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John Chadwick

When the actuator is pressed on an aerosol can, the valve is opened, and a stream of liquid product travels thought the valve and exits the can. (See Fig. 1) Simple, right? Consider this - as the product stream exits the terminal orifice of the actuator, an amazing feat of physics occurs. The liquid stream is instantaneously and explosively converted into tens of thousands of discrete particles every second that the valve is held open. This metamorphosis is attributed to several factors but most notably to an immediate and violent phase change. The propellant (a hydrocarbon, such as propane, butane, isobutane, or a fluorocarbon, such as FC-152, or changes from a liquid to a gas as it exits the can. The explosive nature of this change literally blows apart the liquid stream into discrete particles. Controlling the resultant particle size is the topic of this paper, and one of the factors behind a great spray finish.

What is the Target Particle Size?

First, let's define what we mean by the term "target particle size." In metal coating applications the optimal particle size is that which allows for a uniform coating to be applied to the target surface. The film should be smooth and uniform, and thick enough to provide full coverage yet thin enough to prevent vertical running. In addition, the particle size range should be such that these attributes are consistently delivered while keeping both "fines" and "hot spots" to a minimum. In terms of particle size, I am defining "fines" as the lower end of the particle size distribution, while "hot spots" (Fig. 2) occur when the particle size is well above the average of the distribution. Too much of either is undesirable in spray coating applications.

The size of the discrete particles emitted by a self-professed package is influenced by many factors. Among these are container pressure, the type and amount of solvent present in the formula, the type and amount of propellant used, and, finally, the design of the valve system. From a practical standpoint, all of these factors can be highly customized to produce the desired effect: in this case, the preferred particle size. The control of aerosol particle size is a very complex topic. For this article I will focus on the valve systems' impact on particle size, and, more specifically, how the valve system can be adjusted to control particle size. This control is one of the keys to achieving a magnificent spray finish.

Aerosol Valve Selection

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John Chadwick is an aerosol products technical consultant who also provides a full spectrum of expert witness services including product failure analysis.

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