It is not a coating nor is it plastic. You have "Orangeburg" pipes which are rolled up tar paper. Eventually, if not already, roots will come through the joints which are at every 10 feet, and the pipe is going to become oval or flat from the weight of the ground above it.
Orangburg pipe used here out west was the pipe to be, is it layered tar paper, was supposed to last a life time, boy where they wrong, this pipe is known for root intrusions egg shaping over the years of use. If you don't have it in your neck of the woods feel lucky.
It was also used in the Midwest in the 40's and 50's. It was touted as the perfect replacement for cast-iron because roots would be repelled by the tar coating.