The truck driver attempted to steer to the left to keep from colliding into the car. Then the car traveled into the left lane again, colliding with the truck's right front causing the driver to lose control, police said.
The truck traveled onto the west berm and back into the left lane and began to spin out of control, according to police. The truck then continued north in the northbound lanes with its trailer continuing to spin clockwise, police said.
The truck then flipped onto the driver's side and slid across the northbound lanes approximately 90 feet before coming to rest, police said.
Some of the truck's contents, originally thought to be hazardous, spilled out of the roof of the trailer onto the median.
The back end of the trailer was also twisted and bent and the driver's side of the truck was smashed.
The car received minor damage.
Dennis Monn of Franklin County Emergency Management said the contents, about 700, 50-pound bags of resin produced by a company in Moundville, Ala., were non-hazardous.
"We were told by the manufacturer that it's nothing harmful," Monn said.
Only a small amount of diesel fuel spilled onto the roadway.
"We're looking at several hours before we get this cleaned up," said Bill Little, associate chief of operations for Greencastle, Pa.'s, Emergency Medical Service.
Traffic was slowed almost to a halt for hours with only the east berm open to pass. Pennsylvania and Maryland State Police detoured some of the traffic onto U.S. 11 and other roads.
A loader and empty trailer were brought to the scene to unload the bags of resin from the overturned truck before it could be turned upright and hauled away.
Both lanes on Interstate 81 were opened up at about 8 p.m.
"We're coming from Virginia and going to Connecticut. At this rate we'll never get there," said one person who had been waiting in the traffic jam for a half hour before being able to pass. The couple said earlier in their trip they had waited at another accident.