WO1990012680A1

WO1990012680A1 - Method and apparatus for the manufacture of porous pipe

Info

Publication number: WO1990012680A1
Application number: PCT/US1990/002195
Authority: WO
Inventors: Charles H. Cordewener
Original assignee: Subrain, Ltd.
Priority date: 1989-04-25
Filing date: 1990-04-23
Publication date: 1990-11-01
Also published as: AU5529390A

Abstract

A porous irrigation pipe manufactured by extrusion of a composition comprising vulcanized rubber particles and a thermoplastic binder. The composition is processed under heat and pressure and in the absence of moisture or other volatiles. The total volume of the thermoplastic binder in the composition after processing is lower than the matrix volume between the rubber particles in the composition. After extrusion the composition is permitted to expand in free space allowing the deformed rubber particles to regain their original shape and in so doing shear the thin thermoplastic layers between particles to form a network of pores and interconnected channels.

Description

METHOD AND APPARATUS FOR THE MANUFACTURE OF POROUS PIPE

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to a method for the manufacture of porous pipe by extrusion of a composition comprising vulcanized rubber particles and a thermoplastic binder wherein the total volume of the thermoplastic binder in the composition after processing by extrusion is lower than the matrix volume between the rubber particles in the composition. During extrusion the composition is processed at a temperature higher than the melting temperature of the binder and lower than the temperature at which the rubber particles lose their elastic properties.

2. DESCRIPTION OF THE PRIOR ART

Methods for the manufacture of porous pipes intended for irrigation purposes wherein the pipe is imbedded in the soil are disclosed in Turner U.S. Patents Nos. 4,110,420 and 4,168,799 and in Mason U.S. Patents Nos. 4,517,316, 4,616,055 and 4,615,642. In all of these patents, methods are described wherein the size of the internal and external diameter of the pipe is controlled by the diameter of the die opening or extrusion gate. In addition, the disclosed methods require that the composition contain a certain percentage of water or other non specified volatile matter in order to obtain porosity. None of these patents refer to the elastic properties of the rubber particles or the matrix volume to control porosity.

The adjustment and maintenance of a particular water content in the composition has proven to be difficult because the composition is always exposed to conditions of varying atmospheric humidity. Adjustment of the moisture content by means of a venting device fitted on the extruder, as proposed in U.S. Patent 4,110,420, does not appear to be a successful solution for the manufacture of pipe with uniform porosity over the entire length. The Mason U.S. Patent No. 4,517,316 teaches the use of a two stage process. The composition is first formed into pellets primarily because the water content in the pellets is thought to be more easily controlled. Nevertheless, it is further proposed to pack the formed pellets in tightly closed polyethylene bags and to put these in turn in a protecting container. This procedure is a recognition that the pellets themselves may change their moisture content due to different atmospheric conditions. The second stage involves the feeding of the pellets through an extruder to form a pipe.

It had been shown, as will be discussed in more detail in the following specification, that the use of water as a means to obtain porosity has many serious disadvantages. This is true not only because of the expensive provisions required to maintain the moisture (water) content in the raw material mix but also because of the inability to control the entire production process. Acceptable reproducibility of uniform porosity has not yet been achieved.

In the past, it has been fully impossible to produce a low porosity pipe with a high degree of uniformity throughout the entire pipe production. Low porosity is desirable especially for longer pipes (greater than 100 meters) because of the relatively higher inlet pressures necessary in order to bring the water to the end of the pipe and at the same time ensure the high emission uniformity along the entire length of the pipe. In the case of higher porosities, the necessarily higher inlet pressure in long pipes results in excessive emission levels and too much loss of water in the first part of the pipe. The water may not even reach the end of the pipe.

SUMMARY OF THE INVENTION

The present invent ion provides a method which eliminat es or g reatly diminishes the above-mention ed disadvantages and is based on the insight gained by nψnerous experiments as will be presently described. Crumbed rubber particles in bulk , like most other particles of varying geometrical forms , shapes and sizes, show a certain matrix volume, i.e. hollow spaces between the particles, even at maximum packing rate. In a compressed composition mainly consisting of solid vulcanized rubber particles completely surrounded by molten thermoplastic binder the rubber particles will be subjected to compressive forces but will not be deformed because the particles are not pressured against each other. The rubber particles under compression are initially surrounded with molten plastic material completely filling the matrices, i.e. spaces between particles, the rubber particles will remain distant from each other and, after release of the compressive forces and cooling, no pores are formed. Upon decompression, the rubber particles will simply expand very slightly and return to their original configuration and remain completely surrounded by thermoplastic binder. If, however, the weight ratio of rubber particles and thermoplastic binder is selected such that, when in the molten state, the volume of thermoplastic is smaller than the original matrix volume or space between the uncompressed rubber particles another process occurs. As the composition is compressed and the binder becomes molten, the rubber particles are tightly pressured together and matrix air is driven out, resulting in actual deformation of the particles. Because of the decreased volume of available binder, however, the rubber particles are encompassed with only thin layers of thermoplastic binder. Upon decompression the deformed rubber particles strive to regain their original shape and in doing so shear the thin thermoplastic layers between particles. There is insufficient binder available to fill the increased matrix volume being formed between the rubber particles as the composition expands. The spaces which are not filled with binder now form a network of pores and inter connected channels throughout the composition. There is sufficient binder, however, to maintain the structural integrity of the composition. This method of forming pores is far superior to relying upon a certain moisture content for pore-forming since it can be controlled by utilizing various logical parameters. The formation of a pore network within the composition on decompression is dependent on the weight-ratio between rubber particles and the thermoplastic binder. Stated otherwise, the formation of the pore network is dependent on the ratio of the available decompressed matrix volume or space between the rubber particles to the available volume of binder. The porosity of the pore network is determined in large part by the mentioned weight-ratio or volume-ratio and also by the particle size and particle size distribution of the rubber crumb. Other parameters such as shape of the particles, processing pressure, processing temperature and the type of thermoplastic binder utilized also affect the porosity of the pore network formed.

It has been demonstrated that the presence of any substantial amount of moisture in the composition is a very disturbing factor in the pore forming process. The water content is not a desirable working parameter for obtaining a reproducible porosity. Even if the moisture content is originally known, unknown quantities of moisture escape through the extrusion screw and the feed hopper during processing. Although not yet fully explained, it is known that small differences in the total water content of the starting composition show significantly wide variations in the resulting porosity. It is believed that the compressed water vapor seeks the way of least resistance to the atmosphere after release of the pressure. In doing so, the water vapor acts destructively on the independently proceeding process of pore forming caused by the return of the rubber particles to their original configuration as they push against one another. Any moisture present may also serve to weaken the bond between the binder and the rubber particles thus adversely affecting the uniformity of the porosity. The aim is therefore to obtain a moisture content as low as possible, preferably nil so that the pores are formed in the absence of moisture and other volatiles. If the moisture level rises above 0.45% it actually becomes necessary to dry the starting materials or remove the moisture as by venting before it reaches the extrusion die in order to eliminate adverse effects and obtain uniform porosity. With the concept described, it is now possible to use entirely different factors and parameters to influence porosity.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side elevational schematic of an installation for manufacturing porous pipe according to the present invention; and

Fig. 2 is an axial cross-section of the extrusion die shown in Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION

According to the method of the present invention pores are formed in the absence of moisture and other volatiles, reliance being placed on the control of the weight ratio of thermoplastic binder to elastomeric particles which are extruded under pressures exceeding 200 bar. The hollow extrudate is expanded after leaving the extrusion die in a free space and is thereafter cooled. Preferably the composition is entirely free from materials known to form pores or cells such as moisture or other gas forming or "blowing agents". The extrusion pressure is preferably higher than 250 bar. In order to more or less support the expansion of the hollow extrudate it is recommended that an internal gas pressure be generated and that the extruded material be transported near the end of its expansion through a calibration apparatus while maintaining a relatively thick layer of cooling medium between the interior wall of the calibration tube and the exterior wall of the extrudate.

The resulting pipe possesses a high degree of uniformity in its porosity around its circumference and over its length. The reproducibility of this uniformity is excellent even at low porosities. Upon release of compressional forces on the composition, the initially compressed rubber particles strive to return to their original shape. Because of the action of shearing forces between the rubber particles and the binder matrix, adhesion on the contact-surfaces is decreased. Because the deformed rubber particles return to their original shape, they are pushed away from each other forming interstices thereby increasing the circumference of the pipe and the thickness of the pipe wall simultaneously. The increase in wall thickness can reach 30-40%. In this respect the supporting internal air or other gas pressure is advantageous since it maintains a desired internal diameter of the pipe with the rubber particles being pulled slightly apart in the circumferential direction.

