Plasma Technologies for Textiles

Roshan Shishoo

• Publisher: Woodhead Publishing
• Year: 2007
• Print ISBN: 9781845690731
• eBook ISBN: 9781845692575
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Plasma technologies present an environmentally-friendly and versatile way of treating textile materials in order to enhance a variety of properties such as wettability, liquid repellency, dyeability and coating adhesion. Recent advances made in commercially viable plasma systems have greatly increased the potential of using plasma technology in industrial textile finishing. This pioneering book provides an essential guide to both the technology and science related to plasmas and its practical applications in the textile industry.The first part of the book discusses the science and technology behind plasmas. Chapters give detailed and comprehensive descriptions on the characteristics of plasmas and methods of control and treatment in the processing of textiles. Both low pressure cold plasma and atmospheric pressure cold plasma processes are described as well as the diagnosis and control of plasma parameters in plasma generating reactors. A chapter is devoted to the use of plasma technology to achieve nanoscale treatment of textile surfaces. The second part of the book concentrates on specific applications of plasma technologies. Chapters cover treatments for water and oil repellency of textiles, engineering of biomedical textiles and woollen finishing techniques through the use of plasma technologies. Further chapters cover the modification of fibres for use in composites and the potential use of plasma technologies for the finishing of fabrics made of man made fibres. The final chapter in the book gives a comprehensive analysis of the surface chemical and physical characterisation of plasma treated fabrics.Written by a distinguished international team of experts, Plasma technologies for textiles is an invaluable reference for researchers, scientists and technologists alike. - Summarises both the science and technology of plasma processing, and its practical applications - Discusses how plasma technology improves textile properties such as wettability and liquid repelling - An invaluable reference for researchers, scientists and technologists

Part I

Plasma science and technology

1

The physics and chemistry of plasmas for processing textiles and other materials

W.G. Graham Queen’s University, Belfast, UK

1.1 Introduction

The unique physical and chemical characteristics of the plasma environment make it attractive for textile processing. A plasma is an ionised gas, i.e. it contains electrons, ions and neutral atoms and/or molecules. However, not all of the ionised gases used in textile processing will exhibit the properties associated with plasmas, mainly because of they have low charge state densities compared to the neutral gas density or are produced by transient phenomena. The physical characteristics of plasmas are described in this chapter, along with the general chemical characteristics and surface interactions of partially ionised gases. In Section 1.2 the basic properties of gases in which plasmas are created are introduced. Section 1.3 introduces the basic concepts of plasma creation and their physical structure, along with the conditions that must prevail for an ionised gas to behave as a plasma, and the parameters that describe the plasma. The unique aspects of plasma chemistry are described in Section 1.4 through discussions of the constituent species, their collisions and interactions. In textile processing, the interactions of the plasma generated species with and on the surfaces in contact with the plasma are of great importance and these are discussed in Sections 1.5 and 1.6, respectively.

1.2 Gases

The underlying neutral gas environment in the plasma systems used in textile processing can be quite complex and inevitably involves an initial mixture of atoms and molecules. The gas can be flowing and may have temperature and density gradients. However, the most useful approach to obtain a basic understanding of gas behaviour is through using kinetic theory. This approach makes the following assumptions: the gas consists of identical molecules; individual molecules are small compared to the average space between them; the molecules themselves are relatively incompressible; and the molecules are in constant random motion. So here the gas will be assumed to be single species, uniformly distributed and not flowing. In addition, for simplicity, the term molecule will be used to describe all the neutral particles unless otherwise stated.

The continuous motion of the molecules is quantified in terms of their temperature. The higher the temperature, then the more vigorous is their average motion. Molecules acquire or lose energy through collisions with one another or through contact with solid objects. This energy is in the form of kinetic energy (E_K), related to the mass (M) and velocity (v) of the molecule by the expression

${\mathit{E}}_{\mathit{K}}=\frac{1}{2}\mathit{M}{\mathit{\nu }}^2$

[1.1]

In a large population the molecules have a wide range of energies and the energy of an individual molecule is constantly changing. The energy distribution of such a collection of molecules is statistical and is described by a function known as a Maxwell–Boltzmann distribution The energy distribution can be defined by a single quantity, the temperature (T). The mean speed of molecules $\left(\overline{\mathit{c}}\right)$

in such a gas is given by

$\overline{\mathit{c}}=\sqrt{\frac{8\mathit{kT}}{\mathit{\pi M}}}$

[1.2]

where k is Boltzmann’s constant = 1.38 × 10⁻²³ J K⁻¹, T is in Kelvin and M is the mass of the molecule in kg. So, for example, nitrogen molecules in air at 20 °C (293 K) have a mean speed of 500 m s⁻¹.

