When structural steelwork reaches a critical temperature, it can lose its load bearing capability and buckle or collapse, with drastic consequences for the stability of the building it is supporting. To prevent this, intumescent coatings (means a ‘swelling up’ or ‘expansion’ - one which expands when in contact with heat.) are applied to the steel.

In the event of a fire, the foam formed by intumescent coatings insulates the metal structure from the heat. When they come in contact with a flame, intumescent coatings expand by several times their original thickness, forming a thick, insulating non flammable foam which insulates the metal structure from the fire. It is typically made of a combination of additives which react chemically when they reach a certain temperature.

They are available as paints and coatings which are applied to protect a whole variety of materials, including steel structures, metal sheets, wood, plaster and concrete.

Flame retardants are a major component in the formulation of intumescent paints, coatings, sealants and mastics.

Use of Flame Retardants

To reduce the risk of fires and provide enough time to evacuate a building in the event of a fire, manufacturers and fire safety authorities have promoted the use of ignition resistant and low combustion building materials. Flame retardants are used in building materials because not only do they raise the threshold temperature at which a material ignites, but they also reduce the rate at which it burns, diminish flame spread and in some cases abate smoke.

Flame retardants are critical to meeting regulatory requirements for fire safety in public buildings, and their use helps to ensure that cinemas, theatres and hospitals are safe from the risk of fire and are used in all building structures including steel, rigid foams for insulation, solid wood panelling, structural wood products, wood-based panels and wood flooring.

Polystyrene (PS) insulation foams

HBCD’s (hexabromocyclododecane) main use is in expanded (EPS) and extruded polystyrene (XPS) insulation foam boards, which are widely used by the construction sector. Determined by their difference in formation, ‘closed-cell’  foam (Figure 1) is very strong, with a low density and a high compressive strength as it has no interconnected pores, which gives better  ‘burn-through’ resistance. This is a major consideration in the choice of flame retardant and mostly it is the brominated flame retardants that are used.

EPS and XPS insulation foams play a key contribution in helping governments to meet a major part of global, regional and national energy efficiency targets.
In Europe, polystyrene (PS) insulation foams are indispensable for the implementation of the EU Directive on energy performance in buildings (2002/91/EC).

EPS and XPS foams are processed to meet stringent fire safety regulations. The use of flame retarded EPS and XPS insulation foams is essential for achieving these standards in construction. HBCD provides high degree of flame retardancy when used at very low concentrations.

High impact polystyrenes can also be flame retarded with TBBPA.
(tetrabromobisphenol A)

The main application for chlorinated phosphate esters (Aryl, Bisphosphate, Alkyl and Chlorinated) is the flame retarding of both rigid and flexible polyurethane foam employed in the building industry to reduce ignitability, flammability, burning rate smouldering and ease of flame extinction. The presence of both chlorine and phosphorous is advantageous for optimum effect upon flammability in both the solid and gaseous phases.

Wood

As a building material, wood is one of the most versatile, renewable and easily workable products. However, wood has always been associated with fire. Flame retardants have ensured that we can continue to benefit from wood’s qualities, while being reassured that the risk of fire is minimised. Indeed, also due to the fact that it can now be treated with flame retardants, wood has recently enjoyed renewed popularity, especially in Europe, in the construction and decoration of our homes.

Flame retardant treatments for wood are numerous and can be classified by :

• those incorporated integrally into wood composite products during manufacture
• those pressure-impregnated into solid wood, plywood, particleboard and hardboard industrially, after manufacture

Most existing flame retardants are effective in reducing different reaction-to-fire parameters of wood but cannot make wood non combustible. In all instances they are formulated and applied to control ignition and flame spread across a wooden surface and to lower the rate of heat release from the wood based substrate. This is achieved by the promotion of :

• char formation
• conversion of volatile gases to non ignitable gases such as water vapour and carbon dioxide
• forming a glaze barrier at the surface
• forming an intumescent foam barrier at the surface  
• prevention of release free radicals in the gaseous phase

Examples include flame retardants based on phosphorous, nitrogen, inorganic retardants like boron, silica and their combinations, where their behaviour can be synergistic.
(e.g. melamine phosphate, mono-ammonium phosphate, di-ammonium phosphate, ortho-phosphoric acid, ammonium sulphate, borax/boric acid/boric oxide/disodium octoborate)

(Source : http://www.fireretard.com/main.php?langid=1&itemid=2&subitem=5)

The highest European and national fire classifications for combustible products can be reached though maintenance of high retention levels but this has to be considered alongside ordinary preservation treatments used to protect wood against biological decay.

Source : http://flameretardantlumber.com/

Protective Coatings

Melamine (a nitrogen based flame retardant) can show excellent flame retardant properties and versatility in use with coatings (also intumescent) because of their ability to employ various modes of flame retardant action such as char formation, heat transfer to reduce dripping and heat sink.

In combination with phosphorous synergists, melamine can further increase char stability through formation with nitrogen-phosphorous substances. Melamine can act as blowing agent for the char, enhancing the heat barrier functionality of the char layer.
(Source : specialchem4polymers.com)

Intumescent paints and coatings produce a protective char retarding the spread of flames and the transfer of heat in the event of a fire.