Because a constant speed of pulling of the hollow extrudate material forming the pipe results in a constant pulling force, the return of the rubber particles to their original shape will not only be supported in the circumferential direction but also lengthwise of the pipe. In the radial direction there might be less separation between the rubber particles and it is assumed that the structural integrity of the entire pipe wall is obtained in large part by a network of shifting radial bindings of the thermoplastic binder between the rubber particles.

The maintenance of a layer of cooling medium, for example water, between the interior wall of the calibration tube and the exterior wall of the pipe during the last phase of expansion allows cooling fluid to enter the formed pores thus supporting a rapid freezing-in of the formed pore network. Further cooling to ambient temperature may be accomplished for example in a downstream cooling tank. By contrast, prior art calibrating apparatuses for the manufacture of pipe or hose are known wherein the pipe or hose is held under interior pressure against the interior wall of the calibrating tube. In this type of device, no expansion of pipe or hose is intended as far as applicant knows. The calibration tube has in these devices normally the same interior diameter as the exterior circumference of the extrusion die and is directly linked to the die without forming a free space between the extrusion die and the entrance to the calibration apparatus.

The calibration apparatus according to the present invention has a perforated interior calibration tube with the perforations being connected with a cooling chamber surrounding the calibration tube. The pressure applied by the layer of cooling medium must be so adjusted that the water flows between the calibration tubing interior wall and the exterior pipe wall. The water pressure must not be so high, however, that the extrudate will be deformed against the internal gas pressure. The internal pressure in the formed pipe may be obtained from a gas, for example air, supplied under pressure via a bore in the mandrel of the die. It has been demonstrated that it is not necessary to close the extruded pipe to maintain the internal pressure if the pressure of the compressed gas is high enough at the extrusion die to maintain the desired interior pressure. The compressed air needs to possess a certain pressure at the outset on its way toward the end of the pipe being produced to overcome head loss in the pipe. It has been shown that the tendency of the deformed rubber particles to regain their original shape upon release of compression as they emerge from the extrusion die combine to produce an outward bound force on the hot extrudate. The internal pressure of the compressed gas delivers an additional support of the total forces being re¬ leased for expanding the extruded material.

The thermoplastic binder can be made of a polymer or copolymer of ethene, propenene or butene or mixtures of one of these polymers and/or copolymers. Preferably the binder pos¬ sesses a melt temperature below 160°C for the reason that the processing temperature is then lower than the temperature at which the rubber particles in general start losing their elastic properties. The temperature at which rubber particles start to decompose is around 225°C, at which temperature they start seriously losing their elastic properties also. Especially advantageous is the use of a binder that mainly consists of 30-80% high density and 70-20% low density polyethylene or medium density polyethelene types supplying the desired flexibility of the pipe with sufficient mechanical properties. Certain types of linear low density polyethelene may also be used. The rubber particles may consist of vulcanized natural rubber or of a vulcanized synthetic rubber such as butadiene rubber (BR) , isoprene rubber (IR) or their copolyitjers with polystyrene, isobuthylene or acrylonitril. Terpolymers like ethene-propene-norborneen (EPDM) are also suitable. The rubber particles may be derived from discarded automobile tires, in which case it is strongly recommended that all metal and fiber parts be removed as far as possible since these fibers may interfere with the uniform porosity. The dimensions of the particles are preferably below 600 u (micrqnsj. It has been demonstrated that the particle size and particle size distribution influence porosity, the general •rule being, that the higher the bulk weight or the more densely the particles are packed, the lesser the matrix space and, the lesser the porosity. The particle size and/or the particle size distribution, however, have little or no effect on the uniformity of the relevant porosity per unit area. To ensure that not too many very fine particles are present in the starting composition, it is recommended that these fines e.g. below 30 u be removed by sieving or air classification.

