Development of Renewable Polyester Resins for Coil Coatings Based on 2,5-Furandicarboxylic Acid

Abstract

In this work, the development of FDCA-based polyester resins for coil coatings in industrial environment is presented. The goal of our research was to prepare industrial coatings made from renewable materials with the same performance as the standard coating. Resins with 1%–41% of FDCA on polymer were synthesized and then used in a formulation for primer. Resins were characterized by the determination of non-volatile matter, acid value, hydroxyl value, glass transition temperature, and measurement of viscosity, color and molecular weight. Coatings were characterized by the determination of viscosity, density, non-volatile matter, adhesion, T-test, MEK test, reverse impact, and pencil hardness, as well as the measurement of gloss. FTIR measurements confirmed successful incorporation of FDCA into the polymer. The results showed that resins with up to 31% of FDCA on polymer can be used to prepare coil coating where the properties of resins comply with the requirements and are comparable to the properties of standard resin. Resins had non-volatile matter between 59.0 and 60.1%, an acid value up to 4.6 mg KOH/g, a hydroxyl value of 22.0–24.9 mg KOH/g and viscosity at 23 °C between 6100 and 7500 mPa.s. Nevertheless, with the increase in FDCA in the formulation, discoloration of the resin occurred and incompatibility with the solvents was observed, while up to 10 °C lower glass transition temperatures and up to 28% lower molecular weights of the resins were determined. For coatings prepared from FDCA-based resins, the properties improved or were comparable to the properties of coating prepared from standard resin. Adhesion improved with higher content of FDCA in the resin from 2 Gt to 0 Gt, while all coatings had gloss at 60° of 39%–41%, a reverse impact of 10 J and a pencil hardness of H/2H. T-bend test results varied between 2 T and 0.5 T and the results of the MEK test showed resistance > 100 DR.

1. Introduction

Renewable monomers can be incorporated into existing formulations by the drop-in replacement of a conventional substance with a chemically identical substance derived from biomass or by substitution with a renewable alternative that is chemically similar to the fossil-based one. Additionally, the mass balance approach, according to the ISSC PLUS certification scheme [1], can be used to increase the content of renewable or recycled materials.

2,5-furandicarboxylic acid (FDCA) is a renewable organic compound that consists of two carboxylic groups attached to the furan ring. In general, it is derived by dehydrating polysaccharides via oxidation of 5-hydroxymethylfurfural (HMF) [2,3,4]. Due to its resemblance to fossil-based terephthalic acid, FDCA can be used to replace terephthalic as well as isophthalic acid in different polymer materials [5]. FDCA is currently the most promising bio-derived diacid with an aromatic character from a commercial point of view [6]. One of the most widely investigated uses of FDCA is in poly(ethylene 2,5-furandicarboxylate) (PEF), a renewable alternative to poly(ethylene terephthalate) (PET) [7]. A typical application of PEF is in packaging for drinks and alcoholic beverages, but it can also be used in films and fibers [5,8]. Attempts have also been made to use FDCA in other polyesters [9,10,11,12,13,14,15], alkyds [16], polyamides [9,17], epoxies [18,19,20], polyurethanes [21,22], fire-retardants [23], nanocomposites [24], and elastomers [25], among others [26]. Due to some limiting factors of FDCA, such as low solubility in certain solvents, a high melting point and a high boiling point, the potential use of 2,5-FDCA esters has also been investigated [27]. In addition to FDCA, other furanic monomers, including furanic-based aldehydes, alcohols, amines, epoxides, carbonates, dicarboxylic acids, esters, acrylates and bi-cyclic monomers, could be derived from HMF and used to prepare bio-based polymers [28].

In this study, FDCA was used in coil coatings, which can be used for manufacturing in the building, architecture, home appliance and automotive fields. They are applied on metal coils in several layers, such as the back coat, primer and topcoat, and put through extensive and long-term testing due to the necessity that they are chemically, mechanically and UV resistant. So far, several renewable monomers, such as isosorbide, FDCA, 1,5-pentanediol, succinic acid, 1,3-propanediol, etc., have been used for coil coatings [13,29] but, to the best of our knowledge, such monomers have never been tested and validated in industrial formulations for coil coatings. FDCA-based resins could also be used for can coating applications. Nevertheless, the resins would have to be tested to confirm this statement.

