The gas plasma treatment is an advanced technology that is dry, clean, cost effective and environmentally safe. It increases the surface energy of the substrate and provides reactive groups, resulting in increased bonding strength between the substrate and coating. This technology is replacing more traditional methods, such as wet chemical, flame, abrasion, corona discharge, acid etching and adhesion promotion primers and coatings.

For the treatment of plastics, the plasma is a low pressure or vacuum process so the temperature is only slightly elevated above ambient, preventing thermal distortion of the molded article. Creating a uniform plasma within a large vacuum chamber has been a technical challenge. In recent years, the technical obstacles have been overcome and large plasma systems have been commercially available. With the introduction of equipment sufficiently large to permit treatment of large plastic moldings, such as automobile bumpers, plasma has emerged from the laboratory. Gas plasma equipment has been installed in production facilities and is in operation for the pre-treatment of plastic automotive parts prior to painting.

Plasma processes have been used commercially for several decades to enhance adhesive bond strengths for electronic connectors and medical assemblies, as well as for the modification of the wetting characteristics of medical devices such as catheters and culture trays. Over the past several years, large scale plasma equipment have become available. One driving force for the development of large scale equipment has been the increasing use of thermoplastic olefins (TPO) as a material of choice for automotive bumpers and claddings. While TPO possesses the mechanical properties desired for automotive applications they can not be painted without surface modification. The most common pre-treatment prior to painting has been either flaming (Europe) or the application of an adhesion promoter (North America). Flaming, while effective to a degree, is not practical with more sophisticated designs which have recesses, louvers, or deep accent grooves. Adhesion promoters in general provide a higher level of effectiveness than flame treatment. This is especially true if the part has contours or louvers. Since solvent based adhesion promoters contain a large proportion of volatile organic compounds (VOCs) they have been identified as a destructive environmental agent. Water borne adhesion promoters have not yet proven to be as effective and are more costly. In Europe, a combination of flaming and adhesion promoter has come into use enhancing the effect of either used separately. More recently, high VOC uv primers have also been placed into production.

In laboratory testing on molded plaques, plasma treatment in oxygen containing plasmas has proven to out perform all commercial pre-treatment processes or combinations of processes. Plasma treatment provides improved paint adhesion with resistance to all automotive performance tests such as humidity aging, thermal shock, chip resistance, and fuel soak. Paint adhesion has been shown to exceed the strength of the TPO base material, which has never been demonstrated with any other pre-treatment process. Translation of these laboratory results to full-size automotive bumpers in production worthy plasma equipment has been a desire of the automotive industry. This paper discusses results on one such system.

0.5- 1.0 mbar. In the work presented herein, high frequency (13.56 MHz) rf energy is used to create the plasma. The rf energy dissociates the gas, creating a plasma characterized by a distinctive glow. The color of the discharge is unique for each gas combination, thereby providing convenient spectroscopic quality control measures, if desired. Since the process is conducted at reduced pressures, the bulk temperature of the gas is near ambient temperature. This process is thus also known as cold gas plasma, glow discharge, or cold gas glow discharge. The electrons or ions created in the plasma bombard the polymer's surface, abstracting atoms or breaking bonds creating free radicals. These free radicals are unstable and seek to satisfy a more stable state by reacting with free radicals or groups within the plasma gas, establishing new moieties on the surface of the polymer. In this manner the polymer surface can be molecularly re-engineered to provide functional groups that enhance attraction of the paint and provide reactive sites that will result in covalent chemical bonding of the paint to the polymer. Covalent bonds greatly enhance the permanency and the adhesive tenacity of the paint to the polymer. In most applications the gas or gases used are innocuous, requiring no exhaust gas treatment in order to be in full compliance with "clean air legislation". It is a work place safe and environmentally friendly process.

A gas can be excited to a plasma with almost any type of electrical or thermal energy, but all plasma equipment is not the same and all plasmas are not equal. The work presented herein was performed to investigate the uniformity of the system developed and commercially offered to the automotive industry by HIMONT Plasma Science for the treatment of large automotive moldings.

