WO1993019924A1

WO1993019924A1 - Biaxially oriented tube, process and apparatus for producing it

Info

Publication number: WO1993019924A1
Application number: PCT/GB1993/000626
Authority: WO
Inventors: Alan Gray; Carl David Thorley
Original assignee: Wavin B.V.
Priority date: 1992-03-27
Filing date: 1993-03-26
Publication date: 1993-10-14
Also published as: AU3894393A; GB9206766D0

Abstract

Biaxially oriented tubular products for use, e.g. as gas pipes are made by biaxially orienting a hollow workpiece and then externally cooling the biaxially oriented product while it is still being biaxially oriented, e.g. axially stretched, the cooling preferably being in the form of a cooling ring located at or shortly after the product leaves the mandrel on which it was oriented. The products have an asymmetric internal stress distribution through their walls.

Description

BIAXIALLY ORIENTED TUBE, PROCESS AND APPARATUS FOR PRODUCING IT.

This invention relates to pipes and a process for their manufacture.

Pipe products can be manufactured by the extrusion of a variety of polymers through a die of suitable dimensions. Subsequent solid state deformation, e.g. hydrostatic extrusion or die-drawing orientates the polymer chains in a longitudinal direction parallel to the axis of the pipe. Pipes can be strengthened further by the process of biaxial orientation in the pipe manufacture at the drawing stage. K Richard et al ("Plastics" December 1961) discloses biaxial orientation in

Ziegler polythene pipes and a method by which strengthened Ziegler polythene pipes may be produced through the simultaneous circumferential expanding (by an expanding mandrel or compressed air) and longitudinal stretching; the biaxial orientation increases the creep rupture strength of the polythene pipe. GB-A- 1456222 describes biaxially orientating a pipe involving applying a lubricant between the mandrel and pipe and significantly after the expanding part of mandrel cooling the pipe round a cylindrical draw off part with an external cooling bath. AU-A-70136/91 is a development of this latter method with laser control to test the degree of axial orientation; in addition to the cooling in GB-A-1456222, the tube can be exposed near the conical expanding section of the mandrel substantially to stationary air, but liquid coolant can also be used. We have found that externally cooling the biaxially oriented tube during or after said orientation can drastically increase the production rate of said tube compared to operation without cooling, but without significant loss in physical properties of the tube. The present invention provides a process for the manufacture of a tubular product, said process comprising solid state deforming a material comprising a thermoplastic polymer by increasing the internal cross-sectional area of a hollow workpiece of said material and stretching said workpiece in the axial direction to effect biaxial orientation, characterized by externally cooling during said orientation to produce a biaxially oriented tubular product.

The present invention also provides apparatus for use in the manufacture of a biaxially oriented tubular product, said apparatus comprising means for maintaining a hollow workpiece of a thermoplastic polymer at a temperature for solid state deformation thereof, means for biaxially orienting said hollow workpiece to produce a biaxially oriented tubular product, characterized by comprising means for externally cooling said product during said biaxial orientation.

The thermoplastic polymer may be one with a crystalline melting point, in which case the deformation is performed at a temperature below that melting point, or may be an amorphous polymer with a rubbery or glass transition temperature and the solid state deformation is performed at a temperature above that temperature. An example of a polymer-with crystalline melting point may be an olefin polymer, which is preferably substantially isotropic when subjected to the solid state deformation. An example of an amorphous polymer is a homo-or co-polymer of vinyl chloride.

The polymer may be an ethylene homopolymer but is preferably a copolymer of ethylene.

By using copolymers of ethylene in the manufacture of pipes, biaxial orientation enhances resistance to impact as expressed by Charpy impact strength measured in the axial direction. By using the term "copolymer" we mean a polymer produced from the polymerisation of ethylene with at least one other alpha olefin. The term can cover terpolymers and higher interpolymers. Suitable copolymers of ethylene are those produced with other alpha olefin hydrocarbons of 3-12, especially 4-8 carbon atoms; examples of suitable olefins are propylene, butene-l,4-methylpentene-l, hexene-1, octene-1 and decene-1 and mixtures thereof. The weight percentage of said other olefin to the total of ethylene and other olefin is usually 1-102, e.g. 1-82, in particular 3.-8% with olefins of 3-5 carbons or 1-52 with olefins of 6-10 carbon atoms. Suitable polymers may therefore be those wherein the ethylene comprises not less than 90% by weight of the olefin polymer. The branch content of the copolymer is usually up to 20 branches per 1000 carbon atoms, preferably 1-20 and especially 2-10 branches. The base density of the (co)polymer (i.e. the density of the

