TECHNICAL FIELD

The invention relates to a method for producing a leak-tight vessel having a predetermined permeability for holding a gas and/or liquid, in particular a fibrous reinforced leak-tight vessel, and to a leak-tight vessel produced in this way.

BACKGROUND ART

Leak tight vessels comprising a fiber reinforced material as their wall structure 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 or both, 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).

With “gas and/or liquid tight” is meant that it can be gas tight, or liquid tight, or both, depending on the intended application.

A known method for making leak-tight vessels, in particular pressure vessels, uses filament winding of continuous fibers impregnated with a thermoset resin over an inner bottle (also called “liner”) that will remain in the vessel after the filament winding step. The inner bottle is sufficiently rigid to be tightly overwrapped with continuous fibers, and is quite thick (e.g. 1-4 cm) to act as the gas and/or liquid barrier. A disadvantage of such a method is that the bottle (liner) is heavy and expensive.

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 large pressure is exerted upon the object being wound, the plastic bottle needs to be sufficiently thick (e.g. >1 cm thick for a diameter of about 50 cm). At the same time, such a bottle also acts as the gas and/or liquid barrier for the leak-tight vessel, while the fibers wound around the bottle act as a protection layer. When producing pressure vessels, the bottle is usually made of a thermoplastic material, in order to avoid cracks due to the internal pressure. While such a bottle can provide a high barrier for the gas and/or liquid, it is heavy and expensive.

U.S. Pat. No. 4,760,949 describes a composite container for storage of products at non-atmospheric conditions. The composite container has a high barrier liner layer including a metal layer of vacuum deposited aluminum parallel with and spaced from the longitudinal edge of a synthetic plastic base thereby to define a first web that is helically wound around a cylinder in edge overlapping relation such that one longitudinal edge of the metal strip overlaps the other longitudinal edge of the metal strip by a given constant distance (d). The overlapping edges of the first web are hermetically joined by a heat-sealable bond between an adhesive layer covering the metal strip and the adjacent face of the first web, and a compatible heat sealable layer on the opposite face of the web. The method applies filament winding around a cylinder. After the fibrous cylindrical wall is removed from the mandrel, metal end parts are added to form a leak-tight vessel, and an end sealing compound is provided between the composite wall and the metal end part so as to obtain a hermetical connection.

A disadvantage of this method is that such a leak-tight vessel is not suitable to withstand high pressure (e.g. 2 bar or more). U.S. Pat. No. 3,367,815 entitled method and apparatus for forming filament wound vessels, patented Feb. 6, 1968, describes a method for forming a filament wound vessel comprising following steps:

Forming a first wound shell of resin impregnated filaments about a collapsible mandrel having a detachable fitting;

Curing the first shell to bond the fitting to the shell;

Removing the mandrel;

Mounting the fitting and shell on a shaft;

Forming a second wound shell of resin impregnated filaments amount the first shell while using the latter as a mandrel;

Curing the vessel. (abstract)

The method described above is meant to solve the problems related to the use of an inflatable mandrel (column 1, lines 50-55), or a meltable or soluble mandrel (column 2, line 2).

Another prior art solution was the use of a ‘lost’ mandrel, left behind in the finished vessel (column 2, line 25).

A specific characteristic of this disclosure is that the partially wound shell, resulting from the above two first process steps, serves as a forming mandrel during the subsequent process steps. This procedure is said to permit removal of the relatively bulky mandrel components from the interior of the shell prior to the time that the latter is completely formed (column 2, lines 60-66).

Differently phrased, and as set out in column 3, lines 5-7, the mandrel components are removed from the interior of the vessel during the manufacture process thereof. This essential feature of the invention so disclosed is repeated in other terms again in column 3, lines 25-30 and column 4, lines 66-70, column 8 line 6 and again on line 19, column 10, lines 44-46, column 12, lines 40-41.

At least two disadvantages are imminently related to the process as disclosed in this prior art document.

The first is that although use is made of a reusable mandrel, such mandrel is not used during the entire manufacturing process. It is quite clear that from a mechanical point of view a reusable mandrel is a complicated and hence expensive piece of equipment. Also given this mechanical complication, its assembly and disassembly requires costly production time, and therefore using such piece of equipment during only a part of the manufacturing process of a fibrous vessel is in itself an inefficient operation.

