According to the swedish producer of milk boxes, Tetra Pak, about 3/4 is paperboard and the last 1/4 is plastic (mainly polyethylene, but also polypropylene). In the aseptic products, they also use a thin aluminum film.
Source: Tetra Pak
You can read more about their use of the different commodities here:
Hopefully this somewhat answers your questions! If it doesn't, feel free to contact us again!