Several aliphatic waterborne polyurethane prepolymers are prepared from isophorone diisocyanate

dimethylolpropanic acid

poly(butylene adipate) (PBA) and polypropylene glyco (PPG)．The prepolymers are blocked with 2-methoxyethanol

followed by neutralization pendant carboxyl by triethylamine

then diluted to 50 wt％ with methyl ethyl ketone

emulsified by adding deionized water under constant agitation．The principle of emulsification is explained．The aggregated structure of prepolymers in organic solvent is discussed．The prepolymer molecules with ionic groups can form segment microionic lattices．The emulsification process and phase inversion are characterized by using the variation of viscosity and conductivity．The emulsification process includes the dissociation of the segment microionic lattices and the aggregation of the hydrophobic segments．The inversion process can be clearly divided into three regions depending upon the variation of viscosity and conductivity．The experimental results show that several factors can affect the phase inversion heavily

including the molecular weight and type of soft segments

the[NCO]／[OH] ratio

carboxyl content and neutralization degree．With the increasing of the molecular weight of soft segment

the phase inversion delays．In addition

the phase inversion of PBA-based prepolymer occurs earlier than that of PPG-based prepolymer

while their soft-segment molecular weights are equal．High[NCO]／[OH] ratio not only decreases the viscosity of both prepolymer solution and emulsification system

but also makes phase inversion occur earlier．With the increasing of carboxyl content

the phase inversion occurs early．In the meantime appropriate carboxyl content can decrease the viscosity of phase inversion point and lead to easy phase inversion．The decrease of neutralization degree can contribute to low viscosity and early phase inversion．Selecting appropriate conditions can optimize the emulsification process．