Martin Bastian

Terahertz time-domain spectroscopy as a tool to monitor the glass transition in polymers.

We demonstrate the suitability of terahertz time-domain spectroscopy as a non-destructive, contact-free tool to monitor the glass transition in polymers--a core feature of the amorphous phase. Below the glass transition temperature T(g), segmental motions along the polymer chain are frozen due to the lack of free volume between neighboring macromolecules. We show that this transition also reflects in the temperature dependence of the refractive index at terahertz frequencies. Two domains can be identified, which differ in their sensitivity to temperature changes. To verify the proposed approach, we determine the glass transition temperature T(g) of semi-crystalline poly(oxymethylene) (POM) with terahertz time-domain spectroscopy and validate the results by destructive differential scanning calorimetry (DSC) measurements.

Experimental investigation of the morphology development of polyblends in corotating twin-screw extruders

Compounding extruders are still designed based on experience and time-consuming experimental examinations. This work investigates the morphology development of incompatible polyblends along a mixing zone at the end of a corotating twin-screw extruder. During the process, the samples are taken from the running extruder using special barrel plates. These samples are subsequently examined by means of scanning electron microscopy (SEM). This method allows sampling in less than 1 min and thus extremely fast and almost unaffected. The experimental investigation of the morphology development improves the knowledge about the factors essentially influencing the blending process. It also allows the verification as well as improvement of theoretical models. Polyblends of polypropylene (PP) and polyamide (PA) with 7.5, 15, and 30 wt % PA were examined. As well as the relevance of the mass percentage of the dispersed phase, the influence of the screw geometry, the screw speed, the melt temperature, the melt throughput, and the pressure profile was investigated. Apart from the melt throughput, all varied factors show an influence on the resulting blend morphology that may not be neglected. However, the changes of the mean particle sizes in the observed mixing zones are only gradual (mean particle size ≈ 1–4 μm), which can be attributed to the extremely fine blend morphology already existing during or after the melting. That means that the application of “classic melting zones” generally already produces finely dispersed blend morphologies, thus proving the essential importance of the melting zones regarding the development of the blend morphology. Consequently, the mean particle sizes, calculated by means of quantitative image analyses of SEM micrographs in the mixing zones following the homogenizing section only slightly depend on the compounding conditions (screw speed, melt throughput, screw geometry, melt temperature, and pressure profile). However, the direct visual analysis of the SEM images, especially in the first parts of the mixing zones, shows the simultaneous existence of large PA6 particles in the PP matrix. In addition, a downstream unification of the particle size distribution can be observed. Especially the number and size of the coarser particles decreases in the mixing zones.

Design of a compounding extruder by means of the SIGMA simulation software

The simulation program SIGMA, which can be used to assess the compounding process on tightly-intermeshing, co-rotating twin screw extruders, was developed within the framework of a joint project conducted by the Institut fur Kunststofftechnik (KTP) of the University of Paderborn and fifteen industrial companies of several fields in the polymer industry. The program presented here permits calculations based on physical mathematical models of the pressure, temperature, local degree of filling, melting, residence time, mixed substance characteristics derived therefrom, power consumption, and degree of dispersion of the machine. These results assist the designing process engineer in the optimization of existing equipment or in the designing of new equipment.

Terahertz spectroscopy: A powerful tool for the characterization of plastic materials

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.

Terahertz plastic compound lenses

We present terahertz (THz) lenses made of highly refracting polymeric compounds which provide a better focusing performance and an increased functionality in comparison to conventional THz lenses. Using mixtures consisting of polypropylene (PP) and alumina as well as PP and zinc sulfide allows a significant increase of the refractive index while simultaneously keeping a low extinction and dispersion. With these new material combinations, lenses with an increased focusing capability are realized. This is evaluated by focal plane measurements using a fiber coupled THz time-domain spectrometer.

Melting of Polymer Blends in Co-rotating Twin Screw Extruders

Abstract Part II of the publication describes a model for calculating the melting of polymer blends which has been implemented in the SIGMA simulation software for the design of co-rotating twin screw extruders. The model is based on the findings discussed in Part I and makes it possible to calculate the temperature progression in the solids conveying section and during the subsequent melting process for binary incompatible polymer combinations. The two material components are observed in parallel during the melting process, and the respective degrees of melting over the length of the screw are calculated. The properties of the melt phase, which forms from both components, are calculated with mixing rules implemented in the program. The calculations supply the melting profiles for both components, thereby permitting a comprehensive analysis of the melting of binary polymer combinations. The results additionally form the basis of a calculation to estimate the morphology development of polymer blends in the melting section of the extruder. Part III of the report looks into a means of investigating the melting of binary polymer combinations, and the results of experimental investigations into polypropylene/polyamide blends are discussed. The simulation calculations are compared with the results of experimental studies in order to verify the model presented in this part of the publication.

