FR2642066A1 - Triacylated amidoxime derivatives, synthesis thereof and industrial applications thereof especially as crosslinking agents for polyolefins - Google Patents

Abstract

New triacylated amidoxime derivatives, synthesis thereof and industrial applications thereof, especially as crosslinking agents for polyolefins, in particular of high density polyethylene, are described. These compounds correspond to the general formula: in which R and R<1> are monovalent organic radicals, and R<2> and R<3> are also monovalent organic radicals or form, with each other, an aliphatic, alicyclic or aromatic divalent radical -Z-. They can be prepared by a process which comprises the halogenation of an aldoxime in the hydroxamoyl halide form, which is then acylated and then reacted with an alkaline imide.

Description

The invention relates to triacylated derivatives of amidoximes, their synthesis and their uses, in particular their industrial applications as crosslinking agents for polyolefins, more particularly for high density polyethylenes.

dans laquelle:
R est un radical organique monovalent aliphatique, aromatique,
alkylaromatique ou hétérocyclique,
R1 est un radical organique monovalent aliphatique, aromatique,
alkylaromatique ou hétérocyclique et,
R2 et R3 sont chacun un radical organique monovalent aliphatique ou
aromatique ou sont reliés entre eux pour former un système cyclique avec
I'enchainement

Plues particulièrement dans les composés de l'invention, R peut représe3ter un radical hydrocarboné aliphatique, linéaire ou ramifie, de 1 a 50 atomes de carbone, et de préférence de I a 25 atomes de carbone.A titre d'exemples, on peut citer la série des radicaux alkyles linéaires de Cl a C25 ; des radicaux alkyles ramifiés du type éthyl-hexyle ou ceux correspondant aux acides aliphatiques ramifiés industriels de type iso C8, Cg ou C10 ; ou des radicaux de type polyisobutényle de masse moléculaire moyenne comprise par exemple entre 200 et 1000.The triacylated derivatives of the amidoximes according to the invention can be defined as corresponding to the general formula

in which:
R is a monovalent aliphatic, aromatic organic radical,
alkylaromatic or heterocyclic,
R1 is a monovalent aliphatic, aromatic organic radical,
alkylaromatic or heterocyclic and,
R2 and R3 are each an aliphatic monovalent organic radical or
aromatic or are linked together to form a ring system with
The sequence

More particularly in the compounds of the invention, R can represent an aliphatic hydrocarbon radical, linear or branched, from 1 to 50 carbon atoms, and preferably from I to 25 carbon atoms. the series of linear alkyl radicals from C1 to C25; branched alkyl radicals of the ethyl-hexyl type or those corresponding to industrial branched aliphatic acids of the iso C8, Cg or C10 type; or radicals of polyisobutenyl type with an average molecular mass of, for example, between 200 and 1000.

R may also be an aromatic or alkyl aromatic hydrocarbon radical, the alkyl radical advantageously comprising from 19 carbon atoms, such as, for example, the phenyl, tolyl, octyl-phenyl and nonyl-phenyl radicals. The aromatic radical may consist of two phenyl groups in a chain, such as the biphenyl group, or linked together by a divalent substituent of the alkylene type. for example methylene or isopropylidene, or hetero-atomic such as for example the atoms of oxygen and sulfur. The aromatic radical can also be an enserr.ble of joined rings such as for example naphthalene, phenanthrene or else consist of an alkyl derivative of these rings for which the alkyl radical is of the same nature as above. The R radical can be a nitrogenous heterocycle such as, for example, pyridine or its derivatives, oxygenated such as, for example, furan or its derivatives, or sulfur-containing, for example thiophene or its derivatives. The heterocyclic radical can itself be substituted by an alkyl radical as defined above.

In formula (Ij, the radical. Rl can be defined in the same way as the root R.

The R2 and R3 radicals, when they are not linked together, may more particularly represent movovalent aliphatic hydrocarbon radicals, preferably saturated, of 1 to 5 carbon atoms (from methyl to pentyl).

dans laquelle Z est un radical hydrocarboné divalent aliphatique, alicyclique (monocyclique ou polycyclique) ou aromatique, qui forme alors avec le groupement -CO-N-CO-un cycle imide.When R2 and R3 are linked together, the triacylated derivative of amidoxime can be represented by the formula:

in which Z is a divalent aliphatic, alicyclic (monocyclic or polycyclic) or aromatic hydrocarbon radical, which then forms with the -CO-N-CO-group an imide ring.

où R4 est un radical aliphatique monovalent, linéaire ou ramifié, de 1 à30 atomes de carbone ou un radical aromatique monovalent.When -Z- is aliphatic, it can consist of a polymethylene radical of formula - (CH) -, or n takes the value 2 or 3, or a substituted methylene radical of formula

where R4 is a monovalent, linear or branched aliphatic radical of 1 to 30 carbon atoms or a monovalent aromatic radical.

