Thermal and Mechanical Properties of Novel Composites of Methyl

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Thermal and Mechanical Properties of Novel Composites of Methyl Silicone Polymer and Partially Ceramized Rice Bran M. Mahbubul Hassan,*,†,§ Tatsuhiro Takahashi,†,‡ and Kyohito Koyama†,‡ †

Venture Business Laboratory and ‡Department of Polymer Science & Engineering, Yamagata University, 8-3-5 Jonan, Yonezawa-shi, Yamagata-ken, Japan. ABSTRACT: Rice bran (RB), rich in carbon and silicone, is an agricultural waste that is abundantly available in rice producing countries. In this work, composites of a preceramic methyl silicone resin (MSR) and RB were prepared by blending powdered RB with a molten MSR at 110 °C at various ratios in a Brabender-type static mixer. Composites were made from them by compression molding and cross-linking at 260 °C under 20 MPa pressure. The thermal, mechanical, and chemical properties of the produced composites were assessed by thermogravimetric analysis, scanning electron microscopy, and FT-IR and by measuring compressive strength and hygroscopic expansion. It was found that RB was partially ceramized and decomposed to carbonaceous materials during cross-linking. The water absorption and hygroscopic expansion of the MSR/RB composites were increased but the compressive strength was decreased with an increase in the weight % of RB. The highest compressive strength was shown by the composites made from the 50/50 blends, which was 27.61 MPa. The developed composites showed an excellent compressive strength and also very low water absorption and hygroscopic expansion.

1. INTRODUCTION In the past several decades the world has witnessed remarkable developments in structural material technologies. New materials such as ceramics, ceramics from polymeric precursors, and polymer−ceramic composites are replacing metals in many applications ranging from turbines to tennis rackets. The development of new hybrid and ceramic composites is opening new avenues for material developments. Hybrid composite materials made from organic−inorganic preceramic polymer and inorganic ceramic/metallic fillers recently have drawn attention for their many good properties. The fabrication of this hybrid ceramics has many advantages over conventional ceramics fabrication processes as they can be made in any shape, have low processing temperature and low energy demand, and have an easy fabrication process as they can be processed like thermoplastic polymers by conventional polymer processing machinery. Methyl silicone resins (MSR) are one of the preceramic polymers, and by thermal cross-linking polymethylsilsesquioxane-type glassy polymers can be made from them. They are relatively cheap and therefore have received special attention as precursors of silicone oxycarbide ceramics.1−5 Rice bran (RB), which is a waste from rice processing industries, is abundantly available in many tropical and subtropical countries. The common practice is to discard and burn RB in the field in the open space which releases silica particles in the air. These silica particles are suspended in the air for a long time, which could be environmentally hazardous as over long time exposure they could cause silicosis and other respiratory diseases.6 RB contains 34.0% cellulose, 28.2% hemicellulose, 24.8% lignin, and 3.1% silica. The silica content of RB is quite low compared to rice husk7,8 which has 15−18% silica and is used in the preparation of many types of zeolites.9,10 A number of studies on conversion of RB into activated carbon have also been reported.11,12 In all cases, RB is pyrolyzed in an inert atmosphere in order to remove © 2012 American Chemical Society

combustible organic compounds leaving behind highly carbonaceous materials. As a waste product, RB is cheap filler. Its applications as a filler were investigated to improve the mechanical properties of polymeric and ceramic composites.13,14 Ceramized RB was investigated as reinforcement materials in polyamide and polyacetal plastics to make gears by injection molding, and these gears showed reduction of noise during their use.14,15 Recently, ceramics made from RB (known as RBC) have drawn much attention because of their high mechanical strength and high solid lubrication property.16−19 They are made by carbonizing composites of RB and a phenolic resin20,21 and are being used in oil-free bearings and other applications. Their mechanical properties very much depend on the composite formation methods. The RBC composites show very high compressive strength. It was reported that the compressive strength of uniaxially formed and injection molded RBC reached to 32 and 264 MPa, respectively, with 70% reduction in friction coefficient.22−24 Although fabrication of Si−O−C ceramics and fibers from MSRs by high temperature pyrolysis are well-established,1,2,4 the fabrication of composites, especially with ceramized RB, is quite new. To the best of our knowledge thermoset composites of MSR and ceramized RB were never investigated before, although MSRs were investigated to make glass and ceramic foams.2,3 In the present study, we used MSR as a matrix and partially ceramized RB powder as a reinforcing filler to examine how various weight % of partially ceramized RB affect various properties of the MSR/RB composites. The thermal, mechanical, and morphological properties, according to the Received: Revised: Accepted: Published: 1275

