Background of the invention It is known that in hydro-thermo-sanitary plants made of ferrous pipes in the latter may occur through corrosions due to the action of local leakage currents and/or chemical corrosions caused by acid substances released from building materials like concrete of the floors in which said plant pipes are embedded. Such through corrosions cause, in turn, water leakages with the well known heavy consequences for both the house where said leakages took place and in case the underlying flat, for example in flat blocks and the like.

For repairing said leakages it will be necessary to break the floor starting from the visible moisture spots up to finding out the corrode zones, to substitute the corresponding pipe portions and to complete the floor again.

Beside a remarkable discomfort and high costs, in practice it is not always possible to have at disposa the same tiles like the broken ones so that a footstep of said substitution interventions will always be well visible.

Likewise remarkable could be the damages for repairing the ceiling of the underlying flat.

It has been tried to avoid the drawbacks of the known ferrous pipes by using copper pipes due to their features of, among others, drinkability (as copper does not favor the formation of micro-organisms, does not release toxic substances and it is not gas-permeable), life (practically indefinite), stability (it takes up thermic lapses without any shortcomings), and malleability (by deforming itself it may follow small settlings of the floor or ground in which the copper pipe is embedded, for example in the case of water distribution plants in outside floors like pavements, large squares, terraces, balconies and the like).

However, in practice it has been observe that also copper pipes can undergo through corrosions even though in a smaller extent with respect to ferrous pipes.

Pipes made of a plastic suitable for conveying water are also known, which do not undergo corrosions indeed and are good electric insulators, however, they show the typical drawbacks of undergoing elongations and dilatations under heat action, requiring the use of special metal unions having the shortcoming set fort in the following, undergoing crack formations under exposition to ultraviolet rays, and of being oxygen- permeable, whereby possible oxygen inside said plastic pipes may cause perforations in the boilers connecte therewith, as well as of allowing a micro-organism proliferation in case of water stagnant for long periods.

Composite pipes consisting of metal-plastic in which the metal is an aluminium alloy are also known. Particularly, said pipes consists of an intermediate pipe of aluminium alloy which is longitudinally welded or has overlapped edges, an internal pipe of cross-linked polyethylene, an outer pipe also made of cross-linked polyethylene, as well as two adhesive layers bonding the aluminium alloy pipe to both outer layers.

Therefore, said known composite metal-plastic pipes on the one hand have several layers and an expensive manufacturing and on the other hand aluminuim and aluminium alloys are not suitable for conveying drinking- water. It is also known that aluminium is very sensitive with respect to corrosions.

A contact between drinking-water and the intermediate aluminium layer due to leaky zones or tearings of the internal layer would remain hidden as the plastic outer layers would hinder a water leak. This would induce to suppose an intact composite pipe whereas the delivered water could be no more drinkable.

A further shortcoming of the known composite aluminium-plastic pipes is that they require metal unions which in order to avoid chemical corrosion phenomena have also to be provided with an appropriate preassembled gasket. This increases the manufacturing costs of said metal unions and requires a specific keeping in stock thereof.

A further drawback of the known composite metal-plastic pipes is to be seen in the complexity of the manufacturing method and devices.

Summarv of the invention The object of the present invention is to provide a composite metal-plastic pipe of the mentioned character which is able to eliminate the drawbacks of the prior art pipes and can be produced in a simple and cheap manner.

It is another object to provide a manufacturing method which can be carried out in a simply way and with known devices.

The stated objects are achieved with a composite metal-plastic pipe and a manufacturing method thereof according to this invention, by the features as indicated in claims 1 and 8.

Further advantageous embodiments of the invention are inferable from the sub-claims.

With the composite pipe according to the present invention are achievable several important advantages. First, with the outer pipe-like layer made of a plastic suitable for conveying drinkable-water is assured a double protection, that is: -an outer protection, as the outer plastic pipe protects the copper pipe outwardly against corrosion and leakage current phenomena because the plastic material is a good electric insulator, and -an internal protection as in case of possible leaky zones in the copper pipe both the outer plastic layers close said leakages from the outside, whereby the contact water-plastic of both outer layers maintins the feature of"drinkability"of the water. Due to the fact that the used plastic is not sensitive with respect to corrosion, the considered plant will continue to work in a perfect manner and avoid any leakages. The above discussed drawbacks are, therefore, avoided.

Considered the function assigne to the outer plastic layer, i. e. of a reinforcement and sealing sheath, the thickness of said outer plastic layer could be rather small, and further the presence of both the outer layers which are firmly anchored onto the complete surface of the metal pipe, for example copper or steel, allows to reduce the thickness of the metal pipe, which permits in turn to reduce the weight of the composite pipe with respect to pipes only consisting of metal.

With equal outer pipe diameters said weight reduction could be, in case of copper, up to about 35%, which leads to an advantageously cheaper production. The outer plastic layer has further the avantage of being very resisting against impacts, abrasions and internal pressures so that the composite pipe according to the present invention can be laid without particular precautions.

Also the connection between composite pipes according to the present invention can be effected in a simple and quick way, for example by use of a threaded metal locking union with ogive and a rubber 0-ring.

The composite pipe according to claim 2 has a middle residual carbon content which is lower than the quantity provided for by the specific law norms.

By providing dimensions as stated in claim 3 it is possible to produce particularly light and resisting pipes.

The feature of claim 4 assures the inalterability of the metal, for example copper, in the time.

Composite pipes according to claim 5 have a high uviolet resistance. A high time stability of the composite pipes in every installation environment can be achieved with the feature of claim 6. Adopting the teaching of claim 7 it is possible to achieve an optimum insulation of the propose pipes, for example in heating and conditioning plants.

Also in presence of a thermic gradient with the method according to the invention is obtained a firm, uniform and stable anchoring between the internal metal pipe and the outer plastic pipe with an intermediate very thin layer of plastic acting as a biadhesive layer. A further avantage consists in providing a manufacturing by means of a single step of separate extrusion, whereby is advantageously provided a single co- extrusion onto the metal pipe.

The propose method can be advantageously integrated with the feature of claim 9.

A further avantage consists in the fact that the plurality of features utilized according to the invention allow an advantageous use of the propose composite pipe also in the conditioning fjeld. The outer plastic layer or pipe also contribute also to accomplis an advantageous insulation action which, together with the teaching of claim 10, avoids the formation of the known condensate onto the outer pipe surface.

Brief Description of the drain Farther features, details and avantages of the composite pipe according to the invention and the manufacturing method thereof will become more apparent from the following description in conjunction with the annexed drawing in which for the sake of an easier understanding the shown figures are not on scale, and in which: figure 1 is a longitudinal middle cross-section through a portion of the composite pipe according to the invention, and figure 2 is a cross-section similar to that of figure 1, through a second embodiment of the composite pipe according to the invention.

Detailed Description of the Preferred Embodiments The composite pipe made of metal-plastic according to the invention is denoted by 1. It is formed by an internal metal pipe 2, for example made of copper, steel or other metal suitable for conveying drinking-water, and an outer pipe or layer 3 consisting of a plastic suitable for conveying drinking-water, for example high density cross-linked polyethylene (PE- X). An intermediate layer having the function of a biadhesive layer is denoted by 4. Also said layer 4 consists of a plastic suitable for conveying drinking-water and capable to firmly adhere to both the internal metal pipe 2 as well as the outer plastic layer 3. The"biadhesive"layer 4 consists for example of a linear low density polyethylene.

Optimum resistance and low weight results have been achieved in practice by using copper pipes having an outer diameter in the range of 4 to 54 mm. Particularly, by providing for example copper pipes with an outer diameter of 12 mm, 15 mm and 18 mm, respectively, the pipe thickness is of 0,5 mm, 0,5 mm and 0,6 mm, respectively. The thickness of the intermediate adhesive layer 4 is in the range of 50 to 100 rlm.

The thickness of the outer layer pipe 3 was chosen in the range between 0,1 and 10 mm. With said three copper pipes indicated above the chosen thickness of the outer layer 3 is of 1 to 1,5 mm, 1,5 mm and 2 mm, respectively, included the thickness of the respective tested adhesive layer 4.

Similar sizes are also provided according to the invention for steel pipes and for specific sizes according to the European standards EN 1057, as well as to other national and international standards.

The method for producing the composite metal-plastic pipe 1 provides an application by extrusion of both the bonding plastic layer 4 and the outer plastic layer 3. The extrusion steps onto the metal pipe, for example copper pipe 2, can be effected in separated steps or simultaneously in a single co-extrusion operation. This allows for a high productivity and a reliable and immediate application of both plastic layers 3 and 4 which permits to surely avoid a possible dirt or extraneous material inclusion, an air bubble formation and so on.

According to the invention during the rolling operation the metal pipe is specularly polished onto its internal wall and it is subjected to a known treatment by the pipe manufacturer which allows to reduce the carbon deposits onto the internal pipe wall up to 59% of the maximun value of 0,2 mg/dm2 which is provided for by law for pipes utilized for conveying drinking-water.

In order to avoid an early ageing of the PE. X liner caused by the metal, for example copper, it is suggested the use of anti-oxidizer agents, for example in case of copper phenolic anti-oxidizers of the type IRGANOX 1076 or of the type IRGANOX MD 1024. According to the invention the anti-oxidizer additive is added in small quantities directly to the plastic stock by the plastic manufacturer, and this for at least the layer 4, and preferably for both the layers 4 and 3 in order to assure a uniform distribution of said additive and, therefore, a reliable uniform protective action onto the complete outer surface of the copper pipe 2.

In the embodiment shown in figure 2 the composite pipe 1 of metal-plastic according to the invention is further provided with an external layer 5 of a plastic material acting as a so-called oxygen-tight barrier, for example of EVOH, ethyl-vinyl copolymer (reference test DIN 4726). The thickness of said anti-oxygen barrier 5 is of a few micron, for example of 2-10.

According to the invention also the anti-oxygen barrier 5 can be advantageously applied by means of a separate extrusion or by means of a co-extrusion while manufacturing the composite pipe 1.

Possible leaky points in the metal pipe 2 by using the composite pipe 1 in hydro-thermo-sanitary plants will allow the conveyed water to enter into contact with the plastic layers 4 and 3, whereby layer 3 acts as an outer containing and sealing sheath.

Said outer pipe 3 avoids on the one hand water leakages in the plant and will permit on the other hand to reliably keep the"drinkability"of the conveyed water. At the same time the outer layer 3 will efficiently avoid an access of corrosive substances to the metal pipe 2, for example a copper pipe, while assuring the integrity thereof in the time. The adoption of an anti-oxidizer agent at least in layer 4 uniformly in contact onto the complete outer surface of the metal 2, and preferably in both layers 4 and 3, will permit to surely avoid the early ageing phenomena of said layers 3 and 4.

A dark coloring, for example a black one, of the outer layer 3, which coloring may be achieved for example by adding carbon black, will impart to the pipe a high resistance against the uviolet action.

In the embodiment of figure 2 said anti-oxygen barrier 5 will permit to surely avoid degradation phenomena of the plastic material of layer 3 due to the oxygen action in any installation or laying environment.

By providing the composite pipe 1 with a known not shown outer insulating liner or pad made for example of closed cell foamed polyethylene it is possible to efficiently avoid thermic leakages, for instance in heating plants.

Using the composite pipe 1 in conditioning plants, the outer layer made of plastic material 3 with or without the anti-oxygen barrier layer 5 due to its insulating action will contribute to avoid the formation of condensate onto the pipe outside, and this advantageously in presence of the above stated small thickness of the metal, for example copper, steel and so on, whereby the composite pipe will be therefore light and cheap as stated above.

The condensate suppression action in conditioning plants will be obviously increased by adopting composite pipes 1 with an external insulating liner as set forth above.

From the foregoing description it is apparent that the composite metal- plastic pipe according to the present invention permits to efficiently achieve the stated objects ant to obtain the mentioned advantages. In practice it will be surely possible to use any suitable metal, and for the layers 3 and 4 plastic materials which are suitable for conveying drinking- water different from the stated ones. It will also be possible to vary the thicknesses stated for the different layers, and in case to complete the composite pipe with layers of specific materials for specific uses as known to those skilled in the art without departing from the scope of the present invention.