Phosphate ester compositions and process for preparing them

Abstract

 Flame retarding and plasticizing compositions for vinyl-chloride polymers comprising (i) a first phosphat ester component containing at least one C₆-₁₂ alkyl diphenyl phosphate and (ii) triphenyl phosphate having proportions of (i) to (ii) in the range of 1.8:1 to 5.5:1 are stable homogeneous liquids at low temperatures when prepared by a process in which a phosphorus oxyhalide is reacted with an excess of an alkali metal phenate to form a mixture containing triphenyl phosphate and alkali metal phenate, and the alkali metal phenate in that mixture is reacted with a C₆-₁2 alkyl phosphrordichloridate.

Description

[0001] This invention relates to phosphate ester compositions that are useful as flame retardant plasticizers for vinyl chloride polymers.

[0002] Alkyl diaryl phosphates, for example 2-ethylhexyl diphenyl phosphate, are used as plasticizers for vinyl chloride polymers which are to be used under conditions where fire resistant properties in the polymer are required. Commercial alkyl diaryl phosphates normally contain small amounts, for example not more than 5% by weight of the total phosphate,of dialkyl aryl phosphates and triaryl phosphates. It has also been proposed to use halogen-containing phosphate esters for this purpose, but the halogen-containing esters are generally more expensive than hydrocarbyl esters, and their use is considered by some to be undesirable because of possible adverse environmental effects.

[0003] Triaryl phosphates, for example triphenyl phosphate, impart improved flame retardant properties to vinyl chloride polymers compared with the alkyl diaryl phosphates but the latter are the more effective plasticizers as judged by the physical properties of the vinyl chloride polymers containing them, especially by flexibility at low temperatures. Moreover, triphenyl phosphate is a crystalline solid at ordinary temperatures, and its incorporation into vinyl chloride polymers therefore requires a different 'and generally more complicated procedure than that used for the liquid alkyl diaryl phosphates.

[0004] We have found that certain blends of alkyl diphenyl phosphates and triphenyl phosphate can be prepared containing sufficient triphenyl phosphate to impart improved flame retardant properties to vinyl chloride polymers plasticized with the blends, while not significantly detracting from the physical and processing properties of the thus- plasticized polymers relative to the same vinyl chloride polymers plasticized with alkyl diaryl phosphates alone.

[0005] A simple way of making such blends is to dissolve the triphenyl phosphate in the alkyl diphenyl phosphate at a temperature at which the triphenyl phosphate dissolves readily, but we have found that triphenyl phosphate crystallizes from such blends on storage at temperatures of 0°C. or below. Blends prepared in this way are therefore not suitable as commercial products.

[0006] The present invention provides blends of alkyl diphenyl phosphates having good low temperature storage stability and a process for the production of such blends.

[0007] A phosphate ester composition of the invention is a liquid comprising (i) a first phosphate ester component consisting of at least one C_6-12 alkyl diphenyl phosphate or a mixture of at least one C_6-12 alkyl diphenyl phosphate and at least one di(C_6-12 alkyl) phenyl phosphate in which mixture the amount of the di(C_6-12 alkyl) phenyl phosphate or phosphates is not more than 10 molar percent of the mixture, and (ii) triphenyl phosphate, the ratio by weight of the first phosphate ester component to the triphenyl phosphate in the composition being from 1.8:1 to 5.5:1, the phosphate ester composition-having a low temperature storage stability such that no crystallisation of triphenyl phosphate occurs during storage of the composition during 10 days at -5 C.

[0008] The process of the invention is one for the production of a liquid phosphate ester composition which comprises reacting a phosphorus oxyhalide with a stoichiometric excess of an alkali metal phenate to form a mixture containing triphenyl phosphate and alkali metal phenate, and reacting the alkali metal phenate in that mixture with a dihalidate reactant comprising at least one C_6-12 alkyl phosphorodihalidate, the proportions of the reactants being selected so that the liquid phosphate ester composition contains (i) a first phosphate ester component consisting of at least one C_6-12 alkyl diphenyl phosphate or a mixture of at least one C_6-12 alkyl diphenyl phosphate with at least one di(C_6-12 alkyl) phenyl phosphate in which mixture the amount of the di(C_6-12 alkyl) phenyl phosphate or phosphates is not more than 10 molar percent of the mixture and (ii) triphenyl phosphate, the ratio by weight of the first phosphate ester component to the triphenyl phosphate being from 1.8:1 to 5.5:1.