Two different bumper grade thermoplastic olefin (TPO) formulations (A&B) supplied by Statoil were evaluated. A commercial bumper was used as a test fixture to which test specimens of the same formulation were affixed using double sided adhesive tape. In this manner, the influence of loading effects are eliminated. Two separate series of trials were conducted; molded plaques and cut plaques from molded bumpers. In the first set of trials, molded plaques were affixed to the bumper in a predetermined pattern to evaluate the uniformity of plasma along the length and height of the bumper. Replicate cycles were conducted with each material to determine reproducibility. Thus, this test elucidated both the uniformity of the plasma treatment within the chamber as well as the reproducibility from cycle to cycle. In a second series of trials, bumpers were cut into test plaques and then fixed on their matching locations on the bumper test fixture. In this manner, variation of plasma effectiveness because of molding variations could be evaluated. A 90 ^o T-peel strength test was used for evaluation. After plasma treatment, the treated plaques, approximately 100 x130 x 3 millimeters, were coated with a peelable water-borne coating (Gramos Chemical International SL15/C) to protect the parts from contamination during transport from California, where the plasma treatment was performed, to Europe, where the peel test specimens were prepared and tested. All peel test specimens were prepared into test specimens within a week of plasma treatment, but no sooner than 72 hours.

Plasma treatment provided from 500 to 850% improvement in peel strength versus the control (Tables I & II and Figure II). More importantly, in most instances the failure mechanism shifted from adhesive between the paint layer and the substrate to cohesive within the substrate. A 5% to 12% variation was observed within a trial set, dependent on the material. The lower modulus material exhibited the higher variation within a test set. No pattern could be discerned with respect to position of the plaques in the plasma treatment. In the replicate sets of molded plaque trials, the reproducibility was 5% and 1% for the two TPO formulations respectively. Since all failures were cohesive, any measured variation could as easily be attributed to variations in surface morphology of the molded plaque or to the test procedure itself. Further, since the equipment was not changed in any way from one test to another, one must conclude that the trial sets with the least variation must represent the worst case for the equipment. Thus, uniformity within the plasma chamber was shown to be 95% or higher.

The second set of trials were conducted with test plaques cut from actual production bumpers. The plaques were adhered via double sided pressure sensitive tape to a molded fascia of the same material in the same relative position for treating. Eighteen to twenty-one positions were examined across the face of each bumper. The number of test pieces differed due to design limitations of the molding. Lower peel strengths were obtained for specimens cut from molded bumpers than from molded plaques. This was most significant for Formulation A (Table III), where the peel test was found to be 22 N/15mm, a 30% decrease, albeit the failure mode was cohesive, the same as it was for molded plaques. Formulation B (Table IV) exhibited only a 13% decrease. Variation dependence on position was similar for both materials, 9.9 and 11.0% respectively (Figures II & III). While excellent results were generally obtained, it was apparent that areas were affected by molding conditions, mold design, and/or part handling during specimen preparation.

Molding conditions may have affected the level, and potentially the uniformity of the T-peel strength. Handling and contamination could also be involved. Further studies are required to determine the root cause. Differences, sometimes very significant, were observed between the level of adhesion obtained in panels cut from bumpers versus the level of adhesion exhibited in molded plaques treated, painted and tested in identical manners. Such effects must be due to changes to the surface morphology resulting in a reduction of the bulk strength of the material at the surface, not a reduction in the efficacy of the plasma treatment. Further studies need to be conducted to examine this area in more detail.

The data suggest that molding parameters and material variations will have the most predominant effect on variation from point to point on a given molding. The plasma equipment and the process parameters employed in this study are consistent with equipment necessary for commercial production and provides strong economic justification versus the use of adhesion promoters. Because plasma is a three dimensional process, at least in this equipment, plasma also offers the advantages of complete design freedom, a major limitation with flaming.