*3.5. SEM Analysis*

When biomass is heated beyond 40 to 50 ◦C, its morphology changes, creating flow characteristics that are not just determined by temperature but also by the structure of the biomass, such as surface regression and major fragmentations that are relevant to bonding because natural binders contained in the biomass are activated upon increasing temperature [6,7,34,65]. Figure 5 presents cross-sections of the SEM images of pure and blended samples of NSP and PSP obtained under the same measurement conditions such as those inscribed in the images.

**(a) (b)** 

> **Figure 5.** *Cont.*

**(c)** 

**Figure 5.** The SEM images of pure and blended samples of NSP and PSP: (**a**) NSP (100%); (**b**) PSP (100%); (**c**) NSP/PSP (50%/50%).

Figure 5a–c typically show the fracture surfaces of the pure and blended samples of NSP and PSP. The nature of the fracture surfaces was an indication of the production of pellets with differences in visual appearance. It could also be noted that the surfaces of the pellets were quite rough with interconnected particles and voids that characterized the image of PSP (100%), which were believed to have been created by pelleting process conditions such as compression force and temperature. The observable particle-to-particle interconnection was equally an indication of the promotion of adhesion by increased molecular contact between particles, which also represented the presence of short-range attraction forces such as intra and intermolecular forces including van der Waals forces, attributed to the presence of increased polar functional groups, free chemical bonds, and hydrogen bridges [41,66,67]. It could also be noted from the images that particles were clustered and densely packed, an indication that the materials were milled to relatively small particle sizes of <1 mm prior to pelleting. Surface morphology of the pellet samples equally showed conspicuous differences in the pattern of roughness, particularly between the images of NSP (100%) and PSP (100%), whose SEM image, as earlier stated, showed more void spaces that were believed to have been caused by a pelleting spring-back effect due to the highly reactive oxygen-containing polar functional groups associated with PSP (100%). This spring-back effect was an indication of interlocking of fibers and that bonding in 100% PSP was most likely due to dipole–dipole and hydrogen bonding. According to Mani et al. [68], the compaction of biomass grinds occurs in different stages, such as the formation of void spaces that facilitates inter-particle contacts. Surface roughness enhances bond strength, which is intimately linked to bond energy [69–71]. Therefore, the pattern of roughness is a strong indication of differences in the mechanism of bonding between the particles of the pure and blended pellet samples. However, the voids observed in the image of PSP (100%) were no longer visible in the SEM image of NSP/PSP (50%/50%), a clear evidence of component consolidation and an indication that components of the two materials (Norway spruce and pea starch) interacted to create stronger unions, perceived as a type of intermolecular bonding connected to covalent and hydrogen bonding [72]. Furthermore, the particles of PSP (100%) were more spherical in shape than those of NSP (100%), and also appeared complemented in the blend (NSP/PSP 50%/50%) (Figure 5c). Spherical particles have higher surface areas that are more densely packed; as a result, they are likely to form pellets with greater strength [49].

With regard to the quality of the pellets, the SEM images did not provide clear morphological features that could be linked to differences in the quality of the pellets, whether in terms of strength, bulk density, or of any other quality parameter for that matter; hence, it was difficult to draw conclusions on the quality of the pellets from this analysis.
