Various methods are used to cure or polymerize organic coatings. Heat curing is the most widely used method and is applicable to most epoxies, polyurethanes, silicones, and polyimides. With new government regulations on volatile emissions and energy conservation and the need to shorten curing times to increase production, curing methods such as ultraviolet light polymerization, moisture curing, microwave curing, and e-beam curing are becoming more prevalent.
Heat Curing
All solvent-based coatings require drying which consists of evaporating the solvent at room temperature or at an elevated temperature. If the solvent-based formulation contains the prepolymerized coating, no further polymerization or curing may be necessary. If the solvent contains mixtures of monomers and oligomers further curing will be necessary to polymerize or cross link the smaller molecules. This is usually accomplished by exposure to elevated temperature for a period of time.
Two-part systems (resin and hardener or resin and catalyst) generally require elevated temperatures for complete cure and polymerization. Some epoxies and polyurethanes begin to polymerize quickly after mixing in the hardener or catalyst (short pot lives) while others require a definite elevated temperature to begin the polymerization process. In all cases, exposure to an elevated temperature for a period of time is required to achieve complete cure.
Step Curing. Curing at a single temperature does not always result in optimum properties for a coating. Initial curing at lower temperatures
allows time for moisture and other volatiles to escape. Step curing at progressively higher temperatures is often required to complete the cure, reduce stresses, and reduce subsequent outgassing from the coating. One device manufacturer reported that by doing a five-step cure, each at a progressively higher temperature, the amount of moisture in an encapsulant was reduced by a factor of four.[43] Step curing of some polymer coatings, such as benzocyclobutene at the high temperatures, may have to be performed in a nitrogen ambient or in a vacuum to avoid oxidation and decomposition of the polymer.
Equipment. The choice of equipment to heat-cure coatings depends largely on the desired properties of the cured product and the availability and degree of facility automation. For batch processes and single-temperature curing, a convection oven is all that is necessary. Convection ovens maintain an even temperature by circulating heated air or nitrogen through the chamber.
For assembly-line processes, where parts are produced in series, a conveyorized belt furnace is used. An oven loses its temperature stability each time the door is opened, for example to insert more parts—a problem not encountered with the conveyorized belt furnace. Conveyorized belt furnaces have heated zones that can be maintained at several temperatures so that step curing is easily performed. By changing the belt speed, the dwell time in each heated zone can be adjusted and controlled. A conveyorized belt furnace may also be placed in-line with an automatic spray or dip-coating system.
A variation of the conveyorized belt furnace is one in which the infrared energy is used to cure the coating instead of radiant heat. Infrared energy is absorbed by the carbon-carbon bonds of the polymer coating as well as by the carbon-hydrogen and other bonds. The bonds begin to vibrate at a rapid rate causing localized heating of the coating. If the wavelength of the IR energy is narrow, only the organic coating will be heated, rendering the process cooler overall than radiant heating.
Moisture Curing
Some prepolymer resins or monomers contain functional groups that are blocked and unreactive until exposed to atmospheric moisture. Moisture reacts with the blocked groups freeing the functional groups allowing them to polymerize. Most RTV silicones and some polyurethanes cure in this manner. Optimum electrical and physical properties usually
Coating Materials for Electronic Applications
require 24 to 48 hours of exposure to air at a minimum of 30% RH and sometimes require a short thermal post-cure. For this reason, manufacturing facilities located in hot, dry climates should avoid selecting conformal coatings that rely on moisture curing. The mechanisms and chemistries of moisture curable coatings were treated in Ch. 2.