If you come across black plastic looking substance as seen below then you are are dealing most likely with a refrigerant slugging issue. This black plastic comes from the suction duct inside the compressor.  The most likely thing that caused it is a hard slug of refrigerant. 

Next Steps :

• Blow refrigeration system out with nitrogen to make sure al the plastic is out.
• Install driers to make sure any other plastics are filtered out.
• Replace compressor because it is now compromised

Verify operation, make sure unit can equalize  in the off cycle. Then verify operation in a week to make sure that the driers are not compromised. If you feel that not everything is making it out of system, it mat be time to consider unit replacment.

Below is an example of what you might see: