A method of making a filled resin includes providing functionalized particles; and combining and homogenously mixing the functionalized particles with an organic matrix in a vacuum kneader to provide the filled resin so that the functionalized particles comprise at least about 20% by weight of the filled resin. The step of providing functionalized particles can also include providing a feedstock of (i) untreated particles, (ii) a surface treatment agent reactive with the particles, and (iii) solvent, and directing the feedstock through a continuous reactor maintained at a temperature sufficient to react the particles with the surface treatment agent to provide the functionalized particles in less than about 4 hours; and directing the functionalized particles from the continuous reactor directly into the vacuum kneader. In another aspect, the a finished resin comprises at least about 20% by weight of functionalized particles in an organic matrix.