Index

Production of plastic powder feedstocks for additive manufacturing by precipitation

Funded by Deutsche Forschungsgemeinschaft (DFG) – Project number 409808524

The overall aim of this project within priority program 2122 “Materials for Additive Manufacturing” is the development of novel semi-crystalline polymer powders for selective laser sintering (SLS) via a liquid-liquid phase separation (LLPS) and subsequent crystallization for polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyoxymethylene (POM) and polyvinylidene fluoride (PVDF). Selection criteria for appropriate solvents for LLPS will be deduced by an iterative screening method based on Hildebrand and Hansen solubility parameters. In-situ model LLPS experiments employing dynamic light scattering, optical microscopy and impedance spectroscopy –supported by ex-situ electron microscopy- will allow for deeper insights into the phase separation and particle formation mechanisms. A detailed mechanistic understanding of the underlying mechanism of LLPS, i.e. nucleation, droplet coalescence and growth and elucidation of their dependencies on system composition, time-temperature history will be achieved. LLPS will be implemented on the mini plant scale, where the effect of process parameters (e.g. influence of stirring, cooling regime) on material and bulk solid properties of the obtained polymer particles will be systematically studied. Moreover, in-situ functionalization of the particles with nanoparticulate flowing aids and additive enhancement with thermal stabilizers (antioxidants) will be studied, scale-up criteria will be deduced and powder amounts applicable for studies on SLS processing behavior (several 100 grams to the kilogram scale) will be produced. A detailed structural characterization of the obtained SLS particles with respect to crystallinity, polymorphism, thermal characteristics and morphology will be performed employing amongst others dynamic scanning calorimetry, X-ray diffraction, vibrational spectroscopy (IR, Raman) and electron microscopy. The effect of process parameters on molar mass distributions of the polymer and melt viscosity will be assessed by gel permeation chromatography and melt rheology. Bulk solid characteristics such as the product particle size distribution and the powder flowability which are seen to be most important for processing will be characterized by laser diffraction particle sizing, respectively shear testers and powder application model experiments. The gained understanding of the structure-property relationships and the information on SLS processability provided by the cooperation partners will be utilized for further LLPS process optimization to tailor the desired properties of the novel SLS powders. Novel SLS powders with improved material behavior, i.e. chemical resistance (PET, PBT, PVDF), impact resistance (PBT), high stiffness, excellent dimensional stability (PET, PBT, POM) shall be developed widening the field of application of SLS-manufactured parts.

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Formulation of dispersed systems via melt emulsification

Funded by Deutsche Forschungsgemeinschaft (DFG) – Project number 504809428

The aim of this project within priority program 2364 “Autonomous processes in particle technology” is the automated production of liquid-liquid disperse systems via melt emulsification, whereby in this process emulsification takes place at elevated temperature. The products obtained after cooling are dispersions of spherical nanoparticles or microparticles. Within the scope of this project, a melt emulsification device for the automated production of product particles with a well-defined particle size distribution (PSD) will be further developed. The PSD has a significant influence on the subsequent product properties, such as flow behavior or drug release kinetics. The PSD of the products is determined by the complex interaction of competing mechanisms. These are, in particular, droplet breakup in a rotor-stator device as a result of shear and elongation stress, as well as coalescence and further ripening, which in turn depend on the system composition, i.e. the emulsifier used (type, concentration) and the dispersion phase (viscosity, volume fraction). Therefore, for a better process understanding and an active process control, possibilities for in situ determination of the PSD are urgently required. In this project, a novel fiber-coupled measurement system based on broadband elastic light scattering is developed for in situ measurement of the PSD. The system will be validated on reference particle systems and applied to the emulsification process. Furthermore, a hybrid process model is developed, which is the basis for the design of a model predictive control of the process. The model predictive control in combination with the in situ measurement will provide the possibility for an active process control and the production of emulsions with predefined properties and a simultaneous optimization of the process time.

