Flame retardant composition.

0001This invention relates to flame retardant compositions containing thermoplastic resins and a magnesium hydroxide filler. The compositions are particularly useful as insulation for wire and cable.

0002Thermoplastic resins, which have good electrical insulation characteristics, are widely used to provide insulating jacketing or sheaths for wire and cable. Recently, there has been a demand for improved flame retardant properties, e.g., as high as V-1 to V-0 using Underwriters Laboratories standards.

0003Thermoplastic resin can be made flame retardant by adding to the resin organic halides or antimony oxides, for example, or the resin itself can be halogenated such as polyvinyl chloride or chlorinated polyethylene. These thermoplastic resins, however, on burning, drip, sag, and emit large amounts of smoke and other harmful gases, and also corrode metals.

0004In order to solve these problems, it has been proposed to add metal hydroxides to non-halogenated thermoplastic resins. Aluminum hydroxide was first used because of its low cost, but it has such a low decomposition temperature (170° to 190°C) that the aluminum hydroxide decomposes generating water, which, in turn, causes foaming on interior surfaces. Furthermore, to obtain a flame retardance of V-1 to V-0, aluminum hydroxide has to be added to the thermoplastic resin in amounts of as much as 60 to 65 percent by weight based on the weight of the total composition.

0005Compared with aluminum hydroxide, magnesium hydroxide has a much higher decomposition temperature (about 360°C), and, thus, exhibits less foaming. For this reason, and others, magnesium hydroxide has been widely used as a flame retardant in resins. One disadvantage of a thermoplastic resin/magnesium hydroxide flame retardant composition, however, is that the magnesium hydroxide absorbs carbon dioxide from high humidity, high temperature atmospheres such as the atmosphere found in a cable tunnel. This results in the formation of magnesium hydroxycarbonate (MgCO₃·Mg(OH)₂), a white substance, on, for example, the surface of a wire or cable jacket. This "whitening" not only detracts from the appearance of the jacket, but also has a degrading effect insofar as arc resistance, insulation, mechanical, low temperature brittleness, and other properties are concerned.

0006An object of this invention, therefore, is to provide a composition, which is suitable as an insulating jacket for wire or cable, and, as a jacket, will not be susceptible to whitening caused by the chemical reaction of magnesium hydroxide, carbon dioxide, and water.

0007Other objects and advantages will become apparent hereinafter.

0008According to the invention, a composition has been discovered which meets the above objective. The composition comprises <ul id="ul0001" list-style="none"><li>(i) a thermoplastic resin;</li><li>(ii) magnesium hydroxide; and</li><li>(iii) a hydrogenated styrene-butadiene copolymer.</li></ul>

0009The thermoplastic resin can be any homopolymer or copolymer produced from two or more comonomers, or a blend of two or more of these polymers, conventionally used as jacketing and/or insulating materials in wire and cable applications. Generally, the monomers useful in the production of these homopolymers and copolymers will have 2 to 20 carbon atoms. Examples of such monomers are alpha-olefins such as ethylene, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene; unsaturated esters such as vinyl acetate, ethyl acrylate, methyl acrylate, methyl methacrylate, t-butyl acrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate, and other alkyl acrylates; diolefins such as 1,4-pentadiene, 1,3-hexadiene, 1,5-hexadiene, 1,4-octadiene, and ethylidene norbornene; other monomers such as styrene, p-methyl styrene, alpha-­methyl styrene, vinyl naphthalene, and similar aryl olefins; nitriles such as acrylonitrile and methacrylonitrile; vinyl methyl ketone, vinyl methyl ether, and maleic anhydride; and acrylic acid, methacrylic acid, and other similar unsaturated acids. In addition to polyolefins, included among the polymers can be polyesters, polycarbonates, and polyurethanes. The homopolymers and copolymers of ethylene are preferred. The resins are preferably non-halogenated.

0010Examples of homopolymers and copolymers of ethylene are high pressure, low density Polyethylene; polyethylenes of various densities (high, medium, linear low, very low, and ultra-low) wherein the comonomer is 1-butene, 1-hexene, 4-methyl-1-pentene, or 1-octene; ethylene/propylene rubber; ethylene/propylene/diene monomer rubber; ethylene/vinyl acetate copolymer; ethylene/ethyl acrylate copolymer; isobutylene/isoprene rubber; and polybutene-1.

0011While conventional magnesium hydroxides can be used, a particularly preferred magnesium hydroxide and a method for its preparation are described in United States Patent No. 4,098,762 issued on July 4, 1978. Preferred characteristics for this magnesium hydroxide are (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 surface area, determined by the BET method, of less than 20 square meters per gram.

