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.
Reactive groups
Oxygen
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.
UV radiation
Carbon polymer
Oxygen
Hydrogen
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.
Plasma powder activation
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.
Applications
- 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
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.
FAQ Frequently Asked Questions
Which surfaces can be activated in low-pressure plasma systems?
Diener electronic’s low-pressure plasma system can be used to activate the following surfaces: plastics, metals, glass, ceramics and textiles.
How long can activated parts be stored before further processing?
The curing time of the components depends on the activation time and the material, and varies from a few minutes to several months. It is therefore often necessary to carry out tests on site.
Metals, ceramics, glass and elastomers: a few hours
; plastics (excluding elastomers): several days, weeks or months
How should treated parts be stored?
Following plasma treatment, it is advisable not to store the parts in the open, as they attract dust, organic contamination and moisture.
Sealed parts have a significantly longer shelf life than those stored in the open.
Parts treated by us on a contract basis are packaged in close consultation with the customer, e.g. in certified silicone-free PE bags, ESD packaging or customer-specific packaging material provided to us.
What are the main benefits of activation using atmospheric pressure plasma?
PlasmaBeam technology is suitable for in-line processes, e.g. plasma activation of continuous rubber profiles and hoses prior to printing, bonding, flocking or coating.
This technology is suitable for use with robots, i.e. the 2- or 3-dimensional surfaces can be scanned with the plasma beam using robots.
PlasmaBeam enables localised surface activation without the need for masking – the remaining surface areas are not affected.
How can plasma activation be measured?
Contact angle / edge angle
The contact angle is the angle formed by the tangent to the drop’s contour and the surface of the solid at the three-phase point when observing the projection of the lying drop onto the solid. According to the physical definition, a surface with a contact angle of less than 90° is hydrophilic (wettable), whilst one with a contact angle greater than 90° is hydrophobic (non-wettable). The contact angle can be altered (increased or decreased) by means of plasma treatment. Using a suitable plasma process or by applying a suitable coating via a plasma process, hydrophilic surfaces can be converted into hydrophobic ones (and vice versa using hydrophilic layers).
Test inks from Diener electronic
Instruments for estimating surface energy: If the test ink contracts after being applied to the surface, the surface energy of the solid is lower than that of the ink; if wetting is maintained, the surface energy of the solid is equal to or greater than that of the liquid. By using a series of test inks with graduated surface energies, the total surface tension of a solid can thus be determined. However, the polar and non-polar components of the surface energy cannot be determined using this method.
Cross-cut test
To check the adhesion of paint finishes, a cross-cut test (standards: DIN EN ISO 2409 and ASTM D3369-02) is carried out. After painting, the paint layer on the plastic component is scored in a grid pattern. A standardised adhesive tape is then applied to the cross-hatch pattern, pressed down firmly and pulled off abruptly. If paint remains on the adhesive tape, the adhesion of the paint finish is poor. The cross-hatch test thus indicates the adhesion strength of paint layers on plastic parts.
