1,2-Propanediolizobutyl POSS as a co-flame retardant for rigid polyurethane foams
Journal of Thermal Analysis and Calorimetry
1,2-Propanediolizobutyl POSS as a co-flame retardant for rigid polyurethane foams
Sławomir Michałowski 0 1
Krzysztof Pielichowski 0 1
0 Department of Chemistry and Technology of Polymers, Cracow University of Technology , ul. Warszawska 24, 31-155 Krako ́w , Poland
1 & Sławomir Michałowski
Polyurethane (PU) foams physically modified by two additive phosphorous flame retardants (FR)-phenol isobutylenated phosphate or phenol isopropylated phosphate, and chemically reinforced by functionalized 1,2-propanediolizobutyl POSS (PHI-POSS) have been synthesized and investigated towards thermal and mechanical properties, as well as flammability behaviour. The foamed PU hybrid materials were prepared in a two-step process using a polyether polyol and polymeric 4,40-diphenylmethane diisocyanate. On the basis of the obtained results of mechanical properties, thermal insulation, thermal stability and flammability investigations, the influence of the applied additives-including POSS nanoparticles, on the rigid polyurethane foams was determined. The analysis of thermogravimetric and microcalorimetry data revealed an improved resistance to burning of the PU foams containing hybrid reactive (POSS)/additive (phosphate) FR systems, as evidenced by reduced rate of heat release. Importantly, mechanical properties tests showed that incorporation of bulky silsesquioxane nanoparticles to polyurethane structure via covalent bonds strengthens the foam integrity.
Rigid polyurethane foam
Thermal stability Flammability
Polyurethanes (PU) are widely used materials, obtained in
the reaction of isocyanates with compounds containing
active hydrogen, most often polyols. PU are produced as
foams, elastomers, as well as coatings and adhesives.
Among the porous polyurethane materials, one can
distinguish flexible and rigid polyurethane foams. Flexible
polyurethane foams are used, among others, in the
automotive and furniture industry, as fillings for mattresses and
car seats, whereby rigid polyurethane foams find broad
application as thermal insulation materials in, e.g.
automotive and building sector [
One of the most important problems that is still not
resolved is the polyurethane foams flammability—despite
the very good thermal insulation properties, these materials
do not often show the required values in the latest fire
resistance tests [
]. Different flame retardants, mostly
containing nitrogen and phosphorous, are incorporated into
the foam; however, a satisfactory effect is visible only after
the modification with a large amount of flame retardants,
which in turn may cause deterioration of thermal and
mechanical properties of polyurethane foams [
problem has been partially solved by modifying foams
chemically through the formation of isocyanurate rings,
which in turn improves the properties determined in the fire
Due to the wide use of rigid polyurethane foams in such
industries as construction, automotive, furniture, the issue
of flammability is extremely important. Foamed materials
have a highly developed pore surface, and thus facilitate
the access of oxygen to the material, resulting in easier
combustion. The flammability of polyurethane systems is a
threat to both material consistency and human health, so
looking for suitable flame retardants or synthesis of
inherently non-combustible material is still a serious
In order to meet the requirements for volatile organic
compounds (VOC), flame retardant systems based on
halogen-free phosphorus compounds are getting an
increasing attention. These include, among others,
phosphines and their oxides, phosphonates, phosphates and
phosphites, characterized by low toxicity, lack of release of
toxic gases and production of small amounts of fumes
during combustion. Inorganic phosphorus compounds as
well as red phosphorus are also used effectively. In
addition, flame retardant has been used, which contain, in
addition to the phosphorus atom, additionally nitrogen,
expandable graphite, inorganic compounds or halogen,
through the use of a synergistic effect of their action
Currently, the research attention is focused on reducing
the flammability of foamed polyurethane materials by
organic–inorganic hybrids [
]. This type of compounds
includes polyhedral oligomeric silsesquioxanes (POSS),
which may exist in various structural forms and can be
used as additive and reactive modifiers. POSS are
threedimensional Si–O cages of 1-3 nm diameter which can be
functionalized with organic moieties to yield hybrid
nanoparticles able to form covalent bonds with polymer
In this work, we present results of investigations on the
application of 1,2-propanediolizobutyl POSS (PHI-POSS)
as a reactive co-flame retardant for rigid polyurethane
foams, which were modified by two additive phosphorous
0.25 0.5 0.75 content of phoshorus/mass%
Fig. 1 Thermal properties of polyurethane foams modified by
PHIPOSS and flame retardant: a FR PIBP, b FR PIPP
flame retardants (FR)—phenol isobutylenated phosphate
(PIBP) or phenol isopropylated phosphate (PIPP).
