10.1002/spepro.003569
Is calcium carbonate still
reasonable as cheap and inert
filler?
Nihat Ali Isitman, Mehmet Dogan, Erdal Bayramli, and
Cevdet Kaynak
Calcium carbonate, a widely used, abundant filler, interacts
adversely with halogen-free, flame-retardant additives in polypropylene composites.
Polyolefins are ubiquitous commercial polymers used in a broad range
of products from consumer goods to food packaging. They are, however, inherently flammable, and thus for certain applications must be
treated to meet stringent fire-safety specifications. Halogenated compounds (such as those used in electronics) constitute one well-known
class of flame-retardant additives. But they are subject to environmental regulation, and some compounds have already been restricted
due to their toxic and corrosive decomposition products. A common
halogen-free flame-retardant additive system used in industry is based
on polyphosphates combined with a char-forming agent. (Char is the
solid material that remains after the initial stage of combustion of
carbonaceous compounds). This ensures that, on exposure of plastics
to heat in a fire, an inhibitory mechanism—intumescence—becomes
operational. Intumescence is usually referred to as the formation
of a blown char layer on an exposed material surface that acts as a
physical barrier to transfer of heat, fuel, and oxygen during flaming
combustion.
Calcium carbonate (CaCO3 / is an abundant filler that is widely used
for cost reduction and to provide stiffness (rigidity) to the polyolefin
polypropylene (PP). It also has moderate beneficial effects on mechanical properties such as an increase in modulus and impact strength.
Accordingly, it is worth examining the interactions of intumescent
flame-retardant (IFR) additives with CaCO3 -filled composites. In this
context, inorganic fillers such as talc, CaCO3 , and zinc carbonate do
not interfere with the intumescent process in filled polyamide-6 compounds, yet they increase the thermal shielding efficiency of chars.1
Similarly, the presence of talc increased the fire-retardant performance
of ammonium polyphosphate-filled PP/polyamide-6/ethylene vinyl
acetate blends through char enhancement.2 However, a recent report
on the fire-retarding properties of intumescent PP composites found
Figure 1. Combustion behavior of intumescent (inhibiting flammability) polypropylene (PP) composites with and without calcium carbonate (CaCO3 ). IFR: Intumescent flame-retardant system. IFR1:
Mixture of ammonium polyphosphate and pentaerythritol. IFR2:
Surface-modified ammonium polyphosphate.
that probable interactions between talc and phosphate species lead to
increased flammability.3
In view of these contradictory results, further study is required of
possible interactions between IFR additives and fillers of high commercial importance, such as talc and CaCO3 . Therefore, we investigated the influence of the latter compound on the fire retardancy of
intumescent PP composites.4 We examined two IFR systems: a
Continued on next page
10.1002/spepro.003569 Page 2/3
Table 1. Flammability and fire-retardant properties of PP composites.
PP
PP/IFR1
PP/IFR1/CaCO3
PP/IFR2
PP/IFR2/CaCO3
Limiting
oxygen index
(volume % O2 )
17.5 ˙ 0.2
29.3 ˙ 0.3
19.7 ˙ 0.2
33.1 ˙ 0.2
21.1 ˙ 0.2
UL-94
flammability
rating
No rating
No rating
No rating
V-0
No rating
Maximum rate of
heat release
(kW/m2 )
883 ˙ 28
280 ˙ 11
224 ˙ 18
175 ˙ 25
220 ˙ 33
Figure 2. Scanning electron microscopy images of chars on combustion. (a) PP/IFR. (b) PP/IFR/CaCO3 .
mixture of ammonium polyphosphate and pentaerythritol (IFR1) and a
surface-modified ammonium polyphosphate (IFR2).
Table 1 lists the performance of various composites in flammability
tests (limiting-oxygen index and UL-94 standard ratings) and in a simulated fire scenario. CaCO3 -filled formulations (PP/IFR1/CaCO3 , and
PP/IFR2/CaCO3 / showed only slightly higher limiting-oxygen indices
than unfilled PP. In particular, the UL-94 V-0 rating (the most restrictive flammability criterion) of self-extinguishing PP (PP/IFR2/CaCO3 /
was degraded to ‘no rating’ on incorporation of CaCO3 . The presence of CaCO3 —especially in PP/IFR2/CaCO3 , which contains
surface-modified ammonium polyphosphate—resulted in significantly
increased maximum rates of heat release and mass loss during
combustion in a simulated fire. Accordingly, composites containing
CaCO3 in conjunction with intumescent additives are characterized by
lower fire-performance indices.
Figure 1 compares the heat-release- and mass-loss-rate curves of PP
composites that contain IFR additives with or without CaCO3 filler.
Combustion of intumescent PP composites exhibits two peaks. The first
corresponds to flame spread over the sample surface and the process
of protective barrier-layer formation. Following the first peaks in heatrelease and mass-loss curves, we observed considerable suppression
as an indication of complete establishment of intumescent barriers that
Maximum rate of
mass loss
(g/m2 s)
17.2 ˙ 3.0
7.3 ˙ 0.8
7.4 ˙ 0.2
5.3 ˙ 0.5
6.6 ˙ 0.6
Fire-performance
index
(kW/m2 s) 1
0.23 ˙ 0.04
1.88 ˙ 0.22
0.75 ˙ 0.21
1.75 ˙ 0.28
1.16 ˙ 0.24
greatly influence the combustion process. The protective phosphorouscarbonaceous chars formed in the absence of CaCO3 effectively
impede heat and mass transfer between the condensed and the gas
phases. On the other hand, typical heat-release-rate curves for samples containing CaCO3 differ greatly from those without CaCO3 . The
initial and rapid rises in heat-release and mass-loss rates followed by
gradual smooth decreases are indicative of the absence of intumescence
and formation of a nonexpanded, rigid, ceramic-like residue lacking
protective properties during combustion (see Figure 2).
In conclusion, CaCO3 does not act as an inert filler, but interacts
adversely with IFRs during their decomposition. This strongly hinders formation of protective intumescent barriers and, therefore, causes
drastic losses in flame-retarding effectiveness. We aim to find better
solutions for a halogen-free, flame-retardant additive for use in PP
composites filled with CaCO3 .
Author Information
Nihat Ali Isitman
Polymers and Nanocomposites Lab
Middle East Technical University
Ankara, Turkey
Mehmet Dogan
Department of Polymer Science and Technology
Middle East Technical University
Ankara, Turkey
Erdal Bayramli
Department of Chemistry
Middle East Technical University
Ankara, Turkey
Continued on next page
10.1002/spepro.003569 Page 3/3
Cevdet Kaynak
Materials and Metallurgical Engineering Department
Middle East Technical University
Ankara, Turkey
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c 2011 Society of Plastics Engineers (SPE)