Combustion of Polymers

Retardancy due to char formation and addition of inert fillers

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 or chemical. 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 (Kashiwagi 1994). 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 fragments evolved.
• 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 virgin polymer (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, Equation 1.

where M₁ is the polymer, C₁ 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 Equation 3

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 (in press).

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 sample.
• 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 polymer.
• Obstruction of combustible gases. The inert-filler layer that builds up during combustion may obstruct the flow of combustible gases and oxygen.

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, 1996).

References

1. C.E. Anderson Jr, D.E. Ketchum, and W.P. Mountain 1988. Thermal conductivity of intumescent chars. Journal of Fire Sciences 6 390-410.
2. 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 35 1863-1876.
3. 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) 721-794.
4. T. Kashiwagi 1994. Polymer combustion and flammability - role of the condensed phase. 25th International Symposium on Combustion (Pittsburgh PA: Combustion Institute) pp 1423-37.
5. 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 24(2) 107-130.
6. 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) 255-267.
7. 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, July 2000.