Christopher Dölle

Gradual photochemical-induced conversion of liquid polydimethylsiloxane layers to carbon containing silica coatings by VUV irradiation at 172 nm.

Thin liquid layers of polydimethylsiloxane (PDMS) were irradiated by VUV light under nitrogen atmosphere using a Xe(2)- excimer lamp. The irradiated layers were analyzed with infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS), showing a gradual photochemical-induced conversion of the liquid PDMS to solid SiO(2)-like coatings. IRRAS measurements revealed a smooth frequency shift of the maximal absorption band from 1111 to 1231 cm(-1) with increasing irradiation energy density caused by a gradual shift from the asymmetric Si-O stretching vibration of PDMS to the longitudinal optical (LO) mode of SiO(2). The shift was found to be dependent on the applied irradiation energy density and the O/Si ratio in the film analyzed by XPS measurements. The atomic ratio of O/Si increases from 1:1 to about 2.5:1. At the same time, the atomic ratio of C/Si decreases from 2:1 down to 1:6.5. Images taken by high resolution field emission scanning electron microscopy (FESEM) and scanning force microscopy (SFM) show a smooth surface without cracks or pores. The controllable coating properties in combination with the possibility for local irradiation using masks are promising high potential for the coating technology.

Surface Pretreatment of Fiber-Reinforced Plastics via VUV Radiation

Fiber-reinforced plastics (FRP) are of great importance for the transport sector, the aerospace industry, for wind power plants, in the building sector and in the field of sports and leisure applications. Optimization of the adhesive bonding process for FRP structures, especially the surface preparation prior to bonding, will be of a central importance in forthcoming expansion of FRP use. In this connection the key problem depends on the FRP polymer matrix. In the case of duroplastic matrix the main problem is the presence of release agent on the surface of joining components. For the thermoplastic matrix such as polypropylene (PP), the main problems are the low surface energy and the inertness of its surface. Conventional pretreatment methods, such as manual grinding, shall be replaced by energetic radiation technics like VUV lamps (vacuum ultraviolet spectral range: 100 – 200 nm). This approach is a non-contact method, characterized by high treatment homogeneity and material-saving properties, combined with no further fibers to be released. The surface of the thermoplastics is activated by the incorporation of oxygen, release agent contamination on the thermoset is cleaned or modified [1 - 8]. The results of the VUV surface activation of PP and CFRP with regard to the incorporation of functional groups, increase of surface energy, matrix degradation and the adhesion increase of adhesive bonds are presented. In addition, studies on the release agent coating and the release agent modification by VUV radiation are presented. The work is completed by considerations concerning possibilities to accelerate the process (in particular, wavelength dependence, influence of an inert gas or the moisture content). Finally, an evaluation of the VUV pretreatment is carried out on the basis of two specific applications.