WO2000064952A2

WO2000064952A2 - Non-crosslinked polyethene

Info

Publication number: WO2000064952A2
Application number: PCT/FI2000/000366
Authority: WO
Inventors: Anneli Malmberg; Barbro LÖFGREN
Original assignee: Borealis Technology Oy
Priority date: 1999-04-27
Filing date: 2000-04-27
Publication date: 2000-11-02
Also published as: WO2000064952A3; FI990948A0; AU4299200A

Abstract

The invention relates to non-crosslinked polyethene which value of the normalised difference between its storage modulus G' and its loss modulus G'', determined by dynamic rheological measurement, is continuously below 0,5 under frequencies from 0,01 to 100 rad/s and at the temperature 190 °C. Preferably, said difference is essentially constant under said frequencies. Such polyethenes are preferably prepared by using metallocene catalysts, most preferably based on rac-ethylene bis(3-tri-isopropylsiloxy)indenyl zirconium dichloride or rac-ethylene bis(3-tert-butyldimethylsiloxy)indenyl zirconium dichloride.

Description

Non-crosslinked polyethene

Field of the invention

The object of the invention is a non-crosslinked polyethene with novel rheological properties. The invention also relates to the polymerisation of said non-crosslinked polyethene.

Background of the invention

The narrow molar mass distribution that is characteristic of metallocene polymers is associated with desirable mechanical properties. However, these materials show, in comparison to low density polyethene (PE-LD), unfavourable behaviour in processing, such as melt fracture due to the small shear sensitivity and melt elasticity.

Improvement of the processability can be reached by either broadening the molar mass distribution or by introducing long chain branching to the polymer. Of these two alternatives, increasing the degree of long chain branching while mamtaining the narrow molar mass and comonomer distribution is more desirable. The pseudo- plasticity (shear thinning) and melt elasticity of these polymers are improved, while maintaining the desired mechanical properties.

The patent application EP 676 421 describes a metallocene catalyst and polymerisation conditions leading to polyolefins having at least 0.01 long chain branches per 1000 carbon atoms along the polymer backbone and a breadth of molar mass distribution (M_w/M_n) greater than 2,5. A particularly preferred metallocene complex is a C₂-bridged bis(indenyl)zirconium dichloride.

The polymerisation described in EP 676 421 is preferably carried out in gas-phase in the presence of a cocatalyst such as methyl aluminoxane. The polymers according to this publication show increased shear thinning compared to other single site catalysts. Their shear thinning behaviour is conventional, i.e. the polymers show increased shear rate dependency of the viscosity. The processing window is, however, still rather narrow which can cause problems like melt fracture, extruder high back pressure, bubble instability in blow film processes and sagging in blow molding.

Further, single site polymer gives narrow molecular weight distribution which limits extrudability of those polymers. The processing window can now be broadened by using the novel non-crosslinked polyethene of the present invention. This non-crosslinked polyethene having novel rheological properties shows througout the shear rate range, already at very low shear rates, both high elasticity and increased shear rate dependency of the viscos- ity.

Description of the invention

The non crosslinked polyethene according to the invention is such that the value of the normalised difference between its storage modulus G' and its loss modulus G", determined by dynamic rheological measurement, is continuously below 0,5 under frequencies from 0,01 to 100 rad/s and at the temperature 190°C. The curves G' and G" are thus essentially parallel in said frequency range.

Such rheological behaviour, which does not occur in prior non crosslinked polyethenes, means that the polymers are very elastic at low shear rates. Thus the processing of the polymer is facilitated since also the shear tliinning (or sensitivity) is in- creased and the range of optimal process parameters is widened without modifying the molar mass.

The non crosslinked polyethene according to the invention has preferably been polymerised with a catalyst system comprising a metallocene complex. Said metallocene complex is a metallocene having a sandwich bonding ligand comprising a sandwich bonding moiety having an unsaturated 5-membered ring or having a 6- membered ring fused to an unsaturated 5-membered ring which moiety is covalently substituted by an organic group via a heteroatom.

Preferably, said moiety comprises an indenyl, dihydroindenyl or tetrahydroindenyl ring system. Said moiety is also preferably substituted by a silyloxy or germyloxy group.

