Polypropylene composition with improved impact resistance for pipe applications

Abstract

The present invention relates to a polypropylene composition comprising a multimodal propylene random copolymer with at least one comonomer selected from alpha-olefins with 2 or 4 to 8 carbon atoms, wherein the polypropylene composition has a melt flow rate MFR.sub.2 (2.16 kg, 230° C.) of 0.05 to 1.0 g/10 min, determined according to ISO 1133, a polydispersity index (PI) of 2.0 to 7.0, and a Charpy Notched Impact Strength at 0° C. of more than 4.0 kJ/m.sup.2, determined according to ISO 179/1eA:2000 using notched injection moulded specimens, a process for producing said polypropylene composition, an article comprising said polypropylene composition and the use of said polypropylene composition for the production of an article.

Claims

1. A polypropylene composition suitable for pipe applications comprising: a propylene random copolymer with at least one comonomer selected from alpha-olefins with 2 or 4 to 8 carbon atoms, wherein the propylene random copolymer comprises at least (i) a propylene random copolymer fraction having a low molecular weight, which is a low molecular weight fraction, and (ii) a propylene random copolymer fraction having a high molecular weight, which is a high molecular weight fraction, wherein the weight average molecular weight of the low molecular weight fraction is lower than that of the high molecular weight fraction, wherein the polypropylene composition has a melt flow rate MFR.sub.2 (2.16 kg, 230° C.) of 0.05 to 1.0 g/10 min, determined according to ISO 1133, a polydispersity index (PI) of 2.0 to 7.0, and a Charpy Notched Impact Strength at 0° C. of more than 6.5 kJ/m.sup.2, determined according to ISO 179/1eA:2000 using notched injection moulded specimens; and wherein the polypropylene composition does not comprise a polymeric nucleating agent.

2. The polypropylene composition according to claim 1, wherein the propylene random copolymer does not contain an elastomeric polymer phase dispersed therein.

3. The polypropylene composition according to claim 1, wherein the polypropylene composition has a flexural modulus of at least 700 MPa, determined according to ISO 178 at a test speed of 2 mm/min and a force of 100N on test specimens having a dimension of 80×10×4.0 mm.sup.3 (length×width×thickness) prepared by injection moulding according to EN ISO 1873-2.

4. The polypropylene composition according to claim 1, wherein the polypropylene composition has a tensile stress at yield of at least 15 MPa, determined according to ISO 527-2:1996 using type 1A injection moulded test specimens prepared according to ISO 527-2:1996.

5. The polypropylene composition according to claim 1, wherein the polypropylene composition has a Charpy Notched Impact Strength at 23° C. of at least 45 kJ/m.sup.2, determined according to ISO 179/1eA:2000 using notched injection moulded specimens.

6. The polypropylene composition according to claim 1, wherein the polypropylene composition has a content of xylene cold solubles (XCS) of from 1.0 to 15.0 wt determined at 25° C. according to ISO 16152.

7. The polypropylene composition according to claim 1, wherein the at least one comonomer is ethylene.

8. The polypropylene composition according to claim 1, wherein the comonomer content of the propylene random copolymer is in the range of 4.5 to 9.5 mol %, based on the total content of monomeric units in the propylene random copolymer.

9. The polypropylene composition according to claim 1, wherein the propylene random copolymer having a high molecular weight fraction has a higher content of comonomer, than the propylene random copolymer having a low molecular weight fraction.

10. The polypropylene composition according to claim 1, wherein the low molecular weight fraction is present in the propylene random copolymer in an amount of 35 to 55 wt %, based on the total amount of the propylene random copolymer (100wt %), and the high molecular weight fraction is present in the propylene random copolymer in an amount of 65 to 45 wt %, based on the total amount of the propylene random copolymer (100wt %).

11. A process for producing a polypropylene composition according to claim 1, wherein the propylene random copolymer is produced in a multistage polymerization process in the presence of (I) a solid catalyst component comprising a magnesium halide, a titanium halide and an internal electron donor; and (II) a cocatalyst comprising an aluminium alkyl and optionally an external electron donor, the multistage process comprising the steps of (a) continuously polymerizing propylene together with a first comonomer selected from alpha-olefins with 2 or 4 to 8 carbon atoms, in a first polymerization stage by introducing streams of propylene, hydrogen and said first comonomer into the first polymerization stage at a temperature of 60 to 80° C. and a pressure of 3000 to 6500 kPa to produce a first propylene random copolymer, wherein said first propylene random copolymer has a melt flow rate MFR.sub.2 (2.16 kg; 230° C.; ISO 1133) of from 0.2 to 3.0 g/min; (b) withdrawing from the first polymerization stage a stream comprising said first propylene random copolymer and transferring said stream into a second polymerization stage; (c) polymerizing propylene together with a second comonomer selected from alpha-olefins with 2 or 4 to 8 carbon atoms, in said second polymerization stage by introducing streams of propylene, said second comonomer and optionally hydrogen into said second polymerization stage at a temperature of 70 to 90° C. and a pressure of 1000 to 3000 kPa to produce a propylene random copolymer of said first propylene random copolymer and a second propylene random copolymer; (d) continuously withdrawing a stream comprising said propylene random copolymer from the second polymerization stage and optionally mixing said propylene random copolymer with additives; and (e) extruding said propylene random copolymer mixture into pellets, which have a melt flow rate MFR.sub.2 (2.16 kg; 230° C.; ISO 1133) of from 0.05 to 1.0 g/min.

