Patent Description:
British patent <CIT> discloses a method of making a pressure container comprising the provision of an inflatable bladder, surrounding the bladder with several layers of reinforcing strands impregnated with a heat curable material, inflating the bladder to a degree causing the reinforcing strands to be tensioned, and then heating the resulting body to cure the impregnating material. The strands of fibers, preferably glass fibers are stretched meridionally or helically between the two ends of the bladder on its entire surface. Hoop windings may also be applied either directly on to the meridional strands or in a preformed tube into which the body is slid. A cord or wire is wrapped circumferentially around the bladder to secure the strands in position.

In European patent application <CIT> an inner tank is surrounded by a wire extending continuously, impregnated or not with resin. This winding can be done in different ways, namely polar, hemi-axial or radial.

Also, in European patent application <CIT> a pressure vessel of fiber-reinforced plastic has an essentially cylindrical central portion and two end portions and is preferably provided with a gas-tight liner. The central portion comprises a tangential winding of the fiber material, and an axial winding of fiber material, located outside said tangential winding, and the fiber material of which extends out into and forms a reinforcement of the fiber material in the end portions. The central portion, including its tangential winding and its axial winding, in each of its parts located nearest to an end portion, is formed with a diameter which decreases in the direction towards the end portion.

Slightly different solution is proposed in earlier European patent application <CIT> wherein the filament winding constituting the outer casing is advantageously a cross winding comprising fibers coiled along the two diagonal planes of the cylindrical reservoir and the coiled fibers along its circumference, so that the density of the fibers at the ends of the cylindrical reservoir is greater than that at its wall.

According to European patent application <CIT> a composite pressure vessel has a plastic liner, which is reinforced with a fiber winding. The reinforcing wrap consists of a fiber reinforcement such as carbon, aramid, glass, boron, Al<NUM>O<NUM> fibers or mixtures (hybrid yarns) thereof, in a matrix of thermosetting resins, e.g. epoxy or phenolic resins, or in thermoplastics, like PA12, PA6, PP etc. The fiber composite material consisting of fibers and polymers is applied in both the axial and tangential directions of the container, or the orientation of the fiber longitudinal axes have only a small angle (<NUM>° to <NUM>°) relative to the container longitudinal axis in the case of the axial wrapping in the cylindrical container part.

Another European patent application <CIT> discloses a composite tank and a method of its manufacturing. The composite tank consists of an inner part and a two-piece collar connected with the inner part, and is covered with eight layers of composite material made of carbon fibers (inner layers) and glass fibers (outer layers).

In European patent application <CIT> and international patent application <CIT> a multilayer container for pressurized liquids and gases is disclosed. Its outside surface is reinforced by winding resin-impregnated fibers, preferably glass, carbon, aramid, basalt, ceramide fibers, or several kinds of these fibers. Three methods of winding are applied: <NUM>) cross method consisting in simultaneous rotation of the container or the winding head around the container and shift of the head alongside the length of the container reaching an angle of inclination falling within the range from <NUM>° up to <NUM>°; <NUM>) polar method according to which fiber is wound when the winding head moves between the poles of the container; and <NUM>) helical method, consisting in reinforcement of cylindrical part of the container. Individual packages of fibers are separated with a layer of resin from the set of polyester or epoxy resins. Depending on the kind of the resin used, the container wrapped with fibers passes to the furnace to harden resin or to initiate chemical reaction resulting in hardening of laminate.

European patent application <CIT> also discloses a high pressure composite vessel having an outer composite layer fabricated by making a load-bearing wound wraps of bundles of reinforcing filaments according to three winding patterns: helical, polar, and hoop, followed by thermal hardening.

American patent <CIT> describes a filament-wound pressure vessel which has a reinforced access opening. The pressure vessel comprises a blow-molded one piece liner having an outer surface defined by a cylindrical sidewall and oblate ellipsoidal ends. The liner defines at least one access opening into the vessel and the access opening has a cylindrical neck portion and a liner flange extending radially outwardly from an open distal end of the neck portion. A cylindrical reinforcement member surrounds the cylindrical neck portion and has a supporting flange at one end thereof, which engages and annular face of the liner flange. The reinforcement member has a radially extending supporting foot at the other end thereof, which contacts the outer surface of the liner. The reinforcement member comprises a plurality of separable arcuate segments so that it may be assembled around the cylindrical neck portion of the liner. A resin-impregnated filament-winding covers the liner and the supporting foot of the reinforcement member.

