A new type of geopolymer composite was synthesized from two industrial wastes, red mud (RM) and rice husk ash (RHA), at varying mixing ratios of raw materials and the resulting products characterized by mechanical compression testing, X-ray diffraction, and scanning electron microscopy to assess their mechanical properties, microstructure, and geopolymerization reactions. Prolonged curing significantly increases the compressive strength and Young’s modulus, but reduces the ductility. Higher RHA/RM ratios generally lead to higher strength, stiffness, and ductility, but excessive RHA may cause the opposite effect. The compressive strength ranges from 3.2 to 20.5 MPa for the synthesized geopolymers with nominal Si/Al ratios of 1.68–3.35. Microstructural and compositional analyses showed that the final products are mainly composed of amorphous geopolymer binder with both inherited and neoformed crystalline phases as fillers, rendering the composites very complex composition and highly variable mechanical properties. Uncertainties in the composition, microstructure, the extent of RHA dissolution, and side reactions may be potential barriers for the practical application of the RM–RHA based geopolymers as a construction material.

Synthesis and characterization of red mud and rice husk ash-based geopolymer composites

Rice husk (RH) has now become a source for a number of silicon compounds, including silicon carbide, silica, silicon nitride, silicon tetrachloride, zeolite, and pure silicon. The applications of such materials derived from rice husks are very comprehensive. The methods of synthesizing these silicon-based materials from RHs and their applications are reviewed in this paper.

Silicon-Based Materials from Rice Husks and Their Applications

The effects of chemical modification of fiber surface in sisal–oil palm reinforced natural rubber green composites have been studied. Composites were prepared using fibers treated with varying concentrations of sodium hydroxide solution and different silane coupling agents. The vulcanisation parameters, processability conditions and tensile properties and swelling characteristics of these composites were analysed. The fiber reinforcing efficiency of the chemically treated biocomposites was compared with that of untreated composites. The extent of fiber alignment and strength of fiber–rubber interfacial adhesion was analysed from the swelling measurements. Composites containing chemically treated fibers were found to possess enhanced mechanical properties. The hardness and abrasion resistance of the untreated and treated composites were also analyzed. Surface characterization of treated and untreated sisal fibers by XPS showed the presence of numerous elements on the surface of the fiber. The fracture mechanism of treated and untreated fiber reinforced rubber biocomposites was investigated from SEM studies.

Effect of chemical modification on properties of hybrid fiber biocomposites

Nano and microcomposites of polymers elastomers and their reinforcement

The current study focuses on giving a basic understanding of tubular graphene sheets or carbon nanotubes (CNTs) and points towards their role in fabricating elastomer composites. Since the properties and the performance of CNT reinforced elastomer composites predominantly depend on the rate of dispersion of fillers in the matrix, the physical and chemical interaction of polymer chains with the nanotubes, crosslinking chemistry of rubbers and the orientation of the tubes within the matrix, here, a thorough study of these topics is carried out. For this, various techniques of composite manufacturing such as pulverization, heterocoagulation, freeze drying, etc. are discussed by emphasizing the dispersion and alignment of CNTs in elastomers. The importance of the functionalization technique as well as the confinement effect of nanotubes in elastomer media is derived. In a word, this article is aimed exclusively at addressing the prevailing problems related to the CNT dispersion in various rubber matrices, the solutions to produce advanced high-performance elastomeric composites and various fields of applications of such composites, especially electronics. Special attention has also been given to the non-linear viscoelasticity effects of elastomers such as the Payne effect, Mullin's effect and hysteresis in regulating the composite properties. Moreover, the current challenges and opportunities for efficiently translating the extraordinary electrical properties of CNTs to rubbery matrices are also dealt with.

