The invention relates to a strong thermoplastic composite pipe and to a method for its manufacture. The pipe according to the invention is made up of a thermoplastic core pipe onto which there has been connected seamlessly a composite material layer made up of reinforcement fibers and a thermoplastic.

Pipes are used, for example, for the transport of liquids and gases and as various structural components in machines and devices, in transport vehicles, in the building industry, etc. By the use of plastic pipes, significant advantages can be achieved over metal pipes in a number of applications. Typical advantages of plastic pipes as compared with metal pipes include their light weight, resistance to corrosion, moldability in manufacture, and electrical and thermal insulation capacity.

Plastic pipes are typically manufactured by extrusion. Reinforced plastic pipes are most commonly manufactured by pultrusion, winding, pull winding, or pressure molding.

Non-reinforced plastic pipes are manufactured, for example, from PVC, polyethylene, polypropylene, polybutene, and crosslinked polyethylene. Reinforced plastic pipes are commonly manufactured from glassfiber and a thermosetting plastic, which may be polyester, vinyl ester or epoxy.

It is known that structures light in weight and resistant to corrosion are achieved by using thermoplastic pipes. The problems involved with thermoplastic pipes typically include low mechanical properties and susceptibility to creep when loaded. Furthermore, they have a low impact resistance at low temperatures, and for pressure resistance the pipes must be made thick-walk .

It is known, on the other hand, that pressure-resistant and rigid structures can be obtained by using reinforced-plastic pipes. However, reinforced-plastic pipes are easily damaged by impact, whereupon they lose some of their mechanical strength properties and become susceptible to environmental effects such as corrosion. Furthermore, the wear resistance

of reinforced-plastic pipes is in certain conditions low.

Efforts have been made to rectify the above-mentioned poor properties by making composite-material pipes by depositing a reinforced thermosetting plastic layer onto a thermoplastic pipe. With pipes thus manufactured, high interior resistance to wear and chemicals is achieved, as well as good pressure resistance and rigidity. However, brittleness typical of thermosetting plastics makes the pipe susceptible to rupture under impact. Thus the outer pipe made of a thermosetting plastic may rupture, whereupon its structure becomes susceptible to corrosion and its mechanical strength is reduced. Furthermore, sufficient adhesion is not achieved on the interface between the thermoset¬ ting plastic and the thermoplastic, and therefore delamination will occur on the interface, i.e. the layers will detach from each other when the pipe is subjected to sufficient stress. This phenomenon will cause weakening of both the mechanical and the chemical strength of the pipe.

Furthermore, efforts have been made to rectify the above-mentioned drawbacks of plastic pipes by interconnecting thermosetting plastic and thermoplastic pipes in different orders, or thermoplastic pipes have been interconnected with other thermoplastic pipes or with thermoset pipes so that their interfaces are tightly fitted to each other. However, these constructions do not eliminate from the pipes the discontinuity points due to the interfaces, and may cause weakening of the structure owing to the above-mentioned damage by impact, to material-specific thermal expansion coefficients or elongations of the different pipe types.

To eliminate the discontinuity points at the connection points between the different layers of a pipe, US patent publication 3 900 048 discloses a method of manufacturing a reinforced-plastic pipe wherein a glassfiber-reinforced, thermoplastic, non-crosslinked polymer is connected onto a thermoplastic core pipe by means of a solvent. According to the system disclosed in the publication, the clear interface between the layers can be caused to disappear by means of the solvent.

It is a prerequisite to the success of the method according to the said publication that the thermoplastic pipe and the polymer matrix of the glassfiber-reinforced polymer layer are

soluble. However, materials which are not soluble, or are very poorly soluble, are also used in pipes. The dissolving of a polymer is in many cases time-consuming, and therefore such a method is often not suitable for practical applications. For example, polypropylene, which is used as a preferred alternative in the composite pipe according to the present invention, is a very poorly soluble material. Furthermore, non-desirable solvent residues may be left in the pipe from the solvent used.

State-of-the-art pipes of composite material have, as described above, weaknesses in their chemical-mechanical properties, mainly owing to the discontinuity points between the different layers of the pipe. Interconnecting the different pipe layers by dissolving is also not widely applicable to the manufacture of pipes.

The object of the present invention is to provide a strong thermoplastic composite pipe, the various layers of the pipe being interconnected seamlessly.

The object of the invention is thus to provide a strong thermoplastic composite pipe which has good pressure resistance properties, high impact resistance also at low temperatures, high wear resistance, high rigidity, and low creep.

It is also an object of the invention to provide a thermoplastic composite pipe which has all the other good chemical-mechanical properties typical of a thermoplastic pipe.

Thus it is an object of the invention to provide a strong thermoplastic composite pipe which has no detrimental discontinuity points between the different layers.

Furthermore, it is an object of the invention to provide a strong thermoplastic composite pipe which may contain one or more different types of plastic.

It is a further object of the invention to provide a strong thermoplastic composite pipe of the type described above, by a method which is simple and rapid in terms of manufactu¬ ring technology.

It has now been observed, surprisingly, that it is possible to achieve the objects presented

above with a strong thermoplastic composite pipe according to the invention, which is characterized in that a thermoplastic core pipe and a surrounding composite material made up of a thermoplastic and continuous reinforcement fibers are fused seamlessly to each other by means of heat. Thus there are left between these layers no disadvantageous discontinuity points which would weaken the properties of the pipe. Thus, with a pipe according to the invention it is possible to achieve high pressure resistance, high impact resistance also at low temperatures, high wear resistance, high rigidity, low creep, and additionally all the other good properties typical of a thermoplastic pipe.

The high pressure resistance of a pipe according to the invention is achieved so that the continuous reinforcement phase will bear the stress to which the pipe may be subjected, so that the core pipe will not under pressure expand, stretch, or develop local strictures critical to pressure stress.

The composite materials made up of a thermoplastic and a continuous reinforcement phase, used in the composite pipe according to the present invention, can be manufactured by various impregnation techniques in which continuous reinforcement fibers are im¬ pregnated with a thermoplastic. In the composite materials it is also possible to use fabrics, mats, or continuous reinforcement structures of other types, which have been contacted with some thermoplastic. The reinforcement materials used in the composite structure may be, for example, glassfibers, carbon fibers, aramide fibers, polyethylene fi¬ bers, boron fibers, organic fibers, and other corresponding fibrous reinforcement mate¬ rials. The said composite materials are called thermoplastic prepregs, i.e. pre-impregna- tes.

The above-mentioned composite materials made up of a thermoplastic and a continuous reinforcement phase can be used for the manufacture of a pipe according to the invention, either as a thermoplastic prepreg, i.e. a pre-impregnated intermediate, or by directing the composite material directly from the preparation process onto the thermoplastic core pipe, i.e. by using an in situ method. These methods are described below in the specification.

The said materials of a pipe interconnect seamlessly, their polymer matrices becoming mixed or diffused with each other. Typically, mixing or diffusion occurs when plastics are

in contact with each other in molten state. If mixing or diffusion has not taken place, a discontinuity point can be observed at the interface. When diffusion does occur between the plastics, the thickness of the diffused boundary layer will range from a few tens of Angstrom units (A) up to thick diffused layers of similar polymers the thickness of which may approach the total combined thickness of the said materials, which can be indicated by oo , /Wu, Souheng, Polymer Interface and Adhesion, Marcel Dekker Inc., New York, 1982, p.393/.

In a thermoplastic composite pipe according to the present invention, the thermoplastic core pipe and the thermoplastic matrix of the surrounding composite material made up of a thermoplastic and continuous reinforcement fibers may be of the same or of different types of plastic. Preferably the thermoplastic of the core pipe and the polymer matrix of the composite material are of the same plastic. Usable plastics include any thermoplastics, preferably, for example, polyethylene, polypropylene, polybutene, PVC and ABS plastics. Polypropylene is a plastic very suitable for use in a composite pipe according to the invention.

In a preferred embodiment of the invention, in which the thermoplastic of the core pipe and the matrix of the said composite material are of the same plastic, diffusion of the polymer chains will occur between them under suitable conditions. When the polymer chains become diffused, the connection point will be seamless.

