What do you call a substance added to a resin to add flexibility and reduce brittleness?

A substance added to a resin to enhance its flexibility and reduce brittleness is known as a plasticizer. Plasticizers work by embedding themselves between the polymer chains within the resin, resulting in a more pliable material. This is particularly important in applications where flexibility is necessary to prevent cracking or breaking under stress.

In the context of manicuring and nail products, maintaining flexibility in the resin is critical for achieving durable and long-lasting results, as nails are subject to various stresses and movements. Many professional products use plasticizers to ensure that manicures withstand wear and tear without compromising quality.

While thinners can be used to adjust the viscosity of a substance, and hardening agents are meant to increase the strength and rigidity of materials, they do not specifically address the need for increased flexibility. Colorants, on the other hand, are solely aimed at altering the appearance of a substance without impacting its physical properties related to flexibility or brittleness. Therefore, plasticizers are the correct answer in this context.