BACKGROUND OF THE INVENTION
(a) Field of the Invention
The invention relates to a gastight, pressure-resistant storage and/or transport container for low-molecular-weight reactive filling media, especially hydrogen, oxygen, air, methane and/or methanol, with a high filling pressure, which container is substantially rotationally symmetrical and has at least one connection cap with a sealing device.
(b) Prior Art
It has been usual for some time to fill, store and/or transport low-molecular-weight reactive media, especially gases such as hydrogen, oxygen and air, in thick-walled metal cylinders fitted with a safety sealing cap. In this way, large quantities of gas can be concentrated in the smallest possible space, stored free from leakage for long periods and transported safely. A pressure reducing valve is fitted on the sealing cap of a metal cylinder only immediately before use.
Thick-walled metal cylinders made of steel nevertheless have the disadvantage that they are extremely heavy in comparison with their stored contents. Replacement of steel cylinders by corresponding aluminium cylinders was a first important step in the right direction in relation to the weight, but nevertheless the said contents-container disproportion persists although to a lesser extent.
U.S. Pat. No. 4,073,400 A describes such a gas container made of metal, preferably aluminium or steel, which has an external protective layer of a fibre-reinforced resin/polymer. Optionally, an extra internal anticorrosion layer is provided which also consists of a fibre-reinforced resin/polymer. Evidently no diffusion protection is necessary for such a metal container.
DE 3821852 A1 also describes a pressurised gas cylinder consisting of an internal metal container surrounded by external glass-fibre-reinforced plastic layers. This pressurised glass cylinder which is designed as a fuel tank for motor vehicles is suitable for content pressures up to 340 bar. Thanks to the metal cylinder there are no diffusion problems, and when an anticorrosion aluminium alloy is used for the internal container also no corrosion problems.
Since the oil crisis, natural gas has played an increasing role in both the heating and the motor vehicle sectors. The French company Ullit S.A., F-36400 La Châtre, offers ultra-light high-pressure cylinders for gas-powered motor vehicles, which cylinders basically consist of a one-piece thermoplastic wound body. These cylinders, having a capacity of 126 litres and an operating pressure of 200 bar, are incorporated in a motor vehicle like a battery and serve as a fuel reserve. The plastic cylinders, also plastic composite cylinders, represent a completely new type of gas container in the high pressure range compared with metal cylinders. Plastic cylinders are very low in weight, are not susceptible to corrosion or alternating load fatigue and in particular are sufficiently impervious to higher molecular-weight gases, e.g. natural gas.
Manufacture of a plastic two-layer pressurised gas container is described in WO 00/66939 A1. An internal plastic container is pre-treated in rotation in order to increase its cross-linking and adhesion properties. After application of an adhesive, a fibre-reinforced winding tape is applied spirally and sticks very well onto the internal container to form an effective pressure reinforcement. No problems of diffusion or corrosion are described or mentioned.
With respect to a pressurised gas container for high pressures, a completely different class of double-wall heat storage cylinder (thermos cylinder) made of plastic in accordance with U.S. Pat. No. 3921844 has in a known manner silver layers which reflect the heat radiation and also act as diffusion barriers. The vacuum between the comparatively very thin double walls can thus be maintained for a long period of time and heat convection suppressed. Such a double-wall in a pressure vessel with a vacuum between the two walls would be not only pointless but also counterproductive.
SUMMARY OF THE INVENTION
The inventors have faced the task of providing a gastight, pressure-resistant storage and/or transport container of the type described above, which with a low empty weight is impermeable media-specific and/or if necessary corrosion-proof.
The said task is solved in accordance with the invention in that the container wall comprises in essence a thermoplastic with at least one diffusion barrier system or a diffusion barrier and anticorrosion system. Special and refined embodiments of the container are the subject of dependent claims.
The term “diffusion barrier layer” comprises both layers deposited on the container wall and films applied on or in the container wall, with or without functional layer(s). A diffusion barrier layer can also simultaneously or exclusively form an anticorrosion layer without this being specially mentioned each time. All types of diffusion barrier layers should preferably have approximately the same expansion coefficient as the container wall.
A “diffusion barrier system” or an “anticorrosion system” may comprise a compact layer and/or dispersed, passive or reactive nanoparticles. Reactive nanoparticles react chemically with a permeating gas, passive nanoparticles adsorb (store) such a gas.
Plastic containers with high content pressure, i.e. in the region of at least 50-100 bar, have the external dimensions and forms that are commercially standard. They are preferably substantially cylindrical in shape and in the area of their longitudinal axis have a sealing cap on one or both sides with a seal of the usual design. Suitably, the casing length of large containers lies in the usual range of 1 to 6 m, the internal diameter may be up to 40 cm, in particular approximately 35 cm, the filling pressure is preferably at least 150 bar, in particular at least 250 bar. Portable medicinal cylinders for patients, also included in the invention, for example are in essence smaller.
