Effect of silica nanopowder on the properties of wood flour/polymer composite.
Wood polymer composite (WPC) covers a wide range of area in composite field (1). These are eco friendly, low cost consuming, biodegradable, and renewable (2). The consumption of plastic materials has increased enormously due to their various advantages. The disposal of post-consumer plastic materials in the form of carry bags, packaging films, boxes, etc. causes environmental pollution. The majority of the waste plastic materials consist of a substantial amount of polyethylene, polypropylene and less amount Of poly (vinyl chloride), polystyrene, poly (ethylene terephthalate), etc. Recycling and reusing is one of the processes to reduce the environmental pollution caused by post-consumer plastic materials. The use of recycled plastic materials is restricted due to their poor mechanical properties. The properties can be improved if the waste plastics are combined with cellulosic materials. The nonconventional plant materials can be transformed into value added products by treating with waste plastics. In solution blending, the knowledge regarding. the proportion of different polymers in the waste plastic is essential to optimize the solvent ratio. The situation becomes further critical due to the problems like separation and segregation of different kind of plastics, labels of the container. adhesives in the packaging film. etc. To overcome all these problems. it has been decided to use virgin polymers such as high-density polyethylene. polypropylene, polyvinyl chloride) of different ratio instead of waste plastics for making composites. Ordinary polymer composites are lacking in dimensional stability, thermal stability. mechanical properties, etc. Incorporation of wood flour to the polymer matrix enhances all these properties (3). The wood flour acts as reinforcing agent to the polymer and hence improves various properties. One of the major disadvantages of WPC is the poor interfacial adhesion between inorganic wood flour and organic polymer. This results in poor compatibility among the constituents and hence decreases the properties of the composites. To improve the miscibility between organic and Inorganic phase, certain compatibilizers are used The compatibilizer enhances interaction between hydrophilic wood fibers and hydrophobic polymers and at the same time improves the interfacial adhesion among different thermoplastic materials. The addition of a small amount of maleated polypropylene to PP/cellulose composite has improved the mechanical properties of the composites (4). The uses of glycidyl methacrylate (5) and polyethylene-co-glycidyl methacrylate (6) as compatibilizer for making wood polymer composite have been reported.
In the polymer composites. different types of tillers are used for improving the thermal, mechanical as well as other properties. Among them, nanopowder is widely used as filler. The surface characteristics of nanopowders play a key role in their fundamental properties from phase transformation to reactivity. Due to a large fraction of surface atoms and a higher surface area, a nanopowder would he expected to be more active. A dramatic increase in the interfacial area between fillers and polymer can significantly improve the properties of the polymer (7). Different types of metal oxide nanoparticles such as [Si0.sub.2] [TiO.sub.2], ZnO are widely used for these purposes. These are nontoxic, stable, and highly thermostable inorganic filler. Owing to all these properties, these are widely used in all types of materials like plastics, rubbers.
In polymer composites, [SiO.sub.2], nanopowder is one of the widely used fillers (8). [SiO.sub.2] can enhance the mechanical as well as thermal properties of the composite. Polymer--[SiO.sub.2] composites are of technological importance due to their potential applications in electrochromic windows, fuel cells, chemical separation, electrochemical sensing, and water treatment (9). [SiO.sub.2] nanoparticles increase the tensile and impact strength of epoxy nanocomposite (10). To increase the hydrophobicity of the inorganic silica particles, the surface has been modified by different silane compounds (11)
In this communication, we report the modification of [SiO.sub.2] by treatment with cetyl trimethyl ammonium bromide and study the effect of [SiO.sub.2] on various properties of composites based on wood, PE-co-GMA, and polymer mixture of HDPE/LDPE/PP/PVC.
HDPE and LDPE (Grade: PE/20/TK/CN) were collected from Plast Alloys India Ltd. (Harayana, India). PP homopolymer (Grade: H110MA, MRI 11 g/10 min) and PVC (Grade: SPVC FS: 6701) were collected from Reliance Industries (Mumbai, India) and Finolex Industries (Pune, India), respectively. The compatibilizer poly (ethylene-co-glycidyl methacrylate) (PE-co-GMA) (Otto chemicals, Mumbai, India), N-cetyl-N, N, N-trimethyl ammonium bromide (CTAB) (Central Drug house, Delhi, India) and [SiO.sub.2] nanopowder (5-15 nm, 99.5% trace metals basis) (Aldrich, China) were used as such received. A nonconventional wood, Nals (Phragmites karka) was collected from local forest of Assam. Other reagents used were of analytical grade.
