Radiation and Industrial Polymers

With the advancement of industrialisation, pollution is a crucial problem for mankind. In the Green drive, i.e tomake the world pollution-free, radiation technology takes an important position. Nuclear radiation has made itsentry into many chemical processes. ‘Polymerisation’, ‘grafting’ and ‘curing’, all-important chemical processes inthe polymer field, can proceed through radiation techniques. The radiation technology is preferred over the otherconventional energy resources due to some reasons, e.g. large reactions as well as product quality can becontrolled, saving energy as well as resources, clean processes, automation and saving of human resources etc.Apart from this, radiation is also a good sterilising technique over other conventional sterilising techniques. Theirradiation of polymers can be applied in various sectors. In this review, the attention has focussed primarily to foursectors, i.e. biomedical, textile, electrical and membrane technology. 
From the age of stone and metals, we have come to the age of nuclear energy and polymers. Indeed,we live in the world of polymers. That is why scientists and technologists have termed this era as the‘polymeric age’. In every step of our daily lives, we come across things, which are the fruits of polymerresearch. The ever widening application of polymers in everyday life over the last several decades hasgenerally been acknowledged as a mixed blessing by scientists and technologists. Though started in themiddle of the last century, work in this field of chemistry has been so rapid and the application so usefuland versatile, that the number of polymer systems are enormous.
The last three decades have also witnessed the emergence of nuclear radiation as a powerful source ofenergy for chemical processing applications. Thus, it can be applied in different industrial areas. The factthat radiation can initiate chemical reactions or destroy micro-organisms has led to the large-scale use ofradiation for various industrial processes. Nuclear radiation is ionising, which on passage throughmatter, gives positive ions, free electrons, free radicals and excited molecules. The capture of electronsby molecules can also give rise to anions. Thus, a whole range of reactive species becomes available forthe chemist to play with.
Radiation-based processes have many advantages over other conventional methods. For initiationprocesses, radiation differs from chemical initiation. In radiation processing, no catalyst or additives arerequired to initiate the reaction. Generally with the radiation technique, absorption of energy by thebackbone polymer initiates a free radical process. With chemical initiation, free radicals are broughtforth by the decomposition of the initiator into fragments which then attack the base polymer leading tofree radicals. Sakurada [1] compared the efficiency of the two processes and estimated that the samenumber of initiating radicals are produced in unit time with a radiation dose of 1 rad/s or a chemicalinitiator, e.g. benzoyl peroxide, at a concentration of.01 M is used. Chemical initiation is howeverlimited by the concentration and purity of the initiators. However, in the case of radiation processing,the dose rate of the radiation can be varied widely and thus the reaction can be better controlled. Unlikethe chemical initiation method, the radiation-induced process is also free from contamination. Chemicalinitiation often brings about problems arising from local overheating of the initiator. But in the radiation-induced process, the formation of free radical sites on the polymer is not dependent on temperature but isonly dependent on the absorption of the penetrating high-energy radiation by the polymer matrix,Therefore, radiation processing is temperature independent or, in other words, we may say it is a zeroactivation energy process for initiation.
As no catalyst or additives are required, the purity of the processed products can be maintained. Withradiation processing, the molecular weights of the products can be better regulated. Radiation techniquesalso have the capability of initiation in solid substrates. The finished products can also be modifying bythe radiation technique.
Nuclear radiation energy, however, is expensive though very efficient in bringing about chemicalreactions. The unit cost of installed radiation energy is much higher than that of conventional heat orelectrical energy. Despite this fact, the application of nuclear radiation energy has proved its superiorityand its cost effectiveness in a number of chemical processes over that of other forms of energy such asheat or electrical energy. Radiation techniques have good efficiencies with regard to power and needsonly a small space to be set up.
The application of radiation on polymers can be employed in various industrial sectors, i.e. bio-medical, textile, electrical, membrane, cement, coatings, rubber goods, tires and wheels, foam, footwear,printing rolls, aerospace and pharmaceutical industries. In this review, attention is focused primarily onfour sectors: biomedical, textile, electrical and membrane technologies.

