Journal of Materials Science and Chemical Engineering, 2013, 1, 43-48 Published Online October 2013 (
Copyright © 2013 SciRes. MSCE
Silicon-Basing Ceramizable Composites Containing
Long Fibers
Zbigniew Pędzich1, Rafał Anyszka2, Dariusz M. Bieliński2, Magdalena Ziąbka1,
Radosław Lach1, Magdalena Zarzecka-Napiera ła 1
1Department of Ceramics and Refractory Materials, AGH-University of Science & Technology, Krakow, Poland
2Institute of Polymer and Dye Technology, Lodz University of Technology, Lodz, Poland
Received July 2013
Ceramization is a phenomenon which assures compactness of polymer-based composites in the case of their thermal
degradation caused by open fire or exposure at high temperatures. This phenomenon is based on preventing volatiles of
thermal decomposition of silicone rubber from evacuation by creation of ceramic layer. Thi s ceramized structure is
composed of mineral filer particles, connected by fluxing agentglassy phase. The ceramic barrier created during fir-
ing is aimed to protect copper wire inside the cable from melting, being additionally strong enough to maintain integrity
of electrical circuit. The paper presents experimental data on mechanical properties of silicone rubber composites
strengthened additionally with long fibers of different types—aluminosilicate and polyamide (Kevlar) ones. Fibers were
introduced into composites in oriented way. Mechanical properties were investigated taking into account fiber orienta-
tion anisotropy. Ceramization process of composites was described by observation of morphology and strengthen mea-
surements of samples fired at 1000˚C.
Keywords: Silicone Rubber; Ceramization; Composites; Polyamide Fibe rs ; Aluminosilicate Fibers
1. Introduction
Silicone-basing ceramizable composites are very high
loaded systems, of filler content at the level of 100 - 120
phr (part per hundred of rubber). The characteristic of the
filler particles and their interaction with silicone matrix
decides the mechanical properties of the composites and
it is important from the point of view of mix preparation
and its further processing by extrusion.
The type, the volume, the grain size distribution of
these additives are decisive for shaping of microstruc-
ture of ceramized body [1-8]. Moreover, ceramization
condition can strongly influence its final microstructure
and other properties [9-11].
Three major processes occurring during fire and sup-
porting ceramization can be featured: 1) sint er in g of
mineral filler particles due to hydroxyl groups condensa-
tion which are presented on its surface, what leads to
obtaining larger ceramic domains. 2) creation of silica
bridges between mineral particles as a result of thermal
decomposition of silicone rubber matrix in presence of
oxygen and mineral powders which surfaces can act as a
catalyzer of silica creation. 3) inc orporation of a low sof-
tening point temperature glassy frit (fluxing agent) which
particles melt in elevated temperature and create connec-
tions between mineral filler particles [12-18].
Continuous and porous structure which can be ob-
tained protects covered elements ( for example copper
wires) against thermal and mechanical external stresses
and can maintain functioning of important devices (sprin-
klers, lifts, etc.) even up to three hours under fire condi-
tions (e ven when temperature approaches cooper melting
point) [19-23].
Presented paper concerns the possibility of using long
fibers as the reinforcing agent. These fibers added simul-
taneously with other fillers could improve mechanical
properties of composites in vulcanized state and also they
could influence properties of composite bodies after ce-
ramization. The possible application of investigated ma-
terials is cable industry. The only effective method of
cable manufacturing is extrusion process. Such method
could be utilized to easy orientation of long fibers paral-
lel to extrusion direction and consequently to fabrication
of composites with strong anisotropy of mechanical
2. Experimental
Investigated materials were composites prep ared on the
base of silicone rubber containing 40 phr (part per hun-
Copyright © 2013 SciRes. MSCE
dred of rubber) reinforcing, fumed silica. These premixes
were produced by Silicony Polskie Nowa Sarzyna (Po-
land). The reference material was composed of 40 phr of
wollastonite TER M IN 283 originated from Qarzwerke
(Germany) and also 20 phr of fluxing agent—glass frit
FR2050 (commercial grade, Reimbold & Strick, Ger-
Additionally, two types of composites were elabo-
rated—one containing aluminosilica fibers Rockseal
RS440—Roxul 1000 (Lapinus Fibers, The Netherlands)
and the other containing 1F1417 Kevlar fibers (DuPont,
France). Different composites were prepared containing
5, 10 and 15 phr of each typ e of strengthening fibers.
