Patent Description:
As part of a future energy mix that draws substantially from renewable energy sources (solar, wind, hydropower), gases, such as hydrogen, offer a solution for improved, more flexible power grids. In addition, manufacturers increasingly rely on systems using CNG and hydrogen as an alternative propulsion technology for heavy and light vehicles.

Devices for storing such gases are known in general from the art. Pressure vessels are typically used for storing a variety of liquids under pressure, such as for example for storing oxygen, natural gas, nitrogen, hydrogen, propane and other fuels.

Typically, such pressure vessels are divided into different types. A so-called type II pressure vessel is a pressure vessel provided with an inner container, the so-called liner, made of high-grade steel or stainless steel, and an outer shell surrounding this inner container, for example made of fiber reinforced material. A type III pressure vessel has an aluminum liner and a type IV pressure vessel typically has a liner of a plastic material, such as high density polyethylene (HDPE), surrounded by at least one outer casing of fiber reinforced plastic material. Such pressure vessels serve as devices for storing gases - or liquids - under high pressures. In general, they serve to store the largest possible amount of gas in a relatively manageable volume of the device at pressures in the order of about <NUM> bar, in the order of about <NUM> bar or in the order of <NUM> to <NUM> bar, particularly for light gases such as hydrogen.

Type IV pressure vessels or tanks have a metal-free body that typically comprises a carbon fiber reinforced polymer composite, wrapped and/or braided over a thermoplastic polymeric liner. Suitable pressure vessel materials comprise laminated layers of wrapped glass fiber filaments or other synthetic filaments which are bonded together by a thermal curing or thermoplastic resin. An elastomeric or other non-metal resilient liner is placed inside the composite casing to seal the vessel and to prevent liquids within from contacting the composite material. The composite construction of the vessels offers numerous advantages, such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These properties are due to the high specific strength of the reinforcing fibers or filaments, which are typically oriented in the direction of the major forces in the construction of the pressure vessels.

Each vessel has a valve coupled thereto for filling the vessel with compressed liquid. However, the valve cannot be directly connected to the polymer liner. For this reason, a boss needs to be provided for coupling the valve to the vessel.

<CIT> discloses pressure container wherein an electric insulating layer is formed on the outer surface of a base of an inner liner, and an outer liner is formed while winding a reinforcing yarn made of carbon fibers around the layer. The electric insulating layer is formed by coating resin having electric insulating properties on the peripheral surface of a cylindrical shaft.

<CIT> discloses a plastic pressure vessel that has a metal base inserted and fixed to the end wall of a vessel body molded by rotation molding. The base has a cylindrical shaft allowing the vessel body to communicate with the inside and outside, and a flange protruded from one end of the cylindrical shaft, and they are buried in the end wall.

<CIT> discloses a high pressure gas container or high pressure gas pipeline that contains copolymerization units of vinylidene fluoride and tetrafluoroethylene, and includes a layer comprising a copolymer having a ratio of vinylidene fluoride units to a tetrafluoroethylene units of <NUM>/<NUM>-<NUM>/<NUM> in terms of mole ratio.

<NPL>, discloses a construction of a hydrogen tank.

<CIT> discloses a method for manufacturing a polymer bladder assuring the internal sealing of a tank vis-a-vis a pressurized fluid which is contained therein, wherein said polymer is a thermosetting polymer, and said method comprises at least one step of polymerizing at least two precursor compounds of said thermosetting polymer carried out in a mould in rotation. The document further discloses a tank for storing a pressurized fluid for example a type IV tank comprising said polymer bladder.

<CIT> discloses a pressure-resisting container, wherein a base head part on one end side of a base part is supported by a shoulder part and a cylindrical head part of an inside shell, a disc type burying part on the outside of the one end side is buried between the shoulder parts of the inside shell, and a cylindrical base part head part is held from the outside on a polar part opening part of a shoulder part of an outside shell.

An aim of the invention may be to provide a stable and integrated interface between boss and liner, for example to prevent blow outs and/or accumulations of gas between the boss and liner.

