Methyl ester ethoxylates

Hoechst and Henkel first attempted ethoxylation of these materials in 1989 with alkali/alkali earth and aluminium hydroxycarbonates respectively but these catalyst activities were too low for commercial application [24, 25]. Vista, in 1990, patented [26] the use of activated calcium and aluminium alkoxides and Lion Corporation, in 1994, filed a patent using magnesium oxide [27]. There was a flurry of activity in the 1990s and Michael Cox and his co-workers have written most of the literature [28-30]. The proprietary catalysts are more expensive than those for standard alcohol ethoxylates and generally have to be removed from the final product. They are more reactive than the standard alkali catalysts with the result that the reaction proceeds faster and at lower temperature and uses less catalyst. 

These materials, of course, contain no active hydrogen, so how does the reaction work The mechanism is complex and not fully understood but is thought to involve transesterification. The actual distribution of the ethoxymers depends on the catalyst used but 

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Although there hasn t been a great deal of research on the mechanisms involved in the ethoxylation of esters, one mechanism that has been proposed involves transesterification (7). As shown in Figure 13.12, it is the catalyst (in this case, a mixture of calcium and aluminium alkoxides) that first becomes ethoxylated (forms the metal alkoxyethoxylate). After the catalyst picks up a mole of EO, it then transesterifies with the ester to form methyl ester ethoxylate, alkyl ester ethoxylate, and metal-coordinated methoxide. These steps occur continuously until the available EO is exhausted and a distribution of methyl ester ethoxylate homologues (ethoxymers) is obtained. 

Although very little methyl ester ethoxylates are produced at the present time they are included here because of their potential to have a significant impact on the nonionic surfactants market in the future. The general structure of these surfactants (9) is as follows ... 

Methyl ester ethoxylates (MEEs) are a new introduction to the stable of feedstocks which can be ethoxylated, and have only recently become commercially available (currently being produced and utilized in a detergent formulation in Japan by the Lion Corporation). Based on recent literature, however, a significant amount of effort is being focused on the development of MEEs for detergent applications. 

Littau, Ch., Miller, D. 1998. Methyl ester ethoxylates. SOFW-J. 124 690-697. 

Cox, M. F., Weerasooriya, U. 1998. Impact of molecular structure on the performance of methyl ester ethoxylates. J. Surfact. Deterg. 1 11-22. 

Behler, A., Syldath, A. 2000. Fatty acid methyl ester ethoxylates—a new class of nonionic surfactants. Proceedings of the 5th World Surfactants Congress. Firenze, 1 382-391. 

In 1990, Vista Chemical Company (now Sasol North America Inc.) developed a commercially viable process based on more complex alkoxyla-tion catalysts (activated calcium and aluminum alkoxides) that effectively and efficiently achieved the ethoxylation of esters [4]. Soon after, Lion demonstrated that magnesium oxide-based catalysts also worked well [5]. These discoveries opened the door to ester alkoxylate development and have led to a flurry of research directed at understanding and utilizing these materials, most notably methyl ester ethoxylates [6-23]. 

It is important to note that methyl ester ethoxylates have been produced, primarily for the textile industry, for several years. They have been manufactured by condensing fatty acids with monomethyl-capped polyethylene glycol. This reaction, however, is more complex and more costly than direct ethoxylation [26]. 

Methyl ester ethoxylates are similar in structure to conventional alchol ethoxylates, but the structural differences that do exist have an important impact on their performance. As shown in Fig. 1, methyl ester ethoxylates contain an ester linkage at the hydrophobe-hydrophile boundary of the molecule in place of the ether linkage in alcohol ethoxylates. This ester linkage sterically constrains the molecule, which reduces the tendency of the surfactant to micellize and leads to a higher critical micelle coneentration... 

FIG. 1 Methyl ester ethoxylates vs. alcohol ethoxylates differences in molecular structure. 

CMC). Methyl ester ethoxylates also carry a terminal methoxy group in place of a terminal hydroxyl group. This reduces the hydrogen bonding of the surfactant, which in turn reduces water solubility and the tendency to form aqueous gels. 

Another important characteristic of methyl ester ethoxylate structure is the distribution of the ethoxymers (the relative concentration of unethoxylated feedstock, of 1-mol ethoxymer, of 2-mol ethoxymer, etc.). As discussed later in this chapter, the ethoxymer distribution of methyl ester ethoxylates, like that of their alcohol ethoxylate counterparts, can vary depending on the catalysts used to prepare them. 

