UTP CCA vs UTP CU cables
DS18B20 digital thermometer has maximum supply current of 1.5mA. You can use CCA cable as long as total loop resistance ( 20m + 20m ) of the cable is not greater than say 60-80 ohms. You should also pay attention to the capacitance of the data line. Regardless of the cable used it will form low pass filter to your data.