VESSEL

TECHNICAL FIELD

The invention relates to a method for producing a leak-tight vessel for holding a gas and/or liquid using a partly reusable and removable mandrel, and to a leak-tight vessel produced in this way.

BACKGROUND ART

Leak tight vessels comprising a fibrous material and methods for producing them are known in the art.

With "leak-tight vessel" is meant a substantially liquid-tight vessel or a substantially gas-tight vessel, wherein the permeability of the vessel for the liquid and/or gas to be stored inside the vessel is below a maximum prescribed limit for the given application the vessel is intended for. For example, in case the application is a hot water boiler application, the relevant permeability is the permeability of hot water under the intended storage conditions (e.g. temperature, pressure).

Such a leak tight vessel and a method for producing it is known from EP2000288. The leak-tight vessel described in EP2000288 comprises a hollow body with a main part having e.g. a cylindrical shape, and having at least one opening that is closed by an end part, which is e.g. dome shaped. The end part is connected to the main part along its circumference by means of glueing or welding, such that a closed body is formed. A disadvantage of this leak-tight vessel is that the end part needs to be added in a separate production step, and that it is difficult to make the vessel leak-tight up to elevated pressures (e.g. > 20 bar).

In another known method for producing a leak-tight vessel, in particular a pressure vessel, continuous fibers are filament wound over an inner bottle, also called "liner", that will remain in the leak-tight vessel after the filament winding step. Such an inner bottle (liner) needs to have a wall of sufficient thickness (e.g. >1 cm) to be able to resist the pressure exerted upon the bottle during the filament winding. A disadvantage of such a vessel is that it is heavy and expensive.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method for producing a leak-tight vessel that is capable to resist up to an elevated pressure (e.g. >20 bar), without requiring an inner bottle ("liner").

This is achieved according to the present invention with the method of the first claim. As will become clear further, the disclosed method is also very well suited for producing low-pressure leak-tight vessels or containers.

Thereto the method for producing a leak-tight vessel for holding a gas and/or liquid according to the present invention comprises the steps of: - assembling a partly reusable and removable mandrel suitable for filament winding, the mandrel having a first rounded outer surface around a symmetry-axis and comprising: - reusable and removable parts provided to be arranged in such a manner as to form the first rounded outer surface; - a first end fitting having a second rounded outer surface suitable for filament winding, and having an opening large enough to allow the passage of the reusable and removable parts; - mounting the first end fitting to the mandrel; - forming a shell layer by filament winding a fibrous material over the first rounded outer surface of the mandrel and over at least part of the second rounded outer surface of the first end fitting while leaving the opening for removing the reusable and removable parts of the mandrel; - applying a gas and/or liquid tight layer to an inner surface of the shell layer; - disassembling the mandrel and removing the reusable and removable parts through the opening, while leaving the first end fitting in the shell layer.

With "gas and/or liquid tight layer" is meant substantially gas-tight layer or substantially liquid-tight layer, or both, depending on the intended application.

By filament winding a fibrous material over the first rounded outer surface of the mandrel and over at least part of the second rounded outer surface of the first end fitting, a shell layer forming essentially the wall of the leak- tight vessel is formed, which shell layer extends over at least part of the first end fitting, thereby providing a good mechanical hold of the first end fitting to resist internal pressure from inside the leak-tight vessel. Furthermore, with the method of the invention, a large contact area between the wall and the first end fitting can be obtained. By providing this large contact area between the wall and the first end fitting, the force counteracting the internal pressure acts as a shear force and is spread over a large area, thereby reducing the tension exerted upon the materials of the shell layer and of the first end fitting, thereby decreasing the risk of leakage, and the formation of gaps or cracks.

By overlapping a large portion of the first end fitting, the fibrous material surrounding it acts as a reinforcing shell around the first end fitting, so that leak-tight vessels able to resist an increased internal pressure can be produced suited for some applications, or the structural strength of the first end fitting can be reduced, thereby reducing cost and weight for other applications.

As the first end fitting is left behind in the leak-tight vessel, an extra processing step for adding an end fitting afterwards can be omitted, saving production time, production space and energy, and avoiding the risk of leakage at the location where the first end fitting is connected to the wall structure, as is the case in the prior art.

By overwrapping the second rounded outer surface of the first end fitting, the strength and rigidity of the leak-tight vessel can be determined primarily by the strength of the filament wound fibrous material and less by the strength of the end fitting itself. This is especially true when filament winding with continuous fibers. This allows for several optimizations depending on the application.

By using a partly reusable and removable mandrel, the use of an inner liner (or bottle) that is overwrapped can be avoided, thereby saving considerable material, weight and cost. The inventor has found that, instead of using such a liner, a gas and/or liquid tight layer can be applied on an inside of the fibrous vessel, whereby the thickness of the gas and/or liquid tight layer is not dominated by its mechanical strength, but only by its chemical and/or physical properties, in particular its barrier properties for the gas or liquid to be stored inside the vessel.

By applying the gas and/or liquid tight layer to an inner surface of the shell layer, a tight connection thereof with the shell layer can be established, thereby reducing the risk of disconnection. This is advantageous in applications with an over-pressure inside the vessel, in which case the gas and/or liquid tight layer is pushed against the shell layer, but also in applications with an under-pressure in which case the gas and/or liquid tight layer is prevented from loosening and thus collapsing.

By applying the gas and/or liquid tight layer on the inside of the leak-tight vessel instead of the outside, the risk that the gas and/or liquid tight layer gets accidentally damaged, for example by a sharp object is minimized.

Another advantage of applying the gas and/or liquid tight layer on the inside of the leak-tight vessel is that direct contact between the gas and/or liquid inside the leak-tight vessel and the material of the shell layer can be avoided.

The material of the gas and/or liquid tight layer can be optimally chosen in function of the envisioned application, e.g. to provide an oxygen barrier or smell barrier, or to protect the fibrous material against chemicals inside the vessel. If so desired, the gas and/or liquid tight layer can consist of multiple layers.

