COATED HOLLOW POLYMER PARTICLE LATEX AND A METHOD FOR THE MANUFACTURE THEREOF

Abstract: The invention relates to latexes of hollow polymer particles which are coated with resins in order to improve the heat resistance and light scattering properties of the hollow polymer particles. The invention also relates to a method for coating synthetic hollow polymer particles with resins. The hollow polymer particle latex is coated with a formaldehyde based resin and the formaldehyde based resin comprises melamine, urea or combinations thereof.

Description: Coated hollow polymer particle latex and a method for the manufacture thereof The invention relates to latexes of hollow polymer particles which are coated with resins in order to improve the heat resistance and light scattering properties of the hollow polymer particles. The invention also relates to a method for coating synthetic hollow polymer particles with resins. Hollow polymer particles are useful as opacifying agents in coating applications, such as in paints and paper coatings for high quality printing paper products where colour pictures are presented, such as art books, brochures, annual reports, magazines etc. In such coatings the need for pigments and extenders is reduced due to the use of hollow particle latexes thus reducing the undesirable weight of the coating. The hollow polymer particle latex provides opacity because the hollow structure in a latex particle scatters light more efficiently than a corresponding solid particle with uniform density. Hollow polymer particles have found extensive applications as white pigments because hollow sphere pigments have excellent optical properties such as hiding power, opacity, gloss, brightness and whiteness and they can also be used as control release devices for target compounds. White hollow polymer pigments also find applications as titanium dioxide replacing organic pigment in thermoplastic applications. From FI 970936 a method is known for the preparation of hollow polymer particle latex by emulsion copolymerization. The method comprises the preparation of seed latex with a water-soluble initiator and an anionic surfactant followed by the preparation of latex of a highly carboxylated copolymer on the seed latex, then further preparing an intermediate shell, then swelling the particles and finally preparing a hard shell on the swollen expanded particles and optionally preparing an external shell on the hard shell. US 3,931,063 discloses a method for the manufacture of hollow urea formaldehyde emulsion for absorbent and pigment applications. US 3,428,607 describes a method for the manufacture of melamine formaldehyde emulsion with a particle size less than 1 μm. A method for coating of inorganic mineral material with a resin such as methylolated melamine, urea or fenol is disclosed in PCT/US97/10403. The particle size of the obtained particle emulsion is in the range of 5-75 μm. Thermal resistance and light scattering properties of hollow polymer particles according to the state of the art are not sufficient for more demanding applications. Insufficient thermal resistance is problematic in applications where heat resistance is especially important, such as in super calendering of paper or extrusion of thermoplastics. Based on the above it can be seen that there clearly exists a need for hollow polymer particle latexes with improved thermal resistance and light scattering properties, and for a method for the manufacture thereof. An object of the invention is to provide coated hollow polymer particle latexes with improved thermal properties and a method for the manufacture of hollow polymer particle latexes. Characteristics of coated hollow polymer particle latexes and of a method for the manufacture thereof are stated in the claims. It has now been found that the above-identified objectives can be attained and the disadvantages of the methods and hollow polymer particle latexes according to the state of the art can be avoided or decreased by the solution according to the invention. It has been discovered that thermal resistance and light scattering properties of hollow polymer particles can be significantly improved by coating the polymer particles with a resin or with a mixture of resins. A suitable polymer particle for coating is a hollow organic polymer particle which comprises polystyrene, polystyrene-acrylate, a polymerized particle based on polyacrylate or mixture thereof. An essential feature of the invention is that the hollow particle comprises hydrophilic functional groups in the particle shell, preferably acrylate or me hacrylate groups. This allows the attraction of the resin on the hollow particle resulting in mass transfer into the void and polymerization on the shell. The particle size of the polymer particle is in the range of 100 run - 2 μm, preferably 300-700 nm. A suitable resin for the coating of polymer particles is heat resistant melamine formaldehyde (MF) or urea formaldehyde (UF) condensate or a combination thereof. Due to the high degree of crosslinking, these resins are heat resistant up to 200°C. In order to increase thermal resistance and light scattering properties of hollow polymer particles the shells of the polymer particles are coated with a formaldehyde-based resin. It is essential for the emulsion polymerization coating method that the methylolated monomer, which is melamine or urea or a mixture thereof, is completely methylolated and dissolved in specified concentrations of water. This enables the hollow polymer particles to act as the only seed particles in the emulsion polymerization, to which the monomer is transferred and wherein it is polymerized. The method for the coating of hollow polymer particles with a resin is described in more detail in the following. The method comprises methylolation of the monomer followed by polymerization of the resin onto the hollow polymer particles. The pH of the solution, which comprises formaldehyde and optionally water, is adjusted to 6.7-10, preferably 9- 10 using sodium hydroxide or formic acid and after the pH adjustment the monomer, such as melamine or urea or a mixture thereof, is charged to the reactor and heating is started, and the methylolation is continued at 80-98 °C until the solution clarifies. The molar ratio of formaldehyde to melamine or to urea is 2:1- 3:1. Then the polymerization of the resin onto the hollow polymer particles is performed. The methylolated monomer obtained earlier is added to a hollow polymer particle latex, suitably at 80-99 °C and at the pH of 6.7-10 and the condensation polymerization is continued after the addition of the monomer for some hours. The rate of addition of the methylolated monomer to the hollow polymer particle latex is 1 wt% - 30 wt% of the methylolated monomer calculated from the hollow polymer particle amount per 15 min and preferably 5-15 wt%. After the addition is completed the product is cooled. Alternatively the hollow polymer particle latex may be added to the methylolated monomer at 80-97 °C and at the pH 6.7-10 as described above. The amount of the melamine or urea formaldehyde condensate is 1-80 w-% and preferably 10-60 w-% of the dry content of the hollow polymer particle latex which is coated. The diameter of the hollow void is preferably 50-700 run depending on the application. The glass transition temperature is preferably 120-200°C depending on the application. The solid content of the coated hollow polymer particle latex is in the range of 20-55 % depending on the application. The particle size of the obtained coated hollow polymer particle is 100 nm - 2 μm, preferably 300-700 nm depending on the application. The hollow polymer particle latex coated with MF, UF resin or combinations thereof exhibits optimal light scattering and thermal resistance properties. The coated hollow polymer particle latex is suitable for several applications and specially for demanding applications such as high quality printing paper, decorative papers for plywood, thermoplastic compounds, printing inks and water borne coatings. The external resin coating on the hollow polymer particles improves significantly thermal resistance of the polymer particles and the properties of the coated hollow polymer particles may be modified by varying the diameter of the hollow void in the resin particle, the thickness of the resin shell and by varying the amount of the impregnated resin in the seed hollow polymer particle latex. The specific diameter of the hollow void particle and the particle size of the coated particle for some application areas are presented in the following Table 1. Table 1 In the emulsion polymerization process according to the invention the condensation polymerization of the desired amount of MF or UF resin or combinations thereof to the hollow polymer particles can be performed in a controlled manner so that the transfer rate of the MF or UF resin to the hollow polymer particles is less than the polymerization rate of the MF or UF resin. The methylolated melamine or urea monomer dissolved in water is transferred from the water solution to the hollow polymer particles without forming new polymerizable resin particles. Additionally, because the methylolated MF and UF monomers are completely methylolated and dissolved in water, the hollow polymer particles will act as the only seed particles to which the monomer is transferred and wherein the polymerization takes place. The invention is illustrated in more detail in the following examples which are not meant to narrow the scope of the invention. EXAMPLE 1 Preparation of MF precondensate 139 g of formaline (49 % formaldehyde in water) and 62 g of water are charged to a 1 1 reactor equipped with reflux condenser, stirrer and oil heating mantle. The pH of the solution is then adjusted to 9.2 - 9.4 with 25 wt-% NaOH. After stirring 174 g of melamine and 25 g of water are added and the mixture is heated to 97°C. The metylolisation of the melamine is complete, when all melamine is dissolved, and the solution turns clear without any undissolved melamine particles. This is due to the formation of water-soluble melamine-formaldehyde oligomers. The solid content of the precondensate is 60 wt-%. Coating of hollow polymer particle latexes. To 1 kg of a hollow particle latex with a solid content of 27 wt-% 44 g of the MF precondensate is added under stirring at a temperature of 90 - 95 °C at a feeding rate of 10 % condensate per 15 min. The total feeding time is 2.5 h. The heating and stirring is continued in order to complete the melamine-formaldehyde condensation reaction on the hollow particle shell. This gives a MF coated hollow particle with a 10 wt-% MF content compared to the solid content of the hollow particle. The solid content of the MF coated hollow particle is 28 wt-%, the particle size is 350 nm, and hollow void is 240 nm. The glass transition temperature as measured by DSC is 150°C, which indicates the increased temperature resistance. EXAMPLE 2 A MF coated hollow particle latex with 30 wt-% MF content is prepared in the same way as in example 1, except that 130 g of the MF condensate is used. The solid content of the MF coated hollow particle is 30 wt-%, the particle size is 350 nm, and hollow void is 240 nm. The glass transition temperature as measured by DSC is 170°C, which indicates the increased temperature resistance. EXAMPLE 3 A MF coated hollow particle latex with larger particle size is prepared in the same way as in Example 1, except that a hollow particle latex with a 550 nm particle size and 350 nm hollow void is used. The solid content of the MF coated hollow particle is 25 wt-%, the particle size is 550 nm, and hollow void is 350 nm. The glass transition temperature as measured by DSC is 150°C, which indicates the increased temperature resistance. EXAMPLE 4 Preparation of UF precondensate 298 g of formaline (49 % formaldehyde in water) is charged to a 1 1 reactor equipped with reflux condenser, stirrer and oil heating mantle. The pH of the solution is then adjusted to 7.2 with 25 wt-% NaOH. After stirring 150 g of urea is added. The reaction mixture is then made acidic (pH 5.5) by adding formic acid. The condensation is completed by heating to 95 °C for 5 h. Coating of hollow polymer particle latexes. To 1 kg of a hollow particle latex with a solid content of 27 wt-% 82 g of the UF precondensate is added under stirring at a temperature of 90 - 95 °C at a feeding rate of 10 % condensate per 15 min. The total feeding time is 2.5 h. The heating and stirring is continued in order to complete the urea-formaldehyde condensation reaction on the hollow particle shell. This gives a UF coated hollow particle with a 20 wt-% UF content compared to the solid content of the hollow particle.

