Polyvinyl acetal with improved residual particle size distribution

Abstract

The present invention is directed to polyvinyl acetal having improved residual particle size, a process for making the polyvinyl acetal, and its use as a binder for producing ceramic green sheets.

Description

[Technical field]

[0001] The present invention is directed to polyvinyl acetal having improved residual particle size distribution, a process for making the polyvinyl acetal, and its use as a binder for producing ceramic green sheets.

[0002] Ceramic materials, such as ceramic capacitors for the electronics industry, are generally produced by sintering so-called green sheets, i.e. thin film-like bodies containing the ceramic material. For the production of such green sheets, a suspension of metal oxides, plasticizers, dispersants and binders in an organic solvent is produced. This suspension is then applied to the desired thickness by a suitable process (i.e. a doctor blade process) onto a carrier film and the solvent is removed. The green sheets thus obtained must be free of cracks, exhibit a smooth surface and still have a certain degree of elasticity.

[0003] Polyvinyl acetals, such as polyvinyl butyral (PVB), are frequently used as binders in the production of ceramic green sheets.For this reason, DE 40 03 198 A1 describes the production of casting slips for ceramic green films using PVB as binder with a residual polyvinyl acetate content of 0 to 2% by weight.

[0004] In recent years, multilayer ceramic capacitors (MLCCs) have become increasingly important in the electronics industry. MLCCs are made up of several individual capacitors stacked in parallel and contacted via their terminal surfaces. Due to the increasing miniaturization of electronic components, the size of powder particles, e.g. metal oxide particles, can be in the nanometer range. Residual particles, i.e. particles that are not soluble in the solvent (mixture) used to make the suspension required for the production of green sheets, but have the same size as the metal oxide particles, are of great concern as they are difficult to selectively remove and can lead to defects in the ceramic layer after sintering of the green sheets.

[0005] Moreover, residual particles in the low nanometer range tend to clog the screens used to filter the binder solution, thus creating further problems that make the process uneconomical since the screens need to be replaced or cleaned more frequently. Moreover, even partial clogging of the screens leads to high back pressure during the filtration process. Moreover, polymer particles are generally difficult to filter due to the soft behavior of the swollen polymer particles in the solution.

[0006] It was therefore an object of the present invention to provide a polyvinyl acetal for use as a binder for ceramic green sheets having improved properties with respect to residual particles, and a process for the preparation of said polyvinyl acetal. A further object was to provide such a binder which results in improved dissolution rate, improved adhesion, improved handling, improved elongation at break, improved tensile strength, improved dispersion effect, improved environmental, health and safety effects, and / or a better economic profile in the production and use of ceramic green sheets.

[0007] The inventors have surprisingly discovered that one of the main sources of particles in the range of 2-5 μm is the trace amount of sodium chloride present in the final polyvinyl acetal powder. Sodium chloride is not soluble in the binder solutions used in the production of green sheets or in the organic solvents used in the production of suspensions, and therefore remains as residual particles. Polyvinyl acetal is generally produced by the acetalization process, in which polyvinyl alcohol and aldehydes are converted into the corresponding acetal in the presence of an acid. In this regard, hydrochloric acid is frequently used. After precipitation of polyvinyl acetal, the crude product is washed with water to remove the acid until it is neutral, and then the washed product is dried.

[0008] Regular water used in industrial parks can contain significant amounts of chloride ions. For example, the upper limit for the chloride content of tap water in the Hoechst industrial park in Germany was 250 mg / l in 2020. As a result, even if the polyvinyl acetal is thoroughly washed with water to remove residual hydrochloric acid, the chloride content of the final product will be quite high, since the washing water itself contains chloride ions. This chloride content leads to an increased content of residual particles, especially in the critical low nanometer range.

[0009] Thus, a first aspect of the present invention is a method for producing a polyvinyl acetal powder, the method comprising the steps of: a. reacting polyvinyl alcohol with an aldehyde in the presence of an acid to produce polyvinyl acetal; b. precipitating the polyvinyl acetal obtained in step a; c. washing the precipitate obtained in step b with a washing solution containing water; d. drying the washed precipitate of step c. and the wash solution of step c) contains not more than 100 mg / l chloride ions.

[0010] The chloride content in the cleaning solution is determined by silver nitrate titration (Test Method B) in accordance with ASTM D512-04.

[0011] Preferably, the washing solution in step c contains not more than 80 mg / l, more preferably less than 75 mg / l, even more preferably less than 50 mg / l, most preferably less than 30 mg / l, especially less than 20 mg / l of chloride ions.Also preferably, the washing solution contains 0.1 to 10 mg / l of chloride ions.