(co)ρolymer in the absence of any non miscible solid additives such as fillers) is usually 930-955, preferably 930-940 kg/m^ and the density of the copolymer containing additives is usually 940- 965, preferably 940-955 kg/m^. The number molecular weight (M_JJ) of the copolymer will preferably be in the range 5,000-35,000, more preferably in the range 5,000-20,000. The polydispersity (M_w/M_n) of the copolymer will preferably be in the range 5-60, e.g. 6-35, preferably 7-20 and especially 15-35. The Melt Index (measured according to ASTM D1238 Condition E under 2.16 kg load at 190^SC) is usually 0.01-10, especially 0.05-2 g/min. The crystalline melting point of the copolymer is usually 120-140^aC. A suitable olefin copolymer may have M_n - 16,000, M_w/M_n «= 10, and 5 branches/1000 carbon atoms, a melt index of 0.2 g/10 min and a density of 938 kg/nr measured in accordance with BS3412:1976. Examples of vinyl chloride polymers are those with rubber or glass transition temperatures of 50-90^fiC, and are especially homopolymers of vinyl chloride, or copolymers with up to 20% by weight of a copolymerizable comonomer such as vinyl acetate. The polymers may have M_n of 20,000-80,000 and

of 2.0-6.25, e.g. 2.0-4.5 and a K value (1% by weight solution in cyclohexanone at 25^BC) of 45-80. Preferably the polymer is a homo PVC and is especially mixed with impact modifiers such as acrylic polymers to strengthen it.

The process of pipe manufacture of the present invention may comprise any process which allows introduction of biaxial orientation by solid state deformation of a thermoplastic polymer.

The basic techniques of biaxial orientation are known per se, e.g. as described in the Richard article mentioned above, the disclosure of which is hereby incorporated by reference. The orientation is such that the polymer is oriented in two directions, said directions having a component perpendicular to each other; thus the orientation can be both axial and hoop. Axial or longitudinal orientation is obtained by elongation of the polymer, e.g. without altering the internal diameter of the workpie^'ce, so the walls simply become thinner; this orientation may be achieved by hydrostatic extrusion involving forcing by hydrostatic or hydraulic pressure a former of the (co)polymer, usually a hollow former, over a mandrel either continuously or batch wise, or by drawing a hollow former through a die of reduced external diameter but the same internal diameter as the former. Hoop or circumferential orientation involves expansion of the hollow former to increase its mean circumference (or to increase its mean diameter) ; this orientation may be achieved by internal fluid pressure, e.g. a compressed gas such as air or drawing over a mandrel of larger external diameter than the internal diameter of the hollow former or by rolling over such a mandrel. The drawing may be performed during passage of the former through a die or not, i.e. with or without application of an external force on the tube perpendicular thereto. The axial and hoop orientations may be applied consecutively or concurrently to a substantially unoriented polymer. Thus longitudinal elongation, e.g. by extrusion or drawing may be followed by radial expansion, e.g. with compressed gas batch wise or continuously, but preferably drawing over such a mandrel is used to achieve both orientations simultaneously. In the process of drawing, biaxial orientation of the pipe may be introduced by means of a mandrel positioned in and/or beyond a heating collar, sleeve or die, the external cross-sectional area of the mandrel being greater than the internal cross-sectional area of the input hollow workpiece. By ^internal cross-sectional area" is meant the cross-sectional area of the hollow workpiece defined by the internal surface of the workpiece perpendicular to the direction of drawing and is therefore the aperture of the hollow workpiece. In the process of the invention the principle internal dimension of the hollow workpiece, e.g. its diameter for a circular aperture is increased. Advantageously the product to be biaxially oriented is drawn through an annulus defined internally by the mandrel and externally by the collar, sleeve or die; to aid passage of the former over the mandrel, the mandrel is usually provided with an initial^" upstream part of expanding diameter, which may be conical, frustoconical or curved, e.g. convex or .concavely curved in axial cross-section. Preferably this part is of frustoconical shape with semivertical angle of 5-50^β, usually 10-40°. If desired the product may be drawn over the mandrel without any physical external constraint on the external diameter, e.g. without significant external force of the heating collar being applied to the product, though the product and collar may be in touching or glancing contact. The internal diameter of the die or heating collar or sleeve may be smaller than, the same as or larger than the external diameter of the mandrel. If desired the mandrel may have an expanding, e.g. frustoconical part of section first to expand the former, and then a later part or section of substantially constant diameter which is the same as the widest diameter of the expanding part, to keep the internal diameter of the orientated object substantially constant while it is being pulled further. The expanding, e.g. frustoconical part is heated and is preferably made of a heat conductor, e.g. metal. If desired the later parts or sections of substantially constant diameter may be heated or cooled and may be of heat conductor, e.g. metal, but the later part may instead be a heat insulator, e.g. of high melting polymer and not be separately heated. The expanding, e.g. frustoconical part and later part of the mandrel may be integral or releasably or non-releasably joined to one another. The former is then cooled in the process of the invention. After the product has been drawn over the mandrel and cooled, it is preferably cooled further, e.g. externally via a cooling bath or by exposure to ambient conditions to reduce or stop, i.e. quench any further reduction in the diameter. The drawing over the mandrel and through the cooled area(s) may be achieved by means of a pair or pairs of opposing rubber rollers or belts, acting to pull the object (e.g. a "Caterpillar" arrangement) or may be achieved by a wire or chain attached to a nut, bolt or clamp attached to the expanded object, e.g. by threading or screwing into its end, the other end of the wire being drawn by haul off means, e.g. a powered drum. The drawing applies a tension to the former less than its draw failure level. If desired the drawing involving pulling the former and tube downstream of the mandrel can be aided by means for pushing the former upstream of the mandrel, e.g. by opposing rubber rollers or belts acting to urge the former towards and over the mandrel. The expanded biaxially orientated object is finally collected.

