Analysis of flammability and smoke emission of rigid polyurethane foams modified with nanoparticles and halogen-free fire retardants
J Therm Anal Calorim
Analysis of flammability and smoke emission of rigid polyurethane foams modified with nanoparticles and halogen-free fire retardants
Kamila Salasinska 0 1 2
Monika Borucka 0 1 2
Milena Leszczyn´ ska 0 1 2
Wojciech Zatorski 0 1 2
Maciej Celin´ ski 0 1 2
Agnieszka Gajek 0 1 2
Joanna Ryszkowska 0 1 2
0 Warsaw University of Technology , Wołoska 141, 02-507 Warsaw , Poland
1 Central Institute for Labour Protection - National Research Institute (CIOP-PIB) , Czerniakowska 16, 00-701 Warsaw , Poland
2 & Kamila Salasinska
Using one-step method, rigid polyurethane foams were made, modified with developed fire retardant systems containing halogen-free flame retardants and nanofillers in the form of multi-walled carbon nanotubes or nanoscale titanium dioxide. The materials were subjected to a test using a cone calorimeter and smoke-generating chamber, and selected samples were further analyzed via thermogravimetry and oxygen index. Moreover, the products of thermal degradation of selected samples were identified using gas chromatography with mass spectrometer. Conducted flammability tests confirmed the presence of a synergistic effect between the used nanofillers and halogen-free flame retardants. It has been observed that the carbonized layer, the formation of which favored the presence of nanoadditives, inhibits the combustion process. Furthermore, nanofillers influenced favorably reduction in the amount and the number of occurring products of thermal degradation.
Rigid polyurethane foams; Halogen-free flame retardants; Oxygen index; Cone calorimetry; Smoke chamber; Thermal analysis
Rigid polyurethane foams (RPUF) are used in many areas,
including construction industry as one of the best
commercially available insulation materials. RPUF have very
good mechanical properties, resistance to aging and water
and also atmospheric factors [
]. Unfortunately, rigid
polyurethane foams have also some disadvantages, among
which special attention should be paid to flammability and
toxicity of the gas products emitted during thermal
degradation and combustion .
Combustion of polymeric materials is an exothermic
reaction of the catalytic oxidation of organic compounds
carried by energy supplied in the form of heat and forming
free radicals. This phenomenon is accompanied by heat,
light and combustion products (gases, smoke). The ability
to ignite the polymer depends primarily on the availability
of oxygen, temperature and its physicochemical properties.
In contrast, the combustion process is conditioned, among
others, by composition, chemical structure and density of
the material, porosity, size, shape and structure of the
The thermal decomposition of non-fire retarded
polyurethane foams in air is generally quite well understood.
Generally, the initial decomposition of the foam ([300 C)
results in the volatilization of isocyanates, amines and
‘‘yellow smoke’’, leaving behind polyols in the condensed
phase. These polyols are fragments and volatilize as the
temperature increases ([600 C), leaving behind a char.
This char can decompose further, leaving behind a residue,
to produce simple organic fragments and some polycyclic
aromatic hydrocarbons (PAHs). In the gas phase,
isocyanates, amines and ‘‘yellow smoke’’ are begun to
decompose at [600 C into low molecular weight
nitrogen-containing fragments (such as benzonitrile, aniline and
hydrogen cyanide (HCN)). At [800 C these compounds
further fragment into simple molecules (such as HCN, CO,
CH4 and CH2O) and PAHs [
Rigid polyurethane foams are carbonized during
combustion. This process leads to the reduction in the amount
of heat released during the combustion of polymers and
affects the amount and type of the emission. Formation of
carbon by restricting access of the flame to the deeper
layers of the material prevents the formation of low
molecular weight organic compounds, which support the
fuel process. Unfortunately, the amount of formed carbon
layers in the combustion of rigid polyurethane foams is
relatively low [
Research and development units and chemical
corporations around the world are currently carrying out
numerous works related to the improvement in thermal
stability and fire resistance of rigid polyurethane foams.
