Use of carboxylate sodium salt and nitrogen base chelating agents to restore the controlled degradation performance of n-acyloxyamine in the presence of magnesium metal salt

Abstract

A process for the thermal degradation of a polymer composition comprising polypropylene or propylene copolymers in the presence of a free radical generator and a magnesium salt, wherein the thermal degradation is carried out in the presence of a chelating agent selected from the group consisting of aminocarboxylic acids, hydroxycarboxylic acids, polycarboxylic acid salts, substituted triazoles and substituted salicyloyl hydrazides or salicyloyl amides.

Description

[0001] Use of carboxylate sodium salt and nitrogen base chelating agents to restore the controlleddegradation performance of N-acyloxyamine in the presence of magnesium metal salt The present invention relates to a process for the thermal degradation of polypropylene or pro- pylene copolymers in the presence of a free radical generator and a magnesium salt, and fur- ther relates to a polymer composition comprising polypropylene or propylene copolymers, a free radical generator, a magnesium salt, and a chelating agent (metal deactivator). The controlled preparation of polyolefin grades (polymer types having different molar masses,melt viscosities, densities, molar mass distributions, etc.) by customary compounding methods,for example by extrusion or injection moulding, is a routine process employed by polymer manu-facturers and polymer processors / compounders. The setting of the desired parameters, for ex-ample the melt viscosity, by means of this polymer process step is critically dependent on the controlled reactivity and mode of action of the additives employed.The use of free-radical generators for modifying the melt viscosity (rheology) of polyolefins is agenerally known method. Whether it results in a lowering of the molecular weight (degradation) or an increase in the molecular weight (cross linking) depends primarily on the chemical struc- ture of the polyolefin. The reaction of a polymer of the polypropylene type with a free-radical for- mer during a polymer-processing process generally results in the degradation of the polymer, whereas polymers of the polyethylene type tend to cross-linking. Examples that may be men-tioned here are polyethylene types, which are obtainable by means of Phillips catalysts (LDPE)or metallocene catalysts (LLDPE). Exceptions are the polyethylene types prepared by the Zieg- ler process, which likewise tend to undergo chain degradation when processed in the presence of free-radical formers. In the case of copolymers and terpolymers or copolymer blends, high proportions of propylene produce polypropylene-like behaviour, while high proportions of ethylene result in polyethylene- like behaviour. If the above-mentioned copolymers and terpolymers or copolymer blends com- prise proportions of multiply unsaturated olefins, the probability of cross-linking decreases with decreasing concentration of free double bonds. The controlled degradation of polypropylene (PP) to give a product having a lower molecular weight and a narrower molecular weight distribution is a commercially important process for pro- ducing controlled rheology polypropylene (CR-PP). While specific PP grades ("reactor grades") are obtainable by optimisation of the synthesis process or the catalyst systems (metallocene catalyst, Ziegler catalyst), standard PP grades are frequently modified in process technology by means of a processing step following the synthesis. Known degradation pro-cesses proceed either thermally, in particular at temperatures above 280 °C, or in the presenceof free radical generators. In process technology, the free-radical-induced process is carried out in extruders or injection-moulding machines at temperatures above 180 °C. Suitable free-radical generators are organic peroxides which are added during the processing step in diluted form (PP Mastermix, diluted in oil, stabilized on inorganic supports) or directly as a liquid. Under the given processing conditions, the peroxide disintegrates into free radicals, which initiate the chain cleavage reactions and form polymers having the desired rheological properties (melt vis- cosities). The degradation of a PP to form a product having a lower molecular weight (higher melt flow rate (MFR)) is generally referred to as a viscosity-breaking or vis-breaking process. CR-PP grades are mainly used for fibre applications and injection-moulding applications in which low melt viscosities are a prerequisite for economical processing. A wide range of melt viscosities or molecular weights is nowadays required in process