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Polymer Degradation and Stability 89 (2005) 418e426 www.elsevier.com/locate/polydegstab
Thermal stability and fire retardant performance of photo-oxidized nanocomposites of polypropylene-graft-maleic anhydride/clay M. Diagnea, M. Gue`yea, L. Vidalb, A. Tidjania,* a LRNA, Faculte´ des Sciences et Techniques, Universite´ Cheikh Anta Diop de Dakar, BP 5005, Dakar-Fann, Se´ne´gal Service de Microscopie Electronique, Institut de Chimie des Surfaces et Interfaces CNRS UPR 9069, 15 rue Jean Starcky, BP 2488 F-68057 Mulhouse, France
b
Received 18 November 2004; received in revised form 31 December 2004; accepted 18 January 2005 Available online 7 April 2005
Abstract Polypropylene-graft-maleic anhydride (PPgMA)/montmorillonite nanocomposites have been prepared using direct melt intercalation techniques (extrusion and injection molding). Characterization of the nanocomposites, performed with X-ray diffraction and transmission electronic microscope techniques, suggested an importance of the mode of preparation and the type of organic modification that was used in the montmorillonite on the dispersability of clay in the polymer matrix. Injected mold nanocomposites were generally exfoliated in opposite to extruded samples that presented different structures (exfoliated, immiscible). Thermal degradation and fire behaviour of the corresponding nanocomposites displayed an improvement in comparison to pure PPgMA; this improvement was more important for injected mold specimen than for the extruded ones. It was ascribed to the presence of clay layers which act as diffusion barriers and reduced the mass loss; results that got the support of Limiting Oxygen Index measurements. A deterioration of these properties was observed on UV-irradiated nanocomposites. This was ascribed to the generation of weak points issue from the photo-oxidation reactions that take place mainly on the sample surface as shown in the oxidation profile. The oxygen starvation and reduced radical mobility, due to the presence of clay platelets, are blamed to be responsible for this oxidation profile. On the other hand, pure PPgMA showed an outstanding improvement of the fire behaviour with ageing time. Improvement that was attributed to cross-linking reactions which create a compact structure that is less inclined to volatilize. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polypropylene-graft-maleic anhydride; Nanocomposites; Photo-ageing; Thermal stability; Fire retardancy; Limiting Oxygen Index
1. Introduction Nanocomposite technology has been the topic of several scientific papers in the last decades. This enthusiasm is explained by the potential applications of nanocomposites in several areas such as packaging, film barrier, automotive and so on [1]. The starting point of its expansion occurred when the Toyota group [2] announced remarkable enhancement of polyamide-6
* Corresponding author. Tel.: C221 864 55 39; fax: C221 824 63 18. E-mail address: [email protected] (A. Tidjani). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.01.032
clay nanocomposite properties, containing only 5% clay. They found an increase of 40% in tensile strength, 68% in tensile modulus, 60% in flexural strength and 126% in flexural modulus, while the heat of distortion temperature increases from 65 to 152 C. Today in literature, numerous articles can be found dealing with production of nanocomposites using different techniques and host polymer, thermal stability, fire retardancy and permeability of nanocomposites. The typical methods used to produce polymer nanocomposites are: intercalation of a suitable monomer followed by polymerisation, polymer intercalation from solution, and direct polymer intercalation. Intercalation
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of polymer melts is the most attractive method because of its ease, its environmental benignancy and cost effectiveness. The current favorite nano-clay used is montmorillonite (MMT). The clay exists in a tactoid structure of 20e25 layers, which translates into an aspect ratio of about 10. Because of its hydrophobic nature, MMT is not compatible with most polymers; it must be modified and this is achieved by exchanging the cations in between the silicate layers by usually an alkyl ammonium ions. This operation increases the clay layer or basal spacing and reduces the attractive forces between the clay layers, that make possible the intercalation of the polymer and/or the exfoliation of the clays platelets. Indeed, introduction of an alkyl ammonium surfactant may offer sufficient excess enthalpy for most polar polymers to promote the nanocomposite. In the case of non-polar polymers, an addition of a compatibilizer is requested to enhance the diffusion of polymers into clay galleries. Nanocomposites are described as intercalated or exfoliated. In the first case, the registry between the clay layers is maintained oppositely to the second case in which the clay layers are dispersed leading to the loss of registry. Polypropylene (PP) e one of the most widely used polyolefins e has simulated intensive research in order to produce polypropylene nanocomposite. Besides the effort put to optimize the production of polypropylene nanocomposites (PPCNs), it has been reported an increase of thermal stability and mechanical properties, an improvement of fire retardancy, decrease of gas permeability [3e5]. One aspect which has been neglected so far, is the ageing of nanocomposites [6] and specifically the effect of ageing on the remarkable properties of these materials reported in literature. This investigation is of importance since it can be predicted that nanocomposites will probably replace conventional material. In their future use, they could be submitted to heat and/or light exposition that can alter their properties. In the following paper, investigation of the effect of photoageing on the thermal stability and fire retardancy properties (cone calorimeter, Limiting Oxygen Index) of PPCNs, prepared by extrusion and injection molding, is presented.
2. Experimental 2.1. Materials Polypropylene containing 0.6% of maleic anhydride was commercial product available from Aldrich Chemical Company. The presence of maleic anhydride as compatibilizer in PP is to favor the melt blending with modified clays used. Three modified montmorillonites (MMTs) in which, different organic cations were used to change the hydrophilic nature of layered silicates to make
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them miscible with the hydrophobic character of polypropylene are experienced here in. In Cloisite 30B and Cloisite 20A e coming from Southern Clay Products, Inc. e modification has been done using a quaternary ammonium salt, methyl tallow bis2hydroxethyl (MT2EtOH) and dimethyl dihydrogenatedtallow (2M2HT), respectively. The third, Nanomer I28E from Nanocor, is a natural montmorillonite ion-exchanged with an Octadecyl trimethyl amine (OD3MA). 2.2. Preparation of the nanocomposites The melt compounding of PP with the modified clay was performed using an intermeshing twin-screw extruder Haake Rheomex TW100 for the first series of samples. All the ingredients were mixed simultaneously without any processing stabilizer. The major processing variables were screw speed, barrel temperature profiles; they were set at 50 rpm and, from 433 K at the first barrel to 458 K at the last barrel, respectively. The resulting material was pressed to produce plates or films with the aid of a hydraulic press equipped with two heated plates. Another series of specimen has been produced by extrusion followed by injection molding. The extrusion has been carried out in a ZSK 25 machine (Werner und Pfeiderer, Germany) through nine temperature zones ranging from 448 to 462 K followed by the injection at 453 K under a pressure of 900 bar and a post-pressure of 350 bar for 4 s. Whatever the experiment conditions, PPCNs with clay mass fraction of 5% were produced. 2.3. Instrumentation 2.3.1. Characterization of nanocomposites The commun techniques used to evaluate the state of mixing of nanocomposites are X-ray diffraction (XRD) and transmission electronic microscope (TEM). The first one helps to know if the registry between the clay layers is maintained as the polymer inserts (intercalation structure) or, if it is lost due to exfoliation of the clay layers. The second technique allows a direct observation of the state of org-MMT dispersion. A Philips X’Pert X-ray diffraction apparatus (CueKa radiation, l Z 0.154 nm) operating at a generator voltage of 40 kV and a generator current of 20 mA was used for XRD measurements. The samples were scanned from 1.2 to 10 in 2q at a rate of 0.3 /min. TEM was performed on a Philips CM20 equipment at 200 kV on specimen microtomed with a LKB 8800 Ultrotome III. 2.3.2. UV irradiation For photo-oxidation operations, plates or bar of nanocomposites were exposed to UV light: under accelerated conditions using a SEPAP 12-24 unit working
