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-in-water core comprising a core polymer phase, the core polymer phase comprising at least one polymer phase; the core polymer phase comprises a structural unit of a salt of a carboxylic acid monomer, a structural unit of n-butyl acrylate or 2-ethylhexyl acrylate or a combination thereof, and a structural unit of methyl methacrylate; and a shell comprising one or more acrylate monomers, and styrene. The porous core-shell polymer can be used as a component of an undercoat interlayer of a heat-sensitive paper product.

Description

Background Art

[0001] The present invention relates to a composition comprising an aqueous dispersion of multiphase polymer particles, which multiphase polymer particles can be used as porous hollow sphere pigments for coating applications.

[0002] Thermal paper is a multi-layer recording material that includes a paper web, an intermediate insulating layer containing a binder and hollow sphere pigments (HSPs), and an image-forming layer (see US10,730,334B1). The printing performance in direct thermal printing applications depends largely on maximizing the porosity of the HSPs. The greater the porosity, the less printing energy is required to create an image. Porosity alone does not predict printing performance, however: higher porosity results in thinner particle shells and an increased likelihood of particle collapse, along with a decrease in printing performance. Thus, in the field of thermal printing, it would be an advantage to improve printing performance by providing HSPs with increased porosity and collapse resistance. Summary of the Invention

[0003] In one aspect, the present invention addresses the need in the art by providing a composition comprising an aqueous dispersion of porous core-shell polymer particles having:

[0004] a) a water-entrapped core containing a core polymer phase, the core polymer phase comprising 1) structural units of a salt of 30 wt% to 55 wt% of a carboxylic acid monomer; 2) structural units of 4.5 wt% to 55 wt% of n-butyl acrylate or 2-ethylhexyl acrylate or a combination thereof; and 3) structural units of 4.5 wt% to 55 wt% of methyl methacrylate;

[0005] b) a shell comprising 1) structural units of 3.4 wt% to 16 wt% of one or more acrylate monomers selected from the group consisting of ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate;

[0006] and 2) structural units of 80 wt% to 96.4 wt% of styrene;

[0007] wherein the porous core-shell polymer particles have a number-average particle size in the range of 750 nm to 2 μm.

[0008] The composition of the present invention is a precursor of porous HSPs with high porosity and collapse resistance. Brief Description of the Drawings

[0009] Figure 1 is a scanning electron micrograph of a complete porous hollow sphere pigment.

[0010] Figure 2It is a scanning electron micrograph of collapsed non-porous hollow sphere pigments. Detailed Description of the Invention

[0011] In one aspect, the present invention is a composition comprising an aqueous dispersion of porous core-shell polymer particles having:

[0012] a) a water-entrapped core containing a core polymer phase, the core polymer phase comprising 1) structural units of a salt of a carboxylic acid monomer in an amount of 30 wt% to 55 wt%; 2) structural units of n-butyl acrylate or 2-ethylhexyl acrylate or a combination thereof in an amount of 4.5 wt% to 55 wt%; and 3) structural units of methyl methacrylate in an amount of 4.5 wt% to 55 wt%; and

[0013] b) a shell comprising 1) structural units of one or more acrylate monomers in an amount of 3.4 wt% to 16 wt%, the structural units of the one or more acrylate monomers being selected from the group consisting of ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate;

[0014] and 2) structural units of styrene in an amount of 80 wt% to 96.4 wt%;

[0015] wherein the porous core-shell polymer particles have a number average particle size in the range of 750 nm to 3 μm.

[0016] The weight ranges of the listed structural units of the monomers in the core polymer phase are all based on the weight of the core polymer phase; similarly, the weight ranges of the listed structural units of the monomers in the shell are all based on the weight of the shell.

[0017] The aqueous dispersion of porous water-entrapped polymer particles is advantageously prepared in multiple stages as follows: non-water-entrapped core polymer particles are prepared by copolymerizing a monomer emulsion (ME 1) under emulsion polymerization conditions: the monomer emulsion contains a carboxylic acid monomer, an acrylate monomer, and methyl methacrylate. The core polymer particles can be prepared by polymerizing ME 1 in the presence of seed polymer particles, which are typically a copolymer of methyl methacrylate and methacrylic acid. The ratio of seed polymer particles to ME1 is typically in the range of 1:99 to 50:50 w / w. The core polymer particles can be separated or directly used for the production of core-shell polymer particles.

