The mpp capacitors consist of low loss dielectric formed by pure polypropylene foil. A thin self healing mixture of zinc and aluminium is metallised directly on one side of the pp - foil under vacuum. This technology ensures a long operating life of the capacitors. In some cases additional unmetallised foils are added between the metallised ones. The capacitors elements are dried in a vacuum. After insertion into the capacitors case a patented viscous polyurethane resin , mainly containing castor oil or dried insulation gas , is introduced. This protects the winding from environmental influence and provides an extended life - expectancy and stable capacitance.