Char work: PPCJ has been following investment into Russian fire-protection systems. David...

Fire conjures up many images in our minds, and encompasses many emotions. Fire is arguably the oldest tool that mankind has attempted to control, and yet it is one of the least understood of natural phenomena.

Humans are unusual in their lack of fear of fire, and yet in our attempts

Historical tactics

The first recorded attempts to introduce fire protection go back to ancient Greece, when wooden fortresses were protected against fire in 83 BC.

The first recorded fire protection systems were introduced in mediaeval France in the 16th century. At the time, wooden theatres were illuminated with hundreds of candles, and fires were common, often with significant loss of life. Because the French King was keen on the theatre himself, he instructed that methods to protect curtains against fire were found.

Fire protection has evolved and developed since then, and modern structures are commonly protected with a range of fire resistant and flame retardant technologies.

Recently, however, many of the commonly used products have come under legislative scrutiny, and there is a growing demand for alternative technologies.

Development of fire protection

The development of fire protection has only been possible with increased understanding of how fires burn and spread, and this is an extremely complex process.

One of the leading bodies in fire research is the Semenov Institute of Chemical Physics of the Russian Academy of Science.

At first, soon after its foundation in Moscow in 1931, this Institute led its research into the theory of chain reactions, which was then applied to processes of burning and explosion and later to chain nuclear depletion.

Chain reactions form the basis of polymer chemistry, which is also included into the research programme of the Institute.

One of the key figures in the Semenov institute is Professor Nikolai Khalturinski, who is the Head of the Polymer Burning Laboratory and is widely regarded as one of Russia's leading experts in flame retardancy.

He has worked for more than 30 years on the physics and chemistry of polymer burning and has devoted considerable resources into ways to introduce flame retardation.

In 1999, this work attracted the interest of Flintstone Technologies, who specialise in identifying the commercial potential of new technologies, particularly those in the former Soviet Union. Firestop Chemicals was formed to develop and fund Khalturinski and his team. Firestop is now close to introducing a range of halogen-free flame retardants for textiles, plastics, paints and wood.

'Before ways of fighting fire could be developed, it was important to first understand how fires burn and spread,' explained Khalturinski.

'There are two types of burning. In the first case, premixed substances burn when the products of polymer pyrolysis are mixed with air, and the speed of burning is determined by the reaction rate in the gas phase.

If a halogen containing substance is added to the products of polymer destruction, the oxidation reaction rate drops by the factor of ten. If the same substance is added to the bulk of the polymer, the reaction rate changes insignificantly.

In the second case, with diffusion type flames, the combustible agent and oxygen are fed into the flame area. The speed of burning is determined by the gas feed rate into the flame area.

This can also be described as a mass transfer between the flame and the surface of burning material. The temperature of the fire and the speed of the flames are also determined by the mass transfer feed rate.'

Chemical flame protection

There are two traditional methods of flame protection, each with its own type of flame retardants. The first method acts in the gas phase, where halogen containing substances are commonly used for protection.

The activity of halogen containing FR is usually attributed to its participation in gas-phase reactions that are similar to those with premixed flames.

The rate of the oxidation reaction decreases when halogens are added to the gas mix because they have an effect on the kinetics of process. Halogenated chemicals interfere with chain reaction mechanisms and dilute the concentration of combustible pyrolysis products in the flame.

The effectiveness of that type of FR in polymers is very low. During burning, carcinogenic and toxic substances like dioxins, are produced, which have a maximum permissible concentration of [10.sup.-13]-[10.sup.-14].

Efficient prevention methods

The second method takes place in the condensed phase because the use of FR in the gas phase is not the most efficient process of preventing the spread of fire.

As a consequence of the heat/mass transfer process, if the feed rate is reduced by 10, the speed of burning is reduced by over 1000 times. The reduction in the burn rate is several orders of magnitude greater than the reduction in the feed rate.

This creates a bottleneck in the mass transfer process, dramatically lowering temperatures.

The reasons for this dramatic reduction are because of the various contributing factors in the burning process. In terms of reaction kinetics, a 10[degrees] rise in temperature can double the rate of a chemical reaction, and in a fire, chemical reactions release heat.

The other important factor is that the amount of energy radiated from an object is proportional to the fourth power of the temperature, and a significant amount of this energy will be in the form of infrared radiation, which is heat by another name.

This shows that a small reduction in mass transfer will slow the chemical reactions, reducing the temperature and therefore dramatically reducing the amount of radiated energy in the process, which lowers the temperature still further. Lower temperatures will also reduce the amount of material evaporating from the solid surface, which lowers the mass transfer rate into the gas phase.

Fire protection strategies

'The conclusion of research at the Semenov Institute is that the most efficient way to reduce the flammability is to control the heat and mass transfer between the flame and the surface', explained Khalturinski. 'From this statement, four strategies to combat fire can be explored:'

1. Reduce the amount of combustible substance.

In Pyraeus in ancient Greece, archeological evidence shows that crystal hydrates were painted onto wooden fortresses. These materials emit water when heated, diluting the amount of flammable materials and cooling the surface. Examples include Al[(OH).sub.3], and Mg[(OH).sub.2].

