GB1579779A - Flame-retardant branched polyester compositions - Google Patents

Description

(54) FLAME-RETARDANT BRANCHED POLYESTER COMPOSITIONS (71) We, GENERAL ELECTRIC COMPANY, a corporation organised and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, New York, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to flame-retardant thermoplastic polyester compositions having improved melt strength. More particularly, it pertains to thermoplastic compositions comprising a branched, high molecular weight poly(alkylene terephthalate or isophthalate) and a flame-retardant.

High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. 2,465,319 and in Pengilly, U.S. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers. Articles manufactured from poly(alkylene terephthalates or isophthalates) have many valuable characteristics, including strength, toughness, solvent resistance, and high gloss. These articles may be fabricated by a number of well-known techniques, including injection molding, roto molding, blow molding, and extrusion, depending on the shape of the desired product.

Certain of these techniques, in particular, blow molding and extrusion, require that the molten poly(alkylene terephthalates or isophthalates) have a suitably high melt viscosity, e.g., in excess of 10,000 poises, to prevent collapse or blow-outs in the soft preformed state.

It has been found that poly-(alkylene terephthalates or isophthalates) of such high melt viscosity are obtained only with great difficulty in the conventional bulk melt polymerization processes generally used to prepare the polyester.

A useful family of such compositions are those which are rendered flame-retardant by the incorporation of a flame-retarding amount of a flame retardant and, optionally, the incorporation of a reinforcing agent, such as filamentous glass.

Unfortunately, conventional flame-retardant poly-(alkylene terephthalate or isophthalate) compositions have insufficient melt strength to be successfully extruded into continuous and reproducible profiles. Nor can these conventional flame-retarded polyester compositions be thermoformed without excessive sag or sufficient stability of the dimensions during the forming process.

It has now been discovered that the use of branched, poly(alkylene terephthalate or isophthalate) resins in place of the unbranced resin in these flame-retarded compositions results in compositions having much improved melt strength so as to result in satisfactory processing of both satisfactory thermoformed parts and reproducible extruded profiles.

According to the present invention, there are provided thermoplastic compositions, with improved melt strength, for molding, e.g., injection molding, compression molding, and transfer molding, comprising: (a) a branched, high-molecular weight poly(alkylene terephthalate or isophthalate) resin having a melt viscosity of at least 15,000 poise, comprising a high molecular weight poly(alkylene terephthalate or isophthalate) and from 0.05 to 3 mole percent, based on the terephthalate or isophthalate units, of a branching component which contains at least three ester-forming groups; and (b) a flame retarding amount of a flame-retardant (as hereinafter defined) The term "high molecular weight poly(alkylene terephthalate or isophthalate) resin" includes, in general, saturated condensation products of diols and terephthalic or isophthalic acids or reactive derivatives thereof. Preferably, they will comprise condensation products of terephthalic or isophthalic acids or esters and aliphatic diols. It is understood that it is also possible to include cyclic aliphatic linkages, such as those derived from l,4-dimethylolcyclohexane. In addition to the terephthalic or isophthalic acid units, other dicarboxylic acids, such as adipic acid, naphthalene dicarboxylic acid, isophthalic and o-phthalic acid can also be present e.g., from 0.5 to 15 mole percent of the total acid units.

The alkylene constitutent can likewise be varied, and may comprise units derived from ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexane-diol, 1,10-decanediol, the cyclo aliphatic diols, and mixtures thereof. Preferably, the alkylene units will contain from 2 to 6 carbon atoms, and especially preferably, the alkylene units will be 1,4-butylene units, because these provide polyesters which crystallize very rapidly from the melt.

Preferred polyesters will be of the family consisting of high molecular weight, polymeric glycol terephthalates or isophthalates having repeating units of the general formula

wherein n is a whole number of from two to four, and mixtures of such esters, including copolyesters of terephthalic and isophthalic acids of up to 30 mole percent isophthalic units.

Especially preferred polyesters are poly(ethylene terephthalate) and poly(1,4-butylene tcrephthalatel. Special mention is made of the latter because it crystallizes at such a good rate that it may be used for injection molding without the need for nucleating agents or long cycles, as is sometimes necessary with poly(ethylene terephthalate).

