Decortication and separation of wet or dry hemp stems - What difference does it make?

Hemp decortication, a crucial step in fibre processing, traditionally uses dry stems with low water content. However, processing hemp in a wet state has gained attention as an alternative method, especially with the development of harvesting techniques that decorticate the stems directly in the field. We investigated the effects of stems' initial moisture content on hemp fibre functional quality using freshly harvested hemp stems from different varieties. The stems were subjected to wet and dry decortication processes to evaluate potential differences in fibre damage and biochemical composition. Our experiments involved utilising a laboratory roller-breaking device for the initial decortication of the fresh hemp stems in their wet state and subsequent mechanical separation to assess fibre bundle integrity. As a comparison, we performed the same procedure on stems that were first dried before processing. We found that wet processing resulted in higher lignin levels in the fibre bundles, containing a higher quantity of middle lamellas, where lignin or another biochemical marker is typically found. The analysis revealed significant damage to the fibre bundles during the wet decortication process, as evidenced by the destruction and fibrillation of the bundles, clearly visible in scanning electron microscope (SEM) images. Interestingly, the dry conditions led to more fractures between fibres, while wet conditions allowed fractures between bundles and the surrounding histological structure. These findings suggest that wet processing may render the material more ductile, resulting in more complex crack modes, including combined torsion and delamination, leading to fracture paths passing through the cross-section of individual fibres. Consequently, the fibre bundles from wet processing may exhibit more structural damage than dry-processed fibre bundles. In conclusion, our study's findings are important as they highlight the severe damage that can occur to the fibre bundles during the decortication and separation of freshly harvested, moist hemp stems.