Is your OBJ/MTL model correctly rendered when you reimport it in Sweet Home 3D or in another 3D software like Blender?
Did you try to load it with com.eteks.sweethome3d.j3d.OBJLoader class, the OBJ loader I developed for Sweet Home 3D? Actually, I developed it because Java 3D com.sun.j3d.loaders.objectfile.ObjectFile class had some issues in texture coordinates and transparency management. Even I also needed a few other things for Sweet Home 3D, I managed to keep OBJLoader class independent from other Sweet Home 3D classes, except the small OBJMaterial class.