Thermal stability and flammability of polyurethane foams chemically reinforced with POSS
Thermal stability and flammability of polyurethane foams chemically reinforced with POSS
Sławomir Michałowski 0
Edyta Hebda 0
Krzysztof Pielichowski 0
0 Department of Chemistry and Technology of Polymers, Cracow University of Technology , ul. Warszawska 24, 31-155 Krako ́w , Poland
This work reports on the thermal stability and flammability of a novel class of rigid polyurethane foams chemically modified by functionalized 1,2-propanediolisobutyl POSS (PHI-POSS) as a pendant group and octa (3-hydroxy-3-methylbutyldimethylsiloxy) POSS (OCTAPOSS) as a chemical cross-link. The foamed hybrid materials were prepared in a three-step process using a sorbitol-based polyether polyol, polymeric 4,40-diphenylmethane diisocyanate and dimethyl propane phosphonate as a flame retardant. The addition of the POSS modifier influences the PU cellular structure as evidenced by a change in anisotropy index of the cross section parallel to the growth direction. Based on thermogravimetric data and flammability results, one can suggest that there is char formation at the surface and the formed layer acts as an insulating barrier limiting heat and mass transfer with flame retardant and thus leading to decreased heat release rate, especially for systems containing OCTA-POSS.
Polyurethane foams; POSS; Thermal stability; Flammability
Polyurethanes (PUs) are a class of polymers with broad
range of properties and applications. Their many uses
include rigid foams as insulation in walls and roofs, flexible
& Sławomir Michałowski
foams in upholstered furniture and thermoplastic
polyurethanes, e.g., adhesives and coatings, as well as
biomedical materials [1–4]. Polyurethanes have
increasingly been used during the past thirty years in a variety of
applications due to their ease of synthesis, low cost, energy
efficiency and potential environmental friendliness. The
mechanical, thermal, insulating, etc., properties of PU
foams can be tailored by using appropriate polyisocyanates
and polyols as well as by combining them with various
nanofillers [3, 5–12].
Unmodified PU foams are highly cellular materials that
are easily ignitable and highly flammable, which is a factor
that limits their broader use  and causes a demand for
fire-retarding additives. Apart from traditional halogenated
and non-halogenated fire retardants, new alternatives are
being tested, e.g., polyhedral oligomeric silsesquioxanes
(POSS). They are unique nanobuilding blocks that can be
used to obtain a wide variety of hybrid materials with a
range specific properties [14–16]. POSS consist of a
siliceous cage-like core with Si atoms on the vertices and O
atoms on the edges. Organic ligands are attached on the Si
atoms which allow the cage-like cores to be directly bound
to the polymer chains, via covalent linkage [17, 18]. These
siliceous cages have sizes of 1–3 nm and can be considered
as the smallest possible particles of silica. Reactive POSS
moieties can be bonded with macromolecules during the
polymerization reaction or by a reactive blending approach
. POSS moieties were found to strongly influence
mechanical, thermal and barrier properties of polymer
matrices. Special attention was focused on the
investigations of flame retardancy effects of POSS—for instance,
Lichtenhan et al. have investigated
poly(ether-blockamide)s reinforced by POSS and they found that the peak
of the heat release rate (PHRR) measured by cone
calorimetry was decreased by 77% as compared to the
pristine polymer . Along this line of interest, Fina et al.
 reported on the applications of dimeric and oligomeric
Al- and Zn-isobutyl silsesquioxane (POSS) in
polypropylene (PP)—the incorporation of Al-POSS in PP caused a
decrease in combustion as compared to PP, evidenced by a
decrease of the heat release rate as well as reduction in CO
and CO2 production rates. Bourbigot et al. in a search to
replace traditional halogenated fire retardants with
nonhalogenated, nanostructured alternatives, investigated by
mass loss calorimetry the reaction to fire of thermoplastic
polyurethane (TPU) containing poly(vinylsilsesquioxane).
Large reduction of peak of heat release rate (PHRR) in
comparison with pristine TPU was observed, and the
protection mechanism was found to occur via an intumescent
mechanism . Recently, an attempt to evaluate the
thermal stability and flame retardancy of POSS-containing
polymer nanocomposites, including determination of
structure–property relationships, was made by Qian and
coworkers . Generally, metal-containing POSS show
good catalytic charring ability, whereby vinyl- and
phenylcontaining POSS promote the strength of char. All POSS
materials may be considered as ceramic precursors that are
able to protective surface layer.
