Ester compounds

Abstract

An ester comprises a reaction product of a polycarboxylic acid (or a reactive equivalent) and a polyol. The ester may be a polymer comprising at least four monomer units. The acid may be succinic acid

Description

Silicone compounds, such as polydimethylsiloxane, functionalised polydimethylsiloxanes, organosilicones, and related compounds are used widely in a range of applications and products. They may be provided in concentrate compositions, either for direct use or for later dilution priorto use. However, high concentrations of silicone compounds can cause concentrate compositions to be highly viscous and difficult to pour or pump. Furthermore, high concentrations of the silicone compound can cause concentrate compositions to be unstable when stored for extended periods of time, especially when the silicone compound is poorly soluble or poorly miscible with a diluent comprised in the concentrate composition. There are a number of disadvantages associated with the use of such silicone compounds, including their low biodegradability, which has led to environmental concerns. It is thus an object of the invention to provide an alternative to silicone compounds that can be used in a wide range of applications and uses. It is another object of the invention to provide alternatives to silicone compounds that are more biodegradable whilst having equal or improved properties. It is another object of the invention to improve the stability and / or flowability of concentrate compositions comprising such compounds. Summary of the Invention According to aspects of the present invention, there is provided an ester compound, method and concentrate composition as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and from the description which follows. The inventors have identified that certain ester compounds have properties that are at least comparable to that provided by known silicone compounds, whilst also being more biodegradable. According to a first aspect of the invention, there is provided an ester compound, wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. According to a second aspect of the invention, there is provided a concentrate composition comprising one or more ester compounds and optionally at least one solvent, wherein the concentrate composition comprises at least 20 wt% of the one or more ester compounds, and wherein the or each ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. According to a third aspect of the invention, there is provided a use of a surfactant to emulsify at least one ester compound in an aqueous concentrate composition, wherein the aqueous concentrate composition comprises at least 20 wt% of the one or more ester compounds, and wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. According to a fourth aspect of the invention, there is provided a method of emulsifying at least one ester compound in an aqueous composition to make an aqueous ester compound concentrate composition, wherein the aqueous ester compound concentrate composition comprises at least 20 wt% of the one or more ester compounds, the method comprising admixing a surfactant with the ester compound in an aqueous composition, wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. Other features of the invention will be apparent from the dependent claims, and from the description which follows. Features described in relation to the second, third and fourth aspects may have any of the suitable features and advantages described in relation to the first aspect. Detailed Description of the Invention Unless otherwise stated, the following terms used in the specification and claims have the meanings set out below. The terms “alkyl” and “alkylene” include both straight and branched chain alkyl and alkylene groups respectively unless otherwise stated. References to individual alkyl groups such as “propyl” are specific for the straight chain version only and references to individual branched chain alkyl groups such as “isopropyl” are specific for the branched chain version only. For example, “C3-30 alkyl” includes C6-24 alkyl, Ce-s alkyl, propyl, isopropyl and t-butyl. The term “alkenyl” includes both straight and branched chain alkenyl groups. References to individual alkenyl groups such as “propenyl” are specific for the straight chain version only and references to individual branched chain alkenyl groups such as “isopropenyl” are specific for the branched chain version only. For example, “C3-30 alkenyl” includes C6-24 alkenyl, Ce-s alkenyl, propenyl and isopropenyl. Herein C3-30 means a group having from 3 to 30 carbons atoms therein, for example having 3, 4, 5 etc up to 30 carbon atoms. The term "hydrocarbyl" is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, it refers to a group having a carbon atom directly attached to the remainder of the molecule and having predominantly hydrocarbon character. As used in the specification and the appended claims, the singular forms "a", "an," and "the" include both singular and plural referents unless the context clearly dictates otherwise. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of’ or “consists essentially of’ means including the components specified but excluding other components except for components added for a purpose other than achieving the technical effect of the invention. The term “consisting of’ or “consists of’ means including the components specified but excluding other components. Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to include the meaning “consists essentially of’ or “consisting essentially of’, and also may also be taken to include the meaning “consists of’ or “consisting of’. As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts of percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear. The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1,2,3,4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.70 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein. The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each exemplary aspect of the invention, as set out herein are also applicable to any other aspects or exemplary aspects of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or embodiment of the invention as interchangeable and combinable between different aspects of the invention. As used herein, the term "and / or," when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a list is described as comprising group A, B, and / or C, the list can comprise A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination. The term “reactant” is used herein to refer to a compound comprising at least one reactive functional group. As reported herein, the number average molecular weight was determined by gel permeation chromatography using a polystyrene standard according to ASTM D6579-11 (“Standard Practice for Molecular Weight Averages and Molecular Weight Distribution of Hydrocarbon, Rosin and Terpene Resins by Size Exclusion Chromatography”. UV detector; 254 nm, solvent: unstabilised THF, retention time marker: toluene, sample concentration: 2mg / ml). According to a first aspect of the invention, there is provided an ester compound, wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. Suitable features of the first aspect will now be described. The ester compound of the first aspect is the reaction product of reactants comprising one or more first reactants and one or more second reactants as defined herein. In other words, the ester compound may be obtainable or obtained by reacting reactants comprising one or more first reactants and one or more second reactants as defined herein. The ester compound of the first aspect may be the reaction product of reactants consisting essentially of or consisting of one or more first reactants and one or more second reactants as defined herein. In other words, the ester compound may be obtainable or obtained by reacting reactants consisting essentially of or consisting of one or more first reactants and one or more second reactants as defined herein. The first and second reactants may be reacted in any suitable molar ratio to make the ester compound, as would be appreciated by a person skilled in the art. The first and second reactants may be reacted in a molar ratio of from 1:10 to 10:1, preferably from 1:6 to 6:1, for example from 1:3 to 3:1. Ratios refer to the total amounts of first reactants or second reactants if more than one first reactant or second reactant is present. The first and second reactants may be reacted in a molar ratio of from 1:1.5 to 1.5:1, such as a molar ratio of 1:1. The ester compound may be prepared by any suitable method, as would be known to persons skilled in the art. References herein to a reaction product of reactants comprising the first and second reactants are intended to refer to a product of the reaction of reactants comprising the first and second reactants conducted in any suitable manner. For example, the reaction may occur between the first and second reactants in the absence of other reactant(s) or may occur in the presence of other reactant(s). The or each of the one or more first reactants used to make the ester compound is a polycarboxylic acid or a reactive equivalent thereof. Mixtures of two or more different first reactants (i.e. different polycarboxylic acids or reactive equivalents thereof) may be used to make the ester compound. Any suitable polycarboxylic acid or a reactive equivalent thereof may be used to make the ester compound, as would be understood by the person skilled in the art. The polycarboxylic acid may comprise two or more carboxylic acid groups, such as two or three carboxylic acid groups. Preferred polycarboxylic acids are dicarboxylic acids. The polycarboxylic acid may be aliphatic or aromatic. The aliphatic polycarboxylic acid may be cycloaliphatic. The aliphatic polycarboxylic acid may be saturated or unsaturated. By the term “unsaturated” we mean that the polycarboxylic acid comprises one or more carbon-carbon double bonds. Preferably the polycarboxylic acid is aliphatic and / or saturated. The polycarboxylic acid may comprise one or more heteroatoms other than the oxygen atoms in the carboxylic acid groups. By the term “heteroatoms” we mean atoms other than carbon or hydrogen, such as oxygen, nitrogen and sulfur atoms. For example, the polycarboxylic acid may comprise one or more moieties selected from hydroxy groups, amino groups, ether groups, and / or thioether groups. The polycarboxylic acid may be of the formula HOOC(CR2)nCOOH, wherein n is from 0 to 30; and each R is independently hydrogen ora substituent; and / or two R groups on the same carbon atom may be taken together to form a methylene (=CH2) group; and / or when n is two or more, two R groups on adjacent carbon atoms may be taken together to form a double bond. n is suitably from 1 to 20, preferably from 2 to 16, more preferably from 2 to 12, for example from 2 to 10. Each (CR2) group can be the same or different. Unless otherwise specified, the terms “each R”, “R groups” and the like are intended to refer to all instances of the “R” group in the formula HOOC(CR2)nCOOH, Each R may be hydrogen. Alternatively, one or two R groups may be a substituent and the remaining R groups may be hydrogen. Any suitable substituent may be used as R. The substituent may be a hydrocarbyl group or a heteroatom-containing group. Examples of suitable substituents include hydroxy groups, amino groups, carboxyl groups, alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups, wherein the alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups are optionally substituted with one or more of a hydroxy group, an amino group, and / or a carboxyl group. One or two of the R groups may be a hydroxy group. Another of the R groups may optionally be a carboxyl group or a carboxyl-substituted methyl group, and the remaining R groups are suitably hydrogen. Suitably, one or two of the R groups may be a hydroxy group, another of the R groups may be a carboxyl group or a carboxyl-substituted methyl group, and the remaining R groups may be hydrogen; or one ortwo of the R groups may be a hydroxy group and the remaining R groups may be hydrogen. The polycarboxylic acid may be malic acid, tartaric acid, or citric acid. One or two R groups may be an alkyl or alkenyl group. Preferably, one R group is an alkyl or alkenyl group. The remaining R groups are suitably hydrogen. Each alkyl or alkenyl group may contain from 6 to 30, such as from 8 to 24, carbon atoms. The polycarboxylic acid may be an alkyl or alkenyl substituted succinic acid, preferably an alkenyl substituted succinic acid. Examples of such polycarboxylic acids include C20-24 alkenyl succinic acid, dodecenyl succinic acid (such as (2-dodecen-1-yl)succinic acid), nonenyl succinic acid, octadecenyl succinic acid and octenyl succinic acid. One R group may be a polyisobutenyl group and the remaining R groups may be hydrogen. The polyisobutenyl group suitably has a number average molecular weight of from 100 to 2000, preferably from 100 to 1000, for example 260 or 550. The polycarboxylic acid may be a polyisobutenyl succinic acid, for example wherein the polyisobutenyl group has a number average molecular weight of 260 or 550. Two R groups on the same carbon atom may be taken together to form a methylene (=CH2) group. The polycarboxylic acid may comprise one or more such methylene groups. Two R groups on the same carbon atom may be taken together to form a methylene (=CH2) group and the remaining R groups may be hydrogen. The polycarboxylic acid may be itaconic acid. When n is 2 or more, two R groups on adjacent carbon atoms may be taken together to form a double bond. The polycarboxylic acid may comprise one or more such double bonds. The double bond may be in a cis or trans configuration. Two R groups on adjacent carbon atoms may be taken together to form a double bond and the remaining R groups may be hydrogen. The polycarboxylic acid may be maleic acid or fumaric acid. The polycarboxylic acid may be of the formula HOOC(CH2)nCOOH, wherein n is from 0 to 30. n is suitably from 1 to 20, preferably from 2 to 16, more preferably from 4 to 12, for example from 5 to 10. The polycarboxylic acid may be selected from oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, or dodecanedioic acid (especially from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, or dodecanedioic acid, more especially from succinic acid, pimelic acid, sebacic acid, or dodecanedioic acid, most especially sebacic acid). Preferably, the polycarboxylic acid may be selected from pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, or dodecanedioic acid. The polycarboxylic acid may be of the formula HOOC(CH2)mX(CH2)m2COOH, wherein m+m2 is from 0 to 30 and X is O, S, or NR1 wherein R1 is hydrogen or a hydrocarbyl group. m+m2 is suitably from 1 to 20, preferably from 2 to 16, more preferably from 2 to 12, for example from 2 to 6. m and m2 may each independently be from 0 to 30, suitably from 1 to 19, preferably from 1 to 15, more preferably from 1 to 11, for example from 1 to 5. R1 may be a hydrocarbyl group, such as an alkyl or alkenyl group. The hydrocarbyl group may contain from 1 to 12 carbon atoms, preferably from 1 to 6 carbon atoms. Preferably, R1 is hydrogen. X is preferably O or S (most preferably O). Polycarboxylic acids of the formula HOOC(CH2)mX(CH2)m2COOH, wherein m+m2 is from 2 to 12 and X is O or S (especially O) are preferred. The polycarboxylic acid may be selected from diglycolic acid, thiodiglycolic acid, 3,3’-thiodipropanoic acid, or iminodiacetic acid, preferably from diglycolic acid or thiodiglycolic acid. The polycarboxylic acid may be of the formula HOOCCH2(OCH2CHR2)XOCH2COOH, wherein x is from 1 to 30 and each R2 is independently hydrogen or a hydrocarbyl group. x is suitably from 1 to 20, preferably from 2 to 16, more preferably from 4 to 14, for example from 6 to 12, such as 10. Each R2 may independently be a hydrocarbyl group, such as an alkyl group. The alkyl group may contain from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms. The alkyl group is preferably methyl. In some embodiments, a portion of R2 groups are hydrocarbyl groups (such as methyl groups) and the remaining R2 groups are hydrogen. Preferably, each R2 is hydrogen. The polycarboxylic acid may be a poly(ethylene glycol)bis(carboxymethyl) ether. The poly(ethylene glycol)bis(carboxymethyl) ether may contain from 1 to 20 ethylene oxide units (i.e. -CH2CH2O-), preferably from 2 to 16 ethylene oxide units, more preferably from 4 to 14 ethylene oxide units, for example from 6 to 12 ethylene oxide units, such as 10 ethylene oxide units. The polycarboxylic acid may be a poly(ethylene glycol)bis(carboxymethyl) ether containing 10 ethylene oxide units. The polycarboxylic acid may comprise a cyclic group. The cyclic group may be substituted, for example with alkyl or alkenyl groups. The carboxyl groups of the polycarboxylic acid may be attached directly to the cyclic group, or via an alkyl or alkenyl group. The cyclic group may be a cycloaliphatic group. Examples of cycloaliphatic groups include cyclohexane and cyclohexene (especially cyclohexane). The polycarboxylic acid may be 1,2-cyclohexanedicarboxylic acid or 1,4-cyclohexanedicarboxylic acid. The cyclic group may be an aromatic group. Examples of aromatic groups include benzene and naphthalene. The polycarboxylic acid may be selected from phthalic acid, isophthalic acid, terephthalic acid, homophthalic acid, 1,2,4-benzenetricarboxylic acid, pyromellitic acid, 