A further aim of the invention may be to provide a boss-liner structure showing improved resistance against expansion or contraction.

Another aim of the invention may be to provide a method for forming such a boss-liner structure.

The aim of providing a method for forming a boss-liner structure is achieved according to the invention by a method showing the technical characteristics of the first independent claim.

In a first aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the invention comprises a method for the production, by means of rotational molding, of a boss-liner structure for a type IV pressure vessel from a material containing a curable raw material.

The method comprises providing a mold, for forming a liner comprising a hollow body defined by an elongated cylindrical liner wall having opposed inner and outer surfaces extending between a first end and a second end of the liner,.

The method further comprises providing a sealing element in the boss bore through the boss; filling a mold cavity in the mold with the material; heating the material to a first predetermined temperature; rotating the mold so that the liner is formed from the material, wherein the liner is formed at the inner and outer sides of the flange and in the bores or recesses; and cooling the formed liner to a second predetermined temperature.

Due to the presence of the boss, particularly the flange of the boss, in the interior of the mold during the production of the liner by rotational molding, the boss is easily integrated and centered in the liner. Moreover, a well-sealed structure is formed due to the liner being formed at the inner and outer sides of the flange. Furthermore, the boss is secured against axial rotation relative to the liner by the anchors, i.e., parts of the formed liner extending in the bores or recesses of the flange.

Thus, at least one of the following traditional processing steps may be omitted: drilling out, deburring and removing all dirt from the zone of the liner where the boss is to be provided, applying a seal, mounting the boss and then cleaning up the structure again.

In a first embodiment according to the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the sealing element extends only partially into the boss bore so that the liner formed while rotating the mold extends partially into the boss bore. In a further embodiment according to the invention, the sealing element may be provided with a substantially semicircular end part facing the interior of the mold and arranged in such a way that, in an axial direction of the boss bore through the boss, the axial position of the inner side of the neck of the boss corresponds to the axial position of a point of the semicircular end part.

By partially covering the inner side of the wall surrounding the boss bore, or by integrating it into the liner, a good sealing can be provided up to a connecting element, for example a valve, that is connected to the boss, for example by means of a thread. As a result, the structure or the pressure vessel will suffer less from expansion or contraction.

In a second embodiment according to the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the mold is defined by an elongated cylindrical mold wall having an inner surface that extends between a first end and a second end of the mold, wherein the mold wall comprises a cylindrical main portion, a rounded head portion and an opening near the first end of the mold, wherein the boss is arranged so that the boss bore extends through the opening in the mold and is in fluid communication with the interior of the mold, and so that the outer flange surface faces the inner surface of the mold. In a further embodiment according to the invention, the mold wall may comprise a second opening near the second end of the mold.

In a third embodiment according to the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the mold is provided with a second opening in a second front end and the method further comprises providing a second boss having a boss bore extending axially through the second boss, wherein the boss bore is determined by a cylindrical wall having an inner surface, and a second flange projecting radially outward having inner and outer flange surfaces located opposite to each other, wherein the second boss is arranged so that the boss bore extends into an interior of the mold, and so that the outer flange surface faces the inner surface of the mold, and providing a second sealing element in the boss bore through the second boss. In a further embodiment according to the invention, the boss and the second boss may be facing away from each other.

In a second aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the invention comprises a boss-liner structure for a type IV pressure according to independent claim <NUM>. The structure comprises a polymeric, preferably thermoplastic, liner, comprising a hollow body defined by an elongated cylindrical liner wall having opposed inner and outer surfaces extending between a first end and a second end of the liner. The cylindrical liner wall comprises a cylindrical main portion, a rounded head portion and a cylindrical neck, wherein the cylindrical liner wall has a passage opening near the first end of the liner, wherein the passage opening is determined by an edge of the liner. The structure further comprises a boss having a boss bore extending axially through the boss, wherein the boss bore is determined by a cylindrical wall having an inner surface, wherein the boss bore is in fluid communication with an interior of the pressure vessel.

