Tuesday, March 31, 2009

Critical Cleaning For Contamination Control: Water: A Starting Point

Back in October 2007, we wrote about how significant water is to life, and how hydrogen-bonding intermolecular forces enable that. Essential as it is, however, water alone isn't sufficient for cleaning work. We have to add something to it, or take something away.

Since 1974, the U.S. has had Federal standards for drinking water which have the force of law. Generally, the Safe Drinking Water Act (SDWA, 40 CFR-141) specifies what must not be found in water at all, and the maximum permissible levels at which other impurities can be found. This is a basic level of water quality to which substantial changes are made to enable it to be used in industrial operations.

Typical aqueous cleaning technology starts with water which would comply with the SDWA and then adds something. There are two reasons for this: most surfaces we want to clean have significant hydrocarbon character, and hydrocarbons aren't soluble in water.

So we have to add something which is compatible with both water and the materials we want to clean. That's a surfactant — a bifunctional molecule. An example of such is shown below.

If water weren't so cheap (well, it once was so), so commonly available (also once was so), and innocuous (to humans and the environment), we could do this cleaning work with another base fluid to which we add some bi-functional molecule.

But we can't because we don't know of such an alternate fluid. The boiling point of liquid CO2 is too low. Methanol and heptane are VOCs and flammable. HFCs are too costly. We clean metal with surfactants in water because we don't have a better fluid to which we can add something to make water attractive to what we're trying to clean.

It's just the opposite in critical cleaning. We start with water that would comply with the SDWA, and then remove molecules from it. We don't want added molecules contaminating what it is we are trying to do.

In electronics cleaning, we seek either no ions or a measured amount of ions dissolved in the water we use for cleaning. We produce at least three levels of ion-lean (deionized or DI) water using anion and cation exchange beds.

We characterize DI water by its inability to conduct electrical current — its resistance measured in Ohm-cm. Three common levels of ion-leanness are 50,000 Ohms (50 kOHM) which is easy to produce, 1,000,000 Ohms (1 megohm) which is a fairly standard product, and 20,000,000 Ohms (20 megohm) which is so hungry for ions — it will literally cut through steel to dissolve the Iron.

This cleaning (or processing) work demands a different kind of purity: absence of, at least, organisms and byproducts of disinfection. The U.S. Pharmacopeia lists a variety of types of pure water, all of which are produced starting with a product which would comply with the SDWA. Some of these are intended for special pharmaceutical purposes (including cleaning and processing), for hemodialysis, for injection (possibly into humans), for irrigation, purified for use in analytical testing as well as cleaning, and pure steam. In addition, each of these types is also produced in a version in which sterility is claimed.

Necessary production processes are, at least, filtration, distillation, and deionization.

From the perspective of those doing cleaning work, much of what we ask of water is that it not get in the way. To the contrary, we ask solvents to be the cleaning process: to wash, rinse, and dry by evaporation. That's why cleaning with water is often so difficult; one has to invent the process as well.

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