Progress on wheat straw prototype
Our current research focuses on the characterisation of the straw as a raw material, before studying the straw bale as a prototype insulation material. We have assessed and compared some physical, biochemical and hygric properties of a French straw and an English straw.
The two straws are visibly different in their colour; the French straw is yellow whereas the English straw is greyer in colour. This difference is most likely due to how the straw is stored following harvesting. Their initial moisture contents - around 9% for the English straw and around 24% for the French straw - are also different. We will assess their bio-composition, cellulose, hemicellulose and lignin contents, using the Van Soest method (Van Soest et al., 1991). Only the English straw has been tested so far: it is mainly composed of cellulose (54%) with hemicellulose (27%) and lignin (9%) the other major constituents.
A material’s hygrothermal performance is highly influenced by its porosity, so the focus has been on understanding these properties. We evaluate the straw’s pore size distribution through the Mercury Intrusion Porosimetry (MIP) test. This consists of measuring the volume of mercury penetrating in the pores of a material as a function of the pressure applied. The English and the French straw have a similar pore size distribution with an average diameter of 157 nm for the English straw and 117 nm for the French straw. We can also find the density of an individual piece of straw using the MIP test. We found that the English straw has a true density of 1.43 g.cm-3 while the French straw has a true density of 1.40 g.cm-3. This led to a calculated porosity of 79% and 86% respectively.
This difference in porosity is highlighted by the water absorption of the straw by immersion. After 48 hours of immersion, the French straw absorbed 390% of its weight whereas the English straw absorbed only 290%. While water adsorption can lead to bio-degradation, there are many other beneficial properties for the regulation of the indoor environment, through gaseous moisture sorption.
Finally, we assessed the straw’s capacity to store moisture using the Dynamic Vapour Sorption instrument to measure the sorption-desorption isotherms. This uses very accurate scales to weigh the straw as the relative humidity increases and decreases. With its porosity, straw seems to have a very good hygric property with a good sorption capacity (21% of moisture content at 95% of relative humidity for the English straw). This should play a positive role in the moisture regulation of an indoor environment.
The next steps of the investigation will deal with the baling of the straw and the study of hygrothermal properties according to the straw orientation in the prototype insulation material.