Use of a polyolefin composition for pipes and fittings with increased resistance to chlorine dioxide

1. A method for increasing the lifetime of a pipe or a fitting made of a polyolefin composition, produced in a sequential multistage process utilizing reactors coupled in series, using a combination consisting of two antioxidants (B) and (C), at least one transition metal, at least one pigment, at least one filler, at least one mineral, and at least on lubricant in said polyolefin composition,

wherein said antioxidants are the following:

antioxidant (B) with general formula R 1 —P(OAr) 2 , wherein OAr is according to formula (I):

wherein

R 1 is a non-substituted or substituted aliphatic or aromatic hydrocarbyl radicals which may comprise heteroatoms,

R 2 , R 3 , R 4 , R 5 and R 6 independently are a hydrogen atom or non-substituted or substituted aliphatic or aromatic hydrocarbyl radicals which may comprise heteroatoms,

and

antioxidant (C) according to formula (II):

wherein

R 7 , R 8 and R 9 independently are non-substituted or substituted aliphatic or aromatic hydrocarbyl radicals which may comprise OH-groups,

X 1 , X 2 , and X 3 independently are H or OH, with the proviso that at least one of X 1 , X 2 and X 3 is OH, and

the entire molecule does not comprise an ester group;

wherein the pipe or fitting is in permanent contact with chlorine dioxide-containing water.

2. The method according to claim 1 , wherein the polyolefin composition further comprises a base resin (A), which comprises an ethylene homo- or copolymer.

3. The method according to claim 2 , wherein the base resin (A) of the polyolefin composition consists of an ethylene homo- or copolymer.

4. The method according to claim 1 , wherein R 1 is OAr.

5. The method according to claim 1 , wherein R 3 and R 5 are hydrogen atoms and at least one of R 2 , R 4 and R 6 is an alkyl radical.

6. The method according to claim 1 , wherein R 7 and R 8 are alkyl radicals with 3 to 10 carbon atoms.

7. The method according to claim 1 , wherein R 9 has from 30 to 70 carbon atoms and includes one or more phenyl residues.

8. The method according to claim 1 , wherein the sum of concentration of antioxidants (B) and (C) is between 1500 and 6000 ppm.

9. The method according to claim 1 , wherein said increased lifetime of the pipe or fitting is shown in a hoop stress test according to ASTM F 2263-03 compared to a pipe or fitting made of a corresponding polyolefin composition comprising antioxidants or a combination of antioxidants other than the combination of antioxidants (B) and (C) in an equal concentration.

10. The method according to claim 9 , wherein said increased lifetime of the pipe or fitting is shown by a failure time of at least 2500 hours in a hoop stress test according to ASTM F 2263-03.