CN1717448A - Polyethylene blow molding composition for producing small containers - Google Patents

Abstract

The invention relates to a polyethylene composition with multimodal molecular weight distribution, especially suitable for blow molding small containers with a volume in the range of 200-5000 cm ³ (=ml). The composition has a density at 23°C in the range of 0.955-0.960 g/cm ³ and a MFI _190/5 in the range of 0.8-1.6 dg/min. It contains 45-55% by weight of low molecular weight ethylene homopolymer A, 20-35% by weight of high molecular weight copolymer B made of ethylene and another 1-olefin with 4-8 carbon atoms and 20- 30% by weight of ultra-high molecular weight ethylene copolymer C.

Description

Polyethylene blow molding composition for the production of small containers

The present invention relates to a polyethylene composition having a multimodal molecular weight distribution, which is particularly suitable for blow molding small blow moldings, such as containers having a capacity or volume in the range of 200-5000 cm ³ (=ml), and to the preparation of such polyethylene The process of the ethylene composition is carried out by a multi-step process consisting of successive slurry polymerizations in the presence of a catalyst system consisting of a Ziegler catalyst and a cocatalyst. The invention also relates to small blow moldings produced from the polymer composition by blow molding.

Polyethylene is widely used for the production of blow moldings of various sizes requiring the material to have particularly high mechanical strength and a high resistance to stress cracking. Another particular advantage of polyethylene is that it also has good chemical resistance and is inherently a lightweight material.

EP-A-603,935 has previously described a blow molding composition based on polyethylene and having a bimodal molecular weight distribution and suitable for producing moldings with good mechanical properties.

US-A 5,338,589 describes a material with an even broader molecular weight distribution, prepared by using a high mileage catalyst known from WO91/18934, in which the magnesium alkoxide is in the form of a gel-like suspension use. Surprisingly, it has been found that the use of this material in moldings, especially in pipes, allows simultaneous improvement of properties usually inversely related in semi-crystalline thermoplastics, these are in particular stiffness on the one hand and stiffness on the other hand For stress crack resistance and toughness.

However, such known bimodal products in particular have a relatively low melt strength during processing. This means that the extruded parison often breaks in the molten state, making the extrusion process unacceptably sensitive to processing. Furthermore, particularly when producing thick-walled containers, it was found that the wall thickness was not uniform due to the flow of the melt from the upper to the lower region of the mould.

It was therefore the object of the present invention to develop a polyethylene composition for blow molding which exhibits a further improvement over all known materials in the processing by blow molding to produce small blow moldings. In particular, the high melt strength of the composition should allow prolonged operation of the extrusion process without parison failure, and the precisely tuned swelling ratio of the composition should allow optimization of wall thickness control.

We have surprisingly found that this object is achieved by the composition described at the outset, which is characterized in that it comprises 45-55% by weight of low molecular weight ethylene homopolymer A, 20-35% by weight of ethylene and Another high molecular weight copolymer B made of 1-olefin having 4-8 carbon atoms and 20-30% by weight of ultra-high molecular weight ethylene copolymer C, wherein all percentage values are based on the total weight of the molding composition .

The invention also relates to a process for the preparation of such compositions in a cascaded slurry polymerization process and to a process for the manufacture of small blow moldings, such as capacities (=volumes) in the range of 200-5000 cm ³ (=ml) and having rather excellent Containers with mechanical strength properties.

The polyethylene polymer of the present invention has a density in the range of 0.955-0.960 g/ ^cm3 at 23°C and a broad trimodal molecular weight distribution. The high molecular weight copolymer B contains only a low proportion of other 1-olefin monomer units having 4-8 carbon atoms, ie 0.1-0.6% by weight. Examples of such comonomers are 1-butene, 1-pentene, 1-hexene, 1-octene or 4-methyl-1-pentene. The ultra-high molecular weight ethylene copolymer C also contains a 1-alkene content in the range of 0.5-2.5% by weight of one or more of the aforementioned comonomers.

The pelletized polymer composition of the present invention has a melt flow index (ISO 1133) in the range of 0.8-1.6 dg/min, expressed as MFI _190/5 , and a viscosity number VN _tot of 280-350 cm ³ /g. range, measured according to ISO/R 1191 in decalin at 135°C.