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at 60 C, and under natural conditions at Dakar (Senegal) located at 17 E longitude and 15 N latitude. The profile of oxidation of the samples was done using a Nicolet 800 FTIR spectrometer coupled to a Nic-plan microscope (128 scans summation, nominal resolution 4 cmÿ1). 2.3.3. Thermal analysis Thermogravimetric analysis (TGA) was carried out using a Mettler/Toledo TGA/SDTA 851. The TGA scans were recorded at a heating rate of 10 K minÿ1 under air atmosphere in the temperature range of 293e873 K. Temperatures recorded from TGA data are reproducible to G2% while the amount of char is reproducible to G2%. Sample masses were set between 15 and 20 mg. 2.3.4. Fire resistance Cone calorimeter and Limiting Oxygen Index (LOI) were used for fire retardancy properties. The cone calorimeter is designed to measure a range of important properties which characterize the fire behaviour of materials [7]. It has the ability to measure ignitability, the heat release rate, HRR, the mass loss rate, MLR, the evolution of smoke and the rate of production of various gas species with a reproducibility within 10%. These uncertainties are based upon many runs in which thousands of samples have been combusted [8]. The cone measurements were performed in duplicate with a FTT, UK device according to ASTME-1354-92 at an incident flux of 50 kW mÿ2 using a cone shape heater. Exhaust flow was set at 24 l/s and the spark was continuous until the sample ignited. The sample, in a form of plate (100 mm ! 100 mm, 5 mm thick), was placed in an aluminium foil then in a box having the same dimensions in the horizontal position. As for the LOI, it is designed to provide a precise method for determining the minimum volume concentration of oxygen in a flowing stream of oxygen and nitrogen required to maintain candle-like burning of that sample. LOI test was conducted according to the ASTM D286377/ISO 4589-2. The sample in a form of bar (100 mm long, 7 mm wide and 5 mm thick) was suspended vertically and ignited using a burner. The concentration of oxygen was increased if the specimen was extinguished before burning away for 3 min or 5 cm.
3. Results and discussion 3.1. Characterization of nanocomposites A first glance of the structure of the nanocomposites obtained can be determined using XRD technique from the position, shape and the intensity of the basal reflections in the XRD patterns. The XRD patterns of two specimens are shown in Fig. 1.
Fig. 1. XRD patterns of Cloisite 20A-2M2HT (a, curve 1) and Cloisite 30B-MT2EtOH (b, curve 1) and their nanocomposites produced by extrusion (curve 2) or by injection molding (curve 3).
For the XRD patterns of PP-2M2HT nanocomposites, the d001 peaks were broadened with low intensity and shifted towards lower angles whatever the mode of production. The diffuse nature of the d001 peak makes locating its center difficult to be able to estimate accurately their d-spacings; however their shift to lower angle is unquestionable. These results suggest either a formation of intercalated nanocomposites or a mixed intercalatedeexfoliated system (or simply a disordered system). An additional small peak is observed towards higher angle for samples prepared by injection molding. The TEM pictures displayed further down show that this peak corresponds to stack of platelets in which the d-spacing is reduced. In the case of PP-MT2EtOH, the situation depends on the blending conditions. For samples produced by injection molding, a similar situation that was described for PP-2M2HT is observed. For the extruded blending of PP-MT2EtOH, a small peak towards higher angle is observed indicating an unexpected decrease of the d-spacing since the shear stress of the extruder is supposed to help dispersion of clay platelets and/or intercalation of the polymer chains. Some authors attributed this decrease to a partial
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decomposition of the organic alkyl ammonium modifier during processing, leading to a decrease of the d-spacing. The loss of the ammonium salt and its replacement with a proton has been proposed to explain this observation [9]. Another explanation did not associate this decrease to the thermal degradation but rather to a collapse of the interlayer structure due to the shear force [10]. Such structural variation could be related with the transition from bilayer to monolayer arrangement. To complete the characterization of the nanocomposites produced, TEM micrographs of the corresponding samples e that give actual image of the clay e are presented in Figs. 2 and 3. Poor dispersion of the clay platelets and presence of tactoids are observed in Fig. 2a. The corresponding material should probably be described as immiscible system (the dominant phase) in which some clay layers are dispersed. This got the support of the XRD results. On the other hand, PP-MT2EtOH produced by injection molding displayed surprisingly a total dispersion of the clay platelets (Fig. 3b). Some may suggest that the hydroxyethyl present in MT2EtOH which is supposed to favor wettability of the clay by the maleic anhydride groups grafted onto PP, facilitating the production of nanocomposite, plays its role during the injection molding process and not during the extrusion alone. Partial explanation of this phenomena was given in a previous paper [11]. For PP-2M2HT nanocomposites, a good dispersion and a mixed intercalatedeexfoliated system were attained by extrusion and injection molding, respectively (Figs. 2b and 3a). These confirm the diffuse nature of the peaks obtained in XRD patterns of the corresponding specimen. One may conclude that a satisfactory level of dispersion is obtained with Cloisite 20A whatever the experimental conditions though some stacks were observed in injection molded samples. 3.2. Profile of oxidation It is well known that photo-oxidation of thick plates leads to heterogeneous distributions of the products in the matrix. This is due to difficulty of oxygen to diffuse leading to a starvation of oxygen in the bulk of the material [12]. As a result, distribution of the photoproducts within UV-irradiated specimen is requested to have an exact idea of the level of oxidation in the plate. Fig. 4 presents the plots of the absorbance of nonvolatile carbonyl oxidation products at 1714 cmÿ1 as a function of the distance from the exposed surface at two selected ageing times. The plots were drawn from spectra recorded every 15 mm by moving the analysed area under the axis of the objective. After 150 h of irradiation, no significant difference is noticed from the surface to the core. On the other hand, after 300 h of UV irradiation, one can observe a higher level of oxidation at the surface comparatively to the core of the sample.
Fig. 2. TEM micrographs of PP-MT2EtOH (a) and PP-2M2HT (b) nanocomposites produced by extrusion.