Materials and methods
The rigid polyurethane foams (PUF) were manufactured
using a two-step method—in the first step, the polyol
premix (component A), containing a polyol (Rokopol
RF551from PCC Rokita), PHI-POSS, water and n-pentane as
blowing agents, catalysts (Polycat-9 from Evonik), and
surfactant (SR-321from Momentive) and selected
phosphorus flame retardants (FR), was prepared by mechanical
stirring. The PHI-POSS (1,2-propanediolizobutyl POSS)
reactive additive was introduced into the polyol in an
amount of 10 mass% of the polyol weight and dissolved in
THF and dispersed by the ultrasonic homogenizer. Phenol
isopropylated phosphate (Roflam F12 from PCC Rokita)
and phenol isobutylenated phosphate (3:1) (Roflam B7
from PCC Rokita) in the amount of 0.25, 0.5 and
0.75 mass% based on the phosphorus content in the foam
were used as additive phosphorus flame retardants.
In the second step, polymeric 4,40-diphenylmethane
diisocyanate (pMDI from Minova Ekochem) as component
B was added to component A and the polyurethane system
was mixed using a mechanical stirrer for 10 s. After this
time, the mixture was poured into an open mould where
free foaming occurred in the vertical direction.
Study of apparent density
The apparent density was determined according to PN-EN
ISO 845 standard. From the obtained material, a
rectangular prism of 200 9 200 9 25 mm3 was cut, which was
measured to an accuracy of 0.01 mm, and then weighed
with accuracy 0.01 g.
Content of phosphorus/mass%
PU PIBP PUPOSS PIBP Density 0.75 0.25 0.5
Content of phosphorus/mass% 0.75
Content of phosphorus/mass%
P1 P2 R/Ad P1/Ad
Heat conduction coefficient research
The heat conduction coefficient was measured in
accordance with the PN-ISO 8301 standard using the Laser
Comp Heat Flow Instrument Fox 200, 24 h after the
material was obtained.
In this method, the value of the heat flux flowing
through the foam with the dimensions of
200 9 200 9 25 mm3 is determined. According to
Fourier’s law, if there is a temperature gradient along a given
axis in the material, a certain amount of heat per unit of
time flows through a unit of surface perpendicular to this
axis, at a fixed heat flow. To provide a temperature
gradient, the sample was placed in the apparatus between two
plates at appropriate temperatures of 0 and 20 C.
Compressive strength test
The compressive strength test was carried out in
accordance with the PN-EN ISO 844 standard, using a Zwick
Z005 TH Allround-Line device. The compressive stress
(being the ratio of the maximum compressive force to the
area of the cross-sectional area of the sample) at 10%
deformation of the sample in the direction parallel (R) and
Content of phosphorus/mass%
P1 P2 R/Ad P1/Ad
perpendicular (P1, P2) to the direction of foam growth was
Examination of the content of closed cells
The determination of the content of closed cells was made
on the basis of the PN-EN ISO 4590 standard using
apparatus for measuring the content of closed cells.
Material samples with dimensions 25 9 25 9 100 mm3
were subjected to testing.
Thermogravimetric analysis was performed using a
Netzsch TG 209 F1 Libra thermal analyser to determine the
thermal stability of the obtained foams modified by
PHIPOSS and two additive flame retardants. The samples
(sample mass ca. 5 mg) were heated in an open corundum
pan from 30 up to 800 C at a heating rate of 10 C min-1
under air atmosphere.
