Process for making a cross-linked polyethylene article

Abstract

A process for for making a cross-linked polyethylene article includes: (A) feeding a blend comprising a cross-linkable silane-group containing polyethylene copolymer and at least one additive to an extruder, (B) feeding a liquid comprising a silanol condensation catalyst to said extruder to form a mixture with said blend, (C) extruding said mixture to form an article and (D) cross-linking said article. The article may be a pipe or cable.

Claims

1. A process for making a cross-linked polyethylene article comprising: (A) feeding a blend comprising a cross-linkable silane-group containing polyethylene copolymer and at least one additive to an extruder, (B) feeding a liquid comprising a silanol condensation catalyst to said extruder to form a mixture with said blend, (C) extruding said mixture to form the article and (D) cross-linking said article.

2. The process according to claim 1 wherein the blend and the liquid are separately fed to the extruder.

3. The process according to claim 1 wherein the blend and the liquid are premixed before entering the extruder.

4. The process according to claim 1 wherein the at least one additive is an antioxidant.

5. The process according to claim 1 wherein the at least one additive is a scorch retarder.

6. The process according to claim 5 wherein the scorch retarder is a silane-containing compound.

7. The process according to claim 6 wherein the cross-linkable silane-group containing polyethylene copolymer is a copolymer of ethylene and a tri-alkoxy vinyl silane or vinyl tri-methoxy silane.

8. The process according to claim 1 wherein the cross-linkable silane-group containing polyethylene copolymer is formed in a reactor under a pressure of 500 to 4,000 kg/cm.sup.2.

9. The process according to claim 1 wherein the silanol condensation catalyst comprises a metal and a Lewis acid.

10. The process according to claim 1 wherein the silanol condensation catalyst is dioctyl tin laureate (DOTL) or zirconium acetylacetonate.

11. The process according to claim 1 which is peroxide free.

12. The process according to claim 1 wherein the article is a cable and the extruder is a cable extruder.

13. The process according to claim 1 wherein the blend comprises all the additives of the composition to be formed, except for the silanol condensation catalyst and optionally a drying agent.

14. The process according to claim 1 wherein the liquid consist of the silanol condensation catalyst or the silanol condensation catalyst and a drying agent.

15. The process according to claim 1 further comprising: forming the blend prior to the feeding.

16. The process according to claim 1, wherein the blend is in the form of pellets.

17. The process according to claim 1, wherein the blend is in the form of a powder.

18. The process according to claim 1, wherein the blend is in the form of a second liquid.

19. The process according to claim 1, wherein the liquid consists of dioctyl tin laurate.

20. A process for making a cross-linked polyethylene article comprising: (A) feeding a blend comprising a cross-linkable silane-group containing polyethylene copolymer and at least one additive to an extruder, (B) feeding a liquid comprising a silanol condensation catalyst to said extruder to form a mixture with said blend, (C) extruding said mixture to form the article and (D) cross-linking said article; wherein the blend is in the form of pellets comprising an ethylene vinyl silane copolymer and vinyl trimethoxy silane.

Description

EXAMPLES

(1) The present invention will now be described in more detail by reference to the following inventive examples and reference examples.

Inventive Examples 1 to 6

(2) The polymer compositions of the inventive examples are summarized in Table 1.

(3) The blend used in all the examples is a pelletized reactor made ethylene vinyl silane copolymer (EVS) and includes vinyl tri-methoxy silane (VTMS) as additive. The blend contains 1.3 wt % VTMS, has a melt index of 1.0 g/10 min, and density of 0.923 g/cm.sup.3.

(4) As silanol condensation catalyst, dioctyl tin-laurate (DOTL) is used in example 1 to 5. In examples 1 to 4 the liquid consists of the sole catalyst, whereas in example 5, the liquid consists of the catalyst and vinyl tri-methoxy silane (VTMS). In example 6, dodecyl benzyl sulphonic acid (DBSA) is used as silanol condensation catalyst and the liquid further includes VTMS.

