In some applications, e.g., in electrical devices, molded polyamide compositions need to have good flammability properties and also high resistance to electrical discharge which may result in the carbonization of the plastic. Flammability characteristics are quantifiable according to the method specified by Underwriter Laboratories standard UL 94. The UL 94 ratings assigned to a material subjected to a vertical burning test are V-0, V-l, and V-2 , where the V-0 rating indicates the lowest degree of flammability. The resistance of the polymer to exposure of voltage is evaluated using the Comparative Tracking Index (CTI), which is a measure of the susceptibility of solid insulating materials to surface tracking when exposed to an electrolyte under the application of voltage. CTI indicates the voltage which causes the sample to undergo carbonization. The higher the CTI, the better is the resistance of the sample to electrical discharge etc.

The presence of additives such as reinforcing fillers and halogenated flame retardants in the polyamide results in a significant decrease of the CTI of the polymer. For example, while neat polyamide 6,6 resin exhibits CTI value of about 600 volts, it is noted that glass fibers reinforced polyamide 6,6 which contains also halogenated flame retardants in an amount sufficient to meet the UL 94 V0 demonstrates CTI value of about 300 volts. Hence, polyamide compositions with good mechanical strength, high UL 94 rating and acceptable CTI values are not easily achieved. Brominated compounds with high thermal stability, in particular bromine-containing polymers, are used to reduce the flammability of polyamides . Their action is usually supported by the presence of an inorganic metal oxide (e.g., antimony trioxide ) . For example, US 5,863,974 discloses glass fiber reinforced polyamide composition in which bromine containing polymers, such as poly (pentabromobenzyl acrylate) and brominated epoxy, were combined with antimony trioxide and appreciable amounts of minerals capable of enhancing the CTI values of the resultant polymers. The experimental results reported in US 5,863,974 indicate that polyamide compositions with acceptable CTI values of about 400 volts and above were achieved, but these compositions did not demonstrate flame resistance level sufficiently good to satisfy the UL 94 VO classification .

WO 2005/116139 reports the results of a study where various glass fibers reinforced polyamide compositions containing known flame retardants were prepared and tested for their electrical and flammability characteristics. UL 94 V0 rated compositions exhibiting CTI value around 400 volts are reported, in which bromine-containing polymer was used as a flame-retardant agent (brominated polystyrene), in combination with antimony trioxide.

Fillers such as magnesium hydroxide are sometimes applied in a modified form, i.e., with their surface being treated by means of suitable agents. Gao-xiang et al. [Huagong Kuangwu Yu Jiagong, 34(9), 7-9, 13 (2005)] described superfine Mg(OH) ₂ powders which were surface-treated with silicone oil. JP 2005336398 describes a fatty acid-treated magnesium hydroxide which was kneaded with Ph-containing organopolysiloxane, giving a powder with good dispersibility in HIPS. Kodama et al. [Materials Research Society Symposium Proceedings (2005) studied the effect of Mg(OH) ₂ , modified with several kinds of polysiloxane oils, on the mechanical properties and oxygen index of low density polyethylene. CN 1341669 describes the use of Mg(0H)2 surface treated with silicone oil in polyethylene (PE) or polypropylene (PP). US 6,576,160 describes surface- modified fillers such as magnesium hydroxide, which were coated with several distinct layers, including a polysiloxane layer, and were tested in EVA, polyketons and polypropylene. US 5827906 describes magnesium hydroxide which has been surface-treated with fatty acid derivatives and optionally polydialkylsiloxanes , and was then compounded with polypropylene and ethylene vinyl acetate (EVA). Polymer compositions comprising surface-modified magnesium hydroxide were also described in WO 2007/146289.

It has now been found that the surface treatment of magnesium hydroxide by means of polysiloxanes , in particular polydimethylsiloxanes , results in a surface modified magnesium hydroxide which is especially useful as an additive in polyamides. The addition of a combination comprising (i) polysiloxane-coated magnesium hydroxide and (ii) bromine-containing compounds (e.g., brominated polymers) to a polyamide, such as glass fiber reinforced polyamide, affords an improved polyamide composition characterized by both UL 94 V0 rating and acceptable CTI values {e.g., around 400 volts).