1,2-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid. 1,8-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, or 2,7-naphthalenedicarboxylic acid. In some embodiments, the polycarboxylic acid does not comprise a cyclic group, especially does not comprise an aromatic group. The polycarboxylic acid may be a dimer acid. Such compounds are formed from the dimerisation of unsaturated acids, for example unsaturated fatty acids having from 6 to 50, suitably from 8 to 40, preferably from 10 to 36, for example from 10 to 20 carbon atoms, or from 16 to 20 carbon atoms. The dimer acid may be hydrogenated or unhydrogenated. Preferably, the dimer acid is hydrogenated. The dimer acid may have from 12 to 100 carbon atoms, preferably from 16 to 72 carbon atoms such as from 20 to 40 carbon atoms, for example from 32 to 40 carbon atoms. Preferred dimer acids comprise C36 dimer acids such as those prepared by dimerising oleic acid, linoleic acid and mixtures comprising oleic and linoleic acid, for example, tall oil fatty acids. Preferably, the dimer acid comprises a dimer acid prepared by dimerising oleic acid. The dimer acid may be a hydrogenated C36 dimer acid. Preferably, the one or more first reactants may comprise at least a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, wherein n, R, m, M2, X, R2 and x are as defined herein. Most preferably, the one or more first reactants may comprise at least a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, wherein n is from 2 to 16 (more preferably from 4 to 12, for example from 5 to 10), R is hydrogen or a substituent (preferably hydroxy), X is O or S (preferably O), m+m2 is from 2 to 12, each R2 is hydrogen and x is from 2 to 16 (more preferably from 4 to 14, for example from 6 to 12). A reactive equivalent of the polycarboxylic acid may be used. By the term “reactive equivalent”, we mean a compound that results in the same reaction product as the corresponding polycarboxylic acid. Suitable reactive equivalents include acid chlorides and esters of the polycarboxylic acids described herein. The reactive equivalent may be an acid chloride of the polycarboxylic acid. The acid chloride of the polycarboxylic acid suitably does not comprise any free carboxylic acid groups. The reactive equivalent may be an ester of the polycarboxylic acid. The ester of the polycarboxylic acid suitably does not comprise any free carboxylic acid groups. The ester of the polycarboxylic acid is suitably a hydrocarbyl ester, such as an alkyl ester, an alkenyl ester, an aryl ester, an aralkyl ester, or an alkaryl ester. Preferably the ester is an alkyl ester. The alkyl ester is suitably formed by reacting the polycarboxylic acid with an alkanol. The alkanol suitably contains from 1 to 10 carbon atoms, such as from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms. The alkanol is preferably methanol. The alkyl ester is preferably a methyl ester. Preferably the one or more first reactants used to make the ester compound is a polycarboxylic acid and not a reactive equivalent thereof. Suitably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, maleic acid, fumaric acid, malic acid, tartaric acid, citric acid, C2o-24 alkenyl succinic acid, dodecenyl succinic acid (such as (2-dodecen-1-yl)succinic acid), nonenyl succinic acid, octadecenyl succinic acid, octenyl succinic acid, polyisobutenylsuccinic acid, itaconic acid, 1,2-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, phthalic acid, isophthalic acid, terephthalic acid, homophthalic acid, 1,2,4-benzenetricarboxylic acid, pyromellitic acid, 1,2-naphthalenedicarboxylic acid, 2,3- naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diglycolic acid, thiodiglycolic acid, 3,3’-thiodipropanoic acid, iminodiacetic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a dimer acid, or an acid chloride or ester thereof. Suitably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a dimer acid, or an acid chloride or ester thereof (especially selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, tartaric acid, citric acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a dimer acid, or an acid chloride or ester thereof). Suitably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from oxalic acid, succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, malic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, isophthalic acid, terephthalic acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid or an acid chloride thereof. Suitably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid or an acid chloride thereof (especially selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, tartaric acid, citric acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid or an acid chloride thereof). Suitably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from oxalic acid, succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, malic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, isophthalic acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid, or an acid chloride thereof. Suitably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid, or an acid chloride thereof. Preferably, the or each polycarboxylic acid or the reactive equivalent thereof may be selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, malic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid, oxalyl chloride, isophthaloyl chloride, or terephthaloyl chloride (especially selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, malic acid, tartaric acid, citric acid, diglycolic acid, thiodiglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid, or oxalyl chloride, more especially selected from sebacic acid, diglycolic acid, poly(ethylene glycol)bis(carboxymethyl) ether, or a hydrogenated dimer acid). In some embodiments, mixtures of two or more different first reactants may be used to make the ester compound. For example, a mixture of dodecanedioic acid and tartaric acid may be used. The polycarboxylic acid compounds or reactive equivalents thereof discussed herein may be commercially available or may be prepared using procedures well known in the art. The or each of the one or more second reactants used to make the ester compound is a polyol. Mixtures of two or more different second reactants (i.e. different polyols) may be used to make the ester compound. Any suitable polyol may be used to make the ester compound, as would be understood by the person skilled in the art. The term polyol is used to refer to any compound including two or more hydroxy (OH) functional groups. In some embodiments the or each polyol may comprise carbon, hydrogen and oxygen atoms, and optionally additionally nitrogen atoms. In other embodiments, the or each polyol may consist essentially of or consist of carbon, hydrogen and oxygen atoms. Suitable polyols for preparing the ester compound may be compounds having from 2 to 10, preferably from 2 to 6, more preferably 2 or 3, hydroxy groups. Preferred polyols may have 2 hydroxy groups. Suitable polyols may include one or more of a polyol of formula (I): H-(OR3)P-OH (I) wherein each R3 is independently an optionally substituted hydrocarbylene group and p is an integer of at least 1. When p is an integer of greaterthan 1, the polyol of formula (I) may comprise groups R3 that are all the same or may comprise groups R3that are different. Suitably, each R3 in the formula (I) may be the same. Preferably p is an integer from 1 to 140, such as from 1 to 110, from 1 to 40 or from 1 to 10. For example, suitable polyols may include one or more of a polyol of formula (IA): H-(OR4)q-OH (IA) wherein each R4 is independently an optionally substituted alkylene group and q is an integer of at least 1. When q is an integer of greater than 1, the polyol of formula (IA) may comprise groups R4 that are all the same or may comprise groups R4 that are different. Suitably, each R4 in the formula (IA) is the same. Preferably q is an integer from 1 to 140, such as from 1 to 110, from 1 to 40 or from 1 to 10. When the polyol of formula (I) or (IA) contains a substituted hydrocarbylene or alkylene group, any suitable substituent may be present, such as for example a carboxy or amido substituent. The polyol of formula (IA) may have more than 2 hydroxy groups and the group R4 may be a hydroxy substituted alkylene group. Each hydroxy substituted alkylene group R4 may have 1,2 or more hydroxy groups. Each hydroxy substituted alkylene group R4 may preferably have 1 hydroxy group. The polyol of formula (IA) may have 2 hydroxy groups and the group R4 may be an optionally substituted alkylene group wherein the optional substituent is not hydroxy. The polyol of formula (I) or of formula (IA) may be a sugar derived compound in which R3 or R4 includes one or more hydroxy residues. The or each R3or R4 may represent a cyclic alkylene unit. One or more heteroatoms (for example oxygen atoms) may be present in the cyclic alkylene unit. For example the unit may contain an ether linkage. Suitably the or each R3 or R4 may be one or more saccharide units or may be substituted with one or more saccharide units. Suitably the or each R3 or R4 may be an unsubstituted alkylene group. Preferably the or each R3 or R4 is an optionally substituted alkylene group having from 1 to 50, such as from 1 to 40, preferably from 1 to 30, more preferably from 1 to 20, suitably from 1 to 12 or from 1 to 10, for example from 2 to 6, carbon atoms. Preferably the or each R3 or R4 is an unsubstituted alkylene group having from 1 to 50, preferably from 1 to 20, more preferably from 1 to 12 or from 1 to 10, suitably from 2 to 6 carbon atoms. Each R3 or R4 may be straight chained or branched. Suitably the or each R3 or R4 may be an ethylene, propylene, butylene, pentylene, hexylene or dodecylene group. When R3 or R4 has more than 2 carbon atoms any isomer may be present. Preferably R3 or R4 is an ethylene or a propylene group, most preferably an ethylene group. When q is 1, R4 may be a group of formula (CH2)x wherein x is from 2 to 12, preferably from 3 to 12. Suitably when q is 1, R4 may be a straight chain or branched optionally substituted alkylene group and the polyol may be selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, sorbitol, xylitol, glycerol and neopentyl glycol (preferably from ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, glycerol and neopentyl glycol). Suitably when q is 1, R may be a straight chain or branched alkylene group having from 2 to 12, preferably from 3 to 12, carbon atoms. Suitable compounds of this type include ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, pentaerythritol and neopentyl glycol. Suitably when q is 1, R4 may be a branched alkylene group having from 2 to 12, preferably from 3 to 12, carbon atoms. Suitable compounds of this type include propylene glycol, 1,2-butanediol, 1,3-butanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, pentaerythritol and neopentyl glycol. Suitably when q is 2 or more, the or each R4 may be a straight chain or branched alkylene group and the polyol may, for example, be selected from diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol and polypropylene glycol (especially polyethylene glycol and polypropylene glycol). Suitably when q is 2 or more, the or each R4 may be a straight chain or branched alkylene group having from 2 to 4, preferably 2 or 3, carbon atoms. Suitable compounds of this type include diethylene glycol, triethylene glycol, di propylene glycol, tripropylene glycol, polyethylene glycol (PEG), for example having a number average molecular weight of from 150 to 6000, and polypropylene glycol (PPG), for example having a number average molecular weight of from 400 to 2000. Polyethylene glycol and polypropylene glycol are preferred. Suitable polyols may include one or more of PEG 6000, PEG 1500, PEG 1000, PEG 600, PEG 400, PEG 200, PPG 2000, PPG 1000 and PPG 425. In some embodiments, the polyol may be polyethylene glycol having a number average molecular weight of from 400 to 6000, such as from 1000 to 2000, or polypropylene glycol having a number average molecular weight of from 400 to 2000, such as from 1000 to 2000. The or each R3 or R4 may comprise a mixture of isomers. For example when R3 or R4 is propylene, the polyol may include moieties -CH2CH(CH3)- and -CH(CH3)CH2- in any order within the chain. R3 or R4 may comprise a mixture of different groups for example ethylene, propylene or butylene units. The or each R4 may be an ethylene, propylene or butylene group. For example, the or each R4 may be an n-propylene or n-butylene group or an isopropylene or isobutylene group. For example the or each R4 may be -CH2CH2-, -CH2CH(CH3)-, - CH(CH3)CH2-, CH2CH2CH2-, -CH2C(CH3)2-, -CH2CH2CH2CH2-, -CH(CH3)CH(CH3)- or-CH2CH(CH2CH3)-. Preferably R4 is selected from -CH2CH2-, -CH2CH2CH2-, -CH(CH3)CH2- or -CH2CH(OH)CH2, more preferably from -CH2CH2-, and-CH2CH2CH2-. Suitably the polyol of formula (I) or (IA) may be a sugar derived alcohol, for example, glucose, fructose, trehalose, sucrose, lactose, maltose or sorbitol, preferably sorbitol. Suitably the polyol of formula (I) may be selected from one or more of 1,12-dodecanediol, 1,6-hexanediol, trimethylolpropane, neopentyl glycol, polyethylene glycol (such as PEG 6000, PEG 1500, PEG 1000, PEG 600, PEG 400, PEG 200), polypropylene glycol (such as PPG 2000, PPG 1000, PPG 425), sorbitol, trimethylolpropane and xylitol. Suitably, the polyol, for example of formula (I), may be an ester of glycerol (also known as a glyceride) and a hydroxycarboxylic acid. The ester of glycerol may be a mono-, di- or triglyceride, suitably a tri-glyceride. Suitably, the hydroxycarboxylic acid comprises one or more hydroxyl groups and one or more carboxylic acid groups. The hydroxycarboxylic acid may be a monocarboxylic acid comprising one or more hydroxyl groups. The hydroxycarboxylic acid may be of the formula R5COOH, wherein R5 is a hydroxyl-substituted hydrocarbyl group. R5 is suitably a hydroxy-substituted alkyl, alkenyl or alkaryl group, preferably a hydroxy-substituted alkyl or alkenyl group. R5 suitably comprises from 1 to 25 carbon atoms, preferably from 1 to 20 carbon atoms, more preferably from 1 to 17 carbon atoms. Suitably, the hydroxycarboxylic acid may be selected from glycolic acid, lactic acid, hydroxy butyric acid, hydroxyvaleric acid, hydroxycaproic acid, hydroxystearic acid (preferably 12-hydroxystearic acid), 2,2-bis(hydroxymethyl)propionic acid, mandelic acid or ricinoleic acid. Preferably, the hydroxycarboxylic acid or the cyclic ester thereof may be selected from glycolic acid, mandelic acid, ricinoleic acid, malic acid, tartaric acid or citric acid. More preferably, the hydroxycarboxylic acid or the cyclic ester thereof is ricinoleic acid. Preferably, when the polyol is an ester of glycerol, the polyol may be a mono-, di- or tri-glyceride of ricinoleic acid. Most preferably, when the polyol is an ester of glycerol, the polyol may be a tri-glyceride of ricinoleic acid, such as castor oil. Suitable polyols may include one or more of an alkoxylated polyol of formula (I) or (IA). These polyols may be the reaction product of a polyol of formula (I) or (IA) and one or more alkylene oxides, such as ethylene oxide or propylene oxide. Such alkoxylated polyols may include polyoxyethylene (80) sorbitan monooleate (also known as Tween® 80) and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate. Preferably, the or each polyol may be independently selected from castor oil, 1,6-hexanediol, sorbitol, neopentyl glycol and a polyalkylene glycol (such as PEG 200). When the or each polyol comprises carbon, hydrogen, oxygen and nitrogen atoms, the or each polyol may comprise two or more (such as 2 or 3) hydroxy groups and one or more (suitably one) amine groups. The amine groups may suitably be secondary or tertiary amines. Preferably the amine groups are tertiary amines. These polyols may be referred to herein as nitrogen containing polyols. For example, suitable nitrogen containing polyols may include one or more of a polyol of formula (II) or a derivative thereof: NR6R7R8 (II) wherein R6, R7 and R8 are each independently selected from hydrogen, hydroxyalkyl and hydrocarbyl, provided that at least two of R6, R7 and R8 represents a hydroxyalkyl group. Preferably, the at least two of R6, R7 and R8 that represent a hydroxyalkyl group are the same. For example, in the compounds of formula (II), R6 and R7 may each represent hydroxyalkyl and R8 may represent hydrogen, hydroxyalkyl or hydrocarbyl. In the formula (II), the hydroxyalkyl group may contain from 1 to 8, such as from 1 to 4, such as 2 or 3, carbon atoms. For example, each hydroxyalkyl group may be hydroxyethyl or hydroxypropyl, particularly hydroxyethyl. In the formula (II), the hydrocarbyl group (when present) may represent any suitable such group, such as an alkyl group, for example an alkyl group containing from 1 to 10, such as from 1 to 6 or from 1 to 4, carbon atoms. Thus, the hydrocarbyl group may represent a methyl, ethyl, propyl or butyl group (especially butyl or methyl). For example, in the compounds of formula II), R6 and R7 may each represent hydroxyalkyl and R8 may represent hydrogen, hydroxyalkyl or hydrocarbyl (especially hydrogen or hydrocarbyl, more especially hydrocarbyl). In some embodiments, the R6 and R7 may each represent hydroxyalkyl and R8 may represent hydroxyalkyl or hydrocarbyl. Preferably in the compounds of formula (II), R6 and R7 both represent hydroxyethyl and R8 represents hydrogen or hydrocarbyl, such as a hydrocarbyl group containing from 1 to 6, or from 1 to 