[Insecticide Application] Insecticidal Foams Require Knowledge and Precision

Author’s note: The following was excerpted from The Service Technician’s Field Manual. Learn more about the book on page 108.

The application of foam insecticides requires precision equipment and a knowledgeable applicator. The technology behind the equipment is based on the mechanics of pressure, nozzle flow rate and spray pattern, and droplet size. These features are interrelated, and they influence the delivery, surface coverage and performance of an insecticide.

Foam is typically used to apply insecticide to uneven or difficult-to-reach surfaces, including vertical surfaces. Foam can flow like a liquid, remain in place like a solid and disappear in a short period of time. It is a material in which a gas, usually air, is dispersed in a matrix of bubbles that are separated from each other by a thin film of liquid. The liquid film drains to the surface, and the bubbles become thinner and finally collapse.

Composition. Foams are typically 95 percent air and 5 percent liquid. The liquid content consists of soap surfactants and insecticide active ingredient. The amount of surfactant controls the amount of foaming that takes place in a liquid. The soap surfactants reduce the surface tension, which permits the liquid to accept the air that will produce the foam.

There is a limit to the solubility of surfactant molecules in a liquid. Once the limit is reached, the surface tension can no longer be decreased. The continued addition of surfactant to the original liquid will not increase the amount of foam. Adding more foaming agent does not continue to increase the amount of foam or the expansion ratio of liquid to foam.

The addition of a foaming agent to an insecticide should begin with the lowest recommended amount, which is typically 1 to 2 ounces of foaming agent per finished gallon of insecticide. Increase the amount of foaming agent only as needed.

Expansion Ratio. The expansion ratio is the amount of foam produced by a specific volume of liquid; ratios can range from 5:1 to 35:1. The liquid-to-foam ratio is determined by the equipment, the insecticide formulation and the foaming agent. Foam application equipment is specifically designed to introduce a certain amount of air into the liquid stream, and to move the foam through the delivery hose to the nozzle. Changing any part, such as the length or diameter of hose, valve or nozzle, will likely change the foam.

Because of the active ingredient and solvent system of modern insecticides, some will produce foam more readily than others. This cannot be predicted, but must be determined on a product-by-product basis. Similarly, commercial foaming agents have a specific combination of ingredients, each product is distinct, and each may produce more or less foam with a particular insecticide.

Wet Foam and Dry Foam. Water-based foam is an effective means of delivering household and wood-protection insecticides as the foam will spread around objects and into crevices that conventional liquid sprays would not reach.

The foam is created by adding a foaming agent to the finished dilution of insecticide, then mixing air with the liquid before it is sprayed. Combining air and liquid produces the foam. The combination and quantity of air and surfactant in a liquid produce two types: “wet” foams and “dry” foams.

• In wet foam, the liquid content is high, and the walls of the individual bubbles are thick and have a spherical shape.
• In a dry foam, the liquid content is low and the walls of the bubbles are thin and have an angular shape.

As gravitational forces drain the liquid from the walls of the bubbles, wet foams become dry foams and eventually collapse. Dry foam is generally considered a foam-to-liquid ratio of 25:1 or more.

Regardless of what type of foam you ultimately select, it’s a formulation that offers numerous benefits in those situations where it is appropriate.