Accordingly, the present invention is primarily directed to a flame retarded polyamide composition comprising polyamide, a reinforcing filler, surface-modified magnesium hydroxide and at least one brominated flame retardant, e.g., a bromine-containing polymer, wherein the magnesium hydroxide has on its surface one or more polysiloxanes.

The surface-modified magnesium hydroxide additive is formed upon vigorously mixing together magnesium hydroxide particles with a suitable amount of the polysiloxane. The weight percent of the polysiloxane is not less than 0.1%, preferably not less than 0.5%, and even more preferably not less than 0.8%, e. g. , between 1 and 5 wt% and more specifically between 1 and 3 wt%, of the total weight of the surface-modified magnesium hydroxide.

The neat magnesium hydroxide subjected to the surface treatment is in the form of particles with a specific surface area in the range from 4 to 30 m ² /g. The particles are of high purity and are relatively uniform in shape. Suitable forms of magnesium hydroxide are commercially available, e.g., FR-20-100D S7 and FR-20-100D S10 from ICL- IP.

Polysiloxanes that can be applied for coating the surface of the magnesium hydroxide flame retardant include polydialkylsiloxanes (such as polydimethylsiloxanes ) , polyalkylarylsiloxanes and polydiarylsiloxanes . The polysiloxanes may contain functional groups {e.g., carbonyl or ester) and may be modified in various forms. Polydimethylsiloxanes, especially organo-modified polydimethylsiloxanes such as:

(i) polydimethylsiloxane modified with alkyl groups having 4 to 18 carbon atoms, e.g., from 10 to 18 carbon atoms;

(ii) polydimethylsiloxane modified (e.g., terminated) with carbonyl or ester groups; and (iii) polydimethylsiloxane modified with alkyl groups having 4 to 18 carbon atoms and carbonyl or ester groups.

Thus, for example, polydimethylsiloxane suitable for use according to the invention may be described by the following formula:

wherein R indicates the modified portion of the polydimethylsiloxane, and is preferably a linear alkyl chain consisting of 4 to 18 carbon atoms; R ^' indicates the end group of the polydimethylsiloxane, and is methyl or a carbonyl-containing functionality, such as an ester group (-OC(O)Alk, wherein Alk is alkyl chain); and m and n are integer numbers indicating the degree of polymerization.

The density of useful polysiloxanes is about 0.90-0.92 g/ml and their viscosity is about 200-500 mPa's at 25°C. Suitable forms of polysiloxanes are commercially available from Evonic Industries under the series Tegopren® (e.g., Tegopren® 6875 ) .

One or more polysiloxane ( s ) is (are) applied onto the surface of the magnesium hydroxide particles by means of mixing the ingredients together. To this end, the polysiloxane is gradually fed to a high speed mixer charged with magnesium hydroxide particles. During the portionwise or dropwise addition of the polysiloxane liquid, which preferably lasts a few minutes, the mixer is operated at a relatively low rotation speed (400-600 rpm) . Having completed the polysiloxane addition stage, the rotation speed is increased and the mixing is allowed to continue for a period of time of not less than ten minutes. If needed, the operation of the mixer may be interrupted in order to clean its walls and crush agglomerates formed.

Preferably, the one or more polysiloxane ( s ) is (are) the sole surface modifiers applied onto the surface of the magnesium hydroxide, i.e., the coating on the magenisum hydroxide particles consists solely of one or more polysiloxanes as set out above, said coating being devoid of other surface-modifiers such as fatty acids, organotitantes and organozirconates .

The coated magnesium hydroxide particles exhibit good dispersibility in polyamide. The concentration of the coated magnesium hydroxide in the polyamide composition is from 5% to 40% of the total weight of the composition.

