An experimental analysis of carrier layer flows
This thesis is concerned with flows relating to the continuous coating of multiple layers on moving webs using the slide bead process. The lowermost layer is generally known as a carrier layer when the viscosity and flow rate are both small compared with the corresponding properties of the other layers. This study is predominantly experimental in nature and broad in scope as addressing issues relating to an industrial slide coating process used for the manufacture of photographic products and inkjet media. Novel specialist pieces of equipment have been designed and built for visualizing such flows as part of this work. The studies have been carried out using a pilot coating machine and ancillary flow control facilities currently owned by HARMAN technology Limited. The new techniques enable fresh insight into the interaction between the carrier layer and the surface properties of the substrate, including roughness, surface free energy, electric charge and porosity - an area of investigation that has hitherto been largely ignored. The behaviour of the bead when coating embossed webs showing a "stippled" finish is of particular interest when compared with apparently equally rough substrates of equivalent surface energy. Increasing slide angle is shown to be advantageous to expanding the coating window for difficult substrates. The results show that the widely perceived criteria for a carrier layer needs to be redefined when coating rough surfaces of low surface energy using this process. Charge assisted coating is shown likely to be superior to conventional slide bead coating for minimising waste due to streaks. The studies include the visualisation of flows at the slot exit and on the slide. The methods allow the profile of the interface as well as the free surface to be monitored and give new insight into two major unreported effects limiting the use of a thin low viscosity carrier layer. The scope also extends to the study of waves induced in the surface of wet multi-layer coatings when subjected to the impact of air from an impingement dryer - an area of considerable interest to the coating technologist yet largely ignored by the equipment supplier.