Flame-resistant polyolefinic fibres and films

Abstract: Disclosed are polyolefinic fibres and films which have been made flame-resistant by the addition of a flame-proofing additive, i.e., a copolyester containing units deriving from (a) terephthalic acid; (b) (C₂-C₆) alkylene glycol; and (c) phenyl hydroxymethylene phosphinic acid. These fibres and films are prepared by the transformation of a mixture of polyolefin and additive in the molten state and extrusion of said melt.

Description: The present invention relates to flame-resistant polyolefinic fibres and films, and to a process for their preparation. Polyolefins are widely used in the production of articles such as films and fibres and there is a great necessity of flame-resistant polyolefinic products, particularly for safety reasons, in the various fields where they are used, for example, in the textile industry. Various flame-proofing additives have been proposed in the art to give polyolefins flame-resistant properties. Generally, said additives consist of organic compounds containing phosphorus or halogen or both, or halogenated compounds combined with a metal oxide, as described by J.W. Lyons in "The Chemistry and Uses of Fire Retardants", Wiley Interscience 1970, pages 286-297. The flame-proofing additives already known are not, however, entirely satisfactory, mainly because of their incompatibility with polyolefins and also because of the high amounts required to result in the necessary flame-resistance. Flame-proofing compounds such as phenyl hydroxymethylene phosphinic acid and its alkyl esters of general formula (II): (where R is hydrogen or lower alkyl), are disclosed in US-A-4 517 355, together with the use thereof for making linear polyesters flame-resistant. Oligomeric flame-proofing compounds of general formula (III): (where R has the above meaning and n has an (average) value of from 2 to 50), are described in US-A-4 812 502, together with the use thereof for making linear polyesters flame-resistant. In EP-A-334 423, a linear copolyester is described, which is used as a flame-proofing agent and which contains in its macromolecule units deriving from (a) terephthalic acid; (b) (C 2 -C 6 ) alkylene glycol; and (c) phenyl hydroxymethylene phosphinic acid, in a ratio of from 1:1:0.2 to 1:1:1, and in which the units (c) are distributed at random between units (a) and (b). Said copolyester is obtained by briefly contacting, under melting conditions, a linear polyester derived from terephthalic acid and a (C 2 -C 6 ) alkylene glycol with an oligomer of formula (II). It has now been found that some of the copolyester, which are described in the above EP-A, can be mixed with polyolefins and can render fibres and films thereof flame-resistant. Only small quantities of copolyesters are needed for said purpose. In accordance with this, one of the aspects of the present invention concerns flame-resistant fibres and films comprising a polyolefin and, as flame-proofing additive, a copolyester containing units deriving from (a) terephthalic acid; (b) (C 2 -C 6 ) alkylene glycol; and (c) phenyl hydroxymethylene phosphinic acid and having the general formula (I): wherein: x is a number ranging from 2 to 6; y is a number ranging from 0.5 to 1; and z is a number ranging from 2 to about 7. The additive of formula (I) is, preferably, a copolyester containing units deriving from (a) terephthalic acid; (b) ethylene glycol (x = 2 in formula (I)); and (c) phenyl hydroxymethylene phosphinic acid. This additive has an average molecular weight ranging from about 700 to about 2000, a phosphorus content ranging from 6 to 8.8% by weight (calculated as pure element) and an amorphous or partially crystalline structure, with a softening or partial melting point of lower than 200°C, preferably ranging from 160 to 195°C. The melting point of the additive is an important characteristic for allowing it to homogenize in the molten state with a polyolefin. Another aspect of the present invention concerns a process for the preparation of flame-resistant polyolefinic fibres and films, which comprises mixing and homogenizing, in the molten state (temperatures about 240-280°C), the additive of formula (I) and polyolefinic polymer(s) and the subsequent transformation of the homogeneous mixture into fibre or film. The flame-resistant additive (I) can be added to the polyolefin after drying, for example, at 150°C under vacuum, or during processing in the molten state, before extrusion to form the fibre or film. According to the present invention, flame-resistant fibres and films can be obtained, starting from the usual polyolefins such as low, medium and high density polyethylene, polypropylene and the copolymers of two or more (alpha-)olefins. The most suitable polyolefin for this particular purpose is polypropylene. The amount of additive (I) to give flame-resistant properties should be sufficient to ensure a phosphorus content (calculated as element) in the polyolefinic fibre or film of, preferably, from 0.1 to 3.0% by weight, particularly of from 0.25 to 1.0% by weight. The following examples further illustrate the present invention without limiting the scope thereof in any way. EXAMPLE 1 3200 g of a prepolymer of polyethylene terephthalate having an average molecular weight of about 500 and 0.04% by weight of said prepolymer of phosphoric acid and antimony(III) oxide are charged into a 10 litre reaction vessel. The mixture is heated, under stirring, to 270-280°C, and the pressure is lowered from 760 Torr to less than 1 Torr over a period of 1.5 hours. When a fibre grade polyethylene terephthalate (intrinsic viscosity about 0.65 dl/mg) is obtained, 1400 g of an oligomer of general formula (III), wherein R is partly C 4 -alkyl and partly hydrogen and n has an average value of 4.1, with a phosphorus content of 19.4% by weight are added, using a suitable charging cylinder equipped with a heating jacket (temperature of 150°C). A temporary drop in the viscosity of the mixture and a sharp temperature drop are observed. The high vacuum is re-applied and within 15 minutes, a vacuum of 0.03 Torr and an intrinsic viscosity of about 0.40 dl/g are obtained. The product is discharged, at a temperature of 215°C, through a valve situated at the bottom of the reaction vessel, in the form of a thin strip which, after cooling, is cut into chips. The resulting flame-proofing additive is subjected to elemental analysis and acidimetric titration. The latter is carried out by dissolving the product in ortho-cresol and chloroform (1:1 by weight) and titrating it with an 0.5 M ethanol solution of tetraethylammonium hydroxide. In this manner, the molecular weight is obtained through the acidity of the product which, in its turn, is a function of the values of y and z in formula (I). More specifically, the additive thus obtained has a molecular weight of around 1456, a phosphorus content of 6.53% by weight and can be represented by the formula: The additive which on DSC (differential scanning calorimetry) shows a melting point of 194°C for the crystalline part, is used as a flame-proofing agent for polypropylene in the following example 2. EXAMPLE 2 The additive of example 1 is micronized and a mixture is prepared which contains: Polypropylene flakes (Moplen (R) FL F20; Himont) 943 g Additive of example 1 55 g Anox (R) (Enichem Synthesis) * 1 g Alkanox (R) 240 (Enichem Synthesis) * 1 g The dry blend is mixed for 10 minutes at 70 rpm in a Hentschel mixer and is then extruded in a Brabender extruder (19 mm screw), with a compression ratio of 4:1, equipped with a flat head. The following temperature profile is used: 200, 225, 235 and 250°C. Using a draw ratio of 3:1, an opalescent, 50 µm thick film, which shows good mechanical characteristics, is obtained. A second test (comparative) is carried out with a formulation of polypropylene, Anox (R) 20, Alkanox (R) 240 and a third test (comparative) is conducted with a formulation of polypropylene, Anox (R) 20, Alkanox (R) 240 and a commercial flame-proofing agent containing bromine derivatives and inorganic substances (Sandoflam (R) 5072, Sandoz) with the same quantity of additive as used in example 1. A 50 cm portion of each of the films obtained in the three above-mentioned tests, having a width of 70 mm and a thickness of 50 µm, are fixed to a vertical support at one of their ends. The performance of the films is evaluated by exposing them to a flame obtained by using a 10 mm diameter Bunsen burner, the oxidizing flame having a 19 mm high blue cone, for the time necessary to start burning over the whole width of 70 mm. In the case of the film used in the first test, which contains the additive of the present invention, the fire is extinguished almost immediately after the flame is removed. The film of the second test sample (without any flame-proofing agent) undergoes complete combustion, whereas the film of the third test sample (containing a commercial flame-proofing agent) requires extinguishment times of 2 to 3 seconds. In the latter case, there is also a development of fumes which are somewhat denser than in the other cases.

