Cationic hollow particle latexes with high thermal stability of the hollow polymer are useful as opacifying agents, fillers, binders and extenders in coating and impregnating applications. particu- larly when positive charge and high thermal stability of the hollow polymer particles are required.

E. g. the cationic hollow particle latexes are very useful for acid-catalyzed thermosetting coatings cured at high temperatures. The cationic hollow particle latexes can be used in pulp and paper industry as multifunctional additives (white pigments ; fillers ; density decreasing, retention aids etc.) The cationic hollow particle latexes are useful in the production of filled antistatic latex coatings on the basis of amine containing polymers.

DESCRIPTION OF THE PRIOR ART In the prior art a lot of methods for the production of hollow particle latexes are known. However a majority of them, e. g. US patents Nos. 4, 427, 836, 4, 594, 363, 5, 157, 084, 5, 494, 971 discloses a preparation of anionic hollow particle latexes. In other words these latexes are not very useful for acid-catalyzed thermosetting coatings because the high base content in the latexes can interfere with acid-catalyzed curing reactions.

The Finnish patent application FI 991050 discloses a process for the preparation of cationic hollow particle latexes by recharging of anionic hollow particles with cationic surfactants selected from quarternary ammonium salts. The main drawback of this mode is connected with low heat

stability of the hollow polymer prepared since the glass transition temperature is no higher than 105°C.

US patent 4, 489, 825 discloses a method for the preparation of latexes with cationic hollow polymeric particles consisting of the core comprising crosslinked copolymer of methyl methacrylate and amino-monomer and the shell comprising poly (iso-butyl methacrylate).

According to the invention description an additional external shell comprising up to 50% wt. of a crosslinking monomer such as divinyl benzene can be obtained. Thus there is a possibility to prepare a hollow polymer with high heat stability. However the method has the following disadvantages : 1. In accordance with an example a large coagulate amount (higher than 50% ut.) is formed at the synthesis.

2. Too high cationic surfactant concentrations (about 10% wt.) are used at the stage of latex particle core formation.

3. The solids of the final latex is low (about 8% wt.).

In order to obtain a hollow polymer with high heat stability it is necessary to use a large amount of an expensive crosslinking monomer There is a known process for producing of hollow particles of crosslinked melamine resin as disclosed in US patent 5, 360, 832, said process comprising subjecting a water-soluble methyl- etherified-melamine resin precondensate to condensation reaction in the presence of a curing catalyst in an aqueous solution containing a water-soluble polymer with carboxyl groups or water- soluble copolymer of ethylenically unsaturated carboxylic acid monomer, when the reaction liquid becomes turbid, adding to the reaction mixture a substance which dissolves or swells the melamine resin, and continuing the condensation reaction. The cured melamine hollow particles have high heat resistance and in principle depending on the amount of an acid catalyst for curing they can possess a positive charge. However the hollow particles prepared are too large in particle diameter (about 10 um) and they are not suitable enough as opacifying agents in coating and impregnating applications.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved production of cationic hollow particle latexes with high heat resistance of hollow polymers. It is another object of the present invention to provide a production of waterborne compounds with cationic hollow particles as opacifying agents cured at elevated temperatures and useful in coating and impregnating applications.

These objects which will become apparent from the following disclosure are achieved by the present invention which comprises a process for making cationic hollow particle latex useful for opacifying characterized in that an anionic hollow particle latex is recharged by addition into said latex of an acid colloid of melamine-formaldehyde resin, the weight ratio of said acid colloid of melamine-formaldehyde resin/anionic hollow particle latex based on dry weight of components being equal to from 0. 1 to 0. 7.

In another aspect, the process for making a cationic hollow particle latex characterized in that said acid colloid of melamine-formaldehyde resin is prepared in aqueous medium at molar ratio melamine/formaldehyde/hydrochloric acid as 1 : (2-12) : (0. 3-1. 5) at temperature from 70°C to 97°C.

In one more aspect, the invention comprises a composition for coating and/or impregnating characterized in that said cationic hollow particle latex and water-soluble melamine-formaldehyde resin are mixed at weight ratio based on dry weight of components in the range from 0. 05 to 0. 70, preferably 0. 30, curing being carried out at elevated temperatures up to 130-200°C.

DETAILED DESCRIPTION OF THE INVENTION The cationic hollow particle latex is prepared by mixing of an anionic hollow particle latex and acid colloid of melamine-formaldehyde resin at ambient temperature, said acid colloid of melamine-formaldehyde resin being added fast into anionic hollow particle latex at intensive agitation. During this procedure anionic hollow particles are recharged and converted into cationic ones. The fast mixing of the components makes it possible to avoid the coagulation because the system passes through the isoelectric state quickly.

The recharged hollow particle latexes have positive s-potential in the range from +5 mV to +70 mV, preferably from +20 to +40 mV. Solids of the cationic hollow particle latexes is from 15% wt. to 30% wt., preferably from 20 to 25% wt.

The weight ratio of the said acid colloid of melamine-formaldehyde resin/anionic hollow particle latex based on dry weight of components is equal to from 0. 1 to 0. 7, preferably from 0. 3 to 0. 5. At low values of said weight ratio (below 0. 3) the cationic hollow particle latex is too viscous. At high value of the weight ratio the shelf life of cationic hollow particle latexes is too low because the latex gelation or coagulation occurs.

Commercially available hollow particle latexes of Rohm & Haas Co. or hollow particle latexes prepared according to well-known procedures disclosed e. g. in US patent 4, 427, 836 or PCT patent application WO 98/39372 are used as anionic hollow particle latexes. As a rule a hollow polymer comprises styrene, methyl methacrylate and small amounts of other monomer units such as acrylonitrile, (meth) acrylic acid, crosslinking monomer etc. Negative charge of the hollow particles is provided with the application of usual anionic surfactants and persulfate initiators. The q-potential of anionic hollow particle latexes is in the range from-20 mV to-65 mV, preferably from-40 to-60 mV. Anionic hollow particle latexes have particles from 400 nm to 950 nm in size.

Solids of the latexes are in the range from 30% wt. to 46% wt.

Acid colloids of melamine-formaldehyde resins are prepared in aqueous medium at molar ratio melamine/formaldehyde/hydrochloric acid as 1 : (2-12) : (0. 3-1. 5), preferably as 1 : (4. 0-7. 0) : (0. 4-0. 7) at 70-97°C. The process temperature depends on the amount of hydrochloric acid (the larger this amount the lower the temperature). The procedure of the acid colloids of melamine-formaldehyde resins preparation is as follows. First water and melamine are charged into a reactor. The amount of water charged depends on formalin concentration and is selected so that the final product concentration should be in the range from 10 to 15% wt. The reaction mixture is heated to 70-97°C at agitation. Then concentrated hydrochloric acid (30-35% wt.) is added. When melamine is dissolved completely formalin is charged and the process is carried out for 50-60 min.

A composition for coating and/or impregnating is prepared by mixing of cationic hollow particle latex and water soluble alkyl-etherified melamine-formaldehyde resin as a binder at weight ratio based on dry weight of components in the range from 0. 05 to 0. 30.

The composition may or may not comprise a water-thinnable alkyd resin or aqueous dispersion of a soft polymer, e. g. vinyl acetate-dibutyl maleate copolymer, for plasticization. Curing of the composition is carried out at 150-200°C. Depending on the temperature the curing time varies from several minutes to several hours.

The heat resistance of the hollow polymer is evaluated according to the following test. First a coating on a glass plate is prepared on the basis of hollow particle latex and melamine binder. The coating is dried at ambient temperature for 24 hrs and opacity of the coating is measured. Then the coating is cured for 75 min when temperature is elevated gradually from ambient to 170°C and opacity is measured again. Simultaneously opacity of the coating prepared with usual hollow particle latex free of cationic colloid of melamine-formaldehyde resin treatment is measured. The hollow polymer is considered as heat resistant when opacity of the coating does not decrease or drops slightly after curing at elevated temperature. A significant decrease of opacity after curing is an evidence of destruction of hollow particles and consequently low heat resistance of a hollow polymer.

Opacity is defined as a contrast ratio of the coatings determined by photometric method. The reflection coefficient of the coated glass plate at , =582 nm is determined using a photometer. First the reflection coefficient of a glass plate with coating placed on a black surface with reflection coefficient of less than 1 % is determined. Then the glass plate with the coatings is placed on a white surface with the reflection coefficient 82% and the reflection coefficient is determined again. The contrast ratio is calculated as the ratio of reflection coefficients.

