Plasma modification of the Plasma modification of the surface properties of polymerssurface properties of polymers

Periolatto Monica

Dipartimento di Scienza Applicata e tecnologia

Politecnico di Torino

Plasma : nature and actionPlasma : nature and action

Plasma is a gas which becomes ionized when introduced between two

electrodes maintained at high voltage: it results a mixture of electrons

and ions emitting electromagnetic radiations. Such complex mixture of

ions, electrons and light is able to act on the surface energy of

material to be treated.

For polymer treatment low temperature plasma (LTP) only can be used.

The plasma treatment increases the surface energy (low in synthetic as

well as natural polymers) improving the related properties: adhesion,

wettability, printability, dyeability……

Plasma acts only on the polymer surface without affecting the bulk, hence

in textiles the fiber surface only is modified without damage of

mechanical properties.

It is an “eco-friendly” treatment, since the process is carried out in gas

phase without, or almost without, chemicals.

Plasma : interaction with a polymer substrateAccording to operating conditions and gas the following

surface modifications can be performed:Etching: ablation of the substrateGrafting : inserction of functional group onto substratePECVD: nanometric layer deposition onto substrate

polymer

adhesion, wettability

Wettability increase of cellophane film after an air plasma treatment at low pressure

Dyeability increase Dyeability increase of cellophane film after an air plasma treatment at low pressure

HydrophilicHydrophilic

hydrophobichydrophobic

low pressure plasma : ionized gas (argon, helium, nitrogen…..) and electrodes are contained in a chamber at a pressure of few millibar (20 or even lower): the process is practically carried out under vacuum; such system enables to introduce gases with controlled composition : Ar, He, N2, O2, H2, CH4, CF4, SF6, HMDSO, ecc.

atmospheric plasma: the process is performed in un a ionizing field under atmospheric pressure, that is in contact with the surrounding ambient. In this case the choice of gases is more limited: Ar, He, N2, O2.

Problems with the low pressure plasma Problems with the low pressure plasma

Advantages of atmospheric plasma

Vacuum chambers

Pumping systems

Electronic control of

pressures

Batch process

High maintenance

costs

On-line process

Cost savings: no vacuum

chamber, no vacuum

pumps

High gas

consumption

Flammable or toxic

gases must be

avoided

Types of atmospheric plasmaTypes of atmospheric plasma

DBD : Dielectric Barrier Discharge

CD : Corona Discharge

Plasma Torch

Plasma jet

Corona Corona DischargeDischarge

Corona discharge can be obtained by applying high voltage between two metallic electrodes of different design, as for example a point and a plate placed on the opposite sides. The high electrical fields applied to the point electrodes generate high ionic concentrations in the volume between the electrodes.

Corona discharge is a plasma process which acts in discrete manner on a plane surface yielding a non-uniform treatment. Moreover relatively low power treatment should be applied to the substrate to avoid the formation of high concentrate discharges at high temperature which can cause needlepoint burns on the substrate.

Corona discharge is much utilized in plasma treatment of polymer films.

Dielectric Barrier Discharge and Atmospheric Pressure Glow Discharge

DBD is obtained by insertion of a dielectric material between two metallic flat electrodes placed at few mm of distance. At voltage higher than breakdown tension of the gas, the dielectric function is to block the formation of higly ionized and warm sparks through charge intensification on the surface and generation of an electric field opposite to the external field.

A simple air DBD shows in any case filamentary structure, highly discrete and not useful for homogeneous treatments. An homogeneous treatment can be achieved only if the discharge at atmospheric pressure is generated in diffuse structure called Atmospheric Pressure Glow Discharge (APGD). Such result is obtained by optimization of three parameters: system geometry, gas flow rate (He homogeneous discharge, O2 ed Ar filamentary), power and high voltage source.

Bactericidal action of plasma on fabricBactericidal action of plasma on fabric

Industrial plant for atmospheric DBD plasma Industrial plant for atmospheric DBD plasma treatment of cotton fabrictreatment of cotton fabric

Industrial plant for atmospheric DBD plasma Industrial plant for atmospheric DBD plasma treatment of cotton fabricstreatment of cotton fabrics

Problems arising in industrial plant with 60 m/min treatment speed 2 m size:

materials (cathodes subjected to strong mechanical and thermal stresses), power (1000 times higher than in laboratory scale), hardware modularity, uniformity and control of the process (many cathodes and gas injection in many points)

Discharge between coaxial electrodes and Discharge between coaxial electrodes and plasma-jetplasma-jet

Differences from DBD

Advantages : plasma generation unaffected by material characteristics a plasma-polymerization allowed

Drawback : more consumption of gas (nitrogen and mixtures)

Functionalization through injection of chemicalsFunctionalization through injection of chemicalsin AcXys devicein AcXys device

AcXys roll-to-roll apparatus for plasma treatment of polymer films and fabrics

Plasma effect on wool fabric dyeability : low temperature dyeing Plasma effect on wool fabric dyeability : low temperature dyeing allowedallowed

Electron-beam polymer Electron-beam polymer processesprocesses

Electron beams are a stream of electrons that move at very high speeds.