12. A polypropylene composition according to claim 1 produced by a process for producing a polypropylene composition, wherein the propylene random copolymer is produced in a multistage polymerization process in the presence of (I) a solid catalyst component comprising a magnesium halide, a titanium halide and an internal electron donor; and (II) a cocatalyst comprising an aluminium alkyl and optionally an external electron donor, the multistage process comprising the steps of (a) continuously polymerizing propylene together with a first comonomer selected from alpha-olefins with 2 or 4 to 8 carbon atoms, in a first polymerization stage by introducing streams of propylene, hydrogen and said first comonomer into the first polymerization stage at a temperature of 60 to 80° C. and a pressure of 3000 to 6500 kPa to produce a first propylene random copolymer, wherein said first propylene random copolymer has a melt flow rate MFR.sub.2 (2.16 kg; 230° C.; ISO 1133) of from 0.2 to 3.0 g/min; (b) withdrawing from the first polymerization stage a stream comprising said first propylene random copolymer and transferring said stream into a second polymerization stage; (c) polymerizing propylene together with a second comonomer selected from alpha-olefins with 2 or 4 to 8 carbon atoms, in said second polymerization stage by introducing streams of propylene, said second comonomer and optionally hydrogen into said second polymerization stage at a temperature of 70 to 90° C. and a pressure of 1000 to 3000 kPa to produce a propylene random copolymer of said first propylene random copolymer and a second propylene random copolymer; (d) continuously withdrawing a stream comprising said propylene random copolymer from the second polymerization stage and optionally mixing said propylene random copolymer with additives; and (e) extruding said propylene random copolymer mixture into pellets, which have a melt flow rate MFR.sub.2 (2.16 kg; 230° C.; ISO 1133) of from 0.05 to 1.0 g/m in.

13. An article comprising the polypropylene composition according to claim 1.

14. An article comprising the polypropylene composition according to claim 12.

15. The polypropylene composition according to claim 1, wherein the comonomer content of the low molecular weight fraction is 1.0 to 6.0 mol %, based on the total content of monomeric units in the low molecular weight fraction.

16. The polypropylene composition of claim 1, wherein the at least one comonomer is selected from alpha-olefins with 4 to 8 carbon atoms.

17. The process of claim 11, wherein the first comonomer is selected from alpha-olefins with 4 to 8 carbon atoms.

18. The polypropylene composition of claim 12, wherein the first comonomer is selected from alpha-olefins with 4 to 8 carbon atoms.

19. The process of claim 11, wherein the second comonomer is selected from alpha-olefins with 4 to 8 carbon atoms.

20. The polypropylene composition of claim 12, wherein the second comonomer is selected from alpha-olefins with 4 to 8 carbon atoms.

Description

EXAMPLES

(1) 1. Determination Methods

(2) a) Melt Flow Rate

(3) 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.2 of polypropylene at a temperature 230° C. and a load of 2.16 kg.

(4) The MFR.sub.2 is herein assumed to follow the following mixing rule (equation 1):

(5) $\begin{array}{cc}{\mathrm{MI}}_b={\left(w_1{\mathrm{MI}}_1^{-0.0965}+w_2.Math.{\mathrm{MI}}_2^{-0.0965}\right)}^{\left(-1/0.0965\right)}& \left(\mathrm{eq}.1\right)\end{array}$

(6) Where w is the weight fraction of the component in the mixture, MI is the MFR.sub.2 and subscripts b, 1 and 2 refer to the mixture, component 1 and component 2, respectively.

(7) b) Density

(8) Density of the polymer was measured according to ISO 1183-1:2004 Method A on compression moulded specimen prepared according to EN ISO 1872-2(February 2007) and is given in kg/m.sup.3.