As examples of monovalent aliphatic radicals for R4, mention may be made of linear or branched dodecyl, tetrapropenyl and polyisobutenyl radicals with a molecular mass ranging from about 200 to 1000.

éventuellement substituées par au moins un radical alkyle renfermant par exemple de 1 à 10 atomes de carbone. When the divalent radical -Z- is alicyclic, it can be monocyclic or polycyclic and correspond more particularly to the following structures:

optionally substituted by at least one alkyl radical containing, for example, from 1 to 10 carbon atoms.

When -Z- is aromatic, it may contain one or more benzene rings, the two valences carrying the carbonyl groups being located in the ortho position on a th me ring or in the peri position on two joined rings.

The aromatic radical Z can be substituted by at least one alkyl radical containing, for example, from I to 10 carbon atoms.

The compounds of the invention can be prepared by a process comprising three steps from aldoximes obtained for their part by conventional methods.

This process comprises a first step in which the halogenation of an aldoxime is carried out to yield a hydroxamoyl halide according to the reaction (eq. 1).

R-CH = NOH + X2 -> R-CX = N'OH + HX éa. I where X represents a halogen atom and
R has the same meaning as before.

This halogenation can be carried out in an organic solvent medium (for example ethyl ether or dichloromethane) by following the procedures described for example by i.H. Chiang, Journ. Org. Chem. 36, 2146, 1971, or by G. Casnati, A. Ricca, Tet.

Lett. 327, 1967. The halogenation can also be carried out in an aqueous medium. In the latter case, the recommendations of O. Piloty, H. Steinbock,
Ber., 35, 3101, 1902 or those of French patent 1,559,700.

In a second step, the hydroxamoyl halide obtained is acylated with a RICOX 'or (R1 CO) 2O derivative depending on the reaction (eq. 2): R-CX = StOH + R1COX'R1COX'R-CX = NOCOR1 + HX 'éa. 2 where R and R have the same meaning as before and
X 'represents a halogen atom or an R 1 COO group.

This operation can be carried out for example at room temperature in a solvent, for example dichloromethane, toluene or ethyl acetate, in the presence of a base, for example pyridine, sodium bicarbonate, sodium carbonate. potassium, soda or potash.

The last step of the process consists in substituting the halogen atom of the acylated hydroxamic derivative by an alkali metal imide according to a new reaction represented by equation 3, where M is an alkali metal:

This transformation resembling the classic reaction of Gabriel (WGNigh,
Journ. Chem. Educ. X, 670, 1975) is carried out in a temperature range generally between 20 and 100C and in a polar solvent, such as for example dimethylformamide, dimethylacetamide or N-methylpyrrolidone.

It is also possible to operate with a two-phase reaction system where the hydroxamic derivative is in solution in an organic solvent immiscible with water, for example toluene and the alkali imide is in aqueous solution. The mixture is then heated in the presence of a phase transfer catalyst according to a procedure described by D. Landini, F. Rolla ,. Synthesis 389, 1976.

The third aspect of the invention relates to an industrial application of the products represented by formula (I). This application has been found in the field of the transformation of polyolefins and particularly for the crosslinking of polyolefins or of their copolymers.

It is well known that obtaining molded or extruded parts of crosslinked high density polyethylene cannot be carried out satisfactorily by using conventional peroxides as crosslinking agents. Indeed, the range of temperatures at which these peroxides release effective amounts of radicals is too widely exceeded by the temperatures for processing high density polyethylene. The prior art describes compounds capable of crosslinking high density polyethylene while avoiding this difficulty.

Certain compounds recommended previously are peroxygenated derivatives, such as the acyclic percetals described in US Pat. No. 4,271,279. Others are azo derivatives. such as the azoethers described in US patent 3,776,885 or the azoesters described in US patent 4,129,531 or French patent 2,403,531.

