This invention relates to phosphorus-containing flame retardants particularly, but not exclusively for glass-filled polyamide resins.

BACKGROUND

With current and future market requirements for electrical components trending toward lighter weight plastic parts with improved electrical and mechanical properties, there is a substantial growth in the use of engineering plastics for electronic applications. At the present time, polyamides are the dominant engineering thermoplastic for electronic and other applications, especially when reinforced with a glass filler to increase their structural and impact strength and rigidity.

Polyamides are, in general, characterized as being relatively thermally stable upon long exposure to processing temperatures and shear. Upon exposure to flame, however, they burn quite readily, with the flammability being characterized by a dripping behavior of the burning resin. There is therefore a substantial and increasing demand for flame retardant polyamides and especially flame retardant glass-filled polyamides. Likewise, there is also demand for their polyester resin cousins, typically glass reinforced, with the choice of resin being dependent on several factors such as a balance between cost and mechanical property performance.

One of the major classes of flame retardants for thermoplastics and polyurethane foams is that of organic phosphorus compounds (typically phosphates and phosphonates). These may be non-halogenated or may include phosphorus-halogen compounds and blends of phosphorous compounds with halogenated flame retardants, typically brominated flame retardants.

In general organic phosphorus compounds provide fire retardant activity through a combination of condensed phase reactions, vapor phase reactions, polymer carbonization promotion, and/or char formation. These processes obviously depend on the polymer in which such additive(s) reside. Therefore, specific phosphorus containing structures need to be designed for various polymers types. For example, U.S. Patent No. 3,681 ,281 discloses a shaped structure comprising a polyester, at least 1 percent by weight of the polyester of a tertiary phosphine oxide, and from 10 to 50 percent by weight of the tertiary phosphine oxide of a synergist selected from the group consisting of triphenylmelamine, benzil and dibenzyl. Among the tertiary phosphine oxides exemplified is xylylene bis-diphenylphosphine oxide.

In an article entitled "Phosphorus based additives for flame retardant polyester. 1 . Low molecular weight additives", Industrial & Engineering Chemistry Product Research and

Development, (1982), 21 (2), pages 328-31 , Robert W Stackman evaluates various phosphorus- containing organic compounds, including xylylene bis-diphenylphosphine oxide, as flame retardants for poly(ethylene terephthalate) and poly(1 ,4-butylene terephthalate). The evaluation includes the effect of the additives on melt stability of preformed polymers as well as the effect upon flammability of films, as determined by a non-standard bottom burn, oxygen index method. The oxygen index values for these blends were a function of the phosphorus content of the blend. The efficiency of the phosphorus compounds as flame retardants changed as the nature of the phosphorus structure changed, with the order R ₃ PO > R(R'0) ₂ PO > (R'0) ₃ PO. Polymeric additives were reported to be attractive additives, giving a combination of a high degree of flame retardancy combined with a minor degree of property degradation on blending, even at <20 wt% of the blend.

Other organic phosphorus compounds have also been suggested for use as flame retardants for polyamides. For example, Research Disclosure 168051 (published April 1978) entitled "Improved Nylon" describes a flame-resistant nylon fiber prepared by coating flakes of bis(4- aminocyclohexyl)methane-dodecanedioic acid copolymer with 8-10% p-xylylenebis (diphenyl- phosphine oxide) flame retardant and spinning a yarn using a screw melter equipped with a homogenizing, in-line mixer to supply the molten polymer to a unit for producing a 34-filament yarn. The molten polymer was heated to 300-310° and the holdup time was approximately 15 min. The yarn was drawn 2.3 times on a hot pipe to produce a 100-denier yarn.

In addition, U.S. Patent No. 4,341 ,696 discloses a glass filled thermoplastic polyamide polymer rendered flame retardant by having combined therewith an effective amount of a tris-(3- hydroxyalkyl) phosphine oxide having the formula: R-ι R ₂ R ₃ (HOCHCHCH) ₃ PO

wherein R^ and R ₃ are any radical selected from the group consisting of hydrogen, phenyl and alkyl radicals of 1 to 4 carbon atoms and R ₂ is any radical selected from the group consisting of hydrogen, phenyl and alkyl radicals of 2 to 4 carbon atoms, provided that when R^ and R ₃ are hydrogen radicals, R ₂ is either an alkyl radical of 2 to 4 carbon atoms or a phenyl radical.

