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… 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 Pumping systems Electronic control of pressures Batch process High maintenance costs Advantages of atmospheric plasma On-line process Cost savings: no vacuum chamber, no vacuum pumps High gas consumption Flammable or toxic 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, 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.

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 Many wavelengths Multidirectional Incoherent Monochromatic Directional Coherent

Laser radiation is due to the stimulated emission process: Lasing action Laser radiation is due to the stimulated emission process: M* + hν → M + 2hν 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.

SEM micrographies on a linen fabric laser treated. 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.