Polyethylene composition for drip irrigation pipes or tapes

Abstract

The present invention relates to pellets comprising a polyethylene composition, a pipe or pipe system comprising the pellets as well as a process for the preparation of such a pipe or pipe system.

Claims

1. Pellets comprising a polyethylene composition, the polyethylene composition comprising: a) at least 60.0 wt. % of a multimodal ethylene polymer (a), based on the total weight (100.0 wt. %) of the polyethylene composition, and b) from 0.5 to 13.0 wt. % of carbon black product (b), based on the total weight (100.0 wt. %) of the polyethylene composition, wherein the polyethylene composition has i) a MFR.sub.2 (ISO 1133, 2.16 kg load) of 0.2 to 0.3 g/10 min, and ii) a density measured according to ASTM D792 of at least 959 kg/m.sup.3.

2. The pellets according to claim 1, wherein the multimodal ethylene polymer (a) has: a) a density measured according to ASTM D792 in the range from 950 to 965 kg/m.sup.3, and/or b) a MFR.sub.2 (ISO 1133, 2.16 kg load) of 0.2 to 0.3 g/10 min, and/or c) a MFR.sub.5 (ISO 1133, 5 kg load) of 0.5 to 1.5 g/10 min, and/or d) a MFR.sub.21 (ISO 1133, 21.6 kg load) of 20 to 35 g/10 min, and/or e) a FRR.sub.21/2 (ISO 1133, 21.6 kg load/2.16 kg load) of 90 to 130 g/10 min.

3. The pellets according to claim 1, wherein the carbon black product (b) is carbon black as such or a carbon black masterbatch comprising carbon black and carrier polymer(s).

4. The pellets according to claim 1, wherein the carbon black product (b) is carbon black as such (neat) and present in the polyethylene composition in an amount from 0.5 to 10.0 wt. %, based on the total weight (100.0 wt.-%) of the polyethylene composition, or the carbon black product (b) is a carbon black masterbatch and present in the polyethylene composition in an amount from 0.5 to 10.0 wt., based on the total weight (100.0 wt.-%) of the polyethylene composition.

5. The pellets according to claim 1, wherein the polyethylene composition has: a) a MFR.sub.5 (ISO 1133, 5 kg load) of 0.5 to 1.5 g/10 min, and/or b) a MFR.sub.21 (ISO 1133, 21.6 kg load) of 20 to 35 g/10 min, and/or c) a FRR.sub.21/2 (ISO 1133, 21.6 kg load/2.16 kg load) of 100 to 140 g/10 min.

6. The pellets according to claim 1, wherein the polyethylene composition has: a) an eta (0.05 rad/s) of at least 51 000 Pas, and/or b) a SHI.sub.2.7/210 of 30 to 50, and/or c) a die swell (190 C., 2.16 kg load) of at least 1.25.

7. The pellets according to any one of claims 1 to 6, wherein the polyethylene composition has: a) a stress at yield measured according to ISO 527-2 of at least 28 MPa, and/or b) a stress at break measured according to ISO 527-2 of at least 28 MPa, and/or c) a strain at break measured according to ISO 527-2 of at least 800%.

8. A pipe or pipe system comprising the pellets according to claim 1.

9. The pipe or pipe system according to claim 8, wherein the pipe or pipe system is a drip irrigation pipe or drip irrigation pipe system.

10. The pipe or pipe system according to claim 8, wherein the pipe or pipe system has a wall thickness of less than 0.4 mm.

11. The pipe or pipe system according to claim 8, wherein the pipe or pipe system has a wall thickness of less than 0.2 mm and a burst pressure of more than 0.26 MPa.

12. A process for the preparation of a pipe or pipe system comprising the steps of: a) providing pellets according to claim 1, b) extruding the pellets of step a), whereby a temperature profile of up to 270 C. is maintained over the length of the extruder, such as to obtain a pipe or pipe system, c) introducing perforations for irrigation into the pipe or pipe system obtained in step b).

13. The process according to claim 12, wherein the line speed is in the range from 190 to 280 m/min.

14. The process according to claim 12, wherein the pellets of step a) are prepared by compounding the multimodal ethylene polymer (a) and the carbon black product (b) at a temperature profile of up to 270 C.