Polymer Blends in Co-Rotating Twin-Screw Extruders

Abstract The properties of polymer blends are determined to a decisive extent by the morphological structure of the polymer combinations employed. The design of extruders thus calls for models to calculate the estimated morphology development over the length of the extruder screws in the melt-conveying section. Since the most significant morphological changes are observed in the melting section, however, it is also necessary for the morphology formation and development to be analyzed in this section of the extruder. The melting process of binary material combinations is thus important too. In the context of this research, experimental investigations were conducted using polypropylene/polyamide 6 (PP/PA 6). In the tests, the degree of melting and the morphology development were determined over different screws and compared with calculations. In order to analyze a range of relevant influences, the extruder size, screw configuration, screw speed, weight components and also the viscosity ratio were varied by using different PP types. Apart from the model for calculating the melting of polymer blends, a formulation was developed that can be used to estimate the morphological changes occurring in the melt-conveying section. The model is based on the assumption that morphological changes can be estimated by calculating the probabilities of different drop breakup mechanisms and the coalescence process. The investigations of the blend morphology in the melting section and the melt-conveying section reveal key findings that have to be taken into account for modeling the formation and development of the morphology. First of all, in the melting section, it is very clear that a kind of melt film removal occurs at the surface of the granules of the second component, which melts at higher temperatures, as these granules melt. The drops of second component in the melting section, which are directly adjacent to fractions that have not yet fully melted in some cases, have already assumed dimensions (in the μm range) similar to those seen at the end of the extrusion process. This means that, in the melting section of the twin-screw extruder, small volumes are broken or worn off the already-molten granule surfaces. An evaluation of scanning electron micrographs also shows that, in the melting section of co-rotating twin-screw extruders, virtually all the breakup mechanisms that can essentially be distinguished take place in parallel, such as quasi-steady drop breakup or supercritical breakup, folding, end pinching, tip streaming and breakup through capillary instabilities. Alongside the breakup mechanisms, there are also drops that clearly unite to form bigger drops through coalescence. When comparing the calculations for the melting of polymer blends, relatively good agreement is obtained with the experimental test results. The calculations display a satisfactory level of accuracy, particularly for polymer combinations with similar viscosities and also for bigger extruders. The calculations with the morphology model also show the same trends as the experimental investigations. Hence, for the design or optimization of twin-screw extruders, it is now possible not only to calculate the fundamental process variables (such as pressure, temperature, melting) but also to estimate the morphology that has a decisive influence on the resultant material properties.

Terahertz inline wall thickness monitoring system for plastic pipe extrusion

Conventional and commercially available inline wall thickness monitoring systems for pipe extrusion are usually based on ultrasonic or x-ray technology. Disadvantages of ultrasonic systems are the usual need of water as a coupling media and the high damping in thick walled or foamed pipes. For x-ray systems special safety requirements have to be taken into account because of the ionizing radiation. The terahertz (THz) technology offers a novel approach to solve these problems. THz waves have many properties which are suitable for the non-destructive testing of plastics. The absorption of electrical isolators is typically very low and the radiation is non-ionizing in comparison to x-rays. Through the electromagnetic origin of the THz waves they can be used for contact free measurements. Foams show a much lower absorption in contrast to acoustic waves. The developed system uses THz pulses which are generated by stimulating photoconductive switches with femtosecond laser pulses. The time of flight of THz pul...

Melting of Polymer Blends in Co-rotating Twin Screw Extruders: Part III: Experimental Verification

Abstract Experimental studies of the melting process in extruders generally involve a high outlay and can only be performed with certain restrictions, both in respect of the method employed and with regard to the evaluation of the results. This part of the work describes one possible way of studying the melting of binary incompatible polymer combinations. The method described makes it possible to estimate the degree of melting of the two components over the length of the screws of the twin screw extruder. Experimental studies were performed of polypropylene/polyamide 6 (PP/PA6) blends containing low weight contents of the disperse PA 6 phase. In order to verify the theoretical models set out in Part I, page 124 of this issue, not only were the process conditions of screw speed and throughput varied but also the viscosity ratio. This was achieved by using two different PP grades. In addition, comparable tests were performed on two different sizes of extruders. The tests show that the melting of polymer blends, and particularly the melting of the second component, which melts at a higher temperature, is determined to a significant extent by the screw rotational speed, the throughput, the ratio of the extruder channel depth to granule diameter and also the material combination (viscosity ratio). Comparisons of calculations and experiments show that the melting profiles calculated for different material combinations and process conditions tally well with the experimental studies in overall terms.

Terahertz-Forschung begegnet Kunststofftechnik (Terahertz Research Meets Polymer Technology)

Die Terahertz-Messtechnik kann in Zukunft vermehrt Einzug in die Prozess- und Qualitätskontrolle der Kunststofftechnik halten. Zeitbereichsspektroskopie und ein neuer Algorithmus zur äußerst genauen Datenauswertung ermöglichen es, den Gehalt von Zusatzstoffen präzise inline zu überwachen oder Kunststoffbauteile zerstörungsfrei zu prüfen. Im Gegenzug versprechen aber auch Funktionswerkstoffe auf Polymerbasis Mehrwert als THz-Systemkomponenten. Terahertz technology might soon be introduced into the process and quality control for the polymer processing industry. Time-domain spectroscopy and a new algorithm for highly accurate data evaluation enable the precise in-line monitoring of additive content or the non-destructive testing of polymeric components. In return, materials on polymeric basis can provide valuable terahertz system components.