Process for preparing high melt strength polypropylene

Abstract

The present invention is directed to a process for preparing a high melt strength polypropylene wherein a stabilized mixture comprising linear polypropylene, an acid scavenger, and a stabilizer comprising alpha-tocopherol in an amount of less than 150 ppm is irradiated by electron beam irradiation. The present invention is also directed to a high melt strength polypropylene obtained by the inventive process. The high melt strength polypropylene is suitable for foam applications.

Description

[0001] Process for Preparing High Melt Strength Polypropylene

[0002] Technical Field

[0003] The present invention is directed to a process for preparing a high melt strength polypropylene and to a high melt strength polypropylene obtained by the process. The high melt strength polypropylene is suitable for foam applications.

[0004] Technical Background

[0005] In principle, melt strength of a polypropylene can be increased by reducing the melt flow rate. However, this is not expedient for all commercial applications since they require certain melt flow rate ranges, such as extrusion foaming. Linear polypropylene grades having a rather low melt flow rate, e.g., an MFR2 below 0.3 g / 10 min, can have a rather high melt strength, e.g., above 50 cN, corresponding to a rather high ratio of melt strength to melt flow rate, e.g., above >150. These high melt strength grades are unfavorable for the extrusion foam process due to their high shear viscosity resulting in very high pressure at the die plate. This is a severe limitation for the processing rate as the temperature window for producing a low-density polypropylene foam is narrow. These linear high melt strength polypropylene grades have limited extensibility of the melt, making the production of low-density foam with low open cell content difficult as the foam bubbles burst and combine during the expansion phase.

[0006] Commercial high melt strength products suitable for extrusion foam application (such as Daploy™ WB140HMS from Borealis or Pro-fax PF814 from LyondellBasell) have high melt strength of higher than 40 cN (determined with the measurement method described herein) and a melt flow rate MFR2 of 2-3 g / 10 min corresponding to a ratio of melt strength to melt flow rate of 13-20.

[0007] Hence, the task is to provide high melt strength polypropylene having a melt flow rate which is suitable for the desired application.

[0008] The basic mechanism to produce high melt strength polypropylene with a specific ratio of melt strength to melt flow rate via post-reactor processes is the introduction of long chain branches into a linear polypropylene. Long chain branches are formed by a competitive reaction of visbreaking and combination reactions of macro radical chains. Macro radical chains are formed by hydrogen abstraction reaction that can be triggered by decomposition products of peroxides or ionizing radiation (Beta, Gamma or UV irradiation).

[0009] The key problem of any post reactor production process of high melt strength polypropylene

[0010] HLZ:MAHthat starts from linear feedstock polypropylene is the high sensitivity thereof and tendency towards visbreaking reactions caused by hydroperoxides and / or radicals created during or after the irradiation process. The unwanted visbreaking reaction results in high melt strength polypropylene having a ratio of melt strength to melt flow rate outside the market requirements, e.g., for foam applications, i.e. , the melt strength may be sufficiently high, but the melt flow rate may be too high. If the melt flow rate of a high melt strength polypropylene is too high, the required melt pressure in the foaming process cannot be achieved as a certain pressure is required for foaming with a foaming agent.

[0011] The standard approach to decrease visbreaking is to prevent contact of the linear polypropylene feedstock with oxygen by working under inert conditions over the complete process chain, such as the production and transportation of pellets, irradiation or chemical conversion of pellets including any necessary deactivation steps. Maintaining an oxygen concentration of 400 ppm or lower over the complete process chain is known to be perfectly suitable. However, under industrial conditions for high production rates, meaning for more than 1 ton per hour, maintaining low oxygen conditions is rather cumbersome resulting in high effort and costs.

[0012] WO 2009 / 003930 A1 describes a process for producing high melt strength polypropylene under reduced oxygen conditions by irradiation with electron beam utilizing non-phenolic antioxidants such as a hindered amine, a hydroxylamine, an amine oxide, a benzofuranone, or an organic phosphite and / or organic phosphonite, to improve achievable melt strength at given melt flow rate.

[0013] It is believed that classical phenolic antioxidants cannot be used in the production process as they block macroradicals by a transfer of hydrogen reaction that prevents the coupling reaction and accordingly leads to too low ratio of melt strength to melt flow rate.

[0014] WO 2018 / 028922 A1 describes a process for producing high melt strength polypropylene by irradiation with electron beam utilizing Vitamin E (tocopherol) as antioxidant, to improve achievable melt strength at suitable melt flow rate. In this application it is postulated that the addition of tocopherol before irradiation together with calcium stearate would lead to higher melt strength. Verification experiments suggest that many process parameters are missing or are even incorrect.

[0015] WO 2022 / 238520 A1 describes a process for producing high melt strength polypropylene byprocessing a linear polypropylene with a coupling agent such as linseed oil and irradiating the resulting mixture by electron beam irradiation.

[0016] There is still a need for an improved process for preparing a high melt strength polypropylene, wherein improvements may reside in the process or in the obtained product. For example, a desired improvement would be to deal with the competition of visbreaking and combination reactions of macro radical chains thereby achieving a high melt strength polypropylene with sufficiently low melt flow rate, i.e. , having a high ratio of melt strength to melt flow rate, without the need of addition of a coupling agent and / or with a lower amount of a suitable antioxidant.

[0017] The object of the present invention is to provide a process for producing a high melt strength polypropylene, which addresses at least one of these needs.

[0018] The problem is solved according to the present invention by providing a process for producing a high melt strength polypropylene, wherein a stabilized mixture comprising linear polypropylene, an acid scavenger, and a stabilizer comprising alpha-tocopherol in an unusual low amount is irradiated by electron beam irradiation.

[0019] The problem is also solved according to the present invention by providing a high melt strength polypropylene obtained by the process as outlined above.

[0020] Description of the present Invention

[0021] The present invention is according to a first aspect directed to a process for preparing a high melt strength polypropylene, the process comprising the following steps:

[0022] a) providing a linear polypropylene having a melt flow rate of below 1.0 g / 10 min as determined according to ISO 1133 at 230 °C and a load of 2.16 kg,

[0023] b) optionally reducing the oxygen content of the reaction atmosphere resulting in an oxygen content of below 120,000 ppm,

[0024] c) incorporating an acid scavenger and a stabilizer comprising alpha-tocopherol into the linear polypropylene, thereby obtaining a stabilized mixture, wherein the amount of alpha-tocopherol incorporated into the linear polypropylene is less than 150 ppm with respect to the stabilized mixture,

[0025] d) providing the stabilized mixture in the form of a bed,

[0026] e) irradiating the bed by electron beam irradiation.The term “high melt strength polypropylene” is known by the skilled person. A commonly used abbreviation is “HMS-PP”.

[0027] The term “high melt strength polypropylene” is usually used in the art to refer to a polypropylene having a certain extent of long chain branching. The long chain branching is responsible for the high melt strength, i.e. , if compared to linear polypropylene.

[0028] Thus, a high melt strength polypropylene is branched and, as such, differs from a linear polypropylene in that the polypropylene backbone covers a certain extent of long side chains whereas a non-branched polypropylene, i.e. a linear polypropylene, does not cover long side chains, at least not in a considerable content. The side chains have significant impact on the rheology of polypropylene. Accordingly, linear polypropylenes and high melt strength polypropylenes can be clearly distinguished by their flow behavior under shear or extensional deformations.

[0029] The skilled person knows that this comparison makes merely sense in case the melt strength values of a high melt strength polypropylene, i.e., of a polypropylene with a suitable degree of long chain branching, is compared to a linear polypropylene having more or less the same melt flow rate, MFR. Generally speaking, melt strength increases not only by presence of long chain branched (LCB) structures but also with decreasing melt flow rate.

[0030] High melt strength polypropylene can have several applications depending on its melt flow rate. For coating and film applications, the melt flow rate determined according to ISO 1133 at 230 °C and a load of 2.16 kg, i.e., MFR2, will be usually around 10 to 20 g / 10 min. For foam applications, however, the melt flow rate must be lower to enable establishing a sufficiently high melt strength and at the same time a sufficiently high shear viscosity to be able to build up enough pressure for foam applications, i.e., the melt flow rate being considerably below 10 g / 10 min. The absolute melt strength required for foam applications is accordingly much higher than for coating or film applications.

[0031] The present invention is directed to foam applications. In the context of the present invention, the term “high melt strength polypropylene” is accordingly used in the sense that the HMS-PP has sufficiently high melt strength for foam applications and - connected thereto - sufficiently low melt flow rate for foam applications.

[0032] The skilled person knows linear polypropylene usable as reactant for the preparation of ahigh melt strength polypropylene by electron beam irradiation.

[0033] The term “polypropylene” as used herein is to be understood in a broad sense in that it encompasses propylene homopolymers, (monophasic) propylene copolymers, and heterophasic propylene copolymers (HECOs).

[0034] The term “propylene homopolymer” relates to a polypropylene that consists substantially, i.e. of at least 99.0 wt%, more preferably of at least 99.5 wt%, still more preferably of at least 99.8 wt%, like of at least 99.9 wt%, of propylene units, i.e., units derivable from propylene monomer. Preferably, only propylene units are detectable in the polypropylene, i.e., in the preparation of a propylene homopolymer, merely propylene is added as monomer for polymerization and no other polymerizable monomer (comonomer) is added or grafted after polymerization.

[0035] The term “propylene copolymer” as used herein refers to a copolymer of propylene with ethylene and / or one or more a-olefin(s), for example, ethylene and / or one or more a-olefin(s) chosen from the group of a-olefins having 4 to 8 carbon atoms. The total amount of ethylene and a-olefin units is typically less than 10 wt% (e.g., in a range of 1 to less than 10 wt%), based on the total weight of the propylene copolymer. Such a propylene copolymer is often addressed as propylene random copolymer and consists of a single phase (monophasic propylene copolymer). Preferably, the propylene copolymer is a copolymer of propylene with ethylene, more preferably, the propylene copolymer consists of propylene and ethylene units.

[0036] A “heterophasic propylene copolymer” (HECO) in the meaning of the present disclosure is a propylene polymer composition comprising: I) a matrix being a polymer of propylene, like a propylene homopolymer and / or a propylene copolymer (propylene random copolymer), and II) an elastomer being a copolymer comprising units derived from propylene and ethylene and / or C4 to Cs a-olefin. In that context, the term “heterophasic” indicates that the elastomer is (finely) dispersed in the matrix, i.e., a dispersed phase. In other words, the elastomer forms inclusions in the matrix. Thus, the matrix contains (finely) dispersed inclusions of the elastomer being not part of the matrix phase. The term “inclusion” can be understood in that the matrix and the inclusions form different phases within the propylene copolymer (HECO), said inclusions are for instance visible by high resolution microscopy, like electron microscopy or scanning force microscopy.The linear polypropylene applied in the present invention can be a propylene homopolymer or a propylene copolymer, preferably a propylene homopolymer or a propylene random copolymer.

[0037] The propylene random copolymer is preferably a copolymer of propylene and ethylene, more preferably the propylene copolymer consists of propylene and ethylene units.

[0038] The linear polypropylene is preferably selected from the group consisting of a propylene homopolymer and of a propylene random copolymer with relatively low comonomer content, i.e. , having a total content of ethylene units and C4 to Cs a-olefin units of less than 0.5 mol%, more preferably having a content of ethylene units of less than 0.5 mol%, the propylene copolymer consisting of propylene and ethylene units.

[0039] It is particularly preferred that the linear polypropylene is a propylene homopolymer.

[0040] The term “linear” with regard to the polypropylene means, that branching in the polymer is low. In particular, it is preferred that the amount of branching in the linear polypropylene is in the range from 0 to 10 branches / 1000 carbon atoms, more preferably in the range from 0 to 5 branches / 1000 carbon atoms, or in the range from 1 to 5 branches / 1000 carbon atoms.

[0041] Linear polypropylene is known in the art. A linear propylene homopolymer is obtained by polymerizing propylene under suitable polymerization conditions. A linear propylene copolymer is obtained by copolymerizing propylene with one or more other olefins, preferably ethylene, under suitable polymerization conditions. The preparation of propylene homopolymers and copolymers is, for example, described in Moore, E. P. (1996) Polypropylene Handbook. Polymerization, Characterization, Properties, Processing, Applications, Hanser Publishers; New York.

