Studies on the thermal properties and flammability of polyamide 6 nanocomposites surface-modified via layer-by-layer deposition of chitosan and montmorillonite
Journal of Thermal Analysis and Calorimetry
Studies on the thermal properties and flammability of polyamide 6 nanocomposites surface-modified via layer-by-layer deposition of chitosan and montmorillonite
Tomasz M. Majka 0 1
Marcin Cokot 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 & Tomasz M. Majka
In this work, layer-by-layer assembly technique has been employed to deposit cationic chitosan (CH) and anionic montmorillonite (MMT-Na?) polyelectrolyte coatings on polyamide 6 (PA6). Without an additional surface treatment, 5, 10 and 20 bilayers of CH/MMT-Na? were successfully adsorbed on PA6 matrix as confirmed by XRD, ATR-FTIR and SEM-EDX results. The thermal stability was evaluated using TGA and DSC methods, and it was found that neat PA6 demonstrated better performance (by ca. 5 C) and bilayers caused slightly delayed melting of PA6 crystalline phase. Some interesting results proving imparted flame retardancy by applied assemblies were obtained by vertical (VFT) and horizontal (HFT) flammability tests and microcalorimetric analysis (MCC) methods. It was found that all of the coated PA6 specimens have accomplished V-0 notes in VFT and the intensity of melt dripping reduced. According to MCC results, PA6 covered with 20 bilayers showed the highest reduction-up to 10% relative to reference material-in PHRR.
Polymer nanocomposites; Polyamide 6; Flame retardancy
The origins of thin-film coating of solids by layer-by-layer
(LbL) technique date back to 1960s, when Iler [
pioneering research, followed by Decher and Hong [
Since then, many practical applications of this surface
treatment method have been described, such as in
electrochromic materials [
], controlled drug delivery [
composites with barrier properties towards oxygen [
antireflective  and waterproof coatings [
antibacterial materials [
]. In principle, LbL
assemblies arise from alternative dipping of substrates in, most
often, water-based solutions of polyelectrolytes, which
makes this method eco-friendly. By adjusting the process
parameters, i.e. the kind of polyelectrolytes used [
molecular mass [
], pH [
], the presence of opposite
], ionic strength [
] and temperature [
one can fabricate coatings with desired properties, ready
for scaling up and automatization [
Layer-by-layer self-assembly of polyelectrolytes to
impart flame retardancy to cotton textiles has been applied
by Li et al. [
] who have studied different hybrid
inorganic–organic and all-inorganic systems, which—when
exposed to flame source—build up protective residue over
the surface, limiting the access of oxygen and heat.
Carosio’s and Mannen’s research groups applied nanoparticles
of aluminosilicates and zirconium phosphate which,
deposited on synthetic fibres, completely or almost utterly
diminished dropping of polymer melt [
of the kind of substrate, most of the modifications
minimized heat and gas products release during pyrolysis and
combustion. Further research efforts in this field are
focused on effective intumescent flame retardants
composed of blowing agent, acid source and carbon source,
which altogether can show synergistic effects.
This work reports on the thermal properties and
flammability of polyamide 6 nanocomposites
surfacemodified via layer-by-layer deposition of chitosan and
montmorillonite. It is noteworthy that LbL technique could
be an ecologically friendly alternative for electrospinning
and electrospraying methods.
Tarnamid T-27 polyamide 6 (PA6) in the form of pellets
was received from Grupa Azoty S.A. (Tarno´w, Poland).
According to producer specification, it is medium viscosity
type of polymer, designed for highly durable elements,
including thin-walled elements, formed by injection
moulding as well as processing modified pellets by
compounding method. Cloisite 15A (Southern Clay Products
Inc., the USA) montmorillonite (MMT) organophilized with
125 meq/100 g quaternary ammonium salt chloride
(composed of * 65% C18, * 30% C16, * 5% C14 mixture of
aliphatic chains) was used to prepare PA6 nanocomposite
(PA6/15A). To create a 0.5% (m/v) cationic solution,
chitosan (CH) (Sigma-Aldrich, Iceland) (Mw = 50–190 kDa,
deacetylation degree 75–85%) was dissolved into 18.2 MX
deionized water (DI) (Hipernet, Poland). Sodium
montmorillonite (MMT-Na?) (Dellite LVF, Laviosa Chimica
Mineraria, Italy) was dispersed in DI with 0.5% (m/v) to
obtain an anionic mixture. Both mixtures were magnetically
stirred overnight, and after that, pH of chitosan dispersion
was adjusted to 4.0 ± 0.5 by adding 1 M hydrochloric acid
solution (Chempur, Poland) or 1 M sodium hydroxide
solution (POCH, Poland). pH of MMT-Na? dispersion was
not additionally stabilized (9.8 ± 0.1).