This profile of oxidation products is probably due to the difficulty of oxygen to diffuse in the mass of specimen leading to a starvation of oxygen in the core. The presence of silicate platelets may also drastically reduce the diffusion of oxygen [13] and the mobility of radicals created from the surface to the core of specimen. Some one can also suspect attenuation of the light absorption within the sample due to the presence of silicate layers. Note that the oxidation level at the surface was very much dependent on the modified clay used but not to the mode of nanocomposite production. For instance
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Cracks were observed on PP-OD3MA and PPMT2EtOH; these were commun on the surface of degraded polymers. On the other hand, spherulites were observed on PP-2M2HT surfaces. Similar spherulites were observed in our previous work on UV-degraded polyolefin films using a scanning electron microscope [14]. According to some authors [15,16], clay may serve as an efficient nucleation agent that promote formation of spherulites; nucleation efficiency increases as the degree of clay dispersion improves. Since these spherulites were only visible on UV-irradiated specimen, it can be suggested that the UV irradiations contribute to reveal these spherulites. Our results also indicate that the action of nucleating agent seems to be dependent on the type of clay used. Work is in progress in this laboratory to elucidate these points. There is no doubt that the chemical changes induced by the UV degradation process may have some effect on thermal stability and fire properties of the nanocomposites. The corresponding results are presented in the next sections. 3.3. Thermal degradation
Fig. 3. TEM micrographs of PP-2M2HT (a) and PP-MT2EtOH (b) prepared by injection molding.
after 300 h of UV irradiation in the SEPAP 12-24, the 1714 cmÿ1 absorbance at the specimen surface was 0.4 for PPgMA, 0.65 for PP-MT2EtOH, 0.7 for PPOD3MA and 1.85 for PP-2M2HT. It is worth mentioning that for samples blended by injection molding, the visual aspect of the degraded surfaces revealed changes comparatively to virgin nanocomposites and PPgMA. Photographs of these surfaces taken with Leica Makroskop M420 coupled with a digital camera DC200, !10 show differences (Fig. 5).
During the thermal degradation, the TGA curves displayed a single step degradation process for all samples produced. Table 1 collects the TGA data e corresponding to the temperatures at which 10% and 50% degradation occur, the peak in the derivative curve and, the fraction of non-volatile material at 873 K, referred as residue e of virgin and UV-degraded PPCN. The onset temperatures (T10%, T50%) of the nonirradiated nanocomposites are higher than that of virgin PPgMA. This enhancement observed by several authors [11,17,18], is more important for injection molded nanocomposites (from 16 to 55 C) compared to extruded nanocomposites (from 4 to 27 C). The changes of DTG parallel the changes in onset temperature while the amount of char residue remains almost constant. These findings are ascribed to the presence of clay layers which act as diffusion barriers; that may delay the decomposition process [11]. It can be concluded from these results and the ones on characterization that, the better the dispersion of the clay platelets, the higher the enhancement in thermal oxidation. This hypothesis is not valid for PP-MT2EtOH which, though displaying the better dispersion for samples produced by injection molding, shows a small enhancement of TGA parameters. From previous work in this laboratory, we attributed this behaviour to the sensitivity of EtOH group that may initiate the oxidation leading to a fragile material [11]. However, the presence of clay layers could not prevent deterioration of the thermal properties of the aged nanocomposites as registered in Table 1 with the shift of the onset temperatures towards lower temperature.
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Fig. 4. Variation of absorbance at 1714 cmÿ1 as a function of the distance from the irradiated surface in a 5 mm thick plate UV-irradiated at different times of ageing in an SEPAP 12-24: (-) after 150 h, (A) after 300 h.
Though the surface specimen was the main part of the plate to be apparently affected by the UV irradiation, a decrease in thermal stability is observed. This decrease seems to be similar for both types of samples presented herein. It could be speculated that the photo-ageing creates weak points that facilitate the thermal degradation in the bulk. These weak points could initiate the degradation from the surface to the core. 3.4. Cone calorimetry and LOI The assessment of flammability of nanocomposites is usually performed by cone calorimetry and/or Limiting Oxygen Index (LOI). The time to ignition, tign, the peak heat release rate, PHRR, and the time to PHRR, tPHRR, the average specific extinction area, SEA, a measure of a smoke and the mass loss rate, MLR, were measured with the cone calorimeter. Table 2, including all these parameters and visual observations during the cone tests of virgin specimen, is presented below. Looking first at the reduction in PHRR, the best results have been obtained for the nanocomposites prepared by injection molding apart from PP-MT2EtOH
nanocomposite. This reduction is accompanied by a decrease of the ignition time and the tPHRR without significant changes in the heat of combustion, Hc, smoke yield (SEA) and carbon monoxide yield (not shown in Table 2). This indicates that the improvement of fire behaviour occurs in the solid phase and not in the gas phase. A constant ratio between the heat release and the mass loss was observed for nanocomposites, suggesting that the improved combustion behaviour of nanocomposite is primarily due to the reduced mass loss. This is confirmed by the reduction of the mass loss rate values recorded for nanocomposites in comparison to PPgMA (Table 2). This reduction of the MLR tackles quite well the one observed for the PHRR. Notice that the MLR values were lower for samples produced by injection molding than for those prepared by extrusion. It is of importance to mention the disappointed fire behaviour of PP-MT2EtOH produced by injection molding; partial explanation of this was given previously. Concerning the visual observations of the nanocomposites through the cone tests, it has been extensively described elsewhere [13,18]. In brief, it is observed during the combustion tests that true nanocomposites swell
Fig. 5. From left to right, UV-irradiated surfaces of PP-OD3MA after 300 h in the SEPAP 12-24 and 4 months of natural ageing and, of PP-2M2HT after 300 h of accelerated ageing.
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Table 1 TGA data of virgin and UV-irradiated nanocomposites investigated Time
T10% ( C)
T50% ( C)
DTG ( C)
e
PPgMA
Virgin 450 h 4 months
310 304 314
376 378 380
411 387 392
7 7 7
PPgMAi
Virgin 450 h 4 months
317 328 326
378 394 394
407 408 402
6 6 6
PP-OD3MAe
Virgin 450 h 4 months
330 (C20) 307 318
401 (C24) 387 395
419 (C8) 405 417
9 10 9
PP-OD3MAi
Virgin 450 h 4 months
356 (C39) 331 350
416 (C38) 406 413
425 (C18) 420 424
11 11 10
PP-2M2HTe
Virgin 450 h 4 months
337 (C27) 317 330
408 (C32) 393 407
428 (C17) 414 425
9 9 9
PP-2M2HTi
Virgin 450 h 4 months
372 (C55) 361 375
430 (C52) 427 434
443 (C36) 439 446
10 10 10
PP-MT2EtOHe
Virgin 450 h 4 months
314 (C4) 301 322
387 (C11) 389 394
401 (ÿ10) 406 412
11 10 9
PP-MT2EtOHi
Virgin 450 h 4 months
333 (C16) 337 335
397 (C19) 404 399
416 (C9) 421 419
10 10 10
Samples
Residue (%)
Samples labelled (e) were produced by extrusion, those labelled (i) were injection molded.
