What material is used for the floor panels in the main cabin for Boeing 747-400? Just conducting some research into materials used in construction for Boeing 747-400 and Douglas DC-3. 

I just found out that in a DC-3 the floor panels in the main cabin and companionway are of plywood.  

As such, I tried to find the information with regards to Boeing 747-400, but was unsuccessful. 

I was hoping someone could provide me with information with regards to the material of the floor panels in the main cabin and companionway of Boeing 747-400.
This illustration is earlier than a -400. Later ones might differ. There are two types: the normal one has aluminum skins bonded to a PVC core, for galleys/hold it is titanium skin bonded to a PVC core with greater strength/corrosion resistance.



747 cutaway view



Later a honeycomb sandwich was used, apparently two cross plied glass-epoxy prepreg skins bonded to phenolic coated nomex honeycomb core using an epoxy film adjesive. From:


  Handbook of Adhesives and Sealants: volume 1, 2005