Claims: Flame-resistant fibres and films, comprising a polyolefin and, as flame-proofing additive, a copolyester containing units deriving from (a) terephthalic acid; (b) (C 2 -C 6 ) alkylene glycol and (c) phenyl hydroxymethylene phosphinic acid and having general formula (I): wherein: x is a number ranging from 2 to 6; y is a number ranging from 0.5 to 1; and z is a number ranging from 2 to about 7. Fibres and films according to claim 1, wherein said flame-proofing additive of formula (I) is a copolyester containing units deriving from (a) terephthalic acid; (b) ethylene glycol; and (c) phenyl hydroxymethylene phosphinic acid, and having an average molecular weight of from about 700 to about 2000, a phosphorus content of from 6 to 8.8% by weight, and an amorphous or partially crystalline structure, with a softening or partial melting point of lower than 200°C, preferably of from 160 to 195°C. Films and fibres according to any one of claims 1 and 2, wherein the additive of formula (I) is present in an amount such as to ensure a phosphorus content of from 0.1 to 3.0% by weight, preferably from 0.25 to 1.0% by weight, with respect to the fibre or film. Films and fibres according to any one of claims 1 to 3, wherein said polyolefin is low, medium and/or high density polyethylene, polypropylene or a copolymer of two or more olefins and, preferably, is polypropylene. Process for the preparation of flame-resistant films and fibres according to any one of claims 1 to 4, wherein the polyolefin and the flame-proofing additive of formula (I) are mixed and homogenized in the molten state, at a temperature of from 240 to 280°C, and the mixture thus obtained is extruded in the form of film or fibre.

Inventors: LANDONI GIANLUIGI, NERI CARLO, LANDONI, GIANLUIGI, NERI, CARLO
Applicants: GREAT LAKES CHEMICAL ITALIA, GREAT LAKES CHEMICAL ITALIA S.R.L.
Application year: 1991
Main classification: D01F1
IPC codes: D01F1, C08J5, D06M15, C08L23, C08K5, D01F6, C08G63, D06M101