Surface Properties of Poly(Hydroxyurethane)s Based on Five-Membered Bis-Cyclic Carbonate of Diglycidyl Ether of Bisphenol A

Poly(hydroxyurethane)s (PHU) are alternatives for conventional polyurethanes due to the use of bis-cyclic dicarbonates and diamines instead of harmful and toxic isocyanates. However, the surface properties of poly(hydroxyurethane)s are not well known. In this work, we focus on the analysis of the surface properties of poly(hydroxyurethane) coatings. Poly(hydroxyurethane)s were obtained by a catalyst-free method from commercially available carbonated diglycidyl ether of bisphenol A (Epidian 6 epoxy resins) and various diamines: ethylenediamine, trimethylenediamine, putrescine, hexamethylenediamine, 2,2,4(2,4,4)-trimethyl-1,6-hexanediamine, m-xylylenediamine, 1,8-diamino-3,6-dioxaoctane, 4,7,10-trioxa-1,13-tridecanediamine, and isophorone diamine, using a non-isocyanate route. The structures of the obtained polymers were confirmed by FT-IR, 1H NMR and 13C NMR spectroscopy, and thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed. The rheological characteristic of the obtained polymers is presented. The static contact angles of water, diidomethane, and formamide, deposited on PHU coatings, were measured. From the measured contact angles, the surface free energy was calculated using two different approaches: Owens–Wendt and van Oss–Chaudhury–Good. Moreover, the wetting envelopes of PHU coatings were plotted, which enables the prediction of the wetting effect of various solvents. The results show that in the investigated coatings, a mainly dispersive interaction occurs.


