These plastics have very strong bonds between the different chains. This makes it almost impossible for the chains to slide past each other and result in plastics that are both hard and brittle.
When heated, the chains do not move and tend to burn and char rather than melt. They are technically referred to as thermosetting plastics. These plastics are stronger than the types with little cross linking and are used where strength is the desirable property, such as in the handles of cooking pots. These plastics require a great deal more effort to recycle.Examples include bakelite.