Polypropylene cones tend to be inherently well damped with the result that they can deliver smooth, if not terribly extended, frequency responses. The material itself and the methods used in manufacturing cones with it are such that tight tolerances are easily achieved. In the early days of polypropylene use it acquired some bad reputation due to a fact that it's a difficult material to bond. Modern adhesive technology has completely solved this problem, allthough it is not completelly free of problems. The most common thinking is that it is the viscoelastic creep present in all plastic materials that is responsible.
Viscoelastic creep refers to the tendency of plastic materials to slowly stretch when under stress.
This process may or may not be linear and typically is related to the lossiness in the material. By some opinions that the joint between the voicecoil former and the cone may be to blame. It is suggested that the heat generated by the vc and dissipated by the former may soften either the plastic cone material or the glue at the joint. All these represent attempts at finding combinations of stiffness, lossiness, density, and sound velocity that are somehow optimal for a given application. They generally have the same virtues and potential pitfalls as polypropylene.