[0012] The cleaning liquid may be a mixture of water and a water-miscible solvent, for example an alcohol such as methanol, ethanol or isopropanol. Preferably, the cleaning liquid consists essentially of water.

[0013] Acids which can be used are those known in the process for the preparation of polyvinyl acetals, such as hydrochloric acid, sulfuric acid, nitric acid or mixtures of these acids. Preferably, the acid is hydrochloric acid.

[0014] The chloride content of the wash liquor can be reduced by any method known in the art. Mechanisms of chloride removal mainly include chemical precipitation, adsorption, oxidation, and membrane separation. In chemical precipitation, chloride is removed by the formation of CuCl, AgCl, BiOCl, and Friedel's salt. Adsorbents used for chloride removal mainly include ion exchangers, bimetallic oxides, and carbon-based electrodes. Oxidation for chloride removal includes ozone-based oxidation, electrochemical oxidation, and sulfate radical-based oxidation. Membrane separation for chloride removal consists of diffusion dialysis, nanofiltration, reverse osmosis, and electrodialysis. Preferably, the amount of chloride in the wash liquor is reduced by chemical precipitation, reverse osmosis, adsorption, and / or oxidation.

[0015] By using a washing liquid with a low chloride content, especially water, a final polyvinyl acetal powder with a low chloride content can be obtained. Thus, in one embodiment of the present invention, the polyvinyl acetal powder contains no more than 25 ppm chloride, more preferably less than 20 ppm, even more preferably less than 15 ppm, most preferably less than 10 ppm, especially less than 5 ppm chloride.

[0016] Preferably, the polyvinyl acetal has a molecular weight of ≧20,000 g / mol, as determined by gel permeation chromatography in accordance with DIN ISO 16014 1:2019-05.

[0017] The molecular weight is measured by gel permeation chromatography according to DIN ISO 16014 1:2019-05. Preferably, the molecular weight is 40,000 g / mol or more, more preferably 50,000 g / mol or more. Also preferably, the molecular weight is 200,000 g / mol or less, more preferably 175,000 g / mol or less, most preferably 150,000 g / mol or less, in particular 100,000 g / mol or less.

[0018] Preferably, the acetal groups of the polyvinyl acetal each have from 1 to 7 carbon atoms, ie, the acetal groups are derived from a condensation reaction with an aldehyde having from 1 to 7 carbon atoms. More preferably, the acetal group is methanal (formaldehyde), acetaldehyde, n-propanal (propionaldehyde), n-butanal (butyraldehyde), isobutanal (2-methyl-1-propanal, isobutyraldehyde), n-pentanal (valeraldehyde), isopentanal (3-methyl-1-butanal), sec-pentanal (2-methyl-1-butanal), tert-pentanal (2,2,dimethyl-1-propanal), n-hexanal (capronaldehyde), isohexanal (2-methyl-1-pentanal, 3-methyl-1-pentanal, 4-methyl-1-pentanal), 2,2-dimethyl-1-butanal, 2,3-dimethyl-1-butanal. The list of dimethyl-1-butanal, 3,3-dimethyl-1-butanal, 2-ethyl-1-butanal, n-heptanal, 2-methyl-1-hexanal, 3-methyl-1-hexanal, 4-methyl-1-hexanal, 5-methyl-1-hexanal, 2,2-dimethyl-1-pentanal, 3,3-dimethyl-1-pentanal, 4,4-dimethyl-1-pentanal, 2,3-dimethyl-1-pentanal, 2,4-dimethyl-1-pentanal, 3,4-dimethyl-1-pentanal, 2-ethyl-1-pentanal, 2-ethyl-2-methyl-1-butanal, 2-ethyl-3-methyl-1-butanal, 3-ethyl-2-methyl-1-butanal, cyclohexylaldehyde and benzaldehyde. More preferably, the acetal groups are derived from a condensation reaction with isobutyraldehyde, acetaldehyde and / or n-butyraldehyde. Most preferably, the polyvinyl acetal is a polyvinyl butyral or a mixed polyvinyl acetal derived from the condensation of n-butyraldehyde with acetaldehyde.

[0019] The method for producing the polyvinyl acetal used in this embodiment of the present invention is not particularly limited, but it can be produced by a method in which an aldehyde is added to a polyvinyl alcohol solution under acidic conditions to subject the solution to an acetalization reaction.

