# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 3 Test Methods
### Hyroscopic Properties  
The hygroscopicity (i.e., the ability to adsorb/release moisture under relative humidity (RH) variations [60]) can be determined via several tests, divided into those performed using the step response method and those performed in a flux chamber [61]. In the present study, the step response method was considered, according to ISO 24353 [58]. The variation in mass over time was evaluated for both the materials (the bio-aggregates and adhesive, considered separately: AA, HH, AAHH, and S) and the composites (A, H, and AH). The bio-aggregates were placed in plastic boxes with open-top surfaces and filled up to a height of 2 cm (Figure 4a), as described by [PERSON] et al. [28]. This ensured the exposure of only one sample face (15 cm \(\times\) 11 cm). Three replicates of each bio-aggregate were tested (AA1, AA2, and AA3; HH1, HH2, and HH3) as well as the mixture of 50% _A. donax_ and 50% hazelnut shells (AAHH1, AAHH2, and AAHH3). As for the dried sodium silicate solution and the composites, a moisture barrier was provided using aluminum foil (Figure 4b,c), according to ISO 24353 [58] and the method described by [PERSON] et al. [29; 30]. Thus, only the top surface (area = 0.1 m\({}^{2}\)) was exposed. Four replicates of the dried sodium silicate solution of 10 cm \(\times\) 10 cm and a thickness between 2 and 4 mm (S1, S2, S3, and S4) were tested (Figure 4b). As for the composites, three samples of 10 cm \(\times\) 10 cm and a thickness of 4 cm for each composite--the same employed to evaluate the apparent density--were considered (Figure 4c). The samples were exposed to four cycles of 24 h (a 12 h sorption phase at 75% RH and a 12 h desorption phase at 50% RH) using a climatic chamber (Climatest ARGOLab CH 250) after being pre-conditioned at T = 23 \({}^{\circ}\)C and RH = 63% (Figure 4d) until reaching a constant mass (with a change in mass not higher than 0.5% over 24 h). The samples were weighed every 3 h to determine the moisture adsorption (\(\rho_{\text{A,ac}}\)) and desorption content (\(\rho_{\text{A,dc}}\)), moisture content difference (\(\rho_{\text{A,sc}}\)), and, thus, the sorption/desorption capacity, as indicated in ISO 24353 [58] and described in Equations (1)-(3). \[\rho_{\text{A,ac}}\left[\frac{\text{kg}}{\text{m}^{2}}\right]=\frac{\text{m}_ {\text{an}}-\text{m}_{\text{d(n-1)}}}{\text{Area}} \tag{1}\]  
\[\rho_{\text{A,dc}}\left[\frac{\text{kg}}{\text{m}^{2}}\right]=\frac{\text{m}_ {\text{an}}-\text{m}_{\text{dn}}}{\text{Area}} \tag{2}\]  
Figure 4: Samples for the hygroscopicity test: (**a**) plastic boxes (15 cm \(\times\) 11 cm) filled up to 2 cm with the different aggregates (samples AA, HH, and AAHH); (**b**) air-dried sodium silicate solution covered with aluminum foil, except for the exposure surface (area = 0.1 m\({}^{2}\)); (**c**) composite samples (aggregates + sodium silicate solution) A, H, and AH (10 cm \(\times\) 10 cm \(\times\) 4 cm) covered with aluminum foil, except for the exposure surface (area = 0.1 m\({}^{2}\)); and (**d**) samples in the climatic chamber. \[\rho_{\rm A,sc}\left[\frac{\rm kg}{\rm m^{2}}\right]=\rho_{\rm A,ac}-\rho_{\rm A,dc} \tag{3}\]  
where \(\rm m_{an}\) (kg) is the mass at the end of the sorption phase of the considered cycle, \(\rm m_{d(n-1)}\) (kg) is the mass at the end of the previous desorption cycle, \(\rm m_{dn}\) (kg) is the mass at the end of the desorption phase of the considered cycle, and Area (\(\rm m^{2}\)) is the exposed surface. The moisture buffering value (MBV) quantifies the content of adsorbed and released moisture considering cycling between two levels of RH [62; 63]. It was calculated considering the average value between the MBV of the sorption, \(\rm MBV_{a}\) (Equation (4)), and the desorption phase, \(\rm MBV_{d}\) (Equation (5)), of the last three cycles [29; 30].