NO166651B - FLAMMABLE, TRANSMISSIBLE POLYMER MATERIAL AND THE USE OF IT IN THE COVER FOR ELECTRICAL LEADERS. - Google Patents

Description

This invention relates to a flame retardant, crosslinkable polymer material which exhibits moisture resistance and heat resistance, and which is useful for the manufacture of insulation for wires and cables and for the manufacture of molded products. An important feature of the new polymer material is that it passes the so-called CSA varnish test ("CSA Varnish Test"). The invention also relates to the use of the new polymer material for the formation of a single cross-linked, insulating and encapsulating layer on an electrical conductor.

One of the most important areas where fire-resistant polymer materials are used is the electrical area, where both insulating properties and fire resistance properties are desired, especially with regard to the insulation of conductors. Previously, in order to have flame retardancy properties, extrudable materials were required to contain halogenated polymers, such as chlorinated polyethylene, polyvinyl chloride, chlorobutadiene, chlorinated paraffin, etc., together with antimony trioxide, both components being present in significant amounts. Alternatively, a coating of chlorosulfonated polyethylene was applied to a non-flame retardant insulation material, which entailed an additional manufacturing operation.

For certain types of dry transformers, especially high-voltage transformers, a problem 1 and that electrical failure took place as a result of surface creep currents in the organic insulation component used has arisen. The problem was solved by adding hydrated aluminium-

oxyd to materials in which the organic binder consisted of butyl rubber, epoxy resins or polyester resins. However, these materials do not have the right balance of extrudability properties, physical and electrical properties, heat resistance and flame retardant properties. Such materials are described in US Patent Nos. 2 997 526, 2 997 527 and 2 997 528. The materials described there for such use have low tensile strength, low elongation and a small percentage of retained elongation after ageing.

Flame retardant polymer materials which, among other things, exhibit improved resistance to moisture and heat resistance, and which mainly consist of an intimate mixture of at least one crosslinkable polymer containing as a main component an ethylene-vinyl acetate copolymer, one or more silanes and one or more hydrated inorganic fillers , has found extensive use in the wire and cable industry. Materials of this kind are described in US Patent Nos. 3,832,326 and 3,922,442.

These known polymer materials exhibit a unique combination or balance of improved physical and electrical properties, along with a high degree of flame and fire retardancy. These highly desirable results are achieved without the use of halogenated polymers, such as polyvinyl chloride and chlorosulfonated polyethylene, whereby hydrogen chloride vapors are avoided. They do not contain carbon black, which means they can be used as colored insulation materials. They also do not have any flame-retardant coating, as is often necessary, and thereby a further production step is avoided when the materials are used, for example. as insulating materials that are extruded onto a conductor. They also do not contain antimony trioxide, whereby:, a very expensive compound is eliminated.

Such materials find particular application as white (an intrinsic property) and colored "non-insulating" materials, which can be extruded over metal conductors, e.g. copper or aluminum conductors, to form a single layer of insulating and encapsulating material, which according to the U.S. standards are rated to withstand operation at 90°C, and in some cases to withstand operation at temperatures as high as 125°C, at voltages up to 6OO.1 volts.

The insulation materials according to the above-mentioned US patent documents No. 3,832?. 326 and 3,922,442 have been found to be particularly useful; when insulating system cables, apparatus cables and car cables, where a very special combination of good electrical properties and resistance to the degrading effects of heat and flames is essential,

and how low smoke density and non-corrosive smoke pressure is desirable.

In addition to the three main components, the materials according to US patent documents no. 3,832,326 and 3,922,442 may also contain other additives, such as pigments, stabilizers, lubricants and antioxidants. Among the antioxidants, according to the aforementioned US patents, it was found that polymerized trimethyl-dihydro-quinoline

gave effective oxidation inhibition.

In the CSA varnish test, three samples of polymer-insulated wire are (1) heated in an oven, (2) immersed in insulating varnish, (3) reheated in an oven for an extended period of time,

(4) cooled and (5) bent over a small diameter mandrel.

