Improvement Of Thermo-oxidative Stability Research Articles

Overexpression of tocopherol biosynthesis genes in guayule (Parthenium argentatum) reduces rubber, resin and argentatins content in stem and leaf tissues

Natural rubber produced in stems of the guayule plant (Parthenium argentatum) is susceptible to post-harvest degradation from microbial or thermo-oxidative processes, especially once stems are chipped. As a result, the time from harvest to extraction must be minimized to recover high quality rubber, especially in warm summer months. Tocopherols are natural antioxidants produced in plants through the shikimate and methyl-erythtiol-4-phosphate (MEP) pathways. We hypothesized that increased in vivo guayule tocopherol content might protect rubber from post-harvest degradation, and/or allow reduced use of chemical antioxidants during the extraction process. With the objective of enhancing tocopherol content in guayule, we overexpressed four Arabidopsis thaliana tocopherol pathway genes in AZ-2 guayule via Agrobacterium-mediated transformation. Tocopherol content was increased in leaf and stem tissues of most transgenic lines, and some improvement in thermo-oxidative stability was observed. Overexpression of the four tocopherol biosynthesis enzymes, however, altered other isoprenoid pathways resulting in reduced rubber, resin and argentatins content in guayule stems. The latter molecules are mainly synthesized from precursors derived from the mevalonate (MVA) pathway. Our results suggest the existence of crosstalk between the MEP and MVA pathways in guayule and the possibility that carbon metabolism through the MEP pathway impacts rubber biosynthesis.

Improvement in Thermo-oxidative Stability and Mechanical Strength of Carbon Foams for Thermal Management of Reusable Launch Vehicles

Carbon foams were prepared and coated with silicon carbide (SiC) by chemical vapour infiltration (CVI) technique and tested for improvement of oxidation stability and compressive strength, compared to virgin carbon foams. Microstructural and morphological characterisation of SiC-coated carbon foam samples proved the formation of stoichiometric high strength, crystalline β-SiC over amorphous carbon which enables the ceramic foam to take high compressive loads. The final ceramic foam was successfully tested for oxidation stability up to a maximum temperature of 1650 °C under air atmosphere. The weight loss of SiC-coated carbon foam was found to depend on the final density, which depends on the SiC coating thickness. The functional property of reducing the back wall temperature of ceramic matrix composites (CMC) based light weight thermal protection systems (TPS) using SiC-coated carbon foam as the core material was validated by carrying out the plasma testing under atmospheric re-entry conditions. The process is noted for the cost-effective method of imparting desired ceramic property of SiC to less-expensive carbon foam structure.

Effect of functionalized CNTs and liquid crystalline polymer on thermo-oxidative stability of polyethylene-based hybrid composites

Hybrid composites based on polyethylene (PE) filled with thermotropic liquid crystalline polymer (LCP) and pristine or hydroxyl- or carboxylic-functionalized CNTs were prepared using melt extrusion process. Thermo-oxidative decomposition under dynamic and isothermal conditions was investigated using thermogravimetric analysis (TG). From TG profiles and activation energy analysis, the highest thermo-oxidative stability revealed from both dynamic and isothermal investigations was clearly observed for the PE-based hybrid composite filled with LCP and small amount of carboxylic-functionalized CNT. The melt rheological investigation showed that the effect of LCP and CNTs loading on viscoelastic characteristics of the composites was clearly pronounced at low frequency and the viscous part was found as a dominant factor. Complex viscosity, storage and loss moduli of the hybrid composite containing carboxylic-functionalized CNT showed the highest values compared among all samples examined. The morphological results observed using scanning electron microscopy and transmission electron microscopy indicated the improved dispersion of both LCP and carboxylic-functionalized CNT fillers. The obtained results indicated the importance of LCP-assisted dispersion of carboxylic-functionalized CNT through interface interaction, resulting in the remarkable improvement in thermo-oxidative stability of the composite.

