EP1309737A1

EP1309737A1 - Plasma coating method

Info

Publication number: EP1309737A1
Application number: EP01958165A
Authority: EP
Inventors: Jean-Tristan SIDEL OUTREMAN; Eric SIDEL ADRIANSENS
Original assignee: Sidel SA
Current assignee: Sidel SA
Priority date: 2000-08-01
Filing date: 2001-07-24
Publication date: 2003-05-14
Also published as: WO2002010474A1; US20050019481A1; US20050016459A1; US20030150858A1; FR2812665A1; CA2416521A1; MXPA03000910A; AU2001279897A1; FR2812665B1; JP2004505177A; US20050019577A1; CN1446269A; KR20030033003A; BR0112873A

Abstract

The invention concerns in particular a method for using low pressure plasma to deposit a coating on an object to be treated, whereby the plasma is obtained by partially ionising, under the action of an electromagnetic field, a reaction fluid injected under low pressure into a treatment zone. The invention is characterised in that the method comprises at least two steps: a first step during which the reaction fluid is injected into the treatment zone at a first flow rate and under a given pressure, and a second step during which the same reaction fluid is injected into the treatment zone at a second flow rate lower than the first.

Description

Plasma coating deposition method, device for implementing the method and coating obtained by such a method

The invention relates to methods for depositing thin layer coatings using low pressure plasma. In such a process, a reaction fluid is injected under low pressure into a treatment zone. This fluid, when brought to the pressures used, is generally gaseous. In the treatment zone, an electromagnetic field is established to bring this fluid to the plasma state, that is to say to cause at least partial ionization. The particles from this ionization mechanism can then be deposited on the walls of the object which is placed in the treatment area.

Deposits by low pressure plasmas, also called cold plasmas, make it possible to deposit thin layers on plastic objects sensitive to temperature while guaranteeing good physicochemical adhesion of the coating deposited on the object.

Such deposition technology is used in various applications. One of these applications relates to the deposition of functional coatings on films or containers, in particular with the aim of reducing their permeability to gases such as oxygen and carbon dioxide.

In particular, it has recently become apparent that such technology could be used to coat plastic bottles intended for packaging oxygen-sensitive products, such as beer and fruit juices, or products with a barrier material. carbonates such as sodas.

Document WO99 / 49991 describes a device and a method which makes it possible to cover the internal or external face of a plastic bottle with a coating of highly hydrogenated amorphous carbon by using acetylene as reaction fluid. The process which is described in this document makes it possible, in a single step, to form a particularly effective coating layer.

The object of the invention is to propose an improved process making it possible to obtain coatings having further improved characteristics. To this end, the invention proposes a process using a low pressure plasma to deposit a coating on the object to be treated, of the type in which the plasma is obtained by partial ionization, under the action of an electromagnetic field, of a reaction fluid injected under low pressure into a treatment zone, characterized in that the process comprises at least two steps: - a first step during which the reaction fluid is injected into the treatment zone with a first flow rate and under a given pressure; and

- a second step during which the same reaction fluid is injected into the treatment zone with a second flow rate less than the first flow rate.

According to other characteristics of the invention:

the steps are linked continuously so that, in the treatment zone, the reaction fluid remains in the plasma state during the transition between the two steps; - the second flow is constant;

- the second flow is variable;

- the second flow decreases during the second step; the power of the electromagnetic field is kept substantially constant during the duration of the two stages; the pressure in the treatment zone during the second stage is lower than the pressure in the treatment zone during the first stage;

- The reaction fluid comprises a gaseous hydrocarbon compound;

- The reaction fluid is acetylene; - The part of the coating which is deposited during the second stage has a density greater than that of the part of the coating which is deposited during the first stage;

- The part of the coating deposited during the second step has a density which increases from the interface with the part deposited during the first step to the surface of the coating;

- The coating deposited consists of a hydrogenated amorphous carbon material;

- the part of the coating deposited during the second stage has a proportion of sp3 hybrid carbon atoms which is greater in the vicinity of the surface of the coating compared to the same proportion measured in the vicinity of the interface with the part deposited during the first step;

- The method is implemented for depositing a gas barrier coating on a plastic substrate; - the substrate is a film;

- the substrate is a container;

- the coating is deposited on the internal surface of the container; and

- The coating retains its barrier properties when the substrate undergoes a bi-axial stretch of the order of 5%. The invention also relates to a device for implementing the method incorporating any of the preceding characteristics, of the type comprising a reaction fluid supply device comprising in particular a source of reaction fluid, a flow control valve and a injector which opens into the treatment zone, characterized in that during the transition between the first and the second stage, the control valve is controlled to cause a drop in the flow rate of reaction fluid delivered to the treatment zone.

