Anhydride-Cured Epoxy Powder Coatings from Natural-Origin Resins, Hardeners, and Fillers

Carbon-neutral policy and technological race on the powder coatings market force to develop more advanced, safer, cheaper, and naturally sourced products. To meet the market needs, powder coating compositions and coatings were prepared from safe and natural-origin hardeners, resins, and fillers prepared from rosin, bio-diols, bio-epichlorohydrin, and halloysite, to investigate their thermal, mechanical, and functional properties in comparison with petroleum-based references: cross-linking behavior, glass transition temperature, thermal stability, hardness, cupping resistance, adhesion, chemical resistance, gloss, color, and anti-corrosive behavior in salt chamber. As a result, compositions containing up to 83 wt.% of natural resources, and showing comparable or better properties, as compared to references, were successfully prepared. Their application includes binders for future ecological powder paints for demanding protection of steel substrates.


Introduction
Powder coatings belong to the group of one-component coating materials in the form of fine-grained (<100 µm) powder. They are intended mainly for the protective coating of metal substrates. When a powder paint is applied to a substrate, using special application methods such as electrostatic spraying, the coated detail is heated at high temperature in order to melt the paint and, usually, perform the cross-linking process. Compared to liquid paints, they are characterized by several advantages that can be described as 5E [1]: • "Ecology" (resulting from the lack of volatile organic compounds dangerous to health and the environment); • "Excellence of finish" (excellent properties of the coatings, six times less complaints about painting than in the case of liquid paints); • "Economy" (economy, up to 50% less production costs of powders in relation to other types of paints); • "Energy" (energy efficiency of the production process of powder paints and coatings is lower by approx. 30%); • "Efficiency" (high use of paint, reduction of waste by up to 90%).
The most important challenge in the field of powder coatings is that the coatings market continuously needs better, cheaper, and more ecological products. Due to strong competition between paint producers, the most important thing is offering high-quality products at the lowest possible price. These conditions are met by epoxy-mechanism powder paints, consisting a predominant part of the market [2] however, the increasing costs and formal obstacles, related to the most actual carbon neutrality policy, force to seek biobased alternatives [3][4][5][6][7].

1.
Solid state in ambient conditions with melting temperatures in a range of ca. 100-140 • C, which should be lower than curing temperatures of powder coatings (usually 140-220 • C); 2.
Flexible chains in molecules enabling non-brittle curing of coatings; 3.
The cross-linking mechanism should be as mild as possible to avoid shrinkage and cracking; this requirement is met only by the anhydride mechanism; it is noteworthy that due to considerable toxicity, petroleum-based anhydride-curing powder paints are not developed nowadays, however rosin-based anhydrides are claimed as nontoxic [30].
This work is dedicated to the preparation of such rosin-derived epoxy resins and anhydride hardeners that meet the aforementioned requirements to prepare innovative rosin powder coating materials, and evaluate their mechanical, thermal, and functional properties in comparison with petroleum-based reference samples. The preparation of rosin derivatives included two novel, previously unknown, dianhydrides from rosin and bio-based diols, as well as a rosin-derived epoxy resin [17] obtained in an ecological way, using bio-based epichlorohydrin. Moreover, influence of the addition of a natural filler was also investigated. Such a selection of components allowed to prepare compositions showing extraordinary high level of natural resources content.

Synthesis of Rosin-Derived Dianhydrides
The method of synthesis of maleopimaric acid diesters consists of esterifying rosin with a diol in the presence of an acid as a catalyst and maleinating the resulting rosin diester with maleic anhydride. The esterification was carried out using the Dean-Stark apparatus, at the temperature from 160 to 190 • C, for 24 h in an argon atmosphere, where 1 mole of diol and 2 moles of maleic anhydride are used for 2 moles of abietic acid, the catalyst was p-toluenesulfonic acid in an amount of 10% by weight concerning the abietic acid used. The reaction scheme of rosin conversion into dianhydrides is shown in Figure 1. General synthesis methods of maleopimaric acid (MPA), butylene-1,4-dimaleopimarate (BDR), decylene-1,10-dimaleopimarate (DDR), and maleopimaric acid triglycidyl ester (3GR), and its spectroscopic and some physiochemical properties are presented in Supplementary Data. Two synthesized rosin-derived dianhydride hardeners were reported for the first time, while MPA [26] and 3GR [17] had been reported previously.