Composite mixes were prepared using a Brabender
(Germany) Plasticorder internal micro-mixer, operating
with rotors speed of 20 rpm during incorporation of the
components and 60 rpm during their homogenization (30
min). Sheeting of mixes and orientation of fibers in sili-
cone matrix were made on a David Bridge (UK) two-roll
mill at 40˚C.
The crosslinking process of composites containing
Kev lar fibers demanded parallel using of two agents be-
side dichlorobenzoyl peroxide, dicumyl peroxide were
In this paper investigated materials were described as:
REF—composite containing additives of silica, fluxing
agent and wollastonite; LAPcomposites containing alu-
minosilicate fibers, additionally; KEV—composites con-
taining Kevlar fib ers , additionally.
Mechanical properties (SE100, SE200—stress at 100 and
200% elongation respectively, TS—tensile strength, and
Eb—elongation at break) of vulcanized composites were
investigated using Zwick 1435 instrument.
Microstructures of broken samples were observed us-
ing Hitachi S-3000N scanning electron microscope under
100 Pa vacuum with BSE analysis mode.
Ceramization tests were performed by heating with a
relatively slow temperature increase from 20˚C up to
1000˚C and 20 minutes of soaking time at maximum
Ceramized samples were subjected to compression
tests using a Zwick Roell Z2.5 instrument (Germany).
The maximum force required to destroy a composite
sample was detected. The sample was in shape of disc 15
mm in diameter and 10 mm in height. Strength test was
performed as compression probe when stre ss was applied
perpendicularly to sample diameter. The mechanical
strength of the ceramized materials was calculated as an
average value of 5 probes.
Microstructures of ceramized samples were examined
by scanning electron microscopy using Nova Nano SEM
200 (FEI, UK) apparatus.
The pore size distribution of ceramized materials was
analyzed using mercury porosimetre Poremaster 60
(Quantachrome, USA).
3. Results and Discussion
Mechanical properties of the silicone composites tested
paralelly (||) or perpendicularly () to fibers arrangement
in composites are presented in Table 1.
It is worth to notice that REF material containing
mainly wollastonite as mineral filler has the average lev-
el of mechanical parameters. Incorporation of aluminosi-
licate long fibres significantly increased tensile strength
of composites in the 5 - 10 phr range of incorporated
fibers amount. This strengthening effect was much dis-
tinct when it was measured in direction parallel to load-
ing direction. Composites containing Kevlar fibers were
much more stiffer. Strengthening effect was observed in
the case of fibers addition on 5 phr level but elongation at
bre ak (Eb) was shorter. Addition of higher amounts of
Kev lar fibers (10 and 15 phr) did not lead to tensile
strength increase and stiffn e ss of the samples increased
Figures 1-3 illu strates broken composite surfaces af-
ter tensile strength measurements. In the Figure 1 elon-
gated grains are wollastonite particles. They are rather
brittle and their influence to tensile strength is not so big.
Figure 2 shows the L AP composite fracture surface.
Clearly visible are aluminosilicate fibers broken perpen-
dicularly to fiber diameter. It suggests that adhesion of
fibers to matrix is strong enough to improve composite
Table 1. Mechanical properties of investigated composites.
SE100 [MPa] SE200 [MPa] TS [MPa] Eb [%]
REF 2.0 3.1 3.9 255
LAP 5 2.0 3.2 4.2 265
LAP 5 | | 2.6 3.8 5.4 285
LAP 10 2.1 3.2 4.5 270
LAP 10 | | 2.7 3.8 5.7 295
LAP 15 2.1 3.2 4.2 270
LAP 15 | | 2.6 3.7 4.0 220
KEV 5 2.8 4.1 4.7 240
KEV 5 | | 3.7 4.9 5.1 210
KEV 10 3.4 - 3.4 115
KEV 10 | | - - 3.2 30
KEV 15 - - 3.6 85
KEV 15 | | - - 4.7 35
Copyright © 2013 SciRes. MSCE
Figure 1. SEM micrograph of a typical area of REF compo-
site after the strength test.
Figure 2. SEM micrograph of a typical area of LAP compo-
site after the strength test.
Figure 3. SEM micrograph of a typical area of KEV com-
posite after the strength test.
strength. Moderate in crease of the tensile strength in the
case of composites containing aluminosilicate fibers (5
and 10 phr) arranged perpendicularly to tensile force was
probably caused by easier fracture of long brittle fibers
during loading not parallel to fiber axis. This effect was
not observed in the case of composite containing 15 phr
of aluminosilicate fibers . Its tensile strength measured for
both fiber orientations was similar and bigger about 10%
only than this measured for composite without fibers.
