Glass-fiber reinforced poly ( 1 , 4-butylene terephthalate) , hereinafter abbreviated PBT/GF, is used in various insulating parts for electrical and electronic devices. The commonly used grades are generally reinforced with 30 wt% glass fibers based on the total weight of the composition (hereinafter abbreviated

Many polymers in commercial use contain flame retardants to reduce their flammability and PBT/GF is no exception to this rule. PBT/GF, e.g., PBT/GF30, is more difficult to flame retard than the non-reinforced polymer. The flammability characteristics of plastic materials are usually quantifiable according to a method specified by Underwriter Laboratories standard UL 94, where an open flame is applied to the lowermost edge of a vertically mounted test specimen made of the tested polymer formulation. The specimens used in the UL 94 test method vary in thickness (typical thicknesses are -3.2 mm, -1.6 mm, -0.8 mm and -0.4 mm) . During the test, various features of the flammability of the test specimens are recorded. Then, according to the classification requirements, the polymer formulation is assigned with either V-0, V-l or V-2 rating at the measured thickness of the test specimen. Polymer formulation assigned with the V-0 rating is the less flammable. Furthermore, in the UL-94 burning test, the thinner the specimens are, the longer the burning time. Therefore, the requirements of UL 94 V-0 rating for thin PBT/GF30 test specimens (e.g., 0.8 or 0.4 mm thick samples) are not easily met .

Metal salts of hypophosphorous acid, that is, metal hypophosphite, have been found to be effective flame retardants in polyesters, with the predominance of literature focusing on aluminum hypophosphite . In US 7,700,680 it was shown that aluminum hypophosphite, A1(H2P02)3, can be used in PBT/GF, in combination with melamine cyanurate. Yet, the experimental results reported in Table 2 of US 7,700,680 suggest that the aforementioned combination does not fulfill the requirements of UL 94 V-0 rating for thin PBT/GF30 specimens (e.g., 0.8 mm or 0.4 mm) . UL 94 V-0/0.8 mm rating was reported for PBT/GF30 with the aid of a ternary system consisting of A1(H2P02)3, melamine cyanurate and polycarbonate as a charring agent with 10 wt% , 10 wt% and 7.5 wt% loadings, respectively - see Yang et al . [Industrial & Engineering Chemistry Research 50, p. 11975-11981 (2011)]. Chen et al . [Polymer Degradation and Stability 97, p. 158-165 (2012)] reported that A1(H ₂ P0 ₂ )3 is able to secure UL 94 V-0/1.6 mm rating for PBT/GF30 as a sole flame retardant additive at a concentration of 25% by weight. As to other metal hypophosphite salts, calcium hypophosphite Ca(H ₂ P0 ₂ )2 is mentioned in US 6, 503, 969 (in the name BASF) and WO 2012/113146 (in the name of Rhodia) .

There exists a need for a suitable system of flame retardants to effectively reduce the flammability of thin polyester parts, especially thin PBT/GF30 parts, that is, where the thickness of the test specimen is 0.8 mm or less. We have now found that aluminum hypophosphite in combination with bromine-containing polymers fulfill the UL 94 V-0 requirements for such thin PBT/GF3o-made parts used in electrical engineering and electronics applications (that is, UL 94 V-0/0.8 mm and preferably also UL 94 V-0/0.4mm) . Notably, polyesters such as PBT/GF30 can be flame retarded with the aid of the aforementioned mixture even in the absence of antimony trioxide [Normally bromine-containing flame retardants are added to plastic polymers together with antimony trioxide, which functions synergistically to enhance the activity of the flame retardant, usually at about 2:1-5:1 weight ratio (calculated as the ratio between the concentrations of bromine supplied by the flame retardant and Sb203 in the polymer composition) .

Another method used for evaluating the flammability of polyester compositions of the invention is based on measuring their Glow Wire Ignition Temperature (GWIT) . GWIT is the lowest temperature at which a material ignites and burns for longer than 5 seconds under the glow wire test (IEC 60695-2-13 standard) . For some preferred polyester compositions of the invention the GWIT measured was above 800°C, and even above 850°C (GWIT > 870°C, e.g., from 870-900°C (for plate test thickness of 3.2 mm) ) .

The present invention is therefore primarily directed to a composition comprising polyester and a mixture of flame retardants having at least the following two components:

A) aluminum hypophosphite and B ) bromine-containing polymer.