Preparation of Natural-Sourced Anti-Corrosive Filler
Anti-corrosive filler was prepared in a two-stage process widely described in our previous work [31]. In the first stage, 100 g of natural halloysite was operated in a demineralized water-loaded ultrasonic bath (35 kHz) at room temperature for 24 h. In the second stage, it was ultrasonically mixed with 8-hydroxyquinoline in demineralized water (35 wt.% suspensions) and stirred using a mechanical agitator in the ultrasonic field. Next, the solvent was evaporated, to transform the dry mass into a fine powder using a ball mill. The resulting filler parameters are: specific surface 17 m 2 ·g −1 , average lumen inner diameter 21 nm, average particle diameter 250 nm.

Preparation of Natural-Sourced Powder Coatings
Each powder coating composition was obtained as follows. Components: resin, hardener and filler were ground into fine powders using a knife mill. Next, they are mechanically pre-mixed in stoichiometric proportions (optionally in presence of 2.5 wt.% of filler) before extrusion via twin-screw (16 mm, l/d = 40) co-rotating extruder Prism 16 (ThermoFisher, Waltham, MA, USA) at 90 • C, 60 rpm. After cooling in ambient conditions the extrudate was ground by knife milling (1000 rpm) and classified using a sieve shaker. A fraction of 45-65 µm was loaded to Optiflex-2 corona-charging gun (Gema, Switzerland), applied uniformly onto steel substrates, and then cured in an oven at 180 • C for 15 min. The measured coatings thickness was 100 ± 15 µm. The acronyms and composition of prepared coatings are summarized in Table 1.

Evaluation Methods of Coating Components
Nuclear magnetic resonance (NMR). The 1 H and 13 C NMR spectra were recorded in CDCl 3 on a DPX-400 Avance III HD spectrometer (Bruker, Bremen, Germany) operating at 400.13 MHz ( 1 H) and 100.62 MHz ( 13 C). TMS was used as an internal standard.
Potentiometric titration with 0.1 M perchloric acid in presence of tetraethylammonium bromide/acetic acid was assigned for the determination of epoxy equivalent weight (EEW) of rosin-derived epoxy resin dissolved in chloroform using T50 device (Mettler Toledo, Worthington, OH, USA) in accordance with EN-ISO 3001 standard [32].

Evaluation Methods of Coating Compositions and Coatings
Rheometry of coating compositions was performed using DHR-1 rheometer equipped with Environmental Test Chamber and 25 mm plate-plate measuring system (TA Instruments, New Castle, DE, USA), gap 1 mm, heating from 90 to 200 • C (heating rate 10 • C·min −1 ) followed by the reaction under isothermal conditions. Differential scanning calorimetry (DSC) was performed on a Q-100 device (TA Instruments, New Castle, DE, USA). About 9-13 mg samples were heated from 25 • C to 240 • C with a heating rate of 10 • C·min −1 , standard aluminum pans were used under nitrogen atmosphere 25 cm 3 ·min −1 .
Thermogravimetric analysis (TGA). The analysis of cured coatings was carried out on a Q-5000 device (TA Instruments, New Castle, DE, USA) in a platinum crucible. Samples between 5 and 10 mg were heated from 25 • C to 1000 • C with a heating rate of 10 • C·min −1 , under air atmosphere, flow rate: air-25 cm 3 ·min −1 , nitrogen (as protective gas)-10 cm 3 ·min −1 .
Ultraviolet-visible spectroscopy (UV-Vis) was done using UV9000s device (Biosens, Warsaw, Poland) for determination of coatings color of samples on 5 cm × 5 cm substrates.
Adhesion of coatings to steel was checked in cross-cut test, according to EN ISO 2409 standard [34], 3 measurements.
Performance in the salt chamber was carried out according to PN-EN ISO 9227:2007 [37] in CorrosionBox 400 (Co.Fo.Me.Gra., Milano, Italy) using an aqueous NaCl solution (concentration of 50 ± 5 g/L) sprayed with compressed oil-free air (100 kPa). The backside and edges of steel QD-46 Q-panels (dimensions: 102 mm × 152 mm) with x-cut paints (according to EN ISO 17872:2007) were protected with a special pressure adhesive tape and mounted at an angle of 20 • vertically. The temperature in a spray cabinet was maintained at 35 • C during the test period of 1000 hours.

