PRESENTATION ON NEW GENERATION CABLES & ACCESSORIES
EMBODYING STATE-OF-THE-ART ELECTRON BEAM IRRADIATION
CROSS-LINKING TECHNOLOGY

Organised by M/s. NICCO Corpn.Ltd.

Friday, 26th April, 2002 – 1900 hrs – Hotel Centaur, Santacruz

Address by the Chief Guest

Shri B.Bhattacharjee
Director, Bhabha Atomic Research Centre

As you are aware, the Department of Atomic Energy (DAE) is committed to improve the quality of life of our 1 billion plus population through induction of nuclear science and technology primarily in two ways, namely, generation of electricity that should be safe, reliable, economical and of course eco-friendly and applications of radioisotopes and radiation technology in non-power generation areas like health care, agriculture and food processing, isotope hydrology and industry, including their applications in frontier areas of nuclear science and technology.

In the context of today’s function, we will spend sometime on applications of radiation technology in industry in general and in producing new generation of cables and accessories in particular.

1. Introduction

The major emphasis of radiation processing applications in Industry during the initial years was on the use of ⁶⁰Co as radiation source mainly because it is available as a by-product in nuclear reactors and gamma radiation having large penetration offered an easy means for irradiation of thicker materials. However, in recent years, Electron Beam (EB) accelerators as a source of high energy ionizing radiation have now emerged as preferred alternative for industrial radiation processing for specific purposes since they offer the following advantages:

• Electron beam accelerators provide a dose rate, which is several orders of magnitude higher than that from of radio-isotopes, thereby resulting in very high throughputs as well as minimizing the oxidative degradation of the substrate.

• Ability to amalgamate with the existing industrial infrastructure for online processing.

• Ability to confine treatment to surfaces or upto the chosen depth in materials.

2. Applications of Electron Beam

Electron Beams have played a key role in the field of basic and applied sciences. Over the last decade or so, the focus has shifted mainly to industrial applications, where the electron beams are being employed for various uses. For example, 

This field is growing and expanding at a fast pace with more and more avenue opening up daily. With time, the demand for the beam power is also going up. Today, the accelerators upto a beam power of ~ 400 kW are being conceived, designed and developed for various industrial applications. 

Processing of materials using high energy electron accelerators in the energy range 200 KeV – 10 MeV constitutes the largest commercial radiation application in the industry with over 800 EB accelerators operating world wide and is now a multi billion dollar industry offering high technology products.

3. Types of Accelerators 

To meet different types of demands of industry, we need accelerators differing in energy and beam power. Both DC & RF accelerators have been employed for this purpose. DC accelerators give high average beam power but low energy gain per unit length whereas the RF accelerators being pulses by nature, give low average power but large energy gain per unit length.

Hence, if we need large average power but low energy, DC accelerators are more efficient. Therefore, if the energy range of 3 to 4 MeV is desired coupled with large power of about a 100 kW or so, DC accelerators should be used. Beyond this energy, DC accelerators tend to become bulky and less cost effective.

For 4 to 10 MeV, and a beam power of about 50 kW but less than 100 kW, RF accelerators should be used. They are quire compact.

Therefore, depending upon the requirements of beam, the choice of accelerators is made. As is obvious, for large energy RF accelerators are preferred and for large beam powers, DC accelerators are chosen.

4. Industrial Applications

4.1 EB-Radiations in Polymer Industry

Radiation induced crosslinking of polymers constitutes one of the most important applications of EB radiation processing. As you are aware, the significant advancement in the power cable technology came through the development of crosslink polyethylene cables (XLPE) around mid of 20^th century which are now widely used for voltage levels right from 11 KV to 500 KV. These LXPE cables have very good di-electric properties, good thermal properties with an operating temperature of 90°C and quick cable jointing and end-terminations. Conventional crosslinking of XLPE cables in India are exclusively done through “chemical crosslinking processes” adopting either peroxide crosslinking or silane crosslinking. Peroxide based crosslinking is done either through dry N₂(300-400°C at 10-20 kgs/cm²) or steam (200°C at 15 kgs/cm²) as the curing medium. XLPE cables through dry N2 curing provide better dialectic properties which can be used upto 500 KV voltage level. Presence of micro-rods and moisture (1000-2000 ppm) in steam cured XLPE cables affect seriously the dialectic property and such cables are used upto 33 KV voltage level. Silane cross-linking technology is adopted by most of the Indian XLPE manufacturers to produce XLPE cables for low voltage (11 KV) application as a substitute for PVC cables because this process is very cost effective for Low Voltage applications inspite of its inherent poor dialectic strength.

