Print resistance is the ability of a coating to resist being printed or “indented” by another material
with pressure applied. Print resistance is important to prevent the surface indentations of packing
materials or other materials that may come into contact with the finished surface. Determining
ahead of time whether a finish and curing process will provide print resistance is important to
prevent customers from finding the defects when the finished work is unpacked. What to look
for, how to test, and how to prevent will be discussed in a multi-part blog.
Many factors contribute to if and when a finish and the process used will provide print
resistance. These include; the finish, other finishes used in the finish system, dry times, air flow,
temperatures, humidity, type of material that comes into contact with the finish, the pressure of
the material against the finish, the amount of time the material is in contact with the finish, and
the temperature while the material is in contact with the finish.
Each finish is different. Lacquers cure simply when the solvents evaporate from the coating.
This means that when all of the solvent has evaporated the finish is fully cured. Each lacquer
may have different solvents that evaporate at different rates which will change how soon it
will become print resistant to a certain level. Also, lacquers may contain a variety of resins and
usually a blend of resins. These different resins will each have different properties that affect
print resistance. A common resin found in lacquers is called a primary plasticizer. Plasticizers
provide flexibility to coatings to prevent them from cracking as the wood substrate expands and
contracts during temperature and humidity changes. The down side to primary plasticizers is they
can migrate from the finish over time and possibly “melt” into the packing material. Also, many
packing materials contain these plasticizers and they can migrate from the material and “melt”
into the finish. Either way, if plasticizer migration takes place the finish is affected. Another
problem for lacquers is they are thermoplastic. This means they do not cross-link, which makes
them more susceptible to printing if a material is in contact at elevated temperatures, such as on a
truck during warm weather.
Conversion varnishes require a two step curing process. First the solvent must evaporate to
allow the resins to come in close contact. Then cross-linking happens. This means the different
resins react together forming a new molecular structure which can provide a much more durable
coating. This usually reduces the probability of elevated temperatures causing a printing
problem. Since conversion varnishes use resins that cross-link to obtain their durability, primary
plasticizers may not be used. This reduces the chance of plasticizer migration causing a problem.
Follow the manufacturer’s recommendations on catalyzing the varnish with the correct catalyst,
measured correctly, added correctly, and the finish applied within recommended parameters. Not
following the recommendations can result in a finish with improper cure and one of the problems
can be a reduced resistance to printing.
To be continued in future blogs.