Properties of Rosin-Derived Dianhydride Hardeners
To the best of our knowledge, two of the obtained rosin derivatives: butylene-1,4dimaleopimarate and decylene-1,10-dimaleopimarate were prepared for the first time. They were characterized by lower molecular mass than rosin-polycaprolactone dianhydrides prepared by Wang et al. [28] and higher molecular flexibility than dimaleopimaryl ketone synthesized by El-Ghazawy et al. [23]. Besides presented in the previous section 1 H NMR and 13 C NMR prepared rosin dianhydride hardeners were measured and identified by FTIR-ATR. Figure 2 shows the spectra of maleopimaric acid, butyl dimaleopimarate, and decyl dimaleopimarate, respectively. As shown in Figure 2, the broad absorption peak from 3000 to 3500 cm −1 assigned to stretching vibrations of -OH group from -COOH for MPA almost disappeared after esterification in 2BM-MPA and 2DD-MPA. Figure 2 shows the stretching vibrations originating from O-H, C-H, C=C, and C=O bonds. In all spectra, it can also notice the absorption bands in the range 1776-1778 cm −1 , assigned to stretching vibrations of carbonyl group (C=O) bonds. The absorption bands at range 1841-1843 cm −1 , 1777-1779 cm −1 , and 1693-1697 cm −1 are also noticeable, assigned to the stretching vibrations originating from C-H (bending), C=O (stretching), and C=C (stretching) respectively.
The thermal stability of MPA and its diesters were investigated using the TG analysis ( Figure 3). The degradation onset temperature for obtained rosin-diol anhydride curing agents was in the range of 266.4-291.7 • C, depending on the alkyl chain of used diol, meanwhile, the value for MPA was 314.0 • C. Thus, the synthesized maleopimaric acid diesters showed lower thermal stability compared to the maleopimaric acid. Figure 3 shows the temperature of a 5% mass loss of MPA, BDR, and DDR. As it is shown on the TG curves, the thermal stability increased with the length of the alkyl chain.