TG–MS study on the effect of multi-walled carbon nanotubes and nano-Fe2O3 on thermo-oxidative stability of silicone rubber

In this work, an investigation was focused on how carbon nanotubes and nano-Fe2O3 improved the thermo-oxidative stability of silicone rubber (SR). A series of nanoparticles, including carbon nanotubes (CNTs), Fe2O3 with two different crystalline forms (α-Fe2O3 and γ-Fe2O3), a mixture of CNTs and γ-Fe2O3 (γ-Fe2O3 + CNTs), and γ-Fe2O3-modified CNTs (γ-Fe2O3 − CNTs), were involved in this article. Thermal degradation characteristics of SR samples filled with these nanoparticles were investigated through thermogravimetric analysis equipped with mass spectroscopy system (TG–MS), and the evolutions of prominent volatiles corresponding to different stages of decomposition were recorded. It was discovered that these nanoparticles affected the thermo-oxidative stability of SR in different ways. While CNTs had an effect of delaying every stage of the degradation of silicone rubber, Fe2O3 inhibited the oxidation of the side methyl, in which γ-Fe2O3 had better performance. γ-Fe2O3 − CNTs had the best effect on preventing the side methyl from degradation, which can be seen from the remarkable decreases in methanol, methanoic acid and methane by 58.91, 57.27 and 27.45 %, respectively. This effect was superior to CNTs, γ-Fe2O3 or a mixture of both at the same addition amount. Herein, the reason for the different performance of these nanoparticles, which make a profound contribution to further improvement in thermo-oxidative stability of SR, has been discussed.

The synergistic mechanism of thermally reduced graphene oxide and antioxidant in improving the thermo-oxidative stability of polypropylene

The feasibility of using thermally reduced graphene oxide (TrGO) to improve the antioxidative efficiency of commercial antioxidant (3,5-di-tert-butyl-4-hydroxycinnamic acid, AO) on unstabilized polypropylene (PP) was evaluated in terms of the oxidation induction time (OIT) and the initial degradation temperature (Ti). It was found that a synergistic effect exist between AO and TrGO in retarding the degradation of PP in oxygen-containing atmosphere. Compared with that of PP/AO (100/0.5) sample, the OIT value of PP/AO/TrGO (100/0.5/1) composite was almost doubled at 180°C and the value of Ti in air was improved by 37.2°C. As verified by using radical scavenging assay with the 1,1-diphenyl-2-picrylhydrazyl radical, the TrGO sheets exhibited a good radical scavenging capacity. The synergistic stabilization mechanism was attributed to the enhanced dispersion of TrGO sheets in the PP matrix in the presence of 0.5wt.% AO, which could improve the oxygen barrier effect and radical scavenging efficiency of TrGO. This synergistic effect between AO and TrGO can efficiently reduce the concentrations of oxygen and peroxy radicals in the PP matrix, leading to the significant improvement in thermo-oxidative stability of PP/AO/TrG composite.

Improvement of thermooxidative stability of non‐edible vegetable oils of Indian origin for biodegradable lubricant application

AbstractFor environmental reasons, as well as the dwindling source of petroleum, a new class of environmentally acceptable and renewable lubricants based on vegetable oils is available. Even though vegetable oils possess excellent lubricant‐related properties, there are some concerns about using it as lubricant base oil. Still, unmodified and modified varieties of soybean, rapeseed, sunflower and canola oils have been in use in the USA and Europe. In India, with the shortage of edible oil, alternate sources of vegetable oils stocks are being explored. With this aim, a comprehensive study has been conducted earlier in the authors' laboratory. In this study, numerous options of non‐edible vegetable oil sources were explored, and a few potential vegetable oils were studied in the laboratory. It was found that even though the oils performed much better in comparison with other vegetable oils, it still required improvement in thermooxidative stability. Therefore, in the later part of the study, different options were explored to improve thermooxidative stability. With a background on the initial studies of the authors as described above, the present paper deals with the studies on improvement of these non‐edible candidate vegetable oils of Indian origin for lubricant by treating with selected antioxidants for applying them in lubricants. Copyright © 2010 John Wiley & Sons, Ltd.

Film-Forming Properties of Polyol Esters, Polyphenyl Ethers and Their Mixtures Over a Wide Range of Temperature

There is a growing interest in lubrication at very high temperature and in the development of synthetic lubricants to satisfy the requirements of high temperature systems. Polyolesters, polyphenylethers and their mixtures which have excellent stability at high temperature are possible candidates. In this study, the film-forming properties of these fluids were investigated over a wide range of temperature, from 40° to 240°C. As a result the following findings were made. The effective pressure-viscosity coefficients and thus elastohydrodynamic film-forming properties of PPEs are, on the whole, greater than those of POEs. Considering mixtures of POE and PPE, both thermo-oxidative stability and film forming properties can be improved by the addition of PPE to POE. Mixture of R4P2E and POE result in the effective improvement of thermo-oxidative stability as well as effective pressure-viscosity coefficient and are thus good candidates for high temperature usage. A good correlation between the effective pressure-viscosity coefficient and viscosity was also found Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999