Alternatively, the supply device comprises, downstream of the control valve, a buffer tank capable of storing reaction fluid, and, during the transition between the first and the second step, the control valve is closed, the tank buffer then gradually emptied of the reaction fluid it contains.

The invention also relates to a plastic container, characterized in that it is provided on at least one of its faces with a coating deposited according to a process according to any one of the preceding characteristics.

The invention also relates to a coating, characterized in that it consists of a hydrogenated amorphous carbon material, and in that, in the vicinity of the surface of the coating, the coating has a density (and / or a proportion of hybrid carbon atoms sp3) which is greater than that which it presents in the vicinity of its interface with the substrate.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows as well as in the appended drawings in which: - Figures 1 and 2 are schematic views illustrating two devices for implementing a method according to the invention;

- Figure 3 is a schematic graph illustrating an example of evolution of certain parameters during the course of a method according to the invention.

Illustrated in Figures 1 and 2 schematic views in axial section of two embodiments of a treatment station 1 0 allowing the implementation of a method according to the teachings of the invention. The invention will be described here in the context of the treatment of plastic containers. More specifically, a method and a device will be described making it possible to deposit a barrier coating on the internal face of a plastic bottle.

In both cases, the station 1 0 can for example be part of a rotary machine comprising a carousel driven by a continuous movement of rotation about a vertical axis.

The treatment station 1 0 comprises an external enclosure 1 4 which is made of electrically conductive material, for example metal, and which is formed of a tubular cylindrical wall 1 8 of axis A1 vertical. The enclosure 1 4 is closed at its lower end by a bottom bottom wall 20.

Outside the enclosure 1 4, fixed to the latter, there is a housing 22 which includes means (not shown) for creating inside the enclosure 1 4 an electromagnetic field capable of generating a plasma . In this case, it may be means capable of generating electromagnetic radiation in the UHF domain, that is to say in the microwave domain. In this case, the housing 22 can therefore contain a magnetron, the antenna 24 of which opens into a waveguide 26. This waveguide 26 is for example a tunnel of rectangular section which extends along a radius with respect to the axis A1 and which opens directly inside the enclosure 14, through the side wall 1 8. However, the invention could also be implemented in the context of a device provided with a sou rce of radiofrequency type radiation, and / or the source could also be arranged differently, for example at the lower axial end of the enclosure 1 4. Inside the enclosure 1 4, there is a tube 28 of axis A1 which is made with a transparent material for electromagnetic waves introduced into the enclosure 1 4 via the waveguide 26. It is for example possible to produce the tube 28 in quartz. This tube 28 is intended to receive a container 30 to be treated. Its internal diameter must therefore be adapted to the diameter of the container. I t must further define a cavity 32 in which a vacuum will be created once the container inside the enclosure.

As can be seen in Figure 1, the enclosure 1 4 is partially closed at its upper end by an upper wall 36 which is provided with a central opening of diameter substantially equal to the diameter of the tube 28 so that the tube 28 is completely open upward to allow the introduction of the container 30 into the cavity 32. On the contrary, it can be seen that the metallic lower wall 20, to which the lower end of the tube 28 is tightly connected, forms the bottom of cavity 32.

To close the enclosure 1 4 and the cavity 32, the treatment station 1 0 therefore has a cover 34 which is axially movable between a high position (not shown) and a low closing position illustrated in Figures 1 and 2. In position high, the cover is sufficiently cleared to allow the introduction of the container 30 into the cavity 32. In the closed position, illustrated in FIG. 2, the cover 34 bears in leaktight manner against the upper face of the upper wall 36 of enclosure 1 4.

In a particularly advantageous manner, the cover 34 does not have the sole function of ensuring the tight closure of the cavity 32. It indeed carries complementary members.