PA6 matrix processing
PA6 pellets vacuum-dried at 80 C for 6 h and organo
philized layered silicate 15A were mixed in mass
proportion 97:3 by mechanical shaking of batch in plastic
container for 15 min. Later on, the batch was dosed into
twin-screw counter-rotating laboratory extruder Thermo
Scientific Haake Polylab PTW 16/25 to form a nanocom
posite with 150 rpm rotation speed. As a dosing device,
Brabender DDSR20 feeder compatible with extruder,
working at 45 rpm, was used. The temperature from feed
end to die end ranged from 245 to 260 C. The obtained
strip was cooled in a water bath (18 C) of 1.5 m length
and 27 dm3 capacity, whereupon pelletized in a ZAMAK
device. To reduce the thermal history, the same procedure
was applied to pristine polyamide 6 resin. After one more
vacuum-drying, two types of materials (PA6 and PA6/15A)
were compression-moulded in laboratory press ZAMAK
P-200 to form 15 9 15 9 0.3 cm sheets. 50–55 g of
pellets were filled into mould and pressed at 250 C under
20 MPa for 2.5 min. The sheets were cooled for 20 min
and cut into 15 9 1.3 9 0.3 cm bar specimens for fire
testing. Thermal analysis (TGA and DSC) investigations
were done on non-further-processed pellets.
Before depositing LbL assemblies on pellets, both
materials were washed with DI. Bar specimens had been
additionally immersed into 10% acetone solution prior to DI
washing, and then, either pellets or bars were vacuum-dried
at 80 C for 2 h. All of the specimens were alternatively
dipped into the positively and negatively charged solutions:
to enhance adhesion, first dips in CH and MMT-Na? were
set to 5 min, but subsequent dips lasted for 1 min. In case
of pellets, to avoid material losses, specially designed
polypropylene sieves, closed from top by aluminium
sieves, were used for dipping procedure. After each
adsorption step, the materials were washed with DI for
1 min in order to desorb weakly adsorbed surface modifi
cation and dried for 30 min at 80 C. Some part of
excessive amount of water was taken away using tissue
paper. The scheme of described procedure is shown in
Fig. 1. Each cycle was repeated until 5, 10 and 20 bilayers
(BLs) were built on each specimen type. (For instance,
PA6/15A/10BL denotes PA6/MMT nanocomposite with 10 bilayers deposited.)
Crystalline structure of chitosan, PA6, PA6/15A and
samples after surface treatment was studied by means of
Xray diffraction (XRD) technique. The XRD diffractograms
were obtained using a Bruker D2 PHASER with
LynxEyeTM detector with copper anode Ka as a radiation
source (k = 1.54184 A˚ ). All scans were done
independently in the range of 2h = 1 –10 (space 0.1 mm) and
10 –30 (space 1 mm). The only exception was chitosan
sample, which was measured in the range of 2h = 1 –60
with space 1 mm. 2h angle increment during analysis was
0.02 . The voltage and the current of X-ray tube were
30 kV and 10 mA, respectively. The degree of crystallinity
presented in Table 2 was determined using Eq. (1):
vc ¼ ð1
where xn is nanofiller content (0 for PA6, 0.03 for PA6/
15A), DHm—heat of melting of polymer under
investigation, determined by DSC (J g-1), and DHm,100%—heat of
melting of 100% crystalline polymer (230 J g-1).
ATR-FTIR (attenuated total reflectance–Fourier trans
form infrared) spectra of materials have been recorded
before flammability tests using a PerkinElmer Spectrum 65
FT-IR spectrometer in the range of 4000–600 cm-1 at
ambient temperature. The apparatus was equipped with
diamond crystal, and 32 scans were collected with 4 cm-1
resolution and 1 cm-1 data collection interval.