European patent application <CIT> discloses a container or conduit for compressed gas and/or cryogenic gas constructed with a gas impermeable synthetic polymer forming a gas barrier (liner), and a structural component that provides structural integrity to the container or conduit. The structural overwrap is made of a composite material and surrounds and may be bonded to or be in contact with the inner liner. The composite material is made of structural fibers which are consolidated within a matrix resin. Such structural fibers may be made of graphite, carbon, aramid (e. , Kelvar), S2 glass, E glass, Boron, LCP's, and ultrahigh molecular weight polyethylene. These fibers are consolidated within matrices of thermoplastic or thermosetting matrix resins. Thermosetting matrix resins, however, are most popular and include epoxy, polyester, phenolic, bismaleimide (BMI), and polyimide resins. The composite material of the structural overwrap may be made by any technique for making composites, for example, hand lay-up, tape laying, braiding, or filament winding. Filament winding of continuous fibers, however, is preferred. The gas impermeable synthetic polymer is preferably a thermotropic liquid crystal polymer.

According to <NPL>, the most often employed thermoset technique is wet filament winding, and there are essentially three winding techniques: circumferencial or hoop winding, helical winding, and polar winding. The first technique involves wrapping the materials in either a single band or multiple repeating bands.

Resin matrix used in production of composite vessels is often fiber reinforced. According to European patent application <CIT> combination of lightness in weight and resistance to failure in composite containers is possible due to the high specific strengths of the reinforcing fibers or filaments (carbon, glass, aramid, etc.) suspended in resin which, in the construction of pressure vessels, are typically oriented in the direction of the principal forces.

CNS-infused carbon nanomaterials and process therefor is described in international patent application <CIT>. A composition includes a carbon nanotube (CNT) yarn or sheet and a plurality of carbon nanostructures (CNSs) infused to a surface of the CNT yarn or sheet, wherein the CNSs are disposed substantially radially from the surface of the CNT yarn or outwardly from the sheet. If wet winding is desired, the CNT- infused carbon fiber materials can be passed through a resin bath and wound on a mandrel or spool. The resulting carbon fiber material/resin combination locks the CNTs on the carbon fiber material allowing for easier handling and composite fabrication. In some embodiments, CNT infusion is used to provide improved filament winding. Thus, CNTs formed on carbon fibers such as carbon tow, are passed through a resin bath to produce resin-impregnated, CNT-infused carbon tow. After resin impregnation, the carbon tow can be positioned on the surface of a rotating mandrel by a delivery head. The tow can then be wound onto the mandrel in a precise geometric pattern in known fashion.

In American patent application <CIT> a further step consisting of using a resin matrix with an additive of carbon nanotubes (CNTs) instead of suspended reinforcing fibers is described. A high pressure container according to this invention comprises a hollow liner capable of being sealed, its outer surface being covered with a reinforcement layer including composite carbon fiber bundles wound in laminated multiple layers. These carbon fiber bundles are fixed by a cured product of thermosetting resin. The reinforcement layer contains the cured product of thermosetting resin and a plurality of CNTs between a carbon fiber contained in one composite carbon fiber bundle and a carbon fiber contained in other composite carbon fiber bundle. CNTs suspended in the resin matrix adhere to the surfaces of the carbon fibers. Each of the plurality of continuous carbon fibers contained in the composite carbon fiber bundle is in contact with another carbon fiber via a cured product of thermosetting resin, and the plurality of CNTs. Since a plurality of CNTs are adhered to the surface of each of the plurality of carbon fibers, adhesive force between the carbon fiber and the cured product of thermosetting resin is enhanced due to anchor effects. As a result of that, peeling strength of the interface between the carbon fiber and the cured product of thermosetting resin increases. The method for manufacturing a high pressure container comprises the steps of: winding a composite carbon fiber bundle impregnated with a thermosetting resin enriched with CNTs around the outer surface of the hollow liner while applying a tensile load to the composite carbon fiber bundle, and forming the reinforcement layer by curing the thermosetting resin. Helical and hoop windings are applied.

What is very important in the context of the present invention and will be referred to hereinafter, it is that according to the disclosure contained in the above mentioned patent document <CIT>), as the CNTs, most preferably multi-walled carbon nanotubes (MWCNTs) were used, which were grown to have a diameter of <NUM> to <NUM> and a length of not less than <NUM>. In the present invention a radically different solution has been proposed.

The inventors of the proposed invention found that the micro- and nanobubbles present in resin and filled with various gases like air or vapors of water and organic solvents reduce the reinforcing properties of the composite outer layer. Especially solvents, used for example as suspension media of CNTs are problematic because the vapor bubbles are created in elevated temperature, i.e. during the phase of resin hardening. Also the air bubbles expand as the temperature increases. Finally, the resulting micro- and nanobubbles are freezed in the composite reinforcing layer and reduce its strength.

