PROCESS FOR DEPOSITING A BARRIER FILM ON THREE-DIMENSIONAL ITEMS
FIELD OF THE INVENTION The invention relates to a Chemical Process of Deposition in the Steam phase increased by Plasma < PECVD > whereby a coating of inorganic material is deposited on three-dimensional articles - including low melting temperature polymer articles. The coating has excellent barrier properties against gas and / or water vapor. BACKGROUND OF THE INVENTION With the increased emphasis on the use of medical plastic products, there is a special need to increase the barrier properties of articles made of polymers. Such medical products that would benefit considerably from an improvement in their barrier properties include, but are not limited to, collection tubes and especially tubes used for blood recovery. Additionally, such improvement of the barrier properties of polymer articles may also have applications in terms of food, cosmetics and foods. For example, in terms of recovery tubes, blood recovery tubes require certain performance standards to be acceptable for use in medical applications. Such performance standards include the ability to maintain an extraction volume greater than about 90Y * of the original extraction volume over a period of one year, must be sterilizable by radiation and must not interfere with testing and analysis. Accordingly, there is a need to improve the barrier properties of articles made with polymers and particularly plastic blood collection tubes where certain performance standards would be met and the article would be effective and usable in medical applications. Metal-oxide or glass-type films synthesized from chemical vapor deposition techniques have been used as a thin barrier coating on polypropylene films. However, thin films of the synthesized glass type are substantially granular in terms of their morphology instead of having a substantially continuous glass-like morphology and therefore do not have the oxygen and water vapor barrier characteristics of a truly glass material continuous. It has been observed that to overcome the drawbacks of the morphology of thin films similar to glass, "layers" of glass-like films are "stacked" with a continuous organic polymer film interposed between each layer. Such multilayer layered coatings improve the performance of the oxygen barrier of polypropylene films, however such layered formations do not produce a glass-like barrier and the layered structure simply acts as a laminate of metal oxides and Acrylate polymer coatings. Therefore, it is desired to produce a compound that can be used to achieve a gas and water barrier performance similar to glass. Generally, existing processes that are used to produce PECVD barrier film are mainly suitable for two-dimensional surfaces. Those that have been developed for three-dimensional items do not fit well for increased high density density matrix treatments. The process of the present invention is well suited for such an increase. SUMMARY OF THE INVENTION The present invention relates to a method for applying a chemical coating of barrier film deposited in a vapor phase aided by plasma on the outer wall surfaces of two or more three-dimensional articles (such as, for example, hollow articles), which it comprises: a) supplying an apparatus capable of applying said barrier film coatings on external walls of said articles, said apparatus having: a hermetic chamber, a device for supplying a monomer to said articles; a device for supplying an oxidant to said articles; a device for inserting radiofrequency-activated electrodes into the internal surface of said articles; at least two electrodes; a device for creating and maintaining a vacuum within said chamber containing said articles; and further where said apparatus is mounted on a pumping station, and wherein said chamber is fixed on a device for importing energy into said article where said device is a radiofrequency power generator; b) positioning at least two three-dimensional articles having an open end, a closed end, an external part, an internal part, and an external and internal wall surface such that said open end is on said at least one electrode; c) evacuate said chamber. containing said articles less than 5 mTorr; d > supplying a monomeric gas to said external surfaces of said article from about 1 sccm to 5 sccm and approximately 80 mTorr to 160 mTorr; e > supplying an oxidizing gas to said external surfaces of about 50 to 150 sccm and about 80 mTorr to 160 mTorr; f) supplying a radio frequency power to said electrodes of approximately 1 to 50 MHz and approximately 0.1 to 2 watts / cm2; and g) obtaining reverse barrier films on said article at a rate of 40 to 100 n / min. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a general schematic view of an apparatus of the present invention. Figure 2a-b is a graphic representation of the polystyrene permeation coated as a function of pressure (mTorr) vs. oxygen flow (sccm) during plasma deposition. Figure 3 is a three-dimensional representation of a mode where 10 electrodes are located and the locations of the electrodes are presented, and this electrode array can be used in the apparatus of Figure 1. DETAILED DESCRIPTION OF THE INVENTION The present invention focuses on to a PECVD process in which a coating of inorganic material can be placed in three-dimensional articles in a matrix spaced at small intervals. This inorganic material can be a metal oxide such as for example SiO? where x is from about 1.4 to about 2.5; or a composition based on aluminum oxide. The composition based on silicon oxide is substantially dense and impervious to steam and desirably derived from volatile organosilicon compounds and an oxidant such as oxygen or nitrous oxide. Preferably, the thickness of the material based on silicon oxide is about 50 to 400 nm. Figure 1 shows a schematic of an apparatus 40 in relation to an embodiment of the present invention. In use, the polymer tubes are placed in the electrodes 43 and the chamber 44 is evacuated to a base pressure, preferably less than 5 mTorr. An iron vapor (such as, for example, HMDS0 (hexamethyldisodium loxane)) and an oxidant (such as oxygen) are admitted into the apparatus by 41 and 42, respectively. For a system of approximately 30.48 centimeters in diameter with a vertical flow, an HMDS0 flow of approximately 1 to 5 sccm and an oxygen flow of approximately 50 to 150 sccm is employed. The system is continuously pumped at a rate to maintain a pressure of approximately 80 to 160 mTorr. The apparatus is mounted on a pumping station 46. A radio frequency (RF) power generator and an adaptation system 45 is used to generate a plasma with a frequency of 1 to 50 MHz and a power per electrode area of approximately 0.1. at 2 watts / cm2 according to the meter and the proximity of the electrodes. A deposit of SiO? occurs in this way on the article exposed to a regime in the order of 40 to 100 nm / min. Significant barrier properties can be realized with recesses of 50 to 400 nm thickness. During the deposit, the electrode potentials oscillate with an