cleaning times cleaners from the farm

cieaningtimes CLeaners From the Farm

T oday's news reports speak about global and national interest in

converting from petrochemical- based fuels (and chemicals) to materials based on "renewable resources."

This column is about three general types of cleaning agents, all derived from renewable resources: One is based on corn; another on citrus; and the third is based on soy feedstocks.

Plant-based cleaning agents are as different from petrochemical-based cleaning agents as wood finishing is from metal finishing. Yet plant-based agents offer good value in the right applications. I'll try to identify those applications, and note how each type of cleaning agent is different.

I'll also comment on why there is negligible momen tum in the cleaning industry to use renewable resources, but substantial momen- tum to use such resources to meet energy needs.

C O R N AS A FEEDSTOCK While ethanol being produced from corn for automotive fuel is a relative- ly new activity (see Figure 1), production of lactic acid-based cleaners from renewable resources is not new'

Traditionally derived from milk, lactic acid is now principally produced from corn. Wet-milling and various separation schemes convert raw corn to valuable chemicals. To produce lactic acid from corn, the first significant step is to produce a cooked and wet (hydrated) paste: corn starch.

Chemically, starches consist o f repeating lactose units. The structure of lactose, also known as "milk sugar" is shown in Figure 2.

Lactic acid is produced by fermen-

tation of lactose via specially developed bacteria. The general fermenta- tion process can be described by Equat ion 1:

Lactic acid (CH3CHOHCO2H; Carbohydrates] Bacteria r Alcobolsq + CO + Ener

or sugars a ~- [ or acids ] 2

see Figure 3) is a colorless, liquid organic acid.

By itself, lactic acid is not a useful cleaning solvent. In water, though, with surfactants, it can be an excel- lent descaling agent (vs. citric acid). Lactic acid is completely water soluble, but is not soluble in organics and is not a solvent for organic soils.

As with soy-based fatty acids (see below), the reaction of lactic acid with an alcohol produces a useful solvent for organic soils, one which is soluble in some organics. See Equation 2 (below), in which the product is an ester. Water is also produced.

A very c o m m o n lactate-based cleaning solvent is ethyl lactate, produced by the reaction of lactic acid with ethanol (see Figure 4). Here, both ingredients can be produced from agricultural materials. A vari- ety of other alcohols are also used.

Lactic acid is less useful than esters o f lactic acid because the esters have considerably more hydrogen-bonding intermolecular forces, which makes them match up better with common soils. This is shown in Table 1.

2o00 . . . . . . . .

i

"a ,

° aallll w - -

Figure 1: Conversion of corn to ethanol.

Figure 2: Molecular structure of lactose.

Figure 3: Lactic acid.

Figure4: Ethyl lactate.

ORANGE CITRUS AS A FEEDSTOCK Cleaning products, for industry or for the home, are often refined from citrus feedstocks. When citrus fruits are squeezed and juiced, rind or peel remain, from which oil can be

Equation 2 :

CH3CH(OH)CH(=O)H C H 3 C H 2 O H CH3CH(OH)CH(=O)OCH2CH 3 HOH

[ Lac cAcid ]+[ tha o, [ thy"ac a e ]+[ Water ]

www.meta[finishing.com January 2008 meta|finishing 140

cLeaningtimes pressed. When collected, this is food grade D-limonene.

Later, the rinds and peels are further processed in a steam extractor that extracts more oil from the peel or rind. When the steam is con- densed, a significant layer of oil floats to the surface in the water tank.

This is the most common method of production of D-limonene. The product is called technical grade D- Limonene, and its structure is shown in Figure 5. That is what is used by most formulators to make cleaning products.

Neat (straight) D-Limonene is water-insoluble. Miscibility with water comes only with addition of something else, often a surfactant or another solvent.

Consequently, D-limonene-based cleaners are formulated products. At least one surfactant is always present, as is a scented oil to act as an odor maskant.

Figure 5: Technical grade D-Limonene.