According to the proposed invention and as recited in claim <NUM> the method of manufacturing a highpressure container having a casing reinforced with an outer composite reinforcing layer, comprises subsequent stages;.

As recited in claim <NUM>, the resin composition consists of at least <NUM> wt. %, preferably <NUM>-<NUM> wt. % of bis- [<NUM>- (<NUM>,<NUM>-epoxypropoxy) phenyl] propane, and up to <NUM> wt. %, preferably <NUM>-<NUM> wt. % of <NUM>,<NUM>-bis (<NUM>,<NUM>-epoxypropoxy) butane. The nanoadditive consists of at least <NUM> wt. % of graphene nanotubes (GNTs), at most <NUM> wt. % of iron (Fe) nanoparticles, and at most <NUM>% of other allotropic forms of carbon such as graphene flakes or fullerenes, and some moisture.

As recited in claim <NUM>, GNTs in the form of single-walled graphene nanotubes (SWGNTs) are used. In the proposed invention SWGNTs have a diameter of <NUM>-<NUM> and a length of at most <NUM> micrometers, and length-to-diameter ratio of at least <NUM>. In embodiments not covered by the appended claims, slightly different values can be applied as well, but in any case the SWCNTs diameter should be less than <NUM>. As was mentioned above, in the invention disclosed in <CIT>), mostly multi-walled carbon nanotubes (MWCNTs) were used as the CNTs, having a diameter exceeding <NUM> and a length of not less than <NUM>. This is clearly visible that CNTs used in the here proposed invention are much smaller: one order of magnitude thinner and also one order of magnitude shorter. It should be noted that this results in the CNTs' volume reduction of three orders of magnitude.

It was found, that using SWCNTs instead of MWCNTs one can significantly reduce the content of nanoadditives in the mixture with resin, at least by one order of magnitude, without any loss of the reinforcing power introduced by the CNTs. It was even found that the impact resistance of final products reinforced with the use of SWCNTs increased when compared with the same product but reinforced with MWCNTs, without any loss of pressure inside the container. Microphotograpic inspection of tested containers proved that mean length of propagation of microfractures resulting from impact test was significantly reduced. In the proposed invention the impregnating mixture contains <NUM>-<NUM> wt. % of the resin composition, and <NUM>-<NUM> wt. % of the nanoadditive.

According to an embodiment of the invention, pressure in the mixing device is set below <NUM> hPa, preferably <NUM>-<NUM> hPa. The lower the pressure, the more efficiently the gas dissolved in resin is removed. This is very important to combine reduced pressure and mixing of the impregnating mixture of a resin composition and a nanoadditive, because only during mixing dissolved gases and vapors have easy contact with the mixture surface. In other words, the mean distance of a bubble to surface is significantly reduced.

According to a further embodiment of the invention, mixing of the impregnating mixture consists of three stages:.

The proposed circumferential speed of the disperser blade in stage "a" was experimentally selected to be high enough to trigger cavitation in the mixed material at the blade edges. It is recommended to combine both stages "a" and "b" into one stage, because combination of ultrasonic and mechanical stirring increases efficiency of cavitation which contributes to higher homogeneity of the mixture, helps in eliminating dissolved gases and prevents from diffusing gases from the outside into the mixture.

In a further embodiment of the invention, the curing agent which is added directly to the resin bath consists of <NUM>-<NUM> wt. % of polyoxypropylene diamine, <NUM>-<NUM> wt. % of diamino-methylcyclohexane, and at most <NUM> wt. % of <NUM>-methylcyclohexane-<NUM>,<NUM>-diamine. In an embodiment of the invention, diameter of the carbon filament bundles equals to <NUM>-<NUM> micrometers, and tension of the carbon filament bundles in the winding machine is set to at least <NUM> N.

The winding patterns differ in angles of winding and a degree of surface coverage, they include helical, polar, and hoop patterns.

Thermal hardenin is a final operation of cured resin hardening, and can consist of subsequent stages:.

As recited in claim <NUM>, a high pressure container manufactured with the use of the above described method comprises a casing and an outer composite reinforcing layer made of the carbon filament bundles wound around the outer surface of the casing and fixed with cured impregnating mixture of a resin composition and a nanoadditive. The resin composition consists of at least <NUM> wt. %, preferably <NUM>-<NUM> wt. % of bis- [<NUM>- (<NUM>,<NUM>-epoxypropoxy) phenyl] propane, and up to <NUM> wt. %, preferably <NUM>-<NUM> wt. % of <NUM>,<NUM>-bis (<NUM>,<NUM>-epoxypropoxy) butane. The nanoadditive consists of at least <NUM> wt. % of graphene nanotubes (GNTs), at most <NUM> wt. % of iron (Fe) nanoparticles, and at most <NUM>% of other allotropic forms of carbon such as graphene flakes or fullerenes, and moisture. GNTs are single-walled graphene nanotubes (SWGNTs) and have diameter of <NUM>-<NUM> and length of at most <NUM> micrometers, and length-to-diameter ratio of at least <NUM>. The impregnating mixture contains <NUM>-<NUM> wt. % of the resin composition, and <NUM>-<NUM> wt. % of the nanoadditive.

