POLYMER COMPOSITION FOR CAPS AND CLOSURES

Abstract

Polyethylene composition has a density of 949-955 kg/m 3 and a melt index MI.sub.2 between 15 and 40 g/10 min, and comprises a low molecular weight fraction (A) and a high molecular weight fraction (B), wherein the amount of (A) is 52-58 wt % based on the weight of (A)+(B), and the melt index MI.sub.2 of (A) is 200-600 g/10 min. Injection moulded articles, preferably caps or closures, made from the composition are also described.

Claims

1. Polyethylene composition having a density of 949-955 kg/m.sup.3 and a melt index MI.sub.2 between 15 and 40 g/10 min, which comprises a low molecular weight fraction (A) and a high molecular weight fraction (B), wherein the weight fraction of (A) is 52-58 wt % based on the weight of (A)+(B), and the melt index MI.sub.2 of (A) is 200-600 g/10 min.

2. Composition according to claim 1, which has an MI.sub.2 of 19 to 37 g/10 min, preferably 20 to 35 g/10 min and more preferably 22 to 32 g/10 min.

3. Composition according to claim 1, which has a density of 950-954 kg/m.sup.3, preferably 951-953 kg/m.sup.3.

4. Composition according to claim 1, which has a molecular weight distribution (measured by GPC analysis) Mw/Mn of 5-10, preferably 7-10.

5. Composition according to claim 1, which has a SHI.sub.(1,1000) of 2.5-8, preferably 3-6.

6. Composition according to claim 1, wherein the weight fraction of (A) is 53-57 wt %, preferably 54-56 wt %.

7. Composition according to claim 1, wherein the low molecular weight fraction (A) has an MI.sub.2 of between 250 and 550 g/10 min, preferably between 300 and 500 g/10 min.

8. Composition according to claim 1, wherein the high molecular weight fraction (B) has a density of 918-928 kg/m.sup.3, preferably 920-926 kg/m.sup.3.

9. Composition according to claim 1, which has a density of 950 to 954 kg/m.sup.3, an MI.sub.2 of between 20 and 35 g/10 min, a SHI.sub.(1,1000) of between 2.5 and 8 and an Mw/Mn between 5 and 10, preferably a density of 950 to 954 kg/m.sup.3, an MI.sub.2 of between 22 and 32 g/10 min, a SHI.sub.(1,100) of between 3 and 6 and an Mw/Mn between 7 and 10.

10. An injection moulded article comprising a composition as defined in claim 1.

11. Article according to claim 10 which is a cap or closure.

12. Process for making an injection-moulded article, comprising the steps of polymerising ethylene and optionally comonomer, compounding the polyethylene composition, and then injection moulding the composition to form an article.

13. Process according to claim 12 wherein the step of polymerising ethylene comprises forming a multimodal polyethylene in at least two reactors in series.

Description

EXAMPLES

[0081] 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.

Melt Index

[0082] 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.

Density

[0083] Density of the polyethylene was measured according to ISO1183-1 (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.

Dynamic Rheological Analysis

[0084] 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 anti-oxidant 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.

[0085] 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.

Shear Thinning Index SHI

[0086] Shear thinning index (SHI) is calculated according to Heino (Rheological characterization of polyethylene fractions Heino, E. L., Lehtinen, A., Tanner J., Seppala, 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.)

[0087] The SHI value is obtained by calculating the complex viscosities 111 and moo 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 Ili and moo.

ESCR (on PCO1810 Cap Design)

[0088] 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 1 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.

[0089] All PC01810 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: [0090] Screw diameter: 40 mm [0091] Injection speed values:

TABLE-US-00001 Length (mm) Speed (mm/s) Time (s) 11 30 0.37 21.6 84 0.26 5.3 86 0.06 5.8 53 0.11 3.6 25 0.14 4.1 18 0.23 [0092] Injection pressure: 1150 bar [0093] Temperature of all zones: 200 C. [0094] Mould temperature: 10 C. [0095] Cooling time at 10 C.: 3.5s [0096] Holding pressure: 1150 bar [0097] Holding pressure time: 1.0 s

Top Load Stiffness Test

[0098] Since the injection moulding process for caps can significantly affect the final mechanical properties of the cap, stiffness (like ESCR) is measured directly on the cap rather than on a sample of the polymer used to make the cap. In the top load stiffness test, the increase in force required to push in the top plate of a cap to a maximum displacement of 1.2 mm is measured. The top of the cap is pushed in at a rate of 1 mm/minute by a 13 mm diameter punch tool having an end-point radius of curvature of 6.5 mm), using a Hounsfield H10KS universal testing machine. The force required to achieve a displacement of 0.05, 0.20, 0.25, 0.40, 0.60, 0.08, 1.0 and 1.2 mm respectively is measured.

[0099] The top load stiffness, expressed in N/mm, is the slope of the force curve between the displacements at 1.2 mm and 0.25 mm.

Flow Index

[0100] Flow index is a parameter which indicates the injection melt viscosity, and may be considered to be a measure of the flowability of the molten polymer under injection conditions: a low Flow Index indicates good flowability. It is determined during the injection process for the PC01810 caps, the injection conditions for which are described above for the ESCR measurement (including an injection temperature of 200 C.). The Flow Index is calculated by the Nestal Synergy machine 1000-460 which performs the injection moulding. During the injection process, the screw of an injection moulding machine acts as a plug to push the molten polymer into the cavity. The Flow Index is the calculated mean value of the injection pressure between two positions of the end of the screw.

[0101] FIG. 1 shows the typical variation of injection pressure as the screw is extended. The mean value of the area between positions A and B is the Flow Index. For the machine used in these measurements, A and B are located at 26 mm and 35 mm respectively, where 0 mm is initial position of the end of the screw. The Flow Index is obtained by dividing the area under the curve between these two points by the distance between them (9 mm in this case).

Impact Resistance

[0102] Notched Charpy Impact Resistance was measured according to ISO 179-1/1eA (2000) at a temperature of 23 C. on type 1 specimens (80104 mm) taken from compression moulded plates obtained according to ISO 291:1997 and notched with a Type A notch.

Gel Permeation Chromatography Analysis for Molecular Weight Distribution Determination

[0103] 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 GPC-IR of Polymer Char with 3 columns PL Olexis (13 m) Agilent of 30 cm length and a IRS MCT detector.

[0104] The solvent used was 1,2,4 trichlorobenzene at 160 C., stabilised with BHT, of 0.4 g/litre concentration. Polymer solutions of 0.3 g/litre concentration were prepared at 160 C. for two hours with stirring. The nominal injection volume was set at 200 l and the nominal flow rate was 1 ml/min.

[0105] A relative calibration was constructed using 16 narrow molecular weight linear polystyrene standards:

TABLE-US-00002 PS Standard Molecular Weight (Mp), Da 1 12200000 2 5030000 3 3080000 4 1400000 5 526000 6 250000 7 127000 8 63000 9 34800 10 17600 11 12600 12 5490 13 3500 14 1820 15 672 16 266

[0106] The elution volume, V, was recorded for each PS standards. The PS molecular weight was then converted to PE equivalent using a Q factor: 0.36490.

[0107] The calibration curve Mw Pp=f(V) was then fitted with a 3 fit order equation. All the calculations are done with GPC One software from Polymer Char.

A) Catalyst

[0108] Magnesium diethoxide was reacted with titanium tetrabutoxide for 4 hours at 140 C. in an amount such that the molar ratio of titanium to magnesium was equal to 1. The reaction product thus obtained was subsequently chlorinated and precipitated by bringing the latter into contact with an ethylaluminium dichloride solution (EADC) for 90 minutes at 45 C. The EADC/Mg ratio was 6.5 mole/mole. The obtained slurry was subsequently aged at 60 C. for 45 minutes, and then cooled at ambient temperature (<35 C.). The by-products from the chlorination reaction were removed from the slurry by washing the solid with polymerisation grade hexane at ambient temperature. The catalyst thus obtained, collected from the suspension, comprised (% by weight): Ti: 17; Cl: 41; Al: 2; Mg: 5.

B) Composition

[0109] The manufacture of a composition comprising ethylene polymers was carried out in suspension in hexane in two loop reactors connected in series and separated by a device which makes it possible continuously to carry out the reduction in pressure.

