Combustion of Polymers
Oxygen-Index Methods
Introduction
Oxygen index methods, which describes the tendency of a material to
sustain a flame, are widely used as a tool to investigate the
flammability of polymers. They provide a convenient, reproducible, means
of determining a numerical measure of flammability. A further attraction
is that the test method uses inexpensive equipment and only requires a
small sample size. These methods have been used to systematical
investigate the relative flammabilities of fire-retarded materials,
frequently comparing the effectiveness of fire-retardants and
fire-retardancy mechanisms.
The quintessential feature of oxygen-index methods is that the sample is
burnt within a controlled atmosphere. The standard procedure is to
ignite the top of the sample, using a gas flame which is withdrawn once
ignition has occurred, and to find the lowest oxygen concentration in an
upward flowing mixture of nitrogen and oxygen which just supports
sustained burning. The criticality criterion typically takes the form of
a minimum burning length: either specifying that the sample must burn
for a certain length of time or that a specified length of material be
consumed. The effectiveness of fire retardants is measured by the change
in the critical oxygen concentration that they induce as a function of
their concentration.
The limiting oxygen index (LOI), also called the critical oxygen index
(COI) or oxygen index (OI), is defined as:
LOI = [O2,cr] Equation (1),
-------------
[O2,cr] + [N2]
where [O2,cr] and [N2] are the minimum oxygen concentration in the
inflow gases required to pass the ``minimum burning length'' criterion
and the nitrogen concentration in the inflow gases respectively. If the
inflow gases are maintained at constant pressure then the denominator of
equation~(1) is constant since any reduction in the partial pressure
(concentration) of oxygen is balanced by a corresponding increase in the
partial pressure (concentration) of nitrogen. Limiting oxygen index is
more commonly reported as a percentage rather than fraction.
Since air comprises about 20.95% oxygen by volume, any material with a
limiting oxygen index less than this will burn easily in air.
Conversely, the burning behaviour and tendency to propagate flame for a
polymer with a limiting oxygen index greater than 20.95 will be reduced
or even zero after removal of the igniting source. Self-sustaining
combustion is not possible if LOI>100, such values are not physically
meaningful.
In (Nelson 2001) we investigated how the introduction of a
fire-retardant changes the oxygen index of a material. For this purpose
it is useful to assign materials into experimentally meaningful
groupings depending upon their oxygen index. The minimum level of
retardancy required to increase the classification of a material can
then be calculated. From the preceding paragraph two obvious groupings
are LOI<20.95$ and LOI>100. We refer to materials satisfying these
requirements as being ``flammable'' and ``intrinsically non-flammable''
respectively. Several researchers have suggested that materials with a a
limiting oxygen index greater than 28 are generally self-extinguishing
(Horrocks et al 1989). We describe materials satisfying 28.00 < LOI <
100 as being ``self-extinguishing''. The threshold LOI=20.95 is of great
practical interest and we define materials with a limiting oxygen index
of 20.95 as being ``marginally stable''. We follow Fenimore (1975) and
refer to materials that are between the marginally stable and
self-extinguishing thresholds, i.e. 20.95< LOI< 28, as being
``slow-burning''.
Marginally-stable materials form a natural set for a quantification of
the efficiency of fire-retardant mechanisms. We achieve this by finding
the value of the relevant continuation parameter to increase the LOI of
these materials to 28.0, the transition between slow-burning and
self-extinguishing polymers, and to 100, the threshold for intrinsically
non-flammable materials.
It should be realised that our classification of materials (flammable,
slow-burning, self-extinguishing, intrinsically non-flammable) is
specific to the limiting oxygen index test, i.e. a material that is
self-extinguishing here is not necessarily self-extinguishing in another
test method. The tenet in the limiting oxygen index is that the higher
the value of the LOI the `safer' the material. However, we stress that
results from one test method do not necessarily agree with another
(Emmons 1974). The reasons for this were alluded to in the opening
paragraph. Thus throughout this paper an assignment of a material as
being self-extinguishing is short-hand for ``self-extinguishing in the
limited oxygen index test''.
Additional details of oxygen-index methods and their applications,
particularly to assessing the burning behaviour of textiles, are
provided in the comprehensive review by Horrocks et al (1989).