Figure 6: Linoleic acid, 18 carbons, 3 double bonds.

The flash point of D-Limonene is 124°F. Hi-flash hydrocarbons are often added to raise the flash point above 141°F 2 so that wastes can be classified as non-hazardous.

WOULD YOU LIKE ORANGE OR LEMON WITH THAT? Unfortunately, terpene chemistry is not simple.

Terpenes are formed in both plants and animals. All are formed by assembly of isoprene "units,"

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Figure 7: The methyl ester of linoleic acid.

each containing five carbon atoms. Limonene (C10) has two of the isoprene "units." More complex terpenes are made from three, four, five, and six isoprene "units," which are emitted by the leaves of various plants as a natural by-product of plant metabolism. At five carbon atoms per isoprene "unit," these complex terpenes have 15, 20, 25, and 30 carbon atoms. Terpenes are

© EnviroTech International 2007

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c[eaningtimes also produced synthetically in chemical reactors.

In addition to there being more than one terpene, there are also more than one limonene. The one used in cleaning products is known as D-Limonene, and reeks of its citrus parentage. The other, not used in cleaning products, is L-Limonene 3.

TOFU, BIODIESEL OR CLEANING AGENTS? Soybeans ("Glycine max"), native to Northeastern Asia, were first intro- duced into the United States in 1765. Today, the world's largest pro- ducer of soybeans is the United States, with the majority of cultiva- tion located in the Midwestern and southern United States.

Two main products are processed from soybeans: Oil and the dry solid residue known as meal. The oil con- tent of soybeans is around 20%. Soybeans are the world's largest

oilseed crop, with about 18.8 billion pounds (2.5 billion gallons) of oil produced in the U.S. from 3.1 billion bushels of soybeans in 2004.

Oil is extracted via various methods and refined to produce, among other goods, fatty acids.

Fatty acids are long-chain hydro-

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deaningtimes carbon molecules, refined from an animal or vegetable fat, oil, or wax, usually containing 16 or 18 carbon atoms. Fatty acids commonly found in soy oil include:

• Palmitic acid, 16 carbons, fully saturated;

• Stearic acid, 18 carbons, fully saturated;

• Oleic acid, 18 carbons, 1 double bond;

• Linoleic acid, 18 carbons, 2 double bonds; and

• Lino[eic acid, 18 carbons, 3 double bonds (shown in Figure 6). Fatty acids that contain more

than about ten carbon atoms are solids. Unfortunately, they are the type most found in oil derived from soybeans (or other vegetables), so fatty acids are not generally useful as cleaning agents.

As with lactic acid, fatty acids are reacted with an alcohol to produce an ester, as illustrated in Equation 2.

Methanol is most commonly used because of price.

The methyl ester of linolenic acid is shown in Figure 7.

These structures may appear "ungainly," but they are quite versa- tile. This is because they combine two distinctly different functional structures: (1) the acid/ester group, and (2) the long chain hydrocarbon. This duality allows compatibility with soils containing both types of s t r u c t u r e s .

The same effect of esterification on solubility parameters is shown in Table 2. Note that this is not the same effect as shown in Table 1.

The reason is that the properties of the methyl ester of linolenic acid (a significant component of the commercial product called "methyl soyate") are much more determined by the long hydrocarbon chain than they are by the ester or acid group.

DISTRESSED PRODUCTS? 4 None of these three types of cleaning

agents, refined from corn, citrus, and soy feedstocks, are the primary products for which these feedstocks were planted.

While a farmer may grow corn to produce ethanol as a motor fuel, he doesn't plant corn to produce ethyl lactate for cleaning of metal compo- nents. That farmer would have con- siderable acreage lying fallow!

The same is true for chlorinated solvents. A reason that their price is less than $1/pound is that chlorinated solvents are primarily produced as feedstocks from which other chemicals are produced.

