Fire-Retardant Imide Copolymers

This invention relates to fire-retardant foamed polymers.

U.S. Pat. No. 3,075,944 teaches the use of bromodiphenylethers with metal oxide synergists for making fire-retardant polystyrene and polyethylene.

U.S. Pat. No. 4,151,218 teaches the use of the same synergistic fire-retardant system for copolymers of styrene and maleic anhydride alone and rubber-modified.

The use of volatile hydrocarbon blowing agents to prepare foams of polystyrene is well known. These blowing agents are flammable hydrocarbons such as pentane and hexane.

U.S. Pat. No. 2,911,382 teaches the use of mixtures of volatile hydrocarbon blowing agents and water or hydrated salts.

Canadian Pat. No. 844,652 teaches the use of water or hydrated salts as the sole blowing agent for copolymers of styrene and acrylic acid.

We have now found that fire-retardant foams can be prepared from compositions consisting of a blend of polar polymers with from 2 to 6 percent by weight based on polymer of water as the blowing agent, a brominated diphenylether as flame retardant and a synergist for the flame retardant selected from certain metal oxides. Herein, the polar polymers are copolymers of vinyl aromatic monomers and an imide derivative of an ethylenically unsaturated dicarboxylic acid monomer.

The invention consists of a fire-retardant foamed composition and a process for making said foam.

The foamed composition has a density of between 1 and 10 pounds per cubic foot and consists of a copolymer of 50 to 95 mole percent of a monovinyl aromatic monomer and 5 to 50 mole percent of an imide derivative of an ethylenically unsaturated dicarboxylic acid monomer, from 10 to 20 parts per hundred parts of copolymer of an at least tri-brominated diphenylether, and 4 to 8 parts per hundred parts of copolymer of a synergist for the ether.

The foamed composition is prepared by a process consisting of blending a mixture of (a) a copolymer of 50 to 95 mole percent of a monovinyl aromatic monomer and 5 to 50 mole percent of an imide derivative of an ethylenically unsaturated dicarboxylic acid monomer; (b) from 0.5 to 10 percent by weight of water; (c) from 10 to 20 parts per hundred parts of copolymer of an at least tri-brominated diphenylether; and (d) 4 to 8 parts per hundred parts of copolymer of a metal oxide synergist for the ether until an intimate blend is obtained; heating said blend to a temperature of between 125.degree. and 190.degree. C. under sufficient pressure to prevent foaming of the blend; releasing the pressure to allow the polymer blend to expand to a foam of density between 1 and 10 pounds per cubic foot; and cooling the resultant polymer foam to room temperature.

The imide copolymers useful in the present invention may be prepared by any of the known methods, such as direct copolymerization of a monovinyl aromatic monomer with an ethylenically unsaturated dicarboxylic acid imide by the continuous controlled addition of the more reactive imide monomer. Another method involves the reaction of the corresponding copolymer of monovinyl aromatic monomer and an ethylenically unsaturated dicarboxylic anhydride with anhydrous ammonia or amines. A further method of preparation is that described in U.S. Pat. No. 3,840,499, which is hereby incorporated in its entirety and wherein copolymers are prepared by reaction of aqueous ammonia or amines with an aqueous suspension of a copolymer of a monovinyl aromatic monomer and an ethylenically unsaturated dicarboxylic acid moiety, such as the diacid, its anhydride, or its half ester with an aliphatic alcohol. The reaction is carried out at temperatures between 125.degree. C. and 200.degree. C., at autogenous pressures, for between 3 and 48 hours.

The monovinyl aromatic monomers useful in the copolymers are styrene, alpha-methylstyrene, nuclear-methylstyrenes, ethylstyrene, isopropylstyrene, tert-butylstyrene, chlorostyrenes, dichlorostyrenes, vinylnaphthalene and mixtures of these.

Suitable imides are the imides of maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, ethyl maleic acid, methyl itaconic acid, chloromaleic acid, dichloromaleic acid, bromomaleic acid, dibromomaleic acid, and mixtures thereof and the like.

Imide derivatives, besides the unsubstituted imide of these diacids, are those derived from reaction of an ethylenically unsaturated dicarboxylic acid moiety, as above, and alkyl amines having 1 to 4 carbon atoms in the alkyl group, such as methylamine, ethylamine, propylamine, isopropylamine, and butylamine; ethanolamine; aniline; benzylamine; allylamine and the like.

The imide copolymers may contain from 5 to 50 mole percent of imide or N-substituted imide and 95 to 50 mole percent of monovinyl aromatic monomer.

