temperature = 30°C d) Printhead resistance = 583 ohms at an applied voltage of 20.6V

[0034] A 50% 80x80 checkerboard pattern was printed at print energies of 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, and 0.50 mJ / dot. The optical density of three boxes was measured for each print energy using a handheld X-rite 428 spectrodensitometer.

[0035] Table 2 shows the void fraction (VF), disintegration rating (CR) and particle size (PS) for the HSPs produced from the porous water-occluding dispersions of Examples 1 to 4 and Comparative Examples 1 and 2, as well as the optical density (OD) at 0.25 mJ / dot for the corresponding thermal recording materials.

[0036] [Table 2] * Globular HSPs not produced in this experiment

[0037] In the next series of experiments, dispersions of water-occluded core-shell polymer particles (Examples 5-7 and Comparative Example 3) were prepared as in Example 2, with different weight percentages of STY and BA, or STY and EA, used in the preparation of the shell. The w / w ratio of shell:core was 4:1 for each example. Table 3 shows the relevant properties of the HSPs resulting from these dispersions.

[0038] [Table 3] * Incomplete swelling of water-absorbing polymer particles.

[0039] The data demonstrate that the presence of acrylate monomer structural units in the shell and core improves desired properties. Figures 1 and 2 demonstrate the dramatic difference in HSP integrity between the HSPs prepared from the dispersions of Example 6 and Comparative Example 1. Over 90% of the HSPs resulting from water-occluded core-shell particles made with BA in the core and shell stages are porous and retain spherical integrity. In contrast, a substantial portion of the HSPs resulting from water-occluded core-shell particles prepared without BA in the core stage are non-porous and collapse into cup-shaped hemispheres.

[0040] Example 8-18.7 Alternative Preparation of BA Core / 6 BA Shell Polymer Particles A dispersion of water-occluded porous polymer particles having the composition of Example 2 was prepared by the following alternative process. Deionized water (2600.00 g) and glacial acetic acid (0.20 g) were charged to a 5 L four-neck round-bottom flask and heated to 96 °C under N2. ME1, containing deionized water (134.00 g), NaDDBS (1.54 g, 22.5% in water), STY (330.48 g), BA (21.60 g), ALMA (0.72 g), and MAA (7.20 g), was prepared in a separate first container, and ME2, containing deionized water (15.77 g), NaDDBS (0.18 g, 22.5% in water), and STY (40.00 g), was prepared in a second container. An initiator solution of sodium persulfate (1.51 g) in deionized water (76.80 g) was also prepared. A solution of sodium persulfate (0.76 g) in deionized water (10.40 g) was charged to the reactor and rinsed with deionized water (4 g). A portion of the core polymer dispersion (273.45 g) from Intermediate Example 2 was charged to the reactor and rinsed with deionized water (32 g). ME1 and the initiator solution were then fed to the reactor over 120 minutes at 90°C, after which the vessel was rinsed with deionized water (28 g total). A solution of iron sulfate heptahydrate (5.33 g of a 0.15% solution) and VERSENE™ chelating agent (0.80 g of a 1.0% solution) was added to the reactor, and the reaction temperature was maintained at 90°C for 15 minutes. After the hold, a solution of 4-hydroxy-TEMPO (2.40 g, 5% active) was added to the reactor. ME2 was then charged to the reactor and rinsed with deionized water (24 g). A neutralizer solution was prepared from deionized water (120 g), NaDDBS (10.68 g, 22.5% in water), and ammonium hydroxide (30%, 33.42 g). The neutralizer solution was added to the reactor over 15 minutes, and the container containing the solution was rinsed with deionized water (16 g), which was then added to the reactor. The reactor contents were held at 90°C for 15 minutes, after which a solution of t-BHP (70% solution, 2.40 g in 12 g deionized water) was added to the reactor, followed by IAA (1.33 g in 76.80 g deionized water) over 15 minutes, followed by a deionized water rinse (4 g). The reactor contents were then cooled to room temperature and filtered to remove coagulum. The dispersion had a solids content of 13.1% and a pH of 9.1.The HSP had a porosity of 74.1%, a disintegration rating of 2, and particles of 1.42 μm. The optical density of the resulting thermal recording material at 0.25 mJ / dot was 0.97.

[0041] Examples 9-18. Alternative Preparation of 7BA Core / 12BA Shell Polymer Particles A dispersion of water-absorbing porous polymer particles was prepared by the process described in Example 8, except that the amounts of STY (308.88 g) and BA (43.2 g) in ME1 were changed. The dispersion had a solids content of 13.3% and a pH of 9.2. The HSP had a porosity of 71.4%, a disintegration rating of 1, and a particle size of 1.38 μm. The optical density of the resulting thermal recording material at 0.25 mJ / dot was 0.99.

