POLYMER COMPOSITION

Abstract: A polymer composition, for example for a wire insulation or a heat-shrinkable article, comprises a polymer containing polyether units and a flame retardant package comprising red phosphorus and an ammonium phosphate e.g. ammonium polyphosphate or melamine phosphate. Preferably the polymer is a polyether ester and especially a segmented polyether ester block copolymer derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butanediol. The combination of red phosphorus and the ammonium phosphate exhits a synergistic improvement in flame retardancy in the polyether ester copolymers but no such improvement in similar polymers such as polyester ester copolymers that do not contain polyether units.

Description: Polymer Composition This invention relates to the flame retarding of polymer compositions, and especially to polymers containing polyether units. Polymer compositions have been flame retarded for many years by means of halogenated flame retardants, but these have the disadvantage that toxic and corrosive gases are evolved when the flame retarded compositions are burned. Accordingly a number of halogen-free flame retardants have been proposed for use with polymers, one such flame retardant being red phosphorus. The red phosphorus may be used on its own or may be mixed with co-agents in the flame retardant package. For example Japanese patent application publication No. 61-223045 to Sekisui KK describes the use of red phosphorus and ammonium phosphate to flame retard polyolefins. We have found that the performance of such flame retardants depends critically on the polymer into which they are incorporated and that in the case of certain polymers which contain polyether units a synergistic increase in flame retardancy can be observed when both red phosphorus and an ammonium phosphate are used. Accordingly, the present invention provides a polymer composition which comprises a polymer containing polyether units and a flame retardant comprising red phosphorus and an ammonium phosphate. Although such materials have been proposed as flame retardants in polyolefins, they do not show synergistic effects in polymers that are similar to those employed in the present invention. Furthermore, although red phosphorus has been proposed as a flame retardant in some polymers, on its own it appears to exhibit no significant flame retardant activity whatsoever in the polymers employed in the invention. The polymer is preferably a polyether ester. For example the polymer may be a polyether ester based thermoplastic elastomer and especially one based on a segmented copolyester polymer comprising recurring intralinear long chain ester units and short chain ester units randomly joined head-to-tail, the long chain ester units being represented by the formula: 0 0 O - G - O - C - R - C - and said short chain ester units being represented by the formula: 0 0 - 0 - D - 0 - C - R - C - where G is a divalent radical remaining after the removal of terminal hydroxyl groups from at least one long chain glycol having a molecular weight of about 600 - 6000; R is a divalent radical remaining after removal of carboxyl groups from at least one dicarboxylic acid having a molecular weight less than about 300; and D is a divalent radical remaining after removal of hydroxyl groups from at least one low molecular weight diol having a molecular weight less than 250. Exemplary of the preferred segmented copolyester polymers are the segmented polyether ester copolymers derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butanediol. These are random block copolymers having crystallizable hard blocks with the repeating unit and amorphous, elastomeric polytetramethylene ether terephthalate soft blocks of the repeating unit having a molecular weight of from about 600 - 3000, ie n = 6 - 40. Such polymers are commercially available from the Du Pont Co, under the registered trademark "Hytrel". Pertinent information regarding their structure, properties and methods of preparation are to be found in US Patent Nos. 3,023,192, 3,651,014, 3,763,109, 3,766,146 and 3,784,520 and in Belgian Patent No. 793,332, the disclosures of which are incorporated herein by reference. Additional information is found in "Segmented Polyether Ester Copolymers - A new Generation of Thermoplastic Elastomers" by G K Hoeschele published by the Elastomers Department E I Du Pont D Nemours Inc, Wilmington, De, and references cited therein, all of which are also incorporated by reference. The term "an ammonium phosphate" as used herein is intended to include both inorganic ammonium compounds and ammonium compounds derived from organic amines (provided that the organic moieties do not interfere with the effectiveness of the flame retardant. In addition, the term "phosphate" includes polyphosphates and salts derived from other oxyacids of phosphorus. The preferred ammonium phosphates are ammonium polyphosphate and melamine phosphate. The composition preferably contains at least 5% by weight, and more preferably at least 10% by weight of the ammonium phosphate. The flame retardancy appears to increase with the level of the ammonium phosphate used, so that it is preferably employed at levels of at least 15%, especially at least 20% by weight. Levels greater