montmorillonite composites: Effects of coupling agents and layered silicate

Polymer Degradation and Stability 91 (2006) 2874e2883 www.elsevier.com/locate/polydegstab

Properties of poly(vinyl chloride)/wood flour/montmorillonite composites: Effects of coupling agents and layered silicate Yongsheng Zhao, Kejian Wang*, Fuhua Zhu, Ping Xue, Mingyin Jia Institute of Plastics Machinery and Engineering, Beijing University of Chemical Technology, P.O. Box 60, District of Chaoyang, 15 BeiSanhuan East Road, ChaoYang District, Beijing 100029, China Received 7 July 2006; received in revised form 4 September 2006; accepted 5 September 2006 Available online 11 October 2006

Abstract Organomodified montmorillonite (OMMT) was prepared using cetylalkyl trimethyl amine bromide. OMMT and wood flour (WF) were surface-modified by silane coupling agent. They were melt-blended with polyvinyl chloride (PVC) and extruded into woodeplastic composite samples using one conical twin screw extruder. The effects of their contents on the composite mechanical properties were investigated. X-ray diffraction, transmission electron microscopy and scanning electron microscopy observed intercalation and dispersion of the OMMT. FTIR and X-ray photoelectron spectroscopy were used to analyze the silane-modification effects. The possible reaction mechanisms were proposed. After wood flour was modified by 1.5 phr silane, the impact strength and the tensile strength of wood flourePVC composite were increased by 14.8% and 18.5%, respectively. Mechanical tests showed that the addition of OMMT did not enhance the untreated wood flourePVC composites. However, adding 0.5% OMMT did improve the mechanical properties of the treated ones. The grafting improved the interfacial compatibility between components producing higher properties of the composites. Further addition of OMMT reinforced the composites. Too higher contents of silane and OMMT impaired some properties because of weak interfacial layer and higher concentrated stress. Cone calorimetry showed that the fire flame retardancy and smoke suppression of composites were strongly improved with the addition of OMMT. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Woodeplastic composite; Poly(vinyl chloride) (PVC); Fire retardancy; Montmorillonite

1. Introduction Wood flour and plastics composites (WFPCs) attracted a great interest in recent years owing to their relatively good properties and the lower costs. It is widely applied in various aspects with far-reaching friendly environment influences. Among them, wood flourepoly(vinyl chloride) (WF/PVC) composite has good market because of its good property combination of the mechanical properties, chemical stability and water resistance as well as its low cost. Most WF/PVC composites are used for decking, siding, indoor building material and so on [1]. With the development of WFPCs, highly

* Corresponding author. E-mail address: [email protected] (K. Wang). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.09.001

concerned problems are its interfacial incompatibility, poor fire resistance and weak impact strength for such applications as window profiles in building material area. In general, the impact strength of WFPC decreases rapidly with wood flour loading, extremely when the WFPC contains more wood flour than 50 wt%. In addition, the combination with wood flour would make the PVC material more easily combust because the wood flour burns so easily that the application of WF/PVC composite may be faced with more fire dangers. For these reasons, it is fundamental to develop WFPCs with higher mechanical properties and fire resistance. The incompatibility results in a poor interfacial adhesion between hydrophilic wood and the hydrophobic plastic matrix, which results in poor adhesion and therefore in poor ability to transfer stress from the matrix to the fiber reducing mechanical strengths and ductility. A lot of literature focuses on this

