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Thermal Stability and Fire Retardant Properties of Polyamide 11 Microcomposites containing different Lignins Neeraj Kumar Mandlekar, Aurélie Cayla, Francois Rault, Stéphane Giraud, F. Salauen, Giulio Malucelli, and Jin-Ping Guan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03085 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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Industrial & Engineering Chemistry Research

Thermal Stability and Fire Retardant Properties of Polyamide 11 Microcomposites containing different Lignins N. Mandlekar a,b,c,d*, A. Cayla a,b, F. Rault a,b, S. Giraud a,b, F. Salaün a,b, G. Malucelli c , J. Guan d a

Univ. Lille Nord de France, F-59000 Lille;

b

ENSAIT, GEMTEX, F-59100 Roubaix;

c

Politecnico di Torino, Dept. of Applied Science and Technology, Alessandria, Italy;

d

College of Textile and Clothing Engineering, Soochow University, Suzhou, China.

*Corresponding Author E-mail: [email protected] ABSTRACT This study investigates the influence of various lignins and their content on the thermal stability and fire retardancy of bio-based polyamide 11 (PA). Microcomposites based on PA and containing 5, 10, 15 and 20 wt.% of different lignins were prepared with a twin-screw extruder. Morphological analysis showed good interfacial interaction and uniform distribution of lignin particles within the resulting microcomposites. Further, thermogravimetric analyses carried out in inert atmosphere indicated that, unlike kraft lignin, which is able to give rise to the formation of lower char residue (41 – 48 wt.% at 600 °C), the sulphonated counterpart provides a higher

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thermal stability as well as a higher char residue (55 – 58 wt.%). Furthermore, vertical flame spread tests clearly showed that 15 wt.% is the optimum of kraft or sulphonated lignin loading to achieve improved flame retardant properties and V1 rating. In addition, cone calorimeter was exploited to study forced combustion behaviour; in particular the microcomposites containing sulphonated lignin revealed a significant reduction of peak of heat release rate (-51%), of total heat release (-23%) and a lower average mass loss rate together with a noticeable increase of the final residual mass (about 9 wt.%). Conversely, the microcomposites containing kraft lignins showed opposite effect, since HRR and THR values increased in the presence of kraft lignin. KEYWORDS: Lignin, Polyamide 11, Microcomposites, Thermal stability, Fire retardancy 1. INTRODUCTION Over the past few years, the increasing global environmental concerns and programmed depletion of petroleum resources have driven researchers to bring their attentions towards renewable and sustainable resources which are less harmful for environment. In the same context, lignin is the second most abundant polymer from biomass after cellulose, and the main one as far as aromatic subunits are considered. Industrial lignin is primarily a by-product of wood pulping and paper making industries, its amount is estimated to be 70 million tons per year around the world. However, it is primarily used for the production of energy by burning, and only less than 2% is actually recovered for the utilization as a commercial product comprising about 1,000,000 tons/year lignosulphonate from sulphite pulping and about 100,000 tons/year kraft lignin from kraft process.1,2 Lignin polymers differ each other to a large extent, since they possess different functional groups and physico-chemical properties depending upon source of origin, presence of impurities, extraction process and post-treatments. Lignin as an aromatic polymer is characterised


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by a highly branched, three dimensional phenolic structure including three main phenyl propane units, namely p-coumaryl, coniferyl, and sinapyl. The resulting phenolic substructures are called p-hydroxyphenyl (H, for coumaryl alcohol), guaiacyl (G, for coniferyl alcohol), and syringyl (S, for sinapyl alcohol) moieties.3 These various structures play crucial role when lignins are used as filler in polymer composites. They can affect the thermal stability and compatibility with the polymer matrices.4,5 The chemical structure of lignin is different between hardwood and softwood, thus thermal degradation behaviour and charring properties are greatly influenced by their chemistry.6 In recent years, the valorisation of lignin compounds has attracted growing interests from researchers for its potential applications in polymers because of many advantages including abundance, many reactive functional groups, high carbon content and tailored capability for structure modification. During the last years, lignin has been employed to improve the thermal stability7–9 and fire retardancy10–12 of thermoplastic polymers. Lignin thermally decomposes over a broad temperature range, because various aromatic functional groups have different thermal stability. Furthermore, lignin is also able to generate a high amount (ranging in between 30 to 50 wt.%) of char residue upon heating at high temperature (500 – 700°C ) in inert atmosphere,13 the char-formation can also reduce the heat release rate of the polymeric material during degradation steps. In addition, improvements in thermal and flame retardant properties of lignin-based blends can be further strengthened by addition of intumescent flame retardant additive based on phosphorus and nitrogen elements. Lignin can be exploited in combination with polymers mainly by using two approaches: (i) through chemical modification to obtain a large range of chemical products, building blocks and polymers; and (ii) by direct melt blending with polymers to confer new or improved functional properties. The second route is considered as a convenient way to



