Microfiltration of Polyoxyalkylene Metalworking Fluid Lubricant Additives Using Aluminum Oxide Membranes

Microfiltration is capable of reducing health hazards and environmental pollution associated with metalworking fluids (MWFs) by enabling recycling and microbial removal. This paper investigates chemical characteristics of MWFs that can lead to flux decline during microfiltration using aluminum oxide membranes by studying the family of polyoxyalkylene diblock copolymers comprised of ethylene oxide and propylene oxide. These copolymers are commonly used as lubricant additives in metalworking fluids and serve as a model for beginning to understand the relationship between metalworking fluid formulation and microfiltration flux. It is found that increasing the hydrophobic content of the copolymers can lead to reduced flux. Anionic modification and increasing molecular weight of the copolymers can also lead to reduced flux. Insufficient cleaning of anionic copolymers from the membrane leads to pH-dependent flux of deionized water during subsequent filtration. The pH-dependence of flux arises due to swelling caused by electrostatic repulsions between the aluminum oxide surface and anionic copolymers that remain adsorbed to the surface of the membrane. This swelling serves to resist permeate flow above the isoelectric point of aluminum oxide. This phenomenon is observed directly using Field Emission Environmental Scanning Electron Microscopy (FE-ESEM).