Defoamers: Antifoam Selection Criteria
Foam is generated in most commercial processes. Some of the more common processes are food processing, chemical manufacturing, fermentation, textile, adhesive manufacturing, printing inks, paints, coating and resins, wastewater management. If it were not for a defoamer, the objects that surround our daily lives either may not be possible or would be at such a high cost, they would probably not be obtainable by the average consumer.
Defoamers are used to either control foam formation or eliminate foam from forming during the intended process. When the formation of foam is prevented, the chemical is usually referred to as antifoam. Most often, the terms defoamer and antifoam are used interchangeably.
Foam can be generated by either mechanical agitation or through a chemically influenced mechanism, such as the fermentation process, for example. Defoamers are manufactured and engineered to work in specific environments. Environments in which defoamers are expected to work could include temperature, pH, solubility or its insolubility, under pressure, or chemical constituent’s compatibility. Defoamers may even be required to meet certain regulations such as The Food and Drug Administration Code of Federal Regulations, EPA, National Science Foundation, Kosher or Kosher for Passover Certification, or just be used for industrial applications meeting local, state, and federal guidelines. Depending on each state or country, the laws are specific and govern the defoamer’s selection and application.
Foam formation is the result of dissolved molecules in a liquid. The dissolved molecules alter the surface tension of the liquid and can be viewed as surface active agents (surfactants). The surfactants can be nonionic, cationic, anionic, or amphoteric. The liquid can be either aqueous, nonaqueous, or both (some industrial systems may contain dissolved organics which require special consideration). Different surfactants will generate different types of foam and foam stability. When agitated, bubbles will form, which will immediately encounter gravitational effects pulling liquid along the bubble walls back down into the liquid beneath the bubble. A simplified picture of a bubble can be described as spherical, having both an outer wall and inner wall.
Nonionic surfactant-generated foam is generally depicted as having a hydrophobic head (water-insoluble portion) at the air-liquid interface, and the hydrophilic tail (water-soluble portion) at an aqueous solution. Its orientation would be reversed in nonaqueous liquids.
Anionic surfactant-generated foam would have a negative charge on the hydrophilic tail. As aqueous liquid is pulled down over the bubble’s surface, the negative charges reach a concentration at the bubble at the liquid interface. Most often the negative charge serves to stabilize the bubble and will begin to repel each other at the interface. This phenomenon is known as an electrostatic repulsion.
Cationic surfactant-generated foam would have a positive charge on the hydrophilic tail and would exhibit similar behavior as the anionic surfactant in an ideal aqueous liquid.
When the surface tension is high enough, bubble formation becomes more rigid and stable. If a bubble is subjected to mechanical agitation, bubbles caused by entrained air would form very stable lamellar structures. The Marangoni effect is a major stabilizing factor in foam and is driven by osmotic pressure. In some cases, the aqueous liquid is being pulled through the bubbles’ walls creating regions of low and high surfactant concentrations, which sets up a gradient along the bubbles’ surface. The gradient would pump liquid back onto the bubble walls, where this phenomenon is referred to as a surface transport.
The bulk viscosity also contributed to foam stability. As the viscosity of liquids increase, entrained air, now a bubble, can be trapped below the liquid’s surface. Increasing viscosity of the system also reduces the coalescence capability of smaller bubbles merging to become larger bubbles. If the bubbles become large enough (increasing the diameter), bubble stability decreases. The surface viscosity is also important, as it affects the coalescence formation between bubbles. The higher the bulk viscosity becomes, the lower the coalescence formation is between bubbles.
When the surface tension is lowered on the bubble, it will burst. The resulting interaction of the defoamer to disperse the foam and its bubble formation is a physical interaction with the aqueous liquid. We engineer defoamers to work with specific applications and their systems. We use a checklist to describe the criteria in which form is generated and from this checklist, a defoamer selection is made to begin testing.
DESIGN AND SELECTION (Physical Parameters of the Foam in the System:)
What is the pH?
What is the temperature?
What is the viscosity?
What is the solids content?
What is the volatile content?
What is the dissolved organic content, or chemical composition of the foam? Exactly which type of process is generating foam?
PROCESSING PARAMETERS OF THE FOAMING SYSTEM
What is the flow rate of the liquid?
Is the foam mechanically generated?
Is the foam chemically produced?
Is the foam being generated during the application of your product?
How will the defoamer be introduced into the foaming system?
What is the starting medium of the process involved?
Are there any Kosher or Passover requirements?
Are there any FDA food clearances?
Are there any EPA or other environmental restrictions?
Are there any undesirable ingredients, for example, silicone?
Is there any chemistry incompatible with your process/product?