Particulate filler, coated with material bonded thereto and containing a sulfur-containing group which releases sulfur as a curing agent for s-curable unsaturated polymers

A process for the production of a coated particulate filler which process comprises binding to the surface of a basic particulate filler through reaction with an acidic group which is part of a material comprising at least one acidic group reactive with the filler which acidic group is linked directly or indirectly to at least one sulphur-containing group which is not reactive with the filler but which is decomposable to a sulphur-containing species which is capable of taking part in a sulphur-based curing reaction, in which the acidic group is selected from the group consisting of a saturated aliphatic carboxylic acid group, a salt thereof, and an anhydride thereof, and in which the sulphur-containing group is selected from the group consisting of a polysulphide group, a xanthate group, a thiocarbonate group, a dithiocarbamate, a trithiocarbamate group, a monothiocarbonate group, a dithiocarbonate group, a mercaptol group, a mercaptole group, a thioacetate group, a dithioacetate group, and an isothiouronium group said binding being accomplished by forming a mixture of said filler and said material, and optionally heating the mixture during mixing at a temperature such that premature curing of the mixture and decomposition of the sulphur-containing group is avoided.