Recently Crystalline nanocellulose (CNC) has received increased attention in the scientific community following a universal push for more environmetally conscious engineering and increased sustainability. Due to the combination of high stiffness and low density of CNC, a high specific modulus of approximately 90 MPa/(kg*m³) [1] is achieved. However, during processing via fused filament fabrication (FFF), the material is exposed to a series of thermal and mechanical loads. Therefore, the load history has to be considered when characterizing and predicting the composite’s properties in the final 3D-printed construction part. A recycled 3D-printable polypropylene modified with CNC up to 15 vol% content was examined in this study. Since the CNC’s reinforcement effect is mostly determined by the interactions between the cellulose and the matrix [3], a maleic-acid-anhydride-based compatibilizer was used. To characterize the novel composite shear rheological investigations, dynamic-mechanical analysis (DMA) and measurements regarding shrinkage were performed. An unexpected decrease in viscosity with increasing filler content was observed. This decrease was attributed to (I) the increased use of low viscosity compatibilizer with increased filler content and (II) the thermal degradation of the compatibilizer manifested in a colour change of the tested material. This hypothesis was verified by differential scanning calorimetry (DSC) measurements. This enabled a reliable material characterization, which can help to predict the properties of the final printed part.