Trends in the design of biodegradable non-ionic surfactants

Transcription

1 Pierre Varineau Trends in the design of biodegradable non-ionic surfactants INTRODUCTION There are well over one thousand non-ionic surfactants commercially available (1), based on unique combinations of hydrophobes, linking groups, and hydrophiles, as described in Table 1. Biodegradation has become a key surfactant selection criterion because of increasing concerns about exposure and environmental impact. With a simple set of 15 hydrophobes, 6 linking functional groups and 15 hydrophiles, a simple linear hydrophobe-link-hydrophile structure results in a set of over 1300 possible nonionic surfactants, with a diverse set of properties capable of meeting a broad set of application requirements. Even with the enormous selection of non-ionic surfactants currently available, there is continuing development of high-performance biodegradable surfactants. The universe of non-ionic surfactants contains hydrophobes based on seed oil or synthetic hydrocarbons, poly(dimethylsiloxane) (silicone surfactants) and perfluorinated hydrocarbons. In this article, we will focus only on those non-ionic surfactants based on seed-oil or synthetic hydrocarbon based hydrophobes. The current drivers for surfactant innovation include increased efficacy, performance, and improved affordability while maintaining high biodegradability. As alkylphenol ethoxylates (APEs) continue being phased out of various applications, there is also a need to find high-performance alternatives to these products. APEs have outstanding cost performance balance in laundry, hard surface cleaning, emulsification, paints and coatings, agricultural adjuvants, and a wide variety of other applications. While some readily biodegradable surfactants, such as linear primary alcohol ethoxylates, may work well as APE alternatives in laundry, they may perform poorly in other applications such as hard surface cleaning or freeze-thaw stabilization for paints and coatings. Highly branched surfactants such as the trimethyl nonanol ethoxylates may act as excellent offsets for alkylphenol ethoxylates in select emulsion applications, but these surfactants are not readily Table 1. The universe of commercially available nonionic Surfactants is defined by a broad set of hydrophobes, linking functional groups and hydrophiles biodegradable and thus are not acceptable alternatives to APEs. Our focus in this paper is to discuss the general industry-wide efforts associated with designing new biodegradable non-ionic surfactants with enhanced performance. There are three general areas of surfactant development associated with manipulation of 1) the hydrophobe 2) the linking functional group and 3) the hydrophile. BIODEGRADATION An extensive text on the biodegradation of surfactants is given by Swisher (2). A global standard screening test for the aerobic biodegradation of surfactants is based on the Organization for Economic Cooperation and Development (OECD) day modified Sturm test, which gives results as "readily biodegradable" (>=60 percent biodegradation) "inherently biodegradable" (>=20 percent but less than 60 percent) or "non biodegradable" (<20 percent). For global regulatory compliance, it is broadly perceived that any new surfactants developed and commercialized must meet the "readily biodegradable" classification using the OECD 301 series aerobic tests. Various computer models (3) and group contribution systems (4) have been developed for the prediction of extent of biodegradation based on surfactant structure and measured biodegradation rates. As a general rule, non-ionic surfactants with linear hydrophobes are more biodegradable than surfactants with highly branched hydrophobes. For example, it is widely known that XXVIII

2 linear primary alcohol ethoxylates biodegrade faster than branched primary alcohol ethoxylates (PAEs) or APEs. The presence of quaternary carbons on the hydrophobe backbone is a particular problem for achieving ready biodegradation (2), and it has been suggested that levels of quaternary carbons above about 0.5 mole percent can adversely affect biodegradation (12). Highly branched alcohol non-ionic surfactants, such as the trimethyl nonanol ethoxylates, have low rates of Another general rule is that neither the nature of the linking functional group nor the structure of the hydrophilic group impacts aerobic biodegradation of non-ionic surfactants (2). An extensive review of anaerobic biodegradation of surfactants given by Berna (5) shows that fatty acids, alcohol ethoxylates, sugar-based non-ionic surfactants (alkylpolyglucosides, glucamides), alkylethanolamides, and linear amine oxides are readily biodegradable. Branched APEs are only partially biodegradable. A study of anaerobic degradation of non-ionic surfactants as a function of hydrophobe branching was performed by Moshe (6), where it was shown that almost complete