With the method according to the present invention, compositions may be processed in which the composition comprises 80-50 parts by weight of rubber particles and 20-50 parts by weight of thermoplastic binder. To achieve porous pipe suitable for irrigation purposes it is preferable to choose a weight; ratio of rubber particles versus binder larger

► _ than 1 and in particular about 2.33. In order to take maximum _$ * _. »• - -t ^*~ " advantage of the inherent expansion of the composition caused by the return t© the original configuration of the rubber particles after decompression, the ratio between the largest and the smallest size of each rubber particle should be as much as possible equal to 1. The size of the particles themselves should be nearly the same in order to obtain the largest matrix volume, i.e. space between particles, at the outset. Seen in this light, cryogenic ground rubber is generally better suited than ambient ground rubber. The first method results generally in more squarish compact particles while the second method results generally in crumbed material with relatively many long stretched particles.

With the method according to the present invention pipes can be made with an internal diameter of 4 to 100 mm and a wall thickness of 1-40 mm. Pipes of this character are especially suitable for irrigation purposes where the pipes are installed subsurface. A correct choice of wall thickness relative to the interior diameter is important in subsurface installations for mechanical installation and in order to prevent compression or even damage to the pipe from the weight of the surrounding soil or from vehicles passing over the installed pipe.

The greatest advantage gained by the method of the present invention is the production of pipe which has a con¬ trolled and uniform porosity, especially for lower values of porosity. The porosities of a porous pipe according to the present invention expressed in leak rate per meter per hour may be varied from 0.5 liter at an inlet pressure of 5 bar to more than 600 liter with an inlet pressure of 0.4 bar.

Reference is now made to the drawings for a more detailed description of an example of a typical installation.

Fig. 1 is a schematic illustration of a pipe manufac¬ turing installation which includes in series an extrusion machine 1, a calibration apparatus 2, a cooling tank 3 and a pulling and coiling device 4. The extrusion machine 1 includes a barrel 5 having a screw 6 mounted for rotation therein. The screw 6 is driven by motor 7 via a gear box 8. An extrusion die 9 is located on the front end of the extrusion machine 1 and will be presently described in more detail in connection with Fig. 2. The barrel 5 of the extrusion machine is surrounded by a number of heating and cooling jackets 10-14 for the purpose of controlling the temperature of the barrel. The temperature of the extrusion die 9 is separately controlled by means of a fluid which is pumped through the jacket 15 independent from the jacket 10-14. The temperature of the fluid in the jacket 15 is controlled by means of the thermostat 16 which constitutes the circulating fluid temperature control equipment. The infeed end of the extrusion machine is provided with a hopper 17 equipped with a force feeding screw 18 driven via shaft 19 and motor 20 to transport the contents of the hopper into the barrel.

The calibration apparatus 2 is located immediately downstream from the die 9 and includes a perforated tube 21 having perforations 22. The perforations 22 connect the space 23 between the outer surface of the extruded pipe and the tube 21 with the space 24 in the cooling jacket 25. Cooling water is supplied through the inlet 26. The cooling tank 3 may be from 15 to 35 feet long or longer, its length in any particular installation being dependent upon the desired production rate. The tank 3 is fitted with a cooling water supply 27 and a cooling water outlet 28.

The pulling and coiling installation indicated gener¬ ally at 4 consists of a pulling unit 29 and a motor driven coiler 30. The pulling unit 29 is preferably of the caterpillar type having two moving conveyer belts 31 and 32 between which the manufactured pipe is clamped and moved. The puller 29 will be a variable speed unit with a speed of the Coiler 30 being controlled to conform with the pulling speed and may bύ adjusted according to the diameter of the material as it is coiled.

Referring to Fig. 2, the extrusion die 9 may constitute any well known type of extrusion head. In this type of die a spider 34 supports the mandrel 35 within the die cavity and outlet channel. The mandrel 35 is centrally located in the cylindrical space 36 and serves to form an annular extrusion channel 37. The mandrel 35 is provided with an axial-bore 38 which connects with a radial bore or channel 39 extending through a suitable pipe in the fluid jacket 15 and communicates with the outside surface of the extruder hread. The bore 38 and channel 39 are utilized for supplying a pressurized gas to the outside face of the mandrel 35. The gate of the extrusion die is indicated generally at 40 in Fig. 2 and will have an outside diameter which is smaller than the interior diameter of the perforated calibration tube 21.

With the installation described in Figs. 1 and 2, the method according to the present invention can be illustrated and carried out by the following example of practice.

