Compatibility Improvement of Rice Husk and Bagasse Ashes with Natural Rubber by Molten-State Maleation

Transcription

1 European Journal of Scientific Research ISSN X Vol.43 No.3 (2010), pp EuroJournals Publishing, Inc Compatibility Improvement of Rice Husk and Bagasse Ashes with Natural Rubber by Molten-State Maleation Suwatthana Phrommedetch Center of Excellence on Natural Rubber Technology, Department of Chemical Engineering Faculty of Engineering, Thammasat University, Thailand Cattaleeya Pattamaprom Center of Excellence on Natural Rubber Technology, Department of Chemical Engineering Faculty of Engineering, Thammasat University, Thailand Tel: ext 3125, Fax: Abstract Rice husks and bagasses are prevalent industrial wastes from rice mills and sugar industries, respectively. In Thailand, these industrial wastes were normally used as fuel sources and most of the ashes remained after combustion were disposed of as land fill. Some researches have proposed to use them for various applications including reinforcing fillers in rubber composites to increase their values. Since the surfaces of rice husk and bagasse ashes are hydrophilic, the compatibility of these ashes with rubber is expected to be low. In this study, modification of natural rubber into maleated natural rubber was proposed to improve the hydrophilicity of natural rubber and the compatibility with those agricultural waste fillers. The reinforcing effects of rice husk ashes in natural rubber and maleated natural rubber (MNR) are compared to those of conventional reinforcing fillers, such as silica and carbon black. Brunauer-Emmett-Teller (BET) and X-ray fluorescence (XRF) were employed to study the surface areas and the inorganic compositions of the fillers. The effects of maleated rubber on degree of swelling, as well as, tensile strength of the rubber compounds were also studied. We found that the tensile strengths and moduli of MNR composites filled with bagasse ashes were improved from those without maleic anhydride, together with lower degree of swelling. For rice husk ashes, even though their tensile strengths were comparable, the MNR composites processed significantly higher moduli than the composites of conventional natural rubber. Nevertheless, we found that MNR compounds exhibited a long delayed onset of vulcanization compared to that of natural rubber, which may be due to the reaction of sulphenamide-accelerator (CBS) with free maleic anhydride molecules and/or succinic acid. 1. Introduction Several attempts have been made to use rice husk ash as filler for natural and synthetic rubbers. Most of them indicate that RHA is moderately reinforcing fillers, in that the reinforcing effect is not as good as silica and carbon black but is better than talcum, clay, or calcium carbonate (CaCO 3 )[1,2]. Haxo and Mehta[3] revealed the possibility of using ground RHA obtained by a special burning condition as moderately reinforcing filler for styrene-butadiene rubber (SBR), ethylene-propylene-diene elastomer (EPDM), and natural rubber (NR). The influences of RHA incorporation on mechanical properties and