The q-potential of the latexes is estimated by macroelectrophoresis technique at Barton installation using latexes with the concentration 7% and 0. 025 N KC1 solution as a side liquid. The q-potential was calculated using the Helmholtz-Smolukhovsky equation where n and D-viscosity and dielectric permeability of the dispersion medium (water) are equal to 0. 1 cP and 81 correspondingly, Uo-electrophoretic mobility of the particles determined according to the equation

U # 1<BR> U0 = ------- E where U is latex/side liquid boundary shift rate, E is potential difference equal to 100 V in our experiments and 1 is distance between the electrodes-24 cm.

The following examples are provided for illustrative purposes only, and do not limit the scope of the invention. which is reflected only by the claims.

Example 1. Anionic Hollow Particle Latex Preparation of the anionic hollow particle latex is carried out according to the following procedure.

Stage A. Seed latex preparation The process is carried out in a glass 100 cm3 reactor equipped with a stirrer. reflux condenser, inlet for inert gas and necks for charge of the components.

Recipe : Methyl methacrylate 42 Monomer n-Butyl acrylate..........................4.09 g mixture 1 Methacrylic acid (MAA)....................... 0. 16 g Sodium dodecylbenzene sulfonate........ 0. 038 g (SDBS) in 5. 15 g of water Potassium persulfate (PP).................... 0. 067 g in 5. 0 g of water Distilled water 57. 5 g

This step is carried out as a batch process. 57. 5 g of distilled water is charged into the reactor and heated up to 80°C with stirring and inert gas flow. Then SDBS solution is charged. In 5 min PP solution and monomer mixture 1 are added. The process period (after the monomer mixture charge) is 105 min. The latex prepared is cooled, discharged and filtered.

The following stages are carried out in the only glass 350 cm3 reactor equipped with stirrer, reflux condenser, inlet for inert gas and a device for feeding of the components as a number of successive steps.

Stage B. Highly carboxylated core polymer preparation.

Recipe : Seed latex prepared in accordance with stage A........................................ 3. 53 g MMA................................................ 7. 68 g Monomer MAA................................................... 3. 28 g mixture 2 Ethylene glycol dimethacrylate............ 0. 057 g PP...................................................... 0. 027 g in 5. 0 g of water Distilled water................................... 48. 86 g SDBS............................................... 0. 0187 g Aqueous PP..................................................... 0. 077 g mixture 2 Distilled water................................. 43. 26 g for feeding The seed latex and 48. 86 g of water are charged into the reactor. The reactor content is heated in the inert gas flow up to 80°C. Then the solution of PP (0. 027 g in 5. 0 g of water) is ad- ded followed by monomer mixture 2 and aqueous mixture 2 feeding. Feeding of the both components is carried out with a constant rate for 180 min. Then the process is continued for 25 min.

Stage C. Shell polymer preparation.

Recipe : Latex prepared at stage B...... 111. 79 g Styrene...................................... 84. 73 g Monomer Acrylonitrile................................ 27. 90 g mixture 3 Divinyl benzene........................... 1. 14 g PP.............................................. 0. 28 g in 10. 0 g of water SDBS...................................... 0. 227 g Aqueous PP............................................ 0. 57 g mixture 3 Distilled water......................... 48. 92 g for feeding Stage C starts straight away after completing stage B. PP solution (0. 28 g in 10. 0 g of water) is added followed by monomer mixture 3 and aqueous mixture 3 feeding. The feeding of the both mixtures is carried out with a constant rate for 134 min. Then the process continues for 10 min followed by addition of 0. 62 g of SDBS in 2. 38 g of water.

Stage D. Neutralization and swelling of the core-shell particles.

Recipe : Latex prepared at stage C Ammonia aqueous solution (conc. 14% wt.)....... 6. 94 g Ammonia solution is added into the latex dropwise within 4-5 min and the reaction mixture is heated up to 97-98°C within the next 10-15 min and maintained for 60 min. Then the latex is cooled and filtered. The final diluted latex is examined with an electron microscope. A single void can be observed inside each particle.

Besides as an anionic hollow particle latex commercially available latex of Rohm & Haas Co. Ropaque-HP-543 was used.

Performances of two anionic hollow particle latexes are listed in Table 1.