The example to be described relates to the manufacture of a pipe with an exterior diameter of 24.5 mm. A starting composition is made from 70 parts by weight of rubber particles having a size range of from 30-600 u. The composition has 17 parts by weight of low density polyethylene having a density of 0.922 and a melt index I2 ₀f 0.9 and 13 parts by weight of a high density polyethylene having a density of 0.954 and a melt index I2 of 0.35. The polyethylenes used are in granular form. Any possible moisture present in the original components of the composition is reduced by drying to a weight percentage of moisture of under 0.45%, preferably to nil. The drying occurs immediately before the processing begins. The composition described is fed via the hopper 17 with force feeding by the feed screw 18 into the extruder where it is further transported by the extrusion screw 6.

During this transport, the polyethylene binder will melt due to the internal friction generated resulting in the rubber particles being dispersed in a matrix formed by the polyethylene melt. The extruder used may have an interior diameter (D) of 60 mm for example with a length to diameter (L/D) ratio in the order of 25. With the help of the cooling and/or heating jackets 10-14 a controlled temperature profile is maintained along the length of the extruder. The temperature profile mounts from about 30°C at the beginning of the screw 6 to 160-170°C at the end of the screw. The melt is forced through the extrusion channel 37 at a pressure of approximately 350-400 bar. The interior diameter of the extrusion channel 37 is 23.3 mm and the exterior diameter of the mandrel 35 is 18.3 mm. The channel is kept a temperature of approximately 170°C. Due to the pressure at the end of the screw 6 and partly in the extrusion channel 37, the matrix air is driven out and the rubber particles are tightly forced against one another. The rubber particles deform because of their elasticity as previously described and thus the matrix volume of the pressurized composition is reduced. During the approach of the mass of composition toward the annular extrusion gate 40, the pressure is reduced and drops to zero upon leaving the extrusion, gate. As the rubber particles leave the extrusion gate 40 they are covered with the polyethylene melt or binder and begin to regain their original form and shape which exerts a force on the pipe-formed extrudate resulting _ in the tendency to increase the pipe diameter. This expansion force is supported by the admission of compressed air at a pressure of about 1 bar in the hollow extrudate via channel 39 and bore 38.

In the free space between the extrusion gate 40 and the calibration apparatus 2 the extrudate expands as described forming a labrinth of pores in the walls of the hollow extrudate. Before the extrudate becomes too greatly expanded it is pulled into the calibration tube 21 by means of the take-off installation 29. Because of the tensile force applied by the pulling the rubber particles are not only slightly separated in a circumferential direction by radial expansion but are also slightly separated in the lengthwise or axial direction.

As soon as the radially and axially expanded extrudate arrives in the perforated calibration tube 21 it is surrounded or enveloped by a layer of cooling water at about 15°C which is maintained via perforations 22. The interior diameter of the tube 21 at this point is 25 mm. under these conditions, the cooling water seeps into the formed pores and provides a fast setting of the pore system which contains a

__*

^: great number of wall to wall channels or passages. The extrudate is further hauled or pulled through the cooling tank 3 after which the pipe is coiled on a reel. The wall thickness of the pipe is 3 mm and has thus increased more than 20% compared with that of the extrudate leaving the extrusion gate 40. As illustrated in Fig. 2, the wall thickness increases as from a-b and the interior diameter of the hollow extrudate increases in diameter as from c-d.

The measured uniformity of porosity can be expressed in terms of the coefficient of variation (Cv) . This is the standard deviation divided by the average emission per unit length at the design pressure of the irrigation system at a constant temperature. For a porous pipe the usual unit of length is 1 meter, see ASAE specification EP 405 of the American Society of Agricultural Engineers.

The test method used in measuring samples made by the present invention is as follows.

Samples are taken from production at random or for practical reasons after certain lengths have been produced. For example after each 200 m pipe produced, 10 m long test pieces are taken. These test pieces are placed horizontal on a 10 m long testing tank. The test pieces are placed in such a manner that the emitted quantities of water flow freely and directly into the testing tank. The testing tank is fitted with lengthwise separated compartments of 1 meter each with each compartment having a discharge and a collector in the form of a measuring glass. After one end of the pipe has been plugged, the other end is supplied under exact controlled pressure with water for a given time. A pressure regulator and precision pressure gauges are utilized. The water collected in the measuring glasses is accurately read and recorded. Based on the test results the standard deviation and average emission are calculated according to the statistical methods from which the Cv is obtained.