than 60% and especially 40% are generally not employed since at such high filler levels some deterioration of the physical properties of the polymer will occur. All percentages given herein are based on the total weight of the composition. The blend will normally contain at least 0.1% by weight red phosphorus. Preferably the blend contains at least 1%, more preferably at least 2% and especially at least 3% red phosphorus by weight. The composition will not normally contain more than 10%, preferably not more than 7% red phosphorus by weight. All percentages given herein are percentages by weight based on the total weight of the composition. For some applications the polymer composition may be crosslinked. They may be crosslinked by any convenient method, for example by irradiation or, by chemical crosslinking using, for example, a peroxide. Suitable peroxides are those that decompose rapidly within the range of 150°C - 250°C. These include, for example, dicumyl peroxide, 2,5-bis (t-butylperoxy)-2,5-dimethylhexane, and t , * -bis (t- butylperoxy) di-isopropylbenzene. In a typical chemically crosslinkable composition there will be about 0.5 - 5 parts by weight of peroxide per 100 parts of polymer composition. The peroxide may be absorbed on an inert carrier such as a calcium carbonate, carbon black, or kieselguhr; however, the weight of the carrier is not included in the above range. Preferably, however, the polymer compositions of the present invention are crosslinked using high energy radiation. Radiation dose levels to achieve crosslinking according to the present invention may range from about 20 to 800 kGy or more, but a dose of about 50 to 500 kGy is preferred. For most purposes a dose of about 80 to 200 kGy will be effective. In some cases it may be desirable to add to the crosslinkable polymer composition a co-agent to assist in the crosslinking reaction. Such co-agents usually contain multiple unsaturated groups such as allyl or acrylic esters. While their mode of action is not known with certainty, it is believed that they react with the initial radical formed on the polymer backbone to form a more stable radical, which undergoes a coupling reaction to form crosslinks more readily than chain scission reactions. The co-agent can be for example, N,N'-m(phenylene)- dimaleimide, trimethylolpropane trimethylacrylate, tetra(allyloxy)- ethane, triallyl cyanurate, triallyl isocyanurate, or polyethylene oxide glycol dimethacrylate. The amount of the co-agent is preferably up to about 5 parts by weight per 100 parts of the polymer composition and preferably from 1 to 3 parts by weight per 100 parts of the polymer composition. Crosslinked polymer compositions according to the present invention may be used in a wide range of applications, and the preferred compositions find particular application as electrical insulation or jacketing in electrical wire or cable or in electrical harnesses. For example they may be used for primary insulation and/or primary jackets in electrical wires or they may be used as extruded cable jackets. Compositions according to the invention, especially crosslinked ones, may be used for the production of heat-recoverable articles for a wide variety of purposes. A heat-recoverable article is one which is in a dimensionally unstable condition and is capable of altering its physical form upon the application of heat alone to assume a dimensionally heat stable condition. Preferably the article is elastomeric in both its heat unstable and heat stable forms. Heat-recoverable articles may be produced for example deforming an article under heat and pressure from an original dimensionally heat stable form to a dimensionally heat unstable form from which it is capable of recovery towards it original form upon the application of heat alone. Heat-recoverable articles and methods for their production are described, for example in US Patents Nos. 2,027,962, 3,086,242 and 3,957,372. Other polymers may be blended with the composition according to the invention. In the broadest aspect of the invention any polymer may be blended with the composition provided that it does not destroy the improvement in thermal stability of the polyether polymer given by the red phosphorus. Preferred polymers for blending with the compositions include ethylene homo- and copolymers eg. low, medium or high density polyethylene and ethylene vinyl acetate copolymers (or so-called vinyl acetate ethylene copolymers which have a higher vinyl acetate content), ethylene acrylic acid copolymers, polyamides (including polyether amides), and polyesters for example polybutylene terephthalate. Examples of ethylene acrylic acid copolymers include ternary copolymers such as those sold by DuPont under the tradename "Vamac". In addition to the polymer(s) and red phosphorus, the composition according to the invention may contain additional conventional additives for example fillers, ultraviolet stabilizers, antioxidants, acid acceptors, anti-hydrolyis stabilizers, pigments, fungicides and the like. Such additives will not normally be employed other than in small quantities, eg. up to 30% by weight, although in some instances eg, in the case of halogen-free flame retardants, larger quantities may be employed. The following Examples illustrate