Because three factors compound. The two-unit glycol backbone and the four-carbon butyl group already give DB a very high boiling point, and acetylating the hydroxyl adds further molecular weight while removing the hydrogen bonding that would otherwise pull the molecule into the water phase. The result is strongly oil-soluble and extremely slow to evaporate.
2
Where is such a slow solvent needed?
In high-build and hot-applied coatings that must stay mobile for a long time to level, and in screen printing inks that must not dry in the mesh. It is also used as a retarder, added in small proportion to a faster blend to extend flow time and eliminate blushing in humid conditions.
3
Can DBAC be used in two-pack polyurethane systems?
Yes. The hydroxyl is esterified, so there is no active hydrogen to consume isocyanate and disturb the crosslinking stoichiometry. Providing isocyanate compatibility at this evaporation rate is unusual, and it is the principal reason DBAC is specified rather than a faster acetate.
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What should be watched when using a very slow solvent?
Retained solvent in the film. A solvent this slow can remain in a thick or low-temperature-cured coating for a long time, keeping it soft, delaying full hardness development and potentially causing blocking or print-through. Dose and bake schedule should be established by trial rather than by carrying over from a faster solvent.
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