Other materials emit gases, such as carbon dioxide, to displace oxygen. Carbonates such as MgCa[(C[O.sub.3]).sub.4] are examples of this approach. These materials work at low temperatures, but stop working at elevated temperatures, providing more fuel for the fire.

Halogenated materials react with active oxygen radical as well as other highly reactive radicals producing inactive particles that slow down chain reactions and cool the flame. Various other compounds work in synergy with halogens to increase efficiency, such as antimony trioxide.

2. Use chemicals that take energy from the fire in an endothermic process.

When these materials decompose they also create inert substances. These type of materials include aluminium trioxide, ammonium phosphate and ammonium chloride.

The decomposition of ammonium phosphate and ammonium polyphosphate releases ammonia, which also displaces oxygen from the fire. Ammonium chloride has the highest decomposition temperature discovered so far, at 338[degrees]C, but it is not commonly used for other reasons.

3. Form an impermeable barrier.

This approach is exemplified with zinc borate and other materials that form a glassy char, as well as ceramic materials, such as Ceepree, which vitrify and form an impermeable glaze.

4. Reduce the temperature with catalysts that shift the reaction to char formation.

Khalturinski and the Firestop team has specialised in this area. Phosphorus containing compounds have been used to form chars.

In an early example in 1820, the French scientist Gay-Lussac, who is most famous for his work on gases, invented a treatment for linen fabrics that induced char formation, which then would extinguish the flame.

This mixture contained ammonium phosphate, ammonium chloride and borax. When phosphorus-containing compounds react with cellulose, catalytic dehydration occurs, leaving a layer of carbonised char.

This char contains water and only a small amount of combustible material, which rapidly reduces the rate of mass transfer, with a decrease in flame speed and temperature.

Optical processes

In another dramatic discovery, Khalturinski described how the optical properties of materials can also have a major impact on flame retardation.

'The key is to include inorganic materials with specific optical properties on the surface of the char. Consider that over 50% of the heat at the surface is transferred there from the flame area in the form of IR radiation. By researching optical properties selected chemicals in the IR part of spectrum, and then adding these chemicals to various materials, the absorption and reflection profile of these materials can be changed, cooling the burning process.

Research in this area shows that a number of inorganic oxides have appropriate optical properties. One of these is antimony trioxide, which efficiently reflects IR radiation, cooling down the surface of the material.

'Antimony trioxide acts as a catalyst of free radical recombination in the gas phase, hence inhibits the flame and cools it down.' continued Khalturinski, 'But this material can react with brominated species to form dioxins, which are extremely toxic and damaging to the environment;

Char development

Khalturinski went on to explain how the formation of a surface char has been further expanded to create a carbonised foam.

'The foam reduces the mass transfer into the burning zone and reduces the heat flux to the surface. The thicker the layer, the lower the temperature of the substrate surface.

'The effectiveness of the char foam cap depends on its structure, the size of pores and their position in the cap, because all these factors influence the specific thermal conductivity of the formed char.'

'Firestop's research has further developed the use of phosphorus compounds to create an intumescent char that expands 20-30 times in volume when heated.

'The formulation includes ammonium polyphosphate, pentaerythritol and inorganic materials with optical properties to absorb and reflect the IR radiation created in a fire.'

Ammonium polyphosphate is milled to particles and encapsulated in pentaerythritol. As the coating is heated the N[H.sub.4] groups on the ammonium polyphosphate react with hydroxy groups on the pentaerythritol, releasing water to cool the fire and ammonia gas, which creates, the expanded char.

Inorganic oxides remain physically bonded to the char and absorb infrared radiation because of their optical properties.

'For wood protection, char formation should start at 150-200[degrees]C, while the protection of steel should start at a higher temperature of about 250[degrees]C.

'The challenge for steel protection is to prevent the temperature from reaching 550[degrees]C in 60 minutes. At 400[degrees]C and above, the steel will start to soften and weaken, and the performance of the steel will degrade if subjected to temperatures above 500[degrees]C'.

According to literature, comment has been made in the past that intumescent systems can suffer from adhesion problems to the substrate and the formation of brittle chars that flake away, exposing more substrate to the flames. PPCJ asked Khalturinski to comment on this situation:

'For wood coatings, adhesion is not a problem. We are able to make clear lacquers as well as pigmented systems that have extremely high adhesion (150kPa) to timber. For steel, it becomes more difficult and one of the critical steps is choosing the correct primer and binder systems.

'If this is not optimised properly, the intumescent char can slump away from the surface, but our latest tests show that with the correct primer in place, there is sufficient adhesion to maintain the performance of the char. It is also important to optimise the char strength by ensuring the correct char structure is formed.'

For metal coatings, we have achieved the necessary protective function of the char (Figure 3).