4, carbon atoms. Preferably in the compounds of formula (II), R6 and R7 both represent hydroxyethyl and R8 represents hydrogen or an alkyl group containing from 1 to 6, or from 1 to 4, carbon atoms (especially methyl). Examples of suitable nitrogen containing polyols include N-methyl diethanolamine, N-butyl diethanolamine, triethanolamine and diethanolamine (especially N-methyl diethanolamine and N-butyl diethanolamine), and derivatives thereof. The or each nitrogen containing polyol may be independently selected from N-methyl diethanolamine, N-butyl diethanolamine, triethanolamine and diethanolamine (especially N-methyl diethanolamine and N-butyl diethanolamine). References herein to nitrogen containing polyols include derivatives thereof, such as a corresponding quaternary compound. Thus, as example of a suitable derivative of a nitrogen containing polyol is tris(2-hydroxyalkyl)methylammonium compound, for example tris(2-hydroxyethyl)methylammonium methylsulfate, which is a quaternary ammonium salt of triethanolamine. In some preferred embodiments, the nitrogen containing polyols are not derivatised, for example they are not quaternary ammonium salts. Preferably, the or each nitrogen containing polyol may be independently selected from tris(2-hydroxyethyl)methylammonium methylsulfate, N-butyl diethanolamine, diethanolamine and N-methyl diethanolamine, and derivatives thereof. More preferably, the or each nitrogen containing polyol may be independently selected from N-methyl diethanolamine and N-butyl diethanolamine, and derivatives thereof. Preferably, the or each nitrogen containing polyol may be independently selected from N-butyl diethanolamine, diethanolamine and N-methyl diethanolamine, and derivatives (for example quaternary ammonium salts) thereof. Further examples of suitable nitrogen containing polyols include those formed by reaction of a hydroxy substituted cyclic ester or cyclic carbonate, such as glycerol carbonate or gluconolactone, with a suitable primary or secondary amine compound, such as ethanolamine, dipropylamine, hexylamine, dodecylamine, phenethylamine, dipropylamine, ethylenediamine, or a polyether polyamine. Suitable polyether amines may include polyether monoamines (such as for example Jeffamine M-1000) and polyether polyamines (such as for example Jeffamine ED-600). As the skilled person would appreciate, the reaction of a hydroxy substituted cyclic ester or cyclic carbonate with a suitable primary or secondary amine compound will result in a ring opening reaction to form a compound such as a N-carbamoyl polyol or an amido polyol. The or each polyol may be selected from one or more polyol of the formula (I) or (IA) as defined herein (including an ester of glycerol and a hydroxycarboxylic acid (such as castor oil)), one or more of an alkoxylated polyol of formula (I) or (IA) as defined herein, one or more nitrogen containing polyol as defined herein (including compounds of the formula (II)) and one or more polyol formed by reaction of a hydroxy substituted cyclic ester or cyclic carbonate with a suitable primary or secondary amine compound. Preferably, the or each polyol may be selected from one or more polyol of the formula (I) or (IA) as defined herein (including an ester of glycerol and a hydroxycarboxylic acid (such as castor oil)), one or more of an alkoxylated polyol of formula (I) or (IA) as defined herein, one or more nitrogen containing polyol of the formula (II) as defined herein and one or more polyol formed by reaction of a hydroxy substituted cyclic ester or cyclic carbonate with a suitable primary or secondary amine compound. More preferably, the or each polyol may be selected from one or more polyol of the formula (I) or (IA) as defined herein (such as an ester of glycerol and a hydroxycarboxylic acid , for example castor oil), and one or more nitrogen containing polyol of the formula (II) as defined herein. In one embodiment, the or each polyol may be selected from one or more polyol of the formula (I) or (IA) as defined herein, such as an ester of glycerol and a hydroxycarboxylic acid (for example castor oil) / In another embodiment, the or each polyol may be selected from one or more of an alkoxylated polyol of formula (I) or (IA) as defined herein. In one embodiment, the or each polyol may be selected from one or more polyol of the formula (I) or (IA) as defined herein, such as an ester of glycerol and a hydroxycarboxylic acid (for example castor oil) and from one or more of an alkoxylated polyol of formula (I) or (IA) as defined herein. In another embodiment, the or each polyol may be selected from one or more nitrogen containing polyol as defined herein (including compounds of the formula (II)). In some embodiments, when at least one of the one or more polyols is a nitrogen containing polyol then any further polyol(s) contain only 2 hydroxy groups. In another embodiment, the or each polyol may be selected from one or more polyol formed by reaction of a hydroxy substituted cyclic ester or cyclic carbonate with a suitable primary or secondary amine compound. In one embodiment, the ester compound may be the reaction product of reactants comprising one (i.e. a single) polycarboxylic acid or a reactive equivalent thereof and one (i.e. a single) polyol. In other embodiments, the ester compound may be the reaction product of reactants comprising two different polycarboxylic acids or reactive equivalents thereof and one (i.e. a single) polyol, or the ester compound may be the reaction product of reactants comprising one (i.e. a single) polycarboxylic acid or a reactive equivalent thereof and two different polyols. In one embodiment, the ester compound may be the reaction product of reactants consisting essentially of or consisting of one (i.e. a single) polycarboxylic acid or a reactive equivalent thereof and one (i.e. a single) polyol. In other embodiments, the ester compound may be the reaction product of reactants consisting essentially of or consisting of two different polycarboxylic acids or reactive equivalents thereof and one (i.e. a single) polyol, or the ester compound may be the reaction product of reactants consisting essentially of or consisting of one (i.e. a single) polycarboxylic acid or a reactive equivalent thereof and two different polyols. The combined amount of the one or more first reactants and the one or more second reactants may be at least 50 mol%, suitably at least 75 mol%, preferably at least 90 mol%, for example at least 95 wt% of the reactants that are reacted to obtain the ester compound. The ester compound for use herein may be the reaction product of reactants comprising the one or more first reactants and one or more second reactants as disclosed herein and additionally one or more third reactants. In other words, the ester compound may be the reaction product of reactants comprising one or more first reactants as disclosed herein, one or more second reactants as disclosed herein and one or more third reactants. The one or more third reactants may for example act as end capping groups and / or may introduce additional functional groups to the ester compounds. The one or more third reactants may be selected to impart the desired groups and / or properties to the ester compound by the person skilled in the art. The combined amount of the one or more first reactants, the one or more second reactants, and the one or more third reactants may be at least 50 mol%, suitably at least 75 mol%, preferably at least 90 mol%, for example at least 95 wt% of the reactants that are reacted to obtain the ester compound. The ester compound for use herein may be the reaction product of reactants consisting essentially of or consisting of the one or more first reactants and one or more second reactants as disclosed herein and additionally one or more third reactants. In other words, the ester compound may be the reaction product of reactants consisting essentially of or consisting of one or more first reactants as disclosed herein, one or more second reactants as disclosed herein and one or more third reactants. The ester compound for use herein is suitably the reaction product of no more than four different reactants. Preferably, the ester compound is the reaction product of no more than three different reactants. For example, the ester compound may be the reaction product of one first reactant, one second reactant, and one third reactant, or the reaction product of two different first reactants and one second reactant as disclosed herein. In some preferred embodiments, the ester compound is the reaction product of only two different reactants, i.e. one first reactant and one second reactant as disclosed herein (and no further reactants). Examples of suitable third reactants include one or more of the following: (i) cyclic anhydrides; (ii) monocarboxylic acids or esters thereof; (iii) hydroxycarboxylic acids or cyclic esters thereof; (iv) epoxide compounds; (v) polyfunctional reactants having a reactive amino group; (vi) monoalcohols; and (vii) monofunctional reactants having a reactive amino group. The third reactant may be (i) a cyclic anhydride. By the term cyclic anhydride we mean a compound (or reactant) that comprises at least one anhydride group that is contained within a ring structure. For example, the ring structure that contains the anhydride group may comprise from 4 to 8 atoms, which atoms are typically carbon and oxygen. The ring structure that contains the anhydride group may be saturated or partially unsaturated (and is preferably saturated). The overall cyclic anhydride may typically comprise more than 8 atoms. Examples of suitable cyclic anhydrides include succinic anhydride, maleic anhydride, glutaric anhydride, a Ce-30 alkenyl succinic anhydride such as C20-24 alkenyl succinic anhydride, dodecenyl succinic anhydride (such as (2-dodecen-1-yl)succinic anhydride), nonenyl succinic anhydride, octadecenyl succinic anhydride, octenyl succinic anhydride, octadecyl succinic anhydride, or octyl succinic anhydride, a branched alkenyl succinic anhydride such as tetrapropenyl succinic anhydride or polyisobutenyl succinic anhydride, phthalic anhydride, pyromellitic dianhydride, 1,2,4-benzenetricarboxylic anhydride, 1,2-cyclohexanedicarboxylic anhydride, 1,2-naphthalic anhydride, 2,3-naphthalic anhydride, 1,8-naphthalic anhydride and homophthalic anhydride. Preferably, the cyclic anhydride is selected from C20-24 alkenyl succinic anhydride, dodecenyl succinic anhydride (such as (2-dodecen-1-yl)succinic anhydride), nonenyl succinic anhydride, or phthalic anhydride. C20-24 alkenyl succinic anhydride is preferred. The third reactant may be (ii) a monocarboxylic acid or an ester thereof. Suitable monocarboxylic acids may be aliphatic or aromatic. The aliphatic monocarboxylic acid may be cycloaliphatic. The aliphatic monocarboxylic acid may be saturated or unsaturated. The monocarboxylic acid may contain from 2 to 40 carbon atoms, suitably from 4 to 30 carbon atoms, preferably from 6 to 20 carbon atoms, for example from 8 to 18 carbon atoms. The monocarboxylic acid may be a fatty acid. Examples of suitable monocarboxylic acids or esters thereof include propionic acid, hexanoic acid, octanoic acid, decanoic acid, lauric acid, myristic acid, myristoleic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linolelaidic acid, arachidic acid, arachidonic acid, behenic acid, erucic acid, and esters (preferably methyl esters) thereof. For example, suitable monocarboxylic acids or esters thereof may include hexanoic acid, octanoic acid, decanoic acid, lauric acid, myristic acid, myristoleic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linolelaidic acid, arachidic acid, arachidonic acid, behenic acid, erucic acid, and esters (preferably methyl esters) thereof. Preferably, the monocarboxylic acid is selected from propionic acid, hexanoic acid, lauric acid, stearic acid, oleic acid, erucic acid, and esters (preferably methyl esters) thereof. Preferably, the monocarboxylic acid is selected from propionic acid, hexanoic acid, lauric acid, stearic acid, oleic acid, or erucic acid. In some embodiments, the ester compound is prepared from reactants that do not include a monocarboxylic acid or an ester thereof. The third reactant may be (iii) a hydroxycarboxylic acid or a cyclic ester thereof. Suitable hydroxycarboxylic acids comprise one or more hydroxy groups and one or more carboxylic acid groups. The hydroxycarboxylic acid may be a monocarboxylic acid comprising one or more hydroxy groups. By the term “cyclic ester” of a hydroxycarboxylic acid we mean a compound comprising a cyclic group, wherein the cyclic group comprises one or more ester groups. The ester may be a cyclic monoester or a cyclic diester. The cyclic ester may correspond to a single cyclised molecule of the hydroxycarboxylic acid. This may, for example, be formed by an intramolecular reaction between the hydroxy group and the carboxylic acid group on one molecule of the hydroxycarboxylic acid. Alternatively, the cyclic ester may correspond to a cyclic dimer of a hydroxycarboxylic acid. This may, for example, be formed by an intermolecular reaction between the hydroxy groups and the carboxylic acid groups on two molecules of the hydroxycarboxylic acid. Examples of suitable hydroxycarboxylic acids or cyclic esters thereof include glycolic acid, lactic acid, hydroxy butyric acid, hydroxyvaleric acid, hydroxycaproic acid, hydroxystearic acid (preferably 12-hydroxystearic acid), dihydroxystearic acid, 2,2-bis(hydroxymethyl)propionic acid, mandelic acid, ricinoleic acid, malic acid, tartaric acid, citric acid, y-butyrolactone, 6-valerolactone, s-caprolactone, menthide, D-lactide, L-lactide, or DL-lactide. Preferably, the hydroxycarboxylic acid is selected from ricinoleic acid or citric acid. The third reactant may be (iv) an epoxide compound. Suitable epoxide compounds may comprise one or more than one epoxide group. For example, suitable epoxide compounds may comprise two epoxide groups. Preferred epoxide compounds comprise a single (i.e. only one) epoxide group. Examples of suitable epoxide compounds include 1,2-epoxydodecane, ethyl glycidyl ether, isopropylglycidyl ether, 2-ethylhexyl glycidyl ether, octyl glycidyl ether, nonyl glycidyl ether, decyl glycidyl ether, butyl glycidyl ether (such as n-butyl glycidyl ether), 1,2-epoxyhexane, epichlorohydrin, glycidyltrimethylammonium chloride, cyclopentene oxide, cyclohexene oxide, poly(ethylene glycol) diglycidyl ether, poly(propylene glycol) diglycidyl ether and poly(butylene glycol) diglycidyl ether. The third reactant may be (v) a polyfunctional reactant having a reactive amino group. By the term “polyfunctional reactant” we mean a reactant with at least two reactive groups. By the term “reactive group” we mean a group that reacts with the first reactant and / or the second reactant. At least one of the reactive groups is a reactive amino group. The other reactive groups in the polyfunctional reactant may be reactive amino groups, or may be reactive groups other than amino groups, such as hydroxy groups or carboxyl groups. Reactive amino groups are suitably primary amino groups or secondary amino groups. The polyfunctional reactant may comprise at least one reactive amino group and at least one hydroxy group. The polyfunctional reactant may be an alkanolamine or an alkoxylated alkanolamine. The polyfunctional reactant may comprise at least two reactive amino groups. The polyfunctional reactant may be an aliphatic diamine, a polyether diamine, ora polyalkylene polyamine. The polyfunctional reactant may comprise at least one reactive amino group and at least one carboxyl group. The polyfunctional reactant may be an amino acid, such as a naturally occurring amino acid. Examples of suitable polyfunctional reactants having at least one reactive amino group include ethanolamine, diethanolamine, ethylene diamine, coco propylene diamine, diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), a polyether diamine (preferably Jeffamine ED-600, Jeffamine ED-900, or Jeffamine ED-2003 which are commercially available), alanine, arginine, asparagine, aspartate, cysteine, glutamine, glutamate, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. The third reactant may be (vi) a monoalcohol. Suitable monoalcohols may be aliphatic or aromatic. The aliphatic monoalcohol may be cycloaliphatic. The aliphatic monoalcohol may be saturated or unsaturated. The monoalcohol may contain from 2 to 40 carbon atoms, suitably from 4 to 30 carbon atoms, preferably from 6 to 20 carbon atoms, for example from 8 to 18 carbon atoms. The monoalcohol may be a fatty alcohol. Examples of suitable monoalcohols include hexanol, octanol, 2-ethylhexanol, decanol, dodecanol alcohol, myristyl alcohol, myristoleyl alcohol, palmityl alcohol, palmitoleyl alcohol, stearyl alcohol, oleyl alcohol, elaidyl alcohol, linoleyl alcohol, linolelaidyl alcohol, arachidyl alcohol, arachidonyl alcohol, behenyl alcohol, erucyl alcohol, benzyl alcohol, and choline chloride. Preferably, the monoalcohol is hexanol or oleyl alcohol. The third reactant may be (vii) a monofunctional reactant having a reactive amino group. Suitable monofunctional reactants having a reactive amino group have one reactive amino group and no other reactive groups. The monofunctional reactant may contain other functional groups that are not reactive groups, such as tertiary amino groups or ether groups. The monofunctional reactant may be an aliphatic monoamine (such as a fatty alkyl amine), an N,N-dialkylaminoalkylamine, ora polyether