The composition of the invention is substantially Sb203-free. By "substantially Sb203-free" is meant that the concentration of antimony trioxide in the composition is well below the acceptable amount used in plastic composites in conjunction with halogenated additives, e.g., not more 1.0% by weight, more preferably, up 0.5% by weight, e.g., 0.0-0.3% by weight (based on the total weight of the composition) . Most preferably, the compositions of the invention are totally devoid of antimony trioxide.

The concentration of the mixture of flame retardants in the polyester composition is preferably from 10 to 30% by weight, more specifically from 15 to 25% by weight, for example 17 to 23% by weight based on the total weight of the composition, and the mixture is proportioned such that the ratio between the weight concentrations of bromine (supplied by the brominated additive) and aluminum hypophosphite is preferably not less than 1:1, e.g., from 1.05 to 2.0, e.g., from 1.05 to 1.8 (in favor of bromine) . The concentration of bromine in the polyester composition is calculated by multiplying the bromine content of the flame retardant used (designated herein Br _F r name and expressed as % by weight) by the weight concentration of that flame retardant in the polyester composition, as illustrated below.

The first component of the additive mixture is aluminum hypophosphite. This salt is available on the market from various manufacturers; it can be made by reacting an aluminum salt with hypophosphorous acid (e.g., with slow heating at 80- 90°C (see Handbook of inorganic compounds, second Edition by D. L. Perry; see also US 7, 700, 680 and J. Chem. Soc. P. 2945 (1952) ) .

The second component of the additive mixture is a bromine- containing polymer. Different types of bromine-containing polymers can be used in combination with A1(H2P02)3 according to the present invention, for example: i) Poly (pentabromobenzyl acrylate) , represented by the following formula:

(n=degree of polymerization)

The polymer (abbreviated PBBPA) is produced by polymerizing the corresponding monomer pentabromobenzyl acrylate, either in bulk (in an extruder at a temperature in the range from 120°C to 290°C as described in US 4,996,276), or in solution, see US 4,128,709 or 6,028,156. The polymer is also available on the market, being sold by ICL-IP (FR-1025) . ii) Brominated polystyrene, represented by the following formula :

(n=degree of polymerization; m= 1,2,3,4 or 5)

The polymer is prepared by methods known in the art (see US 4,879,353 and US 5,532,322) . Suitable grades have weight average molecular weight in the range from 500,000 to 600,000, with bromine content preferably exceeding 60 or even 65 % by weight (that is, average of 2-3 bromine atoms per aromatic ring in the polymer backbone chain) . Such polymers, in the form of a free flowing powder or in granular form, are available on the market, e.g., from ICL-IP (FR 803P) . iii) brominated epoxy resins and end-capped derivatives thereof, represented by the Formula (I) :

(m=degree of polymerization) ; wherein Ri and R2 are independently selected from the group consisting of the following:

and

Preferred are epoxy-terminated resins (that is, resin having glycidyl end groups) represented by the following Formula (la) :

with weight-average molar mass between 2500 and 30000, preferably from 15000 to 25000. The epoxy resins are obtainable by reacting tetrabromobisphenol A with epichlorohydrin . Commercially available examples include F- 2100 from ICL-IP, with molecular weight of about 20,000.

The composition of the invention comprises not less than 10% by weight, e.g., from 20 to 70% by weight and preferably from 20 to 60% by weight of a thermoplastic polyester (e.g., 35-55%), in particular polyesters of the classes based on: aromatic dicarboxylic acids and aliphatic dihydroxy compounds; and

aromatic dicarboxylic acids and aromatic dihydroxy compounds.

Of the former class, preferred are linear polyesters obtained from terephthalic acid, isophthalic acid and 2,6 naphathalenedicarboxylic acid, wherein the aliphatic dihydroxy compound is a diol having from 2 to 6 carbon atoms, such as 1 , 2-ethanediol , 1 , 3-propanediol , 1 , 4-butanediol , 1,6- hexanediol, 1 , 4-hexanediol , and mixtures thereof. Particularly preferred are polyalkylene terephthalates derived from alkanediols having from 2 to 6 carbon atoms. Among these, especially preferred are polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) . Copolymers and/or polyblends available in the market for many linear polyesters are also contemplated for use in the invention. Additional details regarding suitable polyesters can be found in US 6,503,969 and US 8,188,172. PBT is available from various manufacturers, e.g., BASF and Ticona. For example, the melt flow index of the PBT (MFI; ISO 1133 250°C/2.16 kg) may vary from 25 g/10 min to 50 g/10 min, e.g., from 20 to 35 g/10 min or from 35 to 50 g/10 min .

