The present invention relates to process for a producing a pipe by a biaxial elongation of a polyethylene composition. The invention further relates to a pipe obtainable by such process.

It is known to improve the physical and mechanical properties of a polymer material by orienting the material. In many cases, orienting a material to improve a property in one direction leads to worsening of the same property in the direction perpendicular to the direction of orientation. In order to adapt the properties in both directions, a biaxial orientation of the material may be applied. The biaxial orientation means that the polymer material is oriented in two directions, perpendicular to one another. A pipe can be oriented in the axial direction and peripheral direction (hoop direction) to improve properties such as tensile strength.

US5910346 describes process for a producing a pipe by a biaxial elongation of a polymer composition. The thickness of the walls of polyolefin product pipe is from 0.1 to 5.0 mm. For polyethylenes, the preferred axial draw ratio is at least 2 and preferably greater than 3. In one of the examples, HDPE 00-240 is biaxially drawn from a billet having an outer diameter of 62.0 mm at an axial draw ratio of 3.8, inner hoop draw ratio of 2.7 and outer hoop draw ratio of 1 .03. In this example, the average hoop draw ratio and the wall thickness can be calculated to be 1 .45 and 3.58 mm, respectively. HDPE 00-240 is a unimodal ethylene-butylene copolymer.

JP09-94867 discloses an extrusion molding of a hollow molded article made of a thermoplastic resin, by biaxial drawing using a tapered mold.. The inner surface temperature of the parison is kept at least at its melting temperature. CA2457430 describes the need for new polyethylene materials which offer an advantageously balanced combination of thermal, mechanical and processing properties to be used in pipe production. CA2457430 discloses a polyethylene multimodal resin having a multimodal molecular weight distribution.

Pipes have different applications depending on their outer diameter and thickness. Pipes with a low wall thickness are sensitive to outside damage that can lead to failure due to any point load. Pipes with a high wall thickness are required in applications in severe environments, for example for use as a buried pipe.

One of the most important properties for pipes is the resistance to crack propagation. Resistance to crack propagation can be determined according to ISO 13479 "Polyolefin pipes for the conveyance of fluids - Determination of resistance to crack propagation - Test method for slow crack growth on notched pipes (notch test)". The test simulates slow crack growth and record time to failure on notched pipes. These pipes are tested at 80°C under constant internal test stress of 4.6 MPa.

It is an objective of the present invention to provide a stable process for producing a biaxially oriented pipe with good resistance to crack propagation.

The invention provides a process for producing a biaxially oriented pipe, comprising the steps of:

a) forming a polyethylene composition into a tube, wherein the polyethylene

composition comprises a bimodal or a multimodal high-density polyethylene (HDPE) and

b) stretching the tube of step a) in the axial direction and in the peripheral direction to obtain the biaxially oriented pipe,

wherein step b) is performed at an axial draw ratio of 1 .1 to 3.2 and an average hoop draw ratio of 1 .1 to 2.0 and the obtained biaxially oriented pipe has an outer diameter of at least 60 mm and a wall thickness of at least 5.5 mm or

step b) is performed at an axial draw ratio of 1 .1 to 1 .9 and an average hoop draw ratio of 1 .1 to 2.0 and the obtained biaxially oriented pipe has an outer diameter of less than 60 mm.

Preferably, the invention is a process for producing a biaxially oriented pipe, comprising the steps of:

a) forming a polyethylene composition into a tube, wherein the polyethylene

composition comprises a bimodal or a multimodal high-density polyethylene (HDPE) and

b) stretching the tube of step a) in the axial direction and in the peripheral direction to obtain the biaxially oriented pipe,

wherein step b) is performed at an axial draw ratio of 1 .1 to 3.2 and an average hoop draw ratio of 1 .1 to 2.0 and the obtained biaxially oriented pipe has an outer diameter of at least 60 mm and a wall thickness of at least 5.5 mm or step b) is performed at an axial draw ratio of 1 .1 to 1 .9 and an average hoop draw ratio of 1 .1 to 2.0 and the obtained biaxially oriented pipe has an outer diameter of less than 60 mm, and

wherein step b) is performed at a drawing temperature which is 1 to 30 ⁵ C lower than the melting point of the polyethylene composition

The terms "pipe" and "tube" are herein understood as a hollow elongated article, which may have a cross section of various shapes. The cross section may e.g. be circular, elliptical, square, rectangular or triangular. The term "diameter" is herein understood as the largest dimension of the cross section.