As used herein, the terms "barrier layer" or "gas and/or liquid tight layer" are used as synonyms.

Preferably the fibrous material is applied by filament winding continuous fibers impregnated with a first plastic material. By filament winding using continuous fibers a very strong yet lightweight shell layer is applied around the mandrel, which shell layer will form the wall of the leak-tight vessel. By winding continuous fibers, the obtained endless filament structure will allow the vessel to withstand higher hydrostatic pressures.

Depending on the order in which the different steps of the method according to the invention are performed, different embodiments of the method and different embodiments of a resulting leak-tight vessel according to the present invention are obtained. In a first embodiment of the method according to the present invention, the gas and/or liquid tight layer is applied to the mandrel before mounting the first end fitting, and the first end fitting is mounted to the gas and/or liquid tight layer before the step of forming the shell layer. By applying a gas and/or liquid tight layer to the outer surface of the mandrel, then placing the first end fitting, and then applying the fibrous material, the gas and/or liquid tight layer is actually applied to an inner surface of the leak-tight vessel in an indirect way, without actually going inside the vessel. This is easier in production, and enhances controllability. In this embodiment the gas and/or liquid tight layer is preferably applied by spraying or coating.

In a second embodiment of the method according to the present invention, the first end fitting is mounted to the mandrel before applying the gas and/or liquid tight layer, and the gas and/or liquid tight layer is applied to the mandrel and to the first end fitting before the step of forming the shell layer. By doing so, the first end fitting is located on the inside of both the shell layer and of the gas and/or liquid tight layer, while the gas and/or liquid tight layer is adjacent to the fibrous layer also near the first end fitting. In this embodiment the gas and/or liquid tight layer is preferably applied in the form of a thermoplastic film or by spraying or coating.

In a third embodiment of the method according to the present invention, the first end fitting is mounted to the mandrel before the step of forming a shell layer, and the gas and/or liquid tight layer is applied to an inner surface of the shell layer and to an inner surface of the first end fitting after disassembling the mandrel and removing the reusable and removable parts of the mandrel. This method offers the advantage that direct contact between the gas and/or liquid inside the vessel and the first end fitting can be avoided. In this embodiment the gas and/or liquid tight layer is preferably applied in the form of rotation moulding, or blow molding, or by spraying or coating.

The inventor has found that the rigidity and mechanical strength of the leak-tight vessel according to the present invention can be further improved by consolidating the material of the shell layer and/or the material of the first end fitting and/or the material of the gas and/or liquid tight layer. Forces exerted upon one part or layer of the vessel are then optimally transferred to the other parts or layers of the vessel, thereby obtaining an overall reduced stress and a more even expansion or compression of the vessel, thus reducing the risk for gaps and cracks and leakage, thus increasing the lifetime of the vessel.

As used herein, with "consolidation" of two or more materials is meant unification; in the context of thermoplastic materials consolidation means uniting by heating or local melting, in the context of thermoset plastic materials consolidation means polymerization also known as curing.

In an embodiment the shell layer comprises a first plastic material and the first end fitting comprises a third plastic material, and the method further comprises a step of consolidating the first and the third plastic materials with each other. This is a very practical way for tightly fastening the first end fitting to the shell structure without any additional process steps, such as drilling holes in the shell structure for fastening screws, and also reduces the risk for damaging the vessel.

In another embodiment the shell layer comprises a first plastic material and the gas and/or liquid tight layer comprises a second plastic material, and the method further comprises a step of consolidating the first and the second plastic materials with each other. This is a very practical way for firmly connecting the gas and/or liquid tight layer to the shell layer, which prevents it from loosening.

In another embodiment the first end fitting comprises a third plastic material and the gas and/or liquid tight layer comprises a second plastic material, and the method further comprises a step of consolidating the third and the second plastic materials with each other. In this way a hermetical connection between the gas and/or liquid tight layer and the first end fitting can be provided without using extra materials.

The invention also relates to a leak-tight vessel produced according to such a method. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention. The drawings are intended to describe the principles of the invention. Embodiments of the invention can use combinations of the different features and elements of different drawings.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein can operate in other orientations than described or illustrated herein.

The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting of only components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Fig 1A shows a partly reusable and removable mandrel with a first rounded outer surface suitable for filament winding, comprising a first and a second end fitting, which mandrel can be used for producing a leak-tight vessel according to the present invention.

Fig 1 B shows the mandrel of Fig 1A without the first and second end fitting. Fig 1 C shows the dome shaped first end fitting of Fig 1A in perspective view.

Fig 1 D shows another partly dome shaped end fitting that can be used in combination with the mandrel of Fig 1A, this end fitting has a flange for connecting external tubing.

Figures 2A-2D show a preferred embodiment of the elongated segments used in the mandrel of Fig 1A. The segment shown consists of three interconnected parts, the middle part being straight, the two end parts being curved.

Fig 2A shows the side of the segment intended to be oriented to the inside of the mandrel of Fig 1A.

Fig 2B shows the segment of Fig 2A in side-view.

Fig 2C shows the side of the segment intended to be oriented to the outside of the mandrel of Fig 1A.

Fig 2D shows a cross section of the segment of Fig 2A, according to line ll-ll.

Fig 3A shows an embodiment of the mandrel of Fig 1 A in a detailed perspective view, whereby only one segment is shown for clarity. The elongated segments are held in position by pulling two spindle parts away from each other.

Fig 3B shows the releasable connection of the segments of the mandrel of Fig 3A in more detail.

Fig 4A shows a fibrous vessel as can be obtained by filament winding around the mandrel of Fig 1A, after removal of the reusable and removable parts of the mandrel.

Fig 4B shows a first embodiment of a leak-tight vessel according to the present invention.

Fig 4C shows a second embodiment of a leak-tight vessel according to the present invention.

Fig 4D shows a third embodiment of a leak-tight vessel according to the present invention.