The branching component used in the polyesters will contain at least three ester forming groups. It can be one which provides branching in the acid unit portion of the polyester, or in the glycol unit portion, or be a hybrid. Illustrative of such branching components are trior tetracarboxylic acids, such as trimesic acid, pyromellitic acid and lower alkyl esters thereof, or, preferably, polyols, and especially preferably tetrols, such as pentaerythritol, triols, such as trimethylolpropane, or dihydroxy carboxylic acids and hydroxydicarboxylic acid and derivatives, such as dimethyl hydroxy-terephthalate.

The range of branching component included in the esterification mixture (and, generally, that included in the product), will be from 0.05 to 3 mole percent based on the terephthalate or isophthalate units. Especially preferably, it will comprise from 0.1 to 1 mole percent, based on the terephthalate or isophthalate component.

Processes for preparing such polyesters will be well known to those skilled in the art. The description in U.S. 2,465,319 and U.S. 3,047,539 are helpful, as are those in U.S.

3,692,744. In general, it is convenient to add small amounts of the branching components to the terephthalic or isophthalic acid or ester and an excess of the alkylene glycol in the presence of a conventional polyester catalyst then to heat to form a pre-polymer and finally to heat under a high vacuum until the desired degree of polymerization is reached.

The molecular weight of the branched polyester should be sufficiently high to provide a melt viscosity of at least 15,000 poise but not in excess of that wherein the branched polyester is no longer thermoplastic. For example, the molecular weight of the branched polyester should be sufficient to provide a melt viscosity in the range of from 15,000 poise to 100,000 poise.

Illustrative flame retardants, useful in the practice of the present invention, are disclosed in U.S. 3,833,685, U.S. 3,341,154, U.S. 3,915,926 and U.S. 3,671,487. Other flame rctardants are disclosed in U.S. 3,681,281, U.S. 3,557,053, U.S. 3,830,771 and U.K.

1,358,080.

In addition to the essential carbonate flame retardants, to be defined in more detail below, it is possible to incorporate other well known flame retardant additives. Generally speaking, the more important of these compounds contain chemical elements employed for their ability to impart flame resistance, e.g., bromine, chlorine, antimony, phosphorus and nitrogen. It is preferred that the flame retardant additive comprises a halogenated organic compound (brominated or chlorinated), a halogen-containing organic compound in admixture with antimony oxide; elemental phosphorus or a phosphorus compound; a halogen-containing compound in admixture with a phosphorus compound or compounds containing phosphorus-nitrogen bonds or a mixture of two or more of the foregoing.

The amount of flame retardant additive used is not critical to the invention, so long as it is present in a minor proportion based on said composition - major proportions will detract from physical properties - but at least sufficient to render the branched polyester resin non-burning or self-extinguishing. Those skillcd in the art are well aware that the amount will vary with the nature of the resin and with the efficiency of the additive. In general, however, the amount of additive will be from 0.5 to 50 parts by weight per 100 parts of resin. A preferred range will be from 3 to 25 parts and an especially preferred range will be from 8 to 12 parts of additive per 100 parts of resin. Smaller amounts of compounds highly concentrated in the elements responsible for flame retardance will be sufficient, e.g., elemental red phosphorus will be preferred at 0.5 to 2.0 parts by weight per 100 parts of resin, while phosphorus in the form of triphenyl phosphate will be used at 25 parts of phosphate per 100 parts of resin and so forth. Halogenated aromatics will be used at 8 to 12 parts and synergists, e.g., antimony oxide will be used at 2 to 5 parts by weight per 100 parts of resin.

Among the useful halogen-containing compounds are those of the formula:

wherein R is an alkenylene, alkylidene or cycloaliphatic linkage, e.g., methylene, ethylene, propylene, isopropylene, isopropylidene, butylene, isobutylene, amylene, cyclohexylene, or cyclopentylidene; a linkage selected from ether, carbonyl, amine; a sulfur-containing linkage e.g., sulfide, sulfoxide, or sulfone; or a phosphorus-containing linkage. R can also consist of two or more alkylene or alkylidene linkages connected by such groups as aromatic, amino, ether, carbonyl, sulfide, sulfoxide, sulfone, or a phosphorus-containing linkage.

Ar and Ar' are mono- or polycarbocyclic aromatic groups such as phenylene, biphenylene, terphenylene, or napthylene. Ar and Ar' may be the same or different.

Y is an organic, inorganic, or organometallic substituent. The substituents represented by Y include (1) halogen, e.g., chlorine, bromine, iodine, or fluorine or (2) hydroxy or ether groups of the general formula OE, wherein E is a monovalent hydrocarbon radical similar to X or (3) monovalent hydrocarbon groups of the type represented by X or (4) other substituents, e.g., nitro, or cyano, said substituents being essentially inert provided there be at least one and preferably two halogen atoms per aryl, e.g., phenyl nucleus.