[0018] The monomers used to form the shell can be polymerized in a single stage or in two stages. The single-stage polymerization can be carried out as follows: When the addition of ME 1 to the reactor is completed, a second monomer emulsion (ME 2) is added to the core polymer dispersion in the reactor under emulsion polymerization conditions. The second monomer emulsion contains 3.4 wt% to 16 wt% acrylate monomer and 80 wt% to 96.4 wt% styrene based on the weight of the monomers in the second monomer emulsion. The weight ratio of the shell to the core polymer phase is preferably in the range of 3:1 to 7:1. When the addition of ME 2 is completed, hot deionized water and a neutralizing amount of base (such as NH4OH) or alkali metal hydroxide (such as NaOH) are added to the mixture. This neutralization step causes particle swelling, accompanied by the entrapment of water in the core.

[0019] The two-stage polymerization can be completed as follows: When the reaction of ME 1 and ME 2 is completed and after a suitable holding time (about 15 minutes), a third monomer emulsion (ME 3) containing styrene is fed into the reactor in the presence of a radical inhibitor (such as 4-hydroxy TEMPO). When the addition of ME 3 is completed, hot deionized water and a neutralizing amount of base are added to the mixture. Advantageously, the dispersion is traced with tert-butyl hydroperoxide (t-BHP) and isoascorbic acid (IAA), and the contents are filtered to remove any coagulum. The weight ratio of ME 3 to ME 2 is generally in the range of 0.08:1 or 0.1:1 to 0.5:1 or 0.3:1 or 0.2:1. The water-entrapped core contains a core polymer phase that contains 30 wt%, preferably 35 wt%, and more preferably 38 wt% to 55 wt%, preferably 45 wt%, more preferably 42 wt% of structural units of the salt of a carboxylic acid monomer, the salt of the carboxylic acid monomer being 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 unit" refers to the residue of the monomer after polymerization. For example, a structural unit of a salt of methacrylic acid, where M + is a counterion, preferably a lithium counterion, a sodium counterion, a potassium counterion, or an ammonium counterion, as shown below:

[0020]

[0021] The total contribution of the structural units of butyl acrylate and 2-ethylhexyl acrylate in the core polymer phase is in the range of 4.5 wt% to 55 wt%. Preferably, the core polymer phase contains 9 wt% or 12 wt% to 55 wt% or 50 wt% or 45 wt% of the structural units of an acrylate, which is preferably n-butyl acrylate. The core polymer phase also contains 9 wt% or 13 wt% or 18 wt% to 55 wt% or 50 wt% or 45 wt% of the structural units of methyl methacrylate. Preferably, at least 90 wt%, more preferably at least 95 wt%, and most preferably at least 99 wt% of the core polymer phase contains the structural units of a salt of methacrylic acid, an acrylate monomer, and methyl methacrylate.

[0022] The shell contains 3.4 wt%, preferably 4 wt%, and more preferably 5 wt% to 16 wt%, preferably 15 wt%, more preferably 13 wt% of the structural units of an acrylate monomer, preferably n-butyl acrylate, and 80 wt%, preferably 84 wt%, more preferably 85 wt%, and most preferably 87 wt% to 96.4 wt%, preferably 94 wt%, and most preferably 93 wt% of the structural units of styrene.

[0023] The shell may also contain the structural units of an acid monomer salt, such as an ethylenically unsaturated carboxylate salt, including lithium salts, sodium salts, potassium salts, and ammonium salts of acrylic acid and methacrylic acid; ethylenically unsaturated sulfonates, such as sodium 4-vinylbenzenesulfonate (sodium styrenesulfonate); and phosphates, such as ethyl phosphate methacrylate (PEM). The concentration of the structural units of the acid monomer salt in the shell is preferably in the range of 0.1 wt% or 0.5 wt% or 1.0 wt% or 1.5 wt% to 5 wt% or 3.5 wt% or 3.0 wt%. The shell preferably contains the salt of methacrylic acid, more preferably the structural units of sodium methacrylate or ammonium methacrylate.

[0024] The shell may also contain the 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 is preferably in the range of 0.05 wt%, more preferably 0.1 wt% to preferably 1 wt%, more preferably 0.5 wt%.

