Fibres and Flammability

Introduction

This is a basic introduction to the flammability of textiles. The principles covered are general to all fibres, specific items may vary in detail but will share the general concepts. A description of textile fibre types can be found here. A “piece” of fabric is constructed from yarn which in turn is composed of fibres. The fibres are made up from long molecules (polymers) the nature of which determines the fibre properties. Polymers are made from smaller molecules called monomers, it is the joining together of many monomer units in the form of a long chain which constitutes polymers.

Production of combustible fuels:

When textile fibres are heated by an ignition source, the polymer molecules start to break up (a process called pyrolysis) into smaller molecules. Some of these pyrolysis products are “fuels” capable of burning. Since the fibres from different textiles are made of different polymers, the type and amount of “fuels” produced is different for each textile type. This can be important with regard to how vigorously a type of textile burns, the amount of smoke and fumes produced and the type of flame retardant system required to treat the fabric. A generalised representation of the “ignition cycle” is shown below (figure 3) and an animation of the process can be seen here.

FireCycleImage

Figure 3. Schematic view of a ‘fire propagation cycle’.

There are three general mechanisms whereby the polymers degrade during pyrolysis. These are (1) random scission, (2) depolymerisation and (3) side group elimination. 1 & 2 involve the formation of highly energetic and reactive chemical species called “free radicals”. These free radicals are important in the polymer breakdown and are therefore involved in the formation of fuels. This is mentioned here simply because the mode of action of many flame retardants (discussed later) involves their interaction with free radicals produced during textile fibre pyrolysis.

Limiting Oxygen Index

The types and amount of fuels produced during pyrolysis vary according to the type of textile. The different fuels produced need to be mixed with oxygen (from the atmosphere) in order to burn. The amount of oxygen required to support combustion varies for each fuel. It is therefore possible to arrange textiles in order of their ease of ignitability with reference to the amount of oxygen each textile requires in order to support combustion. This is referred to as the Limiting Oxygen Index (LOI). The oxygen content of the earth’s atmosphere is about 21%. Materials with an LOI below 21 burn readily whilst those with a value above this do not burn readily. In addition to the amount of oxygen required by the different fuels to support combustion, the temperature at which different fuels ignite also varies. Table 1 below shows a number of fabrics arranged in order of their LOI with an indication of the ease with which they will ignite.

Table 1.  Fibre ignitability in relation to Limiting Oxygen Index

Fibre

Limiting Oxygen Index

Ignition Temperature (°C)

Ease of Ignition

Polyvinyl chloride (PVC)

35-40

575

Flame resistant

Wool

24-25

590

Will not ignite readily

Polyester

20-23

485

combustible

Acrylic

18-20

390

combustible

Polypropylene
17-18
350
Burns easily
Cotton

18-21

390

Burns easily

Flame Retardant Action: Flame retardants act chemically and/or physically in the solid or gas phase. They interfere with combustion by acting on the polymer at the heating, decomposition or ignition stage or by acting on flame spread should a flame be developed.

Physical Action

Cooling: The flame retardant causes an endothermic (heat absorbing) reaction in the fibre polymer thereby cooling the material below the ignition temperature. Example: Aluminium hydroxide.

Formation of protective layer: The solid polymer can be shielded from the immediate environment (i.e. the ignition source and atmospheric oxygen) by a solid or gaseous layer produced by the flame retardant. The polymer is thus cooled, fewer pyrolysis gas fuels are evolved, oxygen required for combustion is excluded and heat transfer is impeded. Example: Phosphorous and boron compounds.

Dilution of fuels: The evolution of inert gases by the flame retardant when it becomes hot dilutes both the fuel and oxygen in the vicinity of the degrading polymer. Example: Aluminium hydroxide.

Chemical Action

Gas phase reactions: The free radicals (described earlier) formed during the polymer pyrolysis are “mopped up” by the flame retardant thereby reducing formation of fuels and the associated exothermic (heat producing) reactions which occur which in turn causes the polymer to cool and in so doing produce less fuel. Example: Halogenated flame retardants (as in backcoating). This type of activity acts at the time of, or very soon after, a flame has developed.

Solid phase reactions: The flame retardant can cause a layer of carbon to form on the polymer surface. This can be induced by the dehydrating action of the flame retardant producing “double bonds” in the polymer. These bonds can induce cross-linking within the polymer leading to the formation of carbonaceous material. Example: Phosphorous compounds (as in the chemicals often used on curtaining). This type of activity would be considered as ‘modifying fuel production’ and thereby attempting to prevent a flame developing in the first place.

Another strategy used in the solid phase is to reduce the melting point of the fibres (principally the thermoplastic polymers such as polyester). This results in the formation of free radicle inhibitors in the flame front (should one develop) and causes the material to recede from the ignition source without burning.

Types of Flame Retardant

Halogenated flame retardants: Halogens are the elements fluorine, chlorine, bromine and iodine. Chlorine and bromine are used in many flame retardants with bromine being the most common and widely used halogen. The pyrolysis of fibre polymers will give rise to free radicals (typically hydrogen and hydroxyl free radicals) which are high in energy and promote the combustion process by propagating the breakdown of the polymer releasing large quantities of combustible fuels. Without getting into too much chemistry, it is sufficient to say that the halogens react with the high energy free radicals formed during pyrolysis to form low energy products (water in the case of the hydroxyl free radical) thereby eliminating the exothermic (heat producing) reactions and so lowering (eventually completely suppressing) the release of flammable gasses. A typical Halogenated flame retardant used for textiles is Decabromodiphenylether (used in backcoating flame retardants) which acts in the gas phase by releasing bromine from the backcoating when the fabric is exposed to an ignition source.

A key factor controlling the efficiency of brominated flame retardants is their thermal stability relative to the fibre polymer they are designed to work with. For a flame retardant to work efficiently, it should breakdown (thereby releasing/activating the flame retardant components) before the fibre polymer it is designed to work with starts to decompose (pyrolise) when exposed to an ignition source. Ideally, the flame retardant should decompose some 50°C below that of the polymer. Typically, brominated flame retardants such as Decabromodiphenylether break down at quite low temperatures (200-300°C). This range overlaps with the decomposition temperature of many common polymers hence the widespread use of this material for flame retarding textiles.

Phosphorus flame retardants: These flame retardants influence the reactions taking place in the condensed (solid) phase. The flame retardant is converted by thermal decomposition to phosphoric acid which extracts water from the pyrolysing polymer causing it to char (form a carbonaceous protective layer). This process modifies the breakdown of the polymer thereby reducing the amount and type of fuel produced.