Implementing Safe and Sustainable by Design (SSbD) principles in the plastic value chain requires integrating heterogeneous information generated across life cycle stages and across safety, environmental, economic, and social dimensions. However, these data remain dispersed, inconsistently structured, and difficult to harmonise, undermining both the transparency and reproducibility of SSbD assessments. Ontologies address these challenges by providing a machine-interpretable structure of concepts and relationships, enabling semantic interoperability, consistent terminology, and harmonised data organisation across tools and datasets. While existing ontologies address specific areas such as chemistry, Life Cycle Analysis, and industrial processes, none bring together the full SSbD framework across all four dimensions or provide a holistic representation of the plastic value chain with Technology Readiness Level (TRL)-adaptive assessment logic. This work introduces one of the first ontologies designed specifically to support SSbD implementation in plastics, integrating substances identification, their role in processes, process context, scoping elements, TRL-dependent dimension assessments, and cross-dimensional interactions into a coherent semantic backbone for structuring databases and enabling decision-support systems.