CH645657A5 - Flame-retarded compositions and their use for coating elongate substrates - Google Patents

Description

The invention presented here relates to flame retardant compositions and more particularly coatings for elongated substrates, for example electrical insulations for wires or cables, formed from these compositions.

Consequently, the invention presented here provides an elongated substrate provided, on at least part of it, with a flame retardant coating as defined in claim 1.

The olefinic component of the vinyl acetate / alkene 65 and polyalkene copolymer, when it is present, preferably consists of a C2 to C6 alkene and more particularly of an a-unsaturated alkene such as ethylene, 1-propylene. and 1-butene, with a preference for ethylene. Alkene / alkene copolymers which can be used

3

645,657

Sees in the compositions of this invention are preferably C2-C6 alkene / C2-C12 alkene copoylmers and more particularly the copolymers of ethylene with C3-C12 alkenes, and more particularly C3-CI2 a alkenes -unsaturated such as 1-n-propylene, 1-n-butene, 1-n-pentene and 1-n-hexene. It is particularly preferred to take ethylene copolymers which contain more than 50% by weight, for example more than 60% by weight of ethylene, and even better those which contain more than 70% by weight, for example more than 85% by weight of ethylene. Particularly interesting copolymers are those which are called linear low density ethylene copolymers, the density of which is between 0.910 and 0.940 g / cm3 at 25 ° C, and which are described in British Patent No. 7,911,916 and in Canadian Patent No. 873828, cited herein as references.

In the case where the vinyl acetate / alkene copolymer is used in the polymer composition in the company of the polyalkene or alkene / alkene copolymer, the vinyl acetate content of said composition is at least 18% by weight. weight, preferably greater than 40% by weight, for example in the range between 45 and 98% by weight, particularly greater than 55% by weight, for example up to 95% by weight, especially between 65 and 95% by weight, for example 65 to 90% by weight, and more especially between 65 and 85% by weight, for example 70 to 80% by weight. Furthermore, or in another way, the ratio of the weight of the vinyl acetate / alkene copolymer to the weight of the other component of the mixture, the polyalkene or alkene / alkene copolymer, is preferably within 1 '' interval 1: 0.2 to 4, preferably 1: 0.2 to 1.5 or, for certain applications, 1: 1.5 to 4.0 or even 1: 2.5 to 4.0.

In addition, or otherwise, it is preferred that the total amount of vinyl acetate in the mixture is at least 18% by weight, in particular that it is greater than 40% by weight, for example in the field between 65 and 75% by weight.

In most cases, it is desirable to include a binder in the composition so as to improve the compatibility between the flame retardant and the polymer (s) of the composition, i.e. to improve the physical properties of the composition. Preferred binding agents include organosilicon and titanium derivatives, such as, for example, silanes and titanates.

As examples of silanes, mention may be made of dimethyldi-chlorosilane, methyltrichlorosilane, vinyltrichlorosilane, y-methacryloxypropyltrimethoxysilane, N, N-bis (P-hydroxyethyl) -y-aminepropyltriethoxysilane, vinylpropyltrimethoxysilane-yyoxy-propyl-yl-propyl vinyltrimethoxysilane, y-glycidoxypropyltrimethoxysilane, ß- (3,4-epoxycyclohexyl) ethyltrimethylsilane and vinyltrimethoxyethoxy-silane.

Other suitable silanes are described in British Patent No. 1284082.

As examples of titanates and other organo-titanium derivatives which can be used as binders, mention may be made of tetra-isooctyltita-nate, isopropyldiisostearylmethacryl titanate, isopropyltriisostearyl titanate, isopropyltriacryl titanate, di (dioct -pyrophosphate) of titanium and more specifically hybrid titanates such as isopropyldimethacrylisostearyl titanate. Other suitable titanium compounds are described in S. J. Monte & G. Suger-man, "J. Elastomers & Plastics ”, vol 8 (1976) pp. 30-49, and in Bulletins KR 0376-4 and 0278-7, "Ken-React Titanate Coupling Agents for Filled Polymers", published by Kenrich Petrochem Inc., listed here for reference.

The ratio of the weight of the binder, if used, to the weight of the flame retardant is respectively in the range of 0.005 to 0.1: 1 and preferably between the range of 0.01 to 0.05: 1.

The flame retardants chosen are those which release water during heating, and in particular the hydrated aluminum oxides of formula A1203-3H20, in which x is between 0.5 and 3, and more especially Al203-3H20, ammonium or sodium dawsonite, hydrated magnesia or hydrated calcium silicate, more especially α-alumina trihydrates. Fire retardants such as tri- alumina

hydrated preferably have a specific surface of at least 0.1 m2 / g, and better still at least 1 m2 / g, that is to say from 1 to 80 m2 / g, especially from 3 to 20 m2 / g, this being measured according to the nitrogen absorption method of Brunauer, Emmett and Teller (BET).

The particle size of the flame retardant is preferably less than 5 n, and more preferably, less than 2 µm. In addition, it may be advantageous to use a flame retardant which is composed of particles of different sizes, so as to reduce the viscosity of the composition. If desired, the flame retardant can be treated chemically so as to improve its compatibility with the polymer, this using one of the above-mentioned binding agents, or alternatively, in the case of certain flame retardants such as alumina. trihydrate, with products that facilitate manufacturing such as stearic acid or stearates, for example calcium stearate.

The flame retardant is preferably used in an amount which varies between 50 and 200 parts per 100 parts of polymer (s).

Significant results are obtained using an amount ranging from 80 to 150 parts by weight of flame retardant per 100 parts by weight of polymer.

Besides the flame retardant, the composition of the invention presented here can include other additional additives, for example fillers, stabilizers such as stabilizers against ultraviolet rays, antioxidants, acid acceptors and hydrolysis stabilizers, foaming agents and dyes, manufacturing aids such as plasticizers, or other polymers. If other additives are used, it is preferable that the composition contains less than 5% by weight of halogen, and even better less than 2% by weight, and in particular that it does not contain halogen.

The polymer compositions can be produced according to the usual methods, for example by mixing the components thereof in a Banbury mixer. They can then be transformed into a coating by extrusion or molding, depending on the nature of the coating and the support, and they can undergo crosslinking at the same time or later.

The polymer composition can be crosslinked for example by including a crosslinking agent or by exposing it to high energy radiation.

Suitable crosslinking agents include free radical promoters such as peroxides, for example dicumyl peroxide, 2,5-bis (t.-butylperoxy) -2,5-dimethylhexane and a, a-bis ( t.-butylperoxy) diisopropylbenzene. In a typical chemical preparation of crosslinking agent, about 0.5 and 5% by weight of peroxide is found, based on the weight of the polymer mixture. The crosslinking agent can be used alone or in combination with a crosslinking facilitating agent, such as a polyfunctional vinyl or allyl compound, for example triallyl cyanurate, triallyl iso-cyanurate or tetramethacrylate. pentaerythritol.

Crosslinking by means of radiation can be carried out by exposure to high energy radiation such as an electron or y-ray beam. Radiation doses varying between 2 and 50 megarads, preferably between 2 and 50 megarads, ie from 2 to 20 megarads and especially between 4 and 12 megarads, are satisfactory. To trigger crosslinking during irradiation, 0.2 to 5% by weight of a radiosensitizer such as a polyfunctional vinyl or allyl compound such as triallyl cyanurate, is incorporated into the composition before irradiation. 'triallyl isocyanurate or pentaerythritol tetramethacrylate.

The degree of crosslinking can be expressed in terms of the gel content (ANSI / ASTM D2765-68) of the composition of crosslinked polymers, i.e. excluding non-polymerized additives which may be present. Preferably, the gel content of the crosslinked composition will be at least 10%, and better still at least 20%, for example at least 30% and especially at least 40%.

When it is desired to apply the coating in the form of a sheath all along the support, it may be practical to extrude the polymer composition, before the crosslinking thereof, directly on the substrate with simultaneous or subsequent crosslinking.

5

10

15

20

25

30

35

40

45

50

55

60

65

645,657

4

In another way, and particularly when the coating must envelop only part of the support, it may be useful to produce the coating before applying it to the substrate, for example in the form of a strip or 'a sheet that will be wrapped around the substrate, or in the form of a sheath in which the support is introduced. When the coating is produced before its application, it is preferable to produce it in a shrinkable form, that is to say in a form such that its dimensional configuration changes during an appropriate treatment, this particularly in a form heat shrink. The coating can be made heat shrinkable by deformation of the thermally stable form thereof into a crosslinked form, this at a sufficiently high temperature, that is to say at a temperature situated above the crystalline melting point or point softening the polymer composition, then cooling it while it is in the deformed configuration, this method being known and described for example in US patents Nos. 2027962, 3086242 and 3957372 cited here by reference.

