In the electrical field, one of the essential things for electrical wires and cables is insulating andjacketing materials. For many years, the pre-eminent insulation material for power cables was oil-impregnated paper due to its excellent electrical properties. It has also the capacity to withstand ahigh degree of thermal overload without excessive deterioration. However, due to its hygroscopicnature, the metal sheath is moisture corroded. There was, therefore, a long-felt need for a powercable insulation material, which had a combination of the non-hygroscopic nature of thermoplasticmaterials.
The preparation of crosslinked polymers can be done by two different methods. One is the chemicalmethod and the other is the ionising method. Though the realisation of this effect of crosslinking is over150 years old, the crosslinking effect of ionising radiation was conclusively demonstrated for the firsttime by Charlesby. The radiation crosslinking method is the most productive for small-sized and thinwall wires and therefore the wires used for electric and electronic equipment have been producedby the radiation crosslinking method. The method is advantageous because of low energy consumptionand needs small space. The radiation process is easily controlled and has the potential for energy savingsas well as pollution controlled. The specific features of radiation crosslinking are summarised as follows:(1) The production line speed can be controlled. High speed covering (extrusion) is possible, as there isno requirement of crosslinking agent. By the use of an accelerator with high power and low energy, rapidcuring can be achieved. (2) Crosslinking uniformity is excellent. The uniform crosslinking by selectingan appropriate machine and adopting optimum design for wire feeding can be carried out. (3) Variouskinds of polymers can be prepared, depending upon the degree of crosslinking by radiation crosslinkingprocess. Moreover, the radiation curing process is more preferable than the steam curing process. In thesteam curing process, water permeating into the polymer layer under a high steam pressure creates anumber of ‘microvoids’, which could induce tree-shaped partial discharge breakdown when the cable isin service. Although the phenomenon is much complicated, the trees can grow and cause a decrease inthe dielectric strength of the cables. Apart from these, the steam curing process has some drawbacksfrom the standpoint of energy consumption: (a) high steam pressure is needed to obtain a high tempera-ture; (b) the efficiency of thermal conduction from outside the cable is low and (c) large amount ofenergy is consumed by the cable conductor, which results in a lower thermal efficiency and also a longertime for crosslinking reaction. Radiation curing is a candidate for the dry processes. However, it has theproblem that the build up of electrons stopped and/or formed in the insulation layer by irradiation canalso induce tree-shaped partial breakdown during and after irradiation. It is completely different from the‘water-free process’. As the polymer cable contains high moisture and large voids, the curing process isnecessary. Apart from the above advantages, semiconductor materials may be introduced easily in the radiation curing process which is not easy in case of steam curing process as most of the materials couldnot withstand the high temperature and pressure.
Radiation grafting technique does also impart the conductivity to the matrix. This is the unique method of combining of conducting matrix on to the insulating one. This technique involves deactiva-tion of backbone polymer with a suitable monomer by grafting and subsequent deposition of theconducting polymer over the active surface of the backbone. Apart from the insulating behaviour, inthis case polymer can behave as conducting one. Though it has not yet established, it can exhibit severalpotential applications such as EMI shielding, conducting coatings and antistatic agents. Bhattacharya etal. have prepared the composites polymer–FEP-g-(AA)–PPY and polymer–FEP-g-(sty)–PPY. At first,polymer–FEP was irradiated form Co-60 source and the film was then dipped in differentpercentage of monomers. PPy was then deposited over the grafted surface by oxidative polymerisationof pyrrole using ferric chloride as oxidant. The surface resistance is decreased and are of the order of104–105ohm/cm2. The surface resistance depends on the percentage of grafting of monomers. Usingthis technique, surface conductivity rather than bulk conductivity can be increased. Photoconductingbehaviour of the film can also be imparted by grafting technique. Cellulose acetate-g-(N-vinyl carbazole) and cellulose acetate-g-(N-vinyl carbazole–methyl methacylate) are the examples of thephotoconducting film.
