Materials are not a part of the ZTL or OBJ file. If you have PolyPainted a material onto the model, the ZTL format will store the material’s index value. That doesn’t store the material; it simply tells ZBrush what slot to look at for the material.
So if you have a custom material that you want to use with the file on multiple computers, you must first load the ZTL into ZBrush on the computer and then you must select a specific material slot. In that slot, you need to load your leather material. If you use the same slot every time you load the model, then the model will display the correct material.