X is a monovalent hydrocarbon group exemplified by the following: alkyl, such as methyl, ethyl, propyl, isopropyl, butyl or decyl; aryl groups, such as phenyl, naphthyl, biphenyl, xylyl, or tolyl; aralkyl groups, such as benzyl or ethylphenyl; cycloaliphatic groups such as cyclopentyl, or cyclohexyl; as well as a monovalent hydrocarbon group containing inert substituents therein. It will be understood that where more than one X is used, they may be alike or different.

The letter d represents a whole number ranging from 1 to a maximum equivalent to the number of replaceable hydrogens substituted on the aromatic rings comprising Ar or Ar'.

The letter e represents 0 or a whole number ranging from 1 to a maximum controlled by the number of replaceable hydrogens on R. The letters a, b and c represent 0 or whole numbers. When b is not 0, neither a nor c may be O. Otherwise either a or c, but not both, may be O. Where b is 0, the aromatic groups are joined by a direct carbon-carbon bond.

The hydroxyl and Y substituents on the aromatic groups, Ar and Ar' can be varied in the ortho, meta or para positions on the aromatic rings.

Included within the scope of the above formula are the following 2,2-bis-(3,5-dichlorophenyl)propane bis-(2-chlorophenyl)methane bis(2,6-dibromophenyl)methane 1, 1-bis-4(4-iodophenyl)ethane 1 ,2-bis-(2,4-dichlorophenyl)ethane 1,1-bis-(2-chloro-4-iodophenyl)ethane 1, 1-bis-(2-chloro-4-methylphenyl)ethane 1 ,1-bis-(3 ,5-dichlorophenyl)ethane 2,2-bis-(3-phenyl-4-bromophenyl)ethane 2,6-bis-(4,6-dichloronaphthyl)propane 2,2-bis-(2,6-dichlorophenyl)pentane 2,2-bis-(3,5-dichlorophenyl)hexane bis-(4-chlorophenyl)phenylmethane bis-(3 ,5-dichlorophenyl)cyclohexylmethane bis-(3-nitro-4-bromophenyl)methane bis-(4-hydroxy-2,6-dichloro-3-methoxyphenyl)methane 2,2-bis-(3,5-dichloro-4-hydroxyphenyl)propane 2,2-bis-(3-bromo-4-hydroxyphenyl)propane The preparation of these components is known in the art. In place of the divalent aliphatic group in the above examples, other groups, e.g. sulfide or sulfoxy, may be substituted.

Included within the above structural formula are substituted benzenes exemplified by 1,3-dichlorobenzene, 1,4-dibromobenzene, 1,3-dichloro-4-hydroxybenzene, hexachlorobenzene, hexabromobenzene, and biphenyls such as 2,2'-dichlorobiphenyl, 2,4'dibromobiphenyl, and 2,4'-dichlornbiphenyl.

The essential flame retardant additives employed according to the invention include aromatic carbonate homopolymers having repeating units of the formula:

wherein R' and R2 are hydrogen, alkyl or phenyl, X' and X2 are bromine or chlorine and m and r are from 1 to 4, and aromatic carbonate copolymers in which from 25 to 75 weight percent of the repeating units comprise chlorine- or bromine substituted dihydric phenol units and the remainder of the repeating units comprise dihydric phenol, glycol or dicarboxylic acid units. See, e.g., A.D. Wambach, U.S. 3,915,926, above-mentioned.

In addition, preferred are aromatic halogen compounds such as chlorinated benzene, brominated benzene, chlorinated biphenyl, chlorinated terphenyl, brominated biphenyl, brominated terphenyl, or a compound comprising two phenyl radicals separated by a divalent alkylene or oxygen group and having at least two chlorine or bromine atoms per phenyl nucleus, and mixtures of at least two of the foregoing.

In general, the preferred optional phosphorus compounds are selected from elemental phosphorus or organic phosphonic acids, phosphonates, phosphinates, phosphonites, phosphinites, phosphine oxides, phosphines, phosphites or phosphates. Illustrative are triphenyl phosphine oxide. This can be used alone or mixed with hexabromobenzene or a chlorinated biphenyl and, optionally, antimony oxide.

