Polymer composition for caps and closures

Abstract

Polyethylene composition having a density of 950-960 kg/m.sup.3, a SHI.sub.(1,100) of 4-12, a melt index MI.sub.2 between 0.2 and 2 g/10 min, and a relationship between spiral flow SF (measured in mm at 250? C./1000 bar/100 mm/s) and ESCR E (measured in hours) of E>200?SF. The composition contains 48-62 wt % of an ethylene polymer (A) and 38-52 wt % of an ethylene copolymer (B). Copolymer (B) has a higher weight average molecular weight than polymer (A), and both of fractions (A) and (B) have a reverse comonomer distribution such that comonomer content increases with increasing molecular weight in the individual fraction.

Claims

1. Polyethylene composition having a density of 950-960 kg/m.sup.3, a SHI.sub.(1,100) of 4-12, a melt index MI.sub.2 between 0.2 and 2 g/10 min, and a relationship between spiral flow SF (measured in mm at 250? C./1000 bar/100 mm/s) and ESCR E (measured in hours) of E>200?SF, wherein the composition comprises 48-62 wt % of an ethylene polymer (A) and 38-52 wt % of an ethylene copolymer (B), copolymer (B) having a higher weight average molecular weight than copolymer (A), and both of fractions (A) and (B) have a reverse comonomer distribution such that comonomer content increases with increasing molecular weight in the individual fraction.

2. Composition according to claim 1, wherein the relationship between spiral flow SF (measured in mm at 250? C./1000 bar/100 mm/s) and ESCR E (measured in hours) is E>370?2SF.

3. Polyethylene composition having a density of 950-960 kg/m.sup.3, of SHI.sub.(1,100) of 4-12, a melt index MI2 between 0.2 and 2 g/10 min, and a relationship between spiral flow SF (measured in mm at 250? C./1000 bar/100 mm/s), ESCR E (measured in hours) and melt index MI2 (measured in g/10 min according to ISO 1133 at 190? C. at load of 2.16 kg) of E>(9800?36SF?1000MI2)/60.

4. Composition according to claim 3, wherein the relationship between spiral flow SF, ESCR E and melt index MI2 is E>(11000?36SF?1000MI2)/60.

5. Composition according to claim 1, which has a SHI.sub.(1,100) between 4 and 10.

6. Composition according to claim 1, which has a molecular weight distribution (Mw/Mn) (measured by GPC analysis) between 5 and 13.

7. Composition according to claim 1, which has a relationship between spiral flow SF (measured in mm at 250? C./1000 bars/100 mm/s) and solubles S (measured in g/kg) of S<0.1SF.

8. Composition according to claim 1, which comprises 50-60 wt % of ethylene polymer (A) and 40-50 wt % of ethylene copolymer (B).

9. Composition according to claim 1, wherein ethylene polymer (A) is a copolymer, and has a density between 969 and 974 kg/m3 and a MI2 of from 10 to 800 g/10 min.

10. Composition according to claim 1, which has a density between 950 to 954 kg/m3 and an MI2 of between 1 and 2 g/10 min.

11. Composition according to claim 1, which has a density between 954 to 960 kg/m3 and an MI2 of between 0.1 and 1 g/10 min.

12. Composition according to claim 1, which has a density between 950 to 954 kg/m3 and an MI2 of between 1 and 2 g/10 min, wherein the density of copolymer (B) is between 919 and 936 kg/m3 and the HLMI of copolymer (B) is from 3 to 6 g/10 min.

13. Composition according to claim 1, which has a density between 954 to 960 kg/m3 and an MI2 of between 0.1 and 1 g/10 min, wherein the density of copolymer (B) is between 929 and 947 kg/m3 the HLMI of the copolymer (B) is from 0.2 to 2 g/10 min.

14. Composition according to claim 3, which has a SHI.sub.(1,100) between 4 and 10.

15. Composition according to claim 3, which has a molecular weight distribution (Mw/Mn) (measured by GPC analysis) between 5 and 13.