Introduction
Knowledge of interfacial phenomena is necessary to understand the interaction and processes occurring at the surface and interface between two phases. Furthermore, this phenomenon is one of the most important in every industrial process including heterogeneous catalysis, manufacturing of composite materials, environmental protection, medical technology and finally technical processes, for example the fabrication of compound systems such as reinforced materials or coated materials [1]. The obtained adhesion between the layers is also influenced by proper surface preparation [2]. A suitable wettability promotes intimate adhesive-substrate contact, and it is an important factor that must be taken into consideration when adhesion can or cannot be achieved. Hence, the focus of intense scientific research is the wettability, surface free energy, and the properties of the surface layer. The contact angle (CA or θ) is allowed to assess the wettability of the solid by the liquid. It is well known that a wetting liquid forms a contact angle with the solid smaller than 90 • ; in contrast, a non-wetting liquid creates a contact angle larger than 90 • [3]. Furthermore, with θ = 0 • , drop spreads and complete wetting of the surface are observed. The surface free energy (SFE, γ) of solid surfaces is determined based on CA measurement. The SFE cannot be directly measured, however, there are hydroxyl groups affect the overall stability of the obtained compounds due to their ability to form intramolecular and intermolecular hydrogen bonds, and at the same time, lead the way for the subsequent modification of polymers [50]. The hydroxyl groups increase the water absorption and at the same time the chemical resistance and improve the adhesion [51]. Five-membered cyclic carbonates can be obtained by carbonation using dimethyl carbonate or carbon dioxide. Dimethyl carbonate is not harmful or mutagenic, but it is a flammable liquid. Additionally, the carbonation reaction by dimethyl carbonate is carried out at the boiling point of dimethyl carbonate. The reaction of epoxide rings with carbon dioxide enables the utilization of waste CO 2 . In our previous work, we have shown the possibility of using an epoxy resin based on diglycidyl ether of bisphenol A as a raw material for bis-cyclic dicarbonate synthesis [52].
To the best of our knowledge, the research focuses on the synthesis of new PHUs based on bio-sourced and ecological raw materials, rather than seeking their novel innovative applications. Hence, the novel applications of PHUs is not popular topic of scientific research. In general, PHUs exhibit lower molar masses and slower kinetics [13], so their application is limited by their properties. However, some of the possible uses of PHUs have been already presented, e.g., coatings [53,54], adhesives and glues [11,16,20,25,26,30,31,54,55] and foams [19]. It should be highlighted that the number of works describing the potential applications of PHUs is constantly growing.
This work is a continuation of our research on improving the properties of PHU and their use as an adhesive or a coating [25,31,32,52,56,57]. The hanging off pendant hydroxyl groups in PHUs macromolecules strongly influence the hydrophilicity that limits the use of PHU coatings in technological applications. The goal of this study is focused on changes in the water sensitivity of PHUs. The novelty of this work is to use carbonated epoxy resin as a raw material changing the hydrophilicity of PHUs coatings. PHUs based on diglycydyl ether of Bisphenol A have been already presented in the literature [57]. In this work, instead of expensive diglycidyl ether of Bisphenol A, we have used commercially available and cheap epoxy resin, Epidian 6, as a raw material for PHUs synthesis. The epoxy resin, which is mainly a mixture of diglycidyl ether of bisphenol A, can be successfully used in the carbonation reaction with waste CO 2 , leading to five-membered bis-cyclic carbonates. The epoxy resin was used as received without any further purification or distillation. Last but not least, the epoxy resin is cheaper than diglycidyl ether of Bisphenol A. Then, to link the coating properties with the structure of the obtained PHUs, various diamines were used. The schematic workflow is shown in Scheme 1.
Materials 2020, 13, x FOR PEER REVIEW  3 of 14 pending hydroxyl groups affect the overall stability of the obtained compounds due to their ability to form intramolecular and intermolecular hydrogen bonds, and at the same time, lead the way for the subsequent modification of polymers [50]. The hydroxyl groups increase the water absorption and at the same time the chemical resistance and improve the adhesion [51]. Five-membered cyclic carbonates can be obtained by carbonation using dimethyl carbonate or carbon dioxide. Dimethyl carbonate is not harmful or mutagenic, but it is a flammable liquid. Additionally, the carbonation reaction by dimethyl carbonate is carried out at the boiling point of dimethyl carbonate. The reaction of epoxide rings with carbon dioxide enables the utilization of waste CO2. In our previous work, we have shown the possibility of using an epoxy resin based on diglycidyl ether of bisphenol A as a raw material for bis-cyclic dicarbonate synthesis [52].
To the best of our knowledge, the research focuses on the synthesis of new PHUs based on biosourced and ecological raw materials, rather than seeking their novel innovative applications. Hence, the novel applications of PHUs is not popular topic of scientific research. In general, PHUs exhibit lower molar masses and slower kinetics [13], so their application is limited by their properties. However, some of the possible uses of PHUs have been already presented, e.g., coatings [53,54], adhesives and glues [11,16,20,25,26,30,31,54,55] and foams [19]. It should be highlighted that the number of works describing the potential applications of PHUs is constantly growing.
This work is a continuation of our research on improving the properties of PHU and their use as an adhesive or a coating [25,31,32,52,56,57]. The hanging off pendant hydroxyl groups in PHUs macromolecules strongly influence the hydrophilicity that limits the use of PHU coatings in technological applications. The goal of this study is focused on changes in the water sensitivity of PHUs. The novelty of this work is to use carbonated epoxy resin as a raw material changing the hydrophilicity of PHUs coatings. PHUs based on diglycydyl ether of Bisphenol A have been already presented in the literature [57]. In this work, instead of expensive diglycidyl ether of Bisphenol A, we have used commercially available and cheap epoxy resin, Epidian 6, as a raw material for PHUs synthesis. The epoxy resin, which is mainly a mixture of diglycidyl ether of bisphenol A, can be successfully used in the carbonation reaction with waste CO2, leading to five-membered bis-cyclic carbonates. The epoxy resin was used as received without any further purification or distillation. Last but not least, the epoxy resin is cheaper than diglycidyl ether of Bisphenol A. Then, to link the coating properties with the structure of the obtained PHUs, various diamines were used. The schematic workflow is shown in Scheme 1. The structure of the obtained PHUs was confirmed by spectral analysis. We have shown the thermal and rheological properties of the obtained polymers. Moreover, we have determined the  The structure of the obtained PHUs was confirmed by spectral analysis. We have shown the thermal and rheological properties of the obtained polymers. Moreover, we have determined the surface free energy of the obtained coatings by the Owens-Wendt, and additionally, by the van Oss-Good-Chaudhury methods, and finally, we have thoroughly analysed the wetting behaviour.
Carbonated epoxy resin was obtained as described in our previous work [52].