Table 1 Performances of anionic hollow particle latexes Performance Latex prepared Ropaque HP-543 according to example 1 Solids, % wt. 43. 0 30. 5 pH 7. 0 7. 1 Particle diameter, m-n 590 470 Brookfield viscosity (50 RPM, &num 2), cP 83. 4 15 s-potential, mV-50-67 Example 2. Acid colloid of melamine-formaldehyde preparation The process is carried out in 200 cm3 glass reactor equipped with stirrer, reflux condenser and neck for charge of components. First water and melamine are charged into the reactor and the reaction mixture is heated to 70-97°C at agitation. Then concentrated hydrochloric acid (conc. 31. 8 % wt.) is added. When melamine is dissolved completely formalin (formaldehyde conc. 36. 0% wt.) is charged and the process is carried out for 60 min. Conditions of the process are listed in Table 2.

Example 3. Cationic hollow particle latex preparation The process is carried out in 200 cm3 glass vessel equipped with stirrer at ambient temperature. Anionic hollow particle latex and water are placed into the vessel and then an acid colloid of melamine-formaldehyde resin is charged fast within 20-30 s with intensive agitation.

Agitation continues for 30 min. Recipes and performances of cationic hollow particle latexes are listed in Table 3.

Example 4A. Preparation of water-thinnable methylated melamine formaldehyde binder for compositions with cationic hollow particle latexes A methyl-etherified melamine-formaldehyde resin only or in combination with aqueous dispersion of vinyl acetate-dibutyl maleate copolymer are used as a water-thinnable binder.

A water soluble methyl-etherified melamine-formaldehyde resin is prepared according to the following procedure.

88. 28 g of formalin (conc. 36%) is charged into a 200 cm glass reactor equipped with reflux condenser and stirrer. Then 2. 90 cm3 of sodium hydroxide solution (conc. 0. 4% wt.) is added to achieve pH 8. 2 of the reaction mixture. The reaction mixture is heated to 80°C followed by melamine (31. 78 g) addition. After dissolving of melamine the process is carried out for 45 min. Then methanol (34. 74 g) and 2. 32 g of sodium hydroxide solution (conc. 40% wt.) are added and the process is carried out for 1 more hour. Content of the methylated melamine- formaldehyde resin in the aqueous solution prepared is 61. 8% wt. pH of the resin solution is 11. 2.

Example 4B. Preparation of a combined binder methylated melamine-formaldehyde resin -vinyl acetate-dibutyl maleinate copolymer Combined binder is prepared by addition of vinyl actetate-dibutyl maleate copolymer aqueous dispersion into methylated melamine-formaldehyde resin with stirring at ambient temperature.

58. 19 g of methylated melamine-formaldehyde resin prepared according to example 4A is placed into 200 cm3 glass vessel. Then the resin is diluted with 49. 35 g distilled water followed by addition of 21. 61 g vinyl acetate-dibutyl maleate copolymer dispersion (solids 51. 3% wt.) at stirring. Stirring continues for 10 min.

Example 5. Composition preparation, casting and curing of the coatings Compositions are prepared by incorporation of a cationic hollow particle latex into a binder produced according to examples 4A and 4B at agitation. The cationic hollow particle latex is introduced dropwise into the binder at stirring.

Coatings are prepared by casting of the compositions prepared onto glass plates followed by drying at ambient temperature for 24 hrs and curing at gradual temperature increase from ambient temperature to 170°C for 75 min. The contrast ratio for the coatings is measured before and after the curing. The composition recipes and opacity of the coatings prepared therefrom characterized by contrast ratio are listed in Table 4.

Table 4 illustrates that opacity of the coatings after curing practically does not change. This result can be considered as an evidence of high heat resistance of the hollow particles incorporated into the coating.