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local rice mill, thoroughly washed with distilled water to remove dust and dirt, and dried in an oven. The dried RB was ground to fine powder by a mechanical grinder. 2.2. Preparation of MSR/RB Composites. The production process of MSR/RB thermoset composites and also the various changes that occur during composite formation are illustrated in Scheme 2. RB powder was dried in a vacuum oven at 80 °C overnight prior to blending with MSR. The RB powder and methyl silicone resin were mixed in a static mixer having two screws that move in opposite directions (Labo Plastomill made by Toyo Seiki Seisaku-sho, Ltd., Japan) at 110 °C at a 70 rpm screw speed for 10 min to get a homogeneous mixture. The RB and MSR were blended at various ratios, 50/ 50, 60/40, 70/30, and 80/20. After mixing, the samples were cooled and ground to granules, and those granules were used to fabricate composites by compression molding. Rectangularshaped plate samples of 50 × 50 × 10 mm size were prepared by thermal cross-linking in a compression molding machine (Mini Test Press, Model MP-S, Toyo-Seiki Company Ltd., Japan) under 20 MPa pressure at 260 °C. The temperature of the mold was raised to 260 °C at 2 °C/min and held for 60 min. After completion of cross-linking, the composites formed were slowly cooled to room temperature under 20 MPa pressure by switching off the heating and circulating cold air through a fan. The molded plates were cut by a circular saw to suitable sizes to assess their various properties. 2.3. Characterization of Thermal Properties. Thermogravimetric (TG) analysis was carried out on a Seiko DSC/TG Analyzer (Model SSC 5000) from room temperature to 600 °C at a heating rate of 5 °C/min in a nitrogen environment. A high purity oxygen-free nitrogen gas was used as an inert gas to purge air in the pyrolysis zone for avoiding unwanted oxidation of samples during pyrolysis. A constant flow of nitrogen was fed at a rate of 20 mL/min to the system from a point just below the samples. The TG curves were recorded simultaneously along with the temperature rise. 2.4. Physical and Mechanical Testing. The density of the porous ceramic materials was measured by measuring a certain volume of sample after conditioning at 20 ± 2 °C and 65 ± 2% relative humidity for 48 h. The density was calculated as the mass of the material divided by the volume and expressed as gm/cm3. The compressive strength of the various MSR/RB composites with different weight % of RB was measured by uniaxial compression testing, on samples of about 10 × 10 × 5

reinforcing RB content in respect to thermoset methyl silicone polymer, were assessed and reported here.

2. EXPERIMENTAL SECTION 2.1. Materials. The MSR used in this work is a commercially available Silres-610 (basically a methyl silsesquioxane) with a general formula [(CH3−Si−O1.5)n, n = 130− 150], Tg = 41.6 °C, Mw = 9.2 × 103 (g/mol), and Mw/Mn = 3.2, which was purchased from Wacker Chemie GmbH (Germany). It is a methyl groups containing solid silicone resin having melting point 35−55 °C. It has 4 mol % hydroxyl (−OH) and ethoxy (−Si−OCH2CH3) reactive groups and they undergo cross-linking reactions at 150−260 °C generating polymethylsilsesquioxane-type polymer and also water and ethyl alcohol, as shown in Scheme 1. The RB (defatted) was procured from a Scheme 1. Formation of Methyl Silicone Polymer from a Methyl Silicone Resin

Scheme 2. Schematic Diagram of Formation of RB/MSR Composites by Cross-Linking and Compression Molding

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3. RESULTS AND DISCUSSION 3.1. Physical Properties of MSR/RB Composites. The physical properties of the MSR/RB composites with various weight % of RB are provided in Table 1. The composites

