Methyl ester ethoxylates esters, ethoxylation

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. 

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

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. 

In practice, inverse cloud point temperature is used more as a quality control measure rather than as a solubility requirement. Although methyl ester ethoxylates are less water soluble than their alcohol ethoxylate counterparts, they can achieve comparable formulatability characteristics at somewhat higher ethylene oxide levels. 

Methyl ester ethoxylates have been shown to have a significantly reduced tendency to form gels [9,16]. For example, the viscosity behavior of a commonly used 7-mol lauryl-range ethoxylate (based on a modified oxo-type C12 15 alcohol) is illustrated in Fig. 7. Gross viscosity was examined as a function of concentration and ionic strength (sodium chloride concentration) at 10, 25, and 40°C. As illustrated, gels (shown in blaek in Fig. 7) form readily, particularly at room temperature and below. 

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