Thermosensitive paper

Abstract

The invention relates to a coated paper product having an intermediate primer layer comprising high-voidage porous hollow-sphere polymer particles (HSP). HSP having very small pore area density and high voidage result in increased optical density at a given print energy in thermal printing applications.

Description

Technical Field

[0001] This invention relates to thermal paper. Background Technology

[0002] The present invention relates to thermal paper having a primer comprising porous hollow sphere pigment, particularly porous hollow sphere pigment with high porosity.

[0003] Thermal paper is a multilayer recording material comprising a paper blank, an intermediate insulating layer containing hollow sphere pigment (HSP), and an image-forming layer (see US 10,730,334 B1). Print performance in direct thermal printing applications largely depends on maximizing the porosity of the HSP. Higher porosity results in less printing energy required for creation and imaging. However, porosity alone cannot predict print performance. Therefore, in the field of thermal printing, providing HSPs that improve print performance would be advantageous. Summary of the Invention

[0004] This invention addresses a need in the art by providing a coated paper article comprising: a) Paper blanks with a thickness of 40µm to 500µm; b) A primer layer of 2µm to 10µm thickness, comprising porous, segmented polymer particles and a binder; and c) Thermal recording layers with a thickness of 1µm to 30µm; The primer layer is disposed between the thermal recording layer and the paper blank; The porous, segmented polymer particles have a shell-hollow core morphology, and the number-weighted average particle size is in the range of 500 nm to 3 µm. The shell comprises 0.5% to 10% by weight of structural units of carboxylic acid monomer salts and 90% to 99.5% by weight of structural units of styrene. The outer shell comprises at least 98% by weight of styrene structural units and structural units of the salt of the carboxylic acid monomer; and The percentage of pore area is in the range of 0.005% to 0.5% of the surface area of ​​the porous, segmented polymer particles.

[0005] The article of the present invention provides thermal paper with improved printing performance. Detailed Implementation

[0006] This invention relates to a coated paper product, comprising:

[0007] a) Paper blanks with a thickness of 40µm to 500µm; b) A primer layer of 2µm to 10µm thickness, comprising porous, segmented polymer particles and a binder; and c) Thermal recording layers with a thickness of 1µm to 30µm; The primer layer is disposed between the thermal recording layer and the paper blank; The porous, segmented polymer particles have a shell-hollow core morphology, and the number-weighted average particle size is in the range of 500 nm to 3 µm. The shell comprises 0.5% to 10% by weight of structural units of carboxylic acid monomer salts and 90% to 99.5% by weight of structural units of styrene. The outer shell comprises at least 98% by weight of styrene structural units and structural units of the salt of the carboxylic acid monomer; and The percentage of pore area is in the range of 0.005% to 0.5% of the surface area of ​​the porous, segmented polymer particles.

[0008] The primer layer comprises a binder and multi-segment polymer particles having: a) a hollow core-shell morphology (HSP) and b) pores, which are channels extending from the particle surface to the hollow core. The porous HSP is preferably prepared as follows: a dispersion of first-segment polymer particles is prepared by reacting methyl methacrylate with methyl acrylate under aqueous emulsion polymerization conditions to form an aqueous dispersion of first-segment polymer particles having a z-average particle size in the range of 200 nm or 300 nm to 750 nm or 600 nm as measured by dynamic light scattering. The dispersion of first-segment polymer particles is then reacted with styrene and carboxylic acid monomers under aqueous emulsion polymerization conditions to form an aqueous dispersion of solid polymer particles comprising a first segment having structural units of methyl acrylate and methyl methacrylate, and a second segment (i.e., a shell) having structural units of styrene and carboxylic acid monomers.