2 Compatibility Improvement of Rice Husk and Bagasse Ashes with Natural Rubber by Molten-State Maleation 412 fatigue behavior of epoxidized natural rubber (ENR) were also investigated [4],[5]. It was found that rice husk ash has potential as a semi-reinforcing filler for ENR and the presence of a silane coupling agent enhanced the cure times, tensile strengths and fatigue properties of the vulcanizates. Moreover, the fatigue properties of RHA-filled vulcanizate are better than carbon black. Sae-oui et al. [1] revealed the possibility of using two grades of rice husk ash (low-and high-carbon contents) in natural rubber. It was found that both grades of rice husk ash provide inferior mechanical properties in comparison commercial silica and carbon black. The addition of silane-coupling agent has little effect on the properties of the ash-filled vulcanizates. Another way to improve rubber-filler compatiblility is by modifying on the hydrophillicity of the rubber matrix itself. One way to do that is by changing its molecular structure into maleated natural rubber. The graft copolymerization of maleic anhydride onto the natural rubber molecules could be carried out in molten or solution states. [6, 7, 8, 9, 10,11] The maleation in molten state was selected for this study because the reaction was initiated simply by shearing action at high temperature in an internal mixer, whereas the solution-state maleation involved toxic solvents and chemicals. The fillers used in this study are rice husk ashes and bagasse ashes. 2. Experimental 2.1. Materials The as-received RHA ash was obtained from atmospheric combustion of rice husk. Prior to use, the ash was milled and sieved on a 200-mesh sieve (< 75 μm) and dried in an oven at 100 O C for 24 hr. Natural rubber (STR 5L) was supplied by Rubber Estate Organization (Thailand). Maleaic Anhydride(MA) was purchased from Merck KGaA Ltd. Hi-Silica 255s silica (SiO 2 ) was received from Rubber Research Institute(Thailand). Carbon black (N-220, particle size 22 nm), Stearic acid and zinc oxide (ZnO) were supplied by Loxley Public Co., Ltd. N-cyclohexyl-2-benzothiazyl sulfenamid (CBS), Tetramethyl Thiuramdisulphide (TMTD) were obtained from Sunny World (1989) Co., Ltd. Sulphur was purchased by Kurusapa business Organization (Suksapan Panit), Thailand and silane-coupling agent, bis(3-triethoxylpropvl)-tetrasulfane or so called (Si-69) were obtained from JJ-Degussa Chemicals (Thailand) Ltd Filler Characterization The solid filler particles were fully characterized both physically and chemically through different techniques. The surface area of fillers was determined by the Brunauer-Emmett-Teller (BET) method on the Quantachrome (AUTOSORB-1) following ASTM D standard test method. The inorganic compositions of the rice husk ash was analyzed by Philips PW2404 (wavelength dispersive) X Ray Fluorescence Spectrometer (XRF). X-ray powder diffraction patterns of fillers particles with and without rubber matrices were obtained using a D8 ADVANCE X-ray diffractometer (XRD). The data were recorded in the range of 2θ = 10 90, using a step size of 0.03 o /min Preparation of Maleated Natural Rubber (MNR) Maleated natural rubber (MNR) was prepared by the grafting reaction of maleic anhydride with natural rubber in molten state following Visconte et al. (1997). A possible reaction mechanism is shown below.

3 413 Suwatthana Phrommedetch and Cattaleeya Pattamaprom Figure 1: A possible reaction between MA and NR during processing. The grafting reaction was prepared by cutting the natural rubber, STR 5L, into small pieces. The rubber was then dried in a vacuum oven at 40 C for 24 h. The internal mixer was then used to masticate STR 5L at 135 C with a rotor speed of 60 rpm for 2 min before adding 6 phr of maleic anhydride into the internal mixer. The mixing was continued for 10 min at 135 C. The blended product was then dumped out onto the two-roll mill, sheeted and cut into small pieces. The presence of the anhydride group grafted on the natural rubber backbone was determined using a Perkin-Elmer FTIR spectrometer model (Prior to the measurement, unreacted maleic anhydride was first removed from the rubber piece by drying overnight at 40 C for 24 h., dissolving and refluxing in toluene. The filtrate was then reprecipitated in acetone, and dried in the vacuum oven at 40 C for 24 h.) 2.4. Rubber Compounding Rubber compounding was carried out in an internal mixer with a rotor speed of 40 rpm and at a starting temperature of 50 o C. Total time taken to complete one mixing cycle was 10 mins. The compounding formulations were listed in Table 1. The curing condition of the rubber samples were identified using a Moving Die Rheometer. For the mechanical property measurement of filled vulcanizates, the compounds were cured at 150 o C in a compression mold using the predetermined curing conditions. Table 1: Rubber compounding formulation Formula Compound A (phr) Unfilled NR B (phr) Unfilled MNR C (phr) NRfiller D (phr) MNRfiller E (phr) NR/MNR-filler # STR 5L MNR* Filler # Stearic Acid Zinc Oxide CBS TMTD Sulphur * MNR = Maleated Natural rubber # Filler = Rice husk ashes(rha), Bagasse Ash(BA) and Carbon black(cb) 2.5. Testing of Rubber Vulcanizates The tensile stress strain properties were measured according to ISO , with type-2 test specimens. The crosslink density of the rubber vulcanizates can not be determined on the basis of rapid solvent-swelling measurements because the difference in chemical structure between NR and MNR leads to different swellability in toluene. Therefore, we assessed the crosslink density qualitatively by

