Foam polystyrene (PS) /ˌpɒliˈstaɪriːn/ is a synthetic aromatic polymer made from the monomer styrene.
Foam polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and rather brittle. It is an inexpensive resin per unit weight.
Foam polystyrene is a rather poor barrier to oxygen and water vapour and has a relatively low melting point.
It is one of the most widely used plastics, the scale of its production being several million tonnes per year. (source)
Can guinea pigs eat foam?
No they can’t at all. It is not good for guinea pigs to eat and should be avoided.
For more things that guinea pigs can and can’t eat, check out our guinea pig food list.