Preparation of Wood Samples
Nals (P. karka), a kind of soft wood, is available in the forest of Assam. It was collected and chopped into small strips. Wood contains lignin and different types of other extraneous materials which may cause problem in the adhesion and improvement in other properties of the final composite. So, to remove all this extraneous materials, it is required to do pretreatment of the wood strips. These were initially washed with 1% soap solution followed by washing with 1% NaOH solution and finally with cold water. The washed wood strips were oven dried at 100 [+ or -] 5 [degrees]C till the attainment of constant weight. These dried wood strips were grinded in a mixer, sieved, and kept for subsequent uses. Modification of [SiO.sub.2]
About 10 g of [SiO.sub.2], was placed at room temperature in a round bottom flask containing 1:1 ethanol--water mixture under stirring condition. The temperature of the flask was raised to 80 [degrees]C and the stirring was continued for 12 h. About 12 g of CTAB was taken in a beaker containing ethanol--water mixture and stirred at 80 [degrees] C for 3 h. Under stirring condition, this mixture was added slowly to the flask containing [SiO.sub.2] mixture. The stirring was continued for 6 h. The mixture was filtered and washed with deionized water for several times. It was collected, dried in vacuum oven at 45 [degrees] C, grinded, and stored in desiccator to avoid moisture absorption. CTAB-modified [SiO.sub.2], was dispersed in a flask containing xylene and kept for checking of any settling of particles. The dispersion of [SiO.sub.2] was found to be stable. This indicated that proper modification occurred.
Preparation of Wood Polymer Nanocomposite
The minimum ratio of xylene and THF, at which a homogenous solution of HDPE, LDPE, PP, and PVC was obtained, was optimized as 70:30. About 6 g each of HDPE, LDPE, and PP (1:1:1) were added slowly to 105 ml of xylene taken in a flask fitted with a spiral condenser at room temperature. This was followed by the addition of the PE-co-0MA (5 phr). The temperature of the flask was increased from room temperature to 130 [degrees] C in order to make a homogenous solution. Now, another solution containing 3 g of PVC in 35 ml of tetrahydrofuran (THF) was prepared. The temperature of the polymer solution containing HDPE, LDPE, and PP was brought down to 120 [degrees] C. To this, PVC solution was added gradually and stirring was continued at 120 [degrees] C (approximately) for 1 h. A known quantity of CTAB-modified [SiO.sub.2], nanopowder (1-5 phr) was dispersed in 15 ml of tetrahydrofuran (THF) solution by sonication. This dispersed mixture was added gradually to the polymer solution under stirring condition. Oven dried wood flour (WF) (40 phr) was added slowly to this solution. The whole mixture was stirred for another 1 h. The mixture was transferred to a tray, dried, and grinded. The composite sheets were obtained by the compression molding press (Santec, New Delhi) at 150 [degrees] C under a pressure of 80 MPa.
Polymer blend (HDPE LDPE + PP + PVC), polymer blend/5 phr PE-co-GMA, and polymer blend/5 phr PE- co-GMA/40 phr wood were designated as PB, PB/G5, and PB/G5/W40, respectively. WPC filled with 1, 3, and 5 phr [SiO.sub.2] were designated as PB/G5/W40/S1, PB/G5/ W40/S3, and PB/G5/W40/S5, respectively.
X-Ray Diffraction (XRD)
The degree of [SiO.sub.2] distribution in the WPC was evaluated by X-ray diffraction (XRD) analysis. It was carried out in a Rigaku X-ray diffractometer (Miniflax. UK) using CuKx. (z = (1.154 mn) radiation at a scanning rate of 1/min with an angle ranging from 2 to 70 .
Transmission Electron Microscopy (TEM)
Thin portion of the WPC samples was prepared by using microtome. This thin portion was used for TEM analysis. The study of the dispersion of [SiO.sub.2], nanoparticles in WPCs was performed by using Transmission Electron Microscope (JEM-100 OK lb at an accelerated voltage of 20-100 kV.
