PLASTIC TUBE WITH IMPROVED ONE, ACOUSTIC CHARACTERISTICS AND MANUFACTURING PROCESSES OF SUCH A PIPE

STANDARD PATENT Application Number: Lodged: Invention Title: PLASTIC PIPE WITH IMPROVED ACOUSTIC PROPERTIES, AND PROCESS FOR MANUFACTURING SUCH A PIPE The following statement is a full description of this invention, including the best method of performing it known to us :- PLASTIC PIPE WITH IMPROVED ACOUSTIC PROPERTIES AND PROCESS OF MANUFACTURING SUCH A PIPE. The invention relates to a plastic tube, obtained by the extrusion or injection-moulding of a plastic, of the type containing fillers, in particular inorganic fillers, in order to improve the acoustic insulation properties of the pipe. Plastic pipes filled with barium sulphate which, compared to conventional pipes, have improved acoustic insulation properties are known from EP-A-0 254,375. The presence of such inorganic fillers in the resin serving for the manufacture of the pipe modifies the rheology and the abrasive properties of the material. This may result in impairment of the surface finish of the endproduct, and more rapid wear of the injection-moulding nozzles and extrusion dies. These problems become more acute when, for example, the proportion of filler increases and/or the filler which is used is more effective for acoustic insulation of the pipe but has a deleterious effect on the rheology and/or abrasive properties. The object of the invention is, above all, to provide a plastic pipe obtained by extrusion or injection-moulding which, while still having good acoustic insulation properties, also has a very good surface finish of the endproduct. It is also desirable for the mechanical strength of the pipe and its fire resistance to be as good as possible. It should be noted that the term "pipe" used in the description and the claims is to be understood in a very general sense as also encompassing pipe parts of special shapes, such as elbows or fittings. The present invention relates to a plastic pipe obtained by co-extrusion of a plastic containing fillers for improving its acoustic insulation properties, the plastic pipe comprising at least two coaxial layers which lie next to each other, the two axial layers being an outer first .ayer made of a conventional plastic, and a second inner layer made of a plastic containing zirconium silicate (ZrSi04) filler, each layer being obtained by co-extrusion, the improvement of the acoustic insulation properties being achieved with the filler of the second inner layer. The second layer contributes not only to improving the acoustic insulation properties but also to improving the fire resistance. Preferably, the pipe includes a third coaxial layer, lying radially on the inside of the second layer containing the fillers, this third layer, or inner layer, also being made of a conventional plastic. The presence of the layer or layers of conventional plastic makes it possible, during manufacture in particular by co-extrusion, to decrease the abrasive effect of the fillers of the second layer on the injection-moulding or extrusion equipment. Furthermore, the appearance of the end-product and its surface finish are very similar to those of conventional pipes, thereby facilitating the joining of pipes by bonding and the forming of bell-and-spigot joints, whilst still benefiting from good acoustic insulation. The thickness of the second layer is approximately 70% of the total thickness of the composite wall of the pipe. In the case of a pipe made of PVC (poly(vinylchloride)), the second layer is filled with zirconium silicate with a filler content by weight which is greater than 50 phr (50 parts per i00 parts of resin), this filler content preferably being between i00 phr and 200 phr, and possibly being as high as 300 phr. The abbreviation "phr" will be used to replace the full expression parts per hundred parts of resin", the filler contents being all expressed in parts by weight. Preferably, the density of the second layer is between 2 and 2.5 kg/dm . The surface density of the end-product exceeds the value 2 2 of 8 kg/m and is preferably at least equal to i0 kg/m . Patent Application JP-A-51-95449 mentions the advantage of zirconium compounds, in particular of zirconium silicate (ZrSiO 4 ), as fillers for improving the acoustic insulation properties of PVC. However, this document makes no mention of the problems created by the use of such a filler when manufacturing plastic pipes by extrusion or injection-moulding, problems which the invention aims to solve, while maintaining the improvements in properties, in particular the acoustic insula- tion properties, due to the use of zirconium silicate as filler. In the case of PVC pipes, it is advantageous, in the case of the second filled layer, to use a mixture of virgin material and recycled material, in particular with contents lying between 50 phr and I00 phr of recycled material. The invention also relates to the use, for manufacturing a pipe, of plastic of the polyolefin (PE = polyethylene, PP = p01ypropylene, copolymer) type filled with ZrSi0 4 , this material being compounded in a preliminary compounding (mixing) step. The material may be obtained from a resin in the form of granules and may undergo a first extrusion followed by granulation and a second extrusion. The invention consists, apart from the arrangements indicated above, of a certain number of other arrangements which will be dealt with more explicitly below with regard to illustrative embodiments which are described with reference to the drawing appended hereto but which are in no way limiting. ., Figure i, of this drawing, is a diagrammatic perspective view of part of a .pipe according to the invention. Figure 2, finally, is an enlarged cross-section of the pipe shown in Figure I. The plastic pipe T shown in Figure 1 is intended for transporting liquids, more particularly for the disposal 'of waste water