The secret is in the Polymer of which there are broadly two types.
THERMOSETS or THERMOPLASTICS

Thermoplastics.
Thermoplastic molecules generally polymerise end to end to form long chain molecules which will soften under heat. Hence the name thermoplastic. They are usually poor in their resistance to short wave radio wave radiation and will gradually degrade under prolonged UV radiation. Examples of Thermoplastics are Polyethylene, Polystyrene, Polypropylene, PVC, Saturated Polyesters and Nylon.

Thermosets.
Thermoset molecules generally polymerise accross the chain (cross linking) and end to end in order to form a 3D molecule which once polymerised cannot be softened or reworked with heat. Once polymerised they are intrinsically resistant to all forms of short wave radiation. Unsaturated polyesters are so good in this respect that they have been specified by BNFL for the encapsulation and disposal of Nuclear waste, when used as a polymer concrete. Examples of Thermosets are Unsaturated Polyesters, Polyurethanes, Phenolics and Epoxies.

Here's the difference. The Cross Linked Thermosets can clearly be seen on the right.