Handbook of Solvents - George Wypych - ChemTech - Ventech!

880 George Wypych
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14.5 COIL COATING
George Wypych
ChemTec Laboratories, Inc., Toronto, Canada
The coil coating industry is under pressure to eliminate the use of solvents. Polyester coil
coatings contain up to 40% of solvents such as glycol esters, aromatic hydrocarbons (e.g.,
Solvesso 150), alcohols, ketones, and butyl glycol. 1,2 A recent book 1 predicted that the solvent-based
technology will not change during the next decade because the industry heavily
invested in equipment to deal with solvents. Such changes in technology require long testing
before they can be implemented. The coil coating industry normally recovers energy
from evaporated solvents either by at-source incineration or by a recycling process which
lowers emissions. Because of the large amount of solvents used, the use of PVC and fluorocarbon
resins in some formulations, and the use of chromates in pretreatments the pressure
remains on the industry to make improvements. 3 The coil coating industry is estimated to
be consuming about 50,000 tons of solvents both in Europe and in the USA. 1 About half of
these solvents are hydrocarbons.
According to the published studies, 3,4 efforts to change this situation did start in the
early 1990s and by mid nineties research data were available to show that the technology
can be changed. Two directions will most likely challenge the current technology: radiation
curing and powder coating.
Coil coats are thin (about 30 μm wet thickness) but contain a high pigment loading.
Consequently, UV curing is less suitable than electron beam curing. The application of this
technology requires a change to the polymer system and acrylic oligomers are the most suitable
for this application. This system can be processed without solvents. If a reduction of
viscosity is required, it can be accomplished by the use of plasticizers (the best candidates
are branched phthalates and linear adipates) and/or reactive diluents such as multifunctional
monomers. Results 3 show that the UV stability of the system needs to be improved by using
a polyester top coat or fluoropolymer. With top coat, material performs very well as learned
from laboratory exposures and exposures in industrial environment. 3 At the time of the
study (about 6 years ago), process of coating was less efficient than solvent-based system
because production speed was about 6 times slower than the highest production rates in the
industry (120 m/min). At the same time, it is known 2 that the quality of solvent-based coatings
suffers from excessive production rates as well. Radiation curing has a disadvantage
because of its high capital investment but it does have an economical advantage because the