For heat shrinkable applications, it is preferable that the polymer composition contains a mixture of vinyl acetate / alkene copolymer and at least one thermoplastic polymer. The weight ratio of the vinyl acetate / alkene copolymer to the weight of the thermoplastic polymer can for example be in the range of 1: 0.2 to 4, i.e. 1: 0.2 to 1 , 5 and more particularly from 1: 0.2 to 0.55.

The thermoplastic polymers chosen for the blends intended for heat shrinkable applications are the thermoplastic copolymers of polyalkenes and of alkene / alkene described above.

In addition, and in certain circumstances, it may be desirable to cover at least part of the surface of the coating which is in contact with the support with a sealant or an adhesive, for example a hot-melt adhesive, activated by heat, pressure-sensitive, or contact-sensitive, or putty, and more particularly with a hot-melt adhesive such as that disclosed in West German Patent No. 2723116 which is mentioned here for reference.

A particularly interesting aspect of this invention relates to electrical insulations for which there are extremely severe non-flammability conditions.

As examples of use of this invention for electrical insulation, the following applications may be cited: primary and secondary insulation of wires, cable sheaths and conduits for wires and cables, in which the coating is applied over the entire length of substrate, terminations and overlap of wire and cable ties where the covering is in the form of a strip, sheet or sheath, cable or pipe breaks in which the covering is in the form of strips or parts molded, for example nipples in which the coating is applied only over part of the length of the cable, wire or pipe.

Another interesting aspect of this invention relates to the sealing or the mechanical and environmental protection of pipes and conduits, or at least of a section thereof, that is to say at joints or on damaged parts. , for example in the 5 public distribution networks for gas or water pipes,

in district heating systems, in ventilation and heating pipes and in pipes carrying domestic or industrial waste. An important example of such an application is the assembly of air conditioning or ventilation pipes in which the covering can be used in the form of a strip or of sheets which are wound around the joint, and more particularly that which is in a heat shrinkable form.

In the uses mentioned above, the coatings according to this invention are characterized by remarkably low flammability, as well as by low emissions of smoke and corrosive gases, this under severe conditions of ignition. It has been found that these low flammability characteristics should be attributed directly to the total level of vinyl acetate in the copolymer of the composition. On the other hand, when comparing this composition with other analogous compositions which do not contain a polyalkene or alkene / alkene copolymer in their mixture and which have the same vinyl acetate content, it is found that the coatings which are formed from compositions including a component of the mixture based on polyalkene or alkene / alkene, have a substantial improvement as regards their inertness towards chemical solvents and in particular vis-à-vis -vis oils, their property of flame retardant and their emission of smoke, that under rigorous conditions of ignition.

This invention is illustrated by the following examples, in which parts and percentages are expressed by weight.

Examples 1 to 12:

The relative compositions, the number of parts of which (rounded to the nearest whole number) of their constituents is given in Table 1, were mixed uniformly in two laboratory roller mills heated to a temperature between 120 and 140 ° C. , pressure molded into plates at 190 ° C and then irradiated under normal atmospheric conditions with an electron beam of 5.8 MeV at the indicated dose. The samples were subjected to the test protocol described below.

Examples 13 to 18:

The compositions reported in Table 1 are mixed in a similar manner to that described in the preceding examples, the two roller mills being cooled so as to prevent the mass from attaching, and plates are produced which will serve as samples of comparison in the following tests.

Table 1

Example No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

DYNH 3 (trade name), low density polyethylene from Union Carbide

0

0

0

0

30

24

20

16

0

17

14

0

0

0

0

0

0

0

Sclair 11-D-l (trade name), low density polyethylene from DuPont

35

28

27.5

22

0

0

0

0

17

0

0

14

0

0

0

0

0

0

Vynathene EY-907 (trade name), vinyl acetate (60% by weight) / ethylene copolymer from USI

15

12

0

0

20

16

0

0

33

33

26

26

0

0

0

0

0

0

Vynathene EY-902 (trade name), copolymer at 40% by weight of vinyl acetate / ethylene from USI

0

0

22.5

18

0

0

30

24

0

0

0

0

0

0

40

50

0

0

5,645,657

Table 1 (continued)

Example No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Elvax 360 (trade name), 75% by weight ethylene / vinyl acetate copolymer from DuPont

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

40

50

Elvax 460 (trade name), 82% by weight ethylene / vinyl acetate copolymer from DuPont

0

0

0

0

0

0

0

0

0

0

0

0

40

50

0

0

0

0

Alumina trihydrate (covered with silane): Hydral 705, available from Alcoa Corp, covered with 1.5% vinyltrimethoxy-ethoxysilane

50

60

50

60

50

60

50

60

50

50

60

60

60

50

60

60

60

50

Radiation dose (megarads)

6

6

6

6

12

12

12

12

6

12

12

6

12

12

12

12

12

12

Test protocol

The plates produced according to the previous examples are subjected to the following tests:

Water resistance measured by water absorption in%: ASTM-D-570-77, except that a disc 25 mm in diameter and 1.5 mm thick is used.

Oil resistance measured by oil absorption in%: 30 ASTM-D-570-77, except that a disc 25 mm in diameter and 1.5 mm thick is used.

Tensile strength: BS 903, part A2

Elongation: BS 903, part A2

Flammability measured by the oxygen limitation index: 35 ASTM-D-2863

Smoke emission: Measured in an Aminco NBS smoke chamber according to the catalog of smoke density chambers Nos J4-5800B and J4-5800BE, instruction 1410 published in November 1975 by American Instrument Co., 8030 Georgia Ave. Silver Spring, Maryland 20910.

The results are reported in Table 2 and they clearly demonstrate, among other things, the improvement in the oil resistance of the blends (Examples 1 to 12) when compared to the polyethylene-free copolymer (Examples 15 to 20) which has the same content of ethylene and vinyl acetate as the mixture, as well as the improvement of the flame retardant properties and the lower emission of fumes in most cases.

Table 2

Example No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

% water absorption at 90 ° C, 24 h

6.0

6.2

6.5

5.8

6.7

5.2

7.7

5.0

9.0

7.3

9.7

7.4

7.3

6.8

8.0

15.4

5.6

7.6

48 h

8.1

7.4

8.4

7.1

8.8

8.4

10.8

7.5

13.4

11.4

14.5

11.8

7.8

8.6

12.5

18.2

8.0

10.3

% oil absorption at 100 ° C, ASTM N ° 2 (mineral oil)

15

14

32

22

20

15

47

25

13

13

27

21

69

100

54

42

42

66

Synthetic lubricant Britisch Spec. D. Eng. R.D. 2487

15

13

37

23

22

12

37

34

33

29

9.7

7.4

42

60

80

77

39

56

Tensile strength (in MPa)

11.6

11.8

11.8

11.8

10.8

10.5

13.1

7.5

12.1

11.2

11.1

9.9

13.3

12.0

12.0

13.5

14.5

16.3

Elongation (%)

130

100

145

60

74

18

110

25

230

102

168

47

40

85

200

240

70

146

Oxygen index (% of 02)

26.0

31.5

27.0

32.5

23.5

33.5

25.5

28.5

29.0

30.0

33.0

30.0

28.0

25.0

34.0

27.0

27.5

24.0

Tie (min)

Mode of ignition

9.5

10

8

11

8

9

7.3

7.3

8.5

9

8.5

8.5

7

6

10.5

1.5

9

4

645,657

Example 19:

Heat shrink tape was produced from the following formulation:

Component %

* Sclair 11-D-l 17 Vinyl acetate / ethylene copolymer containing

60% vinyl acetate 33 A1203-3H20 covered by

1.5% vinyltrimethoxyethoxysilane 50

* Sclair is the trade name for a low density linear ethylene / butene copolymer available from DuPont.

The components are agglomerated in a two-roller mill between 120 and 140 ° C, cooled and transformed into granules. The granules thus formed are introduced into a usual extruder fitted with a ribbon matrix, maintained at 140 ° C., and are extruded into a ribbon 100 mm wide and 1.5 mm thick. The ribbon thus obtained is then irradiated with high energy electrons so as to give a total dose of 6 megarads. After irradiation, the tape is heated to 150 ° C, stretched lengthwise by 50% more than its original length and allowed to cool in this stretched position. The ribbon is then covered with a thin layer (0.5 mm) of a hot-melt adhesive, the composition of which is described in West German patent No. 2723116, example 3, formulation "O". The physical properties of the tape before coating were as follows:

Tensile strength (23 ° C) 12.0 MPa

Elongation at break (23 ° C) 266%

LAW (23 ° C) 29

Tensile strength (150 ° C) 2.0 MPa

Elongation at break (150 ° C) 105%

100% tangent modulus (150 ° C) 1.2 MPa

The heat-shrinkable tape covered with the adhesive thus produced is used to seal the joint of a metal pipe 150 mm in diameter for air conditioning by preheating the pipe in the region of the joint, by wrapping the tape around the pipe on the joint of so that successive turns partially overlap, then by heating the ribbon with a usual propane torch, which causes the ribbon to tighten around the joint and causes the adhesive to melt, which produces a solid bond between the coating thus formed and the driving.

In another version, the granules are extruded in the form of a cuff which is then made heat-shrinkable by irradiation and which is then coated with an adhesive layer in the same manner as that described above.

The cuff thus produced is put around the joint and made to tighten by application of heat.

Examples 19 and 20:

A formulation made of 40% of vinyl acetate / ethylene copolymer, containing 60% of copolymerized vinyl acetate, with 60% by weight of alumina trihydrate treated with 1.5% of its weight of vinyltrimethoxyethoxysilane is agglomerated. , this in a Banbury blender at a temperature of around 125 ° C.

After the material has been agglomerated in a two-roller mill, the material is taken out in the form of a strip, it is cooled and cut into pieces and it is rubbed lightly so as to avoid amalgamation of the granules. The material is then extruded in the form of a cable sheath on a 22.5 mm outer diameter multi-strand cable, so as to provide a sheath having a thickness of 1.25 mm, using an extruder of 8 , 9 cm (3.5 inch), a thread pitch L / D of 25: 1 and at a temperature along the barrel of 60, 8CT, 120, 120,130 ° C, and a temperature profile of the matrix extrusion of 150,140,120 ° C.

The sheathed cable is then irradiated by high-energy electrons up to a total dose of 5 megarads.

The elastomers used in the various examples are the following:

Example 19:

Hytrel - trade name of a thermoplastic polyester elastomer derived from dimethyl terephthalate, a polyglycol and a short chain diol, available from DuPont.

Example 20 - styrene / butadiene rubber

In Example 20, the agglomeration temperature in the Banbury mixer was 130 ° C. In Example 19, the agglomeration temperature in the Banbury mixer was 180 ° C and the temperatures along the barrel were 75, 90, 125,140 and 165 ° C, respectively.

In each case, after agglomeration, the material is removed in the form of a skin which is cut into narrow strips which are introduced into the device to be extruded. The sheathed materials have the following physical properties:

Example 19

Example 20

Tensile strength p 23 ° C (MPa)

11.5

14.7

Elongation at break p 23 ° C (%)

240

174

Fragility on impact (according to ASTM-D-746) ° C

-32

-42

LAW p 23 ° C

38.8

33.2

LAW p 250 ° C

25.1

20.4

6

5

10

15

20

25

30

35

40

45

50

R

Claims (23)