The outer composite reinforcing layer is wrapped using filament winding with at least <NUM> different winding patterns, and wherein each carbon filament bundle contains at least <NUM> thousands, preferably <NUM> to <NUM> thousands of carbon filaments. In an embodiment, curing agent consists of <NUM>-<NUM> wt. % of polyoxypropylene diamine, <NUM>-<NUM> wt. % of diamino-methylcyclohexane, and at most <NUM> wt. % of <NUM>-methylcyclohexane-<NUM>,<NUM>-diamine. In an embodiment, diameter of the carbon filament bundles equals to <NUM>-<NUM> micrometers and the carbon filament bundles are wound on the casing by wrapping impregnated carbon filament bundles in at least <NUM> winding patterns differing in angles of winding and a degree of surface coverage, preferably including helical, polar, and hoop patterns. For most high pressure containers <NUM> reinforcing layers are sufficient, however in more demanding embodiments <NUM> layers are applied.

An embodiment of the high pressure container according to the proposed invention looks the same as in the drawing of the prior art document <CIT> (A1), the only difference being in the structure and composition of the reinforcing layer. The figures in this drawing illustrate the container structure and winding patterns:.

The composite high pressure container for storing pressurized liquids and gases is manufactured in a sequence of operations: i) blow molding a plastic preform into final shape of the casing <NUM>, ii) connecting the casing with a connection fitting <NUM> having a retaining collar <NUM> and an external annular boss <NUM>, iii) connecting the casing with a bottom unit <NUM> having branched wings <NUM> and annular bosses <NUM>, and iv) strengthening the container according to the claimed method by making a composite reinforcement <NUM> on the casing surface. The casing is immobilized in the connection fitting by gaskets (not shown) placed in a sealing groove <NUM> and a fixing groove <NUM>. The composite reinforcing layer is fabricated by winding a bundle of filaments according to three winding patterns: helical, polar, and hoop.

Claim 1:
Method of manufacturing a high pressure container having a casing (<NUM>) reinforced with an outer composite reinforcing layer (<NUM>), comprising subsequent stages:
A) an outer surface of the casing (<NUM>) of the container is covered with a thin anti-adhesive layer preventing the reinforcing layer (<NUM>) from binding to the casing (<NUM>);
B) an impregnating mixture of a resin composition and a nanoadditive is prepared in a mixing device at a pressure lower than a normal pressure of <NUM> hPa;
C) the impregnating mixture is poured into a resin tray of a winding machine;
D) at least one bobbin of carbon filament bundles, each carbon filament bundle containing at least <NUM> thousands, preferably <NUM> to <NUM> thousands of carbon filaments, are mounted on the winding machine;
E) a curing agent is added to the impregnating mixture in a ratio of <NUM>-<NUM> wt.% of the impregnating mixture;
F) the carbon filament bundles are impregnated in a resin bath with use of the resin tray so that the impregnated carbon filament bundle contains at least <NUM> wt.% of carbon filaments and at most <NUM> wt.% of composition of the impregnating mixture and the curing agent;
G) the carbon filament bundles are wound on the casing (<NUM>) by wrapping the impregnated carbon filament bundles in at least <NUM> different winding patterns;
H) the resulting composite reinforcing layer (<NUM>) is thermally hardened,
and wherein the resin composition used for making the impregnating mixture consists of at least <NUM> wt.%, preferably <NUM>-<NUM> wt.% of bis- [<NUM>- (<NUM>,<NUM>-epoxypropoxy) phenyl] propane, and up to <NUM> wt.%, preferably <NUM>-<NUM> wt.% of <NUM>,<NUM>-bis (<NUM>,<NUM>-epoxypropoxy) butane, the nanoadditive consists of at least <NUM> wt.% of graphene nanotubes, GNTs, at most <NUM> wt.% of iron (Fe) nanoparticles, and at most <NUM>% of other allotropic forms of carbon such as graphene flakes or fullerenes, and moisture, wherein GNTs are single-walled graphene nanotubes, SWGNTs, and have diameter of <NUM>-<NUM> and length of at most <NUM> micrometers, and length-to-diameter ratio of at least <NUM>, and wherein the impregnating mixture contains <NUM>-<NUM> wt.% of the resin composition, and <NUM>-<NUM> wt.% of the nanoadditive.