Instrumentation
A Bruker ALPHA FT-IR (Billerica, MA, USA) spectrometer equipped with a Platinum singlereflection diamond ATR module was used to record FT-IR spectra in the wavelength range of 400-4000 cm −1 , with a resolution of 4 cm −1 .
1 H NMR and 13 C NMR spectra were measured with a Varian VXR 400 MHz spectrometer (Palo Alto, CA, USA). Tetramethylsilane as an internal standard and with deuterated solvents (DMSO-d 6 ) was used. Obtained results were analyzed with MestReNova v.6.2.0 (Mestrelab Research S.L, Santiago de Compostela, Spain) software. Basic thermal characteristics of the PHUs was measured with a differential scanning calorimetry (DSC) on TA Instruments Q2000 (New Castle, DE, USA) apparatus. A thermogravimetric analysis (TGA) was carried out with on TA Instruments SDT Q600 (New Castle, DE, USA) apparatus. The applied rate in both cases was 10 K min −1 . Additionally, the weight loss was measured at a temperature of 150 • C as a function of time.
Polymer shear viscosity measurements were carried out with a Malvern Kinexus Pro rheometer (Malvern, England) equipped with parallel plate geometry (gap 0.4 mm using a spindle of diameter 10 mm). In the centre of the plate, a standard mass of the sample (0.3 g) was placed. The measurements were repeated at least twice with new samplings. The shear viscosity was in the specified temperature range (80-160 • C).
A K Paint Applicator RK Prints (Royston, UK) equipped with a 4-sided applicator and heated bed was used to prepare coatings on the cleaned glass substrates. The polymer and the glass substrate was heated up to 120 • C. A coating was prepared using a gap width of 120 µm and the wet film thickness provided was equal to half the gap size. The surface roughness of the coatings was controlled with gauge SRT-220 TestAn roughness meter (Gdańsk, Poland). The roughness value (Ra) was randomly measured 3 times on the surface of coatings prior wettability measurement. The measured surface roughness of the coatings was about 0.1 µm. Prior to the contact angle measurements, the prepared coatings were kept under an argon gas atmosphere. Drop shape analysis system, DSA 30 (Krüss, Germany) was used to measure contact angle of smooth and horizontal sessile drops of the test liquids (water, diiodomethane and formamide) deposited on solid surface-PHU coatings. Needles with a diameter of 0.5 mm were used for all liquids. The contact angle was measured on the static drops 5 s after deposition. The contact angle was measured with the error ±3 • . The drop shape analysis was performed using the tangent method 1. The reported contact angle values for liquids are the mean of six drops deposited on two separate coating samples. The measurements were performed under controlled environmental conditions of 23 • C and 50% relative humidity [58].

General Procedure for the Preparation of PHUs
PHUs were synthesized by the reaction of carbonated epoxy resin with diamine according to a well-known procedure reported elsewhere [28,57]. The formulation is listed in Table 1. The 30 g of carbonated epoxy resin was dissolved in approx. 7 mL of DMSO in a 100 mL round bottom flask equipped with a mechanical stirrer and a nitrogen inlet. The amine was added after heating the mixture up to 100 • C. The reaction proceeded for 48 h. The FT-IR spectroscopy was used to monitor the reaction progress. The FT-IR, 1 H NMR and 13 C NMR results confirming the structure of the obtained PHUs are listed in Supplementary Materials.

PHUs Synthesis
The synthesis of PHUs was carried out using five-membered bis-cyclic dicarbonate obtained from epoxy resin and various diamines. In order to analyze the surface properties of the PHUs, various diamines were used. The synthesis of diglycerol dicarbonate from commercially available epoxy resin, Epidian 6 has been described in our previous work [52]. The formulations of PHU are listed in Table 1. The reaction was monitored by the FT-IR spectroscopy. During the reaction, the strongest peak at 1785 cm −1 , assigned to the cyclic carbonate group, disappears. Next, new peaks typical of hydroxyurethane bonding appear (at 3312 cm −1 , 1687 cm −1 and 1538 cm −1 ), which indicates the consumption of the bis-cyclic dicarbonate. The chemical structures of the PHUs were further confirmed by 1 H NMR and 13 C NMR. Figures S1 and S2 in Supplementary Materials show the 1 H NMR spectra of the monomer and PHU, respectively. The PHUs are not a new group of polymers, hence the detailed description of the PHU's structure based on various bis-cyclic carbonates and diamines can be found elsewhere [32,[59][60][61][62]. The urethane structure of the obtained PHU is confirmed by the signal in the range of 7.7 to 7.8 ppm from urethane proton, depending on the diamine used. A primary hydroxyl group is observed as a broad separate signal in the area of 5 ppm. The signal confirming the secondary hydroxyl group is hidden by the broad multiples in the range of 3.8-4.1 ppm. Furthermore, the presence of urethane groups (C=O) is confirmed by a signal at approx. 156 ppm in the 13 C HNMR spectra.
Because of the lack of solubility of the polymers in typical solvents used in GPC technique, the molecular weight and molecular weight distributions were not determined.