645,657 2 1. Elongated substrate, characterized by the presence, on at least part, of a flame retardant coating formed by a halogen-free polymer composition containing a crosslinked vinyl acetate / alkene copolymer mixed with a styrene / buta rubber -diene, a thermoplastic polyester elastomer or a thermoplastic polyalkene or an alkene / alkene thermoplastic copolymer and including at least 50 parts of a mineral flame retardant free of halogen per 100 parts of polymers, with the following conditions: i) the vinyl acetate / alkene copolymer contains more than 55% by weight of vinyl acetate when mixed with a styrene / butadiene rubber, a thermoplastic polyester elastomer; or ii) when the mixture of vinyl acetate / alkene copolymer and alkene / alkene copolymer or of thermoplastic polyalkene contains less than 66% by weight of vinyl acetate / alkene copolymer, the proportion of vinyl acetate is at least 18% by weight relative to the total weight of said mixture; or iii) when the vinyl acetate / alkene copolymer is mixed with a polyalkene or alkene / alkene copolymer and the mixture contains at least 66% by weight of vinyl acetate / alkene copolymer, the acetate copolymer vinyl / alkene contains more than 40% by weight of vinyl acetate. 2. Covered substrate according to claim 1, characterized in that the coating is an electrical insulator. 3. Covered substrate according to claim 2, characterized in that the substrate is a wire, a cable or an electric ramp. 4. Covered substrate according to claim 1, characterized in that the coating constitutes a seal or a mechanical protection or against the environment. 5. Covered substrate according to claim 4, characterized in that the substrate is a pipe or a pipe. 6. Covered substrate according to one of the preceding claims, characterized in that the coating is located around the substrate and that it covers it. 7. Covered substrate according to one of the preceding claims, characterized in that it contains a sealant or an adhesive between the substrate and the coating. 8. Coated substrate according to one of the preceding claims, characterized in that the vinyl acetate / alkene copolymer of the coating has a vinyl acetate content which is between 60 and 95% by weight. 9. Coated substrate according to claim 8, characterized in that the vinyl acetate / alkene copolymer of the coating has a vinyl acetate content which is between 65 and 85% by weight. 10. Coated substrate according to one of the preceding claims, characterized in that the vinyl acetate / alkene copolymer is a vinyl acetate / ethylene copolymer. 11. Covered substrate according to one of the preceding claims, characterized in that the mineral flame retardant free of halogen is alumina trihydrate. 12. Coated substrate according to one of the preceding claims, characterized in that the vinyl acetate / alkene copolymer is found mixed with at least one other copolymer. 13. Coated substrate according to claim 12, characterized in that the vinyl acetate / alkene polymer is mixed with a thermoplastic polymer. 14. Coated substrate according to one of claims 13 or 14, characterized in that the vinyl acetate / alkene copolymer is mixed with a polyalkene or alkene / alkene copolymer. 15. A coated substrate according to claim 14, characterized in that the vinyl acetate / alkene copolymer is mixed with a polyethylene or ethylene / C3 to C12 alkene copolymer. 16. Substrate according to claim 15, characterized in that the vinyl acetate / alkene copolymer is mixed with a linear ethylene / alkene C3 to C12 copolymer which has a density between 0.910 and 0.940 gm / cm3 at 25 ° C. 17. Coated substrate according to one of claims 12 to 16, characterized in that the weight ratio of the acetate copolymer 5 of vinyl / alkene by weight of the other polymer (s) in the blend is in the range 1: 0.2 to 4. 18. Coated substrate according to claim 17, characterized in that the ratio of the weight of the vinyl acetate / alkene copolymer to the weight of the other or of the other polymers of the mixture is io included in the range 1: 0, 2 to 1.5. 19. coated substrate according to one of claims 12 to 18, characterized in that the total amount of vinyl acetate in the mixture is at least 30% by weight. 20. Coated substrate according to one of the preceding claims. is tes, characterized in that the polymer composition of the coating contains a binder. 21. A heat-shrinkable flame retardant coating for a substrate according to claim 1, formed from a halogen-free polymer composition containing a vinyl acetate / alkene copolymer. 20 bonded as a mixture with a styrene / butadiene rubber, a thermoplastic polyester elastomer or a thermoplastic polyalkene or an alkene / thermoplastic alkene copolymer and including at least 50 parts of a halogen-free mineral flame retardant per 100 parts of polymers, this with the following conditions: I) the vinyl acetate / alkene copolymer contains more than 55% by weight of vinyl acetate when mixed with a styrene / butadiene rubber, a thermoplastic polyester elastomer; or ii) when the mixture of vinyl acetate / alkene copolymer and Of alkene / alkene copolymer or of thermoplastic polyalkene contains less than 66% by weight of vinyl acetate / alkene copolymer, the proportion of vinyl acetate is at least 18% by weight relative to the total weight of said mixture; or iii) when the vinyl acetate / alkene copolymer is mixed 35 to a polyalkene copolymer or to a thermoplastic alkene / alkene copolymer and the mixture contains at least 66% by weight of vinyl acetate / alkene copolymer, the vinyl acetate content of the vinyl acetate / alkene copolymer is more than 40% by weight. 40 22. Heat-shrinkable flame-retardant coating according to claim 21, characterized in that the polymer composition is defined as in one of claims 15 to 19. 23. Heat-shrinkable flame-retardant coating according to claims 21 and 22, characterized in that the weight ratio of the co- 45 vinyl acetate / alkene polymer by weight of the thermoplastic polymer is in the range 1: 0.2 to 0.55. 24. Flame retardant coating according to claims 21 to 23, characterized in that at least one face thereof is covered with a putty or an adhesive. n / a 25. A flame retardant coating according to claim 24, characterized in that the adhesive is a hot-melt adhesive.