# Evaluation of Recycled Paperboard Properties and Characteristics
## 3 Results and Discussion
### Colorimetric Characteristics of Recycled Paperboards
A comparative study demonstrates that the addition of bleached pulp to the top ply of recycled paperboards significantly increased (by four times) the whiteness compared to the unbleached laboratory Sample, while coating improved the whiteness of the paperboard by almost five times compared to the Sample. The human eye tends to describe materials with a slightly bluish tint as \"whiter\". This is why the compensation of yellowish colors of raw materials with the help of blue colorants or OBAs became a common practice. OBAs absorb the energy of the electromagnetic spectrum in the invisible ultraviolet region and emit it in the visible range of 430-470 nm. As a result, the material appears slightly bluish [26]. Usually, the manufacturer does not indicate whether OBAs have been used in the paperboard composition. This can be determined experimentally by measuring the values of the spectral reflectance at the wavelength 400-500 nm. The spectral curves indicate the absence of OBAs in the composition of materials (Figure 2). Neva demonstrates the sharpest rise at 410-430 nm; however, this sample has the lowest whiteness index in its group (Table 3). The shape of the curves of the other paperboards is smoother; they repeat the pattern of the laboratory sample, though they demonstrate a higher spectral reflectance coefficient. After the evaluation of whiteness of the paperboard samples, a qualitative colorimetric assessment of the printed paperboard was carried out. The tristimulus values of the imprints were measured on the test charts. The color gamuts of the samples were displayed  
Figure 2: Spectral curves of the recycled paperboards. on the a*b* chromaticity diagram of the CIE La*b* colorimetric system (Figure 3). After that, the areas of hexagons were calculated (Table 4). The vertices of the hexagons are the coordinates of the primary colors of subtractive synthesis (cyan, magenta, yellow) and the colors obtained by superimposing them in pairs (red, green, blue). As mentioned above, the color gamut of any material depends on the whiteness, as clearly visible in Figure 4, where the areas of the color gamut for each paperboard were matched to its CIE whiteness index. The color gamuts of the samples increase with the improving whiteness values of the paperboards, with the best results corresponding to the coated materials. Neva steps out of the trend, which is explained by the coarse fiber composition and the initially low values of whiteness and smoothness. Consequently, the coated ply emphasizes the texture of the fibers and does not provide proper whiteness. The uncoated paperboard samples (NM, Ladoga) have lower whiteness values, and therefore a smaller color gamut. On the other hand, for samples with bleached fibers in the top ply, it is much higher than for a laboratory sample obtained from 100% wastepaper. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Coating** & **Paperboard** & **Color Gamut Area** \\ \hline \multirow{3}{*}{Uncoated} & Sample & 7590 \\  & Ladoga & 9447 \\  & HM & 9702 \\ \hline \multirow{3}{*}{Coated} & Umka & 12,216 \\  & MCB & 12,574 \\ \cline{1-1}  & MCM & 12,160 \\ \cline{1-1}  & Neva & 11,967 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Color gamut areas for recycled paperboards. Figure 3: Chromaticity diagrams of imprints for recycled paperboards. Tone value increase is the apparent relative increment of a raster element on an imprint to its values on a printing form or in a digital file (ISO 12647-1) [25]. In order to determine the amount of tone enhancement, it is necessary to find the difference between the tone-transmitting characteristic of the imprint and those of the file. In Figure 5, the TVI curves for color increments in the recycled cardboard imprints are presented. Figure 4: Correlation between color gamut and paperboard whiteness (CIE). Figure 5: TVI curves for a laboratory Sample (**a**); uncoated paperboards Ladoga (**b**) and NM (**c**); and coated paperboards Umka (**d**), MCB (**e**), and Neva (**f**). Axes: X—tone value in file; A—total dot area in imprint.