RFUP flame retardancy can be achieved by the addition of
flame retardants [
], the task of which is usually to
delay ignition, slow the process of combustion and
pyrolysis, reduce emissions of smoke, and reduce the
phenomenon of dripping. The group of fire retardants includes,
among others, compounds containing halogens such as
bromine and chlorine, compounds of phosphorus and
nitrogen, and hydroxides of aluminum and magnesium
]. Most flame retardants with built-in halogen atoms
currently attract a lot of controversy, mainly because of
their safety and corrosion properties of gases released [
The most frequently used halogen-free flame retardants,
apart from aluminum hydroxide and magnesium,
containing compounds are phosphorus/nitrogen. Flame retardants
based on Mg(OH)2 and Al(OH)2 prevent heating up of the
material to the ignition temperature, and the water vapor,
along with degradation products, is released into the
combustion zone reducing the concentration of
combustible products and oxygen. As a consequence, the flame
temperature is lowered and the resultant oxides form a
protective layer, on the surface of the material, limiting the
movement of the volatile degradation products to the flame
and oxygen to the burning material. The group of flame
retardants containing phosphorus and nitrogen includes,
among others, phosphaphenanthrene phosphonamidates
], ammonium polyphosphate [
, triphenylphosphate [
], dimethylpropanphosphonate [
] and aluminum phosphinate
4, 20, 21
A relatively new group of measures that increase the
thermal stability and reduce the flammability of polymers,
are nanoparticles. The advantage of nanocomposites is the
possibility to obtain satisfactory results using only a few
percent of the filler, while in the case of traditional flame
retardants these quantities reach up to 60%. Otherwise,
nanomaterials exhibit improved rheological properties,
higher mechanical strength and lower emission of fumes
]. The most commonly used nanofillers are layered
], nano-SiO2 [
], titanium oxide and carbon
nanotubes (CNT) [
4, 27, 28
]. During combustion,
composite nanoparticles can migrate to the surface of the
polymer and assist in the formation of carbon layer
]. The combination of nanomaterials with
conventional flame retardants, leading to the formation of
synergistic effect between those substances, is currently the
subject of numerous studies [
23, 25, 31–33
Wang et al. [
] reported that the introduction of
graphene nanoparticles to polybutylene succinate (PBS)
containing melamine poliphosphate favorably affects the
formation of carbon and increases the thermal stability of
the polymer. On the basis of the cone calorimeter testit was
found that the values of maximum rate of heat release
(pHRR) and total heat generated (THR) of polybutylene
succinate containing 18% melamine phosphate and 2% of
graphene were, respectively, 63 and 23% lower compared
to the results obtained for pure PBS. The paper also
presents the results for the materials modified with polyhedral
oligomeric silsesquioxane (POSS), but the synergistic
effect in combination with melamine poliphosphate was
significantly lower in comparison with graphene. In
addition, increased quantities of fumes are released, which is
not observed in the case of materials of melamine
phosphate and graphene.
These results are contrary to those described by
Didane et al. [
]. The authors reported the results for
flammability testing of polyethylene terephthalate (PET)
using a 9 mass% of flame retardant agent based on the zinc
phosphinate, and three types of POSS, which have referred
to the unmodified PET and PET with 10 mass% a/m
retardant. It has been observed that the introduction of
1 mass% POSS contributed to the reduction in the
maximum HRR from 500 kW m-2 for PET and 365 kW m-2
for PET with flame retardant to 214 kW m-2 in the case of
one of the types of nanofillers. In addition, for the same
kind of POSS emissions of carbon dioxide at a level similar
to the values obtained for unmodified PET was observed.
The aim of this study was to produce nanocomposites of
rigid polyurethane foams modified with halogen-free fire
retardants with reduced flammability and smoke emissions.
Prepared materials were analyzed using the cone
calorimeter and the single-chamber test; under obtained
results, the initial selection of the proposed flame retardant
systems was done. In the next stages of work, for the
selected compositions, thermogravimetric analysis and
oxygen index were performed. Also, specified were the
nature and quantity of the substances present in the exhaust
fumes and emitted during the thermal decomposition. The
analysis made it possible to assess the effectiveness of the
proposed system on the behavior of materials containing
them in the conditions of pyrolysis and combustion.
