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)