The tested polymer material is considered to fail the test

if a crack down to the conductor occurs in the insulation in one or more of the three samples when these are bent over the spin part. The materials that illustrate the invention according to the above-mentioned US patent documents No. 3,832,326 and 3,922,442,

where polymerized trimethyl-dihydro-quinoline is used as an antioxidant, the CSA paint test is not passed.

Many polymers are susceptible to oxidation, which causes a deterioration of their physical properties. This deterioration can be initiated by the impact of heat, light or other forms of energy. In most polymers, the oxidation proceeds according to a free-radical chain mechanism. The free radicals are formed in the polymer under the influence of an internal energy source. These radicals then react with oxygen to form a peroxy radical, which in turn reacts with the polymer to form a hydroperoxide and another radical which then continues the chain reaction.

Antioxidants have been developed to inhibit polymer degradation. These either serve to bind the peroxide radicals, so that these radicals are unable to maintain the chain reaction, or to split the hydroperoxides in such a way that carbonyl groups and other free radicals are not formed. The former, which are termed chain-breaking antioxidants, free-radical scavengers or inhibitors, are usually hindered phenols, amines and the like.

The latter, which are termed peroxide splitters, are usually sulfur compounds (e.g. mercaptans, sulfides, disulfides, sulfoxides, sulfones, thiodipropionic acid esters and the like) or metal complexes of dithiocarbamates and dithiophosphates.

A number of antioxidants are known in the art which have hitherto been used together with olefinic resins.

US Patent Nos. 3,160,680 and 3,282,890 describe an antioxidant combination of a sterically hindered phenol and; a diester of thiodipropionic acid for use in α-olefin hydrocarbon polymers. US Patent No. 3,033,814 mentions the use of a three-component antioxidant consisting of a hindered phenol, a diester of thiopropionic acid and phenyl salicylate in a polymer of a C2_C10a-oleiintVydrocarl:)0n. According to US Patent No. 3,181,971, the combination of a phenolic antioxidant and a primary or secondary aromatic or aliphatic amino compound with propylene homopolymers or copolymers of propylene with other hydrocarbons. According to US Patent No. 3,242,135, a boric acid ester is combined with a hindered phenol and a dithiopropionic acid diester to achieve oxidation inhibition for homopolymers and copolymers of C2-C8 α-olefin hydrocarbons. US Patent No. 3,245,949 describes homo:— and copolymers of C2-Cg aliphatic olefin hydrocarbons and mixtures thereof, where a combination of a phosphorus-containing polyphenolic compound and the dilauryl or distearyl ester of dithiopropionic acid is used as an antioxidant. None of these patents state or suggest that the antioxidant combinations can be incorporated with good results into polymers other than hydrocarbon polymers, i.e. no use is suggested with a polymer containing a larger amount of an ethylene-vinyl acetate copolymer.

It has now been shown that when an antioxidant comprising a diester of thiodipropionic acid is used instead of the polymerized trimethyldihydro-quinoline which is used as an antioxidant in the ethylene-vinyl acetate (EVA) materials according to the above-mentioned US patent documents Nos. 3,832,326 and 3,922,422 , the resulting materials not only exhibit substantially the same moisture resistance, heat resistance, flame retardancy, and oxidation retardance as before, but they also unexpectedly pass the CSA paint test.

With the invention, a flame retardant, crosslinkable polymer material is now provided, containing: (a) a polymer component containing at least 66% by weight

of a copolymer of ethylene and a vinyl ester of a C 2 -C 8 -aliphatic carboxylic acid, a C 1 -C 4 -alkyl acrylate or a C 1 -C 8 -alkyl methacrylate, preferably an ethylene-vinyl acetate copolymer,

(b) a hydrated inorganic filler, in an amount

of from 80 to 400 parts by weight, preferably from 125 to 150 parts by weight, per 100 parts by weight of the polymer component,

(c) an alkoxysilane, in an amount of from 0.5 to 5 parts by weight per 100 parts by weight hydrated inorganic filler, (d) an antioxidant, optionally a mixture containing a sterically hindered phenol, in an amount of 0.5-5.0 parts by weight, preferably 1.0-3.0 parts by weight per 100 parts by weight of the polymer component, and (e) a lubricant in an amount of 0.5-5.0 parts by weight per 100 parts by weight of the polymer component.

