Synthetic fabrics like polyester or nylon are made from plastic polymers. These materials have a low melting point compared to natural fibers like cotton. Applying a standard high-temperature iron setting can cause the polymer chains to break down, resulting in permanent damage such as melting, scorching, or a shiny glaze on the fabric. Therefore, you must use a low heat setting to safely remove wrinkles without harming the garment.

Final Answer: they are damaged at high temperature.