Flame retardant compositions

IS 2 155 443 T3

DESCRIPTION

Flame retardant compositions.

This invention relates to flame retardant compositions containing an ethylene copolymer and a magnesium hydroxide filler. The compositions are useful for the insulation and sheathing of electrical conductors and communication media such as fiber optic cable.

Wires and cables, if used for power or communications transmission, are usually stored outdoors or in unheated areas prior to installation. If insulation or sheathing becomes brittle at low temperatures and cracks, the integrity of the wire or cable disappears and a short circuit with its foreseeable problems becomes a real possibility. Consequently, specifications for insulation or liner layers generally require that these materials pass a low temperature brittle test. Low temperature behavior of minus 50 C or higher is often cited by wire and cable manufacturers as desirable, particularly for installations in, for example, Canadau.

A typical electrical power cable is made up of metallic conductors insulated with a polymeric material. These elements are generally twisted into a core and are protected by another polymeric sheath or lining material. In certain cases, the added protection is achieved by inserting a sheath between the nucleus and the sheath.

Flame retardant compositions comprising polyethylene and magnesium hydroxide are useful in wire and cable applications, with polyethylene providing flexibility and magnesium hydroxide providing flame retardancy. Unfortunately, these compositions are not close to achieving the most desirable low temperature performance in terms of brittleness. It is difficult, for example, to achieve low brittleness at a temperature of 0 ° C with a flame retardant composition of linear low-density polyethylene, let alone at the desirable minus 50 ° C.

An object of this invention, therefore, is to provide a flame retardant composition, which has exceptional resistance to low temperature brittleness, thus being useful in cold climates. Other objects and advantages will become apparent below.

According to this invention, the aforementioned object is fulfilled with a composition that comprises:

(a) polyethylene having a density equal to or less than 0.915 grams per cubic centimeter grafted with unsaturated alifautic diuacid anhydride; and (b) magnesium hydroxide, the surface of which is not treated.

Polyethylene is a very low density polyethylene (VLDPE). VLDPE is a copolymer of ethylene and at least one alpha-olefin having 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms. The density of the copolymer is equal to or less than 0.915 grams per cubic centimeter and preferably not less than 0.860 grams per cubic centimeter. It can be produced, for example, in the presence of (i) a catalyst containing chromium and titanium, (ii) a catalyst containing magnesium, titanium, a halogen, and an electron donor; or (iii) a vanadium-containing catalyst, an electron donor, an alkyl aluminum halide modifier, and a halocarbon promoter. The catalysts and processes for preparing the VLDPE are described, respectively, in US Pat. 4,101,445 and 4,302,565 and in European Patent Application 0120501 published on October 3, 1984.

EP-A-0370517 refers to a composition comprising a thermoplastic resin, an inorganic flame retardant, a copolymer of ethylene and 1-hexene or 1-octene, in which said copolymer has been modified with an unsaturated organic compound. containing at least one carboxylate group and optionally a copolymer of ethylene and one or more alpha-olefins having a crystallinity of less than about 40% by weight.

DE-3813200 refers to a thermoplastically processable plastic composition based on modified polyethylenes for electrical cables and wires consisting of a polyethylene having a specific density below 0.91 g / cm³ (VLDPE) and loads in large quantities.

JP-A-62280242 refers to a polyethylene resin composition comprising 20 to 70% by weight of an LLDPE having ethylene units and 6C alpha-olefins and 80 to 30% by weight of Mg (OH) 2 with a 2.2-8.0 μm mean particle diameter. The composition has a good retardation capacity of

ES 2 155 443 T3 flame, good mechanical properties and low temperature brittleness and does not generate toxic gases when burned.

The melt index of the VLDPE may be in the range of 0.1 to 50 grams per 10 minutes and is preferably in the range of 0.4 to 10 grams per 10 minutes. Melt index is determined in accordance with ASTM D-1238, Condition E, measured at 190^◦C. Examples of suitable alpha-olefin comonoimers are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The portion of VLDPE attributed to the comonoimer (s) other than ethylene, is in the range of 1 to 49 percent by weight based on the weight of the copolymer and is preferably in the range of 10 to 40 percent. percent by weight.

Unsaturated aliphatic diaicide anhydrides are commonly grafted to various polyolefins. These anhydrides can have 4 to 20 carbon atoms and preferably have 4 to 10 carbon atoms. Examples of anhydrides, which are useful in this invention, are maleic anhydride, itacoinic anhydride, and nidic anhydride. The preferred anhydride is maleic anhydride. Excess anhydride, if present after grafting, can be removed by devolatilization at temperatures in the range of 200 ° C to 250 ° C.