15. The process according to claim 14, wherein the multimodal ethylene polymer (a) is prepared by i) polymerizing ethylene such as to form a LMW component (A), and ii) polymerizing ethylene and optionally at least one C3-20 alpha olefin comonomer in the presence of component (A) obtained in step i) such as to form a HMW component (B), and iii) compounding the product obtained in step ii) to yield pellets.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 refers to a micrographic picture of comparative example CE2(a).

(2) FIG. 2 refers to a micrographic picture of comparative example CE2(b).

(3) FIG. 3 refers to a micrographic picture of inventive example IE.

EXAMPLES

1. Test Methods

(4) a) Melt Flow Rate

(5) The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR.sub.5 of polyethylene is measured at a temperature 190 C. and a load of 5 kg, the MFR.sub.2 of polyethylene at a temperature 190 C. and a load of 2.16 kg and the MFR.sub.21 of polyethylene is measured at a temperature of 190 C. and a load of 21.6 kg. The quantity FRR (flow rate ratio) denotes the ratio of flow rates at different loads. Thus, FRR.sub.21/2 denotes the ratio of MFR.sub.21/MFR.sub.2.

(6) b) Density

(7) Density of the polymer was measured according to ASTM; D792, Method B (density by balance at 23 C.) on compression moulded specimen prepared according to EN ISO 1872-2 (February 2007) and is given in kg/m.sup.3.

(8) c) Comonomer Content

(9) Comonomer content in polyethylene was measured in a known manner based on Fourier transform infrared spectroscopy (FTIR) calibrated with .sup.13C-NMR, using Nicolet Magna 550 IR spectrometer together with Nicolet Omnic FTIR software.

(10) Films having a thickness of about 250 m were compression molded from the samples. Similar films were made from calibration samples having a known content of the comonomer. The comonomer content was determined from the spectrum from the wave number range of from 1430 to 1100 cm.sup.1. The absorbance is measured as the height of the peak by selecting the so-called short or long base line or both. The short base line is drawn in about 1410-1320 cm.sup.1 through the minimum points and the long base line about between 1410 and 1220 cm.sup.1. Calibrations need to be done specifically for each base line type. Also, the comonomer content of the unknown sample needs to be within the range of the comonomer contents of the calibration samples.

(11) d) Die Swell

(12) The extrudate swell (die swell) was evaluated by measuring afterwards the strands cut during the MFR measurement according to ISO 1133, at 190 C. with 2.16 kg load. Three pieces of ca 2.5 cm long strands were collected and the diameters were measured with a caliber (readability 0.01 mm).

(13) The die swell results are expressed as swell ratios (SR), i.e. ratios of the diameter of the extruded strand to the diameter of the capillary die (=2.095 mm). The reported swell ratios were calculated from the averages of measured strand diameters.

(14) e) Tensile Properties

(15) Stress at yield, stress at break and strain at break were measured on injection molded samples according to ISO 527-2, Specimen type Multipurpose bar 1A, 4 mm thick. Tensile modulus was measured at a speed of 1 mm/min. Sample preparation was done according to ISO 1872-2.

(16) f) Rheological Parameters Shear Thinning Index SHI.sub.2.7/210

(17) The characterization of polymer melts by dynamic shear measurements complies with ISO standards 6721-1 and 6721-10. The measurements were performed on an Anton Paar MCR501 stress controlled rotational rheometer, equipped with a 25 mm parallel plate geometry. Measurements were undertaken on compression molded plates using nitrogen atmosphere and setting a strain within the linear viscoelastic regime. The oscillatory shear tests were done at 190 C. applying a frequency range between 0.01 and 600 rad/s and setting a gap of 1.3 mm.

(18) In a dynamic shear experiment the probe is subjected to a homogeneous deformation at a sinusoidal varying shear strain or shear stress (strain and stress controlled mode, respectively). On a controlled strain experiment, the probe is subjected to a sinusoidal strain that can be expressed by
(t)=.sub.0 sin(t)(1)
If the applied strain is within the linear viscoelastic regime, the resulting sinusoidal stress response can be given by
(t)=.sub.0 sin(t+)(2)
where .sub.0, and .sub.0 are the stress and strain amplitudes, respectively; is the angular frequency; is the phase shift (loss angle between applied strain and stress response); t is the time.