However, with the availability of highly reliable electron beam accelerators, electron beam radiation crosslinking technology has slowly curved out its place in cable industry because of the wide flexibility it offers along with distinct cost advantages and improved thermal, electrical and chemical properties over chemical crosslinking technologies. 

A polymeric material upon exposure to high energy radiation may undergo cross linking reaction (leading to the increase in the molecular weight) or undergo chain scission reaction (leading to decrease in molecular weight). Generally, both these processes occur simultaneously but in a given radiation dose, one of them is the predominant process. Cross linking reaction improves the thermal, chemical and other properties of inexpensive common thermoplastics extending their utility to demanding applications, which otherwise necessitates use of higher cost engineered plastic materials. Chain scission on the other hand can result in improving the solubility of materials under milder conditions. (e.g., (i) radiation degradation of PTFE at 1-2 MGY dose level to simplify the process of its pulverization which is otherwise extremely difficult, that extends the application of the unique properties of PTFE to a number of areas that call for antifriction, non-stick and non-toxic properties; (ii) degradation of viscose rayon, the most popular and bio-degradable polymer, to make the technology for viscose rayon more environment-friendly by reducing the consumption of toxic chemical CS₂ by 25% and NaOH by 23%. Both the two processes are currently being utilised in an economically beneficial manner to add value to the product. 

BARC has been working with major cable manufacturers in the country to develop indigenously, the EB crosslinking cables in India. 

As compared to crosslinkage initiated by chemical catalysts, radiation crosslinking process enables easy control of reaction rate and results in the finished product with uniform degree of crosslinking throughout the extruded insulation with no residual catalyst. However, for successful exploitation of this new technology, a closer involvement of the polymer chemists, the radiation technologists and accelerator technologists are to be ensured by the industry personnel to overcome the problem involved in development of special EB curable formulations and establishing the radiation dose parameters needed to achieve the superior properties of polymer. Typical accelerators with 2 MeV – 10 MeV beam energy and 10 KW to 150 KW power level are utilised for such applications. Because of restrictions of 10 MeV beam energy (that restricts the insulation thickness) imposed by the available accelerator technology, EB radiated cables are produced for applications upto 66 KV.

4.2 EB Cross linked Wire and Cables

Cross linking of wire and cable by high energy radiation using EB accelerators has been well established and constitutes one of the most important applications of radiation technology. The process involves cross linking of ethylene based polymers such as low density polythylene (LDPE), ethlene-venyl acetates (EVA) and Ethylene-propylene die monomer (EPDM) rubber either alone or in blended form using very high dose rates of EB accelerator and without need of any initiators. LDPE has about 30% crystaline structure with balance 70% being amorphous. If we attempt to polymerise this by conventional higher temperature thermal route, the crystalline part get converted to amorphous phase during the process of cross-linking of the amorphous phase. This results in decreased mechanical properties even though the registivity does not deteriorate because of absence of inhibitors. On the other hand if EB route is followed which is a room temperature process there is no change to the crystaline portion of LDPF while the amorphous part get cross-linked as usual.

4.3 Radiation Crosslinked PVC Wire and Cables

PVC cables are difficult to crosslink using thermal methods as the polymer undergoes de-hydrochlorination at elevated temperatures. That is why PVC wires/cables are generally used as such without any cross-linking for application upto 70°C for a sizeable share of cable market where these cheap PVC cables can be gainfully used. If PVC cables are blended with LDPE/EPDM and then polymerised by conventional chemical route of crosslinking, the temperature limit for application can be extended to 85°C. On the other hand, BARC in collaboration with Sriram Institute of Chemical Research, New Delhi has developed a special formulation of PVC based material which on EB radiation cross-linking, leads to a product that can stand upto 105°C. In this connection it may be mentioned that Indian Railways is looking for huge quantities of such EB-crosslinked LDPE or (LDPE + EPDM) wires/cables for wiring the cabins of diesel locomotives. Because of much improved ageing properties of EB-crosslinked products, the frequency of replacement of these wires/cables are changed from once in ten years to once in thirty years.

Another important area of EB-crosslinked PVC or LDPE or (LDPE+EPDM) wires is in the field of Automobiles and Electronic media whose demands are presently being made by imports from South Korea. Blending with EPDM rubber helps to ensure that crosslinked material does not flow due to temperature rise which is essential for keeping the insulation level constant.