U.S. Patent No. 7,332,534 discloses a flame retardant formulation for thermoplastic and thermoset polymers, including polyesters and polyamides, containing, as flame retardant component A, from 90 to 99.9% by weight of phosphinate salt of the formula:

and/or a diphosphinate salt of the formula

and/or polymers thereof, where R , R ² are the same or different and are each d-C ₆ alkyl, linear or branched, and/or aryl; R ³ is C Ci ₀ alkylene, linear or branched, C ₆ -Ci ₀ arylene, alkylarylene or arylalkylene; M is Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Sr, Mn, Li, Na and/or K; m is from 1 to 4; n is from 1 to 4; x is from 1 to 4 and, as component B, from 0 to 50% by weight of a nitrogen-containing synergist or of a phosphorus/nitrogen flame retardant and, as component C, from 0.1 to 10% by weight of a liquid component.

U.S. Patent No. 8,420,719 discloses a flame-retardant resin composition comprising a base resin, such as a polyester, polyamide or styrenic resin and a polymeric benzylic phosphine oxide polymer or oligomer as flame retardant.

U.S. Patent No. 7,41 1 ,013 discloses a flame-retardant resin composition comprising a base resin (A), such as a polyester, polyamide or styrenic resin, an organic phosphorus compound (B) and a flame-retardant auxiliary (C), wherein the organic phosphorus compound (B) has a uni represented by the following formula:

wherein Ar represents an aromatic hydrocarbon ring or a nitrogen-containing aromatic heterocycle; X ¹ represents an oxygen atom or a sulfur atom; Y and Y2 are the same or different and each represents a hydrocarbon group, an alkoxy group, an aryloxy group, or an aralkyloxy group; Z represents an alkylene group, or a nitrogen-containing bivalent group corresponding to an alkylamine; Y and Y ² may bind to each other, and Y and Y ² together with the adjacent phosphorus atom may form a ring; "a" denotes 0 or 1 ; and "b" denotes an integer of 1 to 6.

According to the present invention, it has now been found that certain benzyl-substituted cyclic phosphinates, especially when combined with specific synergists, are highly effective flame retardants for thermoplastic resins, including polyamides, especially glass filled polyamides.

SUMMARY

Accordingly, the invention resides in one aspect in a flame-retardant resin composition comprising a base resin (A), an organophosphorus compound (B) comprising a unit represented by at least one of the following formulas (I), (II) and (III):

where A is selected from O, S, S0 ₂ , a single bond, and alkyl;

P is a phosphorus-containing group of the formula:

wherein R , R ² and Y, together with the phosphorus atom to which R and R ² are bound form a ring, wherein R and R ² are the same or different and each is selected from O-alkyI, O-aryl, alkyl and aryl, and Y is a linking group selected from direct bond, alkylene and -0-, e.g., Y is a direct bond;

R ³ is H or alkyl;

each a is an integer individually selected from 0 to 4, e.g., 0 1 , 2, 3, or 4 provided that at least one a is at least 1 ;

n is an integer from 1 to 100,000, e.g., 2, 3, 4 or 5 to 100,000, and m is an integer from 0 to 100,000, e.g., 1 , 2, 3, 4 or 5 to 100,000;

and, optionally,

(C) at least one flame retardant adjuvant material.

It should be understood that in formula (I), (la) and (II), where n is 2, 3, 4, 5 or higher, each of the individual variables A, R , R ² , R ³ , Y, M or a for each unit of Formula (I), (la) or (II) may be the same or different. For example, a polymer comprising monomer units of Formula (I) can contain monomer units of Formula (I) where the variable 'a' is 1 , and monomer units of Formula (I) where the variable 'a' is 0, resulting in a polymer comprising phenyl rings linked by a group A, wherein some of the phenyl rings are substituted by a group (CH ₂ P ¹ ) and some of the phenyl rings are not substituted by a group (CH ₂ P ¹ ). Conveniently, the organophosphorus compound (B) is present in an amount of 10% to 40%, e.g., 10% to 30 %, by weight of the flame-retardant resin composition.

Conveniently, at least one flame retardant adjuvant material (C) is present and is selected from melamine, a melamine derivative such as a melamine condensation product or melamine salt, an inorganic metal compound, a clay material, a layered double hydroxide material, and a polyphenylene ether resin.

Conveniently, the at least one flame retardant adjuvant material (C) comprises a melamine salt and an inorganic metal compound in a weight ratio of from 10:1 to 1 :1 or a condensation product of melamine and an inorganic metal compound in a weight ratio of from 10:1 to 1 :1 .

Conveniently, the inorganic metal compound comprises a metal salt such as a borate, particularly zinc borate.

Conveniently, the melamine salt comprises a melamine phosphate, particularly melamine polyphosphate or melamine pyrophosphate; the condensation product of melamine conveniently comprises melam, melem or melon.

