FR2582004A1 - Process for crosslinking polyolefins especially for insulating materials for electrical cables - Google Patents

Abstract

a. Process for crosslinking polyolefins especially for insulating materials for electrical cables. b. Process characterised in that a 1,2-polybutadiene of low molecular weight, whose degree of polymerisation is preferably of the order of 3 to 200, is employed as crosslinking accelerator. c. The invention relates to a process for the crosslinking of polyolefins especially for insulating materials for electrical cables.

Description

Process for the crosslinking of polyolefins in particular
for insulating materials for electrical cables
The invention relates to a process for the crosslinking of polyolefins by using crosslinking sensitizers, which have the effect of improving the thermo-mechanical properties (for example the stability of the form to heat, the thermal expansion) of the products. without having negative effects on the electrical properties. As a result, these products can be used in the cable industry, as insulation, in the building industry as hot water piping, and in other fields.

 The invention can be used not only on polyolefins, but also on copolymers of olefins (for example ethylene and propylene, or ethylene and butene), and on copolymers of olefins and derivatives of olefins (for example ethylene and vinyl acetate) as well as on mixtures of polyolefins of uniform or different densities, with copolymers of olefins and their derivatives. Crosslinking can take place both chemically and radiochemically.

 Crosslinking of polyolefins under the effect of high energy radiation, or by chemical initiation using crosslinking sensitizers, is known.

In general, polyunsaturated compounds which are polymerizable with polar groups, containing oxygen or nitrogen, are used as crosslinking sensitizers. As examples, mention may be made of di- and polvacrylates, polyallyl esters, polyallyl csanurate, etc. (for example, DD-WP 152 566,
DE-AS 15 44 804, DE-AS 15 44 805, DE-AS 12 85 177, GB-PS 865 793).

 Under the usual conditions, the polar compounds migrate, because of their relatively great incompatibility with the non-polar polyolefins, on the surface of the latter, so that a significant part is lost for the sensitization of the crosslinking. The effects of the incompatibility and the significant volatility which frequently exists, in addition, of these compounds can be reduced or eliminated by means of special provisions, such as for example the operation under pressure, or by a beginning of crosslinking, however, this results in significant technology expenditure. In addition, the dielectric properties of polymers are, in general, compromised by polar additives.

 Non-polar compounds have no effect on the dielectric properties. Only a small number of examples of their use as crosslinking sensitizers are known so far. Thus, for example, highly volatile p-divinylben zene has been proposed (DE-AS 12 18 719). In radiochemical crosslinking, the weakness of the sensitizing activity turns out however to be a particular defect.

 Another proposal (US-PS 41 64 458) relates to the use of diethines of general formula R1-C = C-C = C-R2 which embraces both polar and non-polar compounds. Nonpolar compounds, free of free etoatoms, have been described as having little or no sensitization for radiochemical crosslinking. among the polar, only two compounds have been shown to be effective, however they exhibit the previously described disadvantages of this class of compounds.

 The activity of ethine and butadiene gas as crosslinking sensitizers (US-PS 38 35 004) is controlled by diffusion to a large extent and is. therefore, limited to use in powder or thin film products.

In the case of molded parts with thick walls, we can mainly count on a surface crosslinking.

 With the non-polar sensitizers known up to now, it was not even possible to reach the efficiency of the best polar compounds (for example triallyl-cyanurate), which give products whose thermo-mechanical properties are good, their electrical properties are not enough. however for severe requirements such as those posed for example by cables for medium and high voltage.

 The radiation dose required for sufficient crosslinking (in the case of insulation for cables for intense current: gel content approximately 70 X, thermal expansion according to the hot-set test at 473 K less than 200%), is very high. , in all known processes for chemical crosslinking by radiation, (Up to 200 kGy). This does not only mean that the energy expenditure is high, but can lead to deterioration of the material, such as "branching" by radiation, the formation of blisters or the formation of foam.

 For chemical crosslinking, the effectiveness of the sensitizers mentioned is generally sufficient. The polar compounds however cause a deterioration of the dielectric properties of the crosslinked products and limit their possibilities of use.

