Doctor of Philosophy
School of Mechanical, Materials and Mechatronics Engineering
Novareza, Oyong, Tribology of aqueous copolymer lubricants for metal forming applications, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4062
The tribological properties of aqueous symmetrical tri-block 'normal‘ and 'reverse‘ copolymer solutions as metal forming lubricant have been investigated. The copolymer consists of hydrophilic poly-ethylene oxide (PEO) blocks and hydrophobic polypropylene oxide (PPO) blocks. In aqueous environment the different structure has a different tribological behavior due to different physio-chemical characteristics namely cloud point, HLB, critical micelle concentration (CMC) and temperature. The study is confined to two normal (serial) symmetrical tri-block copolymers, L62 (PEO6-PPO32- PEO6) and L64 (PEO13-PPO30-PEO13) that has longer hydrophilic PEO ends. The reverse tri-block copolymers were 17R4 (PPO14-PEO24-PPO14) and 25R2 (PPO21- PEO14-PPO21). The 25R2, which contains a lower percentage of PEO but has longer hydrophobic PPO blocks were selected in order to determine the effect of the hydrophobic chain length. The interaction of phosphate ester, an anionic surfactant, and the aqueous copolymer solutions was also investigated. In aqueous form, copolymers are soluble non-ionic amphiphilic surfactants.
For solutions used at below and above the cloud point, the dynamic friction, wear volume and surface roughness were measured with ball-on-disk tribometer. The copolymers were blended with water in 0.06 g/ml concentration. Low carbon steel and stainless steel surfaces were tested. AFM was used to measure the surface roughness of the worn track, and the wear modes of the track were investigated with SEM images. The solutions when used below the cloud point exist in unimer state for PPOn-PEOm- PPOn and micelle for PEOm-PPOn-PEOm. They formed stronger adsorbed film than that above the cloud point. However, addition of phosphate ester as extreme pressure additive into aqueous solutions produced stronger adsorbed lubricant film protecting the surface. Low COFs were obtained from solutions below and above the cloud point. For the wear volume, the presence of phosphate ester produced lower wear. The copolymers and phosphate ester were found to form a synergistic interaction, which protect the surfaces more effectively than other solutions used in this study.
The different characteristics of these copolymers are expected to affect the adsorption mechanism of the copolymers on mild steel and stainless steel. The adsorbed lubricant film thickness is one of the parameters to understand the adsorption mechanism. Two different thickness measurement techniques using ellipsometer and atomic force microscope (AFM) were used in this thesis. The AFM results of the film thickness can contribute to the understanding of the adsorption of the copolymer and added additive. The majority of film thickness was found thicker when phosphate ester was added into solutions.
The strength of adsorbed lubricant film formed on MS and SS surfaces has been investigated based on the critical load points when the adsorbed film delaminates on surfaces using the progressive load method on a micro-scratch instrument. Two copolymers 17R4 and L64 were investigated with this technique. Both of these copolymers have comparable molecular weight and also similar hydrophobic PPO and hydrophilic PEO blocks number.
The copolymers are in dual-phase above the cloud point and produce thicker adsorbed films compared to those below cloud point. It was found that the adsorption film above the cloud point is weaker than that below the cloud point. Below the cloud point, the micelle of normal copolymers (L64) has been found to form a stronger adsorbed layer compared to the unimer phase of reverse copolymers (17R4). The EP additive phosphate ester when added into the copolymer solutions have produced superior adsorption performance in terms of strength, friction and wear. Different phases due to the temperature and the structure of the copolymers show insignificant effect on the strength of the adsorbed lubricant film.
A proposed adsorption mechanism on the metal for each of copolymers type below and above the cloud point and the mixture of copolymers and phosphate ester was developed based on the results of tribological performances, film thickness and adsorbed copolymer strength. The adsorption of normal copolymers and reverse copolymers on the oxide layer of MS and SS surfaces is primarily driven by hydrophobic interaction between PPO and the oxide layer, the hydrogen bond between PEO and water and the effect of temperature on the size of the PPO and PEO blocks.
Below the cloud point, a reverse copolymer is in unimer phase and adsorbs to the metal surface by the PPO blocks that are expected to form loops, serial loops and trains when they anchor on MS and SS surfaces. The PEO is buoyed in solution, not perfectly folded and tends to create an irregular shape. On the other hand, normal copolymers that exist in micelle are adsorbed by forming hemispherical shape where the core containing the PPO blocks is attached onto the MS and SS surface whereas the PEO is extended outward to the bulk solution.
When the solution temperature increases, the hydrophobic PPO chain becomes more hydrophobic whereas the hydrogen bond between the PEO and water is weakened and starts to dehydrate and becomes less hydrophilic. When the solution reaches the cloud point, the hydrogen bond breaks and the PEO is separated out from the water. The PPO shrinks as it becomes more hydrophobic and minimizes contact with water.
Above the cloud point, the PPO becomes more hydrophobic and is expected to create more interaction with partially hydrophobic surfaces of MS and SS. When the EP additive phosphate ester is mixed with copolymer, two kinds of interaction were found to drive the adsorption. The anionic surfactant phosphate ester adsorbs to the oxide layer by an electrostatic interaction whereas the copolymers form hydrophobic bond with hydrophobe of the phosphate ester. Although the main driver for the adsorption was found the same, the shape of adsorbed components was found to be different. The EP expected to create the first layer on top of the metal surface followed by the copolymers that adsorb onto the top of EP layer. The adsorption of the copolymers to the EP layer is in similar manner with the adsorption of the copolymers to metal surfaces without the added EP.