4.4 Radiation crosslinking of polyethylene “O” rings

Process optimization studies have been carried out for radiation crosslinking of polyethylene “O” rings to impart dimensional stability at 200°C. These “O” rings are used as gaskets for drum closures and need to be exposed to high temperature as the paint of drum is cured after filling and hence the need for “O” rings of higher thermal stability. In order to induce uniform crosslinking of the product on a commercial scale, a rotating multi – spindle conveyor system has been designed that can meet the desired objective. The process has been commercialised and presently, 1,00,000 rings can be irradiated per day using a 16-spindle underbeam geometry. Until now, more than 5 million rings from three different manufacturers have been processed using the 2 MeV – 20 KW ILU6 RF facility of BARC now relocated in BARC, Vashi Complex, Navi Mumbai. 

4.5 Heat Shrinkable Materials

Crosslinking induces “Memory Effect” to Polymer i.e., if the EB-crosslinked LDPE polymer is stretched or expanded by heating and immediately cooled thereafter it retains the memory of pre-stretched state in the sense if the cooled product is again reheated above its crystalline melting point, it retains its original shape. This effect has resulted in a number of industrial applications such as (i) food wrapper films. The wrapper film when subjected to heating in micro-oven goes back to pre-heating state, ensuring excellent wrapping of the food stuff or (ii) sleeves for joining cables specially the LDPE polymer used in telecommunications, etc. This is another area in which BARC is working with Indian industries like M/s. Raychem Industries, to develop this technology. M/s. Raychem is presently coping with the increasing demand of such heat shrinkable polymers by importing from Belgium. 

5. BRNS sponsored Collaborative work done between IIT, Kharagpur, NICCO and BARC

The indigenous development of radiation technology for the Indian cable industries necessitated involvement of technologists from different areas, viz., polymer technologist, radiation technologists and industry personnel to come together and initiate work in this here hereto unexplored area in India. BRNS offered a common platform in the form of sponsoring radiation technology projects involving IIT Kharagpur, NICCO and BARC for development of radiation curable formulations for wire and cables and heat shrinkable materials. During 1990-2000, three such projects were supported by BRNS which laid the foundation for developing such applications using radiation technology. In this project, the ILU-6 accelerator at Isotope Group, BARC was used to gain “hands-on” experience by the industry and for developing a variety of formulations.

5.1 MoU between NICCO and BARC – Achievements

On March 24, 1999, BARC and NICCO signed an MoU for 2 years under which it was agreed to make use of their respective strength by collaborating for drawing up of technical specifications of EB machine, technical evaluation of the offers, conceptual design of the facility and development of EB curable formulations, carrying out the initial trials of the product and providing irradiation services for development trial orders. The MoU has been successfully implemented with following achievements:

• Since signing the MoU, BARC has collaborated with NICCO on the development of EB crosslinked cables besides offering consultancy services regarding formulating technical specifications of EB machine, technical evaluation and conceptual design of the facility as envisaged under MoU.

• Cables of different diameter and measuring more than 100 km were irradiated using the indigenously developed conveyor system. The irradiated cables have been found to possess end-properties close to the desired cable properties and were approved by the end-users.

• A trial development order by Chittaranjan Locomotive Works Ltd., Indian Railways, to NICCO cables for supply of EB crosslinked cables worth Rs.20 lakhs was successfully completed.

5.2 Extension of MoU between NICCO and BARC

The validity of MoU signed between NICCO and BARC has been further extended for from August 1, 2001 and extended MoU will remain in force for 2 years starting from date of NICCO acceptance of completed installation of LINAC at Kolkata. Following are the new objectives of extended MoU:

• BARC will offer expertise for development of EB curable formulations and initial trials (partially completed)

• BARC will provide expert assistance for arranging technical seminars for industry for promotion of this technology.

• Offer expertise in process and product dosimetry trials.

• BARC professionals will get exposed to the large scale EB-accelerators for upgradation of their knowledge base

With the commissioning of the 3 MeV, 150 KW DC-accelerator in the industry, many more industries will be sensitized to adapt this unique technology to produce high quality world class materials.

6. Indigenous development of Accelerator Programme of BARC, Mumbai.

Keeping in mind the enormous potential of electron beams and to exploit their industrial uses, APPD, BARC has taken up an elaborate indigenous programme of designing and developing the electron accelerators.

Both these accelerators are in the advanced stage of development. Barring unforeseen circumstances, the commissioning trials for both of them should start by the end of December 2002.

With the commissioning of all these indigenously developed accelerators, a good technological base in the area of high power electron beam accelerators would have been established in the country.