Improvement of primer adhesion to thermoplastic olefins (TPOs) by methane plasma polymerization with a low-temperature cascade arc discharge was investigated. Methane plasma with a low-temperature cascade are plasma torch can be used for improving the primer adhesion to TPOs. Tape-adhesion tests (ASTM 3359-92a method) demonstrated this improvement, with a rating of 0 for untreated TPOs and 5 for methane plasma-polymerized TPOs at certain plasma conditions even for aging at 60 degreesC and 80% relative humidity for 5 days. The adhesion to primer for the soft, flexible TPOs (ETA-3041c and ETA-3101) was easily enhanced. The adhesion to primer for the hard and brittle TPOs (ETA-3183) needs to optimize the plasma conditions to pass the dry- and wet-adhesion test with methane plasmas. To relate the surface characteristics of methane plasma-polymerized TPOs to adhesion performance with primer, the wettability and polarity of TPOs were evaluated by the contact-angle measurements of primer and deionized water to TPOs. TPO surface morphology was evaluated with scanning electron microscopy. The surface composition was characterized with electron spectroscopy for chemical analysis. (C) 2003 Wiley Periodicals, Inc.