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The role of nanodispersion on the fire performance of organoclay–polyamide nanocomposites
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COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 2882–2891 www.elsevier.com/locate/compscitech
The role of nanodispersion on the fire performance of organoclay–polyamide nanocomposites Russell J. Varley
a,*
, Andrew M. Groth b, K.H. Leong
c
a
c
CSIRO Materials Science and Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia b CSIRO Molecular and Health Technologies, Bag 10 Clayton South, Victoria 3169, Australia Group Research, PETRONAS, R&T Division, Lot 3288-3289 Off Jalan Ayer Itam, Kawasan Institusi Bangi, 43000 Kajang, Selangor, Malaysia Received 17 October 2007; accepted 20 October 2007 Available online 6 November 2007
Abstract This work has investigated the importance of nanoscale dispersion upon the fire performance of Nylon-6 nanocomposites by characterising the role of the char layer and its formation in reducing peak heat release rate during combustion. To do this, a series of layered silicate nanocomposites systems using Nylon-6 were prepared using twin screw extrusion techniques at different levels of clay addition and different processing temperatures. This work has shown that the addition of layered silicates improves the peak heat release rate in a synergistic manner, by forming a tough char layer that prevents or hinders the transfer of combustible products into the gaseous phase. Differences in nanoscale dispersion of the clay as measured using transmission electron microscopy (TEM) correlated strongly with changes in fire performance according to cone calorimetry measurements. Corresponding changes in the thermal decomposition behaviour (thermogravimetric analysis) and microstructure and elemental composition of the char layer (SEM) were used to further understand the fire retardation mechanism in the condensed phase. Comparison of the rate of mass loss with the heat release rates and XRD of the char were further used to investigate the role of the char layer in improving fire performance. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymers; A. Layered structures; B. Thermal properties; D. Scanning electron microscopy (SEM)
1. Introduction Since the discovery of layered silicate polyamide nanocomposites by the Toyota Research Central Laboratory in the 1990s there has been a renaissance of research in both academia and industry, aimed both at understanding the fundamental principles behind nanocomposite formation and also the development of commercial applications. Along with the well known effects upon mechanical properties, concurrent improvements in fire performance for low levels of nanoclay addition have also generated significant interest. Early research, using cone calorimetry showed that the peak heat release rate (PHRR) could be reduced by 63% during combustion, compared to the based *
Corresponding author. Tel.: +61 3 9545 2941; fax: +61 3 9544 1128. E-mail address: [email protected] (R.J. Varley).
0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.10.039
polymer, at levels of only 5 wt% [1,2] for Nylon-6 (amongst other polymers). Comparison of the mass loss rate with the PHRR suggested that the fire retardancy mechanism occurred primarily in the condensed phase through char formation and not to any significant level in the gaseous phase. As a result, attention has also focussed on understanding the role of the chemical structure and morphology of the char layer in controlling fire performance. Investigation of the microstructure of the post-combustion char layer by Qin et al. [3] found increased levels of Si on the surface and attributed this to the formation of a tough, ceramic char layer and subsequent improvements in PHRR. Kashiwagi et al. [4] also investigated the microstructure of the char layer of Nylon-6 nanocomposites and found significant improvement in the fire performance measured using cone calorimetry. They reported that protective ‘‘floccules’’ consisting of 80% of clay particles and
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20% of a carbonaceous graphitic structure, formed on the sample surface shielded the polyamide from the external thermal radiation and heat feedback from the flame. The effect of silicate addition upon polymer degradation and subsequent char formation in determining fire performance has also been highlighted by Song et al. [5] and Le Bras and Bourbigot [6]. The former group of researchers showed that char yields as determined via thermogravimetric analysis (TGA) closely correlated with decreases in PHRR, while the latter reported a similar relationship when using a nanofiller in a ternary blend [7,8]. In both cases, the nano-reinforced char layer was visually observed to be tougher than the unmodified polyamide counterpart, thereby improving barrier performance while also promoting intumescence. The importance of the quality of the dispersion whether a microcomposite, an intercalated and/or an exfoliated nanocomposite is also an important factor in determining fire performance. Currently, there are mixed results in the literature in regard to this. Morgan et al. [9], for example, reported that there was little effect upon fire performance whether the nanocomposite was exfoliated or intercalated. They reported improvements of around 30% regardless of whether the polymer was intercalated or exfoliated. In contrast, Wang et al. [10] reported a 40% decrease in PHRR for a 5 wt% organo-modified montmorillonite nanocomposite but only achieved a 16% improvement for the corresponding microcomposite based on an unmodified clay. This paper seeks to contribute to the understanding of nanoscale dispersion in a layered silicate nylon nanocomposite system on fire performance. The fire performance of clearly defined intercalated and exfoliated Nylon-6 nanocomposites structures will be compared and discussed. Characterisation of the nanocomposites is determined using transmission electron microscopy (TEM), X-ray diffraction (XRD), cone calorimetry and thermogravimetric analysis (TGA). Investigation of the microstructure of the char layer after combustion was investigated using scanning electron microscopy (SEM), electrodispersive spectroscopy (EDS) and XRD. 2. Experimental 2.1. Materials and sample preparation The polyamide used in this work was Toray’s Amilan CM1017-K which is a high rigidity unreinforced Nylon-6. Prior to use, the nylon was dried in a desiccant drier at 80 C until the moisture level was measured to be below 50 ppm. The nanoclay used was the organically modified polymeric layered silicate Nanomer I.34TCN produced by Nanocor which is specifically designed for use with polyamide. As with the Nylon-6, the nanoclay was also dried overnight in a vacuum oven at 80 C and in the presence of P2O5 prior to processing. A co-rotating JSW twin screw extruder with a 42:1 l/d ratio and a maximum throughput of approximately
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40 kg h1 was used to prepare the nanocomposites. Gravimetric feeders connected in series to the extruder equipped with a desiccant drier, were used to ensure precise proportions of sufficiently dried nanoclay during preparation of the samples. The screw configuration employed was based upon the design suggested by the nanoclay supplier, although practical considerations were taken into account for translation to the JSW extruder. The segmented screw therefore consisted of conveying, reversing and mixing elements at different places along the screw with the mixing elements affording dispersive as well as distributive mixing. A 50 Ton Cincinnati Milacron injection moulder was used to prepare test coupons of the samples for analysis and testing. This work prepared samples containing 0, 1, 2, 3 and 5 wt% nanoclay that were processed at an extruder temperature of 240 C. Processing was also performed at 220 C, 255 C and 270 C at concentrations of 0, 3 and 5 wt% nanoclay concentrations. The samples prepared in this work and their corresponding notation are summarised in Table 1. 2.2. Material characterisation X-ray diffraction analysis was carried out on all samples using a Bruker Advance D8 X-ray diffractometer, equipped with a diffracted beam polycrystalline graphite monochromator. The X-ray beam was operated at 40 mA, 40 kV, and data collection between 1 and 70 (2h) with step size of 0.02 at a scan rate of 6 s per step. Phases were identified using the ICDD-JCPDS powder diffraction database. The samples were analysed in ‘‘as-received’’ state, i.e. postextrusion, with no further sample preparation required. Transmission electron microscopy (TEM) was performed without using cryogenic cooling to more comprehensively investigate the nanoscale dispersion of the nanocomposites produced in this work. The char layer was investigated using a Philips XL30 field emission scanning electron microscope (SEM) while elemental analysis was performed using electrodispersive spectroscopy (EDS). Cone calorimetry was performed using a Stanton Redcroft Cone Calorimeter in a horizontal orientation according to the procedure outlined in the ASTM E1354-1992 standard. Samples measuring 75 mm · 100 mm were used and testing was performed in triplicate. In addition, they were carefully wrapped around the bottom and the sides using heavy aluminium foil to ensure that the molten polymer would not flow out of the combustion zone and were held securely in the sample holder. Fire performance was evaluated by subjecting the samples to a heat flux of 35 kW/m2 from above the sample and determining parameters such as time to ignition, peak heat release rate and average heat release rate. Thermogravimetric analysis was performed in an oxidative (air) atmosphere using a Mettler-Toledo 821e TGA/ SDTA. Samples placed in an Alumina crucible, and ramped from 50 C to 800 C at a rate of 10 C min1 while the flow of air was maintained at 50 ml min1.
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Table 1 Summary of samples prepared and tests undertaken in this work, listed alongside the notation used in this paper Sample ID
Processing temperature (C)
Clay concentration (wt%)
TGA
XRD
Unprocessed nanoclay 0%NC240PT 1%NC240PT 2%NC240PT 3%NC240PT 5%NC240PT 0%NC2xxPTa 3%NC2xxPTa 5%NC2xxPTa
– 240 240 240 240 240 220/255/270 220/255/270 220/255/270
100 0 1 2 3 5 0 3 5
p p p p p p p p
p
a
p
Cone calorimetry
Elemental analysis (of char)
p
p
xx refers to the last two numbers of the temperature used for processing the nanocomposites.
dispersion. Consequently it can be suggested that the 5%NC220PT sample contains:
3. Results and discussion 3.1. Nanocomposite formation Fig. 1 shows representative XRD traces of the layered silicate nanocomposites formed during processing at 240 C plotted alongside that of the unmodified nylon. The traces clearly highlight the lack of order in the (postprocess) layered silicate of the nanocomposites and can therefore be described as being at least highly intercalated according to XRD analysis. It should be noted however that with increasing clay concentration, the region where the 0 0 1 reflection peak would be expected to be visible, does exhibit, shoulder regions on the traces highlighting the limitation of using XRD to make specific assertions as to the quality of the nanoscale dispersion. Hence TEM has been used to complement the XRD analyses. In Fig. 2, images of 5%NC220PT and 5%NC255PT samples show that overall both these nanocomposite have a complex mixture of intercalated, highly intercalated and exfoliated clay layers thus agreeing to some degree with the results obtained from the XRD analyses. TEM however provides additional information by facilitating easier differentiation of the different types of clay morphologies present and affords a better judgement of the relative quality of the 2000
1200 5wt% clay 3wt% clay 2wt% clay
1000
1wt% clay clay
800
1200 600 800 400 400
Intensity (counts)
Intensity (counts)
1600
200
0
0 1
7
13
19
25
31
37
2
Fig. 1. XRD spectra showing that the order in the clay layer has been destroyed for the samples of varying concentration prepared at 240 C.
(1) greater separation of clay platelets from their tactoids; (2) much less large agglomerates; and (3) higher levels of alignment of platelets according polymer flow. The varying sizes and thickness of the clay tactoids reinforce that the process of exfoliation is achieved through individual platelets separating or ‘‘peeling’’ off consecutively from the cluster of platelets [11]. The greater exfoliation (and alignment) at lower temperature, reinforces the importance of higher shear forces (from higher viscosities at lower processing temperature) determining the extent of exfoliation [12]. 3.2. Thermal analysis The effect of silicate concentration on thermal stability of the nylon in an oxidative environment for the samples processed at 240 C is shown Fig. 3. Here the decomposition of the nylon can be seen to obey a two-step process consisting primarily of the decomposition of the bulk polymer (90%) followed by degradation of the remaining char layer (10%) through a chain scission volatilisation process. As can be seen, clay concentration appears to have little effect upon the onset and rate of degradation which is consistent with results obtained from polymer systems other than nylon [13]. The char yields however, at 500 C (char formation) and 800 C (char degradation) both increase consistently with clay content above what would be expected in the absence of significant nylon clay interaction. The relative thermal stabilities of the char layer are also represented in the inserted diagram within Fig. 3, which shows the rate of change of degradation in the 500–600 C region. Here, the decomposition of the char layer can be seen to occur more slowly and at higher temperatures with increasing clay concentration. The complete set of results at all temperatures and concentrations, measured in this work are shown in Table 2 and reinforce that the initial onset of degradation is relatively unaffected by
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Fig. 2. TEM images of the (a) 5%NC255PT and the (b) 5 wt%NC220PT samples at two different magnifications.
unmodifed polyamide
100
1wt% clay 2wt% clay
80
Mass Loss (%)
3wt% clay Rate of Mass Loss
5wt% clay
60
40
20
500
0 200
Temperature (ºC)
300
600
400
500
600
700
Temperature (ºC) Fig. 3. TGA traces showing the mass loss behaviour with increasing clay concentration in an oxidative atmosphere. Insert shows the rate of change of mass loss between 500 and 600 C.
clay addition as determined by the 10% degradation temperature, T10%, and the temperature of the peak in the 1st derivative of the mass loss, Tmax1. The results also show that increasing clay concentration generally increases the thermal stability of the char layer at the varying processing temperatures as measured by the second or higher temper-
ature peak in the 1st derivative spectra of the mass loss Tmax2. Thermogravimetric analysis however, was not sensitive enough to measure any differences in thermal stability resulting from the improved nanoscale dispersion of the 5%NC220PT and the 5%NC255PT samples as measured by TEM (see Fig. 2).
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0 3 5 0 1 2 3 5 0 3 5 0 3 5
220 220 220 240 240 240 240 240 255 255 255 270 270 270 a b c
415.3 418.5 425.4 415.3 417.7 416.4 413.4 425.6 418.8 416.5 417.7 418.5 421.2 421.1
453.4 462.3 460.3 457.3 461.3 455.6 453.0 457.8 456.6 455.1 454.4 457.9 456.5 454.4
4.73 7.57 7.50 5.47 5.39 6.79 8.92 8.07 5.12 17.37 8.61 5.03 9.24 8.90
538.4 605.7 570.9 540.1 566.6 604.9 598.7 566.3 533.1 609.6 582.9 536.1 585.0 585.0
0.13 2.11 3.88 0.66 1.33 2.02 3.01 4.18 0.50 4.60 3.88 4.33 6.57 4.75
Temperature at 10% weight loss. Temperature of the 1st peak in the 1st derivative of the mass loss. Temperature of the 2nd peak in the 1st derivative of the mass loss.
3.3. Cone calorimetry The effect of increasing clay concentration on the key fire performance parameter, the peak heat release rate (PHRR), is shown in Fig. 4. The measured decrease in PHRR and time to ignition with increasing clay concentration is consistent with reports of this nature in the literature [14]. Fig. 5 shows the effect of increasing clay concentration on PHRR and the effective heat of combustion which highlight the synergistic performance achieved through the addition of layered silicate to nylon. A 2 wt% addition of an organo-modified clay has reduced the PHRR by 18.2% while a 5 wt% addition resulted in a reduction of 26.7% in the PHRR. It is important to recognise that the organo-modified clay contains about 30 wt% of an alkylammonium compatabiliser which effectively reduces the level of actual silicate content in the blend. The effect of
650
45
600
PHHR (kW/m2)
Clay Temperature T10%a Tmax1b Char yield 1 (%) Tmax2c Char (C) (C) (C) yield 2 content (C) (%) (wt%)
50
700
18.2%
18.8%
40
550 26.7%
500 450
30
400 25 350 20
300 0
1
2
3
4
5
wt/wt clay concentration
Fig. 5. Plot of the reduction in the PHRR and the Effective heat of combustion as a function of clay concentration.
processing temperature upon the PHRR for the 5 wt% samples is shown in Fig. 6 where the sample processed at 220 C clearly exhibits significant further improvement in fire performance compared to the samples processed at 240 C, 255 C and 270 C. The PHRR of the 5%NC220PT sample exhibited a decrease of 41%, while the other two samples displayed an overall average reduction in PHHR of only 20%. The 5%NC220PT sample also displayed a time to ignition similar to that of the unmodified nylon which is in contrast to the other modified samples which all displayed a shorter time to ignition. Furthermore, the PHRR for the 5%NC220PT sample shifted to longer times to complete combustion and displayed a distinctly bimodal mechanism in comparison to the other modified samples and unmodified samples. The behaviour of the 5%NC220PT sample during actual combustion was quite distinct to that of the other samples, which in turn behaved differently to the unmodified polyamide. Combustion occurs in the cone calorimeter when the concentration of the combustible products increases
700
700 unmodified nylon
600
unmodified nylon
2wt% clay
600
5wt% at 220degC
3wt% clay
5wt% at 240degC
5wt% clay
PHHR (kW/m 2)
500
2
PHHR (kW/m )
35
Effective Heat of Combustion (MJ/kg)
Table 2 TGA results in air showing the thermal decomposition properties for the samples prepared at various temperatures under investigation
400 300 200 100
500
5wt% at 255degC 5wt% at 270degC
400 300 200 100 0
0 0
100
200
300
400
500
600
time (seconds)
Fig. 4. Cone calorimetry of the samples processed at 240 C showing the effect of increasing clay concentration.
0
100
200
300
400
500
600
time (seconds)
Fig. 6. Plot of the PHRR showing the effect of processing temperature on the samples containing 5 wt% of clay as well as the unmodified sample.
R.J. Varley et al. / Composites Science and Technology 68 (2008) 2882–2891
sufficiently in the gaseous phase to ignite. In the case of the unmodified nylon, the molten polymer burned and bubbled profusely and continuously until it was completely consumed. In the case of the intercalated nanocomposites, which according to TEM studies were the 5%NC240PT, 5%NC255PT and the 5%NC270PT samples, it was observed that soon after ignition a ‘‘skin’’ had formed on the surface of the polymer. Although this did not prevent combustion from gaining momentum it did however slow down the rate of increase in the intensity of the flame compared to the unmodified nylon. As combustion continued, the surface of the char in these three intercalated samples continued to expand and rise upwards, i.e. intumesce, although it was also apparent that there was significant molten polymer continuing to bubble beneath the surface. At the end of the combustion process, scattered char residues remained, but with little structural integrity. The combustion process of the exfoliated nanocomposites, the 5%NC220PT sample, behaved quite differently from the intercalated samples but similar to that described elsewhere [3,15]. The exfoliated structure differs from the intercalated one in the sense that: (1) the char formed sooner; (2) the char appeared to have greater strength or stability in that it altered very little during the entire combustion process (i.e. the volume did not expand noticeably at all). Fig. 7 shows an image of the char immediately after combustion in the sample holder, which highlights a greatly increased level of structural integrity after combustion compared to the intercalated samples. (3) there was no evidence of any bubbling of molten polymer beneath the surface at all during combustion. The clear differences between nanoscale dispersion as determined using TEM, can therefore be seen to play an
Fig. 7. Photograph of the char layer of the 5 wt%NC220PT showing the structural integrity post-combustion.
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important role here in controlling the level of improvement in fire performance. Given the importance of the char layer in reducing the heat output during combustion, a relationship between the char yield and the PHHR would be expected. The results obtained from the cone calorimetry work support this hypothesis reasonably well in two demonstrable ways as shown in Table 3, which summarises the results obtained from cone calorimetry. Firstly, the char yield increases consistently with increasing clay concentration while at the same time also reducing PHRR. Secondly, the exfoliated 5%NC220PT sample displays a higher char yield (albeit not enormously) than the intercalated 5%NC240PT, 5%NC255PT and 5%NC270PT samples which also correlated with improvement in fire performance. It should be cautioned that with increased char generated it also suggests that more of the base material has been decomposed and in a practical sense this could be disadvantageous from a structural point of view since less of the original structure is left behind to support working loads. Other important points to note are that the times to ignition decreased with increasing clay concentration, irrespective of nano-dispersion. The exfoliated 5%NC220PT sample showed the lowest reduction although it still ignited sooner than the unmodified polyamide. The effect upon different heat output measurements generally follow the trends already discussed while toxic gas emissions such as CO and CO2 gas display inconsistent results. 3.4. Characterisation of char To further investigate the role of nanoscale dispersion (i.e. exfoliated vs intercalated) on fire performance, the char from the 5%NC220PT and 5%NC255PT samples were selected for further analyses using SEM. Representative micrographs are shown in Fig. 8 at three different magnifications to highlight the differences between the microstructures on different length scales. At the lowest magnification (see Fig. 8a) the 5%NC220PT sample displays a unique ‘‘tree root’’ formation which is considered here to be evidence of a faster or ‘‘non-equilibrium’’ formation of the char layer. This morphology was in contrast to the 5%NC255PT sample which displayed a relatively featureless surface at that magnification. On further inspection at intermediate magnification (see Fig. 8b), the 5%NC255PT sample has an array of large craters on the surface of the order of 10 lm to 100 lm in diameter. These craters are formed through molten polymer flow evidenced by the ‘‘ripples’’ around the edges of the craters as gaseous products escape. In contrast, the 5%NC220PT sample shows little evidence of any large craters and contains sharp edges akin almost to a ‘‘brittle fracture surface’’. It is also apparent that the 5%NC220PT sample surface is quite porous, consisting of very fine voids of the order of a few microns in diameter. The differences in the microstructure of the chars are clearer at higher magnification (see Fig. 8c) where the finer voids in the 5%NC220PT sample is more clearly revealed. It is suggested therefore that the origin of these
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Table 3 Raw data obtained from the cone calorimeter for all samples studied Temp. (C)
Clay content (wt%)
Time to ignition (s)
Total heat evolved (kJ)
Peak heat release rate (kJ/m2)
Heat of combustion (MJ/kg)
Specific extinction area (m2/kg)
CO (kg/kg)
CO2 (kg/kg)
Char yield (%)
240 240 240 240 220 220 255 255 270 270
0 2 3 5 3 5 3 5 3 5
137 102 108 102 108 113 102 102 110 104
614.2 (15.3) 634.13 (1.39) 651.00 (16.01) 616.07 (35.82) 648.60 (36.66) 557.37 (42.45) 605.73 (22.61) 642.67 (29.78) 634.87 (13.74) 606.13 (27.95)
618.6 506.0 502.6 453.6 488.8 366.0 474.9 529.8 502.9 464.5
44.6 34.0 36.9 33.3 33.6 32.2 31.4 38.8 33.5 34.3
651.7 776.8 798.9 644.3 714.1 674.2 688.4 651.3 636.1 560.4
0.0259 0.0257 0.0268 0.0194 0.0281 0.0199 0.0231 0.0241 0.0215 0.0194
2.83 2.84 3.01 2.58 2.51 2.30 2.67 3.03 2.79 2.88
1.34 4.02 5.06 6.42 3.36 6.81 3.45 5.84 2.48 4.84
(3) (2) (6) (2) (4) (5) (4) (2) (10) (9)
(33.7) (44.6) (29.4) (37.8) (45.0) (10.0) (28.0) (71.0) (19.6) (45.8)
(9.9) (1.0) (1.8) (2.3) (1.0) (3.4) (3.0) (4.1) (2.0) (3.0)
(111.6) (23.0) (79.7) (55.1) (123.3) (94.6) (50.7) (92.2) (112.6) (1245)
(0.0063) (0.0019) (0.0021) (0.0015) (0.0107) (0.0024) (0.0008) (0.0030) (0.0028) (0.0015)
(0.06) (0.19) (0.28) (0.09) (0.21) (0.35) (0.47) (0.21) (0.39)
(1.7) (0.7) (1.1) (0.9) (0.8) (1.1) (0.6) (1.2) (1.8) (0.5)
Values in parentheses refer to standard deviation.
Fig. 8. SEM images of the microstructure of the (i) 5 wt%NC255PT and (ii) 5 wt%NC220PT at (a) low, (b) medium and (c) high magnifications.
differences in microstructure of the char, arises from the varying nanoscale dispersions which controls the strength and barrier properties of the char, and in turn affects the fire
performance. These results complement those of Wang et al. [15] who investigated the char layer microstructure and also found a relationship between fire performance and the
R.J. Varley et al. / Composites Science and Technology 68 (2008) 2882–2891
into the gaseous phase has been demonstrated in this work. Evidence for the importance of the char layer has also been found when comparing the mass loss rate during combustion with the change in the heat release rate as shown in Fig. 10 for the 5%NC220PT and 5%NC255PT samples. The strong correlation for both of the samples shows that the source of the fire retardancy mechanism remains within the condensed phase regardless of whether the clay platelets can be described as either exfoliated or intercalated. This further infers the importance of the stability and strength of the char layer in determining fire performance. It is important to note that the actual correlation between the heat release rate and the mass loss indicate not only that the fire retardancy mechanism occurs in the gas phase, it can also be concluded that other mechanisms occurring in the gaseous phase can be disregarded as occurring to any significant degree. The morphology of the nanoclay present in the char layer was characterised using XRD. Fig. 11 compares these results of the 5%NC220PT, 5%NC255PT and 5%NC270PT samples with the neat organo-modified clay where it can be seen that the high levels of disorder (i.e. exfoliated or intercalated) have been destroyed and that the order of the layered silicate has been reformed. The revealed d spacings of the layered silicates are seen to be smaller than that of the
400 PHRR
350
Mass Loss Rate
PHRR (kW/m2)
300
3.5. Condensed phase mechanism
0.11 0.09
250 0.07
200 150
0.05
100
0.03
50 0.01
0
The importance of the strength and the structure of the char layer in preventing the transfer of combustible gases
-50 0
20
40
60
80
Mass Loss Rate (mg/s)
formation of a tough, compact and well-dispersed honeycomb char structure. Elemental analysis of the char layer was performed to determine the relative silicon and carbon contents present in the char at the surface and in the bulk. The results of the elemental analyses are shown in Fig. 9 which highlights significant differences between the exfoliated 5%NC220PT sample and the three intercalated samples 5%NC240PT, 5%NC255PT and 5%NC270PT. The two intercalated samples processed at higher temperatures, i.e. 5%NC255PT and 5%NC270PT, have similar Si and C contents both on the surface and in the bulk of the char. For both of these samples, there is an increase in the relative content of Si on the surface of the char suggesting that the char layer has been strengthened by the migration of silicate species to the surface of the char. The prevailing hypothesis that Si species migrate to the surface to act as insulators or ceramic layers has been discussed and demonstrated by Lewin [16] and Vaia et al. [17] recently. The enrichment of the surface of the char with nanoclay was suggested to be driven primarily by the low surface free energy of the clay that helps the migration process. The 5%NC220PT sample has reasonably similar C and Si contents in the bulk polymer to that of the intercalated samples, but radically different behaviour on the surface. In the case of the exfoliated sample, the relative Si content diminishes markedly as the C content increases at the surface of the char layer. This shows that in the case of this sample, the structure of the char layer is able to resist degradation more than the corresponding intercalated samples. As a result, the C content remains high and the Si content correspondingly decreases. This suggests that the formation of an exfoliated nanocomposite is able to promote the formation of a more thermally stable carbonaceous char more efficiently than a nanocomposite that is only intercalated [18].
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-0.01 100 120 140 160 180 200
Time (seconds) C
600
90 80
500
60
Si
0.13 Si
Si
50 40
C C
30
0.15
Mass Loss Rate
400
0.11 0.09
300
0.07 200
0.05 0.03
100
20
0.01 0
10
-0.01
-100
0 220°C
220°C 255°C 255°C 270°C o Processing Temperature ( C)
270°C
Fig. 9. Elemental analysis of char layers for the 5 wt% samples processed at 220 C, 255 C and 270 C.
0
20
40
60
80
100
120
140
Mass Loss Rate (mg/s)
from above - polymer skin
70
%
0.17 PHRR
from below - bulk char
PHRR (kW/m2)
100
-0.03 160
Time (seconds) Fig. 10. Comparison of the Mass loss rate with the PHRR as determined for the (a) 5 wt%NC220PT sample and the (b) 5 wt%NC255PT.
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25 0
5wt% clay at 270degC 5wt% clay at 255degC 5wt% clay at 220degC organoclay
intensity
20 0
15 0
10 0
50
0 2
5
8
11
14
17
20
23
2θ
Fig. 11. XRD traces of the char layers and organo-modified clay showing the collapse of the clay layers during combustion.
neat organoclay highlighting the collapse of the nanocomposites regardless of the original morphology. This result compliments those of Gilman et al. [1,18] who suggested that the reformation of the layered structure at d spacings closer that that of the organo-modified clay is evidence of the formation of a tough ceramic char layer. No doubt this is valid, but this work indicates that it is not able to differentiate between the strength, thermal stability or toughness of the layer, since it is known from this work that the 5%NC220PT sample is clearly stronger than the other samples. 4. Summary and conclusions This work has shown that the addition of layered silicates improve the critical parameter of the peak heat release rate in a synergistic manner due to the formation of a char layer that prevents or hinders the transfer of combustible products into the gaseous phase. Correlation with the heat release rate and the mass loss rate during combustion also reinforce this observation. As well, XRD has shown that the disordered morphology collapses facilitating the ordered silicates to reinforce the char layer at d spacing closer than prior to intercalation. This work has also shown the importance of achieving true exfoliation compared to intercalation in optimising fire performance. Superior fire performance was achieved for the exfoliated 5%NC220PT sample which was shown to be a result of a more thermally stable char layer. Evidence for this was provided by its structural integrity after combustion, the microstructure of the char layer and electro dispersive spectroscopy.
References [1] Gilman JW. Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites. Appl Clay Sci 1999;15:31–49. [2] Gilman JW, Kashiwagi T, Brown JET, Lomakin S, Giannelis EP, Manias E. Flammability studies of polymer layered silicate nanocomposites. Int SAMPE Symp Ex 1998;43:1053–66. [3] Qin H, Su Q, Zhang S, Zhao B, Yang M. Thermal stability and flammability of polyamide 66/montmorillonite nanocomposites. Polymer 2003;44:7533–8. [4] Kashiwagi T, Harris RH, Zhang X, Briber RM, Cipriano BH, Raghavan SR, et al. Flame retardant mechanism of polyamide 6-clay nanocomposites. Polymer 2004;45:881–91. [5] Song L, Hu Y, Lin Z, Xuan S, Wang S, Chen Z, et al. Preparation and properties of halogen-free flame-retarded polyamide 6/organoclay nanocomposite. Polym Degrad Stabil 2004;86:535–40. [6] Le Bras M, Bourbigot S. Use of carbonizing polymers as additives in intumescent polymer blends: A review. Abstracts of Papers, 220th ACS National Meeting, Washington, DC, United States, August 20– 24, 2000. 2000; PMSE-238. [7] Dabrowski F, Le Bras M, Cartier L, Bourbigot S. The use of clay in an EVA-based intumescent formulation. Comparison with the intumescent formulation using polyamide-6 clay nanocomposite as carbonisation agent. J Fire Sci 2001;19:219–41. [8] Dabrowski F, Le Bras M, Delobel R, Gilman JW, Kashiwagi T. Using clay in PA-based intumescent formulations. Fire performance and kinetic parameters. Macromol Symp 2003;194:201–6. [9] Morgan AB, Harris Jr RH, Kashiwagi T, Chyall LJ, Gilman JW. Flammability of polystyrene layered silicate (clay) nanocomposites: Carbonaceous char formation. Fire Mater 2002;26:247–53. [10] Wang S, Hu Y, Li Z, Wang Z, Zhuang Y, Chen Z, et al. Flammability and phase-transition studies of nylon 6/montmorillonite nanocomposites. Colloid Polym Sci 2003;281:951–6. [11] Fornes TD, Yoon PJ, Keskkula H, Paul DR. Nylon 6 nanocomposites: the effect of matrix molecular weight. Polymer 2001;42:09929–40. [12] Dennis HR, Hunter DL, Chang D, Kim S, White JL, Cho JW, et al. Nanocomposites: The importance of processing. Plastics Engineering (Brookfield, CT, United States) 2001;57:56–60.
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[16] Lewin M. Some comments on the modes of action of nanocomposites in the flame retardancy of polymers. Fire Mater 2003;27:1–7. [17] Vaia RA, Price G, Ruth PN, Nguyen HT, Lichtenhan J. Polymer/ layered silicate nanocomposites as high performance ablative materials. Appl Clay Sci 1999;15:67–92. [18] Gilman J, Harris R, Shields JR, Kashiwagi T, Morgan AB. A study of the flammability reduction mechanism of polystyrene-layered silicate nanocomposite: layered silicate reinforced carbonaceous char. Polym Adv Technol 2006;17:263–71.
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 2882–2891 www.elsevier.com/locate/compscitech
The role of nanodispersion on the fire performance of organoclay–polyamide nanocomposites Russell J. Varley
a,*
, Andrew M. Groth b, K.H. Leong
c
a
c
CSIRO Materials Science and Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia b CSIRO Molecular and Health Technologies, Bag 10 Clayton South, Victoria 3169, Australia Group Research, PETRONAS, R&T Division, Lot 3288-3289 Off Jalan Ayer Itam, Kawasan Institusi Bangi, 43000 Kajang, Selangor, Malaysia Received 17 October 2007; accepted 20 October 2007 Available online 6 November 2007
Abstract This work has investigated the importance of nanoscale dispersion upon the fire performance of Nylon-6 nanocomposites by characterising the role of the char layer and its formation in reducing peak heat release rate during combustion. To do this, a series of layered silicate nanocomposites systems using Nylon-6 were prepared using twin screw extrusion techniques at different levels of clay addition and different processing temperatures. This work has shown that the addition of layered silicates improves the peak heat release rate in a synergistic manner, by forming a tough char layer that prevents or hinders the transfer of combustible products into the gaseous phase. Differences in nanoscale dispersion of the clay as measured using transmission electron microscopy (TEM) correlated strongly with changes in fire performance according to cone calorimetry measurements. Corresponding changes in the thermal decomposition behaviour (thermogravimetric analysis) and microstructure and elemental composition of the char layer (SEM) were used to further understand the fire retardation mechanism in the condensed phase. Comparison of the rate of mass loss with the heat release rates and XRD of the char were further used to investigate the role of the char layer in improving fire performance. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymers; A. Layered structures; B. Thermal properties; D. Scanning electron microscopy (SEM)
1. Introduction Since the discovery of layered silicate polyamide nanocomposites by the Toyota Research Central Laboratory in the 1990s there has been a renaissance of research in both academia and industry, aimed both at understanding the fundamental principles behind nanocomposite formation and also the development of commercial applications. Along with the well known effects upon mechanical properties, concurrent improvements in fire performance for low levels of nanoclay addition have also generated significant interest. Early research, using cone calorimetry showed that the peak heat release rate (PHRR) could be reduced by 63% during combustion, compared to the based *
Corresponding author. Tel.: +61 3 9545 2941; fax: +61 3 9544 1128. E-mail address: [email protected] (R.J. Varley).
0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.10.039
polymer, at levels of only 5 wt% [1,2] for Nylon-6 (amongst other polymers). Comparison of the mass loss rate with the PHRR suggested that the fire retardancy mechanism occurred primarily in the condensed phase through char formation and not to any significant level in the gaseous phase. As a result, attention has also focussed on understanding the role of the chemical structure and morphology of the char layer in controlling fire performance. Investigation of the microstructure of the post-combustion char layer by Qin et al. [3] found increased levels of Si on the surface and attributed this to the formation of a tough, ceramic char layer and subsequent improvements in PHRR. Kashiwagi et al. [4] also investigated the microstructure of the char layer of Nylon-6 nanocomposites and found significant improvement in the fire performance measured using cone calorimetry. They reported that protective ‘‘floccules’’ consisting of 80% of clay particles and
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20% of a carbonaceous graphitic structure, formed on the sample surface shielded the polyamide from the external thermal radiation and heat feedback from the flame. The effect of silicate addition upon polymer degradation and subsequent char formation in determining fire performance has also been highlighted by Song et al. [5] and Le Bras and Bourbigot [6]. The former group of researchers showed that char yields as determined via thermogravimetric analysis (TGA) closely correlated with decreases in PHRR, while the latter reported a similar relationship when using a nanofiller in a ternary blend [7,8]. In both cases, the nano-reinforced char layer was visually observed to be tougher than the unmodified polyamide counterpart, thereby improving barrier performance while also promoting intumescence. The importance of the quality of the dispersion whether a microcomposite, an intercalated and/or an exfoliated nanocomposite is also an important factor in determining fire performance. Currently, there are mixed results in the literature in regard to this. Morgan et al. [9], for example, reported that there was little effect upon fire performance whether the nanocomposite was exfoliated or intercalated. They reported improvements of around 30% regardless of whether the polymer was intercalated or exfoliated. In contrast, Wang et al. [10] reported a 40% decrease in PHRR for a 5 wt% organo-modified montmorillonite nanocomposite but only achieved a 16% improvement for the corresponding microcomposite based on an unmodified clay. This paper seeks to contribute to the understanding of nanoscale dispersion in a layered silicate nylon nanocomposite system on fire performance. The fire performance of clearly defined intercalated and exfoliated Nylon-6 nanocomposites structures will be compared and discussed. Characterisation of the nanocomposites is determined using transmission electron microscopy (TEM), X-ray diffraction (XRD), cone calorimetry and thermogravimetric analysis (TGA). Investigation of the microstructure of the char layer after combustion was investigated using scanning electron microscopy (SEM), electrodispersive spectroscopy (EDS) and XRD. 2. Experimental 2.1. Materials and sample preparation The polyamide used in this work was Toray’s Amilan CM1017-K which is a high rigidity unreinforced Nylon-6. Prior to use, the nylon was dried in a desiccant drier at 80 C until the moisture level was measured to be below 50 ppm. The nanoclay used was the organically modified polymeric layered silicate Nanomer I.34TCN produced by Nanocor which is specifically designed for use with polyamide. As with the Nylon-6, the nanoclay was also dried overnight in a vacuum oven at 80 C and in the presence of P2O5 prior to processing. A co-rotating JSW twin screw extruder with a 42:1 l/d ratio and a maximum throughput of approximately
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40 kg h1 was used to prepare the nanocomposites. Gravimetric feeders connected in series to the extruder equipped with a desiccant drier, were used to ensure precise proportions of sufficiently dried nanoclay during preparation of the samples. The screw configuration employed was based upon the design suggested by the nanoclay supplier, although practical considerations were taken into account for translation to the JSW extruder. The segmented screw therefore consisted of conveying, reversing and mixing elements at different places along the screw with the mixing elements affording dispersive as well as distributive mixing. A 50 Ton Cincinnati Milacron injection moulder was used to prepare test coupons of the samples for analysis and testing. This work prepared samples containing 0, 1, 2, 3 and 5 wt% nanoclay that were processed at an extruder temperature of 240 C. Processing was also performed at 220 C, 255 C and 270 C at concentrations of 0, 3 and 5 wt% nanoclay concentrations. The samples prepared in this work and their corresponding notation are summarised in Table 1. 2.2. Material characterisation X-ray diffraction analysis was carried out on all samples using a Bruker Advance D8 X-ray diffractometer, equipped with a diffracted beam polycrystalline graphite monochromator. The X-ray beam was operated at 40 mA, 40 kV, and data collection between 1 and 70 (2h) with step size of 0.02 at a scan rate of 6 s per step. Phases were identified using the ICDD-JCPDS powder diffraction database. The samples were analysed in ‘‘as-received’’ state, i.e. postextrusion, with no further sample preparation required. Transmission electron microscopy (TEM) was performed without using cryogenic cooling to more comprehensively investigate the nanoscale dispersion of the nanocomposites produced in this work. The char layer was investigated using a Philips XL30 field emission scanning electron microscope (SEM) while elemental analysis was performed using electrodispersive spectroscopy (EDS). Cone calorimetry was performed using a Stanton Redcroft Cone Calorimeter in a horizontal orientation according to the procedure outlined in the ASTM E1354-1992 standard. Samples measuring 75 mm · 100 mm were used and testing was performed in triplicate. In addition, they were carefully wrapped around the bottom and the sides using heavy aluminium foil to ensure that the molten polymer would not flow out of the combustion zone and were held securely in the sample holder. Fire performance was evaluated by subjecting the samples to a heat flux of 35 kW/m2 from above the sample and determining parameters such as time to ignition, peak heat release rate and average heat release rate. Thermogravimetric analysis was performed in an oxidative (air) atmosphere using a Mettler-Toledo 821e TGA/ SDTA. Samples placed in an Alumina crucible, and ramped from 50 C to 800 C at a rate of 10 C min1 while the flow of air was maintained at 50 ml min1.
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Table 1 Summary of samples prepared and tests undertaken in this work, listed alongside the notation used in this paper Sample ID
Processing temperature (C)
Clay concentration (wt%)
TGA
XRD
Unprocessed nanoclay 0%NC240PT 1%NC240PT 2%NC240PT 3%NC240PT 5%NC240PT 0%NC2xxPTa 3%NC2xxPTa 5%NC2xxPTa
– 240 240 240 240 240 220/255/270 220/255/270 220/255/270
100 0 1 2 3 5 0 3 5
p p p p p p p p
p
a
p
Cone calorimetry
Elemental analysis (of char)
p
p
xx refers to the last two numbers of the temperature used for processing the nanocomposites.
dispersion. Consequently it can be suggested that the 5%NC220PT sample contains:
3. Results and discussion 3.1. Nanocomposite formation Fig. 1 shows representative XRD traces of the layered silicate nanocomposites formed during processing at 240 C plotted alongside that of the unmodified nylon. The traces clearly highlight the lack of order in the (postprocess) layered silicate of the nanocomposites and can therefore be described as being at least highly intercalated according to XRD analysis. It should be noted however that with increasing clay concentration, the region where the 0 0 1 reflection peak would be expected to be visible, does exhibit, shoulder regions on the traces highlighting the limitation of using XRD to make specific assertions as to the quality of the nanoscale dispersion. Hence TEM has been used to complement the XRD analyses. In Fig. 2, images of 5%NC220PT and 5%NC255PT samples show that overall both these nanocomposite have a complex mixture of intercalated, highly intercalated and exfoliated clay layers thus agreeing to some degree with the results obtained from the XRD analyses. TEM however provides additional information by facilitating easier differentiation of the different types of clay morphologies present and affords a better judgement of the relative quality of the 2000
1200 5wt% clay 3wt% clay 2wt% clay
1000
1wt% clay clay
800
1200 600 800 400 400
Intensity (counts)
Intensity (counts)
1600
200
0
0 1
7
13
19
25
31
37
2
Fig. 1. XRD spectra showing that the order in the clay layer has been destroyed for the samples of varying concentration prepared at 240 C.
(1) greater separation of clay platelets from their tactoids; (2) much less large agglomerates; and (3) higher levels of alignment of platelets according polymer flow. The varying sizes and thickness of the clay tactoids reinforce that the process of exfoliation is achieved through individual platelets separating or ‘‘peeling’’ off consecutively from the cluster of platelets [11]. The greater exfoliation (and alignment) at lower temperature, reinforces the importance of higher shear forces (from higher viscosities at lower processing temperature) determining the extent of exfoliation [12]. 3.2. Thermal analysis The effect of silicate concentration on thermal stability of the nylon in an oxidative environment for the samples processed at 240 C is shown Fig. 3. Here the decomposition of the nylon can be seen to obey a two-step process consisting primarily of the decomposition of the bulk polymer (90%) followed by degradation of the remaining char layer (10%) through a chain scission volatilisation process. As can be seen, clay concentration appears to have little effect upon the onset and rate of degradation which is consistent with results obtained from polymer systems other than nylon [13]. The char yields however, at 500 C (char formation) and 800 C (char degradation) both increase consistently with clay content above what would be expected in the absence of significant nylon clay interaction. The relative thermal stabilities of the char layer are also represented in the inserted diagram within Fig. 3, which shows the rate of change of degradation in the 500–600 C region. Here, the decomposition of the char layer can be seen to occur more slowly and at higher temperatures with increasing clay concentration. The complete set of results at all temperatures and concentrations, measured in this work are shown in Table 2 and reinforce that the initial onset of degradation is relatively unaffected by
R.J. Varley et al. / Composites Science and Technology 68 (2008) 2882–2891
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Fig. 2. TEM images of the (a) 5%NC255PT and the (b) 5 wt%NC220PT samples at two different magnifications.
unmodifed polyamide
100
1wt% clay 2wt% clay
80
Mass Loss (%)
3wt% clay Rate of Mass Loss
5wt% clay
60
40
20
500
0 200
Temperature (ºC)
300
600
400
500
600
700
Temperature (ºC) Fig. 3. TGA traces showing the mass loss behaviour with increasing clay concentration in an oxidative atmosphere. Insert shows the rate of change of mass loss between 500 and 600 C.
clay addition as determined by the 10% degradation temperature, T10%, and the temperature of the peak in the 1st derivative of the mass loss, Tmax1. The results also show that increasing clay concentration generally increases the thermal stability of the char layer at the varying processing temperatures as measured by the second or higher temper-
ature peak in the 1st derivative spectra of the mass loss Tmax2. Thermogravimetric analysis however, was not sensitive enough to measure any differences in thermal stability resulting from the improved nanoscale dispersion of the 5%NC220PT and the 5%NC255PT samples as measured by TEM (see Fig. 2).
