Tribo-charging guns charge powder particles by intimate contact and subsequent separation of the powder particles from the gun wall. When two different materials are brought into contact, there is a transfer of charge from one to the other to eliminate the imbalance of charge. The magnitude and direction of the charge transfer depends on many factors, including the chemical and electronic structure of both materials.

Polytetrafluoroethylene or PTFE is typically used as the powder contact walls in a tribo gun. Powder particles, of course, are a composition of materials comprised of resin, pigment, fillers, and possibly other additives. Experience has shown that most finishing powders become positively charged by contact with PTFE. Theoretically, the gun walls will be left with a charge equal in magnitude but opposite in polarity to the charge accumulated on the powder particles. This charge on the gun walls must be conducted away or else it could build up inside the gun, which would cause the gun to stop charging and could cause it to become a shock hazard.

Tribo guns charge the powder particles within the gun when the powder particles contact the PTFE walls. The more contacts a powder particle makes with the walls and the harder it hits them, the greater the charge on the particles. As long as there’s PTFE in the gun for the powder to contact, the powder will become charged.

The tribo gun does not have an electrode at high potential nor the resulting electric field between the gun and part as does a corona gun. Therefore, airflow from the gun plays a more significant role with a tribo gun transporting the powder particles onto the part.

For even more information and some great pictures, check out this link (edit)