Flame and Heat Resistance
The flame and heat resistance of textiles is concerned with the flammability of such materials, i.e. flammable or non-flammable, and the ability of these materials to reduce heat transfer from a high temperature source, either by direct contact (conduction /convection) or via radiation. The required flame and heat resistance of a textile product will depend on its end-uses in particular applications within a given technical textiles sector . For example in the PPE? Sector (Personal Protective Equipment), regarding clothing &work wear, fire fighters protective clothing will have different technical specifications depending on whether the end use is underwear or outer garments, and if the latter, is it for fire entry, flash fires (in the oil & gas industries ) or just close proximity? In contrast protective clothing for foundry workers or welders may be less demanding. Similar examples for the difference in performance requirements can be cited for transport interiors , comparing aircraft carpets and seat covers with their automotive counterparts; also building interiors, comparing household soft furnishings with similar fabric end-uses in hospital and public buildings. The minimum performance requirements (MPR), and standard testing procedures (Reg&Stand), of products for the various technical textile sectors have been established by various regulatory bodies , but these can differ from sector to sector and from one country to another. In the PPE? Sector, many manufacturers comply with CE mark labelling (abbreviation: Conformité Européenne), which is a mandatory conformity mark for declaring that their products comply with the requirements of the European health & safety directives.
Various terms are used for the way a fabric reacts when in contact with a flame (Terms & Definitions). If negligibly affected it is said to be flame-proof or fire-proof; if it ignites but self-extinguishes on removal from the flame, it is fire resistant – difficult to burn; if the material will not burn but can melt and/or decompose at high temperatures, it is referred to as flame retardant, non-combustible or incombustible - not capable of igniting and burning. A very important aspect to flame/fire-proof textiles is that they are thermally stable: will not readily burn or shrink when exposed to a flame or intense heat (heat proof/heat resistant).
The burning of a material is a chemical process in which atoms of oxygen or oxidizing agents combine with atoms or molecules of flammable gases released by the heated material. The action of combining is exothermic, meaning that energy is released in the form of a fire (heat and light), which further heats the material to liberate more gases to combine with more oxygen and thereby sustain the burning. The basic principle of material combustion therefore involves three factors: (1) temperature /heat, (2) oxidizing agent / oxygen, (3) continuous liberation of flammable gases /fuel; these are often referred to as the 3-sides of the combustion triangle, in which they interact in an uninhibited chemical exothermic reaction.
Combustion may be prevented or will cease by:-
• inhibiting the liberation of flammable gases [i.e. blanking the material with a heat Conductor?-/absorber];
• utilizing a material with high molecular bond strength to resist rupture, as well as containing chemical elements, such as nitrogen and sulphur, so that on rupture of the bonds non-flammable gases are formed;
• introducing an additive that has an endothermic chemical reaction with oxygen (i.e. combines with the absorption of heat).
The chemical and physical reactions of textile materials to the high thermal energy of a fire, or to direct contact or radiant heat, are complex and depend mainly on the fibre type(s) constituting the fabric and the fabric finishing treatments (See Materials). Since cost, environmental regulations and other, non-thermal, required fabric properties have also to be addressed, there is rarely a single solution to developing a fabric that achieves a specified flammability performance.
Fabrics that are considered inherently non-combustible are made from what are termed high-performance fibres (HPFs), either inorganic fibres such as glass or ceramic fibres or fibres spun from thermally resistant synthetic polymers, typically fibres such as Kevlar and Nomex that are made from aramid polymers. A number of other inherently non-flammable synthetic polymers are available from which HPFs are made. The chemical structures of these polymers have strong molecular bonds and contain chemical elements that would form non-flammable gases when the bonds break; importantly also they have a high carbon content, so that when the gases are being liberated a carbon char forms which blankets the material. HPFs made from these polymers, therefore, have the ability to resist fire at the molecular level.
Generally, HPFs are expensive and/or lack the comfort properties and dyeability needed for certain types of fire protective fabrics. In transport and building interiors , where many square metres of fabric may be used for a given project, cost-effectiveness is paramount and the expense of HPFs can restrict their use in 100% form. When specified flammability performance permits it, natural fibres such as wool may be blended with HPFs to overcome certain of these limitations. The chemical structure of wool contains nitrogen and sulphur, but there are other beneficial properties such as a good Moisture content? and the tendency to form a to form a self-insulating char which prevents flame spread; when wool is heated to the point of combustion this structure tends to foam providing an insulating layer of pyrolysed material separating heat and oxygen from the fuel (i.e. the base material).
