The activation of metals is possible in principle, however, the activation of metal is very unstable and therefore short-lived. If metal is activated, it must be further processed (glued, painted ...) within a few minutes or hours, since surfaces bond quickly and permanently with contamination from the surrounding air.
Metal activation makes sense before processes such as soldering or bonding.
Plastics such as polypropylene or PTFE are non-polar. This means that the plastics have to be pretreated before printing, painting and gluing. The same is true for glass and ceramics. The process gas is usually industrial oxygen. However, many activations can also be performed with ambient air.
The parts remain active for from a few minutes to a few months. Polypropylene can be further processed several weeks after treatment. Despite this, it is recommended not to store the parts in the open, as they attract dust, organic contamination and humidity.
The activation can be demonstrated very impressively by dipping a treated and an untreated workpiece in water (polar solution). Drops form on the untreated part as usual. The treated part is completely wetted with water.
Glass and ceramics behave like metals and can be well activated and also etched to some degree.
UHMWPE (ultra-high molecular weight polyethylene) powder activating
For many applications, hydrophilic UHMW polyethylene powder is used as an innovative possibility. It can be used as an additive in rubber, with the result that a higher tear strength is attained. In addition, it is also possible to use hydrophilic PE powder to increase the bond strength between metals and plastic. This hydrophilation is achieved by a plasma treatment in the low-pressure process. The treatment parameters are strongly dependent on the particle size of the powder. The plasma treatment time may be between 30 minutes and several hours. Thermal damage of the powder is excluded, since the process temperature remains consistently low in this treatment method. Likewise, the fundamental properties of the powder such as melting temperature, degree of crystallization and molecular weight are not affected. Due to numerous variable process parameters, a variety of coating properties can be obtained.
The quality of the activation can be tested quickly and easily by two different methods:
In this method, the wetting angle of a water drop to the activated surface is measured. The better the activation, the flatter the drop of water lies on the surface. This method is rarely used because the instrument is relatively expensive and the measurement can generally not be made immediately on site. In particular, large or complex shaped parts can be difficult or impossible to measure with most wetting angle measuring instruments, without first cutting them up.
The treatment materials may be associated with a specific surface energy depending on how the test inks flow. The unit is mN/m [formerly dyn/cm]. Water has a surface energy of 72.6 mN/m. Test inks are available from 28 mN/m - 105 mN/m in 10 steps.
With long processing times (> 15 minutes) and appropriately chosen gases, the surfaces are not only activated, but etched. The surfaces are then rough and matt. The highest adhesion forces are achieved with etched surfaces. Coatings are usually tested with the so-called Cross Hatch Test (standards: EN ISO 2409 and ASTM D3369-02) For this purpose, the plastic is painted and then a lattice(cross hatch) is cut with the Cross Hatch Test device. Finally, a standard specified tape and is stuck on and abruptly removed. If pieces of paint stick to the tape, the paint adhesion is poor. The individual increments are described in the standards.
Plasma indicator tags or metal compounds can be quickly and easily checked to show whether plasma treatment has taken place. For details, see heading "Plasma indicators".
You can find more information under "Applications".