Materials and methods
To prepare the materials, open-cell polyurethane foam
EKOPRODUR 0612 PCC Prodex (PURO), intended for
thermal insulation coatings for water pipes and heating
systems in residential and industrial buildings, was used. A
combination of three halogen-free liquid fire retardants
included: triethylphosphate (TEP) from Minova Ekochem
SA, dimethylpropanphosphonate (DMPP), the trade name
Levagard DMPP from LANXESS GmbH and the cyclic
phosphorus compound named Addroce CT 93 FR (FR CT)
from Walter Thieme Handel GmbH, were used. Also three
flame retardants in the form of powder, i.e.: aluminum
hydroxide under the trade name Reflamal S 30 (ATH) from
Walter Thieme Handel GmbH, ammonium polyphosphate
(APP) named Exolit AP 750 from Clariant GmbH and zinc
borate (ZB) from Nordmann, Rassmann GmbH were
applied. As nanofillers were used: multi-walled carbon
nanotubes (MWNTs) with a diameter of 20 to 30 nm and a
length of 10 to 30 lm from Cheap Tubes Inc. and
nanosized titanium dioxide (TT) with a grain size of 20 nm
from Sigma-Aldrich Co. LLC.
The materials were prepared by a single-stage method of
components A and B, and the isocyanate index equaled 1.1.
Production started with the preparation of mixtures of
nanoparticles and three flame retardants marked as TEP,
FR CT 93 and DMPP. Flame retardant composition was
subjected to a mechanical mixing process using three
speeds corresponding to 7000, 10,000 and 17,000 rpm with
mixing time about 3 min. In order to prevent overheating
of the compound cooling with ice-water bath was applied.
The mixture was then subjected to a process of
homogenization using the ultrasonic disperser Q Sonic 700. The
amplitude of the operation was 50 with the homogenization
time of about 5 min for each mixture. The obtained
mixtures and selected flame retardant powder (ATH, APP or
ZB) were introduced into component A, comprising a
catalyst, a surfactant and a blowing agent and mixed again
via mechanical stirring. The duration of the process was
3–5 min, and the maximum speed of the stirrer did not
exceed 17,000 r min-1. In a final step, the resulting
mixture was added to ingredient B, and after mixing, it was
poured into an open mold. The total proportion of flame
retardant substances and nanofillers was 30 mass% relative
to the component A in individual materials. Additionally, a
polyurethane foam comprising only three liquid-halogen
flame retardants (PURO 7) and a foam with no additives
(PURO 8) were made. The compositions of the materials
produced are summarized in Table 1.
Burning behavior of investigated materials was determined
based on research conducted using the cone calorimeter
device from Testing Technology Ltd. in accordance with
the procedure described in ISO 5660-1. The specimens
with dimensions 100 9 100 9 10 mm were irradiated
horizontally at a heat flux of 35 kW m-2. In addition, the
optical system with silicon photodiode and a helium–neon
laser allowed continuous measurement of the optical
density of smoke. Spark ignition was used to ignite the
pyrolysis products, and the result of the burning test was
the arithmetic average of at least three measurements.
The flammability properties of materials were examined
also by oxygen index (OI) test, according to the summary
procedure (procedure C) described in EN ISO 4589-2,
allowing determination of the minimum OI values. The
samples used for measurement were rectangular-shaped
beams of the measuring 150 9 10 9 4 mm. The bar is
placed in a column for measuring the minimum oxygen
concentration in the oxygen–nitrogen mixture at which the
sample burned. The test was carried out for at least three
samples from each batch.
Optical density of smoke was determined by Smoke
Density Chamber from Fire Testing Technology Ltd. in
accordance with the document ISO 5659-2. During the test,
each of the samples with dimensions of 75 9 75 9 10 mm
was exposed to an external heat flux of 25 kW m-2. The
optical system allowed the continuous measurement of the
optical density of smoke (Ds), and based on the curve of Ds,
parameter VOF 4, which informs how much smoke is
generated in the first 4 min of fire, was determined. The
values were the arithmetic average of three surveys.
Identification of thermal degradation and combustion products
One of the objectives of this work was the determination of
toxic products that can be evolved in the combustion and
thermal degradation of polyurethane materials.
Experiments were carried out in the steady-state tube furnace
(Purser furnace, ISO/TS 19700), which has been used
specifically to generate toxic products from real fires under
different temperature conditions. The furnace allowed the
identification of the products emitted not only directly
during the thermal degradation of test materials, but also as
a result of secondary reactions between the products. The
samples (5 g) of selected materials in special test specimen
boats were delivered into the furnace tube. Then, the
samples were heated from room temperature to 600 C at a
heating rate of 10 C min-1 in air, with the gas flow rate of
20 L min-1. When the furnace temperature reached 300,
450 and 600 C, it was maintained for the 5 min and the
samples of thermal degradation products were collected
from mixing chamber of furnace using solid-phase
microextraction (SPME) technique and the carboxen/
polydimethylsiloxane (CAR/PDMS) fiber coating. After
collection (5 min), the SPME fiber desorbed immediately
in the gas chromatograph injector for analysis. The released
species have been identified using gas chromatograph (GC
7890 A, Agilent Technologies, USA) with mass
spectrometer (MSD 5975, Agilent Technologies, USA).