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Formulation of hetero-aggregates

Funded by Deutsche Forschungsgemeinschaft (DFG) – Project number 46231449

The vision of the project within priority program 2289 “Hetero-Aggregates” is the establishment of a continuous process for formulation of ternary hetero-aggregates from dry primary particles. The functionality of the hetero-aggregates is grounded in the designed composition and distribution of hetero-contacts between constituents achieved by mixing at the primary particle scale (~ 25 nm). Process modelling is an integral part of the project: Firstly, to establish the process function of the formulation process (in the Rumpfian sense). Secondly, to generate tools for model-based sensor fusion, process optimization and control. Comprehensive characterization of hetero-aggregate properties links experiments and the simulation studies, iteratively providing validation and improvement of process models and experimental design as well as revealing the material functions of hetero-aggregate formulation in opposed jet fluidized beds. Having established the processing strategy, characterization methods, fundamental models of mixing of binary nanoscale primary particles into aggregates, and the connection between process parameters and structural parameters of the aggregates, the second funding period will see a continuation and extension of the focus towards functionality of the hetero-aggregates. The main designed functionality of the hetero-aggregates is photocatalytic activity by designing hetero-aggregate structures consisting of TiO2 and ZrO2 primary particles via composition and formation of hetero-junctions. Furthermore, a third component, bismuth vanadate, BiVO4, will be added to form ternary hetero-aggregates, extending the accessible range of the electromagnetic spectrum for “harvesting” electrons required in photocatalysis and to increase the efficiency of the separation of the generated electron-hole pairs, thus allowing design of an improved photocatalyst. The objectives of the project are: i) Establishment of a continuous ternary hetero-aggregate formulation process in opposed jet fluidized beds; elucidation of structure formation towards design functionality; ii) Development of a CFD-informed multivariate population balance model for formulation ternary hetero-aggregates; iii) Development of a model-based soft-sensor for photocatalytic activity of the hetero-aggregates, taking into account property and structure distributions (e.g., size, composition); iv) Model-based analysis, optimization and control of formation dynamics, structural properties and functionality by population balance models and soft-sensor; v) Extension of the SEM-EDX and the Raman mapping approach to characterize the intra- and inter-aggregate composition and mixing of ternary hetero-aggregates; vi) Elucidation of the optical, electronic and photochemical properties of ternary hetero-aggregates formed from semiconductors and characterization of the heterojunction (bulk behavior); vii) Linking of hetero-aggregates properties on the single aggregate level with bulk behavior and photocatalytic performance.

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Quality assurance system for polymer powders used in additive manufacturing

Funded by Deutsche Forschungsgemeinschaft (DFG) – Project number 61375930

Aim of this transfer project of CRC 814 “Additive Manufacturing” is the scientific development of a quality assurance system for polymer powders used in laser beam melting. The quality assurance system is based on two methods. First, an empirical model for ageing of polymers will be established, which considers beside chemical additionally physical ageing mechanism. The model is based on experimentally determined bulk material properties as well as rheological and thermal properties of the present material system. The material model in combination with process data allows the prediction of the ageing state and will lead to a demonstrator software, which will be experimentally validated. Furthermore, a measuring system will be developed, which allows for determination of powder flowability at elevated temperatures and rheological properties of the polymer melt. After validation, the quality assurance system will be transferred to a demonstrator system in close cooperation with the industry partner.

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Functional polymer particles via liquid-based top-down processes

Funded by Deutsche Forschungsgemeinschaft (DFG) – Project number 6137593

Aim of this sub-project of CRC 814 “Additive Manufacturing” is process intensification and optimization of process parameters in view of minimization of alteration of intrinsic polymer properties due to processing. A continuous wet grinding – separation circuit will allow for an increased yield of the target particle size fraction and optimal control of particle residence time. The melt emulsification approach, amongst others, will be performed for low-viscous polymer waxes. The obtained particles will be subjected to solid state polymerization. Precipitation will be utilized for production of microparticle filled systems.

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