the invention: Example 1 The following components were blended using a Baker Perkins MPC V30 twin screw mixer with dump extruder: Component Parts bv Weight *Hytrel 4056/Hytrel 10 MS 9:1 100 50% encapsulated f ed - Phosphorus ' -- Masterbatch 12 50% Hytrel 4056 J Ammonium polyphosphate 0-30 Crosslinking promoter 5 (triallyl cyanurate) *Hytrel: tradename for segmented polyetherester block copolymers having polybutylene ether soft blocks and polybutylene terephthalate hard blocks. "{"Encapsulated in a melamine formaldehyde resin. The components were blended, extruded, injection moulded and then irradiated with high energy electrons to a dose of 100 kGy. The samples were tested for flame retardancy by measuring their limiting oxygen index (LOI) in accordance with ASTM D2863. This test essentially determines the minimum percentage of oxygen in an oxygen/nitrogen mixture that will support a flame when the polymer is burned. The results are given in Table I (with a precision of ±0.5) and are shown graphically in Figure 1 from which it can be seen that the flame retardancy of the composition in accordance with the invention increases with ammonium polyphosphate level more steeply than does the flame retardancy of a composition containing no phosphorus, thereby indicating a synergistic effect of the combination. Table I The LOI of the polymer blend containing no ammonium polyphosphate was determined for various levels of phosphorus. It was found that the LOI remained constant at 19.5 for phosphorus levels between 0% and 10%. Example 2 Example 1 was repeated with the exception that the ammonium polyphosphate was replaced with melamine phosphate. The results are given in Table II and are shown graphically in Figure 2 from which it can be seen that rate of increase in LOI with melamine phosphate content is greater in the presence of red phosphorus, indicating a synergistic effect. Table II Comparative Example Example 1 and 2 were repeated with the exception that the polyetherester polymer were replaced with a polyester ester block copolymer sold by Akzo NV under the trade name Arnitel. The LOI results are given in Tables III and IV and are shown graphically in Figures 3 and 4, from which it can be seen that the presence of red phosphorus alone increases the LOI by about 2 units but the presence of both red phosphorus and the ammonium phosphate does not exhibit any synergism as shown by the fact that the rate of increase of LOI with phosphate level is unchanged by the presence of red phosphorus. Table III Table IV

Claims: Claims: 1. A polymer composition which comprises a polymer containing polyether units and a flame retardant comprising red phosphorus and an ammonium phosphate. 2. A composition as claimed in claim 1, wherein the polymer is a polyether ester. 3. A composition as claimed in claim 2, wherein the polymer comprises a segmented copolyester polymer consisting essentially of recurring intralinear long chain ester units and short chain ester units randomly joined heat-to-tail, the long chain ester units being represented by the formula: 0 0 - O - G - O - C - R - C - and said short chain ester units being represented by the formula: 0 0 - O - D - O - C - R - C - where G is a divalent radical remaining after the removal of terminal hydroxyl groups from at least one long chain glycol having a molecular weight of about 600 - 6000; R is a divalent radical remaining after removal of carboxyl groups from at least one dicarboxylic acid having a molecular weight less than about 300; and D is a divalent radical remaining after removal of hydroxyl groups from at least one low molecular weight diol having a molecular weight less than 250. 4. A composition as claimed in claim 3, wherein the polymer comprises a segmented polyether ester copolymer derived from terephthalic acid, polytetramethylene ether glycol and 1,4-butanediol, having the repeating unit and in which n = 6 to 40. 5. A composition as claimed in any one of claims 1 to 4, wherein the ammonium phosphate comprises melamine phosphate. 6. A composition as claimed in any one of claims 1 to 5, wherein the ammonium phosphate comprises ammonium polyphosphate. 7. A composition as claimed in any one of claims 1 to 6, which contains not more than 10% by weight red phosphorus. 8. A composition as claimed in any one of claims 1 to 7, which contains at least 1% by weight red phosphorus. 9. A composition as claimed in any one of claims 1 to 8, which contains not more than 30% by weight of the phosphate. 10. A composition as claimed in any one of claims 1 to 9, which contains at least 1% by weight of the phosphate. 11. A composition as claimed in any one of claims 1 to 10 which is crosslinked. 12. A dimensionally heat-recoverable article which comprises a composition as claimed in any one of claims 1 to 11. 13. An electrical wire or cable or an electrical harness which includes insulation of jacketing formed from a composition as claimed in any one of claims 1 to 11. 14. The use of red phosphorus and an ammonium phosphate to flame retard a polymer that contains polyether units. φ ^ * *

Inventors: HOOD KATHLEEN, HOOD, KATHLEEN
Applicants: RAYCHEM LTD, RAYCHEM LIMITED
Application year: 1992
Main classification: H01B3
IPC codes: C08K3, C08K5, C08L67, C08L71, H01B3, H01B7