Trimodality is a measure of the position of the center of gravity of three separate molecular weight distributions and can be described by means of the viscosity number VN according to lSO/R 1191 of the polymer formed in successive polymerization stages. Therefore, the relevant band widths of the polymers formed in each stage of the reaction are as follows:

The viscosity number VN ₁ measured on the polymer after the first polymerization stage corresponds to the viscosity number _VNA of the low molecular weight polyethylene A and according to the invention is in the range of 70-90 cm ³ /g.

The viscosity number _VN measured on the polymer after the second polymerization stage is not equal to the viscosity number VN _B of the high molecular weight polyethylene B formed in the second polymerization stage, but represents a mixture of polymer A and polymer B The viscosity number of VN _B can only be determined by calculation. According to the invention, VN ₂ is in the range of 150-200 cm ³ /g.

The viscosity number VN ₃ measured for the polymer after the third polymerization stage is not equal to the viscosity number VN _c of the ultrahigh molecular weight copolymer C formed in the third polymerization stage, but represents polymer A, polymer B and The viscosity number, VN _c , of the mixture of polymers C can only be determined by calculation. According to the invention, VN ₃ is in the range of 260-340 cm ³ /g.

Polyethylene is passed in the slurry at 70-90°C, preferably in the range of 80-90°C, under a pressure of 0.15-1MPa, and in a mixture of transition metal compounds and organoaluminum compounds such as triethylaluminum, triisobutyl It is obtained by polymerizing monomers in the presence of highly durable Ziegler catalysts composed of aluminum bases, alkylaluminum chlorides or alkylaluminum hydrides. The polymerization was carried out in three stages, ie three stages arranged in series, each molecular weight adjusted by means of hydrogen feed.

The polyethylene composition of the invention may comprise other additives alongside the polyethylene. Examples of such additives are heat stabilizers, antioxidants, UV absorbers, light stabilizers, metal deactivators, peroxide-destroying compounds, and bases in amounts of 0-10% by weight, preferably 0-5% by weight Sexual co-stabilizers, and fillers, reinforcing agents, plasticizers, lubricants, emulsifiers, pigments, optical brighteners, flame retardants, antistatic agents, foaming agents, or their combinations, the total amount is 0- 50% by weight, based on the total weight of the mixture.

The composition of the invention is particularly suitable for blow molding processes to produce small blow moldings, by first plasticizing the polyethylene composition in an extruder in the range of 200-250°C, and then extruding through a die into a mould, Here it is cooled and thus solidified.

The composition according to the invention produces particularly good processability in the blow molding process for the production of small blow moldings, since it has a swelling ratio in the range of 115-145%, and the small blow moldings produced thereby have particularly good High mechanical strength, since the molding compositions according to the invention have a notched impact strength (ISO) in the range of 8-14 KJ/m ² . Its stress crack resistance (FNCT) is in the range of 8-20h.

Notched impact strength (ISO) determined at 23°C according to ISO 179-1/leA/DIN 53453. The specimen size is 10 x 4 x 80 mm, and the V-notch is inserted using a 45° angle, with a depth of 2 mm and a base radius of the notch of 0.25 mm.

The stress-crack resistance of the molding compositions according to the invention was determined by an in-house test method and is given in hours (h). This laboratory method is described by M. Fleissner in Kunststoffe 77 (1987), p. 45 ff., and corresponds to ISO/FDIS 16770, which is in force. When ethylene glycol was used as the medium to promote stress cracking and a tensile stress of 3.5 MPa was used at 80°C, the time to fracture was shortened as the stress initiation time was shortened by the notch (1.6mm/insert). Test specimens were produced by sawing three test specimens with dimensions 10 x 10 x 90 mm from pressed panels 10 mm thick. These samples were cut with a center notch using a blade on a punching device specially made for this purpose (see Figure 5 in the publication). The notch depth is 1.6mm.

Example 1

Ethylene is polymerized in a continuous process in three reactors arranged in series. As WO 91/18934, the Ziegler catalyst of the 13.5mmol/h amount prepared relative to the titanium compound described in Example 2, the operable number in WO is 2.2, with 174mmol/h triethylaluminum and sufficient amount of Diluent (hexane), ethylene and hydrogen are fed together into the first reactor. The amount of ethylene (=67.2 kg/h) and hydrogen (74 g/h) was adjusted so that the percentages of ethylene and hydrogen measured in the gas space of the first reactor were 20-23% by volume and 66-71% respectively % by volume, the remainder being a mixture of nitrogen and evaporated diluent.

