Process for producing pipe by biaxial elongation

Abstract

The invention relates to a process for producing a biaxially oriented pipe by a) forming a polyethylene composition into a tube, wherein the polyethylene composition comprises high density polyethylene (HDPE) and a second polyethylene selected from linear low density polyethylene (LLDPE), low density polyethylene (LDPE) and a combination of LLDPE and LDPE and b) stretching the tube of step a) in the axial direction and peripheral direction to obtain the biaxially oriented pipe.

Claims

1. A process for producing a biaxially oriented pipe by a) forming a polyethylene composition into a tube, wherein the polyethylene composition comprises high density polyethylene (HDPE) and a second polyethylene comprising a linear low density polyethylene (LLDPE), wherein the LLDPE has a density of 932 to 948 kg/m.sup.3 and b) stretching the tube of step a) in the axial direction and peripheral direction to obtain the biaxially oriented pipe, wherein step b) is performed at an axial draw ratio of at most 5 and an average hoop draw ratio of 1.1 to 2.0; and wherein the obtained biaxially oriented pipe has an outer diameter and a thickness and wherein the outer diameter is 2 mm to 1 cm and the thickness is 0.3 mm to 2 mm, the outer diameter is 1 cm to 10 cm and the thickness is 1 mm to 3 mm, or the outer diameter is 10 cm to 50 cm and the thickness is 1 mm to 1 cm, or the outer diameter is 50 cm to 2 m and the thickness is 5 mm to 10 cm.

2. The process according to claim 1, wherein the HDPE has a density of 940-960 kg/m3 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).

3. The process according to claim 1, wherein the HDPE has a density of 940-955 kg/m.sup.3 measured according to ISO1183 and a Melt Flow Rate of 0.1-4 g/10 min measured according to ISO1133-1:2011 (190° C/5 kg).

4. The process according to claim 1, wherein the LLDPE has a Melt Flow Rate of 0.1-3.0 g/10 min, determined according to ISO1133-1:2011 (190° C/2.16 kg).

5. The process according to claim 1, wherein the second polyethylene further comprises a low density polyethylene (LDPE), wherein the LDPE has a Melt Flow Rate of 0.1-3.0 g/10 min, determined according to ISO1133-1:2011 (190° C/2.16 kg).

6. The process according to claim 1, wherein the second polyethylene further comprises a low density polyethylene (LDPE).

7. The process according to claim 1, wherein the weight ratio of HDPE to the second polyethylene in the polyethylene composition is more than 1.

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

9. The process according to claim 1, wherein the total amount of HDPE and the second polyethylene with respect to the total polyethylene composition is at least 90 wt %.

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

11. The process according to claim 1, wherein step b) is performed at a drawing temperature which is 1 to 30° C. lower than the melting point of the polyethylene composition.

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

13. The process according to claim 1, wherein the HDPE has a density of 940-960 kg/m.sup.3 measured according to ISO1183 and a Melt Flow Rate of 0.1-1 g/10 min, measured according to ISO1133-1:2011 (190° C/5 kg); wherein the HDPE is bimodal or multimodal; wherein the LLDPE has a density of 932 to 935 kg/m.sup.3, determined according to ISO1872-2 and/or a Melt Flow Rate of 0.3-3.0 g/10 min, determined according to ISO1133-1:2011 (190° C/2.16 kg); wherein the second polyethylene further comprises a low density polyethylene (LDPE) wherein the LDPE has a density of 932 to 935 kg/m.sup.3 determined according to ISO1872-2 and/or a Melt Flow Rate of 0.1-3.0 g/10 min, determined according to ISO1133-1:2011 (190° C/2.16 kg); wherein the second polyethylene is LLDPE or a combination of LLDPE and LDPE; wherein the weight ratio of HDPE to the second polyethylene in the polyethylene composition is 1.2-5; wherein the total amount of HDPE and the second polyethylene with respect to the total polyethylene composition is at least 95 wt %; and wherein step b) is performed at an axial draw ratio of at most 4.

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 1, wherein the HDPE has a density of 940-955 kg/m.sup.3 measured according to ISO1183 and a Melt Flow Rate of 0.1-1 g/10 min, measured according to ISO1133-1:2011 (190° C/5 kg); wherein the LLDPE has a Melt Flow Rate of 0.3-3.0 g/10 min, determined according to ISO1133-1:2011 (190° C/2.16 kg); and wherein the second polyethylene further comprises a low density polyethylene (LDPE), wherein the LDPE has a Melt Flow Rate of 0.3-3.0 g/10 min, determined according to ISO1133-1:2011 (190° C/2.16 kg).

16. The process according to claim 1, wherein the weight ratio of HDPE to the second polyethylene in the polyethylene composition is 2:1 to 3:1.

Description

EXAMPLES

(1) Materials:

(2) 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.

(3) LLDPE: SABIC grade LLDPE 6135BE having a density of 932 kg/m.sup.3 and MFR 2.16 kg/190° C. of 0.8 g/10 minutes and MFR 5kg/190° C. of 2.4 g/10 minutes.

(4) LLDPE: SABIC grade LLDPE 6335BE having a density of 935 kg/m.sup.3 and MFR 2.16 kg/190° C. of 2.8 g/10 minutes.

(5) Procedure:

(6) HDPE and LLDPE 6135BE were compounded using twin screw extruder at the weight ratio of 70/30, 50/50 and 30/70. Processing temperature and screw profile were of standard polyethylene compounding. Load-extension curves of these compositions are shown in FIG. 1, along with pure HDPE and pure LLDPE 6135BE.

(7) FIG. 1 shows that the load-extension curve of pure HDPE has a relatively sharp peak, which indicates a neck with high natural draw ratio. Other samples do not show such behavior, indicating that necking is suppressed.

(8) These compounded granules of blends were used to produce thick tubular profiles of approximate dimensions of outer diameter 30 mm and inner diameter 15 mm. These thick tubes were drawn over an expanding conical mandrel of exit diameter of 32 mm and semi angle 15 degree at temperature of 120° C. Tubes from the blend of HDPE/LLDPE (70/30) were drawn very uniformly in thickness and could be drawn to low axial draw ratio of 1.5 at slow drawing speed of 5 cm/minute.

(9) Hoop tensile strength was measured for the produced pipes. The biaxial drawing increases the hoop tensile strength. It can be understood that the blend of HDPE/LLDPE (70/30) allows a stable manufacturing of a pipe where necking is suppressed, with a hoop tensile strength of the obtained pipe much higher than the non-drawn HDPE.

(10) TABLE-US-00001 TABLE 1 Av. Hoop Axial Hoop Tensile LLDPE Draw Draw Strength HDPE 6135BE Ratio Ratio (MPa) 100 0 1 1 19.5 ± 1.2 100 0 3 1.4 28.5 ± 1.5 70 30 1 1 .sup. 20 ± 1.5 70 30 3 1.4 .sup. 25 ± 0.8 50 50 1 1 16.5 ± 1.6 50 50 3 1.4 20.6 ± 0.5

(11) Production and Testing of Pipes

(12) Preparation of Blends

(13) HDPE and LLDPE 6335BE were compounded using twin screw extruder at the weight ratio of 70/30, 80/20 and 90/10. Processing temperature and screw profile were of standard polyethylene compounding.

(14) Biaxially Oriented Pipes, Inventive Examples (Inv.)

(15) The compounded granules of blends were used to produce pipes having an outer diameter of 32 mm and an average wall thickness of 3 mm. To obtain biaxially oriented pipes (Inventive examples=Inv., table 2), the pipes were drawn over an expanding conical mandrel of exit diameter of 32 mm and semi angle 15 degree at temperature of 120° C. Pipes from the blend of HDPE/LLDPE (70/30, 80/20 and 90/10) were drawn very uniformly in thickness and could be drawn to low axial draw ratio of 1.5 at slow drawing speed of 5 cm/minute.

(16) Pipes, Comparative Examples (Comp.)

(17) The compounded granules of blends were used to produce pipes having an outer diameter of 32 mm and an average wall thickness of 3 mm.

(18) Testing

(19) The resistance to internal pressure of pipes has been determined at different stress levels at 20° C. according to ISO 1167-1 on pipes (Inventive and comparative examples) having an outer diameter of 32 mm and an average wall thickness of 3 mm. The results are shown in Table 2.

(20) TABLE-US-00002 TABLE 2 Results of pipe testing of HDPE/LLDPE blends. LLDPE Sigma16/ Sigma18/ Sigma20/ HDPE 6335BE Draw 20° C. 20° C. 20° C. wt % wt % Ratio [hours] [hours] [hours] Comp. 100 0 1 1.77 Comp. 90 10 1 0.92 0.18 0.05 Comp. 80 20 1 0.73 0.14 0.03 Comp. 70 30 1 0.38 0.08 0.01 Inv. 90 10 2.4 >2500 417 25 Inv. 80 20 2.5 >2500 >2500 73 Inv. 70 30 2.5 >2500 >2500 33

(21) It can be seen from the results presented in Table 2 that biaxially oriented pipes (Inventive samples=Inv.) of the said composition greatly outperformed the as melt extruded pipes (Comparative examples=Comp.) of the same composition.