A process for the preparation of composite filler particles having a degree of polymerization of the monomers of at least 50 % and less than 100 % as determined by any of the method disclosed in the description, comprising:
(a) coating a particulate glass filler having a median particle size (D50) of from 1 to 1200 nm with a coating composition containing a film-forming agent forming a coating layer on the surface of the particulate filler, said coating layer displaying reactive groups on the surface of the coating layer, said reactive groups being selected from addition polymerizable groups and step-growth polymerizable groups, thereby forming a coated particulate filler; subsequently or concurrently
(b) agglomerating the coated particulate filler for providing a granulation of the coated particulate filler wherein the granulation contains the coated particulate filler particles separated from and connected to each other by at least one coating layer, whereby the at least one coating layer may be crosslinked by crosslinking groups obtained by reacting the reactive groups, whereby the agglomeration is carried out by spray agglomeration or growth agglomeration;
for providing composite filler particles having a median particle size (D50) of from 1 to 70 µm, wherein reactive groups are transformed into crosslinking groups obtained by reacting reactive groups and optionally a further crosslinking agent, and wherein the particulate filler is the main component by volume of the composite filler particles.