The goal of this study was to synthesize sustainable industrial resin for coil coatings with an increased share of renewable components. This was achieved by replacing terephthalic and isophthalic acid in our standard industrial resin formulation with FDCA, where the properties of renewable resins and coatings would be comparable to their standard counterparts in terms of mechanical and physical properties. Furthermore, the properties of resins with 1%–41% of FDCA on polymer were synthesized and validated in an industrial chromate-free polyester coil coatings. The different properties of the resins and coatings were determined and compared to standard.

2. Materials and Methods

2.1. Materials

For the synthesis of resins, standard industrial monomers from the market were used: isophthalic acid (99.9%), terephthalic acid (99.8%) and phthalic anhydride (99.9%) obtained from ProChema (Wien, Austria); neopentyl glycol (99.0%) obtained from BASF (Ludwigshafen, Germany); and ethylene glycol (99.9%) obtained from Helm AG (Hamburg, Germany). Additionally, for the synthesis of renewable resins, 2,5-furandicarboxylic acid (FDCA) (99.9%) was obtained from Avantium (Amsterdam, The Netherlands). As a catalyst for color reduction, hypophosphorous acid (50%) obtained from Solchem (Ljubljana, Slovenia) was used. For resin dilution, solvent naphtha 150 (98.0%) and methoxypropyl acetate (99.5%) obtained from Shell Chemicals (London, UK) were used.

The materials used for the preparation of coatings were hexamethoxymethylol melamine resin obtained from Melamin Kočevje (Kočevje, Slovenia), soft acrylic plastification resin obtained from BASF (Ludwigshafen, Germany), dibasic ester (DBE) (99.0%) obtained from Oqema (Kranj, Slovenia), methoxypropanol (99.5%), solvent naphtha 100 (98.0%) and solvent naphtha 150 (98.0%) obtained from Shell Chemicals (London, UK), wetting and dispersing additive obtained from BYK-Chemie (Wesel, Germany), rheology control additive from Evonik (Essen, Germany), talc functional extender obtained from Elementis (London, UK), corrosion inhibiting pigment obtained from SNCZ (Neuville-sur-Escaut, France), titanium dioxide pigment obtained from Kronos (Dallas, TX, USA), amino blocked acidic catalyst obtained from Allnex (Frankfurt am Main, Germany) and internally produced epoxy-phosphate ester resin. All chemicals were used without any further purification.

2.2. Resin Synthesis

The resins were synthesized in an inert atmosphere in a 2 L three-neck glass reactor equipped with a mechanical stirrer, thermometer, condenser, water trap and external heating. Raw materials were charged into the reactor, which was purged with nitrogen. The mixture was heated with constant stirring up to 240 or 200 °C. During the reaction, the acid value and viscosity were checked. Towards the end of the reaction, vacuum was applied to enhance the removal of water. When the required acid value and viscosity values were reached, the resin was cooled down, diluted and filtered through a 190 µm filter. The resin was diluted with a combination of solvent naphtha 150 and methoxypropyl acetate (75/25 or 70/30), or only methoxypropyl acetate.

At first, five polyester resins were synthesized at a temperature of 240 °C: a standard resin (RES-STD)—a saturated polyester resin and that is already commercially produced—which was used as a reference, and four FDCA-based polyester resins, where terephthalic acid was replaced with FDCA in various portions (4.4%, 8.8%, 17.6% and 35.2%) corresponding to 1% (RES-FDCA-1), 2% (RES-FDCA-2), 4% (RES-FDCA-4) and 8% (RES-FDCA-8) of FDCA on polymer. Then, three polyester resins were synthesized at a temperature of 200 °C: first, where 100% of terephthalic acid was replaced with FDCA, corresponding to 22% of FDCA on polymer (RES-FDCA22); second, where 100% of terephthalic acid and 50% of isophthalic acid was replaced with FDCA, corresponding to 31% of FDCA on polymer (RES-FDCA-31); and third, where 100% of terephthalic acid and 100% of isophthalic acid was replaced with FDCA, corresponding to 41% of FDCA on polymer (RES-FDCA-41).

