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The case for re-evaluating our approach to testing for ignition leading to smoulder combustion.

Is smoulder a “different” type of combustion?

This document considers 2 different types of combustion behaviour, Solid and Gas phase, to demonstrate the necessity of assessing both types of hazard when considering the fire safety of upholstery.

It gives guidance on the need to assess different materials according to their thermal data, the implications for fire safety and the requirements for ignition sources used when testing.

What is smoulder?
“We are in the first instance addressing Fire Safety or COMBUSTION processes.
The core of any combustion process is a global exothermic reaction that results in the release of heat, and both gaseous and solid products. Whether smouldering or flaming will be the dominant mode is dictated by which chemical species is oxidized.
If the oxidation takes place in the solid phase, smouldering is dominant.
If the oxidation takes place in the gas phase, then flaming dominates.
Fires that involve upholstered furniture frequently begin as a smouldering combustion and, with time, transition to a flaming combustion, which sharply increases the level of hazard. Therefore, understanding how the compositions of the primary flammable components of the furniture, i.e., flexible foam and upholstery fabric, affect this transition is important for fire safety considerations.
Quoted without permission:B04 Rein Smouldering Combustion SFPE52016.pdf”

Professor Guillermo Rein of Imperial College, London describes
the smouldering behaviour of a cigarette in scientific terms. (Reference 1). It describes Solid Phase Combustion (SPC).
The cigarette as an ignition source was cited in the UK Upholstered Furniture Fire Safety Standards (FFR) and the related British Standards (BS 5852 1978 et al) as “progressive smoulder” ignition source and in the context of “Smokers Materials”.
However, the different behaviour of the 2 types of ignition were not defined resulting in a tendency to consider the testing requirements as an either/or situation. This is a false premise.

Smoulder or Slow Combustion is different to open flame ignition.
The slow combustion process has been shown to transition from smoulder to open flame but the necessary conditions remain the subject of research.
Whilst this phenomenon may be seen in (neglected) smoulder tests it is normally outside the dimensions of the tests as applied.

The most common type of combustion demonstrated by ignition testing takes place as the combustion of gasses. The materials under test are converted from solids to gas by heat whence they react with oxygen (from the air) to emit energy and waste gasses. The addition of oxygen from the air to the chemicals consumed leads to this being recognised as an oxidation process.
The reaction is typically very rapid and, provided the necessary components are freely available e.g., a good air supply, will progress to a completed reaction when one component is fully consumed. The oxidation process is thus also complete and the waste gasses tend to be less toxic.

SPC is, in comparison, a slower process and more likely to remain uncompleted as a reaction and leading to the evolution of gasses that are far more toxic. A relatively small amount of material can create a comparatively large amount of toxic smoke and fumes in the ‘smoulder only’ state.

In a simple terms, the human body can take in Oxygen and breathe out Carbon Dioxide. The partial oxidation stage is Carbon Monoxide and that is highly lethal to the human body.

Modern casualty figures from domestic fire incidents continue to cite Smoke and Toxic Fumes as a major cause of death and injury.

We must pay special regard to the SPC process rather than consider completed burn scenarios.

Textile flammability testing has always noted the progressive smoulder potential in textiles. It is commonly found in what are classed as Natural Fibres and was significantly more important at a time when thermoplastic textile fibres were not in use.

Afterglow (AG) is seen when a textile chars and glowing may be seen in the subsequent black char and, more pertinently, it may be seen to continue after the open flame ignition source has been removed. Many testing standards have guidance on recording and reporting AG.
This is all described in smoulder combustion science. This slow progress leads to a second name: Slow Combustion.

In testing terms (for non-solid phase ignition) AG is observed after the flame has been removed – hence ‘after’ glow.
The differences between an open flame temperature and SPC ignition conditions is significant.