Chromatographic separation was achieved on a HP-5MS
fusedsilica capillary column (30 m 9 0.25 mm 9 0.25 lm film
thickness) using helium as the carrier gas at 1 mL min-1.
The oven temperature was maintained at 40 C for 10 min,
increased by 5 C min-1 to 240 C and held for 8 min. The
GC injector port was 250 C. The MSD was operated by
electronic impact (70 eV) in scan mode (25–450 m/z).
A thermogravimetric analysis (TG) of the prepared
materials was conducted using a TGA Q500 from TA
Instruments. From each of the series, at least one sample of
9.0 ± 1 mg was cut and tested in an atmosphere of
nitrogen or air, with flowing gas at a rate 30 mL min-1 in the
chamber and 70 mL min-1 in the oven. Based on the curves
and using the Universal Analysis software version 4.1 D, the
initial degradation temperature, the temperature of
maximum rates of mass loss and percentage of char residue at
950 C were specified. The samples were heated from room
temperature to 950 C at a rate of 10 C min-1.
Results and discussion
Summary results of the flammability and smoke emission
tests, conducted with cone calorimeter and
smoke-generating chamber, are given in Table 2. Designations of the
individual samples are consistent with the indications given
in Table 1.
A relatively short ignition time of open-cell
polyurethane foams was due to the cell structure of this type of
]. The introduction of flame retardants caused
a shortening of the time, followed by the ignition of the
tested materials from 5 to as much as 1 s, in the case of the
sample labeled PURO 2. The presence of flame retardant
systems contributed to the intensification of the process of
degradation of the polymer, which has been described in
the literature [
]. The assessment of listed values Time to
Ignition (Table 2) shows that the analyzed parameter was
badly affected by the combination of halogen-free flame
retardants with nanofillers in the form of multi-walled
On the graph presented in Fig. 1, heat release curves of
PUR foams nanocomposites with divided into the series
and reference materials were juxtaposed. Heat release rate
is one of the key parameters used to evaluate the burning
behavior of materials. It has been proven that doubling
HRR can lead to more than threefold reduction in survival
time of victims, which was not observed in the case of an
increased time to ignition or toxic potential of plastics [
a The values in parentheses are the standard deviations
For most of the mixtures, except for the sample PURO 5,
there was a reduction in the maximum rate of heat release.
Comparing the results obtained for a reference sample
PURO 7 with the values determined for nanocomposites, it
was found that the replacement of a small amount of flame
retardants with nanoadditives resulted in further lowering
the amount of heat release. These observations may
indicate the occurrence of synergistic effect between
nanofillers and flame retardant. The lowest pHRR values, lower
compared to non-modified polyurethane foams by 37 and
30%, were obtained for samples PURO 3 and PURO 4. The
reason for the reduction in the values was the formation of
permanent char on the composites’ surface, limiting heat
and mass exchange between the materials and the flame
]. Ye et al. [
] observed that the MWNTs introduced to
polymer contained different amount of magnesium
hydroxide as fire retardant caused the formation of
nanofiller network structures and compact charred layers, but
also increase in the thermal stability due to nanotubes
strength and integrity in the charred layers. The synergistic
effect between nanofillers and halogen-free fire retardants
has also been confirmed in the previous literature
34, 40, 41
]. The similar results were observed for the
maximal average heat release emission, basing on which it
is possible to predict the development of fire in full-scale
Another extremely important fire feature is the fumes
emission accompanying the pyrolysis and combustion of
polymeric materials. Numerous studies have shown that
limited visibility resulting from the formation of fumes,
constituting a gas phase along with the suspended products
of incomplete combustion, is the reason for the death of the
prevailing number of victims [
]. The parameters used
to evaluate the emission of fumes according to the research
conducted using the cone calorimeter are specific
extinction area (SEA) and total smoke release (TSR). Parameter
SEA, corresponding to the surface light-absorbing particles
of smoke produced by the combustion of 1 kg of material
, was reduced compared to the unmodified PUR foam
only for the material designated as 6 and slightly for PURO
1. As for the total amount of the emitted fumes, the lowest
values were obtained for mixtures PURO 1, 4 and 6. The
flame retardant based on aluminum hydroxide, which was
introduced to the PURO 1 and PURO 4, is a widely used
means of limiting the emission of smoke. In turn, the flame
retardant comprising zinc borate activated its ability to
suppress the fumes only in combination with nanoparticles
of titanium dioxide.