0012The average particle diameter of the magnesium hydroxide can be in the range of about 0.1 to about 15 microns and is preferably in the range of about 0.5 to about 3 microns. The surface area can be about 1 to about 20 square meters per gram and is preferably about 3 to about 8 square meters per gram. The magnesium hydroxide is preferably surface treated with various fatty acids or metallic salts of fatty acids such as stearic acid, sodium oleate, sodium stearate, sodium lauryl sulfonate, calcium stearate, or zinc stearate as described, for example, in United States patent 4,255,303.

0013The proportions of components in subject composition based on 100 parts by weight of thermoplastic resin are about as follows: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="center">Broad Range</entry><entry namest="col3" nameend="col3" align="center">Preferred Range</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="center">(parts by weight)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">Magnesium hydroxide</entry><entry namest="col2" nameend="col2" align="right">5 to 450</entry><entry namest="col3" nameend="col3" align="right">100 to 250</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Hydrogenated styrenebutadiene copolymer</entry><entry namest="col2" nameend="col2" align="right">1 to 80</entry><entry namest="col3" nameend="col3" align="right">30 to 60</entry></row></tbody></tgroup></table></tables>

0014The weight ratio of magnesium hydroxide to hydrogenated styrene-butadiene copolymer can be in the range of about 0.06:1 to about 450:1 and is preferably in the range of about 1.7:1 to about 8.3:1.

0015The styrene-butadiene copolymer is preferably a block copolymer and is hydrogenated by conventional techniques to essentially eliminate the unsaturation in the polymer chains. The styrene-butadiene copolymers can be prepared by anionic polymerization, cationic polymerization, coordination polymerization, radical polymerization, solution polymerization, or emulsion polymerization. The copolymers include various copolymers resulting resulting from the copolymerization of styrene and butadiene such as styrene-butadiene straight chain block copolymers; styrene-butadiene-styrene block copolymers; and styrene/butadiene/isoprene block copolymers. The weight ratio of styrene to butadiene in the copolymer can be in the range of about 0.4:1 to about 3:1 and is preferably in the range of about 0.7:1 to about 1.3:1. The molecular weight of the copolymer is generally in the range of about 50,000 to about 200,000 and is preferably in the range of about 80,000 to about 150,000. Other characteristics of the styrene-butadiene copolymer are as follows: the density of the styrene/butadiene copolymer can range from about 0.88 to about 0.96 grams per cubic centimeter and the melt index from about 1 to 10 grams per 10 minutes measured at 230°C under a 2.16 kilogram load.

0016Other useful additives for subject composition are surfactants, reinforcing filler or polymer additives, crosslinking agents, ultraviolet stabilizers, antistatic agents, pigments, dyes, slip agents, plasticizers, lubricants, viscosity control agents, extender oils, metal deactivators, water tree growth retardants, voltage stabilizers, flame retardant additives, and smoke suppressants.

0017Examples of antioxidants are: hindered phenols such as tetrakis[methylene(3,5-di-tert­butyl-4-hydroxyhydrocinnamate)]methane and thiodiethylene bis(3,5-di-tert-butyl-4-­hydroxy)hydrocinnamate; phosphites and phosphonites such as tris(2,4-di-tert-butylphenyl)phosphite and di-tert-butylphenylphosphonite; various amines such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline; and silica. Antioxidants are used in amounts of about 1 to about 5 parts by weight per hundred parts by weight of thermoplastic resin.

0018The thermoplastic resin can be crosslinked by adding a crosslinking agent to the composition or by making the resin hydrolyzable, which is accomplished by adding hydrolyzable groups such as -Si(OR)₃ wherein R is a hydrocarbyl radical to the resin structure through copolymerization or grafting.

0019Suitable crosslinking agents are organic peroxides such as dicumyl peroxide; 2,5-dimethyl-­2,5-di(t-butylperoxy)hexane; t-butyl cumyl peroxide; and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane-3. Dicumyl peroxide is preferred.

0020Hydrolyzable groups can be added, for example, by copolymerizing ethylene with an ethylenically unsaturated compound having one or more -Si(OR)₃ groups such as vinyltrimethoxy­silane, vinyltriethoxysilane, and gamma-­methacryloxypropyltrimethoxysilane or grafting these silane compounds to the resin in the presence of the aforementioned organic peroxides. The hydrolyzable resins are then crosslinked by moisture in the presence of a silanol condensation catalyst such as dibutyltin dilaurate, dibutyltin diacetate, stannous acetate, lead naphthenate, and zinc caprylate. Dibutyltin dilaurate is preferred.

0021Examples of hydrolyzable copolymers and hydrolyzable grafted copolymers are ethylene/­vinyltrimethoxy silane copolymer, ethylene/gamma-­methacryloxypropyltrimethoxy silane copolymer, vinyltrimethoxy silane grafted ethylene/ethyl acrylate copolymer, vinyltrimethoxy silane grafted linear low density ethylene/1-butene copolymer, and vinyltrimethoxy silane grafted low density polyethylene.