mm using a mechanical testing machine, Instron 5882 UTM (Instron Danvers, MA, USA), and a cross-head speed of 1 mm/ min. Each data point reported represents the average of 10 individual tests. Tests were carried out in an environmentally conditioned room maintained at 20 ± 2 °C and 65 ± 2% RH. The wettability of the MSR/RB composites was measured by measuring the contact angle, which is a quantitative measure of the wetting of a solid surface by a liquid. The contact angle of the surfaces of plate samples of various MSR/RB composites was measured by a KSV CAM 200 Contact Angle Measurement Apparatus (made by KSV Instruments, Finland). Five microliter volume drops of Milli-Q grade water were deposited on the MSR/RB composite plate with a syringe. Images of the drops were acquired through a digital camera positioned just behind the sample platform of the KSV instrument. The contact angle calculation was performed by applying the spherical approximation of the drop by curve fitting based on the Young−Laplace equation by using the software (KSV CAM 200) supplied with the equipment. The water absorption of the various MSR/RB composites was carried out according to the standard test method ASTM D570-98(2010)e1: Standard Test Method for Water Absorption of Plastics.25 The composite samples were cut to 40 × 40 × 10 mm size and dried in an oven at 50 °C. The samples were then immersed in 100 mL of distilled water at room temperature (20 °C) for a certain time, after which they were removed from the water tank, wiped by tissue paper, and weighed. The weight of the samples was measured to a precision of 1 mg. Finally, the water absorption as percentages was calculated as the mass difference with the following equation: water absorption ratio,

Table 1. Physical Properties of MSR/RB Composites with Various Weight % of RB sample id.

RB (%)

MSR (%)

density (g/cm3)

water absorption after 5 days (%)

hygroscopic expansion after 5 days (%)

1 2 3 4

50 60 70 80

50 40 30 20

1.40 1.27 1.17 1.14

3.17 4.41 4.98 5.48

0.21 0.33 0.44 1.0

fabricated from RB and MSR were crack-free as no surface crack was visible, but they were black in color because of a high percentage of carbonaceous residues. From the color of the composites, it can be anticipated that RB was burnt and partially ceramized during cross-linking and compression molding. It is difficult to prepare three-dimensional structures from 100% MSR as they are very glassy and brittle. On the other hand if the weight % of RB in the composite is higher than 80%, then the produced composites are poor in strength. Therefore, we limited our investigations to 50/50, 60/40, 70/ 30, and 80/20 blending ratios of RB and MSR. The density of MSR/RB composites made with 50 to 80% RB is shown in Table 1. The highest density was shown by the composites made from 50/50 blend of MSR and RB, and the lowest for the 80/20 ratio. The density of the composite made with 50% RB was 1.4, but it was reduced to 1.14 after increasing the weight of RB to 80%. The density of the MSR/RB composites was decreasing with an increase in the weight % of RB. 3.2. Thermal properties. TG analysis (TGA) was carried out to assess the weight loss behavior of composite materials during pyrolysis and also to determine their stability at various temperatures. The TGA curves of 100% RB, 100% MSR, and the various composites made from them are shown in Figure 1. It can be seen that during pyrolysis under nitrogen the weight loss of RB took place at three temperature zones, room temperature to 100, 220 to roughly 350, and 350 to until 600 °C. Similarly, the weight loss of the MSR and MSR/RB composites also occurred at three temperature zones, room temperature to 100, 220 to 360, and 360 to until 600 °C. RB and composites of RB/MSR started losing weight at around 50 °C, but the MSR started losing weight at only above 180 °C. The composites started weight loss at that low temperature because of the moisture absorbed by the cellulosics in the RB, and the weight loss observed until 100 °C could be attributed to the absorbed moisture. At the second stage from 220 to 350 °C the MSR/RB composites rapidly lost weight. The rate of weight loss observed at 350 to 465 °C was slower compared to the weight loss that occurred at 220 to 350 °C, after which the rate of weight loss became very slow. The TG curve of RB shows that it approximately lost 15% of the weight at the crosslinking temperature of MSR and it can be imagined that at long cross-linking time (60 min) most of the RB would convert to carbonceous material by partial ceramization. The weight loss that occurred in the range of 220−350 °C may be associated with the decomposition of organic matters such as cellulose, hemicelluloses, and lignin of the RB and release of gaseous combustion products.26 The weight loss that occurred in the

Wa(t ) = ((Wt − Wo)/Wo) × 100 ([1])

