Sintered bases are made of heated powder with some possible ingredients like carbon (silicone, molybdene, fluorocarbons, teflon...). They mostly have molecular density exceding 1,000,000 daltons (the molecular density should be the function of the pressure used – the higher, the "better“ the base). There are more amorphous zones in sintered bases compared to extruded which guarantees better wax absorption. The ingredients further improve the glide in some special conditions.
(Sorry if the info lacks precision. I´m no chemist, just collected the info from various sources and partially had to translate it into English. There must be those knowing better.)