2.3. Resin Characterization

2.3.1. Non-Volatile Matter Determination

Non-volatile matter was determined according to the SIST EN ISO 3251 standard [30] as follows: a dry and clean metal cap (with a metal clip) was weighed, and then 1.0 g of the sample was added and distributed evenly over the cap using the metal clip. The cap with the sample was put in the oven and heated at a temperature of 150 °C for 30 min. For coatings, 1.0 g of the sample was heated at a temperature of 135 °C for 1 h. Then, the cap and the sample were cooled down in the desiccator and weighed. The non-volatile matter (%) was calculated using Equation (1):

$$\mathrm{N}\mathrm{o}\mathrm{n}–\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{e} \mathrm{m}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{r}={\displaystyle \frac{(\mathrm{m}2-\mathrm{m}0)}{\mathrm{m}1}}\times 100$$

(1)

where m0 is the weight of the metal cap (with the metal clip) (g), m1 is the weight of the sample before heating (g) and m2 is the weight of the metal cap (with the metal clip) and sample after heating (g).

2.3.2. Acid Value Determination

The acid value was determined according to the SIST EN ISO 2114 standard [31]. The sample of the resin (0.4 g) was weighted into a 250 mL Erlenmeyer flask. Then, 50 mL of a neutralized solvent mixture consisting of xylene and ethanol (2:1 v/v) was added to dissolve the sample. The solution was titrated with 0.1 N KOH in methanol to the equivalent point using phenolphthalein (1% in ethanol) as an indicator.

The acid value (mg KOH/g of the sample) was calculated using Equation (2):

$$\mathrm{A}\mathrm{c}\mathrm{i}\mathrm{d} \mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}={\displaystyle \frac{\mathrm{V}\times \mathrm{M}\times 56.1}{\mathrm{m}}}$$

(2)

where V is the volume of 0.1 N KOH in methanol required for the titration of the sample (mL), M is the molarity of KOH in methanol and m is the mass of the sample (g) being analyzed.

2.3.3. Viscosity Measurement

During the reaction, the viscosity of the sample was measured using the Brookfield CAP 2000+ viscometer (200 °C, spindle 2, 100 rpm) (Middleboro, MA, USA), as a smaller sample is needed and it consumes considerably less time to obtain the result, while the final viscosity of the final resin was measured using the Rheolab QC Anton Paar rotational rheometer (Graz, Austria) at 23 °C according to the SIST EN ISO 3219 standard [32].

2.3.4. Hydroxyl Value Determination

The hydroxyl value of the resin was determined according to the SIST EN ISO 4629 standard [33]. The sample of the resin (1.0 g) was weighted into a 250 mL Erlenmeyer flask. Then, 5 mL of ethyl acetate was added to dissolve the sample. After dissolving the sample, 5 mL of acetylating reagent was added, and the mixture was heated for 45 min at 50 °C. After cooling, 10 mL of a mixture of pyridine and water (3:1 v/v) was added and left for 5 min at room temperature. Then, 60 mL of a neutralized mixture of toluene and n-butanol (1:2 v/v) was added. The solution was titrated with 0.5 N KOH in methanol to the equivalent point using phenolphthalein (1% in ethanol) as an indicator.

The hydroxyl value (mg KOH/g of the sample) was calculated using Equation (3):

$$\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{l} \mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}={\displaystyle \frac{\left(\mathrm{B}-\mathrm{A}\right)\times \mathrm{M}\times 56.1}{\mathrm{m}}}+\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d} \mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}$$

(3)

where A is the volume of 0.5 N KOH in methanol required for the titration of the sample (mL), B is the volume of 0.5 N KOH in methanol for the titration of the blank solution (mL), M is the molarity of KOH in methanol and w is the mass of the sample (g) being analyzed.

2.3.5. Color Measurement

The color of the resins was measured spectrophotometrically according to the SIST EN ISO 6271 standard [34] using Hach Lico 690 spectral colorimeter (Loveland, CO, USA). A glass cuvette containing a sample was inserted into the instrument, and the color value according to the Gardner scale was read from the screen of the instrument.

2.3.6. FTIR Measurements

FTIR measurements were performed using a Thermo Scientific Nicolet 6700 FT-IR spectrometer (Waltham, MA, USA) in transmission mode (32 scans, resolution of 4 cm⁻¹). Samples were applied in a thin layer on a KBr crystal and scanned in the wavelength range of 3800 cm⁻¹ to 500 cm⁻¹.