Conveniently, the at least one flame retardant adjuvant material (C) is present in an amount of 1 to 20 wt% of the flame-retardant resin composition. For example, the combination of the organophosphorus compound (B) and the at least one flame retardant adjuvant material (C) may make up from 1 1 to 60 wt% of the total composition, e.g., from 1 1 to 50 wt% or from 15 to 40% of the total composition.

Conveniently, the base resin (A) comprises at least one of a thermoplastic or thermoset resin. A typical thermoset resin is an epoxy resin and a typical thermoplastic resin is polyester, a polyamide, a polycarbonate or a styrenic resin.

In one embodiment, the base resin (A) comprises a polyamide and particularly a glass-filled polyamide, typically containing from 15 to 40 % glass by weight of the total weight of the polyamide and glass. DESCRIPTION OF THE EMBODIMENTS

Described herein is a benzylic-substituted organophosphorus flame-retardant compound, and a flame-retardant resin composition comprising a base resin (A), a benzylic-substituted organophosphorus compound (B) and optionally at least one flame retardant adjuvant material (C).

Base Resin

The base resin can be any organic macromolecular material, such as a polyester-series resin, a styrenic resin, a polyamide-series resin, a polycarbonate-series resin, a vinyl-series resin, an olefinic resin, an acrylic resin, an epoxy resin, a blend of two or more of said resins or a blend of one or more of said resins with a polyphenylene oxide-series resin. The base resin can be a thermoplastic or a thermoset resin and in some embodiments further comprise a reinforcing agent, e.g., a glass reinforced resin. Particularly preferred are engineering resins, such as polyester-series resins, polyamide-series resins and polycarbonates, especially glass-filled polyesters and polyamides.

Polyester-series resins include homopolyesters and copolyesters obtained by, for example, polycondensation of a dicarboxylic acid component and a diol component, and

polycondensation of a hydroxycarboxylic acid or a lactone component. Preferred polyester- series resins usually include a saturated polyester-series resin, in particular an aromatic saturated polyester-series resin, such as polybutylene terephthalate.

Polyamide-series resins include polyamides derived from a diamine and a dicarboxylic acid; polyamides obtained from an aminocarboxylic acid, if necessary in combination with a diamine and/or a dicarboxylic acid; and polyamides derived from a lactam, if necessary in combination with a diamine and/or a dicarboxylic acid. The polyamide also includes a copolyamide derived from at least two different kinds of polyamide constituent components.

Suitable polyamide-series resins include aliphatic polyamides (such as nylon 46, nylon 6, nylon 66, nylon 610, nylon 612, nylon 1 1 and nylon 12), polyamides obtained from an aromatic dicarboxylic acid (e.g., terephthalic acid and/or isophthalic acid) and an aliphatic diamine (e.g., hexamethylenediamine, nonamethylenediamine), and polyamides obtained from both aromatic and aliphatic dicarboxylic acids (e.g., both terephthalic acid and adipic acid) and an aliphatic diamine (e.g., hexamethylenediamine), and others. These polyamides may be used singly or in combination.

The base resin may be composited with a filler to modify its properties, such as mechanical strength, rigidity, thermal stability and electrical conductivity. The filler may be fibrous or non- fibrous. Suitable fibrous fillers include glass fibers, asbestos fibers, carbon fibers, silica fibers, fibrous wollastonite, silica-alumina fibers, zirconia fibers, potassium titanate fibers, metal fibers, and organic fibers having high melting point (e.g., an aliphatic or aromatic polyamide, an aromatic polyester, a fluorine-containing resin, an acrylic resin such as a polyacrylonitrile). Suitable non-fibrous fillers include plate-like (or layered) fillers, such as kaolin, talc, glass flakes, mica, graphite, metal foil, and layered phosphates (e.g., zirconium phosphate, and titanium phosphate). In addition, particulate or amorphous fillers can be used including carbon black, white carbon, silicon carbide, silica, powdered quartz, glass beads, glass powder, milled fibers (such as milled glass fiber), silicates (e.g., calcium silicate, aluminum silicate, clays, diatomites), metal oxides (e.g., iron oxide, titanium oxide, zinc oxide, and alumina), metal carbonates (e.g., calcium carbonate and magnesium carbonate), metal sulfates (e.g., calcium sulfate and barium sulfate), and metal powders.

Preferred fillers include glass fiber and carbon fiber. In one embodiment the base resin comprises a glass-filled polyamide containing from 15 to 40 % glass fiber by weight of the total weight of the polyamide and glass.