so that the falling of the liquids inside this pipe generates noise. It is therefore sought to improve the acoustic insulation properties of the pipes T produced by extrusion or injection-moulding using a thermoplastic by incorporating fillers into the plastic, in particular inorganic fillers, suitable for increasing the surface density and for improving the • acoustic insulation. These fillers are in addition to the conventional fillers such as calcium carbonate CaC0 3 , or zinc oxide ZnO, or having a fire resistance effect, or other fillers. These additional fillers, the content of which is generally high in order to increase the acoustic attenuation effect, modify the rheology of the material and may, on the one hand impair the surface finish of the endproduct and, on the other hand, accelerate the wear of the equipment used to produce the pipe because of the increase in the abrasiveness of the compound. This wear may occur in particular within the injection-moulding nozzles or extrusion dies and within screw/barrel assemblies. According to the invention, the pipe T has a composite wall, as may be seen in Figure 2, comprising at least one outer layer 1 made of a conventional plastic and another layer 2, which is coaxial and lying radially towards the inside, containing the fillers C suitable for improving at least the acoustic insulation properties. Preferably, an inner coaxial layer 3, also made of a conventional plastic, is provided. The filler C of the layer 2 is advantageously formed by zirconium silicate (ZrSiO 4) used either in its natural state (ore having a purity equal to or greater than 50%) or purified. The thermoplastic used is preferably PVC, polypropylene, polyethylene or their respective copolymers, or ABS (acrylonitrile-butadiene-styrene copolymer). The content of the filler C is adjusted so that the Surface density of the end-product exceeds a value of 8 kg/m 2, thus making it possible to confer on the end pipe an effect whereby the noise generated in situ by the flow of liquids is acoustically attenuated considerably. In particular, in the case of a PVC pipe T, the external layer i and the internal layer 3 are made of a conventional rigid PVC formulation which is customary for pipes and fittings used in the building industry, with a filler content by weight of from 0 to 30 phr; the central layer 2, or core, is made of rigid PVC highly filled with zirconium silicate with a filler content which is greater than 50 phr, possibly being as high as 300 phr, and preferably lying between I00 phr and 200 phr. The fire resistance of such a product may enable it to be classified as "MI", according to building specifications, merely by adding from 0.i to 0.2 phr of fire-retarding agents such as ZnO in the outer layer 1 and inner layer 3. The smoke evolved by forced ignition is greatly reduced, the zirconium silicate having a fireretarding effect. The presence of zirconium silicate makes it possible to dispense with introducing fire- retarding additives in the central layer. The PVC/zirconium silicate compound has better extrudability (ease of extrusion) than with other fillers; this property combined with the fire-retarding effect of the zirconium silicate allows the use of high contents of PVC coming from "bottle recycling". It has been noticed that the addition of recycled PVC is beneficial and leads to an additional improvement in the extrudability, in particular in the feed zone of the screw. For the central layer 2, it is recommended to add phr of reground material coming from PVC bottle recycling, in the case of a PVC pipe T. The composition and particle size of this regrind facilitates the flow of the material (within the mixer and in the feed hopper of the extruder), as well as good gelling of the material in the body of the extruder. The presentation (surface appearance) and use (bondabil ty) of the pipes and fittings according to the invention are identical to those of conventional pipes by virtue of the outer layer 1 and, possibly, of the inner layer 3. The outer layer 1 and the inner layer 3, made of rigid or conventional PVC filled with calcium carbonate, have a relative density of approximately 1.5. The central layer 2 filled with zirconium silicate with a filler content by weight of between i00 ph r and 200 phr has a density of between 2 and 2.5 kg/dm 3. The central layer 2 has a thickness (radial dimension) of about 70% of the total thickness of the wall of the pipe T. Preferably, the total surface density is about I0 kg/m 2. By virtue of this structure, the requirements demanded of the pipes are achieved: joining the pipes by bonding, forming bell-and-spigot joints, resistance to chemicals, fire resistance, in addition to acoustic insulation performance characteristics. Furthermore, the impact strength of the pipe is also improved. Zirconium silicate has a high natural hardness which increases the abrasiveness of the compounds incorporating it. By virtue of the composite structure of the invention, the impact on this increase in abrasiveness is moderated in the case of PVC pipe extrusion. In order to obtain a PVC compound with i00 phr of zirconium silicate, starting from a resin which is in the form of a powder, the compounding of the compound is carried out in conventional equipment. In order to obtain a polypropylene (PP) or polyethylene (PE) compound with at least I00 phr of zirconium silicate, starting from a resin which is in the form of granules, the process of passing via a first extrusion, followed by granulation and then a second extrusion, is preferable. In conclusion, the extrusion of the pipes with a composite structure of the invention is possible with standard equipment. In the case of PVC, the incorporation of external reground material makes it easier for the compounds (mixtures) to flow in the extruder. The approach taken by the invention makes it possible to decrease the