Hydrophobic and fire retardant treatment with functionalised silica particles applied on hemp shiv

Abstract

With an ever growing population, increased air pollution, and decrease of natural resources, the construction sector requires new sustainable solutions. Bio-materials, usually crop by-products, are considered a strong alternative to conventional building materials, and their use is not a direct competition in other supply chains like food crops can be. On top of their lower cost of manufacture (from both an economic and resource perspectives), they present attractive acoustic and thermal insulation properties. Specifically, the woody core of the hemp plant (Cannabis sativa L.), also named hemp shiv, acts as a moisture buffer due to its micropores, which allow water vapour to permeate. A wider adoption of this material as part of a building envelope is however hindered by the intrinsic chemical characteristics of hemp shiv. Made up mostly of cellulose and other sugar-derived polymeric groups, hemp shiv deteriorate in warm and humid environments, giving rise to microbial growth, and present a non-negligible fire risk. Their combustibility and predisposition to absorb liquid water are two issues which need to be addressed in order to enable their use in the built environment. This thesis presents the development of a silica particle technology to limit the biodegradation and improve the fire performance of hemp shiv. A rational design approach was followed for the synthesis of silica particles, by changing reactions parameters and the ratios of reagents. Better understanding of the effect of the reagents proportions and temperature on the synthesis allowed the size, porosity, and surface chemistry of the resultant silica particles to be tailored. Specifically, it was found that keeping the quantity of water between two boundary conditions enabled the controlled synthesis of discrete particles which diameters ranged from 120 to 820 nm with a defined pore shape distribution. Select formulations of particles of different sizes were then functionalised to make them hydrophobic, which was successfully assessed when functionalised silica particles were deposited on glass slides. Hemp shiv were then coated with selected formulations of functionalised silica particles, and their water repellence was equally verified, as well as the limitation of their water uptake, which increased from a few seconds to beyond 20 minutes. The samples also demonstrated delayed biodegradation when exposed to a humid environment, whilst retaining their moisture buffering and hydrophobic properties. Finally, silica was shown to act as a heat sink, by absorbing part of the radiant heat thus more energy was required for the onset of pyrolysis to occur. They also protected the hemp shiv against further oxidation, and consequently reduced their combustion rate. Composite structures made from hemp shiv in which the silica particles were introduced displayed improved fire performance, and similar response (although physical rather than chemical) when compared to composites treated with conventional fire retardant solutions. The approach followed here proved that silica particles can prevent moisture and thermally induced deterioration of hemp shiv, at laboratory scale.