Radiation and Industrial Polymers Types of Reactions Involved

Radiation-initiated reactions can be categorically classified as two types: (1) crosslinking and scissionand (2) grafting and curing. 
Crosslinking is the intermolecular bond formation of polymer chains. Thedegree of crosslinking is proportional to the radiation dose. It does not require unsaturated or other morereactive groupings. With some exceptions (as in polymers containing aromatics), it does not vary greatlywith chemical structure. It does not vary greatly with temperature. Although the mechanism of cross-linking by radiation has been studied since its initial discovery, there is still no widespread agreement onits exact nature. The mechanism of crosslinking generally varies with the polymers concerned. Theuniversally accepted mechanism involves the cleavage of a C–H bond on one polymer chain to form ahydrogen atom, followed by abstraction of a second hydrogen atom from a neighbouring chain toproduce molecular hydrogen. Then the two adjacent polymeric radicals combine to form a crosslink.The overall effect of crosslinking is that the molecular mass of the polymer steadily increases withradiation dose, leading to branched chains until, ultimately a three-dimensional polymer network isformed when each polymer chain is linked to another chain.
In contrast, scission is the opposite process of crosslinking in which the rupturing of C–C bondsoccurs. Crosslinking increases the average molecular weight whereas the latter process reduces it. If theenergy of the radiation is high, chain breaking occurs through the cleavage of C–C bond. In aeratedsolution medium, however, the mechanistic way of scission proceeds through indirect manner. Thepolymeric free radicals are generated by solvent-free radicals, which are already formed by radiation.The addition of oxygen with the polymeric free radicals forms the peroxy species, which on decom-position forms smaller molecules. The oxidative degradation of the polymers depends upon the solventused in the system. Actually, the polymer degradation competes with the oxidation of the solvent.
Grafting is a method where monomers are introduced laterally on to the polymer chain where ascuring is the rapid polymerisation of an oligomer monomer mixture to form a coating, which is essen-tially bonded by physical forces to the substrate. In the simplest form, such methods involveheterogeneous systems, the substrate being a film, fibre or even a powder, with the monomer as aneat liquid, vapour or solution. There is a close relationship between grafting and curing althoughthere are certain differences. Actually, there is no time limit for the process of grafting. It can takeminutes, hours or even days, whereas curing is a usually very rapid process occurring in a fraction ofsecond. In grafting, covalent C–C bonds are formed whereas in curing, bonding usually involves weakervan der Waals or London dispersion forces. van der Waals bonding operate at distances where there islittle or no overlap or exchange and it is generally associated with smaller energies. However, covalentbonding, is effective at small internuclear distances and is associated with electron overlap, exchange,and consequently higher energies. Another important aspect of curing reactions is the possibility thatconcurrent grafting with curing occurs leading to improved properties of the finished product, particu-larly in adhesion and flexibility.
Grafting proceeds in three different ways: (a) pre-irradiation; (b) peroxidation and (c) mutual irradia-tion technique. In the pre-irradiation technique, the first polymer backbone is irradiated in vacuum or inthe presence of an inert gas to form free radicals. The irradiated polymer substrate is then treated with the monomer, which is either liquid or vapour or as a solution in a suitable solvent. However, in theperoxidation grafting method, the trunk polymer is subjected to high-energy radiation in the presenceof air or oxygen. The result is the formation of hydroperoxides or diperoxides depending on the nature ofthe polymeric backbone and the irradiation conditions. The peroxy products, which are stable, are thentreated with the monomer at higher temperature, whence the peroxides undergo decomposition toradicals, which then initiate grafting. The advantage of this technique is that the intermediate peroxyproducts can be stored for long periods before performing the grafting step. On the other hand, with themutual irradiation technique the polymer and the monomers are irradiated simultaneously to form thefree radicals and thus addition takes place. Since the monomers are not exposed to radiation in the pre-irradiation technique, the obvious advantage of that method is that it is relatively free from the problemof homopolymer formation which occurs with the simultaneous technique. However, the decideddisadvantage of the pre-irradiation technique is the scission of the base polymer due to its directirradiation, which brings forth predominantly the formation of block copolymers rather than graftcopolymers.