biodegradation was achieved for linear alcohol ethoxylates, poly(ethylene glycol), dodecanol, 2-ethyl-hexanoic acid, and 3-methyl-valeric acid. Conversely, poor biodegradation was observed for highly branched alcohol ethoxylates, 2-butyl branched alcohol ethoxylates, alcohol alkoxylates (i.e. ethoxylated propoxylated alcohols), poly(propylene glycol), and isotridecanol. Even a mildly branched 2- ethyl hexanol ethoxylate was converted to 2-ethylhexyloxy-acetate, which was not further degraded. One may conclude that even moderately branched non-ionic surfactants do not exhibit high rates of anaerobic Historically anaerobic biodegradation has not been as much of a regulatory issue as aerobic biodegradation because the environmental impact has been perceived as less significant (5). Although it is unclear how future anaerobic regulatory requirements will affect surfactant development, the industry should remain aware that surfactants that are aerobically readily biodegradable may not show similar anaerobic This is also observed for tridecyl alcohol ethoxylates, which have been measured as "readily biodegradable" or "inherently biodegradable" depending on ethoxylate chain length and degree of branching in the hydrophobe. Recent surfactant development aimed at creating high performance surfactants have focused on introducing branching on the hydrophobe to a degree that increases target performance, but maintains a status of "readily biodegradable". Even though linear hydrophobe based non-ionic surfactants work well in some applications, surfactants with higher degrees of branching correlate to lower pour points, narrower gel ranges, faster wetting times, lower dispersive energies, lower equilibrium surface tension, faster dynamic surface tension reduction, lower critical micelle concentrations, and enhanced performance (8, 9). There is a critical biodegradation-limited "zone" within which enhanced surfactant hydrophobe design must occur. Too much hydrophobe branching results in enhanced performance, but poor biodegradation; while too little branching results in enhanced biodegradation, but limited performance enhancement. MANIPULATION OF THE HYDROPHOBE Oligomerization of olefins Oligomerization of ethylene via the Ziegler process followed by hydrolysis produces highly biodegradable, linear, even carbon-numbered surfactant alcohols in the C6-C20 range that are chemically identical to seedoil based alcohols (10). Higher olefins produced by either the Ziegler process, or by the Shell Higher Olefins Process, followed by hydroformylation to the alcohol produces a mixture of odd-numbered surfactant alcohols ranging from C7-C21 (typical), that are mixtures of linear alcohols and of 2-methyl branched alcohols. SURFACTANT PERFORMANCE/ STRUCTURE VS. BIODEGRADATION Figure 1 shows a general chart of surfactant hydrophobe structure vs. biodegradation vs. performance. We emphasize general because there is significant variability in biodegradation test results, which depend not only on variability of the measurement method, but also on how the surfactants are produced. One clearly observable trend affecting biodegradation is the size of the hydrophile. For example, a C12-15 secondary alcohol ethoxylate with 5 moles of EO has a biodegradation of 67 percent, while the 40 mole ethoxylate has a biodegradation of 92 percent (7). For the latter case, an increase in the mass percentage of biodegradable hydrophile increases overall Figure 1. Representation of hydrophobe branching vs. extent of The hydrophile is represented by "B" XXIX

3 Typical levels of 2-methyl-branching in highly linear alcohols produced via hydroformylation of alphaolefins range from percent. These alcohols are also readily biodegradable, and have been commercially available for several decades (11). Recently, researchers have developed a modified process in which branching is catalytically introduced along the backbone of the alcohol to give almost 100 percent mono-alkyl branching, with the majority of the branch alkyl groups comprising methyl and ethyl groups (12). Various moderately branched alcohols are commercially available as hydrophobes for non-ionic surfactants (13). New catalysts for ethylene tetramerization produce fractions of higher olefins (C10+) that are mono-alkyl branched (14). For example, a new process developed for ethylene tetramerization produces a C10+ olefin byproduct stream consisting of approximately percent linear C10-14 alpha olefins, and percent mono-branched internal olefins (including percent vinylidines) (15). When these samples are converted to non-ionic surfactants via hydroformylation and hydrogenation followed by ethoxylation with 5-7 moles of ethylene oxide, the resulting products are readily biodegradable with > 75 percent biodegradation using OECD 301 criterion (16). These alcohols are functionally similar to those produced via the catalytic introduction of mono-alkyl branching, as