FUEL ADDITIVES

Abstract: The present invention provides a compound of formula(I) (wherein R<1> is an optionally substituted, saturated or unsaturated, straight-chained or branched hydrocarbyl group, preferably an alkenyl or poly(alkenyl) group; one of R<2> and R<3> represents a hydrogen atom and the remaining one of R<2> and R<3> represents a straight-chained or branched alkyl group; R<4> represents a hydrogen atom or a straight-chained or branched alkyl group, preferably a hydrogen atom; or, alternatively, R<1> and R<4> may be linked together to form an optionally substituted 5- or 6-membered ring) and metal salts and complexes, optical isomers and diastereoisomers thereof.

Description: Fuel Additives The present invention relates to additives for liquid hydrocarbon fuels, and fuel compositions containing them. More specifically the invention relates to additives effective to reduce the particulate and/or unburnt hydrocarbon content of exhaust gas emissions from distillate hydrocarbon fuels such as diesel and heating oils. The invention also relates to additives effective in diesel particulate filter regeneration. Diesel fuels and diesel engines are particularly prone to the emission of small size particulate material in the exhaust gas. These particulates include not only those which are visible as smoke emission, and to which diesel engines are prone especially when the engine is overloaded, worn or badly maintained, but also those which are normally invisible to the naked eye. Particulate emission by diesel engines is a major source of atmospheric pollution. Similar problems can also arise during the combustion of other distillate fuel oils, e.g. heating oils. Another problem associated with liquid hydrocarbon fuels is that of incomplete combustion resulting in the emission of unburnt hydrocarbons which are understood to represent health hazards. Legislation now exists in many countries that is designed to control pollution from diesel engines and more demanding legislation is planned. A number of ways are being examined to enable diesel engines to run and comply with the developing legislation. Engine designs to give effective combustion within the cylinder are being developed. However, the drawbacks to engine management solutions include cost, complexity and the poor capability for retrofitting. Other solutions to the problem of exhaust gas emissions include post-combustion treatments, such as hydrocarbon oxidation catalysts, De-NO x catalysts, and Diesel Particulate Filters (DPFs) . Diesel particulate filters can be used as a means to control particulates emissions without the need to further reduce fuel sulphur levels. Furthermore, there is now evidence to suggest that ultrafine particles, usually regarded as those of below 2.5 μm diameter, exhibit carcinogenic tendency irrespective of their composition. As a result, characterisation of diesel particulates emission by particle size and number distribution is likely to become more important than that by mass. Diesel Particulate Filter (DPF) designs exhibiting high efficiency for particles of aerodynamic diameter 10 μm and below have been demonstrated, e.g. by J-B Dementhon et al . in SAE 972999. One problem associated with the use of DPFs is that of filter blockage. This causes an increase in exhaust back pressure and a loss of engine efficiency and/or "chimney fires" resulting from sudden and intense burn off of soot from highly loaded filters. A number of fuel additives have been proposed in an attempt to solve this problem (see e.g. N Miyamoto et al . SAE 881224, B Martin et al . I Mech E November 1990, G Lepperhoff et al. SAE 950369, V D N Rao et al . SAE 940458, H Ise et al. SAE 860292 and D T Daly et al . SAE 930131) . These additives serve to reduce the soot ignition temperature such that suitable conditions for filter regeneration (i.e. decrease in back pressure) occur at high frequency during normal driving. Succinic acid derivatives, in particular those derived from poly (butenyl) succinic anhydride, are known to be useful as metal carriers in fuel additive compositions. Their metal salts or complexes with alkali and alkaline earth metals are particularly effective in particulates suppression and in the regeneration of diesel particulate filters, as outlined in O-A-96/34074 and O-A-96/34075. Whilst in many respects excellent, these poly (butenyl) succinate carriers exhibit the following limitations: 1) Solutions of the metal-carriers are of high viscosity at modest metal concentrations. 2) The carriers and their metal salts/complexes are difficult to characterise. 3) There is limited availability of low molecular weight poly (butenes) , in turn limiting metal concentration. 4) The metal salt/complex solutions are prone to interact with antifoams. High viscosity of a metal-containing solution can give problems in blending with other additive components. Alternatively, if extra solvent is added to overcome the viscosity problem, associated extra costs for transport arise and the size of the additive package can become unattractively large. The problems of package size versus additive viscosity are even more acute for DPF applications requiring on-board dosing on small passenger cars . Accurate dosing requires a non-viscous solution; a minimum additive tank size to achieve adequate service intervals requires a concentrated solution . Diesel fuel is prone to form foam when pumped into consumers tanks on forecourts. This is undesirable for two reasons. Firstly, customer clothing may be splashed with diesel. Secondly, for safety reasons the delivery pump cuts out when the nozzle tip is covered by fuel or foam. Where a significant "head" of foam occurs, less fuel may be dispensed leading to lost sales at a particular outlet. To avoid these problems, premium diesel brands often employ antifoaming agents (antifoams) , such as those sold by Thomas Goldschmidt AG as the ' Tegopren™ ' range. A range of tests including "Bottle-shaking" and "BNPe" tests have been devised to simulate this behaviour and assess antifoam treatment rates required by particular fuel/additive package combinations. It has been found that metal salts or complexes prepared from poly (butenyl) succinic anhydrides (PIBSAs) are prone to partially or completely destroy the effectiveness of antifoam in diesel additive packages . A metal carrier which does not cause this problem is clearly highly desirable. Surprisingly, it has now been found that succinic acid hemi-esters are particularly effective as metal carriers for fuel additive formulations. Metal salts or complexes of succinic acid