Preparation and properties of halogen-free flame-retarded polyamide 6/organoclay nanocomposite

Transcription

1 Polymer Degradation and Stability 86 (2004) 535e540 Preparation and properties of halogen-free flame-retarded polyamide 6/organoclay nanocomposite Lei Song a, Yuan Hu a, *, Zhihua Lin a, Shanyong Xuan a, Shaofeng Wang a,b, Zuyao Chen b, Weicheng Fan a a State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei , Anhui, PR China b Department of Chemistry, University of Science and Technology of China, Hefei , Anhui, PR China Received 19 May 2004; received in revised form 16 June 2004; accepted 18 June 2004 Abstract Halogen-free flame-retarded polyamide 6/organoclay (PA6/OMT) nanocomposite was prepared by using magnesium hydroxide (MH) and red phosphorus (RP) as a flame retardant and organoclay (OMT) as synergist via a melt blend technique. The morphology was characterized by XRD and TEM. The effects of organoclay on the mechanical properties and flammability of the PA6 were investigated. The results show higher mechanical and flame-retarded properties of the nanocomposite as compared with flame-retarded PA6 and a synergistic effect among OMT, MH and RP. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: PA6; Nanocomposite; Halogen-free; XRD; Magnesium hydroxide; Red phosphorus 1. Introduction Polyamide 6 (PA6) is an important engineering resin, which is widely used in fibre, film and engineering thermoplastics, and its thermal stability and fire resistance properties are always considered first. Magnesium hydroxide (MH) which has a high endothermic decomposition temperature (O340 C), is an effective flame retardant and smoke suppressant additive for polyamide 6 [1]. Many investigations have been done on the use of MH as a flame retardant in thermoplastics [2e4]. Many researchers have found that O50 wt.% loading of MH was required to obtain an adequate level of flame retardance. However, high loadings of MH would decrease the mechanical properties of the filled * Corresponding author. Tel./fax: C address: [email protected] (Y. Hu). polymer. MH combined with synergist red phosphorous (RP), can increase flame retardancy, and this combination can decrease the high loading level of MH needed [5]. In the past two decades, much attention has been focussed on polymer-layered silicate nanocomposites (PLSN), which represent a better choice to conventional filled polymers. Nanocomposite technology has been described as the new great frontier of materials science due to employing addition of a few (!5) wt.% of nanoclays, which enhance mechanical, thermal, dimensional and barrier performance properties significantly [6e9]. The studies about the flame-retarded properties of PLSN mainly demonstrate a significant decrease in the peak heat release rate (PHRR), a change in the char structure, and a decrease in the rate of mass loss during combustion in a cone calorimeter [10e18]. The first PA6/clay nanocomposites were reported as early as 1976 by Fujiwara and Sakomoto [19]. Another research team /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.polymdegradstab

2 536 L. Song et al. / Polymer Degradation and Stability 86 (2004) 535e540 from Toyota reported improved methods for producing PA6/clay nanocomposites using in-situ polymerisation in 1986 [20,21]. It is reported that the addition of conventional flame retardants, such as combination of decabromodiphenyl oxide (DB) and antimony trioxide (AO) or melamine cyanurate (MCA) to PA6/clay nanocomposites leads to more satisfactory performance in both the cone calorimeter and UL-94 test, compared with pure PA6 nanocomposites [10]. However, corrosive and toxic smoke and other gaseous by-products were also generated during the combustion of polymer containing halogen-based flame retardants. So the studies on halogen-free flame-retarded PA6/clay nanocomposites are worthwhile. In this work, flame-retarded PA6/OMT nanocomposite was prepared, in combination with MHeRP and OMT, by direct melt intercalation process. Then we studied mechanical and flame-retarded properties of the nanocomposite. 