Magnetizable "core-shell" microspheres based on a cross-linked organopolysiloxane and a process for their preparation

Magnetizable microspheres comprising:

• (1) a core comprising a magnetizable filler not coated with either a hydrophobic surfactant or a dispersing agent having a size smaller than about 300.times.10.sup.-4 .mu.m;
• and
• (2) shell based on a crosslinked organopolysiloxane derived from hydrosilylation of:
• a) at least one organopolysiloxane SiVi of the following formula I:
• R'R.sub.2 Si O (Si R R" O).sub.m Si R.sub.2 R' (I)
• in which formula
• the R radicals are identical or different and are selected from the group consisting of a C.sub.1 -C.sub.4 alkyl radical and a phenyl radical;
• the R' radicals may be identical or different and are selected from the group consisting of a C.sub.1 -C.sub.4 alkyl radical, a phenyl radical, and a vinyl radical, wherein the number of vinyl radicals is at least 2 per macromolecule;
• with at least 60% of the R and R' radicals being methyl radicals;
• the R" radicals may be identical or different and are selected from the group consisting of a C.sub.1 -C.sub.4 alkyl radical, a phenyl radical, and an -r-X unit, wherein r is a divalent organic radical and X is selected from the group consisting of epoxy, hydroxyl, carboxyl, aldehyde, ester, acetoester, mercapto, mercaptoester, amino, alkylamino, dialkylamino, trialkylamino, quaternary ammonium, aminoalcohol, amido, hydrazide, hydrazino, C.sub.1 -C.sub.3 haloalkyl, halobenzyl, cyano, ##STR4## cyanato, sulphate and sulphonyl;
• n and m can independently be zero, R' being a vinyl radical if m is zero, and n and m having values sufficient to provide a polymer having a viscosity of 20 mPas to 30,000,000 mPas at 25.degree. C.;
• and wherein the total number of -r-X units contributed by the organopolysiloxane SiVi and the organohydrogenpolysiloxane SiH, defined below, ranges from 1:1 to 1,000:1 per molecule obtained from the hydrosilylation of organopolysiloxane SiVi with organohydrogenpolysiloxane SiH;
• said organopolysiloxane SiVi containing SiVi groups comprising a vinyl group bonded to a silicon atom;
• b) with at least one organohydrogenpolysiloxane SiH containing, per molecule, at least three SiH groups with each group comprising a hydrogen atom linked to a silicon atom, said organohydrogenpolysiloxane having a viscosity ranging from 5 to 1,500 mPas at 25.degree. C.