[0009] Examples of carboxylic acid monomers include acrylic acid, methacrylic acid, and itaconic acid, with acrylic acid being preferred. The second segment comprises at least 98% by weight, or at least 99% by weight, or at least 99.5% by weight structural units of styrene and carboxylic acid monomers or their salts. Preferably, the second segment comprises less than 1% by weight, or less than 0.5% by weight, or less than 0.1% by weight, or 0% by weight any C1-C of acrylic acid. 20 Alkyl esters or C1-C methacrylic acid 20 The structural unit of alkyl esters or any acrylic C3-C 20 Cycloalkyl esters or C3-C methacrylic acid 20 The structural unit of the cycloalkyl ester. The weight:weight ratio of the monomer used to prepare the second stage to the monomer used to prepare the first stage (i.e., the ratio of the second stage to the first stage) is preferably in the range of 5:1 or 7:1 or 8:1 to 12:1 or 10:1 or 9:1.

[0010] Then, under conditions sufficient to hydrolyze methyl acrylate into acrylic acid or its salts, the aqueous dispersion of non-porous segmented polymer particles is converted into an aqueous dispersion of segmented porous polymer particles. Hydrolysis can be carried out, for example, by contacting the dispersion of non-porous polymer particles with NaOH under high pressure at a temperature in the range of 100°C to 150°C. Hydrolysis produces swollen porous core-shell polymer particles, wherein the core, as the first polymer segment, contains water and structural units of methyl methacrylate and sodium acrylate, and the shell, as the second polymer segment, contains at least 98% by weight of structural units of styrene and structural units of salts of carboxylic acid monomers. When the water evaporates, the core becomes hollow. The average particle size of the swollen porous segmented polymer particles is in the range of 500 nm, or 750 nm, or 900 nm to 3 µm, or 2 µm, or 1.8 µm, or 1.5 µm, or 1.2 µm.

[0011] The percentage of pore area is in the range of 0.005% or 0.01% to 0.5%, or 0.2%, or 0.1%, or 0.06% of the area of ​​the porous segmented polymer particles. The porosity of porous HSPs is typically in the range of 60% to 75%, or 70%, or 65%.

[0012] Thermal recording materials typically contain leuco dyes and developers (see U.S. Patent 4,929,590) and may also contain a variety of other additives, including binders, fillers, crosslinking agents, surfactants, sensitizers, and heat-fusible materials. Thermal paper can be prepared by methods known in the art, such as those described in the Examples section.

[0013] Example

[0014] Porosity percentage determination

[0015] The following procedure was used to determine the particle porosity. 40 g of latex containing a porous polymer was added to a 50 mL polypropylene centrifuge tube (with a hemispherical bottom). The tube was placed in a centrifuge and centrifuged at 18,500 rpm for 120 min. The clear supernatant was decanted from the compacted packing and weighed. The porosity percentage (VF%) was determined using the following formula based on the latex mass, solids percentage, and supernatant mass:

[0016] W T =Total weight of the sample in the tube. Since the polymer density is close to 1, weight is used instead of volume.

[0017] Solids% = Solid content of latex.

[0018] k = the packing factor of the random packing of unswollen monodisperse spheres, i.e., 0.675. The packing factor indicates that some water will be trapped between the spheres in the compacted packing.

[0019] Particle size measurement

[0020] The size of the hollow spherical particles was measured using scanning electron microscopy (SEM). Two drops of emulsion were applied onto a conductive carbon ribbon on an aluminum SEM slide. After drying at ambient temperature for 2 hours, a thin chromium layer was applied to the sample using a sputtering current of 100 mA for 100 s in an EMS 150T ES metal coater. SEM images were acquired using a Thermo Fisher Nova NanoSEM 630 scanning electron microscope with an Everhart-Thornley secondary electron detector at an accelerating voltage of 5 kV from a Schottky field emission electron source. All images were acquired at 20,000x magnification, with an image size of 1024 × 884 pixels and a bit depth of 8 (grayscale range 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 hollow spherical particles. Two images were analyzed for each sample using ImageJ software (version 1.53c). The diameter of all particles in the image, except those at the image edges, was manually measured. The number-weighted average particle size and standard deviation were calculated.