2.3.7. Glass Transition Temperature (Tg)

The glass transition temperature (Tg) was determined using the differential scanning calorimetry method using a DSC 1 Mettler Toledo differential scanning calorimeter (Columbus, OH, USA). All scans were performed in a nitrogen atmosphere with a 40 mL/min flow rate, a heating rate of 10 °C/min and a temperature ranging from −50 °C to 200 °C. The amount of the sample was 5.0 mg.

2.3.8. Gel Permeation Chromatography (GPC)

Gel permeation chromatographic (GPC) measurements were performed using a GPC/MDS Agilent (1260 Infinity II) system consisting of an isocratic pump (Agilent G7110B 1260 Iso Pump) and a refractive index detector (Agilent 1260 Infinity II RI detector) (Santa Clara, CA, USA). Three Agilent PLgel columns (PLgel 7.5 × 50 mm, 5 µm, guard; PLgel 3 µm MIXED-E, 300 × 7.5 mm; PLgel 3 µm MIXED-E, 300 × 7.5 mm) connected in series were used. The column temperature was 30 °C. Polystyrene standards were used for the calibration curve. The mobile phase was tetrahydrofuran with a flow rate of 1.0 mL/min.

2.4. Validation of Resins in Coatings

All resins were used in a formulation for white primer and applied on a hot dip galvanized steel (HDGS) panel of 0.6 mm in thickness with a manual film applicator for determination of mechanical properties. Each panel was cured to reach a peak metal temperature of 232 °C (37 s on oven temperature of 320 °C). Then, a light grey commercial topcoat was applied using a manual film applicator and cured to obtain a peak metal temperature of 232 °C (37 s on oven temperature of 320 °C). The targeted dry film thickness was 5–7 µm for the primer and 18–20 µm for the topcoat.

The primer and topcoat formulations used for the validation of resins are our standard industrial formulations for chromate-free polyester coil coatings. The content of the resin in the primer is 51%.

2.4.1. Viscosity Determination

The viscosity of the liquid coatings, expressed as flow time, i.e., the time in which the liquid coating flows from the flow cup, was determined at a temperature of 20 °C according to the DIN 53211 standard [35] using a DIN 4 flow cup.

The flow cup was secured to the stand and leveled into a horizontal position. The orifice of the cup was closed with a finger, and the cup was filled with a liquid coating. Then, a glass plate was drawn horizontally across the rim of the cup to remove any potential meniscus. The finger was removed from the orifice and, at the same time, the measurement of flow time started. The flow time was recorded as soon as the first brake in the stream close to the orifice occurred.

2.4.2. Density Determination

The density of the liquid coatings was determined according to the SIST EN ISO 2811-1 standard [36] at a temperature of 20 °C by using a 50 cm³ metal pycnometer. First, the pycnometer was weighed and then filled with a sample. The mass of the pycnometer filled with a sample was recorded, and the density was calculated using Equation (4):

$$\mathsf{\rho }={\displaystyle \frac{\mathrm{m}2-\mathrm{m}1}{\mathrm{V}\mathrm{t}}}$$

(4)

where m1 is the mass of the empty pycnometer (g), m2 is the mass of the pycnometer filled with the sample (g) and Vt is the volume of pycnometer at the temperature of 20 °C (cm³).

2.4.3. Thickness of the Film

The coatings’ thickness was determined according to the SIST EN 13523-1 standard, Method D—optical method [37]. A conical hole was drilled through the coating down to the metal plate with a special steel drill. Then, the thickness was measured with a measuring microscope.

2.4.4. Gloss of the Topcoat

The gloss of the topcoat was measured at a 60° angle according to the SIST EN 13523-2 standard [38]. The glossmeter was placed on the coated surface, and then the gloss value was read from the screen of the apparatus.

2.4.5. MEK Test

The resistance to solvents (rubbing test) was determined according to the SIST EN 13523-11 standard [39] using methyl ethyl ketone as a solvent. The test panel was double-rubbed and then evaluated to verify whether the coating had been removed and the material underneath could be seen. The result is expressed as the number of strokes after which the coating stays intact.