The new polymer material is characterized by the fact that the antioxidant (d) comprises at least 25% by weight distearyl-3,3'-thiodipropionate, and that the lubricant (e) is a mixture of 15-35% by weight lauric acid and 85-65% by weight ethylene-bis -stearamide.

The copolymer, silane and inorganic filler used in the polymer material include those described in US Patent Nos. 3,832,326 and 3,922,442.

The new materials are particularly useful for insulation

of wires and cables.

The invention thus also concerns an application of the new polymer material for the formation of a single cross-linked, insulating and encapsulating layer on an electrical conductor.

All percentages and parts used in this text are calculated on a weight basis, unless otherwise stated.

The crosslinkable copolymer component.

The terms "crosslinkable" and "crosslinkable" are to be understood in the usual meaning they have in the art, i.e. they express the formation of monovalent bonds between the polymer molecules.

Cross-linking can be accomplished by any of the known methods, such as by chemical means, including peroxide cross-linking, by irradiation using cobalt-60, accelerators, 3-rays, Y-rays, electrons, X-rays, etc. , or wood

thermal cross-linking. The basic methods for crosslinking polymers are very well known in the art, and it is not considered necessary to describe them in detail here.

The polymer component of the present material is

a copolymer of ethylene and a comonomer which may be a vinyl ester, an acrylate or a methacrylate. The vinyl ester can be a vinyl ester of a C2~C6 aliphatic carboxylic acid, such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate or vinyl hexanoate. The acrylates and methacrylates can be any of the C 1 -C 8 alkyl esters, including e.g. methyl, ethyl, propyl, butyl, pentyl and hexyl acrylate and methacrylate. The preferred copolymer for use as a polymer component according to the invention is an ethylene-vinyl acetate copolymer containing 9-90%, preferably 9-40%, and most preferably 9-28%, vinyl acetate, the remainder being ethylene.

Although little can be achieved by this, and certain properties can even be degraded, it is possible to incorporate smaller amounts of other crosslinkable polymers or copolymers in the material according to the invention. However, ethylene copolymers, preferably ethylene-vinyl acetate copolymers as described above, must constitute at least 66% of the total amount of polymers present. Representative of such minor amounts of polymer components which may be used in such non-preferred embodiments include polyethylene, copolymers of ethylene and propylene; butene, the acrylates and maleates, polydimethylsiloxane and polymethylphenylsiloxane, copolymers of vinyl acetate with the acrylates, etc. Likewise, mixtures of these in smaller amounts of used polymer components can be used.

Furthermore, terpolymers of ethylene and vinyl acetate, derived from e.g. from any of the corresponding monomer materials listed above (other than ethylene and vinyl acetate). A representative terpolymer would be an ethylene-vinyl acetate-vinyl maleate terpolymer.

The ethylene-vinyl acetate copolymers used according to the invention preferably have a melt index of from 1.0 to 20.0.

Polyethylene which can be used in the materials according to the invention includes mostly all polyethylenes with high, medium and low density, as well as mixtures thereof. The most preferred polyethylenes for use in materials for use as uni-insulation for electrical wires and cables generally have a density of from 0.900 to 0.950 g/cm 2 and a melt index of from 1.0 to 10.0.

More specifically, the materials according to the invention provide a very good and unexpected balance of: 1. Low temperature brittleness, i.e. the material will not crack during movement at low temperature (ASTM D 746). 2. Heat resistance after ageing, i.e. excellent elongation after prolonged use at 90°C and even at 125°C. 3. Resistance to arcing and 'tracking',<1>which is as great as 5 KW, while a material such as porcelain exhibits surface breakdown at 4 KW. However, this property is often not required, when used under preferred conditions, where the voltage is below 600 volts 4.Flame resistance properties and flame retardant properties 5.Resistance to moisture, i.e. little mechanical absorption of water, which gives a good dielectric constant 6.Resistance to industrial chemicals.