hemi-esters with Group I and Group II metals have been found to be effective not only in reducing particulate emissions and the unburnt hydrocarbon content of exhaust gas emissions from liquid hydrocarbon fuels, especially distillate hydrocarbon fuels such as diesel and fuel oil, but also in effecting diesel particulate filter regeneration. In addition, these provide a number of advantages over conventional additives such as reduced viscosity at effective metal concentrations. More importantly, these do not destroy the effects of antifoams present in diesel additive packages. Viewed from one aspect the present invention therefore provides a compound of formula I : (wherein R 1 is an optionally substituted, saturated or unsaturated, straight-chained or branched hydrocarbyl group, preferably containing from 6 to 40 carbon atoms, more preferably 8 to 24 carbon atoms, e.g. 12 to 24 carbon atoms; one of R 2 and R 3 represents a hydrogen atom and the remaining one of R 2 and R 3 represents a straight-chained or branched alkyl group, preferably a Cj_ 10 alkyl, e.g. a Cj_ 6 alkyl group; R 4 represents a hydrogen atom or a straight-chained or branched alkyl group, preferably a Cj.g alkyl, e.g. a Cj . _ 4 alkyl group; or, alternatively, R 1 and R 4 may be linked together to form an optionally substituted, saturated or unsaturated, 5- or 6-membered, preferably 6-membered, carbocyclic ring) and metal salts and complexes, optical isomers and diastereoisomers thereof . In the compounds of the invention, R 1 is preferably an alkenyl or poly (alkenyl) group, preferably a C 2 _ 20 alkenyl or poly (C 2 _ 6 alkenyl) group, for example a C 8 _ 12 alkenyl or poly(C 2 . 4 alkenyl) group. More preferably, R 1 is a poly(C 2 _ 4 alkenyl) group, for example a poly (propenyl) , poly (butenyl) , poly (isobutenyl) , or tetrapropenyl group. Other examples of suitable R 1 groups include 2-octenyl, 2-dodecenyl or 2-octadecenyl . These R 1 groups may optionally be hydrogenated to remove the unsaturation. Thus, other suitable R 1 groups include saturated alkyl groups, preferably C 6 _ 40 alkyl, more preferably C 8 _ 24 alkyl, e.g. C 12 _ 24 alkyl. A wide range of substituents may be present in the compounds of the invention, in particular in group R 1 . Typical substituents include alkyl groups, for example C ± _ 6 alkyl groups, more preferably Cj_ 4 alkyl groups, optionally substituted by hydroxy, alkoxy, cyano, nitrile or halogen groups. Examples of particular substituents include methyl, ethyl, isopropyl, n-propyl, t-butyl and -butyl . In the case where R 1 is a poly (alkenyl) group, in particular a poly(C 2 . 6 alkenyl) group, e.g. poly (butenyl) , R 1 may be substituted with one or more additional groups of the formula: o (wherein R 2 , R 3 and R 4 are as herein defined) . In the compounds of formula I , R 2 and R 3 when not hydrogen are preferably sterically hindered alkyl groups, typically secondary alkyl groups. Suitable alkyl groups include isopropyl, secondary butyl and 2-ethylhexyl . Preferably R 2 or R 3 is an isopropyl group, Preferably, R 4 is a hydrogen atom. As used herein, the terms poly(alkene) and poly (alkenyl) are intended to cover products derived from the polymerisation of an olefin or mixture of olefins, in particular α-olefins, preferably C 2 _ 10 α-olefins, especially butenes . Such compounds will comprise at least one olefinic bond, typically at least one vinylidene group. Particularly preferred for use in the invention are those compounds of formula I in which R 1 is derived from the polymerisation of butenes, i.e. in which R 1 is poly (butenyl) . As used herein, the term poly (butenyl) is intended to cover both oligomers and polymers prepared from isobutene and those from mixed C 4 olefin streams, i.e. comprising isobutene, but-1-ene and cis and trans but-2-ene. The metal carriers formed from the hemi-esters of compounds of formula I may be regarded as salts. Without wishing to be bound by theory, however, it is believed that the ester groups co-ordinate, whether inter- or intra-molecularly, to the metal ions. They may thus fairly be referred to as metal complexes. Preferred metal salts or complexes formed from the hemi- esters of formula I include those of formula II: ( i i : (wherein R 1 and R 4 are as hereinbefore defined; and one of R 2 and R 3 is an alkali or alkaline earth metal M of valency n, and the remaining one of R 2 and R 3 is a straight-chained or branched alkyl group, preferably a Ci_ 10 alkyl, e.g. a C ± 6 alkyl group; wherein when one of R 2 and R 3 is an alkali metal then n=l, and when one of R 2 and R 3 is an alkaline earth metal, n=2; and for whichever of R 2 and R 3 is an alkyl group the electrostatic charges on O and R 2 or R 3 are absent) and the optical isomers and diastereoisomers thereof. Preferably, M is selected from the group consisting of Na, K, Mg, Ca and Sr. Most preferably, M is K or Sr. The compounds of the invention may be prepared by conventional synthetic techniques, conveniently starting from succinic anhydride or succinic acid derivatives and alcohols, preferably sterically hindered alcohols. Thus, viewed from a further aspect the invention also provides a process for the preparation of compounds of formula I, said process comprising at least one of the following steps: (a) reacting a compound of formula III: R-OH (III) (wherein R is a straight-chained or branched alkyl group, preferably a C ! _ 10 alkyl, e.g. a C^ alkyl group) together with a succinic anhydride derivative of formula IV: wherein R 1 and R 4 fined) (b) reacting a compound of formula III as defined in step (a) together with a succinic acid derivative of formula V: (wherein R 1 and R 4 are as hereinbefore defined) ; (c) separating the mixture formed in steps (a) or (b) by conventional separating techniques; (d) resolving a chiral compound of formula I into its isomers; (e) metallating a compound of formula I; and (f) performing at least one of steps (a) to (e) above using reagents with protected functional groups and subsequently removing the protecting groups. The products of the above reactions may be either single compounds or mixtures of discrete compounds . Typically these will comprise mixtures of compounds of formula I . The starting compounds used in the reactions described above are either known from the literature or can be prepared using conventional synthetic techniques. In the reactions described above, the reactants may be substituted with functional groups which are inert under the reaction conditions employed, as would be understood by one skilled in the art. Alternatively, any reactive groups present may