The role of Flame Retardants.
Treatment textiles to reduce ignitability and/or inhibit their burning behaviour may require restriction of afterglow and the specific chemical treatment options for AG are very limited. Certain chemical systems are more effective in dealing with AG and this is undoubtedly due to their mode of action as a flame retardant.
Borates are effective as AG additives (See reference 4) and they are not effective in the gas phase. They rely on forming a molten chemical surface layer that literally “smothers” the burning behaviour. Intumescent FRs have a similar smothering approach (denying the access of heat and oxygen to the test substrate) but their mode of action is based upon a char barrier derived from a carbon source and thus encourages char development rather than inhibiting it.
In this respect, the most effective flame retardants are active in the gas phase and they may be of limited effect in controlling AG. Their action is normally transient whereas the slow combustion process can continue over long periods.* They require the heat from the ignition flame to make them active. The duration of ignition time in the BS 5852 cigarette test is in the order of 12 – 20 minutes. The Risk of ignition by over-heating electronic devices is a more recent phenomenon. The lower temperatures and longer duration of ignition time are significant factors. They have been shown to produce slow combustion behaviour and so may serve as solid phase ignition sources.
* the longest burning fires are subterranean and show SPC. There are examples that have been burning for decades

The role of the cigarette.
Whilst they demonstrate smoulder combustion cigarettes are a useful slow combustion ignition source. Their principal value lies in the slow consumption of materials and a proximity to the substrate.

Thermoplastic materials.
These tend to melt back from heat sources in ignition testing and they do not normally ignite at the temperatures found in smoulder combustion. They are less likely to fail smoulder ignition testing.

The presence of thermoplastic materials in a composite test are likely to prevent the conditions required for SPC from occurring.

The importance of assessing both hazards in upholstery.
The recognition of the smoulder and/or char potential of materials becomes much more important as the nature of upholstery and its combination of filling materials and outer cover is ideal for SC.
The thermal insulation characteristics of upholstery are helpful in maintaining the low heating effect of smoulder ignition sources.
Curtains and drapes more easily demonstrate GPC due to their alignment and tendency to present single layers of textile to the flame. The obvious greater availability of oxygen plays an important role.
Upholstery manufacturers must identify critical combinations of materials. It is just as correct to say some materials will normally demonstrate a potential for SC as is to say they some materials do not show a great potential to demonstrate SC.
This identification of the nature of different materials must become part of manufacturers process control and testing. The ability of any chemical treatments to control different risks is also highly relevant.

The nature of foam.
Polyurethane is the main constituent of foam used in upholstery. It is a thermoset plastic and will demonstrate smoulder combustion. It does , however, demonstrate thermoplastic like behaviour as thermal decomposition leads to the release of liquid polyols that are a major component of its polymeric structure.
However, there is a weight loss requirement in the testing and the ignition source may be relatively heavy and powerful (Crib 5).

The Thermal behaviour of textile fibres.
Simple facts give a good indication of comparative behaviour when an ignition source is applied.

The most important fact here, in the context of this article, is the melting point.

Cotton and wool, natural fibres do not melt and will form a char when ignited.
Man-made, synthetic, “plastic” fibres demonstrate thermoplastic behaviour: they melt as a first step under the influence of heat.

Thermoplastic fibres are unlikely to demonstrate smoulder combustion.

The Consequences.

It is fact that upholstery must be constructed of light materials that provide comfort. The most common method of manufacture involves the use of a textile cover over a lightweight filling comprised of a wide variety of components with a structure that provides strength and lightness and the required characteristics of comfort.

These materials will often ignite and burn easily based on their physical nature alone and without considering the behaviour of the materials used.

Open flame ignition will not satisfactorily test for the potential for Smoulder Combustion.

The choice of materials may significantly reduce the risk of Smoulder Combustion occurring.
There are certain materials that will ignite more easily with an open flame.
There are certain materials that will more likely demonstrate Smoulder Combustion.

Manufacturers need to assess the chosen combinations of materials be aware of those with higher risk to both types of ignition.

It is likely that the choice of materials may have a greater role in avoiding risk with SC, especially as the effectivity of chemical additives cannot always resolve the risk from any problem combinations of materials.

Peter Wragg
Peter Wragg
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