The solid state deformation process of the present invention, i.e. the biaxial orientation can be carried out at a temperature below, e.g. 5-50°C or especially 10-20°C or 15-25^βC below, the crystalline melting point of the polymer or S-SO'C, e.g. 10-20^βC above the glass transition temperature of an amorphous polymer.

Preferably the hollow former is passed through a constant temperature area before reaching the expanding section of the mandrel; in this area, if desired, the former may be calibrated to a special internal and/or external diameter.

The mandrel can have an expanding, e.g. frustoconical part of axial length a and maximum diameter 2c, and may have a substantially constant dimension part contiguous with said maximum diameter part of diameter 2c, said constant diameter part having axial length b and diameter 2c. If desired the constant diameter part may be absent (i.e. b is 0). Alternatively the length ratio of b to a may be 0-10:1, such as 0.01-10:1, preferably 0.02-6:1 or 0.05-5:1 and especially 0.1-2:1 or 0.05-0.2:1. The ratio of c to b may be 0.05-10:1 such as 0.5-8:1, especially 2-6:1, while the ratio of c to a may be 0.05-10:1, e.g. 0.1-1:1 or 0.2-0.7:1. The aspect ratio of the mandrel, i.e. the ratio of length (a + b) to diameter (2c) is usually 0.1-10:1, preferably 0.8-3.0:1 and especially 1-1.5:1. The semivertical angle of the frustoconical section may be 5 to 50", e.g. 10-40° and especially 15-35°.