First of all, the cover 34 carries means for supporting the container. In the example illustrated, the containers to be treated are bottles made of thermoplastic material, for example polyethylene terephthalate (PET). These bottles have a collar protruding radially at the base of their neck so that it is possible to grasp them using a claw bell 54 which engages or snaps around the neck, preferably under the collar. Once carried by the claw bell 54, the bottle 30 is pressed upwards against a bearing surface of the claw bell 54. Preferably, this support is sealed so that when the cover is in position the interior space of the cavity 32 is separated into two parts by the wall of the container: the interior and the exterior of the container.

This arrangement makes it possible to treat only one of the two surfaces (interior or exterior) of the wall of the container. In the example illustrated, it is sought to treat only the internal surface of the wall of the container.

This internal treatment therefore requires being able to control both the pressure and the composition of the gases present inside the container. For this, the interior of the container must be able to be placed in communication with a vacuum source and with a reaction fluid supply device 1 2. The latter therefore comprises a source of reaction fluid 1 6 connected by a tube 38 to a injector 62 which is arranged along the axis A1 and which is movable relative to the cover 34 between a high retracted position (not shown) and a low position in which the injector 62 is immersed inside the container 30, through of the cover 34. A controlled valve 40 is interposed in the pipe 38 between the fluid source 1 6 and the injector 62.

In the device of Figure 2, we can see that the supply device 1 2 also has a buffer tank 58 interposed in the pipe 38 between the valve 40 and the injector 62. So that the gas injected by the injector 62 can be ionized and form a plasma under the effect of the electromagnetic field created in the enclosure, it is necessary that q ue the pressure in the container is lower than atmospheric pressure, for example of the order of 1 0 ^"4 bar To put the interior of the container in communication with a source of vacuum (for example a pump), the cover 34 has an internal channel 64, the main termination of which opens into the underside of the cover, more precisely at the center of the surface d support against which the bottle neck 30 is pressed.

Note that in the proposed embodiment, the bearing surface is not formed directly on the underside of the cover but on a lower annular surface of the claw bell 54 which is fixed under the cover 34. Thus, when the upper end of the container neck is in abutment against the support surface, the opening of the container 30, which is delimited by this upper end, completely surrounds the orifice through which the main termination opens into the lower face of the cover 34. In the example illustrated, the internal channel 64 of the cover 24 has a junction end 66 and the vacuum circuit of the machine has a fixed end 68 which is arranged so that the two ends 66, 68 are opposite the one from the other when the cover is in the closed position.

The machine illustrated in the figures is designed to treat the internal surface of containers which are made of relatively deformable material. Such containers could not withstand an overpressure of the order of 1 bar between the outside and the inside of the bottle. Thus, to obtain inside the bottle a pressure of the order of 1 0 ^"4 bar without deforming the bottle, it is necessary that the part of the cavity 32 outside the bottle is also at less partially depressurized. Also, the internal channel 64 of the cover 34 comprises, in addition to the main termination, an auxiliary termination (not shown) which also opens out through the underside of the cover, but radially outside the annular bearing surface on which the neck of the container is pressed.

Thus, the same pumping means simultaneously create the vacuum inside and outside the container.

To limit the pumping volume, and to avoid the appearance of unnecessary plasma outside the bottle, it is preferable that the pressure outside does not drop below 0.05 to 0.1. bar, against a pressure of approximately 1 0 ^"4 bar inside. It can also be seen that the bottles, even with thin walls, can withstand this pressure difference without undergoing significant deformation. For this reason, provision is made to provide the cover with a controlled valve (not shown) capable of closing off the auxiliary termination.

The operation of the device which has just been described can therefore be as follows.

Once the container has been loaded onto the claw bell 54, the cover is lowered to its closed position. At the same time, the injector lowers through the main termination of channel 64, but without closing it.

When the cover in the closed position, it is possible to draw in the air contained in the cavity 32, which is connected to the vacuum circuit thanks to the internal channel 64 of the cover 34. Initially, the valve is controlled to be opened so that the pressure drops in the cavity 32 both outside and inside the container. When the vacuum level outside the container has reached a sufficient level, the system controls the closing of the valve. It is then possible to continue pumping exclusively inside the container 30.

Once the treatment pressure has been reached, the treatment can start according to the method of the invention.

FIG. 3 is a graph illustrating the variations over time of two important parameters of the method according to the invention, namely the mass flow rate F of reaction fluid injected into the treatment zone and the power of the electromagnetic field applied inside speaker

14.

From the instant t0 when the treatment pressure is reached in the treatment zone, that is to say inside the container, the valve 40 can be opened so that the reaction fluid is injected into the treatment.

