The transition from a linear economic model to a circular model is crucial, due to the limited resources of our planet. The Circular Economy (CE) aims to maintain material continuity through the cycle of production, consumption and recycling. The Digital Product Passport (DPP) is recognised as a critical instrument for advancing CE, serving as a comprehensive digital repository for product lifecycle information. The DPP paradigm fosters transparency and traceability. However, so far there is no agreed-upon standard for technically representing and expressing DPPs. This paper aims to provide a comprehensive analysis of the requirements of a general (cross-sectoral) DPP, and to discuss a core DPP model. We propose to express this in the form of an ontology network, i.e., a formal model serving as a "translation layer" from raw data to interpreted information, along with SHACL shapes for ensuring increased data quality. Despite existing research on DPPs, a comprehensive tool for technically representing DPPs is missing, making this paper a pioneering exploration into the modelling of a DPP core ontology.