## 3 Results  
In Figure 2, the spectra relating to the reflectivity of each orthodontic adhesive resin reveals a lower reflectivity in the Trans XT compared to the other two materials, while for all the testing samples, the reflectivity drops down at 380 nm. Bisco is the resin that possesses the greatest reflectivity. Bisco suddenly reaches its maximum peak around 470 nm and remains constant beyond this wavelength, except for a slight drop between 800 and 900 nm, which is followed by a downward trend. This drop between 800 and 900 nm of wave length is present for all three composite resins, but refers to different percentage values of reflectivity. Light has a curvilinear pattern very similar to Bisco. The surface roughness and the internal composition of the samples have been investigated through the FIB/SEM analysis. A first comparison can be performed that directly evaluates the surface structure of the resins at SEM imaging: as can be seen in Figure 3, the Bisco and Light samples show a quite similar morphology with a homogeneous grainy shape. Conversely, Trans XT shows a smoother surface with the presence of small spikes and troughs. After the milling process, the internal composition of the resins can be evaluated: again Bisco and Light show similar characteristics that present a higher density of grains in the matrix with respect to Trans XT. As can been observed in Figure 4, the SEM analysis performed at a magnification of 60,000 Trans samples presents essentially two types of grains embedded in the matrix: a big one with an average size of tens of microns and a sub-micron class of grains less dense than the other two resins. Vickers microhardness measurements were carried out for three orthodontic composite resins. As recommended by ASTM E381-16 ISO standard [24], to obtain HV hardness values for each sample, the indentations were made at 500 g of the applied load with a dwell time of 20 s. The diagonal length was measured after each indentation and used to calculate the HV value by applying the Crawford equation [25,26]:  
\[\mathrm{HV}=\frac{(1854.4\ \mathrm{F})}{\mathrm{D}^{2}} \tag{1}\]  
where HV is Vickers hardness, F is applied load in grams and D is diagonal length in mm. Experimental values of diagonal lengths and relative calculated mean HV values are presented in Table 2:  
Figure 3: SEM images of the surfaces of (**a**) the Bisco Ortho Bracket Paste LC (Bisco), (**b**) Light-Cure Orthodontic Paste (Leone), and (**c**) Transbond XTM Light Cure Adhesive (Trans) collected at a tilt angle of 52\({}^{\circ}\). The scale bars indicate 10 \(\upmu\)m.  
Figure 2: The spectra relating to the reflectivity of each orthodontic adhesive resin. The obtained HV values followed this order: Bisco > Light > Trans XT and, namely, 96, 83 and 69, respectively. In particular, the value relative to Trans XT resin is in agreement with the literature value [27; 28]. Moreover, a further investigation of the three orthodontic composite resins was performed in order to establish a dependence of HV values on the applied load. Is it known that microhardness does not represent an absolute characteristic of a plastic material and is dependent on the applied load [25; 29]. To this end, a complete set of indentations was carried out at 25, 50, 100, 200, 300, 500 and 1000 g of the applied load. In order to quantify the dependence of HV on the applied load, the applied load and the corresponding diagonal length values were plotted in a logarithmic graph (see Figure 5), as suggested by [PERSON] [25] and [PERSON] [29]. In Figure 5, the sets of values of the applied load F and diagonal D for each sample: Bisco (black squares), Light (red circles) and Trans XT (blue triangles) is plotted. Logarithmic plots allowed us to linearize the F vs. D trends as a result of the reformulation of Equation (1) in the form of Equation (2):  
\[\log\mathrm{F}=\log\left(\frac{\mathrm{HV}}{1854.4}\right)+n\ \mathrm{logD} \tag{2}\]  
where F is applied force in grams, HV is Vickers hardness value, \(n\) is an exponent of D and D is diagonal length.