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increase the content of renewable sources and to develop new polymeric materials. Up to now, few studies have been published on fire behaviour of polymers containing lignin without flame retardant compounds. In this context, Canetti et al. used lignin in polypropylene (PP) and polyethylene terephthalate (PET).14,15 An increase in PP thermal degradation temperature with increasing the lignin content was observed and attributed to the interactions occurring between PP and lignin, which lead to the formation of protective surface layer. In the case of PET, lignin was found to affect the overall crystallization of the polymer due a nucleating effect. Further, Song and coworkers studied the reactive compatibilization of lignin in acrylonitrile butadiene styrene (ABS),16 which can further reduce the flammability of the copolymer due to formation of an improved char layer. Soda lignin was also incorporated in polyhydroxybutyrate (PHB) biopolyester by Mousavioun and Bertini; in particular, notwithstanding a decrease of the degradation onset observed in the presence of lignin, the thermal stability of resulting blends was spread over a wider range of temperatures17,18 Lignin was found to be well dispersed in PHB due to interaction between the functional groups of lignin and the carbonyl group of PHB. Over the past decade, most of the studies concerning the use of lignin as fire retardants in polymer matrices have been carried out by using modified additives.19–21 Few studies proposed the direct lignin incorporation in a polymer matrix; to the best of authors knowledge, no work thoroughly discussing the most effective type of industrial lignin as char former in polymer matrices has been reported so far. Polyamide 11 (PA), one of the available bio-based polymer is used as a polymer matrix.22 Thus, the aim of this work is to investigate the influence of lignin sources and its content on thermal degradation behaviour and char forming ability of PA/lignin microcomposites obtained by direct extrusion method, without any chemical modification and pre-treatment. Chemical structure and morphology of each selected lignin (namely, alkali kraft


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low sulphonate lignin, lignosulphonate lignin, alkali kraft lignin and Domtar BioChoice™ kraft lignin) were first examined by Fourier Infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses. Their effects on the thermal stability and char forming ability of microcomposites have been thoroughly investigated by thermogravimetric (TGA) experiments. Furthermore, vertical flame spread and cone calorimetry tests were exploited for assessing the fire behaviour of the prepared microcomposites. 2. EXPERIMENTAL SECTION 2.1. Material. Four types of lignins were used as carbon source, differing as far as the plant resource, chemical nature and purity are concerned. Two sulphonated lignins; namely Domsjö lignosulphonate lignin provided from Domsjö Fabriker AB (Sweden), and Alkali kraft low sulphonate (4 wt.% sulphur) lignin purchased from Sigma Aldrich, France (hereinafter coded as LL and LS), respectively; two kraft lignins (hereinafter coded as DL and KL) obtained from UPM Biochemicals, Finland (European distributer of Domtar BioChoice™ lignin), and purchased from Sigma Aldrich, respectively. A bio-based Polyamide 11 (Rilsan® BMNO-TLD; Mn=17,000 g/mol, MFI=14-20 g/10 min at 235 ºC), supplied from Arkema (France), was chosen as polymer matrix in melt process. Among two sulphonated lignins, LS consists of alkali kraft lignin and 4% sulphur as sulphonate, belongs to pure grade; conversely, LL, containing Na-lignosulphonate (about 70 %) and small amount of Mg & Ca lignosulphonate and some impurities like ash (about 20%) and carbohydrates, refers to industrial grade. KL belongs to pure grade and consists of alkali kraft lignin with purity level > 98%, whereas DL represents industrial grade and contain mainly alkali



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kraft with 90% purity level The purpose of using these four types of lignin was to assess their effect on the thermal stability and fire retardancy of polyamide 11. Preparation of PA/lignin microcomposites. The different PA/lignin Microcomposites, labelled as PAX-YZ (where X represents the amount of PA, Y the type of lignin used, and Z the amount of lignin in wt.%) were prepared by melt extrusion (Table 1 lists the microcomposite formulations investigated). To minimize the water content for the melt blending with PA, all the additives were previously dried overnight at 80°C. Prior to compounding, x wt.% of PA chips with z wt.% of lignin were first mechanically mixed at room temperature. The mixture was then meltextruded using a co-rotating intermeshing twin-screw extruder (Thermo Haake, screw diameter = 16 mm, L/D = 25). To produce PA microcomposites, the temperatures of the five heating zones ranged from 170 to 220 °C, and the rotation speed was set at 100 rpm. In all cases, the extruded rods were pelletized. Pellets are dried overnight at 80°C prior to use for any analysis. Plates (thickness of 3 mm) were manufactured by compression moulding using a Dolouets hydraulic press, operating at 50 bar and 220 °C. Specimens in accordance with cone calorimeter (100 x 100 mm2) and UL-94 (125 x 13 mm2) tests were used. 2.2. FTIR spectroscopy. The chemical structure of each lignin samples was analysed by FT-IR spectroscopy. Samples were ground and mixed with KBr to make pellets. FTIR spectra were recorded using a Nicolet Nexus, Germany from 4000 cm–1 to 500 cm–1, using 64 scans and with 4 cm-1 resolution. 2.3. Morphology. Mean diameter and particle size distribution of various lignins were first observed by using an optical microscope (Axiolab Pol, Carl Zeiss, Germany) at 10x magnification and equipped with a uEYE camera (IDS, Obersulm, Germany). Further, images


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were processed in ImageJ online available particle analysis software to measure the mean particle diameter. SEM analyses of various lignins and microcomposites were carried out using a LEO1450VP apparatus (Carl Zeiss, Germany), equipped with a X-ray probe (INCA energy oxford, Cu KαX-ray source, k = 1.540562 Ǻ) to perform elemental analysis. Pellets of different microcomposites were fractured in liquid nitrogen, then gold metallized. 2.4. Thermal properties of microcomposites. The thermal behaviour of PA/lignin microcomposite was investigated using a TAQ20 analyzer (DSC) equipped with TA Advantage control software. Indium was used as standard for temperature calibration and the analysis was made under a constant nitrogen flux (50 ml/min). Samples were placed in aluminium pans, which were hermetically sealed before testing and subjected to the following thermal cycle: heating up from 0 to 250°C at 10°C/min, followed by 5 minutes isothermal treatment at 250°C, cooling down from 250 to 0°C at 10°C/min, and final heating up from 0 to 250°C at 10°C/min. The crystallization exotherm recorded from cooling cycle and the second heating cycle was used to determine melting endotherm. The degree of crystallinity (Xc) was calculated according to the following equation: = where ∆H m