(9) c) Comonomer Content

(10) The comonomer content was determined by quantitative Fourier transform infrared spectroscopy (FTIR) after basic assignment calibrated via quantitative .sup.13C nuclear magnetic resonance (NMR) spectroscopy in a manner well known in the art. Thin films are pressed to a thickness of between 100-500 micrmeter and spectra recorded in transmission mode.

(11) Specifically, the ethylene content of a polypropylene-co-ethylene copolymer is determined using the baseline corrected peak area of the quantitative bands found at 720-722 and 730-733 cm.sup.−1. Specifically, the butene or hexene content of a polypropylene copolymer is determined using the baseline corrected peak area of the quantitative bands found at 1377-1379 cm.sup.−1. Quantitative results are obtained based upon reference to the film thickness.

(12) The comonomer content is herein assumed to follow the mixing rule (equation 2):
C.sub.b=w.sub.1.Math.C.sub.1+w.sub.2.Math.C.sub.2  (eq. 2)

(13) Where C is the content of comonomer in weight-%, w is the weight fraction of the component in the mixture and subscripts b, 1 and 2 refer to the overall mixture, component 1 and component 2, respectively.

(14) As it is well known to the person skilled in the art the comonomer content in weight basis in a binary copolymer can be converted to the comonomer content in mole basis by using the following equation

(15) $\begin{array}{cc}{c}_m=\frac{1}{1+\left(\frac{1}{c_w}-1\right).Math.\frac{M{W}_c}{M{W}_m}}& \left(\mathrm{eq}.3\right)\end{array}$

(16) where c.sub.m is the mole fraction of comonomer units in the copolymer, c.sub.w is the weight fraction of comonomer units in the copolymer, MW.sub.c is the molecular weight of the comonomer (such as ethylene) and MW.sub.m is the molecular weight of the main monomer (i.e., propylene).

(17) d) Xylene Cold Solubles

(18) Xylene cold solubles (XCS, wt.-%) content was determined at 25° C. according ISO 16152; first edition; 2005-07-01.

(19) The content of xylene soluble polymer is herein assumed to follow the mixing rule (equation 4):
XS.sub.b=w.sub.1.Math.XS.sub.1+w.sub.2.Math.XS.sub.2  (eq. 4)

(20) Where XS is the content of xylene soluble polymer in weight-%, w is the weight fraction of the component in the mixture and subscripts b, 1 and 2 refer to the overall mixture, component 1 and component 2, respectively.

(21) e) Flexural Modulus

(22) The flexural modulus was determined according to ISO 178. The test specimens having a dimension of 80×10×4.0 mm.sup.3 (length×width×thickness) were prepared by injection molding according to EN ISO 1873-2. The length of the span between the supports was 64 mm, the test speed was 2 mm/min and the force was 100 N.

(23) f) Tensile Stress at Yield, Tensile Strain at Yield

(24) Tensile stress at yield and tensile strain at yield was determined according to ISO 527-1:1996 and ISO 527-2:1996 on test specimen ISO 527-2:1996 type 1A molded specimen, the Injection moulding carried out according to ISO 1873-2:2007.

(25) g) Charpy Notched Impact Strength

(26) Charpy notched impact strength (Charpy NIS) was determined according to ISO 179-1:2000 on notched specimen of 80×10×4 mm, cut from test specimen ISO 527-2:1996 type 1A. Notched impact specimen according to ISO 179-1/1eA:2000 was used. Testing temperature is 23±2° C. for Charpy NIS at 23° C. and 0±2° C. for Charpy NIS at 0° C. Injection moulding carried out according to ISO 1873-2:2007.

(27) h) Rheological Parameters, Polydispersity Index

(28) 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 moulded plates, using nitrogen atmosphere and setting a strain within the linear viscoelastic regime. The oscillatory shear tests were done at T 190° C. applying a frequency range between 0.01 and 600 rad/s and setting a gap of 1.3 mm.

(29) 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
γ(t)=γ.sub.0 Sin(ωt)  (1)

(30) If the applied strain is within the linear viscoelastic regime, the resulting sinusoidal stress response can be given by
σ(t)=σ.sub.0 sin(ωt+δ)  (2)

(31) where

(32) σ.sub.0 and γ.sub.0 are the stress and strain amplitudes, respectively

(33) ω is the angular frequency

(34) δ is the phase shift (loss angle between applied strain and stress response)

(35) t is the time

(36) 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, η*, the dynamic shear viscosity, η′, the out-of-phase component of the complex shear viscosity η″ and the loss tangent, tan δ which can be expressed as follows:

(37) $\begin{array}{cc}{G}^{\prime }=\frac{{\sigma }_0}{{\gamma }_0}\cos \delta \left[\mathrm{Pa}\right]& \left(3\right)\\ G^{\prime }=\frac{{\sigma }_0}{{\gamma }_0}\sin \delta \left[\mathrm{Pa}\right]& \left(4\right)\\ G^{*}=G^{\prime }+{\mathrm{iG}}^{″}\left[\mathrm{Pa}\right]& \left(5\right)\\ {\eta }^{*}={\eta }^{\prime }-i{\eta }^{″}\left[\mathrm{Pa}.s\right]& \left(6\right)\\ {\eta }^{\prime }=\frac{G^{″}}{\omega }\left[\mathrm{Pa}.s\right]& \left(7\right)\\ {\eta }^{″}=\frac{G^{\prime }}{\omega }\left[\mathrm{Pa}.s\right]& \left(8\right)\end{array}$

(38) The values of storage modulus (G′), loss modulus (G″), complex modulus (G*) and complex viscosity (η*) were obtained as a function of frequency (ω). Thereby, e.g. η*.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.

(39) The polydispersity index, PI, is defined by equation 9.

(40) $\begin{array}{cc}\mathrm{PI}=\frac{{10}^5}{G^{\prime }\left({\omega }_{\mathrm{COP}}\right)},{\omega }_{\mathrm{COP}}=\omega \mathrm{for}\left(G^{\prime }=G^{″}\right)& \left(9\right)\end{array}$

(41) where, ω.sub.COP is the cross-over angular frequency, determined as the angular frequency for which the storage modulus, G′ equals the loss modulus, G″.

REFERENCES

(42) [1] Rheological characterization of polyethylene fractions″ Heino, E. L., Lehtinen, A., Tanner J., Seppälä, 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.

(43) 2. Examples

(44) a) Preparation of the Catalyst

(45) First, 0.1 mol of MgCl.sub.2×3 EtOH was suspended under inert conditions in 250 ml of decane in a reactor at atmospheric pressure. The solution was cooled to the temperature of −15° C. and 300 ml of cold TiCl.sub.4 was added while maintaining the temperature at said level. Then, the temperature of the slurry was increased slowly to 20° C. At this temperature, 0.02 mol of diethylhexylphthalate (DOP) was added to the slurry. After the addition of the phthalate, the temperature was raised to 135° C. during 90 minutes and the slurry was allowed to stand for 60 minutes. Then, another 300 ml of TiCl.sub.4 was added and the temperature was kept at 135° C. for 120 minutes. After this, the catalyst was filtered from the liquid and washed six times with 300 ml heptane at 80° C. Then, the solid catalyst component was filtered and dried. Catalyst and its preparation concept is described in general e.g. in patent publications EP 491 566, EP 591 224 and EP 586 390.

(46) For the preparation of Examples Ex1, Ex2, Ex3 and Ex4 as well as Reference Example Ref5 triethylaluminium (TEAL), dicyclopentyldimethoxysilane (DCPDMS) as donor (Do), and the catalyst as produced above were added into oil, like mineral oil, e.g. Technol 68 (kinematic viscosity at 40° C. 62-74 cSt), in amounts so that Al/Ti was 3-4 mol/mol, and Al/Do was as well 3-4 mol/mol. Catalyst concentration in the final oil-catalyst slurry was 10-20 wt-%.

(47) b) Polymerization of Examples Ex1-Ex4 and Reference Example Ref5

(48) For the polymerization of Examples Ex1-Ex4 and Reference Example Ref5 the catalyst was fed together with propylene to a prepolymerization reactor. Triethylaluminium was used as a cocatalyst and dicyclopentyldimethoxysilane as a donor. The polymerization conditions and feeds are listed in Table 1.

(49) The slurry from the prepolymerization stage was directly fed to a loop reactor. Propylene, hydrogen and ethylene were further added to the loop reactor. The polymerization conditions and feeds are listed in Table 1.

(50) The slurry from loop reactor was introduced to a gas phase reactor via direct feed line, i.e. without monomer flashing in-between the reactors. Propylene, ethylene and hydrogen were fed to the gas phase reactor. The polymerization conditions and feeds are listed in Table 1.

(51) In Examples Ex1 to Ex4 the low molecular weight fraction of the propylene random copolymer is polymerized in the loop reactor whereas the high molecular weight fraction of the propylene random copolymer is polymerized in the subsequent gas phase reactor in the presence of the low molecular weight fraction.

(52) In Reference Example Ref5 the high molecular weight fraction of the propylene random copolymer is polymerized in the loop reactor whereas the low molecular weight fraction of the propylene random copolymer is polymerized in the subsequent gas phase reactor.

