Bicyclic phosphate ether, ester, and carbonate intumescent flame retardant compositions

Abstract: A flame resistant composition has (1) an organic polymeric substance (such as polypropylene) in intimate contact with (2) an ether, ester, or carbonate derivative of a bicyclic phosphorous compound (such as pentaerythritol phosphate) and (3) a gas producing compound (such as melamine or ammonium polyphosphate).

Description: This invention relates to flame retardant compounds which may be contacted with organic substances to reduce the tendency of the organic substance to flame. In particular, this invention relates to such flame retardants which act, at least in part, by their ability to intumesce. For reasons of customer demand, government regulations, and product stewardship, those manufacturing and selling organic substances, particularly synthetic resins, have sought to reduce the tendency of those substances to flame. A suitable flame retardant additive must be economical to purchase, relatively nontoxic, easy to incorporate into the organic substance, nonmigrating, thermally stable, and effective at reducing the tendency of the substance to flame. The use of flame retardants is not entirely satisfactory in some instances due to the tendency of many such compositions to be toxic, corrosive, or contributors to smoke formation. One approach toward a workable flame retardant is that of intumescent flame retardants. Organic substances contacted with intumescent flame retardants behave generally as do organic substances contacted with other flame retardants, but upon contact with a flame, they soften, bubble, char, and swell, forming a thick, relatively non-flammable, thermally insulative barrier. Intumescent flame retardants are generally believed to require (a) a polyol component, (b) a dehydration catalyst (eg: phosphoric acid), and (c) a blowing agent. In some instances, a single molecule may provide two or even all three of the components. For instance, pentaerythritol phosphate may be regarded as providing a source of both the polyol component and the dehydration catalyst. In general, intumescent flame retardants are relatively non-toxic, non-corrosive, and low smoke contributors. However, they are also thermally unstable and tend to degrade at the temperature commonly encountered in polymer processing, resulting in discolorization of the polymer. US-A-4,454,064 teaches the synthesis of pentaerythritol phosphate and its usefulness as an intermediate in the preparation of unnamed flame retardant materials. US-A-4,341,694 teaches the use of pentaerythritol phosphate as an intumescent flame retardant in polymers such as polyolefins. US-A-3,808,296 discloses various hindred phenol esters of pentaerythritol phosphate and their use as "stabilizers" (ie: antioxidants), including such use with a multitude of coaditives including nitrogen-containing basic costabilizers such as melamine. US-A-4,584,331 teaches the use of various ester and ether derivatives of pentaerythritol phosphate as (non-intumescent) flame retardants for a specific polyphenylene ether composition. US-A-3,873,496 teaches the use of pentaerythritol phosphate as a non-intumescent flame retardant in polyester. GB 889,338 teaches the synthesis and use of pentaerythritol phosphate as a stabilizer for vinyl halide resins. In one respect, the invention is a flame resistant composition comprising a polyolefin, polyester, polyamide, ABS, polyvinylaromatic, polyacrylate, polyurethane, or polyphenylene oxide having in intimate contact therewith, a synergistic flame retardant amount of: (a) 12 - 30 wt.%, based on the weight of the composition, of a bicyclic phosphorus compound of the formula wherein m is 1 to 3; n is 1 or 2; p is 0 or 1; q is 1 to 3; and R is hydrogen, methyl, ethyl or methoxy, when p is 1 and n is 1; R is methyl, when p is 0 and n is 1, and R is carbonyl, when p is 0 and n is 2; and (b) 4 - 20 wt.