2. Experiment 2.1. Materials The original purified sodium montmorillonite (MMT, with a cation exchange capacity of 122 mmol/ 100 g) and organophilic montmorillonite (OMT) were kindly provided by Ke Yan Company. Organophilic montmorillonite (OMT) with an average particle size of about 5 mm was obtained from MMT by ion exchange reaction using hexadecyl trimethyl ammonium bromide (C16) in water according to a reported method [1]. Nylon 6 (PA6, 1003NW8) was supplied by Yubu Company, Japan. The magnesium hydroxide and red phosphorus with an average particle size of about 2 mm and 5 mm, respectively, are commercial products used as received The preparation of OMT Montmorillonite (MMT) was gradually added to a previously prepared solution of hexadecyl trimethyl ammonium bromide (C16), which was dissolved in hot distilled water at 80 C. The resultant suspension was stirred vigorously for 2 h. Then the treated montmorillonite, a white precipitate, was washed repeatedly with hot deionised water several times to remove C16 residue. The filtrate was titrated with 0.1 mol/l AgNO 3 until no precipitate of AgBr was observed, to ensure complete removal of bromide ions. The product was placed in a vacuum drying oven at 80 C for 12 h. Finally, the dried product was ground and screened to an average particle size of about 5 mm to obtain the OMT The preparation of samples All the samples (Table 1) were prepared by melt mixing PA6 with additives using a twin-roll mill (XK- 160, made in Jiangsu, China) at about 245 C for 10 min to yield samples and the roll speed was maintained at 50 rev/min. These samples were then compression moulded at about 250 C into sheets (3 mm thick) which would be cut into suitable shapes for fire testing Characterization The dispersibility of the silicate layers in the PA6 was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray diffraction experiments were performed on samples at room temperature by using a Japan Rigaku D/max -ra X diffractometer (30 kv, 10 ma) with Cu (l Z A ) irradiation at the rate of 2 /min in the range of 1.5e10. TEM specimens were cut from epoxy blocks with the embedded samples at room temperature using an ultramicrotome (Ultracut-1, U.K.). Thin specimens, 50e80 nm, were collected in a trough filled with water and then placed on 200-mesh copper grids. TEM images were obtained with Hitachi H-800 microscope at an acceleration voltage of 100 kv. Scanning electron microscopy (SEM) studies were performed on the charred residue using a Hitachi X650 scanning electron microscope. The mechanical properties of samples were measured with a Universal Testing Machine DCS-5000 (Shimadzu, Japan) at a cross-head speed of 25 mm/min according to the Chinese Standard GB The flammability was characterized by cone calorimeter, limiting oxygen index (LOI) and UL-94 test. Cone calorimeter experiments were performed in a Stanton Redcroft cone calorimeter according to ASTM under a heat flux of 50 kw/m 2. The experiments were repeated three times and the results were reproducible to within 10%. The cone data reported here are the average of three replicated experiments. The limiting oxygen index (LOI) was determined according to ASTM D The specimens used for the test were of dimension 100! 6.5! 3 mm. The UL-94 test was performed on sheets (3 mm thick) under the condition of the standard test (ASTM D635-77), which provided only a qualitative classification of the samples. Table 1 Sample identification and composition Identification Composition PA6 Pure nylon 6 PA6 C OMT 2 wt.% PA6-2 PA6 C MH 8 wt.% C RP 5 wt.% PA6-3 PA6 C OMT 2 wt.% C MH 6 wt.% C RP 5 wt.%