0022The composition of the invention can be extruded about, or coated on, an electrical conductor or subjected to injection, rotational, or compression molding.

0023In addition to being useful in wire and cable applications, subject composition can be used as a sheath for a glass core in fiber optics applications.

0024Commercial embodiments of the composition of the invention are generally obtained by mixing together the thermoplastic resin, the magnesium hydroxide, and the hydrogenated styrene-butadiene copolymer with one or more antioxidants and other additives in apparatus such as a Banbury mixer, a pressure kneader, a twin screw extruder, a Buss co-kneader, a Henschel mixer, or a roll kneader. The components can be added in any order and the components used in smaller amounts can be added via a masterbatch. The mixtures can then be extruded or subjected to injection molding, rotational molding, or compression molding.

0025In the examples, the performance of subject composition is evaluated by using a dumbbell specimen cut out of a sheet formed by compression molding the composition. The degree of whitening is determined by measuring the weight increase of a specimen exposed to a carbon dioxide gas stream containing moisture. The exposure is effected in a glass chamber having a volume of 50 cubic centimeters. The carbon dioxide is introduced into the chamber after bubbling through water at room temperature to provide a gas stream having a relative humidity higher than 90 percent.

0026The patents mentioned in this specification are incorporated by reference herein.

0027The invention is illustrated by the following examples.

EXAMPLE 1

0028A composition is prepared by mixing the following components in a Banbury mixer at 160°C for 10 minutes: <ul id="ul0002" list-style="none"><li>(i) 100 parts by weight of an ethylene/ethyl acrylate copolymer having a melt index of 0.7 and containing 19 percent by weight ethyl acrylate;</li><li>(ii) 5 parts by weight of magnesium hydroxide surface treated with stearic acid;</li><li>(iii) 0.5 parts by weight tetrakis [methylene-3(3,5-di-tert-butyl-4-hydroxy phenyl) propionate] methane as an antioxidant;</li><li>(iv) 2.5 parts by weight carbon black; and</li><li>(v) 1 part by weight hydrogenated styrene-butadiene block copolymer.</li></ul>

0029The composition is pelletized and the resulting pellets are formed into a sheet measuring 1 millimeter thick, 150 millimeters in length, and 180 millimeters in width. The sheet is punched out to provide dumbbell specimens in accordance with (describe reference). The above whitening test is performed on the specimen. The results are set forth in the Table.

0030It is noted from the Table that it took 21 days for the specimen to increase in weight one percent. The results of the accelerated test suggest that it will take 3700 days (over ten years) for the specimen to incur the same weight increase when exposed to a normal atmosphere containing 0.03 percent by weight carbon dioxide. Thus, it can be said the subject composition has sufficient resistance to whitening caused by carbon dioxide gas for commercial applications.

0031In a visual test, it took 11 days before a white substance appeared on the surface of the specimen, and 25 days before the surface of the specimen was completely white. These periods in the accelerated test translate into 900 days and 4000 days, respectively, in a normal atmosphere containing 0.3 percent by weight carbon dioxide.

EXAMPLES 2 TO 14

0032Example 1 is repeated except that components and amounts are varied. Variables and results are shown in the Table.

EXAMPLES 15 TO 17

0033Example 1 is repeated except that components and amounts are varied. For comparative purposes, the amount of the hydrogenated styrene-butadiene block copolymer is less than 1 part by weight. Variables and results are shown in the Table. <tables id="tabl0002" num="0002"><img file="EP0370518A2_D0001.tif" /></tables>

Notes to Table:&gt;

0034<ul id="ul0003" list-style="none"><li>1. Melt index 0.7; 19% by weight ethyl acrylate.</li><li>2. Polyethylene made in the gas phase having a density of 0.906 gram per cubic centimeter.</li><li>3. Surface treated with stearic acid.</li><li>4. Ratio of styrene to butadiene by weight = 3:7; other characteristics of the copolymer: density = 0.93 g/cc, melt index = 1 g/10 min.</li><li>5. Ratio of styrene to butadiene by weight = 4:6; other characteristics of the copolymer: density = 0.93 g/cc, melt index = 1 g/10 min.</li><li>6. Tetrakis [methylene-3 (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] methane.</li><li>7. Weight increase results from the reaction represented by the following equation: 4Mg(OH)₂+3CO₂→3MgCO₃·Mg(OH)₂·3H₂O Conditions of accelerated test: temperature = 15 to 30°C and relative humidity = 90% +.</li><li>8. The test is conducted by placing the specimen in a dessicator containing water vapor and carbon dioxide at 40°C and 90% relative humidity. The white substance appearing on the specimen is identified as magnesium hydroxycarbonate by infrared spectrometry, and this substance is in the form of needle crystals according to its electron micrograph.</li></ul>