[0021] Hole area percentage measurement

[0022] The pore area percentage was measured using the same SEM used for particle size measurement. Two images were analyzed for each sample using ImageJ software (version 1.53c). All images were preprocessed using the "Smooth" command in ImageJ, followed by the "Enhance Contrast" command with 0.3% saturation pixels and the "Normalize" option. In the micrographs, these pores appear as dark areas on the bright particle surface; a grayscale value of 60 was used as the segmentation threshold to separate the pores for area measurement. Bright pixels with grayscale values ​​between 61 and 255 constitute the particles, and their total area is denoted as [missing value]. A 颗粒 Dark pixels with grayscale values ​​between 0 and 60 constitute hole or gap locations (dark areas between grains). Two additional constraints are used to distinguish between hole and gap locations: the hole size is between 9 and 800 pixels, and the roundness is between 0.70 and 1.00. Any gap locations incorrectly selected by the software using these criteria are manually excluded from the selection. After these adjustments, the total area of ​​the selected holes is denoted as... A 孔 Then, the percentage of the orifice area is defined by the following formula:

[0023] Intermediate Example 1 - Preparation of an aqueous dispersion of porous hollow spherical polymer particles

[0024] A. Preparation of the first-stage polymer dispersion

[0025] Deionized water (1630.00 g) and glacial acetic acid (0.50 g) were placed in a 5 L, 4-necked round-bottom flask and heated to 92 °C under N2. In a separate container, a monomer emulsion containing DI water (335.33 g), sodium dodecylbenzenesulfonate (30.22 g, 22.5% in water), methyl methacrylate (940.40 g), and methyl acrylate (681.00 g) was prepared. An initiator solution of sodium persulfate (3.02 g) in DI water (90 g) and a buffer solution of sodium bicarbonate (0.27 g) in DI water (90 g) were also prepared. A solution of sodium persulfate (1.51 g) in DI water (20 g) was added to a reactor and rinsed with DI water (10 g). An acrylic seed polymer dispersion (55.39 g, 45.6% solids content, 100 nm) was added to the reactor and rinsed with DI water (10 g). The monomer emulsion, initiator solution, and buffer solution were then fed into the reactor over 180 min, while the reaction was maintained at 90 °C. After feeding, the vessel was rinsed with DI water (50 g total), and the reaction was maintained at 90 °C for 15 min, followed by cooling to room temperature. The resulting core polymer dispersion had a solids content of 41.7 wt%.

[0026] B. Preparation of dispersions of non-porous multi-segmented polymer particles

[0027] Deionized water (1668.00 g) and glacial acetic acid (0.50 g) were placed in a 5 L, 4-necked round-bottom flask and heated to 95 °C under N2. In a separate container, a monomer emulsion containing DI water (112.30 g), sodium dodecylbenzenesulfonate (3.15 g, 22.5% in water), styrene (712.73 g), and acrylic acid (29.37 g) was prepared. A solution of sodium persulfate (2.18 g) in DI water (20 g) was added to a reactor and rinsed with water (5 g). A portion of the first stage A polymer dispersion (231.24 g) was added to the reactor and rinsed with DI water (20 g). The monomer emulsion was fed into the reactor over 100 min while maintaining the reaction at 90 °C. After the monomer emulsion feeding was complete, the monomer emulsion container was rinsed with DI water (40 g) and the reaction was maintained at 90 °C for 45 min, then cooled to room temperature. The resulting dispersion of nonporous multi-segmented polymer particles has a solid content of 28.8% by weight.

[0028] C. Preparation of aqueous dispersions of porous polymer particles

[0029] A portion of intermediate 1B (205.9 g) was mixed with DI water (95.8 g) and sodium hydroxide solution (5% in water, 49.5 g). The resulting dispersion was placed in a pressure vessel and heated to 140 °C. After 6 h, the vessel was cooled to room temperature and the swollen porous particle dispersion was removed. The final pH of the porous polymer particle dispersion was 11.5, the solid content was 17.8% by weight, the number-weighted particle size was 1.03 ± 0.06 µm, the porosity was 61.7%, and the pore area percentage was 0.03 area.