Cross section morphology of bar specimens was imaged
using a JEOL JSM-6010LA scanning electron microscope
(SEM) (accelerated voltage 10 kV). Prior to analysis, the
materials were cut with microtome and gold-sputtered onto
cross section surface. Similar procedure was applied to the
sample residues after combustion. Energy-dispersive X-ray
(EDX) analyser (10 kV accelerated voltage, 5009
magnification) was used to investigate the elemental composition
of specimens before combustion.
Thermogravimetric analysis (TGA) was carried out using a
Netzsch TG 209 apparatus at a heating rate 20 C min-1
under inert argon gas atmosphere (gas flow rate
20 cm3 min-1). About 5 mg of the sample was put in an opened
corundum (a-Al2O3) crucible and heated from 30 to 600 C.
Differential scanning calorimetry (DSC) profiles were
recorded using a Mettler Toledo DSC823e instrument.
Calibration was performed with indium and zinc standards.
For the DSC measurements, 5 mg of sample was put into
an alumina crucible, closed by pressing, and then examined
under argon gas atmosphere (gas flow rate 20 cm3 min-1)
by increasing temperature from - 50 to 300 C, followed
by cooling from 300 to - 50 C at a heating/cooling rate of
20 C min-1.
Vertical (VFT) and horizontal (HFT) flammability tests
were conducted according to IEC 60695-11-10:1999
(UL94) standard. Total burning time and rate of burning (unless
material showed self-extinguishing effect in specified test
conditions) were measured, and digital photographs were
taken to depict characteristics of burning together with
residues. Limiting oxygen index (LOI) measurement was
performed according to DIN EN ISO 4589-2 using Concept
Fire Testing Oxygen Index Module apparatus that lets us
adjust concentration of oxygen in gas stream with 0.1% by
volume accuracy. Nitrogen/oxygen mixture rate flow was
stabilized at 10 L min-1.
Combustion properties were also tested using
microscale combustion calorimetry (MCC) according to ASTM
D7309 standard with synthetic air. Mass of the samples
was ca. 5 mg, and the heating rate was set at 1 C min-1.
Results and discussion
Coating growth and characterization
The XRD patterns of examined materials are shown in
Fig. 2. Reflections visible in diffraction spectra in the range
of 2h = 20 –24 are ascribed to crystalline phase of PA6
(Fig. 2c, e) [
]. As it is apparent from Fig. 2a, near 20 a
peak resulting from the CH presence in LbL surface
modification is expected. The observed shift of reflections
at 21.3 for PA6 and at 21.6 for PA6/15A to lower angles
right after deposition of coatings implies effective
polycationic CH adsorption. Moreover, the peak at 23.4
(Fig. 2c), indicative of a002/202, is not well visible in
Fig. 2e, but there is an appearance of a new shoulder near
24 instead. This shoulder shifts towards lower angles for
BL materials and becomes slightly attenuated. Importantly,
a similar shoulder can be seen at below 2h = 20 only in
diffraction patterns of BL samples. Overlapping of three
patterns for BL modifications in Fig. 2e in the wide 2h
range led to the assumption of weak adsorption of
MMTNa? to the PA6 substrate. Coating growth is hindered at
just few nanolayers, which is observed as little or no
intensity changes with consecutive dips, so in order to
achieve expected protective properties, it is not necessary
to utilize long-lasting laboratory or industrial processes.
By analysing diffraction patterns of PA6/15A series
(Fig. 2e), one can see peaks at 22 (PA6/15A) or at 21.5
(BL compositions) which can be deconvoluted to show
c001, c200 and a002/202 crystalline forms. Such a
characteristic pattern is not visible in Fig. 2c, suggesting that
silicate incorporation into polymer matrix facilitates
crystallization of PA6 in less thermodynamically stable,
pseudo-hexagonal c-form [
]. Because of space
confinement, intermolecular hydrogen bonds are impeded to form
as conformation of backbone chains alters from monoclinic
a into c. Coexistence of both forms can be also verified by
analysis of the reflection in the range 10 –11 , shifting to
lower 2h angles for BL materials due to overlaying with
broad CH signal (Fig. 2a), which correspond to c020 plane.
It is worth paying attention to baseline shape of all
LbLmodified materials in the range of 1 –3 , which indicates
that the adsorbed clay exists in the form of randomly
spread, disordered nanoplatelets. Pristine PA6 diffraction
pattern exhibits different behaviours: at 1.1 baseline grows
rapidly, then, up to 2.7 , is similar to the clay-containing
composites, and, finally, up to 3.5 , the rising trend can be
noticed again. Eventually, it seems important to explain
similarities between baseline shape in the 4 –6 region for
PA6/10BL and PA6/15A composites. Apparently, this
number of bilayers (10) may be a limiting number to
physically adsorb MMT-Na? on non-spherical PA6
BL growth on PA6
5 10 15
Number of bilayers/BL
substrate, observed as a series of reflections imitating those
of layered silicates (d & 2 nm).
Supplementary data concerning incorporation of
nanofiller into polyamide matrix were obtained from ATR-FTIR
analysis of PA6 and PA6/15A materials (Fig. 3). In the IR
spectra, band at 1540 cm-1 is assigned to deformation
vibrations of N–H; band at 1634 cm-1 corresponds to
carbonyl moieties; and two peaks at 2800–2950 cm-1 are
due to C–H stretching vibrations in alkyl groups. Nearby
3300 cm-1, there is an absorption band of hydroxyl groups.
Moreover, inter- and intramolecular hydrogen bonds are
formed among PA6 chains between C=O and N–H groups
of amide linkages.
Fig. 6 TG profiles of PA6
(a) and PA6/15A (b) with 5, 10
and 20 bilayers
Formation of nanocomposite (PA6/15A) is confirmed by
shape of signals at 1000–1150 cm-1. In montmorillonite
spectrum, a broad band of Si–O bond stretching is visible
in this region. Besides, a weak signal at 918 cm-1
originates from AlOH scission deformation. Figures 4 and 5
present FTIR spectra of all tested materials, in which some
regular shifts are noticeable. With more bilayers deposited,
the intensities of bands at 3200–3400 cm-1 (hydrogen
bonds and OH groups) and at 2800–2950 cm-1 are
increasing. A distinct change can be observed at
950–1150 cm-1, which corresponds to incorporation of
polyelectrolytes onto polymeric substrates. As signals of
virgin chitosan overlay with those of PA6 (symmetric and
asymmetric stretching of NH4? at 1540 and 1635 cm-1,
respectively; planar deformations of N–H and C–N
stretching at 1540 cm-1; scissoring vibrations of NH4? at
1170 cm-1), LbL modification effects require a detailed
study of the latter region. Actually, band at 1110 cm-1
correlated with C–O–C stretching of glycoside linkage
shows growth of intensity. (In the proximity in MMT
spectrum, a broad Si–OH stretching signal is also visible.)
Moreover, deposition of MMT-Na? can be evidenced by
the appearance of band at 3625 cm-1 (Al–OH and Si–OH
To further describe coating build-up by self-assembly mechanism of nanolayers, relative intensities of
characteristic bands for utilized polyelectrolytes were
calculated. Diagrams of transmittance ratio as a function of
number of bilayers, namely C–O–C/C=O: T1110 cm-1/
Fig. 8 DSC profiles of a PA6
and b PA6/15A with 5, 10 and
T1634 cm-1 and (Al–OH ? Si–OH)/C=O: T3625 cm-1/
T1634 cm-1, are shown in Figs. 4 and 5. For all nanolayers
with exception of MMT-Na? on PA6 substrate (known in
the literature as exponential regime or two linear regimes
with 10 BLs as a limiting bilayer number), values of these
show linear trend. The exponential regime is commonly
explicated by initial island-like evolution, followed by
diffusional growth consisting of diffusion of single
polyelectrolyte in or out of the thin film. This difference
between two basal materials is going to be explained later
in relationship to flame-retardant properties of both.
The thermogravimetric curves for the PA6 and PA6/MMT
nanocomposites with different values of BLs are presented
in Fig. 6a, b, respectively. The analysed results (in an
atmosphere of inert gas) in terms of temperatures at which
5 (T5%), 10 (T10%), 20 (T20%), 50% (T50%) and maximum
(Tmax) mass loss occurs are given in Table 1.