2.4.6. Adhesion of the Coating

The adhesion after indentation (cupping test) was determined according to the SIST EN 13523-6 standard [40]. Six parallel cuts 1 mm apart were made with the crosscut knife, together with six additional parallel cuts 1 mm apart at a 90 °C angle to the previously made cuts. Then, an indentation of 6 mm in depth was made, with cross-hatching centered on the dome. The tape was placed over the lattice in a direction parallel to one set of the cuts and firmly rubbed to ensure good contact. Within the next 1 min, the tape was removed steadily at an angle close to 60 °C. The result is expressed as the percentage of cross-hatched squares removed after taping.

2.4.7. T-Bend Test

The resistance to cracking on bending (T-bend test) was determined according to the SIST EN 13523-7 standard [41] using the folding method. The test specimen was bent several times in a folding device and thoroughly examined immediately after each bend. The result is expressed as the minimum bending radius to which the test specimen can be bent without loss of adhesion and without cracking.

2.4.8. Reverse Impact

The resistance to rapid deformation (impact test) was determined according to the SIST EN 13523-5 standard [42] measuring the loss of adhesion. The test panel was placed in the apparatus with the coated surface facing downward. The mass was dropped from the height required to provide appropriate energy of impact. The tape was placed over the deformation and firmly rubbed to ensure good contact. Within the next 5 min, the tape was removed steadily at an angle close to 60 °C. The result is expressed as the impact energy at which no loss of adhesion occurs.

2.4.9. Pencil Hardness

The pencil hardness was determined according to the SIST EN 13523-4 standard [43]. The pencil was held at 45 °C to the surface of the coating and pushed forward with a force of 7.5 N downward pressure using a mechanical device. The result is expressed as the hardness of the hardest lead which does not remove the coating for a minimum of 3 mm in length.

3. Results and Discussion

3.1. Properties of the Resins

Renewable content was calculated for all resins, consisting of renewable content according to the C-14 method and renewable content according to the biomass balance (BMB) approach as certified by the ISCC PLUS [1]. In

Table 1. Renewable content of standard and FDCA-based resins.

	RES-STD	RES-FDCA-1	RES-FDCA-2	RES-FDCA-4	RES-FDCA-8	RES-FDCA-22	RES-FDCA-31	RES-FDCA-41
Renewable content C-14 (%)	0	1	2	4	8	22	31	41
Renewable content according to ISCC BMB (%)	24	24	24	24	24	24	24	24
Total renewable content on polymer (%)	24	25	26	28	32	46	55	65
Total renewable content on final resin (%)	14	15	16	17	19	28	33	39

, the total renewable content on polymer and on final resin is shown. Renewable content according to the BMB approach is 24% in all resins, while the renewable content according to the C-14 method varies from 0% in the standard resin with no FDCA in the formulation up to 41% in the resin with 41% of FDCA on polymer in the formulation. The total renewable content on polymer spans from 24% in the standard resin up to 65% in the resin with the highest amount of FDCA in the formulation, while total renewable content on final resin ranges from a minimum of 14% up to a maximum of 39%.

The properties and the appearance of standard and FDCA-based resins are summarized in

Table 2. Properties of standard resin and resins with 1%–8% of FDCA on polymer.

	Requirements	RES-STD	RES-FDCA-1	RES-FDCA-2	RES-FDCA-4	RES-FDCA-8
Non-volatile matter (%)	59–61	59.5	60.1	59.8	59.6	59.7
Acid value (mg KOH/g)	≤5	2.5	3.5	2.5	3.1	4.0
Hydroxyl value (mg KOH/g)	20–28	24.7	24.6	23.8	23.7	24.9
Viscosity Brookfield @200 °C (mPa·s)	1900–2400	1900	1976	2055	2014	2108
Viscosity @23 °C (mPa·s)	5500–7500	6200	5950	6150	6350	7250
Color (Gardner)	≤3	0.1	2.2	3.4	4.5	4.9
Tg (°C)	n/a	42.4	39.4	36.1	38.8	35.8
Mn (g/mol)	n/a	6022	5266	5733	5740	5946
Mw (g/mol)	n/a	11,704	11,478	11,875	11,644	11,621
PDI	n/a	1.944	2.180	2.071	2.029	1.954

and Figure 1. Additionally, in Table 2, the requirements for polyester resins for coil coatings are stated. All synthesized resins had an acid value, hydroxyl value and viscosity within the range required, while the color of the resin strongly deteriorated with increasing amounts of FDCA in the formulation from 0.1 Gardner for the standard resin to 4.9 Gardner for the resin with 8% of FDCA on polymer. Since these resins were synthesized at the temperature of 240 °C, the color deterioration can be attributed to the decarboxylation of FDCA, which appears at temperatures higher than 220 °C and gives side products that contribute to the high color of the resin [44,45]. In another study where the synthesis of polyester using FDCA was performed at a temperature of 215 °C, discoloration of the product with increasing FDCA was also observed. [13] In addition to its dark color, the resin with 8% of FDCA on polymer was hazy as a result of incompatibility with the solvents used for the dilution of the resin. For the resin to become clear, the ratio of the solvents had to be adjusted in such a way that it consisted of less solvent naphtha and more methoxypropyl acetate (70/30). This was achieved by trial and error.

Resins with up to 8% of FDCA on polymer exhibited similar molecular weights, a slightly higher polydispersity index and a Tg that was 3 to 7 °C lower compared to the standard resin.

For the synthesis of the standard resin and resins with up to 8% of FDCA on polymer, the temperature was set to 240 °C due to the better solubility of terephthalic acid at higher temperatures. Our preliminary trials showed that if the temperature is lowered to 220 °C or less to avoid decarboxylation of the FDCA, the solubility of the terephthalic acid becomes very poor. Consequently, all the terephthalic acid was replaced with FDCA, corresponding to 22% of FDCA on polymer, and the temperature of the synthesis was lowered to 200 °C. Additionally, 50% and 100% of the isophthalic acid was replaced with FDCA, corresponding to 31% and 41% of FDCA on polymer.

The properties and appearance of these resins are shown in

Table 3. Properties of standard resin and resins with 22%–41% of FDCA on polymer.

	Requirements	RES-STD	RES-FDCA-22	RES-FDCA-31	RES-FDCA-41
Non-volatile matter (%)	59–61	59.5	59.5	60.1	59.0
Acid value (mg KOH/g)	≤5	2.5	4.6	3.4	3.6
Hydroxyl value (mg KOH/g)	20–28	24.7	24.9	22.7	22.0
Viscosity Brookfield (mPa·s)	1900–2400	1900	1924	2318	2329
Viscosity 23 °C (mPa·s)	5500–7500	6200	6100	6100	7500
Color (Gardner)	≤3	0.1	3.8	3.0	2.3
Tg (°C)	n/a	42.4	38.9	32.2	37.8
Mn (g/mol)	n/a	6022	4974	4691	4362
Mw (g/mol)	n/a	11,704	10,046	9582	8620
PDI	n/a	1.944	2.020	2.043	1.976

and Figure 2.

The results showed that all resins exhibited an acid value, hydroxyl value and viscosity within the requirements. The color of the resins synthesized at 200 °C was strongly improved compared to that of the resins synthesized at 240 °C and was also lower when the isophthalic acid was replaced with FDCA. Nevertheless, all resins were diluted with only methoxypropyl acetate, since even a small amount of solvent naphtha induced incompatibility and the resin became hazy. Furthermore, the resin with 41% of FDCA on polymer crystallized after a few weeks (Figure 3) and could not be used to prepare coating.

Compared to standard resin, resins with 22% to 41% content of FDCA on polymer showed somewhat lower Tg (4 to 10 °C) and lower molecular weights (14 to 26%). Wilsens [45] attributed lower molecular weights when using FDCA to its decarboxylation to 2-furancarboxylic acid, which acts as a chain stopper and limits the molecular weight build-up. However, viscosity did not follow that trend. The increase can be attributed to lower solvation and/or side reactions.

In Figure 4, Figure 5 and Figure 6, FTIR spectra of standard and FDCA-based resins are shown where characteristic peaks of the furan ring are evident. They confirm the formation of ester linkages and the successful incorporation of FDCA into the polymer. Characteristic peaks at 3160 and 3128 cm⁻¹ correspond to the =C-H stretching vibration of the furan ring. The area between 3025–2700 cm⁻¹ is attributed to the stretching vibrations of the C-H bonds. The absorption peak at 1726 cm⁻¹ corresponds to the C=O stretching of the ester linkage and is wider at higher amounts of FDCA on polymer. The absorption bands at 1580 and 1530 cm⁻¹ correspond to the -C=C bending vibration of the furan ring. The bands at 1274 and 1136 cm⁻¹ are the characteristic absorption peaks of the asymmetrical and symmetrical stretching vibrations of the ester C-O-C group. The vibrations at 1220 and 1020 cm⁻¹ correspond to the =C-O-C= furan ring vibration. The peaks at 985, 828 and 765 cm⁻¹ correspond to the =C-H out-of-plane deformation vibration of the furan ring and the peak at 618 cm⁻¹ is attributed to the vibration of the furan ring [46].

3.2. Properties of Coatings

In

Table 4. Properties of liquid coatings made from standard resin and resins with 1%–31% of FDCA on polymer.

	COAT-STD	COAT-FDCA-1	COAT-FDCA-2	COAT-FDCA-4	COAT-FDCA-8	COAT-FDCA-22	COAT-FDCA-31
Viscosity DIN4 20 °C (s)	79	80	81	81	80	80	79
Density 20 °C (g/cm3)	1.217	1.217	1.208	1.207	1.209	1.237	1.233
Non-volatile matter (%)	53.7	53.3	52.0	51.9	51.7	51.7	50.8

, the properties of liquid resins are represented. It can be seen from the table that the viscosity and density of all the FDCA-based coatings were comparable to those of the standard resin-based coating. The viscosity ranged from 79 to 81 s and the density ranged from 1.207 to 1.237 g/cm³. On the other hand, the non-volatile matter decreased for coatings with higher amounts of FDCA in the resin as compared to the standard resin coating. Namely, the non-volatile matter of the standard resin-based coating was 53.7%, while the non-volatile matter of the coating prepared from the resin with the highest FDCA amount, i.e., 31% of FDCA on polymer, was 50.8%, corresponding to a 5.4% drop.

The results of testing the coatings prepared from the standard and FDCA-based resins are presented in

Table 5. Properties of coil coatings made from standard resin and resins with 1%–31% of FDCA on polymer.

	COAT-STD	COAT-FDCA-1	COAT-FDCA-2	COAT-FDCA-4	COAT-FDCA-8	COAT-FDCA-22	COAT-FDCA-31
Film thickness primer (µm)	6	6	7	7	7	7	7
Film thickness topcoat (µm)	18	20	19	20	20	20	18
Gloss 60° topcoat (%)	39	41	40	40	39	40	39
Adhesion topcoat (Gt)	2	1	1	0	0	0	0
T-bend adhesion primer (T)	2	0.5	1	0.5	1.5	1	1.5
T-bend adhesion topcoat (T)	1.5	0.5	0.5	0.5	1	1	1.5
T-bend cracking topcoat (T)	1.5	2	1	1	1.5	2	2
MEK test topcoat (DR)	>100	>100	>100	>100	>100	>100	>100
Reverse impact topcoat (J)	10	10	10	10	10	10	10
Pencil hardness topcoat	H/2H	H/2H	H/2H	H/2H	H/2H	H/2H	H/2H

. Due to crystallization of the resin with 41% of FDCA on polymer, the coating using this resin could not be prepared.

It can be seen from the table that the reverse impact, pencil hardness and MEK test results of the FDCA-based coatings are comparable to those of standard coatings, while the results of adhesion and T-bend test vary between the FDCA-based and standard coatings. The adhesion of the coating was improved with increased content of FDCA in the resin, and the same was observed for adhesion in the T-bend test, while the results for cracking in the T-bend test were somewhat scattered.

Based on these results, we can conclude that coil coatings with improved or comparable properties to the standard can be prepared from resins containing up to 31% of FDCA on polymer.

4. Conclusions

The research showed that FDCA can be used as a renewable replacement for terephthalic and isophthalic acid in polyester resins for coil coating applications. Resins with up to 31% of FDCA on polymer can be used to prepare coil coatings with the same performance as their standard versions. The synthesis results showed that the properties of all the FDCA-based resins were comparable to those of the standard resin and complied with all requirements except for color, which strongly deteriorated with increased amounts of FDCA at higher temperatures. To avoid such strong discoloration, the temperature during synthesis should be lowered from 240 °C to 200 °C, and all terephthalic acid should be replaced with FDCA.

Further studies on weather resistance are needed, and the use of FDCA-based resins for other applications, such as can coatings, should be investigated. Furthermore, the use of FDCA esters should be explored.