"聚丙烯腈系纤维" can be translated as "polyacrylonitrile-based fiber" or "acrylic fiber" (the latter being a more common and simplified term in daily usage and some industries, though strictly speaking "acrylic fiber" encompasses fibers made from acrylonitrile copolymers as well).
If you want to emphasize the base material as polyacrylonitrile specifically, "polyacrylonitrile-based fiber" would be more precise.