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Achieving the proper curing rate with the finishes you use is vital to the appearance and durability of the final finish. Coatings have at least one curing mechanism and some have more than one. Consistency of cure helps to keep your production process efficient and the end result the same day to day. We will look at how some of the common types of wood finishes cure and how to keep them consistent.
Lacquer
Lacquer has the simplest curing process. Lacquer is a coating that cures through solvent evaporation. When the solvent evaporates, the coating is finished curing, no other curing mechanism takes place.
Hybrid coatings based on lacquer and another curing mechanism have become popular because they combining the ease of use of lacquers and the durability of another type of finish. These hybrid lacquers have the same issues of solvent evaporation rates as lacquers, plus special curing requirements based on what type of coating has been hybridized with the lacquer.
You can control the variables in your finish room to keep the curing rate consistent; variables include: the temperature of the room, parts and coating; oven use; the airflow through the curing area; the number of coats applied and time between steps.
Problems may arise when these variables are not controlled. For example, the conditions in finish rooms can vary greatly between summer and winter. In summer, temperatures are high and open doors and windows can increase airflow; this leads to greatly accelerated evaporation of the solvents in the finish. In contrast, winter temperatures are lower and the airflow reduced, which leads to slower drying rates.
The curing rates of lacquers are determined by the solvents used in the coating. Solvents can be chosen from very fast to very slow evaporation rates when formulating the coating. Once the coating has been produced, the changes the finisher can make are limited. Retarder can be added to slow the drying of the lacquer, but this will reduce the solids of the coating and the faster solvent in the lacquer is still there. This means the retarder has to be used at a high enough level to overcome it and leave the film "open" and allow flow-out.
Retarder
When using retarder, it is important to realize some retarders are extremely slow evaporating solvents and can linger in the film for some time — reducing the ability to sand the finish or cause blocking or printing problems. Fast evaporating solvents can be added to speed the drying of the lacquer, but the slow solvents used in manufacturing are still in the finish and limit the results. Adding the fast evaporating solvents will reduce the solids and may require an additional coat of finish to get the build desired.
Waterborne lacquers cure when the water and solvents (many have some solvent content) evaporate. This is somewhat similar to the lacquers; however, it's important that the water evaporates first leaving the solvent to "melt" the emulsion particles (resin) together before the solvent finally evaporates.
High humidity and cool temperatures can greatly slow the evaporation of the water and create a problem of the water staying in the film too long, preventing proper film formation. When this happens, the film will be weak and can alter the appearance. In severe cases a clear film can become "milky" looking and flake off. Increasing the temperature and decreasing the humidity is the best solution. Also, a very slow retarder can be added to stay in the film for prolonged times to properly cure the film when the water finally evaporates. Of course, this means the film will be soft or "green" for some time while the retarder is slowly evaporating.
It is important to work with your finish supplier to determine the best adjustments to make if controlling the temperature and airflow is not feasible. Keep records when using retarders or fast thinners to identify if other problems are created by adjusting the dry rate.
Catalyzed alkyd & amino resins
One of the most common curing mechanisms used today in wood coatings is acid catalyzing alkyd and amino resins together; these can be catalyzed varnish, hybrid lacquers, catalyzed lacquer and precatalyzed lacquer.
Coatings containing alkyd and amino resins that are catalyzed or cross-linked at the molecular level in the presence of acid produce very durable finishes. These finishes have similar constraints with solvent evaporation as the lacquers above, plus additional requirements for the proper cross-linking of the alkyd and amino resins.
Always use the acid and amount recommended by the manufacturer of the finish and follow the mixing directions to achieve optimum results. There are many types of acids that can be used to catalyze alkyd and amino resins, and the finish manufacturer has specified the best one for the particular alkyd and amino resin. Also, the amount of acid used is vital as the proper amount will produce a durable finish over a prolonged life, while too little or too much acid will decrease the durability and life of the coating. Adding too little acid will leave the film susceptible to premature damage from wear and household chemicals. Sometimes it may be tempting to add extra acid to speed up the curing process; however, it can produce a brittle film that will not be able to expand and contract with the wood substrate resulting in cracks in the film.
Enamels
Like lacquers, enamels are affected by the solvent evaporation and cure by oxidation. The resin molecules bond with each other in the presence of oxygen. This bonding happens very slowly, but dryers are used to speed up the oxidation process. Manufacturers use specific dryers with specific resins to achieve the desired film properties. Many wiping stains use resins that cure by oxidation to hold the pigments in place after application.
Japan dryer is sometimes used by finishers to speed up the oxidation process; however, it is important to check with the manufacturer to learn if this will create other problems in the film that might show up later.
Two component polyurethane (2K PU) finishes not only have the solvent evaporation issues, but the resin in side "A" and side "B" (hardener) cross-link in the presence of each other. The specific resins used in each of the two sides determine the cure rates and final film properties. Adding more hardener can speed up the dry rate but leave the film brittle or less resistant to household chemicals. Be sure to accurately measure the components for the best results.
Also, tin can be added to speed up the cure rate. It is very important to check with the manufacturer if this is acceptable for the coating. The tin additive can speed up the cure rate, but it may affect other properties such as greatly shortening the pot-life of the coating. The pot-life of 2K PU finishes is also greatly shortened in humid conditions and special care should be taken when using them in humid conditions.
As always, check with your finish supplier about what you can do to improve the consistency of dry and cure rates of the finishes you use. Your specific finishing constraints may require a custom coating formulated for your finish operation; ask about the possibilities.
Ed. note: David Jackson began working for a wood coatings manufacturer in 1986 and has been formulating wood finishes since 1988. He held various titles including laboratory technician, formulator, senior chemist and general manager for two wood coatings companies. Jackson currently is the general manager for Jasper Chemical Coatings in Jasper, Ind.
author: By David Jackson