Fig 4E shows a first embodiment of an end fitting comprising a metal material partly surrounded by a plastic material. Fig 4F shows a second embodiment of an end fitting comprising a metal material partly surrounded by a plastic material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Various methods for producing leak-tight vessels are known in the art. In one of these methods continuous fibers impregnated with a thermoset resin are filament wound over a plastic inner bottle, also called "liner", that will remain in the leak-tight vessel after the production. Because during filament winding of continuous fibers a relatively large pressure is exerted upon the object being wound, the wall of the plastic bottle needs to be sufficiently thick (e.g. >1 cm for a vessel with a diameter of about 50 cm). In the end product such a bottle primarily acts as the gas and/or liquid barrier for the leak-tight vessel, while the fibers wound around the bottle primarily act as a protection layer. The bottle is usually made of a thermoplastic material, in order to avoid cracks e.g. due to the internal pressure or due to impact (e.g. when falling). However, a leak-tight vessel with such a bottle is heavy and expensive.

It is an object of the present invention to provide a method for producing a leak-tight vessel 14 that can resist a pressure of more than 20 bar, without requiring the bottle (liner). After years of experimenting the inventor has found such a method. Surprisingly enough this method is also very well suited for producing leak-tight vessels intended to be subjected to a low pressure, such as e.g. higher than 1.0 bar, as will be explained further.

The method for producing a leak-tight vessel 14 for holding a gas and/or liquid according to the present invention comprises the steps of:

- assembling a partly reusable and removable mandrel 1 suitable for filament winding, the mandrel 1 having a first rounded outer surface 87 around a symmetry- axis 10 and comprising: - reusable and removable parts 6 provided to be arranged in such a manner as to form the first rounded outer surface 87; - a first end fitting 8 having a second rounded outer surface 86 suitable for filament winding, and having an opening 4 large enough to allow the passage of the reusable and removable parts 6; - mounting the first end fitting 8 to the mandrel 1 ; - forming a shell layer 12 by filament winding a fibrous material over the first rounded outer surface 87 of the mandrel 1 and over at least part of the second rounded outer surface 86 of the first end fitting 8 while leaving the opening 4 for removing the reusable and removable parts of the mandrel 1 ; - applying a gas and/or liquid tight layer 49 to an inner surface 15 of the shell layer 12; - disassembling the mandrel 1 and removing the reusable and removable parts 6 through the opening 4, while leaving the first end fitting 8 in the shell layer 12.

As used herein, when referring to "the first and second end fitting 8, 28" or to "the end fittings 8, 28", it should be understood that the second end fitting 28 is optional for the present invention unless explicitly mentioned otherwise. Also, unless explicitly mentioned otherwise, everything that is said about the first end fitting 8 (e.g. material) is also true for the second end fitting 28, if the latter is present. The second end fitting 8 can have an identical shape and size as the first end fitting 8, but can also have a different size and shape. The second end fitting 28 can also have an opening 4 large enough for removal of the reusable parts of the mandrel, but it can also be closed. When the mandrel has two end fittings 8, 28, preferably both end fittings are left behind in the shell layer 12.

The inventor has surprisingly found that the shell layer 12 obtained by filament winding a fibrous material over the mandrel 1 and over at least part of the first end fitting 8 can be made gas and/or liquid tight up to elevated pressures (e.g. > 20 bar) in an inexpensive way, by applying one or more gas and/or liquid barrier layers 49 to the shell layer 12, whereby the material(s) of the gas and/or liquid tight layer(s) 49 can be selected in function of the envisioned application, by taking into account e.g. the chemical properties of the gas and/or liquid to be stored inside the vessel 14 and/or the physical conditions (pressure, temperature, etc) under which they are to be stored. Note that this additional gas and/or liquid tight layer 49 is not required in case the shell layer 12 itself provides a sufficiently low permeability for the liquid and/or gas to be stored inside the leak- tight vessel 14, in which case the gas and/or liquid tight layer is considered part of the shell layer 12.

The main focus of the present invention is the fact that the first end fitting 8 is applied to the mandrel 1 before the filament winding step, and the fact that the filament winding is applied not only to the first rounded outer surface 87 of the mandrel 1 so as to form the wall structure of the vessel 14, but also covers part of the second rounded outer surface 86 of the first end fitting 8. By doing so the wall is extended over at least part of the first end fitting 8, thereby incorporating it into the shell layer 12 at the time as forming the wall of the leak- tight vessel 14 by filament winding. Fig 4A shows the resulting shell structure 41 obtainable by performing these steps. With "shell structure" is meant the shell layer 12 plus the first and second end fittings 8, 28. Thus a separate step for adding the end fitting 8 after forming the wall (as is typically done in the prior art) can be omitted. By filament winding over the first end fitting 8, preferably over the entire outer peripheral edge 82 thereof, and over a predefined portion of the second outer surface area 86, a strong mechanical hold is provided to the first end fitting 8 to resist to an internal pressure inside the vessel 14. An especially strong hold can e.g. be provided by polar winding over the first and second end fitting 8, 28. By adding a gas and/or liquid tight layer 49 to this shell structure 41 , a leak-tight vessel 14 is obtained having similar or even better mechanical properties, such as e.g. higher strength than traditional leak-tight vessels, but is cheaper and more lightweight than known vessels, in particular with respect to vessels having an inner bottle (liner).

As can be seen from the figures 4B-4D, a leak-tight vessel 14 with a very thin wall (e.g. < 4.0 mm) can be produced, resulting in a very lightweight vessel 14. Apart from the top and bottom location, where the first and/or second end fittings 8, 28 are located, the thickness T of the wall can for example be only the thickness of the shell layer 12 comprising filament windings and a first plastic material plus the thickness of the gas and/or liquid tight layer 49, the latter being chosen only in function of the desired permeability characteristics, and not on its mechanical strength, as explained above. However, the thickness of the shell layer 12 can be increased by winding the filaments multiple times around the mandrel to further increase the strength of the vessel 14 if so desired. The thickness of the gas and/or liquid tight layer 49 can be increased by applying multiple barrier layers, if so desired.

