The equipment requiring vacuum systems can be primarily categorized into three types: those needing 'coating', those requiring 'weight reduction', and those requiring 'purification'.

1. Materials requiring surface coating: Inorganic flame retardants such as aluminum hydroxide (ATH), magnesium hydroxide (MH), hydrotalcite, and zinc borate... These inorganic powders demonstrate excellent flame-retardant properties at low cost, yet they suffer from a critical limitation: their incompatible compatibility with plastics and rubber.

Their surfaces are polar, while the polymer substrate is non-polar. Forcing them together is like adding water to oil—completely incompatible, resulting in materials with disastrous mechanical properties.

What to do? Dress them up—perform surface treatment with coupling agents.

This "coating" process is frequently conducted under vacuum conditions. As repeatedly referenced in "Formulation Design and Processing of Flame Retardant Polymer Materials", the "vacuum mixer" or "vacuum kneader" is specifically designed to uniformly coat coupling agents onto powder surfaces under negative pressure, while simultaneously removing adsorbed air and moisture from the powder's pores to achieve a denser coating layer.

2. Phosphorus Microencapsulation for "Weight Loss": Red phosphorus is an effective flame retardant with high efficiency and low addition requirements. However, it has a troublesome drawback: hygroscopicity, self-ignition tendency, and emission of highly toxic phosphine gas.

How to address this? By implementing 'weight reduction' —microencapsulation.

During the development of vacuum equipment, numerous specialized books have been published detailing techniques such as vacuum evaporation coating and vacuum sputtering. These devices can uniformly deposit a layer of polymer or inorganic material (e.g., aluminum hydroxide, phenolic resin) on the surface of red phosphorus particles, effectively encapsulating this "time bomb" with a protective barrier.

Only in vacuum environment can the coating material be deposited uniformly in molecular state and form a dense and non-porous coating layer, which can be truly "isolated from the world".

3. Requirements for "purification": Refining of organic flame retardants, including bromine-based flame retardants (e.g., decabromodiphenyl ether) and phosphorus-based flame retardants (e.g., phosphate esters), which are mostly high-boiling-point organic compounds. After synthesis, they often contain impurities such as unreacted monomers, by-products, and catalyst residues.

How to purify?

Conventional distillation? The temperature is too high, causing the flame retardant to decompose directly.

In such cases, molecular distillation equipment or vacuum distillation apparatus must be employed. As clearly explained in "Vacuum Engineering Design," under extremely high vacuum conditions, the boiling point of substances is significantly reduced. Substances that previously required 300°C to vaporize can now be "reassuredly released" at 150°C, thereby purifying the product while maintaining thermal stability.