Process for the production of highly expanded polyolefin insulated wires and cables

• US 4,204,086 A
• Filed: 02/27/1976
• Issued: 05/20/1980
• Est. Priority Date: 05/23/1972
• Status: Expired due to Term

• Current Assignee: Sumitomo Electric Industries Limited (Sumitomo Corporation)
• Original Assignee: Sumitomo Electric Industries Limited (Sumitomo Corporation)
• Inventors: Suzuki, Fumio
• Primary Examiner(s): Anderson, Philip
• Application Number:
  US05/661,875
• Time in Patent Office: 1,544 Days
• Field of Search: 264/45.9, 264/46.1, 264/45.5, 264/101, 264/174, 264/DIG. 5, 174/28, 174/102 R, 174/110 F
• US Class Current: 174/102.00R
• CPC Class Codes: B29C 44/322 the preformed parts being elongated inserts, e.g. cables
• CPC Class Codes: H01B 13/067 Insulating coaxial cables H01B13/20 takes precedence
• CPC Class Codes: H01B 13/24 by extrusion extrusion of cables with plastic material in general B29C48/15
• CPC Class Codes: Y10S 264/05 Use of one or more blowing agents together

Abstract

In a process for producing a highly expanded polyolefin insulated coaxial cable comprising the steps of feeding a polyolefin, a nucleating agent and a volatile liquid as a blowing agent to an extruder and heat extruding said mixture through said extruder onto an inner conductor passing through said extruder to cover the outer periphery of said inner conductor thereby forming an insulation layer of a highly expanded polyolefin foam having a foaming ratio in excess of 2.5 times (60% expansion degree), the improvement which includes the step of, after extruding the foaming polyolefin mixture around said inner conductor, sizing the outer surface of said foaming polyolefin layer on said inner conductor by means of a sizing die to provide a highly expanded uniform polyolefin insulation layer of a predetermined size, said insulation layer being tightly adhered to said inner conductor.

Claims

1. In a process for producing a highly expanded polyolefin insulated coaxial cable having excellent high frequency characteristics comprising the steps of feeding a polyolefin, a nucleating agent and a volatile liquid blowing agent to an extruder and heat extruding said mixture through said extruder onto an inner conductor passing through said extruder to cover the outer periphery of said inner conductor thereby forming an insulation layer of a highly expanded polyolefin foam having a foaming ratio in excess of 2.5 times (60% expansion degree), the improvement which includes the step of, after extruding the foaming polyolefin mixture around said inner conductor, sizing the outer surface of said foaming polyolefin layer on said inner conductor by means of a sizing die having a parallel land at normal atmospheric pressure to prevent free foaming in the sizing die and to obtain the highly expanded uniform polyolefin insulation layer while providing tight adhesion of said insulation layer to said inner conductor due to the absence of a space between said foaming material and said inner conductor, which space, if present, would permit free expansion on said foaming material and wherein the temperature of the sizing die is maintained lower than the boiling point of the volatile liquid, said volatile liquid blowing agent being in liquid form and providing a lubricating effect for the foamed material being sized.

2. The process for producing a highly expanded polyolefin insulated cable as defined in claim 1, wherein said polyolefin is selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene and mixtures thereof.

3. The process as claimed in claim 1, wherein said nucleating agent comprises a solid blowing agent and a decomposition accelerator for said blowing agent and wherein said process includes compounding each of said blowing agent and said decomposition accelerator individually with a portion of said polyolefin.

4. The process as claimed in claim 1, wherein said volatile liquid is fed into said extruder as in a conventional injection process.

5. The process as claimed in claim 1, wherein said volatile liquid is fed into said polyolefin as in a conventional swelling process, the mixture then being fed into said extruder.

6. The process as claimed in claim 1, wherein the flow rate of said polyolefin mixture from a die at the head of said extruder is greater than the feeding rate of said inner conductor.

7. The process of claim 1, wherein said inner conductor is unheated.

8. The process of claim 1, wherein said foaming polyolefin mixture is applied directly to said inner conductor.

9. The process of claim 1, wherein the temperature of the extruder used for the extruding is maintained at a temperature higher than that of the sizing die.

10. The process as claimed in claim 1, wherein said sizing die has the same diameter as the extruded outer surface of the foaming polyolefin layer.

11. The process as claimed in claim 10, wherein said polyolefin mixture is substantially completely foamed during said extrusion.

12. The process as claimed in claim 1, wherein said polyolefin is a mixture of a low density polyethylene and a high density polyethylene.

13. The process as claimed in claim 12, wherein said mixture comprises about 80 to 20% high density polyethylene, balance low density polyethylene, based on the polyethylene present.

14. The process as claimed in claim 13, wherein said low density polyethylene has a density of less than 0.925 g/cm³ and said high density polyethylene has a density of greater than 0.94 g/cm³.

15. The process as claimed in claim 1, wherein said extruding is into said sizing die without any unconfined expansion of the extruded polyolefin mixture.

16. The process as claimed in claim 1, wherein the compression strength of the coaxial cable is increased due to radial shrinkage being prevented by said sizing die.

17. In a process for producing a highly expanded polyolefin insulated coaxial cable comprising the steps of feeding a polyolefin, a nucleating agent and a volatile liquid blowing agent to an extruder and heat extruding said mixture through said extruder onto an inner conductor passing through said extruder to cover the outer periphery of said inner conductor, thereby forming an insulation layer of a highly expanded polyolefin foam having a foaming ratio in excess of 2.5 times (60% expansion degree), the improvement which includes the step of applying a pressure reduction around said inner conductor up to an area near the contacting point of said foaming polyolefin mixture and said inner conductor but prior to said contacting point, and after extruding the foaming polyolefin mixture around said inner conductor, sizing the outer surface of said foaming polyolefin layer on said inner conductor by means of a sizing die having a parallel land at normal atmospheric pressure to prevent free foaming in the sizing die to obtain a uniform size of the highly expanded uniform polyolefin insulation layer while providing tight adhesion of said insulation layer to said inner conductor.

18. The process of claim 17, wherein the inner conductor is heated to thereby tightly adhere the foaming polyolefin insulation layer thereto.

19. The process of claim 17, wherein said inner conductor is heated prior to said pressure reduction, and after said pressure reduction said inner conductor is passed through a zone cooled to a temperature below the foaming point of the polyolefin, nucleating agent and volatile liquid mixture.

20. The process of claim 19, wherein the foamed polyolefin mixture has a foaming ratio in excess of 3.

21. The process of claim 20, wherein said pressure reduction is a pressure reduction of about 5 mmHg to about 200 mmHg.

22. Highly expanded polyolefin insulated coaxial cable having excellent high frequency characteristics wherein the inner conductor coated with the layer of insulation is produced by a process comprising the steps of feeding a polyolefin, a nucleating agent and a volatile liquid blowing agent to an extruder and heat extruding said mixture through said extruder onto an inner conductor passing through said extruder to cover the outer periphery of said inner conductor, thereby forming an insulation layer of a highly expanded polyolefin foam having a foaming ratio in excess of 2.5 times (60% expansion degree), wherein the improvement includes the step of, after extruding the foaming polyolefin mixture around said inner conductor, sizing the outer surface of said foaming polyolefin layer on said inner conductor by means of a sizing die having a parallel land at normal atmospheric pressure to prevent free foaming in the sizing die to obtain a uniform size of the highly expanded uniform polyolefin insulation layer while providing tight adhesion of said insulation layer to said inner conductor.