Most preferably said metallocene complex is rac-ethylene bis(3-tri- isopropylsiloxy)indenyl zirconium dichloride or rac-ethylene bis(3-tert- butyldimethylsiloxy)indenyl zirconium dichloride.

Further, said metallocene complex is preferably supported, the support being silica, alumina and/or magnesium dichloride.

According to a preferred embodiment of the invention, the polymerisation is carried out in a slurry process, in which a C₃-C₈ inert hydrocarbon e.g. propane, isobutane, pentane, heptane, hexane, toluene or any other aliphatic or aromatic solvent is used as polymerisation medium,.

The reaction temperature is preferably from 60 to 110°C, more preferably from 75 to 100°C.

The polymerisation conditions are preferably such that the ethene partial pressure is from 0,25 to 40,0 bar, preferably from 5,0 to 15,0 bar. Hydrogen is optionally used, preferably in an amount from 0,1 to 10 mol, more preferably from 0,5 to 2 mol per 1000 mol of ethene.

Comonomer can also be used in the polymerisation and preferably the comonomer is an α-olefϊn, preferably a C -C₈ α-olefϊn. The amount of comonomer is such that the comonomer content of the final product is from 1,0 to 10,0 wt-%, preferably from 3,0 to 10,0 wt-%, most preferably from 5,0 to 8,0 wt-%.

When a cocatalyst is needed, an alkyl aluminoxane is preferably used, and most preferably said alkyl aluminoxane is methyl aluminoxane, hexaisobutyl aluminox- ane and/or tetraisobutyl aluminoxane. The amount of alkyl aluminoxane used is such that Al/Metal-ratio is from 50 to 2500, preferably from 100 to 1500.

Consequently, the non crosslinked polyethene according to the invention has the desirable mechanical properties characteristic of metallocene polymers associated to good processability due to long chain branching.

Experimental

Polymerisations

The polymerisations were performed in a 3 L semiflow stainless steel autoclave reactor equipped with a paddle stirrer and an external glycol jacket for temperature control. The reactor was dried and degassed thoroughly prior to use.

Molar mass (Mw in g/mol) and molar mass distribution (MMD) as M_w/M_n, measured by gel permeation chromatography (GPC), and rheological properties at 190 °C of the polymers of the examples are given in Table 1.

Example 1

1800 mL of dry pentane, 0,25 μmol catalyst rac-ethylene bis(3-tri- isopropylsiloxy)indenyl zirconium dichloride and 0,0538 mL 30 wt-% methyl alu- minoxane (MAO) in toluene were added to the reactor. The ratio Al/Zr was 1000. 0,2 bar of hydrogen (vessel size 512,25 mL) was fed batchwise with ethene to the reactor. Ethene flow was adjusted so that the ethene partial pressure in the reactor was 5,0 bar.

The polymerisation was allowed to run for 30 minutes at 80°C and the reactor was let to cool after releasing the pressure. The medium was evaporated off the polymer. 89,7 g of polymer was recovered (Figure 1).

Example 2

The procedure was identical to that in example 1, but the polymerisation time was 15 minutes. 64,6 g of polymer was recovered (Figure 2).

Example 3

The procedure was identical to that in example 1, but now also 25 mL 1-hexene was added to the reactor batchwise with ethene and the polymerisation time was 15 minutes. 82,5 g of copolymer was recovered, which comonomer content was 6,7 wt- % (Figure 3).

Example 4

1800 mL of dry pentane, 0,3 μmol of catalyst rac-ethylene bis(3-tert- butyldimethylsiloxy)indenyl zirconium dichloride and 0, 136 mL 30 wt-% methyl aluminoxane (MAO) in toluene were added to the reactor. 25 mL of 1-hexene was added batchwise to the reactor with ethene. Ethene flow was adjusted so that the ethene partial pressure in the reactor was 2,5 bar.

The polymerisation was allowed to run for 30 minutes at 80°C. The reactor was let to cool after releasing the pressure and the medium was evaporated off the polymer. 17 g of polymer was recovered, which comonomer content was 7,2 w-% (Figure 4).

Example 5

1200 ml of dry pentane and 0,0538 ml 30 wt-% methyl aluminoxane (MAO) in toluene were added to the reactor.