[0042] A propylene homopolymer and a propylene copolymer, i.e., a copolymer of propylene with ethylene or an a-olefin, can be made by any known polymerization technique as well as with any known polymerization catalyst system. Regarding the techniques, reference can be given to slurry, solution, or gas phase polymerizations; regarding the catalyst system, reference can be given to Ziegler-Natta, metallocene, or single-site catalyst systems.

[0043] Preferably, the linear polypropylene, like the linear propylene homopolymer, is isotactic. Accordingly, it is preferred that the linear polypropylene, like the linear propylenehomopolymer, has a rather high pentad concentration (mmmm%), i.e. , more than 94.1 %, more preferably more than 94.4 %, like more than 94.4 to 98.5 %, still more preferably at least 94.7 %, like in the range of 94.7 to 97.5 %.

[0044] The linear polypropylene can comprise one or more linear polypropylene components, which are different. For linear polypropylene comprising more than one, like two, different linear polypropylene components, which are propylene copolymers, it is preferred that all linear polypropylene components contain the same comonomer, preferably ethylene.

[0045] The melt flow rate, determined according to ISO 1133 at 230 °C and a load of 2.16 kg, i.e., MFR2, of the linear polypropylene is suitably low, i.e., below 1.0 g / 10 min, to enable preparing a high melt strength polypropylene for foam applications having a melt flow rate in the desired range after some increase of melt flow rate during the reaction under electron beam irradiation, i.e., the MFR2 of the high melt strength polypropylene being in the range of 2.0 to 5.0 g / 10 min.

[0046] The MFR2 of the linear polypropylene is usually at least 0.05 g / 10 min. The MFR2 of the linear polypropylene is usually in the range of 0.1 to 0.9 g / 10 min, or in the range of 0.2 to 0.8 g / 10 min.

[0047] The melt strength of the linear polypropylene is preferably higher than 40 cN. The melt strength of the linear polypropylene is usually below 100 cN. The melt strength of the linear polypropylene is usually in the range of 45 to 90 cN.

[0048] The linear polypropylene has preferably a ratio of melt strength to melt flow rate of 60 to 250 cN / (g / 10 min).

[0049] The melt extensibility of the linear polypropylene is typically below 60 mm / s. The melt extensibility of the linear polypropylene is typically higher than 10 mm / s. The melt extensibility of the linear polypropylene is typically in the range of 20 to 55 mm / s.

[0050] In an attempt to provide a high melt strength polypropylene suitable for food contact applications, having a content of non-intentional added substances as low as possible, the linear polypropylene is preferably as pure as possible.

[0051] Hence, the linear polypropylene preferably does not comprise antioxidants in a total amountof higher than 50 ppm, more preferably is essentially free of any antioxidants, still more preferably does not comprise any antioxidants in a detectable amount. Preferably, in the preparation of the linear polypropylene including polymerization and optional post-reactor treatment, no antioxidant is added.

[0052] The linear polypropylene may be provided in step a) of the process of the present invention in any suitable form, like in form of a powder, preferably directly obtained from the polymerization reactor. It is preferred that the linear polypropylene is provided in granular form.

[0053] The term “granular form” encompasses polymer powder, pellets, and chips as known in the art. Pellets are preferred.

[0054] In step b) of the process of the present invention, the oxygen content of the reaction atmosphere is optionally reduced resulting in an oxygen content of below 120,000 ppm.

[0055] The term “reaction atmosphere” denotes the gas mixture, which is in contact with the linear polypropylene before incorporation of an acid scavenger and a stabilizer takes place, which continues to be in contact with the linear polypropylene during incorporation of an acid scavenger and a stabilizer, which continues to be in contact with the obtained stabilized mixture, which continues to be in contact with the stabilized mixture during provision of the stabilized mixture in the form of a bed, and which continues to be in contact with the bed during irradiating the bed by electron beam irradiation.

[0056] The oxygen content of the reaction atmosphere is optionally reduced resulting in an oxygen content of below 120,000 ppm and these conditions are maintained throughout the process steps c), d), and e) without any interruption.

[0057] Preferably, the process for preparing a high melt strength polypropylene of the present invention further comprises after step e):

[0058] f) an annealing step at a temperature of higher than 100 °C, more preferably higher than 130 °C.

[0059] The temperature in the annealing step will be usually below 170 °C.

[0060] Preferably, the annealing step is conducted for a time of 20 to 40 minutes.Preferably, the annealing step f) is followed by:

[0061] g) a cooling step.

[0062] A skilled person knows how an annealing step and a cooling step are to be performed.

[0063] Preferably, the oxygen content of the reaction atmosphere is reduced resulting in an oxygen content of below 120,000 ppm and these conditions are maintained throughout the process steps c), d), and e), and in case present, f) and g), without any interruption.

[0064] Preferably, the oxygen content of the reaction atmosphere is reduced resulting in an oxygen content of below 100,000 ppm, more preferably 75,000 ppm, still more preferably below 50,000 ppm.

[0065] Furthermore, connected with more effort but still preferred considering the potential negative impact of oxygen on the desired reactions and considering the fact that irradiation of polypropylene causes the formation of hydrogen, it is very much preferred in an embodiment of the present invention to further reduce (and maintain) the oxygen content of the reaction atmosphere resulting in an oxygen content of below 25,000 ppm, or below 10,000 ppm, or below 5,000 ppm, or below 2,000 ppm, or below 1 ,000 ppm, and most preferred below 500 ppm.

[0066] The resulting oxygen content of the reaction atmosphere will be usually at least 50 ppm.

[0067] The process for preparing a high melt strength polypropylene of the present invention may be preferably performed with an oxygen content of the reaction atmosphere in the range of 100 to 5000 ppm, or in the range of 100 to 2500 ppm, or in the range of 100 to 1000 ppm.

[0068] However, considering the high effort of reducing oxygen content very much and taking into consideration the presence of a stabilizer in the stabilized mixture of the process of the present invention, an alternative preferred embodiment of the present invention is to reduce (and maintain) the oxygen content of the reaction atmosphere resulting in an oxygen content of above 10,000 ppm, or above 25,000 ppm, or above 30,000 ppm, but below 120,000 ppm, or below 100,000 ppm, or below 75,000 ppm, or below 50,000 ppm.

[0069] Hence, the process for preparing a high melt strength polypropylene of the present inventionmay be preferably performed with an oxygen content of the reaction atmosphere in the range of 20,000 to 60,000 ppm, or in the range of 30,000 to 50,000 ppm.