It was surprisingly found that biaxial drawing with low draw ratios, especially a low axial draw ratio, leads to a good resistance to crack propagation. The low axial draw ratio is made possible according to the invention by the use of a bimodal HDPE. Upon solid- state drawing, semi-crystalline polymers such as polyethylene form a neck. This neck has to be drawn out until a product with a uniform thickness is obtained. Therefore the production of a biaxially drawn pipe requires a certain minimum draw ratio. A unimodal HDPE requires a relatively high draw ratio for drawing out the neck. According to the invention, a bimodal or multimodal HDPE composition is used which allows drawing at a low axial draw ratio while necking is prevented. Accordingly, the present invention provides a stable, neck-free production of a pipe having a good time-to-failure property.

The process may be performed as a continuous process or a batch-wise process. A continuous process is herein understood as a process wherein the polyethylene composition is continuously fed for the tube making step a), while the drawing step b) is continuously performed.

Applying a low axial draw ratio has an advantage regarding the ease of production of the pipe in particular when the process is a continuous process. For the continuous production of biaxially oriented pipes with certain dimensions, a tube of a certain outer diameter and inner diameter is provided depending on the draw ratio to be applied. The tube passes through a temperature conditioning unit so that it reaches a uniform stretching temperature and subsequently pulled over a conical expanding mandrel to attain orientation in both hoop and axial direction. This conical mandrel is supported on a rod, which is anchored through the cross-head die of the tube melt extruder. If the wall of the tube is too thick, it will take considerable length of temperature conditioning unit to achieve the desired drawing temperature and length of the mandrel supporting rod. If the inner diameter of the starting tube is too small, it becomes increasingly difficult and in certain cases impossible to hold mandrel supporting rod in place.

Hence, it is advantageous to be able to use a tube with a thickness which is not too large and an inner diameter which is not too small. It is a further advantage of the invention that a low axial draw ratio is used, since a tube with a relatively small wall thickness and a large internal diameter can be used.

The relationship between the pipe dimensions, draw ratios and tube dimensions are given in the table below.

For obtaining a pipe with desired dimensions, applying an axial draw ratio of 4 will require a starting tube with a very thick wall and a very small inner diameter, which may make the production of such pipe impossible. As the axial draw ratio is lowered, the processing of the starting tube becomes easier by the lower thickness and the higher inner diameter.

According to the process of the invention, a relatively large pipe having an outer diameter of at least 60 mm or a relatively small pipe having an outer diameter of less than 60 mm can be obtained. For obtaining the relatively small pipe, the draw ratio is selected to be very small for obtaining a good time-to-failure property. For obtaining the relatively large pipe, the upper limit of the draw ratio is higher than for the relatively small pipe, but the tube dimensions are selected to result in a relatively thick pipe. For obtaining a biaxially oriented pipe having an outer diameter of at least 60 mm, the axial draw ratio is selected to be 1 .1 to 3.2 and an average hoop draw ratio is selected to be 1 .1 to 2.0. The wall thickness is at least 5.5 mm. Preferably, the axial draw ratio is at least 1 .2, at least 1 .3, at least 1 .5 or at least 1 .8 and/or at most 3.0, at most 2.8 or at most 2.5. Preferably, the average hoop draw ratio is at least 1 .2 or at least 1 .3 and/or at most 1 .8 or at most 1 .6. Preferably, the outer diameter is 60 mm to 2000 mm, for example 60 mm to 150 mm or 150 mm to 2000 mm. Preferably, the wall thickness is 5.5 mm to 100 mm, for example 5.5 to 15 mm or 15 mm to 100 mm. In some embodiments, the biaxially oriented pipe according to the present invention has an outer diameter of 60 mm to 150 mm and a wall thickness of 5.5 to 15 mm.