%, based on the weight of the composition, of a gas producing compound which releases a non-flammable gas upon application of a flame. According to the invention preference is given to a bicyclic phosphorus compound wherein m is 1 and q is 1. Compared to the prior art compositions containing pentaerythritol phosphate, the flame resistant compositions of the invention are particularly thermally stable, resistant to solvent leaching, and non-migrating. In view of the fact that the pentaerythritol phosphate derivatives employed in the invention have less phosphorus (weight percent based on the entire molecule) than does pentaerythritol phosphate, the superior performance of these compositions is particularly surprising. The bicyclic phosphorus compounds of the invention may be easily prepared by one skilled in the art. The ester version of the bicyclic phosphorus compound may be prepared by the reaction of pentaerythritol phosphate (2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide) with a corresponding carboxylic acid, anhydride, or acid chloride. The carbonate version of the bicyclic phosphorus compound may be prepared by reaction of pentaerythritol phosphate with chloroformate, or carbonyldimidazole followed by reaction with an alcohol. The ether version of the bicyclic phosphorus compound can be prepared by reacting pentaerythritol phosphate with an alkyl halide. The pentaerythritol phosphate starting material may be prepared by the methods taught in aformentioned US-A-4,341,694, US-A-3,873,496, and GB-A-889,338. A second component of the invention is a gas producing compound which is capable of evolving a non-flammable gas upon the application of a flame. Such compounds are generally nitrogen compounds which thermally degrade to yield a nitrogen-containing gas. Such compounds are conventional in intumescent flame retardants and are well known to those skilled in the art. Several such compounds are described in aforementioned US 4,341,694. Desirable gas producing compounds include amines, ureas, guanidines, guanamines, s-triazines, amino acids, and salts thereof. Preferred compounds include ammonium polyphosphate, melamine pyrophosphate, and melamine. Ammonium polyphosphate is a preferred gas producing compound because of its high level of activity and its complementary effect with the bicyclic phosphorus compound (reducing the level of bicyclic phosphorus compound required for a UL 94 rating of VO). Both melamine pyrophosphate and ammonium polyphosphate contain phosphorus which is believed to be released in the form of phosphoric acid. These compounds therefore contribute to the flame retardance by providing a dehydration catalyst as well as acting as a blowing agent. A mixture of gas producing compounds may be used if desired. The third component of the invention include polyolefins such as polypropylene (especially including polypropylene copolymers containing, for instance, up to 20% comonomer such as polyethylene), polybutylene, and EPDM (ethylene propylene dimer); polyesters such as polybutyleneterephthalate and polyethyleneterephthalate; polyamides such as nylon 6; ABS (acrylonitrile-butadiene-styrene); polyvinylaromatics such as polystyrene; polyacrylates such as polymethylmethacrylate; polyurethanes; and polyphenylene oxides. Preferred polymers are polyolefins, polyesters and polyamides. More preferred are polyolefins and polyesters Still more preferred are polyolefins, and most preferred is polypropylene (including copolymers thereof). The invention requires that the bicyclic phosphorus compound and the gas producing compound (the flame retardant compounds) be intimately contacted with the organic substance. By "intimately contacted" is meant that the flame retardant compounds and the organic substance all be finely divided and uniformly distributed. This is, of course, a matter of degree, and any degree of contact which improves the flame resistance properties of the organic substance is acceptable. Further, the nature of "finely divided" and "uniformly distributed" will vary according to the nature of the organic substance. Obviously, an insoluble polymer which is highly filled with a coarse filler will allow for less uniform distribution than an unfilled polymer which can be solution blended with the flame retardant compounds. A particular advantage of many compounds of the invention is that they have melting points below that used to melt blend most polymers, which permits more even dispersion of the compounds in the organic substance. The flame retardant compounds are incorporated into the organic substance in a synergistic flame retardant amount. By "synergistic flame retardant amount" is meant an amount which will render the organic substance more flame resistant than if the flame retardant compound were totally absent. The precise amount of the flame retardant compounds which are most advantageously