[0025] The weight ratio of the shell to the core polymer phase is preferably in the range of 2.5:1 or 3.0:1 or 3.5:1 or 3.8:1 to 7:1 or 6.5:1 or 6.0:1 or 5.0:1 or 4.5:1 or 4.2:1.

[0026] As measured by scanning electron microscopy, the resulting porous core-shell polymer particles have a number average particle size diameter in the range of 750 nm or 900 nm or 1.1 μm to 2 μm or 1.8 μm or 1.5 μm. The solids content of the aqueous dispersion of the core-shell porous polymer particles is preferably in the range of 10 wt% to 20 wt%.

[0027] It has been unexpectedly found that the inclusion of structural units of acrylate monomers in the core polymer phase and the shell is necessary for the preparation of porous HSPs with high porosity and anti-collapse properties. The inclusion of the structural units of acrylate monomers reduces the T g of the polymer phase, thereby providing a way to prepare the water-encapsulated HSP precursor. However, the reduction of only T g cannot explain the improvement of HSP efficiency, because the plasticized shell due to the presence of unreacted styrene (ME 3) does not produce HSPs with comparable porosity and anti-collapse properties.

[0028] The compositions of the present invention can be used as components for the undercoat intermediate layer of thermosensitive recording materials. The undercoat formulation is prepared by blending an aqueous dispersion of porous water-encapsulated core-shell polymer particles with one or more binders (such as styrene-butadiene latex, styrene-acrylic latex or polyvinyl alcohol). The undercoat formulation optionally contains additional pigments, such as calcined clay. Then the undercoat formulation is coated on the surface of the paper sheet and then dried and conditioned in a manner well known in the art. Then the thermosensitive layer is coated on the undercoat and further dried. Accordingly, another aspect of the present invention is a thermosensitive recording material comprising a paper web, a 1 μm to 20 μm or 10 μm thick undercoat layer comprising the composition of the present invention and a binder, and a 1 μm to 30 μm thick thermosensitive recording layer, wherein the undercoat layer is disposed between the thermosensitive recording layer and the paper web.

[0029] The dry base coat contains a porous HSP with a porosity in the range of 50%, preferably 55%, more preferably 60%, more preferably 65%, and most preferably 70% to 80% or 75%. As used herein, "porous" means one or more channels extending from the surface of the HSP to the void portion. The HSP contains structural units of styrene in an amount of 60% or 64% or 66% to 80% or 78% or 76% by weight based on the total weight of the core polymer phase and the shell, structural units of acrylate monomer in an amount of 1% or 3.5% or 7% or 9% to 20% or 18% by weight, preferably n-butyl acrylate, structural units of methyl methacrylate in an amount of 0.5% or 2% or 4% to 15% or 10% by weight, and structural units of methacrylic acid or its salt in an amount of 5% or 6% to 20% or 15% or 10% by weight, preferably structural units of methacrylic acid or sodium methacrylate.

[0030] Example

[0031] Preparation of seed polymer dispersion

[0032] Deionized water (1948.40 g) and sodium dodecylbenzenesulfonate (NaDDBS, 1.78 g, 22.5%, in water) were charged into a 5 L, 4-necked 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 (10.8%, 165.00 g) of the monomer emulsion was charged into 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 into the reactor. An exotherm was observed and maintained at the peak temperature for 15 minutes. The remainder of the ME was fed into the reactor over 120 minutes with the temperature set at 85 °C. After the feeding was completed, the addition vessel was rinsed with deionized water (50 g) and the reactants were maintained at 85 °C for 20 minutes. Then, the contents of the reactor were cooled to room temperature and filtered to remove any coagulum. The resulting dispersion had a solids content of 31.2% and a particle size of 187 nm.

[0033] Intermediate Example 1 - Preparation of dispersion of core polymer particles

[0034] Deionized water (1301.00 g) and glacial acetic acid (0.51 g) were charged into a 5 L, 4-necked round-bottom flask and heated to 91 °C under N2. In a separate container, a monomer emulsion 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) was prepared. An initiator solution of sodium persulfate (3.50 g) in deionized water (150 g) was prepared separately. The solution of sodium persulfate (3.50 g) in deionized water (45 g) was charged into the reactor and rinsed with deionized water (10 g). A portion of the seed polymer dispersion (157.05 g, 31.2% solids content, 187 nm) was charged into the reactor and rinsed with deionized water (30 g). Then the monomer emulsion and the initiator solution were fed into the reactor over 120 minutes, with the monomer emulsion being fed at a 50% rate in the first 20 minutes while maintaining the reactants at 83 °C. After the feeding was complete, the container was rinsed with deionized water (60 g) and the reactants were kept at 83 °C for 30 minutes. Then, the contents of the reactor were cooled to room temperature and filtered to remove any coagulum. The resulting core polymer dispersion had a solids content of 36.4% and a particle size of 552 nm.