discussed above. Tetramerization of propylene has long been used to produce highly branched surfactant hydrophobes. Propylene trimerization, followed by alkylation with phenol is used to produce the highly branched nonylphenol used for alkylphenol ethoxylates. Propylene tetramerization followed by hydroformylation to the iso-c13 alcohol is the base for the highly branched tridecyl alcohol ethoxylate (TDAs), which are commonly used as alternatives for APEs due to similarity in hydrophobe branching. Hydrophobes based on propylene oligomerization can lead to lower levels of biodegradation because of the potential for a high mole fraction of both tertiary and quaternary carbons. Recent developments in hydrophobe development have focused on the trimerization of butene followed by hydroformylation to form C13 alcohols (17). These products have the potential for fewer branch points, and higher levels of The number of branch points along the hydrophobe backbone can be reduced even further by the dimerization of higher olefins such as hexene or octene, which results in surfactant range alcohols with lower degrees of branching and therefore a higher level of intrinsic biodegradation (18). Catalytic introduction of branching into seed oil hydrophobe Detergent range C8-C18 alcohols derived from seed-oils such as coconut, canola, soy or palm kernel oil are 100 percent linear, and used to produce highly biodegradable surfactants such as the linear primary alcohol ethoxylates. These alcohols are identical those produced via the synthetic Ziegler-hydrolysis process outlined above. To produce seed-oil based products with the enhanced performance of a branched hydrophobe, both saturated and unsaturated fatty acids can be isomerized with Zeolite-type catalysts to form mono-methyl branched iso-fatty acids, such as iso-stearic acid (19). These iso-acids can be further processed to form iso-alcohols, which can then be converted to the "seed oil" version of the highperformance moderately branched PAEs. The degree of branching is roughly one, so that the final products remain generally readily biodegradable. Guerbet alcohols Guerbet alcohols are formed by hydroformylation/aldol condensation/hydrogenation of alpha olefins, typically propylene or butene. These products have a degree of branching of exactly one, and non-ionic surfactants based on these materials are generally readily biodegradable. Examples of these types of surfactants include the 2-ethyl hexanol alkoxylates, 2-propyl heptanol alkoxylates, 2-pentyl nonanol ethoxylates, and a variety of other Guerbet-alcohol derived materials (20). Hydrogenation of APEs Commercial nonyl and octylphenol ethoxylates have been hydrogenated to preserve detergent properties, but eliminate fluorescence and ultraviolet light absorption (21) which can interfere with biological analysis. This approach has also been proposed for producing an alternative to APEs (22). The surfactant properties for hydrogenated nonylphenol ethoxylates are similar to the conventional APE. However, biodegradation measured by the authors on hydrogenated nonylphenol poly(oxyethylene)-9 showed that this material is not readily biodegradable (<60 percent in 28 days) which demonstrates that this technique is not viable for producing readily biodegradable surfactants as alternatives to APEs. Introduction of branching onto the backbone using higher alkylene oxides Figure 2. Introduction of branching onto linear alcohols via block alkoxylation with higher alkylene oxides. B = hydrophile Hydrophobic branching can be introduced onto a linear hydrophobe by block alkoxylation with a higher alkylene oxide such propylene oxide, butylene oxide, styrene oxide, or higher alkylene oxides as shown in Figure 2. This is a convenient method for converting 100 percent linear seed-oil or synthetic based alcohols to pseudo-branched alcohols as a base for ethoxylated nonionic surfactants. These materials can meet the OECD 301B "readily biodegradable" criterion, but the extent of alkoxylation dictates the extent of For example, the authors measured the OECD301F biodegradability of a series 2EH(PO)x(EO)y compounds and found that only materials with a degree of propoxylation of less than about x = 6.0 were classified as readily biodegradable. There are several advantages to using block alkoxylation as a means to introduce branching in the hydrophobe backbone. Block alkoxylation of alcohols XXX

