# Description of Thermochromic Offset Prints Morphologies Depending on Printing Substrate
## 3 Results
### Specific Surface Free Energy of Papers  
The contact angles measurements (Table 3) show that water spreads over the surface of the hydrophilic F paper (\(\Phi\) = 20.9\({}^{\circ}\)). The surfaces of other papers are waterproof (\(\Phi\) > 90\({}^{\circ}\)), so these surfaces are hydrophobic. Wetting of F paper with glycerol (\(\Phi\) = 59.0\({}^{\circ}\)) is also better than the wetting of other paper surfaces (\(\Phi\) > 90\({}^{\circ}\)). Glycerol, as well as water molecules, with the exception of F paper, have the best ability to cover the surface of synthetic paper. Improved surface wetting with water and glycerol may result from a heterogeneous surface of synthetic paper and better adhesion of polar molecules of water and glycerol to areas with higher CaCO\({}_{3}\) content. Diiodomethane spreads the best over the surface of F paper (\(\Phi\) = 27.0\({}^{\circ}\)) due to its large surface free energy and low tension of the diiodomethane surface. Diiodomethane spreads well over the surfaces of other papers, but not as good as over the surface of F paper. This can be explained by their smaller surface free energies. When comparing the R, B and S papers, diiodomethane spreads the best over the surface of R paper due to its higher surface free energy compared to surface free energy of S and B papers (Table 4). Figure 3: FTIR spectra of (**a**) synthetic paper and (**b**) recycled (R), bulky (B), and filter (F) papers. Table 4 shows the values of the surface free energies and the surface free energy components of unprinted paper samples. According to the OWRK method, the largest surface free energy (\(\gamma\)) was calculated for the surface of F paper (60.8 mJ m\({}^{-2}\)). Surfaces of other papers have significantly lower values of surface free energies. The smallest surface free energy has the surface of S paper (23.0 mJ m\({}^{-2}\)). The highest surface free energy with the exception of F paper has the surface of R paper (29.4 mJ m\({}^{-2}\)). In work by [PERSON] et al. [46], it was demonstrated that the nature and the amount of sizing agents influence the paper surface free energy. The combination of the cationic starch with synthetic surface sizing influences the paper surface energy, i.e., it causes the reduction of the polar component of the surface free energy; therefore, the surface becomes less hydrophilic. The application of more hydrophobic surface-sizing agents decreases more significantly the polar component of surface free energy. In the study by [PERSON] and [PERSON] [56], it was shown that the static contact angles of the sized cellulose films increased monotonically with the sizing dosage. For the highest sizing doing level, the film shows a water contact angle of 101.7\({}^{\circ}\). B paper that contains lignin has the smaller surface free energy (26.4 mJ m\({}^{-2}\)). According to [PERSON] and [PERSON] [54], the adhesion of two components with high surface energy, such as cellulose (55 mJ m\({}^{-2}\)) and lignin (47 mJ m\({}^{-2}\)) in the fiber wall, tends to lower the free energy of the system considerably (up to 23 mJ m\({}^{-2}\)). During alkali pulping, lignin is extensively removed and exposure of carbohydrates generates the sites with relative high surface energy (up to 33 mJ m\({}^{-2}\)). At the same time, molecules with low surface energy, e.g., wood extractives, tend to segregate to the surface and minimize the total surface free energy. The cellulose molecules on the surface of F paper are connected by dispersive (\(\gamma^{\text{d}}\) = 32.7 mJ m\({}^{-2}\)) and polar forces (hydrogen bonds) (\(\gamma^{\text{P}}\) = 28.1 mJ m\({}^{-2}\)). Interactions occurring on the surfaces of other papers are mainly dispersive and weaker than the dispersive interactions of cellulose fibers on the surface of F paper. In R and B papers, this is the result of shorter cellulose fibers and addition of additives.