Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.17 Motor vehicle assembly 959
The cleaning process involves acid/alkaline and solvent cleaning. Typical solvents involved
are acetone, xylene, toluene, and 1,1,1-trichloroethylene. The primer bath is water-based
but usually some organic solvents are present (5-10%). These solvents are the
same as those listed above. After the application of primer, the car body is baked and then
undergoes waterproofing with an application of polyvinylchloride sealant which contains a
small amount of solvents. Following waterproofing, the automotive body proceeds to the
anti-chip booth, where urethane or epoxy solvent-coating systems are applied. This process
is followed by application of primer-surfacer coating which is either a polyester or an epoxy
ester in a solvent system. The primer-surfacer coating is applied by spraying and provides a
durable finish which can be sanded. After the sanding step, the primary color coating is applied
also by spraying. These primary color formulations contain about twice as much solvent
as the primer-surfacer coating. Solvents are flashed-off (no heating) and a clear coat is
applied. Then the entire car body is baked for about 30 min. Solvents used include butanol,
isobutanol, methanol, heptane, mineral spirits, butyl acetate ethyl acetate, hexyl acetate,
methyl ethyl ketone, acetone, methyl amyl ketone, toluene, and xylene.
Several finishing operations also employ solvents. After baking, a sound-deadener is
applied to certain areas of the underbody. It is a solvent based material with a tar-like consistency.
A trim is applied with adhesives which contain solvents (see section on adhesives and
sealants). After the installation of trim and after the engine is installed, car undergoes an inspection.
Some repainting is required in about 2% of the production. If damage is minor
then repainting is done by a hand operated spray gun. If the damage is substantial a new
body is installed. Equipment cleaning solvents are also used. Spraying equipment is cleaned
with a “purge solvent” which may consist of a mixture of dimethylbenzene,
4-methyl-2-pentanone, butyl acetate, naphtha, ethyl benzene, 2-butanone, toluene, and
1-butanol.
Tables 14.17.1 and 14.17.2 contain information on the reported solvent releases and
transfers from the motor vehicle assembly industry. The data show that solvent use is very
large compared with all industries covered so far in our discussion except for the steel and
iron industry. The motor vehicle assembly industry is the sixth largest producer of VOC and
also the sixth largest industry in reported emissions and transfers.
The data in Tables 14.17.1 and 14.17.2 are data from 1995 the most recent available.
The automotive industry and associated paint companies conduct extensive work on replacement
of VOC containing paint systems. These efforts are mainly directed to water-based
systems and powder coatings. Until recently, water-based systems were preferred
but now attention is shifting to powder coatings which eliminate VOC. There is no status
quo. Changes are dynamic and kept protected by trade secrets which makes it difficult to
comment on specific progress. Solvent use by the European industry 5 is that the production
of one car requires an average of 10 kg of solvents. Solvents use is not the only problem the
industry is facing. 16% of the total energy used in car production is required by painting and
finishing operations. Both energy conservation and reduction is solvent consumption must
be pursued to meet environmental objectives. Not only can these issues be addressed
through material reformulation but the design of equipment used in applying and drying the
coating can also reduce emission and save energy.
A new trend is apparent as plastics are introduced to automotive production. Plastic
parts must also be painted. Paint systems are difficult to select. Chlorinated polyolefins provide
good adhesion of paints and reduce VOC but are also under scrutiny because of pres-