[0070] Reducing oxygen content of the reaction atmosphere can be done by routine measures known in the art. Conventionally, a sealable containment containing the linear polypropylene provided in step a) is flushed with nitrogen until an atmosphere having the desired oxygen inside the containment is reached.

[0071] Incorporating an acid scavenger and a stabilizer into the linear polypropylene, thereby obtaining a stabilized mixture, means that an acid scavenger and a stabilizer, including more than one stabilizer or a mixture of stabilizers, are independently added to the linear polypropylene such that these compounds are contacted with the linear polypropylene.

[0072] Hence, incorporation of an acid scavenger can take place:

[0073] i. before incorporation of a stabilizer or a mixture of stabilizers;

[0074] and / or

[0075] ii. together with incorporation of a stabilizer or a mixture of stabilizers;

[0076] and / or

[0077] ill. after incorporation of a stabilizer or a mixture of stabilizers.

[0078] It is possible to subject the stabilized mixture to extrusion.

[0079] The acid scavenger is preferably calcium stearate.

[0080] Preferably, the amount of acid scavenger, e.g., calcium stearate, incorporated into the linear polypropylene is in the range of 100 to 2,000 ppm, or 200 to 1 ,000 ppm, or 250 to 750 ppm, with respect to the stabilized mixture.

[0081] The amount of alpha-tocopherol incorporated into the linear polypropylene is preferably at least 10 ppm, or at least 20 ppm, or at least 30 ppm, or at least 40 ppm, more preferably at least 50 ppm, or at least 60 ppm, or at least 70 ppm, with respect to the stabilized mixture.

[0082] The amount of alpha-tocopherol incorporated into the linear polypropylene is preferably not higher than 140 ppm, or not higher than 130 ppm, or not higher than 100 ppm, with respect to the stabilized mixture.In a preferred embodiment of the present invention, the stabilizer incorporated into the linear polypropylene consists of a compound having a content of alpha-tocopherol of at least 90 wt%, more preferably of at least 95 wt%.

[0083] In an alternative preferred embodiment of the present invention, the stabilizer in addition to alpha-tocopherol comprises an amine oxide as further defined below with respect to formula (I).

[0084] In this alternative preferred embodiment of the present invention, the amount of said amine oxide incorporated into the linear polypropylene is preferably in the range of 100 to 1 ,000 ppm with respect to the stabilized mixture, preferably in the range of 250 to 750 ppm with respect to the stabilized mixture, still more preferably in the range of 350 to 650 ppm with respect to the stabilized mixture.

[0085] In this alternative preferred embodiment of the present invention, the stabilizer incorporated into the linear polypropylene more preferably consists of two compounds B1 and B2, the compound B1 having a content of alpha-tocopherol of at least 90 wt%, still more preferably of at least 95 wt%, and the compound B2 having a content of an amine oxide as defined by the formula (I) of at least 90 wt%.

[0086] As already mentioned above, the amine oxide is defined by the following formula (I):

[0087]

[0088] wherein

[0089] G1and G2are identical or different, and are each independently of one another, a C5-C35 straight or branched alkyl, a C6-C12 aryl, a C7-C36 aralkyl, a C7-C36 alkaryl, a C5-C35 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl;

[0090] G3is a C1-C36 straight or branched chain alkyl, a C6-C12 aryl, a C7-C36 aralkyl, a C7-C36 alkaryl, a C5-C36 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl;with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0091] Preferably, G1and G2are identical or different, and are each independently of one another, a C5-C35 straight or branched alkyl, a C5-C35 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl; G3is a C1-C36 straight or branched chain alkyl, a C5-C36 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0092] More preferably, G1and G2are identical or different, and are each independently of one another, a C5-C35 straight or branched alkyl; G3is a C1-C36 straight or branched chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0093] Still more preferably, G1and G2are identical or different, and are each independently of one another, a C5-C35 straight alkyl; G3is a C1-C36 straight chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0094] Yet more preferably, G1and G2are identical or different, and are each independently of one another, a C10-C35 straight alkyl; G3is a C1-C20 straight chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0095] Yet more preferably, G1and G2are identical or different, and are each independently of one another, a C10-C30 straight alkyl; G3is a C1-C10 straight chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0096] Yet more preferably, G1and G2are identical or different, and are each independently of one another, a C10-C30 straight alkyl; G3is a C1-C5 straight chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0097] Yet more preferably, G1and G2are identical or different, and are each independently of one another, a C10-C25 straight alkyl; G3is a C1-C5 straight chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.

[0098] Yet more preferably, G1and G2are identical or different, and are each independently of one another, a C10-C25 straight alkyl; G3is a C1-C3 straight chain alkyl; with the proviso that at least one of G1, G2and G3comprise a 3-carbon-hydrogen bond.Yet more preferably, G1and G2are identical or different, and are each independently of one another, a C12-C25 straight alkyl; G3is methyl; with the proviso that at least one of G1and G2comprise a 3-carbon-hydrogen bond.

[0099] A very preferred compound covered by an amine oxide of formula (I) is N-methyl-N-tetracosyl-N-tetradecylhydroxylamine (CAS Number 204933-93-7).

[0100] In the process for preparing a high melt strength polypropylene according to the present invention, the stabilized mixture has preferably one or more of the following properties: the sum of the linear polypropylene, the acid scavenger, and the stabilizer make up at least 99.99 wt% of the stabilized mixture,

[0101] the stabilized mixture does not comprise a coupling agent,

[0102] the stabilized mixture consists of the linear polypropylene, the acid scavenger, the stabilizer, of non-intentional added substances, and of unavoidable components of a (commercial) acid scavenger or stabilizer,

[0103] the stabilized mixture consists of the linear polypropylene, the acid scavenger, and the stabilizer.

[0104] In the process for preparing a high melt strength polypropylene according to the present invention, the stabilized mixture is provided in the form of a bed.

[0105] Preferably, in step e) irradiating the bed by electron beam irradiation is performed by an accelerator having an acceleration voltage, and the bed in step d) having a bed height, the bed height being set according to the following formula:

[0106] bed height [cm] = (0.4 to 1.5) [cm / MeV] x acceleration voltage [MeV],

[0107] More preferably, the bed height being set according to the following formula:

[0108] bed height [cm] = (0.8 to 1.5) [cm / MeV] x acceleration voltage [MeV],

[0109] In the process for preparing a high melt strength polypropylene according to the present invention, the bed is irradiated by electron beam irradiation preferably with a surface dose of 20 to 150 kGy.

