Low-odor polyethylene blends

The present invention relates to low-odor polyethylene blends comprising (a) from 30 to 90% by weight of a high-molecular-weight ethylene copolymer with a melt flow rate MFR 190/21.6≦1.5 g/10 min, a density ≦0.950 g/cm 3 , a weight-average molecular weight M w ≦300,000 g/mol and a polydispersity Mw/Mn of from 1 to 10, and (b) from 10 to 70% by weight of a low-molecular-weight ethylene homopolymer or ethylene copolymer with a melt flow rate MFR 190/2.16 of from 20 to 100 g/10 min, a density ≦0.95 g/cm 3 , a weight-average molecular weight M w of from 8000 to 80,000 g/mol and a polydispersity M w /M n of from 2.5 to 12, where the Al content of the high-molecular-weight component is from 5 to 60 ppm, the Al content of the low-molecular-weight component is from 0 to 5 ppm and the Al content of the blend is from 1 to 55 ppm. It further relates to a process for preparing polyethylene blends of this type, and also to their use for producing moldings, in particular hollow articles and pressure pipes.

Ever higher requirements are being placed upon the mechanical load-bearing capacity of moldings made from polyethylene. In particular, there is a requirement for highly stress-cracking resistant, impact-resistant and stiff products which are particularly suitable for the production of hollow articles, and also of pressure pipes. The requirement for good stress-cracking resistance at the same time as stiffness is not easy to fulfil, since these properties counteract one another. Whereas stiffness increases with the density of the polyethylene, stress-cracking resistance decreases with increasing density.

For hollow articles and pressure pipes it has therefore proven to be advantageous to use blends made from a high-molecular-weight, low-density ethylene copolymer and from a low-molecular-weight, high-density ethylene homopolymer. These are described, for example, by L. L. Böhm et al., Adv. Mater. 4, (1992) 234-238. Similar polyethylene blends have been disclosed in EP-A 100 843, EP-A 533 154, EP-A 533 155, EP-A 533 156, EP-A 533 160 and U.S. Pat. No. 5,380,807.

Pressure pipes made from polyethylene are used increasingly for conveying drinking water. For this application, besides high stiffness and high creep rupture strength at high pressures it is important that the polyethylene has very low odor and is very taste-neutral. The odor level of a material may be determined by various methods. A necessary, but not sufficient, criterion for a good odor level is a very low proportion of volatile carbon compounds in the material. For example, the test known as the VW Audi test (test specification 3341, Verband Deutscher Automobilbauer, Recommended Standard No. 277), determines volatile carbon fractions of a material at 120° C. However, it is also essential to assess odor. According to DIN 10955 and 10951 Al or EN 1622 a number of test personnel assess the odor of a material on a scale from 0 to 4. The “electronic nose” records volatile constituents of the material by means of various conductivity measurements.

Polymeric Materials Encyclopedia, Ed. J. P. Salamone, CRC Press, New York, 1996, pages 5997-98 lists possible causes for odor in polyethylene. The odor of polyethylene is generally caused by oxidation of the polymer or by catalyst residues, e.g. the triethylaluminum used as cocatalyst in Ziegler catalysts. Other possible causes are additives, e.g. Ca stearate or Zn stearate, and especially decomposition products of these. These additives are used, for example, to bind HCl deriving from Ziegler catalysts. For drinking water applications it is therefore frequently necessary to add odor-trapping additives to the polyethylene or to carry out additional steps, such as deodorization via aeration.

Low-molecular-weight components are a particular problem in the preparation of low-odor polyethylene blends, since their molecular weight should be very low to ensure sufficiently high density/stiffness in the blend. If, however, the low-molecular-weight component has a broad molecular weight distribution there is the risk that the blend will comprise too many oligomers which could cause odor. U.S. Pat. No. 5,350,807 has therefore disclosed polyethylene where a degree of polymerization of only 9 to 125 is a preferred range for the low-molecular-weight fraction. In addition, impact strength of the blend is adversely affected by too high a proportion of oligomers. The molecular weight distribution of the low-molecular-weight component should therefore be very narrow.

Narrow molecular weight distributions are preferably achieved with the traditional Ziegler catalysts and metallocene catalysts. However, these require large amounts of free organic Al cocatalysts, which adversely affect the odor of the polyethylene. The free cocatalysts may moreover form ethylene oligomers via the molecular-weight-increase reaction described by Ziegler, and these increase the proportion of volatile compounds in the polymer. In addition, Ziegler catalysts have only low productivity in the preparation of low-molecular-weight polyethylene, and therefore a large amount of catalyst is required, and therefore also a large amount of cocatalyst. Phillips catalysts give only broad molecular weight distributions and likewise require large amounts of cocatalysts in order to achieve low molecular weights. The problem of preparing low-odor, low-molecular-weight polyethylene components for ethylene polymer blends with high-productivity catalysts has not yet been solved.