Heterogenoeous catalyst was prepared by mixing 34,2 mg of rac-ethylene bis(3-tri- isopropylsiloxy)indenyl zirconium dichloride with 1,98 mL of 30 wt-% methyl aluminoxane and further diluting the mixture with 0,55 mL of toluene. After 1 hour reaction time 1,48 mL of the complex solution obtained was slowly impregnated on activated silica (0,987 g Sylopol 55SJ, activated at 600°C). Impregnation time was 1 hour after which the catalyst was dried by nitrogen purging.

196,3 mg of said heterogeneous catalyst was added to the reactore. The ration Al/Zr was 208. Ethene flow was adjusted so that the ethene partial pressure in the reactor was 5,0 bar. The polymerisation was allowed to run for 60 minutes at 80 °C. The reactor was let to cool after realeasing the pressure and the medium was evaporated off the polymer. 104 g of polymer was recovered (Figure 5).

Comparative examples 6-8 are examples of polymers catalysed with metallocenes and containing some long chain branching.

Comparative example 6

Using the same procedure as in example 1, but 0,5 μ ol of catalyst rac-ethylene (bis)indenyl zirconium dichloride, 0,11 mL 30 wt-% methyl aluminoxane (MAO) in toluene and 25 ml 1-hexene were used.

The polymerisation was allowed to run for 60 minutes at 80°C. 109 g of polymer was recovered (Figure 6).

Comparative example 7

Using the same procedure as in example 5, but 1,0 μmol of catalyst rac-ethylene (bis)indenyl zirconium dichloride and 0,22 mL 30 wt-% methyl aluminoxane (MAO) in toluene were used.

The polymerisation was allowed to run for 60 minutes at 80°C. 74,3 g of polymer was recovered, its comonomer content was 5,5 wt-% (Figure 7).

Comparative example 8

1800 mL of dry pentane, 0,5 μmol of catalyst rac-ethylene (bis)-4, 5, 6, 7- tetrahydroindenyl zirconium dichloride and 0,11 mL 30 wt-% methyl aluminoxane (MAO) in toluene were added to the reactor. 0,2 bar of hydrogen (vessel size 512,25 mL) and 50 mL of 1-hexene were both added batchwise to the reactor with ethene. Ethene partial pressure was 10 bar.

The polymerisation was allowed to run for 60 rninutes at 80°C. 155,6 g of polymer was recovered, which comonomer content was 2,5 wt-% (Figure 8). Comparative examples 9 and 10 are examples of linear polymers catalysed with metallocenes.

Comparative example 9

The same procedure as in example 1 was used, but 1800 mL of dry pentane and 297,8 mg of heterogeneous catalyst based on di-(n-butylcyclopentadienyl)zirconium dichloride prepared above were used. The Al/Zr ratio was 200.

The polymerisation was allowed to run for 60 minutes at 80°C and 205,4 g of polymer was recovered (Figure 9).

Comparative example 10

1800 mL of dry toluene, 0,5 μmol of catalyst di-(n-butylcyclopentadienyl)zirconium dichloride and 0,0538 mL 30 wt-% methyl aluminoxane (MAO) in toluene were added to the reactor. Al/Zr ratio was 1000. 45 mL of 1-hexene was added to the reactor batchwise with ethene. Ethene flow was adjusted so that the ethene partial pressure in the reactor was 5,0 bar.

The polymerisation was allowed to run for 60 minutes at 80°C and the reactor was let to cool after releasing the pressure. The polymer was then precipitated with ethanol, washed and dried in vacuum. 60,0 g of polymer was recovered which comonomer content was 1,2 wt-% (Figure 10).