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problem by using such chemicals as coupling agents to improve the composite properties. Coupling agents include copper amine, silanes, maleic anhydride, and their grafting polymers [2e5]. However, unlike the common polyolefin/ wood-fiber composites, the well-known claim of converting the hydrophilic surface of wood to a hydrophobic one is not effective enough for enhancing the adhesion of PVC to wood fiber. Matuana and Mengeloqlu found that Lewis acidebase interactions for decomposition are significant in enhancing its interfacial adhesion. The aminosilane successfully promoted the interaction between wood fiber and PVC [6]. Therefore, the current authors exploited the effects of g-aminopropylmethyldiethoxysilane as compatibilizer in preparing WF/PVC composite. Another reason for choosing the aminosilane is that the induced acidebase reaction favors fire retardancy and smoke suppression [7]. As one building material, WF/PVC composite must strongly resist fire and suppress smoke. For the relative study progress see Ref. [7]. Regardless of good flame retardancy of pure PVC due to its high chlorine content (56.8%), flame retardancy of its WFPC is still another important topic because of easy flammability of high content of wood flour and other low-molecule additives as well as the subsequent thermal degradation. Besides, PVC can generate high levels of black smoke when it is forced to burn. In order to reduce heat release, to prevent smoke evolution and to drop the extent of burning, flame retardants and smoke suppressants are often incorporated into WF/PVC composites. Until now, many kinds of flame retardants and smoke suppressants have been developed. Metal oxide Sb2O3 flame retardant, halogenated organic flame retardants, molybdenum trioxide (MoO3) and ammonium octamolybdate smoke suppressant are ever typical effective ones. New developments in the area mostly focus on combinations of various flame retardants and smoke suppressants in the search for synergistic effects. For instance, phosphate esters are often used as flame retardant flame and smoke resistance. However, most of them can result in negative environment effects. Moreover, their flame or smoke resistance is very limited. Sb2O3 promotes dehydrochlorination of PVC. Thus, there are reservations about the general use of these retardants. Recently, inorganic flame retardants and fillers such as alumina trihydrate and magnesium hydroxide are widely used for the purpose. But the deployed loading level is more than 60 wt% causing serous disadvantages: the high density and the lack of flexibility of end products, the low mechanical properties and the problematic compounding and processing. Layered silicateepolymer nanocomposites have been proposed as a totally new and promising approach for the fire retardancy and smoke suppression of polymers besides remarkable improvements in some material properties such as mechanical strength, optical properties, electric properties and fire retardancy with as low loading of the nano-montmorillonite particles as only 5e10 wt% content to reduce maximum rate of heat release by 70% [8e10]. Montmorillonite (MMT) is a clay most commonly used in polymer nanocomposite preparation. It is a crystalline 2:1

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layered clay mineral with a central alumina octahedral sheet sandwiched between two silica tetrahedral sheets. When these nanoparticles are dispersed in a polymer, polymer/layered filler nanocomposites obtained are of three different types, namely (1) intercalated nanocomposites, (2) flocculated nanocomposites, and (3) exfoliated nanocomposites. The best performances are commonly observed for the exfoliated nanocomposites. However, previous studies about nanocomposites synthesis show that it is necessary to modify the silicate to strengthen the compatibility of it with polymer. Therefore, the present article reports the mechanical properties, fire retardancy and smoke suppression of the silane-modified WF/PVC composites filled by modified montmorillonite (OMMT). The effects of silane coupling agent for wood flourePVC and modifier of montmorillonite are highlighted. The modification of wood flour is characterized by X-ray photoelectron spectroscopy (XPS). The d-spacing, the dispersion of layered silicate and the interface between wood flour and PVC are observed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Its flammability is characterized by cone calorimeter. 2. Experimental 2.1. Materials PVC (SG-5 type) with number average molar mass being 1000 was supplied by Beijing Second Chemical Plant, Beijing, China. Wood flour from pine species (100 mesh size) was supplied by Beijing Ruize Ltd. Co. The used montmorillonite (MMT) named DKO with purity of 95e98% and density 0.45 g/cm3 was purchased from Zhejiang Fenghong Clay Lmt Co. Cetylalkyl trimethyl amine bromide (CTABr) was from Jiangsu Chemical Plant. Coupling agent of g-aminopropylmethyldiethoxysilane YH-62, NH2CH2CH2CH2SiCH3 (OC2H5)2]. One compound Pb-thermal stabilizer FP511 was purchased from Jiangsu Liyang Chemical Factory. 2.2. Preparation of organomodified MMT (OMMT) Seventy grams of montmorillonite (MMT) was dissolved into water of 1 l. After agitating for 1.5 h, cetylalkyl trimethyl amine bromide was dispersed into the solution by a ratio 110 mmol/100 g with MMT. The mixture was stirred for 3.5 h at 75  C. Homogeneous mixture was cooled to room temperature for deposition and filtration. The filtrate was washed using water three to five times until no light yellow deposit could be detected using 0.1 mol/1 AgNO3 solution. The organomodified MMT was dried in vacuum oven at 70  C to constant weight. The product obtained was grinded into 300 mesh-size powder which is abbreviated as OMMT in the following. 2.3. Treatment of wood flour with aminosilane Wood flour was vacuum-dried at 120  C for 6 h in an oven with circulating air. Before the surface treatment, the wood