Natural Resources Content and Thermal Properties of Coatings
The curing process of coating compositions was investigated via rheological measurements heated up from 90 to 200 • C (for 11 min) followed by the reaction under isothermal conditions. Curves of viscosity changed in function of time are shown in Figure 4. They had a characteristic "square root" shape: their viscosity initially decreased below 1 Pa·s before rapid growth to values of ca. 10 4 Pa·s. The time for reach up maximum viscosity values was shorter for compositions from petroleum-based resources (Figure 4a) than for samples containing rosin derivatives (Figure 4b-f). It can be explained by the lower mobility of relatively large rosin-derived molecules that contains rigid abietane structures (see Figure 1) in comparison with the slender and simpler petroleum-based components.
Moreover, it should be noted, that samples modified with natural filler (featured as dash curves) generally showed a slightly longer time to reach 10 4 Pa·s threshold than their unmodified counterparts (solid curves). The use of natural components extended curing times. Nevertheless, all coating compositions were correctly crosslinked in 20-60 min.    Table 1). (a-c) CER-commercial epoxy resin; (d-f) 3GR-rosin-based triglycidyl ether.
Thermal properties investigation included also glass transition temperatures (T g ) as well as temperatures at 5% mass loss (T 5% ). These parameter values are presented in Figure 5 ordered according to the increasing content of natural resources in the coating. It should be elucidated here, that prepared coatings contained from 0 to 83 wt.% of natural resources. This means that they reached the top level of natural resources content in comparison with other studies [17][18][19][20][21][22][23][24][25][26][27][28][29]. It is obvious that this value would not reach 100 wt.%, because of maleic anhydride built in the structure of MPA. Figure 5. Content of natural resources in prepared coatings and their glass transition temperature (T g ) and temperature at 5% mass loss (T 5% ) values.
As can be seen in Figure 5, coatings with natural resources exhibited generally high T g values, closed to bisphenol-A based samples (ca. 100 • C). Interestingly, samples cured with butanediol-based hardener showed noticeably lower T g (ca. 90 • C) whereas the presence of decanediol-based dianhydride increased T g values of samples over 100 • C. It suggested, that BDR was less effective as epoxy curing agent than its commercial competitor CH, in contrast to DDR, which should be even more effective than CH. The influence of halloysite filler on T g of coatings was rather small, but it was easy to find out, that the presence of this natural filler slightly decreased the T g values.
Analyzing the thermal stability of the coatings, in relation to petroleum-based samples, the discussed parameter was improved for most DDR-cured binders, while for samples with BDR it was noticeably deteriorated. T 5% value of 3GR/DDR/H reached 350 • C which meant a significant increment compared to the 300 • C value of reference CER/CH/H sample ( Figure 5). On the other hand, samples crosslinked with BDR exhibited T 5% values in a range of 220-250 • C. This is in accordance with the analysis presented in Figure 3 that showed higher thermal stability of DDR in comparison with BDR. Furthermore, as can be seen in Figure 5, the addition of halloysite increased T 5% values of samples, in contrast to the aforementioned decrement of T g .
Intriguingly, a rosin-based material cured with MPA dianhydride joined with a short ketone bond prepared by El-Ghazawy et al. [23] achieved T 5% significantly below 200 • C, while Wang et al. materials prepared from a bisphenol A epoxy resin and rosin dianhydride joined with a polymeric chain, reached T 5% values up to 364 • C [28]. It may be preliminary concluded, that a higher molar mass of rosin-based dianhydride can improve the thermal stability of samples.

Mechanical Properties of Coatings
These observations of prepared coatings thermal parameters were accompanied by hardness and cupping resistance results, summarized in Table 2. The prepared coatings were characterized by high pendulum hardness values from 145 to 209 a.u. (compared to 230 a.u. the hardness of borosilicate glass surface). The highest hardness was noted for CER/CH commercial references, as well as for samples cured with DDR. The hardness values of the rest of the samples were noticeably lower. Noticeably lower hardness values were recorded for samples cured with BDR hardener. The addition of halloysite slightly increased coatings hardness up to 7 a.u. for 3GR/DDR/H sample. The strongest cupping resistance values (9,1-9,5 mm) characterized petroleum-based coatings. All layers containing rosin derivatives exhibited weaker cupping resistance that varied between 4,8 and 7,5 mm. Two significant observations in the results of the cupping test should be mentioned here. First, among rosin-contained coatings, samples cured with DDR showed stronger cupping resistance than BDR-cured materials. Second, the halloysite in coatings improved their cupping resistance in all cases.
These mechanical test results revealed that BDR-cured samples showed noticeably lower mechanical performance than other samples. On the other hand, DDR-cured samples exhibited interesting (and very acceptable for potential customers) mechanical features. Especially, 3GR/DDR samples showed the highest hardness values and cupping resistance on the level of ca. 80% of these parameter values for more-stretchable petroleum-based counterparts (7,1-7,5 mm vs. 9,1-9,5 mm). The presence of the natural filler was beneficial to all coating systems, improving both their hardness and cupping resistance.