Organophosphorus Compound

The organophosphorus compound employed in the present flame-retardant resin composition can be represented by at least one of the following formulas (I), (la), (II) and (III):

where A is selected from O, S, S0 ₂ , a single bond, and alkyl;

P is a phosphorus-containing group of the formula:

wherein R , R ² and Y, together with the phosphorus atom to which R and R ² are bound form a ring, wherein R and R ² are the same or different and each is selected from O-alkyI, O-aryl, alkyl and aryl, and Y is a linking group selected from direct bond, alkylene and -0-, e.g., Y is a direct bond;

R ³ is H or alkyl;

each a is an integer individually selected from 0 to 4, e.g., 0 1 , 2, 3, or 4 provided that at least one a is at least 1 ;

n is an integer from 1 to 100,000, e.g., 2, 3, 4 or 5 to 100,000 and m is an integer from 0 to 100,000, e.g., 1 , 2, 3, 4 or 5 to 100,000, e.g., n is an integer from 4 to 100,000, or 5 to 100,000, and m is an integer from 1 to 100,000.

Representative benzylic-substituted organophosphorus compounds within the above formulas include the cyclic phosphinates shown below:

wherein b is 2 or 3,

Conveniently, the organophosphorus compound (B) is present in an amount of from 10% to 40%, e.g., 10% to 30 %, by weight of the flame-retardant resin composition.

Flame Retardant Adjuvant

To enhance its flame retardant properties, the present resin composition can include at least one flame retardant adjuvant material in addition to the organophosphorus compound described above. Suitable flame retardant adjuvant materials comprise melamine and melamine derivatives such as melamine salts and condensation products of melamine, inorganic metal compounds, clay compounds, layered double hydroxide materials, polyphenylene ether resins and mixtures thereof.

Suitable melamine salts include salts of melamine itself as well as salts of melamine derivatives, such as substituted melamines (e.g., an alkylmelamine such as 2-methylmelamine,

guanylmelamine), condensation products of melamine (e.g., melam, melem, melon), and copolycondensed resins of melamine (e.g., melamine-formaldehyde resins, phenol-melamine resins, benzoguanamine-melamine resins and aromatic polyamine-melamine resins). Generally the salts are produced by reaction of the melamine with an oxygen-containing acid, such as nitric acid, a chloric acid (such as perchloric acid, chloric acid, chlorous acid, hypochlorous acid), a phosphorous acid, a sulfuric acid, a sulfonic acid, a boric acid, a chromic acid, an antimonic acid, a molybdic acid, a tungstic acid, stannic acid, or silicic acid.

Examples of suitable melamine salts include melamine orthophosphate, melamine phosphate, melamine pyrophosphates (including melamine pyrophosphate and dimelamine

pyrophosphate), melamine polyphosphates (including melamine triphosphate and melamine tetraphosphate), melamine sulfates (including melamine sulfate, dimelamine sulfate and guanylmelamine sulfate), melamine pyrosulfates (e.g., melamine pyrosulfate and dimelamine pyrosulfate), melamine sulfonates (e.g., melamine methanesulfonate, melam methanesulfonate, and melem methanesulfonate) and melamine orthoborates (e.g., mono- to trimelamine orthoborates).

The preferred melamine salts are melamine pyrophosphates. Preferred condensation products of melamine comprise melam, melem or melon.

Suitable inorganic metal compounds for use in the present synergist combination include metal salts of inorganic acids, metal oxides and hydroxides, and metal sulfides.

Where the inorganic metal compound is a metal salt of an inorganic acid, suitable inorganic acids include phosphorous acids (such as orthophosphoric acid, metaphosphoric acid, phosphorous acid, hypophosphorous acid, hypophosphoric, pyrophosphoric acid,

polyphosphoric acids, anhydrous phosphoric acid and polymetaphosphoric acid), boric acids (such as orthoboric acid, metaboric acid; pyroboric acid, tetraboric acid, pentaboric acid and octaboric acid), a stannic acid (such as stannic acid, metastannic acid, orthostannic acid and hexahydroxostannic acid), a molybdic acid, and a tungstic acid.

Examples of suitable metal salts include calcium pyrophosphate, calcium polymetaphosphate, alkaline earth metal hydrogenphosphates (such as magnesium hydrogen orthophosphate and calcium hydrogen orthophosphate); transition metal hydrogenphosphate (such as manganese hydrogenphosphate, iron hydrogenphosphate, zinc hydrogenphosphate and cadmium hydrogenphosphate); a hydrogenphosphate of a metal of Group 13 of the Periodic Table of Elements (such as aluminum hydrogenphosphate); a hydrogenphosphate of a metal of Group 14 of the Periodic Table of Elements (such as tin hydrogenphosphate), an alkaline earth metal borate (such as calcium orthoborate, calcium metaborate, calcium pyroborate and trimagnesium tetraborate); a transition metal borate (such as manganese orthoborate, manganese tetraborate, nickel diborate, copper metaborate, zinc metaborate, zinc tetraborate, cadmium metaborate and cadmium tetraborate), an alkali metal stannate (e.g., sodium stannate and potassium stannate), an alkaline earth metal stannate (e.g., magnesium stannate), a transition metal stannate (e.g., cobalt stannate and zinc stannate), zinc molybdate and zinc tungstate.