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flour was Soxhlet-extracted with acetone for 24 h to remove contaminants or impurities on the surface of fibers and then dried adequately. Preweighed coupling agent YH-62 was added into 95% aqueous solution of ethanol in a weight ratio of 1:5 and then agitated into homogeneously modified solution for subsequent uses. The well dried wood flour was poured into high-intensity laboratory mixer followed by adding the modified solution. Upon finishing the addition of the required solution, the mixing continued for another 2 min. The silanetreated wood flours (STWF) were dried at 120  C for 1 h. The resultant was the modified wood flour.

2.7. Mechanical testing Tensile tests were conducted according to ASTMD 638-99 with a Universal Testing Machine (INSTRON 1185, UK). The tests were performed at crosshead speeds of 10 mm/min, and temperatures of 23  C. Unnotched Izod impact strength tests were conducted on an impact tester UJ-40, Hebei Chengde Materials Laboratory Machines Plant according to ASTMD 256-97 at room temperature. Each value obtained represented the average of five samples. 2.8. Cone calorimeter tests

2.4. Preparation of WF/PVC mixture and manufacture of WFPC composites Well dried wood flours, PVC and other additives were poured into the high-speed mixer and continued mixing for 8 min to obtain the unloaded mixtures. All samples were prepared by mixing 100 parts PVC resin, 70 phr (per hundred of resin) wood flour, 5 phr Pb-thermal stabilizer compound, 1.5 phr processing aid, 5 phr chloride polyethylene (CPE) and OMMT whose phr (content) depends on the formulations. The dried mixture was incorporated into conical twin screw extruder (KMD 2-50KK, KraussMaffei) with temperatures set at 155  C\165  C\160  C\150  C\170  C from hopper to die. The screw speed was 20 rpm. The extrudates were pelletized and cooled. All pelletized samples were fed into the conical twin screw extruder with temperatures set at 165  C\170  C\165  C\155  C\180  C from hopper to die. The panel (5 mm in thickness and 100 mm in width) was extruded at screw rotation of 15 rpm, and then cut into specimens for testing properties.

Combustion experiments were performed on a cone calorimeter (FTT Company, UK) at an incident heat flux of 50 kW/m2 according to ASTME 1354. Average values of the parameters were calculated from the three tests for each sample. 2.9. Morphological characterization X-ray diffraction (XRD) patterns were obtained using a diffractometer (Vercarl Zeiss Jena HZG4IB, GRD, Germany) with Cu Ka radiation (l ¼ 0.154 nm, 40 kV, 50 mA) at room temperature, scanning rate was 1 /min. The morphology structure of the composites was investigated by a Hitachi (Japan) H-800-1 transmission electron microscope (TEM) with an acceleration voltage of 100 kV. The ultrathin slides were obtained by sectioning the extruded samples. Scanning electron microscopy (SEM) photographs of the surface of the composite were taken on an SEM FP6800173 by FEI, USA. The surfaces were coated with gold at the accelerating voltage of 15 kV.

