Radiation and industrial polymers

Abstract

With the advancement of industrialisation, pollution is a crucial problem for mankind. In the Green drive, i.e to make the world pollution-free, radiation technology takes an important position. Nuclear radiation has made its entry into many chemical processes. ‘Polymerisation’, ‘grafting’ and ‘curing’, all-important chemical processes in the polymer field, can proceed through radiation techniques. The radiation technology is preferred over the other conventional energy resources due to some reasons, e.g. large reactions as well as product quality can be controlled, 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. The irradiation of polymers can be applied in various sectors. In this review, the attention has focussed primarily to four sectors, i.e. biomedical, textile, electrical and membrane technology.

Introduction

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 polymer research. The ever widening application of polymers in everyday life over the last several decades has generally been acknowledged as a mixed blessing by scientists and technologists. Though started in the middle of the last century, work in this field of chemistry has been so rapid and the application so useful and 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 of energy for chemical processing applications. Thus, it can be applied in different industrial areas. The fact that radiation can initiate chemical reactions or destroy micro-organisms has led to the large-scale use of radiation for various industrial processes. Nuclear radiation is ionising, which on passage through matter, gives positive ions, free electrons, free radicals and excited molecules. The capture of electrons by molecules can also give rise to anions. Thus, a whole range of reactive species becomes available for the chemist to play with.

Radiation-based processes have many advantages over other conventional methods. For initiation processes, radiation differs from chemical initiation. In radiation processing, no catalyst or additives are required to initiate the reaction. Generally with the radiation technique, absorption of energy by the backbone polymer initiates a free radical process. With chemical initiation, free radicals are brought forth by the decomposition of the initiator into fragments which then attack the base polymer leading to free radicals. Sakurada [1] compared the efficiency of the two processes and estimated that the same number of initiating radicals are produced in unit time with a radiation dose of 1 rad/s or a chemical initiator, e.g. benzoyl peroxide, at a concentration of.01 M is used. Chemical initiation is however limited 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. Unlike the chemical initiation method, the radiation-induced process is also free from contamination. Chemical initiation 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 is only 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 zero activation energy process for initiation.

As no catalyst or additives are required, the purity of the processed products can be maintained. With radiation processing, the molecular weights of the products can be better regulated. Radiation techniques also have the capability of initiation in solid substrates. The finished products can also be modifying by the radiation technique.

Nuclear radiation energy, however, is expensive though very efficient in bringing about chemical reactions. The unit cost of installed radiation energy is much higher than that of conventional heat or electrical energy. Despite this fact, the application of nuclear radiation energy has proved its superiority and its cost effectiveness in a number of chemical processes over that of other forms of energy such as heat or electrical energy. Radiation techniques have good efficiencies with regard to power and needs only a small space to be set up.

The application of radiation on polymers can be employed in various industrial sectors, i.e. biomedical, 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 on four sectors: biomedical, textile, electrical and membrane technologies.