Introduction

There are several methods of Cross-linking of polyethylene .This article covers only two types of Cross-linked Polyethylene insulation as they constitute largest commercial application in LV & MV – power cable manufacturing.

A. Peroxide XLPE:

Peroxide-XLPE (for CCV) is a thermo set compound of LDPE produced by incorporating

(a) Cross-linking agent such as Di Cumyl Peroxide.

(b) Antioxidant additive cum scorch Retardant.

There are two competitive processes for compounding of XLPE:

• Soaking of Dicumyl Peroxide in molten stage on Clean filtered LDPE.

• Direct injection of DCP and AO in molten LDPE followed by screening before granulation.

B. Moisture Curable XLPE:

The second method of cross-linking Polyethylene viz Silane-Moisture Cross-linking technology was introduced in the 1968 by Dow Corning with the Sioplas-2- step process.

The latest being Ethylene-Vinyl Silane (EVS) copolymeri-zation process developed by Mitsubishi.

Myth Pertaining to Peroxide Curable – XLPE

MYTH: 01: Cleanliness of Polymer and Environment

The myth is that cleanliness of the Polymers can be ensured by close loop system; hence the Polymer producers are the only eligible producers of these grades.

With increased demand it is possible to procure good and clean PE in packages which are clean and can be transported without compromising the cleanliness. These are the packages which avoid any possibility of contamination.
Polymer (LDPE) cleanliness is ensured by further filtration after incorporation of additive by use of very fine filter (multipack) before granulation.
Creation of clean environment for handling of polymer and compounding possible by proper filtrations of air and measurement of cleanliness of air by use of advanced Air Particle Counter meters and maintaining the same by judicious control of filter cleaning schedule.
Online and Offline measurement of contamination in LDPE and XLPE by use of advanced Optical Control System Machine is possible with advancement of process technologies.

MYTH: 02: Consistent Degree of Cross-linking

The myth is that there could be variance in degree of cross linking if the process quantities are not large enough.

Precise dosing of LDPE, Antioxidant and Peroxides by use of state of art Loss-in-Weight-Feeders.
Online monitoring of Rheological properties by Oscillating Disc Rheometer(ODR) and/or Moving Die Rheometer (MDR).
The advancement of process technologies have made it possible to usage of high-tech process control tools by relatively small players and it is no more a tool available only to Polymer producers.

MYTH: 03: Suitability beyond Medium Voltage Application

The myth is that product for application beyond MV may not be produced by direct injection method of compounding.

Since the awareness with regard to requirement of PE for High and Extra High voltage cable have gone up in last couple of years. It is possible to procure suitably designed grades of PE to produce HV Compounds.
Availability of filtration techniques and equipments at low pressure make it further possible to produce the right product for HV applications.
Online testing of product ensures that each and every granule is tested and nothing is left to chance.

MYTH: 04: Life of Cable

The myth is that the product made through injection method may result in decreased service life of the cables.

Cleaner XLPE insulation leads to a much longer in service life for cables – higher level of purification/ filtration of LDPE, Antioxidants and Peroxides ensures cleaner XLPE.
Better ageing properties (1500C for 10 Days) make it possible to achieve by judicious selection of Antioxidants additive and dosage of same. Superior ageing properties will ensure cable life with direct injection compounded material comparable with product coming from PE producers.
Tree Retardant additives (polymer or chemical) could also be incorporated by the direct injection process there by eliminating any impact on cable life made by compounding route.

Myth Pertaining to Moisture Curable – XLPE

MYTH: 01: Not good above 1.1 KV

Easily possible to manufacture up to 11 KV by using right compatible grades of semiconducting compounds.
Possible to make up to 33 KV by use of right grades of PE compound as well as semiconducting compounds and use of Triple Co-extrusion of insulation and Semicon.

MYTH: 02: Shelf-life too short in case of grafted polymers in comparison to Copolymers

It is possible to improve shelf life greatly by following:

By fine tuning of chemistry during grafting.
Packaging of grafted polymer in packages having suitable barrier properties.
Using Nitrogen during packing enhances shelf life.
Advance packaging materials and packaging machines have helped grafted manufacturers a great deal to enable them deliver reasonably safe shelf life span.

MYTH: 03: Higher Cross-linking time when compared with Copolymer

Diffusion rate of water molecule through Semicon and insulation can be controlled by proper design of curing bath and maintaining the temperature.
By use of suitable grade of compound depending on thickness of insulation (voltage grade) the curing time could be achieved up to acceptable / practical levels.

MYTH: 04: Productivity when compared with Copolymer

Productivity (line speed) of Sioplas can be improved by right grade selection of LLDPE and fine tuning of chemistry during grafting.
By modification of base polymer suitably, the high productivity grades can be manufactured through grafting route also.

Conclusion

It is evident that compounded grades of MV / HV Peroxide XLPE and MV Moisture curable products are able to deliver comparable performance with products coming from co-polymer route and also from original polymer suppliers.

This is possible through:

(a) Recent advances in polymer compounding extruders and balancing equipments.

(b) Availability of clean polymers suitable for such application in commercial quantities, and