2.5. XPS study of aminosilane-treated wood flour 3. Results and discussion Fifteen grams of wood flour dried at 120  C for 6 h was added in a solution of YH-62, ethanol and water by ratios of 5 g:200 g:10 g using a flask. After well stirred for 1 h, the mixture solution was heated to 130  C and maintained for 30 min. Thus, the modified flour was maintained after being kept at room temperature for 24 h. The treated or untreated wood flour was mixed in acetone for 72 h. After that, they are extracted in a Soxhlet-extraction apparatus with acetone to remove the impurities and unreacted coupling agents. The samples were taken from it at 10 h interval after being initially distilled for 48 h. The samples were oven-dried at 105  C until constant weight was achieved. The unreacted aminosilane and the extractive on the sample surface were removed for XPS analysis. XPS spectra were recorded on an ESCALAB250 spectrometer (Thermo Company, USA) using Al Ka (1486.6 eV) radiation. 2.6. FTIR of silane-treated OMMT and silane-treated wood flour Using FT-IR60SXB, Nicolet, FTIR were conducted for samples of MMT, OMMT, silane-treated OMMT, wood flour and silane-treated wood flour.

3.1. The effects of amine bromide and silane on MMT Fig. 1 shows the XRD patterns of clays (MMT and OMMT). The MMT diffraction peak is at 2q ¼ 7.058 which corresponds to an interlayer d-spacing around 1.251 nm. After being treated, three peaks appeared at 2q ¼ 1.574 , 3.089 and 4.634 , from which d001 is calculated as 5.607 nm indicating that cetylalkyl trimethyl amine bromide seemed successfully dispersed into the clay gallery. From Fig. 2, it can be seen that the incorporation of OMMT into silane-treated wood flour (STWF)/PVC composite renders one obvious diffraction peak at 2q ¼ 2.082 suggesting that the clay d-spacing broadens to 4.241 nm, which is smaller than that of neat OMMT. This may be due to either further dispersed, even intercalated, clay by PVC chain under intense shear in twin screw extruder or exfoliated clay permitting the amine bromide moving outwards from clay gallery for thermo-decomposition of alkyl quaternary ammonium or reacting with some components of PVC compound. Thus, the XRD patterns indicating the possible larger interlayer spacing of OMMT in STWF/PVC composites prove that organomodified MMT was successfully

Y. Zhao et al. / Polymer Degradation and Stability 91 (2006) 2874e2883

Fig. 1. XRD patterns of (a) MMT and (b) OMMT.

prepared using CTABr as a surfactant. Amine bromide may play a key role in the intercalation of the surfactant into the gallery of MMT. Fig. 3 shows the dispersion state of the composites observed by TEM. When the loading level of OMMT into the WF/PVC is as low as 0.5% (Fig. 3a), OMMT was homogeneously and immiscibly dispersed in the composite or possibly partially exfoliated. Increasing the level of OMMT to 3%, the size of dispersed OMMT became larger or even aggregated in part. Based on the XRD in Fig. 2 and TEM result in Fig. 3, it is concluded that nanoscale component existed in OMMT/ STWF/PVC composites. The FTIR could further show the effects of amine bromide and silane on MMT as shown in Fig. 4. After intercalated by amine bromide, two absorbing peaks emerged at 2919.8 cm1

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Fig. 3. TEM microphotographs of fracture surface OMMT/STWF/PVC composites. (a) 100000 (OMMT 0.5 wt%); (b) 100000 (OMMT 3.0 wt%).

and 2850.3 cm1, which correspond to the tension and vibration of eCH2. Another appearing peak exists at 1470 cm1 illustrating the bending motion of eCH2e or eCH3, the groups belong to the long-chain alkyl of amine bromide. Further silane-modification does not change the three peaks implying that silane treatment did not ill-affect the formed dispersion or intercalation. However, there are some small variations after further silane treatment as shown in the magnified plots. From 3400 cm1 to 3100 cm1, two new tiny absorbing peaks are of the extensions for eNH2. One weak peak appears between 1620 cm1 and 1550 cm1 implying the vibration and bending of NeH. From 1350 cm1 to 1250 cm1, new peak is of the extensionbending for CeN. This means that silane was gratified onto MMT or the amine bromide. The reaction-formed SieOeSi peak is covered by strong SieOeSi peak of MMT at 1080e 1020 cm1. Regardless of this, it is well known that the hydroxy absorbed on MMT is apt to react with hydrolyzed silane. Thus, silane could couple the MMT and PVC to strengthen the interfaces. 3.2. The effects of silane on wood flour

Fig. 2. XRD patterns of composites: (a) WF/PVC and (b) 3.0 wt% OMMT/ STWF/PVC.

