VUV-induced micro-folding of acrylate-based coatings: 2. Characterization of surface properties

Abstract

The surface properties of micro-folded, “physically matted” acrylate coatings prepared via vacuum UV (VUV) treatment are described. By comparing micro-structured and smooth surfaces, static and dynamic friction values were determined. The topography of micro-wrinkled areas was scanned by an optical profiler and correlated to the gloss. In addition, the reproducibility of surface structure formation and their spatial frequency spectrum roughness have been analyzed using the power structural density functions calculated from the measured surface topographies. Since the high energy 172 nm photons (7.2 eV) are able to generate radicals in acrylates even in the absence of any photoinitiator, a highly crosslinked acrylate matrix was formed in the near surface range. As a result of the increased cross-link density, the micromechanical properties such as the surface hardness were improved. Selected data are presented.

Introduction

The creation and characterization of micro- and nanostructured coatings and surface layers are intensively investigated research areas [1], [2], [3], [4]. As a tool for surface structuring, photochemical micro-folding has been discovered in connection with the development of short-wavelength excimer lamps [5], [6], [7] and was regarded so far as a simple method to produce extremely matted decorative surfaces by “physical matting”. During the last years, the application of decorative and functional surface coatings, especially in combination with scratch and abrasion resistant nanocomposite varnishes [8], [9] resulted in the discovery and application of other interesting and beneficial properties of surfaces treated by 172 nm irradiation. In this context it is obvious that different micro-structured acrylate coatings (see Fig. 1), generated by the procedure schematically shown in Fig. 2, do not only exhibit a variable matted appearance, but will also display different haptics and friction coefficients.We recently reported on a device allowing for the real-time recording of the folding kinetics of the micro-folding process [10]. Here, we present our latest results on the topographical description of the random structured surface as well as on the influence of the high energy 172 nm photons on the micromechanical properties of both the surface and the entire coating.

Materials

Materials for the investigation of the micro-folding behavior of acrylic mono- and oligomers were chosen with the objective to cover a wide range of relevant properties given by the type of the compound itself, the molecular weight, viscosity, functionality, glass transition temperature, as well as surface and shrinking tension. Samples of acrylic monomers (EB 80, EB 210, EB 810, EB 830, EB 852, SR 351, tripropylenglycol diacrylate (TPGDA), CN 132, Genomer 2259) were provided by Bayer…

Surface topography

Micro-folded acrylate-based coatings were prepared from both an SL8 and an SD-L4 varnish. Both coatings were applied by roll coating on a lab-coater. Table 1 summarizes the most important parameters of these coatings.

Micro-profiling was carried out for two independent SL8 (only one shown in Fig. 3a) and two independent SD-L4 (only one shown in Fig. 3b) coatings. These measurements revealed the reproducibility of the micro-folding process as also demonstrated in Fig. 4. For quantification of the…

Conclusions

By analyzing the surface with optical profilometry we could demonstrate that the PSD profiles, calculated from the measured surface height profiles, provide excellent information on coarseness, homogeneity and reproducibility of the micro-structured surface texture. Referring to the matting effect it could be shown that there is a correlation between gloss and the aspect ratio. Furthermore, the gloss can be reproducibly manipulated by the process parameters. Unlike in conventional chemical…

Acknowledgments

The authors thank the Technologieförderung of the Sächsische Aufbaubank, Dresden, for funding the project and also all named companies for providing materials.

Autors among others are Dr. Rolf Schubert and Prof. Dr. Rainer Mehnert – VUV-induced micro-folding of acrylate-based coatings: 2. Characterization of surface properties – ScienceDirect

IOT