The present invention relates to a reinforced rigid tubular manufactured article and the method for its manufacture. More particularly, the present invention relates to a rigid and reinforced tubular manufactured article of plastics ma¬ terial used for conveying and transporting fluids and liq¬ uids, so that it can find advantageous application in aque¬ ducts and oil pipelines, gas pipelines, drainage networks and the like.

Hitherto, tubular manufactured articles of reinforced and non-reinforced concrete, of metal materials, such as steel, or of single-wall plastics material have been and are used for such applications.

These manufactured articles in effect have to be laid under¬ ground at a given depth so that they are subjected to both distributed and localised buckling loads (these localised loads are sometimes temporary, as in the case of manufac¬ tured articles laid underground below roadways. They further have to be produced in a leak-tight manner in order to pre¬ vent losses or leakages of the liquid or fluid which passes inside them, under the action of a pressure which can also reach substantial values. Finally, they have to be chemi¬ cally inert with respect to the fluids which pass inside them. It will immediately be appreciated that any loss or fracture of the underground tubular manufactured articles would lead to complex interventions because it would be nec¬ essary to carry out excavations in order to locate and reach the portion of the manufactured article or the junction at which the loss may have occurred, and to carry out the re¬ pair in situ.

Given the relative cost of the steel tubes which need to be adequately protected and the tubes of plastics material with a single wall, the manufactured articles currently in use

are preferably concrete articles, which, however, have the main disadvantage of being very heavy, so that they can be used only in sections of modest length, therefore requiring a large number of leak-tight junctions. It is obvious that each junction is a potential source of losses. A number of solutions have been proposed in the past in or¬ der to produce these types of tubular manufactured article of plastics material since it is thus possible, on the one hand, to use chemically and electrically inert materials (in fact, it should be noted that electrical inertia constitutes a defence against through-corrosion) and, on the other hand, to produce manufactured articles having a specific weight which is far less than the conventional materials previously mentioned.

A solution which has been examined and studied to a greater degree is one which comprises two coaxial cylindrical ele¬ ments, between .which a reinforcing element is interposed which is fixedly joined to the mutually facing surfaces of both of the cylindrical elements. This reinforcing element can be composed of a ribbing of the material that consti¬ tutes the coaxial cylindrical elements, which material is preferably a glass fibre reinforced unsaturated polyester resin. This solution is described in Italian patents numbers 1,006,957, 1,037,746 and 1,058,524, which also make provi¬ sion for the reinforcing ribbing to be helically wound and embedded in a layer of expanded resin which is interposed between the two coaxial cylindrical elements. In other cases (French patent No. 1,308,575), the reinforc¬ ing element is itself constituted by a tubular element which is helically wound in the gap between the two cylindrical elements, the helices of the tubular reinforcing element be¬ ing permanently fixed in the mutual contact zones to the ad¬ jacent surfaces of the coaxial cylindrical elements. Similar

solutions have also been proposed in published German patent application Nos. 2504883 and 2558288.

More recently, there has been proposed in international pat¬ ent application No. WO 02/065011 a composite tubular manu¬ factured article which also comprises at least two coaxial rigid elements of plastics material, forming a gap in which a reinforcing element is arranged which is itself consti¬ tuted by a helically wound tubular element which is charac¬ terised in that it is corrugated or shaped in an indented manner and whose turns, during manufacture, are caused to penetrate the adjacent surfaces of the two coaxial cylindri¬ cal elements, if only to a minimum thickness. This solution produces tubular manufactured articles in which the rigidity in a transverse direction is more than double that of a single-layer tube having a wall of a thick¬ ness equal to the total thickness of the reinforced manufac¬ tured article.

Naturally, a further increase in the buckling resistance in addition to the other mechanical properties of the rein¬ forced tubular manufactured article still remains a highly desirable objective from the industrial viewpoint and the point of view of applications.

This is precisely the object of the present invention, that is to say, to produce a rigid and reinforced tubular manu¬ factured article of plastics material, in which the mechani¬ cal properties, and in particular the buckling resistance in a direction transverse to the axis of the manufactured arti¬ cle, are greatly improved over the prior art. This object and other objects are achieved by the tubular manufactured article according to the present invention, of the type comprising at least two coaxial cylindrical members of plastics material, a spacer element which is accommodated in the gap between the two cylindrical members and which is helically wound around the internal cylindrical member of

the pair of cylindrical members, characterised in that a layer of a weldable material is interposed between each turn, of the spacer element and the directly facing surface of one and of the other of the coaxial cylindrical members, alter¬ nately, in order to form a chemical bond with the surfaces of the coaxial members, the layer being arranged in the gap with a wave-like course, in the concave portion of which the spacer element is accommodated.

The spacer element is preferably a tubular element, and even more preferably is a corrugated or knurled tubular element. In the preferred embodiment of the present invention, the layer of weldable material is of the plastics material that forms the coaxial cylindrical members, in particular of glass fibre reinforced unsaturated polyester resin or an ep- oxy resin which is also reinforced with glass fibres or other thermosetting materials.