Thermal Characterization
The results of TGA and DSC are summarized in Table 2. Table 2. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses of obtained PHUs.

Name
T The obtained polymers exhibited quite high values of the glass transition temperature T g , and the values are comparable with those shown previously for other PHUs, for example see in [63,64]. The highest value of T g was observed for PHU-CC6 (115 • C) that is based on isophorone diamine; the lowest for PHU-C6O2 and PHU-C10O3 (36 • C and 34 • C, respectively). Furthermore, the glass transition temperature is higher than observed for the PHU-Ar based on m-xylylenediamine. The glass transition temperature for PHU-CC6 is also much higher than those observed for PHUs based on aliphatic diamine [56].
The obtained PHUs exhibited an initial degradation (at 5% weight loss) in the range of 239 • C to 283 • C and maximum decomposition rates in the range of 355 • C to 371 • C, values comparable to those observed previously by us [32,55,56]. The PHUs based on aliphatic diamines (NIPU1−NIPU3) exhibited the lowest temperatures of initial degradation (at 5% weight loss). Higher values of initial degradation temperature were observed for NIPU-Ar and PHU-C10O3. Furthermore, the lowest weight loss at maximum decomposition was observed for NIPU5. NIPU1 exhibited the highest thermal stability with maximum rate of degradation occurring at 411 • C, but simultaneously with the highest weight loss.

Rheological Properties
The temperature dependence of the shear viscosity of the obtained poly(hydroxyurethanes) is shown in Figure 1.

Rheological Properties
The temperature dependence of the shear viscosity of the obtained poly(hydroxyurethanes) is shown in Figure 1.  Due to the very high viscosity of PHU-CC6, we did not determine the viscosity of this PHU in the specified temperature range. The highest viscosity was observed by PHU-Ar and the lowest by PHU-C6O2 and PHU-C10O3, which is related with the presence of short rigid aromatic groups and flexible oxyethylene chains in the macromolecule, respectively. A similar tendency was observed by us, as noted in our previous work [32]. Hence, the shear viscosity of PHUs is mainly influenced by the properties of the diamine used, rather than the bis-cyclic carbonate.

Wettability, Surface Free Energy
The determination of the static contact angles (CA) gives important knowledge regarding interactions between the coating and the liquid. Moreover, the absorption of water into the coating was observed, while the water contact angle measurements were performed. This effect was also observed for other NIPU layers [14,24,29] and it is directly related to the structure of the PHUs where pendant hydroxyl groups and urethane groups are built into the macromolecule. The pendant hydroxyl group improves the adhesion of PHUs with various substrates and influences the appearance of hydroxyl bonds [31].
In this work, to calculate SFE, two different methods were used: Owens-Wendt (also known as the Owens-Wendt-Rabel-Kaelble), and for the first time, van Oss-Good-Chaudhury.
The Owens-Wendt, OW, approach [8] was used to calculate the surface free energy of coatings, Equation (1): where θ is the contact angle (CA) of testing liquid (deg), γ li is the surface free energy (SFE) of the testing liquid γ d li , γ p li are the dispersion and polar components of the tested liquid, and finally, γ d s , γ p s are SFE of the coating. The total surface free energy is the sum of polar and dispersive component according to Equation (2): Materials 2020, 13, 5184 8 of 14 The acid base theory by van Oss, Good and Chaudhury, vOCG, was used for the determination of the dispersive, i.e., non-polar Lifshitz-van der Waals γ LW , acid (electron-acceptor) γ + , and base (electron donor) γ − components of the PHU surface energy [9,10], Equation (3): where i refers to the testing liquid and s refers to solid material. The Lewis acid-base component was calculated from, Equation (4): Moreover, the total surface free energy is defined by Equation (5): The values of surface energy, its acid-base components and polar fraction of testing liquids used for OW and vOCG calculation are listed in Table 3. Table 3. Surface energy and its components (in mN·m) of testing liquids used for surface free energy (SFE) calculation. The values of the contact angles of water, and the calculated SFE of various investigated PHU coatings, using the Owens-Wend and van Oss-Good-Chaudhury approaches, are summarized in Table 4. Table 4. Values of contact angle total surface free energy, its components determined by Owens-Wendt and van Oss-Good-Chaudhury method.

