A method of producing PUD by a solvent free process includes drying polyol in vacuum; placing the dried polyol and DMBA in a reaction vessel to mix until a first mixture becomes transparent; adding diisocyanate which having functional group of isocyanate(—NCO) to the first mixture to react until —NCO terminated first prepolymer is formed; decreasing temperature of the —NCO terminated first prepolymer to 65° C.; adding a neutralizing agent to the —NCO terminated first prepolymer until —NCO terminated second prepolymer is formed; adding the remaining diisocyanate having isocyanate functional group to the —NCO terminated second prepolymer to form a second mixture; adding deionized water to the second mixture to form prepolymer dispersion; and adding chain extension agent to the prepolymer dispersion to rotate at a range of 200 to 2,000 rpm for chain extension for 1-3 hours until solid content 40.0 wt. % solvent free PUD is produced.