[0009] In preferred compositions, the ratio by weight of the first phosphate ester component to the triphenyl phosphate is in range 1.85:1 to 5:1. The actual proportion in any particular instance will depend on a balance between increasing the amount of triphenyl phosphate to improve the flame-retardant properties of the compositions and achieving low temperature stability, the latter being improved as the proportion of triphenyl phosphate decreases. Compositions of the invention may, for example, have weight ratios of the first phosphate ester component to triphenyl phosphate within the range 2:1 to 2.5:1, or within the range 3:1 to 5:1.

[0010] Storage stabilities of the compositions of the invention are usually better than the minimum requirement that no crystallisation of triphenyl phosphate should occur during storage for 10 days at -5°C. For example, storage stabilities such that no crystallisation of triphenyl phosphate occurs during storage for 10 days or longer at temperatures below -5°C., for example 10 days at -10°C. or -13°C, or 20 days at -20°C. are often observed.

[0011] The dihalidate reactant for use in the process of the invention can be prepared by the reaction of a C_6-12 alkanol or a mixture of C_6-12 alkanols with phosphorus oxyhalide, using 1 mole of alkanol per mole of phosphorus oxyhalide, with removal of the hydrogen halide generated by this reaction. Under normal process conditions, a small amount of di(C_6-12 alkyl)phosphorohalidate, usually not exceeding one-tenth on a molar basis of the amount of the dihalidate reactant,is formed, as well as the C_6-12 alkyl phosphorodihalidate which is the major product. In practice it is unnecessary to remove the di(C_6-12 alkyl) phosphorohalidate, so that the final liquid phosphate ester composition will normally contain an amount of di(C_6-12 alkyl) phenyl phosphate corresponding to the amount of di(C_6-12 alkyl) phosphorohalidate in the dihalidate reactant.

[0012] The steps of the process as defined above appear to be critical for the production of a product having the required degree of low temperature stability. For example, the concurrent formation of the alkyl diphenyl phosphate and the triphenyl phosphate by the reaction of a mixture of phosphorus oxyhalide and alkyl phosphorodihalidate with an alkali metal phenate in the appropriate proportions gives a product which is significantly less stable.

[0013] In a preferred way of operating the process, an amount of phosphorus oxyhalide corresponding to the required amount of triphenyl phosphate is added with agitation and cooling to an aqueous solution of alkali metal phenate containing sufficient alkali metal phenate to react with the whole of the phosphorus oxyhalide and with the dihalidate reactant to be added subsequently. The reaction of the phosphorus oxyhalide with the alkali metal phenate is highly exothermic, and the temperature is preferably controlled below 50 C., preferably in the range 15-30°C.

[0014] When the addition of the phosphorus oxyhalide is complete, addition of the dihalidate reactant to the reaction mixture is begun, again with agitation and cooling of the reaction mixture to keep the temperature below a preferred 50 C., for example within the range 15-30°C.

[0015] It is preferred to employ a small excess, for example from 2 to 10% more than the stoichiometric amount, of the alkali metal phenate overall. It is also preferred to use an alkali metal phenate solution containing free alkali metal hydroxide, for example up to 20% on a molar basis of the amount of alkali metal phenate.

[0016] When the addition of the dihalidate reactant is complete, stirring may be continued for a further period to ensure completion of the reaction. The reaction mixture is then preferably cooled to 5-15 C., preferably after the addition, if necessary, of sufficient water to retain in solution the alkali metal halide formed during the process, and the agitation discontinued. The reaction mixture thereafter separates into an aqueous layer containing alkali metal halide, and an organic layer comprising the phosphate ester composition. The organic layer is separated from the aqueous layer, and can be purified by conventional techniques, including for example washing with water and vacuum treatment.