with ageing time, whatever the irradiation conditions and their mode of preparation. This result is clearly seen in Fig. 7 that depicted the heat release rate of virgin PPgMA, virgin and UV-irradiated PP-2M2HT nanocomposites. Besides the increase of the PHRR of aged PPCN, a shift of this peak towards lower time and a reduction of the combustion time with ageing time could be noticed. These changes in PHRR are accompanied, under accelerated conditions, by a decrease of the ignition time for PP-OD3MA and PP-2M2HT and, an increase for PP-MT2EtOH and this, for both types of nanocomposites. Under natural conditions, tign decreased for
(or intumesce) with formation of a char layer; on the other hand, immiscible composite tends to boil, bubble, then drip. The cone tests performed on UV-irradiated specimen revealed some changes in the fire behaviour comparatively to virgin materials. The discussion will be focused on the PHRR, the mass loss rate and the time to ignition. The two first parameters are supposed to best describe the fire behaviour of the mass of specimen, as for the ignition time, it could best inform on the state of surface degradation of samples tested. The general tendency drawn from Fig. 6 is that nanocomposites displayed an increase of the PHRR
Table 2 Cone calorimeter average data of the nanocomposites blended by extrusion (first line) and by injection molding (second line) Sample
tign (s) PHRR (kW mÿ2) (% reduction) tPHRR (s) Mean Hc (MJ/kg) SEA MLR (g/s m2) (%reduction) Visual observations
PPgMA
35 32
1010 1012
294 220
34.66 31.14
580 436
PP-2M2HT
29 28
491 (52) 324 (68)
211 78
36.52 39.47
681 445
6.7 (59) 4.7 (68)
Swelling Swelling
PP-OD3MA
27 20
609 (40) 318 (69)
258 43
38.14 42.25
667 455
11.3 (32) 4.6 (67)
Swelling Swelling
PP-MT2EtOH 24 26
837 (17) 776 (23)
259 295
40.37 38.92
585 665
14.2 (13) 14.5 (0)
Small dripping Small dripping
16.5 14.3
Dripping Dripping
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Fig. 7. The HRR variations of virgin and UV-degraded PP-2M2HT nanocomposites comparatively to virgin PPgMA. The order in the box corresponds to order of curves from top to bottom.
PPgMA with ageing time but not for the nanocomposites for which it remains constant (Fig. 8). This behaviour parallels the one observed in the changes of PHRR with ageing time. These results tell that UV irradiation has a negative influence on the fire behaviour of nanocomposites as stated by the increase of the PHRR without significant changes in the mass loss. This could be explained by the presence of small oxidation products issue from the degradation, that are more keen to ignite. Since these oxidation products are located in the first microns of the plate, they can contribute to increase the PHRR but they cannot significantly change the fire behaviour of the bulk material. From these explanations, we should expect a decrease of the ignition time; this was the case for PP-2M2HT nanocomposites whatever the irradiation conditions. In the opposite, for the other nanocomposites, the irradiation conditions appear to play a role on tign. The outstanding fire behaviour improvement of
Fig. 6. PHRR changes of nanocomposites blended by extrusion and degraded under accelerated (a) and natural conditions (c) and, blended by injection molding and degraded under accelerated (b) and natural conditions (d). Correspondence of symbols: PPgMA (,), PPMT2EtOH (-), PP-OD3MA (B), PP-2M2HT (C).
PP-2M2HT and increased for PP-OD3MA and PPMT2EtOH. On the other hand, PPgMA displayed a surprising decrease of the PHRR accompanied by an increase and a decrease of tign under accelerated and natural conditions, respectively. Concerning the mass loss rate changes, a significant decrease is observed for
Fig. 8. Mass loss rate changes of PPgMA and injection molded nanocomposites UV-degraded. Correspondence of symbols: PPgMA (,), PP-MT2EtOH (-), PP-OD3MA (B), PP-2M2HT (C).
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the PHRR decreased with ageing time for nanocomposites but not the mass loss rate. This was due to weak points created by the UV degradation process on the samples. On the other hand, an outstanding improvement of fire properties was achieved on UV-irradiated PPgMA. This was explained by the cross-linking reactions that generate a compact structure which is less easily volatilizable. The LOI results obtained on virgin and UV-degraded specimen got the support of the cone results.
Fig. 9. LOI of virgin and UV oxidized samples investigated: PPCN0 (PPgMA), PPCN1 (PP-OD3MA), PPCN2 (PP-2M2HT), PPCN3 (PP-MT2EtOH).
UV-degraded PPgMA calls for other explanation. The decrease of its PHRR is accompanied by a decrease of its mass loss rate; this indicates that an improvement of fire behaviour occurs in the bulk. This improvement is probably due to cross-linking reactions that occur in the bulk because of the oxygen starvation mentioned previously. The cross-linking reactions generate a structure which is more compact and less easily volatilizable. The question that may be risen is why this cross-linking effect is not visible in UV oxidized nanocomposites. It can be blamed to the presence of clay platelets that reduces the formation of radicals by attenuating the UV lights, and also, prevents radical mobility. The flame retardant characteristics of the virgin and photo-oxidized PPCNs and PPgMA were also examined by measuring their LOIs. Fig. 9 reveals a very small decrease in LOI when silicate layer is incorporated in PPgMA, and whatever the state of mixing. These LOI values remained almost constant (taking into account error bars) for photo-aged nanocomposites under accelerated conditions. For the PPgMA, the LOI values fluctuated with ageing time. All these results are somehow consistent with the cone data.
4. Conclusion The true PP/clay nanocomposites exhibited improved thermal stability and fire retardancy properties at a clay loading of 5% by weight. The extent of the improvement was more for PP nanocomposites produced by injection molding than for those obtained by extrusion. The most interesting finding of this study was the dramatic change of these properties observed in the UV-degraded nanocomposites. The onset temperatures of degradation and
Acknowledgements The authors are indebted to the Vokswagen foundation for the financial support, the Federal Institute for Research and Testing (BAM) for the cone tests, Mr Claude da Silva from the universite´ de Mulhouse, France, for the X-ray measurements and, Dr Be´ne´dicte Mailhot from the Laboratoire de Photochimie de l’universite´ BP de Clermont, France, for the oxidation profile.
References [1] Revue de Sciences physiques, nanoscience, microe´lectronique, mate´riau, Mai 2004, N 11. [2] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. J Mater Res 1993;8:1185e9. [3] Gilman JW. Appl Clay Sci 1999;15:31e49. [4] Gorrasi G, Tortora M, Vittoria V, Galli G, Chiellini E. J Polym Sci Part B: Polym Phys 2002;40:1118e24. [5] Su S, Jiang DD, Wilkie CA. Polym Degrad Stab 2004;83:321e31. [6] Tidjani A, Wilkie CA. Polym Degrad Stab 2001;74:33e7. [7] ISO 5660e1, Part 1, 2002-12-15. [8] Gilman JW, Kashiwagi T, Nyden M, Brown JET, Jackson CL, Lomakin S. In: Al-Malaika S, Golovoy A, Wilkie CA, editors. Chemistry and technology of polymers additives. Oxford: Blackwell Science; 1998. p. 249e65. [9] Zanetti M, Camino G, Thomann R, Mulhaupt R. Polymer 2001;42:4501e7. [10] Kim KN, Kim H, Lee JW. Polym Eng Sci 2001;41:1963e9. [11] Tidjani A, Wald O, Pohl MM, Hentschel MP, Schartel B. Polym Degrad Stab 2003;82:133e40. [12] Jouan X, Gardette J-L. Polym Commun 1987;28:329e31. [13] Tidjani A. Polym Degrad Stab 2005;87:43e9. [14] Tidjani A, Arnaud R. Polymer 1995;36:2841e4. [15] Hambir S, Bulakh N, Kodgire P, Kalgaonkar R, Jog JP. J Polym Sci Part B: Polym Phys 2001;39:446e50. [16] Kodgire P, Kalgaonkar R, Hambir S, Bulakh N, Jog JP. J Appl Polym Sci. 2001;81:1786e92. [17] Su S, Wilkie CA. Polym Degrad Stab 2004;83:347e62. [18] Kashiwagi T, Harris RH, Zhang X, Briber RM, Cipriano BH, Raghavan SR. Polymer 2004;45:881e91.
