A guide to understanding defoamers

1. The Formation of Foam

When surfactants are present in a liquid, the molecules can adsorb onto the bubble surface. Once these molecules, oriented on the bubble surface, reach a certain concentration, they form a robust bubble wall film. The surfactant molecules adsorb at the gas-liquid interface to form a liquid film, reducing the surface tension and thereby increasing the gas-liquid contact area, which makes it difficult for bubbles to coalesce. Air bubbles have a much lower density than water, so they rise to the surface of the liquid. When the rising bubbles pass through the liquid surface, they adsorb the surfactant molecules from the upper layer of the surface. Therefore, the bubble film exposed to the air, with adsorbed surfactant, differs from the bubble film in the solution; it is covered with two layers of surfactant molecules. The adsorbed surfactant provides a protective function for the liquid film, and the hydrophobic groups of the second layer of surfactant molecules face the air, as shown in Figure 1.

2. The Harm of Bubbles

The production capacity is greatly limited: for example, in various biological fermentation processes (such as beer production, alcohol manufacturing, and the production of major antibiotics in medicine), various fermentation tanks, reactors, and cooking vessels. To prevent the occurrence of foam and avoid overflow losses, the feed ratio has to be significantly reduced.

2. Waste of raw materials and products: Due to foam, valuable or useful raw materials may overflow and be lost, resulting in obvious waste.

3. Extended reaction cycle: The presence of gases and liquids among the chemical reaction products causes gas retention due to foam, prolonging the reaction cycle and unnecessarily consuming more energy. Additionally, if foam causes over-fermentation in wine, it may alter the flavor...

4. Impact on product quality: In the textile industry, during the processes of dyeing, printing, and aqueous coating, the retention of bubbles leads to blemishes and defects on finished fabrics. In the case of pulp slurry, foam not only poses a hazard to environmental hygiene and worker health but also results in numerous holes in the finished paper, thereby causing a significant decline in product quality.

5. Not conducive to accurate measurement: In industrial processes, the presence of foam interferes with the accurate measurement of liquid level gauges, leading to measurement errors. The presence of foam in the liquid causes significant fluctuations in liquid density, which can often result in falsely high liquid levels in reactors, absorption towers, and distillation columns, causing operational imbalance and even leading to accidents.

One of the causes of environmental pollution and accidents: Due to foam overflow, it will inevitably pollute the production environment and its surrounding environment, and in some cases, even cause major accidents.

The above is by no means the entirety of the harm caused by bubbles, but it is enough to demonstrate their severity. In short, the existence of bubbles affects various sectors and aspects of the national economy. If not properly addressed, it is no exaggeration to say that "bubbles" will become a roadblock for us and a "bottleneck" in certain processes. It is reassuring that we already have good strategies for eliminating bubbles.

3. Defoaming Mechanism

To eliminate foam, methods such as settling, heating, or decompression are generally used, but these are time-consuming. In various production processes and product applications, it is necessary to quickly eliminate the generated foam, which requires the use of defoamers. Therefore, it is necessary to understand the mechanism of action of defoamers.

Theoretically, eliminating the factors that stabilize foam can achieve defoaming. The primary factor affecting foam stability is the strength of the liquid film. When a defoamer is added to the working solution, it becomes a solution, emulsion, or dispersion that adsorbs onto the foam surface. Since it has a lower surface tension than the foam, it can attract surfactants or foaming substances, reducing the surface viscosity of the foam liquid film, causing local thinning and rupture. Therefore, the main reasons for foam destruction are the thinning of the liquid film and the diffusion of gas within the foam. To prevent the reformation of foam, surface elasticity should be reduced or eliminated, that is, utilizing a brittle surface film instead of an elastic one, resulting in unstable foam.

4. Types and Characteristics of Defoamers

For defoamers, we hope to achieve both defoaming and foam suppression effects, with low solubility in water, and they should not be easily emulsified or solubilized by the working fluid. There are many types of defoamers, but generally, they can be divided into two categories: silicone-containing and non-silicone.

01

Silicone defoamer

Silicone-based defoamers have low surface tension, low solubility, good dispersion, long-lasting effect, and require a small amount, which gives them strong foam-breaking and foam-suppressing capabilities. Additionally, these defoamers are chemically inert, non-toxic, and environmentally friendly. They mainly include silicone oil, silicone oil emulsions, silicone oil solutions (silicone oil dissolved in organic solvents), and silicone oil with other fillers (such as SiO2, Al2O3), among others.

02

Silicone-free defoamer

There are the following types based on different components:

(1) Phosphate ester defoamers: They have good defoaming effects and are widely used.

(2) Alcohols, ethers, fatty acids and their esters, animal and vegetable oils or mineral oils, as well as substances like polyethylene glycol and propylene glycol: The characteristics include easy availability of raw materials and a certain defoaming effect. A small amount is used alone, but most are used in combination.

(3) Polyether-based defoamers: Adducts of alcohols and ethylene/propylene oxide. By controlling the carbon content of the alcohol and the ratio of ethylene/propylene oxide, its defoaming performance can be controlled. This type of defoamer is characterized by the ability to develop some high-temperature resistant products. Since it does not contain silicone, it will not cause contamination or oil stains on equipment walls and fabrics. Some products not only have defoaming and foam suppressing effects but also serve as functional additives in the textile industry, playing roles in penetration, washing, retard dyeing, and leveling, making them more widely applicable than silicone-containing defoamers.

Points to consider when choosing a defoamer:

1. Insoluble or slightly soluble in foaming liquid.

Lower surface tension than the foaming liquid.

3. Has a certain degree of affinity with the foaming liquid.