Mouthguards can be made of acrylic (type of plastic), acetate, vinyl, some kind of rubber, a hybrid of more than one material or metal, such as chromium cobalt.
Better guards are often made of a laminate of two materials – hard material on the outside and soft on the inside, to ensure both durability and comfort.
One of the proprietary laminates is Ethel Vinyl Acetate. The quality of the material will make your guard more durable and resistant to chewing pressure.
If your mouthguard is designed to be boiled before being fitted to your mouth, make sure that it can be heated repeatedly without falling apart, otherwise, it will not last long.