The pipe manufacturer according to the present method and calculated for succeeding 10 m test lengths for several hours on a production run yielded a Cv value of 0.04 which implies a very high uniformity. Variations may appear when testing is made on pipe-lengths made under different production parameters and/or from different batches of raw materials. Nevertheless the method according to the present invention results in production of porous pipe with Cv-values below 0.08 which is excellent compared to known porous pipes. The larger Cv values may be a result of the non-homogenous particle size distribution of the rubber particle component.

While the present invention has been described and illustrated with respect to a specific embodiment, it will be apparent to those skilled in the art that modifications, substitutions and alterations may be made without departing from the spirit of the invention or from the scope of the appended claims.

Claims

What is claimed is:

1. A method of producing a uniformly porous extrudate comprising; forming a composition consisting of elastomeric par¬ ticles evenly dispersed with granular thermoplastic binder, the total volume of said thermoplastic binder being less than the matrix volume of the uncompressed composition defined by the available space between said elastomeric particles, compressing said composition with sufficient force so as to pressure said elastomeric particles against one another causing deformation thereof so as a decrease the matrix volume of the composition, simultaneously subjecting said composition to elevated temperatures above the melting temperature of said binder and below the temperature at which said elastomeric particles lose their elastic properties, whereby said deformed elastomeric particles are surrounded in the compressed state with a thin layer of molten binder, then extruding said compressed composition under pressure through a die opening, supporting said extrudate in free space upon exit from said die opening to permit natural expansion by return of said elastomeric particles to their original configuration, thereby shearing the bond between certain surfaces of said elastomeric particles and said binder to form channel-like pore networks in the body of the extrudate, and then cooling the porous extrudate to congeal the binder to provide structural integrity for the extrudate mass.

2. The method according to claim 1 wherein said die opening is configured to produce a hollow pipe-like extrudate and including the step of; supporting the interior of said hollow extrudate with gas pressure during radial and circumferential expansion thereof to maintain a desired internal diameter of the pipe.

3. The method according to claim 2 including the step of; applying a constant pulling force on said hollow extrudate to support the longitudinal expansion thereof.

4. Tl|e method of claim 3 including the step of transporting the hollow extrudate after initial expansion through an elongated tubular calibration apparatus while maintaining a layer of coςling fluid between the interior wall of the calibration apparatus and the exterior wall of the extrudate, whereby cooling fluid enters the formed pores to promote rapid congealing of the binder in the pore network.

5. The method of claim 1 wherein the moisture content of said composition is no greater than 0.45% by weight, and said composition is extruded under pressure greater than 200 bar.

6. The method of claim 5 wherein the weight ratio of elastomeric particles to binder is greater than 1.

7. The method of claim 6 wherein said elastomeric particles are vulcanized rubber, and the weight ratio of rubber particles to binder is the in the order of 2.33.

8. The method of claim 7 wherein said rubber par¬ ticles have a particle size of 30-600 u, said rubber particles being generally rectangular shaped as generally obtained by cryogenic grinding.

9. The method of claim 8 wherein said binder consists of 30-80% high density polyethelene and 70-20% low density polyethelene with melt indices of I2 between 0.1 and 5.0.

10. The method according to claim 1 including the step of; removing the moisture from said composition prior to extrusion to obtain a moisture content not exceeding 0.45% by weight.

11. The method according to claim 10 wherein said moisture content is 0.0-nil

12. A method of producing a uniformly porous extrudate comprising; forming a composition consisting of elastomeric par¬ ticles and a thermoplastic binder wherein the volume of the binder is less than the matrix volume of the composition defined by the available space between the elastomeric particles in an uncompressed state, extruding said composition through a die under suf¬ ficient pressure and at a temperature above the melting temperature of the binder and below the temperature at which the elastomeric particles loose their elastic properties, whereby said elastomeric particles are surrounded with a layer of molten binder, supporting said extrudate in free space upon exit from said die to permit natural expansion thereof, and then cooling the extrudate after initial expansion to congeal the binder.

13. The method according to claim 12 including the step of; removing the moisture from said composition prior to extrusion to obtain a moisture content not exceeding 0.45% by weight.