 The object of the invention is to obtain crosslinked polyolefins offering good technical, mechanical, thermal and electrical transformation properties.

 To this end, the invention proposes to find crosslinking sensitizers which have good compatibility with polyolefins and with which crosslinking gives products which have the properties mentioned.

 The process according to the invention consists in incorporating the crosslinking sensitizer into the polyolefin which may also contain, if necessary, other auxiliary products, for example peroxides, fillers, release agents, antioxidants, etc. , and where the crosslinking is ensured by a thermal action or by irradiation with high energy radiation: this process is characterized in that one uses, as crosslinking sensitizer, a 1,2-polybutadiene whose rate of polymerization is of the order, preferably of n = 3 ... 200, optionally in combination with non-polar, polyunsaturated, polymerizable hydrocarbons. Polybutadiene1,2 can also be in the form of a mixture of compounds 1,2 and 1,4, where however the part 1,2 must constitute at least 50 X of the whole. The amounts added will be of the order of 0.1 to 10% by weight, preferably to twist from 0.5 to 5% by weight. These indications of proportions are also valid, even if oligobutadiene is introduced in combination with other non-polar, polyunsaturated, polymerizable (aliphatic, cycloaliphatic or aromatic di- or oligo-enes) hydrocarbons, such as for example dodecadilin-5 , 7,2,4,6-trivinylcyclohexane, methylphenylfulvene, or mixtures of di- and trivinylbenzene.

The rates of crosslinking are, when using combinations of this kind, in the same order of magnitude as those obtained by the use of the same proportions doligobutaCiene-i.2. although these hydrocarbons alone. have no satisfactory effect on acceleration.

 It is surprising that the mentioned low molecular weight pol> butadiene accelerates the crosslinking of polyolefins to a significant extent. because. in the literature (for example in the Forschungsbericht des Bundesministerium für Forschung und Technoîcgie der BRG. BMFT-FB-T 81 - 122. page 90), it is expressly pointed out that pols-butadlene-1,2 crosslinking of polyethylene with electronic radiation, because the double bonds of the carbon chain are notoriously not very reactive.

observation which was also made on the plastic ester of oleic acid.

 The low molecular weight 1,2-polybutadiene will be incorporated into the polymer for example by means of a heat treatment operation. If the crosslinking is initiated chemically, the oligobutadiene will be introduced into the polymer. separately, or jointly with the radical former, and possibly other auxiliary products, by a thermoplastic treatment operation. As radical formers, it is particularly the organic peroxides, having a relatively high average reaction temperature such as for example dicumyl- or di-tert.-butylperoxvde which will be suitable and which will be used at concentrations of 0.1 at 3% by weight, calculated on the polsolefin.

The crosslinking will be carried out in the known manner, by a thermoplastic transformation operation giving a shape, by extrusion for example, followed by a heat treatment.

 The advantages of the invention, as regards chemically initiated crosslinking, reside in that the crosslinking operation is clearly accelerated. and cJe the crosslinking yield is appreciably increased without any negative effect on the dielectric properties of the products.

 In the event of crosslinking under the effect of high energy radiation, the sensitization effect depends, apart from the microstructure and the concentration of the sensitizer. also, of the dose of raxonnage. By bringing this parameter to the optimum point, it is possible to reduce the dose necessary for sufficient crosslinking (coefficient of thermal expansion of 100%) in a proportion which can go up to 40%.

 In both cases. (chemical crosslinking, and chemical crosslinking by radiation), products are obtained which, because of their mechanical and electrical properties, can be used for high quality products, in particular also as insulating material for cables for medium and high voltage.

Example of realization
In case of crosslinking initiated chemically. high pressure polyethylene (LDPE) is transformed into granules (density 0.92 g / cm) on a laboratory mixer with two cylinders, at (385 + 5) K into a material in the plastic state. Then, the crosslinking sensitizer, or the combination of the sensitizer and the cosensitizer, is incorporated into this plastic melt for 5 minutes so as to make it homogeneous. The incorporation of the radical former takes place in the same way, but separately from the sensitizer, so that at the end of the mixing operation, two calendered sheets of one or two mm thick are obtained, containing mixes auxiliary products.

