Laundry Detergent Formulations 63Preformed peracids are also known. They offer a weight efficiency advantage sinceno hydrogen peroxide is required and there is no “wasted” leaving group. The main issuewith preformed peracids is stability—both of the raw material itself and in finished product.Phthalimidoperoxycaproic acid (PAP) is probably the best known. Metal bleach catalystsalso offer great promise of weight efficiency. Bleach catalysts are transition metal compounds,typically Mn, Fe, Cu, or Co, with various chelating organic ligands. They reactwith appropriate oxygen sources to form high valent metal oxides that are potent oxidizingagents. Most are designed for use with hydrogen peroxide, but the obvious goal is todevelop a catalyst that works with molecular oxygen. To date there is no successfulcommercial example, but the search continues within many academic and corporate labs.In some regards photobleaches (normally metal phthalocyanines) could be considered asbleach catalysts. They generate singlet oxygen, a powerful bleaching species, on exposureto light and air. Because of the need for sunlight, and because the bleach is only reallyeffective while the clothing is still wet, the utility of photobleach is greatest in areas whereconsumers air dry their laundry outside. Figure 4 presents the exemplary structure ofcommon bleaches and chelants.F. Chelating AgentsChelants are often formulated in detergents because metal ions in the wash are almostalways a detriment to end performance. Many highly colored stains incorporate metals.Removal of the metal can often decolorize the stain and/or make it easier to remove bydestabilizing its structure. Examples include porphyrins found in blood and tannins in tea.Metal ions can also catalytically decompose bleach in a formulation, leading to significantlyreduced performance. Finally, metals often find their way onto fabric surfaces, eitheras insoluble salts as with calcium or magnesium fatty acids, or as metal oxides. Both leadto a multicycle dinginess and fabric feel issues.What chelant is used depends largely on local environmental regulations. Diethylenetriamine pentaacetic acid (DTPA) is commonly used in North America. It’s an analog ofthe more well-known ethylenediaminetetraacetate (EDTA), but has a better environmentalprofile. In Europe, however, DTPA is banned due to concerns with aquatic toxicity. As aresult, European formulations rely more on phosphonate-based chelants such as diethylenetriamine penta(methylene phosphonic) acid (DTPMP) or ethylene diamine tetra(methylene

phosphonic) acid (DDTMP). In an interesting twist, these materials cannot be used inNorth America due to bans on phosphorous in laundry detergents.In environmental terms, ethylene diamine disuccinic acid (EDDS) represents thebest achievement thus far. The molecule has two chiral centers and only the S,S-isomeris fully biodegradable. This makes it a more expensive chelant than the phosphonatechelants mentioned previously. It is used in some European granules today. HEDP, 1-hydroxyethyidene-1,1-diphosphonic acid, is not truly a chelant in the sense of the materialsmentioned above, but because it also helps control deposition of metals on fabrics itdeserves mention in the same section. HEDP works more by inhibiting crystal growth.

Liquid Bleach Formulations 195Radical scavengers work against the same type of decomposition process as chelants,i.e. radical decomposition, but they do it through a different mechanism. Chelants try tooccupy active sites in the coordination sphere of transition metals, thus making themunavailable for interaction with hydrogen peroxide and preventing the formation of OH.

radicals. Instead radical scavengers perform the function of stopping OH. radicals oncethey have been formed. Typical radical scavengers are molecules that can donate a hydrogenatom to a reactive radical, such as OH., thus transforming it into a nonradical specieswith lower reactivity. The radical scavenger molecule, after donating its hydrogen atom,becomes itself a radical, however, the new radical formed is a more stable one with lowreactivity, so the original radical chain reaction is terminated or significantly slowed down.Usually the stable radical obtained from the scavenger is an oxygen-centered radicalderived from a phenolic structure. The phenolic structure allows to delocalize the unpairedelectron on the aromatic ring, which results in lower energy, higher stability and lowerreactivity of the radical molecule (see Fig. 9). Well-known examples of radical scavengersare butylated hydroxy toluene (BHT), mono-tert-butyl hydroquinone (MTBHQ) [81,82],and other phenolic compounds. Primary or secondary aryl amines have also been proposedas effective radical scavengers [83].According to some authors, the amine oxide functionality may perform a radicalscavenging action too. Thus the effectiveness of amino phosphonates as stabilizers maybe related not only to their chelating ability, but also to the formation of amine oxidefunctionalities upon oxidation of their tertiary amine sites [79].The possibility of synergisms between different stabilizers combined together in the

same formulation has been highlighted by several authors [84]. The most frequentlyexploited combination is the one between a chelant and a radical scavenger [81–83],however, the combination of chelants with other classes of compounds, e.g., polyacrylates,has also been mentioned as synergistic [80].Hydrogen peroxide solutions are not significantly affected by exposure to visiblelight, while they are decomposed by radiation in the ultraviolet region, particularly between380 and 200 nm as mentioned above. The rate of decomposition is directly proportionalto the concentration of hydrogen peroxide and to the square root of radiation intensity.Unfortunately, the reproducibility of UV light-induced decomposition experiments is lowand strongly dependent on the particular grade of hydrogen peroxide used, so specificdecomposition data as a function of UV frequency and intensity are not very reliable.However, what’s most important for the formulator from a practical standpoint is thatordinary plastic bottles usually provide a sufficient light barrier for storage on indoorshelves. It may be required to add to the plastic a charge of a UV absorber, if the productneeds to be sold in countries where storage conditions are such that prolonged exposureto direct sunlight is likely to occur.Figure 9 Stabilization of oxygen-centered radical through delocalization of the unpaired electronon the aromatic ring, in a typical radical scavenger with phenolic structure.

D. Phosphonates and ThiophosphonatesPhosphonates and thiophosphonates were introduced as lubricant detergent additivesin 1943 by Loane and others in Standard Oil Company of Indiana [26–29]. Bothphosphonates and thiophosphonates are the reaction products of liquid polyolefins suchas polyisobutene with phosphorus pentasulfide. The reaction products were found toconsist of a mixture of phosphonate, thiophosphonate, and thiopyrophosphonate [30].The commercial products are generally called either phosphonates or thiophosphonatesdepending on the extent to which sulfur has been remove by steam treatment and thelength of hydrolysis. When the detergent was first introduced into the market, it wassupplied as a normal potassium salt. Later, it was changed to a barium salt [31].

The chemical structures of normal phosphonate, thiophosphonate, and thiopyrophosphonateare shown below:R has a molecular weight (MW) ranging from 500 to 2000. Typically, the alkylgroup is a Friedel-Crafts reaction product of isobutylene.Overbased metal salts have been prepared using the same overbasing process for theother detergents such as sulfonates, phenates, and salicylates. The processes developed for

production of overbased phosphonates or thiophosphonates include the use of methanol [32]and phenol [33] as the promoter. An overbasing ratio of at least 4:1 has been reported [34]

.