Influence of different long and short ageing protocols on the mechanical behaviour and damage mechanisms of a hemp fibre reinforced polypropylene

This paper deals with the study of the evolution of the durability of short hemp fibres reinforced composites subjected to long-term ageing. The compounds were stored for ten years in atmospheric conditions (natural ageing) prior to the study. Therefore, two kinds of specimen are studied. First, the aged compounds were used to moulded tensile specimens. The residual mechanical behaviour of natural aged hemp/PP composites (natural aged specimens) were compared to properties obtained 10 years ago. This storage showed that the hemp is more sensitive to time degradation than PP and was confirmed by the initial degradation of compounds pointed out by ATG. Then, injected natural aged polypropylene reinforced with several hemp fibre weight fractions compounds were subjected to two different protocols in order to characterize their durability (aged specimens). The specimens were subjected to water immersion at ambient temperature and storage in a climatic chamber maintained at 40 °C with a relative humidity of 98%. The fracture surfaces were observed with SEM to identify the physical phenomena leading to the observed loss of mechanical properties. A weight water sorption of 14% with respect to hemp weight fraction was, for the first time, determined as a critical water sorption for which the reinforcement effect becomes negligible. The physical phenomenon responsible for this critical absorption was finally investigated with Infrared Spectroscopy, showing the occurring of a dissolution of the fibres.