The present invention relates to a multilayer plastic pipe, preferably intended for water piping in central heating systems. The multilayer pipe comprises an inner core and an outer barrier layer which is surrounding the inner core and is made of a material which prevents oxygen diffusion through the multilayered pipe. The invention also relates to the use of a multilayer plastic pipe.
Plastic pipes have previously been used, for example, for water distribution. Such pipes are preferably made of polyolefins, such as cross-linked polyethylene (PEX), polypropene or polybutylene. Plastic pipes which are to be used for water distribution in heating systems, such as for example, those for floor heating or for radiator systems, must be provided with an oxygen barrier layer so as to prevent oxygen diffusion into the water and thus causing corrosion on the metal components which are part of the heating systems.
In order to prevent oxygen diffusion through the pipe, it is a commonly used method to coat the outer surface of the pipe with a barrier layer of a material which prevents diffusion, such as for example, ethylene vinyl alcohol (EVOH). An intermediate layer of an adhesive material, an adhesive layer, may be provided between the inner core and the barrier layer, said intermediate layer being effective to create or to increase the adhesion between the inner core, such as a PEX-pipe, and the outer barrier layer.
An advantageous technique for connecting plastic pipes with different types of couplings and fittings in water distribution systems is previously known, i.a. from SE-C2-500665 and SE-C2-501555. The described technique is used for pipe couplings for plastic pipes having viscoelastic properties, or memory properties, i.e. plastic pipes having a tendency to automatically shrink back to their original shape after, for instance, an expansion. A pipe material having this property is, for example, cross-linked polyolefin, especially cross-linked polyethylene (PEX). The mechanical properties of the material allows a deformation of the pipe at temperatures as low as at least xe2x88x9220xc2x0 C. In order to mount the plastic pipe on the pipe coupling, the end of the plastic pipe is expanded and a nipple is inserted into the expanded end of the pipe and is kept in the pipe until the pipe end shrinks back over the nipple. This known technique is advantageous as it is quick and easy to preform and provides a safe and water-tight coupling.
In accordance with the technique described above, the pipes are substantailly expanded in a radial direction. This has proven to function very satisfactorily when PEX-pipes are being used. However, this advantageous coupling technique is disadvantageous when it is used on pipes having an oxygen barrier layer as described above. In the process of an substatial, radial expansion of the pipe, cracks may appear in the outer barrier layer, said cracks occurring especially at low temperatures, such as temperatures below about 15xc2x0 C. The cracks which occur in the barrier layer have shown a tendency to propagate into the pipe wall during the expansion process of the pipe, thus causing pipe fractures leading to leaking pipe couplings.
Attempts have been made to solve the above problem by modifying know barrier materials. The object has been to try to find modified barrier materials which may withstand an expansion without cracking. So far, a modified barrier material which provides an acceptable solution to the expansion cracking problem has not been found.
The object of the present invention is thus to provide a solution to the above problem, in that the advantageous coupling technique may be used on the plastic pipes which are specially made for use in heating systems.
This object has been achieved according to the invention by means of a multilayer plastic pipe having the characterizing features as defined in the claims.
The inventive solution provides the advantage that the advantageous coupling technique, which is quick, efficient and inexpensive, may be applied to a hole range of different pipes. This means that plumbers working with installations at the site do not require different couplings for different types of pipes, but may instead use just one type of couplings.
According to the invention the problem has been solved by coating the plastic multilayer pipe with an additional layer of an adhesive material, an adhesive layer. The adhesive layer may consist of the same or similar material as the intermediate adhesive layer which is applied between the inner core and the barrier layer so as to increase the adhesion between the layers. The inventive multilayer pipe which is coated with an outer adhesive layer can be radially expanded without crack formation occurring in the barrier layer or in other parts of the pipe wall.
In a plastic pipe having an outer layer, such as a barrier layer, stress concentrations naturally occur especially in the outer layer, when the pipe is expanded. Therefore, stress cracks appear mainly in the outer layer and these cracks may propagate, as described above, into the pipe wall during the expansion of the pipe and cause pipe fractures. According to the invention the outer barrier layer is coated with an additional layer which in itself is sufficiently pliable to absorb stress concentrations and which also adheres to the brittle barrier layer such that this layer is xe2x80x9csupportedxe2x80x9d. This prevents or reduces the tendency to form cracks in the pipe wall.
The mechanical properties of crystalline materials, and also of semi-crystalline materials such as for example EVOH, depend among other things upon the crystalline structure of the material. The crystalline structure is dependent upon the rate of crystallisation, i.e. the cooling rate. On extrusion of a plastic pipe with a barrier layer, the outer layer is exposed directly to ambient air outside the extruder die. An extruded pipe with no additional outer layer is subjected to rapid cooling and crystallisation. On extrusion of a pipe having an outer adhesive layer according to the invention, the cooling of the barrier layer is slower and the thus obtained crystalline structure of said barrier layer has improved mechanical properties.
The barrier layer of a pipe with no additional outer layer is directly exposed to air when it leaves the extruder die at a temperature of above 200xc2x0 C. An oxidative degradation of the barrier layer takes place at this stage which deteriorates the mechanical properties of-the material. In a pipe coated with an additional layer outside the barrier layer according to the invention, the outer layer protects the barrier layer against said degradation during the cooling period and therefor the mechanical properties of the barrier layer is improved.