∆ × 100 −

° 1 ∆

° is the enthalpy of fusion registered on the heating up, ∆ is the theoretical

enthalpy of fusion of 100% crystalline polyamide 11 (189.05 J/g),23,24 and is the weight fraction of lignin. 2.5. Thermal stability. Thermogravimetric analyses (TGA) were carried out on a TAQ500 analyzer under nitrogen atmosphere at a purge rate of 50 ml/min. For each experiment, a sample of approximately 10 mg was used. A heating rate of 10°C/min was applied, and the temperature



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was raised from 30 to 700°C. Both TG and DTG (derivative) curves were recorded for all the samples. Different degradation parameters were determined from TG curves, such as temperature at 5 wt.% mass loss (T5%), onset of weight loss (Tonset), which corresponds to initial degradation calculated at the inflection point and char residue recorded at 600°C. Furthermore, maximum mass loss rate (MMLR) and the corresponding temperature (Tmax) were obtained from derivative curves (DTG). 2.6. Fire behaviour – flame spread test. Flammability properties of different microcomposites were evaluated on sheets (125 x 13 x 3 mm3) by vertical flame test method according to IEC 60695-11-10,25 which is well known as UL 94 burning flame test. This test is a small-scale laboratory screening procedure for comparing the relative burning behaviour of vertically oriented specimens, which can be classified as non-flammable (V-0), less flammable (V-1), flammable (V-2) and highly flammable (Non-classified) rating. 2.7. Fire behaviour – forced combustion test. Cone calorimetry was employed to assess the forced combustion behaviours of sheets (100 x 100 x 3 mm3) at 35kW m−2 heat flux using spark/piloted ignition, according to ISO 5660 standard.26 Three tests were performed on each formulation and the results averaged. Time to ignition (TTI), peak of heat release rate (PHRR), total heat released (THR), average mass loss rate (AMLR), effective heat of combustion (EHC) values and residual mass were measured. 3. RESULTS AND DISCUSSION 3.1. Structures and morphology of the various lignins. The FTIR spectra of various lignins are shown in Figure 1; the peak positions of the most significant absorption bands, based on assignments given by Faix

27

are summarized in Table S1 (Supporting Information). All lignin


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spectra show a broad absorption around 3411cm−1, assigned to the hydroxyl stretching in phenolic and aliphatic structures. Alkyl C–H are found between 2929 and 2840 cm−1; besides the peaks at 1760 and/or 1695 cm-1 are related to C=O stretching of unconjugated and conjugated, respectively. The shifting of the band towards higher wavelength is ascribed to the presence of the sulphonate group in sulphonated lignins sample. The presence of carbonyl group at 1591 cm-1 also showed that the C=O bonds are in conjugation with aromatic ring. Further, the absorption peaks at 1510, 1450, 848, 810 cm-1 are assigned to C=C stretching and C–H out-of-plane vibrations in position 2, 5 and 6 of guaiacyl units in the aromatic groups, respectively. In LS, DL and KL spectra, the peak at 1261 cm-1 is assigned to C–O stretching of the guaiacyl ring, and the band at 1130-1138 cm–1 is related to aromatic C-H in-plane deformation in the guaiacyl ring; C-O deformations of secondary alcohols and aliphatic ethers are found at 1076 cm–1. Furthermore, in the LL and LS spectra, the peak at about 652 cm-1 (from S–O structure) and two other bands at 1178 and 1130 cm−1 representing asymmetric and symmetric –SO2 vibrations are typical of sulphonate structures. On the other hand, the absence of characteristic absorption band at 1325 and 1108 cm-1, which is related to C–O stretching and aromatic C–H deformation of syringyl ring,28,29 confirms the absence of Hardwood lignin: thus, all lignins have more softwood characteristics. Assignments of the recorded spectra of the lignin samples are in agreement with those reported in other works.30–32 The mean diameter and size distribution of the four lignins mainly depend on their origin, chemical structure, type of extraction process and their molecular weight.33,34 Table 2 reports the lignin mean particle diameter obtain from optical microscopic images. Thus, LL and LS samples (sulphonate products) have a mean diameter higher than the two other samples, i.e. 63 ± 13 and 71 ± 18 µm, respectively, versus 31 ± 9 µm for DL and 39 ± 10 µm for KL samples (Figure 2).



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Furthermore, LS sample has a wider size distribution than the other samples, which is anticipated by higher value of standard deviation. Lignins tend to form some aggregates in various medium, which affects the preparation and the formulation of lignin-based advanced polymers. The aggregated particles are thermally stable and are related to the establishment of cohesive interactions among lignin molecule, i.e. intermolecular hydrogen bonding and π- π stacking of aromatic groups illustrating the effect of the chemical structures. Therefore, it can be noticed that the aggregation degree of the sulphonate containing lignins is less important than for the alkali kraft lignins. SEM images of lignin samples (Figure 3) confirm the OM images: in particular, the lignin samples display significant changes according to their origin. Thus, high pure lignins from Sigma Aldrich (LS and KL samples) have a smooth spherical shape, and a porous inner core, whereas the two industrial samples show a rough and irregular polygonal shape, with the presence of impurities on the particle surface. 3.2. Morphology of PA/lignin microcomposites. The effect of lignin content on the morphology of resulting microcomposites was studied by SEM microscopy. Figure 4 (a-f) shows the typical images of fractured surfaces. It was observed that, during the microcomposite preparation process, lignin particles are broken down and homogenously distributed in PA matrix. Even at high concentrations (below 15 wt.%), lignin particles are reduced to less than 10 µm in size. Furthermore, micrographs show no phase separation up to 15 wt.% lignin contents. Beyond this value, the morphology of the resulting microcomposites changes and lignin particles tend to form aggregates separated from the polymer matrix. In addition, 20 wt.% loading of lignin, irrespective of its type, increases the voids volume,35 as displayed in Figure 4 (f). This finding can be ascribed to increased water absorption as a consequence of higher amount of lignin: in fact, drying does not eliminate the water completely, as also confirmed by the presence of a broad –OH peak


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(3300-3400 cm-1) in IR spectra and by the initial degradation peaks in DTG curves of lignins, attributable to the release of bound water during extrusion, hence giving rise to the formation of large cavities within the extruded monofilament. These findings were also confirmed by Polat et al., where increase in lignin loading leads higher porosity, which affects the mechanical strength of the resulting blends.36 3.3. Morphology and crystallinity. Both melting and crystallization behaviour of PA in the presence of different lignins was investigated through DSC measurements; in particular, enthalpy of fusion (∆Hm), melting temperature (Tm), enthalpy of crystallization (∆Hc) and crystallization temperature (Tc) were evaluated. All these parameters are listed in Table S2 (Supporting information). The degree of crystallinity (Xc) for unfilled PA and its microcomposites with selected lignins was calculated from melting enthalpy, on the basis of the lignin loading. The maximum crystallization rate was found to occur at 157°C for unfilled PA. The microstructure and therefore the thermal properties of the microcomposites can be governed by the nucleation process. The presence of lignin in some polymer acts as a nucleating agent such as in PET and PHB matrices.37,38 It can be observed that the crystallization temperature slightly increases by 1 to 3°C at low lignin content, but it decreases in the presence of higher amounts. Besides, from the Tc onset results, it can be concluded that lignin is not acting as a nucleating agent in PA/Lignin microcomposites. Furthermore, irrespective of the source of lignin, the filler does not affect the melting of the microcomposites, since the Tm onset and Tm peak values are similar to those of unfilled PA. For all the systems, the presence of lignin influences Xc; in particular, Xc decreases with increasing lignin content in PA. It is expected that lignin particle size affects Xc: in this context, SEM images show that lignin particle size in microcomposite decreases (below 10 µm) with decreasing lignin content; smaller lignin size leads to better interaction between lignin and



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polymer chains, favouring the crystal packing, hence increasing Xc. This is the reason for which the maximum Xc is observed at low lignin content for all the PA/lignin microcomposites. It is assumed that smaller DL particle size greatly affects crystal packing, hence higher crystallinity values (i.e. 34%) are obtained for PA-DL microcomposites (with a nanofiller content not exceeding 15 wt.%). However, increasing the lignin amount in PA leads to agglomeration, hence to the decrease of Xc. 3.4. Thermal stability of lignins. The thermal stability of different lignins and their char forming capability were assessed by means of thermogravimetric analyses carried out at a heating rate of 10°C/min in nitrogen. Weight loss (TG) and weight loss rate (DTG) curves are shown in Figure 5; the data are collected in Table 3. The thermal decomposition of lignins occurs in a wide temperature range and can be divided into three main steps: (i) from 60 to 140°C (moisture release), (ii) from 140 to 500°C (main mass loss stage occurring through devolatilization), and (iii) beyond 500°C (continuous devolatilization with charring), where 40 to 60 wt.% of all lignin samples still remain unvolatized due to the formation of condensed aromatic structures.39,40 From an overall point of view, during the first degradation step, the water release or mass loss, related to the loss of free and bound water, varies in the presence of sulphonate groups. Furthermore, it was also noticed by Haz et al. that during this first stage, a plasticization phenomenon occurs.41 The broad range temperature degradation is related to the chemical structure of the lignins, and more specifically to the presence of various oxygen functional groups, the scissions of which occur at different temperatures. Thus, in the earlier stages, there is the scission of weak ether, aryl-alkyl and phenyl glycosides bonds, and unstable C-C bonds. The mass loss observed is mainly attributed to the degradation of components of carbohydrates converted to volatile gases such as CO, CO2 and CH4. At the end of the second stage, above 400°C, the polyaromatic


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compound formed further decomposes to volatile species such as phenolic, alcohols or aldehyde, and char is due to the formation of stronger bonds. Overall, LS has the greatest thermal stability and the highest char yield (58%), followed by LL (55%), KL (48%) and DL (41%). The lower production of char residue from kraft lignin degradation can be attributed to its higher methoxy content than sulphonated lignins.42 As depicted in the DTG curves, the main degradation steps of LL sample occur at lower temperature than the other samples with a Tmax at 250°C. The maximum devolatilization rate temperatures of the three other lignins can be found at 318°C for LS, 358°C for KL, and 385°C for DL. These results also confirmed that all lignins have some differences in their structures and chemical natures. As shown by Jakab et al., the ether bonds between guaiacol units are more stable than those between syringol units, which may explain why LL has the lowest maximum devolatilization rate temperature, unlike kraft lignins that exhibit the highest one.43 3.5. Effect of lignins on thermal stability of PA. The effect of the presence of lignin on the thermal stability of PA was assessed through thermogravimetric analyses carried out in nitrogen. In particular, Tonset (i.e. the inflection point of weight loss), T5% (corresponding to initial 5 wt.% weight loss), Tmax (temperature at maximum rate of weight loss), the corresponding mass loss rate (MMLR) and residue at 600 ºC are collected in Table 4. Figure 6 (a) shows the thermogram of unfilled PA, which exhibits a characteristic single degradation step, occurring between 390 and 440 ºC with sharp DTG profile (Figure 6 (b)), and a rapid mass loss rate, without leaving any char residue: this behaviour is in agreement with what reported in the scientific literature.44 The presence of LS is responsible for the increase of both Tonset and Tmax; at the same time, T5% decreases. A similar behaviour is shown by PA-LL microcomposites. In this case, T5% greatly reduces: this finding could be attributed to rapid mass loss rate of LL at lower temperature region.