(19) Dynamic test results are typically expressed by means of several different rheological functions, namely the shear storage modulus, G, the shear loss modulus, G, the complex shear modulus, G*, the complex shear viscosity, *, the dynamic shear viscosity, , the out-of-phase component of the complex shear viscosity, and the loss tangent, tan , which can be expressed as follows:

(20) $\begin{array}{cc}{G}^{}=\frac{{}_0}{{}_0}\cos \left[\mathrm{Pa}\right]& \left(3\right)\\ G^{}=\frac{{}_0}{{}_0}\sin \left[\mathrm{Pa}\right]& \left(4\right)\\ G^{*}=G^{}+{\mathrm{iG}}^{}\left[\mathrm{Pa}\right]& \left(5\right)\\ {}^{*}={}^{}-i{}^{}\left[\mathrm{Pa}.Math.s\right]& \left(6\right)\\ {}^{}=\frac{G^{}}{}\left[\mathrm{Pa}.Math.s\right]& \left(7\right)\\ {}^{}=\frac{G^{}}{}\left[\mathrm{Pa}.Math.s\right]& \left(8\right)\end{array}$

(21) The determination of so-called Shear Thinning Index, which correlates with MWD and is independent of Mw, is done as described in equation 9.

(22) $\begin{array}{cc}{\mathrm{SHI}}_{\left(x/y\right)}=\frac{{\mathrm{Eta}}^{*}\mathrm{for}\left(G^{*}=x\mathrm{kPa}\right)}{{\mathrm{Eta}}^{*}\mathrm{for}\left(G^{*}=y\mathrm{kPa}\right)}& \left(9\right)\end{array}$

(23) For example, the SHI.sub.(2.7/210) is defined by the value of the complex viscosity, in Pa s, determined for a value of G* equal to 2.7 kPa, divided by the value of the complex viscosity, in Pa s, determined for a value of G* equal to 210 kPa.

(24) The values of storage modulus (G), loss modulus (G), complex modulus (G*) and complex viscosity (*) were obtained as a function of frequency ().

(25) Thereby, e.g. *.sub.300 rad/s (eta*.sub.300 rad/s) is used as abbreviation for the complex viscosity at the frequency of 300 rad/s and *.sub.0.05 rad/s (eta*.sub.0.05 rad/s) is used as abbreviation for the complex viscosity at the frequency of 0.05 rad/s.

(26) The loss tangent tan (delta) is defined as the ratio of the loss modulus (G) and the storage modulus (G) at a given frequency. Thereby, e.g. tan.sub.0.05 is used as abbreviation for the ratio of the loss modulus (G) and the storage modulus (G) at 0.05 rad/s and tan.sub.300 is used as abbreviation for the ratio of the loss modulus (G) and the storage modulus (G) at 300 rad/s. The elasticity balance tan.sub.0.05/tan.sub.300 is defined as the ratio of the loss tangent tan.sub.0.05 and the loss tangent tan.sub.300.

(27) Besides the above mentioned rheological functions one can also determine other rheological parameters such as the so-called elasticity index EI(x). The elasticity index EI(x) is the value of the storage modulus (G) determined for a value of the loss modulus (G) of x kPa and can be described by equation 10.
EI(x)=G for (G=x kPa) [Pa](10)
For example, the EI(5 kPa) is the defined by the value of the storage modulus (G), determined for a value of G equal to 5 kPa.

(28) The viscosity eta.sub.747 is measured at a very low, constant shear stress of 747 Pa and is inversely proportional to the gravity flow of the polyethylene composition, i.e. the higher eta.sub.747 the lower the sagging of the polyethylene composition.

(29) The polydispersity index, PI, is defined by equation 11.

(30) $\begin{array}{cc}\mathrm{PI}=\frac{{10}^5}{G^{}\left({}_{\mathrm{COP}}\right)},{}_{\mathrm{COP}}=\mathrm{for}\left(G^{}=G^{}\right)& \left(11\right)\end{array}$
where .sub.COP is the cross-over angular frequency, determined as the angular frequency for which the storage modulus, G, equals the loss modulus, G.