The hollow former may be drawn over the expanding, e.g. frustoconical part of the mandrel without significant heat loss, but after it has reached the maximum dimension of the expanding part of the mandrel, e.g. the constant dimension part (if present) the expanded biaxially oriented former is cooled. The external cooling is usually localized, and /or cools the outer surface of. the tubular product rapidly, e.g. at a rate of 15-70°C/sec. The cooling can be applied once the workpiece has been transversely oriented and while it is being axially oriented. The cooling may be internal or preferably external or may be both internal and external; very advantageously greater cooling is applied externally to the expanded hollow former than is applied, if any, internally. The cooling may be applied over a band of expanded former with an axial length equal to 0.1-30 mm, e.g. 0.2-20 mm such as 0.5-10 mm especially over a localized band, e.g. of 1-4 such as 1-3 mm axial length. The cooling may be applied externally about or after the point of maximum diameter of said expanding part of the mandrel, in particular from a distance of 0.3a (e.g. 0.04a) or 2.5b (e.g. 0.25b) or 0.6c (e.g. 0.07c) upstream of said point to 10a, 10b or 2c downstream of said point, such as 17 or 20, e.g. 10 mm upstream to 60, 50 or 40 mm downstream. The cooling may also be applied a distance from the point of maximum diameter of the expanding, e.g. frustoconical part equal to O.Olb-lOb, e.g. 0.1b-2b and especially 0.5b-1.5b, or to 0-lOa, e.g. 0-a and preferably 0-0.8a or 0.01a-5a, e.g. 0.1a- 2a, especially 0.5a-1.5a, or alternatively 0-2c, e.g. 0-c or 0- 0.8c such as 0.01c-2c, e.g. 0.05c-0.7c and especially 0.1c-0.5c. The cooling is particularly applied, e.g. for making 60-130 mm and especially 60-75 mm diameter pipe at a distance from said point of 0.04a-0.8a, e.g. 0.07a to 0.67a or 0.07a-0.54a and most preferred 0.10a-0.3a, and/or 0-7b_r e.g. 0.35b-7b or 0.6b-4.7b, especially 0.8b-2.5b and/or 0.1c-1.8c, e.g. 0.16c-1.2c, especially 0.18c- 0.65c; this location of cooling is especially suitable for tube of 1-15 mm, e.g. 2-15 mm, or 4-8 mm, but preferably 1-4 mm wall thickness. ^• The cooling is usually applied at a time from the time when the expanded former leaves the point of maximum diameter of the expanding, e.g. frustoconical part of 0-50 sec, e.g. 0.01-20 sec or 0.05-20 sec such as 1.5-40 sec or 2.5-30 sec or at a time from the time when it leaves the mandrel of 0-20 sec. The cooling may be applied externally about or after the tubular product leaves the mandrel, preferably a mandrel heated to about the same temperature as the expanding section and especially without such cooling significantly before the end of the mandrel. Preferred distances downstream from the end of the mandrel are from -0.45a, -3.5b or -0.9c (preferably -0.15a, -1.25b or -0.3c) (especially 0) to + 0.8a, +6b or + 1.7c, preferably -0.1a to + 0.7a, especially - 0.05a to + 0.5a, or preferably -0.9b to + 5b, especially -0.25b to + 3.7 b, or preferably -0.2c to + 1.3c, especially -0.1c to + lc, the negative signs denoting distances upstream. The cooling may also be applied at a distance downstream of the end of the mandrel (e.g. the end of the constant dimension part) of 0-50 mm, e.g. 0- 35 mm; these distances are especially suitable downstream of the heated part of the mandrel. The above parameters are in general suitable for use with thermoplastic polymers, but are especially suitable when the polymer is an olefin polymer in particular an ethylene alpha olefin copolymer. The cooling may be uniformly applied, e.g. with a constant temperature liquid or gas bath but is advantageously localised, e.g. as a directed jet (e.g. from a cooling ring with internally facing, especially regularly spaced holes) of liquid or gaseous coolant in which the direction is towards a transverse line on the periphery of the expanded former, which is within the ranges from the point of maximum diameter of the expanding frustoconical part or end of the mandrel quoted above. The directed jets can be focussed on to at least one, e.g. 1-4 such lines but especially onto one such line. The cooling ring may therefore have have 1 or a series of lines of inward facing holes around its internal circumference and the holes being spacially distributed, especially uniformly specially distributed, around the internal circumference. The effect of the directed jet is believed to "freeze" the outer surface of the expanded former and reduce subsequent shrinkage of the internal diameter of the expanded former and/or reduction of the wall thickness, once it has left the mandrel. The rate of cooling is usually to reduce the external surface temperature of the former by 5-100, e.g. 15- 70 such^' as 30-50°C/sec so for a period of 2.0-0.2 sec, e.g. 1.3- 0.3 sec, while the surface is moved across the cooling bath or jet, the temperature of the surface can be reduced by 15-50°C, e.g. 15-35°C, such as to give a temperature on the surface of 75- 105°C, e.g. 75-85 or 90-105^βC for olefin polymers and 80-110°C, e.g. 90-100°C for vinyl chloride polymers. As an alternative -to the use of directed jets as cooling means a ring of solid carbon dioxide may be used, especially retained within a cooling ring. In the absence of sufficient internal cooling, once the expanded former has left the area of the external localized cooling, the temperature of its external surface starts to rise again because of the warmer inner part of the former, but then starts to fall under the influence of the external surrounding.

