Functionalised plastics, parts and surfaces

The outstanding properties of plastics such as their unbeatable mechanical behaviour in relation to their weight and their outstanding film-forming capacity mean that they are penetrating ever new fields of application in a world that is shaped by technology and innovation and, increasingly, by considerations of resource efficiency and sustainability. As part of this process, there is a demand for additional properties that were not necessarily found in a traditional plastic, such as electrical and thermal conductivity, non-linear optical characteristics, magnetic addressability, and the ability to convert electrical energy into light and vice versa. In addition, sensorial and actuatorial functions are also targeted, as well as a self-healing ability. Demands made on surfaces could include self-cleaning capability, antistatic behaviour, specific adhesion properties and, if possible, the ability to switch properties on and off. In general, it is often implicitly assumed that the other customary valued properties of the plastic are not impaired. Although this is still far from being classed as reality, there have been a number of fascinating successes that illustrate just what is still possible with the right amount of research and development.

Attempts to functionalise the entire plastic in a component or selectively only its surface predominantly pursue two strategies. Firstly, functionality can be achieved in or on a component by incorporating suitable additives and fillers into the plastic. By adding conductive carbon black, carbon nanotubes or graphene (which has been the subject of much research lately) to the plastic, it is possible, for example, to achieve isotropic and anisotropic electrical conductivity, and in some cases, even thermal conductivity. If the filler is given an opportunity through suitable processing to join together to form closed conductive paths (percolation paths), the material is able to transport electrons or heat. In the other extreme, for the production of particularly effective insulators as are needed in thin layers for electronic components, the conductive particles need to be incorporated into the plastic without mutual contact. It is a major challenge in plastics processing to identify which regulators leave the functional fillers dispersed on one occasion and then, on another occasion, induce them to form continuous percolation paths for transporting charges.

New types of sensors can also be developed on an analogue basis. If the conductive paths contained in the plastics are – depending on certain ambient parameters – reasonably continuous, this can be measured very precisely by the change in conductivity. Related to this are efforts to incorporate magnetic particles, for example based on iron oxides, into the plastic. With the right procedure, this results in materials that can be heated up very efficiently by alternating magnetic fields and, in response to this stimulus, can undergo additional processes such as self-healing or shape changes as in shape memory polymers. With particles that have a very high refractive index, are significantly smaller than the wavelength of visible light (approx. 300 – 800 nm), and are finely dispersed in the plastic, high-refraction lenses can be manufactured for e.g. head-up displays in the automotive sector.

Additives and fillers incorporated into the plastics close to the surface that additionally modify the surface topology in the micro to nanometer scale can have a considerable influence on the wetting and adhesion properties. The lotus effect copied from nature is one commonly known example of this. In addition to soil repellency, the influence of cell adhesion on plastics is attracting a lot of attention as part of the work being conducted on the subject of anti-fouling (e.g. to prevent organisms settling on surfaces). It is frequently useful to combine these concepts with an anti-bacterial treatment of the plastics close to the surface, for example with silver (nano)particles. Near-surface structures that have a periodicity near the wavelength of light can also be used to produce interference colours for decorative applications or as safety labels. They can also be combined with a sensorial function (deformation, stress, swelling etc.).

The second large group of functionalised plastics covers those that have their functionality "built in". These new polymers are currently playing an important role in the fields of medicine and electronics. In the medical sector, for example, they are being developed as biodegradable materials with programmable decomposition behaviour, while also offering a whole set of additional functions (biocompatibility, non-critical metabolism, shape memory effects etc.). Particularly promising examples are polymeric aliphatic hydroxycarboxylic acids such as polylactic acid in which the crystallisation behaviour can be finely controlled through the incorporation of enantiomers. Intensive R&D work is also being conducted on plastics for the pharmaceutical industry with precisely programmable active ingredient release.

The other group are the intrinsically conductive polymers. This conductivity can refer to ions and electrons. Ionic conductivity is needed, for example, in the membranes of fuel cells (proton conductor, e.g. Nafion) or lithium ion batteries. The polymers used for this are noted for their many highly polar to ionic groups that offer the migrating ions continuous paths along which they can migrate in a stabilised condition. The major challenges here are the as yet inadequate mobility of the ions in these polymer membranes, their long-term stability and the reliability of the production processes.

Apart from this, there are other polymers that enable the transport of electrons. They are normally characterised by a structure that allows the electrons, due to the many multiple bonds in the polymer structure, to migrate through the collection of polymer chains via easy-to-activate hopping processes. The conductivity attainable in this way is not usually sufficient to cover long distances and make the entire plastic component conductive – irrespective of the generally low resistance of these plastics to environmental influences – but in thin layers they can produce astonishing results. With these (semi-)conductive polymers, it is possible to produce transistors (OFETs) for electronic tags (RFID), logistic switches or sensors (“electronic nose”). If these materials also offer high absorption of visible light, they can be used in solar cells. Finally, if they have a high electroluminescence quantum yield, they are predestined for light-emitting diodes (OLEDs). The enormous attraction of these materials also comes from their ease of printing, allowing the economical production of large numbers at low cost (printed organic electronics).

With all these systems, the problem lies in the fact that their efficiency and long-term stability is in most cases still inadequate. However, measured against the immense progress that has been achieved in the last few years, it is safe to assume that these difficulties can be overcome. That is why many people say the 21st century will be the century of "plastics electronics".

Functionalised plastics, parts and surfaces - Vita Prof. Dr. Matthias Rehahn