Secondly, as the first wound shell resulting from the first two steps of this manufacturing process is used as a mandrel during the next manufacturing steps, such wound shell must exhibit a sufficient rigidity to effectively serve as mandrel during the subsequent winding process. Hence the thickness of the shell layer wound in the first two process steps must be thick enough to yield such rigidity. As a result the fibrous vessel produced according to this method is rather heavy weight, and the benefits of the expensive reusable mandrel used in this process are only partly used, thereby rendering this manufacturing method and the resulting fibrous vessels rather expensive.

GB Patent 1 255 738 entitled ‘A flexible and collapsible container and method of making the same’, complete specification published Dec. 1, 1971, describes a method for forming a yarn wound container comprising the following steps:

Preparing a metal former including separate component elements and annular clamps;

Applying a first coating of a thermoplastic synthetic resin;

Heat treating the coated former;

Winding on to the coated former a plurality of layers of flexible yarn;

Applying a second coating of thermoplastic resin to form an outer layer;

Heating the outer and inner coating layers so as to integrate the said coatings;

Removing the former by dividing it into its individual elements.

(Page 1, line 31 until line 58)

Page 2, lines 113-117 teach heating the metal former to a selected temperature whereupon a thermoplastic synthetic resin is applied to its surface to form a coating.

The wall of the container so formed is constituted by windings of flexible yarn and an inner and outer layer of a pliable thermoplastic synthetic resin. Therefore the walls of the container itself are flexible and the container is collapsible. (page 2, lines 53-60).

A container manufactured according to this manufacturing process however clearly is not gas and/or liquid tight, as nowhere during the manufacturing process any measure or precaution in this respect has been made, nor have the materials that are used in this manufacturing process been selected with this aim in mind.

Also this manufacturing process implies, apart from the assembly and final disassembly of the former or mandrel, at least five subsequent steps, two coating application steps, one winding step and two heating steps. The overall manufacturing process hence is quite lengthy, and the resulting manufacturing cost of the containers consequently is high.

U.S. Pat. No. 3,334,780 entitled Pressure fluid container, patented Aug. 8, 1967, describes improved pressure fluid container constructions which preclude build-up of excessive internal pressures.

More specifically, the object of this disclosure is the provision of a pressure fluid container which, although fluid-impervious at normal temperature conditions, becomes fluid-pervious upon subjection to ambient temperatures above a known critical level. Thereby controlled escape of the fluid contents of the container and thus control of and limiting of pressure rise within the container is permitted (column 2, lines 6-11).

To this end, filaments are wound about a mandrel, such filaments being either moistened or impregnated with a suitable binder such as epoxy resin before, during or after the winding operation. After such a shell of predetermined thickness is formed, the resin is cured (column 4, lines 9-16). As a final step, the mandrel is removed through a large polar opening (column 4, line 20).

In carrying out the above, provision is made for closing the polar opening by means of a porous or fluid pervious material (column 4, lines 26-28). Also, the outer shell which envelops the inner shell, is porous, at least in the region surrounding the polar opening (column 4, lines 38-40).

The solution proposed in this disclosure however is quite difficult to realize in practical manufacturing conditions.

U.S. Pat. No. 3,508,677 entitled vessel for storing high-pressure gases, patented Apr. 28, 1970, discloses a vessel for storing gases under extreme pressure conditions as required e.g. in military aircraft.

Such vessel comprises three different layers: an inner layer comprising a thermoplastic resin liner; an intermediate diaphragm bonded to the inner liner; an exterior housing composed of various layers of resin impregnated fiber glass strands.

The manufacturing method for this type of vessel is based on the use of a water soluble mandrel (column 2, line 71). Given the specific application field for the vessels manufactured according to the method of this patent (military aircraft, aircraft fuselage), manufacturing cost is not the top priority. Key here is the ability for the vessel to store gases under extremely high pressures ((Column 1, line 26). Hence a one-time soluble mandrel can be used, but it is quite clear that the use of such one time-use mandrels for most commercial applications is not affordable. Also the inner liner consists of pre-formed laminated resin sheet sections that are joined to form an impermeable inner liner, again adding to the overall cost of the vessels manufactured according to this disclosure (FIG. 3).