Smoke-generating chamber is a tool used to assess the
emission of fumes in the cumulative (static) conditions.
The introduction of developed systems of nanofillers and
halogen-free flame retardants reduced the maximum
optical density of smoke (Ds,max). The lowest values of Ds,max
were obtained for samples PURO 2 and 4 (Table 2).
Similar results were observed for parameter VOF 4. The
results do not coincide with those obtained on the basis of
the test performed using the cone calorimeter, which is a
dynamic (flow) method of research used to assess the
emission of fumes of the materials [
Basing on the performed analysis of flammability and
fumes produced, one sample containing the given
nanofiller was selected for further study, including the
determination of thermal stability, oxygen index and toxicity of
fumes emitted. Among the polyurethane foams modified
with multi-walled carbon nanotubes, the composite labeled
PURO 1 was chosen, while PURO 4 was selected among
the series containing titanium dioxide. The results obtained
for the above-mentioned materials were compared to those
defined for reference materials PURO 7 and 8.
Thermal stability of the prepared materials was
investigated using mass loss curves (TG) by determination of the
temperatures corresponding to 5% of mass loss (T5%). The
onset degradation point of foam with no modifications was
147 C, while application of the developed flame retardant
systems resulted in a slight decrease in degradation onset
for experiments in both air and inert atmosphere. Similar
results have been previously reported in the literature
9, 34, 43
]. The phosphorus-containing flame retardants
applied in the study undergo decomposition at increased
temperature and react with the polymer, which contributes
to the formation of a char layer on the materials surface.
The presence of nanofillers additionally favored the
formation of char layer, which was confirmed by TG curves
(Fig. 2) and increased amounts of residue at 950 C in the
case of composite materials (Tables 3, 4). The formation of
char layer limits the mass loss and increases the thermal
stability of the polymer [
]. The samples modified with
the multi-walled carbon nanotubes were characterized with
slightly increased thermal stability among other
composites. The TG and DTG curves obtained in the course of the
study were compared in order to precisely analyze the
processes of thermal degradation in air and inert
atmosphere (Fig. 2b, 3b; Tables 3, 4). As evidenced by the
obtained results, the thermal degradation of examined
foams involves degradation steps with maximum at
temperatures in the range 161–168 C (TNmax1, TAmax1) in
both air and inert atmosphere. This signal is probably
related to the degradation of the urethane rigid segments
]. Furthermore, the analyses in air indicate the
degradation step with maximum at 201–208 C (TAmax2)
characterized by fast degradation rate. The former signal might
correspond to the degradation of the products formed by
decomposition of the rigid segments occurring in the initial
degradation step. Presence of this signal is clearly related
to the effect of oxygen on the decomposition process. The
temperature range related to the first degradation step for
the processes conducted in nitrogen atmosphere is similar
to the range related to two first degradation steps for
analyses performed in air.
Further degradation processes observed in air (TAmax3)
take place in temperature range of 228–360 C, while the
same processes were observed in nitrogen atmosphere at
ca. 220–320 C (TNmax2). Moreover, the degradation in
nitrogen atmosphere involved third step (TNmax3) at
Fig. 4 Total ion chromatograms from analysis of volatile products
obtained during thermal degradation of selected rigid polyurethane
materials at 300 C
Isocyanatobenzene (CAS: 103-71-9)
2-Propanone, 1-hydroxy-, acetate (CAS: 592-20-1)
1,2-Propanediol diformate (CAS: 53818-14-7)
Methoxyformamidine (CAS: 2440-60-0)
Cycloserine (CAS: 68-41-7)
1-Acetyl-3-methylurea (CAS: 623-59-6)
Isopropyl acetate (CAS: 108-21-4)
2-Propanol, 2-nitroso-, acetate (ester) (CAS: 6931-04-0)
Isopropyl glycidyl ether (CAS: 4016-14-2)
temperatures in the range of 320–345 C. Those stages are
most likely related to the degradation of urea rigid
segments and products of their decomposition [
The fourth step (TNmax4) was observed in nitrogen
atmosphere at temperature range of 345–450 C and fifth
step (TNmax5) at range of 450–620 C. For the experiments
performed in air, the fourth degradation step (TAmax4) was
observed at 360–670 C. The mass loss in that stage is
associated with other segments of the remaining structure,
including ether groups. Moreover, in those stages the
byproducts of pyrolysis or oxidization are also decomposed.
