Chromium catalysed ethylene copolymer powder

1. Process for producing a chromium catalysed ethylene copolymer powder wherein the chromium catalysed ethylene copolymer powder is characterised by a particle fragmentation coefficient (“m”) equal or superior to 0.29, wherein the particle fragmentation coefficient (“m”) of the copolymer powder is defined by

m =Ln(APS particle/APS catalyst)/Ln(Productivity)

wherein

“Ln” is the “natural logarithm”,

“APS particle” is the median particle size (D50) of the polymer powder,

“APS catalyst” is the median particle size (D50) of the catalyst, and

“Productivity” is the number of grams of polymer per gram of catalyst, comprising polymerising the corresponding monomers in the presence of an activated supported chromium oxide based catalyst, wherein the process for the activation of said supported chromium oxide based catalyst is performed in a fluidised bed activation reactor which comprises a catalyst bed being fluidised by a fluidisation gas, said activation including treatment at temperatures above 500° C., wherein the fluidisation velocity (Vf1) of the fluidisation gas is initially maintained below 6.5 centimeters per second (cm/sec) until the temperature inside the activation reactor reaches at least 200° C., and said fluidisation gas is then brought to a value (Vf2) which is at least 1 cm/sec higher than Vf1.

2. Process according to claim 1 wherein said ethylene copolymer powder has been obtained by the gas phase polymerisation of ethylene together with at least one other alpha olefin containing from 3 to 12 carbon atoms in the presence of a supported chromium oxide based catalyst.

3. Process according to claim 1 wherein the control of the fluidisation gas velocity during the initial stage of the activation process is maintained until the temperature inside the activation reactor reaches 250° C.

4. Process according to claim 3 wherein the control of the fluidisation gas velocity during the initial stage of the activation process is maintained until the temperature inside the activation reactor reaches 400° C.

5. Process according to claim 1 wherein the fluidisation gas velocity during the initial stage of the activation process is maintained above 1.5 cm/s.

6. Process according to claim 1 wherein the fluidisation gas used during the initial stage of the activation process is an inert gas.

7. Process according to claim 1 wherein the fluidisation gas velocity Vf2 of the fluidisation gas used during the consecutive stage of the activation process is at least 1.5 cm/sec higher than Vf1.

8. Process according to claim 1 wherein the fluidisation gas used during the consecutive stage of the activation process is chosen amongst inert gases, or oxidising gases, or sequentially an inert gas followed by an oxidising gas.

9. Process according to claim 1 wherein said consecutive fluidisation stage includes thermal treatments with holding temperatures higher than 600° C.

10. Process according to claim 1 wherein the fluidisation gas velocity Vf2 is at least 6 cm/s.

11. Process according to claim 1 wherein the fluidisation gas velocity Vf2 is maintained below 12 cm/s.

12. Process according to claim 1 wherein the duration of the initial fluidisation stage lasts at least 1 hour.

13. Process according to claim 1 wherein the duration of the consecutive fluidisation stage lasts at least 1 hour.

14. Process according to claim 1 wherein the consecutive fluidisation stage comprises at least two separate thermal treatment steps wherein

the first step consists in bringing the catalyst from the initial stage under an inert atmosphere to a temperature comprised between 600 and 900° C. and then keeping the catalyst during a holding time of less than or equal to 8 hours under an inert atmosphere at a temperature comprised between 600 and 900° C., and

the second step consists in the treatment of the catalyst coming from step 1 under an oxidizing atmosphere at a temperature comprised between 400 and 700° C. during a holding time comprised between 2 and 10 hours, and

wherein the maximum temperature of the second step is always lower than the maximum temperature of the first step and wherein the difference between the maximum temperature of the first step and the maximum temperature of the second step is comprised between 50 and 250° C.

15. Process according to claim 1 wherein the supported chromium oxide based catalyst used to produce the powder is a titanium modified supported chromium oxide based catalyst.

16. Process according to claim 1 wherein the catalyst support is a refractory oxide support selected amongst silica, alumina, aluminophosphate, metal oxides, or combinations thereof.

17. Process according to claim 1 wherein the catalyst support is a spherical or spheroidal refractory oxide support.

18. Process according to claim 1 wherein the chromium oxide based catalyst contains 0.3 to 2% by weight of chromium being expressed with respect to the weight of final supported catalyst.

19. Process according to claim 1 wherein the chromium oxide based catalyst contains between 0.5 and 5% by weight of titanium being expressed with respect to the weight of final supported catalyst.

20. Process according to claim 1 wherein the fluidization velocity Vf1 of the fluidization gas is initially maintained below 5.5 cm/sec.

21. Process according to claim 1 wherein the fluidization gas velocity Vf2 is at least 7 cm/sec.

22. Process according to claim 18 wherein the chromium oxide based catalyst contains 0.3 to 1% by weight of chromium being expressed with respect to the weight of final supported catalyst.

23. Process according to claim 19 wherein the chromium oxide based catalyst contains between 1.5 and 4% by weight of titanium being expressed with respect to the weight of final supported catalyst.