PLASMA TECHNOLOGIES COMPARISON
Tri-Star Technologies plasma machines stand aside from a wide variety of so called Corona Discharge systems for 3D surface treatment. Although both techniques are based on the same physical phenomenon, namely Electrical Breakdown of Gases at Atmospheric Pressure, or Electrical Discharge, the methods of creation and application of this discharge are completely different.
As can be seen from the preceeding table, the TST Plasma is much more effective and flexible than Corona Discharge 3D Treaters.
All of the Corona Systems require at least 2 electrodes to initiate and to maintain a discharge. In the case of web treatment it is done quite effectively by placing the moving film between High Voltage electrode (metal rod or bar) and a grounded metal roll. However, this would hardly work for 3D objects, since the breakdown distance between web electrodes is limited and usually does not exceed ¼".
Standard 3D corona treater consists of a dielectric enclosure (discharge head) with 2 small electrodes, made of medium thickness bare aluminum or stainless steel wire, connected to a high voltage power supply, and a fan. When a high voltage exceeds the air breakdown value (30KV/cm) electrical arc occurs. This high current arc is then blown out of the enclosure by the stream of air and usually has a length of several inches and about one millimeter diameter.
Striking a solid object the arc goes part of the way across the surface and returns back into the enclosure. The treatment is provided by moving the object under the discharge head or by moving the head over the object. It is obvious, that the arc is essentially 1D object (like a wire), and a treatment of 3D surfaces requires at least 2 heads. It is also obvious, that in this configuration it would be almost impossible to treat objects with complicated geometry, especially with internal cavities or channel structure, including a simple tubing.
The average power required for one head operation is about 500 Watts. If only 1% of this energy goes directly in to the arc, the gas temperature along the arc trajectory would rise up to 2000°F in less than a second. In reality, the arc is constantly cooled by surrounding air flow from the fan, but still has a pretty high core temperature (>1000°F). The efficiency of this system is even lower, if we take into account the fact, that the major role in the plasma surface modification play only high energy (1-10 eV) ions and electrons that directly strike the surface. Vast majority of them, however, travels along the arc trajectory, where the main transfer of energy occurs.
The air stream, created by the fan is very unstable, and the arc trajectory on the surface is constantly changing following the random pulsations of the air. That leads to a highly non-uniform surface treatment. To increase uniformity the exposure time should be significantly increased cutting down the production rate. On the other hand, high current density within the arc makes the arc plasma very hot and limits exposure time by the thermal damage threshold of the material of the surface.
The plasma effect on the material itself strongly depends on the exposure time. In other words, each particular material requires some minimum exposure time to activate the surface. The required level of the surface modification depends on the further application (e.g. printing, bonding, coating, etc.) as well as on the applied ink, adhesives, coatings and curing process.
Very narrow window between plasma treatment and material damage for the 3D corona treaters considerably reduces the applicability. Especially this relates to the "plasmaphobic" tough treating materials when the surface rather gets burned than modified. It is also the case for the heat sensitive materials, thin wall plastic objects, wires with thin insulation, fiberoptics, thin coating layers, etc. The problem might be partly solved by installation of the several discharge heads along the process line, proportionally rising the cost of the system.
3D corona treaters also have an environmental problem. First, having a high current density inside the arc, they partly work as ozone generators. To reduce high ozone concentration special filters are required. Even with the filters it would be hard to comply with clean room environment regulations, that is frequently the case in medical and semiconductor industries. Second, the high voltage signal applied to the electrodes is usually of audible frequency (60Hz and up). Electrical breakdown takes place each half a period of the cycle and produces a small shock wave with distinctive sound. That generates a significant amount of noise, especially in the multiple head discharge systems.
Tri-Star Technologies plasma system creates a uniform plasma cloud that completely surrounds small objects or spreads over in the boundary layer of the surface, it also could be placed in the internal cavities, channels, etc. The same plasma system may be used to treat internal surfaces of the capillary of 50m diameter and to cover the surface area of 2" diameter and up.
The unique design of the machine (US patent No.5,798,146) is based on the well known physical phenomenon, that the strength of the electrical field considerably increases in the vicinity of small radius objects.
Applying a high voltage signal to the sharp edge body (e.g. a needle) causes electron leakage from the edge to the gaseous environment (e.g. air). These free electrons, accelerated by the strong electrical field, have enough energy to ionize neutral gas molecules and produce another free electrons and ions. These electron avalanches, however, do not develop into the arc, but gradually decay moving away from the edge, and create a uniform glowing cloud in the vicinity of the electrode.
Since there is no well defined second electrode, it rather distributed on the infinity, the currents in the plasma cloud are extremely low (100mA) and the plasma is distributed in the finite 3D volume with near to room temperature. The overall power to initiate and maintain this glow discharge in usually does not exceed 100 Watts.
The low temperature plasma cloud may be applied directly on the surface of the treated material, or considerably extended with use of inert gases such as Helium, Argon, etc. This kind of plasma may be in contact with a surface for unlimited period of time, and, on the other hand, is very effective, since most of the microdischarge trajectories ends up on the surface of the treated material.
Specially designed High frequency-Low current power supply significantly increases the quantity of microdischarges per unit of time, comparing with regular sinusoidal voltage waveform. That, in turn, increases the efficiency. The system is practically noiseless, produces very little ozone when operates in the open air, and there is no ozone generation when inert gases are used.
The small amount of reactive gases and gas mixtures may be added to the inert one to obtain plasma with unique properties, that is frequently required for chemical surface modification. Thin film deposition on surface of the material by plasma polymerization process may also be accomplished by adding monomers (e.g.CH4 , C2 H2, etc.) into the plasma cloud.