These results show that by combining OMMT, CH and
MMT it is not possible to thermally stabilize PA6 during
the initial stages of decomposition. The combination of
MMT with CH stabilizer leads to a decrease in the thermal stability of polyamide nanocomposites for both series of materials.
For the composite PA6/10BL, T5% was equal to 321 C.
In contrast, the LbL deposition caused a decrease in the
beginning of the degradation by 37 C in relation to
polyamide 6. In the range of 400–475 C, these PA6
samples with 5, 10 and 20 BLs have decreased stability (in
terms of lower mass loss) compared to the neat polymer.
In the range of 400–500 C, depolymerization process
takes place with emission of large amounts of
]. This is important not only in terms of thermal
stability, but also from the point of recycling of clay/PA6
composites. In the narrow range of 440–460 C, the PA6
thermogravimetric curve overlapped with the TG profiles
100 150 200 250 300
of PA6/5BL, PA6/10BL and PA6/20BL. The main stage of
degradation of these composites ended up near 470 C,
yielding some residues—Table 1. Thermogravimetric
analysis (especially residue formation at 480 C) has
shown that the presence of bilayer inorganic–organic
architectures strongly enhances the char formation.
Furthermore, infrared spectroscopy has revealed that these
coatings are able to promote the formation of a char having
an aromatic nature. Depending on the number of layers, the
organic–inorganic coatings are able to induce the formation
of a thermally stable aromatic char that protects the
polymer substrates from the application of a flame or different
external heat fluxes.
Thermograms presented in Fig. 6 show that within a
temperature range of 350–460 C, the PA6/15A
nanocomposites had very similar mass loss and the similar
thermal stability. The thermal degradation of polyamide
composites (Fig. 6) takes place also in one step. Tmax
values of the composites containing the nanofillers and 5,
10 or 20 BLs are similar to and lower than those materials
without 15A nanofiller.
At lower temperatures up to 120 C, the observed mass
loss is due to evaporation of physically adsorbed water, and
water from hydrated ions is lost in the temperature range of
80–180 C [
]. The main stage between 200 and
470 C is attributed to scission of HN-CO bonds, leading to
the formation of unsaturated and saturated carbon–carbon
bonds along the macrochain. Smaller amounts of residues
in the range of 470–600 C for all of systems reveal that
there is less or no formation of ceramic-like structure by
In order to afford a fair comparison of the effect of
nanoparticles on polyamide, it is useful to define a thermal
analysis parameter that is matrix independent. In this study,
the overall stabilization effect (OSE) for PA6 and PA6/15A
nanocomposites was calculated via integration of the area
under the mass% versus temperature curves and is shown
in Fig. 6a, b [
where T is the degradation temperature.
A positive OSE value indicates an improvement in the
overall thermal stability of the polymer nanocomposite in
the temperature range 20–605 C, while a negative value
suggests that the overall thermal stability of the
nanocomposite is inferior to that of the unmodified matrix.
OSE values for PA6 and PA6/15A systems are presented in Fig. 7.
The OSE results reveal the combined stabilization action
of clay or organoclay and CH stabilizer (5 BLs) on PA6.
However, OSE values of all materials studied herein sug
gest that LbL technique itself is effective enough to yield
PA6 materials with improved thermal stability.
The DSC curves for the PA6 and PA6/15A nanocom
posites with different values of BLs are presented in
Fig. 8a, b, respectively. The analysed results—melting
temperature (Tm), crystallization temperature and degree of
crystallinity—are given in Table 2.
The PA6/15A nanocomposite containing 3 wt% of
organoclay has a higher DHm as well as a higher proportion
of PA6 crystalline phase. Under the processing conditions
applied, the presence of the clay supports polymer
crystallization. For nanocomposite materials, with the BL
increase, the melting enthalpy decreases. In general, the
stack of nanolayers deposited on polymer substrate did not
significantly influence the melting and crystallization
temperatures, whereby the melting enthalpy for PA6/15A/
BL materials is lower than for PA6/15A reference material.