The composition of the invention preferably comprises reinforcing filler, especially glass fibers, which are usually pre-coated by manufacturers in order to improve their compatibility with the polymer in question (e.g., polyester in the present case) . The major constituents of glass fibers applied for reinforcing polyester intended for use in electrical devices are alumino-borosilicates ; such type of glass in known as E-glass. The glass fibers comprise filaments with diameter in the range from 1μ to 30μ, and are applied in the form of chopped strands with length in the range from 2 to 10 mm, e.g., from 3 to 4.5 mm. The concentration of the glass fibers is usually from 5% to 40% by weight, e.g., from 10% to 40% by weight, preferably at least 20% by weight, e.g. from 20 to 35% by weight, in particular from 24 to 35% by weight and especially from 27 to 33% by weight, i.e., around 30% by weight, based on the total weight of the composition. The preferred compositions of the invention are therefore designated PBT/ GF20-35, PBT/ GF24-35 and PBT/ GF27-33, in particular, the aforementioned PBT/ GF30 .

The invention specifically provides a composition comprising: from 20 to 60% by weight of a polyester (e.g., 35-55%), in particular polyalkylene terephthalate, and especially polybutylene terephthalate (PBT) ;

from 20 to 35% by weight of a reinforcing filler, in particular glass fibers (e.g., 24-35% and especially 27-33%); and

a mixture of flame retardants comprising aluminum hypophosphite and bromine-containing polymer selected from the group consisting of poly (pentabromobenzyl acrylate) , brominated polystyrene and brominated epoxy resin of Formula (la), wherein the concentration of said aluminum hypophosphite is preferably in the range from 5% to 10% (e.g., 6 to 9%) by weight based on the total weight of the composition, and the concentration of the bromine-containing polymer is adjusted such that the bromine concentration of the composition is from 7.0 to 12% (e.g., 8 to 11%) by weight based on the total weight of the composition .

The concentration of bromine in the composition is calculated by multiplying the bromine content of a given flame retardant (designated herein Br _F r name and expressed as % by weight) by the weight concentration of that flame retardant in the composition. For example, the bromine contents of commercially available FR-1025, FR-803P and F-2100 are Br _F R-IO25 = 71%, Br FR-803P = 66% and Br F-2100 = 52%, respectively. Therefore, to incorporate 10% by weight bromine into the polyester composition, the corresponding concentrations of these three flame retardants should be 14.1%, 15.1% and 19.3% (by weight relative to the total weight of the composition) .

The compositions according to the present invention also include one or more anti-dripping agents such as polytetrafluoroethylene (abbreviated PTFE) in a preferred amount between 0.1 and 1.0 wt%, more preferably between 0.3 and 0.7 wt%, and even more preferably between 0.3 and 0.5 wt%, based on the total weight of the composition. PTFE is described, for example, in US 6,503,988.

The invention is especially directed to a composition characterized in that it achieves a UL-94 flammability test rating of V-0 at a thickness equal to or less than 0.8 mm, e.g., at 0.8 mm and preferably also at 0.4 mm. For example, compositions comprising: from 40 to 55 % by weight of PBT (e.g., from 45 to 55 %), from 20 to 35 % by weight of glass fibers (e.g., 27 to 33%), from 5 to 10% by weight of A1(H ₂ P0 ₂ ) 3 (e.g., 6 to 9%), from 10 to 15% by weight of poly (pentabromobenzyl acrylate) and from 0.1 to 1.0% by weight of PTFE (e.g., from 0.3 to 0.7); or from 40 to 55 % by weight of PBT (e.g., from 45 to 55 %), from 20 to 35 % by weight of glass fibers (e.g., 27 to 33%), from 5 to 10% by weight of A1(H ₂ P0 ₂ ) 3 (e.g., 6 to 9%), from 10 to 18% by weight of brominated polystyrene (e.g., from 10 to 15%) and from 0.1 to 1.0% by weight of PTFE (e.g., 0.3 to 0.7%); or from 40 to 50 % by weight of PBT (e.g., 40 to 45%), from 20 to 35 % by weight of glass fibers (e.g., 27 to 33%), from 5 to 10% by weight of A1(H ₂ P0 ₂ )3 (e.g., 6 to 9%), from 15 to 20% by weight of brominated epoxy resin of Formula (I) or (la) and from 0.1 to 1.0% by weight of PTFE (e.g., 0.3 to 0.7%); were found to possess the desired flammability properties, that is, fulfilling the requirements for UL 94 V-0/0.8 mm rating (and in some cases also UL 94 V-0/0.4 mm rating) .

