Silicon-containing polymers and composites 2014
San Diego, 14-17 Dec. 2014
NEW CONCEPTS FOR HIGH COHESION OF ASHES:
APPLICATION TO FIRE RESISTANT CABLES
Dr. Remi THIRIA – Science & Technology Leader / Bluestar Silicones USA / York, SC
Email: [email protected]
Dr. David Mariot – Silicone elastomers team leader / Bluestar Silicones France / Lyon
Gerald Guichard – Silicone elastomers team leader / Bluestar Silicones France / Lyon
ABSTRACT
The standard performances of silicone elastomers in a wide range of temperature (-50°C to +300°C) can be
attributed to some very basic properties of the polymer backbone (unique semi-organic structure) as well as
the composition of the formulated product.
When it is about an application in safety electrical cables, the silicone elastomer material, which has the
main role in protecting the wire from short-circuit, must be able to withstand, for a certain period of time, a
temperature over 1000°C (fire conditions). Under these extreme conditions, silicones degrade and generate
ceramic char. The integrity of this ceramic char, or ash cohesion, is critical for the performance of this safety
cable.
In order to have a better understanding of the mechanism of ash formation, we first investigated the
degradation mode of silicones under extreme heat. From what we learned, several formulation paths dealing
with improving the ash cohesion were identified and explored. Also, this technical lecture proposes a review
of our main findings as well as their application at the industrial level.
1. INTRODUCTION
Thanks to Polydimethylsiloxane backbones, Silicone Rubbers have basic intrinsic fire resistance properties.
Those performances are directly related to the Si-O bond energy that is highly resistant to oxidation
compared to organic C-C bonds. During the burning of Silicone Rubbers, some volatile and cyclic molecules
are produced and are transformed into inert silica, H2O, SiC and CO2. Consequently, low smokes emissions
are non toxic, non corrosive and halogen free. This behaviour allows a safe and quick evacuation of people
during the fire.
In addition to that, some safety installations have to continue to run during an extended period of fire (such
as the elevator, smoke aspiration systems, emergency lighting…). For this, safety cables with functional
integrity saving are required. They need to answer to numerous severe norms composed of long exposition
to fire or high temperature, shocks, high voltage, and even sprayed water (NFC 32070 CR1, BS 6387 Z
class, EN 50 200…).
Studying separately the heat and fire degradation mechanisms of silicone elastomers, the ceramization
efficiency, and the cohesion of the ashes, permit to choose the right functional fillers for a highly efficient
action. The understanding of the role of each component is the key to decrease the density of the
formulation by decreasing the dilution filler content and to deeply increase the performance under fire of
safety cables.
2. INCREASE THE ASH QUANTITY BY STUDYING THE THERMAL DEGRADATION MECHANISM
2.1 MECHANISM OF ASH FORMATION
As explained above, silicones elastomers are well known for having high temperature stability. With thermal
stability additives, they can keep relative good mechanical properties after aging. Usually, the choice and the
concentration of the additives depend on the temperature: for example, we can cite the Bluesil THT series®
for outstanding thermal stability up to 300°C (7 days). At these temperatures of work, the thermo-oxidative
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Silicon-containing polymers and composites 2014
San Diego, 14-17 Dec. 2014
degradation is the main mechanism; that is why antioxidant/redox compounds are used1. Stabilisation of
polyorganosiloxanes can be performed with organic compounds such as aromatic amines and phenols, or
with oxide compounds of Fe, Al, Be, Ti…
For safety cable applications, it is considered that cables can be exposed to higher temperature since fire
resistance is required. In this case, others mechanisms occur such as the depolymerisation that is the most
preponderant. Silicone chains are highly mobile and can then easily form cyclic transition state. When
temperature increases, formation of polydisperse cyclosiloxanes can be observed by gas chromatography2.
This lead in a decrease of molecular weight of the chain until almost the whole elastomer is volatilized (see
PDMS degradation curve Figure 1)3. To limit the chains mobility, fillers can be incorporated in the
formulation. For example, pyrogenic silicas creates numerous interactions between silica surface and the
silicone backbone through hydrogen bonds that reduce mobility of silicone chains4. However, it does not
allow changing degradation mechanism since same degradation products are formed, and the residue after
calcination is almost equal to the filler content (Figure 1).
