Boron in Fire Retardancy


U.S. Borax products are used in a variety of fire retardant applications, including:
  • Polymer-based applications including:
    • Electrical and electronics
    • Transportation (mass transit, aerospace, and automobiles)
    • Construction
    • Coatings (wire and cable)
  • Cellulosic products
  • Lumber and plywood

Borates in insulation

Zinc borate in a polymer is known to promote the formation of char.

Zinc borates for fire protection

Specialized borate compounds, such as zinc borates, are used as fire retardants and smoke suppressant additives in polymers, plastics, and rubber applications.

Our Firebrake® family of products can be used as a flame retardant, char promoter, afterglow and smoke suppressant, and anti-arc agent in polymer systems. Firebrake ZB is a unique zinc borate that combines the optimum effects of zinc and boron oxides with water release for flame-retardant formulations processable up to 554°F (290°C). It should be noted that Firebrake ZB does not contain intersitial water in this crystalline material.

  • Wire and cable products
  • Electrical connectors and bobbins
  • Circuit breakers and housings
  • Transformer components
  • Appliance components
  • Wall coverings
  • Flooring products
  • Intumescent coatings
  • Foam insulation
  • Mechanical parts
  • Conveyor belts
  • Carpet backing
  • Automotive upholstery
  • Roofing membranes
  • Skylight panels
Although its applications are extensive and varied, Firebrake ZB is a multipurpose synergist in both halogen-containing and halogen-free polyolefin. Firebrake 500 is an anhydrous form of Firebrake ZB that is specially designed for applications in which no water evolution is permitted. Because it contains no water of hydration, it is stable up to 1112°F (600°C).

Fire retardancy for cellulose materials

Cellulose—the basis of wood, cotton, and most other plant-derived raw materials—is in widespread industrial use. But in many forms, cellulose is inherently flammable. The use of borates in cellulose materials imparts flame retardancy, enabling them to meet stringent safety standards and helping to protect consumers.

The three major uses of borate flame retardants for cellulosic materials are in cellulose insulation, wood products and composites, and cotton batting (used as a filler in mattresses and futons). Because most borates are water-soluble, they are not suitable for clothing or materials that are laundered regularly. The decision to use borates depends on:

  • The type of product 
  • The application methods
  • The compatibility with other additives
  • Compliance with fire test standards

Combustion of cellulosic materials can occur both in a primary mode, where visible flames are present, and in a secondary mode, where flames are absent. The latter case is referred to as glowing or smoldering, depending on whether light is emitted.

In cellulose insulation, wood composites, mattresses, fabrics, and paper, borates are used primarily to prevent flaming combustion and to suppress glowing and smoldering. Optibor® has long been recognized as an effective flame retardant additive due to its ability to prevent smoldering and is often used to help protect cotton-batting infill material in mattresses and futons. The combination of Optibor and Neobor® yields reliable fire and corrosion test performance. Flame-retardant lumber, plywood shingles and shakes, and wood composite board can be made by vacuum or pressure impregnation with Optibor or Polybor® sodium borate solutions. These solutions can also be applied by direct spraying or dipping to drapes or curtains, rugs, ironing-board covers, firefighter uniforms, fabric heat deflectors for stoves or fireplaces, and fire-smothering blankets. Solutions of Optibor, Polybor, or 20 Mule Team® ammonium pentaborate can be applied to paper, such as high-gloss file-storage boxes, although the products have a stiffening effect that must be offset.

The three major uses of borate flame retardants for cellulosic materials are in cellulose insulation, wood products and composites, and cotton batting.


Protection from fire for lumber and plywood

Boron compounds make effective flame retardants in lumber and plywood. Such compounds can be used in conjunction with other chemicals, including ammonium sulfate, diammonium phosphate or zinc chloride, or used alone. Flame retardant-treated lumber and plywood have been successfully evaluated for structural strength and durability at service temperatures per ASTM D5664 and AWPA Standard U1.

Dimensional lumber and plywood are usually treated by vacuum/pressure impregnation with aqueous solutions of flame retardants. Individual plywood veneers can also be soaked with fire retardants before assembly and gluing, so long as compatibility between the flame-retardant additives and the adhesive system is ensured (to promote strong bonding).

20 Mule Team Borax Products

These 20 Mule Team Borax products are developed to retard fire.

Ammonium Pentaborate

Whether inhibiting heat, flame, or corrosion, the purity of this alkaline salt makes it an excellent choice. Learn More


These products are unique zinc borates combining the best of zinc and boron oxides that can be used to develop fire-retardant formulations. Learn More

Firebrake 500

This zinc borate is anhydrous—it contains no water of hydration—which makes it highly stable in fire-retardant applications and in polymer systems that require very high processing temperatures. Learn More

Firebrake ZB

This unique zinc borate combines the best of zinc and boron oxides with water release to provide multi-functional benefits in fire-retardant formulations. Learn More


With lower transportation, handling, and storage costs, this concentrated sodium borate is used in glass, fiberglass, cleaning products, and flame retardants. Learn More

Optibor boric acid

From reducing melting temperatures in fiberglass production to inhibiting corrosion in fuel additives, Optibor has a multitude of uses in numerous industries. Learn More

U.S. Borax is a member of:

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U.S. Borax, part of Rio Tinto, is a global leader in the supply and science of borates—naturally-occurring minerals containing boron and other elements. We are 1,000 people serving 500 customers with more than 1,700 delivery locations globally. We supply 30% of the world’s need for refined borates from our world-class mine in Boron, California, about 100 miles northeast of Los Angeles.  Learn more about Rio Tinto.

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