Expandable vinyl aromatic polymers containing graphite particles having a polymodal particle size distribution
Expandable vinyl aromatic polymers may contain comminuted graphite with a polymodal particle size distribution. Foams obtained from such expandable vinyl aromatic polymers have a reduced thermal conductivity at a reduced foam density.
1. Expandable vinyl aromatic polymers comprising platelet graphite particles having a polymodal particle size distribution, wherein 45% or more of a total amount of the platelet graphite particles have a diameter between 3 μm and 20 μm, and wherein 40% or less of the total amount of the platelet graphite particles have a diameter of less than 3 μm.
2. The expandable vinyl aromatic polymers according to claim 1 , wherein 60% or more of the total amount of the platelet graphite particles have a diameter between 3 μm and 20 μm, and wherein 35% or less of the total amount of the platelet graphite particles have a diameter of less than 3 μm.
3. The expandable vinyl aromatic polymers according to claim 1 , wherein said polymodal particle size distribution is a bimodal or a trimodal distribution.
4. The expandable vinyl aromatic polymers according to claim 1 , wherein the platelet graphite particles are characterized by a BET of 20 m 2 /g or less according to ASTM D-3037-93.
5. The expandable vinyl aromatic polymers according to claim 1 , wherein the platelet graphite particles have an aspect ratio between 1 and 200.
6. The expandable vinyl aromatic polymers according to claim 1 , wherein the expandable vinyl aromatic polymers comprise between 1 and 10% by weight of the platelet graphite particles having the polymodal particle size distribution.
7. The expandable vinyl aromatic polymers according to claim 1 , wherein the expandable vinyl aromatic polymers comprise between 0.2 and 25% by weight of flame retardant agent, between 2 and 10% by weight of blowing agent, and between 0.01 and 5% by weight of foam cell regulator.
8. The expandable vinyl aromatic polymers according to claim 1 , wherein the expandable vinyl aromatic polymers comprise between 0.2 and 25% by weight of flame retardant agent, between 2 and 10% by weight of blowing agent, between 0.1 and 1.0% by weight of polyethylene wax, and between 0.25 and 3.0% by weight, of talc.
9. A process for the preparation of beads or granules of expandable vinyl aromatic polymer comprising platelet graphite particles having a polymodal particle size distribution, wherein 45% or more of a total amount of the platelet graphite particles have a diameter between 3 μm and 20 μm, and wherein 40% or less of the total amount of the platelet graphite particles have a diameter of less than 3 μm, the process comprising:
a) producing a main polymer melt stream after a polymerization process of the vinyl aromatic polymer;
b) introducing a blowing agent into said main polymer melt stream;
c) deriving a part of said main polymer stream and creating a side loop with an additional polymer melt stream;
d) dispersing the platelet graphite particles and foam cell regulator into said additional polymer melt stream;
e) joining the additional polymer stream and the main polymer melt stream;
f) introducing flame retardant agent and synergist into the main polymer melt stream;
g) cooling the polymer melt down to a temperature lower than 200° C.; and
h) discharging the polymer melt through a die plate with holes and pelletizing the polymer melt under water.
10. The process according to claim 9 , wherein between 10 and 30% of the main polymer melt stream is derived in step c) to form the additional polymer melt stream.
11. The process according to claim 9 , wherein, in step d), the platelet graphite particles and the foam cell regulator are dispersed in the additional polymer melt stream by means of an extruder.
12. The process according to claim 9 , wherein, in step f), the flame retardant agent and the synergist are blended, in a separate extruder, with the vinyl aromatic polymer before being introduced in the main polymer melt stream, comprising the platelet graphite particles and the foam cell regulator.
13. Polymer foams obtained from the expandable vinyl aromatic polymers according to claim 1 .
14. The expandable vinyl aromatic polymers according to claim 1 , wherein the platelet graphite particles are characterized by a BET of 10 m 2 /g or less according to ASTM D-3037-93.
15. The expandable vinyl aromatic polymers according to claim 1 , wherein the platelet graphite particles are characterized by a BET of 5 m 2 /g or less according to ASTM D-3037-93.
16. The expandable vinyl aromatic polymers according to claim 1 , wherein the platelet graphite particles have an aspect ratio between 5 and 100.
17. The expandable vinyl aromatic polymers according to claim 1 , wherein the platelet graphite particles have an aspect ratio between 5 and 50.
18. The expandable vinyl aromatic polymers according to claim 1 , wherein the expandable vinyl aromatic polymers comprise between 2 and 6% by weight of the platelet graphite particles having the polymodal particle size distribution.
19. The expandable vinyl aromatic polymers according to claim 1 , wherein the expandable vinyl aromatic polymers comprise between 1 and 15% by weight of flame retardant agent, between 3 and 7% by weight of blowing agent, between 0.1 and 0.5% by weight of polyethylene wax, and between 0.5 and 2.0% by weight of talc.