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Further, because of the slow degradation rate of LS and LL, their microcomposites generate a significant amount of char residue at 600 ºC. It is assumed that the presence of sulphonate group also helps in getting stable char residue because during degradation SO2 is released and Na2SO3 or Na2SO4 is formed by cationic moiety, hence providing an increased thermal stability.43 On the other hand, the addition of KL and DL in PA reduces T5% up to 40 ºC, which is much smaller than what observed in LS and/or LL microcomposites. Unlike LS and LL, kraft lignin microcomposites show a very limited or even no change in Tonset and Tmax as compared to unfilled PA. This leads to large mass loss for these microcomposites and lower char residue at the end of experiment. This behaviour can be explained by the maximum mass loss of KL and DL (in between 350 and 450 ºC), due to the evolution of a higher amount of volatile products during kraft lignins degradation.45 3.6. Influence of lignin content on thermal stability of PA. The lignin loading in the microcomposites also affects their degradation behaviour. Figure 7 (a) shows TG and DTG curves of PA and its microcomposites with LS loading ranging from 5 to 20 wt.%. With respect to unfilled PA, increasing the LS amount reduces T5% but, at the same time enhances the thermal stability by shifting Tonset and Tmax to higher temperatures. In addition, increasing the amount of lignin slows down the mass loss rate, hence resulting in a higher amount of char residue at the end of test. Irrespective of a slight variation in values, a similar trend in TG and DTG curve was observed for PA-LL microcomposites, However, the introduction of 5 wt.% LS or LL in PA marginally increases T5%, as well as the compatibility between the two components. Nevertheless, further lignin loading induces immiscibility in microcomposites and T5% values decrease as a function of the lignin loading. On the contrary, T5% values decrease as a function of KL loading in the corresponding microcomposites (Figure 7 (b)): this finding can be ascribed to


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the poor miscibility and broad temperature range of KL degradation.5,16 Furthermore, mass loss rate and MMLR decrease with increasing lignin loading as compared to neat PA. 3.7. Fire behaviour – flame spread test. UL94 is widely used for evaluating relative flammability phenomenon and melt dripping of polymeric materials. The data collected in Table 5 include, total combustion time, burnt length, weight loss and UL94 rating for the developed formulations. The addition of LS lignin to PA increases combustion time, which further rises in line with the lignin content. This could be attributed to the microcomposite decomposition that occurs at a lower temperature as compared to the polymer matrix; as a result, high flammability is observed and the samples cannot be classified by UL94 test. Conversely, LL limits the microcomposite flammability by reducing total combustion time; in addition, the flame retardant properties increase with increasing the LL content. In particular, the microcomposites containing 15 to 20 wt.% LL show a significant reduction of total combustion time and achieve V1 rating. It is observed that during the test, a protective char layer is formed, which acts as barrier for heat and mass transfer. It is expected that the presence of sulphonate functionality and other impurities in LL lead to formation of a thermally stable compound in the condensed phase, which also contributes to the formation of an effective char layer, leading to self-extinction to LL and improving the rating. Besides, the addition of KL to PA has a detrimental effect; at high lignin contents (15 – 20 wt.%), the samples cannot be classified. Furthermore, the microcomposites containing KL (at low loading, i.e. 5 and 10 wt.%) show the dripping phenomena, which are responsible for V2 rating. Unlike KL, increasing DL content in PA provides a decreased combustion time, hence a further improvement of flammability rating. It is interesting to note that 15 wt.% of LL and DL allows achieving V1 rating: this lignin loading seems the best for enhancing the flame retardant properties of PA.



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3.8. Fire behaviour – Forced combustion test. Cone calorimetry has been widely used to simulate the fire behaviour of materials in real fire scenario. Heat release rate (HRR) and total heat release (THR) are considered key parameters in order to evaluate the combustion behaviour of a material exposed to a certain heat flux. In this study, HRR, THR, average mass loss rate (AMLR) and char residue of the prepared microcomposites were compared with unfilled PA; the data for all four types of lignins are collected in Table 6. Figures 8 (a – d) show the influence of lignin and its content on HRR and THR. It is noteworthy that Figure 8 shows the curves for pure grade LS and KL lignins. Unfilled PA is ignited after 79 s, rapidly increases heat release rate (HRR) and reaches a peak of HRR (PHRR) of about 862 kW/m2 and then decreases, with total heat released (THR) of about 98 MJ/m2. It is worth noticing that all PA/lignin microcomposites anticipate the ignitability: this finding can be attributed to lignin degradation that starts at about 200°C, i.e. below that of PA. Furthermore, the microcomposites containing sulphonated lignins show a higher reduction of PHRR and THR, because of formation of a higher char residue. It is assumed that condense aromatic structure of sulphonate containing lignins generate higher char residue. It is also expected that the presence of sulphonate groups further contribute to lower HRR: in fact, during degradation SO2 is released,46 hence limiting the heat release; furthermore, the formation of thermally stable Na2SO4 in the condensed phase supports the creation of a protective barrier towards the heat and mass transfer from and to the underlying polymer, hence promoting the reduction of PHRR and other fire parameters. On the other hand, kraft lignins containing microcomposites show increased PHRR and THR values in comparison with unfilled PA. This finding is attributed to the high mass loss as their AMLR values are higher than PA (see Table 4). The higher AMLR value could be explained considering the thermal degradation of KL and DL blends: for these latter, Tmax is shifted towards lower temperature, due to presence of less thermal stable guaiacyl units. As a consequence, higher mass loss is observed, hence determining


