Process for the polymerisation of olefins

1. Process for the manufacture of an ethylene co-polymer having, after compounding:

a density between 936 and 941 kg/m 3 and a MI5 between 0.5 and 1.2 dg/min and the pipes produced with this ethylene copolymer meet the requirements of ISO 4427 and ISO 4437 in hydrostatic pressure testing at 20° C. and 9.0 MPa with a ductile failure at more than 100 hours, or

a density between 942 and 946 kg/m 3 and a MI5 between 0.4 and 0.6 dg/min and the pipes produced with this ethylene copolymer meet the requirements of ISO 4427 and ISO 4437 in hydrostatic pressure testing at 20° C. and 9.0 MPa with a ductile failure at more than 100 hours, or

a density between 943 and 947 kg/m 3 and a MI5 between 0.1 and 0.4 dg/min and the pipes made from this ethylene copolymer and tested in hydrostatic pressure testing at 20° C. under 12.0 MPa have a time to failure at more than 100 hours,

the above hydrostatic pressure testing being carried out following ISO1167-1 and 2, and

the process comprising bringing ethylene and one or more other olefins into contact with a titanium modified supported chromium oxide based catalyst under polymerizing conditions,

wherein

the titanium modified supported chromium oxide based catalyst is supported on a refractory oxide and

has been subjected to a two step thermal treatment wherein

the first step consists of bringing the catalyst under an inert atmosphere to a temperature 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 between 600 and 900° C., and

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

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

and further wherein the preparation of the catalyst is not carried out by a cogel method and involves the introduction of a titanium compound on to a support by addition and/or impregnation.

2. Process according to claim 1 wherein the refractory oxide is spherical and/or spheroidal.

3. Process according to claim 2 wherein the spherical and/or spheroidal refractory oxide consists of silica.

4. Process according to claim 1 wherein the titanium modified supported chromium oxide based catalyst contains 0.3 to 2% by weight of chromium.

5. Process according to claim 1 wherein the titanium modified supported chromium oxide based catalyst contains between 0.5 and 5% by weight of titanium.

6. Process according to claim 1 wherein a holding time during the first step of thermal treatment is less than or equal to 6 hours and is at least 30 minutes.

7. Process according to claim 1 wherein the temperature of the first step of thermal treatment is at least 650° C. and is less than or equal to 850° C.

8. Process according to claim 1 wherein the second step has a holding time of at least 3 hours and is less than or equal to 9 hours.

9. Process according to claim 1 wherein the temperature of the second step is between 500 and 700° C.

10. Process according to claim 1 wherein the difference between the maximum temperature of the first step and the maximum temperature of the second step is between 50 and 200° C.

11. Process according to claim 1 wherein the first step is performed under nitrogen.

12. Process according to claim 1 wherein a total duration of any thermal treatment (non oxidising and oxidising steps) above 300° C. of the titanium modified supported chromium oxide based catalyst is less than 24 hours.

13. Process according to claim 1 which is a process for the gas phase polymerisation of ethylene.

14. Process according to claim 1 wherein ethylene is copolymerised with one or more C3 to C8 olefins.

15. Process according to claim 1 wherein the oxidising atmosphere of the second step is air.

16. Process according to claim 4 wherein the titanium modified supported chromium oxide based catalyst contains 0.3 to 1% by weight of chromium.

17. Process according to claim 5 wherein the titanium modified supported chromium oxide based catalyst contains between 1.5 and 4% of titanium.

18. Process according to claim 7 wherein the temperature of the first step of thermal treatment is at least 700° C. and is less than or equal to 800° C.

19. Process according to claim 9 wherein the temperature of the second step is between 600 and 700° C.

20. Process according to claim 10 wherein the difference between the maximum temperature of the first step and the maximum temperature of the second step is between 100 and 200° C.