None of the abovementioned disclosures encompasses ethylene polymer blends which have, besides good mechanical properties, a low odor level and low intrinsic taste.

It is an object of the present invention to provide improved blends of this type.

We have found that this object is achieved by means of the blends defined at the outset. A process for preparing blends of this type has also been found, as has their use for hollow articles and pipes.

The polyethylene blend of the present invention comprises two components.

The low-molecular-weight component (b) is composed of an ethylene homopolymer or ethylene copolymer with a weight-average molecular weight of from 8,000 to 80,000 g/mol, preferably from 20,000 to 70,000 g/mol and particularly preferably from 30,000 to 60,000 g/mol. The polydispersity M w /M n is from 2.5 to 12, preferably from 3 to 10 and particularly preferably from 5 to 8. The melt flow rate MFR 190/2.16 of the ethylene homopolymer or ethylene copolymer is from 15 to 100 g/10 min, preferably from 20 to 60 g/min and particularly preferably from 25 to 40 g/l0 min. The density is at least 0.95 g/cm 3 , preferably from 0.95 to 0.97 g/cm 3 , particularly preferably from 0.96 to 0.97 g/cm 3 .

Besides ethylene, the low-molecular-weight component may also comprise comonomers. The comonomer is selected taking account of the properties desired. However, comonomers preferably used are 1-olefins, particularly preferably propene, 1-butene, 1-pentene, 1-hexene, 1-octene or 4-methylpentene. The amount of the comonomer used is likewise selected taking into account the properties desired, but the amount is preferably not more than 1 mol %, based on the amount of all the monomers used.

The low-molecular-weight polyethylene component of the present invention has only small fractions of volatile carbon compounds. The proportion of volatile carbon, measured at 120° C. by the abovementioned VW-Audi test is not more than 80 mg/kg, preferably not more than 70 mg/kg.

The low-molecular-weight component preferably comprises no aluminum. However, the present invention also encompasses components which comprise traces of Al. Traces of this type may result, for example, from preparation, transport or storage in vessels comprising Al. The Al content of the low-molecular-weight component is, however, never greater than 5 mg/kg, based on polyethylene.

The high-molecular component (a) is composed of an ethylene copolymer with a weight average molecular weight ≧300,000 gimol, preferably from 350,000 to 700,000 g/mol and particularly preferably from 400,000 to 600,000 g/mol. The comonomer used besides ethylene is selected taking into account the properties desired. However, the comonomers used are preferably 1-olefins, particularly preferably propene, 1-butene, 1-pentene, 1 -hexene, 1 -octene or 4-methylpentene. The amount of the comonomer used is also selected taking into account the properties desired, but preference is given to amounts of from 0.2 to 4.0 mol % based on the amount of all the monomers used.

The polydispersity Mw/Mn is from 1 to 10, preferably from 3 to 9 and particularly preferably from 5 to 9. The melt flow rate MFR 190/21.6 of the ethylene copolymer is not greater than 1.5 g/10 min, preferably from 0.5 to 1.5 g/10 min and particularly preferably from 0.6 to 1.2 g/10 min. If the melt flow index MFR 190/21.6 is greater than 1.5 g/10 min the mechanical properties of the blend are impaired. The density is not greater than 0.950 g/cm 3 , preferably from 0.91 to 0.945 g/cm 3 , particularly preferably from 0.92 to 0.94 g/cm 3 .

The high-molecular-weight polyethylene components of the present invention have only small fractions of volatile carbon compounds. The proportion of volatile carbon, measured at 120° C. using the abovementioned VW Audi test is less than 80 mg/kg, preferably less than 40 mg/kg.

The high-molecular-weight polyethylene component of the present invention comprises from 5 to 60 mg/kg of Al, based on polyethylene, particularly preferably from 20 to 50 mg/kg.

The novel blends comprise from 30 to 90% by weight of the high-molecular-weight component and from 10 to 70% by weight of the low-molecular-weight component. They preferably comprise from 40 to 80% by weight of the high-molecular-weight component and from 20 to 60% by weight of the low-molecular-weight component, particularly preferably from 40 to 60% by weight of the high-molecular-weight component and from 60 to 40% by weight of the low-molecular-weight component.

The novel blends comprise not more than 55 mg/kg of Al. If the Al content is greater the polyethylene blends obtained no longer have low odor. The Al content is preferably from 1.5 to 55 mg/kg and particularly preferably from 5 to 30 mg/kg.

The novel blends furthermore have low odor. The odor class determined according to DIN 10955 and 10951 Al or EN 1622 is below 3. The proportion of volatile carbon, measured at 120° C. using the abovementioned VW Audi test is not more than 80 mg/kg, preferably not more than 60 mg/kg, particularly preferably not more than 30 mg/kg.