The stability and bursting pressure of the container can in essence be increased if the thermoplastic material of the container wall, for example polyethylene, polypropylene, acetyl butadiene styrene, polyamide, polyvinyl acetate or a polyester, is reinforced with a tension-resistant material, preferably with carbon-, glass- or ceramic fibres, but also with steel wires.
Dependent on the aggressivity and permeation ability of the contents and the external atmosphere, a diffusion barrier layer is arranged inside and/or outside the container wall, if necessary also or only in the wall itself.
In the case of aggressive contents, in a container stored in inert atmosphere, only an internal diffusion barrier layer is necessary, or the wall of a hydrogen container comprises dispersed reactive nanoparticles.
If a container with aggressive contents is stored in a corrosive atmosphere, an external diffusion barrier layer is also applied which is at the same time an anticorrosion layer.
In the case of a container wall which is inert against a reactive content, a diffusion barrier layer can be integrated in the said wall, for example by coextrusion or corresponding winding technology, both methods known in themselves, or the wall of a hydrogen container comprises dispersed passive or reactive nanoparticles.
At least one diffusion barrier layer can be applied to the container wall by two fundamentally different methods:
as a composite film preferably 10 to 1000 μm thick, with a diffusion barrier layer in the stricter sense, preferably maximum approximately 500 μm, in particular maximum approximately 20 μm thick,
by deposition from the gaseous phase with or without chemical reaction, also as a thin layer in the region of 10 to 600 nm, especially up to 100 nm. This deposition may take place directly onto the container wall and/or onto a carrier film subsequently applied on or in the container wall.
The film may be applied externally for example by winding, preferably with thickly spirally overlapping film strips, by longitudinal application of a film again with thick overlapping of the side edges or by application of a piece of shrink film or film weldable to size. Internal coating or lining with film to form the diffusion barrier layer is carried out by introduction of a custom-sized bag with dimensions corresponding to the container interior, provided with one or two openings depending on the container. The inserted bag is fixed in the region of the filler neck, e.g., by gluing, clamping or screwing-on.
The application or internal extrusion of a metal film, generally an aluminium or steel film, as the diffusion barrier layer is preferably carried out with a composite film. A composite film comprising a 9 μm thick aluminium film with one unilaterally or two bilaterally laminated or extruded film(s) of LLDPE (linear low density polyethylene) of for example approximately 100 μm thickness is sufficiently tear-resistant for all said processes.
Pure plastic composite or laminate films may also be applied or internally extruded, e.g. LLDPE (100 μM)/OPP (20 μm)/PVA (14 μm)/OPP (20 μm) LLDPE (100 μm). OPP is oriented polypropylene, PVA (=PVAL) is polyvinyl alcohol. The PVA layer can also be provided with an SiOx or DLC layer (diamond-like carbon).
A container in accordance with the invention, or a film introduced therein, can also be protected by one or more diffusion barrier layers deposited from the gaseous phase. Deposition from the gaseous phase is carried out in the known manner with or without chemical reaction in the gaseous phase, or also by co-deposition of materials. Concrete examples of this are: arc vaporisation and cathode atomisation (sputtering). Further examples are: deposition by laser, electron, ion or molecular beams or thermal action, in each case with or without plasma excitation and with or without magnetic field support, as well as plasma spraying. The deposited layers form a diffusion barrier layer that, where necessary, is also the anticorrosion layer.
If a plastic container in accordance with the invention or a film to be introduced or applied is required to have a metallic or ceramic diffusion barrier layer, pre-treatment is often advantageous in order to increase adhesion of this diffusion barrier layer. Pre-treatment is suitably performed by plasma activation of the surface to be treated or by means of a very thin polar plasma layer of clearly <1 μm thick. In a first case the coating is deposited immediately after activation and in a second case the polar layer can for years stabilise the surface tension of the plastic surface at >50 mN/m or if necessary even at >70 mN/m.
In plasma activation for pre-treatment, monomer gases containing oxygen and/or nitrogen are introduced in a mixture of noble gases (Ar, He) with radio-frequency discharge (RF), e.g. good results are obtained with CO2, O2, N2, NOx and/or NH3. RF contains low frequency, high frequency and very high frequency.
Plasma activation has been used industrially for some time, for example in the form of corona discharge or low pressure discharge.
Ar and a little O2 are directed at high velocity onto a plastic substrate for less than 1 minute, either continuously or pulsed, at 200-2000 W, 13.56 MHz or 2.45 GHz.
Noble gases containing NH3 are directed at high velocity onto a plastic substrate during high or low frequency discharge. Excellent results are obtained for adhesion of Al to polypropylene.
