Manufacture of fatty acid esters of sorbitan as surfactants

Details Manufacture of fatty acid esters of sorbitan as surfactants Manufacture of fatty acid esters of sorbitan as surfactants

1999-04-22

2002-03-26

Carr, Deborah D. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Fatty compounds having an acid moiety which contains the...

Reexamination Certificate

active

06362353

ABSTRACT:

This application is a 371 PCT/GB97/02047 filed Jul. 30, 1997.
This invention relates to an improved method of making surfactant esters, especially sorbitan esters of fatty acids, to the use of the product esters as surfactants, and to the manufacture of alkoxylated, especially ethoxylated, surfactant esters, in particular the ethoxylated sorbitan fatty acid esters known as polysorbates and to the use of the product alkoxylated esters as surfactants.
Sorbitan esters of fatty acids, such as those sold by various ICI companies under the Trade Mark “Span” are widely used as surfactants and as intermediates in the manufacture of relatively more hydrophilic surfactants by alkoxylation, especially ethoxylation to make so-called polysorbate surfactants e.g. as sold by various ICI companies under the Trade Mark “Tween” Typically, sorbitan fatty acid esters are commercially manufactured of a large scale by reacting sorbitol and the fatty acid in the presence of a catalyst system which promotes the esterification reaction and which also catalyses the internal etherification of the sorbitol to sorbitan. Generally the etherifcation reaction is desired only to progress to the mono-cyclic product although a second internal etherification reaction is possible to form the iso-sorbide moiety. It is believed that the internal etherification takes place after the esterification reaction, but this is not directly important for most large scale manufacturing methods as the reactions are, in practice, carried out batchwise under a single stage or “one pot” protocol. As there are various sites for esterification and internal etherification, the product is usually a mixture of isomers. Further scope for variability in the molecule is provided by the possibility of multiple esterification. The variability of the molecules possible is well known among those who manufacture and use these surfactants.
Esterification is, in principle subject to both general acid and base catalysis and etherification is typically catalysed by acids. Typically, in the manufacture of sorbitan fatty acids esters, the catalyst systems used are a mixture of acidic and basic catalysts. Conventionally explained, the base is used to catalyse the esterification and the acid to catalyse the etherification. With water being present in the system, either from supply of starting materials as aqueous solutions or water formed during the reactions, as expected, the acid and base tend to react to form salts. This may imply that the true catalyst is a salt or combination or acid or base and salt. Typically the reaction temperature is about 240° C., the catalysts are chosen so that they are both chemically stable and non-volatile at the reaction temperatures. Usually conventional catalyst systems use NaOH as the base and a phosphorus oxyacid as the acid. Various phosphorus oxyacids can be used successfully as acid components of the catalyst system, but usually non-condensed phosphorus oxyacids such as phosphoric acid have been preferred historically. Conventionally, the base and acid catalyst components (for a typical NaOH/phosphoric acid system) are used at a weight ratio of about 1:1 corresponding to a molar ratio of about 1.3:1 and at an overall level of between 0.6 and 0.8% by weight of the combined acid and sorbitol reagents equivalent to between about 2.3 and about 3% by weight of the sorbitol reagent.
At the elevated reaction temperatures typically used in the reaction, care needs to be taken to avoid excessive oxidation of the reagents and usually the reaction vessel is blanketed with nitrogen. Despite this some oxidation and/or pyrolysis (possibly oxidative pyrolysis) does usually take place and efforts have been made to reduce the extent and/or effect of these undesired side reactions on the properties of the product. The most obvious effect on the product is that it is typically coloured. Improvements in the process to reduce or remove the coloured side products include the inclusion in the reaction of carbon (“activated carbon”) to absorb coloured side products and the use of reducing varieties of phosphorus acids, particularly phosphorous and/or hypophosphorous acids, to make the reaction environment less oxidising (possibly by the reducing acid acting as a sacrificial anti-oxidant). Often after separation of the activated carbon from the reaction product the product is further decolourised by bleaching. Even using such improvements, the colour of the usually liquid product (as the neat material) is typically about 8 Gardner units having a dark brown colour. In the absence of such process improvements the colour would probably be more than 10 Gardner units. Gardener units are based on visual comparisons and in this context probably represent an approximately logarithmic scale of concentration of the coloured side products.
It is known to make very pure sorbitan fatty acid esters by using specially purified starting materials and separating the etherification and esterification reactions for example as is described in JP 62-142141 A. However, such methods are of little use in the bulk manufacture of sorbitan fatty acid esters as the multiplicity of purification and reaction stages makes them very expensive.
Polyalkoxylated sorbitan fatty acid ester surfactants, particularly of the polysorbate type, are typically manufactured by reacting the corresponding sorbitan esters with alkylene oxide, usually ethylene oxide, typically under alkali catalysis.
The present invention is based on the discovery that the use of a catalyst system in which the relative proportion of acid is greater than that used conventionally can yield sorbitan fatty acid ester products which have significantly improved purity, particularly improved colour (lower Gardner colour) and odour even when no activated carbon is included in the reaction system. Further, using such modified catalyst systems enables a higher level of catalyst to be used giving shorter reaction times, lower reaction temperatures or a combination of both, which can yield further improvements in the properties of the product. The fatty acid esters can be alkoxylated, and in particularly ethoxylated to give polysorbate type products, also showing improved colour and odour as compared with otherwise similar products made with conventionally made sorbitan fatty acid esters.
Accordingly, the present invention provides a method of making fatty acid esters of sorbitan which comprises reacting the fatty acid directly with sorbitol in the presence of a catalyst system which comprises a phosphorus oxyacid, including a reducing phosphorus oxyacid, and an alkali or alkali earth metal strong base in a molar ratio of acid to base of from 0.9:1 to 1.7:1 and at a catalyst system concentration of from about 1.5 to about 30% by weight of the sorbitol.
The invention further enables the manufacture of alkoxylated esters of sorbitan, in particular polysorbate materials, having improved properties and the invention accordingly includes the use of fatty acid esters of sorbitan made by the method of the invention in the manufacture of corresponding alkoxylated esters of sorbitan, in particular polysorbate materials, by alkoxylating and in particular ethoxylating the fatty acid esters of sorbitan made according to the invention. Specifically, the invention includes a method of making alkoxylated esters of sorbitan, in particular polysorbate materials, comprising reacting a fatty acid directly with sorbitol in the presence of a catalyst system which comprises a phosphorus oxyacid, including a reducing phosphorus oxyacid, and an alkali or alkali earth metal strong base in a molar ratio of acid to base of from 0.9:1 to 1.7:1 and at a catalyst system concentration of from about 1.5 to about 30% by weight of the sorbitol to form a fatty acid ester of sorbitol; and subsequently alkoxylating, and in particular ethoxylating, the fatty acid ester of sorbitol by reacting the ester with an alkylene oxide, particularly ethylene oxide.
Molar ratios of acid and base refer to the ratios of the nominal H
+
and OH
−
con

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