14. The method according to claim 13 wherein said moisture content is 0.0-nil.

15. The method according to claim 13 wherein said elastomeric particles are vulcanized rubber having a particle size of from 30-600 u, said particles being obtained by cryogenic grinding.

16. The method according to claim 12 wherein said composition consists essentially of;

70 parts by weight of crumb rubber having a size range of from 30-600 u,

17 parts by weight of low density granular polyethelene having a density of 0.9-0.922mi, 13 parts by weight of high density granular polyethelene having a density of 0.350-0.954 mi. the moisture content of said composition being

0.0-nil.

17. The method according to claim 16 wherein said composition is extruded through a die at a temperature of 170°C under a pressure of approximately 410 bar.

18. The ^'method according to claim 17 wherein said die opening is configured to produce a hollow pipe-like extrudate and including the step of; supporting the interior of said hollow-extrudate with gas pressure during radial and circumferential expansion thereof to maintain a desired internal diameter of the pipe.

19. The method according to claim 18 including the step of applying a constant pulling force on said hollow extrudate to support the longitudinal expansion thereof.

20. The method according to claim 19 including the step of transporting the hollow extrudate after initial expan¬ sion through an elongated tubular calibration apparatus while maintaining a layer of cooling fluid between the interior wall of the calibration apparatus and the exterior wall of the extrudate, whereby cooling fluid enters the formed pores to promote rapid congealing of the binder in the pore network.

21. A uniformly porous extrudate comprising; a major portion by weight of elastomeric particles, a minor portion by weight of thermoplastic binder, said elastomeric particles being evenly dispersed in a matrix of said binder, and channel-like pore networks extending uniformly through the body of said extrudate, the volume of said binder being less than the matrix volume of said extrudate defined by the available space between said elastomeric particles in the uncompressed state, said pores being formed by shearing action between the binder and certain surfaces of said elastomeric particles caused by free expansion and cooling of the extrudate after passing through and extrusion die opening under pressrue and elevated temperatures above the melting temperature of the binder and below the temperature at which the elastomeric particles loose their elastic properties.

22. The extrudate according to claim 21 wherein; the extrusion and expansion of said extrudate is conducted in the absence of any substantial moisture or other volatile substances.

23. The extrudate according to claim 22 comprising a hollow pipe having uniform porosity along its length and having Cv value below 0.08.

24. The extrudate according to claim 23 having a Cv value of 0.04.

25. A product manufactured according to the method of claim 2.

26. A product manufactured according to the method of claim 18.

27. Apparatus for manufacturing porous hollow pipe comprising; extrusion means having an infeed section and an exit die for producing a hollow expanding extrudate with a thermo¬ plastic binder matrix, said extrudate undergoing free expansion simultaneously in the radial, circumferential and longitudinal directions, calibration apparatus spaced a predetermined distance form said die to provide free initial expansion of said extrudate, said calibration apparatus having an internal diameter greater than the outside diameter of said extrudate and including means to maintain a layer of cooling fluid therebetween for introduction into the formed pores of the hollow extrudate to congeal said thermoplastic binder, and means to pull said extrudate through said calibration apparatus with a constant force.

28. The apparatus of claim 27 wherein said extrusion means includes means to introduce gas pressure into said hollow extrudate as it exits said die to support radial and circumferential expansion thereof.

29. The apparatus of claim 28 wherein said calibration means comprises; internal and external cylindrical walls forming a cooling liquid- chamber,

-said internal cylindrical wall including perforations and being spaced from the outside surface of said extrudate to permit said fluid to contact the extrudate surface, and means to supply a cooling fluid to said chamber.

30. The method according to claim 12 wherein said composition consists essentially of;

55-80 parts by weight of crumb rubber having a size range of from 30-600 u,

45-20 parts by weight of a thermoplastic binder, consisting of:

35-65 parts by weight of low density polyethelene having densities of from 0.910-0.930 and melt indices I₂ f_rθm 0.3-2

65-35 parts by weight of high density polyethelene having densities of from 0.945-0.965 and melt indices I₂ from 0.3-2 the moisture content of said composition being 0.0-nil.

31. The method according to claim 30 wherein said composition is extruded through a die at a temperature of 150-185°C under a pressure of 250-450 bar.