In the mid-1950s a series of patents on a new ethylene polymer was issued. These patents were all similar in that solid catalysts were used to produce polyethylene at relatively low ethylene pressures (Table 1). In contrast to the high pressure process which produced highly branched polymers with densities of 0.910 to 0.930 g/cm3, the new resins were linear and had densities of 0.940 to 0.970. Thus, the material made by the early ICI process became known as high pressure or low density (LDPE) polyethylene, and the new materials were called low pressure or high density (HDPE) polyethylene. This early technology for HDPE may be broadly grouped in terms of three different processes: Phillips, Standard Oil of Indiana, and Ziegler.