Table 2 Conditions for the preparation of acid colloids of melamine-formaldehyde resins Example Amount of the components, g Melamine/CH, O/HCI Temperature, pH No. Melamine Formalin HO Water molar ratio °C (36. 0% wt.) (31. 8% wt.) 2A 12. 60 23. 17 10. 33 164. 0 1 : 2. 78 : 0. 90 85 0. 65 2B 12. 60 34. 83 15. 15 197. 2 1 4. 18 : 1. 32 80 0. 45 2C 12. 60 34. 83 13. 54 194. 4 1 4. 18 : 1. 18 80 0. 70 2D 12. 60 34. 83 14. 35 195. 8 1 4. 18 : 1. 25 80 0. 45 2E 12. 60 34. 83 11. 94 191. 6 1 4. 18 : 1. 04 80 0. 70 2F 12. 60 34. 83 10. 33 188. 7 1 4. 18 : 0. 90 80 0. 80 2G 12. 60 34. 83 8. 26 185. 1 1 : 4. 18 : 0. 72 85 0. 90 2H 12. 60 46. 66 10. 33 213. 8 1 : 5. 60 : 0. 90 80 0. 45 21 12. 60 46. 66 7. 92 209. 6 1 : 5. 60 : 0. 69 80 0. 95 2J 12. 60 46. 66 4. 82 204. 2 1 : S. GO : 0. 42 80 1. 55 2K 12. 60 58. 33 10. 33 238. 6 1 : 5. 60 : 0. 90 80 0. 40 2L 12. 60 58. 33 7. 92 234. 4 1 : 7. 00 : 0. 69 80 0. 65 2M 12. 60 58. 33 4. 82 228. 9 1 : 7. 00 : 0. 42 80 1. 10 2N 12. 60 70. 00 4. 82 253. 7 1 : 8. 40 : 0. 42 75 1. 40 Table 3 Recipes and performances of cationic hollow particle latexes

Example Type and amount (g) of the components Hollow polymer/MFR Solids, q-potential, No. Anionic hollow Acid colloid Water weight ratio % wt. mV particle latex of melamine-formaldehyde resin 3A Ropaque HP-543 21. 7 Example 2E 28. 3 1 : 0. 5 19. 6 6 3B Ropaque HP-543 28. 0 Example 2E 22. 0 4 : 0.3 21.9 +31 3C Ropaque HP-543 32. 8 Example 2E 17. 2 1 : 0.2 23.6 +25 3D Ropaque HP-543 39. 6 Example 2E 10. 4 1 0. 1 26. 2 +20 3E Example 1 17. 4 Example 2J 32. 6 1 : 0. 5 22. 2 +33 3F Example 1 20. 1 Example 2J 22. 6 7. 3 1 : 0. 3 22. 2 +26 3G Example 1 24. 3 Example 2J 9. 1 16. 6 1 : 0. 1 22. 2 +22 3H Example 1 17. 4 Example 21 32. 6 1 : 0. 5 22.2 +39 31 Example 1 17. 4 Example 2L 32. 6 1 : 0. 5 22.2 +33 Table 4 The composition recipes and contrast ratio for the coatings Example Recipe (in grams) Contrast ratio for the coatings N. Binder Cationic hollow Before curing After curing particle latex <BR> <BR> 5A Example 4A 39. 8 Example 3A 10. 2 0. 840 0. 814 5B Example 4A 26. 1 Example 3A 23. 9 0. 976 0. 965 5C Example 4A 33. 3 Example 3B 16. 7 0. 966 0. 958 5D Example 4A 34. 2 Example 3C 15. 8 0. 987 0. 944 5E Example 4A 35. 3 Example 3D 14. 7 0. 940 0. 935 5F Example 4A 27. 6 Example 3E 22. 4 0. 718 0. 725 5G Example 4A 15. 2 Example 3E 34. 8 0. 936 0. 937 5H Example 4A 29. 8 Example 3F 202 0. 933 0. 930 5I Example 4B 34. 6 Example 3A 15. 4 0. 923 0. 928 5J Example 4B 34. 6 Example 3H 15. 4 0. 923 0. 923 5Example 4B 34. 6 Example 31 15. 4 0. 853 0. 862

Table 5 The composition recipes and contrast ratio for the coatings Example Recipe (in grams) Contrast ratio for the coatings No. Binder Cationic hollow Before curing After curing particle latex Comparative Example 4A 45. 0 Ropaque HP-543 5. 0 0. 900 0. 413 example A Comparative Example 4A 36. 6 Ropaque HP-543 13. 5 0. 974 0. 683 example B Comparative Example 4A 39. 8 Example 1 10. 2 0. 811 0. 308 example C Comparative Example 4A 30. 9 Example 1 19. 1 0. 857 0. 278 example D