Still other compounds are hydroxylamine derivatives, such as those described in patent EP 00 56 674 or derivatives of hydroxamic acids, such as those described in US patent 4,367,342.

Quite unexpectedly, it turned out that the triacylated N, N ', N1 amidoximes defined by formula (I) decompose, presumably radically, at high rates in a temperature range from 230 to ; 00 C. Since the temperatures for using the polyolefins are generally 140 to 220 ° C., depending on their viscosity and their nature, and the rate of decomposition of the compounds according to the invention being very low in this temperature range, it It is possible to use by all conventional methods (extrusion, injection, molding, etc.) a mixture comprising for example from 0.01 to 10% by weight of a triacylated amidoxime according to the invention, from 0 to 10% a crosslinking coagent belonging to the group of multifunctional vinyl monomers such as, for example, triallyloyanurate, triallylisocyanurate, divinyl benzene, diallyterephthalate, triallyltrimellitate, triallylphosphate, and from 80 to 99.99% by weight of a homopoly mother or copolymer based on C2 to C8 olefins such as polyethylene, polypropylene or ethylene propylene copolymers optionally combined with a diolefin monomer (EPDM).

The polymer can also contain various additives or specific fillers, such as, for example, antioxidants, UV stabilizers, processing agents, plasticizers, carbon black or various other fillers.

After processing, for example at a temperature below 210 ° C., the crosslinking can be carried out by reheating the shaped product in the temperature range 230-300 "C for a time ranging for example from 2 to 30 minutes by any process. usual reheating.

The N, N ', N' triacylated amidoximes of the invention, defined by formula (I), are particularly suitable for the crosslinking of high density polyethylene. Thus, it is possible to manufacture crosslinked high density polyethylene pipes or sheaths by extrusion of mixtures as defined above, this extrusion being followed by continuous heating of the extruded tube or sheath, by any conventional means of re-heating. continuous heating. The following examples will illustrate on the one hand the method of preparation of the products according to the invention and on the other hand their use for the crosslinking of high density polyethylene. These latter examples do not in any way limit the use of the products for the crosslinking of other olefinic polymers, the processing temperature of which would not exceed 250 ° C.

Example I
A solution of sodium phthalimide is prepared by adding 1.6 g of sodium hydride (50% in paraffin) to 7.84 g (0.053 M) of phthalimide in 100 ml of dimethyl formamide. 22.5 g (0.053 M) of benzhydroxamoyl chloride are added
O-benzoyl prepared according to the method described by YH Chiang (reference cited above) and the mixture is left to react at 25 ° C. for 3 hours. The mixture is precipitated in water and the organic products are extracted with dichloromethane. Removal of the solvent This results in a solid which is recrystallized from ethyl alcohol. 19 g of N-benzhydroxamoyl phthalimide benzoate are isolated, melting at 145 ° C.

Example 2:
A solution of sodium succinimide (0.06 M) in 100 ml of dimethyl formamide is prepared and reacted with 25.3 g (0.06 M) of O-benzoyl benzhydroxamoyl chloride under the conditions of the example. 1. By an identical treatment, 15 g of N-benzhydroxamoyl succinimide benzoate, melting at 183 ° C., are isolated after recrystallization from ethanol.

Eemrle 3 A solution of sodium tetrapropenyl succinimide (0.077 M) is prepared in 110 ml of dimethyl formamide. Then added 18.2 g (0.07 M) of O-benzoyl benzhydroxamoyl chloride and allowed to react for 3 hours at 25C. The organic solution is precipitated in water, and the insoluble organic fraction is extracted with a 1: 1 benzene-hexane mixture. Chromatography on silica using the eluent mixture of benzene-hexane 1: 1 allows 19 g of Nbenzhydroxamoyl tetrapropenylsuccinimide benzoate to be isolated in the form of a viscous liquid.

Preparation of O-stearovl benzhvdroxamovl chloride 110 g (0.9 M) of benzaldoxime suspended in 620 ml of 8.7 N hydrochloric acid are chlorinated while maintaining the temperature at +5 ° C. over a period of 30 minutes. chloroxime is then extracted with 400 ml of dichloromethane. 212 g (0.7 M) of stearoyl chloride dissolved in 200 ml of dichioromethane are added to this solution. 80 ml of pyridine are slowly added to this solution while maintaining the temperature. between 0 and + 50 C. The mixture is then treated with water, filtered and the organic phase is decanted, dried and evaporated under vacuum. An oil crystallizing on contact with methanol is isolated. 190 g of O-stearoyl chloride are isolated. benzhydroxamoyl melting at 41-42 C after recrystallization from methanol.