The boss comprises a flange having inner and outer flange surfaces located opposite to each other, wherein the inner flange surface faces the inner surface of the liner and the outer flange surface faces the outer surface of the liner, wherein the flange is integrated in the cylindrical neck of the cylindrical wall.

In an embodiment according to the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the flange is integrated in a monolithic layer of the cylindrical neck.

In a third aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, the invention comprises a pressure vessel configured for storing a pressurized fluid, comprising a boss-liner structure manufactured as described above or a boss-liner structure as described above, and an outer composite shell, wherein the outer composite shell surrounds an outer perimeter of the liner. In an embodiment according to the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, said outer composite shell is formed by braiding a number of strands of fiber around at least part of an outer perimeter of said liner.

The invention will hereafter be further elucidated by reference to an exemplary embodiment shown in the drawings.

The present invention will hereafter be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto and is defined only by the claims. The drawings described are merely schematic depictions and are non-limiting. In the drawings, the dimensions of certain parts may be exaggerated, and not drawn to scale, for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to the actual reductions to practice of the invention.

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

Moreover, the terms "top", "bottom", "over", "under" and the like are used in the description and the claims for descriptive purposes and not necessarily for indicating relative positions. The terms so used are interchangeable under the appropriate circumstances, and the embodiments of the invention may be carried out in other orientations than those described or illustrated herein.

Furthermore, the different embodiments, although referred to as "preferred forms", should rather be understood by way of example, as ways in which the invention may be carried out, and not as limiting the scope of the invention.

The term "comprising", as used in the claims, should not be interpreted as being restricted to the means or steps listed thereafter; the term does not exclude other elements or steps. The term should be interpreted as specifying the presence of said features, elements, steps or components as referred to, without, however, precluding the presence or addition of one or more additional features, elements, steps or components, or groups thereof. What is meant is that with respect to the present invention, only the components A and B are listed, and the claim should further be interpreted as also comprising equivalents of those components.

<FIG> shows an elongated pressure vessel <NUM>, having a main portion <NUM> and end portions <NUM>, cylindrically shaped with a radius and extending over a length d between both ends of the pressure vessel <NUM>. A metal boss <NUM> is arranged at one or both ends of the pressure vessel <NUM> so as to provide a port for communicating with the interior of the pressure vessel <NUM>. The pressure vessel <NUM> is formed by an inner (thermoplastic) polymeric liner <NUM> covered by an outer composite shell. In this case, "composite" refers to a fiber reinforced resin matrix material, such as a fiber wrapped or laminated structure. The pressure vessel <NUM> according to the present invention may be manufactured for storing about one liter, two liters, three liters, five liters, eight liters, thirteen liters, twenty-one liters, thirty-four liters, fifty-five liters and other suitable amounts of hydrogen.

<FIG> show details of improved boss and liner structures for a pressure vessel. The liner <NUM> has a generally cylindrical shape having a substantially semicircular end section <NUM> with a neck-shaped end section <NUM>. The neck-shaped end section <NUM> comprises a passage <NUM> adapted for being aligned within an opening in the outer composite shell, and a hollow wall <NUM> surrounding the passage. The hollow wall <NUM> comprises an inner annular wall part <NUM> and an outer annular wall part <NUM>.

The liner <NUM> according to the present invention may be manufactured from any material available to the person skilled in the art. Suitable thermoplastic (co)polymers are for example polyamides (PA), polyolefins, for example polypropene (PP), polyethene (PE), polytetrafluoroethylene (PTFE), polyphenylene ether (PPE or PPO), etcetera, amorphous and/or crystalline polyesters such as polyalkylene terephthalates, for example polyethene terephthalate (PETP or PET), polybutene terephthalate (PBT), etc., or polyimides (PI), such as for example polyetherimide (PEI), polyamide-imide (PAI), or polymethyl(meth)acrylate (PMMA), polyether methacrylate (PEMA), and polycarbonates.

