PROCESS FOR MODIFYING AND JOINING ORIENTED PIPES

Abstract

The invention relates to a modification process for modifying a biaxially oriented pipe, comprising a) providing a biaxially oriented pipe made by stretching a tube made of a thermoplastic polymer composition in the axial direction and in the peripheral direction, b) placing an insert within an end portion of the pipe, wherein the outer periphery of the cross section of the insert substantially matches the inner periphery of the cross section of the pipe and c) heating the end portion such that the end portion axially shrinks while the inner periphery of the cross section of the end portion is substantially maintained, to obtain a modified biaxially oriented pipe with a thickened end portion

Claims

1. A modification process for modifying a biaxially oriented pipe, comprising a) providing a biaxially oriented pipe made by stretching a tube made of a thermoplastic polymer composition in the axial direction and in the peripheral direction, b) placing an insert within an end portion of the pipe, wherein the outer periphery of the cross section of the insert substantially matches the inner periphery of the cross section of the pipe, and c) heating the end portion such that the end portion axially shrinks while the inner periphery of the cross section of the end portion is substantially maintained, to obtain a modified biaxially oriented pipe with a thickened end portion.

2. The process according to claim 1, wherein step a) involves a1) forming a thermoplastic polymer composition into a tube, and a2) stretching the tube of step a1) in the axial direction at an axial draw ratio of 1.1 to 5.0 and in the peripheral direction at an average hoop draw ratio of 1.1 to 2.0 to obtain the biaxially oriented pipe.

3. The process according to claim 2, wherein step a2) involves drawing the tube at a drawing temperature which is 1 to 30 C. lower than the melting point of the thermoplastic polymer composition.

4. The process according to claim 3, wherein step c) involves heating the end portion at a temperature at or higher than the drawing temperature.

5. The process according to claim 1, wherein the end portion is heated from the inside and/or the outside of the end portion.

6. The process according to claim 1, wherein the insert comprises a thermally conductive portion and a thermally insulating portion, wherein step b) involves placing the insert in the pipe such that the thermally conductive portion is closer to the butt-end of the end portion of the pipe than the thermally insulating portion and step c) involves heating the thermally conductive portion of the insert.

7. The process according to claim 1, wherein the thermoplastic polymer is selected from the group consisting of polyethylene, polypropylene, polyvinylchloride, polyester, polycarbonate, polyamide, polyacetal, polyimide, polyvinylidene fluoride and polyether ether ketone and combinations thereof.

8. The process according to claim 1, wherein the thermoplastic polymer comprises high density polyethylene or random polypropylene.

9. The process according to claim 1, wherein the thickened end portion has a thickness which is 110-250% of the original thickness of the end portion.

10. The process according to claim 1, wherein the original thickness of the end portion is 0.3 mm to 100 mm.

11. The process according to claim 1, wherein the thickened end portion has a minimum ultimate tensile load of at least 80% of the minimum ultimate tensile load of the end portion of the original pipe measured according to ASTM D2290.

12. A pipe joining process, comprising I) performing the modification process according to claim 1 to obtain a first modified biaxially oriented pipe and a second modified biaxially oriented pipe and II) joining the butt-end of the thickened end portion of the first biaxially oriented pipe and the butt-end of the thickened end portion of the second biaxially oriented pipe.

13. The pipe joining process according to claim 12, wherein step II) involveswelding, solvent joining or electrofusion.

14. A joined pipe comprising two biaxially oriented pipes and a joint portion between the two biaxially oriented pipes, wherein the biaxially oriented pipes has an outer diameter of 2 mm to 2000 mm and a thickness of 0.3 mm to 100 mm and are obtained by drawing a tube made of a thermoplastic polymer composition at an axial draw ratio of 1.1-5.0 and a hoop draw ratio of 1.1-2.0, wherein the joint portion is made of the same thermoplastic polymer composition as the tube, wherein the two biaxially oriented pipes and the joint portion have the same inner diameter and the joint portion has a larger thickness than the biaxially oriented pipes.

15. (canceled)

16. The process according to claim 1, wherein the thickened end portion has a minimum ultimate tensile load of at least 100% of the minimum ultimate tensile load of the end portion of the original pipe measured according to ASTM D2290.

17. The process according to claim 1, wherein step a) involves a1) forming a thermoplastic polymer composition into a tube, and a2) stretching the tube of step a1) in the axial direction at an axial draw ratio of 1.1 to 5.0 and in the peripheral direction at an average hoop draw ratio of 1.1 to 2.0, and drawing the tube at a drawing temperature which is 1 to 30 C. lower than the melting point of the thermoplastic polymer composition, to obtain the biaxially oriented pipe, wherein the insert comprises a thermally conductive portion and a thermally insulating portion, wherein step b) involves placing the insert in the pipe such that the thermally conductive portion is closer to the butt-end of the end portion of the pipe than the thermally insulating portion, and wherein step c) involves heating the thermally conductive portion of the insert.

18. The process according to claim 17, wherein the thermoplastic polymer comprises high density polyethylene or random polypropylene.

19. The process according to claim 17, wherein the thickened end portion has a thickness which is 110-250% of the original thickness of the end portion, wherein the original thickness of the end portion is 0.3 mm to 100 mm.

20. The process according to claim 1, wherein the thickened end portion has a minimum ultimate tensile load of at least 90% of the minimum ultimate tensile load of the end portion of the original pipe measured according to ASTM D2290.

Description

EXAMPLES

[0125] Preparation of Die-Drawn Pipes

[0126] Circular HDPE tubes of outer diameter 60 mm and inner diameter 25 mm were melt extruded. These thick isotropic tubes were drawn over a conical mandrel of exit diameter 59 mm at a temperature of 120 C.

[0127] Biaxially oriented pipes were produced using the batch die-drawing facility. A series of biaxially oriented pipes with an outer diameter of 64.9(0.8) mm and an inner diameter of 56.6(0.3) mm were prepared by drawing at an axial draw ratio of about 3.5 and an average hoop draw ratio of about 1.4.

[0128] Preparation of Thickened End Portions

[0129] A cylindrical insert of the same outer diameter as the inner diameter of the oriented pipe was placed inside one end of the biaxially oriented pipe. This insert is made of a cylinder made of steel and a cylinder made of nylon joined together. The insert was inserted in the oriented pipe such that the end of the steel cylinder is flush with the butt-end. The steel cylinder was heated to 130-140 C., close to the melting temperature of the HDPE pipe material while the rest of the pipe in its vicinity was kept well below this temperature over the nylon cylinder. This setup is shown in FIGS. 1 and 2. FIGS. 1 and 2 illustrate the heating tool used for preparation of the end portions of pipes at time t=0 and t=10 minutes, respectively.

[0130] Upon heating the pipe end, the pipe end started shrinking axially while the inner diameter was maintained due to constraint by the insert. The outer and inner diameters of the end portions changed from values of 64.9(0.8) mm and 56.6(0.3) mm to 70.6(0.4) mm and 54.5(0.5) mm. The process ultimately resulted in a significant increase in the wall thickness of the end portion from 4.1 mm to 8 mm.

[0131] Burst Pressures

[0132] The burst pressures of pipes can be calculated via Lame relation

[00004]$P=S.Math.\frac{\left(D^2-d^2\right)}{\left(D^2+d^2\right)}$

[0133] where P represents the burst pressure, S the minimum ultimate tensile strength of pipe material, D the outer pipe diameter and d the inner pipe diameter. The minimum ultimate tensile strength of pipe material is measured as hoop tensile strength of pipes measured using split disk method ASTM D2290. Table 1 shows the variations in burst pressures, calculated for different values of the pipe dimensions to illustrate the influence of the pipe wall thickness.

TABLE-US-00002 TABLE 1 S D d Wall thickness P pipe [MPa] [mm] [mm] [mm] [MPA] isotropic 20.sup. 63 51.4 5.8 4.0 biaxially 35.sup. 63 51.4 5.8 7.0 oriented annealed end 25.sup.1) 70 51.4 9.3 7.5 25.sup.1) 72 51.4 10.3 8.1 20.sup.2) 70 51.4 9.3 6.0 20.sup.2) 72 51.4 10.3 6.5 .sup.1)Partial loss of orientation .sup.2)Complete loss of orientation at the joints

[0134] These calculations demonstrate that the joints having a high wall thickness have a high burst pressure, which will compensate for the loss of the orientation.

[0135] Changes in the Crystalline Morphology Related to Heating

[0136] A series of pole figure diagrams were derived from WAXD experiments on each sample, machined from different positions within the pipe cross sections.

[0137] It was found that the heating resulted in melting and a loss of orientation within a very narrow region near the inner pipe surface. The orientation at the outer pipe surface was maintained to a large degree. There was a higher decrease in the degree of orientation in the axial direction than in the hoop direction.