where Wa(t) is the water absorption at time t, Wo is the original weight of the composite sample, and Wt is the weight of the composite sample at a given immersion time t. The hygroscopic expansion was measured by placing 20 × 10 × 5 mm size samples in 100 mL of distilled water, and the length of the samples was measured after a predetermined time by a digital slide calliper (Mitutoyo Corporation, Japan) up to 5 days. The hygroscopic expansion was calculated as the length difference before and after immersion in water and expressed as a percentage. 2.5. Fourier Transform Infrared (FTIR) Spectroscopy. FT-IR spectra of MSR cross-linked at 260 °C and various MSR/RB composites with various weight % of RB were measured on a Perkin-Elmer System 6000 FT-IR spectrometer. Few milligrams of composite were powdered in a mortar shell, mixed with potassium bromide, and a thin disc was produced by a hydraulic press. The discs were then scanned in a FTIR at a resolution of 4 cm−1 at room temperature, and a minimum of 64 scans were signal-averaged. 2.6. Scanning Electron Microscopy. 20 ×10 × 5 mm size rectangular-shaped plate samples of MSR/RB composites were prepared by cross-linking and compression molding. They were then immersed in a liquid nitrogen bath for 30 min and fractured by impact. The fractured surfaces were sputter coated with gold (in the case of EDX measurement cracked surface were carbon coated) on a JEOL JFC 1500 Fine Coat Ion Sputter (to prevent charging during scanning). The coated surfaces were scanned on a JEOL SEM (Model JSM-6340F) equipped with an energy dispersive X-ray spectrometer. The accelerating voltage was 10 kV for all the samples. 1277

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Figure 1. TG analysis of RB, MSR, and their composites.

Figure 2. Effect of increase in RB weight % on the compressive strength of RB/MSR composites.

worsened the interfacial bonding between the filler and the polymer matrix, this resulted in a decrease in compressive strength. As RB fillers were irregular in shape, they failed to support stresses transferred from the polymer matrix. The poor interfacial bonding produced microspaces between the filler and the matrix, which hindered stress propagation when compressive stress was loaded. The maximum compressive strength was shown by the composite made from a 50/50 blend of MSR and RB which was 27.61 MPa and the lowest by the 80/20 blend which was 20.54 MPa. The compressive strengths of the composites containing 60 and 70% RB were 25.65 and 22.82 MPa, respectively. Colombo and Hellman fabricated ceramic foams by blending polyurethane with various MSRs, and the compressive strength of the produced ceramic foams ranged from 1.2 to 16 MPa depending on the type of MSR.30 The polymeric composites we produced from MSR and RB showed significantly higher compressive strength than the ceramic composites they produced. As their ceramic composites were porous, they showed lower compressive strength than the polymeric composites of the MSR/RB. 3.4. Water Absorption, Hygroscopic Expansion, and Contact Angle. Water absorption of composites relates to their composite properties such as dimensional stability, strength loss, creep behavior, and impact strength. The water absorption by various MSR/RB composites was monitored up to five days. The results of the water absorption tests are shown

region of 350 to 600 °C could be associated to the carbonization process, which may be attributed to further reactions involved to reaction intermediates.26 It is evident that the weight loss of the composites increased with increasing the weight % of RB. The lowest weight loss was observed for the composites containing 50% RB and the highest for the composites having 80% RB. The weight losses for 50, 60, 70, and 80% RB in the composites were 51.5, 56.7, 62.3, and 68.45%, respectively. The weight losses for 100% MSR and 100% RB were 15.1 and 81.3%, respectively. The weight loss of RB observed here was consistent with the published data.27 3.3. Mechanical Properties. The compressive strength of thermoset composites made from MSR and RB with various weight % of RB is shown in Figure 2. The compressive strength of the cross-linked MSR was not measured as it was very glassy and very brittle, and after compression molding its threedimensional structure broken down in the mold. The compressive strength of MSR/RB composites decreased with increasing the weight percentage of RB in the composites. The increase in RB weight % increased the percentage of whiskers (acted as filler) as the RB was converted to carbonaceous materials. It is known that until a certain level (5%) addition of fillers increases the tensile strength of polymeric composites,28 but high levels of fillers actually decrease tensile strength.29 As the filler loading increased with the increase in RB weight % in the composites, thereby increasing the interfacial area which 1278

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Figure 3. Water absorption and hygroscopic expansion properties of MSR/RB composites with various weight % of RB.

chains by reducing the intermolecular interactions, particularly when the water molecules are able to become attached with the hydrophilic groups of polymethylsilsesquioxane and also decomposed RB. The contact angle shown by surfaces of plate samples of MSR/RB composites made with various weight % of RB is shown in Table 2. All the composites showed moderate to high