Figure 1: Thermogravimetric analyses of PDMS, PDMS+SiO2 and PDMS+SiO2+ppm of Platinum3
However, possibilities exist to reduce these high weight losses under thermal treatment. Chemical links
between chains (stronger than the physical ones) reduce this depolymerisation behaviour. This was
observed, for example, with the incorporation of octavinyl polyhedral oligomeric silsesquioxanes that will be
intimately bond to the framework during the crosslinking step5. The final residue was 70 % increased in
comparison to the theoretical solid weight percent. It is worth noting that an increasing crosslinking density
creates high residue under heating, however one could easily imagine that it will also disrupt the mechanical
properties. This has to be keep in mind in order to follow the European Standard EN 50363-1-Ei2 that
indicates the minimal mechanical properties required.
1
T.N. Balykova, V.V. Rode, Progress in the study of the degradation and stabilisation of siloxane polymers,
Russian Chemical Reviews, 38, (4), 1969, p 306-317.
2 Camino, G.; Lomakin, S. M.; Lageard, M., Thermal polydimethylsiloxane degradation. Part 2. The
degradation mechanisms. Polymer 2002, 43, (7), 2011-2015.
3 Delebecq, E.; Hamdani-Devarennes, S.; Raeke, J.; Lopez Cuesta, J.-M.; Ganachaud, F., High Residue
Contents Indebted by Platinum and Silica Synergistic Action during the Pyrolysis of Silicone Formulations.
ACS Applied Materials & Interfaces 2011, 3, (3), 869-880.
4 Berrod, G.; Vidal, A.; Papirer, E.; Donnet, J. B., Reinforcement of siloxane elastomers by silica. Interactions
between an oligomer of poly(dimethylsiloxane) and a fumed silica. Journal of Applied Polymer Science 1979,
23, (9), 2579-2590.
5 D. Yang, W. Zhang, R. Yao, B. Jiang, Thermal stability enhancement mechanism of PDMS composite by
incorporating octavinyl polyhedral oligomeric silsesquioxanes, Polymer Degradation and Stability, 98 (2013)
109-114.
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Silicon-containing polymers and composites 2014
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Figure 2: Schematic representation of PDMS/Vi-POSS crosslinking network5
Another way to increase the final residue, with a limited impact on the mechanical properties, is the
incorporation of platinum complex in the formulation. Platinum has a huge impact on fire retardancy and is
used for improving the fire resistance for many decades6,7. At the beginning, its effect was not fully
understand, but today, many studies explained its efficiency through a temperature induced crosslinking
radical mechanism8 (Figure 3) and the huge impact of the formulation on its efficiency. A recent publication3
represented for example the threshold effect of the platinum quantity (Figure 3). When platinum quantity
increases, residue after calcination increases too. Nevertheless, under a threshold decreasing quantity (30
ppm of platinum in this example), no more impact on the final residue is observed. The red thermogram
presented Figure 1 illustrate the impact of ppm of Platinum on the started temperature degradation and on
the final residue. With only 21% of filler, residue percentage after calcination almost double (40%).
Figure 3: Temperature induced crosslinking radical mechanism in presence of Platinum and
threshold effect3
3
The authors also studied the positive impact of the presence of reinforcing filler and of the silica surface
treatment; filler treatment usually decreases the final residue with the exception of presence of reacting
groups in the treatment (like vinyl groups) that reduce this decrease.
As a result, to increase the ash quantity, one could:
- Increase the filler content
- Increase the crosslinking density
- Incorporate Platinum in the formulation
The two first bullet points are cost effective solutions, but it may badly affect the mechanical properties and
the density. In contrary, the last solution has no impact on the mechanical properties but dramatically
increases the cost of the formulation. The key leads then in being able to increase the platinum activity in
order to decrease the threshold limit. The objective is then to obtain high residue with usual filler content and
without altering the crosslinking density in order to disturb the mechanical properties as less as possible.