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increased HRR and THR values. Combustion efficiency of the volatile species were evaluated by effective heat combustion (EHC) parameter, which reduces with increasing the amount of lignin in PA but no clear trend was observed with respect to lignin content or lignin type. Furthermore, PHRR and THR of PA-LS microcomposites decrease with increasing the lignin content. The presence of 5 wt.% lignin increases PHRR (1040 kW/m2) and THR (106 MJ/m2) with respect to unfilled PA: this detrimental effect may be attributed to low lignin content, which is not enough for triggering the formation of a stable char layer. Besides, the microcomposite containing 10 wt.% LS shows a two-step combustion process: the first step can be associated with the char formation; then a steady state region is observed, which undergoes cracking or damage, giving rise to the second HRR peak. However, with increasing lignin content, AMLR lowers and a higher char amount is observed: as a consequence, the shape of curve changes from non-charring to thick charring behaviour.47

Further increasing the LS content to 20 wt.%

drastically reduces both PHRR (- 48 %) and THR (-23 %). With some improvements in PHRR value, a similar trend is observed with increasing LL loading in PA. Similarly, THR and AMLR value are decreased as a function of LL loading in the corresponding microcomposites. However, the increase of kraft lignins (KL and DL) content up to 15 wt.% does not show any significant improvement in fire retardant properties. Finally, the presence of 20 wt.% of DL is responsible for THR reduction (-14%).

4. CONCLUSIONS In this work, PA microcomposites containing four different types of lignin were prepared by melt mixing; the effect of presence of the lignins on the thermal degradation behaviour and fire retardant properties was thoroughly assessed through TGA, UL94 and cone calorimetry tests. FT-



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IR analyses were exploited for investigating the structure of the different lignins. SEM analyses of the microcomposites revealed that the addition of lignin by melt mixing reduces the particle size; in addition, a good dispersion of the different lignins within the polymer matrix was achieved, when the additive loading was not beyond 15 wt.%. DSC analyses showed that the presence of just 5 wt.% lignin noticeable increases Xc as compared to unfilled PA. Further increasing the lignin loading promotes, decrease of Xc, which for the highest lignin loading approaches that of the unfilled polymer. The presence of sulphonate groups in LL and LS was responsible for the formation of a high amount of char residue; to some extent, p-hydroxyphenyl units in LS also helped in favouring the formation of char. The low char residue derived from kraft lignins (KL and DL) was ascribed to guaiacyl units, which are less thermally stable and generate more volatile products during degradation. Further, thermogravimetric analyses performed on PA/lignin microcomposites showed that sulphonated lignins (LL and LS) lead to degradation process while increasing Tonset and Tmax values: this indicates an improvement in thermal stability, which further favours the formation of a stable char residue at 600°C. Furthermore, the amount of char residue increased with lignin content. Conversely, the microcomposites containing kraft lignins did not show noticeable improvement in thermal stability. As assessed by vertical flame spread tests, only PA-LL and PA-DL microcomposites showed enhancements in flame retardant properties, as their total combustion time and burning rate was reduced and V1 rating was achieved in the presence of 15 wt.% of lignin, thus indicating that this loading is optimum for flame retardant purposes. Results from cone calorimetry tests clearly showed that the addition of sulphonated lignins (LS and LL) significantly reduced PHRR and THR values; in particular, 20 wt.% of LL was able to lower PHRR by 51% and THR by 22%. Furthermore, char residue increased with increasing lignin content, approaching 8.7 % in the microcomposites containing 20 wt.% LS. Conversely, the presence of kraft lignins (KL and


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DL) showed a detrimental effect on both HRR and THR. In conclusion, from an overall point of view, the use of lignin in PA seems to be quite promising, also considering the “green” character of both the polymer matrix and the proposed flame retardant additives. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of this research work by European commission’s Erasmus Mundus joint doctorate program under the framework of Sustainable Management & Design for textiles (SMDTex). The authors also acknowledge Domsjö Fabriker AB, Sweden and UPM Biochemicals, Finland, for supplying the lignins for this research project. Corresponding Author Neeraj Kumar Mandlekar E-mail id – [email protected] ENSAIT-GEMTEX 2 Allée Louise et Victor Champier, 59056 Roubaix, France Phone: +33 3 20 25 64 64 Present address Neeraj Kumar Mandlekar E-mail id – [email protected] Politecnico di Torino Dept. of Applied Science and Technology Viale T. Michel 5, 15121, Alessandria, Italy Phone +390131229356 REFERENCES



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FIGURES CAPTIONS Figure 1. FTIR spectra of the four kind of lignins used in this study. Figure 2. Optical microscopy (OM) images of different lignin powder at 10x magnification for, (a) LS, (b) LL, (c) KL and (d) DL samples. Figure 3. SEM micrographs of different lignins (a) LS, (b) LL, (c) KL, and (d) DL samples. Figure 4. SEM micrographs of (a) unfilled PA, (b) PA85-DL15 (c) PA80-DL20 (d) PA85-LL15 (e) PA80-LL20 and (f) voids structure in PA80-LS20 sample. Figure 5. TG and DTG curves of lignin samples (10℃/min, N2). Figure 6. (a) TG and (b) DTG curves, in nitrogen for PA/lignin microcomposite containing 20 wt.% lignin. Figure 7. TG and DTG thermograms in nitrogen for (a) PA-LS and (b) PA-KL microcomposites. Figure 8. HRR and THR curves at external heat flux of 35 kWm-2 for PA-LS and PA-KL, (a) and (b) HRR, (c) and (d) THR.


