I think polystyrene-foam would work well, with styrofoam as a synonym. People are familiar with Styrofoam, which is much more intuitive to type.
The usage guidance can include:
For questions about extruded (XPS) and expanded (EPS) polystyrene foam materials,
such as Styrofoam (XPS) and bead-board (EPS).
As to the broader case of brand names, I'm not immediately opposed to them. When it comes to certain supplies, the particular brand may be a relevant tag over the generic type. Perhaps prismacolor would make sense on some questions about that specific brand over just colored-pencils.
A digital parallel I can think of is "Intuos" vs "Drawing Tablet". An Intuos tablet is a Wacom-brand drawing tablet, but its compatibility and features and user-base are different than drawing tablets in general. The same appears to be true of Prismacolors, as I'm starting to learn.
In deciding whether or not to use the branded tag, I would ask:
- Is it only being used on questions in a generic way?
- Would a reasonable person use or understand the brand name as a stand in for that type of product?
If the answer to these lean towards "yes", then I would recommend the same as here: fine a suitable alternative for the general product, and synonymize the brand name. styrofoam -> polystyrene-foam, kleenex -> facial-tissue, etc.
If the answer is "no", then I would ask:
- Does this particular brand refer to a specific product?
- Does that product have unique properties or a specialized set of users?
If the answers to the above lean towards "yes", then I would allow the tag to stick around on its own, using my prismacolor example earlier, however it should be refined to prismacolor-pencils.
If the answer is "no", then the tag probably doesn't need to exist on that question. For instance, I have a question that is about colored pencils in general, but happens to also be about Prismacolor pencils:
What can I do to stop my colored pencils from falling apart?
The tag doesn't really belong, and colored-pencils would suffice.
While I'd be okay with prismacolor-pencils, I haven't yet thought of a great-fit question worth adding it to. It's merely an example to illustrate the idea.