For obtaining a biaxially oriented pipe having an outer diameter of less than 60 mm, the axial draw ratio is selected to be 1 .1 to 1 .9 and an average hoop draw ratio is selected to be 1 .1 to 2.0. Preferably, the axial draw ratio is at least 1 .2, at least 1 .3 or at least 1 .5 and/or at most 1 .8 or at most 1 .7. Preferably, the average hoop draw ratio is at least 1 .2 or at least 1 .3 and/or at most 1 .8 or at most 1 .6. Preferably, the outer diameter is 10 mm to less than 60 mm, for example 10 mm to 40 mm or 40 mm to less than 60 mm. Preferably, the wall thickness is 1 mm to 10 mm, for example 1 .5 mm to 5 mm. In some embodiments, the biaxially oriented pipe according to the present invention has an outer diameter of 10 mm to 40 mm and a wall thickness of 1 .5 mm to 5 mm .

Polyethylene composition

The polyethylene composition comprises HDPE. In some embodiments, the

polyethylene composition comprises a further polyethylene other than HDPE. The further polyethylene may e.g. be linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE) or a combination of LLDPE and LDPE. Preferably, the further polyethylene is LLDPE or a combination of LLDPE and LDPE. More preferably, the further polyethylene is LLDPE. In case the further polyethylene is a combination of LLDPE and LDPE, the weight ratio of LLDPE to LDPE may e.g. be at least 0.1 , for example at least 0.2 or at least 0.3 and at most 10, for example at most 5 or at most 3. Preferably, the weight ratio of LLDPE to LDPE is at least 1 , for example 2 to 10.

Preferably, the weight ratio of HDPE to the further polyethylene in the polyethylene composition is more than 1 , preferably 1 .2-5, for example 1 .5-4 or 2-3. In some embodiments, the polyethylene composition essentially comprises no further polyethylene other than HDPE. The amount of HDPE in polyethylene in the

polyethylene composition may be at least 95 wt%, at least 98 wt%, at least 99 wt% or 100 wt%. The polyethylene composition may comprise components other than HDPE and the optional further polyethylene, such as additives and fillers. Examples of the additives include nucleating agents; stabilisers, e.g. heat stabilisers, anti-oxidants, UV stabilizers; colorants, like pigments and dyes; clarifiers; surface tension modifiers; lubricants; flame-retardants; mould-release agents; flow improving agents; plasticizers; anti-static agents; external elastomeric impact modifiers; blowing agents; and/or components that enhance interfacial bonding between polymer and filler, such as a maleated polyethylene. The amount of the additives is typically 0 to 5 wt%, for example 1 to 3 wt%, with respect to the total composition.

Examples of fillers include glass fibers, talc, mica, nanoclay. The amount of fillers is typically 0 to 40 wt%, for example 5 to 30 wt% or 10 to 25 wt%, with respect to the total composition.

Accordingly, in some embodiments, the composition further comprises 0 to 5 wt% of additives and 0 to 40 wt% of fillers.

The polyethylene composition may be obtained by melt-mixing HDPE and the optional further polyethylene, optionally with any other optional components.

Preferably, the total amount of HDPE, the optional further polyethylene and the optional additives and the optional fillers is 100 wt% with respect to the total polyethylene composition.

In some embodiments, the total amount of HDPE and the optional further polyethylene with respect to the total amount of polymers present in the polyethylene composition is at least 95 wt%, at least 98 wt%, at least 99 wt% or 100 wt%.

In some embodiments,, the total amount of HDPE and the optional further polyethylene with respect to the total polyethylene composition is at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt% or 100 wt%.

Preferably, the polyethylene composition has a Melt Flow Rate of 0.1 -4 g/10 min, more preferably 0.1 -1 g/10min, measured according to IS01 133-1 :201 1 (190 ⁵ C/5 kg).

The production processes of HDPE, LLDPE and LDPE are summarised in Handbook of Polyethylene by Andrew Peacock (2000; Dekker; ISBN 0824795466) at pages 43- HDPE

The HDPE is bimodal or multimodal. Such HDPEs have properties suitable for producing a pipe. Furthermore, such HDPEs can be drawn at a low draw ratio without causing the necking problem.

It is understood that a bimodal HDPE has a molecular weight distribution having two peaks corresponding to the first median and the second median of the respective stages in the polymerization. It is similarly understood that a multimodal HDPE has a molecular weight distribution having multiple peaks corresponding to the first median, the second median and one or more further medians of the respective stages in the polymerization.

HDPE may be an ethylene homopolymer or may comprise a comonomer, for example butene or hexene.

Preferably, the HDPE has a density of 940-960 kg/m ³ , more preferably 940-955 kg/m ³ , measured according to IS01 183.

Preferably, the HDPE has a Melt Flow Rate of 0.1 -4 g/10 min, more preferably 0.1 -1 g/1 Omin, measured according to IS01 133-1 :201 1 (190 ⁵ C/5 kg).

In some embodiments, the composition comprises a compound comprising the HDPE and a colorant, wherein the compound has a density of 947-965 kg/m ³ measured according to IS01 183. The colorant may e.g. be carbon black or a pigment having a color of e.g. black, blue or orange. The amount of the colorant is typically 1 -5 wt%, more typically 2-2.5 wt%, with respect to the compound comprising the HDPE and the colorant, the rest typically being the HDPE.

The HDPE can be produced by using low pressure polymerisation processes. For example, pipe materials of the performance class PE 80 and PE 100 are known, which are generally produced in cascade plants by a so called bimodal or multimodal process. The production processes for bimodal HDPE are summarised at pages 16-20 of "PE 100 Pipe systems" (edited by Bromstrup; second edition, ISBN 3-8027-2728-2). Suitable low pressure processes are slurry cascade of stirred reactors, slurry cascade of loop reactors and a combination of different processes such as slurry loop gas phase reactor. It is also possible to use a multimodal polyethylene, preferably trimodal polyethylene, as described for example in WO2007003530, as high density

polyethylene pipe material.

The performance classes PE 80 and PE 100 are discussed at pages 35- 42 of "PE 100 Pipe systems" (edited by Bromstrup; second edition, ISBN 3-8027-2728- 2). The quality test methods are described at pages 51 -62 of "PE 100 Pipe systems".

The production of bimodal high density polyethylene (HDPE) via a low pressure slurry process is described by Alt et al. in "Bimodal polyethylene-Interplay of catalyst and process" (Macromol.Symp. 2001 , 163, 135-143). In a two stage cascade process the reactors may be fed continuously with a mixture of monomers, hydrogen, catalyst/co- catalyst and hexane recycled from the process. In the reactors, polymerisation of ethylene occurs as an exothermic reaction at pressures in the range between for example 0.5 MPa (5 bar) and 1 MPa (10 bar) and at temperatures in the range between for example 75 ⁵ C and 85 ⁵ C. The heat from the polymerisation reaction is removed by means of cooling water. The characteristics of the polyethylene are determined amongst others by the catalyst system and by the applied concentrations of catalyst, co monomer and hydrogen. The concept of the two stage cascade process is elucidated at pages 137-138 by Alt et al. "Bimodal polyethylene-Interplay of catalyst and process" (Macromol.Symp. 2001 , 163). The reactors are set up in cascade with different conditions in each reactor including low hydrogen content in the second reactor. This allows for the production of HDPE with a bimodal molecular mass distribution and defined co monomer content in the polyethylene chains in each reactor.

Preferred examples of the HDPE include a bimodal PE 80, a bimodal PE 100 and a multimodal HDPE. PE 80 is a PE material with an MRS (minimum required strength after 50 years for water at 20 degrees Celsius) of 8 MPa and PE 100 is a PE material with an MRS of 10 MPa. The pipe classification is elucidated at page 35 of "PE 100 Pipe systems" (edited by Bromstrup; second edition, ISBN 3-8027-2728-2).

