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Fly ash (FA) as a by-product of power station plants is known to consist of silicon dioxide similar to precipitated silica. The use of FA as filler in natural rubber (NR) was of interest to reinforce and/or reduce product cost. In this article, viscoelastic properties of FA-filled NR composites with various FA loadings were investigated with the utilization of two different modes of shear flow, namely, oscillatory and steady shear flow. It is found that the addition of FA to NR increases storage modulus (G 0 ) and shear viscosity under both oscillatory and steady shear flow. Moreover, the oscillatory test results exhibit the unexpected increase in magnitude of viscous response with increasing FA loading in FA-filled NR compounds. The explanation is proposed in terms of the ball-bearing effect of FA with spherical shape associated with the occurrence of molecular degradation induced by inorganic constituents particularly manganese, iron, and copper in nonrubber component of NR as well as the small amount of heavy metals including iron, copper in FA. An isoprene rubber (IR) containing no nonrubber component was used to validate the proposed explanation. In addition, with the use of Cox-Merz concept, the results of both complex viscosity under oscillatory shear flow and apparent shear viscosity under steady shear flow can effectively be superimposed in the case of FA-filled compounds, supporting the promotion of viscous response by FA.

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Molasses is an inexpensive byproduct made during the extraction of sugars from sugarcane. Viscous molasses was converted to powder form under heating process. Molasses black (MB), mainly consisting of carbon, was finally obtained. As a result of elemental analyzer, it is evident that MB containing 56.13 % of carbon. Additionally, Fourier Transform Infrared Spectrophotometer (FTIR) result confirmed that function groups of MB were comparable with carbon black (CB). Nevertheless, the large particle size and low specific surface area of MB was less effective for reinforcement. Therefore, in this research, a feasibility study was carried out on the utilization of MB together with carbon black as hybrid filler in natural rubber (NR). Reinforcement of NR with MB/CB hybrid filler at various ratios was studied. The total hybrid filler content was 30 parts per hundred of rubber (phr). The mechanical properties were determined. The results revealed that the NR vulcanizates containing hybrid filler exhibited the better reinforcement than that filled with single MB filler.

Silica extracted from rice husk ash (RHA-Si) was incorporated in deproteinized natural rubber (DPNR) latex compounds up to 20 phr. The cast films were prepared after the prevulcanization at 70°C for 2 hours. The effect of RHA-Si loading on swelling and tensile properties was investigated. Scanning electron microscope (SEM) was used to examine morphology of the prevulcanized films. Dynamic mechanical behaviors of filled prevulcanizates were studied using the dynamic mechanical analyzer (DMA). Two types of tests, namely, temperature sweep and strain sweep test were performed. Swelling test revealed the decrease in crosslink density with increasing RHA-Si loading more than 10 phr. Tensile strength and elongation at break decreased with increasing RHA-Si loading. Conversely, the presence of RHA-Si increased the modulus at 100% strain of the prevulcanized DPNR latex films. The poor filler-rubber distribution was evidenced by SEM image. The DMA results exhibited that storage modulus (G′) and damping factor (tanδ) increased with increasing RHA-Si loading. The narrow linear viscoelastic (LVE) region was noticed for prevulcnizates filled with high RHA-Si content. Glass transition temperature (Tg) of highly filled DPNR shift to low temperature.

In this research, the preparation of carbon powder from molasses, a by-product of the sugar production process or agricultural wastes, was studied as filler in natural rubber. The carbon powder from molasses or molasses black (MB) was subsequently activated using 1:4 by weight (MB: KOH) using heating energy from microwave radiation and comparing with the electrically heating furnace. The result showed the surface area and total pore volume of MB activated by microwave radiation (MB KOH Wave) are clearly enhanced comparable with those activated using an electrically heating furnace. All of KOHactivated MB were used as filler in NR in the amount of 10 phr. It was found that MB KOH Wave filled NR obviously shows the improvement in cure characteristic, tensile properties, tear strength and swelling resistance of NR vulcanizate and is close with that of filled with carbon black (CB).

Rice husk is an agricultural residue from a rice-mill known as a rich source of silica. In this study, the rice husk was extracted for the contained silica by pretreatment with hydrochloric and calcination at 700 °C, and the white rice husk ash (WRA) mainly consisting of the silica was finally obtained. WRA was then investigated for its composition and structure with X-ray fluorescence (XRF) and X-ray diffractometer (XRD). The results showed that WRA contained a large amount of silica (90%) with amorphous structure. After that, WRA was used as filler for the preparation of natural rubber products. Two kinds of the natural rubber products were prepared here: 1) the pure natural rubber (PNR) product and 2) the deproteinized natural rubber (DNR) product. DNR was a protein removal natural rubber which is suitable for use as a dental material with the low allergenic properties caused from the proteins. Additionally, DNR used in this study could be provided by removing proteins from the natural rubber latex with 0.1 % urea in the presence of surfactant at room temperature. It was found that the nitrogen content (main element of proteins) of DNR was lower than PNR under the test condition. For the preparation of natural rubber products, WRA was mixed into the PNR and DNR latexes comparing with the commercial silica (Hisil-233). All finished latexes were compounded to obtain the natural rubber products. The viscoelastic properties and the cure characteristics of the entire products were investigated. It can be observed that WRA can be comparable with the commercial silica for use as filler for the natural rubber products. This is an attractive option to produce dental materials from the agricultural residues, which will reduce the cost of the production and also add value to the agricultural residues.

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