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0 3 5 0 1 2 3 5 0 3 5 0 3 5
220 220 220 240 240 240 240 240 255 255 255 270 270 270 a b c
415.3 418.5 425.4 415.3 417.7 416.4 413.4 425.6 418.8 416.5 417.7 418.5 421.2 421.1
453.4 462.3 460.3 457.3 461.3 455.6 453.0 457.8 456.6 455.1 454.4 457.9 456.5 454.4
4.73 7.57 7.50 5.47 5.39 6.79 8.92 8.07 5.12 17.37 8.61 5.03 9.24 8.90
538.4 605.7 570.9 540.1 566.6 604.9 598.7 566.3 533.1 609.6 582.9 536.1 585.0 585.0
0.13 2.11 3.88 0.66 1.33 2.02 3.01 4.18 0.50 4.60 3.88 4.33 6.57 4.75
Temperature at 10% weight loss. Temperature of the 1st peak in the 1st derivative of the mass loss. Temperature of the 2nd peak in the 1st derivative of the mass loss.
3.3. Cone calorimetry The effect of increasing clay concentration on the key fire performance parameter, the peak heat release rate (PHRR), is shown in Fig. 4. The measured decrease in PHRR and time to ignition with increasing clay concentration is consistent with reports of this nature in the literature [14]. Fig. 5 shows the effect of increasing clay concentration on PHRR and the effective heat of combustion which highlight the synergistic performance achieved through the addition of layered silicate to nylon. A 2 wt% addition of an organo-modified clay has reduced the PHRR by 18.2% while a 5 wt% addition resulted in a reduction of 26.7% in the PHRR. It is important to recognise that the organo-modified clay contains about 30 wt% of an alkylammonium compatabiliser which effectively reduces the level of actual silicate content in the blend. The effect of
650
45
600
PHHR (kW/m2)
Clay Temperature T10%a Tmax1b Char yield 1 (%) Tmax2c Char (C) (C) (C) yield 2 content (C) (%) (wt%)
50
700
18.2%
18.8%
40
550 26.7%
500 450
30
400 25 350 20
300 0
1
2
3
4
5
wt/wt clay concentration
Fig. 5. Plot of the reduction in the PHRR and the Effective heat of combustion as a function of clay concentration.
processing temperature upon the PHRR for the 5 wt% samples is shown in Fig. 6 where the sample processed at 220 C clearly exhibits significant further improvement in fire performance compared to the samples processed at 240 C, 255 C and 270 C. The PHRR of the 5%NC220PT sample exhibited a decrease of 41%, while the other two samples displayed an overall average reduction in PHHR of only 20%. The 5%NC220PT sample also displayed a time to ignition similar to that of the unmodified nylon which is in contrast to the other modified samples which all displayed a shorter time to ignition. Furthermore, the PHRR for the 5%NC220PT sample shifted to longer times to complete combustion and displayed a distinctly bimodal mechanism in comparison to the other modified samples and unmodified samples. The behaviour of the 5%NC220PT sample during actual combustion was quite distinct to that of the other samples, which in turn behaved differently to the unmodified polyamide. Combustion occurs in the cone calorimeter when the concentration of the combustible products increases
700
700 unmodified nylon
600
unmodified nylon
2wt% clay
600
5wt% at 220degC
3wt% clay
5wt% at 240degC
5wt% clay
PHHR (kW/m 2)
500
2
PHHR (kW/m )
35
Effective Heat of Combustion (MJ/kg)
Table 2 TGA results in air showing the thermal decomposition properties for the samples prepared at various temperatures under investigation
400 300 200 100
500
5wt% at 255degC 5wt% at 270degC
400 300 200 100 0
0 0
100
200
300
400
500
600
time (seconds)
Fig. 4. Cone calorimetry of the samples processed at 240 C showing the effect of increasing clay concentration.
0
100
200
300
400
500
600
time (seconds)
Fig. 6. Plot of the PHRR showing the effect of processing temperature on the samples containing 5 wt% of clay as well as the unmodified sample.
R.J. Varley et al. / Composites Science and Technology 68 (2008) 2882–2891
sufficiently in the gaseous phase to ignite. In the case of the unmodified nylon, the molten polymer burned and bubbled profusely and continuously until it was completely consumed. In the case of the intercalated nanocomposites, which according to TEM studies were the 5%NC240PT, 5%NC255PT and the 5%NC270PT samples, it was observed that soon after ignition a ‘‘skin’’ had formed on the surface of the polymer. Although this did not prevent combustion from gaining momentum it did however slow down the rate of increase in the intensity of the flame compared to the unmodified nylon. As combustion continued, the surface of the char in these three intercalated samples continued to expand and rise upwards, i.e. intumesce, although it was also apparent that there was significant molten polymer continuing to bubble beneath the surface. At the end of the combustion process, scattered char residues remained, but with little structural integrity. The combustion process of the exfoliated nanocomposites, the 5%NC220PT sample, behaved quite differently from the intercalated samples but similar to that described elsewhere [3,15]. The exfoliated structure differs from the intercalated one in the sense that: (1) the char formed sooner; (2) the char appeared to have greater strength or stability in that it altered very little during the entire combustion process (i.e. the volume did not expand noticeably at all). Fig. 7 shows an image of the char immediately after combustion in the sample holder, which highlights a greatly increased level of structural integrity after combustion compared to the intercalated samples. (3) there was no evidence of any bubbling of molten polymer beneath the surface at all during combustion. The clear differences between nanoscale dispersion as determined using TEM, can therefore be seen to play an
Fig. 7. Photograph of the char layer of the 5 wt%NC220PT showing the structural integrity post-combustion.