Owens-Wendt (dw) 1 van Oss-Chaudhury-Good (dfw) 2
Water Variance analysis of the results indicated a significant effect of PHU structure on contact angle measurements, as well as the calculated values of SFE and its components (p-value < 0.05).
The values of CA of water varied from 58 to 79 • for PHU-C10O3 and NIPU5, respectively. Lower values of CA are related to the greater wettability and hydrophilicity of the surface. The values of CA decrease when the aliphatic chain in the diamine increases (from C2 to C4 in NIPU1 and NIPU 3, respectively), and the value of CA for water increases when the aliphatic chain in the diamine increases (from C4 to C5 in NIPU 3 and NIPU5). The same tendency was observed by us as shown in our previous work [56]. The CA of water is much lower for PHU-C6O2 and PHU-C10O3, which is related to the presence of oxygen atoms in the diamine chain.
The surface of PHU coatings is populated by polar groups such as urethane bonding, hydroxyl groups and additionally, depending on the PHU structure, oxygen atoms. These groups exhibit basic surface sites and can interact with the weakly polar H δ+ of the water molecule on the exterior. As a result, water absorption into the PHU coating surface is observed (see Figure 2).
Variance analysis of the results indicated a significant effect of PHU structure on contact angle measurements, as well as the calculated values of SFE and its components (p-value < 0.05).
The values of CA of water varied from 58 to 79° for PHU-C10O3 and NIPU5, respectively. Lower values of CA are related to the greater wettability and hydrophilicity of the surface. The values of CA decrease when the aliphatic chain in the diamine increases (from C2 to C4 in NIPU1 and NIPU 3, respectively), and the value of CA for water increases when the aliphatic chain in the diamine increases (from C4 to C5 in NIPU 3 and NIPU5). The same tendency was observed by us as shown in our previous work [56]. The CA of water is much lower for PHU-C6O2 and PHU-C10O3, which is related to the presence of oxygen atoms in the diamine chain.
The surface of PHU coatings is populated by polar groups such as urethane bonding, hydroxyl groups and additionally, depending on the PHU structure, oxygen atoms. These groups exhibit basic surface sites and can interact with the weakly polar H δ + of the water molecule on the exterior. As a result, water absorption into the PHU coating surface is observed (see Figure 2). As shown in Figure 2, the greatest water absorption was observed for NIPU2, whereas the CA decreases from 73° down to 38°. On the contrary, the lowest water absorption was observed for As shown in Figure 2, the greatest water absorption was observed for NIPU2, whereas the CA decreases from 73 • down to 38 • . On the contrary, the lowest water absorption was observed for NIPU1 based on ethylodiamine, and PHU-CC6 and PHU-Ar based on isophorone diamine and m-xylylenediamine, respectively. These PHUs are more "hydrophobic" than the other obtained PHUs.
The values of total SFE determined with the Owens-Wendt approach are the highest for PHU-C10O3, equal to 57 mJ·m −2 ; and the lowest value was calculated for PHU-CC6, equal to 47 mJ·m −2 . The values of total SFE are much lower than those calculated by us in our previous works [25,31,55]. The results show that in the investigated coatings, mainly dispersive interactions occur. The values of the polar component are very low, between 3 and 11 mJ·m −2 for NIPU5 and PHU-C10O3, respectively. This indicates the poor polarity of the obtained PHUs, despite the presence of hydroxyl groups and other polar groups in the PHU molecules. However, the value of the polar part of PHU-C10O3 indicates a stronger polar character of this coating in comparison to the other investigated PHUs, which is related to the lowest value of water CA.
The knowledge of the polar and disperse parts of surface free energy allows us to calculate the curve known as the wetting envelope (Figure 3). other polar groups in the PHU molecules. However, the value of the polar part of PHU-C10O3 indicates a stronger polar character of this coating in comparison to the other investigated PHUs, which is related to the lowest value of water CA.
The knowledge of the polar and disperse parts of surface free energy allows us to calculate the curve known as the wetting envelope (Figure 3). By plotting γ p against γ d , a bow-shaped curve is obtained, which starts at the origin, reaches a maximum value, and then returns to the horizontal axis. If the contour of the wetting envelopes is known, the wettability of the solvent on the material can be predicted. The material is wetted with the contact angle of the wetting envelope, if the polar and dispersive parts of the liquid lie this curve [32]. Hence, the knowledge of wetting envelope curve allow to predict approximate value of contact angle without need for directly measurement.
