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Plasma modification of the
surface properties of polymers
Periolatto Monica
Dipartimento di Scienza Applicata e tecnologia
Politecnico di Torino
Plasma : 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 substrate
According to operating conditions and gas the following surface
modifications can be performed:
Etching: ablation of the substrate
Grafting : inserction of functional group onto substrate
PECVD: nanometric layer deposition onto substrate
polymer
adhesion, wettability
Wettability increase of cellophane film after an air plasma
treatment at low pressure
Dyeability increase of cellophane film after an air plasma
treatment at low pressure
Hydrophilic
hydrophobic
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
Vacuum chambers
Batch process
Pumping systems
High maintenance
Electronic control of pressures
costs
Advantages of atmospheric plasma
On-line process
High gas consumption
Cost savings: no vacuum
Flammable or toxic
chamber, no vacuum pumps
gases must be avoided
Types of atmospheric plasma
DBD : Dielectric Barrier Discharge
CD : Corona Discharge
Plasma Torch
Plasma jet
Corona Discharge
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 fabric
Industrial plant for atmospheric DBD plasma
treatment of cotton fabric
Industrial plant for atmospheric DBD plasma
treatment 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 plasma-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 chemicals
in AcXys device
AcXys roll-to-roll apparatus for plasma treatment of polymer
films and fabrics
Plasma effect on wool fabric dyeability : low temperature dyeing
allowed
Electron-beam polymer
processes
E-beam basics

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 device
E-beam : industrial applications

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 : environmental and economic aspects

Unlike gamma irradiation, which involves the use of a radioactive source, ebeam 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.
Laser 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
1.
Monochromatic
2.
Multidirectional
2.
Directional
3.
Incoherent
3.
Coherent
Lasing action
Laser radiation is due to the stimulated emission process:
M* + hν → M + 2hν
1.
2.
3.
4.
5.
6.
7.
8.
Energy is applied to a medium raising electrons to an unstable energy level.
These atoms spontaneously decay to a relatively long-lived, lower energy,
metastable state.
A population inversion is achieved when the majority of atoms have reached this
metastable state.
Lasing action occurs when an electron spontaneously returns to its ground state and
produces a photon.
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.
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.
The partially reflective mirror allows the transmission of a small amount of
coherent radiation that we observe as the “beam”.
Laser radiation will continue as long as energy is applied to the lasing medium.
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 fibers
SEM micrographies on a linen fabric laser
treated.
Laser effect on fibers
a
b
SEM on a linen fabric. (a) elctron beam (b) hot ironing at 160°C reaching the
same effect obtained by laser treatment.