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Table 1. PA/lignin based microcomposites formulations

Polyamide 11 (wt.%)

Lignin (Y* in wt.%)

PA95-Y5

95

5

PA90-Y10

90

10

PA85-Y15

85

15

PA80-Y20

80

20

sample

*Samples name are coded as PAX-YZ, where Y is the kind of lignin, i.e.; LL (lignosulphonate lignin from Domtar INC), LS (lignosulphonate from Aldrich), KL (kraft lignin from Aldrich) and DL (kraft lignin from Domsjö Fabriker AB).



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Table 2. Lignin mean particle diameter obtain from optical microscopic images

Lignin type

Chemical nature

Source

Mean particle diameter (µm)

LS

Alkali with low sulphonate content

Sigma Aldrich

71 ± 18

LL

Lignosulphonate

Industrial

63 ± 13

KL

Alkali kraft

Sigma Aldrich

39 ± 10

DL

Alkali kraft

Industrial

31 ± 9


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Table 3. Thermogravimetric data for different lignin studied

1st degradation

Mass loss

2nd degradation

(°C)

(%/°C)

LS

75

LL

(°C)

Mass loss (%/°C)

Char at 600 °C (%)

0.07

318

0.23

58

90

0.04

250

0.23

55

KL

-

-

358

0.28

48

DL

62

0.02

385

0.38

41

Samples



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Table 4. Thermogravimetric results for neat PA and microcomposites of PA/lignin

T5%

Tonset

Tmax

(°C)

(°C)

(°C)

PA100

375

394

423

2.5

0.4

PA95-LS5

383

428

473

2.0

2.7

PA90-LS10

355

421

468

2.0

6.2

PA85-LS15

328

415

462

1.8

8.9

PA80-LS20

307

407

461

1.6

13.2

PA95-LL5

367

423

463

2.1

3.7

PA90-LL10

310

409

461

1.9

8.3

PA85-LL15

297

413

456

1.8

9.9

PA80-LL20

278

406

452

1.6

13.7

PA95-KL5

362

391

425

2.6

3.0

PA90-KL10

351

389

427

1.8

6.3

PA85-KL15

338

393

431

1.6

8.5

PA80-KL20

331

386

440

1.4

10.3

PA95-DL5

364

387

423

2.2

2.5

PA90-DL10

361

390

415

2.2

4.5

PA85-DL15

345

386

420

1.8

8.1

PA80-DL20

338

381

428

1.5

9.8

Samples

MMLR Char at (%/°C) 600 °C (%)


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Table 5. Results from UL 94 vertical flame spread tests

1st burning time (sec)

2nd burning time (sec)

Total combustion (sec)

Burn length (cm)

Weight loss (%)

Cotton ignition

Rating

PA100

13.3 ± 0.7

10.6 ± 1.6

23.9 ± 1.2

3.5 ± 0.5

28.0 ± 4.0

Yes

V2

PA95-LS5

25.8 ± 2.9

16.6 ± 3.8

42.4 ± 5.1

4.7 ± 0.5

48.6 ± 4.1

Yes

NC

PA90-LS10

25.2 ± 11.5

43.1 ± 21.6

68.3± 22.9

5.2 ± 0.6

48.6 ± 5.9

Yes

NC

PA85-LS15

37.2 ± 4.2

44.9 ± 7.9

82.1 ± 4.1

6.7 ± 0.7

59.2 ± 6.2

Yes

NC

PA80-LS20

27.3 ± 5.0

57.5 ± 18.4

84.8 ± 15.5

7.0 ± 1.0

62.7 ± 7.3

Yes

NC

PA95-LL5

25.2 ± 1.3

21.9 ± 6.3

47.1 ± 5.3

5.6 ± 0.7

42.2 ± 3.2

Yes

NC

PA90-LL10

9.7 ± 1.6

7.5 ± 1.6

17.2 ± 2.0

4.7 ± 0.8

29.2 ± 5.4

Yes

V2

PA85-LL15

2.4 ± 0.8

3.7 ± 1.3

5.9 ± 2.5

2.8 ± 1.3

17.2 ± 7.1

Yes

V1

PA80-LL20

4.5 ± 0.8

5.1 ± 0.9

9.5 ± 1.7

4.5 ± 0.5

29.6 ± 6.8

Yes

V1

PA95-KL5

17.0 ± 0.9

5.4 ± 1.0

22.4 ± 0.9

3.5 ± 1.0

35.4 ± 1.2

Yes

V2

PA90-KL10

16.3 ± 2.9

8.0 ± 2.4

24.4 ± 0.9

3.0 ± 0.4

25.5 ± 4.2

Yes

V2

PA85-KL15

16.4 ± 1.0

18.0 ± 3.3

34.3 ± 3.6

2.6 ± 0.3

22.9 ± 0.9

Yes

NC

PA80-KL20

28.4 ± 6.8

22.6 ± 6.2

51.1 ± 11.0

2.7 ± 0.4

29.2 ± 3.6

Yes

NC

PA95-DL5

11.8 ± 1.3

17.6 ± 4.8

29.4 ± 5.3

5.9 ± 0.6

46.2 ± 4.3

Yes

V2

PA90-DL10

9.4 ± 3.8

10.1 ± 0.7

19.5 ± 4.1

5.8 ± 0.4

46.2 ± 3.9

Yes

V2

PA85-DL15

4.1 ± 1.3

5.5 ± 1.3

9.5 ± 2.1

4.6 ± 0.4

32.9 ± 2.3

Yes

V1

PA80-DL20

8.5 ± 4.1

9.3 ± 2.9

17.8 ± 6.9

5.7 ± 0.5

42.4 ± 1.8

Yes

V2

samples



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Table 6. Cone calorimetry data for PA/Lignin microcomposites