Example 4
According to the conditions of Example 1, 0.06 M of sodium succinimide is reacted with 25.3 g (0.06 M) of O-stearoyl benzhydroxamoyl chloride in 100 ml of dimethyl formamide. 15 g of Nbenzhydroxamoyl succinimide stearate, melting at 80cc, are isolated after recrystallization from methanol Exemnle -5:
Following the conditions described in Example 1, the reaction of 0.05 M of sodium phthalimide with 22.5 g (0.05 M) of O-stearoyl benzhydroxamoyl chloride in dimethyl formamide makes it possible to isolate 19 g of Nbenzhydroxamoyl phthalimide stearate, melting at 65 "and 90" C after recrystallization from ethanol.

Prenaration of O-benzovl heptanehvdroxamovl chloride 26 g (0.2 M) of heptaldoxime (prepared according to E.W. Bousquet, Org. Syntheses Collect.

Volume II p. 313) dissolved in 200 ml of chloroform are chlorinated between -40 and -; 0 C for a period of 15 minutes. The solution is subjected to strong nitrogen bubbling to remove excess chlorine and hydrochloric acid. 28 g (0.2 M) of benzoyl chloride in 50 ml of chloroform and 30 ml of pyridine are then added while controlling the rise in temperature by an ice bath. The solution is washed several times with water. 55 g of benzoyl heptanehydroxamoyl O chloride are collected.

Example 6:
5.4 g (0.02 M) of O-benzoyl heptanehydroxamoyl chloride, 4 g (0.021 M) of potassium phthalimide and 15 ml of dimethyl formamide are mixed. The solution is stirred at 100 ° C. for 10 hours. The solution is diluted with ether and washed several times with water. After elimination of the ether, the product is filtered through silica gel in methylene chloride. 2.5 g of partially crystallizing N-heptanehydroxamoyl phthalimide benzoate are isolated.

Exemole 7:
6.2 g (0.02 M) of O-stearoyl heptanehydroxamoyl chloride prepared as above is reacted with 5 g (0.027 M) of potassium phthalimide in 15 ml of dimethyl formamide at 100 ° C. for 10 hours. A treatment identical to that of Example 6 makes it possible to isolate 3 g of partially crystallized N-heptanehydroxamoyl phthalimide stearate.

Example 8:
6.8 g (0.02 M) of O-benzoyl undecane hydroxamoyl chloride, 4 g (0.02M) of potassium phthalimide and 15 ml of dimethyl formamide are mixed. The mixture is stirred at 100 ° C. for 10 hours. 4 g of liquid N-undecane hydroxamoyl phthalimide benzoate are isolated by following the procedure of Example 6.

Chromatography on a silica column makes it possible to isolate the two isomers constituting this product: one liquid and the other melting at 45-50 ° C.

The decomposition temperatures of the triacylated amidoximes described in Examples I to 8, determined by enthalpy differential analysis by taking the temperature at the maximum of the exothermic peak, are given in Table 1 below.

TABLE 1

<tb><SEP> Temperature <SEP> of
<tb><SEP> EX <SEP>[<SEP> R <SEP> R1 <SEP> -Z- <SEP> decomposition
<tb><SEP> CsHs <SEP> CgHg <SEP>'À-<SEP> 282ex
<tb> t <SEP> i <SEP> i <SEP> CH2 <SEP>
<tb><SEP> CH
<tb><SEP> ~ <SEP>'<SEP> C6Hf <SEP> i <SEP>240'C
<tb><SEP> CHU
<tb> I <SEP> 3 <SEP> f <SEP> C1H2 <SEP> CH25 <SEP>FH;<SEP> 2510C
<tb><SEP> C6H5 <SEP> C6H5
<tb><SEP> C12H23
<tb><SEP> 4 <SEP> | <SEP> C6H5 <SEP> C <SEP> C17H35 <SEP> CH2 <SEP> 242ex
<tb><SEP> H2%
<tb><SEP> 5 <SEP> C6H5 <SEP> C17H35 <SEP> TO <SEP> 2580C
<tb><SEP> 6 <SEP> C6H13 <SEP> C6H5 <SEP> TO <SEP> 2700C
<tb><SEP> 7 <SEP> C6H13 <SEP> C17H35 <SEP> TO <SEP> 260ex
<tb><SEP> 8 <SEP> C11H23 <SEP> C6H5 <SEP> TO <SEP> 250 C
<tb>
The following examples illustrate the effectiveness of the products according to the invention for crosslinking high density polyethylene which is defined as an ethylene-based polymer whose density measured according to standard ASTM D 1248 is greater than or equal to 0.94. i.

