Combustion of Polymers
Retardancy due to char formation and addition of inert
Two mechanisms of fire retardancy
A conceptually simple, and useful, approach to describe fire retardants is to
classify them, depending upon the phase of their activity,
as either being solid-phase active or gas-phase active.
Fire-retardants work by breaking the combustion cycle in
one of three ways: modification of the thermal degradation process;
quenching the flame;
reduction of the supply of heat from the flame back to the polymer surface.
Their mechanism of activity can be described as being either physical
The mode of action of most fire retardants is understood only in the most
general terms. Although this simplified description is
useful, there is no doubt that the best additive packages work in
more than one way in more than one phase.
Kashiwagi has identified char formation as the most promising
general form of fire-retardancy and has reviewed its benefits in
improving the fire-resistance of polymers
Its advantages are:
- Reduced mass of volatiles.
Part of the carbon (and hydrogen) stays in the
condensed phase, reducing the mass of volatile combustible degradation
- Thermal insulation.
As the polymer degrades, a char layer is formed over the remaining virgin
polymer. The low thermal conductivity of this layer enables
it to act as thermal insulation, absorbing some of the heat input
and therefore reducing the heat flux reaching the
(Anderson et al 1988;
Camino et al 1988).
In addition, re-radiation losses increase significantly as the char
surface temperature increases, further protecting the polymer.
Both of these processes help to prevent thermal degradation.
- Obstruction of combustible gases.
A charred surface may act as a physical barrier, obstructing the flow of
combustible gases generated from the degradation of the underlying
unburnt material, and hindering the access of oxygen to the surface of the
polymer (Camino et al 1988).
Two general mechanisms of char formation can be identified, competitive and
non-competitive. By non-competitive char formation we mean the scheme
M1 --> c C1 +(1-c) V,
where M1 is the polymer,
is char and V represents gaseous volatiles.
By competitive char formation we mean the reaction scheme
M1 --> V Equation 2.
M1 --> c C1 +(1-c) V
Although both the non-competitive and competitive schemes represent
a considerable chemical
simplification they offer the practical advantage that only
one-or-two reactions need be considered.
The mechanism of char-formation has been investigated in detail,
particularly for cellulose
(Kandola et al 1996).
However, kinetic parameters
are unknown for the majority of steps in these detailed schemes. Thus highly
simplified chemical schemes serve a practical purpose. In this paper we
restrict attention to non-competitive char-formation.
A discussion of the modelling of char-formation,
relevant to the combustion of polymers, is
provided elsewhere Nelson et al
An inert filler is material that is chemically inert. Their
advantages are threefold
- Reduced mass of fuel.
Since the size of the sample is specificed by the test
method the addition of an inert filler reduces the mass of fuel in the
- Thermal insulation. As a polymer-filler composite burns,
the polymer at the top of the sample vapourises leaving behind the
filler. Thus a layer of filler is built-up that resides upon virgin
- Obstruction of combustible gases. The inert-filler layer that
builds up during combustion may obstruct the flow of combustible gases
The last two mechanisms are common to both char-formation and inert
fillers. However, char-formation usually results in a consolidated
residue whereas the residue is
unconsolidated for inert fillers. Thus these mechanisms are expected
to be much less
efficient for an inert filler. For the products considered in this
paper the most important mechanism by which an inert filler operates
is through the reduction in the mass of fuel in the sample.
Experimental data is frequently reported using
`percentage of additive in the
sample by mass' as the control parameter.
When the mass of the fuel in the sample is calculated using
this approach it is found to be a nonlinear function of the
polymer density and the additive density. A more attractive way to measure
the role of the additive is to use a fuel mass fraction
(Nelson et al 1995,
- C.E. Anderson Jr, D.E. Ketchum, and W.P. Mountain
1988. Thermal conductivity of intumescent chars.
Journal of Fire Sciences 6 390-410.
- G. Camino, L. Cota, E. Casorati, G. Bertelli, and
R. Locatelli 1988. The oxygen index method in fire retardance studies
of polymeric materials. Journal of Applied Polymer Science
- B. Kandola, A.R. Horrocks, D. Price, and
G.C. Coleman 1996.
Flame retardant treatments of cellulose and their mechanism of
cellulose pyrolysis. Journal of Macromolecular Science - Reviews in
Macormolecular Chemistry and Physics C36(2)
- T. Kashiwagi 1994.
Polymer combustion and flammability - role of the condensed phase.
25th International Symposium on Combustion
(Pittsburgh PA: Combustion Institute) pp 1423-37.
- M.I. Nelson, J. Brindley, and A.C. McIntosh
1995. The dependence of critical heat flux on fuel and additive
properties: A critical mass flux model. Fire Safety Journal
- M.I. Nelson, J. Brindley, and A.C. McIntosh 1996.
Ignition properties of thermally thin thermoplastics - The
effectiveness of inert additives in reducing flammability.
Polymer Degradation and Stability 54(2-3)
- M.I. Nelson, J. Brindley, and A.C. McIntosh
. A critical mass flux model for char formation. I Non-competitive
char formation in thermally thin samples.
Accepted for publication in
Journal of Applied Mathematics and Decision Sciences,
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Last Updated: 19th August 2002.