[0110] In the process for preparing a high melt strength polypropylene according to the present invention, irradiating the bed by electron beam irradiation preferably takes place at a temperature of 10 to 30 °C.In the process for preparing a high melt strength polypropylene according to the present invention, irradiating the bed by electron beam irradiation preferably takes place by at least two irradiation steps, wherein the total surface dose applied in the at least two irradiation steps is 20 to 150 kGy.

[0111] The number of irradiation steps will be usually not higher than 10, or not higher than 5.

[0112] The present invention is according to a second aspect directed to a high melt strength polypropylene obtained by the process according to the first aspect of the present invention, including all preferred embodiments mentioned above.

[0113] Preferably, the high melt strength polypropylene obtained by the process according to the first aspect of the present invention has a melt flow rate in the range of 2.0 to 5.0 g / 10 min as determined according to ISO 1133 at 230 °C and a load of 2.16 kg.

[0114] Preferably, the high melt strength polypropylene obtained by the process according to the first aspect of the present invention has a melt strength of higher than 35 cN. The melt strength will be usually not higher than 80 cN.

[0115] Preferably, the high melt strength polypropylene obtained by the process according to the first aspect of the present invention has a ratio of melt strength to melt flow rate of 9 to 30 cN / (g / 10 min), wherein the melt flow rate is determined according to ISO 1133 at 230 °C and a load of 2.16 kg.Experimental Part

[0116] Measurement Methods

[0117] The following definitions of terms and measurement methods apply to the above general description of the invention, to the claims, as well as to the below examples unless indicated otherwise.

[0118] Melt flow rate

[0119] The melt flow rate of polypropylene is determined according to ISO 1133 at a temperature of 230 °C at a load of 2.16 kg (MFR2).

[0120] Melt strength and melt extensibility (velocity at break)

[0121] The melt strength and extensibility of polymer are measured according to ISO 16790:2005, using capillary rheometer RG75 and Rheotens 71.97 from Gbttfert. The capillary rheometer is equipped with a piston with 12 mm diameter and a round die with L / D ratio of 20 / 2 (mm / mm) and an entrance angle of 180°. The spinline length between the die exit and Rheotens wheels (string length) is 100 mm. The polymer melt is melted for about 10 minutes at 200 °C and extruded with a piston speed of 0.20781 mm / s equivalent with an average linear velocity in the die of V0 = 7.5 mm / s. The starting take-up speed of Rheotens wheels is about Vs = 7.4 mm / s. The take up speed gradually increases with an acceleration of 12 mm / s2until the filament breaks. The tensile force is recorded by drawdown speed using EXTENS program. The final result is the average of 10 single measurements. The maximum force is referred as melt strength and the maximum velocity at maximum force is called extensibility (drawability).

[0122] Oxygen content

[0123] Oxygen concentration was tested by placing spot senor SP-PSt6-NAU (PreSens - Precision Sensing GmbH) inside the glass vessel containing pellets before irradiation step. The oxygen concentration was measured with an optical oxygen sensor (Presens Fibox trace).

[0124] NMR-methods

[0125] Quantification of microstructure by NMR spectroscopy

[0126] Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content and comonomer sequence distribution of the polymers. Quantitative13C{1H} NMR spectra were recorded in the solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for1H and13C respectively. Allspectra were recorded using a13C optimized 10 mm extended temperature probehead at 125 °C using nitrogen gas for all pneumatics. Approximately 200 mg of material was dissolved in 3 ml of 1 ,2-tetrachloroethane-d2 (TCE-d2) along with chromium-(lll)-acetylacetonate (Cr(acac)3) resulting in a 65 mM solution of relaxation agent in solvent (Singh, G., Kothari, A., Gupta, V., Polymer Testing 285 (2009), 475). To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatory oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution and quantitatively needed for accurate ethylene content quantification. Standard single-pulse excitation was employed without NOE, using an optimized tip angle, 1 s recycle delay and a bi-level WALTZ16 decoupling scheme (Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha, A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225; Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R., Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128). A total of 6144 (6k) transients were acquired per spectra.

[0127] Quantitative13C{1H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals using proprietary computer programs. All chemical shifts were indirectly referenced to the central methylene group of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present. Characteristic signals corresponding to the incorporation of ethylene were observed Cheng, H. N., Macromolecules 17 (1984), 1950).

[0128] For polypropylene homopolymers all chemical shifts are internally referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm.

[0129] Characteristic signals corresponding to regio defects (Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253; Wang, W-J., Zhu, S., Macromolecules 33 (2000), 1157; Cheng, H. N., Macromolecules 17 (1984), 1950) or comonomer were observed.

[0130] The tacticity distribution was quantified through integration of the methyl region between 23.6-19.7 ppm correcting for any sites not related to the stereo sequences of interest (Busico, V., Cipullo, R., Prog. Polym. Sci. 26 (2001) 443; Busico, V., Cipullo, R., Monaco, G., Vacatello, M., Segre, A.L., Macromoleucles 30 (1997) 6251).

[0131] Specifically, the influence of regio defects and comonomer on the quantification of the tacticity distribution was corrected for by subtraction of representative regio defect and comonomer integrals from the specific integral regions of the stereo sequences.

[0132] The isotacticity was determined at the pentad level and reported as the percentage of isotactic pentad (mmmm) sequences with respect to all pentad sequences:[mmmm] % = 100 * ( mmmm / sum of all pentads )

[0133] The presence of 2,1 erythro regio defects was indicated by the presence of the two methyl sites at 17.7 and 17.2 ppm and confirmed by other characteristic sites.

[0134] Characteristic signals corresponding to other types of regio defects were not observed (Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev. 2000, 100, 1253).Examples

[0135] The following materials are used:

[0136] L-PP linear propylene homopolymer powderHAXOOl of Borealis, having MFR2 of 0.2 g / 10 min, a melting point of 165.4 °C, a crystallization temperature of 121.5 °C, an isotacticity of 98.7 % (pentad concentration by13C NMR), produced by Borstar Process using a Ziegler-Natta catalyst ZN 180.

[0137] CaSt Calcium stearate, Ceasit Fl by Baerlocher.