(31) The values are determined by means of a single point interpolation procedure, as defined by Rheoplus software. In situations for which a given G* value is not experimentally reached, the value is determined by means of an extrapolation, using the same procedure as before. In both cases (interpolation or extrapolation), the option from Rheoplus Interpolate y-values to x-values from parameter and the logarithmic interpolation type were applied.

(32) References:

(33) [1] Rheological characterization of polyethylene fractions, Heino, E. L., Lehtinen, A., Tanner J., Seppl, J., Neste Oy, Porvoo, Finland, Theor. Appl. Rheol., Proc. Int. Congr. Rheol, 11th (1992), 1, 360-362. [2] The influence of molecular structure on some rheological properties of polyethylene, Heino, E. L., Borealis Polymers Oy, Porvoo, Finland, Annual Transactions of the Nordic Rheology Society, 1995. [3] Definition of terms relating to the non-ultimate mechanical properties of polymers, Pure & Appl. Chem., Vol. 70, No. 3, pp. 701-754, 1998.

2. Examples

Example Ex 1: Production of the Multimodal Ethylene Polymer of the Invention

(34) A loop reactor having a volume of 500 dm.sup.3 was operated at 95 C. and 60 bar pressure for producing the lower molecular weight polymer component (A). Into the reactor were introduced 110 kg/h of propane diluent, ethylene and hydrogen together with the Lynx 200 (TM) catalyst as manufactured and supplied by BASF (SE) and TEAL (triethylaluminium) as the cocatalyst.

(35) The polymer slurry was withdrawn from the second loop reactor and transferred into a flash vessel operated at 3 bar pressure and 70 C. temperature where the hydrocarbons were substantially removed from the polymer. The polymer was then introduced into a gas phase reactor operated at a temperature of 85 C. and a pressure of 20 bar. In addition ethylene, 1-butene, nitrogen as inert gas and hydrogen were introduced into the reactor. The polymerization feeds and conditions are shown in Table 1. The resulting polymer was purged with nitrogen (about 50 kg/h) for one hour, stabilised with conventional UV stabilisers and Ca-stearate and then extruded to pellets in a counter-rotating twin screw extruder CIM90P (manufactured by Japan Steel Works) so that the throughput was 221 kg/h and the screw speed was 349 rpm. The temperature profile is in each zone was 90/120/190/250 C.

(36) TABLE-US-00001 TABLE 1 Polymerisation feeds and conditions IE Reactor 1 - Loop Temperature ( C.) 95 Pressure (kPa) 60 C2 concentration (mol %) 4.0 H2/C2 ratio (mol/kmol) 670 C4/C2 ratio (mol/kmol) 0 Production rate (kg/h) 36 split % 51 MFR.sub.2 (g/10 min) 310 Density (kg/m.sup.3) 970 Reactor 2 - Gas Phase Temperature ( C.) 85 Pressure (kPa) 20 H2/C2 ratio (mol/kmol) 100 C4/C2 ratio (mol/kmol) 50 Production rate (kg/h) 34 Split % 49 MFR.sub.2 (g/10 min) 0.2 MFR.sub.5 (g/10 min) 1.1 MFR.sub.21 (g/10 min) 28 Final density (kg/m.sup.3) 954

(37) Pellets of the inventive example IE and comparative example CE1 were prepared by compounding 93.9 wt.-% of base PE resin, 5.8 wt.-% of a carbon black masterbatch (CB in LLDPE carrier) and 0.3 wt.-% of conventional additives (antioxidants) in KOBE LCM80H-continuous mixer using a temperature profile of 201/199/183/177/185/190/220/225 C. SEI was 0.210 kW/hr.

(38) The pellet properties of the inventive and comparative examples are outlined in table 2.