3 L. Song et al. / Polymer Degradation and Stability 86 (2004) 535e Result and discussion 3.1. Dispersibility of PA6/OMT nanocomposites Fig. 1 shows the XRD patterns of the MMT, OMT, PA6/OMT () nanocomposite and flameretarded PA6/OMT/MHeRP (PA6-3) nanocomposite. The average basal spacing (d 001 ) of MMT and OMT are 1.3 and 2.2 nm, respectively. When MMT is modified with C16, the galleries of MMT are intercalated and expanded by molecular chains of C16. and PA6-3 have broadened (001) plane diffraction peak at about 3.5 nm, 1.3 nm gallery height increase, when compared with OMT, which indicates an intercalatedeexfoliated morphology. The TEM for (Fig. 2) also confirms that mixed morphology has been formed in the PA6 matrix. Individual silicate layers are well-dispersed (Fig. 2a), and some multilayer particles (Fig. 2b) are visible, but it is difficult to determine the exact ratio of the two ingredients in the sample Mechanical properties The mechanical properties of pure PA6, PA6/OMT, PA6/MHeRP, and PA6/OMT/MHeRPare given in Table 2. When MH and RP are added, the tensile strength of sample PA6/MHeRP (PA6-2) decreases. This result indicates that the mechanical properties of PA6/MHeRP were reduced. This reduction is mainly due to agglomeration of flame retardants (MH and RP) and phase separation between flame retardants and bulk PA6. When OMT is loaded, the tensile strength of samples PA6/OMT () and PA6/OMT/MHeRP (PA6-3) increases, compared with pure PA6. More than 25% increase in strength is achieved by loading only 2 wt.% of the OMT. The OMT, which function as the Arbitrary intensity 0 PA6-4 OMT MMT θ (degree) Fig. 1. XRD patterns of MMT, OMT,, and PA6-3. Fig. 2. TEM of nanocomposite. fibre in a fibre-reinforced plastic, could enhance the tensile strength of the polymer. Although the loading of flame retardants reduces the tensile strength of samples, the loading of OMT offsets part of that reduction and improves the total tensile strength Flammability properties The cone calorimeter is one of the most effective bench-scale methods to study the flammability properties of materials. Heat release rate, in particular peak HRR (phrr), has been found to be the most important parameter to evaluate fire safety [22]. Fig. 3 shows that the combustion process of pure PA6 included two stages. The phrrs of PA6 appear at 120 and 170 s, respectively. OMT and MHeRP have distinct effects on the heat release rate (HRR) of PA6. Table 3 lists the maximum values of peak heat release rate (phrr) of samples. The maximum phrr values of PA6/OMT (), PA6/MHeRP (PA6-2) and PA6/OMT/ MHeRP (PA6-3) are reduced, respectively, about 39%, 59% and 73% in comparison with the pure PA6. Fig. 4 and Table 3 shows the char residue weight increases in the order of PA6-3 O PA6-2 O PA6-1 O PA6. This trend is the same as that of the HRR in the cone calorimeter. Thermal decomposition of the OMT takes place at about 200 C and proceeds according to the Hoffman degradation mechanism [23]. C16 loses an olefin and an amine, and leaves an acid proton on the surface of the Table 2 The mechanical properties and the results of UL-94 test of samples Samples Tensile strength (MPa) UL-94 test LOI PA Burning Burning 21.5 PA V-0 29 PA V-0 31

4 538 L. Song et al. / Polymer Degradation and Stability 86 (2004) 535e540 Heat Release Rate (kw/m 2 ) PA6 PA6-2 PA6-3 Weight (wt.%) PA6 PA6-2 PA Time (s) Fig. 3. Heat release rate (HRR) data for pure PA6, PA6/OMT nanocomposite, P6/MH-RP, and PA6/OMT/MH-RP nanocomposite. MMT in the place. These acid protons on the surface of MMT have a catalytic effect on the initial stage of decomposition of organic within the OMT. Furthermore, acids, even in trace amount, effectively accelerate thermal decomposition of aliphatic polyamides [24]. The combustion of includes two stages same as in PA6. The first phrr of appears at 125 s, and is higher than that of PA6. The higher phrr in the early stage of combustion might attribute to the decomposition of C16 and the catalytic effect of catalytic acid sites compared with combustion of pure PA6 which is shown in Fig. 3. The second phrr of appears at 170 s, and is much lower than that of PA6. Studies [10e18] have suggested that the lower flammability of polymer/clay nanocomposites is due to the formation of carbonaceous char in the condensed phase. The nano-dispersed silicate layers drastically influence reaction kinetics, product transfer, and volatilisation, with enhancement of the char formation in the polymer matrix while burning. The nanocomposite forms char with a multilayered carbonaceous-silicate structure. This high-performance carbonaceous-silicate char forms on the surface of burning polymer, insulates the underlying polymeric substrate and slows down heat and mass transfer between the gaseous and condensed phases. Thus, it retards the thermo-oxidative degradation of the polymer. The lower phrr in the later stage of PA6 combustion might be attributed to that barrier Table 3 The values of peak heat release rate and peak mass loss rate of samples Samples Peak HRR (kw/m 2 ) Char residue (wt.