[0138] Toco 2,5,7,8-Tetramethyl-2(4’,8’,12’-trimethyltridecyl)chroman-6-ol, a-Tocopherol > 95% by Sigma Aldrich.

[0139] Genox Genox™ EP stabilizer by SI group.

[0140] Irganoxl 010 Pentaeryth rity l-tetrakis(3-(3’ , 5’-di-tert . butyl-4-hydroxyphenyl)-propionate, Irganox1010 by BASF.

[0141] Irgafos168 Tris (2,4-di-t-butylphenyl) phosphite, Irgafos168, by BASF.

[0142] Inventive examples, comparative examples, and reference examples were prepared as follows:

[0143] In the first step, various dry blends were produced from L-PP and the respective additives in a food processor under nitrogen. The dry blends were stored deep frozen until further processing.

[0144] The composition of the individual dry blends is indicated below in Table 1 ( / wt%).

[0145] Table 1 :

[0146] >

[0147] Blend no. L-PP CaSt Toco Genox Irganox1010 Irgafos168 1 96.0 1.0 - - 2.0 1.0

[0148] 2 98.0 1.0 - - 1.0

[0149] 3 98.0 1.0 - 1.0

[0150] 4 99.0 1.0 - 5 98.80 1.0 0.20 - - 6 98.50 1.0 0.50 - - 7 98.0 1.0 1.0

[0151] 8 97.80 1.0 0.20 1.0

[0152] 9 97.50 1.0 0.50 1.0In the second step, 150 g of each of the frozen dry blends was crushed and homogenized using a knife mill at 8000 rpm for 120 seconds and frozen again.

[0153] In the third step, 5.0 wt% of each of the crushed and homogenized dry blends was mixed with HAX 001 powder using a food processor for 5 minutes. Each of the resulting samples was filled into a 4 liter aluminium tube vessel, and was flushed with nitrogen (15 L / min) for 15 minutes.

[0154] In the fourth step, 2 kg of each powder mix prepared in the third step was compounded into pellets on a Prism TSE 24MC under nitrogen as indicated below. The throughput was 10 kg / h. The temperature setting of the extruder was between 20 °C and 240 °C.

[0155] The pellets were packed under nitrogen and stored in gas tight vessels at -24 °C to prevent oxidation.

[0156] The composition of the individual samples is indicated below in Table 2 ( / ppm; numbering and order being the same as the blends in Table 1 above). The remainder of the composition is L-PP (values omitted in Table 2; see Table 1).

[0157] Table 2:

[0158] Pellets no. CaSt Toco Genox Irganox1010 Irgafos168 1 500 - - 1,000 500

[0159] 2 500 - - 500

[0160] 3 500 500

[0161] 4 500

[0162] 5 500 100 - - 6 500 250 - - 7 500 500 - - 8 500 100 500

[0163] 9 500 250 500

[0164] The electron beam irradiation process was performed with a 10 MeV IBA accelerator in three steps:In the first step, 250 g polymer pellets were filled into a 0.5 L Schlenk glass vessel equipped with spot senor SP-PSt6-NAU (Presens) on the stopcocks and a Bunsen-valve. The oxygen concentration was measured with an optical oxygen sensor (Presens Fibox trace).

[0165] In the second step, a defined oxygen concentration in the Schlenk vessel was adjusted as follows:

[0166] 200-500 ppm oxygen:

[0167] The vessel filled with pellets was flushed 4 times with 5 L / min nitrogen for 10 minutes to remove oxygen and then the vessel was sealed.

[0168] 40,000 ppm oxygen (±20%):

[0169] The vessel filled with pellets was flushed 4 times with 5 L / min nitrogen for 10 minutes to remove oxygen. Then 50 mL air was injected before the vessel was sealed.

[0170] 100000 ppm oxygen (±20%):

[0171] The vessel filled with pellets was flushed 4 times with 5 L / min nitrogen for 10 minutes to remove oxygen. Then 125 mL air was injected before the vessel was sealed.

[0172] In the third step, Irradiation of the pellets placed in a glass vessel was done on a moving belt receiving electrons from 10 MeV IBA accelerator with a surface dose of 90 kGy in two steps of 45 kGy each at 20 °C in reduced oxygen concentration atmosphere. The belt speed was adjusted to apply a defined surface dose, the belt width being 800 mm, the bed height of the irradiated sample being 50 mm (diameter of the glass vessel). The reaction time (waiting time) time between the two irradiation steps was 5 minutes. Dose measurements were performed by RPE / Alanine dosimetry in a traceable manner to an international standard.

[0173] The composition of the reaction atmosphere changes since hydrogen is created during irradiation. The vessels containing 40,000 and 100,000 ppm oxygen, respectively, were flushed with nitrogen after irradiation to remove hydrogen and oxygen before heating.

[0174] The radiated pellets were heated for 30 minutes at 60 °C.

[0175] Afterwards, the radicals were deactivated by heating the pellets for 30 minutes at 140 °C.The properties of the pellets are shown in Tables 3 to 5.

[0176] Table 3 shows the results of irradiation of the pellets at an oxygen content of 200 ppm.

[0177] Table 4 shows the results of irradiation of the pellets at an oxygen content of 40,000 ppm.

[0178] Table 5 shows the results of irradiation of the pellets at an oxygen content of 100,000 ppm.

[0179] What can be derived from these results, is the following.

[0180] As indicated earlier, long chain branches are formed by a competitive reaction of visbreaking and combination reactions of macro radical chains. Table 3 shows the results for an oxygen content of 200 ppm. This oxygen content is very low and will be hardly continuously maintained in an industrial process. At such low oxygen content, impact of oxygen on the competitive reaction of visbreaking and combination reactions of macro radical chains is very much reduced. Hence, Table 3 enables analyzing the impact of stabilizers on the competitive reaction of visbreaking and combination reactions of macro radical chains.

[0181] Indeed, the composition of Example RE4.1, containing no stabilizer at all, results in a product with acceptable melt flow rate and melt strength. Table 4 shows that this is not the case at considerably higher oxygen content of 40,000 ppm. The composition of Example RE4.2, containing no stabilizer at all, results in a product with too high melt flow rate.