The novel blends may be prepared in a manner known per se from the high- and the low-molecular-weight component by intimate mixing and homogenizing at elevated temperatures, e.g. in single- or twin-screw extruders or kneaders. During the mixing and homogenization procedure the exposure of the polymer to temperature is held very low so that very little additional volatile carbon fraction is produced by decomposition. The temperature is therefore kept very low and the mixing time very short. The temperature is preferably from 180 to 270° C. It is also possible to add other components during the mixing and homogenization procedure, for example processing aids, color pigments, dyes, UV stabilizers or antioxidants known per se. The amount of the additives is adjusted in such a way that the proportion of volatile carbon in the blend does not exceed 80 mg/kg and that the odor class obtained is below 3.

The high-molecular-weight ethylene copolymers described with low proportions of volatile carbon are prepared using Ziegler catalysts known per se. Titanium catalysts are particularly preferred. Catalysts of this type, and also the processes for preparing polymers of this type, are described, for example, in DE-A 34 33 468, DE-A 42 17 171 and EP-A 518093, but this selection is not intended to limit the invention. The amount of cocatalysts is restricted in such a way that the proportion of volatile carbon does not exceed 80 mg/kg and that the Al content is not more than 60 mg/kg. The polymerization is usually carried out in suspension, e.g. in a loop reactor, or in the gas phase, e.g. in a fluidized-bed reactor.

The low-molecular-weight ethylene homopolymers or ethylene copolymers described with low proportions of volatile carbon are prepared using a chromocene catalyst on an oxidic support. Chromocene catalysts for preparing polyolefins are known in principle, for example from DE-A 43 23 192 or U.S. Pat. No. 4,424,139. For the purposes of this invention bis(cyclopentadienyl)chromium(II) is particularly suitable.

However, the invention also encompasses chromocene derivatives in which the cyclopentadienyl groups carry inert organic substituents. Examples of possible substituents are alkyl, such as C 1 -C 6 -alkyl, and/or C 6 -C 12 -aryl. Annelated cyclopentadienyl groups, such as indene or fluorene, which may likewise have substitution by the radicals mentioned, are also possible.

Suitable support materials are metal oxides, e.g. the oxides of silicon or of zirconium, silica being preferred. The preparation of supports of this type has been described, for example, in DE-A 36 34 534. These supports preferably have high internal surface of from about 50 to 1000 m 2 /g. The average pore diameter is in the range from 1 to 100 nm. An example of a particularly preferred commercially available product is Silicia Gel 332 from Grace. Contrasting with this, the use, for example, of the chromocene catalyst disclosed by U.S. Pat. No. 4,424,139 on an AlPO 4 support gives low-molecular-weight components which have a marked odor and high proportions of volatile carbon. Chromocene catalysts supported in this way also have lower productivity.

Before loading with the chromocene, the support materials are usually baked in an inert gas atmosphere at from 200 to 900° C. to remove adsorbed water.

The loading of the dried support material is preferably carried out by dissolving the chromocene in a solvent and exposing the support material to the solvent for several hours. Suitable solvents are hydrocarbons, such as n-pentane, n-hexane, cyclohexane or toluene. The amount of solvent is selected in such a way that the support material is completely wetted. The solids are then separated off from the solvent, e.g. by filtration, and the solid is dried.

The amount of chromium in the supported catalyst is generally from 0.1 to 10% by weight, based on the support material.

The polymerization is generally carried out in suspension, e.g. in a loop reactor, or in the gas phase, e.g. in a fluidized-bed reactor, in a manner known per se, for example as described in Ullmanns Enzyklopddie der technischen Chemie, Vol. 19, 4th edition.

Cocatalyts may also be used in addition to chromocene. Cocatalysts which are excluded are aluminum compounds and boron compounds. However, hydrides or organometallic compounds of alkali metals or of alkaline earth metals may be added as cocatalysts if required. Possible metals, besides lithium, are sodium, potassium, beryllium, magnesium, calcium or barium. Preferred organometallic compounds are metal alkyl compounds and metal aryl compounds. Possible hydrocarbon radicals are aliphatic radicals having from 1 to 6 carbon atoms, and also aromatic radicals having from 6 to 15 carbon atoms. Preference is given here to the lithium compounds, e.g. n-butyllithium or phenyllithium. The amount may be selected taking into account the requirements, but is restricted in such a way that the proportion of volatile carbon in the low-molecular-weight components prepared does not exceed 80 mg/kg.

The novel polyethylene blends have low odor and low intrinsic taste. They also have high stress-cracking resistance and high impact strength. They are therefore highly suitable for the production of films and moldings, in particular of pressure pipes for conveying drinking water, and also for hollow articles for the storage and transport of drinks and foods.

The following working examples serve to describe the invention further but are not intended to limit its scope.

The measurements described were determined in the following way:

Preparation of the High-molecular-weight Component: