Good wettability is a prerequisite for the adhesion of binding partners in painting, gluing, printing or bonding. Wetting is not only impaired by contamination with oil and grease; many materials have surfaces which, even if clean, cannot be sufficiently wetted by many liquids such as adhesives and paints. The liquid rolls off. Also after curing or drying, it will not adhere to the surface.
This is due to a low surface energy of the substrate. Substances with a low surface energy wet those with a high surface energy, but not vice versa. The surface energy of the applied liquid, also known as surface tension in the case of liquids, must therefore be lower than that of the substrate.
Plastics such as polypropylene or PTFE have a naturally non-polar structure. This means that these plastics must be pre-treated before printing, painting and gluing. The same is true for glass and ceramics. Activation increases the surface energy of a surface. In this process, attachment sites are created for the applied liquid.
Conventional activation uses chemical primers as liquid adhesion promoters which are highly corrosive and harmful to the environment. On the one hand, they require adequate evaporation time before further processing and on the other hand their duration of action is short in many cases. Non-polar materials such as polyolefines are not sufficiently activated by chemical primers.
During activation in an air or oxygen plasma, the non-polar hydrogen bonds of the plastic polymers are replaced by oxygen bonds which can provide free valence electrons for bonding the liquid molecules.
Plasma activation at low pressure or atmospheric pressure turns even “non-gluable” plastics such as POM, PE and PP into surfaces with excellent gluing and painting characteristics. Thanks to the highly accurate setting options for the desired surface energy, over-activation with the risk of slight surface etching can be avoided.
In low-pressure plasma, other gases besides air and oxygen can be used which, instead of oxygen, accumulate nitrogen (N2), amines (NHx) or carboxyl groups (-COOH) as reactive groups.
The duration of action of these parts is some minutes to several months. For polypropylene, further processing is possible several weeks after treatment. Still, it is advisable not to store the parts in the open, as they attract dust, organic contamination and humidity.
For an impressive demonstration of the activation, immerse a treated and an untreated component in water (polar solution). On the untreated part, drops will form as usual. The entire surface of the treated component will be wetted with water.
Generally, the surface energy of metals, ceramics and glass is higher than that of plastics. Nevertheless, there are also applications where these materials benefits from plasma activation. The surface tension of solder alloys is likewise high, they roll off many metal surfaces. Therefore, plasma activation of metals can also improve wetting during soldering.
However, activation of metals is highly unstable and thus has a short duration of action. Once metal has been activated, further processing (gluing, painting....) must take place within the next minutes or hours because contaminations from the ambient air will become attached to the surfaces fast and permanently.
Metal activation makes sense before processes such as soldering or bonding.
UHMWPE (ultra-high molecular weight) powder activation
For many applications, hydrophilic UHMW polyethylene powder is used as innovative option. It can be used as an additive in rubber, resulting in an increased tear propagation strength. Furthermore, hydrophilic PE powder can increase the bonding strength between metal and plastic. This hydrophiling is made possible by low-pressure plasma treatment. The treatment parameters strongly depend on the particle size of the powder. Plasma treatment can take between 5 minutes and several hours. The risk of thermal damage to the power can be ruled out since the process is constantly carried out at low temperatures for this type of treatment. Likewise, basic properties of the powder such as melting temperature, degree of crystallisation and molar weight are not affected. Via a wide range of variable process parameters, many different coat properties can be achieved.
- Additive in paints (lubricant varnishes)
- Disinfection / germ reduction in seeds, spices
- Minimised slide friction due to addition of treated PTFE powder
- Production of plastic boxes filled with PUR foam to increase PUR adhesion
- Fibre activation, etching
- Plastic granulates activation, etching
UHMWPE (ultra-high molecular weight polyethylene) is a polyethylene with a very high molecular weight. Polyethylene can be regarded as the plastic with the simplest structure: a chain of carbon atoms, only hydrogen at the sides. When this chain is very long, the material is described as UHMWPE.
There are two different processes for quick and easy checking of the activation quality:
Contact angle measurement
In this process, the wetting angle of a drop of water in relation to the activated surface is measured. The better the activation, the flatter the drop of water lies on the surface. However, this process is rarely used because the measuring instrument is quite expensive and measuring usually cannot be done directly on site. Measuring of large or complex fabricated parts is a problem for most wetting angle meters, or altogether impossible without cutting off a section.
Depending on the spreading of these test inks, a certain surface energy can be assigned to the items to be treated. The unit is mN/m [before: dyn/cm]. Water has a surface energy of 72.6 mN/m. The test inks are available from 28 – 105 mN/m in 10 steps.
With long process times (> approx. 15 minutes) and appropriately selected gases, the surfaces are not only activated but etched slightly. This makes the surfaces coarse and matt. Etched surfaces offer the highest adhesion. Paint coats are usually checked with the so-called cross-cut test (standards: DIN EN ISO 2409 and ASTM D3369-02). To this end, the plastic is painted and then cut crosswise with the cross-cut tester. Next, an adhesive tape as specified in the standard is stuck to the cut lattice and pulled off abruptly. If paint particles are visible on the tape, the adhesion of the paint coat is insufficient. The classifications are described in the standard.
PlasmaBeam technology can guarantee a treatment width of approx. 8-12 mm with compressed air as a process gas. Use of pure nitrogen (N2) or pure oxygen (O2) can increase the treatment width.
The treatment width of PlasmaAPC 500 is approx. 60 mm. However, treatment is much less homogeneous than with PlasmaBeam or low-pressure plasma. Multiple treatment increases the homogeneity of the treatment.
The treatment width depends very much on the treatment rate.
The treatment width of one nozzle is approx. 8-12 mm. However, the cleaning width for every application must be checked beforehand (e.g. by measuring the contact angle).
Using pure oxygen (O2) or nitrogen (N2) slightly increases the treatment width.
PlasmaBeam: The treatment rate varies between some cm per minute for the activation of ceramics or metals and some meters per second for the activation of plastics (e.g. activation of folding boxes before gluing).
PlasmaAPC 500: The maximum rate is approx. 100 – 150 mm/s. Any further rate increase will result in an uneven surface treatment.
In plasma activation, mainly nitrogen oxides (NONO2) are generated which must be extracted from the workplace.
PlasmaAPC 500: The formation of ozone(O3) must be anticipated.
- Dry and oil-free compressed air – approx. 2.000 NL/h.
- 230 V/6A power supply
- Extraction for the nitrogen oxides
Extraction is not required for testing devices. The maximum usage without extraction is limited to some minutes per hour.
The workplace must be aired frequently and thoroughly.
This technology is ideal for the following processes:
- Local plasma activation of plastics before gluing
- Plasma activation of elastomers before gluing, flock coating, printing (e.g. rubber profiles in automotive)
- Local plasma activation of metal and ceramic surfaces before gluing or bonding
PlasmaAPC 500 is excellent for treating plastic parts before pad printing directly in pad printing machines.