Polymerization in the first reactor was carried out at 84°C.

The slurry in the first reactor is then transferred to the second reactor, where the percentage of hydrogen in the gas phase has been reduced to 16-20% by volume, 120 g/h of 1-butene and 46.8 kg/h of ethylene together into the reactor. The amount of hydrogen is reduced by the intermediate _H2 decompression. 65-70% by volume ethylene, 16-20% by volume hydrogen and 0.15%-0.20% by volume 1-butene were measured in the gas phase of the second reactor, the remainder being a mixture of nitrogen and evaporated diluent.

Polymerization in the second reactor was carried out at 84°C.

The slurry in the second reactor was transferred to the third reactor and the amount of hydrogen in the gas space of the third reactor was adjusted to 2.0% by volume with further intermediate H2 depressurization.

540 g/h of 1-butene were fed together with 32.1 kg/h of ethylene into the third reactor. Percentages of 81-84% by volume ethylene, 1.9-2.3% by volume hydrogen and 1.2% by volume 1-butene were determined in the gas phase of the third reactor, the remainder being a mixture of nitrogen and evaporated diluent.

Polymerization in the third reactor was carried out at 84°C.

The long-term polymerization catalyst activity required for the cascade process described above is provided by specially developed Ziegler catalysts described starting in WO 91/18934. The usefulness of this catalyst is measured by its exceptionally high hydrogen sensitivity and uniformly high activity over extended periods of 1-8 hours.

The diluent is removed from the polymer slurry leaving the third reactor, the polymer is dried and pelletized at a temperature of 220-250°C with a specific energy loss of 0.2-0.3 kW/h/kg. The polymer powder was stabilized with 0.1% by weight of calcium stearate, 0.08% by weight of Irganox 1010 and 0.16% by weight of Irgafos 168.

Table 1 shown below gives the viscosity numbers and quantitative ratios W A , W B , and W _c of polymers _A , _B , and C in the polyethylene composition prepared in Example 1.

Table 1 Example serial number 1 W _A [% by weight] 46 W _B [weight%] 32 W _C [% by weight] twenty two VN ₁ [cm ³ /g] 75 VN ₂ [cm ³ /g] 188 VN _tot [cm ³ /g] 317 Density [g/cm ³ ] 0.957 MFI _190/5 [dg/min] 0.95 SR[%] 142 FNCT[h] 10 NIS _ISO [kJ/ ^m2l 12.3

The abbreviations of physical properties in Table 1 have the following meanings:

- SR (= swelling ratio) in [%], at 190°C and shear rate 1440S ^-1 in high pressure capillary flow in a 2/2 circular section die with conical inlet (angle = 15°) measured in the instrument.

- FNCT = Stress cracking resistance (Full Notch Creep Test) measured by the in-house test method of M. Fleissner, expressed in [h].

- NIS _ISO = notched impact strength determined according to ISO 179-1/leA/DIN 53453 at 23° C., expressed in [kJ/m ² ].

Example 2

The polymer composition was prepared in the same manner as described in Example 1, however, with the difference that the amount of Ziegler catalyst was added to the first reactor with 180 mmol/h of triethylaluminum relative to the titanium compound It is 14mmol/h instead of 13.5mmol/h as in Example 1.

The amount of ethylene (=72.8 kg/h) and hydrogen (68 g/h) is adjusted so that the percentages of ethylene and hydrogen measured in the gas space of the first reactor are 20-23% by volume and 66% by volume, respectively, The remainder was a mixture of nitrogen and evaporated diluent.

Polymerization in the first reactor was carried out at 85°C.

The slurry in the first reactor is then transferred to the second reactor, where the percentage of hydrogen in the gas phase has been reduced to 6-8% by volume, 45 g/h of 1-butene and 30.8 kg/h of ethylene together into the reactor. The amount of hydrogen is reduced by the intermediate _H2 decompression. 79% by volume ethylene, 6-7% by volume hydrogen and 0.7% by volume 1-butene were determined in the gas phase of the second reactor, the remainder being a mixture of nitrogen and evaporated diluent.

Polymerization in the second reactor was carried out at 82°C.