optionally be protected during the reaction by means of conventional protecting groups which are cleaved again after the reaction. Subsequent cleaving of any protecting group used may be carried out by methods known in the art, for example hydrolysis. Optionally, the final step in the synthesis of compounds of formula I will thus be deprotection of a protected derivative of a compound of formula I and such a process forms a further aspect of the invention. Preferred compounds of formula III for use in step (a) or step (b) include sterically hindered alcohols, more preferably sterically hindered secondary alcohols. Preferred examples of sterically hindered secondary alcohols include secondary butyl alcohol and 2- ethylhexyl alcohol . An even more preferred example is isopropyl alcohol . Preferred reaction conditions for steps (a) and (b) include ensuring that the alcohol is present in excess over either the cyclic succinic anhydride derivative of formula IV (in step (a) ) or the succinic acid derivative of formula V (in step (b) ) . Reaction in step (a) is performed with no catalyst present. Once all the succinic anhydride or succinic acid derivative has been consumed (followed by FTIR analysis) , the excess alcohol is stripped off to leave the product . In preparing the compounds of formula IV and V, suitable poly (butenes) include those distributed by Amoco as ■ Indopol™', by BASF as 'Glissopal™' and by BP as 'Ultravis™' and 'Napvis™' . Typical number average molecular weights of commercially available fractions are of 500 amu and upwards. This limits the maximum possible metal content of a PIBSA-derived material prepared from them. Lower molecular weight fractions, such as ' Indopol L-10™', ' Indopol L-6™' and 'Napvis X- 10™' are available and are preferred for use in the invention. Low molecular weight propylene and ethylene oligomers, such as α-olefins are readily available. Maleinisation can be performed on α-olefins and fractions to produce 2 -alkenyl succinic anhydrides or on isomerised α-olefins or fractions resulting in β-branched 2-alkenyl succinic anhydrides. In contrast to the PIBSA-derived materials, such alkenyl succinic anhydrides (ASA) -derived materials are readily biodegradable. Biodegradation in the environment is frequently a requirement for synthetic chemicals in the modern marketplace. The option to use ASA- as opposed to PIBSA-derived materials is an advantage in the preparation of the metal carriers in accordance with the invention. Within a given poly(butene) fraction a large number of isomers exist. By contrast, ethylene and propylene oligomers are available as well -characterised and tightly specified fractions with almost exclusively terminal unsaturation. Maleinisation to alkenyl succinic anhydrides (ASAs) is consequently better controlled and understood, and the products and their derivatives more readily characterised. Typically, the compounds for use in preparing the compounds of the invention will comprise a mixture of compounds of formula IV or V having different R 1 groups resulting from the maleinisation of isomerised α-olefins or isomerised α-olefin fractions. For example, the compound of formula IV may conveniently be prepared by reaction of maleic anhydride or an α-substituted maleic anhydride with an olefin or polyolefin, preferably a monoolefin or diolefin, e.g. a monoolefin such as poly (butene) . Where the olefin, e.g. poly (butene) , is reacted with a molar excess of maleic anhydride, a proportion of the resulting R 1 groups may react with the maleic anhydride via a so-called '2-centre ene ' reaction. This has the effect of introducing one or more additional succinate groups into the compounds of formula IV and in turn into the compounds of the invention. In> particular, where R 1 is a poly(C 2 _ 6 alkenyl) group, a proportion of the R 1 groups may have linked thereto one or more additional succinate groups, essential corresponding to the compounds of formula I . In any mixture of compounds of formula I , the ratio of succinate groups linked to group R 1 to succinated poly (alkenyl) moieties (e.g. succinated poly (C 2 _ 6 alkenyl) moieties) is preferably from 1:1 to 1.3:1, more preferably from 1.05:1 to 1.15:1. After introduction of the succinate group (s), the unsaturation remaining in R 1 following the '2-centre ene' or Diels Alder reactions may be removed by hydrogenation using techniques well-known in the art. The metal salts of the compounds of formula I may be obtained by reacting a source of the metal M, e.g. the elemental metal, a metal alkyl or hydride, an oxide or hydroxide, with a compound of formula I in a hydrocarbon, preferably an aromatic hydrocarbon solvent such as 'ShellSol AB™ ' . The most convenient source of the metal M will usually be the hydroxide or oxide, preferably the hydroxide . The compounds of the invention may be present in the form of enantiomers or racemates thereof or in the form of their pairs of diastereoisomers . The compounds of formula I may be separated into their diastereoisomers on the basis of their physical/chemical differences by methods known in the art, e.g. by chromatography and/or fractional crystallisation. As mentioned above, the compounds of formula I may be resolved into their optical isomers. Thus, those compounds having at least one optically active carbon atom may be resolved into their enantiomers, e.g. by chiral chromatography. The compounds of the invention are particularly effective as particulate suppressants in liquid hydrocarbon fuels. It has also been found that the compounds of the invention lead to a reduction in unburnt hydrocarbon emission, not only in the exhaust gas emissions from diesel fuels but from other liquid hydrocarbons as well. The additives also serve to remove preformed soot or carbon deposits in internal combustion engines and fuel injectors of all kinds, including exhaust systems used therewith. Thus, in addition to particulate suppression, the additives of the invention have added value as exhaust emission control agents for reducing unburnt hydrocarbon emissions from liquid hydrocarbon fuels, and as clean-up agents for the removal of soot and carbon deposits resulting from the incomplete combustion of liquid hydrocarbon fuels . Viewed from a further aspect the invention provides a fuel additive for liquid hydrocarbon fuels, in particular diesel fuels, comprising at least one metal complex of a compound of formula I, preferably a compound of formula II as hereinbefore defined. Typically, the fuel additives in accordance with the invention may additionally comprise an organic, fuel soluble