Fire and Water – Acetone

I was in the middle of my program “Cleaning Catastrophes” at the Parts Cleaning Expo (PCx) in Columbus. The sea of faces in the audience brightened expectantly when I began the topic of mixing acetone with water or with aqueous cleaners.  I had to squelch everyone’s enthusiasm by explaining that mixing acetone or any other flammable solvent with an aqueous cleaner is a particularly dangerous idea.  In fact, making “do it yourself” improvements in the cleaning process by blending your own cleaning agents from ingredients found around the manufacturing plant is not a good idea.

I explained that we received an inquiry from a concerned engineer who learned about the practice of mixing an aqueous cleaning agent with anywhere from 10% to 30% acetone. The mixture was then used in an ultrasonic cleaning system. The person knew that using acetone in an ultrasonic cleaner was like playing “Russian Roulette.”  After all, you have a low flashpoint material plus an oxidizer plus an ignition source. 

The Question: Is it playing “Russian Roulette” when mixing acetone and water, then using it in an ultrasonic system?

The Answer: Yes, it is playing Russian Roulette.  Do NOT try ultrasonic cleaning with acetone/water mixtures. The mixture is flammable.

Flashpoint
Acetone has a very low flashpoint. The exact flashpoint depends on the method used. For example, one SDS (previously known as MSDS) lists the flashpoint of acetone as  -20 Deg C (-4 Deg F) (closed cup) or -9 Deg C (15.8 Deg F) (Cleveland). Whatever the method, the message is clear: acetone is flammable.

Flammability is a problem even at very low concentrations of acetone. In one study, researchers determined the flashpoint of acetone/water and ethanol/water mixtures at various concentrations ranging from 5 to 100 per cent solvent by volume. Using both the Abel closed cup and the Pensky-Martens closed cup methods, they determined that while adding more water raised the flashpoint: even at 5% acetone, the mixtures were flammable. The flashpoint of a 50% solution is raised to only -12.5 Deg C and a 5% acetone solution is 29.5 Deg C, barely above common ambient temperatures. They concluded that “for most flammable products, such as acetone or ethanol, weakly concentrated aqueous solutions still remain flammable.” (1) 

Flammable limits
Acetone is one of many chemicals that have an upper and lower limit of flammability. This means there is a range of concentrations in air, 2.6 to 12.8 per cent by volume, in which acetone is flammable. The likelihood of acetone within that flammability range are too great – don’t take chances.

Intelligent Cleaning with Acetone
Manufacturers often gravitate toward the use of acetone because, aside from the unfortunate consequence that if it contributes to a fire, you, your workers, and your facility can turn crispy crunchy, acetone has certain advantages.

Acetone is an effective cleaner. It has Hansen solubility parameters that allow it to solubilize a wide range of soils. It evaporates rapidly; this is important where low residue is desirable. Of course, you have to consider materials compatibility issues; you don’t want to dissolve the product you are trying to clean.

Acetone has a relatively favorable worker exposure profile. In the U.S., acetone is exempt as a volatile organic compound (VOC) at the Federal level.  While all VOCs contribute to smog formation, acetone is considered “negligibly” reactive. Therefore, many states and air districts regulate acetone from a VOC standpoint as if it is water; this encourages the use of acetone especially for hand cleaning and bench top cleaning.  (Note: check with your local regulators for applicability in your jurisdiction).

Unlike water, acetone is highly flammable. This means you have to consider the design of the process, including selecting the right equipment. It is possible to use all sorts of cleaning processes, including ultrasonics and liquid/vapor phase degreasing – but you have to choose the correct cleaning equipment with the correct engineering controls. This means investing time, effort, and money – often lots of money – in process development.  Consider the placement of that process within your facility, including other sources of ignition from processes that are performed regularly and intermittently.

Bottom line: don’t play Russian Roulette
Treat both acetone and acetone/water mixtures like the flammable chemicals that they are.

Depending on the reference and the method used for flashpoint determination, you might find differences in the exact flashpoint cited for a particular acetone/water mixture. However, the message remains clear: even in very dilute solutions, acetone has to be treated as a highly flammable solvent. That means you don’t heat it – not even a little bit, you don’t use it in ultrasonic systems. and you keep it away from ignition sources like light switches, motors, spark producing devices, and heat sources.

Thanks to Steve Derman for his helpful comments and suggestions. 

Ref.

1. A. Janes and J. Chaineaux, “Experimental Determination of Flash Points of Flammable Liquid Aqueous Solutions.” Chemical Engineering Transactions, Vol. 31. 2013.

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