monoamine. The polyether monoamine suitably comprises a polyether backbone selected from polyethylene glycol (PEG), polypropylene glycol (PPG), or a copolymer of polyethylene glycol (PEG) and polypropylene glycol (PPG). The copolymer of PEG and PPG may be a block copolymer or a random copolymer. The polyether backbone is suitably capped at one end with the reactive amino group, and capped at the other end with an alkyl group (preferably a methyl group). The polyether monoamine suitably has a molecular weight of from 150 to 6000, preferably from 400 to 3000, for example from 600 to 2000. The polyether monoamine may have a weight average molecular weight of from 150 to 6000, preferably from 400 to 3000, for example from 600 to 2000. Examples of suitable monofunctional reactants having a reactive amino group include propylamine, dipropylamine, butylamine, hexylamine, octylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, octadecylamine, tallow alkyl amine, benzylamine, phenethylamine, 3-dimethylaminopropylamine (DMAPA), or Jeffamine M-1000 (which is commercially available). Preferably, the monofunctional reactant having a reactive amino group is selected from dodecylamine, N,N-dimethylaminopropylamine (DMAPA), or Jeffamine M-1000. In some embodiments, the one or more third reactants are selected from one or more of the following: (i) cyclic anhydrides; (ii) monocarboxylic acids or esters thereof; (iii) hydroxycarboxylic acids or cyclic esters thereof; and (iv) monoalcohols. The ester compounds may be prepared from the first, second and optionally third reactants by any suitable method, as would be known to the person skilled in the art. The esterification reaction will typically be conducted in the presence of a suitable esterification catalyst, such as tin(ll) ethylhexanoate, tin(ll) oxalate, p-toluenesulfonic acid, methanesulfonic acid, or sulfuric acid. The reaction may be carried out for any suitable length of time, such as at least 1 hour, preferably at least 3 hours, for example at least 5 hours. The reaction may be carried out at any suitable temperature, such as from 50 to 300°C, preferably from 100 to 200°C. References herein to a reactant are intended to refer to the compounds that react to form the ester compound and are not intended to include a catalyst used in the reaction. Suitable molar ratios of the first, second and optional third reactants may be used to prepare the ester compounds. Suitably, the ester compound is not further reacted after the reaction of the reactants. When the third reactant is used, the molar ratio of the first reactant to the sum of the second and third reactants is suitably from 1:1 to 1:10, such as from 1:1 to 1:5, such as from 1:1 to 1:3. Suitably, the one or more first reactants may comprise at least a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, a polycarboxylic acid comprising a cyclic group, or a dimer acid; wherein n is from 2 to 10; each R is hydrogen or one or two of the R groups is a hydroxyl group, another of the R groups is optionally a carboxyl group or a carboxyl-substituted methyl group, and the remaining R groups are hydrogen; m+m2 is from 2 to 12; XisO, S, or NR1; R1 is hydrogen ora hydrocarbyl group; x is from 1 to 30; and each R2 is independently hydrogen ora methyl group; and at least one of the one or more second reactants may be selected from polyethylene glycol, polypropylene glycol, trimethylol propane, castor oil, ora nitrogen containing polyol. Suitably, the one or more first reactants may comprise at least a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, ora dimer acid; wherein n is from 2 to 10; each R is hydrogen or one or two of the R groups is a hydroxyl group, and the remaining R groups are hydrogen; m+m2 is from 2 to 12; X is O, S, or NR1 (preferably O or S, especially O); R1 is hydrogen ora hydrocarbyl group; x is from 1 to 30; and each R2 is independently hydrogen ora methyl group; and at least one of the one or more second reactants may be selected from polyethylene glycol, polypropylene glycol, propylene glycol, 1,6-hexanediol, trimethylol propane, castor oil, or a nitrogen containing polyol. Suitably, the one or more first reactants may comprise at least a polycarboxylic acid of the formula HOOC(CH2)nCOOH, wherein n is from 5 to 10 or a polycarboxylic acid of the formula HOOCCH2(OCH2CHR2)xOCH2COOH, wherein x is from 1 to 30 and each R2 is independently hydrogen or a hydrocarbyl group, and the one or more second reactants may comprise at least a nitrogen containing polyol and / or a polyol of formula (I): H-(OR3)P-OH (I) wherein each R3 is independently a straight chain or branched alkylene group and p is an integer of at least 2. Suitably, the one or more first reactants may comprise at least a polycarboxylic acid of the formula HOOC(CH2)nCOOH, wherein n is from 5 to 10 or a poly(ethylene glycol)bis(carboxymethyl) ether and the one or more second reactants may comprise at least N-butyl diethanolamine and optionally polyethylene glycol (PEG). Suitably, the first reactant may be a polycarboxylic acid of the formula HOOC(CH2)nCOOH, wherein n is from 5 to 10 or a poly(ethylene glycol)bis(carboxymethyl) ether and the second reactant may be N-butyl diethanolamine and optionally polyethylene glycol (PEG). Suitably, the first reactant may be sebacic acid or a poly(ethylene glycol)bis(carboxymethyl) ether and the second reactant may be N-butyl diethanolamine and optionally PEG 200. The sebacic acid and the N-butyl diethanolamine may be reacted in a 1:1 molar ratio. The sebacic acid and the N-butyl diethanolamine and PEG 200 may be reacted in a 2:1:1 molar ratio. The poly(ethylene glycol)bis(carboxymethyl) ether and the N-butyl diethanolamine may be reacted in a molar ratio of from 2:1 to 1:2, such as 1:1. Suitably, the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; and the or each second reactant is a polyol, wherein at least one of the one or more second reactants is selected from propylene glycol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, glucose, fructose, trehalose, sucrose, lactose, maltose, sorbitol, xylitol, glycerol, neopentyl glycol, diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, castor oil, polyoxyethylene (80) sorbitan monooleate, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, N-butyl diethanolamine, diethanolamine, derivatives (for example quaternary ammonium salts) of N-butyl diethanolamine or diethanolamine, or a polyol formed by reacting a hydroxy substituted cyclic ester or cyclic carbonate with a primary or secondary amine compound. Suitably, the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; and the or each second reactant is a polyol, wherein at least one of the one or more second reactants is selected from propylene glycol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, glucose, fructose, trehalose, sucrose, lactose, maltose, sorbitol, xylitol, glycerol, neopentyl glycol, diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, castor oil, polyoxyethylene (80) sorbitan monooleate, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, N-butyl diethanolamine, N-methyl diethanolamine, diethanolamine, derivatives (for example quaternary ammonium salts) of N-butyl diethanolamine, N-methyl diethanolamine, or diethanolamine, or a polyol formed by reacting a hydroxy substituted cyclic ester or cyclic carbonate with a primary or secondary amine compound. Suitably, the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; and the or each second reactant is a polyol, wherein at least one of the one or more second reactants is selected from propylene glycol, 1,6-hexanediol, trimethylolpropane, polyethylene glycol, polypropylene glycol, castor oil, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, N-methyl diethanolamine, N-butyl diethanolamine, ortris(2-hydroxyethyl) methyl ammonium methylsulfate (especially selected from polyethylene glycol, polypropylene glycol, castor oil, N-methyl diethanolamine, or N-butyl diethanolamine). Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of (preferably consisting of) one or more first reactants, one or more second reactants, and optionally one or more third reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; the or each second reactant is selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, glucose, fructose, trehalose, sucrose, lactose, maltose, sorbitol, xylitol, glycerol, neopentyl glycol, diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, castor oil, polyoxyethylene (80) sorbitan monooleate, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, N-butyl diethanolamine, triethanolamine, diethanolamine, derivatives (for example quaternary ammonium salts) of N-butyl diethanolamine, triethanolamine or diethanolamine, or a polyol formed by reacting a hydroxy substituted cyclic ester or cyclic carbonate with a primary or secondary amine compound; and the or each third reactant (when present) is selected from: (i) cyclic anhydrides; (ii) monocarboxylic acids or esters thereof; (iii) hydroxycarboxylic acids or cyclic esters thereof; (iv) epoxide compounds; (v) polyfunctional reactants having a reactive amino group; (vi) monoalcohols selected from hexanol, octanol, 2-ethylhexanol, decanol, dodecyl alcohol, myristyl alcohol, myristoleyl alcohol, palmityl alcohol, palmitoleyl alcohol, stearyl alcohol, oleyl alcohol, elaidyl alcohol, linoleyl alcohol, linoelaidyl alcohol, arachidyl alcohol, arachidonyl alcohol, behenyl alcohol, erucyl alcohol, benzyl alcohol, and choline chloride; and (vii) monofunctional reactants having a reactive amino group. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of one or more first reactants, one or more second reactants, and optionally one or more third reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; the or each second reactant is selected from propylene glycol, 1,6-hexanediol, trimethylolpropane, polyethylene glycol, polypropylene glycol, castor oil, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, N-methyl diethanolamine, N-butyl diethanolamine, or tris(2-hydroxyethyl) methyl ammonium methylsulfate (especially selected from polyethylene glycol, polypropylene glycol, castor oil, N-methyl diethanolamine, or N-butyl diethanolamine); and the or each third reactant (when present) is selected from: (i) cyclic anhydrides; (ii) monocarboxylic acids or esters thereof; (iii) hydroxycarboxylic acids or cyclic esters thereof; (iv) epoxide compounds; (v) polyfunctional reactants having a reactive amino group; (vi) monoalcohols selected from hexanol, octanol, 2-ethylhexanol, decanol, dodecyl alcohol, myristyl alcohol, myristoleyl alcohol, palmityl alcohol, palmitoleyl alcohol, stearyl alcohol, oleyl alcohol, elaidyl alcohol, linoleyl alcohol, linoelaidyl alcohol, arachidyl alcohol, arachidonyl alcohol, behenyl alcohol, erucyl alcohol, benzyl alcohol, and choline chloride; and (vii) monofunctional reactants having a reactive amino group. The ester compound may be substantially free of nitrogen atoms. By substantially free of nitrogen atoms we mean that the ester compound contains less than 1 wt% of nitrogen in the ester compound, preferably less than 0.5 wt% of nitrogen in the ester compound. More preferably, the ester compound is free of nitrogen atoms, by which we mean that it is not possible to detect nitrogen in the ester compound. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of (preferably consisting of) one or more first reactants, one or more second reactants, and optionally one or more third reactants, wherein the or each first reactant is selected from a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, a polycarboxylic acid comprising a cyclic group, a dimer acid, or a reactive equivalent thereof; wherein n is from 0 to 30; and each R is independently hydrogen or a substituent selected from hydroxy groups, carboxyl groups, alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups, wherein the alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups are optionally substituted with one or more of a hydroxy group and / or a carboxyl group; and / or two R groups on the same carbon atom may be taken together to form a methylene (=CH2) group; and / or when n is two or more, two R groups on adjacent carbon atoms may be taken together to form a double bond; m+m2 is from 0 to 30; X is O or S; x is from 1 to 30; and each R2 is independently hydrogen or a hydrocarbyl group; the or each second reactant is selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, glucose, fructose, trehalose, sucrose, lactose, maltose, sorbitol, xylitol, glycerol, neopentyl glycol, diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, castor oil, polyoxyethylene (80) sorbitan monooleate, or 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate; and the or each third reactant is selected from: (i) cyclic anhydrides; (ii) monocarboxylic acids or esters thereof; (iii) hydroxycarboxylic acids or cyclic esters thereof; and (iv) epoxide compounds; and monoalcohols selected from hexanol, octanol, 2-ethylhexanol, decanol, dodecanol alcohol, myristyl alcohol, myristoleyl alcohol, palmityl alcohol, palmitoleyl alcohol, stearyl alcohol, oleyl alcohol, elaidyl alcohol, linoleyl alcohol, linoelaidyl alcohol, arachidyl alcohol, arachidonyl alcohol, behenyl alcohol, erucyl alcohol, and benzyl alcohol. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of (preferably consisting of) one or more first reactants and one or more second reactants, wherein the or each first reactant is selected from a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, a polycarboxylic acid comprising a cyclic group, a dimer acid, ora reactive equivalent thereof; wherein n is from 0 to 30; and each R is independently hydrogen or a substituent selected from hydroxy groups, carboxyl groups, alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups, wherein the alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups are optionally substituted with one or more of a hydroxy group and / or a carboxyl group; and / or two R groups on the same carbon atom may be taken together to form a methylene (=CH2) group; and / or when n is two or more, two R groups on adjacent carbon atoms may be taken together to form a double bond; m+m2 is from 0 to 30; X is O or S; x is from 1 to 30; and each R2 is independently hydrogen ora hydrocarbyl group; and the or each second reactant is selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, glucose, fructose, trehalose, sucrose, lactose, maltose, sorbitol, xylitol, glycerol, neopentyl glycol, diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, castor oil, polyoxyethylene (80) sorbitan monooleate, or 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate. Suitably, the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; and the or each second reactant is a polyol, wherein at least one of the one or more second reactants is a nitrogen containing polyol and / or a polyol of formula (IA): H-(OR4)q-OH (IA) wherein each R4 is independently an optionally substituted alkylene group wherein the optional substituent is not hydroxy and q is an integer of at least 1. Suitably, the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof; and the or each second reactant is a polyol, wherein at least one of the one or more second reactants is selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, neopentyl glycol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, N-methyl diethanolamine, N-butyl diethanolamine, diethanolamine, or derivatives (for example quaternary ammonium salts) of N-methyl diethanolamine, N-butyl diethanolamine. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of a first reactant, one or more second reactants, and optionally a third reactant, wherein the first reactant is a polycarboxylic acid or a reactive equivalent thereof, wherein the polycarboxylic acid is selected from pimelic acid, suberic acid, azelaic acid, sebacic acid, tartaric acid, citric acid, diglycolic acid, poly(ethylene glycol) bis(carboxymethyl) ether or a hydrogenated dimer acid (especially selected from sebacic acid, diglycolic acid, poly(ethylene glycol) bis(carboxymethyl) ether or a hydrogenated dimer acid); the one or more second reactants are selected from PEG 200, PEG 400, PEG 600, PEG 1500, PPG 1000, PPG 2000, trimethylol propane, castor oil, N-methyl diethanolamine, N-butyl diethanolamine, tris(2-hydroxyethyl) methyl ammonium methylsulfate, or a polyol formed by reacting glycerol carbonate and ethanolamine (especially selected from PEG 1500, PPG 1000, castor oil, N-methyl diethanolamine, or N-butyl diethanolamine); and the third reactant is propionic acid. The molar ratio of the first reactant and the one or more second reactants is preferably 1:1. When the third reactant is used, the molar ratio of the first, second and third reactants is suitably 1:1.1:1.1. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of (preferably consisting of) a first reactant, one or more second reactants, and optionally a third reactant, wherein the first reactant is a polycarboxylic acid or a reactive equivalent thereof, wherein the polycarboxylic acid is selected from pimelic acid, suberic acid, azelaic acid, sebacic acid, tartaric acid, citric acid, diglycolic acid, poly(ethylene glycol) bis(carboxymethyl) ether or a hydrogenated dimer acid; the one or more second reactants are selected from PEG 200, PEG 400, PEG 600, PPG 1000, PPG 2000, trimethylol propane, castor oil, N-butyl diethanolamine, tris(2-hydroxyethyl) methyl ammonium methylsulfate, or a polyol formed by reacting glycerol carbonate and ethanolamine; and the third reactant is propionic acid. The molar ratio of the first reactant and the one or more second reactants is preferably 1:1. When the third reactant is used, the molar ratio of the first, second and third reactants is suitably 1:1.1:1.1. Preferably, the ester compound is the reaction product of reactants comprising: (i) tartaric acid or a reactive equivalent thereof and PPG 2000, preferably in a molar ratio of 1:1; (ii) thioglycolic acid or a reactive equivalent thereof and 1,6-hexanediol, preferably in a molar ratio of 1:1; (iii) diglycolic acid or a reactive equivalent thereof and PEG 1500, preferably in a molar ratio of 1:1; (iv) dodecanedioic acid ora reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (v) pimelic acid or a reactive equivalent thereof and PPG 2000, preferably in a molar ratio of 1:1; (vi) pimelic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (vii) dodecanedioic acid or a reactive equivalent thereof and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, preferably in a molar ratio of 1:1; (viii) succinic acid or a reactive equivalent thereof and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate., preferably in a molar ratio of 1:1; (ix) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PEG 200, preferably in a molar ratio of 1:1; (x) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and 1,2-propanediol, preferably in a molar ratio of 1:1; (xi) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PPG 200, preferably in a molar ratio of 1:1; (xii) sebacic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (xiii) pimelic acid or a reactive equivalent thereof, N-butyl diethanolamine and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (xiv) pimelic acid or a reactive equivalent thereof, N-butyl diethanolamine and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xv) hydrogenated dimer acid or a reactive equivalent thereof and tris(2-hydroxyethyl)methylammonium methylsulfate, preferably in a molar ratio of 1:1; (xvi) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PPG 1000, preferably in a molar ratio of 1:1; (xvii) diglycolic acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1; or (xviii) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants comprising: (i) citric acid or a reactive equivalent thereof and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (vi) suberic acid or a reactive equivalent thereof and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the suberic acid or the reactive equivalent thereof and the polyol are preferably reacted in a molar ratio of 1:1; (vii) citric acid or a reactive equivalent thereof and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the citric acid or the reactive equivalent thereof and the polyol are preferably reacted in a molar ratio of 1:1; (viii) a hydrogenated dimer acid or a reactive equivalent thereof and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (ix) sebacic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (x) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (xi) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5; (xii) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xiii) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2; (xiv) a hydrogenated dimer acid or a reactive equivalent thereof, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xv) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PPG 1000, preferably in a molar ratio of 1:1; (xvi) azelaic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xvii) diglycolic acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants comprising: (i) citric acid or a reactive equivalent thereof and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (vi) a hydrogenated dimer acid or a reactive equivalent thereof and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (vii) sebacic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (viii) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (ix) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5; (x) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xi) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2; (xii) a hydrogenated dimer acid or a reactive equivalent thereof, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xiii) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PPG 1000, preferably in a molar ratio of 1:1; (xiv) azelaic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xv)diglycolic acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1. More preferably, the ester compound is the reaction product of reactants consisting essentially of or consisting of: (i) citric acid or a reactive equivalent thereof and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (vi) suberic acid or a reactive equivalent thereof and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the suberic acid or the reactive equivalent thereof and the polyol are preferably reacted in a molar ratio of 1:1; (vii) citric acid or a reactive equivalent thereof and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the citric acid or the reactive equivalent thereof and the polyol are preferably reacted in a molar ratio of 1:1; (viii) a hydrogenated dimer acid or a reactive equivalent thereof and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (ix) sebacic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (x) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (xi) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5; (xii) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xiii) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2; (xiv) a hydrogenated dimer acid or a reactive equivalent thereof, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xv) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PPG 1000, preferably in a molar ratio of 1:1; (xvi) azelaic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xvii) diglycolic acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants consisting essentially of or consisting of: (i) citric acid or a reactive equivalent thereof and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid or a reactive equivalent thereof and trimethylol propane, preferably in a molar ratio of 1:1; (vi) a hydrogenated dimer acid or a reactive equivalent thereof and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (vii) sebacic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (viii) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (ix) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5; (x) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xi) sebacic acid or a reactive equivalent thereof, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2; (xii) a hydrogenated dimer acid or a reactive equivalent thereof, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xiii) poly(ethylene glycol) bis(carboxymethyl) ether or a reactive equivalent thereof and PPG 1000, preferably in a molar ratio of 1:1; (xiv) azelaic acid or a reactive equivalent thereof and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xv)diglycolic acid or a reactive equivalent thereof and castor oil, preferably in a molar ratio of 1:1. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of (preferably consisting of) a first reactant, one or more second reactants, and optionally a third reactant, wherein the first reactant is selected from pimelic acid, suberic acid, azelaic acid, sebacic acid, tartaric acid, citric acid, diglycolic acid, poly(ethylene glycol) bis(carboxymethyl) ether or a hydrogenated dimer acid; the one or more second reactants are selected from PEG 200, PEG 400, PEG 600, PPG 1000, PPG 2000, trimethylol propane, castor oil, N-butyl diethanolamine, tris(2-hydroxyethyl) methyl ammonium methylsulfate, or a polyol formed by reacting glycerol carbonate and ethanolamine; and the third reactant is propionic acid. The molar ratio of the first reactant and the one or more second reactants is preferably 1:1. When the third reactant is used, the molar ratio of the first, second and third reactants is suitably 1:1.1:1.1. Suitably, the ester compound may be the reaction product of reactants comprising, consisting essentially of or consisting of a first reactant, one or more second reactants, and optionally a third reactant, wherein the first reactant is selected from sebacic acid, poly(ethylene glycol) bis(carboxymethyl) ether, hydrogenated dimer acid or diglycolic acid; the one or more second reactants are selected from polyethylene glycol (such as PEG 1500), polypropylene glycol (such as PPG 1000), castor oil, N-butyl diethanolamine, or N-methyl diethanolamine; and the third reactant (when present) is selected from one or more of the following: (i) cyclic anhydrides; (ii) monocarboxylic acids or esters thereof; (iii) hydroxycarboxylic acids or cyclic esters thereof; and (iv) monoalcohols. The molar ratio of the first reactant and the one or more second reactants is suitably from 1.2:1 to 1:1.2. Preferably the molar ratio is 1:1. When the third reactant is used, the molar ratio of the first, second and third reactants is suitably 1:1.1:1.1. Preferably, the ester compound is the reaction product of reactants comprising: (i) tartaric acid and PPG 2000, preferably in a molar ratio of 1:1; (ii) thioglycolic acid and 1,6-hexanediol, preferably in a molar ratio of 1:1; (iii) diglycolic acid and PEG 1500, preferably in a molar ratio of 1:1; (iv) dodecanedioic acid and trimethylol propane, preferably in a molar ratio of 1:1; (v) pimelic acid and PPG 2000, preferably in a molar ratio of 1:1; (vi) pimelic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (vii) dodecanedioic acid and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, preferably in a molar ratio of 1:1; (viii) succinic acid and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate., preferably in a molar ratio of 1:1; (ix) poly(ethylene glycol) bis(carboxymethyl) ether and PEG 200, preferably in a molar ratio of 1:1; (x) poly(ethylene glycol) bis(carboxymethyl) ether and 1,2-propanediol, preferably in a molar ratio of 1:1; (xi) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 200, preferably in a molar ratio of 1:1; (xii) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (xiii) pimelic acid, N-butyl diethanolamine and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (xiv) pimelic acid, N-butyl diethanolamine and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xv) hydrogenated dimer acid and tris(2-hydroxyethyl)methylammonium methylsulfate, preferably in a molar ratio of 1:1; (xvi) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 1000, preferably in a molar ratio of 1:1; (xvii) diglycolic acid and castor oil, preferably in a molar ratio of 1:1; or (xviii) poly(ethylene glycol) bis(carboxymethyl) ether and N-butyl diethanolamine, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants comprising: (i) citric acid and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid and trimethylol propane, preferably in a molar ratio of 1:1; (vi) suberic acid and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the suberic acid and the polyol are preferably reacted in a molar ratio of 1:1; (vii) citric acid and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the citric acid and the polyol are preferably reacted in a molar ratio of 1:1; (viii) a hydrogenated dimer acid and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (ix) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (x) sebacic acid, 1:0.5:0.5: N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of (xi) sebacic acid, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5: (xii) sebacic acid, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xiii) sebacic acid, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2: (xiv) a hydrogenated dimer acid, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xv) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 1000, preferably in a molar ratio of 1:1; (xvi) azelaic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xvii) diglycolic acid and castor oil, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants comprising: (i) citric acid and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid and trimethylol propane, preferably in a molar ratio of 1:1; (vi) a hydrogenated dimer acid and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (vii) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (viii) sebacic acid, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (ix) sebacic acid, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5: (x) sebacic acid, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5: (xi) sebacic acid, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2: (xii)a hydrogenated dimer acid, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xiii) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 1000, preferably in a molar ratio of 1:1; (xiv) azelaic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xv)diglycolic acid and castor oil, preferably in a molar ratio of 1:1. More preferably, the ester compound is the reaction product of reactants consisting essentially or consisting of: (i) tartaric acid and PPG 2000, preferably in a molar ratio of 1:1; (ii) thioglycolic acid and 1,6-hexanediol, preferably in a molar ratio of 1:1; (iii) diglycolic acid and PEG 1500, preferably in a molar ratio of 1:1; (iv) dodecanedioic acid and trimethylol propane, preferably in a molar ratio of 1:1; (v) pimelic acid and PPG 2000, preferably in a molar ratio of 1:1; (vi) pimelic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (vii) dodecanedioic acid and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate, preferably in a molar ratio of 1:1; (viii) succinic acid and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate., preferably in a molar ratio of 1:1; (ix) poly(ethylene glycol) bis(carboxymethyl) ether and PEG 200, preferably in a molar ratio of 1:1; (x) poly(ethylene glycol) bis(carboxymethyl) ether and 1,2-propanediol, preferably in a molar ratio of 1:1; (xi) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 200, preferably in a molar ratio of 1:1; (xii) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (xiii) pimelic acid, N-butyl diethanolamine and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (xiv) pimelic acid, N-butyl diethanolamine and PEG 600, preferably in a molar ratio of 1:0.5:0.5: (xv) hydrogenated dimer acid and tris(2-hydroxyethyl)methylammonium methylsulfate, preferably in a molar ratio of 1:1; (xvi) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 1000, preferably in a molar ratio of 1:1; (xvii) diglycolic acid and castor oil, preferably in a molar ratio of 1:1; or (xviii) poly(ethylene glycol) bis(carboxymethyl) ether and N-butyl diethanolamine, preferably in a molar ratio of 1:1. More preferably, the ester compound is the reaction product of reactants consisting essentially of or consisting of: (i) citric acid and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid and trimethylol propane, preferably in a molar ratio of 1:1; (vi) suberic acid and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the suberic acid and the polyol are preferably reacted in a molar ratio of 1:1; (vii) citric acid and a polyol formed by reacting glycerol carbonate and ethanolamine, wherein the citric acid and the polyol are preferably reacted in a molar ratio of 1:1; (viii) a hydrogenated dimer acid and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (ix) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (x) sebacic acid, 1:0.5:0.5: N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of (xi) sebacic acid, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5: (xii) sebacic acid, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5; (xiii) sebacic acid, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2: (xiv) a hydrogenated dimer acid, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xv) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 1000, preferably in a molar ratio of 1:1; (xvi) azelaic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xvii) diglycolic acid and castor oil, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants consisting essentially of or consisting of: (i) citric acid and PPG 2000, preferably in a molar ratio of 1:1; (ii) tartaric acid and castor oil, preferably in a molar ratio of 1:1; (iii) suberic acid and trimethylol propane, preferably in a molar ratio of 1:1; (iv) pimelic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (v) pimelic acid and trimethylol propane, preferably in a molar ratio of 1:1; (vi) a hydrogenated dimer acid and tris(2-hydroxyethyl) methyl ammonium methylsulfate, preferably in a molar ratio of 1:1; (vii) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; (viii) sebacic acid, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5; (ix) sebacic acid, N-butyl diethanolamine, and PEG 400, preferably in a molar ratio of 1:0.5:0.5: (x) sebacic acid, N-butyl diethanolamine, and PEG 600, preferably in a molar ratio of 1:0.5:0.5: (xi) sebacic acid, N-butyl diethanolamine, and PPG 1000, preferably in a molar ratio of 1:0.8:0.2: (xii)a hydrogenated dimer acid, tris(2-hydroxyethyl) methyl ammonium methylsulfate, and propionic acid, preferably in a molar ratio of 1:1.1:1.1; (xiii) poly(ethylene glycol) bis(carboxymethyl) ether and PPG 1000, preferably in a molar ratio of 1:1; (xiv) azelaic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1; or (xv)diglycolic acid and castor oil, preferably in a molar ratio of 1:1. Preferably, the ester compound is the reaction product of reactants comprising, or of reactants consisting essentially of or consisting