This effect may be explained by the protective action of BL influencing the melting behaviour of PA6 matrix.
LOI and UL-94 flammability tests are commonly used to
study flame retardancy and burning characteristics of
polymer materials. Values of both tests are listed in
Tables 3–5. LOI results show that while surface-modified polyamide 6 materials require less of oxygen to steadily combust (compared to PA6), LOI of PA6/15A materials increases.
It has been observed that compositions based on PA6
burn with bright, natural flame and, up to
self-extinguishment, polymer melt including charred products of
degradation runs down slowly. Nanocomposite counterparts
exhibit different behaviours, namely polymer melt distracts
from the sample with flame and there is no
self-extinguishment. Due to high viscosity of nanocomposite melt, it
drops slower than in case of unfilled polymer, as well as
irregularly, and the drops are bigger. Despite that, melt still
burns and is capable of spreading fire. Improved flame
retardancy of LbL modifications of PA6 were further
confirmed by UL-94 tests.
Although PA6 reference material self-extinguishes, the
number of melt drops was at least twice higher than for
LbL compositions. On the other hand, the latter burn
slower, most effectively just with 5 BLs deposited (burning
speed even 6 cm min-1 slower than for PA6/15A). All
PA6 BL modifications can pass V-0 rating at tested spec
imens, but pure PA6 shows V-1 only; in vertical test, both
series present a decline in number of melt drops.
Charred residues of PA6/10BL and PA6/15A/10BL were analysed by SEM as shown in Fig. 9. Unfilled
Fig. 10 HRR curves of a PA6
and b PA6/15A with 5, 10 and
polymer modification has more irregular morphology of
carbonaceous char than nanocomposite. It consists of
loosely linked flakes and bubbles appearing during
evolution of combustion gases and is comparable with original
structure (Fig. 9b).
Microscale combustion calorimeter was used to explain
the impact of surface modification on the intrinsic
combustion properties by direct measurements of the heat
released during controlled heating of samples. Table 6 lists
the values of PHRR (maximum peak in heat release rate
curve), temperature of PHRR, THR (total heat release),
percentage change of these in relation to references and
FIGRA (fire growth rate index—the ratio of PHRR to
temperature of PHRR) as an important index of fire safety
of polymer materials. The HRR curves are shown in
Fig. 10. Of all studied samples, best parameters were found
for PA6/20BL, which showed an improved thermal
stability, as shown by TGA and DSC.
PA6/15A series start burning in lower temperatures (by
about 20 C) than PA6 (340 and 360 C, respectively).
Furthermore, the highest reduction in PHRR was
demonstrated in materials with 20 BLs deposited, though none of
them were higher than 10%. The temperatures of PHRR
were not profoundly shifted with best results for 10BL
modifications. With increasing number of bilayers, the
parameter rises, which may reflect the existence of limiting
thickness of coating, which shows minimum flame
In this work, LbL assembly technique has been
successfully employed to deposit cationic chitosan and anionic
montmorillonite polyelectrolyte coatings on PA6 and its
nanocomposite. It has been found that the coating growth is
hindered at just few nanolayers, which is observed as little
or no intensities change with consecutive dips, so in order
to achieve expected protective properties it is not necessary
to utilize long-lasting laboratory or industrial processes.
Deposition of MMT-Na ? has been confirmed by IR
spectroscopy through appearance of bands at 3625 cm-1
(Al–OH and Si–OH stretching) and at 918 cm-1
originating from AlOH scission deformations. The obtained TG
profiles showed that within a temperature range of
350–460 C, the PA6/15A nanocomposites had very
similar mass loss and hence the similar thermal stability. The
OSE results reveal the combined stabilization action of
clay or organoclay and CH stabilizer (5 BLs) on PA6. The
flammability results revealed that although the number of
melt drops for PA6 reference material was at least twice
higher than for LbL composites, the surface-modified PA6
materials burn considerably slower and can pass V-0
rating, whereas pristine PA6 shows V-1 behaviour only. MCC
test results indicate that the temperatures of PHRR were
maximally reduced for 10BL modifications, but with
increasing number of bilayers, this parameter’s value was
increased. It may suggest that there is a critical thickness of
coating, above which the flame retardancy effect is less
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