16. Composition according to claim 3, which has a relationship between spiral flow SF (measured in mm at 250? C./1000 bars/100 mm/s) and solubles S (measured in g/kg) of S<0.1SF.

17. Composition according to claim 3, which comprises 48-62 wt % of an ethylene polymer (A) and 38-52 wt % of an ethylene copolymer (B), copolymer (B) having a higher weight average molecular weight Mw than polymer (A).

18. Composition according to claim 3, wherein ethylene polymer (A) is a copolymer, and has a density between 969 and 974 kg/m3 and a MI2 of from 10 to 800 g/10 min.

19. Composition according to claim 3, which has a density between 950 to 954 kg/m3 and an MI2 of between 1 and 2 g/10 min.

20. Composition according to claim 3, which has a density between 954 to 960 kg/m3 and an MI2 of between 0.1 and 1 g/10 min.

21. Composition according to claim 3, which has a density between 950 to 954 kg/m3 and an MI2 of between 1 and 2 g/10 min, wherein the density of copolymer (B) is between 919 and 936 kg/m3 and the HLMI of copolymer (B) is from 3 to 6 g/10 min.

22. Composition according to claim 3, which has a density between 954 to 960 kg/m3 and an MI2 of between 0.1 and 1 g/10 min, wherein the density of copolymer (B) is between 929 and 947 kg/m3 the HLMI of the copolymer (B) is from 0.2 to 2 g/10 min.

23. Composition according to claim 2, wherein the relationship between spiral flow SF (measured in mm at 250? C./1000 bar/100 mm/s) and ESCR E (measured in hours) is E>540?3SF.

24. Composition according to claim 4, wherein the relationship between spiral flow SF, ESCR E and melt index MI2 is E>(12000?36SF?1000MI2)/60.

25. Composition according to claim 5, which has a SHI.sub.(1,100) between 4 and 8.

26. Composition according to claim 7, which has a relationship between spiral flow SF (measured in mm at 250? C./1000 bars/100 mm/s) and solubles S (measured in g/kg) of S<0.1SF?2.5.

27. Composition according to claim 26, which has a relationship between spiral flow SF (measured in mm at 250? C./1000 bars/100 mm/s) and solubles S (measured in g/kg) of S<0.1SF?5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further described with reference to the accompanying drawings, in which FIGS. 1 and 2 show the improved balance of properties for the examples of the invention such as high ESCR, low levels of solubles and high values for spiral flow indicative of good processability in the injection molding process.

EXAMPLES

(2) The meanings of the symbols used in these examples and the units expressing the properties mentioned and the methods for measuring these properties are explained below.

(3) Melt Index

(4) Melt indices MI.sub.2 and HLMI are determined according to ISO1133 at a temperature of 190? C. under a load of 2.16 kg and 21.6 kg, respectively, are indicated in g/10 min.

(5) Density

(6) Density of the polyethylene was measured according to ISO 1183-1 (Method A) and the sample plaque was prepared according to ASTM D4703 (Condition C) where it was cooled under pressure at a cooling rate of 15? C./min from 190? C. to 40? C.

(7) Solubles

(8) Solubles were measured on a sample of 1.5 g by extraction with a Kumagawa extractor using n-hexane under reflux at 68? C. for 2 hours. The weight of C.sub.6-solubles is determined by the difference of weight before and after extraction, the sample being dried in an oven to eliminate any trace of n-hexane.

