Indian 10urnalof Fibre & Textile Research Vo1.31, March 2006, pp. 99-115 Plasma polymerization and its applications in textiles Dirk Hegemann" EMPA - Materials Science & Technology, Functional Fibers and Textiles, Lerchenfeldstrasse 5, 9014 St.Gallen, Switzerland Plasma polymerization enables the deposition of thin coatings on all kinds of substrates using electrical monomer discharges. This paper reviews plasma polymerization processes as surface modification (finishing) for textile applications. The dry and ecofriendly plasma technology aims at replacing wet-chemical process steps and adding new values to textile products. Characteristics of hydrocarbon, organosilicon, fluorocarbon, hydrophilic functional, monofunctional and ceramic coatings have been discussed to demonstrate their potential for textiles and fibers. Plasma technology requires adequate reactors for the continuous treatment of fabrics and fibers. To enable the optimization of plasma polymerization on batch reactors, questions of up-scaling are addressed to demonstrate the transfer to an industrial level. Both atmospheric and low pressure plasmas are considered regarding their effectiveness and efficiency. Examples for applications in textiles, such as hydrophobic, oleophobic and permanent hydrophilic treatments, have also been reported. Keywords: Fluorocarbon coating, Hydrocarbon coating, Hydrophobic treatment, Hydrophilic treatment, Oleophobic treatment, Plasma polymerization 8 IPC Code: Int. CI. D06M 10100, H05H 1 Introduction the main part of the plasma-activated gas remains The challenges the European textile industry is close to room temperature. The electrons, however, facing today are enormous. Therefore, the need for a gain just the right energies to excite, dissociate and reorientation is strong. In order to survive, the ionize atoms and molecules. The degree of ionization European textile industry is taking the transition typically lies between lOA and 10-6. The earliest towards a knowledge-based economy focusing on plasma treatments on textiles and fibers date back to high-value added products. Plasma technology is the 1960s, focusing on the improvement of offering an attractive way to add new functionalities, wettability, shrinking resistance, tWIsting and such as water repellency, hydrophilicity, dyeability, desizing? Low pressure plasmas operating between conductivity and biocompatibility, due to the 0.1 Pa and 100 Pa were usually considered, which can nanoscaled modification of textiles and fibers. At the be activated by direct current (DC), alternate current same time, the bulk properties of textiles and fibers (Ae), radio frequency (RF) or microwave (MW). remain unaffected. Moreover, due to a low material Plasma polymerization is performed using different and energy input, while avoiding wet-chemical kinds of plasma-polymerizable gases (monomers).3 processes, the plasma technology provides the These gases might not undergo polymerization by realization of environmentally sound processes. I conventional activation (e.g. methane), showing a The plasma state consists of an equal part of nega­ main difference between plasma and conventional tively and positively charged particles (quasi­ polymerization. A plasma polymer typically results neutrality), excited states, radicals, metastables, and from a rivaling etching and deposition process vacuum ultraviolet (VUV) radiation. Surface treat­ depending on the plasma species present during film ments of materials can be performed by non­ growth yielding a more or less cross-linked structure. equilibrium plasmas which are excited by electric Atmospheric pressure plasmas, such as corona or fields. Since the energy coupling is conducted by dielectric barrier .discharges (DBD), however might electrons which achieve a mean temperature of seve­ be of special interest for the textile industry due to an ral electron volts (corresponding up to 100,000 K), easier processability.4 During the past decade, considerable efforts have been made to generate "E-mail: [email protected] stable atmospheric pressure plasmas.5 While they 100 INDIAN J. FIBRE TEXT. RES., MARCH 2006 show some use for activation and hydrophilization of reactivation of reaction products determine the plasma textiles6-8, it is rather difficult to obtain high quality polymerization. The reactive species are mainly plasma-polymerized coatings on textiles. Thus, low radicals, where cycle 1 consists of reactions of pressure plasma is a serious alternative for reactive species with a single reactive site, and cycle 2 multifunctional coatings for textile applications.9 First is based on divalent reactive species. Cross-cycle attempts to deposit plasma polymers on natural and reactions from 2 to 1 can occur. Furthermore, the synthetic fibers