The invention also provides a particulate magnesium hydroxide having on its surface one or more organo-modified polydimethylsiloxanes selected from the group consisting of:

(i) polydimethylsiloxane modified with alkyl groups having 4 to 18 carbon atoms;

(ii) polydimethylsiloxane modified with carbonyl or ester groups; and

(iii) polydimethylsiloxane modified with alkyl group having 4 to 18 carbon atoms and carbonyl or ester groups. Another aspect of the invention relates to the use of polysiloxane as set forth above as CTI enhancer in polymers (e.g. polyamide, polybutylene terephthalate (PBT) and polyethylene terephthalate (PET)) compositions, wherein the polysiloxane is preferably applied as a coating on the particles of halogen-free flame retarding agent incorporated in the polymer. Thus, the invention also relates to a method for enhancing the comparative tracking index of a polyamide, PBT or PET composition, comprising applying organo-modified polydimethylsiloxanes as set forth above onto the surface of a particulate halogen-free flame retardant agent (e.g., magnesium hydroxide), and incorporating said surface-modified flame retardant into said polyamide, PBT or PET.

The bromine-containing compounds which are suitable for use in the present invention need to have good thermal stability, in view of the high processing temperatures at the polyamide compounding stage. Bromine-containing oligomers and polymers are therefore useful, such as brominated epoxy resins and their end-capped derivatives, for example, those represented by Formula I:

wherein m is the weight average degree of polymerization (e.g., between 3 and 135, or between 6 and 135) and Ri and R ₂ are independently selected from the group consisting of:

and

We have found that intermediate weight average molecular weight (Mw) brominated epoxy polymers of Formula I, and especially their end capped derivatives, with Mw of less than 25,000, and preferably from 5000-25,000 can be used effectively for reducing the flammability of polyamide, achieving UL 94 V0 rating, while also allowing convenient processability (e.g., good flowability) at the compounding stage. However, high molecular weight bromine-containing polymers of Formula (I), with Mw greater than 25,000, e.g., from 30,000 to 70,000 can also be used.

The preferred flame retardants of Formula (I), which are suitable for reducing the flammability of polyamide compositions according to the invention, comprise end- capped resins of the following structure:

with weight average molecular weight of about 15,000-25,000 (e.g., F-3100 from ICL-IP) and brominated epoxy of the following structure:

with molecular weight of 40,000-60,000 (e.g., F-2400, which is commercially available from ICL-IP).

The bromine-containing polymeric flame retardants of Formula I, which are provided in the form of mixtures comprising individual resins (which vary in their chain length) can be prepared by methods known in the art (e.g., US 4,605,708, EP 467364 and EP 1587865). For example, the epoxy-terminated flame-retardants of Formula (I) may be produced by reacting tetrabromobisphenol A with epichlorohydrin, optionally in an inert solvent such as toluene or methyl isobutyl ketone, in the presence of a base (e.g., an aqueous solution of sodium hydroxide) under heating. Following phase separation, the organic phase, which contains the product, is washed with water to remove residual salts therefrom and the product is finally recovered by removing the organic solvent. The average epoxy equivalent weight of the product may be controlled by modifying the ratio of the reactants. The lower the concentration of epichlorohydrin used, the higher the epoxy equivalent weight of the resulting mixture.

The tribromophenol-terminated end-capped flame retardants of Formula (I) can be prepared by reacting the mixture of epoxy resins of Formula (I) with tribromophenol, optionally in a solvent. The reaction is carried out under heating in the presence of a catalyst (e.g., Li based catalyst) or an inorganic base, such as sodium hydroxide or potassium hydroxide, or an organic base, such as tertiary amine, quaternary ammonium salt or a quaternary phosphoniuiti salt. The preparation of tribromophenol-terminated end-capped derivatives of Formula (I) is described in WO 2004/063263.

High molecular weight brominated flame retardants (other than those represented by Formula I) which can be used in the polyamide composition of the invention are poly ( pentabromobenzyl acrylate), described, e.g., in US 4,128,709. Poly (pentabromobenzyl acrylate) is commercially available as FR-1025 from ICL-IP. Brominated polystyrene (e.g., FR-803P from ICL-IP) can also be used. The concentration of the bromine-containing flame retardant in the polyamide composition is from 5% to 40% of the total weight of the composition, with the bromine content of the composition being preferably from 5 to 20 % by weight.

Polyamides suitable for use according to the invention are well known in the art. The polyamides are generally manufactured by the polycondensation reaction of dicarboxylic acid and diamine (e.g., the condensation of hexamethylene diamine and adipic acid to give polyamide 6,6), the ring-opening polymerization of lactam (e.g., the polymerization of carpolactam to give nylon 6) or the reaction of acid chlorides with amines. Polyamides operable in the invention include also aromatic polyamides. Illustrative examples of suitable polyamides include: polyhexamethylene adipamide (polyamide 6,6), polycaprolactam (polyamide 6), poly ( 7-heptaneamide ) (polyamide 7), polycapryllactam (polyamide 8), poly (11- undecanamide ) (polyamide-11 ) , polylauryllactam (polyamide- 12) and polytetramethylene adipamide (polyamide 4,6). The preferred polyamides include polyamide 6,6 and polyamide-6. Mixtures and copolymers of polyamides are also within the scope of the invention. The composition comprises at least 30% polyamide, e.g., between 40% and 80% wt. %.

The reinforcing fillers used in the invention preferably include glass fibers, which are typically pre-coated by methods known in the art prior to their use in order to improve their compatibility with the polyamide matrix. Such modified forms of glass fibers are available in the market, e.g., GF Chop Vantage 3660 from PPG. The glass fibers comprise filaments with diameter in the range from 2μ to 20μ, and are applied in the form of pieces with length in the range from 2 to 10 mm, e.g., from 3 to 4.5 mm. The major constituents of glass fibers applied for reinforcing polyamide intended for use in electrical devices are alumino- borosilicates ; such type of glass in known as E-glass. The concentration of the glass fibers is from 5% to 40% of the total weight of the polyamide composition.

In addition to the polyamide, the reinforcing fillers, the bromine-containing polymer and the surface-modified magnesium hydroxide, the composition of this invention may further contain conventional additives, such as lubricants, antioxidants (e.g., of hindered phenol or phosphite type), pigments, UV stabilizers and heat stabilizers. The concentration of each of the conventional additives listed above is typically in the range between 0.05 and 10 wt% . It should be noted that polymers that are compatible with polyamides can be also included in the composition of the invention at a concentration of up to 30% (e.g. polyolefins, polyolefin copolymers, styrene-containing homopolymers , copolymers and terpolymers ) . Preferably, the polyamide composition of the invention comprises from 7 to 15 wt% bromine supplied by the flame retardant of Formula (I), and from 15 to 25 wt% surface- modified magnesium hydroxide described above. Preferably, the composition is essentially free of antimony trioxide, e.g., it contains less than 2 wt% Sb ₂ 0 ₃ , e.g., less than 1 wt%, and is most preferably free of antimony trioxide.

The polyamide 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 150 to 350°C. It is possible to feed all the ingredients to the extrusion throat together, but it 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 polyamide, the bromine containing flame retardant, one or more of the conventional additives and optionally a portion of the magnesium hydroxide are dry blended and the blend is fed to the extruder throat, followed by the addition of the remaining or entire amount of the magnesium hydroxide into said main feeding port. The Glass fibers are the last to be added, i.e., downstream.

The resultant compositions, in the form of granules or pellets, are dried and are suitable for feed to an article shaping process, such as injection molding. Articles molded from the polyamide compositions form another aspect of the invention. Specific examples of articles include electric and electronic components used in electrical and electronic home appliances and electrical industrial and office appliances . Examples

Materials

The materials employed in the experimental work are set out in Table 1 (the abbreviation "FR" indicates flame retardant ) :

Table 1

Measurements

The methods employed in determining the properties of the polyamide compositions were as follows:

The flammability vertical test according to the Underwriters Laboratories UL-94 standard was carried out in a gas methane operated flammability hood.

The Limiting Oxygen Index (LOI), which indicates the minimal oxygen concentration required to support candlelike combustion of the tested sample, was determined according to ASTM D 2863-00 using FireTesting Technology instrument.

Glow Wire Flairanability Index (GWFI) and Glow Wire Ignition Temperature (GWIT) were measured according to the CEI EN 60695-2-13 method. The instrument used for determining these indexes was the PLT Glow Wire test instrument with pulse timer type T-03-24.