Besides identifying if a fibre has certain elements within its chemical structure that could form non-combustible gases as well as the ability to char, there are a number of other properties that are important to material selection:-
• Ignition temperature - the minimum temperature at which the fibre will burn i) in contact with a spark or flame (forced ignition), ii) without a spark or flame (auto-ignition / self-ignition/spontaneous ignition/autogenic flash point)
• Melt Temperature /Melting point – polymers that melt are likely to drip in the molten state and could
initiate burning of some other material.
• Heat Release /Heat of Combustion – heat generated during burning
• Limiting Oxygen Index – minimum of oxygen required for combustion to take place
• Smoke Emission - volume and optical density for a set quantity of material
• Toxicity of Combustion Gases – Concentration (part-per million) in smoke emission
• Flame spread – the distance over which burn occurs in a set time.
These properties can be determined for a particular fibre type or the resulting fabric using ISO or ASTM standard test methods (See Materials/test methods)
When the inherent flame and heat resistant properties of a material do not attain the required specifications, flame retardant (FR) chemicals (See Materials) may be applied to the fabric as finishing treatments (See Materials – Flame Retardants Chemicals-Case Studies) or FR additives (usually as fine particles - micro or nanoparticles) (See Materials – Flame Retardant Additives) mixed into a polymer applied as a coating or compounded with the polymer that is to be converted into fibres. Conventional finishing and coating technologies may be used, respectively, to apply FR chemicals and additives, and synthetic polymers compounded with additives are conventionally melt-spun. (See Filament?-production/">Technologies) Beyond ‘state-of-the-art’
When the inherent flame and heat resistant properties of a material do not attain the required specifications, flame retardant (FR) chemicals (See Materials) may be applied to the fabric as finishing treatments (See Materials – Flame Retardants Chemicals-Case Studies) or FR additives (usually as fine particles - micro or nanoparticles) (See Materials – Flame Retardant Additives) mixed into a polymer applied as a coating or compounded with the polymer that is to be converted into fibres. Conventional finishing and coating technologies may be used, respectively, to apply FR chemicals and additives, and synthetic polymers compounded with additives are conventionally melt-spun. (See Technologies) Beyond ‘state-of-the-art’ technologies are being developed which involve plasma, laser and microwave systems. As essentially, these are not wet processes and have little environmental impact, in terms of water usage, energy requirement and waste generated, they are classed as clean technologies with significant potential for new product development and more competitive process economics. (See Technologies)
Regulations and Standards
Protective Clothing Regulations
The flammability regulations of the European Union and USA effectively form the basis for many national regulatory bodies throughout the world.
The European Personal Protective Equipment (PPE?) Directives, published in 1989 and obligatory since the mid 1990s, have led to the development and use of a very wide range of European (EN) specifications for PPE?.
CEN, the European Standardisation body, has developed several product specifications published as ENstandards that set out specific methods of tests and related performance levels for clothing. Although the application of the two European PPE? directives, one for CE certification of products and the other for product use, are mandatory it is not mandatory to use EN product specifications for CE certification; however, almost all fire protective clothing in use in Europe is CE certified using EN standards because this is the most straightforward route and therefore understood best by both the PPE? manufacturers and their clients.
In the US the Occupational Health & Safety Administration (OHSA) sets policies [29 CFR 1910.132(a)] on the general industry standard for personal protective equipment (PPE?), incl. fire protective clothing. The National Fire Protection Association designates the standard [NFPA 1971] on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting, which specifies "the minimum design, performance, safety, testing, and certification requirements for structural fire fighting protective garments that include coats, trousers, coveralls, helmets, gloves, footwear, and interface components.”
Fire fighters' protective clothing (pants and jacket) is a three-component ensemble intended to protect the fire fighter from radiant and thermal exposure, unexpected flashover conditions, and puncture and abrasion hazards while still maintaining an adequate level of dexterity and comfort. The performance requirements for the individual components (moisture barrier, thermal liner, and outer shell) and the ensemble are described in NFPA 1971, whereas the selection, care, and maintenance of the "turnout gear" is described in NFPA 1851. The American Society for Testing of Materials (ASTM) provides specifications for the minimum requirements in labelling protective clothing as heat and flame resistant for applications where the potential for flame contact or high heat exposure exist, and defines the heat and flame resistance of materials used in protective clothing .
Textiles for aircraft seats, head rests and curtains must comply with current FAA and JAA regulations. Federal Aviation Regulations (FAR)
Textiles for rail and other non-aerospace public transport should comply with prCEN/TS 45545-2 which applies to combustible materials in respect of their potential heat release. The heat release rate is determined with the Cone calorimeter according to ISO 5660-1.
The basis of CE-marking of construction products and building elements is the European system of fire testing and classification of products and elements. Fire safety classification of products is in accordance with the Euroclass system. Building elements and structures are to be tested and classified in respect of their fire resistance performance and smoke emission. The Euroclass system indicates the performance by a letter - e.g. R, E or I - and an index that indicates the time for which the property is maintained, e.g. RE60.
