they have a paper core with back & front layers of polyethylene. They're not entirely encapsulated though, so this is how chemistry can enter the sides of the paper by prolonged wet time (the paper can become delaminated from excess wet time or mishandling. Chemicals can get into the base and not be removed easily and can cause stains later on. they're not so impervious to heat & humidity either. They can still curl & flex with cycling conditions and when it comes to processing--they can burn up & melt in dryers or get a mottled/hazy look from low heat or air drying).....the emulsion is coated on top of the plastic. If you want to read theories & what-if's about RC papers--look at Henry Wilhelm's book a bit where he theorizes that if a RC paper were made on an opaque melinex (polyester) base like a Cibachrome, that this would be a more stable material than a fiber base paper...it would be like a b&w version of a cibachrome, that could toned for protection. At any rate, polyester is a better material, but is more expensive--polyethylene is cheap.
Here, I did some homework for y'all, I thought maybe you'd maybe just go dig this out yourselves, but there are alot of things like this out there--you could also try the archivist listgroups (look on Archives-L) or the conservators that deal with indoor air pollution. As far as I know, these posts are all copyrighted to the ConsDistlist authors, so I'm going to sort of point in that direction.
oh well, read these, dig around & form your own conclusion. --my opinions only as always--KT