Which additive is used to increase the flexibility of coatings?

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Flexibility in a coating film comes from additives that let the cured film stretch and move without cracking. A flex additive is designed to act as a flexibilizer, increasing the mobility of the polymer chains in the dried film, which lowers the glass-transition temperature and allows greater elongation. This combination helps the coating bend with the substrate and resist cracking during flexing or temperature changes. The other options don’t primarily improve flexibility: a colorant changes appearance, a filler adds bulk and opacity, and a binder additive tweaks film formation or flow but isn’t aimed at making the film more flexible. So the flex additive best achieves the goal of increasing flexibility.