Exploring Lacquer 2/14/2012 10:50:00 AM
Lacquers “cure” by evaporation, which is the definition of why a certain coating is lacquer. Understanding more about how coatings work allows for a better understanding of why we are successful sometimes and not successful other times.
Lacquer has been considered the standard for appearance to which other types of coatings are compared. This along with its ease of use and low cost led to it dominating the market in the past. However, in recent decades its use has declined because its durability is lower than many requirements for kitchen and bath cabinetry and furniture. Lacquer will still be used for years to come but will have a lower market share over time compared to other coatings types. These other coatings types include; catalyzed and pre-catalyzed lacquers, catalyzed varnish, waterborne, two-component polyurethane, UV, etc.
Lacquers like other types of coatings, are composed of four main types of raw materials; resins, pigments, additives and solvents. All four are not required to make a coating, but all four may be used. See Catalyzed Varnish part 1 blog for more information on the four types of raw materials used in coatings.
A true lacquer relies solely on the evaporation of solvent for its cure. Lacquers form a film by the resin molecules becoming entangled together as the solvents evaporate. True lacquers can be applied to a surface and, with the correct solvents, be completely dissolved again because no cross-linking occurred in the coating.
This property is why lacquers are so easy to use and repair or touch-up. Since each coat of a lacquer melts into the previous coats, finishing defects such as scratch marks in the previous coat are reduced or eliminated when the next coat is applied. When multiple coats of lacquer are applied, it results in one continuous coat because of the melting together of the individual coats.
Lacquers use two primary classifications of resins to produce the film. Film forming resins are the first; these resins will form a film on their own. Film forming resins include nitrocellulose, cellulose acetate butyrate (CAB), acrylic, vinyl, etc. These resins will provide much of the hardness and resistance to certain chemicals. The second are modifying resins; these resins do not form a film on their own but can provide certain desirable properties. Modifying resins include some alkyds, oils, plasticizers, etc. These resins are often used to provide flexibility for the coating so it doesn’t crack when stressed and lower the cost and or increase the solids of the coating.
Lacquers usually have more than one type of resin because many types of lacquer resins do not form a durable film by themselves. Therefore many lacquers will have one or more film forming resin coupled with one or more modifying resin. The resins used and specific blend ratios are what allow different lacquers to have different features. These features include; hardness, flexibility, chemical resistance, clarity, viscosity, solids, etc.
Solvents play an important part in the “cure” of the film since the evaporation of the solvent is the only “cure” mechanism. Different resins may dissolve in different solvents. This means that each lacquer may have different solvent blends based on the resins used. It is important to use the correct thinners, reducers and retarders for optimal results. The blend of solvents must be compatible with the specific lacquer and provide the correct dry time for proper flow out. Check with your supplier to learn more about which thinner and how much to use for your specific lacquer.
Lacquers are affected by many variables at application and during drying.
- The temperature of the lacquer, substrate and air will all affect the flow, leveling, dry time, and the durability of the final finish.
- Air flow rates also affect the drying of the solvents from the film. The solvents need to be removed, after the lacquer has flowed out, to allow for proper film formation. Too much air flow and the lacquer will dry too quickly and not flow out evenly. Not enough air flow will slow the dry rate.
- The amount applied per coat of lacquer will also affect its properties. Too thin a coat may not flow out well and not provide the coverage necessary to protect the wood substrate. Too thick a coat will lead to excessive dry times and can leave solvent trapped deep inside the coating. This can cause poor resistance properties and cracking.
- Follow manufacturers recommendations on the number of coats and film build as too thin a film will not protect the substrate and too thick a film will crack.
Some coatings sold as “lacquer” may actually be a hybrid lacquer. Hybrid lacquers rely on more than solvent evaporation alone to “cure” the film. Common hybrid coatings are catalyzed and pre-catalyzed lacquers (which are usually labeled correctly). These coatings are a blend of lacquer and catalyzed varnish. Other hybrid lacquers may have an oxidative cure resin, typically a certain type of alkyd that provides a secondary cure. These may not be marketed as such and can provide certain advantages and disadvantages to pure lacquers. Advantages may include better water and chemical resistance and disadvantages may include susceptibility to lifting or wrinkling.
Lacquers are still widely used today and learning more about them will help you get the best results possible.