(53) a) Compounding and Pipe Extrusion

(54) The polypropylene resins of Examples Ex1 to Ex4 and Reference Example Ref5 emerging from the gas phase reactor (identified as reactor powder in Table 1) were compounded together with conventional antioxidants and Ca-stearate (same amounts were used for Examples Ex1 to Ex4 and Reference Example Ref5) and pelletized in a W&P ZSK 70 twin-screw extruder (Coperion) at a melt temperature of 240° C. and an extruder throughput of 200 kg/h.

(55) The polymer pellets of Examples Ex1 to Ex4 and Reference Example Ref5 were prepared to test specimens for the mechanical and thermal tests as listed below in Table 2 or were extruded to pipes in order to test the processability of the compositions.

(56) TABLE-US-00001 TABLE 1 Polymerization conditions of Examples Ex1-4 and Reference Example Ref5 Ex1 Ex2 Ex3 Ex4 Ref5 Prepolymerisation step Cocatalyst (TEAL) feed 200 200 200 200 200 [g/t C3] Donor (DCPDMS) feed 30 30 30 30 40 [g/t C3] Temperature [° C.] 30 30 30 30 30 Pressure [kPa] 5300 5300 5300 5300 5300 Loop Reactor Temperature [° C.] 70 70 70 70 70 Pressure [kPa] 5300 5300 5300 5300 5300 H2/C3 [mol/kmol] 0.61 0.62 0.65 0.5 0.07 C2 content [wt-% 3.0 3.0 4.0 3.0 4.3 (mol %)] (4.4) (4.4) (5.9) (4.4) (6.3) MFR2 [g/10 min] 0.75 0.75 0.75 0.5 0.1 XCS [wt %] 5.0 5.0 7.0 5.0 7.7 Split [%] 40 40 40 40 60 Gas Phase Reactor Temperature [° C.] 80 80 80 80 85 Pressure [kPa] 1600 1600 1600 1600 1600 H2/C3 [mol/kmol] 1.5 2.0 1.8 1.3 26 C2 content (calc.) 4.7 5.5 4.8 5.5 3.4 [mol %]* XCS [wt %](calc.)* 7.2 10.0 7.5 10.0 2.8 Split 60 60 60 60 40 Final polypropylene composition** C2 content [wt-% 4.0 4.5 4.5 4.5 4.0 (mol %)] (measured) (5.9) (6.6) (6.6) (6.6) (5.9) MFR.sub.2 [g/10 min] 0.24 0.29 0.27 0.22 0.22 XCS [wt %] (measured) 6.3 8.0 7.3 8.0 5.8 C2 content refers to the ethylene comonomer content; C3 refer to the propylene monomer feed. *calculated for the polymer polymerised in the gpr reactor (high molecular weight fraction) **measured from final polypropylene composition after the compounding step (a) as described above

(57) TABLE-US-00002 TABLE 2 Mechanical and thermal properties of Examples Ex1 to Ex4 and Reference Example Ref5 Ex1 Ex2 Ex3 Ex4 Ref5 MFR.sub.2 (pellets) 0.24 0.29 0.27 0.22 0.22 [g/10 min] Flexural modulus [MPa] 935 811 854 783 964 Charpy NIS, 23° C. 62.4 69.7 52.1 75.9 24.3 [kJ/m.sup.2] Charpy NIS, 0° C. 6.9 11.8 6.5 9.6 3.6 [kJ/m.sup.2] Ten. Stress (yield) [MPa] 26.2 24.5 24.8 23.5 28.8 Ten. Strain (yield) [%] 12.8 12.9 12.9 14.0 12.7 PI 3.5 3.3 3.3 3.1 4.1

(58) It can be seen from the results of Tables 2 and 3 that the Examples Ex1 to Ex4 according to the invention show an improved balance of properties in terms of flexural modulus, Charpy notched impact strength at room temperature (23° C.) and cold temperature (0° C.), tensile stress at yield and tensile strain at yield.

(59) By comparing the properties of the examples according to the invention it can be seen that Examples Ex2 and Ex4 which have a higher amount of ethylene comonomer in the HMW fraction show a better impact performance especially at cold temperatures but also at room temperature compared to Examples Ex1 and Ex3.

(60) Pipe Tests:

(61) Test Pipe preparation: The polymers of Inventive Examples were extruded to pipes by using a Reifenhauser 381-1-70-30 pipe extruder. Output of the extruder was 46 to 48 kg/h, melt pressure was 180 to 220 barg and the melt temperature was 180 to 230° C. The test pipes had a diameter of 32.3 mm and wall thickness of 3 mm. The shrinkage of the produced test pipes was clearly less than 5%.