In an example of a cold water tank for storage of water under atmospheric pressure, the shell layer 12 could be e.g. 2.0 - 3.0 mm thick and comprising continuous glass fibers (e.g. 60 weight %) impregnated with polypropylene (e.g. 40 weight %), and the gas and/or liquid tight layer 49 could be e.g. 1 mm thick and comprising polypropylene applied to an inner surface 15 of the shell layer 12.

In an example of a hot water boiler for storage of water under a pressure up to 5 bar, the shell layer 12 could be e.g. 2.0 - 3.0 mm thick and comprising continuous glass fibers (e.g. 60 weight %) impregnated with polypropylene (e.g. 40 weight %), and the gas and/or liquid tight layer 49 could be e.g. 2 mm thick and comprising polypropylene applied to an inner surface 15 of the shell layer 12.

Table 1 gives a comparison of a typical weight for leak-tight vessels 14 according to the present invention, as compared to the typical weight of prior art vessels having an internal bottle, or prior art vessels made of steel. The three vessels are chosen to be suitable to resist an internal pressure of 24 bar. For the vessel of the present invention, it is assumed that the first end fitting 8 is made of glass-fiber reinforced polypropylene, that the inner diameter of the vessel is substantially 45,7 cm (for the vessels of 100 and 150 litre) and is substantially 60,9 cm (for the vessel of 300 litre), and the shell layer 12 comprises continuous glass fibers and polypropylene, which fibers are wound circular and helical. For the prior- art fibrous vessel it is assumed that it comprises a liner of polybutene having a thickness of 4 mm, overwrapped by glass fibers impregnated with a polyester resin.

Table 1 .

Depending on the order in which the different steps of the first claim are performed, different embodiments of the method according to the present invention are obtained, thereby providing different embodiments of the leak-tight vessels 14, as shown in Figures 4B, 4C and 4D respectively.

In a first embodiment of the method according to the present invention, the gas and/or liquid tight layer 49 is applied to the first outer surface 87 of the mandrel 1 before mounting the first end fitting 8, and the first end fitting 8 is mounted to the gas and/or liquid tight layer 49 before the step of forming the shell layer 12. Fig 4B shows a leak-tight vessel 14 obtainable by this method.

In a second embodiment of the method according to the present invention, the first end fitting 8 is mounted to the mandrel 1 before applying the gas and/or liquid tight layer 49, and the gas and/or liquid tight layer 49 is applied to the mandrel 1 and to the first end fitting 8 before the step of forming the shell layer 12. Fig 4C shows a leak-tight vessel 14 obtainable by this method. In both the embodiments of Fig 4B and Fig 4C the gas and/or liquid tight layer 49 can e.g. be applied in the form of a thermoplastic film or by spraying or coating.

Another very interesting way of applying this gas and/or liquid tight layer 49 is by winding a barrier strip around the mandrel 1 in such a way that each strip fragment of the barrier strip shows a first local overlap over at least a lateral overlapping distance with a first substantially parallel strip fragment and shows a second local overlap with a second crossing strip fragment, as described in another application filed by the same applicant on the same day as this application. The inventor has surprisingly found that by winding a barrier strip of width W in an overlapping way as described above, a gas and/or liquid tight layer can be provided having similar barrier properties as an inner bottle ("liner") with a solid wall thickness of approximately W. By choosing proper materials for the barrier strip and by choosing the lateral overlapping distance large enough (e.g. 50% of the width of the strip), the permeability obtained can be determined mainly by the width W of the strip and not by its thickness. In this way a strip with a thickness of e.g. 800 μηη and a width of 4 cm using an overlap of 50% can achieve a similar barrier effect as an inner bottle of 4 cm thickness made of the same material as the first and/or second layer of the barrier strip.

In a third embodiment of the method according to the present invention, the first end fitting 8 is mounted to the mandrel 1 before the step of forming a shell layer 12, and the gas and/or liquid tight layer 49 is applied to an inner surface 15 of the shell layer 12 and to an inner surface 16 of the first end fitting 8 after disassembling the mandrel 1 and removing the reusable and removable parts of the mandrel 1 . Fig 4D shows a leak-tight vessel 14 obtainable by this method. In this embodiment the gas and/or liquid tight layer 49 is preferably applied in the form of rotation moulding, or blow molding, or by spraying or coating. When using rotation moulding (or rota moulding) the process of applying the coating can be fully automated, and the inner surfaces 15 of the shell layer 12, 16 and 36 of the first resp. second end fitting 8, 28 can be coated with a substantially constant predefined thickness, taking into account the desired permeability for the liquid and/or gas to be stored inside the vessel 14. Blow moulding is faster than rota moulding, thus lowering the production time. However, other techniques for applying the coating layer known by the person skilled in the art can also be used.

Although not the main object of the present invention, the method according to the present invention uses a partly removable mandrel 1 and a corresponding first end fitting 8. Several removable mandrels 1 could be used for this purpose, e.g. a mandrel made of plaster, but preferably a mandrel 1 with reusable parts is used. An example of such a mandrel is shown in Figures 1A-3B, but the method of the present invention would also work for other mandrels.

Fig 1A shows a preferred embodiment of a partly reusable and removable mandrel 1 as can be used in the method of the present invention. The mandrel 1 has reusable and removable parts, such as the segments 6, but also has parts that will remain in the vessel to be produced, notably the first end fitting 8 and the optional second end fitting 28. The first end fitting 8 needs to have an opening 4, preferably a central opening, large enough to allow passage of the reusable and removable parts of the mandrel 1 , such as e.g. the elongated segments 6 and the means for holding or fastening them (as will be described further). Because of the correspondence between the reusable mandrel parts 6 and the opening 4 of the first end fitting 8, the first end fitting 8 and optional second end fitting 28 can be seen as non-reusable parts of the mandrel 1 which parts are left behind in the shell layer 12 after the filament winding step, forming together with the shell layer a shell structure 41 , as shown in Fig 4A.