of: (i) sebacic acid and N-butyl diethanolamine, preferably in a molar ratio of 1:1, and wherein the ester compound has a number average molecular weight of from 9,000 to 11,000 Daltons; or (ii) sebacic acid, N-butyl diethanolamine, and PEG 200, preferably in a molar ratio of 1:0.5:0.5, and wherein the ester compound has a number average molecular weight of from 3,800 to 5,800 Daltons. The ester compound may be a polymer. The reactants used to prepare the ester compound may be monomers. The first reactant may be a first monomer, the second reactant may be a second monomer, and the third reactant (when present) may be a third monomer. Thus, the ester compound may be a polymer comprising, consisting essentially of, or consisting of repeat units derived from the one or more first reactants, repeat units derived from the one or more second reactants, and optionally repeat units derived from one or more third reactants as defined herein. Typically, the polymer is a polyester. The polymer may be a random copolymer or a block copolymer, in particular when there is more than one first reactant or more than one second reactant. Preferably, the polymer is a random copolymer. Suitably, the polymer is not in a solid form. For example, the polymer is preferably in the form of a liquid or gel, preferably a liquid. Suitably, the polymer is substantially free or free of cross-linking. The polymer formed from the first, second and optional third reactants may comprise a suitable number of total monomer units (i.e. repeat units). For example, the polymer may comprise at least 4 monomer units. In other words, the polymer may comprise a total of at least 4 monomer units including the first, second and optional third monomers. Suitably, the polymer may comprise from 4 to 50 monomer units, preferably from 4 to 30 monomer units. Suitably, the ester compound has a number average molecular weight of from 1,000 to 150,000 Daltons, preferably from 1,000 to 15,000 Daltons (for example from 2,000 to 7,000 Daltons). Preferably, the ester compound is substantially free of silicon atoms. By substantially free of silicon atoms we mean that the ester compound contains less than 1 wt% of silicon in the ester compound, preferably less than 0.5 wt% of silicon in the ester compound. More preferably, the ester compound is free of silicon atoms, by which we mean that it is not possible to detect silicon in the ester compound. Suitable methods of measuring the amount of silicon in an ester compound are well known to those skilled in art and include elemental analysis and inductively coupled plasma (ICP) spectroscopy. Preferably, the ester compound is substantially free of fluorine atoms. The ester compound may be substantially free of halogen atoms. By substantially free of fluorine or halogen atoms we mean that the ester compound contains less than 1 wt% of fluorine or halogen in the ester compound, preferably less than 0.5 wt% of fluorine or halogen in the ester compound. More preferably, the ester compound is free of fluorine atoms. The ester compound may be free of halogen atoms. By free of fluorine or halogen atoms we mean that it is not possible to detect fluorine or halogen in the ester compound. Suitable methods of measuring the amount of fluorine or halogen in a ester compound are well known to those skilled in art and include elemental analysis and inductively coupled plasma (ICP) spectroscopy. Preferably, the ester compound is substantially free of quaternary ammonium moieties. By substantially free of quaternary ammonium moieties we mean that the ester compound contains less than 1 wt% of quaternary nitrogen atoms in the polymer, preferably less than 0.5 wt% of quaternary nitrogen atoms in the ester compound. More preferably, the ester compound is free of quaternary ammonium moieties, by which we mean that it is not possible to detect quaternary ammonium moieties in the ester compound. According to a second aspect of the invention, there is provided a concentrate composition comprising one or more ester compounds and optionally at least one solvent, wherein the concentrate composition comprises at least 20 wt% of the one or more ester compounds, and wherein the or each ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. The concentrate composition may comprise at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, or at least 80 wt% of the one or more ester compounds based on the total weight of the concentrate composition. The concentrate composition additionally may comprise 99 wt% or less, such as 95 wt% or less, for example 90 wt% or less of the one or more ester compounds based on the total weight of the concentrate composition. The concentrate composition may comprise from 20 to 99 wt%, preferably from 20 to 95 wt%, for example from 50 to 90 wt%, of the one or more ester compounds based on the total weight of the concentrate composition. References herein to the amount of ester compound in the concentrate composition are intended to refer to the total of the or each ester compound as defined herein that is included in the composition, i.e. based on the total weight of the concentrate composition. Suitably, the concentrate composition may be stable for at least 1 week, preferably at least 4 weeks, most preferably at least 8 weeks under ambient conditions (for example, at atmospheric pressure and at a temperature of 20°C). The stability of the concentrate composition may be determined visually. Suitably, the concentrate composition is unstable when it shows creaming, sedimentation, separation, ora combination thereof. Suitably, the concentrate composition may comprise at least 20 wt%, such as at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, or at least 80 wt% of the one or more ester compounds and the concentrate composition may be stable for at least 1 week, preferably at least 4 weeks or at least 8 weeks, most preferably at least 3, 6, 12 or 24 months, under ambient conditions. Suitably, the concentrate composition may be flowable. The meaning of a “flowable” composition is well known to those skilled in the art. Typically, flowable compositions are pourable from a container at 20°C. Flowable compositions may be poured without needing to heat the composition above 20°C or manually scraping the composition. Suitably, the concentrate composition may have a viscosity of 7,000 cP or less, preferably 3,500 cP or less at 20°C. Suitably, the concentrate composition may comprise at least 20 wt%, such as at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, or at least 80 wt% of the ester compound and the concentrate composition may be flowable. Suitably the concentrate composition of the second aspect is substantially free of silicon atoms. By substantially free of silicon atoms we mean that the composition contains less than 1 wt%, preferably less than 0.5 wt%, of silicon in the composition. More preferably, the composition is free of silicon atoms, by which we mean that it is not possible to detect silicon in the composition. Suitable methods of measuring the amount of silicon in a composition are well known to those skilled in art and include elemental analysis and inductively coupled plasma (ICP) spectroscopy. The ester compound referred to in relation to the second aspect may comprise the reaction product of reactants comprising one or more first reactants, one or more second reactants and optionally one or more third reactants as defined herein. The ester compound referred to in relation to the second aspect may be the reaction product of reactants consisting essentially of or consisting of one (preferably consisting of) or more first reactants, one or more second reactants and optionally one or more third reactants as defined herein. Features of the ester compound, and of the first and second reactants (and the third reactants when present), in relation to the second aspect of the invention are as set out herein in relation to the first aspect of the invention. The concentrate composition of the second aspect may optionally comprise any suitable solvent, such as for example a polar solvent, such as a polar protic solvent. Preferably, the concentrate composition comprises at least one solvent. The solvent may be an aqueous solvent. Thus, the concentrate composition may be an aqueous composition. The term “aqueous solvent” herein is considered to mean water or mixtures of water and at least one water miscible solvent. Water miscible solvents may include alcohols and substantially water-miscible organic solvents. Preferably the aqueous solvent is water. When the solvent is an aqueous solvent, the concentrate composition may be in the form of an emulsion. The solvent may be a non-aqueous solvent. The non-aqueous solvent suitably comprises an organic solvent, such as an alcohol. The non-aqueous solvent preferably does not comprise any surfactant. When the solvent is a non-aqueous solvent, it is preferably miscible with water. Preferably, the non-aqueous solvent is not an oil. Suitable such alcohols for use in the solvent include monohydric alcohols, polyhydric alcohols, alkoxy alcohols and aryloxy alcohol. Suitable alcohols are monohydric alcohols, polyhydric alcohols and alkoxy alcohols. Preferred alcohols are miscible with water. Suitable simple monohydric alcohols include methanol, ethanol, isopropanol and butanol. The solvent may comprise a polyhydric alcohol or an alkoxyalcohol. Suitable alkoxy alcohols include diethylene glycol monobutyl ether, 3-methoxy-3-methyl-1-butanol and 2-butoxyethanol. Suitable aryloxy alcohols include 2-phenoxyethanol. Suitable polyhydric alcohols include glycerol, ethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol and 2-methylpentanediol. Suitable polyhydric alcohols include glycerol, ethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol and polypropylene glycol. Preferably, the non-aqueous solvent comprises a monohydric alcohol such as isopropanol. The concentrate composition may comprise at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt% or at least 70 wt% of the at least one solvent (when present) based on the total weight of the concentrate composition. The concentrate composition additionally may comprise 80 wt% or less, such as 70 wt% or less, of the at least one solvent (when present) based on the total weight of the concentrate composition. The concentrate composition may comprise from 20 to 80 wt%, preferably from 20 to 75 wt%, such as from 50 to 80 wt% or from 50 to 75 wt%, of the at least one solvent (when present) based on the total weight of the concentrate composition. The concentrate composition of the second aspect may additionally comprise one or more surfactants (in particular when the solvent is an aqueous solvent). Preferably, the concentrate composition of the second aspect comprises one or more surfactants (in particular when the solvent is an aqueous solvent). Any suitable surfactant(s) may be included, such as for example one or more surfactants selected from anionic surfactants, cationic surfactants, non-ionic surfactants, and amphoteric or zwitterionic surfactants. One or more non-ionic and / or anionic surfactant(s) are preferred. Non-ionic surfactants are most preferred. The concentrate composition may comprise an anionic surfactant. Suitable anionic surfactants may include sulfate surfactants (such as mono- or di-alkyl sulfates or alkyl ether sulfates), sulfonate surfactants, alkyl ether carboxylate surfactants, sarcosinate surfactants, phosphate surfactants, succinate surfactants, sulfosuccinate surfactants, sulfoacetate surfactants, isethionate surfactants, taurate surfactants, amino acid surfactants (such as glutamates and glycinates), lactylate surfactants, and fatty acid salts. Particularly exemplary salts of the above, where applicable, are the sodium, potassium, ammonium, magnesium and triethanolamine salts. An alaninate is another example of a suitable amino acid surfactant. Suitable alkyl ether carboxylate surfactants may include alkoxylated (such as ethoxylated or propoxylated, especially ethoxylated) carboxylic acids. Preferred alkyl ether carboxylate surfactants may be fatty alcohol ether carboxylates. An example of a preferred alkyl ether carboxylate surfactant is laureth-11 carboxylic acid. Alkyl ether carboxylate surfactants are preferred anionic surfactants for including in the concentrate composition. The concentrate composition may comprise a cationic surfactant. The cationic surfactant may be an ammonium salt such as an alkyl trimethyl ammonium salt, a dialkyl dimethyl ammonium salt, an alkyl-(N-hydroxyethyl)-dimethyl ammonium salt, or an alkyldimethylbenzylammonium salt (i.e. a benzalkonium salt). The salt may be a halide, hydroxide, sulfate, hydrogensulfate, phosphate, or carboxylate salt. The salt is suitably a halide salt, such as a fluoride, chloride, or bromide salt. The salt may be a carboxylate salt, such as formate or acetate salt. Preferably, the salt is chloride. The concentrate composition may comprise a non-ionic surfactant. Suitable non-ionic surfactants for use herein include alcohol alkoxylates (such as alcohol ethoxylates, alcohol propoxylates, and ethylene oxide / propylene oxide copolymer derived surfactants), aliphatic esters, aromatic esters, sugar esters (such as sorbitan esters), glycolipids (such as sophorolipids, rhamnolipids and alkyl (poly)glycosides), fatty acid alkoxylates (such as fatty acid ethoxylates and fatty acid propoxylates), or polyethylene glycol esters (including partial esters), glycerol esters (including glycerol partial esters and glycerol triesters), fatty alcohols (such as cetearyl alcohol, lauryl alcohol, stearyl alcohol, behenyl alcohol), castor oil, alkanolamides, fatty amine alkoxylates (such as fatty amine ethoxylates and fatty amine propoxylates), and polyglyceryl fatty acid esters. Suitable non-ionic surfactants for use herein include alcohol alkoxylates (such as alcohol ethoxylates, alcohol propoxylates, and ethylene oxide / propylene oxide copolymer derived surfactants), aliphatic esters, aromatic esters, sugar esters (such as sorbitan esters), alkyl (poly)glycosides, fatty acid alkoxylates (such as fatty acid ethoxylates and fatty acid propoxylates), or polyethylene glycol esters (including partial esters), glycerol esters (including glycerol partial esters and glycerol triesters), fatty alcohols (such as cetearyl alcohol, lauryl alcohol, stearyl alcohol, behenyl alcohol), castor oil and alkanolamides. Suitable sugar esters may include alkoxylated (such as ethoxylated) sugar esters. For example, the sugar ester may comprise an alkoxylated (such as ethoxylated) sugar ester of a fatty acid, such as oleic acid. An example of a preferred sugar ester surfactant is Tween® 80. Suitable fatty amine alkoxylates may include fatty amine ethoxylates and fatty amine propoxylates. Fatty amine ethoxylates are preferred. The fatty amine alkoxylates may be based on any suitable fatty alcohol. An example of a preferred fatty amine alkoxylate is a fatty amine ethoxylate such as Empilan® AMT 11, which is a fatty amine ethoxylate containing 11 moles of ethylene oxide. Fatty amine alkoxylates (especially fatty amine ethoxylates) are preferred non-ionic surfactants for including in the concentrate composition. The concentrate composition may comprise an amphoteric or zwitterionic surfactant. The amphoteric or zwitterionic surfactant may be selected from betaines (such as alkyl betaines and alkylamidopropyl betaines), amphoacetates, diamphoacetates, and amine oxides (such as alkylamine oxides and alkylamidopropyl amine oxides). Suitable amphoteric or zwitterionic surfactants include lauryl betaine, cocamidopropyl betaine, sodium lauroamphoacetate, sodium cocoamphoacetate, disodium cocoamphodiacetate, lauramine oxide, C12-18 alkyldimethylamine oxide, myristamine oxide, and cocamidopropyl amine oxide. The one or more surfactants may be selected from a fatty alkyl amphoacetate (for example cocoyl amphoacetate, lauryl amphoacetate), an alkyl (poly)glycoside (for example lauryl glycoside), an acyl glycinate (for example cocoyl glycinate), a sulfosuccinate, an amphodiacetate and a fatty alcohol ether carboxylate (for example C8-12 alkyl ether (6-11 moles EO) carboxylate). Preferably, the one or more surfactants may be selected from one or more of a fatty amine alkoxylate (for example fatty amine ethoxylate containing 11 moles of ethylene oxide) and a fatty alcohol ether carboxylate (for example Cs-12 alkyl ether (6-11 moles EO) carboxylate). The concentrate composition may comprise at least 1 wt%, at least 3 wt%, at least 5 wt%, of the one or more surfactants (when present) based on the total weight of the concentrate composition. The concentrate composition additionally may comprise 15 wt% or less, such as 10 wt% or less, of the one or more surfactants (when present) based on the total weight of the concentrate composition. The concentrate composition may comprise from 1 to 15 wt%, preferably from 3 to 10 wt% of the one or more surfactants (when present) based on the total weight of the concentrate composition. The concentrate composition of the second aspect may comprise the one or more surfactants in an amount of from 1 to 100 wt%, suitably from 5 to 50 wt%, preferably from 5 to 25 wt% or from 10 to 25 wt% based on the total weight of the one or more ester compounds. When the concentrate composition comprises the one or more ester compounds, the at least one solvent and the one or more surfactants, the composition may comprise: (i) from 20 to 60 wt% of the one or more ester compounds; (ii) from 35 to 60 wt% of the at least one solvent; and (iii) from 1 to 15 wt% of the one or more surfactants; based on the total weight of the concentrate composition. When the concentrate composition comprises the one or more ester compounds, the at least one solvent and the one or more surfactants, the composition may comprise: (i) from 20 to 45 wt% of the one or more ester compounds; (ii) from 50 to 75 wt% of the at least one solvent; and (iii) from 1 to 15 wt% of the one or more surfactants; based on the total weight of the concentrate composition. When the concentrate composition comprises the one or more ester compounds, the at least one solvent and the one or more surfactants, the composition may comprise: (i) from 20 to 40 wt% of the one or more ester compounds; (ii) from 50 to 75 wt% of the at least one solvent; and (iii) from 3 to 10 wt% of the one or more surfactants; based on the total weight of the concentrate composition. According to a third aspect of the invention, there is provided a use of a surfactant to emulsify at least one ester compound in an aqueous concentrate composition, wherein the aqueous concentrate composition comprises at least 20 wt% of the one or more ester compounds, wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. The ester compound referred to in relation to the third aspect may comprise the reaction product of reactants comprising one or more first reactants, one or more second reactants and optionally one or more third reactants as defined herein. The ester compound referred to in relation to the third aspect may be the reaction product of reactants consisting essentially of or consisting of (preferably consisting of) one or more first reactants, one or more second reactants and optionally one or more third reactants as defined herein. Features of the ester compound, and of the first and second reactants (and the third reactants when present), in relation to the third aspect of the invention are as set out herein in relation to the first aspect of the invention. Features of the surfactant in relation to the third aspect of the invention are as set out herein in relation to the second aspect of the invention. According to a fourth aspect of the invention, there is provided a method of emulsifying at least one ester compound in an aqueous composition to make an aqueous ester compound concentrate composition, wherein the aqueous ester compound concentrate composition comprises at least 20 wt% of the one or more ester compounds, the method comprising admixing a surfactant with the ester compound in an aqueous composition, wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol. The ester compound referred to in relation to the fourth aspect may comprise the reaction product of reactants comprising one or more first reactants, one or more second reactants and optionally one or more third reactants as defined herein. The ester compound referred to in relation to the fourth aspect may be the reaction product of reactants consisting essentially of or consisting of (preferably consisting of) one or more first reactants, one or more second reactants and optionally one or more third reactants as defined herein. Features of the ester compound, and of the first and second reactants (and the third reactants when present), in relation to the fourth aspect of the invention are as set out herein in relation to the first aspect of the invention. Features of the surfactant in relation to the fourth aspect of the invention are as set out herein in relation to the second aspect of the invention. Examples The invention will now be further described with reference to the following non-limiting examples. Example 1 - synthesis of polyol reactant from gluconolactone and amine Gluconolactone and an amine (1 eq) were combined in methanol (100 mL, for 70 mmol of gluconolactone) and heated at 65°C for 2 hours. The reaction mass was concentrated in vacuo to provide a polyol, which was used in subsequent reactions without further purification. In cases where a diamine was used (for example ethylenediamine or Jeffamine ED-600) the mole equivalents of diamine were reduced (0.5 eq). Example 2 - synthesis of polyol reactant from glycerol carbonate and amine Glycerol carbonate and an amine (1 eq) were combined and heated at 70°C for two hours. The reaction mass was cooled to provide a polyol, which was used in subsequent reactions without further purification. In cases where a diamine was used (for example ethylenediamine or Jeffamine ED-600) the mole equivalents of diamine were reduced (0.5 eq). Example 3 - general method for synthesis of ester compounds The non cyclic polycarboxylic acid reactant(s) were combined with the polyol(s) and optional third reactants. Tin(ll) ethylhexanoate (0.5 wt% relative to the total weight of reactants) was added. The reaction mass was heated at 160°C for 6 hours. The resulting ester compound was decanted from the reaction flask, and no further purification was carried out. Ester compounds 1 to 157 were prepared according to Example 3, using the reactants and reaction stoichiometries as set out in Table 1. Table 1 Ester compound Polycarboxylic acid 1 Polycarboxylic acid 2 (when present) Polyol 1 Polyol 2 (when present) Further (third) reactant (when present) Molar ratio of reactants 1 Succinic acid PEG 1000 1:1 2 Maleic acid PEG 1000 1:1 3 Tartaric acid Neopentyl glycol 1:1 4 Terephthalic acid PEG 1000 1:1 5 Succinic acid PEG 1500 1:1 6 Dodecanedioic acid Tartaric acid Neopentyl glycol 0.9:0.1:1 7 1,4-cyclohexane dicarboxylic acid Ethylene glycol 1:1 8 Citric acid PEG 1500 1:1 9 Dodecanedioic acid 2,2-Diethyl-1,3-propanediol 1:1 10 Dodecanedioic acid PPG 2000 1:1 11 Succinic acid PPG 2000 1:1 12 Maleic acid PPG 2000 1:1 13 Tartaric acid PPG 2000 1:1 14 1,4-cyclohexane dicarboxylic acid PPG 2000 1:1 15 Citric acid PPG 2000 1:1 16 Thiodiglycolic acid 1,6-Hexanediol 1:1 17 Thiodiglycolic acid PPG 2000 1:1 18 Succinic acid Castor oil 1:1 19 Tartaric acid Castor oil 1:1 20 Sebacic acid PEG 200 1:1 21 Sebacic acid Neopentyl glycol 1:1 22 Sebacic acid 1,2-Propanediol 1:1 23 Sebacic acid PPG 425 1:1 24 Sebacic acid PPG 2000 1:1 25 Sebacic acid Diethanolamine 1:1 26 Sebacic acid N-Methyl diethanolamine 1:1 27 Sebacic acid Trimethylol propane 1:1 28 Sebacic acid Glycerol 1:1 29 Sebacic acid 2,2-Bis(hydroxy methyl) propionic acid 1:1 30 Adipic acid PPG 425 1:1 31 Adipic acid N-Butyl diethanolamine 1:1 32 Diglycolic acid PEG 1000 1:1 33 Diglycolic acid PEG 1500 1:1 34 Diglycolic acid PEG 6000 1:1 35 Diglycolic acid Neopentyl glycol 1:1 36 Diglycolic acid 1,6-Hexanediol 1:1 37 Diglycolic acid PPG 425 1:1 38 Citric acid Glycerol 1:1 39 Suberic acid 2,2-Bis(hydroxy methyl)propionic acid 1:1 40 Dodecanedioic acid Trimethylol propane 1:1 41 Suberic acid Trimethylol propane 1:1 42 Citric acid Trimethylol propane 1:1 43 Pimelic acid PEG 200 1:1 44 Pimelic acid PPG 425 1:1 45 Pimelic acid PPG 2000 1:1 46 Pimelic acid 2-Ethyl-1,3-hexanediol 1:1 47 Pimelic acid Diethanolamine 1:1 48 Pimelic acid N-Butyl diethanolamine 1:1 49 Pimelic acid Trimethylol propane 1:1 50 Pimelic acid 2,2-Bis(hydroxy methyl)propionic acid 1:1 51 Diglycolic acid Reaction product of glycerol carbonate and hexylamine, prepared according to Ex 2 1:1 52 Maleic acid Reaction product of glycerol carbonate and dodecylamine, prepared according to Ex 2 1:1 53 Tartaric acid Reaction product of glycerol carbonate and dodecylamine, prepared according to Ex 2 1:1 54 Citric acid Reaction product of glycerol carbonate and dodecylamine, prepared according to Ex 2 1:1 55 Tartaric acid Tween 80 1:1 56 Dodecanedioic acid Reaction product of glycerol carbonate and ethylenediamine, prepared according to Ex 2 Stearic acid 1:1:1 57 Dodecanedioic acid Reaction product of glycerol carbonate and ethylenediamine, prepared according to Ex 2 Hexanoic acid 1:1:1 58 Sebacic acid Glycerol Hexanol 1:1:1 59 Tartaric acid Reaction product of glycerol carbonate and ethanolamine, prepared according to Ex 2 1:1 60 1,4-cyclohexane dicarboxylic acid Reaction product of glycerol carbonate and ethanolamine, prepared according to Ex 2 1:1 61 Suberic acid Reaction product of glycerol carbonate and ethanolamine, prepared according to Ex 2 1:1 62 Citric acid Reaction product of glycerol carbonate and ethanolamine, prepared according to Ex 2 1:1 63 Suberic acid Reaction product of glycerol carbonate and Jeffamine M-1000, prepared according to Ex 2 1:1 64 Succinic acid Reaction product of gluconolactone and Jeffamine ED-600, prepared according to Ex 2 1:1 65 Citric acid PEG 1000 020-24 ASA 0.5: 1 :0.5 66 Dodecanedioic acid 2,4,7,9-Tetramethyl-5-decyne- 4,7-diol ethoxylate 1:1 67 Succinic acid 2,4,7,9-Tetramethyl-5-decyne- 4,7-diol ethoxylate 1:1 68 Suberic acid 2,4,7,9-Tetramethyl-5-decyne- 4,7-diol ethoxylate 1:1 69 Sebacic acid 2,4,7,9-Tetramethyl-5-decyne- 4,7-diol ethoxylate 1:1 70 Polyethylene glycol)bis(carboxymethyl) ether PEG 200 1:1 71 Polyethylene glycol)bis(carboxymethyl) ether PEG 1000 1:1 72 Polyethylene glycol)bisearboxymethyl) ether Neopentyl glycol 1:1 73 Polyethylene glycol)bisearboxymethyl) ether Tris(2-hydroxyethyl)methylammonium methylsulfate 1:1 74 Polyethylene glycol)bisearboxymethyl) ether 1,2-Propanediol 1:1 75 Polyethylene glycol)bisearboxymethyl) ether PPG 2000 1:1 76 Polyethylene glycol)bisearboxymethyl) ether N-methyl diethanolamine 1:1 77 Polyethylene glycol)bisearboxymethyl) ether Trimethylol propane 1:1 78 Diglycolic acid Castor oil 1:1 79 Adipic acid Castor oil 1:1 80 Diglycolic acid 1,6-Hexanediol Ricinoleic acid 0.05:0.05:0.9 81 Diglycolic acid 1,6-Hexanediol Ricinoleic acid 0.15:0.15:0.7 82 Diglycolic acid 1,6-Hexanediol Ricinoleic acid 0.2:0.2:0.6 83 Diglycolic acid 1,6-Hexanediol Ricinoleic acid 0.25:0.25:0.5 84 Diglycolic acid 1,6-Hexanediol PPG 2000 Ricinoleic acid 0.1:0.05:0.05:0.8 85 Diglycolic acid 1,6-Hexanediol PPG 2000 Ricinoleic acid 0.15:0.075:0.075:0.7 86 Diglycolic acid 1,6-Hexanediol PPG 2000 Ricinoleic acid 0.2:0.1:0.1:0.6 87 Diglycolic acid Trimethylol propane Ricinoleic acid 0.15:0.15:0.7 88 Diglycolic acid Trimethylol propane Ricinoleic acid 0.2:0.2:0.6 89 Thiodiglycolic acid Castor oil 1:1 90 Polyethylene glycol)bis(carboxymethyl) ether Castor oil 1:1 91 Suberic acid Castor oil 1:1 92 Dodecanedioic acid Trimethylol propane Erucic acid 1:1:1 93 Suberic acid Trimethylol propane Erucic acid 1:1:1 94 Sebacic acid Trimethylol propane Erucic acid 1:1:1 95 Thiodiglycolic acid Trimethylol propane Erucic acid 1:1:1 96 Pimelic acid Trimethylol propane Erucic acid 1:1:1 97 Hydrogenated dimer acid Trimethylol propane Erucic acid 1:1:1 98 Polyethylene glycol)bis(carboxymethyl) ether Trimethylol propane Erucic acid 1:1:1 99 Hydrogenated dimer acid PEG 200 1:1 100 Hydrogenated dimer acid PEG 1000 1:1 101 Hydrogenated dimer acid Neopentyl glycol 1:1 102 Hydrogenated dimer acid 1,6-Hexanediol 1:1 103 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate 1:1 104 Hydrogenated dimer acid PPG 2000 1:1 105 Hydrogenated dimer acid 2-Ethyl-1,3-hexanediol 1:1 106 Hydrogenated dimer acid N-butyl diethanolamine 1:1 107 Hydrogenated dimer acid N-methyl diethanolamine 1:1 108 Hydrogenated dimer acid Trimethylol propane 1:1 109 Hydrogenated dimer acid 2,4,7,9-Tetramethyl-5-decyne- 4,7-diol ethoxylate 1:1 110 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Neopentyl glycol 1:0.8:0.2 111 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Neopentyl glycol 1:0.4:0.6 112 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate 1,6-Hexanediol 1:0.8:0.2 113 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate 1,6-Hexanediol 1:0.6:0.4 114 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate 1,6-Hexanediol 1:0.2:0.8 115 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate PEG 1000 1:0.5:0.5 116 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate PPG 425 1:0.5:0.5 117 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Propionic acid 1:1:1 118 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Hexanoic acid 1:1:1 119 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Lauric acid 1:1:1 120 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Oleic acid 1:1:1 121 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Erucic acid 1:1:1 122 Hydrogenated dimer acid Reaction product of glycerol carbonate and ethanolamine, prepared according to Ex 2 1:1 123 Sebacic acid N-butyl diethanolamine 1:1 124 Sebacic acid N-butyl diethanolamine 1,2-Propanediol 1:0.5:0.5 125 Sebacic acid N-butyl diethanolamine Neopentyl glycol 1:0.5:0.5 126 Sebacic acid N-butyl diethanolamine PEG 200 1:0.5:0.5 127 Sebacic acid N-butyl diethanolamine PEG 400 1:0.5:0.5 128 Sebacic acid N-butyl diethanolamine PEG 600 1:0.5:0.5 129 Sebacic acid N-butyl diethanolamine 1,6-Hexanediol 1:0.5:0.5 130 Sebacic acid N-butyl diethanolamine PPG 2000 1:0.5:0.5 131 Sebacic acid N-butyl diethanolamine PPG 425 1:0.5:0.5 132 Sebacic acid N-butyl diethanolamine PPG 1000 1:0.5:0.5 133 Sebacic acid N-butyl diethanolamine PPG 1000 1:0.2:0.8 134 Sebacic acid N-butyl diethanolamine PPG 1000 1:0.8:0.2 135 Polyethylene glycol) bis(carboxymethyl) ether PPG 2000 1:1 136 Polyethylene glycol) bis(carboxymethyl) ether PPG 2000 1:1 137 Hydrogenated dimer acid N-methyl diethanolamine 1:1 138 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Oleic acid 1.1:1:1 139 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Propionic acid 1.1:1:1 140 Polyethylene glycol) bis(carboxymethyl) ether Diethylene glycol 1:1 141 Polyethylene glycol) bis(carboxymethyl) ether 1,3-Butanediol 1:1 142 Polyethylene glycol) bisearboxymethyl) ether 1,4-Butanediol 1:1 143 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Oleic acid 1:1.1:1.1 144 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate Propionic acid 1:1.1:1.1 145 Hydrogenated dimer acid Tris(2-hydroxyethyl) methyl ammonium methylsulfate 1:1 146 Polyethylene glycol) bisearboxymethyl) ether PPG 1000 1:1 147 Azelaic acid Neopentyl glycol 1:1 148 Azelaic acid N-butyl diethanolamine 1:1 149 Azelaic acid PPG 1000 1:1 150 Azelaic acid PPG 2000 1:1 151 Sebacic acid 1,3-Butanediol 1:1 152 Dodecanedioic acid 1,3-Butanediol 1:1 153 Dodecanedioic acid 2,3-Butanediol 1:1 154 Succinic acid Castor oil 1:1 155 Diglycolic acid Castor oil 1:1 156 Polyethylene glycol)bis(carboxymethyl) ether N-butyl diethanolamine 1:1 157 Diglycolic acid PEG 1500 PPG 1000 1:0.5:0.5 Table 2 - abbreviations and chemical names PEG Poly(ethylene glycol) PPG Poly(propylene glycol) Duomeen CD N-cocoyl propylene diamine, commercially available from Nouryon Specialty Chemicals (Amsterdam, Netherlands) Jeffamine M-1000 Polyether monoamine comprising ethylene oxide and propylene oxide derived repeat units and having Mn ~ 1,000. Commercially available from Huntsman Corporation (Texas, United States) Jeffamine ED-600 Polyether diamine comprising ethylene oxide and propylene oxide derived repeat units and having Mn ~ 600. Commercially available from Huntsman Corporation (Texas, United States) 5 Example 4 - preparation of a concentrated, pumpable and flowable composition using ester compound 123 Ester compound 123 was treated with a solvent (up to 20 wt%) and homogenized thoroughly. The composition was allowed to stand for 24 hours at 20°C before visually observing and 10 measuring the dynamic viscosity (cP). Dynamic viscosity was measured using an Anton Paar Stabinger SVM 301 viscometer. The results are shown in Table 5. 15 Tables Proportion of ester compound 123 (wt%) Solvent Proportion of solvent (wt%) Dynamic viscosity @ 20°C (cP) Visual observation 100 None 12,909 Homogenous 90 Isopropanol 10 2,963 Homogenous 80 Isopropanol 20 977 Homogenous 80 Water 20 Not measurable Immiscible layers Example 5 - Preparation of further concentrated aqueous emulsions. An ester compound (20 wt%), surfactant (5 wt% active unless otherwise indicated in Table 6) and deionized water (balance, i.e. 75 wt% when surfactant is 5 wt%) were combined with a vortex mixer. The resulting mixture was left to stand at ambient temperature and the emulsion quality overtime was observed. The results are shown in Table 6. If the resulting mixture formed a stable emulsion, this is entered as “Stable”. If recorded as “Stable”, the stability time indicates the last time-point which was assessed. Table 6 Ester compound Surfactant Amount of surfactant (wt% active) Result Stability time 137 Tallow amine ethoxylate (11 EO) 5 Stable 2 months 78 Tallow amine ethoxylate (11 EO) 5 Stable 3 months 123 Tallow amine ethoxylate (11 EO) 5 Stable 10 weeks 146 Tallow amine ethoxylate (11 EO) 5 Stable 3 months Example 6 - solubility improvement at high concentrations using a surfactant Ester compound 33 (80 wt%) was combined with deionised water (20 wt%) using a vortex mixer and solubility was observed. If the sample was not fully dissolved, additional deionised water was added until the sample fully dissolved. This was repeated with the ester compound 33 and adding 5 wt% of laureth-11 carboxylic acid (and using 15 wt% of water). The results are shown in Table 7. Table 7 Sample Soluble at 80 wt% in DI water? Soluble at 60 wt% in DI water? Ester compound 33 No Yes Ester compound 33 + Laureth-11 Carboxylic Acid (5 wt%) Yes Yes The laureth-11 carboxylic acid surfactant aided the solubility of the ester compound in water at a high concentration, i.e. 80 wt%. Unless otherwise stated herein, the reference to “liquid”, “gel” and “solid” refer to a state at 25 °C and standard pressure (101,325 Pa). Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. An ester compound, wherein the ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol.

