I read mashaAllah a very informative article on whether Hijab is really integral part of Islam or not. Here is the link to that article http://www.salahtimes.com/announcements/view/are-niqab-and-other-forms-of-hijab-simply-traditions-and-cultural-practices
Here are a few important lectures that explain in detail why Niqab is an integral of Islam.
JazkaAllah khair for reading my post and hope it benefits you all inshaAllah :)