Examples of suitable metal oxides and hydroxides include molybdenum oxide, tungstic oxide, titanium oxide, zirconium oxide, tin oxide, copper oxide, zinc oxide, aluminum oxide, nickel oxide, iron oxide, manganese oxide, antimony trioxide, antimony tetraoxide, antimony pentoxide, aluminum hydroxide, magnesium hydroxide, tin hydroxide, and zirconium hydroxide. Mixed oxides such as aluminosilicates, including clays, can also be used.

Examples of suitable metal sulfides include molybdenum sulfide, tungstic sulfide and zinc sulfide. The preferred inorganic metal compounds are borates and particularly zinc borates.

In one embodiment, the flame retardant adjuvant is a combination of a melamine salt and an inorganic metal compound in a weight ratio of from 10:1 to 1 :1 . In another embodiment, the flame retardant adjuvant comprises a combination of a melamine condensation product and an inorganic metal compound in a weight ratio of from 10:1 to 1 :1 .

In another embodiment, the flame retardant adjuvant material is a char-forming organic compound such as a polyphenylene ether (PPE) type resin. A specific PPE resin example would be PPO 803 from Sabic Innovative Plastics. The PPE resin can be used typically at a load level of up to 50%, e.g., from 1 % to 25%, more typically from 5% to 15%.

EXAMPLES

Preparation of DOPO-Methylene-poly(phenylene ether):

Lithium bis(trimethylsilyl)amide-[LiHMDS] (52 g, 0.31 mol) and THF (220 mL) were added to a 1 L round-bottom flask fitted with an over-head stirrer, temperature probe, addition funnel and reflux condenser. The solution was placed under N ₂ , cooled to 5 °C (ice bath) and 9,10- dihydro-9-oxa-10-phosphaphenanthrene 10-oxide, i.e., DOPO, (70 g, 0.32 mol) was added portion-wise over 10 minutes while maintaining an internal temperature below 10 'C. Upon complete addition, bromo-methylated poly(phenylene ether (44 g, 0.27 mol) (as described in Example 3, but with slightly higher molecular weight) in THF (120 mL) was added via addition funnel over 1 hr. During the addition, the internal temperature rose to 25 'C and a yellow/white slurry was obtained. Upon complete addition, the reaction slurry was allowed to stir at room temperature for 16 hours and then concentrated to a solid under reduced pressure. The resulting yellow solid was taken up in methylene chloride (300 mL), washed with H ₂ 0, dried, filtered and concentrated to provide an off white solid. This solid was placed in a vacuum oven (210 'C, 29 in. Hg) for 3 hours, to provide the title compound (87 g) as a glass after cooling to room temperature.

Flame Retardants in Glass-Reinforced PA66:

The flame retardant compound described in the Example above was formulated with PA66 resin by mixing the materials either with a twin screw extruder using a PA66 glass concentrate to produce a target 30% glass fiber reinforcement, or in a vented Brabender Preparation Center. In the Brabender Preparation Center the samples were compounded for four minutes at 265 °C. About 3 minutes after the start of mixing, glass fiber was added over a period of about 15 seconds to produce a target 30% glass fiber reinforcement. The compounded materials were ground on a Thomas Wiley mill and then molded by using a small injection molding machine at 265 °C to form 1/16" thick UL-94 test bars. The test bars were subjected to the UL-94 vertical burn test protocol in which a bar is subjected to two 10 second flame applications. The time for the bar to extinguish after each application is noted and reported as Time 1 (T1 ) and Time 2 (T2) for the bar. The average burn times for 5 test bars were noted for both flame applications (T1 , T2). The total burn times were also summed for each of the two flame applications across all five test bars along with the observations of any dripping behavior. The results are shown in Table 1 .

Table 1 . Testing of Compound XI with Melem in 30% GR PA66.

0.4% PTFE (DYNEON TF 9205); 2% Surlyn

² Burning bar/chunk which separated at the clamp & continued to burn on the chamber floor DELACAL NFR HP is a multi-component material comprised of the two main homologues melem and melam, available from Delamin, Ltd.

This data shows that at certain load levels of FR or adjuvant synergist in combination, a strong flame retardant system can be achieved.