Encapsulation of aluminium hydroxide fillers with poly-methyl-methacrylate

A process was developed for the microencapsulation of inorganic filler part icles with poly-methyl-methacrylate, to increase the interaction between th e hydrophilic filler particles and a polymer matrix. The filler utilised wa s aluminium hydroxide with an average diameter of 1.9 mum and a specific su rface area of 5 m(2)/g. The process comprised a surface modification, in wh ich a monolayer of isopropoxy titanium isostearate was chemically bound to the surface to render it hydrophobic and to ensure a chemical bond between the filler and the organic phase. Then, an encapsulation reaction was carri ed out by means of an emulsion-like polymerization process at monomer starv ed conditions. The modified particles were stabilized in water with sodium- dodecyl-sulphate. A redox system consisting of cumene-hydroperoxide in comb ination with sodium-formaldehyde-sulphoxylate and iron(II) salt was applied for the initiation of the polymerization. Besides surface polymer, free po lymer particles were also formed. The parameters which varied were the fill er concentration, the concentration of the initiator components and the sur factant concentration. At optimum conditions, similar to 50% of the added m onomer polymerized at the modified filler surface, thus forming encapsulate d filler particles. SEM together with TGA analysis indicated that a smooth polymer layer had been formed on the filler surface. At high filler loading , however, coagulation occurred.