PROCESS FOR PRODUCING PIPE BY BIAXIAL ELONGATION

Abstract

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

Claims

1. A process for producing a biaxially oriented pipe, comprising: 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 C. lower than the melting point of the polyethylene composition.

2. The process according to claim 1, wherein the outer diameter of the biaxially oriented pipe is at least 60 mm and the axial draw ratio is at least 1.2 and/or at most 3.0.

3. The process according to claim 1, wherein the outer diameter of the biaxially oriented pipe is 60 to 150 mm and a wall thickness of 5.5 to 15 mm.

4. The process according to claim 1, wherein the outer diameter of the biaxially oriented pipe is less than 60 mm and the axial draw ratio is at least 1.2 and/or at most 1.8.

5. The process according to claim 1, wherein the biaxially oriented pipe has an outer diameter of 10 to 40 mm and a wall thickness of 1.5 to 5 mm.

6. The process according to claim 1, wherein the HDPE has a density of 940-960 kg/m.sup.3 measured according to ISO1183 and/or a Melt Flow Rate of 0.1-4 g/10 min, measured according to ISO1133-1:2011 (190 C./5 kg).

7. The process according to claim 1, wherein the amount of HDPE with respect to polyethylene present in the polyethylene composition is at least 95 wt %.

8. The process according to claim 1, wherein the polyethylene composition has a Melt Flow Rate of 0.1-4 g/10 min, measured according to ISO1133-1:2011 (190 C./5 kg).

9. The process according to claim 1, wherein the composition further comprises 0 to 5 wt % of additives and 0 to 40 wt % of fillers.

10. The process according to claim 1, wherein step b) is performed at a drawing temperature of 115-123 C.

11. The process according to claim 1, wherein the process is a continuous process.

12. The biaxially oriented pipe obtained by the process according to claim 1.

13. The process according to claim 1, wherein the outer diameter of the biaxially oriented pipe is at least 60 mm and the axial draw ratio is at least 1.5 and at most 2.8, wherein the amount of HDPE with respect to polyethylene present in the polyethylene composition is at least 98 wt %, wherein the polyethylene composition has a Melt Flow Rate of 0.1-1 g/10 min, measured according to ISO1133-1:2011 (190 C./5 kg),

14. The process according to claim 13, wherein step b) is performed at a drawing temperature of 115-123 C.

15. The process according to claim 13, wherein the process is a continuous process.

16. The process according to claim 1, wherein the outer diameter of the biaxially oriented pipe is less than 60 mm and the axial draw ratio is at least 1.3 and at most 1.8, wherein the amount of HDPE with respect to polyethylene present in the polyethylene composition is at least 98 wt %, wherein the polyethylene composition has a Melt Flow Rate of 0.1-4 g/10 min, measured according to ISO1133-1:2011 (190 C./5 kg).

17. The process according to claim 16, wherein step b) is performed at a drawing temperature of 115-123 C.

18. The process according to claim 16, wherein the process is a continuous process.

Description

CPROCESS STEPS

[0063] 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.

[0064] Methods for forming the polyethylene composition into a tube and the biaxial elongation of the tube are described in U.S. Pat. No. 6,325,959:

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Conditions for Step b)

[0070] Preferably, step b) is performed at a drawing temperature which is 1 to 30 C. lower than the melting point of the polyethylene composition, for example 2 to 20 C. or 3 to 10 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 C. lower than the highest melting point of the polyethylene composition, for example 2 to 20 C. or 3 to 10 C. lower than the highest melting point of the polyethylene composition.

[0071] In some embodiments, step b) may be performed at a drawing temperature which is 1 to 30 C. lower than the melting point of the HDPE, for example 2 to 20 C. or 3 to 10 C. lower than the melting point of the HDPE.

[0072] In some embodiments, step b) is performed at a drawing temperature of 115-123 C.

[0073] Step b) is performed at a certain axial draw ratio and a certain average hoop draw ratio as described above.

[0074] 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,

[00001]${}_{\mathrm{axial}}=\frac{{\left(\mathrm{Tube}.Math..Math.\mathrm{OD}\right)}^2-{\left(\mathrm{Tube}.Math..Math.\mathrm{ID}\right)}^2}{{\left(\mathrm{Product}.Math..Math.\mathrm{OD}\right)}^2-{\left(\mathrm{Product}.Math..Math.\mathrm{ID}\right)}^2}$

[0075] OD stands for outer diameter and ID stands for inner diameter.

[0076] The average hoop draw ratio can be defined as:

[00002]${}_{\mathrm{average}.Math..Math.\mathrm{hoop}}=\frac{\mathrm{Total}.Math..Math.\mathrm{Draw}.Math..Math.\mathrm{Ratio}.Math..Math.{}_{\mathrm{Total}}}{\mathrm{Axial}.Math..Math.\mathrm{Draw}.Math..Math.\mathrm{Ratio}.Math..Math.{}_{\mathrm{axial}}}$

[0077] Where

[00003]${}_{\mathrm{Total}}=\frac{\mathrm{Tube}.Math..Math.\mathrm{Wall}.Math..Math.\mathrm{Thickness}}{\mathrm{Product}.Math..Math.\mathrm{Wall}.Math..Math.\mathrm{Thickness}}$

[0078] The relatively low draw ratios were surprisingly found to improve the time-to-failure property.

[0079] Biaxially Oriented Pipe

[0080] The invention also relates to the biaxially oriented pipe obtained or obtainable by the process according to the invention.

[0081] 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.

[0082] Examples of suitable biaxially oriented pipes according to the invention have the following outer diameter and inner diameter and wall thickness.

TABLE-US-00002 Outer diameter (mm) Inner diameter (mm) Wall thickness (mm) 110 90 10.0 90 73.6 8.2 75 61.4 6.8 63 51.4 5.8 32 26 3.0

[0083] 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.

[0084] 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.

[0085] 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.

[0086] The invention is now elucidated by way of the following examples, without however being limited thereto.

[0087] Materials:

[0088] HDPE: SABIC grade Vestolen A 6060R having a density of 959 kg/m.sup.3 (black compound density) and MFR 5 kg/190 C. of 0.3 g/10 minutes. Bimodal PE.

[0089] Procedure:

[0090] HDPE was made into granules using twin screw extruder. Processing temperature and screw profile were of standard polyethylene compounding.

[0091] 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-US-00003 TABLE 1 Av Mandrel Tube Tube Tube Product Product Product Axial Hoop dia OD ID wall OD ID wall Draw Draw Ex (mm) (mm) (mm) (mm) (mm) (mm) (mm) Ratio Ratio Isotro 63 51.4 5.8 63 51.4 5.8 1 1 pic Ex 1 61 60 24 18 63.8 51 6.4 2 1.4 Ex 2 63 61 24.6 18.2 63.2 54.5 4.35 3 1.4 CEx 3 65 61 24.6 18.2 63.5 57 3.25 4 1.4

[0092] 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-US-00004 TABLE 2 Test Time-to- temperature Test stress failure Sample [ C.] [MPa] [h] Isotropic 80 4.6 644.8 752.6 757 Ex 1 80 4.6 >16789 stopped >16789 stopped >16789 stopped Ex 2 80 4.6 4659.8 >17171 stopped >17171 stopped CEx 3 80 4.6 2397.5 2581.8 3491

[0093] 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.

[0094] The pipes with low axial draw ratio showed a better resistance to crack propagation.