in Figure 3. It can be seen that water absorption of the composites was increased with increasing the weight % of RB. The absorption of water by various MSR/RB composites started to increase immediately after immersion in water and slowly reached to almost the saturated level after 24 h of soaking. The highest absorption was achieved for the composite made from an 80/20 mixture of RB/MSR, and after 5 days of soaking it reached 5.48%. The lowest absorption was shown by the composite made from a 50/50 mixture of RB/MSR, and after 5 days immersion in water the water absorption reached to 3.17%. The second lowest water absorption was shown by the composite made from 60/40 mixtures. It is evident that water absorption increases with increasing the RB weight %. The decomposition of RB might produce carbonaceous materials with hydrophilic groups such as hydroxyl and carboxyl groups, and those might bind water molecules; therefore, water absorption increased. The linear hygroscopic expansion of the various MSR/RB composites was monitored up to five days. Figure 3 shows the hygroscopic expansion of various MSR/RB composites. After 5 day storage in water at room temperature (20 °C) all composites showed small hygroscopic expansion (0.3 to 1%). The hygroscopic expansion for all the composites was very low but the hygroscopic expansion was increased with increasing the weight % of RB in the composites. The hygroscopic expansion started to increase after 25 min of soaking and reached to almost a saturated level after 24 h for all the composites. The highest hygroscopic expansion was shown by the composite made from 80/20 mixture of RB/MSR, and after 5 days of soaking it was increased to only 1%. On the other hand, the lowest hygroscopic expansion was shown by the composite made from a 50/50 mixture of RB/MSR, and after 5 days of soaking it was increased to only 0.33%. The hygroscopic expansion shown by the composites made from 60/40 and 70/ 30 mixtures of RB/MSR was in between of the hygroscopic expansion shown by the composites made from 50/50 and 80/ 20 mixtures. It can be seen that water absorption and hygroscopic expansion are quite correlated. It should be considered that not only water absorption but also other factors might influence hygroscopic expansion. The absorbed water might have expanded the space between the polymer

Table 2. Contact Angle Shown by the Surface of Plate Samples of MSR/RB Composites with Various Weight % of RB contact angle (deg) time (s)

50% RB

60% RB

70% RB

80% RB

30 31 32 33

118.0 117.9 117.6 117.3

110.8 111.4 111 111

105.4 105 104.4 104.3

99.5 99.5 99.0 99.0

contact angle, but the contact angle decreased with an increase in RB weight %. The MSR/RB composite made with 50% RB showed the highest contact angle (118°). The lowest contact angle was shown by the MSR/RB composites made with 80% RB (99.5°). The contact angle was diminished only slightly with time, which shows that the composites made from MSR and RB were quite hydrophobic, and the contact angle results were consistent with the water absorption data. 3.5. Morphology of Composites. The observation of fracture surfaces were conducted to investigate the mechanism of lower compressive strength with increasing RB percentage in the MSR/RB composites. Figure 4 shows SEM images of the fractured structure of MSR/RB composites with various weight % of RB. It can be seen that the produced composites are not porous which is normal for ceramics made from them. It is evident that the smoothness of the fractured surfaces is affected with increasing the weight % of RB, and surfaces are progressively becoming rough with increasing the weight % of RB. It is evident from SEM images that pyrolysis of RB produces spherical-shaped micrometer-size whiskers, and those act as filler in the composites. It was reported that pyrolysis of rice husk at 1100−1400 °C produced silicon carbide (SiC) 1279

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Figure 4. SEM micrographs of MSR/RB composites with various weight % of RB. (a) 50% RB; (b) 60% RB; (c) 70% RB; (d) 80% RB.

whiskers that were spherical to fibrous in shape.31,32 The long thermal cross-linking at 260 °C for 60 min during the fabrication of composites by compression molding might have converted RB into spherical-shaped Si−O−C whiskers as the production of SiC at that temperature could be impossible. Therefore, the color of the composites was mostly black. The highly magnified fractured surface of the composite with 50% RB loading was quite smooth, but few sphericalshaped whiskers were visible. In the case of 60% RB, more spherical whiskers were visible than the composite with 50% RB, and the surface became progressively rough. It could be seen that the quantity of whiskers increased with increasing weight % of RB, indicating that whiskers were produced from RB. From SEM images it is evident that the increase in weight % of RB produces more whiskers/fillers, and traces of pulled out whiskers are visible at the fractured surfaces indicating that those whiskers are not bonded to the polymethylsilsesquioxane matrix which therefore reduces the compressive strength. The pulled-out traces of spherical whiskers in the SEM micrographs confirm poor interfacial bonding between whiskers and the matrix polymer. It is also evident that RB fillers were homogeneously dispersed in the preceramic polymer matrix; therefore, all the composites showed quite reasonable compressive strength. From Figure 4 it was also evident that the increase of weight % of RB to 80% produced microcracks in