(9) Spiral Flow

(10) Spiral Test is carried out using a FANUC S2000i 150A injection moulding apparatus with a spiral mould. The spiral mould is a conventional mould with a spiral cavity of circular form, a thickness of 1 mm and breadth of 10 mm. The flow length is measured with a long spiral flow channel emanating from the center; notches are typically etched along the flow path to help identify the length the polymer has flowed within the mould. The mould is filled using a rotating screw in the barrel operating at a constant speed (injection speed). During the filling phase of the mould, the specific injection pressure on the screw increases progressively until it reaches 1000 bar, which is set in the injection moulding apparatus as the commutation pressure. At this pressure the screw is stopped and the screw speed falls to zero, ending the filling phase. There is no holding phase following the filling phase (no holding pressure or holding time), and the polymer spiral starts to cool immediately until the mould can be opened to eject the solid spiral of polymer. The behaviour of the polymer is evaluated based on flow length. Flow length data are presented in millimeters. The injection conditions are shown below:

(11) Specific injection pressure of commutation: 1000 bar

(12) No holding pressure

(13) Screw diameter: 32 mm

(14) Screw rotation speed: 80 rpm

(15) Screw injection speed: 100 mm/s

(16) Temperature in pre-chamber and die: 250? C.

(17) Temperature of all zones: 250? C.

(18) Mould temperature: 40? C.

(19) Cooling time: 20 s

(20) Cycle time: 30 s

(21) Dynamic Rheological Analysis

(22) Dynamic rheological measurements are carried out, according to ASTM D 4440, on a dynamic rheometer (e.g., ARES) with 25 mm diameter parallel plates in a dynamic mode under an inert atmosphere. For all experiments, the rheometer has been thermally stable at 190? C. for at least 30 minutes before inserting the appropriately stabilised (with antioxidant additives), compression-moulded sample onto the parallel plates. The plates are then closed with a positive normal force registered on the meter to ensure good contact. After about 5 minutes at 190? C., the plates are lightly compressed and the surplus polymer at the circumference of the plates is trimmed. A further 10 minutes is allowed for thermal stability and for the normal force to decrease back to zero. That is, all measurements are carried out after the samples have been equilibrated at 190? C. for about 15 minutes and are run under full nitrogen blanketing.

(23) Two strain sweep (SS) experiments are initially carried out at 190? C. to determine the linear viscoelastic strain that would generate a torque signal which is greater than 10% of the lower scale of the transducer, over the full frequency (e.g. 0.01 to 100 rad/s) range. The first SS experiment is carried out with a low applied frequency of 0.1 rad/s. This test is used to determine the sensitivity of the torque at low frequency. The second SS experiment is carried out with a high applied frequency of 100 rad/s. This is to ensure that the selected applied strain is well within the linear viscoelastic region of the polymer so that the oscillatory rheological measurements do not induce structural changes to the polymer during testing. In addition, a time sweep (TS) experiment is carried out with a low applied frequency of 0.1 rad/s at the selected strain (as determined by the SS experiments) to check the stability of the sample during testing.

(24) Shear Thinning Index SHI

(25) Shear thinning index (SHI) is calculated according to Heino (Rheological characterization of polyethylene fractions Heino, E. L., Lehtinen, A., Tanner J., Seppiili, J., Neste Oy, Porvoo, Finland, Theor. Appl. Rheol., Proc. Int. Congr. Rheol, 11th (1992), 1, 360-362, and 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.)

(26) The SHI value is obtained by calculating the complex viscosities ?.sub.1 and ?.sub.100 at a constant shear stress of 1 and 100 kPa respectively. The shear thinning index SHI.sub.(1/100) is defined as the ratio of the two viscosities ?.sub.1 and ?.sub.100.

(27) ESCR (on PCO1810 Cap Design)

(28) Environmental stress crack resistance (ESCR) is determined on a cap made according to cap design PCO1810 having a weight of 2.9 g. The cap is screwed onto a PET-preform pre-filled with water with a torque of 25 cm.Math.kg. The hydrostatic pressure in the PET-preform is maintained using a flexible pipe connected to its end. The cap part is entirely submerged in a 10 wt % solution of Igepal CO360. The test is done at 6 bar and 40? C.: the time taken for a pressure drop due to leakage to occur (caused by cracking of the cap) is measured. The test is done on 10 caps, and the ESCR results is calculated using the arithmetic average of the 10 test results.