The grafting is carried out using an organic peroxide catalyst, that is, a generator of free radicals, such as dicumyl peroxide; lauroyl peroxide; benzoyl peroxide; tertiary butyl perbenzoate; di (tertiary butyl) peroxide; Cumene Hydroperoxide; 2,5-dimethyl-2,5-di (t-butylperoxy) hexyne-3; 2,5-dimethyl-2,5-di (t-butyl-peroxy) hexane; tertiary butyl hydroperoxide; isopropyl percarbonate; and alpha, alpha'-bis (tertiary butylperoxy) diisopropylbenzene. The organic peroxide catalyst can be added along with the anhydride.

Grafting temperatures can be in the range of 100 to 300 ° C and are preferably in the range of 150 to 200 ° C.

A typical procedure for grafting maleic anhydride onto polyethylene is described in US Patent 4,506,056.

Grafting can also be carried out by adding an anhydride solution, an organic peroxide catalyst, and an organic solvent to particulate polyethylene. The organic peroxide catalyst is soluble in the organic solvent. Various organic solvents can be used, which are inert to the reaction. Examples of useful organic solvents are acetone, methyl ethyl ketone, methyl propyl ketone, 3-pentanone, and other ketones. Other carrier solvents can be used which allow peroxide and anhydride solubilization, and which separate well under appropriate devolatilization conditions. Acetone is a preferred solvent because it acted as a removal agent for ungrafted anhydride residues or anhydride by-products.

The anhydride solution may contain 10 to 50 weight percent anhydride; 0.05 to 5 weight percent organic peroxide catalyst; and 50 to 90 weight percent organic solvent based on the total weight of the solution. A preferred solution contains 20 to 40 percent anhydride; 0.1 to 2 percent peroxide; and 60 to 80 percent solvent.

The anhydride grafted polyethylene may contain 0.05 to 5 parts by weight of anhydride per 100 parts by weight of polyethylene and preferably contains 0.1 to 2 parts by weight of anhydride per 100 parts by weight of polyethylene.

The VLDPE can also be grafted with an unsaturated monomeric silane having one or more hydrolyzable groups in order to make the polymer hydrolyzable thus allowing moisture cure. Although this grafting can be carried out simultaneously with the grafting of the aliphatic diacid anhydride, the grafting can also be carried out before or after the grafting of the anhydride, if desired.

The silane grafted copolymer can be prepared by the technique described above. In this copolymer, the portion attributed to the silane is present in an amount of 0.5 to 10 percent by weight based on the weight of the copolymer and is preferably incorporated into the polymer in an amount of 0.5 to 4 percent by weight. weight. Generally speaking, any unsaturated monomeric silane containing at least one hydrolyzable group can be employed. The silane used to modify the copolymer may be, among others, an alkenyl-alkoxy-silane such as a vinyl-trialkoxy-silane exemplified by vinyl-trimethoxysilane, vinyl-triethoxy-silane, or vinyl-isopropoxy-silane. If slower water cure or better storage stability is desired, vinyl triisobutoxy silane or vinyl tris (2-ethyl hexosi) silane can be used.

IS 2 155 443 T3

The hydrolyzable silane and anhydride grafted polymer can be crosslinked with moisture in the presence of a conventional silanol condensation catalyst, such as dibutylstane dilaurate, dioctylstane maleate, stannous acetate, stannous octoate, lead naphthenate, zinc octoate,

Iron 2-ethylhexoate, and other metalic carboxylates. Further acceleration of crosslinking can be achieved by adding a catalyst such as tetramethoxy titanate, tetraisopropyl titanate, tetramethyl titanate, or other organotitanates mentioned in US Patent 4,446,279. The catalysts can be introduced into the composition in conventional amounts, typically 0.005 to 1 part by weight of primary catalyst per 100 parts by weight of copolymer and 0.25 to 10 parts by weight of cocatalyst per 100 parts by weight of copolymer.

A typical procedure for preparing a silane grafted polyethylene is as follows: 100 parts of ethylene / 1-butene copolymer having a density of 0.90, 0.2 parts of 1,3-dihydro-2,2,4-trimethylquinoline (an antioxidant), 0.1 part of dicumyl peroxide, and 4 parts of vinyl-tri-2-ethylhexoxysilane are mixed in a Brabender mixer at a temperature in the range of 80 ° C to 115 ° C, a temperature low enough to keep dicumyl peroxide below its decomposition temperature. After mixing for 5 minutes, the temperature rises to a temperature in the range of 150^°C to 220^°C. The batch is then mixed for 5-10 minutes during which grafting of the silane to the copolymer occurs. The antioxidant is used as a radical scavenger to control the amount of crosslinking. The foregoing technique can be repeated, for example, with 3 parts of vinyltriisobutoxysilane and 0.1 part of the antioxidant tetrakis [methylien (3-5-di-tert-butyl-4-hydroxyhydrocinnamate)]; initial mixing takes place in the range of 110^°C to 120^°C; grafting occurs for five minutes at 185^°C.