the composites; therefore, addition of RB up to 70% should be practiced. To identify the compositions of the spherical whiskers that are visible in the SEM images in Figure 4 for the composites with high RB weight %, we carried out EDX analysis of two whiskers. It is evident from the EDX spectra that all the spheres showed similar composition, that is, showed peaks for Si, C, and O (Figure 5). The EDX spectra show that the spheres are made of Si, C, and O, (could be Si−O−C). 3.6. FTIR Analysis. The FTIR spectra of cross-linked MSR, RB pyrolyzed at 250 °C, and composites of MSR/RB with various weight % of RB over the wavenumber range of 450 to 4500 cm−1 are shown in Figure 6. In the case of the IR spectrum of MSR cross-linked at 260 °C, IR bands were shown at 800, 1027, 1123, 1265, and 2964 cm−1 which is consistent with the published IR spectra of polymethylsilsesquioxane.33 Typical polysilsesquioxane absorption bands were shown at 1027 and 1123 cm−1. The presence of methyl groups was shown by the absorption band at 1265 cm−1. It is well-known that oxidation curing of MSR involves the cross-linking of the MSR with oxygen through formation of Si−O−Si by the oxidation of Si−OH and Si−CH3 bonds.32,33 The absorption band shown at 2964 cm−1 could be assigned to the C−H vibrations of −CH, −CH2, and −CH3 groups.34 The two very sharp peaks with almost equal intensity at 1027 and 1123 cm−1 1280

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Figure 5. EDX spectra of spheres visible in the SEM images in Figure 4.

Figure 6. FT-IR spectra of MSR and various RB/MSR composites.

790, 1680−1690, 2860, and 2920, and a broad peak at around 3400 cm−1 other than peaks shown by the cross-linked MSR. RB shows two extra peaks at 1615 and 1710 cm−1, which can be assigned to unsaturated groups such as alkenes and aliphatic acid CO stretching, respectively.35 However, in the case of composites those two peaks are vanished and a new peak was formed at 1680−1690. The peaks shown by MSR/RB composites with various weight % RB at 545−560 cm−1

indicate that cross-linking of MSR produced polymethylsilsesquioxane-type polymer at 260 °C. The peak shown at 800 could be assigned to Si−CH3, which indicates that the polymer formed siloxane network was terminated by a methyl group. RB shows peaks at 560, 758, 1170, 1615, 1710, 2860, 2930, and a broad peak around 3400 cm−1, which are consistent with the published IR spectra of rice bran.35 On the other hand, MSR/ RB composites showed extra peaks at around 545−560, 764− 1281

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could be assigned to silica. The broad peak at 3400 cm−1 is associated with O−H stretching vibrations, and it became broader with increasing RB weight %, indicating the formation of more OH groups.36 The band at around 1680 cm−1 could be assigned to a C−O stretching vibrations layer. An observed increase in the intensity of the bands in the range between 1300 and 900 cm−1 which may be ascribed to the C−O vibrations associated with hydroxyl groups and ether type structures reflects the increase of C−O groups on the surface after increasing RB weight %. The absence of bands of cellulose, hemicellulose, and lignin in the FTIR spectra of composites indicated that all RB were decomposed by prolonged heating at 260 °C but the presence of some organic moieties (−CH3, −COOH) showed that it was only partially ceramized.

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4. CONCLUSIONS We have demonstrated that RB and MSR composites can be fabricated with high compressive strength at low temperatures. The highest compressive strength was 27.61 MPa shown by the composite made from the 50/50 blends of MSR and RB. The compressive strength of the MSR/RB composites decreased with increasing the weight % of RB. The water absorption showed by composites was quite small (≈5%), and even after one week of immersion in water the water absorption capacity of composites increased only marginally. The water absorption capacity of the composites was increased with increasing RB weight %. The produced composites showed very little hygroscopic expansion (≈0.3% to 1%), even after 5 days of immersion in water, and the highest hygroscopic intensity was shown by the composite made from 80/20 blends of RB and MSR. The high compressive strength, low water absorption, and almost no detectable hygroscopic expansion may allow using them in various applications including replacement of ceramic bricks in building constructions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +64-3-3218755. Fax: +64-3-321-8811. Present Address §

Food & Biobased Products Group, Agresearch Limited, Cnr Springs Road & Gerald Street, Lincoln, Christchurch 7647, New Zealand. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge help received from Prof. Hideyuki Tagaya of the Dept. of Chemistry & Chemical Engineering of Yamagata University for thermogravimetric analysis, and Prof. Takeshi Kuriyama of the Department of Polymer Science & Engineering of the same university for giving us the opportunity to use his muffle furnace to carry out pyrolysis.

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REFERENCES

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