Antifoams & Defoamers.
Silicone anti-foam / de-foamer products are used as process aids in the manufacturing process where foaming is prevalent. Foam is generated by agitation of liquids through, stirring, shaking, boiling, mixing, evacuation and filling under high speed conditions. The presence of foam can result in: loss of capacity, as foam displaces product; inefficient mixing and pumping; separation; excessive wastage of product and increased processing time.
What is foam?
Before we look at the solution, we need to have an understanding of the problem – the foam itself! In simple terms, foam is a mass of bubbles that is created when a gas is dispersed into a liquid and this dispersion becomes stabilised. Foam is often difficult to break down because it is stabilised by the surface elasticity and surface viscosity of its film. In a pure liquid, foam usually will not form but this is rarely the case in any system and the presence of surface active agents or surfactants are often responsible for the stabilization.
When gas bubbles form, they take on a spherical shape and will begin to rise to the surface. As they emerge, a liquid lamella is formed – the liquid film that separates the bubbles. It is in this film that the surfactant and/or foam stabilizing molecules will orientate themselves. The resulting intermolecular forces between these stabilizing molecules result in the stabilization of the film and thereby the bubble itself. As liquid drains through the lamella the foam film thins and the bubbles become more tightly packed and take on polyhedral shapes. Without the stabilization, the lamella will continue to thin and eventually will rupture (which would occur in a pure liquid).
What is anti-foam?
An antifoam is an additive designed to prevent the formation of foam. Similarly, a de-foamer is used to remove the foam that has already formed in the system. In many instances, the same additive will perform both functions – removing the foam that is present upon its addition and then preventing any further foam from forming.
There are many different types of antifoams but they are all essentially composed of a synergistic mixture of hydrophobic solids and liquids. The liquid needs to have some degree of incompatibility with the foaming medium and have a lower surface tension than the system it is added to. For use in aqueous systems, an antifoam is usually applied in the form of an emulsion or a dispersion containing a surfactant system, which allows emulsification of the hydrophobic components into the medium. This is generally not required for applications in non-water based systems and can simply be the hydrophobic liquid component with or without the hydrophobic solid.
How does an antifoam work?
The basic principle behind the activity of an anti-foam is the destabilization of the film that forms the bubble surface, causing the foam to collapse.
This is achieved by the hydrophobic fluid component of the antifoam having a lower surface and interfacial tension than the foaming medium, which enables them to flow easily over the bubble film. They then occupy the spaces between the foam stabilizing molecules at the liquid-air interface within the system. The hydrophobic solid component in the antifoam assists in the droplets entry into the foam film more easily. They also help increase the penetration depth of the droplet which in turn facilitates the bridge formation, which is explained below.
The major theories of anti-foam action are bridge de-wetting, where the oil droplet enters the foam film and as this thins, the drop forms into a lens shape and bridges the opposite surfaces. De-wetting then occurs through capillary action that draws the film away from the oil droplet and results in the film rupturing. The alternative theory is bridging-spreading, where that the oil droplet forms the bridge as before but then thins over time and eventually ruptures at its thinnest point, causing the film to rupture. Both are depicted in the diagram beside:
Silicone based antifoam:
One of the most widely used base fluids for antifoam manufacturing are silicone fluids, which consist of a liquid polymerised siloxane (Si–O–Si linkage) backbone as the core structure and may contain different functional groups. Although initially more expensive than other base materials, silicones have proven to be more efficient and thereby more cost effective, often being more active at much lower dosage rates than organic based antifoams.
Silicone fluids are chemically inert, which results in less odour and residue in the systems in which they are applied. This is particularly amenable for use in the food and beverage industry where taste will not be effected. The low surface tension of the silicone fluid base aids in more rapid destruction of the foam and their insolubility in a wide range of systems makes them suitable for use in a much wider range of applications. The non-volatile nature of silicone also prevents evaporation at high temperatures and they are known to be stability over extremes of pH.
Despite all the benefits of silicone based antifoams, there are certain circumstances where silicone cannot be used due to restriction or limitations of the production process. In these cases an alternative, organic/ mineral oil based antifoam, would be needed.
Why use anti-foam?
The existence of foam will causes several problems in any process where it is present. If not controlled, foam can reduce production capacity, by occupying production volume; increase processing time; decrease cleaning efficiency or increase cleaning time; can overflow vessels causing loss of product or possible contamination and could even result in damage to production equipment by clogging filters or distribution lines. These all will ultimately result in increased operational costs through maintenance or the need for higher-capacity equipment to compensate for the loss through the foam.
Foam can be often be controlled by making changes to the process itself, however, chemical foam control agents are the most versatile, effective and economical solution to the foaming problem. Furthermore, the introduction of foam can be caused by many of the essential stages during the production process, making it difficult to alter the production method and eliminate the foam formation.
As such, the use of anti-foam will eliminate or reduce the foam in the system to acceptable levels. In doing so, this can effectively increase the capacity of mixing tanks by reducing the volume that would otherwise be occupied by foam (meaning more space for product). Reduction in waste and increase in production efficiency can also be achieved with the use of anti-foams, along with reduced production times (by eliminating holding times for foam removal from products). In the end, this all comes down to saving you money and helping produce a better end product.
Antifoam and de-foamer products are often interchangeable but generally there is a functional difference:
1. A product acts as an antifoam when it is added to a system prior to foam formation and serves to prevent the generation of foam, i.e. application by pre-dosing.
2. A product acts as a de-foamer when it is added to a system after foam has been generated