The expanded former is preferably cooled while being drawn under tension over the mandrel, e.g. when on the mandrel and especially shortly thereafter.

The cooling bath or cooling ring or other cooling means is preferably separated from the expanding part of the mandrel and especially the collar, sleeve or die, from which the former is emitted, by insulation means, e.g. in the form of a baffle or insulated region, which conveniently is shaped around the former while it is on the expanding part of the mandrel or the portion of mandrel downstream thereof, and may be in touching contact therewith to reduce cooling of the former on the expanding part of the mandrel, and especially cooling of the collar, sleeve or die. The extent of the orientation in the axial and radial directions may be varied and the two degrees of orientation may be the same or different. The axial orientation (or draw ratio) which is defined by the ratio of the solid cross sectional area of the object before orientation to that after orientation is usually 2-10, preferably 2-7.5 and especially 3-7. The average circumferential or hoop orientation, which is defined as the ratio of the mean diameter of the object after orientation to that before orientation, is usually 1.1-2.5, preferably 1.2-2 and especially 1.4-1.8. The biaxiality ratio is the ratio of the hoop to the sxial orientation and is usually 0.18-0.6, e.g. 0.25-0.6 and especially 0.30-0.55. The ratio of the tensile modulus (e.g. 1% secant modulus) of the biaxially orientated product in the hoop direction to that in the longitudinal direction is usually 0.75- 1.05, preferably 0.9-1.05 and especially 0.95-1.02; when the biaxiality ratio is 0.35-0.55 the ratio of tensile moduli in the hoop and longitudinal directions is usually 0.9-1.5 and this tends to give the best impact performance to the biaxially oriented product; this benefit of impact resistance is very useful in particular with pipes under pressure of water or gases ,e.g. natural gas which may be struck during their installation or during their use, the capacity for which is required by regulations to be very lon , at least 50 years.

The hollow biaxially orientated products may have wall thicknesses of 1-25 mm, e.g. ^'3-10 mm and especially have 30-500, e.g. 50-150 mm external diameter. They may be in the form of pipes, as described above, for which continuous drawing is the preferred production route. The pipes may be of any convenient cross sectional area, including polygonal, e.g. square or hexagonal but are preferably ellipsoidal or especially circular; the pipes may be profiled with one or more ridges. The hollow products may also have a closed end, as with a shot gun cartridge, in which case batchwise extrusion between a fixed die and a movable mandrel is preferred; the closure may be formed during the extrusion or added thereafter. The hollow products of the invention may be cut along their length to produce flat sheets. If desired the hollow biaxially orientated products may contain in addition to the polymer, conventional additives, e.g. antioxidants, plasticizers, antiblocks, slip agents and fillers, especially carbon black, e.g. in amount of 0.1-5% by weight. One effect of the cooling is drastically to increase the production rate while increasing to a much smaller extent the load required to achieve it, but in addition the tubular product, e.g. pipe obtained has an asymmetric internal axial and/or hoop stress distribution, which can provide a stronger tube than one without cooling. Greater cooling on the outside of the pipe than on the inside results in a large difference in the residual axial and hoop stress between the outside and inside of the pipe. The present invention therefore also provides a tubular biaxially oriented polymer product having a wall with an inside and an outside portion and an asymmetric internal stress distribution through said wall between said portions. The residual tensile axial stress on the surface of the outside portion and the compressive axial stress on the surface of the inside portion are usually at least 2 MPa, e.g. at least 4 MPa such as 2-12, 5-10 such as 8 MPa, especially for olefin polymers such as those described above. These figures are usually at least 2 times and especially 3-8 times larger than the corresponding figures for a tubular product made in a modification of the process of the invention without cooling. The tubular product of the invention may also be one for which the residual stress in an axial sample is such that the sample curls significantly with the outer surface of the tubular product on the inside. The radius of curvature at 23°C of the sample of dimensions 175 x 6 x 3-mm is usually less than 200 mm, e.g. less than 100 mm such as 200-20 or 80-30 mm, while the restoring force in Newtons necessary at 23°C to flatten the curled sample of these dimensions is usually at least 2, e.g. at least 4 such as 2-15 or 6-13 Newtons in both cases, especially for olefin polymers such as those described above.