6
JW. Harder, Flame resistant silicone elastomers, Dow Corning, British Patent 1161052, 1969.
M.G. Noble, J.R. Brower, Flame retardant composition, General Electric Company, US 3514424, 1970.
8 K. Hayashida, S. Tsuge, H. Ohtani, Flame retardant mechanism of polydimethylsiloxane material
containing platinum compound studied by analytical pyrolysis techniques and alkaline hydrolysis gas
chromatography, Polymer 44 (2003) 5611–5616.
7
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2.2 NEW DEVELOPMENTS
One of the solution applied in our lab was first to enhance the platinum activity by incorporating functional
filler. Only few percentages of these fillers incorporated with platinum induce an increase of more than 100%
of residue in comparison to the theoretical one. Through formulation work, we could measure how great is
the impact of the type of metal oxide in the formulation, the specific surface area and the surface treatment in
order to select the fillers with the highest efficiency.
On Figure 4, the residue percentage is express in function of the platinum quantity added in the formulation.
In a common silicone formulation, the amount of platinum added as no effect on the final residue (red
results), since the residue is just slightly higher than the filler content. With few percent of functional filler
(blue results), at iso-platinum quantity, the residue is almost multiply by two, highlighting a higher platinum
efficiency in presence of such a filler. If platinum quantity decreases, it can be observed that the residue
decreases too. This indicates that thanks to the good choice of functional filler, the threshold effect could be
shifted to the lower platinum quantity.
Figure 4: Platinum efficiency on the residue and methane formation
Cone Calorimeter gas analyses presented Figure 4 corroborate these results. Methane production can be
followed thanks to Infrared analyses. This gas is emitted when the crosslinking radical mechanism occurs,
notably because of presence of platinum complexes. By this way, one can follow the platinum effectiveness
on the residue. Results point out that functional fillers induce a faster and more efficient crosslinking reaction;
the methane production is indeed more important and start earlier than without any.
Fillers have proven their ability to interact with platinum and
enhance its efficiency. This allows to drastically reducing the
platinum content. Additionally, a second way to boost the
platinum efficacy is to change platinum ligands. Recently, new
platinum species were synthesized and patented by Bluestar
Silicones9. These “carbene platinum”, as represented Figure 5,
are particularly interesting for fire retardant applications10,
especially in creating a high residue with low platinum content.
The effect of platinum from this new family is represented
Figure 4 (Green line). When low amount of platinum is used,
residue are clearly higher with carbene platinum than with
Karstedt one. Again, by reducing its quantity, the residue
decreases, but with a new shift of the threshold effect.
Figure 5: New “Carbene platinum”
patented by Bluestar Silicones9
As a result, by studying the degradation mechanism of silicone elastomers at very high temperature, different
ways of increases the residue have been identified. Each of them had been explored thanks to various
characterisation techniques. Enhancing the platinum efficiency allows to drastically increase the residue
percentage without altering the mechanical properties. Formulations can be then loaded with fillers that will
give cohesion to the residue.
3. PROVIDE COHESION TO THE ASH
When silicone elastomers are calcined, residue obtained comes from the thermally stable fillers originally
present in the formulation, but also from the in-situ produced silica that can be formed during the calcination.
9
O. Buisine, I. Marko, S. Sterin, Method for preparing metallic carbene-based catalysts for hydrosilylation of
unsaturated compounds and resulting catalyst, Bluestar Silicones, WO02/098888, 2002.
10 D. Blanc, A. Pouchelon, S. Sterin, R. Thiria, Use of a specific platinum compound in order to improve the
resistance of silicon elastomers to degradation caused by very high temperatures, Bluestar Silicones,
WO2006/095068, 2006.
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In most cases, this residue is brittle and can break just by a smooth finger touch. For safety cable
applications, it is of primarily importance to give cohesion to this residue in order to keep the functional
integrity of the electrical circuit. In other word, the produced ash has to be cohesive enough to form a
protective barrier that prevent short circuits during the tests (with shocks, water spray…).