added to the organic substance will vary with the particular flame retardants, the particular organic substance, and the degree of intimate contact. The bicyclic phosphorus compound will be present at 12 to 30 weight percent based on the weight of the organic substance including all additives. The gas producing compound will be present at 4 to 20 weight percent based on the weight of the organic substance including all additives. For any given combination of bicyclic phosphorus compound and gas producing compound there will be an optimum ratio for achieving the best flame retardancy. This optimum ratio can easily be determined by simple trials. The invention is further described in the following examples. EXAMPLE I 2,6,7-Trioxa-1-phosphabicyclo[2.2.2]octano-4-methanol-1-oxide (45 g, 0.25 mole) and acetic anhydride (100 g, 1 mole) were charged to a 500 ml reactor and heated at reflux for one hour. After cooling the excess acetic anhydride and by-product acetic acid were largely removed by evaporation under reduced pressure to leave a sticky white solid. This solid was recrystallized from dichloromethane and hexane, followed by drying in a fluidized bed at 100°C, to give 47.2 g of white crystals having a melting point of 170-172°C. Carbon-13 and phosphorus-31 NMR spectroscopy confirmed this material to be the desired acetate (Compound A, end group = CH₃C(O)). EXAMPLE II 2,6,7-Trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide (180 g, 1 mole) and 96% formic acid (280 g) were charged to a one litre reactor. Concentrated hydrochloric acid (10 drops) was added and the mixture heated under reflux for six hours. Excess formic acid was then removed under reduced pressure by distillation, leaving 227 g of a white solid. This material was subjected to column chromatography on silica using 9:1 v/v dichloromethane:methanol as eluent, followed by recrystallization from dichloromethane, methanol and diethyl ether to afford a white solid having a melting point of 164-165°C. The structure was confirmed by IR and NMR spectroscopy to correspond to that of the desired formate (Compound B, end group = HC(O)). EXAMPLE III 2,6,7-Trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide (90 g, 0.5 mole), triethylamine (50.5 g, 0.5 mole), and acetonitrile (300 ml) were charged to a one litre reactor and stirred while propionyl chloride (69.4 g, 0.75 mole) was added dropwise over 20 minutes. An exothermic reaction occurred and after first becoming clear a precipitate formed. Stirring was continued for four hours and then the precipitated triethylamine hydrochloride removed by filtration. Solvent was evaporated from the filtrate to leave a sticky solid. This solid was dissolved in dichloromethane and the solution washed (2x300 ml) with water, dried over MgSO₄, filtered, and the solvent evaporated to afford 105 g of solid. This was subjected to charcoal treatment followed by recrystallization from chloroform and diethyl ether. A white solid (77 g) of melting point 96-97°C was obtained which was shown by NMR and IR spectroscopy to be the desired propionate. (Compound C, end group = CH₃CH₂C(O)-). In a similar manner the following esters of 2,6,7-trioxa-1-phosphabicyclo [2.2.2]octane-4-mechanol-1-oxide were prepared from the corresponding acid chlorides: Ester Melting Point (°C) Identification Benzoic 236-238 Compound D , R-A- = C₆H₅C(O)- Adipic 232-235 Compound E , R-A₂- = -(O)C(CH₂)₄C(O)- Succinic 218-220 Compound F , R-A₂- = -(O)C(CH₂)₂C(O)- EXAMPLE IV 2,6,7-Trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide (90 g, 0.5 mole), pyridine (39.5 g, 0.5 mole) and acetonitrile were charged to a one litre reactor and stirred while methyl chloroformate (64 g) was added dropwise. An exothermic reaction to 60°C occurred and a clear solution resulted. Additional quantities of methyl chloroformate (22 g, 60 g, 30 g, 30 g) were added at intervals over a total reaction time of 30 h (dissappearance of starting material monitored by TLC). The solvent was evaporated to afford 161 g of a brown, viscous oil which was subjected to column chromatography on silica gel using dichloromethane as eluent. This gave 36 g of a white solid which was further purified by recrystallization from dichloromethane and diethyl ether to afford 32 g of white solid having a melting point of 148-150°C, which NMR and IR spectroscopy confirmed to be the methyl carbonate derivative. (Compound G, end