In general, good results are obtained when the mixture of flame retardants used consists solely of aluminum hypophosphite and a bromine-containing polymer, devoid of other flame retardants. But in some embodiments, one or more auxiliary flame retardants may be added to the composition, in particular, phosphorous- containing flame retardants, e.g., phosphate esters.

For example, an aryl phosphate ester of hydroquinone (1,4- dihydroxybenzene ) of Formula (II) :

Formula (II ^' wherein R ¹ , R ² , R ³ and R ⁴ each independently is aryl (e.g., phenyl) or alkyl-substituted aryl (e.g., xylenyl), optionally interrupted with heteroatoms, and n has an average value of from about 1.0 to about 2.0, may be used. The compounds of Formula (II) are described in EP 2089402. In general, the hydroquinone bis-phosphates of Formula II are prepared by reacting a diaryl halophosphate with hydroquinone in the presence of a catalyst. For example, diphenylchlorophosphate (DPCP) is reacted with hydroquinone in the presence of MgCl2 to produce hydroquinone bis- (diphenyl phosphate) . Detailed methods for synthesizing compounds of Formula (II) can be found in EP 2089402. One preferred compound of Formula (II) to be used in this invention has Ri=R2=R3= 4=phenyl and 1.0<n≤l.l, that is, hydroquinone bis (diphenyl phosphate) with an n average value of about 1.0<n<1.05. The compound is obtainable in a solid form, see Example 1 of EP 2089402. It is available on the market from ICL-IP in the form of pastilles; the pastilles, when compounded with thermoplastics, avoid various handling problems as well as impart improved thermal properties, such as resin flow. Another phosphate ester flame retardant that can be added to the polyester composition of the invention is resorcinol bis (diphenyl phosphate) , which is a liquid bis-phosphate available from ICL-IP (CAS no.: 57583-54-7; Fyrolflex ^® RDP) ; it can be formulated into masterbatch pellets in PBT carrier for use in the invention.

Apart from a polyester, a reinforcing filler, a mixture of flame retardants and an anti-dripping agent, the composition of this invention may further contain conventional additives, such as UV stabilizers (e.g., benzotriazole derivative), processing aids, antioxidants (e.g., hindered phenol type), lubricants, pigments, dies and the like. The total concentration of these auxiliary additives is typically not more than 3 % by weight.

The preparation of the polyester compositions of the invention may be carried out using different methods known in the art. For example, the polyester compositions are produced by melt- mixing the components, e.g., in a co-kneader or twin screw extruder, wherein the mixing temperature is in the range from 220 to 280°C. It is possible to feed all the ingredients to the extrusion throat together, but it is generally preferred to first dry-mix some of the components, and then to introduce the dry blend into the main feed port of the extruder, with one or more of the ingredients being optionally added downstream. For example, the polyester, aluminum hypophosphite, the bromine containing flame retardant, one or more of the conventional additives are dry blended and the blend is fed to the extruder throat, followed by the addition of glass fibers downstream. Process parameters such as barrel temperature, melt temperature and screw speed are described in more detail in the examples that follow.

The resultant extrudates are comminuted into pellets. The dried pellets are suitable for feed to an article shaping process, injection molding, extrusion molding, compression molding, optionally followed by another shaping method. Articles molded from the polyester compositions form another aspect of the invention. Specific examples of articles include electric and electronic parts, such as connectors, circuit breakers and power plugs .

As pointed out above, flame retarded PBT/GF compositions with UL 94/0.8 mm V-0 rating are difficult to produce, let alone PBT/GF compositions having UL 94/0.4 mm V-0 rating that are flame retarded with a bromine-containing additive absent a conventional synergist, e.g., the antimony trioxide synergist that normally promotes the action of the brominated flame retardant in plastics. But the experimental results reported herein show that the goal can be achieved using a combination of polymeric brominated flame retardant and aluminum hypophosphite (the terms "bromine-containing polymer" and "polymeric brominated flame retardant" are used herein interchangeably) . Quite the opposite, a non-polymeric bromine- containing flame retardant that was tested (decabromodiphenyl ethane, an additive which has gained commercial acceptance for many applications, including PBT) was unable to achieve UL 94 V-0/0.8 mm rating when combined with aluminum hypophosphite. It appears that aluminum hypophosphite is able to offset the absence of antimony trioxide when polymeric brominated flame retardants such as poly (pentabromobenzyl acrylate) , brominated polystyrene and brominated epoxy resins are used in PBT, but not when decabromodiphenyl ethane is used.