For plasma coating as a pre-treatment, mixtures of the noble gases Ar and He and/or, depending on the required surface tension, mixtures of e.g. the monomer gases CO2, O2, N2, NOx, NH3, CH3OH, CH4, CH3CN and C2H2 are introduced. For hydrophilic pressed layers with long-term stability see WO 99/39842, in which for a polar coating a waterless process gas is used containing at least one also substituted hydrocarbon compound with up to eight C atoms and one inorganic gas.
A plasma coating as a pre-treatment is carried out with a mixture of equal parts of: Ar, C2H2, N2 and CO2. This gives a surface tension of >60 mN/m.
An apolar diffusion barrier layer, i.e. having a barrier effect, can also be applied directly i.e. without pre-treatment by means of plasma polymerisation, for example as an 0.01 to 1 μm thick amorphous DLC hydrocarbon layer (diamond like carbon). This is constructed on the basis of carbon and hydrogen, has a 20 to 80 at % content of the two elements and respectively 0.01 to 6 at % of at least one element of the group comprising oxygen, nitrogen, fluorine, chlorine, bromine, boron and silicon. In this connection reference is made to WO 00/32938 (table, item E).
Following the pre-treatment described above, an actual diffusion barrier layer e.g. a metallic, organic metal-containing and/or ceramic layer is deposited. In the sense of the present invention, the metallic layers also include boron and silicon. In this connection, there are several known procedures and combinations thereof. Most of these are suitable for external coating of the container, but only with limitations for the internal coating. At best technical details must be adapted such as enlargement of the outlet and/or miniaturisation of the source.
For deposition of a diffusion barrier layer of submicron thickness on the container wall or on a film to be introduced or deposited, the use of plasma-supported coating processes is particularly suitable since the substrate temperature can be kept lower and good adhesion of the layer to the substrate is achieved by adhesion-promoting interaction with the plasma. In addition, by targeted variation of the plasma parameters including the process gases, a layer structure is achieved which supports adequately the expansion of the container.
Effective ceramic diffusion barrier layers comprise e.g. Al2O3, TiN, TiC, Si3N4, SiC, ZrO2, Cr2O3, SiOx and/or SiOxNy.
In accordance with a variant for a hydrogen container, the diffusion barrier system according to the invention comprises, in the container wall, in the diffusion barrier layer and/or in a composite film with the diffusion barrier layer, finely dispersed passive nanoparticles for storage of hydrogen or reactive nanoparticles for chemical reaction with hydrogen. These nanoparticles preferably contain Ti, Pd, Fe, Al, Mg, Mg2Ni, TiC, TiO2, Ti3Al, TiN, Ti2Ni, LaNi5H6, graphite, silicate and/or nanotubes containing carbon. The nanoparticles may also be embedded in a matrix, for example, passive TiN nanoparticles or active Ti nanoparticles in a Si3N4 matrix with maximum grain size of a few Jim. Similarly, Ti and/or TiC nanoparticles can be embedded in a SiC matrix, or Ti and/or TiO2 nanoparticles in a SiO2 matrix. For other reactive gases, e.g. oxygen, similar diffusion barrier systems exist (table, item 1).
Reactive nanoparticles react chemically with a gas that diffuses through the container wall, e.g. Al nanoparticles with oxygen to form Al2O3. Passive, i.e. non-reactive nanoparticles adsorb a gas diffusing through the container wall, e.g. Ti nanoparticles adsorb H2. They can be incorporated in widely varying geometric form and form a physical diffusion barrier.
A hydrogen-storing component must be selected such that the coefficient of expansion during hydrogen absorption and particle size are adapted to the container dimensions and the pressure variations.
Examples for Diffusion Barrier Layer Systems
Functional layer system 1: container and film, internal and/or external barrier layer
Plasma activation of a plastic substrate is carried out in order to increase adhesion to the subsequent coating. An aluminium metal layer is applied by PVD (physical vapour deposition). The PVD is carried out for example by cathode atomisation (sputtering) and/or internal and external arc vaporisation, external thermal and electron beam vaporisation. If this metal layer is then oxidised by a plasma process, e.g. by RF discharge, a defined additional Al2O3 protective and diffusion barrier layer is formed on the surface. This is imperative e.g. for a methanol container if no additional protective layer is deposited internally.
Functional layer system 2: container or container with film and barrier layer, preferably internal.
A DLC layer can be deposited directly, without pre-treatment as a diffusion barrier layer which also serves as a protective layer, onto a plastic substrate. To achieve the necessary flexibility, a gradient layer from polymer-like to diamond-like or from elastic to dense, can be produced by way of the process control. The electrically non-conductive substrate together with the layer material enables inductive coupling of the radio frequency into the container.
Separately or in addition to at least one barrier layer, by means of the gaseous phase with organic metal components finely dispersed nanoparticles containing metal (e.g. Al, Ti, Mg) can be deposited on or in the container wall or in a film to be introduced, which particles absorb and/or store the diffusing hydrogen or oxygen.