within a continuous low pressure surface takes part in plasma polymerization by third­ plasma reactor, where the fibers are processed air-to­ body reactions and etching processes that lead to air, were performed in the early 1980s.'0 Phosphorus ablation and re-deposition. containing monomers were used to impart flame The concept of chemical quasi-equilibria enables a retardancy to cellulosic fibers and acrylamide plasmas macroscopic approach to plasma polymerization. It is to improve the physico-mechanical properties. In assumed that the gas particles (e.g. monomer) first recent years, plasma polymerization on textiles and enter an active zone, where excitation and fibers was mainly investigated for hydrophobization, dissociation processes are taking place, and then permanent hydrophilization and adhesion I I travel through a passive zone yielding recombination improvement in fiber reinforced composites. and stable products, such as deposition on substrate, Although, within the textile industry up to now electrode or wall. Following this macroscopic merely special applications and niche products have 13 approach, the reaction parameter power input per gas been performed with the help of plasma flow (WIF), which represents the energy invested per polymerization, especially low pressure plasma particle within the active plasma zone, determines the polymerization is on the way to stimulate the textile mass deposition rate (Rill) using the following sector. relationship: 2 Plasma Polymerization When adverting of plasma polymerization, a ... ( 1 ) radical-dominated plasma chemical vapor deposition (plasma CYD) process is thought of, resulting in macromolecule formation, i.e. mainly amorphous, where is the reactor depending geometrical factor; G more or less cross-linked structures. The underlying W. the power input; F, the gas flow; and the Ea, growth mechanism is known as Rapid Step-Growth activation energy corresponding to the used '2 Polymerization (RSGP). As schematically shown in monomer. 14.15 Fig. 1, the recombination of reactive species and If Eq. (1) holds, a linear fit is obtained using an I�.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-., Arrhenius-type plot for the mass deposition rate per gas flow depending on the (inverse) energy input as shown by the straight line in Fig. 2 around the I I I lon·induced : ,Deposition . I , I \, I I I I i Plasma I I I I ! boundary I I I I I I I I Oligomers taking part --,1-._._.1 I in deposition I - _ _ I I � - - I - - - I I - - I I - ! Activation energy : = : Slope of linear fit (Energy input)"' Fig. 1- Plasma polymerization via Rapid Step-Growth Polymerization (RSGP) involving an activation and a Fig. 2- Dependence of deposition rate per gas flow on energy recombination zone (concept of chemical quasi-equilibria) input HEGEMANN: PLASMA POL YMERIZAnON AND ITS APPLICATIONS IN TEXTILES 101 6 activation energy.1 The negative slope of the linear plasma these conditions are given for different fit represents the activation energy, which is found to monomers and gas mixtures within a broad parameter be the minimum energy required to initiate the plasma range, even when additional gases are added to obtain polymerization process and to obtain stable cross­ plasma co-polymerization.18 In the latter situation the linked plasma polymers.17 Deviations from this total gas flow is given by adding the different gases straight line might be found at low specific energies considering a flow factor that weights its contribution due to oligomerization and at high energies due to to the plasma polymerization process using the 8 etching, sputtering or temperature effects. 1 At rather following relationship: energetic plasma conditions or long durations, an . ( 3) increasing temperature during plasma polymerization .. has to be taken into account that might strongly where is the monomer gas; the non­ Fill Fe. influence the deposition rate depending on the type of polymerizable or carrier gas; and the reaction cross­ a, monomer used. section (flow factor) which is smaller than 1. Eqs (1)­ While the actual interaction of plasma particles and (3) enable the up-scaling of plasma polymerization plasma-induced radiation influence the absolute processes. The situation in atmospheric plasma deposition rates depending on the reactor geometry processes, which appears to be less defined, has not described by the geometrical factor in Eq. (1), the yet been examined using this macroscopic approach. activation energy merely depends on the plasma For plasma polymerization, in principle, all types chemistry of the considered monomer and is thus of materials might be treated. The plasma treatment of independent of the reactor design. To demonstrate the different types of textiles and fi bers has already