The embedded printed circuit may be engulfed by a rather thick layer of the embedding material. Some of the less popular embedding materials include polystyrene containing copolymers , polybutadiene copolymers, phenolics and wax . Polystyrene has a rather low Tg, which reduces utility as an embedding material. The elastomers derived from polybutadiene become crosslinked and embrittled with time and heat. The phenolics often contain an acid catalyst that is corrosive to the PCB and in IC's.
Phenolics also provide poor moisture resistance as well as produce water condensate in the cure.
Waxes work fairly well, but have a very low Tg - ca.