Rheological measurements

The viscoelastic parameters, loss modulus G'(ω) and storage modulus G"(ω) were determined on a Rheometric Scientific stress controlled dynamic rheometer SR-500 with the parallel plates geometry under the following conditions:

- polymer sample appropriately stabilised prior to testing (e.g. containing 2000 ppm of a thermal/oxidative stabiliser, e.g. Irganox B 215 commercially available from Ciba-Geigy),

- frequency range from 0,01 or 0,02 to 100 rad/s,

- temperatures 170°C, 190°C and 210°C and

- the strain amplitude was operator-chosen for best signal in linear viscoelastic region. The data treatment of the results includes:

-an appropriate software (e.g. Rheometrics RSI Orchestrator software) used to shift the moduli curves along the frequency axis with compensation for the effect of temperature on melt density to construct the master curves and to determine the shift factor ax,

- fitting the resultant shift factors a_τ to Arrhenius equation for evaluation of flow activation energy E_a from aτ=exp(E_a/RT) and

- displaying the master curve data, the elastic modulus G'(ω) and the viscous modulus G"(ω), versus frequency ω.

The samples tested were those prepared in examples 1-10 as well as three reference materials, that were commercial low density polyethene (PE-LD, Figure 11), high density polyethene (PE-HD, (Figure 12) and linear low density polyethene (PE- LLD, (Figure 13). The results are summarised in Table 1 (Tables la and lb): melt flow rate at weight 21 kg (MFR₂₁, in g/10 min), molar mass M_w, molar mass distri- button MMD, complex viscosity η* at 0,02 rad/s and 190 °C (in Pa.s), ω _cross.over, that is the value of the modulus G (=G^,=G^{, '>}) at which the curves of G' and G" cross each other, polydispersity index PI, that is the value of 10 ⁵ /G in the crossover point, branching index g' and flow activation energy E_a (in kJ/mol). The branching index g' values were calculated as g' = η (branched) / η (linear), where η (branched) and η (linear) were measured by GPC-on-line viscometry.

The PE-LD reference sample was characterised by high molar mass, broad molar mass distribution and a flow activation energy of 50 kJ/mol, typical for low density polyethene. The PE-LLD sample was a conventional Ziegler-Natta catalysed film grade linear low density polyethene, Mw 80 000 g/mol and MMD 4,0, yielding 33 kJ/mol for flow activation energy. The PE-HD sample was a chromium catalysed high density polyethene with few or no long chain branches, characterised by high molar mass (Mw 300 000 g/mol) and broad MMD (15). The flow activation energy of this material was 31 kJ/mol. Table 1. Polymer properties

n.c. no modulus cross-over in the measured frequency range

n.a. not appplicable

n.m. not measured

The frequency dependency of the dynamic moduli of the samples are shown in figures 1-13. Figures 1-5 show the corresponding curves for samples prepared in examples 1-4, according to the invention. Figures 6-10 show the curves for the comparative samples prepared in examples 6-10 and figures 11-13 show the curves for the reference materials PE-LD, PE-HD and PE-LLD.

In figures 1 and 2 the curve representing the storage modulus G'(ω) is parallel to that of loss modulus G"(ω) over the whole measured frequency region, four decades (0,01-100 rad/s). In figures 3-5, the two curves are essentially parallel over the frequency region from 0,3 or 0,4 to 100 rad/s. In figures 6-10 the storage modulus G' (ω) is not parallel to the loss modulus G"(ω) over any part of the frequency region studied. Such is the case also in figures 11-13.

In Table 2 is disclosed calculated values for tan delta (G'VG') and the normalised difference

in the whole frequency range obtained from data of G" and G'. As can be seen from this data, the examples of the invention fulfill the conditions

I. no moduli cross-over in the range of 0,05-100 rad/s, and

B. tan delta values are below 2, 1 and the normalised difference is below 0,5 in the whole frequency range (0,02-100 rad/s).

Table 2. Normalised difference (nd) and calculated tan delta (td) versus frequency

Claims

1. Non crosslinked polyethene, characterised in that the value of the normalised difference between its storage modulus G' and its loss modulus G", determined by dynamic rheological measurement, is continuously below 0,5 under frequencies from 0,01 to 100 rad/s and at the temperature 190°C.

2. Non crosslinked polyethene according to claim 1, characterised in that the absolute value of the difference between its storage modulus G' and its loss modulus G", determined by dynamic rheological measurement, is continuously higher than 0 under frequencies from 0,1 to 100 rad/s and at the temperature 190°C.

3. Non crosslinked polyethene according to claim 2, characterised in that said absolute value of the difference between its storage modulus G' and its loss modulus G" is essentially constant.

