The Role of Aluminum Sulfate (Alum) as an Eco-Friendly Flame Retardant

Aluminum Sulfate, often referred to as alum with the chemical formula Al₂(SO₄)₃, is a versatile inorganic compound employed in fireproofing treatments for materials such as fabrics, wood, and paper products. Its flame-retardant properties stem from its ability to modify thermal decomposition pathways, promote char formation, and inhibit the spread of flames by reducing combustible gas release. Historically, alum has been used since ancient times, with records from early civilizations applying it to reduce the ignitability of natural cellulosic fibers like cotton, jute, and linen. In the 18th century, mixtures of alum, ferrous sulfate, and borax were patented for treating wood and textiles, and alum was even used to enhance the fire resistance of balloons in 1783. In modern applications, aluminum sulfate is valued for its non-halogenated, eco-friendly profile, making it a sustainable alternative to toxic flame retardants. It is particularly effective in synergistic formulations and is applied through soaking, impregnation, or incorporation during manufacturing to achieve fire safety standards like UL-94 or limiting oxygen index (LOI) improvements. While commonly used for cellulosic materials, its role extends to enhancing thermal stability in building materials and archives, contributing to reduced fire hazards in industries like textiles, construction, and packaging.

How Aluminum Sulfate Works as a Flame Retardant

Aluminum sulfate functions as a flame retardant by interfering with the combustion process at multiple stages, primarily through chemical reactions that favor non-flammable outcomes over volatile gas production. Its effectiveness is enhanced in acidic environments, where it hydrolyzes to release species that catalyze protective changes in the substrate.

Acidic Hydrolysis and Charring

Upon heating, aluminum sulfate decomposes to generate acidic species, including sulfuric acid and aluminum hydroxide intermediates. This hydrolysis promotes the dehydration of organic components, such as cellulose in fabrics and paper or lignin/hemicellulose in wood, redirecting thermal degradation toward the formation of a stable carbonaceous char rather than flammable volatiles. For instance, in cellulosic materials, it catalyzes the breakdown of hemicellulose at lower temperatures (around 120–160°C), increasing char yield by up to 63% at higher concentrations and reducing the maximum thermal degradation rate by 68%. This char acts as an insulating layer, lowering ignition risk and slowing flame propagation.

Formation of Protective Coatings

Aluminum sulfate, often in forms like potassium aluminum sulfate (alum), forms inorganic coatings or flocs on the material surface through hydrolysis, creating hydroxyl complexes that fill voids and encapsulate fibers. When exposed to heat, these coatings decompose to produce solid residues (e.g., K₂SO₄ and Al₂SO₄) that harden into a thermal barrier, preventing heat transfer and oxygen ingress. In wood treatments, hydrolyzed aluminum flocs form a protective carbon layer, enhancing dimensional stability and thermostability. Similar effects occur in textiles and paper, where the coating swells or fuses to shield the substrate.

Reduction of Flammable Gas Release

By accelerating dehydration and char formation, aluminum sulfate minimizes the pyrolysis of organic matter, thereby decreasing the emission of combustible vapors essential for sustaining flames. It also releases non-combustible gases like SO₃ and H₂O during decomposition, which dilute flammable pyrolysis products and cool the combustion zone. This gas-phase inhibition is evident in treated cotton fabrics, where flame duration and after-glow time are reduced by up to 60%.

Synergistic Effects with Other Flame Retardants

Aluminum sulfate exhibits strong synergy when combined with compounds like ammonium sulfate, borax, borates, or phosphates, amplifying flame inhibition and smoke suppression. Historical mixtures with ferrous sulfate and borax enhanced wood and textile protection, while modern formulations with melamine or pentaerythritol boost charring in textiles. In wood, it catalyzes degradation alongside other salts for better thermal stability. These combinations achieve higher LOI values and UL-94 ratings, making them suitable for eco-friendly applications. 

Fireproofing Applications by Material

Fabrics (cotton, linen, wool)

• Treatment Method: Fabrics are immersed in aqueous solutions of aluminum sulfate (e.g., 0.1–0.6M concentrations), often with additives, and then dried or cured. For cotton, alum is incorporated during processing for durable protection.

• Effect: It reduces flammability by promoting self-extinguishing behavior, increasing char formation (up to 63%), and decreasing flame propagation rate by 70% at higher concentrations. Ignition time extends from 2 to 11 seconds.

• Uses: Protective clothing, stage curtains, upholstery, tents, and historical textiles, where it provides lasting flame resistance without washing off.
  

Wood

• Treatment Method: Wood is pretreated by soaking in 5–15% aluminum sulfate solutions, followed by heat treatment at 120–160°C, or via pressure impregnation/surface brushing. Historical methods included alum-vinegar solutions for boats.

• Effect: Enhances thermostability with 105% higher residual weight at 500°C and 68% lower degradation rate; forms protective flocs and carbon layers, reducing swelling and ignition.

• Uses: Interior timber, furniture, decorative panels, historical preservation, facades, and urban furniture, promoting sustainable construction.
  

Paper Products

• Treatment Method: Incorporated during pulp processing, spraying, or dipping in aluminum sulfate solutions, leveraging its role in papermaking for even distribution.

• Effect: Minimizes ignition and flame sustenance by promoting dehydration and char in cellulose, similar to fabrics, with increased char yield and reduced volatiles.

• Uses: Book archives, document storage, stage backdrops, and packaging for hazardous goods, ensuring safety in fire-prone environments. 

Advantages of Aluminum Sulfate in Fireproofing

• Non-combustible and Stable: Remains inert under normal conditions, with good thermal stability and low toxicity.

• Water-Soluble for Easy Application: Facilitates uniform treatments via soaking or spraying, compatible with industrial processes.

• Compatible with Other Chemicals: Enhances performance in synergies, reducing smoke and toxicity.

• Cost-Effective and Eco-Friendly: Cheaper than synthetics, non-halogenated, and harmless, aligning with sustainable practices; reduces energy use in treatments.