Improving bonding paint adhesion
Plasma primer envoromental friendly cost effective improvment of bonding and paint adhesion
Plasma processes can be completed in several stages, i.e. the ultra-fine surface cleaning is followed, for example, by activation and/or deposition of a layer.
The transition to each subsequent step is made without intermediate ventilating to prevent uncontrolled reactions.
The plasma primer is a reactive aerosol which is added to the plasma atmosphere following the ultra-fine cleaning and decomposed from the coupled energy into molecule groups.
These deposit homogeneously on the cleaned substrate and remain ready on the surface to form a permanent bond with a coating (paint, printing ink, glue, additional polymer). The functional groups of the plasma primer are physically and chemically adapted to those of the coating. The coating thickness of the precipitated primer is in the nanometer range, i.e. it is extremely thin, but still highly uniform and effective.
In addition, this layer,which is customized for both partners, can replace mechanical pretreatments and those containing solvents. Up to now the metal parts for rubber-metal compounds have been and are roughened by sandblasting to enlarge the surface and free it of organic contaminants. If the required composite adhesive strength is not achieved by this process step, then a primer containing solvent must additionally be applied. This
extensive pretreatment can be replaced by the multi-layer, dry-chemical plasma process.
In any case, the dimensional stability of the components also does not change and the requirements regarding adhesion strength are fulfilled. This was verified in trials which were performed by the customer on a sealing ring. The elastomer was vulcanized onto ring surfaces, which were prepared as follows:
- sandblasted and conventionally primed
- ultra-fine plasma cleaning and plasma primer
The differently modified metal rings were placed in the end customer's mold and the seal molded.
Following a storage period, a tensile test (pulling speed 25mm/min) was performed to determine the detachment force of the rubber.
The diagram (Figure 1) indicates that the sandblasted seals remain far below the required adhesion values. The components modified by plasma cleaning and plasma primer fulfilled the specification to
its full scope. The values determined were somewhat lower than those of the standard sandblasting and conventional primer-coat combination, but greatly above the required value.
When metal parts are pretreated in plasma, the much higher weight as compared to plastic workpieces must be considered in the design of the part mounts and in the loading and unloading of the chamber. For this reason, primarily systems with front doors are used for such pretreatments (Figure 2). This enables the workpiece carrier to be pushed directly from a wagon with a roller conveyor onto the roller track embedded in the chamber floor and routed further to the opposite side in the production flow at the end of the process.
Figure 2 Plasma system with automatically activated front doors for easy handling of the workpiece carrier