Typical of the preferred optional phosphorus compounds to be employed in this invention would be those having the general formula:

in which X = S or 0, and n = O or 1, Y', Y" and Y"' are the same or different and represent alkyl, cycloalkyl, aryl, alkyl substituted aryl, halogen substituted aryl. aryl substituted alkyl, alkyloxy, cycloalkyloxy, halogen substituted alkyloxy, aryloxy, halogen substituted aryloxy, or halogen. Two of the Y's may be combined into a cyclic structure. or one or two of the Y's may be difunctional, in which case the compounds consist of short or long chain compounds containing a plurality of P atoms per molecule. Typical examples of suitable phosphorus compounds include: triphenyl phosphate, diphenyl phenyl phosphonate, phenyl diphenyl phosphinate, triphenyl phosphine, triphenyl phosphine oxide, tris(p-bromophenyl) phos phase neopentyl phenyl phosphonate. tris-(dibromopropyl) phosphate, dibenzyl phenyl phosphonate, poly-(1,4 cyclo hexylene dimethylene) phenyl phosphonate, and pentaerythritol bis(p-hromophenyl) phosphonate. A preferred flame retardant is tris(tribromophenyl) phosphate.

Also suitable as optional flame retardant additives for this invention are compounds containing phosphorus-nitrogen bonds. such as phosphonitrilic chloride, phosphorus ester amides, phosphoric acid amides, phosphonic acid amides, phosphinic acid amides, tris(aziridinyl) phosphine oxide or tetrakis(hydroxymethyl) phosphonium chloride. These flame retardant additives are commercially available.

Reinforcing agents which may be optionally included in the composition of the present invention are well known but illustratively, they may be selected from metals, such as aluminum, iron or nickel particles, and non-metals, such as carbon filaments, silicates, such as acicular calcium silicate, asbestos, titanium dioxide, potassium titanate and titanate whiskers, wollastonite, glass flakes and fibers. It is also to be understood that unless the filler adds to the strength and stiffness of the composition, it is only a filler and not a reinforcing filler as contemplated herein.

Although it is only necessary to have at least a reinforcing amount of the reinforcement present, in general the reinforced compositions will comprise from 10 to 60% by weight of the total composition of the reinforcing agent.

In particular, the preferred reinforcing fillers are of glass, and it is usually preferred to employ fibrous glass filaments comprised of lime-aluminum borosilicate glass that is relatively soda-free. This is known as "E" glass. However, other glasses are useful where electrical properties are not important, e.g., the low soda glass known as "C" glass. The filaments are made by standard processes, e.g., by steam or air blowing, flame blowing and mechanical pulling. The filament diameters range from 0.00012 to 0.00075 inch, but this is not critical to the present invention. Glass fibers may be surface coated in accordance with standard procedures to improve their reinforcing performances. In general, best properties will be obtained from reinforced compositions that contain from 15 to 30 percent by weight of the glass reinforced composition.

The length of glass filaments and whether or not they are bundled into fibers and the fibers bundled, in turn to yarns, ropes or rovings, or woven into mats, are also not critical to the practice of the invention. In preparing the present compositions, it is convenient to use the filamentous glass in the form of chopped strands of from 1/8 inch to 1 inch long, preferably less than 1/4 inch long. In articles that are molded from the compositions of the invention, even shorter lengths will be encountered because, during compounding, considerable fragmentation will occur. This is desirable, however, because the best properties are exhibited by thermoplastic injection molded articles in which the filament lengths lie between 0.000005 inch and 0.12 (1/8 inch).

The compositions of this invention can be prepared by a number of procedures. In one way, the flame-retardant and reinforcement, e.g., glass fibers, if desired, are put into an extrusion compounder with the branched polyester resin to produce molding pellets. The flame-retardant and, optionally, the reinforcing agent, are dispersed in a matrix of the branched polyester resin in the process. In another procedure, the flame-retardant and reinforcing agent, if used, are mixed with the branched polyester resin by dry blending, then either fluxed on a mill and comminuted, or they are extruded and chopped. The flame-retardant and reinforcing agent, if employed, can also be mixed with the powdered or granular branched polyester and directly molded, e.g., by injection or transfer molding techniques.

It is always important thoroughly to free all of the ingredients: resin, flame-retardant and any optional, conventional additives such as reinforcements, from as much water as possible.

In addition, compounding should be carried out to ensure that the residence time in the machine is short; the temperature is carefully controlled; the friction heat is utilized; and an intimate blend between the resin, flame-retardant and optional ingredients, such as the reinforcement is obtained.