Microcalorimetry PCFC method
The analysis of the combustion process of flame retarded
rigid polyurethane foams was carried out using a pyrolysis–
combustion flow calorimeter (PCFC) manufactured by Fire
Testing Technology Ltd. During the measurements, it was
possible to register the amount of heat emitted (HRR), the
rate of heat release by foam materials and the flash point.
The samples tested had masses ranging from 1 to 3 mg.
Results and discussion
In the first stage of the research, an analysis of the impact
of the used flame retardants and PHI-POSS on the thermal
insulation properties and apparent density was carried out.
Rigid polyurethane foams containing PHI-POSS and flame
retardant (PIBP) were characterized by a small increase in
the thermal conductivity coefficient in relation to materials
containing only the flame retardant PIBP for 0.5 and
0.75 mass% content of phosphorus, respectively (Fig. 1a).
The best thermal insulation properties of systems
containing PIBP as flame retardant had a system consisting of
PHI-POSS and 0.25 mass% phosphorus content, which
may be caused by the smallest apparent density of the
obtained material (Fig. 1a).
In the case of systems with PIPP as a flame retardant and
its mixture with PHI-POSS, similar dependencies as for
systems containing PIBP as flame retardant were observed.
Material comprising 0.25 mass% phosphorus and
PHIPOSS had the lowest thermal conductivity coefficient,
which is the result of the smallest apparent density of the
obtained material (Fig. 1b).
For all compositions, an increase in apparent densities
along with an increase in the amount of flame retardants
introduced was observed. However, the introduction of
PHI-POSS to rigid polyurethane foam systems resulted in
an increase in the density of obtained materials, which is
related to the increase in the viscosity of the initial
compositions (Fig. 1).
An important parameter determining the properties of
rigid polyurethane foams is the content of closed cells. All
the produced materials displayed value of this parameter of
over 90%. The highest content of closed cells was found
for materials containing only PIPP as flame retardant and a
mixture of PIBP and PHI-POSS (Fig. 2).
However, the closest to 90% value had a composition
based on the mixture of PIPP and PHI-POSS.
This effect may be caused by changing the initial
viscosity of systems and the use of POSS and flame retardants,
which may contribute to the opening of cells during the
Due to the anisotropic nature of the obtained materials,
compressive strength in the parallel and perpendicular
direction was measured.
For materials obtained with the use of PIBP as flame
retardant, an increase in compressive strength in the
parallel direction along with the increase in the FR content in
the system was observed. However, the compressive
strength in the perpendicular direction changed in such a
way that the lowest values had composition for the
0.5 mass% addition of FR (Fig. 3a).
For systems in which PHI-POSS was added, an increase
in mechanical parameters in each of the measured
directions along with the amount of FR introduced (Fig. 3b) was
found. Due to the changing apparent density of individual
compositions, profiles compensating the influence of
apparent density on this parameter have been presented
The addition of PHI-POSS to the rigid polyurethane
foam system resulted in an improved compressive strength,
especially in the direction perpendicular to the direction of
The introduction of the PIPP as flame retardant into the
composition of the rigid polyurethane foam affects the
deterioration of the mechanical properties in each
measured direction, which is particularly evident in the
relationships taking into account the apparent density (Fig. 4a).
The use of a second modifier in the form of PHI-POSS
improves the mechanical parameters of the obtained
materials both in the parallel and perpendicular directions
The results of the mechanical properties tests clearly
show the influence of PHI-POSS on the properties of the
obtained materials—the reactive bulky silsesquioxane
modifier incorporated into the structure of polyurethane
strengthens its integrity.
The obtained thermogravimetric analysis results show
that both the FR PIBP itself and the mixture with
PHIPOSS affect the earlier thermal degradation of the obtained
materials, which is related to the degradation of the
phosphorus flame retardant. The lowest degradation
temperature was observed for the highest FR PIBP content in both
systems (Fig. 5). However, in the presence of PHI-POSS a
faster mass loss at the initial stage of degradation was
observed (Fig. 5b).
A similar relationship for the second modifier system
was observed, but in this case the changes related to
thermal degradation are rather minor as the decomposition
proceeds in a similar manner (Fig. 6).
Based on DTG analysis, there are four steps during the
thermal decomposition of polyurethane foams containing
phosphoric flame retardants, as well as flame retarded PU
systems with POSS (Figs. 5, 6). The first less intensive
peak is present at ca. 100 C that is probably related to the
evaporation of entrapped tetrahydrofuran which was used
to facilitate POSS dissolution during polyurethane
synthesis. The next degradation step at ca. 230–240 C could
be attributed to the degradation (ignition) of phosphoric
flame retardants which have a relatively low flash point at
ca. 230 C. Noteworthy, no or small residue at 600 C
indicates mainly gas-phase activity of this kind of flame
retardants. The next two stages of degradation at ca. 320
and 550 C in the atmosphere of air involve the reaction of
oxygen to form hydroperoxides which themselves are
unstable and undergo decomposition to form more free
radicals. POSS influence on the initial stage of flame
retarded polyurethane decomposition by restriction of the
molecular mobility of PU chains in the presence of bulky
]. As PHI-POSS show relatively low
thermal stability (Tonset = 244 C), its mode of action at
higher temperatures may be associated with formation of
barrier layer that hinders heat and mass transfer as revealed
later on by flammability studies results [
]. In the
presence of both PIBP and PIPP, one could observe a slight
decrease in the onset temperature of PU composites as the
amount of the flame retardant increases. However, flame
retarded systems containing POSS have a higher onset
temperature than foams without POSS (Table 1). Again,
this may indicate the role of silsesquioxanes as charring
agents during polyurethane decomposition process.
The results of micro-calorimeter pyrolysis and
combustion (PCFC) show that the addition of PIBP to the rigid
polyurethane foam system reduces the flammability of the
obtained materials to a small extent, which is illustrated by
the HRR curves. There is a decrease in the HRR value for
the modified samples, especially visible at 0.25 mass%
content of this flame retardant (Fig. 7).
However, an interesting change was observed for
systems in which PHI-POSS was used. The addition of this
modifier together with the PIBP flame retardant caused a
significant reduction in the HRR peak, assuming the lowest
value for 0.5 mass% of the FR PIBP content (Fig. 7).
The incorporation of flame retardant PIPP at 0.5 and
0.75 mass% content of phosphorus resulted in a reduction
in the rate of heat release; however, these changes were not
significantly large (Fig. 8). Only the application of
PHIPOSS influenced the reduction of the HRR peak of the
obtained materials, especially for the system containing the
highest content of FR PIPP (Fig. 8).
PCFC micro-calorimeter results revealed an interesting
synergy effect of a hybrid system consisting of a selected
phosphoric additive flame retardants and PHI-POSS
leading to the reduction of flammability of rigid polyurethane
foams. This effect may be caused by the specific
interactions of the applied modifiers in the rigid polyurethane
foam, as well as through density changes of the porous
structure. An increase in the apparent density may reduce
the propagation of the smoking process due to the smaller
surface development in the fabricated materials. One can
also postulate the role of POSS as a charring agent; the
layer formed at the PU surface may act as an insulating
barrier which limits heat and mass transfer during
The obtained results have shown that the use of PHI-POSS
with phosphorus additive flame retardants leads to the
reduction of the rigid polyurethane foams flammability,
without significant changes of the foams’ crucial
mechanical and thermal conductivity properties.
The best thermal insulation properties showed
PU/PHIPOSS systems containing an additive fire retardant in the
amount of 0.25 mass% phosphorus as these materials have
the lowest apparent density of all obtained materials.
Modified rigid polyurethane foam systems have
adequate mechanical strength, that is especially visible for
compositions containing PHI-POSS, which is probably the
result of increasing the content of hard segments by
incorporating a reactive modifier into the structure of the
Flammability studies by micro-calorimeter pyrolysis
and combustion revealed that hybrid reactive
(POSS)/additive (phosphate) systems are characterized by improved
resistance to burning. This effect may be linked with
changed apparent density of the composite materials and
more efficient formation of char barrier in the presence of
Importantly, the use of organic–inorganic hybrid
systems with silsesquioxanes can provide perspectives in an
effective protection of polyurethane materials against 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 Creative
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