Why is plastic coated with melamine?

The correct answer is option 2. It becomes a fireproof plastic.

Plastic is coated with melamine primarily because it becomes a fire-proof plastic: It becomes a fire proof plastic.

Melamine resin is a thermosetting plastic known for its fire-resistant properties. When applied as a coating to plastic, it improves the plastic's ability to resist burning or melting at high temperatures. This makes it suitable for applications where heat resistance is important, such as:

Kitchenware:  Melamine coatings are often used on plates, bowls, and utensils to make them more heat resistant and prevent them from melting during use.

Countertops:  Melamine laminate is a popular choice for countertops due to its durability, scratch resistance, and fire retardancy.

Fireproofing fabrics:  Melamine resin can be used to treat fabrics, such as firefighter uniforms, to make them more flame-resistant.

While melamine coatings can offer some benefits like increased strength, they don't make the plastic a good conductor of heat (it becomes an insulator), nor are they remoldable after curing.