Thermal stability and fire retardant performance of photo-oxidized nanocomposites of polypropylene-graft-maleic anhydride/clay M. Diagnea, M. Gue`yea, L. Vidalb, A. Tidjania,* a LRNA, Faculte´ des Sciences et Techniques, Universite´ Cheikh Anta Diop de Dakar, BP 5005, Dakar-Fann, Se´ne´gal Service de Microscopie Electronique, Institut de Chimie des Surfaces et Interfaces CNRS UPR 9069, 15 rue Jean Starcky, BP 2488 F-68057 Mulhouse, France
b
Received 18 November 2004; received in revised form 31 December 2004; accepted 18 January 2005 Available online 7 April 2005
Abstract Polypropylene-graft-maleic anhydride (PPgMA)/montmorillonite nanocomposites have been prepared using direct melt intercalation techniques (extrusion and injection molding). Characterization of the nanocomposites, performed with X-ray diffraction and transmission electronic microscope techniques, suggested an importance of the mode of preparation and the type of organic modification that was used in the montmorillonite on the dispersability of clay in the polymer matrix. Injected mold nanocomposites were generally exfoliated in opposite to extruded samples that presented different structures (exfoliated, immiscible). Thermal degradation and fire behaviour of the corresponding nanocomposites displayed an improvement in comparison to pure PPgMA; this improvement was more important for injected mold specimen than for the extruded ones. It was ascribed to the presence of clay layers which act as diffusion barriers and reduced the mass loss; results that got the support of Limiting Oxygen Index measurements. A deterioration of these properties was observed on UV-irradiated nanocomposites. This was ascribed to the generation of weak points issue from the photo-oxidation reactions that take place mainly on the sample surface as shown in the oxidation profile. The oxygen starvation and reduced radical mobility, due to the presence of clay platelets, are blamed to be responsible for this oxidation profile. On the other hand, pure PPgMA showed an outstanding improvement of the fire behaviour with ageing time. Improvement that was attributed to cross-linking reactions which create a compact structure that is less inclined to volatilize. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polypropylene-graft-maleic anhydride; Nanocomposites; Photo-ageing; Thermal stability; Fire retardancy; Limiting Oxygen Index
1. Introduction Nanocomposite technology has been the topic of several scientific papers in the last decades. This enthusiasm is explained by the potential applications of nanocomposites in several areas such as packaging, film barrier, automotive and so on [1]. The starting point of its expansion occurred when the Toyota group [2] announced remarkable enhancement of polyamide-6
* Corresponding author. Tel.: C221 864 55 39; fax: C221 824 63 18. E-mail address: [email protected] (A. Tidjani). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.01.032
clay nanocomposite properties, containing only 5% clay. They found an increase of 40% in tensile strength, 68% in tensile modulus, 60% in flexural strength and 126% in flexural modulus, while the heat of distortion temperature increases from 65 to 152 C. Today in literature, numerous articles can be found dealing with production of nanocomposites using different techniques and host polymer, thermal stability, fire retardancy and permeability of nanocomposites. The typical methods used to produce polymer nanocomposites are: intercalation of a suitable monomer followed by polymerisation, polymer intercalation from solution, and direct polymer intercalation. Intercalation
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of polymer melts is the most attractive method because of its ease, its environmental benignancy and cost effectiveness. The current favorite nano-clay used is montmorillonite (MMT). The clay exists in a tactoid structure of 20e25 layers, which translates into an aspect ratio of about 10. Because of its hydrophobic nature, MMT is not compatible with most polymers; it must be modified and this is achieved by exchanging the cations in between the silicate layers by usually an alkyl ammonium ions. This operation increases the clay layer or basal spacing and reduces the attractive forces between the clay layers, that make possible the intercalation of the polymer and/or the exfoliation of the clays platelets. Indeed, introduction of an alkyl ammonium surfactant may offer sufficient excess enthalpy for most polar polymers to promote the nanocomposite. In the case of non-polar polymers, an addition of a compatibilizer is requested to enhance the diffusion of polymers into clay galleries. Nanocomposites are described as intercalated or exfoliated. In the first case, the registry between the clay layers is maintained oppositely to the second case in which the clay layers are dispersed leading to the loss of registry. Polypropylene (PP) e one of the most widely used polyolefins e has simulated intensive research in order to produce polypropylene nanocomposite. Besides the effort put to optimize the production of polypropylene nanocomposites (PPCNs), it has been reported an increase of thermal stability and mechanical properties, an improvement of fire retardancy, decrease of gas permeability [3e5]. One aspect which has been neglected so far, is the ageing of nanocomposites [6] and specifically the effect of ageing on the remarkable properties of these materials reported in literature. This investigation is of importance since it can be predicted that nanocomposites will probably replace conventional material. In their future use, they could be submitted to heat and/or light exposition that can alter their properties. In the following paper, investigation of the effect of photoageing on the thermal stability and fire retardancy properties (cone calorimeter, Limiting Oxygen Index) of PPCNs, prepared by extrusion and injection molding, is presented.
2. Experimental 2.1. Materials Polypropylene containing 0.6% of maleic anhydride was commercial product available from Aldrich Chemical Company. The presence of maleic anhydride as compatibilizer in PP is to favor the melt blending with modified clays used. Three modified montmorillonites (MMTs) in which, different organic cations were used to change the hydrophilic nature of layered silicates to make
419
them miscible with the hydrophobic character of polypropylene are experienced here in. In Cloisite 30B and Cloisite 20A e coming from Southern Clay Products, Inc. e modification has been done using a quaternary ammonium salt, methyl tallow bis2hydroxethyl (MT2EtOH) and dimethyl dihydrogenatedtallow (2M2HT), respectively. The third, Nanomer I28E from Nanocor, is a natural montmorillonite ion-exchanged with an Octadecyl trimethyl amine (OD3MA). 2.2. Preparation of the nanocomposites The melt compounding of PP with the modified clay was performed using an intermeshing twin-screw extruder Haake Rheomex TW100 for the first series of samples. All the ingredients were mixed simultaneously without any processing stabilizer. The major processing variables were screw speed, barrel temperature profiles; they were set at 50 rpm and, from 433 K at the first barrel to 458 K at the last barrel, respectively. The resulting material was pressed to produce plates or films with the aid of a hydraulic press equipped with two heated plates. Another series of specimen has been produced by extrusion followed by injection molding. The extrusion has been carried out in a ZSK 25 machine (Werner und Pfeiderer, Germany) through nine temperature zones ranging from 448 to 462 K followed by the injection at 453 K under a pressure of 900 bar and a post-pressure of 350 bar for 4 s. Whatever the experiment conditions, PPCNs with clay mass fraction of 5% were produced. 2.3. Instrumentation 2.3.1. Characterization of nanocomposites The commun techniques used to evaluate the state of mixing of nanocomposites are X-ray diffraction (XRD) and transmission electronic microscope (TEM). The first one helps to know if the registry between the clay layers is maintained as the polymer inserts (intercalation structure) or, if it is lost due to exfoliation of the clay layers. The second technique allows a direct observation of the state of org-MMT dispersion. A Philips X’Pert X-ray diffraction apparatus (CueKa radiation, l Z 0.154 nm) operating at a generator voltage of 40 kV and a generator current of 20 mA was used for XRD measurements. The samples were scanned from 1.2 to 10 in 2q at a rate of 0.3 /min. TEM was performed on a Philips CM20 equipment at 200 kV on specimen microtomed with a LKB 8800 Ultrotome III. 2.3.2. UV irradiation For photo-oxidation operations, plates or bar of nanocomposites were exposed to UV light: under accelerated conditions using a SEPAP 12-24 unit working
