During industrial production, numerous harmful foams are generated, necessitating the addition of defoamers. There are many types of defoamers, including organosilicon compounds, polyethers, silicone–ether grafts, and those containing amines, imines, and amides. These defoamers typically exhibit faster defoaming action, longer foam‑suppressing duration, and broader compatibility with various media, even under harsh conditions such as high temperatures, strong acids, and strong alkalis. They are widely used to eliminate unwanted foam in processes across industries like latex production, textile sizing, food fermentation, biopharmaceuticals, coatings, petrochemicals, papermaking, and industrial cleaning. 1. Natural Oils (e.g., soybean oil, corn oil) Advantages: readily available, low cost, and easy to use. Disadvantages: if improperly stored, they can deteriorate, leading to an increase in acid value. 2. Higher Alcohols Higher alcohols are linear molecules with strong hydrophobicity and weak hydrophilicity, making them effective defoamers in aqueous systems. In the early 1970s, Soviet researchers conducted experiments in aqueous solutions of anionic, cationic, and nonionic surfactants, demonstrating that the defoaming efficacy of alcohols correlates with their solubility and diffusion rate within the foaming liquid. Alcohols with carbon chain lengths of C7–C9 are the most effective. Higher alcohols ranging from C12 to C22, when formulated into water-in-oil emulsions with appropriate emulsifiers—typically at particle sizes of 4–9 μm and concentrations of 20–50%—serve as efficient defoamers for aqueous systems. Additionally, certain esterified derivatives, such as phenethyl oleate and phenethyl laurate, exhibit defoaming properties in penicillin fermentation; the latter can also function as a precursor. 3. Polyether Defoamers A wide variety of polyether-based defoamers exist, including: a. GP‑type defoamers: These are synthesized by adding propylene oxide or a mixture of ethylene oxide and propylene oxide to glycerol as the starting agent. Due to their poor hydrophilicity and limited solubility in foaming media, GP‑type defoamers are best suited for dilute fermentation broths. Their superior foam‑suppressing capability makes them ideal for incorporation into basal media to curb foam formation throughout the entire fermentation process. b. GPE‑type defoamers: Also known as “foam killers,” these are produced by further adding ethylene oxide to the terminal polypropylene glycol chains of GP‑type defoamers, yielding polyoxyethylene‑polyoxypropylene glycerols with hydrophilic end groups. Depending on the degree of ethylene oxide addition—10%, 20%, … up to 50%—they are designated as GPE10, GPE20, … GPE50. GPE‑type defoamers possess better hydrophilicity, spread easily in foaming media, and demonstrate strong defoaming power; however, their higher solubility results in shorter active durations, making them more effective in viscous fermentation broths. c. GPES‑type defoamers: A newer class of polyether defoamers features hydrophobic stearate end groups on the GPE‑type polymer chains, creating a block copolymer structure with hydrophobic ends separated by hydrophilic segments. This molecular architecture facilitates flat, layered aggregation at the gas–liquid interface, enhancing surface activity and improving defoaming efficiency. 4. Silicone‑Based Defoamers The most commonly used is polydimethylsiloxane, also referred to as dimethyl silicone oil. It exhibits low surface energy and surface tension, with limited solubility in water and most oils yet high activity. Its backbone consists of siloxane linkages, rendering it a nonpolar molecule. It shows poor affinity for polar solvents like water and minimal compatibility with common oils. With low volatility and chemical inertness, it is relatively stable and exhibits low toxicity. Pure polydimethylsiloxane, without dispersion treatment, is difficult to employ as a defoamer, likely due to its high interfacial tension with water and low spreading coefficient, hindering uniform distribution in foaming media. Therefore, silicone oil is often blended with SiO₂ aerosols to form composite materials—specifically, dispersing treated SiO₂ aerosols into dimethyl silicone oil and subjecting the mixture to controlled temperature and time treatments to produce a usable defoamer. Organic silicon defoamers are prepared by mechanically emulsifying silicone grease, emulsifiers, water repellents, thickeners, and other ingredients with an appropriate amount of water. They are characterized by low surface tension, high surface activity, strong defoaming power, minimal dosage requirements, and low cost. They are immiscible with water and most organic substances, effectively suppressing foam across a wide range of media. They also offer excellent thermal stability, functioning reliably over a broad temperature range of 5°C to 150°C, and good chemical stability, resisting reactions with other substances. With proper formulation, they can be used in acidic, alkaline, or saline solutions without compromising product quality. Furthermore, they display physiological inertness, with an LD50 of 250 g/kg in rodents, making them suitable for food and pharmaceutical applications. Capable of both suppressing and breaking foam, they fall under the category of broad-spectrum defoamers. They are extensively employed in detergents, papermaking, pulp processing, sugar refining, electroplating, fertilizer production, auxiliary agents, and wastewater treatment. In the petroleum industry, they are widely utilized for desulfurization of natural gas and accelerating oil–gas separation, as well as for controlling or suppressing foam in equipment involved in ethylene glycol drying, aromatic hydrocarbon extraction, asphalt processing, and lubricant dewaxing. In the textile sector, they aid in defoaming during dyeing, scouring, and sizing processes; in the chemical industry, they serve to suppress foam in resin synthesis, latex production, coating, and ink manufacturing; and in the food industry, they help manage foam in various concentration, fermentation, and distillation operations. Silicone grease can be applied directly to pot walls, outlets, or metal screens for defoaming purposes. Alternatively, it can be formulated into solutions for oil-phase defoaming, or mixed with low-viscosity silicone oil to create water-in-oil emulsions suitable for multiple aqueous systems. In medicine, it is frequently used preoperatively and before X-ray or gastroscopic examinations to relieve abdominal bloating or gastric distension. Defoamers can generally be categorized into two types: those that eliminate existing foam, such as ethanol; and those that inhibit foam formation, such as emulsified silicone oil. In China, approved defoamers include emulsified silicone oil, higher alcohol–fatty acid ester complexes, polyoxyethylene–polyoxypropylene pentaerythritol ethers, polyoxyethylene–polyoxypropylene amine ethers, polyoxypropylene glycerol ethers, and polyoxypropylene compounds. 5. Polyether‑Modified Silicon Combining the advantages of polyethers and organic silicon defoamers, this product is non‑toxic, harmless to microbial strains, requires only minimal addition, and offers excellent cost‑effectiveness.