This fact indicated the limit of effective acting of alumi-
nosilicate long fib e rs addition.
The analyze of broken surface of composites contain-
ing the Kevlar fibers showed in all cases the presence of
fibers distinctly pulled apart of the material. In Figure 3
these fibers are clearly visible as elongated gray objects.
The poor adhesion of Kevlar fibers to silicone rubber and
the rest of composite components limited the strengthen-
ing effect only to composition containing 5 phr of fibers.
Addition of higher amounts of Kevlar fibers (10 and 15
phr) caused significant increa se of composite stiffness.
This effect is very strong and could not be profitable ca-
ble for industry demands.
Microstructures of ceramized samples are presented in
Figures 4-6. The analysis of these images indicates that
REF composite after ceramization is composed of big
wollastonite grains and small silica ones. As it was de-
termined by mercury porosimetry the total porosity of
this composite was about 61.6%. The total porosity of
LAP composites was changed between 59.5 and 62.6%
depending on fibers content ( 5 - 15 phr).
The total porosity of KEV composites was changed
between 60.7 and 64.5% depending on fibers content (5 -
15 phr).
The main difference between LAP and KEV compo-
sites microstructures after ceramization was the fact that
aluminosilicate fibers after firing still were present in the
Figure 4. SEM micrograph of a typical area of ceramized
REF composite sample.
Copyright © 2013 SciRes. MSCE
Figure 5. SEM micrograph of a typical area of ceramized
LAP composite sample.
Figure 6. SEM micrograph of a typical area of ceramized
KEV composite sample.
composite microstructure (see Figure 5). Kevlar fibers
after firing at 1000˚C disappeared (see Figure 6).
However, investigation of ceramized composites sam-
ples did not show significant differences between the
total amount of pores volume and their size distribution
among all types of composites (see Figures 7 and 8).
Additionally, no significant differences in porosity were
detected between samples containing 5, 10 or 15 phr of
long fibers. It means that microstructures of ceramized
bodies were very similar. Ceramized bod ies showed
pores bigger than 1 micrometer. It means that fluxing
agent acted very effectively and it joined together wol-
lastonite particles and ultrafine silica particles produced
Figure 7. The cumulative curves of pore size distribution of
ceramized reference material (REF) and containing 5 phr of
mineral (LAP) or Kevlar (KEV) fibers.
Figure 8. The differential curves of pore size distribution of
ceramized reference material (REF) and containing 5 phr of
mineral (LAP) or Kevlar (KEV) fibers.
as silicone rubbe r decomposition effect. Two fractions of
bigger pores (~5 micrometers and ~20 micrometers)
were created as an effect of agglomeration.
The presence of long fibers was not so important for
this process. The aluminosilicate fibers survived the ce-
ramization process and they stayed present in the cera-
mized bodies. The Kevlar fibers disappeared during fir-
ing but it has no significant influence for the microstruc-
ture of ceramized bodies, most probably due to relatively
low viscosity of mixture of fluxing agent and ceramiza-
tion products.
Figure 9 summarized strength of ceramized bodies
Copyright © 2013 SciRes. MSCE
Figure 9. Plots of breaking force dependence vs. fibers con-
tent for mineral (LAP) and Kevlar (KEV) fibers. Samples
were ceramized at 1000˚C.
investigations. Differences observed between elaborated
samples were not significant. One can observe a slight
increase of strength for composites containing 5 phr ad-
ditions of both types of fibers. Such results were proba-
bly the effect of microstructure uniformization at high
temperatures due to flowing of the fluxing agent.
4. Summary
Performed experiment confirms the possibility of rein-
forcing of silicone rubber-basing composites containing
wollastonite particles with addition of long fibers.
The performance of specially oriented microstructures
was successful and it was established that addition of
small (5 - 10 phr) amounts of aluminosilicate fibers sig-
nificantly in creased composite tensile strength.
The Kevlar fibers have no good adhesion to the others
material compounds and only the minimal used level of
fibers gave property improvement.
All investigated materials showed good properties af-
ter firing. Ceramized bodies were tight, uniform and
strong enough to be used in fire-protecting application.
Performed test recommended composites containing
aluminosilicate fibers as more perspective for mentioned
5. Acknowledgements
The work was financially supported by the European
Union within a framework of National Coherence Strat-
egy under Innovative Economy Operating Programme
(grant no. POIG.01.03.01-00-067/08) and by the Polish
State Ministry for Science and High Education (AGH
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