Industrial film drying

Drying processes have been with us for a long time. During the Roman Empire, for example, the sun was used as a heat source in the convective drying of bricks, while during the industrial revolution, moist ceramic products were carried into and out of steam heated rooms by child labourers since it was deemed more cost effective to maintain a constant room temperature than to cool the room down in order to remove the products in a safe and healthy manner. Drying is the removal of volatile substances (moisture) by heat from a mixture that yields a solid product [1]. The volatile substances or solvents used in the coating process can range from simple to handle materials such as water to active organic materials where the active ingredients may be polymers, binders, dyes, hardeners etc. The heat supplied during the drying process vaporizes the solvent and the solvent removal must be performed without adversely affecting the coating formulation or interfering with the physical uniformity of the coating. Note that although drying may involve solvent removal, chemical reactions may also aid solidification and it is possible also to accelerate solidification by using a catalyst or an outside energy source such as ultra-violet radiation. This is also called curing to differentiate it from drying although most coatings experience some form of reaction in the dryer. From Keey [1] it may be observed that drying processes for web type substrates can be categorised into conduction, radiation and convection style processes. Conduction processes are characterised by high levels of solid contact and the subsequent high material temperatures associated with this contact. Radiation style processes are generally limited to the drying of thin films as impinging radiation rarely penetrates beyond the immediate surface of a material. The most efficient and hence most frequently implemented drying method is based on convective drying. In this method a carrier gas is heated before being passed over or through the wet material. The convective method is the only method to simultaneously increase the heat transfer coefficient and the mass transfer coefficient [2]. This paper focuses on the convective style of drying. The following figure shows a schematic of two popular air convection drying processes. Figure 1(a) shows a typical air impingement drying process [3] where hot air jets pass over a coated film and transfer heat into the coating to vaporize the solvent. Figure 1(b) shows a schematic of a air floatation dryer [4], which is used to support webs on an air cushion, thereby avoiding possible damage to the web substrate. Figure 1: Schematic diagrams of (a) air impingement and (b) air floatation drying processes [3,4]. Although most coatings are applied in the fluid state, they are used in the dried state. Thus the drying process is as important as the coating process since it may either improve or degrade the properties of the coating. In practice, drying is also important since it may restrict the optimisation of a coating process since in drying sensitive applications drying considerations may dominate over coating ones in limiting possible line speeds. Drying must be performed without adversely affecting the coating formulation or interfering with the physical uniformity of the coating [5].