solvent, preferably a hydrocarbon, miscible in all proportions with the fuel. Suitable solvents for this purpose include: aromatic kerosene hydrocarbon solvents such as 'ShellSol AB™ ' (boiling range 186°C to 210°C) , 'ShellSol R™ ' (boiling range 205°C to 270°C) , 'Solvesso 150™' (boiling range 182°C to 203°C), toluene, xylene, or alcohol mixtures such as 'Acropol™' (boiling range 216°C to 251°C) . Other suitable solvents miscible with diesel and other similar hydrocarbon fuels will be apparent to those skilled in the art. Viewed from a further aspect the invention thus provides a fuel additive for liquid hydrocarbon fuels, in particular for diesel fuels, comprising an organic, fuel soluble solvent, preferably a hydrocarbon, miscible in all proportions with the fuel, and at least one metal complex of a compound of formula I as hereinbefore defined. In general, the amount of metal in the additive represents the best compromise between high concentration and the need to achieve low viscosity, as will be readily understood by those skilled in the art. Metal concentrations in the additive may range from 1 to 10% or more by weight, preferably 4 to 6% by weight, for example 5% by weight. Viewed from a yet further aspect the invention thus provides a method of reducing particulate or visible smoke emissions and/or unburned hydrocarbons resulting from the incomplete combustion of liquid hydrocarbon fuels, in particular from diesel fuels, said method comprising the step of incorporating into the fuel prior to combustion an effective amount of a metal complex of a compound of formula I . According to a further aspect of the present invention there is provided the use of a metal complex of a compound of formula I as a black smoke, particulate and/or unburned hydrocarbon-reducing additive. The additives in accordance with the invention may be dosed to the fuel at any stage in the fuel supply chain. However, preferably the additives are added to the fuel close to the engine or combustion systems, within the fuel storage system for the engine or combustor, at the refinery, distribution terminal or at any other stage in the fuel supply chain. As used herein, the term "fuel" includes any hydrocarbon that can be used to generate power or heat. The term also covers fuel containing other additives such as dyes, cetane improvers, rust inhibitors, antistatic agents, gum inhibitors, metal deactivators, de- emulsifiers, upper cylinder lubricants, antifoam compositions and anti-icing agents. Preferably, the term covers diesel fuel . As used herein, "diesel fuel" means a distillate hydrocarbon fuel for compression ignition internal combustion engines meeting the standards set by BS 2869 Parts 1 and 2 as well as fuels in which hydrocarbons constitute a major component and alternative fuels such as rape seed oil and rape oil methyl ester. Combustion of the fuel can occur in, for example, an engine such as a diesel engine, or any other suitable combustion system. Examples of other suitable combustion systems include recirculation engine systems, domestic burners and industrial burners. Preferably the additive composition of the invention is fuel-soluble or fuel miscible. A preferred composition of the invention is one which can be supplied in concentrated form in a suitable solvent which is fully compatible with diesel and other hydrocarbon fuels, such that blending of fuel and additive may be more easily and readily carried out . The additives in accordance with the invention may be used in combination with a particulate filter enabling collected material to be more readily oxidised. Many types of filter are known to those skilled in the art including "wall flow" or "cracked wall", "deep bed" ceramic types and sintered metal types. Whilst the additives are suitable for use with all particulate filters, examples of preferred filters include the • Corning EX80™', the 'NoTox' SiC types (both ceramic wall -flow filters), one constructed from ' 3M Nextel™' fibre (a deep bed type) or an SHW sintered metal DPF. Thus viewed from a still yet further aspect the present invention provides the use of a metal complex of a compound of formula I as a part of an additive composition for the regeneration of particulate filters, particularly diesel particulate filters. The additives in accordance with the invention are capable of providing frequent spontaneous regenerations across a wide range of engine operating conditions using a wide range of fuels, including fuels with realistic sulphur concentration, and at low levels of loading of particulate material within the filter. This results in minimum average pressure drop across the filter, representing minimum fuel efficiency penalty. Regeneration with less carbon in the filter also results in lower thermal stresses arising from the exothermic regeneration. Furthermore, the additives of the invention require minimum adjustment to engine operating parameters (e.g. injection timing) and/or minimum energy input to produce a 'forced' regeneration, whether operating within or outside an envelope wherein spontaneous regenerations are likely to be encountered. These specifications can be achieved at a dose level which provides minimum inorganic ash. The key advantages of the present invention are that it provides additives for diesel fuels that are cost- effective and which provide an overall emissions benefit to the environment on combustion by improving the combustion process and therefore reducing black smoke, particulate and/or unburned hydrocarbon emissions. In addition, the additives are of lower viscosity than those of the art and do not interact with antifoams present in diesel fuel. As would be clear to one skilled in the art, the compounds and additives of this invention may be used in diesel fuels together with other materials used as additives to improve various aspects of fuel or engine performance. A non-limiting list of these additives would include: detergents, carrier oils, anti-oxidants, corrosion inhibitors, colour stabilisers, metal deactivators, cetane number improvers, other combustion improvers, antifoams, pour point depressants, cold filter plugging point depressants, wax anti-settling additives, dispersants, reodorants, dyes, smoke suppressants, particulate filter regeneration additives and lubricity agents. Therefore, a further aspect of the present invention provides the use of a metal complex of a compound of formula I as part of a formulation further comprising any of the additives mentioned in the non-limiting list above for use in any of the applications hereinbefore described. The present