amplitude of approximately 500 v 1000 v from peak to peak for RF frequencies of approximately 5 to 15 MHz. For a given radio frequency power the amplitudes decrease with an increase in frequency and increase with a decrease of the frequency. If the adaptation network includes a blocking capacitor and if a part of the electrode circuit is exposed to the plasma, an electron current from the plasma establishes a negative direct current polarization at the electrodes of approximately -100 v to -400 v. This polarization can be reduced or essentially eliminated by minimizing the area of the electrode circuit exposed to the plasma and / or by short-circuiting the DC component of the electrode potential through an inductor that blocks the radio frequency current i. An optimum barrier occurs for the deposition conditions that supply an energy element to the polymer article that fails to produce thermal degradation. This absorbed energy is a product of treatment time, ion current, and electrical potential in the plasma envelope that accelerates these ions. Since the process is carried out under vacuum, little heat conduction or convection is observed and almost all the energy absorbed is conserved. To produce a high quality barrier film, such as an SiOx barrier film, a narrow range of physical and chemical properties must be met. Failure to comply with any of the aspects will result in a highly permeable film. Response surfaces that plot the properties of the oxygen barrier against plasma deposition process parameters (see Figure 2) show that an optimal barrier is observed only for a small area in the matrix space for oxygen and flow regimes of HMDSO and systems pressure. Outside this range, films similar to soft polymers are observed for excessive monomers, fractured and very tight films are observed in the case of an excess of an oxygen, slow deposition rates occur in the case of a low pressure, and a nucleation in Gas phase leading to dusty deposits occurs in the case of high system pressure. Even with adequate chemical conditions, a defective barrier will be observed without adequate ion bombardment of the film during deposit. The electric field near the surface of the substrate is an essential element to increase the deposition rate and, more importantly, to identify the film by ion bombardment to eliminate microvacies and granular structure. However, excessive ionic energy can thermally destroy the film. The balance between these requirements requires an adequate combination of the total number of electrodes, spaces between electrodes, RF frequency, radiofrequency power, and connection of the plasma to a conductor connected to the ground. The radiofrequency power supplied to an electrode simultaneously generates the radiofrequency plasma discharge surrounding the polymer article and produces an electric field that accelerates the ions to the surface. The discharge causes the formation of reactive species that can be combined in the film. The electric field densifies independently the material deposited in a waterproof film. There is no guarantee that for a given system geometry both processes can be carried out simultaneously; there are examples where no barrier can be achieved through the process space. The discharge formed around an electrode is more intense at a distance of approximately 0.3 to 2 cm and decreases in density at greater distances. When additional electrodes are arranged in a matrix with separations of a few cm or less, for example, from 0.5 to 15 cm, these discharge zones are spliced and the density of plasma near the electrodes is increased by the effect of their neighbors. This allows a lower radiofrequency power to be used to achieve a given plasma density which may be necessary to generate the required chemical reaction. As mentioned above, the electrode potential that controls the envelope fields depends on the radiofrequency power and the frequency. The plasma potential is influenced by the above factors and additionally by the flow of charged species to any nearby conductive surface, such as a chamber wall connected to ground. Therefore, a successful system requires a carefully balanced set of process parameters as well as a design geometry. The present invention can be realized in various ways according to the size and shape of the articles to be coated. In the case of small cylindrical vessels, the following configuration that can be used in the apparatus of FIG. 1 is preferred. Ten electrodes that fit inside 13? PET tubes. 100 m of arrange in a matrix as shown in figure 3. The electrodes have location numbers 1-10. The distance from center to center of separation in this case is 4 cm but this does not represent a limitation in any way. A flow of 2.6 sccm of HMDSO and 70 sccm of oxygen is established and the pressure is regulated to 120 mTorr by means of the regulation of the pump. A deposit of SiOx of 3 min is produced with a radio frequency excitation of 120 watt 11.9 MHz. The electrodes undergo a p-p radiofrequency amplitude of 770 v with a direct current bias of -230 v. Since these tubes have a surface area of approximately 40 cm2, this results in an energy load of 0.3 w / cm2. This treatment provides an improved barrier against gas and water vapor of approximately 3 times what is obtained with a 1 mm thick PET tube without treatment. None of the above parameters is independent of the others. For example, less numerous and more remote electrodes require a higher energy per area to produce a barrier; For only four tubes used in the outer corners of the existing matrix, ie electrodes No. 2, 3, 8 and 9 in Figure 3, as the power is increased to produce a suitable plasma density, thermal degradation occurs before achieving a comparable barrier. In this case an amplitude of p-p electrode of 915 v na can equal the deposit produced with the arrangement of 10 p-p electrodes of 770 v. In addition, direct current polarization has been widely discussed in publications related to film deposition by PECVD such as Sibson, Mat. Res. Soc. Sy p. Proc., 223 (1991) and Green, Mat. Res. Soc. Symp. Proc., 165 (1990) as an essential element of sputtering, was irrelevant here: by connecting the electrode circuit to an 8000 μH inductor connected to ground, this polarization can be totally reduced to zero without loss or barrier. By inserting resistors in series with the inductor, the polarization can be reduced in stages. As this occurs, the plasma potential becomes positive, and the surface charge on the substrate is altered to maintain the envelope potential. EXAMPLES EXAMPLE 1 Using the arrangement of the preferred embodiment, there was a treatment with identical gas flows, pressure, and RF frequency and adaptation but with a power of 148 watts for 2 minutes. This treatment provided a barrier against water of approximately 2.5x the barrier obtained in the untreated tube. EXAMPLE II Another example is a treatment identical to the previous one, except that upn 167 watt plasma was used for 1 minute. An improvement of 1.7x was observed regarding the barrier against water.