[0035] The core polymer dispersions of Intermediate Examples 2 to 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 core from the seed polymer dispersion, as well as the solids content and particle size, are shown in Table 1. The particle size is the z-average particle size measured by dynamic light scattering.

[0036] Table 1 - Core Compositions of Intermediate and Comparative Intermediate Examples

[0037]

[0038] Example 1 - Preparation of water - entrapped core - shell polymer particles with 6% BA in the shell

[0039] Deionized water (2650.00 g) and glacial acetic acid (0.20 g) were charged into a 5 L, 4-necked round-bottom flask and heated to 96 °C under N2. In a separate container, a monomer emulsion 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) was prepared. An initiator solution of sodium persulfate (1.51 g) in deionized water (76.80 g) was additionally prepared. A solution of sodium persulfate (0.76 g) in deionized water (10.40 g) was charged into the reactor and rinsed with deionized water (4 g). A portion of the intermediate Example 1 core polymer dispersion (273.45 g, 36.6% solids content, 541 nm) was charged into the reactor and rinsed with deionized water (32 g). Then the monomer emulsion and the initiator solution were fed into the reactor within 120 minutes while maintaining the reactants at 90 °C. After the feeding was completed, the container was rinsed with deionized water (total 28 g), and the reactants were kept at 90 °C for 15 minutes. A solution of ferric sulfate heptahydrate (0.15% solution, 12.12 g) and VERSENE TM chelating agent (a trademark of The Dow Chemical Company or its affiliates, 1.0% solution, 1.90 g) was added to the reactor. A solution of tert-butyl hydroperoxide (t-BHP, 70% solution, 2.40 g, in 20 g deionized water) was added to the reactor, and then isoascorbic acid (IAA 0.66 g, in 38.40 g deionized water) was gradually added within 15 minutes. A neutralizing agent 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 reaction kettle within 15 minutes. The container containing the neutralizing agent solution was rinsed with deionized water (16 g), and the deionized water was added to the reactor. The reaction temperature was maintained at 90 °C for 60 minutes, and then the contents of the reactor 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.

[0040] Examples 2 to 4 and Comparative Examples 1 and 2 were prepared substantially as described in Example 1, except that the corresponding intermediate example and comparative example core polymer dispersions were used. For each example, BA (6 wt%), styrene (91.8 wt%), acrylic acid (2.0 wt%), and ALMA (0.8 wt%) were used to prepare the shell. Based on the weight of the core, the concentration of the structural unit of ammonium methacrylate was kept constant at 39.2 wt%. For each example, the void fraction (V.F.%), optical grade (O.R.), and optical density (O.D.) at 0.25 mJ / point were measured by the following procedure.

[0041] Measurement of particle size and determination of collapse grade

[0042] The size of the HSPs was measured based on scanning electron micrographs (SEM). Two drops of the emulsion were drop-coated onto a conductive carbon tape on an aluminum SEM sheet. After drying at ambient temperature for 2 h, a thin chromium layer was coated on the sample in an EMS150T ES metal coater using a sputtering current of 100 mA for 100 s. In a Thermo Fisher Nova NanoSEM 630 scanning electron microscope, SEMs were obtained using an Everhart-Thornley secondary electron detector at an acceleration voltage of 5 kV from a Schottky field emission electron source. All images were obtained at a magnification of 20,000 times, with an image size of 1024 × 884 pixels and a bit depth of 8 (gray scale ranging from 0 to 255, where 0 is the darkest and 255 is 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 to 60 HSPs. Two images were analyzed for each sample using ImageJ software (version 1.53c). The diameters of all particles in the image except those at the edge of the image were manually measured. The number-average particle size was recorded. The number of collapsed particles was counted and compared with the total number of particles, and a collapse grade was assigned based on the fraction of collapsed particles as follows:

[0043] Grade 1: <10% collapsed particles

[0044] Grade 2: 10% to 50% collapsed particles

[0045] Grade 3: >50% collapsed particles

[0046] Determination of porosity

[0047] The particle porosity is determined as follows. An aqueous dispersion (40 g) of the porous water-entrapping core-shell polymer particles is added to a 50 mL polypropylene centrifuge tube. The tube is placed in a centrifuge and centrifuged at 18,500 rpm for 120 minutes. The clear supernatant is decanted from the compacted packing and weighed. The porosity percentage (VF%) is determined using the following formula based on the latex mass, solids percentage, and supernatant mass:

[0048]

[0049] W T = total weight of the sample in the tube. The polymer density is approximately 1 g / cm 3 , so weight is used in place of volume.