4 with propylene oxide followed by ethylene oxide produces materials with low levels of residual alcohol and odour (23, 24). For standard alcohol ethoxylates, the level of residual alcohol ranges from 3-15 percent depending on the degree of ethoxylation. For the block alkoxylates, a block of only 1 mole of propylene oxide reduces the unreacted alcohol to less than 0.5 weight percent. A block of 3 moles of PO reduces the level of unreacted alcohol to less than 0.1 percent (25). Block alkoxylation with alkylene oxides extends the chain length of the hydrophobe, so that solvent-range alcohols such as 2-ethyl hexanol can be converted to surfactant-range alcohols (26). A variety of performance advantages are observed with a general alcohol(ao)x(eo)y block structure, where AO is a higher alkylene oxide such as propylene oxide or butylene oxide. For example, these products have been shown to be effective alternatives to NPEs for laundry (27). Matsuda (28) found that when compared with C12 alcohol ethoxylates, extended C12 alcohol(po)x(eo)y block alkoxylates showed lowered surface tension, lower critical micelle concentrations and greater dispersing power. Hashimoto (29) found that C12(PO)4(EO)10, when compared with C12(EO)7 (similar cloud points) gave significantly better emulsification capability and solubilization. MODIFICATIONS TO THE LINKING FUNCTIONAL GROUP The typical "linkage" groups for non-ionic surfactants include ethers, esters, amides, sulfides, acetals, carbonates, and orthoesters. In some cases, a separate linking agent is used. For example, for the alkyl-monoglyceride ethoxylates, glycerine can be considered a linking agent, for which both an alkyl carboxylic ester and ether linkage are present. Generally, the "linker" functional groups by themselves generally do not adversely affect However, select "linker" functional groups that exhibit rapid hydrolysis can assist in designing surfactants that have enhanced biodegradation kinetics (30). Groups falling under the "rapid hydrolysis" category include acetals, carbonates, and orthoesters. Examples include the C12-14 glyceryl ketal ethoxylates, which were designed as acid-splittable surfactants with enhanced waste treating capability (31). Ortho-esters and carbonates have also been explored as hydrolysable surfactants exhibiting enhanced biodegradation profiles (32). Finally, traditional hydrolysable linkages such as esters have been proposed as agents enabling biodegradation of traditionally bio-resistant materials. For example, the incorporation of epsiloncaprolactone into the backbone of poly(alkylene oxides) incorporates an ester functional group, which allows for hydrolysis and subsequent biodegradation (33). Although the "linker" group generally does not impact the biodegradation of the surfactant to the same degree as the hydrophobe, there are cases in which a negative impact is observed. For example, the authors determined the extent of biodegradation of two different 2-ethyl hexanol alkoxylates. The first, a standard 2-EH(PO)5(EO)9 showed "ready biodegradability" above 60 percent, whereas 2EH(monoglyceryl ether)(po)5(eo)9 showed less than 60 percent Gemini surfactants are an interesting class of surfactants that exhibit two hydrophobe tails and two hydrophilic head groups. The "linker" for Gemini surfactant falls outside of functional-group linkers described above. For example, the Gemini acetylenicbased diol surfactant 2,7 dimethyl-4-octyn-3,6-diolpoly(oxy-ethylene) really consists of two ether linkage groups. The acetylenic bridge is considered as part of the hydrophobe. This surfactant, which is highly branched, is considered "inherently biodegradable", with OECD 301 biodegradation rates of >20 percent, but less than 60 percent. Other Gemini surfactants have been specifically designed for enhanced biodegradability and performance (34) MODIFICATIONS TO THE HYDROPHILE Common nonionic hydrophiles include alcohols, polyhydric alcohols (such as glycerin, polyglycerin or sorbitol), poly(ethoxylates), poly(alkoxylates), sugars, poly-sugars, carboxylic acids, polypeptides, amides, amine oxides and alkanolamides. A general observation is that the nature of the hydrophile does not significantly adversely affect the biodegradation characteristics of non-ionic surfactants. However, there are obvious exceptions to this rule. For example, if pentaerythritol (which contains a quaternary carbon) is used as a hydrophile, then the final compound may show relatively low biodegradation characteristics. Although the hydrophile does not adversely affect biodegradation, these groups can have a positive effect on the ultimate biodegradability of the non-ionic surfactant. For example, the biodegradation of a 40- mole ethoxylate of a secondary alcohol shows biodegradation of 92 percent, while a 9-mole XXXI