[0030] Intermediate Example 2 - Preparation of Primer Formulation

[0031] A portion of intermediate Example 1C (67.4 g), RHOPLEX™ P-308 styrene-acrylic binder (P-308, 4.5 g, 50 wt% solids, trademark of The Dow Chemical Company or its affiliates) and #67710 polyvinyl alcohol (PVOH, 5 g, 15 wt% in demineralized water, Kremer Pigmente) were mixed in a container using a top-blade mixer and then diluted with DI water (23.1 g) to adjust to 17 wt% solids.

[0032] Comparative Intermediate Example 1 - Preparation of an Aqueous Dispersion of Porous Polymer Particles

[0033] The procedure of intermediate Example 1 was essentially repeated, except that a nonporous segmented dispersion was prepared using styrene (696.50 g), methyl acrylate (37.10 g), and acrylic acid (8.50 g). The resulting dispersion had a solids content of 29.0% by weight. A porous dispersion was prepared by reacting a portion of the nonporous segmented dispersion (203.9 g) with DI water (80.40 g) and sodium hydroxide solution (68.2 g, 5% by weight in water). The final pH of the porous polymer particle dispersion was 12.7, the solids content was 18.4% by weight, the number-weighted particle size was 0.98 ± 0.07 µm, the porosity was 63.2%, and the pore area percentage was 1.06%.

[0034] Comparative Intermediate Example 2 - Preparation of an Aqueous Dispersion of Porous Polymer Particles

[0035] Except for the preparation of a nonporous multi-segmented dispersion using styrene (659.40 g), methyl acrylate (74.20 g), and acrylic acid (8.50 g), the procedure of intermediate Example 1 was essentially repeated. The resulting dispersion was found to have a solid content of 28.3% by weight. A porous dispersion was prepared by reacting a portion of the nonporous dispersion (209.4 g) with DI water (55.48 g) and sodium hydroxide solution (97.3 g, 5% by weight in water). The final pH of the porous polymer particle dispersion was 13.0, the solid content was 18.4% by weight, the number-weighted particle size was 0.93 ± 0.06 µm, the porosity was 62.6%, and the pore area percentage was 1.71%.

[0036] Comparative Intermediate Example 3 - Preparation of Primer Formulation

[0037] A portion of an aqueous dispersion (67.2 g) of porous polymer particles from Comparative Intermediate Example 1, P-308 (4.5 g) and PVOH (5 g) were mixed in a container using a top-blade mixer and then diluted with DI water (23.4 g) to adjust to a solid content of 17% by weight.

[0038] Comparative Intermediate Example 4 - Preparation of Primer Formulation

[0039] A portion of an aqueous dispersion (66.2 g) of porous hollow spherical particles from comparative intermediate Example 2, P-308 (4.5 g) and PVOH (5 g) were mixed in a container using a top-blade mixer and then diluted with DI water (24.3 g) to adjust to a solid content of 17% by weight.

[0040] Intermediate Example 3 - Thermal Recording Layer Formulation

[0041] The materials and formula are from Nissho Kogyo Co, LTD. Place DI water (51.6 g) in an 8 oz container, then add 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 colorimetric agent (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-aniline-6-(dibutylamino)-3-methylfluorane dye (5.7 g, Mitsubishi, 35 wt%), and mix using a top-mounted blade mixer.