3.1 INTERNAL TESTS TO MEASURE THE ASH COHESION
Optical characterization can give a first interesting information on the ash aspect and on its cohesion. Figure
6 represents two different ashes obtained after calcination of silicone elastomers slabs. The one on the right
appears more compact and more cohesive than the other one.
Figure 6: Visual aspect of two different ashes
obtained after calcination
Figure 7: Three point-flexure test to evaluate
the flexibility of the residue
However, in order to quantify cohesiveness, an internal test was set up by adapting a DMA program. A 3point flexure test (as represented Figure 7) was preferred to a single point test with the purpose of effectively
measuring the flexibility.
3.2 MECHANISM OF ASH COHESION
Different mechanisms have been identified to give cohesion to the ash. Again, the choice of the right filler for
a precise role is critical.
3.2.1 Melting fillers
Some kind of fillers are able to melt during the high temperature treatment and then fill the pore created by
the degradation and link particles that compose the residue. Depending on the supplier, glass frits with
different melting temperatures and different frit sizes can be found; especially “low temperature” melting
glasses are particularly interesting. Pedzich et al.11 studied the impact of increasing the amount of glass in a
formulation. Images of ceramized composites in Figure 8 illustrate the main differences between investigated
samples: with the highest quantity of glass, the powder is obviously glued.
Figure 8: SEM analyses of calcined silicone formulations containing two concentrations of glass11
11 Z. Pedzich, J. Dul, Optimization of the ceramic phase for ceramizable silicone rubber based composites,
Advances in Science and Technology Vol. 66 (2010) p 162-167.
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The boron oxide glass seems to be the most used glass family. Same team12 used for example a mixture of
boron oxide (B203) and CaO-B2O3-SiO2 glass to increase considerably the compaction force.
3.2.2 Reacting fillers
The second option is to employ filler that will react during the high-temperature treatment. Our interest was
then oriented on thermally activated solid-state reactions. A well-known example of such reactions is the
reactivity of calcium oxide with silica13. XRD Patterns of the residue attest the presence of new crystalline
structures such as wollastonite and larnite formed at 800°C (Figure 9). New chemical bonds and structures
are then formed during the degradation process, which may link the particles together. Depending on the
initial filler used, compression tests14 corroborate the possibility of strongly reinforce the ash.
Figure 9: High temperature reaction between CaO and SiO2: formation of new crystalline structure
3.2.3 Structural reinforcement
Of course, all previously described reactions and mechanisms depend on the physical chemistry of the filler
(such as the morphology, particle size, the specific surface area, the water or hydroxyl content, the surface
treatment…) and its ability to interact with its environment. However, these parameters also played an
important role on the structural reinforcement of the residue, both on packing efficiency and on the structural
reinforcement. Two different residues obtained after calcination are presented Figure 10. It can be easy to
say which sample contained lamellar fillers.
Figure 10: Ash aspect depending on the reinforcement filler used
4. CONCLUSION
By combining a bibliographic survey on all different aspect of the silicone degradation and the solid cohesion
and R&D developments, a new expertise on ash formation and cohesion was developed. With a low
platinum content, a high percentage of residue can be obtained. Different ways were then explored and
employed to bind together the structure of the ash. Each filler is then incorporate for a precise role, which
allows having formulations with lower density, but equivalent or even better properties.
Capitalising on this expertise, a new portfolio is currently in development and under evaluation, with the
objective to match the wide performance required by the variety of international standards.
12
Z.Pędzich, D. M. Bieliński, R. Anyszka, M. Zarzecka-Napierała, Influence of Boron Oxide on Ceramization
of Silicone-basing composites, Proceedings of the 12th Conference of the European Ceramic Society –
ECerS XII, Stockholm, Sweden - 2011
13 S. Hamdani, C. Longuet, J. Lopez-Cuesta, F. Ganachaud, Calcium and aluminium-based fillers as flameretardant additives in silicone matrices. I. Blend preparation and thermal properties. Polymer Degradation
and Stability, 2010, p 1911-1919.
14 Unpublished results
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