The slurry in the second reactor was transferred to the third reactor and the amount of hydrogen in the gas space of the third reactor was adjusted to 2.8% by volume with further intermediate _H2 depressurization.

270 g/h of 1-butene were fed together with 36.4 kg/h of ethylene into the third reactor. Percentages of 84% by volume ethylene, 2.8% by volume hydrogen and 0.9% by volume 1-butene were determined in the gas phase of the third reactor, the remainder being a mixture of nitrogen and evaporated diluent.

Polymerization in the third reactor was carried out at 85°C.

The polymer leaving the third reactor was then dried, pelletized and stabilized under the same conditions as described in Example 1.

Table 2 shown below gives more details of the polyethylene composition prepared in Example 2.

Table 2 Example serial number 2 W _A [% by weight] 52 W _B [weight%] twenty two W _c [% by weight] 26 VN ₁ [cm ³ /g] 85 VN ₂ [cm ³ /g] 194 VN _tot [cm ³ /g] 305 Density [g/cm ³ ] 0.958 MFI _190/5 [dg/min] 1.0 SR[%] 118 FNCT[h] 10 NIS _ISO [kJ/m ² ] 14

Abbreviations in Table 2 have the same meanings as those in Example 1.

Claims (10)

1. polyethylene composition with multimodal shape molecular weight distribution has at 23 ℃ and is 0.955-0.960g/cm ³Density and the MFI in the 0.8-1.6dg/min scope in the scope _190/5The low-molecular-weight ethylenic homopolymer A that comprises 45-55% weight, the high molecular weight copolymer B that is made by ethene and another kind of 1-alkene with 4-8 carbon atom of 20-35% weight and the ultra-high-molecular-weight polyethylene multipolymer C of 20-30% weight, wherein all percent value are based on the gross weight of moulding compound.

2. polyethylene composition as claimed in claim 1, wherein high molecular weight copolymer B contain based on the low ratio of the weight meter 0.1-0.6% weight of multipolymer B have 4-8 carbon atom comonomer and wherein the ethylene copolymer C of ultra-high molecular weight contain based on the comonomer in the weight meter 0.5-2.5% weight range of multipolymer C.

3. as the polyethylene composition of claim 1 or 2, it contains 1-butylene as comonomer, 1-amylene, 1-hexene, 1-octene, 4-methyl-1-pentene, or the mixture of these comonomers.

4. as one or multinomial polyethylene composition among the claim 1-3, the viscosity number VN that has _TotBe 280-350cm ³/ g, preferred 300-320cm ³/ g, according to ISO/R 1191 in naphthalane 135 ℃ of measurements.

5. as one or multinomial polyethylene composition among the claim 1-4, it has the swelling ratio in the 115-145% scope, at 8-14KJ/m ²Notched Izod impact strength (ISO) and the stress cracking resistance in the 8-20h scope (FNCT) in the scope.

6. produce method as one or multinomial polyethylene composition among the claim 1-5, wherein monomer in slurry in 20-120 ℃ temperature range, under the pressure of 0.15-1MPa scope, with polymerization in the presence of the high-mileage ziegler catalyst that constitutes by transistion metal compound and organo-aluminium compound, this method comprises carries out polyreaction in three stages, and wherein the poly molecular weight for preparing in each stage is regulated by means of hydrogen.

7. method as claimed in claim 6, the concentration of wherein regulating hydrogen in first polymerization stage is so that the viscosity number VN of low molecular weight polyethylene A ₁At 70-90cm ³In/g the scope.

8. as the method for claim 6 or 7, the concentration of wherein regulating hydrogen in second polymerization stage is so that the viscosity number VN of the mixture of polymer A and polymer B ₂At 150-200cm ³In/g the scope.

9. as each method of claim 6-8, the concentration of wherein regulating trimerization hydrogen in the stage is so that the viscosity number VN of the mixture of polymer A, polymer B and polymkeric substance C ₃At 260-340cm ³In/g the scope, 280-320cm particularly ³/ g.

As one or multinomial polyethylene composition among the claim 1-5 be used to produce small-sized blowing product such as capacity at 200-5000cm ³The purposes of the container in the scope of (=ml) wherein at first plastifies polyethylene composition and then is expressed in the mould by die head in 200-250 ℃ of scope in forcing machine, thereby be cooled then and curing in this blowing.