5 ethoxylate shows biodegradation of 67 percent. This may represent an important avenue for the development of high-performance biodegradable surfactants. For example, if a highly branched hydrophobe with limited biodegradation is chosen because it adds performance to a surfactant, the addition of a large poly(oxyethylene) or poly(glucoside) chain could produce a final product that is readily biodegradable. CONCLUSIONS For non-ionic surfactants consisting of a hydrophobe- (linker)-hydrophile structure, the degree of branching in the hydrophobe has the largest impact to the overall biodegradability of the compound. Hydrophobes that are linear or mono-alkyl branched result in nonionic surfactants classified as "readily biodegradable". The nature of the linking functional group does not generally adversely affect biodegradation rates, but can be used to enhance biodegradation rate if hydrolyzable groups such as esters, acetals, or carbonates are used. Incorporation of hydrolyzable groups within the hydrophobe can also increase biodegradation groups. The nature of the hydrophile also generally does not adversely affect biodegradation rates, and can actually enhance biodegradation rates if it dominates the mass of the compound. Future direction for surfactant development will likely continue to focus on developing surfactants that have the performance advantage of highly branched hydrophobes, and yet maintain the status of ready biodegradation and potentially high rates of anaerobic REFERENCES AND NOTES 1. Michael and Irene Ash in Handbook of Industrial Surfactants, An International Guide to More Than 16,000 Products by Tradename, Application, Composition & Manufacturer. Gower Publishing (1993). 2. R.D. Swisher, Surfactant Biodegradation, Marcel Dekker, New York (1987). 3. J.W. Raymon et. al., "A review of Structure-based biodegradation estimation methods", Journal of Hazardous Materials, 84(2-3), pp (2001). 4. R.S. Boethling et. al., "Group Contribution Method for Predicting Probability and Rate of Aerobic Biodegradation" Environ, Sci, Technol., 28, p. 459 (1994). 5. J.L. Berna et. al., "Anaerobic Biodegradation of Surfactants", Scientific Review Tenside Surf. Det., 44, p. 6 (2007). 6. M. Mosche, "Anaerobic degradability of alcohol ethoxylates and related non-ionic surfactants", Biodegradation, 15, pp (2004). 7. Data gathered from The Dow Chemical Company. 8. F.A. Kvietok, et. al., "Detergent Composition Comprising Mid- Chain Branched Surfactants". U.S. Patent B1. PIERRE VARINEAU*, KARA WEBER, ANDRE ARGENTON, R. KIRK THOMPSON * Corresponding author Dow Fabric & Surface Care The Dow Chemical Company 2301 N. Brazosport Blvd Freeport, TX 77541, USA 9. Joseph F. Albert, "Advantages of Branched Secondary Alcohol Ethoxylates", World Conference on Detergents: Reinventing the Industry-Opportunities and Challenges, 5th, Montreux, Switzerland, Oct AOCS Press p H.A. Wittcoff et. al., Industrial Organic Chemicals, John Wiley & Sons, New York (1996). 11. Example: Shell Neodol series or the Sasol LIAL series alcohols. 12. D.M. Singlton et. al., Highly Branched Primary Alcohol Compositions and Biodegradable Detergents Made Therefrom. U.S. Patent See for example the Sasol ISALCHEM alcohols. 14. A. Bollmann et. al. "Ethylene Tetramerization: A New Route to Produce 1-Octene in Exceptionally High Selectivities", J. Am. Chem. Soc., 126, pp (2004). 15. Sasol Technology "The composition of a higher olefin fraction from targeted ethylene oligomerization processes and use in detergent applications" IPCOM D, from the IP.com prior art database. 16. The conversion of these materials to nonionic surfactants, and subsequent testing for biodegradation was performed by the authors. 17. E. Zeller et. al., C13 Alcohol Mixture and Functionalized C13 Alcohol Mixture" United States Patent US B1 (2005). 18. P. M. Ayoub et. al., "Methods for preparing branched aliphatic alcohols" United States Patent Document US A1 (2005). 19. C.J. Kenneally et. al., "Process for the Branching of Saturated and/or Unsaturated Fatty Acids and/or alkyl esters thereof" WO World Intellectual Property Organization, Commercial sources include the Lutensol XL and XP or ISOFOL series. 21. G.E. Tiller et. al., "Hydrogenation of Triton X-100 Eliminates it Fluorescence and Ultraviolet Light Absorption while Preserving Its Detergent Properties", Analytical Biochemistry, 141, p. 262 (1984). 22. R.L. Rayborn, "Alkyl Cyclohexanol Alkoxyaltes and Method for Making Same" U.S (2000). 23. C. Wulff et. al., Alkoxylates Exhibiting Low Residual Alcohol Content United States Patent Office US See for example the ECOSURF series from the Dow Chemical Company. 25. Misubishi Petrochemical Company, Liquid Detergents Comprising Nonionic Surfactants Patent GB (1978). 26. K. Bergstrom, "An Alkoxylate Mixture and Its Use as a Cleaning Agent for Hard Surfaces" WO World Intellectual Property Organization (2004). 27. P. Jeschke et. al., Liquid Nonionic Surfactant Mixtures United States Patent US 4,965,014 (1990). 28. M. Matsuda et. al., Synthesis and the applications of polyalkylene glycol derivatives. I. Synthesis and surface activity of higher alcohol-propylene oxide-ethylene-oxide adducts Yukagaku, 18(2), pp (1969). 29. Y. Hashimoto, Nonionic Surfactant and Nonionic Surfactant Composition Containing the Same Japanese Patent Application JP P. Somasundaran, "Encyclopedia of Surface and Colloid Science", Taylor and Francis, p. 906 (2004). Readers interested in a complete list of references are kindly invited to write to the author at PTVarineau@dow.com XXXII