Process for preparing aluminum salts of dialkylphosphinic acids and diphosphinic acids

A process for preparing aluminum salts (I) of dialkylphosphinic acids or (formula (II)) of alkylated diphosphinic acids ##STR1## where PA1 R.sup.1 and R.sup.2 are a linear or branched C.sub.1 -C.sub.8 -alkyl radical and PA1 R.sup.3 is a linear or branched C.sub.1 -C.sub.10 -alkylene radical or an arylene radical or an alkylarylene radical or an arylalkylene radical, PA1 which comprises reacting a dialkylphosphinic acid ester of the formula (III) or a diphosphinic acid ester of the formula (IV) ##STR2## where PA1 R.sup.1, R.sup.2 and R.sup.3 are as defined above and PA1 R.sup.4 is a linear or branched C.sub.1 -C.sub.8 -alkyl radical, in the presence of a saturated aliphatic monocarboxylic acid having a total of from 1 to 7 carbon atoms, with aluminum hydroxide at a temperature of from 150 to 350.degree. C.

The invention relates to a process for preparing aluminum salts of
 dialkylphosphinic acids and diphosphinic acids.
 Aluminum salts of phosphinic acids are useful flame retardants for
 polyester and polyamide molding compositions. They are prepared from the
 phosphinic acids in aqueous solution with metal carbonates, metal
 hydroxides or metal oxides (EP-A2-0 699 708).
 In the prior art, conversion of phosphinic acid esters to the corresponding
 phosphinic acids to hydrolysis with an excess of water under pressure at
 180.degree. C. gives good yields only if the alcohol formed is removed as
 a mixture with water from the gas phase of the autoclave (Houben-Weyl,
 Methoden der organischen Chemie [Methods of Organic Chemistry], 1982, Vol.
 E2, page 142; DE-A1-27 45 982).
 It was then found that the aluminum salts of phosphinic acids can be
 prepared in good yields from the corresponding esters with water and
 aluminum hydroxide, under pressure (DE 196 294 32.0).
 This industrially useful process has the disadvantage of a relatively long
 reaction time, especially when longer-chained esters are used. For
 example, the hydrolysis of the butyl esters is markedly slower than that
 of the methyl esters, but they are simpler to prepare than the methyl
 esters.
 The object was to find a process which does not have the disadvantages
 mentioned above and which can be implemented industrially without great
 cost and using auxiliary materials which are easily obtainable. The
 process should furthermore allow the desired products to be obtained both
 in high yield and in high purity.
 Surprisingly, this object has been achieved by means of a process for
 preparing aluminum salts (formula (I)) of dialkylphosphinic acids or
 (formula (II)) of diphosphinic acids
 ##STR3##
 where
 R.sup.1 and R.sup.2 are a linear or branched C.sub.1 -C.sub.8 -alkyl
 radical, preferably a linear or branched C.sub.1 -C.sub.4 -alkyl radical,
 such as methyl, ethyl, n-propyl, isobutyl, n-butyl, n-hexyl or phenyl and
 R.sup.3 is a linear or branched C.sub.1 -C.sub.10 -alkylene radical,
 preferably a linear or branched C.sub.1 -C.sub.4 -alkylene radical, such
 as methylene, ethylene, n-propylene, isopropylene, n-butylene, n-decylene,
 or an arylene radical, such as phenylene or naphthylene; or an
 alkylarylene radical, such as methylphenylene, ethylphenylene or
 methylphenylenemethyl, or an arylalkylene radical, such as phenylmethylene
 or phenylethylene,
 which comprises reacting a dialkylphosphinic acid ester of the formula
 (III) or a diphosphinic acid ester of the formula (IV)
 ##STR4##
 where R.sup.1, R.sup.2 and R.sup.3 are as defined above and R.sup.4 is a
 linear or branched C.sub.1 -C.sub.8 -alkyl radical, preferably a linear or
 branched C.sub.4 -C.sub.6 -alkyl radical,
 in the presence of a saturated aliphatic monocarboxylic acid having a total
 of from 1 to 7 carbon atoms, with aluminum hydroxide at a temperature of
 from 150 to 350.degree. C.
 Surprisingly, the reaction times can be markedly reduced by the novel
 process. It gives the aluminum phosphinates in high purity and high yield.
 The process is technically simple, since to achieve the high yield it is
 not necessary to remove any byproducts, such as the esters produced from
 the monocarboxylic acids, from the reaction mixture.
 Dialkylphosphinic acid esters which can be used for preparing the aluminum
 salts are in particular: methyl dimethylphosphinate, ethyl
 ethylmethylphosphinate, isobutyl ethylmethylphosphinate, n-butyl
 methylpropylphosphinate, amyl isobutylmethylphosphinate, isopropyl
 hexylmethylphosphinate, n-butyl methyloctylphosphinate, n-butyl
 methylphenylphosphinate and n-pentyl diphenylphosphinate. Diphosphinic
 acid esters which can be used for preparing the aluminum salts are in
 particular: di-n-butyl hexane-1,6-di(methylphosphinate) and diisobutyl
 benzene-1,4-di(methylphosphinate).
 ##STR5##
 Dialkylphosphinic acid esters and diphosphinic acid esters are both
 referred to below as phosphinic acid esters.
 Examples of saturated, aliphatic monocarboxylic acids are formic acid,
 acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric
 acid, isovaleric acid and mixtures of these acids. Acetic acid or
 propionic acid or mixtures of the same are particularly suitable.
 Phosphinic acid esters, aliphatic monocarboxylic acids and aluminum
 hydroxide are known and familiar compounds, obtainable as widely marketed
 commercial products or easily accessible via preparation processes known
 in the prior art.
 The aliphatic monocarboxylic acids are advantageously used in an amount of
 from about 50 to 300% by weight, preferably from 80 to 150% by weight,
 based in each case on the phosphinic acid ester used. If desired, the
 reaction mixture may also contain water, the ratio of monocarboxylic acid
 to water being in the range from 50:50 to 100:50.
 The process of the present invention is generally carried out by heating
 the corresponding phosphinic acid esters with the aliphatic monocarboxylic
 acid and, if desired, water and stoichiometric amounts of aluminum
 hydroxide to a temperature of from 150 to 350.degree. C., preferably from
 180 to 250.degree. C. in an autoclave, advantageously with continuous
 stirring. The reaction mixture is advantageously stirred for from 5 to 25
 h, preferably from 5 to 10 h, to convert the phosphinic acid esters to the
 corresponding aluminum salts, the reaction times depending on the chain
 length of the ester groups of the phosphinic acid ester. Methyl esters,
 for example, react more readily than the butyl esters. During the
 reaction, the pressure in the autoclave rises to values in the range from
 5 to 250 bar, preferably from 10 to 100 bar, in particular from 10 to 50
 bar. As the end of the reaction approaches, the pressure remains
 approximately constant. After the reaction has ended, the reaction mixture
 is cooled and the aluminum phosphinates are then filtered off with
 suction. Finally, the resultant aluminum phosphinates are dried.
 Surprisingly, it has been found that in order to achieve a good yield of
 the aluminum phosphinates, it is not necessary to remove from the reaction
 mixture the monocarboxylic esters which are produced in the reaction. The
 same is true if additional water is added to the reaction mixture. In this
 case also, it is not necessary to remove the monocarboxylic acid/alcohol
 mixture produced.

