Terahertz Spectroscopy A Powerful Tool for the Characterization of Plastic Materials S. Wietzke*, C. Jansen, N. Krumbholz, O. Peters, N. Vieweg, C. Jördens, M. Scheller, D. Romeike Institut für Hochfrequenztechnik Technische Universität Braunschweig 38106 Braunschweig, Germany steffen.wietzke@ihf.tu-bs.de T. Jung, M. Reuter, S. Chatterjee, M. Koch Fachbereich Physik Philipps-Universität Marburg 35032 Marburg, Germany martin.koch@physik.uni-marburg.de B. Baudrit, T. Zentgraf, T. Hochrein, M. Bastian Süddeutsches Kunststoff-Zentrum, SKZ-KFE gGmbH 97076 Würzburg, Germany Abstract— Terahertz (THz) spectroscopy holds a high potential as non-destructive, contact-free testing tool for the analysis of macromolecules, the monitoring of plastic processing and the inspection of plastic components. Even molecular properties, such as the glass transition temperature of highly-crystalline polymers, can be derived from THz measurements. Furthermore, we have identified a plethora of emerging applications in the plastics industry. To appeal to these upcoming challenges, we developed the first THz time-domain spectroscopy (TDS) system for the inline monitoring of compounding processes, which was awarded the Otto von Guericke Prize 2009. It has been demonstrated that the filler content at the end of the extrusion line could be precisely determined by time-of-flight measurements. In addition, THz technology is capable of measuring the water content sensitively, THz birefringence measurements reveal the fiber orientation in reinforced plastics and the quality of plastic weld joints or adhesive bonds can be inspected by the interference evaluation of the THz data. Due to the outstanding dielectric contrast of materials at THz frequencies, even contaminations invisible to x-rays or ultrasonic measurements can be revealed by THz TDS imaging opening the door for a new generation of non-destructive, contact-free quality control systems. Keywords-terahertz spectroscopy; polymer; plastic; non- destructive testing; monitoring; compound; glass transition; fiber orientation; plastic weld joint; additive content; water content I. INTRODUCTION In the electromagnetic spectrum, the terahertz (THz) region is commonly defined by the frequency range of 0.3 THz to 10 THz. Located between the microwaves and the infrared, the non-ionizing THz radiation combines the great depth of penetration of microwaves with the lateral sub-millimeter resolution of the higher-frequency infrared region, especially when imaging techniques are employed [1]. Due to this exposed position, a variety of metrological applications with high impact on industries exists. However, as the number of optical and electronic devices that could access this frequency range was very limited, the THz gap remained a weakly illuminated spot on the spectral map for a long time. The rapid progress in (femtosecond, fs) laser science and microwave engineering finally allows full access to this long unexplored frequency band [2]. Now, a variety of terahertz sources exist. The recent advances in THz system technology [3] and a plethora of research projects currently pursued suggest that the market introduction is rapidly approaching. More and more industrial applications arise, which realize the high potential of this emerging technology. This contribution serves as an overview of selected applications in the plastics industry, especially focusing on those where existing measuring methods cannot deliver satisfactory solutions. II. TERAHERTZ TIME-DOMAIN SPECTROSCOPY ON POLYMERS We will focus on THz time-domain spectroscopy (TDS) providing lots of information in the time and the frequency domain. THz TDS employs a coherent generation and detection scheme. Therefore, this technique has access to both, the phase and the amplitude of the electric field in contrast to far-infrared absorption spectroscopy. Thus, THz TDS is capable of extracting the material’s complex refractive index without the need for Kramers-Kronig relations [4]. Furthermore, an advanced data extraction algorithm allows for determining the real sample thickness d, the absorption n simultaneously [5]. Near- and mid-infrared spectroscopy considers absorption peaks that mainly arise from highly localized intramolecular deformation such as hindered rotation or stretching oscillation of covalent bonds [6]. However, THz waves interact with collective motions of large molecules. Polymer macromolecules interact much stronger with each other than gas molecules. These interactions result in much broader resonances compared to the sharp peaks of rotational spectra. By far-infrared absorption spectroscopy, complex inter- and intramolecular characteristics of polymers have been identified such as skeletal vibrations, liquid-lattice modes, intermolecular vibrations in the crystalline phase, or hydrogen bonds [7]. 1 2010 International Conference on Solid Dielectrics, Potsdam, Germany, July 4-9, 2010 978-1-4244-7944-3/10/$26.00 ©2010 IEEE