Fig. 5 presents the XPS spectra of untreated wood flour and wood flour treated by silane, respectively. The figures show the number of electron counted versus the binding energy in electron volts (eV). The elemental compositions of all the studied spectra are summarized in Table 1 as the relative mole percentages of the elements. It is seen that the contents of Si and N are increased remarkably after silane treatment. The doubly increases of Si and N derive from the silane YH-62, which reacts with wood flour. Si and N proportionally increase. Silane was hydrolyzed and grafted onto wood flour with appended Si and N. For better clarity, FTIR was used to analyze wood flour. As seen in Fig. 6, two peaks at 3350.3 cm1 and 3290.0 cm1 of eNH2 appear after wood flour was treated by silane. The NeH

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Fig. 4. FTIR of MMT, OMMT and silane-treated OMMT.

bendevibration absorbing peaks are at 1567.9 cm1 and 1548.6 cm1. The weak peaks at 1311 cm1 and 1191.8 cm1 represent the retraction for CeN of alkyl amine. The typical absorbing peaks of SieO bond exist at 1134.5 cm1 and 1042.2 cm1. Thus, it confirms the gratification of silane on

wood flour. Besides, the silane treatment results in such new peaks as those at 1661.2 cm1 for C ¼ O, at 1447.5 cm1 for CeN and at 1483.2 cm1 for C ¼ C of aryl. The peak-overlaps and -offsets may be due to many structures of flour or formed by the complex reactions. There were two major gratification reactions. One is shown in Fig. 7(a). Alkoxy of silane hydrolyzes to silica alkyd. The abundance of hydroxyl group on cellulose, hemicellulose and lignum in wood flour gratifies with silica alkyd. The other is shown in Fig. 7(b). The Lewis acid in wood flour reacts with amidogen of silane into ammonium. The carboxyl becomes amide or carboxymethyl cellulose by further bonding with cellulose. Fig. 8 shows the typical SEM micrographs of WF/PVC. For the fractured composite containing untreated WF, the wood flours were pulled out leaving clear flour cavities as shown in Fig. 8(a). The interphase clearance is obviously observed indicating weak adhesion. This is consistent with its lower mechanical properties as will be shown in the following. As surface-modifier, the coupling agent facilitates the affinity between WF and PVC resulting in stronger adhesion, the phase boundary is unconspicuous in Fig. 8(b). Furthermore, low OMMT-filling in the STWF/PVC exerted the nanoscale effects. The nanoparticles were well dispersed in resin matrix as illustrated in Fig. 8(c).

3.3. Mechanical properties of composites As shown in Fig. 9, the addition of OMMT into WF/PVC impairs both tensile strength and impact strength. Fig. 10 Table 1 Elemental composition mole ratios of untreated wood flour and silane-treated wood flour Elements

Fig. 5. XPS spectra of untreated WF (a) and silane-treated WF (b).

C O Si N

Untreated wood flour

Silane-treated wood flour

Variation

Percent (%)

Ratio to C (%)

Percent (%)

Ratio to C (%)

Percent (%)