Hence, there is also provided by the invention a method of minimizing the amount of antimony trioxide in glass-fiber reinforced poly (butylene terephthalate ) composition which is flame retarded with bromine-containing additive, comprising the steps of using a polymeric brominated flame retardant in the role of said bromine containing additive, and adding aluminum hypophosphite to said composition, thereby achieving a UL-94 flammability test rating of V-0 at a thickness equal to or less than 0.8 mm when the amount of antimony trioxide is from 0.0- 0.5% by weight based on the total weight of the composition, including in antimony trioxide-free composition { [Sb203] =0 } . To this end, the amount of the polymeric brominated flame retardant is adjusted such that the bromine content of the composition is preferably from 7.0 to 12% by weight based on the total weight of the composition, and the amount of aluminum hypophosphite is adjusted such that its concentration is preferably from 5 to 10% by weight by weight based on the total weight of the composition. The polymeric brominated flame retardant is selected from the group mentioned above, with the brominated epoxy resin of Formula (la) :

with weight-average molar mass between 15000 to 25000, being especially preferred.

Examples

Materials and methods

The materials used for preparing the PBT/GF30 formulations are tabulated in Table 1 (FR is abbreviation of flame retardant) :

Table 1

Flammability properties

A direct flame test was carried out according to the Underwriters-Laboratories standard UL 94 in a gas methane operated flammability hood, applying the vertical burn on specimens of 0.8 mm or 0.4 mm thickness.

Glow Wire Ignition Temperature (GWIT) were measured according to the IEC EN 60695-2-13 method. The instrument used was the PLT Glow Wire test instrument with pulse timer type T-03-24 (3.2 mm thick plate) .

Mechanical properties

The Notched Izod impact test was carried out according to ASTM D256-81 using Instron Ceast 9050 pendulum impact system. Tensile properties were determined according to ASTM D638 using Zwick 1435 material testing machine (type 2 dumbbells were used, with a speed test of 5 mm/min) .

Thermal properties

Heat distortion temperature (abbreviated HDT; this is the temperature at which a polymer sample deforms under a specific load) was measured according to ASTM D-648 with load of 1820 MPa and heating rate of 120°C/hour. The instrument was HDT/VICAT -plus Daveport, Lloyd instruments.

Examples 1 to 6 (of the invention) and 7 (comparative) PBT/GF30 flame retarded with aluminum hypophosphite and

bromine-containing polymers

In this set of examples, combinations consisting aluminum hypophosphite and a brominated flame retardant selected from the group consisting of poly (pentabromobenyzl acrylate) , brominated polystyrene and brominated epoxy resin were tested to determine their ability to reduce the flammability of PBT/GF30 in the absence of antimony trioxide. Test specimens with thickness of 0.8 mm or 0.4 mm were prepared. In addition, mechanical and thermal properties were measured.

To prepare the compositions, PBT and additives (with the exception of the brominated FR and glass fibers) were premixed and the blend was fed via Feeder no. 1 into the main port of a twin-screw co-rotating extruder ZE25 with L/D=32 (Berstorff ) . The brominated FR was fed via Feeder no. 2 to the extruder main port. The glass fibers were added downstream, via Feeder no. 3 to the fifth zone of the barrel through a lateral feeder. Operating parameters of the extruder were as follows: Barrel temperature (from feed end to discharge end) : 220°C, 250°C, 260°C, 260°C, 260°C, 265°C, 270°C, die - 275°C.

Screw rotation speed: 350 rpm

Feeding rate: 12 kg/hour.

The strands produced were pelletized in a pelletizer 750/3 from Accrapak Systems Ltd. The resultant pellets were dried in a circulating air oven (Heraeus Instruments) at 120°C for 4 hours. The dried pellets were injection molded into test specimens using Allrounder 500-150 from Arburg under the conditions tabulated below:

Table 2

Specimens of various thicknesses were prepared. The test specimens were conditioned for one week at 23°C, and were then subjected to the tests to determine their properties. The compositions and the results are set out in Table 3. Table 3

The results set out in Table 3 indicate the high efficacy of a combination consisting of aluminum hypophosphite salt and either poly (pentabromobenyzl acrylate) - Examples 1 and 2, brominated polystyrene - Examples 3 and 4, or brominated epoxy resin - Examples 5 and 6 in reducing the flammability of thin polyester test specimens, achieving UL 94 V-0 rating down to 0.4 mm thickness.

A non-polymeric bromine-containing flame retardant tested (FR- 1410; decabromodiphenyl ethane, a flame retardant containing 82% aromatic bromine, which is considered the additive of choice for a large variety of applications, including PBT) was shown to be ineffective in Sb203-free system, seeing that the combination FR-1410/aluminum hypophosphite failed to gain UL 94 V-0/0.8 mm rating.