From time t1, the electromagnetic field is applied in the treatment area. Preferably, the instants t0 and t1 are separated by a time sufficient to carry out a complete sweep of the container 30 with the reaction fluid, this in order to purge as much as possible the treatment area of traces of air which remain despite the vacuum created. initially.

During all the time between instants t1 and t2, a first deposition step is carried out under conditions allowing an optimal deposition rate to be obtained on the internal wall of the container. For example it is thus possible to use a flow rate of about 1 60 sccm (standard cubic centimeter per minute) of acetylene at a pressure of about 1 0 ^"4 bar, with a power consumption micro- waves of the order of 400 watts. Under these conditions, to treat a container of approximately 500 ml, the duration of scanning between the instants t0 and t1 can be of the order of 200 to 600 ms, in any case less than 1 second The duration of the first processing step may vary between 600 ms and 3 s depending on the performance we are trying to achieve. From time t1 begins a second deposition step which, according to the invention, must take place with a flow rate of reaction fluid less than the one used in the first step. The purpose of this reduction in flow rate is to slow down the speed of deposition of the coating in order to obtain a finishing layer which, without excessively increasing the thickness of the deposit, nevertheless allows functional performance to be obtained at a very good level. With such a process, it is possible to obtain, over a comparable time, deposits of thinner thickness which have performances of the same order as the thicker deposits produced in a single step. By way of example, under the conditions of implementation described above, the duration of this second step is substantially between 500 ms and 2.5 s.

In the device illustrated in FIG. 1, the lower flow rate of the second stage is regulated by controlling the valve adequately. We can then be content to use a constant flow level of the order of 60 sccm. It is also possible to control the flow rate so as to vary it during the second step, either in stages or continuously as illustrated in FIG. 3. In this case, the variation can for example be a decreasing linear variation as a function of time. The transition between the two deposition steps can then be "continuous", that is to say without the flow of fluid being cut off, or discontinuous. In the device illustrated in Figure 2, the valve 40 is closed at the end of the first step. However, the reaction fluid contained in the buffer tank 58 is gradually sucked towards the treatment zone so that it can be seen that the plasma deposition can continue during the second step as long as one preserves the electromagnetic field in the treatment area.

The volume of the buffer tank 58 can be relatively small insofar as, if there are pressure drops in the supply device between the buffer tank and the treatment zone, the reaction fluid is stored in the buffer tank at a pressure higher than that prevailing in the treatment zone. The amount of material contained in a small volume can then be sufficient to ensure the supply under reduced mass flow during the second step. It has thus been seen that the buffer tank 58 can be constituted by the supply device itself if the internal volume of the latter is of the order of 20 to 100 cubic centimeters, a volume which is rapidly reached if the valve 40 is not in the immediate vicinity of the injector 62.

This second embodiment of the invention does not allow precise regulation of the mass flow rate of gas injected during the second step. It can however be measured that the flow rate of reaction fluid actually injected into the treatment zone decreases over time during the second stage, at the same time as the pressure in the buffer tank (or in the distribution device itself) gradually balances with the pressure in the treatment area. This second embodiment of the device is advantageous in terms of cost and simplicity.

In all cases, it is possible to envisage maintaining during the second stage the same level of electromagnetic power as during the first stage, or one can on the contrary choose to decrease this level of power. Tests have shown that it is possible to use power levels of the order of 1 00 W both during the first phase and during the second.

If we analyze the material deposited, we can see that the density of the material deposited during the second stage is higher than that of the material deposited during the first stage. More precisely, if the flow rate of the reaction fluid is varied during the second step in the direction of a decrease, it is found that the material deposited sees its density gradually increase. In this way, one obtains, in the part of the coating which is deposited during the second step, an area situated on the surface which is of density greater than the density of the material in an area situated in the vicinity of the interface with the part of the coating which is deposited during the first step.

When the reaction fluid used is a gaseous hydrocarbon compound such as acetylene, the material deposited by the process according to the invention is a hydrogenated amorphous carbon. In this case, it is found that the proportion of carbon atoms which are sp3 hybrids is greater on the surface of the coating compared to the same proportion measured deep in the coating.