The present invention further relates to the method for pro¬ ducing the above-defined tubular manufactured article. In the preferred embodiment, this method includes the steps:

(a) manufacturing, by a method known per se, a tubular cylindrical member, which is referred to as the in¬ ternal cylindrical member, of rigid plastics material on a temporary support mandrel;

(b) winding a tubular spacer element of compatible plas¬ tics material on the outer surface of the internal cylindrical member in a helix which has a predeter¬ mined pitch and which is substantially perpendicular to the axis of the temporary support mandrel which is kept in rotation;

(c) producing, by a known method, before the plastics ma¬ terial which constitutes the internal cylindrical member has completely hardened, a layer of plastics material which is weldable to that of the internal cylindrical member, at such an angle as to allow the

layer to be positioned both against the external sur¬ face of the internal cylindrical member and over the turns of the tubular spacer element in such a manner as to take on a wave-like form;

(d) optional winding of a second tubular spacer element having the same pitch as the first winding, but stag¬ gered axially by a half-pitch, in such a manner as to be interposed between the turns of the winding of the preceding step (b) , and being positioned in the in¬ dentations of the wave which is already formed by the layer produced in the preceding step (c) ;

(e) manufacturing, by a known method, a cylindrical mem¬ ber, which is referred to as the external cylindrical

-member, which is coaxial with respect to the internal cylindrical■ member and which is spaced by the thick¬ ness corresponding to the sum of the diameter of the tubular spacer element and the thickness of the in¬ termediate layer of plastics material, before the resin constituting the layer has completely hardened;

(f) optional repetition of steps (b) to (e) in order to produce tubular manufactured articles having a plu¬ rality of coaxial cylindrical members;

(g) withdrawing the temporary support mandrel.

The above-defined method naturally does not constitute the only method by which it is possible to produce the tubular manufactured article according to the present invention. In the preferred embodiment of this method, the internal and external cylindrical members and/or the layer of plastics material which is interposed between the tubular element and which is helically wound and accommodated in the gap is/are produced by a known technique which is referred to as ^Xλ fila- ment winding" consisting in winding continuous filaments of glass which are impregnated with a resin on a mandrel. A ro¬ tation movement is imparted to the mandrel and the head for

depositing the glass filament impregnated with resin carries out an alternating translational movement which is directed axially relative to the mandrel.

With this technique, it is in fact known that manufactured articles are produced having strength characteristics which are substantially improved over those obtained previously with spray or casting techniques.

As regards the rigid plastics material from which the manu¬ factured article of the present invention itself is pro¬ duced, as already mentioned, this is preferably a polyester resin which is reinforced with glass fibres or an epoxy resin which is also reinforced with glass fibres or other thermosetting materials.

In place of the glass fibres, however, it is possible to use other reinforcing materials of fibre form, such as carbon fibres, or other inorganic or organic fibres, such as ara- mide fibres (for example, Kevlar) .

In the case of the glass fibres, these are contained in the usual quantities for producing the so-called manufactured articles in plastics material reinforced by glass fibres, which quantity in the case of the invention is between 30 and 80% by weight of the total, and preferably in the order of 70% by weight, always with reference to the total weight of the manufactured article.

As regards the thickness of the gap formed between the in¬ ternal and external cylindrical members, this is preferably between 5 and 40 mm, and the thickness of the layer forming the wave is preferably from 1 to 5 mm, even more preferably 2 mm.

The rigid tubular manufactured articles of plastics material according to the present invention can also have very large diameters in the order of 3 metres, and even greater, so that they comply with the requirements set out in the intro¬ duction to this description.

The features and advantages of the present invention will be appreciated from the description below given with reference to the appended drawings, in which:

Figure 1 is a schematic _r partial section in an axial plane of a first embodiment of the tubular manufactured article according to the present invention;

Figure 2 is a view similar to Figure 1 of a second embodi¬ ment of the present invention, and

Figure 3 is a partial section of a third embodiment of the invention.

With reference first to Figure 1, a first embodiment of the tubular manufactured article of the present invention is shown and comprises an internal cylindrical tubular member 10 and an external cylindrical tubular member 12, between which a gap 14 is defined.

A spacer constituted by a tubular element 16 which is heli¬ cally wound around the internal cylindrical member 10 is lo¬ cated in this gap.

A layer of plastics material 18 is further arranged in the gap 14, which layer 18 is produced on the outer side of the turns of the tubular element 16 in such a manner as to bring about a wave-like course, in the indentations of which the turns of the tubular element 16 are located.

As shown in the Figures mentioned, the layer 18 is substan¬ tially in contact with the external wall 20 of the internal cylindrical member 10, to which wall, as will be seen, it remains fixed as a result of the formation of permanent chemical bonds.

The crests of the waves of the layer 18 are also located in contact with the internal wall 22 of the external cylindri¬ cal member 12, being anchored thereto as a result of the formation of permanent chemical bonds.

