Cell wall microstructure, pore size distribution and absolute density of hemp shiv

Abstract

This paper, for the first time, fully characterizes the intrinsic physical parameters of hemp shiv including cell wall microstructure, pore size distribution and absolute density. Scanning electron microscopy revealed microstructural features similar to hardwoods. Confocal microscopy revealed three major layers in the cell wall: middle lamella, primary cell wall and secondary cell wall. Computed tomography improved the visualization of pore shape and pore connectivity in three dimensions. Mercury intrusion porosimetry (MIP) showed that the average accessible porosity was 76.67 ± 2.03% and pore size classes could be distinguished into micropores (3–10 nm) and macropores (0.1–1 µm and 20–80 µm). The absolute density was evaluated by helium pycnometry, MIP and Archimedes' methods. The results show that these methods can lead to misinterpretation of absolute density. The MIP method showed a realistic absolute density (1.45 g cm−3) consistent with the density of the known constituents, including lignin, cellulose and hemi-cellulose. However, helium pycnometry and Archimedes’ methods gave falsely low values owing to 10% of the volume being inaccessible pores, which require sample pretreatment in order to be filled by liquid or gas. This indicates that the determination of the cell wall density is strongly dependent on sample geometry and preparation.

Comparative moisture and heat sorption properties of fibre and shiv derived from hemp and flax

“…The cell wall of a plant is a complex structure, and the chemical composition and microstructure of cell wall vary with the cell type. In previous publications, the single middle lamellae (ML) and primary wall (P) of hemp shiv were found to have thickness of 30-50 nm and 70-110 nm, respectively (Thygesen et al 2006;Jiang et al 2018). The secondary cell wall was found to have a 100-130 nm thick S1 layer and a 3-13 lm thick S2 layer.…”

“…The major layers were constructed of 100 nm thick lamellae. Whereas, the lamellae of flax varied in thickness from 100 to 200 nm (Jiang et al 2018). The thickness of the secondary cell wall was a 5.5-8.2 lm (His et al 2001).…”

“…The particles of hemp shiv and flax shiv had a mean length of 17.6 ± 2.3 mm and a mean width of 8.4 ± 1.8 mm and a mean length of 13.5 ± 1.6 mm and 3.3 ± 0.6 mm, respectively. The protocol used to dry the samples are as reported by Jiang et al (2018). The bulk densities of hemp shiv and flax shiv were in the range of 85-90 kg/m 3 , which is measured based on the followed protocol that was developed by the RILEM Technical Committee 236-BBM (Picandet 2017).…”

“…The microstructure of the fibre and shiv was observed by a scanning electron microscope (JEOL SEM-6480LV, Tokyo, Japan). The sample preparation for SEM measurement has been reported by Jiang et al (2018). FTIR transmittance spectra were collected in an attenuated total reflection (ATR) mode with 20 mm -1 resolution and 10 scans were accumulated for each spectrum in the range 6-40 m -1 .…”

“…Acting as a carbon bank, hemp and flax lock in up to two tonnes of CO 2 per tonne of fibre, making them a particularly sustainable choice for construction applications (Boutin et al 2005). In the stem of natural plants, such as hemp and flax, a barklike layer containing fibres surrounds a woody core (Jiang et al 2018). These components can be separated either by retting including chemical and microbial actions or mechanically, dividing the stem into bark fibres and woody stem (shiv) fractions.…”