Inorganic Flame Retardants

Inorganic flame retardants can also be used in polyurethane foams. Common examples include antimony trioxide, aluminum hydroxide, borates, and ammonium polyphosphate. However, inorganic flame retardants are typically in the form of particulate powders. The finer the powder, the better it is for foam applications. Certain mechanical foaming processes cannot utilize foaming materials containing solid powders, which limits the application of inorganic flame retardants in polyurethane foams.

(1) Antimony Trioxide

Antimony trioxide, also known as antimony oxide or white antimony, has the molecular formula Sb₂O₃. It appears as a white powder with an average particle size of 1–3 μm, a relative density of 5.2–5.7, and a melting point of 652–656°C. It is insoluble in water, alcohol, and organic solvents but soluble in concentrated acids and strong alkaline solutions. When used alone, antimony trioxide has a low flame-retardant effect. It must be combined with halogenated flame retardants to achieve better performance. During polymer combustion, it decomposes to form a dense gas protective layer on the material's surface, isolating oxygen. Additionally, antimony trioxide reacts with halogenated compounds during combustion to produce antimony halides, which absorb significant combustion heat, reduce surface temperatures, and effectively inhibit and delay combustion.

Due to its density difference from polyurethane resin, antimony trioxide is challenging to disperse and is generally used only in specialized applications.

(2) Aluminum Hydroxide

Aluminum hydroxide powder is the most commonly used inorganic flame retardant and filler, applicable to various polymers. Its disadvantage is the need for large quantities, which increases material viscosity. The primary form used as a flame retardant is α-trihydrate aluminum hydroxide, which appears as a fine white crystalline powder with an average particle size of 1–20 μm and a relative density of 2.42.

Aluminum hydroxide contains a large amount of crystalline water (up to 34% by mass). This crystalline water remains stable during foam production but rapidly decomposes at foam combustion temperatures. At around 200°C, it releases significant water vapor, absorbing combustion heat, lowering the material surface temperature, and forming non-combustible barriers between the fire source and the foam. This dilution of oxygen in the combustion zone provides flame-retardant properties. Aluminum hydroxide is also a smoke suppressant, reducing smoke and toxic gas emissions. During combustion, it produces aluminum oxide, which promotes the formation of a carbonized protective layer on the polymer surface.

As a flame retardant for polyurethane foams, it is primarily added via formulation or slurry impregnation for open-cell soft foams. Aluminum hydroxide is non-toxic and safe to use.

(3) Ammonium Phosphate and Ammonium Polyphosphate

Ammonium phosphate and ammonium polyphosphate (APP) contain high phosphorus (30–32%) and nitrogen (14–16%) content, providing excellent flame retardancy, dimensional stability, hydrolysis resistance, and thermal stability for rigid foams. These flame retardants produce minimal smoke during combustion and do not generate hydrogen halides. The production of carbon monoxide and hydrogen cyanide is also significantly lower than halogen-containing polyurethane foams. The toxicity and corrosiveness of gases generated are comparable to those of non-flame-retarded materials.

APP is a white crystalline or amorphous fine powder consisting of unbranched long-chain polymers. Depending on the degree of polymerization (n), APP is categorized as water-soluble (n=10–20) or water-insoluble (n>20). Crystalline APP, commonly used, has excellent thermal stability, low water solubility, minimal hygroscopicity, and good dispersibility. It is chemically stable and does not react with other materials. APP is a safe, efficient, non-halogen phosphorus-based flame retardant with smoke suppression properties. It is widely used in intumescent fireproof coatings, polyethylene, polypropylene, polyurethane, epoxy resins, rubber products, fiberboards, and dry powder fire extinguishing agents. However, as APP is a solid powder, it may clog mixing heads during mechanical foaming, making it unsuitable for block soft foams.

(4) Red Phosphorus

Red phosphorus acts as a flame retardant by thermally decomposing to form layers of phosphoric acid → metaphosphoric acid → polyphosphoric acid. These layers cover the combustible surface, acting as a barrier. Phosphoric acid and metaphosphoric acid exhibit strong dehydration properties, causing polymers to dehydrate into carbon, forming a glassy char layer on the polymer surface. This char isolates oxygen and provides robust flame retardancy at high temperatures. Red phosphorus flame retardants are effective, require small dosages, are cost-efficient, low in toxicity, and synergize with halogenated flame retardants such as TCEP and DMMP. They enhance char formation in conjunction with nitrogen-containing compounds, improving flame retardancy in both the solid and gas phases.

Encapsulation technology produces ultra-fine encapsulated red phosphorus flame retardants, solving moisture absorption issues and increasing fire resistance.

(5) Other Inorganic Flame Retardants

Other inorganic flame retardants include zinc borate, barium metaborate, magnesium hydroxide, and antimony oxide-silica composites.

Solid flame retardants added to liquid raw materials tend to settle. They are typically added during or immediately before foaming. When added to polyether blends, continuous stirring is needed to maintain uniformity. Solid flame retardants increase material viscosity and reduce foam flowability. Adding inorganic flame-retardant fillers may negatively affect foam properties. The finer the particle size, the better the flame retardancy and the less adverse the impact on foam properties.

Due to the presence of solid particles, certain mixing head mechanical foaming processes cannot be used. Some domestic and international companies have developed specialized mixing heads capable of handling solid powder fillers.