In the embodiment shown in Figure 2, the tubular manufac¬ tured article has the same structure as that in Figure 1,

except that in the gap 14 there is provided a second helical winding of a tubular element 24 which is wound with a stag¬ gering of a half-pitch with respect to that of the tubular element 16 so that it is arranged symmetrically relative to the opposite side of the layer 18 with respect to the wind¬ ing 16.

Finally with reference to Figure 3, an embodiment of the tu¬ bular manufactured article is shown according to the present invention, in which the structure of Figure 1 is repeated in order to further increase the transverse rigidity when spe¬ cific performance is desired.

In this case, the cylindrical member 12 acts as an interme¬ diate cylindrical member which serves as the external cylin¬ drical member for the gap 14 but as the internal cylindrical member with respect to another external cylindrical member 26, with which it forms a gap 28, in which a second tubular spacer element 30 preferably having features identical to those of the tubular element 16 is accommodated with helical winding.

Another reinforcing layer 32 having features and a course similar or preferably identical to those of the reinforcing layer 18 is produced over the winding formed by the tubular element 30, so that in this case permanent chemical bonds are also formed between this layer 32 and the surfaces at which it is in contact with the external wall 34 of the cy¬ lindrical member 12 and with the internal wall 36 of the ex¬ ternal cylindrical member 26.

These chemical bonds are produced by virtue of two concomi¬ tant features of the present invention:

(a) firstly, the selection of the resins which constitute the various components of the composite tubular manu¬ factured article of the present invention, which res¬ ins have to be mutually compatible and in particular weldable upon contact, provided that this is effected

before the hardening of each resin is complete. The types of resin already set out above comply with this requirement;

(b) secondly, carrying out the various method steps for manufacturing the tubular manufactured article using methods and times so as to comply with the require¬ ment indicated above, that is to say that (i) the helical winding of the tubular element 16 on the internal cylindrical member 10 is effected before the resin constituting the member has completely hardened, preferably when the resin (or the resin re¬ inforced with glass fibre) ^' which constitutes it is still sufficiently pasty to allow the turns of the tubular element 16 to penetrate to a depth, however small, in the surface of the internal cylindrical member 10;

(ii) similarly, the external cylindrical member 12, also preferably having the same composition as the internal cylindrical member 10, is formed in such a manner that it engages with a given pressure with the .ridge portions of the layer 18 so as to bring about not only the chemical bonds between the internal wall 22 of the member 12 and the regions in which it is in contact with the layer 18, but also to carry out (af¬ ter final hardening) a physical action of mutual clamping of the various components of the tubular manufactured article.

As regards the tubular element 16 (as is also the case for the elements designated 24 and 30) , they can be of a simple type, as illustrated in the Figures, or can be constituted by corrugated tubes (of the type indicated, for example, in international patent application WO 02/065011) , in which case the mechanical strength of the final tubular manufac-

tured article is further increased, and in particular the transverse rigidity is increased.

Tests were carried out with the tubular manufactured arti¬ cles produced according to the present invention in compari¬ son with single-layer tubes having a thickness corresponding to the overall wall thickness of the tubular manufactured article according to the present invention (and therefore having equal resistance to internal pressure) . For the tubular manufactured article according to the pre¬ sent invention, it should be stated that the external and internal cylindrical members were produced by means of "filament winding" with polyester resin reinforced with 70% glass fibre by weight. The tubular element 16 itself was constituted by a corrugated tube of PVC and the layer 18 it¬ self was produced of polyester resin reinforced with glass fibre.

The table below (in which ECOD indicates a tubular manufac¬ tured article according to the invention) shows the results for various internal diameters (DN) of the tubular manufac¬ ture article:

Where S _tru c _tural ⁼ thickness of the cylindrical (i) member (i) , S _c on _n ec _t io _n = thickness of the layer- (18) in the ECOD tubes.

y= deformation imparted along the diameter of the ring in the test expressed as a percentage over the nominal diame¬ ter.

For the single-layer tubes and the ECOD tubes, the nominal pressure PN is equal to 10 bar, while for the tube which is filled with sand PN = 6 bar.

All the ECOD tubes are produced by means of "filament wind¬ ing" with polyester resin.

In the ECOD tubes, the spacer is constituted by a corrugated tube of PVC.

The rigidities of the single-layer tubes are obtained from the tables of the manufacturers.

The rigidities of the ECOD tubes are obtained from tests carried out at the OMECO srl laboratory in accordance with standards UNI EN 1226 and UNI EN 1228; the value indicated is the mean of those measured for the two samples of each diameter.

It will readily be appreciated from the above data that the transverse rigidity is greatly increased in the tubular manufactured article produced according to the present in¬ vention.

These data make the tubular manufactured article according to the present invention highly advantageous both over tubes of concrete (that is to say, those currently used) and over single-layer tubes of plastics material having an equivalent wall thickness.