The influences of pigments on recycled PP and HDPE plastics’ dimensional stability, microstructure, thermal and mechanical properties

Recycling polyolefins such as polyethylene (PE) and polypropylene (PP) has become increasingly important due to the growing volume of plastic wastes, particularly from short-lived packaging and consumer products. These materials are valuable because of their semi-crystalline nature, which enables their use in a wide range of applications. Pigments are colouring agents that generally provide the desired colour effects on virgin plastics, while influence plastics microstructures and consequently, their ultimate mechanical properties. Similarly, the incorporation of pigments into recycled plastics can also influence crystallization and affect mechanical and thermal properties, which are essential for maintaining product quality and stability. However, these effects have not yet been extensively investigated in the literature, particularly for recycled polypropylene (r-PP) and recycled polyethylene (r-PE). Therefore, this study examines the effects of red and black pigments on shrinkage, crystallization behaviour and mechanical properties of r-PP and r-HDPE, sourced from single-use packaging household waste. The effects of both pigments were more pronounced in r-PP than in r-HDPE. The red pigment caused an increase in shrinkage in both r-PP (~0.13%) and r-HDPE (~0.08%) compared to non-pigmented samples, while the black pigment had a minimal effect. X-rd curves showed β-crystals and DSC analysis revealed nucleation effects in red-pigmented r-PP. For r-HDPE, the red pigment showed a linear relationship with increased shrinkage and crystallinity. Red-pigmented r-PP had lower tensile modulus and higher strain, while black-pigmented r-PP showed the opposite. This could be due to β-crystals in red-pigmented r-PP. Both pigments affected tensile properties in r-HDPE based on crystallinity. These findings are useful to process pigments in r-PE or r-PP based products.