When you export an AC composite, formed by different skins with its own Building Material definitions, you only see the "Name/Description" and "Thickness" when you check the IFC model on a viewer.
In another side, Archicad has its own Material Catalogue (XML file) in order to use it in the EcoDesigner tool (or energy evaluation), linked to Building Material database, but without transferring those properties to IFC Scheme.
At the same time, in Graphisoft AC20 help guide appears (see attached image) the information about TAG and CATEGORIES data of Building Materials are shared as standard IFC Material properties.
This 'general' question/problem generates a similar specific problem for me - discussed here: https://archicad-talk.graphisoft.com/viewtopic.php?f=12&p=294263#p294263
(sorry for picking up old topics and kind of hijacking them, but either I'm missing out on some basic setting, or David's question is still very relevant... - admin can merge topics if this is more appropriate)