Although it is not essential, best results are obtained if the ingredients are precompounded, pelletized and then molded. Pre-compounding can be carried out in conventional equipment. For example, after carefully pre-drying the branched polyester resin, the flame-retardant and the reinforcing agent, (if used), e.g., under vacuum at 100"C, for 12 hours, a single screw extruder is fed with a dry blend of the ingredients, the screw employed having a long transition section to ensure proper melting. On the other hand, a twin screw extrusion machine, e.g., a 28 mm Werner Pfleiderer machine can be fed with resin and additives at the feed port and reinforcement downstream. In either case, a generally suitable machine temperature will be 450 to 460"F.

The pre-compounded composition can be extruded and cut up into molding compounds such as conventional granules, or pellets, by standard techniques.

The compositions can be molded in any equipment conventionally used for thermoplastic compositions. For example, with poly(1,4-butylene terephthalate), good results will be obtained in an injection molding machine, e.g., of the Newbury type with conventional cylinder temperatures, e.g., 450"F and conventional mold temperatures, e.g., 1500F. On the other hand, with poly(ethylene terephthalate), because of the lack of uniformity of crystallization from interior to exterior of thick pieces, somewhat less conventional but still well-known techniques can be used. For example, a nucleating agent such as graphite or a metal oxide, e.g., ZnO or MgO can be included and standard mold temperature of at least 230"F will be used.

The following example illustrates the invention.

EXAMPLE A dry blend of 69 parts by weight of lightly branched poly(1,4-butylene terephthalate) resin containing 0.21 mole percent pentaerythritol as a branching component and having a melt viscosity of 40,000 to 50,000 poise, 26 parts by weight of aromatic (copoly-) carbonate of a 50:50 mole ratio bisphenol A and tetrabromobisphenol A, 5 parts by weight of antimony tin oxide and minor amounts of conventional stabilizers is compounded and extruded at 4500 - 500"F in an extruder. After molding in a 3 oz. Van Dorn reciprocating screw injection molding machine, the properties obtained are as follows: Properties Melt viscosity at 482"F, poise 64,068 Heat Deflection Temp. at 264 psi, "F 205 Notched Izod Impact Strength, ft. Ibs./in. 0.88 Gardner Impact Strength at 300 in. Ibs. 10/10 pass Tensile strength, psi 8,704 Tensile Elongation at Break, % 121 Flammability per UL-94 at 1/16" thick 94 V-O When extruded, the composition provides continuous and reproducible profiles. When thermoformed, it produces parts without extensive sag and excellent dimensional stability.

When the example is repeated substituting a linear, i.e., unbranched, poly(1,4-butylene terephthalate), the composition results in molded articles with lower resistance to distortion by heat, lower notched impact strength and lower elongation at break. Most significantly, however, this substitution leads to a complete loss in the ability to be extruded into continuous and reproducible profiles. Moreover, the control composition exhibits excessive sag during thermoforming and thermoformed parts have a wide variety of part-to-part dimensional measurements.

Trimethyolethane, trimethylolpropane or trimethyl trimesate can be substituted for the pentaerythritol as the branching component in the Example.

WHAT WE CLAIM IS: 1. A thermoplastic composition which comprises: (a) a branched high molecular weight poly(alkylene terephthalate or isophthalate) resin, having a melt viscosity of at least 15,000 poise, comprising a high molecular weight poly(alkylene terephthalate or isophthalate) and 0.05 to 3 mole %, based on the terephthalate or isophthalate units, of a branching component which contains at least three ester-forming groups; and (b) a flame-retarding amount of a flame-retardant which is an aromatic carbonate homopolymer having repeating units of the formula:

wherein R' and R2 are hydrogen, alkyl or phenyl, Xl and X2 are bromine or chlorine and m and r are integers from 1 to 4, an aromatic (copoly-) carbonate in which from 25 to 75 wt. % of the repeating units comprise chlorine or bromine-substituted dihydric phenol units and the remainder of the repeating units comprise dihydric phenol, glycol or dicarboxylic acid units, or a mixture thereof.

2. A composition as claimed in Claim 1 wherein the branched poly(alkylene terephthalate or isophthalate) resin comprises from 0.1 to 1 mole % of the branching component.

3. A composition as claimed in Claim 1 or 2 wherein the branching component is a tricarboxylic acid, a tetracarboxylic acid or an alkyl ester thereof.