Preferably, the HDPE or the compound comprising the HDPE and the colorant has one or more of, preferably all of, the following characteristics:

- Tensile modulus of 500-1400 MPa, preferably 700-1200 MPa (according to ISO 527- 2)

- Yield stress of 15-32 MPa, preferably 18-28 MPa (according to ISO 527-2) - Full Notch Creep Test (FNCT): 100 - 20000 h (according to ISO 16770 @ 80 degrees centigrade / 4 MPa)

- Charpy of 10-35 °-C @ 23 °-C, preferably 14-30 °-C (according to ISO 1 eA). LLDPE

The polyethylene composition may comprise LLDPE.

The technologies suitable for the LLDPE manufacture include gas-phase fluidized-bed polymerization, polymerization in solution, polymerization in a polymer melt under very high ethylene pressure, and slurry polymerization.

The LLDPE comprises ethylene and a C3-C10 alpha-olefin comonomer (ethylene- alpha olefin copolymer). Suitable alpha-olefin comonomers include 1 -butene, 1 - hexene, 4-methyl pentene and 1 -octene. The preferred co monomer is 1 -hexene.

Preferably, the alpha-olefin co monomer is present in an amount of about 5 to about 20 percent by weight of the ethylene-alpha olefin copolymer, more preferably an amount of from about 7 to about 15 percent by weight of the ethylene-alpha olefin copolymer.

Preferably, the LLDPE has a density of 900-948 kg/m ³ , more preferably 915-935 kg/m ³ , more preferably 920-935 kg/m ³ , determined according to IS01872-2.

Preferably, the LLDPE has a Melt Flow Rate of 0.1 -3.0 g/10min, more preferably 0.3- 3.0 g/1 Omin, determined according to IS01 133-1 :201 1 (190°C/2.16kg).

LDPE

The polyethylene composition may comprise LDPE.

The LDPE may be produced by use of autoclave high pressure technology and by tubular reactor technology.

LDPE may be an ethylene homopolymer or may comprise a comonomer, for exampl butene or hexene.

Preferably, the LDPE has a density of 916-940 kg/m ³ , more preferably 920-935 kg/m ³ , determined according to IS01872-2. Preferably, the LDPE has a Melt Flow Rate of 0.1 -3.0 g/10min, more preferably 0.3-3.0 g/10min, determined according to IS01 133-1 :201 1 (190°C/2.16kg).

Process steps

The polyethylene composition may be formed into a tube (step a) by any known method, such as extrusion or injection moulding. The biaxial elongation (step b) may be performed by any known method.

Methods for forming the polyethylene composition into a tube and the biaxial elongation of the tube are described in US6325959:

A conventional plant for extrusion of plastic pipes comprises an extruder, a nozzle, a calibrating device, cooling equipment, a pulling device, and a device for cutting or for coiling-up the pipe. By the molten mass of polymer on its way from the extruder through the nozzle and up to calibration, cooling and finished pipe being subjected to shear and elongation etc. in the axial direction of the pipe, an essentially uniaxial orientation of the pipe in its axial direction will be obtained. A further reason that contributes to the orientation of the polymer material in the direction of material flow is that the pipe can be subjected to tension in connection with the manufacture.

To achieve biaxial orientation, this plant can be supplemented, downstream of the pulling device, with a device for temperature control of the pipe to a temperature that is suitable for biaxial orientation of the pipe, an orienting device, a calibrating device, a cooling device, and a pulling device which supplies the biaxially oriented pipe to a cutting device or coiler.

The biaxial orientation can also be carried out in direct connection with the first calibration after extrusion, in which case the above-described supplementary equipment succeeds the first calibrating device.

The biaxial orientation of the pipe can be carried out in various ways, for instance mechanically by means of an internal mandrel, or by an internal pressurised fluid, such as air or water or the like. A further method is the orienting of the pipe by means of rollers, for instance by arranging the pipe on a mandrel and rotating the mandrel and the pipe relative to one or more pressure rollers engaging the pipe, or via internally arranged pressure rollers that are rotated relative to the pipe against an externally arranged mould or calibrating device.