Highly porous lightweight polyurethane foams tend to
have fast flame spread with the oxygen index in the range
between 16 and 18 V V-1 [
]. The value of OI for the
analyzed commercial polyurethane foam was relatively
high and not less than 25% V V-1. The use of the proposed
flame retardant systems resulted in only a slight increase in
the parameter for the other samples. No visible changes in
the results between the reference sample PURO 7 and
composites lead to the conclusion that the use of nanofillers
did not affect the test results. The obtained values allow to
classify the composites as flame-resistant materials
(21 B OI B 28).
All tested rigid polyurethane foams the largest amount
of gases and fumes were emitted when the thermal
degradation and combustion occurred at 300 C. Gas
chromatography data of the volatile samples released from
selected materials during thermal degradation at 300 C are
shown in Fig. 4. The largest quantities of hazardous
substances were created during the thermal degradation of the
sample PURO 7, which contain the addition of three
nonhalogen flame retardants (Table 1). While during
degradation of PURO 2, the smallest quantity of products was
created. Moreover, the amount of products released during
the decomposition of the PURO 2 was also slightly lower
than the quantity of the substance produced during the
thermal decomposition of the foam does not contain any
additives (PURO 8). In Table 5 are presented compounds
identified in samples of gases and fumes, which were
emitted during the thermal degradation of selected rigid
polyurethane foams. The largest number of substances was
detected and identified when the thermal decomposition of
samples PURO 7 and 8 occurred. In the largest quantities,
the products such as isopropenyl acetate,
2-methyl1,3-dioxalone and 1,5,6,7-tetrahydro-4-indolone were
formed. Moreover, during decomposition of the foam
PURO 8, the substances 1,2-benzenedicarbonitrile,
piperonylonitrile and 4-isocyanatobenzonitrile were produced,
not detected in other tested materials. Less of the
compounds were identified in the gases and fumes emitted
during the decomposition of PURO 1 and PURO 4.
However, the type of major products remained unchanged
in comparison with the reference samples.
After comparing the results obtained during the
combustion of the nanocomposite and reference material, it has
been found that the addition of nanofillers reduces the
amount of evolved products and also effects on their type.
Within the framework of this work, the recipes of flame
retardants for rigid polyurethane foams, containing
nanofillers (MWNTs, TT) and halogen-free flame retardants,
were prepared. The produced materials were subjected to a
series of tests allow to characterize their flammability and
smoke emission. Moreover, for selected composites
thermal stability, oxygen index and the identification of
products released during the thermal degradation and
combustion of these materials were also determined.
Increasing the thermal stability and reducing the
flammability of composites were achieved by the formation
of the carbonized coating on the surface of composites,
which limited the access of fire into the deeper layers of the
material and inhibited the formation of radicals. The
formation of char, confirmed by thermogravimetric analysis,
was favored by the presence of nanoadditives. The amount
of emitted smoke was reduced by the applying appropriate
flame retardants, especially the flame retardant based in
aluminum hydroxide. The results of a study conducted by
means of the cone calorimeter confirmed the presence of a
synergistic effect between the used nanofillers and
halogen-free flame retardants. Using the steady-state tube
furnace and gas chromatograph with mass spectrometer
allowed the identification of the major organic substances
presented in the gases and fumes emitted during the
combustion of selected polyurethane foams. It was found that
the addition of nanofillers reduces the amount and the
number of emitted products.
Acknowledgements The research, prepared under the third stage of
the multiannual program, entitled: ‘‘Improving the safety and working
conditions’’ was financed in the years 2014–2016 in the field of
research and development by the Ministry of Science and Higher
Education/National Center for Research and Development.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
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link to the Creative Commons license, and indicate if changes were
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