Polyolefin composition for medium/high/extra high voltage cables comprising benzil-type voltage stabiliser

1. A polyolefin composition comprising

(A) greater than 79 wt % and less than or equal to 99.99 wt % of a cross-linkable polyolefin, and

(B) 0.01 wt % to 5 wt % of a voltage stabilizer comprising a benzil derivative comprising the structural unit according to the following formula (I):

wherein

R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 independently from each other are hydrogen, or a hydrocarbyl group which may contain heteroatoms or at least two of said R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic or non-aromatic ring fused to the ring system of formula (I) and

at least R 1 or R 2 is an ester group.

2. The polyolefin composition according to claim 1 wherein R 1 and R 2 are both ester groups.

3. The polyolefin composition according to claim 2 wherein R 1 and R 2 are the same ester group.

4. The polyolefin composition according to claim 1 wherein the ester group is selected from the group consisting of CH 3 CO 2 —, C 2 H 5 CO 2 —, C 3 H 7 CO 2 —, C 4 H 9 CO 2 —, C 5 H 11 CO 2 —, C 6 H 13 CO 2 —, C 7 H 15 CO 2 —, C 8 H 17 CO 2 —, C 9 H 19 CO 2 —, C 10 H 21 CO 2 — and C 11 H 23 CO 2 —.

5. The polyolefin composition according to claim 1 wherein R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are hydrogen.

6. The polyolefin composition according to claim 2 wherein at least one of R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 is an alkyl group or at least two of said R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic ring fused to the ring system of formula (I), and the remaining of R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are hydrogen.

7. The polyolefin composition according to claim 5 wherein R 1 and R 2 are CH 3 CO 2 — or C 11 H 23 CO 2 — and R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are hydrogen.

8. A master batch comprising

(i) greater than 79 wt % and less than or equal to 99.99 wt % of a cross-linkable polyolefin matrix polymer, and

(ii) 0.01 wt % to 5 wt % of a voltage stabilizer comprising a benzil derivative (B) comprising the structural unit according to the following formula (I):

wherein

R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 independently from each other are hydrogen, or a hydrocarbyl group which may contain heteroatoms or at least two of said R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic or non-aromatic ring fused to the ring system of formula (I) and

at least R 1 or R 2 is an ester group.

9. A cable comprising a layer made of a polyolefin composition according to claim 1 .

10. The cable according to claim 9 wherein the cable is a medium, high or extra high voltage cable comprising an inner semiconductive layer, an insulating layer and an outer semiconductive layer.

11. The cable according to claim 10 wherein at least the insulating layer is made of a polyolefin composition.

12. The polyolefin composition according to claim 3 wherein at least one of R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 is an alkyl group or at least two of said R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic ring fused to the ring system of formula (I), and the remaining of R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are hydrogen.

13. The polyolefin composition according to claim 2 , wherein at least two of said R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic ring fused to the ring system of formula (I), and the remaining of R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are hydrogen.

14. The polyolefin composition according to claim 3 , wherein at least two of said R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic ring fused to the ring system of formula (I), and the remaining of R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 are hydrogen.

15. The polyolefin composition according to claim 1 , wherein compound (B) is present in the polyolefin composition in an amount of 0.05 wt % to 4 wt %.

16. The polyolefin composition according to claim 1 , wherein the cross-linkable polyolefin comprises hydrolysable silane groups for achieving cross-linking; and wherein the benzil derivative will not react or decompose in the cross-linking.

17. The polyolefin composition according to claim 1 ,

wherein the benzil derivative will not react or decompose in the cross-linking.

18. The polyolefin composition according to claim 1 , wherein the cross-linkable polyolefin comprises an unsaturated low density polyethylene homopolymer.

19. The polyolefin composition according to claim 1 , wherein the cross-linkable polyolefin comprises an unsaturated low density polyethylene copolymer.

20. A polyolefin composition consisting of

(A) a cross-linkable polyolefin,

(B) a voltage stabilizer comprising a benzil derivative comprising the structural unit according to the following formula (I):

wherein

R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 independently from each other are hydrogen, or a hydrocarbyl group which may contain heteroatoms or at least two of said R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 and R 10 together with the ring atoms of the ring system of formula (I) they are attached to, form a further aromatic or non-aromatic ring fused to the ring system of formula (I) and

at least R 1 or R 2 is an ester group; and

(C) a cross-linking agent in an amount of less than 10 wt % of the polyolefin composition;

(D) a scorch retarder in an amount of 0.005 wt % to 1.0 wt % of the polyolefin composition; and

(E) further additive(s) in a total amount of 0.001 wt % to 5 wt % of the polyolefin composition;

wherein compound (B) is present in the polyolefin composition in an amount of 0.01 wt % to 5 wt %.