화학공학소재연구정보센터
Chemical Engineering Science, Vol.55, No.18, 3639-3650, 2000
Experimental observation of dip-coating phenomena and the prospect of using motion control to minimize fluid retention
A computer controlled dip-coating apparatus was constructed to investigate physical aspects of transient coating processes and the feasibility of minimizing fluid retention during such operations. Electrobalance measurement of mass retention and drainage has elucidated a number of features which have not received prior notice. Relaxation processes were observed when the substrate motion was halted: the long-time behavior agreed with a combination of Landau-Levich and Jeffreys theories. However, short-time meniscus relaxation produced significant departures which are not fully understood. Comparisons of fluid retention were made for variable draw lengths: good agreement between Landau-Levich theory and experiment was observed even for very short withdrawal lengths, indicating the rapid transition to the steady-state regime. Computer-controlled substrate motion was performed to acquire preliminary information of its effect on mass retention. We investigated constant acceleration and constant speed withdrawal segments over a fixed process time. Experiments involving constant speed withdrawal periods interrupted by drainage periods revealed that more interruptions produced less fluid retention overall; in the limit of an infinite number of interruptions, the retention reached a minimum - equivalent to withdrawing the plate at the time-averaged speed. In addition, ramped speed experiments indicated very little difference over the accessible motion range. As a whole, the study revealed no procedures which could reduce mass retention below that which would be obtained with the slowest allowable constant speed withdrawal.