Inorganic or ceramic particle filled polymer composites for high thermal …

Description (excerpt)

CROSS-REFERNECE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Provisional Application No. 63/702,445 filed Oct. 2, 2024, which is hereby incorporated herein by reference in its entirety. GOVERNMENT FUNDING This invention was made with government support under W911NF-20-2-0155 awarded by the Department of Defense. The government has certain rights in the invention. SUMMARY We conceptualized and executed an ancillary process invention related to tuning the viscosity of the inorganic and/or ceramic particle-filled polymer system to broaden the formulation and process windows. Innovation was necessary to achieve inorganic and/or ceramic particle filler loadings in excess of 30 vol % at melt viscosities within the capabilities of the extrusion equipment (<0.1MPa·s at 10 Hz shear rate). The invention is related to adding a process aid such as microcrystalline wax, plasticizers, or surfactants to reduce the viscosity of the particle-polymer composite and to broaden the formulation and process windows. Specifically, the process entails the following aspects: Pre-wetting dry inorganic and/or ceramic particle fillers with the low-viscosity a wax, plasticizer, or surfactant additive to prevents particle aggregation during the physical mixing process; this facilitates higher volume loadings during subsequent melt-mixing (e.g. twin-screw extrusion) as localized segregation effects such as particle agglomeration and clustering are suppressed. Compounding with a wax, plasticizer, or surfactant additive adds an additional degree of freedom in formulating the thermoplastic binder composition (wax, plasticizer, or surfactant fraction) to the appropriate viscosity. This allows for higher inorganic and/or ceramic particle loading (essential to achieve high thermal conductivity) and to reduce the viscosity of a highly filled inorganic and/or ceramic particle polymer composite system to make otherwise not melt processable composite mixtures extrudable. The wax, plasticizer, or surfactant additive can act as a plasticizer and adds elasticity to the melt without largely affecting the mechanical properties of the extruded material. Primary Applications This formulation and process are to be used in conjunction with coextrusion and other melt-processing methods. In many cases, precise control of the system's melt viscosity is necessary for controlled assembly of the desired microstructures. Specifically, the invention facilitates multilayer coextrusion methods of forming layered composite materials as described and disclosed in U.S. Pat. Nos. 6,582,807, 7,002,754, and 8,902,508, which are incorporated herein by reference in their entirety. Those applications target highly thermally conductive composite materials for thermal management applications. Prior Art and Problem to Be Solved For systems with significant shape anisotropy, exponential viscosity increases can occur at relatively low volumetric loadings when the particles form a percolated network structure. At loadings above this percolation threshold, filled systems become solid-like and are exceedingly difficult to process with conventional extrusion and co-extrusion equipment. On the other hand, many of the unique properties of filled polymer composites rely on loadings in excess of this quantity. To obtain high thermal conductivity (>5 W/m·K) in soft polymer composites, for example, one must achieve a structure in which the rigid fillers bridge across soft polymeric regions to provide channels for heat dissipation. Because the deleterious viscosity increase and beneficial thermal conductivity increase stem from the same underlying structural features, some compromise is generally required to obtain a product that remains processable. For extrusion processes, including the layer-multiplying co-extrusion process, some degree of elongational flow is often experienced by the polymer system. Stresses can be both tensile and compressional at the die-exit and in the squeezing flow channels, respectively. Accordingly, the filled system may additionally be required to withstand some degree of stress in the melt state as imposed by the dies or molds employed in production. Formulations may be devised empirically that strive to achieve certain functional characteristics dictated by end-use (e.g., thermal conductivity, elastic modulus, elongation at break, etc.) while maintaining processing characteristics dictated by production requirements (e.g., viscosity, melt-strength, etc.). In the present case, microcrystalline wax was a unique additive that was successful in both reducing the viscosity of the system while adding to the system's melt elasticity. Accordingly, very high particle loadings (>30 vol %) capable of imparting very high thermal conductivity (&gt