Fourier Transform Infrared Spectroscopy (FTIR) Studies
FTIR spectra of wood flour, [SiO.sub.2], nanopowder, and WPC loaded with [SiO.sub.2] nanopowder were recorded in FTIR spectrophotometer (Impact-410, Nicolet, USA) using KBr pellet.
Scanning Electron Microscopy (SEM)
The compatibility among different polymers as well as morphological features of the WPC was studied by using Scanning Electron Microscope (JEOL JSM-6390LV) at an accelerated voltage of 5-10 kV. Fractured surface of samples for flexural test carried out at room temperature was considered for SEM analysis. The fractured surface deposited on a brass holder and sputtered with platinum were used for this study.
The tensile and flexural tests for polymer blend. PE-co-GMA-treated polymer blend. and WPC loaded with different percentage of [SiO.sub.2] were carried out using Universal Testing Machine (Zwick, model Z010) at a crosshead speed of 10 mm/min at room temperature according to ASTM D-6138 and D-790, respectively. Three samples of each category were tested and their average values were reported.
The hardness of the samples was measured according to ASTM D-2240 using a durometer (model RR 12) and expressed as shore D hardness.
Linuting Oxygen Index (LOI)
LOI of the samples were measured by flammability tester (S.C. Dey. Kolkata) according to ASTM D-2863 method. The total volume of the gas mixture ([N.sub.2] + [O.sub.2]) was kept fixed at 18 cc. To begin with experiment. the volume of nitrogen gas and that of oxygen gas were kept initially at a maximum and minimum level. Now, the volume of nitrogen gas was decreased and that of oxygen as was increased gradually. However, the total volume of gas mixture was kept fixed at 18 cc during the experiment. The sample was placed vertically in the sample holder of the LOI apparatus. The ratio of nitrogen and oxygen at which the sample continues to burn for at least 30 s Was recorded.
Limiting oxygen index (LOI) = volume of [0.sub.2]/ volume of ([O.sub.2] + [N.sub.2]) x 100.
Thermal properties of polymer blend and the WPCs were measured in a thermogravimetric analyzer (TGA) (MA-50, shimadzu) at a heating rate of 10 C/min up to 600 [degrees] C under nitrogen atmosphere at the rate of 30 ml/m.
WPC samples were cut into blocks of (2.5 x 0.5 x 2.5) [cm.sub.3] for this study. Percentage water uptake was measured by submerging the samples in distilled water at room temperature (30 C) and weights were measured after 12, 24, 36, 48 60. and 72 h. It is expressed as
Water uptake(%) = ([W.sub.s] - [W.sub.1])/ [W.sub.1] x 100
where [W.sub.s] is the weight of the water saturated specimen and [W.sub.1] is the weight of the oven dried specimen.
RESULTS AND DISCUSSION
Figure 1 shows the plots al intensity versus scattering angle. 20. for neat polymer blend, the [SiO.sub.2], nanopowder and 1, 3, and 5 phr [SiO.sub.2]-loaded WPCs. The curve representing neat polymer blends (Fig. 1a) showed some sharp diffraction peaks and diffusion peaks which revealed the presence of crystalline and amorphous region, respectively. The most prominent X-ray scattering peaks appeared at 20 = 14.12 (200) (PP), 17.06 (040) (PP, PVC), 18.64 (211) (PP), 21.62 (110) (PE), and 24.02 (200) (PE, PP, PVC) were for the crystalline portion of different polymers in the polymer blend (12-15).
Figure 1b shows a broad diffraction peak at 20 = 23.5 for the amorphous [SiO.sub.2] nanoparticles. In WPC loaded with different percentages of [SiO.sub.2] (Fig. lc--e), a decrease in intensity of the crystalline peak at position 21.62 (110) was observed. The intensity of other crystalline peaks was also found to decrease. The decrease in intensity of the crystalline peaks was attributed to the enhancement of the amorphous portion in the composites caused by the addition of amorphous [SiO.sub.2] nanoparticles and wood flour. Mina et al. (12) prepared isotactic polypropylene/ [TiO.sub.2]] composite and found that the intensity of the crystalline peaks of PP decreased with increasing
the amount of [TiO.sub.2]. This indicated that [SiO.sub.2] particles were infused into the WPC.
