Method for producing a coating layer made of expandable and cross-linkable material in a cable

1. A method for producing a cable comprising:

at least one transmissive element; and

an expanded and cross-linked coating layer in a radially outer position with respect to said at least one transmissive element, said coating layer comprising a composition including an expandable cross-linkable polymeric material;

said method comprising the steps of:

a) extruding said composition via around said at least one transmissive element via an extruder;

b) forming a coating layer made of expandable and cross-linkable polymeric material with the composition thus extruded;

c) expanding said coating layer made of expandable and cross-linkable polymeric material; and

d) cross-linking said coating layer made of expandable and cross-linkable polymeric material;

said expanding and cross-linking steps c) and d) being carried out downstream of said extruder by heating said coating layer made of expandable and cross-linkable polymeric material at atmospheric pressure by means of a heating fluid, and

wherein said method is operated in a manner to avoid expansion and cross-linking of said expandable and cross-linkable polymeric material prior to said expanding and cross-linking steps c) and d).

2. The method according to claim 1 , wherein said heating fluid is subjected to forced circulation.

3. The method according to claim 1 , wherein said heating fluid is subjected to forced circulation at a rate between about 2 and about 80 m/s.

4. The method according to claim 1 , wherein said expanding and cross-linking steps c) and d) are carried out by heating said coating layer made of expandable and cross-linkable polymeric material to a temperature between about 120° C. and abut 400° C.

5. The method according to claim 1 , wherein said heating fluid is selected from the group of air and inert gases.

6. The method according to claim 1 , wherein said composition comprises at least one expanding agent and at least one cross-linking agent.

7. The method according to claim 6 , wherein said at least one expanding agent and said at least one cross-linking agent have respective decomposition temperatures which differ from each other by at most about 50° C.

8. The method according to claim 6 , wherein said at least one cross-linking agent is selected from the group of organic peroxides and sulphur.

9. The method according to claim 8 , wherein said at least one cross-linking agent is selected from the group of 2,5-dimethyl-2,5-bis-(ter-butylperoxy)hexane, 2,5-dimethyl-2,5-bis(ter-butylperoxy)hexine-3-di-ter-butylperoxide, bis-(ter-butylperoxyisopropyl)benzene, ter-butylcumylperoxide, dicumylperoxide, 4,4′-di-terbutylperoxy-n-butylvalerate, ter-butylperoxy-3,5,5-trimethylhexanoate, 1,1-di-ter-butylperoxy-3,3,5-trimethylcyclohexane, ter-butylperoxybenzoate, dibenzoylperoxide, bis-(2,4-dichlorobenzoyl)peroxide, bis-(p-chlorobenzoyl) peroxide, 2,2-di-ter-butylperoxybutane, ethyl-3,3-di-ter-butylperoxybutyrate, and 2,2′-azo-di-(2-acetoxypropane).

10. The method according to claim 6 , wherein said at least one expanding agent is selected from the group of oxydibenzyl sulphonhydrazide, azodicarbamide, paratoluene sulphonylhydrazide, and mixtures of organic acids with carbonates and/or bicarbonates.

11. The method according to claim 1 , further comprising the step of cooling said cable provided with said expanded and cross-linked coating layer.

12. The method according to claim 1 , further comprising the step of providing said cable having said expanded and cross-linked coating layer with a metallic screen.

13. The method according to claim 12 , further comprising the step of coating said metallic screen with an outer sheath.