Functional layer system 3: container and film, barrier layer internally (arc, cathode atomisation) and/or externally (arc/cathode atomisation/ PA-reactive electron beam vaporisation).
Plasma pre-treatment of a plastic substrate is carried out in order to polish the surface where applicable and increase adhesion to the subsequent coating. A ceramic layer of Al2O3, SiOx, SiON, TiO2 and/or ZrO2 can be deposited using the said PVD methods, arc, reactive cathode atomisation (sputtering) and plasma-activated reactive electron beam vaporisation. A sandwich-like structure of the diffusion barrier layer which as a whole is totally gas-impermeable but nevertheless tolerates the expansion of the mechanically stressed container without damage, can be achieved by variation of the process parameters, e.g. a dense, hard layer or a soft, expandable layer. Metal-containing (elementary) nanoparticles can be incorporated in the layer by co-deposition or additional use of a molecular beam.
In addition, a thin diffusion barrier layer, namely a DLC layer with or without passive/ active nanoparticles or a thin ceramic layer, e.g. SiO2, Al2O3, and/or Si3N4 with or without passive/active nanoparticles, can be deposited on the plastic substrate by plasma-excited (organic metal) chemical vacuum deposition from the gaseous phase (PE(MO)CVD). Reference is made here to FIG. 9 below and to WO 01/55489 (despite the different function) in relation to DLC layers of submicron thickness with metallic nanoparticles, i.e. particles in the nm range of a maximum of 50% of the layer thickness of corresponding size.
Functional layer system 4: container and film, internal and/or external barrier layer
A barrier layer comprises a sandwich-like construction with up to seven layers, e.g. the following layers: polymer-metal-polymer-metal oxide-polymer, namely: UV-hardened polyacrylate (1-5 μm)/Al (10-1000 nm)/polyacrylate (0.5 μm),/TiO2 (10-100 nm)/polyacrylate (0.5 μm). The metal and metal oxide layer are deposited by vaporisation. Instead of the TiO2 layer, a DLC, SiON and/or Al2O3 layer can be deposited. The extendability of the coating is thus guaranteed. Thicker layers could be deposited e.g. by plasma spraying (table, item H).
Functional layer system 5: container, barrier layer, preferably internal
A polymer layer comprising e.g. polypropylene is applied as pre-treatment, with a thickness of one or a few μm, in order if necessary to polish the surface which can also be plasma-activated in order to increase adhesion to the subsequent coating. Several metallic and/or ceramic “brick-like” structural layers are applied, e.g. sheet silicates. A final polymer-like protective layer guarantees freedom of movement of the brick-like structure. By way of example, liquid crystal polyesters (LCP) can also be biaxially stretched, thus producing a lamelliform structure.
Functional layer system 6: container, internal and/or external barrier layer
A combination of two different deposition methods, plasma-excited (organic metal) chemical vacuum deposition from the gaseous phase (PE (MO) CVD) and physical vapour deposition from the gaseous phase (PVD), preferably cathode sputtering, results in a composite diffusion barrier layer comprising one inorganic and one organic material or various inorganic materials. The inorganic component is a metal (e.g. aluminium or titanium) or a ceramic (e.g. Si3N4 or Al2O3), the organic component is a plasma polymer of highly cross-linked hydrocarbon or a copolymerised hydrocarbon with oxygen and/or nitrogen.
A smooth transition, i.e. a gradient, can be achieved by variation of the process parameters or with incorporated particles.
For low-molecular reactive media, in particular hydrogen, oxygen, methane and/or methanol, gastight tank systems in accordance with the invention are provided. A pressure-resistant plastic container with a fundamentally lower weight for motor vehicles is lined internally and/or externally with a highly effective diffusion barrier layer which prevents the escape of the filling medium in even the most minute quantities and guarantees its storage in accordance with legal safety specifications.
Combination of the properties of suitable metal films, plastic films and coatings allows production of a such a versatile high barrier film system. Irrespective of dimensions, for internal coating or lining of plastic containers, functional matching of the high barrier film system to the particular specifications of the contents is necessary. In other words, for each filling substance the most suitable film combination can be applied or a layer deposited, or the most suitable nanoparticles can be integrated in the container wall.
When a coating is applied directly onto the plastic container, the coating methods can be scaled up to the applicable dimension. With particularly aggressive filling substances, the nature and combination of the layers can be adapted accordingly. For example, with an aluminium diffusion barrier layer a further layer can be applied if methanol is the filling substance.
Finally, the recycling suitability of the plastic container represents a further advantage of the invention. The diffusion barrier layer can be removed, can consist of an equivalent material to that of the container or has such a low mass fraction that it is negligible in recycling.