[0020] Preferably, the degree of acetalization of the polyvinyl acetal used in the present invention is 50 mol% or more and less than 90 mol%, more preferably the lower limit of the degree of acetalization is more than 60 mol%, even more preferably more than 70 mol%, and most preferably more than 80 mol%. Furthermore, the upper limit of the degree of acetalization is more preferably 90 mol% or less, and most preferably 85 mol% or less.

[0021] The percentage of vinyl alcohol units in the polyvinyl acetal of the present invention is preferably 10 to 25 mol %, more preferably 15 to 20 mol %, based on the total monomer units constituting the resin.

[0022] The vinyl alcohol content and vinyl acetate content of the polyvinyl acetals were determined in accordance with DIN ISO 3681 (acetate content) and DIN ISO 53240 (PVA content).

[0023] In a second aspect, the present invention relates to a suspension composition comprising one or more inorganic pigments, one or more organic solvents, one or more binders, one or more plasticizers and one or more dispersants, wherein the binder is a polyvinyl acetal as described above.

[0024] Preferably, the acetal group has 2 to 7 carbon atoms and is derived from the same aldehydes as mentioned above, and most preferably, the acetal group is derived from n-butyraldehyde or a mixture of n-butyraldehyde and acetaldehyde.

[0025] The inorganic pigments can be selected from finely ground granules of paraelectric or ferroelectric materials, titanium dioxide (rutile), preferably modified with additives of zinc, zirconium, niobium, magnesium, tantalum, cobalt and / or strontium, as well as MgNb2O6, ZnNb2O6, MgTa2O6, ZnTa2O6, (ZnMg)TiO3, (ZrSn)TiO4, BaTiO3 and Ba2Ti9O 20 The inorganic pigment preferably has an average particle size of about 0.01 to 1 μm.

[0026] The organic solvent may be selected from aromatic compounds such as toluene, xylene, alcohol compounds such as methanol, ethyl alcohol, isopropyl alcohol, butyl alcohol, and more preferably mixtures thereof. Most preferably, the organic solvent is a mixture of ethanol and toluene.

[0027] Suitable dispersants include fish oil, phosphate esters, and functional polymers having polyoxyalkylene groups in the side chains, such as the MALIALIM™ series available from NOF America Cooperation.

[0028] In addition to the binder according to the invention, the suspension may contain other components chosen in particular from other binders such as cellulose resins, acrylic resins, vinyl acetate resins, polyvinyl alcohol resins; plasticizers such as polyethylene glycols or phthalates and / or antifoamers.

[0029] The method for producing the suspension composition is not particularly limited. Various dispersion methods can be used, such as a method using a media-type mill such as a bead mill, a ball mill, an attritor, a paint shaker, or a sand mill, a method of kneading a powdered ceramic, a dispersion medium, a dispersant, a binder, a plasticizer, and the like; and a method using a three-roll mill. The method using a three-roll mill includes a method of dispersing a powdered inorganic pigment in an organic solvent (mixture) together with a dispersant, a binder, a plasticizer, and the like. The mixture is compressed and kneaded by passing through a small gap between a first roll and a second roll that roll independently of each other and are adjacent to each other with a small gap therebetween, and then the mixture is further compressed and kneaded by passing through a gap between the second roll and a third roll that rolls and is adjacent to the second roll with a gap smaller than the gap between the first roll and the second roll.

[0030] Preferably, the powdered ceramic, the dispersant and the solvent (mixture) are premixed and dispersed such that the dispersant is adsorbed on the powdered ceramic. In a second step, the binder is added to the mixture thus formed, followed by mixing and dispersion again.

[0031] The dry thickness of the coating film produced by these steps may be 0.25 to 25 μm, and is typically 1 to 15 μm.

[0032] A third aspect of the present invention relates to the use of the polyvinyl acetal powder obtained by the process according to the invention as a binder for the production of ceramic green sheets or ceramic moulded bodies.

[0033] A fourth aspect of the present invention relates to a polyvinyl acetal having a residual particle count of particles having a particle size of 2 to 500 μm of 500 counts / ml or less and a ratio of the residual particle count of particles having a particle size of 2 to 5 μm divided by the residual particle count of particles having a particle size of 2 to 20 μm of 0.8 or less, the residual particle count being measured as described herein in a 2% by weight solution in a 1:1 mixture of toluene and ethanol. Preferably, this ratio is 0.75 or less, more preferably 0.70 or less, even more preferably 0.65 or less, most preferably 0.60 or less, especially 0.50 or less.