Electrons are generated when a current is passed through a tungsten wire

filament within a vacuum. The wires heat up due to the electrical resistance

and emit a cloud of electrons. These electrons are then accelerated by an

electric field to over half the speed of light and move out of the vacuum

chamber through a thin titanium window into the atmosphere. Once outside

the vacuum chamber, the electron beam is a powerful source of energy for

forming or breaking chemical bonds.

Conventional electron beam processes for industrial purposes involve an

electron beam accelerator that directs an electron beam onto the material to be

processed. The accelerator has a large, lead-encased vacuum chamber

containing an electron generating filament, or filaments, powered by a filament

power supply. During operation, the vacuum chamber is continuously

evacuated by vacuum pumps.

E-beam basicsE-beam basics

E-beam deviceE-beam device

Commercial applications for electron beam technology are based broadly on

utilizing the electron beam as a source of ionizing energy in order to initiate

chemical reactions (for example, printing and curing of films) or to break down

more complex chemical structures (for example, air pollution abatement). The

commercial potential of electron beams was first recognized in the 1970s. Since

then, electron beams have been used to a limited extent across some industrial

processes, such as the drying or curing of inks, adhesives, paints and coatings as

well as the crosslinking of rubber tires and the terminal sterilization of medical

devices.

Electron beams are an extremely efficient form of energy for industrial processes

and also, at the same time, reduce energy dependency and eliminate the need for

harmful chemicals, which result in pollution.

E-beam : industrial applicationsE-beam : industrial applications

Unlike gamma irradiation, which involves the use of a radioactive source, e-beam technology neither produces nor stores any radiation in the target materials once those materials are outside of the beam. While ionizing radiation is present when the accelerator is on, workers are separated from this potential hazard by thick concrete walls. However, when the accelerator is switched off, the ionizing radiation stops, just like in a cathodic tube of a TV set.

While the value added to products by using e-beam technology can be quite high, so are the costs of installing and operating a dedicated e-beam plant. The cost for a typical facility, including the beam, shielding, physical plant, conveyor system, safety system, utilities and support equipment can range from $5 million to $9 million, depending on accelerator voltage.

For commercial purposes, electron beams are classified either as high or low voltage. High voltage accelerators achieve MeV in the range 0.5 - 10 MeV, while low voltage accelerators generate electrons with up to 0.3 MeV. Today there are more than 1,000 electron beam systems in commercial operation worldwide. Of these, about 700 are high voltage systems, although now the number of low voltage installations is growing at a much faster rate.

E-beam : environmental and economic aspectsE-beam : environmental and economic aspects

Laser SourcesLaser Sources

Laser = Light Amplification by Stimulated Emission of Radiation

The light emitted from a laser is monochromatic, that is, it is of one

color/wavelength. In contrast, ordinary white light is a combination of many

colors (or wavelengths) of light.

Lasers emit light that is highly directional, that is, laser light is emitted as a

relatively narrow beam in a specific direction. Ordinary light, such as from a light

bulb, is emitted in many directions away from the source.

The light from a laser is said to be coherent, which means that the wavelengths of

the laser light are in phase in space and time. Ordinary light can be a mixture of

many wavelengths.

These three properties of laser light are what can make it more hazardous

than ordinary light. Laser light can deposit a lot of energy within a small

area. Nevertheless it improves the application field of laser: cut, incision or

welding of metals, measuring instruments, information transport by optical fibers.

Incandescent vs. Laser Light

1. Many wavelengths

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent

Lasing actionLasing action

Laser radiation is due to the stimulated emission process:

M* + hν → M + 2hν

1. Energy is applied to a medium raising electrons to an unstable energy level.2. These atoms spontaneously decay to a relatively long-lived, lower energy,

metastable state.3. A population inversion is achieved when the majority of atoms have reached this

metastable state.4. Lasing action occurs when an electron spontaneously returns to its ground state and

produces a photon.5. If the energy from this photon is of the precise wavelength, it will stimulate the

production of another photon of the same wavelength and resulting in a cascading effect.

6. The highly reflective mirror and partially reflective mirror continue the reaction by directing photons back through the medium along the long axis of the laser.

7. The partially reflective mirror allows the transmission of a small amount of coherent radiation that we observe as the “beam”.

8. Laser radiation will continue as long as energy is applied to the lasing medium.

Laser application on textiles and leather Laser application on textiles and leather

Laser applications in textile field are based on surface ablation.

marking and cutting operations on leather, fabrics (natural or synthetic)

and denim, or any other textile item.

Among the applications, marking of textiles with patterns reaches fabrics

not only from an esthetical point of view, but characterizing the fabric in a

unique and refined way.

Good effects are obtained on velvet substrates, with the partial asportation

of naps.

Limitation: no coloured patterns are possible.

Laser effect on fibersLaser effect on fibers

SEM micrographies on a linen fabric laser treated.

a b

Laser effect on fibersLaser effect on fibers

SEM on a linen fabric. (a) elctron beam (b) hot ironing at 160°C reaching the same effect obtained by laser treatment.