## 3 Results and Discussion
### Qualitative Composition of the VOCs from the Obtained Bio-Based Boards
This means that SE processing significantly changes the VOCs profile of lignocellulosic contributing to the drastic formation of furfural and acetic acid because of the degradation of all constituents during the process [26]. There have been reports about the release of acetic acid during the SE process resulting in the furfural formation from degraded C5 sugars, mainly xylose and the formation of 5-HMF from cellulose degradation [27]. Now, it is clear that these substances remain for a long time even in the final composite and proceed to emit, raising concern about the use of the materials. To reduce the emissions from particleboards, different coatings and overlaying are used, including melamine-impregnated paper used in this study. However, the effect of overlaying on VOCs emissions from agricultural biomass-based boards was not definite. In  
Figure 2: The GC–MS chromatograms (TIC—total ion current) of the VOCs from different board samples: 1—acetic acid; 2—hexanal; 3—furfural; 4—2–furanmethanol; 5—1,3-propanediol; 6—\(\alpha\)-pinene; 7—\(\gamma\)-butylrolactone; 8—5-methylfurfural; 9—sylvestrene; 10—nonanal. the case of overlayed HS-based boards, a lower peak area of the main VOCs was observed with a significantly increased peak number (comparison of HS 800-6 and HS 800-6L in Table 2). This means that the overlaying caused new VOCs generated by additional thermal treatment under pressure. However, comparing the overlayed HS-board sample with the non-overlayed HS-board sample of high density, the latter demonstrated an even higher peak number of the VOCs with low concentrations (Table 2 and Figure 2A). Furthermore, a drastic change in the relative area % was observed for the five main compounds: acetic acid, furfuryl acetate, and butylactone increased by 2, 5, and 3 times, respectively, but furfural and 5-methyl-furfural decreased by 2 and 4 times, respectively. In the case of overlayed WS-based boards, the main VOCs remained almost in the same proportions with decreased peak number (comparison of WS 800-6 and WS 800-6L in Table 3). The increased individual VOCs emission after the overlaying could be attributed to the applied additional temperature (145 \({}^{\circ}\)C) during the overlaying and the chemical composition. For example, the higher contents of xylan and formic acid in the WS sample after SE could result in a higher concentration of furfural during the overlaying process. To some extent, a similar tendency was reported by adding scavengers to the particleboards from different raw materials (pine and poplar), concluding the highest VOC emissions (particularly acids) for pine-based boards, indicating the differences in raw materials producing different VOCs [28]. The woody biomass binder-less board VOCs had fewer dominant compounds--acetic acid and furfural were the dominant ones (Table 4 and Figure 2C), as with the previously described samples. Similarly, \(\gamma\)-butyrolactone and 5-methyl furfural were typical for all binder-less boards, but benzyl alcohol was more characteristic for the woody biomass boards. Table 4 shows that the identified compounds covered approximately 85% of the total peak area in GC-MS chromatograms, despite the fact that the overall number of peaks in the case of grey alter and birch wood materials was smaller than in the case of hemp and wheat. This is primarily due to the comparatively low VOC concentration emitted from the wood-based boards, which hinders identification. However, the lower peak number in wood-based boards was possibly affected by different fabrication conditions including SE and hot-pressing (Table 1). A higher peak number was detected for GA-based boards vs. B-based boards independent of the density, indicating the previously mentioned differences between raw materials. Unlike agricultural crops-based boards, a higher content of minor VOCs was detected in the medium-density boards than in the high-density boards.