Silicone defoamer: Advantages, Disadvantages, Main Classifications, and Performance

Advantages of silicone defoamers:

Wide range of applicationsDue to the special chemical structure of silicone oil, it is neither miscible with water or substances containing polar groups, nor with hydrocarbons or organic substances containing hydrocarbon groups. Because of the insolubility of silicone oil in various substances, it has a wide range of applications and can be used for defoaming in both aqueous and oil-based systems.

2) Low surface tension:Silicone oil generally has a surface tension of 20-21 dynes/cm, which is lower than that of water (72 dynes/cm) and most foaming liquids, resulting in good defoaming performance.

3) Good thermal stability: Taking commonly used dimethicone as an example, it can withstand 150℃ for a long time and over 300℃ for a short time, with its Si-O bonds remaining unbroken. This ensures that silicone antifoaming agents can be used in a wide temperature range.

4) Good chemical stability. Due to the stability of Si-O bonds, silicone oil possesses high chemical stability and is hardly reactive with other substances. Therefore, provided the formulation is reasonable, silicone defoamers are permitted to be used in systems containing acids, bases, and salts.

5) Physiological inertia:Silicone oil has been proven non-toxic to humans and animals, with a median lethal dose (LD50) greater than 34 g/kg. Therefore, silicone antifoams (formulated with suitable non-toxic emulsifiers, etc.) can be safely used in the food, medical, pharmaceutical, and cosmetic industries.

Strong defoaming ability. Silicone defoamers can not only effectively break down existing foam, but also significantly inhibit and prevent foam formation. They are used in very small quantities; adding only one part per million (1 ppm) by weight of the foaming medium can produce a defoaming effect. Their common usage range is 1 to 100 ppm. They are not only low in cost, but also do not contaminate the defoamed substances.

Disadvantages:

1) Polysiloxane dispersion is relatively difficult: Polysiloxane is poorly soluble in water, hindering its dispersion in aqueous systems. Dispersants must be added. If too much dispersant is added, the emulsion will be stable, but the defoaming effect will be reduced. Emulsifiers must be used sparingly to achieve a good defoaming effect while maintaining emulsion stability.

2) The oil solubility of silicone reduces its defoaming effect in oil-based systems.

3) Poor long-term high temperature and strong alkali resistance.

Silicone Defoamer Classification and Performance

Oil-based defoamer (oil paste type)

Oil-based defoamers use silicone oil directly as the defoaming agent. They are mainly used in oil-phase foaming systems where dispersants and emulsifiers are not allowed. It is generally believed that silicone oils of different viscosities exhibit different defoaming effects on the same foaming system. Low-viscosity silicone oil defoamers have a fast defoaming effect, but the persistence is poor, while high-viscosity silicone oil defoamers have a slow defoaming effect, but the persistence is better. Paste-like defoamers are defoamers in a paste form made by adding fine powders such as silicon dioxide and aluminum trioxide to silicone oil, and are mainly used for defoaming in non-aqueous systems.

The defoaming effect of dimethyl silicone oil depends on its dispersion in the foaming system. The higher the dispersion, the better the defoaming effect. To improve the dispersion, mechanical stirring can be used, such as high-speed stirring under heating, or a uniform dispersion of silicone oil and mineral oil can be prepared in advance using a colloid mill. Alternatively, the silicone oil can be pre-mixed with fine powders like carbon black to form a paste-type defoamer, utilizing the dispersing action of the fine powders like carbon black.

Solution defoamer

Silicone solution defoamers can be made by dissolving dimethyl silicone oil in a suitable solvent. Silicone defoamers formulated with organic solvents such as polychloroethane, toluene, and xylene can be used for defoaming in oil-soluble media. Silicone defoamers formulated with water-soluble organic solvents such as hexanediol and glycerol can be used for defoaming aqueous solutions. The preparation process of solution-type silicone defoamers is very simple and convenient to use. Silicone oil is dispersed in the foaming liquid with the help of organic solvents to achieve defoaming. At the same time, some solvents also play a certain defoaming role during the diffusion process. However, this method is not widely used, mainly because large-scale use will increase the cost of defoamers and may also cause environmental pollution.

Emulsion-type defoamer

Silicone oil emulsions are produced by dispersing silicone oil or silicone paste in water under vigorous stirring or with the aid of emulsifiers. This effectively improves the dispersibility of silicone oil in aqueous phases, making it widely used as a defoamer in water-based systems. Emulsion-type defoamers are the most widely used and applied type of silicone defoamers.