Polyurethane foams with chemically built-in POSS
cages are a new class of foamed materials that has not been
yet reported in the literature, apart from our just performed
studies [24, 25].
Hence, in this work we report for the first time on the
thermal stability and flammability of novel class of
polyurethane foams chemically modified by functionalized
1,2propanediolisobutyl POSS (PHI-POSS) as a pendant group
and octa (3-hydroxy-3-methylbutyldimethylsiloxy) POSS
(OCTA-POSS) as a chemical cross-link, flame retarded by
dimethyl propane phosphonate (DMPP).
Polyurethane foams (PUF) were synthesized using
(i) Rokopol RF551, a general purpose, sorbitol-based
polyether polyol recommended for the production of rigid
polyurethane foams and acquired from PCC Rokita S.A.
and (ii) polymeric 4,40-diphenylmethane diisocyanate
(polymeric MDI) acquired from Minova Ekochem under
the trade name Ekopur B. As auxiliary agents silicone
SR321 from Momentive Performance Materials Inc. (a
nonhydrolyzable silicone surfactant for rigid foam
formulations, such as those having very low reactivities) and
Polycat 9 catalyst from Air Products, a low odor tertiary
amine that provides a balanced promotion of the urethane
and urea reactions in rigid foam applications, were applied.
Levagard DMPP (dimethyl propane phosphonate) was
used as a halogen free flame retardant with a very high
The POSS: 1,2-propanediolisobutyl POSS (PHI-POSS)
and octa (3-hydroxy-3-methylbutyldimethylsiloxy) POSS
(OCTA-POSS) were purchased from Hybrid Plastics.
For the synthesis of polyurethane foams, a three-step
process was applied—at the first stage, polyol (polyether
RF551) was stirred with POSS (1,2-propanediolisobutyl POSS
and octa (3-hydroxy-3-methylbutyldimethylsiloxy) POSS)
dissolved in THF using an ultrasonic homogenizer. Then,
catalyst (Polycat 9), water, surfactant (SR-321) and flame
retartant (Levagard DMPP) were added in order to
prepare the polyol premix (component A). In the next step,
n-pentane as a physical blowing agent was added to
component A. In the third step, component B (polymeric
4,40diphenylmethane diisocyanate (PMDI)) was added to
component A and the mixture was stirred for 15 s with an
overhead stirrer. Formulation of the obtained PU-POSS
foams is given in Table 1.
Finally, the prepared mixtures were dropped into a
Teflon open mold. All the experiments were performed at
ambient temperature of ca. 20 C.
Measurements of thermal conductivity were carried out by
means of a Laser Comp Heat Flow Instrument Fox 200
designed according to ISO 8301. Method of measuring
coefficient of thermal conductivity (k) determines the
amount of heat flow through a material per unit of time
during a steady flow of heat, at constant temperature
difference on opposite sides of the test sample material. A test
Table 1 Formulation of the obtained PU-POSS foams
piece of dimensions 200 9 200 9 25 mm3 is placed in a
chamber between two isothermal plates, which the lower
plate is movable. The method of measurement is based on
one-way flow of heat, which describes the Fourier
The temperature gradient is expressed by a temperature
difference of hot and cold plates relative to the thickness of
the sample. Measurements were performed after 24 h from
the receipt of foam at an average temperature of 10 C,
setting the appropriate cold temperature of 0–20 C hot
plate. The samples were placed in a dry room at room
temperature. After period of 7 days, the test was repeated.
Measurements of compressive strength were carried out
according to the standard ISO 844. For the obtained foam,
after cutting skins and sides samples were cut in the shape
of cubes with an edge length 25 mm. Then, the samples
were subjected to 10% compressive deformation by means
of testing machine Zwick Allround 7005 and checked for
compressive strength in the direction parallel and
perpendicular to the direction of foam rise.
The contents of closed cell
The method of measuring the content of closed cells is
suitable for determining the percentage by volume of
closed cells in rigid plastics porous according to ISO 4590.
Samples with dimensions of 20 9 20 9 100 mm3, without
deformation, are placed in a sealed chamber of apparatus.
The principle of method is that the volume of gas ejected
from the sample when it is placed in the chamber shall be
determined based on changes in the pressure of the
remaining air surrounding the sample, where it is increased
by a known volume (expansion into a known volume).
