POLYMERIC COMPOSITIONS ENDOWED WITH IMPROVED FLAME-RESISTANCE PROPERTIES

Abstract: Polymeric compositions endowed with improved self-­ extinguishing properties, comprising by weight: (A) from 60 to 98.9% of a thermoplastic polymer; (B) from 0.1 to 10% of an antimony and/or bismuth compound; (C) from 0 to 1% of a promoter of free radicals; (D) from 1 to 30% of melamine bromohydrate.

Description: The present invention relates to compositions based on thermoplastic polymers, endowed with high self-extinguishing properties. Different methods to provide polymers with flame-resistance are known in the art: said methods are generally based on the addition to the polymer of heat-unstable halogenated compounds associated with metallic compounds, expecially antimony and/or bismuth compounds. Generally, the combinations between metallic compound and halogenated compound give satisfactory results as far as the characteristics of flame-resistance are concerned, but they involve more or less serious inconveniences and particularly: corrosion phenomena in the equipment wherein the treatment of polymers containing the above mentioned additives is carried out; worsening in the release of darkening smoke during combustion and/or decomposition of said polymers. It is also known that ammonium salts of hydroacids generally have an effective action as flame-retarders in many fields such as the field of paints and coatings, but they cannot be used directly as additives in thermoplastic polymers due to their high corrosive action with respect to metals, their extreme sensitivity to water and air humidity and their poor thermal and thermo-oxidizing stability. US-A-4 028 333 discloses a flame retardent composition containing melamine dihydrobromide (20% by weight), polypropylene resin and antimony oxide (5% by weight). The Applicant have now surprisingly found that most of the thermoplastic polymers commonly used today can reach good levels of flame resistance without incurring any of the previously described inconveniences when mixed with a combination of melamine bromohydrate with antimony and/or bismuth compounds such as those hereinafter mentioned, optionally combined with small amounts of a promoter of free-radicals. Therefore, the object of the present invention is to provide polymeric compositions endowed with self-extinguishing properties, comprising by weight: A) from 60 to 98.9% of a thermoplastic polymer; B) from 0.1 to 10% of at least one product selected from antimony trioxide, antimony oxychloride, antimony trisulfide, metal bismuth and an organic or inorganic, oxygenated or non-oxygenated bismuth salt; C) a free radical promoter in an amount of up to 1 % selected from the group consisting of 2,3-dimethyl-2,3-diphenyl butane, 2,3-dimethyl-2,3-diphenyl hexane and organic peroxides; D) from 1 to 5 % of melamine bromohydrate. In such compositions, melamine bromohydrate is preferably present in an amount of 3 to 5% by weight. Among bismuth salts it is possible to mention as preferred ones bismuth oxychloride, bismuth nitrate, bismuth basic nitrate, bismuth basic carbonate, bismuth sulfate, bismuth trisulfide, bismuth phosphate, bismuth basic salycilate and bismuth acetate. Examples of thermoplastic polymers to be possibly used in the compositions of the present invention are olefinic polymers such as for instance polypropylene, polypropylene modified with ethylene, ethylene/propylene copolymers, mixtures of polypropylene with up to 20% by weight of ethylene/propylene elastomeric copolymers containing up to 50% by weight of copolymerized ethylene and futhermore polystyrene ( crystal and shockproof), ABS resins, polyamides and polyesters resins. Organic peroxides can be also used as promoters of free radicals in the present compositions, preferably in amounts ranging between 0.05 and 0.1 parts by weight. Melamine bromohydrate ground at a few micron fineness also proves to be used for improving flame-resistance of fibers, raffia and in general of fabrics made of polypropylene and of other thermoplastic polymers hereinabove mentioned, provided that they are suitable to be spun under the usual conditions and according to the techniques known in the art. Melamine bromohydrate can be prepared by allowing melamine to react with hydrobromic acid, in molar ratios of from 1/0.9 to 1/1.2, in an aliphatic alcohol containing 3 or 4 carbon atoms, which is preferably water unsoluble. An example of this preparation on industrial scale can be as follows: 700 kg of powdered melamine are dispersed in n-butyl alcohol ( 3,000 kg) in a glazed reactor and kept under stirring. Thereafter, 960 kg of hydrobromic acid in 47% aqueous solution are gradually added; the temperature must not exceed 45-50 °C. The mixture is kept under stirring for one hour, at room temperature; the product is centrifuged and dried in a worm screw rotary drier; the melamine bromohydrate is thus obtained in form of fine white powder, For the purposes of the present invention, the use of melamine bromohydrate as a composition comprising melamine bromohydrate, containing up to 10% by mols of free melamine not salified with HBr, 3-5% by weight of stearic acid (which can be added during the drying of melamine bromohydrate) and optionally, but not necessarily, from 2 to 3 % by weight of micronized silica, having a 1-2 NM granule size, such as for instance silica known on the market as "Syloid 72"® ( manufacturer Italian Grace Co) has proved particularly useful. The compositions of the present invention can be prepared according to conventional methods, for instance by mixing the polymer with additives in a Banbury mixer, at a temperature equal or exceeding the polymer softening temperature and then by extruding the mixture in an extruder, at the most suitable temperature to obtain a product in granules. To determine the self-extinguishing properties of the compositions, test pieces ( 3 mm thickness) are moulded by means of a Carver type press, by working at a temperature at least equivalent to the softening temperature of the polymer, under a pressure of 4·10⁶Pa (40 kg/cm²), for 7 minutes. The flame-resistance degree is determined on the thus obtained test pieces by means of the "Oxygen index " measure ( according to ASTM-D 2863 specification) which gives the minimum percentage of oxygen in mixture with nitrogen necessary for the test piece to burn continuously, as well as by complying with UL-94 specifications ( published by Underwriters Laboratories-U.S.A) which provide an evaluation of the extinguishing degree of plastic materials. In applying such specifications, the "Vertical Burning test " has been adopted. This test allows classification of the materials at 94V-0, 94V-1 and 94V-2 levels on the basis of test pieces combustion time and on the basis of the fact that they do or not drop flamimg particles. According to said method the test piece is primed, keeping it in vertical position, approaching the flame to its lower end and performing two ignition attempts, each of them lasting 10 seconds. Each test is performed on a group of 5 test pieces, also carrying out measurement of the extinction time for 4 subsequent ignition on the same test piece, as further differentiating criterium. Tables 1 to 5 report Examples which illustrate the compositions of the invention, without any limitative purpose. The "Comparisons" are comparative examples. The compositions of each example were prepared as previously described, using a Dolci screw extruder provided with a 20 mm diameter screw, length/diameter ratio of the screw = 23 and screw operating speed = 20 r.p.m. at a temperature of 200 - 240 °C.

