Issue link: https://iconnect007.uberflip.com/i/1093246
82 DESIGN007 MAGAZINE I MARCH 2019 ops the coating's protective properties. Cure times are typically much longer than for physi- cal drying products, often requiring many hours at 80–90°C to develop optimum properties. Moisture curing coatings are available in both silicone and polyurethane chemistries in solvent-based and solvent-free formulations. The materials absorb water from environ- mental humidity, which initiates cross-link- ing. These materials are used widely because they don't require any extra curing processes, although heat can be used to accelerate the reactions if required. Heat-curing conformal coatings are largely silicone based and require a minimum tem- perature of 100–110°C for 10–15 minutes to achieve full cure. The main advantage of these materials is that they require no additional time to develop properties and are considered to be virtually 100% reacted, enabling coated boards to be safely bagged without fear of out- gassing. With chemical-cure coatings, such as ure- thanes and silicones), reactive oligomers are mixed with cross-linking materials immedi- ately before application. Once these two spe- cies are mixed together in the correct ratio with a suitable catalyst, a chemical reaction occurs to produce a dry, cured coating. Again, heat can be introduced to increase throughput. UV curable materials cure extremely rapidly (within seconds) when exposed to UV radia- tion of a suitable wavelength and intensity. However, the risk of shadowing by tall com- ponents means that a secondary cure mech- anism, such as by heat or moisture, is often necessary. When Coating Failure Is Not an Option When coating failure isn't an option, it is important to consider the two principal failure mechanisms: corrosion and loss of insulation (leading to short circuits). Corrosion is a com- plicated, diffusion-controlled, electrochemical process that takes place on an exposed metal surface, usually in the presence of water and ionic contaminants. Cleaning before conformal coating will go a long way to removing these two prerequisite conditions for corrosion. than that needed for solvent-based materials, resulting in reduced energy bills and reduced CO 2 emissions. From a performance point of view, solvent- free materials can be applied slightly more thickly, improving coverage and protection. Because they're easier to process and more readily compatible with rapid throughput manufacturing operations, solvent-free formu- lations are often technically superior and can meet the demands of challenging applications in the automotive and aerospace sectors where increased condensation and thermal shock resistance is required. It's All About the Cure The cure mechanisms of the main classes of coating materials include drying, oxidative, moisture, heat, chemical, and UV. Your choice will depend on a variety of factors, such as the performance requirements of the application and physical constraints, including the maxi- mum permissible cure temperature and the time allotted for curing. Acrylic polymers in a solvent can be air dried. Once the solvent has evaporated, the residual coating is physically dry, and no fur- ther reaction mechanisms are needed. Heat is often used to speed up solvent evaporation, but care must be taken to avoid solvent-entrap- ment and bubble formation. Oxidative cure coatings based on solvent- based alkyd chemistry are dried as mentioned in the previous paragraph before undergoing a reaction with atmospheric oxygen, which initi - ates a cross-linking reaction that further devel- Heat is often used to speed up solvent evaporation, but care must be taken to avoid solvent-entrapment and bubble formation.