Surfaces and interfaces play a central role in plastic technology. Where surfaces are concerned, a distinction must be made between the technological and the aesthetic perspectives. The visible exterior of a production part or product is its surface. Besides its aesthetic appearance the surface often has an additional technological function (scratch resistance, water repellence, low contamination, antistatic or antibacterial properties…). In the production process, however, the role of component surfaces is purely technological. Here, the key challenges include printability, paintability and adhesion.

Where interfaces are concerned - that is the inner surfaces of materials, where one material comes into direct contact with another - solely the technological aspect must be considered. The most important interfaces include the interfaces between fillers or reinforcing materials in polymer-matrix composites, where the filler and reinforcing material must bond well to the matrix.

Photograph showing good adhesion (left) and poor adhesion (right)

One of the main research focuses is on surface and interface analysis of polymer materials. This is used, for example, to assess the functionality of compounds. The wettability or non-wettability correlates to the cleanability, or mechanical properties can be deduced from the difference in surface energy between the matrix and filler.

The surface tensions of liquid media can be determined using lying or hanging drops or by the Wilhelmy method, while the interface energy of solid plate-like or powdered materials can be investigated.

The best-known properties of photocatalytically active surfaces, such as self-cleaning or antimicrobial properties, will be made available for plastic surfaces. The photocatalytic process is initiated by UV light, and in suitable ambient conditions the photocatalyst (most commonly TiO2) has an oxidising effect, mineralising organic materials and consequently destroying bacteria and fungae. Many conventional wet chemical coating methods for applying TiO2 coatings are not suited to plastic surfaces due to the high processing temperature.

The plastic surface itself must also be protected from the oxidative effect of the photocatalyst.

Furthermore a large variety of application-specific requirements must be fulfilled:

• Endurance / long-term stability
• Adhesive strength
• Appearance
• Scratch resistance
• Ease of application

Different photocatalysts, in particular TiO2, are already commercially available and should be made available for plastic surfaces. To this end, coating designs are being developed and adjusted to the specific application. Examples include embedding of particles in varnish-like systems, or direct input of the particles into plastics while they are being processed into components.

• Application at low temperatures and consequently with low temperature strain on the substrate
• Addition of photocatalytic effects to existing surfaces is possible
• Ease of application, as conventional coating processes are used
• Use of conventional plastic processing methods to generate photocatalytically active surfaces.