An animal product is impure (najis) if both of the following are present:
- At the time of slaughtering the animal, its blood comes out with gush, and
- Such animal (as mentioned above) is not slaughtered according to method prescribed by Islam, nor is there a probability that it may have been slaughtered thus.
Now if these conditions apply to Collagen then it is najis, otherwise it is pure (paak).
Reference: See Tawdheehul Masaa'il of both Ayatullah al-Khu'i (may God have mercy on his ruh) and and al-Seestani in the chapter of Najasat under the discussion of "Dead Body".