[0017] The phosphorus oxyhalide usually employed in practice is phosphorus oxychloride,but phosphorus oxybromide, for example, could be used; and the usual alkali metal phenate is sodium phenate, but potassium phenate could be used.

[0018] In the compositions of the invention, the C_6-12 alkyl group can be a straight or branched-chain group, for example a n-hexyl, n-octyl, 2-ethylhexyl, n-decyl or n-dodecyl group, the corresponding alcohols used to prepare the dihalidate reactant being n-hexanol, n-octanol, 2-ethylhexanol, n-decanol and n-dodecanol. A mixture of alcohols can be used, resulting in the first phosphate ester component containing more than one C_6-12 alkyl diphenyl phosphate.

[0019] .A phosphate ester composition of the invention may contain only the first phosphate ester component and the triphenyl phosphate, but minor amounts of other ingredients? such as diluents, compatible plasticizers or flame retardants may be added if desired.

[0020] The preferred compositions are those that contain no significant amounts of other ingredients, and in such compositions, the weight ratio ranges referred to above include compositions containing from 65% to 75% by weight of the first phosphate ester component, the balance (i.e. from 35% to 25% by weight of the composition) being triphenyl phosphate, and compositions containing from 75 to 85%, for example 80%, by weight of the first phosphate ester component, the balance being triphenyl phosphate. Particularly useful compositions contain 68-72% by weight 2-ethylhexyl diphenyl phosphate, 4-8% by weight di(2-ethylhexyl) phenyl phosphate, the balance being triphenyl phosphate.

[0021] The phosphate ester compositions of the present invention can be used as flame retardaht plasticizers for vinyl chloride polymers in the same manner and in substantially the same amounts as the klkyl diphenyl phosphates used hitherto. The term "vinyl chloride polymer" includes polyvinyl chloride homopolymer and polymers obtained by the copolymerization of vinyl chloride with a minor amount of one or more monomers copolymerizable with vinyl chloride, for example olefins such as ethylene and propylene, vinyl ethers and alkyl acrylates and methacrylates.

[0022] The invention is illustrated by the following Examples.

EXAMPLE I

a) Preparation of alkyl phosphorodichloridate.

[0023] 2-Ethylhexanol (137 grams) was added gradually to a stirred and cooled reactor containing 162 grams of phosphorus oxychloride while keeping the temperature of the reaction mixture at 13-17°C. To remove hydrogen chloride formed during the reaction, the pressure in the reactor was reduced to 70 mbar and the temperature was raised to 25⁰C. These conditions were maintained for 60 minutes and the 2-ethylhexyl phosphorodichloridate was then cooled to 15°C. and the vacuum released.

b) Preparation of sodium phenate solution.

[0024] A 22% by weight solution of sodium hydroxide was prepared by diluting 368 grams of 46% by weight sodium hydroxide solution with 402 grams of water. To this solution,347 grams of phenol were added gradually, keeping the temperature of the solution at 15-20°C.

c) Preparation of phosphate ester composition.

[0025] Phosphorus oxychloride (78.3 grams) was added gradually to the sodium phenate solution in a stirred and cooled reactor, the rate of addition being adjusted to maintain the temperature of the reactants at 20-25°C. The 2-ethylhexyl phosphorodichloridate was then added, again keeping the temperature at 20-25°C., and stirring (without cooling) was continued for 45 minutes after the addition of the phosphorodichloridate was complete. Water (300 grams) was then added and stirring was continued for a further 15 minutes and during subsequent cooling to 15⁰C. Stirring was then stopped, and the mixture was allowed to settle for 60 minutes, forming two layers, an upper aqueous layer and a lower organic layer.

[0026] The two layers were separated, and the organic layer was washed twice at 90°C, with an equal volume of 2% by weight sodium hydroxide solution containing a small amount of a sequestering agent, and a further four times with an equal volume of water at a temperature of 70-80°C. After the final wash the phosphate ester product was steam stripped under vacuum to remove any residual low-boiling material, and then filtered.