[0042] Example 1 and Comparative Examples 2 and 3 - Preparation of paper coated with an image layer

[0043] NewPage fiber-free paper (basic weight: 58g / m³) 2 The paper (roughness: 4.00µm, Gurley porosity: 24.6s) was cut into 37.9cm × 20.1cm pieces with the long side in the longitudinal direction and then placed in a controlled temperature chamber (72℉ (22℃) and 50% humidity) for at least 2 hours. The paper was then attached to a sheet of copy paper using masking tape, which was then attached to a manual squeegee. A drop of primer formulation was then pipetted onto the masking tape positioned above the fibrous paper. The winding rod was then manually moved down onto the primer formulation strip, passing over the paper, to evenly coat it. The paper was then exposed to hot air for 45 seconds, followed by transfer to an oven and further drying at 80℃ for 45 seconds. After drying, the paper was conditioned in a controlled temperature chamber (72℉ and 50% humidity) for 2 hours. An image layer was then applied to the primer-coated paper using the same procedure as for applying the primer and dried at 80℃ for 1 minute.

[0044] Dynamic sensitivity measurement

[0045] Cut the fully coated paper lengthwise into two 2.5” (1cm) wide strips. Connect the two strips end to end and print using an Atlantek Paper Tester Model 200. Measure the dynamic sensitivity using the following printing conditions: a) Sequence point pulse duration = 0.8ms b) Total cycle time (T) 循环 =5.0ms c) Printhead temperature = 30℃ d) At an applied voltage of 20.6V, the printhead resistance is 583Ω. A 50% 80×80 checkerboard pattern was printed using printing 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. The optical density of each printing energy for the three boxes was measured using a handheld X-rite 428 spectrophotometer.

[0046] The paper coated in Example 1 was found to have an optical density of 0.59 at a printing energy of 0.2 mJ. Conversely, the coated papers in Comparative Examples 1 and 2 were found to have optical densities of 0.49 and 0.41, respectively, at the same printing energy. The porous HSPs had similar porosities (between 61.7% and 63.1%); however, the primer containing an HSP with a significantly smaller percentage of pore area resulted in an increase in the optical density of the coated paper at a given printing energy.

Claims

1. A coated paper product, comprising: a) Paper blanks with a thickness of 40µm to 500µm; b) A primer layer of 2µm to 10µm thickness, said primer layer comprising porous, segmented polymer particles and a binder; and c) Thermal recording layers with a thickness of 1µm to 30µm; The primer layer is disposed between the thermal recording layer and the paper blank; The porous, segmented polymer particles have a shell-hollow core morphology and a number-weighted average particle size in the range of 500 nm to 3 µm. The shell comprises 0.5% to 10% by weight of structural units of carboxylic acid monomer salts and 90% to 99.5% by weight of structural units of styrene. The outer shell comprises at least 98% by weight of styrene structural units and structural units of the salt of the carboxylic acid monomer; and The percentage of pore area is in the range of 0.005% to 0.5% of the surface area of ​​the porous segmented polymer particles.

2. The coated paper product according to claim 1, wherein the shell of the porous, segmented polymer particles comprises at least 99% by weight of styrene structural units and carboxylic acid monomer structural units, wherein the carboxylic acid monomer is acrylic acid or a salt thereof or methacrylic acid or a salt thereof.

3. The coated paper product according to claim 1, wherein the outer shell of the porous, segmented polymer particles comprises less than 0.5% by weight of any acrylic C1-C 20 Alkyl esters or C1-C methacrylic acid 20 The structural unit of alkyl esters.

4. The coated paper product according to claim 1, wherein the outer shell of the porous, segmented polymer particles comprises less than 0.5% by weight of any acrylic C3-C 20 Cycloalkyl esters or C3-C methacrylic acid 20 The structural unit of cycloalkyl esters.

5. The coated paper product according to claim 2, wherein the number-weighted average particle size of the porous, segmented polymer particles is in the range of 750 nm to 1.5 µm.

6. The coated paper article according to claim 5, wherein the pore area percentage is in the range of 0.01% to 0.1% of the surface area of ​​the porous segmented polymer particles; and the number-weighted average particle size of the porous segmented polymer particles is in the range of 900 nm to 1.2 µm.

7. The coated paper product according to claim 6, wherein the porosity of the porous, segmented polymer particles is in the range of 60% to 75%.

8. The coated paper product according to claim 6, wherein the porosity of the porous, segmented polymer particles is in the range of 60% to 70%.

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