Process and quality monitoring in polymer foam extrusion with time-resolved te-rahertz spectroscopy

Schober, Giovanni; SKZ - KFE gGmbH; Germany

Shober, G.; SKZ - KFE Gmbh; Germany
Werner, M.; SKZ - KFE Gmbh; Germany
Kolb, C.; SKZ - KFE Gmbh; Germany
Kremling, S.; SKZ - KFE Gmbh; Germany
Hochrein, T.; SKZ - KFE Gmbh; Germany

ID: ECNDT-0025-2018

There are different measurement techniques (e.g. manual visual characterization, digital image evaluation, computed tomography, near infrared radiation, ultrasound technology) which can be used to characterize polymer foams. Unfortunately, none of these methods can be used to determine the raw density as well as the cell size distribution of polymer foams in the context of inline monitoring.
In this paper, a new characterization method for the determination of polymer foam pa-rameters is presented. The terahertz time-domain-spectroscopy (THz-TDS) technique can provide crucial advantages since THz waves can penetrate polymer foams well and provide data in real-time. With the use of a THz-TDS system, reflection and transmission spectra of extruded polymer foams are measured. The raw density and the cell size distribution are the main characteristic parameters of polymer foams, which specify the quality and thus the field of application. Based on a Mie scattering theory, the extinction of the THz beam is used to calculate the cell size distribution. The second important foam parameter, the raw density, is directly correlated with the effective refractive index of the foam in the THz regime.
THz-TDS measurements are contact-free and there is no need for special radiation pro-tection because the emitted energy is in the range of meV. The investigated polymer foams differs in their manufacturing processes (chemical/ physical extrusion, expan-sion, polyaddition), their materials (polyurethane (PUR), polypropylene (PP), polyeth-ylene (PE), polyethyleneterephthalate (PET)) and different foam properties (open pores, closed pores, mixture). This gives a very broad overview of the possibilities of this technique for the investigation of polymer foams.