Uses of Borates in Flame Retardants

History

It has been reported that around 360 B.C. wood was coated with vinegar to protect against fire.  In 83 B.C. the Romans coated their siege towers with alum prior to scaling their enemy’s walls.  According to the American Chemical Society, the history of flame retardant clothing began in 1735 when Obadiah Wyld received a patent for the first flame retardant mixture of alum, ferrous sulfate and borax to be used on fabrics.  Moving forward, it was in the 1820s when Gay-Lussac recommended using mixtures of ammonium phosphate, ammonium chloride and borax to reduce the flammability of textiles used in theatres.  This formulation containing borates in flame retardants still has merit today. 

In general terms a chemical used as a coating for a component of a combustible material to reduce or eliminate a tendency to burn would be considered a fire retardant.  This can be used with textiles, plastics, rubbers, paints, and other materials; however, for this discussion we will feature cellulosic materials (i.e. cotton, hemp, paper, wood and other plant derived materials).  A fire retardant can also be defined as a material that has been chemically treated to self-extinguish. 

Borate Benefits

  Acts as a buffer when neutralizing acid materials

•   Acts as an anticorrosive protecting metals in cellulosic materials
•   Prevents flame combustion
•   Promotes char forming
•   Suppresses glowing, smoldering and smoke
•   Synergistic value with other fire retardants

The physical aspects of a fire can take the form of volatile, flammable gases igniting and further decomposing the solid masses via pyrolysis, thus creating more flammable gases until only a carbon structure is left.  The second method operates at a lower temperature with the carbon to produce a smoldering combustion.

Fire Retardant Process

Combustion of cellulosic materials can occur in two phases.  In the first phase, visible flames are present; in the second phase, flames are absent and are in a gaseous state. In the latter phase, the combustion is referred to as glowing or smoldering, depending on whether or not light is emitted from the burning material.

Flame retardants refer to chemicals added so that the treated material will not support flaming combustion after the igniting flame is removed.  Smoldering retardants refer to chemicals which effectively prevent smoldering combustion, the flameless combustion of materials which occurs after the igniting flame is extinguished.  Smoldering combustion is the heart of the fire hazard problem due to its potential for transition to flaming combustion once more.

Borate Value

Depending on the fire test to be administered borates can offer both advantages to cellulosic materials.  Sodium borates (hydrated alkaline borate) are able to release water from their crystalline structure which will help as a fire retardant.  However, in the case of boric acid, a non-alkaline borate will react both as a fire retardant and smolder suppressant.  Boric acid releases water as well to help extinguish the fire, but provides char forming value on the surface of the cellulose due to the presence of its boron value.

Borates can also act as a buffer when placed in fire retardant formulations (both liquid and dry) where more acidic fire retardant compounds could be used.  Borate’s anticorrosive value is realized when cellulosic materials come in contact with ferrous metals.

Borates can provide synergistic value with other fire retardant compounds that would improve flame retardancy. These compounds include, but are not limited to, aluminum trihydrate, aluminum sulfate, ammonium phosphate, ammonium sulfate, borax/boric acid combined in a species of di-sodium octaborate tetrahydrate, calcium sulfate (gypsum), guanylurea phosphate, urea, zinc chloride, and zinc phosphate.  The following is a partial list of uses of borates in flame retardants.

Applications (partial list)

• Cellulose insulation
• Cotton batting
• Nonwoven polymer/cellulose materials for auto, home, appliances, insulation
• Roofing
  • It has been determined that Colemanite is used in ‘torch-down’ applications to mitigate/suppress fire on the asphaltic system.
• Wood
  • Dimensional lumber
  • Medium density fiberboard
  • OSB Board
  • Plywood

American Borate Company Products:

Refined Borates

• 10 Mol
• Etibor 48 (Borax 5 Mol)
• Etidot 67 (di-sodium octaborate tetrahydrate)
• Boric acid

Mineral Borates

• Colemanite
• Ulexite

Investigate these potentially helpful links:

• National Fire Protection Association  
• North American Flame Retardant Alliance
• European Flame Retardant Association
  
• Phosphorus, Inorganic and Nitrogen Flame Retardants Association (Pinfa)