Dip coating assisted polylactic acid deposition on steel surface: Film thickness affected by drag force and gravity

Dip coating process has the potential of providing an easier and economical way to form a polylactic acid (PLA) layer on metal surface for various applications. The effects of dip coating operating parameters such as the withdrawal velocity, concentration, and viscosity of the solution on the film thickness were investigated in this study. Our experimental results show that the increase of withdrawal velocity leads to the increase of PLA film thickness at the relatively low withdrawal velocity regime. However, at the relatively high withdrawal velocity regime of the dip coating process, it was observed that the PLA layer thickness starts to decrease with increasing withdrawal velocity. It is suggested that the competition of drag force and gravity during the dip-coating process dominates PLA film formation. A mechanism was proposed to explain this special behavior which was never observed in the dip coating process.