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important role here in controlling the level of improvement in fire performance. Given the importance of the char layer in reducing the heat output during combustion, a relationship between the char yield and the PHHR would be expected. The results obtained from the cone calorimetry work support this hypothesis reasonably well in two demonstrable ways as shown in Table 3, which summarises the results obtained from cone calorimetry. Firstly, the char yield increases consistently with increasing clay concentration while at the same time also reducing PHRR. Secondly, the exfoliated 5%NC220PT sample displays a higher char yield (albeit not enormously) than the intercalated 5%NC240PT, 5%NC255PT and 5%NC270PT samples which also correlated with improvement in fire performance. It should be cautioned that with increased char generated it also suggests that more of the base material has been decomposed and in a practical sense this could be disadvantageous from a structural point of view since less of the original structure is left behind to support working loads. Other important points to note are that the times to ignition decreased with increasing clay concentration, irrespective of nano-dispersion. The exfoliated 5%NC220PT sample showed the lowest reduction although it still ignited sooner than the unmodified polyamide. The effect upon different heat output measurements generally follow the trends already discussed while toxic gas emissions such as CO and CO2 gas display inconsistent results. 3.4. Characterisation of char To further investigate the role of nanoscale dispersion (i.e. exfoliated vs intercalated) on fire performance, the char from the 5%NC220PT and 5%NC255PT samples were selected for further analyses using SEM. Representative micrographs are shown in Fig. 8 at three different magnifications to highlight the differences between the microstructures on different length scales. At the lowest magnification (see Fig. 8a) the 5%NC220PT sample displays a unique ‘‘tree root’’ formation which is considered here to be evidence of a faster or ‘‘non-equilibrium’’ formation of the char layer. This morphology was in contrast to the 5%NC255PT sample which displayed a relatively featureless surface at that magnification. On further inspection at intermediate magnification (see Fig. 8b), the 5%NC255PT sample has an array of large craters on the surface of the order of 10 lm to 100 lm in diameter. These craters are formed through molten polymer flow evidenced by the ‘‘ripples’’ around the edges of the craters as gaseous products escape. In contrast, the 5%NC220PT sample shows little evidence of any large craters and contains sharp edges akin almost to a ‘‘brittle fracture surface’’. It is also apparent that the 5%NC220PT sample surface is quite porous, consisting of very fine voids of the order of a few microns in diameter. The differences in the microstructure of the chars are clearer at higher magnification (see Fig. 8c) where the finer voids in the 5%NC220PT sample is more clearly revealed. It is suggested therefore that the origin of these
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Table 3 Raw data obtained from the cone calorimeter for all samples studied Temp. (C)
Clay content (wt%)
Time to ignition (s)
Total heat evolved (kJ)
Peak heat release rate (kJ/m2)
Heat of combustion (MJ/kg)
Specific extinction area (m2/kg)
CO (kg/kg)
CO2 (kg/kg)
Char yield (%)
240 240 240 240 220 220 255 255 270 270
0 2 3 5 3 5 3 5 3 5
137 102 108 102 108 113 102 102 110 104
614.2 (15.3) 634.13 (1.39) 651.00 (16.01) 616.07 (35.82) 648.60 (36.66) 557.37 (42.45) 605.73 (22.61) 642.67 (29.78) 634.87 (13.74) 606.13 (27.95)
618.6 506.0 502.6 453.6 488.8 366.0 474.9 529.8 502.9 464.5
44.6 34.0 36.9 33.3 33.6 32.2 31.4 38.8 33.5 34.3
651.7 776.8 798.9 644.3 714.1 674.2 688.4 651.3 636.1 560.4
0.0259 0.0257 0.0268 0.0194 0.0281 0.0199 0.0231 0.0241 0.0215 0.0194
2.83 2.84 3.01 2.58 2.51 2.30 2.67 3.03 2.79 2.88
1.34 4.02 5.06 6.42 3.36 6.81 3.45 5.84 2.48 4.84
(3) (2) (6) (2) (4) (5) (4) (2) (10) (9)
(33.7) (44.6) (29.4) (37.8) (45.0) (10.0) (28.0) (71.0) (19.6) (45.8)
(9.9) (1.0) (1.8) (2.3) (1.0) (3.4) (3.0) (4.1) (2.0) (3.0)
(111.6) (23.0) (79.7) (55.1) (123.3) (94.6) (50.7) (92.2) (112.6) (1245)
(0.0063) (0.0019) (0.0021) (0.0015) (0.0107) (0.0024) (0.0008) (0.0030) (0.0028) (0.0015)
(0.06) (0.19) (0.28) (0.09) (0.21) (0.35) (0.47) (0.21) (0.39)
(1.7) (0.7) (1.1) (0.9) (0.8) (1.1) (0.6) (1.2) (1.8) (0.5)
Values in parentheses refer to standard deviation.
Fig. 8. SEM images of the microstructure of the (i) 5 wt%NC255PT and (ii) 5 wt%NC220PT at (a) low, (b) medium and (c) high magnifications.
differences in microstructure of the char, arises from the varying nanoscale dispersions which controls the strength and barrier properties of the char, and in turn affects the fire
performance. These results complement those of Wang et al. [15] who investigated the char layer microstructure and also found a relationship between fire performance and the
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into the gaseous phase has been demonstrated in this work. Evidence for the importance of the char layer has also been found when comparing the mass loss rate during combustion with the change in the heat release rate as shown in Fig. 10 for the 5%NC220PT and 5%NC255PT samples. The strong correlation for both of the samples shows that the source of the fire retardancy mechanism remains within the condensed phase regardless of whether the clay platelets can be described as either exfoliated or intercalated. This further infers the importance of the stability and strength of the char layer in determining fire performance. It is important to note that the actual correlation between the heat release rate and the mass loss indicate not only that the fire retardancy mechanism occurs in the gas phase, it can also be concluded that other mechanisms occurring in the gaseous phase can be disregarded as occurring to any significant degree. The morphology of the nanoclay present in the char layer was characterised using XRD. Fig. 11 compares these results of the 5%NC220PT, 5%NC255PT and 5%NC270PT samples with the neat organo-modified clay where it can be seen that the high levels of disorder (i.e. exfoliated or intercalated) have been destroyed and that the order of the layered silicate has been reformed. The revealed d spacings of the layered silicates are seen to be smaller than that of the
400 PHRR
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3.5. Condensed phase mechanism
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formation of a tough, compact and well-dispersed honeycomb char structure. Elemental analysis of the char layer was performed to determine the relative silicon and carbon contents present in the char at the surface and in the bulk. The results of the elemental analyses are shown in Fig. 9 which highlights significant differences between the exfoliated 5%NC220PT sample and the three intercalated samples 5%NC240PT, 5%NC255PT and 5%NC270PT. The two intercalated samples processed at higher temperatures, i.e. 5%NC255PT and 5%NC270PT, have similar Si and C contents both on the surface and in the bulk of the char. For both of these samples, there is an increase in the relative content of Si on the surface of the char suggesting that the char layer has been strengthened by the migration of silicate species to the surface of the char. The prevailing hypothesis that Si species migrate to the surface to act as insulators or ceramic layers has been discussed and demonstrated by Lewin [16] and Vaia et al. [17] recently. The enrichment of the surface of the char with nanoclay was suggested to be driven primarily by the low surface free energy of the clay that helps the migration process. The 5%NC220PT sample has reasonably similar C and Si contents in the bulk polymer to that of the intercalated samples, but radically different behaviour on the surface. In the case of the exfoliated sample, the relative Si content diminishes markedly as the C content increases at the surface of the char layer. This shows that in the case of this sample, the structure of the char layer is able to resist degradation more than the corresponding intercalated samples. As a result, the C content remains high and the Si content correspondingly decreases. This suggests that the formation of an exfoliated nanocomposite is able to promote the formation of a more thermally stable carbonaceous char more efficiently than a nanocomposite that is only intercalated [18].
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Fig. 9. Elemental analysis of char layers for the 5 wt% samples processed at 220 C, 255 C and 270 C.
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Time (seconds) Fig. 10. Comparison of the Mass loss rate with the PHRR as determined for the (a) 5 wt%NC220PT sample and the (b) 5 wt%NC255PT.
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R.J. Varley et al. / Composites Science and Technology 68 (2008) 2882–2891 30 0
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Fig. 11. XRD traces of the char layers and organo-modified clay showing the collapse of the clay layers during combustion.
neat organoclay highlighting the collapse of the nanocomposites regardless of the original morphology. This result compliments those of Gilman et al. [1,18] who suggested that the reformation of the layered structure at d spacings closer that that of the organo-modified clay is evidence of the formation of a tough ceramic char layer. No doubt this is valid, but this work indicates that it is not able to differentiate between the strength, thermal stability or toughness of the layer, since it is known from this work that the 5%NC220PT sample is clearly stronger than the other samples. 4. Summary and conclusions This work has shown that the addition of layered silicates improve the critical parameter of the peak heat release rate in a synergistic manner due to the formation of a char layer that prevents or hinders the transfer of combustible products into the gaseous phase. Correlation with the heat release rate and the mass loss rate during combustion also reinforce this observation. As well, XRD has shown that the disordered morphology collapses facilitating the ordered silicates to reinforce the char layer at d spacing closer than prior to intercalation. This work has also shown the importance of achieving true exfoliation compared to intercalation in optimising fire performance. Superior fire performance was achieved for the exfoliated 5%NC220PT sample which was shown to be a result of a more thermally stable char layer. Evidence for this was provided by its structural integrity after combustion, the microstructure of the char layer and electro dispersive spectroscopy.
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