Investigation of the mechanism of lubrication in starch–oil composite dry film lubricants

Abstract

The boundary coefficient of friction (COF) of starch-oil composite dry film lubricants was investigated as a function of starch type (waxy vs. normal purified food grade corn starch), oil chemistry (hexadecane vs. oleic acid and various vegetable oils), and starch-to-oil ratio. Based on the results, a mechanism of starch-oil interaction in these composites was proposed. According to the proposed mechanism: (a) the oil in the composite is distributed between the bulk and the surface of the starch; and (b) the fraction of the oil trapped in the bulk and that adsorbed on the surface are related to each other by an equilibrium constant, and are functions of the total oil concentration in the composite. In line with the proposed mechanism, an adsorption model was used to quantify the free energy of adsorption (᭝G ads ) of the polar oils onto the starch surface. The analysis gave ᭝G ads values that were higher than those reported for the adsorption of the same polar oils onto steel surfaces. This result is consistent with the effect of the relative surface energies of steel and starch on the adsorption of polar oils. The adsorption property of the non-polar hexadecane relative to the polar oils was estimated by comparing their interfacial tensions with starch. The result showed a higher interfacial tension for hexadecane-starch than that for the polar oil-starch composites. This result predicts a relatively poorer compatibility with, and, hence, poorer adsorption of hexadecane onto starch leading to higher COF, as was observed in the friction measurements.

Friction properties of chemically modified starch

“…The effect of DS is consistent with the expectation that starches with lower DS will have more free hydroxyl groups and, hence, more adsorption sites than those with high DS. This explanation is supported by the data in Table III, which compares ∆G ads from this work with that of soybean oil on starches that were solubilized in water using a steam-jet cooking process called Fantesk TM [21]. It is clear from this comparison that the adsorption of vegetable oils on steam-jet cooked starch, which does not have any of its free hydroxyl groups converted to esters (i.e.…”

“…Equations (4) and (5) can be used to analyse boundary friction data such as that shown in Figure 5 [19][20][21]. This is possible because the fractional surface coverage, θ, can be estimated from boundary friction data using the following relationship [19][20][21]:…”

“…This is possible because the fractional surface coverage, θ, can be estimated from boundary friction data using the following relationship [19][20][21]:…”

Emulsions stabilized using potato starch

“…Fat interactions with starch are only well recognized on a molecular level in systems containing single fatty acids (12)(13)(14)(15)(16). Studies of the effects of starch in emulsions have been carried out mainly in systems containing vegetable fat (17,18).…”

Recent Processing Methods for Preparing Starch‐based Bioproducts

“…Non-food applications demonstrating the efficacy of oil delivery in aqueous starch-based media included biodegradable polyurethane foams (Cunningham et al ., 1997 ), lubricants for water-based oil drilling muds (Fanta et al ., 2002a ;Sifferman et al ., 2003 ) and lubricants for metal working (Biresaw, 2004 ;Biresaw and Erhan, 2002 ;Biresaw et al ., 2007 ;Kenar et al ., 2009 ). The jet-cooked starch-oil technology was also used to deliver a soybean oil-based UV absorbing agent to provide UV protection in cosmetic and agricultural applications, and was shown to increase the efficiency of UV absorption (Compton et al ., 2007 ).…”