Handbook of Solvents - George Wypych - ChemTech - Ventech!

1204 Margot Scheithauer
Despite the introduction of water based and powder varnishes, cellulose nitrate varnishes
with their high solvent content, are still widely used due to their easy application and
their low prices. The current varnish application still requires the use of solvents. The chosen
coating procedure determines the necessary processing viscosity, which may be adjusted
in different ways:
• Thinning the binding agent/varnish with a solvent consisting of one or more organic
compounds without altering the binding agent chemically. Nowadays, the solvent
can partly or almost totally be replaced by water.
• Thinning the binding agent with a reactive thinner, i.e., one or more
monomers/oligomers included in the system, which react with the binding agent
during the hardening process and is built into a molecule. According to definitions,
they do not have characteristics of solvents. They partly take over the solvent
function while they remain in the liquid state.
• Temperature increase of the coating system, which limits the required amount of
solvents.
• Liquid application of a melted solid system
Profiled construction components form parts of complicated shapes, which must rely
on the spraying of coatings for decorative and protective purposes. The spraying requires
low-viscosity material. Viscosity reduction is usually achieved by adding solvents but
sometimes by increasing the material temperature. Parts with large flat surfaces however,
may be coated by roller-coating, which tolerates higher viscosity materials. After fulfilling
their function as solvents, they are expected to completely evaporate from the varnish system.
Emission of solvents from a varnish system occurs in the course of the “life-cycle” of a
varnish in several different locations as the following survey shows (Figure 18.1.1). As
early as in the production stage of the varnish system, solvent emissions contaminate the
ambient air of the factory. About 90 % of the solvents contained in the varnish system evaporates
during its application which affects the air quality in the workplaces. These emissions
are in the milligram per cubic meter of air range. The thinning effect decreases the solvent
concentration in the ambient air at the varnisher’s workplace into the microgram to
nanogram per cubic meter.
Figure 18.1.1. Emission of VOCs in the life-cycle of a varnish.