Finally the inner layer of this type of vessels remains flexible so that it is capable of expanding and contracting when under pressure arises and/or pressure is released during use. Such inner flexibility however often gives rise to damage to the inner layer, whereby the vessel loses its leak-tight characteristic. For one-time use vessels as disclosed in the application field of this invention, this is not a concern, but for most commercial applications, the vessel should retain its gas and/or liquid tight property during extended periods of use.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a method for producing a leak-tight vessel and a leak tight vessel produced according to such method, thereby avoiding the disadvantages and problems associated with the prior art vessels and manufacturing methods described above.

More particularly, it is an object of the present invention to provide leak-tight vessels and their manufacturing methods, able to resist a pressure higher than 2 bar, and that has a similar barrier effect as the bottle, but without using a liner bottle.

After years of experimenting the inventor has found such a method.

This is achieved according to the present invention with the method of the first claim.

As will become clear further, the disclosed method and the disclosed leak-tight vessel thus obtained are also very well suited for producing leak-tight vessels or containers used for storage of liquids under atmospheric conditions.

Thereto, the method of the present invention comprises producing a leak-tight vessel (14) for holding a gas and/or liquid, comprising the steps of:

(a) assembling a reusable and removable mandrel (1) having a rounded outer surface suitable for filament winding;

(b) applying an end fitting (8) to the mandrel (1), said end fitting having an opening (74) large enough for removing the mandrel (1) there through after being disassembled;

(c) forming a barrier layer by winding a barrier strip (60) around the mandrel (1) and the end fitting (8), thereby completely covering a predefined area of the rounded outer surface of the mandrel and the end fitting, while leaving the opening (4) large enough for removing the mandrel (1) after disassembly, whereby the winding of the barrier strip (60) is applied such that the successive barrier strip windings (61) overlap over at least a lateral overlapping distance (66) with a previously wound parallel barrier strip winding (62) and a next wound parallel barrier strip winding (63);

(d) forming a shell layer (41) by winding a fibrous material (12) over the barrier layer while leaving the opening (4) large enough for removing the mandrel (1) after disassembly, thereby exerting pressure upon the wound barrier layer so that the successive windings (61) of the barrier strip (60) are pressed tightly against the mandrel (1) and against each other so that the materials of such successive windings (61,62,63) can be consolidated at their contacting surfaces;

(e) consolidating the successive windings of the barrier strip (61) thereby forming a gas and/or liquid tight layer (49), this step (e) being performed either during and/or following step (c), and/or this step (e) being performed following step (d);

(f) consolidating the gas and/or liquid tight layer formed in step (e) with the end fitting, thereby forming a hermetical gas and/or liquid tight connection with such end fitting, this step (f) being performed either concomitantly with and/or following step (e);

(g) consolidating the wound shell layer (41) formed in step (d) with the gas and/or liquid tight barrier layer formed in step (e) so as to form a consolidated wall structure, this step (g) being performed either concomitantly with and/or following either steps (e) or (f);

(h) disassembling and removing the mandrel (1) through the opening (4).

Further, according to the invention, leak-tight vessels with the following characteristics are produced.

A leak-tight vessel (14) for holding a gas and/or a liquid, comprising an inner barrier layer comprising a heat-sealable thermoplastic material, an outer shell layer comprising a fibre-reinforced heat-sealable thermoplastic material, an end fitting positioned at the inner side of the inner barrier layer, covered at least partly at its outer concave surface with a heat-sealable thermoplastic material, wherein the inner barrier layer, the outer shell layer and the end fitting form a unitary rigid structure able to withstand an overpressure of at least 2 bar.

Preferably, the heat-sealable thermoplastic materials selected for the inner barrier layer, the outer shell layer, and the covering of the concave outer surface of the end fitting, should be selected to as to be compatible during the consolidation steps as described hereafter. More preferably their softening and/or melting points should be compatible, i.e. situated in the same or neighbouring temperature ranges to enable a smooth consolidation.

As used herein, with “consolidation of two or more materials” is meant unification or leak-tight connection, e.g. in the context of two thermoplastic materials consolidation can mean uniting under high temperature by local melting or softening; e.g. in the context of consolidating a plastic material with a metal material, consolidation can mean melting against the metal surface, or gluing to the metal surface.