[0034] Also preferably, the residual particle count of particles having a particle size of 2 to 500 μm is 450 counts / ml or less, more preferably 400 counts / ml or less, even more preferably 350 counts / ml or less, and most preferably 300 counts / ml or less.

[0035] Such polyvinyl acetal, which has a low overall amount of residual particles, and in particular a low ratio of residual particles in the critical range of 2 to 5 μm, is particularly useful for producing ceramic green sheets. EXAMPLES

[0036] [Synthesis of polyvinyl acetal] Polyvinyl Butyral (PVB 1) 100 parts by weight of polyvinyl alcohol with a viscosity of 19 mPas (measured according to DIN 53015 at 20° C., 8 w / w% in aqueous solution) and a degree of hydrolysis of 98 mol % were dissolved in 1000 parts by weight of water while heating to 90° C. At a temperature of 40° C., 65 parts by weight of n-butyraldehyde were added and, under stirring, at a temperature of 5° C., 250 parts by weight of 20% hydrochloric acid were added. The mixture was heated to 40° C. After precipitation of polyvinyl butyral (PVB), the mixture was stirred at this temperature for 48 hours. After cooling to room temperature, the PVB was separated, washed to neutrality with water with a chloride content of 9.4 mg / l and dried. A PVB with a polyvinyl alcohol content of 12.7% by weight (19.1 mol %) and a polyvinyl acetate content of 2.3% by weight (1.8 mol %) was obtained.

[0037] Mixed Polyvinyl Acetal (PVB 2) 100 parts by weight of polyvinyl alcohol with a viscosity of 50 mPas (measured according to DIN 53015 at 20° C., 8 w / w% in aqueous solution) and a degree of hydrolysis of 99 mol % were dissolved in 1000 parts by weight of water while heating to 90° C. At a temperature of 40° C., 200 parts by weight of 20% hydrochloric acid were added and at a temperature of 12° C., first 22 parts by weight of acetaldehyde and then 30 parts by weight of n-butyraldehyde were added while stirring. The mixture was heated to 40° C. After precipitation of polyvinyl butyral (PVB), the mixture was stirred at this temperature for 48 hours. After cooling to room temperature, the PVB was separated, washed to neutral with water with a chloride content of 9.4 mg / l and dried. A PVB with a polyvinyl alcohol content of 12.5% ​​by weight (18.8 mol %) and a polyvinyl acetate content of 1.7% by weight (1.3 mol %) was obtained.

[0038] Polyvinyl Butyral (PVB 3) 100 parts by weight of polyvinyl alcohol with a viscosity of 26 mPas (measured according to DIN 53015 at 20° C., 4 w / w% in aqueous solution) and a degree of hydrolysis of 99 mol % were dissolved in 1075 parts by weight of water while heating to 90° C. At a temperature of 40° C., 57 parts by weight of n-butylaldehyde were added and at a temperature of 10° C., 75 parts by weight of 20% hydrochloric acid were added while stirring. After precipitation of polyvinyl butyral (PVB), the mixture was heated to 70° C. and stirred at this temperature for 1 hour. After cooling to room temperature, the PVB was separated, washed to neutral with water with a chloride content of 9.4 mg / l and dried. A PVB with a polyvinyl alcohol content of 20.0% by weight (28.8 mol %) and a polyvinyl acetate content of 1.4% by weight (1 mol %) was obtained.

[0039] [Measurement of particle size distribution] 2 g of PVB 3 powder was dissolved in 98 g of a solvent mixture (1:1 mixture of ethanol and toluene) using a tilt / rotate mixer. After 24 hours, a 50 ml sample was taken and analyzed by single particle measurement by laser focus using an Accusizer™ 780 from Soliton GmbH. Only particles in the size range of 2-500 μm were counted.

[0040] [Table 1]

Claims

1. A polyvinyl acetal in which the residual particle count of particles having a particle diameter of 2 to 500 μm is 500 counts / ml or less, and the ratio of the residual particle count of particles having a particle diameter of 2 to 5 μm to the residual particle count of particles having a particle diameter of 2 to 20 μm is 0.8 or less, wherein the residual particle count is measured as described herein in a 2% by mass solution in a 1:1 mixture of toluene and ethanol.

2. The polyvinyl acetal according to claim 1, wherein the ratio is 0.75 or less.

3. Use of polyvinyl acetal according to claim 1 or 2 as a binder for manufacturing ceramic green sheets or ceramic molded articles.