technology. A further parameter that influences the processing behaviour of the polymer, in addition to the molecular weight, is the molecular weight distribution (MWD). While polymer grades having broad MWDs display improved orientation behaviour of the polymer chains at low pull-off speeds in a fibre spinning process, the reverse is the case for high pull-off speeds and broad MWDs. For this reason, narrow MWDs are essential at high pull-off speeds in order to achieve improved continuity in the spinning process. The use of peroxides is a drawback, since only a restricted processing temperature window is available because of their decomposition tempera-tures, which are generally below the customary temperatures of polymer processing. In addi-tion, strict safety regulations have to be adhered to during storage, handling and processing of peroxides. A further disadvantage of peroxides is the impossibility of decomposition-free melt compounding with polymers. Apart from peroxides, other sources of free radicals are also known, e.g. C-radical generators based on cumyl systems, but these can be used only at tem- peratures above 280 °C. Electret filter media have long been used in many filtration applications. Electret filter media are those that include a dielectric insulating polymer web that is treated to possess substantially permanent spatially oriented, opposite charge pairs or dipoles. Among the common polymer webs used for electret filter media are polypropylene, polyethylene, polyester, polyamide, poly- vinyl chloride, and polymethyl methylacrylate. Conventional filter media are substantially lacking in electrostatic charge and rely upon impinge- ment, impaction and diffusion for filter performance. Electret filter materials offer improved filter- ing performance over conventional filter materials. The presence of oriented dipoles in the elec- tret filter media is believed to enhance filter performance by allowing the filter media to attract and retain charged and uncharged particles to be filtered. Electret filter materials are made by a variety of known techniques. One technique for manufac- turing electret filter media involves extruding a polymer, typically having a high melt flow index, through a die having a linear array of orifices. An air knife is used to attenuate the extruded pol- ymer fibers by a ratio of about 300:1. The attenuated fibers are then collected on a rotating drum or moving belt using a moderate vacuum. The fiber web is then treated to impart on the fiber web charge pairs or dipoles. The charge pairs or dipoles can be imparted to the fiber, for example, using AC and / or DC corona discharge. One problem associated with electret filter material is that the charge pairs or dipoles imparted to the filter media often are not stable. In some instances, charge or its spatial orientation is lost after filtering certain contaminants for relatively short time periods. The result is a marked de- crease in filter performance over a relatively short period of time (e.g., less than 20 minutes). One other problem associated with electret filter material is their inability to maintain the electro- static charge after being subjected to heat. Manufacturing standards for respiratory products, for example, often mandate that final respiratory mask be subjected to a thermal treatment process to simulate an aged phenomenon. Electret filter media having an increased electrostatic charge that is substantially maintained in the presence of heat include a melt processable charge enhancing additive, such as a fatty acid amide, and a melt processable charge stabilizing additive, such as a fatty acid metal salt. The charge enhancing additive is particularly effective to increase or enhance the electrostatic charge of the filter media when a charge is imparted thereto, and the charge stabilizing additive is particularly effective to stabilize the charge such that, when the filter media is subjected to a heat treatment, the enhanced electrostatic charge is substantially maintained. The meltblown polymer fiber web can also include a variety of melt processable charge enhanc- ing additives. The charge enhancing additive can be, for example, a fatty acid amide that is de- rived from a fatty acid, which includes saturated or unsaturated straight chain carboxylic acids obtained from the hydrolysis of fats. Exemplary fatty acids include lauric acid, myristic acid, pal- mitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and eleostearic acid. The meltblown polymer fiber web can also include a charge stabilizing additive, such as a fatty acid metal salt, which is effective to stabilize the electrostatic charge in the