[0182] The composition of Example RE1.1 , containing a classical stabilizer mixture, results in a product with too high melt flow rate and too low melt strength, showing the negative impact of this stabilizer mixture on the desired reaction. At higher oxygen content, the composition of corresponding Example RE1.2 shows even worse results with strongly increased melt flow rate. The composition of Example RE2.1 , containing only one of the classical stabilizers, results in a product with still acceptable melt flow rate and improved melt strength, at least at low oxygen content.

[0183] The composition of Example RE3.1, containing an alternative stabilizer (Genox), results in a product with fully acceptable melt flow rate and already rather high melt strength. The comparison with the composition of Example R4.1, containing no stabilizer at all, shows that this stabilizer appears to have some impact on the reaction at low oxygen levels.

[0184]

[0185] Table 3: 200 ppm oxygen

[0186] Example Pellets Outline of composition Toco Genox MFR2 MFR2 MFR2 Melt Velocity Melt strength no. no. before after ratio strength at break / melt flow rate irradiation irradiation after / after after after irradiation before irradiation irradiation

[0187] irradiation (standard (standard

[0188] deviation) deviation)

[0189] / ppm / ppm / g / 10 min / g / 10 min / cN / mm / s

[0190] RE1.1 1 Irganox1010, Irgafos168 - - 0.29 6.1 21.0 15 (0.7) 83 (1.9) 2

[0191] RE2.1 2 Irganox1010 - - 0.49 4.2 8.6 36 (9) 82 (5) 9

[0192] RE3.1 3 Genox - 500 0.40 1.8 4.4 42 (4) 74 (5) 24 RE4.1 4 no stabilizer - - 0.64 3.9 6.1 50 (12) 86 (3.5) 13

[0193] IE1.1 5 Toco, low amount 100 - 0.33 4.0 12.1 39 (12) 78.8 (6.8) 10 CE1.1 6 Toco, medium amount 250 - 0.24 4.4 18.3 36 (9) 81 (5) 8

[0194] CE2.1 7 Toco, high amount 500 - 0.18 6.2 34.4 28 (1.7) 84 (1.5) 5

[0195] IE2.1 8 Toco, low amount, Genox 100 500 0.17 2.3 13.5 59 (20) 83 (4) 26 CE3.1 9 Toco, medium amount, Genox 250 500 0.15 3.1 20.7 36 (8) 61 (5) 12

[0196]

[0197] Table 4: 40,000 ppm oxygen

[0198] Example Pellets Outline of composition Toco Genox MFR2 MFR2 MFR2 Melt Velocity Melt strength no. no. before after ratio strength at break / melt flow rate irradiation irradiation after / after after after irradiation (compare before irradiation irradiation

[0199] 200 ppm irradiation (standard (standard

[0200] oxygen deviation) deviation)

[0201] / %)

[0202] / ppm / ppm / g / 10 min / g / 10 min / cN / mm / s

[0203] RE1.2 1 Irganox1010, Irgafos168 - - 0.29 13.8 (126) 47.5 8 (3) 110 (45) 1

[0204] RE4.2 4 no stabilizer - - 0.64 5.9 (51 ) 9.2 58 (5) 89 (0.8) 10

[0205] IE1.2 5 Toco, low amount 100 - 0.33 4.5 (14) 13.8 53 (2) 87.5(1.1 ) 12 CE1.2 6 Toco, medium amount 250 - 0.24 5.8 (33) 24.3 49 (2) 87.2(0.65) 8

[0206] CE2.2 7 Toco, high amount 500 - 0.18 9.7 (56) 53.8 30 (1 ) 87.5(0.97) 3

[0207] IE2.2 8 Toco, low amount, Genox 100 500 0.17 3.8 (67) 22.5 62 (2) 88.4(0.69) 16 CE3.2 9 Toco, medium amount, Genox 250 500 0.15 4.3 (37) 28.4 56 (2) 87.5(0.75) 13

[0208]

[0209] Table 5: 100,000 ppm oxygen

[0210] Example Pellets Outline of composition Toco Genox MFR2 MFR2 MFR2 ratio no. no. before after after / before irradiation irradiation irradiation (compare

[0211] 200 ppm

[0212] oxygen / %)

[0213] / ppm / ppm / g / 10 min / g / 10 min

[0214] RE1.3 1 Irganox1010, Irgafos168 - - 0.29 13.2 (116) 46 CE1.3 5 Toco, low amount 100 - 0.33 17.4 (335) 53 CE2.3 6 Toco, medium amount 250 - 0.24 18.3 (316) 76 CE3.3 7 Toco, high amount 500 - 0.18 15.4 (148) 86 IE1.3 8 Toco, low amount, Genox 100 500 0.17 2.9 (27) 17 CE4.3 9 Toco, medium amount, Genox 250 500 0.15 13.1 (323) 87The composition of Example CE2.1, containing 500 ppm a-tocopherol, results in a product with too high melt flow rate and rather low melt strength, these results being worse than the results of the composition of Example RE3.1, containing 500 ppm of Genox. At higher oxygen content, the composition of Example CE2.2, containing 500 ppm a-tocopherol, results in even worse melt flow rate. A lower amount of a-tocopherol, 250 ppm, does still not result in an acceptable performance, particularly at higher oxygen levels as can be derived from the results of Examples CE1.1 and CE1.2, respectively.

[0215] The composition of inventive Example IE1.1, containing merely 100 ppm a-tocopherol, surprisingly results in a product with fully acceptable melt flow rate and rather high melt strength. The performance is comparable to the composition of Example CE3.1 , containing 500 ppm Genox. At higher oxygen levels, the composition of inventive Example IE1.2, containing merely 100 ppm a-tocopherol as well, surprisingly results in a product with merely slightly higher and still acceptable melt flow rate and even higher melt strength.

[0216] The composition of Example CE3.1, containing 250 ppm a-tocopherol in combination with 500 ppm Genox, results in a product with a performance, which is comparable to the composition of Example CE1.1, containing merely 250 ppm a-tocopherol, and which is worse than the performance of the composition of Example RE3.1 , containing merely 500 ppm Genox. Hence, it appears that the addition of 500 ppm Genox does not have any remarkable effect, and the addition of a second stabilizer would merely waste substance, effort and money.