TTI

(%)

AMLR (g/s)

EHC (MJ/kg)

Residual mass (%)

98 ± 8.5

-

0.101

35.3 ± 1.2

0.6 ± 0.8

+17.1

106 ± 2.4

+7.8

0.111

36.5 ± 0.1

2.1 ± 0.1

688 ± 44

-20.2

99 ± 1.4

+1.1

0.099

34.2 ± 1.2

3.4 ± 0.2

42 ± 4.7

558 ± 19

-35.3

92 ± 2.2

-5.9

0.079

31.6 ± 3.2

5.6 ± 0.2

PA80-LS20

36 ± 3.6

451 ± 53

-47.7

75 ± 14.3

-23.1

0.074

31.0 ± 0.5

8.7 ± 0.2

PA95-LL5

60 ± 3.2

886 ± 8

+2.7

88 ± 5.4

-9.7

0.181

29.3 ± 2.7

2.3 ± 0.3

PA90-LL10

58 ± 6.0

715 ± 70

-17.1

83 ± 8.2

-15.5

0.091

20.6 ± 1.2

3.8 ± 0.1

PA85-LL15

51 ± 1.7

468 ± 36

-45.8

82 ± 0.6

-16.5

0.072

23.8 ± 3.1

5.9 ± 0.3

PA80-LL20

58 ± 6.1

425 ± 12

-50.8

76 ± 2.0

-22.2

0.052

24.8 ± 1.8

8.4 ± 0.2

PA95-KL5

55 ± 7.6

1374 ± 22

+37.3

103 ± 3.7

+5.0

0.150

27.6 ± 3.5

2.0 ± 0.1

PA90-KL10

42 ± 0.6

1166 ± 137

+26.1

98 ± 3.5

-0.1

0.174

21.6 ± 3.3

4.5 ± 1.1

PA85-KL15

47 ± 6.2

1138 ± 134

+24.2

97 ± 3.8

-0.7

0.137

25.5 ± 2.0

5.3 ± 1.1

PA80-KL20

50 ± 3.5

851 ± 35

-1.3

90 ± 7.4

-8.5

0.164

20.0 ± 1.0

7.3 ± 0.3

PA95-DL5

55 ± 1.0

971 ± 73

+11.2

97 ± 0.3

-0.4

0.107

35.3 ± 2.0

1.8 ± 0.1

PA90-DL10

50 ± 3.0

936 ± 48

+7.8

96 ± 0.9

-1.6

0.138

30.9 ± 0.3

3.2 ± 0.1

PA85-DL15

53 ± 9.2

1030 ± 42

+16.3

90 ± 1.6

-8.0

0.125

26.3 ± 3.3

5.5 ± 0.2

PA80-DL20

43 ± 5.7

917 ± 76

+5.9

84 ± 0.8

-14.1

0.089

28.8 ± 2.9

8.3 ± 1.1

(s)

Peak HRR (kW/m2)

PA100

79 ± 11.2

PA95-LS5

∆ (%)

THR (MJ/m2)

862 ± 140

-

56 ± 1.7

1040 ± 61

PA90-LS10

45 ± 2.6

PA85-LS15

Samples

∆


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thermal stability and Fire Retardant Properties of Polyamide 11 Microcomposites containing different Lignins N. Mandlekar a,b,c,d*, A. Cayla a,b, F. Rault a,b, S. Giraud a,b, F. Salaün a,b, G. Malucelli c , J. Guan d Graphical Abstract: for table of content use only.

Synopsis Different lignin based formulations are prepared to study charring efficiency for the development of bio-based intumescent fire retardant system. *Corresponding author – Email address: [email protected]



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. FTIR spectra of the four kind of lignins used in this study. 87x104mm (300 x 300 DPI)


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Figure 2. Optical microscopy (OM) images of different lignin powder at 10x magnification for, (a) LS, (b) LL, (c) KL and (d) DL samples. 80x60mm (300 x 300 DPI)



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Figure 3. SEM micrographs of different lignins (a) LS, (b) LL, (c) KL, and (d) DL samples. 90x67mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. SEM micrographs of (a) unfilled PA, (b) PA85-DL15 (c) PA80-DL20 (d) PA85-LL15 (e) PA80-LL20 and (f) voids structure in PA80-LS20 sample. 125x140mm (300 x 300 DPI)



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Figure 5. TG and DTG curves of lignin samples (10℃/min, N2). 84x59mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) TG and (b) DTG curves, in nitrogen for PA/lignin microcomposite containing 20 wt.% lignin. 84x111mm (300 x 300 DPI)



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Figure 7. TG and DTG thermograms in nitrogen for (a) PA-LS and (b) PA-KL microcomposites. 127x177mm (300 x 300 DPI)


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Figure 8. HRR and THR curves at external heat flux of 35 kWm-2 for PA-LS and PA-KL, (a) and (b) HRR, (c) and (d) THR. 177x127mm (300 x 300 DPI)



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Graphical Abstract: for table of content use only 90x50mm (300 x 300 DPI)


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Thermal Stability and Fire Retardant Properties of ... - ACS Publications

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