Carbon nanotube based elastomer composites – an approach towards multifunctional materials

A laboratory-sized two-roll mill was used to incorporate rice husk ash into natural rubber (NR). A conventional vulcanization system was used for curing and cure studies were carried out on a Monsanto rheometer. Physical testing of the NR vulcanizates involved determining tensile and tear resistances and hardness. Swelling behavior of NR compounds and scanning electron microscopy were used to investigate the interaction between rice husk ash and natural rubber. Also, dynamical mechanical thermal analysis was used to assess filler–rubber interactions in terms of storage modulus (E′) and loss tangent (tan δ). For comparison purposes, two commercial fillers, precipitated silica (Zeosil-175) and carbon black (N774), were also used. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2331–2346, 2002

Mechanical and dynamic mechanical properties of rice husk ash–filled natural rubber compounds

Rice husk ash was incorporated into natural rubber (NR) using a laboratory size two-roll mill. Curing using a conventional vulcanization system (CV) was chosen, and cure studies were carried out on a Monsanto rheometer. Physical testing of the NR vulcanizates involved the determination of tensile, tear, and abrasion resistances, and hardness. Fourier transform infrared spectroscopy (FTIR) analysis was done to verify the presence of the characteristic functional groups of precipitated silica in MHA (milled husk ash) and THA (treated husk ash). The effect of the coupling agent, bis(3-triethoxysilylpropyl)-tetrasulfane (Si-69), on the curing and physical properties of the vulcanizates was investigated. A chemical treatment on a rice husk ash was done, and the effects of this procedure are also reported. For comparison, two commercial fillers, precipitated silica (Zeosil-175) and carbon black (N774), were also used. Although the presence of the silane coupling agent had not brought the expected increase in properties, treated husk ash showed exceptional performance in terms of tensile strength and abrasion resistance of the filled vulcanizates. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1019–1027, 2000

The effect of coupling agent and chemical treatment on rice husk ash‐ filled natural rubber composites

In a previous investigation, we observed that in the presence of a conventional vulcanization system, the addition of white rice husk ash (WRHA) to natural rubber (NR) compounds increased the rate of crosslinking and lowered the apparent activation energy (Ea) of the vulcanization reaction more strongly than the other fillers used. In this work, commercial fillers, such as precipitated silica (Zeosil-175) and carbon black (N762), were partially replaced by black rice husk ash and WRHA. Cure studies were carried out on a TI-100 curometer at 150, 160, 170, and 180°C, and the overall rates and the Ea's for the vulcanization process were calculated for each compound, with the assumption that vulcanization followed first-order kinetics. Again, WRHA showed some catalytic effect on the NR vulcanization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1405–1413, 2003

Rice-husk-ash-filled natural rubber. II. Partial replacement of commercial fillers and the effect on the vulcanization process

In preceding investigations the overall rate and apparent activation energy for the vulcanization process were calculated for natural rubber compounds, assuming that vulcanization follows first-order kinetics. It was ob- served that the addition of white rice husk ash (WRHA) increased the rate of crosslinking and lowered the apparent activation energy more profoundly than commercial fillers, precipitated silica (Zeosil-175) and carbon black (N762), with a conventional vulcanization system. In this work, a specific model for the vulcanization process accelerated by N-cyclo- hexylbenzothiazole sulfenamide was used to investigate the real role of WRHA in crosslink formation. Cure studies were carried out at 150°C, and the kinetics constants were evalu- ated. In relation to the other fillers, WRHA seems to develop catalytic activity, resulting in a positive effect on the specific rate of crosslink formation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1519 -1531, 2003

Rice husk ash filled natural rubber. III. Role of metal oxides in kinetics of sulfur vulcanization

Black rice husk ash (BRHA) and white rice husk ash (WRHA) were incorporated into natural rubber (NR) using a laboratory-size two-roll mill. A conventional vulcanization system (CV) was chosen and cure studies were carried out on a TI-100 Curometer. The torque curves were obtained at 150, 160, 170, and 180°C. The overall rate and the apparent activation energy for the vulcanization process were calculated for each compound assuming that vulcanization follows first-order kinetics. For comparison purposes, two commercial fillers, precipitated silica (Zeosil- 175) and carbon black (N762), were also used. It was ob- served that addition of WRHA to NR compounds increased the cross-linking rate and lowered the apparent activation energy more markedly than the other fillers. © 2002 Wiley

H. M. da Costa

Papers

Mechanical and dynamic mechanical properties of rice husk ash–filled natural rubber compounds

The effect of coupling agent and chemical treatment on rice husk ash‐ filled natural rubber composites

Rice-husk-ash-filled natural rubber. II. Partial replacement of commercial fillers and the effect on the vulcanization process

Rice husk ash filled natural rubber. III. Role of metal oxides in kinetics of sulfur vulcanization

Rice Husk Ash Filled Natural Rubber. I. Overall Rate Constant Determination for the Vulcanization Process from Rheometric Data