[0217] However, surprisingly, at lower content of a-tocopherol, 100 ppm, the combination with 500 ppm Genox shows a synergistic effect as can be derived from the results of inventive Example IE2.1. The melt flow rate is fully acceptable, and the melt strength is comparably high. This impressive performance is present at higher oxygen levels, too, as can be derived from the results of inventive Example IE2.2. Furthermore, even at very high oxygen levels, the impressive performance is maintained as can be derived from the results of inventive Example IE1.3. The fully acceptable melt flow rate and the rather low increase in view of inventive Example IE2.1 indicate that also the obtained melt strength will be acceptable.

[0218] These results are promising since low levels of a-tocopherol may be used to substitute higher levels of Genox or may be used to reduce the content of Genox in the preparation of high melt strength polypropylene. Considering health and environmental aspects, the application of a-tocopherol and the reduction or substitution of Genox are certainly desirable.

Claims

Claims1. A process for preparing a high melt strength polypropylene, the process comprising the following steps:a) providing a linear polypropylene having a melt flow rate of below1.0 g / 10 min as determined according to ISO 1133 at 230 °C and a load of 2.16 kg,b) optionally reducing the oxygen content of the reaction atmosphere resulting in an oxygen content of below 120,000 ppm,c) incorporating an acid scavenger and a stabilizer comprising alphatocopherol into the linear polypropylene, thereby obtaining a stabilized mixture, wherein the amount of alpha-tocopherol incorporated into the linear polypropylene is less than 150 ppm with respect to the stabilized mixture, d) providing the stabilized mixture in the form of a bed,e) irradiating the bed by electron beam irradiation.

2. The process for preparing a high melt strength polypropylene according to claim 1 , wherein the linear polypropylene is a propylene homopolymer or a propylene random copolymer.

3. The process for preparing a high melt strength polypropylene according to claim 1 or 2, wherein the linear polypropylene has one or more, preferably all, of the following properties:a melt flow rate of at least 0.05 g / 10 min as determined according to ISO 1133 at 230 °C and a load of 2.16 kg,a melt strength of higher than 40 cN,a ratio of melt strength and melt flow rate of 60 to 250 cN / (g / 10 min), wherein the melt flow rate is determined according to ISO 1133 at 230 °C and a load of 2.16 kg,a melt extensibility of below 60 mm / s,not comprising antioxidants in a total amount of higher than 50 ppm, being essentially free of any antioxidants.

4. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 3, wherein the linear polypropylene is provided in granular form.

5. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 4, wherein in step b) the oxygen content of the reaction atmosphere is reduced resulting in an oxygen content of below 50,000 ppm.

6. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 5, wherein the acid scavenger is calcium stearate, preferably, wherein the amount of calcium stearate incorporated into the linear polypropylene is in the range of 100 to 2,000 ppm with respect to the stabilized mixture.

7. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 6, wherein the amount of alpha-tocopherol incorporated into the linear polypropylene is at least 10 ppm with respect to the stabilized mixture.

8. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 7, wherein the stabilizer in addition to alpha-tocopherol comprises an amine oxide, wherein the amount of the amine oxide incorporated into the linear polypropylene is in the range of 100 to 1 ,000 ppm with respect to the stabilized mixture, preferably in the range of 250 to 750 ppm with respect to the stabilized mixture, and wherein the amine oxide is defined by the following formula (I):whereinG1and G2are identical or different, and are each independently of one another, a C5-C35 straight or branched alkyl, a C6-C12 aryl, a C7-C36 aralkyl, a C7-C36 alkaryl, a C5-C35 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl;G3is a C1-C36 straight or branched chain alkyl, a C6-C12 aryl, a C7-C36 aralkyl, a C7- C36 alkaryl, a C5-C36 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl; with the proviso that at least one of G1, G2and G3comprise a B-carbon-hydrogen bond.

9. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 7, wherein the stabilizer incorporated into the linear polypropylene is selected from the group consisting of the following two alternatives A) and B):A) the stabilizer consisting of a compound having a content of alpha-tocopherol of at least 90 wt%,B) the stabilizer consisting of two compounds B1 and B2, the compound B1 having a content of alpha-tocopherol of at least 90 wt%, and the compound B2 having a content of an amine oxide as defined by formula (I) of at least 90 wt%, and wherein formula (I) is:whereinG1and G2are identical or different, and are each independently of one another, a C5-C35 straight or branched alkyl, a C6-C12 aryl, a C7-C36 aralkyl, a C7-C36 alkaryl, a C5-C35 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl;G3is a C1-C36 straight or branched chain alkyl, a C6-C12 aryl, a C7-C36 aralkyl, a C7- C36 alkaryl, a C5-C36 cycloalkyl, a C6-C36 alkcycloalkyl, or a C6-C36 cycloalkylalkyl; with the proviso that at least one of G1, G2and G3comprise a B-carbon-hydrogen bond.

10. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 9, wherein the stabilized mixture has one or more of the following properties:the sum of the linear polypropylene, the acid scavenger, and the stabilizer makes up at least 99.99 wt% of the stabilized mixture,the stabilized mixture does not comprise a coupling agent, the stabilized mixture consists of the linear polypropylene, the acid scavenger, and the stabilizer.

11. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 10, wherein in step e) irradiating the bed by electron beam irradiation is performed by an accelerator having an acceleration voltage, and the bed in step d) having a bed height, the bed height being set according to the following formula: bed height [cm] = (0.4 to 1.5) [cm / MeV] x acceleration voltage [MeV],12. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 11 , wherein irradiating the bed by electron beam irradiation with a surface dose of 20 to 150 kGy and at a temperature of 10 to 30 °C.

13. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 12, wherein irradiating the bed by electron beam irradiation takes place by at least two irradiation steps, wherein the total dose applied in the at least two irradiation steps is 20 to 150 kGy.

14. The process for preparing a high melt strength polypropylene according to anyone of claims 1 to 13, further comprising after step e):f) an annealing step at a temperature of higher than 100 °C, preferably higher than 130 °C, for a time of 20 to 40 minutes, preferably followed by:g) a cooling step.

15. A high melt strength polypropylene obtained by the process of anyone of claims 1 to 14, preferably having one or more, more preferably all, of the following properties:a melt flow rate in the range of 2.0 to 5.0 g / 10 min as determined according to ISO 1133 at 230 °C and a load of 2.16 kg,a melt strength of higher than 35 cN,a ratio of melt strength to melt flow rate of 9 to 30 cN / (g / 10 min), wherein the melt flow rate is determined according to ISO 1133 at 230 °C and a load of 2.16 kg.

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