(29) All PCO1810 Caps design caps were produced by injection moulding on a Nestal Synergy machine 1000-460 with an Antonin mould having 12 cap cavities. The injection conditions are displayed below:

(30) Screw diameter: 40 mm

(31) Injection speed values: 8 mm/s for 1.48 s, then 23 mm/s for 0.37 s, then 36 mm/s for 0.1 is, then 48 mm/s for 0.25 s, then 66 mm/s for 0.15 s, then 49 mm/s for 0.09 s, then 16 mm/s for 0.17 s, then 8 mm/s for 0.23 s.

(32) Injection pressure: 1400 bar

(33) Temperature of all zones: 220? C.

(34) Mould temperature: 10? C.

(35) Cooling time at 10? C.: 1.75 s

(36) Holding pressure: 1290 bar

(37) Holding pressure time: 0.25 s

(38) Gel Permeation Chromatography Analysis for Molecular Weight Distribution Determination

(39) Apparent molecular weight distribution and associated averages, uncorrected for long chain branching, were determined by Gel Permeation (or Size Exclusion) Chromatography according to ISO16014-1, ISO 16014-2 and 16014-4, using a PL 220 of Polymer Laboratories with 4 columns WATERS STYRAGEL HMW 6E of 30 cm length and 1 guard column Waters Styragel 4.6?30 mm and a differential refractometer detector.

(40) The solvent used was 1,2,4 trichlorobenzene at 150? C., stabilised with BHT, of 0.2 g/liter concentration. Polymer solutions of 0.8 g/liter concentration were prepared at 160? C. for one hour with stirring only in the last 30 minutes. The nominal injection volume was set at 400 ?l and the nominal flow rate was 1 ml/min.

(41) A relative calibration was constructed using 13 narrow molecular weight linear polystyrene standards:

(42) TABLE-US-00001 PS Standard Molecular Weight 1 7 520 000 2 4 290 000 3 2 630 000 4 1 270 000 5 706 000 6 355 000 7 190 000 8 114 000 9 43 700 10 18 600 11 10 900 12 6 520 13 2 950

(43) The elution volume, V, was recorded for each PS standards. The PS molecular weight was then converted to PE equivalent using the following Mark Houwink parameters:

(44) kPS=1.21 10-4 dl g-1 ?PS=0.707, kPE=3.92.10-4 dl g-1, ?PE=0.725.

(45) The calibration curve Mw Pp=f(V) was then fitted with a first order linear equation. All the calculations are done with Empower 2 software from Waters.

(46) A) Catalyst

(47) Reagents Used

(48) TEA Triethylaluminium

(49) TMA Trimethylaluminium

(50) TiBAl Triisobutylaluminium

(51) Ionic Compound A [N(H)Me(C.sub.18-22H.sub.37-45).sub.2][B(C.sub.6F.sub.5).sub.3(p-OHC.sub.6H.sub.4)]

(52) Complex A (C.sub.5Me.sub.4SiMe.sub.2N.sup.tBu)Ti(?.sup.4-1,3-pentadiene)

(53) To 10.0 kg of silica ES757 (available from PQ Corporation), previously calcined at 400? C. for 5 hours, in 90 liters of hexane was added 19.28 of 0.5 mol Al/liter of TEA in hexane. After 1 hour stirring at 30? C. the silica was allowed to settle and the supernatant liquid was removed by decantation. The residue was then washed five times with 130 liters hexane and reslurried in 130 liters hexane. Then 1 liter of a solution of Statsafe 2500 (available from Innospec) in pentane (2 g/l) was added and the slurry was stirred for 15 mins.

(54) 8.19 kg of a toluene solution of Ionic Compound A (10.94% wt) were cooled to 5? C. and 342 g of a hexane solution of TMA (1 mol/L) were added over 10 mins. After stirring for a further 20 mins at 5? C., the solution was transferred to the slurry containing the TEA-treated silica from the previous step over a period of 80 mins. The resulting mixture was well agitated for 3 hours at 20? C. Then 2.19 kg of a heptane solution of Complex A (9.51% wt) were added over a period of 30 minutes and the mixture was well agitated for another 3 hours at 20? C. Then the slurry was allowed to settle and the supernatant was removed by decantation. The residue was then washed three times with 150 liters hexane and dried in vacuum at 45? C. until a free flowing green powder was obtained. [Al]=1.11 mmol/g [Ti]=38 ?mol/g [B]=48 ?mol/g
B) Composition

(55) The manufacture of a composition according to the invention was carried out in suspension in a multistage reaction in two loop reactors of volume 200 L and 300 L respectively, preceded a prepolymerisation in a 40 L loop reactor. The reactors were connected in series, the slurry from the prepolymerisation reactor was transferred directly to the first loop reactor. The second loop reactor was separated from the first loop reactor by a device making it possible to continuously carry out a reduction in pressure. Examples 1 and CE5 employ hexane as diluents and 1-butene as comonomer, examples 2-4 employ isobutene as diluent and 1-hexene as comonomer.

(56) Diluent, ethylene, hydrogen, TiBAl (10 ppm) and the catalyst prepared in as described above were continuously introduced into the prepolymerisation reactor and the polymerisation of ethylene was carried out in this mixture in order to form the prepolymer (P). The mixture, additionally comprising the prepolymer (P), was continuously withdrawn from the said prepolymerisation reactor and introduced into the first reactor. Additional diluent, ethylene, hydrogen TiBAl (10 ppm) and optionally ?-olefin comonomer were continuously introduced into the first loop reactor and the polymerisation reaction was carried out in this mixture in order to obtain a first ethylene polymer fraction (A). The mixture, additionally comprising the first polymer (A) was continuously withdrawn from said first reactor and subjected to a reduction in pressure (?45? C., 6.0 bar), so as to remove at least a portion of the hydrogen. The resulting mixture, at least partially degassed of hydrogen, was then continuously introduced into a second polymerisation reactor, at the same time as ethylene, comonomer, diluent and hydrogen, and the copolymerisation of ethylene and ?-olefin was carried out therein in order to form the ethylene/?-olefin copolymer fraction (B). The suspension containing the polymer composition was continuously withdrawn from the second reactor and this suspension was subjected to a final reduction in pressure, so as to flash off the diluent and the reactants present (ethylene, comonomer and hydrogen). In the case where hexane was used as diluent, steam was additionally added after the final reduction in pressure to facilitate the evaporation of the diluent. The composition was then dried and degassed to remove residual hydrocarbons and recovered as a dry powder. The other polymerisation conditions and copolymer properties are specified in Table 1 and 2.

(57) The polymer powder was then transferred to a Werner and Pfleiderer ZSK40 twin-screw extruder and compounded with the following additive package:

(58) Tinuvin 622: 0.6 g/kg

(59) Calcium Stearate: 2 g/kg

(60) Irgafos 168: 1.5 g/kg

(61) Comparative examples C6 and C7 are bimodal copolymer compositions comprising a homopolymer fraction (A) and an ethylene/l-butene copolymer fraction (B), and are prepared according to the teachings in EP 1441959A.