In the electrical cable industry, mainly polyethylene, polyvinyl chloride (PVC), EPDM rubbers areused. Polyethylene is used because of its excellent electrical properties and its longer duration. Low-density polyethylene is preferred over the high-densitypolyethylene due to several reasons.The reasonsare as follows: (a) more flexibility; (b) higher dielectric strength than high-density polyethylene; (c)longer life than HDPE; (d) less difficult to process than HDPE and (e) less risk of inclusion of voids inthe insulation of LDPE, which causes ionisation. Despite all such advantages, LDPE has its ownlimitations as a cable insulation material. Being a thermoplastic polymer, it has a softening temperatureat around 105–115⬚C and has the tendency for stress cracking to occur when it is in contact with certainsurface-active agents. Crosslinking of polyethylene molecules improves the thermal as well as physicalproperties while its electrical properties largely remain unchanged. Crosslinked polyethylene is, there-fore, no longer a thermoplastic polymer. It softens at the crystalline melting point of polyethylene andassumes an elastic, rubber-like consistency, a property which it retains during further rises of tempera-ture, until it becomes carbonised without melting at 300⬚C. The tendency to stress-cracking disappearsentirely and very good resistance to ageing in hot air is acquired. Crosslinked polyethylene cables arewidely preferred because of its excellent electrical and physical properties. It is capable of carryinglarge currents, withstands small radius bending and is light in weight, allowing for easy and reliableinstallation, i.e. it is free from height limitations since it is not comprised of any oil and thus is free fromthe failures due to oil migration in oil field cable. It also does not generally require a metallic sheath.Thus, it is free from the failures peculiar to metallic sheathed cables, corrosion and fatigue. Now-a-days,radiation crosslinking is industrially applied to not only polyethylene but also other polymers also suchas polyvinyl chloride, polyisobutylene etc. On its own PVC is extremely unstable polymer. It startedgaining commercial significance only after the development of effective means of stabilisation. With thehelp of modifying agents (stabilisers, plasticisers, fillers and other additives), PVC can be made toexhibit a wide spectrum of properties, ranging from extremely rigid to very flexible. The diversity ofits application and its low cost are responsible for its importance in the world market.
To increase the crosslinking efficiency, polymersare very seldom used intheir pure form. Plasticisers,antioxidants, fillers have their role in their respective way to impart the requisite properties. The additionis better during the crosslinking process. Plasticisers are added to polymers to reduce the brittleness ofthe polymer product. They affect the crosslinking whenever they take part in the generation of freeradicals or enter the propagating reactions. Dibutyl phthalate, tritolyl phosphate and diallyl phosphate are the common examples of the plasticiser to PVC. Flexibility and elasticity, which is veryimportant in electrical insulation, are improved by adding the plasticisers to PVC. Actually in case ofPVC, which is polar due to unbalanced structure, gives rise to strong intermolecular bonds, which jointhe macromolecular chains rigidly, together make it inflexible. Antioxidants are another group ofadditives, which are necessary for any crosslinked mixture designed for the practical purpose of com-paring higher thermooxidative stability on a polymer production. Usually they affect the crosslinking byscavenging radicals, which may form crosslinks. RC (4,4-thio-bis(6-tert-butyl-3-methyl phenol), MB(Mercapto benzoimidazole) are the examples of antioxidants which are used by Ueno et al. Inaddition to plasticisers and antioxidants, colourants are required, as the wire insulation materials haveused especially for appliances. Colourants for plastics include a variety of inorganic and organicmaterials. The discoloured additives are not preferred in this field. Fillers are generally added to improvetheir physico-mechanical properties and processability. A positive effect of fillers may be observedduring irradiation crosslinking. It is found that the yield of radicals in polyethylene was increased by 50%, when a small amount (0.05%) of aerosil is added. It has been assumed that a higherproduction of radicals takes place at the interphase aerosil–polyethylene, where macromolecules canbe in the non-equilibrium state of uncompensated strains. With a higher content of filler, a transfer ofenergy from the filler to the polymer phase may occur and thus contribute to a higher yield of freeradicals. Moreover, combination of irradiation with reactive admixture may affect the localisation ofcrosslinks along the polymer chains.
In short, radiation plays the important role in the polymer processing that is used in electrical field.‘Radiation crosslinking’ is the phenomena by which the properties of the polymers can be improved. It isthe most advanced method such as ‘vulcanisation’ bears some limitations. The crosslinking efficiencycan be improved by choice of suitable monomers. In radiation crosslinking process, the plasticisers,fillers and flame retardant addition is quite effective in radiation crosslinking process. The radiationcrosslinking method is also very useful in preparation of semiconductor materials. Apart from these,radiation grafting technique can also be employed to prepare the conducting composite film and filmswith photoconducting behaviour.
Post time: May-02-2017