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at 60 C, and under natural conditions at Dakar (Senegal) located at 17 E longitude and 15 N latitude. The profile of oxidation of the samples was done using a Nicolet 800 FTIR spectrometer coupled to a Nic-plan microscope (128 scans summation, nominal resolution 4 cmÿ1). 2.3.3. Thermal analysis Thermogravimetric analysis (TGA) was carried out using a Mettler/Toledo TGA/SDTA 851. The TGA scans were recorded at a heating rate of 10 K minÿ1 under air atmosphere in the temperature range of 293e873 K. Temperatures recorded from TGA data are reproducible to G2% while the amount of char is reproducible to G2%. Sample masses were set between 15 and 20 mg. 2.3.4. Fire resistance Cone calorimeter and Limiting Oxygen Index (LOI) were used for fire retardancy properties. The cone calorimeter is designed to measure a range of important properties which characterize the fire behaviour of materials [7]. It has the ability to measure ignitability, the heat release rate, HRR, the mass loss rate, MLR, the evolution of smoke and the rate of production of various gas species with a reproducibility within 10%. These uncertainties are based upon many runs in which thousands of samples have been combusted [8]. The cone measurements were performed in duplicate with a FTT, UK device according to ASTME-1354-92 at an incident flux of 50 kW mÿ2 using a cone shape heater. Exhaust flow was set at 24 l/s and the spark was continuous until the sample ignited. The sample, in a form of plate (100 mm ! 100 mm, 5 mm thick), was placed in an aluminium foil then in a box having the same dimensions in the horizontal position. As for the LOI, it is designed to provide a precise method for determining the minimum volume concentration of oxygen in a flowing stream of oxygen and nitrogen required to maintain candle-like burning of that sample. LOI test was conducted according to the ASTM D286377/ISO 4589-2. The sample in a form of bar (100 mm long, 7 mm wide and 5 mm thick) was suspended vertically and ignited using a burner. The concentration of oxygen was increased if the specimen was extinguished before burning away for 3 min or 5 cm.
3. Results and discussion 3.1. Characterization of nanocomposites A first glance of the structure of the nanocomposites obtained can be determined using XRD technique from the position, shape and the intensity of the basal reflections in the XRD patterns. The XRD patterns of two specimens are shown in Fig. 1.
Fig. 1. XRD patterns of Cloisite 20A-2M2HT (a, curve 1) and Cloisite 30B-MT2EtOH (b, curve 1) and their nanocomposites produced by extrusion (curve 2) or by injection molding (curve 3).
For the XRD patterns of PP-2M2HT nanocomposites, the d001 peaks were broadened with low intensity and shifted towards lower angles whatever the mode of production. The diffuse nature of the d001 peak makes locating its center difficult to be able to estimate accurately their d-spacings; however their shift to lower angle is unquestionable. These results suggest either a formation of intercalated nanocomposites or a mixed intercalatedeexfoliated system (or simply a disordered system). An additional small peak is observed towards higher angle for samples prepared by injection molding. The TEM pictures displayed further down show that this peak corresponds to stack of platelets in which the d-spacing is reduced. In the case of PP-MT2EtOH, the situation depends on the blending conditions. For samples produced by injection molding, a similar situation that was described for PP-2M2HT is observed. For the extruded blending of PP-MT2EtOH, a small peak towards higher angle is observed indicating an unexpected decrease of the d-spacing since the shear stress of the extruder is supposed to help dispersion of clay platelets and/or intercalation of the polymer chains. Some authors attributed this decrease to a partial
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decomposition of the organic alkyl ammonium modifier during processing, leading to a decrease of the d-spacing. The loss of the ammonium salt and its replacement with a proton has been proposed to explain this observation [9]. Another explanation did not associate this decrease to the thermal degradation but rather to a collapse of the interlayer structure due to the shear force [10]. Such structural variation could be related with the transition from bilayer to monolayer arrangement. To complete the characterization of the nanocomposites produced, TEM micrographs of the corresponding samples e that give actual image of the clay e are presented in Figs. 2 and 3. Poor dispersion of the clay platelets and presence of tactoids are observed in Fig. 2a. The corresponding material should probably be described as immiscible system (the dominant phase) in which some clay layers are dispersed. This got the support of the XRD results. On the other hand, PP-MT2EtOH produced by injection molding displayed surprisingly a total dispersion of the clay platelets (Fig. 3b). Some may suggest that the hydroxyethyl present in MT2EtOH which is supposed to favor wettability of the clay by the maleic anhydride groups grafted onto PP, facilitating the production of nanocomposite, plays its role during the injection molding process and not during the extrusion alone. Partial explanation of this phenomena was given in a previous paper [11]. For PP-2M2HT nanocomposites, a good dispersion and a mixed intercalatedeexfoliated system were attained by extrusion and injection molding, respectively (Figs. 2b and 3a). These confirm the diffuse nature of the peaks obtained in XRD patterns of the corresponding specimen. One may conclude that a satisfactory level of dispersion is obtained with Cloisite 20A whatever the experimental conditions though some stacks were observed in injection molded samples. 3.2. Profile of oxidation It is well known that photo-oxidation of thick plates leads to heterogeneous distributions of the products in the matrix. This is due to difficulty of oxygen to diffuse leading to a starvation of oxygen in the bulk of the material [12]. As a result, distribution of the photoproducts within UV-irradiated specimen is requested to have an exact idea of the level of oxidation in the plate. Fig. 4 presents the plots of the absorbance of nonvolatile carbonyl oxidation products at 1714 cmÿ1 as a function of the distance from the exposed surface at two selected ageing times. The plots were drawn from spectra recorded every 15 mm by moving the analysed area under the axis of the objective. After 150 h of irradiation, no significant difference is noticed from the surface to the core. On the other hand, after 300 h of UV irradiation, one can observe a higher level of oxidation at the surface comparatively to the core of the sample.
Fig. 2. TEM micrographs of PP-MT2EtOH (a) and PP-2M2HT (b) nanocomposites produced by extrusion.