Claims

1. 1. A composition comprising an aqueous dispersion of porous core-shell polymer particles, the aqueous dispersion of porous core-shell polymer particles comprising: a) a water occlusive core containing a core polymer phase comprising: 1) 30 to 55 weight percent structural units of a salt of a carboxylic acid monomer; 2) 4.5 to 55 weight percent structural units of n-butyl acrylate, or 2-ethylhexyl acrylate, or a combination thereof; and 3) 4.5 to 55 weight percent structural units of methyl methacrylate; b) a shell comprising: 1) 3.4 to 16 weight percent structural units of one or more acrylate monomers selected from the group consisting of ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate; and 2) 80 to 96.4 weight percent structural units of styrene; A composition wherein said porous core-shell polymer particles have a number average particle size in the range of 750 nm to 2 μm.

2. the core polymer phase comprises 35 to 45 weight percent structural units of a salt of a carboxylic acid monomer, 9 to 50 weight percent structural units of n-butyl acrylate, or 2-ethylhexyl acrylate, or a combination thereof, and 9 to 50 weight percent structural units of methyl methacrylate; the shell comprises 4 to 15 wt. % structural units of a salt of one or more acrylate monomers and 84 to 94 wt. % structural units of styrene; the porous core-shell polymer particles have a number average particle size in the range of 900 nm to 2 μm; 2. The composition of claim 1, wherein the weight to weight ratio of the shell to the core polymer phase is preferably in the range of 2.5:1 to 7:

1.

3. the core polymer phase comprising 38 to 42 weight percent structural units of a salt of a carboxylic acid monomer, wherein the salt of the carboxylic acid monomer is ammonium methacrylate or sodium methacrylate; 12 to 50 weight percent structural units of n-butyl acrylate, or 2-ethylhexyl acrylate, or a combination thereof; and 13 to 50 weight percent structural units of methyl methacrylate; 3. The composition of claim 2, wherein the shell comprises 4 to 13 weight percent of the structural units of one or more acrylate monomers methyl methacrylate and 85 to 94 weight percent of the structural units styrene.

4. the shell further comprises 0.1 to 5 wt. % of structural units of an acid monomer salt and 0.05 to 1 wt. % of structural units of a multi-ethylenically unsaturated monomer, and the porous core-shell polymer particles have a number average particle size in the range of 1.1 μm to 1.5 μm; 4. The composition of claim 3, wherein the weight to weight ratio of the shell to core polymer phase ranges from 3.0:1 to 6.0:

1.

5. the shell comprises 5 to 13 weight percent structural units of n-butyl acrylate, 85 to 93 weight percent structural units of styrene, 1 to 3.5 weight percent structural units of the salt of the acid monomer, wherein the salt of the acid monomer is ammonium methacrylate or sodium methacrylate; and 0.1 to 0.5 weight percent of the multi-ethylenically unsaturated monomer, wherein the multi-ethylenically unsaturated monomer is allyl methacrylate; 5. The composition of claim 4, wherein the weight to weight ratio of the shell to core polymer phase is in the range of 3.5:1 to 4.5:

1.

6. 1. A composition comprising an aqueous dispersion of porous core-shell polymer particles, the aqueous dispersion of porous core-shell polymer particles comprising: a) a water-occlusive core containing a core polymer phase comprising: 1) 35 to 45 weight percent structural units of ammonium or sodium methacrylate; 2) 12 to 45 weight percent structural units of n-butyl acrylate; and 3) 18 to 45 weight percent structural units of methyl methacrylate, based on the weight of the core polymer phase, wherein at least 95 weight percent of the core polymer phase comprises structural units of ammonium or sodium methacrylate, n-butyl acrylate, and methyl methacrylate; b) a shell comprising, based on the weight of the shell, 1) 5 to 15 weight percent of structural units n-butyl acrylate, and 2) 85 to 93 weight percent of structural units styrene; and a shell, wherein the porous core-shell polymer particles have a number average particle size in the range of 1 μm to 2 μm, and the weight to weight ratio of the core polymer phase to the shell is in the range of 3.0:1 to 5.0:

1.

7. 7. The composition of claim 6, wherein at least 99 wt. % of the core polymer phase comprises a) ammonium or sodium methacrylate structural units, b) n-butyl acrylate structural units, and c) methyl methacrylate structural units, and the shell further comprises 0.1 to 0.5 wt. % of allyl methacrylate structural units and 0.5 to 3.5 wt. % of structural units of a sodium or ammonium salt of acrylic or methacrylic acid, based on the weight of the shell, wherein the weight to weight ratio of the shell to the core polymer phase is in the range of 3.5:1 to 4.5:1, and the porous core-shell polymer particles have a number average particle size in the range of 1.0 μm to 1.8 μm.

8. 8. The composition of claim 7, wherein the sodium or ammonium salt of acrylic or methacrylic acid is sodium methacrylate or ammonium methacrylate, and the porous core-shell polymer particles have a number average particle size in the range of 1.1 μm to 1.5 μm.

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