It is not known why the materials according to the invention provide such a good balance of properties. It is possible to assume that there is a synergistic relationship between the ethylene copolymer, the silane, the hydrated inorganic filler and the preferred lubricant, but the invention is not intended to be bound to such a theory. However, it has been determined that at low voltages, namely at voltages below 5000 volts, and to an even greater extent at voltages below 600 volts, the materials according to the invention are equivalent to the previously known materials for use as uni-insulation. The expression "uni-insulation" is used here to denote insulation that is extruded in one layer around the conductor, which one layer serves as electrical insulation and as a jacket to provide physical protection and flame protection. The present materials are particularly adapted for use as uni-insulation in the voltage range below 5000 volts, especially in the range below 60.0 volts, where a single extruded overcoat is used, and it is in this area that a high-quality balance of properties is required. It has also been shown that ethylene-vinyl acetate copolymers will retain very large amounts of filler and equally provide high flexibility and a large degree of cross-linking. The simultaneous achievement of high filler capacity, high flexibility and a high degree of cross-linking is quite surprising, as high flexibility and a high degree of cross-linking are commonly thought to be incompatible, similarly the simultaneous achievement of a high degree of cross-linking and high filler capacity ( which implies a low content of crosslinkable polymer). Ethylene-vinyl acetate copolymers further endow the polymer materials according to the invention with good flame retardant properties.

The above-described ethylene-vinyl acetate copolymers can be cross-linked by irradiation with high-energy electron beams, or by using chemical cross-linking additives. When they: are fully cross-linked, these polymers become thermosetting. For the preferred polymer materials according to the invention, chemical cross-linking is preferred, especially when very good physical strength properties are required.

Chemical cross-linking is carried out by incorporating a cross-linking agent, e.g. dicumyl peroxide or α,α-bis-(t-butylperoxy)-diisopropylbenzene in the ethylene-vinyl acetate copolymer. The peroxide is later activated during the processing process to bind the ethylene-vinyl acetate polymer chains together into a three-dimensional network (and other smaller amounts of crosslinkable polymers, if present).

The chemical cross-linking is carried out according to methods well known in the art, and changes to the general cross-linking conditions given below can be made by the person skilled in the art. Furthermore, the invention is not limited to the use of tertiary organic peroxides to achieve chemical cross-linking, as other materials recognized in the art which decompose to form free radicals can also be used. It goes without saying that such cross-linking agents must not undergo cleavage during the preparation of the material, but the choice of usable cross-linking agents will be easily made by the person skilled in the art.

Generally speaking, the degree of polymer cross-linking will increase when the amount of cross-linking agent is increased. Usually it is not necessary to use more than 10% (calculated on the monomer) of the organic tertiary peroxides, and amounts between 3 and 6% are the most common. Other cross-linking agents may require different amounts, but these will be easily determined. It is often advisable to avoid very small amounts of cross-linking agents, as this can lead to a certain loss of resistance to deformation under sudden or continuous compressive loads. Crosslinking aids such as triallyl cyanurate and the like may also be incorporated to increase the effectiveness of the crosslinking agent.

The tertiary organic peroxides become similar to

most other chemical cross-linking agents are activated by heating to a temperature above their activation temperature, whereby they undergo decomposition. Any of the known methods can be used to bring about activation. For example, the material can be exposed to high-pressure steam.

The technique of cross-linking by irradiation

is so highly developed that little would need to be said about this technique. When using large total doses of irradiation, the degree of cross-linking usually increases, and in order to achieve a preferred cross-linking, a total irradiation dose of 5 - 25 megarads will be used.

The cross-linking is usually carried out at above-atmospheric pressures, e.g. at pressures of the order of magnitude 14.1 - 28.1 kg/cm 2 , although higher or lower pressures can also be used. Pressure is used to: avoid uncontrolled porosity in the polymer, which would be very undesirable in electrical insulation.

The greater the degree of cross-linking, the more resistant the polymer material will generally be to moisture, chemical reagents, etc., and the less wear-resistant the polymer material will be. be. At lower degrees of cross-linking, there will also be a certain loss of heat resistance and likewise a marked effect on the percentage elongation after ageing. The exact degree of cross-linking can therefore be varied to take into account the above factors and their impact on the final product. Although higher or lower values may be used, a percentage cross-linking of the order of 95% for ethylene-vinyl acetate, determined by extraction (<y>ect basis) of soluble components in the cross-linked polymer, is usually preferred for insulation of wires and cables.