group = CH₃OC(O)-). [The inefficient utilization of methyl chloroformate in this reaction indicates side-reactions to be occurring]. EXAMPLE V 2,6,7-Trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol-1-oxide (90 g, 0.5 mole) and 300 ml of acetonitrile were stirred in a one litre vessel under a nitrogen atmosphere. Carbonyl diimidazole (81 g, 0.5 mole) was added portionwise over five minutes (gas was evolved). After stirring at ambient temperature for 30 minutes, ethanol (100 g) was added , followed by a small amount of potassium t-butoxide. The reaction was heated to reflux for two hours, cooled, and the solvent evaporated to leave a viscous yellow oil. This was dissolved in dichloromethane and the solution washed well with water, dried over MgSO₄, filtered, and the filtrate evaporated to afford 90.5 g of a white solid. This material was subjected to column chromatography on silica gel using a 9:1 v/v mixture of dichloromethane:methanol as eluent. Recovered solid was recrystallized from dichloromethane and diethyl ether to give a white solid having a melting point of 132-134°C, which NMR and IR spectroscopy confirmed to be the desired ethyl carbonate derivative. (Compound H , end group = CH₃CH₂OC(O)-). By similar procedures the following carbonates of 2,6,7-trioxa-1-phosphabicyclo [2.2.2]octane-4-methanol-1-oxide were also prepared: Carbonate Melting Point (°C) Identification butyl 81-83 Compound J , R-A- = CH₃(CH₂)₃OC(O)- bis 320 (decomposed) Compound K, R = -C(O)-, p = 0 EXAMPLE Va 2,6,7-Trioxa-1-phosphabicylo[2.2.2]octane-1-methanol-1-oxide (89 g; 0.5 mole), powdered potassium hydroxide (28.2 g. 0.5 mole) and dimethylsulfoxide (DMSO) (500 ml) were charged into a 1 litre flask. Methyl iodide (107 g; 0.15 mole) was added dropwise over 1 hour. An exothermic reaction resulted and the reaction turned brown. The mixture was stirred for 4 hours and the DMSO was then removed by distillation to leave a dark brown oil. Column chromatography on silica eluting with 9:1 dichloromethane:methanol followed by recrystallization from dichloromethane and ether gave 25 g of white crystals having a melting point of 131-132°C. The structure of this methyl ether derivative (Compound L, R = CH₃-, p = 0) was confirmed by ¹³C and ³¹P NMR. EXAMPLE VI Polypropylene was melt blended with various additives in the proportions shown in Table 1 by mixing in a Haake twin-rotor internal bowl mixer operating at 165-180°C. After mixing for five minutes the formulations were removed from the mixer and compression moulded between chromed steel plates at 190°C to give 1/16" (1.6 mm) thick sheets. Specimens were cut from these sheets and subjected to the Underwriters Laboratory UL94 vertical burn test to assess flammability. In the UL94 test the ratings, from best to worst, are V0, V1, V2 and F. The results are shown in Table 1. From Table 1 it will be seen that neither 2,6,7-Trioxa-1-phosphabicyclo[2.2.2] octane-4-methanol-1-oxide (Pentaerythritol phosphate, Compound X, R = H,p=0 [not an sample of the invention]) nor the acetate thereof, (Compound A), are able to impart a V0 rating to polypropylene alone, even at 30% loading. Similarly ammonium polyphosphate, melamine pyrophosphate, and melamine alone are ineffective at flame retarding polypropylene. Both X and A, however, exhibit a synergism, with respect to imparting a VO rating to polypropylene, with ammonium polyphosphate, melamine pyrophosphate and melamine. The optimum ratio of X or A to synergist varies for each particular synergist (see Table 1) but, in general, the total weight loading of X or A and synergist must be above 24% to result in a VO rating. The synergist will always contain nitrogen atoms (eg melamine) but may also contain phosphorous and oxygen atoms (melamine pyrophosphate, ammonium polyphosphate). In addition, mixtures of synergists may also be used effectively. It should be noted that Compound A of the present invention imparts V0 ratings to polypropylene at similar, or slightly lower, loadings than Compound X of US 4,341,694, when both are used with an appropriate synergist. This is surprising in that the acetate contains a lower phosphorous content than Compound X (13.96% versus 17.2% P) so that one might expect a higher loading of acetate than X to be required. EXAMPLE VII Polypropylene was compounded and assessed for UL94 flammability rating as described in Example VI using a variety of ester