Method of cleaning the surface of a material coated with an organic substance and a generator and device for carrying out said method

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DESCRIPTION

Procedure for cleaning the surface of a material coated with an organic substance, generator and device to carry out said procedure.

The invention relates to a method for cleaning the surface of a material coated by an organic substance, a particular generator and a device for the implementation of said method. This procedure is more specifically intended for the cleaning of metal sheets, without being limited thereto.

Indeed, the sheets from the various existing manufacturing stations are generally covered with a film of oil that can have two origins. First, said film may have been applied by means of a protective oil spray, in order to protect the surface of the sheet against corrosion. But it can also be a residual oil film in the case of sheets coming from the cold rolling mill or the skinpass. In both cases, the oil weights are of the order of one hundred mg per m².

The realization of a metallic or organic deposit on said sheets requires the removal of the oil film during a cleaning or degreasing and reviving operation, in order to obtain a correct adhesion of said coating. The techniques generally used for this purpose in industrial lines present the obligation not to heat the sheet more than necessary, in order to preserve the mechanical properties of the steel strip.

For this, the most common of these techniques consists of an alkaline degreasing assisted or not by an electrolytic process. For environmental reasons, this procedure requires the installation of complex annex workshops for the treatment of eco-toxic by-products.

Other technical solutions make it possible to avoid the formation of said by-products, such as laser ablation, which has the effect of resorbing organic compounds by photochemical means, but currently does not yet allow treating bands at speeds greater than a few meters per minute, due to to the lack of power of the lasers.

On the other hand, document US 5 529 631 shows that an advantageous surface treatment technique consists in using a high pressure plasma, produced by discharges with a dielectric barrier in gaseous mixtures containing mostly helium. This rare gas is necessary to achieve a stable glow discharge, thus avoiding switching to an arc regime that would lead to inhomogeneous treatment. The helium content must, in this case, be greater than 70% by volume, which implies that the oxygen content is limited. The examples mentioned in the patent show that a plasma treatment carried out continuously, in said gaseous mixtures, is sufficient to increase the surface energy of a polymer. But, in the case of a plasma treatment used to clean a metal surface, it is only the reactive oxygen species (O °, etc.) formed in the plasma that oxidize the oil that covers the sheet and allow the transformation of the carbon chains in volatile species. Therefore, it is observed that the treatment is not fast enough, probably due to the low density of reactive oxygen species if electric discharges are used with gaseous mixtures with contents equal to or greater than 30% by volume of oxygen.

To solve this problem, patent US 5 968 377 describes a method of surface treatment by plasma at atmospheric pressure in which a pulsed electric field is imposed between the electrodes. The imposition of a pulsed electric field makes it possible to cut off the discharge before it goes into the arc regime and to restart it the next instant. The applied voltage pulsations have the particularity that they are symmetrical. But, the present inventors found that said process was not usable for cleaning a material coated with an organic substance. Indeed, in this case, it is observed that only a part of the organic substance oxidizes and volatilizes, and that another part polymerizes. The film thus formed on the surface can only be partially removed after a long time immersed in plasma.

The object of the present invention is to remedy the drawbacks of the prior art procedures, making available a procedure that allows the continuous cleaning of the surface of a substrate without obtaining ecotoxic by-products, with a treatment speed greater than 10 m / min. .

To this end, a first objective of the invention is constituted by a continuous cleaning process of the surface of a material coated with an organic substance, characterized in that it includes the steps that consist of introducing said material into a treatment zone fed by a gaseous flow that includes oxygen, to place said material in the mass, and in generating a plasma by imposing an electric field between the surface of said material and at least one electrode covered with dielectric, said electric field being pulsed and including a succession of positive and negative voltage pulsations in relation to said material, the voltage being maximum of positive pulsations U + greater than the arc ignition voltage Ua, and the maximum voltage of the negative pulsations being in an absolute value lower than the starting voltage Ua.

The present inventors especially verified that the positive pulsation had to be quite high, that is to say greater in absolute value than the starting voltage of the arc Ua, to generate a plasma dense enough in the treatment zone, to achieve high cleaning speeds.

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They also verified that it was essential that the maximum voltage of the negative pulsations U-, in absolute value, be lower than the starting voltage Ua, so as not to activate an electric discharge between the two electrodes, since the use of a too large negative voltage it leads to a polymerization of the oil that does not allow a good degreasing to be achieved.

The arc ignition voltage value is mainly a function of the gas pressure in the reactor and the distance between the electrodes. These parameters are bound by Parchen's law.

The process of the invention may also have the following characteristics, alone or in combination:

- the voltage rise front of said field is less than or equal to 600 ns, preferably 60 ns,

- the frequency of the positive pulses is greater than or equal to 20 kHz,

- the gaseous flow consists of air or oxygen,

- the material is a metallic material, preferably carbon steel,

- the organic substance is a temporary protective oil against corrosion, or an unstable mechanical emulsion (oil-water mixture) originating, for example, from the laminating operation (skin-pass) of the metallic material,

- the material is in the form of a moving band, and the different stages of the process are carried out continuously by means of installations successively arranged along the path of the moving band.