In calculating the mean kinetic energy of the molecules in the gas, the mean square speed $\overline{{\mathit{c}}^2}$

is required, which for a Maxwell–Boltzmann distribution is given by

$\overline{{\mathit{c}}^2}=\frac{3\mathit{kT}}{\mathit{M}}$

[1.3]

and therefore the average kinetic energy of the molecules is related to the gas temperature by the expression

$〈{\mathit{E}}_{\mathit{K}}〉=\frac{1}{2}\mathit{M}\overline{{\mathit{c}}^2}=\frac{3}{2}\mathit{kT}$

[1.4]

1.2.1 Mean free path

The moving molecules collide with one another. Between collisions, the molecules will travel in a straight line. Since the molecules are randomly distributed within the volume and are moving with different velocities, each one travels different straight line distances between collisions.

While $\overline{\mathit{c}}$

is the mean speed of the molecules, in collisions it is the relative mean velocity between molecules that is significant and this depends on the angle between the respective directions of motion of the molecules. It can be shown that for a Maxwell–Boltzmann velocity distribution and a uniformly dense gas, the mean velocity is $\sqrt{2}\overline{\mathit{c}}$

.

If there are n_g gas molecules per unit volume, then the collision frequency (v) or collision rate is given by

$\mathit{\nu }=\sqrt{2}{\mathit{n}}_{\mathit{g}}{\mathit{\sigma }}_{\mathit{c}}\overline{\mathit{c}}$

[1.5]

where σ_c is known as the cross-section for the collision between the molecules. The cross-section is the effective area that a molecule appears to have when approached by another molecule. The concept will be discussed in more detail in Section. 1.4.3. As the temperature increases, the particle velocity increases and so therefore does the collision frequency.

The collision mean free path (λ_c) in such a gas is therefore

${\mathit{\lambda }}_{\mathit{c}}=\frac{1}{\sqrt{2}{\mathit{\sigma }}_{\mathit{c}}{\mathit{n}}_{\mathit{g}}}$

[.6]

As the gas density increases, the mean distance a molecule moves between collisions decreases. The molecules in the air at atmospheric pressure and room temperature collide with each other with a frequency of about 10⁹ collisions per second, with a mean free path between collisions of about 10⁻⁸ m. At 10 Pa, the mean free path increases to a few mm.

1.2.2 Particle flux and pressure

The particle flux (Γ) of a gas striking a unit surface or crossing an imaginary unit area from one side will depend on the velocity distribution of the gas molecules and the angular distribution of the molecular motion relative to the surface. Considering a static gas and molecules crossing the surface in a direction normal to the surface, it can be shown that

$\mathrm{\Gamma }=\frac{{\mathit{n}}_{\mathit{g}}\overline{\mathit{c}}}{4}$

[1.7]

Then

$\mathrm{\Gamma }={\mathit{n}}_{\mathit{g}}{\left(\frac{{\mathit{k}}_{\mathit{B}}\mathit{T}}{2\mathit{\pi M}}\right)}^{1/2}$

[1.8]

i.e. the particle flux is directly proportional to the particle density and the square root of the gas temperature, and is inversely proportional to the square root of the mass of the molecules.

Pressure (P) is defined as the rate at which momentum is imparted to a unit area of surface. When a molecule bounces off a surface, there is a total change of momentum of $2\mathrm{M}\overline{\mathit{c}}$

. and so the rate of momentum change is $2\mathrm{M}\overline{{\mathit{c}}^2}$

. Molecules can be considered to be moving in six directions corresponding to the six faces of a unit cube, so on average n/6 molecules will cross a unit area in unit time. Therefore, from Equation 1.3

$\mathit{P}=\frac{1}{3}\mathit{M}{\mathit{n}}_{\mathit{g}}\overline{{\mathit{c}}^2}={\mathit{n}}_{\mathit{g}}\mathit{kT}$

[1.9]

1.3 Plasmas

A gas is normally an electric insulator. However, when a sufficiently large voltage is applied across a gap containing a gas or gas mixture, it will breakdown and conduct electricity. The reason is that the electrically neutral atoms or molecules of the gas have been ionised, i.e. split into negatively charged electrons and positively charged ions. The nature of the breakdown and the voltage at which this occurs varies with the gas species, gas pressure, gas flow rate, the materials and the nature, geometry and separation of the surfaces across which the voltage is sustained, the separation distance of the electrodes, the nature of the high voltage supply (e.g. dc, ac, radiofrequency or microwave)...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Introduction – The potential of plasma technology in the textile industry
• Part I: Plasma science and technology
• Part II: Textile applications
• Index