ABSTRACT
Plasma chemistry is a quite complex process and involves a large number of elementary reactions. These are mainly homogenous and heterogeneous reactions. Homogeneous reactions occur between species in the gaseous phases as a result of inelastic collisions between electrons and heavy species. On the other hand, heterogeneous reactions occur between the plasma species and the solid surface immersed in it. Some homogeneous reactions are excitation of atoms or molecules, de-excitation from electronically excited state to ground state, ionization of neutral species through electron detachment to form positively charged particles, dissociation of a molecule, dissociative attachment, dissociative ionization, recombination of ions, electron-ion recombination, ion-ion recombination and Penning dissociation or ionization when neutrals collide with energetic meta-stable species (Chen, 1995). In plasma, there is a possibility of various types of constructive and destructive reactions that may take place among the plasma species themselves or with substrate. Different types of plasma surface reactions that are responsible for hydrophilic surface modification of textile are discussed below. (i) Oxidation: In treatment by oxygen-containing plasma, surface excitation leads to absorption of oxygen and formation of polar surface groups such as ketone, hydroxyl, ether, peroxide and carboxylic acid that are much more hydrophilic (wettable) than the untreated surface. On exposure to air discharge, articles like polyethylene (PE) film, teflon, and polypropylene nonwoven fabrics show significant increase in surface wettability. Corona discharge treatment is routinely applied to PE film to make it printable. Such kind of plasma treatment also leads to improvement in adhesion properties (Karmakar, 1999). (ii) Peroxide formation: When a typical polymer/textile substrate is exposed to argon (Ar) plasma followed by exposure to air, a high proportion of reactive sites are converted to peroxide form. Since peroxides are known to act as initiators for vinyl polymerization, a vinyl monomer under suitable conditions will react with the peroxide groups to produce a graft copolymer with the substrate. Since peroxide groups are indefinitely stable under ambient conditions, they can be stored as reactive sites to be used later on. Such surfaces also show improved interfacial strength. (iii) Radical formation: Carbon-free radicals are formed when the energetic ions from the plasma break the organic bonds at the surface of the substrate with the evolution of gaseous products, e.g. hydrogen from hydrocarbons. Other processes besides radical formation are degradation, cross-linking, solid-state polymerization and etching action. (iv) Polymerization: Initiation of polymerization produces a thin uniform
polymeric film on the electrodes or substrates. Ionization of organic monomers in the vapour phase by bombardment of the electrons under discharge conditions lead to rapid polymerization resulting in formation of thin pin hole-free films with good stability, insulating properties and uniform thickness. Grafting of block co-polymers may also be achieved through plasma polymerization. The free radicals formed on the surface of the fibre by exposure to plasma can be used directly to initiate polymerization with a new polymer bonded firmly to the surface by carbon linkage. This makes coating possible by continuous movement of textiles, paper, etc., through flat plate electrodes.