However, the interface between the core pipe and the surrounding composite material can be made seamless also when the thermoplastic of the core pipe and the matrix plastic of the composite material are of different plastics. This presupposes that diffusion or mixing occurs between these plastic materials. In pipe applications, a seamless structure is achieved, for example, by using a PVC core pipe and a polyethylene-based composite material. By using these materials, a strong diffused layer can be produced, having a thickness of, for example, 90000 A. /Wu, Souheng, Polymer Interface and Adhesion, Marcel Dekker Inc., New York, 1982, p. 393/. There is no clear interface in the said boundary layer; there is a seamless shift from one material to the other. However, this does not exclude the possibility of achieving a seamless structure with boundary layer thicknesses smaller than that cited. In this case it is possible to use as the plastic matrices

of the core pipe and the said composite material also other thermoplastics which are advantageously miscible or diffusible with each other.

In the manufacture of a composite pipe according to the invention, the diffusion between the thermoplastic core pipe and the polymer matrix of the composite material surrounding it, i.e. the seamless interconnecting of the layers, is effected by fusing them together by means of heat. An addition of pressure at the connection point of the melt fronts of the core pipe and the composite material will promote the diffusion. Furthermore, diffusion can be enhanced by using compatibilizers by means of which the mixing of the plastic materials with each other is promoted.

In one embodiment of the invention, a thermoplastic composite pipe according to the invention is prepared in such a manner that a thermoplastic core pipe and a composite layer made up of a thermoplastic and continuous reinforcement fibers are interconnected by means of a third thermoplastic material or thermoplastic composite material by fusing it by means of heat between the aforementioned layers, so that it will interconnect in mol¬ ten state both with the thermoplastic core pipe and with the first mentioned composite material made up of continuous reinforcement fibers and a thermoplastic.

The bond between the core pipe and the said composite material is at its most advanta¬ geous when a potential rupture will not occur at the imagined connection point but the rupture will take place either in the material of the core pipe or in the matrix material of the composite material, or randomly alternately in each.

A strong thermoplastic pipe according to the invention can be manufactured, for example, by winding onto a thermoplastic core pipe the above-mentioned composite material containing a thermoplastic matrix. A seamless structure according to the invention is achieved in this manufacturing method by fusing by means of heat the matrix polymer of the composite material and/or the thermoplastic core pipe, either entirely or in part, and then by interconnecting the layers in molten state.

The composite material to be wound may be wound onto the core pipe by winding it onto the thermoplastic core pipe at a winding angle of 0 - 180°. The selection of the winding

angle depends on the intended use of the pipe and the stresses it will be subjected to. The angle is selected so that the capacity of the pipe to bear axial and radial loads will be optimal. For a pressure pipe the preferred winding angle is 50 - 60°. For a structural pipe the preferred winding angle may be approx. 0°. It is always preferable to use a winding angle by means of which the bands of composite material can be placed adjacent to each other to form an even layer.

A strong thermoplastic composite pipe according to the invention may also be manufactu¬ red by connecting onto a thermoplastic core pipe, in a suitable order and at suitable angles, various composite materials in which the thermoplastic matrix is the same but which may contain different types of continuous reinforcement fibers.

In general it is expedient to wind 3-5 layers of composite materials onto the core pipe, depending on the materials used. Likewise, the order of the composite materials in the various layers depends on the components used for the various layers. Often it is prefera¬ ble to use the same components in the different layers.

After the winding or other manufacturing method, a thermoplastic composite pipe according to the invention may be coated with a shielding layer, such as a layer of a thermoplastic or a thermosetting plastic and/or some other coating material which will adhere to the outermost composite material layer and the purpose of which is to shield the thermoplastic composite pipe from impact, radiation, thermal action, burning, cooling ac¬ tion, corrosion, and/or other environmental effects.

In a composite pipe according to the invention, the composite material made up of continuous reinforcement fibers and a thermoplastic and/or the thermoplastic core pipe may, when necessary, additionally contain some other filler, admixture, and/or rein¬ forcement material.