%) PA PA PA Peak MLR (g/m 2 s) Time (s) Fig. 4. Mass loss data for pure PA6, PA6/OMT nanocomposite, P6/MH-RP, and PA6/OMT/MHeRP nanocomposite. effect of clay compared with combustion of pure PA6 which is shown in Fig. 3. The loading of MH and RP to pure PA6 and has a great effect on the combustion test (shown in Figs. 3e5). When MH and RP were loaded into PA6, the phrr value of PA6-2 decreases from 1120 to 463 kw/cm 2. While 2 wt.% MH was replaced by equivalent OMT, the phrr value of PA6-3 decreases from 463 to 308 kw/cm 2. Considering the flameretarded effect of MH [25,26], OMT and phosphorus species flame retardant [27], the synergistic effects of OMT on MHeRP probably take place as follows. At about 200 C, C16 degrades to form a catalytic acid site (acid proton) on MMT layer. These catalytic acid protons promote the endothermic decomposition of MH (Mg(OH) 2 / MgO C H 2 O). At the same time, the silicate layers of MMT retard the release of water vapour from the decomposition of MH. The water Mass Loss Rate (g/m 2 s) PA6 PA6-2 PA Time (s) Fig. 5. Mass loss rate (MLR) data for pure PA6, PA6/OMT nanocomposite, P6/MHeRP, and PA6/OMT/MHeRP nanocomposite.

5 L. Song et al. / Polymer Degradation and Stability 86 (2004) 535e vapour and acid accelerate decomposition of RP, and then lead to the formation of highly crosslinked polyphosphoric acid derivatives in the flame. The water vapour and acids may further facilitate the thermooxidation degradation of PA6, crosslinking and charring. During combustion, the polyphosphoric acid derivatives react with decomposition of PA6, MgO and OMT to form a glassy coating and stable carbonaceous charred layers. This stable physical protective barrier on the surface of polymer materials may insulate the underlying polymer substrate from the heat source and slow down heat and mass transfer between the gaseous and condensed phases. From 50 s to 125 s, HRR of PA6-3 is higher than that of PA6-2, which is attributed to the decomposition of C16, the catalytic effect of acid proton and the barrier effect of silicate layers of OMT. After 125 s, HRR of PA6-3 is lower than that of PA6-2. This phenomenon is attributed to the barrier effect of the clay layers and char formation. Apparently, there are synergistic effects among clay, MHeRP and PA6. The other primary parameter which is relevant to HRR of the samples is the mass loss rate (MLR) during combustion. Fig. 5 shows that the trend of the evolution of the MLR is the same as the change of the HRR (Fig. 4). This also indicates that the flame retarding mechanism of MHeRPeOMT happens mostly in the condensed phase. SEM studies show that the charred residue forms loose porous net structure after burning of PA6-2 (Fig. 6a). The charred residue of PA6-3 containing organoclay and flame retardants (Fig. 6b) is more compact than that of PA6-2 (Fig. 6a) containing only flame retardants. The number and size of those holes are decreased greatly along with the addition of organoclay. Moreover, the study of the microstructure of the charred residue shows that the surface of the charred residue of PA6-3 (Fig. 6d) is more homogenous and compact than that of PA6-2 (Fig. 6c). These results imply that organoclay promotes the formation of a ceramic-like material with a homogenous compact surface; this homogenous compact surface protects the underlying materials and results in the formation of a compact carbonaceous charred layer and reinforces the mechanical strength of the charred layer. The homogenous compact charred layer more effectively protects the underlying polymeric substrate from the heat source and slows down heat and mass transfer between the Fig. 6. SEM of charred residue of PA6-2 (a, b) and PA6-3 (c, d).

