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

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. ■ INTRODUCTION During conventional textile and fiber manufacturing processes, lubricants are used to reduce friction on the fiber’s surface and to prevent its abrasion. Textile lubricants are usually washed off after a fabric is manufactured either by woven, knit, or nonwoven processes. To be effective, lubricants must adhere well enough to the fibers to reduce friction during processing but be easily removed once the process is completed. In this paper we address lubricant adhesion and lubrication effectiveness on thin films of polymers commonly used in fiber and textile manufacturing. Fiber lubrication can be divided into three categories depending on the gap between the fiber and the sliding surface: boundary lubrication, hydrodynamic lubrication, and semihydrodynamic lubrication. According to the seminal work of Hansen and Tabor, boundary lubrication occurs at low sliding speeds and high contact pressures and in this regime the friction coefficient decreases with an increase of sliding speed, lubricant viscosity, and the reciprocal of pressure. On the contrary, hydrodynamic lubrication occurs when there is large interfacial spacing and a continuous fluid film extends between two surfaces sliding at high speeds. The friction coefficient in the hydrodynamic lubrication increases with an increase in sliding speed, lubricant viscosity, and the reciprocal of pressure. Due to limitations in equipment accuracy to probe the boundary regime, studies on fiber lubrication have primarily focused on hydrodynamic lubrication. As the layer of lubricant in the hydrodynamic regime is thick and continuous, this regime is solely governed by the rheological properties of the lubricant. The transitional regime between the boundary and hydrodynamic lubrication is known as the semihydrodynamic regime. Boundary lubrication plays an important role in fiber processing, particularly when the fiber surface slides over other surfaces at slow relative speeds, and it has recently raised the interest of the scientific community from both fundamental and practical points of view. In the 1960s, Fort et al. studied the boundary regime on synthetic yarns using a conventional boundary friction apparatus. They found that lubricants with carboxylic polar groups significantly reduced friction on synthetic fibers by adsorbing as a thin monolayer onto the surface. This effect was not observed when working with nonpolar lubricants. More recently, boundary lubrication has been linked to process parameters, such as spinning velocity, normal pressure, and lubricant viscosity. It is difficult to access boundary lubrication using friction measurement techniques. For example, the surface force apparatus (SFA) with tribology device and others require the surface to be very smooth. In contrast, textile materials often Received: October 5, 2011 Revised: January 31, 2012 Accepted: January 31, 2012 Published: January 31, 2012 Article