wear of the equipment and to improve the impact strength of the end pipe. EXAMPLES In order to compare the mechanical and physical properties, the fire resistance and the acoustic insulation due to the structure (dimensions) and to the materials (nature of the soundproofing filler), the following pipes were extruded, all with an external diameter of i00 mm: pipe !:-covers PVC pipe having a compact wall (a single layer) with i00 phr of ZrSi0 4 ; total wall thickness: 2.91 mm; surface density: 5.99 kg/m 2. pipe la: the same as pipe I, but with a wall thickness of 4.47 mm. pipe 2: PVC pipe having a compact wall (a single layer) with I00 phr of Ba S0 4 ; total wall thickness: 2.58 mm; surface density: 5.25 kg/m 2. pipe 3: composite pipe according to the invention (three layers: an outer and an inner layer of conventional PVC and a filled central layer or core), the core of which is filled with I00 phr of ZrSi0 4 ; the total wall thickness is 4.28 mm; surface density: 7.97 kg/m 2. pipe 4: composite pipe according to the invention (three layers: an outer and an inner layer of conventional PVC and a filled central layer or core), the core of which is filled with i00 phr of ZrSi0 4 ; the total wall thickness is 5.5 mm; surface density: 10.34 kg/m 2. Mechanical Properties. Tests have shown that the mechanical properties of pipes la, 3 and 4 were superior to those of pipes 1 and 2; this can be explained in part by the total wall thickness being greater in the case of pipes la, 3 and 4. However, one mechanical property which is very substantially improved is that of the impact strength. This strength is determined by a standardized test consisting in dropping a mass of 0.8 kg from a height of 1.20 m, at a temperature of 0°C, onto the pipe to be tested. The number of fractures produced on a series of pipes of the same type subjected to this test is noted. The results are as follows: pipe i: 25% fractures pipe la: 40% fractures pipe 2: 100% fractures pipe 3: 0% fractures pipe 4: 3% fractures. The difference in wall thickness of the pipes tested cannot alone explain this very significant improvement in the impact strength of the pipes according to the invention. The impact strength of a pipe is, however, not necessarily improved by the increase in thickness; on the contrary, a thinner-walled pipe, which is therefore more flexible, may better withstand the impact test, in which a mass falls on an annular test piece, than a thickerwalled pipe subjected to the same test conditions. The improvement in the impact strength is due for the most part to the structure according to the invention, with the presence of an external skin (and generally of an internal skin) of conventional plastic, in particular of lightly fi 1 led standard PVC. Fire Behaviour. Tests have shown that the fire resistance is improved when ZrSiO 4 is used as filler. Adding a soundproofing filler to the core of a composite pipe according to the invention results in a large reduction in smoke production. These results were particularly good with pipe 4, having a surface density Acoustic Insulation (Attenuation Index). Airborne-noise insulation tests were carried out. Principal of the measurement: The difference in sound pressure levels in two rooms separated by an acoustic partition, through which the pipe to be tested passes, is measured. A noiseemitting source is placed in one of the two rooms close to the open end of the pipe. In the other room, a microphone is placed close to the pipe, the end of which is closed off by a plug of glass wool. The difference in the sound pressure levels produced between the two rooms is measured over a spectrum having a wide band ranging from 630 to i0,000 Hz. The measured value is a linear overall level expressed in dB. In order to take into account the ear's sensitivity to the various frequencies, the value is weighted in dB(A). The frequency range involved in water flow lies between 1250 and i0,000 Hz. The values given below are values in dB(A) weighted over this range. pipes Diam. Thick. Wall Surface Insulation in mm in mm Density Density in dB(A) g/cm 3 kg/m 2 from 1250 to 10,000 Hz St. cel. IOO 3.2 0.868 2.78 42.8 PVC St. co. i00 3.0 1.431 4.31 46.2 PVC pipe 3 i00 4.28 1.862 7.97 51.3 pipe 4 I00 5.50 1.880 10.34 53 .2 PP/BaSO 4 ii0 5.64 1.888 10.65 53 .4 Compact The first column in this table defines the type of pipe: standard cellular PVC (St. cel. PVC); standard compact PVC (St. co. PVC) ; pipe 3 according to the invention defined above; pipe 4 according to the invention, defined above; compact pipe made of polypropylene filled with BaSQ 4 . The next columns give: the external diameter of the pipe in mm; the total wall thickness in mm; the wall density 3 2 in g/cm ; the surface density in kg/m and the overall weighted insulation in dB(A) from 1250 Hz to i0,000 Hz. The results of the tests show that pipe 4 made of composite PVC having an overall surface density of approximately 10.30 kg/m provides acoustic insulation very close to that of a pipe made of polypropylene PP filled with i00 phr of BaS0 4 by weight, the thickness of which is however slightly greater (5.64 mm as opposed to 5.50 mm). In pipe 4 of the invention, only 70% of the thickness of the wall is filled. The loss of acoustic insulation in the case of pipe 3 of equivalent structure, but having a thickness of 4.28 mm and a surface density of approximately 8 kg/m , is approximately 2 dB(A). The improvement in insulation provided by pipe 4 compared to conventional pipes on the market is from 8 to I0 dB(A). It should be noted that the results given correspond to a laboratory set-up which does not take full account of the actual configuration in which the pipe is placed during its use. The accessories and parameters, such as Y-branch, clips, flow rate of the flowing water, etc., also have an impact on the results. "Comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.