[0050] Solids% = solids content of the latex.

[0051] k = packing factor for random packing of non-swollen monodisperse spheres, i.e., 0.675. The packing factor accounts for some water being trapped between the spheres in the compacted packing.

[0052] Preparation of undercoat and thermosensitive recording formulation

[0053] The primer formulation is prepared by mixing a portion (154.4 g) of Example 2, RHOPLEX TM P-308 styrene-acrylic binder (7.5 g, 50 wt% solids, a trademark of The Dow Chemical Company or its affiliates) and polyvinyl alcohol (8.3 g, 15 wt%, in demineralized water, catalog #67710 Kremer Pigmente) in a container with an overhead blade mixer, and then diluting with deionized water (22.0 g) to adjust to 13 wt% solids. The primers for the other examples and Comparative Example 1 are prepared using equal dry weights of all components.

[0054] The thermosensitive recording layer is prepared from materials and formulations obtained from Nissho Kogyo Co, Ltd. Deionized water (51.6 g) was placed in an 8 oz container, and then Tunex-E precipitated calcium carbonate (4.4 g), P-603 Mizucasil silica (3.7 g), PVA-203 buffer (1.0 g, Kuraray 15 wt%), D-8 4-hydroxy-4'-isopropoxydiphenyl sulfone color former (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%) were added in sequence and mixed with a top-mounted blade mixer.

[0055] Preparation of thermosensitive recording article

[0056] NewPage freesheet paper (basis weight: 58 g / m 2 , roughness: 4.00 μm, Gurley porosity: 24.6 s) was cut into 37.9 cm × 20.1 cm with the long side in the longitudinal direction and then placed in a controlled temperature chamber (22 °C and 50% humidity) for at least 2 hours. The paper was adhered to a sheet of copy paper with masking tape, and the copy paper was attached to a manual doctor blade with masking tape. Then, a drop of the undercoat formulation was pipetted onto the masking tape located above the freesheet paper. Then the wire wound rod was manually moved down onto the undercoat formulation strip and across the paper to coat the paper evenly. Then the paper was exposed to hot air for 45 s, after which the paper was transferred to an oven and dried at 80 °C for another 45 s. After drying, the paper was conditioned in a controlled temperature chamber (22 °C and 50% humidity) for 2 hours. Then the thermosensitive layer was coated on the paper with the undercoat using the same procedure as for applying the undercoat and dried at 80 °C for 1 minute.

[0057] Measurement of optical density

[0058] The fully coated paper was cut longitudinally into two 2.5” (1 cm) wide strips. The two strips were joined end to end, and a type 200 Atlantek paper tester was used to perform printing under the following conditions:

[0059] a) Sequence dot pulse duration = 0.8 ms

[0060] b) Full cycle time (T 循环 ) = 5.0 ms

[0061] c) Print head temperature = 30 °C

[0062] d) Print head resistance = 583 Ω at an applied voltage of 20.6 V

[0063] Print a 50% 80×80 chessboard pattern with print energies of 0.05 mJ / dot, 0.10 mJ / dot, 0.15 mJ / dot, 0.20 mJ / dot, 0.25 mJ / dot, 0.30 mJ / dot, 0.35 mJ / dot, 0.40 mJ / dot, 0.45 mJ / dot, and 0.50 mJ / dot. Measure the optical density of each print energy for 3 cartridges using a handheld X-rite 428 spectrophotometer.

[0064] Table 2 shows the void fraction (V.F.), collapse grade (C.R.), and particle size (PS) of the HSPs produced from the porous water-inclusion dispersions of Examples 1 to 4 and Comparative Examples 1 and 2, and the optical density (O.D.) of the corresponding thermosensitive recording materials at 0.25 mJ / dot.

