Near infrared spectroscopy as a tool for in-line control of process and material properties of PLA biopolymer

Abstract

Abstract Near infrared (NIR) spectroscopy has become an analytical tool for material and process control, leading to substantial quality improvements in the output material. In the field of polymer processing NIR spectroscopy has been increasingly applied for on-/in-line monitoring, mainly for lab-scale process development but also for the production of high-value materials. For this paper the bio-polymer polylactide (PLA) was investigated by NIRS in the range of 1.2 to 2.4 μm, in order to identify modifications induced by additives of nanofil® and nano titanium oxide. Although these additives do not strongly absorb NIR radiation, their influence can be analysed using chemometric codes, due to their light scattering. A twin screw extruder (Haake Polylab PTW 16) was used to extrude polymer pellets mixed with the additives. The extruder was fed by a single screw metering feeder. The optical sensor probes (glass fibers) were placed between the screw and the nozzle to achieve a good optical throughput and to withstand typical temperature and pressure conditions during the extrusion (up to 220 ℃ and a few 10 MPa). The on-line NIR spectroscopy provided real time information which could be used for continuous process or feeding control. The chemometric data evaluation was able to quantify the measured values, especially the content of the additives in the compounds. The established statistical models predicted the actual values with high correlation coefficients (> 0.99). The additives altered the viscosity of the melt and the mechanical properties (Young's modulus) of injection moulded test samples of the compound.