[0027] The phosphate ester composition prepared as described above contained about 66% by weight of 2-ethylhexyl diphenyl phosphate, about 4% by weight of di(2-ethylhexyl) phenyl phosphate and 30% by weight of triphenyl phosphate. The composition showed no signs of crystallization during 6 weeks storage at -13°C. or at -20°C. In comparison, a composition containing the same proportions of phosphate ester ingredients but prepared by dissolving triphenyl phosphate in the other component showed turbidity after one week and an extensive deposit after two weeks at -13°C.

EXAMPLES 2-4

[0028] Other compositions within the scope of the invention were prepared by the same procedure as that described in Example 1, but with different amounts of reactants in stages b) and c). The amounts of reactants, together with the proportions of 2-ethylhexyl diphenyl phosphate (EDP), di(2-ethylhexyl) phenyl phosphate (DPP) and triphenyl phosphate (TPP) in the products, are shown in the table below.

Claims

1. A liquid phosphate ester composition comprising (i) a first phosphate ester component consisting of at least one C_6-12 alkyl diphenyl phosphate or a mixture of at least one C_6-12 alkyl diphenyl phosphate and at least one di(C_6-12 alkyl) phenyl phosphate in which mixture the amount of the di(C_6-12 alkyl) phenyl phosphate or phosphates is not more than 10 molar percent of the mixture, and (ii) triphenyl phosphate; characterised in that the ratio by weight of the first phosphate ester component to the triphenyl phosphate in the composition is from 1.8:1 to 5.5:1, and the phosphate ester composition has a low temperature storage stability such that no crystallisation of triphenyl phosphate occurs during storage of the composition during 10 days at -5°C. -

2. A composition according to Claim 1 characterised in that the storage stability is such that no crystallisation of triphenyl phosphate occurs during storage of the composition during 10 days at -13°C.

3. A composition according to Claim 1 characterised in that the first phosphate ester component is a mixture of 2-ethylhexyl diphenyl phosphate and di(2-ethylhexyl) phenyl phosphate containing not more than 10 molar percent of the mixture of di(2-ethylhexyl) phenyl phosphate.

4. A composition according to any of Claims 1 to 3 characterised in that the ratio by weight of the first phosphate ester component to the triphenyl phosphate in the composition is within the range 2:1 to 2.5:1.

5. A composition according to any of Claims 1 to 3 characterised in that the ratio by weight of the first phosphate ester component to the triphenyl phosphate in the composition is within the range 3:1 to 5:1.

6. A process for the production of a phosphate ester by the reaction of a phosphorus oxyhalide or an alkyl phosphorodihalidate with an alkali metal phenate., characterised in that, for the production of a liquid phosphate ester composition as defined in Claim 1, phosphorus oxyhalide is reacted with a stoichiometric excess (relative to the phosphorus oxyhalide) of an alkali metal phenate to form a mixture containing triphenyl phosphate and alkali metal phenate, and the alkali metal phenate in that mixture is reacted with a dihalidate reactant comprising at least one C_6-12 alkyl phosphorodihalidate, the proportions of the reactants being selected to give a liquid phosphate ester composition containing the required proportions of the first phosphate ester component and triphenyl phosphate.

7. A process according to Claim 6, characterised in that an amount of phosphorus oxyhalide corresponding to the required amount of triphenyl phosphate is added with agitation and cooling to an aqueous solution of alkali metal phenate containing sufficient alkali metal phenate to react with the whole of the phosphorus oxyhalide and with the dihalidate reactant to be added subsequently and when the addition of the phosphorus oxyhalide is complete, the dihalidate reactant is added to the aqueous reaction mixture with agitation and cooling.

8. A process according to Claim 7, characterised in that the original amount of alkali metal phenate is from 2 to 10% more than the stoichiometric amount relative to the phosphorus oxyhalide and dihalidate reactant together.

9. A process according to either of Claims 7 and 8, characterised in that the reaction temperature during the reaction of the phosphorus oxyhalide with the alkali metal phenate and during the reaction of the dihalidate reactant with the alkali metal phenate is maintained in the range 15-30°C.

10. A process according to any of Claims 6 to 9, characterised in that the phosphorus oxyhalide is phosphorus oxychloride, the alkali metal phenate is sodium phenate, and the alkyl phosphorodihalidate is an alkyl phosphorodichloridate.