Boundary Lubrication of PEO-PPO-PEO Triblock Copolymer Physisorbed on Polypropylene, Polyethylene, and Cellulose Surfaces

Abstract

In situ lateral force microscopy (LFM) and X-ray photoelectron spectroscopy (XPS) were used to probe the lubrication behavior of an aqueous solution of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) (PEO-PPO-PEO) symmetric triblock copolymer on thin films of polypropylene (PP), polyethylene (PE), and cellulose. LFM experiments were carried out while the substrates were immersed in water and in solutions of the copolymer. The friction coefficient on PP and PE was reduced after adsorption from the PEO-PPO-PEO aqueous solution while the opposite effect was observed for cellulose surfaces. A critical normal loading force, at which the friction coefficient of the lubricated and unlubricated surfaces is equal, was identified and related to the affinity of the polymer with the substrate. Further experiments were performed to mimic practical operations involving lubricant addition during manufacturing and postprocessing removal. XPS was used to verify the presence of the lubricant on the polymeric substrates and to evaluate its removal by water washing. The lubricant layer was easily removed by water from the PP and cellulose surfaces while a durable layer was found on PE. The XPS results were in agreement with the highest critical normal loading force measured for PE (52 nN for PE in contrast to a minimum of 10 nN for cellulose). While several reports exist on lubrication on hard surfaces, friction behavior on soft surfaces is still not well documented as the substrates usually deform under loading pressure. Therefore, we also propose a simple lubrication model for PP, PE, and cellulose and the use of critical normal loading force as a parameter to predict lubricity and durability of adsorbed nonionic block copolymers.

Friction and anti-wear property of aqueous tri-block copolymer solutions in metal forming

“…It must be noted that the level of hydrophobicity of the surface also plays important role in the adsorption of the copolymers. As copolymer is known to adsorb well on hydrophobic surface [4,[14][15][16][17][18][19]25], the film formation capability of the aqueous copolymers reported here may be different on other metallic surfaces with different level of hydrophobicity.…”

“…In aqueous room temperature environment, PEO m -PPO n -PEO m copolymers adsorb via PPO middle block anchored to the surface and the two PEO end blocks extend out into bulk solution [4,[15][16][17][18]25]. This may explain the poor adsorption of the normal copolymer at above cloud point.…”

“…However the structural difference of the copolymers types is expected to influence their tribological performance, even though both have the same molecular weight and composition [6]. PEO n PPO m PEO n have been reported to adsorb well on hydrophobic surfaces such as polystyrene latex [14], polypropylene and polyethylene [15,16], poly(ethylene terephthalate), nylon, graphite [17], self-assembled hydrophobic surface [18], and hydrophobic elastomer [19] but the adsorption on metallic surfaces have not been studied.…”

Surface Film Adsorption and Lubricity of Soybean Oil In-Water Emulsion and Triblock Copolymer Aqueous Solution: A Comparative Study

“…It is clearly demonstrated that the adsorbed copolymer layer can be removed from the surface by means of a simple surface cleaning process ( Figure 13 left) while those lubricated by the VO/W emulsions still have residue on the surface (Figure 13 right). While vegetable oils tendency to produce ROOH hydroperoxide through oxidation results in thickening, varnish formation and sludge deposition on surfaces [26,63] and postulates poor cleanability, triblock copolymers have high wetting and spreading ability, thus are easily rinsed with Milli-Q water [8,20]. This direct observation, together with the QCM film removal of Figures 3 and 4 at rinsing stage (above 20 min) demonstrates the superior surface cleanability of the copolymer solutions compared to the VO/W emulsions, albeit with higher friction and wear.…”

“…This makes vegetable oil in water emulsion a cost effective green and renewable lubricant with low oil consumption, in accordance with the minimum quantity lubricant MQL strategy [18,19]. Aqueous triblock copolymer solutions are another class of low cost aqueous alternatives, possessing comparable tribological performance to conventional lubricants and excellent surface cleanability [8,20,21]. …”