Most helmets are made from Aramid Fiber(Kevlar) and UHWM-PE.
Kevlar, as most people were mistaken, is a fiber. Not a hard piece of plastic.
Most helmets have layers upon layers of Kevlar binded together with a resin to give shape to the helmet as well as resist deformation caused by the projectile.
A bulletproof helmet are formed by two essential factors - "Hard"
It is the resin that gives the ‘hardness’ to the helmet, not the Kevlar itself. Kevlar is really nothing more than a cloth that is tear resistant, and its layering design makes it shear resistant to stop bullets from pushing its way through the threads into your noggin.
However, newer materials are beginning to replace Kevlar for helmet manufacturing. Which is Dyneema or Spectra, are both "Ultra High Molecular Weight Polyethylene - UHMW-PE". It is the same stuff they use to make climbing ropes.
These materials have greater tear and shear resistance than Kevlar, and have stood up to 5.56mm rounds. At much longer ranges, it could even withstand 7.62 NATO as a friend of mine tested at the range, although it is NOT rated for such calibers. Most of such helmets are rated IIIA for the minimum even though soldiers have gone on record saying their helmets have saved them from sniper rounds.
Helmets such as the Enhanced Combat Helmet and SPECTRA helmet are such examples of newer materials used in its manufacture.