Fig 1 B shows the mandrel 1 of Fig 1A after the first and second end fitting 8, 28 are removed. The mandrel 1 comprises fourteen elongated segments 6 that are placed side by side to form a first rounded outer surface 87. The mandrel 1 has a rotation symmetrical shape with a varying outer diameter D around a symmetry axis 10, and is suitable for filament winding.

Fig 1 C shows an embodiment of the first and/or second end fitting 8, 28 that can be used in the mandrel of Fig 1A. According to the present invention, the first and/or second end fittings 8, 28 are applied to the mandrel 1 before the filament winding step, so that they are at least partly overwrapped by the filament windings. At least one of the end fittings 8, 28 needs to have an opening 4 large enough to allow passage of the removable parts 6 of the mandrel 1 . In case of the mandrel 1 shown in Fig 3A-3B, all parts except for the end fittings 8, 28 are reusable and removable, that is the segments 6, the segment holders 7, the first spindle part 42 and the second spindle part 43. As shown, the first end fitting 8 has a second rounded outer surface 86, e.g. spherical, or elliptical or dome shaped so as to avoid sharp edges which can cause mechanical stress in the shell layer 12, and should be avoided.

Fig 1 D shows another embodiment of an end fitting 8, 28 having a flange 83 with holes 19 for connection to the outside world, e.g. to connect external piping (not shown). The exact shape of the first end fitting 8 can however be further modified by the person skilled in the art. It can for example have a flange 83 with provisions for O-rings, or holes 19 with internal screw thread, or a V-clamp, or other traditional fastening means.

Figures 2A - 2D show an embodiment of the elongated segments 6 of the mandrel 1 shown in Fig 1A in more detail. The segments 6 preferably have a reinforcement rib 5 at the side which is foreseen to be oriented towards the interior of the mandrel 1.

As shown in Fig 2D, preferably the side wall 24 of the segments 6 is shaped in such a way that the plurality of segments 6 form a substantially closed first rounded outer surface 87 when they are mounted edge to edge in the mandrel 1 .

Preferably the segment 6 is provided as a single part, but it can also consist of multiple elements 2, 3, 32 that can be interconnected using conventional techniques, e.g. using a connection part 18 and screws 17 that fit in screw holes 23 of the elongated segments 6. In Fig 2A-2C the segment 6 consists of three parts: two curved or bended ends 2, 32 located substantially at the extremities of the elongated segments 6, and a straight intermediate part 3 located between the first and second ends 2, 32.

Vessels 14 of different size and shape can be produced by varying the shape and size of the elongated segments 6 and/or the dimensions of the end fittings 8, 28.

Referring to Fig 2C, the segments 6 of the mandrel 1 can all have the same size, or they can vary in size, e.g. they can have different widths W. The number of segments 6 required to form a substantially closed hollow mandrel 1 as shown in Fig 1A, depends on the size of the segments 6. The mandrel 1 of Fig 1A comprises fourteen segments 6, but another number of segments is also possible. In general, the more segments 6 are used for a mandrel 1 of a given size, the smaller the opening 4 in the first end fitting 8 can be.

Fig 3A shows a practical implementation of such a partly reusable and removable mandrel 1 in detail. It comprises a plurality of elongated segments 6 held in position by pulling two spindle parts 42, 43 away from each other, e.g. on a traditional filament winding machine, whereby segment holders 7 are mounted to the spindle parts 42, 43 for engaging with opposite ends 2, 32 of the segments 6. The parts 6, 7, 42, 43 of this mandrel 1 are reusable and removable. The mandrel 1 is shown together with a first and a second end fitting 8, 28, but as already mentioned before, the end fittings 8, 28 can also be mounted to the mandrel 1 after applying the gas and/or liquid layer 49 to the mandrel 1 first. This can easily be done e.g. by shifting the first resp. second end fitting 8, 28 towards the first resp. second end 2, 32 of the segments 6 over the spindle parts 42, 43 after applying the gas and/or liquid tight layer 49.

Preferably the elongated segments 6 of the mandrel 1 are made of metal, preferably a lightweight metal such as aluminum or an aluminum alloy, as this is easy to manipulate during assembly and disassembly of the mandrel 1 , but other metals can also be used, such as e.g. steel or stainless steel, but non-metallic materials can also be used.

In Fig 3A only one out of fourteen segments 6 and only two out of eight segment holders 7, four on each side, are shown for clarity reasons. The person skilled in the art can however choose another number of segments 6 or segment holders 7 using the same principle.

The invention would also work with another kind of removable mandrel 1 , e.g. a non-reusable mandrel made of plaster, but a re-usable mandrel is preferred, because it decreases production costs, produces less waste during the production, and reduces the risk of damaging the shell layer 12 and/or the gas and/or liquid tight layer 49 when disassembling the mandrel, e.g. by breaking the plaster and removing the pieces through the opening 4.

Fig 3B gives an enlarged view on the releasable connection of the first spindle part 42, the segment holders 7 and the elongated segment 6. As shown, the first spindle part 42 has a circumferential groove 44, and the segment holder 7 has a circular protrusion 46 that fits in the groove 44. The segment 6 has a curved or bended edge 47 that engages in a groove 45 of the segment holder 7. The first and second spindle parts 42, 43 are hollow tubes, so that the segment holders 7 can be manually placed on or removed from the first spindle part 42 e.g. by inserting a hand in the tube. After all segments 6 and segment holders 7 are placed on the first and second spindle parts 42, 43, the first and second end fittings 8, 28 each having an opening 4 (see Fig 1 C), can then be shifted over the first resp. second spindle part 42, 43 against the first resp. second ends 2, 32 of the segments 6. The holding and pulling of the first and second spindle parts 42, 43 in opposite directions can e.g. be implemented on a filament winding machine (not shown).