Large quantities of methyl esters are currently produced from oleochemi-cal sources (Fig. 2). Their major use is as intermediates in the production of fatty alcohols. They can also be produced through esterification of fatty acids with methanol. It is safe to assume that commercial quantities of methyl esters could be made readily available as ethoxylation feedstocks for methyl ester ethoxylates. 

This chapter first examines the ethoxylation of esters and the composition of methyl ester ethoxylates. Important aspects related to the formulation of... 

The mechanism of the ethoxylation of esters with these complex catalysts is not well understood. It is thought to involve a transesterification which effectively inserts ethylene oxide into the ester linkage between the carbonyl carbon and the methoxy oxygen [9,15,18]. This mechanism is illustrated in Fig. 4. As shown, the active catalyst (calcium and aluminum alkoxyethoxy-late) first reacts with ethylene oxide to form the ethoxylated version of the metal alkoxythoxylate. This molecule then transesterifies with methyl ester to form the alkyl ester ethoxylate and a metal-coordinated methoxide ion. Addition of more ethylene oxide (step 2) produces progressively more highly ethoxylated versions of the metal-coordinated methoxide ions, which then transesterify with the ester (step 3) to form methyl ester ethoxylate, the alkyl ester ethoxylate, and the metal-coordinated methoxide. Steps 2 and 3 occur continuously with the addition of more ethylene oxide until excess methyl... 

FIG. 3 (A) Ethylene oxide distribution (via supercritical liquid chromatography) of Ci4 methyl ester ethoxylate obtained with conventional (NaOH) catalyst. (B) Ethylene oxide distribution (via supercritical liquid chromatography) of C14 methyl ester ethoxylate obtained with Ca/Al-alkoxide catalyst. (From Ref. 27.) (C) Ethylene oxide distribution (via supercritical liquid chromatography) of C14 alcohol ethoxylate obtained with conventional (NaOH) catalyst. (D) Ethylene oxide distribution (via supercritical liquid chromatography) of C14 alcohol ethoxylate obtained with Ca/Al-alkoxide catalyst. (From Ref. 27.)... 

FIG. 4 Proposed mechanism for the alkoxylation of methyl ester ethoxylates. 

Another feature that dilferentiates methyl ester ethoxylates from alcohol ethoxylates in unsaturatinon. Methyl esters, particularly those in the tallow range, are relatively highly unsaturated, generally more than 50%. Alcohols, in contrast, are typically fully saturated. The impact of unsaturation on performance can be significantly, and is addressed in the last section of this chapter. 

As with all ethoxylates, the relationship between mols of and weight-percent of ethylene oxide is nonlinear for methyl ester ethoxylates. This relationship is also slightly different from that for the corresponding alcohol ethoxylate because of the difference in molecular weight between the feedstocks. The relationship between mols and weight-percent ethylene oxide for various methyl ester, ethoxylates is shown in Fig. 5. 

IV. FORMULATES DETERGENTS WITH METHYL ESTER ETHOXYLATES... 

Methyl ester ethoxylates, however, are inherently less water soluble than their alcohol ethoxylate counterparts because they contain a terminal me-thoxy group in place of the more hydrophilic hydroxyl group. Consequently, it takes a higher degree of ethoxylation for methyl ester ethoxylates to achieve water solubility. Although it is dilScult to determine the precise ethoxylation level needed to achieve water solubility, studies suggest that methyl ester... 

FIG. 6 Water solubility (inverse cloud point temperature) as a function of EO content (moles) for C12-16 alcohol and methyl ester ethoxylates made with Ca/Al-alkoxide catalyst. (Cloud point temperature = temperature at which 1 % aqueous solution turns cloudy upon slow heating C12-16 alcohol ethoxylate is described in Table 1 distribution of methyl ester = 9% Cj, 8% Cio, 46% C12, 18% C14, 9% Cig, and 10% C18.) (From Ref. 27.)... 

Ci2 16 methyl ester ethoxylate requires about two additional moles of ethylene oxide compared to a C12 16 linear alcohol ethoxylate. To obtain an inverse cloud point of 80 °C, approximately eight additional moles of ethylene oxide would be needed. 

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