The foams of the present invention are prepared using water as the blowing agent. The amount of water which is required will depend upon the desired density of the copolymer foam. Sufficient water should be present to expand the copolymer to the desired density with the density being inversely proportional to the amount of water present. Generally, amounts of water between 0.5 and 10 percent by weight of copolymer are sufficient. The water is preferably used in amounts of from 2 to 6 percent by weight based on the copolymer. Greater than 10 percent water may be used, but the foam cell size begins to become too large.

The water may be introduced into the polymer by any of the known methods. These include direct impregnation of water into the polymer pellets, grinds or beads; direct addition of water to the surface of the pellets; direct injection of water into molten polymer in an extruder; and blending the polymer with salts containing water of hydration or water of occlusion. If the imide copolymers are prepared by an aqueous suspension system, the water is incorporated into the copolymer beads.

Hydrated salts suitable for use in the invention include for example gypsum, sodium metasilicate pentahydrate (Na.sub.2 SiO.sub.3.5H.sub.2 O), aluminum sulfate (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O), magnesium sulfate (MgSO.sub.4.7H.sub.2 O), calcium thiosulphate (CaS.sub.2 O.sub.3.6H.sub.2 O), sodium tetraborate (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O), sodium carbonate (Na.sub.2 CO.sub.3.10H.sub.2 O), etc. and mixtures thereof.

It has been found advantageous to disperse in the polymer compositions a nucleating agent which will aid in the formation of tiny bubbles of the vaporized expanding agent and produce a foam having a fine and uniform cell size particularly when the compositions are foamed by extrusion. The combination of bromodiphenylethers and metal oxides has been shown to have the effect of a nucleating agent, so the use of additional agent is optional. Suitable nucleating agents are finely divided inert solids such as talc, diatomaceous earth, soft wood flour and pecan shell flour. Conveniently from 0.5 to 5 percent by weight of polymer of nucleating agent is employed. Preferably 1 to 4 percent by weight based on the copolymer is used.

The diphenylethers suitable as flame retardants for the present invention must contain at least three bromine atoms per molecule. The distribution of the bromines on the two phenyl rings is not critical. Examples of such ethers are tribromodiphenylether, tetrabromodiphenylether, pentabromodiphenylether, hexabromodiphenylether, tribromochlorodiphenylether, tribromodichlorodiphenylether, tribromotrichlorodiphenylether, tetrabromodichlorodiphenylether, octabromodiphenylether, and decabromodiphenylether. The preferred ether is decabromodiphenylether.

The diphenylethers will normally be used in concentrations of 10 to 20 parts by weight of ether per 100 parts of copolymer, preferably 12 to 18 parts per 100 of copolymer.

Various synergists for the bromo-compounds may be added in amounts of from 4 to 8 parts per hundred parts of polymer to improve the fire-retardant properties of the foams. Suitable synergists are the metal oxides, such as antimony trioxide, bismuth oxide, molybdenum oxide, stannic oxide and tungsten oxide, and the organic peroxides decomposing at temperatures higher than the extrusion temperature of the foams. The preferred synergist is antimony oxide.

Other known additives and fillers can be used with the proviso that they not interfere with the foaming action of the water blowing agent or with the flame-retardancy of the bromocompounds. Suitable additives may be the various stabilizers, fillers, smoke-suppressants, lubricants, etc.

The expandable compositions may be expanded to fine cell foams of low density in conventional extrusion equipment by placing the blends in an extruder wherein the material is heat plastified and heated to a temperature between 125.degree. and 190.degree. C. after which the material is extruded through an orifice into the atmosphere. Foams of from 1-10 pcf density are easily obtained. The base polymers have higher second-order transition temperatures than, for example, polystyrene, with the transition temperature increasing with increasing imide content, and hence the foams are useable at higher temperatures.

The invention is further illustrated but not limited by the following example in which parts are parts by weight unless otherwise indicated.

A blend was prepared of 100 parts of a styrenemaleimide copolymer, containing 2 wt-% water and having 86 wt-% styrene and 14 wt-% maleimide, 15 parts of decabromodiphenylether (DBDPE), and 5 parts of Sb.sub.2 O.sub.3. The blend was extruded through a small 6".times.1" L/D laboratory bench type extruder having a 1/8" die orifice at 180.degree.-182.degree. C. extruder temperature. Foam rod, 1/2 to 5/8" diameter, with small cells, hard skin, some voids and a 4.5 pcf density was produced. It should be noted that although no additional nucleating agent was used, the combination of bromocompound and metal oxide acted as nucleating agent to produce the small cells. When the foam was contacted with a flame from a match or a bunsen burner, the foam did not ignite or burn. A repeat run with copolymer pellets containing 3.5 wt-% of water, gave a foam product of 2.8 pcf density which did not burn.

It should be noted that dropping the DBDPE content to 10 parts and the Sb.sub.2 O.sub.3 to 3.3 parts gave a foam product which did ignite and burn.