and carbonate analogues of Compound A. Formulations and results arc shown in Table 2. It can be seen that as the number of carbon atoms in the ester function increases, the relative ability of the additive to impart a V0 rating to polypropylene decreases. Highest activity is thus observed for the formate (Compound B). This is reasonable since the percentage phosphorus and oxygen in the additive decreases with increasing carbon content. Table 2 also illustrates not only that ester derivatives of Compound X, but also carbonic acid esters, are capable of imparting a V0 rating to polypropylene when used in conjunction with a suitable synergist; highest activity is observed when the number of carbon atoms is minimized. When the methyl carbonate (Compound G) was heated alone on a spatula with a flame a large amount of foam and char was produced (i.e. intumescent behaviour); however, this carbonate did not impart a V0 rating to polypropylene in the absence of a synergist. Ether derivative: of Compound X, such as Compound L, are also able to impart V0 ratings to polypropylene when used with synergists. EXAMPLE VIII Various grades of polypropylene homopolymer and co-polymer were processed as described in Example VI, according to the formulations given in Table 3. The UL94 results contained in Table 3 demonstrate that Compound A of the present invention when used at 12% by weight with 18% ammonium polyphosphate is capable of imparting V0 ratings to polypropylene homopolymers of widely varying melt flow indices, and also to polypropylene co-polymers. EXAMPLE IX Polypropylene formulations were prepared according to the proportions set out in Table 4 and processed into 1/16" (1.6 mm) thick sheets as described in Example VI. One set of specimens from each formulation was evaluated in the UL94 test. Two further sets of specimens were cut for each formulation and placed in the thimble of a soxhlet extraction apparatus, deionized water was used as the extraction medium. One set of specimens was extracted for 48 hours and the second set for 100 hours. Each set of specimens was dried and the weight loss was recorded. Results are shown in Table 4. It will be noted that the amount of weight lost after water extraction is less for the formulations containing Compounds A or G of the present invention than for the formulation containing the prior art compound X. EXAMPLE X The volatility/thermal stability of several of the intumescent type flame retardant additives herein considered was examined by thermogravimetric analysis using a temperature rise rate of 20°C/minute and a nitrogen flow rate or 20 ml/minute. Results are shown in Table 5. First it can be noted that the compounds of the present invention exhibit lower weight losses than the commercial intumescent flame retardant C329. The acetate and especially the formate, esters possess improved thermal stability/reduced volatility compared to the prior art Compound X. Intumescent flame retardants of improved thermal stability compared to existing materials are highly desirable since they allow a greater latitude in compounding temperatures to be exercised. EXAMPLE XI Polypropylene polymer was dry blended with either Compound X and APP (13% + 17%) or Compound A and APP (12% + 18%) and added to a Haake twin-screw extruder operating with barrel/die temperatures of 190-220°C. The extrudates were dried and injection moulded in a Newbury 50 ton machine operating with barrel/die temperatures of 225-245°C. The resulting specimens were compared to polypropylene containing no additives for tensile strength and Vicat softening point. The specimens containing the Compound A of the present invention retained 83% of the original tensile strength and a Vicar Softening point of 146°C (147°C for virgin polypropylene). The formulation containing the prior out compound X possessed a lower Vicat softening point (142°C) and retained less of the original tensile strength (79.7%). Moreover the lesser thermal stability of the prior art compound X compared to Compound A of the present invention was manifested in the colour of the specimens obtained in the above injection moulding; the surface of the specimens containing Compound X exhibited brown streaks, while the specimens containing Compound A were not discolored. These data are reported in Table 8. EXAMPLE XII Polypropylene specimens produced as in Example XI by extrusion/injection moulding