59.36 33.35 4.32 2.97

100.00 56.18 7.28 5.00

59.05 27.15 7.59 6.21

100.00 45.98 12.85 10.52

0.31 6.20 þ3.27 þ3.24

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of 1.5%. The impact strength increases from 16.08 kJ/m2 to 19.34 kJ/m2 until silane is loaded up to 2.5%. After the highest optimal loading levels, its tensile strength and impact strength drop with further addition of silane. The interface is critical in improving the mechanical properties of WF/PVC composites. The incompatibility answers for a poor interfacial adhesion between hydrophobic PVC and hydrophilic wood flour without modification. This was worsened by incorporation of OMMT because the added OMMT did not improve the interfacial adhesion but increased stress concentration points. This explains the decreasing trends in tensile strength and impact strength of untreated WF/PVC filled with OMMT in Fig. 9. However, the silane coupling agent promotes the compatibility between wood flour and PVC resulting in the composites with higher strengths after coupling compared with untreated WF/PVC. The strong enhancement trend continues up to the loading levels at 1.5% for tensile strength and 2.5% for impact strength. Above the critical loading levels, overplus silane acted as plastisizer of WF/PVC impairing its strengths. Thus, the 1.5 wt% loading level of silane is chosen for the preparation of OMMT/ STWF/PVC in the following sections. To explore whether there are synergic effects of silane coupling agent and nanoclay, the tensile and impact properties of the OMMT-filled silane-treated wood flour (STWF)/PVC were tested. Fig. 11 illustrates that incorporation of OMMT into STWF/PVC improves the tensile strength in the filling range of 4.5% and the impact strength in the filling range of 1.5%. In fact, the small quantity of OMMT imparts strong amelioration, the composite tensile strength rises by 9.7% and the impact strength by 15.4% with added 0.5% OMMT. The largest values are all higher than those resulted by filling of either coupling agent or OMMT, which means the synergic influences. For the very low OMMT-loaded STWF/PVC composites, the interfacial aminosilane also adhered MMT to STWF and PVC, which helped the unitary structure to endure higher damage exhibiting higher impact strength and tensile strength. However, the impact strength decreased when loading level is above 0.5%, while the tensile strength slightly increases until at 1.5% and then drops. At higher content of OMMT, its

Fig. 6. FTIR spectra of differently treated wood flour: (a) untreated wood flour and (b) silane-treated wood flour.

indicated the effect of silane-treated wood flour on the mechanical properties of STWF/PVC composites. The tensile strength increases from 33.2 MPa to 39.2 MPa when the treated WF/PVC was filled with silane at the loading level

(a)

NH2(CH2)3Si CH3 (OC2H5)2

Hydrolysis

NH2(CH2)3Si CH3 (OH)2 + 2C2H5OH OH

NH2(CH2)3Si CH3 + H2O O Wood flour

(b)

NH2CH2CH2CH2Si CH3 (OC2H5)2+H-L

L- +[NH3CH2CH2CH2Si CH3 (OC2H5)2] C

NH2CH2CH2CH2Si CH3 (OC2H5)2 + —COOH

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C- NH1CH2CH2CH2Si CH3 (OC2H5)2 H2O

Fig. 7. Grafting mechanism of silane to the wood.

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Fig. 8. SEM microphotographs of fractured surface of WF/PVC composites. (a) Untreated WF/PVC; (b) 1.5% STWF/PVC; (c) 0.5% OMMT/STWF/PVC.

higher surface energy induced their re-agglomeration as shown in Fig. 8(b). The feeble large clusters of OMMT, which brought on more possible stress-damage points, cannot sustain higher outside force inducing earlier damage. Therefore, the impact strength was reduced to 16.9 kJ/m2 at 4.5% of OMMT from original 18.46 kJ/m2 of silane-treated WF/PVC. In addition, the impact strength of the composite peaks at 0.5% of OMMT while its tensile strength at 1.5%. Then, the composite tensile strength rises by 9.8% while its impact strength decreases by 7.0% with OMMT loading 3.0% compared with the none-OMMT ones. The obvious differences in tensile strength and impact strength were due to the loading speed in tests. The tensile speed was 10 mm/min, while the hammer momentarily fell in impact test. The longer function period allowed the composite to absorb more energy by creep deformation, thus the deformation could not occur in instantaneous impact so that the composite with more OMMT was apt to impact damage. However, our experiments revealed that the addition of 0.5% OMMT led to the highest impact strength, which was consistent with the results in Refs. [11,12]. Ren et al. showed that the impact strength of OMMT/PVC nanocomposites was still higher than pure PVC up to 5% of OMMT [13]. In contrast, our experiments found that the upper addition limit of OMMT in STWF/PVC was 1.5%. The

Fig. 9. The effects of OMMT content on the tensile strength and impact strength of the WF/PVC.

difference is probably because a large amount of wood flour contained in our composites could not be melted (WF/ PVC ¼ 70/100).

3.4. Flammability of the composites Cone calorimetry provides the detailed information about various parameters such as char residue (CR), time to ignition (TTI), heat release rate (HRR), time to peak heat release rate (TPHRR), smoke production rate (SPR) and total smoke production (TSP). These parameters describe flammability and smoke emission behaviors of OMMT/STWF/PVC in Figs. 12e14. The typical values are summarized in Table 2. Table 2 illustrates the general effects of organic MMT content on the flammability properties of STWF/PVC nanocomposites. CR is improved from 16.8% to 19.8%/21.7% when 0.5%/1.5% OMMT was incorporated into STWF/PVC. TTI rises from original 25 s for unfilled STWF/PVC to 38 s for 0.5% OMMT/STWF/PVC retarding by 52%, to 78 s for 4.5% OMMT/STWF/PVC retarding doubly. A significant improvement in flame retardancy of OMMT was noted especially at very low OMMT content (0.5 wt%).

Fig. 10. The effects of silane on the tensile strength and impact strength of WF/PVC.

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Fig. 11. The effects of OMMT content on the tensile strength and impact strength of OMMT/STWF/PVC.

The HRR parameter is thought to correlate with the heat release in a room burn situation. av-HRR records the average value of the heat release rate during the whole test period, more av-HRR means higher total heat release during burning

Fig. 12. The effect of OMMT content on HRR (kW/m2) versus time relations of OMMT/STWF/PVC.

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Fig. 13. The variation in THR versus OMMT content of OMMT/STWF/PVC.

test. Table 2 shows that the addition of OMMT can reduce the average heat release rate (av-HRR) a little but slightly increase the peak heat release rate (pk-HRR). Higher pk-HRR would accelerate the flame. The HRR plots of STWF/PVC composites with different loading of OMMT are recorded in Fig. 12. The average HRR, the peak HRR and the time to peak HRR are also calculated in Table 2. Comparison of HRR curves of STWF/PVC and those of OMMT/STWF/PVC shows that after adding OMMT, the TPHRR is delayed with increasing TTI. Moreover, a small peak A appears for samples with OMMT. Wood flour contains the cellulose, hemicellulose, lignin and other low molecular extractives, all of which can decompose above 150  C. There are also decomposable components in PVC compounds. Thermally radiant surface layer could be heated up to 150  C during cone calorimetry test in the first 25 s time induced some components in wood flour and PVC compounds to decompose

Fig. 14. The total smoke production (TSP) versus OMMT content of OMMT/ STWF/PVC.

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Table 2 Flame data of OMMT/STWF/PVC by cone calorimeter OMMT content (%)

0.0

0.5

1.5

3.0

4.5

CR (%) TTI (s) TPHRR (s) pk-HRR (kW/m2) av-HRR (kW/m2) av-SEA (m2/kg)