2. The ester compound according to claim 1, wherein the ester compound is a polymer.

3. The ester compound according to claim 2, wherein the polymer comprises at least 4 monomer units.

4. The ester compound according to any preceding claim, wherein the or each polycarboxylic acid is of the formula HOOC(CR2)nCOOH, wherein n is from 0 to 30; and each R is independently hydrogen ora substituent;and / or two R groups on the same carbon atom are taken together to form a methylene (=CH2) group;and / or when n is two or more, two R groups on adjacent carbon atoms are taken together to form a double bond; andn is suitably from 1 to 20, preferably from 2 to 16, more preferably from 2 to 12, for example from 2 to 10.

4. The ester compound according to claim 3, wherein the or each polycarboxylic acid or the reactive equivalent thereof is selected from oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, maleic acid, fumaric acid, malic acid, tartaric acid, citric acid, C20-24 alkenyl succinic acid, dodecenyl succinic acid (such as (2-dodecen-1-yl)succinic acid), nonenyl succinic acid, octadecenyl succinic acid, octenyl succinic acid, polyisobutenylsuccinic acid, itaconic acid, 1,2-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, phthalic acid, isophthalic acid, terephthalic acid, homophthalic acid, 1,2,4-benzenetricarboxylicacid, pyromellitic acid, 1,2-naphthalenedicarboxylic acid, 2,3-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diglycolic acid, thiodiglycolic acid, 3,3’-thiodipropanoic acid, iminodiacetic acid, poly(ethylene glycol)bis(carboxymethyl) ether, a dimer acid, or an acid chloride or ester thereof.

5. The ester compound according to claim 3 or 4, wherein the or each polycarboxylic acid or the reactive equivalent thereof is selected from succinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, maleic acid, malic acid, tartaric acid, citric acid, 1,4-cyclohexanedicarboxylic acid, terephthalic acid, diglycolic acid, thiodiglycolic acid,poly(ethylene glycol)bis(carboxymethyl) ether, a hydrogenated dimer acid, oxalyl chloride, isophthaloyl chloride, or terephthaloyl chloride.

6. The ester compound according to any preceding claim, wherein the or each second reactant is a polyol having from 2 to 10, preferably from 2 to 6, more preferably 2 or 3, hydroxy groups.

7. The ester compound according to claim 6, wherein the or each polyol is selected from one or more polyol of the formula (I), one or more of an alkoxylated polyol of formula (I), one or more nitrogen containing polyol, and one or more polyol formed by reaction of a hydroxy substituted cyclic ester or cyclic carbonate with a suitable primary or secondary amine compound, wherein the polyol of formula (I) is of the formula H-(OR3)P-OH, wherein each R3 is independently an optionally substituted hydrocarbylene group and p is an integer of at least 1.

8. The ester compound according to claim 6 or 7, wherein the or each second reactant is a polyol independently selected from castor oil, 1,6-hexanediol, sorbitol, neopentyl glycol and a polyalkylene glycol (such as PEG 200).

9. The ester compound according to any preceding claim, wherein the ester compound is the reaction product of reactants comprising, consisting essentially of or consisting of (preferably consisting of) one or more first reactants and one or more second reactants, wherein the or each first reactant is selected from a polycarboxylic acid of one of the formulae HOOC(CR2)nCOOH, HOOC(CH2)mX(CH2)m2COOH, or HOOCCH2(OCH2CHR2)xOCH2COOH, a polycarboxylic acid comprising a cyclic group, a dimer acid, or a reactive equivalent thereof;wherein n is from 0 to 30; andeach R is independently hydrogen or a substituent selected from hydroxy groups, carboxyl groups, alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups, wherein the alkyl groups, alkenyl groups, aryl groups, aralkyl groups, and alkaryl groups are optionally substituted with one or more of a hydroxy group and / or a carboxyl group;and / or two R groups on the same carbon atom may be taken together to form a methylene (=CH2) group;and / or when n is two or more, two R groups on adjacent carbon atoms may be taken together to form a double bond;m+m2 is from 0 to 30;X is O or S;x is from 1 to 30; andeach R2 is independently hydrogen ora hydrocarbyl group; andthe or each second reactant is selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, 1,12-dodecanediol, trimethylolpropane, 2-ethyl-1,3,-hexanediol, 2,2-diethyl-1,3-propanediol, 2,2-bis(hydroxymethyl)propionic acid, pentaerythritol, glucose, fructose, trehalose, sucrose, lactose, maltose, sorbitol, xylitol, glycerol, neopentyl glycol, diglycerol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol, polypropylene glycol, castor oil, polyoxyethylene (80) sorbitan monooleate, or 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate.

10. The ester compound according to any preceding claim, wherein the ester compound has a number average molecular weight of from 1,000 to 150,000 Daltons, preferably from 1,000 to 15,000 Daltons (for example from 2,000 to 7,000 Daltons).

11. The ester compound according to any preceding claim, wherein the ester compound is substantially free of silicon atoms.

12. A concentrate composition comprising one or more ester compounds and optionally at least one solvent, wherein the concentrate composition comprises at least 20 wt% of the one or more ester compounds, and wherein the or each ester compound is the reaction product of reactants comprising one or more first reactants and one or more second reactants, wherein the or each first reactant is a polycarboxylic acid or a reactive equivalent thereof and the or each second reactant is a polyol.

13. The concentrate composition according to claim 12, wherein the composition comprises at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, or at least 80 wt% of one or more ester compounds.

14. The concentrate composition according to claim 12 or 13, wherein the composition additionally comprises one or more surfactants.

15. The concentrate composition according to claim 14, wherein the one or more surfactants are independently selected from anionic surfactants, cationic surfactants, non-ionic surfactants, and amphoteric or zwitterionic surfactants.

16. The concentrate composition according to claim 15, wherein the one or more surfactants are independently selected from one or more of a fatty alkyl amphoacetate, an alkyl (poly)glycoside, an acyl glycinate, a sulfosuccinate, an amphodiacetate and a fatty alcohol ether carboxylate.

17. The concentrate composition according to claim 15, wherein the one or more surfactants are independently selected from one or more of a fatty amine alkoxylate and a fatty alcohol ether carboxylate.5 18. The concentrate composition according to any of claims 13 to 17, wherein the concentratecomposition is stable for at least 3, 6, 12 or 24 months under ambient conditions.

19. The concentrate composition according to any of claims 13 to 18, wherein the concentrate composition is flowable.

020. The concentrate composition according to claim 19, wherein the concentrate composition has a viscosity of 7,000 cP or less, preferably 3,500 cP or less at 20°C.