Figure 2 shows the TEM micrographs of WPC loaded with 1, 3, and 5 phr of [SiO.sub.2] nanopowder. A good dispersion of nanoparticles was observed in WPC loaded with 1 and 3 phr [SiO.sub.2] nanoparticles. The [SiO.sub.2] nanoparticles were found to become more agglomerated in WPC containing 5 phr SKI) compared to those of WPC containing 1 and 3 phr [SiO.sub.2], respectively. Higher amount of nanoparticles tend to increase the surface interaction among themselves and thereby enhanced the tendency for agglomeration. Vladimirov et al. (16) studied the morphological properties of isotactic polypropylene/fumed silica nanocomposites and observed that agglomeration of [SiO.sub.2] nanoparticles occurred at higher [SiO.sub.2] content.
FTIR spectra of CTAB, [SiO.sub.2], and CTAB- modified [SiO.sub.2], are shown in Fig. 3. In the spectrum of CTAB (Fig. 3a), the absorption peaks at 2918, 2848, and 1475 [cm.sup.-1] were assigned to asymmetric, symmetric stretching, and scissor modes of --[CH.sub.2] in the methylene chains, respectively (17). The spectrum of unmodified [SiO.sub.2] (Fig. 3b) shows some absorption peaks at 3424 and 1632 [cm.sup.-1] for
--OH stretching and hydroxyl group adsorbed on particle surface, respectively. The other peaks appeared in the range 1087-465 [cm.sub.-1] were due to Si--O--Si group in the [SiO.sub.2] (18). The spectrum of CTAB-modified [SiO.sub.2] (Fig. 3c) exhibited peaks around 3424, 2926, 2857, 1632, 1475 [cm.sub.-1] and 1086-462 [cm.sub.-1] were found. The intensity of -OH stretching in the modified [SiO.sub.2] was found to decrease, indicating an interaction of the hydroxyl group absorbed on [SiO.sub.2], surface with CTAB. Qu et al. (19) modified [TiO.sub.2], with CTAB and reported that the interaction of [TiO.sub.2] occurred with Br--of CTAB through hydrogen bond formation or electrostatic attractions. The other notable peaks appeared in the CTAB-modified [SiO.sub.2] spectrum were the characteristic peaks for CTAB and [SiO.sub.2]. These results indicated the incorporation of CTAB on the surface of [SiO.sub.2] particles.
Figure 4 represents the IR spectra of wood, PB/G5/ W40, and [SiO.sub.2]-loaded WPC. In the spectrum of wood (Fig. 4a), the presence of bands corresponding to 3424, 2924, 1735, 1637, 1161, 1045 [cm.sup.-1], and 1000-650 [cm.sup.-1] were attributed to the--OH stretching,--CH stretching, C=0 stretching,--OH bending, C--0 stretching, and C--H bending vibration (out of plane). PB/G5/W40 (Fig. 4b) exhibited peaks at 2923 [cm.sup.-1] (--CH stretching), 1734 [cm.sup.-1] (C=0 stretching), 1636 [cm.sup.-1] (--OH bending). and 720 [cm.sup.-1] (--[CH.sub.2], stretching). Figure 4c--e represents the FTIR spectra of [SiO.sub.2]-loaded WPC. It was observed that the intensity of hydroxyl as well as the C=0 peaks decreased with the increase in the loading of [SiO.sub.2]. The decrease in peak intensity might he due to the increase in interaction of hydroxyl and carbonyl groups of wood fiber with the hydroxyl groups present in [SiO.sub.2]. Moulting and Luyt (20) prepared LDPE/wax/[SiO.sub.2] nano-composite and reported that the intensity of carbonyl group of wax decreased due to the formation of hydrogen bonds with the hydroxyl groups on the silica surface. In the [SiO.sub.2]-loaded composite, the absorption peak at 3424 [cm.sup.-1] (for -OH stretching) was also found to shift little toward lower wave number suggesting the formation of hydrogen bond between wood surface. [SiO.sub.2]. and polymer matrix. Moreover, the intensity of peak in the composites at 2926 [cm.sub.-1] corresponding to -CH stretching increased. The peak at 1045 [cm.sub.-1] corresponding to C--O stretching of wood was considered as a reference. Deka and Mail (6) reported similar type of shifting of hydroxyl peaks and increase of intensity of -CH stretching during analysis of FTIR spectra of polymer blend/wood/clay nanocomposite. The other characteristic peaks for [SiO.sub.2] appeared in the composites. These results suggested the formation of strong interaction between [SiO.sub.2]. polymer. PE-co-GMA, and wood.