Analysis of the cellular structure
The analysis of the cellular structure was conducted using
AphelionTM software. The program can set parameters
such as surface area of the cells, cell number and the
anisotropy index of the materials obtained.
Attenuated total reflectance-Fourier transform
Fourier transform infrared spectroscopy measurements
were performed on a Nicolet iS5 spectrometer (Thermo
Scientific) equipped with diamond crystal attenuated total
reflectance unit. Spectra were recorded with a resolution of
4 cm-1 from 4000 to 400 cm-1 with an average of 16
Scanning electron microscopy (SEM) micrographs were
recorded with a JEOL In Touch Scope JSM-6010LV
scanning electron microscope (Tokyo, Japan) with
energydispersive X-ray analysis capabilities, operated at 10-kV
Thermogravimetric analysis was performed using a
Netzsch TG 209 thermal analyzer to investigate the thermal
stability of the obtained PU/POSS/DMPP foams. The
samples (about 5 mg) were heated in an open a-Al2O3 pan
from 25 up to 600 C at a heating rate of 10 C min-1
under air atmosphere (air flow 30 cm3 min-1).
Limiting oxygen index—LOI
The oxygen index is defined as the minimum volume
concentration of oxygen in a mixture of oxygen and
nitrogen introduced at 23 ± 2 C that will just support
combustion of a material under specified test conditions,
according to ISO 4589-2.
The pyrolysis combustion flow calorimetry (PCFC)
technique, developed by Fire Testing Technology Ltd. (UK),
uses traditional oxygen depletion calorimetry. The sample
is first heated at a constant rate of temperature rise of
1 deg s-1 in a pyrolyzer. The thermal decomposition
products are swept from the pyrolyzer by an inert gas. After
pyrolysis, the gas stream is mixed with oxygen and enters a
combustor at 900 C, where products after decomposition
are completely oxidized. Oxygen concentrations and flow
rates of the combustion gases are used to determine the
oxygen depletion involved in the combustion process and
the heat release, as well as the heat release capacity.
Results and discussion
Analysis of the cellular structure
Analysis of the cellular structures of obtained material was
performed on cross section both parallel and perpendicular
to the growth direction of the material. Since the
polyurethane foams are anisotropic materials, the cellular
structure needs to be analyzed in two directions relative to
the growth direction of the material. PHI-POSS additive to
the rigid polyurethane foam systems caused an increase in
average cross-sectional area of the cells and a slight
decrease in the number of cells per 1 mm2 in cross section
parallel in comparison with the reference material
The anisotropy index in the cross section increased to
10% of the PHI-POSS, while for 15% of the added
modifier reached a value lower than the reference material.
However, in the perpendicular cross section initial increase
was observed and then decrease in the number of cells with
the highest modifier content that showed the highest mean
cross-sectional surface (Table 2).
For foams modified with OCTA-POSS, no apparent
change with increasing modifier introduced into the foam
system was observed. The anisotropy index was changed
only in the parallel direction, since it has reduced for the
foams containing 10 and 15 mass% of OCTA-POSS
Table 2 Characteristics of cellular structure of foams modified with
of POSS/ of cells/
Table 3 Characteristics of cellular structure of foams modified with
of POSS/ of cells/
area of the cell
area of the cell
Content of closed cells for foams containing PHI-POSS
is comparable to the reference material. In contrast, in the
system comprising OCTA-POSS reduction in the quantity
of closed cells, especially for the modifier content of
10 mass%, was found (Fig. 1).
The addition of the modifier influences the cellular
structure of the obtained materials as evidenced by changes
in anisotropy index of the cross section parallel to the
growth direction of PU foams.
ATR-Fourier transform infrared spectroscopy
Figure 2 shows the spectra of the reference foam (neat
PUF) and the hybrid foams containing 5, 10, 15 mass%.
PHI-POSS and OCTA-POSS. A few characteristic
vibration bands may be observed in the range 1750–1500 cm-1
(stretching of C = O group and C = C in aromatic ring),
1300–1200 cm-1 (stretching of C–O in alcohols functional
groups of POSS) which are capable of reacting with
isocyanate moieties. Moreover, there are characteristic
asymmetric stretching vibrations of C–O–C group in the
range 1200–1000 cm-1, whereby characteristic bands in
the range 920–730 cm-1 arise from vibrations of
siliconcontaining groups. No peaks originating from the unreacted
isocyanate component were observed in the characteristic
spectral range of 2273–2240 cm-1 (Fig. 2).
