August 2012

Anhydrous formulas are not corrosive… most of the time.

Corrosion CornerHello everyone. Free liquid water is needed for metal corrosion and polymer corrosion to occur. Anhydrous spray formulas (or solvent formulas) are typically regarded as being corrosion-proof because they are essentially water-free. However, there are instances when a non-corrosive anhydrous formula with a very small amount of contaminant water becomes a voracious spray package-eater.

When is an anhydrous formula really anhydrous?
There is probably some water contamination in almost every anhydrous formula. Consequently, there is probably no such thing as a completely anhydrous formula.

What are some of the sources of water contamination in anhydrous formulas?
Water is more soluble in some propellants than others, and thus some types of propellants are a potential source of contaminant water in anhydrous formulas. For example, water is approximately 24-times more soluble in HFC 152a than in the LPG propellants, such as butanes and pentanes; and water is approximately 980-times more soluble in DME than in the butane and pentane LPG propellants.

Some raw materials could also provide contaminant water in an anhydrous formula. For example, water is soluble in both acetone and ethanol. The anhydrous versions of acetone and ethanol could absorb atmospheric water during humidity days, inadvertently introducing contaminant water into the anhydrous formula. Manufacturing equipment used to produce multiple types of

products could also be a source of contaminant water. This is particularly true when an aqueous product is manufactured prior to manufacturing an anhydrous product. Residual amounts of water left in tank bottoms, pumps, transfer pipelines, filling equipment
and storage tanks could provide water contamination in an anhydrous formula.

How much contaminant water is too much water?
There is no set concentration for all anhydrous formulas. The amount of water must be large enough to be liquid water. In addition, the water must be free-water and not bound-up as hydration water. In addition, the chemical composition of your formula determines if the free-water is or is not corrosive and what amount of free-water is needed to transform a non-corrosive formula into a corrosive formula.

How do you prevent and control water contamination?
I recommend measuring the contaminant water in the test samples from your storage test or electrochemical corrosion test during the product development and package stability testing stages. The measured water concentration in the test containers or test cells would subsequently be used as the maximum specification concentration for contaminant water in your formula.

I don’t recommend using theoretical water concentrations based on contaminant water in raw materials and/or water-of-hydration for raw materials. Hydration water can be very misleading and provide a contaminant water concentration that is significantly higher than the actual concentration, above which corrosion occurs.

In other words, corrosion testing is the most precise way to determine if there is a critical concentration of contaminant water that transforms a non-corrosive anhydrous formula into a corrosive anhydrous formula and what is the critical concentration of the contaminant water. Consequently, I recommend including corrosion studies on samples that have been spiked with water at a variety of concentrations as part of the development process for new anhydrous products and line extensions of existing anhydrous products.

Such a study helps to determine what the actual specification should be for contaminant water. The corrosion test could be either a long term storage stability test or the appropriate type of electrochemical corrosion test.

During commercial production of anhydrous formulas, the contaminant water concentration should be measured prior to packaging. The Karl Fisher titration is one method measuring water concentration in anhydrous formulas.

Manufacturing equipment, pumps, filling equipment, transfer pipelines and storage tanks should flushed with a suitable drying solvent, such as acetone or ethanol, and thoroughly dried prior to manufacturing an anhydrous formula after the equipment was used
to manufacture an aqueous one.

Please send your questions/comments/suggestions to [email protected] or visit www.pairodocspro.com for more information. Thanks for your interest and I’ll see you in September. Back issues of Corrosion Corner are available on CD from Spray Technology & Marketing.