This profile of oxidation products is probably due to the difficulty of oxygen to diffuse in the mass of specimen leading to a starvation of oxygen in the core. The presence of silicate platelets may also drastically reduce the diffusion of oxygen [13] and the mobility of radicals created from the surface to the core of specimen. Some one can also suspect attenuation of the light absorption within the sample due to the presence of silicate layers. Note that the oxidation level at the surface was very much dependent on the modified clay used but not to the mode of nanocomposite production. For instance
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Cracks were observed on PP-OD3MA and PPMT2EtOH; these were commun on the surface of degraded polymers. On the other hand, spherulites were observed on PP-2M2HT surfaces. Similar spherulites were observed in our previous work on UV-degraded polyolefin films using a scanning electron microscope [14]. According to some authors [15,16], clay may serve as an efficient nucleation agent that promote formation of spherulites; nucleation efficiency increases as the degree of clay dispersion improves. Since these spherulites were only visible on UV-irradiated specimen, it can be suggested that the UV irradiations contribute to reveal these spherulites. Our results also indicate that the action of nucleating agent seems to be dependent on the type of clay used. Work is in progress in this laboratory to elucidate these points. There is no doubt that the chemical changes induced by the UV degradation process may have some effect on thermal stability and fire properties of the nanocomposites. The corresponding results are presented in the next sections. 3.3. Thermal degradation
Fig. 3. TEM micrographs of PP-2M2HT (a) and PP-MT2EtOH (b) prepared by injection molding.
after 300 h of UV irradiation in the SEPAP 12-24, the 1714 cmÿ1 absorbance at the specimen surface was 0.4 for PPgMA, 0.65 for PP-MT2EtOH, 0.7 for PPOD3MA and 1.85 for PP-2M2HT. It is worth mentioning that for samples blended by injection molding, the visual aspect of the degraded surfaces revealed changes comparatively to virgin nanocomposites and PPgMA. Photographs of these surfaces taken with Leica Makroskop M420 coupled with a digital camera DC200, !10 show differences (Fig. 5).
During the thermal degradation, the TGA curves displayed a single step degradation process for all samples produced. Table 1 collects the TGA data e corresponding to the temperatures at which 10% and 50% degradation occur, the peak in the derivative curve and, the fraction of non-volatile material at 873 K, referred as residue e of virgin and UV-degraded PPCN. The onset temperatures (T10%, T50%) of the nonirradiated nanocomposites are higher than that of virgin PPgMA. This enhancement observed by several authors [11,17,18], is more important for injection molded nanocomposites (from 16 to 55 C) compared to extruded nanocomposites (from 4 to 27 C). The changes of DTG parallel the changes in onset temperature while the amount of char residue remains almost constant. These findings are ascribed to the presence of clay layers which act as diffusion barriers; that may delay the decomposition process [11]. It can be concluded from these results and the ones on characterization that, the better the dispersion of the clay platelets, the higher the enhancement in thermal oxidation. This hypothesis is not valid for PP-MT2EtOH which, though displaying the better dispersion for samples produced by injection molding, shows a small enhancement of TGA parameters. From previous work in this laboratory, we attributed this behaviour to the sensitivity of EtOH group that may initiate the oxidation leading to a fragile material [11]. However, the presence of clay layers could not prevent deterioration of the thermal properties of the aged nanocomposites as registered in Table 1 with the shift of the onset temperatures towards lower temperature.
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Fig. 4. Variation of absorbance at 1714 cmÿ1 as a function of the distance from the irradiated surface in a 5 mm thick plate UV-irradiated at different times of ageing in an SEPAP 12-24: (-) after 150 h, (A) after 300 h.
Though the surface specimen was the main part of the plate to be apparently affected by the UV irradiation, a decrease in thermal stability is observed. This decrease seems to be similar for both types of samples presented herein. It could be speculated that the photo-ageing creates weak points that facilitate the thermal degradation in the bulk. These weak points could initiate the degradation from the surface to the core. 3.4. Cone calorimetry and LOI The assessment of flammability of nanocomposites is usually performed by cone calorimetry and/or Limiting Oxygen Index (LOI). The time to ignition, tign, the peak heat release rate, PHRR, and the time to PHRR, tPHRR, the average specific extinction area, SEA, a measure of a smoke and the mass loss rate, MLR, were measured with the cone calorimeter. Table 2, including all these parameters and visual observations during the cone tests of virgin specimen, is presented below. Looking first at the reduction in PHRR, the best results have been obtained for the nanocomposites prepared by injection molding apart from PP-MT2EtOH
nanocomposite. This reduction is accompanied by a decrease of the ignition time and the tPHRR without significant changes in the heat of combustion, Hc, smoke yield (SEA) and carbon monoxide yield (not shown in Table 2). This indicates that the improvement of fire behaviour occurs in the solid phase and not in the gas phase. A constant ratio between the heat release and the mass loss was observed for nanocomposites, suggesting that the improved combustion behaviour of nanocomposite is primarily due to the reduced mass loss. This is confirmed by the reduction of the mass loss rate values recorded for nanocomposites in comparison to PPgMA (Table 2). This reduction of the MLR tackles quite well the one observed for the PHRR. Notice that the MLR values were lower for samples produced by injection molding than for those prepared by extrusion. It is of importance to mention the disappointed fire behaviour of PP-MT2EtOH produced by injection molding; partial explanation of this was given previously. Concerning the visual observations of the nanocomposites through the cone tests, it has been extensively described elsewhere [13,18]. In brief, it is observed during the combustion tests that true nanocomposites swell
Fig. 5. From left to right, UV-irradiated surfaces of PP-OD3MA after 300 h in the SEPAP 12-24 and 4 months of natural ageing and, of PP-2M2HT after 300 h of accelerated ageing.
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Table 1 TGA data of virgin and UV-irradiated nanocomposites investigated Time
T10% ( C)
T50% ( C)
DTG ( C)
e
PPgMA
Virgin 450 h 4 months
310 304 314
376 378 380
411 387 392
7 7 7
PPgMAi
Virgin 450 h 4 months
317 328 326
378 394 394
407 408 402
6 6 6
PP-OD3MAe
Virgin 450 h 4 months
330 (C20) 307 318
401 (C24) 387 395
419 (C8) 405 417
9 10 9
PP-OD3MAi
Virgin 450 h 4 months
356 (C39) 331 350
416 (C38) 406 413
425 (C18) 420 424
11 11 10
PP-2M2HTe
Virgin 450 h 4 months
337 (C27) 317 330
408 (C32) 393 407
428 (C17) 414 425
9 9 9
PP-2M2HTi
Virgin 450 h 4 months
372 (C55) 361 375
430 (C52) 427 434
443 (C36) 439 446
10 10 10
PP-MT2EtOHe
Virgin 450 h 4 months
314 (C4) 301 322
387 (C11) 389 394
401 (ÿ10) 406 412
11 10 9
PP-MT2EtOHi
Virgin 450 h 4 months
333 (C16) 337 335
397 (C19) 404 399
416 (C9) 421 419
10 10 10
Samples
Residue (%)
Samples labelled (e) were produced by extrusion, those labelled (i) were injection molded.