6 540 L. Song et al. / Polymer Degradation and Stability 86 (2004) 535e540 gaseous and condensed phases. These lead to yield the results of HRR and MLR. The limiting oxygen index (LOI) test is still widely used to evaluate flame retardant properties of polymer materials. The LOI values of samples are listed in Table 2. The LOI values of samples increase with loading of MHeRP. The LOI values of samples loaded 2 wt.% OMT increase in the range of 0.5e2.5. The LOI value of increase 0.5 as compared with pure PA6, but the LOI value of PA6-3 increase 2 as compared with PA6-2. It is apparent that OMT has a synergistic effect on the LOI values of flame-retarded PA6 containing MHeRP. It is well-known that the UL-94 makes a qualitative classification, namely, burning or extinguishing of the samples assessed by cone calorimetry. cannot achieve V-0 degree because of its burning. PA6-2 and PA6-3 can achieve V-0 degree in the UL-94 test, indicating some compatibility and synergistic effects among PA6, MHeRP and clay (Table 2). 4. Conclusion Flame-retarded PA6/organoclay nanocomposites can be prepared by melt intercalation blending PA6, OMT and flame retardant additives (MH and RP). Exfoliatedeintercalated morphology of PA6/organoclay nanocomposite forms in the PA6 matrix. The addition of OMT improves the mechanical properties of the nanocomposites. Cone calorimeter experiments, LOI and UL-94 tests show that a synergistic effect occurs as MHeRP and OMT added in PA6. The flame retardant property of PA6 containing MHeRP and OMT is better than PA6 containing MHeRP or OMT. Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No ), the China NKBRSF project (No. 2001CB409600). References [1] Hornsby PR, Wang J, Rothon R, Jackson G, Wilkinson G, Cossick K. Polym Degrad Stab 1996;51:235e49. [2] Hornsby PR, Watson CL. Plast Rubber Compos Process Appl 1989;11:45. [3] Hornsby PR. Fire Mater 1994;18:269. [4] Larcey PA, Redfern JP, Bell GM. Fire Mater 1995;19:283. [5] Wang ZZ, Qu BJ, Fan WC, Huang P. J Appl Polym Sci 2001;81: 206. [6] Hu Y, Song L, Xu JY, Yang L, Chen ZY, Fan WC. Colloid Polym Sci 2001;279:819e22. [7] Wang SF, Hu Y, Song L, Wang ZZ, Chen ZY, Fan WC. Polym Degrad Stab 2002;77:423. [8] Song L, Hu Y, Li BG, Chen ZY, Fan WC. Int J Polym Anal CH 2003;8:317. [9] Alexandre M, Dubois P, Sun T, Garces JM, Jerome R. Polymer 2002;43:2123. [10] Hu Y, Wang SF, Ling ZH, Zhuang YL, Chen ZY, Fan WC. Macromol Mater Eng 2003;288:272. [11] Bourbigot S, Bras ML, Dabrowski F, Gilman JW, Kashiwagi T. Fire Mater 2000;24:201. [12] Bourbigot S, Devaux E, Flambard X. Polym Degrad Stab 2002; 75:397. [13] Zhu J, Wilkie CA. Polym Int 2000;49:1158. [14] Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP, et al. Chem Mater 2000;12:1866. [15] Hartwig A, Putz D, Schartel B, Bartholmai M, Wendschuh- Josties M. Macromol Chem Phys 2003;204:2247. [16] Wang SF, Hu Y, Li ZL, Wang ZZ, Zhuang YL, Chen ZY, et al. Colloid Polym Sci 2003;281:951. [17] Zanetti M, Camino G, Canavese D, Morgan AB, Lamelas FJ, Wilkie CA. Chem Mater 2002;14:189. [18] Tang Y, Hu Y, Wang SF, Gui Z, Chen ZY, Fan WC. Polym Int 2003;52:1396. [19] Fujiwara S, Sakomoto T. Japanese Patent Application No (1976) [assigned to Unichika K.K., Japan]. [20] Okada A, Fukushima Y, Kawasumi M, Inagaki S, Usuki A, Sugiyama S, et al. United States Patent No (1988) [assigned to Toyota Motor Co., Japan]. [21] Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y, Kurauch T, et al. J Mater Res 1993;8:1179. [22] Gilman JW. Appl Clay Sci 1999;15:31. [23] Xie W, Gao ZM, Pan WP, Hunter D. Chem Mater 2001;13:2979. [24] Levchik SV, Weil ED, Lewin M. Polym Int 1999;48:532. [25] Lewin M. Polym Adv Technol 2001;12:215. [26] Wu Q, Lu JP, Qu BJ. Polym Int 2003;52:1326. [27] Dabrowski F, Le BM, Cartier L, Bourbigot S. J Fire Sci 2001; 19:219.