For most parts made of polypropylene or its copolymers, good stiffness and good toughness is required. One of the major problems of such parts is that their physical properties continue to change by physical ageing during their further lifetime. While a certain increase in stiffness is normally regarded as rather positive for their performance, the simultaneous reduction of toughness is certainly negative and can strongly limit the applicability of such materials. This effect is known e. g. from"Journal of Applied Polymer Science"10 (1966) 901-915", where the stiffness increase and parallel embrittlement of a polypropylene film at room temperature is studied, and also from"Polymer Testing 18 (1999) 257-266", where the relation between stiffness and impact strength of polypropylene homopolymers is investigated. In this literature it is shown that ageing at room temperature develops increasing stiffness combined with embrittlement (decreasing toughness). From this literature it is further known that the toughness of such materials can be improved by annealing at elevated temperatures. But unfortunately the gain in toughness by annealing is again lost by physical ageing during the further lifetime.

The object of the invention was to overcome the problem of this high embrittlement and decreasing stiffness of polypropylenes during their lifetime. Unexpectedly it was found that certain specific compositions of heterophasic ethylene-propylene copolymers (HECO), when heated to above about 65 °C exhibit an improvement of stiffness and toughness alike. Moreover, this improved toughness is surprisingly not reduced to the original level by further ageing at room temperature, but remains at much higher levels.

The present invention accordingly relates to a process for improving stiffness and toughness of ethylene-propylene polymer compositions, or of formed parts consisting essentially of said polymer compositions, said process comprising heating said polymer compositions or said parts for 1 to 100 h, preferably for 12 to 48 h, at a temperature of 75 °C to 150 °C, preferably of 100 to 140 °C, said polymer compositions essentially consisting of a) 65 to 95 wt%, preferably 75 to 90 wt%, of crystalline propylene-homopolymers and/or propylene-random copolymers comprising 0 to 10 wt%, preferably 0 to 6 wt%, of alpha-olefins consisting of the group of C2-and C4-to C8-alpha-olefins, the propylene- portion of said homopolymers or copolymers having an isotacticity of greater than 90 %, preferably greater than 95 %, b) 5 to 35 wt%, preferably 10 to 25 wt% of elastomeric copolymers comprising 20 to 80 wt%, preferably 40 to 60 wt% of ethylene and of 80 to 20 wt%, preferably 60 to 40 wt% of alpha-olefins of the group consisting of C3-to C8-alpha-olefins, said elastomeric copolymers being soluble in xylene at room temperature to more than 50 wt%, preferably to more than 70 wt%.

Said compositions are preferably commercial products, e. g. like the heterophasic copolymers (HECO) of Borealis, consisting of 75 to 90 wt% of crystalline propylene- homopolymers or-random copolymers comprising 0 to 6 wt% of ethylene, the propylene- portion of said homopolymers or copolymers having an isotacticity of greater than 95 %, measured with IR spectroscopy according to the method described e. g. in EP-A 255693 (Chisso, 1988), and 10 to 25 wt% of elastomeric copolymers comprising 40 to 60 wt% of ethylene and 60 to 40 wt% of propylene, said elastomeric copolymers having a solubility in xylene at room temperature (about 20 to 25 °C) of about greater than 70 wt%. A preferred heterophasic copolymer is e. g. MC17 XMOD of Borealis, which consists of 80 wt% of propylene homopolymer with an isotacticity of above 97% and 20 wt% of an elastomeric copolymer containing 60 wt% ethylene and 40 wt% of propylene, which is soluble in xylene (23 °C) to more than 80 wt%. These polymer compositions are normally directly produced in serial polymerisations in multiple reactors. It is also possible to obtain these polymer compositions e. g. by mixing said crystalline and said elastomeric polymers.

The compositions can usally also contain conventional polymer additives, among them antioxidants to prevent thermooxidative damage, UV-stabilisers to reduce photooxidative damage and processing aids like internal slip agents and antistatic agents preferably in a total amount of less than 2 wt%. Additionally, the compositions can also contain nucleating agents like talc, sodium benzoate or sodium 2,2'- methylene bis- (4,6-di-tert butylphenyl) phosphate to promote the crystallisation of the polymer in an amount of up to 3 wt%, preferably from 0.01 to 3 wt %. For achieving the necessary mechanical profile, the compositions may also include conventional mineral fillers like talc, calcium carbonate or wollastonite as well as short glass fibers as reinforcing agents in a content of 0-40 wt%, preferably 0-20 wt%.

The stiffness of the polymer compositions may be measured e. g. as the elastic modulus, e. g. the flexural modulus according to ISO 178. The toughness (or inverse brittleness respectively) may be measured e. g. as the impact strength, e. g. according to ISO 179.

