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Abstract

Molecular weight (M w ) effects in poly(vinylidene fluoride) (PVDF) influence both processability and combustion behavior in energetic Al-PVDF filaments. Results show decreased viscosity in unloaded and fuel-lean (i.e., 15 wt% Al) filaments. In highly loaded filaments (i.e., 30 wt% Al), reduced viscosity is minimal due to higher electrostatic interaction between Al particles and low M w chains as confirmed by Fourier-transform infrared spectroscopy. Thermal and combustion analysis further corroborates this story as exothermic activity decreases in PVDF with smaller M w chains. Differential scanning calorimetry and Thermogravimetric analysis show reduced reaction enthalpy and lower char yield in low M w PVDF. Enthalpy reduction trends continued in nonequilibrium burn rate studies, which confirm that burn rate decreases in the presence of low M w PVDF. Furthermore, powder X-ray patterns of post-burn products suggest that low M w PVDF decomposition creates a diffusion barrier near the Al particle surface resulting in negligible AlF 3 formation in fuel-rich filaments.

To probe reaction kinetics and polymer decomposition, energetic filaments composed of aluminum (Al) and poly(vinylidene fluoride) (PVDF) with varying hexafluoropropylene (HFP) content are tested for processability and combustion characteristics. Rheological and crystallization findings indicate that the Al‐binder interfacial interactions are disrupted by HFP content. Thermal analysis shows that fuel consumption scales with Al particle size due to the diffusion‐driven Al‐fluoropolymer reaction regardless of HFP concentration. Char yield analysis shows that more solid product is retained in samples with smaller particle diameters, which further reflects the diffusive nature of both Al‐PVDF and Al‐P(VDF‐HFP) reactions. Burn rates reveal two competing mechanisms for reaction efficiency: 1) accelerated binder decomposition through Al‐PVDF interactions and 2) more energetic fluorination due to higher fluorine content in the P(VDF‐HFP) binders. Finally, powder X‐ray diffraction (PXRD) patterns show that AlF3 is the primary product from self‐propagating burns. However, in larger Al particle sizes, filaments are unable to burn completely and result in high levels of Al2O3 and Al4C3 formation, which indicates that these binders are not amenable with low surface area, metallic fuels. These findings aim to improve fluorinated feedstock selection for potential binder candidates in energetic additive manufacturing (AM).

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