I. Characteristics of Antifreeze Systems and Defoaming Requirements

1. Water-based antifreeze as the mainstream

   Over 90% of antifreeze products on the market are water-based systems, mainly composed of a mixture of ethylene glycol and water. They need to be compatible with low-temperature fluidity and high-temperature stability. Foaming in such systems is mainly caused by shear from circulation pumps, air entrainment, or additive reactions, requiring the defoamer to diffuse quickly and disrupt the stability of the foam film.
2. Specificity of oil-based antifreeze

   Oil-based antifreeze (e.g., propylene glycol-based) has low corrosiveness and high-temperature resistance but is costly and has a narrow application range. It is only used in special scenarios such as turbocharged intercoolers. If 适配 (adapted for) oil-based systems, high-viscosity silicone oil (1000–1,000,000 cSt) must be selected to enhance dispersibility and durability in the oil phase.

II. Core Impact of Dimethyl Silicone Oil Viscosity

1. Low viscosity (50–350 cSt): "Pioneers" for rapid defoaming

• Advantages: Short molecular chains and strong fluidity enable rapid penetration to the foam surface, reducing surface tension and breaking the foam film. For example, silicone oil with 50–200 cSt can eliminate over 90% of foam in water-based systems within 10 seconds.
• Applicable scenarios:
  • Production and use of conventional water-based antifreeze, such as in automotive engine cooling systems.
  • High-shear environments (e.g., when circulation pumps are operating), where rapid response to foam formation is required.
• Typical models:
  • 201-100 (100 cSt): Balances defoaming speed and dispersibility, suitable for formulations with medium solid content.
  • 201-350 (350 cSt): Better viscosity stability at high temperatures, suitable for cooling systems in long-term operation (e.g., construction machinery).

2. Medium-high viscosity (500–1000 cSt): "Guardians" for long-lasting foam suppression

• Advantages: Long molecular chains and strong shear resistance allow the formation of a durable silicone oil film on the foam surface, inhibiting secondary foaming.
• Applicable scenarios:
  • High-temperature working conditions (e.g., long-term high-load operation of engines), where stable defoaming performance needs to be maintained.
  • Complex formulations with high concentrations of surfactants or additives, which require resistance to foam regeneration.
• Typical models:
  • 201-500 (500 cSt): Maintains over 70% defoaming efficiency even at 150°C.
  • 201-1000 (1000 cSt): Suitable for oil-based antifreeze or extreme working conditions, but attention must be paid to its dispersibility.

III. Industry Standards and Practical Application References

1. National standard recommendations

   According to HG/T 2366-2015 Dimethyl Silicone Oil, common industrial-grade defoamer models are 201-100 (100 cSt), 201-350 (350 cSt), and 201-500 (500 cSt). Their viscosity ranges are highly compatible with the defoaming needs of water-based antifreeze.
2. Verification with typical cases

• Automotive antifreeze: A brand’s coolant formulation added 0.05% (by mass) of 350 cSt silicone oil. In cyclic tests from -40°C to 120°C, the foam height dropped from an initial 50 mm to below 5 mm, and the defoaming efficiency remained above 85% after 100 hours.
• Industrial cooling systems: A chemical plant used 100 cSt silicone oil defoamer in its circulating water system, shortening the foam elimination time to 30 seconds and reducing annual maintenance costs by 40%.

IV. Selection Recommendations and Precautions

1. Priority recommended solutions

• Conventional water-based antifreeze: Choose 201-350 (350 cSt). It balances rapid defoaming and high-temperature stability, complying with the technical specifications of most automotive manufacturers (e.g., ASTM D3306).
• High-temperature or complex formulations: A mixture of 201-100 (100 cSt) and 201-12500 (12,500 cSt) (in a 3:1 ratio) can be used. The low-viscosity component enables rapid foam breaking, while the high-viscosity component provides long-lasting foam suppression.

2. Compatibility and process optimization

• It is necessary to test the compatibility of silicone oil with corrosion inhibitors and scale inhibitors in the antifreeze to avoid stratification or precipitation. For example, 100 cSt silicone oil has better compatibility with ethylene glycol than the 500 cSt model.
• The recommended dosage should be controlled at 0.01%–0.1% (by mass). Excessive dosage may reduce the lubricity of the cooling system or affect freezing point performance.

3. Handling of special scenarios

• For oil-based antifreeze, 201-1000 (1000 cSt) is recommended, combined with hydrophobic silica (e.g., Aerosil R972) to enhance dispersibility.
• For equipment with frequent low-temperature starts (e.g., vehicles in northern regions), 201-100 (100 cSt) is preferred. Its viscosity at -50°C is only 2000 cP, which does not affect the pumpability of the coolant.

V. Summary

The selection of dimethyl silicone oil viscosity for antifreeze defoamers follows the principle of "low viscosity for fast defoaming, high viscosity for long-lasting effect, medium-high viscosity preferred for high temperatures". For mainstream water-based systems, 350 cSt is the optimal choice for comprehensive performance. For extreme working conditions, customized effects can be achieved by mixing silicone oils of different viscosities. Finally, it is recommended to verify the selection through laboratory tests to balance defoaming efficiency and the overall performance of the coolant.

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