(39) TABLE-US-00002 TABLE 2 Properties of the pellets of the comparative and inventive examples. CE1 CE2 IE Appearance Black Natural Black Amount of carbon black 2.6 0 2.1 (wt.-%*) MFR.sub.2 (g/10 min) 0.5 0.3 0.24 MFR.sub.5 (g/10 min) 1.9 1.3 1.1 MFR.sub.21 (g/10 min) 36 33 28 FRR 21/2 70 110 120 Eta at 0.05 rad/s 26 000 44 000 53 000 (Pa .Math. s) SHI (2.7/210) 20 44 40 Stress at yield (MPa) 24 31 30 Stress at break (MPa) 24 16 30 Strain at break (MPa) 700 850 820 Die swell at 190 C., 1.2 1.3 1.3 2.16 kg Density (kg/m.sup.3) 959 959 965 *wt.-% of carbon black was based on the total amount of the polyethylene composition (100.0 wt.-%) and refers to the amount of carbon black as such. CE1 comprises a polymer being a natural base resin having a density of 949 kg/m.sup.3. CE2 is a natural base resin having a density of 959 kg/m.sup.3. Carbon black was added prior pipe extrusion.

(40) Both natural and black pellets were produced. The pellets denoted natural were pellets without carbon black, whereas the pellets denoted black contained carbon black.

(41) Drip Irrigation Pipe Production

(42) The thin wall drip irrigation pipe production of CE1 was carried out by using a single screw extruder having screw diameter of 75 mm and L/D ratio of 40. The temperature profile was 245-245-245-245-245 C.

(43) CE2(a), CE2(b) and IE samples of thin wall drip irrigation pipes were prepared using an extruder having a screw diameter of 65 mm and L/D ratio of 38. The temperature profile was 180-250-265-265-265-265 C. Table 3 summarises the pipe production results and the tape properties. The CE2(a) and CE2(b) pipes were produced using a dry blend consisting of 3 wt.-% of carbon black masterbatch (CB in LLDPE carrier) and 97 wt.-% of the base resin of the CE2 pellets. CE2 pellets and CBMB pellets were mixed prior tape production.

(44) TABLE-US-00003 TABLE 3 Pipe production results and pipe properties CE1 CE2(a) CE2(b) IE Amount of CB added (wt.-%) 0 3.0 3.0 0 during pipe production Tape wall thickness (mm) 0.2 0.16 0.14-0.15 0.15 Line speed used in tape 130 190 200 200 production (m/min) Elongation at pull-out 9.5 nm 3.0 3.5 strength of 110N (%) Elongation at pull-out nm nm 7.0 12.0 strength of 130N (%) Burst pressure (MPa) 0.28 nm 0.36 0.29 Density (kg/m.sup.3) 959 965 963 965 Amount of carbon black 2.6 1.2 1.2 2.1 (wt.-%*) homogeneity nm poor poor excellent Tensile at yield (MPa) nm 26 27 27 Tensile strength at break nm 21 27 23 (MPa) Nominal strain at break (%) nm 90 190 240 *wt.-% of carbon black was based on the total amount of the polyethylene composition (100 wt.-%) and refers to the amount of carbon black as such.

(45) FIGS. 1-3 show the micrographs of cross sections cut from CE2(a), CE2(b) and IE. CE2(a) and CE2(b) show poor mixing of carbon black and white domains are clearly visible. On the other hand, the IE sample is homogenenous and good carbon black distribution is evident.

(46) Compared to CE1, the pipes produced from IE show excellent mechanical properties. The pull out test elongation (110N/15 min) of CE1 is 9.5%, whereas the IE shows elongation of 3.5%. The burst pressure of CE1 and IE is comparable, eventhough the wall thickness of CE1 is 0.2 mm and the wall thickness of IE is 0.15 mm. This demonstrates that IE shows superior mechanical properties compared with CE1, which enables thinner walls in drip irrigation tapes while maintaining the mechanical properties.

(47) IE shows excellent homogeneity and carbon black distribution compared to the comparative examples leading to improved processability and UV resistance. On the other hand, the high density of the IE leads to superior mechanical properties of drip irrigation tapes enabling thinner walls while keeping sufficient mechanical performance of the tapes. The inventive example IE thus shows better homogeneity and mechanical properties over the comparative examples of the prior at and highly feasible processability when producing the pipes.