The silane component

One or more substituted silanes constitute the second main component of the polymer materials according to the invention.

Any silane which does not adversely affect the desired balance of properties and which will help bind the polymer and the inorganic filler used in accordance with the invention may be used, provided that the silane is not flammable and does not act too - affecting the polymer cross-linking or breaking down during the processing of the polymer. Examples are alkoxy- and amine-silanes.

The preferred silanes for use in the production of the insulating materials are the alkoxysilanes, e.g. the lower-alkyl-, alkenyl-, alkynyl- and aryl-alkoxysilanes as well as the lower alkyl-alkenyl-, alkynyl- and aryl-alkyloxy silanes or -aryloxyalkyl silanes. Specific examples of such silanes are methyltriethoxy-, methyltris-(2-methoxyethoxy)-, di-methyldiethoxy-, alkyltrimethoxy-, vinyltris-2-(methoxy-ethoxy)-, phenyltris-(2-methoxyethoxy)-, vinyltrimethoxy-

and vinyltriethoxy silane.

To achieve the best results, it is preferred to use the vinyl silanes, and among the vinyl silanes the following are particularly preferred:

-y -me thac ry loxy pro py 1 tr ime thoxy - s il an

and

vinyl-tris-(2-methoxyethoxy)-silane H2C = CHSi (OCH2CH2OCH3)

The hydrated inorganic filler

The fillers used according to the invention are the hydrated inorganic fillers, e.g. hydrated aluminum oxides (Al203 • 3H2° or Al(OH)3), hydrated magnesium oxide and hydrated calcium silicate. Among these compounds, hydrated aluminum oxide is particularly preferred.

In order to achieve the described good balance of properties, it is absolutely necessary that a hydrated inorganic filler is used in the production of the polymer mixtures. It is emphasized that larger amounts of another type of filler, whether this filler is inert or not, cannot be added to the materials while maintaining the high-quality balance of properties.

The inorganic filler's water of hydrate must be released when sufficient heat is applied to cause combustion or ignition of the ethylene-vinyl acetate copolymer. The hydrate water that is chemically bound to the inorganic filler is released endothermically. It has been shown that the hydrated inorganic filler increases the flame retardant properties far better than other fillers previously used in the art to give insulation materials flame retardant properties, such as carbon black, clay materials, titanium dioxide, etc. Even more surprising is that the flame retardant properties are combined with excellent electrical insulation properties at the large amounts of filler used, as the copolymer material at these large amounts of filler contains a large amount of bound water.

The particle size of the filler should be as previously used in the subject.

The antioxidant component

An antioxidant constitutes the fourth component

of the polymer materials according to the invention, and it is this component that unexpectedly causes the materials to pass the CSA paint test. A diester of thiodipropionic acid forms a main component of this antioxidant. The preferred diester is distearyl-3,31-thiodipropionate (DSTDP). Although this material is a known antioxidant, acting as a peroxide scavenger, its effect in causing the materials of the invention to pass the CSA paint test is not fully understood. The action of this particular diester is so much more surprising that a related diester, dilauryl-3,3<1->thiodipropionate (DLTDP) - although it is often used interchangeably with or in combination with DSTDP as an antioxidant, will not produce a polymeric material with the ability to pass the CSA paint test when used instead of DSTDP in the materials according to the invention.

In addition to DSTDP, which forms a main component of the antioxidant, other antioxidants can also be used together with this. It has been shown that the use of two different types of antioxidants provides effective oxidation inhibition. Thus, a mixture of an antioxidant of the chain breaking type and an antioxidant which is a peroxide scavenger provides an effective antioxidant. Therefore, together with DSTDP, which is a known peroxide splitter, an amine or a hindered phenol can be used as an antioxidant with good results. Among these free-radical scavengers, the sterically hindered phenols are particularly effective. Useful phenols include the alkylated phenols, the alkylidene-bis-alkylated phenols and the polyphenols. Specific examples of such phenols are 2,6-di-tert-butyl-para-cresol, octadecyl-3,5-di-t-butyl-4-hydroxy-hydrocinnamate, 2,2'-methylene-bis-(6- t-butyl-4-methyl-phenol), 4,4'-butylidene-bis-(6-t-butyl-3-methyl-phenol), 1,3,5-trimethyl-2,4-6-tris- (3,5-di-t-butyl-4-hydroxybenzyl)-benzene and tetrakis-[methylene-(3,5-di-t-butyl-4-hydroxy-hydrocinnamate)]-methane, among which the latter is particularly preferred.

