Life Cycle Inventory is one of the longest part of the LCA and yet we cannot always get all the necessary data. This study is a methodological point of view on getting a quality results from a case study of polypropylene recycling, based on an intersection of currently used methods of benchmarking the unavailable data, using the sensitivity analysis and the good practices in LCA. The principle is in classification of unavailable data based on their relevance to the results and to the goal and scope definition. The goal is to let the analysts focus on the most important parameters before spending resources on the least relevant missing data. The approach was realized on a case study of High Impact Polypropylene (HIPP) recycling. The LCA study is completed with study of impacts on the mechanical properties of the HIPP.