(5) The EVS blend and the liquid are preheated at 60 C. Then, the liquid is added over the EVS pellets followed by intensive dry blending in order to distribute the catalyst all over the pellets surface. Immediately after the mixing of the blend and the liquid, under humidity controlled conditions, the material is added to the extruder and tapes 0.2 mm thick are extruded at 190 C. A normal extruder screw is used (compression ratio 1:3.6).

(6) Then, crosslinking is performed in a water bath at temperatures of 90 C. for 24 hours. After, the samples are placed in a constant room set up at 24 C. and 50% humidity for 24 h. Then samples of desired length are cut out.

(7) TABLE-US-00001 TABLE 1 Component Ex1 Ex2 Ex3 Ex4 Ex5 Ex6 Blend 99.92 wt % 99.88 wt % 99.825 wt % 99.90 wt % 99.89 wt % 99.89 wt % EVS-VTMS EVS-VTMS EVS-VTMS EVS-VTMS EVS-VTMS EVS-VTMS Liquid 0.08 wt % 0.12 wt % 0.175 wt % 0.10 wt % 0.11 wt % 0.11 wt % DOTL DOTL DOTL DOTL 9DOTL/1VTMS 9DBSA/1VTMS

Reference Examples 1 to 4

(8) The polymer compositions of the reference examples are summarized in Table 2.

(9) In Reference examples 1 and 2, the same blend as in the inventive example is used.

(10) Instead of a liquid comprising a catalyst, a master batch is used and added to the EVS polymer. The master batches used contain either 0.175 wt % DOTL or 0.075 wt % DBSA as silanol condensation catalyst. The master batch further contains ethylene-butyl acrylate-copolymer produced in a high-pressure tubular process having a butyl acrylate content of 17 wt %, an MFR.sub.2 (ISO 1133, 190 C., 2.16 kg) of 7.0 g/10 min and a density of 924 kg/m.sup.3.

(11) In reference 3 and 4 a linear low-density polyethylene (LLDPE), having melt index of 2.8 g/10 min and density of 0.918 g/cm.sup.3 is used.

(12) A liquid mixture of VTMS, dicumylperoxide (DCP), and DBTL is added to the LLDPE and e reactive extrusion is performed. The used mixture is Dynasylan Silfin 06 from Evonik.

(13) TABLE-US-00002 TABLE 2 Component Ref. 1 Ref. 2 Ref. 3 Ref. 4 Blend 95 wt % 95 wt % EVS-VTMS EVS-VTMS Master batch 5 wt % 5 wt % DOTL- DBSA- MB MB Olefin 99 wt % 98.4 wt % LLPDE LLPDE Liquid blend 1 wt % DCP- 1.6 wt % VTMS-DBTL DCP-VTMS- DBTL

(14) All the mixtures are extruded at 190 C. Then, cross-linking is performed in a water bath at temperatures of 90 C. for 24 hours. After, the samples are placed in a constant room set up at 24 C. and 50% humidity for 24 h. Then samples of desired length are cut out.

(15) The results of the tests performed on the cross-linked tapes are reported in Table 3 and compared to the results obtained for the inventive examples.

(16) All the inventive examples feature a hot set elongation value below 100, making the articles of the invention suitable for cable applications.

(17) All the analyzed samples, inventive examples and references, feature a surface quality which is acceptable for cable applications. The best combination of hot set elongation and tape surface quality are obtained for Ex6.

(18) In conclusion, with the simplified process of the present invention, cross-linked polyethylene articles suitable for cable applications are obtained.

(19) TABLE-US-00003 TABLE 3 Test Ex3 Ex4 Ex5 Ex 6 Ref. 1 Ref. 2 Ref. 3 Ref. 4 Hot set 34 72.9 71.5 37 30 26 157.2 35.8 Tape surface quality rough smooth very very very very rough very (stable conditions) smooth smooth smooth smooth rough No and size of 10-15 10-15 5 10-15 10-15 gels/ cm.sup.2 ~1 mm <0.1 mm <0.1 mm ~1 mm ~1 mm Tape surface quality smooth, some smooth, some rough, many very rough, after 5 min stop small gels small gels large gels many large gels Tape surface quality rough, many smooth, some very rough, after 30 min stop small gels, some small gels many large gels uneven edges Easy of cleaning Neutral Easy Easy Hard Very hard