Handbook of Solvents - George Wypych - ChemTech - Ventech!

1028 Mohamed Serageldin, Dave Reeves
predominant method is still particulate blasting (using abrasive media), which is used to remove
mil scale, extra weld material, rust, and old coatings.
The angle at which the surface is blasted is chosen to generate the desired peaks and
valleys on the substrate, that will accommodate the viscosity, chemistry (polar groups) of
the primer coating. The surface profiling will also help the primer coating adhere mechanically
to the substrate, contributing to the longevity of the coating system. 15 Pre-construction
primers are sometimes used immediately following surface preparation (blasting) to prevent
steel from oxidizing (rusting). This primer is removed by particulate blasting, before
the protective coating system (one or more coatings) is applied to the assembled parts or
blocks. Removal of such primers (when they cannot be welded-through) can result in emissions
of VOCs and HAPs.
14.26.4.2 Cleaning operations after coatings are applied
Surface coating operations at shipyards use predominantly solvent-based coatings. Hence,
relatively large amounts of organic solvents are used for cleaning and thinning activities.
Table 14.26.1 shows the most common organic solvents used for thinning and cleaning,
based on 1992 data. 16 Table 14.26.2 gives examples of solvent products that can be used for
both thinning coatings and for cleaning surfaces after coatings are applied and for maintenance
cleaning. The solvent products are listed in decreasing order of evaporative rate. Acetone,
a ketone solvent is commonly used for cleaning and thinning polyester resins and gel
coats. However, it is also used in formulating low-VOC and low-HAP products. Methyl
ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) are fast evaporative solvents that
are used for thinning and cleaning vinyl coatings, epoxy coatings, and many other high performance
coatings. Fast evaporative coatings that can improve application properties for a
good finish may also be formulated by blending different solvents. Examples are shown in
Table 14.26.2. The fast evaporative mix includes solvents varying in polarity and solubility
parameters. They include an oxygenated solvent (MIBK), aromatic hydrocarbon solvents
that contain less than 10 percent (by mass) HAPs, and aromatic hydrocarbons like xylene
that are 100 percent HAPs as will be shown later. Together they produce the correct solvency
for the polymer (resin).
Table 14.26.1. Predominant solvents used in marine coatings [from ref. 16 ] and EPA
regulatory classifications
Organic solvent
ALCOHOLS
VOC HAP, Sec. 112 (d) Toxic chemicals, Sec. 313
Butyl alcohol Y Y Y
Ethyl alcohol Y N N
Isopropyl alcohol Y N Y a
AROMATICS
Xylene Y Y Y
Toluene Y Y Y
Ethyl benzene Y Y Y