Disassembly of the mandrel 1 after the filament winding step can be done as follows: pushing the spindle parts 42, 43 slightly inside the vessel 14, removing the segment holders 7 from the spindle parts 42, 43 e.g. by inserting a hand inside the hollow spindle part, extracting the spindle parts 42, 43 out of the vessel 14, removing the segment holders 7 and the segments 6 out of the vessel 14 through the opening 4, while leaving the end fittings 8, 28 inside the vessel 14.

Fig 4A shows the vessel 14 of Fig 4D at an intermediate stage of production, after disassembly and removal of the mandrel 1 but before one or more gas and/or liquid tight layers 49 are applied for example to provide an extra oxygen barrier or smell barrier, or to protect the fibrous material against chemicals to be stored inside the vessel 14. The materials of the gas and/or liquid tight layer(s) and of the first end fitting 8 can be optimally chosen in function of the envisioned application.

Fig 4B shows a first preferred embodiment of a leak-tight vessel 14 according to the present invention, comprising a first end fitting 8 located between the gas and/or liquid tight layer 49 and the shell layer 12 comprising fibrous material. Preferably the material of the first and second end fittings 8, 28 is consolidated with the material of the gas and/or liquid tight layer 49 and with the material of the shell layer 12 so that all materials of the leak-tight vessel 14 are connected to each other. The gas and/or liquid tight layer 49 can e.g. be a sheet or foil of a thermoplastic material. In a variant of this embodiment an additional such gas and/or liquid tight foil or sheet can be applied after mounting the first and second end fittings 8, 28 but before the filament winding step, or as an intermediate layer in between two filament winding steps. To increase the impermeability (barrier effect) through the material of the first end fitting 8, several techniques are possible: such as e.g. using a first end fitting 8 made of a metal material, or using an end fitting 8 comprising a metal inner core as shown in Fig 8A, or using a first end fitting 8 made of any material having a sufficient thickness, or using a first end fitting 8 made of a plastic material coated with an aluminum layer, or any other way known by the person skilled in the art.

Fig 4C shows a second preferred embodiment of a leak- tight vessel 14 according to the present invention, whereby the first end fitting 8 is located on the inside of the gas and/or liquid tight layer 49. The gas and/or liquid tight layer 49 forms a first shell layer around the inner volume 73 of the vessel 14 and is preferably consolidated to the first and second end fittings 8, 28. A second shell layer 12 comprising a fibrous material is then wrapped around the gas and/or liquid tight layer 49, and is preferably consolidated thereto to improve the mechanical characteristics of the leak-tight vessel 14, such as rigidity, strength, impact resistance.

Fig 4D shows a third preferred embodiment of a leak-tight vessel 14 according to the present invention, whereby the gas and/or liquid tight layer 49 is applied to an inner surface 15 of the shell layer 12 and to an inner surface 16, 36 of the first resp. second end fitting 8, 28 after disassembly and removal of the mandrel 1. This gas and/or liquid tight layer 49 can e.g. by applied as a coating layer using techniques such as rota moulding or painting or spraying or blow-moulding, or a combination of these techniques, or by any other technique for applying coating layers known in the art. This coating can act as a gas and/or liquid barrier, or as a sealing layer, or as a protecting layer between the gas and/or liquid in the leak-tight vessel 14 and the fibrous material of the leak-tight vessel 14, or a combination thereof. Depending on the application, one or more gas and/or liquid tight layers 49 can be applied, optionally with different mechanical and/or chemical properties.

Preferably the method of the present invention further comprises a step of hermetically connecting said gas and/or liquid tight layer 49 to the first end fitting 8. This is not required in all embodiments, e.g. when the gas and/or liquid tight layer 49 is applied by blow moulding whereby the gas and/or liquid tight layer 49 extends through the opening 4 where it is hermetically sealed by an O-ring mounted in the flange 83 (not shown).

Preferably the shell layer 12 is applied in such a way that an outer peripheral 82 of the first end fitting 8 is completely covered by the filament windings, as this helps to provide a stronger connection between the wall structure and the first end fitting 8.

Preferably the first end fitting 8 is at least partly dome shaped. By mounting a first end fitting 8 which is at least partly dome shaped, stress concentrations in the first end fitting 8 and in the fibrous material overlapping the first end fitting 8 can be avoided, so that the risk of formation of cracks is minimized. This is especially important for gas-tight vessels in high pressure applications, such as e.g. more than 50 bar, but also for liquid-tight vessels subjected to vibrations such as e.g. in transportation applications.

Preferably the fibrous material is applied by filament winding continuous fibers impregnated with a first plastic material.

The fibers can be pre-impregnated, or can be impregnated during the filament winding step, or the first plastic material can even be applied after the filament winding step. The continuous fibers can be impregnated with a thermoset or a thermoplastic material. If the fibers are pre-impregnated with a thermoplastic resin such as polypropylene, polybutene or polybutylene, etc, they typically have to be heated up to a temperature between 120°C - 200°C during the filament winding step. Alternatively thermoset resins such as e.g. unsaturated polyester, vinyl ester, epoxy, phenol, polyurethane can be applied e.g. at room temperature, but other resins are also possible.

The continuous fibers can e.g. be applied using winding techniques known in the art, such as "circular" or hoop winding, "helical" winding, or "polar" winding, or combinations thereof. Polar winding is especially suited to counteract pressure exerted on the end fittings 8, 28, while circular winding is especially suited to counteract pressure in the radial direction of the leak-tight vessel 14. Depending on the application and the geometry of the leak-tight vessel 14 different winding strategies and different winding angles can be used. As a first example, a leak-tight vessel 14 could be produced by applying first a circular winding near the equatorial 72 of the mandrel 1 , followed by a helical winding to cover the entire mandrel 1 , followed by a polar winding as an extra reinforcement of the end fittings 8, 28. With equatorial 72 is meant the ring-shaped outer boundary of the cross-section of the rotation symmetric three dimensional mandrel 1 , perpendicular to its symmetry axis 10, at the mandrel's midpoint or point of greatest radius (as in the equator of the Earth). Either the same or different continuous fiber materials can be used in successive winding steps. As a second example, a leak- tight vessel 14 can be produced using only one helical winding step. The person skilled in the art can choose which winding strategy is the best depending on the application.

