Process for the surface activation of materials

Details Process for the surface activation of materials Process for the surface activation of materials

2001-03-06

2002-07-16

Pianalto, Bernard (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Pretreatment of substrate or post-treatment of coated substrate

C427S322000, C427S327000, C427S331000, C427S419200, C427S402000, C427S404000, C427S444000, C427S532000, C427S569000

Reexamination Certificate

active

06419995

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for the surface activation of materials in web form in particular films of plastic and/or metal, by means of 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.
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 pulverulent 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 minimization 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 (polarized 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 localized 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.
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 vacuo 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.
To avoid pin-holed coatings over a part area, such as occur in corona coating, atmospheric plasmas can also be generated by arc discharges in a plasma torch. With conventional torch types only virtually circular contact areas of the emerging plasma jet on the surface to be processed can be achieved because of the electrode geometry with a pencil-like cathode and concentric hollow anode. For uses over large areas the process requires an enormous amount of time and produces very inhomogeneous surface structures because of the relatively small contact point.
DE-A-195 32 412 describes a device for pretreatment of surfaces with the aid of a plasma jet. By a particular shape of the plasma nozzle, a highly reactive plasma jet is achieved which has approximately the shape and dimensions of a spark plug flame and thus also allows treatment of profile parts with a relatively deep relief. Because of the high reactivity of the plasma jet a very brief pretreatment is sufficient, so that the workpiece can be passed by the plasma jet with a correspondingly high speed. For treatment of larger surface areas, a battery of several staggered plasma nozzles is proposed in the publication mentioned. In this case, however, a very high expenditure on apparatus is required. Since the nozzles partly overlap, striped treatment patterns can moreover occur in the treatment of materials in web form.
DE-A-298 05 999 U1 describes a device for plasma treatment of surfaces which is characterized by a rotating head which carries at least one eccentrically arranged plasma nozzle for generation of a plasma jet directed parallel to the axis of rotation. When the workpiece is moved relative to the rotating head rotating at a high speed, the plasma jet brushes over a strip-like surface zone of the workpiece, the width of which corresponds to the diameter of the circle described by the rotation of the plasma nozzle. A relatively high surface area can indeed be pretreated rationally in this manner with a comparatively low expenditure on apparatus. Nevertheless, the surface dimensions do not correspond to those such as are conventionally present in the processing of film materials on an industrial scale.
DE-A-195 46 930 and DE-A-43 25 939 describe

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