Effect of Liquid Properties on Curtain Stability in Low Flow Rate

摘要

Curtain coating, which can achieve very high speed coating and smooth film without defects, is recently used to produce pre-painted steel sheets for home appliances and utensils. One of the difficulties limiting curtain coating operation arises from a low limit in flow rate, below which the falling liquid sheet breaks apart and we can not operate.We measured the minimum flow rate for curtain formation for polyester resin solutions in very low Reynolds number (Re) experimentally, and found that the minimum flow rate decreased with decreasing surface tension and density, and with increasing viscosity of the liquids.Relationship between the physical properties of the liquids and the minimum flow rate for curtain formation (curtain stability) was analyzed by using Reynolds number (Re) which represented the flow rate and Physical property number (Ka) which included surface tension, viscosity and density. As a result, the calculating formula by which the minimum flow rate can be estimated was given.Further more, effects of surfactant on the minimum flow rate were studied. It is suggested that homogeneity of the surface is very important to stabilize the curtain, and if we use surfactants which decrease surface tension of the liquids uniformly, the curtain stability increases. In the range of this study, curtain stability is mainly dominated by liquid properties, and neither edge guides nor disturbances like bubbles in the liquids seem key factors for curtain stability.