Claims: Polymeric compositions endowed with self-extinguishing properties, comprising by weight: A) from 60 to 98.9% of a thermoplastic polymer; B) from 0.1 to 10% of at least one product selected from antimony trioxide, antimony oxychloride, antimony trisulfide, metal bismuth and an organic or inorganic, oxygenated or non-oxygenated bismuth salt; C) a free radical promoter in an amount of up to 1 % selected from 2,3-dimethyl-2,3-diphenyl butane, 2,3-dimethyl-2,3-diphenyl hexane and organic peroxides; D) from 1 to 5 % of melamine bromohydrate. Compositions according to claim 1, wherein the bismuth salt is selected from oxychloride, silicate, basic carbonate, basic nitrate, basic salycilate. Compositions according to claims 1 or 2, wherein the thermoplastic polymer is an olefinic polymer, polystyrene, polyamide, polyester resin, ABS copolymer. Compositions according to claims from 1 to 3, wherein melamine bromohydrate is present in amounts ranging from 3 to 5% by weight. Process for preparing polymeric compositions according to claims 1 to 4 by mixing the polymer with additives in a Banbury mixer, at a temperature equal or exceeding the polymer softening temperature and then extruding the mixture in an extruder, at the most suitable temperature to obtain a product in granules.

Inventors: BERTELLI, GUIDO, LOCATELLI, RENATO
Applicants: HIMONT INCORPORATED
Application year: 1986
Main classification: C08K5
IPC codes: C09K21, C08K3, C08K13, C08K5, C08L101