Figure 5 illustrates the SEM micrographs of polymer blend. WPC, and WPC loaded with different percentage of [SiO.sub.2]. The fractured surface of untreated polymer sample is shown in Fig. 5a. The figure shows the immiscibility among different polymers. The immiscibility appeared due to the poor compatibility among the polymers which could he decreased by addition of compatibilizer 1211. Compatibility among the polymer blend was improved by adding wood flour and compatibiliter (Fig. 5b) to the composite. Figure 5c--e shows the SEM micrographs of 1. 5. and 3 phr [SiO.sub.2]-loaded WPC. The roughness of the fractured surface of the composites decreased up to addition of 3 phr [SiO.sub.2] into WPC. Beyond that the agglomeration of [SiO.sub.2], particles occurred and hence the roughness of the fractured surface increased again. Generally silica particles tend to agglomerate due to formation of hydrogen bond between the surface hydroxyl groups. Modification of the surface of silica decreased the chance for agglomeration. The agglomeration of silica particles decreased due to addition of PP-g-MA as compatibilizer to the PP/ [SiO.sub.2] composites (16). The less roughness might be attributed to the increase in interaction of the modified silica with the polymer and wood.
TABLE 1. Hexural, tensile, hardness, and limiting oxygen index (LOI) values of polymer blend and WPC loaded with different percentage of [SiO.sub.2]. Flexural Tensile properties properties Sample Strength Modulus Strength (MPa) (MPa) (MPa) PB 14.53 [+ or -] 771.28 [+ or -] 6.51 [+ or -] 1.02 1.05 1.42 PB/G5 16.42 [+ or -] 1 104.91 [+ or -] 10.72 [+ or -] 0.97 1.12 1.43 PB/G5/W40 19.27 [+ or -] 3915.94 [+ or -] 20.28 [+ or -] 0.95 1.12 1.47 PB/G5/W40/S1 21.56 [+ or -] 4337.12 [+ or -] 24.74 [+ or -] 1.32 1.32 1.14 PB/G5/W40/S3 26.46 [+ or -] 4812.48 [+ or -] 33.74 [+ or -] 0.94 0.86 1.61 PB/G5/W40/S5 24.52 [+ or -] 4616.52 [+ or -] 29.56 [+ or -] 1.23 1.09 1.05 Sample Modulus Hardness LOI (%) (MPa) (Shore D) PB 91.44 [+ or -] 66.0 ([+ or -] 23 ([+ or -] 19.63 0.4) 1.0) PB/G5 119.93 [+ or -] 68.5 ([+ or -] 37 ([+ or -] 18.45 0.3) 2.0) PB/G5/W40 273.31 [+ or -] 67.0 ([+ or -] 46 ([+ or -] 16.86 0.6) 1.0) PB/G5/W40/S1 375.19 [+ or -] 71.0 ([+ or -] 54 ([+ or -] 17.82 0.5) 3.0) PB/G5/W40/S3 597.58 [+ or -] 78.0 ([+ or -] 62 ([+ or -] 19.49 0.3) 1.0) PB/G5/W40/S5 556.13 [+ or -] 75.0 ([+ or -] 68 ([+ or -] 21.87 0.4) 2.0)
The flexural and tensile properties of polymer blend and WPC loaded with different percentage of [SiO.sub.2] loading are shown in Table 1. The data presented in the table are the average of three readings. The addition of PE-co-GMA to the polymer blend increased the flexural as well as tensile properties of the polymer blend. PE-co-GMA enhanced interfacial adhesion among different polymers and hence both flexural and tensile properties improved. The properties were further improved by adding wood flour (WF) to the polymer blend. Natural fillers are generally stiffer than polymer matrix and hence they will increase the properties of WPC (22). WF acted as load carrier. Therefore, it would reinforce the polymer blend and enhance the flexural and tensile properties. PE-co-GMA improved the interfacial adhesion between hydrophilic WF and hydrophobic polymer blend through its glycidyl group and hydrocarbon backbone, respectively. The incorporation of [SiO.sub.2], particles to the WPC further enhanced the flexural and tensile properties. Flexural and tensile properties of the composites increased up to 3 phr [SiO.sub.2] and decreased at higher loading (5 phr). The interaction between [SiO.sub.2], polymer, PE-co-GMA, and wood would restrict the movement of the polymer chains and as a result the flexural and tensile properties would be improved. At lower percentage of [SiO.sub.2], the nanoparticles were well dispersed and thereby increased the surface area for interaction. Hence improvements in properties were observed. The agglomeration of [SiO.sub.2] nanoparticles was responsible for lowering in tensile and flexural properties at higher [SiO.sub.2] loading. Lee et al. (23) prepared polypropylene/polyolefin elastomer blends by the incorporation of nanosilica and found a significant improvement in mechanical properties.