Functional Properties of Coatings
Presented in Table 2 results of adhesion showed that, expect 3GR/CH samples, the prepared coatings exhibited satisfactory adhesion of 0 • -1 • . The best results were noted for CER/CH, CER/DDR, and 3GR/DDR coating systems. The natural filler did not visibly influence the discussed parameter. These results partially correlated with cupping resistance values, because 0 • of adhesion characterized samples with the highest cupping resistance. Such a correlation was particularly visible for the chemical resistance test, whose results were set in Table 2. Mentioned samples with the best adhesion, exhibited also maximum barrier performance against methyl-ethyl ketone (MEK), according to EN 13523-11 standard (400 double rubs) [36]. On the other hand, samples having worse, non-zero adhesion classes were characterized by significantly weaker chemical resistance (between 210 and 270 double rubs).
As was mentioned above, several samples showed outstanding protective behavior against MEK. The other essential test of organic coatings was an anti-corrosive performance, examined in a 1000 h salt spray test. The results are presented in Figure 6. Petroleumbased samples CER/CH and CER/CH/H showed reference anti-corrosive performance: rusty X-scratch with medium trickles and some black corrosion centers. 3GR/CH and 3GR/CH/H samples were characterized by slightly worse anti-corrosive behavior in comparison with the reference samples: probably a significant amount of corrosion products in X-scratch had been caused by insufficient adhesion (see Table 2). All four samples with BDR hardeners exhibited poor anti-corrosive properties. They were delaminated over a large area, combined with extensive underneath corrosion of substrates. On the other hand, the anti-corrosive performance of DDR-cured samples was surprisingly better than the reference coatings. The amount of corrosion products in four DDR-cures samples was noticeably lower. Furthermore, 3GR/DDR and 3GR/DDR/H samples had considerably less corrosion centers than other tested samples. Profitably, the natural filler improved the anti-corrosive performance of all samples, except BDR-cured ones. Therefore, the following factors: good adhesion, presence of DDR curing agent, and modified halloysite filler proved to be beneficial for protective properties of the prepared coatings. It is noteworthy, that a choice of salt chamber test to investigate anti-corrosive features of coatings is preferred by commercial customers more than indirect methods, e.g., electrochemical impedance spectroscopy (EIS). Nevertheless, EIS is appreciated in basic research, so the prepared coatings could be examined via EIS as a part of our future experiments. Investigation of the prepared coatings functional properties was completed by the determination of samples gloss and color, as well. The results are presented in Table 2. Colors of unfilled samples, matching the RAL color chart, were described as shades of yellow, gold, green, beige, grey and brown, while coatings with modified halloysite showed grey, beige, and brown shades. In general, the presence of natural components, both rosin derivatives and halloysite contributed to the noticeable darkening of coatings color. On the other hand, the influence of these components on coatings gloss was not so visible, but measurable. All coatings with halloysite showed reduced gloss in comparison with unfilled samples. Beneficially, all results of color and gloss were within the discretionary acceptability of the potential end user.
Summarizing the functional properties of prepared materials, 3GR/DDR/H, 3GR/DDR, CER/DDR/H, CER/DDR as well as both CER/CH/ references proved the most acceptable for end users compilation of good hardness and adhesion, acceptable cupping resistance, significant protection from corrosion and solvent, as well as acceptable visual properties, that made them applicable not only as primers but also as topcoats in an environment that is not exposed to UV radiation.

Conclusions
Powder coatings components from natural resources, including innovative non-toxic anhydride rosin-based hardeners were synthesized in order to prepare sustainable prototype coatings with natural resources content up to 83 wt.%. Coatings crosslinked with decylene-1,10-dimapeopimarate showed functional, mechanical, and thermal properties competitive with the petroleum-based references. The presence of modified halloysite in prepared coatings gave them extra benefits of improvement of their key properties. From the point of view of an end-user, the most interesting set of properties (in comparison with petroleum-based references CER/CH and CER/CH/H) characterized 3GR/DDR and 3GR/DDR/H samples. They showed several advantages, i.e., natural resources content up to 83% (vs. 0%), 50 • C higher thermal stability, better hardness, and comparable/slightly better anti-corrosive performance in industrial salt chamber. These coatings showed also not deteriorated glass transition temperatures (ca. 100 • C), adhesion (the best possible results), and chemical resistance (the best measurable results). On the other hand, they showed longer cross-linking time (over 40 min), lower gloss, and cupping resistance, nevertheless these three parameters were within the discretionary acceptability of potential end users. In final conclusion, the prepared innovative compositions, especially 3GR/DDR/H, can be strong and ecological replacements of commercial petroleum-based epoxy binders in powder coatings formulation. Formulation of paints from the best prepared binders and anti-corrosive fillers/pigments should be a next step in the development of these promising materials.