C SA paint test

This test was developed by Canadian Standards

Association and is used to assess the insulation of coil connection wires (paragraph 6.7 of CSA Standard C22.2

No. 116) and electrical wires and cables (paragraph 4.25

in CSA Standard C22.2 No. 0.3). The tests for coil-lead wires and electrical wires and cables are essentially the same. Samples of insulated wire are heated in an oven with air circulation at 104 - 106°C for 30 minutes, after which they are taken out of the oven and immediately afterwards immersed in insulating varnish and kept immersed for 1 hour at room temperature. After removal from the varnish, the samples are suspended at room temperature for 1 hour, and then they are kept in an oven for 20 hours at either 149 - 151°C, 159 - 161°C or 20 3 - 2 05°C, depending on the type of wire. After cooling each sample at room temperature for 2 hours, it is bent once around a small diameter mandrel. An insulated cable fails in this test if the insulation in one or more of the three samples of the cable cracks all the way to the conductor.

The materials according to the invention quite unexpectedly pass this rigorous test, while the previously known materials, e.g. those described in US Patent Nos. 3,832,326 and 3,992,442 do not pass the CSA paint test, although they have properties that make them suitable as very good flame retardant insulation materials for wires and cables. Not only do the materials according to the invention pass the CSA paint test, but they also exhibit the excellent flame retardant properties and the resistance to moisture and heat that characterize the polymer materials according to the aforementioned US patent documents.

The quantity ratio between the components

The amounts of the polymer and the filler in the material according to the invention can be varied within wide limits. However, the amount of silane must be in the range from 0.5 to 5 parts per 100 parts filler. Smaller amounts may be insufficient to provide the desired surface treatment, while larger amounts may have an adverse effect on some of the physical properties, such as the elongation, of an extruded insulation material after cross-linking.

The best results are achieved when when applied to electrical wires and cables, e.g. by extrusion,

are present from 80 to 400 parts by weight of filler (most preferably at least 125 - 150 parts by weight), from 0.5 to 5.0 parts by weight of silane and 100 parts by weight of polymer.

The antioxidant must be used in a quantity that

will provide effective oxidation inhibition, and which will also add a sufficient amount of DSTDP for the polymer material to be able to pass the CSA paint test. If DSTDP is the only component of the antioxidant, no problems arise with regard to the CSA paint test. When other antioxidants are used together with DSTDP, DSTDP should make up at least 25% of the total antioxidant. The antioxidant must be present in an amount of from 0.5 to 5.0, preferably from 1.0 to 3.0, parts per 100 parts polymer.

The materials according to the invention can be formed in several ways. However, it is always necessary that the filler and the silane be brought into intimate contact with each other. For example, the preferred method for filler treatment is direct addition of the silane to the polymer with subsequent addition thereto of the filler, antioxidant and other additives, if desired. This can be done in a built-in mixer, such as a Banbury mixer or a Werner&Pfleiderer mixer. Alternatively, the silane can be added directly to the filler, and dispersed in this, and the polymer and antioxidant then added.

Any treatment device known in the art which ensures intimate mixing of all three main components can be used, provided that the silane is dispersed intimately and thoroughly on the surface of the hydrated inorganic filler.

It will be understood that in addition to the main components of the materials according to the invention, other additives may also be present, e.g. pigments and stabilizers,

as long as these do not have an adverse effect on the cross-linking or the desired properties. Such materials are present in very small amounts, up to 10% of the polymer, usually in amounts of less than 5%. There are two reasons why particularly large amounts of other components are not desirable. Firstly, the material according to the invention itself has very good properties, and secondly, significant amounts of e.g. other fillers only serve to reduce or upset the balance of properties.

For the application of insulation to conductors by extrusion, which represents the most preferred embodiment of the invention, an additional component is required,

namely a lubricant. Such an addition is like-

is conducted important to improve the tear-off properties of the wire insulation and thereby enable the user to easily remove the insulation, which facilitates work with splicing and connections. However, it is necessary to avoid soaps that interfere with the cross-linking reaction (free-radical mechanism), such as zinc stearate, which will react with organic peroxides.

The lubricant used is a mixture of 15-35% lauric acid and 85-65% ethylene-bis-stearamide.

The following example serves to illustrate certain aspects of the invention.

A number of crosslinkable EVA copolymer materials were produced and these were subjected to the CSA paint test. Each of the samples contained the same copolymer, the same hydrated aluminum oxide, the same silane and the same cross-linking agent in the same proportions, namely as follows:

17 samples were prepared, each of which contained, in addition to the above-mentioned components, a one-component or two-component antioxidant and a lubricant. Four commercially available antioxidants were assessed,

therein included two diesters of thiopropionic acid which in the art are often used interchangeably with each other due to their very similar antioxidation properties. The other two antioxidants were of the amine type and the phenol type. The two lubricants assessed were also commercially available products.

The silane, the filler and the other components were added to the polymer and mixed with it. Precautions were taken to regulate the temperature rise during mixing,

so that the peroxide would not be activated before the mixture was finished. After mixing, the polymer material was extruded onto a copper wire using a Brabender extruder,

and its temperature was raised to the activation temperature of the peroxide by vulcanization in steam under high pressure.

The composition of these samples and the results obtained in the CSA paint test are listed in Table I below.

Sample No. 1 is an example of the materials of US Patent Nos. 3,832,326 and 3,922,442. This sample did not pass the CSA paint test. Six other samples also failed this test. Of the seven samples containing DSTDP, six passed the test (samples no. 3, 4, 7, 8, 14 and 16). Of the eighteen specimens that made up these six DSTDP samples, only one specimen showed a deep crack in the insulation. Each of the fifteen samples containing the polymerized quinoline antioxidant showed deep cracks (Samples 1, 2, 11, 12 and 13) as did all nine samples containing the DLTDP antioxidant (Samples Nos. 12, 15 and 17). It may also be significant that only the DSTDP sample (sample no. 11) that failed the test also contained the polymerized quinoline antioxidant, and that all three samples showed deep cracks.

These experiments show that DSTDP was not effective with the polymerized quinoline antioxidant. Also, DLTDP cannot replace DSTDP when the CSA paint test is required to pass.

From these tests, it appears that among the additives tested, DSTDP makes the most significant contribution to passing the CSA paint test. Furthermore, the best antioxidant combination was DSTDP plus the hydrocinnamate, while the most effective system contained this oxidation system plus the "Mold Wiz" lubricant. Samples of this latter system (samples no. 4, 14 and 16) showed no or only weak cracks.

Claims (2)

1. Flame retardant, crosslinkable polymer material, containing: (a) a polymer component containing at least 66% by weight of a copolymer of ethylene and a vinyl ester of a C 2 -C 3 aliphatic carboxylic acid, a C 3 -C 8 alkyl acrylate or a C 3 -C 6 -alkyl methacrylate, preferably an ethylene-vinyl acetate copolymer, (b) a hydrated inorganic filler, in an amount of from 80 to 400 parts by weight, preferably from 125 to 150 parts by weight, per 100 parts by weight of the polymer component, (c) an alkoxysilane, in an amount of from 0.5 to 5 parts by weight per 100 parts by weight hydrated inorganic filler, (d) an antioxidant, optionally a mixture containing a sterically hindered phenol, in an amount of 0.5-5.0 parts by weight, preferably 1.0-3.0 parts by weight per 100 parts by weight of the polymer component, and (e) a lubricant in an amount of 0.5-5.0 parts by weight per 100 parts by weight of the polymer component, characterized in that the antioxidant (d) comprises at least 25% by weight of distearyl-3,3'-thiodipropionate, and that the lubricant (e) is a mixture of 15-35% by weight of lauric acid and 85-65% by weight of ethylene-bis -stearamide. 2. Use of the polymer material according to claim 1, for the formation of a single cross-linked, insulating and encapsulating layer on an electrical conductor.