Table 1 also shows the hardness results of polymer blend, PE-co-GMA-treated polymer blend, and WPC with different percentage of [SiO.sub.2] loading. From the table, it was observed that after the addition of compatibilizer to the polymer blend, hardness value of the blend increased. The incorporation of WF to the polymer blend did not improve the hardness. The hardness values increased to a large extent after addition of [SiO.sub.2]. The value was improved up to addition of 3 phr of [SiO.sub.2] beyond that it decreased. The reason for increase in hardness values could be explained and stated earlier. Jinshu et al. (24) prepared wood/urea formaldehyde composite using nano- [SiO.sub.2] and found that nano-[SiO.sub.2] treated composite had higher value of hardness compared to untreated composite.
Limiting Oxygen Index (L0I) Study
The LOI values of polymer blend and different percentage of [SiO.sub.2]-loaded WPC are shown in Table 1. From the table, it was observed that LOI value increased on addition of PE-co-GMA to the polymer blend. The increase in interfacial adhesion among polymers by PE-co-GMA resulted in higher value of LOI. Surface treatment of [SiO.sub.2] particles improved the dispersion and enhanced the interfacial adhesion between WF and polymer blend. The higher LOI values observed might be due to the formation of high performance carbonaceous-silica char on the surface. The higher the amount of [SiO.sub.2], the higher was the amount of char formation.
Polymer blend produces very little amount of char. It generates small, black smoke having candle-like flame. With addition of compatibilizer, it burns with small localized flame with little char. The char formation was increased after the addition of wood and [SiO.sub.2] nanopowder. The reason has been explained earlier. All the [SiO.sub.2]-loaded WPC samples burn with a small localized flame, producing small, black smoke.
Figure 6 represents the thermograms for the polymer blend, WPC samples loaded with or without [SiO.sub.2]. Table 2 shows the initial decomposition temperature ([T.sub.i]), maximum pyrolysis temperature ([T.sub.m]), decomposition temperature at different weight loss (%) ([T.sub.D]), and residual weight (RW, %) for WPC and [SiO.sub.2]-treated WPC. Polymer blend exhibited lowest [T.sub.i] value. [T.sub.i] value increased after the addition of WF to the composites. The higher value of [T.sub.i] was attributed to the presence of WF which delayed the degradation of the polymer matrix. Wood contains cellulose, hemicellulose, lignin. pectin, and wax. Besides this, it contains elements mainly carbon. oxygen, and small amount of silicon. The lignin present in the blend could produce a high char. The char was carbon-based residue which undergoes slow oxidative degradation. "Ihis could form a protective layer and reduce the diffusion of oxygen toward wood polymer composite (25). The presence of compatibilizer also played a significant role as it improved the adhesion of WF to the polymer matrix. Awal et al. (26) reported that the thermal degradation of the polypropylene composite delayed due to incorporation of wood pulp and maleated polypropylene compatibilizer to the composite.
[T.sub.m] values for [SiO.sub.2]-treated composites were more compared to silica untreated composite and polymer Mend alone. [T.sub.m] allies for the first stage in polymer blend. WPC. and [SiO.sub.2]-treated WPC were due to the decomposition of cellulose (27) and dehydrochlorination of PVC (28), (29). The second stage decomposition peak was due to decomposition of HDPE and PP (30). [T.sub.m] value shined toward higher temperature in both the stages of decomposition when Sift, was added. The Shilling was more for the composites treated with 3 phr [SiO.sub.2] values were also found to increase with the increase in the percentage of [SiO.sub.2] upto 3 phr. Beyond that it decreased. The well-dispersed [SiO.sub.2] nanoparticles enhanced the interaction with wood and polymer matrix and thereby delayed the decomposition of degradable volatile components. The dispersed silicate layers also produced difficulty in heat conduction and acted as a mass transport harrier to the volatile product during decomposition (31). At higher [SiO.sub.2] loading, the agglomeration of [SiO.sub.2] nanoparticles decreased the interaction and hence decomposition started early. RW value also followed an increasing trend. On the basis of the above observation, it can be concluded that thermal stability of WPC improved due to addition of [SiO.sub.2]. The increase in thermal stability might be due to the enhancement in interaction between polymer matrix, wood flour, and [SiO.sub.2] Katsikis et al. (32) prepared PMMA/silica nanocomposite and found an improvement in thermal stability of the composite.
TABLE 2. Thermal analysis of polymer blend and wood polymer composite loaded with different percentage of [SiO.sub.2] nanopowder. Temperatures of decomposition ([T.sub.D] in C at different weight loss (%) Sample [T.sub.1] [T.sub.m] (a) [T.sub.m] (b) 20 PB 23.2 252 407 295 PB/G5/W40 238 257 455 302 PB/G5/W40/S1 244 262 475 328 PB/G5/W40/S3 260 280 483 360 PB/G5/W46/S5 249 270 478 345 RW % at Sample 40 60 80 600 PB 389 442 470 6.2 PB/G5/W40 395 445 473 7.3 PB/G5/W40/S1 447 470 487 10.3 PB/G5/W40/S3 456 478 496 14.3 PB/G5/W46/S5 453 475 492 12.1 [T.sub.1] value for initial degradation. (a.) [T.sub.m] value for first step. (b.) [T.sub.m] value for second step.
Water Uptake Study
Figure 7 shows the percentage of water uptake results for polymer blend and WPC samples. From the figure, it was observed that polymer blend absorbed less water than the composites, as expected. The addition of compatibilizer decreased the water absorption. The compatibilizer improved the interfacial adhesion between the polymers and thus decreased the water uptake. The addition of GMA as compatibilizer to WPC decreased the water absorption (33). The water uptake increased significantly when WF was added to the blend. This was due to the hydrophilic nature of wood. But the water uptake capacity decreased on inclusion of modified [SiO.sub.2] to the WPC. Water uptake was found to decrease with the increase in the percentage of [SiO.sub.2] in composite. The strong interaction between silica particles, wood, and polymer matrix decreased the accessibility of water through WPC. Minelli et al. (34) studied the barrier properties of organic-inorganic hybrid coatings based on polyvinyl alcohol and found an increase in water barrier properties after incorporation of [SiO.sub.2]. Composite loaded with 3 phr [SiO.sub.2]showed lowest water uptake. The reason was discussed earlier.
The ratio of xylene and THE for solution blending of HDPE, LDPE, PP, and PVC (1:1:1:0.5) was optimized as 70:30. Nanosilica was modified and mixed with the solution blended polymer and wood flour. The influence of nanosilica on the mechanical and thermal properties, flame retardancy, and water absorbency was investigated. X-ray studies showed that [SiO.sub.2] nanoparticles were infused into the WPC. At lower loading of [SiO.sub.2] (1-3 phr), the dispersion was found to improve as revealed by TEM study. SEM study showed that the surface of the composite became smooth up to the addition of 3 phr [SiO.sub.2] nanoparticles. FTIR study indicated the interaction between [SiO.sub.2], polymer, PE-co-GMA, and wood. Tensile, flexural, and hardness properties of the WPC increased with increasing [SiO.sub.2] content up to 3 phr, beyond that it decreased. The incorporation of nanosilica in wood flour/ polymer composite significantly improved the thermal stability. Flame retardancy of WPC nanocomposite enhanced due to formation of silica char on the surface of composite. Water absorption of the composite decreased with the incorporation of [SiO.sub.2] in the composite.
Correspondence to: T.K. Maji: e-mail: email@example.com
Contract grant sponsor Council of Scientific and Industrial Research (CSIR)-New Delhi: contract grant number: 01(2287)/08/EMR-11.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2012 Society of Plastic Engineers
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Biplab K. Deka, Tarun K. Maji
Department of Chemical Sciences, Tezpur University, Assam-784028, India