invention will now be described by way of the following non-limiting examples. Other modifications will be apparent to those skilled in the art without departing from the scope of the present invention. Examples Example 1 Hemi-ester of Isopropanol with Octenylsuccininc Anhydride Octenyl succinic anhydride (Aldrich, 202.52 g, 0.963 mol) and propan-2-ol (135.5 g, 2.258 mol) were charged to a one litre jacketed Soverel™ reactor fitted with overhead stirrer and reflux condenser. The heating oil supplied to the jacket was warmed to 90°C and the solution stirred and heated for 3 hours. FTIR analysis (ATR device) indicated complete conversion of anhydride to a mixture of ester and carboxylic acid. The mixture was stirred overnight with the oil at 70°C before stripping unreacted propan-2-ol at the rotary evaporator to constant weight of involatiles. Titration against lithium methoxide for residual acidity gave an acid content of 3.81 mmol H + /g. Theoretical value for starting material and hemi-ester product are, respectively, 4.75 and 3.70 mmol H + /g. Example 2 Hemi-ester of Isopropanol with Dodecenylsuccinic Anhydride Dodecenyl succinic anhydride (Pentagon, 352.49 g, 1.323 mol) and propan-2-ol (182.27 g, 3.038 mol) were charged to the one litre jacketed Soverel™ reactor, as above. Oil heated to 85°C was circulated through the jacket for a total of 6 hours. FTIR indicated that very little unreacted DDSA remained and that a mixture of ester and acid had formed. After a further hour the heating was turned off and the reactor contents allowed to cool. Excess propanol was removed at the rotary evaporator to constant weight of involatiles. Acid content of the product was 3.12 mmol H + /g versus 3.52 for the diacid and 3.06 theoretical for hemi-ester. Example 3 Hemi-ester of Isopropanol with Tetrapropenyl Succinic Anhydride Tetrapropenyl succinic anhydride (Pentagon, 582.82 g, 2.187 mol) and propan-2-ol (282.37 g, 4.706 mol) were allowed to react as described above, overnight, at an oil temperature of 85°C. FTIR showed complete consumption of the starting anhydride. Acid value of the stripped material was 3.15 mmol H + /g, versus 3.52 for the diacid and 3.06 theoretical for hemi-ester. Example 4 Hemi-ester of Isopropanol with Poly (butenyl) Succinic Anhydride A sample of poly (butenyl) succinic anhydride (654.15 g) prepared by thermal maleinisation of BP Napvis™ X-10 and having an unreacted PIB content of 21.3 wt% and 23.8 wt% reacted maleic anhydride content, was charged to the Soverel™ apparatus described above . The PIBSA was heated to 80°C and propan-2-ol (198.21 g, 3.305 mol) added. FTIR showed that a minimal conversion of the anhydride to ester and acid occurred on mixing. Warming to 95°C over 2 hours gave substantially complete conversion. The reaction mixture was left at 95°C over 16 hours before removal of volatiles at the rotary evaporator, to constant weight of involatile material. Acid content of the product was 2.12 mmol/g, corresponding to 2.12 mmol/g theoretical for the hemi- ester. Example 5 Hemi-ester of Isopropanol with Isomerised C16/C18 Succinic Anhydride A sample of Pentasize™ 68F (269.73 g) , an alkenyl succinic anhydride prepared from an isomerised mixture of C16 and C18 α-olefins and having an acid content equivalent to 336.9 mg KOH/g, was reacted with propan-2- ol (64.65 g, 1.078 mol) in the Soverel™ apparatus at 90°C over 16 hours. FTIR (ATR) showed the reaction to be complete and excess propanol was removed under vacuum. The product had an acid value of 2.48 mmol/g, versus 2.54 theoretical. Example 6 Hemi-ester of 2-Ethylhexanol with Poly (butenyl) Succinic Anhydride A further sample of the PIBSA used in Example 4, above, (667.69 g) was reacted with 2-ethylhexanol (259.95 g, 1.996 mol) in the Soverel™ reactor. The oil temperature was set to 105°C, internal temperature rose initially to 115°C, cooling back to 105°C. After 3 hours at this temperature, FTIR showed the PIBSA to have been consumed . Example 7 Potassium Salt Complex of Hemi-ester of Isopropanol with Octenylsuccininc Anhydride. The one litre Soverel™ jacketed reactor fitted with reflux condenser and overhead stirrer was charged with the product of Example 1 (127.22 g, 0.485 mol H + ) and Shellsol™ (214.43 g) . The two were stirred and warmed to 80°C, forming a homogenous solution. KOH flake (29.83 g, 0.478 mol) was added resulting in an exotherm to 106 °C within about 15 minutes. A colour change from colourless to orange-brown was noted. After stirring for 16 hours at 80°C FTIR showed carbonyl signals at 1717 c "1 (C=0 as ester) and 1575 cm "1 (C=0 as carboxylate salt) . The C=0 as acid peak at 1708 cm "1 was no longer detectable. The resulting solution had an acid value of 0.1 mmol/g. Example 8 Potassium Salt Complex of Hemi-ester of Isopropanol with Dodecenylsuccininc Anhydride. The one litre reactor was charged with the product of Example 2 (106.66 g, 0.333 mol H + ) and Shellsol™ (214.80 g) . The two were stirred and warmed to 76 °C, forming a homogenous solution. KOH flake (12.35 g, 198 mmol) was added, causing an exotherm to 85°C, the solution cooling back to 77°C within 35 minutes, accompanied by dissolution of the solids. Further KOH (8.03 g, 129 mmol) was added, dissolving within 40 minutes to yield a clear, bright solution. FTIR of the carbonyl region detected only absorbtions characteristic of ester and carboxylate salt. Acid value of the product was 0.074 mmol/g. Example 9 Potassium Salt Complex of Hemi-ester of Isopropanol with Tetrapropenylsuccininc Anhydride. Product of Example 3 (199.33 g) was dissolved in Shellsol AB™ (256.48 g) and reacted at an initial temperature of 85°C with KOH (39.39 g) . A clear solution was obtained within one hour. Example 10 Potassium Salt Complex of Hemi-ester of Isopropanol with Poly (butenyl) succinic Anhydride. Product of Example 4 (603.70 g, 1.28 mol acid equivalent) and Shellsol AB™ (323.32 g) were charged to the reactor described previously. The contents were warmed to 85°C and KOH flake (79.69 g, 1.28 mol base) was added. Within one hour virtually all of the KOH had dissolved. The oil temperature was reduced to 50°C and the mixture left to stir overnight. FTIR of the carbonyl stretching region showed peaks ascribable to ester at 1715 cm "1 and carboxylate salt at 1575 cm "1 . Titration against lithium methoxide solution showed the solution to have 0.1 mmol H + /g. Example 11 Potassium Salt Complex of Hemi-ester of Isopropanol with Isomerised C16/C18 Succinic Anhydride. The product of Example 5 (286.22 g, 0.71 mol acid equivalent) and Shellsol AB™ (229.95 g) were charged to the reactor used