What makes a garment "wrinkle-free?"

What makes a shirt wrinkle-free?

One of the more popular wrinkle-free treatments in clothing is formaldehyde. Formaldehyde resins have been used to treat textiles, particularly cotton, since the 1920s. Formaldehyde treatments are usually applied to combat wrinkling, improve stain resistance/colorfastness, and prevent mildew. Essentially, it has been used to make apparel fabrics easier to care for, transport, and manufacture over the last 100 years.