The International Organization for Standardization (ISO), The National Fire Protection Agency (NFPA), The American Society for Testing of Materials (ASTM) and their national counterparts have established a consistent set of performance criteria and test methods for flammability related PPE? materials. The following reference is a guide to such test standards. (See 1 to 5)
For aviation flammability requirements, evaluations are carried out in accordance with ASTM, CGSB, CAN/ULC, NFPA, ISO, MVSS, Bombardier and Boeing standards- Fire testing for textiles including testing in accordance to CAN/CGSB, FF 5-74, NFPA 701, and UFAC. (covers: Furnishings and Contents; Smoke and Combustion Products; Transportation)
49 CFR Appendix B to Part 238 - Test Methods and Performance Criteria for the Flammability and Smoke Emission Characteristics of Materials Used in Passenger Cars and Locomotive Cabs
Standard Test Method for Determining the Heat Release Rate and Other Fire-Test-Response Characteristics of Wall Covering or Ceiling Covering Composites Using a Cone Calorimeter
Terms & Definitions
Terms and their definitions are intended to provide a precise understanding and interpretation of the fire-test-response standards, the fire-hazard-assessment standards, and the fire-risk-assessment standards in which they appear. This terminology covers terms, related definitions, and descriptions of terms used or likely to be used in fire-test-response standards, fire-hazard-assessment standards, and fire-risk-assessment standards.
Basic Principles of Combustion
Burning or combustion is a chemical process which occurs when oxygen combines /reacts with another substance producing sufficient heat and light (exothermic reaction) to cause ignition. The chemical process is called oxidation Oxidation of materials takes place continuously so long as the material is exposed to oxygen (or an oxidizing agent – e.g. air) . At normal temperatures, the rate at which oxidation occurs is slow, and the heat generated negligibly small and is naturally conducted away from the material by the immediate environment. As the temperature rises above ambient, the oxidation rate increases, the heat being released becomes significant, and pyrolysis takes place; this is the decomposition of materials by the action of heat. HPFs have very strong bonds and require high heat energy to break them. As bonds break, the bond fragments can form combustible substances liberated as a gas, depending on the initial chemical composition of the base material. The amount of gas liberated increases with temperature, and when its ignition temperature is reached (forced ignition or auto-ignition) burning occurs. However, there must be sufficient oxygen present to combine with the gas molecules to generate the amount of heat that will raise the temperature to the point of ignition.
The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical will ignite decreases as the pressure increases or oxygen concentrationincreases. It is usually applied to a combustible fuel mixture.
If the heat released is sufficient to sustain or increase the oxidation rate then burning will continue until the material is consumed. The heat released from continual burning can reach a level which causes neighbouring flammable materials to ignite; flash over is said to have occurred.
Flashover from Heat Release
A stage during combustion within an enclosed space when surfaces exposed to thermal radiation (heat release) from already burning items reach ignition temperature and the resulting flames spread rapidly throughout the space (flashover) resulting in a full room fire
If the oxygen present is made to react, preferably with some other substance added that gives low heat release and does not combust at the temperature reached, the flame will go out and the base material will cool. Alternatively, if the base material is shielded by a heat absorbing or conducting material, the reacting gases will lose heat faster than they generate it, the temperature will drop and the effect of shielding will be to prevent further liberation of gases. The fire would then be extinguished.
Often the factors involved in the process of first chemical oxidation followed by pyrolysis and combustion are depicted by what is called the ‘fire triangle’, when all sides ( the factors) of the triangle are in place, burning occurs; when any one of the sides is removed, burning stops. For non-HPFs-textiles, flame retardant finishes or additives have the effect of removing one or more of the sides. They work by either a chemical or physical action.
Physical Action: Two mechanisms may be induced:- a) heat absorbing reaction (endothermic) can be initiated by the rise in temperature leading to the release of water and/or carbon dioxide, diluting the flammable gases; b) the flame retardant physically combines with the base material to effect a char layer during the early stage of burning, thereby reducing the temperature and inhibiting further release of the flammable gas.
For examples of the above retardants see Materials/FR chemicals & Additives
The principle mechanism of combustion fire may be summarized as follows :
- There must be an oxidizing agent, combustible material, and a source of ignition for combustion to take place.
- Combustible material must be heated to its ignition temperature before it will burn.
- Combustion will continue until-
(a) The combustible material is removed or consumed.
(b) The oxidation agent concentration is lowered below that
(c) The combustible material is cooled below its ignition
(See Case Studies)
Recommended Additional Readings: