A Study About Water/Alkali Treatments of Hemp Fiber on Ultraviolet Ageing of the Reinforced Polypropylene Composites

Abstract

Ultraviolet (UV) ageing is one main environmental factor affecting the service performance and life of plant fiber reinforced composites. Plant fiber treatment or modification is an essential step in composite preparation to deal with the poor interfacial strength between plant fiber and thermoplastic matrix. Our previous study shows that water treatment is one green fiber treatment method and has potential industrial application. In this paper, effects of water and alkali treatment of hemp fiber on UV resistance of the reinforced polypropylene (PP) composites were studied. Hemp fiber was treated with water and alkali solution firstly, and then blended with PP, respectively. UV ageing of the PP composites was performed. Digital microscopy, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were employed to characterize composite degradation. Water uptake and tensile tests were also conducted to deeply understand the effects of water and alkali treatment on UV ageing of the composites. The surface observation results showed that UV ageing occurs around the fiber locations on composite surface in early stage, both water and alkali treatment of fiber could retard surface whitening and cracking of the composites. After 4 weeks, carbonyl index of the composite reinforced with untreated fiber reached up to 29.3 while that of the composites reinforced with water or alkali treated fiber was only 19.6. The XRD patterns also confirmed that both water and alkali treatment have less crystallization after 8 weeks. Water uptake of composites is greatly decreased from 2.5 to 1.0% by water or alkali treatment of hemp fibers. However, either water or alkali treatment didn’t give obvious contribution to resist the deterioration of composite tensile properties. The hydrophilicity and water absorption of hemp fiber play an important role in UV degradation of composites. Water and alkali treatment could greatly decrease water absorption, which is beneficial to improve UV resistance of plant fiber reinforced composites. This study is helpful to deeply understand UV ageing mechanism and develop related anti-UV ageing technology of plant fiber reinforced composites.