The boss <NUM> extends in the hollow wall <NUM> of the neck-shaped end section <NUM> and comprises a neck <NUM> and flange <NUM> projecting radially outward. The neck <NUM> defines a passage <NUM> aligned with the passage <NUM> of the liner <NUM>, allowing high-pressure fluid to be communicated to the interior of the pressure vessel <NUM>. The flange <NUM> has an inner side <NUM> and an outer side <NUM>. A bottom opening <NUM> is defined at the inner side of the flange <NUM>. The bottom opening is connected to an upper opening <NUM> at the outer side of the flange <NUM> by means of a number of discrete bores <NUM>. The bores <NUM> may for example be circular or elongated in cross section.

<FIG> show the boss-liner structure provided with a thread <NUM> on the inner surface of the neck <NUM>. <FIG> further shows a layered liner <NUM> consisting of several layers of at least two different materials. The inner and outer monolithic layers are manufactured from the same material, which is preferably molded by rotational molding. The flange is integrated in the outer layer.

A method for connecting the liner <NUM> and the boss <NUM> comprises allowing a liquid polymer material for the liner <NUM> to flow at the inner and outer sides <NUM>, <NUM> of the flange <NUM> and into the bores <NUM> so as to fill these. After solidifying, the liner material forms an anchor <NUM> in the bores <NUM>. The liner <NUM> is mechanically interlocked with the boss <NUM> by the anchors <NUM> formed in the bores <NUM>, which in this embodiment connect the liner material at the inner side <NUM> to the liner material at the outer side <NUM>. Alternatively, a "bore" does not need to extend all the way through a member. As such, the bore <NUM> may extend from its openings <NUM>, <NUM> to a closed end thereof. In both embodiments, separation of the liner <NUM> from the boss <NUM> is prevented even under extreme pressure circumstances.

A method for forming a pressure vessel <NUM> comprises inserting a boss <NUM> into het front part of a mold. The protruding part of the boss is secured at the outer side of the mold so that the boss can no longer move. The boss is positioned in such a way in het front part of the mold that liner material may come between the mold and the boss (i.e., at the outer side of the boss). Further, the boss receives a sealing means provided with an air-permeable filter or Teflon plastic membrane extending through the boss and ensuring that no material can fall out of the mold, and also ensuring that a liner layer can adhere to the inner side of the boss. As a result, the boss is completely surrounded in the mold, both at the inner and at the outer side, so that that no further membrane sealing is required.

Claim 1:
Method for the production, by means of rotational molding, of a boss-liner structure for a type IV pressure vessel from a material containing a curable raw material, comprising the steps of:
▪ providing a mold, for forming a liner (<NUM>) comprising a hollow body defined by an elongated cylindrical liner wall having opposed inner and outer surfaces extending between a first end and a second end of the liner, wherein the cylindrical liner wall comprises a cylindrical main portion, a rounded head portion (<NUM>) and a neck-shaped end section (<NUM>), wherein the cylindrical liner wall has a passage opening near the first end of the liner, wherein the passage opening is determined by an edge of the liner;
▪ providing a boss (<NUM>) having a boss bore extending axially through the boss, wherein the boss bore is determined by a cylindrical wall (<NUM>) having an inner surface, and a flange (<NUM>) projecting radially outward having inner and outer flange surfaces located opposite to each other, wherein the boss (<NUM>) is arranged so that the bore at least partially extends into an interior of the mold, and so that the outer flange surface faces the inner surface of the mold;
▪ providing a sealing element in the boss bore through the boss;
characterized in that the method comprises the steps of:
▪ filling a mold cavity in the mold with a first thermoplastic material;
▪ heating the material to a first predetermined temperature;
▪ rotating the mold so that an outer monolithic layer of the liner is formed from the first thermoplastic material, wherein the liner is formed at the inner and outer sides of the flange and in bores or recesses of the flange and wherein the flange is integrated into the outer monolithic layer;
▪ forming at least one further layer of the liner inside the outer monolithic layer, such that the liner is a layered liner consisting of a plurality of monolithic layers of at least two different thermoplastic materials; and
▪ cooling the formed liner to a second predetermined temperature.