4 Compatibility Improvement of Rice Husk and Bagasse Ashes with Natural Rubber by Molten-State Maleation 414 comparing the percent increase in torque (T) from the cure characteristic test ( Tmax Tmin ) ( ΔT % = Χ 100 ) T min 3. Results and discussion From the characteristic test of fillers in Table 2, we found that the surface areas of RHA and BA are lower than those of the commercial reinforcing fillers (carbon black and silica) and the silica content in RHA and BA were slightly less than that of commercial silica. Table 2: The surface areas and silica contents of fillers used in this study compared to commercial fillers Type of filler Surface area (m 2 /g) Silica content (%) Carbon black (N-220) (CB) Silica (Hi-Silica 255) (Si) Rice husk ash (RHA) Bagasse Ash (BA) The IR spectra of NR and MNR in the range of wave number cm -1 are shown in figure 2. For natural rubber (cis-1,4-polyisoprene), the characteristic bands for the stretching of C-H attached to C=C bonds appears at 835 cm -1 [12] and the characteristic stretching of alkenes (C=C) is at 1604 cm -1 [13]. For MNR, the characteristic bands of the anhydride group grafted on the natural rubber backbone were observed. A peak at 1710 cm -1 is attributed to the carbonyl group formed by the reaction of the anhydride functional group with moisture and the characteristic band of succinic anhydride symmetrical C=O stretch was observed at 1743 cm -1. Figure 2: Infrared spectra of STR5L and MNR using 6 phr of maleic anhydride by dissolving in toluene. The crosslink density was measured from the increase in torque from the cure characteristic test, where ΔT% of unfilled NR and unfilled MNR are 556% and 535%, respective. This indicated that crosslink density of the unfilled MNR is lower than that of the unfilled NR. For RHA-filled samples (NR-RHA and MNR-RHA), we found that the crosslink density of MNR-RHA (ΔT% = 588%) is higher than that of NR-RHA (ΔT% = 568%). Nevertheless, we found that all MNR compounds exhibited a long delayed onset of vulcanization compared to that of natural rubber, which may be due to the reaction of sulphenamide-accelerator (CBS) with free maleic anhydride molecules and/or succinic acid.

5 415 Suwatthana Phrommedetch and Cattaleeya Pattamaprom The tensile properties of unfilled NR and MNR compounds are compared with those of the compounds using RHA as reinforcing fillers in figure 3. As can be seen, the tensile stresses at low strains are comparable for the unfilled NR and MNR. However, above 700% strain, the unfilled NR exhibits abrupt strain hardening, whereas strain hardening of the unfilled MNR occurs at a slower rate. This may be due to lower degree of strain-induced crystallization. Nevertheless, the %elongation at break of unfilled MNR is higher than MR by about 150% indicating a tougher compound. When mixing RHA with these compounds, RHA shows much stronger reinforcing effect in MNR than in NR, as can be seen by the dramatic change in the stain and degree of strain hardening. This is due to the formation of chemical interaction between polar groups in MNR molecules and in RHA molecules. When we compared MNR-RHA with NR/MNR-RHA (formula E in table 1), %elongation of NR/MNR-RHA was better than MNR-RHA because the compound adopted good strain hardening from NR coupled with good compatibility from MNR. Figure 3: Stress-Strain curves of natural rubber and maleated natural rubber compound mixed with and without rice husk ashes. In figure 4, the stress-strain curves of natural rubber and maleated natural rubber compounds using RHA and BA as reinforcing fillers were compared to that of carbon black, a conventional reinforcing fillers. As can be seen, in conventional NR compounds, carbon black provided much higher strength than BA and RHA. When the hydrophilicity of NR was improved through maleation, RHA and BA composites (MNR-RHA and MNR-BA) provided better reinforcing effect. Besides, the stresstrain properties of MNR-RHA increase even beyond that of the conventional NR-CB compound, especially when the % strain was higher than 400%. Nevertheless, improvement is still needed to increase %elongation at break for this compound. Figure 4: Stress - Strain curves of natural rubber and maleated natural rubber compounds using RHA and BA as reinforcing fillers compared to those of conventional fillers.