(62) TABLE-US-00002 TABLE 1 polymerisation conditions EXAMPLE 1 2 3 4 CE5 Diluent Hx i-C.sub.4 i-C.sub.4 i-C.sub.4 Hx Comonomer 1-C.sub.4 1-C.sub.6 1-C.sub.6 1-C.sub.6 1-C.sub.4 Prepolymerisation reactor Pressure (bars) 29.4 37.7 36.8 36.9 28.5 Diluent (l/h) 108 108 108 108 108 C.sub.2 (kg/h) 0.4 0.6 0.5 0.8 0.7 H.sub.2 (g/h) 1.0 0.7 0.7 0.6 0.6 T (? C.) 29 35 28 29 28 Residence time (h) 0.37 0.45 0.49 0.48 0.37 wt % prepolymer (P) 2 2 2 2 2 Reactor 1 Pressure (bars) 29.4 38.1 37.0 37.3 28.7 Diluent (l/h) 158 158 158 158 158 C.sub.2 (kg/h) 21.0 21.5 21.0 21.0 20.5 Comonomer 1-C.sub.4 1-C.sub.6 1-C.sub.6 1-C.sub.6 Comonomer (g/h) 41.5 53.9 56.8 33.2 0 H.sub.2 (g/h) 11.7 13.0 13.4 14.0 12.0 T (? C.) 70 70 70 70 70 Residence time (h) 1.12 1.14 1.19 1.17 1.12 wt % polymer (A) 54 54 59 54 49 Reactor 2 Pressure (bars) 29.5 37.8 34.3 34.5 29.1 Diluent (l/h) 220 220 220 220 220 C.sub.2 (kg/h) 19.5 22.5 17.1 21.1 23.3 Comonomer 1-C.sub.4 1-C.sub.6 1-C.sub.6 1-C.sub.6 1-C.sub.4 Comonomer (kg/h) 0.36 1.21 0.38 0.35 0.52 H.sub.2 (g/h) 2.00 1.20 0.64 0.80 3.5 T (? C.) 80 80 85 85 80 Residence time (h) 1.17 1.07 1.09 1.07 1.14 wt % polymer (B) 44 44 39 44 49 i-C.sub.4 = isobutane, Hx = hexane, 1-C.sub.4 = 1-butene, 1-C.sub.6 = 1-hexene

(63) TABLE-US-00003 TABLE 2 polymer properties EXAMPLE solubles 1 2 3 4 CE5 CE6 CE7 Properties polymer fraction A MI.sub.2 (A) (g/10 min) 391 433 403 399 277 239 147 Density (A) (kg/m.sup.3) 970.5 970.5 970.3 972.1 975.1 972.0 972.0 wt % polymer (A) 54 54 59 54 49 50 45 Properties polymer fraction B* MI.sub.2 (B) (g/10 min) 0.16 0.15 0.02 0.03 0.32 0.28 0.17 HLMI (B) (g/10 min) 4.9 4.5 0.7 0.9 9.7 8.5 5.2 Density (B) (kg/m.sup.3) 932 928 933 936 931 934 937 Properties polymer composition (measured after pelletisation) MI.sub.2 (g/10 min) 1.8 1.7 0.5 0.4 2.2 1.8 0.8 Density (kg/m.sup.3) 953.6 952.0 956.0 956.4 953.5 953.0 952.5 Spiral flow (mm) 170 165 158 145 155 165 125 ESCR (h) 44.2 77.2 62.7 98.9 34.7 30.2 70.0 C.sub.6 Solubles (g/kg) 11.7 5.6 10.7 10.8 9.0 18.2 13.5 SHI.sub.1/100 6.4 5.5 7.7 6.2 4.9 6.9 6.1 Comonomer content (mol %) 0.4 0.4 0.2 0.2 n.d. 0.5 0.4 Mn (kDa) 12.2 12.4 n.d. 13.3 n.d. n.d. n.d. Mw (kDa) 113 109 n.d. 157 n.d. n.d. n.d. Mz (kDa) 403 357 n.d. 532 n.d. n.d. n.d. Mw/Mn 9.3 8.8 n.d. 11.8 n.d. n.d. n.d. *calculated, n.d. = not determined

(64) FIGS. 1 and 2 show the improved balance of properties for the examples of the invention such as high ESCR, low levels of solubles and high values for spiral flow indicative of good processability in the injection moulding process.