Analysis of SEM pictures
In order to determine the effect of the additives on the
microstructure of rigid polyurethane foams, scanning
electron microscopy with EDS microanalysis was
For systems containing PHI-POSS, no major changes
resulting from the amount of introduced POSS were
observed. POSS moieties show very good distribution on
the surface of the foam, the same as for flame retardant
applied (Fig. 3).
Fig. 1 Content of closed cells of foams modified with POSS
Fig. 2 ATR-FTIR spectra neat PUF and foams modified by PHI- and
In the case of compositions containing OCTA-POSS,
fine distribution of silicon can be seen; however, at POSS
level of 15 mass% agglomeration of silsesquioxane takes
place, as evidenced in Fig. 4f. Distribution of phosphorus
present in DMPP is uniform on the surface of polyurethane
foam (Fig. 4).
Thermal conductivity studies
Insulating properties of PU foams containing POSS were
measured after 24 h. The measurement was repeated after
seven days in order to check the changes in thermal
conductivity. The largest increase in the coefficient of thermal
Fig. 3 SEM micrographs
showing cell structure of PUF/
PHI-POSS and EDS mapping of
P and Si in PUF/PHI-POSS
conductivity was found for foam containing the largest
amount of silsesquioxane (Fig. 5). Repeated testing after
seven days showed an increase in this parameter, which is
caused by the diffusion of a physical foaming agent, whose
place is taken by the air affecting thus the heat-insulating
Apparent density of foams containing 5 and 10 mass%
of PHI-POSS was decreased due to the introduction of
small amounts of tetrahydrofuran THF, which was used to
dissolve the POSS in the polyol premix—Fig. 6. However,
for silsesquioxane content of 15 mass% an increase in
apparent density can be observed—this effect is caused by
viscosity increase of the polyol premix in the presence of
For the foams modified by OCTA-POSS, there is an
increase in the thermal conductivity for all the filler
concentrations. After seven days, a slight increase in the
coefficient of thermal conductivity, caused by the
replacement of physical blowing agents, has been recorded.
Foam containing 10 mass% of POSS modifier was
characterized by a significant deterioration of thermal
insulation properties resulting primarily from the low content of
closed cells contributing to the increased exchange of
physical blowing agents.
The apparent density of the foams containing
OCTAPOSS is lower than density of the reference material which
might be associated with the presence of additional
physical foaming agent—THF that was used to facilitate the
introduction of POSS into the polyol premix.
Mechanical properties studies
In order to determine the effect of the silsesquioxane
modifiers on mechanical properties of anisotropic
Fig. 4 SEM micrographs cell structure of PUF/OCTA-POSS and EDS mapping P and Si of PUF/OCTA-POSS
Fig. 5 Thermal insulation properties of foams modified with
Fig. 6 Thermal insulation properties of foams modified with
polyurethane foams, compressive strength was measured in
the direction parallel (X) and perpendicular (Y, Z) to the
Fig. 7 Mechanical properties of foams modified with PHI-POSS
PHI-POSS modified foams were characterized by
reduced mechanical strength, regardless of the direction of
the study which can be linked with a reduction in the
apparent density of the materials obtained (Fig. 7). The
smallest change in the compressive strength in the parallel
direction was observed for hybrid composite containing
5 mass% of PHI-POSS. In the perpendicular direction,
composite material containing 15 mass% of reactive filler
was characterized by comparable mechanical properties as
the reference material.
For foams containing OCTA-POSS, similar changes in
the mechanical properties can be observed. The obtained
composite materials are characterized by a reduced
compressive strength regardless of the amount of modifier
incorporated (Fig. 8). Changes in compressive strength are
primarily associated with changes in the apparent density
caused by introduction of the modifier together with a
small amount of THF which increases the amount of a
5% OCTA 10% OCTA 15% OCTA
Y Z Density
Fig. 8 Mechanical properties of foams modified with OCTA-POSS
physical blowing agent and leads to plasticization of the
Thermal stability investigations
The results of the thermogravimetric analysis of modified
PU/POSS/DMPP foams under air atmosphere are shown in
Fig. 9 and Table 4. The degradation process runs in three
distinct stages with maximum of mass loss at ca.
316–322 C and 537–546 C.
First step of TG analysis could be attributed to the
vaporization of DMPP which has a relatively low boiling
point of 182 C). No or small residue at 600 C indicates
mainly the gas phase activity of DMPP. However, the next
two stages of degradation in the atmosphere of air PU
materials involves the reaction of oxygen to form
hydroperoxides which themselves are unstable and
succumbing to decomposition to form more free radicals
POSS in combination with flame retardant may affect to
stabilize the polyurethane material in the process of
degradation that can influence the thermal stability .
Table 4 Thermogravimetric parameters of modified PU foams under
T5%—temperature at 5 mass% of mass loss. T10%—temperature at 10
mass% of mass loss. T50%—temperature at 50 mass% of mass loss
Moreover, the addition of POSS to polyurethane foams
with DMPP causes an increase in temperature at 50 mass%
of mass loss and increases the amount of residue after the
thermogravimetric analysis which is probably due to the
formation of char containing polyphosphoric acids. This
observation can indicate that POSS moieties stabilize the
PU chains more effectively than the DMPP flame retardant
Table 5 LOI results of modified PU foams
Fig. 9 Thermogravimetric curves of modified PUF in air atmosphere
The silsesquioxanes addition caused a slight decrease in the
LOI value compared to the reference material, regardless
of the amount of the introduced modifier (Table 5).
Microcalorimetry results revealed that POSS moieties
caused a change in the heat release rate (HRR) during
burning of polyurethane foams. Interestingly, for PUF/
DMPP/PHI-POSS system a decrease in HRR was observed
which can be the result of a synergistic effect between the
additives (Fig. 10a). In comparison with foams that contain
DMPP flame retardant, only a substantial decrease in the
peak HRR for all the compositions containing POSS was
registered (Fig. 10b). Reduction of the heat release rate
indicates that POSS species are supporting the flame
retardant’s action probably thorough formation of a
protective surface layer .
The microcalorimetry profiles of all the foams exhibit
two decomposition peaks. The larger one occurs at the
temperature range 300–400 C and the weaker around
500 C (Fig. 10); both steps correspond to the
decomposition steps detected by TG under air atmosphere . The
incorporation of OCTA-POSS causes a systematic shift of
both complex peaks to higher temperatures, indicating that
the hybrid foams are more stable. One could explain this
effect by the cross-linking abilities of OCTA-POSS leading
to the formation of more dense network. The incorporation
of pendant PHI-POSS moieties does not impose such a
prominent effect due to its more loose arrangement in the
Taking into account thermogravimetric data  and
flammability results, one can suggest that POSS moieties
facilitate char formation at the surface and the formed layer
acts as an insulating barrier limiting heat and mass transfer
and thus leading to decreased heat release rate. It is in
accordance with the results of previous investigations
[23, 28, 29]. However, the fire behavior of foamed
polyurethane materials is strongly influenced by their cellular
structure that is modified in the presence of POSS
Introduction to the rigid polyurethane foam of the two
types of POSS and DMPP as a flame retardant influences
the physical and mechanical properties of the materials
obtained. This is caused by changes in apparent density and
cell structure, which are closely related to the compressive
strength. The materials obtained are characterized by small
changes in the thermal insulation properties and a good
distribution of applied additives. Only at the highest
content of OCTA-POSS, a non-uniform dispersion of
silsesquioxane in the polyurethane matrix can be observed.
In the PUF/POSS foamed materials, thermal
stabilization effect due to the presence of POSS particles and
DMPP is evidenced by a shift of the initial decomposition
temperature. This may be explained as stabilization of the
PU macrochains by bulky covalently linked substituents
that reinforce the polymer against thermally induced
motions leading to fragmentation and formation of
macroradicals. It has been found that the POSS moieties
stabilize the PU chains more effectively in the absence of
reactive oxygen-containing species which are unlikely to
interact with silica-based nanocages. Taking into account
thermogravimetric data and flammability results, one can
suggest that there is char formation at the surface and the
formed layer acts as an insulating barrier limiting heat and
mass transfer with flame retardant and thus leading to
decreased heat release rate, especially for systems
containing OCTA-POSS. One could explain this effect by the
cross-linking abilities of OCTA-POSS leading to the
formation of more dense network, whereby incorporation of
pendant PHI-POSS moieties does not impose such a
prominent effect due to its more loose arrangement in the
Acknowledgements This project was financed by the Polish National
Science Centre under contract No. DEC-2011/02/A/ST8/00409.
Open Access This article is distributed under the terms of the
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