Conditions for step b)

Preferably, step b) is performed at a drawing temperature which is 1 to 30 ⁵ C lower than the melting point of the polyethylene composition, for example 2 to 20 ⁵ C or 3 to 10 ⁵ C lower than the melting point of the polyethylene composition. When more than one melting point can be measured for the polyethylene composition, step b) is preferably performed at a drawing temperature which is 1 to 30 ⁵ C lower than the highest melting point of the polyethylene composition, for example 2 to 20 ⁵ C or 3 to 10 ⁵ C lower than the highest melting point of the polyethylene composition..

In some embodiments, step b) may be performed at a drawing temperature which is 1 to 30 ^g C lower than the melting point of the HDPE, for example 2 to 20 ^g C or 3 to 10 ^g C lower than the melting point of the HDPE.

In some embodiments, step b) is performed at a drawing temperature of 1 15-123 ⁵ C.

Step b) is performed at a certain axial draw ratio and a certain average hoop draw ratio as described above.

The axial draw ratio of the drawn pipe is defined as the ratio of the cross-sectional area of the starting isotropic tube to that of the biaxially oriented pipe (i.e. product), that is,

(Tube OD) ² - (Tube ID) ²

λ -a. xial (Product OD) ² - (Product ID) ²

OD stands for outer diameter and ID stands for inner diameter.

The average hoop draw ratio can be defined as:

Total Draw Ratio ^■ Total

λ a. verage hoop

-axial

Where

Tube Wall Thickness

^■ Total ^— Product Wall Thickness The relatively low draw ratios were surprisingly found to improve the time-to-failure property.

Biaxially oriented pipe

The invention also relates to the biaxially oriented pipe obtained or obtainable by the process according to the invention.

The biaxially oriented pipe according to the present invention may be a pressure pipe or a non-pressure pipe. The preferred pipe is a pressure pipe.

Examples of suitable biaxially oriented pipes according to the invention have the following outer diameter and inner diameter and wall thickness.

It is noted that the invention relates to all possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

It is further noted that the term 'comprising' does not exclude the presence of other elements. However, it is also to be understood that a description on a

product/composition comprising certain components also discloses a

product/composition consisting of these components. The product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process. When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed. The invention is now elucidated by way of the following examples, without however being limited thereto.

Materials:

HDPE: SABIC grade Vestolen A 6060R having a density of 959 kg/m ³ (black compound density) and MFR 5kg/190 ⁵ C of 0.3 g/10 minutes. Bimodal PE.

Procedure:

HDPE was made into granules using twin screw extruder. Processing temperature and screw profile were of standard polyethylene compounding.

These compounded granules were used to produce thick tubular profiles of

approximate dimensions of an outer diameter of about 60 mm and an inner diameter of about 24 mm. These thick tubes were drawn over an expanding conical mandrel of exit diameter of 61 -65 mm and semi angle 15 degree at temperature of 120°C at a draw speed of 100 mm/min. Axial draw ratio was varied as summarized in Table 1 and the average hoop draw ratio was 1 .4.

Table 1

The time-to-failure of the pipes so obtained was measured according to ISO 13479, as well as of the 'isotropic pipe'. Three pipe samples were tested for each example.

Table 2

Test Time-to-

Sample temperature Test stress failure

[°C] [MPa] [h]

644.8

Isotropic 80 4.6 752.6

757

> 16789

stopped

> 16789

Ex 1 80 4.6

stopped

> 16789

stopped

4659.8

> 17171

Ex 2 80 4.6 stopped

> 17171

stopped

2397.5

CEx 3 80 4.6 2581 .8

3491 The sample 'Isotropic' was made from the same material as Ex 1 , Ex 2 and CEx 3 into a pipe having an outer diameter of 63 mm and an inner diameter of 51 .4 mm by an extrusion without the stretching step. The pipes with low axial draw ratio showed a better resistance to crack propagation.