[0065] Table 2 - Property dependence on changes in acrylates and MMA in the core

[0066]

[0067] * Spherical HSPs not produced in this experiment

[0068] In the next series of experiments, prepare dispersions of water-inclusion core-shell polymer particles as described in Example 2 (Examples 5 to 7 and Comparative Example 3), where different weight percentages of STY and BA or STY and EA are used to prepare the shell. For each example, the w / w ratio of shell:core is 4:1. Table 3 shows the relevant properties of the HSPs produced from these dispersions.

[0069] Table 3 - Property dependence on changes in acrylates and STY in the shell

[0070]

[0071] * Incomplete swelling of water-inclusion polymer particles

[0072] Data shows that the properties of interest are improved by providing structural units of acrylate monomers in the shell and core. Figure 1 and Figure 2Shows a significant difference in the HSP integrity of the HSPs prepared from the dispersions of Example 6 and Comparative Example 1. More than 90% of the HSPs produced from the water-in-oil core-shell particles prepared using BA in both the core and shell stages are porous and retain spherical integrity. In contrast, most of the HSPs produced from the water-in-oil core-shell particles prepared without using BA in the core stage are non-porous and collapse into cup-shaped hemispheres.

[0073] Example 8 - Alternative preparation of 18.7BA core / 6BA shell polymer particles

[0074] A dispersion of water-in-oil porous polymer particles having the composition of Example 2 was prepared by the following alternative method. Deionized water (2600.00 g) and glacial acetic acid (0.20 g) were charged into a 5 L, 4-necked 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 additionally prepared. A solution of sodium persulfate (0.76 g) in deionized water (10.40 g) was charged into the reactor and rinsed with deionized water (4 g). A portion (273.45 g) of the core polymer dispersion of Intermediate Example 2 was charged into the reactor and rinsed with deionized water (32 g). Then, at 90 °C, ME1 and the initiator solution were fed into the reactor over 120 minutes, after which the container was rinsed with deionized water (total 28 g). Ferric sulfate heptahydrate (0.15% solution, 5.33 g) and VERSENE TMA solution of a chelating agent (1.0% solution, 0.80 g) was added to the reaction vessel, and the reaction temperature was maintained at 90 °C for 15 minutes. After holding, a solution of 4-hydroxy-TEMPO (2.40 g, 5% active) was added to the reactor. Then ME2 was charged into the reactor and rinsed with deionized water (24 g). The neutralizing agent solution was prepared from deionized water (120 g), NaDDBS (10.68 g, 22.5%, in water), and ammonium hydroxide (30%, 33.42 g). The neutralizing agent solution was added to the reactor within 15 minutes, and the container containing the solution was rinsed with deionized water (16 g) and added to the reactor. The contents of the reactor 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, then IAA (1.33 g, in 76.80 g deionized water) was added within 15 minutes, and rinsed with a deionized water rinse (4 g). Then, the contents in the reactor were cooled to room temperature and filtered to remove any condensate. The dispersion had a solids content of 13.1% and a pH of 9.1. The HSP had a porosity of 74.1%, a collapse grade of 2, and a particle size of 1.42 μm. The resulting thermosensitive recording material had an optical density of 0.97 at 0.25 mJ / dot.

[0075] Example 9 - Alternative preparation of 18.7BA core / 12BA shell polymer particles

[0076] A dispersion of water-entrapped porous polymer particles was prepared by the method 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 collapse grade of 1, and a particle size of 1.38 μm. The resulting thermosensitive recording material had an optical density of 0.99 at 0.25 mJ / dot.

Claims

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

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

1.

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

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

1.

5. The composition according to claim 4, wherein the shell comprises structural units of n-butyl acrylate in an amount of 5% to 13% by weight; structural units of styrene in an amount of 85% to 93% by weight; structural units of the salt of the acid monomer in an amount of 1% to 3.5% by weight, wherein the salt of the acid monomer is ammonium methacrylate or sodium methacrylate; and structural units of the polyethylenically unsaturated monomer in an amount of 0.1% to 0.5% by weight, the polyethylenically unsaturated monomer being allyl methacrylate; and wherein the weight ratio of the shell to the core polymer phase is in the range of 3:5:1 to 4.5:

1.

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

1.

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

8. The composition according to claim 7, wherein the sodium or ammonium salt of acrylic acid or methacrylic acid is sodium methacrylate or ammonium methacrylate, and wherein 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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