Paint stripping and cleaning of surfaces with plasma
Published: Galvanotechnik 10-2018
Author. Stefan Nettesheim
Conventional paint stripping and surface cleaning processes have a number of different drawbacks, including the possibility of damage to the workpiece, dust pollution or environmental pollution caused by chemicals. In addition, there are modern but very expensive processes which are not used for economic reasons. This paper presents a new type of plasma treatment that can also remove coarse dirt or thicker layers using a special pulse technique.
In most industrialised countries, the cost caused by corrosion is around 3-5% of the gross domestic product (GDP). This also includes maintenance and paint stripping. Often, processes such as welding, soldering, gluing or painting require the selective removal of old paints or varnishes from components. The removal of the varnish is often more expensive and more polluting for the environment than the original varnishing. Indirectly, the removal of graffiti smears, spray paints and fibre pens on a wide variety of surfaces is also part of maintenance in the public and private sectors. Many simple chemical processes, such as pickling with dichloromethane, are no longer permitted.
Typically, thick layers (> 1/100 mm) were always first removed mechanically, e.g. by grinding, sandblasting or brushing, and then cleaned. This produces large quantities of dust and the product surface can be damaged. Another well-established process is combustion with flame or hot air. Although this does not produce any dust, it does produce a high emission of harmful combustion gases, depending on the composition of the layer. Even wet-chemical processes with aggressive pickling are not harmless during application. Residues of the pickling can later lead to component corrosion and must be completely removed after the process. If a paint layer has to be removed using a wet chemical process, it is important to know the paint composition and the substrate, as good results can only be achieved with a specific formulation of the stain. The frequently advertised mild universal solution does not exist. Modern processes such as laser cleaning or dry ice blasting (with CO2) are not applicable in all cases and may be very cost-intensive. Dry ice blasting requires a continuous supply of CO2 blasting media.
Up to now, atmospheric plasma treatment has been considered to be more of a fine cleaning technique that only works on the immediate surface. A clean removal of coarse dirt or thick layers was not possible until now. A pulse technique in which the power density is raised above the critical ablation threshold now allows higher ablation rates to be achieved. A qualitative overview of the common processes is summarized in Table 1.
2 Principle of pulsed plasma ablation
The basic idea of pulsed plasma paint stripping is to let a short intensive discharge pulse act on the layer to be removed. In the case of short pulses with high intensity, the undesired thermal depth effect is eliminated and only the upper layers are strongly influenced. In order to effectively remove a layer from a workpiece via an electrically ignited plasma, the interface between the two materials is in principle the best point of attack. If it is possible to “focus” the power of the ablation mechanism exactly on this inner surface, the efficiency of the process is highest. In this case, it is not necessary to remove the entire layer thickness step by step, but the interface is stressed to such an extent that the layer is detached.
An atmospheric pressure plasma torch, at which the voltage source can generate a high voltage swing, can now be operated in such a way that an electrical breakthrough occurs in an insulating or poorly conducting layer on a conductive material and a pulsed high energy is released at the transition from the insulating layer to the conductive carrier. With short pulses, a thermomechanical pressure wave is released at the interface and the layer is blown at a well-defined point. Only a small amount of burn-up occurs and the product surface is hardly thermally stressed or mechanically damaged. The result is a clean surface with a fine roughening which is ideally prepared for further processing steps. Bonding, contacting or coating can be used here.
All conductive substrates such as sheet metal, aluminium, steel, copper or conductive carbon fibre structures from which a poorly conductive layer is to be removed can be processed without additional aids. In the following some material combinations are shown. CFC materials are particularly sensitive to mechanical processes, such as grinding or blasting, as the fibres are easily damaged.
In addition to complete layer removal, roughening or partial layer removal can also be set by changing the machining intensity. The properties of the exposed surface are decisive for further processing. If the component is to be repainted or coated, the surface must be clean, dust-free, well wettable, dry and may be slightly roughened. Also an efficient gluing on the exposed surfaces is often aimed at. In the case of metals, spot welding or soldering is also an option.
4 Application engineering
Plasma stripping via non-thermal ablation processes offers great flexibility in production, maintenance and service applications in almost all industrial sectors. Depending on the task, automated total solutions or manual solutions can be used.
Pulsed plasma processes will establish themselves in cleaning and paint stripping technology, as very high power densities can be generated with relatively little effort.
The function on conductive substrates such as metals and CFRP materials has been proven. Special solutions for cleaning non-conductive substrates such as glass and concrete are under development.