That their purity depends upon their source and their method of production is not true for chlorinated solvents. However, that is true for citrus- and soy-based cleaning solvents. Soy-based products are: • Mixtures of all the acids (and their

esters) listed above. Methyl "soyate" is not a pure material. It is a mixture of esters.

• Produced from different sources

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c[eaningtimes and types of soybeans. The dis- tribution of soy acids and esters between sources wi[[ be different (somewhat).

Both of the above are true for many terpene-based cleaners. The refined Orange Oil, from which their active ingredient is produced, is a blend of terpenes whose structures and molecular weights are similar but somewhat different. Terpenes of higher molecular weight are often left on cleaned parts as non-volatile residue.

Terpenes can be directly synthe- sized in chemical reactors. These materials are more expensive than the product from renewable resources, but their composition is less variable and more secure. This author believes that terpene-based cleaners based on renewable resources are less desirable for this reason.

Cleaning agents used in precision cleaning work are not likely to be produced from renewable resources,

due to concerns about reproducibil- ity of cleaning quality.

This situation does not occur for cleaners based on esters of lactic acids. These products, like chlorinated solvents, are single chemicals of high purity. That's why ethyl lactate is used in electronics cleaning operations, but orange-based cleaners seldom are.

PHYSICAL PROPERTIES Values are given in Table 3. Several aspects deserve consideration: • None of these three cleaning

agents derived from renewable resources is suitable for vapor degreasing. Their boiling points are too high.

• None is without concern for flammability.

• None has the values of solubility parameters which match chlorinated solvents (18; 3; 5).

• None has the low surface tension values which match fluorinated solvents,

CLEANING APPLICATIONS Data are available s. The data were developed in an unbiased and scien- tific manner, and allow comparison between cleaning solvents among soils. They are not predictive of quantitative performance in a cleaning machine, because mechanical force and temperature are not exploited as process elements. They should not be used to propose that one cleaning solvent (or biological source) is superior (or not) to another, because that depends upon the specific details of the cleaning job, the equipment available, the staff who will do it, and other factors.

SUMMARY It's this author's opinion that the source of feedstock should not matter in the selection of a cleaning agent. One should choose the one best-matched to the application, mainly by matching Hansen Solubility Parameters (HSP).

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deaningtimes issue affecting choices, this column suggests three major choices and points out how they are different by feedstock source and properties.

NOTES 1. In the interest of full disclo- sure, I must note that in 1993, one of my first professional con- suiting assignments was develop- ment of opportunities for use of cleaning agents derived from lactic acid. I currently have no professional involvement in develop- ments involving cleaning agents derived from lactic acid. 2. 141° F is the flash point temperature at which wastes having that value are not considered hazardous. 3. L-Limonene is found in several pine needle oils, some turpentine oils, star anise, peppermint, and spearmint oils. L-limonene is widely used in perfumery and in citrus fragrances, to give lift and freshness. It is also used as a fla- vor (cloves, lemon, mint) and fra- grance additive in food, house- hold cleaning products, and per- fumes. Its odor is pleasant and mild. The reason L-Limonene is not used in cleaning products is that it is less commonly available, and, therefore, more costly. 4. A distressed product is one that is produced in error but is still useful for some service, or inten- tionally or otherwise produced in excess of the current need. 5. See Marshall, J. and Wilcox, H., "Bio-Based Solvents... How Well Do They Work? Considerations When Choosing Alternative Solvents," CleanTech Magazine, October 2003. p 17-23. This material was also presented by the authors at the ASM International Surface Engineering Congress and Exhibition, Orlando, Fla., August 2004.

John Durkee is the author of the book "Management of Industrial Cleaning Technology and Processes," published by Elsevier (ISBN 0-0804.48887). He is an independent consultant specializing in

metal and critical cleaning. You can contact him at P.O. Box 847, Hunt, TX78024 or 122 Ridge Road West, Hunt, 7X 78024; (830) 238-7610; Fax (612) 677-3170; or (e-mail) jdurkee@precisioncleaning, com.

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