The invention is further described by way of Example with reference to the accompanying drawing in which Figures 1 and 2 are a schematic cross sections of the drawing apparatus with a hollow product being produced.

Inside a circular hole 1 in a heated sleeve or collar 2 there is a mandrel 3 having a frustoconical part 4 of maximum diameter less than [or more than (not shown)] the internal diameter of hole 1, an outer cylindrical part 5 of said mandrel of said maximum diameter and an inner cylindrical part 6 inside the hole 1, the gap between sleeve or collar 2 and mandrel defining an annulus usually of circular cross section. Sleeve or collar 2 has internal heating elements (not shown) as does mandrel 3. Inner part 6 is in a heating chamber 7. A hollow polymer billet 8 has an internal diameter the same or slightly greater than the external diameter of Inner part 6 of mandrel 3, and billet 8 has an external diameter less than that of the internal diameter of the chamber 7 and sleeve or collar 2. The billet 8 is initially in the heating chamber 7 and is finally in the form of an elongate tube 9. Surrounding said outer part 5 of mandrel 3 but spaced therefrom is a circular cooling ring 10 of square cross section with a multiplicity of inward facing holes 11 in a line going completely round the internal circumference of the cooling ring with jets focussed on the opposing surface of tube 9. The cooling ring may have one ring of holes 11 (see Figure 1) and or an axially extending series of rings of holes capable of independent or co-ordinated, e.g. simultaneous emission of jets (see Figure 2) enabling the location of the focussed cooling to be changed. An insulating baffle 12 separates heated sleeve or collar 2 from cooling ring 10, and may just touch the tube 9 as it passes over the outer part 5 of the mandrel 3. Means (not shown) is also provided for feeding coolant to said ring 10. Initially a frustoconical section may be shaped in the end of billet 8 to accommodate the corresponding end 4 of the mandrel which is inserted into the billet 8. The billet 8 is then heated to a temperature for solid state deformation, e.g. a temperature below the crystalline melting temperature for an olefin polymer by means of the heating elements in chamber 7, sleeve or collar 2 and inner part 6 of mandrel 3. The temperature of the chamber 7 sleeve or collar 2 is usually less than that of the mandrel 3, e.g. up to 10^fiC less such as 0.5-5^aC or l-3^aC less. Billet 8 is then pulled over the frustoconical end 4 of the mandrel through the annulus causing the interior and exterior diameters of billet 8 to expand and its wall thickness to contract. After passage through the annulus, then over parts 4 and 5 of the mandrel, the elongated tube 9 is pulled by draw-off means not shown. Just as the tube 9 leaves the outer cylindrical part 5 of the mandrel, tube 9 ^"is cooled by coolant gas, e.g. air emitted from jets 11 of ring 10; the air jet is directed co-linear with the end surface of part 5 of mandrel 3 and is stopped from cooling the tube 9 significantly upstream by baffle 12. The tube 9 may if desired then be pulled through a heated vacuum sizing chamber (not shown) having an internal diameter the same as that of the desired external diameter of the tube in order to adjust the dimensions of the tube 9, the heating being such as not to cause loss of the orientation. A floating plug mandrel may be used to dimension correctly the internal diameter of the tube 9. Finally the tube 9 is collected on draw-off means (not shown) or may be sliced axially and collected as a flat sheet.

The invention is illustrated in the following Examples in which the apparatus shown in Figure 1 was used, unless otherwise stated. Examples 1-3 and Comparative Examples A-C

An ethylene hexene-1 copolymer had 3% w/w hexene-1 and a linear structure with 5 branches per 1000 carbon atoms. The copolymer had a base density of 938 kg/rn^, a Melt Index (2.16 kg load) of 0.2 g/10 min, an M_n of 16000 and Mw/Mn of 10, a crystalline melting point of 127°C. The copolymer contained 2% carbon black as a filler and had overall density of 948 kg/n .

A hollow billet 8 of this copolymer had external diameter of 58 mm and internal diameter of 23 mm and was contained in chamber 7 maintained at 110°C. A mandrel 3 had frustoconical part 4 of longitudinal axial length 70 mm (i.e. a + b) and maximum diameter 62 mm (i.e. 2c) which was the diameter of outer cylindrical part 5, which was 8 mm long (i.e. b) . Inner part 6 of 21 mm diameter of mandrel 3 was positioned inside the chamber 7 and inside the 66 mm hole 1 of sleeve or collar 2. The front section of the billet was machined to produce a frustoconical hole adapted to correspond with frustoconical part 4.

The mandrel and collar were heated to 112°C and 110°C respectively, and billet 8 was drawn by means of haul-off mechanism (not shown) over mandrel 3 to produce tube 9 of about 63 mm external diameter and 58 mm internal diameter. Just when the tube 9 left outer cylindrical part 5 of mandrel 3, continuous jets of cold air were focussed at it from holes 11 in cooling ring 10 to cool the tube across a 2 mm band sufficient to stop shrinkage of the internal diameter of the tube thereafter when the tube passes through the ambient atmosphere to the draw off means. The rate of cooling of the outer tube surface was as given in Table 1. The draw off speed was measured and the draw load needed to achieve this speed noted. The axial orientation was 4 and the hoop orientation was 1.5. The process was repeated without operation of the cooling jets and the speed and load adjusted to give a tube of the same external diameter and the same average hoop ratio as before and the speed and load measured.

The whole procedure was then repeated with the mandrel and collar heated to 117°C and 115°C respectively, and also to 122°C and 120°C respectively and the draw speeds and loads remeasured when producing the tubes of same external diameter and same average hoop ratios. Table 1

The draw speed increased to 6-13.5 times for a 38-58% increase in draw load.

The tube 9 from Example 1 was tested for yield stress (MPa) by the Test Method ISO/R527 and Pipe Stress Rupture time (failure time in hours at 25 MPa) according to Test Method ISO 1167 and compared to those from Example A without cooling and an isotropic unoriented pipe of the same dimensions. The results were as follows.

Table 2

An axial sample 175 x 6 x 3 mm was taken from each of the pipe of Example 1 and also the pipe of Example A. Each was tested for axial residual stress. In each case the sample was tested at 23°C 100 hr after cutting. The radius of curvature of each sample was tested; the results (averaged over 5 samples from each pipe) were 56.4 mm for Example 1 pipe and 254 mm for Example A pipe. In both cases the outer surface of the pipe became the inside of the curled sample with tensile stress in the outside surface of the pipe and compressive stress in the inside surface of the pipe. The two samples were also tested for the size of the restoring force needed to flatten out the curl in the sample; the results (again averaged over 5 samples from each pipe) were 9.20 Newtons for Example 1 pipe and 1.21 Newtons for Example A pipe. By linear approximation from the above data the axial residual stress appears to be about 5.2 MPa (tensile) in the outer pipe surface and 5.2 MPa (compressive) in the inner pipe surface for Example 1; the corresponding figures for Example A appear to be about 1.3 MPa (tensile) and 1.3 MPa (compressive) respectively. Example 4 The procedures of Examples 1 and A were repeated with a hoop orientation of 1.4 achieved by converting a billet of 23 and 63 mm internal and external diameters respectively into a tube of about 57-58 mm and 63 mm internal and external diameters respectively. The collar temperature was 110°C. The results were shown In Table 3.

Table 3

Thus for a 14.2% increase in load, the draw speed increased 6.25 times. Examples 5-10 and Comparative Example E

The process of Example 1 was repeated with the mandrel and collar at 112^βC and 110°C respectively but the location of the cooling ring jets was moved axially upstream and downstream with respect to the line parallel to the transverse end of the mandrel (called the transverse line). In each case of Examples 5-10 the speed of drawing and load were adjusted until a tube of the same 63 mm external and 58 mm internal diameter was made, while in Comparative Example E the tube had the same external diameter and same average hoop ratio. The cooling rate and cooling time were as in Example 1. The draw speeds in each case were as shown in Table 4.

Table 4

In a comparative experiment Example E without the cooling from the cooling ring being applied, the draw speed was 25 mm/min. Examples 11-13 and Comparative Examples F-H

The process of Examples 1-3 and A-C were repeated with identical conditions, apart from the nature of the copolymer and the draw load and draw speed. The copolymer was an ethylene hexene-1 copolymer with a linear structure with an average of 1.7 branches per 1000 carbon atoms. The copolymer had a base density of 947 kg/m², a Melt Index (2.16 kg load at 190°C) of 0.06 g/10 min, a Mw of 330,000 and Mw/Mn of 25, and a crystalline melting point of 131-132°C. The results were as shown in Table 5. Table 5

The draw speed increased 3.7-6 times for an 8-19% Increase in draw load.

The tube 9 from Example 11 was tested for yield stress (MPa) by the Test Method ISO/R527 and compared to those from Example A without cooling and an isotropic unoriented pipe of the same dimensions. The results were as shown in Table 6.

Table 6

Example 14 and Comparative Example J

The process of Example 2 was repeated with a different location of the cooling, namely 23 mm axially upstream of the end of the mandrel, i.e. 15 mm axially upstream of the transverse plane where the frustoconical part 4 meets the outer cylindrical part 5. The direction of the cooling air was transverse to the longitudinal axis of the mandrel, as in Example 2, but because It was impinging on the frustoconical section of the mandrel, provided a slightly broader cooling band than in Example 2. The hollow billet used as feed had external diameter 63 mm and a 20 mm

SUBSTITUTE SHEET wall thickness (i.e. 23 mm internal diameter) to give a biaxiallyoriented tube of 63 mm external diameter.

The process was repeated in Comparative Example J without the applied cooling. The results were as follows. With Cooling Without Cooling

Product Tube Wall Thickness mm 4.5 3.1

Internal Diameter mm 54 58.8

Draw Ratio Axial 3.5 4.6

Average Hoop 1.4 1.4 Draw Speed mm/min 50 26

Draw Load kg 800 750

The cooling increased the production rate compared to no cooling and gave a thicker walled tube than in Example 2.

SUBSTITUTE SHEET

Claims

1. A process for the manufacture of a tubular product, said process comprising solid state deforming a material comprising a thermoplastic polymer by increasing the Internal cross-sectional area of a hollow workpiece of said material and stretching said workpiece in the axial direction to effect biaxial orientation, characterized by externally cooling during said orientation to produce a biaxially oriented tubular product.

2. A process according to claim 1, characterized in that external cooling is at least one of localised cooling and cooling of the outer surface of the tubular product at a rate of 15-70^βC/sec.

3. A process according to claim 1 or 2, characterized in that the hollow workpiece has been fabricated from a substantially isotropic olefin polymer with a crystalline melting point.

4. A process according to claim 3, characterized in that said polymer is a copolymer of ethylene and at least one olefin of 3-8 carbons.

5. A process according to any one of the preceding claims, characterized by continuously drawing said workpiece over the expanding cross section area of part of a mandrel.

6. A process according to claim 5, characterized by externally cooling said tubular product about or after the point of maximum diameter of said expanding part of said mandrel.

7. A process according to claim 6, characterized by externally cooling said product about or after it leaves the mandrel.

8. A process according to claim 7, characterized by externally cooling between 5 mm upstream and 35 mm downstream of the end of the mandrel .

9. A process according to claim 6, characterized by externally cooling between 6 mm upstream and 45 mm downstream of the point of maximum diameter of said expanding part.

10. Apparatus for use in the manufacture of a biaxially oriented tubular product, said apparatus comprising means for maintaining a hollow workpiece of a thermoplastic polymer at a temperature for solid state deformation thereof, means for biaxially orienting said hollow workpiece, to produce a biaxially oriented tubular product, characterized by comprising means for externally cooling said product during said biaxial orientation.

11. Apparatus according to claim 10, characterized by comprising means for effecting cooling as specified in any one of claims 2 and 6-9.

12. A tubular biaxially oriented polymer product, having a wall with an inside and outside portion and an asymmetric internal stress distribution through said wall between said portions.

13. A product according to claim 11, characterized by a residual tensile axial stress on the surface of the outside portion and a compressive axial stress on the surface of the inside portion of at least 2 MPa.

SUBSTITUTE SHEET