Dynamical systems models for polymer combustion
A complete description of the mechanisms leading to the establishment of
a flame over a burning surface requires consideration of mass and heat
transport in both the gas and solid phases. Although the overall
phenomena are complicated, two salient processes, one in each phase,
must occur if a material is to ignite. The solid must first decompose to
release volatiles into the boundary layer. These gases must then mix
with surrounding air to produce a flammable mixture, which then either
autoignites or is ignited by an external source, such as a pilot flame.
Traditionally fire scientists have used highly simplified models which,
typically, examine these key processes in isolation. Recently non-linear
dynamical systems models have been developed describing these processes
(Rychlý and Rychlá 1986; Búcsi and Rychlý 1992; Rychlý and Costa 1995;
Rychlý and Rychlá 1996; Nelson 1998)
Rychlý and co-workers have developed a two-phase dynamical systems model
describing the transient burning behaviour of polymers in the limiting
oxygen index test and the cone calorimeter (Rychlý and Rychlá 1986;
Búcsi and Rychlý 1992; Rychlý and Costa 1995; Rychlý and Rychlá 1996).
This model has been used to investigate the action of certain types of
fire retardants (Rychlý and Rychlá 1986; Rychlý and Rychlá 1996) and it
has been established that there is a good coincidence between calculated
and experimental values (Rychlý and Costa 1995). It has been validated
as a suitability tool to investigate polymer ignitability and burning,
capturing the essence of the two test methods.
Nelson et al introduced a revision of the Rychlý limiting oxygen index
model. The essential features of the model were retained, some
inconsistencies in the modelling of certain physical and chemical
processes being eliminated. It was shown that a limiting oxygen index
can be defined in a steady-state formulation as an extinction limit
point. In (Nelson 2001) the revised Rychlý model was extended to
consider two solid-phase fire-retardant mechanisms: non-competitive char
formation and dilution by addition of an inert filler. We investigated
how effective these mechanisms are at increasing the oxygen-index,
paying particular attention to the retardation of marginally-stable
materials.
References
A. Búcsi and J. Rychlý 1992. A theoretical approach to understanding the
connection between ignitability and flammability parameters of organic
polymers. Polymer Degradadtion and Stability 38 33-40.
H.W. Emmons 1974. Fire and fire protection. Scientific American, 231(1)
21-27.
C.P. Fenimore 1975. Candle-type test for flammability of polymers. In
Flame-retardant polymeric materials volume 1, editors M. Lewin, S.M.
Atlas, and E.M. Pearce (New York: Plenum) pp 371-397.
A.R. Horrocks, M. Tunc, and D. Price 1989. The burning behaviour of
textiles and its assessment by oxygen-index methods. Textile Progress
18(1-3) 1-205.
M.I. Nelson 1998. Ignition mechanisms of thermally thin thermoplastics
in the cone calorimeter. Proceedings of the Royal Society of London A
454 789-814.
M.I. Nelson 2001. A dynamical systems model of the limiting oxygen index
test: II. Retardancy due to char formation and addition of inert
fillers. Combustion Theory and Modelling 5 59-83.
M.I. Nelson, H.S. Sidhu, R.O. Weber, and G.N. Mercer 2001. A dynamical
systems model of the limiting oxygen index test. ANZIAM Journal, 43(1)
105-117.
J. Rychlý and L. Costa 1995. Modelling of polymer ignition and burning
adopted for cone calorimeter measurements: The correlation between the
rate of heat release and oxygen index. Fire and Materials 19 215-220.
J. Rychlý and L. Rychlá 1986. Effect of flame retardants on
polyolefines. Fire and Materials 10 7-10.
J. Rychlý and L. Rychlá 1996. Modelling of heat-release rate-time curves
from cone calorimeter for burning of polymers with intumescence
additives. Polymer Degradation and Stability, 54, 249-254.
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Mark Hargreaves
>Sent: Monday, April 28, 2003 3:57 PM
>To: [log in to unmask]
>Subject: [TN] oxygen index
>
>
>Hi All,
>Our customer's spec requires our bare board (with soldermask)
>to have an "oxygen index" of at least 28% (tested IAW ASTM D2863-77).
>
>Is this a common requirement? Is it part of a UL listing?
>I'm wondering if I'll have to send out samples for testing.
>
>Thanks,
>Mark
>
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