The manufacture of a strong thermoplastic composite pipe by heat fusing can be carried out advantageously, as set forth earlier in the present specification, by using the so-called prepreg method or the in situ method, which methods are described in greater detail below.

Winding of a pipe by the prepreg method

A strong thermoplastic composite pipe according to the invention may be manufactured by the prepreg method by connecting onto a selected thermoplastic core pipe a composite material made up of a thermoplastic and a continuous reinforcement phase, in such a manner that a tape-form composite material of a suitable width, selected according to the diameter of the core pipe and the selected winding angle, is directed from a roll onto the circumference of the rotating core pipe. The seamless fusion of the composite material tape and the thermoplastic core pipe is effected by heating the composite material to its softening or melting point before directing it onto the surface of the core pipe. In addition, the surface of the core pipe may also be heated at the fusion point so that the outermost surface of the pipe will be at a temperature at which softening and/or melting may occur. The fusion of the molten-state thermoplastic phases to each other is ensured by tensioning the composite material tape being wound over the core pipe; the tensioning will produce a pressure advantageous for the fusion at the interconnection point of the said molten phases. Fusion will occur when the melted interconnection point of the composite material and the core pipe cools from the molten-state temperature, while the said composite material tape is still subject to tension. The fusing of the composite material layers subsequent to the first composite material layer on the circumference of the strong thermoplastic pipe blank is carried out in a corresponding manner. The fusion may also be ensured by pressure molding the pipe at the fusion point by means of a pressure roll or the like.

Winding of a pipe by the in situ method

A composite material of a thermoplastic and a continuous fiber reinforcement phase is prepared by an impregnation technique, known per se. suitable for the making of a thermoplastic tape with continuous-fiber reinforcement. The composite material tape coming from impregnation, in molten state with respect to its thermoplastic matrix, is directed onto the circumference of a rotating thermoplastic core pipe at a selected angle so that the rotating core pipe will pull the tensioned composite material tape from the impregnation process, whereby a pressure advantageous for fusion is produced at the interconnection point of the said tensioned composite material and the core pipe. In this

method, also, it is advantageous additionally to heat the core pipe to its softening temperature at the interconnection point of the core pipe and the composite material.

A thermoplastic composite pipe according to the present invention may also be manufactu- red by using other pipe coating methods known in the manufacturing technology, such as pultrusion, pull winding, braiding, flat winding, or pressure molding. Regardless of the method used, it is essential that the interconnection point between the composite material and the core pipe is heated to the softening or melting temperature of the thermoplastic in order to produce a seamless interconnection point.

The invention is described below in greater detail with reference to Figures 1 and 2 and to Example 1.

Figure 1. A cross section of the structure of a strong thermoplastic composite pipe according to the invention.

Figure 2. A partial enlargement of the wall structure of a thermoplastic composite pipe according to the invention.

Figure 1 shows a thermoplastic composite pipe 1 according to the invention, which has, around the thermoplastic core pipe 2 indicated with horizontal hatching, a composite material layer 3, indicated by vertical hatching, made up of thermoplastic and a con¬ tinuous reinforcement phase.

The partial enlargement of the pipe wall depicted in Figure 2 shows how the core pipe 2 and the composite material layer 3 surrounding it are interconnected seamlessly within their interconnection zone 4, which is indicated by cross hatching. The interconnection zone is in reality not a zone clearly delimited by the inner and the outer layers; all interfaces have disappeared owing to diffusion.

Example 1

A strong thermoplastic composite pipe was prepared by the prepreg method by winding onto a polypropylene core pipe having a diameter of 110 mm and a wall thickness of 5 mm a tape-form composite material made up of polypropylene and parallel continuous glassfiber and having a glassfiber/polypropylene ratio of 70 wt. %/30 wt. % . The width of the tape was 6 mm, and its thickness was 0.3 mm. Three layers of the tape were wound onto the core pipe at winding angles of +53°, -53° and 90°. At the fusion stage the composite material was heated by hot air blowing at its interconnection point to a temperature of 195-205 °C. The pressure resistance of the composite pipe thus manufactu- red was 120 bar.