[FIGURE 3 OMITTED]

Char performance

'With all intumescent chars, there are a number of factors that influence char formation and performance.

'First consider the mechanisms of heat transfer. Heat transfer can take place by conduction, convection and radiation. The structure of the char, in terms of void size and distribution will affect the characteristics of each of these three mechanisms.

PPCJ also asked how the modification of the ingredients could affect char performance.

'The size and distribution of the ammonium polyphosphate determines the basic foam structure in terms of void size and distance between voids.

'It is important for the polyphosphate particles to have as narrow particle size distribution as possible, to ensure a regular foam structure. The particles range from 5-1[micro]m in size, which is about the same size as many pigment particles.

'Large particles make the foam irregular in nature, removing efficiency, but it is also important not to have too many fine particles because of the corrosive nature of the dust.'

Although theoretically, smaller par tides could be of benefit, the manufacturing process is limited by the additional cost of making the particles smaller and the more challenging problem of keeping the milling temperature below 30[degrees]C.

Perhaps innovations in milling and classification techniques could further enhance the performance of the char.

Practical demonstrations

The practical demonstration of the technology is as impressive as the theory. Wooden blocks coated in pigmented paints and clear lacquers were subjected to a gas-fired blowtorch for five minutes (Figure 4a).

[FIGURE 4 OMITTED]

The pigmented paint rapidly expanded to form a char that protected the wooden substrate. Although the char glowed under the intense heat of the blowtorch, there were no signs of flame. When the block was removed from the heat, and the char (approx 2cra thick) was cut away, more paint could clearly be seen, and the test was repeated with similar results (Figure 4b).

Development of clear lacquer protection is also underway. This is a much more challenging process, because the requirements on optical clarity of the lacquer severely limit the choice of available materials.

A demonstration on an unoptimised clearcoat offered about three minutes of resistance to the blowtorch. This is certainly a move in the right direction.

A steel panel coated with Intuman also resisted the heating effects of the blowtorch. A thermocouple registered over 900[degrees]C at the point where the flame of the blowtorch touched the coating surface, but the heating effect was reduced to the point where the rear of the steel panel only registered about 200[degrees]C. Although the tests were a simple demonstration of how the intumescent system works, it was impressive visually (photo left).

Extensive testing is currently underway, and the products currently being marketed for the textiles and plastics markets have been classified as Class 4 (non-notifiable)materials and are certified for use under EINECS regulations and are also registered for the US TSCA as well in the Russian Federation.

Although these successes only relate to textile and plastic applications for fire retardant materials, this can be seen as a promising validation of the technology concept, but time will tell if this can be converted into a proven technology for the paints and coatings markets.

Acknowledgements to:

Galina Dudareva, Technical Manager, Firestop, for translating.

Kinetic and diffusion pyrolysis

The dependence of the polymer surface temperature (Ts) during the pyrolysis process on the environmental temperature ([T.sub.[infinity]]) is shown in Figure 1.

[FIGURE 1 OMITTED]

In the low temperature region (up to 300[degrees]C) the degradation process occurs in the kinetic region (area A), or when the sample heating time is far shorter than the chemical reaction time, then the polymer temperature is equal to the environmental temperature.

As the environmental temperature increases and pyrolysis occurs in the thin outer layer of the polymer, the process moves into the diffusion region (area C), When the reaction is rapid, the thickness of the heated polymer layer is small and the rate of the burning process depends on the diffusion rate of combustible pyrolysis products and of the oxidant into the reaction zone.

The burn rate of the process is determined by the heating wave velocity, the thermophysical properties of the polymer and the oxidant diffusion rate.

Figure 2 illustrates the outer diffusion area, where the rate of the process is determined by the heat transfer rate to the surface of the burning material.

[FIGURE 2 OMITTED]

Curve (1) characterises the rate of heat consumption by chemical reaction; curves (2) and (3) are the radiation and convection respectively.

Dashed curves (4) represent the total heat supply to the polymer sample. The point of intersection of curves (1) and (2) is the temperature of the sample during its pyrolysis (or the surface temperature). The surface temperature can therefore be changed by altering the gradient of curve (1).

For example, forming a char cap on the surface could reduce the heat flux to the surface of the burning material. This would therefore cooling the surface temperature, reducing pyrolysis rate and minimising diffusion of combustible products to the flame area.

About Firestop

Firestop technologies are targetted at materials such as plastics, textiles, wood and metal. Applying the principle of including a surface char to cut off the supply of combustible materials to the flames, Firestop has created a range of FR technologies that can be applied almost universally.

Noflan      for textiles
Noflan E    for plastics (<200[degrees]C)
Bizon       for plastics (<230[degrees]C)
Intuman     for metal/wood

The products are non-brominated and antimony free.

For further information:

Professor Nikolai

Khalturinski, c/o Firestop Chemicals Ltd Unit 9, Wedgewood Road, Bicester, Oxon OX26 4UL

Tel/Fax: refer to website

Email: gdudareva@firestop.uk.com

Website: www.firestopchemicals.com