Example 9
Polyethylene powder with a density of 0.953, melt index 2 (at 1900C under ^ 1.6 kg according to ASTM D 1238) is mixed in a rapid mixer at 1500 revolutions / minute for 7 minutes with the triacylamidoxime prepared in Example 6 in the following proportions: per 100 g of polyethylene, 1.5 g of compound of Example 6.

The powder mixture thus obtained is crosslinked in a press at 250 ° C. for 25 minutes at 200 bars to obtain a plate on which the following characteristics have been measured - degree of gel: 73.6 3 - rate of swelling: 1135% (measured in the xylene at l40'C)
The degree of gel is defined by the weight ratio of insoluble polymer to the weight of the sample after 8 h of extraction with toluene at reflux. It is a quantity which characterizes the degree of progress of the crosslinking and therefore the effectiveness of the products according to the invention.

The swelling rate is defined by the weight ratio of polymer swells minus weight of the sample to weight of the sample multiplied by 100 measured after 8 h of swelling of the polymer 9 xylene reflux. It is a quantity related to the density of the crosslinking sites in the material.

Exernole 10:
According to a procedure identical to that of Example 9, the same polyethylene powder is mixed with triallylcyanurate and the compound of Example 6 in the following proportions: for 100 g of polyethylene, 1.5 g of compound of example 6 and 1 g of triallylcyanurate.

After crosslinking in a press at 250 "C for 10 minutes at 200 bars, a plate is obtained on which the following characteristics were measured - degree of gel: 93.6% - rate of swelling (xylene 1400C): 310,% o
Example ll:
The powder mixture prepared in Example 10 is extruded in the form of a rod in a single-screw machine with a diameter of 19 mm and a screw length 25 times the diameter.

The temperatures displayed for the barrel were 160, 170 and 180 ° C respectively for the feed, melting and pumping zones. The die temperature was 185 ° C. For an extrusion screw rotational speed of 40 rpm, the screw end material temperature was 190 ° C.

After cooling, the rod was granulated and then molded in a press. The results obtained according to the molding conditions are shown in Table 2.

TABLE 2: Characterization of the extruded rod

<tb><SEP> Conditions <SEP> of <SEP> molding <SEP>% <SEP> gel <SEP>% <SEP> swelling
<tb> Temperature <SEP> Time <SEP> Pressure
<tb><SEP> 160 C <SEP> 5 <SEP> min <SEP> 200 <SEP> bars <SEP> 0.1
<tb><SEP> 160C <SEP> 20 <SEP> min <SEP> 200 <SEP> bars <SEP> 0.3
<tb><SEP> 250 "C <SEP> 10 <SEP> min <SEP> 200 <SEP> bars <SEP> 93.9 <SEP> 310 <SEP>%
<tb><SEP> 250 "C <SEP> 20 <SEP> min <SEP> 200 <SEP> bars <SEP> 94.3 <SEP> 300 <SEP>%
<tb><SEP> 250 C <SEP> 5 <SEP> min <SEP> 200 <SEP> bars <SEP> 86.4 <SEP> 430 <SEP>% <SEP>
<tb>

It clearly appears that after extrusion under the conditions described above, the decomposition of the compounds according to the invention has not occurred since the gel levels measured (less than 0.5%) are in the uncertainty range. of the measurement method. On the other hand, after reheating at 250 ° C. for a few minutes, the gel values obtained are greater than 80 vo.

Example! 2:
A polyethylene of density 0.961, of melt index 20 (å 190 C under 5.16 kg of load) is mixed according to the procedure of Example 9 with 1.5% (based on 100 g of polyethylene) of the compound of Example 4 and 1% triallylisocynnurate.

The powder mixture crosslinked under the conditions of Example 9 (250 ° C., 25 minutes, 200 bar leads to a gel level of 85% and a swelling rate in xylene at 140 ° C. of 835%.