Similar weights of these two sheets are subjected, under the same conditions which have been described, to a mixture on the laboratory mixer and removed five minutes after a calendered sheet of approximately 1 to 2 mm thick. From this calendered sheet ground into small pieces, 27 g are brought to the wafer form in a pressing frame (10 cm x 6 cm x 0.4 cm) by a progressive pressure operation (final pressure around 40 MPa). at (433 + 5) K for 5 minutes, and cross-links at the same time. After these plates have cooled, the test pieces necessary to determine the coefficient of thermal expansion are prepared (75 mm long bars, 20 mm measurement length cross section 4 mm x 4 mm). The results obtained are collated in Table 1.

 In chemical crosslinking by radiation, the incorporation of the crosslinking sensitizer takes place by means of an extruder. The platelets are prepared according to the procedure described above. These platelets are subjected to irradiation with electrons (E = 1.2 MeV) up to a radiation dose of 120 kGy. The manufacture of the test pieces is consistent with the process already mentioned. The results of the examples having undergone radiochemical crosslinking are collated in Table 2.

Table 1
Zero 1 2 3 4 5
LDPE (% by weight) 98 96 96 97 97 97
Dicumyl peroxide (% by weight) 2 2 2 1.5 1.5 1
Oligobutadiene (% by weight) - 2 2 1.5 1.5 1 of which constituent 1-2 (% by weight) - 80 90 80 95 80
Polymerization rate - 33 25 33 25 33
Cosensitizer
Dodecadein-5.7 (% by weight) - 1
Coefficient of thermal expansion of the crosslinked product (%) 115 50 30 35 20 45
Table 2 ::
Zero 1 2 3 4 5
LDPE (X by weight) 100 98 98 98 98 98
Oligobutadiene (X by weight) - 2 2 2 0.7 1 of which constituent 1.2 () - 60 80 90 90 90
Polymerization rate n - 40 33 25 25 25
Cosensitizer
Dodecadiene (% by weight) - - - - 0.7 1
Methylphenylfulvene - - - - 0.7
Coefficient of thermal expansion after irradiation (X) 290 250 110 90 95 100
In another test, 98 X by weight of a mixture of 90 X by weight of LDPE (n = 463, density 0.919 g / cm 3) and 10 X by weight of HDPE (n = 241, density 0.956 g) were mixed. cm 3, with 2 X by weight of polybutadiene (constituent -1.2, 95 X, polymerization rate 25). The irradiation was carried out with electron radiation (E = 1 MeV) up to a dose of 100 kGy. The coefficient of thermal expansion was 100.

Claims (5)

 10) Process for the crosslinking of polyolefins with the use of crosslinking sensitizers, consisting in incorporating the crosslinking sensitizer, and possibly other auxiliary products in the polyolefin, and in crosslinking by treatment with high energy irradiation, or with peroxides organic under thermal action, process characterized in that, as crosslinking accelerator, a low molecular weight 1,2-polybutadiene is used, the polymerization rate of which is preferably of the order of 3 to 200.  20) Process according to claim 1, characterized in that the polybutadiene can also be in the form of a mixture of the compounds 1.2 to 1.4, provided that the proportion of -1.2 amounts at least to 50%.  30) Process according to claim 1, characterized in that the polybutadiene can be used in combination with polyunsaturated, polymerizable, non-polar hydrocarbons, such as aliphatic, cycloaliphatic or aromatic di- or oligo-enes.  40) Process according to claim 1, characterized in that the proportions used of polybutadiene, or of the combination of polybutadiene and of hydrocarbons, are preferably from 0.1 to 10% by weight, or better still of the order of 0 , 5 to 5% by weight.  50) Process according to claims 1 and 3, characterized in that, as hydrocarbons, methylphenylfulvene, dodecadilin-5,7,2,4,6-trivinylcyclohexane, or mixtures of di- and trivinylbenzene, can be used, alone or as combinations.