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tends <strong>to</strong> oxidize rapidly, even at a moderately<br />

increased operating temperature. As the tem‐<br />

perature increases, the oils thicken due <strong>to</strong> its<br />

tendency <strong>to</strong> enter in<strong>to</strong> viscosity‐increasing reac‐<br />

tions in the presence of atmospheric oxygen.<br />

Similar reaction occurs when the temperature<br />

drops as the oil will begin <strong>to</strong> solidify. Rapeseed<br />

oil, corn oil, and sunflower oil have a solidifica‐<br />

tion point of ‐16 o C, ‐20 o C and ‐17 o C respectively<br />

[4, 7, 10]. Palm oil is even worst, solidifying at a<br />

relatively high temperature of 34.1 o C [11]. Even<br />

as the temperatures drop and approaching the<br />

solidification temperatures, the oils will experi‐<br />

enced a marked increase in viscosity and may<br />

cause problem in cold weather [8]. These prob‐<br />

lems however can be easily fixed by mixing the<br />

vegetable oil with synthetic esters and/or by<br />

adding additives <strong>to</strong> improve its anti oxidant and<br />

pour point properties [7]. While the cost of syn‐<br />

thetic ester is very high, by mixing it with vegeta‐<br />

ble oil base will bring the <strong>to</strong>tal cost of the base<br />

oil down compared <strong>to</strong> a fully synthetic solution.<br />

New antioxidants that are suitable for vegetable<br />

oil yet harmless <strong>to</strong> the environment are also<br />

needed as current antioxidants are designed for<br />

mineral oils and some are quite <strong>to</strong>xic.<br />

Oxidative stability is dependant on the predomi‐<br />

nant fatty acids present in the vegetable oil. Oils<br />

containing mostly saturated fatty acids will have<br />

good oxidative stability compared <strong>to</strong> a vegetable<br />

oil containing oleic acid or other monounsatu‐<br />

rated fatty acids. Oils that contain mostly poly‐<br />

unsaturated fatty acids exhibit poor oxidative<br />

stability [8]. In other words, the oxidative stabil‐<br />

ity is inversely proportional <strong>to</strong> the degree of un‐<br />

saturation. The three most cultivated vegetable<br />

oils, the palm oil, soybean oil, and the rapeseed<br />

oil consist mainly of monounsaturated and poly‐<br />

unsaturated fatty acids. These lead <strong>to</strong> a general<br />

consensus of vegetable oils poor oxidative stabil‐<br />

ity compared with petroleum based oil and also<br />

<strong>MIMET</strong> Technical Bulletin Volume 1 (2) 2010<br />

the fully saturated synthetics such as synthetic<br />

esters, organophosphate and polyalphaolefin<br />

(PAOs) [8]. So as <strong>to</strong> provide for comparable per‐<br />

formance, vegetable oils formulations generally<br />

require higher doses of antioxidants [6]. Due <strong>to</strong><br />

the oxidative instability of these major vegetable<br />

oils, vegetable oils with high saturated acids is <strong>to</strong><br />

be used due <strong>to</strong> the high solidification points.<br />

On the positive side, vegetable oils offer excel‐<br />

lent lubricity and have a high intrinsic viscosity<br />

and extreme‐pressure properties. Well‐<br />

formulated vegetable oil‐based hydraulic fluids<br />

can pass the demanding Vickers 35VQ25 or Deni‐<br />

son T5D‐42 vane pump wear tests. Vegetable oil<br />

can perform satisfac<strong>to</strong>rily for years under mild<br />

climate and operating conditions, provided the<br />

oil are kept free of water contamination [10].<br />

Klein et al suggested that vegetable oil used as<br />

hydraulic fluid base oil can exhibit better low‐<br />

temperature stability without the need for the<br />

addition of pour point depressant or synthetic<br />

esters by adding ethylene oxide and/or propyl‐<br />

ene oxide in<strong>to</strong> the base oil. Among the base oil<br />

tested for this process are the coconut oil, palm<br />

oil, palm kernel oil, peanut oil, cot<strong>to</strong>n oil, soy‐<br />

bean oil, sunflower oil and rapeseed oil. The<br />

resultant mixture produced ethoxylated and/or<br />

propoxylated base oil has been proven <strong>to</strong> have<br />

better pour point characteristic. This develop‐<br />

ment can result in inexpensive base oil for hy‐<br />

draulic fluid as fewer additives are needed <strong>to</strong><br />

make the fluid suitable for hydraulics applica‐<br />

tions [7].<br />

Aside from chemical processes <strong>to</strong> increase the sta‐<br />

bility of the vegetable oils, t<strong>here</strong> is an alternative<br />

method w<strong>here</strong> genetic modifications is employ on<br />

oil producing crops. Recent advances in genetic<br />

| MARINE FRONTIER @ <strong>UniKL</strong><br />

65

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