previously. The contents were stirred and warmed to 56°C, forming a homogenous solution. KOH flake (23.57 g, 0.378 mol) was then added causing an exotherm to 67°C over about 20 minutes. The solution was allowed to cool back to 56°C and a second portion of KOH flake (21.07 g, 0.338 mol) was added. The mixture was left to stir overnight at 56°C. A few small flakes of KOH remained, lithium methoxide titration showed the acid value of the solution to be 0.12 mmolH + /g. Example 12 Strontium salt complex of Hemi-ester of Isopropanol with Poly (butenyl) succinic Anhydride. Hemi-ester of isopropanol with poly (butenyl) succinic anhydride was prepared as described in Example 4, a final acidity of 2.20 mmol/g versus 2.12 theoretical was found. The hemi-ester (498.23 g) and Shellsol™ AB (358.56 g) were charged to the Soverel™ reactor set-up and heated to 70 °C internal (oil temperature 90 °C) before charging anhydrous Sr(OH) 2 (96% active 69.60 g, 549 mmol) and approximately 5cm 3 of water. The mixture was heated and stirred overnight to yield a faintly cloudy, brown solution. Filtration through a short column of Celite™ produced a clear solution of acid value 0.1 mmol/g. The hemi-ester (496.62 g) and Shellsol™ AB (357.85 g) were charged to the Soverel™ set-up and heated with stirring to 70°C before addition of Sr(OH) 2 .8H 2 0 (96% active, 151.45 g, 547 mmol). The resulting suspension was left overnight at 90 °C to produce a faintly cloudy, brown solution which was cleared by filtration through Celite™. The final product had an acid value of 0.13 mmol/g. FTIR indicated the solution to contain some residual acid, ester groups and carboxylic acid salts (1730 cm-1 (wsh) , 1704 (m) and 1560 cm-1 (s) , respectively) . Example 13 Effects of Additives on the Foaming Characteristics of Diesel Fuel . 250 cm 3 screw-cap glass bottles were thoroughly cleaned of all traces of additives from previous tests. The procedure followed was to rinse five times with toluene then twice with acetone before washing with detergent then placing for 10 minutes in an ultrasound bath containing a glass-cleaning solution such as Decon90™. The bottles were then thoroughly rinsed with water, then distilled water before oven drying at 100 °C during 1 hour and cooled to ambient temperature before use. Samples of test fuel (100+1 cm 3 ) were then measured into a measuring cylinder and transferred with minimum agitation into ■ clean bottles, then sealed. Individual test bottles were then shaken vigorously for 5 seconds before placing on a flat, vibration- free surface. The surface of the sample was then observed. The time taken for the first appearance of a clean liquid surface from beneath any foam formed was then measured to the nearest second and recorded. Packages were blended using each of Examples 10, 7, 9 and 8. The formulation for each package was as follows: %w/w Diesel detergent (OMA301) 13.00 T 9318 0.70 MR2057 antifoam 0.80 CAROMAX 20 solvent 55.90 COMBUSTION IMPROVER 29.60 The treat rate for this formulation is such that 1000 mg/kg provides 15 ppm K. The additives were dosed into diesel basefuel and tested for antifoam performance by bottle shaking tests, and compared with 'base package' - a formulation which contains the same components in the same ratios but no combustion improver. Fuel was treated with this base package to equivalent detergent concentration. The following results were obtained: From these results, it is clear that for the two additives wherein R 1 is branched, the antagonism between combustion improver and antifoam is at least reduced and potentially removed. Example 14 Effects of Combustion Improvers on Diesel Foaming Characteristics as Determined by Foam Height and Stability The additives shown by Example 13 to be non-antagonistic towards antifoam were re-tested by a foam-height method which is kept under review by a BNPe working group (October 1991, Revision 1) . Essentially, a 100 cm 3 sample of diesel fuel is injected at a constant pressure from a fixed height, into a graduated collection vessel. The volume and disappearance time of the foam produced are measured and recorded. Thorough cleaning of all glassware and apparatus is essential in order to remove all traces of additive from previous tests . This was carried out much as shown above in Example 13. The diesel fuel under test must be homogenous, but should not be stirred too vigorously. Samples must be tested at a temperature of 20±2°C. A sample which has undergone one such test may not be reused. The packages prepared in Example 13 and containing the products of Examples 10 and 9 were used to prepare fuel samples for the so-called BNPe test. These were compared, as before, to a sample of basefuel and to a sample of 'base package' containing all components except combustion improver. The products of Examples 9 and 10 clearly provide a good antifoam performance, i.e. minimum antagonism towards the antifoam. Example 15 Emissions Tests and Smoke Measurements Material prepared as in Example 10 was blended into a standard European diesel fuel such that the potassium content of the fuel was 15 ppm m/m. Emissions tests were then performed in a modern IDI engine and according to the standard EEC 91/114 protocol consisting of City and Extra-Urban Dive Cycles. Additional 'Hot-start' Urban Drive Cycles were incorporated for illustrative purposes. The values obtained therein are not reflected in the 'Overall' result, which is calculated in the normal way . URBAN PHASE COLD Hydrocarbon CO C0 2 NOx Particulate Distance Fuel gm gm gm gm gm km l/lOOkm Base #1 2.481 498.27 1.85 0.2168 2. .037 9 .3457 Base #2 0.4306 2.5983 496.02 1.8279 0.2073 2. .031 9 .3602 Base #3 0.5756 2.9814 506.24 1.756 0.2552 2. .029 9 .5803 Base #4 0.6238 2.9696 498.18 1.7606 0.2496 2. ,024 9 .4555 spread 0.1932 0.5004 10.22 0.094 0.0479 0 .2346 spread% 35.6 18.1 2.0 5.2 20.6 2.5 mean 0.5433 2.7576 499.68 1.7986 0.2322 9 .4354 Additive #1 0.4185 2.5303 493.01 1.8402 0.2154 2. ,027 9 .3197 Additive #2 0.4549 2.632 495.4 1.7874 0.2155 2. ,027 9 .3696 Additive #3 0.5043 2.6739 497.22 1.801 0.2208 2 .03 9 .3939 spread 0.0858 0.1436 4.21 0.0528 0.0054 0 .0742 spread% 18.7 5.5 0.9 2.9 2.5 0 .7921 mean 0.4592 2.6121 495.21 1.8095 0.2172 9 .3610 Benefit (%) 15.5 5.3 0.9 -0.6 6.5 0.8 URBAN PHASE HOT Hydrocarbon CO CO, NOx Particulate Distance Fuel used gm gm gm gm gm km 1/lOOkm Base #1 0, .2835 1.8254 404.48 1.5883 0.1857 2, .029 7 .6277 Base #2 0 .3122 1.819 406.67 1.6349 0.1854 2 .029 7. .6701 Base #3 0 .3838 1.9725 409.92 1.5722 0.2064 2 .028 7 .7434 Base #4 0 .3908 1.9909 411.16 1.6119 0.2246 2 .025 7 .7791 spread 0 .1073 0.1719 6.68 0.0627 0.0392 0 .1514 spread% 31.3 9.0 1.6 3.9 19.5 2.0 mean 0 .3426 1.9020 408.06 1.6018 0.2005 7 .7051 Additive #1 0.3135 1.7852 407.48 1.6533 0.1827 2.029 7.6844 Additive #2 0.3568 1.8023 411.82 1.6585 0.1977 2.029 7.7685 Additive #3 0.3207 1.7968 408.5 1.6468 0.1786 2.025 7.7194 spread 0.0433 0.0171 4.34 0.0117 0.0191 0.0842 spread% 13.1 1.0 1.1 0.7 10.3 1.0896 mean 0.3303 1.7948 409.27 1.6529 0.1863 7.7241 Benefit 3.6 5.6 -0.3 -3.2 7.1 (%) EUDC PHASE Hydrocarbon CO C0 2 NOx Particulate Distance Fuel use gm gm gm gm gm km 1/lOOkm Base #1 0.3403 1.6821 943.85 3.918 0.4426 6. .97 5. .1539 Base #2 0.3726 1.6153 947.88 3.9681 0.4402 6. .968 5. .1772 Base #3 0.4237 1.8088 961.54 3.9223 0.4755 6. .963 5. .2579 Base #4 0.4337 1.7091 951.53 4.0553 0.487 6. .964 5, .2019 spread 0.0934 0.1935 17.69 0.1373 0.0468 0, .1040 spread% 23.8 11.4 1.9 3.5 10.1 2 .0 mean 0.3926 1.7038 951.20 3.9659 0.4613 5. .1977 Additive #1 0.3779 1.6247 957.36 4.1387 0.4134 6, .958 5 .2365 Additive #2 0.3932 1.5788 965.75 4.1543 0.4002 6, .979 5 .2662 Additive #3 0.3991 1.6526 959.25 4.1609 0.3924 6 .97 5 .2384 spread 0.0212 0.0738 8.39 0.0222 0.021 0 .0297 spread% 5.4 4.6 0.9 0.5 5.2 0 .6 mean 0.3901 1.6187 960.79 4.1513 0.402 5 .2470 Benefit (%) 0.6 5.0 -1.0 -4.7 12.9 OVERALL RESULT HydroCO C0 2 NOx PartiFuel used carbon culates g/km g/km g/km g/km g/km 1/lOOkm Base #1 0.5426 167.33 0.6666 0.0766 6.3756 Base #2 0.1011 0.5470 167.81 0.6738 0.0755 6.4063 Base #3 0.1255 0.6137 170.39 0.6579 0.0850 6.5111 Base #4 0.1315 0.6056 168.97 0.6745 0.0873 6.4575 spread 0.0304 0.0710 3.07 0.0165 0.0118 0.1355 spread% 25.4 12.3 1.8 2.5 14.5 2.1 mean 0.1194 0.5772 168.62 0.6682 0.0811 6.4376 Additive 0.1008 0.5393 168.68 0.6930 0.0737 6.4389 #1 Additive 0.1092 0.5449 169.73 0.6887 0.0737 6.4800 #2 Additive 0.1110 0.5554 169.16 0.6901 0.0718 6.4592 #3 spread 0.0103 0.0161 1.05 0.0042 0.0019 0.0411 spread% 9.6 2.9 0.6 0.6 2.6 0.6 mean 0.1070 0.5465 169.19 0.6906 0.0731 6.4594 Benefit 10.4 5.3 -0.3 -3.4 9.9 -0.3 ( ) The effectiveness of the product of Example 10 as a suppressant of hydrocarbon, carbon monoxide and particulates emissions is thus clearly demonstrated. Smoke measurements were also obtained. This was done using the Sun™ MoT Smoke Testing kit and according to the standard procedure . The procedure was carried out following pre-conditioning of vehicle and tunnel with three EUDC cycles following each complete set of emissions tests. Smoke Reading Base #1 Base #2 2 . , 06 Base #3 1 . 5 Base #4 1 . , 11 spread 0 . , 95 spread% 61 . 0 mean 1 . , 56 Additive #1 1.61 Additive #2 1.24 Additive #3 1.18 spread 0.43 spread% 32.0 mean 1.34 Benefit (%) 13.7 The effectiveness of the product in suppressing visible smoke is thus also confirmed. Example 16 Regeneration of a DPF A Peugeot XUD-9A engine was selected for this work. The engine design was four cylinder, in-line with a single overhead camshaft operating two valves per cylinder. The engine was of the indirect injection (IDI) type, employing a Ricardo Comet type pre-chamber design. The total swept volume of the engine was 1905 cm 3 . The engine was naturally aspirated and had a 23.5 : 1 compression ratio. The engine was fitted with a Roto- Diesel fuel pump and Bosch pintle type fuel injectors. The engine was mounted on a pallet arrangement which was equipped with appropriate heat exchangers, electrical connections and connectors for instrumentation signals. This pallet arrangement was then connected to the engine test bench. The engine dynamometer was a Froude AG150 eddy current machine controlled by the CP Engineering Cadet system. , The engine temperatures were controlled automatically by suitable 3 -term controllers integrated into the secondary coolant system supplies. The test bench was controlled, and data was logged using a CP Engineering Cadet system. Fuel flow was measured using an AVL 730 gravimetric fuel balance. The difference of the mass of fuel, in a vessel, is monitored over a set period of time. For this work fuel flow was measured over a 20 second interval . The engine exhaust system was modified to allow ready interchange of a centre section which could incorporate a selection of DPFs. For the work reported here a NoTox™ SiC wall-flow DPF was used. The base fuel used throughout the testing reported here was an EN 590 specification fuel labelled as SD2. Analysis of the fuel is given below. The engine was run at a series of constant speed/load conditions over 12 to 16 hours per test fuel, as detailed below. Gauge pressure in the exhaust downpipe in advance of the DPF was measured using a pressure transducer at 1Hz with logging of average readings over 10 second periods. Regeneration of the DPF, i.e. burnoff of the trapped material, was detected as a decrease in pressure. Comparison of mean pressure or of mean pressure plus two standard deviations therein is thus a direct comparison of the relative efficiency of any two additives present in the fuel at causing burnoff of trapped material . Two additives were compared. The first, as used in WO- A-96/34075, was the strontium salt formed from reaction of Sr(OH) 2 .8H 2 0 with a poly (butenyl) succinic anhydride prepared from BP Napvis™ X-10 (Sr (PIBSA) The second was the additive of Example 10. The dose rate of strontium in the fuel was 20 ppm m/m. Speed is engine rpm and load is in Nm. Mean pre-DPF exhaust pressure Speed/load Sr (PIBSA) 2 Sr (PIBSA) 2 Sr (PIBSA) 2 Example 10 1260/5 74 69 58 106 1550/10 131 116 118 115 1550/20 96 96 85 2710/30 150 141 141 143 3000/30 169 169 152 Mean + 2 standard deviations pre-DPF exhaust pressure Speed/load Sr ( PIBSA) 2 Sr ( PIBSA) 2 Sr ( PIBSA) 2 Example 10 1260/ 5 113 108 91 183 1550 /10 321 200 211 201 1550 /20 147 156 125 2710 /30 220 222 238 220 3000 /30 278 278 237 Overall, the product of Example 10 can be seen to be at least as effective as the previously employed material. Example 17 Comparative viscosities Within the range of compounds described in WO-A-96/34074 and WO-A-96/34075, the most practical potassium compounds found were the reaction products of poly (butenyl) succinic anhydride with KOH in the presence of a single equivalent on N-methyl pyrrolidinone (NMP) pre potassium ion (KPIBSA 360 .NMP) . These materials gave the lowest viscosity of any PIBSA-based material on a wt:wt K basis and so are used to compare viscosities of the hemi-esters of the current application. Viscosities were determined on a Bohlin Instruments CVO rheometer . Clearly, materials prepared according to the method in the examples have substantially lower viscosities than materials previously considered.