Emulsion-type silicone defoamers are generally formulated from dimethyl silicone oil, emulsifiers, emulsion stabilizers, and deionized water. The emulsifiers used are mainly low-foaming nonionic emulsifiers, such as Span, Tween, and polyethylene glycol. Mixed emulsifiers perform better than single emulsifiers. The particle size of the silicone oil emulsion is the most important control index for emulsion-type silicone defoamers. To obtain a silicone oil emulsion with high defoaming efficiency and good storage stability, the particle size is usually required to be less than 10 μm.

Therefore, in addition to selecting suitable emulsifiers and mixing and grinding process conditions, thickeners such as polyvinyl alcohol and methyl cellulose can also be added to increase the viscosity of the continuous phase.

When using emulsion-type silicone defoamers, it is important to first understand the temperature and pH of the foaming system, as these will affect the stability of the emulsion and may even lead to demulsification.

Although emulsion-type silicone defoamers have drawbacks such as difficulty in long-term storage, easy stratification, and easy deterioration, they are still popular with users due to their advantages of convenient use, wide applicability, significant defoaming effect, and moderate price. Furthermore, with the advancement of process technology, emulsion-type silicone defoamers still have considerable development potential.

Solid defoamer

Solid silicone antifoams are characterized by good storage stability, ease of transportation, and convenience of use. They can be used not only in the production of non-foaming and low-foaming laundry detergents, but also in other applications that require defoaming.

There are three methods for preparing solid silicone antifoams: (1) directly dispersing silicone oil on the surface of a solid carrier; (2) melting silicone oil together with substances with low softening points such as fatty alcohols, fatty acids, fatty amides, fatty acid esters, or paraffin wax, and then adhering the melt to the surface of a solid carrier; (3) mixing silicone oil with a film-forming substance, so that the film-forming substance encapsulates the silicone oil antifoam components to form dispersed solid particles.

Modified silicone defoamer

Modified silicone oil, which introduces hydrophilic polyether segments into methyl silicone oil, can be transformed into a self-emulsifying defoamer. This allows it to maintain dispersion in foaming systems for an extended period, prolonging the lifespan of the defoamer. Currently, commonly used modified silicone oils for defoamers include polyether silicone oil, fluorocarbon silicone oil, and long-chain alkyl silicone oil, all of which are attracting significant attention due to their applications and effectiveness.

End-terminated, side-chain, and branched polyether silicone oils can all be used as defoamers. They all have a distinct cloud point, exhibiting strong defoaming properties only above the cloud point, while promoting foaming below it. Furthermore, leveraging the strong emulsifying ability of polyether silicone oils, they can be compounded with silicone paste to self-emulsify in water systems, forming stable and highly effective defoamers. This has been widely used for defoaming in high-temperature dyeing of polyester fibers, various lubricating oils, cutting oils, antifreeze, and strongly acidic systems.

Long-chain alkyl silicone oil defoamers can be used in fields such as fermentation, food processing, medical treatment, textiles, petroleum, synthetic rubber and resins, coatings, and inks. Due to their good affinity for organic materials, long-chain alkyl silicone oil defoamers do not affect the post-processing performance of products. However, their surface tension increases with the increase in carbon chain length, and the defoaming performance decreases accordingly, so it is important to choose the appropriate carbon chain length. Long-chain alkyl silicone oils are used in defoaming in both non-aqueous and aqueous systems.

Fluorosilicone oils have lower surface tension than other silicone oils. Therefore, antifoams formulated with fluorosilicone oils are more effective at reducing the surface tension of various foaming systems, outperforming other silicone oil antifoams. They are primarily used in non-aqueous foaming media where methyl silicone oil or methyl phenyl silicone oil have high solubility, such as aliphatic hydrocarbons, aromatic hydrocarbons, and naphtha.

Silicone defoamers, with their excellent defoaming and foam-inhibiting properties, low dosage requirements, good chemical inertness, and ability to perform under harsh conditions, have experienced rapid development in recent years. New varieties and models suitable for different application environments are constantly emerging, and their application areas are continuously expanding. In the future, new high-efficiency silicone defoamers with strong applicability and the ability to further improve product quality and equipment utilization will be further developed, especially high-performance polyether-modified silicone defoamers and emulsion-type defoamers, which will hold a major position in the market.