web, particular whenthe fiber web is subjected to heat. The fatty acid portion of the fatty acid metal salt can be, forexample, lauric acid, palmitic acid, stearic acid, oleic acid, etc., and the metal portion of the fatty acid metal salt can be, for example, magnesium, zinc, aluminum, etc. In particular, the fatty acid metal salt is magnesium stearate. The meltblown polymer fiber web can be formed using a variety of techniques, but in one exem- plary embodiment the charge enhancing additive, e.g., a fatty acid amide, and the charge stabi-lizing additive, e.g., a fatty acid metal salt, are mixed with a polymer resin to form a compositionthat is extruded into fibers to form a polymer fiber web. An exemplary process for forming a meltblown polymer fiber web is described in more detail in U.S. Pat. No.6,780,226.The charge enhancing additive and the charge stabilizing additive can be combined with thepolymer resin in a number of ways. The additives can be combined with the resin using a two-screw extruder, yielding polymer pellets with a concentrated amount of each additive. These concentrated pellets, alone or combined with other polymer pellets, are then passed through an extrusion process that yields the desired polymer fiber web. It is an object of the present invention to provide an improved process for the thermal degrada- tion of a polymer composition comprising polypropylene or propylene copolymers in the pres- ence of a free radical generator and a magnesium salt, such as magnesium stearate. In particu- lar, the thermally degraded polymer composition having a modified melt viscosity should be suit-able for the manufacture of electret filter materials. It is a further object of the present inventionto provide a polymer composition comprising polypropylene or propylene copolymers, a freeradical generator, a magnesium salt, which is suitable for the manufacture of electret filter mate- rials.The object is achieved by a process for the thermal degradation of a polymer composition com-prising polypropylene or propylene copolymers in the presence of a free radical generator and a magnesium salt, characterized in that the thermal degradation is carried out in the presence of achelating agent selected from the group consisting of aminocarboxylic acids, hydroxycarboxylicacids, polycarboxylic acid salts, substituted triazoles and substituted salicyloyl hydrazides orsalicyloyl amides.The further object is achieved by a polymer composition comprising polypropylene or propylenecopolymers, a free radical generator, a magnesium salt, and a chelating agent, wherein the che-lating agent is selected from the group consisting of aminocarboxylic acids, hydroxycarboxylicacids, polycarboxylic acid salts, substituted triazoles and substituted salicyloyl hydrazides orsalicyloyl amides, and the use of the degraded polymer composition for the manufacture ofelectret filter materials by extrusion of the degraded polymer composition.Suitable polycarboxylic acid salts are derived from aliphatic, cycloaliphatic or aromatic polycar- boxylic acids. In general, the polycarboxylic acids have 2 to 4 carboxylic acid groups. In pre-ferred embodiments, the chelating agent (metal deactivator) is a polycarboxylic salt selectedfrom the group consisting of disodium succinate, disodium n-dodecylsuccinate, trisodium salt of methylglycine diacid, tetrasodium ethylendiamine tetraacetate, disodium salt of malic acid, triso-dium salt of citric acid, trisodium salt of nitrilotriacetic acid, 2,6-pyridine dicarboxylic acid diso-dium salt, disodium phthalate, 1,2-cyclohexanedicarboxylic acid disodium salt, and tetrasodium benzene-1,2,4,5-tetracarboxylate.Preferred salts of aminocarboxylic acids are derived from glycine, alanine, and anthranilic acid.Preferred salts of hydroxycarboxylic acids are derived from lactic acid, alpha-hydroxyisobutyricacid and mandelic acid. Preferred are in all cases the sodium salts.Suitable substituted triazoles are 3- or 5-aminotriazole, tolyltriazole, benzotriazole carboxylicacid, alkylbenzotriazole (e. g., methyl-, ethyl-, propyl-, butyl-, octyl-benzotriazoles), cycloalkylbenzotriazole and 4-amino-1,2,4-triazole.A particular preferred substituted aminotriazole is a compound of formula (Ia) Suitable substituted salicyloyl hydrazides are compounds of formula (II) wherein Ra and Rb each represent a hydrogen atom, a hydroxyl group, an alkyl group having 1 to 8 carbon atoms, an alkoxy group having 1 to 8 carbon atoms or an aryl group having 6 to 18 carbon atoms, or they are connected to each other to form a ring to make a condensed ring having a total carbon atom number of 10 to 18;R represents -NH–C(O)-(CH2)x-C(O)-NH- or -NH-(CH2)x-NH-;n = 2;x = 1 – 10.A particular preferred salicyloyl hydrazide is a compound of formula (IIa) i.e., dodecandioic acid bis(2-(2-hydroxybenzoly)hydrazide).A particular preferred salicyloyl amide is the compound of formula (Ia) i.e., 3-salicylamido-1H-1,2,4-triazole.Preferably, the free radical generator is an N-acyloxy-amine. Suitable N-acyloxy-amines are, forexample, compounds 1 to 34 as disclosed on pages 12 to 14 of WO2006 / 027327.More preferably, the free radical generator is a N-acyloxyamine of formula (III) wherein n is 1 or 2; Ra is acyl; R1', R2' and R3' are, independently of one another, hydrogen or methyl;and, when n = 1 G3, represents C2-C10-alkylene, C2-hydroxyalkylene or C4-C32-acyloxy-C2-C10-alkylene, C4-C32-acyloxy-C1-C4-alkyl-C2-C10-alkylene or, when n = 2, represents the group (- CH2)2C(CH2-)2. Still more preferably, the free radical generator is a N-acyloxyamine of formula (IIIa) wherein Rais acyl; R1', R2' and R3' are, independently of one another, hydrogen or methyl;and ALK is C2-C10-alkylene or C3-C10alkylene substituted by at least one substituent selectedfrom the group consisting of hydroxy, C4-C32-acyloxy and C4-C32-acyloxy-C1-C4-alkyl. Examples are compounds 24, 25, 26, 27, 28 and 30 as disclosed on pages 13 and 14 of WO2006 / 027327, as follows: Further suitable structures are structures 1-9 and 14-15, 17, 18 and 23 disclosed on pages 12to 14 of WO2006 / 027327, as follows: Very particular preferred is the compound (30).The magnesium salt is general derived from a fatty acid, preferably lauric acid, myristic acid,palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and eleostearic acid. In a partic- ular preferred embodiment, the magnesium salt is magnesium stearate.In general, the chelating agent is present in an amount of from 0.05 to 1.0 % by weight, prefera-bly from 0.1 to 0.8 % by weight based on the total amount of the polymer composition.In general, the magnesium salt is present in an amount of from 0.1 to 2.0 % by weight, prefera-bly from 0.1 to 1.0 % by weight based on the total amount of the polymer composition.In general, the radical generator is present in an amount of from 0.01 to 0.2 % by weight, prefer-ably from 0.015 to 0.1 % by weight, based on the total amount of the polymer composition.Further additives may be included. Examples are colorants (pigments, carbon black, dyes), anti- agglomerants (e.g., glycerol monostearate, calcium stearate), antiblocking agents, antifogging agents, matting agent (e.g. TiO2) and surface modifier (e.g., repellency hydrophobic agents, orhydrophilic agents). In general, the further additives are present in a total amount of from 0.1 %to 5 % by weight, preferably from 0.2 to 2 % by weight based on the total amount of the polymercomposition. The polypropylene or the propylene copolymers are processed in the presence of the additivesincluding the radical generator, the magnesium salt and the metal deactivator at an elevatedtemperature, in general in the range of from 160 to 280 °C. Preferred processing machines are single-screw extruders, contra-rotating and co-rotating twin- screw extruders, planetary-gear extruders, ring extruders or co-kneaders. It is also possible to use processing machines provided with at least one gas removal compartment to which a vac- uum can be applied.Suitable extruders and kneaders are described, for example, in Handbuch der Kunststoffex- tru-sion, Vol.1 Grundlagen, Editors F. Hensen, W. Knappe, H. Potente, 1989, pp.3-7, ISBN: 3-446-14339-4 (Vol.2 Extrusionsanlagen 1986, ISBN 3-446-14329-7). For example, the screwlength is 1 - 60 screw diameters, preferably 35-48 screw diameters. The rotational speed of thescrew is preferably 10 - 600 rotations per minute (rpm), very particularly preferably 25 - 300rpm. The maximum throughput is dependent on the screw diameter, the rotational speed and the driving force. The process of the present invention can also be carried out at a level lower than maximum throughput by varying the parameters mentioned or employing weighing machines delivering dosage amounts.If a plurality of components is added, these can be premixed or added individually. The poly-mers may need to be subjected to an elevated temperature for a sufficient period of time, so that the desired degradation occurs. The temperature is generally above the softening point of the polymers. The process is preferably carried out by premixing the polypropylene or the propylene copoly-mers and the additives including the radical generator, the magnesium salt and the chelatingagent, or by directly introducing the additives including the radical generator, the magnesiumsalt and the chelating agent into an extruder during processing