For the first time, the SFE of PHU coatings was calculated using the van Oss, Chaudhury and Good method. This model has been known for nearly 30 years and is increasing in importance. However, to the best of our knowledge, there are no publications where the values of PHU SFE calculated with the vOCG approach have been presented. The values of total SFE determined with OW and vOCG approaches differ significantly. This tendency has already been observed in other works for various materials, for example PHU [31], wood [57], and plastics [7], where the values of SFE calculated with OW are slightly higher than those calculated by vOCG. The negligible difference between total SFE calculated with both approaches was observed for NIPU1; the greatest difference is for PHU-C6O2 and is equal to 6 mJ·m −2 .
The Lifshitz-van der Waals component, γ LW , is responsible for non-polar interactions on the surface energy of a solid. The γ LW surface energy component is a result of the London dispersion, the Debye dipole-dipole interaction, and the Keesom orientation [9]. The values of γ LW are greater than the values of the acidic and basic part, γ AB , of SFE.
The values of γ LW varied from 41 mJ· m −2 to 46 mJ· m −2 for PHU-CC6 and PHU-C10O3, respectively, and the values of γ AB varied from 1 mJ· m −2 to 9 mJ· m −2 for NIPU5 and PHU-C10O3, respectively. Furthermore, the values of the basic components are higher than the values of the acidic By plotting γ p against γ d , a bow-shaped curve is obtained, which starts at the origin, reaches a maximum value, and then returns to the horizontal axis. If the contour of the wetting envelopes is known, the wettability of the solvent on the material can be predicted. The material is wetted with the contact angle of the wetting envelope, if the polar and dispersive parts of the liquid lie this curve [32]. Hence, the knowledge of wetting envelope curve allow to predict approximate value of contact angle without need for directly measurement.
For the first time, the SFE of PHU coatings was calculated using the van Oss, Chaudhury and Good method. This model has been known for nearly 30 years and is increasing in importance. However, to the best of our knowledge, there are no publications where the values of PHU SFE calculated with the vOCG approach have been presented. The values of total SFE determined with OW and vOCG approaches differ significantly. This tendency has already been observed in other works for various materials, for example PHU [31], wood [57], and plastics [7], where the values of SFE calculated with OW are slightly higher than those calculated by vOCG. The negligible difference between total SFE calculated with both approaches was observed for NIPU1; the greatest difference is for PHU-C6O2 and is equal to 6 mJ·m −2 .
The Lifshitz-van der Waals component, γ LW , is responsible for non-polar interactions on the surface energy of a solid. The γ LW surface energy component is a result of the London dispersion, the Debye dipole-dipole interaction, and the Keesom orientation [9]. The values of γ LW are greater than the values of the acidic and basic part, γ AB , of SFE.
The values of γ LW varied from 41 mJ·m −2 to 46 mJ·m −2 for PHU-CC6 and PHU-C10O3, respectively, and the values of γ AB varied from 1 mJ·m −2 to 9 mJ·m −2 for NIPU5 and PHU-C10O3, respectively. Furthermore, the values of the basic components are higher than the values of the acidic components, because of the presence of electron pairs of oxygen and nitrogen atoms containing ether, carbonyl, and urethane parts of PHUs macromolecules. However, comparing the acidic components, it can be seen that γ + gives the following tendency: NIPU1>NIPU2>NIPU3>NIPU4>NIPU5, so the γ + increases with the increase of the alkyl chain in diamine, and NIPU1 exhibited the highest "acidic" surface character.
In conclusion, all investigated PHUs exhibited various values of γ AB and γ LW due to various amounts and types of functional groups (ether, carbonyl, and urethane) and free end-groups (hydroxyl) in the PHUs, regardless of their similar structure.

Conclusions
In this paper, the synthesis of non-isocyanate poly(hydroxyurethane)s based on carbonated epoxy resin and various amines and diamines is presented. The obtained PHUs were characterized by FT-IR, 1 H NMR and 13 C NMR spectroscopy and additionally by DSC and TGA.
The obtained PHUs were used as coatings. An extensive study on the surface properties of the PHU coatings was carried out. The surface free energies of the obtained PHU coatings were calculated using two different models: the well-known Owens-Wendt and additionally, for the first time, the van Oss, Chaudhury and Good. The values of contact angles and surface free energy and their disperse and polar components, as well as the Lifshitz-van der Waals and acid-base components, indicate solvent wettability. Moreover, the wetting envelopes of the PHU coatings were plotted, which allows the prediction of the wettability of various organic solvents. It should be highlighted that all the investigated PHU coatings shown water absorption-observed by us as a change in the water contact angle as a function of time-which is related to the great hydrophilic nature of the PHUs. However, the incorporation of hydrophobic chains in the diamine provides an easy way to modify the water affinity of the PHUs.

Conflicts of Interest:
The authors declare no conflict of interest.