It should be noted that filament winding with a fibrous material comprising a thermoplastic material was not possible in the art when the filament winding was applied over a liner (bottle) made of a thermoplastic material, since such a liner would weaken and collapse during the filament winding step. Since the method according to the present invention uses a mandrel 1 , the shell layer 12 can now comprise a thermoplastic material, which is a great advantage over the prior art as it allows that a larger range of materials can be used. When using thermoplastic materials a leak-tight vessel 14 with a high impact resistance can be obtained, and such a vessel is better recyclable. When using thermoset plastic materials, the vessel 14 is better suited for high temperature applications, such as e.g. hot water boilers. The shape, size and material of the end fittings 8, 28 can be optimized depending on the application.

In an embodiment the first end fitting 8 comprises a metal material, such as e.g. steel, stainless steel, aluminum or an aluminum alloy, but other metals can be used as well. This is especially suited for applications in which a large pressure is exerted upon the first end fitting 8, such as e.g. a leak-tight vessel 14 intended to be subjected to a pressure larger than 50 bar.

In another embodiment the first end fitting 8 consists of metal. A metal end fitting can e.g. be used when the gas and/or liquid tight layer 49 is applied by blow molding.

In another embodiment the first end fitting 8 comprises a third plastic material. Preferably the third plastic material is consolidated with the first plastic material of the shell layer 12 and/or to the second plastic material of the gas and/or liquid tight layer 49. In an embodiment the first end fitting 8 consists of a third plastic material.

In another embodiment the first end fitting 8 comprises a metal material at least partly covered by the third plastic material, as shown in Fig 4E and 4F. In these examples, the first end fitting 8 has a metal inner core partly surrounded by the third plastic material 88. As shown in Fig 4E, the metal core can e.g. have a plurality of blind holes 89 with internal screw thread wherein the third plastic material is applied so that there is a good mechanical connection of the third plastic material and the metal core, together forming the first end fitting 8. These holes 89 can be applied on the convex and/or on the concave side of the metal core, or on both sides. Instead of blind holes, also grooves or other mechanical provisions can be used for the same purpose. In Fig 4F the metal inner core has a bowl shape comprising through holes 90 so that the plastic material 88 on the convex side is connected to the plastic material on the concave side of the metal inner core. In another embodiment (not shown), the metal inner core is completely surrounded by the third plastic material. In yet another example, the first end fitting 8 has a metal inner core whereto a circumferential strip of a third plastic material is applied, e.g. by glueing. An advantage of an end fitting 8 comprising metal is that it is easy to provide through mounting holes 19 (as shown in Fig 1 D) or holes 19, 89 with internal screw threat, which can be used for the connection of the plastic, but also for the connection of external pipes (not shown) during actual use of the leak- tight vessel 14.

In another embodiment the first end fitting 8 comprises a third plastic material and reinforcing fibers. Such a composite material is usually called "fiber reinforced plastic" material, and such third plastic material would be called "matrix" material. Such a first end fitting 8 can e.g. be produced by injection moulding of a fiber reinforced plastic material, e.g. polypropylene reinforced with chopped glass fibers. Such an end fitting 8 is considerably stronger than an end fitting 8 without fibers, and is suited for a wide range of applications where a plastic end fitting 8 is not strong enough but a metal core is not required.

Preferably the first and second and third plastic materials (resp. of the shell layer 12, the gas and/or liquid tight layer 49 and the first end fitting 8) are compatible thermoset plastic materials, or compatible thermoplastic materials. By choosing compatible plastic materials and by consolidating them, an excellent connection of the shell layer 12 and the first end fitting 8 and the gas and/or liquid tight layer 49 can be obtained. In this way a leak-tight vessel 14 with a unified structure is formed which is mechanically a single entity.

In an alternative embodiment instead of consolidating the materials, the materials can also be joined together by glueing, in which case the permeability through the material of the glue needs to be taken into account, as wall as possible leakage between the glue layer and the plastic materials.

A good cohesion between the shell layer 12 and the gas and/or liquid tight layer 49 is especially important when the leak-tight vessel 14 needs to be vacuum-proof, in order to prevent the gas and/or liquid tight layer 49 from coming loose from the wall. Such loosening e.g. caused by temporary underpressure is a problem in some prior art vessels, rendering them useless.

The thermoplastic material can e.g. be selected from the group consisting of polypropylene, polybutylene, polyethylene, but the invention is not limited thereto, and other thermoplastic materials can also be used.

The thermoset material can e.g. be selected from the group consisting of polyester, vinylester, epoxy, phenol, polyurethane, but the invention is not limited thereto, and other thermoset materials can also be used. The material of the continuous fibers is not essential for the invention. They can e.g. be selected from the group of fibers consisting of: glass fibers, carbon fibers, metal fibers, mineral fibers, wool, cotton, flax, polyester, polypropylene, polyethylene, polyamide, basalt, Kevlar®, aramide or a mix of two or more of these fibers, but the invention is not limited thereto, and other fibers can also be used. When using particularly strong fibers such as carbon fibers, a leak- tight vessel 14 can be provided that can possibly withstand a pressure of up to 500 bar.

Optionally the method further comprises a step of mounting a second end fitting 28 to the mandrel, opposite the first end fitting 8, and the fibrous material is applied in such a manner as to also overlap at least part of the second end fitting 28. When mounting a second end fitting 28, a vessel 14 with a symmetrical shape can be provided (see figures 4A - 4D), which is easier in production and in the actual application of the vessel. As mentioned before, the second end fitting 28 can be open or closed.

The present invention is very well suited for producing a leak-tight vessel 14 having an internal volume in the range of 5 - 1000 litre, preferably in the range of 10 - 500 litre, more preferably in the range of 20 - 250 litre, but the invention is not limited thereto.

In an embodiment the difference between the outer diameter 77 of the first end fitting 8 and the diameter d1 of the opening 4 of the first end fitting 8 is less than 10 cm, preferably less than 8 cm, more preferably less than 6 cm. By providing such a first end fitting 8 the weight of the leak-tight vessel 14 can be further reduced, if so desired.

EXAMPLES

Using the method of the present invention, the person skilled in the art can produce a wide variety of leak-tight vessels 14 with different characteristics optimized for specific applications. More specifically he can select the materials for the first, second and third plastic materials, and for the continuous fibers and for the first end fitting 8, he can select one of the three proposed embodiments Fig 4B-4D, the number of filament winding layers, the number of gas and/or liquid tight layers 49, and select a proper winding strategy as explained above.

As a first example of a method for producing a leak-tight vessel 14 according to the invention, a mandrel 1 as shown in Fig 1A is assembled, whereby the first end fitting 8 comprises polypropylene as a matrix material reinforced with 40 weight % chopped glass fibers, then optionally a mould release is applied to the mandrel 1 , then a thermoplastic film consisting of polypropylene is applied to the mandrel 1 before the filament winding step to form the gas and/or liquid tight (barrier) layer 49, then the mandrel 1 is filament wound using continuous glass fibers pre-impregnated with polypropylene as the first plastic material (e.g. 60 weight % glass fibers, 40 weight % polypropylene), then the matrix material of the first end fitting 8 (polypropylene) and the thermoplastic film material of the barrier layer 49 (polypropylene) and the first plastic material (polypropylene) of the shell layer 12 are consolidated at a temperature of approximately 160°C for approximately 30 minutes, then the whole is cooled off to room temperature, then the mandrel 1 is disassembled and removed by extracting the elongated segments 6 through the opening 4 of the first end fitting 8 while leaving the first end fitting 8 in the vessel 14, and the leak-tight vessel 14 is ready for use.

A second example is very similar to the first example, except that the first end fitting 8 comprises polypropylene without fiber reinforcement.

As a third example of a method for producing a leak-tight vessel 14 according to the present invention, a mandrel 1 as shown in Fig 1A is assembled, whereby the first end fitting 8 comprises polyester (thermoset) as a matrix material reinforced with 50 weight % chopped glass fibers, then optionally a mould release is applied to the mandrel 1 , then an epoxy resin (thermoset) layer of approximately 1 mm thickness is sprayed upon the mandrel 1 (or upon the mould release layer) to form a gas and/or liquid tight layer 49, then the mandrel 1 is filament wound at room temperature using continuous glass fibers pre-impregnated with epoxy resin (thermoset) as the first plastic material, then the matrix material (polyester) and the second plastic material of the gas and/or liquid tight layer 49 (epoxy resin) and the first plastic material of the shell layer 12 (epoxy resin) are consolidated at approximately 80°C for approximately 2 hours, then the whole is cooled off to room temperature, then the mandrel 1 is disassembled and removed as explained above, and the leak-tight vessel 14 is ready for use.

A fourth example is very similar to the first example, except that the first end fitting 8 comprises an aluminum core with blind holes with internal screw threat (see Fig 4E), which aluminum is coated with polypropylene (thermoplastic), which polypropylene is consolidated with the first plastic material (polypropylene) of the shell layer 12 and with the second plastic material (polypropylene) of the gas and/or liquid tight layer 49.

It is clear to the person skilled in the art that many more combinations and alternatives are possible, and that the materials and process can be optimized for specific applications.

By the above description and figures it can be understood that by using a partly reusable and removable mandrel 1 , the need for a heavy and expensive inner bottle ("liner") that would remain inside the leak-tight vessel 14 can be avoided. After consolidation of the first plastic material of the filament windings forming the shell layer 12 and the third plastic material of the first end fitting 8, a leak-tight vessel 14 with a very thin yet very strong wall is obtained, thereby saving material, cost and weight. The latter is especially important in transportation applications.

The invention can be used to produce a wide variety of leak-tight vessels 14 for different applications, such as e.g. containers for storing potable water, milk, soft drinks, beer, wine, or other liquids, hot water boilers, fuel tanks, gas tanks, hydrogen tanks, oxygen tanks, chemical tanks, etc. Dimensions can range from about 20 cm in height H and/or maximum diameter 78 for portable containers such as e.g. oxygen bottles, up to several meters, e.g. 2 m in height and/or diameter for large leak-tight vessels such as e.g. storage tanks, and all sizes in between. The height H can e.g. be 20 cm, 35 cm, 50 cm, 75 cm, 1 m, 1.25 m, 1 .50 m, 1.75 m, 2.0 m or higher. The maximum diameter 78 can e.g. be 20 cm, 35 cm, 50 cm, 75 cm, 1 m, 1 .25 m, 1 .50 m, 1.75 m, 2.0 m or higher. The height H can be the same as the diameter 78, or the height H can be larger than the diameter, or vice versa.

The described method for producing a leak-tight vessel 14 basically only requires a filament winding machine and means for applying a gas and/or liquid tight layer 49. A lot of factory space can be saved with respect to traditional approaches where additional processing steps and machinery are required. This is advantageous for the price of the leak-tight vessel 14 and for the environment. Another advantage of the method according to the invention is that it causes practically no material waste during the production. When for all plastic materials, including the gas and/or liquid tight layer 49 a thermoplastic material is used, a 100% recyclable leak-tight vessel 14 is obtained. When carbon fibers are used, leak-tight vessels for extremely high pressure can be produced. Except for the end fittings 8, 28, which remain behind in the leak-tight vessel 14 during production, the segments 6 and the segment holders 7 of the mandrel 1 can be reused for producing other leak-tight vessels 14. The leak-tight vessel 14 can be produced in a fast and easy and highly economical way, thanks to the fast and easy assembling and disassembling method of the mandrel 1. Either thermoplastic or thermoset materials can be used as the plastic materials, each having its advantages, depending on the application.

Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader scope of the invention as set forth in the claims. Accordingly, the description and drawings are to be regarded in an illustrative sense rather than a restrictive sense.