6 Compatibility Improvement of Rice Husk and Bagasse Ashes with Natural Rubber by Molten-State Maleation Conclusion This study proposed an alternative in improving rubber-filler compatibility of natural rubber and agricultural waste composites by modifying on the molecular structure of natural rubber through maleation. The maleation technique used here was reactive melt-mixing technique, where the rubber was compounded directly with maleic anhydride. The results showed that the the moduli of maleated composites using rice husk ashes and bagasse ashes as fillers were improved from the conventional composites. Nevertheless, the fact that MNR compounds still exhibited a long delayed onset of vulcanization compared to that of natural rubber should be further improved. 5. Acknowledgement The authors are grateful to the financial support from Thammasat University. References [1] Sae-oui, P., C. Rakdee, and P. Thanmathorn, Use of Rice Husk Ash as Filler in Natural Rubber Vulcanizates: In Comparison With Other Commercial Filler, Journal of Applied Polymer Science, 83: [2] Arayapranee, W., N. Na-Ranong, G. L. Rempel, Application of Rice Husk Ash as Fillers In The Natural Rubber Industry, Journal of Applied Polymer Science 98: [3] Haxo, H. E. and P. K. Mehta, Ground Rice-Hull Ash as A Filler For Rubber, Rubber Chemistry and Technology, 48(2): [4] Ishak, Z. A. M. and A. A. Bakar (1995). An Investigation On The Potential Of Rice Husk Ash As Fillers For Epoxidized Natural Rubber (ENR), European Polymer Journal 31(3): [5] Ishak, Z. A. M., A. A. Bakar, U.S. Ishiaku, A.S. Hashim, and B. Azahari (1997). An Investigation of the Potential Of Rice Husk Ash As A Filler For Epoxidized Natural Rubber - II. Fatigue Behaviour. European Polymer Journal, 33(1): [6] Visconte, L.L.Y., C.T. Andrade, C.J. Azuma, UV-Light Induced Crosslinking Reaction of Cinnamate Natural Rubbers: Effect Of The Spacer Length, Counter-Ion Nature, and Content. Journal of Applied Polymer Science, 69(5): [7] Visconte, L. L. Y., C. T. Andrade, C.J. Azuma, Photosensitivity of Modified Natural Polyisoprenes As Function of the Alipathic Side Chain. Polymer Bulletin 25(2): [8] Pyke, J.B., Bauer, R.G., Cohen, M.P., Handa, P.K., US Patent no. 4,996,263. [9] Saedan, M., T. Navarat, A. Sombat, Grafting Of Maleic Anhydride on Natural Rubber Molecules in A Molten State, Research Report of the National Research Council of Thailand (NRCT), Bangkok, pp [10] Carone Jr, E., U. Kopcak, M.C. Concalves, S.P. Nunes, In Situ Compatibilization of Polymide 6/Natural Rubber Blends With Maleic Anhydride. Polymer 41(15): [11] Nakason, C., A. Kaesman, S. Homsin, and S. Kiatkamjornwong, Preparation and Properties of Organosoluble, Colorless, and High-Pretilt-Angle Polyimides Based on an Alicyclic Dianhydride and Long-Main-Chain Alkyl-Group-Containing Diamines. Journal of Applied Polymer Science 81(11): [12] Nakason, C., A. Kaesaman, and P. Supasanthitikul, the Grafting of Maleic Anhydride onto Natural Rubber. Polymer Testing 23(1): [13] Brown, W., T. Poon, Introduction to Organic Chemistry; 3 rd edition; John Wiley & Sons, Inc.: United States, p 31.