By providing a removable mandrel it is possible to apply filament winding without the need for a bottle (or “liner”) with a strong (thus thick and heavy and expensive) wall to enable filament winding thereto. By winding a barrier strip around the mandrel, the thickness of the gas and/or liquid tight layer can be chosen independently of the size of the vessel, in contrast to the traditional approach, where the thickness of the bottle wall needs to increase for larger vessels in order to maintain sufficient stability for the winding process.

Further, thanks to the use of the mandrel, the filament wound material can be a thermoplastic material, which was not possible when using a thermoplastic bottle (liner), because it would weaken. This allows more materials to be used for the leak-tight vessel.

Due to the fact that the mandrel can be disassembled, the parts of the mandrel can be removed through the opening (74) after disassembly, irrespective of the shape the mandrel had during the winding step. This allows a mandrel with a shape different from a cylindrical shape while still being able to separate the vessel from the mandrel.

By providing a mandrel with a varying outer diameter the top and bottom parts can have a diameter smaller than the maximum outer diameter of the vessel, and can be overwrapped by the fibrous material thereby so as to be able to resist high pressure (e.g. more than 2 bar, or even 10 bar, or even 25 bar, or even 50 bar).

The inventor has surprisingly found that by winding a three-layer 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 μm 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!

The inventor has surprisingly found that a high barrier effect can be achieved even when the strip fragments are inter-woven in a way that substantially parallel strip fragments are separated by crossing strip fragments, sometimes only several windings later. Experiments have shown that the high barrier effect is still achieved by providing the above mentioned minimum overlapping distance, even though the overlapping distance is not constant on a non-cylindrical mandrel, provided that the strip fragments are consolidated to each other, which the inventor has observed can be achieved by exerting pressure upon the barrier strip fragments by filament winding the fibrous material at a larger tension than usual (in case the filament winding would be applied directly around a mandrel), so that the strip fragments are tightly pressed against the mandrel and to each other. This is important when using a non-cylindrical mandrel because the strip fragments do not in themselves show perfect contact with the rounded outer surface area, especially at their longitudinal edges.

As an additional advantage, the method of winding the barrier strip as described above can be fully automated, using the same equipment as used for the filament winding of fibrous material, thereby avoiding extra investment costs and factory space.

Another advantage of winding a barrier strip instead of using a bottle (liner) is that a barrier strip of a given width W can be used for vessels of different sizes, which is not the case when using “bottles”, which have a fixed size. This offers a great advantage in logistics, stock and flexibility in production.

In addition, vessels with a higher barrier (also called impermeability) can be produced in a very fast and economical way by simply repeating the winding process so as to cover the predefined area multiple times, without noticeably increasing the weight of the vessel. In this way the useful lifetime of the product (e.g. a hot water boiler before leakage takes place) can be largely increased at only a minor additional cost.

Another advantage of the method according to the present invention is that the entire process for producing a leak-tight vessel including the consolidation can be performed in about 1-3 hours, as compared to 1-3 days required for a blow molded bottle (liner) of 4 mm thickness to completely shrink before it can be filament wound

In a preferred embodiment the end fitting is applied in the form of a dome shaped end fitting having an outer peripheral (82) larger than the first opening and having an opening large enough for removing the mandrel there through after being disassembled. In a further preferred embodiment, the opening is positioned in alignment with the opening, and whereby the fibrous material is applied in such a way as to overlap at least the outer peripheral of the end fitting.

By winding the fibrous material in such a way as to overlap at least the outer peripheral of the end fitting, an excellent mechanical hold is provided of the end fitting against internal pressure from inside the leak-tight vessel. In this way a pressure vessel can be produced that can withstand an elevated internal pressure (e.g. 2, 10, 25 or even 50 bar).

By providing a large overlapping area, preferably the entire end fitting except for the opening, the force exerted upon the end fitting to counteract the internal pressure can be distributed over a larger area, thereby reducing the stress exerted upon the end fitting.

By applying the end fitting before the filament winding of the fibrous material, the end fitting is integrated into the wall structure during the construction of the wall, and an extra processing step for adding a top and/or bottom part afterwards can be omitted, thus reducing the risk of leakage, but also saving considerable time, production space and energy.

