Abstract
Unlike many mineral-based insulation materials bio-based lignocellulosic fibre and shiv offer a number of benefits including thermal and hygroscopic properties. The microstructure, porosity and chemical compositions of the plant cell walls play a major role in the moisture exchange process. In this paper, the effects of microstructure, chemical composition, porosity and pore size distribution of both fibre and shiv, from hemp and flax plants, on both moisture and heat sorption were studied for the first time. The physical and chemical characteristics of the fibre and shiv from hemp and flax were studied by using scanning electron microscopy, mercury intrusion porosimetry and Fourier transform infrared spectroscopy. Water moisture sorption and heat of sorption were studied using a dynamic vapour sorption apparatus and a simultaneous thermal analysis system, combined with a humidity generator and using a copper furnace. Results showed that both the fibre and shiv of hemp and flax adsorbed a similar amount of moisture at a given relative humidity, which was dependent on the availability of hydroxyl groups for water in the cell wall. The macroscopic surface area and porosity of the specimen on a large scale had little influence on the availability of hydroxyl groups in the cell wall. The water molecules bound with cell wall molecules through hydrogen bonds over the full range of relative humidities, with a consistent hysteresis difference between the first sorption isotherm and subsequent sequential sorption cycles found in the hemp shiv specimens. For both hemp and flax, the isothermic hysteresis of the shiv was much higher than that of the fibre, which was shown to be dependent on the lignin content. The result of heat sorption indicated that some blocked sorption sites become available to water vapour molecules due to the change in molecular structure of the cell wall during the adsorption process. This study has improved understanding of the hemp and flax sorption behaviour and is important for optimal application of bio-based insulation materials for construction.Graphical abstract
Highlights
Agro-industrial lignocellulosic materials such as hemp, flax, corn cob and straw are becoming more widely recognised for their excellent hygric and excellent insulating properties (Latif et al 2014; Walker and Pavıa 2014; Collet et al 2017)
This paper aims to study the effects of the microstructure, chemical composition, pore size distribution and porosity on the hygroscopic behaviour of fibre and shiv
It was clear that the cell wall microstructure of the hemp and flax shiv was significantly different from the microstructure of the hemp and flax fibres
Summary
Agro-industrial lignocellulosic materials such as hemp, flax, corn cob and straw (wheat, rice) are becoming more widely recognised for their excellent hygric and excellent insulating properties (Latif et al 2014; Walker and Pavıa 2014; Collet et al 2017). 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. These materials have the potential to act as a building material to moderate humidity levels in an indoor environment due to their inherent hygroscopic properties (Latif et al 2015; Laborel-Preneron et al 2016; Hurtado et al 2016; Bourdot et al 2017). Fibres had a higher content of cellulose compared with that of shiv
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