Highly filled (HF) composites or concentrated suspensions are typically polymers with a large volume fraction of particulate fillers (inorganic or organic) that could be macro-, microor nano-sized particles. These particles could be of a wide variety of chemical composition, shapes, sizes and size distributions. They are generally aimed to impart specific properties of the polymer composite, which could include rheological, mechanical, dielectric, conducting, optical, luminescence, and other properties. Due to their versatility, HF composites are found in many industries, including automotive, additive manufacturing, biomedical, batteries, ceramics, composites, magnetics, electronics packaging, solid propellant, sand adhesives and others.
The processing of HF composites can be challenging, since high temperatures and shear rates are generally needed to process these materials and obtain the required structure. The filled molten polymers present nonlinear rheological behavior, such as wall slip, particle binder segregation, swelling and surface instabilities. Several studies have been carried out concerning filled polymers, especially for low viscosity polymers (suspensions), however for high viscosity polymers the findings are often contradictory and mechanisms for their behavior are not well understood. At present there are also no mathematical models that could accurately predict the flow of molten HF composites, since the existing models do not consider stress rate dependency and particle loading on rheological behavior.
Therefore, the subject of our proposal is a systematic experimental study of individual components of HF composites and their effect on the rheological properties and processing, as well as on properties of solid HF composites. Moreover, the obtained experimental data will be used for development of improved flow models. More specifically, the aim of this proposal is to find and identify mechanisms governing the rheological behavior of HF composites based on the properties of individual components. Identifying the governing mechanisms would enable the proper selection of individual components and processing conditions for better processability of HF composites leading to more homogeneous distribution of particles inside polymeric matrix and hence higher efficiency of final (mechanical and eventual functional) properties of such materials. The properties of individual components, as well as the processing conditions, affect also the mechanical properties of solid HF composites. Thus, the second goal of the proposed project includes the investigation of the effect of individual components and their rheological properties (i.e flow behavior) during processing on mechanical properties of solid composites.
The third part of the project includes modeling of the flow behavior, since this enables obtaining an insight into the effects of individual properties of the components to the overall flow behavior of HF composites. In this part, the goal is to improve the existing models by including not only the volume fraction, occupied by the filler particles, and the maximum packing fraction, but also the processing parameters such as shear rate, temperature and pressure. The improved model will also take into account the particle-particle and matrixparticle interactions due to different surface characteristics of the filler particles.
In summary, the findings of the project will help with the understanding on how individual components of HF composites influence the rheological (flow) behavior and at the same time mechanical properties of solid composites. Improved flow models will enable more accurate prediction of flow behavior of HF polymers and thus the development of new or improved composite materials and manufacturing processes will be more reliable and faster.
