Synthetic Oxidation Stability

Other high performance polymer backbones have been explored as PEM materials in addition to poly-(arylene ether)s and polyimides. Ductile copolymers with high modulus and glass transition values are desirable PEM candidates. The hydrolytic and oxidative stability of many of these materials remains to be determined. Nevertheless, interesting synthetic methodologies have been employed to investigate these materials, which have been instructive in the search for new PEM candidates. 

Lubricants are formulated products composed of a base stock, which is either a mineral or synthetic oil, and various specialty additives designed for specific performance needs. Additive levels in lubricants range from 1 to 25% depending on the application. Synthetic base stocks are oligomers of small molecules, synthesized to a defined molecular weight. Important performance indicators include viscosity index which measures the viscosity index behavior over a temperature range, oxidative stability, and pour point. The performance of synthetic and mineral oils (Morse, 1998 Shubkin, 1993) is summarized in Table 2.7. 

These full synthetics have been all PAO or ester based, and a mixture of ester plus PAO. The addition of PAO or ester to petroleum based engine oils for improved oxidative stability has shown average quality results. The addition of lower levels of PAO or ester base stock <15 wt % to petroleum based formulations show little or no improvement in the thermo-oxidative engine test. The predominant automotive synthetic base stocks (PAO, diesters, polyol esters) do not show any hydrolytic instability in engine oil applications. 

Although there are numerous publications on the effect of natural and synthetic antioxidants on the stability of oils and fats used as food and feed, until recently relatively little publicly available information was available on the effect of antioxidants on the oxidative stability of biodiesel. One of the earliest studies reporting of the effects of antioxidants on biodiesel was that of Du Plessis et aL (1985), which examined storage stability of sunflower oil methyl esters (SFME) at various temperatures for 90 d. Effects of air temperature, presence of light, addition of TBHQ (see Figure 1.1) and contact with steel were evaluated by analysis of free fatty acid content, PV, kinematic viscosity, anisidine value, and induction period. Addition of TBHQ delayed oxidation of samples stored at moderate temperatures (<30°C). In contrast, under unfavorable (50°C) conditions, TBHQ was ineffective. 

Schober and Mittelbach (2004) also investigated the impact of concentration of eleven synthetic phenolic antioxidants on oxidative stability of RME, UCOME and TME) by EN 14112 (Anon., 2003c) and on other relevant fuel... 

Liang et al. (2006) investigated the effect of natural and synthetic antioxidants on oxidative stability of crude and distilled palm oil FAME (PME). Crude palm oil contains minor components such as carotenes and a-toco-pherols that upon transesterification, yields crude PME that exhibit superior oxidative stability (OSI > 25h) compared to distilled PME (OSI - 3.5h), which does not contain minor components. Liang and colleagues therefore endeavored to improve the oxidative stability of distilled PME so that it meets the minimum OSI = 6h as specified in EN 14214 (Anon., 2003b). Natural (a-tocopherol) and synthetic (BHT and TBHQ) antioxidants were investigated in distilled PME. It was discovered that both natural and synthetic antioxidants exhibited beneficial effects on the oxidative stability of distilled PME... 

Namral as well as synthetic antioxidants can improve the oxidative stability of edible oils. Most common natural antioxidants are mixed tocopherols. Tertiery-butylhydroquinone (TBHQ) is one of the most common synthetic antioxidants used in commercial frying oils. This also costs only a fraction of the natural antioxidants. 

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