INVESTIGATIONS ON THE STABILITY OF THE LI QUID BRIDGE IN THE SLIDE HOPPER TYPE COATING TECHNIQUE .4. EFFECT OF VISCOELASTIC FLOW PROPERTIESON THE STABILITY OF THE LIQUID BRIDGE

In the method of simultaneously applying multiple layers of coating li quids to a moving web with a slide hopper type coating apparatus the l iquid bridge, formed between the lower edge of the slide hopper and th e moving web has to be stabilized against oszillations. The stabilizin g effect of a suction to the lower surface of the liquid bridge and th e stabilizing action of added anionic surfactants are known. In this p aper the additional stabilizing effect of viscoelastic flow properties is investigated by coating experiments. The viscoelastic now properti es are measured using a further developed oszillatory capillary rheome ter. Coating experiments show that already in a lower range of viscoel asticity (angle of viscoelasticity Phi about 3(.)-5(.)) the stability of the liquid bridge is increased markedly. The degree of viscoelastic ity is modified by adding small amounts of a water-soluble polyacrylam ide.