Claims: Claims 1. Method for the manufacture of coated hollow polymer particle latex, characterized in that hollow polymer particle latex is coated with a formaldehyde based resin. 2. A method according to claim 1, characterized in that the formaldehyde based resin comprises melamine, urea or combinations thereof. 3. A method according to claim 1 or 2, characterized in that hollow polymer particle latex comprises polystyrene, polystyrene-acrylate, or a polymerized particle based on polyacrylate or mixtures thereof. 4. A method according to any one of claims 1-3, characterized in that the shell of the hollow particle comprises hydrophilic functional groups, preferably acrylate or methacrylate groups. 5. A method according to any one of claims 1-4, characterized in that the particle size of the hollow polymer particle latex is in the range of 100 nm - 2 μm, preferably 300 - 700 nm. 6. A method according to any one of claims 1-5, characterized in that the method comprises steps: a) methylolation of a monomer comprising melamine or urea or combinations thereof with formaldehyde, b) polymerization of the condensate obtained in a) on the shell of hollow polymer particle latex. 7. A method according to any one of claims 1-6, characterized in that the molar ratio of formaldehyde to melamine or urea is 2:1-3:1. 8. A method according to any one of claims 1-7, characterized in that the step a) is performed at the pH of 6.7-10, at the temperature of 80-98°C, and step b) is performed at the pH of 6.7-10, at the temperature of 80-98°C with the addition rate of the monomer of 1-30 wt%/15 min. 9. A coated hollow polymer particle latex, characterized in that the coating com- prises a formaldehyde based resin. 10. A coated hollow polymer particle latex according to claim 9, characterized in that the formaldehyde based resin comprises melamine, urea or combinations thereof. 11. A coated hollow polymer particle latex according to claim 9 or 10, characterized in that hollow polymer particle latex comprises polystyrene, polystyrene-acrylate, or a polymerized particle based on polyacrylate or mixtures thereof. 12. A coated hollow polymer particle latex according to any one of claims 9-11, characterized in that the coated hollow polymer particle latex comprises 1-80 wt% of formaldehyde based resin, the particle size of the coated hollow polymer particle latex is 100 nm - 2 μm and the solid content is 20-55 wt%. 13. A coated hollow polymer particle latex according to any one of claims 9-12, characterized in that the coated hollow polymer particle latex comprises 10-60 wt% of formaldehyde based resin and the particle size of the coated hollow polymer particle is 300-600 nm and the solid content is 20-55 wt%. 14. A coated hollow polymer particle latex according to any one of claims 9-13, characterized in that the coated hollow polymer particle latex comprises 1-80 wt% of formaldehyde based resin and the particle size of the coated hollow polymer particle is 100 nm - 2 μm and the diameter of the hollow void is 50-700 i nm. 15. Use of a coated hollow polymer particle latex according to any one of claims 9-13 in printing, calendering, decorative paper, thermoplastic compound, and in granulated product applications.

Inventors: KAERNAE TOIVO, KOSKINEN JUKKA, LAAMANEN HANNA, SKRIFVARS MIKAEL, KAERNAE, TOIVO, KOSKINEN, JUKKA, LAAMANEN, HANNA, SKRIFVARS, MIKAEL
Applicants: KAERNAE TOIVO, KOSKINEN JUKKA, LAAMANEN HANNA, NESTE CHEMICALS OY, SKRIFVARS MIKAEL, KAERNAE, TOIVO, KOSKINEN, JUKKA, LAAMANEN, HANNA, SKRIFVARS, MIKAEL
Application year: 2000
Main classification: C08J9
IPC codes: B01J13, D21H21