with ageing time, whatever the irradiation conditions and their mode of preparation. This result is clearly seen in Fig. 7 that depicted the heat release rate of virgin PPgMA, virgin and UV-irradiated PP-2M2HT nanocomposites. Besides the increase of the PHRR of aged PPCN, a shift of this peak towards lower time and a reduction of the combustion time with ageing time could be noticed. These changes in PHRR are accompanied, under accelerated conditions, by a decrease of the ignition time for PP-OD3MA and PP-2M2HT and, an increase for PP-MT2EtOH and this, for both types of nanocomposites. Under natural conditions, tign decreased for
(or intumesce) with formation of a char layer; on the other hand, immiscible composite tends to boil, bubble, then drip. The cone tests performed on UV-irradiated specimen revealed some changes in the fire behaviour comparatively to virgin materials. The discussion will be focused on the PHRR, the mass loss rate and the time to ignition. The two first parameters are supposed to best describe the fire behaviour of the mass of specimen, as for the ignition time, it could best inform on the state of surface degradation of samples tested. The general tendency drawn from Fig. 6 is that nanocomposites displayed an increase of the PHRR
Table 2 Cone calorimeter average data of the nanocomposites blended by extrusion (first line) and by injection molding (second line) Sample
tign (s) PHRR (kW mÿ2) (% reduction) tPHRR (s) Mean Hc (MJ/kg) SEA MLR (g/s m2) (%reduction) Visual observations
PPgMA
35 32
1010 1012
294 220
34.66 31.14
580 436
PP-2M2HT
29 28
491 (52) 324 (68)
211 78
36.52 39.47
681 445
6.7 (59) 4.7 (68)
Swelling Swelling
PP-OD3MA
27 20
609 (40) 318 (69)
258 43
38.14 42.25
667 455
11.3 (32) 4.6 (67)
Swelling Swelling
PP-MT2EtOH 24 26
837 (17) 776 (23)
259 295
40.37 38.92
585 665
14.2 (13) 14.5 (0)
Small dripping Small dripping
16.5 14.3
Dripping Dripping
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Fig. 7. The HRR variations of virgin and UV-degraded PP-2M2HT nanocomposites comparatively to virgin PPgMA. The order in the box corresponds to order of curves from top to bottom.
PPgMA with ageing time but not for the nanocomposites for which it remains constant (Fig. 8). This behaviour parallels the one observed in the changes of PHRR with ageing time. These results tell that UV irradiation has a negative influence on the fire behaviour of nanocomposites as stated by the increase of the PHRR without significant changes in the mass loss. This could be explained by the presence of small oxidation products issue from the degradation, that are more keen to ignite. Since these oxidation products are located in the first microns of the plate, they can contribute to increase the PHRR but they cannot significantly change the fire behaviour of the bulk material. From these explanations, we should expect a decrease of the ignition time; this was the case for PP-2M2HT nanocomposites whatever the irradiation conditions. In the opposite, for the other nanocomposites, the irradiation conditions appear to play a role on tign. The outstanding fire behaviour improvement of
Fig. 6. PHRR changes of nanocomposites blended by extrusion and degraded under accelerated (a) and natural conditions (c) and, blended by injection molding and degraded under accelerated (b) and natural conditions (d). Correspondence of symbols: PPgMA (,), PPMT2EtOH (-), PP-OD3MA (B), PP-2M2HT (C).
PP-2M2HT and increased for PP-OD3MA and PPMT2EtOH. On the other hand, PPgMA displayed a surprising decrease of the PHRR accompanied by an increase and a decrease of tign under accelerated and natural conditions, respectively. Concerning the mass loss rate changes, a significant decrease is observed for
Fig. 8. Mass loss rate changes of PPgMA and injection molded nanocomposites UV-degraded. Correspondence of symbols: PPgMA (,), PP-MT2EtOH (-), PP-OD3MA (B), PP-2M2HT (C).
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the PHRR decreased with ageing time for nanocomposites but not the mass loss rate. This was due to weak points created by the UV degradation process on the samples. On the other hand, an outstanding improvement of fire properties was achieved on UV-irradiated PPgMA. This was explained by the cross-linking reactions that generate a compact structure which is less easily volatilizable. The LOI results obtained on virgin and UV-degraded specimen got the support of the cone results.
Fig. 9. LOI of virgin and UV oxidized samples investigated: PPCN0 (PPgMA), PPCN1 (PP-OD3MA), PPCN2 (PP-2M2HT), PPCN3 (PP-MT2EtOH).
UV-degraded PPgMA calls for other explanation. The decrease of its PHRR is accompanied by a decrease of its mass loss rate; this indicates that an improvement of fire behaviour occurs in the bulk. This improvement is probably due to cross-linking reactions that occur in the bulk because of the oxygen starvation mentioned previously. The cross-linking reactions generate a structure which is more compact and less easily volatilizable. The question that may be risen is why this cross-linking effect is not visible in UV oxidized nanocomposites. It can be blamed to the presence of clay platelets that reduces the formation of radicals by attenuating the UV lights, and also, prevents radical mobility. The flame retardant characteristics of the virgin and photo-oxidized PPCNs and PPgMA were also examined by measuring their LOIs. Fig. 9 reveals a very small decrease in LOI when silicate layer is incorporated in PPgMA, and whatever the state of mixing. These LOI values remained almost constant (taking into account error bars) for photo-aged nanocomposites under accelerated conditions. For the PPgMA, the LOI values fluctuated with ageing time. All these results are somehow consistent with the cone data.
4. Conclusion The true PP/clay nanocomposites exhibited improved thermal stability and fire retardancy properties at a clay loading of 5% by weight. The extent of the improvement was more for PP nanocomposites produced by injection molding than for those obtained by extrusion. The most interesting finding of this study was the dramatic change of these properties observed in the UV-degraded nanocomposites. The onset temperatures of degradation and
Acknowledgements The authors are indebted to the Vokswagen foundation for the financial support, the Federal Institute for Research and Testing (BAM) for the cone tests, Mr Claude da Silva from the universite´ de Mulhouse, France, for the X-ray measurements and, Dr Be´ne´dicte Mailhot from the Laboratoire de Photochimie de l’universite´ BP de Clermont, France, for the oxidation profile.
References [1] Revue de Sciences physiques, nanoscience, microe´lectronique, mate´riau, Mai 2004, N 11. [2] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. J Mater Res 1993;8:1185e9. [3] Gilman JW. Appl Clay Sci 1999;15:31e49. [4] Gorrasi G, Tortora M, Vittoria V, Galli G, Chiellini E. J Polym Sci Part B: Polym Phys 2002;40:1118e24. [5] Su S, Jiang DD, Wilkie CA. Polym Degrad Stab 2004;83:321e31. [6] Tidjani A, Wilkie CA. Polym Degrad Stab 2001;74:33e7. [7] ISO 5660e1, Part 1, 2002-12-15. [8] Gilman JW, Kashiwagi T, Nyden M, Brown JET, Jackson CL, Lomakin S. In: Al-Malaika S, Golovoy A, Wilkie CA, editors. Chemistry and technology of polymers additives. Oxford: Blackwell Science; 1998. p. 249e65. [9] Zanetti M, Camino G, Thomann R, Mulhaupt R. Polymer 2001;42:4501e7. [10] Kim KN, Kim H, Lee JW. Polym Eng Sci 2001;41:1963e9. [11] Tidjani A, Wald O, Pohl MM, Hentschel MP, Schartel B. Polym Degrad Stab 2003;82:133e40. [12] Jouan X, Gardette J-L. Polym Commun 1987;28:329e31. [13] Tidjani A. Polym Degrad Stab 2005;87:43e9. [14] Tidjani A, Arnaud R. Polymer 1995;36:2841e4. [15] Hambir S, Bulakh N, Kodgire P, Kalgaonkar R, Jog JP. J Polym Sci Part B: Polym Phys 2001;39:446e50. [16] Kodgire P, Kalgaonkar R, Hambir S, Bulakh N, Jog JP. J Appl Polym Sci. 2001;81:1786e92. [17] Su S, Wilkie CA. Polym Degrad Stab 2004;83:347e62. [18] Kashiwagi T, Harris RH, Zhang X, Briber RM, Cipriano BH, Raghavan SR. Polymer 2004;45:881e91.