[0110] Hexane, ethylene, hydrogen, triethylaluminium and the catalysts were continuously introduced into the first loop reactor and the polymerization of ethylene was carried out in this mixture in order to form the homopolymer (A). This mixture, additionally comprising the homopolymer (A), was continuously withdrawn from the said reactor and was subjected to a reduction in pressure (70 C., 0.11 MPa), 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 polymerization reactor, at the same time as ethylene, butene, hexane and hydrogen, and the polymerization of the ethylene and of the butene was carried out therein in order to form the ethylene/1-butene copolymer (B). The suspension comprising the composition comprising ethylene polymers was continuously withdrawn from the second reactor and this suspension was subjected to a final reduction in pressure in the presence of steam, so as to evaporate the hexane and the reactants present (ethylene, 1-butene and hydrogen) and to recover the composition in the form of a powder, which was subjected to drying in order to complete the degassing of the hexane.

[0111] The polymer powder was then transferred to a Werner and Pfleiderer ZSK40 monomodal profile screw PEO8 and compounded with an additive package described below. Additives incorporated with the resins in the Table below during compounding were 70 ppm of calcium stearate (acid neutraliser), 1500 ppm of Irgafos 168 (process antioxidant).

[0112] The other polymerisation conditions and copolymer properties (as measured on pellets) are specified in Table 1. The properties of the compositions are presented in Table 2.

TABLE-US-00003 TABLE 1 polymerisation conditions EXAMPLE 1 2 CE1 Reactor 1 C2 (g/m.sup.3 solvent) 9.9 10.1 6 H2/C2 (mole/mole) 0.321 0.349 0.26 T ( C.) 85 85 84 Residence time (h 2 1.9 1.3 Reactor 2 C2 (g/m.sup.3 solvent) 8.1 8.7 7 C4/C2 (mole/mole) 0.803 0.849 0.37 H2/C2 (mole/mole) 0.041 0.042 0.17 T ( C.) 85 85 80 Residence time (h) 1.6 1.6 1.4

TABLE-US-00004 TABLE 2 polymer properties EXAMPLE 1 2 CE1 CE2 Properties of polymer fraction (A) wt % (A) 55 55 55 MI.sub.2(A) (g/10 min) 420 400 255 Density (A) (kg/m.sup.3) 974 974 974 Properties of polymer fraction (B) MI.sub.2(B) (g/10 min) * 5.0 4.0 0.45 Density (B) (kg/m.sup.3)** 922.9 923.3 944.1 Properties of polymer composition (after pelletisation) Density (kg/m.sup.3) 952.2 952.1 961.0 953.4 MI.sub.2 (g/10 min) 28.7 23.7 3.8 28.7 SHI.sub.1/100 3.7 3.4 25.6 2.1 Mw/Mn 8.8 9.0 12.3 5.3 ESCR 1 bar, 40 C. (h) 22 62 >200 16 Charpy @ 23 C. (kJ/m.sup.2) 2.3 2.3 3.1 2.2 Charpy @ 20 C. (kJ/m.sup.2) 2.2 1.7 2.7 2.3 Top load stiffness (N/m) 19.0 19.0 31.9 19.8 Flow index 937 981 2020 1136 * calculated using the equation described in Hagstrm, Conference of Polymer Processing in Gothenburg, 19-21/08/1997 **calculated using Equation 1

[0113] Comparative Example 1 is included as an example of a resin suitable for caps and closures which has a relatively low melt index and therefore a high ESCR.

[0114] Comparative Example 2 is Rigidex HD5226EA, a monomodal resin commercially available from INEOS which also has a high melt index.

[0115] The above results show that the compositions of the invention display similar stiffness and impact resistance to Comparative Example 2, but superior ESCR and flowability under injection conditions. This shows that the specific bimodal design of the inventive resins can give improved ESCR whilst maintaining or even improving flowability.

[0116] Compared with Comparative Example 1, it can be seen that the much higher melt index of the inventive resins results in substantially better flowability. Although stiffness is poorer due to the lower density, ESCR is still satisfactory.