Further objects of the invention are ethylene-propylene polymer compositions, which can be obtained by the above described process, and formed parts or articles, which consist essentially of said compositions. Such formed parts or articles like e. g. blow-moulded bottles and containers, cast-extruded or blown films, injection-moulded packaging components or technical parts may be manufactured by any thermopolastic forming technique, e. g. by injection or blow moulding, extrusion etc. It is also possible to improve the stiffness and toughness of such formed parts or articles by heating them as described above for 1 to 100 h at 75 °C to 150 °C. This heating may also be performed, e. g., in the course of a sterilisation, drying or other heating process of said containers or bottles together with their contents, e. g. food or medicines.

The main advantage of said polymer compositions and of said parts consisting essentially of said compositions is that, besides an increase in stiffness, also their toughness is increased, and additionally that the toughness is maintained to a high extent also in storage and/or utilisation at room temperatures for a long time.

A further object of the invention is the use of heterophasic copolymers essentially consisting of a) 65 to 95 wt%, preferably 75 to 90 wt%, of crystalline propylene- homopolymers and/or propylene-random copolymers comprising 0 to 10 wt%, preferably 0 to 6 wt%, of alpha-olefins consisting of the group of C2-and C4-to C8-alpha-olefins, the propylene-portion of said homopolymers or copolymers having an isotacticity of greater than 90 %, preferably greater than 95 %, and b) 5 to 35 wt%, preferably 10 to 25 wt% of elastomeric copolymers comprising 20 to 80 wt%, preferably 40 to 60 wt% of ethylene and of 80 to 20 wt%, preferably 60 to 40 wt% of alpha-olefins of the group consisting of C3-to C8-alpha-olefins, said elastomeric copolymers being soluble in xylene at room temperature to more than 50 wt%, preferably to more than 70 wt%, to improve stiffness and toughness of crystalline propylene-hompolymers or-random-copolymers by heating said heterophasic copolymers or parts consisting essentially of said heterophasic copolymers for 1 to 100 h at 75 °C to 150 °C.

A further object of the invention is the use of said elastomeric copolymers as additional components to said crystalline propylene-polymers to improve stiffness and toughness of said crystalline propylene-polymers, by bringing together said elastomeric copolymers and said crystalline polymers and heating the thus obtained heterophasic copolymers, or parts constisting essentially of said heterophasic copolymers, for 1 to 100 h at 75 °C to 150 °C.

Example 1 (Comparative): Samples of 80x10x4 mm were injection molded from a propylene homopolymer Daplen K2 XMOD (Borealis, MFR 230 °C/2, 16 kg: 8 g/10min, isotacticity above 97%) and stored at room temperature for 24 h, 336 h and 1563 h. Further samples were annealed after the injection molding for 24 h and 168 h at 80 °C, 110 °C and 140 °C and then also stored for 24 h, 336 h and 1563 h. Then the modulus (flexural modulus according to ISO 178 at 23 °C) and impact strength (notched impact strength according to ISO 179 leA at 23 °C) were measured: Modulus of Propylene Homopolymer (in MPa) storage (23 °C no heating 80°C/24h 110°C/24h 140°C/24h 140°C/168h 24 h 2037 2174 2234 2539 2606 336 h 2224 2310 2379 2739 2839 1563 h 2248 2327 2502 2903 3075 impact Strength of Propylene Homopolymer in (kJ/m2) storage (23 °C) no heating 80 °C/24 h 110 °C/24 h 140 °C/24 h 140 °C/168 h 24 h 2,68 2,08 2,17 2,63 2,85 336 h 2,00 2,02 2,02 2,26 2,45 1563 h 1,98 1,94 1,98 2,07 2,11 Example 2: Analogously to example 1, samples were injection molded, annealed, stored and then their modulus and impact strength were measured, with the difference that instead of the homopolymer Daplen K 2 XMOD, the heterophasic copolymer Daplen MC 17 XMOD (Borealis, MFR 230 °C/2, 16 kg: 12 g/10min, isotacticity of matrix component greater than 97%, xylene solubles of elastomeric component greater than 80 wt%) was used.

Modulus of Propylene Copolymer (in Mpa) storage (23 OC) no heating 80 °C/24 h 110 °C/24 h 140 °C/24 h 140 °C/168 h 24 h 1369 1509 1599 1778 1833 336 h 1538 1584 1648 1913 2007 1563 h 1562 1592 1716 1994 2096 Impact Strength of Propylene Copolymer in (kJ/m2) storage (23 °C no heating 80 °C/24 h 110 °C/24 h 140 °C/24 h 140 °C/168 h 24 h 8,72 9,64 10,18 17,19 17,12 336 h 8,43 9,00 9,22 14,89 14,58 1563 h 8,28 8,47 9,16 14,67 13,73