Various processes for preparing silane grafted polymers and numerous unsaturated silanes suitable for use in preparing these polymers and supporting hydrolyzable groups such as alkoxy, oxyaryl, oxyaliphatic, and halogen are mentioned in US Patents 3,075,948; 3,225,018; 4,412,042;

4,413,066; 4,574,133; and 4,593,071.

It would be noted that the same organic peroxide catalyst is used in the silane graft as in the anhydride graft.

Magnesium hydroxide is a catalyst conventionally used in wire and cable applications except that it is not surface treated. This means that the surface of the magnesium hydroxide is not treated or coated with a chemical, it does not matter whether the chemical reacts with the surface or not, nor are the usual surface treatment agents such as carboxylic acids or their salts used in the composition in which magnesium hydroxide is introduced. It should be understood by those skilled in the art that once the magnesium hydroxide is mixed into the cable composition, the cable would then be coated with the mixture of resin and other additives.

A preferred magnesium hydroxide has the following characteristics: (a) a strain in the <101> direction of no more than 3.0 x 10^-3; (b) a crystallite size in the <101> direction of more than 800 angstroms; and (c) a specific surface area, determined by the BET method, of less than 20 square meters per gram. The preferred magnesium hydroxide and a method for its preparation are described in US Patent 4,098,762. A preferred characteristic of magnesium hydroxide is that the specific surface area, determined by the BET method, is less than 10 square meters per gram.

The amount of magnesium hydroxide employed in the composition may be in the range of 100 to 1,200 parts by weight of magnesium hydroxide per hundred parts by weight of anhydride grafted polyethylene and is preferably in the range of 100 to 180 parts by weight of magnesium hydroxide per hundred parts by weight of anhydride grafted polyethylene, the optimum being 120 to 160 parts.

Unmodified polyethylenes including VLDPE, linear low density polyethylenes, and other unmodified polyethylenes of varying densities up to 0.965 grams per cubic centimeter can be introduced into the composition of the invention, if desired, with some sacrifice in low brittleness. temperature as the amount of unmodified polyethylene increases. On the basis of 100 parts by weight of modified polyethylene, the unmodified polyethylene can be introduced in a range of 0 to 750 parts by weight, preferably no more than 325 parts by weight, but the closer to zero the unmodified polyethylene, the better. .

Various conventional additives can be added in conventional amounts to the composition of the invention. Typical additives are antioxidants, ultraviolet absorbers, antistatic agents, pigments, colorants, other fillers including carbon black and aluminum silicate, slip agents,

ES 2 155 443 T3 fire retardants, stabilizers, crosslinking agents, halogen scavengers, smoke inhibitors, crosslinking stimulators, process aids, lubricants, plasticizers, and viscosity control agents.

The invention is illustrated by the following examples.

Examples 1 to 4

Polyethylene I is an unmodified linear low-density linear polyethylene, which is a copolymer of ethylene and 1-hexene that has a density of 0.917 grams per cubic centometer and a flow onyx of 3.4 grams per 10 minutes, for comparison. .

Polyethylene II is a maleinized VLDPE, which is a copolymer of ethylene and 1-butene having a density of 0.900 grams per cubic centometer and a melt onyx of 0.4 grams per 10 minutes. The VLDPE is grafted with 0.3 parts by weight of maleic anhydride per 100 parts by weight of VLDPE according to the procedure mentioned above.

The magnesium hydroxide employed in the examples falls within the definition of preferred magnesium hydroxide mentioned above, and is not surface treated.

The antioxidant is tetrakis [methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane.

Four compositions are prepared for test samples as required by the Low Temperature Brittleness (LTB) test procedure, that is, ASTM D-746-73, and the test procedure is carried out. LTB is given in degrees centigrade.

The variables and the results are given in the Table.

TABLE

<td>Example</td><td>1 (comp.)</td><td> 2</td><td> 3</td><td> 4</td>

<td>Formulation (in parts by weight): Polyethylene I</td><td> 100</td><td> 88</td><td> 76</td><td></td>

<td>Polyethylene II</td><td> —</td><td> 12</td><td> 24</td><td> 100</td>

<td>Magnesium hydroxide</td><td> 139,4</td><td> 139,4</td><td> 139,4</td><td> 139,4</td>

<td>Antioxidant</td><td> 1</td><td> 1</td><td> 1</td><td> 1</td>

<td>Results: LTB (° C)</td><td> +18</td><td> -12,5</td><td> -31,5</td><td> -45</td>

Note to table: Example 4 in which the resin is 100 percent maleinized VLDPE and the magnesium hydroxide is not surface treated results in an LTB of minus 45 ° C.