4. A composition as claimed in Claim 3 wherein the branching component is trimethyl trimesate.

5. A composition as claimed in Claim 1 or 2 wherein the branching component is a poly.

6. A composition as claimed in Claim 5 wherein the branching component is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. resin containing 0.21 mole percent pentaerythritol as a branching component and having a melt viscosity of 40,000 to 50,000 poise, 26 parts by weight of aromatic (copoly-) carbonate of a 50:50 mole ratio bisphenol A and tetrabromobisphenol A, 5 parts by weight of antimony tin oxide and minor amounts of conventional stabilizers is compounded and extruded at 4500 - 500"F in an extruder. After molding in a 3 oz. Van Dorn reciprocating screw injection molding machine, the properties obtained are as follows: Properties Melt viscosity at 482"F, poise 64,068 Heat Deflection Temp. at 264 psi, "F 205 Notched Izod Impact Strength, ft. Ibs./in. 0.88 Gardner Impact Strength at 300 in. Ibs. 10/10 pass Tensile strength, psi 8,704 Tensile Elongation at Break, % 121 Flammability per UL-94 at 1/16" thick 94 V-O When extruded, the composition provides continuous and reproducible profiles. When thermoformed, it produces parts without extensive sag and excellent dimensional stability. When the example is repeated substituting a linear, i.e., unbranched, poly(1,4-butylene terephthalate), the composition results in molded articles with lower resistance to distortion by heat, lower notched impact strength and lower elongation at break. Most significantly, however, this substitution leads to a complete loss in the ability to be extruded into continuous and reproducible profiles. Moreover, the control composition exhibits excessive sag during thermoforming and thermoformed parts have a wide variety of part-to-part dimensional measurements. Trimethyolethane, trimethylolpropane or trimethyl trimesate can be substituted for the pentaerythritol as the branching component in the Example. WHAT WE CLAIM IS:

1. A thermoplastic composition which comprises: (a) a branched high molecular weight poly(alkylene terephthalate or isophthalate) resin, having a melt viscosity of at least 15,000 poise, comprising a high molecular weight poly(alkylene terephthalate or isophthalate) and 0.05 to 3 mole %, based on the terephthalate or isophthalate units, of a branching component which contains at least three ester-forming groups; and (b) a flame-retarding amount of a flame-retardant which is an aromatic carbonate homopolymer having repeating units of the formula:

wherein R' and R2 are hydrogen, alkyl or phenyl, Xl and X2 are bromine or chlorine and m and r are integers from 1 to 4, an aromatic (copoly-) carbonate in which from 25 to 75 wt. % of the repeating units comprise chlorine or bromine-substituted dihydric phenol units and the remainder of the repeating units comprise dihydric phenol, glycol or dicarboxylic acid units, or a mixture thereof.

2. A composition as claimed in Claim 1 wherein the branched poly(alkylene terephthalate or isophthalate) resin comprises from 0.1 to 1 mole % of the branching component.

3. A composition as claimed in Claim 1 or 2 wherein the branching component is a tricarboxylic acid, a tetracarboxylic acid or an alkyl ester thereof.

4. A composition as claimed in Claim 3 wherein the branching component is trimethyl trimesate.

5. A composition as claimed in Claim 1 or 2 wherein the branching component is a poly.

6. A composition as claimed in Claim 5 wherein the branching component is

pentaerythritol or trimethylolpropane.

7. A composition as claimed in Claim 1 or 2 wherein the branching component is a hydroxydicarboxylic acid, a dihydroxycarboxylic acid or an ester thereof.

8. A composition as claimed in any preceding Claim wherein the flame retardant is an aromatic (copoly-)carbonate of substantially equimolar amounts of bisphenol A and tetabromobisphenol A.

9. A composition as claimed in any of Claims 1 to 7 wherein the flame retardant includes antimony trioxide.

10. A composition as claimed in any preceding Claim which further includes a reinforcing amount of a reinforcing agent.

11. A composition as claimed in Claim 10 wherein said reinforcing agent is glass fibers.

12. A composition as claimed in any preceding Claim wherein the high molecular weight poly(alkylene terephthalate or isophthalate) is a polymeric glycol terephthalate or isophthalate ester having repeating units of the formula:

wherein n is 2, 3 or 4, or a mixture of such esters.

13. A composition as claimed in Claim 12 wherein the high molecular weight poly(alkylene terephthalate or isophthalate) is poly-(1 ,4-butylene terephthalate).

14. A composition as claimed in Claim 1 and substantially as hereinbefore described with reference to the Example.