PROCESS FOR PREPARING SPHERICAL SILICONE RESIN PARTICLES

Abstract

A process for preparing spherical silicone resin particles in which alkoxysilanes are reacted with water to form a hydrolysate is provided. The resulting silicone resin particles are isolated from a mixture. The silicone resin particles are dried and the particles are deagglomerated by ultrasonic sieving.

Claims

1-11 (canceled)

12. A process for preparing spherical silicone resin particles, in which alkoxysilanes are reacted with water to form a hydrolyzate, the resulting silicone resin particles are isolated from a mixture, the silicone resin particles are dried and the particles are deagglomerated by ultrasonic sieving.

13. The process as claimed in claim 12, wherein a sieve mesh having a mesh size of 10 to 40 μm is used for the ultrasonic sieving.

14. The process as claimed in claim 12, in which the ultrasonic sieving is carried out in a frequency range from 30 to 38 kHz.

15. The process as claimed in claim 12, in which in the ultrasonic sieving, the particles on the sieve mesh are thrown to a height of 0.3 to 10 cm as a result of the ultrasound excitation.

16. The process as claimed in claim 12, in which the ultrasonic sieving is carried out at an area-specific power of 10 to 500 W/m.sup.2.

17. The process as claimed in claim 12, in which the particles are isolated by filtration or centrifugation.

18. A process for preparing spherical polysilsesquioxane particles as claimed in claim 12, in which in a first step trialkoxysilanes of general formula (I)
RSi(OR.sup.1)3   (I), in which R is a hydrocarbon radical having 1 to 16 carbon atoms, the carbon chain of which may be interrupted by non-adjacent —O— groups, R.sup.1 is a C.sub.1- to C4-alkyl radical, are reacted with acidified water with a pH of at most 6 with mixing to form a hydrolyzate, in a second step the hydrolyzate is mixed with a solution of a base in water or C.sub.1- to C4-alkanol, in a third step the mixture is kept for at least 2 hours, in a fourth step the polysilsesquioxane particles are isolated from the mixture, in a fifth step the polysilsesquioxane particles are dried and in a sixth step the particles are deagglomerated by ultrasonic sieving.

19. The process as claimed in claim 18, in which R is an ethyl radical or methyl radical.

20. The process as claimed in claim 18, in which R.sup.1 is an ethyl radical or methyl radical.

21. The process as claimed in claim 18, in which in the first step the reaction to form the hydrolyzate is carried out at a pH of 4.5 to 2.

22. The process as claimed in claim 18, in which in the second step a solution of alkali metal hydroxide in water or in an alkanol having 1 to 3 carbon atoms is used.

Description

EXAMPLES

General Procedure 1: Preparation of Polymethylsilsesquioxane Particles

[0117] An initial charge of 32 kg of demineralized water having a conductivity of 0.1 μS/cm in a glass-lined 50 liter stirred tank with jacket cooling is kept at a controlled temperature of 20° C. The contents are stirred at 150 rpm. The pH is adjusted to 4.40 by adding 0.1 molar hydrochloric acid. 7.0 kg of methyltrimethoxysilane are metered in over 1 hour, the temperature being kept at 20° C. On completion of the metered addition, the mixture is stirred at 20° C. for 30 minutes. (Step 1)

[0118] The pH is corrected (step 1a).

[0119] After the correction is complete, the mixture is stirred at 20° C. for a further 30 minutes. 363 g of 0.5 molar methanolic KOH solution are added within 1 min at 20° C. and the mixture is mixed homogeneously for a total of 3 min (step 2). The stirrer is then switched off. After 21 hours (step 3), the precipitated particles are filtered off, washed with demineralized water and dried at 150° C. for 18 h.

Example 1

[0120] Polymethylsilsesquioxane particles were prepared according to general procedure 1. In step 1a the pH was corrected to 2.8. The particles obtained have a median particle size d50 of 5.0 μm.

Example 2

[0121] The deagglomerating sieving of the particles from example 1 was carried out using a VRS 600 vibrating round sieving machine with ultrasonic excitation of the sieve mesh at 35 kHz (mesh size 20 μm, sieve diameter 600 mm) from Allgaier, available from Allgaier Process Technology GmbH, Ulmer Strasse 75, 73066 Uhingen, Germany, using abrasion-resistant hollow cylinder tapping aids. The coarse material outlet was removed in order to avoid losses when feeding the product via the outlet. 17 kg of the particles from Example 1 were continuously applied to the sieve so that the sieve always remained covered with raw material. The particles on the sieve mesh were thrown to a height of ca. 1-2 cm. The mean mass throughput was ca. 60 kg/h, corresponding to ca. 0.21 kg/h per cm.sup.2 of sieve area. There was no visible accumulation of coarse material on the sieve. Complete material throughput and thus a 100% fine fraction <20 μm were thus achieved.

Comparative Example C1

[0122] The particles from Example 1 were sieved using a conventional Retsch AS 200 basic throwing sieve machine from Retsch, available from RETSCH GmbH, Retsch-Allee 1-5, 42781 Haan, Germany, without a coarse material outlet, over a sieve with a mesh size of 20 μm and sieve diameter of 200 mm. 100 g of the particles from Example 1 were applied and sieved at an amplitude of 100% (corresponding to deflection ca. 2 mm) without additional tapping aid. After 10 minutes, 44 g of particles have passed the sieve, corresponding to an average mass throughput of ca. 0.009 kg/h per cm.sup.2 of sieve area. The sieve shaker gives only 44% fines <20 μm. No separation of the agglomerated particles can be achieved.

Comparative example C2

[0123] The particles from example 1 were sieved as described in comparative example C1, but with the use of an abrasion-resistant hollow cylinder tapping aid. After 10 minutes, 52 g of particles have passed the sieve, corresponding to an average mass throughput of ca. 0.01 kg/h per cm.sup.2 of sieve area. Even with a tapping aid, the sieve shaker only gives 52% fines <20 μm. Only a slight separation of the agglomerated particles can be achieved.