Surface characterization and flotation behavior of virgin PP and HDPE in the presence of diethylene glycol dibenzoate and lignin alkaline

Plastics are widely used materials whose global production has increased dramatically, reaching 413.8 million tons, and is projected to triple by 2060. This growth has led to a significant rise in plastic waste, posing environmental challenges. Conventional disposal methods such as landfilling and incineration are not environmentally friendly, as they may cause ecosystem damage and toxic gas emissions. Therefore, plastic recycling has become a crucial option from both waste management and production perspectives. Plastic recycling generally involves collection, separation, size reduction, washing, and reprocessing stages, all of which strongly influence final product quality. Effective recycling requires accurate separation of plastics. Although manual and density-based separation methods are commonly used, they are insufficient for systems with similar densities, such as HDPE–LDPE or PVC–PET–POM mixtures. In such cases, flotation offers an effective alternative. Flotation exploits differences in surface chemical properties and wettability, enabling selective separation through surface modification using appropriate reagents. The efficiency of flotation is influenced not only by surface chemistry but also by the density, size, and shape of plastic particles. In this study, the selective separation of mixed plastic systems consisting of HDPE and PP was investigated using flotation-based techniques. Surface physicochemical properties of the plastics and reagents were systematically characterized using surface tension measurements, contact angle measurements, and micro-flotation tests to evaluate their wettability and surface charge behavior. The effects of surface conditioning and flotation parameters on separation efficiency were examined, and the relationship between surface characteristics and flotation response was discussed in detail.