Preferably the barrier strip is applied as a single continuous strip, as this saves time in production, and avoids leakage at the location where the inner barrier layer would otherwise be interrupted.

Preferably the barrier strip has a predefined width W, and is applied in such a way that the lateral overlapping distance measured at the equatorial of the mandrel is 10%-90% of the width of the barrier strip, preferably 20%-80%, more preferably 30%-70%, even more preferably 40%-60%, even more preferably 45%-55%, most preferably about 50%. The inventor has found that the value of 50% is an optimal overlapping distance in terms of barrier effect versus the amount of strip-material required to achieve that effect.

With equatorial is meant the ring-shaped outer boundary of the cross-section of the rotation symmetric three dimensional mandrel, perpendicular to its symmetry axis, at the mandrel's midpoint or point of greatest radius (as in the equator of the Earth).

The end fitting is applied to the mandrel before winding the barrier strip, and the barrier strip is applied in such a way as to overlap the entire outer peripheral of the end fitting, and the method further comprises a step of consolidating the plastic material of the barrier strips with the material of the end fitting so as to form the hermetical connection.

In this way the end fitting is mounted on the inside of the gas and/or liquid tight layer, and is hermetically joined thereto by consolidation.

Alternatively the end fitting is applied on top of the wound barrier strip but before the step of winding the fibrous material. So, according to this alternative embodiment of the invention, the barrier strip material is first wound on the mandrel, followed by the step of applying the end fitting on this wound barrier layer, followed by the step of winding the fibrous material hereupon.

In this alternative way the end fitting is mounted partially between the fibrous layer and the gas and/or liquid tight layer, and is hermetically joined to both by consolidation.

Preferably the fibrous material is applied by filament winding of continuous thermoplastic fibres. These fibres may be thermoplastic fibres reinforced with carbon, glass or stretched thermoplastic fibres. By winding continuous fibres, the obtained endless filament structure will allow the vessel to withstand higher hydrostatic pressures. In this way a leak-tight vessel can be produced able to withstand very high internal pressure, e.g. up to 100 bar or even 200 bar or more.

The method further comprises a step of consolidating the plastic materials of the barrier layer and the shell layer, so as to create a consolidated wall structure. By consolidating the plastic material of the gas and/or liquid tight layer to the plastic material of the fibrous wall, good fastening of the gas and/or liquid tight layer to the fibrous outer wall is obtained, which prevents it from coming loose e.g. in case of under-pressure or even vacuum inside the vessel.

Preferably the method further comprises a step of consolidating the plastic material of the shell layer and the material of the end fitting, so as to obtain a consolidated leak-tight vessel. By choosing compatible materials, such as thermoplastic sealable materials, the gas and/or liquid tight layer, the end fitting and the fibrous wall can all be consolidated together, resulting in a consolidated leak-tight vessel with excellent mechanical properties.

It is also an object of the present invention to provide such a leak tight vessel.

Preferably the varying outer diameter D has a maximum outer diameter Dmax, and the width W of the barrier strip is 4%-20% of the maximum outer diameter, preferably 6%-15%, more preferably 8%-12%, most preferably about 10%. By choosing a strip with such a width a good compromise is reached between low production time and quality of the vessel. A wider strip would require less time to wind, but would require more strip-deformation resulting in an increased risk of air inclusion, resulting in a lower barrier effect, and an increased risk of leakage.

According to a preferred embodiment, the barrier layer structurally comprises three distinct layers. Such a multilayer strip then comprises a first and second layer, and an inner layer of the barrier strip. The material chosen for the first and second layers on the one hand, and the material chosen for the inner layer must not necessarily be one and the same.

In one very interesting combination using only plastic materials the barrier strip is a multi-layer strip and the first resp. second plastic material is a first resp. second thermoplastic material with a first resp. second melting temperature, and the third material is a third thermoplastic material having a melting temperature higher than the first melting temperature and higher than the second melting temperature. A vessel having only thermoplastic materials has a higher impact resistance, and is better recyclable. The thermoplastic material chosen for the first and second layer may be identical, having a melting point lower than the melting point of the thermoplastic material chosen for the inner layer.