A second objective of the invention is constituted by a generator that can be used to implement the method of the invention and that includes a low-voltage power supply that sends low-voltage pulses, of a voltage lower than 1,000 V, to a frequency between 1 and 200 kHz, and components that make it possible to transform said low-voltage pulsations into high-voltage pulsations. The surge edge of said generator is preferably less than or equal to 600 ns and, more especially preferably, less than or equal to 60 ns.

Said generator differs from that described in US patent 5 968 377, since it makes it possible to achieve asymmetric voltage pulsations. This is possible since, in contrast to the generator described in US 5 968 377, the high voltage pulsations are not cut, but low voltage, and the signal is amplified thanks to the transformers. In the context of the present invention, low voltage is understood to be a voltage lower than 1,000 V.

A third objective of the invention is constituted by a device for putting into practice the method of the invention that includes means for moving the band attached to the mass, a series of electrodes covered with dielectric arranged in front of the surface to be treated of said strip, said electrodes having been connected to a generator according to the invention, gas supply means arranged near the surface of the strip, and means for extracting the decomposition gases from the organic substance that covers the strip.

In the context of the present application, a dielectric is understood as a material having a dielectric constant greater than 6. An organic substance is also understood to mean any compound that contains at least carbon, hydrogen and oxygen. The rising front is defined as the time during which the voltage continues to increase until it reaches its maximum.

The invention is illustrated by the description of an embodiment provided by way of indication, with reference to the accompanying drawings, in which:

figure 1 represents a schematic view of a treatment device according to the invention,

- figure 2A represents the principle diagram of the electrical power supply of the device, and figure 2B represents its synoptic diagram,

- figure 3 represents an oscillogram of the voltage variations obtained with a generator according to the invention,

- Figure 4 represents the evolution of the reflectivity percentage (% R) of calibrated samples in oil grammage in the wavelength band corresponding to the stretchings of the CH band,

- Figure 5 represents the calibration curve established from the mathematically treated IRRAS records,

- Figure 6 indicates the evolution of the residual oil grammage G as a function of the treatment time in samples initially coated at 100 mg / m² of oil,

- Figure 7 indicates the evolution of the residual oil grammage G as a function of the treatment time in initially coated samples of 53 mg / m² of oil,

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- Figure 8 indicates the evolution of the residual oil grammage G as a function of the treatment time in samples initially coated with 110 mg / m² of oil.

Figure 1 represents a treatment device that includes a rotating support cylinder 1 for a steel strip 2 coated with a protective oil against corrosion, which it is desired to degrease. This cylinder 1 rotates in the direction indicated by the arrow F and can eventually be cooled, if necessary. It is attached to the mass by means of band 2.

In front of the cylinder 1 are arranged several electrodes 3 cooled and coated with a dielectric. A ceramic, such as alumina or stumatites, will preferably be chosen, since they are capable of withstanding high temperatures. A dielectric will be chosen whose dielectric constant is greater than 6, as is the case of alumina, whose dielectric constant is between 8 and 10, as well as stumatites, whose constant is between 6 and 8.

Each electrode 3 is powered by a high voltage generator 4 according to the invention. The gas or the gas mixture for treatment can be fed in different ways and, in particular, can be introduced to each side of the electrodes 3 by a ramp 5. A device for extraction of gases and volatile species originating from decomposition is also provided. of the oil film, on each side of the device (not shown). In order to facilitate the supply of gas to the area, it could prove advantageous to enclose the treatment area in an enclosure surrounding the plate and the electrodes.

The steel strip 2 is bonded to the ground and therefore plays the role of a counter electrode. It runs through cylinder 1 and exposes one of its surfaces to the action of the reactive species generated by the action of the discharge in the treatment gas, which are especially species or oxygenates of the O ° type.

The electric discharge is fed by means of the generator 4 which delivers, for a frequency that can vary from 1 to 200 kHz, monopolarity voltage pulses, the shape of which depends on the load at which said power is supplied.

Figure 2A represents the type of electrical circuit of the supply with pulsed voltage, which uses the power MOS transistor, connected to a step-up transformer.

Figure 2B shows the synoptic diagram of the power supply designed specifically for this application. It is made up of a block of fast diodes whose role is to manage voltage and current reversals in power transistors and transformers, in order to reduce chemical losses. The transformers are the object of a specific assembly in order to obtain a low conductance, an absence of saturation of the magnetic material and a low parasitic capacity.

Figure 3 includes a curve representing the voltage variations during a succession of two pulses as delivered by a generator according to the invention.