Claims (12)

1 - Compound consisting essentially of a triacylated derivative of amidoxime corresponding to -a the general formula

in which R is a monovalent, aliphatic, aromatic, alkylaromatic or heterocyclic organic radical; R1 is a monovalent, aliphatic, aromatic, alkylaromatic or heteroyclic organic radical; R2 and R3 each represent an aliphatic or aromatic monovalent organic radical or are linked together to form with the sequence

a cyclic system. 2 - A compound according to claim 1, characterized in that, in formula (I), R and R1 are each an aliphatic radical of I to 50 carbon atoms, an aromatic radical comprising one or more rings placed side by side or linked together by a single bond or by a divalent atom or group, an alkylaromatic radical in which the alkyl group contains I to 20 carbon atoms, a nitrogenous, oxygenated or sulfur-containing heterocyclic radical or a heterocyclic radical substituted by an alkyl group of 1 to 20 carbon atoms. 3 - A compound according to one of claims 1 and 2 characterized in that R2 and R3 each represent a saturated aliphatic radical of 1 to 5 carbon atoms. 4 - A compound according to one of claims 1 and 2, characterized in that R2 and R3 are queens between them forming a divalent radical -Z- aliphatic, alicyclic or aromatic. 5 - A compound according to claim 4, characterized in that the divalent radical -Z- is chosen from: - The alkylene radicals of formula - (CH2) -n or n takes the value 2 or 3 and the substituted ethylene radicals of formula

where R4 is a monovalent aliphatic radical of 1 to 30 carbon atoms or a monovalent aromatic radical; - The monocyclic and polycyclic alicyclic radicals corresponding to the structures

optionally substituted with at least one alkyl radical, and - aromatic radicals formed from one or more benzene rings, the two values carrying the two carbonyl groups being in the ortho position on the same ring or in the peri position on two adjacent rings. 6 - Process for preparing a compound according to one of claims 1 to 5, characterized in that it comprises a- a first step in which the halogenation of a aldoxime of general formula R-CH = NOH so as to obtain the corresponding hydroxamoyl halide, of general formula R-CX = NOH; b- a second step in which said hydroxamoyl halide is acylated by reaction with a compound of general formula R1COX 'or (R1CO) 2O, so as to form the corresponding acylated hydroxamic derivative, of general formula R-CX = NOCOR1 ;; and c- a third step in which the said derivative is reacted hydroxamic acylated with an alkali imide of the general formula

so as to form the desired compound, R, R1, R2 and R3 having the same meanings as in each of claims 1 to 5, X being a halogen atom and M representing a alkali metal. 7 - Process according to claim 6, characterized in that step (c) is carried out at a temperature of 20 to 100 "C in a polar solvent. 8 - Method according to claim 6, characterized in that one brings into play a system biphasic wherein said acylated hydroxamic derivative is in solution in a organic solvent immiscible with water and said alkali metal imide is in aqueous solution, The assembly being heated in the presence of a phase transfer catalyst. 9 - Use of a triacylated amidoxime according to one of claims I to 5, as Crosslinking agent for homo- and co-polymers of C2 to C8 olefins. 10 - Use according to claim 9 characterized in that said homo- or co olefin polymer is polyethylene, polypropylene, ethylene co-polymer propylene, or an ethylene - propylene - diolefin monomer co-polymer. It - Use according to one of claims 9 and 10 characterized in that a mixture is used at a temperature below 210 C comprising: from 80 to 99.998 by weight of said olefin homo- or co-polymer from 0.01 to 10 Fó by weight of at least one triacylated amidoxime: and from 0 to 10% by weight of a multifunctional vinyl crosslinking co-agent; and in that said mixture is heated to a temperature of 230 to 300 C. 12 - Use according to one of claims 9 to 1I characterized in that said olefin polymer is a high density polyethylene. 13 - Crosslinkable mixture characterized in that it is defined as in one of claims II and 12. 14 - Method of manufacturing crosslinked high density polyethylene pipes or sheaths characterized in that a mixture is extruded comprising: from 80 to 99.99% by weight of high density polyethylene; from 0.01 to 10 by weight of triacylated amidoxime according to one of the claims 1 to 5; and from 0 to 10 bt by weight of a multifunctional vinyl co-crosslinking agent; The extrusion being followed by continuous reheating.