Combination of silica and graphite and its use for decreasing the thermal conductivity of vinyl aromatic polymer foam

Expandable polymer granulate comprising one or more propellants, a) silica, b) graphite and c) polystyrene homopolymer or copolymer, wherein

• a) the silica is amorphous and has
• a BET surface of from 1 to 100 m2/g,
• an average particle size in a range of from 3 nm to 1,000 nm, and
• the silica is present in an amount of from 0.01 to less than 2 wt. %, based on the weight of the polymer (inclusive of solid and, if any, liquid additives, but exclusive of propellant), and
• b) the graphite has
• a carbon content in a range of from 50 to 99.99 wt. % and
• a particle size in a range of from 0.01 to 100 μm, and
• the graphite is present in an amount in a range of from 0.01 to 10 wt. %, based on the weight of the polystyrene homopolymer or copolymer (inclusive of solid and, if any, liquid additives, but exclusive of propellant)
• wherein the silica and the graphite are present in a weight ratio in a range of from 1:1.5 to 1:10.