The first voltage pulse is observed to be positive and lasts about 1.8 ¿ns, followed by a negative pulse of lesser amplitude, which lasts 48.2 ¿ns. The maximum voltage of the positive pulsation U + is in this case 12.7 kV, and the maximum value of the negative pulsation in absolute value U- is 1.8 kV. The treatment reactor uses a discharge with a dielectric barrier (Al₂OR₃) and the electrode gap is set to 3 mm.

During the pulsation of positive voltage delivered by the electrical generator on the dielectric-coated electrode, a positive current pulsation is recorded followed, 4 ps later by a negative current pulsation, of smaller amplitude. Then the current is practically zero when the voltage measured across the dielectric is negative. The rising edge of the positive voltage is of the order of 400 ns. Said value of the voltage rise front allows the discharge to be primed with a minimum voltage of 5 kV.

Example 1

Two samples of a mild steel strip coated with a corrosion protection oil (Quaker Tinnol N200) are treated, subjecting them to a pulsed electric field according to the invention in order to degrease them. The oil weight in each of the plates is 100 mg / m² and 53 mg / m² respectively. The treatment is carried out in the presence of a flow of 30 l / min of oxygen and at atmospheric pressure.

The treatment reactor uses a discharge with a dielectric barrier (Al₂OR₃) that can contain two rectangular electrodes of dimensions 25 x 200 m². The distance between electrodes is 3 mm.

Different durations of plasma treatment are carried out on samples collected from each of the two plates. The residual grammage of protective oil in each treated sample is measured by means of grazing incidence infrared absorption spectroscopy (IRRAS).

Prior to said experimental measurements, a calibration curve is established from samples calibrated in terms of grammage with the same oil (Quaker Tinnol N200) on the same IRRAS analyzer.

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Figure 4 represents the evolution of the reflectivity percentage (% R) of calibrated samples in oil grammage, in the wave number range (expressed in cm^-1) corresponding to the stretchings of the CH band. The calibrated samples include, starting from the curve closest to the horizontal, 10 mg / m², 32 mg / m², 50 mg / m², 71 mg / m², 100 mg / m² and 150 mg / m² of oil. The absence of oil in the sample induces a reflectivity percentage of 100%.

Figure 5 presents the calibration curve established from the IRRAS recordings made for each calibrated sample.

Figure 6 shows the evolution of the residual oil grammage in the samples extracted from the plate with 100 mg / m²of oil after different times of treatment with plasma, at a frequency of 100 kHz. It is observed that a time of 7 to 8 seconds is sufficient to clean the sheet.

Figure 7 shows the evolution of the residual oil grammage in the samples extracted from the sheet with 53 mg / m²of oil after different times of treatment with plasma, at a frequency of 100 kHz. It is observed that a time of between 3 and 4 seconds is sufficient to clean the sheet.

Example 2

An oiled mild steel sheet with skin-pass is treated in order to clean it with the same reactor and under the same experimental conditions as those described in example 1. The oil weight in the sheet is 110 mg / m².

Different durations of treatment with plasma are carried out on samples extracted from the sheet with skin-pass. Next, according to the method described in Example 1, the residual oil weight in each treated sample is measured by means of infrared absorption spectroscopy in grazing incidence (IRRAS).

Figure 8 shows the evolution of the residual oil grammage in the samples extracted from the plate after different times of treatment with plasma. It is observed that a time of 20 seconds is sufficient to clean the sheet.

Example 3

The test of Example 1 is repeated by coating the steel sheet with a 150 mg / m layer² of Quaker Tinnol N200 oil.

The samples taken from the sheet are treated by applying different electric fields to them. The XPS spectra of the surfaces of said samples are obtained, and of reference samples, and the Fe / C and O / C ratios are calculated by integrating the corresponding peaks.

The results obtained and the conditions of the tests are shown in the following table:

<td></td><td>Treatment time (s)</td><td>Oxygen (l / h)</td><td>Fe / C</td>

<td>Oiled reference</td><td></td><td> -</td><td> 0</td>

<td>Solvent degreased reference</td><td></td><td> -</td><td> 0,30</td>

<td>Plasma with current generator</td><td> 75</td><td></td><td> 0,19</td>

<td>continuous pulsed at 10 kHz</td><td> 180</td><td> 650</td><td> 0,23</td>

<td>Plasma with pulsed direct current generator at 20 kHz</td><td> 45</td><td> 650</td><td> 0,20</td>

<td>Plasma with pulsed direct current generator at 40 kHz</td><td> 22</td><td> 650</td><td> 0,26</td>

<td>Plasma with pulsed direct current generator at 100 kHz</td><td> 10</td><td> 650</td><td> 0,23</td>

The higher the Fe / C ratio, the cleaner the surface of the material.

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If the three results are compared with the pulsed direct current generator, the remarkable improvement in the speed of the degreasing treatment is observed when the positive voltage pulses have a frequency of at least 20 kHz.

Furthermore, it is observed that, for a frequency of 40 kHz, the sheet is fully degreased after 22 seconds, while at a frequency of 100 kHz, only 10 seconds are needed to achieve the same result.