EXAMPLE 1
 49.2 g (0.3 mol) of n-butyl ethylmethylphosphinate, 55 ml of acetic acid,
 25 ml of water and 7.8 g (0.1 mol) of aluminum hydroxide are held at
 220.degree. C. for 20 hours in a 200 ml Hastelloy autoclave. The pressure
 rises to 25 bar. The reactants are then cooled, filtered off with suction,
 washed with acetic acid and dried at 140.degree. C. in a vacuum drying
 cabinet. This gives 31.5 g of aluminum salt of ethylmethylphosphinic acid.
 The filtrate is freed from solvents in vacuo, and the residue is digested
 with water, giving a further 2 g and 33.5 g in total, corresponding to a
 yield of 96% of theory.
 EXAMPLE 2
 49.2 g (0.3 mol) of n-butyl ethylmethylphosphinate, 80 ml of acetic acid
 and 7.8 g (0.1 mol) of aluminum hydroxide are held at 220.degree. C. for
 10 hours in a 200 ml Hastelloy autoclave. The pressure rises to 24 bar.
 The reactants are then cooled, filtered off with suction, washed and dried
 at 140.degree. C. in a vacuum drying cabinet. This gives 32 g of aluminum
 salt of ethylmethylphosphinic acid, corresponding to a yield of 92% of
 theory.
 EXAMPLE 3
 53.4 g (0.3 mol) of n-butyl methylpropylphosphinate, 55 ml of acetic acid,
 25 ml of water and 7.8 g (0.1 mol) of aluminum hydroxide are held at
 200.degree. C. for 20 hours in a 200 ml Hastelloy autoclave. The pressure
 rises to 16 bar. The reaction is worked up as in Example 2. This gives 33
 g of aluminum salt of methylpropylphosphinic acid, corresponding to a
 yield of 85% of theory.
 EXAMPLE 4
 65.6 g (0.4 mol) of n-butyl ethylmethylphosphinate, 55.2 g of formic acid
 and 10.4 g (0.133 mol) of aluminum hydroxide are brought to 190.degree. C.
 over a period of 1.5 hours in a 200 ml Hastelloy autoclave. The pressure
 at 190.degree. C. is 24 bar. The reaction mixture is then held for 3 hours
 at 200.degree. C. and during this the pressure rises to 132 bar. The
 reactants are then cooled, and the pressure at room temperature is now 56
 bar. After the customary work-up, this gives 35 g of aluminum salt of
 ethylmethylphosphinic acid. The filtrate is freed from low boilers in
 vacuo. After drying, a further 6 g remain behind. The total yield is
 therefore 89% of theory.