Thanks to the method according to the invention, the coating deposited has an increased mechanical resistance compared to a coating of the same nature deposited according to the previously known methods. Thus, when the material deposited is a hydrogenated amorphous carbon, it is found that, in addition to the properties already known of this type of material, namely gas impermeability, hardness, resistance to chemical agents, the coating deposited according to the he invention retains a good part of its properties even after having undergone mechanical stresses in bending, stretching or biaxial stretching.

Such a method has been used to coat the internal surface of PET containers and it has been found that these containers retain good barrier properties, even after having undergone a relatively large creep corresponding to an increase in the volume of the container of the order of 5%.

Claims

1. Process using low pressure plasma to deposit a coating on an object to be treated, of the type in which the plasma is obtained by partial ionization, under the action of an electromagnetic field, of a reaction fluid injected under low pressure in a treatment area, characterized in that the method comprises at least two steps:

a first step during which the reaction fluid is injected into the treatment zone with a first flow rate and under a given pressure; and

2. Method according to claim 1, characterized in that the steps are linked continuously so that, in the treatment zone, the reaction fluid remains in the plasma state during the transition between the two steps.

3. Method according to one of claims 1 or 2, characterized in that the second flow is constant.

4. Method according to one of claims 1 or 2, characterized in that the second flow is variable.

5. Method according to claim 4, characterized in that the second flow decreases during the second step.

6. Method according to any one of the preceding claims, characterized in that the power of the electromagnetic field is kept substantially constant during the duration of the two stages.

7. Method according to any one of the preceding claims, characterized in that the pressure in the treatment zone during the second stage is lower than the pressure in the treatment zone during the first stage.

8. Method according to any one of the preceding claims, characterized in that the reaction fluid comprises a gaseous hydrocarbon compound.

9. Method according to any one of the preceding claims, characterized in that the reaction fluid is acetylene.

1 0. Method according to any one of the preceding claims, characterized in that the part of the coating which is deposited during the second step has a density greater than that of the part of the coating which is deposited during the first step .

1 1. Method according to any one of the preceding claims, characterized in that the part of the coating deposited during the second stage has a density which increases from the interface with the part deposited during the first stage up to the surface of the coating.

1 2. Method according to any one of the preceding claims, characterized in that the coating deposited consists of a hydrogenated amorphous carbon material.

1 3. Method according to any one of the preceding claims, characterized in that the part of the coating deposited during the second step has a proportion of sp3 hybrid carbon atoms which is greater in the vicinity of the surface of the coating relative at the same proportion measured in the vicinity of the interface with the part deposited during the first step.

1 4. Method according to any one of the preceding claims, characterized in that it is implemented to deposit a gas barrier coating on a plastic substrate.

1 5. Method according to claim 14, characterized in that the substrate is a film.

1 6. Method according to claim 14, characterized in that the substrate is a container.

1 7. Method according to claim 1 6, characterized in that the coating is deposited on the internal surface of the container.

1 8. Method according to any one of the preceding claims, characterized in that the coating retains its barrier properties when the substrate undergoes a bi-axial stretching of the order of 5%.

1 9. Device for implementing the method according to any one of the preceding claims, of the type comprising a device (1 2) for supplying reaction fluid comprising in particular a source of reaction fluid (1 6), a valve ( 40) for flow regulation and an injector (62) which opens into the treatment zone, characterized in that during the transition between the first and the second stage, the regulation valve (40) is controlled to cause a drop in the flow rate of reaction fluid delivered to the treatment zone.

20. Device for implementing the method according to claim 1 9, of the type comprising a device (1 2) for supplying reaction fluid comprising in particular a source of reaction fluid (16), a control valve (40) flow and an injector (62) which opens into the treatment zone, characterized in that the supply device (1 2) comprises, downstream of the control valve (40), a buffer tank (58) capable of store the reaction fluid, and in that during the transition between the first and the second step, the control valve (40) is closed, the buffer tank (58) then being gradually emptied of the reaction fluid it contains.

21. Plastic container, characterized in that it is provided on at least one of its faces with a coating deposited according to a process according to any one of claims 1 to 1 8.

22. Coating, characterized in that it consists of a hydrogenated amorphous carbon material, and in that, in the vicinity of the surface of the coating, the coating has a density which is greater than that which it presents in the vicinity of its interface with the substrate.

23. Coating, characterized in that it consists of a hydrogenated amorphous carbon material, and in that, in the vicinity of the coating surface, the coating has a proportion of hybrid carbon atoms sp3 which is greater than that that it presents in the vicinity of its interface with the substrate.