Influence of lignin accessibility on chemical and biological decomposition of lignin/polyethylene composite thermoplastics

Kraft lignin has been widely investigated for blending with a large variety of polymers to produce thermoplastic composites. The ratio of lignin and polymer affects the accessibility of lignin to small molecules and microorganisms, thus influencing the degradation of lignin/polymer composites. In this present work, the stability and degradation of lignin-polyethylene composites of different compositions in different environments, including water, acidic solution, alkaline solution, and soils have been investigated. The effects of composite composition (with PE content in the range of 30-80 wt%) and in turn lignin accessibility on the degradation have been elucidated, with some valuable findings disclosed. The degradation/dissolution of lignin creates new void spaces and water fully or partially fills into these void spaces, depending on the specimen size. Because water does not have any interaction with lignin and PE, most of the lignin degradation behaviour does not affect the dimensional size of lignin/PE composites. However, the dissolution of lignin in alkaline solution leads to significant shrinkage of the lignin/PE composite bar, especially in terms of thickness. The lignin degradation behaviour has different effects on the mechanical strength of lignin/PE composites. The composites with low lignin accessibility degrade minimally in all environments, thus retaining most of the mechanical strength. This study provides insight regarding extending/shortening the life cycle of lignin/polymer composites.