I am also waiting to hear back from them on this.
Hard to find on their website, but I was able to get this little blurb from within the “contact us” form-dialouge:
Most of the covers in our collections are made of synthetic material. In particular, the hard covers are made of polypropylene while the soft covers are polyurethane. Other collections are made of cardboard. Some special editions include different materials (eg. fabric).