of the polypropylene or the pro-pylene copolymers, and melt compounding the mixture in the extruder, preferably into pellets. To facilitate the dosing of the different additives they are preferably in the form of concentrated masterbatches. In a preferred embodiment of the process of the present invention, a temperature range from about 160 °C to 280 °C is employed. In a particularly preferred process variant, the temperature range from about 200 °C to 270 °C is employed. The period of time necessary for degradation can vary as a function of the temperature, the amount of material to be degraded and the type of, for example, extruder used. It is usually from about 10 seconds to 20 minutes, in particular from 20 seconds to 10 minutes. The invention is further illustrated by the following examples. Examples These examples describe the use of multiple carboxylic acid sodium salts, substituted aminotria-zole and substituted salicyloyl hydrazide to improve the melt flow properties of polypropylenecontaining the radical generator N-acyloxyamines for viscosity modification in the presence of amagnesium salt. The following examples illustrate the significant improvement in Melt flow index and reduction in melt viscosity after reactive extrusion when the compounds of the invention are employed. The materials employed in these examples were as follows:a) Polymer Component^ PP-1: The polymer used is a high molecular weight polypropylene homopolymer (PP):HA10XT from Polychim Industriel. The polymer is in the grinded form.b) Metal Salt^ MS-1: Magnesium stearate technical grade from Sigma Aldrichc) Chelating agents^ MD-1: ADK STAB^ CDA 1 from Adeka Polymer Additives Europe having the structure ^ MD-2: ADK STAB^ CDA 6 from Adeka Polymer Additives Europe having the structure ^ MD-3: disodium succinate (99%) from Sigma Aldrich^ MD-4: disodium phthalate (>95.0%) from TCI Europe^ MD-5: Trilon M^, trisodium salt of methylglycinediacid from BASF SE^ MD-6: tetrasodium benzene-1,2,4,5-tetracarboxylate, from ECHEMI.com^ MD-7: 1,2-cyclohexanedicarboxylic acid disodium salt from ECHEMI.com^ MD-8: n-dodecylsuccinate disodium salt obtained from the corresponding dodecylsuccinicanhydride by treatment with a solution of NaOH in water.^ MD-9: Trilon B^, tetrasodium EDTA from BASF SEd) Radical generator^ RG-1: N-acyloxyamine compound (30)The above products and PP polymers are used in powder form and compounded before singlescrew extrusion. Below is a description of the compounding preparation with the relative amount of the different products. Production of the different compounding formulations: Unless stated otherwise, polypropylene, the metal salt, the chelating agents (metal deactivators)and the radical generator were mixed using a high speed mixer (3000 rpm for 30’) in theamounts as indicated in Table 1 and then melt compounded into pellets on a 25 mm co-rotating twin-screw extruder Berstorff ZE25A x 47D, operating at 160 revolutions per minute (rpm) andat a set temperature of 200 °C, in order to achieve good homogenization of the different rawmaterials. All formulations also contain 0.1% of Irganox^ B215 and 300ppm of Ca(stearate)2.TABLE 1: Composition of the compounded formulations Ref.1 Ref.2 Ref.3 Ref.4 Inv. 5 Inv. 6 Inv. 7 Inv. 8 Inv. 9PP-1 99.87% 99.47% 99.83% 99.47% 99.47% 99.47% 99.47% 99.47% 99.47%MS-1 0.4% 0.4% 0.4% 0.4% 0.4% 0.4% 0.4%RG-1 0.04% 0.04% 0.04% 0.04% 0.04% 0.04% 0.04%MD-1 0.15%MD-2 0.15%MD-3 0.15%MD-4 0.15%MD-5 0.15%Inv. 10 Inv. 11 Inv. 12 Inv. 13PP-1 99.47% 99.47% 99.47% 99.47%MS-1 0.4% 0.4% 0.4% 0.4%RG-1 0.04% 0.04% 0.04% 0.04%MD-6 0.15%MD-7 0.15%MD-8 0.15%MD-9 0.15%Performance of the formulations Compounded fully formulated granules were melt processed through a 1.5 mm die on a 20 mmsingle screw extrusion Extrusionmeter 20D at a set temperature of 295°C for around 20 minutes,and then taken for compression molding at 200^C for 3 minutes to produce A5 size 1.0 mm thickplaquette.- Melt flow index (MFI, g / 10min.) of the plaquette is measured at 230^C and 190^C accordingto ASTM D1238. If not stated differently a 1 mm capillary is used. See Table 2- Formulation viscosity at 10 rad / sec and 100 rad / sec at 190^C have also been measured usinga rotational rheometer ARES-G2 on 1 mm thick A5 plate after compression molding of theextruded material. See Table 3 TABLE 2: Melt flow Index of the formulations at 230^C and 190^C Ref.1 Ref.2 Ref.3 Ref.4 Inv. 5 Inv. 6 Inv. 7 Inv. 8MFI56* 28* 600 335 741 521 585 754(230^C / 2.16 Kg) MFI19 9* 307 113 386 210 237 321(190^C / 2.16 Kg)Inv.9 Inv.Inv. Inv. Inv.13 10 11 12 MFI1715 912 926 1090 1471(230^C / 2.16 Kg) MFI707 259 359 464 605(190^C / 2.16 Kg)*Measured with 2mm ^ capillary

[0002] TABLE 3: Viscosities of the different formulations at 10 rad / sec and 100 rad / sec measured at190^C ViscosityRef.1 Ref.2 Ref.3 Ref.4 Inv. 5 Inv. 6 Inv. 7 Inv. 8(Pa*sec) 10 rad / sec 362 567 25 37 27 32 23 15100 rad / sec 193 257 23 33 25 30 22 14ViscosityInv.9 Inv.Inv. Inv. Inv.13 (Pa*sec) 10 11 12 10 rad / sec 7.3 19 13 12 8.7100 rad / sec 7.2 18 12 12 8.6Test Condition: Frequency sweep ω = 0.1-400 rad.s-1 / Strain: 5% / T=190°C / Gap=1.0 mm.25mm plateThe data from Table 2 clearly show the effect of using the inventive chelating additives in restoringor even enhancing the MFI level in presence of Mg2+ from the one of Ref. 4 to the level andbeyond the one of Ref. 3 where only the vis-breaking product RG-1 is used.To achieve the optimal controlled degradation of PP for meltblown application in the presence of magnesium salt it is necessary to optimize the loading (%) of the inventive additives. In this way, the best performance for long and stable production output could be achieved.Data from Table 3 show a similar trend than the data for MFI. Also in this case the use of chelatingadditives in the presence of Mg2+ and RG-1 is able to fully restore the viscosity enhancing activityof the N-acyloxyamine by producing formulations which have lower viscosity similar if not evenlower than the viscosity of Ref. 3.

Claims

Claims1. A process for the thermal degradation of a polymer composition comprising polypropyleneor propylene copolymers in the presence of a free radical generator and a magnesiumsalt, characterized in that the thermal degradation is carried out in the presence of a che- lating agent selected from the group consisting of aminocarboxylic acids, hydroxycarbox-ylic acids, polycarboxylic acid salts, substituted triazoles and substituted salicyloyl hydra-zides or salicyloyl amides.

2. The process according to claim 1, wherein the chelating agent is a polycarboxylic salt se-lected from the group consisting of disodium succinate, disodium n-dodecylsuccinate, trisodium salt of methylglycine diacid, tetrasodium ethylendiamine tetraacetate, disodium salt of malic acid, trisodium salt of citric acid, trisodium salt of nitrilotriacetic acid, 2,6-pyri-dine dicarboxylic acid disodium salt, disodium phthalate, 1,2-cyclohexanedicarboxylic acid disodium salt, and tetrasodium benzene-1,2,4,5-tetracarboxylate.

3. The process according to claim 1, wherein the chelating agent is a salt of an aminocar-boxylic acid selected from the group consisting of glycine, alanine, and anthranilic acid, ora salt of a hydroxycarboxylic acid selected from the group consisting of lactic acid, alpha-hydroxyisobutyric acid, and mandelic acid.

4. The process according to any one of claims 1 to 3, wherein the free radical generator is aN-acyloxyamine.

5. The process according to claim 4, wherein the free radical generator is a N-acyloxyamineof formula (III)wherein nis 1 or 2;Rais acyl; R1', R2' and R3' are, independently of one another, hydrogen or methyl;and, when n = 1 G3represents C2-C10-alkylene, C2-hydroxyalkylene or C4-C32-acyloxy-C2- C10-alkylene, C4-C32-acyloxy-C1-C4-alkyl-C2-C10-alkylene or, when n = 2, represents the group (-CH2)2C(CH2-)2.

6. The process according to any one of claims 1 to 5, wherein the magnesium salt is magne-sium stearate.

7. The process according to any one of claims 1 to 6, wherein the chelating agent is presentin an amount of from 0.05 to 1.0 % by weight, based on the total amount of the polymercomposition.

8. The process according to any one of claims 1 to 7, wherein the magnesium salt is presentin an amount of from 0.1 to 2.0 % by weight, based on the total amount of the polymer composition.

9. The process according to any one of claims 1 to 8, wherein the radical generator is pre-sent in an amount of from 0.01 to 0.2 % by weight, based on the total amount of the poly- mer composition.

10. The process according to any one of claims 1 to 9, wherein the process is carried out bypremixing the polypropylene or the propylene copolymers and additives including the radi- cal generator, the magnesium salt and the chelating agent, or by directly introducing addi- tives including the radical generator, the magnesium salt and the chelating agent into anextruder during processing of the polypropylene or the propylene copolymers, and melt compounding the mixture in the extruder.

11. A polymer composition comprising polypropylene or propylene copolymers, a free radicalgenerator, a magnesium salt, and a chelating agent, wherein the chelating agent is se-lected from the group consisting of aminocarboxylic acids, hydroxycarboxylic acids, poly- carboxylic acid salts, substituted triazoles and substituted salicyloyl hydrazides orsalicyloyl amides.

12. The polymer composition according to claim 11, wherein the chelating agent is a polycar-boxylic salt selected from the group consisting of disodium succinate, disodium n-do- decylsuccinate, trisodium salt of methylglycine diacid, tetrasodium ethylendiamine tetraac- etate, disodium salt of malic acid, trisodium salt of citric acid, trisodium salt of nitrilotri-acetic acid, 2,6-pyridine dicarboxylic acid disodium salt, disodium phthalate, 1,2-cyclo-hexanedicarboxylic acid disodium salt, and tetrasodium benzene-1,2,4,5-tetracarboxylate.

13. The polymer composition according to claim 11 or 12, wherein the free radical generatoris a N-acyloxyamine.

14. The polymer composition according to any one of claims 11 to 13, wherein the free radicalgenerator is a N-acyloxyamine of formula (IIIa)wherein Ra is acyl; R1', R2' and R3' are, independently of one another, hydrogen or methyl; and ALK is C2-C10-alkylene or C3-C10alkylene substituted by at least one substituent selectedfrom the group consisting of hydroxy, C4-C32-acyloxy and C4-C32-acyloxy-C1-C4alkyl.

15. The polymer composition according to any one of claims 11 to 14, wherein the magne-sium salt is magnesium stearate.

16. The use of aminocarboxylic acids, hydroxycarboxylic acids, polycarboxylic acid salts, sub-stituted triazoles and substituted salicyloyl hydrazides or salicyloyl amides as chelating agent in a process for the thermal degradation of a polymer composition comprising poly-propylene or propylene copolymers in the presence of a free radical generator and a mag- nesium salt.

17. The use of the polymer composition of any one of claims 11 to 15 for the manufacture of adegraded polymer composition for the manufacture of electret filter materials by extrusion of the degraded polymer composition.

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