# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 4 Results and Discussion
### Hygroscopicity Properties
This is in line with past studies that demonstrated that the adhesives (binder/glue) strongly affected the final properties of the composites [19; 21; 60]. The advantage here, in comparison with other more common binders, is that the sodium silicate solution has a significative positive effect on the hygroscopic capacity. Comparing the hygroscopicity of the raw materials and the composites, there was no correspondence. Indeed, the most hygroscopic raw materials (hazelnut shells) did not produce the most hygroscopic composite (which was the _A. donax_-based one). Hence, a correlation between the aggregates and composites cannot be defined. This was caused by the combination of the sodium silicate-aggregate, in particular, by the different sodium silicate distributions on the exposed surfaces. Indeed, according to Figure 5, the _A. donax_-based samples had a greater amount of sodium silicate on one side, and this could have significantly affected their hygroscopic behavior. The results and the practical experience suggest that the greater the surface area covered by sodium silicate, the greater the capacity to absorb moisture. Furthermore, the mismatch between the materials and composites demonstrates, again, the strong influence of the adhesive on the performance of the final composites. It is important to underline that the sodium silicate distribution in the composites was assessed only qualitatively, and further analysis should be carried out. However, the results already indicate this correlation between the two properties. Table 4 reports the moisture buffering values and the classification, following [PERSON] et al. [64], for both the materials--considering the aggregates and adhesive individually--and the composites (with the average values of the last three cycles and standard deviation), calculated as described in Equations (4) and (5) (Section 3.3). Figure 8: Moisture adsorption/desorption contents for four cycles of the tested samples: comparison between materials and composites (average values). Considering the materials, both the _A. donax_ and the dried sodium silicate solution were rated as good. Hence, they had lower MBVs than the hazelnut shells. Coherently, the addition of _A. donax_ to the hazelnut shells resulted in a lower MBV. As for the sodium silicate result, a higher value of MBV was expected. However, this result was evaluated by considering the average values between the MBV\({}_{\text{a}}\) and MBV\({}_{\text{d}}\) of the last three cycles. Considering these separately, the MBV of the sodium silicate solution was MBV\({}_{\text{a}}\) = 2.05 \(\pm\) 0.10 g/(m2%RH) and MBV\({}_{\text{d}}\) = 1.79 \(\pm\) 0.01 g/(m2%RH). Hence, as expected, the MBV of the sorption phase was higher than that of the desorption phase (Table 4 reports its average value). The MBVs of the composites are coherent with the previous results (Figure 7). Again, the values of _A. donax_ were influenced by the distribution of the sodium silicate, which determined higher values. As for the mixture, the results are in line with the expectation: the combination of the two materials determined a lowering of the moisture buffering capacity, and then of the MBV.