30% Vectran, 70% SK75 Dyneema. BCY doesn't call it white, but rather "natural". Look close and you can see the off-white part (that's the Vectran).
One of the more stable (very low stretch/creep)materials, although it has a soft feel to it. 450 Premium came before it...there was one other, but I can't remember the name off hand. The one I can't remember was the very first HMPE blended bow string material (invented by Bob Destin of BCY). 450+ is the predecessor of 452X and BCY-X.
Good material, not the absolute fastest (all else being equal) due to the Vectran, which is heavier; but the difference is minimal. Very quiet in my experience. I didn't like to use less than 10-12 strands. Static breaking strength was around 155# per strand.
Down-side is it gets fuzzy faster than most other materials, and in colors the colors aren't as bright and get dull quicker.
Chad