4. Non crosslinked polyethene according to any preceding claim, characterised in that it has been prepared by selecting for the polymerisation a catalyst system comprising a metallocene complex.

5. Non crosslinked polyethene according claim 4, characterised in that said metallocene complex is a metallocene having a sandwich bonding ligand comprising a sandwich bonding moiety having an unsaturated 5-membered ring or having a 6- membered ring fused to an unsaturated 5-membered ring which moiety is covalently substituted by an organic group via a heteroatom.

6. Non crosslinked polyethene according to claim 5, characterised in that said moiety comprises an indenyl, dihydroindenyl or tetrahydroindenyl ring system.

7. Non crosslinked polyethene according to claim 5 or 6, characterised in that said moiety is substituted by a silyloxy or germyloxy group.

8. Non crosslinked polyethene according to claim 7, characterised in that said metallocene complex is rac-ethylene bis(3-tri-isopropylsiloxy)indenyl zirconium dichloride.

9. Non crosslinked polyethene according to claim 7, characterised in that said metallocene complex is rac-ethylene bis(3-tert-butyldimethylsiloxy)indenyl zirco- nium dichloride.

10. Non crosslinked polyethene according to any of claims 4-9, characterised in that said metallocene complex is supported.

11. Non crosslinked polyethene according to claim 10, characterised in that said metallocene complex is supported on silica, alumina and/or magnesium dichloride.

12. Non crosslinked polyethene according to any of claims 4-11, characterised in that said polymerisation is carried out in a slurry process.

13. Non crosslinked polyethene according to claim 12, characterised in that a C₃- C₈ inert hydrocarbon e.g. propane, isobutane, pentane, heptane, hexane, toluene or any other aliphatic or aromatic solvent is used as medium in said slurry polymerisa- tion.

14. Non crosslinked polyethene according to claim 12 or 13, characterised in that said slurry polymerisation is carried out at a temperature from 60 to 110°C, preferably at a temperature from 75 to 100°C.

15. Non crosslinked polyethene according to any of claims 12-14, characterised in that the ethene partial pressure in the polymerisation is from 0,25 to 40,0 bar, preferably from 2,0 to 15,0 bar, most preferably from 5,0 to 7,0 bar.

16. Non crosslinked polyethene according to any of claims 12-15, characterised in that hydrogen is optionally used in said slurry polymerisation.

17. Non crosslinked polyethene according to any of claims 12-16, characterised in that comonomer is used in said slurry polymerisation.

18. Non crosslinked polyethene according to claim 17, characterised in that said comonomer is an α-olefϊn, preferably a C₄-C₈ α-olefϊn.

19. Non crosslinked polyethene according to claim 17 or 18 characterised in that the amount of comonomer is such that the comonomer content of the final product is from 1,0 to 10,0 wt-%, preferably from 3,0 to 10,0 wt-%, most preferably from 5,0 to 8,0 wt-%).

20. Non crosslinked polyethene according to any of claims 12-19, characterised in that an alkyl aluminoxane is used as cocatalyst in said slurry polymerisation.

21. Non crosslinked polyethene according to claim 20, characterised in that said alkyl aluminoxane is methyl aluminoxane, hexaisobutyl urninoxane and/or tetraisobutyl aluminoxane.

22. Non crosslinked polyethene according to claim 20 or 21, characterised in that the amount of alkyl aluminoxane used is such that Al/Metal-ratio is from 50 to

2500, preferably from 100 to 1500.

23. Slurry polymerisation process for the preparation of non crosslinked polyethene, which value of the normalised difference between its storage modulus G' and its loss modulus G", determined by dynamic rheological measurement, is continu- ously below 0,5 under frequencies from 0,01 to 100 rad/s and at temperature 190 °C, wherein the process parameters are chosen from the following:

(i) reaction medium is an inert C₃-C₈ hydrocarbon or toluene,

(ii) catalyst is based on a metallocene complex, and is optionally supported,

(iii) reaction temperature is 60 - 110 °C,

(iv) ethene partial pressure is 0,25 - 40,0 bar,

(v) hydrogen is optionally used,

(vi) comonomer is optionally used,

(vii) cocatalyst is an alkyl aluminoxane, and Al/Metal-ratio is 50-2500.