Effect of retting on hemp shiv physicochemical properties

Abstract

Hemp (Cannabis sativa L.) retting is an essential process for the defibrating process to separate hemp shiv and fibres. The goal of this study is to investigate how retting level affects hemp shiv properties that are useful when such particles are used as a building material for insulation applications. Hemp straw samples were taken on the field at 0, 8, 22, 42 and 62 days of retting. Shiv and fibres were manually separated in order to obtain hemp shiv to be analysed in the laboratory. Different properties of hemp shiv were measured: colour, density of both hemp shiv (envelope density) and hemp shiv powder (particle walls density) and chemical composition (content of ash, lignin, cellulose, hemicellulose and soluble). Colour changes were observed: brightness, greenness and yellowness decreased during retting. During the first 22 days of retting, greenness and yellowness showed the greatest variations, while brightness decreased after 22 days. Envelope density of hemp shiv significantly increased (over 65 %) between 22 and 42 days of retting. Nevertheless, particle walls density only increased ≈6 % throughout the whole period of retting. This indicates that the degradation process gives access to a part of the closed porosity of unretted hemp shiv. Hemp shiv mass loss during retting achieved ≈26 % at 62 days of retting. Soluble and hemicellulose were the most affected compounds with a decrease of, respectively, 42 and 45 % whereas changes for cellulose and lignin were less pronounced. In conclusion, the retting process changes colour, density and chemical composition of hemp shiv and some possible consequences for building applications were highlighted.