Claims: Claims 1. A compound of formula I : (wherein R 1 is an optionally substituted, saturated or unsaturated, straight-chained or branched hydrocarbyl group; one of R 2 and R 3 represents a hydrogen atom and the remaining one of R 2 and R 3 represents a straight-chained or branched alkyl group, preferably a C λ . 10 alkyl group; R represents a hydrogen atom or a straight -chained or branched alkyl group, preferably a hydrogen atom; or, alternatively, R 1 and R 4 may be linked together to form an optionally substituted 5- or 6-membered carbocyclic ring) and metal salts and complexes, optical isomers and diastereoisomers thereof. 2. A compound as claimed in claim 1 wherein R 1 is substituted with one or more groups of the formula: (wherein R 2 , R 3 and R 4 are as defined in claim 1 ) 3. A compound as claimed in claim 1 or claim 2 wherein R 1 is an alkenyl or poly (alkenyl) group. 4. A compound as claimed in claim 3 wherein R 1 is a C 2 _ 20 alkenyl or poly(C 2 _ 6 alkenyl) group. 5. A compound as claimed in claim 3 wherein R 1 is a poly(C 2 _ 4 alkenyl) group. 6. A compound as claimed in claim 3 wherein R 1 is a poly (butenyl) group. 7. A compound as claimed in claim 3 wherein R 1 is selected from the group consisting of 2-octenyl, 2- dodecenyl and 2-octadecenyl . 8. A compound as claimed in any one of claims 1 to 7 wherein when not hydrogen, R 2 and R 3 are straight-chained or branched C^ o alkyl groups . 9. A compound as claimed in any one of claims 1 to 7 wherein when not hydrogen, R 2 and R 3 are secondary alkyl groups . 10. A compound as claimed in claim 9 wherein R 2 or R 3 is an isopropyl group. 11. A metal salt or complex as claimed in any one of the preceding claims comprising a compound of formula (10 R 1 and R 4 are as defined in any one of claims 1 to 7; and one of R 2 and R 3 is an alkali or alkaline earth metal M of valency n, and the remaining one of R 2 and R 3 is a straight -chained or branched alkyl group, preferably a Ci. K , alkyl; wherein when one of R 2 and R 3 is an alkali metal, n=l, and when one of R 2 and R 3 is an alkaline earth metal, n=2 ; and for whichever of R 2 and R 3 is an alkyl group the electrostatic charges on 0 and R 2 or R 3 are absent) and the optical isomers and diastereoisomers thereof. 12. A metal salt or complex as claimed in claim 11 wherein M is selected from the group consisting of Na, K, Mg, Ca and Sr. 13. A metal salt or complex as claimed in claim 12 wherein M is selected from the group consisting of K and Sr. 14. A process for the preparation of compounds of formula I as defined in any one of claims 1 to 10, said process comprising at least one of the following steps: (a) reacting a compound of formula III: R-OH (III) (wherein R is a straight-chained or branched alkyl group, preferably a alkyl) together with a succinic anhydride derivative of formula IV: (wherein R 1 and R 4 are as defined in any one of claims 1 to 7) ; (b) reacting a compound of formula III as defined in step (a) together with a succinic acid derivative of formula V: (wherein R 1 and R 4 are as defined in any one of claims 1 to 7) ; (c) separating the mixture formed in steps (a) or (b) by conventional separating techniques; (d) resolving a chiral compound of formula I into its isomers; (e) metallating a compound of formula I; and (f) performing at least one of steps (a) to (e) above using reagents with protected functional groups and subsequently removing the protecting groups. 15. A process as claimed in claim 14 wherein in steps (a) and (b) the alcohol is present in excess over either the cyclic succinic anhydride derivative of formula IV (in step (a) ) or the succinic acid derivative of formula V (in step (b) ) . 16. A process for the preparation of compounds of formula I as defined in any one of claims 1 to 10, said process comprising deprotection of a protected derivative thereof . 17. A fuel additive for liquid hydrocarbon fuels comprising at least one metal complex of a compound of formula I as claimed in any one of claims 1 to 10. 18. A fuel additive as claimed in claim 17 wherein said complex is a compound of formula II as defined in any one of claims 9 to 13. 19. A fuel additive as claimed in claim 17 or claim 18 further comprising an organic, fuel soluble solvent miscible in all proportions with the fuel. 20. A fuel additive as claimed in claim 19 wherein said solvent is a hydrocarbon. 21. A fuel additive as claimed in any one of claims 17 to 20 wherein the amount of metal in the additive is from 1 to 10 wt%. 22. A fuel additive as claimed in any one of claims 17 to 20 wherein the amount of metal in the additive is from 4 to 6 wt%. 23. A fuel additive as claimed in any one of claims 17 to 20 wherein the amount of metal in the additive is 5 wt%. 24. A method of reducing particulate or visible smoke emissions and/or unburned hydrocarbons resulting from the incomplete combustion of liquid hydrocarbon fuels, said method comprising the step of incorporating into the fuel prior to combustion an effective amount of a metal complex of a compound of formula I as defined in any one of claims 1 to 10. 25. Use of a metal complex of a compound of formula I as claimed in any one of claims 1 to 10 as a black smoke, particulate and/or unburned hydrocarbon-reducing additive. 26. Use of a metal complex of a compound of formula I as claimed in any one of claims 1 to 10 as a part of an additive composition for the regeneration of particulate filters. 27. Use of a metal complex of a compound of formula I as claimed in any one of claims 1 to 10 as part of a formulation further comprising any of the additives selected from the group consisting of detergents, carrier oils, anti-oxidants, corrosion inhibitors, colour stabilisers, metal deactivators, cetane number improvers, other combustion improvers, antifoams, pour point depressants, cold filter plugging point depressants, wax anti-settling additives, dispersants, reodorants, dyes, smoke suppressants, particulate filter regeneration additives and lubricity agents. 28. Use of a metal complex as claimed in any one of claims 25 to 27 wherein the metal dose rate to the fuel is up to 30 ppm. 29. Use of a metal complex as claimed in any one of claims 25 to 27 wherein the metal dose rate to the fuel is 10-20 ppm.

Inventors: COOK STEPHEN LEONARD, REID JACQUELINE GLEN, RICHARDS PAUL JOSEPH, COOK, STEPHEN, LEONARD, REID, JACQUELINE, GLEN, RICHARDS, PAUL, JOSEPH
Applicants: ASS OCTEL, COOK STEPHEN LEONARD, REID JACQUELINE GLEN, RICHARDS PAUL JOSEPH, COOK, STEPHEN, LEONARD, REID, JACQUELINE, GLEN, RICHARDS, PAUL, JOSEPH, THE ASSOCIATED OCTEL COMPANY LIMITED
Application year: 1999
Main classification: C10L1
IPC codes: C07C69, C10L1, C10L10