Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2001-03-06
2003-09-02
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S250000, C427S255180, C427S255270, C427S255370, C427S367000, C427S371000, C427S383100, C427S393500, C427S404000, C427S409000, C427S534000, C427S535000, C427S536000, C427S569000
Reexamination Certificate
active
06613394
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to method of finishing films of plastic and/or metal with an atmospheric plasma.
BACKGROUND OF THE INVENTION
Many finishing steps, such as, for example, printing, coating, lacquering, gluing etc., are possible on films of plastic or metal only if an adequate wettability with solvent- or water-based printing inks, lacquers, primers, adhesives etc. exists. A corona treatment is therefore in general carried out in- or offline with the film processing.
As described e.g. in the publications DE-A-42 12 549, DE-A-36 31 584, DE-A-44 38 533, EP-A-497 996 and DE-A-32 19 538, in this process the materials in web form are exposed to a uniformly distributed electrical discharge. Two working electrodes are a prerequisite, one of which is sheathed with a dielectric material (silicone, ceramic). A high alternating voltage with a frequency typically of between 10 and 100 kHz is applied between the two electrodes, so that a uniform spark discharge takes place. The material to be treated is passed between the electrodes and exposed to the discharge. A “bombardment” of the polymer surface with electrons occurs here, the energy of which is sufficient to break open bonds between carbon-hydrogen and carbon—carbon. The radicals formed react with the corona gas and form new functional groups here. Cleaning of the polymer or metal surface furthermore takes place, since film additives and rolling oils are oxidized and distilled off.
In spite of the broad spectrum of use and the constant further development, corona treatment has significant disadvantages. Thus, a parasitic corona discharge on the reverse occurs, especially at higher web speeds, if the materials in web form do not lie on the cylindrical electrode. The corona treatment furthermore causes a significant electrostatic charging of the materials in web form, which makes winding up of the materials difficult, obstructs the subsequent processing steps, such as lacquering, printing or gluing, and in the production of packaging films in particular is responsible for particulate materials, such as coffee or spices, adhering to the film and in the worst case contributing towards leaking weld seams. Finally, corona treatment is always a filament discharge which does not generate a homogeneously closed surface effect. Moreover, it is found in time that a loss in the surface properties occurs, because of migration of film additives, and that molecular rearrangement based on minimisation of surface energy takes place.
Corona treatment is limited here to thin substrates, such as films of plastic and papers. In the case of thicker materials the overall resistance between the electrodes is too high to ignite the discharge. However, individual flashovers can then also occur. Corona discharge is not to be used on electrically conductive plastics. Dielectric electrodes moreover often show only a limited action on metallic or metal-containing webs. The dielectrics can easily burn through because of the permanent exposure. This occurs in particular on silicone-coated electrodes. Ceramic electrodes are very sensitive towards mechanical stresses.
In addition to corona discharge, surface treatments can also be carried out by flames or light. Flame treatment is conventionally carried out at temperatures of about 1,700° C. and distances of between 5 and 150 mm. Since the films heat up briefly here to high temperatures of about 140° C., effective cooling must be undertaken. To further improve the treatment results, which are in any case good, the torch can be brought to an electrical potential with respect to the cooling roll, which accelerates the ions of the flame on the web to be treated (polarised flame). The process parameters which have to be adhered to exactly are to be regarded as a disadvantage in particular for surface treatment of films. Too low a treatment intensity leads to minor effects which are inadequate. Too high intensities lead to melting of the surfaces, and the functional groups dip away inwards and are thus inaccessible. The high temperatures and the necessary safety precautions are also to be evaluated as disadvantages. For example, the safety regulations in force do not allow pulsed operation of a flame pretreatment unit. It is known that the choice of torch gas allows only certain reactive species (ions and radicals) and that the costs of flame treatment are significantly higher than in the case of corona treatment.
The main disadvantage of corona treatment, the localised microdischarges (filaments), can be bypassed by using a low-pressure plasma. These usually “cold” plasmas are generated by means of a direct, alternating or high-frequency current or by microwaves. With only a low exposure to heat of the—usually sensitive—material to be treated, high-energy and chemically active particles are provided. These cause a targeted chemical reaction with the material surface, since the processes in the gas phase under a low pressure proceed in a particularly effective manner and the discharge is a homogeneous volume discharge cloud. With microwave excitation in the giga-Hz region, entire reactor vessels can be filled with plasma discharge. Extremely small amounts of process means are needed compared with wet chemistry processes.
In addition to targeted activation (modification) of surfaces, polymerizations (coating) and graftings can also be carried out in such processes. As a result of the action of the plasma, conventional polymerization monomers, such as ethylene, acetylene, styrenes, acrylates or vinyl compounds, and also those starting substances which cannot polymerise in conventional chemical reactions can be excited to undergo crosslinking and therefore formation of a polymer or layer. These starting substances are, for example, saturated hydrocarbons, such as methane, silicon compounds, such as tetramethylsilane, or amines. Excited molecules, radicals and molecular fragments which polymerise from the gas phase on to the materials to be coated are formed here. The reaction usually takes place in an inert carrier gas, such as argon. Reactive gases, such as hydrogen, nitrogen, oxygen etc., can advantageously be added in a targeted manner for various purposes.
Established physical and chemical plasma coating processes, such as cathodic evaporation (sputtering) or plasma-activated chemical deposition from the gas phase (PACVD), as a rule take place in vacuum under pressures of between 1 and 10
−5
mbar. The coating processes are therefore associated with high investment costs for the vacuum chamber required and the associated pump system. Furthermore, the processes are as a rule carried out as batch processes because of the geometric limitations due to the vacuum chamber and the pump times needed, which are sometimes very long, so that long process times and associated high piece costs arise.
Coating processes by means of corona discharge advantageously require no vacuum at all, and proceed under atmospheric pressure. Such a process (ALDYNE™) is described in DE 694 07 335 T 2. In contrast to the conventional corona, which operates with the ambient air as the process gas, a defined process gas atmosphere is present in the discharge region in corona coating. By selected precursors, layer systems of the following structure can be obtained: e.g. layers based on SiOx from organosilicon compounds, such as tetramethylsilane (TMS), tetraethoxysilane (TEOS) or hexamethyidisiloxane (HMDSO), polymer-like hydrocarbon layers from hydrocarbons, such as methane, acetylene or propargyl alcohol, and fluorinated carbon layers from fluorinated hydrocarbons, such as, for example, tetrafluoroethene.
A serious disadvantage of the existing processes is, however, the non-closed surface deposition caused by the filament-like discharge characteristics of the corona. The process is accordingly unsuitable for application of barrier coatings. For surface polarisation by introduction of functional groups, in contrast to simple corona discharge, the process is too expensive.
To avoid pin holed coatings over a part a
Brandt Rainer
Hartmann Ralf
Jacobsen Sven
Kuckertz Christian
Landes Klaus
Norris & McLaughlin & Marcus
Pianalto Bernard
Wolff Walsrode AG
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