Comparative Tracking Index (CTI) was measured according to the International Electrotechnical Commission (IEC) STANDARD publication 112. The test was performed with a solution of NH ₄ C1 0.1% (solution A) and Pt electrodes. The instrument employed was Surface sliding currents code 6265/000 manufactured by Ceast.

The Notched Izod impact test was carried out according to ASTM D256 using Zwick 5102 pendulum.

Tensile properties were determined according to ASTM D638 using Zwick 1435 materials testing machine (type 2 dumbbells were used, with a speed test of 5 mm/min) .

The spiral flow test, which determines the length (in inches) of a part of a round spiral mold filled at injection molding was measured using Allrounder 500-5150- 320S (Arburg) with set temperature profile of 240-255-275- 275-280°C.

The 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. Preparation 1

Surface Treated Mg(OH) ₂

Magnesium hydroxide (FR-20 100D S7 ) and polysiloxane (Tegopren 6875) were combined together in a high speed mixer. The amounts (in grams) and the respective concentrations (wt. %) of the two ingredients are set out in Table 2.

Table 2

The working procedure used for preparing the surface-coated magnesium hydroxide consists of the following steps.

Magnesium hydroxide was weighed and added to a high speed mixer (manufactured by Gmbh & Co Maschinen and Apparatebau Masch Typ. : TGHKV 8) equipped with a lid provided with an opening adapted for the addition of liquids. Tegopren 6875 was weighed and poured to a beaker. The mixer was operated at a speed of 500 rpm and the Tegopren 6875 was added dropwise through the aforementioned opening. The addition of the Tegopren 6875 lasted about 5 minutes, following which the rotation speed of the mixer was increased to 2200 rpm. The mixing was allowed to continue at 2200 rpm for ten minutes. Then the mixing was halted, the lid was opened and the inner walls of the cylinder were cleaned and agglomerates were crushed. The mixture was allowed to operate for additional ten minutes at 1800-2000 rpm. The speed of rotation was then reduced to 500 rpm, the drain piston was opened and the rotation speed was increased to 1000 rpm to ensure that the mixer is emptied. The mixing operation was completed and the surface-modified magnesium hydroxide was removed from the mixer and transferred to a metallic vessel.

The surface-modified magnesium hydroxide identified in preparation 1 is hereinafter designated FR-20(98:2), indicating the weight proportion between the magnesium hydroxide and the polysiloxane coating. The particle size distribution of the uncoated and surface-coated magnesium hydroxides is tabulated in Table 3 below.

Table 3

Examples 1 (of the invention) and 2-3 (comparative)

The surface-modified magnesium hydroxide flame retardant of Preparation 1 was incorporated into glass fiber reinforced polyamide compositions. The polyamide composition prepared and its properties are set out in Table 5 below. The properties of a reference glass fiber reinforced polyamide composition (free from flame retardants) and comparative compositions in which either a non-modified magnesium hydroxide or a commercially available aminosilane surface- treated magnesium hydroxide were tested (Examples 2 and 3, respectively) are also given in Table 5.

In order to prepare the polyamide compositions, the ingredients were compounded in a vented twin-screw co- rotating extruder {Berstorff ZE25) with L/D=32 and a set temperature profile of 210-270-270-270-275-280-280-285°C . The screw speed was 350 rpm, and the feeding rate was 12 kg per hour. The polyamide, the bromine-containing flame retardant, the lubricant and the antioxidant were first weighed and mixed, and the resultant blend was fed into the main feeding port of the extruder. The magnesium hydroxide was charged into the extruder through the main feeding port. The glass fibers were fed into the fifth section of the extruder via lateral side feeding.

The resultant pellets were dried in a circulating air oven at 120°C for four hours. The dried pellets were injection molded into test specimens using Allrounder 500-150 ex. Arburg. The conditions of the injection molding are set out in Table 4.

Tabl 4

The specimens produced were conditioned at 23°C for a week and were then subjected to a series of tests. The compositions (in wt%) and the results are given in Table 5.

Table 5

The results set out in Table 5 indicate that the polyamide composition in which the polysiloxane-coated magnesium hydroxide is present exhibits superior LOI and CTI values. The mechanical properties of the exemplified compositions are essentially comparable.