16.8 25 32 189.1 93.9 386.2

19.8 38 42 190.7 93.0 348.6

21.7 60 65 194.6 89.3 318.0

23.3 71 78 208.6 87.6 344.3

24.4 78 87 220.1 86.6 376.8

into a kind of small molecule corresponding to the small peak (the A-peak in the Fig. 12) in heat release rate curve. It is worth noting that the STWF/PVC started to burn at this time while OMMT/STWF/PVC did not. Obviously, the addition of OMMT delayed TTI and TPHRR. In Table 2, the TTI was 60 s and 71 s, TPHRR was 65 s and 78 s for the samples containing 1.5% and 3.0% OMMT, respectively. Therefore, two peaks appear (first small peak A at short time about 25 s and then pk-HRR at the TPHRR) for OMMT/WF/PVC, the latter corresponds to the ignition of the samples. The homogeneously dispersed or even intercalated/exfoliated clay structure may form inorganic shell-like till the ignition by heating. Previous experiments showed that the carbonaceous silicate char formed at the surface of the nanocomposite from start before ignition [14]. The inorganic-rich surface had better barrier property in result that the combustion of the nanocomposite was remarkably hindered. The inorganic overcoat acts as an excellent insulator of heat and oxygen transport barrier so that the ignition of the composites with low loading of OMMT was effectively delayed. Besides, aminosilane couples PVC and WF with OMMT, the stronger adhesion prevents the fire advance. The time to destruction of the char shield strongly depends on the formulation and the dispersed/intercalated structure of the compounds. More of OMMT incrassated formation of the char shield delays TTI and TPHRR and the thickened char shield needs higher temperature to break down. The longer TTI means the higher temperature of samples. Consequently the higher temperature makes the larger maximum combustion for combustible gases release violently and immediately after the time to collapse of the char shield on the sample surface. This explains why the TPHRR gets remarkable increase and the HRR becomes a little higher with more OMMT. However, non-flammability of MMT does not produce more heat, which implies the lower total flame heat for the composite with higher OMMT content as shown in Fig. 13. Average specific extinction area (av-SEA) (m2/kg) is a measure of smoke obscuration averaged over the whole test period. Smoke production rate (SPR) can be expressed as the SEA. The relationship of total smoke production (TSP), average SEA (av-SEA) and mass loss (ML) can be expressed as the following equation [15]: TSP ¼ av  SEA  ML

ð1Þ

The TSP data are calculated in Fig. 14. The TSP decreases first by 11.0% for OMMT content up to 0.5% or even 19.3%

up to 1.5%, and then increases with OMMT increment but still being lower than that of composite without OMMT. The smoke suppression by the ceramic skin on the surface of the samples was produced by condensed inorganic nano-dispersed layered silicate lamellae in the cone calorimeter which effectively protect nanocomposite from thermo-degradation. The TSP rises with more OMMT at loading level higher than 1.5%, which may be due to the re-aggregation of OMMT particles. In this case, cetylalkyl trimethyl amine bromide easily decomposes producing hydrogen bromide which enforces the PVC catalytic decomposition effects corresponding to higher TSP. The decompositions can be alleviated by more well dispersed when adding small quantity of OMMT [16]. OMMT effectively boosts flame retardancy and smoke suppression with increase in CR, remarkably postponing TTI and TPHRR and decreasing average HRR as well as effectively reducing TSP when OMMT being below 1.5%. Thus, a small quantity of OMMT reduces the smoke emission, which is favorable for visibility from around and helpful for persons to escape from fire. It also reduces the toxic smoke quantity, which is especially important for PVC. 4. Conclusions OMMT/STWF/PVC composites were successfully prepared by modification and melt-blending. Both the mechanical properties and the fire retardancy of the composites were improved by loading OMMT. (1) It was proved that organomodified MMT was successfully prepared using CTABr as a surfactant. Amine bromide may play a key role in the intercalation of the surfactant into the gallery of MMT. (2) Wood flour was successfully modified by silane for higher contents of Si and N as analyzed by XPS. (3) OMMT was well dispersed into STWF/PVC by twin screw extruder to prepare the nanocomposites. (4) OMMT did not reinforce the untreated WF/PVC. However, the reinforcement effects were appreciable after the surface of wood flour was modified by silane, the impact strength was improved by 20%. Addition of the organically intercalated MMT further enhanced STWF/PVC composites. Impact strength and tensile strength of STWF/PVC composites were boosted by 15.4% and by 9.7%, respectively, when 0.5% OMMT was added. (5) A small quantity of OMMT effectively improved the flame retardancy and the smoke suppression of STWF/PVC.

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