containing Compound X, Compound A, or the commercial additive Charguard C329 were placed in an Atlas weatherometer and subjected to alternating periods of UV irradiation and humidity for 304 hours. The samples were then assessed for blooming of the additives to the surface of the specimen (blooming is generally considered to be an undesirable feature in a flame retardant additive). The results are shown in Table 6 and clearly demonstrate that Compound A of the present invention is much superior to the prior art compound X with respect to blooming. EXAMPLE XIII Polybutylene (Shell DP8510) was melt blended with additives in the proportions shown in Table 7 by mixing in a Haake twin-rotor internal bowl mixer operating at 110-120°C. After mixing five minutes the formulations were removed and compression moulded into sheets of 1/16" (1.6 mm) thickness. Specimens cut from these sheets were assessed for flammability by the UL94 vertical burn test. Results are shown in Table 7; it can be seen that Compound A of the present invention imparts V0 ratings to polybutylene when used in conjunction with a suitable synergist. EXAMPLE XIV Polybutylene terephthalate (General Electric Valox 420) containing 30% by weight of glass fiber was melt blended with additives, in the proportions shown in Table 7, by mixing in a Haake twin-rotor internal bowl mixer at 225°C for 5 minutes. The formulations were removed and compression moulded to 1/16" (1.6 mm) thickness at 235°C. Specimens cut from these sheets were assessed for flammability by the UL94 vertical burn test. The results are shown in Table 7 and demonstrate that compounds of the present invention can impart V0 ratings to polyester polymers. Inclusion of the flame retarding additives caused no discoloration of the polymer. Solvent Extraction Sample 4-1 4-2 4-3 % PP 70 70 70 % APP 17 18 18 % Compound X 13 -- -- % Compound A -- 12 -- % Compound G -- -- 12 UL94 Rating at 1/16" (1,6 mm) Unextracted V0 V0 V0 % Wt. loss 48 h 5.3 2.4 1.2 % Wt. loss 100 h 7.2 3.0 2.4 Weather Test Sample 6-1 6-2 6-3 % PP 70 70 70 % Compound X 13 - - % Compound A - 12 - % Charguard C329* - - 30 % APP 17 18 - Bloom after 304 hrs in Atlas Weatherometer Heavy Light Light/Moderate Fire Rating Sample 7-1 7-2 7-3 7-4 7-5 7-6 7-7 % Polybutylene 73 70 70 70 --- --- --- % PBT --- --- --- --- 70 70 70 % Compound A 18 18 15 12 18 15 12 % APP 9 12 15 18 12 15 18 UL94 Rating at 1/16" (1,6 mm) thickness V2 V0 V0 V0 V2 V0 V0 Physical Performance Sample Additive % Tensile Strength Retained Loss of Vicat (°C) Discoloration 8-1 Compound A 83.0 1 None 8-2 Compound X 79.7 5 Brown streaks The forgoing data demonstrate the utility and unexpected results of the present invention. Specifically, the data show that the compositions of the invention are as effective or more effective than the compositions of the prior art containing pentaerythritol phosphate. The data further show that the compositions of the invention are more thermally stable and more resistant to solvent extraction, weather degradation, discolorization, loss of tensible strength, and loss of viscat softening point, than the prior art compositions employing pentaerythritol phosphate.

Claims: A flame resistant composition comprising a polyolefin, polyester, polyamide, ABS, polyvinylaromatic, polyacrylate, polyurethane, or polyphenylene oxide having in intimate contact therewith, a synergistic flame retardant amount of: (a) 12 - 30 wt.%, based on the weight of the composition, of a bicyclic phosphorus compound of the formula wherein m is 1 to 3; n is 1 or 2; p is 0 or 1; q is 1 to 3; and R is hydrogen, methyl, ethyl or methoxy, when p is 1 and n is 1; R is methyl, when p is 0 and n is 1, and R is carbonyl, when p is 0 and n is 2; and (b) 4 - 20 wt.%, based on the weight of the composition, of a gas producing compound which releases a non-flammable gas upon application of a flame. The composition of claim 1, wherein m is 1 and q is 1. The composition of claim 1 or 2, wherein the gas producing compound is a nitrogen compound which thermally decomposes. The composition of claim 3, wherein the nitrogen compound is melamine, melamine pyrophosphate, or ammonium polyphosphate.

Inventors: MACK ARTHUR G, MOY PAUL Y Y, PARR WILLIAM J, MACK, ARTHUR G., MOY, PAUL Y.Y., PARR, WILLIAM J.
Applicants: AKZO NOBEL NV, AKZO NOBEL N.V.
Application year: 1988
Main classification: C08K5
IPC codes: C08K5, C07F9, C08K13